Log Β2 Research Articles

Spectrophotometric determination of formation constants of copper(II) complexes with 2,2′-bipyridyl, 1,10-phenanthroline and their halides in methanol

The redox mediator used in dye-sensitized solar cells present several disadvantages; several research groups have theorized the possible use of blue copper proteins as electron mediators, despite their high costs. Copper complexes that mimic blue copper proteins could represent an alternative as electronic transfer agent with lower costs.In this work, we conducted a spectrophotometric study of the speciation of copper(II) complexes with 2,2′-bipyridyl, 1,10-phenanthroline and their ternary complexes with chloride and bromide, at 303 K. For the copper–2,2′-bipyridyl complexes, the formation constants obtained are: log β110 = 7.57 ± 0.08, log β120 = 13.26 ± 0.11 and log β130 = 17.21 ± 0.14. For the ternary systems of copper–2,2′-bipyridyl with chloride or bromide, are: log β111 = 12.06 ± 0.06 and log β111 = 11.23 ± 0.01, respectively. Also, the formation constants obtained for the copper–1,10-phenanthroline complexes are log β110 = 4.21 ± 0.01 and log β120 = 7.24 ± 0.02. For the ternary systems of copper(II) with 1,10-phenanthroline and chloride or bromide, are: log β111 = 6.14 ± 0.04 and log β111 = 7.46 ± 0.03, respectively. Finally, the individual calculated electronic spectra of these species were obtained and compared to other copper complexes mimicking to blue copper proteins.

Molecular Aspects of a Robust Assay for Ferroxidase Function of Ceruloplasmin.

Ceruloplasmin (Cp) is one of the most complex multicopper oxidase enzymes and plays an essential role in the metabolism of iron in mammals. Ferrous ion supplied by the ferroportin exporter is converted by Cp to ferric ion that is accepted by plasma metallo-chaperone transferrin. Study of the enzyme at the atomic and molecular level has been hampered by the lack of a suitable ferrous substrate. We have developed the classic chromophoric complex FeIIHx(Tar)2 (H2Tar, 4-(2-thiazolylazo)resorcinol; x = 0-2; overall charge omitted) as a robust substrate for evaluation of the ferroxidase function of Cp and related enzymes. The catalysis can be followed conveniently in real time by monitoring the solution absorbance at 720 nm, a fingerprint of FeIIHx(Tar)2. The complex is oxidized to its ferric form FeIIIHx(Tar)2 via the overall reaction sequence FeIIHx(Tar)2 → FeII-Cp → FeIII-Cp → FeIIIHx(Tar)2: i.e., Fe(II) is transferred formally from FeIIHx(Tar)2 to the substrate docking/oxidation (SDO) site(s) in Cp, followed by oxidation to product Fe(III) that is trapped again by the ligand. Each Tar ligand in the above bis-complex coordinates the metal center in a meridional tridentate mode involving a pH-sensitive -OH group (pKa > 12), and this imposes rapid Fe(II) and Fe(III) transfer kinetics to facilitate the catalytic process. The formation constants of both the ferrous and ferric complexes at pH 7.0 were determined (log β2' = 13.6 and 21.6, respectively), as well as an average dissociation constant of the SDO site(s) in Cp (log KD' = -7.2).

Constraining spatial variations of the fine-structure constant in symmetron models

We introduce a methodology to test models with spatial variations of the fine-structure constant α, based on the calculation of the angular power spectrum of these measurements. This methodology enables comparisons of observations and theoretical models through their predictions on the statistics of the α variation. Here we apply it to the case of symmetron models. We find no indications of deviations from the standard behavior, with current data providing an upper limit to the strength of the symmetron coupling to gravity (log⁡β2<−0.9) when this is the only free parameter, and not able to constrain the model when also the symmetry breaking scale factor aSSB is free to vary.

A detailed spectrophotometric investigation of the complexation of palladium(II) with chloride and bromide

As the speciation of metal species, especially the noble metals, are crucial for recovery and separation purposes, the complex formation of Pd(II), with chloride and bromide respectively, has been investigated employing spectrophotometry at 25°C and an ionic strength of 1.0M Hyperquad Simulation and Speciation (HySS) was employed to determine the ideal titration conditions and an autotitrator (equipped with a flow-through cell) was used to accurately vary the chloride and bromide concentrations over an increased number of intervals, i.e. 128 and 300 respectively. All of these data points were used to calculate the formation constants of both palladium systems employing HypSpec (software specifically developed for the determination of equilibrium constants from spectrophotometric data). The formation constants, βn, for the palladium(II)-chloride and -bromide complexes, have been determined. For [PdCln(H2O)4−n]2−n (n=0–4), the log β values are: log β1=4.49, log β2=7.80, log β3=10.18 and log β4=11.54, while the log β values for [PdBrn(H2O)4−n]2−n (n=0–4), are: log β1=5.04, log β2=9.12, log β3=12.38 and log β4=14.55. The formation constants for the palladium-chloride complexes agree well with the published data, while the formation constants for the bromide complexes differ from somewhat from published data suggesting a different, but more accurate speciation. Molar absorption spectra for all the complexes in question have been obtained. Pourbaix diagrams were constructed from the new speciation data showing the predominant species for each system.

Stability constant determinations for technetium (IV) complexation with selected amino carboxylate ligands in high nitrate solutions

Abstract The stability constants for Tc(IV) complexation with the ligands IDA, NTA, HEDTA, and DTPA were determined in varied nitrate concentrations using liquid-liquid extraction methods. The determined log β101 stability constants at 0.5 M NaNO3 were found to be 9.2±0.3, 10.3±0.3, and 15.3±0.3 for IDA, NTA, and HEDTA, respectively. The log β111 stability constant for DTPA was determined to be 22.0±0.6. These determined stability constants show a slight decrease in magnitude as a function of increasing NaNO3 concentration. These stability constants were used to model the total dissolution of Tc(IV) in acidic aqueous solutions in the presence of each ligand. The results of these predictive models indicate that amino carboxylic ligands have a high potential for increasing the aqueous dissolution of Tc(IV); at pH 2.3, 0.01 M ligand yield dissolved Tc(IV) concentrations of 1.42·10−5 M, 1.33·10−5 M, 6.07·10−6 M, 9.65·10−7 M, for DTPA, HEDTA, NTA, and IDA, respectively.

Ti(IV) and the Siderophore Desferrioxamine B: A Tight Complex Has Biological and Environmental Implications.

The siderophore desferrioxamine B (DFOB) binds Ti(IV) tightly and precludes its hydrolytic precipitation under biologically and environmentally relevant conditions. This interaction of DFOB with Ti(IV) is investigated by using spectro-potentiometric and spectro-photometric titrations, mass spectrometry, isothermal titration calorimetry (ITC), and computational modeling. The data from pH 2-10 suggest two one-proton equilibria among three species, with one species predominating below pH 3.5, a second from pH 3.5 to 8, and a third above pH 8. The latter species is prone to slow hydrolytic precipitation. Electrospray mass spectrometry allowed the detection of [Ti(IV) (HDFOB)]2+ and [Ti(DFOB)]+; these species were assigned as the pH < 3.5 and the 3.5 < pH < 8 species, respectively. The stability constant for Ti(IV)-DFOB was determined by using UV/vis-monitored competition with ethylenediaminetetraacetic acid (EDTA). Taking into consideration the available binding constant of Ti(IV) and EDTA, the data reveal values of log β111 = 41.7, log β110 = 38.1, and log β11-1 = 30.1. The former value was supported by ITC, with the transfer of Ti(IV) from EDTA to DFOB determined to be both enthalpically and entropically favorable. Computational methods yielded a model of Ti-DFOB. The physiological and environmental implications of this tight interaction and the potential role of DFOB in solubilizing Ti(IV) are discussed.

A DFT study on the complex formation between desferrithiocin and metal ions (Mg2+, Al3+, Ca2+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+)

In recent years, Metal-chelating compounds, namely siderphores have been considered very much because of their crucial role in various fields of the environmental researches. Their importance lies in the fact that they are able to be bonded to a variety of metals in addition to iron. A theoretical study on the structures of desferrithiocin siderphore coordinated to Mg2+, Al3+, Ca2+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+ and Zn2+ metal ions was carried out, using the CAM-B3LYP/6-31G(d) level of the theory in the water. In order to understand the factors which control the stability, reactivity and the strength of toxic metals excretion as well as microbial uptake of the metal-siderphore complexes, we examined the stability and binding energies of the desferrithiocin and various metal ions with different spin states. The binding affinity of desferrithiocin to Fe3+ (log β2=23.88) showed that the desferrithiocin can scavenge the excess iron(III) from the labile sources. Also, the binding energy values were well described by addition of the dispersion-corrected D3 functional. Because of the importance of the charge transfer in the complex formation, donor-acceptor interaction energies were evaluated. Based on this analysis, an increase in the effective nuclear charge increases E(2) values. Vibrational analysis showed that the critical bonds (CO stretching and CH bending) are in the range of 1300–1800cm−1. Finally, some probable correlations between the complexation behavior and quantum chemistry descriptors have been analyzed.

Spectrophotometric determination of the formation constants of Calcium(II) complexes with 1,2-ethylenediamine, 1,3-propanediamine and 1,4-butanediamine in acetonitrile

In this work, with the purpose to explore the coordination chemistry of calcium complexes which could work as a partial model of manganese–calcium cluster, a spectrophotometric study to evaluate the stability of the complexes: Calcium(II)-1,2-ethylendiamine, Calcium(II)-1,3-propanediamine and Calcium(II)-1,4-butanediamine in acetonitrile, were carried on. By processing the spectrophotometric data with the HypSpec program allows the determination of the formation constants. The logarithmic values of the formation constants obtained for Calcium(II)-1,2-ethylendiamine, Calcium(II)-1,3-propanediamine and Calcium(II)-1,4-butanediamine were log β110 = 4.69, log β110 = 5.25 and log β110 = 4.072, respectively.

Thermodynamic stability of mercury(II) complexes formed with environmentally relevant low-molecular-mass thiols studied by competing ligand exchange and density functional theory

Environmental contextThe chemical speciation of mercury (Hg) largely controls its biogeochemical cycling and exposure to biota. Here, we investigate the thermodynamic stabilities of complexes formed between inorganic divalent Hg (HgII) and 15 biogeochemically relevant low-molecular-mass (LMM) thiol ligands. This information is critical for accurate modelling of the chemical speciation of HgII and to clarify the role of HgII–LMM thiol complexes in the cycling of Hg in the environment. AbstractInorganic divalent mercury (HgII) has a very high affinity for reduced sulfur functional groups. Reports from laboratory experiments suggest that HgII complexes with specific low-molecular-mass (LMM) thiol (RSH) ligands control rates of HgII transformation reactions. Because of methodological limitations for precise determination of the highly stable HgII complexes with LMM thiol ligands, constants reported in the literature remain inconsistent. This uncertainty impedes accurate modelling of the chemical speciation of HgII and the possibility to elucidate the role of HgII complexes with LMM thiols for Hg transformation reactions. Here, we report values of thermodynamic stability constants for 15 monodentate, two-coordinated HgII complexes, Hg(SR)2, formed with biogeochemically relevant LMM thiol ligands. The constants were determined by a two-step ligand-exchange procedure where the specific Hg(SR)2 complexes were quantified by liquid chromatography–inductively coupled plasma mass spectrometry. Thermodynamic stability constants (log β2) determined for the Hg(SR)2 complexes ranged from 34.6, N-cysteinylglycine, to 42.1, 3-mercaptopropionic acid, for the general reaction Hg2++2RS– ⇌ Hg(SR)2. Density functional theory (DFT) calculations showed that electron-donating carboxyl and carbonyl groups have a stabilising effect on the HgII–LMM thiol complexes, whereas electron-withdrawing protonated primary amino groups have a destabilising effect. Experimental results and DFT calculations demonstrated that the presence of such functional groups in the vicinity of the RSH group caused significant differences in the stability of Hg(SR)2 complexes. These differences are expected to be important for the chemical speciation of HgII and its transformation reactions in environments where a multitude of LMM thiol compounds are present.

Spectrophotometric determination of the formation constants of calcium(II) complexes with 2,2’-bipyridyl and 1,10-phenanthroline in acetonitrile

The oxygen-evolving complex (OEC), which consists of a calcium-manganese cluster, is the reaction center of the Photosystem II. At this catalytic site, the water-splitting reaction in dioxygen and hydronium ions occurs. In order to partially reproduce the water splitting process, several studies have reported the synthesis of functional model complexes. Nevertheless, there is a small amount of reports, concerning the spectralbehavior of calcium complexes or the calcium role in the cluster. In this work, in order to explore the absorption spectrum of calcium species in acetonitrile, an equilibrium study of the calcium complexes with 2,2’-bipyridyl or 1,10-phenanthroline, was carried out. The formation constants and the calculated electronic spectrum of each complex was obtained by a modified method of continuous variations consisting in a correlation of the experimental spectrophotometric data with the HypSpec software. The values of the formation constants for the calcium(II) complexes with 2,2’-bipyridyl and 1,10-phenanthroline, are Log β110 = 4.39 ± 0.02 and Log β110 = 5.94 ± 0.05, respectively.

Evaluation of Immobilized Metal-Ion Affinity Chromatography and Electrospray Ionization Tandem Mass Spectrometry for Recovery and Identification of Copper(II)-Binding Ligands in Seawater Using the Model Ligand 8-Hydroxyquinoline

Complexation by organic ligands dominates the speciation of iron (Fe), copper (Cu), and other bioactive trace metals in seawater, controlling their bioavailability and distribution in the marine environment. Several classes of high-affinity Fe-binding ligands (siderophores) have been identified in seawater but the chemical structures of marine Cu-complexing ligands remain unknown. Immobilized metal-ion affinity chromatography (IMAC) allows Cu ligands to be isolated from bulk dissolved organic matter (DOM) in seawater and separated into fractions which can be characterized independently using electrochemical and spectroscopic techniques. Attempts have been made to combine IMAC with electrospray ionization mass spectrometry (ESI-MS) to characterize marine Cu ligands, but results have proven inconclusive due to the lack of tandem mass spectrometry (MS/MS) data to confirm ligand recovery. We used 8-hydroxyquinoline (8-HQ), a well-characterized model ligand that forms strong 1:2 metal:ligand complexes with Cu2+ at pH 8 (log β2 = 18.3), to evaluate Cu(II)-IMAC and ESI-MS/MS for recovery and identification of copper(II)-complexing ligands in seawater. One-litre samples of 0.45µm-filtered surface seawater were spiked with 8-HQ at low concentrations (up to 100 nM) and fractionated by IMAC. Fractions eluted with acidified artificial seawater were desalted and re-suspended in methanol via solid-phase extraction (SPE) to obtain extracts suitable for ESI-MS analysis. Recovery of 8-HQ by Cu(II)-IMAC was confirmed unambiguously by MS/MS and found to average 81% based upon accurate quantitation via multiple reaction monitoring (MRM). Cu(II)-IMAC fractionation of unspiked seawater using multiple UV detection wavelengths suggests an optimal fraction size of 2 mL for isolating and analyzing Cu ligands with similar properties.

Spectrophotometric determination of the formation constants of calcium(II) complexes with 2,2'-bipyridyl and 1,10-phenanthroline in acetonitrile

Abstract The oxygen-evolving complex (OEC), which consists of a calcium-manganese cluster, is the reaction center of the Photosystem II. At this catalytic site, the water-splitting reaction in dioxygen and hydronium ions occurs. In order to partially reproduce the water splitting process, several studies have reported the synthesis of functional model complexes. Nevertheless, there is a small amount of reports, concerning the spectral behavior of calcium complexes or the calcium role in the cluster. In this work, in order to explore the absorption spectrum of calcium species in acetonitrile, an equilibrium study of the calcium complexes with 2,2'-bipyridyl or 1,10-phenanthroline, was carried out. The formation constants and the calculated electronic spectrum of each complex was obtained by a modified method of continuous variations consisting in a correlation of the experimental spectrophotometric data with the HypSpec software. The values of the formation constants for the calcium(II) complexes with 2,2'-bipyridyl and 1,10-phenanthroline, are Log β110 = 4.39 ± 0.02 and Log β110 = 5.94 ± 0.05, respectively.

Engineered Recognition of Tetravalent Zirconium and Thorium by Chelator-Protein Systems: Toward Flexible Radiotherapy and Imaging Platforms.

Targeted α therapy holds tremendous potential as a cancer treatment: it offers the possibility of delivering a highly cytotoxic dose to targeted cells while minimizing damage to surrounding healthy tissue. The metallic α-generating radioisotopes 225Ac and 227Th are promising radionuclides for therapeutic use, provided adequate chelation and targeting. Here we demonstrate a new chelating platform composed of a multidentate high-affinity oxygen-donating ligand 3,4,3-LI(CAM) bound to the mammalian protein siderocalin. Respective stability constants log β110 = 29.65 ± 0.65, 57.26 ± 0.20, and 47.71 ± 0.08, determined for the EuIII (a lanthanide surrogate for AcIII), ZrIV, and ThIV complexes of 3,4,3-LI(CAM) through spectrophotometric titrations, reveal this ligand to be one of the most powerful chelators for both trivalent and tetravalent metal ions at physiological pH. The resulting metal-ligand complexes are also recognized with extremely high affinity by the siderophore-binding protein siderocalin, with dissociation constants below 40 nM and tight electrostatic interactions, as evidenced by X-ray structures of the protein:ligand:metal adducts with ZrIV and ThIV. Finally, differences in biodistribution profiles between free and siderocalin-bound 238PuIV-3,4,3-LI(CAM) complexes confirm in vivo stability of the protein construct. The siderocalin:3,4,3-LI(CAM) assembly can therefore serve as a "lock" to consolidate binding to the therapeutic 225Ac and 227Th isotopes or to the positron emission tomography emitter 89Zr, independent of metal valence state.

Photoluminescence and Coordination Behaviour of Lanthanide Complexes of Tris (Aminomethyl)Ethane-5-Oxine in Aqueous Solution.

Photophysical properties of a multidentate tripodal ligand, 5,5'-(2-(((8-hydroxyquinolin-5-yl) methylamino)methyl)-2-methylpropane-1,3-diyl) bis (azanediyl)bis (methylene)diquinolin-8-ol, (TAME5OX), with La3+ and Er3+ ions have been examined for photonics applications. The change in behavior in electronic spectra of these complexes reveals the use of TAME5OX as a sensitive optical pH based sensor to detect Ln3+ ions whereas indication of strong green fluorescence allows simultaneous sensing within the visible region in competitive medium. The intense fluorescence intermittently gets quenched under acidic and basic conditions due to photoinduced intramolecular electron transfer from the excited 8-hydroxyquinoline (8-HQ) moiety to the metal ion. This renders these compounds the OFF-ON-OFF type of pH-dependent fluorescent sensor. The thermodynamic stability and coordination behaviour of the chelator with the said lanthanide ions have also been probed by potentiometric, UV-visible and fluorescence spectrophotometric method. TAME5OX forms protonated complex [Ln (H4L)]4+ below pH ~4.0 which sequentially deprotonates through one proton process with increase of pH. The stability constants of neutral complexes have been determined to be in the range log β110=32-34 and pLn in the range of 14-20, indicating TAME5OX is a good synthetic lanthanide chelator. Theoretical spectra were also calculated by ZINDO/s methodology at single excitations (CIS) level on PM7 as sparkle energy-minimized geometries.

A Macrocyclic Chelator That Selectively Binds Ln4+ over Ln3+ by a Factor of 1029.

A tetravalent cerium macrocyclic complex (CeLK4) was prepared with an octadentate terephthalamide ligand comprised of hard catecholate donors and characterized in the solution state by spectrophotometric titrations and electrochemistry and in the crystal by X-ray diffraction. The solution-state studies showed that L exhibits a remarkably high affinity toward Ce4+, with log β110 = 61(2) and ΔG = -348 kJ/mol, compared with log β110 = 32.02(2) for the analogous Pr3+ complex. In addition, L exhibits an unusual preference for forming CeL4- relative to formation of the analogous actinide complex, ThL4-, which has β110 = 53.7(5). The extreme stabilization of tetravalent cerium relative to its trivalent state is also evidenced by the shift of 1.91 V in the redox potential of the Ce3+/Ce4+ couple of the complex (measured at -0.454 V vs SHE). The unprecedented behavior prompted an electronic structure analysis using L3- and M5,4-edge X-ray absorption near-edge structure (XANES) spectroscopies and configuration interaction calculations, which showed that 4f-orbital bonding in CeLK4 has partial covalent character due to ligand-to-metal charge transfer (LMCT) in the ground state. The experimental results are presented in the context of earlier measurements on tetravalent cerium compounds, indicating that the amount of LMCT for CeLK4 is similar to that observed for [Et4N]2[CeCl6] and CeO2 and significantly less than that for the organometallic sandwich compound cerocene, (C8H8)2Ce. A simple model to rationalize changes in 4f orbital bonding for tri- and tetravalent lanthanide and actinide compounds is also provided.

Speciation Studies of Metals in Trace Concentrations: The Mononuclear Uranyl(VI) Hydroxo Complexes.

A direct luminescence spectroscopic experimental setup for the determination of complex stability constants of mononuclear uranyl(VI) hydrolysis species is presented. The occurrence of polynuclear species is prevented by using a low uranyl(VI) concentration of 10–8 M (2.4 ppb). Time-resolved laser-induced fluorescence spectra were recorded in the pH range from 3 to 10.5. Deconvolution with parallel factor analysis (PARAFAC) resulted in three hydrolysis complexes. A tentative assignment was based on thermodynamic calculations: UO22+, UO2(OH)+, UO2(OH)2, UO2(OH)3–. An implementation of a Newton–Raphson algorithm into PARAFAC allowed a direct extraction of complex stability constants during deconvolution yielding log(β1M,1°C)1:1 = −4.6, log(β1M,1°C)1:2 = −12.2, log(β1M,1°C)1:3 = −22.3. Extrapolation to standard conditions gave log(β0)1:1 = −3.9, log(β0)1:2 = −10.9, and log(β0)1:3 = −20.7. Luminescence characteristics (band position, lifetime) of the individual mononuclear hydroxo species were derived to serve as a reference data set for further investigations. A correlation of luminescence spectroscopic features with Raman frequencies was demonstrated for the mononuclear uranyl(VI) hydroxo complexes for the first time. Thereby a signal-to-structure correlation was achieved and the complex assignment validated.

Amidoximes as ligand functionalities for braided polymeric materials for the recovery of uranium from seawater

The formation constants of the UO22+ cation with the amidoximate ligands bzam (benzamidoxime) and acetam (acetamidoxime) are reported. These are of interest in light of their proposed use as the functional groups of extractants for uranium in seawater. The formation constants of bzam with UO22+ were measured by monitoring the absorbance of the π→π∗ transitions in the UV spectrum of the bzam ligand in the presence of 1:1 UO22+ as a function of pH. This yielded log K1=12.4 for UO22+ with bzam, and log K=6.9 for the equilibrium UO2(bzam)++OH−=UO2(bzam)OH at 25°C and ionic strength zero. The bzam complexes were also studied monitoring the fluorescence of the UO22+ system. Analysis of the intense fluorescence that occurs in 5×10−6M UO22+ solutions between pH 5 and 9 suggested that this was due to the [(UO2)3O(OH)3]+ trimer. Monomeric species such as UO22+ and [UO2(OH)4]2−, and dimers such as [(UO2)(OH)2]2+, fluoresce only weakly. Titration of such solutions with bzam supported the above log K values measured by absorbance, and with higher bzam concentrations yielded log β2=22.3. The acetam ligand does not have any absorbance, so that complex-formation was monitored by fluorescence only. Formation constants measured by fluorescence may differ from those measured by other techniques such as absorbance. The agreement obtained between log K values measured by absorbance and fluorescence for the bzam complex of UO22+ supported the log K values measured for the acetam complexes by florescence alone were reliable: log K1=13.6, log β2=23.7, and log K UO2(acetam)++OH−=UO2(acetam)OH=6.8. The high log K values found for the bzam and acetam complexes of UO22+ were analyzed using DFT calculations. These log K values are related to the ability of polymer-based extractants bearing bzam or acetam type functional groups to extract UO22+ at a concentration of 1.3×10−8M and in the competing 0.0025M CO32− present in the oceans.

Calcium complexation and acid-base properties of l-gulonate, a diastereomer of d-gluconate.

The Ca(ii)-complexation and acid-base properties of l-gulonic acid (HGul), a diastereomer of d-gluconic acid (HGluc) differing only in the configurations of C2 and C5 have been investigated via1H and 13C NMR spectroscopies, Ca-ISE- and pH-potentiometry, polarimetry and freezing point depression. Data obtained for Gul-/HGul have been compared with those of Gluc-/HGluc. It was found that some properties (acid dissociation constant, the stoichiometry and formation constants of the Ca(ii)-complexes) were insensitive to the difference in the configuration. In solutions with pH close to neutral, the presence of the complexes CaGul+ and CaGul20 was unambiguously proven, with formation constants of log K1,1 = 0.88 ± 0.02 and log β1,2 = 1.51 ± 0.03 (I = 1 M, T = 25 °C). The formation of Ca(Gluc)20 was also observed by others, which implies that the formation of the charge neutral 1 : 2 Ca(ii)-complex of sugar carboxylates is more common than was previously believed. The stability of these species was found not to vary significantly in the ionic strength range of 1-4 M. Polarimetric measurements attested that the structure of Gul- did not change markedly upon complexation. NMR experiments suggest the coordination of C2-OH and C3-OH groups (beside COO-). DFT calculations support the existence of two coordination isomers, in which Ca2+ is attached to the COO-, C2-OH and C3-OH (in agreement with NMR), as well as to the COO-, C3-OH and C4-OH groups.

Complex formation equilibria of 2,2′-bipyridyl and 1,10-phenanthroline with manganese(II) in methanol

Water photolysis to molecular hydrogen and oxygen is one of the most promising chemical reactions for a sustainable hydrogen generation. The search for catalysts for this process is an important research area; some of the most suggested catalysts are manganese complexes. However, several studies propose the use of the complex: tris(bipyridine)ruthenium(II) chloride to be used as catalyst in artificial photosynthesis. In spite of this, the conferred properties by the 2,2′-bipyridyl ligand, in manganese or ruthenium complexes has not yet been discussed extensively.In this work, in order to obtain more insight of the behavior of manganese(II) with several ligands such as 2,2′-bipyridyl or 1,10-phenanthroline, a spectrophotometric study of the speciation in methanol of these systems at 298K, was performed. The formation constants obtained for the manganese(II)-2, 2′-bipyridyl system are: log β110 = 7.81 and log β120 = 14.68; for the manganese(II)-1,10-phenanthroline system are log β110 = 6.94 and log β120 = 12.86. These results were obtained by refining the experimental spectrophotometric data using the HypSpec software. In addition, a brief discussion of the comparison of the calculated individual electronic spectra of the complexes reported in this work with the spectrum of the tris(bipyridine)ruthenium(II) chloride and the manganese-calcium oxygen-evolving complex of photosystem II is presented.

Mono-, Bi-, and Trinuclear Bis-Hydrated Mn2+ Complexes as Potential MRI Contrast Agents

We report a series of ligands containing pentadentate 6,6′-((methylazanediyl)bis(methylene))dipicolinic acid binding units that form mono- (H2dpama), di- (mX(H2dpama)2), and trinuclear (mX(H2dpama)3) complexes with Mn2+ containing two coordinated water molecules per metal ion, which results in pentagonal bipyramidal coordination around the metal ions. In contrast, the hexadentate ligand 6,6′-((ethane-1,2-diylbis(azanediyl))bis(methylene))dipicolinic acid (H2bcpe) forms a complex with distorted octahedral coordination around Mn2+ that lacks coordinated water molecules. The protonation constants of the ligands and the stability constants of the Mn2+, Cu2+, and Zn2+ complexes were determined using potentiometric and spectrophotometric titrations in 0.15 M NaCl. The pentadentate dpama2– ligand and the di- and trinucleating mX(dpama)24– and mX(dpama)36– ligands provide metal complexes with stabilities that are very similar to that of the complex with the hexadentate ligand bcpe2–, with log β101 values in the range 10.1–11.6. Cyclic voltammetry experiments on aqueous solutions of the [Mn(bcpe)] complex reveal a quasireversible system with a half-wave potential of +595 mV versus Ag/AgCl. However, [Mn(dpama)] did not suffer oxidation in the range 0.0–1.0 V, revealing a higher resistance toward oxidation. A detailed 1H NMRD and 17O NMR study provided insight into the parameters that govern the relaxivity for these systems. The exchange rate of the coordinated water molecules in [Mn(dpama)] is relatively fast, kex298 = (3.06 ± 0.16) × 108 s–1. The trinuclear [mX(Mn(dpama)(H2O)2)3] complex was found to bind human serum albumin with an association constant of 1286 ± 55 M–1 and a relaxivity of the adduct of 45.2 ± 0.6 mM–1 s–1 at 310 K and 20 MHz.