The technique described was elaborated for studying the kinetics of ligand rearrangements of transition metal complexes (porphin and eorrin type metal complexes) (1).

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Abstract

The irreversible-reduction potentials of 26 alkylcob(II1)alamins (RCbl"'; la-z) and 26 alkylcob(I1I)yrinates (RCby'"'; 2a-2) (E, la-z and Ep 2a-2, resp.) have been measured in situ by single-scan voltammetry of hydroxocob(II1)alamin hydrochloride (vitamin BIZb. HCl; 1) or heptamethyl cob(1I)yrinate perchlorate (ClO.,'Cby'"; 2) in presence of the corresponding alkyl halides (RX; 3-2) in DMF. The reduction potentials of alkylcobalt complexes exhibiting half-life times as short as a few seconds become measurable by this technique. Thermodynamic cycles prove that the observed reduction potentials are closely related to the standard reduction potentials E"(R-Co"' + eC%R' + Co'). Electron-withdrawing groups and/or an increased degree of substitution at the Co-bound C-atom in RCbl"' and RCby'"' shift E,(la-z) and E (2a-2) towards positive potentials. Linear correlations have been found between Ep(la-z) (EP(2a-z)) of RCbl" (R'Cby'"') and the pK, of RH (or the Tuft v*-or the Hummeft o--values of R) within each class of R, i.e. MeCbl"' (Me'Cby'"'), primary RCbl"' (RCby'"') and secondary RCbll'P(R'Cby'"L). The correlations allow to distinguish between electronic effects of the Co-bound alkyl residues and their steric interactions with the corrin side chains. The correlations have further been used to visualize the light-induced formal insertion of an olefin into the Co,C-bond of an alkylcobalamin (Scheme 2, la-lu), a key step in the vitamin-B,,-catalized C,C-bond formation.

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