Decarboxylation of fatty acids by ternary zinc cationic complexes studied by mass spectrometry and theoretical calculations

We have shown that fatty acids can be decarboxylated in the gas phase via collision-induced dissociation (CID) of ternary zinc complexes of the general formula [(L)Zn(OOCR)]⁺, where RC₃H₇, C₆H₁₃, C₇H₁₅, C₁₇H₃₅, C₁₇H₃₃, C₁₇H₃₁ and L = 1,10-phenanthroline or 2,2′:6′,2″-terpyridine. These complexes were formed by electrospray ionization of a solution mixture of zinc(II) salt, fatty acid, and the ligand. Two pathways were mapped for the resulting [(L)ZnR]⁺ organozinc ion: i) a gas-phase ion-molecule reaction with a neutral carboxylic acid R’COOH in the linear quadrupole ion trap resulted in the production of the alkane RH and formation of [(L)Zn(OOCR’)]⁺; ii) a second CID stage produced an alkene and the ion [(L)ZnH]⁺ which underwent a subsequent ion-molecule reaction with R’COOH to form [(L)Zn(OOCR’)]⁺ ion accompanied by the loss of H₂. In the case of RR’ = C₃H₇ (butyric acid), both processes presented formal catalytic cycles with the overall equations C₃H₇COOHC₃H₈+ CO₂ or C₃H₇COOHC₃H₆ + H₂+ CO₂. Theoretical DFT calculations provided the energetics of reactants/products as well as transition state energies and were in agreement with the experimental data. The work identifies Zn as a potential catalyst for deoxygenation of fatty acid with no fragmentation of the alkyl chain.