Difference in the Stabilities of the Diastereoisomers of the Tervalent Nickel Complex with 5,7,7,12,14,14-Hexamethyl-1,4,8,11 -tetraazacyclotetradecane in Sulfate- and Perchlorate-Containing Aqueous Solutions. An Electrochemical and Pulse Radiolysis Study

The electrochemical oxidation of two isomers of the complex of divalent nickel with rac-5,7,7,12,14,14-hexamethyl- 1,4,8,11-tetraazacyclotetradecane in aqueous solutions containing perchlorate and sulfate was studied. These oxidations are less reversible than that of the analogous meso complex. Furthermore the kinetic stabilization of the tervalent meso complex by sulfate, through the formation of NiL(SO₄)₂^-, is ca. 5000 times larger than that of the racemic complexes. The oxidation of a racemic complex by hydroxyl radicals was studied. The pK_H of the axial water bound to the tervalent nickel complex thus formed is 3.7. The stability constant for complexation by the first sulfate is 2200 M^-1, i.e., ca. one order of magnitude less than that observed for the corresponding meso complex. The rate-determining step in the anation by sulfate of the tervalent racemic complex is an isomerization of the latter, the rate of which is independent of sulfate concentration. The reasons for the large differences in properties of these diastereoisomers are discussed. The decomposition of several tervalent nickel complexes in alkaline media is first-order in hydroxide ion concentration. Kinetic salt effects on this reaction indicate that at pH 10.0 the tervalent complexes have the formula NiLOH²⁺ for L = the racemic and meso isomers as well as for the corresponding hexamethyl[14]diene ligand and NiL(OH)₂⁺ for L = cyclam. This means that in the latter complex the tervalent nickel is in an octahedral environment whereas in the former ones it is pentacoordinated.