Oxidation of (hydroxyethyl)ferrocene by [2-pyridylmethylbis(2-ethylthioethyl)/amine]copper(II)

A kinetic study of the oxidation of (hydroxyethyl)ferrocene (HEF) by 2-pyridylmethylbis(2-ethyl-thioethyl)ainine]copper(II) (Cu(pmas)²⁺) is reported, with the objective of documenting the influence of the two thioether sulfur ligands on the electron transfer rate. Both reactants exhibit a first-order dependence at pH 6, I = 0.1 M(NaNO₃); k(25°C) = 1.3 × 10⁴M⁻¹sec⁻¹, ΔH3 = 10.1 kcal/mole, ΔS3 = −6 eu. The apparent Cu(pmas)^2+/+ self-exchange electron transfer rate constant calculated from this reaction on the basis of relative Marcus theory (4.7 × 10¹M⁻¹ sec⁻¹) agrees well with previous findings on ferrocytochrome c, Fe(CN)₆⁴⁻, and Ru(NH₃)₅py²⁺ oxidations. Spectrophotometric titrations of Cu(pmas)²⁺ and Cu(tmpa)²⁺ (tmpa = tris(2-pyridylmethyl)amine) with azide ion showed that both Cu(pmas)N₃)⁺ (K_f1 = 3.1 × 10³M⁻¹) and Cu(pmas)(N₃)₂ (K_f2 = 3.5 × 10¹M⁻¹) but Cu(tmpa)(N₃)⁺ (K_f = 6.6 × 10²M⁻¹) are formed up to 0.15 M N₃⁻ (25°C, pH 6, I = 0.2 M), suggesting that a thioether sulfur atom is displaced in the uptake of a second N₃⁻ ion by Cu(pmas)(N₃)⁺. The effect of thioether sulfur displacement by azide ion on the HEF-Cu(pmas)²⁺ reaction rate may be understood entirely through the tendency of N₃⁻ to shift the position of the redox equilibrium towards the reactant side, without invoking any special role for the sulfur ligand in promoting electron transfer reactivity.