Reverse, reversible, and hysteretic shape change of a Ni(II) molecular crystal induced by thermal phase transition

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A nickel(II) molecular complex [Ni(L4Cl,nPen)2] (1, HL4Cl,nPen = 4-chloro-2-pentyliminomethyl-phenol) was synthesized and was found to exhibit reversible and hysteretic crystalline shape change at around 353 K. This shape change has an unusual reverse nature, i.e., sudden contraction along the crystal’s long axis at 361 K (ca. −8%) upon heating and subsequent elongation to the initial length at 343 K upon cooling. X-ray crystallography and differential scanning calorimetry revealed that the thermomechanical deformation of 1 was triggered by structural phase transition.