High oxidation-state (Formally d0) tungsten silylene complexes via double Si-H bond activation [15]

Full text of DOI:10.1021/ja0165436

9702
J. Am. Chem. Soc. 2001, 123, 9702-9703
High Oxidation-State (Formally d0) Tungsten Silylene
Complexes via Double Si-H Bond Activation
Scheme 1. Syntheses of Tucked Cp* Complex 1 and Doubly
Tucked Cation 2
Benjamin V. Mork and T. Don Tilley*
Department of Chemistry
UniVersity of California, Berkeley
Berkeley, California 94720-1460
ReceiVed July 3, 2001
Transition metal silylene complexes are of interest as silicon
analogues of metal carbenes, and as possible intermediates in a
number of metal-catalyzed transformations involving organosili-
con compounds.1 In metal carbene chemistry, it is well established
that the reactivity and electronic structure of the CR2 ligand (R
) H, alkyl, or aryl) are highly dependent on the nature of the
metal fragment to which it is bound.2 Late transition metal carbene
complexes are typically electrophilic at carbon, while early metal
centers often give rise to nucleophilic carbene ligands. To date,
a number of base-free transition metal dialkyl and diaryl silylene
complexes have been synthesized, and these feature ruthenium,3
osmium,4 iridium,5,6 and platinum7,8 in relatively high dn con-
figurations (n g 6). In general, these late metal silylene complexes
have been shown to be very electrophilic at silicon. Thus, it is of
interest to investigate silylene complexes involving strongly
π-donating early transition metal centers with low dn configura-
tions. Such species may exhibit novel chemical properties for the
metal silylene fragment, opening pathways to new transformations
for organosilicon compounds. Our efforts to synthesize base-free
silylene complexes of the more electropositive early transition
metals initially focused on the group 6 metals, and herein we
report base-free tungsten silylene complexes of the type [Cp*-
(dmpe)(H)2WdSiR2][B(C6F5)4], (Cp* ) η5-C5Me5, dmpe ) Me2-
PCH2CH2PMe2). These compounds were prepared with use of a
new, reactive tungsten-based synthon featuring a doubly metalated
Cp* ligand. Previously, base-stabilized silylene complexes of the
Cp′W(CO)2 fragment (Cp′ ) C5H5, C5Me5) have been reported.9,10
We have established routes to platinum and iridium silylene
complexes involving activation of two silicon-hydrogen bonds
of a hydrosilane, via oxidative addition and subsequent R-elimina-
tion.5,8 Significantly, this route requires access to coordinatively
unsaturated metal centers (eq 1).
Given the promise of this silylene-extrusion reaction as a general
route to metal silylene complexes, we sought to synthesize d0
examples via reactions of hydrosilanes with coordinatively
unsaturated but inherently electron-rich fragments of the type
[Cp*L2W]+.11 A possible precursor to such a fragment is Cp*-
(Me3P)(η2-Me2PCH2)W(H)Cl, which appears to be in equilibrium
with Cp*(Me3P)2WCl.12 In an attempt to develop a route to the
[Cp*(dmpe)W]+ fragment, Cp*WCl413 was reduced with 3.0 equiv
of Na/Hg (0.8% w/w) in the presence of dmpe. This procedure
afforded crystalline orange-red (η6-C5Me4CH2)(dmpe)W(H)Cl (1)-
in 75% yield after recrystallization from pentane (Scheme 1).
Thus, unlike Cp*(Me3P)2WCl, which undergoes intramolecular
C-H activation of a phosphine ligand, 16-electron Cp*(dmpe)-
WCl degrades via metalation of the Cp* ligand.14,16 The NMR
spectra of 1 reflect the absence of mirror or rotational symmetry.
For example, the four methyl groups of the dmpe ligand give
rise to individual doublet resonances at δ 0.98, 1.23, 1.29, and
1.53 in the 1H NMR spectrum. Compound 1 was also character-
ized by X-ray crystallography.15
Reaction of 1 with [Li(OEt2)2.5][B(C6F5)4] in fluorobenzene
under argon gives [(η7-C5Me3(CH2)2)(dmpe)W(H)2][B(C6F5)4], 2,
(Scheme 1), a rare example of a “doubly tucked” Cp* metal
LnM + R2SiH2 f Ln(H)2MdSiR2
(1)
(1) (a) Tilley, T. D. In The Silicon-Heteroatom Bond; Patai, S., Rappoport,
Z., Eds.; Wiley: New York, 1991, pp 245. (b) Tilley, T. D. In The Chemistry
of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New
York, 1989, p 1415. (c) Tilley, T. D. Comments Inorg. Chem. 1990, 10, 37.
(d) Zhang, Z. Y.; Sanchez, R.; Pannell, K. H. Organometallics 1995, 14, 2605.
(e) Sharma, H. K.; Pannell, K. H. Chem. ReV. 1995, 95, 1351. (f) Pannell, K.
H.; Cervantes, J.; Hernandez, C.; Cassias, J.; Vincenti, S. Organometallics
1986, 5, 1056. (g) Pannell, K. H.; Rozell, J. M.; Hernandez, C. J. Am. Chem.
Soc. 1989, 111, 4482. (h) Pannell, K. H.; Wang, L. J.; Rozell, J. M.
Organometallics 1989, 8, 550. (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) 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. (l)
Tanaka, Y.; Yamashita, H.; Tanaka, M. Organometallics 1995, 14, 530. (m)
Mitchell, G. P.; Tilley, T. D.; Yap, G. P. A.; Rheingold, A. L. Organometallics
1995, 14, 5472. (n) Mitchell, G. P.; Tilley, T. D. Organometallics 1996, 15,
3477. (o) Tamao, K.; Sun, G. R.; Kawachi, A. J. Am. Chem. Soc. 1995, 117,
8043.
(2) (a) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 2nd ed.; Wiley Interscience: New York, 1994. (b) Collman, J. P.;
Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of
Organotransition Metal Chemistry, 2nd ed.; University Science Books: Mill
Valley, California, 1987. (c) Nugent, W. A.; Mayer, J. M. Metal-Ligand
Multiple Bonds; Wiley Interscience: New York, 1988.
(3) Grumbine, S. K.; Mitchell, G. P.; Straus, D. A.; Tilley, T. D.; Rheingold,
A. L. Organometallics 1998, 17, 5607.
(4) Wanandi, P. W.; Glaser, P. B.; Tilley, T. D. J. Am. Chem. Soc. 2000,
122, 972.
(5) Peters, J. C.; Feldman, J. D.; Tilley, T. D. J. Am. Chem. Soc. 1999,
121, 9871.
(6) Klei, S. R.; Tilley, T. D.; Bergman, R. G. J. Am. Chem. Soc. 2000,
122, 1816.
(7) Feldman, J. D.; Mitchell, G. P.; Nolte, J. O.; Tilley, T. D. J. Am. Chem.
Soc. 1998, 120, 11184.
(8) Mitchell, G. P.; Tilley, T. D. Angew. Chem. Int. Ed. 1998, 37, 2524.
(9) Ueno, K.; Sakai, M.; Ogino, H. Organometallics 1998, 17, 2138.
(10) Sakaba, H.; Tsukamoto, M.; Hirata, T.; Kabuto, C.; Horino, H. J. Am.
Chem. Soc. 2000, 122, 11511.
(11) In this discussion, a d0 silylene complex is one in which invocation
of two metal-based electrons in bonding to the silylene ligand results in a d0
electron configuration. Of course, it should also be possible to obtain silylene
complexes in which the metal atom uses all of its valence electrons in bonding
to other ligands (as with the novel silylenes that are stable as free species and
act as primarily σ-donors).
(12) Baker, R. T.; Calabrese, J. C.; Harlow, R. L.; Williams, I. D.
Organometallics 1993, 12, 830.
(13) Murray, R. C.; Blum, L.; Liu, A. H.; Schrock, R. R. Organometallics
1985, 4, 953.
(14) Examples of metallated Cp* complexes: (a) Bercaw, J. E. J. Am.
Chem. Soc. 1974, 96, 5087. (b) Bulls, A. R.; Schaefer, W. P.; Serfas, M.;
Bercaw, J. E. Organometallics 1987, 6, 1219. (c) Schock, L. E.; Brock, C.
P.; Marks, T. J. Organometallics 1987, 6, 232.
(15) See Supporting Information for details of X-ray crystallographic
characterization.
(16) Cloke, F. G. N.; Green, J. C.; Green, M. L. H.; Morley, C. P. J. Chem.
Soc., Chem. Commun. 1985, 14, 945.
10.1021/ja0165436 CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/01/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 39, 2001 9703
dihydride complex.16,17 Compound 2 can be generated and used
in situ or isolated as an orange crystalline solid in 70% yield by
crystallization from fluorobenzene/pentane. Consistent with the
structure shown in Scheme 1, the 1H NMR spectrum of 2 contains
two resonances assigned to the Cp* methyl groups, at δ 1.68 and
1.85. Two singlets are observed for the diastereotopic hydrogens
of the two equivalent methylene groups at δ 2.64 and 1.77, and
tungsten hydride ligands are observed as a doublet of doublets at
δ -2.11 (JHP ) 38 Hz, JHP′ ) 58 Hz, JHW ) 49 Hz). Complex 2
was also characterized crystallographically, and an ORTEP
diagram of the cation is displayed in Scheme 1. This species
appeared to be a potential source of the [Cp*(dmpe)W]+ fragment,
providing the facile reductive elimination of two C-H bonds.
At room temperature in fluorobenzene, 2 reacts with dimeth-
ylsilane over 3 days under argon to form the silylene complex
[Cp*(dmpe)(H)2WdSiMe2][B(C6F5)4] (3a), isolated in 65% yield
after crystallization from fluorobenzene/pentane (eq 2). The
Figure 1. ORTEP diagram of one of the crystallographically independent
cations in the X-ray structure of dimethylsilylene complex 3a. Hydrogen
atoms were omitted for clarity.
suggest classical dihydride character, rather than the presence of
a dihydrogen ligand in these complexes.18 To investigate the
possibility of W(H)‚‚‚Si interactions in these compounds, the 2JHSi
1
coupling constants were determined from H-detected, 1D 29Si
HMQC BIRD filtered NMR experiments.19 Since these coupling
constants (7, 17, and 17 Hz for 3a, b, and c, respectively) are
less than 20 Hz, it appears that there is little or no interaction
between the hydride and silylene ligands in these complexes.
The new compounds reported here represent high oxidation-
state silylene complexes in a formally d0 electron configuration.
Alternatively, these complexes may be viewed as d2 in character,
if tungsten-based electrons are not invoked in bonding to the
silylene ligand. In preliminary reactivity studies, we have found
that 3a reacts with 1 equiv of pyridine to form an isolable adduct,
the dark red complex [Cp*(dmpe)(H)2WSiMe2(py)][B(C6F5)4] (4).
Whereas this behavior is similar to what is expected for a late
metal silylene complex, the reaction of 3a with MeCl in
fluorobenzene at 90 °C for 20 min produces Me3SiCl, formally
by insertion of dimethylsilylene into the C-Cl bond. This latter
reaction contrasts with what has been found for the late metal
complex [Cp*(Me3P)2OsdSiMe2][B(C6F5)4], which reacts with
alkyl chlorides to form [Cp*(Me3P)2OsSiMe2Cl][B(C6F5)4]. Future
work will explore further concepts of the reactivity of these new
tungsten silylene complexes.
analogous diphenyl- and phenylmethylsilylene complexes (3b and
3c, respectively) were synthesized in the same fashion from the
corresponding silanes. In preparations of 3a, b, and c it is most
convenient to use a one-pot synthesis in which the chloride is
abstracted from 1 in the presence of the silane. These reactions
likely proceed via Si-H oxidative addition to a 16-electron
“mono-tucked” cation, which appears to be thermally accessible
at room temperature but was not observed spectroscopically.
The characterization of 3a as a silylene complex was confirmed
by a downfield resonance in the 29Si{1H} NMR spectrum at δ
314. Silylene complexes 3b and 3c exhibit similarly downfield-
shifted 29Si{1H} NMR resonances at δ 297 and 277, respectively.
X-ray quality crystals of the orange dimethylsilylene dihydride
complex 3a were obtained from a fluorobenzene/pentane solution
of the complex. The structure contains two independent cation-
anion pairs in the asymmetric unit, and one of the cations is shown
in Figure 1. In each cation, the silylene ligand is planar with the
sum of the angles around the silicon center being within error of
360° (359.4(6)° and 359.5(8)°). The two crystallographically inde-
pendent cations possess very short tungsten-silicon bond dis-
tances (2.358(2) and 2.354(3) Å). A search of the Cambridge
structural database revealed a range of 2.389-2.708 Å for W-Si
bond lengths.
Acknowledgment is made to the National Science Foundation for their
generous support of this work, and to Dr. Rudy Nunlist for valuable
discussions. The authors would also like to thank Drs. Fred Hollander
and Dana Caulder of the CHEXRAY X-ray crystallography facility at
U.C. Berkeley for obtaining the crystal structure of compound 1.
Supporting Information Available: Procedures for the synthesis and
characterization of 1, 2, 3a-c, and 4 (PDF) and X-ray crystallographic
information for 1, 2, and 3a (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
To investigate possible H-H and Si-H interligand bonding
interactions in complexes 3a-c, various spectroscopic methods
JA0165436
1
were employed. On the basis of H NMR T1 relaxation experi-
(18) Crabtree, R. H.; Lavin, M.; Bonneviot, L. J. Am. Chem. Soc. 1986,
108, 4032.
ments, the hydride ligands in the silylene dihydride complexes
appear to be classical in nature. Using the null method, the T1-
(min) values for the dihydride ligands in 3a, b, and c were
determined to be in the range of 600-800 ms. Such T1 values
(19) The specific parameters for the 2JSiH coupling constant determination
NMR experiments will be described fully in a future report. The experiment
employed is adapted from the 2D BIRD and double quantum filtered 1H,29Si
HMQC NMR experiment, background for which can be found in the following
references: (a) Bax, A.; Submaranian, S. J. Magn. Reson. 1986, 67, 565. (b)
Braun, S.; Kalinowski, H. -O., Berger, S. 150 and More Basic NMR
Experiments: A Practical Course, 2nd expanded ed.; Wiley-VCH: Weinheim,
1998.
(17) (a) Carter, S. T.; Clegg, W.; Gibson, V. C.; Kee, T. P.; Sanner, R. D.
Organometallics 1989, 8, 253. (b) Gibson, V. C.; Kee, T. P.; Carter, S. T.;
Sanner, R. D.; Clegg, W. J. Organomet. Chem. 1991, 418, 197.