Tantalum-Polyhedral Oligosilsesquioxane Complexes as Structural Models and Functional Catalysts for Epoxidation

Article abstract of DOI:10.1021/acscatal.7b00020

Tantalum-based supported catalysts have been shown to be very selective for epoxidations with aqueous hydrogen peroxide. To gain information relative to the active site on the surface, access to molecular complexes that mimic the active site of the catalyst on the surface is of great interest. In this contribution, several new Ta-polyhedral oligosilsesquioxane (POSS) complexes with POSS ligands are used to model silica-bound Ta sites and are shown to be active as epoxidation catalysts. Notably, a Ta-POSS complex modified by germanoxy ligands and possessing a dinuclear Ta(μ-O)(μ-OH)Ta core is observed to be very efficient for the epoxidation of cyclooctene using aqueous hydrogen peroxide.

Full text of DOI:10.1021/acscatal.7b00020

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Article
Tantalum–polyhedral oligosilsesquioxane (POSS) complexes
as structural models and functional catalysts for epoxidation
Pascal Guillo, Michael I. Lipschutz, Meg E. Fasulo, and T. Don Tilley
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00020 • Publication Date (Web): 14 Feb 2017
Downloaded from http://pubs.acs.org on February 14, 2017
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Scheme 1. (a) Trisilanol 1. (b) The Cp'TiCl-POSS series of complexes (R = alkyl groups) (c) Cp*Ta(X)-POSS
complexes
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Scheme 2. Ta-SAB15 catalyst and a possible molecular model
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Scheme 3. Syntheses of 6 and 7. a) diethyl ether, 23 °C, 6 h; b) 1, diethyl ether, 23 °C, 15 h.
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Scheme 4. Syntheses of 8 and 9. a) diethyl ether, 23 °C, 2 h; b) 1, toluene, 23 °C, 15 h.
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Scheme 5. Synthesis of 10 and 11. a) pentane, 23 °C, 3 h, then vacuum, 110 °C, 4 h; b) 1, diethyl ether,
23 °C, 15 h.
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Scheme 6. Access to 12 by a degradation process of 7 or by direct synthesis
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Figure 1. Molecular structure of 7 displaying thermal ellipsoids at the 50 % probability level. H-atoms and i-
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Figure 2. Molecular structures of 8 and 9 displaying thermal ellipsoids at the 50 % probability level. H-atoms
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Figure 3. Molecular structures of 10 an 11 displaying thermal ellipsoids at the 50 % probability level. H-
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Figure 4. Molecular structure of 12 displaying thermal ellipsoids at the 50 % probability level. H-atoms, i-
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graphical abstract
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Tantalum–polyhedral oligosilsesquioxane (POSS)
complexes as structural models and functional
catalysts for epoxidation
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Pascal Guillo,1,2,† Michael I. Lipschutz,1 Meg E. Fasulo,1 and T. Don Tilley1,2,*
1Department of Chemistry, University of California, Berkeley, Berkeley, CA 94720
2Chemical Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road,
Berkeley, CA 94720
ABSTRACT
Tantalum-based, supported catalysts have shown to be very selective for epoxidations with
aqueous hydrogen peroxide. In order to gain information relative to the active site on the surface,
access to molecular complexes that mimic the active site of the catalyst on the surface is of great
interest. In this contribution, several new Ta-POSS complexes with polyhedral oligosilsesquioxane
(POSS) ligands are used to model silica-bound Ta sites, and are shown to be active as epoxidation
catalysts. Notably, a Ta-POSS complex modified by germanoxy ligands and possessing a dinuclear
Ta(µ–O)(µ-OH)Ta core is observed to be very efficient for the epoxidation of cyclooctene using
aqueous hydrogen peroxide.
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KEYWORDS. Silsesquioxane, tantalum, catalysis, epoxidation, hydrogen peroxide.
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Introduction
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Single-site, supported catalysts have attracted considerable attention in the context of probing
catalytic mechanisms and optimizing efficiencies for a wide range of chemical transformations.1-6
Despite tremendous progress in this area,7-9 there remains a markedly incomplete understanding
of the relationship between reaction mechanisms and structural properties for the active sites of
heterogeneous catalysts. One approach for identifying crucial structural features for supported
catalysts is based on the study of accurate molecular models, which may be interrogated with a
wide array of spectroscopic methods to establish structure-activity relationships. For this purpose,
polyhedral oligosilsesquioxane (POSS) ligands10-13 and their metal complexes have been
extensively employed to mimic the structure and chemistry of silica-bound metal species.14-19
Indeed, silsesquioxane compounds can be used to mimic the different types of silanol groups found
on a silica surface (isolated, vicinal and geminal), and can provide information on the role of
nearest-neighbor atoms on structure and reactivity.14 Furthermore, silsesquioxanes may possess
pKa values for Si-OH units that are close to those of silica.20 Thus, the study of silsesquioxane-
based homogeneous models for single-site heterogeneous catalysts can contribute to a better
understanding of heterogeneous catalysis at the molecular level. Moreover, they may function as
related and efficient homogeneous catalysts.14
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Titanium-polyhedral oligosilsesquioxane (Ti-POSS) complexes, mainly based on the trisilanol
1 (Scheme 1a), have been extensively studied as molecular models for silica-supported titanium
epoxidation catalysts such as microporous TS-1 and Ti-MCM-41.21-27 Whereas TS-1 with its
hydrophobic pore structure is useful for the industrial-scale epoxidation of propene with H2O2,28
mesoporous catalysts such as Ti-MCM-41 or Ti-SBA15 have been developed for higher molecular
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weight olefins with alkylhydroperoxides as the oxidant.29-31 However, although aqueous hydrogen
peroxide is a more attractive choice for the oxidant, due to its low cost and environmental appeal,
silica-supported titanium centers in mesoporous structures are rapidly degraded under aqueous
conditions.30,32,33 Among the different strategies to improve the catalyst stability, one of the most
promising is the incorporation of the catalytic center into a hydrophobic environment.32-37
Development of new materials such as the mesoporous mesophase material Ti-MMM, or the slow
addition of H2O2, represent promising additional approaches for overcoming the problem of
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catalyst degradation.38,39
The Ti-POSS homogeneous catalysts are efficient with
alkylhydroperoxides as the oxidant, but only modest activity is observed in aqueous H2O2.21-25
Only two reports describe homogeneous Ti-POSS complexes that efficiently catalyze epoxidation
with H2O2 (Scheme 1b).26,27 In these examples involving a Cp*Ti center, it is proposed that the
sterically demanding Cp* ligand stabilizes reactive species through the catalytic cycle.
Immobilization of Ti-POSS complexes in a hydrophobic environment such as a PDMS membrane
has also been shown to be an effective way to utilize Ti-POSS complexes for epoxidations with
H2O2.40 Also, organo-bridged silsesquioxane titanates synthesized by a sol-gel process are efficient
catalysts for heterogeneous epoxidation with hydrogen peroxide.41
Research in this laboratory established the utility of site-isolated tantalum centers on silica as
inherently more selective than analogous titanium catalysts for epoxidations with aqueous
hydrogen peroxide.33,42-46 This interesting difference in catalytic behavior points to different
catalytic structures and mechanisms, and highlights the need to obtain useful structural information
and molecular models for the tantalum catalysts.33,44 In addition to the Ta-based systems, Nb-based
silica catalysts also exhibit superior performance with hydrogen peroxide (vs. organic
peroxides).47-52 In this context, it is notable that Ta complexes of silsesquioxane ligands, i.e Ta-
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POSS complexes, have received little attention. To the best of our knowledge, only four reports
have described Ta-POSS complexes, only three of which involve Ta as part of the silsesquioxane
framework.53-56 In order to gain information and to improve Ta-based supported catalysts, access
to Ta-POSS complexes is important for the design and development of more efficient supported
catalysts. This laboratory reported the synthesis and characterization of four Cp*Ta(X)POSS
complexes (X = Cl (2), OTf (3), Me (4), BArf (5); Scheme 1c).55 Interestingly, while Notestein
and coworkers have reported that silica-supported Cp*Ta is an epoxidation catalyst with aqueous
hydrogen peroxide as oxidant,45 the molecular Cp*Ta(X)-POSS complexes (X = Cl (2), OTf (3),
Me (4), BArf (5)) exhibit no activity for this reaction.
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Scheme 1. (a) Trisilanol 1. (b) The Cp'TiCl-POSS series of complexes (R = alkyl groups) (c)
Cp*Ta(X)-POSS complexes
The originally reported Ta-SBA15 epoxidation catalysts, obtained by anchoring the molecular
precursor Ta(OiPr)2[OSi(OtBu)3]3 onto SBA15 silica, efficiently utilize aqueous hydrogen
peroxide as the oxidant.42 Importantly, the efficiency of the H2O2 conversion in this catalysis is
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significantly enhanced by the incorporation of hydrophobic substituents onto the silica surface.43
Although the resulting organic-inorganic hybrid tantalum catalysts are far superior to analogous
titanium catalysts in promoting selective epoxidations, they are about an order of magnitude lower
in catalytic activity. Thus, an important goal in developing useful, supported tantalum catalysts in
selective oxidations is the realization of higher conversion rates. This might be achieved by
chemical modifications of the catalytic center, and it has been established that Ti and Ta catalysts
exhibit higher activities when M–O–Si linkages are replaced by M–O–Ge moieties.57-61 Although
the precise origin of this "germanium effect" is yet to be thoroughly understood, it is of interest to
develop structure-activity relationships for tantalum catalysts with well-defined structures. As
structural and functional models for supported tantalum catalysts, Ta-POSS derivatives should
prove useful given identification of catalytically active examples. Currently, no tantalum-based
homogeneous epoxidation catalysts possessing Ta–O–Ge linkages have been described. In this
contribution, several new Ta-POSS complexes are presented along with their activities as
epoxidation catalysts. Notably, a Ta-POSS complex modified by germanoxy ligands and
possessing a dinuclear Ta(µ–O)(µ-OH)Ta core is observed to be very efficient for the epoxidation
of cyclooctene using aqueous hydrogen peroxide.
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Results and Discussion
Synthesis and characterization of Ta-POSS complexes. To mimic the silica-supported Ta
catalysts previously studied (Scheme 2),42 a Ta-POSS complex with –OSi(OtBu)3 ligands was
initially targeted. While Ta(OiPr)2[OSi(OtBu)3]3 was used as the Ta precursor in the synthesis of
Ta-SBA15 materials, Ta(NMe2)3[OSi(OtBu)3]2 was envisioned as a particularly useful starting
material for incorporating
a
Ta[OSi(OtBu)3]2 group, since the condensation of
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Ta(NMe2)3[OSi(OtBu)3]2 with trisilanol 1 was expected to occur with efficient elimination of 3
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equiv of Me2NH and formation of the fully condensed POSS with Ta in the POSS framework.
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Scheme 2. Ta-SAB15 catalyst and a possible molecular model
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The tris(amido) complex Ta(NMe2)3[OSi(OtBu)3]2 (6) was obtained in 86 % yield as a
white powder from the reaction between Ta(NMe2)3Cl2 and 2 equiv of KOSi(OtBu)3 (Scheme 3).
Reaction of 6 with trisilanol 1 led to formation of Ta[OSi(OtBu)3]2-POSS•HNMe2 (7) in 94 %
yield as a white powder after precipitation from a diethyl ether solution with acetonitrile. The X-
ray crystal structure of 7 reveals an octahedral coordination environment at Ta, with a molecule of
dimethylamine (Me2NH) as a ligand (Figure 1). This retention of an equivalent of Me2NH as a
ligand has been observed for other Ta-NMe2 precursors employed in protonolysis reactions.62-65
The Ta center in 7 is in a distorted octahedral environment, in which the O atom trans to the amine
ligand is more tightly bound (the Ta–O4 distance is 1.909(6) Å), whereas other Ta–O distances
are 1.939(6)-1.947(6) Å). The relatively long Ta–N distance of 2.337(6) Å is consistent with a
dative interaction.62,63 The Ta–OSi(OtBu)3 distances (1.937 and 1.938 Å) are in the range of those
observed for previously reported Ta(OiPr)2[OSi(OtBu)3]3 (1.860(5) to 2.041(7) Å).42
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Scheme 3. Syntheses of 6 and 7. a) diethyl ether, 23 °C, 6 h; b) 1, diethyl ether, 23 °C, 15 h.
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Figure 1. Molecular structure of 7 displaying thermal ellipsoids at the 50 % probability level. H-
atoms and i-butyl groups have been omitted for clarity.
An accepted mechanistic step for the activation of a hydroperoxide in a metal-catalyzed
epoxidation is protonolysis of an M–O–Si linkage to produce an (SiOH)M–OOR intermediate.
Indeed, this type of process has been postulated for Ti-POSS complexes, where cleavage of a Ti–
O bond (in preference to Cp–Ti bonding) has been proposed.21,25,66 Thus, to stabilize the Ta–O
bonds toward engagement in this type of reversible process, a chelating bidentate bis(aryloxide)
ligand was employed. Synthesis of the bidentate ligand LH2 was adapted from the synthesis of a
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related compound,67 and reaction of Ta(NMe2)5 with 1 equiv of LH2 in diethyl ether afforded
Ta(L)(NMe2)3 (8) in 73 % yield as a pale yellow powder (Scheme 4). Subsequent reaction of 8
with 1 equiv of the trisilanol 1 in toluene led to formation of Ta(L)-POSS•HNMe2 (9) in 60 %
isolated yield as a white solid.
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Scheme 4. Syntheses of 8 and 9. a) diethyl ether, 23 °C, 2 h; b) 1, toluene, 23 °C, 15 h.
The X-ray crystal structures of 8 and 9 (Figure 2) reveal the presence of a chelating binaphtholate
ligand with dihedral (twist) angles of 50.3° and 52.0°, respectively (Figure 2). Complex 8
possesses a distorted trigonal bipyramidal geometry while in 9 the tantalum center is in an
octahedral coordination environment, with a dimethylamine molecule coordinated to tantalum. As
for related Ta compounds with a binaphtholate ligand, 8 features an oxygen atom from L in an
equatorial position (with Ta–Oeq = 1.937(1) Å) while the other oxygen is in an axial position with
a Ta–Oax distance of 2.052(1) Å.62
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Figure 2. Molecular structures of 8 and 9 displaying thermal ellipsoids at the 50 % probability
level. H-atoms and i-butyl groups have been omitted for clarity.
To access a molecular precursor with a Ta–O–Ge linkage, we focused on introduction of –
OGeiPr3 ligands as previously reported for a Ti–Ge precursor.60 Initial attempts to achieve this via
reaction of Ta(NMe2)3Cl2 with 2 equiv of KOGeiPr3 did not provide the anticipated
Ta(NMe2)3(OGeiPr3)2 complex, but instead gave a mixture of products that could not be separated.
The reaction of Ta(NMe2)5 with 5 equiv of HOGeiPr3 did not give the expected products
Ta(OGeiPr3)5 or Ta(NMe2)x(OGeiPr3)5-x, but instead provided the ditantalum complex 10 with μ–
O and two μ–OH bridges (Scheme 5). Monitoring the reaction by 1H NMR spectroscopy indicated
clean formation of the initial product Ta(NMe2)(OGeiPr3)4, with 1 equiv of HOGeiPr3 still present
in solution. After removal of solvent under vacuum at room temperature and redissolution in
1
benzene-d6, a mixture of Ta(NMe2)(OGeiPr3)4 and 10 was observed by H NMR spectroscopy.
Subsequent removal of solvent and heating of the resulting solid at 110 °C under vacuum led to
the exclusive and clean formation of [(iPr3GeO)3Ta]2(μ-OH)2(μ-O) (10) as the sole product. This
transformation is associated with thermal decomposition of HOGeiPr3 into unidentified volatile
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products. The synthesis of 10 was accomplished by reaction of Ta(NMe2)5 with 5 equiv of
HOGeiPr3 in pentane, followed by removal of solvent and heating of the resulting solid at 110 °C
under vacuum for 4 h. Compound 10 was then isolated as a white powder in 85 % yield after
precipitation of a solution of 10 into acetonitrile. Suitable crystals for X-ray crystallography were
obtained by slow evaporation of a solution of 10 in diethyl ether at – 30 °C.
Scheme 5. Synthesis of 10 and 11. a) pentane, 23 °C, 3 h, then vacuum, 110 °C, 4 h; b) 1, diethyl
ether, 23 °C, 15 h.
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The μ-oxo and μ-hydroxo ligands of 10 can be identified by the X-ray crystallographic data
(Figure 3). Thus, two distinctly shorter Ta–O distances of 2.024(9) and 2.027(10) Å are attributed
to the Ta–O–Ta group, and four longer distances between 2.113(10) and 2.183(11) Å are assigned
to the Ta–OH–Ta linkages. The Ta–O–Ta bond angle is 6-8° greater than the Ta–OH–Ta bond
angles. This assignment of a Ta(μ-O)(μ-OH)2Ta core is also consistent with electroneutrality in
the complex, and the presence of the hydroxo groups was confirmed by infrared spectroscopy
which revealed the presence of a sharp band at 3647 cm-1 for the O–H stretching mode. Moreover,
based on comparisons to another dinuclear Ta compound with the same core, [Cp*TaCl2]2(μ-
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OH)2(μ-O), 2 bands at 564 and 644 cm-1 were attributed to Ta–O–Ta vibrations for 10 (vs. 577 and
629 cm-1 for [Cp*TaCl2]2(μ-OH)2(μ-O)).68,69
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Complex 10 appears to represent an interesting precursor to olefin epoxidation catalysts, as it
contains Ta–O–Ge linkages as well as a ditantalum core. The latter feature presents the opportunity
to probe possible cooperative effects between metal centers in a bimetallic active site, and few
detailed studies on such catalysts have been realized. Previous studies on supported dititanium
catalysts suggest that catalysis by such species is possible.30,70,71 Additionally, it has been shown
that the catalytically active surface-bound species from the reaction of Ti(OiPr)4 with silica is an
oxo-bridged Ti-O-Ti dimer.70,71 In related work, a monomeric POSS complex with two Ti centers
linked with an oxo bridge has been reported.72 The latter complex results from the condensation
of CpTiCl3 with Cy6Si6O7(OH)4 to give Cy6Si6O12Ti2Cp2. However, for the oxidation of
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t
cyclohexene with BuOOH and a catalyst loading of 2 mol%, the catalytic activity of
Cy6Si6O12Ti2Cp2 was much lower than for its analog with only one Ti center.
Reaction of 10 with 1 equiv of the trisilanol 1 in diethyl ether led to formation of 11, isolated in
95 % yield by precipitation from a diethyl ether solution with acetonitrile (Scheme 5). The crystal
structure of 11 revealed that the ditantalum unit is preserved in the product, with μ-oxo and μ-
hydroxy bridges, while one of the hydroxyl bridges in 10 is replaced by a siloxide bridge (Figure
3). The structure of 11 is very unusual for a metal-POSS compound, in that to the best of our
knowledge, it is the first example of a bimetallic complex derived from 1 equiv of trisilanol 1. As
for 10, the Ta centers in 11 are in a highly distorted octahedral environment with roughly trans–
O–Ta–O angles between 152.3(4)° and 170.3(4)°. The presence of the μ-OH group is also
confirmed by the FTIR spectrum, which possesses a characteristic band at 3577 cm-1.
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Figure 3. Molecular structures of 10 an 11 displaying thermal ellipsoids at the 50 % probability
level. H-atoms; i-butyl groups on siloxide ligands and i-propyl groups on the germoxy ligands
have been omitted for clarity.
Catalysis. The previously reported TaCp*-POSS complexes 2-555 and the new Ta-POSS
compounds 7, 9, and 11 were studied as catalysts for epoxidation reactions. The catalytic
efficiencies of these tantalum complexes (2 mol %) in epoxidations of cyclooctene were
determined for reactions at 65 °C in toluene, with various oxidants and an olefin:oxidant ratio of
12:1 (Table 1). As mentioned in the introduction, the TaCp*X-POSS (X = Cl, Me, OTf, Barf)
complexes 2-5 are analogues of titanium-based TiCp*modifiedCl-POSS that have been reported to
be efficient epoxidation catalysts with H2O2 as oxidant.26,27 However, attempts to observe catalytic
activity for 2-5 were unsuccessful and in all cases no conversion of the cyclooctene was observed
after 24 h at 65 °C in toluene.
Complex 9, with the bidentate bis(aryloxide) ligand, is also inactive for the epoxidation of
cyclooctene with tert-butyl hydroperoxide (TBHP), cumene hydroperoxyde (CHP) and hydrogen
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peroxide (H2O2) (Table 1). For Ta[OSi(OtBu)3]2-POSS•HNMe2 (7), a yield of ca. 30 % was
observed with all oxidants. Complex 11, with the germoxy ligands, presented the most promising
results; a yield of 94 % was obtained after only 1 h with hydrogen peroxide. A lower conversion
(22 %) was observed with CHP as oxidant (Table 1). Interestingly, supported Ta heterogeneous
epoxidation catalysts have also shown better results with hydrogen peroxide in comparison to CHP
or TBHP.43,44
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Table 1. Cyclooctene epoxidation catalyzed by 2-5, 7, 9 and 11.a
Catalyst
Oxidant
CHP
Time, t(h) Yieldb(%)
TaCp*X-POSS
24
24
0
X = Cl (2), Me (3), OTf (4), B[C6F5]- (5) TBHP
0
H2O2, 30 % 24
0
POSSTa[OSi(OtBu)3]2•HNMe2 (7)
CHP
2
2
24
30
30
0
TBHP
H2O2, 30 % 2
Ta(L)-POSS•HNMe2 (9)
CHP
24
24
TBHP
0
H2O2, 30 % 24
0
CHP
22
24
nd
94
11
TBHP
H2O2, 30 % 1
aAll reactions were performed in toluene at 65 °C with [catalyst] = 0.78 mM and a
catalyst:substrate:oxidant of 1:600:50. bYield determined by gas chromatography. CHP: cumene
hydroperoxyde, TBHP: tert-butyl hydroperoxide
To further investigate the catalytic behavior of 7 and 11, additional conditions and substrates
were utilized, and the results are summarized in Table 2. For 7, an excess of olefin substrate is
required for conversion of the alkene into the corresponding epoxide. When a 1:1 or 1:3
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substrate:oxidant ratio was used, no conversion of the cyclooctene was observed after 24 h at 65
°C. Moreover, even with an excess of alkene, when no further conversion of alkene was observed
by GC, an additional aliquot of 50 equiv of oxidant led to no further reaction. This behavior
strongly suggests that decomposition of the catalyst occurs during reaction.
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To examine the behavior of 7 in the presence of a peroxide reagent, this complex was allowed
to react in the presence of 3 equiv of H2O2•urea in benzene-d6 at 65 °C (Scheme 6). Note that this
oxidant allows monitoring of the reaction by NMR spectroscopy, without the complications
associated with a large excess of water (as in 35% H2O2). This experiment revealed the
transformation of 7 into a new complex (12), identified by crystallization of the product from the
reaction mixture. The crystal structure of 12 shows that this complex contains two POSS ligands,
bridged by an octahedral TaO6 center (Figure 4). Complex 12 was independently synthesized via
reaction of 2 equiv of 1 with 1 equiv of Ta(NMe2)5 (Scheme 6), and shown to have the same 1H
NMR spectrum that the one obtained during the degradation reaction, confirming that 12 is the
product of decomposition and not simply formed during the crystallization process. Attempts to
observe epoxidation of cyclooctene with 12 as catalyst showed that there was no conversion of the
substrate. For this system and in the conditions tested, a labile site on the Ta center seems to be
necessary for binding and activation of the oxidant.
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Scheme 6. Access to 12 by a degradation process of 7 or by direct synthesis
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Figure 4. Molecular structure of 12 displaying thermal ellipsoids at the 50 % probability level. H-
atoms, i-butyl groups and [HNMe2]+ have been omitted for clarity.
Of course, the decomposition of 7 to 12 is not possible for a silica-supported Ta catalyst.
However, during the decomposition 7, half of the Ta is likely transformed into tantalum oxide after
decomplexation from the POSS core. A deactivation of the supported catalyst by reaction with
water may hinder formation of the intermediate tantalum-hydroperoxide complex, as occurs with
related Ti-based catalysts with aqueous hydrogen peroxide as oxidant.32,33
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For 11, with cyclooctene as substrate and a catalyst:alkene:H2O2 ratio of 1:600:50, the observed
conversions were the same at 65 °C or at room temperature (94 %) but the reaction at 65 °C was
much faster (1 h vs. 7 h at room temperature). When a catalyst:alkene:H2O2 ratio of 1:100:100 was
used, the reaction was more efficient at room temperature (67 % conversion) than at 65 °C (31 %
conversion). With an excess of oxidant (catalyst:alkene:H2O2 ratio of 1:100:200), low conversion
of cyclooctene was observed (39 % at room temperature), suggesting a slow decomposition of the
catalyst. This catalyst was efficient even at 0.5 % loading, which gave a 70 % yield for the
epoxidation of cyclooctene. It is also interesting that at room temperature the catalyst is efficient
with a cyclooctene:H2O2 ratio of 1:1 (94 % conversion with a catalyst:alkene:H2O2 ratio of
1:50:50). Notably, this is in contrast to Ti-POSS complexes, for which an excess of the alkene was
necessary when alkyl peroxides were used as oxidants.25 Another interesting feature of 11 is that
it is still efficient after the first run. Indeed, addition of H2O2 after the conversion of the substrate
ceased led to a subsequent formation of epoxide. This operation was repeated twice. A loss in
conversion was observed after each run indicating a potential slow degradation of the catalyst.
However, interestingly, with a catalyst:substrate:H2O2 ratio of 1:600:50, no further loss was
observed during the second and third run. All these results might indicate that 11 is not the actual
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catalyst but a pre-catalyst for the epoxidation reaction. Monitoring the reaction by H NMR
spectroscopy, by addition of 10 equiv of cyclooctene then 5 equiv of H2O2 (30 %) to a solution of
11 in benzene-d6, and then heating to 65 °C, revealed formation of 5 equiv of cyclooctene oxide
(100% yield based on H2O2) and the release of 2 equiv of HOGeiPr3. Attempts to isolate the
resulting compound, which might be the active catalyst, were unsuccessful.
The reactivity of 11 as a catalyst is strongly influenced by the solvent. All experiments involved
biphasic conditions with the catalyst and the alkene in toluene and the oxidant in water. With THF
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as the solvent, in which both aqueous hydrogen peroxide and 11 are soluble, no conversion of
cyclooctene was observed. This suggests that 11 is decomposed in the presence of an excess of
oxidant, and that this decomposition is diminished under biphasic conditions in which the catalyst
is exposed to a low concentration of oxidant. However, as observed with the lower yield after the
first run, a slow decomposition process seems to occur. For cyclohexene and 1-octene as substrate,
lower conversions were observed even with an excess of the substrate. However, only the epoxide
was detected and no enone, enol or diol products were detected by GC.
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Table 2. Olefin epoxidation catalyzed by 7 and 11.a
Catalyst substrate
T (°C) Cat/substrate/H2O2 Time Yieldb (%)
(2nd run, 3rd run)
30
TONc
cyclooctene 65 °C 1/600/50
cyclohexene 65 °C 1/600/50
1-octene 65 °C 1/600/50
2 h
15
11
2
7
2 h
22
24 h
24 h
24 h
1h
4
cyclooctene 65 °C 1/100/100
cyclooctene 65 °C 1/100/300
Cyclooctene 65°C 1/100/100
Cyclooctene 65 °C 1/600/50
Cyclooctene 65 °C 1/600/100
Cyclooctene 65 °C 1/600/200
0
0
0
0
31 (21, nd)
94 (79, 76)
31
47
80
158
43
67
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80
139
11
1h
80min 80 (63, 50)
23h
24 h
7h
79
94
67
94
80
70
Cyclooctene RT
Cyclooctene RT
Cyclooctene RT
Cyclooctene RT
Cyclooctene RT
1/50/50
1/100/100
1/600/50
1/600/100
1/600/200
7h
11h
8h
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Cyclooctene RT
Cyclohexene RT
Cyclohexene RT
1/100/200
24h
23h
7h
39
13
60
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1/50/50
1/600/50
1/50/50
1/600/50
30
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1-octene
1-octene
RT
RT
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b
chromatography. cTON, Turnover number expressed as mol epoxide / mol catalyst
The fact that 11 is an efficient catalyst for epoxidations while 7 rapidly decomposes under the
same conditions provides information that may be utilized for the design of more efficient Ta-
based catalysts. First of all, Ta compound 12 is more stable toward oxidizing conditions than the
monometallic compound 7. Also, the Ta2O2 core of 11 with the µ–oxo/hydroxo bridges is
reminiscent of a structural feature for the active site of hydroxylase methane monooxygenase
(MMO) which possesses 2 Fe(III) centers with µ–hydroxo bridges in the resting state of the
catalytic cycle and a di(μ-oxo)diiron(IV) Fe2O2 “diamond core” structure responsible for oxidation
of methane to methanol.73-77 However, in the case of 11 the bridging oxygen atom is apparently
not transferred to the substrate, as indicated by the absence of reaction between 11 and cyclooctene
in benzene-d6 at 65 °C for 4h in the absence of oxidant. Moreover, 2 equiv of HOGeiPr3 are
released when H2O2 is added, suggesting that oxidizing species are formed on the Ta center(s) by
exchange of two germanoxy ligands by (hydro)peroxo ligand(s). As in MMO enzymes, the
presence of the oxo bridge probably stabilizes the oxidizing species and prevents rapid
decomposition of the catalyst. As mentioned in the introduction, the presence of germanium has
been described in the literature to enhance the catalytic activity of catalysts. In the absence of a
catalyst possessing the same Ta2O2 core and no germanoxy ligands, the influence of the
germanium is difficult to evaluate for 11. However, as previously mentioned, only 2 equiv of
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