Ruthenium(ii) arene complexes bearing simple dioxime ligands: effective catalysts for the one-pot transfer hydrogenation/N-methylation of nitroarenes with methanol. Issue 7 (9th March 2023)

• Record Type: Journal Article
• Authors: Colaiezzi, Roberta; Saviozzi, Chiara; di Nicola, Nicola; Zacchini, Stefano; Pampaloni, Guido; Crucianelli, Marcello; Marchetti, Fabio; Di Giuseppe, Andrea; Biancalana, Lorenzo
• Is Part Of: Catalysis science & technology. Volume 13:Issue 7(2023)
• Page Start: 2160
• Page End: 2183
• Publication Date: 2023-03-09
• Subjects: Catalysis -- Periodicals; 541.395
• DOI: 10.1039/d3cy00218g
• Languages: English
• ISSNs: 2044-4753

Abstract

Abstract : Ruthenium(ii ) η 6 -arene complexes with easily-available dioxime ligands are efficient catalytic precursors for the selective reduction/ N -methylation of aromatic nitrocompounds 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 ) η 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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(κ 2 N -dioxime)(η 6 - p -cymene)] + (Cl/dimethylglyoxime, [1 ] + ; Cl/nioxime, [2 ] + ; I/nioxime, [3 ] + ), [RuCl(κ 2 N -dioxime)(η 6 -C6 Me6 )] + (dimethylglyoxime, [4 ] + ; nioxime, [5 ] + ; diphenylglyoxime, [6 ] + ) and [RuCl(κ 2 N -nioxime)(η 6 -1, 3, 5-C6 H3 Me3 )] + ([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, t BuOK, 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 η 6 -arene ligand is proposed as the activation step of the pre-catalyst.