Experimental and Computational Investigations of Chalcogen Bonding

Abstract

Chalcogen bonding (ChB) is a particular case of secondary bonding centred on heavy group-16 elements. It is almost exclusively identified through crystallography by measuring interatomic distances intermediate between single-bond averages and the sum of van der Waals radii. However, there is significant recent progress in discerning its signatures using spectroscopic techniques such as multinuclear NMR. This M.Sc. thesis describes progress in two research projects on chalcogen bonding. The first examined the effect of halogenation on the aggregation of 3-methyl-5-phenyl 1-2-tellurazole 2-oxide. The second examined the strengthening of ChB interaction between molecules of benzo-1,2-chalcogenazole 2-oxides by chlorination. The bromination of 3-methyl-5-phenyl 1-2-tellurazole 2-oxide yielded 3,3,3-tri-bromo-3-methyl-5-phenyl-1,2-tellurazole-2-anole. Four unique crystal structures were obtained with the most promising being the dimeric structure. Deprotonation was unsuccessfully attempted although yielded 2 unique crystal structures co-crystallized with proton-sponge. Iodination of 3-methyl-5-phenyl 1-2-tellurazole 2-oxide was also performed, resulting in a mixed tetrameric aggregate containing two molecules of 3-methyl-5-phenyl 1-2-tellurazole 2-oxide and two 1,1-di-iodo-3-methyl-5-phenyl 1-2-tellurazole 2-oxide molecules. DFT investigations into the electronic properties, thermodynamics of aggregation, and basicity were performed. Similar to the chlorinated derivative, the most favourable aggregate to form is the hetero-tetramer with two brominated or iodinated molecules and 2 non-halogenated molecules. The reaction of benzo 1,2-sellenazole 2-oxide with SO2Cl2 and benzo 1,2-tellurazole 2-oxide with HCl followed by SO2Cl2 yielded halogenated derivatives of each molecule in which the chalcogen was oxidized from Ch(II) to Ch(IV). In the selenium derivative, an unexpected chlorination occurred on the heterocycle of the molecule. Crystal structures were obtained for each chlorinated product where dimeric interactions were observed. DFT calculations show how the electronic and orbital mixing contributions to the ChB interactions are enhanced upon halogenation. Gibbs free energy of aggregation is most negative for a mixed structure in which two chlorinated molecules and two unchlorinated molecules are linked.