1.1.1 Hydrogen Bonding Motifs
In 1990, Margaret Etter introduced empirical hydrogen-bonding rules which describe the nature of the bonding, e.g., dimer vs. intramolecular, and the number of donor and acceptor atoms involved.18 These bonding rules can be simplified by the notation Nyx(n), where N represents the H-bonding motif and whether it is infinite or finite. This can be represented by C (Chain), D (Dimer), R (Ring), or S (intramolecular). Nx and Ny are the number of acceptors and donors, respectively, and (n) is the number of atoms involved in the repeat unit.
In Figure 1 are two examples of H-bonding motifs, R22(8) and S(6). Following from the development of these empirical rules, hydrogen bonded assemblies have been studied in great detail and these interactions are an
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important driving force in supramolecular self-assembly.19 For example, in 2008 Mu et al. demonstrated that Aryl-F---H H-bonding drove the formation of a self-assembled chiral network. The spontaneous formation of these rigid molecular structures was attributed to the H-bonding motif present.20 Supramolecular aggregates have also been shown to form as a result of H- bonding interactions, which promote molecular recognition and self- organisation.21
Figure 1 A) A ring H-bonding motif with 2 acceptors and donors and 8 atoms
involved; B) Intramolecular H-bonding motif with 6 atoms involved.
1.1.2 Microporous Hydrogen-bonded Organic Frameworks
Desiraju introduced the concept of hydrogen-bonded frameworks in 1995, by taking advantage of H-bonding tectons for crystal engineering in supramolecular materials.22 Conceptually, this opened up the possibility of a new method to design of supramolecular assemblies, and with so many potential tectons for H-bonding a new field emerged.23–25
HOFs are formed using hydrogen-bonding building blocks, described here as tectons, which can be recrystallised from solvents forming 1-, 2-, or 3- dimensions networks. These networks can be designed by considering the bond angles, bond distances, and strength of the hydrogen bond interactions between the tectons.26,25 Although a material may contain voids or pores, the framework structures need to be energetically stable so they are maintained after desolvation. In some cases, guest removal will result in the collapse of the framework which has been seen in a number of reported HOFs.27–30 This
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can arise from limited rigidity or weaker h-bonding interactions, hence, although the material may contain guest accessible channels or voids within the material, the HOF would not behave as a porous material.
One of the first microporous solids, which was structurally held together with hydrogen bonds, was reported in 1994 (Figure 2).31 The macrocycle had six hydroxy groups and a calculated density of 1.29 g cm-3. The HOF was crystallised by slow diffusion of methanol into a solution of the macrocycle in
Figure 2 and ethanol. These H-bonding solvents interacted with the
macrocycle and interrupted bonding between the macrocycles. The crystal structure formed stacks, arising from both Van der Waals (VdW) and − interactions, generating channels throughout the material.
Figure 2 The macrocyclic hydrogen-bonding tecton used for the first
potentially microporous HOF in 1994 by Venkataraman et al.
In 1997, HOF-1 was reported by Brunet et al., which they quoted as showing unprecedented structural integrity, with the monomer shown in Figure
3.32 The crystal structure formed an R 2
2(8) hydrogen bonding motif, resulting in hexagonal channels throughout the structure. The largest channel in the resulting structure was 11.8 Å at its widest, however they never fully removed guests from the pores. They did however show that when 63% of the guests were removed, the network most likely remained intact which was determined through NMR studies. It wasn’t until 2011 that HOF-1 was reported to be
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permanently porous, and furthermore showed selective separations for C2H- 2/C2H4.5 The selectivity arises from the favourable adsorption enthalpy for C2H2 (58.1 kJmol-1) compared with C2H4 (31.9 kJmol-1), which is higher than MOFs which exhibit the same selective separation.33,34
Figure 3 H-bonding tecton used for HOF-1, first reported in 1997, and further
used for selective C2H2 and C2H4 separations.
Proton conduction has been hailed as one of the most promising technologies for clean and efficient power generation, prompting the discovery of more materials with this functionality.35 MOFs have been shown to exhibit proton conduction through cooperative H-bonding, leading to the theoretical application of HOFs in proton conduction.36,373839
In 2012, Mastalerz et al. published a HOF which had an exceptionally high Brunauer Emmett Teller Surface Area (SABET) of 2796 m2 g-1. The tecton used to form the microporous network was a triptycene derivative with H- bonding benzimidazolone functionality. Triptycentrisbenzimidazolone (referred to here as T2). Shown in Figure 4 is the crystal structure of T2, which has two H-bonding motifs, forming two distinctive pores in the structure. The R22(8) motif forms ribbons throughout the crystal structure and is responsible for the larger pore A, with a diameter of 14.5 Å at its widest. The N-H---O bond angle was 169 °, and the bond length 2.88 Å. The linear hydrogen bonds result in pore B, with an N-H---O bond length of 2.739 Å and angle 139.28 °. The slits forming pore B have diameters of 3.8 Å at its shortest point, and 5.8 Å at its widest. The self-assembled cooperative H-bonding contributes to the strength of the framework, with the result being a permanently microporous HOF.40
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Figure 4 The left-hand side shows the molecular structure of TTBI, and the
right-hand side shows the way in which these pack together in a single crystal.
Multiple research groups are now starting to take advantage of these triptycene derivatives.41–43 Triptycene is a large and bulky building block with trigonal geometry and a rigid core. These derivatives are not only being used in crystalline materials, such as COFs and HOFs, but also in polymeric materials for adsorption, separations and even water treatment.44–49 In 2014, the Mastalerz group also published porous molecular cube, further illustrating the rigidity of triptycene building blocks in porous material synthesis.50