Eu(III) and Cm(III) Complexation by the Aminocarboxylates NTA, EDTA, and EGTA Studied with NMR, TRLFS, and ITC-An Improved Approach to More Robust Thermodynamics.

The complex formation of Eu(III) and Cm(III) was studied via tetradentate, hexadentate, and octadentate coordinating ligands of the aminopolycarboxylate family, viz., nitrilotriacetate (NTA3-), ethylenediaminetetraacetate (EDTA4-), and ethylene glycol-bis(2-aminoethyl ether)-N,N,N',N'-tetraacetate (EGTA4-), respectively. Based on the complexones' pKa values obtained from 1H nuclear magnetic resonance (NMR) spectroscopic pH titration, complex formation constants were determined by means of the parallel-factor-analysis-assisted evaluation of Eu(III) and Cm(III) time-resolved laser-induced fluorescence spectroscopy (TRLFS). This was complemented by isothermal titration calorimetry (ITC), providing the enthalpy and entropy of the complex formation. This allowed us to obtain genuine species along with their molecular structures and corresponding reliable thermodynamic data. The three investigated complexones formed 1:1 complexes with both Eu(III) and Cm(III). Besides the established Eu(III)-NTA 1:1 and 1:2 complexes, we observed, for the first time, the existence of a Eu(III)-NTA 2:2 complex of millimolar metal and ligand concentrations. Demonstrated for thermodynamic studies on Eu(III) and Cm(III) interaction with complexones, the utilized approach is commonly applicable to many other metal-ligand systems, even to high-affinity ligands.

Mononuclear high-spin iron(III) phthalocyanines.

2,9(or 10),16(or 17), 23(or 24)-Tetradecyloxycarbonylphthalocyaninatoiron, FeTDPc, and 2,3,9,10,16,17,23,24-octadecyloxycarbonylphthalocyaninatoiron, FeODPc, were synthesized and characterized. These compounds seem to be in trivalent iron high-spin state in solvents such as chloroform, dichloromethane, benzene, and chlorobenzene, although their counter anion could not be detected by elemental analyses. They react with strong bases such as pyridine and imidazoles to form their mono- and subsequently their di-base complexes with formation constant of >106 and < 200 dm3 mol-1, respectively, in dichloromethane at 20 °C. The resultant mono-adducts appear to be trivalent iron low-spin while the di-base adducts are bivalent iron low-spin state complexes. The addition of ca. 10-30 equivalent of tetrabutylammonium-chloride or -bromide (electrolyte) to the solution containing FeTDPc or FeODPc, causes their spin-state change from iron(III) high to low-spin state. In a solid power state, however, both FeTDPc and FeODPc exist as a mixture of high-spin iron(III)- and intermediate-spin iron(II) species. Strangely, when these compounds are dissolved in polystyrene, i.e. each molecules are isolated from each other, the signals originated from the iron(II) component disappear.

Complex formation equilibria of polyamine ligands with copper(II) and zinc(II).

A comprehensive study of the protonation equilibria of a series of polyamine ligands along with their complex formation equilibria with Cu2+ and Zn2+ is reported in this work. The primary aim of this study has been the achievement of homogeneous thermodynamic data on these ligands, in order to evaluate their influence on the homeostatic equilibria of essential metal ions (Cu2+ and Zn2+) in biological fluids. These polyamines are largely used as linkers in the building of chelating agents for iron overload. Potentiometric and spectrophotometric techniques were used for the characterization of protonation and complex formation constants. In addition, the characterization of the formed complexes is discussed together with selected solid-state crystal structures, remarking the influence of the length of the chain and of the linear or tetradentate tripod nature of the polyamine ligands on the stability of the complexes.

Efficient Alkaline Isomerization of Lactose to Lactulose in the Presence of an Organogermanium Compound.

Lactulose, a disaccharide widely used in pharmaceuticals and functional foods, is produced by lactose isomerization. Lactose and lactulose have an aldose-ketose relationship. Less than 25 % conversion of lactose into lactulose is achieved using the Lobry de Bruyn-Alberda van Ekenstein transformation with heating, whereas the conversion is increased to 80 % by the addition of an approximately equimolar concentration of the organogermanium compound 3-(trihydroxygermyl)propanoic acid (THGP) to the reaction mixture. To further understand this phenomenon, in this study, we analyzed the affinity between THGP and sugar isomers using 1H nuclear magnetic resonance spectroscopy. For the dimethyl derivative of THGP with lactose and lactulose, the complex formation ratios at 0.1 M (1:1 mixing ratio) were 14 and 59 %, respectively, with complex formation constants of 1.8 and 43 M-1, respectively. The complex formation capacity was approximately 24-fold higher for lactulose than for lactose. Moreover, THGP is considered to protect lactulose from alkaline degradation, resulting in high production yield of lactulose. Therefore, we concluded that high affinity for the isomerization product may promote isomerization and that promotion of sugar isomerization using an organogermanium compound is an effective method for converting lactose to lactulose.

Efficient Conversion of D-Glucose to D-Fructose in the Presence of Organogermanium Compounds.

D-Glucose and D-fructose are isomers of commonly consumed monosaccharides. The ratio of conversion of D-glucose to D-fructose by glucose isomerase (xylose isomerase) is not more than 50 %. However, addition of an equimolar ratio of the organogermanium compound poly-trans-[(2-carboxyethyl)germasesquioxane] (Ge-132) or its derivative increases the conversion ratio to 80 %. In contrast, use of the Lobry de Bruyn-Alberda van Ekenstein transformation with heating results in a lower conversion ratio, less than 30 %, whereas addition of an equimolar concentration of Ge-132 or its derivative to this reaction mixture increases the ratio to 73 %. Therefore, in this study, we aimed to further analyze the affinity between organogermanium compounds (i.e., Ge-132 and its derivatives) and sugar using 1H-nuclear magnetic resonance (NMR) spectrometry. For the dimethyl derivative of Ge-132, the complex formation ratios at 0.25 M (mixing ratio 1:1) were 19 and 74 % for D-glucose and D-fructose, respectively. Additionally, the complex formation constants between monosaccharides and Ge-132 were 1.2 and 46 M-1 for D-glucose and D-fructose, respectively. The complex formation capacity was approximately 40-fold higher for D-fructose than for D-glucose. Therefore, we concluded that the high affinity for the product of isomerization may promote isomerization, and that promotion of sugar isomerization using organogermanium compounds is an effective method for conversion of D-glucose to D-fructose.

Iron(III) and aluminium(III) complexes with substituted salicyl-aldehydes and salicylic acids.

The chelating properties toward iron(III) and aluminium(III) of variously substituted salicyl-aldehydes and salicylic acids have been evaluated, together with the effect of methoxy and nitro substituents in ortho and para position with respect to the phenolic group. The protonation and iron and aluminium complex formation equilibria have been studied by potentiometry, UV-visible spectrophotometry and (1)H NMR spectroscopy. The overall results highlight that salicyl-aldehydes present good chelating properties toward iron(III), with pFe ranging from 14.2 with nitro to 15.7 with methoxy substituent, being ineffective toward aluminium; the pFe values for salicylic acids are generally lower than those for salicyl-aldehydes, and about 4 units higher than the corresponding pAl values. The effect of the two substituents on the chelating properties of the ligands can be rationalized in terms of the Swain-Lupton treatment which accounts for the field and resonance effects. The structural characterization of the 1:2 iron complex with p-nitro salicylic acid shows that iron(III) ion exhibits an octahedral surrounding where two salicylate chelating ligands supply two O-phenolate and two O-carboxylate donor atoms in a roughly equatorial plane. The trans-apical sites are occupied by two aqua ligands.