Statistically controlled kinetics for the formation and decomposition of binuclear complexes of CuII with a large octaaza cryptand†

Abstract

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 upon addition of acid excess occurs with two separate kinetic steps; the rate constant for the faster step shows saturation behaviour, whereas a simple linear dependence on [H⁺] is observed for the slower step. Under similar conditions, 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-nuclear complexes indicates that the rate constants for the successive dissociation of both metal centres are statistically controlled, the value for the 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-nuclear complexes have been determined in very basic solutions, and the rate of co-ordination of the first metal ion is also found to be double that of the second one, showing again statistically controlled kinetics. These results clearly indicate that both tren sub-units of L behave independently during complex formation and decomposition, i.e. there is no significant kinetic effect caused by the proximity of the metal centres and the cryptand is flexible enough to undergo rapidly any reorganisation required to accommodate them. The kinetic data for the formation and decomposition of these compounds are also compared with literature data for related complexes, and possible reaction mechanisms are discussed.