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, ΔH‡ = 10.1 kcal/mole, ΔS‡ = -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-1sec-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)+(Kf1= 3.1 × 103M-1) and Cu(pmas)(N3)2(Kf2= 3.5 × 101M-1) but Cu(tmpa)(N3)+(Kf= 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. © 1987.
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