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

ABSTRACT The low‐valent 15e − monovalent chromium complex, Cp*Cr(CNAr Dipp2 ) 2 ( 2 ) (CNAr Dipp2 = 2,6‐(2,6‐( i Pr) 2 C 6 H 3 ) 2 C 6 H 3 ), 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 N 2 binding to afford the 17e − low‐spin ( S = 1/2) complex, Cp*Cr(N 2 )(CNAr Dipp2 ) 2 ( 2‐N 2 ), which is a rare example of an N 2 ‐induced spin state change. Complex 2 demonstrates distinct redox activity, which allowed for a stepwise investigation of its N 2 binding ability as a function of its redox state. While 1e – reduction of 2 or 2‐N 2 under an N 2 atmosphere suppresses N 2 dissociation to give a saturated dinitrogen‐bound monoanionic species, KCp*CrN 2 (CNAr Dipp2 ) 2 ( 4 ), further reduction leads to complete N 2 dissociation to afford the dianionic unsaturated metalloradical, K 2 Cp*Cr(CNAr Dipp2 ) 2 ( 5 ). This demonstrates how N 2 binding at metal centers is affected by subtle changes to electronic structure, a concept of critical importance to catalytic N 2 reduction.