Base-Free Transfer Hydrogenation with an Ionic-Liquid-Supported Ruthenium eta(6)-Arene Bis(pyrazolyl)methane Catalyst

Ruthenium complexes of the type [RuCl(eta(6)-arene)-(NN)]X (X = BF4 or BPh4) featuring an eta(6)-arene ligand modified with an imidazolium ring and incorporating the bis(azo) chelating (NN) ligands 2-hydroxyphenylbis(3,5-dimethylpyrazol-1-yl) methane or 2-nitrophenylbis(3,5-dimethylpyrazol-1-yl) methane are reported. The catalytic activities of the complexes were evaluated in the transfer hydrogenation of benzaldehyde to benzyl alcohol with iPrOH as the hydrogen donor in the absence of a base and under biphasic conditions; one of the two phases was iPrOH, and the other was an ionic liquid. The catalysts featuring the imidazolium-modified eta(6)-arene could be recycled and reused with only a minor decrease in substrate conversion after multiple reaction cycles. The reaction rate was strongly influenced by the type of ionic liquid employed in the biphasic system.