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

The coordination behaviors of chloro conphycenatoiron(III), a novel hemin i somer 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-hexamethylcorrphycenatoironcm,. Titration of 2-methylimida zole and imidazole proceeded with and without accumulation of an intermedia te mono imidazole species, respectively. On the other hand, a significant a mount of an intermediate adduct was found to accumulate during 1-methylimid azole 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 analysi s of the mono 1-methylimidazole binding to conphycenatoiron(III) afforded a formation constant closely similar to that obtained from the visible spect ra. EPR definitely assigned the iron(III) high-spin state for the intermedi ate. Accumulation of the mono 1-methylimidazole intermediate was explained in terms of the contracted core in corrphycene. The narrow iron cavity weak ens the axial iron-chloride bond facilitating coordination of the first 1-m ethylimidazole. The four non-orthogonal iron(III)-N(pyrrole) bonds formed i n the trapezoidal cavity further disturb the incoming motion of the iron(II I) atom into conphycene plane upon second 1-methylimidazole ligation. Accum ulation of mono 1-methylimidazole intermediate was more evident in ferric c onphycenatoiron(III) with bromide and iodide, supporting the proposed chang es in iron reactivity upon deformation of the equatorial coordination envir onment.