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

Abstract

AbstractUsing 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.