Paloma Arranz-Mascarós

Thermodynamics of Anion−π Interactions in Aqueous Solution

Thermodynamic parameters (ΔG°, ΔH°, TΔS°), obtained by means of potentiometric and isothermal titration calorimetry (ITC) methods, for the binding equilibria involving anions of high negative charge, like SO(4)(2-), SeO(4)(2-), S(2)O(3)(2-) and Co(CN)(6)(3-), and nitroso-amino-pyrimidine receptors in water suggested that anion-π interactions furnish a stabilization of about -10 kJ/mol to the free energy of association. These anion-π interactions are almost athermic and favored by large entropic contributions which are likely due to the reduced hydrophobic pyrimidine surface exposed to water after anion aggregation, and the consequent reduced disruptive effect on the dynamic water structure. The crystal structure of the {H(4)L[Co(CN)(6)]}·2H(2)O complex showed strong anion-π interactions between Co(CN)(6)(3-) and the protonated H(4)L(3+) receptor. The CN···centroid distance (2.786(3) Å), occurring with a cyanide N atom located almost above the centroid of the pyrimidine ring, is the shortest distance till now reported for the interaction between CN(-) ions and heteroaromatic rings.

Solution–solid-state study of the system Cu(II)/N-2-(4-amino-1,6-dihydro-1-methyl-5-nitroso-6-oxopyrimidinyl)glycine.: Mononuclear and polynuclear ladder-like chain complex crystalline structures

An aqueous solution-solid-state study of the Cu(II) / HL system (HL5N-2-(4-amino-1,6-dihydro-1-methyl-5-nitroso-6-ox- opyrimidinyl)glycine) is reported. The solution studies have been carried out on the protonation equilibria of HL and the Cu(II) / HL system, at molar ratios of 1:1 and 1:5. Two complexes were obtained in the solid state, Cu(II) / HL molar ratio51:1, and were characterized by spectroscopic and X-ray single-crystal diffraction techniques. Mononuclear complex 1 ((CuCl (HL) ) crystallizes with 22 n a hexacoordinated coordination mode around the Cu(II) ion, while complex 2 (CuCl(L)(H O) ) is an infinite polynuclear ladder-like 22 chain, with a distorted asymmetric square-based bipyramid. None of those crystalline complexes were detected in the solution study.

Binding and recognition of AMP, ADP, ATP and related inorganic phosphate anions by a tren-based ligand containing a pyrimidine functionality

The interaction of AMP, ADP, ATP and related phosphate, diphosphate and triphosphate anions with the positively charged forms of the HL ligand, constituted by a tren molecule functionalized with a pyrimidine residue, was studied by potentiometric titrations, affording the stability constants of the formed anion complexes, 31P and 1H NMR spectroscopy in aqueous solutions and modelling calculations. The equilibrium data show that electrostatic forces make an important, but not decisive contribution, to the stability of the anion complexes. For nucleotides, 1H NMR spectra showed that the formation of π-stacking interactions between nucleotide adenine and ligand pyrimidine moieties contribute to the stability of these complexes, being very important for those complexes of higher stability. The synergistic action of the different binding groups of the polyfunctional HL receptor leads to a marked recognition of ADP over ATP with a selectivity coefficient as high as 116 at pH 3.3, which is unprecedented in the context of synthetic receptor in water. According to modelling calculations, such exceptional binding selectivity has a strong relation with the increase of stability experienced by ADP complexes upon ADP protonation. A similar phenomenon is observed for HAMP− whose interaction with H3L2+ is at least as strong as that of the more charged HATP3− anion, which is another rare occurrence.

Solution study and 2-D layered structures of zinc(II) and cadmium(II) complexes with N-2-(6-amino-3,4-dihydro-3-methyl-5-nitroso-4-oxopyrimidinyl)-l-methionine as ligand

Abstract The solution study of the N-2-(6-amino-3,4-dihydro-3-methyl-5-nitroso-4-oxopyrimidinyl)- l -methionine has revealed that it acts as a monoprotic acid in the pH range 2.5–9.5. In solution the complexes detected with Zn(II) and Cd(II) at 1:1 and 1:4 metal-to-ligand molar ratios were the mononuclear species [ZnL2], [CdL2] and [Cd(HL)]2+, with the ligand coordinating through the carboxylate group in the two former and through the pyrimidine ring in the later. From these, two layered complexes of Zn(II) and Cd(II) with HL were obtained. {[Zn(μ-L)2]·2H2O}n, monoclinic, space group P21, a=10.882(3), b=7.473(2), c=18.534(4) A, β=90.92(2)°, Z=2; {[Cd(μ-L)2]·3H2O}n, triclinic, space group P1, a=7.615(2), b=10.427(2), c=11.129(2) A, α=86.29(3), β=78.87(3), γ=71.13(3)°, Z=1. The structures of both complexes are very similar except for the coordination numbers of five and seven for Zn(II) and Cd(II), respectively. These facts point out the versatility of this family of N-pyrimidine amino acids as ligands, as well as the importance of the crystallization processes in obtaining supramolecular compounds.

Solution and solid study of Zn(II) and Cd(II) complexes with N-(6-amino-3,4-dihydro-3-methyl-5-nitroso-4-oxo-pyrimidin-2-yl)glycine as ligand. Crystal structures of [ZnL2(H2O)4]·6H2O and {[Cd(μ-L)Cl(H2O)2]·H2O}n

Abstract Reactions of N -(4-amino-1,6-dihydro-1-methyl-5-nitroso-6-oxo-pyrimidin-2-yl)glycine with Zn(II) and Cd(II) (1:1 and 1:4 metal/ligand ratios, in aqueous media at 35°C and 0.1 M KCl ionic strength) were studied by potentiometric methods. This study has revealed a similar qualitative behaviour to that found by us for this family of N -pyrimidine aminoacids, the primary coordination site being either the pyrimidine or the carboxylate group depending on whether the ligand acts in neutral form or deprotonated, respectively. In solid state two complexes were obtained by working in 1:1 metal-to-ligand ratio: [Zn(L) 2 (H 2 O) 4 ]·6H 2 O and {[Cd(μ-L)Cl(H 2 O) 2 ]·H 2 O} n , which were characterised by IR and NMR spectroscopies, TG and DSC techniques and single crystal X-ray diffraction. The former complex is mononuclear with the Zn(II) ion hexacoordinated in a distorted octahedral geometry. The coordination sphere is formed by four water molecules and two ligands coordinating in a monodentate fashion through the carboxylate group. The Cd(II) complex consists of a 1D infinite chain, with the metal ion heptacoordinated in a distorted pentagonal bipyramid. The ligand bridges the metal ions coordinating in a bis-didentate fashion through the pyrimidine and the carboxylate group. The nature of these complexes is very different to that shown by the Zn(II) and Cd(II) complexes with the related methionine derivative, which is attributed to he effect of the R substituent on the aminoacid moiety.

Binding and removal of octahedral, tetrahedral, square planar and linear anions in water by means of activated carbon functionalized with a pyrimidine-based anion receptor†

Binding of S2O32−, SeO42−, Pt(CN)42−, Co(CN)63−, Au(S2O3)23− and Fe(CN)64− anions by the protonated (positively charged) forms of tren (tris(2-aminoethyl)amine) and of the tren-derivative (HL) containing a pyrimidine residue was studied by means of potentiometric measurements in 0.1 M NMe4Cl solutions at 298.1 ± 0.1 K. Both ligands form stable complexes with these anions which appear to be mostly stabilized by electrostatic forces. In the case of HL, an anion–π interaction with the pyrimidine residue of the ligand also affords a significant contribution to complex stability. Some shape preference for tetrahedral and octahedral anions over square planar ones is observed. A hybrid AC/HL material obtained by adsorption of HL on commercial activated carbon (AC) was used to study the extraction of these anions from water. AC/HL shows enhanced adsorption capacity toward all the anions studied with respect to AC. This behavior is ascribed to the stronger interaction of anions with the HL function of AC/HL than with the Cπ-H3O+ sites of unfunctionalized AC. Of special interest is the enhancement of the adsorption capacities found for Au(S2O3)23− and Pt(CN)42−, two anions of great relevance for the extraction of platinum and gold from ores and from metallic wastes.

Bifunctional pyrimidine-amino-acid ligands: solution study and crystal structure of a Mn(II) chain alternating six- and sevenfold coordination environments

The acid � /base characterization in aqueous solution of the N -2-[4-amino-1,6-dihydro-1-methyl-5-nitroso-6-oxopyrimidinyl)methionine, a member of a family of bifunctional N -pyrimidine a-amino acids ligands, has been carried out by potentiometric and UV � /Vis techniques in the 2.5 � /9.0 pH range, indicating a quasi -zwitterionic structure. The solution study of the HL/Mn(II) system at 1:2, 2:1 and 4:1 molar ratios (25 8C and pHB/6), reveals that both moieties are active in coordinating to the metal ion, through the carboxylate group when the ligand is deprotonated or through the pyrimidinic fragment when acting in its molecular form. A solid complex with MnL2/6 / 1 /H2O stoichiometry was isolated from an aqueous 1:3 [HL]/[Mn(II)] mixture at pH 6. The X-ray single-crystal characterization has revealed that this complex can be formulated as {[Mn(H2O)4(m-L)2Mn(L)2(H2O)]/8H2O}n ,a n infinite chain in which two different Mn(II) ions having six- and sevenfold coordination environments alternate along the chain. The asymmetric unit contains two ligands coordinating in different fashion. One of these coordinates monodentately through the oxygen atom belonging to the exocyclic nitroso group while the other exhibits a 3 h-bridging pattern between the six- and sevenfold Mn(II) arrangements. The versatile coordination modes of this ligand is discussed and compared to other complexes of this bifunctional family of ligands. # 2003 Elsevier B.V. All rights reserved.

Coordination modes of N-2-(4-amino-1-methyl-5-nitroso-6-oxo-1,6-dihydropyrimidinyl) potassium L-alaninate toward Ag(I), Cd(II) and Pd(II). Crystal structure of {Ag((η4-μ3-L)]·3H2O}n, first example of a silver 3D-polymer with a pyrimidine-derivative ligand

The acid-base behaviour of the N-2-(4-amino-1-methyl-5-nitroso-6-oxo-1,6-dihydropyrimidinyl) potassium L-alaninate (KL) and its reactivity with Ag(I), Cd(II) and Pd(II) metal ions have been studied. Three solid species with formulae AgL·3H 2 O, CdL 2 ·6H 2 O and PdClL·H 2 O have been obtained and characterized by IR, 1 H and 13 C NMR spectroscopic methods and thermal, conductivity and magnetic measurements. The structure of {Ag((η 4 -μ 3 -L)]·3H 2 O} n has been established by single-crystal X-ray diffraction and consists in a 3D-polymer. Within the polymer each Ag(I) ion is tetrahedrally coordinated to three differents L anions, while each ligand is coordinated in a rather unusual η 4 -μ 3 coordination mode. Spectroscopic data show a similar coordination mode for the Cd(II) complex, while for the Pd(II) one a different coordination mode is proposed.

The reactivity of N-2-(4-amino-1,6-dihydro-1-methyl-5-nitroso-6-oxo-pyrimidinyl)-L-histidine towards silver(I) and zinc(II) ions

Reactions of N-2-(4-amino-1,6-dihydro-1-methyl-5-nitroso-6-oxopyrimidinyl)-L-histidine with AgI and ZnII ions in 1/1 and 4/1 ligand-to-metal molar ratios, in aqueous media at 25 °C and 0.1 M KNO3 and KCl ionic strength, respectively, were followed by potentiometric techniques in order to determine formation equilibria for the complex species. The spectral curves, as a function of ligand-to-metal molar ratio at established pH values, were studied and four solid phases of stoichiometry: AgL·4H2O, Ag(HL)NO3·1.5H2O, AgL(OH)2K2·H2O and Zn(HL)2Cl2·7H2O were isolated and characterised by analytical, spectroscopic and thermal measurements.

The reactivity of N-2-(4-amino-1,6-dihydro-1-methyl-5-nitroso-6-oxopyrimidinyl)-L-histidine towards copper(II) ions

The acid–base properties of the N-substituted amino acid HL [HL = N-2-(4-amino-1,6-dihydro-1-methyl-5-nitroso-6-oxopyrimidinyl)-L-histidine] and its reactivity towards the CuII ion have been measured by potentiometric and spectrophotometric techniques in aqueous solution at 25 °C and 0.1 M KCl ionic strength. These studies show that the neutral HL compound is present in aqueous solution (3–7 pH range) in the zwitterionic form with the diprotonated imidazolic residue. Studies of HL/CuII mixtures reveal the existence of complex species with the neutral ligand, HL and containing the deprotonated L ligand. By controlling the reaction conditions, four solid phases of stoichiometry: CuLCl(H2O)6, Cu2LCl3(H2O)8, CuL2(H2O)6 and Cu(HL)2Cl2(H2O)6 were isolated and characterised by i.r. and electronic spectroscopy, thermal techniques and magnetic measurements.