Hemilabile and Bimetallic Coordination in Rh and Ir Complexes of NCN Pincer Ligands

Two new pincer ligands have been developed that contain a central N-heterocyclic carbene (NHC) moiety linked to two pendant pyrazole groups by either a methylene (NCN(me)) or ethylene (NCN(et)) chain. The coordination of these two ligands to rhodium and iridium resulted in a variety of binding modes. Tridentate coordination of the ligands was observed in the complexes [Rh(NCN(me))(COD)]BPh4 (8), [Ir(NCN(me))(COD)]BPh4 (10), [Rh(NCN(et))(CO)2]BPh4 (13), and [Ir(NCN(me))(CO)2]BPh4 (14), and monodentate NHC coordination was observed for [Ir(NCN(me))2(COD)]BPh4 (11) and [Ir(NCN(et))2(COD)]BPh4 (12). Both tridentate and bidentate coordination modes were characterized for [Rh(NCN(et))(COD)]BPh4 (9) in the solution and solid state, respectively, while an unusual bridging mode was observed for the bimetallic complex [Rh(μ-NCN(me))(CO)]2(BPh4)2 (15). The impact of this diverse coordination chemistry on the efficiency of the complexes as catalysts for the addition of NH, OH, and SiH bonds to alkynes was explored.