Multinuclear Magnetic Resonance and X-Ray Crystallographic Scrutinization of Crystal-Engineered Chalcogen-Bonded Cocrystals

Abstract

Chalcogen bonds, a growing class of noncovalent interactions, have been extensively studied in recent years due to their rising utility and applications in the fields of catalysis, crystal engineering, pharmaceuticals, active anion recognition, and materials chemistry. A chalcogen bond's physical properties, reactivity, and electronic structure are very much under study and development by chemists of today. A chalcogen bond consists of a chalcogen bond donor which features an electron-depleted region on the chalcogen atom (σ-hole) and a chalcogen bond acceptor, which features a region of rich electron density in an atom or functional group of another molecule or the same molecule that contains the chalcogen bond donor. The σ-hole of the chalcogen bond donor imparts high directionality and tuneability as compared to a simple covalent bond, resulting in a versatile class of noncovalent interactions. This thesis presents work where chalcogen-bonded cocrystals were synthesized and analyzed using techniques such as single-crystal X-ray diffraction, powder X-ray diffraction, multinuclear solid-state magnetic resonance spectroscopy and nuclear quadrupole resonance spectroscopy. The beginning of the research work involved working with Te and Se based chalcogen bond donors, namely 3,4-dicyano-1,2,5-selenodiazole (selenodiazole) and 3,4-dicyano-1,2,5-telluradiazole (telluradiazole) with a variety of chalcogen bond acceptors such as hydroquinone, tetraphenylphosphonium chloride, and tetraethylphosphonium chloride. Their electronic structure and geometry were explored using solid-state NMR and X-ray diffraction techniques. The chemical shift (CS) tensors obtained from solid-state NMR experiments on the chalcogen bonded cocrystals featuring selenodiazole-hydroquinone, telluradiazole-tetraphenylphosphonium chloride and telluradiazole-tetraethylphosphonium chloride were used to probe the chalcogen bond donors, selenodiazole and telluradiazole. The principal components of the chalcogen atom chemical shift tensors, as well as their spans and skews, were compared to previous results from our group and a trend in these SSNMR parameters was established. The ¹²⁵Te chemical shift tensor span was found to be sensitive to the ChB formation. The small changes observed in the chemical shift tensor of ⁷⁷Se SSNMR for the selenodiazole-hydroquinone cocrystal are consistent with the retention of self-complementary chalcogen bonds between two selenodiazole molecules. The next project revealed how chemical shift tensors served as simultaneous and complementary probes of both the ChB donor and the acceptor of chalcogen-bonded salt cocrystals. ChB salt cocrystals featuring [K(18-crown-6)]⁺ 3,4-dicyano-1,2,5-telluradiazole-XCN⁻ (X = O, S, Se) were synthesized and compared to pure ChB donor 3,4-dicyano-1,2,5-telluradiazole. The cocrystal of 18-crown-6, KOCN (ChB acceptor part) and telluradiazole (ChB donor), highlighted a ChB between Te and the N atom of the cyanate anion. The further enrichment of KOC¹⁵N helped in inspecting the trend of chemical shift tensor components of the ChB acceptor, in this case, the nitrogen of cyanate ion with the help of ¹⁵N SSNMR. The reduced spans of the ¹²⁵Te and ¹⁵N chemical shift tensors upon ChB formation with respect to pure ChB donor, telluradiazole, are complemented by DFT computation studies on cluster models and corroborates the experimental finding that upon formation of [K(18-crown-6)]⁺[1-OC¹⁵N]⁻, the ¹⁵N isotropic CS and CS tensor span both decrease relative to the values for pure KOC¹⁵N, and the axial symmetry of this tensor is lost. The works in Chapter 7 and Chapter 8 deal with experimental NMR measurements carried out on ChB cocrystals featuring interactions between spin-½ chalcogen nuclei and a quadrupolar nucleus (I>1/2). Chapter 7 provides experimental evidence of non-Fermi contact (FC) spin-spin coupling across the ChB between Te and Br in two ionic polymorphic cocrystals of telluradiazole and tetraphenylphosphonium bromide due to large anisotropic J(¹²⁵Te, ⁷⁹ᐟ⁸¹Br) coupling tensors extracted with the aid of SSNMR. The crystal structures and phase purity of the two polymorphs were characterized by X-ray diffraction (XRD) techniques prior to SSNMR experiments. Chapter 8 revealed higher-order quadrupolar effects of a spin-5/2 ¹²⁷I nucleus when coupled to spin-1/2 ¹²⁵Te in a ChB cocrystal of telluradiazole (ChB donor) and tetraphenylphosphonium iodide (ChB acceptor) featuring a ChB between tellurium and iodine. The increase in the value of the ¹²⁷I quadrupolar coupling constant in the cocrystal of telluradiazole and tetraphenylphosphonium iodide as compared with that for the pure ChB acceptor, tetraphenylphosphonium iodide, implied a reduction of local symmetry. ¹²⁷I nuclear quadrupole resonance (NQR) experiments were carried out to confirm the findings from ¹²⁷I SSNMR spectroscopy.