Osmium(II) Complexes Bearing Chelating N‑Heterocyclic Carbene and Pyrene-Modified Ligands: Surface Electrochemistry and Electron Transfer Mediation of Oxygen Reduction by Multicopper Enzymes

We report the synthesis of original osmium­(II) complexes bearing chelating N-heterocyclic (NHC) and bipyridine ligands. The pincer ligand 1,1′-dimethyl-3,3′-methylenediimidazole-2,2′-diylidene was used to tune the redox properties of osmium complexes. Bipyridine ligands modified with pyrene groups were chosen to study the electrosynthesis of OsII-NHC-based metallopolymers as well as the noncovalent immobilization of these complexes on carbon-nanotube (CNT) electrodes. Poly-[OsII-NHC] polypyrene polymer was electrogenerated on a GC electrode, whereas the pyrene-modified [OsII-NHC] could interact with the CNTs’ sidewalls through π–π interactions, allowing the immobilization of the NHC complexes at the surface of π-extended nanostructured electrodes. Furthermore, an OsII-NHC complex was studied in water, showing electron transfer mediation with multicopper enzymes. UV–visible and electrochemical experiments demonstrate that redox properties of the OsII-NHC complex provide sufficient driving force for electron transfer with bilirubin oxidase from Myrothecium verrucaria while achieving high potential electroenzymatic oxygen reduction at E = +0.45 V vs Ag/AgCl at pH 6.5.