Activation of an Open Shell, Carbyne-Bridged Diiron Complex Toward Binding of Dinitrogen

Binding of N2 by nitrogenase requires a reductive activation of the FeMo-cofactor, but the precise structure and atomic composition of FeMoco in its activated form is not well understood. However, recent crystallographic studies suggest that N2 reduction may occur at a carbon-bridged diiron subunit of FeMoco. Toward modeling the activation of a Fe–(μ-C)–Fe site toward N2 binding, we synthesized a new dinucleating, hexaphosphine ligand derived from a 2,6-disubstituted toluene platform. Activation of the central methyl group of the ligand affords the diiron μ-carbyne complex (P6ArC)­Fe2(μ-H) featuring a biologically relevant Fe­(μ-carbyne)­(μ-H)Fe motif. SQUID magnetometry, Mössbauer spectroscopy, and DFT calculations reveal that (P6ArC)­Fe2(μ-H) has a well-isolated S = 1 ground state, distinguishing it from all other diiron μ-carbyne complexes which are diamagnetic. Upon the addition of sources of H+/e– (H2, TEMPO-H or HCl), (P6ArC)­Fe2(μ-H) is activated toward N2 binding, with concomitant protonation of the carbyne ligand. Although reaction with H2 ultimately leads to complete protonation of the carbyne moiety, mechanistic investigations indicate that formation of a single C–H bond, with concomitant cleavage of one Fe–C bond, generates an iron-carbene intermediate capable of coordinating N2.