Ce-IV(DOTA) (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) was produced both radiolytically and electrochemically in the presence of halides and azide anions. Only fluoride, the hardest Lewis base studied, ligates to both [Ce-III(DOTA)(H2O)](-) and Ce-IV(DOTA), stabilizing the high oxidation state by two orders of magnitude versus [Ce-IV/III(DOTA)](0/-) in its absence. The Ce-IV(DOTA) complexes are long-lived in the dark (decompose photochemically). The tetravalent complexes decompose mainly via kinetics which obeys a first order rate law. The first step is de-carboxylation of the DOTA ligand followed by the formation of CH2O. The kinetics of oxidation of [Ce-III(DOTA)(H2O)](-) by Cl-2(-), Br-2(-) and N-3 were studied. Cl-2(-), the strongest oxidizing agent studied, reacts mainly via H atom abstraction from the DOTA ligand. Br-2(-) at both acidic and neutral pH oxidizes [Ce-III(DOTA)(H2O)](-) with rate constants of <3 x 10(6) M(-1)s(-1), as measured by pulse radiolysis, via an inner sphere mechanism. N-3 oxidizes [Ce-III(DOTA)(H2O)](-) to its tetravalent analogue as well, as proved by UV-vis data after steady-state radiolysis. The presence of F- increased the yield of the Ce-IV(DOTA) formed. The oxidation rate by N-3 is probably lower than that measured for Br-2(-).

Chelation of lanthanide and actinide cations within a suitable macrocyclic ligand often results in a rigid, kinetically inert, and thermodynamically stable complex. A benchmark for such cation-ligand suitability are cyclen-derived macrocyclic ligands, frequently used as large cation hosts for various applications. Herein, a comprehensive study of the 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane ligand (DOTAM) chelates of U-IV and Ce-III and their properties in aqueous solutions is presented. By employing multiple analysis techniques, including X-ray crystallography, UV-vis absorbance, H-1 NMR, UPLC-MS, cyclic voltammetry, and differential pulse voltammetry, the study has revealed that the two aqueous complexes undergo a spontaneous, gradual, and stepwise hydrolysis of each of the coordinated amides toward carboxylates. The coordination of U-IV in the studied reaction has been shown to significantly enhance the reaction rate, leading to an acceleration of up to 6 orders of magnitude compared to the natural process of simple aqueous amides at room temperature. An attempt to describe the unusual chelated metal cation amide-activation feature, based on the relatively lower rigidity of the complex structure, is presented. Additionally, the electrochemical properties of the complex series are discussed in detail, along with the limitations of the analytical methods employed.

A group of researchers from NRCN, Ben Gurion University and the CEA shows that cyclen-derived, hard-bonding ligands facilitate the formation of tetravalent cerium cations bearing oxo/hydroxo clusters in aqueous solutions. The clusters eventually decompose to form the thermodynamically favoured “in-cage” cerric macrocyclic complex. More information can be found in the Research Article by S. Pevzner, E. Maimon, Y. Ben-Eliyahu, P. Moisy, A. Bettelheim, I. Zilbermann and co-workers (DOI: 10.1002/chem.202201868).

The mechanism of reaction of DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) with ·CH3, CH3O2· and ·OH radicals were studied. The radicals were formed in situ radiolytically. The methyl radicals react orders of magnitude slower with DOTA and with MIII(DOTA)- than the hydroxyl radicals. The various final products were identified and mechanisms for their formation are proposed. CH3O2· radicals do not react, or react too slowly to be observed, with DOTA and with MIII(DOTA)- as long as the central cation is not oxidized by the peroxyl radical. The results imply that synthesis of the MIII(DOTA)-(MIII = radioisotope) complexes in a water-organic solvent (ethanol or 2-propanol or acetonitrile) mixture is not only kinetically desired but the so formed complex also decreases the radiolytic decomposition of DOTA.

The CuI/IIATP react with methyl radicals to form methane and methanol, where CuIATP reacts with •CH3 in a process that is surprisingly slow. The low-rate constant of this process is attributed to the significant rearrangement of the chelating ligand required for the transient's formation. These results were corroborated by DFT calculations of the relevant compounds.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The coordination and redox chemistry of aqueous CeIV/III macrocyclic compounds were studied by using the ligands DOTA and DOTP (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(methylene phosphonic acid), respectively). The hydrolysis tendency of the tetravalent cation in the presence of DOTA is shown to result in the formation of a highly ordered, fluorite-like [CeIV 6 (O)4 (OH)4 (H2 O)8 (DOTAH)4 ] oxo-hydroxo structure both in solution and in the solid state. The lifetime of the analogous species formed in the presence of DOTP was found to be much shorter. Spectroscopic measurements of the latter suggest its similarity to the former. Its gradual decomposition in solution leads to the accumulation of the in-cage complexes [CeIV DOTP] and [CeIII DOTP(H2 O)], which were crystallographically characterized in this study. The redox energetics and spectroscopic characteristics for the transition between these two in-cage complexes in aqueous solutions were studied as well. Together with the crystallographic structures of the above-mentioned species, the in-cage [CeIV DOTA(H2 O)] complex structure is presented herein for the first time. An elaborative analysis of the X-ray crystallographic structural data obtained for the in-cage complexes studied herein and similar structures published previously suggests that hard-bonding cyclen-derived ligands are, counter-intuitively, better suited for encapsulating, and perhaps kinetically stabilize softer cations than harder ones with DOTP, marked as a possible adequate chelator for the study of the aqueous properties of LnII and AcIII cations.

Two novel aqueous U IV complexes were synthesized by the interaction between the tetravalent uranium cation and the ( 1,4,7,10-Tetraazacyclododecane-1,4,7,10-tetra(methylene phosphonic acid) (DOTP) macrocyclic ligand. Two distinct homonuclear complexes were identified- the first was characterized by X-ray crystallography as a unique "out-of-cage", [U(DOTPH 6 ) 2 ] complex, in which the U IV cation is octa-coordinated to 4 phosphonic arms from each ligand in a square anti-prism geometry, with a C 4 symmetry. The second is the "in-cage" [U(DOTPH 4 )] complex where the tetravalent cation is located between the macrocycle O 4 and N 4 planes. With the help of UV-vis absorption, 1 H/ 31 P-NMR, ATR-IR and MALDI-TOFMS analytical techniques, the chemical interchange between both species is presented. It is shown that the one-way transition is governed by the formation of a multiple number of soluble oligomeric species consisting of a varied stoichiometric ratios of both characterized homonuclear complexes.

The redox chemistry of CeIIIDOTA in cage in carbonate solutions was studied using electrochemistry and radiolysis techniques (continuous radiolysis and pulse radiolysis). Spectroscopic measurements point out that the species present in the solutions at high bicarbonate concentrations are [CeIIIDOTA(CO3)]3- (or less plausible [CeIIIDOTA(HCO3)]2-) with the carbonate (bicarbonate) anion as the ninth ligand versus [CeIIIDOTA(H2O)]- present in the absence of bicarbonate. Electrochemical results show a relatively low increase in the thermodynamic stabilization of the redox couple CeIV/III in the presence of carbonate versus its aqueous analogue. [CeIVDOTA(CO3)]2- and [CeIVDOTA(H2O)], prepared electrochemically, decompose photolytically. However, kept in the dark, both are relatively long lived; [CeIVDOTA(H2O)], though, is orders of magnitude kinetically more stable (a considerably longer half-life). Thus, one concludes that the carbonate species have a different mechanism of decomposition depending also on the presence of dioxygen after its preparation (in deaerated/aerated solutions). The [CeIVDOTA(CO3)]2- species is produced radiolytically by oxidation of the trivalent species by CO3•- with a rate constant, measured using pulse radiolysis, of 3.3 × 105 M-1 s-1. This rate constant is at least 1 order of magnitude smaller than most of the rate constants so far reported for the reaction of CO3•- with transition metal/lanthanide (cerium)/actinide complexes. This result together with the bulkiness of the reactants might suggest an outer-sphere electron transfer rather than the inner-sphere one so far proposed. The lifetime of the tetravalent cerium species obtained radiolytically in the presence of carbonate is shorter than the electrochemical one, suggesting a different conformer involved.

The 1,4,7,10-tetrazacyclodecane-1,4,7,10-tetraacetic acid (DOTA) aqueous complex of U-IV with H2O, OH-, and F- as axial ligands was studied by using UV/Vis spectrophotometry, ESI-MS, NMR spectroscopy, X-ray crystallography, and electrochemistry. The U-IV-DOTA complex with either water or fluoride as axial ligands was found to be inert to oxidation by molecular oxygen, whereas the complex with hydroxide as an axial ligand slowly hydrolyzed and was oxidized by dioxygen to a diuranate precipitate. The combined data set acquired shows that, although axial substitution of fluoride and hydroxide ligands instead of water does not seem to significantly change the aqueous DOTA complex structure, it has an important effect on the electronic configuration of the complex. The U-IV/U-III redox couple was found to be quasi-reversible for the complex with both axially bonded H2O and hydroxide, but irreversible for the complex with axially bonded fluoride. Intriguingly, binding of the axial fluoride renders the irreversible one-electron U-V/U-IV oxidation of the [U-IV(DOTA)(H2O)] complex quasi-reversible, which suggests the formation of the short-lived pentavalent form of the complex, an aqueous non-uranyl chelated U-V cation.

The reactions of the radicals center dot R, center dot R = center dot CH3 or center dot CH2OH, with the complex Cu-II(ntp)(H2O) were studied. The kinetics of formation of the transient complexes (ntp)Cu-III-R fit pseudo first order rate laws. The mechanism of decomposition of the transient complex (ntp)Cu-III-CH3 at pH >= 7 involves homolysis of the Cu-III-C bond. However, at pH < 6 the decomposition mechanisms of the transient complexes (ntp)Cu-III-R involve inter-molecular electron transfer to Cu-II(ntp)(H2O) and slow intra-molecular electron transfer.

The reactions of methyl radicals with M-II-(ntp)(H2O)(2) complexes were studied. The formation of an unstable intermediate (ntp)(H2O)M-III-CH3 is observed. This reaction is an equilibrium process, i.e. the M-III-C bond decomposes via homolysis. The (ntp)(H2O)M-III-CH3 complexes isomerize to a more stable form. The results compared to those obtained in analogous reactions of the M-II-(nta)(H2O)(2) complexes were shown to be similar with one exception - for the nta complexes no isomerization process is observed.

Background: Nitroxide antioxidants (RNO) protect from injuries associated with oxidative stress. Tyrosine residues in proteins are major targets for oxidizing species giving rise to irreversible cross-linking and protein nitration, but the mechanisms underlying the protective activity of RNO on these processes are not sufficiently clear. Methods: Tyrosine oxidation by the oxoammonium cation (RN+=O) was studied by following the kinetics of RNO formation using EPR spectroscopy. Tyrosine oxidation and nitration were investigated using the peroxidase/H2O2 system without and with nitrite. The inhibitory effect of RNO on these processes was studied by following the kinetics of the evolved O-2 and accumulation of tyrosine oxidation and nitration products. Results: Tyrosine ion is readily oxidized by RN+ = O, and the equilibrium constant of this reaction depends on RNO structure and reduction potential. RNO catalytically inhibits tyrosine oxidation and nitration since it scavenges both tyrosyl and NO2 radicals while recycling through RN+=O reduction by H2O2, tyrosine and nitrite. The inhibitory effect of nitroxide on tyrosine oxidation and nitration increases as its reduction potential decreases where the 6-membered ring nitroxides are better catalysts than the 5-membered ones. Conclusions: Nitroxides catalytically inhibit tyrosine oxidation and nitration. The proposed reaction mechanism adequately fits the results explaining the dependence of the nitroxide inhibitory effect on its reduction potential and on the concentrations of the reducing species present in the system. General significance: Nitroxides protect against both oxidative and nitrative damage. The proposed reaction mechanism further emphasizes the role of the reducing environment to the efficacy of these catalysts.

Hydroxamic acids (RC(O)NHOH) form a class of compounds that display interesting chemical and biological properties The chemistry of RC(O)NHOH) is associated with one- and two-electron oxidations forming the respective nitroxide radical (RC(O)NHO•) and acyl nitroso (RC(O)N═O), respectively, which are relatively unstable species. In the present study, the kinetics and mechanism of the •NO2 reaction with nitroxide radicals derived from acetohydroxamic acid, suberohydroxamic acid, benzohydroxamic acid, and suberoylanilide hydroxamic acid have been studied in alkaline solutions. Ionizing radiation was used to generate about equal yields of these radicals, demonstrating that the oxidation of the transient nitroxide radical by •NO2 produces HNO and nitrite at about equal yields. The rate constant of •NO2 reaction with the nitroxide radical derived from acetohydroxamic acid has been determined to be (2.5 ± 0.5) × 109 M-1 s-1. This reaction forms a transient intermediate absorbing at 314 nm, which decays via a first-order reaction whose rate increases upon increasing the pH or the hydroxamic acid concentration. Transient intermediates absorbing around 314 nm are also formed during the oxidation of hydroxamic acids by H2O2 catalyzed by horseradish peroxidase. It is shown that HNO is formed during the decomposition of these intermediates, and therefore, they are assigned to acyl nitroso compounds. This study provides for the first time a direct spectrophotometric detection of acyl nitroso compounds in aqueous solutions allowing the study of their chemistry and reaction kinetics.

Recently it was reported that copper bicarbonate/carbonate complexes are good electro-catalysts for water oxidation. However, the results did not enable a decision whether the active oxidant is a CuIII or a CuIV complex. Kinetic analysis of pulse radiolysis measurements coupled with DFT calculations point out that CuIII (CO3 )n3-2n complexes are the active intermediates in the electrolysis of CuII (CO3 )n2-2n solution. The results enable the evaluation of E°[(CuIII/II (CO3 )n )aq ]≈1.42 V versus NHE at pH 8.4. This redox potential is in accord with the electrochemical report. As opposed to literature suggestions for water oxidation, the present results rule out single-electron transfer from CuIII (CO3 )n3-2n to yield hydroxyl radicals. Significant charge transfer from the coordinated carbonate to CuIII results in the formation of C2 O62- by means of a second-order reaction of CuIII (CO3 )n3-2n . The results point out that carbonate stabilizes transition-metal cations at high oxidation states, not only as a good sigma donor, but also as a non-innocent ligand.

Ethylene-di-amine-tri-acetate was covalently bound to a Silica-gel matrix (ED3A-SG). The resin thus obtained was examined as a possible Ion Exchange Resin for pre-concentration of actinides and lanthanides. ED3A-SG was prepared by reacting Sodium N-(tri-methoxy-silyl-propyl)-ethylene-di-amine-tri-acetate (ED3A-Silane coupler) with silica-gel. The porosity of the system was measured using BET analysis. The capacity of the resin was measured by two techniques: a. Determination of the carbon content of the Resin using Carbon Sulfur Analyzer (CS Analyzer). b. Saturating the Resin with nickel ions and determining their amount by inductively coupled plasma optical emission spectrometer (ICP-OES). The behavior of the ED3A-SG resin as function of pH was measured by determining the recovery value of cations solutions buffered at different pH values. The molecular structure of the resin was examined by 13C NMR and 29Si NMR.

The reaction kinetics of eight amino-carboxylate complexes of Fe(III) and Mn(II) with carbonate radical anion were studied using the pulse radiolysis method and UV-vis spectroscopy. Difference spectra revealed the formation of Fe(IV) and Mn(III) after reaction with CO3 center dot-. Spectral measurements revealed the first step to be the coordination of carbonate to the metal center. All of these led to the conclusion that the role of coordinated carbonate is essential to the electron transfer process by carbonate radical anion. [GRAPHICS] .

Oxidation of hydroxamic acids (HXs) generates HNO, and it is not clear whether it is formed also in the presence of metal ions. The kinetics of the oxidation of HXs, such as acetohydroxamic acid, suberohydroxamic acid, and suberoylanilide hydroxamic acid (SAHA), by compounds I and II of horseradish peroxidase (HRP) at pH 7.0 and 25 degrees C have been studied using rapid-mixing stopped-flow. The kinetics of these reactions were compared to those observed in the presence of Cu(ClO4)(2), NiSO4, or ZnSO4. The rates decrease upon increasing [Cu-II] at constant [HXs], and no oxidation of HX occurs when [HX]/[Cu-II] approximate to 2, implying that HX oxidation in the presence of Cu-II proceeds through the free ligand since the predominant complex is CuX2. In the case of Ni-II, the oxidation rate decreases upon increasing the ratio [Ni-II]/[HX] beyond 1, where the predominant complex is (NiX+)-X-II, implying that its oxidation is feasible. The effect of Zn-II could be studied only on the rate of HXs oxidation by compound II demonstrating similar behavior to that of Ni-II. HXs were also oxidized catalytically by HRP/H2O2 at pH 7.0, demonstrating that metal ions facilitate the formation of HNO while hardly affecting its yield and the extent of HX oxidation. [GRAPHICS] .

Acyl nitroso compounds or nitrosocarhonyls (RC(O)N═O) are reactive short-lived electrophiles, and their hydrolysis and reactions with nucleophiles produce HNO. Previously, direct detection of acyl nitroso species in nonaqueous media has been provided by time-resolved infrared spectroscopy demonstrating that its half-life is about 1 ms. In the present study hydroxamic acids (RC(O)NHOH) are oxidized electrochemically in buffered aqueous solutions (pH 5.9-10.2) yielding transient species characterized by their maximal absorption at 314-330 nm. These transient species decompose via a first-order reaction yielding mainly HNO and the respective carboxylic acid and therefore are ascribed to RC(O)N═O. The sufficiently long half-life of RC(O)N═O in aqueous solution allows for the first time the study of the kinetics of its reactions with various nucleophiles demonstrating that the nucleophilic reactivity follows the order thiolate > hydroxamate > amine. Metal chelates of CH3C(O)NHOH catalyze the hydrolysis of CH3C(O)N═O at the efficacy order of CuII > ZnII > NiII > CoII where only CuII catalyzes the hydrolysis also in the absence of the hydroxamate. Finally, oxidation of hydroxamic acids generates HNO, and the rate of this process is determined by the half-life of the respective acyl nitroso compound.

During an effort to synthesize the trans-III-copper(II) complex with 1,4,8,11-tetramethyl-pyro-phosphonate-1,4,8,11-tetra-aza-cyclo-tetradecane, using only perchlorate salts, it was noted that the perchlorate is reduced to chloride. Analysis of the reactions leading to this fsurprising result points out that Cu(H2O)(4)2(+) catalyzes the reduction of perchlorate by H-2 and by CH2O. These reactions are slow at room temperature and ambient pressures. A plausible mechanism, supported by DFT calculations, is proposed pointing out that the role of CuH thorn under mild conditions cannot be ignored. Cu(H2O)(4)(2+) + CH2O reversible arrow Cu(H2O)(3)H+ + HC(O)OH + H+ CuH(aq)+ + ClO4- -> Cu-(aq)(2+) + ClO3- + OH- Perchlorate reduction