Competing Effects of Chlorination on the Strength Te…O Chalcogen Bonds Select the Structure of Mixed Supramolecular Macrocyclic Aggregates of Iso-Tellurazole N-Oxides.

Chlorination of 3-methyl-5-phenyl-1,2-tellurazole-2-oxide yielded the λ4 Te dichloro derivative. Its crystal structure demonstrates that the heterocycle retains its ability to autoassociate by chalcogen bonding (ChB) forming macrocyclic tetramers. The corresponding Te…O ChB distances are 2.062 A, the shortest observed to date in aggregates of this type. DFT-D3 calculations indicate that while the halogenated molecule is stronger as a ChB donor it also is a weaker ChB acceptor; the overall effect is that the ChBs in the chlorinated homotetramer are not significantly stronger. However, partial halogenation or scrambling selectively yield the 2:2 heterotetramer with alternating λ4 Te and λ2 Te centers, which calculations identified as the thermodynamically preferred arrangement.