A spectrophotometric and thermodynamic study of the sitting-atop complex formation from reaction between free base meso-tetraarylporphyrins and zirconyl nitrate in chloroform solution

The sitting-atop complexation of meso-tetraarylporphyrins and its para-substituted derivatives (H₂t(4-X)pp, X:H, Br, Cl, CH(CH₃)₂, OCH₃, CH₃), as electron donors, with zirconyl, as an electron acceptor, have been investigated spectrophotometrically in chloroform. The mole ratio studies based on physicochemical techniques were employed clearly and revealed the formation of 1:1 sitting-atop complexes which was confirmed by UV–vis, ¹H NMR and IR spectroscopic data. The value of the formation constant was estimated for each complex using a nonlinear optimization of the complex absorbance vs. mole ratio data by package KINFIT. The results showed that the stability of these complexes decreases with the temperature enhancement. Thermodynamic parameters, ΔG°, ΔH° and ΔS°, of the SAT complexes have been determined from the temperature dependence of formation constants by Van’t Hoff equation. Also, the influence of the substituents of the aryl rings in H₂t(4-X)pp on the stability of the SAT complexes is discussed.