Acid‐, Water‐ and High‐Temperature‐Stable Ruthenium Complexes for the Total Catalytic Deoxygenation of Glycerol to Propane

• Published in: Chemistry : a European journal 2009-10, Vol.15 (39), p.10132-10143
• Main Authors: Taher, Deeb, Thibault, Michelle E., Di Mondo, Domenico, Jennings, Michael, Schlaf, Marcel
• Format: Article
• Language: English
• Subjects: Acids - chemistry; biomass; Catalysis; Crystallography, X-Ray; deoxygenation; glycerol; Glycerol - chemistry; homogeneous catalysis; Hydrogenation; Ketones - chemistry; Molecular Structure; Organometallic Compounds - chemistry; Propane - chemical synthesis; Propane - chemistry; Propylene Glycol - chemical synthesis; Propylene Glycol - chemistry; Ruthenium - chemistry; Temperature; Water
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200901427

The ruthenium aqua complexes [Ru(H2O)2(bipy)2](OTf)2, [cis‐Ru(6,6′‐Cl2‐bipy)2(OH2)2](OTf)2, [Ru(H2O)2(phen)2](OTf)2, [Ru(H2O)3(2,2′:6′,2′′‐terpy)](OTf)2 and [Ru(H2O)3(Phterpy)](OTf)2 (bipy=2,2′‐bipyridine; OTf−=triflate; phen=phenanthroline; terpy= terpyridine; Phterpy=4′‐phenyl‐2,2′:6′,2′′‐terpyridine) are water‐ and acid‐stable catalysts for the hydrogenation of aldehydes and ketones in sulfolane solution. In the presence of HOS(O)2CF3 (triflic acid) as a dehydration co‐catalyst they directly convert 1,2‐hexanediol to n‐hexanol and hexane. The terpyridine complexes are stable and active as catalysts at temperatures ≥250 °C and in either aqueous sulfolane solution or pure water convert glycerol into n‐propanol and ultimately propane as the final reaction product in up to quantitative yield. For the terpy complexes the active catalyst is postulated to be a carbonyl species [(4′‐R‐2,2′:6′,2′′‐terpy)Ru(CO)(H2O)2](OTf)2 (R=H, Ph) formed by the decarbonylation of aldehydes (hexanal for 1,2‐hexanediol and 3‐hydroxypropanal for glycerol) generated in the reaction mixture through acid‐catalyzed dehydration. The structure of the dimeric complex [{(4′‐phenyl‐2,2′:6′,2′′‐terpy)Ru(CO)}2(μ‐OCH3)2](OTf)2 has been determined by single crystal X‐ray crystallography (Space group P$\bar 1$ (a=8.2532(17); b=12.858(3); c=14.363(3) Å; α=64.38(3); β=77.26(3); γ = 87.12(3)°, R=4.36 %). Going all the way on the first date! Using triflic acid in combination with a high‐temperature‐, acid‐ and water‐stable homogeneous ruthenium hydrogenation catalyst, the reduction of glycerol cannot be stopped at alcohol intermediates, but yields the total deoxygenation product propane (see scheme).