Synthesis and catalytic performance of ruthenium complexes ligated with rigid o-(diphenylphosphino)aniline for chemoselective hydrogenation of dimethyl oxalate

Abstract

A series of new ruthenium complexes with rigid ligand o-(diphenylphosphino)aniline, including [(PPh₃)(o-PPh₂C₆H₄NH₂)RuCl₂]₂ (1), (o-PPh₂C₆H₄NH₂)₂RuCl₂ (2), [(o-PPh₂C₆H₄NH₂)₂(o-PPh₂C₆H₄NH)Ru]⁺Cl⁻ (3), Ph₃P(η²-H₂)Ru(μ-H)(μ-o-PPh₂C₆H₄NH)₂RuH(PPh₃) (4), (o-PPh₂C₆H₄NH₂)(o-PPh₂C₆H₄NH)RuCl(CO) (5), (o-PPh₂C₆H₄NH₂)(o-PPh₂C₆H₄NH)RuH(CO) (6), and [(o-PPh₂C₆H₄NH)₂Ru(CO)]₂ (7) were synthesized and employed as catalysts for chemoselective hydrogenation of esters. Among them, complexes 1, 2, and 5 exhibited excellent performance in hydrogenation of dimethyl oxalate to methyl glycolate, in comparison with the ruthenium complexes with a flexible aminophosphine ligand, such as (Ph₂P(CH₂)₂NH₂)₂RuCl₂, (Ph₂P(CH₂)₃NH₂)₂RuCl₂, and (o-Ph₂PC₆H₄CH₂NH₂)₂RuCl₂, under identical conditions. Complexes 1 and 2 also displayed good activities in the hydrogenation of other aliphatic and cyclic esters. The catalytic mechanism of hydrogenation was discussed according to the results of NMR spectroscopic studies and control experiments.