Abstract
Two oxygen adducts of different geometric isomers of the tetrakis(μ-2-anilinopyridinato) dirhodium unit, Rh_2(ap)_4~+, have been synthesized and characterized. Rh_2(ap)_4O_2(4,0) is the oxygen adduct 1 of 4, 0 isomer; and Rh_2(ap)_4O_2(2, 2) is the oxygen adduct 2 of 2, 2 isomer. The ESR spectra of both oxygen adducts show that there is a characteristic signal of a superoxide ion, g=2.03. The 4, 0 isomer with different axial ligands gives the order of A_Ⅱ values; O_2~-≈ClO_4~-Cl~-CN~-. However, the A_Ⅱ value of the 2, 2 isomer is independent of axial ligands. The half wave potentials E_(1/2)~1 for different axial ligands are: O_2~-Cl~-CO, which is consistent with the least A_Ⅱ value of the superoxide. There are plain differences in electrochemical redox process of the two isomers, that is the 2, 2 trans isomer reacting with oxygen involves reversible oxidation, but the oxygen adduct of the 4, 0 isomer is formed irreversibly. The XPS spectrum of the adduct displays a new peak in the region of Ols with low binding energy, 528.7 eV. All characteristict from the spectra show that there is a superoxide group O_2~- in Rh_2(ap)_4O_2 which is dependent on the symmetry of the isomers and bonding ability with oxygen.
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