Bimetallic Co-M (M = Cu, Ag, and Au) Carbonyl Complexes Supported by N-Heterocyclic Carbene Ligands: Synthesis, Structures, Computational Investigation, and Catalysis for Ammonia Borane Dehydrogenation

The reaction of Na[Co(CO)4] with M(IPr)Cl (M = Cu, Ag, and Au; IPr = C3N2H2(C6H3iPr2)2) affords the neutral heterometallic complexes [Co(CO)4{M(IPr)}] (M = Cu, 1; Ag, 2; and Au, 3). Formation of 2 is accompanied by traces of [Ag(IPr)2][Ag{Co(CO)4}2] (4). The reaction of Na[Co(CO)4] with M(IMes)Cl (IMes = C3N2H2(C6H2Me3)2) results in mixtures of [Co(CO)4{M(IMes)}] (M = Cu, 5; Ag, 6; and Au, 7) and [M(IMes)2][M{Co(CO)4}2] (M = Cu, 8; Ag, 9; and Au, 10). In the cases of Cu and Ag, ionic complexes 8 and 9 are the major products, whereas neutral species 7 is the major product for Au. All species 1-10 have been spectroscopically characterized by IR and 1H and 13C{1H} NMR spectroscopy. Moreover, the molecular structures of 2, 3, and 8 have been determined by single-crystal X-ray diffraction (SC-XRD). Bimetallic Co-M-NHC complexes 1-3 and 7-9 have been tested as catalysts for the dehydrogenation of ammonia-borane (AB) in THF as solvent, and their performances compared to [Fe(CO)4{M(NHC)}2], M(NHC)Cl, and Na[Co(CO)4]. DFT computations have been performed to provide information on the structure, IR spectroscopy, and the thermodynamics of Co-M carbonyl clusters.