Hermas R. Jiménez

A Ferromagnetic [Cu3(OH)2]4+ Cluster Formed inside a Tritopic Nonaazapyridinophane: Crystal Structure and Solution Studies†

Chemists working in coordination and/or supramolecular chemistry find a continuous source of inspiration in biomolecules and enzyme active sites. In this respect, trinuclear Cu centers have attracted a lot of interest due to their resemblance to multicenter blue copper oxidases like ascorbate oxidase, ceruloplasmin, lacase oxidase, and particulate methane monooxygenase. These enzymes contain at least four copper centers, which are necessary for four-electron reduction of molecular oxygen to water. For instance, the high-resolution structure of ascorbate oxidase shows that the mononuclear electron-transfer copper site of type 1 (T1) is connected to the trinuclear site through a patch formed by a cysteine residue from which two histidine residues diverge to bind the T3 copper ions. In the oxidized form of the enzyme, the T3 Cu atoms, apart from the imidazole moiety of the histidine residues of the patch, are coordinated by another two imidazole units and by an OH group that bridges the two atoms, which are 3.71 far apart from each other. The geometry around each metal ion can be best described as a trigonal bipyramid with a vacant equatorial position oriented towards the T2 copper atom, which completes the trinuclear center. In the T2 site the Cu center is coordinated in a very particular T-shaped geometry by two histidine residues and a monodentate hydroxo group or a water molecule. The mechanism by which these centers catalyze the fourelectron reduction of molecular dioxygen to water and the magnetic properties of these particular arrangements of copper ions have attracted great interest from both the biochemical and magnetochemical points of view. Therefore, small-molecule studies aimed at mimicking their properties are of great relevance in this respect.

Grafted squaramide monoamine nanoparticles as simple systems for sulfate recognition in pure water

New simple systems formed by a chain containing a squaramide function and a quaternised amine group attached to boehmite or silica-coated boehmite nanoparticles are able to discriminate anions in pure water.

A solution study of the protonation and deprotonation equilibria of 5,10,15,20-tetra(p-sulphonatophenyl)porphyrin. Stability constants of its magnesium(II), copper(II) and zinc(II) complexes

The protonation and deprotonation equilibria of 5,10,15,20-tetra(p-sulphonatophenyl)porphyrin, H2tpps4–, as well as its complex formation with Mg2+, Zn2+ and Cu2+ metal ions have been investigated spectrophotometrically in a dimethyl sulphoxide (dmso)–water (80:20, v/v) solution. The following equilibrium constants have been determined at 25 °C and 0.1 mol dm–3(KClO4+ KOH)(i) tpps6–+ 2H+⇌H2tpps4– log K= 32.80 ± 0.04 (ii) H2tpps4–+ 2H+⇌ H4tpps2– log K= 3.58 ± 0.02(iii)H2tpps4–+ Mg2+⇌[Mg(tpps)]4–+ 2H+ log K=–3.96 ± 0.04 (iv) H4tpps2–+ Zn2+⇌[Zn(tpps)]4–+ 4H+ log K=–1.80 ± 0.04(v) H4tpps2–+ Cu2+⇌[Cu(tpps)]4–+ 4H+ log K= 1.68 ± 0.04 [(i)] 0.1 mol dm–3(KClO4+ HClO4)[(ii), (iv), (v)] and 0.1 mol dm–3 KClO4[(iii)], affording for the first time the stability constants of three metalloporphyrins, [Mg(tpps)]4–(log β= 28.8), [Zn(tpps)]4–(log β= 34.6) and [Cu(tpps)]4–(log β= 38.1). A Novel and more effective procedure for the isolation of high-purity Na4[H2tpps]·4H2O is reported.

Preparation of Hg2+ selective fluorescent chemosensors based on surface modified core–shell aluminosilicate nanoparticles

A synthetic procedure for the preparation of functional structured inorganic–organic hybrid materials consisting of boehmite-silica core–shell nanoparticles and anthracene-containing amines covalently attached to the nanoparticles surface is reported. The system functionalised with the monoamine chain shows a very high sensing performance for Hg2+ detection in pure water reaching a detection limit of 0.2 ppb. Two additional advantages of these systems are their stability over a wide pH window and the feasibility to be recovered by a simple procedure.

1H NMR and UV-vis spectroscopic characterization of sulfonamide complexes of nickek(II)-carbonic anhydrase. Resonance assignments based on NOE effects

The binding of acetazolamide, p-fluorobenzensulfonamide, p-toluenesulfonamide, and sulfanilamide to nickel(II)-substituted carbonic anhydrase II has been studied by 1H NMR and electronic absorption spectroscopies. These inhibitors bind to the metal ion forming 1:1 complexes and their affinity constants were determined. The 1H NMR spectra of the formed complexes show a number of isotropically shifted signals corresponding to the histidine ligands. The complexes with benzene-sulfonamides gave rise to very similar 1H NMR spectra. The NMR data suggest that these aromatic sulfonamides bind to the metal ion altering its coordination sphere. In addition, from the temperature dependence of 1H NMR spectra of the p-fluorobenzenesulfonamide adduct, a conformational change is suggested. The T1 values of the meta-like protons of the coordinated histidines have been measured and resonance assignments based on NOE experiments were performed.

Equilibrium and kinetic studies on complex formation and decomposition and the movement of Cu2+metal ions within polytopic receptors

Potentiometric studies carried out on the interaction of two tritopic double-scorpiand receptors in which two equivalent 5-(2-aminoethyl)-2,5,8-triaza[9]-(2,6)-pyridinophane moieties are linked with 2,9-dimethylphenanthroline (L1) and 2,6-dimethylpyridine (L2) establish the formation of mono-, bi- and trinuclear Cu(2+) complexes. The values of the stability constants and paramagnetic (1)H NMR studies permit one to infer the most likely coordination modes of the various complexes formed. Kinetic studies on complex formation and decomposition have also been carried out. Complex formation occurs with polyphasic kinetics for both receptors, although a significant difference is found between both ligands with respect to the relative values of the rate constants for the metal coordination steps and the structural reorganizations following them. Complex decomposition occurs with two separate kinetic steps, the first one being so fast that it occurs within the stopped-flow mixing time, whereas the second one is slow enough to allow kinetic studies using a conventional spectrophotometer. As a whole, the kinetic experiments also provide information about the movement of the metal ion within the receptors. The differences observed between the different receptors can be interpreted in terms of changes in the network of hydrogen bonds formed in the different species.

1D- and 2D-NMR studies of the pH effects on the metal-site geometry in nickel(II)–azurin from Pseudomonas aeruginosa

The isotropically shifted proton resonances in nickel(II)–azurin have been assigned on a firm basis by using 1D- and 2D-NMR methods which has allowed the subtle structural changes on the metal binding site associated with the pH induced conformational change in this protein to be revealed.

Spectroscopic studies of nickel(II) carbonic anhydrase and its adducts with inorganic anions

Nickel(II) carbonic anhydrase, NiBCA II, and its adducts with nitrate, acetate, cyanate and azide, have been investigated through 1H NMR and electronic absorption spectroscopies. From the pH dependence of the molar absorbance the acidity constants of NiBCA were determined. The anions bind to the metal ion forming 1 : 1 adducts, and the corresponding affinity constants have been determined. The 1H NMR spectra of NiBCA and its adducts have been recorded, and the signals corresponding to the meta-like protons of the co-ordinated histidines followed by 1H NMR titration. The T1 values of these signals were measured and resonance assignments made based on nuclear Overhauser enhancement experiments. The co-ordination geometry of the metal ion in NiBCA and its adducts is discussed on the basis of the temperature dependence of the isotropic shifts, molar absorbance, and longitudinal relaxation times.

Stability constants of metalloporphyrins. A study of the protonation, deprotonation, and formation of copper(II) and zinc(II) complexes of meso-tetra-(p-sulphonatophenyl)porphyrin

Quantitative study of the protonation and deprotonation equilibria of meso-tetra-(p-sulphonatophenyl)porphyrin, H2TPPS4–, as well as the formation equilibria of the Cu2+ and Zn2+ complexes, in a Me2SO-water (80 : 20) solution, has allowed us to determine for the first time the stability constants of two metalloporphyrins, CuTPPS4–(log β 38.1) and ZnTPPS4–(log β 34.6) at 25 °C and 0.1 mol dm3–(KClO4+ HClO4).

A Binuclear MnIII Complex of a Scorpiand-Like Ligand Displaying a Single Unsupported MnIII–O–MnIII Bridge

The crystal structure of a binuclear Mn(III) complex of a scorpiand-like ligand (L) displays an unsupported single oxo bridging ligand with a Mn(III)-O-Mn(III) angle of 174.7°. Magnetic susceptibility measurements indicate strong antiferromagnetic coupling between the two metal centers. DFT calculations have been carried out to understand the magnetic behavior and to analyze the nature of the observed Jahn-Teller distortion. Paramagnetic (1)H NMR has been applied to rationalize the formation and magnetic features of the complexes formed in solution.