CYCLIC VOLTAMMETRIC EVALUATION OF RATE CONSTANTS FOR CONFORMATIONAL TRANSITIONS ACCOMPANYING ELECTRON-TRANSFER - EFFECT OF VARYING STRUCTURAL CONSTRAINTS IN COPPER(II I) COMPLEXES WITH DICYCLOHEXANEDIYL-SUBSTITUTED MACROCYCLIC TETRATHIAETHERS/

Variable-temperature slow-and rapid-scan cyclic voltammetry has been a pplied in a solvent system of 80% methanol-20% water (w/w) to both the Cu(II) and Cu(I) complexes formed with a series of five ligands in wh ich both of the ethylene bridges in the cyclic tetrathiaether [14]aneS (4) (i.e., 1,4,8,11-tetrathiacyclotetradecane) have been replaced by t rans- and/or cis-cyclohexane. All five substituted complexes exhibit e lectrochemical behavior which is consistent with the type of dual-path way electron-transfer mechanism previously observed for the parent Cu- II/I([14]aneS(4)) system in which a conformational change is proposed to occur sequentially to the electron-transfer step. The kinetic param eters associated with the formation of the metastable (CuL)-L-II inter mediate cannot be accurately established under the experimental condit ions used. However, for the formation of the corresponding metastable (CuL)-L-I intermediate, both the equilibrium constant and rate constan ts for the presumed conformational interconversion have been determine d with reasonable accuracy. Of the five systems studied, the meso-tran s, trans-and dl-trans, trans-dicyclohexanediyl-substituted ligands sho w the extremes of behavior in terms of the relative stabilities of the (CuL)-L-I and (CuL)-L-II intermediate species. This behavior is shown to be consistent with molecular mechanical calculations for the possi ble metastable intermediates with these two systems. On the basis of t he data obtained in this work, the two electron-transfer pathways are expected to be reasonably competitive for the dl-trans,trans derivativ e but extremely divergent for the meso-trans, trans derivative, the re lative differences in behavior being attributed to the tendency of the cyclohexane moieties to predispose the four sulfur donor atoms toward the various planar or tetrahedral conformations which can exist for t hese species. Consideration of the differences to be expected in the i nternal strains of the various possible conformations of the two oxida tion states leads to the hypothesis that these Cu(II/I) systems may ac tually involve a three-rung ladder mechanism rather than a simple squa re scheme, although it is doubtful that more than two rungs will ever be experimentally observable.