Exploiting High Pressures to Generate Porosity, Polymorphism, And Lattice Expansion in the Nonporous Molecular Framework Zn(CN)₂

Systematic exploration of the molecular framework material Zn­(CN)₂ at high pressure has revealed several distinct series of transitions leading to five new phases: four crystalline and one amorphous. The structures of the new crystalline phases have been resolved through <i>ab initio</i> structural determination, combining charge flipping and direct space methods, based on synchrotron powder diffraction data. The specific transition activated under pressure depends principally on the pressure-transmitting fluid used. Without fluid or in large molecule fluids (e.g., isopropanol, ethanol, or fluorinert), the high-pressure behavior intrinsic to Zn­(CN)₂ is observed; the doubly interpenetrated diamondoid framework structure transforms to a distorted, orthorhombic polymorph, Zn­(CN)₂-II (<i>Pbca</i>) at ∼1.50–1.58 GPa with asymmetric displacement of the bridging CN ligand and reorientation of the Zn­(C/N)₄ tetrahedra. In small molecule fluids (e.g., water, methanol, methanol–ethanol–water), the nonporous interpenetrated Zn­(CN)₂ framework can undergo reconstructive transitions to porous, non-interpenetrated polymorphs with different topologies: diamondoid (<i>dia</i>-Zn­(CN)₂, <i>Fd</i>3<i>m</i>, <i>P</i>_trans ∼ 1.2 GPa), londaleite (<i>lon</i>-Zn­(CN)₂, <i>P</i>6₃<i>/mmc</i>, <i>P</i>_trans ∼ 0.9 GPa), and pyrite-like (<i>pyr</i>-Zn­(CN)₂, <i>Pa</i>3, <i>P</i>_trans ∼ 1.8 GPa). Remarkably, these pressure-induced transitions are associated with near 2-fold volume expansions. While an increase in volume with pressure is counterintuitive, the resulting new phases contain large fluid-filled pores, such that the combined solid + fluid volume is reduced and the inefficiencies in space filling by the interpenetrated parent phase are eliminated. That both <i>dia</i>-Zn­(CN)₂ and <i>lon</i>-Zn­(CN)₂ phases were retained upon release to ambient pressure demonstrates the potential for application of hydrostatic pressures to interpenetrated framework systems as a novel means to generate new porous materials