Isostructural Series of a Cyclometalated Iron Complex in Three Oxidation States.

An isostructural series of Fe, Fe, and Fe complexes [Fe(ImP)] utilizing the ImP 1,1'-(1,3-phenylene)bis(3-methyl-1-imidazol-2-ylidene) ligand, combining -heterocyclic carbenes and cyclometalating functions, is presented. The strong donor motif stabilizes the high-valent Fe oxidation state yet keeps the Fe oxidation state accessible from the parent Fe compound. Chemical oxidation of [Fe(ImP)] yields stable [Fe(ImP)]. In contrast, [Fe(ImP)], obtained by reduction, is highly sensitive toward oxygen. Exhaustive ground state characterization by single-crystal X-ray diffraction, H NMR, Mössbauer spectroscopy, temperature-dependent magnetic measurements, a combination of X-ray absorption near edge structure and valence-to-core, as well as core-to-core X-ray emission spectroscopy, complemented by detailed density functional theory (DFT) analysis, reveals that the three complexes [Fe(ImP)] can be unequivocally attributed to low-spin d, d, and d complexes. The excited state landscape of the Fe and Fe complexes is characterized by short-lived MLCT and LMCT states, with lifetimes of 5.1 and 1.4 ps, respectively. In the Fe-compound, an energetically low-lying MC state leads to fast deactivation of the MLCT state. The distorted square-pyramidal state, where one carbene is dissociated, can not only relax into the ground state, but also into a singlet dissociated state. Its formation was investigated with time-dependent optical spectroscopy, while insights into its structure were gained by NMR spectroscopy.