Supramolecular Crystal Chemistry with Porphyrin Tinkertoys. Hydrogen-Bonding and Coordination Networks with the Chair and Table Conformers of Tetra(3-carboxyphenyl)porphyrin

This study explores further the supramolecular reactivity of the meso-tetra(3-carboxyphenyl)porphyrin ((TCPP)-C-3) building block in the context of crystal engineering. (TCPP)-C-3 has coordination as well as hydrogen bonding tetradentate functionalities, and exhibits orientational versatility of the 3-carboxylic substituents with respect to the porphyrin core; chair- and table-like conformers have been expressed in this study. In the chair variant of (TCPP)-C-3, two adjacent carboxylic functions are oriented upward and the other two downward, while in the table isomer all four carboxylic arms are oriented in the same direction. Solvothermal reactions of the (TCPP)-C-3 with cadmium and zinc ions afforded metalation of the porphyrin core and hybrid coordination compounds with uniquely interesting and novel architectures. This includes a discrete 4:2 Cd/Zn:porphyrin assembly (4), where the four metal ions link (as mononuclear connectors) between the two metalloporphyrins (present in a table conformation) into a molecular-box, as well as coordination polymers of one-dimensional (5) and two-dimensional (6-8) connectivity. Additional experiments led to the syntheses of hydrogen-bonded networks between the (TCPP)-C-3/Co-(TCPP)-C-3 moieties (given in the chair conformation) and various amine ligands present in the reaction mixtures (1-3). These were associated with either partial or full proton-transfer from the porphyrin tetra-acid to the amine species to further stabilize the supramolecular networking by added electrostatic attraction. The above findings confirm that the (TCPP)-C-3 scaffold may effectively engage in diverse supramolecular constructs, through direct multiple-coordination to metal-ion connectors, as well as in extended hydrogen-bonding networks. While the occurrence of the table conformer of (TCPP)-C-3 in the above context has been observed here for the first time, new supramolecular materials are expected to emerge in the future with the still missing tetrahedral form (with the carboxylic substituents oriented in alternating directions around the porphyrin macrocycle) of this versatile ligand.