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Homogeneous Catalysis of the Photoreduction of Water by Visible Light. 3. Mediation by Polypyridine Complexes of Ruthenium(II) and Cobalt(II)
Author(s) -
Krishnan C. V.,
Creutz Carol,
Mahajan Devinder,
Schwarz Harold A.,
Sutin Norman
Publication year - 1982
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.198200020
Subject(s) - chemistry , cobalt , quantum yield , photochemistry , aqueous solution , quenching (fluorescence) , reaction rate constant , catalysis , yield (engineering) , radiolysis , hydride , medicinal chemistry , ruthenium , inorganic chemistry , kinetics , hydrogen , fluorescence , organic chemistry , quantum mechanics , physics , materials science , metallurgy
Irradiation of polypyridineruthenium(II) complexes (RuL 2+ 3 ) in aqueous solutions containing ascorbate (HAs − ) and Lî (2,2′‐bipyridine or other polypyridines) with visible light yields reduced polypyridine LîH 2 and H 2 . The quantum yield of H 2 is increased greatly when cobalt(II) is also present and is optimal, Φ = 0.13, with L = bpy, Lî = 4,4′‐(CH 3 ) 2 bpy (2 × 10 −3 M), [Co(II)] = 0.01 M, [HAs − ] = 0.7 M at pH 4.5–5.5. The quantum yield of LîH 2 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 *RuL 2+ 3 by HAs − to give RuL + 3 and ascorbate radical, spans the range 0.2 × 10 7 M −1 s −1 (L = 4,4‐(CH 3 ) 2 bpy) to 1.1 × 10 9 M −1 s −1 (L = 5‐(Cl)phen). In the absence of Co(II), RuL + 3 reduces LîH + and formation of LîH 2 (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 H 2 O, H + or Co(Lî) 2 H 2+ to produce H 2 . At high pH, however, these reactions are too slow to compete with oxidation of cobalt(I) ( k ∼ 10 10 M −1 s −1 ) and the hydride complexes by ascorbate radical. By contrast, at pH < 4 the H 2 yield drops because of oxidation of Co 1 (Lî) + n (or its hydride) by LîH + .