Synthesis, structure elucidation, and redox properties of 99Tc complexes of lacunary Wells-Dawson polyoxometalates: insights into molecular 99Tc-metal oxide interactions

The isotope (99)Tc (β(max), 293.7; half-life, 2.1 × 10(5) years) is an abundant product of uranium-235 fission in nuclear reactors and is present throughout the radioactive waste stored in underground tanks at the Hanford and Savannah River sites. Understanding and controlling the extensive redox chemistry of (99)Tc is important in identifying tunable strategies to separate (99)Tc from spent fuel and from waste tanks and, once separated, to identify and develop an appropriately stable waste form for (99)Tc. Polyoxometalates (POMs), nanometer-sized models for metal oxide solid-state materials, are used in this study to provide a molecular level understanding of the speciation and redox chemistry of incorporated (99)Tc. In this study, (99)Tc complexes of the (α(2)-P(2)W(17)O(61))(10-) and (α(1)-P(2)W(17)O(61))(10-) isomers were prepared. Ethylene glycol was used as a "transfer ligand" to minimize the formation of TcO(2)·xH(2)O. The solution structures, formulations, and purity of Tc(V)O(α(1)/α(2)-P(2)W(17)O(61))(7-) were determined by multinuclear NMR. X-ray absorption spectroscopy of the complexes is in agreement with the formulation and structures determined from (31)P and (183)W NMR. Preliminary electrochemistry results are consistent with the EXAFS results, showing a facile reduction of the Tc(V)O(α(1)-P(2)W(17)O(61))(7-) species compared to the Tc(V)O(α(2)-P(2)W(17)O(61))(7-) analog. The α(1) defect is unique in that a basic oxygen atom is positioned toward the α(1) site, and the Tc(V)O center appears to form a dative metal-metal bond with a framework W site. These attributes may lead to the assistance of protonation events that facilitate reduction. Electrochemistry comparison shows that the Re(V) analogs are about 200 mV more difficult to reduce in accordance with periodic trends.     

Water exchange reactivity and stability of cobalt polyoxometalates under catalytically relevant pH conditions: insight into water oxidation catalysis

Water exchange on a molecular, purely inorganic cobalt-based water oxidation catalyst, [Co(4)(II)(H(2)O)(2)(α-P(1)W(9)O(34))(2)](10-) (1), in the catalytically relevant pH region (pH 6-10) is studied using (17)O-NMR spectroscopy and ultrahigh-resolution electrospray ionization mass spectrometry. The results are compared with those of the inactive [Co(II)(H(2)O)(1)Si(1)W(11)O(39)](6-) (2), which is stable in the same pH region. The results obtained provide mechanistic details of the elementary reaction step related to the water oxidation on homogeneous metal oxide catalysts under catalytically relevant conditions. It is shown that the structural integrity of 1 and 2 is maintained, no deprotonation of the aqua ligands on the Co(II) centers occurs, and the water exchange does not undergo any mechanistic changeover at the catalytic pH conditions. We have demonstrated that the water exchange process is influenced by the cluster environment surrounding the water binding sites and is fast enough to not be rate-limiting for the water oxidation catalysis.     

Sorption of Tc(VII) and Am(III) by carbon materials: effect of oxidation

In this work we studied the effect of oxidation on the sorption properties of carbon materials, produced on the basic of activated carbons FAS and VSK in relation to radionuclides Am(III) and Tc(VII). It has been found that the oxidation increases the distribution coefficients for the extraction of radionuclides of sorption nitric acid raising agents. As to identify the mechanism of the oxidation influence on the sorption properties the physical and chemical properties of the synthesized samples (porous surface structure, IR-spectrums etc.) were studied. The possibility of removal of radionuclides from the real industrial solutions of complex composition using of the obtained carbon nanomaterials was investigated.     

Photoreduction of 99Tc pertechnetate by nanometer-sized metal oxides: new strategies for formation and sequestration of low-valent technetium

Technetium-99 ((99)Tc) (β(-)(max): 293.7 keV; t(1/2): 2.1 × 10(5) years) is a byproduct of uranium-235 fission and comprises a large component of radioactive waste. Under aerobic conditions and in a neutral-basic environment, the pertechnetate anion ((99)TcO(4)(-)) is stable. (99)TcO(4)(-) is very soluble, migrates easily through the environment and does not sorb well onto mineral surfaces, soils, or sediments. This study moves forward a new strategy for the reduction of (99)TcO(4)(-) and the chemical incorporation of the reduced (99)Tc into a metal oxide material. This strategy employs a single material, a polyoxometalate (POM), α(2)-[P(2)W(17)O(61)](10-), that can be photoactivated in the presence of 2-propanol to transfer electrons to (99)TcO(4)(-) and incorporate the reduced (99)Tc covalently into the α(2)-framework to form the (99)Tc(V)O species, (99)Tc(V)O(α(2)-P(2)W(17)O(61))(7-). This occurs via the formation of an intermediate species that slowly converts to (99)Tc(V)O(α(2)-P(2)W(17)O(61))(7-). Extended X-ray absorption fine structure and X-ray absorption near-edge spectroscopy analysis suggests that the intermediate consists of a (99)Tc(IV) α(2)- species where the (99)Tc is likely bound to two of the four W-O oxygen atoms in the α(2)-[P(2)W(17)O(61)](10-) defect. This intermediate then oxidizes and converts to the (99)Tc(V)O(α(2)-P(2)W(17)O(61))(7-) product. The reduction and incorporation of (99)TcO(4)(-) was accomplished in a "one pot" reaction using both sunlight and UV irradiation and monitored as a function of time using multinuclear nuclear magnetic resonance and radio thin-layer chromatography. The process was further probed by the "step-wise" generation of reduced α(2)-P(2)W(17)O(61)(12-) through bulk electrolysis followed by the addition of (99)TcO(4)(-). The reduction and incorporation of ReO(4)(-), as a nonradioactive surrogate for (99)Tc, does not proceed through the intermediate species, and Re(V)O is incorporated quickly into the α(2)-[P(2)W(17)O(61)](10-) defect. These observations are consistent with the periodic trends of (99)Tc and Re. Specifically, (99)Tc is more easily reduced compared to Re. In addition to serving as models for metal oxides, POMs may also provide a suitable platform to study the molecular level dynamics and the mechanisms of the reduction and incorporation of (99)Tc into a material.     

Arsenic(III) sorption on nanostructured cerium incorporated manganese oxide (NCMO): a physical insight into the mechanistic pathway

Arsenic(III) sorption was investigated with nanostructured cerium incorporated manganese oxide (NCMO). The pH between 6.0 and 8.0 was optimized for the arsenic(III) sorption. Kinetics and equilibrium data (pH=7.0±0.2, T=303±1.6 K, and I=0.01 M) of arsenic(III) sorption by NCMO described, respectively, the pseudo-second order and the Freundlich isotherm equations well. The sorption process was somewhat complicated in nature and divided into two different segments, initially very fast sorption followed by slow intraparticle diffusion process. Sorption reaction of arsenic(III) on NCMO was endothermic (ΔH°=+13.46 kJ mol(-1)) and spontaneous (ΔG°=-24.75 to -30.15 kJ mol(-1) at T=283-323 K), which took place with increasing entropy (ΔS°=+0.14 kJ mol(-1)K(-1)) at solid-liquid interface. Energy of arsenic(III) sorption estimated by analyzing the equilibrium data using the D-R isotherm model was 15.4 kJ mol(-1), indicating the ion-exchange type mechanism. Raman, FT-IR, pH effect, desorption, etc. studies indicated that arsenic(III) was oxidized to arsenic(V) during the sorption process.     

Dioxygen-initiated oxidation of heteroatomic substrates incorporated into ancillary pyridine ligands of carboxylate-rich diiron(II) complexes

Progress toward the development of functional models of the carboxylate-bridged diiron active site in soluble methane monooxygenase is described in which potential substrates are introduced as substituents on bound pyridine ligands. Pyridine ligands incorporating a thiol, sulfide, sulfoxide, or phosphine moiety were allowed to react with the preassembled diiron(II) complex [Fe(2)(mu-O(2)CAr(R))(2)(O(2)CAr(R))(2)(THF)(2)], where (-)O(2)CAr(R) is a sterically hindered 2,6-di(p-tolyl)- or 2,6-di(p-fluorophenyl)benzoate (R = Tol or 4-FPh). The resulting diiron(II) complexes were characterized crystallographically. Triply and doubly bridged compounds [Fe(2)(mu-O(2)CAr(Tol))(3)(O(2)CAr(Tol))(2-MeSpy)] (4) and [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(2-MeS(O)py)(2)] (5) resulted when 2-methylthiopyridine (2-MeSpy) and 2-pyridylmethylsulfoxide (2-MeS(O)py), respectively, were employed. Another triply bridged diiron(II) complex, [Fe(2)(mu-O(2)CAr(4)(-)(FPh))(3)-(O(2)CAr(4)(-)(FPh))(2-Ph(2)Ppy)] (3), was obtained containing 2-diphenylphosphinopyridine (2-Ph(2)Ppy). The use of 2-mercaptopyridine (2-HSpy) produced the mononuclear complex [Fe(O(2)CAr(Tol))(2)(2-HSpy)(2)] (6a). Together with that of previously reported [Fe(2)(mu-O(2)CAr(Tol))(3)(O(2)CAr(Tol))(2-PhSpy)] (2) and [Fe(2)(mu-O(2)CAr(Tol))(3)(O(2)CAr(Tol))(2-Ph(2)Ppy)] (1), the dioxygen reactivity of these iron(II) complexes was investigated. A dioxygen-dependent intermediate (6b) formed upon exposure of 6a to O(2), the electronic structure of which was probed by various spectroscopic methods. Exposure of 4 and 5 to dioxygen revealed both sulfide and sulfoxide oxidation. Oxidation of 3 in CH(2)Cl(2) yields [Fe(2)(mu-OH)(2)(mu-O(2)CAr(4)(-)(FPh))(O(2)CAr(4)(-FPh))(3)(OH(2))(2-Ph(2)P(O)py)] (8), which contains the biologically relevant {Fe(2)(mu-OH)(2)(mu-O(2)CR)}(3+) core. This reaction is sensitive to the choice of carboxylate ligands, however, since the p-tolyl analogue 1 yielded a hexanuclear species, 7, upon oxidation.     

Development of metal oxide incorporated Al-Zn-Sn sacrificial anodes processed by stir casting and heat treatment

Aluminum sacrificial anodes are currently the first choice for cathodic protection in numerous applications. The galvanic performance of aluminum-based sacrificial anodes is considerably enhanced by addition of certain alloying elements called activators. Recent researches proved that incorporation of specific metal oxides like MnO₂, CeO₂, RuO₂, and IrO₂ into the aluminum matrix could enhance the galvanic efficiency of aluminum anodes; however, the mechanism by which metal oxides improve galvanic properties of aluminum is still subject to discussion. The present work investigates the effect of incorporating commercially available low-cost manganese dioxide concentrate into Al-5Zn-0.1Sn sacrificial anodes in different volume fractions. It also studies the influence of heat treatment on anode’s galvanic performance by performing solution treatment at 3 different temperatures (250 °C, 400 °C, 550 °C). The electrochemical testing results proved an increase in efficiency of anodes incorporated with metal oxides and solution treated at 550 °C. The SEM imaging and EDX elemental mapping declared that the presence of SiO₂ particles in the anode matrix which might cause effective and uniform corrosion of Al anodes and decreased non-coulombic losses.

Graphical abstract

     

Photosensitized and photostabilized oxidation of poly(2,6-dimethyl- 1,4-phenylene oxide) with metal isopropylxanthates

This work describes the results of the Cd(II) isopropylxanthate-stabilized and Mn(III) isopropylxanthate-sensitized photo-oxidation of poly(2,6-dimethyl-1,4-phenylene oxide) film in air at low temperatures (?10 to 80°). The oxidation was followed by light scattering, potassium ferri-oxalate actinometry and by measuring gel formation. The weight-average molecular weight, degree of degradation, rate of scission of links, energy of activation and quantum yield of the process depend on several factors, e.g. temperature, xanthate concentration. Various oxygen-containing groups (hydroperoxides, carbonyls, etc.) are formed in the polymer. For the determination of the content of these groups, iodometry and spectroscopy were applied. The initially present or photo-induced hydroperoxides are directly responsible for subsequent oxidative reactions which occur during 254-nm irradiation. The absorption spectra of the degraded materials in the u.v. and i.r. regions were also studied to substantiate a possible mechanism of the oxidation process.     

Synthesis of a re(VI)-catecholato complex by air oxidation of oxorhenium(V)

The complex [Re(ddcat)₃] (H₂ddcat?=?3,5-di-tert-butylcatechol) was prepared by the reaction of either cis-[Re^VO₂I(PPh₃)₂] or (n-Bu₄N)[Re^VOCl₄] with H₂ddcat in toluene in air. X-ray structure determination of the product unequivocally illustrates that the bidentate chelates are in the catecholato rather than the semiquinone form and that the metal has a formal oxidation state of +VI.     

Biological characterization of 99m/99Tc(V)-cysteine complexes: A potential renal functional imaging agent

Cysteine was chelated with ^99m/99Tc in a freeze-dried kit containing tin(II) ions, and the green ^99m/99Tc-cysteine complex (complex III) was separated to study the biodistribution. The biodistribution of ^99m/99Tc-cysteine complexes was performed in mice. The renal excretion patterns of rats were studied with and without the renal tubular transport inhibitor 2,4-dinitrophenol. The carrier of Tc-cysteine complex in the blood and also the radioactive compounds in the urine were studied by HPGFC and SDS-PAGE electrophoresis. The kidney was confirmed as the target organ; serum albumin functions as a carrier to transport Tc-cysteine complex to the kidney. The protein-bound Tc-cysteine complex was the primary form in excreta, and renal tubular secretion was the foremost excretory pathway.     

Activation of thiocyanogen and selenocyanogen by low oxidation state transition metal complexes

Thiocyanogen and selenocyanogen react with Ru(CO)₃(PPh₃)₂ to give respectively the complexes Ru(CO)₂(PPh₃)₂(NCS)₂ and Ru(CO)₂(PPh₃)₂(NCSe)₂. (M—NCS and M—SCN represent N- and S-thiocyanato groups, M—NCSe and M—SeCN represent N- and Se-selenocyanato groups respectively, while M—CNS indicates the bridging coordination mode of thiocyanate.) Only the thiocyanogen reacts with Ru₃(CO)₁₂ giving [Ru(CO)₂(CNS)₂]_n, which dissolves in hot coordinating solvents, such as pyridine, to form Ru(CO)₂(py)₂(NCS)₂. Selenocyanogen is less effective than thiocyanogen in the oxidative addition reactions with rhodium(I) and iridium(I) complexes; in fact selenocyanogen does not react with Rh(CO)(PPh₃)₂Cl while with Ir(CO)(PPh₃)₂Cl the former gives Ir(CO)(PPh₃)₂(SeCN)₂Cl by an equilibrium reaction. The coordination number of the metal and the charge on the complex do not change the bonding mode of the thiocyanate and selenocyanate groups in the iridium(III) complexes; in the Ir(PPh₃)₂ClX₂ and [Ir(Ph₂PC₂H₄PPh₂)₂X₂]⁺ (X = SCN and SeCN) complexes the pseudohalogens are S- and Se-bonded.The complexes trans-M(PPh₃)₂(SeCN)₂ (M = Pd, Pt) have been obtained by reacting M(PPh₃)₄ with selenocyanogen.     

Adsorption and oxidation of aromatic amines by metal hexacyanoferrates(II)

Interaction of aniline, p-toluidine and p-chloroaniline with nickel, cadmium and manganese hexacyanoferrates(II) has been studied. Nickel and cadmium hexacyanoferrates(II) showed maximum adsorption at neutral pH, whereas, manganese hexacyanoferrate(II) reacts with aniline, p-toluidine and p-chloroaniline forming the colored oligomers on its surface. The adsorption data obtained at neutral pH is fitted in Langmuir adsorption isotherm. The adsorption behavior of the studied aromatic amines followed the order: p-toluidine>aniline>p-chloroaniline, which is related to the basicities of the amines. Results of the present study suggest the importance of metal hexacyanoferrates(II) and metal ions in stabilization of aromatic amines during the processes of prebiotic condensation reactions.     

Coordination of nitrogen(II) oxide by metal prophyrins

The equilibrium constants have been determined for the extra coordination of nitrogen(II) oxide and its metabolite formed in biochemical reactions (the nitrite ion) by cobalt complexes of porphyrins with various structures in ethanolic and aqueous solutions with near-physiological pHs. The far higher values of the thermodynamic stability constants of (NO)CoP nitrosyl complexes compared to those for nitrite complexes argue for a considerable covalence of the Co-NO bond.     

Visible light-induced water oxidation catalyzed by molybdenum-based polyoxometalates with mono- and dicobalt(III) cores as oxygen-evolving centers

The Mo-based polyoxometalates containing mono- and dicobalt(III) catalyst cores, [CoMo(6)O(24)H(6)](3-) and [Co(2)Mo(10)O(38)H(4)](6-), were found to serve as O(2)-evolving catalysts in a system consisting of tris(2,2'-bipyridine)ruthenium(II) (Ru(bpy)(3)(2+)) and sodium persulfate (S(2)O) in an aqueous borate buffer solution at pH 8.0. Kinetics of O(2) evolution reveals that the higher cobalt nuclearity is not necessary to attain the highly active nature of the catalyst.     

Preparation of (99m)Tc labeled vitamin C (ascorbic acid) and biodistribution in rats

The aim of this study was to label ascorbic acid with (99m)Tc and to investigate its radiopharmaceutical potential in rats. Ascorbic acid was labeled with (99m)Tc using the stannous chloride method. The radiochemical purity of [(99m)Tc]ascorbic acid ((99m)Tc-AA) was determined by RTLC, paper electrophoresis, and RHPLC methods. The labeling yield was found to be 93+/-5.0%. The maximum labeling yield of (99m)Tc-AA was determined at pH 5 and 25 degrees C. The biodistribution studies related to (99m)Tc-AA were done in male albino Wistar rats. (99m)Tc-AA, which has a specific activity of 13.02 GBq/mmol, was administered into the tail vein of the rats. The rats were sacrificed at 15, 30, 60, and 120 min after the injection by heart puncture under ether anaesthesia. The organs were weighed after removal. Their activities were counted using a Cd(Te) detector equipped with a RAD 501 count system. The %ID/g (% of injected dose per gram of tissue weight) in each organ and in blood was calculated. Maximum uptake of (99m)Tc-AA was observed in prostate and kidneys at the 60th min. (99m)Tc-AA may be a promising radiopharmaceutical for the imaging of prostate and kidneys.     

The complete oxidation of a volatile organic compound (toluene) over supported metal oxide catalysts

The complete oxidation of a volatile organic compound (toluene) was carried out over oxides of various metals (Cu, Mn, Fe, V, Mo, Co, Ni and Zn) supported with -Al₂O₃ catalyst. It was found that Cu/-Al₂O₃ was the best catalyst among them. With increasing calcination temperature, the specific surface areas and the surface oxygen of the catalyst decreased and, as a result, the catalytic activity decreased. Copper loadings on -Al₂O₃ had an influence on catalytic activity, and it was observed that 5 wt% Cu/-Al₂O₃ catalyst was the most active. Using TiO₂ (rutile) and SiO₂ as support instead of -Al₂O₃, the sequence of copper crystallinity degree with respect to supports was SiO₂ > TiO₂ (rutile) > -Al₂O₃, which was the opposite of the catalytic activity sequence.     

Excited state reactions of amphiphilic Zn(II) porphyrin incorporated into liposomes as models for biological electron transfer

The amphiphilic porphinato zinc(II) complex (ZnP) containing four substituted amphiphilic alkyl chains was embedded in the liposomes of l,2-dipalmitoyl-sn-glycero-3phosphocholine. The distance from the embedded ZnP to the outer phase was changed from 9 to 27 ? by changing the substituted alkyl chain length. The electron transfer from the excited Zn complex to methylviologen (MV²⁺) or benzoquinone (BQ) added in the outer aqueous phase was studied. At first, quenching reactions were analyzed based on dynamic and static reaction models of the excited state. For the MV²⁺ quencher, only the triplet excited state of the embedded ZnP reacted, and electron transfer occurred at a distance less than 12 ?. In BQ both the singlet and the triplet excited states reacted, and the reaction of the singlet state was a static one indicating that BQ is incorporated into the liposomes. The distribution of the BQ molecule in the quenching sphere of ZnP was presented based on calculations assuming a stepwise incorporation into the quenching sphere.     

Synthesis and characterization of novel oxotechnetium (99Tc and 99mTc) and oxorhenium complexes from the 2,2'-bipyridine (NN)/thiol (S) mixed-ligand system

The synthesis and characterization of oxotechnetium and oxorhenium mixed-ligand complexes of the general formula MO[NN][S](3) (M = (99)Tc and Re), where NN represents the bidentate ligand 2,2'-bipyridine and S represents a monodentate thiophenol, is reported. The complexes were prepared by ligand exchange reactions using (99)Tc-gluconate and ReOCl(3)(PPh(3))(2) as precursors for the oxotechnetium and oxorhenium complexes, respectively. Compound 1 (M = (99)Tc, S = 4-methylthiophenol) crystallizes in the monoclinic space group P2(1)/a, a = 23.12(1) A, b = 14.349(6) A, c = 8.801(4) A, beta = 94.81(2) degrees, V = 2918(2) A(3), Z = 4. Compound 3 (M = Re, S = 4-methylthiophenol) crystallizes in the monoclinic space group P2(1)/a, a = 23.018(9) A, b = 14.421(5) A, c = 8.775(3) A, beta = 94.78(1) degrees, V = 2903(2) A(3), Z = 4. Compound 4 (M = Re, S = 4-methoxythiophenol) crystallizes in the orthorhombic space group Pbca, a = 16.32(1) A, b = 24.55(2) A, c = 16.94(1) A, V = 6788(9) A(3), Z = 8. In all cases, the coordination geometry around the metal is distorted octahedral with the equatorial plane being defined by the three sulfur atoms of the thiophenols and one nitrogen atom of 2,2'-bipyridine, while the apical positions are occupied by the second nitrogen atom of 2,2'-bipyridine and the oxygen of the M=O core. The complexes are stable, neutral, and lipophilic. Complete (1)H and (13)C NMR assignments are reported for all complexes. The analogous oxotechnetium complexes have been also synthesized at tracer level ((99m)Tc) by mixing the 2,2'-bipyridine and the corresponding thiol with Na(99m)TcO(4) generator eluate using NaBH(4) as reducing agent. Their structure was established by chromatographic comparison with authentic oxotechnetium and oxorhenium complexes using high performance liquid chromatography techniques.     

Sorption of Tc(IV) and Tc(VII) on Soils: Influence of Humic Substances

The technetium sorption behaviour in different samples of soils was studied under aerobic conditions. Tc(VII) was reduced to Tc(IV) by Sn²⁺ ions. About 99% of reduced technetium is absorbed by the soils under investigation. Sorption of TcO₄ ^– was studied in short-term (1-hour) and long-term (1-month) experiments. Sorption of TcO₄ ^– in presence of sodium humate (Aldrich) was generally lower than from pure water (from 99% to 12%) and depends on the depth of origin of the ground. Immobilisation of TcO₄ ^– after sorption on superficial sample of soils was studied by paper chromatography. Oxidation of Tc(IV) in presence of NO₃ ^– and NO₂ ^– (concentration range 10^–1–10^–5 mol·dm^–3) ions was studied as a function of time and concentration of NO₃ ^– and NO₂ ^– ions. The content of Tc(IV) in NO₃ ^– and NO₂ ^– solutions decreases with time (46 hours) relatively slowly.