Maria Louloudi

Electron spin echo envelope modulation (ESEEM) spectroscopy as a tool to investigate the coordination environment of metal centers

Abstract The applications of electron spin echo envelope modulation (ESEEM) spectroscopy to study paramagnetic metal centers in metalloproteins and bioinorganic complexes are reviewed, with special attention to the novel spectroscopic techniques applied and the structural information obtained. We summarize the physical principles and experimental techniques of ESEEM, the spectral shapes and the methods for their analysis. The physical meaning of the spin Hamiltonian parameters is highlighted in conjunction with their utilization for obtaining structural information.

Highly ordered mesoporous zirconia-polyoxometalate nanocomposite materials for catalytic oxidation of alkenes

A series of well-ordered mesoporous ZrO2-based heteropoly acid nanocomposite frameworks has been prepared through a surfactant-assisted sol–gel copolymerization route. The pore walls of these materials consist of nanocrystalline tetragonal ZrO2 and Keggin-type 12-phosphomolybdic acid (PMA) components with different PMA loadings, i.e. 12, 22 and 37 wt%. Small angle X-ray scattering, high-resolution TEM and N2 physisorption measurements indicated mesoporous property in hexagonal p6mm symmetry with large internal BET surface areas and narrow-sized pores. The incorporated PMA clusters preserve intact their Keggin structure into the mesoporous frameworks according to EDX, FT-IR and diffuse-reflectance UV/vis/NIR spectroscopy. The obtained ZrO2–PMA nanocomposites demonstrated great application potential in oxidative catalysis, exhibiting exceptional stability and catalytic activity in oxidation of alkenes using hydrogen peroxide as oxidant.

Eight-coordination in nitrato manganese(II) complexes with tetradentate di-Schiff bases derived from 2-pyridyl ketones: preparation, characterization and catalytic activity for alkene epoxidation

Abstract The reactions of Mn(NO3)2·6H2O with the tetradentate Schiff bases N,N′-bis[1-(pyridin-2-yl)ethylidene]ethane-1,2-diamine (LA) and N,N′-bis[1-(pyridin-2-yl)benzylidene]ethane-1,2-diamine (LB) afforded the novel eight-coordinate complexes [Mn(NO3)2(LA)] (1) and [Mn(NO3)2(LB)] (2), which have a distorted dodecahedral coordination geometry. The catalytic activity of 1 and 2 for alkene epoxidation is reported and discussed.

Homogeneous and heterogenized copper(II) complexes as catechol oxidation catalysts

Abstract Two macroacyclic ligands represented as L1 and L2 with 3N2O and 5N donor atoms, respectively, have been synthesized by Schiff base condensation. They were subsequently grafted on a silica surface via covalent bonds. The organic ligands L1 and L2 as well as the heterogenized ligands L1·SiO2 and L2·SiO2 reacted with copper(II) leading to the formation of dinuclear copper(II) complexes. Catalytic oxidation of 3,5-di-t-butylcatechol (DTBC) by dioxygen was studied using as catalysts the homogeneous Cu2(L1) and Cu2(L2) and the heterogenized Cu2(L1)·SiO2 and Cu2(L2)·SiO2 complexes. These complexes were found to be very effective catalysts for DTBC oxidation producing mainly 3,5-di-t-butylquinone (DTBQ). During the catalytic process the formation of an o-semiquinone radical has also been confirmed. The immobilized on modified silica surface copper(II) complexes gave significantly higher DTBC conversion than the homogeneous copper(II) complexes.

Heterogeneous clay-manganese(II) oxidation catalyst

A manganese(II)-Schiff base complex has been heterogenized by its intercalation into clay minerals. The incorporation of the homogeneous manganese(II) complexes in the interlayer space of aluminosilicate mineral is accomplished by a cation exchange process. The obtained clay-manganese(II) composite has been studied by chemical analysis, X-ray diffraction and FT-IR spectroscopy. The new catalytic material has been evaluated as oxidation catalyst. Our results showed that this composite exhibits significant catalytic activities for alkene epoxidation using hydrogen peroxide as oxidant.

Alkene epoxidation catalysed by binuclear manganese complexes

Binuclear manganese(II) complexes with macrocyclic ligands have been synthesized by template Schiff base condensation of diethylenetriamine and pentane-2,4-dione or 1,3-diphenyl-propane-1,3-dione. Catalytic epoxidation of simple olefins with hydrogen peroxide and t-BHP were studied using the above manganese complexes in the presence of a base. The influence of reaction temperature, the additive methanol and the cocatalyst had been investigated. The major products of the oxidations were the epoxides. The new manganese complexes showed significant catalytic activities for the epoxidation of alkenes using hydrogen peroxide as oxidant and ammonium acetate as cocatalyst.

EPR study of a novel [Fe–porphyrin] catalyst

This paper reports on an EPR and catalytic study of a novel [Fe-porphyrin] catalyst, [FeR4P], used for the catalytic decomposition of pentacholorophenol (PCP). The porphyrin complex [FeR4P] bears 2,6-di-tert-butylphenols (where R = di-ButPhen) at each meso-aryl position of the porphyrin ring. The FeR4P-SiO2 catalyst was found to be very efficient for the degradation of PCP, using NaIO4 as oxidant plus imidazole as cocatalyst. EPR spectroscopy shows that in the presence of the oxidant NaIO4, at a redox potential of Eh = 250 mV, [FeR4P] forms a high valent [FeIV = O Por+· ] state with an EPR spectrum characterized by effective g-values g ⊥ eff  = 3.41 and g || eff  = 2, similar to Compound I of Horseradish Peroxidase. The [FeIV = O Por+· ] state can described by a S = 1 state of the oxoferryl FeIV = O weakly coupled by exchange interaction J to a S′ = 1/2 porphyrin cation radical. The zero field splitting of the FeIV = O is D = +16.5 K and the coupling parameters are J/D = 0.31, J = +5.2 K. We suggest that the formation of [FeIV = O Por+· ] specie to be the key intermediate responsible for the observed efficient catalytic decomposition of PCP.

Thiamine models and perspectives on the mechanism of action of thiamine-dependent enzymes.

Thiamine dependent enzymes catalyze ligase and lyase reactions near a carbonyl moiety. Chemical models for these reactions serve as useful tools to substantiate a detailed mechanism of action. This tutorial review covers all such studies performed thus far, emphasizing the role of each part around the active site and the conformation of the cofactor during catalysis.

On the mechanism of action of thiamin enzymes, Crystal structure of 2-(α-hydroxyethyl)thiamin pyrophosphate (HETPP). Complexes of HETPP with zinc(II) and cadmium(II)

Abstract The crystal structure of the 2-(α-hydroxethyl) thiamin pyrophosphate (LH2) was solved by X-ray diffraction. Crystallographic data: space group F2dd, a=7.922(4) Å, b=33.11(2) Å, c=36.232(10) Å, V=9503(9) Å3, z=16. Metal complexes of the general formula K2{[M(LH)Cl2]2} (M=Zn2+, Cd2+) were isolated from methanolic solutions and characterized by elemental analysis, IR, Raman, and 13C CP MAS NMR spectra. They were also characterized by 13C NMR, 31P NMR, 113Cd NMR, ES-MS, and 1H NMR ROESY spectra in D2O solutions. The data provide evidence for the bonding of the metals to the N(1′) atom of the pyrimidine ring and to the pyrophosphate group. The free ligand and the metal-coordinated ligand adopt the S conformation. Since thiamin cofactor, substrate, and metal ions are present in our system, the extracted results directly refer to thiamin catalysis and possible functional implications are correlated and discussed.

Metal complexes with 2-(α-hydroxy-benzyl) thiamin pyrophosphate (HBTPP). Models for metal binding of thiamin enzymes

The reactions of MCl2 (M = Zn2+, Cd2+, Hg2+) with 2-(α-hydroxy-benzyl)thiamin pyrophosphate (HBTPP) at various pH values (different protonation states) were studied in methanolic solutions. Solid complexes of formulae K[Zn(HBTPP) −Cl2 · H2On, K2[Cd(HBTPP)2−Cl2 · 3H2On, K2[Hg(HBTPP)2−Cl2 · 3H2O and Zn(HBTPP)20Cl2 were isolated and characterized by elemental analysis and various NMR techniques, namely 13C NMR, 31P NMR, 113Cd NMR, 199Hg NMR and 1H NMR ROESY spectra in D2O. The data provide evidence that Zn(II) in K[Zn(HBTPP) −Cl2 · H2On, and Cd(II) in K2[Cd(HBTPP)2−Cl2 · 3H2On, are coordinated both to the pyrimidine N(1′) and to the pyrophosphate group. In contrast, Hg(II) in K2[Hg(HBTPP)2−Cl2 · 3H2O and Zn(II) in Zn(HBTPP)20Cl2 are bound only to the N(1′) atom or to the pyrophosphate group, respectively.