Combination Catalysts Consisting of a Homogeneous Catalyst Tethered to a Silica-Supported Palladium Heterogeneous Catalyst: Arene Hydrogenation

Transition metal complex catalysts tethered to organic or inorganic supports1 have received much attention in the past few decades because they can, in principle, combine the advantages of homogeneous and heterogeneous catalysts. Such complexes can be easily tethered on silica surfaces through a ligand in the complex which has alkoxyor chlorosilane functional groups that react with surface hydroxyl groups on the SiO2. Silica-supported heterogeneous metal catalysts such as Pd-SiO2, Rh-SiO2, and Pt-SiO2 also have surface hydroxyl groups that could be used to tether transition metal complex homogeneous catalysts. These combination catalysts consisting of a tethered complex on a supported metal (TCSM) catalyst (Figure 1) could function by synergistic action of both catalyst components. For hydrogenation reactions of unsaturated organic substrates, one might imagine that these TCSM catalysts could function in a way that H2 is activated on the supported metal (e.g., Pd, Rh, or Pt) with the resulting hydrogen atoms spilling over onto the silica where they could react with the unsaturated organic substrate that is simultaneously coordinated and activated by the tethered complex. This mechanism for the functioning of a TCSM catalyst depends on the well-known phenomenon of hydrogen spillover on supported metal catalysts.3 In other mechanisms, the tethered complex may interact more directly with molecules that are activated on the supported metal. In this paper, we report an example, the first to our knowledge, of a tethered complex on a supported metal (TCSM) catalyst, whose activity for the hydrogenation of arenes is substantially higher than that of the tethered complex or the supported metal separately. In fact, its activity is higher than that of any reported homogeneous or immobilized metal complex catalyst under the mild conditions of 1 atm of H2 and 40 °C. Two TCSM catalysts were prepared by tethering either of the rhodium isocyanide complexes, RhCl[CN(CH2)3Si(OC2H5)3]3 or RhCl(CO)[CN(CH2)3Si(OC2H5)3]2, to a silica-supported palladium metal catalyst (Pd-SiO2). The rhodium isocyanide complex RhCl(CO)[CN(CH2)3Si(OC2H5)3]2 (Rh-CNR2) was prepared by the reaction of [Rh(CO)2Cl]2 with 4 equiv of CN(CH2)3Si(OC2H5)3 in toluene, in a reaction similar to that described for the synthesis of RhCl(CO)[CNBu]2. The complex RhCl[CN(CH2)3Si(OC2H5)3]3 (Rh-CNR3) was prepared in the reaction of [Rh(COD)Cl]2 (COD ) cyclooctadiene) with 6 equiv of CN(CH2)3Si(OC2H5)3 according to a procedure used for the preparation of RhCl[CN(2,6-xylyl)]3. The toluene solution containing RhCl(CO)[CN(CH2)3Si(OC2H5)3]2 or RhCl[CN(CH2)3Si(OC2H5)3]3 was refluxed with the silica-supported palladium catalyst Pd-SiO2 (Pd, 10 wt %) for 4 h. After filtration, the solid was washed with toluene and then dried in vacuum at room temperature. The resulting tethered catalysts, Rh-CNR2/Pd-SiO2 (Rh content, 1.10 wt %) and Rh-CNR3/PdSiO2 (Rh content, 1.35 wt %), gave IR spectra (DRIFTS) with ν(CN-) and ν(CO) bands (2197 (s) and 2017 (s) cm-1 for RhCNR2/Pd-SiO2; 2176 (s) and 2124 (w) cm-1 for Rh-CNR3/PdSiO2) that are very similar in position and relative intensity to those of the untethered Rh-CNR2 and Rh-CNR3 complexes,4,8 which indicates that the complexes retain their structures after being tethered to the Pd-SiO2 surface. The rates of hydrogenation (Table 1) of toluene to methylcyclohexane at 40 °C while being stirred under 1 atm of H2 in the presence of the TCSM catalysts or the separate homogeneous and heterogeneous catalysts were determined by following the rate of H2 uptake. The catalysts are active from the outset but the TOF (turnover frequency) values increase to a maximum value of 4.8 for Rh-CNR2/Pd-SiO2 after 1 h and to 5.5 for RhCNR3/Pd-SiO2 after 6.5 h. After several hours at the maximum TOF levels, the activities decrease slightly. From the data in Table 1, it can be seen that the Rh-CNR2/Pd-SiO2 catalyst activity (as measured by the maximum TOF, turnover number (TO), or H2 uptake) is at least 7 times greater than that of the simple heterogeneous SiO2-supported Pd (Pd-SiO2), the RhCNR2 complex tethered to just SiO2(Rh-CNR2/SiO2), just the ligand (CN(CH2)3Si(OC2H5)3) tethered to Pd-SiO2(CNR/PdSiO2), or the homogeneous catalyst (Rh-CNR2) even with relatively large amounts of Rh (20 μmol) as compared with 6.3 μmol in Rh-CNR2/Pd-SiO2. Similarly, Rh-CNR3/Pd-SiO2 is at least 9 times more active than Pd-SiO2, homogeneous Rh-CNR3, tethered Rh-CNR3/SiO2, or CNR/Pd-SiO2. The most active TCSM catalyst, Rh-CNR3/Pd-SiO2, has a maximum turnover frequency of 5.5 mol H2/(mol of Rh min) and a turnover number (1) (a) Hartley, F. R. Supported Metal Catalysts; Reidel: Dordrecht, The Netherlands, 1985. (b) Iwasaka, Y. Tailored Metal Catalysts; Reidel: Tokyo, 1986. (c) Cornils, B.; Hermann, W. A. Applied Homogeneous Catalysis with Organometallic Compounds; VCH: Weinheim, 1996; p 351. (2) (a) Blumel, J. Inorg. Chem. 1994, 33, 5050. (b) Capka, M.; Czakova, M.; Wlodzimierz, U.; Schubert, U. J. Mol. Catal. 1992, 74, 335. (c) Allum, K. G.; Hancock, R. D.; Howell, I. V.; McKenzie, S.; Pitkethly, R. C.; Robinson, P. J. J. Organomet. Chem. 1975, 87, 203. (d) Capka, M.; Hetflejs, J. Collect. Czech. Chem. Commun. 1974, 39, 154. (e) Pugin, B. J. Mol. Catal. A: Chem. 1996, 107, 273. (f) Czakova, M.; Capka, M. J. Mol. Catal. 1981, 11, 313. (3) (a) Pajonk, G. M.; Teichner, S. J.; Germain, J. E. SpilloVer of Adsorbed Species; Elsevier: Amsterdam, 1983. (b) Conner, W. C., Jr.; Pajonk, G. M.; Teichner, S. J. AdV. Catal. 1986, 34, 1. (c) Conner, W. C., Jr.; Falconer, J. L. Chem. ReV. 1995, 95, 759. (d) Inui, T.; Fujimoto, K.; Uchijima, T.; Masai, M. New Aspects of SpilloVer Effects in Catalysis; Elsevier: Amsterdam, 1993. (4) Selected data for RhCl(CO)[CN(CH2)3Si(OC2H5)3]2: 1H NMR (CDCl3) δ 3.82 (q, 12H, OCH2CH3), 3.67 (t, 4H, CNCH2), 1.90 (m, 4H, CH2CH2CH2), 1.21 (t, 18H, OCH2CH3), 0.75 (t, 4H, SiCH2); IR (in toluene) ν(CN-) 2192 (s) cm-1, ν(CO) 1996 (s) cm-1. (5) McCleverty, J. A.; Wilkinson, G. Inorg. Synth. 1990, 28, 84. (6) (CH3CH2O)3SiCH2CH2CH2NC was prepared from (CH3CH2O)3SiCH2CH2CH2NHCHO and Cl3COC(dO)Cl following a procedure developed for the synthesis of other alkyl isocyanides (Skorna, G.; Ugi, I. Angew. Chem., Int. Ed. Engl. 1977, 16, 259); IR (in CH2Cl2), ν(CN-) 2150 cm-1; 1H NMR (CDCl3) δ 3.81 (q, 6H, OCH2CH3), 3.38 (m, 2H, CNCH2), 1.78 (m, 2H, CH2CH2CH2), 1.20 (t, 9H, OCH2CH3), 0.72 (t, 2H, SiCH2). (7) Deeming, A. J. J. Organomet. Chem. 1979, 175, 105. (8) Selected data for RhCl[CN(CH2)3Si(OC2H5)3]3: 1H NMR (CDCl3) δ 3.82 (q, 18H, OCH2CH3), 3.58 (t, 4H, CNCH2), 3.46 (t, 2H, CNCH2), 1.85 (m, 6H, CH2CH2CH2), 1.23 (t, 27H, OCH2CH3), 0.73 (t, 6H, SiCH2); IR (in toluene) ν(CN-) 2158 (s), 2119 (m) cm-1. Anal. Calcd for C30H63O9N3Si3ClRh: C, 43.28; H, 7.63; N, 5.05. Found: C, 42.70; H, 7.37; N, 4.57. (9) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1990, 28, 88. (10) Yamamoto, Y.; Yamazaki, H. J. Organomet. Chem. 1977, 140, C33. (11) Pd-SiO2 was prepared by the incipient wetness method by impregnation of SiO2 using an aqueous solution of H2PdCl4, calcining at 500 °C for 4 h and reducing with H2 at 380 °C for 4 h. Figure 1. Conceptual illustration of a TCSM catalyst consisting of a tethered homogeneous complex catalyst on a supported metal heterogeneous catalyst. 6937 J. Am. Chem. Soc. 1997, 119, 6937-6938