Redox State-Dependent Dinitrogen Binding by an Isocyanide Analogue of 15e^- CpCr(CO)₂

The low-valent 15e^- monovalent chromium complex, Cp*Cr(CNAr^Dipp2)₂ (2) (CNAr^Dipp2 = 2,6-(2,6-(ⁱPr)₂C₆H₃)₂C₆H₃), was isolated and serves as an isolobal analogue of the unstable, monomeric CpCr(CO)₂ fragment. Complex 2 was found to have an unusual intermediate spin ground state (S = 3/2), which enables reversible N₂ binding to afford the 17e^- low-spin (S = 1/2) complex, Cp*Cr(N₂)(CNAr^Dipp2)₂ (2-N₂), which is a rare example of an N₂-induced spin state change. Complex 2 demonstrates distinct redox activity, which allowed for a stepwise investigation of its N₂ binding ability as a function of its redox state. While 1e^- reduction of 2 or 2-N₂ under an N₂ atmosphere suppresses N₂ dissociation to give a saturated dinitrogen-bound monoanionic species, KCp*CrN₂(CNAr^Dipp2)₂ (4), further reduction leads to complete N₂ dissociation to afford the dianionic unsaturated metalloradical, K₂Cp*Cr(CNAr^Dipp2)₂ (5). This demonstrates how N₂ binding at metal centers is affected by subtle changes to electronic structure, a concept of critical importance to catalytic N₂ reduction.