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Cobaloximes with Dimesitylglyoxime:  Synthesis, Characterization, and Spectral Correlations with the Related Cobaloximes

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posted on 28.03.2005, 00:00 by Debaprasad Mandal, B. D. Gupta
Alkyl and non-alkyl cobaloximes with dimesitylglyoxime have been synthesized and characterized with 1H and 13C NMR, UV−vis, and X-ray diffraction. The X-ray structures of MeCo(dmestgH)2Py, ClCo(dmestgH)2Py, and BrCo(dmestgH)2Py are reported. The cistrans influence has been studied by 1H and 13C NMR, UV−vis, and X-ray diffraction and is correlated with the reported cobaloximes. The spectral correlations are much better understood when both cobalt anisotropy and ring current are considered operating together. The trans influence of R/X has been monitored by the coordination shift of the Pyγ proton/carbon, and its chemical shift is a net result of the interplay of cobalt anisotropy and the trans effect of the R/X group. Two factors have been considered to study cis influence:  (a) the effect of axial ligands on the equatorial dioxime moiety and (b) the effect of dioxime on the axial ligands. It is found that CN and Pyα are the most sensitive to any change in the molecule. A change in the axial R/X and dioxime moieties affects the CN resonance, whereas Pyα is sensitive to the change in R/X (trans effect) and the ring current of the dioxime (cis influence). A good correlation between δ(13C, CN) and Δδ(1H, Pyα) suggests the presence of ring current throughout the Co(dioxime) metallabicycle, and the negative slope indicates that they are effected in opposite directions. It is found that dmestgH complexes have the maximum cis influence among all the reported cobaloximes. A cyclic voltammetry study for both alkyl and non-alkyl cobaloximes is reported. The reduction from Co(III) to Co(II) and from Co(II) to Co(I) is found to be more difficult in ClCo(dmestgH)2Py as compared to the other chlorocobaloximes (gH, dmgH, dpgH).

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