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 (UIII(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 UIV(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.