Molecular Tectonics: Design of Hybrid Networks and Crystals Based on Charge‐Assisted Hydrogen Bonds

Abstract

used because of the possibility of accurate structural studies using X-ray diffraction methods. Among many factors governing the formation of crystals, recognition patterns or supramolecular synthons, which appear during the crystallization process, are of prime importance allowing, to a certain extent, to predict the final crystal structure [11]. This aspect has been theoretically investigated using different approaches [12]. However, it is worth noting that owing to our limited knowledge of all subtle intermolecular interactions governing the formation of the crystalline phase, the complete understanding of the formation of molecular networks and their packing leading to the crystal remains so far unreachable [13]. Nonetheless, our current level of knowledge allows to control some of the intermolecular interactions with precision, that is, the appearance of recognition patterns with a good degree of reliability, by properly designing molecular tectons. Thus, one may predict the formation of molecular networks and in some cases their packing in the crystalline phase. The three main features governing the design of molecular networks are (i) the design of recognition patterns linking a self-complementary or two or several complementary tectons, (ii) the geometrical aspects dealing with the localization of the interaction sites, and (iii) the nature of intermolecular forces allowing the interconnection of tectons. The latter, by principle, must be reversible in order to allow self-repairing processes to take place during the construction events. Different reversible intermolecular interactions such as H-bonding [14] and coordination bonding [15] have been widely explored over the last two decades. Other forces such as p-stacking interactions [16] or inclusion processes based on van der Waals interactions [17] have also been used for the construction of inclusion molecular networks in the solid state. For the design of the structural nodes, a possible reason for the extensive use of Hbond interaction is its rather directional nature [18]. In terms of strength, H-bond ranges from weak (about a fraction of kcal mol À1) to moderate interactions (about 5-10 kcal mol À1) [19]. Consequently, in order to increase the robustness of the architecture, one needs to enhance the interaction energy between the tectons. This may be achieved by combining H-bonding with less directional but more energetic electrostatic charge-charge interactions. This type of recognition pattern is called charge-assisted H-bonds (CAHBs) [20]. It results from the interaction between charged hydrogen bond donors (D) and acceptors (A) leading to the (þ DÀH Á Á Á A À) supramolecular synthon. The majority of molecular networks reported to date are based on nonionic Hbonds [14]. However, several cases of charge-assisted hydrogen bonding have been also described in the literature. Some of these cases will be presented in the following section. Although at the initial steps of this approach, investigators were interested in the understanding and control of...

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