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 N2 fixation reactions, but
the reactivity of late-transition-metal
N2 complexes, besides iron N2 complexes, has
remained poorly understood as their N2 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)–N2 complexes, [(ICy)3Co­(N2)] (1) and [(ICy)2Co­(N2)2M]n (M = K, 2a; Rb, 2b; Cs, 2c), respectively, were synthesized from the stepwise reduction
of (ICy)3CoCl by the corresponding alkaline metals under
a N2 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 N2 activation of the end-on N2 ligands
in 2a–c. One electron oxidation of 1 by [Cp2Fe]­[BF4] gave the cobalt­(I)
complex devoid of N2 ligand [(ICy)3Co]­[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)2Co­(η2-R3SiNNSiR3)] (R = Me, 6a; Et, 6b) that are
the first diazene complexes of late transition metals prepared from
N2 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 [η2-R3SiNNSiR3]2–. Complexes 1, 2a–c, 6a, 6b, and (ICy)2CoCl2 proved to be effective catalysts for the reductive
silylation of N2 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
N2-functionalization reactions points out the utility of
low-valent low-coordinate group 9 metal species for N2 fixation.