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THESIS

 

 

ABSTRACT

THE PREPARATION OF SOME
TETRACHLORODIALKOXOMOLYBDENUM(V) COMPLEXES

by David A. McClung

Four compounds, all having the general composition BH[Mo(OR)2Clg],
have been prepared and characterized. BH+ was pyridinium, quinolinium,
or tetramethylammonium ion. The alkoxo group was either methoxo or
ethoxo.

The tetrachlorodimethoxomolybdate(V) compounds were prepared by
reacting molybdenum pentachloride with dry methanol, followed by the
addition of a mixture containing the organic base or (CH3)nNCl in
alcohol. All attempts to prepare the analogous ethoxo compounds by
this method resulted in the formation of mixed products.

The pyridinium tetrachlorodiethoxomolybdate(V) complex was pre-
pared by alkoxo exchange between the methoxo complex and ethanol.

The infrared spectra of the compounds show intense absorption in
the region 1040-1050 cmfl, which is typical of the C-0 stretching
vibration in alkoxides. The 0-H vibrational mode was not found in
the infrared spectra of any of the compounds. The compounds are para—
magnetic and have magnetic moments corresponding to the spin-only
value of 1.73 B.M., which is characteristic of the 4d1 configuration
of molybdenum(V).

Each of the compounds exhibit two bands in the visible region.

The bands are assumed due to the reduction of symmetry from octahedral

David A. McClung
to tetragonal. A third band is apparently masked by a charge trans-

fer peak.

THE PREPARATION OF SOME

TETRACHLORODIALKOXOMOLYBDENUM(V) COMPLEXES

By

David A. McClung

A THESIS

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

MASTER OF SCIENCE

Department of Chemistry

1966

ACKNOWLEDGMENT

The author wishes to express his appreciation to Professor Carl
H. Brubaker, Jr., for his guidance and inspiration during this in-
vestigation.

Sincere thanks are extended to my wife, Marcella, for her pa-
tience, understanding, and talent which helped to make this work
possible.

Appreciation is also extended to the National Science Founda-

tion for financial assistance.

ii

I.

II.

III.

INTRODUCTION .

EXPERIMENTAL

Preparation and Standardization

Materials .

TABLE OF CONTENTS

Analytical Methods

Apparatus . . .

Preparation of Compounds

Spectroscopic Measurements

Magnetic Susceptibility Measurements

RESULTS AND DISCUSSION .

APPENDIX .

REFERENCES

iii

Page

11
11
14
30

35

10.

LIST OF TABLES

The comparison of the infrared spectra of pyridinium
ion and ligand pyridine with that of C5H6N[Mo(0CH3)ZClq]

The comparison of the infrared spectra of quinolinium
ion and ligand quinoline with that of C9H3N[Mo(OCH3)2014]

The comparison of the infrared spectra of pyridinium
ion and ligand pyridine with that of C5H6N[Mo(OC2H5)2C1q]

Visible absorption maxima of the tetrachlorodialkoxomolyb-
dates 0 O O O O O O O O O O O O O O O O O O O O O O O 0

Magnetic susceptibilities and moments of the alkoxo
molybdenum(V) compounds . . . . . . . . . . . . . . . .

Electron spin resonance data for powder samples of the
tetrachlorodialkoxomolybdates(V) . . . . . . . . . . . .

Electron spin resonance data for liquid samples of
pyridinium tetrachlorodimethoxomo1ybdate(V) . . . . . .

Electron spin resonance data for liquid samples of
quinolinium tetrachlorodimethoxomolybdate(V) . . . . . .

Electron spin resonance data for liquid samples of
tetramethylammonium tetrachlorodimethoxomolybdate(V) . .

Electron spin resonance data for liquid samples of
pyridinium tetrachlorodiethoxomolybdate(V) . . . . . . .

iv

Page

15

16

17

18

28

32

32

33

33

34

LIST OF FIGURES

Figure Page
1. Infrared spectrum of C5H5NIMo(OCH3)2C14] (Nujol mull) . . . l9
2. Infrared spectrum of C9H8NIMo(OCH3)2C1g] (Nujol mull) . . . 20
3. Infrared spectrum of (CH3)uNIMo(OCH3)2Clq] (Nujol mull) . . 21
4. Infrared spectrum of 05H6N[Mo(OC2H5)2Cls] (Nujol mull) . . 22
5. Visible absorption spectrum of C5H6N[Mo(OCH3)ZClg] . . . . 23
6. Visible absorption spectrmm of C9H3N[Mo(OCH3)ZClg] . . . . 24
7. Visible absorption spectrum of (CH3)uN[Mo(OCH3)ZClq] . . . 25

8. Visible absorption spectrum of C5H6N[Mo(OCZH5)2Clq] . . . . 26

INTRODUCTION

Molybdenum, the congener of chromium, has a variety of oxidation
states which range from +2 to +6. However, the higher oxidation states
are more common and more stable with the most common one being the
+6 state. One of the common compounds containing molybdenum in the
hexavalent state is the trioxide, M003.

Molybdenum pentachloride, a compound in which molybdenum has a
4d1 configuration, has a spin-only moment of 1.73 B.M. Compounds of
molybdenum in the di-, tri-, and tetravalent states are not very nu—
merous.

Alkoxides of a few transition metals have been reported, with
those of Groups IV B and V B the most extensively studied. Bradley
has published review articles on a number of metal alkoxides (1,2,3).

Titanium tetraalkoxides of Group IV B were first synthesized
by Demarcay (4). The first results of structural significance of
these titanium complexes were given by Claughan, who found that the
degree of polymerization was concentration dependent, increasing
from unity at low concentration to a maximum of three (5). On the
other hand, Bradley found that the degree of polymerization was con—
stant at 2.4 over a reasonable concentration range in boiling benzene
(6). Recently, Ibers has reported preliminary details of the crystal
structure of Ti(OCZH5)s as determined by X—ray diffraction and con-
cluded that the substance is tetrameric in the solid state (7).

Bradley observed that there was an increase in volatility of

1

the zirconium tetraalkoxides as the alkyl group was changed from
primary to secondary to tertiary (8) and that the volatility was
directly related to the molecular complexity (9). The molecular
complexities of the zirconium tetraalkoxides were the same as those
of the analogous hafnium derivatives (10). The mechanism for the
reaction involved in the preparation of the zirconium and hafnium
tetraalkoxides has been studied (11).

A number of alkoxides of the Group V B have been prepared.

The syntheses of vanadium trialkoxides (12) and tetraalkoxides (13)
has been reported. Vanadium tetraethoxide was found to have a

value of 2.04 for the degree of polymerization, suggesting that it

is predominantly in the dimeric form (14). The value of 2.04 for
vanadium tetraethoxide is slightly lower than for the corresponding
titanium alkoxide. The lower value is attributed to a difference in
steric effects (15). Thus, in the case of a given alkoxide group,
shielding of the central atom and consequent opposition to polymer-
ization will depend on the size of the central atom, if other factors
are equal.

Pentaalkoxides of niobium were prepared and found to be dimeric
in boiling benzene (16). Wentworth and Brubaker reported a number of
complexes of niobium, including (C5H5N)2[Nb(0R)C15] (R - CH3, CZHS, or
i-C3H7), [NbC1(OC2H5)2(C5H5N)]2, and Nb(0C2H5)u (17,18,19). The
pyridinium pentachloroalkoxoniobate compound was prepared by electro-
1ytic reduction of niobium pentachloride in HCl-saturated alcohols,
followed by addition of alcoholic pyridinium salts and showed normal,
spin-only paramagnetism. The other two compounds, Nb(0C2H5)u and

[NbC1(0C2H5)2(C5H5N)]2, were diamagnetic and the dimer consisted of

two octahedra joined by bridging chloride ligands.

Tantalum pentaalkoxides were investigated by Bradley and were
found to exist as dimers in benzene (20).

Chromium triethoxide was one of the first alkoxides of the Group
VI B metals to be prepared. This compound was obtained by the re-
action of chromium trichloride with sodium ethoxide (21).

A few molybdenum alkoxides have been reported, all of which
contain molybdenum in the +5 oxidation state. The compound
Mo(OCH3)2013-3CH30H was reportedly prepared by reaction of molybdenum
pentachloride with methanol at -78°C. The compound precipitated in
large green crystals after standing in a stoppered vessel for several
days (22). Upon treatment of this compound with an excess of a
methanol-pyridinium chloride solution, light green needle—shaped
crystals of 05H6N[Mo(OCH3)2Cls] were reportedly formed (23). Accord-
ing to Funk (24), reaction of molybdenum pentachloride with methanol
followed by addition of a pyridinium chloride solution resulted in a
mixed product if the intermediate compound was not isolated. The
compound Mo(0CH3)3C12 supposedly resulted from the addition of an
excess of methanol and pyridine to pyridinium tetrachlorodimethoxo-
molybdate(V) (25). Funk (26) also obtained black crystals of
Mo(0CH3)qC1 when an excess of pyridine was added to a solution of
molybdenum pentachloride in methanol. Unfortunately, no properties
or characterization other than elemental analyses were reported for
any of these compounds.

Chloride alkoxides of tungsten have been reported (27,28).
Compounds of the composition WCl3(0R)2 (R= CH3, CZHS) were obtained

by reaction of tungsten hexachloride with the corresponding alcohol.

 

 

From a solution of WC13(0C2H5)2 the diamagnetic dimer, W2C1u(0C2H5)6,
was obtained. A bi-octahedral structure with two bridging chloride
ligands has been proposed for the structure of the dimer (29). Re—
cently compounds of the type W0(0R)4 have been synthesized by the
reaction of WOCls with ammonia in alcohol (30).

Alkoxides and chloroalkoxides of copper (II) were reported by
Brubaker and Wicholas (31). These complexes were found to have
magnetic moments lower than the spin—only moment of 1.73 B.M. Other
known alkoxides of transition elements include Mn(OCZH5)2 (32) and
Fe(0C2H5)3 (33,34).

Also of interest are the vanadyl (V0+2) derivatives, where
vanadium exists as V+4, with the electronic structure [Argon]3d1.

The most significant feature of the electronic structure seems to be

the existence of considerable oxygen to vanadium n-bonding. In general,
the magnetic moments of powdered samples and aqueous solutions of
vanadyl complexes are all approximately equal to the spin-only value

of 1.73 B.M. for one unpaired electron (35).

The chromyl and molybdenyl ions, CrO+3 and M00+3, are electron—
ically equivalent to VO+2, since they can be formulated as containing
Cr+5 (3d1) and Mo+5 (4d1). “Compounds containing these ions are rare,

and the only well characterized complexes containing CrO+3 and MoO+3

are of the types M’2[M0C15], where M’ is NHn+, Na+, K+, Rb+, or Cs+
and M is Cr(V) or Mo(V) (36). These complexes are assumed to have a
tetragonal structure, with a short M-O bond. The electronic tran-
sitions observed for compounds containing the [MoOCls]-2 ion showed

bands at 14,050 and 22,500 cm-l, corresponding to the transitions

b2--+e and b2-—-#b1 respectively (37). The b2-—+a1 transition is

apparently masked by the charge transfer band. Charge transfer bands
were found at 32,200 and 41,700 cmfl. The electron spin resonance
spectra and magnetic susceptibilities of (NHg)2[MoOC15] and Rb2[Cr0015]
were obtained and reported by Hare (38). The average effective moment
for (NHa)2[M00C15] after correction for temperature independent para-
magnetism was 1.67 B.M.

Although there is a slight difference in the symmetry of compounds
containing the [MoOC15]-'2 ion and the tetrachlorodialkoxomolybdate
complexes, the spectra of the compounds containing the [MoOC15]-2 ion

can be used qualitatively in interpreting the spectra of the tetra-

chlorodialkoxomolybdates.

EXPERIMENTAL
Preparation and Standardization g£_Reagentsm

Silver Nitrate.-- The 0.1 N solution of silver nitrate was prepared
by dissolving 8.5 g of "Baker Analyzed" reagent grade silver nitrate
in 500 m1 of demineralized distilled water. The concentration of
this solution was determined by titration with a standard sodium

chloride solution to the potassium chromate end point (39).

Ammonium Thiocyanate.—-To prepare a solution of 0.1 N ammonium thio-
cyanate, 7.6 g of Mallinckrodt reagent grade ammonium thiocyanate

was dissolved in distilled water to give one liter of solution. The
concentration was determined by titrating with standard silver nitrate

solution to the ferric alum end point (40).

Cerium(IV) Sulfate.--A 0.1 N ceric sulfate solution in sulfuric acid
was prepared by dissolving 40 g of cerium(IV) sulfate in 56 ufl.of a
1:1 sulfuric acid solution. After the addition of 400 ml<of distilled
water, the solution was warmed until the material had dissolved come
pletely. After filtering, the solution was diluted to one liter (41).
The concentration of the solution was determined by titration with a

standard A5203 solution (42).
Materials

Molybdenum Pentgchloride.--Molybdenum pentachloride was obtained from

6

K & K Laboratories and was used without further purification. The

manufacturers claim that the purity of the compound was 99.8%.

Pyridine and Quinoline.-—These were purchased from Matheson, Coleman,

and Bell. Both were dried by refluxing over barium oxide and distilling.

Tetramethylammonium Chloride.--Tetramethy1ammonium chloride was East-
man White Label grade. It was dried for eight hours at 100°C before

use.

Nitrogen and Hydrogen Chloride.-—Both the anhydrous hydrogen chloride
and nitrogen were in cylinders and obtained from the Matheson Co. The
hydrogen chloride was bubbled through concentrated sulfuric acid before

use. Nitrogen was used without further purification.

Methanol.--Methanol was dried by reacting it with magnesium turnings.

It was then refuxed and distilled.

Ethanol.--"Absolute" ethanol was dried by reacting it with sodium and

subsequent distillation.

Solvents.--0ther solvents were used for washing and purification.
Pentane was distilled over phosphorus pentoxide. Chloroform was

Fischer reagent grade and was used without further purification.
Analytical Methods

Molybdenum Analyses.--Molybdenum analyses were done according to the
procedure given by Kolthoff (43) and Busev (44). Enough hydrochloric
acid was added to the sample in water to make the acidity about 3 N.

The solution was then titrated with 0.1 N cerium(IV) sulfate.

Chloride Analyses.--Chlorides were determined by the Indirect Volhard
Method with excess standard silver nitrate and back titration with

ammonium thiocyanate (45).

Carbon, Hydrogen, and Nitrogen Analyses.--These analyses were performed

by Spang Microanalytical Laboratory, Ann Arbor, Michigan.

Apparatus

The reaction vessel consisted of a 3-necked round bottom flask.
Inserted into the middle neck was a ground glass mechanical stirrer.
In another neck of the flask was an adaptor with a vacuum outlet into
which was inserted a dropping funnel, and in the third neck was a 90°
connecting tube with a standard taper joint and stopcock, through which
nitrogen, or hydrogen chloride, entered the flask. The nitrogen served

to exclude air and moisture from the system.

Preparation.gf.Compounds

Pyridinium tetrachlorodimethoxomolybdate(V).

A complete description of the preparation of this compound will
be given and this procedure applies to the preparation of all similar
tetrachlorodimethoxo complexes. In the dry box 2.75 g (0.01 mole) of
molybdenum pentachloride was weighed and put into the 3-necked round
bottom flask. The flask was stoppered and brought out of the dry box
where nitrogen was passed through it. From a dropping funnel 8.1 ml
(0.2 mole) of dried methanol was added slowly, the flask being cooled
in an acetone-dry ice bath. A vigorous reaction occurred with HCl gas
being evolved. A dark green solution resulted after all of the greenish-

black molybdenum pentachloride had dissolved in the methanol. Anhydrous

HCl gas was passed through the flask and 8.1 ml of a mixture containing
10 ml of pyridine and 90 ml of methanol (0.01 mole pyridine) was added
slowly. A lime—green precipitate formed immediately. Nitrogen gas

was again passed through the flask to remove the excess HCl gas. The
flask was stoppered and taken into the dry box where filtration was
carried out under a nitrogen atmosphere. The lime-green crystals

were washed with three 20 ml portions of anhydrous ether and dried

under nitrogen for approximately three hours.

CSHSN + 2 CH30H + MoC15 ——+ C5H5N[Mo(0CH3)2Cl.+] + HCl

Analysis: Calculated for 05H6N[Mo(0CH3)2Clq]:
C, 22.13%; Cl, 37.33%; H, 3.18%; Mo, 25.25%;
N, 3.69%. Found: C, 21.12%; C1, 37.33%;

H, 2.88%; Mo, 24.87%; N, 3.66%.

Quinolinium tetrachlorodimethoxomolybdate(V).--Yellow crystals formed
immediately.
Analysis: Calculated for CgH3N[Mo(OCH3)2C1g]:
C, 19.27%; Cl, 32.98%; H, 4.79%; Mo,
22.31%; N, 3.75%. Found: C, 18.26%;
Cl, 32.90%; H, 4.74%; Mo, 22.01%; N,

3.63%.

Tetramethylammonium tetrachlorodimethoxomolybdate(V).--Yellow-green
crystals were formed immediately.
Analysis: Calculated for (CH3)uN[Mo(OCH3)2C1q]:
Cl, 37.93%; Mo, 25.66%. Found: C1,

37.68%; Mo, 25.64%.

10

Physical Properties gf_the Tetrgchlorodimethoxomolybdates.--The come
pounds are all somewhat sensitive to air, turning brown after being
exposed for a short period of time. They are soluble in water producing
reddish-brown solutions. The pyridinium compound is very soluble in
methanol and the quinolinium and tetramethylammonium compounds are
moderately soluble. All complexes give green solutions. All three

compounds are moderately soluble in acetone but insoluble in ether.

Attempted Preparetion‘g£_Analogous Ethoxo Compounds.

Attempts to prepare the analogous ethoxo complexes by the pro-
cedure given above resulted in the isolation of what appeared to be
mixed products. A number of solvents and mixtures of solvents were

used for purification without success.

Preparation of Pyridinium Tetrachlorodiethoxomolybdate(V).—-This com-
pound was prepared by alcohol exchange with pyridinium tetrachloro-
dimethoxomolybdate(V). A small amount of pyridinium tetrachlorodi-
methoxomolybdate(V) was put into a vial and enough ethanol was added
so that the compound dissolved upon heating. A dark green solution
was produced when all of the complex dissolved. Light-green needle
shaped crystals formed when the solution was cooled. The resulting
pyridinium tetrachlorodiethoxomolybdate(V) compound was filtered, washed
with ether, and dried under a nitrogen atmosphere.
Analysis: Calculated for 05H5N[Mo(0C2H5)2Clg]:

C, 26.49%; Cl, 34.76%; H, 3.95%; Mo,

23.52%; N, 3.43%. Found: C, 26.45%;

C1, 34.35%; H, 3.86%; Mo, 23.52%; N,

3.56%.

11

The low carbon and hydrogen values obtained for the pyridinium
and quinolinium tetrachlorodimethoxomo1ybdate(V) compounds were
apparently due to their air sensitivity. Partial hydrolysis evidently
occurred during shipment of the samples to Spang Laboratory which
resulted in the low values. Extra precautions were taken for shipment
of the pyridinium tetrachlorodiethoxomolybdate(V) complex. The re-
sults obtained for the carbon and hydrogen analyses of the ethoxo

complex were in good agreement with the theoretical values.

Attempted Preparation g£_the Monochloropentamethoxomolybdates.
Numerous attempts were made to prepare compounds containing the
[Mo(OCH3)5C1]- ion. A large excess (0.5 mole) of methanol was re-
acted with molybdenum pentachloride (.01 mole), followed by the
addition of .01 mole of pyridine in methanol. A sodium methoxide
solution was used instead of methanol in further attempts to obtain
the monochloropentamethoxomolybdate compounds. In all cases only

the pyridinium tetrachlorodimethoxomolybdate(V) compound was obtained.
Spectroscopic Measurements

The infrared spectra of the compounds were obtained in Nujol
mulls with the use of a Unicam SP-200 spectrophotometer. The mulls
were prepared in the dry box under a nitrogen atmosphere.

The visible solution spectra were obtained with a Cary Model
14 recording spectrophotometer and methanol, ethanol, or a methanol—

acetone mixture as the solvent.
Magnetic Moment Measurements

Magnetic susceptibilities were measured by the Gouy method.

12

The calculations were made by use of the equation:

106X a a + BF
w

(46)
where x is the gramrsusceptibility of the sample, a is a constant
allowing for the displaced air and is equal to 0.029 (specimen volume),
8 is the tube calibration constant, w is the weight of the specimen,
and F’ is the force on the specimen, i.e., (F-G), with F being the
observed force and 6 the force produced by the Gouy tube.

The constant a must be determined for the specimen tube by use
of a material of known susceptibility. For these measurements,
Hg[Co(SCN)g], with a susceptibility of 16.44 x 10"6 c.g.s. units
at 20° C. was used.

To calculate the gram-susceptibility of the calibrant at any

given temperature, the following equation is used:

' 2
x-1/M[LEL2_:_8_‘LL._C]

T + 6
In this equation, M is the molecular weight (481.9), C, 177 x 10‘6,

is the diamagnetic correction for Hg[Co(SCN)g], 6, 10°, is the Curie—
Weiss temperature, and u equals 4.44 B.M. Substitution yields the

following:

-3
5.092 x 10 -
x - T + 10 — 0.37 x 10

At 24°, the gramesusceptibility of Hg[Co(SCN)g] is 16.22 x 10'.6 c.g.s.

6

units (47).

In most co-ordination compounds, the metal ions, which are the
only components of the molecule to give rise to a paramagnetic effect,
are kept separated from each other by magnetically inert ligand atoms.
Although the diamagnetic susceptibilities of the ligands are small
compared to paramagnetic ones, it often happens that the number of

ligands in a complex ion so outweigh the single metal atom, to which

13

they are co-ordinated, that their diamagnetism forms an appreciable
proportion of the susceptibility of the complex. It is then nec-
essary to correct the observed susceptibility for this contribution.
The corrections for the diamagnetism of the ligands and cations were
obtained from Pascual's constants (48). The values for the pyridinium
and quinolinium ions were obtained from Holm and Cotton (49).

The molar susceptibility of the sample is obtained by multiplying
the gram—susceptibility, x,by the molecular weight. The susceptibility
of the metal ion, xfi, is obtained by correction of the molar suscepti-
bility for diamagnetic species present.

The magnetic moment, u, can be defined by the following:

1. - (Ag-E) "“2 (xgr) 1’2 - 2.84 (xgr) 1’2 B.M.
In this equation, N is Avagadro's number, 8, the Bohr Magneton, k
is Boltzmann's constant, and T is the absolute temperature.

After the sample tube was calibrated, it was filled with a sample
of one of the alkoxo compounds and was sealed. The inert atmosphere
was necessary because the compounds are sensitive to moisture. The
tube was removed from the dry box and the susceptibility was then
measured. In all cases, the measured moments of the tetrachlorodi-
alkoxomolybdates corresponded to the spin-only moment of 1.73 B.M.,

which is characteristic of a d1 metal ion.

RESULTS AND DISCUSSION

Four compounds, all having the general stoichiometry BH[Mo(0R)2C14],
were prepared and characterized during this investigation. One of these
compounds, C5H6N[Mo(OCH3)2Clu], was reported previously but was supposedly
prepared by a somewhat different method (23). B+ was pyridine, quinoline,
or tetramethylammonium ion. The alkoxo group was either methoxo or
ethoxo.

If the infrared spectra of the tetrachlorodialkoxomolybdates are
compared with those of (C5H5N)C1, (C5H5N)2CoClz (50), (CgHeN)Cl,
(C9H8N)2CoC1n (51), and (C9H7N)2CoClz, it can be seen that pyridinium
and quinolinium ions are present rather than pyridine or quinoline
ligands. In Tables 1, 2, and 3, the infrared frequencies are given
for pyridinium and quinolinium ions and ligands as well as those for
the tetrachlorodialkoxomolybdates. In each case, the frequency of
the alkoxo complex corresponds to that of the pyridinium or quinolinium
ion.

The infrared spectra of methoxides and ethoxides have a strong
band in the region where the C-0 stretching vibration occurs. This
band occurs between 1020-1100 cm.1 for the methoxides and at 1025-

1070 cm-1 for the ethoxides. The C—O stretching vibration is present
in the infrared spectrum of each of the tetrachlorodialkoxomolybdates
in the region between 1040 and 1050 cmfl. Since there is no evidence
of the O—H vibrational band, it is assumed that the alkoxide oxygen
is coordinated directly to the metal ion. The exact position of the

14

15

Table l. The comparison of the infrared spectra of pyridinium ion
and ligand pyridine with that of CSHGNIMo(0CH3)2Clu].

 

 

 

05H5N+ (51) ligand C5H5N C5H6N[Mo(OCH3)ZClu]
1630 s 1600 s 1630 s
1600 s 1570 w 1600 s
1530 s 1490 s 1530 s
1480 s 1440 s 1480 s
1325 ---- 1325 s
1240 m 1240 vw 1240 w
1200 m 1215 s 1195 m
1160 m 1150 m 1160 m

 

vw, very weak; w, weak; m, medium; 6, strong

16

Table 2. The comparison of the infrared spectra of quinolinium ion
and ligand quinoline with that of C9H8N[Mo(0CH3)2Clg].

 

 

 

C9H8N+ ~ ligand C9H7N C9H8N[Mo (0CH3)2C11,]
1630 s 1590 s 1630 s
1590 s 1580 m 1590 s
1550 s 1540 s 1555 s
1400 s 1400 m 1410 s
1300 s 1320 s 1300 s
1220 s 1310 s 1225 s
1160 m 1140 m 1155 m
1140 m 1130_m, 1135 m
1080 m 1080 m 1090 m

 

vw, very weak; w, weak; m, medium; 3, strong

17

Table 3. The comparison of the infrared spectra of pyridinium ion
and ligand pyridine with that of CSHGNIMo(0C2H5)2Clu].

 

 

 

05H5N+ ligand 0 5H5N c 5H6N[Mo (0c 2115) 2c1..]
1630 s 1600 s 1630 s
1600 s 1570 w 1600 s
1530 s 1490 s 1530 s
1480 s 1440 s 1480 s
1325 ---- 1325 w
1240 m 1240 vw 1235 m
1200 m 1215 s 1190 m
1160 m: 1150 m 1160 m

 

vw, very weak; w, weak; m, medium; 3, strong

 

 

18

C-0 stretching band for each of the compounds is: C5H5N[Mo(0CH3)2014],
1040 cm’l; C9H8N[MO(0CH3)2C11+], 1050 cm‘l; (CH3)L,N[Mo(0CH3)2ClL,], 1040
cmfl; and C5H6N[Mo(0C2H5)2C1n], 1040 cmfl. The infrared spectra of
these compounds are given in Figures 1-4.

It has been reported that some polymeric ethoxides have two C-O
stretching bands (52). The compound, Nb(0C2H5)5, which was found to
be dimeric, had C-O stretching vibrations at 1029 and 1063 cm-1 (53).
It was concluded that the two absorptions are due to the presence of
both bridging and terminal ethoxide groups. Since only one C-O stretch—
ing band was found for C5H6N[Mo(OCZH5)2014], there appears to be no
bridging ethoxide groups.

The visible spectra of solutions of the tetrachlorodialkoxomo1yb-
dates have been obtained and the absorption maxima are given in Table
4. For C9H8N{M0(0CH3)2C14] and (CH3)QN{MO(OCH3)2C1Q], an acetone-methanol
mixture was used because of the limited solubility of these complexes
in methanol. Sketches of the visible spectra are shown in Figures 5-8.

Table 4. Visible absorption maxima of the tetrachlorodialkoxomolyb-
dates. (in cmTl)

 

 

Compound Absorption frequencies Solvent
: V1 V2
(cm‘l)
C5H5N[MO (0CH3)2C11+] 13,900 23,100 Methanol
chgNIMo(0CH3)2C1u] 14,200 23,150 Methanol-Acetone
(CH3)4N{Mo(0CH3)2C14] 13,900 22,075 Methanol-Acetone

C5H5NIMo(0C2H5)2Cln] 14,150 23,525 Ethanol

 

19

 

 

 

 

 

 

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27

Just as for the compounds containing the [M00015].2 ion, the tetra-
chlorodialkoxo complexes are assumed to be tetragonal with some n-bonding
between the alkoxide oxygen and the metal ion. Although there is a
difference in the symmetry between the‘gragg-BHIM0(0R)201u] (D4h) and
the R2[M00015] (Csv) complexes, the energy level diagram of the
[MoOC15]-2 ion should be similar and can be used, at least qualitatively,
as a guide in interpreting the spectra of the tetrachlorodialkoxo come

pounds. The splittings on increased tetragonal distortion are given

 

below:
””’-’— 31(d22)
:22 ,w’
x2_y2 __y“‘
“ - —______ 'bl (dx2_y2)
I...- ’2 e (dxz’ dyz)
dxz fl,» I”
dyz
xy -——-—-~§~ ~~
2 xy
Oh Duh
<471ncreased_Tetragonal Distortion a;

 

 

There should be three transitions, bg--+ e, bz--# b1, and
b2--*-a1. However, only two of these, probably the'bg--+ e and
bz--+ b1 transitions, are present in the spectra. The bz--+ a1
transition is apparently masked by the charge transfer band at approx-
imately 33,000 cmfl. For the R2[M00C15] compounds, the first two
transitions occur at 14,050 and 22,500 cmfl. Since absorption maxima

for the tetrachlorodialkoxo complexes are approximately at the same

28

frequency as those of the R2[M00C15] compounds, similar assignments
have been made.

Molybdenum in the +5 oxidation state has one unpaired 4d electron
and should have a magnetic moment close to the spin-only value of 1.73
B.M. It was found that all of the compounds prepared and examined
during this research had moments close to this spin-only value. These
moments are listed in Table 5. It is assumed that the value of 1.81
B.M. obtained for C9H3N[Mo(0CH3)2C14] is higher than the other come
pounds because of difficulty in packing the sample tube. It was much
more difficult to get uniform packing for this compound than for the
others. The susceptibility of the ethoxo compound was measured at
Wayne State University and a small experimental error was produced by
not being able to load the sample into the tube under conditions

normally required.

Table 5. Magnetic susceptibilities and moments of the alkoxo molybdenum(V)

 

 

 

compounds.
Compound Temp. (°A) Susceptibility Moment (B.M.)
x’ x 106
(c.g.s. units/mole)
C5H6N[Mo(OCH3)2C1:,] 297 1225 1.71
CgH8N[MO(OCH3)2C11+] 297 1370 1.81
(CH3)L+N[MO(OCH3)2C1L,] 298 1235 1.72
C5H5N[MO(0C2H5)2C11+] 296 1469 1.88

 

The data obtained indicate that all of the tetrachlorodialkoxo-

molybdate(V) compounds are monomeric and contain molybdenum in the +5

29

oxidation state with a 4d1 configuration. The C-O stretch is present
in the infrared spectra while the O-H vibrational mode is not, which
indicates the presence of the alkoxide ion and not the free alcohol.
The alkoxide oxygen should be coordinated directly to the metal ion.
The structure of the tetrachlorodialkoxomolybdate(V) ion appears to
be that of a tetragonally-distorted octahedron with some n-bonding
between the alkoxide oxygen and the metal ion.

It is assumed that only the Eyggs isomer is present for all
of the tetrachlorodialkoxomolybdate(V) compounds. For all of the

have been resolved; if there weren't axial

compounds, 3" and gL

symmetry, gll and EL. could not be resolved.

In the electron spin resonance study of the tetrachlorodialkoxo-
molybdate(V) compounds in dimethylsulfoxide, it was observed that the
line shape changed corresponding to the replacement of the chloride
ligands by dimethylsulfoxide ligands. Upon addition of HCl, the line
patterns for the spectra of the chloroalkoxo complexes were obtained again.

When pyridinium tetrachlorodimethoxomolybdate(V) was put into
ethanol, it was observed that the line shape of the electron spin
resonance spectrum changed with time, indicating possible alkoxo ex—
change. The spectrum of the pyridinium tetrachlorodimethoxomolybdate(V)
compound in ethanol was identical to that of the corresponding ethoxo

complex in the same solvent.

APPENDIX

Electron spin resonance is the branch of spectroscopy in which
molecules containing electrons with unpaired spins absorb radiation
of radio-frequency when placed in a magnetic field. The absorption
of a quantum of radiation produces a transition between different
electron spin energy states of the unpaired electron.

An electron of spin 3 = 1/2 can have spin angular momentum quantum
numbers of ms - f 1/2. In the absence of a magnetic field, a doubly
degenerate spin energy state exists; if a magnetic field is applied,
the degeneracy is removed. The low energy state corresponds to the
quantum number, m8 = -l/2, and has the spin magnetic moment aligned
with the field. The high energy state, mS = +1/2, has its moment
opposed to the field. Upon absorption of a quantum of radiation in
the radio-frequency or microwave region, the transition between two
different electron spin energy states occurs. The energy of the tran-
sition is given by:

E = hv = gBHo
where h is Planck's constant, v the radiation frequency, 8 the Bohr
magneton, g the spectroscopic splitting factor, and Ho the field
strength.

The value of g for a free electron is 2.0023. However, metal
ions often have g values much different from that of the free electron.
The magnitude of g generally depends on the orientation of the molecule
with respect to the magnetic field.

30

31

The g value obtained when the z axis of the ion or radical is
parallel with the external magnetic field is designated by g1| ,
while the g values along the x and y axes are referred to as gL‘,
and result when the external magnetic field is perpendicular to the
z axis.

When an electron comes in the vicinity of a nucleus with a
spin I, an interaction takes place which causes the absorption signal
to be split into 21 + 1 components. The isotopes 95Mo and 97Mo, both
have I a 5/2, and thus six components should result. The isotope
96Mo, with I - 0, would only have one component.

Hare reported a strong central line centered at g = 1.947 for
(NH4)2[M00015] in 10-12 M HCl (38). On each side of the central line
were three weak satellites. The strong central line was assumed due

to the 96Mo (I - 0) isotope, and the weaker satellites due to the

95Mo (I - 5/2) and 97

Mo (1 - 5/2) isotopes. The g" and gL values
for this compound were observed to be 1.915 and 1.965 respectively.
However, Kon and Sharpless (54) found that g" was greater than 81
with g“ = 1.970 and gL = 1.936. The results obtained by Kon for gll
and gL were in good agreement with those of Garifyanov (55).

A number of other electron spin resonance studies on Mo+5 com-
pounds have recently been reported (56, 57, 58).

The electron paramagnetic resonance studies on the tetrachloro-
dialkoxomolybdate(V) compounds were performed by LarryDalton. Work
is still in progress on the pyridinium tetrachlorodiethoxomolybdate(V)
complex. The spectra of both powder and liquid samples were obtained.

The data obtained thus far is listed in Tables 6-10.

It is assumed that only the trans isomer is present for all of

 

32

Table 6. Electron spin resonance data for powder samples of the
tetrachlorodialkoxomolybdates(V).

 

 

 

Compound g gJ_ <g>
($.0002) (1.0002) (f.0003)
C5H6N[Mo(0CH3)2C1s] 1.96897 1.92488 1.93958.
CgHaNIMo(OCH3)2C1n] 1.96008 1.92316 1.93546
(CH3)QNIMO(OCH3)2C1“] 1.97056 1.92431 1.93972
C5H5N[Mo(0C2H5)2Cln] 1.96093 1.93285 1.94221

 

Table 7. Electron spin resonance data for liquid samples of pyridinium
tetrachlorodimethoxomolybdate(V).

 

—v

 

Solvent <g> <a>

I (gauss)
Methanol 1.94626 t .0003 52.49 t 0.3
Acetone 1.94777 1 .0004 51.34 t 0.3
Pyridine 1.93743 1 .0003 47.35 i 0.3
Dimethylformamide 1.94529 1 .0003 51.67 t 0.3
Ethanol 1.94614 1 .0006 52.50 t 0.6

Dimethylsulfoxide 1.94364 + .001 54.35 1.0

H-

 

33

Table 8. Electron spin resonance data for liquid samples of quinolinium
tetrachlorodimethoxomolybdate(V).

 

 

 

Solvent <g> (a)
(gauss)
Methanol 1.94558 1 .0003 52.67 t 0.3
Acetone 1.94798 i .0003 51.40 t 0.3
Ethanol 1.94556 1 .0003 52.18 i 0.3
Dimethylformamide 1.94666 i .0003 51.56 t 0.3
Chloroform 1.94859 + .0003 51.01 t 0.3

 

Table 9. Electron spin resonance data for liquid samples of tetra—
methylammonium tetrachlorodimethoxomolybdate(V).

 

 

 

Solvent <g> <a>
(gauss)
Methanol 1.94587 1' .0005 . 52.83 i' 0.3
Acetone 1.94856 : .0003 51.70 i 0.3
Dimethylformamide 1.94696 1 .0003 51.56 t 0.3

Ethanol 1.94651

1+

.0005 52.51 t 0.3

 

34

Table 10. Electron spin resonance data for liquid samples of pyridinium
tetrachlorodiethoxomolybdate(V).

 

 

 

Solvent <g> <a>
(gauss)

Ethanol 1.94599 f .0003 52.75 t 0.3

Methanol-ethanol 1.94600 + .0003 52.72 + 0.3

 

the tetrachlorodialkoxomo1ybdate(V) compounds. For all of the compounds,
g“ and gJ_ have been resolved; if there weren't axial symmetry, g"

and E1. could not be resolved. Also, according to Dalton, the line
shapes would be different if the‘gig isomer were present.

In the electron spin resonance study of the tetrachlorodialkoxo-
molybdate(V) compounds in dimethylsulfoxide, it was observed that the
line shape changed corresponding to the replacement of the chloride
ligands by dimethylsulfoxide ligands. Upon addition of HCl, the line
patterns for the spectra of the chloroalkoxo complexes were obtained
again.

When pyridinium tetrachlorodimethoxomolybdate(V) was put into
ethanol, it was observed that the line shape of the electron spin reson-
ance spectrum changed with time, indicating possible alkoxo exchange.
The ultimate spectrum of the pyridinium tetrachlorodimethoxomolybdate(V)
compound in ethanol was identical to that of the corresponding ethoxo
complex. It was found chemically that there was alkoxo exchange: this
was the method used for the preparation of the pyridinium tetrachloro-

diethoxomolybdate(V) compound.

10.

11.

12.
13.
14.
15.

16.

17.

18.

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37

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