Helices, Chirality and Interpenetration: the Versatility and Remarkable Interconversion of Silver−Copper Cyanide Frameworks

The structural transformations between cesium silver−copper cyanides under modest conditions, both in solution and in the solid state, are described. Three new cesium silver(I) copper(I) cyanides with three-dimensional (3-D) framework structures were prepared as single crystals from a one-pot reaction initially heated under hydrothermal conditions. The first product to appear, Cs3Ag2Cu3(CN)8 (I), when left in contact with the supernatant produced CsAgCu(CN)3 (II) and CsAgCu(CN)3·1/3H2O (III) over a few months via a series of thermodynamically controlled cascade reactions. Crystals of the hydrate (III) can be dehydrated to polycrystalline CsAgCu(CN)3 (II) on heating at 100 °C in a remarkable solid-state transformation involving substantial breaking and reconnection of metal−cyanide linkages. Astonishingly, the conversion between the two known polymorphs of CsAg2Cu(CN)4, which also involves a major change in connectivity and topology, occurs at 180 °C as a single-crystal to single-crystal transformation. Structural features of note in these materials include the presence of helical copper−cyanide chains in (I) and (II), which in the latter compound produce a chiral material. In (II) and (III), the silver−copper cyanide networks are both self- and interpenetrating, features also seen in the known polymorphs of CsAg2Cu(CN)4.