Hydroxo Ions Research Articles

Mononuclear Ni(II) Complexes with a S3O Coordination Sphere Based on a Tripodal Cysteine-Rich Ligand: pH Tuning of the Superoxide Dismutase Activity.

The superoxide dismutase (SOD) activity of mononuclear NiII complexes, whose structures are inspired by the NiSOD, has been investigated. They have been designed with a sulfur-rich pseudopeptide ligand, derived from nitrilotriacetic acid (NTA), where the three acid functions are grafted with cysteines (L3S). Two mononuclear complexes, which exist in pH-dependent proportions, have been fully characterized by a combination of spectroscopic techniques including 1H NMR, UV-vis, circular dichroism, and X-ray absorption spectroscopy, together with theoretical calculations. They display similar square-planar S3O coordination, with the three thiolates of the three cysteine moieties from L3S coordinated to the NiII ion, together with either a water molecule at physiological pH, as [NiL3S(OH2)]-, or a hydroxo ion in more basic conditions, as [NiL3S(OH)]2-. The 1H NMR study has revealed that contrary to the hydroxo ligand, the bound water molecule is labile. The cyclic voltammogram of both complexes displays an irreversible one-electron oxidation process assigned to the NiII/NiIII redox system with Epa = 0.48 and 0.31 V versus SCE for NiL3S(OH2) and NiL3S(OH), respectively. The SOD activity of both complexes has been tested. On the basis of the xanthine oxidase assay, an IC50 of about 1 μM has been measured at pH 7.4, where NiL3S(OH2) is mainly present (93% of the NiII species), while the IC50 is larger than 100 μM at pH 9.6, where NiL3S(OH) is the major species (92% of the NiII species). Interestingly, only NiL3S(OH2) displays SOD activity, suggesting that the presence of a labile ligand is required. The SOD activity has been also evaluated under catalytic conditions at pH 7.75, where the ratio between NiL3S(OH2)/ NiL3S(OH) is about (86:14), and a rate constant, kcat = 1.8 × 105 M-1 s-1, has been measured. NiL3S(OH2) is thus the first low-molecular weight, synthetic, bioinspired Ni complex that displays catalytic SOD activity in water at physiological pH, although it does not contain any N-donor ligand in its first coordination sphere, as in the NiSOD. Overall, the data show that a key structural feature is the presence of a labile ligand in the coordination sphere of the NiII ion.

Redox-Assisted Self-Assembly of a Water-Soluble Cyanido-Bridged Mixed Valence {CoIII /FeII }2 Square.

The facile redox-assisted assembly of a water-soluble, extremely robust, cyanide-bridged mixed-valence [{CoIII {(Me)2 (μ-ET)cyclen}}2 {(μ-NC)2 FeII (CN)4 }2 ]2- square is reported. The preparation process involved the use of the enhanced lability of inert CoIII synthons triggered by outer-sphere redox processes. Characterization of the final compounds has been carried out by NMR, UV/Vis, electrochemistry, and ICP analyses. DFT calculations have been conducted to optimize a structure that has the same hydrodynamic radius as that obtained from DOSY experiments. The new compound is extremely robust, surviving in aqueous solution within the 0-12 pH range for months. The species shows a high affinity for both protons and hydroxo ions in aqueous medium.

Effects of Modification of Calcium Hydroxyapatites by Trivalent Metal Ions on the Protein Adsorption Behavior

The effects of modification of calcium hydroxyapatites (Hap; Ca10(PO4)6(OH)2) by trivalent metal ions (Al(III), La(III), and Fe(III)) on protein adsorption behavior were examined using bovine serum albumin (BSA; isoelectric point (iep) = 4.7 and molecular mass (M(s)) = 67,200 Da). The Al(III)-, La(III)-, and Fe(III)-substituted Hap particles were prepared by the coprecipitation method with different atomic ratios, metal/(Ca + metal), abbreviated as X(metal). The particles precipitated at X(metal) = 0 (original-Hap) were rod-like and 10 x 36 nm2 in size. The short, rod-like original-Hap particles were elongated upon adding metal ions up to X(metal) = 0.10, and the extent of the particle growth was in the order of La(III) < Al(III) << Fe(III). The crystallinity of the materials was slightly lowered by increasing X(metal) for all systems. The adsorption isotherms of BSA onto the Al(III)-, La(III)-, and Fe(III)-substituted Hap particles showed the Langmuirian type. The saturated amounts of adsorbed BSA (n(s)(BSA)) values were strongly dependent on X(metal) in each system. The n(s)(BSA) values for the Fe(III)-substituted Hap system were increased with an increase in X(Fe) (X(metal) value of Hap particles substituted with Fe(III)); the n(s)(BSA) value obtained at X(Fe) = 0.10 was 2.7-fold more than that for the original-Hap particle, though those for the La(III) system were decreased to ca. 1/5. On the other hand, the n(s)(BSA) values for the Al(III) system were decreased with substitution of small amounts of Al(III), showing a minimum point at X(Al) = 0.01, but they were increased again at X(Al) over 0.03. Since the concentrations of hetero metal ions dissolved from the particles exhibited extremely low values, the possibility of binder effects of trivalent cations dissolved from the particle surface for adsorbing BSA to trivalent-ion-substituted Hap particles was excluded. The increase of n(s)(BSA) by an increase in X(Fe) was explained by elongation of mean particle length along with the production of surface hydroxo ions, such as Fe(OH)2+ or Fe(OH)2+, to induce the hydrogen bond between the Fe(III)-substituted Hap surface and BSA molecules, though the number of original C sites established by Ca(II) atoms was reduced. In the case of La(III)-substituted Hap particles, the number of original C sites established by Ca(II) atoms was reduced by La(III) substitution but the mean particle length remained almost constant. Furthermore, surface hydroxo La(III) groups were absent. Therefore, the reduction of n(s)(BSA) was explained by both the unaltered mean particle length and their low surface hydrophilicity. The change of n(s)(BSA) values by X(Al) resembled that of the mean particle length. These results implied that both the mean particle length and surface hydrophilicity of Al(III)-, La(III)-, and Fe(III)-substituted Hap particles are determining factors of the adsorption amounts of BSA.

Structural variability in uranyl–lanthanide heterometallic complexes with DOTA and oxalato ligands

Four novel 4f–5f heterometallic complexes could be obtained from the reaction of uranyl and lanthanide nitrates with DOTA (H4L) under hydrothermal conditions. In all cases, as in the previous examples reported, additional oxalato ligands are formed in situ. Variations in the stoichiometry of the final products and the presence of hydroxo ions in some cases appear to result in a large structural variability. In the two isomorphous complexes [(UO2)2Ln2(L)2(C2O4)] with Ln = Sm (1) or Eu (2), the lanthanide ion is located in the N4O4 site and is also bound to a carboxylate oxygen atom from a neighbouring unit, to give zigzag chains which are further linked to one another by [(UO2)2(C2O4)]2+ dications, resulting in the formation of a 3D framework. In [(UO2)4Gd2(L)2(C2O4)3(H2O)6]·2H2O (3), 2D bilayer subunits of the “double floor” type with uranyl oxalate pillars are assembled into a 3D framework by other, disordered uranyl ions. [(UO2)2Gd(L)(C2O4)(OH)]·H2O (4) is a 2D assembly in which cationic {[(UO2)2(C2O4)(OH)]+}n chains are linked to one another by the [Gd(L)]−groups. The most notable feature of this compound is the environment of the 4f ion, which is eight-coordinate and twisted square antiprismatic (TSA′), instead of nine-coordinate monocapped square antiprismatic (SA), as generally observed in DOTA complexes of gadolinium(III) and similarly-sized ions.

Highly Symmetrical Tetranuclear Cluster Complexes Supported by p‐tert‐Butylsulfonylcalix[4]arene as a Cluster‐Forming Ligand

AbstractSquare‐planar tetranuclear clusters [M4(L)(AcO)4(μ4‐OH)]– (M = MnII, CoII, and NiII) are synthesized using tetra‐anionic p‐tert‐butylsulfonylcalix[4]arene (L4–) as a cluster‐forming ligand. Three complexes are crystallographically isostructural, being crystallized in the triclinic crystal system with space group P$\bar {1}$. The calix[4]arene acts as a tetrakis fac‐tridentate ligand through four phenoxo and four sulfonyl oxygen atoms to form square arrangement of four metal ions, which are further bridged by four chelating acetate ions and one hydroxo ion in a μ4 manner to complete the hexacoordination of each metal center. Although the whole molecule of each complex is crystallographically independent, the molecule is highly symmetrical with a pseudo‐four‐fold axis lying on the μ4‐OH– group. The tetranuclear clusters are stable enough to maintain the core structures even in highly dilute solution (≈10 μM), which was confirmed by mass spectroscopic study, however, bridging acetates were easily exchanged by other carboxylate chelates to form derivatives such as [M4(L)(BzO)4(OH)]–. Metal–metal interactions were investigated by means of magnetic susceptibility, and it was revealed that both ferro‐ and antiferromagnetic interactions occur in the NiII complex depending on the bridging angles of Ni–O–Ni. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2006)

On the Existence of the Elusive Monomethyl Ester of Carbonic Acid [CH3OC(O)OH] at 300 K: 1H‐ and 13C NMR Measurements and DFT Calculations

AbstractThe elusive monomethyl ester of carbonic acid [CH3OC(O)OH] has been prepared at 300 K by protonation of the sodium salt NaOC(O)OCH3 with anhydrous HCl or water and characterized by 1H‐ and 13C NMR spectroscopy. The stability of the acid and its reactivity towards hydroxo ions and methylating agents under ambient conditions are discussed. The energetics and the mechanism of the investigated reactions are examined on the basis of density functional calculations. For kinetic and thermodynamic reasons CH3OC(O)OH is unlikely to be formed by insertion of CO2 into the O–H bond of methanol. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2006)

Reactivities of coordinated phosphodiesters*

The amminolysis and hydrolysis of the ester moieties in [(NH3)5Co(bnpp)]2+ and cis-[(en)2Co(OH)(bnpp)]+ (where bnpp = bis(4-nitrophenylphosphate) and en = 1,2-ethanediamine) in aqueous solution at 25°C have been followed over a range in pH. In each instance, cleavage of 4-nitrophenol occurred in two observable events triggered by the coordinated nucleophiles, amido ion and hydroxo ion. The mechanistic pathways have been analyzed and the rate enhancements evaluated relative to uncoordinated substrates and other relevant literature data. The analysis indicates requirements for the metal reagent to be effective in phosphate ester hydrolysis and phosphoryl transfer.

Formation Constants of Chromium(III), Scandium(III) and Yttrium(III) Complexes of Some Hydroxy Naphthoic Acids

The complexes of chromium(III), scandium(III) and yttrium(III) formed by 1-hydroxy-2-naphthoic acid (1,2-HNA: H2L) and 3-hydroxy-2-naphthoic acid (3,2-HNA: H2L) were investigated by potentiometry and spectroscopy at 25+/-0.1 degrees C and at an ionic strength of 0.1 M KNO3 in 50% ethanol-water (v/v) medium. The stoichiometries of these three M(III) complexes formed with these hydroxy-naphthoic acids and with hydroxo ion were defined and their formation constants were determined and compared. Thus, the removing capacities of these ligands could be examined by calculating the equilibrium concentration of Cr(III) that exists in the discharge water of various industries since Cr(III) ions are the main pollutants present during waste water treatment in our city, Bursa.

Synthesis, Structure, and Kinetics and Stereochemistry of Base-Catalyzed Hydrolysis of meso- and rac-[Co2(tmpdtne)Cl2]4+, Bis(pentaamine) Complexes Devoid of Deprotonatable NH Centers

The bis(pentadentate) ligand tmpdtne binds two Co(II) centers, and the entity is readily oxidized to the dicobalt(III) derivative [Co(2)(tmpdtne)Cl(2)](4+) which has been separated into two isomeric forms. NMR studies establish these as meso and rac isomers arising from the different or same absolute configurations for the asym configuration about each Co(III) center. Each dinuclear ion base hydrolyses to the dihydroxo derivative [Co(2)(tmpdtne)(OH)(2)](4+) with retained asym configurations about each metal ion and also retained rac or meso configurations. The kinetics for the stepwise loss of the two Cl(-) ligands is uniphasic, and data are presented to show that the loss of the first chloride is rate determining and is followed by very rapid intramolecular and loss of the second Cl(-) via a hydroxo-bridged species to yield the observed dihydroxo derivative. Meso and rac forms of the latter have been crystallized. The X-ray crystal structure of the rac-dihydroxo complex is reported, and it establishes the configurations of all the complexes reported. The (1)H NMR spectra for the hydroxo ions show very high field Co-OH resonances (ca. delta-0.5 ppm) not observed previously for such ions, and this result is discussed in the context of published (1)H NMR data for bridged Co-OH-Co species. The base hydrolysis kinetics for the dichloro ions are first order in [OH(-)], and deprotonation at an alpha-CH(2) center (alpha to a pyridyl) is identified as the source of the catalysis, since there is no NH center available for deprotonation on the ligand. These data further support the new pseudoaminate base hydrolysis mechanism first reported in 2003. The values of k(OH) for the second-order base-catalyzed reaction are ca. 4.0 M(-1) s(-1) for both the rac and meso isomers, and these results are discussed in terms of the increased acidities of these 4+ cations compared to their 2+ ion counterparts.

Outer-sphere electron transfer kinetics of metal ion oxidation by molecular oxygen

Density functional theory molecular orbital calculations and Marcus theory have been combined to assess the rates and physicochemical factors controlling the outer-sphere oxidation of divalent V, Cr, Mn, Fe, and Co aquo and hydroxo ions by O 2 in homogeneous aqueous solution. Key quantities in the elementary oxidation step include the inner-sphere component of the reorganization energy, the thermodynamic driving force, and electrostatic work terms describing the interactions occurring, in this case, between the net charges on the product species. Collectively, these factors and their interplay have a large influence on the rate of the oxidation cross-reaction. An inner-sphere pathway for the self-exchange reactions and oxidation by O 2 of Mn 2+ and Cr 2+ ions has been supported indirectly in this study by comparing predicted outer-sphere rates with the results of previous experiments. Likewise, an outer-sphere pathway is suggested for the similar sets of reactions involving the V, Fe, and Co ions. An assessment of the self-exchange reaction for the oxygen/superoxide couple has led to predicted rates in excellent agreement with direct measurements. Predicted rates of oxidation for the hexaquo Fe ion are also in agreement with experiment, while the predicted rates for the outer-sphere oxidation of its hydrolysis products are ∼2 to 3 (monohydroxo) and ∼4 (dihydroxo) orders of magnitude slower than the observed rates. This suggests an inner-sphere pathway is appropriate to explain the relatively fast rates observed for the hydrolyzed Fe species.

Electrospray mass spectrometry of isomeric tetrakis(N-alkylpyridyl)porphyrins and their manganese(III) and iron(III) complexes

Manganese(III) complexes of isomeric tetrakis(N-alkylpyridyl)porphyrins (N- alkyl = N- methyl , M or N- ethyl , E ), MnTM ( E )-2(3,4)- PyP5+, are being developed as superoxide dismutase (SOD) mimics. Simultaneously, techniques for their purification, identification and characterization are being pursued. Electrospray mass spectrometry ( ESMS ) proved to be an excellent method for identification and characterization of this group of water-soluble cationic porphyrins. The multiply charged parent ion is observed for both the metal-free ligands and their corresponding manganese complexes. The other major peaks in the mass spectra result from loss of N-alkyl groups, reduction of the metal center, axial coordination of chloride or hydroxo ion in the case of the Fe porphyrin, loss of metal and deprotonation of pyrrolic nitrogens. As a result of inductive and resonance effects, which stabilize the ortho isomer, almost no loss of N-alkyl groups from the manganese complex or from its parent ligand was observed. The relative intensity of the multiply charged molecular ion MnIIITM -3(4)- PyP5+/5 was 100% in the case of the meta and para isomers. Although manganese porphyrins display a low preference toward axial ligation, favorable electrostatics at the metal center of the ortho isomer gives rise to 100% relative intensity of the species that has chloride axially ligated at the manganese site, MnIIITM ( E )-2- PyPCl4+/4. When the stronger preference of iron porphyrins toward axial ligation combines with the ortho effect, the monohydroxo iron porphyrin FeIIITM -2- PyP ( OH )4+/4 dominates the ESMS of an aqueous acetonitrile solution at pH 7.8.

Surface structure and composition of calcium hydroxyapatites substituted with Al(III), La(III) and Fe(III) ions

Synthetic calcium hydroxyapatite (CaHap) particles were modified by ionexchange with Al(III), La(III) and Fe(III) by the coprecipitation and immersion methods. The growth of CaHap particles was observed on substituting with these metal ions by coprecipitation. The exchange ratios of Ca(II) with the metal ions revealed that Fe(III) replaces Ca(II) as hydroxo ions and the substitution with La(III) accompanies OH −-deficiency. In the substitution with immersion the exchange ratios for all the metal ions were less than 1.5 of the theoretical ratio, verifying the exchange with metal hydroxo ions. IR spectra of the CaHap particles substituted with Al(III) by both the methods gave rise to several bands due to surface Al-OH besides the surface P-OH bands, whereas the CaHap substituted with La(III) showed no surface La-OH band. The surface Fe-OH was found on the CaHaps substituted by the coprecipitation but not on those substituted by the immersion.

Mixed-valent μ-oxo-bridged diiron complexes produced by radiolytic reduction at 77 K studied by EPR

Several dinuclear diferric complexes were prepared, bridged by only an oxo ion, by an oxo ion and additionally one or two carboxylate groups, or by two hydroxo ions. Frozen solutions of these EPR-silent diferric complexes were radiolytically reduced by γ-irradiation at 77 K. The reduced forms gave EPR spectra typical of mixed-valent diiron species with S = 1/2. The yield was about 50% at high doses. With 57Fe replacing 56Fe the spectral features were broadened because of hyperfine interaction with one or both of the 57Fe nuclei. All oxo-bridged complexes give mixed-valent species with unbroadened EPR spectra observable up to 110 K. The anisotropy is small, Δg = g 1–g 3<0.07, and g av = (g 1+g 2+g 3)/3 falls in the range 1.93–1.96. Annealing at T≥115 K causes the EPR spectra to change and finally become more anisotropic, 0.14<Δg<0.32 and to have smaller g av, 1.78<g av<1.89. These spectra are observable only at T≤35 K. Radiolytic reduction of the diferric complexes with hydroxo bridges leads directly without annealing to this second type of mixed-valent species. The annealing-induced transition from a primary to a secondary mixed-valent species in the case of an oxo-bridged diferric starting complex is suggested to be a structural relaxation including protonation of the bridge. Annealing at temperatures above ca. 180 K and final thawing causes the mixed-valent dinuclear species to disappear, probably by disintegration to mononuclear species. These mixed-valent species are useful models for similar species in proteins, where primary-type species may take part in transient electron transfer, and the secondary-type species are usually more persistent forms in redox reactions. For proteins, available data show that there is a tendency for both primary and secondary species to have larger g anisotropy, Δg, and smaller g av, than obtained with the models. These differences suggest that the diiron centers of proteins are affected by their specific surroundings. Our results also show that the nature of the bridge, oxo or hydroxo, may not always safely be determined from a measured J value. Measurement of the EPR parameters, Δg, g av and limiting temperature of observation (relaxation properties) of a mixed-valent complex, or, in the case of a diferric complex, of the primary mixed-valent species induced by radiolysis at 77 K, offers an alternative empirically based method to determine the nature of the bridge.

Study of lanthanum-based colloidal sols formation

Lanthanum-based colloidal sols have been studied in order to synthesize lanthanum oxychloride inorganic coatings or thin layers for catalytic applications. The singularity of this process leading to microporous coatings is based on the polymerization of lanthanum acetate species in aqueous solution. Sols are prepared from lanthanum chloride modified by acetate ions. The sol formation mechanism can be explained by La3+ hydrolysis, giving basic species (La (OH)X with x = 1 or 2) which condense and lead to polycondensed hydroxo ions. In the pH range of sol formation, OH− and CH3COO− are ligands competitive towards La3+; when acetate ions are present, the condensation rate is limited by lanthanum acetate complexation. Several distributions of lanthanum hydroxide and acetate species are given and related to experimental results (pH, Fourier transform infrared (FTIR) spectroscopy, turbidity and stability of sols, SEM and TEM analysis of coatings). FTIR spectroscopy has been revealed as useful to evidence a polymerization of acetate species in sols leading to translucent gels. These gels allow the preparation of almost fully dense LaOCl coatings after thermal treatment above 450°C. These results confirm the possible polymerization of lanthanum acetate complexes during the drying step and emphasize the great effect of acetate species on the final material texture.

The reactivity of different iron(III) species in the formation of monoacethydroxamatoiron(III) complex

The dependence of the rate constant of the monoacethydroxamatoiron(III) complex formation on ionic strength in acid solution 25 °C has been studied. The observed ‘secondary salt effect’ strongly supports the assumption that an iron(III) hydroxo complex is the reactive species. Evidence is presented showing that the iron(III) hydroxo dimer (Fe 4(OH0 2(H 2O) 4+ 8 exhibits approximately equal reactivity toward acethydroxamic acid as the monomer does. This fact, and lack of evidence for ‘primary salt effect’ strongly confirms the previous findings that the reactive form of ligand is the undissociated rather than dissociated hydroxamic acid. The rate of the reaction of hydroxamic acid with iron(III) hydroxo ion is estimated to be of the same order of magnitude as the rate of water exchange of the same ion.

Kinetics of process for desorption of erbium from a cationite by solutions of iminodiacetic acid salts

The desorption of erbium from a cationite by solutions of iminodiacetic acid salts is limited by the inner diffusion rate of either erbium aquo ions (when the pH of the equilibrium solution is below 6) or hydroxo ions (when the pH is greater than 6).

The specific adsorption of inorganic Hg(II) species and Co(III) complex ions on goethite

The specific adsorption of Hg(II), as Hg 2+ and HgCl 2 0 , on goethite has been measured as a function of pH. The results show that the mercuric ion is not a typical heavy metal cation since its adsorption is considerably less than for other cations at pH values near their respective p*K values. Complexing the mercuric ion with chloride to give HgCl 2 0 was found to reduce adsorption to negligible amounts. It is concluded that hydroxo ligands are essential for Hg(II) adsorption and it is probable that two hydroxo bridges are formed between the oxide surface and the adsorbed Hg(II) atom. Reduction of the positive charged surface. The considerable covalency of the Hg-OH bonds is believed to cause the relatively low affinity of the mercuric ion for the oxide by polarizing the surface-OH-Hg bonds and therefore minimizing the strength of the surface-OH bonds. The complex ions [Co(NH 3) 6] 3+, [Co(NH 3) 5H 2O] 3+, and [Co(en) 2(H 2O) 2] 3+ were not specifically adsorbed even when the latter two were hydrolyzed to hydroxo species. Their inability to adsorb is again attributed to the covalency of the Co(III)-OH bonds of the hydroxo ions which prevents the formation of stable hydroxo bridges between the oxide surface and the Co(III) species.