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

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 degrees C. This investigation has included a re-examination of Cu(II/I)(Me(2)-2,3,2-S(4)) (Me(2)-2,3,2-S(4) = 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 (Me(2)-3,2,3-S(4) = L12b) and two cyclohexanediyl-substituted derivatives, viz., cis-1,2-bis[(3-methylthiopropyl)thio]cyclohexane (cis-cyhx-Me(2)-3,2,3-S(4) = L14) and trans-1,2-bis[(3-methylthiopropyl)thio]cyclohexane (trans-cyhx-Me(2)-3,2,3-S(4) = L15). The corresponding phenylene derivative, 1,2-bis[(3-(methylthio)propyl)thio]benzene (bz-Me(2)-3,2,3-S(4) = L13), was also synthesized but did not form a measurable copper complex. The conditional stability constants for Cu(II)L (K(Cu)()II(L)(')) and Cu(I)L (K(Cu)()I(L)(')) and the Cu(II/I)L formal redox potentials (E(f)) vs NHE at 25 degrees C (generally at &mgr; = 0.10 (NaClO(4))) are as follows: for L12b, 15 M(-)(1), 1.0 x 10(13) M(-)(1), 0.83 V; for L14, 2.8 x 10(2) M(-)(1), 0.9(5) x 10(13) M(-)(1), 0.75 V; for L15, 8.8 x 10(2) M(-)(1), 6.(3) x 10(13) 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 Cu(II/I)L 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.