Crystalline bilayers unzipped and rezipped: solid-state reaction cycle of a metal-organic framework with triple rearrangement of intralayer bonds

We present a series of remarkable structural transformations for a family of layered metal-organic frameworks (MOFs) in a three-step solid-state reaction cycle. The cycle represents new dynamic behavior of 2D coordination polymers and involves the sequence of reactions: {[Mn-2(ina)(4)center dot H2O)(2)]center dot 2EtOH}(n) (JUK-1) -> {(NH4)(2)[Mn(ina)(2)NCS)(2)]}(n)center dot xH(2)O(JUK-2) -> {[ Mn-2(ina)(2)(Hina)(2)(NCS)(2)]}(n)(JUK-3) -> JUK-1 (Hina = isonicotinic acid), each accompanied by rearrangement of intralayer coordination bonds and each induced by a different external stimulus. In situ investigation of the first step of the cycle by combined synchrotron X-ray diffraction and Raman spectroscopy reveals direct mechanochemical unzipping of JUK-1 bilayers to respective JUK-2 layers with reaction rates dependent on the milling conditions. In contrast, the reverse zipping of JUK-2 layers involves two steps and proceeds through a new MOF (JUK-3) whose structure was elucidated by powder X-ray diffraction. Magnetic measurements confirm conversions of manganese nodes in the reaction cycle. The findings indicate the possibility of developing coordination-based assemblies with large structural responses for use in smart stimuli-responsive systems and sensor technologies.