Pentacoordinated rhodium(I) complexes supported by coumarin-functionalized N-heterocyclic carbene ligands

Abstract

New coumarin-tethered benzimidazolium (BzICouRHCl) and imidazolium (ICouRHCl) salts have been prepared as precursors for coumarin–NHC rhodium(I) complexes RhCl(NHC)(cod). Trigonal bypiramidal pentacoordinated bis-coumarin–NHC rhodium(I) species, RhCl(NHC)2, can be obtained by heating rhodium-cod derivatives in the presence of coumarin–azolium salts and a base. These unusual species are stabilized by coordination of the unsaturated bond of both coumarin moieties by the same enantioface. The allyl substituent on doubly functionalized NHC competes for coordination with coumarin wingtips. DFT calculations upon coordination of the olefin moieties support the experimental results.