In situ generation of water-stable and -soluble ruthenium complexes of pyridine-based chelate-ligands and their use for the hydrodeoxygenation of biomass-related substrates in aqueous acidic medium

Article abstract of DOI:10.1016/j.molcata.2015.08.026

The complexes [Ru(2,2′-dipicolylamine)(OH₂)₃](OTf)₂ and [Ru(6,6′-bis(aminomethyl)-2,2′-bipyridine)(OH₂)₂](OTf)₂ can be prepared by reaction of 2,2′-dipicolylamine or 6,6′-bis(aminomethyl)-2,2′-bipyridine with [Ru^III(DMF)₆](OTf)₃ in aqueous medium. During the reaction an in situ reduction from a paramagnetic Ru^III to a diamagnetic Ru^II-complexes occurs with one equivalent of DMF acting as the reducing agent for two ruthenium centres by its reaction with water and decomposition to dimethylammonium triflate and CO₂ generating an additional equivalent of HOTf in the process. The complex solutions are active as catalysts for the hydrogenation of 2,5-hexanedione and 2,5-dimethylfuran to 2,5-hexanediol and 2,5-dimethyltetrahydrofuran with both complexes realizing very high yields (>95% combined yield of the two products with the selectivity determined as a function of added acid co-catalyst). The 2,2′-dipicolylamine complex is stable to 150?°C, while the 6,6′-bis(aminomethyl)-2,2′-bipyridine complex is stable to 200?°C allowing the in situ hydrolysis of 2,5-dimethylfuran to the 2,5-hexanedione and thus direct conversion to the same products in up to 78% combined yield. The effects of co-solvents, acid co-catalysts and temperature on catalyst activity, decomposition and stability are explored.