The Identity in Atomic Structure and Performance of Active Sites in Heterogeneous and Homogeneous, Titanium-Silica Epoxidation Catalysts

Article abstract of DOI:10.1021/jp991991+

The nature of the Ti(IV) active site in heterogeneous titano-silica epoxidation catalysts is shown to be same as that of a Ti(IV)-centered site bound to soluble homogeneous silsesquioxane catalysts. A range of techniques, encompassing pre- and near-edge X-ray absorption spectroscopy and molecular dynamics calculations, provides quantitative information pertaining to the four-coordinated site that is tripodally anchored to silica. The catalytic performance of a series of engineered active sites in [(c-C₅H₉)₇Si₇O ₁₂TiOXPh₃], where X = Si, Ge, or Sn, shows that the order of enhancement is Ge > Sn > Si. This is compared with previously published⁷ performances of engineered active sites in solid titano-silicate catalysts. The key importance of accessibility to the four-coordinated Ti(IV) site is highlighted here.

Full text of DOI:10.1021/jp991991+

J. Phys. Chem. B 1999, 103, 8809-8813
8809
The Identity in Atomic Structure and Performance of Active Sites in Heterogeneous and
Homogeneous, Titanium-Silica Epoxidation Catalysts
,
⊥
‡
§
‡
John Meurig Thomas,* Gopinathan Sankar, Marek C. Klunduk, Martin P. Attfield,
Thomas Maschmeyer, Brian F. G. Johnson,* and Robert G. Bell‡
|
,§,#
DaVy-Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street,
London, W1X 4BS, U.K., Department of Materials Science, The UniVersity of Cambridge, Pembroke Street,
Cambridge, CB2 3QZ, U.K., The UniVersity Chemical Laboratories, The UniVersity of Cambridge,
Lensfield Road, Cambridge, CB2 1EW, U.K., and Laboratory of Organic Chemistry and Catalysis,
Waterman Institute, Faculty of Applied Science, Delft UniVersity of Technology, Julianalaan 136,
2
628 BL Delft, The Netherlands
ReceiVed: June 16, 1999; In Final Form: August 26, 1999
The nature of the Ti(IV) active site in heterogeneous titano-silica epoxidation catalysts is shown to be same
as that of a Ti(IV)-centered site bound to soluble homogeneous silsesquioxane catalysts. A range of techniques,
encompassing pre- and near-edge X-ray absorption spectroscopy and molecular dynamics calculations, provides
quantitative information pertaining to the four-coordinated site that is tripodally anchored to silica. The catalytic
5 9 7 7 3
performance of a series of engineered active sites in [(c-C H ) Si O12TiOXPh ], where X ) Si, Ge, or Sn,
7
shows that the order of enhancement is Ge > Sn > Si. This is compared with previously published
performances of engineered active sites in solid titano-silicate catalysts. The key importance of accessibility
to the four-coordinated Ti(IV) site is highlighted here.
Introduction
our attention on a set of soluble model compounds based on
the family of titanium(IV)-derivatized silsesquioxanes (with the
8
Titanium-based heterogeneous catalysts, involving silica
supports, are employed in the production of epoxides using alkyl
hydroperoxides (typically ethylbenzyl- or tert-butyl hydroper-
general structure shown in Figure 1b). The great advantage of
using these titanosilsesquioxanes (which have been extensively
9,10
11,12
13-15
16-23
studied by Feher, Roesky,
ourselves,
and others
)
1
oxide) as oxidants. High-performance, laboratory-scale variants
is that the immediate environment of the titanium (single-site)
active center may be readily modified in a highly controlled
fashion. The titanosilsesquioxanes are extremely soluble in
solvents such as chloroform, enabling them to be used as
homogeneous catalysts with catalytic activities comparable to
of such catalysts, prepared from titanocene dichloride and
mesoporous silica (MCM-41),2 have been shown by X-ray
absorption spectroscopy and a range of other spectroscopic
studies to possess active centers that are “single-site”, in the
sense that such centers are isolated Ti(IV)-OH groups tripodally
anchored, via covalent bonds to oxygen, to the silica support
-5
those of heterogeneous catalysts such as the so-called TivMCM-
3-7
4
1 and TifMCM-41 forms described elsewhere.
These
(
see Figure 1a).
factors facilitate an analysis of the root-cause of the catalytic
enhancement of the next-nearest atom sites on the titanium
Recently, it has been shown5 that, when these single-site
-7
active centers are modified slightly so that the tripodal anchorage
is no longer HOTi-(OSi)3 but rather HOTi-(OSi)2(OGe), there
is a significant improvement in the catalytic activity. This result
is a little surprising, in view of the fact that the general features
of the chemistries of silicon and germanium are so very similar.
In contrast, substitution of Sn in a like manner greatly depletes
the activity. Modification, in a systematic fashion, of the
immediate environment of the active site in heterogeneous Ti/
SiO2 epoxidation catalysts is not easy to accomplish. To study
systematically the effect of varying the immediate environment
of the titanium center on its catalytic activity, we have focused
24
centers in titanosilicate catalysts in general, as well as shed-
14,15
ding further light on the influence of steric congestion
on
the catalytic epoxidizing performance of Ti(IV) active centers.
The studies described here entail the synthesis, characteriza-
tion (by X-ray absorption spectroscopy), and catalytic testing
of the following compounds: [(c-C5H9)7Si7O12Ti(OXPh3)]
where X is Si, Ge, or Sn (see Figure 1c) and comparison with
the molecular precursor compounds Ti-(O-SiPh3)4 and Ti-
25,26
(O-GePh3)4.
Experimental Section
Unless otherwise stated, all experiments were performed
under an atmosphere of dry, oxygen-free nitrogen using
conventional Schlenk techniques and solvents that were freshly
*
Corresponding authors.
Davy Faraday Research Laboratory, The Royal Institution of Great
†
Britain.
27
‡
Department of Materials Science, The University of Cambridge.
The University Chemical Laboratories, The University of Cambridge.
Waterman Institute, Delft University of Technology.
distilled before use. [(c-C5H9)7Si7O9(OH)3] was synthesized
§
|
⊥
2
8
following the method of Feher et al. Ti-(O-SiPh3)4 (4) and
Davy Faraday Research Laboratory, the Royal Institution of Great
Ti-(O-GePh3)4 (5) were synthesized using standard proce-
Britian, 21 Albemarle Street, London, W1X 4BS, U.K. Fax: (+44) 171
25,26
dures.
and these two materials not only serve as possible
4
957395.
#
precursors to the catalyst preparations, but also serve as good
model compounds for X-ray absorption studies, since we have
The University of Chemical Laboratories, The University of Cambridge,
E-mail: bfgj1@cam.ac.uk.
1
0.1021/jp991991+ CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/01/1999

8810 J. Phys. Chem. B, Vol. 103, No. 42, 1999
Letters
n
Ti(OBu )4 (0.34 mL, 1.05 mmol), and the resulting solution was
refluxed for 24 h. On cooling, the solvent was removed in vacuo
to yield a colorless liquid. A solution of [(c-C5H9)7Si7O9(OH)3]
(
0.765 g, 0.875 mmol) in THF (20 mL) was then added to this
liquid, and the subsequent solution was refluxed for a further
8 h. On cooling, the solvent was removed in vacuo and the
4
crude product was redissolved in CH2Cl2 (25 mL). After
filtration and addition of MeCN, a white powder precipitated,
which was filtered off, washed with pyridine, and then with
acetone to finally yield (2) as a white powder (0.774 g, 71%).
1
Selected Spectroscopic Data for (2). H NMR (400 MHz,
3
CDCl3, 25 °C, TMS): δ ) 7.65 (d, J(H,H) ) 10 Hz, 6H;
3
GePh3), 7.41 (dd, J(H,H) ) 10 Hz, 7.5 Hz, 3H; GePh3), 7.38
3
(
t, J(H,H) ) 7.5 Hz, 6H; GePh3), 1.73 (m, 20H; cyclopentyl-
13
H), 1.52 (m, 36H; cyclopentyl-H), 0.98 (m, 7H; ipso-H);
C
NMR (400 MHz, CDCl3, 25 °C, TMS): δ ) 134.08 (s, CH,
GePh3), 130.05 (s, CH, GePh3), 128.35 (s, CH, GePh3), 29.72,
2
7.52, 27.41, 27.32, 27.18, 27.06, 26.97 (s, CH2, unassignable),
+
2
2.33, 22.20, 21.98 (s, 3:3:1, CH, ipso-H); m/z (FAB ): 1032
+
+
(
M -3C5H9), 918 (M -OGePh3); Anal. Calcd for C53H78O13-
Si7GeTi (found): C, 51.3 (50.4), 6.30 (6.63).
Synthesis of [(c-C5H9)7Si7O12TiOSnPh3] (3). To a solution
of [(c-C5H9)7Si7O9(OH)3] (0.664 g, 0.760 mmol) in toluene (40
mL) was added TiCl4 (0.10 mL, 0.912 mmol), and the resulting
pale yellow solution was stirred at 25 °C for 4 h. A solution of
Ph3SnOH (0.334 g, 0.912 mmol) in toluene (15 mL) was then
added dropwise, and the resulting colorless solution was stirred
at 25 °C for a further 24 h. After filtration, the solvent was
removed in vacuo, and the crude product was redissolved in
CH2Cl2 (10 mL). After filtration and addition of MeCN, a white
powder precipitated, which was filtered off, washed with
pyridine, and then with acetone to finally yield (3) as a white
powder (0.210 g, 22%).
Figure 1. Schematic representation of the Ti(IV) centers (a) tripodally
attached to silica, (b) within a titanosilsesquioxane [R Si 12Ti-OX],
where R ) cycloalkyl and X ) Si, Ge, or Sn, and (c) anchored onto
silica as (∞Si-O) -Ti-OXPh
7
7
O
1
3
3
.
Selected Spectroscopic Data for (3). H NMR (400 MHz,
3
CDCl3, 25 °C, TMS): δ ) 7.66 (d, J(H,H) ) 10 Hz, 6H;
.
3
solved their crystal structures, recently All other reagents were
used as obtained commercially.
SnPh3), 7.48 (dd, J(H,H) ) 10 Hz, 7.5 Hz, 3H; SnPh3), 7.45
3
(t, J(H,H) ) 7.5 Hz, 6H; SnPh3), 1.73 (m, 20H; cyclopentyl-
H), 1.52 (m, 36H; cyclopentyl-H), 0.98 (m, 7H; ipso-H); 13
C
Synthesis of [(c-C5H9)7Si7O12TiOSiPh3] (1). In a procedure
similar to that employed by Crocker et al. for the corresponding
trimethyl derivative,16 [(c-C5H9)7Si7O9(OH)3] (0.237 g, 0.271
mmol) and Ti-(O-SiPh3)4 (4) (0.374 g, 0.325 mmol) were
added to 50 mL of THF, and the resulting mixture was refluxed
for 4 days (the initial suspension of Ti-(O-SiPh3)4 dissolved
to give a colorless solution after 24 h). On cooling to room
temperature, the solution was filtered to remove excess Ti-
NMR (400 MHz, CDCl3, 25 °C, TMS): δ ) 135.77 (s, CH,
SnPh ), 130.24 (s, CH, SnPh ), 128.51 (s, CH, SnPh ), 29.72,
3
3
3
2
7.54, 27.42, 27.35, 27.18, 27.05, 27.01, (s, CH2, unassignable),
+
2
2.97, 22.65, 22.24 (s, 3:3:1, CH, ipso-H); m/z (FAB ): 1079
+
+
(
M -3C5H9), 932 (M -SnPh3); Anal. Calcd for C53H78O13-
Si7SnTi (found): C, 49.5 (47.9), 6.07 (5.65).
Catalysis. Tests were performed under argon using samples
-
4
(O-SiPh3)4, the solvent was removed in vacuo, and the crude
of catalyst that were predried at 80 °C, 10 Torr for 12 h and
either (a) 1 equiv of catalyst to 70 equiv of cyclohexene, TBHP,
and TMS (internal standard), in 0.5 mL of CDCl3 at 45 °C or
(b) 3 mg of catalyst, 0.03 mL of TBHP, 0.12 mL of cyclohexene,
0.05 mL of TMS (internal standard), and 0.42 mL of CDCl3 at
product redissolved in CH2Cl2 (10 mL). After filtration and
addition of MeCN, a white powder precipitated, which was
filtered off, washed with pyridine, and then with acetone to
finally yield (1) as a white powder (0.050 g, 15%).
1
7
Selected Spectroscopic Data for (1). H NMR (400 MHz,
30 °C for 1 h, in sealed vials that were magnetically stirred.
3
CDCl3, 25 °C, TMS): δ ) 7.62 (d, J(H,H) ) 10 Hz, 6H;
Data quoted are derived from at least two runs. Conversions
(based on TBHP consumed) and selectivities (based on the
3
SiPh3), 7.43 (dd, J(H,H) ) 10 Hz, 7.5 Hz, 3H; SiPh3), 7.39 (t,
3
1
J(H,H) ) 7.5 Hz, 6H; SiPh3), 1.73 (m, 20H; cyclopentyl-H),
epoxide formed) were determined by H NMR spectroscopy.
1
3
1
.52 (m, 36H; cyclopentyl-H), 0.98 (m, 7H; ipso-H); C NMR
X-ray Absorption Spectra. Room temperature Ti K-edge
X-ray absorption data were collected at station 8.1 of Daresbury
Laboratory, which operates at 2 GeV with a typical current in
the range of 130 to 250 mA. This station is equipped with a
Si(111) monochromator, ionization chambers for measuring
incident and transmitted beam intensities, and a thirteen-element
Canbera detector for measurements in fluorescence mode. The
data were processed using the suite of programs available at
the Daresbury Laboratory, namely EXCALIB (for converting
the raw data to energy vs absorption coefficient), EXBROOK
(
400 MHz, CDCl3, 25 °C, TMS): δ ) 135.00 (s, CH, SiPh3),
1
2
2
3
30.14 (s, CH, SiPh3), 127.88 (s, CH, SiPh3), 29.72, 27.46,
7.39, 27.32, 27.18, 27.04, 26.97 (s, CH2, unassignable), 22.74,
+
+
2.23, 21.98 (s, 3:3:1, CH, ipso-H); m/z (FAB ): 988 (M -
+
C5H9), 918 (M -OSiPh3); Anal. Calcd for C53H78O13Si8Ti
(found): C, 53.3 (54.2), H 6.53 (6.91).
Synthesis of [(c-C5H9)7Si7O12TiOGePh3] (2). To a solution
of Ph3GeOH (obtained from the reaction of Ph3GeCl and
2
9
water ) (0.336 g, 1.05 mmol) in toluene (10 mL) was added

Letters
J. Phys. Chem. B, Vol. 103, No. 42, 1999 8811
TABLE 1: Catalytic Performance in Epoxidation at 45 °C
of a Series of Soluble Silsesquioxanes with Well-Defined
(
single) Ti(IV) Active Sitesa
pre-edge
intensity
TBHP conversion (%)
catalyst
t ) 30 min t ) 1 h (arb. units)
[
[
[
(c-C
(c-C
(c-C
5
5
5
H
H
H
9
9
9
)
)
)
7
7
7
Si
Si
Si
7
7
7
O
O
O
12Ti(OSiPh
12Ti(OGePh
12Ti(OSnPh
3
)] (1)
)] (2)
)] (3)
9.6
33
15
<1.0
<1.0
30
70
28
<1.0
<1.0
0.66
0.36
0.57
0.79
0.76
3
3
Ti-(O-SiPh
Ti-(O-GePh
3
)
4
(4)
(5)
3
)
4
a
1
:70:70 of catalyst:TBHP:cyclohexene; 0.5 mL of CDCl
3
; 45 °C.
Selectivities > 98%.
TABLE 2: Comparison of the Performance of Insoluble
2
Heterogeneous, Single-Site Ti(IV)/SiO Epoxidation
7
Catalysts with Their Homogeneous Soluble Molecular
Analogues
turnover turnover
Figure 2. Ti K-edge XANES spectra for the molecular compounds
(4) and (5).
frequency frequency heterogeneous
homogeneous catalysta
-1
-1
catalystb
(h
)
(h
)
2
3
40
12
6
4
TivSiO2c
TivMCM-41
TivGevMCM-41
TivSnvMCM-41
(
1) [(c-C5H9)7Si7O12Ti(OSiPh3)]
18
metal center under investigation, for example, titanium-contain-
ing MCM-41, zeolite â, and TS-1 all show an intense pre-edge
(
(
2) [(cC5H9)7Si7O12Ti(OGePh3)]
3) [(c-C5H9)7Si7O12Ti(OSnPh3)]
52
33
3
8
peak in the dehydrated form which decreases markedly upon
hydration or interaction with coordinating molecules.2
The Ti K-edge XANES of the two model compounds, Ti-
,33-36,38
a
3
mg of catalyst:0.03 mL of TBHP:0.12 mL of cyclohexene; 0.42
b
mL of CDCl
catalysts prepared as described in ref 7 (typical Si:Ti ratios of 1:44).
0 mg of catalyst:0.5 mL of TBHP:2 mL of cyclohexene:7 mL of
3
; 30 °C, 1 h. Selectivities > 98%. Heterogeneous
(O-SiPh3)4 (4) and Ti-(O-GePh3)4 (5) (displayed in Figure
5
2) show almost identical pre-edge peak intensities, but exhibit
some differences in their edge features. This suggests that the
local coordination, in particular the oxygen environment in these
two materials, is identical. Indeed the intense pre-edge peak
representative of a tetrahedral coordination is consistent with
c
MeCN; 30 °C, 1 h. Selectivities > 95%. Degussa Aerosil 200 silica.
(
for pre- and post-edge background subtraction), and EX-
CURV98 (for detailed analysis to extract the structural param-
eters).
Molecular Dynamics. The molecular dynamics runs were
carried out using the Discover code (MSI, San Diego). A 5
ps duration run, with a 1 fs time step, at 500 K was employed.
25
their recently solved crystal structures. Although these materi-
als contain titanium in tetrahedral coordination, exposure to
atmospheric moisture (note that Ti K-edge XAS experiments
were performed in ambient conditions) or liquid reactants, used
in a typical catalytic test, did not alter the pre-edge peak
intensity, unlike the catalytically active TivMCM-41, Ti-zeolite
3
0
Results and Discussion
3
3
First, we present the catalytic performance of the titanosil-
sesquioxane catalysts in the epoxidation of cyclohexene. Table
summarizes the results of the homogeneous catalysts; and
Table 2 compares the performance, expressed in turnover
frequency (i.e., per single Ti(IV) site), of a variety of homo-
geneous and heterogeneous catalysts. Clearly, the trend in
activity of the homogeneous silicon (1) and germanium (2)
silsesquioxane catalysts follows that found earlier for the
corresponding heterogeneous catalysts. The reality of the
enhancement of activity of a central Ti(IV) active site by a
juxtaposed germanium (in place of silicon) is apparent. How-
ever, the activity of the tin-containing homogeneous analogue
â, or TS-1. The molecular dynamics calculations (performed
25
using the crystal structure information concerning the isolated
molecule as the starting point) allow us to understand why this
is so. It is clear from Figure 3 that the twelve phenyl groups
shroud the central, potentially catalytically active (and coordi-
natively unsaturated) Ti(IV) atom making access of reactants
to the central Ti(IV) atom in the molecular Ti-(O-GePh3)4
very limited. This is consistent with the catalytic inactivity of
the Ti-(O-GePh3)4 and Ti-(O-SiPh3)4 materials (see Table
1).
1
7
Comparison of the XANES of the catalytically active silicon
(1) and germanium (2) titanosilsesquioxane with model com-
pounds (4) and (5) (see Figures 2 and 4 and Table 1) shows
less intensity in the pre-edge peak (note that all the measure-
ments were carried out ex situ, at ambient conditions). This
behavior is similar to that exhibited by the other titanium-
containing heterogeneous catalysts mentioned previously and
indicates that substitution of three Ph3XO- ligands by the silanol
groups of the silsesquioxane now renders the central, unsaturated
titanium(IV) accessible to interaction with water or other reactant
molecules. This interpretation is confirmed by their observed
catalytic activity and by the molecular simulation shown in
Figure 3. It is apparent from Figure 3 that by anchoring the Ti
centers tripodally and retaining one O-Ge-Ph3 group, it is
possible to create an accessible catalytically active species. Using
a similar synthetic strategy, it is also possible to graft compound
(4) onto the inner, silanol-rich surfaces of MCM-41 in the
presence of a solvent which dissolves both compound (4) and
(
3) lies between the silicon and the germanium forms, in conflict
with the observations found for the heterogeneous catalysts. We
believe that the reason the same order of catalytic activity of
the homogeneous catalysts is not observed for the equivalent
heterogeneous catalysts is because in the TivSnvMCM-41
material it is only possible to form a mixture of active sites,
such as those shown in Figure 1a, and inactive ones in which
7
Ti(IV) is grafted onto particles of SnO2. The inactive sites will
lead to a reduction in the turnover frequency of the TivSnvMCM-
4
1 material.
In an attempt further to understand the catalytic performance
of these materials, we probed their titanium centers using X-ray
absorption near-edge spectroscopy (XANES) which is known
to be sensitive to local coordination geometry, oxidation state,
In particular, the pre-edge peak is
very sensitive to the local coordination geometry around the
31,32
and electronic effects.

8812 J. Phys. Chem. B, Vol. 103, No. 42, 1999
Letters
Figure 3. Results of molecular dynamics simulations for the isolated (top) and “anchored” (bottom) Ti-(O-GePh
3
)
4
molecular clusters. For the
anchored cluster, the Ti and the three pendant OH groups were kept fixed during simulation. These representations consist of superposition of
frames from the simulation taken at 200 fs intervals, roughly the time scale for libration of the phenyl groups. Comparison of the top and bottom
(right) views shows that access of reactants is far greater for the “anchored” than for the free Ti-centered entity. In the free state, the four triphenyl
groups shroud the central, potentially catalytically active (and coordinatively unsaturated) Ti(IV) ion.
A comparison of the XANES data of the silicon-(1),
germanium-(2), and tin-(3) derivatized titanosilsesquioxane
catalysts (see Figure 4) provides an insight into their relative
catalytic activity. Unlike the model compounds (Figure 2), the
pre-edge intensities of these three catalysts are markedly
different from one another (see Table 1), with the germanium
analogue (2) having the least and the silicon analogue (1) having
the most intense pre-edge peak. This could either be due to (a)
the presence of some octahedrally coordinated titanium centers,
similar to the ones found in TiO2 or in ETS-10, along with
tetrahedral species or (b) interaction of Ti centers with atmo-
spheric water, thereby resulting in a distorted, octahedral
coordination similar to that present in heterogeneous titano-
silicates. The former can be clearly ruled out, since the
germanium-containing silsesquioxane shows the minimum pre-
edge intensity (and maximum catalytic activity). Furthermore,
it is well-known that both TiO2 or ETS-10 are not active in
catalytic epoxidation. The latter is the more likely possibility,
since this behavior is similar to that of the heterogeneous
catalysts. (It is not sensible to dehydrate these materials at
temperatures similar to those used for heterogeneous titanium-
Figure 4. Room-temperature Ti K-edge XANES spectra for the
titanosilsesquioxanes catalysts (1), (2), and (3). Note that the data were
collected using the as-synthesized solids.
the triphenylsilanol byproduct. This leaves behind a surface rich
in (mSi-O)3-Ti-OSiPh3 groups (as depicted in Figure 1c) and
a catalyst that we find exhibits greater catalytic activity than
2
,3
those prepared by grafting titanocene onto MCM-41.

Letters
J. Phys. Chem. B, Vol. 103, No. 42, 1999 8813
containing silicate catalysts i.e., in the range 400-500 °C since
these titanosilsesquioxanes are unstable at such temperatures.)
To examine if any loosely bound water molecules are present
in the silsesquioxane catalysts, we separately dehydrated the
Ge form at 55 °C, in a vacuum, for 2 h and we find that the
pre-edge intensity peak does indeed increase (from 0.36 to 0.54).
This mirrors the behavior of titanium-containing heterogeneous
catalysts, where the pre-edge intensity decreases significantly
during reaction upon attachment of the reactant species. Thus
our XANES data show that, in the solid state, the degree of
accessibility of the Ti active center decreases in the order Ge
(13) Maschmeyer, T.; Klunduk, M. C.; Martin, C. M.; Shephard, D. S.;
Thomas, J. M.; Johnson, B. F. G. J. Chem. Soc., Chem. Commun. 1997,
847.
1
(14) Klunduk, M. C.; Maschmeyer, T.; Thomas, J. M.; Johnson, B. F.
G. Eur. J. Chem. 1999, 5, 1481.
(15) Klunduk, M. C.; Thomas, J. M.; Johnson, B. F. G.; Maschmeyer,
T. J. Organomet. Chem., submitted.
(16) Crocker, M.; Herold, R. H. M.; Orpen, A. G. J. Chem. Soc., Chem.
Commun. 1997, 2411.
(
(
17) Crocker, M.; Herold, R. H. M. Pat. Appl. PCT/EP96/05873, 1997.
18) Abbenhuis, H. C. L.; Krijnen, S.; van Santen, R. A. J. Chem. Soc.,
Chem. Commun. 1997, 331.
19) Duchateau, R.; Abbenhuis, H. C. L.; van Santen, R. A.; Thiele, S.
K. H.; van Tol, M. F. H. Organomet. 1998, 17, 5222.
20) Duchateau, R.; Abbenhuis, H. C. L.; van Santen, R. A.; Meetsma,
A.; Thiele, S. K. H.; van Tol, M. F. H. Organomet. 1998, 17, 5663.
21) Buys, I. E.; Hambley, T. W.; Houlton, D. J.; Maschmeyer, T.;
Masters, A. F.; Smith, A. K. J. Mol. Catal. 1994, 86, 309.
22) Field, L. D.; Lindall, C. M.; Maschmeyer, T.; Masters, A. F. Aust.
(
>
Sn > Si which closely follows with the order of catalytic
(
activity.
(
Acknowledgment. We acknowledge with gratitude support
from the EPSRC (U.K.) for a rolling grant to J.M.T., a regular
one to B.F.G.J., an ICI (Wilton) CASE award to M.C.K., and
Shell International for a maintenance grant to M.P.A.. We would
also acknowledge Drs. M. Crocker and I. Maxwell for stimulat-
ing discussions.
(
J. Chem. 1994, 47, 1127.
(23) Wada, K.; Nakashita, M.; Bundo, M.; Ito, K.; Kondo, T.; Mitsudo,
T. Chem. Lett. 1998, 659.
(24) Notari, B. AdV. Catal. 1996, 41, 253.
(25) Klunduk, M. C.; Sankar, G.; Martin, C. M.; Thomas, J. M.; Johnson,
B. F. G. Manuscript in preparation.
References and Notes
(
(
26) Zeitler, V. A.; Brown, C. A. J. Am. Chem. Soc. 1957, 79, 4616.
27) Shriver, D. F.; Drezdon, M. A. The Manipulation of Air-SensitiVe
(
1) Murugavel, R.; Roesky, H. W. Angew. Chem., Int. Ed. Engl. 1997,
3
3
6 (6), 477.
Compounds, 2nd ed.; Wiley: New York, 1986.
(2) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Nature 1995,
(
28) Feher, F. J.; Budzichowski, T. A.; Blanski, R. L.; Weller, K. J.;
Ziller, J. W. Organomet. 1991, 10, 2526.
29) Dannley, R. L.; Farrant, G. C. J. Org. Chem. 1969, 34, 2428.
78, 159.
(3) Oldroyd, R. D.; MacFaul, P. A.; Maschmeyer, T.; Thomas, J. M.;
(
Snelgrove, D. W.; Ingold, K. U.; Wayner, D. D. M. Angew. Chem., Int.
Ed. Engl. 1996, 35, 2787.
(30) Discover Code, MSI, San Diego, 1996.
(31) X-ray Absorption, Principles, Applications, Techniques of EXAFS,
SEXAFS and XANES; Koningsberger, D. C., Prins, R., Eds.; Wiley: New
York, 1988.
(4) Marchese, L.; Gianotti, E.; Dellarocca, V.; Maschmeyer, T.; Rey,
F.; Coluccia, S.; Thomas, J. M. PCCP 1999, 1, 585.
(
5) Thomas, J. M. Eur. J. Chem. 1997, 3, 1557.
(6) Oldroyd, R. D.; Thomas, J. M.; Sankar, G. J. Chem. Soc., Chem.
(32) Iwasawa, Y. Proceedings of the 11th International Congress on
Commun. 1997, 2025.
7) Oldroyd, R. D.; Sankar, G.; Thomas, J. M.; Ozkaya, D. J. Phys.
Chem. B 1998, 102, 1849.
8) Feher, F. J.; Newman, D. A.; Walzer, J. F. J. Am. Chem. Soc. 1989,
11, 1741.
Catalysis, Baltimore, 1996; Hightower, J. W., Delgass, W. N., Bell, A. T.,
Iglesia, E., Eds.; Elsevier: Amsterdam, 1996; p 21.
(
(
33) Sankar, G.; Rey, F.; Thomas, J. M.; Greaves, G. N.; Corma, A.;
Dobson, B. R.; Dent, A. J. J. Chem. Soc., Chem. Commun. 1994, 2279.
34) Blasco, T.; Camblor, M. A.; Corma, A.; Prez-Pariente, J. J. Am.
Chem. Soc. 1993, 115, 11806.
35) Tozzola, G.; Mantegazza, M. A.; Ranghino, G.; Petrini, G.; Bordiga,
S.; Ricchiardi, G.; Lamberti, C.; Zulian, R.; Zecchina, A. J. Catal. 1998,
79, 64.
(
1
(
(
9) Feher, F. J.; Gonzales, S. L.; Ziller, J. W. Inorg. Chem. 1988, 27,
3
440.
(
(
10) Feher, F. J.; Budzichowski, T. A.; Rahimian, K.; Ziller, J. W. J.
Am. Chem. Soc. 1992, 114, 3859.
11) Murugavel, R.; Voigt, A.; Walawalkar, M. G.; Roesky, H. W. Chem.
ReV. 1996, 96, 2205.
12) Voigt, A.; Murugavel, R.; Montero, M. L.; Wessel, H.; Liu, F. Q.;
1
(
(36) Thomas, J. M. Faraday Discuss. 1996, 105, 1.
(
(37) Sankar G.; Thomas, J. M. Topics in Catal. 1999, 8, 1.
Roesky, H. W.; Uson, I.; Albers, T.; Parisini, E. Angew. Chem., Int. Ed.
Engl. 1997, 36, 1001.
(38) Sankar. G.; Thomas. J. M.; Catlow. C. R. A. Submitted for
publication.