Phase switching and shape-memory effect in a molecular material: revisiting the Werner complex [Ni(4-MePy)4(NCS)2

Shape-memory effects and stimuli-induced phase transformations are observed across many types of materials but remain understudied in molecular materials. In this contribution, we revisit the Werner complex, [Ni(4-MePy)4(NCS)2] (4-MePy = 4-methylpyridine), a molecular material that has long been known to form Werner clathrates and possess a porous polymorph. Whereas earlier work implied that [Ni (4-MePy)4(NCS)2] could be a shape-memory material, we confirmed this to be the case through p-xylene (PX) vapor sorption studies. Specifically, [Ni(4-MePy)4(NCS)2] transformed from its reported nonporous α phase to the PX-loaded β phase induced by PX vapour. β reversibly transformed to its porous β’ phase upon PX removal over at least three PX sorption cycles. β’ reverted to α upon heating at 80 °C in a closed vessel. The porosity of β’ was explored by examining its C3Hx (x = 4, 6, and 8) gas sorption properties, which revealed guest size/shape-dependent sorption behaviour with uptakes of 0.6, 0.9 and 1.6 mol mol−1 for C3H8, C3H6 and C3H4, respectively. Insight into the structural transformations of the α, β, and β’ phases is provided by analysis of their previously reported crystal structures and new density functional theory calculations