Structural criteria for the activity of rhodium(I) phosphine complexes in the catalytic dehydrocoupling of di-n-hexylsilane

The activity of several neutral, rhodium(I) bis(phosphine) complexes for the dehydrogenative coupling of di-n-hexylsilane to two- and three-silicon chains was investigated and discussed in the context of the coordination numbers and geometries likely to be critical to catalyst activity. Wilkinson’s dimer, [Rh(μ-Cl)(PPh3)2]2, shows the highest activity, while [Rh(μ-Cl)(dppe)]2, with its rigidly cis-chelating diphosphine ligand, shows negligible activity for the dehydrocoupling reaction, even when the reaction is carried out in methylene chloride, in which both catalyst and silane are soluble. A dramatic decrease in the activity of Wilkinson’s dimer when the reaction is carried out in toluene or methylene chloride solution instead of neat silane is attributed to rate-limiting escape of the product hydrogen gas away from the solvated active catalyst.