PHOTOCHEMICAL REDOX REACTIONS OF INNER-SPHERE COPPER(II)-DICARBOXYLATE COMPLEXES - EFFECTS OF THE DICARBOXYLATE LIGAND STRUCTURE ON COPPER(I) QUANTUM YIELDS

Studies were carried out on the photochemical redox reactions of a ser ies of structurally related inner-sphere Cu(II)/dicarboxylate complexe s in aqueous solution (N-2-purged), using steady-state illumination. C u(I) quantum yields (313 nm) of these systems were characterized with respect to the effects of copper(II) speciation, mainly considering th e effect of the dicarboxylate structure on the Cu(I) quantum yield. Fo r each solution composition, which corresponds to a unique Cu(II) spec iation, the Cu(II)-based molar absorptivity (epsilon(Cu(II)), M-1 cm(- 1)) and the quantity Phi(Cu(I)epsilon Cu(II)) (where Phi(Cu(I)) repres ents the experimental Cu(I) quantum yield) were determined from experi ments. This experimental information (epsilon(Cu(II)) and Phi(Cu(I)eps ilon Cu(II))) was explicitly combined with the calculated equilibrium Cu(II) speciation (based on critically reviewed thermodynamic data) in a quantitative model to determine molar absorptivities (epsilon(CuL)) and Cu(I) quantum yields (Phi(Cu(I),CuL)) of the Cut complexes, where L represents the dicarboxylate ligand. For Cu(dicarboxylate)(o), the observed relative reactivity of Cu(I) quantum yields at 313 nm (Phi(Cu (I),CuL)) varies by 50-fold and is affected by the dicarboxylate struc ture as follows: oxalate (0.42) > succinate (0.10) much greater than m aleate (0.008). This trend in relative reactivity parallels the trend in expected relative stability of the carbon-centered radicals derived from decarboxylation of the carboxylate (i.e., acyloxyl) radicals for med in the initial photoinduced ligand-to-metal charge (electron) tran sfer reaction of the Cu(II)/dicarboxylate complex.