Bis(acyl)phosphide Complexes of U(III)/U(IV): A Case of a Hidden Redox-Active Ligand

The recently reported tris(bis(2,4,6-triisopropylbenzoyl)-phosphide)uranium (U^III(^trippBAP)₃, 2) complex (Inorg. Chem. 2022, 61 (32), 12508-12517) demonstrated a silent ³¹P NMR spectrum. This complex was described as a U(III) complex with an organic radical ligand fragment. Moreover, the EPR spectrum of 2 was indicative of an organic radical in the ligand framework complexed to uranium, in contrast to that of U^IV(^mesBAP)₄, 1. Herein, with the help of relativistic density functional theory (DFT) calculations, the electronic structures of 1, 2, and U(^mesBAP)₃ (4) are examined in an effort to understand the unusual ³¹P NMR spectrum of 2. Results indicate the reduction of the carbonyl bonds and delocalization of the electrons over the ligands, indicative of U → L backbonding. Additionally, the reduced acyl carbons are found to exist as ketyl radicals [O═C^• -] that are responsible for the silent ³¹P NMR spectra of 2. These findings demonstrate the redox noninnocent nature of BAP^- in 2 and 4, causing uranium to exist in a formal oxidation state of +4.