Redox and Spin States Series of an Organometallic Heme Analogue Based on a Non-Innocent NHC/N-Donor Hybrid Macrocycle

Iron complexes of tetradentate macrocyclic ligands containing <i>N</i>-heterocyclic carbene (NHC) donors have been referred to as organometallic heme analogues, but they usually lack the redox noninnocence under oxidizing conditions that is characteristic of porphyrins. Here we report a novel NHC/N-donor hybrid macrocyclic ligand containing two <i>trans</i> NHC moieties, a pyridine and a redox active carbazolide fragment. Its Fe^II, Fe^III and formal Fe^IV complexes have been isolated and comprehensively characterized, where UV/vis and ⁵⁷Fe Mössbauer spectroscopies, SQUID magnetometry and density functional theory (DFT) calculations reveal that the latter are best described as Fe^III systems antiferromagnetically coupled to a carbazolide-based organic π-radical. Two different redox series are obtained depending on the axial ligands: nitriles such as MeCN give low-spin (LS) configurations of the metal ion, while in case of weakly coordinating solvents and triflate anions the iron adopts an intermediate-spin (IS) configuration; MeCN binding constants have been determined. As in other heme analogues with NHC-based macrocycles, the strong equatorial σ-donor character raises the energy of the Fe(d_{<i>x</i>²-<i>y</i>²}) orbital, making high-spin (HS) iron species inaccessible. The combined features of equatorial ligand redox noninnocence, restriction to LS/IS surfaces and tunability via the axial coligands makes this a promising platform for bioinspired reactivity such as the generation of reactive Fe/O_<i>x</i> intermediates.