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-2 fixation reactions, but the reactivity of late-transition-metal N-2 complexes, besides iron N-2 complexes, has remained poorly understood as their N-2 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-2 complexes, [(ICy)(3)Co(N-2)] (1) and [(ICy)(2)Co(N-2)(2)M](n) (M = K, 2a; Rb, 2b; Cs, 2c), respectively, were synthesized from the stepwise reduction of (ICy)(3)CoCl by the corresponding alkaline metals under a N-2 atmosphere. Complexes 2a-c in their solid states adopt polymeric structures. The N-N distances (1.145(6)-1.162(5) angstrom) and small N-N infrared stretchings (ca. 1800 and 1900 cm(-1)) suggest the strong N-2 activation of the end-on N-2 ligands in 2a-c. One electron oxidation of 1 by [Cp2Fe] [BF4] gave the cobalt(I) complex devoid of N-2 ligand [(ICy)(3)Co] [BF4] (3). The bis(dinitrogen)cobalt(-1) complexes. 2a-c undergo protonation reaction with triflic acid to give N2H4 in 24-30% yields (relative to cobalt). Complexes 2a-c could also react with silyl halides to afford diazene complexes [(ICy)(2)Co(eta(2)-R3SiNNSiR3)] (R = Me, 6a; Et, 6b) that are the first diazene complexes of late transition metals prepared from N-2 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 [eta(2)-R3SiNNSiR3](2-). Complexes 1, 2a-c, 6a, 6b, and (ICy)(2)CoCl2 proved to be effective catalysts for the reductive silylation of N-2 to afford N(SiMe3)(3). 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-2-functionalization reactions points out the utility of low-valent low-coordinate group 9 metal species for N-2 fixation.