σ-CAM Mechanisms for the Hydrogenation of Alkenes by cis- and trans-Disilametallacyclic Carbonyl Complexes (M = Fe, Ru, Os): Experimental and Theoretical Studies

Abstract

Abstract The hydrogenation of alkenes catalyzed by disilametallacyclic carbonyl complexes of iron, ruthenium or osmium was studied experimentally and theoretically. The disilaruthenacycle 2 with two CO ligands in the trans-configuration was prepared, characterized, and its ability to catalyze hydrogenation was studied. Similar to the corresponding iron analogue 1 in which the CO ligands are in the cis-configuration, 2 contains a H2MSi4 core with Si⋯H⋯Si SISHA (secondary interaction of silicon and hydrogen atoms) and catalyzed the hydrogenation of several alkenes under mild conditions. DFT calculations of 1 and 2 with cis- and trans-CO configurations (cis-1, trans-1, cis-2 and trans-2) revealed that the mechanism of ethylene hydrogenation comprises three catalytic cycles, and a key step involves the H-H bond of H2 being activated by an M-Si bond through oxidative hydrogen migration. These mechanisms are a variety of σ-CAM (σ-complex-assisted metathesis) mechanisms. Further calculations suggest that these catalytic cycles can apply to the catalytic hydrogenation of ethylene by osmium analogues of 1 and 2 (cis-3 and trans-3). Some of the elementary reactions in the cycles are dependent on the metal, and the osmium complexes show different performance from the iron and ruthenium analogues due to the characteristic natures of the third-row transition metals.