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

Abstract

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. The molecular 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, 2, 3, 4 are all active and highly selective catalysts for the hydrogenation of FFR to FFA under modest reaction conditions, 3 and 4 showed decreased activity. This is best explained in terms of reduced Lewis acidity of the Ru2+ 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.