Antimony-antimony bond formation by reductive elimination from a hafnium bis(stibido) complex

Article abstract of DOI:10.1021/ic061826q

The bis(stibido) complex CpCp*Hf(SbMes₂)₂ (2) was prepared and structurally characterized. Complex 2 reacts with 2 equiv of xylylisocyanide to give the bis-insertion product CpCp*Hf[C(SbMes ₂)= N(2,6-MeC₆H₃)]₂ (4). The reaction of 2 with oxidants (I₂ and O₂) or donors (carbon monoxide and diphenylacetylene) or thermolysis promotes the reductive elimination of Sb₂Mes₄.

Full text of DOI:10.1021/ic061826q

Inorg. Chem. 2006, 45, 9625−9627
Antimony−Antimony Bond Formation by Reductive Elimination from a
Hafnium Bis(stibido) Complex
Rory Waterman† and T. Don Tilley*
Department of Chemistry, UniVersity of California, Berkeley, Berkeley, California 94720-1460
Received September 25, 2006
The bis(stibido) complex CpCp*Hf(SbMes2)2 (2) was prepared and
structurally characterized. Complex 2 reacts with 2 equiv of
xylylisocyanide to give the bis-insertion product CpCp*Hf[C(SbMes2)d
N(2,6-MeC6H3)]2 (4). The reaction of 2 with oxidants (I2 and O2)
or donors (carbon monoxide and diphenylacetylene) or thermolysis
promotes the reductive elimination of Sb2Mes4.
reactions appear to operate by a mechanism involving σ-bond
metathesis steps, which break Si-H bonds and form Si-Si
bonds via concerted, four-centered transition states.5 The
dehydrocoupling of hydrostannanes to polystannanes is also
catalyzed by group 4 metallocene complexes, and mecha-
nistic work has implicated a process involving R elimination
of a low-valent main-group fragment, which then undergoes
catenation via insertions into Sn-H or M-Sn bonds (Scheme
1).6 The R-stannylene elimination step of this dehydrocou-
pling process involves migration of hydrogen to the metal,
and related aryl and alkyl migrations (e.g., Hf-SnPh3 to Hf-
Ph and :SnPh2) have also been observed.6b,c The latter
observations suggest that early-metal complexes of the
heavier elements might be convenient sources of low-valent
species and may provide new routes to catenated compounds.
There has been considerable recent interest in the use of
transition-metal reagents and catalysts for the formation of
bonds between the heavier elements.1-8 The development
of this field has resulted in the discovery of numerous
compounds with metal-main group element bonds and the
identification of several chemical pathways for element-
element bond formation. For example, rhodium complexes
catalyze the dehydrocoupling of primary and secondary
phosphines to diphosphanes, probably by a simple mecha-
nism involving oxidative additions and reductive elimina-
tions.2 Similar dehydrocoupling reactions produce Si-Si
bonds,3 and extended chains of silicon atoms (polysilanes)
are obtained by dehydrocoupling reactions of primary silanes
catalyzed by group 4 metallocene complexes.1,4 The latter
Scheme 1. Dehydrocoupling Based on R Elimination
In search of new routes to Sb-Sb-bonded species, we have
investigated the chemistry of stibido complexes of zirconium
and hafnium. For related phosphorus systems, Harrod7 and
Stephan8 have reported the catalytic dehydrocoupling of
primary phosphines to cyclic oligomers. Very little is known
about related antimony compounds, although Cp2Zr(SbPh2)2
was reported in 1984.9 More recently, we found that Cp2-
ZrHCl and Cp2ZrMe2 are catalysts for the dehydrocoupling
of MesSbH2 (Mes ) mesityl) to [SbMes]4, and the stibido
complexes CpCp*Hf(SbHAr)Cl (Ar ) Mes, dmp ) 2,6-
Mes2C6H3) undergo R-stibinidene elimination to give
CpCp*HfHCl and [SbMes]4 or dmpSbdSbdmp.10 The gen-
eration of CpCp*Hf[SbH(dmp)]Me results in rapid elimina-
tion of methane and the formation of the stibinidene complex
CpCp*HfdSb(dmp), which was trapped by PMe3 and
2-butyne.11 To further probe the inherent chemical properties
of stibido complexes of hafnium, the bis(stibido) derivative
* To whom correspondence should be addressed. E-mail: tdtilley@
berkeley.edu.
† Current address: Department of Chemistry, University of Vermont,
Burlington, VT 05405.
(1) Gauvin, F.; Harrod, J. F.; Woo, H. G. AdV. Organomet. Chem. 1998,
42, 363-405.
(2) (a) Bohm, V. P. W.; Brookhart, M. Angew. Chem., Int. Ed. 2001, 40,
4694-4696. (b) Han, L.-B.; Tilley, T. D. J. Am. Chem. Soc. 2006, in
press.
(3) (a) Ojima, I.; Inaba, S.; Tetsuo, K.; Matsumoto, M.; Matsumoto, H.;
Watanabe, H.; Nagai, Y. J. Organomet. Chem. 1973, 55, C7-C8. (b)
Brown-Wensley, K. A. Organometallics 1987, 6, 1590-1591. (c)
Rosenberg, L.; Davis, C. W.; Yao, J. J. Am. Chem. Soc. 2001, 123,
5120-5121.
(4) (a) Corey, J. Y. AdV. Organomet. Chem. 2004, 51, 1-52. (b) Tilley,
T. D. Comments Inorg. Chem. 1990, 10, 37-46.
(5) Tilley, T. D. Acc. Chem. Res. 1993, 26, 22-29.
(6) (a) Neale, N. R.; Tilley, T. D. J. Am. Chem. Soc. 2002, 124, 3802-
3803. (b) Neale, N. R.; Tilley, T. D. Tetrahedron 2004, 60, 7247-
7260. (c) Neale, N. R.; Tilley, T. D. J. Am. Chem. Soc. 2005, 127,
14745-14755.
(7) Xin, S.; Woo, H. G.; Harrod, J. F.; Samuel, E.; Lebuis, A.-M. J. Am.
Chem. Soc. 1997, 119, 5307-5313.
(8) (a) Fermin, M. C.; Stephan, D. W. J. Am. Chem. Soc. 1995, 117,
12645-12646. (b) Stephan, D. W. Angew. Chem., Int. Ed. 2000, 39,
314-329.
(9) Wade, S. R.; Wallbridge, M. G. H.; Willey, G. R. J. Organomet. Chem.
1984, 267, 271-276.
(10) Waterman, R.; Tilley, T. D. Angew. Chem., Int. Ed. 2006, 45, 2926-
2929.
(11) Waterman, R.; Tilley, T. D. Chem. Commun. 2006, 4030-4032.
10.1021/ic061826q CCC: $33.50
Published on Web 11/03/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 24, 2006 9625

COMMUNICATION
Scheme 2
CpCp*Hf(SbMes2)2 (2) has been synthesized and character-
ized. Preliminary results on the chemistry of this complex
are reported, including reductive elimination of the distibine
Mes2SbSbMes2.
Figure 1. Perspective view of 2 with thermal ellipsoids drawn at the 35%
probability level. Hydrogen atoms, solvent molecules, and mesityl rings
except ipso carbon atoms have been omitted for clarity. Select bond lengths
(Å) and angles (deg): Hf-Sb(1) ) 2.782(1), Hf-Sb(2) ) 3.013(1), Sb-
(1)-C(31) ) 2.170(9), Sb(1)-C(41) ) 2.197(9), Sb(2)-C(21) ) 2.185-
(9), Sb(2)-C(11) ) 2.199(8); Sb(1)-Hf-Sb(2) ) 88.72(3), C(31)-Sb(1)-
C(41)) 95.9(3), C(31)-Sb(1)-Hf ) 129.4(2), C(41)-Sb(1)-Hf )
133.1(2), C(21)-Sb(2)-C(11) ) 94.7(3), C(21)-Sb(2)-Hf ) 109.5(2),
C(11)-Sb(2)-Hf ) 126.3(2).
Treatment of an ethereal slurry of CpCp*HfCl2 (1) with
2 equiv of LiSbMes2, generated in an Et2O solution, resulted
in a green solution that over a period of ca. 30 min evolved
to a violet color. From this solution, analytically pure purple
crystals of 2 were obtained in 63% isolated yield (Scheme
2). Characterization of complex 2 followed from 1H and 13
C
NMR, IR, and UV-vis spectroscopy. The violet color of 2
results from a ligand-to-metal charge-transfer band at 630
nm (ꢀ ) 1.5 × 104 L cm-1 mol-1). Complex 2 shows pseudo-
complex tentatively assigned as CpCp*HfCl(SbMes2) based
1
on H and 13C NMR spectra. Efforts to separate the two
1
Cs symmetry from temperatures of -95 to +45 °C by H
stibido complexes have thus far failed, and variation of the
reaction conditions (e.g., solvent, temperature, addition rate)
has not yet favored the monosubstituted hafnocene product.
Treatment of a benzene solution of bis(stibido) complex
2 with 2 equiv of xylylisocyanide (CNxyl) afforded the
insertion product CpCp*Hf[C(SbMes2)dN(2,6-Me2C6H3)]2
(4) as analytically pure brown crystals in 85% yield (Scheme
and 13C NMR spectroscopy. Additionally, the o-methyl
substituents of the mesityl rings are equivalent, indicating
rapid Sb-C rotation throughout this temperature range.
Single crystals of 2 were grown from a concentrated Et2O
solution cooled to -35 °C. An X-ray diffraction study was
performed, and the molecular structure of complex 2 is
shown in Figure 1. Notably, two crystallographically unique
antimony centers were located. One antimony center, Sb-
(2), is pyramidal with the sum of the angles about Sb(2)
being 330.4°. The Sb(2)-Hf bond length of 3.014(1) Å is
slightly longer than that for CpCp*HfCl[SbH(dmp)] [3;
3.0035(8) Å].10 The slight lengthening in the Sb-Hf bond
of 2 is likely a result of greater steric congestion associated
with the SbMes2 ligand. The other antimony center is
essentially planar [Σ angles at Sb(1) ) 358.4°] and has a
considerably shortened Sb(1)-Hf bond length of 2.782(1)
Å, which is 0.22 Å shorter than the Sb(2)-Hf bond of 3.
Additionally, the plane of this stibido ligand [C(31)-Sb-
(1)-C(41)] is approximately orthogonal to the Hf-Sb(1)-
Sb(2) plane, which allows for lone-pair donation into the
vacant a1 orbital of the Cp2MX2 fragment.12 These structural
features are consistent with multiple-bond character in
complex 2, similar to that observed in complexes of the type
Cp2M(PR2)2 (M ) Hf, Zr; R ) Et, Cy) and related
derivatives observed first by Baker and co-workers.13
The reaction of hafnocene dichloride 1 with 1 equiv of
LiSbMes2 resulted in a mixture of 2, unreacted 1, and a
1
2). Monitoring the reaction in benzene-d6 by H NMR
spectroscopy showed quantitative conversion to 4. Key
spectroscopic features of 4 include an IR stretching vibration
of νCN 1647 cm-1, and the R-carbon resonance of the
iminioacyl ligand appears at δ 254.7 in the 13C NMR
spectrum. When 2 was treated with 1 equiv of CNxyl, only
a mixture of 4 and 2 was observed, with no evidence of the
single insertion product. A related insertion of phenyliso-
cyanide into the Hf-As bond of a hafnocene-arsenido
complex has been reported by Hey-Hawkins and co-
workers.14 Double insertions of the type observed for 2 are
quite rare for group 4 metallocene derivatives,15 and this may
reflect a high reactivity for Hf-Sb bonds.
Complex 2 reacted with 2 equiv of ethereal HCl to give
quantitatively the secondary stibine HSbMes2 and dichloride
1 (Scheme 2). These two products can be separated by
fractional crystallization from hexane in 77% and 89% yields,
respectively.
The reaction chemistry of 2 is dominated by a reductive
coupling of the SbMes2 fragments to form tetramesityldis-
(14) Lindenberg, F.; Mu¨ller, U.; Pilz, A.; Sieler, J.; Hey-Hawkins, E. Z.
Anorg. Allg. Chem. 1996, 622, 683-688.
(15) Martins, A. M.; Ascenso, J. R.; de Azevedo, C. G.; Dias, A. R.; Duarte,
M. T.; da Silva, J. F.; Veiros, L. F.; Rodrigues, S. S. Organometallics
2003, 22, 4218-4228.
(12) Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98, 1729-
1742.
(13) Baker, R. T.; Whitney, J. F.; Wreford, S. S. Organometallics 1983, 2,
1049-1051.
9626 Inorganic Chemistry, Vol. 45, No. 24, 2006

COMMUNICATION
Scheme 3
to separate products and unreacted diphenylacetylene gave
reduced isolated yield. Products were identified by compari-
son to authentic samples prepared by literature methods.16,18
These studies indicate that the reductive elimination of
distibine from 2 is relatively facile and may occur by an
associative, ligand-induced mechanism.19 Oxidants (O2 or I2)
also induce reductive elimination of Sb2Mes4 from 2 by
unknown mechanisms. Presumably, the reaction of 2 with
I2 initially forms CpCp*Hf(I)(SbMes2) and ISbMes2, which
then react via nucleophilic displacement of iodide by a stibido
group at the antimony center of ISbMes2. Group 4 metal-
locene derivatives are known to undergo reductive elimina-
tion, most notably alkyl hydride species, which produce the
corresponding alkane.19 Related reductive eliminations that
form Si-Cl,20 Si-H,18 and, most recently, C-C bonds21 have
also been reported.
In summary, a new hafnium stibido complex 2 has been
prepared and structurally characterized. CNxyl reacts rapidly
with 2 to insert into the Hf-Sb bond, but other ligands such
as carbon monoxide or diphenylacetylene induce reductive
elimination of Sb2Mes4. Thermolysis of 2 or the addition of
oxidants such as O2 or I2 also result in Sb-Sb bond formation
to give distibine 5.
tibine Sb2Mes4 (5). Treatment of a benzene-d6 solution of 2
with ca. 1 atm of dry O2 resulted in decolorization of the
solution and precipitation of a solid. Analysis of the 1H and
13C NMR spectra of the reaction reveals quantitative conver-
sion to Sb2Mes2, identified by comparison to an authentic
sample (Scheme 3).16 The insoluble residues consisted of
an uncharacterizable hafnium product. Reaction of 2 with 1
equiv of I2 in benzene afforded Sb2Mes2 and CpCp*HfI2 in
71 and 83% isolated yields, respectively, after fractional
crystallization from hexane (Scheme 3).17 This reactivity is
directly analogous to that of Cp2Zr(SbPh2)2, which reacts
with I2 to give Sb2Ph4 and Cp2ZrI2.9 It was observed that
heating benzene-d6 solutions of 2 for >12 h at 90 °C also
gives Sb2Mes4 and a complex mixture of hafnium-containing
products, some of which were insoluble. These reactions
point to a reductive elimination process; therefore, additional
evidence for a resulting hafnium(II) product was sought.
Treatment of complex 2 with carbon monoxide (ca. 2
equiv, 1 atm) gave a complex mixture of hafnium-containing
products, most of which were insoluble, and Sb2Mes4
(Scheme 3). Heating benzene solutions of stibido complex
2 at 90 °C for 4 h in the presence of excess diphenylacetylene
cleanly afforded Sb2Mes4 and CpCp*Hf(C4Ph4) in 54 and
78% isolated yields, respectively (Scheme 3). This reaction
is nearly quantitative, as monitored by 1H NMR spectroscopy
(benzene-d6). The successive fractional crystallization steps
Acknowledgment. This work was funded by the U.S.
National Science Foundation and by the Miller Institute for
Basic Research in Science through a Research Fellowship
(R.W.).
Supporting Information Available: X-ray crystallographic data
(CIF) and experimental details for the synthesis and characterization
of new compounds, tables of unit cell, data collection, and
refinement parameters for 2 (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
IC061826Q
(18) Casty, G. L.; Lugmair, C. G.; Radu, N. S.; Tilley, T. D.; Walzer, J.
F.; Zargarian, D. Organometallics 1997, 16, 8-12.
(19) (a) McAlister, D. R.; Erwin, D. K.; Bercaw, J. E. J. Am. Chem. Soc.
1978, 100, 5966-5968. (b) Gell, K. I.; Schwartz, J. J. Am. Chem.
Soc. 1981, 103, 2687-2695. (c) Baumann, R.; Stumpf, R.; Davis, W.
M.; Liang, L. C.; Schrock, R. R. J. Am. Chem. Soc. 1999, 121, 7822-
7836. (d) Spence, R. E. V.; Piers, W. E.; Sun, Y. M.; Parvez, M.;
MacGillivray, L. R.; Zaworotko, M. J. Organometallics 1998, 17,
2459-2469. (e) Raoult, Y.; Choukroun, R.; Blandy, C. Organome-
tallics 1992, 11, 2443-2446. (f) Pool, J. A.; Lobkovsky, E.; Chirik,
P. J. J. Am. Chem. Soc. 2003, 125, 2241-2251. (g) Pool, J. A.;
Lobkovsky, E.; Chirik, P. J. Organometallics 2003, 22, 2797-2805.
(20) Imori, T.; Heyn, R. H.; Tilley, T. D.; Rheingold, A. L. J. Organomet.
Chem. 1995, 493, 83-89.
(16) Ates, M.; Breunig, H. J.; Soltani-Neshan, A.; Tegeler, M. Z. Natur-
forsch., B: Anorg. Chem., Org. Chem. 1986, 41B, 321-326.
(17) Gassman, P. G.; Winter, C. H. Organometallics 1991, 10, 1592-1598.
(21) Haneline, M. R.; Heyduk, A. F. J. Am. Chem. Soc. 2006, 128, 8410-
8411.
Inorganic Chemistry, Vol. 45, No. 24, 2006 9627