A study of cis influence in alkyl cobaloximes

Article abstract of DOI:10.1016/S0022-328X(97)00210-6

Thirty-nine alkylcobaloximes have been synthesized (many of them new) and characterized with three different dioxime ligands, dmgH, dpgH and chgH. The chgH cobaloximes have been synthesized for the first time from ClCo(chgH)₂Py complexes. A rapid purification procedure using column chromatography affording analytically pure products has been established for all the three series of cobaloximes. Methanol has been found to be the best solvent for the study of cobalt-carbon charge transfer (Co-C CT) band as the alkylcobaloximes exhibit a prominent maxima in this solvent. For MeCo(dpgH)₂Py the Co-C CT band is not resolved; a λ_max value of 453. 6 nm is proposed for it. The literature value of 473 nm is doubtful. The variation of λ_max values with increasing alkyl chain length is surprisingly similar for all the three series of cobaloximes. ¹³C-Spectroscopy has revealed that the P_α experiences the most cis influence and P_β the least. The cis influencing ability of the chgH ligands have been found to be negligible (as compared to dpgH) on the alkyl chain as well as on the P_β and P_γ carbons. ¹H-NMR studies indicate that the cis influence is felt most by the cobalt bound methylene protons followed by P_α and P_β. Interestingly, the O-H-O resonance for the chgH cobaloximes appear ～ 0.5 ppm upfield than the analogous dmgH complexes in all the thirteen alkylcobaloximes. The order of cis influencing ability has been found to be dpgH > chgH > dmgH by all the three spectroscopic techniques. This sequence is exactly the reverse order of the corresponding Co(I) nucleophilicities. C=N and N-O stretching frequencies in the IR studies for three series of cobaloximes follow the order dmgH > chgH > dpgH. The order can be explained if one invokes the electronic effect of the substituents on the dioximic moiety.