Formation, characterization, and reactivity of metal-oxygen adducts of copper, silver, and gold
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Date
1989
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Abstract
The coinage metals are oxidised to their respective cations (Cu+, Ag+, and Au+) at potentials that range from -0.20 V vs SCE for Cu to +1.30 V for Au. At glassy carbon hydroxide ion (-OH) is oxidized to O (+0.35 V vs SCE) and then to O by an ECE mechanism. In contrast, with freshly polished metal eletrodes the oxidation of -OH to O and O is facilitated via formation of metal-s/oxygen-p covalent bonds to give a series of surface compounds. The shift to less positive oxidation potentials has been used to estimate the metal-oxygen covalent bond energies (about 40- 60 kcal). On the basis of the redox thermodynamics and the substrate reactions, the oxygen in these metal-oxygen surface compounds is essentially neutral (atomic O) and stabilized via covalent s-p bonds with neutral-charge metal atoms. Concurrent reduction of Cu+ plus O2 and of Ag+ plus O2, yields a series of surface compounds that parallel those that result from 'OH oxidation at the metal surfaces. The electron-transfer redox chemistry for the porphyrin and bipyridyl complexes of copper and silver [(TPP)Cu, (TPP)Ag, (Cl8TTP)Cu, (CI8TPP)Ag, (bpy)2Cu(ClO4), and (bpy)2Ag(ClO4), have been characterized by cyclic voltammetry and controlled potential electrolysis. ESR measurements confirm that the neutral porphyrin complexes have their spin density localized in the porphyrin ring with three metal-nitrogen covalent bonds; their metal centers do not interact with dioxygen (O2) or -OH. In contrast the ligand-centered reduction of [(bpy)2CuOCu[bpy)2] in the presence of dioxygen yields [(bpy)2CuOCu(bpy)2] that is oxidized Cu(bpy)2[^+2] at +1.80 V vs SCE.
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Typescript (photocopy).
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Major chemistry