The pH and temperature-dependent hydrolyses of [N,N-bis(2-aminoethyl)-1,2-ethanediamine]bis(solvent)cobalt(III) and bis(dimethylformamide)(1,4,7,10-tetra-azacyclododecane)cobalt(III) complexes

Abstract

The pH dependence of the hydrolysis of the stereochemically rigid [N,N-bis(2-aminoethyl)-1,2-ethanediamine]bis(solvent)cobalt(III) ions (solvent = dimethyl sulphoxide, dimethylformamide, or trimethyl phosphate) in water has been studied. The two successive observed rate constants show, respectively, dependences of the form k₁+k₁′[OH^–] and k₂+k₂′K_a/(K_a+[H⁺]), where a limiting deprotonation of a water molecule operates in the second stage of the reaction. Whereas ¹H n.m.r. of the dimethylformamide complex in acid shows two intermediates, indicating that loss from the two non-equivalent (p and t) sites occurs at similar rates, only one intermediate is observed under base hydrolysis, and lability at one site is apparently greater in this case. Comparisons of data with published results available for other amine complexes are made. The complex bis(dimethylformamide)(1,4,7,10-tetra-azacyclododecane)cobalt(III) is also stereochemically rigid, but both leaving sites are equivalent, and two simple sequential rate constants with k₁/k₂ca. 5 : 1 are observed in aqueous acid. This ratio is similar to the value of 5.5 : 1 determined for the composite observed k₁ and k₂ for the N,N-bis(2-aminoethyl)-1,2-ethanediamine analogue, and much smaller than ratios reported when ionic (halogeno) leaving groups are involved.