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

Although a cis 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 beta-heptakis(trifluoromethyl)-meso-tetraarylporphyrin. Reported herein is the molecular structure of a beta-octaiodo-mesotetraarylporphyrin solvate, which also exhibits a cis tautomeric structure. Both structures implicate two factors as critical to the stabilization of the cis 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 center dot center dot center dot X-H center dot center dot center dot 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 cis porphyrin tautomers in a predictable manner may be within reach.