Influence of Coordination Geometry upon Copper(II/I) Redox Potentials. Physical Parameters for Twelve Copper Tripodal Ligand Complexes

Abstract

Twelve related tripodal ligands have been synthesized in which the three legs linked to a bridgehead nitrogen are 2-methyl- or 2-ethylthioethyl and/or 2-pyridylethyl or -methyl. Utilization of both terminal methyl and ethyl groups on the thiaether legs was designed to determine whether slight differences in solvation or steric effects might cause detectable changes in properties. Inclusion of both methyl and ethyl linkages of the pyridines to the bridgehead nitrogen provides a comparison of the effect of five- versus six-membered chelate rings, respectively. For each of the tripodal ligands included in this work, the protonation constants and Cu(II) complex stability constants were carefully determined in aqueous solution at 25 °C, μ = 0.10 M (ClO4-). The CuII/IL redox potentials were also determined using slow-scan cyclic voltammetry, thereby permitting the stability of the Cu(I) complexes to be calculated. The stability constants for the twelve Cu(II) complexes range from 106 to 1017, increasing by 104−105 as the first and second alkylthioethyl substituents are replaced by 2-pyridylmethyl groupswith only a slight increase upon the introduction of a third pyridyl leg. When 2-pyridylethyl groups are introduced, much smaller trends are noted. For the corresponding Cu(I) complexes, the calculated stability constants are relatively constant (at ∼1015) regardless of the donor set or the length of the pyridyl linkages to the bridgehead. Combination of these data with previous measurements on related macrocyclic and acyclic ligands containing both thiaether sulfur and amine nitrogen donor atoms reveals that, for 35 different uncharged terdentate, quadridentate and quinquedentate ligands, the stabilities of the CuIL complexes lie within the narrow range of about 1012−1016, with few exceptions, regardless of large differences in coordination geometry and donor strength. For these same 35 ligands, the CuIIL stability constants span 26 orders of magnitude. 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 CuIIL stability constants for a wide range of ligand types. Future strategies for manipulating the redox behavior of Cu(II/I) systems should recognize that alteration of the ligand coordination geometry primarily impacts the properties of the Cu(II) complex with almost no effect upon the Cu(I) properties.