Substituent effects in the decarboxylation reactions of coordinated arylcarboxylates in dinuclear copper complexes, [(napy)Cu2(O2CC6H4X)]+

A combination of gas-phase ion trap mass spectrometry experiments and density functional theory (DFT) calculations have been used to examine the role of substituents on the decarboxylation of 25 different coordinated aromatic carboxylates in binuclear complexes, [(napy)Cu₂(O₂CC₆H₄X)]⁺, where napy is the ligand 1,8-naphthyridine (molecular formula, C₈H₆N₂) and X = H and the ortho (o), meta (m) and para (p) isomers of F, Br, CN, NO₂, CF₃, OAc, Me and MeO. Two competing unimolecular reaction pathways were found: decarboxylation to give the organometallic cation [(napy)Cu₂(C₆H₄X)]⁺ or loss of the neutral copper benzoate to yield [(napy)Cu]⁺. The substituents on the aryl group influence the branching ratios of these product channels, but decarboxylation is always the dominant pathway. Density functional theory calculations reveal that decarboxylation proceeds via two transition states. The first enables a change in the coordination mode of the coordinated benzoate in [(napy)Cu₂(O₂CC₆H₄X)]⁺ from the thermodynamically favoured O,O-bridged form to the O-bound form, which is the reactive conformation for the second transition state which involves extrusion of CO₂ with concomitant formation of the CO₂ coordinated organometallic cation, [(napy)Cu₂(C₆H₄X)(CO₂)]⁺, which then loses CO₂ in the final step to yield [(napy)Cu₂(C₆H₄X)]⁺. In all cases the barrier is highest for the second transition state. The o-substituted benzoates show a lower activation energy than the m-substituted ones, while the p-substituted ones have the highest energy, which is consistent with the experimentally determined normalised collision energy required to induce fragmentation of [(napy)Cu₂(O₂CC₆H₄X)]⁺.