Solid-Phase "Self-Hydrolysis" of [Zn(NH3)4MoO4@2H2O] Involving Enclathrated Water-An Easy Route to a Layered Basic Ammonium Zinc Molybdate Coordination Polymer.

An aerial humidity-induced solid-phase hydrolytic transformation of the [Zn(NH3)4]MoO4@2H2O (compound 1@2H2O) with the formation of [(NH4)xH(1-x)Zn(OH)(MoO4)]n (x = 0.92-0.94) coordination polymer (formally NH4Zn(OH)MoO4, compound 2) is described. Based on the isostructural relationship, the powder XRD indicates that the crystal lattice of compound 1@2H2O contains a hydrogen-bonded network of tetraamminezinc (2+) and molybdate (2-) ions, and there are cavities (O4N4(µ-H12) cube) occupied by the two water molecules, which stabilize the crystal structure. Several observations indicate that the water molecules have no fixed positions in the lattice voids; instead, the cavity provides a neighborhood similar to those in clathrates. The @ symbol in the notation is intended to emphasize that the H2O in this compound is enclathrated rather than being water of crystallization. Yet, signs of temperature-dependent dynamic interactions with the wall of the cages can be detected, and 1@2H2O easily releases its water content even on standing and yields compound 2. Surprisingly, hydrolysis products of 1 were observed even in the absence of aerial humidity, which suggests a unique solid-phase quasi-intramolecular hydrolysis. A mechanism involving successive substitution of the ammonia ligands by water molecules and ammonia release is proposed. An ESR study of the Cu-doped compound 2 (2#dotCu) showed that this complex consists of two different Cu2+(Zn2+) environments in the polymeric structure. Thermal decomposition of compounds 1 and 2 results in ZnMoO4 with similar specific surface area and morphology. The ZnMoO4 samples prepared from compounds 1 and 2 and compound 2 in itself are active photocatalysts in the degradation of Congo Red dye. IR, Raman, and UV studies on compounds 1@2H2O and 2 are discussed in detail.

Effect of the Microstructure of the Semiconductor Support on the Photocatalytic Performance of the Pt-PtOx/TiO2 Catalyst System.

The influence of the semiconductor microstructure on the photocatalytic behavior of Pt-PtOx/TiO2 catalysts was studied by comparing the methanol-reforming performance of systems based on commercial P25 or TiO2 from sol-gel synthesis calcined at different temperatures. The Pt co-catalyst was deposited by incipient wetness and formed either by calcination or high-temperature H2 treatment. Structural features of the photocatalysts were established by X-ray powder diffraction (XRD), electron spin resonance (ESR), X-ray photoelectron spectroscopy (XPS), optical absorption, Raman spectroscopy and TEM measurements. In situ reduction of Pt during the photocatalytic reaction was generally observed. The P25-based samples showed the best H2 production, while the activity of all sol-gel-based samples was similar in spite of the varying microstructures resulting from the different preparation conditions. Accordingly, the sol-gel-based TiO2 has a fundamental structural feature interfering with its photocatalytic performance, which could not be improved by annealing in the 400-500 °C range even by scarifying specific surface area at higher temperatures.

Study of PtOx/TiO2 Photocatalysts in the Photocatalytic Reforming of Glycerol: The Role of Co-Catalyst Formation.

In this study, relationships between preparation conditions, structure, and activity of Pt-containing TiO2 photocatalysts in photoinduced reforming of glycerol for H2 production were explored. Commercial Aerolyst® TiO2 (P25) and homemade TiO2 prepared by precipitation-aging method were used as semiconductors. Pt co-catalysts were prepared by incipient wetness impregnation from aqueous solution of Pt(NH3)4(NO3)2 and activated by calcination, high temperature hydrogen, or nitrogen treatments. The chemico-physical and structural properties were evaluated by XRD, ¹H MAS NMR, ESR, XPS, TG-MS and TEM. The highest H2 evolution rate was observed over P25 based samples and the H2 treatment resulted in more active samples than the other co-catalyst formation methods. In all calcined samples, reduction of Pt occurred during the photocatalytic reaction. Platinum was more easily reducible in all of the P25 supported samples compared to those obtained from the more water-retentive homemade TiO2. This result was related to the negative effect of the adsorbed water content of the homemade TiO2 on Pt reduction and on particle growth during co-catalyst formation.

Catechol oxidase and phenoxazinone synthase activity of a manganese(II) isoindoline complex.

The mononuclear [Mn(6'Me(2)indH)(H(2)O)(2)(CH(3)CN)](ClO(4))(2) (6'Me(2)indH: 1,3-bis(6'-methyl-2'-pyridylimino)isoindoline) complex has been prepared and characterized by various techniques such as elemental analysis, IR, UV-visible and ESR spectroscopy. The title compound was suitable as catalyst for the catalytic oxidation of 3,5-di-tert-butylcatechol (3,5-DTBCH(2)) to 3,5-di-tert-butyl-1,2-benzoquinone (3,5-DTBQ) (catecholase activity), and o-aminophenol (OAPH) to 2-aminophenoxazine-3-one (APX) (phenoxazinone synthase activity) with dioxygen at ambient condition in good yields. Kinetic measurements revealed first-order dependence on the catalyst and dioxygen concentration and saturation type behavior with respect to the corresponding substrate. It was also found that the added triethylamine in both systems accelerates the reaction.

Cerium(IV)-mediated oxidation of flavonol with relevance to flavonol 2,4-dioxygenase. Direct evidence for spin delocalization in the flavonoxy radical.

The cerium(IV)-mediated oxidation of 3-hydroxy-4'-methylflavone (1) proceeds by H-atom abstraction forming the flavonoxy radical (7), and the subsequent combination of its resonance forms leads to the 3-hydroxy-4'-methylflavone dehydro dimer (9). The above system serves as direct evidence for the intermediacy of the flavonoxy radical, its spin delocalization, and also indirect evidence for valence tautomerism as a key step on the substrate activation both in the quercetinase and its biomimic model system.

Biomimetic oxidation of 3,5-di-tert-butylcatechol by dioxygen via Mn-enhanced base catalysis.

The 6-coordinate dioximatomanganese(II) complex [Mn(HL)(CH3OH)]+ (2, where H2L is [HON=C(CH3)C(CH3)=NCH2CH2]2NH), formed by instant solvolysis of [Mn2(HL)2](BPh4)2 (1) in methanol, accelerates the triethylamine (TEA)-catalyzed oxidation of 3,5-di-tert-butylcatechol (H2dtbc) by O2 to the corresponding o-benzoquinone. Significantly, 2 alone has no catalytic effect. The observed rate increase can be explained by the interaction of 2 with the hydroperoxo intermediate HdtbcO2- formed from Hdtbc- and O2 in the TEA-catalyzed oxidation. The kinetics of the TEA-catalyzed and Mn-enhanced reaction has been studied by gas-volumetric monitoring of the amount of O2 consumed. The initial rate of O2 uptake (V(in)) shows a first-order dependence on the concentration of 2 and O2 and saturation kinetics with respect to both H2dtbc and TEA. The observed kinetic behavior is consistent with parallel TEA-catalyzed and Mn-enhanced oxidation paths. The 3,5-di-tert-butylsemiquinone anion radical is an intermediate detectable by electron spin resonance (ESR) spectroscopy. The dimeric catalyst precursor has been characterized by X-ray diffraction and electrospray ionization mass spectrometry and the monomeric catalyst by ESR spectroscopy.

Reinvestigation of the copper(II)-carcinine equilibrium system: "two-dimensional" EPR simulation and NMR relaxation studies for determining the formation constants and coordination modes.

The equilibria and solution structure of complexes formed between copper(II) and carcinine (beta-alanyl-histamine) at 2< or = pH< or =11.2 have been studied by EPR and NMR relaxation methods. Beside the species that have already been described in the literature from pH-potentiometric measurements, several new complexes have been identified and/or structurally characterized. The singlet on the EPR spectrum detected in equimolar solutions at pH 7, indicates the formation of an oligomerized (CuL)n(2n+) complex, with [NH2,Nim] coordination. The oligomerization is probably associated with the low stability of the ten-membered macrochelate ring, which would form in the mononuclear complex CuL2+. In presence of moderate excess of ligand the formation of four new bis-complexes (CuL2Hn(2+n), n=2,1 and 0/-1) was detected with [Nim][Nim], [NH2,Nim][Nim] and [NH2,N-,Nim][Nim] type co-ordination modes, respectively. At higher excess of ligand ([L]/[Cu2+]>10) and at pH approximately 7, the predominant species is CuL4H2(4+). The 1H and 13C relaxation measurements of carcinine solutions (0.6 M) in presence of 0 mM< or = [Cu2+](tot)< or = 5 mM at pH=6.8, allowed us to extract the carbon-to-metal distances, the electronic relaxation and tumbling correlation times, as well as the ligand exchange rate for the species CuL4H2(4+). According to these results, the metal ion is [4Nim] co-ordinated in the equatorial plane, while the neutral amino groups are unbounded. Since naturally occurring carcinine shows in vivo antioxidant property, the SOD-like activity of the copper(II)-carcinine system has also been investigated and the complex CuLH(-1) was found to be highly active.

Great structural variety of complexes in copper(II)-oligoglycine systems: microspeciation and coordination modes as studied by the two-dimensional simulation of electron paramagnetic resonance spectra.

A series of isotropic EPR spectra recorded at various concentrations and pH (in the range 2-12) on equilibrium systems containing copper(II) and diglycine, triglycine, or tetraglycine were analyzed. A purely mathematical method, matrix rank analysis gave the number of independent EPR-active species. Two-dimensional evaluation then resulted in the formation constants and magnetic parameters of 14 metal complexes (including microspecies) in each system. The independent paramagnetic species formed with each ligand are as follows: Cu(2+) (aqua complex), [CuLH](2+), [CuL](+), [CuLH(-1)], [CuLH(-2)](-), [CuL(2)H(2)](2+), [CuL(2)H](+), [CuL(2)], [CuL(2)H(-1)](-), and [CuL(2)H(-2)](2-). Moreover, for diglycine, the diamagnetic complex [Cu(2)L(2)H(-3)](-), and for triglycine and tetraglycine, the EPR-active species [CuLH(-3)](2-) were identified. Further, equilibria of two microspecies were demonstrated for [CuL(2)], [CuL(2)H(-1)](-), and [CuL(2)H(-2)](2-). The magnetic parameters allowed a detailed description of the coordination modes. The most important findings: (1) For the mono complexes, the in-plane sigma-bonds between copper(II) and the equatorial N donors are particularly strong when the same ligand forms several adjacent chelate rings with the participation of amino N, deprotonated peptide N(s), and the carboxylate group. (2) Structures with coupled chelate rings are likewise favored in the bis complexes. Different protonation states of the two ligands are observed in the major isomer of [CuL(2)] ((LH(-1) + LH) coordination), and in the isomers of [CuL(2)H(-2)](2-) ((LH(-2) + L) coordination) for triglycine and tetraglycine.

Synthesis, structure and catecholase activity of dinuclear copper and zinc complexes with an N(3)-ligand.

The preparation and characterization of dinuclear [M(II)(dbcat)(idpa)](2) (M[double bond]Zn (1), Cu (3); dbcat[double bond]3,5-di-tert-butylcatecholate; idpa[double bond]3,3'-iminobis(N,N-dimethylpropylamine)) complexes are described. Crystallographic characterization of the complex [Cu(II)(dbcat)(idpa)](2) has shown that the co-ordination geometry around copper(II) ions is distorted square pyramidal (triclinic, P-1, a=10.576(1) A, b=11.927(1) A, c=12.621(1) A, alpha=77.89(1) degrees, beta=88.65(1) degrees, gamma=70.21(1) degrees, V=1462.7(2) A(3), Z=2, R=0.0387). Both 1 and 3 were suitable catalysts for the catalytic oxidation of dbcatH(2) to dtbq (dtbq=3,5-di-tert-butyl-1,2-benzoquinone) with dioxygen at ambient conditions in good yields. However, on the basis of kinetic studies the copper- and zinc-catalyzed reactions showed different mechanisms. In the first case valence tautomerism [Cu(II)(dbcat)(idpa)]<==>[Cu(I)(dbsq)(idpa)] precedes the reaction with O(2), while with the zinc complex metal-bound catecholate reacts directly with O(2) with the formation of free superoxide anion.

Microspeciation in the copper(II)-L-histidylglycine system. An ESR study by the two-dimensional computer simulation method.

Twelve ESR-active (and one inactive) copper(II) complexes of L-histidylglycine (HL) were characterized via their formation (micro)constants and ESR parameters obtained by two-dimensional ESR spectroscopic evaluation in aqueous solution. In strongly acidic media, the ligand is coordinated through its N-terminal donor groups: the complex [CuLH(2)](3+) involves monodentate imidazole binding, whereas [CuLH](2+) involves bidentate ligation through the amino and imidazole N atoms. This histamine-like bonding mode also predominates in the isomers of [CuL(2)], formed at ligand excess near pH 7: in the major 4N isomer, both ligands occupy two equatorial sites, while in the 3N isomer, the second dipeptide is coordinated equatorially by the amino and axially by the imidazole groups. At above pH 3-4, deprotonation of the peptide group also starts: in approximately 60% of the molecules of [CuL](+), the peptide group is deprotonated, while in the minor isomer histamine-like coordination occurs. At higher pH, the active dimer [Cu(2)L(2)H(-2)], the mixed hydroxo complexes (the inactive [Cu(2)L(2)H(-3)](-) and the active [CuLH(-2)](-)), and the bis complexes [CuL(2)H](+) and [CuL(2)H(-1)](-) all involve tridentate equatorial ligation of the backbone by the amino and deprotonated peptide N and the carboxylate O atoms. In the active dimer, the neutral imidazole groups form bridges between CuLH(-1) units. In [CuL(2)H](+), the second ligand is bound equatorially via its imidazole group; in [CuL(2)H(-1)](-), the L ligand occupies the fourth equatorial site and an axial site through its amino and imidazole N atoms, respectively.