Synthesis and structural characterization of anion complexes with azacalix[2]dipyrrolylmethane: effect of anion charge on the conformation of the macrocycle

Abstract

Tetrahomodiazacalix[2]dipyrrolylmethane 1, [-CH2(C4H3N)CR2(C4H3N)CH2N(Me)-]2 an expanded version of the calix[4]pyrrole system, has the tendency to change its ring conformation (1,3-alternate) upon anion binding analogous to calix[4]pyrrole. However, owing to its tertiary amine nitrogen atoms in the ring, it can be protonated and becomes a powerful cationic receptor for anions, besides its inherent hydrogen bonding features. Macrocycle 1 binds with a series of monoanions BF4(-), Cl(-), PhCOO(-) and ClO4(-), and their X-ray structures showed that the ring conformation constitutes the 1,2-alternate form. Upon binding with dianions SO4(2-), CrO4(2-), SiF6(2-) and S2O3(2-), the ring conformation changes to the cone conformation. The intermediate partial cone conformation results for complexes with NO3(-) and Cr2O7(2-) ions. The change in the orientation of the pyrrole NH groups depending on the charge of the anion demonstrates the flexibility of the macrocycle and the dicationic macrocycle stabilizes the anions via both hydrogen bonding and electrostatic interactions.

Supramolecular Chemistry of Anionic Dimers, Trimers, Tetramers, and Clusters

“…90 A number have been found to support complexes containing concurrently more than two anions. For instance, Guchhait et al 91 described the synthesis and structural characterization of several polyanion complexes of the azacalix[2]dipyrrolylmethanes 64 and 65 . In analogy to what is seen in the case of calix[4]pyrroles, these receptors have a tendency to undergo a change in conformation upon anion binding.…”

Does Larger Cavity‐Size Really Help Bigger Anions to Bind? A Scrutiny on Core‐Expanded Calix[4]pyrroles and Their Properties

“…As can be seen from the crystal structures, the dicationic macrocycles adopted 1,2‐alternate conformations with mononegative anions, whereas they adopted cone conformations with dinegative anions (Figure 10). [43] The intermediate partial cone conformations were also observed in some cases. All these conformations are possible owing to the flexible nature of the receptors 18 .…”

“…To investigate the anion binding behavior of receptor 18 in solid state, a number of complexes of receptors 18 a and 18 b was synthesized with various mono‐ (NO 3 − , BF 4 − , Cl − , PhCOO − , and ClO 4 − ) and dinegative (SO 4 2− , CrO 4 2− , Cr 2 O 7 2− , SiF 6 2− , and S 2 O 3 2− ) anions [41,43] . The receptors bound the anions in their diprotonated forms.…”

Mannich Reaction: An Alternative Synthetic Approach for Various Pyrrole‐Based Anion Receptors and Chelating Ligands

“…It was also found that upon anion binding with receptor 37, the conformation of the macrocycle changes. [37] Depending on the anion charge, the dipositively charged macrocycles [H 2 37] 2 + or [H 2 38] 2 + arranged themselves in a way where pyrrole rings directed differently. The complexes of dinegatively charged anions such as sulfate (47), hexafluorosilicate (48) etc.…”