INTERCALATION OF CATALYTICALLY ACTIVE METAL-COMPLEXES IN MICA-TYPE SILICATES - RHODIUM HYDROGENATION CATALYSTS

Cationic hydrogenation catalyst precursors of the type RhLn+ (L = triphenylphosphine; n = 2, 3) have been intercalated in a swelling mica-type silicate (hectorite) by the reaction of triphenylphosphine with Rh2(CH3CO2)4-xx+ (x = 1,2) ions on the interlamellar cation exchange sites of the layered silicate. The reduction in methanol of a terminal olefin (1-hexene) with the intercalated complexes occurs without isomerization, whereas extensive isomerization to internal olefins is observed with the analogous catalyst system in homogeneous solution. The difference in specificity between the intercalated and homogeneous catalyst systems is accounted for in terms of the position of an equilibrium, previously proposed by Schrock and Osborn, between catalytically active dihydride and monohydride rhodium complexes: RhH2Ln+ ⇌ RhHLn + H+. The dihydride is a good terminal olefin hydrogenation catalyst but a poor isomerization catalyst, whereas the monohydride is both a good hydrogenation and isomerization catalyst. Relative to homogeneous solution, the dihydride is favored in the intercalated state because of a surface Brønsted acidity that is believed to arise from the dissociation of hydrated Na+ ions which are also present on the interlamellar surfaces. A dramatic enhancement in substrate selectivity is observed for reduction of alkynes to cis olefins with the intercalated catalyst. The initial rates of reduction of relatively small alkynes (1-hexyne, 2-hexyne) in interlayers swelled with methanol to an average thickness of 7.7 Å (= Δd001) are comparable to those observed with the homogeneous catalyst. With larger alkynes, such as diphenylacetylene, the intercalated rates may be up to 100 times lower than homogeneous rates. The spatial requirements of the substrate in the swelled interlayers are important in determining its reactivity with the intercalated catalyst. For example, the ratios of intercalated to homogeneous rates for the reduction of 2-decyne are 0.85 and 0.02, respectively, with CH2Cl2 (Δd001 = 10.0 Å) and C6H6 (Δd001 = 5.7 Å) as the swelling solvent. A binding model is proposed for the intercalated substrate-catalyst complex in which the spatial requirements of the substrate are determined by the minimum distance it must span when the coordinated C≡C bond is oriented perpendicular to the silicate sheets. © 1979, American Chemical Society. All rights reserved.