TEMPERATURE INDUCED STRUCTURAL EVOLUTION OF POROUS IMIDAZOLATE COMPOUNDS VIA SYNCHROTRON XRPD

Sanja Burazer¹, Milan Dopita¹, Yaroslav Filinchuk², Radovan Černy³, Jasminka Popović⁴

¹ Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic

² Institute of Condensed Matter and Nanosciences, Université catholique de Louvain, Louvain-la-Neuve, Belgium

³ Laboratory of Crystallography, DQMP, University of Geneva, Geneva, Switzerland

⁴ Laboratory for Synthesis and Crystallography of Functional Materials, Division for Materials Physics, Ruđer Bošković Institute, Zagreb, Croatia

Zeolitic imidazolate frameworks (ZIFs) are an interesting class of metal-organic frameworks, structured by tetrahedrally configured transition metal cations bridged by imidazolate (Im). ZIFs are able to reproduce the zeolitic topology but also incorporate the electronic properties of the transition metal ions.[1]

In this research, novel high-temperature polymorph of sodium imidazolate, HT-NaIm was discovered. Solid-state NMR was used for initial elucidation of structural features, the crystal structure was determined by single-crystal X-ray diffraction, while the in-situ HT-XRPD experiments utilizing synchrotron radiation have been performed in order to gain the insight into the structural evolution and thermal stability which was additionally analized by differential thermal analysis and hot stage microscopy measurements. HT-NaIm exhibits pores of 50 ų that suggest possible application for gas sorption/separation. Once formed, high-temperature polymorph of NaIm retains its structure and remains stable at room temperature, what is important application-wise.

Additionally, new family of mixed bimetallic imidazolates AMIm₃ (A=Na, K; M=Mg, Mn) has been synthesized and crystal structures were determined from powder X-ray diffraction data. Temperature aided decomposition during in-situ SR HT-XRPD experiments gave the information about structural changes ad thermal stability of the prepared samples. All compounds have the imidazolate ligand connected to four metal cations forming a complex 3D network with channels running along the c-direction, thus showing the similar sorption potential because of the empty volume of around 30 ų incorporated inside the channels (Figure 1).[2]

Figure 1. Extended crystal packing of KMgIm₃ showing channels along the c-direction.

1. O. M. Yaghi, M. J. Kalmutzki, and C. S. Diercks. in Introduction to Reticular Chemistry: Metal-Organic Frameworks and Covalent Organic Frameworks (Wiley-VCH Verlag GmbH & Co. KGaA), 2019, 463-479.

2. S. Burazer, F. Morelle, Y. Filinchuk, R. Černý, J. Popović, Inorganic Chemistry, 58, (2019) 6927-6933.

The authors acknowledge the Swiss-Norwegian Beamlines of ESRF for the allocation of beamtime and excellent support with the data collection.