Research Article pubs.acs.org/acscatalysis
Aerobic Epoxidation of Olefin by Platinum Catalysts Supported on Mesoporous Silica Nanoparticles Dominik Munz,† Daoyong Wang,‡ Megan M. Moyer,§ Michael S. Webster-Gardiner,† Pranaw Kunal,§ David Watts,‡ Brian G. Trewyn,*,§ Andrei N. Vedernikov,*,‡ and T. Brent Gunnoe*,† †
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, United States Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States § Department of Chemistry, Colorado School of Mines, Golden, Colorado 80401, United States ‡
S Supporting Information *
ABSTRACT: We report platinum catalysts for the efficient aerobic oxidation of olefins to form epoxides and/or derived glycol monoethers. The catalystsdiaqua and dichloro PtII complexes supported by the ligand di(2-pyridine)methanesulfonate (dpms)are most active when they are covalently tethered to mesoporous silica nanoparticles (MSNs). Supporting the molecular Pt complexes on the MSNs prevents bimolecular catalyst deactivation. Using this strategy, >40 000 turnovers are achieved for the aerobic oxidation of norbornene in 2,2,2-trifluoroethanol. The position of the tether and the nature of other ligands in the metal coordination sphere (aqua, hydroxo, or chloro) are shown to affect the catalyst activity. The new MSN-supported Pt materials were characterized by nuclear magnetic resonance (NMR) spectroscopy, nitrogen physisorption, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). KEYWORDS: immobilized catalysts, platinum, epoxidation, olefins, oxygen
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INTRODUCTION Despite over 40 years of steady progress in the field of homogeneous hydrocarbon functionalization,1 the development of stable and active catalysts for economically viable functionalization of lower hydrocarbons remains an important challenge.2 A key consideration for large-scale hydrocarbon oxidation, such as oxidation of alkanes or olefins derived from petroleum or natural gas, is the identity of the oxidant. In most cases, the most economically viable oxidant is air (i.e., dioxygen in air) or purified dioxygen. However, examples of hydrocarbon oxidations using environmentally benign and inexpensive oxidants such as molecular oxygen remain scarce.3 In contrast, high-energy oxidants or waste-generating reagents such as peroxides or K2S2O8 are commonly employed.4 Even hydrogen peroxide, which often produces water as a byproduct and, as a result, is claimed as a green oxidant, has issues, because of its production using dihydrogen from synthesis gas (CO and H2), which is derived from the steam reforming of methane. As a result, significant efforts have been devoted to exploring the reactivity of late-transition-metal complexes toward molecular oxygen in recent years.5 Previously, it was demonstrated that the facially chelating hemilabile ligand di(2-pyridine)methanesulfonate (dpms) enables functionalization of PtII−R bonds in protic media with dioxygen as the oxidant.5c,g,h,6 The reaction proceeds via the intermediacy of the derived hydroxo-platinum(IV) species © XXXX American Chemical Society
and can be used to produce alcohols, ethers, aminoalcohols, and epoxides.5f,6,7 In particular, (dpms)PtII(olefin)(OH) complexes can be oxidized with dioxygen (olefin = norbornene, cis-cyclooctene) in 2,2,2-trifluoroethanol (TFE) solution to produce the corresponding PtIV-oxetanes, which reductively eliminate olefin-derived epoxides in quantitative yields with high selectivity (see Scheme 1). Attempts at catalytic aerobic epoxidation of norbornene using the (dpms)PtII(OH) (DMSO) complex resulted in only stoichiometric yields of the epoxide, because of competing catalyst deactivation, which leads to the insoluble and catalytically inactive dinuclear species [(dpms)PtII(μ-OH)]2.6 These observations prompted us to consider the possibility of developing a robust Pt catalyst for aerobic epoxidation of olefins through site isolation on a solid support. The development of catalysts for aerobic epoxidations of olefins that do not involve the sacrificial oxidation of coreductants is highly desirable.8 The industrial use of oxygen as a direct oxidant for lower olefin epoxidation is currently limited to ethylene oxide.9,10 Very few systems are capable of using O2 as the terminal oxidant,11 and examples of late transition-metal catalysts9c,12 are especially rare. To the best of our knowledge, there is only one Pt-based catalyst, reported in a patent, suitable Received: May 31, 2016
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ACS Catalysis Scheme 1. Previously Reported Stoichiometric and Stepwise Aerobic Epoxidation of Norbornene and cis-Cyclooctene Mediated by (dpms)PtIIL(OH)
Scheme 2. Synthesis of dpms Ligand with Tether Attached at the Bridging Carbon Atom
removed through repeated washings of the MSN material. Thus, we used a mixture of 10% 2 and 90% 3 to generate 3MSN. The new 4-(5-(trimethoxysilyl)pentyl-substituted dpms proligand 7 was prepared as shown in Scheme 3, also as a mixture for direct aerobic olefin oxidation.13 Hence, the development of aerobic catalytic procedures involving platinum14 would constitute substantial progress. In this work, we report that, by using mesoporous silica nanoparticles (MSNs) to immobilize molecular Pt complexes, we can achieve an efficient catalytic oxidation of norbornene by dioxygen using (dpms)PtII-MSN materials.15 Some of the resulting PtII catalysts demonstrate high catalytic activity in the aerobic oxidation of norbornene with turnovers (TOs) of >40 000. Since the immobilization typically shuts down bimolecular catalyst deactivation pathways,16 these results support our original hypothesis that the incompetence of (dpms)PtII(OH) (DMSO) and similar soluble complexes in the aerobic olefin epoxidation is related to their bimolecular deactivation. Although we do not demonstrate broad olefin substrate scope in this work, the catalyst immobilization strategy and results of olefin oxidation that we report here may advance future development in the field of aerobic hydrocarbon functionalization. The effect that catalyst immobilization, the nature of the “secondary” ligands (e.g., OH2, Cl), and the position of the tether has on the activity of the catalysts are discussed.
Scheme 3. Synthesis of dpms Ligand with Tether Attached at the C4-Position
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with precursor 6 and with product 6-H2 resulting from hydrogenation of the CC bond present in 6. To minimize formation of the hydrogenated product, the hydrosilylation of 6 was not brought to completion, leaving up to 33% of unreacted 6 in the mixture containing 62% 7 and 5% 6-H2. Since 6, as well as its hydrogenated derivative 6-H2, do not interfere with MSN immobilization of 7, this mixture was used to generate the MSN-tethered material 7MSN. The molecular pro-ligands 3 and 7 were immobilized by a cocondensation method, whereby the pro-ligand was added in situ during the MSN synthesis.15h The co-condensed material was synthesized by hydrolysis of the desired ligand, along with tetraethyl orthosilicate (TEOS), followed by hydrothermal treatment in the presence of nonionic surfactant P104 under acidic conditions (HCl).17 The MSN materials with tethered ligands were obtained after acidic extraction of the P104 surfactant, yielding 3MSN and 7MSN. The resulting MSN materials were analyzed and characterized by powder X-ray diffraction (PXRD), nitrogen sorption (Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) methods), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and solid-state NMR spectroscopy.18 The ligand loadings were measured by
RESULTS AND DISCUSSION Synthesis of Molecular Precursors and Attachment to Mesoporous Silica Nanoparticles. Our strategy for catalyst immobilization involved the incorporation of silyl ether groups into the dpms pro-ligand. The syntheses of two new ω(trimethoxysilyl)alkyl-substituted dpms pro-ligands 3 and 7 are illustrated in Schemes 2 and 3, respectively. The position of the tether at the bridging carbon atom of the dpms ligand, as in 3, or at the C4-position of one of the pyridine rings, as in 7, is dictated by the relative simplicity of the synthesis in the first case and considerations of minimal interference of the tether with the ligand coordination to the metal in the second case. The new 1,1-di(2-pyridyl)-5-(trimethoxysilyl)pentanesulfonate pro-ligand 3 was prepared as shown in Scheme 2 with >90% purity, according to 1H and 13C NMR spectroscopy (see the Supporting Information). The major contaminant was the olefin precursor 2. Performing the hydrosilylation reaction for longer times to achieve better conversion of 2 led to the formation of significant amounts of unidentified impurities. Compound 2 cannot be covalently attached to the MSN, because of the absence of a siloxy group; hence, 2 was easily 4585
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ACS Catalysis thermogravimetric analysis, whereas the metal loadings were determined by solution-phase NMR after digesting the MSN catalysts in concentrated KOH. The SEM of 3MSN and 7MSN in Figure 1 show similar amorphous morphology of the overall
Table 1. Nitrogen Physisorption Data and Ligand Loading of MSN Catalysts material
BET surface area [m2 g−1]
BJH adsorption pore volume [cm3 g−1]
ligand loading [mmol g−1]
3MSN 7MSN MSNunfunct
669 758 623
0.72 0.75 0.99
0.7 0.3 N/A
provide clear evidence that the ligands are incorporated on the MSN. The 13C peaks from the alkyl tether and aromatic peaks from the pyridine residues are clearly observed in the spectra in approximate ranges of −10 to −40 ppm and −120 to −160 ppm, respectively (Figure S35 in the Supporting Information). The functionalization of the MSN materials via co-condensation was directly verified by 29Si solid-state NMR and TGA. Figure 2 shows the 1H → 29Si ssNMR spectra. The resonances
Figure 1. (a and c) Scanning electron microscopy (SEM) and (b and d) transmission electron microscopy of 3MSN (panels (a) and (b)) and 7MSN (panels (c) and (d)) prior to coordinating Pt to the tethered ligands. Red arrows indicate mesopores within the MSN structure in panels (b) and (d). Scale bars represent 1 μm in panels (a) and (c) and 100 nm in panels (b) and (d).
particle structure (MSNunfunct; see Figure S33 in the Supporting Information). This morphology is a direct consequence of the synthesis method. Unlike post-synthetic grafting, in the case of the co-condensation method, the pro-ligands are added in situ with the silica precursor. Because bulky ligands interfere with the pore-forming surfactant during the acid catalyzed hydrolysis and condensation, amorphous materials are typically obtained.19 However, the pores of both catalysts are clearly visible in the TEM (indicated by red arrows in Figures 1b and 1d) and the high surface areas of both also indicate that they are accessible, which is necessary for high catalytic activity.19 The surface areas, pore volumes, and pore size distributions of both catalysts were analyzed by nitrogen sorption techniques. The powder XRD spectra of these materials, plus that of MSNunfunct (see Figure S30 in the Supporting Information) featured an intense (100) diffraction peak.20 Higher-order peaks are observed in the unfunctionalized MSN (Figure S30b) but are not observed in the functionalized 3MSN and 7MSN catalysts (Figure S30a). The loss of resolution of the higherorder peaks is typical for MSN functionalized by the cocondensation method. The observed XRD results of MSN functionalized by co-condensation have been described previously.19 As shown in Figure S31 in the Supporting Information, both 3MSN and 7MSN exhibited characteristic Type IV BET isotherms.21 The BJH average pore diameters for both materials increased from the average pore diameter of an unfunctionalized MSN, as shown in Figure S32 in the Supporting Information.22 Details of the nitrogen sorption data of the immobilized MSN materials can be found in Table 1 and in the Supporting Information. The incorporation of pro-ligands 3 and 7 in both 3MSN and MSN 7 was measured by TGA and solid-state 13C and 29Si NMR spectroscopies. The 1H → 13C cross-polarized magic angle spin solid-state NMR spectra (ssNMR) of both 3MSN and 7MSN
Figure 2. 1H → 29Si CPMAS solid-state NMR of (a) 3MSN and (b) 7MSN, after surfactant extraction.
ranging from between −50 ppm and −70 ppm represent Si atoms that are covalently bound to organic groups, (SiO)2Si(OH)R and (SiO)3SiR. The presence of these “T sites” confirms the covalent attachment of the ligands. The Qsite resonances located at approximately −85 ppm to −120 ppm represent the Q4−Q2 silicon resonances that make up the interior framework and the unfunctionalized surface of the MSN. Detailed information regarding Q- and T- sites in 29Si CPMAS NMR has been previously described.23 These spectral positions agree with literature values.19 The ligand loading was quantified by TGA, and the measured loadings for 3MSN and 7MSN were in the range of 0.3−0.7 mmol g−1 (Table 1; also see Figure S34 in the Supporting Information), which matches well with the results from digestion experiments with NaOH (0.2− 0.6 mmol g−1, vide inf ra). Upon immobilization (vide supra), both 3MSN and 7MSN were metalated using K2PtIICl4 and converted to the Pt-diaqua materials 9MSN and 11MSN (see Schemes 4 and 5). The diaqua complexes 9MSN and 11MSN were converted to dihydroxo derivatives, 9MSN(OH) and 11MSN(OH), respectively, by washing them with aqueous 0.5 M Na2CO3. Based on the pKa of 9.3 ± 0.1 for [(dpms)Pt(OH2)2][NO3] (14),24 we believe that the basicity of an aqueous Na2CO3 solution with an estimated pH of 12 is sufficient for deprotonation of the immobilized aqua complexes. The optimal conditions for the metalation of 3MSN and 7MSN were found by using soluble ligands with similar substitution patterns: K(dpms) for 7 and Li(1,1-di(2-pyridine)ethanesulfonate) Li(dpes) for 3 (see Schemes 6 and 7). 4586
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than for 3MSN and Li(dpes), which was attributed to steric interference of the alkyl group at the ligand bridging carbon atom in the latter cases. The resulting steric congestion at the bridging carbon would push the sulfonate group closer to the pyridine donor atoms, thus increasing the effective bulkiness of these dipyridine ligands and slowing the ligand substitution reaction at the PtII center. The ligand loadings for 3MSN and 7MSN, as well as those of the derived immobilized dichloro and diaqua PtII materials 8MSN− 11MSN, were determined by cleaving the ligand−MSN bond and releasing molecular PtII complexes into solution. This was accomplished by placing a known amount of MSN-immobilized complexes in aqueous KOH and using 1H NMR spectroscopy to integrate characteristic signals of complexes 15−18 and the corresponding nonmetalated ligands with 1,4-dioxane as an internal standard (Schemes 8 and 9). As determined by the 1H
Scheme 4. Metalation of 3MSN with PtII and Preparation of Dihydroxo Material 9MSN(OH)
Scheme 5. Metalation of 7MSN with PtII and Preparation of Dihydroxo Material 11MSN(OH)
Scheme 8. Determination of Catalyst Loading of 8MSN and 9MSN
Scheme 6. Optimization of Metalation Reaction with Molecular Catalyst for Tethering at Bridging Carbon Atom
Scheme 9. Determination of Catalyst Loading of 10MSN and 11MSN
Scheme 7. Optimization of Metalation Reaction with Molecular Catalyst for Tether in the C4-Position
NMR analysis, the typical ligand loading in samples of 3MSN and 7MSN used in this work was in the range of 0.2−0.6 mmol g−1 and 60%−80% of the immobilized ligand present in 8MSN and 10MSN was coordinated to PtII. These values match well with the measured TGA results. The identities of the derived ω(trihydroxysilyl)alkyl substituted sulfonated complexes 15 and 17 were confirmed by ESI-MS. Catalysis. The MSN-Pt heterogeneous catalysts were tested in the aerobic oxidation of norbornene (19), using TFE as a solvent with 8 atm of O2. All starting material used in ∼0.5 M concentration was converted with 0.2 mol % of 11MSN (the amount of Pt is relative to 19) after 15 h at 80 °C. Under these conditions, oxidation products formed in at least 80% yield, with 2,2,2-trifluoroethyl ether (22) being the main product in
The metalation of 7MSN and the related model compound K(dpms) required shorter reaction time and lower temperature 4587
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ACS Catalysis overall 59% yield (Scheme 10). The intermediate formation of the epoxide 21 was observed at reaction times shorter than 5 h Scheme 10. Chemoselectivity of the Aerobic Oxidation of Norbornenea
Figure 3. Optimization on catalytic activity of 11MSN and comparison to molecular catalysts. Turnovers (TOs) corresponds to the sum of all oxidized products as detected by gas chromatography−mass spectroscopy (GC-MS), in the case of the metal-free systems, relative to 1/505 of the norbornene loading. Reaction conditions: 2.1 μmol catalyst, 8 atm O2, 80 °C, 1.06 mmol norbornene, 2 mL TFE, 15 h. Optimized conditions: 100 °C, 0.21 μmol catalyst (based on Pt loading), 21 mmol norbornene, 4 mL TFE, 15 min, 6 h, or 15 h.
Reaction conditions: 2 mL TFE, 1.1 mmol norbornene, 2.1 μmol catalyst (based on Pt loading), 8 atm O2, 80 °C, 15 h. a
by GC-MS. The origin of 22 from reaction of 21 and 2,2,2trifluoroethanol was confirmed by using independent solvolysis of exo-epoxy norbornene in TFE at 80 °C. Besides 22, 2-norbornanol 20 and the ketone norcamphor also formed as minor byproducts (3%). Unidentified products with greater GC−retention time than 22 accounted for the remainder of the mass balance. These products are due to degradation of 21, because they were also obtained when stirring 21 in TFE at 80 °C. Therefore, we conclude that the intermediate epoxide 21 formed in at least 80% yield. In an effort to suppress the solvolytic ring opening of epoxide 21, an evaluation of other solvents was performed. Whereas hexafluoroisopropanol reacted in the same way as TFE, we either obtained very high background reactivity in diglyme, dioxane, 1,2-dimethoxyethane, or ethyl acetate or very low overall activity in acetonitrile or 1,2-dichloroethane at 80 °C. When substituting O2 with tert-butyl hydroperoxide as an oxidant, the reaction was unselective with the formation of many byproducts. For ethanol, a mixture of epoxide and ring opened product after reaction with ethanol was observed. A brief investigation toward the applicability of this oxidation procedure toward different olefins revealed that cis-cyclooctene was oxidized to the epoxide at 100 °C in 21% yield (25% conversion). Unfortunately, we observed deactivation of the catalyst, presumably due to the formation of inactive allylic platinum species.25 For 1-octene, a mixture of products was obtained at 100 °C, because of isomerization of the double bond. In order to determine the limits of the catalytic activity of the systems studied, the reaction conditions were optimized (Figure 3). Changing the oxygen pressure from 8 bar to 4 bar led to equivalent yields (i.e., no meaningful change in yield) and selectivity after 15 h of reaction time. Using 11MSN, the concentration of norbornene was increased up to 3.5 M and the temperature increased to 100 °C. The reaction mixtures containing 2.0 g of norbornene in 4.0 mL of TFE and 0.21 μmol 11MSN have demonstrated 17 500 TOs after 15 min, which correspond to an apparent turnover frequency (TOF) of 19 s−1. After 6 h of reaction, 43 300 TOs were observed. This is in strong contrast to the soluble molecular PtII diaqua complex [(dpms)Pt(OH2)2][NO3] (14), which was deactivated after 3 h with a turnover number (TON) of only 139 after 15 h under the same reaction conditions. We obtained even smaller TON values for the binuclear complex [(dpms)Pt(μ-OH)]2 (TON = 35),7c which is in accordance with the deactivation of 14 via a
bimetallic mechanism leading to the latter dinuclear species.6 These data highlight the noticeable activity and stability of the supported platinum catalyst and the remarkable enhancement in productivity, compared to molecular variants. A comparison with the background reaction in the absence of Pt but in the presence of the MSN-immobilized ligand 7MSN showed the equivalent (compared to an equivalent amount of 11MSN) of 3 TOs at 80 °C after 15 h. MSN material without tethered ligand, which was treated with K2PtCl4 under metalation conditions led to the equivalent of 18 TOs. These experiments reveal that the immobilized platinum complex 11MSN is indeed responsible for the catalysis (Figure 4).
Figure 4. Comparison of catalysis for 11MSN, 7MSN, K2PtCl4, Pt/MSN, and Pt(acac)2. Turnovers correspond to the sum of all oxidized products as detected by GC-MS, in the case of the metal-free systems, relative to 1/505 of the norbornene loading. Reaction conditions: 2.1 μmol catalyst (based on Pt loading), 8 atm O2, 80 °C, 1.06 mmol norbornene, 2 mL TFE, 15 h.
Interestingly, simple platinum complexes such as K2PtCl4 (TON = 110) and Pt(acac)2 (acac = acetylacetonate) (TON = 88) showed moderate catalytic activity as well. This was an unexpected observation, since aerobic epoxidation by simple PtII complexes has little precedence.13 The analysis of the kinetic profile of the reaction catalyzed by 11MSN (conditions given in Figure 5 caption) indicated that norbornene was consumed quantitatively within a reaction time of 7 h at 80 °C. When more substrate was added to the reaction mixture after 6 or 12 h, the reaction resumed with a comparable reaction rate (Figure 6). An ESI-MS analysis of the reaction mixture 4588
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complexes 8MSN−10MSN with chloride ligands were also analyzed. Tethering these (dpms)PtII complexes via the bridging carbon turned out to lead to inferior catalytic results (TO = 279 after 15 h for 9MSN; see Figure 7). This fact may be related to some excessive steric bulk produced by the tether attached to the ligand 3 bridging carbon atom, which, in turn, may affect the rate of the ligand exchange and/or C−O reductive elimination reactions occurring at the Pt center. Previously, it was shown qualitatively that the rates of olefin coordination to PtII (1−2 days to completion at 20 °C), oxidation of the resulting olefin complexes (∼1 day to completion at 20 °C) and C−O reductive elimination of the intermediate PtIV oxetanes (∼12 h to completion at 45 °C) are of comparable magnitude.6 In turn, it may be expected that the additional steric bulk at the bridging carbon of the dpms ligand would accelerate the oxidation step by pushing the sulfonate group closer to the PtIV center emerging during the oxidation. The chloro PtII complexes 10MSN and 8MSN (TO = 206, TO = 111) showed good catalytic activity but, once again, with a bridging carbon-tethered analogue 8MSN being less active than pyridine-tethered material 10MSN. The ligand effects, kinetic profile, deactivation of catalyst in the case of cis-cyclooctene, solvent effects, and rapid deactivation of molecular catalyst do not suggest a radical chain autoxidation mechanism. However, when adding the radical initiator AIBN instead of the MSN platinum catalyst, we also observed the formation of epoxidation product. We therefore evaluated the effect of adding the well-known radical transfer or scavenger reagents CBrCl3, [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl] (TEMPO), and butylated hydroxytoluene (BHT) in stoichiometric amounts relative to norbornene (2.1 μmol catalyst (based on Pt loading), 8 atm O2, 80 °C, 0.53 mmol norbornene, 0.53 mmol additive, 2 mL TFE, 15 h). In all cases, the yield was reduced (TEMPO, 59%; CBrCl3, 43%; BHT, 31%), but we did not observe the formation of products from trapping of C-radicals, such as brominated products, as has been reported for radical reaction mechanisms.26 Since the inhibition can be attributed to slowing the reaction of the platinum catalyst with oxygen,5g,h we conclude that carbon radicals likely do not play a major role in the oxidation reactions. Similarly, in the presence of the free-radical inhibitor CuCl29 (8 atm O2, 80 °C, 0.53 mmol norbornene, 0.53 CuCl, 2 mL TFE, 15 h), a full conversion of norbornene and the formation of 22 as the main oxidation product (37% yield) was observed. Although autoxidation pathways are often difficult to definitively eliminate from consideration,29 we believe the experimental results are most consistent with a nonautoxidation reaction.
Figure 5. Kinetic profile for formation of oxidized products. TO corresponds to the sum of all oxidized products as detected by GCMS. Reaction conditions: 4.2 μmol catalyst (based on Pt loading), 8 atm O2, 80 °C, 4 mL TFE, 2.06 mmol norbornene.
Figure 6. Demonstration of stability of catalyst 11MSN by addition of more norbornene after 6 and 12 h of reaction time. TO corresponds to the sum of all oxidized products as detected by GC-MS. Reaction conditions: 4.2 μmol catalyst (based on Pt loading), 8 atm O2, 80 °C, 4 mL TFE, 2.06 mmol norbornene, and then another 2.06 mmol of norbornene added after 6 and 12 h, respectively.
containing 11MSN after the reaction and the determination of the platinum content of the MSN material after filtration revealed that Pt did not leach into the solvent phase. Hence, we conclude that the immobilized catalyst does not suffer from catalyst deactivation phenomena as in the molecular systems containing 14 or (dpms)Pt(DMSO) (OH).6 The catalytic activities of the hydroxo analogues of the immobilized diaqua complexes 9MSN(OH) and 11MSN(OH) were found to be very low, which is consistent with the expected poor ability of hydroxo ligands to be involved in ligand exchange with norbornene (Figure 7). The catalytic activities of 9MSN and the MSN-immobilized platinum
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CONCLUSION We have shown that new platinum complexes immobilized on mesoporous silica nanoparticles are effective catalysts of aerobic oxidation of norbornene. No signs of catalyst decomposition or deactivation were detected at 80 °C after 15 h, whereas the related soluble molecular complexes suffer from rapid deactivation. In fact, 11MSN gives >40 000 TOs for oxidation of norbornene. This work demonstrates that supported platinum complexes allow for the efficient aerobic epoxidation of olefins. Further work is directed toward the application of those catalysts for the aerobic oxidation of other hydrocarbons and an extension of the substrate scope reported herein.
Figure 7. Comparison of MSN-supported materials for the catalytic oxidation of norbornene. TOs corresponds to the sum of all oxidized products as detected by GC-MS. Reaction conditions: 2.1 μmol catalyst (based on Pt loading), 8 atm O2, 80 °C, 1.06 mmol norbornene, 2 mL TFE, 15 h. 4589
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EXPERIMENTAL SECTION Catalysis. Each reported yield is the mean of at least two runs. The reactions were conducted in 180 mL Fisher−Porter glass reactors. Glass reactors were always heated behind blast shields. In a typical reaction, the glass reactor was filled with MSN material containing 2.1 μmol of tethered molecular catalyst. A stir bar, 100 mg (1.06 mmol) of norbornene, and 2 mL of TFE were added. The reactor was sealed quickly and purged three times with 8 atm of dioxygen. The reactor was filled with 8 atm of dioxygen and stirred at 80 °C. It was cooled to room temperature after 15 h and analyzed by GC-MS after the addition of n-decane as a standard. The products obtained were identified by comparison of the retention times and fragmentation pattern with known, commercially available samples (exo-norborneol, exo-epoxy norbornene) and quantified by calibration curves in the presence of the standard decane. (2-exo, 3-endo) 3-(2,2,2-trifluoroethoxy)-bicyclo[2.2.1]heptane-2-ol was synthesized and characterized according to the Supporting Information (page S35). Especially at higher norbornene loadings, the additional formation of several compounds with higher boiling points were observed, which could not be identified. The very same products formed after stirring exo-epoxy norbornane in TFE at 80 °C. In order to obtain a calibration curve for those unknown oxidation products, a calibration factor proportional to the conversion of a known amount of exo-epoxy norbornene was used. For selected runs, the products were isolated by column chromatography on silica (gradient eluent CH2Cl2:EtOAc = 9:1, PMA stain), analyzed by 1H, 13C and 19F NMR, and compared to the respective spectra of the separately synthesized or commercially available samples. Synthesis and Characterization of Molecular Ligands. 2-Pent-4-enyl-pyridine,27 4-(pent-4-enyl) pyridine,27 and K(dpms)28 were prepared as reported previously. 5,5-di(2-pyridyl)-1-pentene (1). A solution of 2-pent-4-enylpyridine (1.40 g, 9.5 mmol) in 20 mL of dry THF was cooled to −78 °C in a 50 mL Schlenk flask using a dry ice-acetone bath. n-BuLi (2.5 M in hexanes, 3.8 mL, 9.5 mmol) was slowly added to the stirring solution, resulting in a color change from light yellow to dark red. After stirring the solution for an additional hour, 2-fluoropyridine (0.46 g, 4.8 mmol) was added via a syringe. The dry ice bath was subsequently removed and the reaction mixture was allowed to warm to room temperature and stirred overnight. Twenty milliliters (20 mL) of water was added to quench the reaction. The organic and aqueous layers of the resulting yellow solution were separated, the aqueous layer was extracted with Et2O (3 × 20 mL), and the combined organic phase was dried over MgSO4. After filtration, the solvents were removed under reduced pressure, and the residual oil was purified by flash chromatography to yield 0.90 g of product (84% yield). Lithium 1,1-di(2-pyridyl)-4-pentene-1-sulfonate (2). 5,5Di(2-pyridyl)-1-pentene (0.75 g, 3.3 mmol) was mixed with 9 mL of dry THF in a 25 mL Schlenk flask under argon. The solution was cooled to −78 °C and 2.5 M n-BuLi in hexanes (1.3 mL, 3.3 mmol) was added. The resulting dark red solution was stirred for 30 min, and then trimethyl amine sulfonate (0.47 g, 3.3 mmol) was added. The Schlenk flask was then closed with a Teflon seal and the mixture was heated for 24 h at 120 °C. The mixture was then cooled to 5 °C and the reaction quenched with water. The product was extracted in water and washed with CH2Cl2 (3 × 10 mL). The water layer was
concentrated, and the solid product was washed with acetone. The product was dissolved in dry ethanol, filtered through a paper filter and concentrated, and then recrystallized from a mixture of methanol and acetonitrile to obtain 0.8 g of product (77% yield) as a colorless solid. Lithium 1,1-di(2-pyridyl)-5-trimethoxysilylpentane-1-sulfonate (3). Lithium 1,1-di(2-pyridyl)-4-pentene-1-sulfonate (620 mg, 2 mmol), triethoxysilane (656 mg, 4 mmol), and PtCl2(SMe2)2 (8 mg, 0.02 mmol) were mixed with 15 mL of dry methanol in 100 mL Schlenk flask under argon. After being sealed, the reaction mixture was stirred at 60 °C. 1H NMR spectroscopy indicated that >95% of starting material was consumed after 2 h. The reaction mixture was cooled to room temperature before the solvent was removed under vacuum. The resulting residue was washed with dry diethyl ether to remove excess triethoxysilane and derived impurities. Lithium 1,1-di(2-pyridyl)-5-trimethoxysilylpentane-1-sulfonate was obtained as light yellow solid in 93% yield (>90% pure based on 1 H NMR spectrum). 2-Methyl-4-(pent-4-enyl)-pyridine (4). Compound 4 was prepared from 4-(pent-4-enyl)-pyridine in two steps via the intermediate synthesis of 2-methyl-4-(pent-4-enyl)-2H-pyridine-1-carboxylic acid methyl ester. Step 1: Formation of 2-Methyl-4-(pent-4-enyl)-2Hpyridine-1-carboxylic acid methyl ester. To a solution of 4-pent-4-enyl-pyridine (3.68 g, 25 mmol) in 100 mL THF cooled to −60 °C was added methyl chloroformate (2.36 g, 25 mmol). After stirring the slurry for 1 h, MeMgI (2 M in Et2O, 12.5 mL, 25 mmol) was added slowly. The mixture was warmed to room temperature over 2 h, and then quenched with water. The reaction mixture was extracted with diethyl ether (2 × 80 mL), and the combined organic phase was dried over Na2SO4 and then concentrated under vacuum to give 5.3 g of a light yellow oil (85% NMR yield). The oil was used directly for the next step. Step 2: Formation of 2-Methyl-4-(pent-4-enyl)-pyridine (4). To a solution of 2-methyl-4-(pent-4-enyl)-2H-pyridine-1carboxylic acid methyl ester (4.4 g, 20 mmol) in 200 mL THF cooled to 0 °C was added a solution of o-chloranil (4.9 g, 20 mmol) in 25 mL THF. The mixture was stirred for 5 h before being treated with 1 M NaOH solution. The aqueous phase was separated and extracted with diethyl ether (2 × 50 mL). The combined organic phase was dried over Na2SO4, and the solvents were removed under vacuum. The resulting residue was purified by flash chromatography to afford 1.42 g of a light brown oil (44% yield). 1-[4-(Pent-4-enyl)-2-pyridyl]-1-(2-pyridyl)methane (5). A solution of 2-methyl-4-(pent-4-enyl)-pyridine (1.21 g, 7.5 mmol) in 15 mL of dry THF was cooled to −78 °C in a 50 mL Schlenk flask using a dry ice−acetone bath. n-BuLi (2.5 M, 3.0 mL, 7.5 mmol) was slowly added to the stirring solution, resulting in a color change from light yellow to dark red. After stirring the solution for an additional hour, 2-fluoropyridine (0.36 g, 3.8 mmol) was added via a syringe. The dry ice bath was subsequently removed and the reaction mixture was allowed to warm to room temperature and stirred overnight. Twenty milliliters (20 mL) of water was added to quench the reaction. The organic and aqueous layers of the resulting yellow solution were separated, the aqueous layer was extracted with Et2O (3 × 20 mL), and the combined organic phase was dried over MgSO4. After filtration, the solvents were removed under reduced pressure, and the residual oil was purified by flash chromatography to yield 0.70 g of product (78% yield). 4590
DOI: 10.1021/acscatal.6b01532 ACS Catal. 2016, 6, 4584−4593
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ACS Catalysis
Metalation of Immobilized 7MSN (10MSN). 7MSN [200 mg, 0.042 mmol (ligand loading: 0.21 mmol g−1)] was mixed with a solution of K2PtCl4 (33 mg, 0.08 mmol) in water (3 mL). The mixture was heated at 60 °C for 10 h. The solid was isolated by filtration, washed with water, and dried under vacuum to give a light gray material. Diaquaplatinum(II) Derivative 11MSN. 10MSN (200 mg) was mixed with a solution of Hg(NO3)2 (14 mg, 0.042 mmol) and HNO3 (0.08 mmol) in water (2 mL). The mixture was heated at 60 °C for 2 h, filtered, washed with water, and dried under vacuum to give a light gray material. Dihydroxoplatinum(II) Derivative 11MSN(OH). 11MSN (200 mg) was mixed with 2 mL of Na2CO3 aqueous solution (pH 11). The mixture was stirred at room temperature for 15 h, filtered, washed with water, and dried under vacuum to give a light gray material. Also a potential carbonate complex would be converted under the catalytic conditions to the hydroxo complex (trifluoroethanolate complex, respectively). Synthesis and Characterization of Molecular Pt Complexes. Lithium 1,1-di(2-pyridyl)ethane-1-sulfonate, Li(dpes) (12). 1,1-Di(2-pyridyl)ethane (1.8 g, 10 mmol) was mixed with 20 mL of dry THF in a 50 mL Schlenk flask under argon. The solution was cooled to 0 °C and 2.5 M (in hexanes) n-BuLi (4 mL, 10 mmol) was added. The resulting dark red solution was allowed to reach room temperature and after half an hour, trimethyl amine sulfur trioxide complex (1.4 g, 10 mmol) was added. The Schlenk flask was then closed with a Teflon seal and the mixture was heated overnight (18 h) at 120 °C. The mixture was then cooled to 5 °C, and the reaction quenched with water. The product was extracted in water and washed with CH2Cl2 (3 × 20 mL). The water layer was concentrated and the solid product washed with acetone. The product was dissolved in dry ethanol and filtered. The solid from evaporation of the volatiles from the filtrate was recrystallized from a mixture of methanol and acetonitrile to obtain 1.2 g of Li(dpes) (44% yield) as a white solid. (dpes)Pt(OH2)2NO3 (13). Complex 13 was synthesized in two steps from Li(dpes) via Li(dpes)PtCl2. Step 1: Formation of Li(dpes)PtCl2. . Li(dpes) (7 mg, 0.025 mmol) and K2PtCl4 (10 mg, 0.025 mmol) were dissolved in 0.5 mL D2O. The resulting light orange solution was heated at 80 °C. The solution turned dark after 30 min. After being heated for 18 h, the dark solution faded to a pale green color. A 1 H NMR spectrum was recorded using dioxane as an internal standard (70% yield). Step 2: Formation of (dpes)Pt(OH2)2NO3 (13). Hg(NO3)2 (17.1 mg, 0.05 mmol) and HNO3 (0.1 mmol) were added to the mixture described above. The mixture was heated at 60 °C for 10 min. A 1H NMR spectrum was recorded using dioxane as an internal standard. 86% yield. (dpms)Pt(OH2)2NO3 (14). Complex 14 was synthesized in two steps from K(dpms) via K(dpms)PtCl2. Step 1: Formation of K(dpms)PtCl2. K(dpms) (7 mg, 0.025 mmol) and K2PtCl4 (10 mg, 0.025 mmol) were dissolved in 0.5 mL D2O. The resulting light orange solution was heated at 60 °C for 2 h. The solution turned pale yellow and a colorless solid formed. A 1H NMR spectrum was recorded using dioxane as an internal standard. Step 2: Formation of (dpms)Pt(OH2)2NO3 (14). Hg(NO3)2 (17.1 mg, 0.05 mmol) and HNO3 (0.1 mmol) were added to the mixture described above. The mixture was heated at 60 °C for 10 min. A 1H NMR spectrum was recorded using dioxane as an internal standard (94% yield).
Lithium [4-(pent-4-enyl)-2-pyridyl](2-pyridyl)methane-sulfonate (6). 1-[4-(Pent-4-enyl)-2-pyridyl]-1-(2-pyridyl)methane (0.64 g, 2.7 mmol) was mixed with 8 mL of dry THF in a 25 mL Schlenk flask under argon. The solution was cooled to −78 °C and 2.5 M (in hexanes) n-BuLi (1.1 mL, 2.7 mmol) was added. The resulting dark red solution was stirred for additional half hour before trimethyl amine sulfonate complex (0.38 g, 2.7 mmol) was added. The Schlenk flask was then sealed with a Teflon stopper, and the mixture was heated for 24 h at 120 °C. The mixture was then cooled to 5 °C and the reaction quenched with water. The product was extracted in water and washed with CH2Cl2 (3 × 10 mL). The water layer was concentrated under vacuum, and the solid product washed with acetone. The product was dissolved in dry ethanol and filtered. The solid from evaporation of the filtration was recrystallized from a mixture of methanol and acetonitrile to obtain 0.4 g of a white solid (46% yield). Lithium [4-(5-trimethoxysilylpentyl)-2-pyridyl](2-pyridyl)methanesulfonate (7). In glovebox, lithium [4-(pent-4-enyl)2-pyridyl](2-pyridyl)methanesulfonate (0.324 g, 1.0 mmol), trimethoxysilane (12.2 g, 100 mmol), and PtO2 (0.011 g, 0.05 mmol) were mixed in a 25 mL Schlenk tube. The mixture was heated at 85 °C for 24 h, allowed to cool, and concentrated under vacuum. The resulting solid was dissolved in dry methanol to give a light brown suspension, which was filtered to remove PtO 2 and/or derivatives. The filtrate was concentrated and dried to produce a pale yellow solid, which was a mixture of target product, starting material, and hydrogenated product in a ratio of 62:33:5. This mixture was used for immobilization without purification. Synthesis of 3MSN and 7MSN by Co-condensation. P104 surfactant (2 g) was dissolved in 64.1 mL of 1.6 M HCl by stirring at 56 °C for 1 h in a lightly covered Erlenmeyer flask. After the surfactant was dissolved, 4.46 mL (4.16 g) TEOS was added dropwise. This solution was allowed to stir for 40 min, after which 228 mg of 3 or 7, respectively, dissolved in 1 mL ethanol, was added. The solution was stirred at 56 °C for 24 h, hydrothermally treated at 100 °C for 24 h in a Parr reactor, collected by filtration and washed with copious amount of ethanol. The P104 surfactant was removed by extraction in ethanol with concentrated HCl [ratio of 100:1 (v/v)] by stirring for 6 h at 65 °C. The colorless product was isolated by filtration and dried in vacuo. Metalation of Immobilized 3MSN (8MSN). 3MSN [200 mg, 0.076 mmol (ligand loading: 0.38 mmol g−1)] was mixed with a solution of K2PtCl4 (63 mg, 0.15 mmol) in water (3 mL). The mixture was heated at 80 °C for 18 h. The solid was isolated by filtration, washed with water, and dried under vacuum to give a dark gray material. Diaquaplatinum(II) Derivative 9MSN. 8MSN (200 mg) was mixed with a solution of Hg(NO3)2 (26 mg, 0.076 mmol) and HNO3 (0.15 mmol) in water (2 mL). The mixture was heated at 60 °C for 2 h, filtered, washed with water, and dried under vacuum to give a dark gray material. Dihydroxoplatinum(II) Derivative 9MSN(OH). 9MSN (200 mg) was mixed with 2 mL of Na2CO3 aqueous solution (pH 11). The mixture was stirred at room temperature for 15 h, filtered, washed with water, and dried under vacuum to give a dark gray material. A referee pointed out that one could have also obtained the carbonate complex. We believe that also a potential carbonate complex, similar to the hydroxide complex, should be converted under the catalytic conditions to the trifluoroethoxide complex (and water). 4591
DOI: 10.1021/acscatal.6b01532 ACS Catal. 2016, 6, 4584−4593
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ACS Catalysis
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01532. Detailed experimental procedures, synthesis of molecular precursors, NMR data, physisorption isotherms, powder XRD data, TGA analyses, SEM and TEM images of MSNunfunct, and exemplary GC spectra (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*Tel.: +1 (303) 384 2224. E-mail:
[email protected] (B. G. Trewyn). *Tel.: +1 (301) 405 2784. E-mail:
[email protected] (A. N. Vedernikov). *Tel.: +1 (434) 982-2692. E-mail:
[email protected] (T. B. Gunnoe). Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge support from the Center for Catalytic Hydrocarbon Functionalization, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001298. M.S.W.-G. acknowledges support from AES for a graduate student fellowship and the Jefferson Scholars Foundation for a dissertation year fellowship. T.B.G. acknowledges support from the U.S. Department of Energy, Office of Basic Energy Sciences (No. DE-SC0000776) for mechanistic studies. The authors would like to thank Prof. Ranjit Koodali and Chia-Ming Wu for their assistance in obtaining XRD spectra.
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REFERENCES
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DOI: 10.1021/acscatal.6b01532 ACS Catal. 2016, 6, 4584−4593
Research Article
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DOI: 10.1021/acscatal.6b01532 ACS Catal. 2016, 6, 4584−4593
