Database Investigation of Halogen Bonding and Halogen···Halogen Interactions between Porphyrins: Emergence of Robust Supramolecular Motifs and Frameworks

Abstract

We provide a Cambridge Structural Database (CSD) analysis of the role of halogen bonds and halogen···halogen interactions in the solid-state structures of porphyrins. This survey provides a detailed view of halogens in porphyrin self-assembly, highlighting interaction motifs beyond the often considered iodine-based halogen bonds. Particularly notable are the observations of self-assembly through β-positioned substituents, the role of the pyrrole π-system as a halogen bond acceptor, and an abundance of chlorine-based halogen···halogen and halogen-bonding motifs. This overview enabled the observation and systematic classification of complex, multipoint recognition motifs in halogen-based architectures. Similarly to synthons in hydrogen-bonded crystal engineering, these motifs appear in discrete molecular assemblies and can also underlie the formation of two- and three-dimensional frameworks. The systematic categorization of these robust motifs provides a foundation for the design of complex halogen-based supramolecular architectures.

1,4-Dibromo-2,5-bis(phenylalkoxy)benzene Derivatives: C–Br...π(arene) Versus C–H...Br and Br...Br Interactions in the Solid State

“…Pertinent distances (defined in Figure 5) are C34-H34...Br2ii = 2.87Å, angle C34-H34...Br2ii = 159°, Br1ii...Br1iii = 3.4584( 6) Å. This Br...Br separation is less than the sum of the van der Waals radii (3.70 Å) [15], and lies within the range of contacts found in the CSD [11]. The C-Br...Br-C interaction shown in Figure 5 belongs to the type I category (Scheme 1), and each C-Br-Br angle is 150.6(1)°.…”

“…However, this becomes less well defined for Br...Br contacts, and even less so for Cl...Cl contacts [13]. Spilfogel et al have similarly analysed plots of θ1 against θ2, and (with a restricted set of halogenated porphyrins) also demonstrated the difficulties in unambiguously assigning the structure type [11].…”

“…The relatively recent recognition of halogen bonding as an important supramolecular interaction in crystal engineering has spawned numerous reviews and data-mining overviews in the past few years [3][4][5][6][7][8][9][10]. Of particular note are discussions which demonstrate the distinction between a halogen bond and a halogen...halogen interaction [11,12]. When a halogen atom X forms a covalent bond, e.g., a C-X bond, the resulting electron distribution is such that atom X possesses an 'electrophilic cap' (also referred to as a 'σ-hole') located zenithal to the C-X covalent bond.…”

“…Such interactions typically involve Cl, Br and I atoms and may be classified as type I or type II interactions as depicted in Scheme 1 [13]. Interactions are not confined to homonuclear interactions, although analysis of entries in the Cambridge Structural Database (CSD) reveal that Cl...Cl and Br...Br are the most common halogen...halogen contacts [11,13]. Analyses of scatter plots of angles θ 1 against θ 2 (defined in Scheme 1) by Desiraju and coworkers have illustrated that there is a clear distinction between type I and II interactions for I...I contacts.…”

Interplay of Hydrogen and Halogen Bonding in the Crystal Structures of 2,6‐Dihalogenated Phenols

“…[25] Halogen bonds have been studied for a long time and are still a research topic of highest interest. [26][27][28][29] The number of contributions in this field during the past few years is significant and covers a wide area, from understanding the nature of this kind of noncovalent interactions [30][31][32][33][34][35][36][37][38] to the targeted design of halogene-based supramolecular entities [39][40][41][42][43][44] and applications in organocatalysis. [45,46] Studies on halogenated molecules interacting with biomolecular systems have opened up entirely new approaches in drug development.…”