Towards Iron(II) Complexes with Octahedral Geometry: Synthesis, Structure and Photophysical Properties.

The control of ligand-field splitting in iron (II) complexes is critical to slow down the metal-to-ligand charge transfer (MLCT)-excited states deactivation pathways. The gap between the metal-centered states is maximal when the coordination sphere of the complex approaches an ideal octahedral geometry. Two new iron(II) complexes ( and ) prepared from pyridylNHC and pyridylquinoline type ligands, respectively, have a near-perfect octahedral coordination of the metal. The photophysics of the complexes have been further investigated by means of ultrafast spectroscopy and TD-DFT modeling. For it is shown that-despite the geometrical improvement-the excited state deactivation is faster than for the parent pseudo-octahedral complex. This unexpected result is due to the increased ligand flexibility in that lowers the energetic barrier for the relaxation of MLCT into the MC state. For , the effect of the increased ligand field is not strong enough to close the prominent deactivation channel into the metal-centered quintet state, as for other Fe-polypyridine complexes.