Mössbauer spectroscopic study
of the structure of an iron(III) complex
with indole-3-acetic acid
in acidic aqueous solutions
K. Kovács1, A. A. Kamnev2, E. Kuzmann1, A. Vértes1
1Laboratory
of Nuclear Chemistry, Chemical Institute, Eötvös Loránd University,
Pázmány P. s. 1/a, Budapest 1117, Hungary
2Laboratory of Biochemistry, Institute of Biochemistry and Physiology of Plants
and Microorganisms, Russian Academy of Sciences, 410049, Saratov, Russia
E-mail: kkriszti@bolyai.elte.hu
http://www.chem.elte.hu/departments/magkem/hun/index.html
Indole-3-acetic acid (IAA) is one of the most powerful
natural
plant-growth-regulating
substances
(phytohormones of the auxin series) that are capable of
stimulating cell division and promoting cell elongation.
It is well documented to be synthesized also by many
soil microorganisms, in particular, in the rhizosphere
where it plays an essential role in plant–microbe
interactions. The excretion of auxins directly into the
soil, along with their phytoregulating effects, can lead to
chemical interactions involving metal ions. Among
these, iron(III) is an essential microelement ubiquitous
in various soils. Our earlier studies have shown the
possibility of redox processes involving ferric ions as
well as IAA in slightly acidic aqueous solutions.
Indole-3-acetic acid (IAA):
OH
O
N
H
This could be of ecological significance, since Fe3+ has a poor biological availability over a wide
pH range owing to its full hydrolysis and extremely low solubility of ferric (oxy)hydroxides, but
it can be reductively solubilised under appropriate conditions (in slightly acidic media) giving a
more bioavailable iron(II).
On the other hand, studying the mode of coordination of indolic compounds to iron(III) can
provide additional information helpful in understanding the enzymatic degradation of auxins.
The formation of a triple complex (peroxidase–IAA–oxygen) has been proposed for the oxidative
degradation mechanism of IAA including as a key step a simple one-electron transfer from the
IAA molecule to the ferric moiety of the peroxidase heme.
In earlier papers an attempt was made to consider the aqueous Fe3+–IAA system as an inorganic
model for the peroxidase–IAA complex. Ferric chloride and IAA-containing aqueous solutions
were studied in detail, but the structure of the complex formed in the reaction was not
characterised.
In our previous works, we studied several iron(III)–(indole-3-alkanoic acid) systems, including
iron(III)–(indole-3-carboxylic, -acetic, -propionic, -butyric acid) complexes. Here, we wish to
present our work concerning the FeIII-(indole-3-acetic acid) complex. The chemical composition
and coordination structure of the complex were investigated both in solution and in the solid state
using Mössbauer spectroscopy and some additional techniques such as elemental analysis, Fourier
transform infrared and Raman spectroscopies and solution X-ray diffraction. For details, see the
following papers with references therein:
-Kovács, K.; Kamnev, A.A. ; Mink, J.; Németh, Cs.; Kuzmann, E.; Megyes, T.; Grósz, T.;
Medzihradszky-Schweiger, H. and Vértes, A.; Struct. Chem. 2006, 17, 105-120.
-Kamnev, A.A.; Shchelochkov, A.G.; Perfiliev, Yu.D.; Tarantilis, P.A. and Polissiou, M.G.; J. Mol.
Struct. 2001, 563-564, 565-572.
-Kovács, K.; Kamnev, A.A.; Shchelochkov, A.G.; Kuzmann, E.; Medzihradszky-Schweiger, H.;
Mink, J. and Vértes, A.; J. Radioanal. Nucl. Chem. 2004, 262, 151-156.
-Kovács, K.; Kamnev, A.A.; Kuzmann, E.; Homonnay, Z.; Szilágyi, P.Á.; Sharma, V. K. and
Vértes, A.; J. Radioanal. Nucl. Chem. 2005, 266, 513-517.
As it is well known, the condition of the Mössbauer effect is the recoilless emission and absorption
of the gamma rays and it requires a solid state. This means that Mössbauer spectroscopy is a
structural investigation technique for solid materials. Consequently, the application of Mössbauer
spectroscopy for solution chemistry needs an adequate freezing (quenching) of the liquid solution.
Several publications demonstrate that the coordination environment, the chemical bonding
conditions, and the oxidation states in solutions are reflected in the Mössbauer spectra recorded
after rapid freezing.
In high-spin iron(III) salt solutions the Mössbauer study of the paramagnetic spin relaxation gives
information about the structure of the solution. The Mössbauer spectra of samples containing
paramagnetic iron(III) can show magnetic splitting if the average time of their paramagnetic spin
relaxation (τPSR) is longer than the average time of Larmor precession of the magnetic moment of
the atomic nucleus (τL). 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 (τSSR) and spin-lattice (τSLR) relaxations will be long.
1
Consequently: 

PSR
1

1
 SSR  SLR
will be longer than τL. Under these conditions, the Mössbauer spectra show magnetic structure. If
dimer formation takes place in the solution, the Fe3+ spins will be close to each other, thus the spinspin interaction gets stronger, τSSR decreases and the magnetic splitting collapses in the Mössbauer
spectrum. This effect gives a possibility to use the Mössbauer technique to follow the dimerization
of iron(III) in solutions.
Vértes, A.; Korecz, L. and Burger, K.; Mössbauer Spectroscopy, Elsevier, Amsterdam–Oxford–New
York, 1979, pp. 230-344.
T = 100 K
Mössbauer spectra of iron(III)-nitrate
solutions,
recorded
at
different
temperatures, at pH~1.0, showing a slow
paramagnetic spin relaxation. The
concentration of Fe3+ is 0.01 M where iron
is in monomeric form (Figure A, B).
relative transmission
1,00
0,99
0,98
0,97
0,96
A
0,95
-6
-4
-2
0
2
-1
v / (mm s )
B
T = 4.2 K
Vértes, A.; Korecz, L. and Burger, K.; Mössbauer
Spectroscopy, Elsevier, Amsterdam–Oxford–New
York, 1979, pp. 230-344.
4
6
relative transmission
1,00
0,99
0,98
0,97
C
0,96
-6
-4
-2
0
2
4
6
-1
v / (mm s )
The materials for Mössbauer measurements in aqueous
solutions were prepared using iron(III) solutions
containing enriched (ca. 90% 57Fe) iron dissolved in
nitric acid at elevated temperature. IAA was dissolved
in water adding KOH to the water solutions up to pH
6–7. The concentration of the ligand was 0.03 M.
Addition of iron(III) nitrate to IAA in solutions (up to
the 1:3 metal-to-acid molar ratio) resulted in the colour
change of the solutions and the formation of cocoabrown precipitates indicating complexation of Fe3+
with the IAA. The final pH values of the mixtures were
2.0–2.5. The precipitates were filtered out after 15 min,
dried on the filter paper. Mössbauer spectra of the solid
material and frozen solutions filtered after 15 min and 2
days, were also recorded.
1,01
relative transmission
1,00
0,99
Mössbauer spectra of:
0,98
A: frozen solution of 57Fe(NO3)3 + IAA
mixture (frozen after 15 min)
0,97
0,96
B: frozen solution of 57Fe(NO3)3 + IAA
mixture (frozen after 2 days)
0,95
D
0,94
-6
-4
-2
0
2
-1
v / (mm s )
4
6
The Mössbauer spectrum of the frozen aqueous solution of iron(III) nitrate (0.01 M) shows a line
broadening which is a sign of magnetic relaxation due to a slow paramagnetic spin relaxation.
Namely, the spin-spin and spin-lattice interactions are weak because of the low concentration of
Fe3+ and the relatively low temperature (80 K), respectively (Figure A). (At the temperature of 4.5
K, the Mössbauer spectrum of the same solution shows a magnetic splitting as it is shown in
Figure B.) Adding the ligand to the iron component results in significant changes of the spectra.
Figures C and D suggest the existence of parallel reactions between Fe3+ and IAA:
FeIII complex (precipitate)
Fe3+ + L
Fe2+ + oxidised L
The Mössbauer parameters of the resulting Fe2+ species (isomer shifts δ=1.39 mm/s and
quadrupole splittings ΔEQ=3.35 mm/s) show that it has a hexaaquo coordination environment.
Comparing the spectral intensities, it can be seen that both after 15 min and after 2 days of contact
of the IAA with iron(III), in the solutions there appears a significant amount of ferrous iron. This
is related to the ease of the IAA side-chain decarboxylation and oxidation and, in parallel, to the
reduction of iron(III). The other two components of the spectra represent the iron(III) complex
with the corresponding ligand (doublet with δ=0.52 and ΔEQ=0.58 mm/s) and the remaining
unreacted Fe3+ ions (broad single line).
The results of elemental analysis of the poorly soluble Fe–IAA complex supposes that its
composition most closely corresponds to the μ-(OH)2-bridged complex [L2Fe<(OH)2>FeL2]
(where L is the deprotonated IAA moiety).
FeIAA solid
relative transmission
1,00
0,99
0,98
0,97
0,96
E
-6
-4
-2
0
2
4
6
-1
v / (mm*s )
To study the structure and possible structural changes of the
complexes by dissolving them in an organic solvent, e.g. in
acetone, and adding a small amount of water to the solutions, ca.
0.1 M samples (with regard to total Fe) were measured using the
rapid-freezing method. For these experiments, as well as for the
FTIR, FT-Raman measurements, the solid complexes were
synthesized using natural (not enriched with 57Fe) iron(III) nitrate,
the conditions being the same as described above. The precipitates
were filtered out, washed three times with distilled water and dried
in air.
FeIAA, acetone
relative transmission
1,00
0,99
0,98
F
-6
=0.54 mm/s
-4
-2
0
2
4
6
-1
v / (mm*s )
FeIAA, acetone+water
relative transmission
1,00
0,99
0,98
G
-6
=0.73 mm/s
-4
-2
0
2
-1
v / (mm*s )
4
6
The solid complexes give an intensive symmetric quadrupole
doublet with the parameters typical for high-spin Fe3+ in distorted
octahedral coordination (Figure E, δ=0.52 and ΔEQ= 0.56 mm/s).
The Mössbauer spectra of the complex in acetone solutions shows
one well-resolved quadrupole doublet as well. The lack of a
magnetic structure is an evidence that the iron(III) species has a
dimeric structure with a fast spin-spin relaxation. The doublet for
the acetone solutions has the isomer shift and quadrupole splitting
very close to those for the solid material, which indicates similar
structures of the iron(III) microenvironment both in the solid state
and in the acetone solution, see Figure F. After adding more water
to the acetone solutions, structural changes become visible, with
the quadrupole splittings increasing by 0.19 mm/s (Figure G). This
indicates the formation of a more asymmetric coordination of
iron(III) upon adding water to the system. This can be explained by
hydrolytical replacement of the COO– moieties, possibly giving
free indole-3-acetic acid ligands, which could also be confirmed by
FTIR spectroscopic measurements.
In the FTIR and Raman spectra, one can easily follow the structural changes due to complex
formation:
−C=O stretching band at 1700 cm-1 characteristic of the carboxylic group disappears and, in
parallel, several new bands at 1580, 1525, 675, 625, 550 cm–1 confirm the presence of bidentate
carboxylic groups
−NH stretching bands show weak perturbation
the nitrogen moiety does not contribute to
the coordination
229
347
324
227
205
281
278
179
347
473
324
383
425
(b)
501
539
(a) Fe–IBA complex
206
385
480
501
536
(a)
578
Absorbance
One example of the frequency shifts
due to the deuteration is shown in
the Figure for iron(III) complex
with indole-3-butyric acid (IBA)
which is a structural analog of IAA:
582
424
The existing –OH groups in the complex could be identified with the help of a deuteration
experiment, where the exchangeable protons of the Fe2OH moiety showed strong frequency shifts
in the far infrared region (Fe–O stretching, Fe2OH out-of-plane bending, Fe–O–Fe deformation
modes).
(b) Deuterated Fe–IBA complex
600
500
400
Wavenumber (cm-1)
300
200
100
According to the results discussed above
and with the help of solution X-ray
diffraction, the schematic ball-and-stick
representation of the [Fe2(OH)2(IA)4]
complex (IA represents indole-3-acetate)
can be given as shown in the Figure. The
structural parameters for the ligand were
obtained from a preliminary single-crystal
study of the ligand.
Kovács, K.; Kamnev , A.A.; Mink, J.; Németh, Cs.; Kuzmann, E.; Megyes, T.; Grósz, T.;
Medzihradszky-Schweiger, H. and Vértes, A.; Struct. Chem. 2006, 17, 105-120.