Coordination of Aza Heterocycles with Macroheterocyclic Metal Complexes in Amphiprotic Media: I. Reaction of Imidazole with Chromium(II) (Acetate)tetraphenylpophyrinate

Abstract

Coordination of imidazole with chromium(II) (acetate)tetraphenylpophyrinate involves consecutive substitution of alcohol molecules in the sixth coordination position of the complex. Axial coordination of imidazole produces no charged species.The recent interest in macroheterocyclic complexes generally and in metal porphyrins specifically is explained by their practical importance. Of particular mention is modeling natural biocoordination systems by porphyrin metal complexes. In addition, metal porphyrins are used as catalysts in chemical engineering. Considerable researcher's attention has been given to porphyrin complexes of transition metals, such as iron(II), iron(III), chromium(III), and cobalt(III) [1,2]. Notwithstanding the fact that these complexes contain an axial acido ligand, the coordination ability of the metal ion in them is still unexhausted and can coordinate additional ligands (or solvent molecules) that possess electron-donor properties.The choice of chromium(III) porphyrin complexes as objects for study was motivated by their weaker tendency to formation of m-oxo dimers [1] and also by the fact that Cr(III), unlike Fe(II) and Fe(III), has a stable spin state unaffected by coordination of an axial ligand. It should be noted that chromium(III) porphyrinates are only appreciably soluble in sufficiently solvating solvents whose molecules occupy the sixth coordination site of the octahedral complex. Among organic solvents the most interesting are alcohols as low-molecular models most similar to the macromolecular surrounding of biological macrocyclic complexes [3] in terms of polarity, polarizability, hydrophilic3lypophilic balance, and amphiprotic nature.The thermodynamics, kinetics, and mechanism of axial coordination with chromium(III) porphyrin complexes were studied in low-polarity aromatic hydrocarbons, halohydrocarbons, and other solvents [438] unable to specific interactions and not possessing proton-donor properties. Processes that occur in water, alcohols, and other amphiprotic solvents have scarcely been studies.The present work deals with the regularities of axial ligand exchange in porphyrin complexes of transition metals in alcohols.The first step of the research on the mechanisms of ligand exchange in transition metal complexes involves assessment of the starting and final states in solutions. Ligand exchange in chromium(III) porphyrin complexes has been studied in [4312]. According to [9,10], in aqueous solutions Cr(III) complexes tend to lose the acido ligand localized in the fifth coordination position to give charged bisaqua complexes whose reaction with imidazole gives rise to bisimidazole derivatives. Conductometric and spectral studies [4,13] showed that in organic solvents (acetonitrile, DMSO) coordination of heterocycles involves no dissociation of acido ligand and formation of charged complexes, whereas analysis of the electronic absorption spectra of chromium(III) (acetate)tetraporphyrinate (AcO)CrTPP in toluene made Tipugina and Lomova [8] to suggest that axial coordinati...

Oxidation of Cobalt(II) Tetraphenylporphyrinate with Molecular Oxygen

“…For the sake of comparison we have measured the conductivity of cetyltrimethylammonium bromide CTAB solution, the organic electrolyte is known to exist in the form of free solvated ions in ethanol at concentration of about 10 -5 mol/l. [10] The data collected in Table 1 have evidenced that the excess conductivity was close to zero in the case of Co II TPP, the background value of 0.09 Ω -1 ·cm 2 ·mol -1 had apparently caused by the presence of admixtures.…”

Coordination of Aza Heterocycles with Macroheterocyclic Metal Complexes in Amphiprotic Media: II. Kinetics of Outer-Sphere Ligand Exchange in Chromium(III) Tetrphenylporphyrinate

“…The tendency for association (dimerization inclusive [10]) in the alcohol series should be preserved not only in solvent bulk, but also in the coordination field of the complexing ion. The difference is in stronger H bonding [1]. Thus, the data in Table 1 and Fig.…”