Synthesis and characterization of two binuclear nickel(II) complexes of thiophenol-based “end-off” compartmental ligands and their application as catalysts for selective oxidation of sulfides

<div><p></p><p>Reaction of the binucleating S-protected ligand precursors 2-(<i>N,N</i>-dimethylthiocarbamato)-5-methylisophthalaldehyde di-2′-hydroxy anil (<b>I</b>) and 2-(<i>N,N</i>-dimethylthiocarbamato)-5-methylisophthalaldehyde di-2′-hydroxy 5′-methyl anil (<b>II</b>) with nickel(II) acetate tetrahydrate in the presence of pyrazole afforded the binuclear nickel(II) complexes [L^<b>I</b>Ni₂(pz)] (<b>1</b>) and [L^<b>II</b>Ni₂(pz)] (<b>2</b>), respectively. The complexes have been characterized by routine physicochemical studies as well as by X-ray single crystal structure analysis. In both complexes, Ni(II) ions are doubly bridged by the thiophenolic sulfur of the pentadentate Schiff base ligand and a pyrazolate group. Efficient protocols for the oxidation of sulfides to sulfoxides with high selectivities, catalyzed by binuclear μ-thiophenolato-μ-pyrazolatonickel(II) in the presence of urea hydrogen peroxide (UHP) were explored. We obtained predominantly the monooxygenated product. The resulting products are obtained in good to excellent yields within a reasonable time.</p>