Valence delocalization in ketenylidene bridged mixed-valence iron complexes

The carbide ligand in the iron-molybdenum cofactor (FeMoco) in nitrogenase bridges iron atoms in different oxidation states, yet it is difficult to discern its ability to mediate magnetic exchange interactions due to the structural complexity of the cofactor. We aim to understand how effectively carbon donors can mediate magnetic exchange interactions between iron sites in weak field environments. Here, we report the synthesis and characterization of two mixed-valent diiron complexes with C-based ketenylidene bridging ligands. The [Fe2(-CCO)2]+ complex with two bridges (4) displays valence delocalization on the Mössbauer timescale and a low-spin S = 1/2 ground state. In contrast, the [Fe2(-CCO)]+ complex with one bridge (5) shows a high-spin ground state (S = 7/2). Spectroscopic, magnetic, and computational studies of the latter reveal an Fe–Fe bonding interaction that leads to complete valence delocalization. Analysis of a near-IR intervalence charge transfer transition in 5 indicates a double exchange constant (B) of 950 cm–1, which is amongst the largest spectroscopically determined B values reported to date. These results show that carbon donors are extremely effective at bringing about valence delocalization in mixed-valent iron dimers, which may also be the case in the nitrogenase cofactors.