ELSEVIER Journal of Organometallic Chemistry 543 (1997) 125-134 __Journal otOrgang. metalliC Chemistry A study of cis influence in alkyl cobaloximes B.D. Gupta *, Kushal Qanungo Chemist©' Department, I.I.T. Kanpur, Kanpur, U.P. 208016, India Received 9 December 1996 Abstract 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 C1Co(chgH)2Py comple~es. 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 Am~ , value of 453.6 nm is proposed for it. The literature value of 473 nm is doubtful. The variation of Ama , values with increasing alky[ chain length is surprisingly similar for all the three series of cobaloximes. ~3C-Spectroscopy has revealed that the P~, experiences the most cis influence and Pt~ 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 Pt~ and P~ carbons. 1H-NMR studies indicate that the cis influence is felt most by the cobalt bound methylene protons followed by P~ and PI~" 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 corre,sponding 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. ~ 1'997 Elsevier Science S.A. 1. Introduction Organocobaloximes, RCo(L)2B (where R is an or- ganic group, L = dmgH and B is a coordinating base) have been extensively studied [1], ever since Schrauzer first highlighted their importance as models of coen- zyme B I2 [2]. Since then it has outgrown its initial relevance and has gained significance of its own, mainly because of its rich chemistry [3]. It was found to be a convenient source of free radicals [4], even in aqueous medium [5], and its ability to serve as catalysts [6] and as templates [7] in organic reactions are well docu- mented in the literature. Applications apart, these are ideal systems, like platinum(II) complexes, to study the Abbreviations: dmgH, dimethylglyoxime; gH, glyoxime; dpgH, diphenylgyloxime; mpgH, methyphenylglyoxime; chgH, 1,2-cyclo- hexanedionedioxime, all mono anions * Corresponding author. 0022-328X/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0022-328X(97)00210-6 cis and trans effects [8]. The trans influence has been extensively studied [9], whereas examples of cis influ- ence have been rather few [10]. This is primarily due to (a) the trans influence is much more pronounced as compared to cis influence and (b) paucity of cobaloximes with equatorial ligand other than dmgH. The literature survey further reveals that most of the study, however, has been confined to dmgH as the equatorial ligand. Cis-trans influence studies on cobaloximes with other equatorial ligands like gH [11], dpgH [12], and mpgH [13] are few and scattered. The importance of organocobaloximes with ligands other than dmgH has recently been perceived in many reactions; for example, it has been demonstrated by Welker et al. [7] that in cobaloxime mediated Diels Alder reactions, the use of sterically demanding dpgH as the equatorial ligand instead of dmgH, results in marked improvement of the exo/endo selectivity. We have observed a faster oxygen insertion in RCo(chgH) 2 B complexes as compared to RCo(dmgH)2B complexes