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

The uranyl(VI) benzyne complex (η 2 -C 6 H 4 )UO 2 Cl - 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/CO 2 from the uranyl dibenzoate precursor (C 6 H 5 CO 2 ) 2 UO 2 Cl - . This contrasts the known strategy for the formation of gas-phase benzyne complexes that would result from CO 2 /HCl elimination from (C 6 H 5 CO 2 )UO 2 Cl 2 - , 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 (η 2 -C 6 H 4 )UO 2 Cl - features a metallacyclopropene structure with the C 6 H 4 2- ligand coordinated to uranium(VI) through two polarized U-C benzyne σ bonds, in accordance with the reactivity test toward water. Dehydrochlorination of the benzyne complex (η 2 -C 6 H 4 )UO 2 Cl - from (C 6 H 5 )UO 2 Cl 2 - that originates from decarboxylation of (C 6 H 5 CO 2 )UO 2 Cl 2 - with a single benzoate ligand is neither kinetically nor thermodynamically favorable than simple C 6 H 5 radical loss to give U V O 2 Cl 2 - . 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.