Complex Formation of Tetraphenylporphyrinatozinc(II) and Nucleophilic Substitution Reactions with Pyridines and Pyridine N-Oxides

Abstract

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

Determination of Stability of Molecular Complexes of Zinc(II) meso-Tetraphenylporphyrin with Heterocyclic N-Oxide and Pyridine by Different Methods

“…In general, the stability constants of the complex ZnTPP•Py calculated by various methods, within the error limits are the same (K = 3876 ± 374 l·mol -1 ), which is consistent with published data. [19] In contrast to the stability constants of the complex ZnTPP•4-MePyO, the differences are so significant (from 800 to 15000 l·mol -1 ), which makes the problematic choice of the mean value (Table 4). Additional studies by thin layer chromatography (eluent -chloroform, stationary phase -a plate "Sorbifil" Al 2 O 3 ) of solutions ZnTPP and its complex with 4-MePyO, kept at room Table 4.…”

Quantitative relations holding in coordination of (Tetraphenylporphyrinato)zinc(II) and nucleophilic substitution with anilines

“…Using the extended Taft equation (1), modified by Litvinenko et al [4] for amines, we succeeded [5] in obtaining correlation (2) between log K in benzene (determined by calorimetry), Σσ*, E N , and log k for the reaction of phenacyl bromide with amines in benzene at 25°C. log k = log k 0 + ρ* Σσ* + δ E N ,…”

“…We previously showed [1][2][3] that shifts of absorption maxima (Δλ) in the electronic spectra of Zn-TPP in chloroform upon reaction with 3-and 4-substituted pyridines, as well as with pyridine, quinoline, and acridine N-oxides, are linearly related to logarithms of the stability constants (K) of the complexes, pK a of ligands (L) in water and other solvents, and Hammett constants σ of substituent in the heteroring.…”

“…log(K/k) = 2.77 + 2.73 Σσ* -0.94 E N ; r = 0.983. (2) The goal of the present work was to analyze kinetic and thermodynamic parameters of coordination of Zn-TPP with substituted anilines Ia-Iv in chloroform. DOI: 10.1134/S1070428012060048 We found that shifts of the absorption maxima (Δ λ) in the electronic spectra observed in reactions of Zn-TPP with anilines in chloroform are linearly related (r = 0.95-0.98) to logarithms of the stability constants of the complexes (Δ λ II = 3.26 log K + 6.85), pK a values of the ligands in water (Δ λ II = 1.11 pK a + 9.15), and Hammett constants σ of substituents in the benzene ring of anilines (Δ λ II = 2.92 σ + 14.12) provided that steric factors are lacking (see table).…”

“…Unlike pyridines and heteroaromatic N-oxides [1][2][3], the behavior of anilines upon complexation with Zn-TPP is described much better with the use of substituent constants σ + (log K = -0.57σ + + 2.13; r = 0.99, n = 11) rather than Hammett constants σ (r = 0.91). It was specially noted [6] that the necessity of using constants σ + is determined by the presence of just vacant orbital rather than positive charge on the reaction center.…”