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Introduction to Technetium Chemistry

Introduction to Technetium Chemistry. Technetium: Group VII second row transition metal Lightest radioactive element (no stable isotopes). Historic. 1867 : Mendeleev  Prediction of Element 43, Eka-manganese, M = 100. Tc metal. 1937 : Discovery of Technetium by Perrier and Segre

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Introduction to Technetium Chemistry

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  1. Introduction to Technetium Chemistry

  2. Technetium: Group VII second row transition metal Lightest radioactive element (no stable isotopes)

  3. Historic • 1867 : Mendeleev Prediction of Element 43, Eka-manganese, M = 100

  4. Tc metal • 1937 : Discovery of Technetium by Perrier and Segre Irradiation Mo Target at Berkeley Cyclotron, Mo target shipped to Italy Technetium isolated in Palermo TcO4- TcO4- Mo  97mTc (90 d ) and 95mTc (61 d) Tc2S7 Tc2S7 Mn HTcO4 Flow diagram of element 43 separation Chemical behavior of element 43 is different from Mo, Re and Mn. Re TcCl62-

  5. 1938 : Discovery of 99mTc by Segre and Seaborg • 1952 : Discovery of Tc spectra in star and report of 99Tc in a note from Oak Ridge NL • 1961 : Isolation of 99Tc in pitchblende

  6. Isotopes 30 isotopes from 85Tc to 115Tc 115Tc : T1/2 = 130 ms (shortest) 98Tc : T1/2 = 4.6.106 year (most stable isotope) Two major isotopes of interest: 99Tc (Fission product of nuclear industry) T1\2 = 2.13. 105 year,b- = 294 keV Specific activity : 0.63 Giga-Becquerel by gram 99mTc (Imaging agent in nuclear medicine) T1/2 = 6.02 h, g = 141 keV 194000 Tera-Becquerel by gram

  7. Production 99mTc : 98Mo target neutron irradiation 99Mo b decay of 99Mo 99mTc

  8. 99Tc : Fission product of nuclear industry yield ~ 6 % from fission of 235U Each year, 2 tons of 99Tc are produced by US nuclear industry In spent fuel: Tc is present as metal or alloys with Mo-Ru-Pd -Rh

  9. Applications 99mTc: Imaging agent in nuclear medicine ( 90% of Radio- Diagnostic :brain, heart...) Cardiolite (Heart imaging) : 40 million patients treated since 1991 Cardiolite® – [Tc(CNCH2C(Me)2OMe)6]+

  10. Potential applications Catalyst : Tc efficient catalytic properties for Aldehyde production Anti corrosive agent : 55 ppm in steel avoid corrosion (Passivation by insoluble Tc oxide). Tank in nuclear reactor. Electronic compounds : Tc metal is super-conductor at 7.46 °K. Source of ruthenium (catalyst) : Transmutation of 99Tc to 100Ru

  11. Transmutation Transmutation of 99Tc by neutron capture 99Tc + n 100Tc 100Ru (Stable) + b- Exp. demonstration performed in 2003- 2008 in a fast neutron reactor Transmuted Tc Tc/Ru alloys Assembly for transmutation Phenix fast neutron reactor, France • 5 years irradiation: ~25 % of Tc transmuted into Ru

  12. Tc in Environment • 99Tc in environment as the TcO4- anion 1. Nuclear fuel reprocessing : ~3 tons rejected in the Sea (since 1984) - Sellafield (U.K): 140 kg by year - La Hague (France): 1.6 kg by year 2. Atomic Weapons Tests : ~ [390 – 470 ] kg 3. Chernobyl Explosion : ~ 2 kg 4. Nuclear Medicine (Low)

  13. Acquisition of Fundamental Dataon Tc chemistry • Understand behavior of Tc in the nuclear fuel cycle • Development of new imaging agents • Predict Tc behavior in environment

  14. Inorganic Chemistry 1. Characteristic 2. Solid-state compounds 3. Low valent molecular complexes 4. Aqueous solution chemistry 5. Summary

  15. 1. Characteristic

  16. Tc and Re expected to have similar chemistry

  17. 9 oxidations state: from +7 to -1 tetrahedral TcO4- octahedral TcL6 anion: L = Cl, NCS, CN Square pyramidal TBA: Tetrabutyl-ammonium, (Bu4N)+ TcLCl4 anion L = N, O

  18. 2. Solid-state compounds A. Metal B. Binary oxides C. Binary halides D. Other binary compounds

  19. A. Metal Structure: Hexagonal (Hcp) Density : 11.49 Melting point : 2200 °C Boiling point : 4270 °C Synthesis : Thermal reduction of NH4TcO4 under H2 at T> 500 °C NH4TcO4 + 2H2 Tc + 4H2O + (1/2)N2 Electro-reduction of TcO4- in H2SO4 Hcp structure Tc metal on Cu electrode

  20. B. Binary Oxides Transition metal binary oxides MOn (n = 1- 4) ~70 are known (e.g., 5 for Mn, 3 for Re) 2 technetium binary oxides:

  21. 1. TcO2 Decomposition of NH4TcO4 under Ar atmosphere at 750 °C NH4TcO4 TcO2 + 2H2O + (1/2)N2 T = 750 ºC Ar NH4TcO4 TcO2 Set-up Extended structure: Characterization by NPD Tc2 Tc1 Tc Tc-Tc bonding • TcO2 is insoluble in H2O (solubility: ~ 10-8 M)

  22. 2. Tc2O7 Oxidation of TcO2 by O2 at 450 °C in a sealed tube 2 TcO2 + (3/2)O2 Tc2O7 450 °C O2 Molecular dimer Similar to Mn2O7 Tc2O7 TcO2 Properties 120 °C 311 °C solid liquid gas • Tc2O7 is highly volatile and soluble in H2O

  23. C. Binary Halides • Transition metal binary halides MXn (X = halide, and n = 1-7) • ~Two hundred are known (e.g., 13 for Mo, 14 for W and Re) • 10 Tc binary halides phases: TcCl4, TcF5 and TcF6 synthesized in the 60 s TcClx(x = 2, 3) , TcBrx(x = 3, 4) and synthesizedat UNLV in 2009-2011

  24. 1. Binary fluorides TcF6 synthesized in 1961: Tc + xs F2→ TcF6 TcF5 synthesized in 1963 Tc + xs N2/F2→ TcF5 Tc(+6), d1 Iso structural to ReF6 Chain of TcF6 octahedra Tc(+5), d2 Iso structural to CrF5 Molecular TcF6

  25. 2. Binary chlorides a. Tetrachloride Technetium tetrachloride synthesized in 1957: Tc + xs Cl2→ TcCl4 Cl1 Cl3 Cl2 Tc(+4), d3 No Tc-Tc bond Precursor for synthesis Ex: TcCl4L2 complexes; L= THF Infinite chain of edge-sharing TcCl6 octahedra.

  26. b. alpha- Trichloride Reaction between Tc2Cl2(OAc)4 and HCl(g) at 300 °C ~300 °C ~100 °C HCl(g) HCl(g) Tc2Cl4(OAc)2 TcCl3 Tc2Cl2(OAc)4 TcCl3: Infinite layers of Tc3Cl9 bridged by Cl ligands Tc3Cl9 cluster in TcCl3 Tc-Tc (2.444 Å) : double bond View of a layer • TcCl3 crystallize with the “ReCl3” structure-type

  27. b-Technetium trichloride Reaction between Tc metal and Cl2 in sealed tube b-TcCl3 [Tc2Cl10] unit in b-TcCl3 b-TcCl3 layer b-TcCl3: Infinite layers of edge -sharing TcCl6 octahedra b-TcCl3 converted to a-TcCl3 at 280 °C in a sealed tube.

  28. c. Dichloride Reaction between Tc metal and Cl2 in sealed tube (Tc:Cl ~ 1:2) 450 °C 24 h Tc metal and Cl2 Dark-red film TcCl2 needles Needles + metal TcCl2: Infinite chains of face-sharing Tc2Cl8 rectangular prisms TcCl2 chain [Tc2Cl8] unit in TcCl2 Tc-Tc = 2.129 Å Tc≡Tc triple bond • TcCl2 exhibits a new structure-type

  29. 3. Binary bromides a. Tetrabromide Reaction between Tc metal and Br2 in sealed tube (Tc:Br ~ 1:4) 400 ºC 6 hours TcBr4: Infinite chains of edge-sharing TcBr6 octahedra Br1 Br3 3.791 Br2 TcBr4: air stable • First group 7binary tetrabromide characterized • Crystallizes in the TcCl4 structure-type

  30. b. Tribromide Reaction between Tc metal and Br2 in sealed tube (Tc:Br ~ 1:3) TcBr3: Infinite chains of face-sharing TcBr6 octahedra Br3 Br1 2.828 3.143 TcBr3: air stable Br3 Br4 Tc(+3) d4 Oh coordination Alternation short/long d(Tc-Tc) · deformation of octahedron Weak Tc-Tc interaction • TcBr3 is isostructural to MoBr3 and RuBr3

  31. 4. Binary iodides (last week) TcI3: Reaction between Tc2Cl2(OAc)4 and HI(g) at 300 °C Elemental analysis and EDX spectroscopy : composition TcI3 TcI3 is amorphous: structure not determined Next week EXAFS spectroscopy : TcI3 has the Re3Cl9 or TcBr3 structure ?

  32. D. Other binary compounds 1. Binary Carbides Reaction between Tc metal and graphite at 1050 °C Product depend on Tc:C ratio: For C< 1% : Solid solution of Carbon in Tc metal For 1 <C< 9%: Formation of a new phase TcC. Structure not reported 2. Binary Nitrides Tc nitride formed by thermal decomposition of [NH4]2TcCl6 under N2.  Stoichiometry TcN0.75. Structure unknown T = 300 ºC N2 [NH4]2TcCl6 TcN0.75

  33. 3. Binary Sulfides Two sulfides reported :Tc2S7 and TcS2. Tc2S7: Reaction between TcO4- and H2S in acidic solution Structure unknown. TcS2: formed by decomposition of Tc2S7 at 1000 °C Structure characterized, isostructural to ReS2 4. Phosphides Reaction between Tc metal and phosphorous in a sealed tube (1000 °C):  4 binary phosphides. Tc3P, Tc2P3, TcP3 and TcP4 Structure of TcP3 : Edge sharing TcP6 octahedron

  34. 3. Low valent molecular complexes A. Generality B. Preparation and structure

  35. 1. Generality • High tendency to form metal-metal bonds in low oxidation state Concept of multiple metal-metal bond introduced in 1964 for Re2Cl82- Overlapping of metal d orbital to form a chemical bond • 600 Re and 30 Tc complexes with multiple M-M bonds reported

  36. Complexes with multiple Tc-Tc bonds reported for Tc in the +3, +2, and +1 oxidation state X =Cl, Br; TBA: (Bu4N)+; Tc8I12 cluster Tc2X8n- ion Tc2X4(PMe3)4

  37. 2. Preparation and structure a) (TBA)2Tc2X8 12 M HCl T = 100 °C, H2O2 (TBA)OH (TBA)TcO4 cold TcO2/NH4TcO4 (TBA)TcOCl4 (n-Bu4N)BH4 THF & HBr gas HCl, acetone T = 30 °C (TBA)2Tc2Br8 (TBA)2Tc2Cl8

  38. Crystallization from acetone / ether for single crystal XRD  Formation of an acetone solvate: (TBA)2Tc2X8. 4[(CH3)2CO] Tc2X82- ion Eclipsed TcCl4 units • Steric effect induced by bromide in Tc2Br82- ion Increase of Tc-Tc separation and Tc-Tc-X angle

  39. b) Cs3Tc2X8 1. Disproportionation of Tc2X82- anion in concentrated HX at 80 °C 3 Tc2X82- + 4 X- → 2 TcX62- + 2 Tc2X83- 2. Selective precipitation of TcX62- and Tc2X83- with CsX CsBr (Cs:Tc : 3:1) Transfer in centrifuge tube Precipitation TcBr62- (TBA)2Tc2Br8 in HBr at 80 °C xs CsBr Precipitation Tc2Br83-

  40. Crystallization in concentrated HBr for single crystal XRD  Formation of hydrate: [Cs(2+x)][H3O(1-x)]Tc2Br8·4.6H2O (x = 0.221) Tc2X83- ion Tc2Br83- ion 4.86° • Steric effect induced by bromide in Tc2Br83- anion • Internal rotation angle in Tc2Br83- (4.86°) • d(Tc-Tc) in Tc2Br83- is ~ 0.04 Å shorter than in Tc2Br82-

  41. c) Tc2X4(PMe3)4 Disproportionation of (TBA)2Tc2X8 in CH2Cl2/ PMe3 2(TBA)2Tc2X8 + 6PMe3Tc2X4(PMe3)4 + 2TcX4(PMe3)2 + 4(TBA)X 3. Extraction and Recrystallization in hexane 1. Five min under Ar 2. Pumping todryness (TBA)2Tc2Br8 in CH2Cl2/ PMe3 Tc2Br4(PMe3)4

  42. X P Tc TcX2(PMe3)2 units rotated by 105  Isomorphous to M2X4(PMe3)4 (M = Re, Mo) • Tc24+ core not sensitive to the nature of coordinating halide

  43. 4. Aqueous solution chemistry A. Non-complexing media • Complexing media 1. chloride, 2. carbonate

  44. A. Non-complexing media

  45. Aqueous species Speciation depends on redox and acidity of the solution Tc(+7) Tc(+4) metal  3 species are thermodynamically stable: Tc(+7), Tc(+4) and Tc metal

  46. Disproportionation • Tc(+6), Tc(+5), Tc(+3) are thermodynamically unstable

  47. B) Complexing media

  48. 1. Chloride media 4 Chloro- species identified in concentrated HCl TcOCl5 anion • TcO4- isunstable in 12 M HCl: reduction of Tc by Cl- Cold HCl Cold HCl Warm HCl TcO4- TcOCl5- TcOCl52- TcCl62- Further reduction of Tc(IV) in warm HCl by Zn: TcCl62- + Zn  Tc2Cl83- + Zn2+

  49. 2. Carbonate media • Presence of carbonate in geological formation • Tc(IV) and Tc(III) complexes reported in carbonate solution • Structure not characterized • No solid of Tc carbonate synthesized

  50. 5. Summary

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