Abstract
Single crystal X-ray diffraction is arguably the most definitive method for molecular structure determination, but the inability to grow suitable single crystals can frustrate conventional X-ray diffraction analysis. We report herein an approach to molecular structure determination that relies on a versatile toolkit of guanidinium organosulfonate hydrogen-bonded host frameworks that form crystalline inclusion compounds with target molecules in a single-step crystallization, complementing the crystalline sponge method that relies on diffusion of the target into the cages of a metal-organic framework. The peculiar properties of the host frameworks enable rapid stoichiometric inclusion of a wide range of target molecules with full occupancy, typically without disorder and accompanying solvent, affording well-refined structures. Moreover, anomalous scattering by the framework sulfur atoms enables reliable assignment of absolute configuration of stereogenic centers. An ever-expanding library of organosulfonates provides a toolkit of frameworks for capturing specific target molecules for their structure determination.
Highlights
Single crystal X-ray diffraction is arguably the most definitive method for molecular structure determination, but the inability to grow suitable single crystals can frustrate conventional X-ray diffraction analysis
Johannes Martin Bijvoet first reported the determination of absolute configuration of sodium rubidium (+)-tartrate tetrahydrate using single crystal X-ray diffraction[9], wherein anomalous X-ray scattering by the rubidium heavy atoms enabled definitive structure determination[10]
crystalline sponge method (CSM), often requires specific intermolecular host-guest interactions or covalent fixation and is hindered by slow absorption kinetics that can require weeks for complete target incorporation, the need to use non-polar solvents, an upper size limit on target molecules imposed by the size of the pore apertures, and challenges in structure determination presented by disorder, partial occupancy and retention of solvent molecules in the framework cages
Summary
Single crystal X-ray diffraction is arguably the most definitive method for molecular structure determination, but the inability to grow suitable single crystals can frustrate conventional X-ray diffraction analysis. Molecular structure determination of such stubborn target molecules has been realized by single-crystal X-ray diffraction following their adsorption into a limited number of metal organic frameworks[11,12,13,14,15] and others (a.k.a. crystalline sponges)[16,17].
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