Responsive structural adaptability in ultra-microporous frameworks: guest recognition and macroscopic shape transformations induced by spin transitions within single crystals

Abstract

The selection and modulation of the static and dynamic units within molecular-scale actuators, particularly regarding framework structures, are critical for initiating synergistic inter- and intramolecular motions. However, achieving controllable macroscopic mechanical switching through the collective transfer of microscopic motion while minimizing energy dissipation presents a significant challenge. This paper reports a two-dimensional magnetic coordination framework, Fe(tpe)(NCBH₃)₂ (tpe = 1,1,2,2-tetra(pyridin-4-yl)ethene, 1), synthesized by the minimally sized, planar tetradentate pyridine-based tpe ligand in conjunction with the precursor Fe(NCBH₃)₂. A single-crystal-to-single-crystal (SCSC) transformation, occurring via the adsorption and desorption of C₂Cl₄ guest molecules, is directly associated with the activation of spin crossover (SCO) and CO₂/C₂H₂ separation performance, correlating with the overall elastic frustration and toughness present in the framework. The establishment of multiple C–H^δ+⋯H^δ−–B dihydrogen bonds surrounding the tpe ligand facilitates its effective function as a static unit, simultaneously augmenting cooperativity with the Fe(NCBH₃)₂ acting as a dynamic unit, thereby enabling a macroscopic shape change in the free single crystal with reduced energy dissipation. The spin transition of the Fe²⁺ metal center, activated by SCSC transformation and serving as a driving force for crystal macroscopic deformation, provides new insights into the mechanism of structural adaptability in frameworks and designing novel molecular machines.