Variable-temperature slow- and rapid-scan cyclic voltammetry has been applied 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 which both of the ethylene bridges in the cyclic tetrathiaether [14]aneS(4) (i.e., 1,4,8,11-tetrathiacyclotetradecane) have been replaced by trans- and/or cis-cyclohexane. All five substituted complexes exhibit electrochemical behavior which is consistent with the type of dual-pathway 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 parameters associated with the formation of the metastable Cu(II)L intermediate cannot be accurately established under the experimental conditions used. However, for the formation of the corresponding metastable Cu(I)L intermediate, both the equilibrium constant and rate constants for the presumed conformational interconversion have been determined with reasonable accuracy. Of the five systems studied, the meso-trans,trans- and dl-trans,trans-dicyclohexanediyl-substituted ligands show the extremes of behavior in terms of the relative stabilities of the Cu(I)L and Cu(II)L intermediate species. This behavior is shown to be consistent with molecular mechanical calculations for the possible metastable intermediates with these two systems. On the basis of the data obtained in this work, the two electron-transfer pathways are expected to be reasonably competitive for the dl-trans,trans derivative but extremely divergent for the meso-trans,trans derivative, the relative 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 these species. Consideration of the differences to be expected in the internal strains of the various possible conformations of the two oxidation states leads to the hypothesis that these Cu(II/I) systems may actually involve a three-rung ladder mechanism rather than a simple square scheme, although it is doubtful that more than two rungs will ever be experimentally observable.

No abstract

No abstract

Extensive cyclic voltammetric measurements on the copper complexes formed with 1,4,8,11 -tetrathiacyclotetradecane ([14]aneS4) and related systems have been carried out in both water and 80% (w/w) methanol in order to characterize the unusual electron-transfer behavior previously observed for the former system and related copper complexes. The methanolic media have permitted the use of temperatures as low as -77 O C , thereby making it possible to demonstrate properties of the Cu"/' ([ 14]aneS4) system that would otherwise require inaccessibly large scan rates. The results are shown to be consistent with a square scheme in which two CUI ([ 14]aneS4) conformers exist, the less stable of which is proposed to approximate more closely the geometry of the stable CUI'( [ 14]aneS4) species. Of the two corresponding Cu"( [ 14]aneS4) conformers, only the reduction of the more stable species can be observed directly, apparently due to the very large equilibrium constant favoring this species. The values of the equilibrium and rate constants relating the two Cu' ([14]aneS4) and the two CUI' ([ 14]aneS4) conformers have been estimated for 25 O C . With these values, computer-simulated cyclic voltammograms have been generated that closely approximate the experimental data. It is proposed that square schemes of this type exist for most Cu(II)/(I) complexes and account for the discrepancies previously observed in homogeneous cross-reaction electron-transfer reactions involving both inorganic copper complexes and copper enzymes.The role of copper proteins in biological electron-transfer processes' has stimulated a high level of interest in the kinetics and mechanisms of copper redox reactions. Many kinetic studies have attempted to evaluate the characteristic electron self-exchange rate constant, k , , , for the CU"L/CU'L redox couple (eq 1) k