Uranyl‐Bound Tetra‐Dentate Non‐Innocent Ligands: Prediction of Structure and Redox Behaviour Using Density Functional Theory

Computational methods have been applied to understand the reduction potentials of [UO2‐salmnt‐L] complexes (L=pyridine, DMSO, DMF and TPPO), and their redox behavior is compared with previous experiments in dichloromethane solution. Since the experimental results were inconclusive regarding the influence of the uranyl‐bound tetra‐dentate ‘salmnt’ ligand, here we will show that salmnt acts as a redox‐active ligand and exhibits non‐innocent behavior to interfere with the otherwise expected one‐electron metal (U) reduction. We have employed two approaches to determine the uranyl (VI/V) reduction potentials, using a direct study of one‐electron reduction processes and an estimation of the overall reduction using isodesmic reactions. Hybrid density functional theory (DFT) methods were combined with the Conductor‐like Polarizable Continuum Model (CPCM) to account for solvation effects. The computationally predicted one‐electron reduction potentials for the range of [UO2‐salmnt‐L] complexes are in excellent agreement with shoulder peaks (∼1.4 eV) observed in the cyclic voltammetry experiments and clearly correlate with ligand reduction. Highly conjugated pi‐bonds stabilize the ligand based delocalized orbital relative to the localized U f‐orbitals, and as a consequence, the ligand traps the incoming electron. A second reduction step results in metal U(VI) to U(V) reduction, in good agreement with the experimentally assigned uranyl (VI/V) reduction potentials. Which is reduced? Uranyl bound tetra‐dentate ‘salmnt’ ligand interferes and participates in the reduction event and exhibits redox‐active (non‐innocent) behaviour. Highly conjugated pi‐bonds stabilize the ligand based delocalized molecular orbital (MO) relative to the metal (U) based localized f‐orbitals and traps the incoming electron. However, if L=none or L=water, and addition of a counter cation results in U reduction.