Efforts to replace precious metal photocatalysts like Ru(bpy)<sub>3</sub> with more sustainable solutions had resulted in reports of numerous first row transition metal complexes, with a special emphasis on Fe complexes. Fe(II) complexes have same electronic structure as Ru(II) photocatalysts, however, due to the smaller ligand field strength Fe(II) complexes have worse photophysical properties compared to Ru photocatalysts. Because of that finding better Fe based alternatives remain important step in this field. One approach to improve properties of Fe complexes is using strong σ donor and π acceptor ligands to make high-symmetry complexes in order to induce high ligand fields. Additionally, it was shown lately that higher oxidation states complexes like Fe(III) exhibit better properties than Fe(II) analogues. Here a bidentate C-N type ligand was used to synthesize a homoleptic, distorted octahedral Fe(III) complex. Chemical reduction and oxidation yielded Fe(II) and Fe(IV) analogues of this complex. <sup>57</sup>Mössbauer, EPR, NMR, UV-Vis-NIR, SC-XRD spectroscopy as well as DFT calculations were used to gain insight into electronic structure and bonding in these complexes. Photophysical properties as well as influence of different oxidation states of the metal center were studied by transient absorption spectroscopy. Obtained results indicate that used ligand produces strong field which leads to the low spin Fe(III) complex with distorted octahedral symmetry. Even though ligands can potentially act as redox non-innocent, reduction and oxidation are metal centered, with preserving high symmetry of these complexes. Additionally, changing oxidation states of the metal center leads to the dramatic changes in photophysical properties. In general, this method demonstrates a new approach toward synthesizing homoleptic cyclometalated Fe complexes with C-N type ligands.

Synthesis and characterization of homoleptic cyclometalated iron complexes

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