Bimetallic Cobalt-Dinitrogen Complexes: Impact of the Supporting Metal on N₂ Activation

Expanding a family of cobalt bimetallic complexes, we report the synthesis of the Ti(III) metalloligand, Ti[N(o-(NCH₂P(ⁱPr)₂)C₆H₄)₃] (abbreviated as TiL), and three heterobimetallics that pair cobalt with an early transition metal ion: CoTiL (1), K(crypt-222)[(N₂)CoVL] (2), and K(crypt-222)[(N₂)CoCrL] (3). The latter two complexes, along with previously reported K(crypt-222)[(N₂)CoAlL] and K(crypt-222)[(N₂)Co₂L], constitute an isostructural series of cobalt bimetallics that bind dinitrogen in an end-on fashion, i.e. [(N₂)CoML]^-. The characterization of 1-3 includes cyclic voltammetry, X-ray crystallography, and infrared spectroscopy. The [CoTiL]^0/- reduction potential is extremely negative at -3.20 V versus Fc⁺/Fc. In the CoML series where M is a transition metal, the reduction potentials shift anodically as M is varied across the first-row period. Among the [(N₂)CoML]^- compounds, the dinitrogen ligand is weakly activated, as evidenced by N-N bond lengths between 1.110(8) and 1.135(4) Å and by N-N stretching frequencies between 1971 and 1995 cm^-1. Though changes in ν_N2 are subtle, the extent of N₂ activation decreases across the first-row period. A correlation is found between the [CoML]^0/- reduction potentials and N₂ activation, where the more cathodic potentials correspond to lower N-N frequencies. Theoretical calculations of the [(N₂)CoML]^- complexes reveal important variations in the electronic structure and Co-M interactions, which depend on the exact nature of the supporting metal ion, M.