Hydrolysis of PNPP Catalyzed by Cu (II), Ni (II) Schiff Base Complexes in CTAB Micellar Solution

The kinetics of hydrolysis of p‐nitrophenyl picolinate(PNPP) catalyzed by metallomicelles formed from Cu (II), Ni (II) Schiff base complexes (CuL, NiL) and CTAB micelle were investigated in the pH range of 6.0–9.0 at 30°C. For the Cu (II) Schiff base complex CuL, the apparent rate constants (k _obsd) of PNPP hydrolysis initially increased with the increasing pH of reaction media, then fell off. For the Ni (II) Schiff base complex NiL, the k _obsd always increased with the increasing pH. The kinetic and thermodynamic parameters were calculated. The hydrolysis rate of PNPP catalyzed by Cu (II) complex was much larger than that by Ni (II) complex in CTAB micellar solution. The catalytic mechanism of the PNPP hydrolysis was discussed in detail, and the possibly active specie for the catalytic hydrolysis of PNPP was the monohydroxo metal complex.     

Reactivity of Schiff Base Manganese(III) Complexes with Different Pendants toward the Hydrolysis of p‐Nitrophenyl Picolinate

This paper describes the catalytic performance of two manganese(III) complexes with mono‐Schiff base ligands as artificial hydrolases towards the hydrolysis of p‐nitrophenyl picolinate (PNPP). Observations reveal that the one complex (MnL₂²Cl) containing morpholine pendants exhibits 1.2–1.7 fold kinetic advantage over the other one (MnL₂¹Cl) containing benzoaza‐15‐crown‐5 group. Especially, optimum molecule structures using a Gaussian 03 software confirm that MnL₂²Cl indeed possesses a relatively open linked site for the approaching of PNPP, resulting in higher efficiency due to a convenient association between substrate (PNPP) and MnL₂²Cl. In addition, the steric hindrance of two pendants, i.e., benzoaza‐15‐crown‐5 and morpholine, may be a main influencing factor for tuning catalytic activities of the synthesized Mn(III) catalysts. Both Mn(III) catalysts used here were found to have fine tolerance to the operated temperature and pH. Related kinetic and thermodynamic analyses were also given to demonstrate their structure‐activity relationships (SAR) of both catalysts used.     

Studies on PNPP Hydrolysis Catalyzed by Schiff Base Cobalt（Ⅱ） Complexes

Two cobalt（Ⅱ） complexes of the Schiff base with morpholino or aza-crown ether pendants, CoL^1 and CoL^2, as mimic hydrolytic metalloenzyme, were used in catalytic hydrolysis of carboxylic ester （PNPP）. The analysis of specific absorption spectra of the hydrolytic reaction systems indicates that key intermediates, made up of PNPP and Co（Ⅱ） complexes, have been formed in reaction processes of the PNPP catalytic hydrolysis. The mechanism of PNPP catalytic hydrolysis has been proposed based on the analytic result of specific absorption spectrum. A kinetic mathematical model, applied to the calculation of the kinetic parameter of PNPP catalytic hydrolysis, has been established based on the mechanism proposed. The acid effect of buffer solution, structural effect of the complexes, and effect of temperature on the rate of PNPP hydrolysis catalyzed by the complexes have been also discussed.     

Uncharged Co(lll) Complexes: Transfer Chemical Potentials and Base Hydrolysis in Water-Methanol Mixtures

Solubilities are reported for three Co(III) complexes of 2-aminophenol, 2-amino-4-nitrophenol and 6-amino-2,4-dimethylphenol. Transfer chemical potentials have been derived from appropriate solubility measurements in aqueous methanol. The trends in transfer chemical potentials are discussed in terms of the nature of the coordinated ligands and are compared with those for selected of other inorganic Co(III) complex ions. The kinetics of base hydrolysis of Co(III) complexes in aqueous methanol solutions are described and the change in the activation barrier δ_mΔG^≠ is determined. Solvent effects on reactivity trends are analysed into initial state and transition state components. The small decrease in the rate constant, with increasing proportion of methanol, is ascribed to a greater effect of solvent on the initial state.     

Reactivity Trends and Kinetic Inspection of Hydroxide Ion Attack and DNA Interaction on Some Pharmacologically Active Agents of Fe(II) Amino Acid Schiff Base Complexes at Different Temperatures

The reactivity of few novel high‐spin Fe(II) complexes of Schiff base ligands derived from 2‐hydroxynaphthaldehyde and some variety of amino acids with the OH^? ion has been examined in an aqueous mixture at the temperature range from 10 to 40°C. Based on the kinetic investigations, the rate law and a plausible mechanism were proposed and discussed. The general rate equation was suggested as follows: rate = k_obs[complex], where k_obs. = k₁ + k₂[OH^?]. Base‐catalyzed hydrolysis kinetic measurements imply pseudo–first‐order doubly stage rates due the presence of mer‐ and fac‐isomers. The observed rate constants k_obs are correlated with the effect of substituent R in the structure of the ligands. From the effect of temperature on the rate base hydrolysis reaction, various thermodynamic parameters were evaluated. The evaluated rate constants and activation parameters are in a good agreement with the stability constants of the investigated complexes. Moreover, the reactivity of the investigated complexes toward DNA was examined and found to be in a good agreement with the reported binding constants.     

Kinetic Screening for the Acid‐Catalyzed Hydrolysis of Some Hydrophobic Fe(II) Schiff Base Amino Acid Chelates and Reactivity Trends in the Presence of Alkali Halide and Surfactant

Kinetics of acid‐catalyzed hydrolysis of some high‐spin Fe(II) Schiff base amino acid complexes were followed spectrophotometrically at 298 K under pseudo–first‐order conditions. The studied ligands were derived from the condensation of 5‐bromosalicylaldehyde with different four amino acids (phenylalanine, aspartic acid, histidine, and arginine). The acid hydrolysis reaction was studied in aqueous media and in the presence of different concentrations of the alkali halide (KBr) and cationic surfactant (cetyl‐trimethyl ammonium bromide, CTAB). The general rate equation was suggested to be rate = k_obs[complex], where k_obs = k₂[H⁺]. The increase in [KBr] enhances the reactivity of the reaction, and the addition of CTAB to the reaction mixture accelerates the reaction reactivity. The obtained kinetic data were used to determine the values of δ_mΔG^# (the change in the activation barrier) for the studied complexes when transferred from “water to water containing different [KBr]” and from “water to water containing altered [CTAB].”     

Hydrolysis of BNPP Catalyzed by the Crowned Schiff Base Co(II) Complexes in Micellar Solution

Two symmetrical double aza‐crowned Schiff base cobalt(II) complexes were synthesized and characterized, and the metallomicelle made up of the cobalt(II) complexes and surfactant, as mimic hydrolytic metalloenzyme, was used in catalytic hydrolysis of bis(4‐nitrophenyl) phosphate (BNPP). The analysis of specific absorption spectrums of the hydrolytic reaction systems indicated that key intermediates made up of BNPP and Co(II) complexes are formed in reaction processes of the BNPP catalytic hydrolysis. In this article, the mechanism of BNPP catalytic hydrolysis has been proposed based on the analytic result of specific absorption spectrum. A kinetic mathematical model, for the calculation of the kinetic parameter of BNPP catalytic hydrolysis has been established based on the mechanism proposed. The acid effect of reaction system, the structural effect of the complexes, the effect of surfactant micelles and the effect of temperature on the rate of BNPP hydrolysis catalyzed by the complexes have been discussed.     

Kinetic and Mechanistic Aspects of Reactivity of the Oxidation of Some Fe(II)–Tris Schiff Base Complexes by Peroxydisulfate: Spectrophotometric Tracer and Solvent Effect on the Reactivity

The kinetics of the oxidation of some Fe(II)–Tris Schiff base complexes by peroxydisulfate was studied spectrophotometrically in the aqueous medium and in the organic–aqua binary mixture. The inspected complexes were derived from the condensation of 2‐acetylpyridine and substituted benzylamines. The oxidation reaction of the studied complexes was followed at 303 K under pseudo–first‐order conditions. It was found that the oxidation reaction by S₂O₈^2? consists of two steps. The first step is the formation of an ion pair from the reactants, and the second step is an electron transfer from the metal center to the peroxydisulfate oxidant, with an associated peroxo bond fissure. A mechanism, based on the experimental results, was proposed, and the rate law was derived. The effect of organic solvent on the reaction rate was studied in the presence of different ratios (v/v) of methanol–water and acetone–water mixtures. Moreover, the changes in the activation barrier from water to water–methanol and water–acetone mixtures were estimated from the kinetic data. The transfer chemical potentials of the initial and transition states from water into mixed solvents were determined from solubility measurements. Solvent effects on the reaction rate were discussed in terms of initial state versus transition state solvation.     

Catalytic Kinetics of the Schiff Base Metal Complexes Bearing Side Chain of Cyclic morpholine in Carboxylic Ester Hydrolysis

It has been reported that two Schiff base transition metal complexes bearing the side chain of the morpholine ring were synthesized and characterized, and two complexes with the same base agent but different metal ions were used as a simulant hydrolase in the catalytic hydrolysis of p-nitrophenyl picolinate in this paper. The mechanism of PNPP catalytic hydrolysis is proposed and supported by the results of the spectral analysis and the kinetic calculation. A kinetic mathematical model, applied to the calculation of the kinetic and thermodynamics parameters of PNPP catalytic hydrolysis, has been established on the foundation of the mechanism proposed. The result of the study shows that the two complexes have a good catalytic activity in PNPP catalytic hydrolysis, and the rate of the PNPP catalytic hydrolysis was increased with the increase of the pH values in the buffer solution and affected by the polarization effect of metal ion of the complexes.     

Ag(Ⅰ)Schiff碱配合物的合成及其对黄瓜花叶病毒的抑制作用

Ag(Ⅰ)Schiff碱配合物的合成及其对黄瓜花叶病毒的抑制作用     

Mononuclear and Tetranuclear Copper(II) Complexes Bearing Amino Acid Schiff Base Ligands: Structural Characterization and Catalytic Applications

Two new glycine-Schiff base copper(II) complexes were synthesized. Single crystal X-ray diffraction (SCXRD) allowed us to establish the structure of both complexes in the solid state. The glycine-Schiff base copper(II) complex derived from 2′-hydroxy-5′-nitroacetophenone showed a mononuclear hydrated structure, in which the Schiff base acted as a tridentate ligand, and the glycine-Schiff base copper(II) complex derived from 2′-hydroxy-5′-methylacetophenone showed a less common tetranuclear anhydrous metallocyclic structure, in which the Schiff base acted as a tetradentate ligand. In both compounds, copper(II) had a tetracoordinated square planar geometry. The results of vibrational, electronic, and paramagnetic spectroscopies, as well as thermal analysis, were consistent with the crystal structures. Both complexes were evaluated as catalysts in the olefin cyclopropanation by carbene transference, and both led to very high diastereoselectivity (greater than 98%).     

Transfer Chemical Potentials,Solubility and Reactivity of Psoralen and 8-Hydroxypsoralen in Binary Aqueous Methanol Mixtures

Solubilities are reported for psoralen and 8-hydroxypsoralen in binary aqueous mixtures at ambient pressure and 298 K, with organic cosolvent methanol. Gibb's energies of transfer of there two compounds are calculated from their solubilities. Transfer chemical potentials of both compounds are stabilised by increasing prapartion of organic cosolvent because of their markedly hydrophobic character. Basic hydrolysis of psoralen and 8-hydroxypsoralen in aqueous methanol mixtures was examines kinetically, yielding Gibb's energies of activation. Derived transfer parameters are used in analysis of kinetic data to describe the influence of solvent composition on initial and transition states. Derived transfer chemical potentials for a single ion such as hydroxide ion lead to an interesting comparison of the effects of extrathermodynamic assumptions, Wells estimates and TATB assumption, on the initial and transition states. The analysis comfirm the important role played by the choice of extrathermodynamic assumption in determining the overall kinetic pattern. We describe misgivings concerning procedures used by wells to derive transfer chemical potential for single ions.     

Syntheses and Crystal Structures of Two Novel Linear Trinuclear Schiff Base Nickel(II) and Cadmium(II) Complexes

Two novel linear trinuclear Schiff base complexes, [Ni{Ni(C₁₇H₁₄Br₂N₂O₂)(NO₃)(H₂O)}₂] · 2MeOH · 2H₂O ( 1 ), and [Cd{Ni(C₂₅H₂₀N₂O₂)(CH₃COO)}₂] ( 2 ), were synthesized and characterized by elemental analyses, infrared spectroscopy, and X‐ray single crystal determinations. There are three bridges across the Ni‐M atom pairs (M is Ni for 1 , and Cd for 2 ) in each complex, involving two phenolate O atoms of a Schiff base ligand (N,N′‐bis(5‐bromosalicylidene)‐1,3‐propanediaminate (BSPD) for 1 and N,N′‐bis(2‐hydroxynaphthylmethenylimino)‐1,3‐propanediaminate (HNPD) for 2 ), and an O‐N‐O moiety of a μ‐nitrato group for 1 or an O‐C‐O moiety of a μ‐acetato group for 2 . In each of the complexes, the central M²⁺ is located on an inversion center and has an octahedral coordination involving four bridging O atoms from two Schiff base ligands in the equatorial plane and one O atom from each bridging nitrate or acetate group in the axial positions. The coordination around the terminal Ni²⁺ ions is also octahedral for 1 , but square pyramidal for 2 . The nitrate or acetate bridges linking the central and terminal metal ions are mutually trans. The Ni···M distances are 3.006(2) Å in 1 , and 3.175(2) Å in 2 .     

Metallomicellar Catalysis: Hydrolysis of PNPP Catalyzed by Copper(II), Zinc(II), Cerium(IV) Complexes with Long Alkyl Pyridine Ligands in CTAB Micellar Solution

A kind of new functional surfactant with substituted long alkyl pyridine was synthesized and its Ce(IV), Zn(II), and Cu(II) complexes were used as hydrolytic metalloenzyme models. The hydrolysis of p-nitrophenyl picolinate (PNPP) catalyzed by the metallomicelles in CTAB micellar solution was investigated at different pH and 30°C. Kinetic parameters of catalytic hydrolysis were obtained by employing the ternary complex kinetic model for metallomicellar catalysis. Effects of the structure of ligands and microenvironment of reaction on the hydrolytic reaction of PNPP have been discussed in detail. From the apparent rate constants (k _obsd ) of the catalytic hydrolysis of PNPP, it can be seen that the catalytic effect of complexes of ligand L² with long hydrocarbon chain was stronger than that of ligand L¹, and complex CuL² showed higher catalytic efficacy on the hydrolytic reaction than those of ZnL² and CeL². However, experiment results in this article showed that ZnL² and CeL² are more susceptible to environment than CuL². The catalytic mechanism was proposed, and the possibly active species for the catalytic hydrolysis of PNPP was determined to be the hydroxylated metal complex.     

DSC Monitoring of the Spin Crossover in Fe(II) Complexes

Heat flow to [Fe(bzimpy)²](ClO₄)₂⋅0.25H₂O complex (bzimpy=2,6-bis(benzimidazol-2-yl)pyridine) (I) was measured between 300 and 460 K by differential scanning calorimetry. This exhibits a well-developed peak characteristic of the first-order phase transitions at temperature 403 K. The enthalpy and entropy of transition from low-spin to high-spin state has been determined to be ΔH=17 kJ mol⁻¹ and ΔS=43.0 Jmol⁻¹ K⁻¹. Heat flow to [Fe(bzimpy_−1H)₂]⋅H₂O complex (bzimpy _−1H=deprotonated bzimpy) (II) was measured between 300 and 580 K. The spin crossover in this system is accompanied with liberation of crystal water on the first heating. To monitor the structural changes during the spin crossover, powder diffraction data have been collected as a function of temperature. This revised version was published online in August 2006 with corrections to the Cover Date.     

Salt Effects on Reactivity of Some Fe(II)-Azo Complexes Catalyzing Disproportionation of Hydrogen Peroxide

Summary. Salt effects on kinetics of oxidation and decomposition of novel low-spin Fe(II) complexes of azo amino acids with hydrogen peroxide have been investigated in aqueous medium. The ligands are derived from amino acids (DL-phenylalanine, DL-tryptophane, histidine, and alanine) and p-nitroso aromatic substituted amines (N,N-dimethyl-4-nitrosoaniline and N,N-diethyl-4-nitrosoaniline). Hydrophobic salts of alkali metal and tetraalkylammonium halides, e.g., KBr, tetrabutylammonium bromide (TBAB), tetraethylammonium bromide (TEAB), and tetramethylammonium bromide (TMAB) have been used. Dilute salt solutions exhibit little effects on the reactivities of oxidation and decomposition of the title compounds. This behaviour would be ascribed to a decrease in the activity coefficients of the reacting species in these solutions. Moreover, these effects support the reported pathway in these reactions occurring via the association of dicationic complexes and undissociated H₂O₂. On the other hand, higher salt concentration shows considerable effects on the reactivities. This behaviour is explained in terms of hydrophilicity of the complexes, ion pair formation, salting out, and substituent effects. Hyrophilic nature of the recent complexes enhances reactivity with increasing [KBr] due to salting out effects in the aqueous medium. Increasing [TBAB] retards reactivity via ion pair formation with the reactants. In the presence of R₄N⁺ ions, the reactivity increases with changing R in the following order; R = C₄H₉ < C₂H₅ < CH₃. The bulky R groups in the added tetraalkylammonium salts, exert medium steric hindrance against the approach of reactants.     

Synthesis of Ahod Moiety of Ralstonin A Using Amino Acid Schiff Base Ni(II)-Complex Chemistry

Reported here is the asymmetric synthesis of N-Boc-protected (2S,3S)-3-amino-2-hydroxyoctadecanoic acid, a component of ralstonin A and ralstoamide A. Key synthetic steps include alkylation of chiral Ni(II) complex of glycine Schiff base, conversion of COOH to keto acid (CO−COOH) and reduction of the carbonyl group to generate α-hydroxy functionality. The structure and absolute configuration of (2S,3S)-N-Boc-3-amino-2-hydroxyoctadecanoic acid was shown to be identical to that of the naturally occurring compound.