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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 228x101mm (300 x 300 DPI)

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Scheme 2. Ta-SAB15 catalyst and a possible molecular model 220x78mm (300 x 300 DPI)


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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. 230x79mm (300 x 300 DPI)


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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. 227x89mm (300 x 300 DPI)


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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. 185x69mm (300 x 300 DPI)


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Scheme 6. Access to 12 by a degradation process of 7 or by direct synthesis 252x119mm (300 x 300 DPI)


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Figure 1. Molecular structure of 7 displaying thermal ellipsoids at the 50 % probability level. H-atoms and ibutyl groups have been omitted for clarity. 119x132mm (150 x 150 DPI)


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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. 231x145mm (300 x 300 DPI)


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Figure 3. Molecular structures of 10 an 11 displaying thermal ellipsoids at the 50 % probability level. Hatoms; i-butyl groups on siloxide ligands and i-propyl groups on the germoxy ligands have been omitted for clarity. 278x170mm (300 x 300 DPI)


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Figure 4. Molecular structure of 12 displaying thermal ellipsoids at the 50 % probability level. H-atoms, ibutyl groups and [HNMe2]+ have been omitted for clarity. 174x113mm (150 x 150 DPI)


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graphical abstract 103x61mm (300 x 300 DPI)


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Tantalum–polyhedral oligosilsesquioxane (POSS) complexes as structural models and functional catalysts for epoxidation Pascal Guillo,1,2,† Michael I. Lipschutz,1 Meg E. Fasulo,1 and T. Don Tilley1,2,* 1

2

Department of Chemistry, University of California, Berkeley, Berkeley, CA 94720

Chemical 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.


1

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KEYWORDS. Silsesquioxane, tantalum, catalysis, epoxidation, hydrogen peroxide. Introduction 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 silsesquioxanebased 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 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 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-


3

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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. 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. 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


5

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Ta(NMe2)3[OSi(OtBu)3]2 with trisilanol 1 was expected to occur with efficient elimination of 3 equiv of Me2NH and formation of the fully condensed POSS with Ta in the POSS framework. Scheme 2. Ta-SAB15 catalyst and a possible molecular model

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 Xray 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.

Figure 1. Molecular structure of 7 displaying thermal ellipsoids at the 50 % probability level. Hatoms 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


7

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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. 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


8

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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 benzene-d6, a mixture of Ta(NMe2)(OGeiPr3)4 and 10 was observed by 1H 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


9

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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.

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-


10

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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 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 cyclohexene with tBuOOH 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 μhydroxy 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.


11

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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 Table 1. Cyclooctene epoxidation catalyzed by 2-5, 7, 9 and 11.a Catalyst

Oxidant

Time, t(h) Yieldb(%)

TaCp*X-POSS

CHP

24

0

24

0

H2O2, 30 % 24

0

CHP

2

24

TBHP

2

30

H2O2, 30 % 2

30

CHP

24

0

TBHP

24

0

H2O2, 30 % 24

0

CHP

24

X = Cl (2), Me (3), OTf (4), B[C6F5]- (5) TBHP

POSSTa[OSi(OtBu)3]2•HNMe2 (7)

Ta(L)-POSS•HNMe2 (9)

11

22

TBHP

nd

H2O2, 30 % 1

94

a

All 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. 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

Figure 4. Molecular structure of 12 displaying thermal ellipsoids at the 50 % probability level. Hatoms, 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 catalyst but a pre-catalyst for the epoxidation reaction. Monitoring the reaction by 1H 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. Table 2. Olefin epoxidation catalyzed by 7 and 11.a Catalyst substrate

T (°C) Cat/substrate/H2O2 Time

Yieldb (%)

TONc

(2nd run, 3rd run) 7

11

cyclooctene

65 °C

1/600/50

2h

30

15

cyclohexene 65 °C

1/600/50

2h

22

11

1-octene

65 °C

1/600/50

24 h

4

2

cyclooctene

65 °C

1/100/100

24 h

0

0

cyclooctene

65 °C

1/100/300

24 h

0

0

Cyclooctene 65°C

1/100/100

1h

31 (21, nd)

31

Cyclooctene 65 °C

1/600/50

1h

94 (79, 76)

47

Cyclooctene 65 °C

1/600/100

80min 80 (63, 50)

80

Cyclooctene 65 °C

1/600/200

23h

79

158

Cyclooctene RT

1/50/50

24 h

94

43

Cyclooctene RT

1/100/100

7h

67

67

Cyclooctene RT

1/600/50

7h

94

46

Cyclooctene RT

1/600/100

11h

80

80

Cyclooctene RT

1/600/200

8h

70

139


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Cyclooctene RT

1/100/200

24h

39

39

Cyclohexene RT

1/50/50

23h

13

6

Cyclohexene RT

1/600/50

7h

60

30

1-octene

RT

1/50/50

23h

8

4

1-octene

RT

1/600/50

16h

52

26

a

All reactions were performed in toluene with [catalyst] = 0.78 mM. bYield determined by gas 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 Tabased 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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germanol are released during the catalytic cycle, suggesting that 2 germanoxy ligands are still coordinated to the Ta centers during the catalysis. In conclusion, four new Ta-POSS complexes have been synthesized, characterized and examined as catalysts for epoxidation reactions. In the context of development of new homogeneous and supported oxidation catalysts based on Ta, 7 and 11 are most interesting. Indeed, 7 with one Ta center and two -OSi(OtBu)3 ligands, decomposes after a few catalytic cycles into inactive 12 possessing two POSS ligands. The decomposition of 7 into 12 suggests that the Ta center can be extracted from the POSS ligand and a similar process might be possible on the silica surface, leading to deactivation of a supported catalyst. To the best of our knowledge, 11 is the first Ta-POSS complex that possesses catalytic epoxidation activity, and it is an efficient catalyst for the epoxidation of cyclooctene using aqueous hydrogen peroxide. Moreover, only a small loss of activity is observed even after three runs of catalysis. The presence of µ–oxo and µ–hydroxo bridges, and perhaps the germanoxy ligands, are possible keys for stabilization of Ta during the catalysis.

Experimental section General considerations. All manipulations of air sensitive compounds were conducted under a nitrogen atmosphere using standard Schlenk techniques or using a nitrogen atmosphere glovebox. Solvents were stored in PTFE-valved flasks after drying using solvent purification systems or by distillation under nitrogen from appropriate drying agents. Benzene-d6 was purchased from Cambridge Isotope Laboratories, dried over Na/K alloy, and then degassed by several freezepump-thaw cycles. NMR spectra were recorded on Bruker spectrometers at room temperature unless otherwise noted. Spectra were referenced internally by solvent peaks for 1H and 13C{1H}


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NMR, and tetramethylsilane for 29Si-1H HMBC experiments. X-ray analyses were carried out at the UC Berkeley CHEXRAY crystallographic facility. Measurements of 7, 8, 9, 10, 11 and 12 were made on an APEX CCD area detector with Mo Kα radiation (λ = 0.71069 Å) monochromated with QUAZAR multilayer mirrors. Elemental analyses were performed by the College of Chemistry Microanalytical Laboratory at the University of California, Berkeley. Infrared spectra were collected using a Thermo Nicolet 6700 FTIR spectrometer. GC analyses were performed on an HP 6890N system using a phenylmethyl polysiloxane DB-5 capillary column (30.0 m × 320 m × 1.00 m), and integration was performed relative to a dodecane internal standard. No diols were detected in the reaction mixtures by GC analyses. HOSi(OtBu)3,78 KOSi(OtBu)3,78 Ta(NMe2)3Cl2,79 HOGeiPr3,60 and 2-555 were prepared according to literature methods. Incompletely condensed silsesquioxane 1 was purchased from Hybrid Plastics Inc. and dried overnight under vacuum at 50 °C prior to use. Ta(NMe2)5 was purchased from Strem Chemicals, Inc. and freshly sublimed prior to use. Cyclohexene and 1-octene were purchased from Aldrich and dried over CaH2 prior to use. Tert-butyl hydroperoxide (TBHP, 5.5M in decane), cumene hydroperoxide (CHP, 80 %, technical grade), hydrogen peroxide (H2O2, 30 wt. % in H2O), urea hydrogen peroxide, 2-tert-butyl-4-methylphenol and cyclooctene were purchased from Aldrich and used as received. All other chemicals were purchased from commercial sources and used without further purification. Ta(NMe2)3[OSi(OtBu)3]2 (6). To 518 mg (1.35 mmol) of a suspension of Ta(NMe2)3Cl2 in 5 mL of diethyl ether, KOSi(OtBu)3 (816 mg, 2.7 mmol) in 5 mL of diethyl ether was added. The mixture was stirred at room temperature for 6 h. The solution turned yellow and a white precipitate of KCl appeared. The suspension was filtered on a pad of celite and the pale yellow filtrate was concentrated to about 1 mL and stored at -30 °C for crystallization. After 24 h, crystals of 6 were


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isolated by filtration and dried under vacuum to give 980 mg (86 %) of the product as a white solid. This compound is not stable at room temperature and had to be stored at -30 °C under nitrogen atmosphere.1H NMR (C6D6, 600 MHz, 25 °C): δ = 3.56 (s, 18H), 1.46 (s, 54H). 13C{1H} NMR (C6D6, 600 MHz, 25 °C) δ = 72.7, 47.5, 32.2. 29Si{1H} NMR (C6D6, 600 MHz, 25 °C) δ = 98.10. Anal. Calcd for C30H72N3O8Si2Ta: C, 42.89; H, 8.64; N, 5.00. Found: C, 42.58; H, 8.73; N, 4.73. Ta[OSi(OtBu)3]2-POSS•HNMe2 (7). To 500 mg (0.63 mmol) of the trisilanol 1 in 5 mL of diethyl ether was added a solution of 6 (530 mg, 0.63 mmol) in 5 mL of diethyl ether. The resulting solution was stirred at room temperature for 15 h. After filtration of a small precipitate on a short pad of celite, the solution was evaporated under reduced pressure. The residue was dissolved in 2 mL of diethyl ether and acetonitrile was then added dropwise to the pale yellow solution to precipitate 7 and this mixture was kept at -30 °C for 12 h to achieve complete precipitation. Ta[OSi(OtBu)3]2-POSS•HNMe2 (7) was isolated by filtration and dried under vacuum to give 820 mg (84 %) of the product as a white powder. Crystals suitable for X-ray diffraction were obtained by cooling a saturated solution of 7 in pentane at -30 °C. 1H NMR (C6D6, 600 MHz, 25 °C) δ = 4.37 (quin, 1H, 5.4 Hz), 2.75 (d, 6H, 5.4 Hz), 2.16 (m, 7H), 1.53 (s, 54H), 1.10-1.27 (m, 42H), 0.90-0.95 (m, 6H), 0.84 (dd, 6H, 16.6 Hz, 6.6 Hz), 0.71 (dd, 2H, 15 Hz, 7.8 Hz). 13C{1H} NMR (C6D6, 600 MHz, 25 °C) δ = 73.35, 41.69, 32.60, 25.44, 25.20, 24.96, 24.93, 24.61, 24.32, 23.97, 23.90, 23.56. 29Si{1H} NMR (C6D6, 600 MHz, 25 °C) δ = -98.4, -68.9, -67.8, -65.9, -64.8. Anal. Calcd for C54H124NO20Si9Ta: C, 42.08; H, 8.11; N, 0.91. Found: C,41.83; H,8.07; N,1.15. LH2 (adapted from 67). 2-tert-butyl-4-methylphenol (3.73 g, 22.7 mmol) was dissolved in 23 mL of glacial acetic acid. Concentrated sulfuric acid (4.5 mL) was added to a solution of K2Cr2O7 (2.27 g, 7.72 mmol) in 24 mL of water. This solution was added dropwise to the solution of the


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phenol over 30 min at 50 °C. The orange solution turned green and an orange oil appeared. The solution was stirred at 50 °C for 3 h. After cooling at room temperature, 100 mL of water were added and the aqueous phase was extracted with 200 mL of dichloromethane. The operation was repeated two more times (until the organic phase is colorless). After evaporation of the organic phase, the orange oil remaining was purified by column chromatography (eluant : hexane/ diethyl ether = 99 : 1) and after concentration gave 1.2 g (33 %) of the product as a white foam. 1H NMR (C6D6, 600 MHz, 25 °C) δ = 7.18 (s, 2H), 6.73 (s, 2H), 5.12 (s, 2H, OH), 2.11 (s, 6H , Me), 1.52 (s, 18H, tBu). 13C{1H} NMR (C6D6, 600 MHz, 25 °C) δ = 150.75, 130.19, 129.66, 129.10, 123.62, 35.50, 30.24, 21.17. Anal. Calcd for C22H30O2: C, 80.94; H, 9.26. Found: C, 80.65; H, 9.11. Ta(L)(NMe2)3 (8). To a solution of LH2 (200 mg, 0.61 mmol) in 5 mL of diethyl ether, a solution of Ta(NMe2)5 (246 mg, 0.61 mmol) in 5 mL of diethyl ether was added via canula transfer. After stirring at room temperature for 2 h, the resulting yellow/orange solution was concentrated under vacuum. The light orange solid was washed with 5 mL of cold diethyl ether and give 284 mg (73 %) of the product as a pale yellow solid. Crystals of 8 suitable for X-ray analysis can be obtained by slow diffusion of pentane into a solution of 8 in toluene at -30 °C. 1H NMR (C6D6, 600 MHz, 25 °C) δ = 7.25 (s, 2H), 7.11 (s, 2H), 3.18 (s, 18H, NMe2), 2.30 (s, 6H , Me), 1.54 (s, 18H, tBu). C{1H} NMR (C6D6, 600 MHz, 25 °C) δ = 159.07, 132.72, 131.97, 128.85, 126.75, 45.81, 35.50,

13

30.94, 21.47. Anal. Calcd for C28H46N3O2Ta: C, 52.74; H, 7.27; N, 6.59 . Found: C, 52.98; H, 7.14; N, 6.52. Ta(L)-POSS•HNMe2 (9). Toluene (10 mL) was added to a solid mixture of 8 (100 mg, 0.16 mmol) and 1 (118 mg, 0.15 mmol). After stirring at room temperature for 15 h, the resulting pale yellow solution was evaporated under vacuum. The resulting foam was dissolved in 1 mL of diethyl ether and 5 mL of hexamethyldisiloxane was added. After 24 h at -30 °C, a white precipitate


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was isolated by filtration and dried under vacuum to give 129 mg (60 %) of the product as a white foam. Crystals suitable for X-ray diffraction were obtained by cooling a saturated solution of 9 in pentane at -30 °C. 1H NMR (C6D6, 600 MHz, 25 °C) δ = 7.28 (s, 2H), 6.99 (s, 2H), 2.27 (s, 6H), 2.26 (s, 6H), 2.22-2.10 (m, 7H), 1.70 (s, 18H), 1.62 (sept, 1H, 6.6 Hz), 1.21 (d, 6H, 6.6 Hz), 1.18 (d, 6H, 6.6 Hz), 1.13 (d, 12H, 6.6 Hz), 1.11 (m, 12H), 1.06-1.02 (m, 2H), 0.93-0.84 (m, 16H), 0.55 (d, 2H, 7.2 Hz).

13

C{1H} NMR (C6D6, 600 MHz, 25 °C) δ = 156.30, 138.56, 133.43, 131.85,

129.87, 127.48, 42.01, 35.59, 26.91, 26.62, 26.43, 26.42, 26.28, 26.20, 26.13, 25.22, 25.04, 24.98, 24.90, 24.87, 24.37, 23.72, 23.59, 23.48, 23.42, 21.26. 29Si{1H} NMR (C6D6, 600 MHz, 25 °C) δ = -62.10, -65.38, -67.57, -68.73. Anal. Calcd for C52H98NO14Si7Ta: C, 46.65; H, 7.38; N, 1.05 . Found: C,46.28; H,7.44; N,0.91. [(iPr3GeO)3Ta]2(μ-OH)2(μ-O) (10). To a solution of Ta(NMe2)5 (0.500 g, 1.25 mmol) in 20 mL of pentane, HOGeiPr3 ( 1.36 g, 6.23 mmol) in 20 mL of pentane was added via canula transfer. After stirring at room temperature for 3 h, the resulting pale yellow solution was evaporated under vacuum at room temperature then at 110 °C for 4 h. The resulting white solid was then dissolved in 2 mL of diethyl ether and acetonitrile was added dropwise to precipitate 10. After 24 h at -30 °C to achieve precipitation, a white precipitate was isolated by filtration and dried under vacuum to give 2.15 g (85 %) of the product as a white powder. Crystals suitable for X-ray diffraction were obtained by cooling a saturated solution of 10 in diethyl ether at -30 °C. 1H NMR (C6D6, 600 MHz, 25 °C) δ = 1.73 (sept, 18H, 7.8 Hz), 1.40 (d, 108H, 7.8 Hz). 13C{1H} NMR (C6D6, 600 MHz, 25 °C) δ = 20.32, 19.43. Anal. Calcd for C54H128Ge6O9Ta2: C, 37.72; H, 7.50. Found: C, 38.07; H, 7.28. [(iPr3GeO)2Ta]2(μ-OH)(μ-O)(μ-OSi)-POSS•HNMe2 (11). To 0.500 g (0.63 mmol) of the trisilanol 1 in 5 mL of diethyl ether was added a solution of 10 (1.09 g, 0.63 mmol) in 5 mL of


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diethyl ether. The resulting solution was stirred at room temperature for 15 h. After filtration of a small precipitate on a short pad of celite, the solution was evaporated under reduced pressure. The residue was dissolved in 1 mL of diethyl ether and acetonitrile was then added dropwise to the pale yellow solution to precipitate 11 and this mixture was kept at -30 °C. After 12h, a white precipitate was isolated by filtration and dried under vacuum to give 1.23 g (95 %) of the product as a white powder. Crystals suitable for X-ray diffraction were obtained by cooling a saturated solution of 11 in diethyl ether at -30 °C. 1H NMR (C6D6, 600 MHz, 25 °C) δ = 4.52 (s, 1H), 2.112.26 (m, 7H), 1.75-1.83 (m, 12H), 1.36-1.43 (m, 78H), 1.18-1.24 (m, 30H), 1.12 (d, 6H, 6.6 Hz), 0.86-0.98 (m, 14H). 13C{1H} NMR (C6D6, 600 MHz, 25 °C) δ = 26.90, 26.81, 26.76, 26.67, 26.61, 26.51, 26.30, 25.63, 25.34, 25.19, 25.02, 24.96, 24.64, 24.37, 24.13, 23.49, 20.48, 20.21, 20.15, 20.00, 19.98, 19.94, 19.63. 29Si{1H} NMR (C6D6, 600 MHz, 25 °C) δ = -69.91, -67.59, -65.81, 62.14. Anal. Calcd for C64H148Ge14O18Si7Ta2: C, 37.41; H, 7.26. Found: C,37.44; H,7.28. [Ta-(POSS)2][H2NMe2] (12). To 200 mg (0.25 mmol) of the trisilanol 1 in 5 mL of diethyl ether was added a solution of Ta(NMe2)5 (51 mg, 0.13 mmol) in 5 mL of diethyl ether. The resulting solution was stirred at room temperature for 12 h. After filtration of a small precipitate on a short pad of celite, the solution was evaporated under reduced pressure. The residue was dissolved in 1 mL of diethyl ether and acetonitrile was then added dropwise to the pale yellow solution to precipitate 12 and this mixture was kept at -30 °C. After 12 h, a white precipitate was isolated by filtration and dried under vacuum to give 150 mg (65 %) of the product as a white powder. Crystals suitable for X-ray diffraction were obtained by slow evaporation of a saturated solution of 12 in benzene at room temperature. 1H NMR (C6D6, 600 MHz, 25 °C) δ = 8.49 (s, 2H), 2.11-2.26 (m, 14H), 2.05 (s, 6H), 1.25 (d, 36H, 6.6 Hz), 1.16 (d, 36H, 6.6 Hz), 1.12 (d, 12H, 6.6 Hz), 0.87 (m, 16H), 0.81 (d, 12H, 7.2 Hz).

13

C{1H} NMR (C6D6, 600 MHz, 25 °C) δ = 35.28, 26.94, 26.54,


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26.35, 25.13, 24.93, 24.90, 24.04, 23.63.

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Si{1H} NMR (C6D6, 600 MHz, 25 °C) δ = -67.87, -

64.46, -65.00. Anal. Calcd for C58H134NO24Si14Ta: C, 38.62; H, 7.48; N, 0.78. Found: C,38.38; H, 7.38; N, 0.75. Catalytic alkene epoxidation. Under a flow of nitrogen, to a 0.78 mM solution of the complex in 5 mL of toluene in a 25 mL three-necked roundbottom flask fitted with a reflux condenser and two septums, the alkene and dodecane (68 L) as an internal standard were added. The vessel was immersed in an oil bath where it was allowed to equilibrate for 15 min at the desired temperature. The oxidant was added via syringe to the rapidly stirred solution to access the targeted catalyst:alkene:oxidant ratio. The course of the reaction was monitored by gas chromatography (GC) by taking aliquots in the organic phase, and products were assigned based on known samples analyzed under the same conditions.

ASSOCIATED CONTENT Supporting Information. Crystal data for compounds 7, 8, 9, 10, 11 and 12. CIF files can also be obtained free of charge from the Cambridge Crystallographic Data Centre under reference numbers 1525464, 1525465, 1525466, 1525467, 1525468, and 1525469. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Corresponding Author email address: [email protected]


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Present Addresses † Univ Toulouse, UPS, INPT, CNRS, LCC, 205 Route Narbonne, F-31077 Toulouse, France. Univ Toulouse, Inst Univ Technol Paul Sabatier, Dept Chim, Ave Georges Pompidou, BP 20258, F81104 Castres, France Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENTS

This work was primarily funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division under Contract No. DE-AC02-05CH11231.

REFERENCES (1) Coperet, C.; Chabanas, M.; Saint-Arroman, R. P.; Basset, J. M. Angew. Chem., Int. Ed. 2003, 42, 156-181. (2) Thomas, J. M. Proc.R.Soc.A 2012, 468, 1884-1903. (3) Thomas, J. M.; Raja, R. Top. Catal. 2006, 40, 3-17. (4) Thomas, J. M.; Raja, R.; Lewis, D. W. Angew. Chem., Int. Ed. 2005, 44, 6456-6482. (5) Dal Santo, V.; Liguori, F.; Pirovano, C.; Guidotti, M. Molecules 2010, 15, 3829-3856. (6) Coperet, C.; Comas-Vives, A.; Conley, M. P.; Estes, D. P.; Fedorov, A.; Mougel, V.; Nagae, H.; Nunez-Zarur, F.; Zhizhko, P. A. Chem Rev 2016, 116, 323-421.


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(7) Lelli, M.; Gajan, D.; Lesage, A.; Caporini, M. A.; Vitzthum, V.; Mieville, P.; Heroguel, F.; Rascon, F.; Roussey, A.; Thieuleux, C.; Boualleg, M.; Veyre, L.; Bodenhausen, G.; Coperet, C.; Emsley, L. J. Am. Chem. Soc. 2011, 133, 2104-2107. (8)

Rossini, A. J.; Zagdoun, A.; Lelli, M.; Gajan, D.; Rascon, F.; Rosay, M.; Maas, W. E.;

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