Statistically controlled kinetics for the formation and decomposition of binuclear complexes of Cu-II with a large octaaza cryptand

Kinetic studies have been made on the formation and decomposition of mono- and bi-nuclear copper(II) complexes with a symmetrical binucleating octaaza cryptand L containing two tris(2-aminoethyl)amine (tren) moieties bridged by three m-xylyl groups. The decomposition of binuclear Cu-II-L complexes u pon addition of acid excess occurs with two separate kinetic steps; the rat e constant for the faster step shows saturation behaviour, whereas a simple linear dependence on [H+] is observed for the slower step. Under similar c onditions, the mononuclear complex also decomposes in two steps, with rate constants very close to those found for the binuclear species. The whole of the kinetic data for the acid-promoted decomposition of the mono- and bi-n uclear complexes indicates that the rate constants for the successive disso ciation of both metal centres are statistically controlled, the value for t he first Cu-II being twice as large as that corresponding to the second one . The second order rate constants for the formation of the mono- and bi-nuc lear complexes have been determined in very basic solutions, and the rate o f co-ordination of the first metal ion is also found to be double that of t he second one, showing again statistically controlled kinetics. These resul ts clearly indicate that both tren sub-units of L behave independently duri ng complex formation and decomposition, i.e. there is no significant kineti c effect caused by the proximity of the metal centres and the cryptand is f lexible enough to undergo rapidly any reorganisation required to accommodat e them. The kinetic data for the formation and decomposition of these compo unds are also compared with literature data for related complexes, and poss ible reaction mechanisms are discussed.