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

Abstract

Thermodynamic parameters (ΔG°, ΔS°) of quasi-isoenthalpic coordination of (tetraphenylporphyrinato)zinc(II) with anilines (except for 4-halo derivatives) in chloroform at 273-313 K in the absence of steric factors are linearly related to shifts of their absorption bands in the electronic spectra in reactions with anilines, as well as with logarithms of the stability constants of the complexes, pK a values of the ligands in water, and Hammett substituent constants σ + . Linear relations were also found between thermodynamic and kinetic parameters of some nucleophilic substitution reactions and complex formation of (tetraphenylporphyrinato)zinc(II) with anilines.The present article continues our studies on quantitative relations between the stability constants of complexes formed by (tetraphenylporphyrinato)zinc(II) (Zn-TPP) with various organic compounds, rate constant of nucleophilic substitution, and physicochemical parameters characterizing these processes. We previously showed [1-3] 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.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.Here, k is the rate constant, the term ρ* Σσ* describes the inductive effect of all substituents on the nitrogen atom, and δE N characterizes steric factor (δE s in the Taft equation). 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. R 2 = X = H, R 1 = Me (p), Et (q); Ph (r); R 1 = R 2 = Et, X = H (s);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).The experimental stability constants (K) of the Zn-TPP complexes with aniline derivatives in chloro-

Crystal structure of molecular complexes of zinc(II) tetraphenylporphyrin with pyridine and quinoline N-oxides

“…Unlike observations found for pyridine and its derivatives, the complex formation of Zn-TPP with heteroaromatic N-oxides and aniline with substituents in the 2, 3, and 4 positions and at the nitrogen atom (except for 4-halo derivatives) is accompanied by a low heat release and can be considered, to a first approximation, as isenthalpic processes [3,4].…”

“…Thus, in each of the two series of heteroaromatic N-oxides (for aniline with substituents in the 3 and 4 positions, ΔH 0 av -14.76±0.14 kJ/mol [3]), the coordination with Zn-TPP is a quasi-isenthalpic process.…”

“…As we showed in [3], the ΔS 0 of the coordination of the members of the isenthalpic series of aniline derivatives, including ortho-substituted ligands, with Zn-TPP in chloroform is linearly related to log K [Eq. (3)].…”