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

Abstract

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 C₃H_x (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.