Iso-tellurazole N-oxides yield a remarkable variety of structures with coinage-metal monocations.

Chlorination of 3-methyl-5-phenyl-1,2-tellurazole-2-oxide yielded the λ 4 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 Å, the shortest observed to date in aggregates of this type. DFTÀ 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 λ 4 Te and λ 2 Te centers, which calculations identified as the thermodynamically preferred arrangement.Chalcogen bonding (ChB), [1][2][3][4][5][6][7] i. e. the attractive supramolecular interaction of electrophilic sites on atoms of heavy group 16elements (S, Se, Te) with electron-rich centers, has unveiled an array of fascinating chemistry in fields as diverse as Lewis-acid catalysis, [8][9][10][11][12][13][14][15] optical probes, [14,16,17] functional materials, [5,[18][19][20][21][22][23][24] crystal engineering, [25][26][27][28][29][30] ion recognition and transport, [22,[31][32][33][34][35][36] and protein folding. [37] Amongst the many molecules capable of forming ChBs, isotellurazole N-oxides (1, Scheme 1) stand apart because of their ability to spontaneously assemble cyclic tetramers (1 4 ) and hexamers (1 6 ) that exist in equilibrium in solution. [38][39][40] There are two electrophilic regions on the tellurium atom of 1, antipodal to each the NÀ Te and the CÀ Te bonds. The former has a larger V max and, consequently, the Te•••O ChB interaction in all known cases is formed trans to the N atom (e. g. 1 4,6 in Scheme 1). The Te•••O ChBs are reversible, scrambling is[a] P.

Formal substitution of the oxygen atom of an isotellurazole N-oxide with deprotonated (ortho, meta, and para)hydroxyphenyl groups generated molecules that readily aggregate through Te•••O chalcogen bonding (ChB) interactions. The molecules undergo autoassociation in solution, as shown by variable temperature (VT) 1 H NMR experiments and paralleling the behavior of iso-tellurazole N-oxides. Judicious adjustment of crystallization conditions enabled the isolation of either polymeric or macrocyclic aggregates. Among the latter, the ortho compound assembled a calixarene-like trimer, while the para isomer built a macrocyclic tetramer akin to a molecular square. The Te•••O ChB distances in these structures range from 2.13 to 2.17 Å, comparable to those in the structures of iso-tellurazole N-oxides. DFT calculations estimate that the corresponding Te•••O ChB energies are between −122 and −195 kJ mol −1 in model dimers and suggest that macrocyclic aggregation enhances these interactions.