Reactivity of {(Ph3P)Pt[μ-η2-H-SiH(Ar)]}2 (Ar=2-isopropyl-6-methylphenyl) with phosphines. X-ray crystal structure of trans-{(dppe)Pt[μ-SiH(Ar)]}2

Article abstract of DOI:10.1016/S0020-1693(01)00710-1

The dinuclear Pt-Si complex {(Ph₃P)Pt{μ-η²-H-SiH(IMP)]}₂ (trans-1a-cis-1b=3:1; IMP=2-isopropyl-6-methylphenyl) reacted with basic phosphines such as 1,2-bis(diphenylphosphino)ethane (dppe) and dimethylphenylphosphine (PMe<

Full text of DOI:10.1016/S0020-1693(01)00710-1

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Inorganica Chimica Acta 330 (2002) 82–88
Reactivity of {(Ph3P)Pt[m-h2-HꢀSiH(Ar)]}2
(Ar=2-isopropyl-6-methylphenyl) with phosphines. X-ray crystal
structure of trans-{(dppe)Pt[m-SiH(Ar)]}2
Janet Braddock-Wilking *, Yanina Levchinsky, Nigam P. Rath
Department of Chemistry, Uni6ersity of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, MO 63121, USA
Received 6 July 2001; accepted 28 August 2001
Dedicated to the memory of Professor Luigi Venanzi
Abstract
The dinuclear PtꢀSi complex {(Ph3P)Pt{m-h2-HꢀSiH(IMP)]}2 (trans-1a–cis-1b=3:1; IMP=2-isopropyl-6-methylphenyl) re-
acted with basic phosphines such as 1,2-bis(diphenylphosphino)ethane (dppe) and dimethylphenylphosphine (PMe2Ph) to afford
different dinuclear PtꢀSi complexes with loss of H2, {(P)2Pt[m-SiH(IMP)]}2 [P=dppe, trans-2a (major), cis-2b (trace); PMe2Ph, 3
(trans only)]. Complexes 2 and 3 were characterized by multinuclear NMR spectroscopy and X-ray crystallography (2a). In
contrast, the reaction of 1a,b with the sterically demanding tricyclohexylphosphine (PCy3) afforded {(Cy3P)Pt{m-h2-
HꢀSiH(IMP)]}2 (trans-4a–cis-4b 2:1) analogous to 1a,b where the central Pt2Si2(m-H)2 core remains intact but the PPh3 ligands
have been replaced by PCy3. Complexes 4a and 4b was characterized by multinuclear NMR and IR spectroscopies. © 2002
Elsevier Science B.V. All rights reserved.
Keywords: Dinuclear complexes; Platinum phosphine complexes; Silylmetallic complexes; Phosphine substitution
Dinuclear complexes containing bridging-silylene
moieties are believed to play a role in catalytic dehydro-
1. Introduction
coupling reactions of silanes to oligomers and polymers
The study of transition metal complexes containing
[3]. For example, diplatinum bis(m-silylene) complexes
have been shown to catalyze the oligomerization (step-
wise) of primary silanes to small chain oligosilanes [4].
Recently, a series of dinuclear rhodium (m-silylene) and
(m-h2-silyl) complexes have been prepared and involve-
ment of the latter in the catalytic dehydrocoupling of
diphenylsilane to 1,1,2,2-tetraphenyldisilane was pro-
posed [5].
nonclassical M···H···Si interactions has been an active
area of research over the last few decades [1,2]. A
number of mononuclear, and to a lesser extent, dinu-
clear complexes containing these interactions are
known. The ‘arrested’ addition of the SiꢀH bond to the
metal center in these complexes has been established by
NMR spectroscopy and/or X-ray crystallography. Sur-
prisingly, the reactivity of complexes containing non-
classical M···H···Si interactions has been essentially
unexplored. Dinuclear complexes containing bridging-
silylene moieties with (or without) nonclassical
M···H···Si interactions may serve as exceptional dinu-
clear catalysts as well as model systems for developing
an understanding of bimetallic reactivity.
We were interested in examining the effect of added
phosphine to solutions of dinuclear complexes contain-
ing bridging-silylene moieties especially those with non-
classical hydrides, [(R3P)Pt(m-h2-HꢀSiR%)]2. The
2
possible products that one would expect from such
reactions could be monoplatinum species, dinuclear
complexes with changes in the central core, or simply
an exchange of phosphines. We were also interested in
determining how the nature of the phosphine affects the
course of the reaction and whether basicity or steric
* Corresponding author. Tel.: +1-314-516 6436; fax: +1-314-516
5342.
E-mail address: jwilking@umsl.edu (J. Braddock-Wilking).
0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0020-1693(01)00710-1

J. Braddock-Wilking et al. / Inorganica Chimica Acta 330 (2002) 82–88
83
constraints play a role. Herein, we report the reaction
of 1,2-bis(diphenyphosphino)ethane (dppe) with the
dinuclear Pt complex, {(Ph3P)Pt[m-h2-HꢀSiH(Ar)]}2
(1a,b) [trans-1a, cis-1b, ratio 3:1; Ar=2-isopropyl-6-
methylphenyl (IMP)] [6,7] which contains two nonclas-
trace) isomers. After 24 h, large X-ray quality round
yellow crystals of {[(dppe)2Pt][m-SiH(IMP)]}2 (2a) had
formed. The crystals were washed with C6H6 (3×1 ml)
and dried in vacuo to give 17 mg of 2a. A second crop
of 2a,b was obtained from the mother liquor providing
an overall yield of 23 mg (94%).
sical Pt···H···Si interactions, to form
a
new
1H{31P} NMR (C6D6, 300 MHz) for 2a and 2b:1 l
four-membered ring {(PhMe2P)2Pt[m-SiH(Ar)]}2 with
loss of H2 [8].
In contrast, the reaction of tricyclohexylphosphine
(PCy3) with 1a,b resulted in the overall replacement of
PPh3 by PCy3 while the central Pt2Si2(m-H)2 core re-
mained intact.
1.31 (d, 12H, 3JHH=7 Hz, ArCH(CH3)2), 1.74 (bm,
3
3
4H, JPtH=21 Hz, JHH=9 Hz, P(CH2)2P), 1.95 (bm,
4H, 3JPtH=21 Hz, 3JHH=9 Hz, P(CH2)2P), 2.13 (s, 6H,
ArCH3), 4.57 (bm, 2H, ArCH(CH3)2), ArH region
6.42–8.10 (46H), 7.28 (s, 2H, 2JPtH=29 Hz, SiH).
31P{1H} NMR (C6D6, 121 MHz) for 2a: l 56.5 (1JPtP
=
3
4
4
1565 Hz, JPtP=272 Hz, JPP=11 Hz (cis), JPP=24
1
Hz (trans)); for 2b: l 57.8 (Js not resolved). H–29Si
2. Experimental
HMQC NMR (C6D6, 99 MHz) for 2a: l −132. IR
(KBr, cm−1): w 2011 (s, SiꢀH, trans), 2040 (b, SiꢀH,
cis).2 A sample of 2a,b was placed under vacuum for 24
h before submitting for analysis. Anal. Calc. for
C72H76P4Pt2Si2·C6H6: C, 58.93; H, 5.20. Found: C,
58.92; H, 5.22% (consistent with one molecule of C6H6
per molecule of 2a,b). Crystals of 2a used for X-ray
structural analysis were not dried in vacuo and were
found to contain five molecules of benzene per unit cell
(drying in vacuo resulted in solvent loss). It was not
possible to determine the exact ratio of benzene to 2a,b
(before drying) by NMR due to overlapping aromatic
All reactions and manipulations were performed in
dry glassware under an Ar atmosphere in an inert
atmosphere drybox or with standard Schlenk tech-
niques. All solvents were distilled under an atmosphere
of N2 and dried before use: C6H6 (CaH2); Et2O
(sodium/benzophenone ketyl). Solvents were degassed
by freeze–pump–thaw degassing cycles (liquid N2) be-
fore transfer to the drybox. Toluene-d8 and C6D6 were
dried over activated alumina and Linde molecular
2
,
sieves (4 A) before use. (Ph3P)2Pt(h -C2H4) was com-
mercially available (Aldrich Chemical Co.) and used as
received. The phosphine ligands: 1,2-bis(diphenylphos-
phino)ethane, tricyclohexylphosphine, dimethylphenyl-
phosphine, and tris(2,4,6-trimethoxyphenyl)phosphine
were purchased from Strem Chemicals and were used
as received.
1
resonances in the H NMR spectrum.
2.2. Reaction of {(Ph3P)Pt[v-p2-HꢀSiH(IMP)]}2 (1a,b)
with PMe2Ph
1
1
All H, H{31P}, 31P{1H} and 29Si{1H} NMR data
were recorded in either a Bruker ARX-500 MHz spec-
trometer or Varian Unity Plus 300 MHz WB spectrom-
eter at ambient temperature (unless noted otherwise)
using a 5 mm tunable broadband probe. Chemical
shifts (l) are reported in parts per million and coupling
constants (J) in hertz. The solution 29Si spectra were
A slurry of {(Ph3P)Pt[m-h2-HꢀSiH(IMP)]}2 (1a,b) (16
mg, 0.013 mmol) in 0.5 ml C6D6 was reacted with a
solution of PMe2Ph (8 mg, 0.05 mmol) in 0.5 ml C6D6
resulting in vigorous gas evolution. The cloudy color-
less reaction mixture changed to a clear intense yellow
1
within 30 min. The H and 31P{1H} NMR data indi-
cated the quantitative formation of {[(PhMe2P)2Pt][m-
SiH(IMP)]}2 (3) exclusively as the trans isomer. Diethyl
ether (1.5 ml) was added to the reaction mixture caus-
ing slow precipitation of 3 as a yellow solid. The solid
was washed with cold Et2O (1×1 ml) then dried in
vacuo to give 14 mg of 3 (86%). Spectroscopic data are
identical with the data for 3 obtained from the
Me2Pt(PMe2Ph)2/(IMP)SiH3 route [8].
1
acquired using a DEPT pulse sequence [9] or a H–29Si
HMQC sequence [10]. Infrared spectra were recorded
on a Perkin–Elmer 1600 Series FT-IR. Elemental
analyses were obtained from Atlantic Microlab Inc.,
Norcross, GA.
2.1. Reaction of {(Ph3P)Pt[v-p2-HꢀSiH(IMP)]}2 (1a,b)
with dppe
A slurry of {(Ph3P)Pt[m-h2-HꢀSiH(IMP)]}2 (1a,b) (20
mg, 0.016 mmol) in 1.0 ml C6H6 was treated with a
solution of dppe (14 mg, 0.04 mmol) in 1.0 ml C6H6
accompanied by vigorous evolution of gas. The reac-
tion mixture turned bright yellow, was shaken until the
solid dissolved, and then set aside overnight. Complex 2
was isolated as a mixture of trans-(2a) and cis-(2b,
1 cis and trans isomers show overlapping resonances.
2 Assignment of cis versus trans was made on the assumption that
the SiꢀH stretching for 2b will occur at higher energy due to a more
sterically hindered environment.

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J. Braddock-Wilking et al. / Inorganica Chimica Acta 330 (2002) 82–88
2
2.3. Reaction of {(Ph3P)Pt[v-p2-HꢀSiH(IMP)]}2 (1a,b)
1JPtSi=199 Hz, JPSi=78 Hz). IR (KBr, cm−1): w 2101
(s, SiꢀH), 1642 (mb, PtꢀH). Anal. Calc. for
C56H96P2Pt2Si2: C, 52.64; H, 7.57. Found: C, 51.99; H,
7.47%.
with PCy3
To a slurry of {(Ph3P)Pt[m-h2-HꢀSiH(IMP)]}2 (1a,b)
(18 mg, 0.015 mmol) was added a solution of PCy3 (9
mg, 0.03 mmol) in 1.5 ml C6H6. After 1 h the solution
became clear and was set aside for 24 h. The solution
was layered with Et2O (2 ml) and after several days, an
off-white solid precipitated. The solid was washed with
Et2O (3×1 ml) and dried in vacuo to give 16 mg of
{(Cy3P)Pt[m-h2-HꢀSiH(IMP)]}2 (4a,b) (86% yield as a
mixture of trans-(4a) and cis-(4b) isomers (ratio of
ꢀ2:1 by NMR)).3
1H NMR (C6D6, 500 MHz) for 4b: l 0.9–2.1 (m,
66H, PCy3),1 1.18 (bs, 2H, JPtH=615 Hz, Pt···H···Si),4
1
3
1.39 (d, 12H, JHH=6.9 Hz, ArCH(CH3)3), 3.06 (br s,
6H, ArCH3), 4.08 (bm, 1H, ArCH(CH3)2), 4.14 (bm,
1H, ArCH(CH3)2), 7.01–7.13, 7.18–7.33 (m, 6H,
ArH), 8.43 (s, 1H, 2JPtH=105 Hz, SiH). 31P{1H} NMR
(C6D6, 121 MHz): l 58.4 (Js not resolved). 29Si{1H}
NMR (C6D6, DEPT, 99 MHz): l 139.4 (d, JPtSi=1576
Hz, JPtSi=241 Hz, JPSi=64 Hz).
1
1
2
1H NMR (C6D6, 500 MHz) for 4a: l 0.9–2.1 (m,
1
66H, PCy3),1 1.26 (s, 2H, JPtH=635 Hz, Pt···H···Si),4
2.4. Reaction of {(Ph3P)Pt[v-p2-HꢀSiH(IMP)]}2 (1a,b)
with P(TMP)3
1.57 (d, 12H, 3JHH=6.6 Hz, ArCH(CH3)2),5 2.72 (s,
3H, ArCH3), 2.84 (s, 3H, ArCH3), 4.29 (bm, 1H,
ArCH(CH3)2), 4.42 (bm, 1H, ArCH(CH3)2), 7.01–7.13,
A sample of {(Ph3P)Pt[m-h2-HꢀSiH(IMP)]}2 (1a,b) (8
mg, 0.006 mmol) was added to a solution of P(TMP)3
(23 mg, 0.04 mmol) in 1 ml C6D6. No visible reaction
was observed (e.g. no color change or evolution of gas).
2
7.18–7.33 (m, 6H, ArH), 9.12 (m, 2H, JPtH=88 Hz,
SiH). 31P{1H} NMR (C6D6, 121 MHz): l 58.8 (1JPtP
=
2
3
3985 Hz, JPtP=272 Hz, JPP=51 Hz). 29Si{1H} NMR
1
The reaction was monitored by H and 31P{1H} NMR
1
(C6D6, DEPT, 99 MHz): l 139.7 (d, JPtSi=1467 Hz,
and the spectra remained unchanged for 2 days at room
temperature and after heating to 60 °C for 24 h. Even-
tual decomposition of 1a,b occurred.
Table 1
Crystal data and structure refinement parameters for 2a
2.5. X-ray crystallography
Empirical formula
Formula weight
Temperature (K)
Crystal size (mm)
Crystal system
C51H53P2PtSi
1951.05
223 (2)
0.38×0.30×0.16
triclinic
X-ray crystal structure determinations were per-
formed in a Bruker SMART diffractometer equipped
with a CCD area detector at 233 K. Crystals of 2a were
grown from a dilute C6D6 solution by slow evaporation
over several days. Preliminary examination and data
collection were performed using a Bruker SMART
CCD Detector system. Preliminary unit cell constants
were determined with a set of 45 narrow frames (0.3° in
‚ scans). The data sets collected consisted of 4028
frames with a frame width of 0.3° in ‚ and counting
time of 15 s per frame at a crystal to detector distance
of 4.930 cm. The double pass method of scanning was
used to exclude any noise. SMART and SAINT software
packages [11] were used for data collection and data
integration. Analysis of the integrated data did not
show any decay. Final cell constants were determined
by a global refinement of xyz centroids. Collected data
were corrected for systematic errors using SADABS [12]
based on the Laue symmetry using equivalent
reflections.
(
Space group
Unit cell dimensions
P1
,
a (A)
13.7171 (8)
13.8672 (8)
14.1798 (8)
84.520 (4)
69.640 (3)
60.632 (3)
2194.4 (2)
2
1.439
3.331
0.0689
0.0959
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
3
,
V (A )
Z
Dcalc (mg m−3
)
v (mm−1
)
R1 (observed data)
wR2 (all data)
Goodness-of-fit on F2
1.115
1.190 and −0.904
−3
,
Largest difference peak and hole (e A
)
3 The reaction was quantitative by 1H and 31P{1H} NMR spectro-
Crystal data and intensity data collection parameters
are listed in Table 1. Structure solution and refinement
were carried out using the SHELXTL-PLUS software
scopies.
4 The chemical shifts for Pt···H···Si were determined from a 2D
EXSY experiment; resonances for the cis and trans bridging hydrides
were found to overlap with the PCy3 resonances.
5 Chemical shift and coupling constant was determined from 2D
1H–1H COSY experiment.
6 No coupling to the cis 31P signal was observed in the 1H–31
P
7 Estimated from a horizontal slice from the 2D-EXSY data
through the terminal hydride (4a) and the cross peak correlation to
the bridging hydrides.
COSY experiment, possibly due to the weak intensity of the 31P peaks
for cis-(2b).

J. Braddock-Wilking et al. / Inorganica Chimica Acta 330 (2002) 82–88
85
basic phosphines. Reaction of 1a,b with dppe (cone
angle 125°) afforded the dinuclear complex 2 in 94%
yield (Eq. (1)) with concomitant loss of H2. Complex 2
was formed as a mixture of trans-(2a) and cis-(2b,
trace) isomers and was characterized by multinuclear
NMR, IR, elemental analysis, and X-ray crystallogra-
phy. Several Pt2Si2 ring systems analogous to 2a,b have
been prepared by Tessier [4,17], Fink [18], Tilley [19],
and Tanaka [20].
(1)
Fig. 1. Molecular structure of 2a (without benzene solvates).
package [11]. The structures were solved by direct
(
method and refined successfully in the space group P1.
Compounds 1a,b and 2a both display 1H NMR
chemical shifts for the terminal SiꢀH proton resonances
at very low field. The 1H{31P} NMR spectrum for
complex 2a exhibited the SiꢀH resonance at 7.28 ppm
as a singlet flanked by platinum satellites. The NMR
resonances associated with 2b were too weak to be
observed except in the 31P{1H} spectrum. The terminal
SiꢀH resonances for 1a,b appeared at 8.42 and 8.92
ppm flanked by two sets of Pt satellites [6,7]. The
terminal SiꢀH resonances for 1a,b and 2a appear sig-
nificantly further downfield than the typical region for
SiꢀH chemical shifts in free silanes or other reported
MꢀSiH moieties [1].
Full matrix least-squares refinement was carried out by
minimizing ꢀw(Fo2−Fc2)2. The non-hydrogen atoms
were refined anisotropically to convergence. The SiꢀH
hydrogen was located and refined freely. All the re-
maining hydrogen atoms were treated using the appro-
priate riding model. Two and one-half molecules of
benzene were found in the crystal lattice for 2a. Projec-
tion view of the molecules with non-hydrogen atoms
represented by 50% probability ellipsoids, and showing
the atom labeling is presented in Fig. 1.
3. Results and discussion
The dppe ligand in 2a exhibited two separate methyl-
1
ene peaks in the H{31P} spectrum as broad multiplets
There are only a few complexes of the Group 10
metals that contain a nonclassical M···H···Si interaction
and these include several diplatinum derivatives with
with platinum satellites. The possibility that these two
peaks is due to two different isomers have been ex-
1
cluded by a H–31P COSY experiment, that indicated
the basic structure {(R%P)Pt(m-h2-HꢀSiR2)}2 (R%=alkyl
3
coupling of both resonances to the signal for the trans
isomer. In addition, the 1H–31P COSY experiment indi-
cated a strong correlation of the trans isomer to two
different resonances of the ortho-P(C6H5)2 unit on the
dppe ligand. Thus, it appears that in solution the
molecule is unsymmetrical.6
or aryl; R=Me, Ph) prepared by Stone and coworkers
[13]. Tessier and coworkers synthesized the novel dinu-
clear platinum complex {(Pr3P)Pt[m-h2-HꢀSi(Hex)-
PtH(PPr3)]}2 (Pr=n-propyl) [14]. More recently, Kim
and coworkers reported several dinuclear Pd complexes
containing bridging-silylene groups bridging the Pd cen-
The 31P{1H} NMR spectra for 1a,b and 2a,b display
a characteristic pattern for a dinuclear platinum com-
plex but differ in the splitting of the satellite peaks due
to the number of phosphines bound to Pt. The 31P{1H}
NMR spectrum for 2a,b shows part of an
AA%A¦A§XX% spin system pattern centered at 56 ppm
ters, [(R3P)Pd(m-h2-HꢀSiR%R¦)]2 [15]. Little is known of
2
the reactivity of complexes of this structural type. A
recent density functional study by Lin and coworkers
has shown that dinuclear transition metal bis(m-h-si-
lane) complexes display noticeably shorter Si···H dis-
tances compared to mononuclear metal (m-h-silane)
complexes [16]. The shorter SiꢀH bonds imply a weaker
metal(d)“SiH(s*) interaction [16].
1
(1JPtP=1565 Hz). The value of JPtP for 2a decreased
by approximately 200 Hz relative to the related dinu-
clear complexes, [(R3P)2Pt(m-SiR%R¦)]2 [8]. The angle
constraints associated with a chelating phosphine lig-
and have been shown to cause a reduction in the value
The dinuclear complex,{(Ph3P)Pt[m-h2-HꢀSi(IMP)]}2
(1a,b) was prepared by reaction of (Ph3P)2Pt(h2-C2H4)
with (IMP)SiH3 [6,7]. The nonclassical Pt···H···Si inter-
action in 1a,b was supported by multinuclear NMR
spectroscopy and X-ray crystallography (trans-1 only).
Recently, we examined the reactivity of 1a,b with
1
of JPtP [21]. Unlike the 31P spectra for related deriva-
tives, [(PhnMe3−nP)2Pt[m-SiH(IMP)]2 (n=0–2) [8]
which exhibited platinum satellites as pseudo-triplets, a
second-order doublet of doublets pattern was seen for

86
J. Braddock-Wilking et al. / Inorganica Chimica Acta 330 (2002) 82–88
the platinum satellites for complex 2a. The low solubility
of 2a precluded resolution of the remaining peaks for the
case where both Pt nuclei are NMR active.
In fact, the reverse reaction of a bridging silylene com-
plex containing no metal–metal bond to a system with a
metal–metal bond is fairly common [3]. In contrast,
mononuclear complexes containing M···H···Si interac-
tions have been reported to undergo reductive elimina-
tion of HSiR3 upon reaction with phosphines. For
example (h6-C6Me6)(OC)2Cr(h2-HꢀSiHPh2) reacted
with P(nBu)3 to give Ph2SiH2 and (h6-C6Me6)(OC)2-
Cr[P(nBu)3] [22].
A dramatic difference is seen in the 29Si NMR chemi-
cal shifts for 1a,b and 2a. Compounds 1a,b exhibits a low
field chemical shift of 131 (1a) and 126 (1b) ppm [6,7]
which falls in the typical range for silylene moieties
bridging the two metal centers (MꢀSiR2ꢀM; with and
without bridging hydrides) [3]. In sharp contrast, a high
field resonance is seen for 2a at −132 ppm. The reason
for the nearly 250 ppm difference in chemical shifts is
currently unknown.
X-ray quality crystals of 2a were grown from a dilute
C6D6 solution by slow evaporation and five benzene sol-
vate molecules were found per dimer (Fig. 1). Crystal
data and structure refinement parameters are listed in
Table 1 and selected bond distances and angles are listed
in Table 2. The hydrogens bound to Si were located and
refined freely. The isopropyl substituents on the IMP
group were found to be positioned over the Pt2Si2 core,
while hydrogens at silicon were pointing away from the
central plane. Complex 2a exhibited similar structural
features to those found in the solid state structures of
[(PhnMe3−nP)2Pt(m-SiH(IMP)]2 (n=0–2) [8], i.e. short
Si···Si cross ring distances, acute SiꢀPtꢀSi angles, and ob-
tuse PtꢀSiꢀPt angles (see Table 2). Tessier and coworkers
have structurally characterized several related complexes
with the formula, [(R3P)2Pt(m-SiR%R¦)]2 and have ob-
served similar trends [4,14,17]. The main difference be-
tween these structures is the PꢀPtꢀP angles and the
spatial arrangement of substituents on the phosphorus
center. For example, due to the configuration associated
with the five-membered chelate ring, the PꢀPtꢀP angles
in 2a were significantly smaller than the corresponding
angles in [(PhMe2P)2Pt(m-SiH(IMP)]2 (86.26(8)° vs.
99.28(3)°) [8].
A different reactivity pattern was exhibited when 1a,b
was treated with the more basic and bulky phosphine
(PCy3) (4 equiv., cone angle 170°). No gas evolution was
observed, in contrast to the dppe and PMe2Ph reactions
described above. The reaction mixture was analyzed by
NMR spectroscopy which indicated that the central
Pt2Si2(m-H)2 core remained intact (Eq. (2)) and that ex-
change of the phosphine ligands had taken place. Com-
plex 4 {(Cy3P)Pt[m-h2-HꢀSiH(IMP)]}2 was formed as a
mixture of trans-(4a) and cis-(4b) isomers (4a major iso-
mer; exact ratio could not be determined but an upper
limit of 2:1 could be established from NMR spec-
troscopy from the terminal SiꢀH region) and 4a,b was
isolated as an off-white solid in 86% yield. Complexes
4a,b is quite soluble in aromatic solvents and was charac-
terized by multinuclear NMR and IR spectroscopies.
Dinuclear complexes of the structural type,
[(R3P)2Pt(m-SiR%R¦)]2 are generally prepared by reaction
of a primary or secondary hydrosilane with an appropri-
ate platinum phosphine complex. Results from our stud-
ies [8] as well as those of Fink and coworkers [18] and
Tilley and coworkers [19] have found that the precursors
to these dimers are bis(phosphine)platinum bis(silyl)
complexes
(R3P)2Pt(SiHR%R¦)2.
Although
(Phn-
Me3−nP)2Pt(SiH2IMP)2 (n=0–2) reacts rapidly with
the starting platinum precursor (PhnMe3−nP)2-
PtMe2 to form the dinuclear complexes, {(R3P)2Pt[m-Si-
H(IMP)]}2 [8], the bis(silyl) precursor (dppe)Pt(SiH2-
IMP)2 did not react.
The reaction shown in (Eq. (1)) is uncommon and we
have now shown that the two types of phosphines can
initiate this type of transformation of a dinuclear bis(m-
h2-HꢀSiR2) complex to a bis(m-silylene) species. Earlier,
we found that compound 1a,b undergoes reaction with
the more basic, less sterically demanding phosphine,
dimethylphenylphosphine (PPhMe2; cone angle 122°) to
form {(PhMe2P)2Pt[m-SiH(IMP)]}2 trans-(3) in 87%
yield along with the elimination of H2 [8]. Complex 3 was
independently prepared by reaction of (PhMe2P)2PtMe2
with (IMP)SiH3 and was characterized by multinuclear
NMR, X-ray crystallography, and elemental analysis [8].
Table 2
,
Selected bond lengths (A) and bond angles (°) for 2a
Bond lengths
Pt1ꢀSi1
2.412(2)
2.395(2)
2.315(2)
2.328(2)
4.019
Pt1ꢀSic1
Pt1ꢀP1
Pt1ꢀP2
Pt1···Ptc1
Si1···Sic1
2.637(5)
Bond angles
P1ꢀPt1ꢀP2
86.26(8)
168.12(7)
169.62(7)
101.74(8)
105.60(7)
113.44(9)
56.41(8)
57.04(8)
66.56(9)
P1ꢀPt1ꢀSic1
P2ꢀPt1ꢀSi1
P1ꢀPt1ꢀSi1
P2ꢀPt1ꢀSic1
Pt1ꢀSi1ꢀPtc1
Pt1ꢀSi1ꢀSic1
Ptc1ꢀSi1ꢀSic1
Si1ꢀPt1ꢀSic1
(2)

J. Braddock-Wilking et al. / Inorganica Chimica Acta 330 (2002) 82–88
87
1
This study has shown that at least two different
types of reactivity are observed with phosphines and
the dinuclear complexes 1a,b: (1) formation of a dinu-
clear complex with change in the central core (with
loss of H2); and (2) simple exchange of phosphines
(leaving the central Pt2Si2(m-H)2 core intact). There are
two features of phosphines that can influence their
reactivity, basicity and the size of the phosphine (cone
angle). The results described in this paper indicate that
the basicity of the phosphine at the Pt center did not
seem to have a significant influence on the formation
of a particular type of a dimer. For example, the
dinuclear Pt2Si2 complexes 4a,b and [(Me3P)2Pt(m-Si-
H(IMP)] [8] contain PCy3 and PMe3, ligands, respec-
tively, that exhibit similar pKa values (9.7 vs. 8.65). On
the other hand, complexes 1a,b and 4a,b contain PPh3
and PCy3 ligands with very different pKa values (2.73
vs. 9.7). One significantly different feature found in
these phosphine ligands is the cone angle [21]. The
phosphines with smaller cone angles, dppe (127°) and
PMe2Ph (122°) form the dinuclear complexes with the
general formula [(P)2Pt(m-SiH(IMP))]2. In contrast,
when the cone angle is 145]x5170° as with PPh3
and PCy3, respectively, then steric constraints favor
the formation of the nonclassical dimer {(R3P)Pt[m-h2-
HꢀSiH(IMP)]}2. Phosphines with cone angles larger
than ꢀ170° appear to be too large to fit into the
dinuclear core.
The H NMR spectrum of 4a,b showed two termi-
nal SiꢀH resonances at low field. The signal for 4a
appeared as an unresolved multiplet centered at 9.12
ppm and seems to indicate that two unique terminal
hydride resonances are overlapping, suggesting that 4a
is unsymmetrical in solution. The terminal hydride
resonance for 4b was found at 8.43 ppm as a singlet
flanked by one set of platinum satellites. The bridging
Pt···H···Si resonances were located by a H–1H EXSY
1
experiment and were found at 1.26 ppm for 4a and
1.18 ppm for 4b. Data from the EXSY experiment
showed that both bridging hydride signals were
flanked by one set of Pt satellites.7 The H–1H EXSY
1
experiment also revealed that the terminal and bridg-
ing hydrides of 4a were exchanging with each other in
addition to the exchange with 4b: the mechanism for
exchange is believed to be analogous to that proposed
for 1a,b [7] where exchange between terminal and
bridging hydrides as well as concomitant cis–trans iso-
merization was observed.
Variable temperature H and 31P{1H} NMR experi-
1
ments for 4a,b gave inconclusive results and the coa-
lescence temperature could not be determined (appears
to be \90 °C). Attempts are in progress to obtain a
X-ray quality crystals of 4a and/or 4b to examine
closely the environment around the central Pt2Si2(m-
H)2 core.
The 31P{1H} NMR spectrum of 4a,b exhibited a
characteristic pattern for a dinuclear platinum complex
(part of an AA%XX% spin system) and two central
single lines were observed for both trans-(4a) and cis-
(4b) isomers centered around 58 ppm. These data are
similar to those of 1a,b and related dinuclear com-
plexes [6,7].
4. Supplementary material
Atomic coordinates, anisotropic displacement
parameters, and a complete list of geometrical parame-
ters are deposited with the Cambridge Crystallo-
graphic Data Center. Complete listings of the atomic
coordinates for the non-hydrogen atoms, the geometri-
cal parameters positional and isotropic displacement
coefficients for hydrogen atoms, anisotropic displace-
ment coefficients for the non-hydrogen atoms for 2a
and NMR spectroscopic data for 2a,b and 4a,b are
available from the corresponding author on request.
The 29Si{1H} spectrum of 4a,b showed a central
doublet for both cis and trans isomers of 4 at 139 ppm
with each exhibiting two sets of Pt satellites (1JSiPt
ꢀ
1500 and ꢀ200 Hz). The larger coupling constant
corresponds to Pt that is bound directly to Si, whereas
1
the smaller value of JPtSi is attributed to the Pt that
participates in the 3c–2e Pt···H···Si interaction with Si.
The chemical shift of 4a,b was in good agreement with
that for complexes 1a,b [6,7].
In addition, the reactivity of 1a,b was explored with
the more sterically demanding and basic phosphine,
tris(2,4,6-trimethoxyphenyl)phosphine (P(TMP)3) (cone
angle 184°). When a benzene-d6 solution of 1a,b was
treated with P(TMP)3, no reaction was detected by
NMR spectroscopy. The spectrum remained un-
changed for 2 days at room temperature and after
heating to 60 °C for 24 h. Thus, it appears that
P(TMP)3 is too sterically hindered to undergo an ex-
change reaction with PPh3 to give the complex,
{[(TMP)3P]Pt[m-h2-HꢀSiH(IMP)]}2.
Acknowledgements
This work is supported by the University of Mis-
souri-St. Louis Research Award. The support of the
NSF and the University of Missouri-Research Board
are gratefully acknowledged for the purchase of a
Varian Unity Plus 300 and Bruker ARX-500 and
Bruker Avance 300 NMR spectrometers (CHE-
9974801). The UM-St. Louis X-ray Crystallography
Facility was funded in part by a NSF Instrumentation
Grant and the UM-St. Louis Research Award.

88
J. Braddock-Wilking et al. / Inorganica Chimica Acta 330 (2002) 82–88
[13] M. Auburn, M. Ciriano, J.A.K. Howard, M. Murray, N.J.
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