Titanium-catalyzed dehydrocoupling of silanes: Direct conversion of primary monosilanes to titanium(0) oligosilane complexes with agostic α-Si-H...Ti interactions

Article abstract of DOI:10.1021/ja067503a

The reaction of TiMe₂(dmpe)₂ with excess PhSiH₃ affords methane, PhSiMeH₂, and the titanium trisilane complex Ti(Si₃H₅Ph₃)(dmpe)₂ (1), which slowly reacts with an addi

Full text of DOI:10.1021/ja067503a

Published on Web 01/30/2007
Titanium-Catalyzed Dehydrocoupling of Silanes: Direct Conversion of
Primary Monosilanes to Titanium(0) Oligosilane Complexes with Agostic
r-Si-H‚‚‚Ti Interactions
Michael D. Spencer, Quinetta D. Shelby, and Gregory S. Girolami*
School of Chemical Sciences, The UniVersity of Illinois at Urbana-Champaign, 600 South Mathews AVenue,
Urbana, Illinois 61801
Received October 19, 2006; E-mail: girolami@scs.uiuc.edu
Polysilane high polymers have been attracting increasing interest
owing to their potential applications as photoresists, photoinitiators,
thermochromic materials, and precursors to silicon carbide.1 In
1985, Harrod and co-workers discovered that group 4 metallocenes
such as Cp2TiMe2 could catalyze the dehydrogenative coupling of
primary organosilanes to linear poly(hydrosilanes), H-(SiHR)n-H
with ∼140 SiHR units.2 Since then, this reaction has been
intensively studied, and many new catalysts have been discovered.
The Si-Si bond-forming step in these dehydrocoupling reactions
is thought to involve σ-bond metathesis reactions of metal silyl
complexes with an entering hydrosilane.2,3 Several group 4 silyl
complexes have been isolated that serve as models of the intermedi-
ates responsible for the catalysis.2,4 Silane dehydrocoupling reactions
have found additional use in the derivatization of silicon surfaces.5
Figure 1. Molecular structure of Ti(Si4H6Ph4)(dmpe)2, 2. Thermal ellipsoids
are represented by the 30% probability surfaces. Selected bond distances
(Å) and angles (deg): Ti-Si(1) 2.539(2), Ti-Si(3) 2.519(2), Ti-P(1) 2.529-
(2), Ti-P(2) 2.652(2), Ti-P(3) 2.511(2), Ti-P(4) 2.657(2), Ti-H(1A) 1.67-
In the course of investigating the reaction chemistry of the 14-
electron titanium(II) alkyl TiMe2(dmpe)2,6 we found that it is a
catalyst for the dehydrogenative oligomerization of primary aryl-
silanes. More importantly, we have been able to isolate several of
the titanium-containing species that are formed in this reaction; these
species document consecutive steps in the dehydrocoupling reaction
and thereby provide intriguing insights into the inner workings of
this process. Specifically, we find that treatment of solutions of
TiMe2(dmpe)2 with 4 equiv of PhSiH3 -20 °C for 4 h quantitatively
yields a diamagnetic species of stoichiometry Ti(Si3H5Ph3)(dmpe)2
(1), which can be isolated as a purple solid.7 The conversion of
TiMe2(dmpe)2 to the trisilane complex 1 is accompanied by the
formation of ∼1 equiv each of methane and PhSiMeH2.8 When
solutions of 1 are incubated with PhSiH3 in Et2O at -20 °C for
several weeks, a new compound of stoichiometry Ti(Si4H6Ph4)-
(dmpe)2 (2) is generated nearly quantitatively.9 This tetrasilane
species can also be isolated as a pure material.
(2), Ti-H(3A) 1.68(2), Si(1)-Si(2) 2.385(2), Si(2)-Si(3) 2.369(2), Si(2)-
Si(4) 2.326(2), Si(1)-H(1A) 1.83(2), Si(1)-H(1B) 1.54(3), Si(3)-H(3A)
1.83(2), Si(3)-H(3B) 1.54(3), Si(4)-H(4A) 1.46(4), Si(4)-H(4B) 1.45-
(3), Si(3)-Ti-Si(1) 97.95(5), Si(2)-Si(1)-Ti 76.67(6), Si(3)-Si(2)-Si-
(1) 106.75(7), Si(2)-Si(3)-Ti 77.33(6).
The NMR spectra of 1 are consistent with a PhSiH2-SiPhH-
SiPhH2 structure for the trisilane ligand, in which one hydrogen
atom on each terminal silicon atom is involved in an agostic
interaction with the titanium center.10 The 1H NMR spectrum of 2
is consistent with the presence of a tetrasilane ligand of stoichi-
ometry PhSi(SiH2Ph)3, with a structure similar to that of 1 except
that the hydrogen atom on the central silicon center is replaced
with a SiH2Ph group.
These conclusions have been verified by a single-crystal X-ray
diffraction study of the tetrasilane complex 2 (Figure 1).11 The
geometry about the titanium center is a distorted octahedron in
which the two dmpe groups and a tetrasilane molecule act as
chelating bidentate ligands. The tetrasilane is branched, PhSi-
(SiPhH2)3, and two of the SiPhH2 groups are involved in agostic
interactions with the titanium center. The Ti-Si(1) and Ti-Si(3)
distances of 2.529(2) Å are shorter than those found in other
titanium silyl (2.583(2)-2.765(8) Å)4,12 or titanium silane (2.597-
(2)-2.891(2) Å)4,13 complexes.14 The Ti‚‚‚Si(2) distance of 3.056-
(2) Å is probably too long for there to be a significant bonding
interaction between these two atoms. Instead, the principal interac-
tion is with the Si-H groups: the Ti-H(1A) and Ti-H(3A)
distances, respectively 1.67(2) and 1.68(2) Å, are within the 1.45-
1.96 Å range reported for terminal Ti-H distances15 and the 1.59-
1.82 Å range found for Ti‚‚‚H-Si distances4,13 in other complexes.
The geometries around Si(1) and Si(3) are distorted trigonal
bipyramids in which the two hydrogen atoms occupy the axial sites.
The H-Si(1)-H angle is 167.9(15)°, and the H-Si(3)-H angle
is 163.8(16)°. For both Si(1) and Si(3), the Si-H distance to the
The NMR spectra of 1 and 2 show that these two molecules are
closely related. Both complexes give 31P{1H} NMR spectra that
correspond to ABCD spin systems; the chemical shifts and coupling
constants for 1 and 2 are very similar, though not identical. In both
compounds, the presence of only one large JPP coupling constant
(∼50 Hz) suggests that the two dmpe ligands are arranged in a
cis-octahedral fashion. In both compounds, there are eight PMe
environments, a result which confirms that both compounds lack
symmetry elements.
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J. AM. CHEM. SOC. 2007, 129, 1860-1861
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C O M M U N I C A T I O N S
J. Y. AdV. Organomet. Chem. 2004, 51, 1-52.
bridging hydrogen, 1.83(2) Å, is considerably longer than the Si-H
distance to the terminal hydrogen, 1.54(3) Å. The Si(4)-H
distances, 1.46(4) and 1.45(3) Å, are typical of Si-H distances in
hydrosilanes.16 The 0.3 Å lengthening of the Si-H distances to
the bridging hydrogen atoms reflects the agostic interactions with
the Ti center.16 The 0.1 Å lengthening of the Si-H distances to
the terminal hydrogen atoms on Si(1) and Si(3) can be ascribed to
the strong trans influence of their respective agostic hydrides, or
to the higher effective coordination number of these silicon atoms.
The axial phosphorus atoms are bent away from the tetrasilane
group to give a P(1)-Ti-P(3) angle of 157.53(6)°. The average
Ti-P(axial) distance, 2.520(2) Å, and the average Ti-P(equatorial)
distance, 2.654(2) Å, fall near the extremes of the 2.51-2.66 Å
range observed for Ti-P distances in other six-coordinate com-
plexes.17 The longer Ti-P(equatorial) distances may be due to the
trans influence of the tetrasilane group.
(4) (a) Samuel, E.; Mu, U.; Harrod, J. F.; Dromzee, Y.; Jeannin, Y. J. Am.
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(7) One equivalent of PhSiH3 remains unchanged in this step.
(8) Presumably H2 is also formed, but quantification was hindered because
its chemical shift is the same as the Si-H resonance for PhMeSiH2.
The NMR spectra of 2 afford additional information about the
bonding between titanium and the tetrasilane molecule. The1JSiH
coupling constants for the non-agostic SiH2Ph group, 186 and 189
Hz, are consistent with those found in free silanes.16 The1JSiH
coupling constants for Si(1)-H(1B) and Si(3)-H(3B) are 127 and
(9) No SiH4 is formed in these reactions, but this product has been seen in
some other systems.4i See also: Wu, F.; Jordan, R. F. Organometallics
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(10) The 1H NMR spectrum of 1 contains five equal-intensity Si-H resonances
with complex line shapes. Two (at δ 4.50 and 4.75), due to hydrogen
atoms that bridge between Si and Ti, are strongly coupled to all four
phosphorus nuclei. The δ 4.50 resonance is coupled to a Si-H resonance
at δ 6.25, and the δ 4.75 resonance is coupled to a Si-H resonance at δ
6.35. The H-H coupling constants of 14.5 Hz between these sites are
consistent with gem H-Si-H couplings. The fifth Si-H resonance at δ
3.27 is a broad singlet and is assigned to the single Si-H group on the
central silicon atom of the trisilane unit.
1
133 Hz; these smaller JSiH coupling constants are consistent with
the 0.1 Å elongations of these bonds as described above. In contrast,
the 1JSiH coupling constants for Si(1)-H(1A) and Si(3)-H(3A) are
1
less than 40 Hz; JSiH coupling constants reported for M‚‚‚H-Si
(11) Crystal data for C36H58P4Si4Ti (T ) 198 K): monoclinic, space group
P21/n, a ) 11.8357(3) Å, b ) 18.8226(5) Å, c ) 18.9559(6) Å, â )
95.102(1)°, V ) 4206.2(2) Å3, Z ) 4, wR2 ) 0.0779 for 433 variables
and 5847 unique data.
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complexes engaged in agostic interactions fall between 20 and 100
Hz.16 The geminal JH(A)H(B) coupling constants (13.7 Hz) are also
in agreement with the structural data; these 2JHH values are unusually
large because the H(A)-Si-H(B) angles are nearly 180°.
The present results show that TiMe2(dmpe)2 reacts with PhSiH3
to afford two titanium(0) products bearing coordinated oligosilane
ligands; the latter are generated by dehydrogenative coupling
promoted by the titanium center. An interesting mechanistic finding
in this system is that the catalysis converts a linear trisilane to a
branched tetrasilane. The titanium-containing products, Ti(Si3H5-
Ph3)(dmpe)2 and Ti(Si4H6Ph4)(dmpe)2, are the first molecules in
which oligosilanes serve as chelating ligands and are rare examples
of compounds in which a metal center is involved in two agostic
M‚‚‚H-Si interactions.16-19
Acknowledgment. We thank the National Science Foundation
(Grants CHE 00-76061 and CHE 04-20768) for support of this
work. X-ray diffraction data were collected by Dr. Scott Wilson
and Ms. Teresa Prussak-Wieckowska at the Materials Chemistry
Laboratory of the University of Illinois. NMR spectra were obtained
in the Varian Oxford Instrument Center for Excellence at the
University of Illinois. Funding for this instrumentation was provided
in part from the W. M. Keck Foundation, the National Institutes of
Health (PHS 1 S10 RR10444-01), and the National Science
Foundation (NSF CHE 96-10502).
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Supporting Information Available: Details of the characterization
of 1 and 2; X-ray crystallographic files for 2 in CIF format. This
material is available free of charge via the Internet at http://pubs.acs.org.
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