Cyclic Voltammetric Evaluation of Rate Constants for Conformational Transitions Accompanying Electron Transfer....

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both of the ethylene bridges in the cyclic tetrathiaether [14]aneS(4) (i.e., 1,4,8,11-tetrathiacyclotetradecane) have been replaced by trans- and/or cis-cyclohexane.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.