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Introduction to Technetium Chemistry
Technetium: Group VII second row transition metalLightest radioactive element (no stable isotopes)
1869: Predicted by D. Mendeleev
Irradiation of molybdenum plate at Berkeley cyclotron
1934: Predicted to have no stable isotope (Mattauch rule)
1937: Discovered by E. Segre and C. Perrier
Historic
Lightest radioactive element (no stable isotopes)
Why there are no stable isotopes ?
1. If there was stable isotope they should located in the “ valley of stability “ in the chart of the nuclides: 97Tc, 98Tc, 99Tc, 100Tc, 101Tc
A = 42Mo 43Tc 44Ru
97 -87.54 -87.22 -86.11
98 -88.11 -86.43 -88.22
99 -85.97 -87.32 -87.62
100 -86.18 -86.02 -89.22
101 -83.51 -86.34 -87.95
2. These isotopes should have the lowest binding energy (Eb) among element with same Z
For Z = 97 to 101:Technetium isotopes do not have the lowest Eb
Eb: Energy required to dissociate the nucleus into its constituent nucleons
Eb = {ZMH – (A-Z)MN} –M Nucleus
Table of binding energy Most stable isotopes are in red
IsotopesIsotopes32 isotopes from 85Tc to 115Tc
115Tc : T1/2 = 130 ms (shortest)98Tc : T1/2 = 4.6.106 y (longest)
Two isotopes of interest: 99Tc (Fission product of nuclear industry)
T1\2 = 2.13. 105 year, - = 294 keVSpecific activity : 0.63 Giga-Becquerel by gram
99mTc (Imaging agent in nuclear medicine)
T1/2 = 6.02 h, = 141 keV194000 Tera-Becquerel by gram
99mTc produced from the decay of 99Mo
99Mo produced by fission of 235U
Production
Fission of 235U
99Tc : Fission product of nuclear industryyield ~ 6 % from fission of 235U
→ 0.8 kg/ MT of spent fuel
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 (-phase)
-phase: Micron-sized particles
ApplicationsApplications
Cardiolite® : [Tc(CNCH2C(Me)2OMe)6]+
99mTc: Imaging agent in nuclear medicine; ~90% of all radiodiagnostic tests
Optimal nuclear properties: Energy allows imaging deep organs without damage Versatile chemistry: Coordination with suitable functional group (brain, heart, bone,..)
Cardiolite ® : Heart imaging , 40 million patients treated since 1991
Catalyst : Tc efficient catalytic properties for Aldehyde production
Anti-corrosive agent : 55 ppm in steel avoid corrosion (Passivation by insoluble Tc oxide).
Electronic compounds : Tc metal is super-conductor at 7.46 °K.
Source of ruthenium : Transmutation of 99Tc to 100Ru
Potential applications
Transmutation
Transmutation of 99Tc by neutron capture99Tc + n 100Tc 100Ru (Stable) + -
Exp. demonstration performed in 2003- 2008 in a fast neutron reactor
• 5 years irradiation: ~25 % of Tc transmuted into Ru
Transmuted TcTc/Ru alloysAssembly for transmutationFast neutron reactor, France
Tc in EnvironmentTc 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)
Improve nuclear fuel cycle applications
Development of new imaging agents
Predict Tc behavior in environment
Understand fundamental chemistry of technetium
Inorganic Chemistry
1. Characteristic2. Solid-state compounds3. Complexes with multiple metal-metal bonds4. Aqueous chemistry5. Summary
1. Characteristic
Characteristic Tc Re
Atomic Number 43 75
M 98 186.20
Electronic Structure [Kr]4d55s2 [Xe]4f145d56s2
Tc and Re share similar electronic structure Similar coordination complexes
Binary halides Metal-metal bonded dimers Heptavalent complexes
Tc coordination chemistry less developed than Re (as of 2008)
Binary halides Metal-metal bonded dimers
Heptavalent complexes
Tc 3 25 30
Re 15 500 150
MF5 M2Cl82- MO4-
Ox. state Tc config. Compound
+7 [Kr]4d0 KTcO4
+6 [Kr]4d1 (TBA)TcNCl4
+5 [Kr]4d2 (TBA)TcOCl4
+4 [Kr]4d3 K2TcCl6
+3 [Kr]4d4 (TBA)3Tc(NCS)6
+2 [Kr]4d5 TcCl2(PMe3)4
+1 [Kr]4d6 K5Tc(CN)6
0 [Kr]4d7 Tc2(CO)10
-1 [Kr]4d8 [Tc(CO)5]-
tetrahedral
octahedral
9 oxidations state: from +7 to -1
TcO4-
TBA: Tetrabutyl-ammonium, (Bu4N)+
TcL6 anion:L = Cl, NCS, CN
Square pyramidal
TcLCl4 anionL = N, O
2. Solid-state compoundsA. MetalB. Binary oxidesC. Binary halidesD. Other binary compounds
A. Metal
Structure: Hexagonal (Hcp)Density : 11.49Melting point : 2200 °CBoiling 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
Ox. state Solid Characteristic
+7 Tc2O7 Molecular dimer
+4 TcO2 Extended structure
Isostructural to ReO2
Transition metal binary oxides MOn (n = 1- 4)~70 are known (e.g., 5 for Mn, 3 for Re)2 technetium binary oxides:
B. Binary Oxides
Extended structure of TcO6 octahedron
TcO2NH4TcO4
T = 750 ºC
Ar
Set-up
1. TcO2
Decomposition of NH4TcO4 under Ar atmosphere at 750 °C
NH4TcO4 TcO2 + 2H2O + (1/2)N2
•TcO2 is insoluble in H2O (solubility: ~ 10-8 M)
Molecular dimerSimilar to Mn2O7
2. Tc2O7
Oxidation of TcO2 by O2 at 450 °C in a sealed tube
2 TcO2 + (3/2)O2 Tc2O7
TcO2
450 °C
O2
•Tc2O7 is highly volatile and soluble in H2O
Tc2O7
Properties
solid liquid gas
120 °C 311 °C
Transition metal binary halides: MXn (X = halide; n = 1-7)Prior to 2008, only three technetium binary halides were known
TcCl4 in 1957: TcF6 in 1961: TcF5 in 1963: Tc + xs Cl2 → TcCl4 Tc + xs F2 → TcF6 Tc + xs N2/F2 → TcF5
No binary iodides and bromides reportedNo trivalent or divalent Tc binary halides reported prior 2008
C. Binary halides
In 2014: Ten binary halides are reported (worked performed at UNLV)
Tube isflame-sealed
Tc + Br2 Tc + Cl2
Tc metal in glass tube
Reaction between halides and Tc metal at elevated temperature in sealed tubes
Furnace250-500 °C 6 h-16 days
Tc + I2
+ X2
-TcCl3
-TcCl2-TcCl2
TcI3
-TcCl3
TcBr4 TcBr3
Tc + X2
Tc:Br ~
1:4
450 °C
400 °
C
Seven new phases reported
Tc:
Br
~ 1:
3
400
°C
Tc:Cl ~ 1:2.5450 °C
AlCl3
280 °C
Tc:I ~ 1:4
1 mtorr
400 °C
New structure-type
1. Binary Carbides
2. Binary Nitrides
Tc nitride formed by thermal decomposition of [NH4]2TcCl6 under N2. Stoichiometry TcN0.75. Structure unknown
N2
T = 300 ºC
[NH4]2TcCl6 TcN0.75
D. Other binary compounds
Reaction between Tc metal and graphite at 1050 °CProduct depend on Tc:C ratio:For C< 1% : Solid solution of carbon in Tc metalFor 1 <C< 9%: Formation of a new phase TcC. Structure unknown
Structure of TcP3 : Edge-sharing TcP6 octahedron
4. Phosphides
Four binary phosphides : Tc3P, Tc2P3, TcP3 and TcP4
Reaction between Tc metal and phosphorous in a sealed tube (1000 °C)
3. Binary sulfidesTwo 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
3. Complexes with multiple metal-metal bondsA. GeneralityB. Preparation and structure
For Tc26+
For Tc25+ For Tc2
4+
Metal-metal bonded dimers: M2n+ units coordinated to ligands
In the M2n+: d orbitals can overlap and form , and bonds
Technetium: Tc2
6+, Tc25+ and Tc2
4+ unit
Tc2Cl83- Tc2Cl8
2-
quadruple bond electron-rich triple bond
Tc2Cl4(PMe3)4
Bond order of 3.5
1. Generality
(TBA)TcO4 (TBA)TcOCl4
(TBA)2Tc2Cl8 (TBA)2Tc2Br8
TcO2/NH4TcO4
T = 100 °C, H2O2
(TBA)OH
12 M HCl
cold
(n-Bu4N)BH4
THF
HCl, acetone
HBr gas
T = 30 °C
&
a) Tc26+: (TBA)2Tc2X8
2. Preparation and structure
Anions Tc-Tc (Å) <Tc-X> (Å) <Tc-Tc-X> (°)
Tc2Br82- 2.1625(9) 2.4734(7) 105.01(3)
Tc2Cl82- 2.1560(3) 2.3223(7) 103.92(2)
Crystallization from acetone / ether for single crystal XRD Formation of an acetone solvate: (TBA)2Tc2X8. 4[(CH3)2CO]
Tc2X82- ion
• Steric effect induced by bromide in Tc2Br82- ion
Increase of Tc-Tc separation and Tc-Tc-X angle
Eclipsed TcCl4 units
b) Tc25+: Cs3Tc2X8
1. Disproportionation of Tc2X82- anion in concentrated HX at 80 °C
3 Tc2X82- + 4 X- → 2 TcX6
2- + 2 Tc2X83-
2. Selective precipitation of TcX62- and Tc2X8
3- with CsX
Precipitation Tc2Br83-
(TBA)2Tc2Br8 in HBr at 80 °C
Transfer in
centrifuge tubePrecipitation TcBr6
2-
CsBr (Cs:Tc : 3:1)
xs CsBr
Anion Tc-Tc (Å) <Tc-X> (Å) <Tc-Tc-X> (°)
Tc2Br83- 2.1265(9) 2.473[2] 106.07(3)
Tc2Cl83- 2.117[4] 2.363[15] 104.82[7]
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
• 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 Tc2Br8
2-
Tc2Br83- ion
4.86°
3. Extraction and Recrystallization
in hexane
c) Tc24+ : Tc2X4(PMe3)4
Disproportionation of (TBA)2Tc2X8 in CH2Cl2/ PMe3
2(TBA)2Tc2X8 + 6PMe3 Tc2X4(PMe3)4 + 2TcX4(PMe3)2 + 4(TBA)X
1. Five min under Ar2. Pumping to dryness
(TBA)2Tc2Br8 in CH2Cl2/ PMe3
Tc2Br4(PMe3)4
compound Average distances (Å)
Tc-Tc Tc-X Tc-P
Tc2Br4(PMe3)4 2.1316(5) 2.520[1] 2.441[1]
Tc2Cl4(PMe3)4 2.1318(3) 2.3858[5] 2.4356[4]
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
X
Tc
P
4. Aqueous solution chemistry
A. Non-complexing mediaB. Complexing media 1. chloride, 2. carbonate
A. Non-complexing mediaA. Non-complexing media
Speciation depends on redox and acidity of the solution
Aqueous species
3 species are thermodynamically stable:Tc(+7), Tc(+4) and Tc metal
Tc(+7)
Tc(+4)
metal
DisproportionationDisproportionation
Tc Reaction
Tc(+6) 2Tc(+6) Tc(+7) + Tc(+5)
Tc(+5) 3Tc(+5) Tc(+7) + 2Tc(+4)
Tc(+3) 5Tc(+3) 4Tc(+4) + Tc(0)
•Tc(+6), Tc(+5), Tc(+3) are thermodynamically unstable
B) Complexing media
Ox. Media Species
+6 HCl/H2SO4 TcOCl5-
+5 Cold HCl TcOCl52-
+4 Warm HCl TcCl62-
+2, +3 Warm HCl +
Zn powder
Tc2Cl83-
TcO4- TcOCl5
- TcOCl52- TcCl6
2- Warm HClCold HCl Cold HCl
Further reduction of Tc(IV) in warm HCl by Zn: TcCl6
2- + Zn Tc2Cl83- + Zn2+
TcOCl5- anion
4 Chloro- species identified in concentrated HCl
•TcO4- is unstable in 12 M HCl: reduction of Tc by Cl-
1. Chloride media
O.N Media Structure
IV Neutral
HCO3- = 0.5 M
Reducing media
Tc(CO3)q(OH)n(4-n-2q)+ ?
III Tc(CO3)q(OH)n(3-n-2q)+ ?
• No solid of Tc carbonate synthesized
• Presence of carbonate in geological formation • Tc(IV) and Tc(III) complexes reported in carbonate solution Structure unknown
2. Carbonate media
5. Summary
Group VII second row transition metalPredicted 145 years ago and discovered 77 years ago
Lightest radioactive element (no stable isotopes)Two isotopes of interest99Tc (-) : fission product of nuclear industry (6% yield from 235U, 2 tons/y produced) 99mTc(): produced from 99Mo decay, imaging agent: cardiolite 40 millions treated
Presence in environment from nuclear fuel reprocessing and atomic weapon test
Basic inorganic chemistryRich redox- chemistry with 9 oxidations states, chemistry similar to Re Solid state compounds: rich and unique halides chemistry (i.e., TcCl2 structure-type)Lack of data for nitrides, sulfides and carbides materialsHigh tendency to form multiple metal-metal bonds in low valent states (i.e., Tc2X8
n-)
Aqueous chemistryNon complexing: 3 oxidation states are thermodynamically stable (+7, +4 and 0)Con. HCl: Tc(+6), Tc(+5), Tc(+4) and Tc(+2.5) complexes characterizedCarbonate: Tc(+4) and Tc(+3) reported but not characterized
Technetium
Questions
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