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( i Pr)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 νN2 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.