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 by 1H 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/RuII couple at 0.4 V vs Cp2Fe+/0 initially, 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//RuII couple (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/RuIII couple (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 E1/2 of the RuIII /RuII and RuIV/RuIII couples 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.