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_2-purged), based on steady-state illuminations. For Cu(dicarboxylate)^0, the Cu(I) quantum yields at 313 nm (Φ_(Cu(I),CuL)) 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 1998, 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_2C(O)O-)^0] by H(malonate)-/H_2(malonate)^0 is proposed. Evidence is presented for Cu(I) photoformation from Cu(malonate)^0.