Oxygen activation by a macrocyclic chromium complex. Mechanism of hydroperoxo-chromium(III) to oxo-chromium(V) transformation

The transformation of the hydroperoxo complex L1(H2O)CrOOH2+ (L1 = 1, 4 8, 11-tetraazacyclotetradecane) to an oxo-chromium(v) species is a first-order process throughout the pH range examined, 1.7 < pH < 9.2. The pH dependence of the rate constant (k1) yielded an apparent pKa of 5.6 for L1(H2O)CrOOH2+. In the acidic range, (pH <4), the value of k1 is 0.191 s(-1). At the other extreme, pH >7.5, k(1)= 0.025 s(-1). No [H+]-dependence is observed within the two limiting regimes, clearly ruling out a simple attack by H+ at the hydroperoxo group. The temperature dependence of k1 in 0.020 M HClO4 yielded the activation parameters DeltaH++ = 53.7 kJ mol(-1) and DeltaS++ = -80.5 J mol(-1) K(-1).     

Collision-induced dissociation of a series of coumarins studied by positive- and negative-ion electrospray triple quadrupole tandem mass spectrometry

Aryl-substituted 4-hydroxycoumarins (1-57) were investigated by electrospray ionisation (ESI) mass spectrometry. Their fragmentation in the ion source or in the collision cell of a triple quadrupole mass spectrometer was investigated. The effect of the substituents in the aromatic ring on the fragmentation of the 4-hydroxycoumarin derivatives is shown. The influence of the tautomerism on the formation of quasimolecular ions and mass spectral fragmentation was explained. Mass spectral studies on some deuterated compounds proved some of the proposed fragmentation pathways. Results obtained are very useful in the process of detection and characterisation of 4-hydroxycoumarins, as well as for structural elucidation of their more complex derivatives.     

Reduction and oxidation of hydroperoxo rhodium(III) complexes by halides and hypobromous acid

Oxygen atom transfer from trans-L(H(2)O)RhOOH(2+) [L = [14]aneN(4) (L(1)), meso-Me(6)[14]aneN(4) (L(2)), and (NH(3))(4)] to iodide takes place according to the rate law -d[L(H(2)O)RhOOH(2+)]/dt = k(I)[L(H(2)O)RhOOH(2+)][I(-)][H(+)]. At 0.10 M ionic strength and 25 degrees C, the rate constant k(I)/M(-)(2) s(-)(1) has values of 8.8 x 10(3) [L = (NH(3))(4)], 536 (L(1)), and 530 (L(2)). The final products are LRh(H(2)O)(2)(3+) and I(2)/I(3)(-). The (NH(3))(4)(H(2)O)RhOOH(2+)/Br(-) reaction also exhibits mixed third-order kinetics with k(Br) approximately 1.8 M(-)(2) s(-)(1) at high concentrations of acid (close to 1 M) and bromide (close to 0.1 M) and an ionic strength of 1.0 M. Under these conditions, Br(2)/Br(3)(-) is produced in stoichiometric amounts. As the concentrations of acid and bromide decrease, the reaction begins to generate O(2) at the expense of Br(2), until the limit at which [H(+)] 2(NH(3))(4)(H(2)O)RhOH(2+) + O(2); i.e., the reaction has turned into the bromide-catalyzed disproportionation of coordinated hydroperoxide. In the proposed mechanism, the hydrolysis of the initially formed Br(2) produces HOBr, the active oxidant for the second equivalent of (NH(3))(4)(H(2)O)RhOOH(2+). The rate constant k(HOBr) for the HOBr/(NH(3))(4)(H(2)O)RhOOH(2+) reaction is 2.9 x 10(8) M(-)(1) s(-)(1).     

Kinetics of acid-catalyzed O-atom transfer from a hydroperoxorhodium complex to organic and inorganic substrates

Oxygen atom transfer from (NH(3))(4)(H(2)O)RhOOH(2+) to organic and inorganic nucleophiles takes place according to the rate law -d[(NH(3))(4)(H(2)O)RhOOH(2+)]/dt = k[H(+)] [(NH(3))(4)(H(2)O)RhOOH(2+)][nucleophile] for all the cases examined. The third-order rate constants were determined in aqueous solutions at 25 degrees C for (CH(2))(5)S (k = 430 M(-)(2) s(-)(1), micro = 0.10 M), (CH(2))(4)S(2) (182, micro = 0.10 M), CH(3)CH(2)SH (8.0, micro = 0.20 M), (en)(2)Co(SCH(2)CH(2)NH(2))(2+) (711, micro = 0.20 M), and, in acetonitrile-water, CH(3)SPh (130, 10% AN, micro = 0.20 M), PPh(3) (3.74 x 10(3), 50% AN), and (2-C(3)H(7))(2)S (45, 50% AN, micro = 0.20 M). Oxidation of PPh(3) by (NH(3))(4)(H(2)O)Rh(18)O(18)OH(2+) produced (18)OPPh(3). The reaction with a series of p-substituted triphenylphosphines yielded a linear Hammett relationship with rho = -0.53. Nitrous acid (k = 891 M(-)(2) s(-)(1)) is less reactive than the more nucleophilic nitrite ion (k = 1.54 x 10(4) M(-)(2) s(-)(1)).     

Synthesis of some N-substituted isonicotinamides

A one-pot transformation of some cyclic 1,3-dicarbonyls 1 with N-(isonicotinoyl)glycine 2 and one-carbon synthons in acetic anhydride to the corresponding N-substituted isonicotinamides (pyridine-4-carbox-amides) 7-9 containing a fused pyran-2-one ring is described. Compound 8 was further converted with some nitrogen-containing nucleophiles either to the corresponding quinoline- 2,5 -diones 10-11 or 5-hydra-zonobenzopyran-2-ones 14-15. Under more severe conditions the compound 8 gave with hydrazine 5-hydrazonoquinoline derivative 12 or even 13 . Hydrazoic acid transformed compound 8 to the pyrano-[3,2-c]azepine system 16 . Diazotization of 1-amino derivative 10 gave deaminated product 11.     

Kinetics of oxidation of nitroxyl radicals with superoxometal complexes of chromium and rhodium

In acidic aqueous solutions, nitroxyl radicals (X)TEMPO (X = H, 4-OH, and 4-oxo) and 3-carbamoyl-PROXYL readily reduce CraqOO2+ and Rh(NH3)4(H2O)OO2+ to the corresponding hydroperoxo complexes. The kinetics are largely acid independent for CraqOO2+, but acid catalysis dominates the reactions of the rhodium complex. This emerging trend in oxidations with superoxometal complexes seems to be directly related to the thermodynamics of electron transfer. The weaker the oxidant, the more important the acid-assisted path. The rate constants for the oxidation of (X)TEMPO by CraqOO2+ are 406 M(-1) s(-1) (X = H), 159 (4-OH), and (20. 6 + 77.5 [H+]) (4-oxo). For the rhodium complex, the values are (40 + 2.20 x 10(3) [H+]) (X = H), (25 + 1.10 x 10(3) [H+]) (4-HO), and 2.21 x 10(3) [H+] (4-oxo). An inverse solvent kinetic isotope effect, kH/kD = 0.8, was observed in the reaction between (O)TEMPO and (NH3)4(H(D))2O)RhOO2+ in 0.10 M H(D)ClO4 in H2O and D2O.     

Simultaneous determination of trace levels of ethylmercury and methylmercury in biological samples and vaccines using sodium tetra(<Emphasis Type="Italic">n</Emphasis>-propyl)borate as derivatizing agent

Because of increasing awareness of the potential neurotoxicity of even low levels of organomercury compounds, analytical techniques are required for determination of low concentrations of ethylmercury (EtHg) and methylmercury (MeHg) in biological samples. An accurate and sensitive method has been developed for simultaneous determination of methylmercury and ethylmercury in vaccines and biological samples. MeHg and EtHg were isolated by acid leaching (H₂SO₄–KBr–CuSO₄), extraction of MeHg and EtHg bromides into an organic solvent (CH₂Cl₂), then back-extraction into Milli-Q water. MeHg and EtHg bromides were derivatized with sodium tetrapropylborate (NaBPr₄), collected at room temperature on Tenax, separated by isothermal gas chromatography (GC), pyrolysed, and detected by cold-vapour atomic fluorescence spectrometry (CV AFS). The repeatability of results from the method was approximately 5–10% for EtHg and 5–15% for MeHg. Detection limits achieved were 0.01 ng g⁻¹ for EtHg and MeHg in blood, saliva, and vaccines and 5 ng g⁻¹ for EtHg and MeHg in hair. The method presented has been shown to be suitable for determination of background levels of these contaminants in biological samples and can be used in studies related to the health effects of mercury and its species in man. This work illustrates the possibility of using hair and blood as potential biomarkers of exposure to thiomersal.     

NMR and HPLC study of chiral selectors with naphthyl unit

Doubling of resonances in NMR spectra of chiral selectors with naphthyl group attached to the tertiary amide nitrogen atom has been noticed what revealed the presence of two isomers. To test the enantiorecognition ability of these chiral selectors they are covalently bonded to the HPLC silica gel. Those kind of chiral stationary phases were compared with analogous commercial leucine chiral stationary phase. They exhibit the better enantioseparation results which indicate that the existence of cis/trans isomers does not have the negative influence on their enantioselective ability.

A note on representation of lattices by weak congruences

A weak congruence is a symmetric, transitive, and compatible relation. An element u of an algebraic lattice L is ??-suitable if there is an isomorphism ?? from L to the lattice of weak congruences of an algebra such that ??(u) is the diagonal relation. Some conditions implying the ??-suitability of u are presented.     

Solid-state synthesis of undoped and Sr-doped K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript>

The solid-state synthesis of undoped K_0.5Na_0.5NbO₃ (KNN) and KNN doped with 1, 2 and 6 mol% Sr, from potassium, sodium and strontium carbonates with niobium pentoxide, was studied using thermal analysis and in situ high-temperature X-ray diffraction (HT-XRD). The thermogravimetry and the differential thermal analyses with evolved-gas analyses showed that the carbonates, which were previously reacted with the moisture in the air to form hydrogen carbonates, partly decomposed when heated to 200 °C. In the temperature interval where the reaction was observed, i.e., between 200 and 750 °C, all the samples exhibited the main mass loss in two steps. The first step starts at around 400 °C and finishes at 540 °C, and the second step has an onset at 540 °C and finishes with the end of the reaction between 630 and 675 °C, depending on the particle size distribution of the Nb₂O₅ precursor. According to the HT-XRD analysis, the perovskite phase is formed at 450 °C for all the samples, regardless of the Sr content. The formation of a polyniobate phase with a tetragonal tungsten bronze structure was detected by HT-XRD in the KNN with the largest amount of Sr dopant, i.e., 6 mol% of Sr, at 600 °C.