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.