AN INVESTIGATION OF SOME
TRIII’IETHYLPLATINUM- COMPOUNDS.

Thesis for the Degree of M. S.
MICHIGAN STATE UNIVERSITY
GLEN RICHARD HOFF
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ABSTRACT
AN INVESTIGATION OF SOME TRIMETHYLPLATINUM COMPOUNDS

by Glen Richard Hoff

Two forms of trimethylplatinum iodide have been re-
ported in the literature: the more common tetrameric yellow
form and the dimeric white form. -As a dimeric structure
would require unusual coordination for platinum(IV) or addi-
tional ligands, experiments were carried out the more
clearly to identify the structure of the white form. The
molecular weight of the white form has been detennined more
accurately than it had been previously and the earlier value
is shown to be in error. Infrared spectra and x-ray powder
diffraction patterns were also determined and are reported
here. The molecular weights, infrared spectra, and x-ray
powder diffraction patterns are the same for both forms,
which indicates that they have the same tetrameric structure.

The infrared spectra, x-ray powder diffraction patterns,
and equivalent conductance in aqueous solution of trimethyl-
platinum sulfate and nitrate were obtained and are re-
ported. While the structures have not been determined,
the results suggest that trimethylplatinum sulfate has
four aquo groups and a bridging sulfate group and that
trimethylplatinum nitrate has a unidentate nitrate group

and two aquo groups.

AN INVESTIGATION OF SOME TRIMETHYLPLATINUM COMPOUNDS

BY

Glen Richard Hoff

A THESIS

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

MASTER OF SCIENCE
Department of Chemistry

1967

To Janet

P.
P.

ACKNOWLEDGMENT

The author wishes to express sincere appreciation to
Trofessor Carl H. Brubaker, Jr. for his advice, guidance
and encoragement throughout this project and to my wife,
Janet, for her patience and understanding.

The author also wishes to thank the faculty, staff,
and graduate students of the Department of Chemistry for

the many helpful suggestions and advice.

iii

TABLE OF CONTENTS

Page

INTRODUCTION . . . . . . . . . . . . . . . . . . . . 1
EXPERIMENTAL . . . . . . . . . . . . . . . . . . . . 6
Materials . . . . . . . . . . . . . . . . . . . 6
Methods of Elemental Analysis . . . . . . . . . 7
Methods of Analytical Analysis . . . . . . . . 8
Preparation of the Compounds . . . . . . . . . 9
RESULTS AND DISCUSSION . . . . . . . . . . . . . . . 16
SUMMARY . . . . . . . . . . . . . . . . . . . . . . 28

BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . 31

iv

LIST OF TABLES

TABLE Page
1. Infrared spectrum of trimethylplatinum iodide 17
"d" spacings for trimethylplatinum iodide . . 18
Infrared spectrum of trimethylplatinum sulfate 21

2
3
4. Infrared spectrum of trimethylplatinum nitrate 22
5. "d" spacings for trimethylplatinum sulfate . . 24
6

. "d" spacings for trimethylplatinum nitrate . . 25

Figure

1. Equivalent
sulfate in

2. Equivalent
nitrate in

LIST OF FIGURES

Page
conductance of trimethylplatinum
aqueous solution . . . . . . . . . 26
conductance of trimethylplatinum
aqueous solution . . . . . . . . . 27

vi

INTRODUCTION AND HISTORICAL BACKGROUND

There are a large number of sigma-bonded alkyl and
aryl complexes of the transition metals3‘9:13 but most re-
quire that pi-bonding ligands, such as carbonyl groups or
phosphine derivatives, also be present to lower the energy
levels of the vacant d-orbitals and increase the stability
of the metal organic compound‘. However, platinum(IV) and
gold(III) belong to a group of metals, including thallium(II),
tin(IV), and lead(IV), whose alkyl derivatives do not need
this stabilization and which give stable metal organic aquo
cation812I18'35.

The first of the platinum alkyl derivatives, yellow
trimethylplatinum iodide, was prepared in 1909 by Pope and
Peachey from the reaction of anhydrous platinum(IV) chloride
with an excess of methylmagneSium iodide39. ~The yellow
product was soluble in benzene and was shown by elemental
analysis to have the formula C3H9Pt1. .By metathetical re-
placement of the iodine, POpe and Peachey obtained trimethyl-
platinum cyanide, chloride, and hydroxide, which were soluble
in benzene, and sulfate and nitrate, which were soluble in
water. They also prepared trimethyldiamminoiodoplatinum(IV)
which has six ligands around the central atom as would be
expected for normally octahedrally coordinated platinum(IV).

In 1933 Menzies and Overton found that molecular .
weight measurements indicated that the yellow form of

1

2
trimethylplatinum iodide, prepared by a method similar to
that of Pope and Peachey, was tetrameric but the white form,
prepared by adding potassium iodide to an aqueous solution
of trimethylplatinum sulfate, was dimeric34. Rundle and
Sturdivant, in 1947, determined the crystal structure of
trimethylplatinum chloride and found it to be a tetramer
with platinum atoms and chlorine atoms at alternate corners
of a distorted cube43. The halogens are bridging so that
each platinum atom is octahedrally coordinated.

In all reported cases platinum(IV) forms octahedral
complexes. This is readily apparent in the case of deriva-
tives such as trimethyldiamminoiodoplatinum and trimethyl-
bis(pyridine)iodoplatinum but in some cases very unusual
structures must be adopted. Trimethylplatinum chloride has
three-way halogen bridges between platinum atoms. In acetyl-
acetonate complexes of trimethylplatinum the acetylacetone
may be mono—, bi- or tri-dentate; in the last case a dimer
is formed in which the central carbon of acetylacetone oc-
cupies the sixth position around a platinum atom14. Cyclo—
pentadiene occupies three positions of the octahedron in
monomeric cyclopentadienyltrimethylplatinum4°. All other
reported trimethylplatinum compounds have octahedrally co-
ordinated platinum and the three methyl groups are always
gig to one another12v43.

It is possible to prepare many derivatives of trimethyl-
platinum by exchange of the non-methyl ligand316I25I29. How-

ever, in all reported cases there is no migration of the

3

methyl groups from positions gig to one another and it is
not possible to remove any of the methyl groups without
immediate decomposition of the compound.

In most cases of sigma-bonded metal organic compounds
the stability of the metal aryls is greater than that of
the metal alkyls. However, attempts to prepare platinum(IV)
aryls have been unsuccessful because there is a greater
steric hindrance between the aryls than there is between
the alkyls in the tetrameric structure of the organoplatinum
compound13. The only alkylplatinum(IV) compounds which have
been synthesized have been those containing either three
methyl or three ethyl groups. Triethylplatinum iodide was
first prepared by S. F. A. Kettle by the reaction of diethyl-
mercury with anhydrous platinum tetrachlorideza.

In 1952 Gilman, Lichtenwalter, and Benkeser carefully
studied the reaction used by POpe and Peachey to prepare tri-
methylplatinum iodide and reported that in addition to the
major product, small amounts of tetramethylplatinum, di-
methylplatinum diiodide, methylplatinum(VI) pentaiodide,
and several unidentified organoplatinum Compounds were also
formedzo. The synthesis of these compounds has not been
successfully repeated but it seems unlikely that a platin-
um(VI) compound would be obtained from platinum(IV) in the
reducing medium of a Grignard reagent13. Tetramethylplatinum
had been previously prepared by Lichtenwalter in 1938 by
the reaction of trimethylplatinum iodide with methylsodium19.

He also prepared hexamethyldiplatinum by reacting

4

trimethylplatinum iodide with powdered potassium. Rundle
and Sturdivant determined that tetramethylplatinum was tetra—
meric and isostructural with trimethylplatinum chloride and
that hexamethyldiplatinum crystallized in polymeric chainsl3I43.

Attempts to duplicate the synthesis of tetramethyl-
platinum have not been successful. Hoechstetter repeated
the preparation according to the description given by
Lichtenwalter and found that the products were white tri-
methylplatinum iodide, trimethylplatinum hydroxide, and
trimethylplatinum methoxide33. Modifications and improve-
ments in the synthetic technique gave either the starting
material or a mixture of trimethylplatinum methoxide and
trimethylplatinum hydroxide. In light of the evidence,
Hoechstetter compared the available spectral data and found
that the X-ray powder diffraction patterns of trimethyl-
platinum hydroxide and that reported for tetramethylplatinum
were very similar in "d"-spacings and intensities. It would
appear from the evidence presented by Hoechstetter and others
that tetramethylplatinum and hexamethyldiplatinum have not

yet been synthesized.

Trimethylplatinum derivatives have only been partially
characterized. Infrared spectral data are avilable for
mixed derivatives with halogens, ammonia, pyridine, and
several other ligands21'34'31I40. The structures of several
trimethylplatinum acetylacetonates and related ligands have
been determined29'41'45. Kite, Smith, and Wilkins examined
the nuclear magnetic resonance spectra and structures of

twenty-three trimethylplatinum compounds and demonstrated

5
that the Spin-Spin coupling constant between methyl protons
and 195Pt is sensitive to the chemical nature of the non-
methyl ligandszs. Raman spectra of trimethylplatinum sul—
fate, nitrate, and perchlorate show that these compounds
are extensively dissociated in aqueous solution11. Other
than nuclear magnetic resonance data, no further studies on

these ionic trimethylplatinum derivatives have been reported.

EXPERIMENTAL

Materials

Platinum.- Scrap wire and sheet platinum were obtained from

 

departmental stores and cleaned by boiling in 37% hydrochloric

acid.

Methyl Iodide.— Reagent grade methyl iodide was refluxed
and distilled from anhydrous calcium chloride immediately

before use.

Ether.- Anhydrous diethyl ether was obtained by refluxing
reagent grade anhydrous ether with lithium aluminum hydride

for twenty-four hours and distilling immediately before use.

Benzene.- Benzene, used for the determination of molecular
weights, was obtained by twice distilling reagent-grade,
thiOphene-free benzene from sodium and collecting the frac-
tion which distilled at 80.10i0.1° (uncorrected). Benzene
for other purposes was reagent grade and was freshly distil-

led once from sodium.

Chloroform.- Reagent grade chloroform was used without

 

further purification.

Potassium Chloride.- Potassium chloride used for the cali-
bration of the conductance cell had been previously purified
by the method of Scathard, Hamer and Wood44 in which reagent

grade potassium chloride was dissolved in conductance water

7
and precipitated by hydrogen chloride gas. The powder thus
obtained was dried at 300° and then at room temperature by
passing through it a stream of air dried by calcium chloride.

Immediately before use it was dried at 110° for several days.

Potassium Bromide.- Finely divided potassium bromide for

infrared spectroscopy was dried at 110° for several days.

Other Materials.- Other materials used in this study were

reagent grade and used without further purification.
Methods of Elemental Analysis

P1atinum.- A weighed quantity of sample in a porcelain
crucible was wetted with chloroform and sulfuric acid (1:4)
and a few crystals of iodine were added. The sample was
carefully heated in a muffle furnace at 100° until decompo-
sition was complete and then the temperature was slowly in-
creased to 900° and held there five hours. The metallic

residue was weighed as platinum.

Iodine.— Iodine was determined by fusion of a sample with
sodium bicarbonate, carefully dissolving the cake in dilute
nitric acid, and titrating the acid solution for iodine by
adding silver nitrate- to precipitate silver iodide and
titrating the excess silver nitrate with potassium thio-

cyanate.

Sulfate.- Sulfate was determined gravimetrically as barium

sulfate by adding barium nitrate to an aqueous solution of

8
the sample in a fine porosity porcelain filter crucible,
carefully washing the precipitate to remove c0precipitated

nitrates, and igniting the precipitate at 900°.
Methods of-Analytical Analysis

Infrared Analysis.- Infrared analysis in the 650—5000 cm—1
region were obtained with a Unicam SP-200 spectrophotometer
with sodium chloride optics. Samples were run in potassium
bromide pellets and mulls of Nujol, Fluorolube, and 1,3-
hexachlorobutadiene. Analysis in the 200-650 cm.1 region
were obtained with a Beckman IR-7 spectrophotometer with
cesium iodide optics. Nujol mulls of the sample were

mounted in polyethylene plates.

X-ray Powder Diffraction.- The X-ray powder diffraction pat-
terns were made by the Debye-Scherrer method with a Phillips
114.59 mm diameter camera. The source of radiation was a cop—
per target (20ma; 40 kv) with a nickel filter which provides a
KG x-ray of 1.5418 R (unresolved). Samples were run in 0.3

mm glass capillaries for twenty hours.

Molecular Weights.- The molecular weights were determined with

 

a Mechrolab vapor pressure osmometer with a non-aqueous probe
designed to operate at 37°. The solvent employed was benzene

and the vapor pressure osmometer was calibrated with benzil.

Conductance Measurements.- A Wayne Kerr Universal Bridge

B221 was used with a modified Shedlovsky cell. Trimethylplat-
inum sulfate was studied by adding a series of weighed portions
of the solid to a known volume of water already in the cell.
Trimethylplatinum nitrate was analyzed by preparing an aqueous
solution of the nitrate directly from the sulfate, diluting

it to a known volume, and making subsequent dilutions to com-

plete the study.

9

Preparation of the Compounds
Potassium HexachloroplatinatedIyl.

Scrap platinum was dissolved in aqua regia and after
reduction of the volume to 25 ml by evaporation, hydrochloric
acid was added and the volume again reduced. This latter
step was repeated until all of the nitrogen oxides had been
removed. Potassium chloride was added to the solution and
potassium hexachlor0platinate(IV) was collected on a Buchner
filter as a yellow powder, washed with dilute hydrochloric

acid and water, and dried at 110°.
Trimethylplatinum Iodide (yellow form).

The yellow form of trimethylplatinum iodide was pre-
pared according to the directions of Clegg and Hallll. The
Grignard reagent methylmagnesium iodide was prepared by the
slow addition of 14.5 ml (0.23 mole) of methyl iodide to
3.5 g (0.14 mole) of magnesium in 100 ml of ether, during
which the mixture was cooled in an ice bath and stirred
continually. The mixture was refluxed twenty minutes, again
cooled in an ice bath, and the Grignard reagent decanted
into a dropping funnel. The use of excess Grignard reagent
in the subsequent preparation of trimethylplatinum iodide
increases the yield. It has been reported that the excess
methyl iodide oxidizes platinum(II) compounds which are formed

by the reduction of platinum(IV) by methylmagnesium iodidel.

10

Ten grams (0.02 mole) of potassium hexachlorOplatinate(IV)
was added to a mixture of 100 ml of benzene and 50 ml of
ether previously distilled into a 500 ml three-necktflask
which was fitted with a stirrer, a reflux condenser, and
the drOpping funnel containing the methylmagnesium iodide.
Nitrogen was swept over the mixture which was also protected
with a calcium chloride guard tube. The mixture was cooled
in an ice bath and the Grignard reagent was slowly added
over a ten minute period while the solution was stirred con-
tinually. The ice bath was then removed and stirring was
continued for several hours. The mixture then stood at am-
bient temperature for twenty hours during which an off-white
solid settled and the solvent layer became clear and faintly
orange-colored. On the basis of several preparations by
the technique of Clegg and Hall, it is recommended that the
stirring after removal of the ice bath be continued for three
hours after the mixture lightens to a very pale yellow or
off-white, which first occurs from four to six hours after
the addition of the Grignard reagent.

The mixture was cooled in an ice bath and an ice
slurry of 100 ml of 10% hydrochloric acid was slowly added
with stirring. The organic layer became orange and the
aqueous layer became a very deep orange. 'The organic layer
was decanted and 100 ml of benzene were added to the aqueous
layer. The mixture was refluxed and stirred for twenty
minutes and the benzene layer was decanted. This step was

repeated twice and the combined extracts and original organic

11
layer were placed over anhydrous sodium sulfate. After
twenty hours the mixture was filtered and the clear fil-
trate was evaporated to dryness in a stream of air. The
residue was washed by decantation once with five milliliters
of ethanol to remove the dark-colored material and then dis-
solved in hot chloroform. The chloroform was evaporated in
an air stream to half the volume of solution present when
the first crystals appeared, and an equal volume of acetone
was added. -After the mixture was cooled and filtered, the
yellow-brown powder collected on the filter was recrystal-
lized from a chloroform-acetone mixture and dried at 110°.
Yields were 75-85%, higher than those reported by Clegg and
Hall. Analysis. Calculated for C3H9PtI: Pt, 53.14%; I,
34.06%. Found: Pt, 53.06%; I, 33.4%.

Trimethylplatinum iodide is soluble in benzene, chloro-
form, dimethylsulfoxide, tetrahydrofuran, and dimethylform-
amide. It is insoluble in water, acetone, methanol, and
ethanol. The color of the original product may be lightened
slightly by dissolving the powder in the minimum amount of
hot chloroform, evaporating the solution to half the original
volume, and adding to it an equal volume of acetone. The
powder obtained after cooling and filtering is finely di—
vided and bright yellow. Evaporation of the filtrate and
wash solutions gives a coarse powder darker than the original
starting material.

A very light yellow or beige form of trimethylplatinum ,

iodide was obtained by refluxing a chloroform solution of

12
dark yellow trimethylplatinum iodide with silver sulfate.
After the mixture had refluxed several hours, during which
the initial yellow color became lighter, it was filtered to
remove the green powder containing largely silver salts and
the volume of the filtrate was reduced by evaporation in
a stream of air to half the volume present when crystals
first appeared. An equal volume of acetone was added, the
mixture was cooled, and the light yellow trimethylplatinum
iodide powder was collected on a Bfichner filter, washed with
acetone, and dried at 110°. A white form of trimethylplat-
inum iodide has not been prepared by this method.

When dark yellow trimethylplatinum iodide was dis-
solved in chloroform or benzene a small quantity of a dark
powder appeared after a short time. The solid was insoluble
in hot chloroform and gave an orange-brown smear on filter
paper. Analysis. -Calculated for PtIaz Pt, 43.4%. Found:
Pt, 44.8%. It was also observed that dry yellow trimethyl-
platinum darkened with age and exposure to light but that
the color could be lightened by recrystallization or reflux-

ing the solid with silver sulfate in chloroform.
»Trimethylplatinum Sulfate.

Trimethylplatinum sulfate was prepared by the reaction
between silver sulfate and trimethylplatinum iodide39:25.
Trimethylplatinum iodide (4.73 g4 0.013 mole) was dissolved
in 100 ml of hot benzene and 2.5 g (0.008 mole) of silver

sulfate and 100 ml of moist acetone were added. The mixture

13
was refluxed with stirring for about eight hours until the
initially yellow solution had turned colorless. The mixture
was filtered to remove the silver salts and the filtrate was
evaporated to partial dryness in an air stream. The water
in the moist acetone prevented complete drying in the stream
of air. The partially dry solid was dissolved in acetone
and a black powder was removed by filtration. The solvent
was evaporated to half the original volume when Crystals
first reappeared and an equal volume of chloroform was added.
The finely divided crystals were collected on a filter and
washed with chloroform. The crystals of trimethylplatinum
sulfate were then recrystallized from water and dried at
room temperature in a vacuum desiccator with phosphorun
pentoxide. The yield was 0.84 g (85%) of white feathery
plates of trimethylplatinum sulfate [(CHahPt13SO4°4 H20.

Analysis. Calculated: Pt, 60.54%; 804, 14.91%. Found:

 

Pt, 60.29%; 304, 15.04%.

Trimethylplatinum sulfate is soluble in water and ace—
tone and insoluble in benzene and chloroform. The compound
decomposes before the waters of crystallization can be driven
off by heating. The dry compound was heated to 110° without
decomposition but partially dry compounds decomposed to
platinum and unidentified residues at temperatures as low
as 40°. It is difficult to purify completely the trimethyl-
platinum sulfate by crystallization owing to its ready solu-
bility, so impurities may color the product cream or light

brown. Acetone-chloroform filtrates from recrystallization

14
and other solutions of trimethylplatinum sulfate which had
partially decomposed were green and contained a small quantity
of black powder. If the powder was removed by filtration,
more appeared in the filtrate and crystalline trimethylplat-
inum sulfate could not be obtained free of green or black

discoloration.
Trimethylplatinum Nitrate.

Trimethylplatinum nitrate was prepared by the metatheti—
cal reaction between barium nitrate and trimethylplatinum
sulfate in an aqueous solution35t39. After the barium sul-
fate was removed by filtration, the fitrate was evaporated
to partial dryness in an air stream and drying was completed
in a vacuum desiccator with phosphorous pentoxide. The
cream colored crystals, reported to be (CH3)3PtN03-2 H20,
could not be further purified to remove the color. -Tri-
methylplatinum nitrate is soluble in water and acetone and
insoluble in benzene and chloroform. The crystals were
very deliquescent so an elemental analysis was not obtained

for trimethylplatinum nitrate.
TrimethylplatinumLIodide (white form).

The white form of trimethylplatinum iodide was easily
obtained by adding potassium iodide to an aqueous solution
of trimethylplatinum sulfate or trimethylplatinum nitrate.
A white precipitate immediately formed and was collected

on a filter after the mixture had been cooled in an ice

15

bath for ten minutes. The white packed powder was washed
with cold water and dried at 110°. Analysis. Calculated
for C3H9PtI: Pt, 53.14%; I, 34.06%. Found: Pt, 54.10%;
I, 34.0%.

White trimethylplatinum iodide is similar to the yel-
low form in properties except for the difference in color.
The white form is converted to the yellow form readily in
chloroform but in benzene it is converted slowly and with
heating. A solution of white trimethylplatinum iodide in
benzene did not give the dark solid found in similar solu-
tions of the yellow form.

If precipitated trimethylplatinum iodide is not re-
moved from the original aqueous solution, oxidation of
excess iodide ion in the solution colors the solid a faint
brown. A similarly colored product is obtained if white
trimethylplatinum iodide is precipitated from a solution
containing iodine as I; . Most of the color is removed

by washing the solid with water and drying it at 110°.

RESULTS AND DISCUSSION

Trimethylplatinum chloride and yellow trimethyl-
platinum iodide are tetrameric, but white trimethylplatinum
iodide is reported to be dimeric. A dimeric structure for
trimethylplatinum iodide would require that either the
platinum(IV) atom not be octahedrally coordinated or that
additional ligands be present. Since no data other than
the molecular weight and elemental analysis have been pub-
lished for the white compound, one objective of the present
investigation was to compare the two forms of trimethyl-
platinum iodide and identify the structure of the white
form.

The experimental infrared spectrum of yellow trimethyl-
platinum iodide (Table 1) is similar to that reported by
Hoechstetter24 and Gribov et al.21 and spectral assignments
are made accordingly. There are no observable differences
in the spectra for the white and yellow form of trimethyl-
platinum iodide.

There is a close correlation between the x-ray powder
diffraction patterns (Table 2) for the two forms of tri—
methylplatinum iodide although there are a few anomalies.
The number of lines and the absence of regular spacing shows
that trimethylplatinum iodide does not have the body-centered
cubic structure of trimethylplatinum chloride and that the

structure cannot be determined from the powder pattern alone.

16

17

 

 

Table 1. Infrared spectrum of trimethylplatinum iodidea
Experi- Litera- .
mental ture21,24 As51gnment
(cm-1)
554 w 562 vw Pt-C stretch
862 vw 852 vw Pt-C bending
--- 882 w Methyl rocking
1218 s 1218 s Symmetric C-H deformation
1257 s 1253 s Degenerate (methyl rocking) C-H
deformation
1410 m 1406 m Unsymmetric (degenerate) C-H
deformation.
2770 w 2775 w Overtone from unsymmetric C-H
deformation
2870 m 2874 m Symmetric C-H stretch
2940 m 2950 m Unsymmetric (degenerate) C-H

stretch

 

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

18

Table 2. "d"-spacings for trimethylplatinum iodidea

Yellow Form White Form

 

 

8.59 m 8.60 w
8.13 s 8.15 s
7.74 m 7.75 m
7.24 m 7.26 m
6.32 s 6.30 s
5.06 w 5.07 w
4.61 w 4.64 w
4.36 w 4.34 w
---- 4.04 vw
3.79 vw 3.79 vw
3.67 vw —---
3.43 vw 3.47 vw
3.33 vw 3.34 vw
3.22 w 3.23 s
3.14 s 3.15 m
3.03 m 3.05 m
2.98 m -—--
2.91 m 2.91 s
2.83 m 2.82 s
2.69 vw ----
2.62 w 2.62 w
2.57 vw 2.57 vw
2.54 vw —--—
2.49 vw 2.49 s
2.41 s 2.41 m
2.38 vw ,-—--
2.36 vw 2.36 m
2.30 w 2.30 vw
.2.22 w 2.22 w
2.20 w 2.19 m
2.16 w —---
2.11 w 2.10 w
2.05 m 2.06 w
2.03 vw 2.03 vw
1.98 m 1.97 m
1.89 vw 1.90 w
1.88 vw -—--
1.87 vw 1.86 w
1.84 vw ----
1.78 vw 1.76 vw
———- 1.73 vw
---- 1.71 vw
~-—- 1.65 vw
1.62 vw 1.62 vw
1.60 vw 1.60 vw
1.58 vw
Other very faint lines 1.55 vw
as, strong; m, medium; w, weak; v, very; b, broad.

19

Foss and Gibson16 determined the molecular weights of
both forms of trimethylplatinum iodide by ebullioscopic
and Barger‘s method and reported association factors of
3.20—3.40 for the yellow compound and 2.42 for the white
compound. A redetermination of the molecular weights by the
author indicates that both forms of trimethylplatinum iodide
are tetrameric in solution. The association factors are
4.01 for the yellow form and 4.07 for the white form. As
the white form,in a benzene solution, is converted to the
yellow with time and heat, the data collected by vapor pres-
sure osmometry should be more accurate than that of the more
time consuming methods used by Foss and Gibson.

The results of these experiments indicate that the
white and yellow forms of trimethylplatinum iodide have
identical structures and differ only in color. Octahedral
coordination has been found in all reported structures of
platinum(IV) compounds and the present investigation has un-
covered no evidence for the presence of additional ligands.
The color of the yellow compound is believed to be caused
by adsorbed iodine since most of it may be removed when the
compound is refluxed with a silver salt and the white com-
pound is obtained when methylsodium is used in the synthesis
instead of methylmagnesium iodide.24

Trimethylplatinum sulfate and nitrate are of interest
because of their ionic character as compared to the highly
covalent character of most of the trimethylplatinum deriva-

tives. Raman spectroscopy was used by Clegg and Hall12 to

20
show the extensive dissociation of these compounds in aqueous
solution. An objective of the present study was to deter—
mine the structures of the solid compounds and find if, in
trimethylplatinum sulfate, two trimethylplatinum groups are
joined together.

The presence of three sulfate sulfate peaks in the
900-1200 cm.1 region of the infrared spectrum of trimethyl-
platinum sulfate (Table 3) shows that in the crystalline
state the sulfate group has a sz symmetry which indicates.
that it is bridging or bidentate.38 Water is present but
it could not be determined from the spectrum whether it is
coordinated or lattice water. Resolution in the far infra-
red region has not been great enough to provide any addi—
tional information about the structure of the molecule.

With potassium bromide pellets a shift in the spectrum of

the c—H frequencies and the appearance of a peak correspond-
ing to ionic sulfate indicates that trimethylplatinum bromide
is formed when the pellet is prepared.

Two different spectra are obtained for trimethylplatinum
nitrate (Table 4) depending upon the technique used. The
spectrum of the sample in a potassium bromide pellet cor-
responds to that of trimethylplatinum bromide and free
nitrate ion although the correlation with the published
values for the nitrate group38 is not good and some of the
peaks may be caused by impurities. A Nujol (mineral oil)
mull of the solid gives a similar spectrum but with several

additional peaks which are attributed to a coordinated

21

 

 

Table Infrared spectrum of trimethylplatinum sulfate.
i:§::i- Literature24'38 Assignment
(Cm'l)
554 m 562 vw Pt-C stretch
985 m 995 m Sulfate (v1)
1080 s 1050-1060 s Sulfate (v3)
1110 s 1105 s Sulfate (v3)
1160 s 1170 s Sulfate (v3)
1218 s Sym. C-H deformation
1240 s Comb.
1253 5 Deg. C-H deformation
1410 m 1406 m Unsym. C-H bending
1630 s 1630-1600 5 Water
2150 w 2150 m Sulfate
2790 w 2775 w Overtone unsym. C-H bending
2890 m 2874 m Sym. CeH stretch
2960 m 2950 m Unsym. C-H stretch
3370 s
3200-3550 8 Water

3450

 

22

 

 

Tabre 4. Infrared spectrum of trimethylplatinum nitrate.
KBr Nujol Literature Assignment
—-- 720 w 721 vs Pt-O stretch (water)
732 730 m 700-800 Nitrate (v4)
822 812 w
815-830 Nitrate (v2)
832 822 m
885 897 m 882 Methyl rock
—-— 1040 5 970-1035 ,Nitrate(coordinated)(v1)
1225 1230 w
1218 C-H deformation
1247 ' 1235 s
1265 1285 m 1253 C-H deformation
--— 1320 m Nitrate(coordinated)(v3)
1385 1360 s a Nitrate. I
1410 1410 vw 1406 C-H deformation
1420 1420 m a Nitrate
1637 1635 m 1600-1635 Water
1755 1755 w a Nitrate
1770 1772 m a Nitrate
2350 1340 w
2450 2450 m
2780 2780 w 2770 C-H deformation
2880 2880 m 2874 .C-H stretch
2950 2930 m 2930 C-H stretch
3350 3380 s
3200-3550 Water
3450 s

 

aNot reported in literature, but assumed to be caused by
nitrate as it only appears in the Spectra of trimethyl-
platinum nitrate.

23
nitrate group. When hexachlorobutadiene is used as the mull-
ing agent the spectrum initially shows the presence of the
coordinated nitrate group, but over a short period of time
a reaction takes place and the spectrum changes to one re-
sembling a trimethylplatinum halide. Infrared spectra alone
are not sufficient to establish whether the nitrate is uni-
dentate or bidentate.

The x-ray powder diffraction patterns of trimethyl—
platinum sulfate and nitrate are given in Tables 5 and 6.
There is no similarity between the patterns for the iodide,
nitrate, or sulfate. vThe structures cannot be determined
from the powder patterns because of the large number of lines
and single crystals could not be grown.

Trimethylplatinum sulfate and nitrate readily dissolve
in water and the shapes of the equivalent conductance
curves (Figures 1 and 2) show that they are strong electro-
lytes. A comparison of graphs of experimental data with
theoretical Onsager graphs indicates that trimethylplatinum
sulfate is a 2:1 electroltye and trimethylplatinum nitrate
is a 1:1 electrolyte.

-While the structure of trimethylplatinum sulfate has
not been determined, the experimental results suggest that
the sulfate group bridges two platinum atoms. -The bridge
is readily brOken and the sulfate group replaced by other
anions. The remaining two positions in the coordination
sphere of each platinum atom are probably occupied by aquo

groups. The probable structure of trimethylplatinum nitrate

24

 

Tatre 5. "d"-spacings for trimethylplatinum sulfate
11.30 s 2.20 w
10.40 s 2.15 s

6.41 vw 2.08 vw
6.17 m 2.03 w
5.55 vs 1.99 w
5.27 vs 1.96 m
5.05 s 1.90 vw
4.91 s 1.87 w
4.45 m 1.80 w
4.18 w 1.73 m
3.38 vw 1.66 vw
3.58 vw 1.62 w
3.42 vw 1.60 w
3.38 m 1.58 w
3.26 m. 1.56 w
3.05 s(b) 1.54 w
2.84 vw 1.50 w
2.73 m 1.48 vw
2.68 s 1.43 w(b)
2.53 vw 1.40 vw
2.47 m 1.38 vw
2.43 m 1.35 w
2.40 vw 1.34 w
2.35 w 1.30 w(b)
2.29 w 1.25 vw

"d"-Spacings for trimethylplatinum nitrate

 

5.91
5.72
5.26
4.64
4.03
3.81
3.61
3.30
3.23
3.17
2.95
2.90
2.85
2.78
2.74
2.70
2.51
2.47
2.44
2.34
2.30
2.20

m

S

S

2 E :5 2 i

E

£

2%?

S

In

W

W

2.15
2.07
2.02
1.98
1.93
1.86
1.83
1.81
1.65
1.59
1.56
1.52
1.47
1.45
1.43

.1.37

1.35
1.28
1.24
1.14

1.04

W

VVW

m

E

2‘

26

Figure 1. Equivalent conductance of trimethylplatinum
sulfate in aqueous solution.

II6 A; "5.7 Onsager Slopes

     

1:1 electrolyte
2:2 electrolyte
2:1 electrolyte

3
O
T

2

3
r
F

CDO CQIHV

I02 '-

IOO '

13k

qr

A (ohmfl

q2_

qu ..

eat

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27

Figure 2. Equivalent conductance of trimethylplatinum
nitrate in aqueous solution

   

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\ \
.. \
\
II
I
9
I
E: I
23
0"
Q)
N l
E Onsager slope
U \ \1:1 electrolyte
1- lo:-
, Onsager slopd\
' E 2:1 electrolyte \
.9. wo—
< \

4,2
/

q... \

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pant? . I02 (eCIUN/fl)

 

 

 

28
is a unidentate nitrate group and two aquo groups occupy-

ing the three non-methyl ligand positions.

SUMMARY

Trimethylplatinum derivatives are of interest because
of their high degree of stability and the non—mobility of
the methyl ligands. Many trimethylplatinum compounds have
been prepared but only limited characterization has been
achieved for most of them. The purpose of the present in-
vestigation was to characterize the white trimethylplatinum
iodide, trimethylplatinum sulfate and trimethylplatinum
nitrate. I

The white form of trimethylplatinum iodide has been
reported to be dimeric, in contrast to the more common yel-
low form which is tetrameric. A dimeric structure would
require that either additional ligands be present or an un-
usual coordination for platinum(IV). Only the molecular
weights and elemental analysis of both forms of trimethyl-
platinum iodide and the infrared spectrum of the yellow
form have been reported in the literature. For a comparison
of the two structures, the previously reported analysis and
spectrum were confirmed and the infrared spectrum of the
white form and x-ray powder diffraction patterns of both
forms were determined. Molecular weight measurements, more
accurate than those which had been reported previously, in—
dicate that both forms are tetrameric in solution. The
infrared spectrum and x—ray patterns are the same for both
white and yellow trimethylplatinum iodide which indicates

that they have the same tetrameric structure.

 

30

No structural information has been reported for the
ionic compounds trimethylplatinum sulfate and nitrate except
elemental analysis. In the present study, the infrared
spectra, x-ray powder diffraction patterns, and equivalent
conductance in aqueous solution were determined. In aqueous
solution, both trimethylplatinum sulfate and nitrate are
strongly dissociated. Trimethylplatinum sulfate is a 2:1
electrolyte and trimethylplatinum nitrate is a 1:1 electro-
lyte. The structures of both compounds cannot be determined
from the diffraction patterns because of the large number of
lines, but the infrared spectra and elemental analysis
indicate that trimethylplatinum sulfate has a sulfate group
bridging two platinum atoms and two aquo groups on each of
the platinum atoms and that trimethylplatinum nitrate has
two aquo groups and an unidentate nitrate group attached to

the platinum atom.

4.

5.

8.
9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

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