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 La 3+ , Ce 3+ , Fe 3+ , Cu 2+ , and UO 2 2+ were investigated by electrospray ionization mass spectrometry, and the corresponding reaction mechanisms were explored by density functional theory (DFT) calculations. Fluoride transfer product La III FCl 3 - /Ce III FCl 3 - and decarboxylation product La III Cl 3 (C 6 H 4 F) - /Ce III Cl 3 (C 6 H 4 F) - were observed when the carboxylate precursors La III Cl 3 (C 6 H 4 FCO 2 ) - /Ce III Cl 3 (C 6 H 4 FCO 2 ) - 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 CO 2 /C 6 H 4 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 Fe III Cl 3 (C 6 H 4 FCO 2 ) - /Cu II Cl 2 (C 6 H 4 FCO 2 ) - are dominated by the reduction product FeCl 3 - /CuCl 2 - regardless of the fluorobenzoate isomer. DFT/B3LYP calculations show that the two-step CO 2 /C 6 H 4 F 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 U VI O 2 Cl 2 (C 6 H 4 FCO 2 ) - isomers, the ortho isomer behaves more similarly to La III Cl 3 (C 6 H 4 FCO 2 ) - /Ce III Cl 3 (C 6 H 4 FCO 2 ) - as evidenced by the formation of U VI O 2 FCl 2 - , and the appearance of U V O 2 Cl 2 - in the cases of the meta and para isomers indicates the similarity with Fe III Cl 3 (C 6 H 4 FCO 2 ) - /Cu II Cl 2 (C 6 H 4 FCO 2 ) - . The shorter U-F distance in U VI O 2 Cl 2 ( o -C 6 H 4 F) - causes the decrease in the fluoride transfer barrier and thus makes this process more favorable over o -C 6 H 4 F radical loss to give U V O 2 Cl 2 - .