Alkene Hydrogenation on an 11-Vertex Rhodathiaborane with Full Cluster Participation

摘要

The facile synthesis of the metallaheteroborane [8,8-(PPh_3)_2-nido-8,7-RhSB)9H_(10)] (1) makes possible the systematic study of its reactivity. Addition of pyridine to 1 gives in high yield the 11 -vertex nido-hydridorhodathiaborane [8,8,8-(PPh_3)_2H-9-(NC_5H_5)-nido-8,7-RhSB_9H_9] (2). 2 reacts with C_2H_4 or CO tl-form [1,1-(PPh_3)(L)-3-(NC_5H_5)-closo-RhSB_9H_8] [L = C_2H_4 (3), CO (4)]. In CH_2CI_2 at reflux temperature 2 undergoes a nido to closo transformation to afford [1,1-(PPh_3)_2-3-(NC_5H_5)-closo-1,2-RhSB9H8](5). Reaction of 2 with alkenes leads to hydrogenation and isomerization of the olefins. NMR spectroscopy indicates the presence of a labile phosphine ligand in 2, and DFT calculations have been used to determine which of the two phosphine groups is labile. Rationalization of the hydrogenation mechanism and the part played by the 2 → 3 nido to closo cluster change during the reaction cycle is suggested. In the proposed mechanism the classical hydrogen transfer from hydride metal complexes to olefins occurs twice: first upon coordination of the alkene to the rhodium centre in 2, and second concomitant with formation of a closo-hydridorho-dathiaborane intermediate by migration of a BHB-bridging hydrogen atom to the metal. Reaction of H_2 with 3 or 5 regenerates 2, closing a reaction cycle that under catalytic conditions is capable of hydrogenating alkenes. Single-site versus cluster-bifunctional mechanisms are discussed as possible routes for H_2 activation.