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

Abstract

AbstractIrradiation 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+.