Development of ruthenium complexes with S-donor ligands for application in synthesis, catalytic acceptorless alcohol dehydrogenation and crossed-aldol condensation

Abstract

The reaction of [Ru(dmso)₄Cl₂] with a potassium salt of four xanthate (RO–C(S)S⁻; R = Me, Et, ⁱPr and ^tBu) ligands (depicted as Lⁿ; n = 1–4) in hot methanol afforded a group of mixed-ligand complexes of type [Ru(Lⁿ)₂(dmso)₂]. The crystal structures of all the four complexes have been determined, which show that the xanthate ligands are bound to the metal center forming four-membered chelates and dmso is coordinated through sulfur and they are mutually cis. The relative thermodynamic stability of this cis and the other possible trans-isomers of these complexes has been assessed with the help of DFT calculations, which have revealed that the cis-isomer is the more stable isomer. The coordinated dmso in the [Ru(Lⁿ)₂(dmso)₂] complexes could be easily displaced by chelating bidentate ligands (depicted as L′) to furnish complexes of type [Ru(Lⁿ)₂(L′)], as demonstrated through isolation of two such complexes, viz. [Ru(L³)₂(bpy)] and [Ru(L²)₂(phen)] (bpy = 2,2′-bipyridine and phen = 1,10-phenanthroline). The crystal structure of [Ru(L³)₂(bpy)] has been determined and the structure of [Ru(L²)₂(phen)] has been optimized by the DFT method. The electronic spectra of the four [Ru(Lⁿ)₂(dmso)₂] complexes and the two derivatives ([Ru(Lⁿ)₂(L′)]; n = 3, L′ = bpy; n = 2, L′ = phen), recorded in dichloromethane solutions, show intense absorptions spanning the visible and ultraviolet regions, which have been analyzed by the TDDFT method. The [Ru(Lⁿ)₂(dmso)₂] complexes are found to serve as efficient catalyst precursors for the acceptorless dehydrogenation of 2-propanol followed by crossed-aldol condensation with substituted benzaldehydes (and related aldehydes), using tert-butoxide as the co-catalyst, producing dibenzylideneacetone derivatives in good yields.