Thermal decomposition of platinum(IV)-silicon, -germanium, and -tin complexes

Article abstract of DOI:10.1021/om970204x

The thermal decomposition of a number of complexes of the type [PtMe₂(Me₃E)X(diimine)] (E = Si, Ge, Sn; X = Cl, Br, I) has been studied. The thermal stability of complexes, as determined by thermogravimetric analysis (TGA), varies depending on the diimine ligand in the order 2,2′-bipyridyl (bpy) > 4,4′-di-tert-butyl-2,2′-bipyridyl (bpy-^tbu₂) > N-(2-(dimethylamino)ethyl)pyridine-2-aldimine (paen-me₂) > (2-imino-n-propyl)pyridine (py-n-pr). Stability also varies according to the trends E = Sn ≈ Ge > Si and X = I > Br > Cl. The products of thermal decomposition have also been determined by ¹H NMR and three distinct modes of decomposition are evident: reductive elimination of Me₃EX, reductive elimination of Me₄E, and α-elimination of Me₂E. The competition between reductive elimination of Me₃EX and Me₄E depends primarily on the halide, X, with the ratio Me₃EX:Me₄E highest for X = Cl and lowest for X = I. The competition between reductive elimination and α-elimination depends primarily on E, with the tendency to α-elimination of Me₂E increasing as E = Si < Ge < Sn. Differential scanning calorimetry (DSC) has been used to estimate D(Pt-Si) for the complex [PtIMe₂(Me₃Si)(bpy)] as 233 ± 14 kJ mol^-1.

Full text of DOI:10.1021/om970204x

Organometallics 1997, 16, 4115-4120
4115
Th er m a l Decom p osition of P la tin u m (IV)-Silicon ,
-Ger m a n iu m , a n d -Tin Com p lexes
Christopher J . Levy and Richard J . Puddephatt*
Department of Chemistry, The University of Western Ontario,
London, Ontario, Canada N6A 5B7
Received March 12, 1997X
The thermal decomposition of a number of complexes of the type [PtMe2(Me3E)X(diimine)]
(E ) Si, Ge, Sn; X ) Cl, Br, I) has been studied. The thermal stability of complexes, as
determined by thermogravimetric analysis (TGA), varies depending on the diimine ligand
in the order 2,2′-bipyridyl (bpy) > 4,4′-di-tert-butyl-2,2′-bipyridyl (bpy-tbu2) > N-(2-(di-
methylamino)ethyl)pyridine-2-aldimine (paen-me2) > (2-imino-n-propyl)pyridine (py-n-pr).
Stability also varies according to the trends E ) Sn ≈ Ge > Si and X ) I > Br > Cl. The
products of thermal decomposition have also been determined by 1H NMR and three distinct
modes of decomposition are evident: reductive elimination of Me3EX, reductive elimination
of Me4E, and R-elimination of Me2E. The competition between reductive elimination of
Me3EX and Me4E depends primarily on the halide, X, with the ratio Me3EX:Me4E highest
for X ) Cl and lowest for X ) I. The competition between reductive elimination and
R-elimination depends primarily on E, with the tendency to R-elimination of Me2E increasing
as E ) Si < Ge < Sn. Differential scanning calorimetry (DSC) has been used to estimate
D(Pt-Si) for the complex [PtIMe2(Me3Si)(bpy)] as 233 ( 14 kJ mol-1
.
In tr od u ction
bond dissociation energies.5 One method which has
been used extensively to find heats of reactions in
inorganic and organometallic chemistry is differential
scanning calorimetry (DSC). We have used this tech-
nique to study the decomposition of [PtIMe2(Me3Si)-
(bpy)] and have obtained a value for D(Pt-Si) in this
complex. A preliminary report on this part of the
research has been published,4 and the synthesis of the
platinum(IV) complexes has been described.6
Although oxidative addition of alkyl halides has been
studied in depth, there are surprisingly few examples
of selective reductive elimination of alkyl halides from
alkylhalometal complexes.1 In 1959, Chatt and Shaw
reported that reductive elimination from halomethyl-
platinum(IV) complexes generally produces mixtures of
methyl halides and ethane, while Ruddick and Shaw
found that platinum(IV) complexes containing at least
two halogens and one or two methyl groups yield methyl
halide on thermolysis (185-210 °C).2 In dihalodimeth-
ylplatinum(IV) complexes, the selectivity for reductive
elimination of MeX or C2H6 depends on the nature of
the halogen and the geometry of the complex. Recently,
a report of competitive C-C and C-I reductive elimina-
tion from [PtIMe3(dppe)] has been published.3 When
the platinum(IV) complex is heated in acetone-d6, initial
reductive elimination of (predominantly) methyl iodide
was observed by NMR spectroscopy, but with extended
heating, all material was converted to ethane and
[PtIMe(dppe)]. Since the oxidative addition of Me3EX
to [PtMe2(diimine)] (E ) Si, Ge, Sn; X ) Cl, Br, I) is
rapid and reversible in solution, a study of the solid-
state thermolysis of the products [PtMe2(Me3E)X-
(diimine)] was carried out.4 Three different decompo-
sition modes were identified, as reported below.
Resu lts
TGA of P la tin u m (IV) Com p lexes. The TGA curves
obtained for platinum(IV) complexes are similar in
appearance to that shown in Figure 1 (20-800 °C) for
[PtClMe2(Me3Sn)(bpy-tbu2)]‚Me3SnCl, a weak complex
of [PtClMe2(Me3Sn)(bpy-tbu2)]‚with Me3SnCl. This com-
plex is thermally stable to 120 °C, above which it loses
substantial mass in a fairly narrow temperature range
(120-160 °C). This initial mass loss (42.7%) could be
due to loss of two molecules of Me3SnCl (44.7%) or one
molecule each of Me3SnCl and Me4Sn (42.4%). In either
case, it is interesting that reductive elimination from
the platinum(IV) complex appears to occur nearly
simultaneously with loss of the weakly bound Me3SnCl
molecule. The decomposition product is stable up to 220
°C, and then it loses mass in a second decomposition
step (220-290 °C). The mass loss (19.2%) does not
correspond to ethane (3.4%), MeCl (5.7%), or bpy-tbu2
(30.1%). Above 290 °C the mass is lost gradually and,
at 800 °C, the remaining residue has a mass percent
Bond dissociation energies of organometallic com-
plexes are usually difficult to obtain, and there are few
experimental or theoretical estimates of metal-silicon
X Abstract published in Advance ACS Abstracts, August 15, 1997.
(1) Stille, J . K. In The Chemistry of the Metal-Carbon Bond: The
Nature and Cleavage of Metal-Carbon Bonds; Hartley, F. R., Patai,
S., Eds.; Wiley: Toronto, 1985; p 745.
(2) (a) Chatt, J .; Shaw, B. L. J . Chem. Soc. 1959, 705. (b) Ruddick,
J . D.; Shaw, B. L. J . Chem. Soc., A 1969, 2969.
(3) (a) Goldberg, K. I.; Yan, J .; Winter, E. L. J . Am. Chem. Soc. 1994,
116, 1573. (b) Goldberg, K. I.; Yan, J .; Breitung, E. M. J . Am. Chem.
Soc. 1995, 117, 6889.
(5) (a) Nolan, S. P.; Porchia, M.; Marks, T. J . Organometallics 1991,
10, 1450. (b) J iang, Q.; Pestana, D. C.; Carroll, P. J .; Berry, D. H.
Organometallics 1994, 13, 3679. (c) Ziegler, T.; Tschinke, V.; Versluis,
L.; Baerends, E. J .; Ravenek, W. Polyhedron 1988, 7, 1625. (d) Tilley,
T. D. In The Silicon-Heteroatom Bond; Patai, S., Rappoport, Z., Eds.;
Wiley: New York, 1991; pp 245, 309.
(4) Levy, C. J .; Puddephatt, R. J . Organometallics 1995, 14, 5019.
(6) Levy, C. J .; Puddephatt, R. J . Organometallics 1996, 15, 2108.
S0276-7333(97)00204-5 CCC: $14.00 © 1997 American Chemical Society

4116 Organometallics, Vol. 16, No. 19, 1997
Levy and Puddephatt
overlapping signals, indicating multiple decomposition
pathways over a small temperature range. This is in
accord with the TGA and 1H NMR results presented
above. Due to the complexity of most DSC results,
analysis of reaction thermodynamics is not straightfor-
ward or practical. For this reason we have concentrated
our efforts on the decomposition of complexes of the type
[PtIMe2(Me3Si)(diimine)], as we have shown these spe-
cies to decompose almost entirely by a single pathway
(Me4Si elimination). The production of a single, non-
reactive volatile component is well suited to thermody-
namic analysis of the reaction by DSC. The clean
decomposition of [PtIMe2(Me3Si)(bpy)] allows for a
straightforward estimation of the Pt-Si bond dissocia-
tion energy through the use of DSC. The enthalpy of a
reaction can be approximated as the difference in
dissociation energies between the bond formed and
bonds broken. For the decomposition of [PtIMe2(Me3-
Si)(bpy)] this gives rise to eq 1, from which D(Pt-SiMe3)
F igu r e 1. TGA curve of [PtClMe2(Me3Sn)(bpy-tbu2)].
(23%) close to the platinum content in the original
complex (21.9%), indicating almost complete decomposi-
tion of the sample. Most platinum(IV) complexes stud-
ied in this work undergo an initial decomposition step
in a fairly narrow temperature range, similar to that
observed for [PtClMe2(Me3Sn)(bpy-tbu2)]‚Me3SnCl. How-
ever, subsequent decomposition often occurs in multiple,
overlapping steps. Table 1 contains TGA data for the
first decomposition step of those platinum(IV) complexes
studied. There is a clear trend for increased thermal
stability with increased halide size, as can be seen in
the TGA curves for the complexes [PtMe2X(Me3Sn)(bpy)]
(X ) Cl, Br, I) in Figure 2.
∆Hrxn
)
D(Pt-CH3) + D(Pt-SiMe3) - D(Me3Si-Me) (1)
can be estimated, though several approximations and
assumptions are necessary.9 This is why we consider
this an estimate rather than a determination of D(Pt-
Si).
Figure 3 shows the DSC result for the decomposition
of [PtIMe2(Me3Si)(bpy)]. Decomposition occurs over the
range 177-195 °C, corresponding closely to the 172-
194 °C range observed by TGA for the same process.
The signal shows exothermic and endothermic regions.
This can be accounted for in terms of an exothermic
reaction which is accompanied by an endothermic
transition, partially due to the phase transition of the
solid and, perhaps, partially due to volatilization of Me4-
Si. The heat of reaction was determined to be -8 ( 5
kJ /mol from eight experiments.
1H NMR An a lysis of Decom p osit ion P r od u ct s.
Platinum(IV) complexes were heated under vacuum in
NMR tubes, and solutions of the solid and evolved
volatiles were examined by 1H NMR spectroscopy. The
temperature of pyrolysis was that of the first decompo-
sition step determined by TGA. The product ratios
1
obtained from H NMR are presented in Table 2.
1
The H NMR spectra of the solid resulting from the
decomposition of [PtClMe2(Me3Sn)(bpy)] shows several
Me-Pt resonances. The most intense of these is due
2
to [PtMe2(bpy)] (δ 0.97, J (PtH) ) 86 Hz), indicating
major decomposition by the reductive elimination of
Discu ssion
Me3SnCl. A minor Me-Pt signal due to [PtClMe(bpy)]
2
(δ 1.05, J (PtH) ) 79 Hz),7 is also present, consistent
Inspection of the TGA data reveals a number of
trends. The thermal stability of complexes varies,
depending on the diimine ligand, in the order bpy > bpy-
tbu2 > paen-me2 > py-n-pr, likely the result of packing
effects, since no substantial electronic differences are
expected. Stability also varies according to the group
14 element, Sn ≈ Ge > Si, in accordance with the results
of previous studies.10 There is also a clear trend for
increased thermal stability with increased halide size
(Figure 2). In most cases, the TGA results are not
consistent with a single mode of decomposition, as
indicated by the theoretical mass losses presented in
Table 1. Despite the fact that specific decomposition
modes cannot generally be established from the TGA
with Me4Sn reductive elimination as a minor decompo-
sition pathway. The spectrum shows two other Me-Pt
2
2
signals (δ 0.38, J (PtH) ) 72.4 Hz; δ 1.45, J (PtH) )
70.7 Hz) with the high-frequency signal having twice
the intensity of the low-frequency signal. These reso-
nances are due to [PtMe3Cl(bpy)],8 the product of
R-elimination of dimethylstannylene, Me2Sn, from
[PtClMe2(Me3Sn)(bpy)].
A
1H NMR analysis of the
volatile products of decomposition shows both Me3SnCl
(δ 0.58) and Me4Sn (δ 0.05) to be present along with a
number of minor unidentified products. These uniden-
tified volatiles are likely products of indiscriminate
reactions of the unstable Me2Sn. Similar pyrolysis
experiments have been carried out on a number of other
platinum(IV) complexes, and the results are sum-
marized in Table 2.
DSC Stu d ies of P la tin u m (IV) Com p lexes: De-
ter m in a tion of th e P t-Si Bon d En er gy in [P tIMe2-
(Me3Si)(bp y)]. DSC results for the first decomposition
step of a series of platinum(IV) complexes are presented
in Table 3. In many cases there are several close or
(9) There are a number of approximations made in this type of
treatment. The energies of bonds that are not directly involved in the
reaction are assumed to remain constant, and no account of the Pt(IV)-
Pt(II) promotional energy is made. Note that in the present case it is
necessary to assume that the remaining Pt-C, Pt-N, and Pt-I bond
dissociation energies are the same as in the Pt(IV) precursors and that
the heats of sublimation of precursor and product are the same. As
well, there are a number of difficulties associated with the practical
aspects of DSC. For a discussion, see: MacKenzie, R. C. Differential
Thermal Analysis; Academic Press: New York, 1970; Vol. 1, pp 55-
120.
(7) Clark, H. C.; Manzer, L. E. J . Organomet. Chem. 1973, 59, 411.
(8) Kuyper, J . Inorg. Chem. 1978, 17, 77.
(10) Hartley, F. R. The Chemistry of Platinum and Palladium;
Applied Science: London, 1973; p 81.

Platinum(IV)-Silicon, -Germanium, and -Tin Complexes
Organometallics, Vol. 16, No. 19, 1997 4117
Ta ble 1. TGA Da ta for th e F ir st Decom p osition Step of Com p lexes of th e Typ e [P tMe2(Me3E)X(d iim in e)]
(E ) Sn , Ge, X ) Cl, Br , I; Si, X ) Br , I)
diimine ligand
Me3EX
onset temp (°C) final temp (°C) 1st mass loss (%) theor Me3EXa (%) theor Me4Eb (%) theor Me2Ec (%)
bpy
bpy
bpy
bpy
Me3SnCl
Me3SnBr
Me3SnI
Me3SiBr
Me3SiI
147
166
171
161
172
145
147
149
131
140
140
117
127
82
180
215
217
176
194
165
176
192
165
176
226
131
167
199
150
161
141
158
162
181
183
127
157
29.2
33.8
33.3
24.4
18.7
42.7
45.6
35.9
21.6
28.5
28.9
17.1
17.2
28.7
23.4
20.7
23.3
16.1
26.4
35.6
30.1
21.7
15.3
34.3
39.0
50.0
28.7
34.4
44.7e
49.7e
54.1e
23.7
28.6
33.1
23.7
28.9
30.0
39.5
43.7
29.1
34.9
33.1
37.7
41.9
27.6
33.2
30.8
28.6
30.7
16.5
15.2
42.4f
43.1f
43.7f
20.5
19.2
18.0
13.6
12.7
26.9
29.0
26.9
16.8
15.4
29.7
27.7
25.8
15.9
14.6
25.6
23.8
25.5
13.7
12.6
39.0g
40.0g
40.9g
15.9
14.9
13.9
9.0
bpy
bpy-tbu2
bpy-tbu2
bpy-tbu2
bpy-tbu2
bpy-tbu2
bpy-tbu2
bpy-tbu2
bpy-tbu2
py-n-pr
py-n-pr
py-n-pr
py-n-pr
py-n-pr
paen-me2
paen-me2
paen-me2
paen-me2
paen-me2
Me3SnCld
Me3SnBrd
Me3SnId
Me3GeCl
Me3GeBr
Me3GeI
Me3SiBr
Me3SiI
Me3SnCl
Me3SnBr
Me3SnI
Me3SiBr
Me3SiI
Me3SnCl
Me3SnBr
Me3SnI
8.4
22.4
24.1
22.4
11.1
10.1
24.7
23.0
21.5
10.5
9.6
98
91
116
139
116
136
153
114
136
Me3SiBr
Me3SiI
a
b
Theoretical mass change upon loss of Me3EX. Theoretical mass change upon loss of Me4E. c Theoretical mass change upon loss of
Me2E. Compelx has formula [PtMe2X(Me3Sn)(bpy-tbu2)]‚Me3SnX. e Theoretical mass change upon loss of two molecules of Me3SnX.
d
f Theoretical mass change upon loss of one molecule each of Me3SnX and Me4Sn. Theoretical mass change upon loss of one molecule
g
each of Me3SnX and Me2Sn.
products indicate three decomposition pathways for
[PtClMe2(Me3Sn)(bpy)] (Scheme 1).11 The decomposi-
tion pattern of [PtClMe2(Me3Sn)(bpy)] has features
similar to those reported for other platinum complexes.
Decomposition by the competitive reductive elimination
of Me3SnCl and Me4Sn is analogous to that observed
for the thermolysis of [PtIMe3(dppe)], which undergoes
reductive elimination of C2H6 (82%) and MeI (18%).4 In
this case, reductive elimination of ethane is the major
decomposition pathway, while for [PtClMe2(Me3Sn)-
(bpy)] the corresponding reductive elimination of tet-
ramethyltin is only a minor pathway. The third de-
composition pathway for [PtClMe2(Me3Sn)(bpy)] is the
elimination of Me2Sn, a process which is not a reductive
elimination and gives a platinum(IV) product. Similar
eliminations of germylene and silylene fragments have
been observed by Tanaka and co-workers12,13 for certain
platinum(II) complexes, but in these cases the platinum
center was coordinatively unsaturated.
From a mechanistic viewpoint, it is likely that the
first step in thermolysis is the dissociation of the Pt-X
bond, as illustrated for the thermolysis of [PtClMe2(Me3-
Sn)(bpy)] in Scheme 2.4,14,15 The reductive elimination
of Me3SnCl from [PtClMe2(Me3Sn)(bpy)] presumably
occurs by a mechanism which is the reverse of the SN2
process. This is a very rapid, reversible process in
solution16 but is clearly much slower in the solid-state
thermolysis, probably because the chloride ion cannot
easily migrate from platinum, as required (Scheme 2,
F igu r e 2. TGA curves of [PtXMe2(Me3Sn)(bpy)] (X ) Cl,
Br, I).
Ta ble 2. Decom p osition P r od u cts of P la tin u m (IV)
Com p lexes
[PtMe2
[PtMeX
[PtMe3X
platinum(IV) complex
(N-N) (%) (N-N)] (%) (N-N)] (%)
[PtClMe2(Me3Sn)(bpy)]
[PtBrMe2(Me3Sn)(bpy)]
[PtIMe2(Me3Sn)(bpy)]
51
Ma
∼0
37
50
10
41
64
3
26
mb
∼0
54
50
90
59
36
97
23
m
∼100
[PtClMe2(Me3Ge)(bpy-tbu2)]
[PtBrMe2(Me3Ge)(bpy-tbu2)]
[PtIMe2(Me3Ge)(bpy-tbu2)]
[PtBrMe2(Me3Si)(bpy)]c
[PtBrMe2(Me3Si)(bpy)]d
[PtIMe2(Me3Si)(bpy)]
9
∼0
∼0
∼0
∼0
∼0
a
b
d
Major product. Minor product. c 190 °C. 210 °C.
data, certain trends are evident and are confirmed by
NMR studies of pyrolysis products. The degree of
decomposition resulting from Me4E and Me2E elimina-
tion generally increases with increasing halide size, and
there is a corresponding decrease in the reductive
elimination of Me3EX. This may be a result of the
increasing difficulty in breaking the Pt-X bond in the
order Cl < Br < I, as evidenced by the higher onset
temperatures in the same series.
(11) The percentages given in Scheme 1 represent product ratios of
complexes in solution. It must be noted that there was a small amount
of insoluble material resulting from decomposition.
(12) Kobayashi, T.; Hayashi, T; Yamashita, H.; Tanaka, M. Chem.
Lett. 1988, 1411.
(13) Yamashita, H.; Kobayashi, T.; Tanaka, M.; Samuels, J . A.;
Streib, W. E. Organometallics 1992, 11, 2330.
(14) Roy, S.; Puddephatt, R. J .; Scott, J . D. J . Chem. Soc., Dalton
Trans. 1989, 2121.
(15) Brown, M. P.; Puddephatt, R. J .; Upton, J . J . Chem. Soc., Dalton
Trans. 1974, 2457.
(16) Levy, C. J .; Puddephatt, R. J . J . Chem. Soc., Chem. Commun.
1995, 2115.
TGA results along with NMR studies of pyrolysis

4118 Organometallics, Vol. 16, No. 19, 1997
Ta ble 3. DSC Sign a ls for P la tin u m (IV) Com p elxesa
Levy and Puddephatt
complex
T1 (°C)
T3 (°C)
T3 (°C)
T4 (°C)
[PtClMe2(Me3Sn)(bpy-tbu2)]‚Me3SnCl
[PtBrMe2(Me3Sn)(bpy-tbu2)]‚Me3SnBr
[PtIMe2(Me3Sn)(bpy-tbu2)]‚Me3SnI
[PtBrMe2(Me3Si)(bpy-tbu2)]
[PtIMe2(Me3Si)(bpy-tbu2)]
[PtBrMe2(Me3Si)(bpy)]
[PtIMe2(Me3Si)(bpy)]
[PtIMe2(Me3Si)(py-n-pr)]
[PtIMe2(Me3Si)(paen-me2)]
161, ex
173, ex
174, ex
103, ex
134, ex
169, en
187, en
147, ex
137, ex
172, en
186, en
188, en
115, ex
169, ex
174, ex
192, ex
153, en
152, ex
176, ex
192, ex
198, ex
120, en
197, ex
135, ex
165, ex
a
T1-T4 are labels for DSC signals (in terms of increasing temperature) arising from initial decomposition of the platinum(IV) complexes.
The terms en and ex indicate endothermic and exothermic signals, respectively.
Sch em e 2
F igu r e 3. DSC signal for the reductive elimination of
Me4Si from [PtIMe2(Me3Si)(bpy)]. Conditions: scan rate,
10.0 °C min-1; sample weight, 1.477 mg; area, -8.9 J g-1
()-5.2 kJ mol-1).
Sch em e 1
favored in the order Cl < Br < I, with a corresponding
decrease in reductive elimination of Me3EX (E ) Si, Ge).
The decrease in Me3EX with increasing halide size is
consistent with the results obtained for similar plati-
num(IV)-tin complexes. The germanium complexes do
not show significant decomposition by the elimination
of Me2Ge. This is only a minor pathway for the chloro
complex, and it is not observed for the bromo and iodo
complexes. Silylplatinum(IV) complexes do not show
any decomposition via the elimination of Me2Si. These
results are in contrast to those obtained for the plati-
num(IV)-tin complexes, which show an increasing
degree of decomposition by this pathway in the order
Cl < Br < I. The decrease in decomposition by the
elimination of Me2E in the series Sn > Ge > Si is
consistent with the clear trend of decreased E-Me bond
dissociation energies in the order E ) C > Si > Ge >
Sn.17 Average E-Me bond energies for Me4E are 317,
265, and 226 kJ /mol for E ) Si, Ge, and Sn, respectively.
The pyrolyses of [PtBrMe2(Me3Si)(bpy)] at 190 and
210 °C clearly show an increased proportion of decom-
position by Me3SiBr reductive elimination with in-
creased temperature and indicates higher activation
barrier for this process than for the reductive elimina-
tion of Me4Si.
DSC studies have provided the first experimental
estimation of a Pt-Si bond energy. Literature values
for D(Me-Pt) and D(Me-Si) were required for this
analysis. The Me3Si-Me bond energy has been derived
by Walsh17 to be 376 ( 8 kJ mol-1. Several literature
values are available for the Me-Pt(IV) bond dissociation
energy. A value of 134 kJ mol-1 is predicted by the
route A). Such a pathway is rare for alkylplatinum(IV)
complexes.1 The elimination of Me4Sn probably follows
the more common mode of reductive elimination from
Pt(IV) complexes, namely a concerted 1, 1-reductive
elimination (Scheme 2, route B). The elimination of
Me2Sn from [PtClMe2(Me3Sn)(bpy)] requires methyl
group transfer from the trimethyltin ligand to the
platinum center along with Pt-Sn bond cleavage.
Because the platinum(IV) center is coordinatively satu-
rated, it seems unlikely that this occurs directly. One
possible mechanism involves dissociation of the chloro
ligand followed by R-migration of a methyl group from
the tin to the platinum center with concomitant elimi-
nation of Me2Sn, then followed by displacement of
“Me2Sn” by Cl- (Scheme 2, route C). The strong trans
effect of the Me3Sn ligand makes the Pt-Cl bond
relatively easy to cleave.
Our studies of silyl- and germylplatinum(IV) com-
plexes show that Me4E (E ) Si, Ge) elimination is
(17) Walsh, R. In The Chemistry of Organic Silicon Compounds;
Patai, S., Rappaport, Z., Eds.; Wiley: Toronto, 1989; p 376-385.

Platinum(IV)-Silicon, -Germanium, and -Tin Complexes
Organometallics, Vol. 16, No. 19, 1997 4119
generalized valence bond theory for [PtCl2Me2(PH3)2],18
values of 137 and 125 kJ mol-1 have been found for cis-
[PtMe4L2] (L ) MeCN, 2,6-Me2C6H3NC),14 a value of 144
kJ mol-1 was found for [PtIMe3(PMe2Ph)2],15 and a
value of 132 kJ mol-1 was determined for [PtIMe3(dppe)]
(dppe ) Ph2PCH2CH2PPh2).3b We have used an average
of the experimentally determined bond dissociation
C, which makes transfer of a methyl group from the
group 14 element to platinum more difficult with lighter
group 14 elements. The reductive elimination of Me3EX
from many platinum(IV) complexes studied in this work
is in agreement with the observed trend that trans RX
reductive elimination is predominant when a ligand of
strong trans effect is trans to the eliminated halide.2,3b
For the first time we have experimentally estimated
a Pt-Si bond dissociation energy. This value of the Pt-
Si bond energy, while approximate, has obvious rel-
evance to catalytic processes, such as hydrosilylation,
which involve the making and breaking of Pt-Si bonds.
As well, new bond energy data are valuable for the
prediction of reaction thermodynamics and reaction
feasibility. Our DSC results give a D(Pt-SiMe3) value
of 233 ( 14 kJ mol-1 for [PtIMe2(Me3Si)(bpy)]. This is
energies (135 ( 10 kJ mol-1 19
for our calculations.
)
Using the literature bond dissociation energy data, we
obtain an estimated value for D(Pt-SiMe3) in
[PtIMe2(Me3Si)(bpy)] of 233 ( 14 kJ mol-1. This is 98
kJ mol-1 larger than the D(Pt-Me) value used in our
calculations, clearly showing the stronger nature of the
Pt-Si bonding interaction compared to Pt-Me. This
result is consistent with the general stability trend Pt-
Si > Pt-C.20
Literature data on transition-metal-group 14 bond
energies are sparse.5 Beauchamp and co-workers21 have
determined TM-Si bond energies for metal ion-silylene
complexes in the gas phase. The Ni-Si bond energy in
[NidSiH2]+ was found to be 280 ( 25 kJ mol-1. Sakaki
and Ieki22 have used ab initio calculations to estimate
D(Pt-SiH3) values for simple platinum(II)-phosphine
complexes. These authors obtained values which range
from 249 to 269 kJ mol-1, compared to 165-177 kJ
mol-1 for D(Pt-Me) in these model complexes. The
calculated Pt-Si bond lengths associated with these
bond energies are in the range 2.366-2.399 Å, similar
to the 2.337(3) Å found for [PtIMe2(Me3Si)(bpy)].23 The
value of D(Pt-Si) obtained for [PtIMe2(Me3Si)(bpy)] (233
kJ mol-1) agrees well with those obtained from the ab
initio calculations, and both studies indicate a Pt-Si
bond which is approximately 100 kJ mol-1 stronger than
the Pt-CH3 bond. An extensive chemistry of silylplati-
num complexes can therefore be anticipated.24
considerably larger (∼100 kJ mol-1) than for the PtIV
-
CH3 bond, demonstrating the dramatic increase in Pt-E
(E ) C, Si, Ge, Sn) bond energy on going to softer group
14 elements.
Exp er im en ta l Section
DSC experiments were conducted using a Perkin-Elmer
DSC7 with a TAC7/DX thermal analysis controller. Samples
were weighed into empty aluminum pans, with the weight of
each (0.5-3 mg) being determined using a microbalance
(supplied with the Perkin-Elmer TGA7). Pans were not sealed
in order to allow for the escape of volatile products.15 The
sample and reference compartments were purged with N2
during the experiments. Scan rates of 5-20 °C/min were used.
Thermogravimetric analysis (TGA) experiments were con-
ducted using a Perkin-Elmer TGA7 with a TAC7/DX thermal
analysis controller. Dry samples were weighed (1-4 mg) into
a platinum pan and were heated at 15-20 °C/min under a
stream of N2. All pyrolysis experiments were carried out in
NMR tubes which were fitted with J . Young valves (Aldrich
Catalog No. Z18,397-0) and could be sealed under vacuum. Dry
platinum(IV) complexes (ca. 10 mg) in evacuated NMR tubes
were heated for 20 min in a temperature-controlled oil bath.
Con clu sion s
TGA studies show increased stability for [PtX(Me3E)-
1
Integrals for H NMR signals are assigned with respect to the
Me2(diimine)] complexes in the order X ) Cl < Br < I
1
major Me-Pt signal. The H NMR parameters for [PtXMe3-
1
and E ) Si ≈ Ge < Sn. TGA and H NMR studies of
(bpy)] (X ) Br, I) have been previously reported.8,25
the decomposition of platinum(IV) complexes indicate
that there are up to three competitive decomposition
processes: 1,1-reductive elimination of Me4E, reductive
elimination of Me3EX, and R-elimination of Me2E. The
generation of Me2Sn and Me2Ge from platinum(IV)
complexes has no precedent in organoplatinum(IV)
complexes. We attribute this to the increasing Me-E
bond dissociation energies in the order Sn < Ge < Si <
P yr olysis of [P tClMe2(Me3Sn )(bp y)]. Two samples of
[PtClMe2(Me3Sn)(bpy)] were heated at 190 °C in NMR tubes.
The solid darkened from yellow to brown, and colorless crystals
were formed on the sides of the NMR tubes (above the level of
the oil bath). A N2 atmosphere was introduced into one tube,
and acetone-d6 was then added. The other tube was placed
under dynamic vacuum to remove any volatile material, and
the residue was dissolved in acetone-d6. Both tubes contained
yellow solutions along with dark insoluble material. The 1H
NMR spectrum (acetone-d6) of the solid residue with volatiles
removed was consistent with a mixture of 51% [PtMe2(bpy)],
26% [PtClMe(bpy)], and 23% [PtClMe3(bpy)]. Methyl-plati-
(18) Low, J . J .; Goddard, W. A., III. J . Am. Chem. Soc. 1986, 108,
6115.
(19) The error is estimated from the spread of the literature bond
energy data.
(20) McKay, K. M.; Nicholson, B. K. In Comprehensive Organome-
tallic Chemistry, Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;
Pergamon: Oxford, U.K., 1982; Vol. 6, pp 1096-1097.
(21) Kang, H.; J acobson, D. B.; Shin, S. K.; Beauchamp, J . L.;
Bowers, M. T. J . Am. Chem. Soc. 1986, 108, 5668.
(22) Sakaki, S.; Ieki, M. J . Am. Chem. Soc. 1993, 115, 2373.
(23) Levy, C. J .; Vittal, J . J .; Puddephatt, R. J . Organometallics
1994, 13, 1559.
(24) (a) Heyn, R. H.; Tilley, T. D. J . Am. Chem. Soc. 1992, 114, 1917.
(b) Chang, L. S.; J ohnson, M. P.; Fink, M. J . Organometallics 1991,
10, 1219. (c) Grumbine, S. D.; Tilley, T. D.; Arnold, F. P.; Rheingold,
A. L. J . Am. Chem. Soc. 1993, 115, 7884. (d) Latif, L. A.; Eaborn, C.;
Pidcock, A.; Weng, N. S. J . Organomet. Chem. 1994, 474, 217. (e)
Ozawa, F.; Hikida, T.; Hayashi, T. J . Am. Chem. Soc. 1994, 116, 2844.
(f) Yamashita, H.; Kobayashi, T.; Hayashi, T.; Tanaka, M. Chem. Lett.
1990, 1447. (g) Yamashita, H.; Tanaka, M.; Goto, M. Organometallics
1992, 11, 3227. (h) Yamashita, H.; Tanaka, M.; Goto, M. Organome-
tallics 1993, 12, 988.
2
num signals were as follows: [PtMe2(bpy)] δ 0.97 [s, J (PtH)
) 86.1 Hz]; [PtClMe(bpy)] δ 1.05 [s, 2J (PtH) ) 79.2 Hz];
2
[PtClMe3(bpy)] δ 0.37 [s, 3H, J (PtH) ) 74.1 Hz, axial Me], δ
2
1
1.22 [s, 6H, J (PtH) ) 70.2 Hz, equatorial Me]. The H NMR
spectrum (acetone-d6) of the solid residue in the presence of
volatiles is very similar, with two extra signals, corresponding
to an 11:89 ratio of Me4Sn:Me3SnCl: Me4Sn δ 0.05 [s, 2J (SnH)av
) 54 Hz]; Me3SnCl δ 0.58 [s, 2J (119SnH) ) 64.9 Hz, 2J (117SnH)
) 62.2 Hz]. The aromatic region of both spectra is complicated,
with overlapping signals arising from the three major platinum
products. The same procedure was followed for the pyrolyses
of [PtBrMe2(Me3Sn)(bpy)] and [PtIMe2(Me3Sn)(bpy)], except
that the oil bath temperature was maintained at 220 °C for
(25) Crespo, M.; Puddephatt, R. J . Organometallics 1987, 6, 2548.

4120 Organometallics, Vol. 16, No. 19, 1997
Levy and Puddephatt
the former and 230 °C for the latter. For the decomposition
of [PtBrMe2(Me3Sn)(bpy)], the H NMR spectrum (acetone-d6)
Pt], δ 1.22 [s, 6H, 2J (PtH) ) 69.5 Hz, equatorial Me-Pt]. The
1H NMR spectrum of the volatiles (C6D6) shows signals due to
Me3GeCl and Me4Ge, as well as two unidentified compounds:
δ 0.17 [s, Me4Ge], δ 0.31 [s, unidentified volatile], δ 0.36 [s,
Me3GeCl], δ 0.40 [s, unidentified volatile]. [PtBrMe2(Me3Ge)-
(bpy-tbu2)] was pyrolyzed at 210 °C. The 1H NMR spectrum
(CD2Cl2) of the solid showed [PtBrMe2(Me3Ge)(bpy-tbu2)] (not
decomposed), [PtMe2(bpy-tbu2)], and [PtBrMe(bpy-tbu2)]. The
ratio [PtMe2(bpy-tbu2)]:[PtBrMe(bpy-tbu2)] was 1:1. Signals
due to Me-Pt and Me-Ge were as follows: [PtBrMe2(Me3-
1
of the solid with volatiles removed is consistent with a mixture
of [PtBrMe2(Me3Sn)(bpy)] (not decomposed), [PtMe2(bpy)] (ma-
jor product), and [PtBrMe3(bpy)] (minor product). Signals due
to Me-Pt were as follows: [PtBrMe2(Me3Sn)(bpy)] and [PtMe2-
(bpy)] (equilibrium mixture) δ 1.05 [s (br), 2J (PtH) ) 80.5 Hz];
[PtBrMe3(bpy)] δ 0.46 [s, 3H, 2J (PtH) ) 74.2 Hz], δ 1.31 [s,
2
1
6H, J (PtH) ) 70 Hz]. The H NMR spectrum (acetone-d6) of
the solid in the presence of volatiles indicates a Me3SnBr:
Me4Sn ratio of 86:14. 1H NMR spectra of volatiles: δ 0.05 [s,
2J (SnH)av ) 54 Hz, Me4Sn], δ 0.15 [s, unidentified volatile], δ
2
Ge)(bpy-tbu2)] δ -0.22 [s, J (PtH) ) 17.7 Hz, Me-Ge], δ 1.28
[s, 2J (PtH) ) 63.3 Hz, Me-Pt]; [PtMe2(bpy-tbu2)] δ 0.92 [s,
2J (PtH) ) 85.4 Hz, Me-Pt]; [PtBrMe(bpy-tbu2)] δ 1.08 [s,
2
0.60 [s (br), J (SnH)av ) 61 Hz, averaged Me3Sn signal]. For
the decomposition of [PtIMe2(Me3Sn)(bpy)], the 1H NMR
spectrum (acetone-d6) of the residue with volatiles removed is
consistent with a mixture of [PtIMe2(Me3Sn)(bpy)] (not de-
composed), [PtIMe3(bpy)], and [PtIMe(bpy)]. The relative
percentages of [PtIMe3(bpy)] and [PtIMe(bpy)] are 93% and
7%, respectively. Signals due to Me-Pt and Me-Sn were as
follows: [PtIMe2(Me3Sn)(bpy)] δ -0.20 [s(br), 9H, Me-Sn], δ
1
2J (PtH) ) 76.8 Hz, Me-Pt]. The H NMR spectrum (C6D6) of
the volatiles shows essentially pure Me3GeBr (δ 0.48).
[PtIMe2(Me3Ge)(bpy-tbu2)] was pyrolyzed at 230 °C. The 1H
NMR spectrum (CD2Cl2) of the solid showed [PtIMe2(Me3Ge)-
(bpy-tbu2)] (not decomposed), [PtIMe(bpy-tbu2)], and [PtMe2(bpy-
tbu2)]. The ratio [PtIMe(bpy-tbu2)]:[PtMe2(bpy-tbu2)] was 9:1.
Signals due to Me-Pt and Me-Ge were as follows:
2
3
[PtIMe2(Me3Ge)(bpy-tbu2)] δ -0.28 [s, 9H, J (PtH) ) 18.0 Hz,
1.43 [s (br), 6H, J (PtH) ) 64.8 Hz, Me-Pt]; [PtIMe3(bpy)] δ
0.57 [s, 3H, 2J (PtH) ) 72.4 Hz, axial Me-Pt], δ 1.45 [s, 6H,
2J (PtH) ) 70.7 Hz, equatorial Me-Pt]; [PtIMe(bpy)] δ 1.27 [s,
2J (PtH) unresolved]. The 1H NMR spectrum of the solid in
the presence of volatiles indicates that Me4Sn is the major
volatile component, with a minor signal due to an unidentified
compound (intensity of 17% with respect to the Me4Sn signal).
P yr olysis of [P t Br Me2(Me3Si)(b p y)]. A sample of
[PtBrMe2(Me3Si)(bpy)] was heated in an oil bath at 210 °C.
The solid darkened from yellow to brown-yellow. The tube was
attached to a vacuum manifold, and the volatile components
were distilled into a second NMR tube. CD2Cl2 was distilled
into each tube by vacuum transfer. The solution of the solid
residue was yellow, with a small amount of dark solid. 1H
NMR spectrum (CD2Cl2) of the solid, with volatiles removed,
shows signals due to [PtMe2(bpy)], [PtBrMe(bpy)], and a small
amount of [PtBrMe2(Me3Si)(bpy)] (not decomposed). The
[PtMe2(bpy)]:[PtBrMe(bpy)] ratio is 41:59. Signals due to Me-
Pt were as follows: [PtMe2(bpy)] δ 0.99 [s, 2J (PtH) ) 85.5 Hz];
[PtBrMe(bpy)] δ 1.14 [s, 2J (PtH) ) 79.6 Hz]. The 1H NMR
spectrum (CD2Cl2) of the volatile products shows Me4Si (δ
0.00), as well as a number of other unidentified resonances (δ
0.07, 0.13, 0.21). Pyrolysis at 190 °C was carried out using
the same procedure as at 210 °C. Similar results were
obtained, except that the [PtMe2(bpy)]:[PtBrMe(bpy)] ratio had
increased from 41:59 (210 °C) to 64:36. The pyrolyses of
[PtIMe2(Me3Si)(bpy)] and [PtClMe2(Me3Ge)(bpy-tbu2)] (X ) Cl,
Br, I) were carried out. [PtIMe2(Me3Si)(bpy)] was pyrolyzed
at 155 °C. The 1H NMR spectrum (CD2Cl2) of the solid residue
shows [PtIMe2(Me3Si)(bpy)] (not decomposed) as well as
[PtIMe(bpy)] and [PtMe2(bpy)]. The [PtIMe(bpy)]:[PtMe2(bpy)]
ratio was 97:3. Signals due to Me-Pt and Me-Si were as
Me-Ge], δ 1.44 [s, 6H,2J (PtH) ) 64.2 Hz, Me-Pt]; [PtIMe-
2
(bpy-tbu2)] δ 1.11 [s, J (PtH) ) 76.1 Hz, Me-Pt]; [PtMe2(bpy-
tbu2)] δ 0.92 [s, 2J (PtH) ) 85.4 Hz, Me-Pt]. The 1H NMR
spectrum (C6D6) of the volatiles shows Me3GeI (δ 0.64) and
Me4Ge (δ 0.12) to be the major components. Signals of lesser
intensity are observed at δ 0.31, 0.36, and 0.40.
P yr olysis of [P t ClMe2(Me3Sn )(b p y-t b u 2)]‚Me3Sn Cl.
[PtClMe2(Me3Sn)(bpy-tbu2)]‚Me3SnCl was heated at 175 °C,
and the solid changed from light yellow to orange. A colorless
solid formed in the tube above the level of the oil bath. The
tube was attached to a vacuum manifold, and the volatiles
were vacuum-transferred to a second NMR tube. The tubes
were placed under an atmosphere of nitrogen, and acetone-d6
1
was added to both. The H NMR spectrum (acetone-d6) of the
solid residue shows a mixture of [PtClMe2(Me3Sn)(bpy-tbu2)],
[PtMe2(bpy-tbu2)], [PtClMe(bpy-tbu2)], and [PtClMe3(bpy-tbu2)].
[PtClMe2(Me3Sn)(bpy-tbu2)] and [PtMe2(bpy-tbu2)] show aver-
aged signals due to rapid exchange. Integration shows the
following relative percentages of decomposition products:
[PtMe2(bpy-tbu2)], 58%; [PtClMe(bpy-tbu2)], 24%; [PtClMe3(bpy-
tbu2)], 18%. Signals due to Me-Pt were as follows: [PtMe2(bpy-
tbu2)] δ 0.93 [s, 2J (PtH) ) 83.7 Hz, Me-Pt]; [PtClMe(bpy-tbu2)]
δ 0.99 [s, 2J (PtH) ) 78.4 Hz, Me-Pt]; [PtClMe3(bpy-tbu2)] δ
0.36 [s, 3H, 2J (PtH) is unresolved, axial Me-Pt], δ 1.19 [s, 6H,
2J (PtH) ) 70.6 Hz, equatorial Me-Pt]. The 1H NMR spectrum
(acetone-d6) of the volatile components shows a 95:5 ratio of
Me3SnCl:Me4Sn: δ 0.05 [s, 2J (SnH)av ) 52.9 Hz, Me4Sn]; δ 0.61
[s, 2J (SnH)av
) 63.5 Hz, Me3SnCl]. The pyrolysis of
[PtBrMe2(Me3Sn)(bpy-tbu2)]‚Me3SnBr was carried out using
the same procedure, except that the oil bath was maintained
at 185 °C. The 1H NMR spectrum (acetone-d6) of the solid
residue in acetone-d6 (orange solution and orange solid)
corresponds to that of [PtBrMe2(Me3Sn)(bpy-tbu2)]: δ 0.41 [s
(v br), 9H, Me-Sn], δ 0.94 [s, 6H, 2J (PtH) ) 83.7 Hz, Me-Pt],
3
follows: [PtIMe2(Me3Si)(bpy)] δ -0.33 [s, 9H, J (PtH) ) 19.0
Hz, Me-Si], δ 1.45 [s, 6H, 2J (PtH) ) 64.3 Hz, Me-Pt]; [PtIMe-
(bpy)] δ 1.15 [s, 6H, 2J (PtH) ) 75.8 Hz, Me-Pt]; δ 0.98 [s,
2J (PtH) ) 86.1 Hz, Me-Pt]. The 1H NMR spectrum of the
volatiles (C6D6) showed predominantly Me4Si (δ 0.00) as well
as a minor signal at 0.10 ppm (5% intensity compared to the
Me4Si resonance) which is not, as yet, assigned. [PtClMe2(Me3-
Ge)(bpy-tbu2)] was pyrolyzed at 210 °C. The 1H NMR spectrum
(CD2Cl2) of the solid shows a 37:54:9 mixture of [PtMe2(bpy-
tbu2)]:[PtClMe(bpy-tbu2)]:[PtClMe3(bpy-tbu2)], respectively:
[PtMe2(bpy-tbu2)] δ 0.92 [s, 2J (PtH) ) 85.4 Hz, Me-Pt];
[PtClMe(bpy-tbu2)] δ 1.05 [s, 2J (PtH) ) 78.4 Hz, Me-Pt];
[PtClMe3(bpy-tbu2)] δ 0.39 [s, 3H, 2J (PtH) ) 75.5 Hz, axial Me-
δ 1.43 [s, 18H, bu], δ 7.80 [dd, J (HaHb) ) 6.3 Hz, J (HbHd) )
t
2
3
2.1 Hz, Hb], δ 8.61 [d, 2H, Hd], δ 8.90 [d, 2H, J (PtH) ) 14.8
2
Hz]. The 1H NMR spectrum of the volatiles (acetone-d6) shows
2
Me3SnBr: δ 0.72 [s, J (SnH)av ) 62.9 Hz].
Ack n ow led gm en t. We thank the NSERC of Canada
for financial support.
OM970204X