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 <i>N</i>-(3-bis­(2-pyridyl)­aminopropyl)­pyrrole with the general formula [Ru­(tpy)­(PPP)­X]^<i>n</i>+ (<b>1</b>, X = Cl^–; <b>2</b>, X = H₂O; <b>3</b>, X = CH₃CN; tpy = 2,2′:6′,2″-terpyridine) have been synthesized and characterized by ¹H NMR, IR, UV–vis, mass spectrometry, and elemental analysis. <b>1</b> and <b>2</b> have been structurally characterized by X-ray crystallography. Both <b>1</b> and <b>2</b> 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 <b>1</b> in acetonitrile revealed a Ru^III/Ru^II couple at 0.4 V vs Cp₂Fe^+/0 initially, but another redox couple resulting from chloride substitution by acetonitrile developed at <i>E</i>_1/2 = 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 <b>2</b> in aqueous medium allowed the observation of the formation of Ru^IVO species in polypyrrole film. As the film grew thicker, the size of the Ru^III/^/Ru^II couple (<i>E</i>_1/2 = 0.8 V vs SCE at pH 1) of poly­[Ru­(tpy)­(PPP)­(OH₂)]^<i>n</i>+ increased accordingly, whereas the growth of the Ru^IV/Ru^III couple (<i>E</i>_1/2 = 0.89 V vs SCE at pH 1) leveled off after the film had reached a certain thickness. The Pourbaix diagram of the <i>E</i>_1/2 of the Ru^III /Ru^II and Ru^IV/Ru^III couples vs pH of the electrolyte medium has been obtained. The resulting poly­[Ru­(tpy)­(PPP)­(OH₂)]^<i>n</i>+ film is electrocatalytically active toward the oxidation of benzyl alcohol