Colourimetric carboxylate anion sensors derived from viologen-based receptors.

A series of tri- and tetrapodal viologen-based anion receptors showing a colourimetric response to carboxylates, such as acetate, have been synthesised. Alteration of the anion binding sites allows for binding site competition within a receptor. This results in a delayed colourimetric response for urea derivatives compared with pyridinium systems because the anions are initially bound to the periphery of the receptor, away from the viologen unit. DFT calculations and experimental measurements allow the colour change to be assigned to an anion-receptor charge-transfer process, facilitated by the exceptionally low reduction potential of the cationic host compounds. Evidence for electron transfer to give the viologen radical cation is also seen in some cases.

Enantioselective lactate binding by chiral tripodal anion hosts derived from amino acids.

Chiral, tripodal anion hosts derived from either S-phenylalanine or S-leucine bind D-lactate enantioselectively. The nature of the host-guest interaction has been probed by solution NMR methods and by DFT calculations. The calculations suggest that the D-lactate may form an additional hydrogen bond in the host-guest complex while the L-lactate complex contains an intramolecular hydrogen bond. Anion binding is in competition with host dimerisation, as demonstrated by DOSY and (1)H NMR spectroscopy, and DFT calculations.

Induced fit interanion discrimination by binding-induced excimer formation.

The synthesis, photophysical, and anion-binding properties of a series of di-, tri-, and tetrapodal anion-binding hosts based on aminopyridinium units with pyrenyl reporter groups are described. The ditopic mesitylene-derived calix[4]arene-based host 4 binds strongly to dicarboxylates, particularly malonate, in a 2:1 anion:host ratio but is essentially nonemissive in the presence of all anions except chloride because of intramolecular quenching by the pyridinium units. Addition of chloride results in a conformational change, giving an initial increase in emission assigned to intramolecular excimer formation. Further chloride addition also results in an increase in the intensity of the pyrenyl monomer emission as chloride binding reduces the acceptor ability of the pyridinium groups. This behavior is not exhibited by control compounds 5 and 6, which lack the ditopic geometry and calixarene spacer unit; however, tripodal 6 forms 1:2 anion:host complexes with a range of anions.

Anion binding in (arene)ruthenium(II)-based hosts?

Asymmetric ruthenium(II) complexes of a flexible aminomethylpyridine derivative exhibit diastereotopic ligand methylene protons, as measured by NMR spectroscopy; binding of external anions renders these protons equivalent possibly by increasing dynamically averaged symmetry; the amount of anion needed to raise average symmetry correlates to the anion binding constant.

Anion binding and luminescent sensing using cationic ruthenium(II) aminopyridine complexes.

The synthesis of a series of ruthenium(II) based anion sensors of the type [Ru(eta(6)-C(6)H(4)MeCHMe(2))Cl(L)(2)][BF(4)] (2) is reported in which ligand L represents a series of substituted pyridinylmethyl-amine derivatives. The carbazole based ligand L(3) exhibits a fluorescent intraligand charge-transfer (ILCT) state that is quenched by ligand-to-metal charge transfer (LMCT) upon coordination to ruthenium in the 1:1 complex [Ru(eta(6)-C(6)H(4)MeCHMe(2))Cl(2)(L(3))] (1 c). The 1:2 complex 2 c is fluorescent, however, and acts as a fluorescent anion sensor because of the mixing of an anion-dependent charge-transfer component into the excited state. The 1:2 complexes of type 2 all exhibit interesting low symmetry (1)H NMR spectra that also are a useful handle on anion complexation. The electronic structures of L(3), 1 c and 2 c have been probed by time-dependent DFT calculations.