Synthesis and structural characterisation of tripodal titanium silsesquioxane complexes: A new class of highly active catalysts for liquid phase alkene epoxidation

Article abstract of DOI:10.1039/a704969b

Titanium silsesquioxane complexes [Ti(L)(R₇Si₇O₁₂)], possessing tripodal geometry, represent models for heterogeneous titanosilicate catalysts; the new complexes are exceptionally active and selective catalysts for liquid

Full text of DOI:10.1039/a704969b

View Article Online / Journal Homepage / Table of Contents for this issue
Synthesis and structural characterisation of tripodal titanium silsesquioxane
complexes: a new class of highly active catalysts for liquid phase alkene
epoxidation
Mark Crocker,*a Ruud H. M. Herolda and A. Guy Orpenb
a Shell Research and Technology Centre Amsterdam, Postbus 38000, 1030 BN Amsterdam, The Netherlands
b School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK BS8 1TS
R
O
OH
R
L
Titanium silsesquioxane complexes [Ti(L)(R7Si7O12)], pos-
sessing tripodal geometry, represent models for hetero-
geneous titanosilicate catalysts; the new complexes are
exceptionally active and selective catalysts for liquid phase
alkene epoxidation reactions.
Si
Si
Si
Ti
O
OH
R
R
R
O
O
R
Si
Si
Si
Si
Si
O
Si
Si
O
Si
O
O
O
O
TiL4
OH
R
–3 HL
R
O
O
Si
Si
R
O
Si
O
O
O
R
R
O
O
The remarkable catalytic properties of titanosilicates such as
titanium silicalite-1 (TS-1) have generated considerable scien-
tific interest, particularly with regard to the nature of the active
titanium species present. It is now generally accepted that a
four-coordinate lattice titanium site is the catalytically active
species in these materials.1 Furthermore, recent studies point
toward the involvement of titanium sites possessing tripodal
geometry, representing so-called open lattice sites,2,3 although
the presence of closed lattice sites (‘tetrapodal’ sites) and
bipodal sites has also been inferred (see Fig. 1).1,2 With this in
mind we have prepared soluble analogues of these different
titanium species based on incompletely condensed silsesquiox-
ane precursors and have studied the catalytic properties of the
resulting complexes.4 Recently we became aware of related
studies aimed at modelling heterogeneous titanium-based
epoxidation catalysts, which also employ silsesquioxane deriva-
tives;5,6 prompted by these reports we now describe our own
findings.
Incompletely condensed silsesquioxanes have recently at-
tracted interest as models for the silica surface.7,8 Elegant work
by Feher and coworkers has revealed that the molecular
structure of 1 (see Scheme 1) shares many structural similarities
with b-cristobalite (111) and b-tridymite (0001). Although the
exact nature of the silica surface is a matter of great controversy,
the occurrence of cristobalite- and tridymite-like surface
structures has been frequently postulated on the basis of both
experimental and theoretical studies. On this basis, we have
prepared titanium complexes of 1 in which the silsesquioxane
ligand functions as a model for the siliceous surface in a tripodal
titanium site. Similarly, the monosilylated form of 1, disilanol
species 2 (Scheme 2) has been used to model the siliceous
surface in bipodal and tetrapodal titanium sites.
R
R
R
1
3–6
Scheme 1 Synthesis of compounds 3–6: L = CH2Ph 3, NMe2 4, OSiMe3 5,
OPri 6; R = cyclohexyl
for a tripodal TiL(silsesquioxane) species. In the case of 6, 29Si
NMR data indicate the presence of a second species containing
five distinct silicon environments (with relative ratios
2:2:1:1:1), corresponding to the dimer [Ti{(c-
C8H11)7Si7O12}(m-OPri)]2 recently reported by Maschmeyer
et al.7 Freshly prepared solutions of 6 were found to contain
exclusively the monomer, while at longer equilibration times a
monomer:dimer ratio of ca. 5:2 was observed in C6D6.
The monomeric nature of 5 was confirmed by the results of a
single crystal X-ray diffraction study (see Fig. 2). The crystal
structure of 5 consists of isolated molecules separated by
normal van der Waals contacts. Molecules of 5 lie in
¯
crystallographic special positions of 3 site symmetry and are
consequently disordered; only one of the two centrosymmet-
rically related images of the disordered molecule is shown. The
molecular structure of 5 consists of a pseudo-tetrahedral
titanium(iv) centre coordinated by the tridentate silsesquioxane
ligand and siloxy (OSiMe3) ligand such that the complex has
exact C3 symmetry [disregarding the cyclohexyl substituent at
Si(1A), the silicon opposite titanium]. As a consequence of the
disorder the details of the apparent molecular geometry must be
treated with scepticism. However, the gross geometry of 5 in the
OPri
OPri
OSiMe3
R
O
OSiMe3
R
O
Si
Si
Ti
R
R
OH
R
O
R
Si
O
Si
Si
O
Si
O
Si
O
Si
Si
O
Si
O
O
Ti(OPri)4
–2 PriOH
O
O
OH
R
Si
R
O
Si
O
Si
R
O
Si
O
O
R
Reaction of 1 with homoleptic titanium(iv) complexes TiL4
(L = CH2Ph, NMe2, OSiMe3, OPri) occurs with protonolysis,
affording in each case a product with the empirical formula
[TiL{(c-C6H11)7Si7O12}] (Scheme 1) on the basis of elemental
O
R
R
O
O
R
R
2
7
R
R
1/2 TiL4
1
Si
Si
analysis and solution H NMR data. 29Si NMR spectra of the
O
–2 HL
R
O
O
products are particularly informative, the observation in each
case of three resonances for the silsesquioxane Si atoms in a
3:1:3 ratio being consistent with the C3v symmetry expected
Si
O
Ti
Si
O
Si
O
R
O
O
R
Si
Si
O
OSiMe3
O
R
O
R
R
R
Si
R
R
O
Si
Me3SiO
O
OH
Ti
OH
Ti
O
Si
Si
O
Si
O
O
O
R
Ti
SiO
OH
SiO
OSi
SiO
OSi
O
R
O
O
Si
O
Si
O
Si
Si
Si
O
O
R
Si
O
a
b
c
8
Fig. 1 Possible framework titanium sites in titanosilicates: (a) bipodal site,
(b) tripodal (open lattice) site, (c) tetrapodal (closed lattice) site
Scheme 2 Synthesis of 7 and 8 (L = CH2Ph, R = cyclohexyl)
Chem. Commun., 1997
2411

View Article Online
Table 1 Epoxidation of oct-1-ene with TBHP catalysed by titanium
silsesquioxane complexesa
C(10)
Si(4)
O(3)
102 k2/
Selectivity to
epoxideb (%)
Catalyst
dm3 mol21 s21
C(3)
C(4)
C(5)
C(2)
C(1)
Ti
O(1)
3
4
5
6
7
8
123
63
97
149
9.3
4.7
2.6
95
99
97
93
75
83
94
Si(2)
O(2)
C(6)
Ti-MCM-41c
Si(1′)
C(7′)
a Conditions: T = 80 °C, Ti = 0.2 mmol, TBHP = 30 mmol, oct-1-ene (75
g) as solvent. Selectivity = (mol 1,2-epoxyoctane formed/mol TBHP
b
consumed) 3 100; selectivities were determined at 90% TBHP consump-
c
tion. Data quoted are derived from at least two runs. Ti content = 1.43
mass%.
Fig. 2 Molecular structure of 5 showing atom labelling scheme. All
hydrogen atoms have been omitted for clarity. Only one of the two
centrosymmetrically related images of the disordered molecule is shown.
Selected distances (Å) and angles (°): Ti–O(1) 1.658(6), Ti–O(3) 1.84(2),
Si(2)–O(1) 1.625(6), Si(2)–O(2) 1.641(6), O(3)–Si(4) 1.79(2); O(1A)–Ti–
O(1) 107.4(2), O(1)–Ti–O(3) 111.4(2), O(1)–Si(2)–O(2) 108.0(3), Si(2)–
O(1)–Ti 151.0(4), Si(4)–O(3)–Ti 180.0.
Table 1. Most striking is the finding that the measured values of
k2 for the tripodal complexes 3–6 are ca. one order of magnitude
greater than for 7 and 8, while the former complexes also show
superior selectivity. Complexes 3–6 are also considerably more
active under these test conditions than titanosilicate catalysts
such as Ti-MCM-41,11 when compared on the basis of k2
values. Our conclusion is that the (most) active site in these
materials, as far as epoxidation catalysis is concerned, corre-
sponds to the tripodal, open-lattice type of titanium site. The
comparatively low epoxidation activity of Ti-MCM-41 can be
ascribed to the fact that only a small fraction of the titanium sites
in the material are exposed at the surface and possess the correct
coordination environment.
solid state is clearly established, and by inference so is the
structure of the monomer form of the isopropoxy analogue 6,
¯
which also crystallises in space group R3 with similar cell
dimensions to 5, and has molecules lying at the same special
positions as 5. For 6 the disorder is less well resolved and
refinement less satisfactory.
The reaction of disilanol 2 with [Ti(OPri)4] was examined as
a possible route to a model bipodal titanium site. Addition of a
diethyl ether solution of 2 to the titanium complex resulted in a
smooth reaction and the subsequent isolation of [Ti(OPri)2{(c-
C6H11)7Si7O11(OSiMe3)}] 7 as the sole product (Scheme 2). 1H
NMR data are consistent with the presence of two inequivalent
isopropoxy groups in 7, while the observation of the five signals
in the 29Si NMR spectrum corresponding to the silsesquioxane
Si7O11 core indicates that, as for the free ligand, a plane of
symmetry runs through the silsesquioxane ligand, with the
alkoxide ligands lying in the plane.
Footnotes and References
* E-mail: m.crocker@siop.shell.nl
† NMR data for selected compounds: 5: 1H NMR (C6D6, 250.1 MHz): d
2.13–0.97 (m, 77 H, c-C6H11), 0.21 (s, 9 H, SiMe3). 29Si NMR (C6D6, 79.5
MHz): d 2.76 (s, OSiMe3), 265.80, 267.90, 268.85 (s, 3:1:3). 7: 1H
NMR (C6D6, 500.1 MHz); d 4.62 [spt, 1 H, CH(CH3)2, J 6.1 Hz], 4.57 [spt,
1 H, CH(CH3)2, J 6.1 Hz], 2.17–1.05 (m, 77 H, c-C6H11), 1.32 (d, 6 H, CH3,
J 6.1 Hz), 1.30 (d, 6 H, CH3, J 6.1 Hz), 0.47 [s, 9 H, Si(CH3)3]. 29Si NMR
(C6D6, 99.4 MHz): d 10.94 (s, OSiMe3), 265.64, 266.37, 266.58, 267.49,
268.86 (s, 1:1:1:2:2). 8: 1H NMR (C6D6, 250.1 MHz): d 2.14–1.00 (m,
154 H, c-C6H11), 0.52 (s, 18 H, SiMe3). 29Si NMR (C6D6, 79.5 MHz): d
10.42 (s, SiMe3), 266.35, 266.79, 266.92, 268.22, 269.26, 269.50,
269.72 (s, all 3 1). All compounds analysed satisfactorily.
The reaction of
2 equiv. of silsesquioxane 2 with
[Ti(CH2Ph)4] was chosen as a convenient means of preparing a
model for the closed lattice site. In aprotic solvents a rapid
reaction occurs with the formation of a colourless solution, from
which bis(silsesquioxane) complex 8 can be isolated in
quantitative yield (Scheme 2).
‡ Crystal data for 5: C45H86O13Si8Ti, Mr = 1107.8, rhombohedral, space
¯
group R3 (no. 148), a = 16.955(4), c = 17.853(6) Å, a = b = 90, g =
1H and 29Si NMR spectroscopy reveal the presence of only
one type of –OSiMe3 group in 8, suggesting equivalence of the
{(c-C6H11)7Si7O11(OSiMe3)} ligands. The presence of mole-
cular C2 symmetry in 8 is confirmed by the results of X-ray
crystallography.9 Consistent with tetrahedral coordination of
the TiIV centre, seven resonances are observed in the 29Si NMR
spectrum for the framework silsesquioxane Si atoms, indicating
that the local mirror symmetry of the silsesquioxane ligands is
not retained at titanium.
120°, U = 4445(2) Å3, Z = 3, Dc = 1.24 g cm23, l = 0.710 73 Å, m(Mo-
Ka) = 0.36 mm21, F(000) = 1782, T = 200 K. The structure was solved
by direct and Fourier methods: 1296 unique reflections collected; 113
parameters (11 restraints); wR2 = 0.287, R1 = 0.107 for the 775 data with
I > 2s(I). CCDC 182/626.
1 G. Bellussi and M. S. Rigutto, Stud. Surf. Sci. Catal., 1994, 85, 177.
2 L. Le Noc, D. Trong On, S. Solomykina, B. Echchahed, F. Beland, C.
Cartier dit Moulijn and L. Bonneviot, Stud. Surf. Sci. Catal., 1996, 101,
611.
3 T. Maschmeyer, F. Rey, G. Sankar and J. M. Thomas, Nature, 1995,
378, 159.
4 M. Crocker and R. H. M. Herold, Pat. Appl., PCT/EP96/05873.
5 H. C. L. Abbehuis, S. Krijnen and R. A. van Santen, Chem. Commun.,
1997, 331.
The epoxidation of oct-1-ene with ButOOH (TBHP) was
chosen as a convenient test of the epoxidation activity of
complexes 3–8. At the alkene/TBHP molar ratio of ca. 20
employed, pseudo-first order kinetics are observed corre-
sponding to the rate equation:
6 T. Maschmeyer, M. C. Klunduk, C. M. Martin, D. S. Shephard, J. M.
Thomas and B. F. G. Johnson, Chem. Commun., 1997, 1847.
7 F. J. Feher, D. A. Newman and J. F. Walzer, J. Am. Chem. Soc., 1989,
111, 1741.
8 F. J. Feher, S. H. Phillips and J. W. Ziller, Chem. Commun., 1997,
829.
d[epoxide]/dt = k1[TBHP]
(where k1 = k2[Ti] = k3[Ti][alkene])
At high TBHP conversions (generally > 80%), some deviation
from first order kinetics is observed, corresponding to a
decrease in the reaction rate. This can be attributed to the fact
that the reaction is autoretarded by the tert-butanol co-product,
a phenomenon observed previously for epoxidation cata-
lysts.10
9 A. G. Orpen, unpublished work.
10 R. A. Sheldon and J. A. van Doorn, J. Catal., 1973, 31, 427.
11 A. Coma, M. T. Navarro and J. Perez-Pariente, J. Chem. Soc., Chem.
Commun., 1994, 147.
The calculated second order rate constants for TBHP
conversion (k2 = k1/[Ti]) for complexes 3–8 are shown in
Received in Basel, Switzerland, 10th July 1997; 7/04969B
2412
Chem. Commun., 1997