The electrochemical behavior and electronic structure of non- transition metal chelate radical anions

The reaction of substituted acetylacetonates with R₃B compounds leads to the production of an interesting series of chelates containing the pseudo-metal ion moiety, BR₂.

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When the substituents on the acetylacetone are capable of extensive delocalization with the chelate ring (e.g., benzoylacetomethane) and/or when the R groups on boron are sufficiently withdrawing (R = F, C₆H₅), these species are capable of reversible one-electron reduction to the radical ion. These ion radicals are extremely stable, having been kept for many months at room temperature under argon atmosphere.

Previous studies have indicated that the dianion radical of the substituted acetylacetonate ligand itself is extremely unstable. It has been trapped at extremely low temperatures and its spectral properties have been reported. Obviously, the main group chelate radical ions that we have studied represent a case where the inherently unstable dianion radical system is stabilized by coordination to the pseudometal ion.

The spin densities and charge distribution in the boron chelate radicals and the neutral parent have been calculated by Pariser-Parr-Pople SCF techniques, extended Hückel molecular orbital calculations, and CNDO and INDO methods. The ligand dianion radicals have also been successfully handled. In all cases, good agreement between experimentally observed ESR hyperfine coupling constants, assigned by satisfactory simulation, and calculated hyperfine constants have been obtained. The spin density calculations on the radical anions indicated that there are negligible spin delocalization over the moiety, BR₂. The net charge distribution in the neutral and the radical anions demonstrate that the effect of BR₂ on the radical stability is due to its withdrawing effect. The electronic energy levels and electronic transitions allowed in the neutral and the radical ions have been calculated by closed-shell and open-shell (RSCF) methods of Longuet-Higgins and Pople with the CI treatment of Ishitani and Nagakura. Good agreement with the experimental data is observed. Correlation diagrams connecting the MO levels for the neutral and radical ion species reveal information about the perturbations of energy levels caused by the injection of an extra electron into the previous spin-paired system.