Influence of coordination geometry upon copper(II/I) redox potentials. Physical parameters for twelve copper tripodal ligand complexes

Twelve related tripodal ligands have been synthesized in which the three le gs linked to a bridgehead nitrogen are 2-methyl- or 2-ethylthioethyl and/or 2-pyridylethyl or -methyl. Utilization of both terminal methyl and ethyl g roups on the thiaether legs was designed to determine whether slight differ ences in solvation or steric effects might cause detectable changes in prop erties. Inclusion of both methyl and ethyl linkages of the pyridines to the bridgehead nitrogen provides a comparison of the effect of five- versus si x-membered chelate rings, respectively. For each of the tripodal ligands in cluded in this work, the protonation constants and Cu(II) complex stability constants were carefully determined in aqueous solution at 25 degrees C, m u = 0.10 M. (ClO4-). The (CuL)-L-II/I redox potentials were also determined using slow-scan cyclic voltammetry, thereby permitting the stability of th e Cu(I) complexes to be calculated. The stability constants for the twelve Cu(II) complexes range from 10(6) to 10(17), increasing by 10(4)-10(5) as t he first and second alkylthioethyl substituents are replaced by 2-pyridylme thyl groups-with only a slight increase upon the introduction of a third py ridyl leg. When 2-pyridylethyl groups are introduced, much smaller trends a re noted. For the corresponding Cu(I) complexes, the calculated stability c onstants are relatively constant (at similar to 10(15)) regardless of the d onor set or the length of the pyridyl linkages to the bridgehead. Combinati on of these data with previous measurements on related macrocyclic and acyc lic ligands containing both thiaether sulfur and amine nitrogen donor atoms reveals that, for 35 different uncharged terdentate, quadridentate and qui nquedentate ligands, the stabilities of the (CuL)-L-I complexes lie within the narrow range of about 10(12)-10(16), with few exceptions, regardless of large differences in coordination geometry and donor strength. For these s ame 35 ligands, the (CuL)-L-II stability constants span 26 orders of magnit ude. Thus, the Cu(II/I) potentials, which cover a range of 1.5 V, are shown to be inversely related to the logarithmic values of the (CuL)-L-II stabil ity constants for a wide range of ligand types. Future strategies for manip ulating the redox behavior of Cu(II/I) systems should recognize that altera tion of the ligand coordination geometry primarily impacts the properties o f the Cu(lI) complex with almost no effect upon the Cu(I) properties.