3074 J. Am. Chem. SOC. zyxwvu 1983, 105, 3074-3082 Surface Organometallic Chemistry: Reactivity of Silica-Bound Rhodium Allyl Complexes and the Genesis of Highly Dispersed Supported Rhodium Catalysts H. C. Foley,+ S. J. DeCanio,* K. D. Tau,+ K. J. Chao,+ J. H. Onuferko,+ Cecil Dybowski,**and B. C. Gates*+ Contribution from the Center for Catalytic Science and Technology, Departments of Chemical Engineering and Chemistry, University of Delaware, Newark, Delaware 1971 1. Received August zyxwvutsr 16, 1982 Abstract: The surface organometallic chemistry of silica-supported diallylrhodium, (SiJ-O-Rh(allyl)2 ((Si) is used to indicate surface Si), was characterized by 'H NMR, IR, and XP spectroscopy. The complex was reduced by H2 under mild conditions, with the elimination of propene, to give highly dispersed rhodium metal aggregates. Reaction of the surface with CO led to the formation of Rhl(CO)* species, formed not from the {SipRh(allyl)2 groups but from other groups, presumably incorporating Rh(II1) ions. The reaction of ( S i p R h ( a l l y l ) , with hydrogen was inhibited by CO bonded to these other Rh centers. Evidently the reduction of the diallyl groups by hydrogen proceeded via reduction of Rh(II1) ions with spillover of hydrogen to the support, which led to further protolysis of (Si)-O-Rh(allyl)2 and eventually to rhodium metal. The small metal aggregates, observed by transmission electron microscopy, were catalytically active for toluene hydrogenation. One of the central challenges of catalysis is to prepare surfaces with well-defined structures for determination of relations between structure, reactivity, and catalytic activity. One of the methods of preparing well-defined supported-metal catalysts involves the protolysis reaction of a transition-metal allyl complex and the OH groups terminating the surface of an inorganic oxide: Catalyst precursors prepared in this way have been reported by Yermakov and co-workers' for transition metals including Zr, Hf, Nb, Cr, Mo, W, Re, Ni, Pd, and Pt. Ward et a1.2" and W a r d and Schwartz2b,crecently reported the synthesis of a diallyl complex of rhodium anchored through a Rh-0 bond to SiOz (and (Si)-OH zyxwvutsrqpon + Rh(all~1)~ - (Si)-O-Rh(allyl), + CH2=CHCH3 The diallyl complex of Rh was suggested to be bound initially to only one oxygen, the suggestion being based mainly on the observed stoichiometry of the synthesis reaction (eq 2).2a,b3d Reactions of the supported complex with H2, HCl, olefins, and phosphines were investigated; structures of the resulting surface species were formulated on the basis of the observed reaction stoichiometries and infrared spectra of the surface species. The supported di- allylrhodium complex was found to be a precursor of a catalyst for hydrogenation of olefins and of arenes.zb-d The objective of the research described here was to use a battery of surface spectroscopic methods, namely, FTIR, XPS, and 'H NMR, to characterize the structures, reactivities, and catalytic activities of the silica-supported species derived from [Rh(allyl),]; a specific goal was to establish the nature of the catalytically active species and the chemistry of their formation from the diallyl- rhodium complex present initially on the surface. Experimental Methods Catalyst Synthesis. The silica support, Cab-0-Si1 (M-5 grade, Cabot Corp.), was slurried with deionized water, dried in an oven, transferred to a fritted glass tube, heated to 200 OC under flowing oxygen for 12 h, and then dried under vacuum at 200 OC for 12 h. The calcined silica was transferred to a glovebox, where it was stored under nitrogen. The solvents toluene and diethyl ether were refluxed over sodium/ benzophenone and then distilled under nitrogen immediately prior to use. to Ti02).3 (2) 'Department of Chemical Engineering. *Department of Chemistry. Table I. Comparison of Vibrational Frequencies of [Rh(allyl), ] and {Si}-0-Rh(a1lyl) KBr disk of IRh(allyl), I, allyl vibrations of {SibO-Rh(allyl),, cm-' a b C cm-' assignment 3055 3055 3050 3061 m A"v(CH ,) 2992 3000 m A'v(CH,) 2942 2960 w 2900 3000-2800 2928 w A' + a"v(CH,) 2851 2850 vw 2350 vw 2350 w, br 1760 w A"6 (CH ,) 1491 1490 1483 m 1463 1460 1453 m A'6 (CH, ) 1473 a This work. Reference 2a. Reference 2d. Reference 7. The complexes [Rh(l,5-hexadiene)Cll2 and [C,HSMgBr] were prepared by literature method^;^^^ [Rh(allyl),] was prepared by a literature me- thod,6a slightly modified, as follows: [C,HSMgBr] was added to [Rh- (1,5-hexadiene)C1I2 under nitrogen and stirred at -78 zyx OC, and the re- sultant mixture was warmed to 0 OC. A stream of air, having passed through a CaSO, trap at -78 OC, flowed into the solution, which was allowed to warm to room temperature. The product was purified by sublimation, and its purity was confirmed by comparison of the observed and reported infrared and 'H NMR The silica-supported rhodium allyl complex, represented as (Si]-0- Rh(allyl)*, was synthesized by the method of Ward et aL2"with each of the steps carried out under an inert atmosphere. In one preparation, giving the lowest loading of Rh on the surface, [Rh(allyl)3](0.0752 g) was dissolved in dry toluene (40 mL), and the solution was added drop- wise over a 2-h period to a toluene slurry of silica. During this time, the [Rh(allyl)J was adsorbed onto the silica, the supernatant solution be- coming colorless. The resulting golden-yellow solid was filtered and (1) Yermakov, Y. I.; Kuznetsov, B. N.; Zakharov, V. A. In "Catalysis by Supported Complexes"; Elsevier: Amsterdam, 198 1. (2) (a) Ward, M. D.; Harris, T. V.; Schwartz, J. zyxwv J. Chem. zyx SOC., Chem. Commun. 1980, 357. (b) Ward, M. D.; Schwartz, J. J. Mol. Catal. 1981, 11, 397. (c) Ward, M. D.; Schwartz, J. J. Am. Chem. SOC. 1981, 103, 5253. (d) Ward, M. D. Ph.D. Dissertation, Princeton University, 1981. (3) [R~(T-C~H~)CO(PP~~)~] on y-alumina has also been reported: Spek, Th. G.; Scholten, J. J. F. J. Mol. Cutul. 1977, 3, 81. (4) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1979, 19, 218. (5) Crummitt, zyxwv 0.; Budewite, E. P.; Chudd, C. C. Org. Synth. 1963, 4, (6) (a) Powell, J.; Shaw, B. L. J. Chem. SOC. A 1968, 583. (b) Powell, (7) Andrews, D. C.; Davidson, G. J. Organomet. Chem. 1973, 55, 383. 1748. J.; Shaw, B. L. J. Chem. SOC., Chem. Commun. 1966, 323. 0002-7863/83/1505-3074$01.50/0 0 1983 American Chemical Society