C2-Symmetric Ru(ii) bimetallic complex catalysed selective degradation and value addition of azo dyes

Abstract

The textile industry pollutes water with persistent azo dyes, yet their complete hydrogenation remains underexplored. Herein, we report for the first time the use of bimetallic Ru(II) complexes for catalytic azo bond activation, achieving complete degradation in both aqueous and organic media, offering a sustainable remediation approach. The bimetallic complexes Ru4 [(p-cymene)₂(RuCl)₂L⁴] and Ru5 [(p-cymene)₂(RuCl)₂L⁵] were derived from the precursor complex [(p-cymene)₂RuCl₂]₂ and Schiff base ligands 6′-((1E,1′E)-((3,3′,5,5′-tetraisopropyl-[1,1′-biphenyl]-4,4′-diyl)bis(azaneylylidene))bis(methaneylylidene)) bis(3,5-dibromophenol) (H₂L⁴) and 2-((E)-((4′-(((E)-(2-hydroxynaphthalen-1-yl)methylene)amino)-3,3′,5,5′-tetraisopropyl-[1,1′-biphenyl]-4-yl)imino)methyl)naphthalen-1-ol (H₂L⁵), respectively. This Ru(II) bimetallic catalytic system enables complete hydrogenation of azoarenes in water using formic acid/formate buffer as a hydride source. With a catalyst loading of just 0.1 mol% (Ru1 and Ru3–Ru5), azoarenes are efficiently converted to anilines in excellent yields. Furthermore, these catalysts facilitate the dehydrogenative coupling of azoarenes with benzyl alcohols, selectively yielding secondary amines or imines at a low catalyst loading (0.5 mol%). Mechanistic investigations using NMR, ESI-MS, and detailed density functional theory (DFT) calculations provide detailed insights into Ru(II)-mediated azo bond activation. This work broadens the scope of Ru(II)-catalyzed hydrogenation under mild, aqueous conditions and establishes a foundation for sustainable catalytic strategies in environmentally friendly transformations.