Self-Assembly of Supramolecular Porphyrin Arrays by Hydrogen Bonding:  New Structures and Reflections

This study relates to the self-assembly of the free-base 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (TCPP), six-coordinate manganese-5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin with molecules of water or methanol as axial ligands [Mn^III(H₂O)₂-TCPP or Mn^III(CH₃OH)₂-TCPP, respectively], and manganese chloride-5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin [Mn^III(Cl)-TOHPP] into multiporphyrin hydrogen-bonding “polymers”. In the first case, the porphyrin units hydrogen bond directly to each other through their carboxylic acid functions. The two-dimensional (2D) square-grid polymeric arrays thus formed are sustained by characteristic intermolecular cyclic dimeric (COOH)₂ hydrogen-bond synthons between a given porphyrin unit and four other neighboring species along the equatorial directions. They stack tightly one on top of the other in an offset manner along the normal direction, yielding channeled lattice architecture. In an aqueous basic environment insertion of Mn^III into the porphyrin core involves deprotonation of one of the carboxylic groups, to balance the charge, and attraction of two water molecules as axial ligands. The Mn^III(H₂O)₂-TCPP units hydrogen bond, however, directly to one another through their carboxylic/carboxylate functions in the equatorial plane in a catemeric (rather than cyclic dimeric) manner. The 2D networks that form in this case are interconnected in the normal direction by additional hydrogen bonds through the axial water ligands, yielding a three-dimensional (3D) hydrogen-bonding architecture. In a methanolic solution of H₃PO₄, the methanol molecules replace water as axial ligands to the metal ion. The phosphate anions balance the extra positive charge of the trivalent metal, and they act also as effective bridges between adjacent Mn^III(CH₃OH)₂-TCPP moieties by interacting as proton acceptors with the peripheral carboxylic functions of four different porphyrins. This affords open square-grid-type layers with alternating porphyrin and phosphate components. The 2D arrays stack in an offset manner, interconnecting to one another through hydrogen bonds involving the methanol axial ligands. The Mn(Cl)-TOHPP building blocks arrange in a unique tetragonal structure around axes of 4-fold rotation and self-assemble through multiple O−H···Cl^- attractions between neighboring units into a single-framework hydrogen-bonding polymer. The directional asymmetry introduced to them by the Cl-axial ligand, which is amplified by the preferred hydrogen-bonding scheme, induces the formation of a noncentrosymmetric crystal structure. The nanoporous nature of TCPP-based multiporphyrin assemblies and the chirality of the Mn(Cl)-TOHPP structure are highlighted.