Electron-Transfer Kinetics and Thermodynamic Characterization of Copper(II/I) Complexes with Acyclic Tetrathiaethers in Aqueous Solution

• Published in: Inorganic chemistry 1997-09, Vol.36 (20), p.4484-4489
• Main Authors: Dunn, Brian C, Wijetunge, Prabodha, Vyvyan, James R, Howard, Turner A, Grall, Andrew J, Ochrymowycz, L. A, Rorabacher, D. B
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic970441s

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.