Kristin Bowman-James

Ammonium based anion receptors

Abstract Selective anion recognition has been a tremendous challenge to chemists over the decades. However, with the advent of the concepts inherent in ‘supramolecular chemistry,’ the field now flourishes. Of the major types of receptors, polyamines have been studied widely as anion hosts by a number of researchers. Both electrostatic interactions and hydrogen bonds between protonated amines and the anion guests govern the binding in these systems. Exceptions to this rule are found in the quaternary ammonium systems, which utilize primarily electrostatic interactions and topological complementarity for binding purposes. This review focuses only on amine-based hosts, and is divided into acyclic and macrocyclic categories, the latter of which are based on cyclic dimension. The resulting four categories are: acyclic, monocyclic, bicyclic, and polycyclic. Within the major categories, binding is discussed according to the nature of the anion target, i.e. ‘simple’ inorganic anions, organic anions, and anionic metal complexes.

Supramolecular Chemistry of Environmentally Relevant Anions

Publisher Summary This chapter discusses supramolecular chemistry of environmentally relevant anions. The ultimate goal of the studies on the basic chemical aspects of anion receptor design of functional pH-independent systems is to target selective binding of anions of environmental significance. In applications-oriented aspects of the research on the design of separations, strategies for selective and efficient removal of targeted anions have been a major long-term goal. The chapter discusses (1) some of the first synthetic sulfate-selective anion-binding agents; (2) simple, structure-based methods for modifying the intrinsic anion selectivity of a given class of anion receptors; and (3) the first system capable of extracting sulfate anion from acidic, nitrate-containing aqueous media. These initial findings point the way to more effective systems involving receptors with higher selectivity, binding strength, and solubility. The evolution of anion-binding ligands from simple ammonium-based systems through mixed amide/amine and thioamide/amine receptors is reviewed. Thus, the emphasis is largely on synthetic design strategy carried out in the Bowman–James group.

Cyclophane Capsule Motifs with Side Pockets

Neutral and charged multitopic cyclophane-capped anion hosts connected by three or four diamide/monoamine chains and a decomposition product with two chains have been synthesized and characterized. The chains in the two former hosts fold together to form one or two binding pockets, respectively, and FHF(-) and several phosphate complexes have been obtained with the anions nestled in these pockets. The decomposition product also shows propensity for binding dicarboxylates, as evidenced by an isophthalate crystal structure.

Fluoride: Solution- and Solid-State Structural Binding Probe

Solid-state and solution studies were performed to determine if F(-) is encapsulated by anion hosts in both media. X-ray crystal structure determinations were compared with both (1)H and (19)F solution NMR data. Three hosts were studied: (1) two polyamide hosts, one with isophthaloyl spacers and the other with pyridine spacers, and (2) a polythioamide host with pyridine spacers. Binding studies showed that the pyridine-containing amide cryptand shows the highest affinity (K(a) > 10(5) in DMSO-d(6)), with the other hosts at least a factor of 10 lower. All of the cryptands appear to encapsulate F(-) in solution, where a deuterium-exchange reaction with DMSO-d(6) can be monitored by (19)F NMR. Four crystal structures are reported and compared: two for the pyridine-containing free base hosts and two for encapsulated F(-) complexes of the two amide-based cryptands.

The Influence of Amine Functionalities on Anion Binding in Polyamide-Containing Macrocycles

Mixed amide/amine macrocyclic anion hosts of varying sizes and with different amine substituents have been synthesized and characterized. Host 2, containing a 28-membered ring and secondary amines, has shown selective binding for HSO(4)(-) over other oxo anions and halides in DMSO-d(6) using NMR titrations. Crystal structures of SO(4)(2-), HPO(4)(2-), H(2)PO(4)(-), and H(2)P(2)O(7)(2-) with the 28-membered ring hosts indicate different macrocyclic conformations depending on the N-substituent. Anion affinities appear to be correlated with macrocycle conformation.

Molecular Thioamide ↔ Iminothiolate Switches for Sulfur Mustards

SNS platinum(II) pincer complexes reversibly bind and release the surrogate half sulfur mustard, 2-chloroethyl ethyl sulfide (CEES). The switch-like behavior of the pincers is attributed to a reversible transformation between the thioamide and iminothiolate forms of the pincer skeleton under slightly acidic and basic conditions, respectively. An amide-based palladium(II) pincer complex also binds CEES, as confirmed crystallographically and by NMR.

Tricyclic host for linear anions.

A tricyclic host for anions consisting of two tetraamide monocycles attached by two ethylene chains was designed and synthesized. Structural and binding results indicate that the receptor is selective for linear triatomic anions. Crystallographic data for two hydrated free bases, along with FHF(-), N(3)(-), and SO(4)(2-) complexes indicate that there are at least two preferred gross conformations for the host, one of which possesses pseudo-D(2) symmetry and the other pseudo-C(2h) symmetry. Both FHF(-) and N(3)(-) are encapsulated in the pseudo-D(2) symmetric complex, bridging the two tetraamido macrocyclic halves. The pseudo-C(2h) octahydrate structure shows an ice-like H-bonded (H(2)O)(6) array of water molecules embedded in the host cavity. The SO(4)(2-) structure has a nearly superimposable host conformation to the octahydrate but with the SO(4)(2-) anions lying outside the host. Binding studies in DMSO-d(6) indicate selectivity for FHF(-), with lesser affinity for other inorganic anions.

Fluoride Ion Receptors: A Comparison of a Polyammonium MonocycleVersusits Bicyclic Corollary

Abstract Two polyammonium macrocyclic receptors, the monocyclic 3,6,9,17,20,23-hexaazatricyclo [23.3.1.111,15]triaconta-1(29), 11,13,15(30),25,27-hexaene (L1) and its bicyclic analog 1,4,12,15,18,26,31,39-octaazapentacyclo[13.13.13.16,10. 120,24.133,37]-tetratetraconta-6,7,9,20(43),21,23,33(42),34,36-nonaene (L2), have been synthesized as their hexatosylate salts. The propensity for binding fluoride ion was examined using both NMR and potentiometric techniques. The fluoride salts of both receptors have been characterized by X-ray crystallographic methods. For the monocycle, the complex crystallized as the mixed fluoride-bifluoride salt, [H6L1]6+ ·4F− ·2FHF− ·4H2O, and the bicycle crystallized as a complex salt, [H6L2]6+ ·F− ·2FHF− ·1.5SiF6 2− ·7H2O.

Enhanced liquid–liquid anion exchange using macrocyclic anion receptors: effect of receptor structure on sulphate–nitrate exchange selectivity

When certain macrocyclic anion receptors are added to a chloroform solution of the nitrate form of a lipophilic quaternary ammonium salt (methyltri-C8,10-ammonium nitrate, Aliquat 336N), the extraction of sulphate from an aqueous sodium nitrate solution via exchange with the organic-phase nitrate is significantly enhanced. Eight macrocycles were surveyed, including two derivatives of a tetraamide macrocycle, five derivatives of calix[4]pyrrole and β-decafluorocalix[5]pyrrole. Under the hypothesis that the enhancement originates from sulphate binding by the anion receptors in the chloroform phase, it was possible to obtain reasonable fits to the sulphate distribution survey data based on the formation of 1:1 and 2:1 receptor:sulphate complexes in the chloroform phase. Apparent 1:1 sulphate-binding constants obtained from the model in this system fell in the range . Comparison of the results for the various anion receptors included in this study reveals that sulphate binding is sensitive to the nature of the substituents on the parent macrocycle scaffolds in a way that does not follow straightforwardly from simple chemical expectations, such as electron-withdrawing effects on hydrogen-bond donor strength.

Encapsulated sulfates: insight to binding propensities

Two crystal structures of sulfate inclusion complexes in an aza- and amido-cryptand represent the first examples of encapsulated sulfate in synthetic cryptand receptors and indicate penta- and octa-coordination, respectively.