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A novel mononuclear copper(II)-flavonolate complex [Cu II L(fla)] (LH: 2-{[bis(pyridine-2-ylmethyl)-amino]methyl}-4-methoxy benzoic acid, flaH: flavonol) was prepared as a structural and functional model of the ES (enzyme-substrate) complex of Cu II -containing flavonol 2,4-dioxygenase. Its structure, properties and reactivity towards molecular dioxygen have been investigated in details. The complex shows the best reactivity in the copper(II)-flavonolate ES model system, may it could be contributed to the approximately monodentate coordination of the substrate flavonolate. [ABSTRACT FROM AUTHOR]

The syntheses, structures, and magnetic properties of two series of Cr-Ln heterometallic clusters based on the 2,2'-bipyridine (bipy) ligand are reported; binuclear [LnCr(bipy)2-(µ2-OH)2(H2O)6](ClO4)4·2H2O {Ln = Gd (1Gd) or Dy (1Dy)} and trinuclear [LnCr2(bipy)4(µ2-OH)4(H2O)4](ClO4)5·H2O {Ln = Gd (2Gd), Dy (2Dy), or Tb (2Tb)}. Their syntheses could be controlled by the amount of lanthanide perchlorate used. The binuclear [CrLn] clusters could be formed with 24 equivalents of Ln(ClO4)3, whereas 12 equivalents of Ln(ClO4)3 gave the trinuclear [Cr2Ln] species with a CrLnCr core. The Cr3+ and Ln3+ ions are bridged by µ2-OH groups in the two systems, and the [Cr2Ln] cluster is further assembled into a 1D supramolecular architecture by weak intermolecular p-p interactions. Magnetic analyses reveal that the Cr···Gd interactions in 1Gd and 2Gd are both antiferromagnetic. [ABSTRACT FROM AUTHOR]

In order to get insights into the metal ion effects and the carboxylate effects on enzymatic activity, a series of the carboxylate ligand supported transition metal complexes [MIIL(OAc)] (M = Mn (1), Fe (2), Co (3), Ni (4), Cu (5) and Zn (6); LH = 2-{[bis-(pyridin-2-ylmethyl)amino]methyl}-4-methoxy benzoic acid) were synthesized and characterized as structural and functional models for the active sites of various MII-substituted resting quercetin 2,3-dioxygenases (2,3-QD). Their structures, spectroscopic features, redox properties, as well as the catalytic reactivity toward the substrate flavonol and O2 have been investigated in detail. The model complexes show higher enzymatic reactivities in the catalytic dioxygenation (oxidative ring opening) of the substrate flavonol at lower temperatures (55–100 °C), presumably caused by the carboxylate group in the supporting model ligand, which could lower the redox potential of the bound substrate flavonolate by electron donation. The catalytic reactivity of [MIIL(OAc)] exhibits notable differences and it is in a metal ion dependent order of Co (3) ＞ Ni (4) ＞ Zn (6) ＞ Fe (2) ＞ Mn (1) ＞ Cu (5). The differences in the reactivities among them could be ascribed to the redox potential of the bound substrate flavonolate, which was drastically influenced by the metal ions via tuning the electron density of flavonolate, providing important insights into the metal ion effects and the carboxylate effects on the enzymatic activity of various MII-substituted 2,3-QD. Our model complexes [MIIL(OAc)] are the first examples of a series of structural and functional models of various MII-substituted resting 2,3-QD. [ABSTRACT FROM AUTHOR]

Two inhibitor-containing ‘half-sandwich’ cobalt(II) complexes [(TpPh)Co(X)(CH3OH)m]·nCH3OH ((TpPh)=hydrotris(3-phenylpyrazolyl)borate; 1: X−=N3−, m=1, n=2; 2: X−=NCS−, m=0, n=0) have been synthesized and used as the catalysts in the bicarbonate dehydration reaction. The structures of 1 and 2 were determined by X-ray diffraction analysis, which shows that N3− and NCS− coordinate to the Co(II) ions of 1 and 2, respectively, with the Co&z.sbnd;N bond lengths of 1.992(6) A˚ and 1.901(3) A˚. The coordination geometries of the Co(II) complexes in solution are five-coordinated trigonal bipyramid as revealed by the spectroscopic measurements. The dehydration kinetic measurements of HCO3− are performed by the stopped-flow techniques at pH<7.9. The apparent dehydration rate constant kobs varies linearly with Co(II) complex and H+ concentrations, respectively, and the catalytic activity of 2 is lower than that of 1. The aqua Co(II) complex must be the reactive catalytic species in the catalyzed dehydration reaction and the rate-determining step is the substitution of the labile water molecule by HCO3−. The kobs values increase with increasing reaction temperature, and the large negative entropy of activation also indicates the associative activation mode. The inhibition ability of NCS− is stronger than that of N3−, which can be rationalized by the decreases in the Co&z.sbnd;N(N3−/NCS−) bond lengths and effective atomic charges of the Co(II) ions based on the X-ray crystallographic data and theoretical calculations in this work. [Copyright &y& Elsevier]

This communication reports the dehydration kinetic measurements of HCO3− catalyzed by three inhibitor-containing half-sandwich nickel(II) complexes [TpMe]NiX ([TpMe]=hydrotris(3,5-dimethylpyrazolyl)borate; X−=η2-NO2− (1), N3− (2), NCS− (3)) by employing the stopped-flow techniques at pH <7.9. The five- or six-coordinated aqua complexes must be the reactive catalytic species in the catalyzed bicarbonate dehydration reaction and the rate-determining step is the substitution of the labile water molecule by HCO3−. The dehydration rate constants k of the rate-determining step decrease in the order: 1 (186 M−1 s−1) > 2 (169 M−1 s−1) > 3 (120 M−1 s−1), which is correlated with an increase of the activation energy barrier heights, this being the origin of the variation in the inhibition abilities among the different inorganic anions. [Copyright &y& Elsevier]