posted on 2020-05-07, 09:02authored byChristopher
A. O’Keefe, Cristina Mottillo, Jogirdas Vainauskas, László Fábián, Tomislav Friščić, Robert W. Schurko
We
demonstrate the combined use of NMR-enhanced crystallography
and solvent-free synthesis by accelerated aging (AA), for the discovery
and structural characterization of a novel cadmium-based open metal–organic
framework (MOF) belonging to the class of zeolitic imidazolate frameworks
(ZIFs). Although solid-state NMR spectroscopy has been used to assist
in structural characterization of crystalline solids by powder X-ray
diffraction (PXRD), typically through quantification of the contents
of the asymmetric unit, this work highlights how it can take a more
active role in guiding structure determination, by elucidating the
coordination environment of the metal node in a novel MOFs. Exploration
of AA reactions of cadmium oxide (CdO) and 2-methylimidazole (HMeIm) enabled the synthesis of not only the previously reported yqt1-topology framework but also a new material (1) exhibiting a Cd/MeIm ratio of 1:3, contrasting the
1:2 ratio expected for a ZIF. Structural characterization of 1 was enabled by using 111Cd solid-state nuclear
magnetic resonance (SSNMR) to provide information on the coordination
environment of the cadmium node. Specifically, 111Cd SSNMR
experiments were conducted on a series of model compounds to correlate
the cadmium coordination environment to the observed isotropic chemical
shift, δiso(111Cd), followed by multinuclear
SSNMR experiments on 1 to determine the nature of the
metal coordination environment and the number of distinct chemical
sites. This information was used in refinement of the molecular level
structure from the available PXRD data, a technique termed NMR-enhanced
crystallography, revealing that 1 is an open diamondoid
(dia) topology Cd(MeIm)2 framework
based on Cd2+ ions tetrahedrally coordinated with MeIm– ligands and additional
HMeIm guest molecules within the framework pores. Although
AA was initially devised as a clean, mild route for making MOFs, these
results provide a proof-of-principle of how, by combining it with
SSNMR spectroscopy as a means to overcome limitations of PXRD structure
determination, it can be used to screen for new solid phases in the
absence of solvents, high temperatures, or mechanical impact that
are inherent to other thermally-, solution-, or mechanochemically-based
techniques.