Flexibility in a ferroelastic MOF

One of most exciting features of metal-organic frameworks (MOFs) is their potential to be incredibly flexible. In this paper we report a new MOF, copper(I) tricyanomethanide, Cu(tcm) that illustrates this very well. Cu(tcm) undergoes a ferroelastic phase transition at T_f = 240 K and below this temperature shows thermal expansion an order-of-magnitude larger than above the transition. In fact, the low-temperature phase α-Cu(tcm) shows ‘colossal’ positive and negative thermal expansion that is the strongest ever reported for a framework material. We also found that Cu(tcm) can be converted just by exposure to gaseous acetonitrile to another new phase (acetonitrilo-copper(I) tricyanomethanide), and that this reaction can be reversed by putting copper tricyanomethanide under vacuum Infrared spectroscopy measurements are sensitive to the phase change, suggesting that Cu(tcm) might perhaps find application in solid-phase acetonitrile sensing.