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(<i>o-</i>(NCH₂P­(^<i>i</i>Pr)₂)­C₆H₄)₃] (abbreviated as TiL), and three heterobimetallics that pair cobalt with an early transition metal ion: CoTiL (<b>1</b>), K­(crypt-222)­[(N₂)­CoVL] (<b>2</b>), and K­(crypt-222)­[(N₂)­CoCrL] (<b>3</b>). 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 <b>1</b>–<b>3</b> 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