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.     

Ultrafast photochemical dissociation of an equatorial CO ligand from trans(X,X)-[Ru(X)2(CO)2(bpy)] (X = Cl, Br, I): a picosecond time-resolved infrared spectroscopic and DFT computational study

Ultrafast photochemistry of the complexes trans(X,X)-[Ru(X)(2)(CO)(2)(bpy)] (X = Cl, Br, I) was studied in order to understand excited-state reactivity of equatorial CO ligands, coordinated trans to the 2,2'-bipyridine ligand (bpy). TD-DFT calculations have identified the lowest electronic transitions and singlet excited states as mixed X -->bpy/Ru --> bpy ligand to ligand/metal to ligand charge transfer (LLCT/MLCT). Picosecond time-resolved IR spectroscopy in the region of nu(CO) vibrations has revealed that, for X = Cl and Br, subpicosecond CO dissociation is accompanied by bending of the X-Ru-X moiety, producing a pentacoordinated intermediate trans(X,X)-[Ru(X)(2)(CO)(bpy)]. Final movement of an axial halide ligand to the vacant equatorial position and solvent (CH(3)CN) coordination follows with a time constant of 13-15 ps, forming the photoproduct cis(X,X)-[Ru(X)(2)(CO)(CH(3)CN)(bpy)]. For X = I, the optically populated (1)LLCT/MLCT excited state undergoes a simultaneous subpicosecond CO dissociation and relaxation to a triplet IRuI-localized excited state which involves population of an orbital that is sigma-antibonding with respect to the axial I-Ru-I bonds. Vibrationally relaxed photoproduct cis(I,I)-[Ru(I)(2)(CO)(CH(3)CN)(bpy)] is formed with a time constant of ca. 55 ps. The triplet excited state is unreactive, decaying to the ground state with a 155 ps lifetime. The experimentally observed photochemical intermediates and excited states were assigned by comparing calculated (DFT) and experimental IR spectra. The different behavior of the chloro and bromo complexes from that of the iodo complex is caused by different characters of the lowest triplet excited states.     

Spectroscopic and theoretical studies on axial coordination of bis(pyrrol-2-ylmethyleneamine)phenyl complexes

Metal (M=Zn(II), Ni(II), Cu(II)) complexes with tetradentate Schiff base ligand, bis(pyrrol-2-ylmethyleneamine)phenyl, has been synthesized and characterized by elemental analyses, (1)H NMR, mass spectra and UV-vis spectra. The standard association constants (K(theta)) and the thermodynamic parameters (Delta(r)H(m)(theta),Delta(r)S(m)(theta),Delta(r)G(m)(theta)) for axial coordination of imidazole derivatives with these Shiff base complexes were measured with UV-vis spectrophotometric titration. The decrease of enthalpy is found to be the drive of the axial coordination. Our Schiff base complexes can incorporate two axial ligands, except 2-Et-4-MeIm with two big substituents of great steric bulk according to stoichiometry of 1:1. ZnL displays high selectively binding to imidazole due to the steric bulk effect. Supporting density functional theory (DFT) calculations have been undertaken on B3LYP/6-31G(d) level.     

Six‐coordinate Co2+ with imidazole,NH3, and H2O ligands: Approaching spin crossover

Octahedral, six‐coordinate Co²⁺ can exist in two spin states: S = 3/2 and S = 1/2. The difference in energy between high spin (S = 3/2) and low spin (S = 1/2) is dependent on both the ligand mix and coordination stereochemistry. B3LYP calculations on combinations of neutral imidazole, NH₃, and H₂O ligands show that low‐spin isomers are stabilized by axial H₂O ligands and in structures that also include trans pairs of equatorial NH₃ and protonated imidazole ligands, spin crossover structures are predicted from spin state energy differences. Occupied Co d orbitals from the DFT calculations provide a means of estimating effective ligand strength for homoleptic and mixed ligand combinations. These calculations suggest that in a labile biological system, a spin crossover environment can be created. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

A mononuclear, mixed (salicylato) (nicotinamide) complex of Zn(II) with penta- and hexa-coordination sites: a novel framework structure

The title compound, diaqua(bissalicylato-??O)(bisnicotinamide-??N)zinc(II)][bis(triaqua (monosalicylato-??O)(mononicotinamide-??N)zinc(II)salicylate, includes three Zn(II) ions, four nicotinamide ligands, six salicylate ligands and eight coordinated aqua ligands in the asymmetric unit in complex structure. The geometry around one of the Zn(II) ions is a slightly distorted octahedron, of which the equatorial plane is formed by two carboxylate oxygens and two aqua oxygens, while the axial positions are occupied by two pyridyl nitrogen atoms. The other Zn(II) ions adopt fivefold coordinations with one carboxylate oxygen atom from salicylate ligand, one N atom from nicotinamide ligand and three oxygen atoms from aqua ligands. In addition, there are two salicylate anions in the unit cell that are not coordinated. They provide charge balance as counter-ions in the complex framework.     

Oxo complexes of osmium(IV) formed via dioxygen activation. X-ray structures of [OsX(dcpe)2]PF6 (X = Cl, Br), [OsCl(eta 2-O2)(dcpe)2]BPh4, and [OsCl(O)(dcpe)2]BPh4 (dcpe = 1,2-bis(dicyclohexylphosphino)ethane)

Dioxygen addition to the 16-electron complexes [OsX(P-P)2]+ (3) gives the dioxygen adducts [OsCl(eta 2-O2)(P-P)2]+ (3), which in turn react with HCl gas to give the novel osmium(IV) oxo complexes trans-[OsX(O)(P-P)2]+ (5) (X = Cl, Br; P-P = 1,2-bis(dicyclohexylphosphino)ethane (dcpe), 1,2-bis(diethylphosphino)ethane (depe), 1,2-bis((2R,5R)-2,5-dimethylphospholano)benzene (Me-duphos)). The complexes [OsX(dcpe)2]+ (X = Cl, Br) (3) are studied by X-ray crystallography and are shown to have a "Y-shaped" coordination geometry in the equatorial plane. The X-ray structural analysis of [OsCl(eta 2-O2)(dcpe)2]+ (4a) reveals an exceptionally short O-O bond (1.315(5) A). trans-[OsCl(O)(dcpe)2]+ (5a), the first oxo complex of osmium(IV) investigated crystallographically, exhibits a long Os-O distance of 1.834(3) A. The reactivity of 4 and 5 as oxidants is described. The dioxygen complex 4a transfers one oxygen atom to PPh3 (to give Ph3PO) or oxidizes iodide ions to triiodide ions in the presence of anhydrous HCl. In both reactions, the corresponding oxo species 5a is quantitatively formed as the only metal-containing product. Oxo complexes 5 are surprisingly stable and unreactive toward standard reducing agents such as phosphines.     

An ENDOR and DFT analysis of ‘solvatochromic’ effects in an oxovanadium (IV) complex

The ‘solvatochromic’ effects of a vanadyl salen complex [VO^IV(salen)] in frozen solution was studied by ENDOR and DFT calculations. In a non-coordinating solvent (dichloromethane), both ENDOR and DFT were in excellent agreement on the expected square pyramidal structure, where V^IV=O is positioned out of the equatorial ligand xy-plane (as determined from calculated VH distances). In a coordinating solvent (dimethylformamide), a subtle perturbation from the square pyramidal structure was observed, suggesting that DMF coordinates trans to the vanadyl oxo-ligand, pulling V^IV=O back into the ligand plane. The axial coordination of DMF was confirmed by ENDOR and in the DFT optimised [VO^IV(salen)]–DMF complex.     

Synthesis, structure, and physical properties for a series of trigonal bipyramidal M(II)-Cl complexes with intramolecular hydrogen bonds

A series of transition metal chloro complexes with the tetradentate tripodal tris(2-amino-oxazoline)amine ligand (TAO) have been synthesized and characterized. X-Ray structural analyses of these compounds demonstrate the formation of the mononuclear complexes [M(II)(TAO)(Cl)](+), where M(II) = Cr, Mn, Fe, Co, Ni, Cu and Zn. These complexes exhibit distorted trigonal-bipyramidal geometry, coordinating the metal through an apical tertiary amine, three equatorial imino nitrogen atoms, and an axial chloride anion. All the complexes possess an intramolecular hydrogen-bonding (H-bonding) network within the cavity occupied by the metal-bound chloride ion. The metal-chloride bond distances are atypically long, which is attributed to the effects of the H-bonding network. Nuclear magnetic resonance (NMR) spectroscopy of the Zn complex suggests that the solid-state structures are representative of that observed in solution, and that the H-bonding interactions persist as well. Additionally, density functional theory (DFT) calculations were carried out to probe the electronic structures of the complexes.     

Spectroscopic and computational evaluation of the structure of the high-spin Fe(IV)-oxo intermediates in taurine: alpha-ketoglutarate dioxygenase from Escherichia coli and its His99Ala ligand variant

The Fe(II)- and alpha-ketoglutarate (alphaKG)-dependent dioxygenases activate O2 for cleavage of unactivated C-H bonds in their substrates. The key intermediate that abstracts hydrogen in the reaction of taurine:alphaKG dioxygenase (TauD), a member of this enzyme family, was recently characterized. The intermediate, denoted J, was shown to contain an iron(IV)-oxo unit. Other important structural features of J, such as the number, identity, and disposition of ligands in the Fe(IV) coordination sphere, are not yet understood. To probe these important structural features, a series of models for J with the Fe(IV) ion coordinated by the expected two imidazole (from His99 and His255), two carboxylate (succinate and Asp101), and oxo ligands have been generated by density functional theory (DFT) calculations, and spectroscopic parameters (M?ssbauer isomer shift, quadrupole splitting, and asymmetry parameter, 57Fe hyperfine coupling tensor, and zero field splitting parameters, D and E/D) have been calculated for each model. The calculated parameters of distorted octahedral models for J, in which one of the carboxylates serves as a monodentate ligand and the other as a bidentate ligand, and a trigonal bipyramidal model, in which both carboxylates serve as monodentate ligands, agree well with the experimental parameters, whereas the calculated parameters of a square pyramidal model, in which the oxo ligand is in the equatorial plane, are inconsistent with the data. Similar analysis of the Fe(IV) complex generated in the variant protein with His99, the residue that contributes the imidazole ligand cis to the oxo group, replaced by alanine suggests that the deleted imidazole is replaced by a water ligand. This work lends credence to the idea that the combination of M?ssbauer spectroscopy and DFT calculations can provide detailed structural information for reactive intermediates in the catalytic cycles of iron enzymes.     

Even-numbered metal chain complexes: synthesis, characterization, and DFT analysis of [Ni4(mu4-Tsdpda)4(H2O)2] (Tsdpda2- = N-(p-toluenesulfonyl)dipyridyldiamido), [Ni4(mu4-Tsdpda)4]+, and related Ni4 string complexes

The synthesis and the X-ray structure of two complexes exhibiting a linear chain of four nickel atoms is reported, following Ni4(mu4-phdpda)4 (1), which had been characterized previously. [Ni4(mu4-Tsdpda)4(H2O)2], where H2Tsdpda is N-(p-toluenesulfonyl)dipyridyldiamine (2), is axially coordinated to two water molecules, at variance with 1. One-electron oxidation of 2 resulted in the loss of the axial ligands, yielding [Ni4(mu4-Tsdpda)4]+, [3]+, which was also structurally characterized. Finally, we report the structure of Ni4(mu4-DAniDANy)4 (4), a complex synthesized starting from the new ligand N,N'-bis-p-anisyl-2,7-diamino-1,8-naphthyridine. Magnetic measurements concluded that 4 is diamagnetic, like 1, whereas 2 is antiferromagnetic (-2J(14) = 80 cm(-)(1), using the Heisenberg Hamiltonian H = -2J(14) S(1).S(4)), as are other axially coordinated chains with an odd number of nickel atoms. DFT calculations are reported on these complexes in order to rationalize their electronic structure and their magnetic behavior. The magnetic properties of the [Ni4]8+ complexes are governed by the electronic state of the Ni(II) atoms, which may be either low-spin (S = 0), or high-spin (S = 1). DFT calculations show that the promotion to high spin of two Ni atoms in the chain, either external or internal, depends on the interplay between axial and equatorial coordination. The synergy between axial coordination and the presence of electron-withdrawing toluenesulfonyl substituents in 2 favors the promotion to the high-spin state of the terminal Ni atoms, thus yielding an antiferromagnetic ground state for the complex. This is at variance with complexes 1 and 4, for which the lowest quintet state results from the promotion to high spin of the internal nickel atoms, together with an important ligand participation, and is destabilized by 9 to 16 kcal mol(-1) with respect to the diamagnetic ground state.     

FT-Raman, FT-IR and NMR spectra, vibrational assignments and density functional studies of 1,3-bis(benzimidazol-2-yl)-2-thiapropane ligand and its Zn(II) halide complexes

1,3-bis(benzimidazol-2-yl)-2-thiapropane (L) ligand and its zinc halide ZnX₂ (X = Cl, Br, I) complexes have been synthesized. The compounds were characterized using the elemental analysis, molar conductivity, FT-Raman, FT-IR (mid i.r., far i.r.), ¹H and ¹³C NMR spectra, and quantum chemical calculations performed with Gaussian 03 package program set. The optimized geometries and vibrational frequencies of the ligand and [Zn(L)Cl₂] complex were calculated using the DFT/B3LYP method with a 6–31g(d) basis set. The geometry optimization of [Zn(L)Cl₂] yields a slightly distorted tetrahedral environment around Zn ion, while the molecule clearly reveals the Cs symmetry. The molar conductivity data reveals that the complexes are neutral. The ligand is bidentate, via two of the imine nitrogen atoms in the bis-imidazole ring units, and together with the monodentate coordination of the two halides to the metal centre.     

The X-ray absorption spectroscopic model of the copper(II) imidazole complex ion in liquid aqueous solution: a strongly solvated square pyramid

Cu K-edge extended X-ray absorption fine structure (EXAFS) and Minuit X-ray absorption near-edge structure (MXAN) analyses were combined to evaluate the structure of the copper(II) imidazole complex ion in liquid aqueous solution. Both methods converged to the same square-pyramidal inner coordination sphere [Cu(Im)(4)L(ax)](2+) (L(ax) indeterminate) with four equatorial nitrogen atoms at EXAFS, 2.02 ± 0.01 ?, and MXAN, 1.99 ± 0.03 ?. A short-axial N/O scatterer (L(ax)) was found at 2.12 ± 0.02 ? (EXAFS) or 2.14 ± 0.06 ? (MXAN). A second but very weak axial Cu-N/O interaction was found at 2.9 ± 0.1 ? (EXAFS) or 3.0 ± 0.1 ? (MXAN). In the MXAN fits, only a square-pyramidal structural model successfully reproduced the doubled maximum of the rising K-edge X-ray absorption spectrum, specifically excluding an octahedral model. Both EXAFS and MXAN also found eight outlying oxygen scatterers at 4.2 ± 0.3 ? that contributed significant intensity over the entire spectral energy range. Two prominent rising K-edge shoulders at 8987.1 and 8990.5 eV were found to reflect multiple scattering from the 3.0 ? axial scatterer and the imidazole rings, respectively. In the MXAN fits, the imidazole rings took in-plane rotationally staggered positions about copper. The combined (EXAFS and MXAN) model for the unconstrained cupric imidazole complex ion in liquid aqueous solution is an axially elongated square-pyramidal core, with a weak nonbonded interaction at the second axial coordination position and a solvation shell of eight nearest-neighbor water molecules. This core square-pyramidal motif has persisted through [Cu(H(2)O)(5)](2+), [Cu(NH(3))(4)(NH(3),H(2)O)](2+), (1, 2) and now [Cu(Im)(4)L(ax))](2+) and appears to be the geometry preferred by unconstrained aqueous-phase copper(II) complex ions.     

Crystal structure of [[mu-bis(salicylidene)-1,3-propanediaminato]copper(II)]dibromozinc(II).

The title compound is a double oxygen-bridged dimeric heteronuclear metal complex. The coordination around the Cu atom is distorted square-planar involving two O and two N atoms from the bis(salicylidene)-1,3-propanediamine ligand. The Zn atom in the molecule has a distorted tetrahedral coordination sphere consisting of the two O atoms of the ligand and the two Br atoms. The bridging plane between the metal atoms is not planar.     

Infrared spectroscopy of phenylalanine Ag(I) and Zn(II) complexes in the gas phase

Infrared multiple-photon dissociation (IR-MPD) spectroscopy has been applied to singly-charged complexes involving the transition metals Ag(+) and Zn(2+) with the aromatic amino acid phenylalanine. These studies are complemented by DFT calculations. For [Phe+Ag](+) the calculations favor a tridentate charge solvation N/O/ring structure. The experimental spectrum strongly supports this as the predominant binding geometry and, in particular, rules out a significant presence of the salt-bridge conformation. Zn(2+) forms a deprotonated dimer complex with Phe, [Zn+Phe(2)-H](+), in which the +2 oxidation state serves as a useful biomimetic model for zinc protein sites. A number of low-energy conformations were located, of which the lowest-energy conformer predicted by the calculations involves a Phe ligand deprotonated on the carboxylic acid, while the other Phe ligand is in the tridentate charge solvation conformation. The calculated IR spectrum of this conformer gives a close fit to the experimental spectrum, strongly supporting this as the predominant binding geometry. This most stable calculated complex is characterized by N/ O/ring metal chelation with a tetrahedral-type coordination core of Zn(2+) to N and O of both ligands. Another similar tightly chelated structure shows a square-planar-type coordination core, but this structure is computed to be less stable and gives a less satisfactory match to the experimental spectrum. This preference for the tetrahedral geometry of the Lewis-basic atomic ligands parallels the common Zn(II) coordination geometry in proteins. The number of clearly identifiable peaks resolved in the IR-MPD spectra as well as the much-improved matches between the observed spectra and the DFT-calculated spectra of the most stable geometries compared to previous studies are noteworthy for systems of this size and complexity. These results demonstrate that IR spectroscopy of transition metal-amino acid complexes in combination with DFT calculations is a very powerful structural tool, readily applicable to biomimetic systems that model, for example, the reaction centers of proteins in the solvent-free environment. In addition, we present a novel ion-capturing method for Fourier transform ion cyclotron resonance mass spectrometry which removes the necessity of a buffer gas pulse, while allowing ion trapping at moderate voltages with apparently reduced collisional excitation of the ions.     

Interactions of 1-methylimidazole with UO2(CH3CO2)2 and UO2(NO3)2: structural, spectroscopic, and theoretical evidence for imidazole binding to the uranyl ion

The first definitive high-resolution single-crystal X-ray structure for the coordination of the 1-methylimidazole (Meimid) ligand to UO2(Ac)2 (Ac = CH3CO2) is reported. The crystal structure evidence is confirmed by IR, Raman, and UV-vis spectroscopic data. Direct participation of the nitrogen atom of the Meimid ligand in binding to the uranium center is confirmed. Structural analysis at the DFT (B3LYP) level of theory showed a conformational difference of the Meimid ligand in the free gas-phase complex versus the solid state due to small energetic differences and crystal packing effects. Energetic analysis at the MP2 level in the gas phase supported stronger Meimid binding over H2O binding to both UO2(Ac)2 and UO2(NO3)2. In addition, self-consistent reaction field COSMO calculations were used to assess the aqueous phase energetics of combination and displacement reactions involving H2O and Meimid ligands to UO2R2 (R = Ac, NO3). For both UO2(NO3)2 and UO2(Ac)2, the displacement of H2O by Meimid was predicted to be energetically favorable, consistent with experimental results that suggest Meimid may bind uranyl at physiological pH. Also, log(Knitrate/KAc) calculations supported experimental evidence that the binding stoichiometry of the Meimid ligand is dependent upon the nature of the reactant uranyl complex. These results clearly demonstrate that imidazole binds to uranyl and suggest that binding of histidine residues to uranyl could occur under normal biological conditions.     

Biomimetic zinc funnel complexes based on calix[6]N3ArO ligands: an acid-base switch for guest binding

The coordination chemistry of Zn in an N(3)ArOH environment has been explored. The ligands are based on calix[6]arenes that present two imidazole arms and an amino phenol moiety at the narrow rim. Three different types of complexes have been characterized. One is dicationic with Zn(2+) coordinated to the three nitrogen atoms and to the oxygen of the phenol group of the calix[6]ligand. This complex is very sensitive to exogenous coordinating molecules and exists as a 5-coordinate species due to the endo-complexation of a guest. The second species is a monocationic complex for which the phenol group has been deprotonated. The resulting N(3)ArOZn complex can also bind a guest ligand albeit with a lower affinity than the dicationic complex. The third species is neutral. It can be obtained upon reaction with a base to yield a hydroxo complex or with an anion such as a chloride that coordinates the metal center from the outside of the calixarene cavity. The simultaneous binding of two anionic donors decreases the Zn Lewis acidity, allowing an impressive conformational reorganization of the system. One imidazole arm is released by the metal center. The other one undergoes self-inclusion into the pi-basic calixarene cavity because the low affinity of the metal center for neutral ligand does not allow the endo-coordination of an exogenous guest. Hence, the calix[6]N(3)ArOH-based Zn complexes act as an acid-base switch for guest binding. Several aspects of this system appear reminiscent of Zn-peptidases of the astacin and serralisin families.     

New insights into the coordination chemistry and molecular structure of copper(II) histidine complexes in aqueous solutions

Aqueous solutions of Cu2+/histidine (his) (1:2) have been analyzed in parallel with infrared, Raman, ultraviolet/visible/near-infrared, electron spin resonance, and X-ray absorption spectroscopy in the pH range from 0 to 10. Comprehensive interpretation of the data has been used to extract complementary structural information in order to determine the relative abundance of the different complexes. The formation of six different, partly coexisting species is proposed. Structural proposals from literature have been unambiguously confirmed, refined, or, in several cases, corrected. At highly acidic conditions, Cu2+ and his are present as free ions, but around pH = 2, coordination starts via the deprotonated carboxylic acid group. This results in the intermediate species Cu2+[H3his+(Oc)] and Cu2+[H3his+(Oc)]2. The coordination via Oc is attended with a drop in the pKa value of the other receptor groups resulting in a concomitant conversion to the bidentates Cu2+[H2his0(Oc,Nam)] and Cu2+[H2his0(Oc,Nam)]2, with the latter being dominant at pH = 3.5. Coordination of the imidazole ring begins around pH = 3 and leads to the formation of the mixed ligand complexes Cu2+[H2his0(Oc,Nam)][Hhis-(Oc,Nam,Nim)] and Cu2+[Hhis-(Nam,Nim)][Hhis-(Oc,Nam,Nim)] around pH = 5. It is demonstrated that coordination of the imidazole ring occurs predominantly via the N(pi) atom. At pH > 7, the double-tridentate ligand complex Cu2+[Hhis-(Oc,Nam,Nim)]2 is the major species with the N atoms in the equatorial plane and the O atoms in the axial position. This complex decomposes at pH > 10 into a copper oxide/hydroxide precipitate. The overall results provide a consistent picture of the mechanism that drives the coordination and complex formation of the Cu2+/his system.     

Imidazolate bridged Cu(II)-Cu(II) and Cu(II)-Zn(II) complexes of a terpyridinophane azamacrocycle: a solution and solid state study

The dinuclear Cu2+ and Zn2+ as well as the mixed Cu2+-Zn2+ complexes of a 5,5'-pentaazaterpyridinophane ligand (L) are able to incorporate imidazolate (Im-) as a bridging ligand. The crystal structure of [Cu(2)L(Im)(Br)(H2O)](CF(3)SO(3))(2).3H2O (1) shows one copper coordinated by the three pyridine nitrogens of the terpyridine unit, one nitrogen of the imidazolate bridge (Im-) and one bromide anion occupying the axial position of a distorted square pyramid. The second copper atom is coordinated by the remaining imidazolate nitrogen, the three secondary nitrogens at the centre of the polyamine bridge and one water molecule that occupies the axial position. Magnetic measurements have been performed in the 2.0-300.0 K temperature range. Experimental data could be satisfactorily reproduced by using an isotropic exchange model H = -JS(1)S(2) with J = -52.3 cm(-1) and g = 2.09. Potentiometric studies have provided details of the speciation and stability constants for the mixed Cu2+-L-HIm, Zn2+-L-HIm (HIm = imidazole) and Cu2+-Zn2+-L-HIm systems. The apparent stability constant obtained at pH = 9 for the addition of imidazole to the dinuclear Cu2+ complexes is one of the highest so far reported (log K = 7.5). UV-Vis spectroscopy and paramagnetic NMR data show that imidazole coordinates to the Cu2+ ions as a bridging imidazolate ligand from pH 5 to 10. Electrochemical reduction of the Cu2+-Zn2+-L complex occurs in two successive one-electron per copper ion quasi-reversible steps. The formal potential of the Cu2+-Zn2+-L/Cu+-Zn2+-L couple is close to that of SOD. The IC50 values measured at pH 7.8 by means of the nitro blue tetrazolium method show significant SOD activity for the dinuclear Cu2+ complexes (IC50 = 2.5 microM) and moderate activity for the Cu2+-Zn2+ mixed systems (IC50 = 30 microM).     

Spectroscopic and theoretical study of Cu(II), Zn(II), Ni(II), Co(II) and Cd(II) complexes of glyoxilic acid oxime

The paper presents a detailed experimental and theoretical study of five metal complexes of glyoxilic acid oxime (gaoH2), Cu(gaoH)2(H2O)2 (1), Zn(gaoH)2(H2O)2 (2), Co(gaoH)2(H2O)2 (3), Ni(gaoH)2(H2O)2 (4) and [Cd(gaoH)2(H2O)2].H2O (5). The electronic and vibrational spectra were measured and discussed as to the most sensitive to the M-L binding bands. Two different types of coordination were considered for gaoH- ligand: bidentate through the carboxylic oxygen and oxime nitrogen in 1-4 and mixed bidentate and bridging through the COO group in 5. It is shown that the spectral behavior of the nu(COO) modes can be used to predict bridging ligand coordination. DFT(B3LYP/6-31++G(d,p)) calculations on model compounds: neutral, anionic and radical forms of gao and Cu(gaoH)2, have been carried out to correlate geometries, electronic and vibrational structures. The results obtained were used to assist the electronic and vibrational analysis of the complexes studied. The effect of the metal-ligand interactions (electrostatic and covalent) on the geometry structure of the ligand was investigated.     

Coordinative flexibility of 1,2-bis[1,4,7-triazacyclonon-1-yl]propan-2-ol in mononuclear and binuclear Ni(II) complexes

Reaction of 1,2-bis[1,4,7-triazacyclonon-1-yl]propan-2-ol hexabromide (T(2)PrOH.6HBr) with Ni(ClO(4))(2)[middle dot]6H(2)O and adjustment of the pH to 7 resulted in the crystallization of pink and blue products from the one reaction mixture. The analytical data and X-ray structure determinations establish compositions corresponding to [Ni(T(2)PrOH)]Br(ClO(4))xH(2)O (pink crystals) and [Ni(2)(T(2)PrO)(OH(2))(3)Br]Br(ClO(4))x2H(2)O (blue crystals). A repeat synthesis of the latter yielded the diperchlorate monohydrate [Ni(2)(T(2)PrO)(OH(2))(3)Br](ClO(4))(2)xH(2)O. In the mononuclear complex, the 2-propanol group connecting the two 1,4,7-trizacyclononane (tacn) rings is protonated, the six nitrogen donors from the T(2)PrOH ligand coordinating to a single Ni(II) centre in a distorted octahedral geometry. In the binuclear complexes and, three coordination sites on each distorted octahedral Ni(II) centre are occupied fac by three nitrogen donors from the one tacn ring, the two metal centres being linked by an endogenous alkoxo bridge. A notable common feature of the two identical cations is that for one Ni(II) centre the remaining two sites are occupied by two water ligands, while in the other a bromo ligand replaces one ligated water. Similar binuclear systems have been recently defined [Zn(2)(T(2)PrO)X(H(2)O)(2)](ClO(4))(2)(X = Cl, Br), two complexes that exhibit coordination asymmetry with one pseudo-octahedral and one pseudo-square pyramidal Zn(ii) centre. The weak antiferromagnetic coupling in and is discussed and compared to di-phenoxo-bridged Ni(II) examples.