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.