Effect of conformational constraints on gated electron transfer kinetics .2. Copper(II/I) complexes with phenyl-substituted [14]aneS(4) ligands in acetonitrile

Kinetic studies have been conducted in acetonitrile on the electron-transfer reactions of five copper(II/I) complexes involving ligands in which either a benzene or a cyclohexane ring, or both, have been substituted into the ligand backbone of the 14-membered tetrathiamacrocycle [14]aneS(4). The specific ligands utilized in this work include 2,3-benzo-1,4,8,11-tetrathiacyclotetradecane (bz-[14]aneS(4)), 2,3-trans-cyclohexano-1,4,8,11-tetrathiacyclotetradecane cane (trans-cyhx-[14]aneS(4)), 2,3-benzo-9,10-trans-cyclohexano-1,4,8,11-tetrathiacyclotetradecane (bz,trans-cyhx-[14]aneS(4)), 2,3-benzo-9,10-cis-cyclohexano-1,4,8,11-tetrathiacyclotetradecane (bz,cis-cyhx-[14]aneS4), and 2,3-cis-9,10-trans-dicyclohexano-1,4,8,11-tetrathiacyclotetradecane (cis, trans-dicyhx-[14]aneS(4)). Each (CuL)-L-II/I system has been reacted with three separate reducing agents and three separate oxidizing agents to examine the effect of driving force upon the kinetic parameters. The Marcus relationship has been applied to each crossreaction rate constant to estimate the apparent self-exchange rate constant, k(11), for each (CuL)-L-II/I system. For all but one of the five systems, the k(11) values obtained from the three reduction reactions are in virtual agreement with the corresponding value obtained for the oxidation reaction with the smallest driving force. As the driving force for (CuL)-L-I oxidation increases, a smaller k(11) value is calculated for each system. This behavior is consistent with our previously proposed dual-pathway square scheme mechanism in which a significant conformational change occurs as a separate step preceding electron transfer in the case of (CuL)-L-I oxidation. Although direct observation of conformationally gated electron transfer was not attained for any of the five systems included in the current work, limits for the rate constant for conformational change have been estimated from the conditions required to change the apparent pathway for the oxidation kinetics. These limits show that the (CuL)-L-I complex involving a single phenyl substituent (bz-[14]aneS(4)) exhibits a much slower conformational change than do any of the other systems included in this study. The implications of this observation are discussed.