A Lewis acid catalyst anchored on silica grafted with quaternary alkylammonium chloride moieties

Article abstract of DOI:10.1002/1521-3773(20010803)40:15<2881::AID-ANIE2881>3.0.CO;2-P

Resistance to leaching and re-usability are characteristic of the novel heterogeneous Lewis acid catalyst that was prepared by anchoring tin(IV) chloride on silica grafted with tetraalkylammonium or pyridinium chloride groups. The catalyst displays high activity and selectivity in the synthesis of 3-methyl-3-buten-1-ol by the Prins condensation of isobutene with formaldehyde [Eq. (1)].

Full text of DOI:10.1002/1521-3773(20010803)40:15<2881::AID-ANIE2881>3.0.CO;2-P

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[4] R. J. Francis, D. OꢁHare, J. Chem. Soc. Dalton Trans. 1998, 3133.
[5] J. Rocha, M. W. Anderson, Eur. J. Inorg. Chem. 2000, 801.
[6] J. Rocha, P. Brandao, Z. Lin, A. P. Esculcas, A. Ferreira, M. W.
Anderson, J. Phys. Chem. 1996, 101, 14978.
[7] W. T. A. Harrison, T. E. Gier, G. D. Stucky, J. Solid State Chem. 1995,
115, 373.
mechanism by which disorder can be introduced in this
direction that also accounts for the observed niobium
deficiency. If all of the Nb(1) atoms in a (101) ªlayerº are
removed, the remaining structural motifs can be connected
through corner-shared Nb(2) and Si(Ge) polyhedra to form
structures containing six-membered channels along the [010]
direction. Modeling studies suggest that if such layers of
Nb(1) atoms are removed at random, order is lost along the
[101] direction but maintained in the [011] direction.
Thermal analysis of NSH-1 and NGH-1 revealed that loss
of the template occurs between about 200 and 5208C. Powder
X-ray diffraction patterns of samples of NSH-1 and NGH-1
heated for 1 ± 2 h at 350 and 4408C, respectively, indicated
that the frameworks remain intact following loss of the
template, although a substantial loss of crystallinity was
evident. IR spectroscopy confirmed complete loss of the
template from the calcined materials. Heating at substantially
higher temperatures produced amorphous materials.
[8] J. Rocha, P. Brandao, A. Phillippou, M. W. Anderson, Chem.
Commun. 1998, 2687.
[9] Powder diffraction data on NGH-1 were collected on beamline X7A
of the NSLS, Brookhaven National Laboratory, l ˆ 0.70117 Š, over
the range 2 ± 478 2q. The pattern was indexed by using the
orthorhombic cell a ˆ 13.0861(2), b ˆ 7.4937(2), c ˆ 11.4967(4) Š,
space group Imma (no. 74). The structure was solved by using the
direct methods package EXPO.[13] This solution was then used in the
Rietveld analysis of the structure using the software package GSAS.[14]
The template atom positions were determined from difference Fourier
syntheses. Soft constraints were applied to the C C and C N bond
lengths in the refinement. The final refinement consisted of 70
structural and profile parameters and proceeded smoothly to con-
vergence, with Rp ˆ 0.0605, Rwp ˆ 0.0716, RF ˆ 0.0552, RF2 ˆ 0.0778,
c2 ˆ 7.732. Powder diffraction data on NSH-1 were collected on a
Scintag 2000 diffractometer using Cu radiation over the range 8 ± 1008
2q. Unit cell refinement gave a ˆ 12.9041(4), b ˆ 7.4967(3), c ˆ
11.4471(5) Š, space group Imma. The structure solution of NGH-1
was used as the starting model in the Rietveld analysis. The refinement
proceeded smoothly to convergence, with Rp ˆ 0.0590, Rwp ˆ 0.0459,
RF ˆ 0.0609, RF2 ˆ 0.0722, c2 ˆ 0.9493. Soft constraints were applied to
the C C and C N bond lengths. Crystallographic data (excluding
structure factors) for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no.ª CCDC-163346 (NSH-1) and CCDC-
163347 (NGH-1). Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax:
(‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
Ion-exchange experiments revealed that the piperazinium
cations in NSH-1 and NGH-1 could be successfully exchanged
‡
‡
for Na and K ions, as demonstrated by small changes in the
powder X-ray diffraction patterns and the diminution in the
intensity of the carbon ± carbon and carbon ± nitrogen stretch-
es in the IR spectra. The materials largely retained their
crystallinity following the exchange procedure, although some
line broadening was observed.
In summary, we have described the low-temperature
hydrothermal synthesis and structure of the first examples
of organically templated open-framework niobium silicates
and germanates. The materials display thermal stability and
ion-exchange capability. Given the large variety of organic
templates that could be used in this synthetic regime, and the
range of compositions seen in condensed niobium silicates
and germanates, the scope for the synthesis of further novel
materials in this class appears to be very large.
[10] N. E. Brese, M. OꢁKeeffe, Acta Crystallogr. Sect. B 1991, 47, 192.
[11] R. D. Shannon, Acta Crystallogr. Sect. A 1976, 32, 751.
[12] R. C. Weast, Handbook of Chemistry and Physics, Chemical Rubber
Company, Cleveland, 1972.
[13] A. Altomare, M. C. Burla, M. Camalli, B. Carrozzini, G. L. Cascarano,
C. Giacovazzo, A. Guagliardi, A. G. Giuseppina-Moliterni, G. Poli-
dori, R. Rizzi, J. Appl. Crystallogr. 1999, 32, 339.
[14] A. C. Larson, R. B. V. Dreele, Los Alamos Natl. Lab. Rep. 1987, La-
UR-86-748.
Experimental Section
Syntheses were conducted hydrothermally in a Teflon-lined autoclave
(23 mL).
NSH-1: Nb2O5 (0.089 g,99.5%, Aldrich) was dissolved in aqueous HF
(0.167 g 48 wt%, Aldrich) and heated to 1108C for 4 h. After cooling, this
solution was combined with SiO2 (0.120 g, fumed, 99.8%, Aldrich),
piperazine (0.459 g, 99%, Aldrich), H2O (2.88 g), and ethylene glycol
(5 g) and heated at 1608C for 25 days. A fine white powder (particle size ca.
0.2 mm) of NSH-1 was recovered. NGH-1 was synthesized following the
same procedure, by using 0.209 g of GeO2 (99.99%, Aldrich) instead of the
SiO2. A fine white powder of NGH-1 (ca. 0.4 mm) was obtained after
heating at 1608C for six days.
A Lewis Acid Catalyst Anchored on Silica
Grafted with Quaternary Alkylammonium
Chloride Moieties**
Thundi M. Jyothi, Mark L. Kaliya, and
Miron V. Landau*
The development of novel catalytic materials based on
mesoporous supports by chemically binding the active species
on their surfaces has become of profound importance in
recent years owing to the benefits of catalyst heterogeniza-
Ion-exchange reactions were performed by stirring 100 mg samples in 2m
aqueous solutions (10 mL) of either NaCl or KCl at 608C for 12 h.
Thermogravimetric analyses were performed in flowing dry air on a
Thermal Instruments TGA 2950 instrument at a ramp rate of 5Kmin 1. IR
spectra were collected on a Mattson FTIR 5000 spectrometer (KBr
method).
[*] Prof. M. V. Landau, Dr. T. M. Jyothi, Dr. M. L. Kaliya
Blechner Center for Industrial Catalysis and Process Development
Department of Chemical Engineering
Ben Gurion University of the Negev
Received: March 12, 2001 [Z16758]
[1] P. B. Venuto, Microporous Mater. 1994, 2, 297.
[2] T. J. Barton, L. M. Bull, W. G. Klemperer, D. A. Loy, B. McEnaney, M.
Misono, P. A. Monson, G. Pez, G. W. Scherer, J. C. Vartuli, O. M.
Yaghi, Chem. Mater. 1999, 11, 2633.
[3] W. M. Meier, D. H. Olson, C. Baerlocher, Atlas of Zeolite Structure
Types, Elsevier, Boston, 1996.
Beer Sheva-84105 (Israel)
Fax : (‡972)8-6472902
E-mail: mlandau@bgumail.bgu.ac.il
[**] T.M.J. is grateful to the Blechner fund for a postdoctoral fellowship.
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tion.[1] This can potentially lead to enhanced activity and/or
product selectivity while avoiding the drawbacks of catalyst
instability and limited reusability. Although polymer-support-
ed catalysts have been widely used in synthesis, catalysts
based on high-surface-area inorganic supports are preferred
due to their higher thermal stability and better rates of
molecular diffusion.[2] The corrosivity and potential environ-
mental hazards associated with common Lewis acid catalysts
such as AlCl3, BF3, and SnCl4, as well as difficulties in catalyst
recycling, have aroused great interest in finding alternatives.
Direct immobilization of such catalysts on inorganic supports
appears to be easy, but their application is rather limited due
to possible leaching of the supported species during the
reaction.[2b,3] Also, the mineral acid evolved in the reaction of
surface hydroxy groups with the Lewis acid is sometimes
retained on the catalyst and can even change the nature of the
support.[4]
Preliminary experiments on the Prins condensation of
isobutene with formaldehyde indicated that complexation of
SnCl4 with a tetraalkylammonium chloride leads to improved
selectivity for the unsaturated alcohol MBOH. Such com-
plexes were grafted onto silica by two methods (Scheme 2).
Scheme 2. Preparation of SnCl4 anchored on tetraalkylammonium chlor-
ide functionalized silica. Py ˆ pyridine, TPA ˆ tetrapropylammonium
chloride, R ˆ alkyl.
In connection with our ongoing research on the Prins
condensation of alkenes and formaldehyde to give valuable
unsaturated alcohols (Scheme 1), we developed a new catalyst
Formation of a complex between SnCl4 and the tetraalkyl-
ammonium chloride moiety was confirmed by 119Sn NMR
spectroscopy. A shift in the NMR signal from d ˆ 665.0 to
725.2 after the reaction with tetraalkylammonium chloride
reflects the change in the coordination number of tin.
Moreover, the tin/chlorine atomic ratio, determined by
elemental analysis, was in accordance with formation of such
a complex.
The results of the Prins condensation of isobutene and
formaldehyde on various complexes are summarized in
Table 1. Clearly, complexation of SnCl4 with tetraalkylammo-
nium chloride improves the selectivity for the unsaturated
alcohol. Tin(iv) chloride alone under anhydrous conditions
displayed high activity, but the selectivity for MBOH was
poor. The solvent (acetonitrile, chloroform, or dichlorome-
thane) had little influence on formaldehyde conversion and
MBOH selectivity. However, the use of an aqueous form-
aldehyde solution instead of paraformaldehyde adversely
Scheme 1. Prins condensation of isobutene with formaldehyde. MBD ˆ
3-methylbutane-1,3-diol, DMD ˆ 4,4-dimethyl-1,3-dioxane, MBOH ˆ
3-methyl-3-buten-1-ol.
by anchoring the active SnCl4 catalyst on silica functionalized
with organic quaternary ammonium chloride moieties. Our
methodology is based on the fact that the nucleophilic
addition of tetraalkylammonium halides to SnCl4 leads to
the formation of pentacoordinate anionic tin species.[5] Such
Table 1. Prins condensation of isobutene and formaldehyde over NR4Cl ±
SnCl4 complexes.[a]
‡
complexes of general formula [NR4] [SnCl5] resemble ionic
liquids or molten salts.[6] Ionic liquids, which can be prepared
Entry Catalyst
Solvent/
HCHO
Conver-
sion of
MBOH
MBOH
‡
by the reaction of a tetraalkylammonium halide NR4 X with
selectivi- yield [%][h]
formalde- ty [%][g]
hyde [%]
a Lewis acid MX, are effective Friedel ± Crafts catalysts.
Holderich et al. immobilized such ionic liquids directly on
various inorganic supports and used them in alkylation
reactions.[7]
1
2
3
4
5
6
7
8
SnCl4
CHCl3/pf[b]
CHCl3/pf
CH2Cl2/pf
CH3CN/pf
97.4
77.1
77.3
75.9
54.6
89.0
88.5
87.7
40.0
87.1
89.4
±
53.0
68.7
68.4
66.7
22.0
66.7
62.8
±
SnCl4-TPAC[c]
SnCl4-TPAC
SnCl4-TPAC
SnCl4-TPAC
SnCl4-BTEAC[e]
A recent patent described the use of SnCl4 anchored to an
ion-exchange resin containing tetraalkylammonium chloride
substituents in the Prins condensation of isobutene and
formaldehyde for the selective synthesis of 3-methyl-3-
buten-1-ol (MBOH), which is an important synthesis block
for industrially valuable terpenes.[8] Zeolite catalysts such as
ZSM-5 and FeMCM-22 were used in the selective synthesis of
MBOH, but the yield was unsatisfactory.[9] Also, MBOH was
prepared from isobutene and formaldehyde by using metal
chlorides such as SnCl4 and ZnCl2 under mild conditions, but
the yield was low due to the formation of many side
products.[10]
CH3CN/aq[d] 59.7
CHCl3/pf
76.6
70.3
±
SnCl4-TMSPAC[f] CHCl3/pf
No catalyst CHCl3/pf
[a] 56 g isobutene, 3 g paraformaldehyde, catalyst containing 4 mmol of SnCl4,
and 40 g of chloroform solvent were introduced into the reactor and stirred for
2 h at 608C. [b] Paraformaldehyde. [c] Tetrapropylammonium chloride ± SnCl4
complex. [d] 30% aqueous solution of formaldehyde. [e] Benzyltriethylammo-
nium chloride ± SnCl4 complex. [f] Complex prepared from [3-(trimethoxy-
silyl)propyl]octadecyldimethylammonium chloride and SnCl4. [g] Other prod-
ucts included 4,4-dimethyl-1,3-dioxane and traces of polycondensation prod-
ucts. [h] Yield was determined by using 2-butanol as an internal standard.
2882
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affected the conversion and MBOH selectivity. Under these
conditions, larger amounts of side products such as 4,4-
dimethyl-1,3-dioxane (DMD) and 3-methylbutane-1,3-diol
(MBD) were formed. Hence, to maximize the yield of
MBOH, anhydrous conditions were maintained in all other
experiments.
different organic reactions. This type of anchoring could be
advantageous for obtaining catalysts with high stability and
longer lifetime. Unlike catalysts with a directly attached Lewis
acid, prepared by the reaction of surface hydroxy groups of
the support with hydrolyzable groups in the Lewis acid, the
present catalyst is stable under the experimental conditions
due to the formation of strong Si O Si bonds on the catalyst
surface. Moreover, we believe that the the active sites will be
more readily accessible to the reactants, since they are further
from the silica surface.
Various silica-based catalysts were tested in the selective
synthesis of MBOH (Table 2). For comparison, the
‡
NR4 SnCl5 complex immobilized directly on silica and
In conclusion, we have demonstrated a new supported
Lewis acid catalyst which is resistant to leaching and shows
high selectivity in the synthesis of 3-methyl-3-butene-1-ol by
the Prins condensation of isobutene and formaldehyde.
Further work is underway to exploit these supported Lewis
acid catalysts for developing clean technology in fine-chem-
ical synthesis.
Table 2. Condensation of isobutene and formaldehyde to give 3-methyl-3-buten-
1-ol (MBOH) over silica-based catalysts and with catalyst recycling.[a]
Entry Catalyst
SnCl4 loading Conversion MBOH
MBOH
yield [%]
1
[mmolg
support]
of formalde- selec-
hyde [%]
tivity [%]
‡
[b]
[c]
1
2
3
4
5
6
7
8
9
SIL-TPA SnCl5
0.32
0.32
0.95
0.78
0.78
±
0.78
0.48
0.48
64.1
57.0
77.9
64.6
63.3
61.9
100
88.1
79.8
63.2
92.8
91.1
91.0
76.1
91.9
82.6
56.4
45.5
49.2
59.9
57.6
56.3
76.1
60.9
80.7
‡
SIL/TPA SnCl5
Silica/SnCl4
‡
[d]
SIL-NR4 SnCl5
Experimental Section
recycle 1[e]
recycle 2
All manipulations were performed under dry and inert conditions in a
glove box, and all solvents were dried by standard procedures. SnCl4 ±
TPAC complex was prepared by the dropwise addition of anhydrous SnCl4
(4 mmol) to a solution of tetrapropylammonium chloride (4 mmol) in
dichloromethane (40 g) with constant stirring. The mixture was stirred
overnight at ambient temperature, and the solution was concentrated in
vacuum to give a white crystalline material, which was recrystallized from
dichloromethane/hexane and stored under moisture-free conditions. Ele-
mental analysis (%): calcd: Sn 24.61, Cl 36.74; found: Sn 24.78, Cl 36.40.
Silica gel (surface area: 500 m2 g 1) with an average pore diameter of 6 nm
and particle size between 0.063 and 0.20 mm (Aldrich), preheated at 473 K
for 6 h, was used as support. In method 1, silica (5 g) was treated with a
solution of [3-(trimethoxysilyl)propyl]octadecyldimethylammonium chlor-
ide (4.82 g, 7 mmol, Aldrich) in chloroform to give tetraalkylammonium
chloride functionalized silica. The excess of the reagent was removed by
exhaustive Soxhlet extraction with dichloromethane and finally dried in
vacuum. In method 2, silica support (5 g) was first treated with a solution of
trimethoxysilylpropyl chloride (3 g, 14.64 mmol, Aldrich) in toluene under
reflux for 5 h. The excess of the reagent was removed as explained above,
and the propylated silica was treated with an excess of pyridine or
tripropylamine (6 ± 7 g) in toluene for 24 h under reflux to give pyridinium
chloride functionalized and tetrapropylammonium chloride functionalized
silica, respectively.
‡
[f]
SIL-NR4 SnCl5
‡
[g]
SIL-Py /SnCl5
SIL-Py /SnCl5
66.3
97.8
‡
[f]
[a] Reaction conditions were as in Table 1, and in all cases the amount of catalyst
containing 4 mmol of SnCl4 was used. [b] Tetrapropylammonium chloride
functionalized silica complex. [c] Direct immobilization of complex on silica.
‡
[d] Catalyst prepared by method 1. [e] Recycling of SIL-NR4 SnCl5 after
exhaustive washing with dichloromethane. [f] Reaction continued for 3.5 h.
[g] Pyridinium chloride functionalized silica complex.
silica-supported SnCl4 were also tested. The catalysts immo-
bilized on silica that was functionalized with organic quater-
nary ammonium moieties showed the highest selectivity for
MBOH (88 ± 93%). X-ray fluorescence (XRF) analysis of the
filtrate after the reaction (detection limit: >0.5 ppm of Sn)
indicated no leaching of tin from the catalyst. Furthermore,
when a subsequent reaction was performed with the filtrate
after separating the catalyst no formaldehyde conversion
occurred. Recycling of the catalyst led to no appreciable loss
in activity. Increasing the reaction time to 3.5 h increased the
MBOH yield to 76% (entry 7) and 81% (entry 9). The
catalyst prepared by treating a solution of SnCl4 in chloroform
with silica (silica/SnCl4) is prone to leaching during the
reaction. Analysis of the filtrate after the reaction showed that
around 39% of the tin chloride had been leached out.
Moreover, the performance of this catalyst in the selective
synthesis of MBOH is poor (entry 3) compared to immobi-
lized complexes. Direct immobilization of the complex on
The amount of chloride ions present on the functionalized silica was
determined by treating the solid with 0.1n nitric acid, followed by titration
with a standard AgNO3 solution and potassium chromate indicator.
The tin chloride was anchored on functionalized silica by treating a
suspension of the amount of modified silica containing 4 mmol of chloride
anion in dicholoromethane with a solution of anhydrous SnCl4 (1.04 g,
4 mmol) with stirring for 12 h (Scheme 2). After the reaction, the catalyst
was collected by filtration and subjected to Soxhlet extraction to remove
loosely bound species.
Silica/TPA-SnCl4 catalyst was prepared by adding a chloroform solution of
the complex prepared as mentioned above to 4 g of predried silica support
suspended in chloroform with constant stirring. The excess solvent was
removed after stirring overnight, and the material was extracted with
chloroform for 12 h in a Soxhlet apparatus. The material was finally dried
and kept under moisture-free conditions before use.
‡
silica (SIL/TPA -SnCl5 ) also resulted in lower catalytic
activity and MBOH yield relative to the pure complex
‡
SnCl4 ± TPAC and the SIL-TPA SnCl5 catalyst. The turnover
number (TON), that is, the number of moles of formaldehyde
converted per mole of catalyst per unit time, for the SIL-
The Prins condensation of isobutene and formaldehyde was carried out at
608C under autogenous pressure by introducing isobutene (56.1 g, 1.0 mol),
paraformaldehyde (3 g, 0.1 mol), the solvent (40 g), and the amount of
catalyst containing 4 mmol of SnCl4 into a 250-mL Büchi autoclave. After
stirring the mixture for 2 h, isobutene was released, the solid catalyst was
separated, and the filtrate was analyzed on a gas chromatograph equipped
with a capillary column (DB WAX) and thermal conductivity detector.
‡
TPA SnCl5 catalyst (TON ˆ 8.0) was 83% of that obtained
in a homogeneously catalyzed reaction (TON ˆ 9.6, Table 1,
entry 2) at similar MBOH selectivities. Direct immobilization
of the complex reduced the relative TON to 74% (TON ˆ 7.3).
The present methodology of immobilizing a Lewis acid is of
wide practical utility due to the possibility of anchoring
various metal halides to give novel supported catalysts for
Received: February 23, 2001 [Z16673]
Angew. Chem. Int. Ed. 2001, 40, No. 15
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COMMUNICATIONS
[1] a) J. H. Clark, D. J. Macquarrie, Chem. Commun. 1998, 853; b) A.
Cauvel, G. Renard, D. Brunel, J. Org. Chem. 1997, 62, 1325; c) Y. V. S.
Rao, D. E. De Vos, P. A. Jacobs, Angew. Chem. 1997, 109, 2776;
Angew. Chem. Int. Ed. Engl. 1997, 36, 2661; d) I. Rodriguez, S. Iborra,
A. Corma, F. Rey, J. L. Jorda, Chem. Commun. 1999, 593.
[2] Chemistry of Waste Minimization (Ed.: J. H. Clark), Chapman and
Hall, London, 1995, p. 141; b) T. W. Bastock, J. H. Clark in Speciality
Chemicals (Ed.: B. Pearson), Elsevier, London, 1992.
[3] a) A. Krzywicki, M. Marczewski, J. Chem. Soc. Faraday Trans. 1 1980,
1311; b) R. S. Drago, S. C. Petrosius, P. B. Kaufman, J. Mol. Catal.
1984, 89, 317.
[4] J. B. Butrille, T. J. Pinnavaia, Catal. Today 1992, 14, 141.
[5] S. E. Johnson, C. B. Knobler, Organometallics 1992, 11, 3684.
[6] G. A. Parshall, J. Am. Chem. Soc. 1972, 94, 8716.
supports for a great variety of reactions, and several well-
defined soluble metallasiloxanes[4, 5] have been used to mimic
and facilitate the understanding of the situation in solid
materials, until now no molecular models have been reported
for late transition metal catalysts supported on titania ± silica.
We previously reported[6] the synthesis of the complex
[Cp*Ti(m3-O)3{Rh(cod)}3] (cod ˆ 1,5-cyclooctadiene, Cp* ˆ
h5-C5Me5), which can be regarded as a model of rhodium
supported on titania. We have now extended our studies to
modeling a rhodium complex supported on titania ± silica.
Here we report on the synthesis of the dimetallic titanium
alkyl siloxide complexes [{Cp*TiMe(O2SiPh2)}2] (1) and
[Cp*Ti(O2SiPh2)3TiCp*] (2), which can be regarded as models
for the above-mentioned titania ± silica systems. The reaction
of 1 with [{Rh(m-OH)(cod)}2] leads to the formation of
[{Cp*Ti(O2SiPh2)(m3-O)Rh(cod)}2] (3), which can be regarded
as an unprecedented molecular model of a late transition
metal supported on titania ± silica.
[7] a) P. Wasserscheid, W. Keim, Angew. Chem. 2000, 112, 3926; Angew.
Chem. Int. Ed. 2000, 39, 3772; b) C. DeCastro, E. Sauvage, M. H.
Valkenberg, W. F. Holderich, J. Catal. 2000, 196, 86.
[8] Kuraray Co. Ltd., Jpn. Pat. H09-262478, 1997.
[9] a) T. Yashima, Y. Katoh, T. Komatsu, Stud. Surf. Sci. Catal. 1999, 125,
507; b) C. D. Chang, N. J. Mogan, Jpn. Pat. 55-113732, 1980; c) H.
Fujiwara, H. Shinohara, T. Yoshida, Jpn. Pat. 58-164534, 1983.
[10] L. A. Mikeska, E. Arundale, US Pat. 2308192 [Chem. Abstr. 1943, 37,
3450]; Canadian Pat. 417, 600; British Pat. 545, 191 [Chem. Abstr. 1942,
36, 7030].
The titanium complex [Cp*TiMe3] reacts with diphenylsi-
lanediol to afford the yellow solid 1 (Scheme 1), which was
isolated in good yield (76%) as a mixture of two isomers. By
comparison with the previously reported analogous titanium
Molecular Models of Titania ± Silica Systems
and a Late Transition Metal Complex Grafted
Thereon**
Rosa Fandos,* Antonio Otero,* Ana Rodríguez,
Â
María Jose Ruiz, and Pilar Terreros
Titania ± silica materials are the focus of much attention on
account of their unique properties:[1, 2] 1) they can act as
catalysts for a wide variety of processes; 2) they are suitable
supports for late transition metal catalysts because they
improve the mechanical strength, thermal stability, and
surface area relative to TiO2 supports.[3] Although titania ±
silica materials have been extensively used as catalysts and
[*] Dr. R. Fandos, Dr. M. J. Ruiz
Â
Â
Departamento de Química Inorganica, Organica y Bioquímica
Facultad de Ciencias del Medio Ambiente
Universidad de Castilla-La Mancha
Avda. Carlos III, s/n 45071 Toledo (Spain)
Fax : (‡34)926-25-53-18
Prof. Dr. A. Otero
Â
Â
Departamento de Química Inorganica, Organica y Bioquímica
Facultad de Químicas, Universidad de Castilla-La Mancha
Campus de Ciudad Real, 13071 Ciudad Real (Spain)
Fax : (‡34)926-29-53-18
E-mail: aotero@qino-cr.uclm.es
Scheme 1. Synthesis of 1 and 2.
Dr. A. Rodríguez
ETS Ingenieros Industriales, Universidad de Castilla-La Mancha
complexes,[7] we propose that 1 is a dimer in which the siloxide
ligands act as bridges between two titanium atoms, and the
two isomers are the cis and trans forms.[8] According to
variable-temperature (VT) NMR experiments in [D8]toluene,
the ratio of the two isomers does not change up to 353 K. It is
also independent of the concentration of the sample and the
solvent ([D8]toluene, C6D6, CDCl3, [D8]THF). According to
Â
Avda. Camilo Jose Cela, 3, 13071 Ciudad Real (Spain)
Dr. P. Terreros
Â
Instituto de Catalisis y Petroleoquímica, CSIC
Cantoblanco, 28049 Madrid (Spain)
Â
Ä
[**] Financial support from the Direccion General de Ensenanza Superior
Â
e Investigacion, Spain (Grant. No. D.G.E.S. PB98-0159-C02-01) is
gratefully acknowledged. We thank Prof. Dr. J. L. G. Fierro, Instituto
Â
de Catalisis y Petroleoquímica, for XPS measurements.
2884
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4015-2884 $ 17.50+.50/0
Angew. Chem. Int. Ed. 2001, 40, No. 15