NMR and X-ray Study Revealing the Rigidity of Zeolitic Imidazolate Frameworks

NMR relaxation studies and spectroscopic measurements of zeolitic imidazolate framework-8 (ZIF-8) are reported. The dominant nuclear spin−lattice relaxation (T1) mechanism for ZIF-8 in air arises from atmospheric paramagnetic molecular oxygen. The C T1 measurements indicate that the oxygen interacts primarily with the imidazolate ring rather than the methyl substituent. Similar relaxation behavior was also observed in a ZIF with an unsubstituted ring, ZIF-4. Single-crystal X-ray diffraction was used to provide data for the study of the thermal ellipsoids of ZIF-8 at variable temperatures from 100 to 298 K, which further confirmed the rigid nature of this ZIF framework. These results highlight a rigid ZIF framework and are in contrast with dynamic metal−organic frameworks based on benzenedicarboxylate linking groups, for which the relaxation reflects the dynamics of the benzenedicarboxylate moiety. ■ INTRODUCTION Zeolitic imidazolate frameworks (ZIFs) are porous, crystalline frameworks in which the tetrahedral metal ions are linked by imidazolate (Im) units. ZIF structures may be compared with those of zeolites, as the bridging angle formed by this linkage is analogous to that between silicon oxygen and aluminum oxygen units in zeolites. ZIFs are of great interest for numerous applications including gas separation and storage. In contrast with metal−organic frameworks (MOFs), ZIF frameworks have exceptional chemical stability. Highlighting the chemical stability and the importance of ZIF frameworks, ZIF-8 (Zn(C4H5N2)2) is now commercially produced in large quantities by BASF. The framework mobility of MOFs, particularly the isoreticular metal−organic framework IRMOF series, based on Zn with benzenedicarboxylate linkers, has been shown to affect the diffusion of guest molecules within the framework. Placing various substituents on the benzenedicarboxylate linkers provides a degree of control of the internal dynamics within MOFs. As specific examples, an activation energy of 11.3 kcal/mol was obtained for the ring-flipping motion for the 1,4-benzenedicarobylate linker. IRMOF-2, containing a bromine substituent on the aromatic ring of the linker, has a rotational barrier of 7.3 kcal/mol. IRMOF-3, with an amino group substituent, lowers the rotational activation energy to only 5.0 kcal/mol. Such efforts may open opportunities for the development of functional materials and artificial molecular machines. In addition to mobility, framework flexibility appears to play a role, as guest molecules have entered framework openings that appear to be smaller than the guest molecules; therefore, an understanding of mobility in ZIFs is essential for accessing the role of diffusion in ZIF frameworks. The purpose of this work is to investigate the framework mobility in ZIF-8 and ZIF-4 using variable temperature nuclear magnetic resonance spectroscopy and single-crystal diffraction. Previous studies have demonstrated that proton and carbon NMR relaxation in MOFs reflects the local dynamics of the benzenedicarboxylate; however, no studies as of yet have focused on ZIFs. The present experiments demonstrate that in contradistinction to MOF frameworks, NMR relaxation in ZIF8 has a much different origin and suggests that the linking groups in ZIFs are much more rigid than the linking groups in MOFs. Furthermore, substitution of the imidazolate in ZIF-8 does not alter the dynamics of the ring. Received: April 23, 2012 Revised: May 31, 2012 Published: June 1, 2012 Article