Two-dimensional hydrogen-bonding patterns in a series of salts of terephthalic acid and the cyclic amines C_nH_2n-1NH ₂, n = 3, 4, 5, 6, 7, 8, and 12

The assembly of two molecules into a specific dimensionality is one of the tenets to make new functional materials. Ammonium carboxylate salts, which make use of charge-assisted N-H · · · O hydrogen bonds, can form either one-dimensional or two-dimensional (1-D or 2-D) assemblies depending on the number of functional groups on the individual molecules. Terephthalic acid, with two carboxylic acid functional groups, and the cyclic amines with general formula C_nH_2n+1NH₂ (n = 3, 4, 5, 6, 7, and 12), containing one amine functional group, undergo double proton transfer to form a 2-D network of hydrogen bonds. The hydrogen-bonded interactions between the functional groups result in ring-shaped patterns, and form two types of nodes between the cations and anions: Type II nodes have a repeating R ₄^3M(10) ring, and type III nodes have alternating R ₄²(8) and R₄⁴(12) rings. There are two types of 2-D hydrogen-bonded sheets, depending on the type of node used. The type V hydrogen-bonded sheet has nodes consisting of type II, and the type VI hydrogen-bonded sheet consists of type III nodes. Using the cation cyclopropylammonium and cycloheptylammonium, the type V assembly is observed, and with the cations cyclooctylammonium and cyclododecylammonium, the type VI assembly is observed. Cyclohexylammonium is a special case as it combines both types of assemblies into one structure. Cyclopropylammonium forms a different 2-D assembly, whereas the salt with cyclobutylammonium incorporates a water molecule to form a three-dimensional (3-D) assembly. The salt with cyclohexylammonium features a disorder of the cations, where the ammonium group occupies the equatorial and the less favored axial position in the ring. The frequency of the two types of 2-D assemblies is quantified using similar ammonium carboxylate salts retrieved from the Cambridge Structural Database.