Silylene extrusion from a silane: Direct conversion of Mes2SiH2 to an iridium silylene dihydride [1]

Full text of DOI:10.1021/ja992367d

J. Am. Chem. Soc. 1999, 121, 9871-9872
9871
Communications to the Editor
Scheme 1
Silylene Extrusion from a Silane: Direct Conversion
of Mes2SiH2 to an Iridium Silylene Dihydride
Jonas C. Peters,† Jay D. Feldman, and T. Don Tilley*
Department of Chemistry, UniVersity of California at Berkeley,
Berkeley, California 94720-1460
ReceiVed July 8, 1999
The activation of silanes by transition metals is a key
component of many catalytic reactions. These activations are
generally viewed as involving the oxidative addition of H-SiR3
bonds to a metal center, but it is often unclear to what extent
further rearrangements of the resulting metal silyl derivative may
contribute to observed transformations.1 Considerable speculation
has focused on the possibility of 1,2-migrations between the metal
center and silicon, which could produce reactive transition metal
silylene derivatives of the type LnM)SiR2.1,2 To better understand
the potential role of silylene complexes in catalytic processes,
we have developed syntheses of such species3 and have recently
reported the first direct observations of 1,2-shifts that convert a
metal silyl derivative M-SiHR2 to silylene hydrides (H)M)SiR2.
These processes were found to occur in cationic, 3-coordinate
platinum silyl complexes for which the 1,2-migration leads to
the formation of a more stable, square planar complex (eq 1).4 In
complex. Herein we introduce a new, anionic, tripodal phosphine
ligand, [PhB(CH2PPh2)3]-, and demonstrate its utility in the facile
preparation of a neutral iridium silylene complex, [PhB(CH2-
PPh2)3](H)2Ir)SiMes2 (3), directly from dimesitylsilane (H2-
SiMes2; Mes ) 2,4,6-Me3C6H2). The formation of a related
cationic species, [(PMe3)3(H)2Ir)SiMes2][MeB(C6F5)3] (5), pro-
vides insight into the mechanism of this silane activation.
The key starting iridium complex was obtained by reaction of
[Li(TMED)][PhB(CH2PPh2)3] (1), readily prepared by addition
of 3 equiv of [Li(TMED)][CH2PPh2] to dichlorophenylborane,5
with [(COE)2IrCl]2.6 As shown in Scheme 1 this reaction gave
[PhB(CH2PPh2)3]Ir(H)(η3-C8H13) (2), the product of C-H activa-
tion in a cyclooctene ligand, obtained as white crystals in 65%
yield. A related process has been observed in the reaction of
[(COE)2IrCl]2 with KTp (Tp ) tris(pyrazolyl)hydroborate) to give
the vinyl hydride TpIrH(COE)(C8H13), which on warming con-
verts to TpIr(H)(η3-C8H13) with loss of cyclooctene.7 The η3-
binding mode of the cyclooctenyl ligand of 2 was confirmed by
X-ray crystallography (Scheme 1). The hydride ligand was not
located by X-ray diffraction; however its presence is confirmed
(1)
an attempt to extend this concept to the highly common case
involving octahedral coordination, we have targeted generation
of a 5-coordinate Ir-SiHR2 complex, which might be expected
to spontaneously rearrange to a d6 octahedral (H)Ir)SiR2 silylene
† Current address: Division of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, CA 91125.
(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) Pannell, K. H.; Sharma, H.
K. Chem. ReV. 1995, 95, 1351. (e) Corey, J. Y.; Braddock-Wilking, J. Chem.
ReV. 1999, 99, 175.
(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) Tobita, H.; Ueno, K.;
Ogino, H. Bull. Chem. Soc. Jpn. 1988, 61, 2797. (d) Ueno, K.; Tobita, H.;
Ogino, H. Chem. Lett. 1990, 369. (e) 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. (f) Reichl, J. A.; Popoff, C. M.; Gallagher, L. A.;
Remsen, E. E.; Berry, D. H. J. Am. Chem. Soc. 1996, 118, 9430. (g) Seyferth,
D.; Shannon, M. L.; Vick, S. C.; Lim, T. F. O. Organometallics 1985, 4, 57.
(h) Tanaka, Y.; Yamashita, H.; Tanaka, M. Organometallics 1995, 14, 530.
(i) Mitchell, G. P.; Tilley, T. D.; Yap, G. P. A.; Rheingold, A. L.
Organometallics 1995, 14, 5472. (j) Mitchell, G. P.; Tilley, T. D. Organo-
metallics 1996, 15, 3477.
1
2
by its H NMR resonance appearing at -12.6 ppm (dt, JHPtrans
2
) 150 Hz; JHPcis ) 14.3 Hz). Notably, 2 exhibits an elongated
Ir-P bond (2.408(2) Å for Ir-P(1); compare 2.304(2) and 2.311-
(2) Å for Ir-P(2) and Ir-P(3), respectively), presumably due to
the trans influence of the hydride ligand.
Addition of H2SiMes2 to 2 in benzene afforded a clear, colorless
solution at ambient temperature, which turned bright yellow on
warming to 95 °C. Within 24 h, quantitative conversion to the
silylene complex [PhB(CH2PPh2)3](H)2Ir)SiMes2 (3) was ob-
(3) (a) Feldman, J.; Mitchell, G. P.; Nolte, J.-O.; Tilley, T. D. J. Am. Chem.
Soc. 1998, 120, 11184. (b) Grumbine, S. K.; Mitchell, G. P.; Straus, D. A.;
Tilley, T. D.; Rheingold, A. L. Organometallics 1998, 17, 5607, and references
in the above.
1
served (by NMR spectroscopy in benzene-d6). The H NMR
spectrum of 3 exhibits two distinct methylene resonances in a
2:1 ratio, a single set of resonances corresponding to the mesityl
groups, and a hydride signal at δ -9.47 (JHPtrans ) 70 Hz).
(4) (a) Mitchell, G. P.; Tilley, T. D. J. Am. Chem. Soc. 1998, 120, 7635.
(b) Mitchell, G. P.; Tilley, T. D. Angew. Chem., Int. Ed. Engl. 1998, 37, 2524.
(5) See Supporting Information for experimental details. This method is
analogous to that reported by Riordan for the synthesis of related
[PhB(CH2SR)3]- reagents. See, for example: Schebler, P. J.; Riordan, C. G.;
Guzei, I. A.; Rheingold, A. L. Inorg. Chem. 1998, 37, 4754. Also, the
[PhB(CH2PPh2)3]- ligand has recently been employed by D. Nocera and co-
workers (personal communication).
(7) (a) Ferna´ndez, M. J.; Rodriguez, M. J.; Oro, L. A.; Lahoz, F. J. J. Chem.
Soc., Dalton Trans. 1989, 2073. (b) Tanke, R. S.; Crabtree, R. H. Inorg. Chem.
1989, 28, 3444. (c) Alvarado, Y.; Boutry, O.; Gutierrez, E.; Monge, A.;
Nicasio, M. C.; Poveda, M. L.; Perez, P. J.; Ruiz, C.; Bianchini, C.; Carmona,
E. Chem. Eur. J. 1997 3, 860.
(6) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15, 18.
10.1021/ja992367d CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/08/1999

9872 J. Am. Chem. Soc., Vol. 121, No. 42, 1999
Communications to the Editor
Consistent with the structure shown in Scheme 1, the 31P NMR
spectrum exhibits a doublet and a triplet in a 2:1 ratio (JPP ) 18
Hz). The silylene ligand gives rise to a doublet of triplets in the
29Si NMR spectrum (δ ) 241.2), presumably resulting from strong
coupling to a trans phosphine group (172.4 Hz) and weaker
coupling to the cis phosphines (4 Hz).
The P-Ir-P angles in 3 are close to 90° (86°-91°), and
although the hydride ligands were not located during refinement,
their positions are indicated by the inequivalent P-Ir-Si angles
which vary from 112.3° to 141.5°. Thus, it seems that the
molecular structure is best described as a highly distorted
octahedron, as might be expected for a complex with ligands of
vastly different steric properties. The markedly short Ir(1)-Si(1)
bond length of 2.260(3) Å suggests multiple bond character, as
most structurally characterized iridium silyl complexes exhibit
longer Ir-Si bonds.1e For example, a highly related silyl complex
with an sp3 silicon center, fac-(Me3P)3(H)2IrSiHPh2, possesses an
Ir-Si bond length of 2.361(3) Å.8 The sp2 Si center sits in a plane
containing its substituent Ir and carbon atoms, and the sum of
the angles about Si is 359° (these angles vary considerably:
C-Si-C 103.5(4)°; Ir-Si-C 122.5(3), 133.0(3)°).
Although the detailed mechanism of the conversion of 2 to 3
is yet to be established, we assume that H2SiMes2 oxidatively
adds to iridium to produce a transient Ir-SiHMes2 species that
rapidly undergoes a 1,2-hydrogen migration to give the final
silylene product. Results that are consistent with this hypothesis
were found in a related iridium system. Addition of H2SiMes2 to
(Me3P)4IrMe9 afforded the neutral iridium(III) complex fac-
(Me3P)3Ir(Me)(H)(SiHMes2) (4). An attempt to generate the
cationic 5-coordinate complex [(Me3P)3Ir(H)(SiHMes2)]+, by
abstraction of the methyl ligand of 4 by B(C6F5)3, led directly to
the silylene complex [(Me3P)3(H)2Ir)SiMes2][MeB(C6F5)3] (5),
isolated as a yellow solid (eq 2). By 1H and 31P NMR
spectroscopy, this reaction is quantitative. The 29Si NMR reso-
nance of δ 254.1 (JSiPtrans ) 168 Hz; unresolved cis-phosphine
coupling) is in the region expected for a silylene ligand3,4 and is
300.1 ppm downfield from the shift for the silyl precursor 4 (δ
-46.7 ppm). The similarity between the 29Si NMR chemical shifts
for 5 and 3 lends considerable support to the formulation of 5 as
a discreet salt, rather than an ion-paired, donor-stabilized silylene
complex.
In conclusion, the activation of both Si-H bonds in a secondary
silane has provided the first synthetic pathway to iridium silylene
complexes.10 This process is related to the conversion of a
saturated carbon center to a carbene ligand in the activation of
tBu2P(CH2)5PtBu2 by iridium.11 Also, Banaszak Holl has reported
the reaction of H2Ge[N(SiMe3)2]2 with a Pt(0) complex, which
proceeds via H2 elimination to give a germylene complex.12 The
process reported here, by which a silane directly delivers a silylene
fragment to a transition metal center, is particularly relevant to
postulated mechanisms for the metal-catalyzed dehydrogenative
coupling of silanes to polysilanes.1c Observation of this chemistry
was facilitated by the use of a novel, tris(phosphino)borate ligand,
which was used to obtain a neutral silylene complex. The
+
zwitterionic nature of 3 allows the study of an L3H2Ir)SiR2
fragment as a neutral species, soluble in a wide range of
hydrocarbons and therefore amenable to mechanistic investiga-
tions. Preliminary results indicate that 3 extrudes silylene frag-
ments from a variety of silanes, and this chemistry is currently
under investigation.
Acknowledgment. Acknowledgment is made to the National Science
Foundation for their generous support of this work and to Dr. Fred
Hollander for determination of the crystal structures. J.C.P. thanks the
Miller Institute, University of California, for a postdoctoral fellowship.
Supporting Information Available: Detailed experimental proce-
dures for the preparation and spectroscopic characterization of complexes
1-5, tables of crystal, data collection, refinement parameters, atomic
coordinates, bond distances, bond angles, and anisotropic displacement
parameters for 2 and 3. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA992367D
(8) Zarate, E. A.; Kennedy, V. O.; McCune, J. A.; Simons, R. S.; Tessier,
C. A. Organometallics 1995, 14, 1802.
(9) Thorn, D. L. Organometallics 1982, 1, 197.
(10) A related iridium silylene complex, [Cp*(Me3P)(H)Ir)SiMes2][OTf],
has been obtained by a similar process: Klei, S. R.; Tilley, T. D.; Bergman,
R. G., submitted for publication.
(2)
(11) Empsall, H. D.; Hyde, E. M.; Markham, R.; McDonald, W. S.; Norton,
M. C.; Shaw, B. L.; Weeks, B. J. Chem. Soc., Chem. Commun. 1977, 589.
(12) Litz, K. E.; Bender, J. E.; Kampf, J. W.; Banaszak Holl, M. M. Angew.
Chem., Int. Ed. Engl. 1997, 36, 496.