Structural transitions during the water sorption process in two layered metal hydrogen-bonded organic frameworks and the effect of the H-bond strength between the layers

Abstract

The supramolecular network material [Co(amp)₃][Cr(ox)₃] (amp = 2-aminomethylpyridine, ox = oxalate) (I′) shows in its powder X-ray diffraction (PXRD) carpet plot during the water adsorption process two sets of diffraction lines related to its structure and to that of [Co(amp)₃][Cr(ox)₃]·6H₂O (I) having the same symmetry (space group P2₁/n). The first set occurs in the range of 0 to 39% relative humidity (r.H.), and the second set is observed in the range of 30 to 90% r.H. During the desorption process, this second set is observed from 90 to 5% r.H., and the first set in the range of 15 to 0% r.H. indicating that the stepped water sorption isotherm and the large hysteresis loop for this material are coordinated with the discontinuous phase transitions occurring with water adsorption–desorption on this material. During these transitions, the rate of conversion depends on the water pressure in the system, and the structural change is mainly observed along the b-axis, which is the stacking direction of the layers. The same observations are made for the supramolecular material [Cu₂(amp)₄Cl][Cr(ox)₃]·1H₂O (II′), which is converted to [Cu₂(amp)₄Cl][Cr(ox)₃]·6H₂O (II) during the water adsorption process. But in this case, the discontinuous phase transition starts at 10% r.H. during both the adsorption and desorption processes, and lens-shaped diffraction lines are observed in the range of 10 and 90% r.H. indicating a continuous phase transition in this range. During the discontinuous phase transition, the space group of the framework is changed from P2₁/n to C2/c, and the latter is maintained during the continuous phase transition. The lens-shaped diffraction lines are related to the gradual increase of the b parameter (stacking direction of layers) with increasing humidity. The hydrogen bonds found in the bilayer space of I′ are stronger than those found in that of II′, which explains the abrupt transition in the former compared to the continuous phase transition in II′ and the higher quantity of water adsorbed in I′ compared to that of II′ during the gate effect.