Can 3D electron diffraction provide accurate atomic structures of metal-organic frameworks?

Zhehao Huang,Xiaodong Zou,Christian Doonan,Meng Ge,Francesco Carraro,Paolo Falcaro

doi:10.1039/d0fd00015a

Abstract

Many framework materials such as metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) are synthesized as polycrystalline powders, which are too small for structure determination by single crystal X-ray diffraction (SCXRD). Here, we show that a three-dimensional (3D) electron diffraction method, namely continuous rotation electron diffraction (cRED), can be used for ab initio structure determination of such materials. As an example, we present the complete structural analysis of a biocomposite, denoted BSA@ZIF-CO3-1, in which Bovine Serum Albumin (BSA) was encapsulated in a zeolitic imidazolate framework (ZIF). Low electron dose was combined with ultrafast cRED data collection to minimize electron beam damage to the sample. We demonstrate that the atomic structure obtained by cRED is as reliable and accurate as that obtained by single crystal X-ray diffraction. The high accuracy and fast data collection open new opportunities for investigation of cooperative phenomena in framework structures at the atomic level.

Highlights

Metal–organic frameworks (MOFs) or porous coordination polymers (PCPs) offer large surface areas, tuneable pore structures, adjustable chemical functionality and structural exibility.[1,2] Because of these unique structural properties, they have tremendous potential in a wide range of applications such as catalysis, gas storage and separation, ion exchange, bio-medical and bio-technological applications.[3,4,5,6,7,8,9] Notably, under different synthesis conditions, various metal–organic frameworks (MOFs) structuresPaper with different topologies and pore sizes can be obtained using the same metal ions and organic linkers
Continuous rotation electron diffraction data collection Different from the step-wise data collection in ADT/PEDT and RED where no crystal rotation is applied during the exposure, the crystal is rotated continuously at a constant speed during the continuous rotation electron diffraction (cRED) data collection (Fig. 2).[53]
The anisotropic atomic displacement parameters are reasonable. This shows that despite the lower crystallinity of Bovine Serum Albumin (BSA)@zeolitic imidazolate framework (ZIF)-CO3-1 and high R1 values, the structural model obtained from cRED data is consistent with that of ZIF-CO3-1 obtained by single crystal X-ray diffraction (SCXRD)

Summary

Introduction

Metal–organic frameworks (MOFs) or porous coordination polymers (PCPs) offer large surface areas, tuneable pore structures, adjustable chemical functionality and structural exibility.[1,2] Because of these unique structural properties, they have tremendous potential in a wide range of applications such as catalysis, gas storage and separation, ion exchange, bio-medical and bio-technological applications.[3,4,5,6,7,8,9] Notably, under different synthesis conditions, various MOF structuresPaper with different topologies and pore sizes can be obtained using the same metal ions and organic linkers. This has been studied in stable crystals such as zeolites and metal oxides,[52,53] it has not yet been applied to MOFs. Here, we report an ab initio structure determination of a ZIF biocomposite by continuous rotation electron diffraction (cRED). 3D electron diffraction data are collected at arbitrary orientations of a crystal, and most of the ED frames are off the zone axes which largely minimizes the chance of multiple scattering in the crystal.[34,59] The RED data processing program can process 3D electron diffraction data collected by both step-wise and continuous rotation methods, and can reconstruct the 3D reciprocal lattice (Fig. 3) from which the unit cell parameters and space group can be determined.[37] Because cRED data collection resembles SCXRD data collection, existing so ware for data processing developed for SCXRD can be directly used.

Results

Conclusion