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 (NCNme) or ethylene (NCNet) 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(NCNme)­(COD)]­BPh4 (8), [Ir­(NCNme)­(COD)]­BPh4 (10), [Rh­(NCNet)­(CO)2]­BPh4 (13), and [Ir­(NCNme)­(CO)2]­BPh4 (14), and monodentate NHC coordination
was observed for [Ir­(NCNme)2(COD)]­BPh4 (11) and [Ir­(NCNet)2(COD)]­BPh4 (12). Both tridentate and bidentate coordination
modes were characterized for [Rh­(NCNet)­(COD)]­BPh4 (9) in the solution and solid state, respectively,
while an unusual bridging mode was observed for the bimetallic complex
[Rh­(μ-NCNme)­(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.