Isolation and characterization of neutral platinum silylene complexes of the type (R3P)2Pt=SiMes2 (Mes = 2,4,6-trimethylphenyl) [2]

Full text of DOI:10.1021/ja981882t

11184
J. Am. Chem. Soc. 1998, 120, 11184-11185
complex (Cy3P)2PtdSiMes2 (1; Scheme 1). After workup, this
complex was isolated in 54% yield as green microcrystals.
Similarly, red solutions of (iPr3P)2PtdSiMes2 (2) were quantita-
tively produced from Pt(PiPr3)37 and Mes2Si(SiMe3)2 in benzene-
Isolation and Characterization of Neutral Platinum
Silylene Complexes of the Type (R3P)2PtdSiMes2
(Mes ) 2,4,6-Trimethylphenyl)
1
d6 (by H and 31P NMR spectroscopy), but removal of solvent
Jay D. Feldman, Gregory P. Mitchell, Jo¨rn-Oliver Nolte, and
T. Don Tilley*
resulted in complete conversion back to Pt(PiPr3)3, along with a
variety of unidentified silicon-containing products. Complex 1
is stable indefinitely in the solid state, but decomposes in solution
over several days to Pt(PCy3)2 and uncharacterized silicon-
containing compounds. For sparingly soluble 1, a long accumula-
tion time was required to observe the 29Si NMR resonance (δ
358; JSiP ) 112 Hz), which confirmed the presence of the silylene
ligand.3 For the soluble complex 2 the 29Si NMR signal was more
readily observed, at δ 367 (JSiP ) 107 Hz; JSiPt ) 2973 Hz). For
comparison, the only other platinum silylene complexes to be
reported have 29Si shifts of δ 309, for [trans-(Cy3P)2(H)PtdSi-
Department of Chemistry
UniVersity of California at Berkeley
Berkeley, California 94720-1460
ReceiVed June 1, 1998
Unlike the analogous transition-metal carbene complexes,
silylene complexes (LnMdSiR2) are extremely rare and poorly
understood.1 Nonetheless, such species have generated consider-
able interest since they appear to be involved in numerous
transformations of organosilicon compounds.2 For this reason,
many synthetic efforts have targeted the isolation and study of
complexes featuring silylene ligands. In recent years, these efforts
have culminated in the syntheses of a number of transition metal
compounds containing sp2-hybridized silicon.3 For the most part
these compounds are cationic, having been obtained via an “anion-
abstraction” method.3a,4 Even among the limited number of
silylene complexes known, a rich variety of bonding modes and
reactivities have been observed. Of particular interest are neutral
silylene complexes of the type (R3P)2MdSiR2 (M ) Pd, Pt),
which have historically been the primary focus of speculation
regarding catalytic silylene intermediates.5 In this communication,
we describe isolation of the first neutral silylene complexes of
this type and initial reactivity studies with them.
(SEt)2][BPh4],3c and
δ
338, for [(iPr2PCH2CH2PiPr2)(H)-
PtdSiMes2][MeB(C6F5)3].8
X-ray quality crystals of 1 were grown from dilute (<0.04 M)
reaction solutions. The molecular structure is shown in Scheme
1. The Pt-Si bond distance of 2.210(2) Å is the shortest yet
reported, and is about 6% shorter than typical Pt-Si single bonds
(2.30-2.40 Å).1a,b For comparison, the Pt-Si distance in [trans-
(Cy3P)2(H)PtSi(SEt)2][BPh4] is 2.270(2) Å, but note that the latter
complex was characterized as being “Fischer-like”, with signifi-
cant Si-S, but very little Pt-Si, π-bonding.3c The summation
of angles about Si, 359.8(6)°, confirms the presence of a planar,
sp2-hybridized silicon atom, and the angles about platinum also
sum within experimental error to 360°. The least-squares plane
of the silylene ligand (including Pt, Si, and the two ipso carbons)
and the coordination plane of platinum (Si, Pt, P, P) intersect at
a dihedral angle of 68.6°, which is not optimal for Pt-to-Si
π-donation. This deviation appears to result from the sterically
hindered environment about the Pt-Si bond, as the expected
dihedral angle of 90° is observed in structures of the related
6
7
Photolysis of a mixture of Mes2Si(SiMe3)2 and Pt(PCy3)2 in
hexanes resulted in a green, supersaturated solution of the silylene
(1) (a) Tilley, T. D. In The Silicon-Heteroatom Bond; Patai, S.; Rappoport,
Z., Eds.; Wiley: New York, 1991; Chapters 9 and 10, pp 245 and 309. (b)
Tilley, T. D. In The Chemistry of Organic Silicon Compounds; Patai, S.,
Rappoport, Z., Eds.; Wiley: New York, 1989; Chapter 24, p 1415. (c) Tilley,
T. D. Comments Inorg. Chem. 1990, 10, 37. (d) Lickiss, P. D. Chem. Soc.
ReV. 1992, 271. (e) Pannell, K. H.; Sharma, H. K. Chem. ReV. 1995, 95, 1351.
(f) Zybill, C. Top. Curr. Chem. 1991, 160, 1.
9
germylene and stannylene complexes (Et3P)2PtGe[N(SiMe3)2]2
and (iPr2PCH2CH2PiPr2)PdSn[CH(SiMe3)2]2,10 respectively.
Initial reactivity studies show that 1 quantitatively transfers its
silylene group to water and alcohols via insertion into O-H bonds
(Scheme 1). Thus, addition of ROH (R ) H, Me, Et) to 1 resulted
in the clean formation of Pt(PCy3)2 and the corresponding silane
Mes2Si(OR)H. No intermediates were observed while monitoring
(2) (a) Pannell, K. H.; Cervantes, J.; Hernandez, C.; Cassias, J.; Vincenti,
S. Organometallics 1986, 5, 1056. (b) Pannell, K. H.; Rozell, J. M., Jr.;
Hernandez, C. J. Am. Chem. Soc. 1989, 111, 4482. (c) Pannell, K. H.; Wang,
L.-J.; Rozell, J. M. Organometallics 1989, 8, 550. (d) Pannell, K. H.; Sharma,
H. Organometallics 1991, 10, 954. (e) Jones, K. L.; Pannell, K. H. J. Am.
Chem. Soc. 1993, 115, 11336. (f) Hernandez, C.; Sharma, H. K.; Pannell, K.
H. J. Organomet. Chem. 1993, 462, 259. (g) Pannell, K. H.; Brun, M.-C.;
Sharma, H.; Jones, K.; Sharma, S. Organometallics 1994, 13, 1075. (h) Pannell,
K. H.; Sharma, H. K.; Kapoor, R. N.; Cervantes-Lee, F. J. Am. Chem. Soc.
1997, 119, 9315. (i) Tobita, H.; Ueno, K.; Ogino, H. Bull. Chem. Soc. Jpn.
1988, 61, 2797. (j) Ueno, K.; Tobita, H.; Ogino, H. Chem. Lett. 1990, 369.
(k) Ueno, K.; Kakano, K.; Ogino, H. Chem. Lett. 1996, 459. (l) Okazaki, M.;
Tobita, H.; Ogino, H. J. Chem. Soc., Dalton Trans. 1997, 3531. (m) Haynes,
A.; George, M. W.; Haward, M. T.; Poliakoff, M.; Turner, J. J.; Boag, N. M.;
Green, M. J. Am. Chem. Soc. 1991, 113, 2011. (n) Nlate, S.; Herdtweck, E.;
Fischer, R. A. Angew. Chem., Int. Ed. Engl. 1996, 35, 186. (o) Pestana, D.
C.; Koloski, T. S.; Berry, D. H. Organometallics 1994, 13, 4173. (p) Mitchell,
G. P.; Tilley, T. D.; Yap, G. P. A.; Rheingold, A. L. Organometallics 1995,
14, 5472. (q) Mitchell, G. P.; Tilley, T. D. Organometallics 1996, 15, 3477.
(r) Clarke, M. P.; Davidson, I. M. T. J. Organomet. Chem. 1991, 408, 149.
(3) Silylene complexes with sp2 silicon atoms: (a) Straus, D. A.; Grumbine,
S. D.; Tilley, T. D. J. Am. Chem. Soc. 1990, 112, 7801. (b) Grumbine, S. D.;
Tilley, T. D.; Rheingold, A. L. J. Am. Chem. Soc. 1993, 115, 358. (c)
Grumbine, S. D.; Tilley, T. D.; Arnold, F. P.; Rheingold, A. L. J. Am. Chem.
Soc. 1993, 115, 7884. (d) Grumbine, S. K.; Tilley, T. D.; Arnold, F. P.;
Rheingold, A. L. J. Am. Chem. Soc. 1994, 116, 5495. (e) Denk, M.; Hayashi,
R. K.; West, R. J. Chem. Soc., Chem. Commun. 1994, 33.
1
the reaction of 1 with ethanol at -60 °C (by H and 31P NMR
spectroscopy in toluene-d8). Related reactivity was previously
reported for the base-stabilized silylene complex [Cp*(Me3P)2-
RuSiPh2(NCMe)]+, which was presumed to dissociate acetonitrile
prior to reaction with an alcohol.11 This reactivity contrasts with
what has been observed for (iPr2PCH2CH2PiPr2)PdSn[CH-
(SiMe3)2]2, which reversibly adds H2O across the Pd-Sn bond
to form a palladium hydride species.12 Reactions of 1 with various
phosphines (iPr2PCH2CH2PiPr2, Ph2PCH2CH2PPh2, and PMe3, in
excess) led to PCy3- and SiMes2-displacement, with formation
of the corresponding PtL4 complex (quantitative by 31P NMR
spectroscopy). In these processes, the displaced silylene ligand
is observed to decompose to a number of species (but not
significant amounts of Mes2SidSiMes2).
(6) Fink, M. J.; Michalczyk, M. J.; Haller, K. J.; West, R.; Michl, J.
Organometallics 1984, 3, 793.
(7) (a) Yoshida, T.; Otsuka, S. Inorg. Synth. 1990, 28, 113. (b) Yoshida,
T.; Matsuda, T.; Otsuka, S. Inorg. Synth. 1990, 28, 119.
(8) Mitchell, G. P.; Tilley, T. D. Angew. Chem. Intl. Ed. In press.
(9) Litz, K. E.; Henderson, K.; Gourley, R. W.; Banaszak Holl, M. M.
Organometallics 1995, 14, 5008.
(10) Krause, J.; Haack, K.-J.; Po¨rschke, K.-R.; Gabor, B.; Goddard, R.;
Pluta, C.; Seevogel, K. J. Am. Chem. Soc. 1996, 118, 804.
(11) Zhang, C.; Grumbine, S. D.; Tilley, T. D. Polyhedron 1991, 10, 1173.
(12) Schager, F.; Seevogel, K.; Po¨rschke, K.-R.; Kessler, M.; Kru¨ger, C.
J. Am. Chem. Soc. 1996, 118, 13075.
(4) Grumbine, S. K.; Straus, D. A.; Tilley, T. D.; Rheingold, A. L.
Polyhedron 1995, 14, 127.
(5) L2MdSiR2 intermediates: (a) Yamamoto, K.; Okinoshima, H.; Kumada,
M. J. Organomet. Chem. 1971, 27, C31. (b) Yamashita, H.; Tanaka, M.; Goto,
M. Organometallics 1992, 11, 3227. (c) Seyferth, D.; Shannon, M. L.; Vick,
S. C.; Lim, T. F. O. Organometallics 1985, 4, 57. (d) Tanaka, Y.; Yamashita,
H.; Tanaka, M. Organometallics 1995, 14, 530. (e) Tamao, K.; Sun, G.-R.;
Kawachi, A. J. Am. Chem. Soc. 1995, 117, 8043. (f) Palmer, W. S.; Woerpel,
K. A. Organometallics 1997, 16, 4824.
10.1021/ja981882t CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/16/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 43, 1998 11185
Scheme 1
Hydrogen (1 atm) reacts with a benzene solution of 1 over 2
days to form trans-(Cy3P)2Pt(H)(SiHMes2) (3, Scheme 1; by H
elimination-oxidative addition couple could also account for this
observation. An intermolecular mechanism was suggested by the
1
and 31P NMR spectroscopy). With this slow conversion there is
competing decomposition of 1 via loss of the silylene ligand to
produce trans-(Cy3P)2PtH2 (ca. 50%; without formation of Mes2-
SiH2). At higher temperatures (refluxing benzene-d6), 3 is
hydrogenated to trans-(Cy3P)2PtH2 and Mes2SiH2. In contrast
to the germylene complex (Et3P)2PtGe[N(SiMe3)2]2,13 the addition
of hydrogen to 1 is irreversible. Complex 3 is fluxional in solution
as observed by variable-temperature 31P NMR spectroscopy. At
-29 °C, two doublets (δ 26.9, JPtP ) 2632 Hz, JPP ) 346 Hz; δ
33.6, JPtP ) 2715 Hz) are observed which, upon heating, broaden
and coalesce to a singlet (δ 28.9, JPtP ) 2707 Hz) at 91 °C. The
1H NMR EXSY spectrum, which contains cross-peaks corre-
sponding to the exchange of 195PtH hydrogens with complexes
of 4 containing other Pt isotopes. More significantly, EXSY
experiments revealed the exchange of PtH hydrogens with the
SiH hydrogens of free H2SiMes2 (25 equiv), also with a rate
constant of 23(4) Hz. In addition, 31P EXSY NMR experiments
provided a similar rate for phosphorus exchange. This exchange
process was shown to be intramolecular based on the absence of
cross-peaks between phosphorus signals in 4 and free PCy3 in
the 31P EXSY spectra. Thus, hydrogen exchange in cis-(Cy3P)2-
Pt(H)(SiHMes2) occurs via the rate-determining reductive elimi-
nation of silane, which does not require prior phosphine disso-
ciation. These results are consistent with our recent observation
that hydrogen appears not to migrate to platinum in 4-coordinate
(iPr2PCH2CH2PiPr2)Pt(Me)SiHMes2, whereas the 3-coordinate
cation [(iPr2PCH2CH2PiPr2)PtSiHMes2]+ rapidly converts to
[(iPr2PCH2CH2PiPr2)(H)PtdSiMes2]+.8
1
PtH and SiH H NMR shifts are also temperature dependent;
however, coalescence is obscured by the Cy resonances. This
behavior is consistent with restricted rotation about the Pt-Si
bond (∆Gq ) 15.8(4) kcal/mol). The mechanism of hydrogen
addition to 1 could involve oxidative addition to the platinum
center, followed by migration of hydride to the silylene ligand,
or direct addition of hydrogen to the PtdSi double bond. At this
point the mechanism is unknown, but it is interesting to note that
whereas the trans silyl hydride is formed by this reaction, the
oxidative addition of Mes2SiH2 to Pt(PCy3)2 exclusively gives
cis-(Cy3P)2Pt(H)(SiHMes2) (4). The cis-complex 4 isomerizes
slowly to 3 at 90 °C, where its decomposition is more rapid
(t1/2 ) ca. 45 min).
Acknowledgment is made to the National Science Foundation for their
Interestingly, the PtH and SiH hydrogens of 4 undergo rapid
exchange with a rate constant of 23(4) Hz (27 °C), as determined
by H NMR EXSY experiments. Given the stability of silylene
ligands in this system, it seemed possible that this exchange could
occur via an R-H migration process (eq 1). However, a reductive
generous support of this work.
Supporting Information Available: Synthetic details and charac-
terization data for new compounds; crystal, data collection, and refinement
parameters, bond distances and angles, and anisotropic displacement
parameters for 1 (16 pages, print/PDF). See any current masthead page
for ordering information and Web access instructions.
1
(13) Litz, K. E.; Bender, J. E., IV; Kampf, J. W.; Banaszak Holl, M. M.
Angew. Chem., Int. Ed. Engl. 1997, 36, 496.
JA981882T