Electronic and steric effects of substituents on the coordinating properties of porphyrins

Abstract

Kinetics of the formation of zinc complexes of porphyrin with various substituents was studied in acetic acid and pyridine. The introduction of strong electron-withdrawing substituents (nitro groups) to the meso-positions of the porphyrin macrocycle was shown to change the reaction rate by an order of magnitude or less compared to the phenyl-substituted analogs. However, the introduction of a large number of bulky substituents leads to the deformation of the porphyrin ligand and thus affects much stronger the coordination properties of porphyrins, decreasing or increasing the rate of the complexation reaction by several orders of magnitude.The deformation of the porphyrin macrocycle due to the introduction of a large number of bulky substituents affects greatly the basic properties of the porphyrin ligand and, depending on the solvent nature, increases (in basic solvents) or decreases (in the solvents of acidic nature) the reaction rate of complex formation by these ligands by several orders of magnitude [1,2]. However, the substithuents themselves can more or less affect the process rate as the electron donors or acceptors.In this regard, the purpose of this study was to determine the ratio of the contributions of the substituents electronic and steric effects to the kinetic parameters of the formation of metalloporphyrins.In this work we studied the kinetics of the reactions of the zinc complexes formation with porphyrins containing strong electron-withdrawing substituents in one or two meso-positions of the macrocycle: 10-nitro-5,15-diphenyl--octamethylporphin (VI), in acetic acid and pyridine.The following porphyrins were synthesized: 7,13,8,12,7,13,8,12, were obtained by nitration of 5, 7,13,8,12, with a mixture of sodium nitrite and trifluoroacetic acid, according to [3]. Porphyrin VII was synthesized in 38% yield by the condensation of dipyrrolylmethane (VIII) with benzaldehyde under the influence of chloroacetic acid in methylene chloride in an inert atmosphere, followed by oxidation of the intermediate porphyrinogen with p-chloranil [3].

Acid–base equilibria and coordination chemistry of the 5,10,15,20-tetraalkyl-porphyrins: implications for metalloporphyrin synthesis

“…Acetic acid has been chosen for the titration procedure since it forms relatively labile solvated complexes of transition metals, which can be easily broken in the presence of the porphyrin macrocycles to form metallocomplexes: thus, for example, 2,3,7,8,12,13,17,18octamethylporphyrin complexation rate constant with Cu 2+ was found to be about 10 5 times higher compared to that measured in pyridine. 32,33 The obtained data on kinetic parameters of Cu-porphyrins formation indicate that electronic effects play the important role in the complexation. Replace of the electron donating substituents in porphyrin 1 for the electron withdrawing ones in the porphyrin 2 leads to almost 100-fold increase in the efficient rate constant k eff of the complexation.…”

Coordination and acid-base properties of meso-nitro derivatives of β-octaethylporphyrin

“…On the other hand an introduction into a macrocycle of a great number of band meso-substituents which possess a definite volume forces a porphyrine molecule to take nonplanar conformation as it was shown in references [6,[23][24][25]30]. A nonplanar distortion of a porphyrins macrocycle structure results in decrese of molecule aromaticity and in some separation of pyrrole and pyrrolenin fragments.…”

“…To date, some attempts have been made in this direction. In our previous works it was shown that nonplanar porphyrin macrocyclic distortions have a great influence on the acid-base properties of porphyrinligand and depend on a solvent nature increase or reduce reaction rate of formation of metalloporphyrins within several orders of magnitude [6,8,[22][23][24][25].…”