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 groupswith 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.

Furin Inhibition by Compounds of Copper and Zinc

“…In analogy to our proposed mechanism, the transfer of copper between the yeast copper chaperone ATX1 and the copper transporter CCC2 is suggested to take place by thiol exchange, following the associative sequence (Cys ATX1 ) 2 (30). In our case, reduction to Cu(I) is likely to take place upon binding of the solvated ion to Cys 198 of furin (31,32). Bonds to Cu(I) are much more labile than bonds to Cu(II) (24).…”

“Green” Atom Transfer Radical Polymerization: From Process Design to Preparation of Well-Defined Environmentally Friendly Polymeric Materials

“…Again, as in the case of N-donor ligands, there was a pronounced increase of the redox potential (decrease of the reducing strength) as an additional methylene group was added between the donor atoms, causing the formation of 6-instead of 5-membered chelate rings. 238 Similarly, the E 1/2 value of Cu I complexes with N 2 S 2 -type ligands increases as a chelate ring in the complexes expands from 5-to 6-and then to 7-membered. 239 A detailed review on the Cu complexes with ligands containing the N 2 S 2 -donor set 240 demonstrates that the electrochemical properties can serve as a "probe" for the structural characteristics of the complexes.…”

“…237 It was shown that, in ligands analogous to TPMA with mixed donor atoms (N and S; Figure 10), the increase of the number of sulfur atoms led to formation of less reducing Cu I complexes. 238 Consequently, although sulfur-only containing ligands are not appropriate as components of active ATRP catalysts, "heterodonor" ligands could possess a better catalytic activity. Such complexes are of interest because they may be sufficiently stable in the presence of acids and therefore may be used to mediate the ATRP of acidic monomers.…”

“…The stability constants of the Cu I and Cu II complexes of TPMA, 238 DETA, 225 PMDETA, 225 TETA, 225 HMTETA, 225 Me 6 TREN, 268,269 and cyclam and its S-containing analogues 359,360 are taken from the literature. The Cu I complexes of PMDETA and Me 6 -TREN are not suitable for aqueous ATRP due to very fast disproportionation.…”

Catalytic catechol oxidation by copper complexes: development of a structure–activity relationship

“…For example L 1 stands for the fragment N,N-bis(2-pyridylmethyl)aminoethyl and Py for 2-pyridylmethyl. Ligand L 1 SCH 3 was synthesized by reductive amination of 2-(methylthio)ethylamine, avoiding the use of toxic 2-chloroethyl methyl sulfide, 25 and increasing the yield from 53% to 99%. A similar synthesis was used for the formation of L 1 OH increasing the reported yield from 85% to 99%.…”