Supramolecular Behaviour of N,N '-Bridged Guanidinium Nitrates in the Crystalline State: Identification of Privileged Hydrogen Bond Networks

Using the marine natural product ptilocaulin as a starting point, the supramolecular behavior of N,N '-bridged guanidinium nitrates in the crystalline state was investigated. For this purpose, various N,N '-bridged guanidines were synthesized (typically by condensation of a 1,2- or 1,3-diamine with cyanogen bromide) and subsequently converted into their nitrate salts with nitric acid. The crystal structures of 28 samples were determined by single crystal X-ray crystallography and revealed the existence of a few privileged, recurring supramolecular motifs (hydrogen bond networks) formed by charge-assisted self-assembly. A topological analysis of possible pseudo-2D-patterns derived from the 1 : 1 ion pairs (generated by salt bridge formation between a N,N '-bridged guanidinium cation and a nitrate anion) through additional H-bonding uncovered only five conceivable regular structural motifs: (1) a linear chain, (2) an alternating (zig-zag) chain, (3) a double chain, (4) a tape/ribbon-type structure, and (5) a cyclotrimer. Experimentally, only the first four of these motives were observed while the cyclotrimer, as a non-infinite and therefore particular super-structure, did not occur. Other H-bonding patterns were found (in three cases) when water was incorporated in the crystals. In two cases, polymorphism was observed. The identified supramolecular preferences prove the value of N,N '-bridged guanidines as useful and easily tunable building blocks for supramolecular chemistry and crystal engineering.