Au(III)-complexes of the alanyl-containing peptides glycylalanine and glycylalanylalanine - Synthesis, spectroscopic and structural characterization

As part of an investigation on the coordination ability of peptides, the dipeptide glycylalanine (H-Gly-Ala-OH), tripeptide glycylalanylalanine (H-Gly-Ala-Ala-OH) and their Au(III)-complexes have been characterized structurally. The quantum chemical calculations and linear-dichroic infrared (IR-LD) spectroscopy predict structures of the compound studied, which are compared with a single crystal X-ray diffraction of H-Gly-Ala-OH. The coordination processes with Au(III) are supported by data for ¹H NMR, ESI-MS, HPLC-MS-MS, TGV and DSC methods. The [Au(Gly-Ala)H₋₁Cl] and [Au(Gly-Ala-Ala)H₋₂] · 2H₂O complexes are formed via -NH₂, N⁻amide/s and groups of the peptides. One Cl⁻ ion is attached to the metal center as terminal ligand in the first complex. In both cases a near to square-planar geometry of the chromophors AuN₂OCl and AuN₃O is yielded.     

Metal-metal stacking patterns between and with [Pt(tpy)X] cations

A comparative study of metallophilic interactions of [Pt(tpy)X]⁺ cations (tpy = 2,2′:6′,2″-terpyridine) in the presence of two different types of anions, (i) []⁻ anions that form double salts and (ii) simple p-block anions, is reported. Single-crystal X-ray diffraction data, solution-state ¹⁹⁵Pt NMR spectra, and variable temperature solid-state luminescence spectra are reported. Three [Pt(tpy)Cl]Y derivatives (Y = SbF₆, 1, SbF₆·CH₃CN, 4, PF₆, 2) and the [Pt(tpy)Br]PF₆ analog, 3, as well as two new double salts [Pt(tpy)CN][Au(CN)₂], 5, and [Pt(tpy)CN]₂[Au(C₆F₅)₂](PF₆), 6, have been synthesized and characterized. Structural analysis shows consistent patterns in Pt···Pt interactions that vary slightly depending on the coordinating halogen or pseudo-halogen X, counter anion Y, and lattice solvent. Metallophilic interactions are seen between [Pt(tpy)X]⁺ cations with all types of X ligands, but only with π-accepting X′ ligands from []⁻ anions are Pt?Au metallophilic interactions seen to be favored over Pt?Pt interactions. The [Au(CN)₂]⁻ anion consistently forms Pt···Au metallophilic contacts, unlike [Au(C₆F₅)₂]⁻. The ¹⁹⁵Pt NMR chemical shifts are ∼−2750 ppm for π-donor ligands and near −3120 ppm for π-acceptor ligands in [Pt(tpy)X]PF₆ compounds. Luminescence data show an unusual blue shift in [Pt(tpy)CCPh][Au(C₆F₅)₂] versus [Pt(tpy)CCPh]PF₆ ascribed to an intermolecular charge transfer.     

Multinuclear nuclear magnetic resonance and X-ray crystallographic investigation of some mixed ligand alkylisocyanide platinum(II) complexes

Single crystal X-ray diffraction structure determination of tetra-n-butylammonium tricyanomethylisocyanideplatinate(II) (1) show that the complex does not feature stacking of the anions or significant Pt-Pt orbital interactions. The cis-dicyanobismethylisocyanideplatinum(II) (2) and cis-dicyanobisethylisocyanideplatinum(II) (3) complexes do crystallize with the platinum atoms collinear with one another but with a Pt-Pt separation distance on the order of 3.5 Å, which is too great for significant orbital overlap. In each of the complexes studied, the Pt-CNR bond lengths of the isocyanides are shorter than the Pt-CN bond lengths of the cyanide ligands. Additionally, each of these complexes have Pt-CNR bond distances marginally shorter than in the parent complex, [Pt(CNR)₄][BF₄]₂ (5). The shortened Pt-CNR distances in the mixed complexes are consistent with the isocyanide ligand being a stronger π-acid than the cyanide ligand, resulting in a preferred cis configuration of the mixed ligand complexes. In solution, the NMR spectra of these complexes are unusual because they display ¹⁹⁵Pt-¹⁴N and ¹H-¹⁴N coupling with high resolution. The NMR parameters of these complexes are compared with those of and (R = CH₃ or C₂H₅).     

Structural and spectroscopic characterisation of the holmium double-decker partially oxidised and bi-radical phthalocyanine complexes with iodine

Two crystals of holmium(III) double-decker iodine doped phthalocyanines, HoPc₂I_5/3 (I) and HoPc₂I (II), were grown directly in the reaction of holmium chips with 1,2-dicyanobenzene under versatile quantity of iodine at 180-160 °C. The complex I crystallises in the P4/mcc space group of tetragonal system, while the complex II crystallises in the P2/c space group of monoclinic system. The space group of P4/mcc and z = 1 requires that the Ho(III) atom is statistically disordered in the HoPc₂I_5/3 structure. The iodine atoms form linear symmetrical triiodide ions in I, while the I⁻ ions in II. Assignment of iodine species as in the HoPc₂I_5/3 and I⁻ in HoPc₂I complexes point to the +5/9 and +1 oxidation state of the HoPc₂ unit in these complexes. Thus one Pc macrocycle of the double-decker HoPc₂ units has a non-integer oxidation state of −1.222 in I, while both Pc-rings are one-electron oxidised radical Pc⁻ in II. Magnetic susceptibilities of HoPc₂I_5/3 and HoPcI at room temperature are 4.56 × 10⁻² and 5.12 × 10⁻² emu/mol and the calculated magnetic moments are 10.46 and 11.08 μ_B, respectively. UV-Vis spectroscopic measurement of I and II in benzene solution were carried out and discussed.     

Suppression of L-type Ca current by fluid pressure in rat ventricular myocytes: Possible role of Cl–OH exchange

The application of fluid pressure (FP) in ventricular myocytes using pressurized fluid flow inhibits L-type Ca²⁺ current (I_Ca), with approximately 80% of this effect coming through the enhancement of Ca²⁺ releases from the sarcoplasmic reticulum. In the present study, we explored the remaining mechanisms for the inhibition of I_Ca by FP. Since FP significantly increases H⁺ concentration and H⁺ is known to inhibit I_Ca, we examined whether pH regulation plays a role in the inhibitory effect by FP on I_Ca. A flow of pressurized (∼16.3 dyne/cm²) fluid, identical to that bathing the myocytes, was applied onto single rat ventricular myocytes for which the I_Ca was monitored using whole-cell patch-clamp under HEPES-buffered conditions. Extracellular application of the alkalizing agent, NH₄Cl (20 mM), enhanced I_Ca by ∼34% in the control conditions while increasing I_Ca significantly less (by ∼21%) in FP-pretreated myocytes, suggesting an inhibition of the effect of NH₄Cl on I_Ca possibly by FP-induced acidosis. Application of DIDS (4,4′-diisothiocyanatostilbene-2,2′-disulphonic acid, 500 μM), which blocks exchange but not Cl⁻–OH⁻ exchange, did not alter the inhibitory effect of FP on I_Ca. Replacement of external Cl⁻ with aspartate attenuated the inhibitory effect of FP on I_Ca. In highly Ca²⁺-buffered cells, where Ca²⁺-dependent inhibition of I_Ca was minimized, the external Cl⁻ removal eliminated the inhibitory effect of FP on I_Ca. These results suggest that the decrease of I_Ca in the presence of FP is at least partly caused by intracellular acidosis via activation of Cl⁻–OH⁻ exchange in rat ventricular myocytes.     

pH-responsive switching of near-infrared absorption of a diradical complex of Pt and 3,4-diaminobenzoate formed in aqueous solutions

In alkaline aqueous solutions, 3,4-diaminobenzoate (H₂(²L^PDA)⁻) reacts with Pt^II to form a 1:2 (Pt:L) complex that intensely absorbs near-infrared (NIR) light at 713 nm (ε = 8.0 × 10⁴ M⁻¹ cm⁻¹). The absorption disappeared at pH < 3 (in DMSO), showing pH-responsive switching of the NIR absorption. By comparing the NIR-absorbing behavior of this complex to that of a complex, [Pt^II(¹L^ISQ)₂]²⁻, containing the analogous phenylenediamine ligand [(¹L^ISQ)²⁻ = o-diiminobenzosemiquinonate radical], the complex can be formulated as [Pt^II(²L^ISQ)₂]²⁻. The assignment of the entity was consistent with the redox and spectroelectrochemical behaviors and electronic spin resonance (ESR) spectroscopy. First, one-electron oxidation of [Pt^II(²L^ISQ)₂]²⁻ formed an ESR-silent complex assignable to the dimeric complex [{Pt^II(²L^ISQ)(²L^IBQ)}₂]²⁻ [(²L^IBQ) = o-iminobenzoquinone form] in which the two radical centers at were antiferromagnetically coupled. Second, the one-electron reduced complex of [Pt^II(²L^ISQ)₂]²⁻ exhibited an ESR signal attributed to [Pt^II(²L^ISQ)(²L^PDA)]³⁻; 34% of the electronic spin was located at the Pt^II center rather than on the moiety. The pH-responsive switching-off of the NIR absorption was thus rationally explained by oxidation of [Pt^II(²L^ISQ)₂]²⁻ to [{Pt^II(²L^ISQ)(²L^IBQ)}₂]²⁻ by the increase of the rest potential of the solution in the lower pH region.     

Thermodynamics and laser luminescence spectroscopy of binary and ternary complexation of Am, Cm and Eu with citric acid, and citric acid + EDTA at high ionic strength

The binary complexation of Am³⁺, Cm³⁺and Eu³⁺ with citrate has been studied at I = 6.60 m (NaClO₄), pcH 3.60 and in the temperatures range of 0-60 °C employing a solvent extraction technique with di-(2-ethylhexyl)phosphoric acid/heptane. Two complexes, MCit and , were formed at all temperatures. For the three metal ions, the log β₁₀₁ was between 5.9 and 6.2 and log β₁₀₂ between 10.2 and 10.6 at 25 °C. The thermodynamic parameters for the Am-Cit system have been calculated from the temperature dependence of the β₁₀₁ and β₁₀₂ values. Positive enthalpy and entropy values for the formation of both complexes are interpreted as due to the contributions from the dehydration of the metal ions exceeding the exothermic cation-anion pairing. The formation of the ternary complex M(EDTA)(Cit)⁴⁻ (M = Cm and Eu) was measured to have large stability constants (log β₁₁₁ between 20.9 and 24.4) at 25 and 60 °C. Time resolved laser luminescence spectroscopy and lifetime measurement data validated the nature of the complexes of Eu(III) formed in the presence of Cit and EDTA + Cit in 6.60 m (NaClO₄) solution.     

Copper·Lys-Gly-His-Lys mediated cleavage of tRNA: Studies of reaction mechanism and cleavage specificity

The reactivity of [Cu²⁺·Lys-Gly-His-Lys-NH₂]²⁺ and [Cu²⁺·Lys-Gly-His-Lys]⁺ toward tRNA^Phe has been evaluated. The amidated and carboxylate forms of the copper peptides display complex binding behavior with strong and weak sites evident (, for the amide form; and , for the carboxylate form), while Cu²⁺(aq) yielded and . The time-dependence of the reaction of [Cu²⁺·Lys-Gly-His-Lys]⁺ and [Cu²⁺·Lys-Gly-His-Lys-NH₂]²⁺ with tRNA^Phe yielded k_obs ∼ 0.075 h⁻¹ for both complexes. HPLC analysis of the reaction products demonstrated guanine as the sole base product. Mass spectrometric data shows a limited number of cleavage fragments with product peak masses consistent with chemistry occurring at a discrete site defined by the structurally contiguous D and TΨC loops, and in a domain where high affinity magnesium centers have previously been observed to promote hydrolysis of the tRNA^Phe backbone. This cleavage pattern is more selective than that previously observed by Long and coworkers for nickel complexes of a series of C-terminally amidated peptides (Gly-Gly-His, Lys-Gly-His, and Arg-Gly-His), and may reflect variations in structural recognition and a distinct reaction path by the nickel derivatives. The data emphasizes the optimal positioning of the metal-associated reactive oxygen species, relative to scissile bonds, as a major criterion for development of efficient catalytic nucleases or therapeutics.     

Macrocyclic dinuclear gold(I) and silver(I) NHCs complexes

The ligands bis-(imidazolium) hexafluorophosphate (Himy = -C₃N₂H₃-, imidazolium; R = 1-naphthylmethylene, 1a; 9-anthracenylmethylene, 1b) with an oxoether chain were easily prepared by the reaction of substituted imidazole with the diglycol diiodide, followed by exchange of anions with . 1a and 1b reacted with Ag₂O in DMSO or CH₃CN to yield [2 + 2] dinuclear Ag(I) NHCs macrocyclic complexes 2a and 2b, which showed much different conformation in solid corresponding to the R- substituent. Carbene transmetalation reactions of 2a-b with Au(SMe₂)Cl give dinuclear Au(I) analogs 3a and 3b. The new NHCs complexes were characterized by elemental analyses, ¹H NMR, ¹³C NMR and the structures of 2a-b and 3a were confirmed by X-ray diffraction determination.     

Coordination versatility of 1,3-bis[3-(2-pyridyl)pyrazol-1-yl]propane: Co(II) and Ni(II) complexes

The ligand 1,3-bis[3-(2-pyridyl)pyrazol-1-yl]propane (L⁸) has afforded six-coordinate monomeric and dimeric complexes [(L⁸)Co^II(H₂O)₂][ClO₄]₂ (1), [(L⁸)Ni^II(MeCN)₂][BPh₄]₂ (2), [(L⁸)Ni^II(O₂CMe)][BPh₄] (3), and . The crystal structures of 1, 2 · MeCN, 3, and 4 revealed that the ligand L⁸ is flexible enough to expand its coordinating ability by fine-tuning the angle between the chelating fragments and hence folds around cobalt(II)/nickel(II) centers to act as a tetradentate chelate, allowing additional coordination by two trans-H₂O, cis-MeCN, and a bidentate acetate affording examples of distorted octahedral , , and coordination. The angles between the two CoN₂/NiN₂ planes span a wide range 23.539(1)° (1), 76.934(8)° (2), and 69.874(14)° (3). In contrast, complex 4 is a bis-μ-1,3-acetato-bridged (syn-anti coordination mode) dicobalt(II) complex [Co?Co separation: 4.797(8) Å] in which L⁸ provides terminal bidentate pyridylpyrazole coordination to each cobalt(II) center. To our knowledge, this report provides first examples of such a coordination versatility of L⁸. Absorption spectral studies (MeCN solution) have been done for all the complexes. Complexes 1-3 are uniformly high-spin. Temperature-dependent (2-300 K) magnetic studies on 4 reveal weak ferromagnetic exchange coupling between two cobalt(II) (S = 3/2) ions. The best-fit parameters obtained are: Δ (axial splitting parameter) = −765(5) cm⁻¹, λ (spin-orbit coupling) = −120(3) cm⁻¹, k (orbital reduction factor) = 0.93, and J (magnetic exchange coupling constant) = +1.60(2) m⁻¹.     

Conformational isomerism and hydrogen-bonded motifs of anion assisted supramolecular self-assemblies using Cu/Co salts and pyridine-4-acetamide

Six novel metal-organic complex assemblies constructed from a conformation-flexible ligand - pyridine-4-acetamide (PAT) and inorganic Cu^II and Co^II salts have been synthesized and structurally characterized by single crystal X-ray diffraction analysis. Crystal structure analysis reveals five types of architectures by variation of metal salts. In {[Cu(PAT)₂Cl₂]}_n (1) and {[Co(PAT)₂Cl₂]}_n (3), PAT ligands bridge metal centers to form one-dimensional chains. The chains are extended to three dimensions with the aid of two types of hydrogen bonded motifs () and (12)). {[Cu(PAT)₂(NO₃)](NO₃)(THF)}_n (5) which exhibits two-dimensional coordinating layers forms open channels filled with solvent molecules. In [Cu(PAT)₂Cl₂] (2), [Co(PAT)₂Cl₂] (4) and [Co(PAT)₄(H₂O)₂](NO₃)₂(THF)₂ (6), PAT is observed as a monofunctional ligand. Complex 2 forms one-dimensional hydrogen bonded chains. Crystal structure of complex 4 has a two-dimensional infinite hydrogen-bonded network with and motifs formed by complementary amide-amide hydrogen bonds. [Co(PAT)₄(H₂O)₂](NO₃)₂(THF)₂ (6) crystallizes in centrosymmetric I4₁/a space group. Complex 6 forms chiral channels which are filled with twisted solvent helices and anion helices. Within each channel the solvent helix and the anion helix have the same handedness; and adjacent channels have opposite handedness. Complexes 1, 2 and complexes 3, 4 illustrate examples of conformational supramolecular isomerism in {[Cu(PAT)₂Cl₂]}_∞ and {[Co(PAT)₂Cl₂]}_∞, respectively. In these complexes, changes of PAT conformations and coordination geometry of metal center induced the structural versatility.     

Kinetic studies of the reduction of hexaaquacobalt(III) perchlorate by a cobaloxime, [Co(dmgBF2)2(H2O)2]

The reaction of with Co(dmgBF₂)₂(H₂O)₂ in 1.0 M HClO₄/LiClO₄ was found to be first-order in both reactants and the [H⁺] dependence of the second-order rate constant is given by k_2obs = b/[H⁺], b at 25 °C is 9.23 ± 0.14 × 10² s⁻¹. The [H⁺] dependence at lower temperatures shows some saturation effect that allowed an estimate of the hydrolysis constant for as K_a = 9.5 × 10⁻³ M at 10 and 15 °C. Marcus theory and the known self-exchange rate constant for Co(OH₂)₅OH^2+/+ were used to estimate an electron self-exchange rate constant of k₂₂ = 1.7 × 10⁻⁴ M⁻¹ s⁻¹ for .     

Syntheses and electrochemical properties of ruthenium(II) complexes with 4,4-bipyrimidine and 4,4-bipyrimidinium ligands

The syntheses and electrochemical properties of novel ruthenium(II) polypyridyl complexes with 4,4^′-bipyrimidine, [Ru(trpy)(bpm)Cl](X) ([1](X; X=PF₆⁻, BF₄⁻)) and with a quaternized 4,4^′-bipyrimidinium ligand, [Ru(trpy)(Me₂bpm)Cl](BF₄)₃ ([2](BF₄)₃) (trpy=2,2^′:6^′,2″-terpyridine, bpm=4,4^′-bipyrimidine, Me₂bpm=1,1^′-dimethyl-4,4^′-bipyrimidinium) are presented. The bpm complex [1]⁺ was prepared by the reaction of Ru(trpy)Cl₃ with 4,4^′-bipyrimidine in EtOH/H₂O. The structural characterization of [1]⁺ revealed, that the bpm ligand coordinated to the ruthenium atom with the bidentate fashion. Diquaternization of the non-coordinating nitrogen atoms on bpm of [1]⁺ by (CH₃)₃OBF₄ in CH₃CN gave [2](BF₄)₃. The electrochemical and spectroelectrochemical properties of the complexes are described.     

Synthesis and characterization of a dinuclear ferric complex with redox-active ligands

A dinuclear ferric complex with the redox-active ligand (L^Cl₂)^2- (H₂L^Cl₂ = N,N′-dimethyl-bis(3,5-dichloro-2-hydroxybenzyl)-1,2-diaminoethane), was synthesized and characterized. The two iron(III) ions are six-coordinate in a distorted octahedral environment of the donor set of one (L^Cl₂)²⁻ and one amine and one phenolate donor of a second (L^Cl₂)²⁻, which bridges the two complex halves. The relatively low-symmetric complex 1 crystallizes in the space group R. The crystal structure contains hexagonal, one-dimensional channels parallel to the c axis with diameters of ∼13 Å. The absorption spectrum of 1 exhibits strong characteristic features of p_π → d_π^∗, p_π → d_σ^∗, phenolate-to-metal CTs, and π → π^∗ ligand transitions. Electrochemical studies on 1 reveal the redox-activity of the coordinated ligand (L^Cl₂)²⁻ by showing irreversible oxidative electron-transfer waves. The reductive electron transfers at negative potentials seem to originate from metal-centered processes. A detailed comparison to complexes with similar donor sets provides new insights into the electrochemical properties of these kinds of complexes.     

The new helical zigzag chain {[Ag(Pepy)(NO3)]}∞ and mononuclear [HPepy(FeCl4)] with 3D network based on hydrogen bonds as well as π-π interactions

The title complexes, {[Ag(Pepy)(NO₃)]}_∞ (1) and [HPepy(FeCl₄)] (2) (Pepy = trans-2-(2-phenylethyleneyl)pyridine), had been synthesized and characterized structurally by single-crystal X-ray diffraction analysis, FTIR as well as thermal analysis. The geometrical structure of complex 1 is a one-dimensional helical zigzag chain. Each Ag^I center is five-coordinated by two anions and one Pepy ligand while the anion is an uncommon tetra-dentate coordinating to two Ag^I by O atoms, respectively. The anions connecting with Ag^I as bridge blocks resulted in the formation of crystalline of helical polymeric chain. In the mononuclear complex 2, the Fe^III center is four-coordinated by four Cl⁻ as [FeCl₄]⁻ anion, which connected with hydrogen atoms from the protonated N atom and the C atoms from HPepy⁺ to form multivalent hydrogen bonded network through N-H?Cl and C-H?Cl interactions. The two frameworks can be both considered as a 3D structure driven by diverse hydrogen bonds as well as π-π stacking interactions.     

Two cyano-bridged heterotrinuclear complexes built from [(Tp)Fe(CN)3] (Tp = hydrotris(pyrazolyl)borate): synthesis, crystal structures and magnetic properties

Using an anionic precursor [(Tp)Fe^III(CN)₃]⁻ (1) as a building block, two cyano-bridged centrosymmetric heterotrinuclear complexes, (2) and (3) (en = ethylenediamine), have been synthesized and structurally characterized. In each complex, [TpFe(CN)₃]⁻ acts as a monodentate ligand toward a central [Mn(C₂H₅OH)₄]²⁺ or [Ni(en)₂]²⁺ core through one of its three cyanide groups, the other two cyanides remaining terminal. The intramolecular Fe-Mn and Fe-Ni distances are 5.2354(4) and 5.0669(11) Å, respectively. The magnetic properties of complexes 2 and 3 have been investigated in the temperature range of 2.0-300 K. A weak antiferromagnetic interaction between the Mn(II) and Fe(III) ions has been found in complex 2. The magnetic data of 2 can be fitted with the isotropic Hamiltonian: where J and J′ are the intramolecular exchange coupling parameters between adjacent and peripheral spin carriers, respectively. This leads to values of J = −1.37 cm⁻¹ and g = 2.05. The same fitting method is applied to complex 3 to give values of J = 1.2 cm⁻¹ and g = 2.25, showing that there is a ferromagnetic interaction between the Fe(III) and Ni(II) ions.     

Synthesis and characterization of tetraphenylporphyrin iron(III) complexes with substituted phenylcyanamide ligands

Several five coordinate complexes of [(TPP)Fe^III(L)] in which TPP is the dianion of tetraphenylporphyrin and L is the monoanion of phenylcyanamide (pcyd) (1), 2,5-dichlorophenylcyanamide (2,5-Cl₂pcyd) (2), 2,6-dichlorophenylcyanamide (2,6-Cl₂pcyd) (3), and 2,3,4,6-tetrachlorophenylcyanamide (2,3,4,6-Cl₄pcyd) (4) have been prepared by the reaction of [(TPP)Fe^IIICl] with appropriate thallium salt of phenylcyanamide. Each of the complexes has been characterized by IR, UV-Vis and ¹H NMR spectroscopic data. Dark red-brown needles of [(TPP)Fe^III(2,6-Cl₂pcyd)] (C₅₁H₃₁Cl₂FeN₆ · CHCl₃) crystallize in the triclinic system. The crystal structure of Fe(III) compound shows a slight distortion from square pyramidal coordination with the 2,6-dichlorophenylcyanamide anion in the axial position through nitrile nitrogen atom. Iron atom is 0.47(1) Å out of plane of the porphyrin toward phenylcyanamide ligand. In non-coordinating solvents, such as benzene or chloroform, these complexes exhibit ¹H NMR spectra that are characteristic of high-spin (S = 5/2) species. The X-ray crystal structure parameters are also consistent with high-spin iron(III) complexes. The iron(III) phenylcyanamide complexes are not reactive toward molecular oxygen; however, these complexes react with HCl and produce TPPFe^IIICl.     

Polynuclear complex formation of trivalent lanthanides by 5-sulfosalicylate in an aqueous system—potentiometric, H NMR, and TRLIFS studies

The complex formation between several trivalent lanthanides (Ln) and 5-sulfosalicylate, (SSA)³⁻, was investigated by potentiometry, ¹H NMR, and time resolved laser-induced fluorescence spectroscopy (TRLIFS). The potentiometric data were used to deduce the stoichiometry and equilibrium constants for the reactions pLn³⁺ + rL ? Ln_pH_−qL_r + qH⁺ at 298 K in an ionic medium with a constant concentration of Na⁺ equal to 1.00 M. Note that “L” denotes the SSA ligand where all protons are dissociated. Two mononuclear chelating complexes, LnL(aq) and , were identified. Their stability constants obtained by least-squares refinement of the potentiometric data agree well with previously published information. In addition, two additional dinuclear complexes, and , which have very different ¹H NMR and fluorescence characteristics, were identified by least-squares refinement in the −log[H⁺] range of 6.0-10.0. ¹H NMR spectra from the ligand in the complex showed separate peaks having two different rates of exchange with free ligand in the bulk solution besides a signal from the free and carboxylate-coordinated ligands. This indicates that the dinuclear complex, , consists of two different types of chelating ligands: μ-{OR}-type chelating ligands between metals to form the {Ln₂L₂}-type core structure and the bidentate ligands outside the {Ln₂L₂}-type core. This core structure is different from the An(IV)-SSA case (An(IV): tetravalent actinide), in which hydroxides play the role of forming the {An₂(OH)₂}-type core structure. TRLIFS measurement gave further information about the dynamics and molecular structures of the complexes.     

Synthesis and structure determination of two new dinuclear end-to-end doubly bridged azido- and thiocyanato-copper(II) complexes derived from diethyldiethylenetriamine

Herein, we report the synthesis and characterization of two new dinuclear bridged azido and bridged thiocyanato complexes: [Cu₂(Et₂dien)₂(μ_1,3-N₃)₂](ClO₄)₂ (1) and [Cu₂(Et₂dien)₂(μ_N,S-NCS)₂]-(ClO₄)₂ (2) where Et₂dien = N,N-diethyldiethylenetriamine. In both complexes, the two copper centers are linked by two azide or two thiocyanate groups in end-to-end bonding fashion. The copper ions are penta-coordinated by three N-atoms of the Et₂dien ligand, one N atom from the bridging azido in 1 or from the thiocyanato group in 2. The fifth coordination site is occupied by N or S atom from the second bridging azide or thiocyanate ligand, respectively. The coordination geometry around the Cu(II) ions in the two complexes may be described as close to square pyramidal (SP) stereochemistry with severe distortion to trigonal bipyramidal (TBP) stereochemistry. The intradimer Cu?Cu distances are 5.264(1) and 5.571(1) Å for the azido and thiocyanato complex, respectively. The IR stretching frequencies of the azido, and the thiocyanato, ν(NCS⁻) groups in the 2030-2120 cm⁻¹ region are discussed in relation to other related species. The visible spectra of the complexes studied in different solvents reveal the assigned predominant SP stereochemistry in solution with the presence of a pronounced amount of TBP geometry in the thiocyanato complex. Moreover, the complexes undergo solvolysis through bond rupture and displacement of one of the bridged azido or thiocyanato ligands.     

Kinetics and mechanism of the oxidation of oxalic acid by tetrachloroaurate(III) ion

The oxidation of oxalic acid by tetrachloroaurate(III) ion in 0.005 ? [HClO₄] ? 0.5 mol dm⁻³ is first order in and a fractional order in [oxalic acid], the reactive entities being AuCl₃(OH)⁻ and ions. The pseudo first-order rate, k_obs, with respect to [Au(III)], is retarded by increasing [H⁺] and [Cl⁻]. The retardation by H⁺ ion is caused by the dissociation equilibrium . A mechanism in which a substitution complex, is formed from AuCl₃(OH)⁻ and ions prior to its rate limiting disproportionation into products is suggested. The rate limiting constant, k, has been evaluated and its activation parameters are reported. The equilibrium constant K₁ for the formation of the substitution complex and its thermodynamic parameters are also reported.