Non-Innocent Base Properties of 3- and 4-Pyridyl-dithia- and Diselenadiazolyl Radicals: The Effect of N-Methylation

2Ch-2N Square Chalcogen Bonds between Pairs of Radicals: A Case Study of 1,2,3,5-Dichalcogenadiazolyl Derivatives

Specific 2Ch-2N square interactions between pairs of heterocyclic rings have been the target of many recent crystallographic and computational studies. According to our search of the Cambridge Structural Database (CSD), a number of crystal structures of the derivatives of 1,2,3,5-dichalcogenadiazolyl (DChDA) radicals, which consist of 2Ch-2N square motifs in the dimer units, were extracted. On the basis of the CSD survey results, a set of dimeric complexes of DChDA-based radicals with diverse aryl substituents at the 4-position were selected to model such squares. Similar to that in conventional chalcogen bonds, 2Ch-2N square interactions become stronger as the atomic size of chalcogens increases. Both the orbital term and electrostatics contribute significantly to the attraction of these interactions, while the dispersion contribution is small but unneglectable. Some five-membered aryl substituents, such as imidazole, thiazole, and oxazole, produce markedly enhanced square interactions, leading to a pronounced influence on the distribution of spin populations on DChDA rings.

Room-Temperature Magnetic Bistability in a Salt of Organic Radical Ions

Cocrystallization of 7,7',8,8'-tetracyanoquinodimethane radical anion (TCNQ^-•) and 3-methylpyridinium-1,2,3,5-dithiadiazolyl radical cation (3-MepyDTDA^+•) afforded isostructural acetonitrile (MeCN) or propionitrile (EtCN) solvates containing cofacial π dimers of homologous components. Loss of lattice solvent from the diamagnetic solvates above 366 K affords a high-temperature paramagnetic phase containing discrete TCNQ^-• and weakly bound π dimers of 3-MepyDTDA^+•, as evidenced by X-ray diffraction methods and magnetic susceptibility measurements. Below 268 K, a first-order phase transition occurs, leading to a low-temperature diamagnetic phase with TCNQ^-• σ dimer and π dimers of 3-MepyDTDA^+•. This study reveals the first example of cooperative interactions between two different organic radical ions leading to magnetic bistability, and these results are central to the future design of multicomponent functional molecular materials.

Radicalradical chalcogen bonds: CSD analysis and DFT calculations

This manuscript reports a combination of crystallographic analysis (Cambridge Structural Database) and theoretical DFT calculations in chalcogen bonding interactions involving radicals in both the Ch bond (ChB) donor and acceptor. As a radical ChB acceptor (nucleophile) we have used benzodithiazolyl radical (BDTA) and as Ch bond donors (electrophile) we have used dithiadiazolyl and diselenadiazolyl radicals of the general formula p-X-C6F4-CNChChN (Ch = S, and Se). We have evaluated how the para substituent (X) affects the interaction energy, spin density and charge/spin transfer from the electron rich BDTA radical to the electron poor dichalcogenadiazolyl ring. The ability of the latter rings to form ChBs in the solid state has been examined by a comprehensive search in the CSD; several cases are used to exemplify the preferred geometric features of the complexes and they are compared with the theory. The molecular surface electrostatic potentials calculated for these ChB donors allow for a very precise rationalization of the self-assembly motifs most frequently adopted in the crystalline state and of their relative robustness.

Experimental and Computational Evidence for "Double Pancake Bonds": The Role of Dispersion-Corrected DFT Methods in Strongly Dimerized 5-Aryl-1λ2,3λ2-dithia-2,4,6-triazines

Crystal structures are reported for bicyclic 3-CF₃C₆H₄CN₅S₃ and monocyclic 3-CF₃C₆H₄CN₃S₂, the latter of which is strongly dimerized in a cis-cofacial geometry [3-CF₃C₆H₄CN₃S₂]₂. The title compounds have previously been characterized in solution by NMR, displaying spectra that are consistent with the structure of [3-CF₃C₆H₄CN₃S₂]₂ in the crystal with anti-oriented CF₃ substituents. The interannular binding was investigated using density functional theory (DFT) methods. However, the DFT-optimized geometry spreads the aryl rings too far apart (centroid-centroid distances of ≥4.353 Å versus experimental distance of 3.850 Å). Significant improvements are obtained with dispersion-corrected DFT functionals B3LYP-D3, B3LYP-D3BJ, M062X, and APFD using the 6-311+G(2d,p) basis set. However, all of these overbind the aryl rings with centroid-centroid distances of 3.612, 3.570, 3.526, and 3.511 Å, respectively. After selecting B3LYP-D3BJ/6-311+G(2d,p) as the best method, five alternative dimer geometries were tested, and all were found to be binding; however, anti cofacial-4 (matching the structure in the solid state) is the most stable. Computed energies of the remainder are as follows: +7.0 kJ mol^-1 (syn-cofacial-5), +26.7 kJ mol^-1 (anti-cofacial-64), +27.0 kJ mol^-1 (syn-cofacial-150), +102.0 kJ mol^-1 (S,S-antarafacial), and +103.7 kJ mol^-1 (S,N-antarafacial), where the suffixes are torsional angles around the CN₃S₂ thiazyl ring centroids. The binding in the four most stable cofacial dimers may be described by "double pancake bonding".