No abstract

Chlorophylls, owing to their adjustable pi-electron system and intense, well-separated electronic transitions, can serve as convenient intrinsic spectroscopic probes of ligand-metal center interactions. They are also interesting for their photosensitizing properties. In order to examine the heavy-atom effects on the chlorophyll triplet state, a key intermediate in chlorophyll-photosensitized reactions, the synthesis of a novel Pt(II)-substituted chlorophyll a was carried out, and the effects of the substitution on steady-state and transient photophysical properties of chlorophyll were studied by absorption and fluorescence spectroscopies, and by laser flash photolysis. The presence of highly electronegative platinum as the central ion increases the energies of the chlorophyll main absorption transitions. As laser flash photolysis experiments show, in air-equilibrated solutions, chlorophyll triplets are efficiently quenched by molecular oxygen. Interestingly, this quenching by oxygen is more effective with metal-containing pigments, in spite of the increased spin-orbit coupling, introduced with the central metals. This points to occurrence of nonspecific interactions of molecular oxygen with metallochlorophylls. The differences in the effects exerted on the pigment triplet by the central metal become distinct after the removal of oxygen. The lifetime of a Pt-chlorophyll triplet remains very short, in the range of only a few microseconds, unlike in the free-base and Mg- and Zn-substituted chlorophylls. Such drastic shortening of the triplet lifetime can be attributed to a large heavy-atom effect, implying that strong interactions must occur between the central Pt(II) ion and the chlorophyll macrocycle, which lead to a more efficient spin-orbit coupling in Pt-chlorophyll than in Pt-porphyrins.

High-valent iron-oxo species have been invoked as reactive intermediates in catalytic cycles of heme and nonheme enzymes. The studies presented herein are devoted to the formation of compound II model complexes, with the application of a water soluble (TMPS)Fe(III)(OH) porphyrin ([meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphinato]iron(III) hydroxide) and hydrogen peroxide as oxidant, and their reactivity toward selected organic substrates. The kinetics of the reaction of H2O2 with (TMPS)Fe(III)(OH) was studied as a function of temperature and pressure. The negative values of the activation entropy and activation volume for the formation of (TMPS)Fe(IV)=O(OH) point to the overall associative nature of the process. A pH-dependence study on the formation of (TMPS)Fe(IV)=O(OH) revealed a very high reactivity of OOH(-) toward (TMPS)Fe(III)(OH) in comparison to H2O2. The influence of N-methylimidazole (N-MeIm) ligation on both the formation of iron(IV)-oxo species and their oxidising properties in the reactions with 4-methoxybenzyl alcohol or 4-methoxybenzaldehyde, was investigated in detail. Combined experimental and theoretical studies revealed that among the studied complexes, (TMPS)Fe(III)(H2O)(N-MeIm) is highly reactive toward H2O2 to form the iron(IV)-oxo species, (TMPS)Fe(IV)=O(N-MeIm). The latter species can also be formed in the reaction of (TMPS)Fe(III)(N-MeIm)2 with H2O2 or in the direct reaction of (TMPS)Fe(IV)=O(OH) with N-MeIm. Interestingly, the kinetic studies involving substrate oxidation by (TMPS)Fe(IV)=O(OH) and (TMPS)Fe(IV)=O(N-MeIm) do not display a pronounced effect of the N-MeIm axial ligand on the reactivity of the compound II mimic in comparison to the OH(-) substituted analogue. Similarly, DFT computations revealed that the presence of an axial ligand (OH(-) or N-MeIm) in the trans position to the oxo group in the iron(IV)-oxo species does not significantly affect the activation barriers calculated for C-H dehydrogenation of the selected organic substrates.

The presented results cover a comparative mechanistic study on the reactivity of compound (Cpd) I and II mimics of a water-soluble iron(III) porphyrin, [meso-tetrakis(2,4,6-trimethyl-3-sulfonatophenyl)porphinato]iron(III), Fe(III)(TMPS). The acidity of the aqueous medium strongly controls the chemical nature and stability of the high-valent iron(IV) oxo species. Reactivity studies were performed at pH 5 and 10, where the Cpd I and II mimics are stabilized as the sole oxidizing species, respectively. The contributions of ΔH(‡) and ΔS(‡) to the free energy of activation (ΔG(‡)) for the oxidation of 4-methoxybenzaldehyde (4-MB-ald), 4-methoxybenzyl alcohol (4-MB-alc), and 1-phenylethanol (1-PhEtOH) by the Cpd I and II mimics were determined. The relatively large contribution of the ΔH(‡) term in comparison to the -TΔS(‡) term to ΔG(‡) for reactions involving the Cpd II mimic indicates that the oxidation of selected substrates by this oxidizing species is clearly an enthalpy-controlled process. In contrast, different results were found for reactions with application of the Cpd I mimic. Depending on the nature of the substrate, the reaction at room temperature can be entropy-controlled, as found for the oxidation of 4-MB-alc, or enthalpy-controlled, as found for 1-PhEtOH. Importantly, for the first time, activation volumes (ΔV(‡)) for the oxidation of selected substrates by both reactive intermediates could be determined. Positive values of ΔV(‡) were found for reactions with the Cpd II mimic and slightly negative ones for reactions with the Cpd II mimic. The results are discussed in the context of the oxidation mechanism conducted by the Cpd I and II mimics.

Vanadium-based catalysts are used in many technological processes, among which the removal of nitrogen oxides (NOx) from waste gases is one of the most important. The chemical reaction responsible for this selective catalytic reaction (SCR) is based on the reduction of NOx molecules to N2, and a possible reductant in this case is pre-adsorbed NH3. In this paper, NH3 adsorption on Brønsted OH acid centers on low-index surfaces of V2O5 (010, 100, 001) is studied using a theoretical DFT method with a gradient-corrected functional (RPBE) in the embedded cluster approximation model. The results of the calculations show that ammonia molecules are spontaneously stabilized on all low-index surfaces of the investigated catalyst, with adsorption energies ranging from −0.34 to −2 eV. Two different mechanisms of ammonia adsorption occur: the predominant mechanism involves the transfer of a proton from a surface OH group and the stabilization of ammonia as an NH4+ cation bonded to surface O atom(s), while an alternative mechanism involves the hydrogen bonding of NH3 to a surface OH moiety. The latter binding mode is present only in cases of stabilization over a doubly coordinated O(2) center at a (100) surface. The results of the calculations indicate that a nondirectional local electrostatic interaction with ammonia approaching a surface predetermines the mode of stabilization, whereas hydrogen-bonding interactions are the main force stabilizing the adsorbed ammonia. Utilizing the geometric features of the hydrogen bonds, the overall strength of these interactions was quantified and qualitatively correlated (R = 0.93) with the magnitude of the stabilization effect (i.e., the adsorption energy).FigureTwo different modes (NH3/NH4 +) of ammonia adsorption on the (001)V2O5 net plane.Electronic supplementary materialThe online version of this article (doi:10.1007/s00894-013-1951-4) contains supplementary material, which is available to authorized users.

Oxidative dehydrogenation (ODH) of light alkanes to olefins-in particular, using vanadium-based catalysts-is a promising alternative to the dehydrogenation process. Here, we investigate how the activity of the vanadium phase in ODH is related to its dispersion in porous matrices. An attempt was made to synthesize catalysts in which vanadium was deposited on a microporous faujasite zeolite (FAU) with the hierarchical (desilicated) FAU as supports. These yielded different catalysts with varying amounts and types of vanadium phase and the porosity of the support. The phase composition of the catalysts was confirmed by X-ray diffraction (XRD); low temperature nitrogen sorption experiments resulted in their surface area and pore volumes, and reducibility was measured with a temperature-programmed reduction with a hydrogen (H 2 -TPR) method. The character of vanadium was studied by UV-VIS spectroscopy. The obtained samples were subjected to catalytic tests in the oxidative dehydrogenation of propane in a fixed-bed gas flow reactor with a gas chromatograph to detect subtract and reaction products at a temperature range from 400-500 • C, with varying contact times. The sample containing 6 wt% of vanadium deposited on the desilicated FAU appeared the most active. The activity was ascribed to the presence of the dispersed vanadium ions in the tetragonal coordination environment and support mesoporosity.

The aim of this study is to elucidate the geometry and electronic structure of various adducts that may be formed between manganese(III) (Mn(III)) porphyrin and hydrogen peroxide. Hydrogen peroxide may interact with Mn(III) porphyrin either as H2O2 or, after dissociation, as OOH–. In the former, it may decompose into two hydroxo groups, which acquire OH– character or an oxo group (=O) and a water molecule. Therefore, the following systems are considered: MnP(H2O2)+, MnP(H2O2)(OH), MnP(OH)3, [Formula: see text], MnPO+, MnPO(OH), MnP(OOH), MnP(OOH)(OH)–, and the possible transformations between them are taken into account. The reported studies are performed within the Density Functional Theory (DFT) method with the GGA-BP functional. The geometry and electronic structures of the structures found along the studied reaction pathways are discussed in terms of interatomic distances, valence angles, Mulliken charges, and spin densities. It was found that different active oxygen species may be formed in the reaction between Mn(III) porphyrin and hydrogen peroxide. As manganese is a transition metal, numerous possible spin states for each of the studied structures are found, where the relative energies of different multiplicities depend strongly on the ligands present in the complex. In view of the catalytic properties, all oxygen-containing ligands are negatively charged, which results in their behaviour as nucleophiles towards hydrocarbons. Finally, the analysis of charge and spin populations on different parts of the studied systems indicate the porphyrin ligand as active in charge transfer processes.

Three non-native derivatives of vitamin B12 with imidazole, ethylenediamine, and pyrazine as cobalt(III) β-ligands were characterized by applying the BP/def2-TZVP density functional method. The binding of all three ligands is thermodynamically favorable. It is proposed that their synthesis might be possible from aquacobalamin as a starting form of vitamin B12, as has been done in the case of an imidazole derivative of B12 (Hannibal et al. Inorg. Chem. 2007, 46, 3613-3618). Furthermore, the possibility of the formation of their conjugates with cisplatin is investigated. The proposed β-ligands may serve as bridging ligands, binding to the platin ion as N-donors. In parallel, the calculations are done for the previously synthetized B12-cisplatin adduct with CN(-) as a bridging ligand and are compared with available experimental data, allowing assessment of the applied computational protocol. A good agreement between the computed and experimental structural parameters is obtained. In each of the studied structures, the Co-β-ligand bond is weaker than the Pt-β-ligand bond.