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) Å) 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 [Cp 2 Fe][BF 4 ] gave the cobalt(I) complex devoid of N 2 ligand [(ICy) 3 Co][BF 4 ] ( 3 ). The bis(dinitrogen)cobalt(−1) complexes 2a – c undergo protonation reaction with triflic acid to give N 2 H 4 in 24–30% yields (relative to cobalt). Complexes 2a – c could also react with silyl halides to afford diazene complexes [(ICy) 2 Co(η 2 -R 3 SiNNSiR 3 )] (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 [η 2 -R 3 SiNNSiR 3 ] 2– . Complexes 1, 2a – c, 6a, 6b, and (ICy) 2 CoCl 2 proved to be effective catalysts for the reductive silylation of N 2 to afford N(SiMe 3 ) 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.