Three-Coordinate and Four-Coordinate Cobalt Hydride Complexes That React with Dinitrogen

We report the formation and N2 reactivity of novel cobalt hydride complexes supported by bulky β-diketiminate ligands (L). Addition of KHBEt3 to LCoCl gives [LCo(μ-H)]2 (1) or K2[LCoH]2 (2), depending on the amount of borohydride used. Compound 2 is the first example of a crystallographically characterized hydride complex in which a transition metal is three-coordinate. Both 1 and 2 react with N2 at room temperature to give dinuclear N2 complexes with loss of H2. Complexes of a transition metal (defined here as a metal with a partially filled d shell) having only three bonds to the metal are interesting because the orbital energies, spin states, and reaction pathways can be different than traditional complexes.1 Isolating three-coordinate complexes invariably depends on the use of extremely bulky supporting ligands, which protect the metal center through steric effects. However, this strategy might seem to be incompatible with three-coordinate hydride complexes, because H – is the smallest possible ligand. Accordingly, no three-coordinate hydride complexes are known.2 New kinds of hydride complexes are desired because of the many roles for hydrides in organometallic chemistry and catalysis.