CONTROL OF INTRAMOLECULAR ELECTRON-TRANSFER BY PROTONATION - OLIGOMERS OF RUTHENIUM PORPHYRINS BRIDGED BY 4,4'-AZOPYRIDINE

The association of pentaammineruthenium(II) with the reducible ligand 4,4'-azopyridine leads to a pH-induced redox reaction in which ruthenium is oxidized to the III state, while 4,4'-azopyridine is reduced to hydrazopyridine. In this process, the conjugated ligand is transformed in a nonconjugated one, with loss of its intramolecular electron-transfer properties. In order to exploit this control of an intramolecular electron transfer by a protonation process, we have prepared 'shish kebab'' oligomers by first inserting a ruthenium chloro carbonyl complex in tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin. The resulting Ru(CO)(porphyrin) complex is photochemically decarbonylated in the presence of bridging ligands (4,4'-azopyridine or pyrazine). Oligomers are thus obtained, which can be oxidized by iodine, giving rise to intervalence transitions between ruthenium(II) and -(III) in the near-infrared. This provides a convenient way to monitor electron transfer along the oligomer chain. In the case of 4,4'-azopyridine, the pH-induced redox reaction is again observed. Starting from a homovalent ruthenium(II) chain, this gives the possibility to switch ''on'' or ''off'' the intervalence transition by a protonation/deprotonation reaction.