Molecular Structure of a Free-Base β‑Octaiodo<i>-meso</i>-tetraarylporphyrin. A Rational Route to <i>cis</i> Porphyrin Tautomers?

Although a <i>cis</i> tautomer has long been invoked as an intermediate in porphyrin tautomerism, the first such species was only recently isolated and structurally characterized in the form of a β-heptakis­(trifluoromethyl)-<i>meso</i>-tetraarylporphyrin. Reported herein is the molecular structure of a β-octaiodo-<i>meso</i>-tetraarylporphyrin solvate, which also exhibits a <i>cis</i> tautomeric structure. Both structures implicate two factors as critical to the stabilization of the <i>cis</i> tautomeric forma free-base porphyrin that is naturally strongly saddled on steric grounds and a hydroxylic or amphiprotic solvent that can provide hydrogen-bonded N–H···X-H···N (X = O in both the above examples) straps connecting the central NH groups with the antipodal unprotonated nitrogens. The results raise the prospect that a rational strategy affording <i>cis</i> porphyrin tautomers in a predictable manner may be within reach