José M. Llinares

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.

Multifunctional molecular recognition of ATP, ADP and AMP nucleotides by the novel receptor 2,6,10,13,17,21-hexaaza[22]metacyclophane

The novel cyclophane receptor 2,6,10,13,17,21-hexaaza[22]metacyclophane L presents a molecular architecture which enables recognition in aqueous solution of ATP, ADP and AMP through electrostatic, hydrogen bonding and π-stacking interactions; electrostatic interactions occur between the polyammonium sites of L and the phosphate chain of the nucleosides, and π-stacking interactions occur between the m-phenylene subunit incorporated in the receptor as a non-pendant integral part of the macrocyclic framework and the adenine ring of the nucleotides.

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.

Anion detection by fluorescent Zn(II) complexes of functionalized polyamine ligands.

The Zn(2+) coordination chemistry and luminescent behavior of two ligands constituted by an open 1,4,7-triazaheptane chain functionalized at both ends with 2-picolyl units and either a methylnaphthyl (L1) or a dansyl (L2) fluorescent unit attached to the central amino nitrogen are reported. The fluorescent properties of the ZnL1(2+) and ZnL2(2+) complexes are then exploited toward detection of anions. L1 in the pH range of study has four protonation constants. The fluorescence emission from the naphthalene fluorophore is quenched either at low or at high pH values leading to an emissive pH window centered around pH = 5. In contrast, in L2 the fluorescence emission from the dansyl unit occurs only at basic pH values. In the case of L1, a red-shifted band appearing in the visible region was assigned to an exciplex emission involving the naphthalene and the tertiary amine of the polyamine chain. L1 forms Zn(2+) mononuclear complexes of ZnH(p)L1((p+2)+) stoichiometry with p = 1, 0, -1. Formation of the ZnL1(2+)species produces a strong enhancement of the L1 luminescence leading to an extended emissive pH window from pH = 5 to pH = 9. Addition of several anions to this last complex leads to a partial quenching effect. On the contrary, the fluorescence emission of L2 is partially quenched upon complexation with Zn(2+) in the same pH window (5 < pH < 9). The lower stability of ZnL2(2+) with respect to ZnL1(2+) suggests a lack of involvement of the sulfonamide group in the first coordination sphere. However, there is spectral evidence for an interesting photoinduced binding of the sulfonamide nitrogen to Zn(2+). While addition of diphosphate, triphosphate, citrate, and D,L-isocitrate to a solution of ZnL2(2+) restores the fluorescence emission of the system (lambda max ca. 600 nm), addition of phosphate, chloride, iodide, and cyanurate do not produce any significant change in fluorescence. Moreover, this system would permit one to differentiate diphosphate and triphosphate over citrate and d, l-isocitrate because the fluorescence enhancement observed upon addition of the first anions is much sharper. The ZnL2(2+) complex and its mixed complexes with diphosphate, triphosphate, citrate, and D,L-isocitrate have been characterized by (1)H, (31)P NMR, and Electrospray Mass Spectrometry.

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.

Structural and anion binding aspects of the tiny octaazacryptand

Abstract The pH sensitivity of the binding of anions with a small octaazacryptand, 1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosane (L), has been examined. NMR studies were performed comparing the binding of a variety of anions: fluoride, chloride, bromide, iodide and nitrate, with the hexatosylate salt of L at pD 2.0 and 4.0. Results indicate that significant complexation only occurs between L and fluoride at pD 4.0 and with chloride and fluoride at pD 2.5 and lower. No binding was observed at either pD with the other anions examined. Crystallographic findings obtained for the chloride (1), [H6L(Cl)][Cl]5·2.75H2O, iodide (2), [H4L(H2O)][I]4·4H2O, and mixed chloride/nitrate (3), [H6L(Cl)][NO3]5·1.5H2O·CH3OH, complexes of L were obtained. For 1 and 3, L is hexaprotonated and a chloride ion is inside the cavity. In the iodide salt, 2, a molecule of water is encapsulated and L is tetraprotonated.

Thermodynamic, NMR and photochemical study on the acid–base behaviour of N,N′-dibenzylated polyamines and on their interaction with hexacyanocobaltate(III)

The synthesis and protonation behaviour of the dibenzylated open-chain ligands 1,4-dibenzyl-1,4-diazabutane (L1), 1,7-dibenzyl-1,4,7-triazaheptane (L2), 1,10-dibenzyl-1,4,7,10-tetraazadecane (L3), 1,12-dibenzyl-1,5,8,12-tetraazadodecane (L4) and 1,13-dibenzyl-1,4,7,10,13-pentaazatridecane (L5) are presented. The protonation behaviour has been studied by means of pH-metric measurements, NMR, absorption UV–VIS and steady-state fluorescence emission spectroscopy. The influence of the aromatic ring on the acid–base behaviour of these ligands is discussed. The association constants of these ligands with [Co(CN)6]3– have been calculated by steady-state fluorescence emission. The reduction in the quantum yield for the photoaquation reaction of [Co(CN)6]3– in the presence of L5 suggests that three out of the six cyanide groups of the anion are involved in the formation of the hydrogen-bond network of the supramolecular adduct.

Geometric isomerism in pentacoordinate Cu2+ complexes: equilibrium, kinetic, and density functional theory studies reveal the existence of equilibrium between square pyramidal and trigonal bipyramidal forms for a tren-derived ligand.

A ligand (L1) (bis(aminoethyl)[2-(4-quinolylmethyl)aminoethyl]amine) containing a 4-quinolylmethyl group attached to one of the terminal amino groups of tris(2-aminoethyl)amine (tren) has been prepared, and its protonation constants and stability constants for the formation of Cu(2+) complexes have been determined. Kinetic studies on the formation of Cu(2+) complexes in slightly acidic solutions and on the acid-promoted complex decomposition strongly suggest that the Cu(2+)-L1 complex exists in solution as a mixture of two species, one of them showing a trigonal bipyramidal (tbp) coordination environment with an absorption maximum at 890 nm in the electronic spectrum, and the other one being square pyramidal (sp) with a maximum at 660 nm. In acidic solution only a species with tbp geometry is formed, whereas in neutral and basic solutions a mixture of species with tbp and sp geometries is formed. The results of density functional theory (DFT) calculations indicate that these results can be rationalized by invoking the existence of an equilibrium of hydrolysis of the Cu-N bond with the amino group supporting the quinoline ring so that CuL1(2+) would be actually a mixture of tbp [CuL1(H(2)O)](2+) and sp [CuL1(H(2)O)(2)](2+). As there are many Cu(2+)-polyamine complexes with electronic spectra that show two overlapping bands at wavelengths close to those observed for the Cu(2+)-L1 complex, the existence of this kind of equilibrium between species with two different geometries can be quite common in the chemistry of these compounds. A correlation found between the position of the absorption maximum and the tau parameter measuring the distortion from the idealized tbp and sp geometries can be used to estimate the actual geometry in solution of this kind of complex.

Synthesis, protonation and co-ordination abilities of the open-chain polyamine 4,8,11,15-tetraazaoctadecane-1,18-diamine

The new open-chain polyamine 4,8,11,15-tetraazaoctadecane-1,18-diamine has been synthesised and characterized. The acid–base behaviour and the co-ordination abilities towards Cu2+, Ni2+, Co2+, Zn2+, Cd2+ and Pb2+ have been studied by potentiometry at 298.1 K in 0.15 mol dm–3 NaClO4, as well as by 1H and 13C NMR spectroscopy. The protonation pattern shows the effects produced by the presence of primary and secondary nitrogen atoms as well as ethylenic and propylenic hydrocarbon chains within the molecule. The stability and spectroscopic data show that at most four of the six nitrogens present are involved in co-ordination to the different metal ions. The contrast to other hexaamines, this compound is able to form binuclear species with all the metal ions studied. Formation of predominant binuclear hydroxo species has been detected in a number of systems.

Polyamine Linear Chains Bearing Two Identical Terminal Aromatic Units. Evidence for a Photo Induced Bending Movement

Abstract Several chemosensors bearing a fluorescent unit at both ends of a linear polyamine chain were synthesised. The protonation as well as the association constants with Cu2+ and Zn2+ were determined by potentiometry in 0.15 mol dm−3 NaCl at 298.1 K. In the case of 1,16-bis(1-naphthylmethyl)-1,4,7,10,13,16-hexaazadecane hexahydrochloride (L1), formation of an excimer emission in aqueous acidic solutions was observed. The system was characterized by steady state fluorescence emission and by time resolved fluorescence. In the ground state the molecule is expected to adopt a more or less linear conformation, while in the excited state a bending movement of the chain must occur in order to allow the excimer emission. The system can be viewed as an elementary machine driven by light.