Co-crystals with Acetylene: Small Is not Simple!

Acetylene is an amazingly versatile component for the formation of co-crystals. It requires careful handling and special techniques for crystallisation, but the efforts seem to be rewarding when attaining co-crystals with small molecules as partners. Many basic questions such as the dominance of specific heterogeneous intermolecular interactions, their driving force for the formation of multicomponent crystals instead of neat ones are expected to be easily analysed. The underlying packing patterns and resulting stoichiometries based on the known supramolecular synthons seem to be straightforward for such small molecules and crystal engineering, considered as the prototype of supramolecular synthesis, should be a simple task. Nineteen co-crystals with acetylene are presented in this paper, some of which have been previously reported individually. An attempt has been made to find features shared by the groups of co-crystals, including those that could not be co-crystallised. But in spite of clear ideas and experiences from previous experiments, surprisingly almost none of systems reached our expectations. Our intuitive approach was not fulfilled, which demonstrates that multicomponent crystals even of small molecules will remain a great challenge for theoretical methods and the crystal structures shown herein represent good candidates for future testing. On the other hand, we wish to encourage other groups to present their views on the crystal structures with an unbiased approach that may offer a better explanation than we are able to outline in this article.