K. M. Davies, K. D. Whyte, A. H. Gilbert
Nov 1, 1990
Citations
0
Influential Citations
5
Citations
Journal
Inorganica Chimica Acta
Abstract
Abstract The kinetics of electron transfer between 1,8-bis(2-pyridyl)-3,6-dithiaoctane copper(II), Cu(pdto) 2+ , and ferrocene, FeCp 2 , in acetonitrile have been studied by stopped-flow spectrophotometry, as a function of ferrocene concentration and the concentration of added electrolytes, n-Bu 4 NPF 6 , n-Bu 4 NBF 4 and NaCF 3 CO 2 . The reactions exhibit different rate dependencies with the three salts. The second-order rate constant, k 2 =(2.1±0.2)x10 4 M −1 S −1 , measured in CH 3 CN at 25 °C with excess ferrocene between 1.2x10 −3 and 7.5x10 −3 M is unaffected by n-Bu 4 NPF 6 added up to 0.10 M. A lowered second-order rate constant, k 2 =(1.56±0.15)x10 3 M −1 S −1 is obtained with [NaCF 3 CO 2 ] between 0.01 and 0.10 M. Inner-sphere association of CF 3 CO 2 − with the Cu(II) center is proposed to explain the rate inhibition in trifluoroacetate solutions. Values of 15 and 0.25 M −1 S −1 have been estimated for the Cu(II)/Cu(I) self-exchange electron transfer rate constant in n-Bu 4 NPF 6 and NaCF 3 CO 2 media, using the Marcus cross relations. With added n-Bu 4 NBF 4 , the measured second order rate constant, k 2 , increases with [BF 4 − ] reaching 1.75x10 5 M −1 S −1 at 0.050 M salt concentration. The rate data in BF 4 − solutions is fitted to a rate law involving ion-pair formation between BF 4 − and Cu(pdto) 2+ , with the ion-paired species showing enhanced electron transfer reactivity. Spectral changes that accompany the addition of BF 4 − to Cu(pdto) 2+ solutions suggest that the association of BF 4 − facilitates electron transfer by modifying the energy of the electron acceptor orbital in the redox change.