Observation of Carbodicarbene Ligand Redox Noninnocence in Highly Oxidized Iron Complexes.

To probe the possibility that carbodicarbenes (CDCs) are redox active ligands, all four members of the redox series [Fe(1)2 ]n+ (n=2-5) were synthesized, where 1 is a neutral tridentate CDC. Through a combination of spectroscopy and DFT calculations, the electronic structure of the pentacation is shown to be [FeIII (1.+ )2 ]5+ (S= 1 / 2 ). That of [Fe(1)2 ]4+ is more ambiguous, but it has significant contributions from the open-shell singlet [FeIII (1)(1.+ )]4+ (S=0). The observed spin states derive from antiferromagnetic coupling of their constituent low-spin iron(III) centres and cation radical ligands. This marks the first time redox activity has been observed for carbones and expands the diverse chemical behaviour known for these ligands.