Bis(dinitrogen)cobalt(-1) Complexes with NHC Ligation: Synthesis, Characterization, and Their Dinitrogen Functionalization Reactions Affording Side-on Bound Diazene Complexes.

Late-transition-metal-based catalysts are widely used in N fixation reactions, but the reactivity of late-transition-metal N complexes, besides iron N complexes, has remained poorly understood as their N complexes were thought to be labile and hard to functionalize. By employing a monodentate N-heterocyclic carbene (NHC), 1,3-dicyclohexylimidazol-2-ylidene (ICy) as ligand, the cobalt(0)- and cobalt(-1)-N complexes, [(ICy)Co(N)] (1) and [(ICy)Co(N)M] (M = K, 2a; Rb, 2b; Cs, 2c), respectively, were synthesized from the stepwise reduction of (ICy)CoCl by the corresponding alkaline metals under a N atmosphere. Complexes 2a-c in their solid states adopt polymeric structures. The N-N distances (1.145(6)-1.162(5) Å) and small N-N infrared stretchings (ca. 1800 and 1900 cm) suggest the strong N activation of the end-on N ligands in 2a-c. One electron oxidation of 1 by [CpFe][BF] gave the cobalt(I) complex devoid of N ligand [(ICy)Co][BF] (3). The bis(dinitrogen)cobalt(-1) complexes 2a-c undergo protonation reaction with triflic acid to give NH in 24-30% yields (relative to cobalt). Complexes 2a-c could also react with silyl halides to afford diazene complexes [(ICy)Co(η-RSiNNSiR)] (R = Me, 6a; Et, 6b) that are the first diazene complexes of late transition metals prepared from N functionalization. Characterization data, in combination with calculation results, suggest the electronic structures of the diazene complexes as low-spin cobalt(II) complexes containing dianionic ligand [η-RSiNNSiR]. Complexes 1, 2a-c, 6a, 6b, and (ICy)CoCl proved to be effective catalysts for the reductive silylation of N to afford N(SiMe). These NHC-cobalt catalysts display comparable turnover numbers (ca. 120) that exceed the reported 3d metal catalysts. The fine performance of the NHC-cobalt complexes in the stoichiometric and catalytic N-functionalization reactions points out the utility of low-valent low-coordinate group 9 metal species for N fixation.