GATED ELECTRON-TRANSFER BEHAVIOR IN COPPER(II/I) SYSTEMS - COMPARISON OF THE KINETICS FOR HOMOGENEOUS CROSS REACTIONS, NMR SELF-EXCHANGE RELAXATION, AND ELECTROCHEMICAL DATA FOR A COPPER MACROCYCLIC TETRATHIOETHER COMPLEX IN AQUEOUS-SOLUTION

The kinetics of electron-transfer reactions involving CuII/I([14]aneS4) reacting with a series of selected counterreagents have been measured in aqueous solution at 25-degrees-C, mu = 0.10 M (ClO4-). The reagents utilized include four oxidants [Ru(NH3)4bpy3+, Ni([14]aneN4)3+, Ru(NH3)2(bpy)23+, and Fe(4,7-Me2phen)3(3+)] and four reductants [Co(Me4[14]tetraeneN4)2+, Ru(NH3)4bpy2+, Ru(NH3)5isn2+, and Ru(NH3)5py2+], which were selected to provide a variety of reagent self-exchange rate constants and overall reaction potentials, thereby yielding a range of cross reaction rate constant values. Application of the Marcus equation to the cross reaction rate constants for the four reduction studies yielded consistent self-exchange rate constant values of log k11(red) = 3.78 (+/-0.26) for CuII/I([14]aneS4). By contrast, the two oxidation kinetic studies having the largest reaction potentials [i.e., using Ru(NH3)2(bpy)23+ and Fe(4,7-Me2phen)3(3+) as oxidants] yielded log k11(ox) almost-equal-to 0 (+/-0.26), while the oxidation reactions with smaller reaction potentials [i.e., using Ru(NH3)4bpy3+ and Ni([14]aneN4)3+ as oxidants] yielded an apparent value of log k11(ox) almost-equal-to 2.5 for CuII/I([14]aneS4) when relatively small concentrations of the counteroxidant were used. Upon an increase in the Ni([14]aneN4)3+ Concentration, however, k11(ox) appeared to decrease until, at relatively high concentrations of the Ni(III) reagent, limiting first-order kinetic behavior was observed. In an independent study, NMR line-broadening measurements were made on CuI([14]aneS4) solutions containing variable amounts of CuII([14]aneS4) to obtain a direct measurement of log k11(ex) almost-equal-to 3.88 +/- 0.09 at 25-degrees-C (DELTAH(double dagger) = 20.7 +/- 4.5 kJ mol-1; DELTAS(double dagger) = -101 +/- 13 J K-1 mol-1). This is in direct agreement with the values calculated from the reduction reactions. The patterns in the kinetic behavior of CuII/I([14]aneS4) are shown to be consistent with a dual-pathway square scheme, as characterized by rapid-scan cyclic voltammetry. For Cu(I)L oxidation reactions, the conformational change to form a metastable Cu(I)L species becomes rate-limiting under specific conditions, resulting in the appearance of ''gated'' electron-transfer kinetics; ultimately, the oxidation reaction switches to the alternate reaction pathway.