The dissociation rates of axially coordinated imidazole in bis-ligated low spin ferric complexes of synthetic porphyrins such as tetraphenylporphyrin (TPP) and tetramesitylporphyrin (TMP) were measured by NMR method. In both TPP and TMP complexes, the axial lability of imidazoles increased in the order 1-methylimidazole < 2-methylirnidazole < 2-ethylimidazole ∼ 1,2-dimethylimidazole. The results were explained in terms of the steric repulsion between the 2-alkyl group of imidazole and the porphyrin ring. The dissociation rates of TPP complexes were then compared with those of TMP complexes carrying the same axial ligands. In every case examined, imidazole dissociated faster from the TPP complex than from the TMP complex. The results were ascribed to the stability of the bis-ligated TMP complex relative to the corresponding TPP complex; the formation constant of the TMP complex having 2-Melm as axial ligand was larger than that of the corresponding TPP complex by a factor of c. 600. A hypothesis has been proposed to explain the stability of the sterically hindered porphyrin complex relative to the less hindered complex.
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