Carbonylation chemistry of the tantalum silyl hydride Cp*(2,6- (i)Pr2C6H3N=)Ta[Si(SiMe3)3]H: The unexpected formation of a Ta(V) carbonyl complex and the complete reduction of CO [12]

Full text of DOI:10.1021/ja9905849

6328
J. Am. Chem. Soc. 1999, 121, 6328-6329
The X-ray crystal structure of 2 revealed the molecular
geometry illustrated in Scheme 1, with the silyl ligand positioned
cis to the carbonyl and hydride ligands. The hydride ligand was
not observed, but its approximate position is evident from the
otherwise empty coordination site between Si(1) and N(1). The
Ta-Si distance is considerably longer than in 1 (2.809(2) vs
2.689(1) Å6), and the Ta-C(carbonyl) distance of 2.101(7) Å is
longer than analogous distances found in Ta(III) carbonyl
complexes such as Cp2Ta(CO)(SiHtBu2)8 (2.009(5) Å) and Cp2-
Ta(CO)H9 (2.034(7) Å).
Carbonylation Chemistry of the Tantalum Silyl
Hydride Cp*(2,6-iPr2C6H3Nd)Ta[Si(SiMe3)3]H: The
Unexpected Formation of a Ta(V) Carbonyl Complex
and the Complete Reduction of CO
Urs Burckhardt and T. Don Tilley*
Department of Chemistry
UniVersity of California at Berkeley Berkeley
California 94720-1460
The formation of a d0 metal carbonyl complex would seem
surprising, although a few examples now exist.10,11 Two possible
bonding schemes could account for the stability of 2. For a d0
Ta(V) center, donation from a ligand-based orbital into the π*
orbital of the CO ligand could lead to a stable complex.11
Alternatively, significant interaction between the silyl and hydride
ligands could result in a silane σ-complex12,13 of d2 Ta(III) and d
f π* back-bonding to the CO ligand. The observed JSi(Ta)H coupl-
ing constant of 28.5 Hz is intermediate between ranges expected
for predominant η2-silane character (50-80 Hz) and classical silyl
hydrides (e20 Hz),12a but is virtually identical to the correspond-
ing value of 28 Hz reported for Cp2Ti(H)(SiHPh2)(PMe3), which
was shown to have considerable η2-silane character.3b However,
the small difference in ν(TaH) infrared stretching frequencies for
1 and 2 (1785 and 1793 cm-1, respectively) indicates the presence
of a normal, terminal Ta-H bond in 2. In addition, the bond
angles about Si(1) in 2 are similar to those in related Ta-Si-
(SiMe3)3 complexes6 and reveal none of the distortions that would
be expected for a significant H‚‚‚Si(1) interaction.
ReceiVed February 25, 1999
Studies on the dehydropolymerization of silanes, as catalyzed
by early transition metal complexes, have focused on the possible
intermediacy of d0 metal silyl hydrides LnM(SiR3)H.1 Isolated
compounds of this type are quite rare2-4 and represent an
intriguing class of chemical species, since they contain two
reactive σ-bonds. We have therefore concentrated efforts on the
synthesis and study of such compounds, and have found that
whereas the hafnium complex CpCp*Hf[Si(SiMe3)3]H (Cp* )
η5-C5Me5) is isolable,4 the silyl hydride (ArNd)2Mo[Si(SiMe3)3]H
(Ar ) 2,6-iPr2C6H3) appears to be highly unstable with respect
to elimination of HSiMe3.5 More recently, we have achieved the
synthesis and isolation of the 16-electron silyl hydride complex
Cp*(ArNd)Ta[Si(SiMe3)3]H (1).6 Here we report the surprising
carbonylation chemistry of 1, which leads initially to the formally
d0 carbonyl complex Cp*(ArNd)Ta(CO)[Si(SiMe3)3]H (2).
A pentane solution of 1 reacted rapidly with carbon monoxide
(1 atm, -40 °C) to produce the adduct 2, obtained as orange
crystals from cold pentane in 72% yield (Scheme 1). The infrared
spectrum of 2 displays a strong carbonyl absorption at 1986 cm-1
and a weak band at 1793 cm-1 assigned to the Ta-H bond. Upon
isotopic substitution, these bands shift to νCO ) 1942 cm-1 (for
2-13C) and νTaD ) 1263 cm-1 (for 2-d). In the 1H NMR spectrum,
the hydride resonance for 2 appears at 8.11 ppm, which is
considerably upfield with respect to the hydride signal of 1 (δ )
21.49 ppm). In contrast to the latter resonance for 1, the hydride
resonance for 2 exhibits well-resolved satellites from coupling
to 29Si (JSiH ) 28.5 Hz). In the 13C NMR spectrum, the carbonyl
resonance is found at 245.7 ppm (JCH ) 2.5 Hz for 2-13C), and
the 29Si NMR spectrum contains a doublet (JSiH ) 28.5 Hz) for
the Ta-Si(SiMe3)3 resonance at -102.7 ppm (compare the singlet
at -22.9 ppm observed for 1). The above data is inconsistent
with the η2-silaacyl structure originally expected for 2, given the
reactivity of other d0 metal silyl complexes7 and the fact that xylyl
isonitrile inserts cleanly into the Ta-Si bond of 1 to produce a
stable η2-silaimine complex.6
The infrared νCO stretching frequency for 2, 1986 cm-1
,
indicates donation of electron density into the π* orbital of the
CO ligand, as this value is much lower than that observed for
+
the cationic Ta(V) carbonyl complex Cp2Ta(CO)H2 (2112
cm-1 14 and is close to values reported for related d1 metal
)
carbonyl species.15 The possibility that electron density from the
Ta-Si bond donates into the carbonyl π* orbital is suggested by
the unusually acute Si-Ta-C(carbonyl) angle of 67.6(2)°, which
results in a Si(1)‚‚‚C(11) interatomic distance of only 2.79 Å.
This situation is therefore analogous to that described for the Zr-
(IV) silanimine carbonyl complex Cp2Zr(η2-Me2SidNtBu)(CO)
(8) Jiang, Q.; Pestana, D. C.; Carroll, P. J.; Berry, D. H. Organometallics
1994, 13, 3679.
(9) Gagliardi, J. A.; Teller, R. G.; Vella, P. A.; Williams, J. M. Cryst. Struct.
Commun. 1982, 11, 861.
(10) Cationic d0 carbonyls: (a) Demerseman, B.; Pankowski, M.; Bouquet,
G.; Bigorgne, M. J. Organomet. Chem. 1976, 117, C10. (b) Guram, A. S.;
Swenson, D. C.; Jordan, R. F. J. Am. Chem. Soc. 1992, 114, 8991. (c)
Antonelli, D. M.; Tjaden, E. B.; Stryker, J. M. Organometallics 1994, 13,
763. (d) Guo, Z.; Swenson, D. C.; Guram, A. S.; Jordan, R. F. Organometallics
1994, 13, 766. (e) Brackemeyer, T.; Erker, G.; Fro¨hlich, R. Organometallics
1997, 16, 531. (f) Calderazzo, F.; Pampaloni, G.; Tripepi, G. Organometallics
1997, 16, 4943.
(1) (a) Tilley, T. D. Acc. Chem. Res. 1993, 26, 22. (b) Corey, J. In AdVances
in Silicon Chemistry; Larson, G., Ed.; JAI Press: Greenwich, Conn., 1991;
Vol. 1, p 327. (c) Gauvin, F.; Harrod, J. F.; Woo, H. G. AdV. Organomet.
Chem. 1998, 42, 363.
(2) (a) Samuel, E.; Mu, Y.; Harrod, J. F.; Dromzee, Y.; Jeanin, Y. J. Am.,
Chem. Soc. 1990, 112, 3435. (b) Woo, H. G.; Harrod, J. F.; Henique, J.;
Samuel, E. Organometallics 1993, 12, 2883.
(3) (a) Kreutzer, K. A.; Fisher, R. A.; Davis, W. M.; Spaltenstein, E.;
Buchwald, S. L. Organometallics 1991, 10, 4031. (b) Spaltenstein, E.; Palma,
P.; Kreutzer, K. A.; Willoughby, C. A.; Davis, W. M.; Buchwald, S. L. J.
Am. Chem. Soc. 1994, 116, 10308.
(11) Neutral d0 carbonyls: (a) Manriquez, J. M.; McAlister, D. R.; Sanner,
R. D.; Bercaw, J. E. J. Am. Chem. Soc. 1976, 98, 6733. (b) Procopio, L. J.;
Carroll, P. J.; Berry, D. H. Polyhedron 1995, 14, 45. (c) Howard, W. A.;
Parkin, G.; Rheingold, A. L. Polyhedron 1995, 14, 25. (d) Howard, W. A.;
Trnka, T. M.; Parkin, G. Organometallics 1995, 14, 4037. (e) Brennan, J. G.;
Andersen, R. A.; Robbins, J. L. J. Am. Chem. Soc. 1986, 108, 335. (f) Parry,
J.; Carmona, E.; Coles, S.; Hursthouse, M. J. Am. Chem. Soc. 1995, 117,
2649.
(4) Casty, G. L.; Lugmair, C. G.; Radu, N. S.; Tilley, T. D.; Walzer, J. F.;
Zargarian, D. Organometallics 1997, 16, 8.
(5) Casty, G. L.; Tilley, T. D.; Yap, G. P. A.; Rheingold, A. L.
Organometallics 1997, 16, 4746.
(12) (a) Schubert, U.; Scholz, G.; Mu¨ller, J.; Ackermann, K.; Wo¨rle, B.;
Stansfield, R. F. D. J. Organomet. Chem. 1986, 306, 303. (b) Schubert, U.
AdV. Organomet. Chem. 1990, 30, 151.
(6) Burckhardt, U.; Casty, G. L.; Tilley, T. D., manuscript in preparation.
Details on the synthesis of 1 are given in the Supporting Information.
(7) Typical values for η2-silaacyl complexes: νCO ) ca. 1500 cm-1; δ(13-
COSi) ) ca. 390 ppm. For example, see: (a) Tilley, T. D. J. Am. Chem. Soc.
1985, 107, 4084. (b) Campion, B. K.; Falk, J.; Tilley, T. D. J. Am. Chem.
Soc. 1987, 109, 2049. (c) Elsner, F. H.; Tilley, T. D.; Rheingold, A. L.; Geib,
S. J. J. Organomet. Chem. 1988, 358, 169. (d) Arnold, J.; Tilley, T. D.;
Rheingold, A. L.; Geib, S. J.; Arif, A. M. J. Am. Chem. Soc. 1989, 111, 149.
(e) Campion, B. K.; Heyn, R. H.; Tilley, T. D. J. Am. Chem. Soc. 1990, 112,
2011.
(13) (a) Tilley T. D. In Chemistry of Organic Silicon Compounds; Patai,
S., Rappoport, Z., Eds.; Wiley: New York, 1989; p 1415. (b) Tilley, T. D. In
The Silicon-Heteroatom Bond; Patai, S., Rappoport, Z., Eds.; Wiley: New
York, 1991; p 309. (c) Corey, J. Y.; Braddock-Wilking, J. Chem. ReV. 1999,
99, 175.
(14) Reynoud, J. F.; Leboeuf, J.-F.; Leblanc, J.-C.; Mo¨ıse, C. Organome-
tallics 1986, 5, 1863.
(15) (a) DeBoer, E. J. M.; Ten Cate, L. C.; Staring, A. G. J.; Teuben, J. H.
J. Organomet. Chem. 1979, 181, 61. (b) Van Raaij, E. I.; Schmulbach, C. D.;
Brintzinger, H. H. J. Organomet. Chem. 1987, 328, 275.
10.1021/ja9905849 CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/19/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 26, 1999 6329
Scheme 1
Scheme 2
(νCO ) 1797 cm-1), which exhibits a Si‚‚‚C(carbonyl) distance
of only 2.35 Å.11b This interaction in the latter complex leads to
a significant 2JSiC coupling constant of 24 Hz, which is somewhat
larger than that observed for 2-13C (15 Hz).
the alcohol HOCH2Si(SiMe3)316 as the main product (>90%, by
NMR spectroscopy and GC-mass spectrometry).
The results described here represent new and unexpected
transformations in the carbonylation chemistry of d0 metal silyl
derivatives, which typically react rapidly with CO to give silaacyl
insertion products (including, for example Cp*Cl3TaSiMe37d). The
silyl hydride 1 reacts differently in that it initially forms a stable
carbonyl complex, which is best described as a Ta(V) complex
which gains stability via a back-bonding interaction involving
donation of electron density from the Ta-Si σ-bond to the π*
orbital of the carbonyl. Under relatively mild conditions, this
species undergoes a series of migrations to produce 3, which
contains a completely reduced C-O bond. The latter result
suggests new possibilities for the use of carbon monoxide as a
synthon in hydrosilation chemistry, and these are currently under
investigation.
On standing at room temperature, pentane solutions of 2
quantitatively transform within 24 h to a new compound 3, as
indicated by a color change to light yellow (Scheme 1). Complex
3, isolated in 96% yield, was identified as a six-membered
tantalacycle with the former carbonyl and silyl ligands incorpo-
rated into the ring. The molecular structure features a chairlike
geometry for the metallacycle, with all bonding parameters in
the expected ranges (Scheme 1). Transformation of the deuteride
2-d to 3-d occurs with selective incorporation of deuterium into
the axial position of the methylene group derived from the
carbonyl ligand (from NOE experiments). A possible mechanism
for the formation of 3 is given in Scheme 2 (intermediates were
not observed by monitoring the reaction at -40 °C).
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. U.B. thanks the Schweiz Nationalfonds and the
Novartis Stiftung for postdoctoral fellowships.
Evidence in support of part of the mechanism of Scheme 2
was obtained by trapping of the silaaldehyde intermediate 2b. In
the presence of an excess of CO (1 atm), the decomposition of 2
led to only a small amount of 3, and the red adduct Cp*(ArNd
)Ta(CO){η2-OCH[Si(SiMe3)3]} (4) was isolated in 58% yield. On
the basis of NMR data for the silaaldehyde ligand (δ(CHSi) )
Supporting Information Available: Detailed experimental proce-
dures for the preparation and spectroscopic characterization of complexes
1-5, tables of crystal, data collection, and refinement parameters, atomic
coordinates, bond distances, bond angles, and anisotropic displacement
parameters for 2 and 3 (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
2
2.15 ppm, JSi,H ) 9.2 Hz; δ(13CHSi) ) 77.3 ppm), 4 is best
described as having metallacycle character. Furthermore, the high
value of 2006 cm-1 for the CO infrared stretching frequency (1964
cm-1 for 4-13C2) appears to reflect the presence of a Ta(V) (rather
than a Ta(III)) center. Similarly, the adduct Cp*(ArNd)Ta(PMe3)-
{η2-OCH[Si(SiMe3)3]} (5) formed as the main product (61%, by
NMR spectroscopy) when the decomposition occurred in the
presence of 1 equiv of PMe3. Hydrolyses of 4 and 5 produced
JA9905849
(16) Elsner, F. H.; Woo, H.-G.; Tilley, T. D. J. Am. Chem. Soc. 1988,
110, 313.