Spectroscopic and kinetic studies of perturbed trinuclear copper clusters: the role of protons in reductive cleavage of the O-O bond in the multicopper oxidase Fet3p

The multicopper oxidase Fet3p couples four 1e(-) oxidations of substrate to the 4e(-) reduction of O2 to H2O. Fet3p uses four Cu atoms to accomplish this reaction: the type 1, type 2, and coupled binuclear type 3 sites. The type 2 and type 3 sites together form a trinuclear Cu cluster (TNC) which is the site of O2 reduction. This study focuses on mutants of two residues, E487 and D94, which lie in the second coordination sphere of the TNC and defines the role that each plays in the structural integrity of the TNC, its reactivity with O2, and in the directional movement of protons during reductive cleavage of the O-O bond. The E487D, E487A, and D94E mutants have been studied in the holo and type 1 depleted (T1D) forms. Residue E487, located near the T3 center, is found to be responsible for donation of a proton during the reductive cleavage of the O-O bond in the peroxide intermediate and an inverse kinetic solvent isotope effect, which indicates that this proton is already transferred when the O-O bond is cleaved. Residue D94, near the T2 site, plays a key role in the reaction of the reduced TNC with O2 and drives electron transfer from the T2 Cu to cleave the O-O bond by deprotonating the T2 Cu water ligand. A mechanism is developed where these second sphere residues participate in the proton assisted reductive cleavage of the O-O bond at the TNC.     

Variable-temperature, variable-field magnetic circular dichroism studies of tris-hydroxy- and mu3-oxo-bridged trinuclear Cu(II) complexes: evaluation of proposed structures of the native intermediate of the multicopper oxidases

Multicopper oxidases catalyze the 4e- reduction of O2 to H2O. Reaction of the fully reduced enzyme with O2 produces the native intermediate (NI) that consists of four oxidized Cu centers, three of which form a trinuclear cluster site, all bridged by the product of full O2 reduction. The most characteristic feature of NI is the intense magnetic circular dichroism pseudo-A feature (a pair of temperature-dependent C-terms with opposite signs) associated with O --> Cu(II) ligand-to-metal charge transfer (LMCT) that derives from the strong Cu-O bonds in the trinuclear site. In this study, the two most plausible Cu-O structures of the trinuclear site, the tris-mu2-hydroxy-bridged and the mu3-oxo-bridged structures, are evaluated through spectroscopic and electronic structure studies on relevant model complexes, TrisOH and mu3O. It is found that the two components of a pseudo-A-term for TrisOH are associated with LMCT to the same Cu that are coupled by a metal-centered excited-state spin-orbit coupling (SOC), whereas for mu3O they are associated with LMCT to different Cu centers that are coupled by oxo-centered excited state SOC. Based on this analysis of the two candidate models, only the mu3-oxo-bridged structure is consistent with the spectroscopic properties of NI. The Cu-O sigma-bonds in the mu3-oxo-bridged structure would provide the thermodynamic driving force for the 4e- reduction of O2 and would allow the facile electron transfer to all Cu centers in the trinuclear cluster that is consistent with its involvement in the catalytic cycle.     

Decay of the peroxide intermediate in laccase: reductive cleavage of the O-O bond.

Laccase is a multicopper oxidase that contains four Cu ions, one type 1, one type 2, and a coupled binuclear type 3 Cu pair. The type 2 and type 3 centers form a trinuclear Cu cluster that is the active site for O(2) reduction to H(2)O. To examine the reaction between the type 2/type 3 trinuclear cluster and dioxygen, the type 1 Cu was removed and replaced with Hg(2+), producing the T1Hg derivative. When reduced T1Hg laccase is reacted with dioxygen, a peroxide intermediate (P) is formed. The present study examines the kinetics and mechanism of formation and decay of P in T1HgLc. The formation of P was found to be independent of pH and did not involve a kinetic solvent isotope effect, indicating that no proton is involved in the rate-determining step of formation of P. Alternatively, pH and isotope studies on the decay of P revealed that a proton enhances the rate of decay by 10-fold at low pH. This process shows an inverse k(H)/k(D) kinetic solvent isotope effect and involves protonation of a nearby residue that assists in catalysis, rather than direct protonation of the peroxide. Decay of P also involves a significant oxygen isotope effect (k(16)O(2)/k(18)O(2)) of 1.11 +/- 0.05, indicating that reductive cleavage of the O-O bond is the rate-determining step in the decay of P. The activation energy for this process was found to be approximately 9.0 kcal/mol. The exceptionally slow rate of decay of P is explained by the fact that this process involves a 1e(-) reductive cleavage of the O-O bond and there is a large Franck-Condon barrier associated with this process. Alternatively, the 2e(-) reductive cleavage of the O-O bond has a much larger driving force which minimizes this barrier and accelerates the rate of this reaction by approximately 10(7) in the native enzyme. This large difference in rate for the 2e(-) versus 1e(-) process supports a molecular mechanism for multicopper oxidases in which O(2) is reduced to H(2)O in two 2e(-) steps.     

Nature of the intermediate formed in the reduction of O(2) to H(2)O at the trinuclear copper cluster active site in native laccase

The multicopper oxidases contain at least four copper atoms and catalyze the four-electron reduction of O(2) to H(2)O at a trinuclear copper cluster. An intermediate, termed native intermediate, has been trapped by a rapid freeze-quench technique from Rhus vernicifera laccase when the fully reduced form reacts with dioxygen. This intermediate had been described as an oxygen-radical bound to the trinuclear copper cluster with one Cu site reduced. XAS, however, shows that all copper atoms are oxidized in this intermediate. A combination of EXAFS, multifrequency EPR, and VTVH MCD has been used to understand how this fully oxidized trinuclear Cu cluster relates to the fully oxidized resting form of the enzyme. It is determined that in the native intermediate all copper atoms of the cluster are bridged by the product of full O(2) reduction. In contrast, the resting form has one copper atom of the cluster (the T2 Cu) magnetically isolated from the others. The native intermediate decays to the resting oxidized form with a rate that is too slow to be in the catalytic cycle. Thus, the native intermediate appears to be the catalytically relevant fully oxidized form of the enzyme, and its role in catalysis is considered.     

Basic and applied features of multicopper oxidases, CueO, bilirubin oxidase, and laccase

Multicopper oxidases (MCOs) such as CueO, bilirubin oxidase, and laccase contain four Cu centers, type 1 Cu, type II Cu, and a pair of type III Cu's in a protein molecule consisting of three domains with homologous structure to cupredoxin containing only type I Cu. Type I Cu mediates electron transfer between the substrate and the trinuclear Cu center formed by a type II Cu and a pair of type III Cu's, where the final electron acceptor O(2) is converted to H(2)O without releasing activated oxygen species. During the process, O(2) is reduced by MCOs such as lacquer laccase and bilirubin oxidase; the reaction intermediate II with a possible doubly OH(-)-bridged structure in the trinuclear Cu center has been detected. The preceding reaction intermediate I has been detected by the reaction of the lacquer laccase in a mixed valence state, at which type I Cu was cuprous and the trinuclear Cu center was fully reduced, and by the reaction of the Cys --> Ser mutant for the type I Cu site in bilirubin oxidase and CueO. An acidic amino acid residue located adjacent to the trinuclear Cu center was proved to function as a proton donor to these reaction intermediates. The substrate specificity of MCO for organic substrates is produced by the integrated effects of the shape of the substrate-binding site and the specific interaction of the substrate with the amino acid located adjacent to the His residue coordinating to the type I Cu. In contrast, the substrate specificity of the cuprous oxidase, CueO, is produced by the segment covering the Cu(I)-binding site so as to obstruct the access of organic substrates. Truncating the segment spanning helix 5 to helix 7 greatly reduced the specificity of CueO for Cu(I) and prominently enhanced the low oxidizing activity for the organic substrates, indicating the success of protein engineering to modify the substrate specificity of MCO.     

Syntheses and structures of isoindoline complexes of Zn(II) and Cu(II): an unexpected trinuclear Zn(II) complex

Deprotonation of the tridentate isoindoline ligand 1,3-bis[2-(4-methylpyridyl)imino]-isoindoline, 4'-MeLH, and reaction with hydrated zinc(II) perchlorate produces an unexpected trinuclear Zn(II) complex, [Zn(3)(4'-MeL)(4)](ClO(4))(2).5H(2)O (1), whereas reaction with hydrated copper(II) perchlorate in methanol produces the expected mononuclear product, [Cu(4'-MeL)(H(2)O)(2)]ClO(4) (2). X-ray diffraction shows that the trinuclear Zn(II) complex (1) contains a linear zinc backbone, and the arrangement of ligands about the outer chiral zinc(II) atoms is helical. The two terminal zinc ions exhibit approximate C(2) site symmetry, with tetrahedral coordination by two pyrrole and two pyridyl nitrogen atoms of the potentially tridentate isoindoline ligands. The central zinc ion exhibits approximate tetrahedral symmetry, with coordination by four pyridyl nitrogen atoms of four different isoindoline ligands. Pyridyl-pyrrole intramolecular pi-stacking interactions contribute to the stability of the trinuclear cation. The structure of the mononuclear copper(II) complex cation in 2 is best described as a distorted trigonal bipyramid. The isoindoline anion binds Cu(II) in both axial positions and one of the equatorial positions; water molecules occupy the other two equatorial positions.     

Assembly of trinuclear and tetranuclear building units of Cu2+ towards two 1D magnetic systems: synthesis and magneto-structural correlations

Two new 1D coordination polymers, [Cu(3)(μ(3)-OH)(ppk)(3)(μ-N(CN)(2))(OAc)](n) (1) and {[Cu(4)(pdmH)(2)(pdm)(2)(μ(2)-OH)(H(2)O)]·ClO(4)}(n) (2) based on two different blocking ligands phenyl-2-pyridylketoxime (ppk) and pyridine-2,6-dimethanol (pdmH(2)) have been synthesized and were characterized by X-ray single crystal structural analysis. In compound 1, the hydroxido-bridged trinuclear core, {Cu(3)(μ(3)-OH)(ppk)(3)(OAc)}, acts as secondary building units and are connected by the N(CN)(2)(-) anions resulting in a one dimensional (1D) coordination polymer. The 1D coordination chains undergo π-π interactions giving rise to a 3D supramolecular framework. In compound 2, tetrameric [Cu(4)(pdmH)(2)(pdm)(2)(H(2)O)](2+) cores are linked via hydroxido groups forming a zigzag 1D coordination chain where non-coordinated ClO(4)(-) ions are intercalated between the chains. Variable temperature magnetic susceptibility study of suggests that Cu(II) ions in the trinuclear Cu(3)(μ(3)-OH) cores are antiferromagnetically coupled with J = -459.7 cm(-1) and g = 2.11 and the trinuclear cores are further weakly coupled antiferromagnetically (zj' = -5.25 cm(-1)) through the N(CN)(2)(-) bridging ligand. Investigation of the magnetic properties of reveals that Cu(II) ions are coupled antiferromagnetically in the tetranuclear core with J = -27.1 cm(-1) and g = 2.17; the Cu(II)(4) building units are further coupled antiferromagnetically with zj' = -9.65 cm(-1). The experimental magnetic behaviours of 1 and 2 are correlated by first principle DFT calculations which provide a qualitative understanding of the origin of antiferromagnetic interactions in both cases.     

Trinuclear, antiferromagnetically coupled Cu(II) complex with an EPR spectrum of mononuclear Cu(II): effect of alcoholic solvents

A rigid trinuclear copper pyrazolato framework supports the solvolytic exchange of mu3-X by mu3-OR ligands (X = Cl and Br; R = alkyl group), converting the trinuclear ferromagnetically coupled S = 3/2 system to antiferromagnetically coupled S = 1/2 in the solid state. In contrast, we propose that, in alcoholic solutions, solvolysis results in unsymmetrical coordination of the Cu3 cluster, magnetically decoupling one Cu center from the other two. This disguises the intact triangular Cu(II)3 system as a mononuclear Cu(II) complex in its electron paramagnetic resonance spectrum.     

Self-assembly and characterization of copper 3,4-pyridinedicarboxylate complexes based on a variety of polynuclear hydroxo clusters

Self-assembly of copper(ii) ion, 3,4-pyridinedicarboxylate (PDC), and 1,10-phenanthroline (phen) under basic conditions at 100 °C affords four PDC linked copper(ii) complexes, [Cu(4)(μ(2)-OH)(3)(μ(3)-OH)(PDC)(phen)(4)](n)·n(PDC)·11.5 nH2O (1), [Cu(4)(μ(2)-OH)(2)(μ(3)-OH)(2)(PDC)(phen)(4)](n)·n(PDC)· 11.5 nH(2)O (2), [Cu(8)(μ(2)-OH)(2)(μ(3)-OH)(6)(PDC)(2)(phen)(8)]·2(PDC)·23 H(2)O (3), and [Cu(3.5)(μ(2)-OH)(3) (PDC)(2)(phen)](n) (4). 1-4 are copper hydroxo complexes, and 1, 2 and 3 co-crystallized from the one-pot reaction. X-ray single crystal diffraction analyses indicate that complexes 1 and 2 are linkage isomers and contain tetranuclear copper cluster cores with different geometry, and that PDC links the cluster core to form a one-dimensional chain. Complex 3 is a discrete step-like octanuclear copper hydroxo cluster complex. The involvement of hydroxo and phen in the coordination makes some coordination sites of PDC idle, which leads to rich hydrogen bonds and π-π interactions in complexes 1, 2 and 3. Complex 4 contains two types of copper hydroxo cluster cores: chair-like tetranuclear and linear trinuclear units, and the cluster cores are linked by PDC to a double-layer metal-organic framework. Magnetic properties of 1, 3 and 4 were investigated. The results reveal that complexes 3 and 4 exhibit strong antiferromagnetic interactions whereas ferromagnetic coupling is predominant for complex 1. The magnetic properties are analyzed in connection with their structures.     

Ferromagnetic Behavior of an Uncommon Trinuclearcopper(II) Coordination Polymer Based on Tartarate and 1,2-bis(4-Pyridyl)ethane Linker

A new Cu(II) based metal-organic framework (MOF) having formula [Cu₃(H₂Tar)(Tar)-(H₂O)(Bpa)] · 3H₂O (I) (H₄Tar = tartaric acid, Bpa = 1,2-bis(4-pyridyl)ethane) has been characterized by elemental analysis, FT-IR spectra, thermal analysis, and single-crystal X-ray diffraction (CIF file CCDC no. 1575136). In I, the Cu(II) center are connected by four symmetry-related tartrate ligands into a 2D layer encapsulating trinuclear cluster, which are further bridged by Bpa molecules into a 3D framework. Complex I exhibits strong ferromagnetic behavior between metal centers.     

Cyanide-Bridged Fe(III)-Cu(II) Complexes: Jahn-Teller Isomerism and Its Influence on the Magnetic Properties

We report here the synthesis and characterization of four dinuclear cyanide-bridged Fe(III)-Cu(II) complexes, based on a tetra- or a pentadentate bispidine ligand (L(1) or L(2), respectively; bispidines are 3,7-diazabiyclo[3.3.1]nonane derivatives) coordinated to the Cu(II) center, and a tridentate bipyridineamide (bpca) coordinated to the low-spin Fe(III) site, with cyanide groups completing the two coordination spheres, one of them bridging between the two metal ions. The four structurally characterized complexes [{Fe(bpca)(CN)(3)}{Cu(L(1)·H(2)O)}]BF(4), [{Fe(bpca)(CN)(3)}{Cu(L(2))}][Fe(bpca)(CN)(3)]·5H(2)O, [{Fe(bpca)(CN)(3)}{Cu(L(2)·MeOH)}]PF(6)·MeOH·H(2)O, and [{Fe(bpca)(CN)(3)}{Cu(L(2))}]PF(6)·2H(2)O belong to different structural isomers. The most important differences are structurally and electronically enforced (direction of the pseudo-Jahn-Teller mode) strong or weak interactions of the copper(II) center with the cyanide bridge. The related strength of the magnetic coupling of the two centers is analyzed with a combination of experimental magnetic, electron paramagnetic resonance (EPR), electronic spectroscopic data together with a ligand-field theory- and density functional theory (DFT)-based analysis.     

Four New Cu（Ⅱ） Complexes with Benzotriazole-based Ligands： Syntheses,Crystal Structures and Theoretical Investigations

Four new Cu（Ⅱ） complexes with two benzotriazole-based ligands, [Cu2（L^1）2（NO3）2]· 2H2O （1）, [Cu2（L^1）2]·2ClO4·2H2O （2）, [Cu2（HL^2）2（NO3）4]·2CH3COCH3 （3） and [Cu（HL^2）2（Cl）]·Cl·2CH2Cl2 （4）, where HL^1 = 1,3-bis（benzotriazol-2-yl）-2-propanol and HL^2 = 1,3-bis（benzotriazol-1-yl）-2-propanol, were synthesized and structurally characterized by elemental analyses, IR and single-crystal X-ray diffraction analyses. It is revealed that complexes 1～3 have dinuclear structures, while 4 possesses a one-dimensional （1-D） chain structure, which extends in two orthogonal orientations. In 1～4, the coordination numbers of Cu（Ⅱ） centers range from four to six, which may be attributed to the different geometries and coordination abilities of the ligands and anions. The L^1 ligand in complexes 1 and 2 adopts a tridentate di-chelating coordination mode, whereas ligand HL^2 in complexes 3 and 4 has a bidentate bridging coordination mode. The different coordination modes of these two ligands may be explained by the different charges of nitrogen donor atoms in the benzotriazole ring, which has been investigated by density functional theory （DFT） calculations.     

Multireference ab initio calculations on reaction intermediates of the multicopper oxidases

The multicopper oxidases (MCOs) couple the four-electron reduction of dioxygen to water with four one-electron oxidations of various substrates. Extensive spectroscopic studies have identified several intermediates in the MCO catalytic cycle, but they have not been able to settle the structures of three of the intermediates, viz. the native intermediate (NI), the peroxy intermediate (PI), and the peroxy adduct (PA). The suggested structures have been further refined and characterized by quantum mechanical/molecular mechanical (QM/MM) calculations. In this paper, we try to establish a direct link between theory and experiment, by calculating spectroscopic parameters for these intermediates using multireference wave functions from the multistate CASPT2 and MRDDCI2 methods. Thereby, we have been able to reproduce low-spin ground states (S = 0 or S = 1/2) for all the MCO intermediates, as well as a low-lying (approximately 150 cm-1) doublet state and a doublet-quartet energy gap of approximately 780 cm-1 for the NI. Moreover, we reproduce the zero-field splitting (approximately 70 cm-1) of the ground 2E state in a D3 symmetric hydroxy-bridged trinuclear Cu(II) model of the NI and obtain a quantitatively correct quartet-doublet splitting (164 cm-1) for a mu3-oxo-bridged trinuclear Cu(II) cluster. All results support the suggestion that the NI has an O2- atom in the center of the trinuclear cluster, whereas both the PI and PA have an O22- ion in the center of the cluster, in agreement with the QM/MM results and spectroscopic measurements.     

Hydrothermal reaction of Cu(II)/pyrazine-2,3,5-tricarboxylic acid and characterization of the copper(II) complexes

Four copper(II) complexes [Cu3(PZHD)2(2,2'-bpy)2(H2O)2].3H2O (1), [Cu3(DHPZA)2(2,2'-bpy)2] (2), [Cu(C2O4)phen(H2O)].H2O (3), and [Cu3(PZTC)2(2,2'-bpy)2].2H2O (4) were synthesized by hydrothermal reactions, in which the complexes 1-3 were obtained by the in situ Cu(II)/H3PZTC reactions (PZHD3- = 2-hydroxypyrazine-3,5-dicarboxylate, 2,2'-bpy = 2,2'-bipyridine, DHPZA3- = 2,3-dihydroxypyrazine-5-carboxylate, C2O42- = oxalate, phen = 1,10-phenanthroline, and H3PZTC = pyrazine-2,3,5-tricarboxylic acid). The Cu(II)/H3PZTC hydrothermal reaction with 2,2'-bpy, without addition of NaOH, results in the formation of complex 4. The complexes 1-4 and transformations from H3PZTC to PZHD3-, DHPZA3-, and C2O4(2-) were characterized by single-crystal X-ray diffraction and theoretical calculations. In the complexes 1, 2, and 4, the ligands PZHD3-, DPHZA3-, and PZTC3- all show pentadentate coordination to Cu(II) ion forming three different trinuclear units. The trinuclear units in 1 are assembled by hydrogen-bonding and pi-pi stacking to form a 3D supramolecular network. The trinuclear units in 2 acting as building blocks are connected by the carboxylate oxygen atoms forming a 2D metal-organic framework (MOF) with (4,4) topology. While the trinuclear units in 4 are linked together by the carboxylate oxygen atoms to form a novel 2D MOF containing right- and left-handed helical chains. The theoretical characterization testifies that electron transfer between OH- and Cu2+ and redox of Cu 2+ and Cu+ are the most important processes involved in the in situ copper Cu(II)/H3PZTC reactions, forming complexes of 1-3.     

Copper(II) complexes with new polypodal ligands presenting axial-equatorial phenoxo bridges {2-[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol, 2-[(bis(2-pyridylmethyl)amino)methyl]-4-methyl-6-(methylthio)phenol}: examples of ferromagnetically coupled bi- and trinuclear copper(II) complexes

Two new ligands, 2-[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol (HL) and 2-[(bis(2-pyridylmethyl)amino)methyl]-4-methyl-6-(methylthio)phenol (HSL), were synthesized and were used to prepare the trinuclear copper(II) complex {[CuSL(Cl)]2Cu}(PF6)2.H2O (1) and the corresponding binuclear complexes [Cu2(SL)2](PF6)2 (2) and [Cu2L2](PF6)2 (3). The crystal structure of 1 shows two different coordination environments: two square base pyramidal centers (Cu1 and Cu1a, related by a C2 axes), acting as ligands of a distorted square planar copper center (Cu2) by means of the sulfur atom of the SCH3 substituent and the bridging phenoxo oxygen atom of the ligand (Cu2-S = 2.294 A). Compounds 2 and 3 show two equivalent distorted square base pyramidal copper(II) centers, bridged in an axial-equatorial fashion by two phenoxo groups, thus defining an asymmetric Cu2O2 core. A long copper-sulfur distance measured in 2 (2.9261(18) A) suggests a weak bonding interaction. This interaction induces a torsion angle between the methylthio group and the phenoxo plane resulting in a dihedral angle of 41.4(5) degrees. A still larger distortion is observed in 1 with a dihedral angle of 74.0(6) degrees. DFT calculations for 1 gave a ferromagnetic exchange between first neighbors interaction, the calculated J value for this interaction being +11.7 cm-1. In addition, an antiferromagnetic exchange for 1 was obtained for the second neighbor interaction with a J value of -0.05 cm-1. The Bleaney-Bowers equation was used to fit the experimental magnetic susceptibility data for 2 and 3; the best fit was obtained with J values of +3.4 and -16.7 cm-1, respectively. DFT calculations for 2 and 3 confirm the nature and the values of the J constants obtained by the fit of the experimental data. ESR and magnetic studies on the reported compounds show a weak exchange interaction between the copper(II) centers. The low values obtained for the coupling constants can be explained in terms of a poor overlap between the magnetic orbitals, due to the axial-equatorial phenoxo bridging mode observed in these complexes.     

Highly efficient phosphodiester hydrolysis promoted by a dinuclear copper(II) complex

The interaction of Cu(II) with the ligand tdci (1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol) was studied both in the solid state and in solution. The complexes that were formed were also tested for phosphoesterase activity. The pentanuclear complex [Cu(5)(tdciH(-2))(tdci)(2)(OH)(2)(NO(3))(2)](NO(3))(4).6H(2)O consists of two dinuclear units and one trinuclear unit, having two shared copper(II) ions. The metal centers within the pentanuclear structure have three distinct coordination environments. All five copper(II) ions are linked by hydroxo/alkoxo bridges forming a Cu(5)O(6) cage. The Cu-Cu separations of the bridged centers are between 2.916 and 3.782 A, while those of the nonbridged metal ions are 5.455-5.712 A. The solution equilibria in the Cu(II)-tdci system proved to be extremely complicated. Depending on the pH and metal-to-ligand ratio, several differently deprotonated mono-, di-, and trinuclear complexes are formed. Their presence in solution was supported by mass, CW, and pulse EPR spectroscopic study, too. In these complexes, the metal ions are presumed to occupy tridentate [O(ax),N(eq),O(ax)] coordination sites and the O-donors of tdci may serve as bridging units between two metal ions. Additionally, deprotonation of the metal-bound water molecules may occur. The dinuclear Cu(2)LH(-3) species, formed around pH 8.5, provides outstanding rate acceleration for the hydrolysis of the activated phosphodiester bis(4-nitrophenyl)phosphate (BNPP). The second-order rate constant of BNPP hydrolysis promoted by the dinuclear complex (T = 298 K) is 0.95 M(-1) s(-1), which is ca. 47600-fold higher than that of the hydroxide ion catalyzed hydrolysis (k(OH)). Its activity is selective for the phosphodiester, and the hydrolysis was proved to be catalytic. The proposed bifunctional mechanism of the hydrolysis includes double Lewis acid activation and intramolecular nucleophilic catalysis.     

Metal-dependent assembly of a helical-[Co3L3] cluster versus a meso-[Cu2L2] cluster with O,N,N',O'-Schiff base ligand: structures and magnetic properties

Self-assembly of a tetradentate ligand, N, N'-bi(salicylidene)-2,6-pyridinediamine (H 2L), with Cu(II) or Co(II), affords a dinuclear [Cu 2L 2] complex ( 1) or a trinuclear [Co 3L 3] complex ( 2), which were characterized by the single crystal X-ray diffraction study. The coordination geometry of the Cu (II) centers in 1 is between square planar and tetrahedral, with the ligand adopting a cis-cis conformation to give a centrally symmetric structure, which can be regarded as a mesocate. However, the coordination geometry of Co (II) centers in 2 is distortedly tetrahedral, and the ligand adopts a cis-trans conformation. The whole complex of 2 is of a pseudo- C 3 symmatrical, torus-like structure, which can be regarded as a circular helicate. Both the mesocate and the helicate exhibit expanded supramolecular structures due to elaborate intercomplex pi-stacking interactions. These two complexes were also characterized by element analysis, IR spectra, and TGA. To verify the stability of 2, ESI-MS was carried out on both the crystal and the powdered samples. Variable temperature magnetic susceptibility measurements reveal that both 1 and 2 display antiferromagnetic properties. DFT calculations were carried out on 1 to verify the antiferromagnetic coupling between intracluster metal centers.     

一类新的多铜酶的模型化合物

本文报道一中新的.柔顺的十齿三核配体N,N,N'N',N",N"-六[2'-苯并咪唑甲基]三乙基四胺(TTHB)及其几种含铜配合物的合成,组常和性质,推测配合物有一对抗铁磁性偶合的Cu(Ⅱ)离子和一个处于畸变四方锥配位几何环境中未偶合的Cu(Ⅱ)离子,这类配合物可作多铜酶三核簇模型化合物.