Porphyrin xanthene amides: anchoring distal H-bonding in heme models

Distal H-bonding to axial ligand is strategically introduced in metalloporphyrin models by modifying porphyrin xanthene acid in which the carboxyl proton is preferentially H-bonded to the xanthene oxygen. Xanthene carboxamide is able to deliver intramolecular H-bonding and simultaneously lock into a well-defined conformation. Multipoint recognition of ligand is demonstrated in an amidinium–acetate–Zn porphyrin complex, mimicking a structural feature found in sulfite reductase.