Photochemical Formation of Copper(I) from Copper(II)-Dicarboxylate Complexes:  Effects of Outer-Sphere versus Inner-Sphere Coordination and of Quenching by Malonate

Copper(I) quantum yields are reported for Cu(II) complexes of aliphatic dicarboxylates in aqueous solution (N<INF>2</INF>-purged), based on steady-state illuminations. For Cu(dicarboxylate)⁰, the Cu(I) quantum yields at 313 nm (Φ<INF>Cu(I),CuL</INF>) exhibit the following trend (25 °C, ionic strength = 0.10 M):  malonate (0.15 ± 0.07) > succinate (0.10 ± 0.02) (Sun, L.; Wu, C.-H.; Faust, B. C. J. Phys. Chem. A <BO>1998</BO>, 102, 8664−8672) > glutarate (0.054 ± 0.005) > adipate (0.042 ± 0.004) ≈ pimelate (0.046 ± 0.009). The systematic decrease in Cu(I) quantum yield observed for these Cu(dicarboxylate)⁰ complexes parallels the increasing degree of outer-sphere coordination of the complexes. Free uncomplexed malonate species quench the photoformation of Cu(I) from Cu(malonate)⁰ in a way that cannot be explained solely based on the Cu(II) speciation. An interpretation based on the quenching of the intermediate [Cu(^·CH<INF>2</INF>C(O)O^-)⁰] by H(malonate)^-/H<INF>2</INF>(malonate)⁰ is proposed. Evidence is presented for Cu(I) photoformation from Cu(malonate)⁰.