Effect of steric strains in the macroring on the structure and properties of molecular complexes of (chloro)[5,15-(p-butoxyphenyl)-2,8,12,17-tetramethyl-3,7,13,17-tetrabutylporphinato]aluminum

Abstract

The chelate (Cl)AlP was prepared by complexation of porphine (P) with aluminum(III) chloride in refluxing pyridine. Equilibrium coordination of nitrogen-containing ligands (L = 2-methylimidazole, imidazole, pyridine, 3,5-dimethylpyrazole, dimethylformamide) with (Cl)AlP in benzene was studied by spectrophotometric titration and computer simulation. Quantitative and qualitative characteristics of the reaction were obtained. The structure of the mixed-ligand complex formed by intermolecular interaction of the metal porphyrin with a base was determined spectrophotometrically and by quantum-chemical calculations. An effect of additional molecular ligand and of steric strain in the macroring on the stability of the complex was noted. The stability constant (Ks) increases with an increase in the basicity (KBH+) of the extra ligand and is proportional to the shift of the main bands (†λ) in the electronic absorption spectra. The geometric and energy characteristics of hexacoordinated aluminum porphyrin were calculated by the PM3 method. Correlations were found between the calculated energy of the interaction of the aluminum atom with the base molecule (Eb) and stability of the mixed-ligand complexes (Cl)Al(L)P. The cis and trans effects in the complexes (Cl)Al(L)P were analyzed. The dependence of the strength of the Al-L bond on the nature of the porphyrin and the basicity of the additional molecular ligand was determined from the experimental data and calculation results.