EFFECT OF MACROCYCLIC LIGAND CONSTRAINTS UPON THE KINETICS OF COMPLEX-FORMATION AND DISSOCIATION AND METAL-ION EXCHANGE - COPPER(II) COMPLEXES WITH CYCLOHEXANEDIYL DERIVATIVES OF THE CYCLIC TETRATHIAETHER [14]ANES(4) IN 80-PERCENT METHANOL

Complex formation kinetics have been measured for solvated Cu(II) reac ting with an entire series of cyclohexanediyl derivatives of the 14-me mbered cyclic tetrathiaether [14]aneS(4) (1,4,8,11-tetrathiacyclotetra decane) using stopped-flow spectrophotometry. In this ligand series, c is- or trans-1,2-cyclohexanediyl has been substituted for one or both of the ethylene bridges in [14]aneS(4) resulting in two monocyclohexan ediyl and five dicyclohexanediyl derivatives. The complex formation ra te constants in 80% methanol-20% water (by weight) for the reaction of Cu(II) with the parent [14]aneS(4) ligand and all seven cyclohexanedi yl-substituted derivatives at 25.0 +/- 0.2 degrees C, mu = 0.10 M (HCl O4), are found to Lie within the narrow range of (2-5) x 10(4) M(-1) s (-1). The consistency in these values implies that the introduction of cyclohexanediyl substituents does not induce significant steric effec ts relative to the first two bond formation steps. Using solvated Hg(I I) ion as a ligand scavenger, the first-order dissociation rate consta nts have been measured independently under the same conditions and are found to vary over a range of 10(4). The ratios of the formation and dissociation rate constants are shown to agree closely with the previo usly determined stability constants, thereby establishing the fact tha t the kinetically observed products are the thermodynamically stable s pecies. Strain optimization calculations indicate that one of the liga nd conformational changes, which must precede the first Cu-S bond rupt ure, represents the most unfavorable step in the dissociation process. The rate constants for direct mercury(II) exchange provide additional insight into the dissociation behavior of the Cu(II) complexes.