Gas-phase models for the nickel- and palladium-catalyzed deoxygenation of fatty acids

Using fatty acids as renewable sources of biofuels requires deoxygenation. While a number of promising catalysts have been developed to achieve this, their operating mechanisms are poorly understood. Here, model molecular systems are studied in the gas phase using mass spectrometry experiments and DFT calculations. The coordinated metal complexes [(phen)M(O₂CR)]⁺ (where phen=1,10‐phenanthroline; M=Ni or Pd; R=C_nH_2n+1, n≥2) are formed via electrospray ionization. Their collision‐induced dissociation (CID) initiates deoxygenation via loss of CO₂ and [C,H₂,O₂]. The CID spectrum of the stearate complexes (R=C₁₇H₃₅) also shows a series of cations [(phen)M(R’)]⁺ (where R’ < C₁₇) separated by 14 Da (CH₂) corresponding to losses of C₂H₄‐C₁₆H₃₂ (cracking products). Sequential CID of [(phen)M(R’)]⁺ ultimately leads to [(phen)M(H)]⁺ and [(phen)M(CH₃)]⁺, both of which react with volatile carboxylic acids, RCO₂H, (acetic, propionic, and butyric) to reform the coordinated carboxylate complexes [(phen)M(O₂CR)]⁺. In contrast, cracking products with longer carbon chains, [(phen)M(R)]⁺ (R>C₂), were unreactive towards these carboxylic acids. DFT calculations are consistent with these results and reveal that the approach of the carboxylic acid to the “free” coordination site is blocked by agostic interactions for R > CH₃.