Synthesis and structure of ruthenium-silylene complexes: Activation of Si-Cl bonds in N-heterocyclic silanes

Article abstract of DOI:10.1021/ja0580744

Ru(0) complexes of bis(imino)pyridine ligands, [η²-N₃]Ru(η⁶-Ar) and {[N₃]Ru}₂(μ-N₂), where Ar = C₆H₆ or C₆H₅Me and [N₃] = 2,6-(MesN=CMe)₂C₅H₃N, react with N-heterocyclic silicon(IV) compounds to yield Ru(II) silylene complexes of the type [N₃]Ru(X)(Cl){Si(NN)} (X = H, Cl, and Si(NN) = N,N'-bis(neopentyl)-1,2-phenylenedi(amino)silylene). The activation of two groups on the silane occurs in a stepwise fashion: initial oxidative addition of a Si-X bond, followed by 1,2-migration (α-elimination) of the Si-Cl group to the metal. Reversible dissociation from the Ru(II) center leads to free silylene, which can be preferentially trapped with Ru(0) complexes to generate a zero-valent silylene complex, [N₃]Ru(N₂){Si(NN)}, which also contains a terminal dinitrogen ligand. Copyright

Full text of DOI:10.1021/ja0580744

Published on Web 04/19/2006
Synthesis and Structure of Ruthenium-Silylene Complexes: Activation of
Si-Cl Bonds in N-Heterocyclic Silanes
Hyojong Yoo, Patrick J. Carroll, and Donald H. Berry*
Department of Chemistry and Laboratory for Research on the Structure of Matter, UniVersity of PennsylVania,
Philadelphia, PennsylVania 19104-6323
Received November 28, 2005; E-mail: berry@sas.upenn.edu
Transition metal silylene complexessspecies with formal double
bonds between metal and silicon atomsshave been the subject of
considerable interest for many years.1 Although several different
synthetic approaches have been employed,2,3 potentially the most
general route to metal silylene complexes is the sequential addition
of two groups from a tetravalent silane to a metal center. The first
process, intermolecular oxidative addition, is well-known for silicon
hydride bonds.1b The second step, intramolecular 1,2-migration or
R-elimination of a second group from silicon to a metal, has been
observed or implied for substituents, such as hydrides,4 silyl,5 or
alkyl groups6 in the formation of metal silylene complexes.
Our group has been interested in the activation of silicon chloride
bonds as a route to reactive silyl and silylene complexes. There
have been only a few previous reports of silicon chloride oxidative
addition,7 and R-halide elimination from a silyl ligand has not
been previously described. Our initial studies have employed highly
reactive zero-valent ruthenium complexes, [η2-N3]Ru(η6-Ar) (1) and
Figure 1. ORTEP drawing of complex 4a (30% thermal ellipsoids).
Hydrogen atoms other than Ru-H are omitted for clarity.
{[N3]Ru}2(µ-N2) (2), where Ar ) C6H6 or C6H5Me and [N3] )
2,6-(MesNdCMe)2C5H3N.8 Tetravalent heterocyclic silanes (X)-
(Cl)Si(NN) (X ) H, Cl; Si(NN) ) N,N′-bis(neopentyl)-1,2-
phenylenedi(amino)silylene, Si[(NCH2But)2C6H4-1,2], were chosen
as substrates because of the relative stability of the corresponding
silylene9 and expected weakening of the Si-Cl bonds in this system.
ment of hydride and chloride ligands. The mesityl ligands are
oriented approximately perpendicular to the [N3] ligand plane and
form a pocket surrounding the silylene ligand.
The silicon atom in 4a is slightly pyramidal and lies 0.240 Å
out of the plane defined by the three attached atoms (Ru, N, N).
The metal silicon bond distance (2.304(1) Å) lies at the short end
of the range of Ru-Si bonds located in the CSD11 (ca. 2.20-2.50
Å, 2.39 Å average). For the most part, heterocyclic silylenes have
been proposed to be excellent σ-donors and poor π-acids.3c,d,12
Complex 4a is fluxional in solution on the 1H NMR time scale.
The 1H NMR spectrum of 4a at 285 K exhibits diastereotopic
neopentyl CH2 groups and no symmetry along either the H-Ru-
Cl or N-Ru-N axes. This is comparable to the solid state structure
and indicates hindered Ru-Si bond rotation and slow inversion at
the slightly pyramidal silicon. The symmetrical spectrum observed
at 350 K is consistent with both inversion and rotation of the silylene
ligand. Coalescence of the various pairs of imine, methylene, and
neopentyl ligand resonances is observed between 313 and 345 K.
Similar conformational interconversions have been reported by
Lappert and co-workers in palladium and platinum complexes of
the silylene ligand.3a
Formation of silylene complex 4a most likely proceeds by
sequential Si-H oxidative addition and Si-Cl 1,2-migration to
ruthenium. Various polar mechanisms are possible, but seem less
likely for a reaction that is rapid in both pentane and THF. The
putative five-coordinate Ru(II) silyl complex, [N3]Ru(H)-Si(Cl)-
(NN) (5a), was not observed, but reaction of 4a with excess PMe3
at 25 °C results in the rapid formation of a phosphine complex
tentatively identified as chlorosilyl complex [N3]Ru(H)(PMe3)(Si-
(NN)(Cl)) (6).10 Complex 6 is not stable in solution at 25 °C and
reacts with a second equivalent of PMe3 to generate free silane
(3a) and the bis(phosphine) complex, [N3]Ru(PMe3)2.8 This provides
We now report the synthesis of ruthenium complexes with cyclic
diaminosilylene ligands, [N3]Ru(X)(Cl){Si(NN)}, formed by se-
quential and reversible activation of silicon hydride and chloride
bonds of tetravalent silanes (X)(Cl)Si(NN). Furthermore, the silylene
reversibly dissociates from the Ru(II) center and can be preferen-
tially trapped with a more electron-rich metal complex, such as 2,
to generate a Ru(0) silylene complex, [N3]Ru(N2){Si(NN)}.
Treatment of either [N3]Ru(0) complex 1 or 2 with cyclic diami-
nohydrochlorosilane 3a in pentanes at 25 °C leads to rapid formation
of ruthenium silylene complex, [N3]Ru(H)(Cl){Si(NN)} (4a), which
has been isolated as a red crystalline solid (77%, eq 1).10
The structure of 4a in the solid state was determined by a single-
crystal X-ray diffraction study. As illustrated in Figure 1, 4a exhibits
a six-coordinate, pseudo octahedral geometry with a trans arrange-
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10.1021/ja0580744 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Proposed Mechanism
Figure 2. ORTEP drawing of complex 8 (30% thermal ellipsoids).
Hydrogen atoms are omitted for clarity.
good evidence for reversible Si-H activation and the intermediacy
of 5a, as well as the reversibility of R-Cl migration.
migration to generate Ru(II) silylene complexes. This is the first
example of the formation of metal silylene complexes from a silane
via this type of R-chloride abstraction process. The activation of
simple chlorosilanes, such as Me2SiCl2 with [N3]Ru(0), will be
described in future reports.
Surprisingly, THF solutions of the Ru(II) silylene complex 4a
react slowly (days at 25 °C) with Ru(0) complexes, such as 1 or 2,
under N2 to yield [N3]Ru(H)(Cl)(THF) (7a) and the ruthenium(0)
silylene dinitrogen complex [N3]Ru(N2){Si(NN)} (8, eq 2).13
Presumably, this indicates that the silylene ligand can reversibly
dissociate from 4a and be trapped with a second metal center.
Acknowledgment. We are grateful to the National Science
Foundation and Laboratory for Research on the Structure of Matter
for support of this work.
Supporting Information Available: Details of synthetic proce-
dures, spectroscopic details, and X-ray crystallographic data for 4a,
4b, 7b, and 8 in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
The geometry of 8 has been confirmed by a single-crystal X-ray
diffraction study to be approximately square pyramidal (Figure 2),10
with the Si(NN) ligand trans to the vacant site and the linear
dinitrogen ligand trans to the pyridine. The Ru-Si bond distance
(2.230(2) Å) is much shorter than in six-coordinate Ru(II) complex
4a and is one of the shortest reported in the CSD.11 However, it is
difficult to attribute this shortening to any single factor, as both
the reduced steric hindrance and increased electron density in the
five-coordinate Ru(0) complex could favor closer Ru-Si contact.
The dichlorosilane 3b (Cl2Si(NN)) also reacts with Ru(0)
complex 1, but in the absence of N2, the reaction stops after initial
Si-Cl activation, and the 16 e- silyl complex, [N3]Ru(Cl)-Si(Cl)-
(NN) (5b), can be isolated (50%) and characterized by multinuclear
NMR spectroscopy.10 Of particular note is the resonance in the 29Si-
{1H} NMR at δ -10.42 for the silyl ligand, which is more than
130 ppm upfield of silylene complexes 4a (δ 126.9) and 8 (δ
133.69) and indicative of a four-coordinate silicon center. The silyl
complex 5b reacts with Ru(0) complexes under N2 to form 8 and
[N3]Ru(Cl)2(THF) (7b), consistent with 1,2-migration of chloride
from Si to Ru, and silylene dissociation, although the intermediate
ruthenium silylene complex, [N3]Ru(Cl)2{Si(NN)} (4b), is not
observed under these reaction conditions. However, silylene
complex 4b has been prepared in 81% yield from the reaction of
[N3]Ru(Cl)2(C2H4) (10) with the free silylene, 9.10
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As in case of 4a, treatment of the Ru(II) complex 4b with 1 or
2 under N2 leads to transfer of the silylene to the Ru(0) center and
formation of 8 and 7b. A likely mechanism for this series of
transformations is shown in Scheme 1.
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