Large breathing effect induced by water sorption in a remarkably stable nonporous cyanide-bridged coordination polymer

Abstract

While metal–organic frameworks (MOFs) are at the forefront of cutting-edge porous materials, extraordinary sorption properties can also be observed in Prussian Blue Analogs (PBAs) and related materials comprising extremely short bridging ligands. Herein, we present a bimetallic nonporous cyanide-bridged coordination polymer (CP) {[Mn(imH)]₂[Mo(CN)₈]}_n (1Mn; imH = imidazole) that can efficiently and reversibly capture and release water molecules over tens of cycles without any fatigue despite being based on one of the shortest bridging ligands known – the cyanide. The sorption performance of {[Mn(imH)]₂[Mo(CN)₈]}_n matches or even outperforms MOFs that are typically selected for water harvesting applications with perfect sorption reversibility and very low desorption temperatures. Water sorption in 1Mn is possible due to the breathing effect (accompanied by a dramatic cyanide-framework transformation) occurring in three well-defined steps between four different crystal phases studied structurally by X-ray diffraction structural analysis. Moreover, the capture of H₂O by 1Mn switches the EPR signal intensity of the Mn^II centres, which has been demonstrated by in situ EPR measurements and enables monitoring of the hydration level of 1Mn by EPR. The sorption of water in 1Mn controls also its photomagnetic behavior at the cryogenic regime, thanks to the presence of the [Mo^IV(CN)₈]⁴⁻ photomagnetic chromophore in the structure. These observations demonstrate the extraordinary sorption potential of cyanide-bridged CPs and the possibility to merge it with the unique physical properties of this class of compounds arising from their bimetallic character (e.g. photomagnetism and long-range magnetic ordering).