Kinetic studies on the reductive elimination of cis-PtMe(SiPh3)(PMe2Ph)2 and cis-PtMe(GePh3)(PMe2Ph)2. The first comparison between C-Si and C-Ge reductive elimination

Article abstract of DOI:10.1021/om970841n

The novel complex cis-PtMe(GePh₃)(PMe₂Ph)₂ (2b) was prepared by the treatment of trans-PtCl(GePh₃)(PMe₂Ph)₂ (3) with 2 equiv of MeLi in THF followed by methanolysis of the reaction system.

Full text of DOI:10.1021/om970841n

1018
Organometallics 1998, 17, 1018-1024
Kin etic Stu d ies on th e Red u ctive Elim in a tion of
cis-P tMe(SiP h 3)(P Me2P h )2 a n d
cis-P tMe(GeP h 3)(P Me2P h )2. Th e F ir st Com p a r ison
betw een C-Si a n d C-Ge Red u ctive Elim in a tion
Fumiyuki Ozawa,* Toshihiko Hikida, Koh Hasebe, and Takuya Mori
Department of Applied Chemistry, Faculty of Engineering, Osaka City University,
Sumiyoshi-ku, Osaka 558, J apan
Received September 25, 1997
The novel complex cis-PtMe(GePh3)(PMe2Ph)2 (2b) was prepared by the treatment of trans-
PtCl(GePh3)(PMe2Ph)2 (3) with 2 equiv of MeLi in THF followed by methanolysis of the
reaction system. The X-ray and NMR data of 2b, together with the NMR data of related
cis-PtMe(SiPh3)(PMe2Ph)2 (1b) and cis-PtMe2(PMe2Ph)2 (4), indicated the following order of
trans influence: SiPh3 . GePh3 > Me. While the trans influence of the GePh3 ligand is
much lower than that of the SiPh3 ligand and only slightly higher than that of the Me ligand,
the GePh3 ligand was found to possess a much higher trans effect than the methyl ligand,
comparable to the SiPh3 ligand. Thermolysis of 1b and 2b in toluene-d8 in the presence of
an excess amount of diphenylacetylene provided quantitative yields of reductive-elimination
products MeSiPh3 and MeGePh3, respectively. Kinetic studies revealed that the methyl-
germyl complex 2b is much more stable than the methyl-silyl complex 1b. The reductive
elimination proceeds for both complexes via a process involving a prior ligand displacement
of one of the PMe2Ph ligands with diphenylacetylene, followed by elimination of MeSiPh3 or
MeGePh3 from the resulting acetylene-coordinated species. Factors governing the reactivity
of C-Si and C-Ge reductive elimination from a Pt(II) center are discussed.
In tr od u ction
elimination of aryl(amido)- and aryl(thiolato)palladium
complexes.3b,d On the other hand, we reported a kinetic
study on the reductive elimination of cis-PtMe(SiPh3)-
(PMePh2)2 to give MeSiPh3 and a platinum(0) species.4
This type of reaction, which forms a carbon-silicon
bond, has been assumed as the product-forming step in
many catalytic silylation reactions of organic molecules,6
while definitive examples of C-Si reductive elimination
are rare5 and their mechanisms have been investigated
only for a few instances.5a,b
We have found that the reductive elimination of cis-
PtMe(SiPh3)(PMePh2)2 (1a ) readily proceeds even at
room temperature.4 The reactivity observed was un-
expectedly high as compared with that of common
diorganoplatinum(II) complexes such as cis-PtMe2-
(PMePh2)2, which is thermally inactive toward reductive
elimination.7 Since theoretical and thermochemical
data have indicated that the Pt-Si bond is generally
Reductive elimination of organotransition-metal com-
plexes is a crucial process in catalytic reactions.1 In the
past decade, a great deal of research has been carried
out to elucidate the mechanism and kinetics of the
reductive elimination that provides a carbon-carbon or
a carbon-hydrogen bond.2 In contrast, studies of the
reductive elimination that causes carbon-heteroatom
bond formation are still limited and their mechanistic
details remain to be explored.3-5 Recently, Hartwig and
co-workers examined the mechanisms of reductive
(1) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis; Wiley-
Interscience: New York, 1992.
(2) (a) Brown, J . M.; Cooley, N. A. Chem. Rev. (Washington, D.C.)
1988, 88, 1031. (b) Yamamoto, A. Organotransition Metal Chemistry,
Fundamental Concepts and Applications; Wiley-Interscience: New
York, 1986. (c) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R.
G. Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987.
(3) For recent examples of reductive elimination giving a carbon-
heteroatom bond other than the carbon-silicon bond, see: (a) Villan-
ueva, L. A.; Abboud, K. A.; Boncella, J . M. Organometallics 1994, 13,
3921. (b) Driver, M. S.; Hartwig, J . F. J . Am. Chem. Soc. 1995, 117,
4708. (c) Hartwig, J . F.; Richards, S.; Baran˜ano, D.; Paul, F. J . Am.
Chem. Soc. 1996, 118, 3626. (d) Baran˜ano, D.; Hartwig, J . F. J . Am.
Chem. Soc. 1995, 117, 2937. (e) Han, R.; Hillhouse, G. L. J . Am. Chem.
Soc. 1997, 119, 8135. (f) Koo, K.; Hillhouse, G. L. Organometallics
1996, 15, 2669. (g) Fryzuk, M. D.; J oshi, K.; Chadha, R. K.; Rettig, S.
J . J . Am. Chem. Soc. 1991, 113, 8724. (h) Han, L.; Choi, N.; Tanaka,
M. Organometallics 1996, 15, 3259. (i) Nakazawa, H.; Matsuoka, Y.;
Nakagawa, I.; Miyoshi, K. Organometallics 1992, 11, 1385. (j) Komiya,
S.; Akai, Y.; Tanaka, K.; Yamamoto, T.; Yamamoto, A. Organometallics
1985, 4, 1130. (k) Thompson, J . S.; Randall, S. L.; Atwood, J . L.
Organometallics 1991, 10, 3906.
(5) (a) Tanaka, Y.; Yamashita, H.; Shimada, S.; Tanaka, M. Orga-
nometallics 1997, 16, 3246. (b) Aizenberg, M.; Milstein, D. J . Am.
Chem. Soc. 1995, 117, 6456. (c) Akita, M.; Hua, R.; Oku, T.; Tanaka,
M.; Moro-oka, Y. Organometallics 1996, 15, 4162. (d) Okazaki, M.;
Tobita, H.; Ogino, H. Organometallics 1996, 15, 2790. (e) Mitchell, G.
P.; Tilley, T. D. Organometallics 1996, 15, 3477. (f) Schubert, U.
Angew. Chem., Int. Ed. Engl. 1994, 33, 419. (g) Lin, W.; Wilson, S.
R.; Girolami, G. S. Organometallics 1994, 13, 2309. (h) Schubert, U.;
Mu¨ller, C. J . Organomet. Chem. 1989, 373, 165. (i) Brinkman, K. C.;
Blakeney, A. J .; Krone-Schmidt, W.; Gladysz, J . A. Organometallics
1984, 3, 1325.
(6) (a) Brunstein, P.; Knorr, M. J . Organomet. Chem. 1995, 500, 21.
(b) Recatto, C. A. Aldrichimica Acta 1995, 28, 85. (c) Tilley, T. D. In
The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport,
Z., Eds.; Wiley: Chichester, U.K., 1989; p 1415.
(7) Low, J . J .; Goddard, W. A., III. J . Am. Chem. Soc. 1986, 108,
6115 and references therein.
(4) Ozawa, F.; Hikida, T.; Hayashi, T. J . Am. Chem. Soc. 1994, 116,
2844.
S0276-7333(97)00841-8 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 02/10/1998

C-Si and C-Ge Reductive Elimination
Organometallics, Vol. 17, No. 6, 1998 1019
13C{1H} NMR
Ta ble 1. Ch a r a cter istic NMR Da ta for 1b, 2b, a n d 4a
complex
31P{1H} NMR
2
1
2
1
(1)
PhMe2P
P(1): δ -4.6, J P-P ) 19 Hz, J Pt-P ) 1955 Hz
PtMe: δ -0.37 (dd), J P-C ) 83 and 7 Hz, J Pt-C ) 486 Hz
Me
2
1
P(2): δ -12.2 (d), J P-P ) 19 Hz, J Pt-P ) 1972 Hz
Pt
PhMe2P
(2)
GePh3
2b
2
1
2
1
(1)
PhMe2P
P(1): δ , -3.7 (d), J P-P ) 19 Hz, J Pt-P ) 1330 Hz,
PtMe: δ 1.3 (dd), J P-C ) 81 and 8 Hz, J Pt-C ) 508 Hz
2J Si-P ) 198 Hz
Me
2
1
P(2): δ -10.9 (d), J P-P ) 19 Hz, J Pt-P ) 2048 Hz
Pt
PhMe2P
(2)
SiPh3
1b
1
1
PhMe2P
Me
Me
δ )10.1 (s), J Pt-P ) 1844 Hz
PtMe: δ 2.9 (AXX′), J Pt-C ) 589 Hz
Pt
PhMe2P
4
a
In CD2Cl2, at -20 °C. Complete NMR data for 2b are reported in the Experimental Section.
Sch em e 1
much stronger than the Pt-C bond and the C-Si bond
is weaker than the C-C bond,8 the much higher
reactivity of methyl(silyl)platinum compared to that of
dimethylplatinum is attributed to kinetic factors, prob-
ably reflecting the size of the silicon atom being greater
than that of the carbon atom, which facilitates orbital
interaction between silicon and carbon at the transition
state, and the low electronegativity of silicon, which
reduces the energy barrier attendant on the reduction
of platinum(II) to platinum(0) during the reductive-
elimination process. To obtain further information on
the C-Si reductive elimination, we compared in this
study the reactivities of methyl(silyl)platinum and the
ment of trans-PtCl(SiPh3)(PMePh2)2 with an excess
amount of MeLi gives the platinate complex
Li+[PtMe2(SiPh3)(PMePh2)]- with liberation of 1 equiv
of PMePh2. Methanolysis of the reaction system leads
to the selective formation of 1a .
related methyl(germyl)platinum species. Kinetic stud-
ies revealed the germyl complex to be much less reactive
than the silyl complex toward reductive elimination.
Resu lts a n d Discu ssion
On the other hand, a cis-methyl(germyl)platinum
compound bearing two PMe2Ph ligands (2b) could be
prepared in good yield, starting from trans-PtCl-
(GePh3)(PMe2Ph)2 (3) (Scheme 1). The reaction was
examined by 31P{1H} NMR spectroscopy. When 2 equiv
of MeLi was added to a THF solution of 3 at room
P r ep a r a tion of Meth yl(ger m yl)p la tin u m (II). To
compare the C-Si and C-Ge reductive-elimination
reactions, cis-PtMe(EPh3)L2 (E ) Si, Ge) complexes
having the same tertiary phosphine ligand (L) are
needed. Since the silyl derivatives have already been
prepared,4,9 syntheses of the germyl analogues were
examined in this study.
1
temperature, the signal of 3 at δ -5.6 (s, J Pt-P ) 2667
Hz) instantly disappeared, and a singlet at δ -9.7
(1J Pt-P ) 2028 Hz) assignable to Li+[PtMe2(GePh3)-
(PMe2Ph)]- and a broad signal arising from free PMe2-
Ph (δ -44.8) appeared in a 1:1 ratio. Addition of an
excess amount of methanol to the system at -20 °C led
to the formation of cis-PtMe(GePh3)(PMe2Ph)2 (2b) with
93% selectivity, which was isolated in 65% yield by
recrystallization from CH2Cl2-Et2O.
First of all, we tried to synthesize cis-PtMe(GePh3)-
(PMePh2)2 (2a ), which is the germyl analogue of cis-
PtMe(SiPh3)(PMePh2)2 (1a ) employed in the previous
study of reductive elimination.4 Treatment of trans-
PtCl(GePh3)(PMePh2)2 with 2 equiv of MeLi in THF
followed by methanolysis of the resulting solution,
however, did not form 2a but instead provided a
platinum(II) complex having a trans geometry: 31P{1H}
Table 1 summarizes the characteristic NMR data for
2b. Also listed are the data for the related complexes
cis-PtMe(SiPh3)(PMe2Ph)2 (1b)9 and cis-PtMe2(PMe2-
Ph)2 (4). The 31P{1H} NMR spectrum of 2b exhibited
two sets of doublets at δ -12.2 and -4.6. Since the
doublet at δ -12.2 was observed in a region similar to
that for the signals arising from the PMe2Ph ligand
trans to the methyl ligand in 1b and 4, this signal was
assigned to P(2), which is trans to the methyl ligand in
2b . Consequently, the other doublet at δ -4.6 was
1
NMR δ 6.9 (s, J Pt-P ) 2769 Hz). This result differs
from the synthetic reaction of 1a , in which the treat-
(8) (a) Sakaki, S.; Ieki, M. J . Am. Chem. Soc. 1991, 113, 5063. (b)
Sakaki, S.; Ieki, M. J . Am. Chem. Soc. 1993, 115, 2373. (c) Levy, C.
J .; Puddephatt, R. J . Organometallics 1995, 14, 5019. (d) Pilcher, G.;
Skinner, H. A. In The Chemistry of the Metal-Carbon Bond; Hartley,
F. R., Patai, S., Eds.; Wiley: Chichester, U.K., 1982; Vol. 1, p 43.
(9) Ozawa, F.; Hikida, T. Organometallics 1996, 15, 4501.

1020 Organometallics, Vol. 17, No. 6, 1998
Ozawa et al.
Complex 2b was treated with PMe3 in CD2Cl2 at -20
°C, and the reaction system was examined by 31P{1H}
NMR spectroscopy at -70 °C. On addition of 1 equiv
of PMe3 to the system, complex 2b was selectively
converted to the new methyl-germyl complex 2c with
liberation of 1 equiv of PMe2Ph. Thus, as the reaction
proceeded, the doublet at δ -4.1 due to the PMe2Ph
ligand trans to the GePh3 ligand in 2b (P(1)) instantly
disappeared, to be replaced by a new doublet at δ -14.1
(2J P-P ) 20 Hz, 1J Pt-P ) 1906 Hz), which was assignable
to the PMe3 ligand at the coordination site trans to the
GePh3 ligand. Simultaneously observed were a broad
singlet of free PMe2Ph (δ -44.9) and a doublet at δ
F igu r e 1. Molecular structure of cis-PtMe(GePh3)(PMe2-
Ph)2 (2b). Thermal ellipsoids are drawn at the 30% prob-
ability level. Selected bond distances (Å) and angles (deg):
Pt-Ge ) 2.4495(7), Pt-C(1) ) 2.127(5), Pt-P(1) ) 2.330-
(1), Pt-P(2) ) 2.308(2); Ge-Pt-C(1) ) 85.6(2), P(1)-Pt-
P(2) ) 94.28(5), Ge-Pt-P(2) ) 93.41(4), P(1)-Pt-C(1) )
87.0(2), Ge-Pt-P(1) ) 166.48(4), P(2)-Pt-C(1) ) 178.2-
(2).
1
-12.5 (2J P-P ) 20 Hz, J Pt-P ) 1960 Hz). Since the
latter signal exhibited almost the same chemical shift
as the P(2) signal of the starting 2b (δ -12.1), this
doublet was assigned to the PMe2Ph ligand trans to the
methyl group. Therefore, we concluded that complex
2c is PtMe(GePh3)(PMe3)(PMe2Ph), in which PMe3 and
PMe2Ph are oriented trans and cis toward the GePh3
ligand, respectively, and the ligand substitution took
place only at the coordination site trans to the GePh3
ligand.
The site-selective ligand displacement with phosphine
was also observed for methyl-silyl complexes 1a 4,9 and
1b.11 In these cases the ligand substitution proceeded
instantly at -20 °C at the site trans to the SiPh3 ligand.
It was further confirmed that the phosphine ligand
trans to the methyl ligand is inactive toward ligand
substitution, even in the presence of an excess amount
of added phosphine at room temperature.9
Kin etic Stu d ies on Red u ctive Elim in a tion . (a )
cis-P tMe(SiP h 3)(P Me2P h )2 (1b). The thermolysis
reaction of 1b was examined in toluene-d8 in the
presence of an excess amount of diphenylacetylene. The
reaction gave MeSiPh3 and Pt(PhCtCPh)(PMe2Ph)2 as
the reductive-elimination products in quantitative yields
(eq 2). In the presence of 0.20 M (10 equiv/1b) of
assigned to P(1) trans to the GePh3 ligand. It is seen
1
from Table 1 that the J Pt-P(1) value (1955 Hz) is
1
somewhat smaller than the J Pt-P(2) value (1972 Hz).
On the other hand, in the 31P{1H} NMR spectrum of
methyl-silyl complex 1b, the Pt-P coupling constants
were much smaller at the trans position (P(1), 1330 Hz)
than at the cis position (P(2), 2048 Hz) of the SiPh3
ligand. Hence, the following order of trans influence
was evidenced: SiPh3 . GePh3 > Me. This order is
consistent with previous observations.10
X-r a y Str u ctu r e of 2b. Figure 1 shows the ORTEP
drawing of 2b. The platinum atom had a slightly
distorted square planar geometry, and the sum of four
angles about platinum was 360.3°. The Ge-Pt-P(2)
angle (93.41(4)°) was significantly wider than the P(1)-
Pt-C(1) angle (87.0(2)°), probably reflecting the steric
bulkiness of the GePh3 ligand. The Pt-P(1) bond
(2.330(1) Å), which is situated trans to the GePh3 ligand,
was somewhat longer than the Pt-P(2) bond (2.308(2)
Å) at the site trans to the methyl ligand, indicating that
the germyl ligand has a greater trans influence than
the methyl ligand. This result is consistent with the
1J Pt-P values of 2b. However, reflecting the modest
trans influence of the GePh3 ligand as compared with
the SiPh3 ligand, the difference between the two Pt-P
distances of 2b (0.022 Å) was much smaller than that
observed for cis-PtMe(SiPh3)(PMePh2)2 (1a ) (0.268 Å).4
Rea ctivity tow a r d Liga n d Su bstitu tion . The
NMR and X-ray diffraction data for 2b indicated that
the GePh3 ligand possesses only a slightly higher trans
influence than the methyl ligand. On the other hand,
the following investigation demonstrated that the GePh3
ligand has a much greater trans effect than the methyl
ligand, comparable to that of the SiPh3 ligand (eq 1).
diphenylacetylene at 30 °C, the reductive elimination
proceeded by obeying the first-order rate law with
respect to the concentration of 1b over 70% conversion
(kobsd ) 2.2 × 10-4 s-1). The reaction was somewhat
slower than the reductive elimination of cis-PtMe-
(11) The reaction of 1b with PMe3 (1 equiv) in CD2Cl2 at -20 °C
instantly formed PtMe(SiPh3)(PMe3)(PMe2Ph), having the PMe3 and
PMe2Ph ligands trans and cis to the SiPh3 ligand, respectively. 31P-
(10) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev.
1973, 10, 335. Bresciani-Pahor, N.; Forcolin, M.; Marzilli, L. G.;
Randaccio, L.; Summers, M. F.; Toscano, P. J . Coord. Chem. Rev. 1985,
63, 1.
1
2
{1H} NMR (-70 °C): δ -10.8 (d, J Pt-P ) 2027 Hz, J P-P ) 22 Hz),
1
2
-12.5 (d, J Pt-P ) 1306 Hz, J P-P ) 22 Hz).

C-Si and C-Ge Reductive Elimination
Organometallics, Vol. 17, No. 6, 1998 1021
line (Figure 2b). On the other hand, the rate of
reductive elimination increased as the concentration of
diphenylacetylene increased and a linear correlation
between 1/kobsd and 1/[PMe2Ph] values was observed.
These kinetic features are essentially the same as the
previous observations for the reductive elimination of
1a , except that the reaction of 1b is almost entirely
suppressed by addition of only a small amount of free
PMe2Ph (>5 mol %) to the system, while the reductive
elimination of 1a proceeds to some extent even in the
presence of 1 equiv of added PMePh2.
(b) cis-P tMe(GeP h 3)(P Me2P h )2 (2b). While the
methyl-germyl complex 2b also underwent reductive
elimination cleanly in solution, the reaction progress
was much slower than that of methyl-silyl complex 1b.
For example, the thermolysis of 2b in toluene-d8 in the
presence of diphenylacetylene (0.20 M, 10 equiv) at 85
°C proceeded with the first-order rate constant kobsd
)
1.4 × 10-4 s-1 to give MeGePh3 and Pt(PhCtCPh)(PMe2-
Ph)2 in quantitative yields (eq 3). Except for the higher
stability, kinetic features of the reductive elimination
of 2b were very similar to those of 1b. Thus, the
reaction was effectively retarded by addition of free
PMe2Ph to the system (Figure 4) and accelerated at a
higher concentration of diphenylacetylene (Figure 5).
Rea ction Mech a n ism s. The kinetic observations for
1b and 2b are consistent with the reductive-elimination
process involving a prior displacement of one of the
PMe2Ph ligands with diphenylacetylene (Scheme 2).
Considering the extremely high trans effects of SiPh3
and GePh3 ligands, which were suggested by the ligand
substitution reactions of 1b and 2b with PMe3, the
coordination of diphenylacetylene was assumed to take
place selectively at the site trans to the EPh3 ligand.
This mechanism is almost identical with that previously
proposed for 1a , except for the absence of the direct
reductive-elimination pathway from the cis-PtMe(EPh3)-
L2 species. Assumptions of the ligand displacement to
be a reversible process and of the rate-determining
elimination of MeEPh3 from the acetylene-coordinated
species lead to the kinetic expressions in eqs 4 and 5,
where [PtMe(EPh3)]total is the sum of the concentrations
of the cis-PtMe(EPh3)L2 complex and the acetylene-
coordinated species at time t.
F igu r e 2. Effect of added PMe2Ph on the reductive
elimination of 1b in toluene-d8 in the presence of diphe-
nylacetylene at 35 °C. Initial concentrations: [1b] ) 0.020
M, [PhCtCPh] ) 0.20 M.
F igu r e 3. Effect of diphenylacetylene on the reductive
elimination of 1b in toluene-d8 in the presence of added
PMe2Ph at 35 °C. Initial concentrations: [1b] ) 0.020 M,
[PMe2Ph] ) 1.6 × 10-4 M.
d[MeEPh3]
kK[PhCtCPh]
)
[PtMe(EPh3)]total
dt
[L] + k[PhCtCPh]
(4)
(5)
(SiPh3)(PMePh2)2 (1a ) under the same reaction condi-
tions (kobsd ) 5.0 × 10-4 s-1).
[L]
kK[PhCtCPh]
1
kobsd
1
+
)
k
The reaction progress was severely hindered by
addition of free PMe2Ph to the system (Figure 2a). The
plot of reciprocals of the rate constants (1/kobsd) against
the concentration of PMe2Ph ([PMe2Ph]) gave a straight
These equations are consistent with the kinetic data
for 1b and 2b represented in Figures 2 and 3 and

1022 Organometallics, Vol. 17, No. 6, 1998
Ozawa et al.
Sch em e 2
Ta ble 2. Kin etic P a r a m eter s for th e Red u ctive
Elim in a tion of 1b a n d 2ba
1b (at 35 °C)
k ) 4.5 × 10-4 s-1
∆Hq ) 20.8 kcal mol-1 ∆Sq ) -6.8 eu
(∆Gq ) 22.8 kcal mol-1
)
)
K ) 1.2 × 10-4
2b (at 100 °C) k ) 1.1 × 10-3 s-1
(∆Gq ) 27.1 kcal mol-1
K ) 1.4 × 10-4
∆Hq ) 28.5 kcal mol-1 ∆Sq ) 3.4 eu
a
The k and K values were calculated on the basis of the plots
in Figures 2a and 3 for 1b and Figures 4a and 5 for 2b,
respectively. Activation enthalpy and entropy were estimated
from the Eyring plots of the rate constants measured at five
different temperatures ranging from 20 to 45 °C for 1b and from
80 to 100 °C for 2b, respectively: the reactions were carried out
in toluene-d8 in the presence of 0.20 M diphenylacetylene.
values, which were calculated on the basis of the slopes
in Figures 2 and 3, were in good agreement with each
other: (5.5 ( 0.1) × 10-8 s-1. Similarly, the k and kK
values for the reductive elimination of 2b at 100 °C were
successfully determined on the basis of the plots in
Figures 4 and 5. The constants thus obtained are
summarized in Table 2 together with the activation
parameters estimated from Eyring plots.
F igu r e 4. Effect of added PMe2Ph on the reductive
elimination of 2b in toluene-d8 in the presence of diphe-
nylacetylene at 100 °C. Initial concentrations: [2b] ) 0.020
M, [PhCtCPh] ) 0.20 M.
The activation parameters listed in Table 2 indicate
that the large difference in reactivities between 1b and
2b toward reductive elimination is mainly due to the
difference in the activation enthalpies. Since 1b and
2b have almost the same structural features, the
activation enthalpies may be dependent predominantly
upon the Pt-E bond energies in the ground states and
the bond energies of partially formed C-E bonds in the
transition states. Generally, the C-Si bond is stronger
than the C-Ge bond by 10-15 kcal mol-1 12
While the
.
energy difference seems smaller for the incompletely
formed C-Si and C-Ge bonds in the transition states,
the order D(C-Si) > D(C-Ge) should be kept in the
transition states as well. On the other hand, the data
comparing the Pt-Si and Pt-Ge bond strengths are still
lacking.8 However, experimental data for the related
silyl and germyl complexes of late transition metals
indicated the M-Si bond to be weaker than the M-Ge
bond.13 Furthermore, a series of experiments regarding
the metathesis reactions of cis-Pt(EMe3)2(dppe) or cis-
F igu r e 5. Effect of diphenylacetylene on the reductive
elimination of 2b in toluene-d8 in the presence of added
PMe2Ph at 100 °C. Initial concentrations: [2b] ) 0.20 M,
[PMe2Ph] ) 2.0 × 10-4 M.
Figures 4 and 5, respectively. For example, reciprocals
of the values of intercepts in Figures 2 and 3, which
correspond to the rate constants (k) for the reductive
elimination of acetylene-coordinated methyl-silyl spe-
cies at 35 °C, were in fair agreement with each other:
(4.5 ( 0.05) × 10-4 s-1. On the other hand, the kK
(12) For example, the mean bond dissociation enthalpies (D(E-C))
in SiMe4 and GeMe4 are 76.5 and 61.7 kcal mol-1, respectively.8d
(13) Burnham, R. A.; Stobert, S. R. J . Chem. Soc., Dalton Trans.
1977, 1489. Cardin, D. J .; Lappert, M. F.; Litzow, M. R.; Spalding, T.
R. J . Chem. Soc. A 1971, 2262. Spalding, T. R. J . Organomet. Chem.
1978, 149, 371. Novak, I.; Huang, W.; Luo, L.; Huang, H. H.; Ang, H.
G.; Zybill, C. E. Organometallics 1997, 16, 1567.

C-Si and C-Ge Reductive Elimination
Organometallics, Vol. 17, No. 6, 1998 1023
Ta ble 3. Cr ysta l Da ta a n d Deta ils of th e Str u ctu r e
Deter m in a tion for 2b
PtCl(EMe3)(dppe) with HE′Me3 (E, E′ ) Si, Ge, Sn)
suggested the following stability order of silyl, germyl,
and stannyl complexes of platinum: E ) Sn . Ge >
Si.14 Therefore, we are convinced that the higher
reactivity of silyl complex 1b than germyl complex 2b
toward reductive elimination is mainly due to the
thermodynamic factors; that is, the Pt-Si bond is
weaker than the Pt-Ge bond and the C-Si bond is
stronger than the C-Ge bond.15
In conclusion, we have confirmed for the first time
that methyl(silyl)platinum(II) is much more reactive
than methyl(germyl)platinum(II) toward reductive elimi-
nation. Because the related dimethylplatinum(II) com-
plexes are known to be inactive toward reductive
elimination, the following order of reactivities is now
clearly observed: C-Si > C-Ge . C-C. The reactivity
order is apparently inconsistent with the order of
elements in the periodic table. The C-Si reductive
elimination seems more thermodynamically favorable
than the C-Ge reductive elimination and more kineti-
cally favorable than C-C reductive elimination.
formula
fw
C35H40GeP2Pt
790.33
habit
prismatic
0.30 × 0.25 × 0.15
triclinic
P1h (No. 2)
11.683(3)
13.979(2)
11.453(5)
109.37(2)
113.36(3)
82.88(2)
1619.9(9)
2
1.620
53.44
780
cryst size, mm
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
d
calcd, g cm-3
µ(Mo KR), cm-1
F(000)
radiation
Mo KR (λ ) 0.710 69 Å)
graphite
+h, (k, (l
4.0-54.9
ω-2θ
1.15 + 0.30 tan θ
16, fixed
293
monochromator
data collected
2θ range, deg
scan type
∆ω, deg
scan speed, deg min-1
temp, K
Exp er im en ta l Section
linear decay, %
abs cor
26.7
empirical
0.535, 1.00
7717
7358 (Rint ) 0.008)
6402 (I g 3σ(I))
352
0.041
0.052
1.61
0.00
Gen er a l P r oced u r e a n d Ma ter ia ls. All manipulations
were carried out under a nitrogen atmosphere using conven-
tional Schlenk techniques. Nitrogen gas was dried by passage
through P2O5 (Merck, SICAPENT). NMR spectra were re-
corded on a J EOL J NM-A400 spectrometer (1H NMR, 399.65
MHz; 13C NMR, 100.40 MHz; 31P NMR, 161.70 MHz). Chemi-
cal shifts are reported in δ (ppm), referenced to an internal
SiMe4 standard for 1H and 13C NMR and to an external 85%
H3PO4 standard for 31P NMR.
min and max transmissn factors
no. of rflns collected
no. of unique rflns
no. of obsd rflns
no. of variables
R
Rw
goodness of fit
max ∆/σ in final cycle
max and min peak, e Å-3
+1.00, -2.15 (near Pt)
THF, Et2O, benzene, and hexane were dried over sodium-
benzophenone ketyl and distilled just before using. CH2Cl2
was dried over CaH2 and distilled just before using. Benzene-
d6 and toluene-d8 were dried over LiAlH4, vacuum-transferred,
and stored under a nitrogen atmosphere. PMe2Ph was pre-
pared by the reaction of MeMgBr with PCl2Ph. cis-PtMe-
(SiPh3)(PMe2Ph)2 (1b) was prepared as previously reported.9
All other compounds used in this study were obtained from
commercial sources and used without purification.
Methanol (0.8 mL) was slowly added, and the mixture was
concentrated to dryness by pumping. The resultant white solid
was dissolved in benzene, filtered through a short Celite
column, and then concentrated to dryness. The crude product
was dissolved in CH2Cl2, and Et2O was carefully layered on
the CH2Cl2 solution. The solvent layers were allowed to stand
at -70 °C overnight, giving colorless crystals of 2b (0.38 g,
3
65%). 1H NMR (CD2Cl2, -20 °C): δ 0.29 (dd, J P-H ) 11.7
and 6.3 Hz, 2J Pt-H ) 60.5 Hz, 3H, PtCH3), 1.19 (d, 2J P-H ) 8.3
Hz, 3J Pt-H ) 21.5 Hz, 6H, PCH3), 1.41 (d, 2J P-H ) 8.3 Hz, 3J Pt-H
) 21.5 Hz, 6H, PCH3), 7.1-7.4 (m, 15H, Ph), 7.4-7.6 (m, 10H,
Ph). 13C{1H} NMR (CD2Cl2, -20 °C): δ -0.37 (dd, 2J P-C ) 83
P r ep a r a tion of tr a n s-P tCl(GeP h 3)(P Me2P h )2 (3).
A
mixture of trans-PtHCl(PMe2Ph)2 (750 mg, 1.2 mmol) and
HGePh3 (553 mg, 1.8 mmol) was heated at 90 °C without
solvent. Upon heating with stirring, the mixture changed into
a homogeneous solution and gradually turned orange with
evolution of hydrogen gas. After 3 h, the mixture was cooled
to -20 °C, washed with Et2O (2 mL × 3), and dried under
vacuum to give a white solid of the title compound (667 mg,
65%). This product was spectroscopically pure and was
employed in the synthesis of 2b without further purification.
1H NMR (CDCl3, 23 °C): δ 1.54 (virtual triplet, J ) 3.4 Hz,
3J Pt-H ) 28.3 Hz, 12H, PCH3), 7.4-7.6 (m, 25H, Ph). 31P{1H}
1
1
and 7 Hz, J Pt-C ) 486 Hz, PtCH3), 14.2 (d, J P-C ) 28 Hz,
2J Pt-C ) 26 Hz, PCH3), 17.9 (dd, J P-C ) 30 and 3 Hz, J Pt-C
)
2
30 Hz, PCH3), 127.0 (s, GePh), 127.6 (s, GePh), 128.7 (d, 3J P-C
3
) 8 Hz, PPh), 128.8 (d, J P-C ) 8 Hz, PPh), 130.2 (s, PPh),
2
3
2
131.6 (d, J P-C ) 12 Hz, J Pt-C ) 20 Hz, PPh), 131.8 (d, J P-C
) 12 Hz, 3J Pt-C ) 17 Hz, PPh), 137.0 (s, 3J Pt-C ) 13 Hz, GePh),
137.9 (d, 1J P-C ) 41 Hz, 2J Pt-C ) 20 Hz, PPh), 140.2 (dd, 1J P-C
2
3
) 43 and 3 Hz, J Pt-C ) 10 Hz, PPh), 148.8 (d, J P-C ) 10 Hz,
2J Pt-C ) 66 Hz, GePh). 31P{1H} NMR (CD2Cl2, -20 °C): δ
1
NMR (CDCl3, 23 °C): δ -5.6 (s, J Pt-P ) 2667 Hz).
2
1
2
-12.2 (d, J P-P ) 19 Hz, J Pt-P ) 1972 Hz), -4.6 (d, J P-P
19 Hz, J Pt-P ) 1955 Hz). Anal. Calcd for C35H40P2PtGe: C,
53.19; H, 5.10. Found: C, 52.75; H, 5.14.
)
P r ep a r a tion of cis-P tMe(GeP h 3)(P Me2P h )2 (2b). To a
solution of trans-PtCl(GePh3)(PMe2Ph)2 (0.60 g, 0.74 mmol) in
THF (10 mL) was added an ethereal solution of MeLi (Aldrich,
low halide; 1.4 M, 1.1 mL, 1.5 mmol). The solution was stirred
for 30 min at room temperature and then cooled to -20 °C.
1
X-r a y Diffr a ction Stu d y of cis-P tMe(GeP h 3)(P Me2P h )2
(2b). A single crystal of dimensions ca. 0.30 × 0.25 × 0.15
mm was sealed in a glass capillary tube. Intensity data were
collected on a Rigaku AFC7R four-circle diffractometer. Unit
cell dimensions were obtained from a least-squares treatment
of the setting angles of 25 reflections in the range 39.60 e 2θ
e 39.91°. The cell dimensions suggested a triclinic cell, and
a statistical analysis of intensity distribution indicated the
space group P1h (No. 2). Diffraction data were collected at 20
°C in the range 4.0 < 2θ < 55° using the ω-2θ scan technique
at a scan rate of 16.0° min-1 in ω. Three standard reflections,
(14) Clemmit, A. F.; Glockling, F. J . Chem. Soc. A 1971, 1164.
(15) After submission of this paper, the following decreasing order
of PtIV-E bond energies was established on the basis of thermochemi-
cal experiments: E ) Si > Ge > Sn. Levy, C. J .; Puddephatt, R. J . J .
Am. Chem. Soc. 1997, 119, 10127; Organometallics 1997, 16, 4115. If
this order is applied to the present Pt(II) systems, one must consider
that the higher reactivity of 1b compared to that of 2b is mainly due
to the stronger C-Si bond rather than C-Ge bond. The transition
states may strongly resemble the reductive-elimination products.

1024 Organometallics, Vol. 17, No. 6, 1998
Ozawa et al.
monitored at every 150 reflection measurements, showed a
linear decay in the intensity by 26.7%. The data were
corrected for Lorentz and polarization effects, decay, and
absorption (empirical, based on azimuthal scans of 3 reflec-
tions). Of the 7358 unique reflections measured, 6402 were
classified as observed (I > 3σ(I)), and these were used for the
solution and refinement of the structure.
9.51 mg, 12.0 µmol) and diphenylacetylene (21.4 mg, 0.120
mmol) were placed in an NMR sample tube equipped with a
rubber septum cap, and the system was replaced with nitrogen
gas at room temperature. A solution of PMe2Ph in toluene-d8
(14.5 mM, 24.8 µL, 0.360 µmol) was added and then total
volume of the solution was adjusted to 0.60 mL using neat
toluene-d8. The sample was placed in an NMR sample probe
controlled to 100.0 ( 0.1 °C and examined by 1H NMR
spectroscopy. The time course of the reductive elimination was
followed by measuring the relative peak integration of the
residual methyl signal of toluene-d8 (δ 2.05) and the methyl
signal of Pt(PhCtCPh)(PMe2Ph)2 (δ 1.45) at intervals. The
kinetic study for 1b was similarly conducted. In this case,
however, the time course of the reaction was followed by
measuring the relative peak integration of the methyl signals
of MeSiPh3 (δ 0.74) and Me2SiPh2 (δ 0.45); the latter was used
as an internal standard.
All calculations were performed with the TEXSAN crystal
structure analysis package provided by Rigaku Corp., Tokyo,
J apan. The scattering factors were taken from ref 16. The
structure was solved by heavy-atom Patterson methods
(PATTY) and expanded using Fourier techniques (DIRDIF92).
The structure was refined by full-matrix least squares with
anisotropic thermal parameters for all non-hydrogen atoms.
In the final cycles of refinement, hydrogen atoms with isotropic
temperature factors (Biso ) 1.20Bbonded atom) were located at
idealized positions (d(C-H) ) 0.95 Å) and were included in
calculations without refinement of their parameters. The
function minimized in least squares was Σw(|Fo| - |Fc|)2 (w )
1/[σ2(Fo)]). The final R index was 0.041 (Rw ) 0.052, S ) 1.61).
Ack n ow led gm en t. This work was supported by a
Grant-in-Aid for Scientific Research on Priority Area
“The Chemistry of Inter-element Linkage” (No. 09239105)
from the Ministry of Education, Science, Sports and
Culture of J apan.
2
R ) Σ||Fo| - |Fc||/Σ|Fo|, and Rw ) [Σw(|Fo| - |Fc|)2/Σw|Fo| ]1/2. S
) [Σw(|Fo| - |Fc|)2/(No - Np)]1/2, where No is the number of
observed data and Np is the number of parameters varied.
Crystal data and details of data collection and refinement are
summarized in Table 3. Additional information is available
as Supporting Information.
Su p p or tin g In for m a tion Ava ila ble: Details of the struc-
ture determination of 2b, including a figure giving the atomic
numbering scheme and tables of atomic coordinates, thermal
parameters, and all bond distances and angles (9 pages).
Ordering information is given on any current masthead page.
Kin etic Stu d ies. A typical procedure for the reductive
elimination of 2b is as follows. cis-PtMe(GePh3)(PMe2Ph)2 (2b;
(16) Cromer, D. T.; Waber, J . T. International Tables for X-ray
Crystallography; Kynoch Press: Birmingham, U.K., 1974; Vol. IV.
OM970841N