Equilibrium Constants For Coordination Research Articles

Nanoconfinement boosts affinity of hydrated zirconium oxides to arsenate: Surface complexation modeling study

Nanoscale hydrated zirconium oxide (HZO) holds great potential in groundwater purification due to its ability to form inner-sphere coordination with arsenate. Despite being frequently used, especially as encapsulations in host materials for practical application in water treatment, the adsorption mechanisms of solutes on HZO are not appropriately explored, in particular for arsenate adsorption. In this study, we investigated the Zr–As coordination configuration and identified the most credible Zr–As configuration using surface complexation modeling (SCM), XPS and FT-IR analysis. The corresponding intrinsic coordination constants (Kintr) values was calculated by SCM, and the nanoconfinement effects were distinguished by comparing bare HZO with the HZO nanoparticles (NPs) encapsulated inside the strongly basic anion exchanger D201. Potentiometric titration suggests that the surface Zirconium hydroxyl groups (≡ZrOH) mainly exist in protonated form (≡ZrOH2+). Batch adsorption experiments demonstrate that the D201 hosts could adsorb As(V) through ion exchange by the quaternary ammonium groups under the low ionic strength (≤0.01 M NaNO3) and at pH > 6. The nanocomposite (HZO@D201) exhibits a higher adsorption capacity in a wide range of pH (3-10) and ionic strength (0.001–0.1 M NaNO3) than bare HZO. SCM simulations reveal that the coordination configuration of diprotonated monodentate mononuclear (MM-H2) dominates at pH 3–6, while deprotonated bidentate binuclear (BB–H0) dominates at pH > 7. For each configuration, the intrinsic coordination constants (Kintr) of HZO@D201 (10−0.66 and 10−16.10, respectively) are significantly higher than those of bare HZO (10−12.24 and 10−44.42, respectively), indicating a superior chemical bonding affinity caused by nanoconfinement. The obtained Kintr values are used to predict arsenate adsorption isotherms in pH 3 and 9, and the results align with the SCM simulation outcomes. This study may offer a feasible method for investigating the nanoconfinement effect of emerging nanocomposite adsorbents from a thermodynamic perspective, and provide reference coordination equilibrium constants of HZO for research and practical application.

Influence of the Coordination of Donor Solvent Molecules to 1,4,7,10-Tetramethyl-1,4,7,10- tetraazacyclododecanenickel(II) and Analogous Nickel(II) Complexes on Their Steric Factors

Coordination equilibrium constants (K NiS) of some donor solvent molecules to 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecanenickel(II) ([Ni(Me4[12]aneN4)]2+) were determined in nitrobenzene (a noncoordinating bulk solvent). The first (K NiS1) and second stepwise coordination equilibrium constants (K NiS2) for 1,4,7,10-tetraazacyclododecanenickel(II) ([Ni([12]aneN4)]2+), 1,4,8,11-tetraazac yclotetradecane- nickel(II) ([Ni([14] aneN4)]2+), 1,4,8,11-tetrathiacyclotetra-decanenickel(II) ([Ni([14]aneS4)]2+) were also reinvestigated. The K NiS values for [Ni(Me4[12]aneN4)]2+ were compared to those of [Ni([12]aneN4)]2+, (1R,4S, 8R,11S)-1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecanenickel(II) (R,S,R,S-[Ni(Me4[14]aneN4)]2+), R,R,S,S-[Ni(Me4[14]aneN4)]2+, [Ni([14]aneN4)]2+, and [Ni([14]aneS4)]2+. Coordination of pyridine (Py), N,N,N′,N′-tetramethylurea (TMU), and N,N-dimethylacetamide (DMA) to [Ni(Me4[12]aneN4)]2+ was observed, although these donor solvent molecules did not coordinate to R,S,R,S-[Ni(Me4[14]aneN4)]2+. The K NiS values for Py, TMU, and DMA are 7.9, 2.8, and 9.0 dm3⋅mol−1, respectively. Some hydrogen-bonding waters were coordinated to R,S,R,S-[Ni(Me4[14]aneN4)]2+, but such waters did not coordinate to [Ni(Me4[12] aneN4)]2+. Also, the K NiS2 values were larger than the corresponding K NiS1 values for [Ni([14]aneS4)]2+. Furthermore, the K NiS1 values for [Ni([12]aneN4)]2+ were the largest among these nickel(II) complex cations. The K NiS, K NiS1, and K NiS2 values are discussed in terms of properties of the donor solvents and steric strains of these nickel(II) complex cations.

The coordination of imidazole and substituted pyridines by the hemeoctapeptide N-acetyl-ferromicroperoxidase-8 (Fe IINAcMP8)

The N-terminus acetylated ferric hemeoctapeptide from cytochrome c, N-acetylmicroperoxidase-8 (Fe III-NAcMP8) can be reduced by dithionite in aqueous solution to produce Fe II-NAcMP8. The UV–Vis spectrum has a broad Soret band and relatively poorly defined Q bands which is consistent with a mixture of a five-coordinate high spin species with His as the axial ligand and a six-coordinate, predominantly high spin species with His/H 2O as axial ligands. There are two spectroscopically observable p K as at 8.7 ± 0.1 and 10.9 ± 0.2 which are attributed to ionization of a heme propionic acid group and coordinated H 2O, respectively; a p K a ⩾14 is due to ionization of the proximal His ligand. Equilibrium constants were determined by UV–Vis spectrophotometry at 25.0 ± 0.2 °C and 0.5 M ionic strength (NaClO 4) for the coordination of imidazole and a number of substituted pyridines, and complement available data for the ferric hemepeptide, allowing a comparison to be made of the affinity of an iron porphyrin with Fe in the +2 and +3 oxidation states towards these ligands. Imidazole is coordinated more strongly by the ferric porphyrin (log K = 4.08) than by the ferrous porphyrin (log K = 3.40). The equilibrium constants for coordination of pyridines by the ferric and ferrous porphyrins increase and decrease, respectively, with increasing ligand basicity. Values determined by cyclic voltammetry show the same dependence on the identity of the ligand. In the ferric porphyrin, the stability of the complex increases with the basicity of the ligand and hence its ability to donate electron density onto the metal. In the case of the more electron rich ferrous porphyrin, greater stability occurs with pyridine ligands that have an electron withdrawing group and hence can accept electron density from the metal. This is consistent with the midpoint reduction potentials E 1/2 of the pyridine complexes determined by cyclic voltammetry; E 1/2 is linearly dependent on, and becomes more negative with an increase in, ligand basicity. Log K for coordination of pyridines by the ferrous hemepeptide correlates well with the energy of the ligand frontier orbital with π symmetry, suggesting that π-bonding effects are significant in determining the strength of binding of pyridines by a ferrous porphyrin.

Capillary electrophoretic determination of coordination equilibrium constants of carboxylate ions with copper(II) complexed with several bis(1,10-phenanthroline) derivatives and bis(bipyridine)

A method for determining coordination equilibrium constants of some carboxylate ions (RCOO −) with Cu(II) complexed with phen (phen=1,10-phenanthroline), bpy (bpy=bipyridine), 4-methyl-, 5-methyl-, 5-chloro-, 5-nitro-, 4,7-dimethyl- and 5,6-dimethyl-phen was developed by capillary zone electrophoresis. The electrophoretic mobilities ( μ ep) of [Cu(phen derivative) 2] 2+ and [Cu(bpy) 2] 2+ were obtained from the migration time of 3 μg ml −1 Cu(II) complexed with 1×10 −3 mol dm −3 phen derivatives or bpy in the carboxylate buffers of pH 4.7 at 20 and 10 kV in the concentration range of ≦0.020 and >0.020 mol dm −3 carboxylate buffers, respectively. The μ ep value decreased with increasing the carboxylate concentration in the buffers. This indicates coordination of the carboxylate ion to [Cu(phen derivative) 2] 2+ or [Cu(bpy) 2] 2+. The typical first coordination equilibrium constants ( K CuS1) of formate, acetate, propionate, n-butyrate, i-butyrate, n-valerate, and i-valerate ions for [Cu(phen) 2] 2+ were 620±80, 620±30, 1530±60, 540±40, 580±70, 560±30 and 380±20, respectively. All the second coordination equilibrium constants were near zero. Therefore, all the complexes formed the five-coordinating species, [Cu(phen derivative) 2(RCOO)] + and [Cu(bpy) 2(RCOO)] +, in the aqueous carboxylate solutions.

Coordination of N-Donor Ligands by the Monomeric Ferric Porphyrin N-Acetylmicroperoxidase-8

The monomeric ferric hemepeptide from cytochrome c, N-acetylmicroperoxidase-8 (NAcMP8), retains the proximal His ligand and has a coordinated water molecule in the second axial site. It provides an opportunity for quantitatively studying in aqueous solution the coordination chemistry of an iron porphyrin with a single accessible coordination site. An examination of the dependence of the spectrophotometrically determined equilibrium constants for coordination of imidazole and cyanide as a function of pH provides a procedure for correcting conditional equilibrium constants for the ionization of coordinated H2O in NAcMP8 and protonation of the entering ligand, L. An examination of the coordination by NAcMP8 of L = primary amines and imidazoles (25 °C, μ = 0.1 M) and pyridines (as a function of temperature, μ = 0.1 M) has shown that the process of binding L to NAcMP8 may be complex. With some ligands such as pyridines and some primary amines there is evidence for formation of an outer-sphere complex before di...

Toward an Understanding of the High Enantioselectivity in the Osmium-Catalyzed Asymmetric Dihydroxylation. 4. Electronic Effects in Amine-Accelerated Osmylations

Electronic effects in osmylation reactions accelerated by pyridine and quinuclidine derivatives were investigated by varying the substituents on the amine ligand as well as on the alkene substrate. Ligand substituent effects were gauged by determination of the equilibrium constants for coordination of the amines to OsO4, evaluation of structural properties and reduction potentials of the amine−OsO4 complexes, and analysis of the kinetics of osmylations in the presence of the amines. Substrate substituent effects were gauged by kinetic Hammett studies using several different amine/alkene combinations. Nonlinear Hammett relationships resulting from alkene substituent effects were observed, and the deviation from a linear free energy relationship was found to depend on the structure, binding capacity, and concentration of the amine. The results were evaluated in terms of the contending “[3 + 2]” and “[2 + 2]” mechanisms currently under consideration. A change in mechanism that depends on the structural and e...

Coordination of donor solvent molecules to nickel square-planar complexes

The basicities of donor solvents were investigated for coordination of the donor solvent molecules to nickel square-planar complexes by spectrophotometry. It was observed that coordination equilibrium constants of the donor solvent molecules to [Ni(2,3,2-tet)] 2+ 2,3,2-tet = 1,4,8,11-tetraazaundecane) and [Ni(cyclen)] 2+ (cyclen = 1,4,7,10-tetraazacyclododecane) were larger than those to [Ni(cyclam)] 2+ (cyclam=1,4,8,11-tetrazacyclotetradecane). Specific coordinations of acetonitrile (AN) to [Ni(2,3,2-tet)] 2+ and water to [Ni(cyclen)] 2+ were observed. The first and second coordination equilibrium constants of [Ni(2,3,2-tet)] 2+ and [Ni(cyclen)] 2+ are discussed with π- backdonation of AN, the hydrogen bond of water molecules, and steric hindrance between the nickel tetraamine complexes and the donor solvent molecules. Also, coordination equilibrium constants of the nickel tetraamine complexes are compared with those of [Ni(ttp)] 2+ (ttp = 1,4,8,11-tetrathiacyclotetradecane).

Nitrile hydration catalysed by palladium(II) complexes

The palladium(II) complexes [Pd(H2O)4]2+, cis-[Pd(en)(H2O)2]2+, cis-[Pd(Met-OMe)(H2O)2]2+, cis-[Pd(dtcol)(H2O)2]2+ and [Pd(dien)(H2O)]2+, which contain aqua, ethane-1,2-diamine (en), methionine methyl ester (Met-OMe), 1,5-dithiacyclooctan-3-ol (dtcol), and diethylenetriamine (dien) ligands, catalysed selective hydration of various nitriles, yielding the corresponding carboxamides. Further hydrolysis to carboxylic acids was not detected. The catalysed reactions are enhanced as much as 106-fold over the uncatalysed ones. Equilibrium constants for co-ordination of nitriles to palladium(II) complexes were determined or estimated. Since carboxamides do not detectably co-ordinate to palladium(II) in solutions containing water, the product of hydration does not inhibit the reaction. Carboxamidate anion, however, co-ordinates to palladium(II) in acetone solution. Kinetic effects of the following factors were examined: catalyst concentration, substrate concentration, pD value, water concentration, electrophilicity of the nitrile group in the substrate, and electron donation and trans effect of the ancillary ligands in the catalyst. In the reaction catalysed by the four chelate complexes no intermediates were detected. In the hydration of CHCl2CN catalysed by [Pd(H2O)4]2+ palladium(II)–iminol complexes were observed as intermediates. In aqueous solutions only bidentate iminol co-ordination was detected. In acetone solutions the more labile unidentate iminol co-ordination was observed as well. The substrate (nitrile), the product (carboxamide), and an iminol intermediate were monitored in the cycle catalysed by [Pd(H2O)4]2+. This reaction was analysed in terms of the Michaelis–Menten model of enzyme kinetics, and microscopic rate constants were determined for the pathways involving and not involving the iminol intermediate. Internal attack on the nitrile ligand by the aqua (not hydroxide) ligand and external attack on the nitrile ligand by solvent water occur at similar rates. A general scheme of catalysis is devised on the basis of the kinetic experiments.

Coordination Behavior of O2 and CO in a Solid Film Consisting of Hemoglobin and Maltose

Abstract Solid hemoglobin (Hb) films were prepared by casting from a mixed solution of human hemoglobin and maltose (wt ratio of Hb/maltose is 60/40). The behavior of O2 and CO in coordinating to Hb in the films was then observed. The coordination equilibrium constants of both ligands were smaller by about an order of magnitude, and the rate constants (kon and koff) were smaller by about 13 orders of magnitude, than those of aqueous solutions of Hb. Interestingly, O2 and CO saturation was limited to 52% and 86%, respectively, under a partial pressure of 760 Torr (1 Torr = 133.322 Pa) of each gas, in spite of the one-to-one ligand binding ratio in aqueous solutions. This is due to the restraint of the structural change from a tense to a relaxed state in the solid phase. The limited saturation of the coordination in the solid phase was found to increase to 100% when a small amount (5.9%) of water was added; a significant structural change from the tense to the relaxed states followed.

Role of Carriers in the Selective Hydrogenation of Cyclopentadiene Over Supported Palladium Catalysts

Several supported palladium catalysts were prepared and thier effects on the kinetic behaviors in the selective hydrogenation of cyclopentadiene were studied. Experiments show that cyclopentadiene inhibits the hydrogenation of cyclopentene. Data based on kinetic analysis indicate that the coordination of cyclopentadiene is much stronger than that of cyclopentene which should be the main factor of this inhibition reaction. Catalyst carriers via thier ligand effects can influence the ratio of the coordination equilibrium constants K-D/K-E and the rate constants K-2/K-4, which in turn change the selectivities and activities of catalyets.

Synergistic extraction studies of uranium(VI) and plutonium(VI) with some β-diketones and heterocyclic N-oxides

Abstract The equilibrium constants for coordination of methyl substituted pyridine N-oxides with plutonium(VI) thenoyl trifluoroacetonate in chloroform (Ks) follow an order similar to those of the analogous uranium(VI) complexes indicating steric hindrance to bonding in the case of ortho substituted pyridine N-oxides. The extraction constants (k) of Pu(VI) chelates with various β-diketones are found to be only marginally higher than the values for the corresponding uranium(VI) chelates which is in conformity with the close similarity of the ionic radii of PuO 2 2+ and UO 2 2+ .

The Ligation of Phosphine Derivatives and the Oxygen-binding Properties of the Cobalt Mesoporphyrin IX Dimethyl Ester in Toluene

Abstract The coordination of phosphine derivatives and some other bases to the cobalt mesoporphyrin IX dimethyl ester in toluene has been spectrophotometrically investigated. The coordination equilibrium constants were determined for each base, and found to be of this sequence (25 °C) : P(C4H9)3>>P(OCH3)3>P(C6H5)3>S(CH3)2>As(C6H5)3. The reversible oxygenation of the penta-coordinate complexes formed as above was investigated at lower temperatures (−20–−80 °C): the corresponding equilibrium constants were found to be in the following sequence (−80 °C): P(C4H9)3>P(OCH3)3>S(CH3)2>P(C6H5)3>As(C6H5)3. These relations were discussed in terms of the σ-donating ability of a ligand coordinated trans to oxygen.