Kinetics and equilibrium studies of the reduction of copper(II) with a tetradentate bis(pyridyl)-dithioether ligand

The reduction of [Cu(pmas)]+2 by hydroxyethylferrocene [HEF] has been studied in the presence of azide, imidazolate and pyridine. Both reactants exhibit a first order dependence at 25.0°C, pH 6.0, I = O.IM (NaNO^) with second order rate constant of 1.3 x 10 M~ s~ . +2/+ The apparent Cu(pmas) self-exchange electron transfer rate constant calculated on the basis of relative Marcus theory (k,, = 4.7 x 10 M s ) agrees well with previous findings on ferrocyto--4 +2 chrome £, Fe(CN)g and Ru (NH^)^ py reactions with +2 [Cu(pmas)] . The copper complex reduction has been studied as a function of temperature, and the enthalpy and entropy of activation are 10.1 Kcal/mol and -6 e.u., respectively. The formation constants of the 1:1 and 2:1 complexes of Cu(pmas)+ 2 with azide were measured spectrophotometrically. The values (K^, = 3.3 x 10 M ) and 5 -1 +2( K ^ ^ = 3 . 5 x l O M ) were compared with Cu(tmpa) [tris(2- yridylmethyl)amine] copper(II) binding constants. The latter 2-1 only presented one value (K^ = 6.6 x 10 M ), up to 0.15 M N^ (25.0°C, pH 6.0, I = 0.2 M), which demonstrates that a thioether sulfur atom is displaced in the uptake of a second N^ nit by Cu(pmas)(N^) . The effect of thioether sulfur displacement by azide ion on the redox reaction rate may be understood entirely through the tendency of N^ to shift the position of the redox equilibrium towards reactant side. Cyclic voltammetry measurements showed that the eduction potentials of Cu(pmas) +2 in the presence of 5 and 150 mM N^ are 307 and 254 mV respectively, as compared with that of free complex, 398 mV. The rate constant is decreased when azide is present (k (25.0°C) = 3 x IO^M'-'-S"-'- pH 6.0, I = 0.2 M). This demonstrates the importance of bond breaking in the activated complex for reduction of five-coordinate Cu(II) to four-coordinate (Cu(I) because the aqueous form has a weakly-held coordinated water molecule, but the azide form has a strongly-retained nitrogen-donor ligand. Also the dependence of the reaction rate on acetonitrile concentration will be described.