Previous kinetic and electrochemical studies of copper complexes with macrocyclic tetrathiaethers-such as 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4)-have indicated that electron transfer and the accompanying conformational change occur sequentially to give rise to a dual-pathway mechanism. Under appropriate conditions, the conformational change itself may become rate-limiting, a condition known as "gated" electron transfer. We have recently hypothesized that the controlling conformational change involves inversion of two donor atoms, which suggests that "gated" behavior should be affected by appropriate steric constraints. In the current work, two derivatives of [14]aneS4 have been synthesized in which one of the ethylene bridges has been replaced by either cis- or trans-1,2-cyclopentane. The resulting copper systems have been characterized in terms of their Cu(II/I)L potentials, the stabilities of their oxidized and reduced complexes, and their crystal structures. The electron self-exchange rate constants have been determined both by NMR line-broadening and by kinetic measurements of their rates of reduction and oxidation with six or seven counter reagents. All studies have been carried out at 25 degrees C, mu = 0.10 M (NaClO4 and/or Cu(ClO4)2), in aqueous solution. Both Cu(II/I) systems show evidence of a dual-pathway mechanism, and the electron self-exchange rate constants representative of both mechanistic pathways have been determined. The first-order rate constant for gated behavior has also been resolved for the Cu(I)(trans-cyclopentane-[14]aneS4) complex, but only a limiting value can be established for the corresponding cis-cyclopentane system. The rate constants for both systems investigated in this work are compared to values previously determined for the Cu(II/I) systems with the parent [14]aneS4 macrocycle and its derivatives involving phenylene and cis- or trans-cyclohexane substituents. The results are discussed in terms of the influence of the fused rings on the probable conformational changes accompanying the electron-transfer process.

The kinetics of a series of Cu(II/I)−acyclic tetrathiaether complexes reacting with several oxidizing and reducing reagents have been examined in aqueous solution at 25 °C. This investigation has included a re-examination of CuII/I(Me2-2,3,2-S4) (Me2-2,3,2-S4 = 2,5,9,12-tetrathiatridecane = L12a), containing the ethylene−trimethylene−ethylene bridging sequence, plus three newly synthesized ligands containing an alternate bridging sequence of trimethylene−ethylene−trimethylene:  2,6,9,13-tetrathiatetradecane (Me2-3,2,3-S4 = L12b) and two cyclohexanediyl-substituted derivatives, viz., cis-1,2-bis[(3-methylthiopropyl)thio]cyclohexane (cis-cyhx-Me2-3,2,3-S4 = L14) and trans-1,2-bis[(3-methylthiopropyl)thio]cyclohexane (trans-cyhx-Me2-3,2,3-S4 = L15). The corresponding phenylene derivative, 1,2-bis[(3-(methylthio)propyl)thio]benzene (bz-Me2-3,2,3-S4 = L13), was also synthesized but did not form a measurable copper complex. The conditional stability constants for CuIIL (K Cu II L ‘) and CuIL (K Cu I L ‘) and the CuII/IL formal redox potentials (E f) vs NHE at 25 °C (generally at μ = 0.10 (NaClO4)) are as follows:  for L12b, 15 M-1, 1.0 × 1013 M-1, 0.83 V; for L14, 2.8 × 102 M-1, 0.95 × 1013 M-1, 0.75 V; for L15, 8.8 × 102 M-1, 6.3 × 1013 M-1, 0.77 V. Application of the Marcus relationship to the experimentally determined cross-reaction rate constants yielded self-exchange rate constants for all four CuII/IL acyclic systems which were relatively constant for both oxidation and reduction under a wide range of conditions. This contrasts sharply with previous results obtained for corresponding macrocyclic ligand systems.

The two ethylene bridges in the macrocyclic tetrathiaether 1,4,8,11-tetrathiacyclotetradecane ([14]aneS(4)) have been systematically replaced by cis- or trans-1,2-cyclopentane to generate a series of new ligands that exhibit differing preferences for the orientation of the sulfur donor atoms while maintaining constant inductive effects. The resulting five dicyclopentanediyl derivatives, along with two previously synthesized monocyclopentanediyl analogues, have been complexed with Cu(II), and their stability constants, formation and dissociation rate constants, and redox potentials have been determined in 80% methanol/20% water (by weight). The crystal structures of the Cu(II) complexes with the five dicyclopentanediyl-[14]aneS(4) diastereomers as well as the structures for a representative Cu(I) complex and one of the free ligands have also been determined. The properties of these complexes are compared to previous data obtained for the corresponding cyclohexanediyl derivatives in an attempt to shed additional light on the influence of sterically constraining substituents upon the properties of macrocyclic ligand complexes.