Mass spectrometric and theoretical study on the formation of uranyl hydride from uranyl carboxylate

Abstract

Uranyl hydride in the form of HUO₂Cl₂⁻ was prepared upon collision-induced dissociation of (RCO₂)UO₂Cl₂⁻ (R = H, CH₃CH₂, CH₃CH₂CH₂, CH₃CHCH, (CH₃)₂CH, C₅H₉, C₆H₁₁ and C₆H₅CH₂CH₂) in the gas phase. It was found that uranyl hydrides result from alkene and alkyne elimination with concomitant β-hydride transfer of uranyl alkylides RUO₂Cl₂⁻ following decarboxylation of the carboxylates with the exception of (HCO₂)UO₂Cl₂⁻, and formation of HU^VIO₂Cl₂⁻ through alkene/alkyne loss is in competition with neutral ligand loss to give U^VO₂Cl₂⁻. According to the calculations at the B3LYP level, loss of a neutral ligand is slightly less favorable in the cases of (CH₃CH₂)UO₂Cl₂⁻ and (CH₃CH₂CH₂)UO₂Cl₂⁻, and the situations of (CH₃CHCH)UO₂Cl₂⁻, ((CH₃)₂CH)UO₂Cl₂⁻, (C₅H₉)UO₂Cl₂⁻, (C₆H₁₁)UO₂Cl₂⁻ and (C₆H₅CH₂CH₂)UO₂Cl₂⁻ with β-hydrogen atoms should be similar despite the fact that the yield of uranyl hydride depends on the nature of the ligand. Although no uranyl hydride was observed when β-hydrogen is not available in the carboxylate precursor, there is no HUO₂Cl₂⁻ generated from (C₆H₅CO₂)UO₂Cl₂⁻, (2-C₆H₄FCO₂)UO₂Cl₂⁻ and (CH₂CHCH₂CO₂)UO₂Cl₂⁻ with β-hydrogen either. This is attributed to the much more favorable formation of UO₂Cl₂⁻ over HUO₂Cl₂⁻ as revealed by the B3LYP calculations, which is similar to the absence of HUO₂Cl₂⁻ in the (CH₃CO₂)UO₂Cl₂⁻ case where highly reactive CH₂ would be formed.