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.