Dehydrogenation of formic acid by first-row transition-metal/crown ether complexes studied by mass spectrometry and theoretical calculations

We have investigated ternary cationic complexes of several first-row transition metals M²⁺ (M = Mn, Fe, Co, Ni, Cu and Zn) with three crown ethers L (where L = 12-crown-4, 15-crown-5, and 18-crown-6) with the goal of hydrogen production from formic acid. The ions of the general formula [LM(OOCH)]⁺ can be easily formed by electrospray ionization and decarboxylated in the gas phase via collision-induced dissociation (CID) with virtually no side reactions to form the corresponding hydride complexes [LMH]⁺. These hydride complexes were reacted with the neutral formic acid via a gas-phase ion-molecule reaction in the linear quadrupole ion trap which resulted in the production of the molecular hydrogen and re-formation of the initial [LM(OOCH)]⁺ ion. Thus, the process represented a formal catalytic cycle with the overall equation HCOOH = H₂+ CO₂. The experimental collision efficiencies of these reactions for 12-crown-4 complexes ranged from 0.2 % (Zn) to 100 %(Co) and followed the trend Co ≥ Ni ≥ Mn > Fe > Cu ≫ Zn. These results correlate very well with the theoretical DFT calculations for the transition state energies (barriers). Additional theoretical calculations looked at the structure of the complexes for the explanation of the observed trends. It was shown for Zn complexes that bigger crowns, especially 18-crown-6, can be significantly distorted resulting in one of O-Zn bonds being significantly shorter than the others. The work shows that crown ethers can serve as effective, flexible ligands for dehydrogenation of formic acid by first-row transition metal ions.