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-tr ansfer reactions of five copper(II/I) complexes involving ligands in w hich either a benzene or a cyclohexane ring, or both, have been substi tuted into the ligand backbone of the 14-membered tetrathiamacrocycle [14]aneS(4). The specific ligands utilized in this work include 2,3-be nzo-1,4,8,11-tetrathiacyclotetradecane (bz-[14]aneS(4)), ans-cyclohexa no-1,4,8,11-tetrathiacyclotetradecane cane (trans-cyhx-[14]aneS(4)), a ns-cyclohexano-1,4,8,11-tetrathiacyclotetradecane (bz,trans-cyhx-[14]a neS(4)), cis-cyclohexano-1,4,8,11-tetrathiacyclotetradecane (bz,cis-cy hx-[14]aneS4), and s-dicyclohexano-1,4,8,11-tetrathiacyclotetradecane (cis, trans-dicyhx-[14]aneS(4)). Each (CuL)-L-II/I system has been rea cted with three separate reducing agents and three separate oxidizing agents to examine the effect of driving force upon the kinetic paramet ers. The Marcus relationship has been applied to each crossreaction ra te 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 vi rtual agreement with the corresponding value obtained for the oxidatio n 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 conformati onal change occurs as a separate step preceding electron transfer in t he case of (CuL)-L-I oxidation. Although direct observation of conform ationally gated electron transfer was not attained for any of the five systems included in the current work, limits for the rate constant fo r conformational change have been estimated from the conditions requir ed to change the apparent pathway for the oxidation kinetics. These li mits show that the (CuL)-L-I complex involving a single phenyl substit uent (bz-[14]aneS(4)) exhibits a much slower conformational change tha n do any of the other systems included in this study. The implications of this observation are discussed.