The structure of Fe(iii) ions in strongly alkaline aqueous solutions from EXAFS and Moessbauer spectroscopy

Article abstract of DOI:10.1039/b806937a

To establish the structure of ferric ions in strongly alkaline (pH > 13) environments, aqueous NaOH solutions supersaturated with respect to Fe(iii) and the solid ferric-hydroxo complex salts precipitating from them have been characterized with a variety of experimental techniques. From UV measurements, in solutions of pH > 13, only one kind of Fe(iii)-hydroxo complex species was found to be present. The micro crystals obtained from such solutions were proven to be a new, so far unidentified solid phase. Moessbauer spectra of the quick-frozen solution and that of the complex salt indicated a highly symmetrical ferric environment in both systems From the EXAFS and XANES spectra, the environment of the ferric ion in these solutions (both native and quick-frozen) and in the complex salt was found to be different. In the complex salt, the bond lengths are consistent with an octahedral coordination around the ferric centres. In solution, the coordination geometry of Fe(iii) is most probably tetrahedral. Our results demonstrate that in strongly alkaline aqueous solutions, ferric ions behave very similarly to other structurally related tervalent ions, like Al(iii) or Ga(iii).

Full text of DOI:10.1039/b806937a

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The structure of Fe(III) ions in strongly alkaline aqueous solutions from
¨
EXAFS and Mossbauer spectroscopy†
Pa´l Sipos,*a Dalma Zeller,a Erno˝ Kuzmann,b Attila Ve´rtes,b Zolta´n Homonnay,b Monika Walczakc and
Sophie E. Cantonc
Received 24th April 2008, Accepted 30th June 2008
First published as an Advance Article on the web 27th August 2008
DOI: 10.1039/b806937a
To establish the structure of ferric ions in strongly alkaline (pH > 13) environments, aqueous NaOH
solutions supersaturated with respect to Fe(III) and the solid ferric-hydroxo complex salts precipitating
from them have been characterized with a variety of experimental techniques. From UV measurements,
in solutions of pH > 13, only one kind of Fe(III)-hydroxo complex species was found to be present. The
micro crystals obtained from such solutions were proven to be a new, so far unidentified solid phase.
Mo¨ssbauer spectra of the quick-frozen solution and that of the complex salt indicated a highly
symmetrical ferric environment in both systems From the EXAFS and XANES spectra, the
environment of the ferric ion in these solutions (both native and quick-frozen) and in the complex salt
was found to be different. In the complex salt, the bond lengths are consistent with an octahedral
coordination around the ferric centres. In solution, the coordination geometry of Fe(III) is most
probably tetrahedral. Our results demonstrate that in strongly alkaline aqueous solutions, ferric ions
behave very similarly to other structurally related tervalent ions, like Al(III) or Ga(III).
been supported by a large number of observations.10–14 For
Introduction
example, from the UV-spectroscopic measurements, besides an
The study of structural aspects of tervalent hard metal ions
unspecified polynuclear ferric-hydroxo complex, the formation
of [Fe(OH)4]- was deduced.10,11 Solubility measurements also
strongly support that [Fe(OH)4]- is the last member of the stepwise
Fe(III)-hydroxo complexes.12–14 However, Cotton and Wilkinson15
mention the possible formation of [Fe(OH)6]3- in strong base
solutions. In a recent paper, from a large number of accurate UV-
vis spectroscopic measurements, Perera and Hefter16 reported the
consecutive formation of [Fe(OH)5]2- and [Fe(OH)6]3- as well as
[Fe(OH)4]- with the exclusive presence of [Fe(OH)6]3- at pH ≥
13. As far as solid ferric hydroxo complexes are concerned,
some naturally occurring minerals17–19 are proven to consist of
[Fe(OH)6]3- structural building blocks and various salts containing
[Fe(OH)6]3-, [Fe(OH)7]4- and [Fe(OH)8]5- synthesized under labo-
ratory conditions and have been described both in the literature20–22
and in university textbooks.23 Moreover, to the best of our
knowledge, solid state complex salts consisting of [Fe(OH)4]-
units have not been prepared and characterized (see below). It
is to be noted here, that the relationship between the chemical
species observed in a given solution, and the structure of the
solid complex salt crystallizing from it, is not straightforward.
For example, a number of solid complexes either with [M(OH)4]-
or with [M(OH)6]3- (M = Al(III) and Ga(III)) building blocks are
knownfrom the literature,24–26 but in the solutions from which these
complexes are obtained, neither [Al(OH)6]3- nor [Ga(OH)6]3- are
present in experimentally detectable quantities.5,8
hydrolysis, such as Fe(III), Al(III) or Ga(III), is a classical theme
in inorganic coordination chemistry1–4 and the topic has consid-
erable commercial importance. For example, technologies used
by the hydrometallurgical industry (where hydrolytic processes
are invariably involved and in some cases dominate the process
chemistry) require such knowledge on a daily basis. The funda-
mental question of whether these tervalent metal ions are four-
or six-fold coordinated in the hydroxo complex(es) formed in
strongly alkaline aqueous solutions, is still intensely investigated
today. For example, from Raman,4,5 solution X-ray diffraction6
and EXAFS7 measurements it has been established that Al(III)
and Ga(III) are predominantly four-fold coordinated even at the
highest concentrations of base. 27Al and 71Ga NMR measurements
support these findings as the observed variations in the NMR
patterns were fully accounted for in terms of the formation of
contact ion-pairs,5,8 without the need to invoke even the existence
of a six-fold coordinated species.
The identity and structure of the water soluble Fe(III)-hydroxo
complexes forming in strongly alkaline solutions is more unclear
and controversial. The well known book of Baes and Mesmer9
states that the only water soluble mononuclear ferric hydroxo
complex at pH ≤ 13 is the tetrahedral [Fe(OH)4]-. This has
aUniversity of Szeged, Department of Inorganic and Analytical Chemistry,
PO Box 440, H-6701, Szeged, Hungary. E-mail: sipos@chem.u-szeged.hu
Kamnev et al. made a remarkable effort27–29 to elucidate the
Mo¨ssbauer spectrum of the Fe(III)-hydroxo complex species form-
ing in strongly alkaline aqueous solutions. In concentrated (15–
17 M) NaOH solution, ferric ions were found to form a reddish-
orange (supposedly colloidal and/or polynuclear) compound.
When the solvent was evaporated, the colour of the complex
bMTA-ELTE Research Group on the Application of Nuclear Techniques
in Structural Chemistry, Roland Eo¨tvo¨s University Pa´zma´ny Pe´ter Se´ta´ny
1/A, H-1117, Budapest, Hungary
cChemical Physics Department, Chemical Centre, Lund University, PO Box
124, SE-22100, Lund, Sweden
† Electronic supplementary information (ESI) available: PXRD and
derivative XANES spectra. See DOI: 10.1039/b806937a
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disappeared. The singlet in the Mo¨ssbauer spectrum of the solid
obtained (isomer shift is 0.68 0.02 mm s-1 and 0.72 mm s-1
half width) indicated the presence of a ‘symmetrically coordinated
mononuclear hydroxo complex, statistically distributed in the
NaOH·H2O matrix’.27 Upon heating, the symmetry of the complex
was retained and the observed increase in the line width was
attributed to the non-uniformity of the complexes’ environment
due to non-uniform drying.28 The colloidal and/or polynuclear
complex (during a 3.5 years ageing period) was found to transform
to a reddish-brown solid complex with Mo¨ssbauer spectral param-
eters very similar to those of the solid obtained upon evaporation.29
From this, Kamnev et al. concluded that (in accordance with
their UV-vis observations10,11 obtained for solutions) the species
responsible for the unique Mo¨ssbauer spectrum is most likely
to be built up from [Fe(OH)4]- units in the solid state. This
explanation is, however, not unequivocal for several counts. Firstly,
the presence of [Fe(OH)4]- both in solution and in the solid
state has not been supported by direct structural information
(i.e., by using techniques providing data for the coordination
number and bond lengths). Secondly, [Fe(OH)6]3- is also of high
symmetry, and from a species like this, very low quadrupole
splitting would be expected, similar to that of [Fe(OH)4]-, therefore
for the Mo¨ssbauer spectroscopy the two micro-environments
are indistinguishable. Thirdly, proving the direct correspondence,
between the complex structures existing in solution and in the solid
state, would require further experimental observations.
which, as a practical shorthand, FeOOH will be used hereafter)
was obtained from a diluted FeCl3 solution (100 mL, 1 mM)
precipitated with an excess of NaOH (~0.1 M, 5 mL added
dropwise during vigorous stirring). The precipitate was collected
on a filter paper and washed with distilled water, until the
conductivity of the filtrate decreased to a value similar to that of
the distilled water. The precipitate was used for further syntheses
within one hour. All the other chemicals were of analytical grade.
Methods
UV-vis spectra. The UV-vis spectra of Fe(III)-containing al-
kaline solutions were recorded in the 200 nm < l < 800 nm
wavelength range on a Hewlett-Packard diode array spectropho-
tometer, using a 1 mm and a 10 mm quartz cuvette. Blank solutions
were prepared for each individual sample and were subtracted
from the spectra in question. The cuvette was stored in KMnO4–
H2SO4 solution to remove impurities, and was cleaned with a
HNO3–H2O2 mixture and distilled water prior to the experiments.
Caution! Both mixtures are of an explosive nature, and special
attention has to be taken to observe the work safety regulations
during their use.
Elemental analyses. The Na+ and Fe3+ content of the solid
ferric-hydroxo complex salts were determined by flame photome-
try and by permanganometric titration, using the Zimmermann–
Reinhardt method, respectively. For the flame photometric mea-
surements, a PFP7-type instrument was used. A calibration series
in the range of 0–10 ppm (linear instrument response range)
was prepared by accurate dilution of a NaCl primary standard
solution. The flame photometer was calibrated before and after
the measurement with three independently prepared parallels for
each sample. The corrections for the matrix effect caused by the
constituents of the samples other than sodium were considered to
be unnecessary.
The aim of the current paper is to elucidate the structure of ferric
ions in aqueous solutions of pH > 13. For this, we will describe
the preparation of strongly alkaline aqueous solutions with
relatively high concentrations of dissolved Fe(III), characterize
the local structure of Fe(III) in these solutions (native and quick-
frozen state) via Mo¨ssbauer and EXAFS/XANES spectroscopies,
describe the isolation and chemistry of the solid ferric-hydroxo
complex salts precipitating from such solutions and compare the
structure of Fe(III) in the solutions and also the solid state.
pH-metric titrations. The presence of a carbonate impurity
in the solid ferric-hydroxo complex salts was checked by pH-
metric titration. The instrumentation has been described in detail
elsewhere.31 During the measurement, an accurately weighed
portion (50–100 mg) of the sample was suspended in a I = 0.1 M
NaClO4 supporting electrolyte and was titrated with standardized
~0.1 M HClO4 solution in an inert atmosphere (Ar).
Experimental
Materials
Concentrated NaOH (~20 M) and KOH (~12 M) stock solutions
were prepared from Millipore MilliQ water and a.r. (analytical
reagent) grade NaOH (Hungaropharma, 99% purity) and KOH
(Reanal, a.r. grade), respectively. Their carbonate content was
minimized as described previously.30 The density of the solution
was determined picnometrically. The exact concentration of the
stock solution was determined by acid–base titration, following an
accurate gravimetric dilution of the base solution. For the titration,
HCl solution (‘Convol’ volumetric standard, accuracy 0.1%) and
phenolphthalein indicator was used. The NaOH solution was
stored in an airtight, caustic resistant Pyrex bottle.
Ferric stock solutions ([FeCl3]T ~0.5 M, [HCl]T ~2.5 M, where
the subscript T denotes analytical or total concentration) were
prepared from FeCl3·6H2O (Reanal, a.r. grade) and concentrated
HCl (ca. 37 wt%, Merck, a.r. grade). The exact concentration
of the solution was determined by EDTA titration. Fe(ClO4)3
solutions were made of solid Fe(ClO4)3·12H2O (prepared and
kindly donated by Dr A. Szorcsik) and HClO4cc (ca. 30 wt%,
Merck, a.r. grade). Freshly prepared ferric-oxy-hydroxide (for
Thermogravimetric analyses. (TG and DTG) measurements
were performed using a MOM Derivatograph Q-1500D instru-
ment, using a 2.5 and a 10 ◦C min-1 heating rate.
IR and Raman spectra. Mid and far IR measurements on the
solid complexes and reference materials (Na2CO3 and NaOH)
were performed using a FTS 65/A896 BioRad Digilab Division
spectrometer equipped with a Mtech 200 type photoacoustic
detector. The Raman spectra were measured with a BioRad
Dedicated Raman spectrometer using a Spectrophysics T10-106C
Nd:YVO4 laser operating at 1064 nm and a germanium detector
cooled with liquid nitrogen. For both the Raman and the IR
spectra, 1024 scans at a 4 cm-1 resolution were used.
Powder X-ray diffraction. XRD patterns of the solid samples
and some of the reference compounds (Na2CO3 and NaOH)
were recorded in the 2q = 2–50◦ range using a Philips PW1710
instrument, employing Cu Ka radiation.
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Mo¨ssbauer spectroscopy. 57Fe Mo¨ssbauer spectra of the salts
and the frozen solutions were recorded with conventional
Mo¨ssbauer spectrometers (Wissel and Ranger) in a transmission
geometry at 300 and 80 K in a bath-type cryostat. The solution
samples were quenched in the form of drops in a copper container
situated on an aluminium slab almost completely immersed in
liquid nitrogen. This freezing technique is expected to provide
a cooling rate, which is safely over the critical cooling rate
necessary to preserve the original structure of the solution in the
liquid state. However, for the samples, where precipitation had
already commenced before quick-freezing (e.g., for the heavily
supersaturated solutions studied in this work), only the fraction,
which is truly in solution can be considered as the one with the
original structure. Isomer shifts are given relative to a-iron. A
57Co/Rh g-radiation source of 3 ¥ 109 Bq activity was used. The
Mo¨ssbauer spectra were analyzed by least-squares fitting of the
Lorentzian lines with the help of the MOSSWINN program.32
to the spectrum described in ref. 16, recorded for ferric containing
solutions at pH > 13. In this work16 this shoulder was suggested
to correspond to the [Fe(OH)6]3- species.
On the basis of our measurements, the intensity of this shoulder,
as expected, increases with increasing [NaOH]T, corresponding
to ca. ten-fold increase in [Fe(III)]T within the series. After
background subtraction and normalization, the spectra obtained
are perfectly superimposable (Fig. 1). From the [NaOH]T and
[Fe(III)]T independence of the spectra, in solutions of [NaOH]T
≥ 1 M there is no speciation change with the increasing caustic
concentrations. Furthermore, there is only one ferric-hydroxo
complex species present. As the formation of the polynuclear
hydroxo complex species can also be excluded, this sole species
9–14
is either [Fe(OH)4]-
or [Fe(OH)6]3-.15,16
XANES and EXAFS spectroscopy. The Iron K edge (7112 eV)
XANES and EXAFS spectra were measured on beamline I811 at
the MAXlab facility, Lund, Sweden.33 This is a superconducting
multipole wiggler beamline equipped with a water cooled channel
cut Si(111) monochromator delivering at 10 keV, approximately
2 ¥ 1015 photons s-1 (0.1% bandwidth) with horizontal and vertical
FWHM of 7 and 0.3 mrad. A beamsize of 0.5 mm ¥ 1.0 mm
(width ¥ height) was used. The incident beam intensity I0 was mea-
sured with an ionization chamber filled with a mixture of He–N2.
The data were measured in transmission mode for the crystalline
samples and in fluorescence mode for the solution and quick-
frozen samples, respectively. The cell-to-detector distance was
varied with concentration in order to avoid detector saturation.
Fig. 1 Background corrected and normalized absorbances as a function
of the wavelength for aqueous NaOH solutions (8 solutions in the range
of 1 M ≤ [NaOH]T ≤ 20 M) saturated with Fe(III).
Results and discussion
Preparation and analysis of the solid ferric-hydroxo complex salt
UV spectra of caustic ferric solutions
After several weeks of rest, an off-white solid precipitated from the
ferric-containing [NaOH]T = 20 M solution used for recording the
UV spectra. When the solid was separated from the mother liquor,
it turned brown immediately due to exposure to air moisture,
suggesting, that it contained some Fe(III). The experiment was
repeated several times, and the experimental parameters were
systematically changed, in order to achieve a synthesis method
for producing a sufficiently large amount of this solid substance
with a well-defined composition. The largest yield was achieved,
when 20 mL FeCl3 solution (0.1 M) was added dropwise (i.e.,
0.2 mL portions) to 200 mL of hot (80–100 ◦C), vigorously
stirred NaOH solution (~20 M). The vast majority of the initially
formed reddish-brown precipitate instantaneously dissolved and
an apparent ferric concentration of up to 10 mM (several orders of
magnitude larger, than the equilibrium solubility of Fe(III) under
these conditions) was attained. The filtration of the hot solution
resulted in a colourless filtrate. The solution thus obtained was
surprisingly stable. An off-white-to-pale green solid appeared in
the resting solution usually after a couple of days. Upon shaking,
stirring or when in contact with a rough surface was found
to significantly speed up the formation of these crystals. The
precipitate/crystals thus formed were very quickly filtered until
air-dry and rapidly transferred to a vacuum desiccator (drying
material: P2O5). During the systematic optimization experiments,
the following observations were made:
The solubility of ferric-oxides or oxy-hydroxides in alkaline
solutions under ambient conditions is estimated to be in the
range of 10-5–10-6 M.14 When FeCl3 solution is added to a
[NaOH]T ≥ 1 M solution, various forms of Fe-oxy-hydroxides
(e.g., amorphous ferric-hydroxide, ferric-hydrite or more often
goethite34) are precipitated and only a small fraction of the ferric
ions remain dissolved in the solution. To a NaOH solution series
(1 M ≤ [NaOH]T ≤ 20 M) an FeCl3 solution was added until
precipitate formation (i.e., [FeCl3]T ≥ 1 mM) and the solutions
were vigorously stirred for a couple of weeks. The precipitates
were separated from the solutions via filtration (using a unit
specifically designed to filtrate strong caustic solutions, consisting
of a polysulfonate made Nalgene vacuum filter equipped with
a Versapor membrane filter with a pore size of 0.45 mm). The
filtrates were found to be invariably colourless, indicating that
the coloured complexes observed in similar solutions by Kamnev
et al.10,11 may have been due to colloidal and not to dissolved iron-
containing species. The UV spectra of the filtrates were recorded
and no significant baseline shifts were observed, suggesting that
practically all the colloidal precipitate was removed via filtration.
A shoulder in the range of l = 250–300 nm was observed for
each solution (the region at l < 250 nm is masked by the intense
absorption of NaOH35). This feature is generally thought to be
associated with ferric ions in caustic solutions35 and is very similar
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1. Adding an amount of NaCl to the solution, equivalent to
the saline formed during the reaction with the FeCl3 added,
no precipitation was observed. Therefore, the precipitate is not
expected to contain significant amounts of NaCl.
2. When the reaction was repeated with a Fe(ClO4)3 solution
or freshly precipitated, amorphous FeOOH instead of FeCl3,
the formation of the compound was again observed as before,
indicating that the phenomenon is largely independent of the anion
accompanying the ferric ion.
3. When the reaction was performed by using a saturated
(~12 M) KOH solution or solutions with [NaOH]T < 18 M,
practically all the ferric ions precipitated in the form of a reddish-
brown gel, and after filtration no precipitation from the colourless
mother liquor could be separated.
For the solid compound obtained from five parallel optimized
syntheses, variable Na (26–34 wt%) and Fe(III) contents (7.8–
10.8 wt%) were found, corresponding to Fe : Na molar ratios
of 1 : 7.6–9.6. We observed that the visibly more crystalline
pale green specimens had a larger iron concentration, than
those with less well-defined (i.e., off-white) colour. From this, we
concluded, that the slightly variable colour of the solid products
is most likely to be associated with the inevitable residuals of the
mother liquor, (which is an almost saturated aqueous solution
of NaOH with some NaCl). If not otherwise stated, the pale
green microcrystalline sample most likely to contain the least
mother liquor residuals was used for further studies. The elemental
analysis data for these specimens obtained from two independent
syntheses: Fe 10.67 0.07 wt% and Na 34.07 0.08. Fe : Na molar
ratio: 1 : 7.66.
FeOOH or Fe2O3 and NaOH. A constant mass is attained from
300 ◦C, and from 800 ◦C the gradual dehydration of NaOH starts.
XRD patterns of the samples prepared indicated an appreciable
crystallinity with well-defined XRD patterns (see Figure S1 in
the ESI†). The XRD patterns of the pale green and off-white
samples were found to be perfectly superimposable. This means,
that the crystalline phase in the various samples is identical and
the variation in the colour is associated with the varying amount
of residual NaOH, which is present in the samples in an X-ray
amorphous form.
From the XRD, a significant amount of crystalline Na2CO3 is
also unlikely to be present, consistent with the results of the pH-
metric titrations. The XRD patterns obtained did not match any
of those registered in the JCPD database, this is another indication
that our sample is not a physical mixture of the constituents and
it might be a solid phase not so far described.
From the data described in this section, the powder obtained is
a solid Na salt of an anionic Fe-hydroxo complex, embedded in
an amorphous NaOH matrix. It can be described with the general
formula of Nax[Fe(OH)3+x]·yNaOH·zH2O.
A comparison of the Raman and the far-IR spectra of the same
specimen (in particular, the comparison of the peaks in the M–O
stretching range, i.e., between 300–550 cm-1), in principle, enables
us to distinguish between tetrahedral and octahedral environments
around the metal ion.36 For the ferric-hydroxo complex salt, a
relatively well-defined Raman spectrum with a well-defined peak
at ~490 cm-1 and a broad feature at ~340 cm-1 has been obtained.
However, the IR spectrum recorded turned out to be featureless,
with no peaks that are clearly distinguishable from the baseline
noise. This prohibited the determination of the coordination
geometry of Fe(III) from IR and Raman measurements.
The carbonate content of the samples from pH-metric titrations
were found to be ≤ 2 mol% of the NaOH. This is consistent with
some carbonate impurities formed during the short air exposures
of the samples during the manipulations. Therefore, carbonate is
unlikely to be an integral component of the sample.
Mo¨ssbauer spectra of the solid ferric-hydroxo complex salt and the
quick-frozen solution from which it was obtained
From the TG/DTG measurements (one representative exper-
imental curve is shown in Fig. 2) the compound loses water in
two consecutive, slightly overlapping steps. The first (between
100–200 ◦C) is likely to correspond to crystalline water (as the
contribution of adsorbed water is unlikely) and constitutes ca.
22 wt% of the total sample mass. The second weight loss (between
200–300 ◦C) is also likely to be water related, and most probably
corresponds to the decomposition of the Nax[Fe(OH)3+x] units to
The Mo¨ssbauer spectra of the solid ferric-hydroxo complex salt
were recorded at room temperature (~20 ◦C) and at liquid N2
temperature (-193 ◦C) (Fig. 3 and Table 1). To compare the
solution and solid structure of Fe(III) in an alkaline environment,
the Mo¨ssbauer spectra of quick-frozen solutions from which the
solid ferric-hydroxo complex salts can be obtained were also
recorded. During the solution preparation, 3.6 mg of 57Fe2O3
was first completely dissolved in 1 mL of hot HClcc (~1 M).
The solution was evaporated to obtain ca. 0.1 mL volume and
it was added dropwise to 5 mL of hot NaOH solution (~20 M).
When the dissolution was complete, the heating was turned off.
The solution was placed in a syringe and quickly transferred
to a copper container, situated in liquid N2. This treatment
provided an approximate freezing rate of ≥8 K s-1 and ensures
that the solutions’ structure is retained in the frozen state. Two
preparations were made. During the first preparation (resulting
in quick-frozen solution 1 in Table 1 and Fig. 3) the solution
was immediately quick-frozen after the complete dissolution of
Fe(III) ions. During the second preparation (yielding quick-frozen
solution 2 in Table 1 and Fig. 3), sufficient time was allowed to
start the precipitation of the ferric-hydroxo complex salt, and a
seemingly non-homogeneous solution was quick-frozen.
On the basis of the Mo¨ssbauer spectra, the solid product
formed has a highly symmetrical geometric structure, which could
Fig. 2 TG/DTG curves of the solid ferric-hydroxo complex salt, obtained
at a heating rate of 10 ◦C min-1.
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Table 1 Mo¨ssbauer spectral parameters (IS: isomer shift, QS: quadrupole splitting) for various samples containing ferric species at various temperatures
Sample
T/◦C
ISa/mm s-1
QSb/mm s-1
Line width/mm s-1
Ferric-hydroxo complex salt
20
-193
-193
-193
-193
-193
-193
0.310
0.409
0.462
0.458
0.68e
0.62
0.289
0.310
0.444
0.367
—
—
—
0.73
0.80
0.65
0.71
0.72
0.79
0.59
Quick-frozen solution 1c
Quick-frozen solution 2d
Fe(III)-complex in a NaOH matrix27
Heat-treated [FeOH)4]- in a NaOH matrix28
Reddish-orange [FeOH)4]- from solution29
0.62
a Estimated standard deviation 0.010 mm s-1. b Estimated standard deviation 0.020 mm s-1. c Quick-freezing performed immediately after complete
dissolution of Fe(III) in ~20 M caustic; the data relate to the broad central singlet; the description of the sextet can be found in the text. d Quick-freezing
performed when the solid ferric-hydroxo complex salt had already started to precipitate from a solution identical to that used for preparing quick-frozen
solution 1. e Identical IS value is reported for room temperature measurements in ref. 27.
micro-conditions and the heavily supersaturated solution was
subjected to a range of transfer procedures, which facilitated the
onset of crystal formation). The Mo¨ssbauer parameters of the
solid sample are similar to those published by Chappert et al.37
In this publication a 3d5.704s0.169 electronic structure was calcu-
-9
lated for iron(III) in the FeO6 environment. However, Kamnev
et al. claimed,10,11,27–29 that such Mo¨ssbauer traces correspond to
tetrahedral [Fe(OH)4]- complexes. Thus, the central paramagnetic
part of the Mo¨ssbauer spectrum represents the small (super-
paramagnetic) particles of [Fe(OH)4]- or [Fe(OH)6]3- with particle
sizes less than ~6–8 nm.38,39
It is interesting to note that the Mo¨ssbauer spectrum of quick-
frozen solution 1 contained a magnetic sextet (with 53.34 T
effective magnetic field) as well. The appearance of this component
can be explained by the slow paramagnetic spin relaxation due
to the low concentration, which results in a weak spin–spin
interaction (a slow spin flip–flop) and to the low temperature,
which results in a weak spin–lattice interaction.
The Mo¨ssbauer spectra of samples containing paramagnetic
iron(III) can show magnetic splitting if the average time of their
paramagnetic spin relaxation (tPSR) is longer than the average
time of Larmor precession of the magnetic moment of the atomic
nucleus (tL). If the concentration of iron(III) species in the solution
and the actual temperature are low (< 0.05 M and < 100 K,
respectively) the spin–spin and spin–lattice interactions are weak
and the average times of spin–spin (tSSR) and spin–lattice (tSLR
)
relaxations will be long.38,39 Consequently, tPSR, which reads
1
tPSR
=
1
1
+
tSSR tSLR
Fig. 3 Mo¨ssbauer spectra of the ferric-hydroxo complex salt (upper
spectrum) and that of the quick-frozen solution from which solid fer-
ric-hydroxo complex salts are obtained (middle and lower spectra). Middle
spectrum: quick-freezing was performed immediately after complete
dissolution of 57Fe(III) in ~20 M caustic. Lower spectrum: quick-freezing
was performed when the solid ferric-hydroxo complex salt had already
started to precipitate from the same solution.
will be longer than tL. Under these conditions, the Mo¨ssbauer
spectra show magnetic structure.
On the basis of the Mo¨ssbauer spectra, we can suppose
that the freshly prepared solution contains both mononuclear
and super-paramagnetic particles and the aged solution has no
more mononuclear iron(III) species. (Note, that in the case of
mononuclear iron(III) containing units, the iron ions are far apart
from each other and this results in a weak spin–spin interaction,
a slow paramagnetic spin relaxation and consequently, magnetic
splitting. In the super-paramagnetic particles we have an opposite
situation; iron(III) ions will be close to each other, the spin–spin
be both octahedral and tetrahedral. (Note, that the Mo¨ssbauer
trace characteristic to the solid product was also observed in
quick-frozen solution 1, most probably due to the onset of its
crystallization prior to the commencement of rapid cooling. This
is not really surprising, as preparative work was performed under
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interaction will be stronger, the paramagnetic spin relaxation will
be faster and the magnetic structure disappears.)
of three features: a pre-edge peak at 7113–7113.7 eV (1s → 3d
transition), a broad maximum or shoulder in 7122–7123.3 eV
(1s → 4s transition) and an edge crest at 7128.8–7132.1 eV (1s →
4p transition).42–44
For complexes in a centrosymmetric (e.g., octahedral) envi-
ronment, the 1s→3d transition is an electric dipole forbidden
transition by parity considerations.45 For example, both for the
metallic iron and for the [Fe(CN)6]3-, no low energy absorption
was observed.46 The small intensity that is nevertheless reported
for certain compounds with an octahedral environment, can be
attributed to vibronically allowed transitions47 or to an electric
quadrupole coupling.45
For complexes in a non-centrosymmetric (e.g., tetrahedral)
environment, mixing of the 3d to the dipole allowed 4p in the
molecular orbital along the distortion provides oscillator strength
to the transition from a Fe 1s orbital. The role of ligand orbitals
is considerably smaller than that of the 1s and 4p because of the
exponential decay of the atomic orbitals on the two separated
atoms.
The total pre-edge intensities of the octahedral high-spin ferric
complexes range from 4–7.3, with an average intensity of 4.9 and
increases with decreasing coordination number for the model
complexes, reflecting the loss of symmetry inversion.41 It was
found to vary inversely with coordination number, 8 units for
6-coordinated, 16 units for 5-coordinated and 24 units for 4-
coordinated.47
In the samples studied, the areas of the pre-edge peaks of
the 1s → 3d transition were calculated from the position of the
inflection point, (which gives the photon energy necessary to excite
a K electron into the first empty level of the Fermi distribution).
The background function was determined by a least-squares fit
of an arctangent (initial absorption close to threshold) and a first
order polynomial below the inflection point. The edge jump was
obtained by fitting a first order polynomial. The difference between
these 2 lines at the inflection point was used as the normalization
factor for the pre-edge peak.
Based on the direct comparison of the line intensities, the coor-
dination number of the ferric ions is smaller in the solution and in
the quick-frozen solution than in the solid ferric-hydroxo complex
salt. These findings are consistent with a six-fold coordinated iron
in the solid and a four-fold coordinated iron in the solution/quick-
frozen solutions.
XANES and EXAFS spectra of the solid ferric-hydroxo complex
salt and the solution (both native and quick-frozen) from which it
was obtained
To compare the local structure of Fe(III) in the different states,
several XANES and EXAFS spectra of the crystalline solid
ferric-hydroxo complex salt and the solution were recorded. A
20 M NaOH solution heavily supersaturated with ferric ions
([Fe(III)]T ~10 mM), was prepared and investigated. Given that
such solutions are prone to decompose (yielding the solid ferric-
hydroxo complex salt), only the XANES spectrum could be
recorded. Since no precipitation occurred during the 15 min
long measurement, the structural information extracted from the
XANES spectrum is that of the ferric ions in solution. (Note,
that for the EXAFS measurements, a sufficiently larger volume
of sample was prepared, than for the Mo¨ssbauer measurements.
Therefore, the lack of precipitate in the former could readily
be checked. This was not that straightforward for the samples
prepared on microscale for the Mo¨ssbauer experiments.)
For the XANES/EXAFS measurements, 10 mL FeCl3 solution
(~0.1 M) was added dropwise (i.e., 0.2 mL portions) to a 100 mL,
hot (80–100 ◦C), vigorously stirred NaOH solution (~20 M). The
hot solution was filtrated and in the colourless filtrate the ferric
concentration was ca. 9 mM. A fraction of the supersaturated
solution was quickly transferred to a custom made Teflon sample
holder via a piston pipette, was quick-frozen in liquid N2 and the
EXAFS spectrum was recorded. Providing that the local structure
of ferric ions is retained during quick-freezing, the structure of
Fe(III) in solution could be deduced from the EXAFS analysis.
XANES spectra. The XANES spectra of the three samples
investigated are shown in Fig. 4. The qualitative characteristics
(i.e., positions and intensities) of the XANES region are sensitive
to the coordination geometry, oxidation state of the absorbing
atom and to the chemical identity of the surrounding ligands.40,41
The Fe K edge spectrum of the ferric ion is known to consist
The breadth of the XANES spectra is determined by the energy
resolution of the beamline. The monochromator band pass is
between 1.5–2 eV at the Fe K edge and together with a core hole
width of 1.25 eV this results in about 2.15 eV resolution. Nuclear
vibration amplitudes at room temperature are of the order of
46
˚
0.1 A, so line broadening of the order of a few tenths of an
eV will result, this being small compared to the 2 eV breadth
from other cumulative causes. The fine structure of the pre-edge
is nevertheless observable and further confirms the difference in
coordination for the solid and for the solutions (both native and
quick-frozen).
The combined use of ligand field theory and DFT calculations48
has demonstrated that the width reflects the ligand field splitting
of the 3d in different geometries.
Fig. 4 XANES spectra of the solid ferric-hydroxo complex salt (lower
spectrum), the strongly alkaline ([NaOH]T = 20 M) solution supersat-
urated with ferric ions (middle spectrum) and the same supersaturated
solution after quick-freezing (upper spectrum).
5
In the high-spin ferric d5 in octahedral geometry, the A1g
ground state has a (t2g)3(eg)2 configuration. Promotion of a 1s
electron into the 3d manifold produces 2 excited configurations
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(t2g)2(eg)2 and (t2g)3(eg)1 and coupling of the holes in these
configurations give the 5T2 and 5E states, respectively. As a result,
the pre-edge appears to split. The energy separation of the two
features can be related to ligand strength, and their approximate
electric quadrupole intensity ratio should be 3 : 2, with any
deviations reflecting differences in covalency. For the solid sample,
the 1s → 3d pre-edge peak has been fitted with two pseudo-Voight
functions, following the procedure described in the literature.48
The difference in peak energy (1.59 eV) and the ratio between
their intensities (0.56) are within the interval found for high-spin
octahedral ferric complexes described in Table 1 of ref. 48
For a tetrahedral geometry, there are two one-electron allowed
excited hole configurations (t2)2(e)2 and (t2)3(e)1 that produce a
triply degenerate 5T2 and a doubly degenerate 5E. The 4p orbital
transforms as t2 so that only mixing into the 3d t2 set contributes to
the intensity of the transition to the 5T2 state. Such an assignment
could hold for the solution as seen in Fig. 5, but the case of quick-
frozen solutions is clearly more complex.
coordination numbers of the Fe(III) in the solid sample and in
the quick-frozen solution. Models were rejected on the basis of
high residuals. The data used during the refinements included the
raw radial distribution function distances, empirically determined
phase shift corrections (+0.64 for Fe and +0.56 for O) and expected
bond lengths for possible metal–ligand pairs as a function of
oxidation state and coordination number.
For the central Fe, the Z + 1 core hole approximation was
chosen. A value of 1 was chosen for the amplitude reduction factor.
Curve fitting of the k3-weighted fine structure was done on the raw
data. The best fit was searched by minimization of the fit index
defined as the sum of the residuals squared divided by the number
of data points.
The structural parameters extracted from the EXAFS analysis
(in accordance with the conclusions from the XANES) strongly
support the presence of an octahedrally coordinated complex in
the solid sample. When the structural function was fitted with six
˚
Fe–O contributions, rFe–O = 2.036–2.040 A and a Debye–Waller
factor of 0.015 gave the best fit.
Although the general shape of the fitted line matches well
with the experimental data, the fitting factor was found to be
unsatisfactory. Therefore a model with two additional iron (rFe–Fe
=
˚
˚
3.51 A) and two sodium (rFe–Na = 3.27–3.29 A) atoms was tested.
This caused a significant improvement in the fit (Fig. 6a), but left
˚
the rFe–O distance (2.030 A) and the Debye–Waller factor (0.015)
unaltered. From this observation, it seems that the contribution
of additional heavy metals is significant in determining the shape
of the EXAFS structure function.
To fit the EXAFS spectrum of the frozen-liquid sample, guided
by the XANES results, a tetrahedral model was used. The general
shape was satisfactorily reproduced (Fig. 6b), but unfortunately
the unfavourable signal-to-noise level did not allow for obtaining
a very good fitting factor in this case. The rFe–O values were
˚
found to be in the range of 1.86–2.01 A, consistent with the first
neighbour interatomic distances for oxygen coordinated ferric ions
41
˚
in a tetrahedral environment (1.8–1.9 A ).
Values for the bond distances can be checked vs.the coordination
number using valence bond sum analysis,49 where the bond valence
is expressed as: sij = exp[(Rij - dij)/b], where dij is the distance
between atom i and j, Rij is the empirically determined distance
and b is generally set to 0.37. Values of Rij that give bond valence
sums near the oxidation state have been tabulated.50–52
Fig. 5 Pre-edge spectra of the solid ferric-hydroxo complex salt (lower
spectrum), the strongly alkaline ([NaOH]T = 20 M) solution supersat-
urated with ferric ions (upper spectrum) and the same supersaturated
solution after quick-freezing (middle spectrum).
When considering the other spectral regions, the edge and
edge crest are systematically shifted towards lower energy for
the solution samples as shown in Fig. 5. The amplitude of the
crest edge decreases and the slope within the peak changes.40 The
differences are clearly seen in the derivative XANES spectra for
the solid, solution and frozen solution (Figure S2 in the ESI†).
The breadth is the largest for the frozen solution sample due to a
shoulder at the energy expected for the 1s → 4 s transition. The
shift by about 2 eV40can be related to a decrease in the effective
ionic charge of the metal,40 since the binding energy of the 1s
electron decreases with a decreasing number of nearest neighbours.
All these findings corroborate with a lower coordination number
of iron in solution than in solid.
Conclusion
From strongly alkaline aqueous solutions, a solid ferric-hydroxo
complex salt has been isolated. Mo¨ssbauer spectroscopic data
proved that the environment of iron is highly symmetrical and from
XANES and EXAFS measurements it has been demonstrated,
that the coordination number of the ferric ion is six, with a primary
˚
Fe–O bond distance of 2.03 A. Thus, the elementary building block
of the solid complex is [Fe(OH)6]3-. The structure of the ferric ions
in solutions, from which the solid hydroxo complex salt has been
isolated, was found to be markedly different from that of the solid
compound. Both the EXAFS and XANES data suggest, that in
solution the highest coordination number stepwise ferric-hydoxo
complex formed is the tetrahedral [Fe(OH)4]-. To the best of our
knowledge, our work is the first direct structural observation,
which unequivocally proves that ferric-hydroxo complexes with
EXAFS spectra. Analysis by mathematical refinement of the
phases, amplitudes and frequencies of the EXAFS structure
function was performed by using the EXCURVE software pack-
age to obtain accurate interatomic distances and estimates of
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´
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patterns), to Mrs Katalin Barna (for the TG/DTG measurements)
to Mrs Katalin Szu˝cs (for assisting with the pH-potentiometric
measurements) and to Dr Katharina Nore´n and Dr Stefan Carlson
(for their assistance in recording the XANES/EXAFS spectra).
The research was supported by grants of the Hungarian Science
Foundation (OTKA K62691, K68135 and K67835) and by the
TARI Grant from MaxLab (Experiment No. I811-072).
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