C2 -Symmetric Bissulfoximine-Based Metal Complexes: Synthesis, Characterization and Their Electrocatalytic Activity in Oxygen Reduction Reaction.

Sulfoximine-based metal complexes have gained much interest in recent years owing to their promising properties in synthetic organic chemistry. These sulfoximines have been recently introduced in the field of organometallics. However, the coordination properties of pincer sulfoximine ligands and their metal complexes have not been reported. Here, a C-2-symmetric bissulfoximine-based pincer ligand ((pyridine-2,6-diylbis(methylene))bis(azaneylylidene))bis(diphenyl-lambda(6)-sulfanone), namely S-P ligand and its metal [M(SP)(2)](2+) {M=Fe (SC-1), Co (SC-2), Ni (SC-3), Cu (SC-4)} complexes, were synthesized. The complexes were characterized by different spectroscopic techniques like UV-Vis, IR, NMR, ESI-MS, and single-crystal XRD. Complexes SC-1 and SC-3 exhibited octahedral geometry irrespective of the metal center. The complexes were systematically explored for electrocatalytic oxygen reduction reaction (ORR) by immobilizing them on a glassy carbon electrode using Nafion. These complexes showed selective two-electron catalytic reduction of O-2 to H2O2 in which complex SC-2 showed the highest selectivity (approximate to 90%) and complex SC-4 showed the highest onset potential (0.75 V).