Autocatalytic modification of the prosthetic heme of horseradish but not lactoperoxidase by thiocyanate oxidation products. A role for heme-protein covalent cross-linking

The mammalian peroxidases eosinophil peroxidase, lactoperoxidase (LPO), and myeloperoxidase oxidize thiocyanate to the antimicrobial agents hypothiocyanous acid (HOSCN) and (SCN)2 and are part of a defense system that protects the host from infections. Horseradish peroxidase (HRP), a plant enzyme, also oxidizes thiocyanate. We report here that the prosthetic heme vinyl groups of HRP react with the catalytically generated HOSCN and (SCN)2 to form at least nine vinyl-modified heme adducts. Mass spectrometry combined with analysis of the equivalent reactions of HRP reconstituted with 2- or 4-cyclopropylheme, or mesoheme-d4, shows that all of the prosthetic heme modifications result from addition of oxidized thiocyanate to the heme vinyl groups. No delta-meso-substitution of the heme was observed, in contrast to what is observed with radical agents. Model studies show that incubation of either HRP with preformed HOSCN or a solution of heme with preformed (SCN)2 gives rise to the same products obtained in the HRP-catalyzed reaction. Model studies also demonstrate that the SCN* radical, if formed, should add to a meso-carbon. These findings implicate an electrophilic addition mechanism. In contrast, oxidation by LPO of thiocyanate, the normal substrate of this enzyme, does not result in heme modification. In view of the demonstrated intrinsic reactivity of the heme group, LPO must actively suppress heme modification. As the key difference between LPO (and other mammalian peroxidases) and HRP is the presence of two covalent ester links between the heme and the protein, we propose that these links contribute to steric protection of the adjacent heme vinyl groups.     

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).     

Radical energies and the regiochemistry of addition to heme groups. Methylperoxy and nitrite radical additions to the heme of horseradish peroxidase

The heme of hemoproteins, as exemplified by horseradish peroxidase (HRP), can undergo additions at the meso carbons and/or vinyl groups of the electrophilic or radical species generated in the catalytic oxidation of halides, pseudohalides, carboxylic acids, aryl and alkyl hydrazines, and other substrates. The determinants of the regiospecificity of these reactions, however, are unclear. We report here modification of the heme of HRP by autocatalytically generated, low-energy NO2* and CH3OO* radicals. The NO2* radical adds regioselectively to the 4- over the 2-vinyl group but does not add to the meso positions. Reaction of HRP with tert-BuOOH does not lead to heme modification; however, reaction with the F152M mutant, in which the heme vinyls are more sterically accessible, results in conversion of the heme 2-vinyl into a 1-hydroxy-2-(methylperoxy)ethyl group [-CH(OH)CH2OOCH3]. [18O]-labeling studies indicate that the hydroxyl group in this adduct derives from water and the methylperoxide oxygens from O2. Under anaerobic conditions, methyl radicals formed by fragmentation of the autocatalytically generated tert-BuO* radical add to both the delta-meso carbon and the 2-vinyl group. The regiochemistry of these and the other known additions to the heme indicate that only high-energy radicals (e.g., CH3*) add to the meso carbon. Less energetic radicals, including NO2* and CH3OO*, add to heme vinyl groups if they are small enough but do not add to the meso carbons. Electrophilic species such as HOBr, HOCl, and HOSCN add to vinyl groups but do not react with the meso carbons. This meso- versus vinyl-reactivity paradigm, which appears to be general for autocatalytic additions to heme prosthetic groups, suggests that meso hydroxylation of the heme by heme oxygenase occurs by a controlled radical reaction rather than by electrophilic addition.     

Oxidation of carboxylic acids by horseradish peroxidase results in prosthetic heme modification and inactivation

Hemoproteins are powerful oxidative catalysts. However, despite the diversity of functions known to be susceptible to oxidation by these catalysts, it is not known whether they can oxidize carboxylic acids to carboxylic radicals. We report here that incubation of horseradish peroxidase (HRP) at acidic pH with H(2)O(2) in acetate buffer results in rapid modification of the heme group and loss of catalytic activity. Mass spectrometry and NMR indicate that an acetoxy group is covalently bound to the delta-meso-carbon in the modified heme. A heme with a hydroxyl group on the 8-methyl is also formed as a minor product. These reactions do not occur if protein-free heme and H(2)O(2) are co-incubated in acetate buffer, if the HRP reaction is carried out at pH 7, in the absence of H(2)O(2), or if citrate rather than acetate buffer is used. A similar heme modification is observed in incubations with n-caproic and phenylacetic acids. A mechanism involving oxidation of the carboxyl group to a carboxylic radical followed by addition to the delta-meso-position is proposed. This demonstration of the oxidation of a carboxylic acid solidifies the proposal that a carboxylic radical mediates the normal covalent attachment of the heme to the protein in the mammalian peroxidases and CYP4 family of P450 enzymes. The hemoprotein-mediated oxidation of carboxylic acids, ubiquitous natural constituents, may play other roles in biology.     

Kinetics and mechanism of oxidation of N, N'-dimethylaminoiminomethanesulfinic acid by acidic bromate

The major metabolites of the physiologically active compound dimethylthiourea (DMTU), dimethylaminoiminomethansesulfinic acid (DMAIMSA), and dimethylaminoiminomethanesulfonic acid (DMAIMSOA) were synthesized, and their kinetics and mechanisms of oxidation by acidic bromate and aqueous bromine was determined. The oxidation of DMAIMSA is much more facile and rapid as compared to a comparable oxidation by the same reagents of the parent compound, DMTU. The stoichiometry of the bromate-DMAIMSA reaction was determined to be 2BrO 3 (-) + 3NHCH 3(NCH 3)CSO 2H + 3H 2O --> 3SO 4 (2) (-) + 2Br (-) + 3CO(NHCH 3) 2 + 6H (+), with quantitative formation of sulfate. In excess bromate conditions, the stoichiometry was 4BrO 3 (-) + 5NHCH 3(NCH 3)CSO 2H + 3H 2O --> 5SO 4 (2) (-) + 2Br 2 + 5CO(NHCH 3) 2 + 6H (+). The direct bromine-DMAIMSA reaction gave an expected stoichiometric ratio of 2:1 with no further oxidation of product dimethylurea (DMU) by aqueous bromine. The bromine-DMAIMSA reaction was so fast that it was close to diffusion-controlled. Excess bromate conditions delivered a clock reaction behavior with the formation of bromine after an initial quiescent period. DMAIMSOA, on the other hand, was extremely inert to further oxidation in the acidic conditions used for this study. Rate of consumption of DMAIMSA showed a sigmoidal autocatalytic decay. The postulated mechanism involves an initial autocatalytic build-up of bromide that fuels the formation of the reactive oxidizing species HBrO 2 and HOBr through standard oxybromine reactions. The long and weak C-S bond in DMAIMSA ensures that its oxidation goes directly to DMU and sulfate, bypassing inert DMAIMSOA.     

Regioselective intramolecular oxidation of phenols and anisoles by dioxiranes generated in situ

A novel method for regioselective oxidation of phenols and anisoles has been developed in which dioxiranes, generated in situ from ketones and Oxone, oxidize phenol derivatives in an intramolecular fashion. A series of ketones with electron-withdrawing groups, such as CF(3), COOMe, and CH(2)Cl, were attached to phenols, anisoles, or aryl rings via a C(2) or C(3) methylene linker. In a homogeneous solvent system of CH(3)CN and H(2)O, oxidation of phenol derivatives 1-10 afforded spiro 2-hydroxydienones in 24-55% yields regardless of the presence of other substituents (ortho Me, meta Me or Br) on the aryl ring and the length of the linker. Experimental evidences were provided to support the mechanism that involves a regioselective pi bond epoxidation of aryl rings followed by epoxide rearrangement and hemiketal formation.     

哌啶氧铵盐对醇氧化反应的活性和选择性

系统地研究了12种具有不同4-位取代基(R=H、CH~3O、Cl)及反离子(X=Cl、BF~4、ClO~4、Br或Br~3)的2, 2, 6, 6-四甲基哌啶氧铵盐对醇的氧化反应, 发现这些氧铵盐都能以很高的由率将一级醇氧化为醛, 二级醇氧化为酮。氧化反应的活性与4-位取代基及反离子有关。当反离子相同时, 反应活性的顺序为Cl>CH~3O>H;当4-位取代基相同时, 反应活性的顺序为Cl^-》BF~4^->ClO~4^->Br^-。氧化反应的选择性主要与反离子有关, 当反离子为Cl^-时, 主要氧化一级醇; 当反离子为BF~4^-、ClO~4^-、Br^-或Br~3^-时, 主要氧化二级醇。     

Kinetics and mechanisms of the reactions of hypochlorous acid,chlorine, and chlorine monoxide with bromite ion

The reaction between BrO2(-) and excess HOCl (p[H+] 6-7, 25.0 degrees C) proceeds through several pathways. The primary path is a multistep oxidation of HOCl by BrO(2)(-) to form ClO(3)(-) and HOBr (85% of the initial 0.15 mM BrO(2)(-)). Another pathway produces ClO(2) and HOBr (8%), and a third pathway produces BrO(3)(-) and Cl(-) (7%). With excess HOCl concentrations, Cl(2)O also is a reactive species. In the proposed mechanism, HOCl and Cl(2)O react with BrO(2)(-) to form steady-state species, HOClOBrO(-) and ClOClOBrO(-). Acid facilitates the conversion of HOClOBrO(-) and ClOClOBrO(-) to HOBrOClO(-). These reactions require a chainlike connectivity of the intermediates with alternating halogen-oxygen bonding (i.e. HOBrOClO(-)) as opposed to Y-shaped intermediates with a direct halogen-halogen bond (i.e. HOBrCl(O)O(-)). The HOBrOClO(-) species dissociates into HOBr and ClO(2)(-) or reacts with general acids to form BrOClO. The distribution of products suggests that BrOClO exists as a BrOClO.HOCl adduct in the presence of excess HOCl. The primary products, ClO(3)(-) and HOBr, are formed from the hydrolysis of BrOClO.HOCl. A minor hydrolysis path for BrOClO.HOCl gives BrO(3)(-) and Cl(-). An induction period in the formation of ClO(2) is observed due to the buildup of ClO(2)(-), which reacts with BrOClO.HOCl to give 2 ClO(2) and Br(-). Second-order rate constants for the reactions of HOCl and Cl(2)O with BrO(2)(-) are k(1)(HOCl) = 1.6 x 10(2) M(-1) s(-1) and k(1)(Cl)()2(O) = 1.8 x 10(5) M(-)(1) s(-)(1). When Cl(-) is added in large excess, a Cl(2) pathway exists in competition with the HOCl and Cl(2)O pathways for the loss of BrO(2)(-). The proposed Cl(2) pathway proceeds by Cl(+) transfer to form a steady-state ClOBrO species with a rate constant of k(1)(Cl2) = 8.7 x 10(5) M(-1) s(-1).     

Kinetics and mechanisms of bromine chloride reactions with bromite and chlorite ions

Chloride ion catalyzes the reactions of HOBr with bromite and chlorite ions in phosphate buffer (p[H(+)] 5 to 7). Bromine chloride is generated in situ in small equilibrium concentrations by the addition of excess Cl(-) to HOBr. In the BrCl/ClO(2)(-) reaction, where ClO(2)(-) is in excess, a first-order rate of formation of ClO(2) is observed that depends on the HOBr concentration. The rate dependencies on ClO(2)(-), Cl(-), H(+), and buffer concentrations are determined. In the BrCl/BrO(2)(-) reaction where BrCl is in pre-equilibrium with the excess species, HOBr, the loss of absorbance due to BrO(2)(-) is followed. The dependencies on Cl(-), HOBr, H(+), and HPO(4)(2)(-) concentrations are determined for the BrCl/BrO(2)(-) reaction. In the proposed mechanisms, the BrCl/ClO(2)(-) and BrCl/BrO(2)(-) reactions proceed by Br(+) transfer to form steady-state levels of BrOClO and BrOBrO, respectively. The rate constant for the BrCl/ClO(2)(-) reaction [k(Cl)(2)]is 5.2 x 10(6) M(-1) s(-1) and for the BrCl/BrO(2)(-) reaction [k(Br)(2)]is 1.9 x 10(5) M(-1) s(-1). In the BrCl/ClO(2)(-) case, BrOClO reacts with ClO(2)(-) to form two ClO(2) radicals and Br(-). However, the hydrolysis of BrOBrO in the BrCl/BrO(2)(-) reaction leads to the formation of BrO(3)(-) and Br(-).     

Individual microdetermination of chlorine, bromine and iodine in halogenated organic compounds by the oxygen-flask method

A simple and accurate micro-method for the individual determination of Cl, Br and I in halogenated organic compounds by the oxygen-flask method is described. Addition of potassium bromate to oxidize bromide, and of hydrogen peroxide to oxidize iodide, followed by boiling, is used to eliminate these two halides in mixtures with chloride or each other.     

4-取代-2,6-二(羟甲基)苯酚的选择氧化

酚类由于本身容易氧化,仅在非常温和的条件下才能直接使4-取代-2,6-二(羟甲基)苯酚中的羟甲基氧化成醛基。文献报道了用活性二氧化锰可以将2,6-二(羟甲基)-4-甲基苯酚氧化成2-羟基-5-甲基-1,3-苯二甲醛,但要使两个羟甲基中只有一个被氧化却是困难的。文献报道了由芳氧基溴化镁与甲醛作用制备水杨醛类化合物的方法,并认为中间产物是邻羟甲基苯酚的镁盐,后者与甲醛之间通过负氢离子转移的分子间氧化还原反应生成相应     

New members of an old family: isolation of IC(O)Cl and IC(O)Br and evidence for the formation of weakly bound Br*...CO

The photochemically induced reactions of a dihalogen, XY, with CO isolated together in an Ar matrix at about 15 K lead to the formation of carbonyl dihalide molecules XC(O)Y, where X and Y may be the same or different halogen atoms, Cl, Br, or I. In addition to the known compounds OCCl2, OCBr2, and BrC(O)Cl, the carbonyl iodide chloride, IC(O)Cl, and carbonyl iodide bromide, IC(O)Br, compounds have thus been identified for the first time as products of the reactions involving ICl and IBr, respectively. The first product to be formed in reactions with Cl2, BrCl, or ICl is the ClCO* radical, which reacts subsequently with a second halogen atom to give the corresponding carbonyl dihalide [OCCl2, BrC(O)Cl, or IC(O)Cl]. The analogous reaction with Br2 affords, in low yield, the unusually weakly bound BrCO* radical, better described as a van der Waals complex, Br*...CO. The changes have been followed and the products characterized experimentally by their infrared spectra, and the spectra have been analyzed in light of the results afforded by ab initio (Hartree-Fock and Moeller-Plesset second-order) and density functional theory calculations.     

Uncatalyzed reactions in the classical Belousov-Zhabotinsky system. I. Reactions of bromomalonic acid with acidic bromate and with HOBr

The uncatalyzed reactions of bromomalonic acid (BrMA) with acidic bromate and with hypobromous acid were studied in 1 M sulfuric acid, a usual medium for the oscillatory Belousov-Zhabotinsky (BZ) reaction, by following the rate of the carbon dioxide evolution associated with these reactions. In addition, the decarboxylation rate of dibromomalonic acid (Br2MA) was also measured to determine the first-order rate constant of its decomposition (4.65 x 10(-5) s(-1) in 1 M H2SO4). The dependence of that rate constant on the hydrogen ion concentration suggests a carbocation formation. A slow oligomerization of BrMA observed in sulfuric acid solutions is also rationalized as a carbocationic process. The initial rate of the BrMA-BrO3- reaction is a bilinear function of the BrMA and BrO3- concentrations with a second-order rate constant of 3.8 x 10(-4) M(-1) s(-1). When a great excess of BrO3- is applied, then BrMA is oxidized mostly to CO2. A reaction scheme compatible with the experimental finding is also given. On the other hand, when less BrO3- and more organic substrate - BrMA or malonic acid (MA)--is applied, then addition reactions of various carbocations with the enol form of the organic substrates should be taken into account in later stages of the reaction. It was discovered that HOBr, which brominates BrMA to Br2MA when BrMA is in excess, can also oxidize BrMA when HOBr is in excess. As Br2MA does not react with HOBr, it is assumed that the acyl hypobromite, formed in the first step of the HOBr and BrMA reaction, can react with an additional HOBr to give oxidation products. It was found that the initial rate of the reaction can be described by the following experimental rate law: k(BHOB)[BrMA]0[HOBr]0(2), where k(BHOB) = 5 M(-2) s(-1). A reaction scheme for the oxidation of BrMA by HOBr is given for conditions where HOBr is in excess. Model calculations illustrate qualitatively that the suggested reaction schemes are able to mimic the experiments. (More quantitative simulations are prevented by kinetic data missing for the various carbocation intermediates.) Finally, the effects of these newly observed reactions on oscillatory BZ systems are discussed briefly.     

FeCl3-activated oxidation of alkanes by [Os(N)O3]-

Although the ion [Os(VIII)(N)(O)(3)](-) is a stable species and is not known to act as an oxidant for organic substrates, it is readily activated by FeCl(3) in CH(2)Cl(2)/CH(3)CO(2)H to oxidize alkanes efficiently at room temperature. The oxidation can be made catalytic by using 2,6-dichloropyridine N-oxide as the terminal oxidant. The active intermediates in stoichiometric and catalytic oxidation are proposed to be [(O)(3)Os(VIII)N-Fe(III)] and [Cl(4)(O)Os(VIII)N-Fe(III)], respectively.     

Coupled oxidation vs heme oxygenation: insights from axial ligand mutants of mitochondrial cytochrome b5

Mutation of His-39, one of the axial ligands in rat outer mitochondrial membrane cytochrome b(5) (OM cyt b(5)), to Val produces a mutant (H39V) capable of carrying out the oxidation of heme to biliverdin when incubated with hydrazine and O(2). The reaction proceeds via the formation of an oxyferrous complex (Fe(II)(-)O(2)) that is reduced by hydrazine to a ferric hydroperoxide (Fe(III)(-)OOH) species. The latter adds a hydroxyl group to the porphyrin to form meso-hydroxyheme. The observation that catalase does not inhibit the oxidation of the heme in the H39V mutant is consistent with the formation of a coordinated hydroperoxide (Fe(III)(-)OOH), which in heme oxygenase is the precursor of meso-hydroxyheme. By comparison, mutation of His-63, the other axial ligand in OM cyt b(5), to Val results in a mutant (H63V) capable of oxidizing heme to verdoheme in the absence of catalase. However, the oxidation of heme by H63V is completely inhibited by catalase. Furthermore, whereas the incubation of Fe(III)(-)H63V with H(2)O(2) leads to the nonspecific degradation of heme, the incubation of Fe(II)(-)H63V with H(2)O(2) results in the formation of meso-hydroxyheme, which upon exposure to O(2) is rapidly converted to verdoheme. These findings revealed that although meso-hydroxyheme is formed during the degradation of heme by the enzyme heme oxygenase or by the process of coupled oxidation of model hemes and hemoproteins not involved in heme catabolism, the corresponding mechanisms by which meso-hydroxyheme is generated are different. In the coupled oxidation process O(2) is reduced to noncoordinated H(2)O(2), which reacts with Fe(II)-heme to form meso-hydroxyheme. In the heme oxygenation reaction a coordinated O(2) molecule (Fe(II)(-)O(2)) is reduced to a coordinated peroxide molecule (Fe(III)(-)OOH), which oxidizes heme to meso-hydroxyheme.     

Kinetics and mechanisms of the oxidation of iodide and bromide in aqueous solutions by a trans-dioxoruthenium(VI) complex

The kinetics and mechanisms of the oxidation of I (-) and Br (-) by trans-[Ru (VI)(N 2O 2)(O) 2] (2+) have been investigated in aqueous solutions. The reactions have the following stoichiometry: trans-[Ru (VI)(N 2O 2)(O) 2] (2+) + 3X (-) + 2H (+) --> trans-[Ru (IV)(N 2O 2)(O)(OH 2)] (2+) + X 3 (-) (X = Br, I). In the oxidation of I (-) the I 3 (-)is produced in two distinct phases. The first phase produces 45% of I 3 (-) with the rate law d[I 3 (-)]/dt = ( k a + k b[H (+)])[Ru (VI)][I (-)]. The remaining I 3 (-) is produced in the second phase which is much slower, and it follows first-order kinetics but the rate constant is independent of [I (-)], [H (+)], and ionic strength. In the proposed mechanism the first phase involves formation of a charge-transfer complex between Ru (VI) and I (-), which then undergoes a parallel acid-catalyzed oxygen atom transfer to produce [Ru (IV)(N 2O 2)(O)(OHI)] (2+), and a one electron transfer to give [Ru (V)(N 2O 2)(O)(OH)] (2+) and I (*). [Ru (V)(N 2O 2)(O)(OH)] (2+) is a stronger oxidant than [Ru (VI)(N 2O 2)(O) 2] (2+) and will rapidly oxidize another I (-) to I (*). In the second phase the [Ru (IV)(N 2O 2)(O)(OHI)] (2+) undergoes rate-limiting aquation to produce HOI which reacts rapidly with I (-) to produce I 2. In the oxidation of Br (-) the rate law is -d[Ru (VI)]/d t = {( k a2 + k b2[H (+)]) + ( k a3 + k b3[H (+)]) [Br (-)]}[Ru (VI)][Br (-)]. At 298.0 K and I = 0.1 M, k a2 = (2.03 +/- 0.03) x 10 (-2) M (-1) s (-1), k b2 = (1.50 +/- 0.07) x 10 (-1) M (-2) s (-1), k a3 = (7.22 +/- 2.19) x 10 (-1) M (-2) s (-1) and k b3 = (4.85 +/- 0.04) x 10 (2) M (-3) s (-1). The proposed mechanism involves initial oxygen atom transfer from trans-[Ru (VI)(N 2O 2)(O) 2] (2+) to Br (-) to give trans-[Ru (IV)(N 2O 2)(O)(OBr)] (+), which then undergoes parallel aquation and oxidation of Br (-), and both reactions are acid-catalyzed.     

Counteranion-controlled water cluster recognition in a protonated octaamino cryptand

Structural aspects of binding of water cluster and halides in the octaamino cryptand L (1,4,11,14,17,24,29,36-octaazapentacyclo[12.12.12.2.(6,9)2.(19,22)2(31,34)]tetratetraconta-6(43),7,9(44),19(41),20,22(42),31(39),32,34(40)-nonaene, N(CH2CH2NHCH2-p-xylyl-CH2NHCH2CH2)3N) in a protonated state were examined. Crystallographic results show binding of the acyclic quasiplanar water tetramer [H4L(H2O)4](I)4.2.57H2O (1) in a tetraprotonated cryptand L having an iodide counteranion, where two water molecules reside inside the two tren-based cavity, bridged by a third water molecule, and a fourth external water molecule is hydrogen bonded to the bridged water molecule. In the case of complexes [H6L(Br)][(Br)6H].4H2O.2HBr (2) and [H6L(Cl)][(Cl)6H].10.86H2O (3), a single bromide and chloride occupied, respectively, the inside of the cryptand cavity, where L is in a hexaprotonated state. Monotopic recognition of bromide/chloride was observed at the center of the cryptand cavity where halides show C-H...halide interactions instead of the N-H...halide interactions reported in the ditopic complexes of halides with the same cryptand, 5 and 6. Thermal analyses on 1-3 were carried out, and the data obtained distinctly differentiate water cluster complex 1 from the anion-encapsulated cryptates 2 and 3. This study represents the first example of anion-controlled cluster formation inside the cavity of a cryptand.     

Production and collision-induced dissociation of gas-phase, water- and alcohol-coordinated uranyl complexes containing halide or perchlorate anions

Electrospray ionization was used to generate mono-positive gas-phase complexes of the general formula [UO2A(S)n]+ where A = OH, Cl, Br, I or ClO4, S = H2O, CH3OH or CH3CH2OH, and n = 1-3. The multiple-stage dissociation pathways of the complexes were then studied using ion-trap mass spectrometry. For H2O-coordinated cations, the dissociation reactions observed included the elimination of H2O ligands and the loss of HA (where A = Cl, Br or I). Only for the Br and ClO4 versions did collision-induced dissociation (CID) of the hydrated species generate the bare, uranyl-anion complexes. CID of the chloride and iodide versions led instead to the production of uranyl hydroxide and hydrated UO2+. Replacement of H2O ligands by alcohol increased the tendency to eliminate HA, consistent with the higher intrinsic acidity of the alcohols compared to water and potentially stronger UO2-O interactions within the alkoxide complexes compared to the hydroxide version.     

Observation of dihalide elimination upon electron attachment to oxalyl chloride and oxalyl bromide, 300-550 K

Rate coefficients have been measured for electron attachment to oxalyl chloride [ClC(O)C(O)Cl] and oxalyl bromide [BrC(O)C(O)Br] in He gas at 133 Pa pressure over the temperature range of 300-550 K. With oxalyl chloride, the major ion product of attachment is Cl2(-) at all temperatures (66% at 300 K); its importance increases slightly as temperature increases. Two other product ions formed are Cl- (18% at 300 K) and the phosgene anion CCl2O- (16% at 300 K) and appear to arise from a common mechanism. With oxalyl bromide, the Br2(-) channel represents almost half of the ion product of attachment, independent of temperature. Br- accounts for the remainder. For oxalyl chloride, the attachment rate coefficient is small [(1.8 +/- 0.5) x 10(-8) cm3 s(-1) at 300 K], and increases with temperature. The attachment rate coefficient for oxalyl bromide [(1.3 +/- 0.4) x 10(-7) cm3 s(-1) at 300 K] is nearly collisional and increases only slightly with temperature. Stable parent anions C2Cl2O2(-) and C2Br2O2(-) and adduct anions Cl- (C2Cl2O2) and Br- (C2Br3O2) were observed but are not primary attachment products. G2 and G3 theories were applied to determine geometries of products and energetics of the electron attachment and ion-molecule reactions studied. Electron attachment to both oxalyl halide molecules leads to a shorter C-C bond and longer C-Cl bond in the anions formed. Trans and gauche conformers of the neutral and anionic oxalyl halide species have similar energies and are more stable than the cis conformer, which lies 100-200 meV higher in energy. For C2Cl2O2, C2Cl2O2(-), and C2Br2O2(-), the trans conformer is the most stable conformation. The calculations are ambiguous as to the oxalyl bromide geometry (trans or gauche), the result depending on the theoretical method and basis set. The cis conformers for C2Cl2O2 and C2Br2O2 are transition states. In contrast, the cis conformers of the anionic oxalyl halide molecules are stable, lying 131 meV above trans-C2Cl2O2(-) and 179 meV above trans-C2Br2O2(-). Chien et al. [J. Phys. Chem. A 103, 7918 (1999)] and Kim et al. [J. Chem. Phys. 122, 234313 (2005)] found that the potential energy surface for rotation about the C-C bond in C2Cl2O2 is "extremely flat." Our computational data indicate that the analogous torsional surfaces for C2Br2O2, C2Cl2O2(-), and C2Br2O2(-) are similarly flat. The electron affinity of oxalyl chloride, oxalyl bromide, and phosgene were calculated to be 1.91 eV (G3), and 2.00 eV (G2), and 1.17 eV (G3), respectively.     

Chelating or bridging? Halide-controlled binding mode of diamido donor ligands in iron(III) complexes

In a series of iron(III) halide complexes of the form {FeX[MesN(SiMe(2))]2O}2 (Mes = mesityl; X = Cl, Br, I), the ancillary diamidosilylether ligand can either chelate to one metal or instead bridge two metal centers, as a function of the halide coligand. The complexes are prepared from diamidosilyletheriron(II) precursors, which are oxidized with iodine, benzyl bromide, or PhICl2 to yield the appropriate iron(III) halide. The bromide analogue can also be synthesized by reacting the iron(II) precursor with a bromonium transfer agent (stabilized by adamantylideneadamantane). The latter reaction may proceed via an iron(IV) intermediate, which can oxidize the normally noncoordinating, inert [B(ArF)4]- counteranion [ArF = 3,5-(CF3)2Ph].