Agostic Si-H bond coordination assists C-H bond activation at ruthenium in bis(phosphinobenzylsilane) complexes

Article abstract of DOI:10.1039/b709408f

Reaction of a phosphinobenzylsilane compound with ruthenium complexes leads to C-H and/or Si-H activation. The new complex Ru{η²-H-SiMe ₂CH(o-C₆H₄)PPh₂}₂ (5) was isolated and X-ray, NMR and DFT studies reveal that 5 displays two agostic Si-H interactions and two carbon-metallated bonds. The Royal Society of Chemistry.

Full text of DOI:10.1039/b709408f

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Agostic Si–H bond coordination assists C–H bond activation at
ruthenium in bis(phosphinobenzylsilane) complexes{
Virginia Montiel-Palma,*a Miguel A. Mun˜oz-Herna´ndez,a Tahra Ayed,b Jean-Claude Barthelat,b
Mary Grellier,c Laure Vendierc and Sylviane Sabo-Etienne*c
Received (in Cambridge, UK) 20th June 2007, Accepted 8th August 2007
First published as an Advance Article on the web 23rd August 2007
DOI: 10.1039/b709408f
spectrum of 3 displays one broad signal in the hydride region at
27.8 ppm. Decoalescence is observed at 273 K and two broad
signals in a 1 : 1 ratio can be seen at 25.2 ppm (v1/2 = 40 Hz) and
210.4 ppm (AA9XX9 pattern, v1/2 = 94 Hz with the two main
lines separated by D = 30 Hz) at the low temperature limit (193 K).
The observed decoalescence is attributable to the Si–H/Ru–H
exchange and characterized by a DG{ of 48.2 kJ mol21, a value
within the range of those previously reported for the exchange
between two types of hydrides (M–(g2-Si–H) and M–H).12,13 The
29Si NMR spectrum shows a singlet at d = +8.3 ppm with small
JSi–H coupling constants of 28 and 15 Hz for the agostic and
classical hydrides, respectively.14 The 31P{1H} NMR spectra at all
temperatures show only one signal at d = 39.2 ppm in agreement
with two equivalent phosphines. As expected, 3 shows similar
spectroscopic properties to those some of us previously reported
for a series of bis(silane) compounds of general formula
RuH2{(g2-H–SiMe2)2X}(PCy3)2 (X = C6H4, (CH2)2, (CH2)3,
OSiMe2O).15 These data lead us to propose the structure shown
in Scheme 1 with a cis disposition of the phosphine ligands. This is
also consistent with DFT calculations performed at the B3PW91
level on the model RuH2{g2-H–SiMe2CH2(o-C6H4)PH2}2. Two
Reaction of a phosphinobenzylsilane compound with ruthe-
nium complexes leads to C–H and/or Si–H activation. The new
complex Ru{g2-H–SiMe2CH(o-C6H4)PPh2}2 (5) was isolated
and X-ray, NMR and DFT studies reveal that 5 displays two
agostic Si–H interactions and two carbon-metallated bonds.
Significant advances appeared concomitantly in the early 1980s in
the general field of bond activation. (i) Bergman and Graham
reported the first examples of intermolecular addition of alkane
C–H bonds;1 (ii) Brookhart and Green developed the concept of
agostic interactions;2 (iii) Kubas et al. established the unambiguous
existence of a s-dihydrogen complex.3 Since these findings, the
development of new systems inducing E–H bond activation (E =
H, C, Si …) has witnessed tremendous growth.4,5 Understanding
E–H bond activation remains a prerequisite for progress in the
field if one wants to achieve selective transformations. Moreover,
tandem bond activation such as C–H/Si–H activation could offer
unique properties in catalysis.6,7 As part of our ongoing program
on E–H bond activation,8 we decided to explore the coordination
of phosphinosilanes toward ruthenium complexes. We reasoned
that in ruthenium chemistry, these compounds could act as
bidentate hemilabile ligands through
P and agostic Si–H
coordination.9,10 Furthermore, among the several strategies used
to activate C–H bonds, chelating assistance by the use of substrates
bearing directing groups (e.g. ketones, nitriles etc.) to promote
cyclometallation is an attractive method.11 In this contribution, we
describe C–H activation derived from the reactions of RuH2(g2-
H2)2(PCy3)2 (1) and Ru(COD)(COT) (4) towards the potentially
chelating phosphinobenzylsilane ligand Ph2P(o-C6H4)CH2SiMe2H
(2).
Complex 1 reacts with 2 equiv. of 2 affording RuH2{g2-H–
SiMe2CH2(o-C6H4)PPh2}2 (3) as the main product (Scheme 1).
The reaction results from the formal substitution of the two
dihydrogen and the two tricyclohexylphosphine ligands in 1 by
two phosphinobenzylsilane ligands, with the Si–H bonds bound to
1
Ru in an agostic fashion. At room temperature, the H NMR
aCentro de Investigaciones Qu´ımicas, Cuernavaca, Morelos, Me´xico,
Av. Universidad 1001, Col. Chamilpa, Mexico.
E-mail: vmontielp@ciq.uaem.mx
bLaboratoire de Physique Quantique, IRSAMC/UMR 5626, Universite´
Paul Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex 04, France
cLaboratoire de Chimie de Coordination du CNRS, 205 route de
Narbonne, 31077 Toulouse Cedex 04, France.
E-mail: sabo@lcc-toulouse.fr
{ Electronic supplementary information (ESI) available: Experimental
details for the synthesis of 3, 5 and 6; optimized structures of complexes 3a,
3b, 5a, 5b and 6. See DOI: 10.1039/b709408f
Scheme 1 Reactivity of the phosphinobenzylsilane compound 2 with
ruthenium precursors. Synthesis of the bis(carbometallated) complex 5.
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Chem. Commun., 2007, 3963–3965 | 3963

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2.2 ppm, whereas the Ru–C resonance appears shielded at
24.8 ppm (JC–H = 141 Hz) in the 13C NMR spectrum. One
31P{1H} NMR singlet is observed at 56.9 ppm at all accessible
temperatures.17
Complex 5 is also obtained quantitatively upon dihydrogen loss
from solutions of 3. Moreover, even under a dihydrogen
atmosphere, THF, benzene or toluene solutions of 3 are not
stable over a period of days and 5 is formed presumably via the
mixed complex RuH{g2-H–SiMe2CH2(o-C6H4)PPh2}{g2-H–
SiMe2CH(o-C6H4)PPh2} (6) as detected spectroscopically.
Independently, 6 can also be obtained as the major product from
the reaction of 4 with 2 equiv of 2 under 3 bar of dihydrogen for
two hours. The main NMR features of complex 6 are two 29Si
NMR signals at d = 11 ppm and at d = 213 ppm for the non-
metallated and the carbometallated ligands respectively, and two
31P{1H} NMR doublets (d 56.5 and 42.7 ppm, 2JP–P 21 Hz). The
three hydride resonances appear as a broad signal at d = 26.0 ppm
(JSi–H = 76 Hz), a pseudo-triplet at 28.0 ppm (JP–H = 21 and
27 Hz) and a doublet of doublets at 29.4 ppm (JP–H = 24 Hz and
54 Hz). The JSi–H value is in agreement with the presence of at least
one agostic Si–H bond and a SISHA interaction.5f The proposed
structure is consistent with the ground-state structure of 6
computed at the DFT/B3PW91 level (see ESI{).
Agostic C–H interactions preluding C–H activation are well
known,4,18 and Berry et al. have described tandem b-C–H
activation/Si–H elimination reactions.6 We show here that the
bis(agostic) Si–H complex 3, displaying rare high order e-agostic
Si–H interactions (to the best of our knowledge no precedent
higher than d-agostic is known) leads finally to 5, with two
b-agostic interactions as a result of C–H activations. The increased
acidity of the methylene groups of the ligand in complex 3 coupled
with the presence of agostic Si–H bonds, which can easily
decoordinate, induce a stepwise H2-loss process resulting ulti-
mately in the formation of the stable bis(carbometallated) complex
5. Whilst C–H activation occurring a to a heteroatom could be
expected,4 it is worth noting that in our system such a reaction
proceeds with final preservation of the agostic Si–H bonds in 5 and
6. A facile dissociation–recoordination pathway could end in the
formation of the carbometallated species whilst reforming the
agostic Si–H bonds in the final stable products. Although no
conclusive mechanistic evidence could be found, the agostic Si–H
interactions favour in some way the C–H activation process.
This work was supported by CONACYT grant 43540 and
CNRS/CONACYT grant 16873. We thank the CINES
(Montpellier, France) for a generous allocation of computer time
and Miss Hanit Trevin˜o for laboratory assistance.
Fig. 1 X-Ray structure of 5 (top) and computed DFT/B3PW91 structure
of 5b (bottom) (non-relevant H atoms omitted for clarity). Ellipsoids in 5
are shown at the 50% probability level. Selected bond lengths (s) and
angles (u), with the DFT data in parenthesis for comparison: Ru–Si(1)
2.4587(13) (2.454), Ru–Si2 2.4367(13) (2.454), Ru–C19 2.263(4) (2.241),
Ru–C24 2.244(4) (2.241), Ru–H100 1.71(4) (1.709), Ru–H101 1.68(4)
(1.709), Si2–H100 1.76(4) (1.709), Si1–H101 1.65(3) (1.709), Si1–Ru–Si2
120.01(5) (121.02), P1–Ru–P2 107.10(4) (102.50), H100–Ru–H101
175.1(18) (178.94), Ru–H101–Si1 95.2(19) (91.80), Ru–H100–Si2
89.4(18) (91.80).
isomers (see ESI{) with the phosphorus in a trans (3a) or cis (3b)
disposition were optimized. 3a being the highest in energy
(+33.6 kJ mol21).
More interestingly, when the addition of 2 was performed
on Ru(COD)(COT) (4), the new species Ru{g2-H–SiMe2CH(o-
C6H4)PPh2}2 (5) was detected as the sole product by 1H and
31P{1H} NMR spectroscopy, and isolated in 92% yield (Scheme 1).
Its formulation as a bis(cyclometallated) species, resulting from
C–H activation of the methylene groups of the starting ligand 2,
was ascertained from NMR, X-ray{ and consistent with DFT
data. The molecular structure of 5 is shown in Fig. 1. The
geometry around the ruthenium centre is a bicapped pseudo-
octahedron with Si atoms capping and a cis disposition of the
other heavy atoms. Two modified phosphinobenzylsilane ligands
coordinate to the metal each through a phosphorus atom, an
agostic Si–H bond and a carbon atom resulting from C–H
activation of the methylene group of the starting ligand. The Si–H
bond lengths of ca. 1.70 s, as determined from X-ray data and
consistent with B3PW91 calculations on the model RuH2{g2-H–
SiMe2CH(o-C6H4)PH2}2 (5b),16 are typical of g2-H–Si bonds. The
metallated Ru–C bond lengths of ca. 2.24 s are characteristic of
Ru–C single bonds. Multinuclear NMR data highlight the unusual
coordination of the ligand. In particular, the two equivalent
Notes and references
{ CCDC 650812. For crystallographic data in CIF or other electronic
format, see DOI: 10.1039/b709408f
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=
9 Hz) with satellites due to coupling with silicon (JSi–H = 67 Hz).
The metallated C–H proton resonates as a broad singlet at
3964 | Chem. Commun., 2007, 3963–3965
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