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

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 (H-2 donor), methylating agent and solvent. Ruthenium(ii) eta(6)-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, {RC(sic)N(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(kappa N-2-dioxime)(eta(6)-p-cymene)](+) (Cl/dimethylglyoxime, [1](+); Cl/nioxime, [2](+); I/nioxime, [3](+)), [RuCl(kappa N-2-dioxime)(eta(6)-C6Me6)](+) (dimethylglyoxime, [4](+); nioxime, [5](+); diphenylglyoxime, [6](+)) and [RuCl(kappa N-2-nioxime)(eta(6)-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, (BuOK)-Bu-t, 130 ?, 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 C-1 and H-2 source and gave insights about organic intermediates. Following in situ bis-deprotonation to the dioximato complex [3(-2H)](-), displacement of the eta(6)-arene ligand is proposed as the activation step of the pre-catalyst.