Electronic and Steric Influence of Axial and Equatorial Ligands in Vitamin B12 Model Complexes Derived from Cobaloxime: Electrochemical and 59Co‐NMR Studies

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AbstractIn four series of strictly related organocobalt complexes, derived from cobaloximes by replacement of the O…H…O with O…‐BF2…O and/or (CH2)3 groups, the trends of 59Co‐NMR shielding and electrochemical data are discussed. A largely parallel behaviour of the plots of E1/2(I) values for the first Co(III)/Co(II) electron transfer vs. the 59Co chemical shifts reflects the similar sensitivity of the two parameters to a change in electron affinity of the central metal ion due to a variation of the organic group R. E1/2(II) values for the second Co(II)/CO(I) electron transfer are less sensitive to the change of R, but the trend of the plot vs. δ(59Co) is still parallel in the four series. Consistent deviations from a roughly linear dependence of E1/2(I) on pKa of the hydrocarbon acid corresponding to R, on Taft constant s̀* and on 59Co shielding are noticed for the isopropyl derivatives and attributed to a steric effect. This was confirmed in a series of RCo(DMG)pyridine complexes in which 59Co shielding decreases steadily with increasing steric parameter Es (Taft) of the alkyl group. There is experimental evidence from X‐ray data that δ(59Co) decreases with an increase of the CoC bond length, illustrating steric hindrance in alkyl coordination to be responsible for the decreased shielding of the 59Co nucleus. The relative displacements of the graphic displays for the different series reflect the effect of changes in electron affinity of the redox center, due to the equatorial ligand, which, in turn, is caused by variations in the electron‐withdrawing power due to the introduction of the BF2 group and by the change from −2 to −1 valence of the (CH2)3‐capped ligands.