Ruthenium(ii) arene complexes bearing simple dioxime ligands: effective catalysts for the one-pot transfer hydrogenation/N-methylation of nitroarenes with methanol

Abstract

The reductive N-monomethylation of nitroarenes is a reaction of wide scientific interest that can be conveniently realized in one-pot using methanol as the reductant (H₂ donor), methylating agent and solvent. Ruthenium(II) η⁶-arene complexes are increasingly investigated as efficient catalytic precursors for (transfer) hydrogenation and dehydrogenation processes. In this framework, amino or alcohol groups working as proton-relay units are essential to realize a metal–ligand cooperative catalysis. Herein, starting from commercially available and inexpensive dioxime ligands, {RCN(OH)}₂, we developed novel ruthenium(II) arene complexes that efficiently catalyze the tandem reduction/N-methylation of aromatic nitrocompounds with methanol. Thus, the complexes [RuX(κ²N-dioxime)(η⁶-p-cymene)]⁺ (Cl/dimethylglyoxime, [1]⁺; Cl/nioxime, [2]⁺; I/nioxime, [3]⁺), [RuCl(κ²N-dioxime)(η⁶-C₆Me₆)]⁺ (dimethylglyoxime, [4]⁺; nioxime, [5]⁺; diphenylglyoxime, [6]⁺) and [RuCl(κ²N-nioxime)(η⁶-1,3,5-C₆H₃Me₃)]⁺ ([7]⁺) were isolated as nitrate or hexafluorophosphate salts in high yields and characterized by analytical (CHN content, conductivity) and spectroscopic (IR, NMR) techniques. Furthermore, three zwitterionic oxime–oximato derivatives were prepared (2^−H–4^−H) and the crystal structures of both dioxime ([3]⁺, [4]⁺) and oxime–oximato (4^−H) complexes were elucidated by X-ray diffraction. Following optimization of the reaction conditions for the one-pot reduction/N-methylation of nitrobenzene as benchmark substrate (MeOH, ^tBuOK, 130 °C, 18 h), the catalytic activity of the dioxime complexes [1–7]⁺ and selected ruthenium(II) compounds for comparative purposes was assessed. The best catalytic precursor, [3]NO₃, was effective in the conversion of a series of aromatic nitrocompounds into the respective N-methyl anilines, which were isolated as hydrochloride salts following silica chromatography. NMR and MS experiments carried out on the model catalytic system (nitrobenzene/[3]NO₃) confirmed the role of methanol as C₁ and H₂ source and gave insights about organic intermediates. Following in situ bis-deprotonation to the dioximato complex [3^−2H]⁻, displacement of the η⁶-arene ligand is proposed as the activation step of the pre-catalyst.