Thermodynamic parameters of complexation of tetraphenylporphyrinatozinc(II) with pyridines in chloroform at 273-313 K are linearly related to the corresponding shifts of their absorption maxima in the electronic spectra, logarithms of the stability constants of the complexes, pK a values of the ligands in water, and Hammett substituent constants σ provided that steric factors are absent. Linear correlations also exist between thermodynamic and kinetic parameters of some nucleophilic substitution reactions and complex formation of tetraphenylporphyrinatozinc(II) with pyridines and pyridine N-oxides.Processes involving axial coordination of various ligands are responsible for biological activity of many porphyrin metal complexes [1,2]. However, studies on complex formation of metal porphyrins in vivo are very difficult to perform. Therefore, such studies are often carried out in vitro using readily accessible from the synthetic viewpoint 5,10,15,20-tetraphenylporphyrinatozinc(II) (Zn-TPP) whose IR, electronic absorption and emission, and NMR spectra are fairly simple and very sensitive to interactions with nucleophiles, ligands, and bases.We previously showed [3] that shifts of absorption maxima (Δλ) in the electronic spectra of Zn-TPP in chloroform, resulting from interaction with pyridine, quinoline, and acridine N-oxides (provided that steric factors are absent), are linearly related to the logarithms of the stability constants of the complexes, pK a values of the ligands in water and other solvents, and Hammett constants σ of substituents in the heterocycle. In the present article we report on complex formation of Zn-TPP with pyridines containing substituents in positions 3 and 4 of the heteroring.Complex formation of pyridine and its derivatives with zinc porphyrins in various solvents was studied in [4][5][6][7][8], and several complexes with pyridine were isolated as individual crystalline substances and analyzed by X-ray diffraction [9] (CSD refcodes: HPORN10, VIXGEO, PRPHZN, EPOPZN10). The goal of the present work was to determine thermodynamic parameters of coordination of substituted pyridines with Zn-TPP and analyze these parameters in terms of the linear free energy relationship principle.The experimental stability constants K of molecular complexes formed by Zn-TPP with pyridine derivatives in chloroform are concentration parameters. On the other hand, taking into account low concentration of the initial compounds (2 × 10 -5 M for Zn-TPP and 10 -4 -10 -3 M for ligands) and the absence of ionic species in solution, we presume that the obtained experimental values insignificantly differ from thermodynamic. We found that, as with heteroaromatic N-oxides [3], in the absence of steric factors shifts of
