2-Mercaptobenzoxazole pentacyanoferrate(II/III) complexes—kinetics of formation, substitution and the thermodynamic cycle

Abstract

The kinetics of substitution and formation of [Fe II/III(CN) 5(bzoxsh)] 3−/2− (bzoxsh=2-mercaptobenzoxazole) complexes were investigated in a MeOH/H 2O (75:25%) solvent mixture at μ=1.00 NaClO 4 mol l −1. The rates of bzoxsh dissociation for the Fe II/III ions, k d II =3.7×10 −4 and k d III =4.5×10 −3 s −1 at 25 °C, were determined through conventional kinetics in the presence of a large excess (10 2–10 3-fold) of the attacking reagents (L=dimethylsulfoxide (dmso), pyridine (py) and N-methylpyrazinium) over the iron complexes. Second-order rate constants, k f II =2.9×10 2 and k f III =3.9×10 6 dm 3 mol −1 s −1 and equilibrium constants, K f II =7.8×10 5 and K f III =8.6×10 8 dm 3 mol −1, were also calculated using the construction of a thermodynamic cycle. The formal reduction potential of the Fe III/Fe II complexes were determined as E f bzoxsh=0.19 V versus EPH. Activation parameters for the dissociation reactions of [Fe III(CN) 5(bzoxsh)] 2− were found with Δ S #=+11.4 J K −1 mol −1 and Δ H #=93.3 kJ mol −1. Specific rate constant for the dissociation of [Fe III(CN) 5(L)] 2−, L=dmso and py, were obtained as 2.5×10 −5 and 1.5×10 −5 s −1, respectively. 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.