Electrochemical synthesis of Costa-type cobalt complexes

Due to the difficulty encountered in chemical preparations, [LCo( (DO)(DOH)pn)R]X species [whereL = neutral ligand, R = alkyl group, (DO)(DOH)pn = IP,P'-propanediylbis(2,3-butanedione 2-imine3-oxime)l have been prepared previously with only a limited range of alkyl ligands in comparison to othermodel systems. In this connection, the feasibility of electrochemical methods in the synthesis of theseorganocobalt Costa-type B12 model complexes was examined. The Co(1) species produced from cathodicreduction of Co((DO)(DOH)pn)Brza t ca. -0.8 to -0.9 V (versus SCE) in acetonitrile at 25 "C reacted quicklywith methyl iodide and more slowly with 2,2-bis(ethoxycarbonyl)propyl bromide (C2H5O2CC(CH3)-(CH2Br)CO2CzH6d,i ester bromide). Although the product of the reaction with the diester bromide couldbe reduced at only slightly more negative potentials, we were able to prepare [H,OCo((DO)(DOH)pn)-diesterlClO,, a useful synthon, by controlled potential electrolysis of Co((DO)(DOH)pn)Br2a t -0.8 V. Thisaqua complex, which we could not prepare by NaBH, reduction, was isolated and characterized. Theelectrochemical results are consistent with the view that both reduction of the organocobalt product andthe slowness of the alkylation of the Co(1) species are responsible for previously observed difficulties instandard chemical preparations. The 1,5,6-trimethylbenzimidazole(M e3Bzm)a nd pyridine (py) adductswere prepared by ligand substitution of the aqua complex and were characterized by NMR spectroscopyand elemental analysis. Rates of ligand dissociation of these adducts were found to be - 10 times fasterthan for the corresponding Me3Bzm and py complexes in which R = methyl. In contrast, the y-13C pyshifts of the two py derivatives were similar. These results can be rationalized if the alkyl ligands areelectronically similar but sterically different, with the more bulky diester group favoring L dissociation.