Synthesis and characterisation of titanium silasesquioxane complexes: Soluble models for the active site in titanium silicate epoxidation catalysts

Article abstract of DOI:10.1039/a905887g

Titanium silasesquioxane complexes have been prepared as models for the catalytically active centres in titanium silicate oxidation catalysts. Complexes [TiL(R₇Si₇O₁₂)] [R = c-C₆H₁₁, L = CH₂Ph 5, NMe₂ 6, OSiMe₃ 7, OPrⁱ 8 or OBu^t 9; R = c-C₅H₉, L = CH₂Ph 13 or OPrⁱ14] were prepared from the reactions of incompletely condensed silasesquioxanes R₇Si₇O₉(OH)₃ 1, 2 with homoleptic complexes TiL₄. Aryloxy derivatives [TiL(R₇Si₇O₁₂)] [R = c-C₆H₁₁, L = OPh 10, O-C₆H₄F-p 11 or O-C₆H₄NO₂-p 12] were prepared from the reaction of 8 with the corresponding aryl alcohols. The ²⁹Si and ¹³C NMR spectroscopic data obtained on 5-14 indicate that the local C_3v symmetry of the silasesquioxane ligand is retained at titanium, consistent with the formation of monomeric complexes possessing tripodal geometry. The monomeric nature of 7 was confirmed by X-ray crystallography. For complexes 8-12 solution NMR spectroscopy reveals the presence of a dimer, containing μ-alkoxy ligands, in equilibrium with the monomer. The zirconium analogue of 9, [Zr(OBu^t){(c-C₆H₁₁)₇Si ₇O₁₂}] 15, was similarly isolated as a monomer-dimer mixture from the reaction of the incompletely condensed silasesquioxane (c-C₆H₁₁)₇Si₇O₉(OH) ₃ with [Zr(OBu^t)₄]. Reaction of the disilanol (c-C₆H₁₁)₇Si₇O ₉(OSiMe₃)(OH)₂ 4 with an excess of [Ti(OPrⁱ)₄] afforded [Ti(OPrⁱ)₂{(c-C₆H₁₁) ₇Si₇O₁₁(OSiMe₃)}] 16, containing a bidentate silasesquioxane ligand, while reactions with TiL₄ (L = CH₂Ph, NMe₂ or OSiMe₃) afforded [Ti{(c-C₆H₁₁)₇Si₇O ₁₁(OSiMe₃)}₂] 17, independent of the stoichiometry of the reactants. Complexes 5-14 serve as soluble models for putative tripodal (open lattice) sites in titanium silicates, while 16 and 17 represent models for bipodal and tetrapodal (closed lattice) sites, respectively. From a study of the catalytic properties of complexes 5-17 in the epoxidation of oct-1-ene with tert-BuOOH (TBHP), revealing high activity for 5-14 and low activity for 16 and 17, it is concluded that the most active site in titanium silicate epoxidation catalysts corresponds to a four-co-ordinate site possessing tripodal geometry. Studies using IR and NMR spectroscopy show that, in the absence of olefins, putative alkylperoxo complexes formed by the addition of TBHP to tripodal complexes decompose rapidly at ambient temperature. Based on the high TBHP-to-epoxide selectivities observed under epoxidising conditions, it is apparent that the rate of epoxidation is significantly greater than that of alkylperoxo intermediate decomposition. The Royal Society of Chemistry 1999.

Full text of DOI:10.1039/a905887g

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Synthesis and characterisation of titanium silasesquioxane
complexes: soluble models for the active site in titanium silicate
epoxidation catalysts†
Mark Crocker,*a Ruud H. M. Herold,a A. Guy Orpenb and Martijn T. A. Overgaaga
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
Received 21st July 1999, Accepted 9th September 1999
Titanium silasesquioxane complexes have been prepared as models for the catalytically active centres in titanium
silicate oxidation catalysts. Complexes [TiL(R7Si7O12)] [R = c-C6H11, L = CH2Ph 5, NMe2 6, OSiMe3 7, OPri 8
or OBut 9; R = c-C5H9, L = CH2Ph 13 or OPri 14] were prepared from the reactions of incompletely condensed
silasesquioxanes R7Si7O9(OH)3 1, 2 with homoleptic complexes TiL4. Aryloxy derivatives [TiL(R7Si7O12)]
[R = c-C6H11, L = OPh 10, O-C6H4F-p 11 or O-C6H4NO2-p 12] were prepared from the reaction of 8 with the
corresponding aryl alcohols. The 29Si and 13C NMR spectroscopic data obtained on 5–14 indicate that the local
C3v symmetry of the silasesquioxane ligand is retained at titanium, consistent with the formation of monomeric
complexes possessing tripodal geometry. The monomeric nature of 7 was confirmed by X-ray crystallography.
For complexes 8–12 solution NMR spectroscopy reveals the presence of a dimer, containing µ-alkoxy ligands, in
equilibrium with the monomer. The zirconium analogue of 9, [Zr(OBut){(c-C6H11)7Si7O12}] 15, was similarly isolated
as a monomer–dimer mixture from the reaction of the incompletely condensed silasesquioxane (c-C6H11)7Si7O9(OH)3
with [Zr(OBut)4]. Reaction of the disilanol (c-C6H11)7Si7O9(OSiMe3)(OH)2 4 with an excess of [Ti(OPri)4] afforded
[Ti(OPri)2{(c-C6H11)7Si7O11(OSiMe3)}] 16, containing a bidentate silasesquioxane ligand, while reactions with TiL4
(L = CH2Ph, NMe2 or OSiMe3) afforded [Ti{(c-C6H11)7Si7O11(OSiMe3)}2] 17, independent of the stoichiometry of
the reactants. Complexes 5–14 serve as soluble models for putative tripodal (open lattice) sites in titanium silicates,
while 16 and 17 represent models for bipodal and tetrapodal (closed lattice) sites, respectively. From a study of the
catalytic properties of complexes 5–17 in the epoxidation of oct-1-ene with tert-BuOOH (TBHP), revealing high
activity for 5–14 and low activity for 16 and 17, it is concluded that the most active site in titanium silicate
epoxidation catalysts corresponds to a four-co-ordinate site possessing tripodal geometry. Studies using IR and
NMR spectroscopy show that, in the absence of olefins, putative alkylperoxo complexes formed by the addition
of TBHP to tripodal complexes decompose rapidly at ambient temperature. Based on the high TBHP-to-epoxide
selectivities observed under epoxidising conditions, it is apparent that the rate of epoxidation is significantly greater
than that of alkylperoxo intermediate decomposition.
The catalytic properties of the synthetic titanium-contain-
ing zeolite, titanium silicalite-1 (TS-1),1 are of considerable
scientific and commercial interest:2,3 using hydrogen peroxide as
oxidant, TS-1 is active for a range of oxidation reactions includ-
ing the epoxidation of alkenes to epoxides, the oxidation
of primary alcohols to aldehydes and secondary alcohols to
ketones, the hydroxylation of aromatics, the ammoxidation of
ketones and the oxidation of alkanes to alcohol–ketone mix-
tures. The remarkable properties of TS-1, coupled with a desire
to overcome the pore size limitations imposed by the MFI
structure, have in turn generated significant interest in the
catalytic properties of amorphous titania–silica mixed oxides4–8
and amorphous forms of titania supported on silica,9–12 as
well as research directed at the synthesis of large13–15 and
ultralarge16–18 pore zeolites in which titanium is incorporated in
the framework.
spond to isolated titanium sites occupying tetrahedral positions
in the silicate lattice.2 Furthermore, recent studies point toward
the involvement of titanium sites possessing tripodal geometry,
representing so-called open lattice sites (structure (b) in Scheme
1). Titanium species possessing this geometry were first postu-
lated by Shell workers to be the active site in amorphous titania
on silica epoxidation catalysts.19 More recently, Maschmeyer
In recent years particular attention has been focused on
elucidating the nature of the active titanium species present in
titanium zeolites in order to achieve a better understanding of
their unique catalytic properties. It is now generally accepted
that the catalytically active species in these materials corre-
Scheme
1
Possible framework titanium sites in titanosilicates:
† Supplementary data available: rotatable 3-D crystal structure diagram
in CHIME format. See http://www.rsc.org/suppdata/dt/1999/3791/
(a) tetrapodal (closed lattice) site, (b) tripodal (open lattice) site,
(c) bipodal site.
J. Chem. Soc., Dalton Trans., 1999, 3791–3804
This journal is © The Royal Society of Chemistry 1999
3791

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et al.20 have shown that tripodal titanium species, prepared by
the grafting of a suitable titanium precursor onto the inner
surface of MCM-41 (a mesoporous silica), show high activity
in alkene epoxidation using alkyl hydroperoxides. Enichem
researchers21 have proposed that an analogous tripodal species
is readily formed by hydrolysis of lattice TiO4 species in TS-1,
the driving force for its formation being the release of lattice
strain around the titanium site, and have presented EXAFS and
photoluminescence data which support the presence of both
closed lattice sites (“tetrapodal” sites, structure (a) in Scheme 1)
and open lattice sites in dehydrated TS-1 samples.22 Le Noc et
al.23 have reached similar conclusions on the basis of EXAFS,
occurrence of cristobalite- and tridymite-like surface structures
has frequently been postulated on the basis of both experi-
mental and theoretical studies.36 On this basis, silasesquioxanes
appear to represent the best molecular models for the silica
surface prepared to date. Indeed the vanadium silasesquioxane
complex [{(c-C6H11)7Si7O12VO}2] has been shown to be a useful
model for vanadia supported on silica, comparison of 51V
NMR and Raman spectroscopic data revealing structural
similarities to the extent that the vanadia species found at the
silica surface could be unambiguously assigned as a tripodal
(SiO) V᎐O unit.37
᎐
3
Following this rationale, we have prepared titanium com-
plexes of compounds 1 and 2 in which the silasesquioxane lig-
and functions as a model for the siliceous surface in a tripodal
titanium site. Similarly, the monosilylated form of 2, disilanol
(c-C6H11)7Si7O9(OSiMe3)(OH)2 4 has been used to model the
siliceous surface in bipodal and tetrapodal titanium sites. In this
paper we report on the preparation and characterisation of
these complexes, together with the results of a study into their
catalytic properties in alkene epoxidation. The implications
these results hold for titanosilicate epoxidation catalysts are
discussed.
1
UV-vis and H MAS NMR spectroscopic data and have sug-
gested that the open lattice site may even predominate in
dehydrated TS-1 samples.
In addition to the closed and open lattice sites, the presence
of bipodal sites (structure (c) in Scheme 1) can also be inferred.
For crystalline titanosilicates the closed lattice site might be
expected to be predominant in dehydrated samples, with tri-
podal and bipodal titanium species occurring at defect sites in
the silicate lattice. In the presence of water, formation of addi-
tional tripodal and bipodal sites is anticipated to occur via suc-
cessive hydrolysis of the Ti–O(Si) bonds of the closed lattice
titanium sites,24 a process which appears to be energetically
favourable according to a recent computational study.25 Despite
the many studies on heterogeneous titanium silicate catalysts,
the relative importance of these different titanium sites in select-
ive oxidation catalysis has yet to be ascertained.
Results and discussion
Preparation and characterisation of tripodal titanium
silasesquioxane complexes
Reaction of silasesquioxane 2 with homoleptic titanium()
complexes TiL4 (L = CH2Ph, NMe2, OSiMe3, OPri or OBut)
occurs with protonolysis in diethyl ether or toluene solution,
affording in each case a product with the empirical formula
[TiL{(c-C6H11)7Si7O12}] (L = CH2Ph 5, NMe2 6, OSiMe3 7, OPri
8 or OBut 9; see Scheme 2) on the basis of elemental analysis
and solution 1H NMR data. The 29Si NMR spectra of the
products (measured over the range ϩ30 to Ϫ70 ЊC) are particu-
larly informative, the observation in each case of three reson-
ances for the silasesquioxane Si atoms in a 3:1:3 ratio being
consistent with the C3v symmetry expected for a tripodal
TiL(silasesquioxane) species. As observed previously for
titanium silasesquioxane complexes,38,39 the resonance for the Si
atoms bearing OH groups (δ Ϫ60.2 for 2) shifts upfield by ca. 5
ppm upon co-ordination of the oxygen with titanium. Further
evidence for the tripodal geometry of complexes 5–9 is provided
by 13C NMR spectra, which likewise exhibit three resonances
for the ipso-carbons of the cyclohexyl groups with integrated
intensities of 3:1:3.
With this in mind we have prepared soluble analogues of
these different titanium species based on silasesquioxanes and
have studied the catalytic properties of the resulting titanium
complexes in an effort to elucidate structure–activity effects.26,27
Related work, also employing silasesquioxane precursors,
has recently been reported by Abbenhuis et al.28 and by
Maschmeyer et al.,29 while Roesky and co-workers30–32 have
described the use of silanetriol precursors for the preparation
of a number of titanium compounds which model titanium
silicates.
Incompletely condensed silasesquioxanes, R7Si7O9(OH)3
{R = c-C5H9 1, c-C6H11 2 or c-C7H13 3} prepared by hydrolysis
of the corresponding trichlorosilanes, RSiCl3, have recently
attracted considerable interest as models for the silica sur-
face.33,34 Feher et al.35 have established that the molecular struc-
ture of 2 (see Scheme 2) shares many structural similarities with
β-cristobalite (111) and β-tridymite (0001). Although the exact
nature of the silica surface is a matter of great controversy, the
In the case of compound 8 29Si NMR data indicate the
presence of a second species containing five distinct silicon
environments in a 1:1:2:1:2 pattern (in the low to high field
direction), corresponding to the dimer [Ti(µ-OPri){(c-C6H11)7-
Si7O12}]2 (Scheme 3) recently reported by Maschmeyer et al.29
Scheme 3 Monomer–dimer interconversion for titanosilasesquioxane
alkoxide complexes.
Freshly prepared solutions of 8 were found to contain exclus-
ively the monomer, while at equilibration times of several days
a monomer:dimer ratio of ca. 5:2 was observed in C6D6 by
29Si NMR spectroscopy. In the case of 9, syntheses in toluene
or diethyl ether similarly afforded the product exclusively
as a monomer, 13C and 29Si NMR spectra of the monomer
measured after 2 weeks in benzene solution showing the
absence of any dimer. However, syntheses performed in THF
consistently afforded a mixture of monomer and dimer, in ratios
(monomer:dimer) of ca. 4:1. In this context it is pertinent
Scheme 2 Synthesis of compounds 5–14: R = c-C6H11; L = CH2Ph
5, NMe2 6, OSiMe3 7, OPri 8 or OBut 9; X = H 10, F 11 or NO2 12;
R = c-C5H9; L = CH2Ph 13 or OPri 14.
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Table 1 Bond lengths [Å] and angles [Њ] for compound 7a
In order to extend the range of titanium silasesquioxane
complexes available, the reactions of 5 and 8 with phenol were
studied. In both cases addition of 1.0 equivalent of phenol to
a diethyl ether solution of the titanium complex resulted in
Ti(1)–O(1)
Si(2)–O(1)
O(3)–Si(4)
1.658(6)
1.625(6)
1.79(2)
Ti(1)–O(3)
Si(2)–O(2)
1.84(2)
1.641(6)
1
protonation of the capping ligand, as evidenced by H NMR
spectroscopy, and formation of the phenoxy complex 10.
Similarly, the reactions of 4-fluorophenol and 4-nitrophenol
with 8 afforded the corresponding phenoxy derivatives 11 and
12 (Scheme 2). As for 8, solution 29Si NMR data obtained on
10–12 in each case reveal the presence of a dimer, in addition to
the main monomeric product. For the dimers of 10–12 there
are, as for 8, five distinct environments for the silicon atoms,
giving rise to the same 1:1:2:1:2 pattern. The observed mon-
omer:dimer ratio for 10, measured in C6D6 (0.06 M solution)
after equilibration for ca. 24 h at room temperature, was typic-
ally around 5:1, the corresponding ratios for 11 and 12 being
respectively 10:1 and 28:1. For syntheses of 10–12 performed
in THF, approximately equimolar mixtures of the monomer
O(1Ј)–Ti(1)–O(1)
O(1)–Si(2)–O(2)
Si(4)–O(3)–Ti(1)
a Primed atoms related to unprimed equivalents by Ϫy, x Ϫ y, z.
107.4(2)
108.0(3)
180.0
O(1)–Ti(1)–O(3)
Si(2)–O(1)–Ti(1)
111.4(2)
151.0(4)
1
and dimer were obtained in each case. The H and 13C spectra
revealed the presence of co-ordinated THF in the mixtures;
based on the integrated intensities of signals in 1H NMR
spectra and the number of 29Si NMR signals observed for the
silasesquioxane silicons, the THF appears to be associated with
the dimer. In this context we note that Maschmeyer et al.29
have reported the synthesis and structural characterisation of
the methanol-solvated dimer of 8, i.e. [Ti(µ-OPri){(c-C6H11)7-
Si7O12}(MeOH)]2. By analogy, we suggest that the dimers of
10–12, when prepared in THF, exist as THF solvates of the type
[Ti(µ-OR){(c-C6H11)7Si7O12}(THF)]2.
Compounds 10–12 were also characterised using mass
spectrometry, field desorption (FD) mass spectra containing a
strong signal corresponding to the parent ion. In the case of 7
the parent ion is not observed in FD or electron impact mass
spectra, while for 8 it is a very weak signal only. For both these
complexes an intense signal is observed at m/z 1970, corre-
sponding to the species {(c-C6H11)7Si7O12Ti}2(µ-O) formed in
the mass spectrometer via bimolecular ether elimination from
the parent complex.
Reaction of silasesquioxane 1 with homoleptic complexes
TiL4 in diethyl ether solution likewise affords complexes
[TiL{(c-C5H9)7Si7O12}] (L = CH2Ph 13 or OPri 14), which
were found to be monomeric in solution. The preparation of
13 from 1 and [Ti(CH2Ph)4] has recently been reported by
Duchateau et al.40 As for complexes 5–9, 29Si and 13C NMR
data for 13 and 14 show that the complexes possess C3v sym-
metry. The successful synthesis of tripodal complexes of 1 is
significant, owing to the ready availability of silasesquioxane 1,
in comparison with 2. Thus 30–40% yields of 1 can be obtained
from the hydrolysis of (c-C5H9)SiCl3 after 3 d at reflux,41
whereas reaction times of around 2–3 years are required
for comparable yields of 2 to be attained from the hydrolysis
of (c-C6H11)SiCl3.35 Unfortunately, complexes 13 and 14 are
only sparingly soluble in organic solvents, as is the case for the
parent silasesquioxane 1,41 rendering them rather unsuitable for
liquid phase studies.
Fig. 1 Molecular structure of compound 7 showing the 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.
that while the reaction of [Ti(η-C5H5)Cl3] with 2 cleanly affords
the monomer [Ti(η-C5H5){R7Si7O12}] (R = c-C6H11),38,39 the
reaction of 2 with [Ti(η-C5H5)2Cl2] has been shown to afford
[Ti(η-C5H5){R7Si7O12}] as the thermodynamic product, the
dimer [Ti(η-C5H5){R7Si7O12}]2 being the initially formed
(kinetic) product.39 Similarly, the monomer of 9 appears to
represent a thermodynamic product: heating the monomer in
the presence of 10 equivalents of THF at 55 ЊC for 10 d failed to
afford any trace of the dimer, there being no discernible inter-
action between the complex and the added THF. On this basis
it appears that the relative thermodynamic stability of the
monomer and dimer in these systems is determined principally
by the steric properties of the ancillary ligand, bulky ligands
favouring formation of the monomer.
The monomeric nature of compound 7 was confirmed by
the results of a single crystal X-ray diffraction study (Fig. 1;
Table 1). The crystal structure consists of isolated molecules
separated by normal van der Waals contacts. Molecules lie in
crystallographic special positions of Ϫ3 site symmetry and are
consequently disordered; only one of the two centrosym-
metrically related images of the disordered molecule is shown in
Fig. 1. The molecular structure consists of a pseudo-tetrahedral
titanium() centre co-ordinated by the tridentate silasesqui-
oxane ligand and siloxy (OSiMe3) ligand such that the complex
has exact C3 symmetry (disregarding the cyclohexyl substituent
at Si(1Ј), 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 7 in the solid state is clearly established, and by inference
so is the structure of the monomer form of the isopropoxy
Preparation and characterisation of zirconium silasesquioxane
complexes
Zirconium-containing silicates have recently been shown to
possess catalytic properties in several selective oxidation
reactions,42–44 making the preparation of zirconium silasesqui-
oxane complexes of interest. Whilst the synthesis and structural
characterisation of a monomeric species [Zr(η-C5Me5){(c-C6-
H11)7Si7O12}] has been reported by Feher,45 the reaction of 2
with [Zr(CH2Ph)4] has been shown to afford a dimeric species
containing bridging siloxy groups.40 Similarly, attempts in this
work to prepare a simple tripodal species from the reaction of 2
with [Zr(NEt2)4] in THF were unsuccessful, an intractable
mixture of products being obtained. However, the use of
¯
analogue 8, which also crystallises in space group R3 with
similar cell dimensions to those of 7, and has molecules lying at
the same special positions as 7. For 8 the disorder is less well
resolved and refinement less satisfactory.
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[Zr(OBut)4] did afford a product with the empirical formula
[Zr(OBut){(c-C6H11)7Si7O12}] 15, solution NMR spectroscopy
indicating that it exists as a mixture of monomer and dimer
in a ratio (monomer:dimer) of ca. 4:1. As for the titanium
analogue, 9, bridging tert-butoxy groups are present in the
dimer.
Preparation and characterisation of titanium complexes derived
from disilanol 4
The reaction of (c-C6H11)7Si7O9(OSiMe3)(OH)2 4 with [Ti-
(OPri)4] was examined as a possible route to a model bipodal
titanium epoxidation catalyst. Slow addition of a diethyl ether
solution of 4 to the titanium complex resulted in a smooth
reaction and the subsequent isolation of [Ti(OPri)2{(c-C6H11)7-
Si7O11(OSiMe3)}] 16 as the sole product (Scheme 4). The 1H and
Fig. 2 Molecular structure of the first independent molecule of
compound 17 showing the atom labelling scheme. All hydrogen atoms
have been omitted for clarity. Both orientations of the disordered atoms
are shown.
Fig. 3 Molecular structure of the second independent molecule of
compound 17. Details as in Fig. 2.
of the silasesquioxane ligands is not retained at Ti. This is
confirmed by 29Si NMR spectroscopy, seven resonances being
observed for the framework silasesquioxane Si atoms in the
range δ Ϫ66.35 to Ϫ69.72, together with one at δ 10.42 corre-
sponding to the OSiMe3 group.
Scheme
4
Synthesis of compounds 16 and 17 (R = c-C6H11,
L = CH2Ph).
Further information concerning the molecular structure of
compound 17 was provided by the results of a single crystal
X-ray diffraction study on its toluene solvate (see Table 2).
Perspective views of the molecular structure of the two
independent molecules of [Ti{(c-C6H11)7Si7O11(OSiMe3)}2] 17
in the crystal structure are given in Figs. 2 and 3. The crystal
structure consists of isolated molecules of 17 and toluene
separated by normal van der Waals contacts. Molecules of 17
lie in crystallographic general positions and consist of a
pseudo-tetrahedral titanium() centre co-ordinated by two
silasesquioxane {R7Si7O11(OSiMe3)} ligands such that the
complex has approximate C2 symmetry (but see below). The
two independent molecules have very similar Ti{Si8O12}2
cores with both having significant distortions away from C2
symmetry. Displacement ellipsoid plots indicate considerable
variation in the motion of the Ti{Si8O12}2 cores, consistent with
considerable framework flexibility (and possibly dynamic or
static disorder).
The co-ordination geometry at titanium is close to tetra-
hedral (O–Ti–O angles range from 107.2(5) to 111.2(6)Њ), with
Ti–O distances between 1.786(6) and 1.828(11) Å with mean
1.797(5) Å (Table 2). The Si–O–Ti angles range from 138.5(6) to
159.9(7)Њ with mean 148.4(28)Њ, the largest variations in angles
and distances being around Ti(2). The Si–O(Ti) distances range
from 1.594(13) to 1.661(7) with mean 1.628(7) Å. Other Si–O
distances range from 1.540(12) to 1.654(8) with mean 1.619(2) Å
13C NMR data are consistent with the presence of two inequiv-
alent isopropoxy groups in 16, while the observation of
five signals in the 29Si NMR spectrum corresponding to the
silasesquioxane Si7O11 core indicates that, as for the “free”
ligand, a plane of symmetry runs through the silasesquioxane
ligand, with the alkoxide ligands lying in the plane.
Interestingly, reaction of one equivalent of compound 4 with
the complexes TiL4 (L = CH2Ph, NMe2 or OSiMe3) does not
afford the corresponding mixed ligand complex [TiL2{(c-C6-
H11)7Si7O11(OSiMe3)}], but yields instead equimolar amounts
of the bis(silasesquioxane) complex [Ti{(c-C6H11)7Si7O11-
(OSiMe3)}2] 17 (Scheme 4; see below) and the unchanged TiL4
complex.
The reaction of two equivalents of disilanol 4 with [Ti(CH2-
Ph)4] was chosen as a convenient means of preparing a model
for the closed lattice titanium site. In aprotic solvents a rapid
reaction occurs with the formation of a colourless solution,
from which the bis(silasesquioxane) complex [Ti{(c-C6H11)7-
Si7O11(OSiMe3)}2] 17 can be isolated in quantitative yield
(Scheme 4). Proton and 13C NMR spectroscopy reveal the
presence of only one type of OSiMe3 group in 17, suggesting
equivalence of the {(c-C6H11)7Si7O11(OSiMe3)} ligands. Con-
sistent with tetrahedral co-ordination of the titanium()
centre, seven resonances are observed in the CH region of the
13C NMR spectrum, indicating that the local mirror symmetry
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Table 2 Selected bond lengths [Å] and angles [Њ] for compound 17
(molecule 1)
Ti(1)–O(1)
Ti(1)–O(2)
1.786(6)
1.788(7)
Ti(1)–O(13)
Ti(1)–O(14)
1.796(6)
1.787(6)
O(1)–Ti(1)–O(2)
O(1)–Ti(1)–O(13)
O(1)–Ti(1)–O(14)
Si(2)–O(2)–Ti(1)
Si(10)–O(14)–Ti(1)
109.7(3)
109.9(3)
108.6(3)
140.6(5)
151.0(4)
O(2)–Ti(1)–O(13)
O(14)–Ti(1)–O(2)
O(14)–Ti(1)–O(13)
Si(1)–O(1)–Ti(1)
Si(9)–O(13)–Ti(1)
109.8(4)
110.5(3)
108.4(3)
153.7(5)
146.7(5)
Table 3 The IR and UV-vis spectroscopic data for titanium silasesqui-
oxane complexes and related materials
Complex/material
ν[Ti(OSi)x]/
λmax/nm (ε/dm3
a
cmϪ1
molϪ1 cmϪ1
)
5
941 (Nujol)
946 (KBr)
932 (KBr)
938 (KBr)
939 (KBr)
942 (KBr)
967 (KBr)
932 (Nujol)
918 (Neat)
960 (KBr)
945 (KBr)
222 (45 000)b
215 (46 500)c
212 (47 200)
217 (46 100)
n.d.
6
7
8
10
11
n.d.
16
214 (46 700)
228 (43 900)
234 (42 700)
210 (47 600)
215, 260, (46 500, 38 500)
17
[Ti(OSiMe3)4]
TS-1 (Ti = 1.5 wt%)
Amorphous TiO2–SiO2
(Ti = 8.2 wt%)
Fig. 4 The IR spectra (800–1050 cmϪ1) of (A) compound 2, (B) 7,
a Absorption measurements made on pentane solutions (ca. 1 mM),
except for TS-1 and amorphous TiO2–SiO2 which were measured in the
reflectance mode. b A CT band also observed at 376 nm associated with
the benzyl ligand. c A CT band also observed at 312 nm associated with
the dimethylamido ligand.
(C) amorphous titania–silica, (D) TS-1.
observed in the region 940–960 cm
.
Ϫ1 4–6 For amorphous titania
supported on silica some workers report an absorption band at
960 cmϪ1 47,48
although the presence of this band appears to
,
depend on the preparation procedure employed.49 Assignment
of this band has led to considerable debate: some workers
initially attributed it to the stretching vibration of a titanyl
and Si–O–Si angles range from 138.3(4) to 163.4(9)Њ with mean
150.2(11)Њ.
The Ti–O distances in the seven tetrahedral titanium()
complexes in the Cambridge Structural Database (May 1993
version), in which titanium is co-ordinated by four oxygens
only, average 1.783(6) Å (range 1.730 to 1.853 Å). The most
similar of these to 17 is [Ti{(SiPh2)4O5}2].46 Additionally, we
note that the mean Ti–O distance in 17 of 1.797(5) Å is very
similar to the Ti–O distance of 1.793(7) Å in TS-1 determined
in a recent EXAFS study,22 this figure representing the averaged
value from the two main site types thought to be present: the
closed lattice site, Ti(OSi)4, and a five-co-ordinate open lattice
species, SiOH ؒ ؒ ؒ Ti(OH)(OSi)3.
group, (SiO) Ti᎐O, by analogy with the spectra of molecular
᎐
2
titanyl complexes,47,50 while others have assigned it to a Si–O–Ti
stretching vibration51,52 or to an Si–OϪ absorption.53 We have
recently shown, on the basis of both synthetic studies and IR
spectroscopy (employing SO2 as a reactive probe molecule),
that titanyl groups do not exist in titanium silicates in detectable
concentrations.49,54 In our opinion Roesky’s observations,
coupled with the above findings for titanium silasesquioxane
complexes, provide convincing evidence that the 960 cmϪ1
absorption band of titanium silicates corresponds to a Si–O–Ti
stretching vibration. This of course does not exclude the possi-
bility that some contribution to this absorption may arise from
Si-OH stretching, particularly for titanosilicates possessing
highly hydroxylated surfaces.6
Comparison of IR and UV-vis spectra of titanium
silasesquioxane complexes with titanium silicates
UV-vis and IR spectroscopic methods have been extensively
used to characterise the titanium phases present in titano-
silicates,2 making a comparison of the spectroscopic properties
of the titanium complexes described above with those of
titanosilicate catalysts of interest (Table 3). The IR spectra of
tripodal complexes 5–14, as well as 16 and 17, contain a band
of medium intensity falling in the range 932–967 cmϪ1, which is
not present for the parent silasesquioxane ligands (Fig. 4). We
note that the complex [Ti(OSiMe3)4] similarly shows an absorp-
tion at 918 cmϪ1, not observed for Me3SiOH. This band must
therefore be associated with the presence of Si–O–Ti linkages,
and is most readily assigned to a Si–O–Ti stretching vibration.
Roesky and co-workers31 have similarly reported the observ-
ation of an intense absorption band at 960–970 cmϪ1 for a
number of cubic titanasiloxane compounds, assignment of the
band to a Si–O–Ti stretching vibration being supported by the
results of 18O labelling.
The UV-vis spectra of the (silasesquioxane)titanium com-
plexes are also informative, containing an intense absorption in
the range 212–228 nm, absent for the corresponding silasesqui-
oxanes. These absorptions are assigned to a ligand to metal
charge transfer transition involving four-co-ordinated titanium
bearing oxygen ligands. A similar band is observed for TS-1
(Fig. 5; Table 3), being assigned to tetrahedrally co-ordinated
titanium species which occupy sites in the titanosilicate
lattice.50,55 In the case of amorphous titania–silica mixed oxides
two absorption maxima are typically observed: a band at ca.
215 nm is similarly assigned to titanium species in tetrahedral
co-ordination, while a band at longer wavelength (ca. 260 nm)
corresponds to the presence of highly dispersed titanium
species which are five- or six-co-ordinate49 (note that some con-
tribution to this latter band may arise from the presence of
titanium sites with increased co-ordination number formed by
the interaction of water with the tetrahedral sites).56
For titanium silicates such as TS-1 and Ti-MCM-41 an
absorption band is found at 960–970 cmϪ1 (Fig. 4),2,3 while for
amorphous titania–silica mixed oxides a similar band is
Recently several groups have attempted to use UV-vis reflect-
ance and/or photoluminescence spectroscopy to differentiate
between closed and open lattice titanium sites in titanium
J. Chem. Soc., Dalton Trans., 1999, 3791–3804
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Table 4 Epoxidation of oct-1-ene with TBHP catalysed by titanium
silasesquioxane complexes and related materials
102 k2/dm3
Selectivity to
epoxidea (%)
Catalyst
molϪ1 sϪ1
5
123
63
97
95
99
97
93
93
88
92
89
90
0
75
83
91
94
98
6
7
8
149
184
157
138
206
249
1.0
9.3
4.7
18.2
2.6
4.2
10
11
12
13
14
15
16
17
[Ti(η-C5H5){(c-C6H11)7Si7O12}]
Ti-MCM-41 (Ti = 1.43 wt%)
Amorphous TiO2–SiO2
(Ti = 3.91 wt%)
TiO2–SiO2 (Ti = 8.2 wt%)
No catalyst
2.3
83
0
b
—
Conditions: T = 353 K, Ti = 0.2 mmol, TBHP = 30 mmol, oct-1-ene
(75 g) as solvent. a Selectivity = (mol 1,2-epoxyoctane formed/mol
TBHP consumed) × 100; selectivities were determined at 90% TBHP
consumption. Data quoted are derived from at least two runs. b 10%
TBHP conversion measured after 240 min.
Fig. 6 Selected pseudo-first order rate plots for the epoxidation of
oct-1-ene with TBHP catalysed by titanium silasesquioxane complexes:
᭜ 5, ᭿ 6, ᭡ 7, × 8, ᮀ 16 and ᭹ 17.
forward by Le Noc and co-workers. Studies employing photo-
luminescence spectroscopy are currently in progress in order to
verify these suggestions.
Epoxidation of oct-1-ene using tert-BuOOH
The epoxidation of oct-1-ene with tert-BuOOH (TBHP) was
chosen as a convenient test of the epoxidation activity of com-
plexes 5–17. At the olefin:TBHP molar ratio of ca. 20:1
employed pseudo-first order kinetics are observed correspond-
ing to rate equation (1). At high TBHP conversions (generally
d[epoxide]/dt = k1[TBHP]
(1)
(where k1 = k2[Ti] = k3[Ti][olefin])
>80%), however, 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 ButOH co-product, a phenomenon observed previously for
a variety of homogeneous and heterogeneous titanium and
vanadium catalysts.59 The calculated second order rate con-
stants for TBHP conversion (k2 = k1/[Ti]) for complexes 5–17,
determined at a reaction temperature of 353 K, are shown in
Table 4. Representative rate plots are collected in Fig. 6. For
selected tripodal titanium silasesquioxane complexes additional
epoxidation experiments were performed in the temperature
range 293–353 K. In all cases plots of ln k against 1/T afforded
straight lines, the resulting kinetic parameters being collected in
Table 5.
Most striking is the finding that the measured values of k2 for
the tripodal complexes 5–14 are approximately an order of
magnitude greater than for 16 and 17, while the former com-
plexes also show superior selectivity. For all the titanium com-
plexes tested 1,2-epoxyoctane was the only product formed
under the reaction conditions employed, selectivity referring to
the yield of epoxide based on TBHP consumed. In contrast, the
zirconium complex 15 showed only slight activity for TBHP
decomposition, without formation of the epoxide, this presum-
ably being a consequence of its lower Lewis acidity relative to
titanium.
Fig. 5 The UV-vis spectra of (A) compound 7, (B) TS-1 and (C)
amorphous titania–silica.
silicalites.22,23,57,58 Opening the Ti–O–Si bond angle shifts the
hybridisation of the oxygen from sp3 to sp2 and eventually sp,
thereby favouring π donation into the empty “e” level of
titanium in Td symmetry. According to Le Noc et al.,23
increased π donation should lead to a blue shift of the ligand to
metal electron transfer (LMET) band, i.e. the LMET absorp-
tion for the open lattice site should occur at lower wavelength
than that of the closed lattice site. In this context it is interesting
that the LMET absorption maxima of the tripodal silasesqui-
oxane complexes are found in the range 212–222 nm, while
that for 17 corresponds to a wavelength of 228 nm. Comparison
of the crystal structures of 7 and 17 shows the Ti–O–Si
(silasesquioxane) bond angles to be similar, being 151(4) and
ca. 148Њ, respectively (the latter figure being the average of four
Ti–O–Si angles). The most significant difference between 7 and
17, in terms of Ti–O–Si bond angles, is the presence of the
(apparently) linear Ti–O–SiMe3 group in 7. The lower wave-
length of the absorption maximum observed for 7, relative to
17, appears, therefore, to be consistent with the explanation put
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J. Chem. Soc., Dalton Trans., 1999, 3791–3804

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Table 5 Kinetic parameters for oct-1-ene epoxidations with TBHP
overrule any steric effects associated with the para substituent.
catalysed by titanium silasesquioxane complexesa
Somewhat unexpectedly, the presence of electron-withdrawing
substituents (F or NO2) results in a decrease of the reactivity of
the complex relative to the simple phenoxy complex, this des-
pite the fact that electron-withdrawing substituents might be
expected to increase the Lewis acidity of the titanium() centre
and thereby the epoxidation activity of the complex. In explan-
ation we suggest that under epoxidising conditions the higher
Lewis acidity of 11 and 12 favours a higher degree of solvation
in these complexes relative to 10 (i.e. favouring a dimer of the
type [Ti(µ-OC6H4X-p){(c-C6H11)7Si7O12}(ButOH)]2), thereby
rendering the metal less accessible to the TBHP reactant and
effectively reducing the Lewis acidity of the metal centre. From
this it follows that there exists an optimum in Lewis acidity with
respect to the epoxidation activity of titanium silasesquioxane
catalysts, an observation which may hold implications for the
design of new heterogeneous titanium-based epoxidation
catalysts.
Finally, we note that complexes 5–12 are considerably more
active as epoxidation catalysts than is [Ti(η-C5H5){(c-C6H11)7-
Si7O12}] (see Table 4), previously prepared by Feher38 and by
Maschmeyer39 and co-workers. Abbenhuis et al.28 have recently
reported that this complex is a catalyst for alkene epoxidation
with TBHP and have shown using NMR spectroscopy that the
cyclopentadienyl ligand remains bound to titanium under
epoxidising conditions. Based on the present work, together
with the findings of Maschmeyer et al.,29 it is apparent that the
low activity of the cyclopentadienyl complex can be ascribed to
the steric bulk of the cyclopentadienyl ligand.
Complex
Ea/kJ molϪ1
ln A
5
6
8
10
11
12
13
42.5
42.4
49.8
55.4
57.2
59.4
41.0
14.56
13.80
17.31
19.30
19.88
20.55
14.51
a Epoxidation conditions as for Table 4 (T = 295–353 K).
Complexes 5–14 are also considerably more active under
these test conditions than titanosilicate catalysts such as Ti-
MCM-41 and amorphous titania–silica mixed oxides, when
compared on the basis of k2 values (i.e. activity per titanium
site). The Ti-MCM-41 sample tested showed low activity
coupled with good selectivity (94%), while an amorphous
titania–silica sample containing 3.91 wt% titanium showed
slightly higher activity and selectivity. In line with literature
reports,6 an amorphous titania-silica sample prepared by the
same procedure but containing 8.20 wt% titanium showed
lower activity per titanium site and also lower selectivity than
the material with the lower titanium content, a reflection of the
increased proportion of octahedrally co-ordinated titanium in
the sample with the higher titanium content.
Consideration of the k2 values obtained for the tripodal
complexes 5–14 raises two important points. First, the fact that
the catalytic properties of 5–12 show significant differences
implies that the ligand, L, occupying the fourth co-ordination
site on titanium, must be reasonably stable with respect to
ligand exchange with the tert-butyl hydroperoxide anion, at
least for the limited number of turnovers (150) in the epoxid-
ation test. The exception to this would appear to be complexes
5 and 6, the basic benzyl and dimethylamino ligands in these
complexes being readily protonated by weak acids such as
alcohols, with the formation of the corresponding alkoxy
complexes. Addition of TBHP to 5 or 6 would therefore be
anticipated to result in formation of the peroxometal species
[Ti(OOBut){(c-C6H11)7Si7O12}], which after one epoxidation
cycle is converted into tert-butoxy complex 9 (see below). Note
that the lower epoxidation activity of 6, relative to 5, can be
rationalised by the inhibiting effect that Lewis bases such as
NHMe2 (formed by protonation of 6) exert on TiIV-catalysed
epoxidation (due to co-ordination to the TiIV).59
Secondly, it is apparent that steric factors significantly influ-
ence the epoxidation activity of titanium silasesquioxane com-
plexes. For 7, 8 and 10 the measured k2 values decrease with
increasing steric bulk of the alkoxo ligand, i.e. OPh > OPri
> OSiMe3. (Note that if the k2 value for 5 is taken as being
representative of the tert-butoxy complex 9, which was itself
not measured in this study, then the OBut ligand can be seen to
conform to this ordering, i.e. OPh > OPri > OBut > OSiMe3.)
A similar observation has been made by Maschmeyer et al.,29
who found that the activity of the complexes [TiL{(c-C6H11)7-
Si7O12}] (L = OMe, OPri or OBun) in cyclohexene epoxidation
with tert-BuOOH followed the order OMe > OBun ӷ OPri.
On this basis it was reasoned that linear alkoxy groups allow
better access to the titanium centre than the non-linear iso-
propoxy group, indicating that accessibility to the TiIV is the
main parameter controlling reactivity. The greater reactivity of
13 and 14 in epoxidation catalysis relative to 5 and 8 can simi-
larly be rationalised in terms of the greater accessibility of
the titanium centre in complexes based on the cyclopentyl-
substituted silasesquioxane ligand, relative to complexes con-
taining the bulkier cyclohexyl-substituted ligand.
Mechanism of TiIV-catalysed olefin epoxidation
The first detailed studies to be reported concerning the mechan-
ism of TiIV-catalysed epoxidation using alkyl hydroperoxides
were performed on the Sharpless epoxidation catalyst.60 A key
feature of the mechanism proposed by Sharpless and co-
workers is the formation of an alkylperoxo species, Ti(OOR),
formed by protonolysis of the alkoxo ligand by the hydro-
peroxide, eqns. (2) and (3). The η2 bonding mode of the peroxo
M(OR) ϩ ROOH → M(η2-OOR) ϩ ROH
(2)
M(η2-OOR) ϩ olefin → M(OR) ϩ epoxide (3)
ligand is supported by the results of quantum mechanical calcu-
lations,61 as well as by the recent report by Boche et al.62 of a
structurally characterised titanium alkylperoxo complex con-
taining the Ti(η2-OOR) unit. Significantly, the authors demon-
strated the capacity of the complex to oxidise nucleophiles,
consistent with the electrophilic character of the peroxo group.
Note that oxygen transfer from the peroxide to the olefin is
proposed to occur via attack of the olefinic bond on the
σ-bound oxygen of the Ti(η2-OOR) moiety and proceeds via a
“bi-triangular” transition state,60 with the generation of a labile
titanium (epoxo)(alkoxide) complex (Scheme 5). A recent study
by Sinclair and Catlow63 employing density functional theory
(DFT) calculations supports the main features of this
In the case of complexes 10–12, containing para-substituted
phenoxy ligands, electronic effects would be anticipated to
Scheme 5 Proposed transition species for oxygen transfer from peroxo-
titanium species.
J. Chem. Soc., Dalton Trans., 1999, 3791–3804
3797

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proposition: ethene epoxidation by H2O2 over titanium silicate
catalysts was found to proceed via the formation of a Ti(η2-
OOH) species, transfer of the oxygen being governed largely by
the ethene HOMO (π) to catalyst LUMO (σ*) interaction.
In the case of TS-1, the most frequently invoked mechanism
for olefin epoxidation with H2O2 (in alcoholic solvents)
resembles that discussed above, in that a metal peroxide species
is proposed as the key intermediate, proton abstraction from
the hydroperoxide occurring at a co-ordinated hydroxide or
siloxide group. The chief difference arises in the oxygen transfer
step: based on the observation of strong solvent and pH
effects in TS-1 and titanium β-catalysed epoxidation,64–68 it
is suggested that hydrogen bonding between co-ordinated
solvent (ROH) and the terminal OH group of the bound
hydroperoxide enhances the stability of the intermediate and
the transition state required to form the epoxide (Scheme 6).
Calculated transition states (using DFT and ab initio methods)
OBut ligand in 9 undergoes exchange with free ButOH.
Although complex 8 was not detected in the product mixture,
the signals for the PriOH formed were broad, implying that it
also exchanges with the OBut ligand in 9. A similar result was
obtained upon addition of 1.0 equivalent ButOH to 8, the
observed PriOH:ButOH ratio at equilibrium being 0.8:1. In the
29Si NMR spectrum of the product mixture obtained from 8
and TBHP the 3:1:3 pattern of the silasesquioxane silicons is
retained, although the signal at δ Ϫ66.3, corresponding to the
silicon atoms nearest to titanium (i.e. Ti–O–Si), is considerably
broadened. A similar effect is observed when 1 equivalent of
ButOH or water is added to 8, the peak broadening being
indicative of a fluxional process whereby the protic species adds
to titanium, with the proton migrating between the oxygen
atoms of the Ti–O–Si linkages (see Scheme 7).
Addition of 3 or more equivalents of TBHP to compound 8
led to the formation of the same products as above, albeit that
the evolution of oxygen from the reactant mixture was clearly
visible. Additionally, at high TBHP:titanium mole ratios some
decomposition of the titanium complex occurred, as evidenced
by the presence of 29Si NMR signals corresponding to the
free silasesquioxane. Logically, this is formed via sequential
protonation of the Ti–O–Si linkages by TBHP. Thus addition
of 10 equivalents of TBHP to 8 resulted in 33% decomposition
to the “free” ligand after 24 h. The fate of the titanium is
unclear, although the final product is presumably a titanium
alkoxide species.
Similar results were obtained with compound 7, although the
reaction with 1 equivalent TBHP was found to be consider-
ably slower than for the less sterically crowded 8, TBHP
disappearance occurring with a half-life of approximately 12 h
at room temperature. Principal reaction products were complex
9, ButOH and ButOOBut, together with two trimethylsilyl-
containing species (based on the observation of broad signals
at δ 0.190 and 0.196 in the 1H NMR spectrum). The 29Si
NMR spectrum contained the expected 3:1:3 pattern for the
silasesquioxane silicon atoms, the signal at the low field side
again being somewhat broadened. In the OSiMe3 region of the
spectrum a single broad resonance was observed, being slightly
shifted relative to the starting material (δ 16.30 vs. 16.52 for 7).
By analogy with 8, it is concluded that Me3SiOH is formed
which undergoes rapid exchange with the tert-butoxy ligand
in 9.
Scheme 6 Proposed mechanism for oxygen transfer from peroxo-
titanium species in TS-1 and Ti-β.
reported in two recent papers broadly support this picture,69,70
although co-ordination of a protic solvent does not appear to
be a pre-requisite.63
When applying the generalised mechanism summarised in
eqns. (2) and (3) to tripodal titanium silasesquioxane catalysts
it is apparent that proton abstraction from the alkyl hydro-
peroxide can occur at either the silasesquioxane ligand or the
ancillary ligand, L, thereby generating peroxometal species of
the type [Ti(OOBut)L{R7Si7O11(OH)}] or [Ti(OOBut)(HL)-
(R7Si7O12)], respectively, where HL may or may not be
dissociated from the metal. In an effort to assess the relative
importance of these species, the reactions of complexes 5, 7 and
8 with tert-BuOOH were studied.
The formation of Me3SiOH was confirmed by IR spectro-
scopy (Fig. 7). The disappearance of the TBHP absorption
band, ν(OH), at 3556 cmϪ1 was accompanied by the simul-
taneous appearance of new absorptions at 3617 and 3695
cmϪ1 assigned to the OH stretching vibration of ButOH and
Me3SiOH, respectively. Additionally, weak, new absorptions
were observed at 3630 and 3683 cmϪ1; whilst definitive assign-
ment of these bands is not possible, the position of the latter is
characteristic for an organic silanol species. In fact, assuming
the absence of intra- or inter-molecular hydrogen bonding,
either signal may correspond to a silasesquioxane silanol group,
formed via protolysis of a Ti–OSi bond.
Reaction of tripodal titanium silasesquioxane complexes with
tert-BuOOH
Addition of 1.2 equivalents of TBHP (5.0 M in decane) to a
0.06 M solution of compound 8 in C6D6 at ambient temper-
ature resulted in a fast reaction, as evidenced by the complete
disappearance within 20 min of 1H and 13C NMR signals
corresponding to the starting materials. Concomitantly, the
formation of tert-butoxy complex 9, PriOH, ButOH, and
ButOOBut (trace) was observed. Signals corresponding to a
putative alkylperoxotitanium complex were not detected. In the
13C NMR spectrum separate, broad resonances were observed
for the quaternary carbons of 9 and ButOH, while their methyl
carbons appeared as one averaged signal, indicating that the
Finally, the reaction of compound 5 with TBHP was exam-
ined. Upon addition of 1.2 equivalents of TBHP the orange
Scheme 7 Reaction of compound 8 with ButOH. Only one of the three possible intermediates containing a co-ordinated silanol group is shown.
3798
J. Chem. Soc., Dalton Trans., 1999, 3791–3804

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Fig. 7 Reaction of compound 7 with 1.0 equivalent TBHP in CCl4: IR spectra measured at 45, 1000 and 2500 min reaction time.
Scheme 8 Generalised scheme for the reaction of (alkoxo)titanium silasesquioxane complexes with ButOOH.
solution immediately decolourised, 1H NMR spectroscopy
indicating the formation of toluene and complex 9. Upon
standing, the excess of TBHP was slowly converted into
ButOH, ButOOBut and oxygen.
Based on the above observations, the reaction of TBHP with
tripodal titanium silasesquioxane complexes can be summar-
ised as shown in Scheme 8. It is apparent that proton abstrac-
tion from the alkyl hydroperoxide can indeed occur at either the
silasesquioxane ligand or anciliary ligand. The resulting alkyl-
peroxotitanium species formed are not stable at room temper-
ature and decompose rapidly with loss of oxygen, presumably
via pathways involving homolytic fission of the Ti–peroxo
bond, as observed previously in molybdenum chemistry,71 eqns.
(4)–(7). The titanium product, 9, formed according to eqn. (6),
epoxide selectivities observed, it is clear that the kinetics of
olefin attack on the peroxo complex is much faster than that of
decomposition to ButOH or ButOOBut.
Based on the generalised mechanism summarised in eqns. (2)
and (3), combined with the above observations, the mechanism
of olefin epoxidation catalysed by tripodal titanium silasesqui-
oxane complexes can be described by the sequence of events
depicted in Scheme 9. We note that the suggestion that
formation of the peroxotitanium complex can occur either via
t
t
ؒ
ؒ
TiOOBu → Ti ϩ Bu OO
(4)
(5)
(6)
(7)
t
t
ؒ
ؒ
2 Bu OO → 2 Bu O ϩ O2
t
t
ؒ
ؒ
Ti ϩ Bu O → TiOBu
t
t
t
ؒ
2 Bu O → Bu OOBu
similarly catalyses the decomposition of excess of TBHP to
ButOH and oxygen (although note that radical chain decom-
position of the TBHP may also play a role). The formation of
ButOOBut was also consistently observed, albeit in lower yield
than that of ButOH, consistent with the generation of alkyl-
peroxo radicals. Under epoxidising conditions, decomposition
of the peroxotitanium complex competes, in principal, with the
epoxidation reaction. However, based on the high TBHP-to-
Scheme 9 Proposed mechanism for olefin epoxidation catalysed by
titanium silasesquioxane complexes.
J. Chem. Soc., Dalton Trans., 1999, 3791–3804
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cleavage of an alkoxy ligand or via cleavage of a siloxy group is
supported by the results of recent DFT calculations reported by
Sinclair and Catlow: 63 for the case of ethene epoxidation with
H2O2 over the putative site Ti(OH)(OSiH3)3, little energetic
preference was found for either pathway, the calculated
(absolute) activation barrier in both cases being estimated at
to that of the heterogeneous analogue, due to the greater
uniformity of catalytic centres. Indeed, tripodal titanium
silasesquioxane complexes are the most active titanium-based
epoxidation catalysts reported to date, and are among the most
active and selective of all known homogeneous epoxidation
catalysts. On this basis, we conclude that the most active site
in titanosilicates, as far as epoxidation catalysis is concerned,
corresponds to the tripodal, open-lattice type of titanium site.
The tripodal site appears to represent an optimum in high
Lewis acidity versus steric accessibility. The comparatively low
epoxidation activity of heterogeneous titanosilicate catalysts
such as Ti-MCM-41 and amorphous titania–silica 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 co-ordination environment.
48–56 kJ molϪ1
.
In a broader context, we conclude that the most active site in
titanium silicates, as far as epoxidation catalysis is concerned,
corresponds to the tripodal, open-lattice type of titanium site.
The comparatively low epoxidation activity of Ti-MCM-41 and
amorphous titania–silica 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 co-ordination environ-
ment. The question then arises as to why the tripodal site is so
much more active in epoxidation catalysis than are the bipodal
or closed lattice sites. Based on the significant difference in the
rates of reaction of compounds 7 and 8 with TBHP, steric
factors can most obviously explain the low epoxidation activity
of 17 and, by implication, closed lattice sites in titanosilicates:
the more sterically congested co-ordination sphere of the
titanium site relative to tripodal and bipodal titanium sites, can
be expected to result in a lower reactivity towards alkyl
hydroperoxides. In the case of the bipodal titanium site steric
arguments are clearly not applicable. That bipodal sites show
low activity in epoxidation catalysis is confirmed by previous
studies in this laboratory.49 Model epoxidation catalysts
(prepared by grafting Ti(CH2Ph)4 onto silica, followed by
hydrolysis), containing predominantly bipodal sites, (SiO)2-
Ti(OH)2, were found to possess low epoxidation activity under
the same test conditions employed in this work (k2 < 6 × 10Ϫ2
dm3 molϪ1 sϪ1 for oct-1-ene epoxidation with TBHP). In the
absence of steric factors, electronic effects would appear to play
the determining role. In this context we note that Feher and
Budzichowski33 have estimated, on the basis of the correlation
between 13C chemical shifts and Hammett substituent param-
eters, that the electron-withdrawing tendency of the Si8O12
framework in a completely condensed silasesquioxane is similar
to that of a CF3 group. Based on simple electronic effects alone,
therefore, the order tetrapodal > tripodal > bipodal would be
anticipated regarding the epoxidation activity of four-co-
ordinate titanium() sites, since this follows the expected order
of Lewis acidity of the titanium centre. If this line of reasoning
is extrapolated to simple titanium alkoxides, Ti(OR)4, one
would therefore expect these compounds to show low epoxid-
ation activity, as is indeed the case.59 According to this logic, the
tripodal site therefore represents the best compromise between
high Lewis acidity and a sterically accessible titanium()
centre.
Experimental
All manipulations of air- and/or moisture-sensitive materials
were performed under an atmosphere of argon using standard
Schlenk techniques, or in a nitrogen-filled Braun model MB-
200 glove-box. Argon and nitrogen were purified by passage
through columns filled with manganese() oxide supported on
vermiculite. Glassware was dried in a vacuum oven (110 ЊC)
before use.
Diethyl ether and THF were distilled from purple solutions
of sodium–benzophenone under nitrogen. Pentane was distilled
from purple solutions of potassium–benzophenone containing
tetraglyme (2,5,8,11,14-pentaoxapentadecane). Toluene was
distilled from sodium, acetonitrile from CaH2, carbon tetra-
chloride from 3 Å molecular sieves and C6D6 was vacuum dis-
tilled (25 ЊC, 10Ϫ4 mbar) from CaH2 and degassed by repeated
freeze–pump–thaw cycles. Anhydrous tert-butyl hydroperoxide
(TBHP, 3 M in isooctane or 5.0–6.0 M in decane) was pur-
chased from Aldrich and stored over 4 Å molecular sieves. Oct-
1-ene (>98%) was obtained from Merck and dried over 3 Å
molecular sieves before use. Phenol (99ϩ%), 4-fluorophenol
(99%) and 4-nitrophenol (99ϩ%) were obtained from Aldrich
Chemical Co. and dried in a vacuum desiccator over P2O5.
Silasesquioxanes 1, 2 and 4 were prepared according to the
literature procedures.35,41 In the case of 2 it was found that
removal of resinous material from the crude product could
most efficiently be performed by washing with pentane, as
opposed to the literature method (washing with acetone). The
compound [Ti(OPri)4] (97%) was purchased from Aldrich
Chemical Co. and [Ti(OSiMe3)4] and [Ti(NMe2)4] from ABCR
GmbH; [Ti(CH2Ph)4] was synthesized according to a literature
method.72 TS-1 and Ti-MCM-41 were prepared by direct
hydrothermal synthesis.1,16 Amorphous titania–silica samples
were prepared by a homogeneous precipitation procedure using
[Ti(OPri)4] and [Si(OEt)4];73 characterisation of these materials
has been reported elsewhere.73,74
Concluding remarks
Identification of the active site in heterogeneous catalysts is a
task which is typically complicated by the presence of a broad
spectrum of chemically inequivalent sites. For this reason,
knowledge of the mechanism of action of most solid catalysts
remains extremely limited. One means of circumventing this
problem is to prepare well defined model catalysts, such that
direct correlations can be drawn between the structure of a
particular site and its catalytic properties. In this respect
incompletely condensed silasesquioxanes have proven to be
structurally well suited to the preparation of co-ordination
complexes modelling the different sites present in silicate-based
metal oxide catalysts. Based on the structural and spectroscopic
similarities exhibited by titanium silasesquioxane complexes
and titanosilicates, we conclude that the former are valid
models for this important class of heterogeneous catalysts.
A further benefit of this approach is the opportunity it
affords to prepare new types of homogeneous catalyst, possess-
ing in the most favourable case activity and selectivity superior
The NMR spectra were recorded either on Bruker AM-500
(1H, 500.1; 13C, 125.8; 29Si, 99.4 MHz), AM-400 (13C, 100.6; 29Si,
79.5 MHz), AM-300 (1H, 300.2 MHz) or AM-250 (1H, 250.1;
13C, 62.9 MHz) spectrometers. All chemical shifts are reported
in units of δ (downfield from tetramethylsilane) and were refer-
enced to residual protio solvent (1H) and solvent (13C) reson-
ances (C6D6; δ 7.16 for 1H, 128.0 for 13C). The 29Si NMR spectra
were generally recorded with inverse-gated proton decoupling
in order to increase resolution and minimise nuclear Over-
hauser enhancement effects. To ensure accurate integrated
intensities, [Cr(acac)3] (0.05 M) was added to 13C and 29Si NMR
samples as a shiftless relaxation agent and a delay of at least 5 s
was used between observation pulses for 13C measurements and
10 s for 29Si measurements.
Field desorption (FD) mass spectra were recorded on a
JEOL JMS HX110 magnetic sector instrument. Acceleration
and cathode voltages were set to 10 and Ϫ1 kV, respectively.
An activated emitter was dipped into C6D6 solutions of the
3800
J. Chem. Soc., Dalton Trans., 1999, 3791–3804

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samples, mass spectra being obtained by heating the emitter
linearly from 0 to 60 mA (ramp 1 mA minϪ1). The resulting
spectra were summed to afford one mass spectrum showing
predominantly molecular ion masses of the desorbed species.
The FT-IR spectra of solids were collected as Nujol mulls or
KBr pellets on a Bio-Rad FTS 50 instrument. Solution spectra
were collected on carbon tetrachloride solutions contained in a
NaCl cell on the same instrument. Stock solutions of the sila-
sesquioxane complexes were prepared on a 1% m/m basis; TBHP
was diluted in carbon tetrachloride to the same molarity as the
silasesquioxane solutions. Equal volumes of the two solutions
were mixed in a glove-box and immediately transferred to the
IR cell. Spectra were recorded at a resolution of 2 cmϪ1. The
UV-vis spectra were collected where appropriate in the absorp-
tion mode on pentane solutions sealed under nitrogen in 1 cm
cuvettes, using a Hitachi U-3300 spectrophotometer, or in the
reflectance mode (TS-1) using a Perkin-Elmer LS-50 spec-
trometer. In the latter case the vacuum dried solids were meas-
ured in air. Elemental analyses were performed by Analytische
Laboratorien, D-5270 Gummersbach, Germany, or by the
analytical group at Shell Research and Technology Centre,
Amsterdam. The oxide WO3 was routinely added to samples to
aid combustion and suppress the formation of silicon carbide.
g, 2.18 mmol) to compound 2 (2.050 g, 2.11 mmol), followed by
work-up as above, afforded 8 as a white microcrystalline solid
1
(2.240 g, 99%). H NMR (C6D6, 250.1 MHz): δ 4.41 (septet,
1 H, CH(CH3)2, J = 6.1), 2.15–1.00 (m, 77 H, c-C6H11) and 1.20
(d, 6 H, CH(CH3)2, J = 6.1 Hz). 13C NMR (C6D6, 100.6 MHz):
δ 80.09 (s, CH(CH3)2), 27.83, 27.81, 27.48, 27.44, 27.33, 27.28,
27.22 (s, CH2), 25.80 (s, CH3), 23.91, 23.85 and 23.78 (s, 3:1:3
for CH). 29Si NMR (C6D6, 79.5 MHz): δ Ϫ66.24, Ϫ68.09,
Ϫ69.08 (s, 3:1:3, monomer); Ϫ67.24, Ϫ68.08, Ϫ68.36, Ϫ69.01
and Ϫ70.00 (5 × s, 1:1:2:1:2, dimer). Mass spectrum (FD):
m/z 1969.9 (100%, [Ti{(c-C6H11)7Si7O12}]2Oϩ). Found: C, 49.93;
H, 7.70. Calc. for C45H84O13Si7Ti: C, 50.16; H, 7.86%.
[Ti(OBut){(c-C6H11)7Si7O12}] 9. Addition of [Ti(OBut)4] (0.99
g, 2.72 mmol) to a solution of compound 2 (2.66 g, 2.74 mmol)
in toluene (100 ml), followed by work-up as above, afforded 9
as a white microcrystalline solid (1.72 g, 57%). 1H NMR
(C6D6, 300.2 MHz): δ 2.15–1.24 (m, 77 H, c-C6H11) and 1.30 (s,
9 H, OC(CH3)3). 13C NMR (C6D6, 100.6 MHz): δ 85.22 (s,
OC(CH3)3), 31.44 (s, OC(CH3)3), 27.92, 27.72, 27.53, 27.50,
27.33, 27.19, 27.15, 26.95 (s, CH2), 23.83, 23.75 and 23.64 (s,
3:1:3 for CH). 29Si NMR (C6D6, 79.5 MHz): δ Ϫ68.32, Ϫ70.36
and Ϫ71.36 (s, 3:1:3). Found: C, 50.52; H, 7.88. Calc. for
C46H86O13Si7Ti: C, 50.61; H, 7.94%.
Repetition of the above procedure using THF instead of
toluene afforded compound 9 as a mixture of monomer and
dimer (and containing THF of crystallisation). 1H NMR
(C6D6, 300.2 MHz): δ 3.54 (m, THF, α-CH2), 2.14–0.86 (m, Cy;
Preparations
[Ti(CH2Ph){(c-C6H11)7Si7O12}] 5. Pentane (20 ml) was added
to a solid mixture of [Ti(CH2Ph)4] (0.500 g, 1.21 mmol) and
silasesquioxane 2 (1.179 g, 1.21 mmol). Stirring at room tem-
perature for 30 min afforded a deep yellow solution, which was
filtered and reduced to dryness under vacuum. The residue was
dissolved in toluene and acetonitrile added dropwise to give a
precipitate of 5. The yellow microcrystalline solid was isolated
by filtration, washed with acetonitrile (3 × 5 ml) and dried
THF, β-CH2), 1.30 (s, OC(CH3)3) and 1.05 (s, µ-OC(CH3)3). 13
C
NMR (C6D6, 100.6 MHz): δ 85.29 (s, OC(CH3)3), 67.85 (s,
THF, α-CH2), 31.48 (s, OC(CH3)3), 31.31 (s, µ-OC(CH3)3),
27.98, 27.86, 27.82, 27.78, 27.69, 27.59, 27.57, 27.42, 27.40,
27.34, 27.28, 27.24, 27.20, 27.11 (s, CH2), 25.76 (s, THF,
β-CH2), 24.72, 24.31, 23.89, 23.82, 23.71 and 23.44 (s, CH). 29Si
NMR (C6D6, 97.5 MHz): δ Ϫ68.29, Ϫ70.31, Ϫ71.36 (3:1:3,
monomer), Ϫ69.49, Ϫ70.36, Ϫ70.60, Ϫ71.27 and Ϫ72.27
(1:1:2:1:2, dimer).
1
under vacuum (1.194 g, 89%). H NMR (C6D6, 250.1 MHz):
δ 7.31–7.03 (m, 5 H, C6H5), 3.10 (s, 2 H, CH2Ph) and 2.20–0.97
(m, 77 H, c-C6H11). 13C NMR (C6D6, 100.6 MHz): δ 142.98 (s,
ipso-C of Ph), 124.33 (s, Ph, other signals obscured by solvent),
81.17 (s, CH2Ph), 27.77, 27.70, 27.60, 27.32, 27.30, 27.27, 27.21
(s, CH2), 23.75, 23.72, 23.51 (s, 3:1:3 for CH). 29Si NMR
(C6D6, 79.5 MHz): δ Ϫ65.55, Ϫ67.67 and Ϫ68.73 (s, 3:1:3).
Found: C, 52.79; H, 7.68. Calc. for C49H84O12Si7Ti: C, 53.11;
H, 7.64%.
[Ti(OPh){(c-C6H11)7Si7O12}] 10. A solution of phenol (0.082
g, 0.87 mmol) in diethyl ether (30 ml) was added dropwise,
using a flexible stainless steel cannula, to a stirred solution of
compound 8 (0.844 g, 0.78 mmol) in diethyl ether over a period
of 30 min. The resulting mixture was stirred for 1 h at room
temperature. The volume of solvent was then reduced under
vacuum (to ca. 30 ml) and acetonitrile (30 ml) added. The
resulting white precipitate was isolated by filtration, washed
with acetonitrile (5 × 20 ml) and dried under vacuum to
[Ti(NMe2){(c-C6H11)7Si7O12}] 6. The compound [Ti(NMe2)4]
(0.20 g, 0.89 mmol) was added via syringe to a stirred solution
of 2 (0.833 g, 0.86 mmol) in pentane–diethyl ether 1:1 (60 ml).
Work-up of the resulting solution (as for 5) afforded 6 as a
1
afford 10 (0.800 g, 92%). H NMR (C6D6, 250.1 MHz): δ 7.50
1
yellow powder (0.882 g, 97%). H NMR (C6D6, 250.1 MHz):
(m, 2 H, o-H of C6H5), 7.41 (m, 2 H, m-H of C6H5), 6.98 (m,
1 H, p-H of C6H5), 2.08–0.94 (m, 77 H, c-C6H11). 29Si NMR
(C6D6, 79.5 MHz): δ Ϫ66.40, Ϫ67.60, Ϫ68.34 (s, 3:1:3, mono-
mer); Ϫ66.68, Ϫ67.50, Ϫ67.79, Ϫ68.43 and Ϫ69.43 (5 × s,
1:1:2:1:2, dimer). Mass spectrum (FD): m/z 1110.3 (100, Mϩ)
and 555.3 (10%, M2ϩ). Found: C, 50.10; H, 7.51. Calc. for
C48H82O13Si7Ti: C, 51.86; H, 7.43%.
δ 3.05 (s, 6 H, NMe2), 2.12–0.97 (m, 77 H, c-C6H11). 13C NMR
(C6D6, MHz): δ 44.97 (s, NMe2), 27.97, 27.88, 27.83, 27.69,
27.52, 27.37, 27.33 (s, CH2), 24.15, 24.09, 23.91 (s, 3:1:3 for
CH). 29Si NMR (C6D6, MHz): δ Ϫ65.1, Ϫ67.1 and Ϫ68.1 (s,
3:1:3). Found: C, 48.15; H, 7.61; N, 1.19. Calc. for C44H83-
NO12Si7Ti: C, 49.73; H, 7.87; N, 1.32%.
Repetition of the above procedure using THF instead of
diethyl ether afforded compound 10 as a mixture of monomer
and dimer, containing co-ordinated THF. 1H NMR (C6D6,
300.2 MHz): δ 7.50 (m, 2 H, o-H of C6H5), 7.41 (m, 2 H, m-H of
C6H5), 6.98 (m, 1 H, p-H of C6H5), 3.75 (s, co-ordinated THF,
α-CH2), 2.19–0.92 (m, 77 H, c-C6H11; THF, β-CH2). 13C NMR
(C6D6, 100.6 MHz): δ 166.20 (ipso-C of C6H5), 129.07 (C6H5),
122.00 (C6H5), 121.71 (C6H5), 69.71 (s, co-ordinated THF,
α-CH2), 27.98, 27.91, 27.86, 27.78, 27.70, 27.55, 27.44, 27.33,
27.27, 27.21, 27.12 (s, CH2), 25.66 (s, THF, β-CH2), 24.73,
24.32, 23.89, 23.85, 23.82, 23.72, 23.70, 23.49, 23.45 (s, CH).
29Si NMR (C6D6, 79.5 MHz): δ Ϫ66.97, Ϫ67.10, Ϫ67.51,
Ϫ67.85, Ϫ68.08 (2 signals), Ϫ68.70 and Ϫ69.76.
[Ti(OSiMe3){(c-C6H11)7Si7O12}] 7. Addition of [Ti(OSiMe3)4]
(1.08 g, 2.67 mmol) to compound 2 (2.595 g, 2.67 mmol),
followed by work-up as above, afforded 7 as a white micro-
1
crystalline solid (2.817 g, 95%). H NMR (C6D6, 250.1 MHz):
δ 2.13–0.97 (m, 77 H, c-C6H11) and 0.21 (s, 9 H, SiMe3). 13C
NMR (C6D6, 100.6 MHz): δ 27.80, 27.76, 27.42, 27.40, 27.32,
27.25, 27.18 (s, CH2), 23.88, 23.85, 23.61 (s, 3:1:3 for CH)
and 1.59 (s, SiMe3). 29Si NMR (C6D6, 79.5 MHz): δ 16.52 (s,
OSiMe3), Ϫ65.80, Ϫ67.90 and Ϫ68.85 (s, 3:1:3). Mass spec-
trum (FD): m/z 1969.9 (100, [Ti{(c-C6H11)7Si7O12}]2Oϩ) and
1107.6 (15%, Mϩ). Found: C, 48.56; H, 7.69. Calc. for C45H86-
O13Si8Ti: C, 48.79; H, 7.82%.
[Ti(OPri){(c-C6H11)7Si7O12}] 8. Addition of [Ti(OPri)4] (0.62
[Ti(OC6H4F-p){(c-C6H11)7Si7O12}] 11. Dropwise addition of
J. Chem. Soc., Dalton Trans., 1999, 3791–3804
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a solution of 4-fluorophenol (0.142 g, 1.27 mmol) in diethyl
ether (30 ml) to compound 8 (1.285 g, 1.19 mmol) dissolved in
diethyl ether (30 ml) as described for 10, followed by the same
work-up procedure, afforded 11 as a pale yellow powder (1.14 g,
p-H of C6H5, J = Hz), 3.02 (s, 2 H, CH2Ph) and 1.99–1.13 (m,
63 H, c-C5H9). 13C NMR (C6D6, 125.8 MHz): δ 142.90 (s, ipso-C
of Ph), 124.34 (s, Ph, other signals obscured by solvent), 81.53
(CH2Ph), 27.92, 27.90, 27.87, 27.78, 27.46 (s, CH2), 22.73, 22.65
and 22.40 (s, 3:1:3 for CH). 29Si NMR (C6D6, 99.4 MHz):
δ Ϫ62.72, Ϫ64.74 and Ϫ65.88 (s, 3:1:3). Found: H, 6.80. Calc.
for C42H70O12Si7Ti: H, 6.98%; satisfactory carbon analysis
could not be obtained.
1
85%). H NMR (C6D6, 250.1 MHz): δ 7.42 (m, 2 H, o-H of
C6H5), 7.15 (m, 2 H, m-H of C6H5) and 2.10–0.95 (m, 77 H,
c-C6H11). 29Si NMR (C6D6, 79.5 MHz): δ Ϫ66.34, Ϫ67.55,
Ϫ68.28 (s, 3:1:3, monomer); Ϫ66.68, Ϫ67.50, Ϫ67.79, Ϫ68.44
and Ϫ69.44 (5 × s, 1:1:2:1:2, dimer). Mass spectrum (FD):
m/z 1129.6 (100, Mϩ) and 564.5 (8%, M2ϩ). Found: C, 52.12; H,
7.37. Calc. for C48H81FO13Si7Ti: C, 51.04; H, 7.23%.
Repetition of the above procedure using THF instead of
diethyl ether afforded compound 11 as a mixture of monomer
and dimer, containing co-ordinated THF. 1H NMR (C6D6,
300.2 MHz): δ 7.46 (m, 2 H, o-H of C6H5), 7.15 (m, 2 H, m-H of
C6H5), 3.86 (m, co-ordinated THF, α-CH2) and 2.09–0.91 (m,
77 H, c-C6H11; THF, β-CH2). 13C NMR (C6D6, 100.6 MHz):
δ 162.08 (s, ipso-C of C6H5), 158.57 (d, p-C of C6H5,
1JC-F = 239), 122.39 (s, o-C of C6H5), 115.38 (d, m-C of C6H5,
2J = 21 Hz), 69.41 (s, co-ordinated THF, α-CH2), 27.95, 27.89,
27.85, 27.80, 27.77, 27.71, 27.68, 27.57, 27.54, 27.41, 27.31,
27.24, 27.16 and 27.09 (s, CH2), 25.68 (s, THF, β-CH2), 24.71,
24.30, 23.87, 23.78 (2 signals), 23.71, 23.67 and 23.43 (s, CH).
29Si NMR (C6D6, 79.5 MHz): δ Ϫ66.95, Ϫ67.33, Ϫ67.80
(2 signals), Ϫ68.06, Ϫ68.58, Ϫ68.73 and Ϫ69.73.
[Ti(OPri){(c-C5H9)7Si7O12}] 14. The compound [Ti(OPri)4]
(0.48 g, 1.69 mmol) was added dropwise via syringe to a vigor-
ously stirred suspension of 1 (1.350 g, 1.54 mmol) in ether (120
ml). The mixture was stirred at room temperature for 20 h, after
which time work-up (as described for 13) afforded 14 as a white
1
microcrystalline solid (1.291 g, 85%). H NMR (C6D6, 500.1
MHz): δ 4.40 (septet, 1 H, CH(CH3)2, J = 6.0), 1.94–1.11 (m, 63
H, c-C5H9) and 1.18 (d, 6 H, CH(CH3)2, J = 6.0 Hz). 13C NMR
(C6D6, 100.6 MHz): δ 80.09 (s, CH(CH3)2), 28.05, 27.97, 27.87,
27.51, 27.44 (s, CH2), 25.67 (s, CH3), 22.84, 22.77 and 22.64
(s, 3:1:3 for CH). 29Si NMR (C6D6, 79.5 MHz): δ Ϫ63.81,
Ϫ65.80, Ϫ66.83 (s, 3:1:3). Found: C, 45.79; H, 6.96. Calc.
for C38H70O13Si7Ti: C, 46.60; H, 7.20%.
[Zr(OBut){(c-C6H11)7Si7O12}] 15. A solution of [Zr(OBut)4]
(1.29 g, 3.36 mmol) in THF (20 ml) was added dropwise via
cannula to a stirred solution of compound 2 (2.99 g, 3.07
mmol) in THF (70 ml) and stirring continued for 90 min. The
volume of liquid was then reduced to 10 ml and acetonitrile (80
ml) added. The resulting precipitate was isolated by filtration,
washed with acetonitrile (3 × 10 ml) and dried under vacuum
to afford 15 (3.15 g, 92%) as a mixture of monomer and dimer.
1H NMR (C6D6, 300.2 MHz): δ 2.29–1.03 (m, c-C6H11, µ-OC-
(CH3)3) and 1.07 (s, OC(CH3)3). 13C NMR (C6D6, 100.6 MHz):
δ 78.74 (OC(CH3)3), 76.45 (µ-OC(CH3)3), 32.11 (OC(CH3)3),
31.55 (µ-OC(CH3)3), 28.48–27.30 (m, CH2), 25.54, 24.90, 24.72,
24.54, 24.23, 24.15 and 24.02 (s, CH). 29Si NMR (C6D6, 79.5
MHz): δ Ϫ70.06, Ϫ70.24, Ϫ70.36, Ϫ71.82, Ϫ72.03 (dimer,
1:1:2:1:2), Ϫ70.19, Ϫ71.03 and Ϫ71.77 (monomer, 3:1:3).
Found: C, 48.84; H, 7.65. Calc. for C46H86O13Si7Zr: C, 48.68; H,
7.64%.
[Ti(OC6H4NO2-p){(c-C6H11)7Si7O12}] 12. Dropwise addition
of a solution of 4-nitrophenol (0.400 g, 2.88 mmol) in diethyl
ether (30 ml) to compound 8 (2.969 g, 2.76 mmol) dissolved in
diethyl ether (30 ml) as described for 10, followed by the same
work-up procedure, afforded 12 as a bright yellow powder (2.90
1
g, 91%). H NMR (C6D6, 250.1 MHz): δ 8.47 (m, 2 H, o-H of
C6H5), 7.53 (m, 2 H, m-H of C6H5) and 2.11–0.95 (m, 77 H,
c-C6H11). 29Si NMR (C6D6, 79.5 MHz): δ Ϫ66.39, Ϫ67.48,
Ϫ68.18 (s, 3:1:3, monomer); Ϫ66.68, Ϫ67.50, Ϫ67.79, Ϫ68.43
and Ϫ69.44 (5 × s, 1:1:2:1:2, dimer). Mass spectrum (FD):
m/z 1155.6 (100, Mϩ) and 577.9 (8%, M2ϩ). Found: C, 50.20;
H, 7.09; N, 0.87. Calc. for C48H81NO15Si7Ti: C, 49.84; H, 7.06;
N, 1.21%.
Repetition of the above procedure using THF instead of
diethyl ether afforded compound 12 as a mixture of monomer
and dimer, containing co-ordinated THF. 1H NMR (C6D6,
300.2 MHz): δ 8.46 (m, 2 H, o-H of C6H5), 7.52 (m, 2 H, m-H of
C6H5), 4.26 (m, co-ordinated THF, α-CH2) and 2.10–0.90 (m,
77 H, c-C6H11; THF, β-CH2). 13C NMR (C6D6, 100.6 MHz):
δ 169.79 (ipso-C of C6H5), 143.50 (C6H5), 125.64 (C6H5), 121.88
(C6H5), 72.31 (s, co-ordinated THF, α-CH2), 28.02, 27.92,
27.84, 27.77, 27.70, 27.65, 27.41, 27.33, 27.26, 27.21, 27.16,
26.98 (s, CH2), 25.58 (s, THF, β-CH2), 24.78, 24.37, 23.94,
23.72, 23.62 and 23.48 (s, CH).
Alternatively, a solution of 4-nitrophenol (0.108 g, 0.78
mmol) in diethyl ether (15 ml) was added dropwise to a slurry
of compound 5 (0.862 g, 0.78 mmol) in diethyl ether (15 ml).
The resulting deep orange solution was stirred for 30 min and
then filtered. The volume of solvent was reduced under vacuum
(to ca. 15 ml) and acetonitrile (25 ml) added to afford a precipi-
tate. The solid was isolated by filtration, was washed with
acetonitrile (5 × 10 ml) and dried under vacuum to afford 12
(0.436 g, 48%), identified by 1H and 29Si NMR spectroscopy.
[Ti(OPri)2{(c-C6H11)7Si7O11(OSiMe3)}] 16. A solution of
compound 4 (2.500 g, 2.39 mmol) in diethyl ether (50 ml) was
added dropwise (via cannula) over a period of 2 h to a vigor-
ously stirred solution of [Ti(OPri)4] (1.38 g, 4.85 mmol) in
diethyl ether (50 ml). Stirring was continued for 30 min, after
which time the volume of liquid was reduced under vacuum
until ca. 10 ml remained. Acetonitrile (15 ml) was then added,
resulting in the precipitation of 16 as a white microcrystalline
solid. The product was isolated by filtration, washed with
acetonitrile (4 × 10 ml) and dried under vacuum (2.622 g, 91%).
1H NMR (C6D6, 500.1 MHz): δ 4.62 (septet, 1 H, CH(CH3)2,
J = 6.1), 4.57 (septet, 1 H, CH(CH3)2, J = 6.1), 2.17–1.05 (m, 77
H, c-C6H11), 1.32 (d, 6 H, CH3, J = 6.1), 1.30 (d, 6 H, CH3,
J = 6.1 Hz) and 0.47 (s, 9 H, Si(CH3)3). 13C NMR (C6D6, 125.8
MHz): δ 79.32 (s, OCH), 79.05 (s, OCH), 28.12, 28.08, 28.03,
27.87, 27.81, 27.79, 27.71, 27.67, 27.63, 27.54, 27.48, 27.40,
27.36, 27.32, 27.23 (s, CH2), 26.29 (br s, CH3), 25.67, 25.13,
24.96, 24.11, 23.86 (s, 1:2:2:1:1, CH) and 2.19 (s, Si(CH3)3).
29Si NMR (C6D6, 99.4 MHz): δ 10.94 (s, Si(CH3)3), Ϫ65.64,
Ϫ66.37, Ϫ66.58, Ϫ67.49 and Ϫ68.86 (s, 1:1:1:2:2). Found: C,
50.36; H, 8.21. Calc. for C51H100O14Si8Ti: C, 50.63; H, 8.33%.
[Ti(CH2Ph){(c-C5H9)7Si7O12}] 13. A solution of [Ti(CH2-
Ph)4] (0.919 g, 2.23 mmol) in diethyl ether (20 ml) was added
dropwise to a vigorously stirred suspension of compound 1
(1.950 g, 2.23 mmol) in ether (100 ml). The resulting deep
yellow solution was stirred for 2 h. The volume of solvent was
then reduced under vacuum (to ca. 15 ml) and acetonitrile (15
ml) added to afford 13 as a yellow precipitate. The solid was
isolated by filtration, washed with acetonitrile (3 × 10 ml) and
dried under vacuum (1.85 g, 82%). 1H NMR (C6D6, 500.1
MHz): δ 7.18–7.15 (m, 4 H, o-, m-H of C6H5), 6.87 (m, 1 H,
[Ti{(c-C6H11)7Si7O11(OSiMe3)}2] 17. Diethyl ether (30 ml)
was added to a solid mixture of [Ti(CH2Ph)4] (0.316 g, 0.77
mmol) and compound 4 (1.601 g, 1.53 mmol). Work-up of the
resulting solution (as above) afforded 17 as a white micro-
1
crystalline solid (1.590 g, 97%). H NMR (C6D6, 250.1 MHz):
δ 2.14–1.00 (m, 154 H, c-C6H11) and 0.52 (s, 18 H, Si(CH3)3). 13
C
NMR (C6D6, 100.6 MHz): δ 28.04, 27.93, 27.80, 27.74, 27.60,
3802
J. Chem. Soc., Dalton Trans., 1999, 3791–3804

View Article Online
Table 6 Crystal and other structural data for compounds 7 and
17ؒC6H5Me
For compound 17 three cyclohexyl groups suffered disorder
(those attached to Si(6), Si(7) and Si(31)) and were refined with
two site models in which some carbons were common to both
ring conformations and the occupancies of the two orientations
were refined. One trimethylsilyl group was also disordered
(50:50 by refinement) and modelled in a similar way with two
silicon positions (Si(32) and Si(33)) and having one methyl
carbon position in common (C(180)). All ordered non-
hydrogen atoms were assigned anisotropic displacement
parameters and restraints (ISOR in the terminology of
SHELXL 93)75 applied ensuring that the parameters of Si, O
and C were reasonably near to isotropic. Disordered and
solvate Si or C atoms were assigned freely refined isotropic
displacement parameters. All Si–Me distances were weakly
restrained to be close to 1.9 Å, and C–C bond distances in the
disordered cyclohexyl rings and those attached to Si(3), Si(5),
and Si(19) were also weakly restrained to be close to 1.5 Å. The
toluene solvate molecules were assigned rigid fixed geometries.
Finally a correction was applied for unassigned solvent
scattering density (SWAT in the terminology of SHELXL 93,75
parameter value = 1.71(13)). The relatively high residuals
(although they are quite low by macromolecule standards; there
are 276 non-hydrogen atoms in this structure) are in part a
consequence of reflection overlap problems noted during data
collection. Other factors include the difficulties in treatment of
the less well ordered cyclohexyl groups and the solvate
molecules.
7
17ؒC6H5Me
Empirical formula
M
C45H86O13Si8Ti
1107.76
C97H180O24Si16Ti
2227.75
Crystal system
Space group (no.)
Rhombohedral
R3 (148)
Triclinic
P1 (2)
¯
¯
a/Å
b/Å
c/Å
α/Њ
16.955(4)
16.955(4)
17.853(6)
21.085(8)
22.498(8)
27.786(10)
73.86(3)
77.13(3)
87.37(3)
12342(8)
4
0.286
32278
32278
—
β/Њ
γ/Њ
V/Å3
4445(2)
3
0.360
1448
1296
0.022
0.107
Z
µ/mmϪ1
Reflections collected
Independent reflections
Rint
Final R1 [I > 2σ(I)]
0.134
27.47, 27.34, 27.27, 27.18 (s, CH2), 25.62, 25.23, 24.87, 24.85,
24.51, 23.80, 23.78 (s, CH) and 2.42 (s, Si(CH3)3). 29Si NMR
(C6D6, 79.5 MHz): δ 10.42 (s, Si(CH3)3), Ϫ66.35, Ϫ66.79,
Ϫ66.92, Ϫ68.22, Ϫ69.26, Ϫ69.50 and Ϫ69.72 (s, all × 1).
Found: C, 50.39; H, 7.96. Calc. for C90H172O24Si16Ti: C, 50.69;
H, 8.13%.
CCDC reference number 186/1646.
See http://www.rsc.org/suppdata/dt/1999/3791/ for crystallo-
graphic files in .cif format.
Structural analyses of compounds 7 and 17ؒC6H5Me
Suitable crystals of compound 7 were obtained by vapour diffu-
sion of acetonitrile into a diethyl ether solution of the titanium
complex cooled to 4 ЊC. Crystals of 17 were prepared by vapour
diffusion of acetonitrile into a pentane solution of 17ؒC6H5Me
cooled to 4 ЊC. All diffraction measurements were made at
200 K on a Siemens R3m/V diffractometer fitted with an LT-1
crystal cooling device, using graphite monochromated Mo-Kα
X-radiation.
Crystal data and other details of the structure analyses are
presented in Table 6. Standard four-circle diffractometry and
data analysis procedures were used with unusual features as
detailed below.
Epoxidation of oct-1-ene
Epoxidation tests were performed batchwise in a magnetically
stirred 250 ml glass reactor, equipped with a condenser, therm-
ometer probe and septum for withdrawing samples. All runs
were performed under an atmosphere of dry nitrogen. Typic-
ally, toluene (3 g, 0.03 mol as internal standard), oct-1-ene (73
g, 0.6 mol), a quantity of catalyst (equivalent to 0.2 mmol of Ti)
and a stirrer bar were placed in the reactor. The mixture was
warmed to 80 ЊC and 10 ml of TBHP solution were added via
syringe. Immediately a sample was taken for analysis (GLC and
iodometric titration), further samples for analysis being taken
at regular intervals. GLC Analyses were performed on a
Hewlett-Packard HP 5890 instrument, with flame ionisation
detection (FID), a 25 m × 0.32 mm (0.52 mm film thickness)
HP-1 (cross linked methyl silicone gum) fused silica capillary
column, and helium as carrier gas. An injection temperature of
140 ЊC was employed, which was found to be sufficiently low to
avoid the occurrence of secondary reactions in the injection
port. The TBHP was determined by iodometric titration with
sodium thiosulfate. Rate constants for TBHP consumption (k1,
k2 = k1/[Ti]) were determined from pseudo-first order rate plots
(Ϫln[TBHP] versus t).
Careful inspection of symmetry equivalent reflections for this
crystal of compound 7 (and for other samples of 7 and of
[Ti(OPri){(SiR)7O12}] 8, which also crystallises in space group
¯
R3 with similar cell dimensions to those of 7) revealed no evi-
dence for lower diffraction symmetry. Attempts to refine 7 in a
lower symmetry space group (e.g. R3) led to much less reason-
able molecular geometries and very unstable refinements, as
might be expected. Similar behaviour (with worse outcome) was
noted for data collected on the pseudo-isostructural 8. The final
model (of many employed) for 7 (see Fig. 1) has the molecule
disordered about a site of crystallographic Ϫ3 symmetry. Pairs
of atoms related by disorder across the inversion centre [Ti(1)
and Si(1); C(9) and C(10); O(3) and C(7)] were constrained to
have identical positional and displacement parameters. Atoms
Ti(1), Si(1), O(3), C(7) and Si(4) all lie on the crystallographic
3-fold axis. All non-hydrogen atoms (except C(11)) were
assigned anisotropic displacement parameters. The atoms with-
in the cyclohexyl group [C(1)–C(6)] were restrained to have
similar displacement parameter components along the C–C
bond directions and to have C–C bond lengths following C2
local symmetry (the DELU and SAME restraints in SHELXL
93).75 The other single-site atoms [O(1), O(2), Si(2)] were refined
without constraints on their positional parameters. The
cyclohexyl group [C(7), C(8), C(10), C(11)] attached to Si(1) is
disordered about the crystallographic 3-fold axis. Models
including merohedral twinning with twin plane (0001) gave no
improvement to the residuals, as might be expected given the
Rint value for 6/m Laue symmetry (0.388).
Acknowledgements
The authors thank T. Couperus and J. Frijns for performing the
13C and 29Si NMR measurements, M. Sonnemans for obtaining
FD mass spectra and Mrs K. Searcy for performing sup-
plementary experiments. Shell International Chemicals B.V. is
thanked for allowing publication of this work.
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