Synthetic indole, carbazole, biindole and indolocarbazole-based receptors: applications in anion complexation and sensing

Sulfonamide carbazole receptors for anion recognition

Carbazole-based receptors functionalized with two sulfonamide groups have been synthesized and their properties as anion receptors have been evaluated. The receptor with bis(trifluoromethyl)aniline groups has shown a very high affinity for halide ions, especially remarkable as only two hydrogen bonds are formed in the complexes. (1)H NMR and fluorescence titrations have been carried out and binding constants up to 7.9 × 10(6) M(-1) have been reached. X-ray structures have been obtained and a modelling study has shown the possible reasons for the large affinity of these compounds for halide anions.

From anion receptors to transporters

Cystic fibrosis is the most well-known of a variety of diseases termed channelopathies, in which the regulation of ion transport across cell membranes is so disrupted that the threshold of a pathology is passed. The human toll exacted by these diseases has led a number of research groups, including our own, to create compounds that mediate ion transport across lipid bilayers. In this Account, we discuss three classes of synthetic compounds that were refined to bind and transport anions across lipid bilayer membranes. All of the compounds were originally designed as anion receptors, that is, species that would simply create stable complexes with anions, but were then further developed as transporters. By studying structurally simple systems and varying their properties to change the degree of preorganization, the affinity for anions, or the lipophilicity, we have begun to rationalize why particular anion transport mechanisms (cotransport or antiport processes) occur in particular cases. For example, we have studied the chloride transport properties of receptors based on the closely related structures of isophthalamide and pyridine-2,6-dicarboxamide: the central ring in each case was augmented with pendant methylimidazole groups designed to cotransport H(+) and Cl(-). We observed that the more preorganized pyridine-based receptor was the more efficient transporter, a finding replicated with a series of isophthalamides in which one contained hydroxyl groups designed to preorganize the receptor. This latter class of compound, together with the natural product prodigiosin, can transport bicarbonate (as part of a chloride/bicarbonate antiport process) across lipid bilayer membranes. We have also studied the membrane transport properties of calix[4]pyrroles. Although the parent meso-octamethylcalix[4]pyrrole functions solely as a Cs(+)/Cl(-) cotransporter, other compounds with increased anion affinities can function through an antiport process. One example is octafluoro-meso-octamethylcalix[4]pyrrole; with its electron-withdrawing substituents, it can operate through a chloride/bicarbonate antiport process. Moreover, calix[4]pyrroles with additional hydrogen bond donors can operate through a chloride/nitrate antiport process. Thus, increasing the affinity of the receptor in these cases allows the compound to transport an anion in the absence of a cation. Finally, we have studied the transport properties of simple thioureas and shown that these compounds are highly potent chloride/bicarbonate antiport agents that function at low concentrations. In contrast, the urea analogues are inactive. The higher hydrophobicity (reflected in higher values for the logarithm of the water-octanol partition constant, or log P) and lower polar surface areas of the thiourea compounds compared to their urea analogues may provide a clue to the high potency of these compounds. This observation might serve as a basis for designing future small-molecule transporters.

Amide- and urea-functionalized pyrroles and benzopyrroles as synthetic, neutral anion receptors

This critical review is dedicated to the anion complexation chemistry of amide- and urea-functionalized (benzo)pyrroles, such as pyrroles, indoles, carbazoles, isoindoles, naphthalenodipyrroles and benzodipyrroles. It provides a comprehensive overview of these simple and neutral anion sensors from the first examples up to very recent studies. There is a discussion of a gradual progress made over time, often based on careful analysis of the properties of former generations that have subsequently led to obtaining excellent effectivities and selectivities. The influence of these species on other fields of chemistry and their applications there are also discussed (57 references).

Nitro-substituted 3,3'-bis(indolyl)methane derivatives as anion receptors: electron-withdrawing effect and tunability of anion binding properties

A series of nitro-substituted 3,3'-bis-indolyl phenylmethane derivatives were synthesized and their anion binding properties were investigated in detail. The introduction of the electron-withdrawing nitro group into indole unit and/or meso-phenyl ring, which leads to the increased acidity of indole NH and meso-position CH proton, has a positive effect on anion binding. The nitro-substituted bis(indolyl)methane receptors exhibited selective colorimetric sensing of F- anion, as revealed by the notable color and spectral changes, rationally due to the deprotonation of the indole NH of the receptor. Meanwhile, the additive introduction of the nitro substituents on the meso-phenyl ring of bis(indolyl)methane can lead to the deprotonation of the meso-position CH and further induce an irreversible oxidation process obtaining bis(indolyl)methene product in the F- anion sensing system.

Fluoride ion sensing by an anion-π interaction

We report the discovery of a supramolecular interaction (anion-π and charge/electron transfer, CT/ET) involving fluoride ion and π-electron deficient colorless naphthalene diimide (NDI) receptors. Strong electronic interactions between lone-pair electrons of F(-) ion and π*-orbitals of the NDI unit lead to an unprecedented F(-)→NDI ET event, which produces an orange colored NDI(•-) radical anion. Further reduction of NDI(•-) by another F(-) ion produces a pink colored NDI(2-) dianion, rendering NDI a colorimetric F(-) sensor. Preorganization of two NDI units in overlapping positions using folded linkers improves their selectivity and sensitivity for the F(-) ion significantly, allowing F(-) detection at nM concentration in 85:15 DMSO/H(2)O solutions.

Spectroscopic properties of the receptors based on 2- and 3-linked tri(indolyl)methanes for anion binding

The anion binding properties of two title tri(indolyl)methane compounds have been evaluated by using UV-vis absorption and fluorescence spectroscopy. Different from 3-linked receptor which exhibited faint even no obvious spectral response to anions, 2-linked receptor can bind F(-), AcO(-) and H(2)PO(4)(-), with the bathochromic shifts of the UV absorption and fluorescence quenching. This selectivity will be attributed to suitable steric array of NH binding sites of 2-linked receptor, which helps to form multiple hydrogen bonding interactions with anions.

Cryptand-like anion receptors

The design of supramolecular hosts for anions began with simple diaza bicycles, named katapinands, and has evolved over the last 40 years to a number of elegantly designed receptors capable of binding many different anions. About the same time the term cryptand appeared in reference to another bicyclic compound that was selective for alkaline-earth ions. Since the first report these simple bicycles, a vast arena of hosts has appeared, including acyclic, monocyclic, and other multicyclic supramolecular receptors. Studies of these systems have revealed considerable information about anion coordination chemistry, including the fact that many of these complexes mimic their transition-metal corollaries in terms of coordination numbers. However, for anions interactions occur via H-bonding most often, rather than the coordinate covalent or dative bonds observed in transition-metal coordination. This critical review examines the design of enclosed, primarily bicyclic cryptands as hosts for anions, with a small scattering of higher polyhedra when deemed appropriate to the discussion. In order to show the development (evolution) of the field, key examples of early work will be noted and compared with more recent developments (136 references).

Anion receptor chemistry: highlights from 2008 and 2009

This critical review covers advances in anion complexation in the year 2008 and 2009. The review discusses anion receptors that employ hydrogen bond donors (both NH and CH), electrostatic interactions, Lewis acidic centres and combinations of these three types of binding interaction to complex anions. Additionally nanotechnological approaches to anion sensing in aqueous solution, lipid bilayer transporters and recent work on the use of anions to drive conformational change are highlighted (130 references).

Hosting anions. The energetic perspective

Hosting anions addresses the widely spread molecular recognition event of negatively charged species by dedicated organic compounds in condensed phases at equilibrium. The experimentally accessible energetic features comprise the entire system including the solvent, any buffers, background electrolytes or other components introduced for e.g. analysis. The deconvolution of all these interaction types and their dependence on subtle structural variation is required to arrive at a structure-energy correlation that may serve as a guide in receptor construction. The focus on direct host-guest interactions (lock-and-key complementarity) that have dominated the binding concepts of artificial receptors in the past must be widened in order to account for entropic contributions which constitute very significant fractions of the total free energy of interaction. Including entropy necessarily addresses the ambiguity and fuzziness of the host-guest structural ensemble and requires the appreciation of the fact that most liquid phases possess distinct structures of their own. Apparently, it is the perturbation of the intrinsic solvent structure occurring upon association that rules ion binding in polar media where ions are soluble and abundant. Rather than specifying peculiar structural elements useful in anion binding this critical review attempts an illumination of the concepts and individual energetic contributions resulting in the final observation of specific anion recognition (95 references).

Squaraine rotaxane as a reversible optical chloride sensor

A mechanically interlocked squaraine rotaxane is comprised of a deep-red fluorescent squaraine dye inside a tetralactam macrocycle. NMR studies show that Cl(-) binding to the rotaxane induces macrocycle translocation away from the central squaraine station, a process that is completely reversed when the Cl(-) is removed from the solution. Steady-state fluorescence and excited-state lifetime measurements show that this reversible machine-like motion modulates several technically useful optical properties, including a three-fold increase in deep-red fluorescence emission that is observable to the naked eye. The excited states were characterized quantitatively by time-correlated single photon counting, femtosecond transient absorption spectroscopy, and nanosecond laser flash photolysis. Cl(-) binding to the rotaxane increases the squaraine excited singlet state lifetime from 1.5 to 3.1 ns, and decreases the excited triplet state lifetime from >200 to 44 micros. Apparently, the surrounding macrocycle quenches the excited singlet state of the encapsulated squaraine dye and stabilizes the excited triplet state. Prototype dipsticks were prepared by adsorbing the lipophilic rotaxane onto the ends of narrow, C18-coated, reverse-phase silica gel plates. The fluorescence intensity of a dipstick increased eighteen-fold upon dipping in an aqueous solution of tetrabutylammonium chloride (300 mM) and was subsequently reversed by washing with pure water. It is possible to develop the dipsticks for colorimetric determination of Cl(-) levels by the naked eye. After dipping into aqueous tetrabutylammonium chloride, a dipstick's color slowly fades at a rate that depends on the amount of Cl(-) in the aqueous solution. The fading process is due primarily to hydrolytic bleaching of the squaraine chromophore within the rotaxane. That is, association of Cl(-) to immobilized rotaxane induces macrocycle translocation and exposure of the electrophilic C(4)O(2) core of the squaraine station, which is in turn attacked by the ambient moisture to produce a bleached product.

Anion receptors containing thiazine-1,1-dioxide heterocycles as hydrogen bond donors

The synthesis, X-ray crystal structures and anion recognition properties of two receptors containing thiazine-1,1-dioxide heterocycles as hydrogen bond donating subunits are reported. The newly synthesized receptors display much different anion selectivities in acetone-d₆ than N,N'-diphenyl-1,3-disulfonamidobenzene that was used as a comparison. The selectivity exhibited by one of the new receptors for chloride anions can be attributed to greater steric demand in the cleft formed, in part, by its terminal phenyl rings; an effect that is absent in the comparison receptor.

Bis-amidopyrrolyl Receptors Based on Anthracene and Carbazole

A new set of diamide receptors containing anthracene and carbazole bridging subunits and either pyrrole or phenyl substituents were synthesized. The four systems produced in this way were shown to bind representative anions in DMSO-d(6) solution and in the solid state. A higher relative affinity for two test oxoanions, namely dihydrogen phosphate and benzoate, over chloride anion was seen in solution, with the anions in question being studied in the form of their respective tetrabutylammonium salts. However, the specifics of the anion recognition process were seen to depend on structure, with the pyrrole-containing systems displaying higher relative affinities than their corresponding phenyl-containing congeners, and the carbazole receptors proving more effective than the anthracene analogues. Such observations provide support for the notion that both the carbazole NH and the pyrrolic NH protons play an important role in stabilizing the receptor-bound anions in solution. Structural analyses of several anion complexes of the diamidopyrrole carbazole receptor reveal that this is not necessarily the case in the solid state; specifically, the pyrrole NH protons are seen to interact with the amide oxygen of another molecule. The net result is an extended a one-dimensional coordinaton polymer.