Porphyrin network solids: examples of supramolecular isomerism, noncentrosymmetric architectures and competing solvation

Abstract

Self-assembly of functionalized tetraarylporphyrins into 2-D and 3-D supramolecular arrays may exhibit structural isomerism when carried out in different experimental conditions. This study shows that the open 2-D hydrogen bonding quadrangular grid networks of free-base tetra(4-carboxyphenyl)porphyrin (TCPP) form an interweaved crystalline architecture, in addition to the non-interweaved offset-stacked porous arrangement observed before. In the two structures the individual arrays are similarly stabilized by multiple cyclic-dimeric (COOH)₂ hydrogen bonding synthons, differing slightly in the grid shape. The Mn-metalled TCPP building blocks afford in a lipophilic environment 2-D coordination polymers by direct Mn⋯⁻OOC/HOOC association between the tetra-acid metalloporphyrin units, without incorporation of any external bridging auxiliaries. This polymerization is further enforced by interporphyrin O–H⋯O hydrogen bonding within and between the 2-D arrays, yielding a solvent-free 3-D architecture. The latter is related to an isomeric crystalline organization of similar layered Mn–TCPP coordination polymers obtained earlier from hydrophilic crystallization mixtures, wherein intercalated solvent separates the layered polymeric arrays. The non-acidic Mn(Cl)–TOHPP compound yields a chiral 3-D assembly sustained by multiple inter-porphyrin hydrogen bonding. Its formation is promoted by the non-centrosymmetric (axially polarized) shape of this porphyrin unit, as well as by the directional hydrogen bonding interactions, and it involves coordination of an additional axial ligand to the manganese ion. Effects of competing solvation, which interfere with the supramolecular aggregation of the TCPP scaffolds into network arrays, are demonstrated also by structures obtained from solvent mixtures containing dimethylsulfoxide.