Tuning the Electronic Properties of Tetravalent Cerium Complexes via Ligand Derivatization

Molecular cerium complexes are of interest due to their remarkable redox and photophysical properties. We have investigated the ligand tunability of the electronic structure and properties of cerium(IV) complexes with functionalized tetradentate N₂O₂-donor ligands: [Ce^IV(L_tBu)₂] (1), [Ce^IV(L_H)₂] (2) and [Ce^IV(L_NO2)₂] (3), where H₂L_tBu = bis(2-hydroxy-3,5-di-tert-butylbenzyl)(2-pyridylmethyl)amine, H₂L_H = bis(2-hydroxybenzyl)(2-pyridylmethyl)amine and H₂L_NO2 = bis(2-hydroxy-5-nitrobenzyl)(2-pyridylmethyl)amine. These compounds all exhibit a quasi-reversible one-electron reduction to cerium(III), with the redox potential correlating with the electron donor-acceptor characteristics of the ligand substituents. This correlation is rationalized by energy stabilization of the HOMO, as determined by density functional theory calculations, and is consistent with arene π → Ce 4f* ligand-to-metal charge transfer bands. The L₃-edge XANES exhibits minimal variation in Ce 4f occupation for the three compounds, which suggests that the 4f covalent character and composition of the ground-state character do not vary significantly across the series. However, M_4,5-edge XAS shows charge transfer satellites that subtly differ in shape and energy, indicating small distinctions in ligand-to-metal charge transfer for the compounds, consistent with small differences in temperature-independent magnetism. The ability to modulate the redox and optical properties of cerium complexes through ligand derivatization highlights the potential for customizable molecular cerium catalysts and photocatalysts.