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INVESTIGATION OF HYDROBORATION AND METAL—BORON BOND

FORMATION IN EARLY TRANSITION METAL BENT METALLOCENE
SYSTEMS

presented by

DEAN R. LANTERO

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1m chiRC/DdeDmpfi-p.“

INVESTIGATION OF HYDROBORATION AND METAL—BORON BOND
FORMATION IN EARLY TRANSITION METAL BENT METALLOCENE
SYSTEMS

By

Dean R. Lantero

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

1 999

ABSTRACT

INVESTIGATION OF HYDROBORATION AND METAL—BORON BOND
FORMATION IN EARLY TRANSITION METAL BENT METALLOCENE
SYSTEMS

By

Dean R. Lantero

Hydroboration of unsaturated organic substrates under catalytic conditions is
accomplished with the use of transition metal systems. Most of the known transition
metal boron complexes, which are proposed hydroboration intermediates, have contained
a late transition metal often supported with phosphine ligands. The lack of early transition
metal-boron complexes prompted study within the well understood bent metallocene
framework. Judicious use of early transition metals within the bent metallocene
framework, Cp’zM (Cp’ = cyclopentadienyl (Cp) or pentamethylcyclopentadienyl (Cp*);
M = Nb or Ta), has led to accessible early transition metal complexes that display unique
structural connectivity and interesting chemical reactivity.

Two primary synthetic methodologies have been investigated for metal—boron
bond formation. The first method involved the reaction of an anionic metal complex,
{szTaHZLi}x, with a haloborane, B—chlorocatecholborane (Cl—BCat) to form
szTaH2(BCat). NMR spectroscopy revealed the formation of two isomers, exo- and
endo—szTaH2(BCat), in which the BCat fragment occupies the outer and central
positions of the metallocene wedge, respectively. For the first time, group 5 regioisomers

have been successfully separated and structurally characterized.

The second method involves the reaction of olefin hydride complexes
Cp’zMH(n2-CH2=CH(R)) (R = H or CH3) with substituted catecholboranes (HBCat’)
and pinacolborane (HBPin) yielding complexes with the general formulae
Cp’2M(H2B(OR)2) ((OR)2 = Cat’ or Pin). Products unique to the individual metals are
isolated when Cp* ligands are employed. Niobium complexes ligated with Cp* form
metal—boron compounds best described as “borane adducts” of “Cp*szH” while the
tantalum complexes are best described as Tav metal—dihydridoboryls based on NMR
spectroscopy, X—ray crystallography and reactivity.

The Cp based metallocenes also yield compounds whose distinct reactivity and
structural characteristics are dependent upon the transition metal. A new niobium diboryl,
szNbH(BCat)2, as well as szNbH2(BCat) are produced and isolated from the reaction
of szNbH(n2-CH2=CH(R)) with HBCat. Detailed 1H NMR studies of this reaction, at
low temperatures, revealed the presence of an intermediate which isomerized to
szNbH2(BCat) upon warming. Kinetic studies revealed independent pathways operate
for the formation of each niobium boron compound. 1H and 1'B NMR spectroscopy and
X-ray crystallography support the formulation of CpngH(BCat)2 as a Nbv hydrido
diboryl complex. The analogous reaction with the tantalum olefin complex yielded the
aforementioned endo—szTaH2(BCat). Deuterium labeling, utilizing DBCat, provided
valuable insight in the mechanisms operating for these systems.

Cp*2M(H2BCat’) compounds react cleanly with CO forming HBCat’ and
Cp*zMH(CO). The clean reactivity, observed over a range of temperatures, allowed
kinetic studies to be performed. The activation parameters, AH: and A81, are consistent
with a preequilibrium step in the Nb system and a three center intermediate in the Ta
system. Isotopic substitution at the hydride positions yielded an inverse isotope effect

(kH/kD < l) for both metal boron complexes.

ACKNOWLEDGMENTS

A project of this magnitude is simply not complete by one’s self: there is much
time and effort put forth by others that has resulted in a successful conclusion of this
undertaking, and for that I am deeply grateful. I would like to thank Mitch for the
Opportunity to learn chemistry from a gifted scientist and the chance to investigate some
really remarkable compounds. But probably more importantly, teaching the qualities
necessary to pursue the truth in chemistry. Carl, Doug and Baixin were crucial in keeping
the atmosphere in the lab fun and to the other group members who would always offer
advice, make suggestions and listen at times to incoherent ramblings. My many new
friends were instrumental in making graduate school a much more enjoyable endeavor.
Special thanks to the guys in the NMR facility, who were always willing to try an new
experiment and the glass shop guys who always seemed to find a way to get it fixed by
the next day. Finally, and most importantly, to my parents and sister who supported me

through the entire process.

TABLE OF CONTENTS

LIST OF TABLES ............................................................................................................ vii
LIST OF FIGURES .......................................................................................................... viii
LIST OF SYMBOLS AND ABBREVIATIONS ............................................................... xi
CHAPTER 1 ........................................................................................................................ 1
INTRODUCTION ............................................................................................................... 1
Reasons For Studying Metal—Boron Chemistry .............................................................. 1
Metal-Boron and Metal—Carbon Bonds .......................................................................... 2
Synthesis of Metal—Boron Complexes ............................................................................ 3
Metal Catalyzed Hydroboration ....................................................................................... 5
CHAPTER 2 ...................................................................................................................... 12
SYNTHESIS ..................................................................................................................... 12
Reaction of CpgTaHz” with chlorocatecholborane ......................................................... l2
Isotopic Perturbation of Equilibria ................................................................................. 21
Cp*szH(olefin) Chemistry .......................................................................................... 28
Isotopic Perturbation of Equilibria ................................................................................. 35
Synthesis of a Weakly Bound Borane Adduct Complex: Cp*sz(H2BPin) ................. 36
Cp*gTaH(olefin) Chemistry ........................................................................................... 39
szNbH(olefin) Chemistry ............................................................................................ 42
Isotopic Perturbation of Equilibria ................................................................................. 48
Structure of 20 ................................................................................................................ 50
Single Crystal X—ray Structure of 18 ............................................................................. 52
CpgTaH(olefin) Chemistry ............................................................................................. 56
CHAPTER 3 ...................................................................................................................... 63
ELIMINATION CHEMISTRY FROM BENT METALLOCENE SYSTEMS ............... 63

Reactions of Cp*sz(HzBCat’) ..................................................................................... 63

Niobium Rate Law Determination for the Reaction of 4 plus CO ................................ 65
Deuterium Isotope Eeffect ............................................................................................. 68
Tantalum Rate Law Determination for the Reaction of 17 plus CO ............................. 70
Deuterium Isotope Effect ............................................................................................... 71
Mechanism Determination ............................................................................................. 72
Comparison Between 8-H and H2 Eliminations ........................................................... 74
Thermodynamic and Kinetic Data ................................................................................. 76
CHAPTER 4 ...................................................................................................................... 79
EXPERIMENTAL ............................................................................................................ 79
General Considerations .................................................................................................. 79
Synthesis ........................................................................................................................ 80
NMR Tube Reactions .................................................................................................... 85
Niobium Kinetics ........................................................................................................... 90
Tantalum Kinetics .......................................................................................................... 92
Equilibrium Measurements ............................................................................................ 92
Single Crystal X—ray Structural Determinations and Refinement. ................................ 93
APPENDIX A ................................................................................................................. 101
Derivation of Model for Possible endo—l Equilibrium ................................................... 101
APPENDIX B ................................................................................................................. 102
Derivation of the Rate Law for Pathways C and D for the Reaction of 17 plus CO ...... 102
APPENDIX C ................................................................................................................. 104
IR Spectroscopic Data for Cp’2M(H2BX2) and Cp’zMH3 Compounds .......................... 104
REFERENCES ................................................................................................................ 106

vi

LIST OF TABLES

Table 1. 1H NMR data for the hydride ligands of exo—l in toluene-(18. ........................... 25
Table 2. l H NMR data for the hydride ligands of endo-l in toluene—dg. ......................... 27
Table 3. Modeling results for endo—l. .............................................................................. 28
Table 4. Selected bond distances and angles ..................................................................... 34
Table 5. NMR data for the hydride resonances of 4—d0_2. ................................................ 35
Table 6. 1H NMR data for the hydride ligands of 17. ....................................................... 40
Table 7. AAG values for the equilibria: ............................................................................. 68

vii

LIST OF FIGURES

Figure 1. Isolobal relationship between boron and carbon fragments. ............................... 2
Figure 2. Lanthanum hydroamination and hydroboration comparison. .............................. 3

Figure 3. Catalyzed and uncatalyzed reaction of 5—hexen—2—one with catecholborane ..... 5

Figure 4. Two possible hydroboration mechanisms ............................................................ 6
Figure 5. Olefin insertion mechanism. ................................................................................ 6
Figure 6. Proposed ruthenium o—bond metathesis pathway. .............................................. 6
Figure 7. Proposed lanthanum metal—catalyzed hydroboration cycle ................................. 7
Figure 8. CpZZr based hydroboration of alkynes. ............................................................... 8
Figure 9. Bis(mesityl)niobium catalyzed “hydroboration.” ................................................ 9
Figure 10. Titanium catalyzed addition of HBCat to alkenes. .......................................... 10
Figure 11. Deactivated catalyst resulting from ‘high’ catalyst loadings. .......................... 10
Figure 12. Deactivated catalyst resulting from ‘typical’ catalyst loadings. ...................... 11
Figure 13. Catalytic dehydrogenative boration with Cp*zTi(n2—CH2=CH2). ................... 11
Figure 14. Frontier molecular orbitals of the bent metallocene fragment. ........................ 13
Figure 15. Reaction of CpQZrMez with CO. ...................................................................... 13

Figure 16. LUMO of metallocene fragment oriented with chlorocatecholborane to form
an exo isomer (top) and an endo isomer (bottom). ..................................................... 14

Figure 17. 1H (top) and lH{”B} (bottom) NMR spectra of exo—l in C6D6 (500 MHz, 22
°C). .............................................................................................................................. 15

Figure 18. Electron contour mapping diagram for exo—1 (top) and endo—l (bottom). Data
with 29 < 30° were used. ............................................................................................ 16

Figure 19. ORTEP diagram of endo—1. Ellipsoids represent 25% probability for electron
density ......................................................................................................................... 18

Figure 20. ORTEP diagram of exo-l. Ellipsoids represent 25% probability of electron
density ......................................................................................................................... 19

Figure 21. 2H spectrum (top, 46 MHz, toluene, —40 °C) and 1H spectrum (bottom, 500
MHz, IOl-dg, —40 °C) of the hydride region of exo and endo—1410.2 .......................... 23

viii

Figure 22. 1H NMR spectra for the endo hydrogen ligand of exo—l (toluene—dg, 500

MHz) ........................................................................................................................... 24
Figure 23. 1H NMR spectra for the hydride ligands of endo—l (downfield) and the exo
hydrogen ligand of exo—1 (upfield) taken in toluene—(lg (500 MHz). ........................ 24
Figure 24. Chemical shift temperature dependence for the exo hydride of exo—1 ............ 26
Figure 25. Chemical shift temperature dependence for the endo hydride of exo—l. ........ 26

Figure 26. Coupling constant temperature dependence for the hydride ligands of exo—1.27

Figure 27. Chemical shift temperature dependence for the hydride chemical shift of
endo—1. ....................................................................................................................... 27

Figure 28. Formation of 7 leading to the synthesis of 4 .................................................... 30
Figure 29. ORTEP diagram of 12. Thermal ellipsoids are shown at 50% probability. 32
Figure 30. Electron density contour map for 12 ................................................................ 33
Figure 31. ORTEP diagram of 13. Thermal ellipsoids are shown at 50% probability. 33
Figure 32. NMR chemical shift data for 4—d0_2. ............................................................... 35

Figure 33. Possible equilibria that can account for the isotopic perturbation of resonance.
.................................................................................................................................... 36

Figure 34. lH NMR spectrum of the Cp* region of the isolated compound after
recrystallization. ......................................................................................................... 37

Figure 35. Stick figure diagram displaying the different chemical environments of the
Cp* ligands. ................................................................................................................ 38

Figure 36. Ball and stick diagram of 15 after recrystallization of 14 ................................ 39
Figure 37. ORTEP diagram of 17. Thermal ellipsoids are shown at 50% probability. ....41
Figure 38. Dihedral angle comparison between 12 and 17. .............................................. 42
Figure 39. 1H spectrum of the catecholate region for 18 in C6D6 (300 MHz, 22 °C). ...... 43

Figure 40. lH{”B} spectrum selectively decoupled at 64 ppm (top), IH{ HB} spectrum
selectively decoupled at 60 ppm (middle) and H NMR spectrum (bottom) of 18

taken in C6D6 (500 MHz, 60 °C). ............................................................................... 44
Figure 41. lH (top) and 2H (bottom) NMR spectra of 20 taken in toluene—dz; (500 MHz
and -20 °C). ................................................................................................................ 46
Figure 42. Reaction of 19 with HBCat .............................................................................. 47

Figure 43. Equilibrium between a d0 exo—borylhydride and a d2 exo—borane adduct. ..... 49
Figure 44. Limiting valence bond descriptions for 18. ..................................................... 49

ix

Figure 45. Possible structures for 20. ................................................................................ 50

Figure 46. lH (top) and 1H{ llB} (bottom) NMR spectra for 20 and exo—1. ..................... 51
Figure 47. Equilibrium between 2 and 20. ........................................................................ 52
Figure 48. ORTEP diagram of 18 with the hydride omitted. Ellipsoids represent 30%

probability for electron density ................................................................................... 53
Figure 49. Isomers with possible B-H interactions. ......................................................... 55
Figure 50. Boryl site exchange through a symmetrical intermediate. ............................... 56

Figure 51. Boryl site exchange through reductive elimination and oxidative addition of
HBCat. ........................................................................................................................ 56

Figure 52. Hydroboration of free propylene. .................................................................... 58

Figure 53. Deuterium label scrambling prior to propylene insertion in the metal hydride

bond. ........................................................................................................................... 59
Figure 54. Mechanism for endo-1 formation .................................................................... 60
Figure 55. Reactivity of 4. ................................................................................................. 63
Figure 56. Mechanism and rate law of 4 plus CO ............................................................. 65
Figure 57. Plot of l/knbs versus [HBCat’]. ......................................................................... 66
Figure 58. Plot of Ilka), versus 1/CO pressure. ................................................................. 66
Figure 59. Eyring plot for the reaction of 4 plus CO. ....................................................... 67
Figure 60. Plot of 4—d0,2 plus CO. ..................................................................................... 69
Figure 61. Mechanism and rate law of 17 plus CO ........................................................... 70
Figure 62. Eyring plot for the reaction of 17 plus CO. ..................................................... 71
Figure 63. Plot of l7—do,2 plus CO. ................................................................................... 71
Figure 64. Possible pathways for compound 32 formation ............................................... 73
Figure 65. Eyring plot for the reaction of 33 plus CO. ..................................................... 75
Figure 66. Plot of 33—d0.3 plus CO. ................................................................................... 75
Figure 67. Free energy diagram for 4 plus CO at 20 °C .................................................... 76
Figure 68. Free energy diagram for 17 plus CO at 70 °C .................................................. 77
Figure 69. Free energy diagram for 33 plus CO at 70 °C .................................................. 78

BDE

Cat

endo

CXO

HBCat
HBPin

N OESY
ORTEP
Pin

ppb

psi

LIST OF SYMBOLS AND ABBREVIATIONS

Angstrom

Bond dissociation energy

1,2—02C6H42'

wave number

ns-C5H5 (cyclopentadienyl)

nS—C5(CH3)5 (pentamethylcyclopentadienyl)
day

doublet

Deuterium

deuterium substitution (x = number of substitutions)
entropy units

the “inner” position of the metallocene wedge
the two “outer” positions of the metallocene wedge
hour

catecholborane

pinacolborane

coupling constant

multiplet

Nuclear Overhauser Exchange Spectroscopy
Oak Ridge Thermal Ellipsoid Plot

1 ,2—OgC2Me42‘

parts per billion

pounds per square inch

terr butyl

xi

AH: change in enthalpy

ASi change in entropy

Tp hydridotris(pyrazolyl)borate ligand
Avl/z full width at half height

5 ppm (parts per million)

It reduced mass mImZ/(m. + m3)

v frequency

xii

CHAPTER 1
INTRODUCTION

Reasons For Studying Metal-Boron Chemistry

While a wealth of chemistry involving metallacarborane complexes exists, 1
considerably less is known concerning simple boron bonds between transition metals,
particularly when contrasted to the chemistry of metal—carbon, metal-nitrogen and
metal—oxygen bonds. In stark contrast to chemistry of metal-carbon bonds, entire classes
of metalloboron compounds have not yet been discovered. For instance, there are no
examples of homoleptic boryl complexes and only recently has a terminal metal—
borylene been reported.2 This is somewhat surprising since metal alkyl, alkylidene and
alkylidyne compounds are well known, and a terminal carbide has recently been reported
as well.3

The development of metalloboron compounds is of interest a variety of reasons,
both fundamental and practical. A practical reason for studying metal—boron interactions
is their role in metal-mediated/catalyzed hydroboration reactions‘i'6 and cross—coupling
reactions of organic halides and boronic esters.7~8 An understanding of the mechanisms
of these processes can lead to the development of more efficient catalysts. These efforts
have generally focused on the use of late transition metal complexes with little emphasis
placed on early transition metal systems. The reactivity in these systems is potentially
quite different and from a fundamental point of view, the scarcity of characterized metal-
boron complexes provides an impetus for their synthesis. Prior to the start of this work
late transition metal compounds, typically stabilized with phosphine ligands, composed
the majority of the known metal-boryl complexes while few early transition metal

examples had been isolated.

Metal-Boron and Metal-Carbon Bonds

A key difference between boryl and alkyl ligands is the hybridization of B (spz)
compared to C (sp3). Thus, appropriate carbon analogs to boryl ligands are carbenes. A
similar analogy can be drawn between borylene (M=BR) and alkylidyne (MECR)
compounds. These isolobal relationships are displayed in Figure 1. Synthetic strategies
used to stabilize carbenes may translate to boron compounds due to the isolobal analogy
that can be drawn between boron and carbon fragments. One of the most successful
strategies has been the use of hetero—atom substitution on the carbene. The homoleptic
carbene fragment, CH2, is a ground state triplet whereas the hetero—atom substituted

species C(OR)R are singlets.9

0 o
>5—MLn ‘T’ >5—MLn

00 00
05 MLn T’ EB—MLn

Figure 1. Isolobal relationship between boron and carbon fragments.

 

In spite of the general analogy that can be drawn between carbon—containing
fragments and the related boron—containing fragments, a significant difference between
the two atoms that may affect bonding is observed in their electronegativities. The less
electronegative boron atom has a value of 2.0 on the Pauling scale while that of carbon is
2.5. The relatively large difference in the electronegativities between the two atoms will
likely affect the polarity of M—B bonds relative to M—C bonds. Since boron’s
electronegativity approaches typical values for transition elements, M—B bonds should be

more covalent than M—C bonds.

Hydroamination chemistry with lanthanide metals”)''3 can be loosely compared
with hydroborationl‘i-15 chemistry in regard to the electronegativity of the atoms
involved in the bond making and bond breaking processes. The bond between lanthanum
and carbon has a dipole situated such that the lanthanum is electropositive and the carbon
atom electronegative (Figure 2).15 Using electronegativity arguments for the metathesis
of a lanthanum—carbon bond with an amine, a new lanthanum—nitrogen bond is formed
along with an alkane. Similar arguments for the metathesis between a borane and a
lanthanum—carbon bond predict the formation of a lanthanum—hydrogen bond and an

organoborane.

5+ 5— 5+ 5—
\ \

La—C La—C
/: ; /E ;
\: i 5 5/
/N_—H H \
6— 5+ a 5+

Figure 2. Lanthanum hydroamination and hydroboration comparison.

Synthesis of Metal-Boron Complexes

Several synthetic approaches have utilized the Lewis acidic nature of the borane
fragment to generate new coordination compounds. The first successful attempt used
triphenyl boron as the Lewis acid reacting with the Lewis basic lithium triphenylgermane
(equation 1).16 Similar compounds were prepared by reacting tin and germanium anions

with boron triflouride. 17

(Ph)3MLi + (Ph)3B > Li[(Ph)3MB(Ph)3] M = Si, Ge (1)

 

Noth and Schmid reported the first transition metal boryl complexes in which the
boryl group was bound directly to the metal in 1963 (equation 2). The general approach
involved a metathesis of a halo—borane with a metal anion.18 This reaction induced

formation of a metal—boron bond with concomitant loss of salt. The second method used

[MezNhBCl + NaMn(CO)5 —» [MezN]zB—Mn(CO)5 + NaCl (2)

for metal—boron bond formation involves oxidative addition of a borane to the metal.
This method was first utilized in 1975 in the reaction between catecholborane (HBCat,
Cat 5 OzC6H42') and Wilkinson’s catalyst (Cth(PPh3)3) forming a rhodium—boron
bonded species (equation 3).19 However, it wasn’t until 1980 when interest was rekindled

as metal—boryl complexes were postulated as intermediates in catalytic hydroborations of

@1131»,th + HBCat

 

CatBRhHX(PPh3)2 (3)

olefins with catecholborane in rhodium—based systems. A third method for metal—boron
bond formation is a o—bond metathesis pathway. An example of this method of bond

formation is pictured in equation 4.20

 

szNbH3 + HBCat szNBH2(BCat) + H2 (4)

Extreme care must be exercised in characterizing compounds where M—B
bonding is believed to be present. This is highlighted by an early report of the reaction of
CpZWHz with BFyOEtz, which led to the formation of an insoluble compound.21 This
compound was originally postulated as containing a tungsten boron bond. However,
subsequent reinvestigation of this reaction showed that the product actually formed is a

cationic trihydride compound (equation 5).sz23

' ‘ +
Cp\ .\“\\H

/ \H _’ W\ H BF4 (5)

C / H
P Cp

 

 

 

One aspect that hampered early investigations was the scarcity of single crystal
X—ray diffraction studies to confirm the actual bonding nature of the boron-containing
fragment to the metal center. The first such study examined fac—[IrH2(PMe3)3(BC3H14)]
in 1990.24 Following this report, several other groups reported X—ray crystal structures of

metal—boryl containing compounds from groups 540203547

Metal Catalyzed Hydroboration

In 1985, the discovery that Wilkinson’s catalyst mediates olefin hydroboration
revived metal—boron chemistry. In the absence of a catalyst, catecholborane reacts
sluggishly with olefins, even at elevated temperatures. In addition to rate acceleration,
metal-catalyzed reactions can alter the regioselectivity of the B—H addition. For example,
the C—C double bond of 5-hexen—2—one is hydroborated in the catalytic reaction in stark
contrast to the uncatalyzed reaction where the ketone group is hydroborated (Figure 3).28

Since this initial report, metal—catalyzed hydroboration reactions have been extensively

- M

O
\BCm

Rh > CatBWY

0

studied. 1429

 

W + HBCat

O

 

 

Figure 3. Catalyzed and uncatalyzed reaction of 5—hexen—2—one with catecholborane.

There have been two potential pathways proposed to account for rhodium-based
hydroboration catalysis (Figure 4). Hydroboration through the first mechanism occurs
with olefin insertion between a metal—hydrogen bond and subsequent loss of alkylborane.
Hydroboration via the second mechanism occurs with olefin insertion between a metal—

boron bond. Results from theoretical calculations have not clarified the controversy

RZBCZHS L.LRhCl R BC _H5 LIR'hCl

+ L ’ L RQBH kdfl RZBH
LthCl Y LnRhCl JJY
CQH’ il l i
L (\L \Cl \\L
RB—R§ Rfi—R“ H—RM RB—R“
[ML I L I BR; - lh‘L
Cl Cl L C1

+ L H L \ C2114
H transfer '0‘“ T L LS

RaB-fim‘F Run;;r-H
I
Figure 4. Two possible hydroboration mechanisms.

surrounding the two mechanisms.30»3l Olefin diboration by (Ph3P)2RhCl(BCat)2 provides
compelling evidence for olefin insertion into M—B bonds (Figure 5).32 Although
oxidative-addition/reductive—elimination mechanisms are commonly invoked for late

transition metal systems, o—bond metathesis pathways have also been established (Figure

6).33
Ar
+ [L2Cth]2
BCat
Ar
E
i BCat
‘/
CanRh \BC
m BCm //BCm
+ 0th \
H

 

L ’“viu‘MC L'uvfu: ‘ ::BC3I > L ,“,.}{u\

CL CL Me CL H

Figure 6. Pr0posed ruthenium G—bond metathesis pathway.

 

 

 

Most metal-catalyzed hydroboration studies have centered on the late transition
metal systems; however, catalytic hydroboration reactions have been reported for early
transition metal systems. Marks has examined the viability of a
bispentamethylcyclopentadienyl lanthanum compound, Cp*zLa(CH(SiMe3)2), in
hydroboration reactions.15 This compound demonstrated the ability to hydroborate a
wide variety of terminal, terminal or internal disubstituted and trisubstituted olefins with
high anti-Markovnikov regioselectivity (>98% in all cases) and with minimal substrate
hydrogenation. It was not effective, however, for the tetrasubstituted olefin 2,3—
dimethyl—Z-butene. Labeling studies with DBCat exhibited exclusive incorporation (by
NMR) of the deuterium label in the [3—C position of the alkylborane. A proposed scheme

for lanthanum catalyzed hydroboration is shown in Figure 7.

H
Cp2*La+SiMe3
SiMe3
H
G—ch CatB—<'SiMe3
C atB SiMe3
R
CP2*L3_® R
G—Bca‘X Y —/
Cp2*La R

Figure 7. Proposed lanthanum metal—catalyzed hydroboration cycle.

Srebnik has examined the capacity of Schwartz’s reagent, Cp22r(H)(Cl), to

hydroborate both alkynes and alkenes.34 In these studies pinacolborane (HBPin, Pin a

02C2(CH3)42_) was used in place of HBCat for the hydroboration reactions with the major
benefit being that the products are air and water stable and can be purified by column
chromatography, whereas the analogous catecholborane products decompose. The
proposed mechanism of alkyne hydroboration by this catalyst is shown in Figure 8. As in
the lanthanum system described above, the reaction proceeds via insertion of the olefin

between the metal—hydrogen bond.

 

H ZGCzCl

H BPin
Figure 8. Cp2Zr based hydroboration of alkynes.

HBCat can disproportionate to form BH; and B2Cflt3, therefore, when HBCat is
used as the hydroborating reagent, care must be exercised when determining whether B—
C bond formation occurs at the metal center or as a result of “traditional” organic
hydroboration.35 For example, addition of HBCat to cyclohexene catalyzed by (mes)2Nb
(mes = mesityl) does not afford cyclohexylcatecholborane as the major product. Instead,

the di and trialkylboranes are the major organoborane products (Figure 9).36 HB NMR

 

OBCat G
B
OBCm 3

6 mol % thf, 6 h.
(mes)2Nb 25 °C

HBCat : 4v 0—30“

Figure 9. Bis(mesityl)niobium catalyzed “hydroboration."

 

experiments established that the products did not result from disproportionation of
cyclohexylcatecholborane to tricyclohexylborane and B2C3I3. Instead, the alkylboranes
are hydroboration products of cyclohexene and transient BH3 generated by metal—
catalyzed disproportionation of HBCat. In some early metal systems, catalysis is better
behaved and B—C bond formation likely occurs at the metal. Hartwig has shown that
conditions must be carefully controlled in order to achieve true metal—catalyzed
hydroboration.37

Titanocene based systems have also been investigated. Bijpost, et. al., found that
most titanocene complexes are inactive or experience rapid catalyst decomposition.38 In
the investigation of the two titanium complexes, szTi(CO)2 and szTiMez, Hartwig
examined hydroboration of alkynes and alkenes and found near quantitative yields with
good to exclusive anti—Markovnikov regioselectivity.37 A proposed mechanism for
titanium catalyzed addition of HBCat to alkenes that proceeds through a ring—opening 0'—

bond metathetic step is shown in Figure 10.

 

 

r _
C
p». I —— CDT-W
— ~———— 1
BCat / k /
/_/ CP R Cp R
R HBCm

R Cp\

.\‘\\\

Figure 10. Titanium catalyzed addition of HBCat to alkenes.

The deactivated form of the catalyst was isolated and subjected to single crystal
X—ray diffraction studies. Depending on the amount of catalyst loading during the
hydroboration reaction, two forms of deactivated catalyst could be isolated. Under ‘high’
catalyst loading environments, a borohydride type complex can be isolated (Figure 11).37

The second form of the deactivated catalyst can be isolated under a more ‘typical’

Cp

\ .H, O
T"‘\ /,,B/

1
/ ‘H' \0
CP
Figure 11. Deactivated catalyst resulting from ‘high’ catalyst loadings.

catalyst loading. A single crystal X—ray diffraction study of this compound indicated a
titanium(III) borate structure (Figure 12).37 The two deactivated forms of the catalyst
were tested for their viability of hydroborating olefins. Each was able to catalyze the
transformation; however, the rate of the reaction was considerably slower than for the

original complex.

10

C
p\T.,.\\‘\\O/”h. ’0
/ 1‘0/3 [0/0

Figure 12. Deactivated catalyst resulting from ‘typical’ catalyst loadings.

Dehydrogenative boration of alkenes can also proceed through a ring—opening 0'—
bond metathesis pathway with use of a different titanium catalyst, Cp*zTi(n2-CH2=CH2).
Reaction of excess ethylene with various boranes led to formation of vinyl borane
products and ethane with regeneration of the original olefin complex.39°41 Figure 13
depicts the mechanism for this catalytic transformation.

Cp*>

,ri</CH2
\an

C *
>Ti—/ p \ ./\/BEZC6H4

Cp*/ B1=,~,C6H4 H

V E=O,NH

A

(3sz —
Figure 13. Catalytic dehydrogenative boration with Cp*gTi( nz-CH2=CH2).

ll

CHAPTER 2
SYNTHESIS

Reaction of szTaHf with chlorocatecholborane

szTaHg can be deprotonated with n—BuLi to form a bright yellow—orange solid,
{CpgTaHzLi},,,42 which has been shown to be a reactive source of CpgTaHg'. In a
reaction comparable to the addition of chlorocatecholborane to Cp2WH',20 one
equivalent of chlorocatecholborane, dissolved in toluene, was added to a szTaHf slurry
in toluene at -78 °C. Upon warming to room temperature, the suspension gradually
lightened in color from bright yellow—orange to pale yellow with formation of an off-—
white precipitate. The reaction mixture was filtered, and a nearly colorless
microcrystalline precipitate was collected after solvent removal. 1H and HB NMR spectra
of the crude solid strongly indicated that two products were formed. The 1H NMR
spectrum displayed two major cyclopentadienyl resonances, two pairs of catecholate
regions as well as several upfield hydride signals. The llB NMR spectrum also exhibited
two resonances, 5 65 and 5 70. Through fractional crystallization in toluene at —78 °C,
two products, exo and endo-szTaH2(BCat)43 (exo and endo—1) could be separated from
the crude reaction mixture. Formation of regioisomeric products for group 5 metallocene
products is rare, but precedent for formation of endo and exo isomers exists as the
addition of silanes to CpgTaHg gives a mixture of endo and exo isomers observed by 1H

NMR spectroscopy.44

12

 

Y

Figure 14. Frontier molecular orbitals of the bent metallocene fragment.

Regioselective reactivity has been observed in zirconium metallocene chemistry.
CO adds preferentially to the central lobe of the la] LUMO45 (Figure 14) of Cp*ZZer to
generate a product with chemically equivalent hydride ligands.46 In the analogous
reaction between CO and szZrMe2,47 the kinetic product is an exo acyl isomer, which
argues strongly for initial attack of the carbon monoxide molecule to the exo lobe of the
la. orbital (Figure 15). Formation of the boryl complexes can be viewed in terms of
nucleophilic addition of the Ta anion to the boron electrophile. Since the filled Ta 1a]
orbital has electron density distributed along the y and z axis (Figure 14), attack at

distinct lobes can give either endo or exo products. (Figure 16).“

21' "‘\\\Mc -“\\\\Me A ,.\\\\MC

CO ——" Zr —Me

‘Me + \ C ZVMC
\\O 0

Figure 15. Reaction of CpZZrMez with CO.

13

 

0 Ta.
\
¢ Td—H
C I \
0,0,0 P BCat

 

/ I 0,.
Ta . ,B < i : Ta—BC t
.1 Q \ C’ a
Cp H 6‘ o P H

Figure 16. LUMO of metallocene fragment oriented with chlorocatecholborane to form

an exo isomer (top) and an endo isomer (bottom).

For the less soluble isomer, the hydride ligands resonated as a singlet at 5 —4.24,
which integrated as two protons versus the cyclopentadienyl resonance. Assuming that
both the hydrides and the boron ligand are attached to tantalum, the chemical equivalence
of the hydrides requires that the hydrogen atoms bind to the metal in flanking positions
(or exo positions) of the metallocene wedge. This relegates the boryl ligand to the central
position (or endo position) of the metallocene wedge.

The high field region of the more soluble isomer exhibited two resonances that
each integrated as one proton with respect to the cyclopentadienyl resonance with a
doublet at 5 —4.20 (IJHHI = 5.6 Hz) and a very broad “singlet” at 5 -5.15. Selective
decoupling of the resonance at —5.15 ppm resulted in the collapse of the doublet at —4.20
ppm indicating that the two hydrides are coupled to each other. The chemically
inequivalent hydrides should ideally appear as two doublets; however, the broad nature of
the hydride resonance at —5.15 ppm can be attributed to coupling to the quadrapolar 11B

and 10B nuclei. The lH{”B} NMR spectrum (Figure 17) exhibited a well—resolved

l4

doublet at -5.15 ppm, which clearly shows that boron coupling is responsible for the
observed line broadening. The only model consistent with the above NMR data requires
that hydride ligands occupy exo and endo positions and the boryl fragment occupies the

other exo position in the metallocene wedge.

J /-\\s
a—a m—a- --. ‘M’ M a.

-—--——--p ——m.—-- ov-,-—...-— . K.“ g“...— M- -....._,_.-. -.-...,-«-

 

 

.‘ .—.. __._-.-———— M

 

-4.0 -4.2 -4.4 -4.6 -4.8 -5.0 -5.2 -5.4 -5.6 PPM
Figure 17. 'H (top) and 'H{ ”13} (bottom) NMR spectra of exo—l in C6D6 (500 MHz, 22

°C).

The two isomers were also crystallographically characterized, and the proposed
binding modes for the boryl fragments in each isomer was confirmed. In spite of bonding
to heavy tantalum nuclei, the hydride positions were located in difference Fourier maps.

Locating hydrogen atom position proximal to heavy atoms (metals) is inherently
complicated. Ghost peaks, resulting from the thermal motion of heavier atoms, can arise
in positions that are structurally reasonable for hydride ligands. This situation is

exacerbated by including high angle data since the decrease in scattering factor with

15

increasing 0 is most pronounced for hydrogen as compared to heavier elements.48 Should
the peak in question be an artifact of a nearby heavy atom, the peak will often vanish at

lower thresholds of (sin 9y). (Figure 18). The way to circumvent this problem is to use

 

 

 

 

 

 

 

Figure 18. Electron contour mapping diagram for exo-1 (top) and endo—1 (bottom). Data

with 20 < 30° were used.

low angle data where contributions from hydrogen are enhanced. Refinement of the

lower angle data resulted in hydride peaks that were, within experimental error, identical

to the refined hydride positions. Thus, the hydride positions in Figure 18 almost certainly
represent “true” positions, despite the fact that the esd’s for their positions are moderately
large.

The two areas of unaccounted electron density in each contour diagram can be
ruled out as reasonable hydride locations for two reasons. The first is the short Ta—H
bond length required as a result of hydrogen occupation in those positions. The second
reason deals with the actual position of the regions of electron density: each unaccounted
region of electron density is located on the bent metallocene “side” of the wedge.

The solid state structure of the endo isomer (Figure 19) adopts a configuration in
which the plane defined by the O(l)—B—O(2) atoms and the plane defined by the Cp
centroids and Ta are coplanar when viewed down the Ta—B vector. At first glance, there
are two possible valence bond descriptions that fit the crystallographic data: (i) a Tam—
borohydride complex with bridging hydrides between Ta and B nuclei, or (ii) a Tav—
dihydrido boryl complex. The formal bonding scheme of this isomer can be determined
by examining certain aspects of the structure: the tantalum-boron distance, the boron—
hydrogen distances and the H-Ta—H angle. The Ta—B distance of 2.263(6) A is shorter
than the Nb—B distance of 2.411(5) A in Cpng(H2BC3H14), which adopts a borohydride
structure.20 Due to the lanthanide contraction, tantalum and niobium atoms and ions are
nearly identical in size, and thus any differences in bond lengths can be directly
compared. Also, the B"‘H distances (1.75(5) A and 2.11(6) A) are substantially longer
than those typically found in borohydride complexes (B-H = 1.10—1.20 A) and
significantly longer than B"‘H distances in the related niobium compound endo—
szNbH2(BCat) (2) (169(4) A and 1.62(5) A), which has been described as a d0
dihydride boryl compound.20 Lastly, the H—Ta—H angle of the endo isomer (113(3)°) is
closer to Hem—Ta—cho bond angle found in szTaH3 (l25.8(5)°) than the corresponding
angles found in Cpng(H2BCng4) (70(3)°) and 2 (92(2)°).20 Thus, the crystallographic

l7

data favor that of a TaV—boryl complex. This bonding description is further corroborated

by spectroscopic data and chemical behavior of endo—1.

Cp2

 

 

Figure 19. ORTEP diagram of endo—1. Ellipsoids represent 25% probability for electron

density.

The exo isomer (Figure 20) ad0pts a configuration similar in which the plane
defined by the O(l)—B—O(2) atoms is perpendicular to the plane defined by the H(1),
H(2), B and Ta atoms. Fourier maps using data from 26 < 30° reveal two peaks at
chemically reasonable positions for the exo (H(1)) and endo (H(2)) hydride atoms. Again,
two structures can be considered. The first is an authentic TaV—dihydrido boryl

m agostic borane complex. Although the esd’s are

compound, and the second is a Ta
larger for the exo structure, the chemical position of H(l) seems reasonable when
comparing the H(1)—Ta—B angle (120(4)°) and the Ta—H(l) distance of l.76(12) A to the
neutron data for szTaH3 (Hexo—Ta—Hexo = 125.8(5)°, Ta—Hexo = 1.769(8) and 1.777(9)

A).49

18

 

Figure 20. ORTEP diagram of exo—1. Ellipsoids represent 25% probability of electron

density.

The position of H(2) can be viewed as unusual both with regard to its position in
the metallocene wedge and to the boryl ligand. The H(1)—Ta—H(2) and H(2)—Ta—B
angles, 81(6)° and 39(3)°, respectively, and the relatively short B—H(2) distance (1 .45(10)
A) indicates unsymmetric coordination of H(2). The Ta—B distance of 2.30(1) A is only
slightly longer than the endo compound discussed above. Although the sum of the angles
about the boron atom is 360.0(22)°, it has been demonstrated that this value is not
significantly affected by hydrogen interactions.50 The B—H coupling, and other
spectroscopic data, also support the interaction between H(2) and B. Based on
spectroscopic differences between exo—l and a congeneric Nb borane adduct (see below),
exo—1 is best described as a Tav complex where the boron atom interacts with the

adjacent hydride ligand.

19

Factors leading to the observed orientation of the boryl fragment in the solid state
structures are not obvious. If all electronic contributions are ignored, steric based
calculations (SPARTAN) predict that the preferred geometry of endo—1 would not be the
observed eclipsed geometry present in the solid state structure. However, the barriers for
rotation about the Ta—B bond appear to be small so the observed eclipsed geometry can
be accessed fairly easily. The observed boryl geometry of the two isomers could be
explained by interactions between boron and the hydrogen ligands essentially “locking”
the boryl ligand in place. In both cases, filled metal—hydride bonding orbitals with the
proper symmetry could back-bond to the p orbital of boron, forcing a geometry where
the boryl oxygen atoms are eclipsed with the cyclopentadienyl ligands. However, this
explanation appears to be of minor importance for the endo isomer as there is no
evidence of interaction between the hydride ligands and the boron atom of endo—1.

To determine whether a rigid, eclipsed structure is maintained in solution, the C2
symmetry about the Ta—B bond must be broken. With this intent, analogs were
synthesized utilizing ,B—chloro—S-methyl catecholborane. Broadening of the
cyclopentadienyl resonances was observed upon cooling; however, the low temperature
limit for rotation was not observed. Even though the slow exchange limit was not
reached, some rotational information about the Ta-B bond can be deduced. Examination
of the 1H NMR spectra revealed the cyclopentadienyl resonances of exo—1 broadened to a
greater degree than the corresponding resonances of the endo isomer, denoting a higher
temperature for coalescence of the exo isomer relative to the endo isomer. This
observation can be rationalized by considering the interaction of boron with the hydride
ligands. Based on bond lengths, a greater interaction of the endo H with the boron atom
(1.45(10) A) would be expected in exo—1 than for either of the two hydride ligands with
the boron atom in endo—1 (l.75(5) A and 2.11(6) A). The higher coalescence temperature
for the exo isomer is consistent with a larger rotational barrier resulting from a higher

degree of B-H interaction for exo—1 compared to endo—1.

20

Isotopic Perturbation of Equilibria

Isotopic perturbation of equilibrium resulting from deuterium substitution has
been extensively applied in NMR studies of equilibria involving hydrogen transfer.51'53
For two rapidly equilibrating species, isotopic substitution shifts the equilibrium as a
result of the difference between H and D zero—point energies.54 Since the observed
chemical shift is the average of limiting shifts for the equilibrating structures,
perturbation in equilibrium results in variations of observed chemical shifts for different
isotopomers. It is crucial to recognize that large chemical shift differences have been
attributed exclusively to intrinsic isotope effects.55 Thus, a large chemical shift difference
is not rigorously diagnostic for structural equilibration. If the chemical shift of a
resonance in question is temperature independent, exclusive contributions from intrinsic
isotope effects could account for the observed shift. Hence, temperature dependent
chemical shifts provide stronger evidence for equilibrium origins when large isotopic
shifts are observed.

There are two other possible explanations producing temperature dependent NMR
resonance shifts.56 The first possibility is chemical exchange between atoms in
magnetically different environments. In this scenario, a barrier to rotation that equates the
chemical environments must be overcome to produce magnetically equivalent
environments on the NMR time scale. In the fast exchange limit, the resulting chemical
shift will be the average of the two limiting chemical shifts (when the two chemically
distinct sites are equally populated) or a weighted average of the two limiting chemical
shifts when the two chemically distinct sites are unequally populated.

The second possible explanation is the interaction of a molecule with a
paramagnetic impurity in the sample. Temperature dependence can be explained by the
degree of interaction of the molecule with any unpaired electron(s) found in the

paramagnetic impurity. The large magnetic moment of an unpaired electron (—1836 x the

21

nuclear magneton for an electron with g = 2.00)57 can alter the localized magnetic field
thereby inducing a shift in the resonance.

The isotopic perturbation of equilibrium that results from deuterium substitution
has been applied by Hartwig and coworkers to probe B—H interactions in 2—d0_2.20
Equilibrium between boryl and borohydride structures was invoked to account for
consecutive upfield shifts of hydride and deuteride resonances upon deuterium
substitution. Temperature dependence for the chemical shift perturbation was not
demonstrated in this study. Thus, chemical shift differences arising from intrinsic isotope
effects cannot be excluded. A related example involving equilibrium between
Ir(H2)H/IrH3 species has been recently reported by Heinekey and co—workers.58 In this
study, the evidence for chemical shift differences arising from isotopic perturbation of
equilibrium is more convincing. In addition to the temperature dependent shifts for the
H3, HgD, and HDz isotopomers, temperature dependent JH_D values for tritiated
compounds correlate with equilibrium constants extracted from the chemical shift data.

For group 5 metallocene complexes with H and BCat ligands in the coordination
sphere, large chemical shift differences from isotopic substitutions arise when B—H
interactions are inferred from X—ray crystal structures.20»59t60 Spectroscopic data for
structurally characterized compounds show no obvious correlation between binding mode
(boryl-hydride vs. borane adduct) and magnitude or direction of the isotopic shift, which
limits interpretations based on isotopic probes. Although it is not clear whether the
observed shifts arise from perturbation of equilibrium or intrinsic isotope effects in group
5 metallocene complexes, isotopic labeling experiments provide data that can be directly
compared to spectroscopic data of closely related compounds which have been
structurally characterized.

Deuterium isotopomers were synthesized for both exo—1 and endo—1. Figure 21

displays the hydride region of the 1H NMR spectrum. A significant perturbation of the

22

resonance noted for the endo hydride of exo—1 and with little perturbation of the

resonances for the other hydride ligands of the two isomers.

‘1 Cp\ MNBCat

 

endo—Cp2 TaDz (BCat) i l T a' D
I .
II it C p/ D
i i,
1 K, C
i \ /' ‘
,2 \\ /
“fif/ xx,” / \Mg
endo—Cp2 TaHz (Bcat) C p \T 33: C at
\_\ C / \H
\\ p

 

lI .3
JUL 2 A.._______
-3.8 -4r.o -4.2 -4.4 -4.6 -4.8 -5.o -5.2 7-5.4 -516 ppin
Fi re 21. 2H s ctrum (to , 46 MHz, toluene, -40 °C) and 1H s ctrum (bottom, 500
gu P6 P P6

MHz, 101-613, -40 °C) of the hydride region of exo and endo—l—dog.

The temperature dependence on the chemical shift for the hydride ligands of exo-
] and endo—1, was investigated over a broad temperature range. The 1H NMR data for the
hydride ligands of exo—1 and endo—1 is found in Table 1 and Table 2, respectively.
Significantly, the chemical shift for the highest field resonance (the endo hydride ligand
of exo—1) was nearly temperature independent (Figure 22). The resonances of the exo
hydride of exo—1 and the hydrides of endo—1 demonstrated a large temperature
dependence (Figure 23). The only resonance that exhibited a larger isotopic perturbation

of the chemical shift is also the only resonance that exhibited very little temperature

dependence.

23

 

 

 

 

 

 

 

 

 

 

 

 

/¥ \1\\\

MI!» kWh

40 0C /M\
M/ ,\ \M#
H \\\M

0 0C //,‘\

/ \x—
—20 °C
wwwwwmx :W»
—40 °C Ax
L MW vA _ Ml] M _.__w_ inn“ “#1:,“ fi_ _

—60 °C -
W“ ‘- :2.“ .w J \‘x‘A‘ i“::‘: m‘“. z: a, A.
-80 °C 1’“
_-—, C .....-:W’/ jM WW3 - ._

 

 

-5.10 515 -5.20 -5.25 -5.30 -5.35 -5.40 -545 550 55513151“
Figure 22. 1H NMR spectra for the endo hydrogen ligand of exo—l (toluene—r13, 500

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MHz).
6 0 ° C l ,1
' 'l I
‘1 "K _ _ _../"i l|fi‘\_
o i i“ ii
4 0 C i i .‘i i i
____./’ ‘U \- ”fit/i i“
2 0 o C If“: ‘nl ("‘2
#J‘, 5’ liq): i‘
0 ° C ,1":
,I), l
#2: ~ \_
it
o l
-2 O C ; i
:‘l t, {va
fl
—4 0 0 C ‘in
___,./’" KW” g
-6 0 ° C ,1
l t
__../' ‘¥___,_/M¥
[1
'7 8 0 0C j] i
r

-4.00 405 410-415 420 -425 -430 -.435 4.40 -445pprn
Figure 23. 1H NMR spectra for the hydride ligands of endo—1 (initially downfield) and

the exo hydrogen ligand of exo—1 (initially upfield) taken in toluene—dg (500 MHz).

24

Temperature dependence for the coupling constant for the exo hydride of exo-1
was investigated over the same temperature range. There was a —18.2% change in the
coupling constant, from the highest coupling constant value to the lowest value, over the
investigated 140 °C temperature range, but only a -8.0% change from the two
temperature extremes. A coupling constant change of 8.0% over 65 °C was observed for
[TpIr(PMe3)(H2)H]BF4—dg and a coupling constant change of —1.1% over the same
temperature range for [TpIr(PMe3)(H2)H]BF4—d. by Heinekey”,61 These two
experiments, taken together, suggest that the observed perturbation and temperature
dependencies result from intrinsic isotope effects rather than a perturbation of equilibria

between limiting structures.

Table l. IH N MR data for the hydride ligands of exo—l in toluene—dg.

 

Hydride position Temperature (°C) Chemical shift (ppm) Coupling constant (Hz)

 

Exo 60 —4.28 5.50
40 —4.28 5.50
20 —4.28 5.00
0 -4.27 4.50
—20 —4.26 5.06
-40 —4.25 4.96
—60 —4.23 4.90
—80 -4.20 4.88
Endo 60 —5.28
40 -5.29
20 —5.31
O —5.32
—20 —5.33
—40 —5.32
—60 —5.31
-80 —5.29

 

25

 

-4.18 1

-420 {L

-4.22 a

—4.24 a

-4.26 4 o

-4.28 1‘ 9 0

-430 i t I 1
-8O -30 20 7o

 

chemical shift (ppm)

 

temperature (°C)

 

 

 

Figure 24. Chemical shift temperature dependence for the exo hydride of exo—l.

 

~5.27 7

-5.28 o

-5.29 ’

-5.30

-5.31 ‘ o 0

-532 1 . .

-5.33 41 I x 1
-80 -3O 20 7O

chemical shift (ppm)

 

temperature (°C)

 

 

 

Figure 25. Chemical shift temperature dependence for the endo hydride of exo—1.

26

 

5.40
5.201

5.001 . ’ o
4.80]

4.60
4.40
4.20j
4.00 +

5.60 1

coupling constant (Hz)

 

T l 1

-80 ~30 20 7O

temperature (°C)

 

 

 

Figure 26. Coupling constant temperature dependence for the hydride ligands of exo—1.

Table 2. 1H NMR data for the hydride ligands of endo—l in toluene-dg.

 

Temperature (°C) Chemical shift (ppm)

 

 

 

 

60 —4.37
40 -4.34
20 —4.31
O —4.28
-20 —4.22
—40 —4.18
—60 —4.14
—80 -4.10
A 4.05 1
g 4.10 J
3 .4.15 «
E -420 —
I?) ~4.25 ‘
g -4.30 ~
,5 -4.35 -
-4.40 r I a
~80 -30 20 7O
temperature (°C)

 

 

 

Figure 27. Chemical shift temperature dependence for the hydride chemical shift of

endo—l .

The temperature dependence of the chemical shift and the coupling constants for
[TpIr(PMe3)(H2)H]BF4—dz and [TpIr(PMe3)(H2)H]BF4—d. were successfully fit to a
model consisting of two equilibrating species in solution.58 The data for the temperature
dependent nature of the hydride resonance of endo—l can be successfully modeled as two
rapidly equilibrating species in solution, a Tav dihydridoboryl (szTaH2(BCat), A) and a
Tam borohydride complex (CpgTa(H2BCat), B) in the low temperature extreme. While
the model (see Appendix A for complete derivation of the model) does successfully
explain the temperature dependence of the hydride chemical shift, the limiting hydride
resonance for the borohydride structure (Table 3) is not typical for this type of complex
and is therefore not considered to be an accurate representation for this compound in
solution.62 An isotopic perturbation of equilibria explanation does not appear likely for
the observed temperature dependence for exo and endo-l. Intrinsic isotope effects can
also be used to explain the observed temperature dependent nature of the chemical shift.
Contributions to the temperature dependence of the chemical shielding (and therefore the
chemical shift) arise from anharmonic vibrations and rotational distortions have been
shown to be prevalent in gaseous systems.63~64 Other possible explanations are
conceivable but do not appear to be likely.65

Table 3. Modeling results for endo-1.
Temp. (°C) exp. (H2, 6) A (H2, 6) B (H2, 6) calc. (H2, 6) % A % B

 

 

—20 —4.22 —4.19 —4.24 —4.22 44 56
—40 —4.18 —4.15 —4.20 —4.18 44 56
—60 —4.14 —4.12 —4.16 —4.14 43 57
—80 —4. 10 -4.08 —4.11 —4. 10 42 58

 

Cp*szH(olefm) Chemistry
Our inability to cleanly synthesize metal anions of pentamethylcyclopentadienyl
(Cp*) analogs required an alternate synthesis for the Cp* derivatives. In broad terms, the

chemistry of molecules containing the Cp* ligand is similar to those containing a Cp

28

ligand, but there are some important differences that can potentially make the reactivity
of Cp and Cp* compounds distinct. First, the Cp* ligand is bulkier than the Cp ligand
which can affect the sterics of the coordination sphere. Secondly, the electron donating
ability of Cp* is greater than that of the Cp ligand thereby altering the electronic structure
of the metal.66 For these reasons, we developed synthetic routes to Cp*zMHngg
compounds from Cp*gMH(olefin) and borane reagents.

Cp*szH(n2—CH2=CH(Me)) (3) and two molar equivalents of HBCat’
(HB02C6H3—4—'Bu) react to give Cp*szH2BCat’ (4) and the alkylboronate ester,
”PrBCat’. Compound 4 could be crystallized from cold pentane solutions giving
analytically pure material. The compound was formulated as the endo form based on a
single upfield hydride resonance that was observed in the 1H NMR spectrum (—8.19 ppm
Avm = 64 Hz in C6D6) which integrated as two protons versus the methyl protons of the
Cp* ring.

Analysis of the 1H NMR spectrum of the isolated compound is straightforward. A
sharp singlet at 1.85 ppm and was assigned to be the Cp* methyl protons. Another sharp
singlet at 1.30 ppm integrated as nine protons versus the Cp* methyl protons and was
attributed to the ten—butyl methyl protons of the catecholate ring. Three resonances in the
aromatic region each integrated as one proton versus the Cp* methyl protons
corresponding to the hydrogen atoms of the catecholate ring. A single peak was also
found in the “B NMR located at 60 ppm with Avuz = 300 Hz.

Some potential mechanisms that could account for the observed products from the
reaction of 3 with HBCat’ are shown in Figure 28. Pathway A incorporates reactivity
through a propyl intermediate, (5). In this route, HBCat’ oxidatively adds to 5 generating
6. Reductive elimination of the organoborane from 6 generates a monohydride
intermediate, (7), which is common to all three mechanisms. The metal-borane adduct, 4,
forms when 7 is trapped with HBCat’. Alternatively, o—bond metathesis between 5 and

HBCat’ could generate 7 directly, circumventing 6 entirely. Pathway B invokes ring-

29

opening of the metallacyclopropane ring similar to that proposed in reactions of Ti olefin
complexes.37t39v4O Reductive elimination of "PrBCat from 8 generates 7. In pathway C,
metathesis between metal-hydride and B—H bonds generates 9 with loss of H220 Olefin
insertion into the metal—boron bond produces 10, which reacts with H2 to yield 11 that

can reductively eliminate alkylborane to yield 7.

H
3
icy] HBCat' NW .
'M 3 > ,Ma-{BCat
5 PathwayA 6
T "PrBCat

.4-
fig , y H CH % .
\\
% ,//\CH1 W , We»
‘ BCat

3 Pathway B 8 7

 

 

 

 

 

 

 

HBCBI' -H2
M % BCat’ W CECE”
.nBCat’ H2 ._,.\\ CH3
,M\ ——> M ____> M—H -
3%. I/ 3%. CH. @fn
CH3 . J
9 Pathwayc 10 11

Figure 28. Formation of 7 leading to the synthesis of 4.

These mechanisms all assume that B—C bond formation occurs at the metal. A
common problem that plagues hydroboration mechanisms in early metal systems is the
tendency for the metal to effect disproportionation of HBCat to BH3 and B2(Cat)3.35t36
Hence, hydroboration by transient BH3 can masquerade as hydroboration where B-C

bond formation occurs at the metal center. This seems unlikely because B"Pr3 was not

30

observed in the reaction mixture. Nevertheless, participation of BH3 can be definitively
addressed by labeling experiments.

Efforts to probe this transformation by a rigorous kinetic analysis were
complicated by irreproducible rate constants. Therefore, our mechanistic hypotheses were
tested by isotopic labeling and trapping experiments. The reaction of the propylene
hydride compound with catecholborane-d (DBCat) provided considerable information
with regard to the pathways in Figure 28. When the reactions were monitored by NMR,
scrambling of the deuterated borane into the hydride or olefin sites of the olefin
complexes was not observed. Thus, reversible B-H/M—H exchange by interaction
between catecholborane and olefin complex does not occur. Furthermore, some important
conclusions can be drawn from the following observation: the nascent alkylborane
products show no deuterium incorporation. Since hydroboration of olefins by DBCat or
BD3, generated from disproportionation of DBCat, would lead to deuterium incorporation
in the borane, scenarios where the organoborane products arise from hydroboration of
free olefins can be excluded. The isotopic labeling experiment also has significant
consequences regarding the mechanistic possibilities in Figure 28. For pathways B and C,
deuterated borane products would be expected, given that eliminations from dihydride-d]
intermediates are common to both mechanisms.67 Therefore. pathways B and C can be
eliminated.

The fate of the deuterium label is revealed by 2H and IH NMR spectra. For the Nb
complex, 4, deuterium is incorporated at niobium—deuteride and Cp* methyl positions in
a 90:10 ratio (2H NMR). This is also confirmed by integrating the resolvable Cp* methyl
resonances of the do, (I. and d2 isotopomers in the lH NMR spectra. Label scrambling in
the Nb compound is not surprising as deuterium incorporation at the Cp* methyl
positions has been observed when Cp*szHg is treated with D;;.68 This scrambling was
attributed to reversible C-H activation of the Cp* methyl group in 7.68 Similar

participation of 7 in Figure 28 would account for scrambling of the isotopic label in the

31

borane adduct complex. When crystalline samples of labeled 4-d2 were stored at room
temperature for extended periods, the isotopic label at the hydride position gradually
diminished and incorporation at the cyclopentadienyl methyl positions was enhanced ('H
and 2H NMR). The experimental results from the labeling experiment are most consistent

with the mechanism in pathway A.

\
C C28
.‘. 7'

 

Figure 29. ORTEP diagram of 12. Thermal ellipsoids are shown at 50% probability.

Suitable crystals of endo—Cp*2Nb(H2B02C6H3—3—’Bu) (12), prepared from 3 and
HB02C6H3—3—'Bu, were obtained and the molecular structure was determined (Figure
29). Electron density contour maps were generated to see if the isotropically refined
hydride ligands are located in regions of high electron density, and therefore reside in
chemically reasonable positions in the crystal structure. Figure 30 shows the electron
density contour map for 12 corresponding to the low angle data from the collected data

set (26 < 30°). This clearly shows that the hydride atoms are in the center of high electron

32

density regions and at structurally reasonable positions. Isotropic refinement of the

hydrogen atoms resulted in minimal shifts of their positions.

 

 

 

 

(k 3
A [X

Figure 30. Electron density contour map for 12.

 

 

Figure 31. ORTEP diagram of 13. Thermal ellipsoids are shown at 50% probability.

33

Since Cp*ngBH4 (13) is a known and readily synthesized compound, its
structure provides a benchmark for a bona fide borohydride complex. Suitable single
crystals of 13 were grown and the X—ray molecular structure is shown in Figure 31. The
Nb—B length in 12 (2.348(4) A) is identical to the corresponding Nb—B length in 13 and
is significantly longer than M—B distances 17 (Table 4). The metal—hydrogen distances
for 12 (1.63(4) and 186(4) A) are marginally shorter than those found in 13 (179(8) and
188(7) A). The B—Hbddging distances for 13 (1.17(7) and l.16(8) A) lie in the expected
range for B—Hbddging bond distances for niobium borohydrides.69t70 The corresponding
distances for 12 are significantly longer (136(4) and 1.46(4) A) but not as long as the
corresponding distances for 2 (1.69(4) and 1.62(5) A)20 and endo—1 (l.75(5) and 2.11(6)
A).60 The B-H bond distances of 12 are in good agreement with B—H bond distances of
structurally characterized borane heteroatom—dimers.7"74 Also, the H—Nb—H angle for
12 (73(2)°) is about 15° greater than that found in 13 (58(3)°) and about 20° less than the
corresponding angle in 2 (92(2)°)20 and significantly less than the H—Ta—H angle for
endo-1 (1 l3(3)°).60 After comparing the metrical data for compound 12 to structurally
characterized borane adduct, boryl and borohydride complexes, 12 is better formulated as

a Nb"l endo borane adduct.

Table 4. Selected bond distances and angles.

 

 

Compound M—B distance M-H distance B—H distance H-M—H angle
(A) (A) (A) (°)

Cp*zTaH2(B02 2.28(1) 1.69(6) 206(6) 1 13(3)
C6H3—4-'Bu) 1.68(6) 184(6)

Cp*sz(H2B02 2.348(4) 186(4) 1.46(4) 73(2)
C6H3-3—'Bu) 1.63(4) 1.36(4)

Cp*szBH4 2.346( 10) 188(7) 1 . 17(7) 58(3)
1.79(8) l.16(8)

 

34

Isotopic Perturbation of Equilibria

There is also a significant perturbation of resonance in the hydride resonance of 4
when deuteria are introduced in the hydride positions. The hydride/deuterium resonance
shifts to higher field with consecutive substitution of deuteria for hydrogen and the
magnitude of the perturbation is similar to that seen in the perturbation in 2.20 The NMR
data for the temperature dependence of the hydride resonances for 4—do_2 are listed in
Table 5. Figure 32 gives a graphical representation of the data displaying that the
resonances vary with temperature from —60 °C to 40 °C. The fact that the
hydride/deuteride resonances shift to higher field with each deuterium substitution

excludes any equilibrium between degenerate structures.

Table 5. NMR data for the hydride resonances of 4—do_2.

 

 

 

'H NMR data 2H NMR data
Temp (°C) H2 resonance HD resonance HD resonance D2 resonance

mm) (ppm) (ppm) (ppm)

40 -8.20 —8.56 —8.77 —9.17

20 —8.36 —8.77 —8.96 —9.39

0 —8.48 -8.93 —9. 13 —9.58

~20 —8.62 —9. 10 —9.31 —9.79
—40 —8.76 —9.26 —9.49 -9.99
—60 —8.89 -9.43 —9.66 —10. 16

 

 

chemical shift (ppm)

 

 

”10.2 " T I l l 1 m
—70 -50 -30 -10 10 30 50

 

 

temperature (°C)

 

 

Figure 32. NMR chemical shift data for 4—dm2.

35

An equilibrium between a borylhydride structure and a hydridoborate structure
may explain the isotopic perturbation; however, other nondegenerate equilibria could also
account for the observed spectroscopic features. Figure 33 displays the possible equilibria

that are viable.

 

 

 

 

Cp *\ .\\\H \ , Cp*\ ..~“‘\\H
Nb BCat C ~ Nb BCat’
/ ‘ ’
* H */ \
Cp Cp H
Cp*\ ,\\\ \ Cp*\ .~‘ ‘\\
V'kH/BC‘“ ~ ‘ §b\/BCat'
Cp* Cp* H
Cp*\ .\\\H \ , Cp *\ Iii
N b‘ / BCat a ~ Nb / BCat’
/ H ‘H
Cp* Cp*

Figure 33. Possible equilibria that can account for the isotopic perturbation of resonance.

Synthesis of a Weakly Bound Borane Adduct Complex: Cp*sz(HzBPin)

The reaction of 3 with HBPin was investigated to determine if there was a
substantial influence on the nature of the metal—boron interaction based on substitution of
the boryl fragment. The reaction was carried out in an identical fashion to the reaction of
3 with HBCat’. A 1H NMR spectrum of the deep red crude material revealed a singlet
attributed to the Cp* methyl protons, a singlet corresponding to the methyl protons of
pinacol fragment and an upfield hydride resonance. The integrated intensities were
consistent with the formula Cp*sz(H2BPin), (14). The lH NMR spectrum of the
recrystallized material differed significantly from that of the crude product. There were
two completely new Cp* methyl resonances, which integrated in a 1:1 ratio (Figure 34).

The resonance attributed to the methyl groups of the pinacol fragment was absent and the

36

hydride resonance shifted approximately 7 ppm downfield. Additionally, the HB NMR
spectrum for this compound was silent. Elemental analysis of this compound was

consistent with the formula (Cp*ngHN)x.

"1

1 f,

 

“—.m
a.._____,__._———
. _____.
-m—

/ \ 1
// \\. // \

.--’
,._ ,,..— - _— -"_/ \_‘—‘ M-‘______.' s_ fl- / “-———.—._._'

 

 

Y“ I ‘t"‘7" r" 1 ’1'"V“ Y"W’T‘"r 7’ 1 ‘ T ‘W“'" 'V‘fi“f- Y T'T’ "T‘_T 'T"T' '1 "' r_T‘—"“T_‘T"'T‘ VT T "Y_"Y- V‘YTT—‘T—Y—‘T‘ T’T‘Y 'f'W—‘Y" V"T— T—‘T—‘T-T’Tfl—‘Y‘TfimT '

2032(22012m199118 197 L961961941931921N
Figure 34. 1H NMR spectrum of the Cp* region of the isolated compound after

recrystallization.

The lH NMR spectrum can be readily explained by invoking a bridging
dinitrogen dimer of Cp*szH, (Cp*ngH)2—p—N2, (15). While not immediately obvious,
two distinct Cp* resonances should be observed for this compound. From Figure 35,
diastereotopic Cp* ligands are labeled either “A” or “B.” The inequivalency arises from
the relative orientation of the Cp* and hydride ligands. The Cp* ligands labeled “A” are
syn to the hydride ligand on the adjacent metal center while the Cp* ligands labeled “B”
are anti to the hydride ligand on the adjacent metal center. Thus, two distinct Cp*

resonances should be observed in the 1H NMR spectrum.

37

 

Figure 35. Stick figure diagram displaying the different chemical environments of the

Cp* ligands.

In hopes of resolving the N2 binding environment, an attempt to determine the
molecular structure from single crystal X—ray diffraction data was undertaken. A crystal
was selected and a data set was collected. A ball and stick diagram of the compound can
be seen in Figure 36. Unfortunately, the quality of the data did not allow for a full
determination of the crystal structure; however, some comparisons can be made to other
bridging dinitrogen compounds. The N1-N2 distance (1.184(7) A), is longer than the N—
N bond distance in molecular N2 (1.0976 A)75, and is similar to the N-N distances for [.1—
N2 ligands in {Cp*2ZrN2}2-p—N2 (u—N2 distance of 1.182(5) A),76 {Cp*2Ti}2-u—N2
( l.16(1) and l.16(l) A for the two independent molecules)77 and {Cp2Ti(PMe3)}2-p—N2
(1.191(8) A).78 Other group 5 metal compounds with bridging N2 fragments have been
structurally characterized that have significantly longer nitrogen-nitrogen bond lengths
corresponding to N=N79‘87 and N—N38'90 complexes. The Nbl—N 1—N2 angle (177.6(5)°)

and Nb2—N2—N1 angle (177.4(5)°) are nearly linear and the Cp* rings are arranged in a

38

staggered configuration about the two metal centers, also in agreement with the

structurally characterized complexes mentioned above.

A

l
‘t /_/ /\

 

Figure 36. Ball and stick diagram of 15 after recrystallization of 14.

Cp*2TaH(olefin) Chemistry

A toluene solution of Cp*2TaH(n2—CH2=CH(Me)) (16) reacted cleanly with
HBCat’ to generate a single metal containing product and the alkylborane product n—
PrBCat. The isolated metal—containing compound was formulated as endo—
Cp*2TaH2BO2C6H3—4-’Bu (17) based on ‘H and ”B NMR data that were similar to the
niobium derivative. The only noteworthy differences in the 1H NMR spectra are the
lower field (-2. 05 ppm) and the narrower line width (Aw/2: 3 Hz) displayed by the
hydride resonance. The B resonance shifts to lower field in the Ta complex and is
similar to that in endo—l (73 ppm, Avm = 610 Hz). Deuterium labeling studies resulted in
exclusive incorporation of the label in the hydride positions of the complex. Scrambling
from the hydride positions into the methyl resonances of the Cp* ring was not observed
in this compound in contrast to the niobium analog. The isotopic labeling results were

similar to those for the niobium derivative and are consistent with pathway A in Figure

39

28. Unlike the Nb analog, a temperature dependence for the hydride resonance was not

observed over the temperature range investigated (-40 °C to 40 0C).

Table 6. 'H N MR data for the hydride ligands of 17.

 

Temp (°C) Chemical shift (ppm)

 

—40 —2.16
—20 —2.17

0 —2.17
20 —2.17
40 -2.16

 

The structural assignment of the decamethylniobocene derivative raises the
question as to whether the binding of the H2BCat’ fragment resembles that in endo—1 or
12. Single crystals were grown and isolated from a concentrated toluene solution cooled
to -35 °C. The structure was solved and an ORTEP diagram of the molecule is shown in
Figure 37. In the solid state, the boryl fragment is no longer eclipsed with the
pentamethylcyclopentadienyl ligands, in contrast to endo—l mentioned above (Figure 19).
The tantalum—boron distance in this structure (2.28 (1) A) is similar to the Ta—B distance
(2.263(3) A) in endo—1. The boron—hydrogen distances (206(6) A and 184(6) A) are
substantially longer than those found in the Cp niobium compound (1.69(4) A and
l.62(5) A)20 and more closely resemble those found in endo-1 (l.75(5) A and 2.11(6)
A),60 consistent with little boron—hydrogen interaction. The H(1)—Ta—H(2) angle
(113(3)°) is identical within experimental error to the corresponding angle in endo—1
(1 l3(3)°) and is much larger than the angle for 2 (92(2)°).20 The metrical parameters for

17 are consistent with a TaV boryl formulation.

40

 

Figure 37. ORTEP diagram of 17. Thermal ellipsoids are shown at 50% probability.

The indication that there are significant B—H interactions in the niobium
compound which are absent in the corresponding tantalum boryl compound is reinforced
by the orientation of the BCat fragment in the metallocene wedge. To maximize orbital
overlap between the boron p orbital and the hydride ligands a dihedral angle (Cp*ccm—Ta—
B—O) of nearly 0° is required. Figure 38 displays a view down the metal-boron vector of
the two compounds, with the Cp* methyl carbons and the catecholate carbon atoms
removed for clarity. As seen in Figure 38, the dihedral angle for the niobium compound
is nearly 0° (24°) while the same dihedral angle in the tantalum compound is
substantially larger, 188°. Thus, the orientations of the BCat’ fragment are consistent
with the conclusions based on B—H distances. Namely, B—H interactions are present in
the niobium compound and absent in the tantalum analog. Since the geometric
parameters for the Cp*2M fragments are virtually identical, these derivatives differ
primarily in the ligation of the H2BCat’ fragment. Valence bond descriptions for the
niobium and tantalum redox isomers reflect tantalum’s relative preference for a (10

configuration.

41

 

Cp*,NB(H,BCat’) Cp*zTaH2(BCat')
Figure 38. Dihedral angle comparison between 12 and 17.

Cp2NbH(olef'm) Chemistry

Addition of HBCat to a stirred pale yellow toluene solution of Cp2NbH(n2—
CH2=CH2) at room temperature led to the formation of an orange-red solution. During
the course of the 18 h reaction, a bright yellow microcrystalline solid precipitated from
solution. The bright yellow solid was dried after the filtrate was removed by cannula
filtration. The 1H NMR spectrum contained a single cyclopentadienyl resonance along
with an upfield resonance typical of a metal—hydride bond. Two pairs of catecholate
resonances could be identified in the 1H NMR spectrum and the 11B NMR spectrum
contained two distinct resonances. The isolated solid was tentatively assigned as a diboryl
compound, Cp2NbH(Cat)2 (18). The 'H and ”B NMR spectra of the red solid isolated
from toluene evaporation of the mother liquor were identical to those the previously

reported compound, 2.20

42

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MJ ”(MW UL‘MWJ

l lfi I l l T T I T

 

7.0 6.9 6.8 6.7 6.6 65 ppm
Figure 39. 1H spectrum of the catecholate region for 18 in C6D6 (300 MHz, 22 °C).

The hydride resonance in the 1H NMR (C6D6) spectrum of 18 appeared as a very
broad singlet at 5 -7.71 that integrated as one proton versus the cyclopentadienyl
resonance. Four equal intensity multiplets at 6 7.00, 6.93, 6.63 and 6.57 (Figure 39)
suggest that the two BCat fragments are bound to the metal in a non-symmetric fashion.
Coordination of the hydride ligand in an exo position with the boryl fragments bound at
endo and exo positions of the metallocene wedge provides a structural model that is
consistent with the spectroscopic data. A 11B NMR spectrum also revealed two
resonances at 5 64 (Aw/2 = 800 Hz) and 5 60 (Aw/2 = 310 Hz). 'H{”B} NMR
experiments were undertaken in order to assign each of the 1'B resonances. The hydride
resonance was monitored as the boron resonances were selectively decoupled (Figure
40). The broader resonance (5 64) was initially thought to be the result of the endo boryl
fragment since an interaction with hydrogen was thought to broaden the resonance.

However, the hydride peak shape was unchanged when the resonance at 64 ppm was

43

selectively decoupled. The hydride peak did sharpen moderately when the boron
resonance at 5 60 was selectively decoupled. The line width of the boron resonance at 5
60 also decreased when the 2H isotopomer was synthesized. This is consistent with the
relative gyromagnetic ratios of 1H and 2H and confirms the interaction between the boryl
fragment, whose resonance is located at 5 60, and the hydride. As was observed in exo—l,
the HB resonance coupled to the hydride ligand shifts to higher field as compared to the
HB resonance for endo—1. The shift of the resonance coupled to the hydride ligand can
potentially be rationalized by increased shielding of the boron nucleus by the hydride

hgand.

”MN
WWW M ,

-7. 1 -7. 2 -7.3 - 7.4 -7. 5 -7.6 -7 .8 -7. 9 8 0 ppm

 

Figure 40. 1H{”B} spectrum selectively decoupled at 64 ppm (top), IH{ llB} spectrum
selectively decoupled at 60 ppm (middle) and 1H NMR spectrum (bottom) of 18 taken in
C6D6 (500 MHz, 60 °C).

When the reaction of Cp2NbH(n2-CH2=CH(Me)) (19) and one equivalent of
HBCat in toluene—(13 was monitored by lH NMR spectroscopy, propylene gas and traces
of propane were observed at early reaction times. In addition to Cp resonances for 2 and
18, new Cp (5 4.88) and hydride resonances (5 —4.40, s, Av1/2 = 6 Hz and 5—6.00, Avu2 =
64 Hz) were detected in a 10:1:1 ratio. A NOESY spectrum revealed a cross—peak
between the new hydride resonances, and the 1H{ '1B} spectrum showed the resonance at
5 —6.00 was coupled to boron. Two new catecholate resonances were also detected at 5
7.08 and 5 6.83. The spectroscopic data are consistent with the catecholate, Cp, and
hydride resonances belonging to a single intermediate (20) formulated as Cp2NbH2BCat
The llB NMR spectrum indicated a resonance at 5 59 (Aw/2 = 490 Hz). While this may
be due to the boron nucleus of 20, the shift is coincident with that of 2. It has been shown
that chemical shifts in Cp’2NbH2BCat (Cp’ = Cp or Cp*) complexes can be insensitive to
changes in the boron environmentzov59 Hence, l'B chemical shifts for 20 and 2 could be
indistinguishable.

At lower temperatures (—20 °C), a 1:1 molar ratio of HBCat and 19 generates 20
as the major Cp containing product (50% of integrated Cp intensity). Significant
quantities of endo—19 (27%), exo—l9 (12%), and 2 (11%) were also present, but the
intensities of the resonances corresponding to 18 were insignificant. In addition to
propylene, small quantities of propane were detected in the initial spectrum. The ratio
between the sum of the integrations for the Cp resonances of 20 and 2, and the integration
for the propylene methyl resonance is approximately 10:3. The intensities of the Cp
resonances for exo—19 were substantially diminished from its equilibrium value,
indicating preferential propylene loss from this isomer.91 At longer reaction times, the
intensity of the propane resonance increases, and resonances for the diboryl compound
appear. At 90% conversion, the Cp resonance for 18 and the propane methyl resonance
integrate in a 10:7.5 ratio, and the ratio between 2 and 18 was determined to be 1.5:1.

Reliable integration could not be made at higher conversion due to precipitation of 18.

45

The reaction between 19 and one equivalent of DBCat gave 20 with exclusive deuterium
incorporation at 5 —6.21 (2H NMR), which corresponds to the hydride resonance at 5 —
6.00 in the 1H NMR spectrum (Figure 41). Gradually, a statistical distribution between

the positions corresponding to resonances at 5 —4.40 and 5—6.21 was reached.

 

1W rT—YfY—rjv Ivrvvrrrfimrvy v r

.4.2 -4.4 -4.6 .43 .5.0 -5.2 -5.4 -5.6 -5.8 -6.0 -6.2 -6.4 ppm

Figure 41. 1H (top) and 2H (bottom) NMR spectra of 20 taken in toluene—dg (500 MHz

Ivvvv‘vyv‘rvYYY‘Vvvvrvvvv', vvrrfivavvvvrrv VIrvYrerVVYYVVTYVrrverrYj'vrrryvvrirrvvvyvvvrl

and —20 °C).

The products from the reaction of exo and endo—l9 with HBCat are distinct from
those in the decamethylmetallocene Nb systems and the Ta chemistry described above.
Extensive isotopic scrambling in the reaction between exo and endo—19 with DBCat
limits the application of labeling experiments to probe the mechanism. Nevertheless,
Figure 42 provides rationalization for the observed products.92 As in the
decamethylmetallocene mechanism in Figure 28, an alkyl intermediate that arises from
olefin insertion into the metal—hydride bond of endo—l9 is invoked; however, a key
difference between Figure 28 and Figure 42 is the orientation of the borane in the
transition state corresponding to B-H activation. Endo and exo orientations of the BCat
fragment in the transition state give rise to intermediates with the boryl group in endo and

exo positions of the metallocene wedge. In pathway A of Figure 42, B-H activation gives

46

a single intermediate where the boryl ligand is in the endo position. The loss of
regioselectivity for B—H activation for Cp relative to Cp* derivatives parallels that

observed for olefin binding in Cp (endo or exo) versus Cp* (endo) systems.

 

H 1
““j‘Me "P BC @ HBCat 4: a)“
Nb-BCat :__r__>at Nb“H —. Nb:BCat
H % % H
‘ 2
HBCat! Pathway A I

H
gibfm gnu flan HBCat fl.“

 

 

 

 

~— --— Nb Nb + /=
% y % ‘/ % ‘ H Me
Me /
It"; BCat
endo—19 exo-19 20
HBCatl Pathway B
H r -
@ We H
..~\“ _n A“
Niel-3121: _PF_H- rob-Bea. fiB—C‘l Nb—BCat
% at w> % BCat
23 b 24 d 18

Figure 42. Reaction of 19 with HBCat.

In a sequence analogous to the mechanism in Figure 28, alkylborane elimination
from 21 generates 22, which is trapped by HBCat to generate 2. This accounts for the
formation of n—PrBCat in the reaction mixture. This event is clearly metal—mediated
because the uncatalyzed addition of HBCat to propylene is prohibitively slow at ambient
temperature. Direct conversion of 20 to 2 provides an alternative to the hydroboration
route and accounts for the mass balance between 2 and the sum of propylene and n—
PrBCat concentrations.

B—H activation along pathway B generates 23. Propane elimination from this

isomer produces 24 which is trapped by HBCat to form 18. The 10:7.5 ratio for the

47

integration of the Cp protons in 18 to the methyl protons of propane is slightly higher
than predicted from Figure 42. Two potential complications could account for the
deviation from the expected 10:6 ratio. First, small quantities of propane were generated
before 18 was detected. Second, the low solubility of 18 resulted in some precipitation at
later reaction times. Since the solutions were cooled to ensure dissolution of the propane,
it is possible that precipitation of 18 was responsible for the lower ratio of 18 to propane.
Significantly, the potential formation of 18 from 2 and HBCat can be excluded since 18
cannot be independently prepared from 2 and HBCat under similar reaction conditions.
Isotopic Perturbation of Equilibria

1H NMR data revealed that the exo-19 reacts preferentially with HBCat to form
20 and propylene. This is consistent with the previous observation that exo—19 reacts with
CO to generate Cp2NbH(CO), (25), and propylene, while endo—19 reacts with CO to give
the olefin insertion product, Cp2Nb(n—C3H7)CO;93 however, this empirical evidence does
not prove that the mechanisms by which propylene is eliminated in reactions of 19 with
CO and HBCat are necessarily similar.

Compound 20 reacted with CO (100 psi at 25 °C) to yield compound 25.94 When
the reaction was complete, the ratio between 25 and 2 was 1022.2, and residual endo—l9
was converted to the propyl carbonyl complex, Cp2Nb(n—Pr)(CO).93 Independent
reactions of 2 and 18 with CO did not generate 25 under similar reaction conditions.

The 200 ppb upfield shift of the boron—coupled deuteride resonance in 20 raises
the possibility of equilibrium between a d0 exo—borylhydride structure and a d2 exo—
borane adduct structure (Figure 43). Variable temperature NMR experiments indicate
small variations in the chemical shifts for the hydride resonances. If contributions from
the exo—borylhydride isomer to the spectrum of 20 are significant, two—bond coupling
between the hydride ligands should be observed. Since 121"”! cannot be resolved in

spectra of 20 at any temperature, the temperature dependent shifts for the hydride

48

resonances cannot be conclusively attributed to an equilibrium between an exo—

borylhydride and an exo—borane adduct.

 

Cp st Cp sCp
/Nb\ /
H I B—o H ,B—o

\

H \o‘ H \_‘

Figure 43. Equilibrium between a d0 exo—borylhydride and a d2 exo—borane adduct.

Compound 18—d was synthesized and the isotopic shift for the hydride resonance
(180 ppb) is consistent with significant B—H interactions in 18. The magnitude is similar
to isotopic shifts in compounds 2 and exo—1, where B—H interactions are present, and
substantially larger than isotopic shifts in endo—1. The chemical shift for the hydride
resonance was temperature independent between —80 °C and 25 °C (1H NMR: 300 MHz,
Ihf-dg).95

Two limiting valence bond descriptions that are consistent with B—H interactions
inferred from the spectroscopic data are shown in Figure 44. The first is a borane adduct
of Nb'” where the borane coordination is similar to that observed in Cp2Ti(HBCat)2.50
The second is a diboryl hydride structure that is similar in ligand displacement to exo and
endo—1. The spectroscopic data clearly illustrate limitations of inferences based solely on
crystallographic data. Specifically, 1H and '1B NMR spectra clearly show that the boron
environments are chemically distinct, which contrasts the crystallographic symmetry

(discussed below) that requires equivalent boron positions.

Cp ,Cp Cp\ sCp
mi ,Nii
H / \ H’\ / \
\B 13" [B i3
. ,,/ a ,,,
1 ,, O O a O O
0Q J

Figure 44. Limiting valence bond descriptions for 18.

49

Structure of 20

‘ Although we could not isolate 20, sufficient concentrations of this species could
be generated to collect relevant spectroscopic data. Since lH{ llB} spectra prove that the
resonance at —6.00 ppm is coupled to boron, formulations for 20 require two chemically
inequivalent hydride resonances, one of which is boron coupled. Potential structures that
satisfy this requirement are shown in Figure 45. Structures A and B are borane adducts of
Cp2NbH, structure C is the Nbv analog of the Ta compound, exo—l, and structure D is an

O—bound borane adduct of Cp2NbH.

Cp‘ 391) 010‘ gp Cp‘ p Cp‘ gp
Nb Nb Nb Nb
H/ HEB—‘0 H/ \/,H H/ ' \B—O H/ \ /H

.B H \ OTB
s' 1
“Q 0‘ ‘0 “Q O 0

A B C D
Figure 45. Possible structures for 20.

Isomer D can be excluded for the following reasons. First, two resonances are
assigned to the catecholate protons in 20. Four chemically inequivalent resonances would
be expected for the aromatic protons in D96 Second, IJHBI should be similar to that in
HBCat; however, the coupling in 20 is not resolved. Lastly, the high—field shift for the
proton coupled to boron is typical for a compound with a metal hydride interaction, and
atypical for a three-coordinate dialkoxyborane.

Distinguishing between the remaining possibilities is more difficult. The ease
with which borane is eliminated from 20 compares favorably with borane eliminations

from borane adducts, and contrasts the reactivity of group 5 boryl complexes. This

50

circumstantial evidence suggests formulation as a borane adduct (A or B). A closer
examination of the spectroscopic data provides a more rigorous determination.

Since C is analogous to the structurally characterized Ta compound, exo—1, it
should exhibit spectroscopic similarities if it adopted an identical structure.60 Figure 46
shows 1H and IH{”B} spectra for 20 and exo—1. The chemical shifts for the hydride
ligands, and the magnitude of the isotopic perturbation for the upfield resonance, parallel
those in exo-l. The only disparity between the spectroscopic data for 20 and exo—l is that
IZJHHI, which is clearly resolved in exo-1 (121ml = 5.6 Hz), is unresolved for 20. This
difference is significant since similar 121ml values for Cp2MH3 (M = Nb, 9—11 Hz;97v98
M = Ta, 10 H299) predict that IZJHHI should be resolved for structure C. In Cp2NbH3,
normal magnetic coupling is masked by exchange coupling, which has equal magnitude
and opposite sign at certain temperatures.98 The fact that 12./”HI was not resolved in
spectra from —60 °C to 20 °C ensures that 121ml is not similarly obscured for 20. Based

on these observations, isomer C can be discounted.

\BCat

 

 

 

 

 

 

IIIIIIIIIIIIIIIIIIIIIIIIII

4.2 4.4 4.6 4.8 -50 .52 .54 .515 .51 -6.0 42 -6.4ppm 4'0" '32 "3.4464350 52 5.456,,“

Figure 46. 1H (top) and 1H{ ”13} (bottom) NMR spectra for 20 and exo—1.

Having eliminated isomers C and D, the formulations that are most consistent

with the spectroscopic data are A and B. The cross peak between the hydride resonances

51

that is observed in the NOESY spectrum of 20 could arise from nuclear Overhauser
effects or chemical exchange. The latter possibility can be rigorously eliminated since
exclusive deuterium incorporation at 5 —6.21 in the 2H NMR spectrum of 20—d proves
that exchange is slow on the NMR time-scale. The NOESY data are consistent with
some contribution from A, but a cross peak between the hydride resonances would also
be expected if equilibrium between A and B were rapid on the NMR time-scale.

Given that there is no evidence for an equilibrium between isomers A and B, and
the NOESY data support an interaction between the hydride ligands, we favor the exo
structure, A, for 20. The irreversible conversion of 20 to 2 suggests a mechanism
involving isomerization of A to B, followed by B—H oxidative addition. It is not clear
whether the rate—limiting step should be isomerization or oxidative addition. This could
be difficult to probe experimentally; however, evaluation of putative reaction coordinates
by computational methods could provide some mechanistic insight.

Compound 20 persisted in solution and gradually converted to 2. When 2 was
prepared from Cp2NbH3 and HBCat at room temperature, resonances for 20 were not
detected. However, when 2 was heated to 50 °C for 1 h, small quantities of 20 formed.

The equilibrium constant calculated for the equilibrium shown in Figure 47 is Keq =

O. 120. 100
cp\Nb..o,,H Cp\ Cat‘B‘\
\ /
Cp H CP H

Figure 47. Equilibrium between 2 and 20.

Single Crystal X-ray Structure of 18
Single crystals were obtained from a concentrated acetone solution cooled to -—78
°C as an acetone solvate, and the unique quadrant of the full reflection sphere was

collected. Systematic absences were consistent with space groups Cc and C2/c with

52

parallel refinements in both space groups converging to similar values of wR2. However,
abnormal deviations in the thermal parameters for a majority of the atoms of the Cc
refinement suggested the correct space group is CZ/c. An ORTEP diagram of the
structure is shown in Figure 48. In this setting, the boryl ligands are symmetrically related

and the hydride is disordered since the niobium center lies on a C2 axis.

 

Figure 48. ORTEP diagram of 18 with the hydride omitted. Ellipsoids represent 30%

probability for electron density.

The quality of the diffraction data for 18 was marginal. After the final least-
squares cycle, the most intense peak in the difference map was located 1.06 A from B and
1.51 A from Nb. The peak is oriented exo to the B and B(A) positions, and lies near the
plane defined by Nb, B and B(A).101 The location of this peak is not a reliable indication
of the true hydride position since the marginal quality of the data set is further

exacerbated by the C2 crystallographic symmetry at Nb that reduces the hydride intensity

53

to half its normal value. The heavy atom positions are more precisely determined. Hence,
the Nb—B distance and the B—Nb—B(A) angle in compound 18 merit comparison to
values in related structures.

The Nb—B distance (229(1) A) compares to M—B distances in 2 (Nb—B =
2.292(5)}1) and exo and endo-l (exo—l, Ta—B = 230(1); endo—1, Ta—B 2.263(6) A).20.60
The Nb—B distance is shorter than M-B distances in the borane adducts, 12 (Nb—B =
2.348(4) A)59 and Cp2Ti(HBCat)2 (Ti—B = 2.335(5) A).50 The B—Nb—B(A) angle (61°) in
18 is considerably smaller than the B-W—B angle (78°) in Cp2W(BO2C6H3-3,5—t-
Bu)2,102 and approaches the value for B—Ti-B angle (54°) in the Ti borane adduct,
Cp2Ti(HBCat)2. The B—Ti—B angle in Cp2Ti(HBCat)2 is more acute than those typically
found in d2 Cp2MX2 species; however, the authors noted that the angle formed by the
midpoints of the B—H vectors and the Ti center (81°) falls within the range of those
measured for d2 Cp2MX2 compounds.'03 Since the hydride location in 18 is poorly
defined, an analogous argument cannot be reliably applied to 18.104 The O(l)—B—O(2)
plane is rotated by 9.8(7)° from the orthogonal wedge plane.

Compound 18 joins a relatively small family of structurally characterized diboryl
complexes.32t105'Ill The fact that the spectroscopic data for 18 indicate chemically
distinct boryl units initially suggested the three ligands in the metallocene wedge would
adopt a typical geometry for a Cp2MX3 compound, such as that found for Cp2NbH3.49
The chemically distinct boryl environments are accounted for if one boryl ligand
occupies the endo position of the wedge with the remaining boryl and hydride ligand
constrained to exo positions in the metallocene wedge. This situation contrasts the
bonding in niobocene and tantalocene disilylhydride complexes, where the silyl ligands
are chemically equivalent and occupy exo positions in the metallocene wedge“;113
Despite the fact that the hydride position is not resolved in the crystal structure for 18, the
possibility that the hydride bridges the boron atoms can be excluded since resonances

corresponding to a single BCat environment would be expected in 1H and HB NMR

54

spectra. The crystallographic equivalence of the boryl ligands in 18 was not anticipated
and it is somewhat surprising that the presence of an adjacent hydride ligand apparently
has no structural consequences.

Two limiting valence bond descriptions that are consistent with B—H interactions
inferred from the spectroscopic data are in Figure 49. Isomer E is a borane adduct of
Nb'”, where borane coordination is similar to that observed in Cp2Ti(HBCat)250 and

Cp*2Nb(H2BCat). Isomer F is a borylhydride structure that compares to exo—1 and 2

Cp Cp Cp Cp

\

‘0’”,

w/ O— “I’,’//

H/ Nb
H/\N 3V... H\W/ \B
V 7°—

E F

BY

\\\““'

Figure 49. Isomers with possible B—H interactions.

We examined the chemical reactivity of 18 to determine whether its behavior was
more consistent with formulation as a Nb"l borane adduct or as a Nbv diborylhydride
The borane adducts, Cp2Ti(HBCat)2 and 4, eliminate borane readily to generate the
trapping products Cp2Ti(CO)2 and Cp*2NbH(CO)”4 (26) when CO is present. Under
Similar conditions, HBCat elimination from exo and endo-1 and 2 is prohibitively slow
When the reaction between 18 and CO (100 psi, 50 °C, 12 h) was examined, 25 and
HBCat were not detected.

Additional information regarding potential elimination reactions is provided by
H and HB NMR data. There is no evidence for boron site exchange via a symmetrical
1ntermediate arising from B—H reductive elimination/oxidative addition since coalescence

of BCat resonances is not observed (Figure 50). Rapid HBCat reductive elimination and

55

CP\ s‘H Cp\ .\\\B Cat Cp\ WQB Cat
NE—BCat :— Nb°—H Nb"—BCat
/ \BC / \BC \H

Figure 50. Boryl site exchange through a symmetrical intermediate.

stereospecific B—H oxidative addition, shown in Figure 51, would not lead to
coalescence; however, this possibility can be eliminated by the fact that chemical shifts

for the BCat resonances are not perturbed when HBCat is added to solutions of 18.

C C

p\ s)“ p\ °

/Nb<BCat ; Nb—BCat + HBCat
Cp BCat C p

Figure 51. Boryl site exchange through reductive elimination and oxidative addition of

HBCat.

The valence bond description for the Nbv diborylhydride structure is favored for
two reasons. First, the Nb—B bond distance is similar to M—B bond distances in other
group 5 boryl complexes, and shorter than M-B distances in borane adducts of Nb and
Ti. For the borane adduct structure, we would anticipate that the Nb—B distance in 18
would be similar to those for d2 Nb and Ti complexes. Second, we do not observe borane
elimination chemistry that would be expected for this isomer. Although correlations
between structure and reactivity of complexes with the generic formulae
Cp’2MHx(BCat)3-x (x = 1, 2) can be rationalized in valence-bond terms, there are obvious
limitations to applying valence—bond models in systems with three—center/two—electron
interactions.

Cp2TaH(olefin) Chemistry
Due to the unexpected reactivity of exo and endo-19 seen above, the analogous

tantalum complex was also investigated. For the niobium olefin complex, rotation around

56

the niobium—olefin bond must be rapid relative to boration of Cp2Nb(iPr) since secondary
alkyl products are not observed. For the tantalum analog, the barrier for rotation is
considerably higher. Because of this, secondary boration products might be observed for
the tantalum system if olefin rotation is slower than rates of boration of the secondary
alkyl intermediate. In contrast to the niobium analog, only endo-Cp2TaH(n2—
CH2=CH(Me)) (27) is produced upon reaction of Cp2TaCl2 and n—propylmagnesium
chloride, and not the exo/endo mixture. Instead, an isomeric mixture (41% exo, 59%
endo) can be generated by irradiating a toluene solution of pure endo—27 under an
atmosphere of propylene gas for 4 h using a Rayonet reactor equipped with 300 nm
lamps.

An isomeric mixture of exo and endo—27 reacted with HBCat to yield endo—l and
n-PrBCat as the major products. Although traces of the other possible regioisomer, exo-1
were present in the reaction mixture, endo—l could be isolated in 78% yield. When the
reaction was monitored by 1H NMR, the intensity of resonances assigned to endo—27
diminished leaving exo—27 as the predominant isomer. When the reaction was complete,
exo-27 was also consumed. The only organic hydroboration product observed was the
anti—Markovnikov product, n-PrBCat. Presumably the exo isomer isomerizes to the endo
form which then reacts with HBCat, accounting for the single hydroboration product.

The analogous reaction employing DBCat gave endo—1410.2 and n—PrBCat.
Integration of a 2H NMR spectrum gave a 40:60 ratio for deuterium incorporation into the
alkylborane and the deuteride resonances of endo—l-d._2, respectively. For the
alkylborane product, the deuterium label is almost exclusively incorporated at the a-
position. When the reaction was monitored by 2H NMR, exchange into the hydride
positions of exo and endo—27 was not detected.

Clearly, HID exchange occurs by some mechanism in the Ta system. Three
potential mechanisms could account for the exchange, and by examining the

experimental data the likelihood for these possibilities can be assessed. First, potential

57

hydroboration of free propylene by DBCat or transient B2D6 must be addressed (Figure
52). Either possibility should give alkylborane products with deuterium incorporation at
the B—positions. Therefore, a—deuteration cannot be reconciled by uncatalyzed additions.
Clearly, the experimental data are inconsistent with hydroboration of uncoordinated

olefin.

H

C T'/

[’2 d\/__
/

endo-27

 

C pzTa/\
6
exo—27

Figure 52. Hydroboration of free propylene.

In the second scenario (Figure 53), scrambling between B—D and Ta—H positions
in 27 could occur prior to propylene insertion into the Ta—H/D bond. Incorporation at the
a position could be accounted for by B—D/Ta—H scrambling in exo-27. Insertion of
coordinated propylene into a Ta-D bond gives an isopropyl intermediate with deuterium
at a methyl position. If rotation about the Ta—C bond is sufficiently rapid, fi—H
elimination from this intermediate will give exo—27—d. with deuterium incorporation at a
and ypositions.115 For endo—27, propylene insertion into a Ta—D bond generates an n—
propyl intermediate with deuterium at the 6 position. Subsequent B—D elimination from
the propyl intermediate will generate the starting isotopomer with a Ta—D bond, while 5—

H elimination generates the Ta—H isotopomer with deuterium at the propylene B

Siteilsilo

58

,BCat
I?’ '.

W t —HBC3I i Cp_Ta/ C T
cpzTé’R’j/ 7 \/ p2 aVK

 

 

H /D

D
/
C 0T3 C Ta
szTa/ p“ B/\ p2 \/

\//-

endo-27

szTa
/H c T /H
CPZTa [)2 a\

( can
DBCat \ /
. .13 Cat
' CH2D

. ,H D
CP2T3’\ — HBCat #- szTa/\ Cp2Ta <
/ / CH3

exo—27

 

 

Figure 53. Deuterium label scrambling prior to propylene insertion in the metal hydride

bond.

The previous pathway can be excluded as a significant contributor in the
deuterium scrambling process. Although the small quantity of fi—deuterium incorporation
in n—PrBCat—d could result from Ta—H/B—D exchange, this mechanism does not account
for predominant incorporation at the a site since a nearly statistical distribution of the

label between a and ysites of coordinated propylene would be anticipated. Furthermore,

59

exchange between Ta—H and B—D positions in the propylene hydride complexes should
result in significant deuterium incorporation at the hydride positions in exo and endo—27.
This possibility was readily tested by using 2H NMR to monitor the reaction as the
deuteride shifts for exo and endo—27 and the boryl products exo and endo—1 are clearly
resolved. Deuterium exchange into the hydride or propylene sites of exo and endo—27
was not detected. Since selective deuterium incorporation at the a—position and an
absence of deuterium incorporation into the exo and endo—27 hydride positions cannot be
reconciled by Ta—H/B—D scrambling between exo and endo—27 and DBCat, this

mechanism can be excluded.

 

@ H fix! CH3 4%:(‘3‘3
+ DBCat

 

 

‘D
Pathway A BC‘“
28 29
+ DBCat,
1 _ HBCat Pathway B n
PchaI—d0_1
gram fl») CH3
\ Ta\ _
% ”CH3 H Ta D/H
endo—27 30
DBCat/ DBCat
H HBCat
"PrBCat-dml

gum @
%\K g? C“: %'D

CH3 D

exo—27

31

Figure 54. Mechanism for endo—1 formation.

60

The exclusive incorporation at the a-position of the borane can be reconciled by
considering mechanisms that involve an alkylidene intermediate (28) in Figure 54. The
fact that alkylidene complexes Cp2Ta(=CH2)(CH3)l ‘7 and Cp"‘2TaH(=CH2)1 13 have been
isolated supports the accessibility of 28. Two pathways that involve this intermediate
could account for deuterium incorporation at the a position of n-PrBCat. In pathway A,
B—C bond formation proceeds via o—bond metathesis between DBCat and the Ta=C bond
in 28 to generate an alkylborane intermediate, 29. Reductive elimination from 29
generates n—PrBCat—dm. and Cp2TaH—do_i, (30), which is then trapped by DBCat to give
endo—1.

Pathway B invokes scrambling between B—D and Ta—H sites in 28. Reversible a—
migration generates 31 with deuterium incorporation in the a—position. Alkylborane
formation via B—H oxidative addition and subsequent B—C reductive elimination
generates 30, which is trapped by catecholborane—do.) to yield endo-140.2. For pathway
B, deuterium scrambling should be reflected by diminished relative intensities for the a-
proton resonances of the coordinated propylene ligand in endo-27, and it should be
possible to detect HBCat by llB NMR due to the fact that IJHBI is clearly resolved.

When the reaction between DBCat and endo—27 was monitored by 1H NMR, the
ratio between the integration of the a—proton resonances of the olefin and the Cp
resonance remained constant through two half—lives. In addition, formation of HBCat
could not be detected by 11B NMR. These observations are inconsistent with scrambling
via pathway B. Thus, the most reasonable mechanism that accounts for the isotopic
incorporation is pathway A. Integration of 2H NMR spectra gave a 40:60 ratio between
the a—methylene resonance in n—PrBCat and the Ta—D resonances. However, deuterium
incorporation is more accurately determined by comparing a—methylene and methyl
resonances in 1H NMR spectra of n-PrBCat.119 Since 2H NMR spectra did not indicate
deuterium incorporation at the methyl position of the alkylborane, the methyl resonance

provides a reliable internal standard. The ratio between a—methylene and methyl

61

resonances (1.5:3) implies that the effect of kH/kD for reductive elimination from the

alkylborane intermediate is negligible.

62

CHAPTER 3
ELIMINATION CHEMISTRY FROM BENT METALLOCENE SYSTEMS

Reactions of Cp*2Nb(H2BCat’)

The niobium-boron linkage of 4 is reactive with various small molecules. The
reactions shown in Figure 55 proceed at room temperature generating HBCat’ as one of
the products. Compound 4 reacts with CO, discussed in detail below, to form 26.
Compound 4 also reacts with BH3 (B2H6) and H2 forming 13 and Cp*2NbH3,
respectively.114 The reaction with H2 gas is interesting because 4 can be synthesized
from addition of catecholborane to Cp*2NbH3.

Another interesting reaction occurs upon addition of olefins. Compound 4 reacts
with CH2=CH(R) (R = H, Me) to form free catecholborane and Cp*2NbH(n2—
CH2=CH(R)) (‘H and “B NMR spectra). The formation of Cp*2NbH(n2-CH2=CH(R)) is
significant since the original niobium boryl complex is synthesized from the olefin
complex with HBCat. The reaction of 4 with propylene was monitored in a sealed NMR
tube. 1H and 11B NMR spectra revealed a gradual decrease in [HBCat’] and a
corresponding increase in alkylborane concentration. Upon consumption of all the

HBCat’, the metal containing products were 3 and 13.

 

 

CP*\ .sH/o. /H CP*\ ..~‘H
/Nb ’B\ /Nb\—H
Cp* ‘1] H Cp* H
Bzflo “2
C13*\ \H CP*\ 4“
..~\‘ CO ..~\
Nb 1 olefin ; Nb
*/ y Cp*ZNb(H2BCat’) */ \
CP / R CP C0

Figure 55. Reactivity of 4.

63

The presence of significant quantities of 13 prompted the question whether olefin
hydroboration was occurring at the metal center or was accomplished by some other
pathway that utilizing BH3. To address this concern, the reaction of 3 with HBCat’ was
performed in the presence of excess cyclooctene and propylene. Cyclooctene is used
because it coordinates poorly to the Cp*2NbH intermediate due to steric factors.120 The
uncatalyzed hydroboration rates for propylene and cyclooctene are competitive (the ratio
for uncatalyzed hydroboration products of propylene:cyclooctene = 0.93: 1).121 Thus, any
B—C forming pathway that does not involve the metal should give appreciable quantities.
of cyclooctene hydroboration products. In this case, 3 is converted to 4 and "PrBCat’ is
the primary alkylborane product observed (the ratio for catalyzed hydroboration products
of propylene:cyclooctene = 4.53:1).

Two aspects for an effective catalytic hydroboration cycle must be addressed. The
first is the rate for B-C bond formation and the second is the rate for B-H elimination to
regenerate the olefin complex. Boron—carbon bond formation is rapid for the reaction of 3
with any HB(OR)2 functionalized borane ((OR)2 5 Cat or Pin). For this aspect of catalytic
hydroboration, the choice of borane is not important under the investigated conditions.

In contrast, the borane utilized greatly affects the rate for B—H elimination and
therefore plays a major role in the effectiveness of catalytic hydroboration in these
systems. In order for the catalytic cycle to be viable, the coordinated borane adduct must
be easily replaced by an olefin. As will be discussed below in quantitative terms, borane
adduct coordination to the niobium metal center is dependent on the borane used.
Qualitatively, this is evident by the formation of 15 by placing 14 under an atmosphere of
N2. The HBCat family of boranes does not yield 15 even when placed under high
pressures of N2 (up to 200 psi N2). This simple experiment clearly demonstrates the
weaker borane adduct binding of HBPin compared to the catecholate family of boranes.
This parallels the tendency for borane coordination of Lewis bases where HBCat

derivatives form stable Lewis acid—base adducts with amines, whereas HBPin does not.

64

Niobium Rate Law Determination for the Reaction of 4 plus CO

The reaction of 4 with CO proceeded cleanly with formation of 26 and free
HBCat’. The clean reactivity witnessed for this reaction makes these compounds ideally
suited for mechanistic investigation. The lack of decomposition products or competing
side reactions allows rate dependencies to be definitively established. It was evident from
initial experiments that the reaction was inhibited by excess borane since plots of ln[4]
versus time were non—linear at lower CO pressures. This suggested a mechanism were
preequilibrium dissociation of HBCat’ generates the sixteen electron hydride complex,
Cp*2NbH, which is subsequently trapped by CO. Application of the steady—state

approximation yields the rate law shown in Figure 56.

 

 

 

 

- 1
flu k1 fi
N' ‘—=-——— Nb—H + HBCat
g1 \IBCEII $1
H k-)
4
k2 CO
-d[41
= kobs [4]
dt

 

k ,k2[CO] 7%
[(0175 = ..\\\H
k_,[HBCat] + k2[CO] wgéko

at high CO pressures:

kobs = kl

 

 

 

Figure 56. Mechanism and rate law of 4 plus CO.
Rearrangement of the rate law from Figure 56 yields equation 8. From this
equation, plots of 1/k,,b,. versus [HBCat’] and 1/[CO] should be linear. Figure 57 and

Figure 58 show this to be the case. Also, the y—intercept in Figure 57 yields a value for k1

65

when [HBCat’] = 0 that is very close to those measured in independent runs with no

added borane.

 

 

 

 

 

 

 

 

 

 

 

 

k _ klk2[CO]
0’” ' k_,[HBCat1 + k2[CO]
1 k_l[HBCat’] k.[co1
k0)... ‘ k1k2[CO] + k1k2[CO]
1 _ k_1[HBCat’] 1
1.0,, ‘ klk2[CO] + k1
2.013+04 -
1.SE+O4 ~
é
a 1.0E+04 1
out-1
5.0E+03 ~
0.0E+OO 1 . . ,
0 0.2 0.4 0.6
[HBCat’] (M)
Figure 57. Plot of l/k,,b_. versus [HBCat’].
ZOE-+04 ‘1
1.554.041
.3 1.03041
= I
5.0E+03
0.0E+00 1 1 . .
0 0.005 0.01 0.015 0.02
1/CO pressure (psi'l)

 

 

 

Figure 58. Plot of l/km versus l/CO pressure.

66

(6)

(7)

(8)

Based on the rate law in Figure 56, under sufficiently high CO pressures, km
should approach a limiting value of k; if k_1[HBCat’] « k2[CO]. This was confirmed when
running the reactions at C0 pressures > 50 psi. Competitive reversible coordination of
HBCat’ results in non—linear first order decay plots when running the reaction under
lower CO pressures.

Since measurement of [6,1,5 at high CO pressures gives k1, which relates to borane
adduct dissociation, the temperature dependence of the rate was examined in order to
extract activation parameters. The Eyring plot in Figure 59 yielded activation parameters
of AH1 = 23(1) kcal/mol and AS1 = 12(4) e.u. for the elimination reaction. These values
are consistent with a dissociative step; however, k. was not determined over a
substantially large temperature range due to rapid reaction rates above 25 °C and
prohibitively slow reaction rates at temperatures below -20 °C which may contribute to

the relatively large errors associated with AH: and A81.

 

-10 w

-12 a

-14 -

In (k/T)

-16 z

 

 

'18 l I l l f 1
0.0034 0.0035 0.0036 0.0037 0.0038 0.0039 0.004

III (16)

 

 

 

Figure 59. Eyring plot for the reaction of 4 plus CO.

The thermodynamics of borane binding were examined in the Nb system by
measuring equilibria between alkyl substituted catecholborane derivatives. AAG values
relative to Cp*2Nb(H2BCat) are given in Table 7. If the assumption is made that

substitution on the catechol backbone has negligible effects on B—H bond energies, AAG

67

reflects the relative ground state energies of the borane adducts, Cp*2Nb(H2BCat’). The
observed trend is consistent with an increase in steric interactions between the borane
adduct and the pentamethylcyclopentadienyl groups following the order HBO2C6H4 <
HBO2C6H3—4—'Bu < HBO2C6H3—3—'Bu. The equilibrium between HBPin and 4 could not
be measured, indicating that borane adduct association in 14 is much weaker than in the

catecholate based systems.

Table 7. AAG values for the equilibria:

Cp*2Nb(H2BCat) + HBCat’ ~——=———-—- Cp*2Nb(H2BCat’) + HBCat (25 °C, C6D6).

 

HBCat’ AAG (kcal/mol)
HBO2C6H3—4—’Bu 026(2)
HB02C6H3-3-IBU 096(2)
HBPin > 5.8

 

 

Deuterium Isotope Eeffect

The labeled compound, 4—d2, was synthesized using the same method used for the
unlabelled compound using DBCat’ in place of HBCat’. Obtaining an isotopically pure
compound was not trivial due to scrambling between the Nb—D and methyl hydrogen
positions of the Cp* ring system.68 Deuterium incorporation was enhanced by
recrystallizing 4—d2 in the presence of excess DBCat’ (~ 84% of all Nb—X bonds were
Nb—D bonds). Over the course of several weeks, even in the solid state, slight
incorporation of deuterium into the methyl positions of the Cp* ring could be detected in
2H NMR spectra.

Kinetic reactions were performed on freshly prepared deuterium labeled
compound under identical conditions as those of the unlabeled compound. An inverse
isotope effect was noted for the reaction with CO (kH/kD = 068(3)), Figure 60. It should
be noted that the observed inverse isotope effect is the result of both primary and

secondary isotope effects. Since the mixed hydride species 4—d. could be

68

 

 

In {[Cp*2Nb(XzBCat’)]/
[Cp*2Nb(XzBC3t’)]°}

 

 

2 1 I l T l
0 1 000 2000 3000 4000

time (sec)

 

 

 

Figure 60. Plot of 4—dn2 plus CO.

synthesized by mixing equal quantities of 4 and H2 resulting in rapid scrambling in the
hydride positions generating a nearly statistical mixture of 4, 4—d1 and 4—d2 (all three
isomers are readily discemable by their Cp* resonances), there is a possibility the two
effects can be separated. Within experimental error, the reaction rates for 4—dl and 4-d2
with C0 are identical. The reaction was not monitored by HB NMR spectroscopy which
would potentially give the relative rate of HBCat’ elimination to DBCat’ elimination
from 4—d 1. The relative reaction rate could be used to calculate a secondary isotope effect
should one be present. Secondary isotope effects usually occur when deuterium is
substituted on an atom that changes hybridization. In this case, the secondary isotope
effect would appear to be negligible since the boron and niobium atoms do not
rehybridize from reactants to products and the identical reaction rates for 4—dr and 4—d2.
In addition, the zero—point energy of DBCat’ is lower than HBCat’ shifting a
preequilibrium favoring [Cp*2NbD] and free borane when deuteria are bound to the metal
center which explains why 4—d2 reacts faster than 4. Transition metal catalyzed olefin

hydrogenation reactions may also be explained in terms of a preequilibrium isotope

effect. 122430

69

Tantalum Rate Law Determination for the Reaction of 17 plus CO

The reaction of 17 with CO resulted in clean formation of HBCat’ and
Cp"‘2TaH(CO),68 (32). An investigation of the reaction mechanism was conducted since
the ground states energies for 4 and 17 may differ leading to distinct pathways.

Though chemically analogous to the reaction between CO and 4, the kinetic
behavior of 17 differs considerably. When 17 was reacted with excess CO, disappearance
of the Ta complex was first—order. The reaction was monitored under various
concentrations of free HBCat’ with surprisingly no observed dependence of [HBCat’]
upon the reaction rate. The effect of CO pressure on the reaction rate was also
investigated, and no [CO] dependence was observed. Based on reaction rate
independence on [C0] and [HBCat’], the rate determining step for the mechanism in
Figure 61 is presumed to be B-H reductive elimination leading to a [Cp*2TaH] transition

state. The mechanistic details are discussed in more detail below.

 

 

 

[TaW Bear + co _’£1__. ’Ta' + HBCat
%\ H $2 co
17
-d[l7]
= kobs [17]
dt
kobs = k]

 

 

 

Figure 61. Mechanism and rate law of 17 plus CO.

For the reaction of 17 with CO, k. is kob, (Figure 61). The reaction was monitored
over a range of temperatures to determine the activation parameters, AHIt and A81. The

Eyring plot in Figure 62 resulted in AH* = 23.7(5) kcal/mol and 713* = —10(1) e.u. for the

70

reductive elimination reaction. The value for AS: does not support a dissociative step, and

is typical for reductive elimination via a three—centered transition state.131vl32

 

-12
-13 I

-14

-15

-161

-17 ‘

-181 . . . . .

0.0026 0.0027 0.0028 0.0029 0.003 0.0031
1/T(K")

ln(k/T)

 

 

 

Figure 62. Eyring plot for the reaction of 17 plus CO.

 

 

In {[Cp*2TaX2(BCat’)]/
[Cp*2TaX2(BCat’)]°}

 

 

2.0 1 I 1 l l
0 5000 10000 15000 20000 25000

time (sec)

 

 

 

Figure 63. Plot of 17—do2 plus CO.

Deuterium Isotope Effect
The failure to observe borane or CO dependence was surprising; however, rate-
limiting, irreversible borane elimination could account for the observed kinetic behavior.

To determine whether B-H elimination follows the rate limiting step, the isotope effect in

71

the reaction between 17—d2 and CO was examined. Isotopically pure 17 can be
synthesized by the reaction of 16 with DBCat’ yielding 17—d2 with deuterium
incorporation only in the hydride positions. Observation of an isotope effect could lend
support to a mechanism where B-H bond formation occurs during (or prior to) the rate—
limiting step.

Figure 63 clearly shows rate acceleration upon deuterium substitution in the
hydride positions. In fact, kH/kD = 044(9) for 17 is substantially smaller than that
observed for 4 (see above). Unlike the results seen for deuterium incorporation in 4, there
are only normal isotopic perturbations of resonance for partially deuterated 17 which
argue against a rapid preequilibration dissociation.

Mechanism Determination

The simplest mechanism that is consistent with the kinetic data is shown in
pathway A of Figure 64. This mechanism describes simple B—H reductive elimination
from the tantalum metal center forming a Ta"l intermediate that is subsequently trapped
by CO. Figure 64 also offers alternate pathways that can account for the observed kinetic
data. Pathway B incorporates a preequilibrium step between 17 and a borane adduct
similar to the niobium compound. The rate limiting step could be either the formation of
the G—complex, or borane elimination from the o—complex to generate borane and the
intermediate hydride, Cp*2TaH. It is attractive to hypothesize that the forward step in the
equilibrium between 17 and the o—complex is rate limiting, as elimination from
Cp*2M'"(H2BCat’) o—complexes should proceed at similar rates. It is worth mentioning
that kH/kD = 044(9) is slightly smaller than values measured for related methane
eliminations from Cp’2WH(CH3) species.133t134 While the magnitude of kH/kD is small
for a kinetic isotope effect, it is similar to equilibrium isotope effects observed by Bercaw
for preequilibrium between Cp*2TaH(n2—CH2O) and Cp"‘2TaOCH3,l ‘3 and by Feher and
Jones in benzene reductive elimination from Cp*Rh(PMe3)(H)(Ph).135 Pathway C

invokes 175—9173 ring—slip and subsequent CO coordination, followed by irreversible

72

borane elimination and 773—9715 coordination.136'139 In pathway D, hydride migration to a
Cp* ring carbon precedes CO coordination. Irreversible borane elimination is then
followed by hydride migration to Ta. Observation of a primary kinetic isotope effect
almost certainly would require borane elimination as rate limiting. For this mechanism,
primary isotope effects might be expected for the hydride to
pentamethylcyclopentadienyl migration and/or the B—H reductive elimination. If CO
coordination precedes the rate limiting step, CO dependence would be expected;
however, the absence of an inverse dependence on borane could prevent entering a

regime that is first order in [CO].

' (If: E g” . —HBCat % co T .4”
1T ‘30“ #510,, ’Ta: BCat :——-’ ,Ta-H , aWO PathwayA
33$. H 3%;
fig” fill - HBCat’ # CO MJ“
,Ta-BCat’ *—— Ta BCat ‘ --------- Ta-H _‘ ,Ta PathwayB
‘11 Ewig 3%?‘c0

I
is if,
I

O

O
/2
\
I‘ano
Jul
1%
:0
0'0:
o."
O.

 

 

’Tzi:BCat’ v—- 1 e
H Edy
H , H -H8Cat'.
// CO /\ . . H
”H \ /H T H H—rmgratlon .\\ PathwayD
1

' —.—‘ / ’
Ta:BCat ' ‘—:'— Ta ‘— 3—BC at

Ta'
%‘ BCat’ ‘co w‘co

Figure 64. Possible pathways for compound 32 formation.

H 6.

l

76

Distinction between pathways A and B cannot be made definitively from the
experimental data. The activation parameters support pathway A as the value for AS1 is
consistent with an ordered transition state. This would appear to argue against pathway B

since AS‘t = 8(2) e.u. for the related dissociative reactions for 4; however, ordering from

73

the preequilibrium between 17 and a o—borane complex could mask dissociative
contributions to ASi. In pathway B, the equilibrium between 17 and the borane adduct
complex would be rate limiting if borane elimination from Cp*2M1"(H2BCat’) adduct
complexes proceed at similar rates.

For Pathways C and D, application of the steady—state approximation to the
concentrations for the intermediates gives identical rate laws that predict saturation
behavior in [CO].140 At low CO pressure, there is no indication of a dependence on [CD]
as predicted for these mechanisms. It is conceivable that if CO coordination is
sufficiently rapid that there is no regime where a [CO] dependence can be observed.
Hence, these mechanisms cannot be rigorously excluded; however, the comparison to H2
elimination from Cp*zTaH3 (33) make these pathways unlikely (deuterium should have a
similar inverse isotope effect, see below).

Comparison Between B—H and H; Eliminations

Compound 33 was synthesized in order to investigate the differences in B—H and
H—H elimination from 17 and 33, respectively. This compound was chosen as direct
structural analog to the tantalum boryl complex in terms of placement of the ligands
within the metallocene wedge and oxidation state of the metal. In both instances, the
metal must undergo a formal reduction from Tav to Tam. In terms of oxidation states,
these reactions can be directly compared unlike the reaction of 4 with CO since the
niobium metal formally does not undergo a reduction in oxidation state. The effect of the
boron fragment bound directly to the metal can therefore be addressed for the tantalum
compounds.

The reaction of 33 with CO proceeds cleanly yielding 32 and hydrogen gas. The
reaction was monitored over a range of temperatures to determine activation parameters.
An Eyring plot was constructed (Figure 65) from the rate constants of individual kinetic
experiments yielding AH* = 27.2(9) kcal/mol and AS: = —11(2) e.u. The value for A81

obtained for this reaction is identical, within experimental error, to the value of A81

74

obtained for the reaction of 17 plus CO that could indicate a similar reductive elimination

pathway.

 

42.01
425 ~
43.0 ~ _
-135 .
44.0 .
-145 ~

I
-15.0 I I T I 1
0.00225 0.0023 0.00235 0.0024 0.00245

ll'l‘ (K‘l)

In (k/T)

 

 

 

 

 

Figure 65. Eyrin g plot for the reaction of 33 plus CO.

 

 

 

 

In {[Cp*2TaX3]/[Cp*2TaX3I°}

0 5000 10000 15000 20000 25000 30000

time (sec)

 

 

 

Figure 66. Plot of 33—d0,3 plus CO.

In contrast to the reaction of 17 plus CO, an inverse isotope effect was not
observed for the reaction of 33 plus CO. The reaction clearly proceeds at a faster rate
with 33 than with 33—d3 (Figure 66) with a calculated kH/kD = l.67(6) for this reaction.

Although the end product for the metal is identical as the reaction with 17, the normal

75

isotope effect witnessed for 33 precludes preequilibrium formation of [Cp*zTaD] and D2.
Calculations might provide useful insight concerning intermediates involved in the
eliminations of B—H and H-H from 17 and 33, respectively.
Thermodynamic and Kinetic Data

The kinetic data allow for construction of partial energy level diagrams in Figure
67, Figure 68 and Figure 69. For niobium, neither the relative barriers for H2 and HBCat’
activation by a common intermediate, nor the barrier for H2 elimination from Cp*2NbH3
has been determined. The equilibria measurements between 4 + H2 and Cp*2NbH3 +
HBCat’, which will allow determination of the relative free energy difference between 4

and Cp*2NbH3, are currently under investigation.

H

\

ll Cp2*Nb" 3C3"

\

  
  

  
 
 

[Cp*2NbH] +
HBCat’ + co

19(2) kcal/mol

    

energy

 

  

l

Cp*2NB(H2BCat)

Cp*2NbH(CO)

 

Figure 67. Free energy diagram for 4 plus CO at 20 °C.

The reactive intermediate, [Cp*zTaH], for the reactions of 17 with CO (Figure 68)
and 33 with CO (Figure 69) is identical, which allows for calculation of BDE values for
reactions up to the common intermediate by using the bond dissociation energy (BDE)
for H2 (104.2 kcal/mol)141 and a calculated BDE for HBCat (1 11.3 kcal/mol)142. For the

reaction depicted in Figure 69, an H-H bond is formed while two Ta—H bonds are

76

broken. AHi is the energy of the bonds formed subtracted from the energy of the bonds
broken. In this case, AHit 2 2 BDE(Ta—H) — BDE(H—H). The maximum value for the
BDE(Ta—H) is calculated to be 65.7 kcal/mol from using the appropriate values since the
value for AH1t is to the barrier of the energy level diagram not to the intermediates energy
position.

For the reaction depicted in Figure 68, an B—H bond is formed while a Ta—H and
a Ta—B bond are broken. For this reaction, AHi S BDE(Ta—B) + BDE(Ta-H) - BDE(B—
H) for the maximum value of the BDE(Ta—H). From the appropriate BDE values listed
above along with the newly calculated BDE(Ta—H), the minimum value for the BDE(Ta—
B) is calculate to be 69.3 kcal/mol. The BDE’s for Ta—B and Ta—H bonds are related as
follows: BDE(Ta—B) 2 BDE(Ta—H) + 3.6 kcal/mol. Also from this relationship, the
BDE(Ta—B) can be bracketed by 69.3 kcal/mol S BDE(Ta—B) S 135.0 kcal/mol should
the BDE(Ta—H) value be 0 kcal/mol. The equilibria measurements between 17 + H2 and
33 + HBCat’, which will allow determination of the relative free energy difference

between 17 and 33, are currently under investigation.

H
ll Cp2*Ta’-\--BCat’ ,CO
Cp2*Ta’\
H

   
  
 

  

[Cp*2TaH] +
HBCat’ + C0

27. 1 (6) kcal/mol

   
     

energy

 

..... i
Cp*ZTaHzBCat’

 

Cp*2TaH(CO)

Figure 68. Free energy diagram for 17 plus CO at 70 °C.

77

  

[Cp*zTaH] +
H2 + co

31(1) kcal/mol

 
     

energy

 

Cp*zTaH(CO)

 

Figure 69. Free energy diagram for 33 plus CO at 70 °C.

78

CHAPTER 4
EXPERIMENTAL

General Considerations

All manipulations were performed using glove box, Schlenk and vacuum-line
techniques. Pentane, heptane, THF, hexanes and toluene were predried over sodium.
Toluene was distilled from NaK/benzophenone ketyl. All other solvents were distilled
from sodium/benzophenone ketyl. Acetone was dried over activated 3 A molecular sieves
and distilled prior to use. Benzene—d6 and toluene—dg were dried over activated 3 A
molecular sieves and then vacuum transferred into a sodium mirrored container. THF—dg
was dried over activated 3 A molecular sieves and vacuum transferred prior to use. Argon
and nitrogen gases were purified by passage over MnO on silica.

Ethylene (Matheson) and propylene (Matheson) were subjected to several freeze—
pump—thaw cycles and then transferred in vacuo to reaction vessels. Carbon monoxide
(Matheson) and dihydrogen (AGA Gas Inc.) were used as received. Diborane gas was
synthesized by the following method: NaBI-I4 (Aldrich) was placed in an air free
container, slurried with distilled dimethoxyethane (DME, Aldrich) and frozen with liquid
N2. Distilled BF3~Et2O (Mallinckrodt) was then added to the frozen slurry. The contents
of the air free container were then evacuated. The system was allowed to slowly warm to
room temperature in the process generating B2H6.

Catecholborane (Aldrich) was vacuum transferred prior to use. Preparation of
DBCat was accomplished by reaction NaBD4 (Aldrich) and BF3-Et20 (Mallinckrodt) to
produce BD3 which was then bubbled through a THF solution of catechol (Aldrich). The
THF was then distilled from the solution at atmospheric pressure and the remaining
solution was vacuum distilled to give the borane as a clear, viscous liquid.

{Cp2TaH2Li}x,42 27,143 Cp2NbH(n2—CH2=CH2),144 19,93 3,120 13114 and 1668 were all

79

prepared by literature methods. Exo and endo—27 were prepared by irradiating pure
endo—27 with 300 nm light for 4 h under an atmosphere of propylene gas generating 41%
exo—27 and 59% endo-27.

lH NMR spectra were recorded on Varian Gemini—300 (300.0 MHz), Varian
VXR-BOO (299.9 MHz), and Varian VXR—SOO (499.9 MHz) spectrometers and
referenced to residual proton solvent signals. 2H NMR spectra were recorded on a Varian
VXR—300 spectrometer operating at 46 MHz or on a Varian VXR—SOO MHz
spectrometer operating at 77 MHz; and spectra were referenced to residual deuterium
signals of the solvent. 1'B NMR spectra were recorded using a Varian VXR—300
spectrometer operating at 96 MHz or on a Varian VXR—SOO MHz spectrometer operating
at 160 MHz; and spectra were referenced to a BF3-Et2O external standard. Infrared
spectra were recorded as Nujol mulls in KBr plates using a Nicolet IR/42 spectrometer.
GC samples were run on a Hewlett Packard 5890A gas chromatograph equipped with a
Hewlett Packard 3392 integrator and a Foxboro GB-l capillary column (25 m x 0.25 mm
ID. with a 0.25 pm film). Elemental Analyses were performed by Desert Analytics,
Tucson, Arizona or in house on the MSU department of chemistry elemental analyzer.
Synthesis

exo and endo-Cp2TaH2(BO2C5H4) (exo and endo-l). A 10 mL toluene solution
of B—chloro—catecholborane (418 mg, 1.31 mmol) was added dropwise to a vigorously
stirred toluene suspension (25 mL) of {Cp2TaH2Li}x (202 mg, 1.31 mmol) at -78 °C. The
mixture was gradually warmed and stirred for 30 min at room temperature during which
time the orange solids were replaced with an off—white precipitate (LiCl) accompanied by
the formation of a pale amber solution. The solvent from the filtrate was stripped and the
crude product (321 mg, 57%) was analyzed by 1H NMR where integration revealed a
32:68 mixture of isomers. The isomers were separated by fractional crystallization from
toluene. endo—Cp2TaH2(BCat) was isolated as pale yellow microcrystals (80 mg, 19%) as

the less soluble isomer. Concentration of the mother liquor and further cooling afforded

80

exo—Cp2TaH2(BCat) as pale microcrystals (132 mg, 23%). endo-Cp2TaH2(BCat): 1H
NMR (C6D6) 8 7.14 (m, 2 H, O2C6H4), 6.82 (m, 2 H, O2C6H4), 4.85 (s, 10 H, C5H5), -
4.24 (s, 2 H, TaH2); “B NMR (C6D6) 5 70 (A v.,2 = 250 Hz); IR (em") 1768 (vm) (1278
(Vran)); Anal. calcd for C14,2H20_3BO2Ta: C, 49.8; H, 4.27. Found: C, 49.6; H, 4.17. exo—
Cp2TaH2(BCat): 5 7.11 (m, 2H, O2C6H4), 6.84 (m, 2 H, O2C6H4), 4.83 (s, 10 H, C5115), —
4.20 (d, J: 5.5 Hz, 1 H, TaH2);—5.17(s, 1 H, TaH2); llB NMR (C6D6) 5 65 (A VH2 = 190
Hz); IR (cm—1) 1771, 1703 ((vm) 1283 Mm». endo-l-d2 and exo-l-d2 were similarly
prepared from {Cp2TaD2Li }x.

endo—Cp2TaH2(BO2C6H4) (endo—1). HBCat (805 mg, 6.76 mmol) was dissolved
in 10 mL toluene and then added via cannula to a 100 mL toluene solution of 27 (950 mg,
2.70 mmol). The solution was stirred for two h during which time the color gradually
darkened to a dark yellow color. The solvent was then placed overnight in a -15 °C
freezer. The solvent was removed by filtration and the yellow microcrystals dried under
vacuum (915 mg, 78%): mp 141—143 °c (dec); ‘H NMR (c6136) 8 7.15 (m, 2 H, O2C6H4),
6.83 (m, 2 H, 02cm», 4.85 (s, 10 H, C5H5), —4.23 (s, 2 H, TaH); ”B NMR (C6D6) 5 70
(A v.,2 = 250 Hz); IR (em") 1751 (vm).

endo-Cp*2Nb(H2BO2C6H4). A 15 mL toluene solution of 3 (219 mg, 0.540
mmol) was cooled to -—78 °C. HBCat (0.16] mL, 1.35 mmol) was added to this stirred
solution via syringe. The resulting solution was warmed to room temperature and stirred
for two h. During the course of the reaction, the color of the solution gradually lightened
from deep red to orange—red, and solvent evaporation gave a deep orange solid. The
reaction mixture was dissolved in a minimal volume of toluene and cooled to —80 °C.
The product was isolated as orange microcrystals (207 mg, 79%): mp 171—173 °C; IH
NMR (C6D6) 5 7.07 (m, 2 H, 02C6H4), 6.85 (m, 2 H, O2C6H4), 1.83 (s, 30 H, C5(CH3)5),
-8.27 (br, 2 H, NbH2); “B NMR (c606) 5 60 (A v.,2 = 240 Hz); IR (cm"') 1653 (vaH);
Anal. calcd for C26H36BNbO2: C, 64.5; H, 7.44. Found: C, 64.1; H, 7.26.

81

endo-Cp*2Nb(H2BO2C6H3-4-'Bu) (endo-4). A 20 mL toluene solution of 3
(400 mg, 0.985 mmol) was cooled to —78 °C. HBO2C6H3-4—'Bu (0.360 mL, 2.07 mmol)
was then added via syringe to the stirred solution, and the solution was warmed to room
temperature. After two h, the toluene was removed in vacuo, leaving a viscous red oil.
The oil was dissolved in a minimal volume of heptane and placed in a —80 °C freezer.
Upon standing overnight, deep red microcrystals were deposited. After the filtrate was
removed, the crystals were washed with cold pentane and dried in vacuo (396 mg, 75%):
mp 154-157 °C; 1H NMR (C6D6) 5 7.30 (d, J = 2.1 Hz, 1 H, O2C6H3), 7.04 (d, J = 8.1
Hz, 1 H, O2C6H3), 6.90 (dd, J = 7.8, 1.95 Hz, 1 H, O2C6H3), 1.85 (s, 30 H, C5(CH3)5),
1.30 (s, 9 H, O2C6H3(4-C(CH3)3)), -8.19 (br, 2 H, NbH2); llB NMR (C6D6) 6 60 (A v1/2 =
300 Hz); IR (cm") 1653 (vaH). endo—4-d2 was similarly prepared from 3 and
DBO2C6H3—4—'Bu: 1H NMR (C6D6) 5 7.28 (d, J = 1.8 Hz, 1 H, O2C6H3), 7.03 (d, J = 8.1
Hz, 1 H, O2C6H3). 6.89 (dd, J = 8.1, 1.95 Hz, 1 H, O2C6H3), 1.83 (s, 30 H, C5(CH3)5),
1.30 (s, 9 H, O2C6H3(4-C(CH3)3)); 2H NMR (C6H6) 5 —9.29 (br, NbD2); “B NMR (C6D6)
8 57 (A VH2 = 270 Hz).

endo-Cp*2Nb(HzBO2C6H3-3-’Bu) (endo-12). A 10 mL toluene solution of 3
(251 mg, 0.618 mmol) was cooled to -78 °C. HBO2C6H3—3-'Bu (229 mg, 1.30 mmol)
was dissolved in 5 mL toluene, and the borane solution was transferred to the propylene-
hydride complex via cannula. Once the addition was complete, the resulting solution was
allowed to warm to room temperature and stir for three h. The toluene was then
evaporated, leaving a tacky, deep orange—red solid. The crude mixture was dissolved in a
minimal volume of pentane and placed in a —80 °C freezer. Orange—red microcrystals
were deposited overnight. The product was collected, washed with pentane, and dried in
vacuo (218 mg, 65%): mp 141—142 °C; ‘H NMR (cbné) 8 7.04 (m, 1 H, O2C6H3), 6.96
(m, l H, O2C6H3), 6.91 (m, 1 H, O2C6H3), 1.86 (s, 30 H, C5(CH3)5), 1.58 (s, 9 H,
O2C6H3(3-C(CH3)3)), —7.91 (br, 2 H, NbH2); “B NMR (C6D6) a 61 (A vm = 280 Hz); IR
(cm'l) 1649 (vaH).

82

endo-Cp*2Nb(H2BO2(C(CH3)2)2) (endo-l4). All manipulations for this reaction
were carried out under an Ar atmosphere. Complex 3 (401 mg, 0.988 mmol) was
dissolved in 10 mL toluene and cooled in a dry ice/isopropanol bath. HBPin (316 mg,
2.47 mmol) was dissolved in 5 mL toluene. The borane solution was then added via
cannula to the stirring propylene—hydride solution. The resulting solution was allowed to
warm to room temperature. The volatiles were removed under vacuum after a two h
reaction time. The rust—orange solid was dissolved in toluene, concentrated and placed in
a -80 °C freezer overnight. The mother liquor was removed by .filtration and the rust-
orange microcystalline compound (286 mg, 59%) was dried under vacuum: 1H NMR
(c6136) 5 1.88 (s, 30 H, C5(CH3)5), 1.18 (s, 12 H, C(CH3)2)2), —9.24 (br, 2 H, NbH2); ”B
NMR (C6D6) 5 54 (A V172 = 200 Hz).

(Cp*2NbH)2-u-N2 (15). A 20 mL toluene solution of 3 (775 mg, 1.91mmol) was
cooled to —78 °C. HBPin (536 mg, 4.19 mmol) was dissolved in 10 mL toluene. The
HBPin solution was then added via cannula to the cooled propylene-hydride complex
solution. The stirring solution was allowed to warm to room temperature. After stirring
for 2.5 h at room temperature, the volatiles were removed under vacuum. The resulting
solid was dissolved in pentane, concentrated and placed in a —80 °C freezer. After 16 h,
the mother liquor was removed by filtration and the deep red microcrystalline solid was
washed once with cold pentane (—78 oC) and vacuum dried (357 mg, 49%): mp 128—130
°C (dec); lH NMR (C6D6) 5 1.99 (s, 30 H, C5(CH3)5), 1.94 (s, 30 H, C5(CH3)5), -—2.68
(br, 2 H, NbI-I); Anal. calcd for C401162N2Nb2: C, 63.5; H, 8.2, N, 3.7. Found: C, 63.5; H,
8.6, N, 4.0.

endo-Cp*2TaH2(BO2C6H4). Catecholborane (0,120 mL, 0.973 mmol) was added
to a 10 mL toluene solution of 16 (193 mg, 0.389 mmol) at room temperature. The
solution was then stirred for two h during which time the initial pale yellow color
gradually turned to a lime green color. Evaporation of the toluene yielded a lime green

solid. Washing this solid with pentane followed by drying under vacuum yielded a pale

83

yellow solid (160 mg, 72%): mp 159—162 °c (dec); 'H NMR (C6D6) 5 7.19 (m, 2 H,
O2C6H4), 6.89 (m, 2 H, O2C6H4), 1.96 (s, 30 H, C5(CH3)5), —2.12 (br, 2 H, TaH2); llB
NMR (c6136) 5 74 (A v1/2 = 390 Hz); IR (cm'l) 1774, 1709 (va15.
endo-Cp*2TaH2(BO2C6H3-4-'Bu) (endo-17). Toluene (5 mL) was used to
dissolve 16 (155 mg, 0.313 mmol). HBO2C6H3—4—'Bu (0.140 mL, 0.784 mmol) was then
added via syringe to the stirred solution. The resulting solution was stirred for two h at
room temperature, giving a rust orange solution. Evaporation of the toluene yielded a
viscous, deep orange oil. Dissolving this oil in pentane and concentrating the resulting
orange solution yielded pale yellow microcrystals when placed in a —80 °C freezer
overnight. Separation of the filtrate and drying the microcrystals under vacuum gave the
pure product (112 mg, 57%): mp 156—158 °C; 1H NMR (C6D6) 5 7.42 (d, J = 2.1 Hz, 1 H,
O2C6H3), 7.19 (d, J = 11.4 Hz, 1 H, O2C6H3), 6.95 (dd, J = 8.4, 1.95 Hz, 1 H, O2C6H3),
1.98 (s, 30 H, C5(CH3)5), 1.29 (s, 9 H, O2C6H3(4-C(CH3)3)), —2.05 (br, 2 H, TaH2); llB
NMR (C6D6) 5 73 (A vm = 610 Hz); IR (cm") 1767, 1720 (vaH); Anal. calcd for
C301144BO2Ta: C, 57.4; H, 7.01. Found: C, 57.6; H, 7.29. eudo-l7—d2 was similarly
prepared from 16 and DBO2C6H3-4-'Bu: 1H NMR (C6D6) 5 7.41 (d, J = 1.8 Hz, 1 H,
02C6H3), 7.18 (d, J = 8.1 Hz, 1 H, O2C6H3), 6.95 (dd, J = 8.1, 1.8 Hz, 1 H, O2C6H3), 1.97
(s, 30 H, C5(CH3)5), 1.29 (s, 9 H, O2C6H3(4—C(CH3)3)); 2H NMR (C6H6) 5 --2.02 (br,
TaD2); “B NMR (c6136) 5 72 (A v”2 = 660 Hz). endo—l7-do-d2 was prepared as a
mixture of isotopomers by adding equal volumes HBO2C6H3—4—'Bu and DBO2C6H3—4-
'Bu of to a toluene solution of 16. The work up was analogous to that described above: 1H
NMR (C6D6) 5 7.41 (m, l H, 02C6H3), 7.17 (m, 1 H, O2C6H3), 6.82 (m, l H, O2C6H3),
1.98 (s, 30 H, C5(CH3)5), 1.29 (s, 9 H, O2C6H3(3—C(CH3)3)), —2.06 (br, 2 H, TaHD); 2H
NMR (C6116) 5 —2.02 (br, TaHD); ”B NMR (c6136) 5 70 (A m = 650 Hz).
Cp2NbH(BO2C6H4)2 (18). A 10 mL toluene solution of Cp2NbH(n2-CH2=CH2)
(274 mg, 1.09 mmol) was added to catecholborane (522 mg, 4.36 mmol). The resulting

solution was stirred overnight at room temperature. The color gradually changed to a

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orange—red and during the course of the reaction a yellow solid precipitated out of
solution. The solids were then allowed to settle and the filtrate was removed via cannula.
The solids were then washed with cold toluene. The product was isolated as yellow
microcrystals (137 mg, 27%): mp 178—181 °C (dec); IH NMR (C6D6) 5 7.00 (m, 2 H,
O2C6H4), 6.93 (m, 2 H, O2C6H4), 6.63 (m, 2 H, O2C6H4), 6.57 (m, 2 H, O2C5H4), 5.11 (s,
10 H, c5115), —7.71 (br, 1 H, NbH); ‘H NMR (THF—dg) 5 6.89 (m, 2 H, ozcer), 6.82 (m,
2 H, O2C6H4), 6.69 (m, 2 H, O2C6H4), 6.66 (m, 2 H, O2C6H4), 5.59 (s, 10 H, C5H5), -7.81
(br, 1 H, NbH); “B NMR (c606) 5 64 (A v.2 = 800 Hz), 5 60 (A v.,2 = 310 Hz); 1R (cm‘
I) 1651 (vaH). l8-d was similarly prepared from Cp2NbH(n2-CH2=CH2) and
catecholborane—d]: lH NMR (TI‘IF—dg) 5 6.85 (m, 2 H, O2C6H4), 6.77 (m, 2 H,
O2C6H4), 6.66 (m, 2 H, O2C6H4), 6.62 (m, 2 H, O2C6H4), 5.55 (s, 10 H, C5113), 2H NMR
(THF) —7.98 (br, NbD); llB NMR (THF—dg) 5 645 (A V172 = 880 Hz), 5 60 (A vr/2 = 270
Hz).

Phenethyl Alcohol Formation. Complex 3 (15.3 mg, 0.038 mmol) was dissolved
in 0.6 mL benzene—dé along with styrene (63.1 mg, 0.607 mmol) and HBO2C6H3—4-'Bu
(106 mg, 0.602 mmol). The solution was placed in an NMR tube and flame sealed. The
tube was then maintained at 75 °C for four (1 in a constant temperature oil bath.
Conversion of free borane to the alkyl borane product was monitored by lH NMR.
Integration revealed 64% hydroboration products along with 11% of a boron
decomposition product35»36 and 25% unreacted borane. The contents were then converted
to the corresponding alcohol by literature method.”5 The products of the oxidative
workup were analyzed by gas chromatography with phenethyl alcohol and sec-phenethyl
alcohol detected in a 3.72:1 ratio.

N MR Tube Reactions

3 + ctr-cyclooctene + HBO2C6H3-4-'Bu. Complex 3 (11.8 mg, 0.029 mmol)

was dissolved in 0.6 mL benzene—d6 with cis—cyclooctene (3.2 mg, 0.029 mmol) and

HBCat’ (4.4 mg, 0.025 mmol). The resulting solution was transferred to a NMR tube

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which was flame sealed. The was kept at room temperature for 2.5 h. A 1H NMR
spectrum showed primary hydroboration occurred with the bound propylene fragment on
the niobium complex with some hydroboration of the cis—cyclooctene. Integration of the
appropriate lBu resonances established the ratio of cis—cyclooctene2propylene
hydroboration as 125.08. The tube was held at room temperature overnight and
integration of another lH NMR spectrum determined the ratio of cis—
cyclooctenezpropylene hydroboration to be 1:3.81.

3 + cis-cyclooctene + propylene + HBO2C6H3-4—‘Bu. Complex 3 (9.8 mg,
0.029 mmol) was dissolved in 0.6 mL benzene—d6. HBCat’ (51.0 mg, 0.290 mmol) and
cis—cyclooctene (16.0 mg, 0.145 mmol) were added to the solution which was then
placed in a NMR tube. The was then frozen with liquid nitrogen and evacuated.
Propylene (0.145 mmol) was then admitted with the aid of a calibrated volume vessel.
The NMR tube was then flame sealed and the contents of the tube heated at 60 °C for
three (1. The rate of hydroboration for propylene was faster than that of cis—cyclooctene
based on integrated IH spectra (propylene:cis—cyclooctene = 4.53:1). Repeating the
reaction without the use of the niobium complex required heating the reaction at 120 °C
for three (I and resulted in nearly identical rates of hydroboration; however, the rate for
cis—cyclooctene was slightly faster (propylene:cis—cyclooctene = 0.93:1).

Cp*2Nb(H2BO2C6Ha) + CO. A 10 mg (0.210 mmol) sample of
Cp*2Nb(H2BO2C6H4) was dissolved in 0.8 mL benzene—d6. This solution was placed in
an NMR tube and frozen with liquid nitrogen. The NMR tube was evacuated and one atm
of CO gas admitted. The tube was then flame sealed and allowed to warm to room
temperature. 1H and 1]B NMR spectra indicated elimination of HBCat, and 26 was the
only Nb—containing product detected. Integration of the 11B spectrum indicated 91%
catecholborane (5 29, d, JHB = 180 Hz) and 9% of a B2Cat3 (5 23, s).

Cp*2Nb(H2BO2C6H4) + B2H6. A 15 mg (0.031 mmol) sample of the niobium

complex was dissolved in 0.6 mL toluene—dg and placed in 3 NMR. The solution was

86

frozen with liquid nitrogen and then evacuated. Diborane generation was performed in
the following way: A Schlenk tube was charged with NaBD4 (130 mg, 3.43 mmol) and
DME (0.875 mL). This slurry was cooled in a liquid nitrogen bath, and BF3-Et2O (0.580
mL, 3.52 mmol) was added under a counter flow of N2 gas. The Schlenk tube was fitted
with an adapter and the assembly was attached to a high vacuum manifold. After the
reaction vessel had been evacuated, the suspension was allowed to slowly warm to room
temperature with generation of B2H6. From a calibrated bulb, approximately 0.150 mmol
of diborane was condensed into a NMR tube. Once the addition of diborane was
completed, the NMR tube was flame sealed. IH and HB NMR spectra were initially taken
at —5 °C, with the temperature gradually raised to 20 °C over the course of three h. The
reaction cleanly formed 13 and catecholborane, with 1'B spectra indicating 92%
catecholborane (5 28, d, JHB = 190 Hz) and 8% of a boron decomposition product (5 22,
s).

4 + C2H4. A 15 mg (0.031 mmol) sample of the niobium complex was dissolved
in 0.6 mL toluene—(lg and placed in a NMR. This tube was then frozen with liquid
nitrogen and evacuated. Ethylene (one atm) was then admitted to the frozen contents of
the NMR tube, which was then flame sealed. 1H and 11B spectra were initially taken at 10
°C and increased to 40 °C over the course of the three h reaction time. The products of
the reaction were Cp*2NbH(n2—CH2=CH2) and free borane. Integration of the final llB
spectrum showed 18% EtBO2C6H3—4-'Bu (5 36, s), 74% of free borane (5 29, d, JHB =
190 Hz) and 8% of a boron decomposition product (5 23, s).

4 + C3H6. A 15 mg (0.028 mmol) sample of 4 was dissolved in 0.6 mL toluene—
d3. This solution was placed in a NMR tube and frozen with liquid nitrogen, followed by
evacuation of the gases. Approximately one atmosphere of propylene was then admitted
to the frozen contents of the NMR tube, after which it was flame sealed. Initial lH and
HB NMR spectra (taken at 0 °C) showed the formation of 3 and free borane. However, as

the reaction proceeded the concentration of the free borane diminished and the

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concentration of n-propylborane increased. After the three h reaction time, during which
the temperature had been gradually raised to 40 °C and then lowered to 25 °C, a llB
spectrum indicated clean conversion to n—propylborane (93%, 5 36, s) and a boron
containing decomposition product (7%, 5 22, s). A final lH spectrum revealed the
presence of 3.

4 + H2. A benzene—(16 (0.6 mL) solution of 4 (15.0 mg, 0.028 mmol) was placed
in a NMR tube. This solution was frozen using liquid nitrogen. Addition of one atm of H2
gas was preceded by evacuation of the frozen NMR tube. IH and HB spectra, initially
taken at 20 °C and gradually raised to 50 °C over the four h reaction time, indicated
formation of Cp*2NbH3 and free borane. Integration of the final 11B NMR spectrum
showed 80% of free borane (5 28.5, d, JHB = 190 Hz) and 20% of a boron decomposition
product, B2Cat3 (5 22.2, s).

exo and endo—27 + HBCat. A 21 mg (0.059 mmol) sample of 27 was dissolved in
benzene-d6 along with HBCat (15 mg, 0.12 mmol). The resulting solution was placed in
a NMR tube and flame sealed. An immediate 1H NMR demonstrated that endo—27 is
used preferentially at early reaction times with the formation of only one set of
alkylborane resonances with predominant formation of endo—1 and slight formation of
exo—l. Only exo—27 remained (44% of the original exo—27 present) upon standing at
room temperature for 12 h in the dark. Another IH NMR spectrum, after the tube was
heated at 40 °C for 2.5 h, revealed one set of alkylborane resonances, endo—1 (84%) and
exo—l (16%) with no detection of remaining exo—27.

endo-27 + DBCat. A 12 mg (0.035 mmol) sample of endo—27 was dissolved in 0.6
mL toluene with 8 mg DBCat (0.07 mmol) and placed in an NMR tube. The resulting
solution was allowed to react for several hours at room temperature. A 2H NMR spectrum
was recorded. Deuterium incorporation was witnessed only the hydride position of endo-

] and the a—carbon position of n—PrBCat. Integration of the 2H NMR spectrum revealed

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40% of the deuterium label was located in the alkyl borane and 60% of the label was
located in the hydride position of the tantalum boryl complex.

endo-27 + DBCat. Complex 27 (12 mg, 0.034 mmol) was dissolved in 0.6 mL
toluene—dg along with 8 11L DBCat (0.07 mmol) and placed in an NMR tube. The
resulting solution was allowed to react at room temperature for several hours. Integration
of the n—PrBCat alkyl resonances resulted in the ratio between a, [3, and yprotons as
1.52223.

Generation of 20. Complex 19 (20 mg, 0.076 mmol) was dissolved in 0.6 mL
toluene—dg and placed in a J—Young NMR tube. HBCat (9 11L, 0.08 mmol) was added to
the headspace of the NMR tube. The NMR tube was kept horizontal to prevent mixing of
the reagents. Upon removal from the glove box, the NMR tube was immediately placed
in a dry ice/isopropanol bath. Once the tube had been cooled, the contents of the NMR
tube were allowed to mix. The reaction was monitored at —20 °C. 1H NMR (toluene-dg) 5
7.08 (m, 2H, O2C6H4), 6.83 (m, 2H, O2C6H4), 4.88 (s, 10H C5H5), -4.40 (s, 1H, NbH), —
6.00 (br, 1H, NbH); llB NMR (toluene—<13) 5 59 (Aw/2 = 490 Hz). A NOESY spectrum
revealed a cross peak between the two hydride positions of 20.

19 + DBCat. A 23 mg (0.085 mmol) sample of 19 was dissolved in 0.6 mL toluene
and added to a J—Young NMR tube. DBCat (10 11L, 0.08 mmol) was added to the head
space of a J—Young NMR tube which was then capped. The tube was kept horizontal to
prevent mixing of the two reagents. Upon removal from the glove box, the tube was
immediately cooled in a dry ice/isopropanol bath. The contents of the NMR tube were
allowed to react at -20 °C as periodic 2H NMR spectra were recorded. Exclusive
deuterium incorporation was observed at the high—field resonance of 20 (2H NMR 5 -
6.21) at early reaction times. A gradual mixing of the deuterium label was seen between
the two hydride positions of 20 over 12 h at room temperature (2H NMR 5—4.42 and 5 —
6.21)

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In a separate experiment, 19 (10 mg, 0.039 mmol) was loaded into a J—Young NMR
tube. A solution of DBCat (10 mg, 0.078 mmol) in 0.6 m1 toluene was then added to the
NMR tube. The resulting NMR sample was quickly capped and kept at room temperature
for 6 h. A 2H NMR spectrum revealed the formation of 2—d0_2, 18—dp_1, propane—do_2 and
n-PrBCat—do_2. The ratio for deuterium incorporation at n-PrBCat, propane and hydride
positions was found to be 29:28:43 by integrating 2H NMR spectra (C7H3, -20 °C). The
isotopic distribution of deuterium between 01, fl, and ycarbons in n—PrBCat was 57:20:23.
For propane, deuterium was preferentially incorporated at the methyl positions as the
methyl and methylene resonances integrate in a 10:1 ratio.

19 + HBCat + CO. An 18 mg (0.067 mmol) sample of 19 was dissolved in 0.6 mL
toluene—dg and placed in a high pressure NMR tube. HBCat (8 1.1L, 0.07 mmol) was then
added to the head space of the NMR tube which was sealed with a teflon plug. The tube
is maintained as close to horizontal as possible to prevent a reaction from immediately
occurring. The tube is immediately placed in a dry ice/isopropanol bath upon removal
from the glove box. Once the toluene solution is cooled, the contents of the tube are
allowed to react at —20 °C for 30 m. A IH NMR (—20 °C) is then taken to assure 20 had
formed. The tube was then placed back in a dry ice/isopropanol bath and pressurized to
100 psi with CO. The contents of the tube were allowed to warm to room temperature. A
lH NMR taken at 25 °C revealed the formation of 25 as the major product along with 2
and 18.

Niobium Kinetics

endo-4 + CO. Typical sample preparation was as follows: endo—4 (5 mg, 0.00926
mmol) was dissolved in 0.550 mL toluene—dz; with a gas free syringe. The solution was
then placed in a high pressure NMR tube and closed. The NMR tube was removed from
the glove box and cooled in a dry ice/isopropanol bath. The NMR tube was then loosely
attached to a high pressure manifold while still immersed in the bath. The entire system

was purged with CO for several minutes. The manifold/NMR tube adapter was then

90

tightened and the NMR tube opened to the CO manifold. The tube was pressurized to 200
psi. The pressure was released to roughly 30 psi by cracking at the NMR tube
cap/manifold adapter junction and bleeding off excess CO. This cycle was repeated four
times. The NMR tube was closed following the final pressurization. The NMR tube was
removed from the manifold and kept in the cold bath. The NMR tube was quickly
transferred from the cold bath to the NMR probe precooled to 0°C. The reaction was
monitored through three half-lives with a kinetics program loaded on the NMR
instrument. Data were analyzed using KaleidaGraph.

endo-4—d2 + CO. The preparations and subsequent kinetic reactions were
performed in the same manner as that described above.

endo-4 + X 4-tert-butyl catecholborane + CO. A typical preparation is as
follows: endo—4 (5 mg, 0.00926 mmol) was dissolved in a minimal amount of toluene—dg.
To this solution varying quantities of 4—tert—buty1 catecholborane was added followed by
enough toluene—dg so the final volume was 0.550 mL. This solution was placed in a high
pressure NMR tube which was then removed from the glove box. The pressurization of
the NMR tube and the subsequent kinetics reaction was accomplished exactly as was
above.

endo-4 + 4-tert-butyl catecholborane + X CO. A typical preparation is as
follows: endo—4 (5 mg, 0.00926 mmol) was dissolved in a minimal amount of toluene-dz;
along with 10 mg of 4—tert—butyl catecholborane (0.0569 mmol). Additional toluene-(13
was added so the final volume was 0.550 mL. This solution was placed in a high pressure
NMR tube which was then removed from the glove box. The pre—pressurization of the
NMR tube was followed as explained above with the final pressure cycle halted at
various CO pressures.

endo-4 + CO at various temperatures. A typical sample preparation was as
follows: endo—4 (5 mg, 0.00926 mmol) was dissolved in 0.550 mL toluene—(13 with a gas

free syringe. The solution was placed in a high pressure NMR tube which was then

91

removed from the glove box. Pressurization of the NMR tube was accomplished as
described above. The NMR probe was cooled to different temperatures prior to the tube
transfer. Data analysis was performed in the same manner as described above.
Tantalum Kinetics

endo-l7 + CO. Typical sample preparation was as follows: 5 mg endo—l7
(0.00797 mmol) was dissolved in 0.550 mL toluene-dg with a gas free syringe. The
solution was then placed in a high pressure NMR tube and closed. The NMR tube was
removed from the glove box and cooled in a dry ice/isopropanol bath. The NMR tube
was then loosely attached to a high pressure manifold while still immersed in the bath.
The entire system was purged with CO for several minutes. The manifold/NMR tube
adapter was then tightened and the NMR tube opened to the CO manifold. The tube was
pressurized to 100 psi. The pressure was released to roughly 30 psi by cracking at the
NMR tube cap/manifold adapter junction and bleeding off excess CO. This cycle was
repeated four times. The NMR tube was closed following the final pressurization. The
NMR tube was removed from the manifold and heated in a constant temperature oil bath
maintained at 70 °C. At recorded time intervals, the reaction was quenched by rapid
cooling in an ice water bath. NMR were then recorded at ambient temperature. The
progress of the reactions was monitored by the ratio of the intensity of the Cp* resonance
of endo—l7 versus the total “intensity” of the Cp* resonances of endo—17 and 32. Data
were analyzed using KaleidaGraph.
Equilibrium Measurements

endo-4 + HBO2C6H3-3-'Bu. Three independent NMR tubes were prepared in the
same manner. endo-4 (10.6 mg, 0.0196 mmol) was dissolved with 4.6 mg HBO2C6H3—3—
'Bu (0.0262 mmol) and 2 [IL hexamethyldisiloxane (as a convenient internal standard) in
0.550 mL benzene-d1; The resulting solution was loaded in a NMR tube and allowed to
stand in the glove box. At three different time periods, ranging from 8 h to 2 (1, NMR

spectra were taken at 25 °C of each tube. Once equilibrium had been achieved, the

92

equilibrium constant was calculated using the integrated area of the respective methyl
resonance of the four species in solution. The NMR tubes were stored in a glove box
while spectra were not being acquired. All other equilibria experiments, using different
catecholboranes and borane adduct compounds, were conducted in the same manner as
described here, utilizing the integrated areas of convenient, well resolved resonances.
Single Crystal X-ray Structural Determinations and Refinement.

Determination and Refinement of endo and exo-l. Diffraction quality crystals
of endo—1 and exo—1 were grown by slowly cooling concentrated toluene solutions to —80
°C. Crystals of endo-1°(C7Hg)1/2 were coated with Paratone—n and a suitable crystal was
chosen and mounted on a glass fiber. Data were collected at -100 °C on a Rigaku AFC6
diffractometer using Mo Kat radiation (A = 0.71069 A). Cell parameters were determined
from a list of reflections collected using an automated random search program. Three
standard reflections were recollected after every 150 scanned reflections with no
significant change in intensity noted. Crystals of exo—l were mounted in similar fashion
and data were collected at —85 °C on a Nicolet (Siemens P3) Autodiffractometer using
Mo K01 radiation (A = 0.71069 A). Cell parameters were determined by least—squares
analyses of 30 centered reflections chosen from precession photographs. Three standard
reflections were recollected after every 150 scanned reflections with no significant
change in intensity noted.

Structures were solved using direct methods and Patterson techniques available in
the teXsan package of crystallographic programs and SHELXS—86. Positional and
thermal parameters were refined using SHELXL—93 and the TEXSAN software. ‘IJ—scan
based empirical absorption corrections were applied to each data set. All non—hydrogen
atoms were refined anisotropically and difference maps revealed over 70% of the
hydrogen atoms on the cyclopentadienyl and catecholate groups which justified using

their calculated positions in later least—squares cycles. In both cases, hydride positions

93

were located in the final difference maps and positional and isotropic thermal parameters
were refined in the last least-squares cycles.

endo—l crystallized in a monoclinic crystal system with systematic absences
indicating the space group P21/n (#14) with the following cell parameters: a = 10.594(6)
A, b = 10.902(8) A, c = 14.545(8) A, 13 = 92.07(5)°, V = 1679(2) A3, 2 = 4. Density
(calculated) = 1.892 mg/cm3. The 2686 independent reflections were collected between
4.66 S 26 S 49.9° using 00/26) scans. Full—matrix least—squares refinement on F2 (2686
data, 100 restraints, 353 parameters) converged to give the following values: R1 =
0.0211, wR2 = 0.0503, GOF = 1.137 for [I > 20(1)]. For all data: R1 = 0.0357, wR2 =
0.1348. R1 = 207,2 — Fcz)/ZF,,2, wR2 = [(w(F,,2 — F.2)2/):(lr,,2)2)]“2 and GOF = (2018,21 —
IF,.2|)/O')/(n - m) where w = 1/02(F(,2), n is the number of reflections used and m is the
number of variables.

exo—1 crystallized in a orthorhombic crystal system with systematic absences
indicating the space group Pbca (#61) with the following cell parameters: a = 15.859(4)
A, b = 18. 131(5) A, c = 10.025(3) A, v = 2882.6(14) A3, 2 = 8. Density (calculated) =
1.991 mg/cm3. The 3325 independent reflections were collected between 5.14
S 26) S 55. 12° using (1)/29 scans. Full—matrix least—squares refinement on I"2 (3325 data,
0 restraints, 246 parameters) converged to give the following values: R1 = 0.0452, wR2 =
0.0962, GOF = 1.062 for [I > 20(1)]. For all data: R1 = 0.0752, wR2 = 0.1098. R1 = 20702
— F,,2)/2F,,2, wR2 = [(w(F.,2 — 12.2)2/2:(F,,2)2)]"2 and GOF = (2018,31 — ch2|)/0')/(n - m)
where w = 1/02(F,,2), n is the number of reflections used and m is the number of variables.

Determination and Refinement of endo-12. Data were collected at 173 K on a
Rigaku AFC6 diffractometer using Mo Ka radiation (A = 0.71069 A). Final unit cell
parameters were obtained by least—squares refinement of 25 reflections that had been
accurately centered. Three standard reflections were recorded after every 150 scans, with
no significant change in intensity noted. The structures were solved using SHELXL—86.

Atomic coordinates and thermal parameters were refined using the full—matrix least—

94

squares program, SHELXL—93, and calculations were based on F2 data. All non—
hydrogen atoms were refined using anisotropic thermal parameters. All crystallographic
computations were performed on Silicon Graphics Indigo computers.

Orange—red crystals of endo—12 (fw 539.7) were grown from a toluene/heptane
solution cooled to —35 °C. The crystals were coated with Paratone—N and a suitable
crystal was selected and mounted on a glass fiber. Compound 12 crystallized in a triclinic
crystal system with systematic absences indicating the space group PT (#2) with the
following cell parameters: a = 9.923(3) A, b = 12.109(4) A, c = 13.588(4) A, 01 =
97.74(2)°, [3 = 100.86(2)°, y: 107.09(2)°, v = 1500.9 A3, 2 = 2. Density (calculated) =
1.298 mg/cm3. The 8555 independent reflections were collected between 4.0
S 26 S 60.0° using 00/26 scans.

All non-hydrogen atoms except one Cp* methyl carbon and the toluene solvent
molecule were found using SHELXS—86. DIRDIF WFOUR was used to extend and
initially “refine” the structure. The toluene solvent molecule was found and modeled later
to fit the difference map peaks. SHELXL—93 was used to refine the structure. The
hydrogen atoms of each Cp* methyl group were calculated as two sets of half—occupancy
positions, rotated initially about the C—C bond by 60° from each other, and refined with
occupancy factors of x and l—x and with a common isotropic displacement parameter.
Later, a single set of hydrogen atoms for each Cp* methyl group was placed at the
occupancy—weighted average H positions. Restraints were applied separately to each
methyl group to make all C—H distances the same, all H—H distances the same, all C—C—
H angles the same, and to use a common isotropic displacement parameter. The hydrogen
atoms of the boryl ring and its ten—butyl group were placed in calculated positions and
refined independently without restraints. The toluene solvent molecule is disordered
about an inversion center with both half—occupancy molecules coplanar and some atoms
(e.g., C32 and C36*, etc.) nearly overlapping each other. The ring protons were

restrained to lie 0.95 A from their carbon atoms and equidistant from the two adjacent

95

carbon atoms. The seven carbon atoms and the five ring hydrogen atoms of the toluene
molecule were restrained to lie in a plane. The methyl hydrogen atoms were treated as the
Cp* methyl hydrogens above. Full-matrix least-squares refinement on F2 (8548 data,
181 restraints, 462 parameters) converged to give the following values: R1 = 0.0502, wR2
= 0.1220, GOF = 1.067 for [I > 20(1)]. For all data: R1 = 0.0730, wR2 = 0.1402. R1 =
209,? — 18257217,}, W = [(w(F,,2 — li‘.2)2/>:(F,,2)2)]”2 and GOP = (2019,21 — IF,.2|)/0')/(n —
m) where w = 1/02(F,,2), n is the number of reflections used and m is the number of
variables.

Determination and Refinement of 13. Data were collected at 173 K on a Rigaku
AFC6 diffractometer using Mo K01 radiation (A = 0.71069 A). Final unit cell parameters
were obtained by least—squares refinement of 25 reflections that had been accurately
centered. Three standard reflections were recorded after every 150 scans, with no
significant change in intensity noted. The structures were solved using SHELXL—86.
Atomic coordinates and thermal parameters were refined using the full—matrix least-
squares program, SHELXL—93, and calculations were based on I72 data. All non—
hydrogen atoms were refined using anisotropic thermal parameters. All crystallographic
computations were performed on Silicon Graphics Indigo computers.

Green crystals of 13 (fw 377.7) were grown from a concentrated solution of
heptane cooled to -35 °C. The crystals were coated with Paratone—N and a suitable
crystal was selected and mounted on a glass fiber. Compound 13 crystallized in a
monoclinic crystal system with systematic absences indicating the space group P21/c
(#14) with the following cell parameters: a = 8.482(2) A, b = 25.306(6) A, c = 9.924(3)
A, [3 = 1 l3.84(2)°, v = 1948.39(9) A3, 2 = 4. Density (calculated) = 1.298 mg/cm3. The
3423 independent reflections were collected between 4.0 S 29 S 50.0° using a) scans. All
non—hydrogen atoms were located in the solution found by SHELXS—86. Methyl
hydrogens for the Cp* ligand were placed in calculated positions (0.95 A) and their

isotropic thermal parameters were allowed to ride on the parent carbon atoms. The

96

hydrides were located in the difference-fourrier map and were refined isotropically. Full—
matrix least—squares refinement on F2 (3402 data, 30 restraints, 335 parameters)
converged to give the following values: R1 = 0.0455, wR2 = 0.1110, GOF = 1.139 for [I
> 20(1)]. For all data: R1 = 0.1 125, wR2 = 0.1566. R1 = 2m} — FEVER}, wR2 = [(w(F,,2
— I~1.2)?-/2(lr.,2)2)]”2 and GOF = (2(1F021— |F2.2|)/0')/(n — m) where w = 1702(F02), n is the
number of reflections used and m is the number of variables.

Determination and Refinement for 18. Data were collected at 173 K on a
Rigaku AFC6S diffractometer using Mo K01 radiation (A = 0.71069 A). Final unit cell
parameters were obtained by least—squares refinement of 25 reflections that had been
accurately centered. Three standard reflections were recorded after every 150 scans, with
no significant change in intensity noted. The structures were solved using SHELXL—86
in the teXsan software package. Atomic coordinates and thermal parameters were refined
using the full—matrix least—squares program, SHELXL—93, and calculations were based
on F 2 data. All non—hydrogen atoms were refined using anisotropic thermal parameters.
All crystallographic computations were performed on Silicon Graphics Indigo computers.

Lemon yellow crystals of 18 (fw 461.5) were grown from a concentrated acetone
solution cooled to —80 °C. The crystals were coated with Paratone—N and a suitable
crystal was selected and mounted on a glass fiber. Compound 18 crystallized in a
monoclinic crystal system with systematic absences indicating the space group C2/c
(#15) with the following cell parameters: a = 15.585(6) A, b = l3.789(9) A, c = 12.701(7)
A, p = 120.86(3)°, v = 2343(2) A3, 2 = 4. Density (calculated) = 1.309 mg/cm3. The 2029
independent reflections were collected between 4.0 S 28 S 50.0° using (1)/29 scans.

All non—hydrogen atoms were located in the solution found by SHELXS—86. The
hydrogen atoms of the Cp ring and the boryl ring were placed in calculated positions and
refined independently without restraints. The hydride was located in the difference-
fourrier map and was refined isotropically. Full-matrix least—squares refinement on I“2

(2029 data, 0 restraints, 184 parameters) converged to give the following values: R1 =

97

0.0542, wR2 = 0.1411, GOF = 1.077 for [I > 20(1)]. For all data: R1 = 0.1167, wR2 =
0.1771. R1 = 208,2 — PCS/2:18,}, wR2 = [(w(F.,2 — F,.2)2/2(F,,2)3)]m and GOF = (2012,21 -—
IF,.2l)/0)/(n — m) where w = 1/02(F,,2), n is the number of reflections used and m is the
number of variables.

Determination and Refinement for endo-l7. Data were collected at 143 K on a
Siemens CCD diffractometer using Mo K01 radiation (A = 0.71073 A). Data were
collected as 90 second frames in a hemisphere of space. Final unit cell parameters were
obtained by least—squares refinement of accurately centered reflections obtained from 60
frames of collected data. Smart was employed to obtain a unit cell and Saint was utilized
to integrate the 1024 collected frames. SADABS was then used to apply absorption
corrections to the data. The structures were solved using SHELXL—86. Atomic
coordinates and thermal parameters were refined using the full—matrix least—squares
program, SHELXL—97, and calculations were based on F 2 data. All non—hydrogen atoms
were refined using anisotropic thermal parameters. All crystallographic computations
were performed on Silicon Graphics Indigo computers.

Colorless crystals of endo—17 (fw 628.4) were grown from a concentrated pentane
solution cooled to —35 °C. The crystals were coated with Paratone—N oil and a suitable
crystal was selected and mounted on a glass fiber. Compound 17 crystallized in a
monoclinic crystal system with systematic absences indicating the space group C2/c
(#15) with the following cell parameters: a = 36.608(7) A, b = 9.974(2) A, c = 15.702(3)
A, [3 = 103.29(3)°, v = 5579.8(19) A3, 2 = 8. Density (calculated) = 1.496 mg/cm3. The
16046 independent reflections were collected between 4.14 S 26) S 56.72°.

All non—hydrogen atoms were found using SHELXS-86. DIRDIF WFOUR was
used to extend and initially “refine” the structure. SHELXL—97 was used to refine the
structure. The hydrogen atoms of each Cp* methyl group were placed in calculated
positions and allowed to “ride” their parent carbon atoms without restraints. Likewise,

hydrogen atoms of the catecholate ring and the tert—butyl group were placed in calculated

98

positions and allowed to “ride" their parent carbon atoms without additional restraints.
The two hydrogen atoms bound to tantalum were found through difference maps and
were refined isotropically. Full—matrix least—squares refinement on F2 (6466 data, 0
restraints, 315 parameters) converged to give the following values: R1 = 0.0716, wR2 =
0.1220, GOF = 1.053 for [I > 20(1)]. For all data: R1 = 0.0876, wR2 = 0.1462. R1 = 2(F02
— F,.2)/>:F,,2, wR2 = [(w(F,,2 — riff/20,551"?- and GOF = (2(IF,,2| — 1F62|)/0)/(n — m)

where w = 1/02(F,,2), n is the number of reflections used and m is the number of variables.

99

APPENDICIES

100

APPENDIX A

Derivation of Model for Possible endo-1 Equilibrium

C /H C /H\
A

Cl is defined as e’AE/RT where R = 1.9872 cal/mol K

a is the mole fraction of A and b is the mole fraction of B

C1=bla

a+b=1

b=1—a

C,=(l—a)/a

C1=1/a—1 b=1—[l/(Cr+1)]

C1+1=1Ia b=(Cl+1)/(C,+1)—1/(C]+1)
1/(C,+l)=a b=C,/(C,+1)

5Ob5 -_- 3(6A) + b(6B)
5obs = 1/(Cl + 1)(5A) + Cl/(C1+ ”(53)
50m = [6A + CI(6B)]/(CI + I)

5

0b, = [5A + .«a‘AE’RR5,3)1/(e‘AE’RT + l)

101

APPENDIX B

Derivation of the Rate Law for Pathways C and D for the Reaction of 17 plus CO

 

 

 

 

 

 

"SE“ = kllAl-k_1[Bl
dfi] = k. [Al-L1 [Bl—k2 [B11C01+k-2 [C1=0
dict” = kleHCOl-k-lel-k3[Cl=0
klellCOl = k-2[C1+k3[C]
k2 [B][CO] = [C](k_2+k3)
k211311C0]
[C] = k_2+k3
k B CO
kl{Al-kn[Bl-kleO1lBl+k-z< 1:113 1) =

 

k—l [81+ 1., [c0] [B] —< k"2 k2 [B] [C(31) = k. [A]

 

 

 

 

k_2 + k3
k_2 k2 [B]+ k_ k3 [Bl+ k_ 2k2 [B _k_ 2k2 [B +k2 k3 [B] [CO]
k_2+k3 k_ 2+st3 _2+k3 _2+k3 k_2+k3

= k A
[B] lt_2k. + k_,k3 + k2k3[CO] _ k [A] 1”
k_+2 k3 k_ 2+k3 k__2+k3 - ‘

k_2 k2 + k__] k3 + k2 k3 [CO]
[Bl{ k_2+k3 = k. [A]

[B] = k, 1Al{ k"? + k3 }

 

k_2 k2 + lr_l k3 ,1 k2 k3 [CO]

 

102

 

 

 

 

 

—d A k—2+k3
—d—[_] = k‘[A]—k" MA] k k co }
t k_2k2 +.k_] k3 + 2 3[ I
—d[A] k__2+k3
—— = k A—k k A
dt 1” “ ‘[ ]{ I(_2I(2 + lt_,lt3 + k2k3[CO]
k k k +1: k_ k
—d[A] = [A] kl— 1 -1 —-2 1 1 3
dt k_2 k2 + k_1k3 + k2 k3 [CO]
at high [CO]: ’ddltA] = k] [A]

103

APPENDIX C

IR Spectroscopic Data for Cp’2M(H2BX2) and Cp’2MH3 Compounds

 

 

Compound VBHM(cm'l)“ VMH(cm")"
Cp*2Nb(H2BCat)59 1653
Cp*2Nb(H2BCat’) (4)59 1653
Cp*2Nb(H2BO2C6H3—3—'Bu) l 649
(12)59
Cp2NbH2(BCat)20 1742, 1675b
Cp2Nb(H2BCgH14)20 1605
Cp2NbH(BCat)2 (18)"16 1651
Cp*2NbBH4 (13)114 1728, 1620
Cp2NbBH4'47 1745, 1628
Cp*2TaH2(BCat)59 1774, 1709
Cp*2TaH2(BCat') (17)59 1767, 1720
endo—Cp2TaH2(BCat) (endo—l)6O 1768
exo—Cp2TaH2(BCat) (exo—l)60 1771, 1703
Cp*CpTaBH..137 1747, 1621
Cp2TaB1-14148 1882-1860
Cp*2NbH3H4 1747, 1695
Cp2NbH3149 1710
Cp*2TaH3 (33)68 1750, 1720
Cp2TaH3150 1735

 

104

“ Recorded as Nujol mulls unless otherwise indicated. b KBr pellet.

REFERENCES

105

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107

B—C of the endo isomer occupies the central position in the metallocene wedge, while the
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(44) The isomeric silylhydrides were not separated as pure compounds.
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(54) Vibrational zero point energies can be expressed as a function of the force constant
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the equilibrium between the two structures. In a limiting structure, the change of nuclear
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a downfield shift in resonance. There is not a static structure of the boryl fragment with
the boryl fragment plane orthogonal to the metallocene wedge plane in solution which
limits the utility of this explanation. Another possible explanation deals with the ring
current from the catecholate ligand interacting with the hydride ligands. The boryl
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the solid state which would serve to further shield the hydride ligands and shift the
resonance upfield. Because the chemical shift moves downfield, this explanation would
also be of limited usefulness.

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(91) The assignment of the resonances due to each isomer is not trivial. The original
assignment of the Cp resonances for the two isomers was incorrect (ref 93). A subsequent
reinvestigation of these complexes correctly identified the resonances of the each isomer
(ref 115).

110

(92) Although B—H activation in Figure 42 is depicted as preceding through oxidative
addition/reductive elimination sequences, a mechanism predicated on 0-bond metathesis
is also plausible.

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(95) A conceivable, but unlikely, scenario could lead to temperature independent shifts
for equilibrating structures. If entropic contributions to AG for the borane adduct and the
diborylhydride (or any other equilibrating structures) are identical, or the limiting hydride
chemical shifts for different structures are coincident, the equilibrium could be
temperature dependent.

(96) A process that rapidly exchanges coordinated and uncoordinated oxygen atoms
would give two resonances. This exchange would not effect arguments based on IIJHBI,
chemical shift or isotope effects on chemical shifts.

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(104) Using an average estimate for a Nb—H distance (1.65 A), and the extrema for B-H
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range for the angle defined by the midpoint of the B—H vector, Nb, and the exo boryl
ligand is 80 > 0 > 74. This value is typical for a d2 complex and smaller than the range
for X—M—X angle in (10 complexes (94-97°). Clearly this comparison suffers from the ill-
defined hydride position in the diboryl compound. Furthermore, the electronic—based
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diboryl compound may limit direct comparison to Cp2Ti(n2—HBCat)2.

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(116) [3—H elimination from the intermediate Cp2TaH(CH2CHDCH3) will yield endo—
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Cp2TaH(CHD=CH(Me)) or exo—Cp2TaH(CH2=CH(CH2D)) upon fl—H elimination.

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(119) Integration of the Ta deuteride and alkylborane resonances in the 2H NMR
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adventitious proton sources makes integration of the 2H resonances unreliable for
determining the extent of deuterium incorporation in the alkylborane product. Thus, the
deuterium incorporation is more accurately determined by integrating the propyl
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