Synthesis and reactivity of new complexes of rhodium and iridium with bis(dichloroimidazolylidene) ligands.: Electronic and catalytic implications of the introduction of the chloro substituents in the NHC rings

The preparation of a new bis(dichloroimidazolylidene) ligand has provided chelate-N-heterocyclic complexes of Rh(I) and Ir(I), which have been fully characterized. The crystal structures of three of the new complexes are described. The study of the electronic properties of the new ligands was made on the basis of the nu(CO) stretching frequencies of the carbonyl derivatives, showing that the chloroimidazolylidene ligand is significantly less sigma-donating than the related nonchlorinated analogue. This electronic modification of the ligand has important implications for the catalytic properties of the compounds obtained, as observed from enhanced activity shown in catalytic hydrosilylation of terminal acetylenes and cyclization of acetylenic carboxylic acids, compared to the complexes with the nonchlorinated NHC analogue ligand.