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 alpha phase to the PX-loaded beta phase induced by PX vapour. beta reversibly transformed to its porous beta ' phase upon PX removal over at least three PX sorption cycles. beta ' reverted to alpha upon heating at 80 degrees C in a closed vessel. The porosity of beta ' 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 alpha, beta, and beta ' phases is provided by analysis of their previously reported crystal structures and new density functional theory calculations.