Dual-Ligand Strategy for the Preparation of Gas-Phase Uranyl(VI) Benzyne Complexes from Uranyl(VI) Benzoates.

The uranyl(VI) benzyne complex (η2-C6H4)UO2Cl- was prepared in the gas phase by electrospray ionization mass spectrometry coupled with collision-induced dissociation. It was formed via a dual-ligand strategy that requires the elimination of benzoic acid or benzene/CO2 from the uranyl dibenzoate precursor (C6H5CO2)2UO2Cl-. This contrasts the known strategy for the formation of gas-phase benzyne complexes that would result from CO2/HCl elimination from (C6H5CO2)UO2Cl2-, during which only one benzoate ligand is involved. Such dual-ligand strategy can be extended to the preparation of a series of methyl- and halo-substituted benzyne complexes of uranyl(VI). Density functional theory calculations at the B3LYP level reveal that the benzyne complex (η2-C6H4)UO2Cl- features a metallacyclopropene structure with the C6H42- ligand coordinated to uranium(VI) through two polarized U-Cbenzyne σ bonds, in accordance with the reactivity test toward water. Dehydrochlorination of the benzyne complex (η2-C6H4)UO2Cl- from (C6H5)UO2Cl2- that originates from decarboxylation of (C6H5CO2)UO2Cl2- with a single benzoate ligand is neither kinetically nor thermodynamically favorable than simple C6H5 radical loss to give UVO2Cl2-. This arises from the presence of an accessible V oxidation state for uranium and accounts for the necessity for the dual-ligand strategy in the preparation of uranyl(VI) benzyne complexes from uranyl benzoate precursors.