The preparation of pyridylimine Mn(I) and Ru(II) complexes as catalyst precursors in formic acid dehydrogenation

Abstract

A series of known and new pyridylimine ligands L1 – L9 were synthesized in high yields (79 –96%) from the condensation reaction between the appropriate pyridine-2-carboxaldehyde and amine derivatives. All the ligands were fully characterized by a range of analytical techniques. Reactions of ligands L1 – L9 with MnBr(CO)5 yielded the new neutral manganese(I) complexes C1 – C9 in moderate to good yields (50 – 84%). The complexes were characterized by NMR and FT-IR spectroscopy, mass spectrometry, and elemental analysis. Single-crystal X-ray diffraction analysis confirmed the molecular connectivity for the targeted ligands and Mn(I) complexes. Evidence of light-induced Mn-CO bond dissociation accompanied by an increase in the Mn metal center's oxidation state (+1 to +2) was shown using FT-IR and SCXRD. The pyridylimine Mn(I) complexes C1 – C9 were evaluated and found to exhibit poor to no activity toward the dehydrogenation of FA. Varying several reaction parameters such as solvent, base, temperature, and catalyst loading, was futile. The poor activity of these complexes was attributed to their light-sensitive nature in solution and the generation of unproductive Mn(II) catalytic species during the reaction. The reaction of ligands L1 – L9 with [RuCl2(p-cymene)]2 generated cationic, half-sandwich ruthenium complexes C10 – C20 in good yields (73 – 87%). The complexes were characterized by FT-IR and NMR spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray diffraction analysis. The cationic Ru(II) complexes were found to be active precatalysts for the base/additive-assisted dehydrogenation of FA. The performance of C10 – C18 was largely influenced by the pyridylimine ligand framework, as shown by catalyst screening studies. Varying the nature of the counterion on the cationic complexes did not affect FA dehydrogenation in DMSO. Complex C1 displayed solvent-dependent catalytic activities in DMSO and water. The activation energies were estimated to be 84 and 108 kJ/mol for reactions performed in DMSO and water, respectively. The key Ru-monohydride species (d = -11 ppm) in the catalytic cycle was identified using 1 H NMR spectroscopy. KIE experiments indicated decarboxylation as the rate-determining step in both DMSO and water. Based on the kinetic and spectroscopic investigations, plausible pathways for the dehydrogenation of FA over C10 in DMSO and water were proposed. Catalyst recyclability studies indicated increased reusability of C10 in water with TON = 6748 after six FA addition cycles compared to DMSO (TON ~ 3000).