A STUDY OF CIS INFLUENCE IN ALKYL COBALOXIMES

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 f rom ClCo(chgH)(2)Py complexes. A rapid purification procedure using co lumn chromatography affording analytically pure products has been esta blished for all the three series of cobaloximes. Methanol has been fou nd to be the best solvent for the study of cobalt-carbon charge transf er (Co-C CT) band as the alkylcobaloximes exhibit a prominent maxima i n this solvent. For MeCo(dpgH)(2)Py the Co-C CT band is not resolved; a lambda(max) value of 453.6 nm is proposed for it. The literature val ue of 473 nm is doubtful. The variation of lambda(max) values with inc reasing alkyl chain length is surprisingly similar for all the three s eries of cobaloximes. C-13-Spectroscopy has revealed that the P-alpha experiences the most cis influence and P-beta the least. The cis influ encing ability of the chgH ligands have been found to be negligible (a s compared to dpgH) on the alkyl chain as well as on the P-beta P-gamm a carbons. H-1-NMR studies indicate that the cis influence is felt mos t by the cobalt bound methylene protons followed by P-alpha and P-beta . Interestingly, the O-H-O resonance for the chgH cobaloximes appear s imilar to 0.5 ppm upfield than the analogous dmgH complexes in all the thirteen alkylcobaloximes. The order of dis influencing ability has b een found to be dpgH > chgH > dmgH by all the three spectroscopic tech niques. This sequence is exactly the reverse order of the correspondin g Co(I) nucleophilicities. C=N and N-O stretching frequencies in the I R studies for three series of cobaloximes follow the order dmgH > chgH > dpgH. The order can be explained if one invokes the electronic effe ct of the substituents on the dioximic moiety. (C) 1997 Elsevier Scien ce S.A.