Hydrogenation and hydrogenolysis of furfural and furfuryl alcohol catalyzed by ruthenium(II) bis(diimine) complexes

The catalytic activity of ruthenium(II) bis(diimine) complexes cis-[Ru(6,6′-Cl2bpy)2(OH2)2](Z)2 (1, Z=CF3SO3; 2, Z= (3,5-(CF3)2C6H3)4B, i.e. BArF) and cis-[Ru(4,4′-Cl2bpy)2(OH2)2](Z)2 (3, Z=CF3SO3; 4, Z=BArF) for the hydrogenation and/or the hydrogenolysis of furfural (FFR) and furfuryl alcohol (FFA) was investigated. Themolecular structures of cis-[Ru(4,4′-Cl2bpy)2(CH3CN)2](CF3SO3)2 (3′) and dimeric cis-[(Ru(4,4′-Cl2bpy)2Cl)2](BArF)2 (5) were characterized by X-ray crystallography. The structures are consistent with the anticipated reduction in steric hindrance about the ruthenium centers in comparison with corresponding complexes containing 6,6′-Cl2bpy ligands.While compounds 1–4 are all active and highly selective catalysts for the hydrogenation of FFR to FFA undermodest reaction conditions, 3 and 4 showed decreased activity. This is best explained in terms of reduced Lewis acidity of the Ru centers and reduced steric hindrance about the metal centers of catalysts 3 and 4. cis-[Ru(6,6′-Cl2bpy)2(OH2)2](BArF)2 (2) also displayed high catalytic efficiency for the hydrogenation of FFA to tetrahydrofurfuryl alcohol. Presumably, this is because coordination of C=C bonds of FFA to the ruthenium center is poorly inhibited by non-coordinating BArF counterions. Interestingly, cis-[Ru(6,6′-Cl2bpy)2(OH2)2] (CF3SO3)2 (1) showed some catalytic activity in ethanol for the hydrogenolysis of FFA to 2-methylfuran, albeit with fairly modest selectivity. Nonetheless, these results indicate that ruthenium(II) bis(diimine) complexes need to be further explored as catalysts for the hydrogenolysis of C-O bonds of FFR, FFA, and related compounds. Copyright © 2012 John Wiley & Sons, Ltd.