Temperature dependence of the formation of Tris[2,3-bis(2-pyridyl)pyrazine]-iron(II)

Abstract

The overall formation constant, β₃, of tris[2,3-bis(2- pyridyl)pyrazine]iron(II) was determined spectrophotometrically at 25, 40, 60 and 80°C as 8.9 x 10⁷, 2.9 x 10⁷, 1.0 x 10⁷ and 1.8 x 10⁶ (mol l^-1)^-3 respectively at pH > 5.5. The pK_a of the ligand is 3.1 at 25°C and decreases to about 2.0 at 80°C. The variation of β₃ with temperature is attributed to differences in the temperature dependence of the rate constants of the complex formation process and of dissociation. The overall energy of activation for complex formation is -870 J mol^-1. The complex dissociates according to first order kinetics with rate constants of 4.3 x 10^-3 s^-1 and 6.8 x 10^-2 s^-1 at 25 and 35°C respectively, and the activation energy of the dissociation reaction is about 3000 J mol^-1. The molar absorptivity of the complex, 11000 1. mol^-1 cm^-1 at the absorption maximum (λ_max 535 nm), is independent of temperature from 25 to 80°C.