Influence of Metal Coordination on the Gas-Phase Chemistry of the Positional Isomers of Fluorobenzoate Complexes

The fragmentation behaviors of the o-, m-, and p-fluorobenzoate complexes of La3+, Ce3+, Fe3+, Cu2+, and UO22+ were investigated by electrospray ionization mass spectrometry, and the corresponding reaction mechanisms were explored by density functional theory (DFT) calculations. Fluoride transfer product LaIIIFCl3-/CeIIIFCl3- and decarboxylation product LaIIICl3(C6H4F)-/CeIIICl3(C6H4F)- were observed when the carboxylate precursors LaIIICl3(C6H4FCO2)-/CeIIICl3(C6H4FCO2)- were subjected to collision-induced dissociation. The variation in product ratios, which is not obvious in the meta and para cases, qualitatively follows the increasing overall energy barrier and reaction endothermicity of the two-step CO2/C6H4 elimination mechanism, and this aligns with the increase in U-F distance in the ortho, meta, and para decarboxylation product isomers. In contrast, the mass spectra of FeIIICl3(C6H4FCO2)-/CuIICl2(C6H4FCO2)- are dominated by the reduction product FeCl3-/CuCl2- regardless of the fluorobenzoate isomer. DFT/B3LYP calculations show that the two-step CO2/C6H4F elimination pathways are comparable in energy for all three positional isomers. It is energetically more favorable to give the reduction product than the fluoride transfer product, which is opposite to the lanthanum cases. Although the decarboxylation product was observed for all three UVIO2Cl2(C6H4FCO2)- isomers, the ortho isomer behaves more similarly to LaIIICl3(C6H4FCO2)-/CeIIICl3(C6H4FCO2)- as evidenced by the formation of UVIO2FCl2-, and the appearance of UVO2Cl2- in the cases of the meta and para isomers indicates the similarity with FeIIICl3(C6H4FCO2)-/CuIICl2(C6H4FCO2)-. The shorter U-F distance in UVIO2Cl2(o-C6H4F)- causes the decrease in the fluoride transfer barrier and thus makes this process more favorable over o-C6H4F radical loss to give UVO2Cl2-.