Organo-bridged dicobaloximes: Synthesis, structure and nuclear magnetic resonance study

This short review describes the synthesis and structural characteristics of methylene, xylylene and biphenyl-bridged dicobaloximes. The bridged dicobaloximes are easily synthesized by oxidative alkylation method and the yield of dicobaloxime depends on many factors like reaction time, solvent, dihalide:Co(I) ratio and also on the nature of dihalide used. The NMR and structural studies have been described. The cobalt-bound CH2 and dioxime protons in xylylene-bridged dicobaloximes become nonequivalent at sub-zero temperature and this is caused by the restricted rotation of the Co–C bond. However, the nonequivalence in biphenyl-bridged dicobaloximes is caused by atropisomerism and/or restricted rotation of C–Ph/Co–C bond. Two cobaloxime units of biphenyl-bridged dicobaloximes are symmetrical, and can be looked at as two independent benzyl cobaloximes and there is no direct or indirect interaction between them. In the xylylene-bridged dicobaloxime, one cobaloxime unit affects the structural parameters of the other unit through axially bridged benzyl group, which is sandwiched between two dioxime wings.