Solvent-free formic acid dehydrogenation in pure base catalyzed by p-cymene-Ru complexes with methyl-piperidines

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Abstract
Consumption of formic acid (FA) is observed in neat triethylamine (TEA) in the presence of 2-, 3-, or 4-methylpiperidine-coordinated Ru-(η6-p-cymene) complexes, with the formation of molecular hydrogen and carbon dioxide as the only gaseous products, as confirmed by CG-TCD analysis. The reactions were kinetically monitored by measuring the gas volume, which increased with increasing complex concentration (4–13 μmol) and temperature (70–100 °C). No significant difference in the initial reaction rates was observed upon changing the amine ancillary ligand. All reactions reached the same saturation yields, which decreased in the presence of water compared with reactions carried out in neat TEA. A reaction mechanism is proposed based on in situ 1H NMR measurements. The chloride ion in the precursor complex is replaced by a formate ion through reaction with an FA/TEA adduct, followed by CO2 elimination to form a metal hydride species. Subsequently, the hydride ion is protonated by another FA/TEA adduct, leading to H2 evolution and regeneration of the cyclic formate complex. The most novel finding is that, in the latter step, the η6-p-cymene ligand undergoes a hapticity shift to η2-p-cymene, which may facilitate its displacement. Nevertheless, the resulting lower-coordinate amine complex remains catalytically active through a parallel catalytic cycle.

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