A cyclic pseudopeptide containing 1,5-disubstituted 1,2,3-triazole subunits adopts a very similar conformation as the anion-binding cyclopeptide and therefore also efficiently interacts with inorganic anions, even in aqueous solution.¹ This pseudopeptide is, however, less prone to form sandwich-type 2:1 complexes.

[For an interactive version of the crystal structure click here]

Analogous cyclic pseudopeptides with 1,4-disubstituted 1,2,3-triazole moieties prefer other conformations, because such triazole units structurally mimic trans- and not cis-amides. The 1,4-disubstituted triazole units also increase the ring diameter with respect to the pseudopeptide with 1,5-disubstituted triazoles and decrease the solubility in aqueous solvents.

The cyclic trimer shown below binds oxoanions, such as sulfate or dihydrogen phosphate, in 2.5 vol% water/DMSO.² The complexes have different stoichiometries, which shows that the structural complementarity of this pseudopeptide for oxoanion recognition is not optimal. The higher homolog, the cyclic tetramer, exhibits particular high affinity for dihydrogen phosphate and dihydrogenpyrophosphate anions. Due to the size of the pseudopeptide, no 1:1 complexes are formed, however. In the case of dihydrogenpyrophosphate anions, two anions are bound between two pseudopeptide rings and in the case of dihydrogen phosphate, a cyclic aggregate of four anions.³ The stability of these complexes is so high that they can be detected in solution and even in the gas phase. The crystal structure of the dihydrogen phosphate complex of this pseudopeptide is shown below.

[For an interactive version of the crystal structure click here]

The cyclic trimer with 5-iodo-1,2,3-triazole units shown below has a smaller cavity than the analogous pseudopeptide with protonated triazole units. It therefore mainly binds via halogen bonding to smaller halides.⁴ The highest affinity is observed for chloride anions.

References

1. M. R. Krause, R. Goddard, S. Kubik J. Org. Chem. 2011, 76, 7084-7095.
2. D. Mungalpara, H. Kelm, A. Valkonen, K. Rissanen, S. Keller, S. Kubik Org. Biomol. Chem. 2017, 15, 102-113.
3. D. Mungalpara, A. Valkonen, K. Rissanen, S. Kubik Chem. Sci. 2017, 8, 6005-6013.
4. D. Mungalpara, S. Stegmüller, S. Kubik Chem. Commun. 2017, 53, 5095-5098.