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

The reaction of [Ru(dmso) 4 Cl 2 ] with a potassium salt of four xanthate (RO-C(S)S - ; R = Me, Et, i Pr and t Bu) ligands (depicted as L n ; n = 1-4) in hot methanol afforded a group of mixed-ligand complexes of type [Ru(L n ) 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(L n ) 2 (dmso) 2 ] complexes could be easily displaced by chelating bidentate ligands (depicted as L') to furnish complexes of type [Ru(L n ) 2 (L')], as demonstrated through isolation of two such complexes, viz. [Ru(L 3 ) 2 (bpy)] and [Ru(L 2 ) 2 (phen)] (bpy = 2,2'-bipyridine and phen = 1,10-phenanthroline). The crystal structure of [Ru(L 3 ) 2 (bpy)] has been determined and the structure of [Ru(L 2 ) 2 (phen)] has been optimized by the DFT method. The electronic spectra of the four [Ru(L n ) 2 (dmso) 2 ] complexes and the two derivatives ([Ru(L n ) 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(L n ) 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.