When Donors Turn into Acceptors: Ground and Excited State Properties of FeII Complexes with Amine-Substituted Tridentate Bis-imidazole-2-ylidene Pyridine Ligands.

In search of new ligand motifs for photoactive iron(II) complexes with long-lived MLCT states, a series of six complexes with tridentate amine-functionalized bis-n-heterocyclic carbene (NHC)-pyridine ligands is presented. In the homoleptic complexes imidazole-, methylimidazole-, or benzimidazole-2-ylidene, NHC donors are employed in combination with pyridine, functionalized in the 4-position by dimethylamine or dibenzylamine. The effects of these different functionalities on the electronic structure of the complexes are examined through detailed ground state characterization by NMR, single crystal X-ray diffraction, as well as electrochemical and spectroscopic methods. The net influence of these different functionalities on orbital-orbital and electrostatic ligand-iron interactions is investigated thoroughly by density functional theory, and changes in the excited state behavior and lifetimes are finally examined by ultrafast optical spectroscopy. Great deviations of the initially expected effects by substitution in 4-position on the photochemical properties are observed, together with a significantly increased π-acceptor interaction strength in the benzimidazole-2-ylidene functionalized complexes.

Electronic Structure of the Hieber Anion [Fe(CO)3(NO)]- Revisited by X-ray Emission and Absorption Spectroscopy.

While the Hieber anion [Fe(CO)3(NO)]- has been reincarnated in the last years as an active catalyst in organic synthesis, there is still a debate about the oxidation state of the central Fe atom and the resulting charge of the NO ligand. To shed new light on this question and to understand the Fe-NO interaction in the Hieber anion, it is investigated in comparison to the formal 3d8 reference Fe(CO)5 and the formal 3d10 reference [Fe(CO)4]2- by the combination of valence-to-core X-ray emission spectroscopy (VtC-XES), X-ray absorption near-edge structure spectroscopy (XANES), and high-energy-resolution fluorescence-detected XANES. In order to extract information about the electronic structure, time-dependent density functional theory and ground-state density functional theory calculations are applied. This combination of experimental and computational methods reveals that the electron density at the Fe center of the Hieber resembles that of the isoelectronic [Fe(CO)4]2-. These observations challenge recent descriptions of the Hieber anion and reopen the debate about the experimentally and computationally determined Fe oxidation state and charge on the NO ligand.