Ruthenium terpyridine complexes containing a pyrrole-tagged 2,2′-dipyridylamine ligand-synthesis, crystal structure, and electrochemistry

Ruthenium(II) terpyridine complexes containing the pyrrole-tagged 2,2′-dipyridylamine ligand PPP (where PPP stands for N-(3-bis(2-pyridyl) aminopropyl)pyrrole with the general formula [Ru(tpy)(PPP)X]n+(1, X = Cl-; 2, X = H2O; 3, X = CH3CN; tpy = 2,2′:6′,2′-terpyridine) have been synthesized and characterized by1H NMR, IR, UV-vis, mass spectrometry, and elemental analysis. 1 and 2 have been structurally characterized by X-ray crystallography. Both 1 and 2 were successfully immobilized onto glassy carbon electrode via anodic oxidation of the pyrrole moiety on the PPP ligand to give stable and highly electroactive polymer films. Cyclic voltammetric studies of 1 in acetonitrile revealed a RuIII/RuIIcouple at 0.4 V vs Cp2Fe+/0initially, but another redox couple resulting from chloride substitution by acetonitrile developed at E1/2= 0.82 V upon repetitive potential scan. This ligand substitution was induced by the acidic local environment caused by the release of protons during pyrrole polymerization. The electropolymerization of 2 in aqueous medium allowed the observation of the formation of RuIV=O species in polypyrrole film. As the film grew thicker, the size of the RuIII//RuIIcouple (E1/2= 0.8 V vs SCE at pH 1) of poly[Ru(tpy)(PPP)(OH2)]n+increased accordingly, whereas the growth of the RuIV/RuIIIcouple (E1/2= 0.89 V vs SCE at pH 1) leveled off after the film had reached a certain thickness. The Pourbaix diagram of the E1/2of the RuIII/RuIIand RuIV/RuIIIcouples vs pH of the electrolyte medium has been obtained. The resulting poly[Ru(tpy)(PPP)(OH2)]n+film is electrocatalytically active toward the oxidation of benzyl alcohol.