Isostructural\nSeries of a Cyclometalated Iron Complex\nin Three Oxidation States

An isostructural series of Fe<sup>II</sup>, Fe<sup>III</sup>, and\nFe<sup>IV</sup> complexes [Fe(ImP)<sub>2</sub>]<sup>0/+/2+</sup> utilizing\nthe ImP 1,1′-(1,3-phenylene)bis(3-methyl-1-imidazol-2-ylidene)\nligand, combining <i>N</i>-heterocyclic carbenes and cyclometalating\nfunctions, is presented. The strong donor motif stabilizes the high-valent\nFe<sup>IV</sup> oxidation state yet keeps the Fe<sup>II</sup> oxidation\nstate accessible from the parent Fe<sup>III</sup> compound. Chemical\noxidation of [Fe(ImP)<sub>2</sub>]<sup>+</sup> yields stable [Fe<sup>IV</sup>(ImP)<sub>2</sub>]<sup>2+</sup>. In contrast, [Fe<sup>II</sup>(ImP)<sub>2</sub>]<sup>0</sup>, obtained by reduction, is highly\nsensitive toward oxygen. Exhaustive ground state characterization\nby single-crystal X-ray diffraction, <sup>1</sup>H NMR, Mössbauer\nspectroscopy, temperature-dependent magnetic measurements, a combination\nof X-ray absorption near edge structure and valence-to-core, as well\nas core-to-core X-ray emission spectroscopy, complemented by detailed\ndensity functional theory (DFT) analysis, reveals that the three complexes\n[Fe(ImP)<sub>2</sub>]<sup>0/+/2+</sup> can be unequivocally attributed\nto low-spin d<sup>6</sup>, d<sup>5</sup>, and d<sup>4</sup> complexes.\nThe excited state landscape of the Fe<sup>II</sup> and Fe<sup>IV</sup> complexes is characterized by short-lived <sup>3</sup>MLCT and <sup>3</sup>LMCT states, with lifetimes of 5.1 and 1.4 ps, respectively.\nIn the Fe<sup>II</sup>-compound, an energetically low-lying MC state\nleads to fast deactivation of the MLCT state. The distorted square-pyramidal\nstate, where one carbene is dissociated, can not only relax into the\nground state, but also into a singlet dissociated state. Its formation\nwas investigated with time-dependent optical spectroscopy, while insights\ninto its structure were gained by NMR spectroscopy.