Unique aspects of the 1-methylimidazole ligation to corrphycenatoiron(III)

The coordination behaviors of chloro corrphycenatoiron(III), a novel hemin isomer with a trapezoidal metallo core, against imidazole, 1-methylimidazole, and 2-methylimidazole were followed by spectrophotometry in chloroform at room temperature. The examined compound is chloro 12, 17-bis(ethoxycarbonyl)-2,3,6,7,11,18-hexamethylcorrphycenatoiron-(III). Titration of 2-methylimidazole and imidazole proceeded with and without accumulation of an intermediate mono imidazole species, respectively. On the other hand, a significant amount of an intermediate adduct was found to accumulate during 1-methylimidazole binding despite the absence of any steric hindrance in the iron(III)-N(1-methylimidazole) coordination bond. This is in remarkable contrast with the conventional results for porphyrinatoiron(III). The proton NMR analysis of the mono 1-methylimidazole binding to corrphycenatoiron(III) afforded a formation constant closely similar to that obtained from the visible spectra. EPR definitely assigned the iron(III) high-spin state for the intermediate. Accumulation of the mono 1-methylimidazole intermediate was explained in terms of the contracted core in corrphycene. The narrow iron cavity weakens the axial iron-chloride bond facilitating coordination of the first 1-methylimidazole. The four non-orthogonal iron(III)-N(pyrrole) bonds formed in the trapezoidal cavity further disturb the incoming motion of the iron(III) atom into corrphycene plane upon second 1-methylimidazole ligation. Accumulation of mono 1-methylimidazole intermediate was more evident in ferric corrphycenatoiron(III) with bromide and iodide, supporting the proposed changes in iron reactivity upon deformation of the equatorial coordination environment.