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

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.