Homogeneous Catalysis of the Photoreduction of Water by Visible Light. 3. Mediation by Polypyridine Complexes of Ruthenium(II) and Cobalt(II)

Irradiation of polypyridineruthenium(II) complexes (RuL²⁺₃) in aqueous solutions containing ascorbate (HAs⁻) and Lî (2,2′‐bipyridine or other polypyridines) with visible light yields reduced polypyridine LîH₂ and H₂. The quantum yield of H₂ is increased greatly when cobalt(II) is also present and is optimal, Φ = 0.13, with L = bpy, Lî = 4,4′‐(CH₃)₂bpy (2 × 10⁻³ M), [Co(II)] = 0.01 M, [HAs⁻] = 0.7 M at pH 4.5–5.5. The quantum yield of LîH₂ is ≧ 0.01 when Lî = bpy at ≧ 10⁻³ M, [HAs⁻] = 0.7 M at pH ∼ 5 but diminishes greatly at pH ≧ 6 or when cobalt(II) is present. The photochemical experiments and pulse‐radiolysis studies indicate that the quantum yields reflect the relative rates of some twenty elementary reactions. The rate constant for the first, quenching of *RuL²⁺₃ by HAs⁻ to give RuL⁺₃ and ascorbate radical, spans the range 0.2 × 10⁷ M⁻¹ s⁻¹ (L = 4,4‐(CH₃)₂bpy) to 1.1 × 10⁹ M⁻¹ s⁻¹ (L = 5‐(Cl)phen). In the absence of Co(II), RuL⁺₃ reduces LîH⁺ and formation of LîH₂ (in competition with oxidation of LîH by ascorbate radical) ensues. When Co(II) is present, Co(Lî)²⁺_n complexes are reduced by RuL⁺₃ to cobalt(I) complexes Co(Lî)⁺_n. These may react with H⁺ to form cobalt hydride complexes which undergo further reaction with H₂O, H⁺ or Co(Lî)₂H²⁺ to produce H₂. At high pH, however, these reactions are too slow to compete with oxidation of cobalt(I) (k ∼ 10¹⁰ M⁻¹ s⁻¹) and the hydride complexes by ascorbate radical. By contrast, at pH < 4 the H₂ yield drops because of oxidation of Co¹(Lî)⁺_n (or its hydride) by LîH⁺.