Guest-Induced Reversible Single-Crystal-to-Single-Crystal Transformation Involving Displacement of 2D Layers and Spin Crossover Behavior Change in a Hofmann-Type Coordination Polymer

A novel two-dimensional (2D) Hofmann-type coordination polymer, {Fe^II(PyHbim)₂[Pd(CN)₄]}·2CH₃OH [1·2CH₃OH, PyHbim = 2-(4-pyridyl)benzimidazole], has been synthesized, which can undergo a spontaneous guest exchange, transforming to 1·2H₂O in a single-crystal-to-single-crystal (SCSC) manner, shifting from orthorhombic Cmmm to monoclinic C2/m involving the displacement of 2D layers. The solvent-induced SCSC transformation process was reversible and verified through powder X-ray diffraction (PXRD) and single-crystal X-ray crystallography analyses. Both 1·2CH₃OH and 1·2H₂O exhibit complete and abrupt spin crossover (SCO) behaviors in two steps, while their SCO temperature ranges drastically shift by ca.100 K, spanning room temperature, owing to different intermolecular interactions resulting from diverse interlayer packing manners and host-guest interactions. Besides, a structural phase transition is observed in 1·2CH₃OH, contributing to the two-step spin transition.