FT-IR spectrum of this material, acetyl acetonate bands have
disappeared whereas the signal of the carbonyl ligands are clearly
visible (2002 and 2074 cm21) indicating that the grafted solid acts
as a bidentate LX ligand toward the rhodium centre. This is
confirmed by 31P MAS CP solid-state NMR measurements on
2@Pm3n and 2@p6m treated with [Rh(cod)2]PF6. A broad signal
at 58 ppm in good agreement with the chemical shift of 3 and the
fact that HPF6 was washed away are consistent with the assumption
that the solid acts as a P^O2 ligand. This is, to the best of our
knowledge, the first example of a complex including the metallic
Rh centre in a five membered ring where the siloxane moiety of the
ring is grafted on a silica surface (Scheme 2).
Scheme 3 Hydrogenations with 3@p6m
The catalytic activity of the grafted complexes towards hydro-
genation of 1-hexene was tested under standard conditions (0.1
mol% of catalyst per run calculated on the basis of the rhodium
precursor, in 20 ml MeOH, under 7 bars of H2, at room
temperature). As a control test, 2 was also grafted (8% in mass) on
an amorphous silica gel (Geduran® Si 60 from Merck) yielding
2@SiO2. After impregnation of 2@SiO2 with the Rh precursor, the
resulting catalytic material was labelled 3@SiO2. The main results
concerning the Rh catalysed hydrogenation of 1-hexene are
gathered in Table 1. After a given number of runs, the catalyst is
isolated by filtration and washed before using it an additional time
for hexene hydrogenation under the same conditions. This step
corresponds to the recycling of the catalytic powder (Table 1). As
the homogeneous catalyst 3 was inactive, we chose [Rh(cod)1-
phospha-2-ethoxydimethylsilyl-3,6-diphenyl-4,5-dimethylnorbor-
nadiene] 4 synthesised the same way as 3 as a reference. Kinetic
studies of the 1-hexene hydrogenation catalysed by 4 showed that
it is the hydrolysed complex 4A which is active. In this case the
PNBD derivate is a P^O2 ligand corroborating the postulated
structure of the grafted complex.
3@Pm3n and 3@p6m proved to have a high activity and to be
recyclable. It is interesting to point out that the 3@SiO2 (grafted on
commercial amorphous silica gel) exhibited smaller turn over
frequencies than 3@Pm3n and 3@p6m catalysts and did not
exhibit any activity towards hexene hydrogenation after the first
recycling. The difference of activity between 3@Pm3n and
3@p6m can be assigned to a better diffusion of reactants and
products in the larger pores provided by the 3@Pm3n mesoporous
catalyst. (see Table 1)
Scheme 4 One-pot Knoevenagel condensation followed by reduction on
[3+N]@p6m
been done to catalyse Knoevenagel condensation in a heteroge-
neous way.15 Therefore, an amino function was chosen to be added
to 2@p6m materials by grafting a well known silica coupling agent
such as the aminopropyltriethoxysilane. Following the same
methodology as the one described above,13 a new catalyst labelled
[2+N]@p6m with additional 14 mass% of amino compound was
obtained. A rough calculation yields a ratio of 1 phosphanorborna-
diene for 10 amino groups tethered on the surface as confirmed by
elemental chemical analysis (N 2.3, P 0.15%) the corresponding Rh
loaded material was labelled [3+N]@p6m, and this catalyst was
used in the one-pot reaction described in Scheme 4. The reaction
proceeded in two steps, 1 hour under argon allowing the
Knoevenagel condensation to occur and then 12 hours under 7 bars
hydrogen for the reduction. This latter approach conceptually opens
new possibilities in the search for innovative polyfunctional
catalysts.
Notes and references
1 S. Mann, S. L. Burkett, S. A. Davis, C. E. Fowler, N. H. Mendelson, S.
D. Sims, D. Walsh and N. T. Whilton, Chem. Mater., 1997, 9, 2300.
2 G. J. de A. A. Soler-Illia, C. Sanchez, B. Lebeau and J. Patarin, Chem.
Rev., 2002, 102, 4093.
3 A. Corma, Chem. Rev., 1997, 97, 2373.
3@p6m was also tested on substrates, which are known to be
more difficult to hydrogenate, namely 1-methyl-1-cyclohexen and
an imine (Scheme 3). Regarding the smooth reaction conditions,14
those results are, to the best of our knowledge, the best reported so
far for hydrogenation of methylcyclohexene.
At this point, it was interesting to add a new functionality to our
heterogeneous PNDB-silica based materials to test their potential-
ity as bifunctionnal catalysts. In the past decade much work has
4 R. Anwander, Chem. Mater., 2001, 13, 4419.
5 C. Copéret, M. Chabanas, R. Petroff Saint-Arroman and J.-M. Basset,
Angew. Chem. Int. Ed., 2003, 42, 156.
6 F. Gelman, J. Blum and D. Avnir, J. Am. Chem. Soc., 2000, 122,
11999.
7 A. Choplin and F. Quignard, Coord. Chem. Rev., 1998, 178–180,
1679.
8 For example see: C. M. Crudden, D. Allen, M. D. Mikoluk and J. Sun,
Chem. Commun., 2001, 1154.
9 F. Mathey, F. Mercier, C. Charrier, J. Fischer and A. Mitschler, J. Am.
Chem. Soc., 1981, 103, 4595.
Table 1 Hydrogenation of 1-hexene with grafted Rh catalyst
10 Adapted from: H. W. Post and H. M. Norton, J. Org. Chem., 1942, 7,
528.
11 Adapted from: V. Goletto, V. Dary and F. Babonneau, Mater. Res. Soc.
Symp. Proc., 1999, 576.
Number of
recyclings
Run
Catalyst
TOFa
TONb
0
1
2
3
4
3
4A
0
7
15
48
72
X
X
0
3
2
0
12 D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka
and G. D. Stucky, Science, 1998, 279, 548.
13 D. Brunel, A. Cauvel. F. Di Renzo, F. Fajula, B. Fubini, B. Onida and
E. Garrone, New J. Chem., 2000, 24, 807.
14 Compare with A. Zsigmond, K. Bogar and F. Notheisz, J. Catal., 2003,
213, 103.
15 D. J. Macquarrie, J. H. Clark, A. Lambert, J. E. G. Mdoe and A. Priest,
React. Polym., 1997, 35, 153.
1000
1000
> 100000
5600
3@SiO2
3@pm3n
3@p6m
a Turn over frequency in cycle min21
between the amount of hexene converted and the amount of catalyst.
.
b Turn over number molar ratio
C h e m . C o m m u n . , 2 0 0 4 , 1 2 4 0 – 1 2 4 1
1241