Questions for crystal engineering of halocuprate complexes: concepts for a difficult system

We evaluate prospects for the crystal engineering of anionic halocuprate complexes [CunXm](z-), X = Cl, Br, I, crystallised with polyatomic cations that possess alkyl or aryl coated surfaces and do not form hydrogen bonds. In solution, halocuprate complexes are involved in kinetically fast dissociative and associative equilibria, as well as redox variation between Cu(I), mixed Cu(I)/Cu(II) and Cu(II), and so the species that crystallise need not be those that predominate in solution: we describe instances of this complication, and the trapping of complexes which are not evident in solution. Our survey includes the relationships between cation properties and the identity and structure of the crystallised halocuprate complex, and the occurrence of polymorphism and de-facto polymorphism. A key aspect is the packing arrangement of anions and cations in these crystals: we illustrate the primary influence of electrostatic energies in determination of crystal packing, and the secondary but significant influences of cation..cation motifs and embraces in controlling details of crystal structure. In the context of crystal engineering, for which fundamental understanding and control of the crystallisation process and of the crystal structure are prerequisites, we conclude that even a chemically simple coordination system such as [CunXm](z-) needs additional knowledge and physico-chemical insight before there can be confident design and reliable fabrication of desired crystals. Progress will be aided by the publication ( in crystal structure papers) of more information about crystallising solutions and the effects of crystallisation variables, as well as ability to repeat crystallisation.