Stabilities and enthalpies of formation of base adducts of vitamin B 12 model complexes

Abstract

Thermodynamic data, obtained by calorimetric titration, are reported for the addition of bases to alkylcobalt disalicylidene-1,2-diaminoethane complexes in acetonitrile and methylcobaloxime in benzene. The complexes RCo(salen) form 1 : 1 adducts of moderate stability with N-donors; adduct stabilities are greater when R = Me than when R = Et, Pr or Bu and are also affected by base structure: heterocyclic bases form less stable adducts than amines and in the latter chain branching lowers adduct stability and ring formation raises it; some of the most stable adducts are formed by pyrollidine and imidazole; enthalpies of adduct formation for all bases are aobut −30 kJ mol −1 and variations are apparently entropy controlled. Methylcobaloxime is a much stronger Lewis acid and forms 1 : 1 adducts of very high stability ( K > 10 5 1 mol −1) with N-, O-, S- and P-donors; enthalpies of adduct formation are mostly about −50 kJ (g-at. Co) −1 with the dimeric form of the cobaloxime.