Base-Free Transfer Hydrogenation of Ketones Using Arene Ruthenium(II) Complexes

Ruthenium derivatives of the type [RuCl(arene)(NN)][BPh₄] (arene = benzene, p-cymene) have been synthesized with NN = 2-hydroxyphenylbis(pyrazol-1-yl)methane (bpzmArOH) or 2-hydroxyphenylbis(3,5-dimethylpyrazol-1-yl)methane (bpz*mArOH). In the p-cymene derivative containing bpzmArOH, activation of the hydroxy group is observed and a scorpionate complex is obtained with the ligand behaving in a tridentate manner, [Ru(bpzmArO-κ³-N,N,O)(p-cymene)][BPh₄]. The structure of this derivative and that of [RuCl(p-cymene)(bpz*mArOH)][BPh₄] were determined by X-ray diffraction. The new derivatives, along with other compounds previously described by us that contain similar ligands, were tested in the transfer hydrogenation of benzophenone and other carbonyl compounds under base-free conditions without any other additive to promote the reaction. The precursors were active in this process, and the p-cymene derivatives exhibited a higher activity than the benzene complexes. For the p-cymene complexes the activity was even higher than that found in the presence of base (KOH). The effect of the substituents on the methylene carbon was also studied. Mechanistic and kinetic studies were carried out, and hydride species were observed after a pretreatment of the precatalyst in 2-propanol. Taking into account all of the results, a proposal for the mechanism taking place during the hydrogenation of carbonyl groups under base-free conditions has been made.