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 La^IIIFCl₃^–/Ce^IIIFCl₃^– and decarboxylation product La^IIICl₃(C₆H₄F)⁻/Ce^IIICl₃(C₆H₄F)⁻ were observed when the carboxylate precursors La^IIICl₃(C₆H₄FCO₂)⁻/Ce^IIICl₃(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 Fe^IIICl₃(C₆H₄FCO₂)⁻/Cu^IICl₂(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 U^VIO₂Cl₂(C₆H₄FCO₂)⁻ isomers, the ortho isomer behaves more similarly to La^IIICl₃(C₆H₄FCO₂)⁻/Ce^IIICl₃(C₆H₄FCO₂)⁻ as evidenced by the formation of U^VIO₂FCl₂^–, and the appearance of U^VO₂Cl₂^– in the cases of the meta and para isomers indicates the similarity with Fe^IIICl₃(C₆H₄FCO₂)⁻/Cu^IICl₂(C₆H₄FCO₂)⁻. The shorter U–F distance in U^VIO₂Cl₂(o-C₆H₄F)⁻ causes the decrease in the fluoride transfer barrier and thus makes this process more favorable over o-C₆H₄F radical loss to give U^VO₂Cl₂^–.