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 ( tripp BAP) 3 , 2 ) complex ( Inorg. Chem . 2022 , 61 (32), 12508-12517) demonstrated a silent 31 P 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 ( mes BAP) 4 , 1 . Herein, with the help of relativistic density functional theory (DFT) calculations, the electronic structures of 1 , 2 , and U( mes BAP) 3 ( 4 ) are examined in an effort to understand the unusual 31 P 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 31 P 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.