Investigating the Impact of Conformational Molecular Engineering on the Crystal Packing of Cavity Forming Porphyrins

Abstract

Herein we report the synthesis of 5,10,15,20-tetraaryl-(X)-substituted- 2,3,7,8,12,13,17,18-octaethylporphyrins (OETArXPs) and a structural investigation of their solid-state properties via small molecule X-ray diffraction. A series of halogen (fluorine to iodine), nitrogenous (azido, cyano), alkyl (TMS-acetylene and acetylene) and chained (benzyloxy) porphyrins were chosen as the initial target molecules. Following this, a selection of tetravalent metal complexes [Cu(II), Ni(II) and Pd(II)] based on these porphyrins were synthesized to allow for an investigation of the effects of metal complexes on the structural properties of these highly substituted porphyrins. The size of the halogen atom affects the potential of intermolecular interactions and the resulting crystal packing in these 4-halo-OETArXP complexes. The fluorine series have an equal preference for alkyl or aryl groups (ortho-hydrogen), the chlorine series favoring interactions between the alkyl groups, and the bromine appears to favor the aryl (ortho- and meta-hydrogens). This results in an extensive cupping pattern in the unit cell. For the 2,6-halo-OETArXP it was established that the change in position alters the types of the intermolecular contacts towards face-to-edge or face-to-face interactions and altering the packing patterns observed. Within the 4-benzyloxy- OETArXP series the meso-substituent favors interacting with the core of the porphyrin macrocycle. The 4-cyano-OETArXP is a suitable hydrogen-bond acceptor and resulted in an interesting Z-shape network. Additionally, it was highlighted that solvent effects play a much larger role in crystal packing than intermolecular/intramolecular interaction or metal(II) center substitution. This is accompanied by a study using both the azide- and acetylene- OETArXPs as a base molecule to allow for a quick one-step reaction for the generation of a variety of functional groups. Using a copper(I)-catalyzed azide-alkyne cycloaddition reaction we were able to append hydrogen bonding functionalities to the OETArXPs framework in high yields. The crystals packing of this work shows the potential to create selective and functional receptor sites based on free base porphyrins. However, in so far of analytical measurements indicate, the design such free base porphyrin through crystal engineering has not yet been realized. The variety of porphyrin packing arrangements herein indicates the need for further studies.