Sulfate recognition by a hexaaza cryptand receptor

Abstract

A hexamine macrobicycle with pyrrolyl spacers was evaluated as an anion receptor in its protonated forms. The protonation constants of the receptor, as well as its association constants with Cl(-), NO3(-), AcO(-), ClO4(-), H2PO4(-), and SO4(2-) were determined by potentiometry at 298.2 ± 0.1 K in H2O-MeOH (50 : 50 v/v) and at an ionic strength of 0.10 ± 0.01 M in KTsO. These studies revealed that the Hnpyrr(n+) receptor has a very high effective association constant value for the SO4(2-) at pH 4.0 (log Keff = 6.42), and it is selective for the uptake of this anion in the presence of the other studied anionic substrates. In particular, the receptor showed very high SO4(2-)/NO3(-) selectivity. Using the indicator-displacement approach the receptor is able to signal the presence of sulfate by a change of color. Single crystal X-ray diffraction determination of [(H6pyrr)(SO4)(H2O)3](SO4)2·9.3H2O revealed the presence of one sulfate anion inside the receptor cavity and showed that the encapsulation of the anion is favored by an array of nine hydrogen bonding interactions, including N-HO, C-HO and water-mediated ones.

Extraction and transport of sulfate using macrocyclic squaramide receptors

“…A further important challenge lies in the release of sulfate following extraction to allow facile recycling of the receptors and enable commercially viable industrial processes. 11 While a number of receptors for selective sulfate recognition have recently been reported, [13][14][15][16][17][18][19][20][21][22][23][24][25] there are relatively few examples of suitable receptors that overcome the Hofmeister bias to allow LLE of sulfate. [26][27][28][29][30][31][32][33] Sessler and co-workers have successfully employed calix[n]pyrroles to extract sulfate into organic media in the presence of methyltrialkylammonium ions.…”

Effects of Solvent Properties on the Anion Binding of Neutral Water-Soluble Bis(cyclopeptides) in Water and Aqueous Solvent Mixtures

“…Indeed, existing sulfate receptors active in aqueous media combine directed hydrogen-bond donors, which often completely surround the anion, and Coulomb attraction. 7,16 Examples are the polyammonium cages described by Delgado and Mateus, 17 the coordination cages developed by Custelcean and Hay, which feature 12 urea NH groups converging toward the cavity, 18 You’s tetrakis(imidazolium)-derived receptor, which forms a sandwich-type 2:1 complex with sulfate anions, in which the anion is bound by eight C–H···O hydrogen bonds, 19 and the pyrrole-based triazoliumphane described by Hay and Sessler. 20 Strong Coulombic interactions are, however, not necessarily required to complex sulfate or other anions in aqueous media, as demonstrated by the bis(cyclopeptides) developed in our group.…”

Macrocyclic squaramides: anion receptors with high sulfate binding affinity and selectivity in aqueous media

“…Following Nature's lead, numerous receptors have been developed for the recognition and/or extraction of sulfate using hydrogen bonding interactions from ureas, thioureas, pyrroles, indoles, and squaramides, 11 – 24 but very few of these show significant sulfate affinity in aqueous solution. To overcome this, charged interactions are often incorporated into sulfate receptors to provide increased binding energies to compensate for the dehydration ‘penalty’.…”