Dynamic Covalent Bonds in the Ebselen Class of Antioxidants Probed by X-ray Quantum Crystallography.

Dynamic bonds are essential structural ingredients of dynamic covalent chemistry that involve reversible cleavage and formation of bonds. Herein, we explore the electronic characteristics of Se-N bonds in the organo-selenium antioxidant ebselen and its derivatives for their propensity to function as dynamic covalent bonds by employing high-resolution X-ray quantum crystallography and complementary computational studies. An analysis of the experimentally reconstructed X-ray wavefunctions reveals the salient electronic features of the Se-N bonds with very low electron density localized at the bonding region and a positive Laplacian value at the bond critical point. Bond orders and percentage covalency and ionicity estimated from the X-ray wavefunctions, along with localized orbital locator (LOL) and electron localization function (ELF) analyses show that the Se-N bond is unique in its closed shell-like features, despite being a covalent bond. Time-dependent DFT calculations simulate the cleavage of Se-N bonds in ebselen in the excited state, further substantiating their nature as dynamic bonds.

The Ambiguous Origin of Thermochromism in Molecular Crystals of Dichalcogenides: Chalcogen Bonds versus Dynamic Se-Se/Te-Te Bonds.

We report thermochromism in crystals of diphenyl diselenide (dpdSe) and diphenyl ditelluride (dpdTe), which is at variance with the commonly known mechanisms of thermochromism in molecular crystals. Variable temperature neutron diffraction studies indicated no conformational change, tautomerization or phase transition between 100 K and 295 K. High-pressure crystallography studies indicated no associated piezochromism in dpdSe and dpdTe crystals. The evolution of the crystal structures and their electronic band structure with pressure and temperature reveal the contributions of intramolecular and intermolecular factors towards the origin of thermochromism-especially the intermolecular Se⋅⋅⋅Se and Te⋅⋅⋅Te chalcogen bonds and torsional modes of vibrations around the dynamic Se-Se and Te-Te bonds. Further, a co-crystal of dpdSe with iodine (dpdSe-I2 ) and an alloy crystal of dpdSe and dpdTe implied a predominantly intramolecular origin of the observed thermochromism associated with vibronic coupling.

Non-Classical Synthons: Supramolecular Recognition by S⋠⋠⋠O Chalcogen Bonding in Molecular Complexes of Riluzole.

Classical examples of supramolecular recognition units or synthons are the ones formed by hydrogen bonds. Here, we report the ubiquity of a S⋅⋅⋅O chalcogen bonded synthon observed in a series of supramolecular complexes of the amyotrophic lateral sclerosis drug riluzole. Although the potential of higher chalcogens such as Se and Te to form robust and directional chalcogen bonded motifs is known, intermolecular sulfur chalcogen bonding is considered to be weak owing to the lower polarizability of S atoms. Here, the robustness and electronic nature of a S⋅⋅⋅O chalcogen bonding non-classical synthon, and the origin of its exceptional directionality have been explored. Bond orders of the drug-coformer chalcogen bonding are found to be as high as one third of a single bond, and they are largely ionic in nature. The contribution of the S⋅⋅⋅O chalcogen bonded motifs to the lattice energies of a series of crystals from the Cambridge Structural Database has been analyzed, showing they can be indeed significant, especially in molecules devoid of strong hydrogen bond donor groups.

Bond orders for intermolecular interactions in crystals: charge transfer, ionicity and the effect on intramolecular bonds.

The question of whether intermolecular interactions in crystals originate from localized atom⋯atom interactions or as a result of holistic molecule⋯molecule close packing is a matter of continuing debate. In this context, the newly introduced Roby-Gould bond indices are reported for intermolecular 'σ-hole' interactions, such as halogen bonding and chalcogen bonding, and compared with those for hydrogen bonds. A series of 97 crystal systems exhibiting these interaction motifs obtained from the Cambridge Structural Database (CSD) has been analysed. In contrast with conventional bond-order estimations, the new method separately estimates the ionic and covalent bond indices for atom⋯atom and molecule⋯molecule bond orders, which shed light on the nature of these interactions. A consistent trend in charge transfer from halogen/chalcogen bond-acceptor to bond-donor groups has been found in these intermolecular interaction regions via Hirshfeld atomic partitioning of the electron populations. These results, along with the 'conservation of bond orders' tested in the interaction regions, establish the significant role of localized atom⋯atom interactions in the formation of these intermolecular binding motifs.