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 HUVIO2Cl2− through alkene/alkyne loss is in competition with neutral ligand loss to give UVO2Cl2−. 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.