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