Influence of the Axial Alkyl Ligand on the Reduction Potential of Alkylcob(III)alamins and Alkylcob(III)yrinates.

Zhou, De-Ling; Tinembart, Olivier; Scheffold, Rolf; Walder, Lorenz

The irreversible-reduction potentials of 26 alkylcob(III)alamins (RCblIII 1a- z) and 26 alkylcob(III)yrinates (R'Cby'III; 2a- z) ( E p 1a- z and E p 2a- z, resp.) have been measured in situ by single-scan voltammetry of hydroxocob(III)alamin hydrochloride (vitamin B12b- HCl; 1) or heptamethyl cob(II)yrinate perchlorate (ClO4'Cby'II; 2) in presence of the corresponding alkyl halides (RX; 3a- z) 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CoIII e−⇆R⋅ CoI). Electron-withdrawing groups and/or an increased degree of substitution at the Co-bound C-atom in RCblIII and, R'Cby'III shift E p( 1a- z) and E p ( 2a- z) towards positive potentials. Linear correlations have been found between Ep( 1a- z) ( Ep( 2a- z)) of RCblIII (R'Cby'III) and the p Ka of RH (or the Taft σ*- or the Hammett σ

Helvetica Chimica Acta, 1990, Vol 73, Issue 8, p2225

0018-019X

Academic Journal

10.1002/hlca.19900730820