Zita Csendes

Using low-frequency IR spectra for the unambiguous identification of metal ion–ligand coordination sites in purpose-built complexes

One of the aims of our long-term research is the identification of metal ion-ligand coordination sites in bioinspired metal ion-C- or N-protected amino acid (histidine, tyrosine, cysteine or cystine) complexes immobilised on the surface of chloropropylated silica gel or Merrifield resin. In an attempt to reach this goal, structurally related, but much simpler complexes have been prepared and their metal ion-ligand vibrations were determined from their low-frequency IR spectra. The central ions were Mn(II), Co(II), Ni(II) or Cu(II) and the ligands (imidazole, isopropylamine, monosodium malonate) were chosen to possess only one-type of potential donor group. The low-frequency IR spectra were taken of the complexes for each ion-ligand combination and the typical metal ion-functional group vibration bands were selected and identified. The usefulness of the obtained assignments is demonstrated on exemplary immobilised metal ion-protected amino acid complexes.

Multinuclear complex formation in aqueous solutions of Ca(II) and heptagluconate ions.

The equilibria and structure of complexes formed between the Ca(2+) ion and the heptagluconate (Hglu(-)) ion in both neutral and alkaline solutions have been studied. In alkaline solutions an uncharged, multinuclear complex is formed with the composition of Ca3Hglu2(OH)4 (or [Ca3Hglu2H(-4)](0)) with an unexpectedly high stability constant (lg β(32-4) = 14.09). The formation of the trinuclear complex was deduced from potentiometry and confirmed by freezing-point depression measurements and conductometry as well. The binding sites of Hglu(-) were determined from NMR measurements. Besides the carboxylate group, the O atoms on the second and third carbon atoms proved to be the most probable sites for Ca(2+) binding.

Superoxide dismutase mimicking Cu(II)-mixed amino acid complexes covalently grafted onto silica gel--an FT-IR study.

AbstractOur recent work concerning the synthesis, characterisation and testing of bioinspired electron transfer catalysts is described in this contribution. The catalysts were various Cu(II) complexes having mixed C- or N-protected amino acids (l-histidine and l-tyrosine) as ligands covalently grafted onto surface-modified silica gel. The resulting materials were structurally characterised by FT-IR spectroscopy, and their superoxide dismutase activities were tested. The covalently anchored Cu(II) complexes displayed appreciable activities in the test reaction; thus, they may be considered as promising candidates as durable electron transfer catalysts approaching the efficiency of the enzyme mimicked. FigureAlthough the active site mimicking, covalently anchored Cu-mixed amino acid complexes are less effective catalysts than the superoxide dismutase enzyme, but they can work under harsh experimental conditions as well

Synthesis of high-quality, well-characterized CaAlFe-layered triple hydroxide with the combination of dry-milling and ultrasonic irradiation in aqueous solution at elevated temperature

The combination of mechanochemical and ultrasonic treatment was applied to synthesize CaAlFe-layered triple hydroxides with carbonate or chloride anions in the interlamellar space. The optimal parameters of the preparation were explored by altering the initial ratio of the metal ions and the temperature of ultrasonic irradiation. The resulting triple hydroxides were characterized by X-ray diffractometry, infrared and X-ray absorption spectroscopies, thermogravimetric analysis and scanning electron microscopy. The products were close-to-phase-pure CaAlFe-layered triple hydroxides. Elevation of the temperature transformed the CaAlFe-Cl(-)-layered triple hydroxide to rare oxyhalides (Ca2FeO3Cl and Ca12Al14O32Cl2).

Calcium L-tartrate complex formation in neutral and in hyperalkaline aqueous solutions

The complex formation reaction between the l-tartrate (Tar2-) and calcium ions taking place in neutral and in hyperalkaline (pH > 13) aqueous solutions has been investigated. It was demonstrated that upon NaOH addition the solubility of the CaTar(s) precipitate significantly increases. Conductometric and freezing point depression measurements further confirmed that in this process water soluble species are formed as a result of a reaction between the CaTar(s) and the hydroxide ion (or, conversely, between Ca(OH)2(s) and the Tar2- ion). 13C NMR spectroscopic measurements yielded the value of pK3 = 15.4 ± 0.2 for the proton dissociation of one of the alcoholic OH groups of Tar2- (at 25.0 °C and 4 M Na(Cl) ionic strength). Upon addition of calcium ions to an alkaline Tar2- solution, the 1H NMR signal gradually broadened and the 13C-satellite peaks split to two components, which also indicate complexation. From H2/Pt potentiometric titrations performed with solutions in the 13.6 ≤ pH ≤ 14.4 range, it was observed, that this complex formation is accompanied by a hydroxide ion consuming process. The titration curves can be best described via assuming the formation of the CaTarH-1-(aq) (lg β11-1 = -11.2 ± 0.1) and CaTarH-22-(aq) (lg β11-2 = -25.3 ± 0.1) complexes. In hyperalkaline solutions, these two species account for more than 90-99% of the calcium ions present and the contribution of the other reasonable and well-established calcium-containing solution species is rather small. The possible structures of the above complexes have been modeled via ab initio calculations. The stoichiometries are consistent both with species containing coordinated alcoholate group(s) and with mixed Ca(ii)-hydroxo-tartrato complexes. From the data available at present, both types of structures can be considered as chemically reasonable.

Fe-amino acid complexes immobilized on silica gel as active and highly selective catalysts in cyclohexene epoxidation

In this work, the syntheses, structure, superoxide dismutase (SOD) activity, and the catalytic use in the oxidative transformations of cyclohexene of covalently grafted Fe(III)-complexes formed with various or various combinations of C-protected amino acid (l-histidine, l-tyrosine, l-cysteine and l-cystine) ligands are presented. The structural features of the surface complexes were studied by XANES/EXAFS and mid/far-IR spectroscopies. The compositions of the complexes were determined by ICP-MS and the Kjeldahl method. The SOD activities of the materials were evaluated in a biochemical test reaction. The obtained materials were used as catalysts for the oxidation of cyclohexene with peracetic acid in acetone. Both covalent grafting and building the complex onto the surface of the chloropropylated silica gel were successful in most cases. In many instances, the obtained structures and the coordinating groups were found to substantially vary upon changing the conditions of the syntheses. All the covalently immobilized Fe(III)-complexes displayed SOD activities, and most of them were found to be capable of catalyzing the oxidation of cyclohexene with appreciably high activities and outstanding epoxide selectivities.

Bioinspired covalently grafted Cu(II)–C protected amino acid complexes: selective catalysts in the epoxidation of cyclohexene

In this work, the syntheses of covalently grafted C-protected Cu(II)–amino acid (methylesters of l-histidine and l-cystine) uniform and mixed ligand complexes with two different amino acid esters are described using chloropropylated silica gel as the support. The conditions of the syntheses were systematically altered. The structural features of the substances obtained were studied by the Kjeldahl method, ICP-MS, X-ray absorption and mid/far infrared spectroscopies. The superoxide dismutase-like activities of the materials were determined in a biochemical test reaction and these substances were also tested as catalysts in the oxidation of cyclohexene. It was possible to prepare metal ion-amino acid complexes grafted with covalent bonds onto the supports. All the covalently anchored materials displayed superoxide dismutase-like activity and most of them were active in the oxidation of cyclohexene, providing the epoxide with high selectivity.