posted on 2006-03-01, 00:00authored bySumod George, Israel Goldberg
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
[MnIII(H2O)2-TCPP or MnIII(CH3OH)2-TCPP, respectively], and manganese chloride-5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin
[MnIII(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 MnIII 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 MnIII(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
MnIII(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.