Selective Catalytic Cyclohexene Oxidation Using Titanium

Nov 6, 2007 - J.M.E. Matos , F.M. Anjos Júnior , L.S. Cavalcante , V. Santos , S.H. Leal , L.S. Santos Júnior , M.R.M.C. Santos , E. Longo. Material...
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J. Phys. Chem. C 2007, 111, 17570-17579

Selective Catalytic Cyclohexene Oxidation Using Titanium-Functionalized Silicone Nanospheres Christopher A. Bradley, Meredith J. McMurdo, and T. Don Tilley* Department of Chemistry, UniVersity of California at Berkeley, Berkeley, California 94720-1460, and the Chemical Sciences DiVision, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720 ReceiVed: June 11, 2007; In Final Form: September 9, 2007

Synthesis of methyl silicone-substituted nanospheres was accomplished by an emulsion condensation polymerization of methyltrimethoxysilane using conventional stirring. The spheres, ranging from 14 to 20 nm in diameter, are soluble in a variety of organic solvents and exhibit high surface areas for materials in this size regime. Silanol groups on the particles undergo reaction with titanium isopropoxide to give siteisolated, four-coordinate titanium centers, as determined by UV-visible spectroscopy. Grafting of Ti(OiPr)4 onto the spheres yields a material competent in the selective epoxidation of cyclohexene using both cumene and tert-butyl hydroperoxide as oxidants. The nanospheres represent a support material that can be functionalized much like related oxide surfaces to give site-isolated oxidation catalysts with activities and selectivities superior to those of comparable homogeneous systems.

Introduction Oxidations catalyzed by transition metals play an important role in both industrial and academic settings.1-5 Epoxidation reactions are particularly significant, as they are used in diverse applications to prepare high-volume chemical intermediates as well as fine chemicals.6,7 Titanium-based epoxidation catalysts represent some of the most active and selective catalysts known. Examples include the homogeneous Sharpless catalyst for enantioselective epoxidations and the heterogeneous titanosilicalite-1 (TS-1) catalysts used industrially for propylene epoxidation.8-15 Previous investigations from this laboratory have focused on a variety of Ti-SBA15 epoxidation catalysts obtained by the thermolytic molecular precursor route.16 Site-isolated materials derived from the grafting of molecular Ti(IV) siloxides yield active (conversions >70%) and selective (>95%) catalysts for the epoxidation of cyclohexene with either cumene (CHP) or tert-butyl (TBHP) hydroperoxide as the oxidant.17 Further surface and active-site modifications of such SBA15 materials have resulted in improved catalysts for epoxidation using aqueous H2O2 as the oxidant (eq 1).18 Although water coordination poisons titanium centers in these and related catalysts, active-site and surface silylation gave significant increases in both the yield and selectivity for epoxidation.19-22 Surface modification creates a more hydrophobic material that repels water from the active sites, and direct silylation of the titanium centers results in more efficient catalysis.

search for readily modified, alternative supports that might offer novel coordination environments. Recently, a procedure was developed to prepare silicone nanospheres on the 10-15 nm size regime that are soluble in a variety of common organic solvents.26 The solubility of these particles and their potential to serve as “macromolecular” ligand supports prompted an examination of these materials in the development of functionalized nanospheres. Related silicone-based systems, both spherical and polymeric in nature, have shown promise as catalyst supports for both epoxidation27 and olefin polymerization,28-32 suggesting that silicone nanospheres could provide adequate stabilization of surface-bound species. It is hoped that the nanosphere-supported catalysts might combine the useful properties of soluble, molecular catalysts with the active sites obtained by the immobilization of single-site species on an oxide support. Here we report the synthesis of silicone nanospheres containing surface silanol functionalities, the grafting of titanium(IV) centers via molecular precursors, and selective cyclohexene oxidation catalysis promoted by the resulting, soluble particles (eq 2). Observed conversions and selectivities for epoxidations rival those of comparable heterogeneous materials, and are superior to those of the corresponding molecular, homogeneous catalysts. The solubility of the nanospheres also allows application of a wide range of spectroscopic methods for the characterization of reaction intermediates. These studies demonstrate that silicone nanospheres can provide useful surface supports for homogeneous chemistry, and that these supports give rise to catalytic behavior that is distinct from that of related molecular and heterogeneous systems.

A continued interest in the development of synthetic routes to well-defined, site-isolated catalysts23-25 has motivated a * Corresponding author. E-mail: [email protected].

10.1021/jp074512c CCC: $37.00 © 2007 American Chemical Society Published on Web 11/06/2007

Selective Catalytic Cyclohexene Oxidation Experimental General Information. All air- and moisture-sensitive manipulations were carried out using standard Schlenk and cannula techniques or in a Vacuum Atmospheres inert atmosphere drybox containing an atmosphere of purified nitrogen. Solvents for air- and moisture-sensitive manipulations were initially dried and deoxygenated using literature procedures.33 Benzene-d6 (Cambridge Isotopes) for NMR spectroscopy was distilled from sodium metal under an atmosphere of nitrogen prior to use. Methyltrimethoxysilane and hexamethyldisilazane were obtained from Wacker Chemie and used as received. Methoxytrimethylsilane and titanium(IV) isopropoxide (99.999%) were purchased from Aldrich and used as received. Deionized water was stirred with charcoal overnight prior to distillation. Cyclohexene was purchased from Aldrich and refluxed over CaH2 prior to distillation. Acetonitrile was purchased from Aldrich and refluxed over P2O5 before distillation and use. CHP (technical grade), TBHP (5-6 M in decane), urea hydrogen peroxide (UHP), and hydrogen peroxide (30 wt % in water) were purchased from Aldrich and used as received. D2O was purchased from Cambridge Isotopes and used as received. Hydrogen peroxide was titrated to determine the exact concentration, as described previously.34 Solution 1H NMR spectra were recorded on a Bruker AVQ400 spectrometer operating at 400.13247 MHz. All chemical shifts are reported relative to SiMe4 using 1H (residual) NMR chemical shifts of the solvent as a secondary standard. Deuterium NMR spectra were recorded on a Bruker DRX-500 spectrometer operating at 76.773 MHz. All solid-state NMR experiments (unless noted) were performed on a Bruker Avance 500 spectrometer equipped with an 11.75 T magnet and a Bruker 4 mm cross-polarization magic-angle spinning (CP-MAS) probe with a MAS rate of 14.5 kHz. The 1H MAS NMR spectrum was acquired with a Hahn-echo pulse sequence (90-τ-180acquisition), with τ being synchronized with rotor time. The 90° pulse width was 2.2 µs, the data size was 32 k, the spectrum width was 500 kHz, the relaxation delay was 5 s, and the number of scans was 80. The 13C CP-MAS NMR spectrum was recorded with a ramped-CP power level and a two-pulse phase-modulated (TPPM) decoupling technique, with a contact time of 1.5 ms, a relaxation delay of 1.2 s, a spectral width of 100 kHz, a data size of 8 k, and 20 000 scans. The chemical shift was referenced to the carboxyl group in glycine at 176 ppm. The 29Si CP-MAS NMR spectrum was recorded with a Bruker 7 mm CP-MAS probe with a zirconia rotor spinning at 5 kHz. The data size was 4 k, the number of scans was 14 000, the contact time was 2 ms, and the relaxation time was 1 s. The spectrum was Fourier transformed with 80 Hz line broadening. A ramp-contact power and TPPM decoupling pulse sequence was used for the experiment. The 2D 1H-13C HETCOR experiment was carried out with a Bruker 7 mm CP-MAS probe and a 7 mm zirconia rotor spinning at 6 kHz. Transmission electron microscopy (TEM) was performed using a Phillips Tecnai 12 instrument with samples visualized at a magnification of 43 K. Samples were prepared by dissolution in methylene chloride (3 mL) followed by sonication and deposition on carbon-Cu grids (Electron Microscopy Services). The grids were then allowed to air-dry overnight prior to analysis. Diffuse reflectance UV-visible spectra were acquired using a Labsphere DRA-CA-30I diffuse-reflectance attachment on a Varian-Cary 300 Bio spectrophotometer with a spectral bandwidth of 2 nm and a collection speed of 600 nm/min. A MgO sample was used as a reference material prior to data collection.

J. Phys. Chem. C, Vol. 111, No. 47, 2007 17571 Solution UV-visible spectra were acquired on the same instrument using similar conditions. Diffuse reflectance infrared spectroscopy (DRIFTS) IR spectra were collected on a Nicolet Nexus 6700 FTIR spectrometer with a liquid nitrogen-cooled MCT-B detector using a Pike Technologies EasiDiff diffusereflectance attachment and a 10 mm-diameter sample cell. A total of 512 scans were performed on each sample with a resolution of 4.0 cm-1. KBr was used as the background. Thermal analyses (thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC)) were performed on a TA Instruments SDT 2960 Integrated TGA/DSC analyzer with a heating rate of 10 °C/min under a flow of oxygen. Nitrogen and oxygen adsorption isotherms were collected at 77 K on a Micromeritics ASAP2020 instrument. Samples were outgassed at 150 °C for 12 h prior to measurement. For all isotherms, warm and cold free space correction measurements were performed with ultrahigh-purity He gas (99.999%). Dynamic light scattering (DLS) samples were prepared at 10 mg/mL concentrations and filtered to remove particulates prior to data collection. Measurements were performed on a Malvern Instruments spectrometer. The refractive index of SiO2 was estimated as the value for the nanospheres. A total of 15 scans were performed with each run. Six experiments were averaged to determine particle size and polydispersity. Gas chromatography (GC) analyses were performed with an HP 6890N system using an HP-5 methyl siloxane capillary (30.0 m × 320 µm × 0.25 µm nominal). Catalysis batch reactions were performed in sand baths temperature-controlled using an Omega CSC32 benchtop controller. Elemental analyses were performed by the College of Chemistry microanalytical laboratory at the University of California, Berkeley, or by Galbraith Laboratories (Knoxville, TN). Methyl Nanosphere Synthesis. The spheres were prepared by a modified literature procedure.26 A 250 mL Schlenk flask was charged with benzthonium chloride (0.37 g, 0.83 mmol) and NaOH (0.005 g, 0.13 mmol). Deionized water (100 mL) was added, and the solution was stirred vigorously for 5 min. Trimethoxymethylsilane (5.00 g, 0.037 mol) was then added dropwise over the course of 5 min. The emulsion was stirred vigorously for 5 h before methoxytrimethylsilane (1.6 mL, 0.012 mol) was added to the reaction. After stirring for an additional 12 h, the emulsion was added to methanol (200 mL). Filtration of the corresponding precipitate followed by copious rinsing with methanol gave a colorless powder. (Note: extensive drying of the solid causes sphere aggregation and insolubility.) The material was then transferred to a 100 mL round-bottomed flask and dissolved in toluene (60 mL). Hexamethyldisilazane (2.1 mL, 0.010 mol) was then added, and the solution was stirred for an additional 16 h. The solution was then poured onto methanol (200 mL). Collection of the precipitate on a Bu¨chner funnel followed by drying on a vacuum line at 120 °C for 12 h gives the nanospheres as a white powder (2 g), which may be redispersed in organic solvents. Elemental analysis found (%): C 20.70, H, 5.39. 1H NMR (400 MHz, benzene-d6): δ ) 0.32 (br, SiMe), 3.70 (br, SiOMe). 1H MAS NMR: δ ) 0.07 (SiMe), 3.54 (br, OMe). 13C CP-MAS NMR: δ ) -3.4, 1.4 (SiCH3), 49.5 (OCH3). 29Si CP-MAS NMR: δ ) -66.2, -57.5, -48.8 (O3SiMe), 8.1 (OSiMe3). IR (DRIFTS, cm-1): 803 (br, SiMe3), 863 (br, SiMe3), 3445 (v br, SiOH), 3778 (br, SiOH). Dibenzyl Magnesium Titration of the Methyl Nanospheres.35 A sample of the methyl nanospheres (10 mg) was placed in a J. Young NMR tube. Excess dibenzyl magnesium (typically 3 mg) was added to the tube, and a 0.05 M benzened6 ferrocene solution (0.4 mL) was added by syringe. The tube

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SCHEME 1: Reaction Sequence for Nanosphere Preparationa

a

(i) Emulsion polymerization. (ii) Partial silanol capping. (iii) Further endcapping.

was shaken, and the reaction was monitored by 1H NMR spectroscopy. Integration of the methyl peak of toluene versus the ferrocene standard gave the number of Si-OH functional groups. The experiment was performed on four samples, using 8-12 mg of nanospheres per sample, and the value for Si-OH sites was averaged. Procedure for Monitoring of Grafting by 1H NMR Spectroscopy. In a typical run, the methyl nanospheres (8-12 mg) were placed in a J. Young NMR tube. A 0.01 M benzened6 ferrocene stock solution (0.4 mL) was then added by syringe, the desired amount of Ti(OiPr)4 was also added (between 0.3 and 1 µL) by microliter syringe, and the reaction was monitored by 1H NMR spectroscopy. Tubes were then degassed, and volatile materials were vacuum transferred into a J. Young NMR tube containing a 0.01 M benzene-d6 (0.1 mL) ferrocene solution. Integration of free isopropanol before and after vacuum transfer gave the average number of Ti-O surface bonds. Three trials were performed for each measurement and averaged. Isolated Catalyst Preparation (TiMN(1)). To a thick-walled glass vessel, methyl nanospheres (0.500 g) and Ti(OiPr)4 (31 µL, 0.10 mmol) were added. The mixture was dissolved in toluene (∼10 mL), and the vessel was sealed and then transferred to a 65 °C sand bath. After heating for 6 h, solvent was removed in vacuo to give a colorless powder. Heating of the solid for 12 h at 120 °C under dynamic vacuum yielded the isolated material (0.400-0.450 g isolated per batch) used for subsequent spectroscopic and catalytic studies. Elemental analysis was used to determine the exact titanium loading. Elemental analysis found (%): Ti, 0.94. Typical Catalytic Run Using TiMN(1). A thick-walled glass vessel was charged with TiMN(1) (0.035 g). On a Schlenk line, toluene (5 mL) and dodecane (50 µL) were added. Cyclohexene (2.5 mL) was added by syringe, and the mixture was allowed to equilibrate at 65 °C for 10 min. Oxidant (CHP or TBHP) was then added by syringe (1.0 mL). Aliquots (ca 0.1 mL) were removed from the reaction by syringe after 1, 2, 4, 6, 12, and 24 h. Following filtration through silica gel and cooling, the filtrate was analyzed by GC by comparison with authentic samples. Catalysis runs using aqueous hydrogen peroxide as the oxidant were performed in a similar manner, using 30% H2O2 (aq) (0.62 mL) and toluene (25 µL) as an internal standard in acetonitrile (5.0 mL). Aliquots were removed from the reaction mixture after 0.25, 1, 2, and 3 h. Typical Catalytic Run with Ti(OiPr)4 or In Situ Grafted Titanium Nanospheres (TiMN(2)). These conditions were identical to the ones described for isolated catalyst runs, except for the method of catalyst addition. With no nanosphere, Ti(OiPr)4, enough to represent 1% Ti weight loading on the nanospheres, was added by syringe to the reaction vessel prior to addition of the other reagents. For in situ catalyst preparation, methyl nanospheres (ca. 0.035 g) and Ti(OiPr)4 (2.2 µL, 0.007

mmol) were added to the reaction vessel and dissolved in solvent. The solution was then heated to 65 °C for 1 h prior to olefin or oxidant addition. Results and Discussion Preparation and Characterization of Methyl Silicone Nanospheres. The methyl nanospheres were prepared by a modified literature procedure (Scheme 1).26 Synthesis of the particles relies on a base-catalyzed emulsion polymerization of methyltrimethoxysilane in water using benzthonium chloride as the surfactant. Prior to isolation, partial capping of the surface silanol functionalities using methoxytrimethylsilane is necessary to avoid sphere agglomeration and irreversible precipitation. Methanol quenching of the condensation followed by filtration gave a colorless powder. Overdrying of the particles at this point results in an insoluble material no longer dispersible in organic solvents. Instead, dissolution in toluene followed by the addition of hexamethyldisilazane affords further silanol capping. A second methanol precipitation allows the methyl nanospheres to be isolated as a fluffy, colorless powder in gram quantities. It should be noted that the condensation conditions were scaled at a specific monomer/surfactant ratio; altering of the ratio led to gelation and insoluble products upon workup. Also, surfactant appears to be washed away during filtration steps, as combustion analysis of the nanospheres indicate 100 nm) were not detected, indicating minimal aggregation in solution. The small discrepancy between TEM and DLS diameters has been noted previously in related systems and follows the expected trend (i.e., DLS diameters are larger,

possibly due to particle shrinkage under the electron beam or the solution measurement indicates a hydrodynamic diameter).26 DRIFTS spectra of the nanospheres (Supporting Information) reveal characteristic stretches attributed to Si-O and CH3 moieties. The Si-O peaks, in the 1200-1300 cm-1 region, are slightly blue-shifted relative to those observed in SBA15.40 Peaks with decreased intensity in the 800-900 cm-1 region are assigned as OSiMe3 vibrations. Methyl C-H stretches are observed in the typical 2800-3000 cm-1 region. Also, a broad Si-OH stretch is observed at 3445 cm-1 along with an accompanying small but noticeable peak at 3778 cm-1; this stretch is attributed to unbridged surface silanol groups and is in accord with that observed for the isolated silanols of SBA15.40 TGA/DSC data collected on the nanospheres (Figure 3) indicate a hydrophobic material, as no significant endothermic water loss is observed. A loss of 17.57 wt % with a corresponding exotherm is observed above 300 °C, which is likely due to a loss of surface Me and SiMe3 groups. Allowing the nanospheres to remain in a closed container under an atmosphere saturated with water vapor for two weeks produced no change in the TGA/DSC trace, suggesting a highly hydrophobic surface. This contrasts with typical mesoporous silica materials, which can absorb up to 30 wt % water under ambient conditions.18

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Figure 3. TGA (b) and DSC (0) traces of the methyl silicone nanospheres. Markers denote every 10th point of data collection.

Figure 4. Nitrogen (solid line) and oxygen (dashed line) adsorption isotherms for the methyl silicone nanospheres. Pore size distributions are shown in the inset.

However, as a number of surface silanols are capped by trimethylsilyl groups, the nanospheres more closely resemble silylated MCM 41 and SBA15 surfaces, which exhibit similar hydrophobic character.18 Porosimetry experiments using either nitrogen or oxygen as the adsorbate gas were used to determine surface area and pore structure for the silicone nanospheres. The adsorption isotherms for the nanospheres under both gases, along with a plot of pore size distribution, are shown in Figure 4. Brunauer-EmmettTeller (BET) surface areas of 345 and 318 m2/g were obtained for the material under O2 and N2, respectively.41,42 The nanospheres are microporous; calculation of pore size distribution using the Horvath-Kawazoe method revealed an average pore size of 7 Å.43 Compared to the expected surface area of 177 m2/g for a hard sphere of 17 nm diameter (Supporting Information), the experimental value is in good agreement with the calculated surface area. The larger observed surface area is likely due to the access of adsorbate to the interior regions of the microporous nanospheres. Titration of the silanol groups on the nanospheres was accomplished by the addition of excess dibenzyl magnesium and determination of the toluene produced, as described

previously.35,44 A value of 1.9(3) × 10-4 OH sites/mg was determined after averaging four independent trials. With the surface area and density of OH sites, a value of 0.4 OH sites/ nm2 can be calculated for the material. As the silanol groups on the nanosphere have been partially capped, taking into account the percentage of sites capped on related silica materials with a similar capping reagent (Me3SiNMe2), back-calculation gives 1.3 OH sites/nm2 prior to surface modification of the nanospheres, which is well in accord with values obtained for SBA15-type materials.18 It should be noted that the titration reagent likely cannot gain access to very small, most likely chemically insignificant, micropores of the material. In summary, characterization of the silicone nanospheres by a variety of techniques reveals soluble, high surface area particles with silanol densities comparable to silylated mesoporous silica. Furthermore, the spheres appear hydrophobic, which is likely due to silylation, indicating that these materials could provide an adequate support for inorganic or organometallic species. Grafting Experiments of the Nanospheres with Ti(OR)4 (R ) iPr or Si(OtBu)3). Studies of the grafting chemistry of transition metal precursors onto the sphere surface have focused

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SCHEME 2: Grafting Chemistry of the Nanospheres with Ti(OR)4

on Ti(OiPr)4 and Ti[OSi(OtBu)3]4.45 The addition of excess titanium precursor to benzene-d6 solutions of the nanospheres was monitored for formation of the appropriate alcohol or silanol as an indication of successful grafting (Scheme 2). No reaction occurred using Ti[OSi(OtBu)3]4 over 12 h at 100 °C. The difficulty in grafting the bulky molecular precursor likely stems from steric incompatibility between the complex and the high coverage of trimethylsilyl capping groups on the sphere surface. However, addition of Ti(OiPr)4, enough to give 1 wt % Ti loading, resulted in the immediate formation of isopropanol as judged by 1H NMR spectroscopy at ambient temperature. A new but broad resonance is also observed in the 1.3-1.6 ppm region; this peak is assigned to the methyl groups on the surfacebound Ti-OiPr groups. On the basis of integration, approximately half of the molecular precursor added to the solution was grafted. Integration of the isopropanol produced in these reactions versus an internal standard by 1H NMR yielded 2.8(3) equivalents of isopropanol per titanium grafted, indicating the formation of multiple titanium surface bonds. On the basis of the amount of titanium added and the experimental surface area of the nanospheres, approximately 1:1 Ti/SiOH grafting should be observed. The grafting procedure results in the release of significant quantities (∼1 equiv) of Me3SiOH, observed as its condensation product hexamethyldisiloxane (by GC mass spectrometry). The loss of capping groups could account for the apparent formation of multiple Ti surface linkages and may result from reaction with adventitious water, or from further condensation of the polysiloxane network. Note that the grafting of Ti(OiPr)4 onto silica has been observed to result in aggregate structures, with Ti-O-Ti linkages formed via the elimination of propene.46 However, no propene was observed upon grafting of Ti(OiPr)4 onto the silicone nanospheres, and the UV-visible data support the formation of site isolated, mono-titanium centers. This difference in reactivity is presumably a function of the quite different surface properties of silica and the silicone nanospheres. Because of the relative ease of grafting for the isopropoxide precursor, it was used to prepare all titanium-functionalized materials. Isolated catalytic materials, designated TiMN(1) (MN denotes methyl nanosphere), were synthesized by the addition of Ti(OiPr)4, enough to give a loading of 1 wt % Ti, to a toluene solution of the nanospheres with subsequent heating to 65 °C for 6 h. Solvent removal followed by drying at 120 °C (to remove excess Ti(OiPr)4 if present) yielded the material used for subsequent catalysis and spectroscopic characterization. Elemental analysis of TiMN(1) gave titanium weight loadings of 0.94%. Successful grafting of titanium onto the nanosphere

surface was further corroborated by DRIFTS spectroscopy. When comparing the DRIFTS spectra of TiMN(1) with spectra of unfunctionalized nanospheres (Figure 5), a new stretch at 930 cm-1 is observed in the titanium-grafted material, indicating the formation of Ti-O surface linkages.17,40 Both solution and solid-state UV-visible spectroscopy were used to examine the geometry and coordination environment of titanium on the nanosphere surface. Solution data of TiMN(1) in hexanes (Figure 6) reveal a ligand-to-metal charge transfer (LMCT) band with a maximum at 213 nm. A shoulder at 262 nm is attributed to a population of surface oligomeric titanium centers. Such behavior has been noted in related heterogeneous materials.16,47-49 The presence of higher coordinate, site-isolated surface titanium centers could also give rise to such a red shift in the charge-transfer band and cannot be ruled out. Similarly, diffuse reflectance (DR)-UV-visible spectra of the material display a broad absorbance maximum at 207 nm with a shoulder at 250 nm. The significant blue shift observed in both spectra compared to titania and other oligomeric titanium materials provides support for the generation of predominantly siteisolated, four-coordinate metal atoms on the nanosphere surface. Selective Cyclohexene Oxidation with Titanium-Functionalized Nanospheres. With the titanium functionalized nanospheres in hand, catalytic competency for cyclohexene oxidation was investigated. Two organic (CHP and TBHP) and one aqueous oxidant (H2O2) were employed. In control experiments for the CHP and TBHP oxidations, reactions performed in toluene without oxidant and catalyst present yielded no

Figure 5. DRIFTS spectra of (a) methyl nanospheres and (b) TiMN(1).

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Figure 6. Solution UV-visible spectrum of TiMN(1) in hexanes.

Figure 7. Cyclohexene oxidation with organic oxidants using (a) TiMN(2), (b) TiMN(1), and (c) molecular precursor (1 wt % Ti) as catalysts. All reactions were performed at 65 °C in toluene.

oxidation products. Similarly, the addition of oxidant to substrate without catalyst produced no oxidation products, except in the case of hydrogen peroxide (vide infra). Performing the oxidations with methyl nanosphere and no catalyst present also resulted in no conversion to epoxide. Three types of experiments were conducted with each oxidant (Figure 7). One catalyst, denoted TiMN(2), was generated by in situ heating of Ti(OiPr)4 (1 wt %) and the nanospheres prior to the addition of substrate and oxidant (Figure 7a). TiMN(1) was also investigated as a catalyst (Figure 7b). A third experiment, using the same amount of Ti(OiPr)4 as used to generate 1 wt % Ti nanosphere catalysts, was also screened as a control to compare conversion and selectivity for the homogeneous precursor to those of the supported catalysts. Plots of cyclohexene oxide conversion versus time with both CHP (Figure 8) and TBHP (Figure 9) as oxidants reveal excellent selectivities (>98%). Both TiMN(1) and TiMN(2) clearly outperform the molecular species in the epoxidation of cyclohexene. When TBHP is used as the oxidant, less than 2% of the epoxide is observed after 24 h with Ti(OiPr)4 as the catalyst, indicating a 20-fold increase in the activity of titanium when grafted onto the nanospheres. The difference in conversion by the molecular catalyst as a function of oxidant likely correlates with oxidant strength, as TBHP is considered a weaker oxygen-transfer agent relative to CHP because of its stronger O-O bond.50-52

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Figure 8. Plot of cyclohexene oxide conversion versus time with CHP as the oxidant for TiMN(2) (9), TiMN(1) (2), and molecular (b) catalysts.

Figure 9. Plot of cyclohexene oxide conversion versus time with TBHP as the oxidant for TiMN(2) (9), TiMN(1) (2), and molecular (b) catalysts.

TABLE 1: Comparison of Homo- and Heterogeneous Titanium Catalysts for Cyclohexene Oxidationa catalyst

% conversion

% selectivity for cyclohexene oxide

TOFb

TiMN(1) TiMN(2) Ti(OiPr)4 Ti2SBA(1.58)c X-G2Ti/Sid,e

24.5 14.7 98 >98 >98 >98 >98

88 52 < 17 2000 175

a Using CHP at 65 °C in toluene for 2 h. b TOF ) mol cyclohexene oxide/mol Ti/hour (at 2 h). c Taken from Reference 17. d Taken from Reference 53. e Ti loading (>3 wt %) results in the increased conversion.

Epoxidations with CHP at higher temperatures (85 °C) led to increased initial conversions using TiMN(2) (25% after 2 h), but longer reaction times gave conversions that were not significantly improved (39% after 6 h) relative to reactions conducted at 65 °C. This, coupled with a slight drop in epoxide selectivity (95% for epoxide), suggests that higher temperature does not dramatically improve catalyst performance. As CHP is known to degrade at temperatures above 70 °C, decreased conversion over time may also be affected by oxidant decomposition. Although superior to the homogeneous catalyst in conversion, comparison of the nanosphere materials to related heterogeneous17 and soluble high surface area dendrimeric materials53

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TABLE 2: Ti-Catalyzed Oxidation of Cyclohexene Using H2O2a catalyst TiMN(1) TiMN(2) Ti(OiPr)4 Ti1SBA15b Ti2SBA15b MecapTi1SBA15b MecapTi2SBA15b

selectivity for selectivity for selectivity for total yield cyclohexene cyclohexenol cyclohexenone based on oxide (%) (%) (%) H2O2 (%) 29.2 17.5 8.6 19.2 11.6 30.3 29.4

25.8 26.5 25.4 30.8 37.7 22.3 27.2

45.0 56.0 66.0 50.0 50.7 47.4 43.4

2.8 2.1 3.4 5.8 13.2 9.4 23.1

a At 65 °C in CH3CN with a reaction time of 2h. b Taken from Reference 18.

SCHEME 3: Proposed Oxidation Mechanism with H2O2 Using Ti-SBA15 Catalysts

(Table 1) reveals interesting similarities and differences. In all cases, for both homo- and heterogeneous catalysts, epoxide selectivity is excellent (>98%) with organic oxidants. However, epoxide conversions for SBA15-supported and dendrimeric catalysts were consistently better than those for the nanosphere catalysts. Accounting for the different weight loadings in these systems, a much higher turnover frequency (TOF) is observed for the Ti2SBA15(1.58) catalyst relative to the nanosphere materials that are comparable to related Ti-grafted xerogels (Table 1, Supporting Information). The nanospheres, though much slower, are fairly robust, as the material continues selective epoxidation even after 12 h. One factor that may account for the lower rates of epoxidation associated with the nanospheresupported catalysts is the active site structure. As the proposed mechanism for epoxidation involves a ligand-stabilized titanium hydroperoxo complex (vide infra), altering the sterics and electronics of the capping group on titanium from a hydroxide to an isopropoxide group likely effects the electronic properties of the titanium intermediate, thereby decreasing the rate of oxygen transfer relative to that of the SBA15-grafted catalyst. Moreover, the catalytic sites on the nanosphere surface may be more sterically hindered, leading to robust but less efficient catalysts. Another interesting comparison can be made between TiMN(1) and TiMN(2). With both oxidants, TiMN(1) consistently

outperforms TiMN(2). This difference could stem from several factors. Although TiMN(2) appears to be competent in performing selective cyclohexene oxidation, as judged by its performance relative to Ti(OiPr)4 with TBHP as the oxidant, some Ti(OiPr)4 is likely present during the epoxidation as a result of incomplete grafting (vide supra). Thus, the nanospheres of TiMN(2) should possess fewer active sites, and decreased conversions for this catalyst might be expected. Moreover, oligomerization between the ungrafted molecular precursor and single site centers on the surface could also reduce activity. Regardless, active and selective oxidation catalysts are produced by either method of catalyst preparation. The nanosphere catalysts were also screened using 30% aqueous hydrogen peroxide as the oxidant. Conversions and product selectivities after 2 h are shown in Table 2. Catalysis data from related heterogeneous materials and Ti(OiPr)4 are also included for comparison. Conversions are much lower for the nanosphere catalysts, but significantly higher than background oxidation, which results in less than 0.5% conversion over the same time period. Also, comparing TiMN(1) to the homogeneous precursor shows that both give similar conversions, but the nanospheres impart a reasonable increase in selectivity for epoxide. Moreover, higher selectivities are observed for TiMN(1) versus TiMN(2), which is consistent with more grafted titanium for the former catalyst. The lower activities for the nanosphere-supported catalysts, relative to those of related catalysts with silica supports, are likely due to several factors. In particular, some of the most efficient, silica-supported catalysts possess siloxy-substituted titanium centers, which are thought to be highly reactive toward electrophilic oxo transfer. Interaction with an ROOH reactant appears to produce a Ti(OOR)(HOSi≡) intermediate, and hydrogen bonding between the -OOR and relatively acidic silanol ligands may promote oxo-transfer to the olefinic substrate.18 Analogous, isopropoxy-capped sites could be significantly less reactive in this regard. In addition, isopropoxysubstituted titanium centers may be more susceptible to deactivation by water, for steric reasons and because of a decreased hydrophobicity imparted by the capping group. Although conversions are relatively low for TiMN(1) and TiMN(2), the catalytic data demonstrates that silicone nanospheres can support soluble, site-isolated catalysts that exhibit improved selectivities relative to those of analogous molecular catalysts.

An alternative source of hydrogen peroxide may be preferred if water reduces catalyst lifetime. UHP was utilized as an oxidant, with TiMN(1) as the catalyst at 65 °C in toluene (eq 3). This gave a low conversion to epoxide (only 5% conversion after 7 days), but the selectivity is improved relative to that for aqueous H2O2 (epoxide selectivity >60%). Background oxidation of cyclohexene by UHP under the same conditions gave only minimal conversion (30%). Several factors contribute to the decreased reactivity observed with UHP; solubility issues of the oxidant in toluene likely play a role in lower conversion. Also, a ureastabilized titanium hydroperoxo derivative might decrease

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Bradley et al.

catalytic turnover. Monitoring the addition of 1 equiv of UHP to TiMN(1) at ambient temperature indicates evolution of 0.8(2) equiv of isopropanol per titanium as judged by 1H NMR spectroscopy. This supports the formation of a urea-stabilized titanium hydroperoxo via displacement of alcohol at titanium (e.g., the structure shown in eq 4).

Active Site Characterization and Mechanism of Oxidation Catalysis. The mechanism of epoxidation for the nanospheresupported titanium catalysts may be similar to that suggested for related silica-supported catalysts (Scheme 3).54-57 The oxidation is thought to involve a titanium hydroperoxo ligand stabilized by hydrogen bonding with an -OR ligand (R ) H, alkyl, silyl) present in the resting state of the catalyst.18 The hydroperoxo can then interact with the olefin through a productive oxygen atom transfer to give epoxide or by radical chemistry that results in oxo formation, ultimately leading to allylic oxidation products. To test for the presence of radicals during the oxidation, a radical trap (butylated hydroxytoluene (BHT), 5 equiv per Ti) was added to the reaction of cyclohexene with aqueous H2O2 and TiMN(1) (eq 5). Monitoring selectivity and conversion over time indicated a conversion (2.5%) similar to that in the absence of BHT, but selectivity was markedly improved at 2 h (>70% for the epoxide). This increased epoxide selectivity suggests the presence of radicals during the catalysis and competing oxidation pathways involving oxygen transfer and radical chemistry.

Attempts to observe the titanium hydroperoxo intermediate were first made by examining solution UV-visible spectra of TiMN(1) in the presence of different oxidants. These spectra were complicated by the absorbance of water or reagent (CHP/TBHP) on the nanosphere surface as judged by control experiments involving water/oxidant addition to ungrafted methyl nanospheres. On the basis of previous difference DR-UV-visible characterization of Ti-hydroperoxo intermediates,18 similar experiments were attempted with TiMN(1) and UHP. The addition of UHP to a hexane solution of TiMN(1) and stirring for 1 h resulted in the precipitation of a bright yellow solid. Drying in vacuo followed by acquisition of the DR-UV-visible spectrum revealed a red shift of the LMCT band from 207 to 350 nm (Figure 10). The band observed at 277 nm is attributed to surface adsorption of isopropanol released from the Ti sites, as the control experiment of isopropanol addition to the

Figure 10. DR-UV-visible spectra of (a) TiMN(1), (b) TiMN(1) after UHP addition, and (c) the normalized difference between spectra a and b.

nanospheres gives a material with a DR-UV-visible maximum at 273 nm. On the basis of comparisons to spectra of related hydroperoxide intermediates on Ti-SBA15 and TS-1 materials, the maximum at 350 nm is assigned as the titanium hydroperoxo LMCT band.18,58,59 Furthermore, exposure of the yellow solid to an equivalent of cyclohexene in acetonitrile for 24 h at 65 °C gave a 15% yield of cyclohexene oxide, indicating that the isolated material can participate in oxygen atom transfer, as would be expected for a hydroperoxo intermediate. Conclusions Methyl silicone-substituted nanospheres were synthesized and characterized as high surface area, hydrophobic materials. The nanoparticles provide good supports for titanium species introduced by reaction with Ti(OiPr)4, to give predominantly site-isolated titanium centers. These soluble materials promote cyclohexene oxidation with organic oxidants to give epoxide with high selectivities and reasonable yields. Epoxidation using aqueous H2O2 results in reduced conversion, but selectivity is significantly improved relative to that of the homogeneous molecular precursor. Examination of the reaction mechanism suggests that competition between oxygen transfer to form epoxide and unproductive radical chemistry to form allylic oxidation products is operative. Although the nanosphere catalysts are less active than their heterogeneous counterparts, these studies provide evidence that reactive metal complexes can be supported on functionalized silicone nanospheres to achieve selective, homogeneous catalysis. Future efforts will be directed toward the development of nanospheres with additional ligand functionalities (phosphines, amines, thiols, etc.). In addition, these systems appear to offer simple means for the preparation of multifunctional soluble supports for several different metal centers on a single surface. Acknowledgment. This work was made possible by the generous support of Wacker Chemie AG. Dr. Michael Lucarelli and Dr. Herbert Barthel (Wacker Chemie AG) are acknowledged for helpful discussions. Simon Humphrey is also thanked for assistance with TEM microscopy. Steven Kaye (Long Group) is acknowledged for assistance in acquiring porosimetry data on the nanospheres. Ping Yu (U.C. Davis) is thanked for acquiring solid-state 1H, 13C, 29Si, and 1H-13C HETCOR NMR data. Andrew Goodwin (Fre´chet Group) is acknowledged for

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