Synthesis, characterization, and catalytic activity of a well-defined rhodium siloxide complex immobilized on silica

Article abstract of DOI:10.1002/anie.200704362

(Chemical Equation Presented) Rhodium siloxide surface complex 2, which was obtained directly by reaction of molecular precursor 1 with aerosil silica and characterized inter alia by solid-state NMR spectroscopy, is a highly active and stable catalyst for hydrosilylation reactions.

Full text of DOI:10.1002/anie.200704362

Angewandte
Chemie
DOI: 10.1002/anie.200704362
Supported Catalysts
Synthesis, Characterization, and Catalytic Activity of a Well-Defined
Rhodium Siloxide Complex Immobilized on Silica**
Bogdan Marciniec,* Karol Szubert, Marek J. Potrzebowski, Ireneusz Kownacki, and
Kinga Łe¸szczak
Molecular compounds incorporating a TM O Si group by reaction of organometallic precursors (e.g., [Rh(h3-
À À
(TM = transition metal) are of great interest, particularly as C3H5)3][16a,c] and [Rh(CO)(Me)(PMe3)2][16b]
) with silica.
models of metal complexes immobilized on silica and silicate Immobilized complexes have been characterized by IR, UV,
surfaces.[1–3] Since 1982, more than 100 new TM siloxide and 31P MAS-NMR spectroscopy.
complexes have been synthesized and characterized by X-ray
We report herein the synthesis and characterization of the
and spectroscopic methods to determine their molecular well-defined rhodium siloxide complex obtained directly by
structure.[1a]
reaction of molecular rhodium siloxide precursor [{Rh(m-
The properties of siloxide as ancillary ligand in the system OSiMe3)(cod)}2] (1, cod = 1,5-cyclooctadiene)[17] with aerosil
À À
TM O SiR3 can be exploited in molecular catalysis, partic- silica.
ularly in such reactions as polymerization[4] olefin meta-
When a mixture of SiO2 (aerosil 200 dried at 3508C,
thesis,[5] epoxidation of alkenes[6] and dehydrogenative cou- 1.27 mmol SiOH per gram, determined by the hydrothermal
pling of silanes.[7] In addition, in the last few years the method[18]) and 1 was stirred in pentane (benzene) for 24 h at
synthesis and characterization of well-defined surface room temperature, surface organometallic complex 2 (2a)
[8]
ꢀ
=
siloxy complexes [( SiO)Re(tBu)( CHtBu)CH2tBu)] and was formed (see Scheme 1 and the Supporting Information).
[9]
ꢀ
=
=
[( SiO)Mo( NAr)( CHtBu)(CH2tBu)] as highly active
heterogeneous olefin metathesis catalysts have been pub-
lished.
In contrast to the abundance of data published on the
early TM siloxides, information on late TM siloxide com-
plexes is scarce.[1a] During the last decade we have developed
a synthetic method to obtain and characterize a variety of
molecular rhodium and iridium siloxide complexes that are
efficient catalysts for hydrosilylation,[10] silylative coupling,[11]
and other reactions of silicon compounds.[12] Tilley et al.
recently reported the synthesis of well-defined tri-tert-butoxy-
siloxy iron(III) complex,[13] as well as analogous molecular
Scheme 1. Synthesis of surface siloxide rhodium complex.
siloxide complexes of Co[14a] and Cu,[14b] which act as The orange solid thus prepared contained 0.217 mmol Rh per
precursors for their grafting onto silica and use as catalysts gram of solid 2 (0.410 mmol Rh per gram of solid 2a), which is
for oxidation of alkanes, alkenes, and arenes by hydrogen in agreement with consumption of the surface silanol groups
peroxide.[13]
(35% 2 and 66% 2a). Moreover, compound 2a contained on
The groups of Basset and Santini[15] and Schwartz[16] average 8.9 C per grafted Rh (as obtained from elemental
reported a strategy for forming surface rhodium complexes analysis), consistent with the theoretically proposed structure,
for which 8 C/Rh was predicted.
Rhodium siloxide complexes, in particular 1, appeared to
[*] Prof. Dr. B. Marciniec, K. Szubert, I. Kownacki, K. Łe¸szczak
be much more effective then the respective chloro complex
Department of Organometallic Chemistry
(as well as the commercially available Pt Karstedt catalyst) in
Faculty of Chemistry
Adam Mickiewicz University
hydrosilylation of a variety of olefins such as 1-hexene,[19]
Grunwaldzka 6, 60-780 Poznan (Poland)
Fax: (+48)618-291-508
vinyl silanes,[20] and allyl alkyl ethers[21] (for a recent review,
see reference [10]). Therefore, the well-defined silica-sup-
ported heterogeneous catalysts 2 and 2a were tested in
selected hydrosilylation reactions of 1-alkenes and vinyl-
siloxanes by heptamethylhydrotrisiloxane as a model of
(poly)hydrosiloxanes.
´
E-mail: Bogdan.Marciniec@amu.edu.pl
Prof. Dr. M. J. Potrzebowski
NMR Laboratory and Department of Structural Studies
Centre of Molecular and Macromolecular Studies
Polish Academy of Sciences
The results (Table 1) show the high effectiveness of these
catalysts, which moreover were subjected to recycling tests.
After completion of each test cycle, the post-reaction mixture
was decanted and the catalyst left on the bottom of the
reaction vessel was used in subsequent reactions. In the
majority of reactions, the catalytic activity of the silica-
´
Sienkiewicza 112, 90–363 Łódz (Poland)
[**] Financial support from the Ministry of Science and Higher
Education (Poland), Grant N204 162 32/4248 is gratefully
acknowledged.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2008, 47, 541 –544
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
541

Communications
Table 1: Hydrosilylation of 1-alkenes and vinyl siloxane by heptamethyl-
hydrotrisiloxane.[a]
R
Cat.
Cycles
Yield[b] [%] (TOF[c] [minÀ1])
(CH2)4CH3
2
1
5
97 (162)
98
10
15
1
95
87
97 (162)
(CH2)13CH3
2a
5
98
10
15
1
5
10
1
95
88
90 (150)
97
60
88 (160)
90
CH2C6H3-3,4-OMe
SiMe(OSiMe3)2
2a
2
5
10
15
20
86
83
91
[a] [SiH]/[H2C CHR]/[Cat.]=1:1:10À4, T=1008C, t=1 h. [b] Yields were
determined by GC analysis. [c] Initial turnover frequency (TOF) mea-
sured after 1 h of reaction and expressed as moles of heptamethylhy-
drotrisiloxane per mole of Rh per minute.
=
Scheme 2. Mechanism of hydrosilylation catalyzed by soluble binuclear
rhodium siloxide complex.
supported rhodium complexes remained practically
unchanged even after more than ten cycles. In particular,
catalyst 2 was shown to maintain a similar catalytic activity in
the hydrosilylation of vinylsiloxanes even after 20 cycles. As
shown by inductively coupled plasma (ICP) analysis, the
content of rhodium in the sample of catalyst 2 after 20 cycles
in this reaction was 156 mmol per gram of silica, that is, 72%
of the initial amount of this element in catalyst 2.
It is well known from earlier studies that in the presence of
Scheme 3. Oxidative addition of silicon hydride to surface rhodium
siloxide complex.
À À
a silicon hydride the Rh O SiMe3 complex undergoes
oxidative addition and subsequent elimination of disiloxane
to generate a low-coordinate complex (16e hydride complex
with already coordinated alkene molecule) with initiation of a
direct hydrosilylation process according to Scheme 2.
À
If such initiation of an Rh H complex containing no
siloxide ligand also took place in the heterogeneous system,
leaching of rhodium diene complex should be observed.
Therefore, in view of the unusually successful recycling of
heterogeneous rhodium catalyst 2 in selected hydrosilylation
reactions, it is necessary to consider a different mechanistic
pathway for the heterogeneous catalyst, which remains stable
and active for reuse.
A number of experimental techniques can be used for
structural studies on heterogeneous catalysts. High-resolution
solid-state NMR spectroscopy has become an indispensable
method for characterization of products and pathways of
organometallic syntheses and catalysis.[9] We used this method
to confirm the reaction pathway shown in Scheme 3and fully
characterize product 3.
Figure 1 shows 13C CP/MAS spectra of 2 and 3 recorded at
a spinning rate of 10 kHz at ambient temperature. Two well-
defined 13C NMR signals at d = 29 and 75 ppm correspond
respectively to methylene and olefinic carbon atoms of 2
Figure 1. 75.46 MHz 13C NMR solid-state CP/MAS spectra of 2 (a) and
3 (b) at a spinning rate of 10 kHz.
542
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(Figure 1a). The spectrum of 3 (Figure 1b) contains five
resolved peaks, which can be assigned as follows: d = 0
((CH3)2Si), 30 (CH2 of cod), 76 (CH of cod), 128, and 135 ppm
(Ph). The 29Si CP/MAS spectra of both samples confirmed
formation of product 3 [d = 99.68 (Q2), 106.88 (Q3), 110.38
(Q4)]. The NMR spectra of the above compounds were
compared with the data for parent complex 1. (The 29Si CP/
MAS and 13C CP/MAS spectra of 1 are provided as
Supporting Information.) From analysis of the spectra it is
clear that compared to samples of 2 and 3, resolution for the
parent system is much better. The four resonances at d = 77.1,
74.6, 72.6, and 70.2 ppm can be assigned to crystallographi-
cally and magnetically non-equivalent vinyl carbon atoms of
the cod ligand. In contrast, three of the crystallographically
nonequivalent methylene groups are magnetically equivalent.
In consequence, we observed only two lines at d = 30.8 and
29.5 ppm with 3:1 integration. The methyl groups bonded to
Si resonate at d = 9.3ppm. The 29Si chemical shift is d =
9.3ppm.
at moderate frequencies (10 kHz) is usually sufficient to
obtain fairly well-resolved proton spectra. This is because the
grafted compounds, although covalently linked to the surface,
still have several degrees of motional freedom that partially
average out the dipolar interactions. Moreover, single-site
catalysts are usually dispersed on the surface, typically
separated by more than 10 Š, which significantly reduces
the proton density at a given site. In our project, proton SS
NMR spectroscopy was employed for characterization of
sample 3. A full description of the 1H CRAMPS FSLG
spectra is provided as Supporting Information.
The results of solid-state NMR spectroscopy have shown
ꢀ
the presence of ( SiO)Rh(H)SiMe2Ph surface organometallic
complexes, formation of which is strong evidence for the
absence of the siloxane elimination observed in the homoge-
neous system catalyzed by 1. Therefore, the catalytic cycle of
heterogeneously catalyzed hydrosilylation is different from
that of the homogeneous case and occurs by the well-known
Chalk–Harrod mechanism initiated by stable immobilized
I
ꢀ
Further evidence for formation of 3 and assignment of the
chemical shifts of cod and trisubstituted silane were obtained
by heteronuclear 13C,1H FSLG HETCOR 2D correlation.
Figure 2 shows the 2D spectrum recorded at a spinning rate of
[( SiO)Rh (cod)] complex 2 according to Scheme 4.
Figure 2. 1H,13C FSLG HETCOR MAS spectrum of 3 recorded at a
spinning rate of 10 kHz and a contact time of 2 ms.
Scheme 4. Mechanism of hydrosilylation catalyzed by surface rhodium silox-
ide complex.
10 kHz and a contact time of 2 ms. The cross-peaks are clear-
cut, and each proton signal can be assigned to a specific
carbon atom. It is noteworthy that the chemical shifts of the
methylene and vinyl protons of cod coordinated to rhodium
are very similar. The spread of proton signals for the
methylene carbon atoms is much larger than that of the
vinyl protons.
In conclusion, we have shown that the rhodium siloxide
surface complex 2 (2a), prepared from molecular precursor 1
as a well-defined rhodium siloxide complex immobilized on
silica and characterized by solid-state NMR spectroscopy and
elemental analysis, is a highly effective catalyst for hydro-
silylation of 1-alkenes and vinylsiloxanes. We believe that
silanol groups in 2 are responsible for the high stability of the
heterogeneous catalyst, which can be recycled at least 10–20
times without a decrease in the yield and selectivity.
The groups of Emsley[22] and Schrock[23] recently reported
that for molecules on surfaces, the magic-angle spinning alone
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543

Communications
[10] a) B. Marciniec, Silicon Chem. 2002, 1, 155; b) “Hydrosilylation
and Related Reactions of Silicon Compounds”: B. Marciniec in
Applied Homogeneous Catalysis with Organometallic Com-
pounds, Vol. 1, 2nd ed. (Eds.: B. Cornils, W. A. Herrmann),
Wiley-VCH, Weinheim, 2002, p. 491.
Experimental Section
All experiments were performed under dry and oxygen-free argon by
using standard Schlenk techniques for organometallic synthesis.
See the Supporting Information for full experimental procedures,
1H, 13C, and 29Si NMR spectra, GC–MS, and elemental analyses of
products.
´
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Keywords: heterogeneous catalysis · hydrosilylation · rhodium ·
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