Redox- and thermally-induced linkage isomerization of thieno[2,3-d]pyrimidin-4-one pentacyanoferrate(II/III) complexes

Abstract

Thieno[2,3-d]pyrimidin-4-one (LN,S) and pyrimidone (LN) pentacyanoferrate(II/III) complexes, [FeII/III(CN)5(L)]3−/2−, were prepared in H2O/DMF (3:1) solutions and characterized by spectroscopic (UV–vis), electrochemical (cyclic voltammetry), and spectroelectrochemical techniques. Results indicate a slight preference of iron(II) to coordination through the nitrogen atom, while iron(III) prefers sulfur, instead. Electrochemical experiments support linkage isomerization, Fe-κN → Fe-κS, in both oxidation states, but the phenomenon is clearly coupled to the electron transfer processes. Conventional kinetics in the presence of a large excess (102–103 fold) of the attacking reagent (N-methylpyrazinium) over the iron(II) complexes afforded the rate of dissociation of the ligand LN,S for the [FeII(CN)5]3− ions as kdII = 3.84 × 10−3 s−1 at 20 °C. Kinetic constant of isomerization was evaluated as kIINS = 5.82 × 10−2 s−1, and since it is fifteen times higher than the dissociation constant (kdII) we concluded that the mechanism of isomerization is intramolecular. Second-order rate constants, kfII = 3,40 × 102 L mol−1 s−1 and kfIII = 5.31 × 103 L mol−1 s−1 and equilibrium constants, KfII = 6.54 × 103 L mol−1 and KfIII = 6.72 × 105 L mol−1, were also calculated using the construction of a thermodynamic cycle and showed a higher stabilization of the iron(III)-LN,S complex by two orders of magnitude over the reduced species. The formal reduction potential of the redox couple [FeII/III(CN)5(LN,S)]3−/2− was determined as E½ (LN,S) = 0.25 V vs SHE. The kinetic and thermodynamic stabilities of the complexes are discussed and correlated with the metal-ligand bond type and the electronic properties of the ligands.