MACROCYCLIC LIGAND COMPLEXATION KINETICS - SOLVENT, RING SIZE, AND MACROCYCLIC EFFECTS ON THE FORMATION AND DISSOCIATION REACTIONS OF COPPER(II) CYCLIC POLYTHIAETHER COMPLEXES

The kinetics of solvated Cu(II) reacting with a series of open-chain and macrocyclic polythiaethers have been studied in methanol-water mixtures at 25°C using the stopped-flow and temperature-jump methods. From a determination of the dependence of the formation rate constant values on the solvent composition, it has been established that Cu(H2O)62+ represents the only reactive species of solvated Cu(Il) over the range of 0-60% CH3OH (by weight). Based on this observation, aqueous formation and dissociation rate constants have been generated for two open-chain tetrathiaethers, five cyclic tetrathiaethers, and one cyclic pentathiaether (i.e., Me2- and Et2-2,3,2-S4; 12-, 13-, 14-, 15-, and 16-ane-S4, and 15-ane-S5). The 25°C aqueous formation rate constants range from a low of 6.5 X 103 s-1 for the smallest macrocycle, 12-ane-S4, to a high of 4.2 X 106 M-l s-1 for the least sterically hindered open-chain ligand, Me2-2,3,2-S4, while the dissociation rate constant extremes are 2.6 and 4.5 X 104 s-1 for these same two ligand systems, respectively. Aqueous activation parameters have been obtainable for only one ligand system, 15-ane-S4, for which the formation and dissociation values are ΔHf(25°C) = 6.1 kcal mol-1, ΔHf = -11 eu, ΔH0= 10 kcal mol-1, ΔS0= -13 eu. It is concluded that second-bond formation is the probable rate-determining step for all of the cyclic polythiaether complexes studied with the variable difficulty in achieving ring closure accounting for observed differences in the formation rate constants. The primary manifestations of the ring-size effect and the macrocyclic effect are associated with the final steps in the complexation process and, thus, tend to parallel the dissociation rate constants. Copyright © 1979, American Chemical Society. All rights reserved.