Light-Induced Electron-Transfer Reactions Involving the Tris(2,2'-Bipyridine)Ruthenium Dication and Related Complexes. 3. Improved Synthesis of 2,2'-Bipyridine-4,4'-Dicarboxylic Acid and Photoreduction of Water by Bis(2,2'-Bipyridine)(2,2'-Bipyridine-4,4'-Dicarboxylic Acid)Ruthenium(II)

Abstract

The oxidation of 4,4′-dimethyl-2,2′-bipyridine with potassium permanganate in water gives 2,2′-bipyridine-4,4′-dicarboxylic acid and 4′-methyl-2,2?-bipyridine-4-carboxylic acid. The latter acid is oxidized to the diacid by boiling nitric acid. Complexes of the type Ru ( bpy )₂L²⁺ have been prepared where L is 2,2′-bipyridine-4,4′- dicarboxylic acid, diethyl 2,2′-bipyridine-4,4′-dicarboxylate, 4′- methyl-2,2′-bipyridine-4-carboxylic acid and ethyl 4′-methyl-2,2′- bipyridine-4-carboxylate. These complexes have been compared with [ Ru ( bpy )₃]²⁺ as sensitizers for the photoreduction of water. Stern- Volmer analysis has been applied to the quenching of their luminescence by methylviologen (mv²⁺), [Co(sep)]³⁺ (sep is 1,3,6,8,10,13,16,19- octaazabicyclo [6.6.6] icosane ) and [Co( CLsar )]³⁺ ( CLsar is 1-chloro- 3,6,10,13,16,19-hexaazabicyclo[6.6.6] icosane ). Changes in the Stern-Volmer constants have been related to the free energy changes associated with the oxidative quenching and the overall charges of the ruthenium complexes. The rates of formation of hydrogen compared favourably in sacrificial cycles with the ruthenium complexes as sensitizers, mv²⁺, Co(sep)³⁺ as electron-transfer agents, platinum/poly(vinyl alcohol) as catalyst, and ethylenediaminetetraacetic acid as electron donor. The results obtained have been discussed in terms of variations in the efficiencies of cage escape in the oxidative quenching and competition between electron transfer and energy transfer.