Organo-f-Element-Based Heterogeneous Catalysts. Kinetics and Mechanism of Olefin Hydrogenation by Supported Pentamethylcyclopentadienyl Actinide Complexes

This contribution reports a detailed kinetic and mechanistic study of heterogeneous olefin hydrogenation by Cp'2Th(CH3)2 (Cp'= 7j5-(CH3)5C5) adsorbed on dehydroxylated alumina (DA). For both propylene and isobutylene, the kinetics can be accommodated by a two-step sequence: (i) olefin insertion into Th-H (rate constant k2); (ii) hydrogenolysis of the resulting Th-alkyl bond (rate constant V3). Kinetic data can be fit to the rate law Nt (turnover frequency) = [formula-omitted], where the reaction is zero-order in olefin and where i/2/x3 = 0.55 bar-1 (-63°C) and 0.47 bar-1 (0°C) for propylene and isobutylene, respectively. Kinetic isotope measurements indicate [formula-omitted] for propylene hydrogenation at -63°C and 2.0(2) for isobutylene hydrogenation at 0°C. In reaction with D2, propylene yields exclusively l,2-propane-d2. Arrhenius activation energies for catalytic hydrogenation are 3.6(2) kcal mol1 (propylene, -63 to -23°C) and 5.3(2) kcal mol1 (isobutylene, -23 to +64°C); for isobutylene, it could be determined that the activation enthalpies for olefin insertion and alkyl hydrogenolysis are comparable (6.2(10) and 4.2(10) kcal mol'1, respectively). Protonolytic poisoning experiments indicate that <4 ± 1% and >2 ± 1 % of the thorium surface sites are responsible for the bulk of the propylene and isobutylene hydrogenation, respectively. As a function of olefin, the relative rates of Cp'2Th(CH3)2/DA-catalyzed hydrogenation are cis-2-butene > trans-2-butene > propylene > isobutylene; 1,3-butadiene yields only 1-butene and cis-2-butene as initial hydrogenation products. For several organoactinides supported on DA, the relative ordering of propylene hydrogenation activity is [formula-omitted]. As a function of support, the relative rates of Cp'2Th(CH3)2catalyzed propylene hydrogenation are DA > dehydroxylated Si02-Al203 > dehydroxylated MgCl2 dehydroxylated Si02-Mg0. Protonolytic poisoning experiments indicate that 35 ± 10% of the Cp'2Th(CH3)2/MgCl2 sites are catalytically significant. Informative parallels can be drawn between these observations and homogeneous solution phenomenology for similar types of compounds.