(Phosphinoalkyl)silanes. 3.1 Poly(o-(diphenylphosphino)benzyl)silanes: Synthesis, Spectroscopic Properties, and Complexation at Platinum or Iridium

Full text of DOI:10.1021/ic941230f

Inorg. Chem. 1996, 35, 1729-1732
1729
(Phosphinoalkyl)silanes. 3.1
Poly(o-(diphenylphosphino)benzyl)silanes:
Synthesis, Spectroscopic Properties, and
Complexation at Platinum or Iridium
benzyl (Ph2PC6H4CH2-) carbanion to synthesize chelate-
stabilized benzyl complexes of nickel, palladium, and platinum
[M(CH2C6H4PPh2)2, M ) Ni, Pd, Pt]. At about the same time,
reactivity of the isomeric (phosphinomethyl)phenyl (Ph2-
PCH2C6H4-) system was investigated9 and found to yield (o-
((diphenylphosphino)methyl)phenyl)trimethylsilane (o-Ph2P-
CH2C6H4SiMe3) on quenching with chlorotrimethylsilane. Very
recently, by the adoption of the reverse approach to P-C bond
formation, treatment of chlorodiphenylphosphine with the
(silylmethyl)phenyl Grignard reagents o-(RCH2)C6H4MgBr (R
) SiMe2H, SiMeH2) was examined by Ang et al.10 For either
R, however, it was found that this procedure resulted in
formation of an inseparable mixture of two positional isomers,
Viz the (o-phosphinobenzyl)silane (Ph2PC6H4CH2R) together
with the corresponding (o-(phosphinomethyl)phenyl)silane (Ph2-
PCH2C6H4R). Subsequent addition of these isomeric mixtures
to triosmium dodecacarbonyl afforded altogether six novel (o-
phosphinobenzyl)silyl- and (o-(phosphinomethyl)phenyl)si-
lyl-triosmium carbonyl clusters {Os3(µ-H)(CO)10(µ-SiR1R2C6H4-
CH2PPh2), Os3(µ-H)(CO)10(µ-SiR1R2CH2C6H4PPh2), or Os3(µ-
H)3(CO)8(µ-SiR1R2C6H4CH2PPh2) for R1 ) R2 ) Me or R1 )
Me, R2 ) H}, which were separated by preparative thin-layer
chromatography and then high-performance liquid chromatog-
raphy. Four of these products were characterized by using
single-crystal X-ray diffraction, revealing that in each case
phosphino-organosilane addition had occurred across a single
Os-Os bond. This established that each modified silyl group
so derived is able to function as a bridging PSi ligand (though
not chelate (cf. eq 1), as was claimed10).
Robert A. Gossage, Geoff D. McLennan, and
Stephen R. Stobart*
Department of Chemistry, University of Victoria,
Victoria, British Columbia, Canada V8W 2Y2
ReceiVed October 26, 1994
Introduction
Earlier papers1,2 have discussed the synthesis and properties
of modified silanes PPh2(CH2)nSiR2H (n ) 1-3; R ) Me, Ph;
e.g. “chelH” for n ) 2, R ) Me), in which a diorganophosphino
donor site is linked through a polymethylene backbone to a silyl
group that possesses an Si-H functionality. Such molecules
will undergo oxidative addition at low-valent transition metal
centers, affording3-5 (phosphinoalkyl)silyl (“PSi”) complexes
in which Si-M bond formation is accompanied by multidentate
coordination that involves one or more phosphorus donors. Thus
the use of SiH(CH2CH2PPh2)3 (i.e.1 “triPSiH”) leads to encap-
sulation of a metal center in a cagelike assembly (e.g. A, triPSi
We report below that, by using appropriate8 organolithium
reagents, it is possible to prepare both pure (o-(diphenylphos-
phino)benzyl)dimethylsilane (1) and pure (o-((diphenylphos-
phino)methyl)phenyl)dimethylsilane (2), i.e. independently,
attachment at M), by which means the silyl group is anchored
in complexes that are structurally interesting5 and are active6
in catalysis. To reduce the conformational flexibility of this
kind of ligand framework, we have synthesized PSi precursors
that use a benzyl skeleton to connect silicon to phosphorus
through a three-carbon backbone that incorporates a planar
benzenoid unit. The new (phosphinobenzyl)silanes so con-
structed have been used to isolate model “modified chelate”
(mcPSi) complexes formed by “chelate-assisted”7 oxidative
addition (see eq 1) at platinum or iridium.
uncontaminated by one another; that the same methodology may
be extended to the synthesis of poly(o-phosphinobenzyl)silanes;
and that ligand precursors belonging to this (phosphinobenzyl)-
silane family will indeed add at a single metal center in a
chelating mode, to afford analogues of the prototypal6 PSi
complexes. The trifunctional ligand precursor SiH(Me)[CH2-
(o-(PPh2)C6H4)]2 (i.e. “mcbiPSiH”) is also shown to be acces-
sible Via dilithiation followed by phosphination of the new
bis(bromobenzyl)silane SiH(Me)[CH2(o-BrC6H4)]2. Restricted
conformational mobility of the backbone connecting Si to P in
these mcPSi systems may be expected to engender steric
constraints on substrate approach in the vicinity of a reactive
metal site that will modify catalyst selectivity.6
Metalation of diphenyl-o-tolylphosphine P(C6H4Me)Ph2 and
of benzyldiphenylphosphine P(CH2Ph)Ph2 was first explored 20
years ago by Chini et al.,8 who used the o-(diphenylphosphino)-
(1) Part 2: Joslin, F. L.; Stobart, S. R. Inorg. Chem. 1993, 32, 2221.
(2) Holmes-Smith, R. D.; Osei, R. D.; Stobart, S. R. J. Chem. Soc., Perkin
Trans. 1 1983, 861.
(3) Auburn, M. J.; Stobart, S. R. Inorg. Chem. 1985, 24, 318.
(4) Auburn, M. J.; Holmes-Smith, R. D.; Stobart, S. R. J. Am. Chem.
Soc. 1984, 106, 1314.
(5) Joslin, F. L.; Stobart, S. R. J. Chem. Soc., Chem. Commun. 1989,
504.
(6) Stobart, S. R.; Grundy, S. L.; Joslin, F. L. U.S. Patent 4 950 798, 1990;
Canadian Patent 1 327 365, 1994.
Experimental Section
All manipulations were conducted under dry dinitrogen gas using
standard inert-atmosphere techniques. Manipulation of solid lithium
salts was carried out in an argon-filled glovebox. Solvents were dried
and redistilled immediately prior to use. Chlorosilanes, aryl bromides,
(7) Holmes-Smith, R. D.; Stobart, S. R.; Cameron, T. S.; Jochem, K. J.
Chem. Soc., Chem. Commun. 1981, 937. Grundy, S. L.; Holmes-Smith,
R. D.; Stobart, S. R.; Williams, M. A. Inorg. Chem. 1991, 30, 3333.
(8) Fantucci, P.; Chini, P.; Canziani, F. Gazz. Chim. Ital. 1974, 104, 249.
(9) von Abicht, H.-P.; Issleib, K. Z. Anorg. Allg. Chem. 1976, 422, 237.
(10) Ang, H. G.; Chang, B.; Kwik, W. L. J. Chem. Soc., Dalton Trans.
1992, 2161.
0020-1669/96/1335-1729$12.00/0 © 1996 American Chemical Society

1730 Inorganic Chemistry, Vol. 35, No. 6, 1996
Notes
chlorodiphenylphosphine, and N,N,N′,N′-tetramethylethylenediamine
(tmeda) were commercial products (Aldrich Co.). NMR spectra were
recorded using a Bruker WM250 Fourier transform spectrometer.
Synthesis of Compounds. A. Ligand Precursors. 1. (o-
(Diphenylphosphino)benzyl)dimethylsilane, 1. To a slurry of o-
tolyldiphenylphosphine (2.34 g, 8.47 mmol) in hexanes (30 mL) was
added from a dropping funnel a mixture of tmeda (1.5 mL) and LiBun
(5.6 mL, 9.0 mmol) in hexanes (20 mL). Appearance of an orange
color was followed by precipitation of a bright yellow solid; after stirring
(2 h), solvent was removed and the residue was washed with hexanes.
This material (1.00 g, 2.84 mmol) was suspended in hexanes (25 mL),
and after the mixture was cooled to 0 °C, chlorodimethylsilane (1.3
mL, 11.7 mmol) was run in, discharging the color and precipitating a
white solid. After stirring (14 h), volatile material was pumped away,
leaving a cloudy, pale yellow oil. This was dissolved in hexanes, the
mixture was filtered, and the filtrate was then reduced in volume and
finally distilled under reduced pressure (200 °C/0.5 mmHg) to afford
the product as a colorless, viscous oil (52%), shown by IR and NMR
spectroscopy to be one component of the mixture (i.e. of compounds
1 and 2) reported elsewhere.10
2. (o-((Diphenylphosphino)methyl)phenyl)dimethylsilane, 2. To
a solution in Et2O (20 mL) of9 (o-bromobenzyl)diphenylphosphine (0.63
g, 1.78 mmol) was added drop by drop n-butyllithium (3 mL, 1.6 M in
hexanes) at 20 °C. During this operation, the initially colorless solution
became cloudy and turned yellow, ultimately affording a deep chrome-
yellow pigmented suspension in an orange-red solution. The reaction
mixture was subsequently stirred for 3 h, and then the ether was
removed under reduced pressure to leave an orange powder; this was
washed with dry hexanes (35 mL) and then suspended in dry benzene
(30 mL), whereupon chlorodimethylsilane (3.0 mL, 35 mmol) was run
in over 5 min, during which the color was discharged and a white
precipitate was deposited. The resulting mixture was heated to 45 °C
and then stirred (4 h); after filtration through a glass frit and removal
of volatiles, the product remained as a clear, colorless oil (0.25 g, 58%),
shown by IR and NMR spectroscopy to be the second (Vs 1, see
preparation A1 above) component of the isomeric mixture reported
elsewhere.10
yellow solution, to which was added PPh2Cl (4.5 mL, 25 mmol),
resulting in an exothermic reaction. After reflux (12 h), stirring (12
h), filtration, and removal of solvent, a pale yellow viscous liquid
remained which was shown by 31P NMR spectroscopy to be the product
(3) contaminated with unchanged PPh2Cl, together with a minor
proportion (ca. 20% Vs 3) of a third constituent (identified by its
chemical shift as PBunPh2), traces of which persisted after repeated
distillation at reduced pressure.
4. Tris(o-(diphenylphosphino)benzyl)silane, mctriPSiH, 4. Using
procedures that paralleled those detailed in preparation A3, (method I)
above, lithiation of o-MeC6H4PPh2 (3.00 g, 10.9 mmol) followed by
addition of trichlorosilane (0.2 mL, 2.0 mmol) afforded a product which
after purification was recovered as a pale yellow wax (1.02 g, 56%).
Anal. Found: C, 79.7; H, 5.7. Calcd for C57H49P3Si: C, 80.1; H, 5.7.
B. Platinum and Iridium Complexes. 1. Bis[(o-(diphenylphos-
phino)benzyl)dimethylsilyl]platinum(II), Pt(mcchel)2, 5. Drop by
drop addition of a solution of silane 1 (150 mg, 0.44 mmol) in benzene
(4 mL) to Pt(cod)Cl2 (81 mg, 0.22 mmol) dissolved in a mixture of
benzene (7 mL) and NEt3 (4 mL) was accompanied by a change from
colorless to yellow and deposition of a fluffy precipitate. After agitation
for 90 min, removal of volatiles under reduced pressure left a yellow
oil, which was redissolved in hexanes (5 mL). Elution with hexanes
through a Florisil column (1 × 3 cm) was followed by evaporation of
solvent to give an off-white crude product, which was recrystallized
from 1:1 pentane/CH2Cl2 as colorless crystals (25 mg, 13%). Anal.
Found: C, 58.8; H, 5.2. Calcd for C42H44P2PtSi2: C, 58.5; H, 5.4.
2. [Bis(o-(diphenylphosphino)benzyl)methylsilyl]platinum(II) Chlo-
ride, Pt(mcbiPSi)Cl, 6. In a manner paralleling that described in
preparation B1 above, admixture of Pt(cod)Cl2 (35 mg, 0.09 mmol),
NEt3 (0.7 mL), and silane 3, i.e. mcbiPSiH (56 mg, 0.1 mmol), in
benzene (8 mL) yielded a light yellow, oily material, which was
extracted with CH2Cl2 (10 mL); filtration through a glass sinter led to
isolation of a sticky yellow solid, which was recrystallized (1:1 hexanes/
CH2Cl2) to afford the pure product as a white powder (20 mg, 27%).
Anal. Found: C, 54.6; H, 4.3. Calcd for C39H35ClP2PtSi‚0.5CH2Cl2:
C, 54.7; H, 4.2.
3. Chlorohydrido[tris(o-(diphenylphosphino)benzyl)silyl]iridium-
(III), Ir(mctriPSi)H(Cl), 7. Silane 4, i.e. mctriPSiH (88 mg, 0.10
mmol), was added directly to a solution of [Ir(cod)Cl]2 (35 mg, 0.05
mmol) in THF (7 mL). The mixture was stirred for 2 h, during which
the color faded from orange to light yellow. After evacuation to remove
volatiles, a light yellow product remained, which was recrystallized
from chloroform/hexanes as a white powder (101 mg, 91%). Anal.
Found: C, 60.0; H, 4.5. Calcd for C57H49ClIrP3Si‚0.5CHCl3: C, 60.4;
H, 4.6.
3. Bis(o-(diphenylphosphino)benzyl)methylsilane, mcbiPSiH, 3.
Method I. n-Butyllithium (2.5 mL, 1.6 M in hexanes) was added
dropwise to a solution of o-tolyldiphenylphosphine (1.0 g, 3.6 mmol)
in dry hexanes (40 mL) and tmeda (0.6 mL). The mixture turned deep
orange and deposited an orange solid within 5 min. After stirring (4
h, 20 °C), volatile material was removed, leaving an orange solid;
redissolution of the latter in benzene (30 mL) was followed by rapid
addition of a stoichiometric deficit of dichloromethylsilane (0.1 mL,
0.74 mmol), whereupon the mixture became warm and faded in color
to light orange. After overnight stirring, removal of volatiles left an
orange oil; this was redissolved in 2:1 benzene/hexanes (30 mL), leaving
a white precipitate and affording a yellow solution, which was filtered
Results
Synthesis of (phosphinoaryl)silanes using the reaction of the
Grignard reagent o-BrMgC6H4CH2SiMe2H with PPh2Cl is
reported10 to lead Via exchange metalation to formation of an
inseparable mixture of compounds 1 and 2, which are positional
isomers. By contrast, we find that lithiation of o-tolyldiphen-
ylphosphine in the presence of tmeda or of (o-bromobenzyl)-
diphenylphosphine, followed by quenching with chlorodimeth-
ylsilane, provides each of these two compounds independently
(Scheme 1), i.e. uncontaminated by one another. In either
reaction, the required organolithium reagent, which is not very
soluble, is formed as a deep orange-yellow suspension: in each
case, discharge of this characteristic color is a good indicator
of the progress of the reaction. Each product (1 or 2) was
recovered as a colorless, rather viscous oil in ca. 50% yield;
survival of the silyl (i.e. Si-H) functionality was obvious from
strong IR absorption near 2120 cm-1 and by observation of 1H
NMR signals at δ 4.14 (1) and 4.60 (2), δ(SiH), as multiplets
that integrated correctly Vs alkyl protons. Further characteriza-
tion was provided by 13C NMR (alkyl carbons) as well as by
29Si and 31P NMR chemical shifts (Table 1). The assignments
proposed earlier by Ang et al.10 on the basis of NMR data
(quoted to 5 significant figures) for the isomer mixture 1/2 are
consistent with the spectra observed for 1 and 2, apart from the
and then reduced in volume to a brown-yellow oil.
A
31P NMR
spectrum of the latter revealed the presence of regenerated o-MeC6H4-
PPh2, which was removed by sublimation (150 °C/10-2 mmHg). This
left a dark brown oil, which was dissolved in benzene (20 mL); the
solution was then filtered through a plug of 1:1 Celite/silica gel.
Removal of volatiles yielded the product as a pale yellow oil (0.41 g,
38%) that set to a glassy semisolid below 20 °C.
Method II. To a solution in Et2O (20 mL) of (2-bromobenzyl)-
magnesium chloride (23.4 mmol) (prepared by Grignard synthesis from
commercial 2-bromobenzyl bromide) was added drop by drop dichlo-
romethylsilane (1.0 mL, 9.6 mmol) also in Et2O (15 mL), and the
mixture was then refluxed (12 h). Filtration and then removal of solvent
under vacuum left orange material, which was extracted with hexanes
(50 mL). Evaporation of the solvent left a yellow, oily liquid, which
was distilled under reduced pressure (180 °C/10-2 mmHg) to yield
colorless, oily bis(2-bromobenzyl)methylsilane (72%), which was
characterized by IR, NMR, and mass spectroscopy: νSi-H 2125 vs cm-1
;
3
1H NMR (CDCl3) δ 7.55-6.91 (C6H4), 4.07 (SiH), 2.41 (CH2, J 2.9
3
Hz), 0.09 (CH3, J 2.6 Hz); 29Si NMR δ -11.22 Vs SiMe4; MS M+
381 (Mcalc 381). This precursor (11.7 mmol) was dissolved in Et2O
(40 mL), the solution was cooled to -78 °C, and LiBun (15.6 mL, 25
mmol) was slowly added by syringe. Warming to room temperature
followed by gentle reflux (3 h) and then stirring (3 h) afforded a clear

Notes
Inorganic Chemistry, Vol. 35, No. 6, 1996 1731
Scheme 1
Table 1. NMR Data for (Phosphinobenzyl)silanes and (Phosphinobenzyl)silyl Complexesa
1H NMRc
13C NMRc
cmpd
δ(31P)b
δ(29Si)c
δ(CH2Si)
3J(CH2SiH)d
δ(CH3Si)
3J(CH3SiH)d
δ(SiH)
δ(CH2Si)
δ(SiCH3)
1J(CH2P)d
1
3
4
5
-14.1
-11.2
-7.2
-3.7
+23.1h
n.m.
2.51
2.64
2.48
1.88i
2.30l
2.38
3.3
0.24
0.02
n.a.f
3.6
3.6
n.a.
n.a.
n.a.
n.a.
4.14
4.01
4.21
n.a.
n.a.
n.a.
23.0
22.00
20.8
31.9
n.m.
n.m.
-4.0
-5.7
n.a.
5.2
n.m.
n.m.
-14.0
3.0e
3.4
21.9
24.6
n.r.
n.m.
n.m.
-14.1
+20.9g
n.a.
n.a.
n.a.
0.25j
-0.53m
n.a.
6
+22.3k
7n
-5.3,o -11.4p
n.m.
a All spectra recorded in CDCl3 solution: resonances due to phenyl substituents were observed in the range δ 8.0-6.4 (1H) or δ 135-125 (13C),
showing multiplet structure which was not analyzed in detail. b Relative to external 85% H3PO4. c Relative to external SiMe4. d Coupling constants
g 1
h 2
in Hz. e 4J(CH2SiCH3) ) 10.3 Hz. f n.a. ) not applicable, n.r. ) not resolved, n.m. ) not measured.
J
) 1458 Hz.
J
) 142 Hz;
PtP
m 3
SiP(trans)
i 3
4
j 3
k 1
l 3
n
2JSiP(cis) ) 22 Hz.
J
) 31.0 Hz; JPH ) 3.7 Hz.
J
) 13.2 Hz.
J
) 2908 Hz.
J
) 54.6 Hz.
J
PtH
) 25.6 Hz. δIr-H ) -9.55;
PtH
PtH
PtP
PtH
2
2JPH(trans) ) 126 Hz; JPH(cis) ) 20.2 Hz. o P trans to P; non-first-order spectra; see text. p P trans to H.
29Si chemical shift for compound 2, which is at -22.1 ppm
(not -10.6 as suggested10). The downfield shift in δ(29Si) of
10 ppm (2 Vs 1) reflects the structural change from a trialkyl-
silane to a dialkylarylsilane.
initially characterized by IR and NMR spectroscopy (1H, 13C,
29Si, 31P): the data are collected in Table 1, and the chemistry
is summarized in Scheme 1.
Compounds 1-4 were further characterized by using mass
spectrometry, which provided in particular an obvious distinction
between the positional isomers 1 and 2, which have previously
only been handled10 as an inseparable mixture. Exact mass
determination Via peak matching further confirmed the identi-
fication of 3 (found 594.2046; calcd 594.206 16) and 4 (found
854.2815; calcd 854.2816). Parent ions were weak but observ-
able in all four spectra. Consistent with similar magnitudes of
Si-C Vs P-C bond strengths, no dominant process is apparent
in subsequent fragmentation: base peaks at m/z 135 (1), 149
(2), 319 (3), and 275 (4) correspond to nominal loss of PPh2-
Our interest in precursors to cagelike coordination environ-
ments led us to examine the possibility of polysubstitution at
Si, using the (diphenylphosphino)benzylcarbanion formed by
lithiation of o-tolyldiphenylphosphine to effect alkylsilyl syn-
thesis. With both dichloromethylsilane and trichlorosilane,
analogues of biPSiH and triPSiH were obtained in moderate
yield. An alternate route to mcbiPSiH (3) is provided by the
action of (2-bromobenzyl)magnesium chloride on dichloro-
methylsilane; this affords in good yield bis(2-bromobenzyl)-
methylsilane as a colorless oil, which after dilithiation at low
temperature followed by reaction with chlorodiphenylphosphine
gave the product. The two compounds (3 and 4, “mcbiPSiH”
and “mctriPSiH”, respectively) are waxy semisolids, which were
•
•
•
CH2 , PPh2 , PPh2C6H4CH2 , and Si(CH2C6H4PPh2)2H•, respec-
tively. Thus for 2 the most abundant fragment is nominally an
o-silylbenzyl cation (although PPh2+ is also prominent, m/z 185,

1732 Inorganic Chemistry, Vol. 35, No. 6, 1996
Notes
•
arising from Me2SiHC6H4CH2 elimination). For mcbiPSiH (3),
Quadridentate coordination of the mctriPSi unit was antici-
pated5,6 and so was investigated around octahedral Ir(III).
Addition of compound 4 to a solution of the well-known labile
dimeric precursor [Ir(cod)Cl]2 led to isolation in virtually
quantitative yield of the pale yellow complex IrH(mctriPSi)Cl
(7: chloroform hemisolvate, NMR). The 31P NMR spectrum
loss of one phosphinoaryl “arm” gives rise to by far the most
important ion in the spectrum, while for mctriPSiH (4), the base
peak corresponds to an o-phosphinobenzyl ion although there
are several other major fragments, notably at m/z 197, 183, and
165. The latter are also present in the spectrum of 1, where
they may arise through loss of PhH (to m/z 256) and then
•
SiMe2H• or SiMe2HCH2 and where the base peak may arise
from PPh2 cleavage to give a reactive phenylcarbonium ion
•
that undergoes benzylic rearrangement involving CH2 loss to
C6H4CH2SiMeH+ (m/z 135). This provides the most obvious
analytical difference between the spectrum of 1 and that of 2:
m/z 149 which is missing completely for 1 is ca. 4 times as
intense as m/z 135 for 2. Interestingly PPh2C6H4CH2+, which
is the base peak for 4, is reduced for 3 and can only be detected
by using chemical ionization for 1.
Under conditions identical with those that lead7 to formation
of Pt(chel)2 from chelH (i.e. PPh2CH2CH2SiMe2H), reaction of
compound 1 with Pt(cod)Cl2 led to isolation of the colorless,
crystalline analogue cis-Pt(SiMe2CH2C6H4PPh2)2, 5 (cf. eq 1),
of this compound showed an A2B pattern (Table 1) that was
not exactly first-order but was recognizable as a distorted
doublet/triplet with a small (i.e. cis) JPP. Accordingly, the δ-
(IrH) (which at -9.5 ppm is in the range for H trans to P rather
than Cl) is split into a doublet of triplets by coupling to both
trans (2J 126 Hz) and cis (20 Hz) P atoms, so that the
stereochemistry is unambiguous with mctriPSi occupying four
all-cis sites and H, Cl trans to P, Si, respectively.
2
Discussion
The lithiation reactions described provide efficient steps for
synthesis of a new family of PSi ligand precursors6 in which
the connectivity between Si and P is made through a benzyl
framework. Polysubstitution at Si affords mcbiPSiH and
mctriPSiH (i.e. 3 and 4), which can lead Via hydrosilylation1-6
to polydentate complexation. The ability of silanes 1, 3, and 4
to undergo such chemistry (eq 1) has been demonstrated by
synthesis of model complexes of Pt(II) and Ir(III). PSi
coordination established in this way is clearly chelate, rather
than10 bridging; the NMR properties of compounds 5-7 render
the stereochemistry in each case unequivocal, with the arrange-
ments adopted by 5 and 7 resembling those reported earlier for
prototypal PSi analogues.5,7 We expect that the polydentate silyl
attachment at a transition metal center of mcbiPSi or mctriPSi
will present a more rigid ligand profile than is imparted by the
prototypal polymethylene systems5,6 biPSi and triPSi: constrain-
ing the carbon atoms â and γ (to Si) to planarity (sp2 character)
should drastically reduce the mobility of the connecting
backbone. The effect of such rigidity on substrate approach,
reactivity, and catalysis6 is under investigation.
for which the low value of 1JPtP ) 1459 Hz is fully consistent7
with P trans to Si. The cis stereochemistry was also obvious
from the 29Si spectrum, in which both trans (142 Hz) and cis
(22 Hz) coupling to P were resolved. Coupling of silylmethyl
hydrogens (3J) to 195Pt was also7 evident (13 Hz). A similar
reaction using mcbiPSiH (3) afforded a further white product
(as a dichloromethane hemisolvate, NMR) identified by mi-
croanalysis and NMR spectroscopy as another d8 Pt(II) complex
Pt(mcbiPSi)Cl, 6. The value for 1JPtP of 2908 Hz is typical for
phosphorus nuclei trans to one another. The benzyl CH2 proton
resonances are also structurally informative for compounds 5
and 6: in the former (δ 1.88, Table 1) coupling to Pt and to
Acknowledgment. We thank the NSERC of Canada for
funding.
3
trans P are obvious, while for the mcbiPSi complex JPtH is
nearly twice as large (i.e. disposed trans to the weak ligand Cl
Vs P in 5) whereas cis coupling to P is small and is not detected.
IC941230F