Rhodium(I) Complexes of New Ferrocenyl Benzimidazol-2-ylidene Ligands: The Importance of the Chelating Effect for Ketone Hydrosilylation Catalysis

Graphical Abstract

Precursors of new ferrocenylphosphane-benzimidazol-2-ylidene bifunctional ligands and monodentate ferrocenyl benzimidazol-2-ylidene ligands have been prepared. The cationic and neutral rhodium(I) complexes showed good activities for the catalytic hydrosilylation of ketones. The use of bidentate ligands proved to be essential for obtaining good selectivities of the desired alcohol.

Abstract

N-[(1-Phosphanylferrocen-1′-yl)methyl]-N′-[(2,4,6-trimethylphenyl)methyl]-5,6-di-X-benzimidazolium tetrafluoroborate salts (X = H, 5a and Me, 5b), precursors of new phosphane-benzimidazol-2-ylidene bifunctional ligands, and related ferrocenyl 5,6-di-X-benzimidazol-2-ylidene iodide salts (X = H, 6a and Me, 6b), precursors of monodentate benzimidazol-2-ylidene ligands, have been prepared for the first time. Cationic rhodium(I) complexes 7a and 7b and neutral rhodium(I) complexes 8a and 8b have been obtained in good yields and have been fully characterised. Cationic rhodium(I) complexes 10a and 10b were prepared from 8a and 8b, for comparison with complexes 7, but not isolated. All complexes showed good activities for the catalytic hydrosilylation of acetophenone derivatives. The activities are much greater than for related imidazol-2-ylidene systems, and the cationic complexes are more active than the neutral complexes; the highest activity is observed for the more soluble complex 7b. The use of bidentate ligands proved to be essential for obtaining good selectivities of the desired alcohol.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)