Reactivity of HTcO4 with methanol in sulfuric acid: Tc-sulfate complexes revealed by XAFS spectroscopy and first principles calculations

Article abstract of DOI:10.1039/c3dt32951h

The reaction between HTcO₄ and MeOH in 13 M H₂SO ₄ was investigated by ⁹⁹Tc NMR, UV-visible and X-ray absorption fine structure (XAFS) spectroscopy. Experimental results and first principles calculations show th

Full text of DOI:10.1039/c3dt32951h

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Reactivity of HTcO4 with methanol in sulfuric acid:
Tc-sulfate complexes revealed by XAFS spectroscopy
and first principles calculations†
Frederic Poineau,*a Philippe F. Weck,b Benjamin P. Burton-Pye,c Ibthihel Denden,d
Eunja Kim,e William Kerlin,a Konstantin E. German,f Massoud Fattahi,d
Lynn C. Francesconi,c Alfred P. Sattelbergerg and Kenneth R. Czerwinskia
Cite this: Dalton Trans., 2013, 42, 4348
Received 11th September 2012,
Accepted 20th January 2013
DOI: 10.1039/c3dt32951h
www.rsc.org/dalton
The reaction between HTcO4 and MeOH in 13 M H2SO4 was inves- on Tc(+7) includes the synthesis and characterization of com-
tigated by 99Tc NMR, UV-visible and X-ray absorption fine struc- plexes with the [TcO3]+ core for potential radiopharmaceutical
ture (XAFS) spectroscopy. Experimental results and first principles applications.5 The redox-chemistry of these trioxo-complexes
calculations show the formation of Tc(+5) sulfate complexes. The has been poorly studied. It has been shown that these com-
results expand the fundamental understanding of Tc in high acid plexes do oxidize olefins, first by Davison et al. and more
solutions.
recently by Braband et al.6a,b
Recently, we studied the behavior of pertechnetate in sulfu-
ric acid and showed that HTcO4 dihydrate forms above 8 M
H2SO4 and is the predominant species above 13 M H2SO4.7 In
solution, pertechnetic acid is an octahedral complex with the
formula [TcO3(OH)(H2O)2]. The redox properties of this
complex have not been examined. A solution of HTcO4 is
stable for months, but is very reactive in the presence of redu-
cing agents.7 Studying the redox chemistry of HTcO4 in the
presence of organic substrates will permit a better understand-
ing of high valent transition metal oxidative properties. These
studies could also provide information about the redox
behavior of Tc(+7) complexes with potential biomedical
applications.
In this context, we investigated the behavior of HTcO4 in
the presence of organics in H2SO4, beginning with methanol.
The Tc complexes formed from the reaction of HTcO4 with
MeOH in sulfuric acid were characterized by spectroscopic
(NMR, EPR, UV-visible and XAFS) techniques and first prin-
ciples calculations.
Transition metal complexes with multiple MvO bonds (M =
Mn, Tc, Re) exhibit oxidative properties that are of interest in
organic and bioinorganic chemistry. The permanganate anion,
E0([MnO4]−/MnO2) = 1.6 V, oxidizes alcohols, alkanes, alkenes,
aldehydes, ketones, and carboxylic acids and has been used as
a degradation agent of organics in the purification of water.1
For rhenium, E0([ReO4]−/ReO2) = 0.52 V; complexes with the
[ReO3]+ core show high activity for the oxidation of alcohols.2
Rhenium heptaoxide (Re2O7) has been mentioned as a catalyst
for the electro-oxidation of methanol (MeOH) in sulfuric acid
and could be used for fuel cell applications.3 Located between
manganese and rhenium, heptavalent technetium, E0([TcO4]−/
TcO2) = 0.74 V, is a stronger oxidizing agent than Re(+7) but
weaker than Mn(+7); its redox-behavior in the presence of alco-
hols has been less studied than its congeners.4 Current work
aDepartment of Chemistry, University of Nevada Las Vegas, Las Vegas, NV 89154,
USA. E-mail: poineauf@unlv.nevada.edu
bSandia National Laboratories, Albuquerque, NM 87185, USA
cDepartment of Chemistry, Hunter College of the City University of New York,
NY 10065, USA
dLaboratory Subatech, Ecoles des Mines de Nantes, Nantes, 44, France
eDepartment of Physics and Astronomy, University of Nevada Las Vegas, Las Vegas,
NV 89154, USA
fA. N. Frumkin Institute of Physical Chemistry and Electrochemistry,
Russian Academy of Sciences, 119071 Moscow, Russian Federation
gEnergy Engineering and Systems Analysis Directorate, Argonne National Laboratory,
Ammonium pertechnetate (∼10 mg) was placed in glass
vials and 1 mL of sulfuric acid (6 M and 13 M) was added with
a glass pipette. The solution in 6 M H2SO4 remains colorless
while a yellow color (HTcO4) was observed in 13 M H2SO4.
Then, 20 μl of methanol was added to the solutions. In 6 M
H2SO4, no reaction was observed8,9 while in 13 M H2SO4, the
solution turned green.
The reaction between HTcO4 and MeOH in 13 M H2SO4 was
further studied by UV-visible spectroscopy. The reaction is
characterized by the disappearance of the bands at 243 nm
and 337 nm of HTcO4 and the appearance of the bands at
Lemont, IL 60439, USA
†Electronic supplementary information (ESI) available: Additional details on
NMR, UV-visible, XAFS experiment and computational methods. See DOI:
10.1039/c3dt32951h
190 nm (εmax = 8540 M−1 cm−1), 255 nm (εmax = 5060 M−1 cm−1
)
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Tc(+5) species. For the reaction with MeOH, an initial reaction
rate of 2.9
×
10−5 moles L−1 min−1 was determined
(Fig. S4_UV†). For the reaction with formaldehyde, the com-
plete conversion of HTcO4 to the Tc(+5) species occurs in less
than 2 minutes and indicates that HTcO4 reacts at least
20 times faster with formaldehyde than with MeOH
(Fig. S5_UV†).14 Finally, no reaction occurs between HTcO4
and formic acid (Fig. S6_UV†). These results indicate that
formic acid is likely the kinetic product of oxidation of MeOH
by HTcO4. In order to probe the presence of formic acid after
the reaction with MeOH, the solution was reacted with mercu-
ric acetate.15 The appearance of the band at 236 nm is consist-
ent with the presence of formic acid after the reaction of
HTcO4 with MeOH (see ESI,† V. Formic acid titration). In
summary, during the reaction of HTcO4 with MeOH in 13 M
H2SO4, Tc(+7) is reduced to Tc(+5) and MeOH is oxidized to
formic acid. The structure of the Tc(+5) species is further
studied by XAFS spectroscopy and DFT techniques.
The solution obtained from the reaction of HTcO4 with
MeOH in 13 M H2SO4 was studied by X-ray absorption fine
structure spectroscopy. The X-ray Absorbance Near-Edge Struc-
ture (XANES) spectrum (Fig. S1_XAFS†) exhibits a weak pre-
edge feature in the region 21 038–21 050 eV. For technetium,
the pre-edge is due to the 1s → 4d transition and is forbidden
in complexes with inversion symmetry. This indicates that the
compound under investigation would not have an inversion
center; further EXAFS and DFT results show that the Tc(+5)
species exhibits a deformed octahedral geometry (vide infra).
The position of the Tc K-edge depends on various parameters
such as oxidation state of the absorbing atom and nature of
the ligands.16 Positions of the Tc K-edge (eV vs. [TcO4]−) for
complexes are reported in Table 1. In our case, the position of
the absorption edge of the Tc(+5) species (21 055 eV, −8 eV vs.
[TcO4]−) is consistent with the one of Tc(+4) and Tc(+5) com-
plexes. This result is in good agreement with the one obtained
from cerium titration which indicates an oxidation state of
Tc(+5) (vide supra).
Fig. 1 UV-visible spectra of the solution (A) before and (B) after the reaction of
HTcO4 with MeOH in 13 M H2SO4. The solution was prepared by dissolving
KTcO4 (26.9 mg, 0.133 mmol) in 13 M H2SO4 (10 mL), then 1 mL of the solution
was reacted with 20 μl of MeOH.
and 695 nm (εmax = 33 M−1 cm−1) of the green Tc species
(Fig. 1). During the reaction an isosbestic point (328 nm) is
observed (Fig. S1_UV†).
The oxidation state of the green Tc species was determined
by cerium titration. Results indicate an average oxidation state
of 4.93(10) (see ESI,† I. Cerium titration), which is consistent
with the presence of Tc(+5).
The Tc(+5) species was analyzed by 99Tc NMR and EPR
spectroscopy. The absence of NMR signals in the region
0–800 ppm (see ESI,† II. NMR spectroscopy) indicates the
absence of Tc(+7) in solution.10 The EPR measurement did not
show any resonance signal which is consistent with the pres-
ence of Tc(+5) species and the absence of Tc(+4) (see ESI,† III.
EPR spectroscopy).
The reactivity of HTcO4 with MeOH was compared with the
one of KMO4 (M = Mn, Re). Solutions of KMnO4 and KReO4
(0.01 M; 1 mL) in 13 M H2SO4 were reacted with MeOH
(20 μL). For manganese, a reaction immediately occurs and a
color change from yellow-brown to pink is observed. The spec-
trum of the pink solution (Fig. S2_UV†) exhibits the bands
characteristic of Mn(+2). For rhenium, no reaction occurs over
the course of 2 days (Fig. S3_UV†).
Reactions between HMnO4 and MeOH in mineral acids
have been studied. Experiments performed in 1 M H2SO4 indi-
cated that the oxidation of MeOH by [MnO4]− leads to formic
acid.11 Other experiments in sulfuric acid, in the temperature
range 45–60 °C, have shown that the oxidation of MeOH with
[MnO4]− in the presence of catalysts leads to carbon dioxide; it
was proposed that the total oxidation of MeOH by [MnO4]−
proceeds successively to formaldehyde, formic acid and CO2.12
Studies in perchloric acid have shown that HMnO4 oxidizes
MeOH to formaldehyde.13 In this study, the mechanism of oxi-
dation of alcohols to aldehydes by HMnO4 has been discussed:
the alcohol (RCH2OH) initially reacts with HMnO4 to form the
RCH2O·MnO3 complex which further decomposes to the alde-
hyde (RCHO) and HMnO3 (i.e., Mn(+5)). The Mn(+5) complex
is unstable and disproportionates to Mn(+2) and Mn(+7).
In order to understand the mechanism of oxidation of
MeOH by HTcO4, kinetic studies were performed. Solutions of
HTcO4 (0.97 mM, 1 mL) were reacted with MeOH (20 μL), for-
maldehyde (20 μl) and formic acid (20 μL). The reactions were
monitored over one hour using the band at 695 nm of the
The Extended X-ray Absorption Fine Structure (EXAFS) spec-
trum was k3-weighted and Fourier transform (FT) performed in
the k range [3–12] Å−1. The FT (Fig. 2) shows one main peak
centered at R + Δ ∼ 1.5 Å and a second peak at R + Δ ∼ 2.65 Å.
The position of the first peak is higher than the one reported
Table 1 Shift of Tc K-edge (eV vs. NH4[TcO4]) versus Tc oxidation state (Tc(+x))
for NH4[TcO4],16a (n-Bu4N)TcNCl4,16b [Tc(ArN)3]2Hg,16a (n-Bu4N)TcOCl4,16c
2− 16c
TcO2,16a TcCl6
metal16a
,
(n-Bu4N)2Tc2X8 (X = Cl, Br),16c Tc2Cl4(PMe2Ph)4,16a and Tc
Tc(+x) Compound
Shift (eV) Tc (+x) Compound
Shift (eV)
Tc(+7) NH4[TcO4]
Tc(+6) (n-Bu4N)-
TcNCl4
Tc(+5) (n-Bu4N)-
TcOCl4
Tc(+5) [Tc(ArN)3]2Hg −4.61
Tc(+4) TcO2 −6.95
0
−6.5
Tc(+4) [TcCl6]2−
−9
Tc(+3) (n-Bu4N)2Tc2Cl8 −11
Tc(+3) (n-Bu4N)2Tc2Br8 −12
Tc(+2) Tc2Cl4(PMe2Ph)4 −13.24
−8
Tc(0)
Metal
−19.85
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Table 2 EXAFS fit parameters obtained for the solution obtained after the
reaction of HTcO4 with MeOH in 13 M H2SO4. ΔE0 = 6.5 eV
Atoms
C. N.
Distance (Å)
σ2 (Å2)
O
O
Sbid
Smono
1.0(2)
4.4(9)
0.5(1)
1.9(4)
1.65(2)
2.07(2)
2.89(3)
3.30(3)
0.0022
0.004
0.006
0.009
The EXAFS results are consistent with the presence of octa-
hedral complexes with [TcvO]3+ cores coordinated to sulfates
in bidentate and monodentate mode. The average TcvO and
Tc–O distances found by EXAFS are similar to the one found
in octahedral Tc(+5) complexes with the [TcvO]O5 core (i.e.,
TcvO = 1.640(6) Å and Tc–O = 2.031(6) Å for [Ph4As]2[TcO-
(ox)2(Hox)]·3H2O).20 The only technetium complex coordinated
to sulfates that has been characterized by X-ray diffraction is
K2[Tc2(SO4)4·2H2O].21 In this compound, the [Tc2]6+ core is
coordinated to bridging sulfates (Tc–O(S) = 2.02(2) Å). The
Tc–S distances found by EXAFS (i.e., Tc–Sbid = 2.89(3) Å and
Tc–Smono = 3.30(3) Å) are larger than the Mo–S distance in
K4[MoO2(SO4)3] (vide supra).
Fig. 2 Fitted k3 EXAFS spectra and Fourier transform of the k3 EXAFS spectrum
of the solution obtained after the reaction of HTcO4 with MeOH in 13 M H2SO4.
Experimental data in red and fit in black.
for complexes with several TcvO bonds (i.e., ∼1.4 Å)7 and
more similar to the one found in complexes with several Tc–O
single bonds (i.e., 1.6 Å).17 The position of the second peak is
higher than the one found for polynuclear complexes with
[Tc(μ-O)2Tc] cores (i.e., 2.4 Å); in these polynuclear species, the
peak at 2.4 Å is due to Tc ⇄ Tc scattering.17 These obser-
vations preclude the presence of complexes with [TcO3]+ and
[Tc(μ-O)2Tc] cores in the green solution. The experimental FT
is similar to the simulated one reported for the putative
EXAFS analysis indicates that the preferential coordination
mode for sulfate to the [TcvO]3+ core is monodentate. It is noted
that in 13 M H2SO4, sulfates are present as the [HSO4]− anion.22
Based on the EXAFS results, the following stoichiometries
are proposed for the complexes: (A) [TcO(HSO4)2(H2O)2(OH)]
[TcO2(SO4)3]3− species.7 In the simulated FT of [TcO2(SO4)3]3−
,
the first peak, centered at R + Δ ∼ 1.5 Å, is due to Tc–O and
TcvO scatterings while the second peak (R + Δ ∼ 2.7 Å) to
Tc–S scatterings.
The EXAFS spectrum was fit using the scatterings calcu-
lated in [TcO2(SO4)3]3−. The species was constructed using the
crystallographic structure of K4[MoO2(SO4)3] by replacing the
Mo by Tc atoms.18 In K4[MoO2(SO4)3], the sulfate ligands are
and/or (B) [TcO(HSO4)3OH]− It cannot be excluded that the
.
solution obtained from the reaction of HTcO4 with MeOH in
13 M H2SO4 is a mixture of complexes (A) and (B).
These complexes are the first technetium oxo-sulfate
species characterized to date. In order to get more information
on their structure, first principles calculations were performed.
Theoretical calculations confirm the stability of the proposed
complexes; the optimized structures of these complexes are
presented in Fig. 3. The distances found by DFT (Table 3) are
coordinated to the metal in monodentate (Mo–Smono
3.228 Å) and bidentate (Mo–Sbid = 2.854 Å) mode.
=
Various adjustments (see ESI,† VI. XAFS spectroscopy) were
performed considering: (1) Tc–O, (2) TcvO, Tc–O, (3) TcvO,
Tc–O, Tc–Smono and (4) TcvO, Tc–O, Tc–Smono and Tc–Sbid
scatterings. The best adjustment, defined as a lower value of
the reduced-chi2, was obtained with the TcvO, Tc–O, Tc–
Smono and Tc–Sbid scatterings. Under our conditions, the
spatial resolution (π/2Δk ∼ 0.17 Å) can distinguish the TcvO
from the Tc–O contributions, and the Tc–Sbid from the Tc–
Smono contributions. Unfortunately, it will not allow us to dis-
criminate the Tc–O(H2) (i.e., ∼2.23(3) Å) from the Tc–O(S) (i.e.,
∼2.16 Å) contributions.
For the adjustment, the Debye–Waller factors (DWF) of the
TcvO and Tc–O scatterings were fixed to the one determined
in HTcO4 (i.e., σ2
= 0.0022 Å2, σ2
= 0.004 Å2).7 For the
Tc–O
TcvO
Tc–S scatterings, the values were fixed to those determined in
uranium(+6) sulfate complexes (i.e., σ2 = 0.006 Å2 and
U-Sbid
σ2
= 0.009 Å2).19 The ΔE0 was constrained to be the
U-Smono
same value for each wave; all the other parameters were
allowed to vary. The results of the adjustment (Fig. 2, Table 2)
indicate the environment of the absorbing atom to be consti-
tuted by 1.0(2) O atoms at 1.65(2) Å, 4.4(9) O atoms at 2.07(2) Å,
Fig. 3 Proposed structure of the complexes obtained from the reaction of
HTcO4 with MeOH in 13 M H2SO4: (A) [TcO(HSO4)2(H2O)2(OH)] and (B) [TcO-
(HSO4)3OH]–. Color of atoms: Tc in grey, O in red, S in yellow, H in light blue.
0.5(1) Sbid atoms at 2.89(3)
at 3.30(3) Å.
Å and 1.9(4) Smono atoms
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laboratory operated by Sandia Corporation, a wholly owned
subsidiary of Lockheed Martin Company, for the United States
Department of Energy’s National Nuclear Security Adminis-
tration under Contract DE-AC04-94AL85000. The work con-
ducted at Hunter College was supported by NSF-CHE-0959617
(for purchase of the 400 MHz NMR spectrometer),
NSF-CHE-0750118, and DE-FG02-09ER16097 (SISGR, Heavy
Element Chemistry, Office of Science, Department of Energy).
The authors thank Trevor Low, and Julie Bertoia for outstand-
Table 3 Average inter-atomic distances (Å) found by first principles calcu-
lations for the complexes (A) [TcO(HSO4)2(H2O)2(OH)] and (B) [TcO(HSO4)3OH]−.
Distances found by EXAFS are in bold
Complexes
TcvO
Tc–O
Tc–Smono
Tc–Sbid
(A)
(B)
1.69
1.69
1.65(2)
2.12
2.08
2.07(2)
3.27
3.35
3.30(3)
—
2.92
2.89(3)
in good agreement with the experimental results. The TcvO ing health physics support and Dr Sungsik Lee at the APS for
distances (i.e., 1.69 Å) are identical in the two complexes, support during the EXAFS experiment.
which indicates that the number and coordination mode of the
sulfate ligands do not affect the [TcvO]3+ core. For complex A,
the two monodentate sulfates are in trans-configuration, while
they are in cis-configuration in complex B. An increase of the
Tc–Smono distance is observed when moving from complex A to
Notes and references
B. This phenomenon is likely due to steric effects induced by
the presence of the bidentate sulfate ligands.
1 A. Fatiadi, Synthesis, 1987, 85.
2 H. S. Lacheen, P. J. Cordeiro and E. Iglesia, J. Am. Chem.
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In summary, the behavior of HTcO4 with methanol has
been investigated in sulfuric acid. In 13 M H2SO4, HTcO4 is
reduced to Tc(+5) and MeOH is oxidized to formic acid. The
solution obtained from the reduction of HTcO4 with MeOH in
13 M H2SO4 was studied by EXAFS spectroscopy, the results
are consistent with the presence of octahedral complexes
with [TcvO]3+ cores coordinated to sulfates in bidentate and
monodentate mode. Complexes with stoichiometry [TcO-
(HSO4)2(H2O)2(OH)] and/or [TcO(HSO4)3OH]− are proposed.
First principles calculations show these complexes to be
stable, the distances found by DFT calculations are in good
agreement with the one found by EXAFS spectroscopy. Pertech-
netic acid appears to be a very reactive species. It is anticipated
that it will react with other organic (e.g., alkene) and inorganic
species (e.g., H2O2). The isotope 99Tc is a major fission product
of the nuclear industry. Because spent fuel reprocessing uses
nitric acid that potentially contains H2O2 (from radiolysis), the
study of the speciation of Tc(+7) in the presence of H2O2 is rel-
evant to the chemistry of this element in the nuclear fuel
cycle. In this context, we note that Tc peroxo-complexes have
3 J. A. Shropshire, J. Electrochem. Soc., 1967, 114, 773.
4 R. Alberto, Technetium, in Comprehensive Coordination
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8 Three months after the addition of MeOH, the solution of
[TcO4]− in 6 M H2SO4 exhibits a brown color. The UV-
visible spectrum of the brown solution [Fig. S7_UV†] exhi-
bits the bands at 244 nm and 288 nm characteristic of
[TcO4]− (∼85%) and the band at 490 nm characteristic of
polymeric Tc(+4) complexes (∼15%).9.
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the U.S. Department of Energy, Office of Nuclear Energy,
through INL/BEA, LLC, 89445. Further support was provided
by NSF-IGERT contract 40A70-A/01010670 through Hunter
College. Use of the Advanced Photon Source at Argonne was
supported by the U.S. Department of Energy, Office of Science,
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