Second generation N-heterocyclic carbene-Pt(0) complexes as efficient catalysts for the hydrosilylation of alkenes

Article abstract of DOI:10.1039/b506369h

A new class of benzimidazolylidene carbene-Pt(0) complexes was developed and used to efficiently catalyse the hydrosilylation of alkenes. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b506369h

View Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Second generation N-heterocyclic carbene–Pt(0) complexes as efficient
catalysts for the hydrosilylation of alkenes{
c
Olivier Buisine, Guillaume Berthon-Gelloz, Jean-Fran c¸ ois Bri e` re, S e´ bastien St e´ rin, G e´ rard Mignani,
a
d
c
c
c
b
b
Paul Branlard, Bernard Tinant, Jean-Paul Declercq and Istv a´ n E. Mark o´ *
a
Received (in Cambridge, UK) 6th May 2005, Accepted 27th May 2005
First published as an Advance Article on the web 21st June 2005
DOI: 10.1039/b506369h
A new class of benzimidazolylidene carbene–Pt(0) complexes
was developed and used to efficiently catalyse the hydrosilyla-
tion of alkenes.
benzimidazolylidene carbenes exhibit the topology of unsaturated
N-heterocyclic carbenes but show structural properties and
reactivity akin to those of carbenes containing a saturated
14
N-heterocyclic ring system.
The hydrosilylation of alkenes, the second most important C–Si
1
bond forming reaction after the Rochow process, is of paramount
2
importance for the silicon industry. This transformation is
Complexes 3a–d were synthesised (Scheme 1, Table 1) in good
to high yields, by in situ deprotonation of the corresponding
t
benzimidazolium salts, in the presence of BuOK, followed by
efficiently catalysed by platinum complexes, such as the Speier
reaction of the carbene with Karstedt’s catalyst. Complex 3e was
prepared in a different manner. The benzimidazolylidene carbene
was generated from the corresponding thiourea by reduction over
Na–K amalgam, in toluene, for 20 days then reacted with
i
catalyst (H PtCl –PrOH) or the Karstedt catalyst (Pt {(g -
3
2
2
6
2
4
2 2 3
ViSiMe ) O} where Vi 5 vinyl). Although this reaction requires
only ppm quantities of these platinum-containing species, it is
plagued by numerous undesired side-reactions, such as isomerisa-
tion of the terminal alkene, hydrogenation of the C–C double
bond and coloration of the reaction medium by formation of
platinum colloids. It has therefore been a longstanding endeavour
to minimize the formation of these unwanted by-products, and
several efforts in this direction, employing modified platinum(0)
14
Karstedt’s catalyst.
Crystals of complex 3e{ suitable for X-ray diffraction analysis
were grown by slow evaporation of a dichloromethane solution.
The molecular structure is shown in Fig. 1. Complex 3e crystallises
as a pair of enantiomers. The folding of the neopentyl arms
around the benzimidazole core defines a C symmetric environ-
2
5–7
complexes have been reported. We have previously described a
ment. The divinyltetramethylsiloxane (dvtms) ligand and the
carbene fragment adopt a trigonal planar geometry around the
platinum(0) centre. The bidentate dvtms ligand exists in a
new class of N-heterocyclic carbene platinum(0) complexes
8
which catalyse efficiently the hydrosilylation of alkenes. These
novel organometallic derivatives tolerate a wide range of
functionalities and protecting groups, produce low amounts of
isomerised olefins (,1%) and lead to no detectable formation
pseudochair conformation which is a common coordination mode
15–18
for this ligand.
the C
This complex owes its chirality to the fact that
2
symmetry of the carbene ligand is broken by the
9
of colloids.
pseudochair conformation adopted by the dvtms ligand. The
benzimidazole carbene is tilted by 68u (C20–Pt1–C1–N9 torsion
angle) from the plane formed by the Pt centre and the alkene
ligands. The strong s-donating ability of the carbene substituent is
evidenced by the amount of backbonding exerted on the dvtms
As part of an ongoing research effort aimed at exploring the
vast potential offered by N-heterocyclic carbene-derived ligands in
the hydrosilylation of alkenes, we investigated the use of the
analogous benzimidazolylidene substituents in this transformation.
Although a plethora of imidazolylidene ligands have been
18
˚
ligand C20–C21 1.432(6) A vs. 1.39(2) in the parent complex 1.
1
0
synthesised and used in homogeneous catalysis, the applications
This strong backbonding, which is responsible for the remarkable
activity of these complexes, renders them also inactive at room
temperature.
1
1
of their benzimidazolylidene counterparts in metal-catalysed
12,13
processes have remained relatively unexplored.
These
N-heterocyclic carbene derivatives display several interesting
features. From an electronic point of view, they possess a
s-donating ability which is intermediate between that of
imidazolylidene and imidazolinylidene carbenes. In addition,
Having obtained the desired carbene–platinum complexes, we
next investigated their catalytic activity. As a model reaction, the
hydrosilylation of 1-octene by silane 5 (a suitable polysiloxane
backbone mimic) was selected [eqn (1)]. The reaction was
performed at a concentration of 0.5 M and with a catalyst loading
a
Unit e´ de Chimie Organique et M e´ dicinale D e´ partement de chimie,
23
of 5 6 10 mol%.
Universit e´ catholique de Louvain Place Louis Pasteur 1, 1348, Louvain-
la-Neuve, Belgium. E-mail: marko@chim.ucl.ac.be; Fax: +32 1047 2788;
Tel: +32 10478773
b
ð1Þ
Unit e´ d’Analyse Structurale, Universit e´ catholique de Louvain Place
Louis Pasteur 1, 1348, Louvain-la-Neuve, Belgium
Rhodia Centre de Recherche de Lyon, Av. des Fr e` res Perret 85,
c
F-69192, Saint-Fons Cedex, France
Laboratoire de Chimie Mol e´ culaire et Thio-organique UMR CNRS
Representative curves of the catalytic activity of selected
complexes are displayed in Fig. 2. Examination of Fig. 2
reveals that, apart from 3e, all the other benzimidazolylidene
carbene complexes 3a–d are more active than the parent
d
6507, ENSICAEN 6, Boulevard du Mar e´ chal Juin, 14 050, Caen, France
{ Electronic Supplementary Information (ESI) available: Experimental
details. See http://dx.doi.org/10.1039/b506369h
3
856 | Chem. Commun., 2005, 3856–3858
This journal is ß The Royal Society of Chemistry 2005

View Online
Scheme 1 Synthesis of complexes 3a–d.
a
Table 1 Synthesis of benzimidazolylidene Pt(0) complexes
Entry
Complex
Yield (%)
80
1
3a
3b
2
60
Fig. 1 Solid-state molecular structure of complex 3e. Hydrogen atoms
3
4
3c
3d
78
77
are omitted for clarity, thermal ellipsoids at 40% probability. Selected
˚
bond lengths (A) and angles (u): C(1)–Pt(1) 2.059(4), C(20)–Pt(1) 2.143(4),
C(21)–Pt(1) 2.147(4), C(22)–Pt(1) 2.150(4), C(23)–Pt(1) 2.144(4), C(26)–
C(25) 1.432(6), C(22)–C(23) 1.415(7), C(1)–N(2) 1.365(5), C(1)–N(9)
1.361(5), N(2)–C(1)–N(9) 106.0(3), C(26)–Pt(1)–C(1)–N(9) 110.0(3),
C(22)–Pt(1)–C(20)–C(21) 0.0(4), C(21)–Pt(1)–C(23)–C(22) 1.4(4).
b
5
3e
50
a
Reaction conditions: benzimidazolium salt (1 mmol), 1 (1 mmol),
BuOK (2 mmol), THF (50 ml), RT. The carbene was obtained by
t
b
reduction of the corresponding thio benzimidazolidinone over Na–K
in toluene for 20 days.
14
Me-imidazolylidene-Pt(dvtms) catalyst. For example, after 100 min
reaction, only 65% conversion is observed for the latter catalyst
whilst 83% conversion is obtained in the case of 3b and 3c. The
initial reaction rate is also faster for the benzimidazole-containing
complexes 3a–d as compared with their imidazole counterpart.
Interestingly, the complex that displayed the highest activity
contains the n-propyl substituted benzimidazolylidene ligand (3b).
This is seemingly due to a good compromise between the steric and
the electronic properties of the NHC substituent. In the second
most active complex, 3d, the remote ester function on the
Fig. 2 Kinetic curves for the hydrosilylation of 1-octene with silane 5
23
catalysed by complexes 3a–e. Reaction conditions: [Pt] 5 5 6 10 mol%,
2
1
[
1-octene] 5 [5] 5 0.5 mol L , o-xylene, T 5 70 uC. (A) Me-NHC-
Pt(dvtms). (B) 3a. (C) 3b. (D) 3c. (E) 3d. (F) 3e.
coordinate to the platinum centre after the loss of the dvtms
ligand, efficiently inhibiting the approach of the reactants.
Hydrosilylation of these substituents is thus required before the
catalyst could exert its full activity. The catalytic behaviour of 3e
can be understood in part by its solid-state structure. The bulky
neopentyl groups wrap around the platinum, effectively blocking
both sides and significantly increasing the induction period. Even
after the departure of the dvtms ligand, the neopentyl substituents
6-position of the benzimidazole ring effectively reduces the electron
density on the platinum centre, thus weakening the coordination of
the dvtms ligand.
Of the complexes described above, both 3c and 3e display a
slightly different catalytic behaviour, involving a prolonged
induction period. For 3c, this induction period can be attributed
to the presence of the pendant alkene side-chains, which
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 3856–3858 | 3857

View Online
271531. See http://dx.doi.org/10.1039/b506369h for crystallographic data in
CIF or other electronic format.
1
2
E. G. Rochow, J. Am. Chem. Soc., 1945, 67, 963–965.
B. Marciniec, J. Gulinski, W. Urbaniac and Z. W. Kornetka,
Comprehensive Handbook on Hydrosilylation, Pergamon, Oxford, UK,
1
992.
J. L. Speier, J. A. Webster and G. H. Barnes, J. Am. Chem. Soc., 1956,
9, 974.
3
7
4
5
B. D. Karstedt, General Electric, US Pat. 3,715,334, 1973.
J. W. Sprengers, M. J. Agerbeek and C. J. Elsevier, Organometallics,
2004, 23, 3117–3125.
6
7
8
J. W. Sprengers, M. J. Mars, M. A. Duin, K. J. Cavell and C. J. Elsevier,
J. Organomet. Chem., 2003, 679, 149–152.
J. W. Sprengers, M. D. Greef, M. A. Duin and C. J. Elsevier, Eur. J.
Inorg. Chem, 2003, 3811–3819.
I. E. Mark o´ , S. St e´ rin, O. Buisine, G. Mignani, P. Branlard, B. Tinant
and J.-P. Declercq, Science, 2002, 298, 204–207.
9 I. E. Mark o´ , G. Michaud, G. Berthon-Gelloz, O. Buisine and S. St e´ rin,
Fig. 3 Proposed catalytic cycle.
are sufficiently bulky to reduce significantly the rate of the
hydrosilylation of 1-octene. These experiments also indicate that
the loss of the dvtms ligand, probably by hydrosilylation, might be
the rate determining step of these reactions. Finally, it is important
to note that, in all cases, colloidal platinum species were never
detected.
Adv. Synth. Catal., 2004, 346, 1429–1434.
0 W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1290–1309.
1
¨
1
1 (a) K. Ofele, W. A. Herrmann, D. Mihalios, M. Elison, E. Herdtweck,
T. Priermeier and P. Kiprof, J. Organomet. Chem., 1995, 498, 1–14; (b)
B. Bildstein, M. Malaun, H. Kopacka, K.-H. Ongania and K. Wurst,
J. Organomet. Chem., 1998, 552, 45–61; (c) F. E. Hahn and M. Foth,
J. Organomet. Chem., 1999, 585; (d) F. M. Rivas, U. Riaz, A. Giessert,
J. A. Smulik and S. T. Diver, Org. Lett., 2001, 3, 2673–2676; (e)
F. E. Hahn, C. G. Plumed, M. Munder and T. Lugger, Chem. Eur. J.,
2004, 10, 6285–6293; (f) H. Sakurai, K. Sugitani, T. Moriuchi and
T. Hirao, J. Organomet. Chem., 2005, 690, 1750–1755.
Though the mechanism of the hydrosilylation reaction catalysed
by platinum carbene complexes 3a–e has not yet been investigated
in detail, the similarity in kinetic behaviour between these
complexes and the parent imidazolylidene complexes prompted
us to draw a parallel. A thorough kinetic study on the parent
imidazolylidene platinum(0) complexes has revealed a unique
mode of action of these organometallic derivatives (Fig. 3).
Carbene complexes 3a–e are precatalysts that initially undergo
hydrosilylation–dissociation of the dvtms substituent. The depar-
ture of this ligand, the slowest step of the whole process, requires
the presence of the alkene 4 and the silane 5 and generates the
12 H. K u¨ c u¨ kbay, B. Cetinkaya, S. Guesmi and P. H. Dixneuf,
Organometallics, 1996, 15, 2434–2439.
1
3 Recent examples: (a) E. Mas-Marza, M. Poyatos, M. Sanau and
E. Peris, Inorg. Chem., 2004, 2213–2219; (b) F. E. Hahn, C. Holtgrewe,
T. Pape, M. Martin, E. Sola and L. A. Oro, Organometallics, 2005, 24,
2
203–2209.
4 F. E. Hahn, L. Wittenbecher, R. Boese and D. Bl a¨ ser, Chem. Eur. J.,
999, 5, 1931–1935.
15 A. R. Bassindale, S. S. D. Brown and P. Lo, Organometallics, 1994, 13,
38–740.
6 P. B. Hitchcock, M. F. Lappert, C. MacBeath, F. P. E. Scott and
N. J. W. Warhurst, J. Organomet. Chem., 1997, 528, 185–190.
1
1
9
transient intermediate—or transition state—7. Insertion of the
Si–H bond into the olefinic linkage leads to the platinum(II)
derivative 8 which, after reductive elimination of 6, affords the
platinum(0) species 9, probably stabilised by coordination to one
1
7
1
20
or two alkenes. Addition of the silane then regenerates the active
17 G. Chandra, P. Y. Lo, P. B. Hitchcock and M. F. Lappert,
2
1
complex 7 and a new catalytic cycle ensues. The variation in the
induction period observed with different precatalysts 3a–e reflects
the respective difficulty in hydrosilylating the chelating dvtms
ligand and correlates with the steric encumbrance provided by the
N-heterocyclic carbene substituents.
Organometallics, 1986, 6, 191.
8 P. B. Hitchcock, M. F. Lappert and N. J. W. Warhurst, Angew. Chem.,
Int. Ed., 1991, 30, 438–440.
1
1
9 The rate of the hydrosilylation reaction appears to be first order in the
alkene 4 and in the silane 5, implying that both partners are involved at
the same time in the ‘‘rate determining step’’ of the catalytic cycle.
Interestingly, incubation of the Cy-NHC-Pt(dvtms) complex with excess
alkene does not promote the exchange between the dvtms ligand and the
olefin. In contrast, treatment of the same complex with excess silane
generates the corresponding N-heterocyclic platinum(II) silyl hydride
dimer. This dimer is, however, a less efficient catalyst than the parent
complex. For the preparation and structure determination of the
analogous phosphine-containing dimers, see: M. Ciriano, M. Green,
J. A. K. Howard, J. Proud, J. L. Spencer, G. A. F. Stone and C. A.
Tsipis, J. Chem. Soc., Dalton Trans., 1978, 801–808. For the
hydrosilylation of alkenes using these dimers, see: M. Green, J. L.
Spencer, G. A. F. Stone and C. A. Tsipis, J. Chem. Soc., Dalton Trans.,
In summary, a new family of benzimidazolylidene platinum(0)
complexes has been synthesized and characterised. These novel
species display interesting catalytic activity in the hydrosilylation of
alkenes. Their activity, which is superior to that of the parent
imidazolylidene carbenes, can be finely tuned by the judicious
modulation of their steric and electronic properties.{
Financial support by the Universit e´ catholique de Louvain,
Rhodia Silicones and Rhodia Corporate is gratefully acknowl-
edged. I.E.M. is thankful to Rhodia for receiving the 2002 Rhodia
Outstanding Award.
1977, 1519.
2
0 The reaction displays saturation kinetics. Whilst increasing the
concentration of the alkene leads initially to an enhanced reaction rate,
the presence of a large excess of olefin eventually results in a partial
inhibition of the hydrosilylation reaction, probably arising from the
saturation of the coordination sphere of the platinum complex 9.
Notes and references
{
parallelepiped, triclinic, a 5 12.915(4), b 5 13.155(4), c 5 18.436(7) A,
44 2 2
Crystallographic details for 3e: C25H N OPtSi , M 5 639.89, colourless
˚
21 The addition of a second portion of silane and alkene, at the end of the
first hydrosilylation reaction, leads to a second complete hydrosilylation.
Therefore the catalyst is not deactivated at the end of the reaction but
remains in a stable resting-state. This is in contrast to Karstedt’s catalyst
which remains inactive upon addition of fresh reactants.
3
˚
a 5 96.35(2) , b 5 93.73(2), c 5 111.56(2)u, U 5 2876(2) A , T 5 293(2) K,
21
¯
space group P1 (no. 2), Z 5 4, m(Mo–Ka) 5 4.98 mm , 61207 reflections
measured, 12925 unique (Rint 5 0.063) which were used in all calculations.
2
2
1
The final R (F ) and wR(F ) were 0.046 and 0.12 respectively. CCDC
3
858 | Chem. Commun., 2005, 3856–3858
This journal is ß The Royal Society of Chemistry 2005