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Bond-valence analysis of charge distribution in the spin ladders of [M2Cu2O3]m[CuO2]n-type cuprates with m = n = 1
Abstract
The bond-valence-sum method is used to calculate the hole distribution in the spin ladders of [M2Cu2O3]m[CuO2]n-type cuprates with m = n = 1 (where M are divalent or trivalent cations). It is shown that the appearance of superconductivity in this system is related to hole transfer from chain planes to the spin-ladder plane. The role of the valency and size of the M atoms is discussed in detail.
... 21a Both the color and the charge balance indicate that the Fe ion is +3 state, as also proved also by BVS (bond valence sum) calculation (2.89). 28 The eventually produced crystals of the enzyme-type dioxygenation reaction product sal complex [Fe III L OMe (sal)]· H 2 O (1′) during recrystallization of [Fe II L OMe (fla)] (1) without N 2 protection at room temperature are likely an indication that [Fe II L OMe (fla)] (1) very easily reacts with O 2 even under air at room temperature, which is consistent with its high enzymetype reactivity (see below). ...
With the aim of revealing the catalytic role of atypically coordinated (3His-1Glu) active site mononuclear non-heme Fe(II)-dependent quercetin 2,4-dioxygenase (Fe-2,4-QD) and the electronic effects of the model ligands on the reactivity toward dioxygen, a set of p/m-R-substituted carboxylate-containing ligand-supported Fe(II)-3-hydroxyflavonolate complexes, [FeIILR(fla)] (LRH: 2-{[bis(pyridin-2-ylmethyl)amino]methyl}-p/m-R-benzoic acid; R: p-OMe (1), p-Me (2), m-Br (4), and m-NO2 (5); fla: 3-hydroxyflavonolate), were synthesized and characterized as structural and functional models for the ES (enzyme–substrate) complexes of Fe-2,4-QD. [FeIILR(fla)] show relatively high enzyme-type reactivity (dioxygenative ring opening of the coordinated substrate fla, single-turnover reaction) at low temperatures (30–65 °C). The reaction shows a linear Hammett plot (ρ = −1.21), and electron donating groups enhance the reaction rates. The notable difference on the reactivity can be rationalized from the electronic nature of the substituent in the ligands, which could tune the reactivity via tuning Lewis acidity of the Fe(II) ion, electron density, and the redox potential of fla. The properties and the reactivity show approximately linear correlations between λmax or E1/2 of fla and the reaction rate constant k. This work sheds light not only on understanding of electronic effects of the ligands and the property–reactivity relationship but also on the role of the catalytic reaction by Fe-2,4-QD.
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.
A series of mononuclear Co(II)-flavonolate complexes [Co(II)L(R)(fla)] (L(R)H = 2-{[bis(pyridin-2-ylmethyl)amino]methyl}-p/m-R-benzoic acid; R = p-OMe (1), p-Me (2), m-Br (4), and m-NO2 (5); fla = flavonolate) were designed and synthesized as structural and functional models for the ES (enzyme-substrate) complexes to mimic the active site of the Co(II)-containing quercetin 2,3-dioxygenase (Co-2,3-QD). The metal center Co(II) ion in each complex shows a similar distorted octahedral geometry. The model complexes display high enzyme-type dioxygenation reactivity (oxidative O-heterocyclic ring opening of the coordinated substrate flavonolate) at low temperature, presumably due to the attached carboxylate group in the ligands. The reactivity exhibits a substituent group dependent order of -OMe (1) > -Me (2) > -H (3)14b > -Br (4) > -NO2 (5), and the Hammett plot is linear (ρ = -0.78). This can be explained as the electronic nature of the substituent group in the ligands may influence the conformation and redox potential of the bound flavonolate and finally bring different reactivity. The structures, properties, and reactivity of the model complexes show some dependence on the substituent group in the supporting model ligands, and there is some relationship among them. This study is the first example of a series of structural and functional ES models of Co-2,3-QD, with focus on the effects of the electronic nature of substituted groups and the carboxylate group of the ligands to the dioxygenation reactivity, that will provide important insights into the structure-property-reactivity relationship and the catalytic role of Co-2,3-QD.
A series of flavonolate complexes [MIIL(fla)] (M = Mn, Fe, Co, Ni, Cu, and Zn) are reported as structural and functional ES (enzyme−substrate) models of various MII-containing quercetin 2,3-dioxygenase (2,3-QD), which exhibit notably different high enzyme-like dioxygenation reactivity (oxidative ring-opening of substrate flavonolate) due to the existing carboxylate group in the ligand, providing important insights into the metal ion effects on the enzymatic reactivity of metal-substituted 2,3-QD.
The series of cuprates [M2Cu2O3]m[CuO2](n) (where M are alkaline earth or trivalent metals) consisting of two sublattices was studied. The first members [M2Cu2O3][Cu1+deltaO2+gamma] of the series were synthesized. The structural investigations of the obtained single crystals have shown that the latter structure can be interpreted as the commensurate version of two fragments due to the reconstruction of the Cu-O ribbons. Depending on the chemical composition, the crystals studied were either superconducting (T-c = 85 K) or non-superconducting.
Parameters for the calculation of bond valence (s) from bond length (r) have been determined for 750 atom pairs using the Inorganic Crystal Structure Database. In the relation s = exp((ro - r)/B), it is found that B = 0.37 was consistent with most of the refined values and the 141 most accurate values of ro for this value of B are tabulated. An algorithm for the calculation of ro in terms of position of the two atoms in the periodic table is given. Graphical bond-valence-bond-length correlations are presented for hydrogen bonds.-J.E.C.
A bond valence analysis of the title compound indicates that Cu1+ is found only on the Cu1 site but Cu3+ is found on both sites with a 25% occupancy on Cu2 and a 50% occupancy on Cu1 for x = 7. The analysis also predicts how the electron holes would be distributed over the four crystallographically distinct O atoms if they reside on the O rather than the Cu atoms. Modeling the two end members using Kirchoff-like network equations and a distance-least-squares refinement shows that the bonds most sensitive to Cu oxidation are those formed by Ba and that the structure is subject to internal stresses that are close to a ferroelastic instability at x = 6 and a ferroelectric instability at x = 7.
Magnetization, resistivity and electron spin resonance (ESR) measurements have been performed on single crystals of A10Cu17O29 (A=Ca5.9, Sr3.5, Bi0.3, Pb0.1, Y0.1, Al0.1) of the S=1/2 quasi-one-dimensional system, which has both simple chains and two-leg ladders of copper ions. Substantial hole doping has been achieved in the studied crystals, which led to superconductivity with a high critical temperature (Tc≈80 K). The values of the penetration depth have been estimated for temperatures in the range 30–60 K using the reversible magnetization data. A rough estimation of the Ginzburg–Landau parameter, κ, indicates that the superconductivity in the investigated ladder material should be described as an extreme type-II limit. It has been suggested that the superconductivity in the studied system should be related to the two-leg ladders rather than to the chains.
We have investigated the transport and magnetic properties of the ladder system with two legs Sr14−xAxCu24O41 (A Ba and Ca). For x = 0, both electrical resistivity and thermoelectric power exhibit semiconductive behavior. It becomes more insulating with the increase in x(Ba), while it becomes more conductive with the increase in x(Ca). A metal-insulator transition is found at x(Ca) = 6.0–8.4 from the thermoelectric power measurements. The magnetic susceptibility exhibits a broad peak around 80 K for x = 0. Subtracting the Curie component at low temperatures, the remainder of the susceptibility χs(T) decreases toward zero with decreasing temperature below ∼80 K, which implies the presence of a spin gap. The temperature at which the broad peak shows the maximum changes little through the partial substitution of Ba and Ca for Sr. However, the value of χs(T) decreases with the increase in x(Ba), while it tends to increase with the increase in x(Ca). These properties are discussed in terms of the redistribution of holes between the two different sites of Cu. It appears that the observed spin gap behavior is due to Cu2+ spins in the CuO2 chain rather than in the Cu2O3 plane.
The concept of ‘net holes’ as calculated from the existing crystallographic data via the bond-valence-sum method is utilized for discussing the charge transfer balances in layered cuprates. When focusing on the net holes, i.e. summing the excess positive charge residing either on copper or on oxygen in the CuO2 planes, one notices that in the resulting p(CuO2) parameter all the contributions from the planar Cu–Opl bonds are counteracted and only the vertical bonds from Cu to the apical oxygen (Oapi) and from Opl to the nearest-neighbouring cations have net effects on the hole concentration. Based on this observation, three different ways of doping the CuO2 planes are defined and illustrative examples of each doping route are given. Furthermore, in order to estimate the distribution of holes between inequivalent CuO2 planes in the n≥3 members of homologous series of superconducting cuprates, expressed by MmA2Qn−1CunOm+2+2n±δ or M-m2(n−1)n, the p(CuO2)’s for the individual CuO2 planes were calculated. As a surprising result, the holes were found to be confined in the innermost CuO2 planes in all the systems analyzed. This observation is discussed in comparison with the results obtained for n-type and p-type doped infinite-layer compounds. Finally, the roles of the apical anion and the Cu–O bond length in the CuO2 plane are considered.
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