![]() Data obtained from the CSD crystal structure database accompanied by data obtained from our group are discussed. Herein we present a comparative analysis of the crystallographic structural characteristics of 13 meso x-halo-substituted porphyrins. However, with all the advances made in the crystal engineering of planar porphyrins, investigation through exploration into the effect of meso-halogenation on the macrocycle architecture have, to date, not been carried out. Studies on the effect of meso-halogenation have focused on the use of crystal data as a means of characterization, discriminating between enantiomers, or as a self-contained structural discussion. With the continuing interest in nonplanar porphyrins and their relevance for the in vivo functioning of porphyrin cofactors, there is a significant importance of the knowledge of the conformation when designing supramolecular materials. As recently illustrated in a review by Kielmann and Senge, the conformation of the porphyrin core can play a key role in the binding of small molecules or on its efficiency as an organocatalyst as demonstrated by Roucan et al. Due to their relatively large size and high electronegativity, halogens induce a conformational distortion of the porphyrin core that is significant compared to those induced by other substitution groups. Another reason to investigate the effect of meso-halogenation of porphyrins is due to the potential of induced distortion of the macrocyclic core. Recent studies from our group have been carried out on the use of halogens as a binding motif in cubanes, bicyclopentane, and nonplanar porphyrins. These traits allow for the design of novel supramolecular architectures which are directive and reproducible, without relying on the hydrogen-bonding functionality which in some cases can be undesirable. This is due to their relative versatility in areas such as directionality, the tunability of the σ-hole, hydrophobicity, and donor atom size. In recent years there has been a strong uprising interest for substituting hydrogen-bonding motifs with their halogen-bonding counterparts. π interactions was formed through halogen bonding in the lattice of self-assembly of meso-tetraarylporphyrins. ![]() reported that a chiral architecture based on C–I Such complexes have been reported for potential application in materials sciences (e.g., molecular sieves) as they form a three-dimensional lattice where more than 50% of the crystal capacity consists of open straight channels. Thereafter, research over the crystal engineering of porphyrins has focused on the noncovalent interactions, such as hydrogen bonds and halogen bonds, or metal coordination interactions. They reported that 5,10,15,20-tetraphenylporphyrin could act like a ‘host’ which could trap a variety of solvent ‘guests’ within the crystal lattice, through hydrogen-bonding and van der Waals forces. were the first to observe that porphyrins have a high propensity to form “porous” clathrates and suggested that they can be used as a “porphyrin sponge”. Keywords: conformational analysis crystal engineering halogenation macrocycles porphyrinsĬrystal engineering using porphyrins as a scaffolding unit has been a topic of increasing interest over the past few decades. Furthermore, we have designed a method to predict the conformation of halogenated porphyrins, without need of crystallization, using DFT calculations with a high degree of accuracy. By using the methods outlined above we were able to deduce the impact a meso halogen has on a porphyrin, for example, meso-halogenation is dependent on the type of alternate substituents present when forming supramolecular assemblies. This contrasts conformational analysis with existing X-ray structures and gives a method to characterize samples that are difficult to crystallize. Additionally, we have used density functional theory calculations to determine the structure of several halogenated porphyrins. In this study we have used the Hirshfeld fingerprint plots together with normal-coordinate structural decomposition and determined crystal structures to elucidate the conformation, present intermolecular interactions, and compare respective contacts within the crystalline architectures. This is accompanied by a detailed conformational analysis of all deposited meso-halogenated porphyrins in the CSD. With this in mind we have characterized a series of direct meso-halogenated porphyrins using single crystal X-ray crystallography. The use of halogens in the crystal engineering of supramolecular porphyrin assemblies has been a topic of strong interest over the past decades.
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