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Fe(III) and Al(III) heteronuclear coordination compounds: A combined experimental and quantum chemical study
Accepted Manuscript Fe(III) and Al(III) Heteronuclear Coordination Compounds: a Combined Experimental and Quantum Chemical Study Irina D. Sorokina, Tamara T. Zinkicheva, Renat R. Nazmutdinov, Alexander F. Dresvyannikov PII: DOI: Reference:
S0277-5387(16)00107-8 http://dx.doi.org/10.1016/j.poly.2016.02.021 POLY 11837
To appear in:
Polyhedron
Received Date: Accepted Date:
10 November 2015 10 February 2016
Please cite this article as: I.D. Sorokina, T.T. Zinkicheva, R.R. Nazmutdinov, A.F. Dresvyannikov, Fe(III) and Al(III) Heteronuclear Coordination Compounds: a Combined Experimental and Quantum Chemical Study, Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.02.021
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Fe(III) and Al(III) Heteronuclear Coordination Compounds: a Combined Experimental and Quantum Chemical Study
Irina D. Sorokina#, Tamara T. Zinkicheva, Renat R. Nazmutdinov and Alexander F. Dresvyannikov
Kazan National Research Technological University, K. Marx Str. 68 420015 Kazan, Republic of Tatarstan, Russian Federation
Abstract A possibility to synthesize heteronuclear compounds from aqueous solutions containing the Fe(II), Fe(III), Al(III), Cl− and OH− ions is explored. The XRF analysis, IR-spectroscopy, a model based on the potentiometric titration data, as well as molecular modelling and density functional theory are employed to determine the elemental and phase composition and the structure of synthesized compounds. The most probable forms of the heteronuclear complexes are proposed on the basis of a combined quantum chemical and spectroscopic analysis.
Keywords: aluminum, iron, poly- and heteronuclear coordination compounds, synthesis, IRspectroscopy, density functional theory.
#
Corresponding author, E-mail: [email protected]
1. Introduction Nowadays, there is a growing interest in chemistry of polynuclear, heteroligand, and heteronuclear coordination structures formed from metal hydroxo- and oxocompounds in aqueous multicomponent solutions. Systems based on iron and aluminium oxocompounds attract special attention because they can be used as coagulants in water decolouration and as catalyst precursors. The presence of aluminium(III) is known to stabilize the necessary Fe(II)/Fe(III) concentration ratio. Solutions with such well-defined ratios are used, for example, for preparing ironing electrolytes for electroplating. Experimental data on potentiometric titration can be used for the development of models of equilibrium processes in order to predict the composition of compounds in solutions and deposits, to estimate equilibrium constants, and to set up the synthesis of target products. Polynuclear and heteronuclear compounds formed in Fe(II), Fe(III), Al(III), Cl− − H2O−OH− systems, however, are not properly identified so far, and the description of their properties is still contradictive [1-5]. The aim of this work is to investigate and to determine the structure of the polynuclear and heteronuclear coordination compounds formed in the Fe(II), Fe(III), Al(III), Cl− − H2O −OH− systems. This paper is organized as follows: Some details of synthesis and DFT calculations are given in Section II. The main results are reported in Section III. Concluding remarks can be found in Section IV. 2. Material and Methods 2.1 Materials The experiments were performed on the compact and narrowly fractionated samples of dispersed aluminum (purity not less than 99.0%), surface areas range from 63.5 cm2/g (fraction of particles 2
350 ± 55 microns) to 1000 cm /g (25 ± 15 microns). FeCl3•6H 6H2O (iron(III) concentration 0.1-2.0 M) is used as the main reagent without further purification.
2
2.2 Experimental Procedure Iron(III) chloride hexahydrate, aluminum foil (А 999 with the Al content 99.9%) and aluminum powder with particle size of 10-405 microns were used to obtain iron-aluminum poly- and heteronuclear compounds. An excess of metallic aluminum was dissolved at room temperature in the 100 cm3 of Fe(III) chloride solution (the FeCl3 concentration amounts to 0.1-1.5 M) and stirred for 10-15 minutes until the termination of intensive gas generation. The process kinetics was studied by sampling at fixed intervals and subsequent potentiometric titration to determine the iron(II) and iron(III) with the use of complexometric methods. The supplementary control of the total content of iron in solution was performed by the X-ray fluorescence analysis using VRA-20L (Carl Zeiss). The hydrogen amount produced at the oxidation of Al by Fe(III) was determined by the pH measurement which gives full information on the gas evolution. 2.3 Potentiometric studies Ionic equilibria in the M − H2O − OH− systems (M = Fe(II), Fe(III), Al(III)) were addressed by potentiometric titration using the methods described in work [6]. We employed a mathematical model resulting from the material balance equation, which is suitable for the most probable species formed in the system under consideration. Their stoichiometry and the equilibrium constants were determined from the experimental curve nOH- = f (pH), nOH- = CNaOH · VNaOH / CM · VM (CM and VM are the concentration and the initial volume of salt solution, respectively). The solution pH was measured using high-resistance millivoltmeter (pH Seven Multi S80-K, Mettler Toledo, Switzerland). The pH value was adjusted by adding a stock solution of an appropriate quantity of hydrochloric acid. The iron(III) chloride solution was passed through purified argon to remove dissolved oxygen. The pH value corresponding to the precipitation onset was recorded during titration. The beginning of precipitation was determined visually at the time of the solution turbidity. Then we built the potentiometric titration curves of iron(III) and aluminium(III) in the coordinates nOH- vs pH.
3
2.4 Spectral measurements The deposits of the synthesized compounds were removed from the mother solution into a Schott funnel under vacuum and dried at 80°С for for 22 h. h. A A weighed weighed portion of poorly soluble compound was stirred with a prescribed weighed portion of boric acid in an agate mortar for 10 min. Then a mixture was placed in a mold of 12 mm in diameter, pressed into tablets under the pressure 2000 MPa. The content of Fe, Al and Cl elements was determined by the X-ray fluorescence method in vacuum (Energy-dispersive X-ray spectrometer TXRF S2 PICOFOX with High Efficiency Module; X-ray tube voltage 15 kV, current 800 mA, time 50 s). The tablets were additionally analyzed by using an infrared spectrometer Shimadzu FTIR-8400S (Japan). To obtain reliable experimental data each experiment was performed at least three times. 2.5 Quantum chemical modelling The quantum chemical calculations were performed at the DFT level with the hybrid exchangecorrelation functional B3LYP as implemented in the Gaussian 03 program suite [7]. The valence orbitals of the Fe and Al atoms were described on a basis set of double-zeta (DZ) quality, while the effect of inner electrons was included in the effective core potential developed by Hay and Wadt (LanL2) [8]. The standard basis set D95V was used to describe the electrons in the Cl, O, and H atoms. The open shell systems were treated in terms of unrestricted formalism. Anti-ferromagnetic states of the model molecules may compete in general with paramagnetic states. For simplicity we did not consider the anti-ferromagnetic state which takes a special analysis based on the broken symmetry technique. The geometry of species was fully optimized without symmetry restrictions. The natural population analysis (NPA) was employed to calculate the atomic charges. Water molecules were not included directly into the first coordination sphere; aqueous medium was addressed by the PCM (Polarized Continuum Model) as implemented into the Gaussian 03 package [7]. A value of 78 was used for the static dielectric constant of electrolyte solutions. Nonelectrostatic (cavitation, dispersion and repulsion) contributions to the solvation free energy were
4
calculated as well [7]. The solvation effects were found in further calculations to be important. The program ChemCraft [9] was used for the visualization of model IR spectra; the computed frequencies were not scaled. The stability of Kohn-Sham orbitals was checked with the help of a standard routine in the Gaussian 09 package. To demonstrate the accuracy of computational level we have performed the test calculations of AlO and Fe-O molecules using the standard basis sets of double- and triple-zeta (including diffuse and polarization functions) quality: LanL2DZ, 6-311++G(d, p), TZVP and the B3LYP functional [7]. Spin multiplicity (s) of Fe-O molecule was varied from 1 to 11, the value s = 5 was found to characterise the ground state (i.e. gives the lowest energy)1. Vibration frequencies were calculated using harmonic approximation. As can be seen from the data compiled in Table 1S, the results obtained with the less CPU time consuming Lanl2DZ basis set show a reasonable agreement with experimental data and some theoretical predictions from literature. 3. Results and discussion 3.1 Chemical equilibrium in Water Solutions of Fe(II), Fe(III), Al(III) Cations. IR spectra. Oxocompounds of iron and aluminum attract interest, because they are used as coagulants for water purification, as well as precursors for the preparation of sorbents. It is known that the presence of aluminum favours to keep a certain Fe(II)/Fe(III) concentration ratio. Such systems are employed, for example, in the preparation of iron plating electrolytes in galvanotechnics. Fig. 1 demonstrates the typical kinetics curves of the Fe(III) reduction by disperse aluminum at room temperature. The process takes place at the Fe(III) excess in the solution which is reduced to Fe(0) through a multistep electron transfer. Due to the high reaction heat and the small size of Al particles (and, therefore, high reaction rates) a self-heating usually occurs, which leads to a significant increasing the solution temperature. The graphic and Ostwald-Noyes methods showed that for aluminum surplus, a decrease of the Fe(III) concentration in the solution is described by the 1
Information on the total spin number of molecules can be extracted, for example from the Electronic Paramagnetic Resonance (EPR) measurements. We dit not find, however, relevant experimental data.
5
first order equation (or pseudo-first order). Temperature was found to affect not only the phase separation rate, but also the mechanism of the process. At low temperature (278 K) the first step Fe(III) → Fe(II) dominates. The polyheteronuclear structures were formed at the redox processes with subsequent aging, i.e. molecular oxygen oxidizes Fe(II) up to Fe(III) again. The presence of aluminum(III) makes the iron(II) oxidation with the air oxygen noticeably slower.
1.0
2
C, M
0.8
0.6
0.4
0.2
1 0.0 0
200
400
600
800
1000
1200
τ, s Fig. 1 Kinetic curves describing the reduction of iron(III) ions from water solution on the aluminium particles of 70-100 µm size at 278 К; 1 − Fe(III); 2 − Fe(II).
Fig. 2 exhibits the curves of the potentiometric titration of mother liquor after the interaction between FeCl3 solution and elemental aluminum of relative concentration CAl = 1.64 M, CFe(II) = 0.21 M. We have concluded previously using the X-ray fluorescence (XRF), Nuclear Magnetic Resonance (NMR) and potentiometric measurements that it is possible to distinguish two heteronuclear compounds with presumable stoichiometric composition [FeAl2(OH)5Cl4] (pH = 4) and [Fe2Al4(OH)15Cl3] (pH = 9) [15]. The both complexes contain Fe(III), the presence of Fe(II) is less
6
favourable from the view point of thermodynamics. This can be explained in terms of the Lewis acid concept: Fe(III) is a stronger Lewis acid than Fe(II) and, therefore, favours the formation of “bridges” which stabilizes the heteronuclear complex forms. It is interesting to note that these compounds with above-mentioned stoichiometric composition cannot be obtained by simple mixing of the salt solutions (Al(III), Fe(II) and Fe(III)). Now we are focused on determining the molecular structure of such species resting on the ex situ IR spectroscopy (see Section 2.4) and a quantum chemical approach.
7 6
nOH
-
5 4 3 2 1 0
2
4
6
8
10
12
14
pH Fig. 2 The solution pH as a function of hydroxyl ion (nOH−) during the titration of mother solution after the reaction between FeCl3 solution and elemental aluminium
The experimental IR spectra of the [FeAl2(OH)5Cl4 ] and [Fe2Al4(OН ))15Cl3] complexes species are shown in Fig. 3. The second specie was additionally investigated in a more detail using the Fourier Transform IR spectrospopy (FTIR). There are two main groups of peaks: in high- (3000 – 3500 cm-1) and low frequency (1000 – 1500 cm-1) regions. To interpret the data, we calculated IR spectra for several model forms of the hereronuclear complexes (see below).
7
Fig. 3 Experimental FTIR spectra, obtained for the [FeAl2(OH)5Cl4] (blue) and [Fe2Al4(OН)15Cl3] (red) complexes 3.2 Quantum chemical modelling of Fe(III) and Al(III) heteronuclear compounds (i) [FeAl2(OH)5Cl4 ] When performing DFT calculations we assumed that the polyheteronuclear complexes in aqueous solutions can be modelled as small clusters. As a starting point the most energy favourable monoatomic Fe(III) and Al(III) aqua-chlorocomplexes were examined. Then a set of several complexes of chained structure (also linked through one or two bridges) was investigated as well. The solvent effects were addressed on the basis of the PCM and five water molecules was directly included in the model heteronuclear structures. The optimized geometry of polyheteronuclear complexes and the calculated heats of some model processes are collected in Table 1 and shown in Fig. 4. As showen the geometry analysis the Fe and Al atoms are linked either by double chloride or hydroxyl bridges (with an additional stabilization due to hydrogen bonding between the hydroxyl ion and the water molecules, forms (i) – (iii)) or single chloride bridge and hydrogen bonds (form (iv)). In the complexes (i) – (iii) the Al-Fe separation was found to be smaller as compared with the form (iv); moreover, these complexes are more stable as well.
8
form (i): c.n. Fe = 6, Al = 5; r(Fe-Al1 ) = 3.32Å; r(Fe-Al2) = 3.586Å; raverage(Fe-Al) = 3.45Å
form (ii): c.n. Fe = 6, Al = 5 and 6; r(Fe-Al1)=3.35Å; r(Fe-Al2 )=3.96Å; raverage(Fe-Al)=3.65Å
form (iii): c.n. Fe = 6, Al = 5; r(Fe-Al1) = 3.34Å; r(Fe-Al2) = 3.58Å; raverage(Fe-Al) = 3.46Å
form (iv): c.n. Fe = 5, Al = 5; r(Fe-Al1)=4.17Å; r(Fe-Al2 )=3.81Å; raverage(Fe-Al)=3.99Å
Fig. 4 The optimized geometry of several forms of the [FeAl2(OH)5Cl4] complex obtained using the results of DFT calculations.
The model processes of the formation of [FeAl2(OH2)5Cl4(H2O)5] polyheteronuclear complexes from aqua and aqua hydroxo complexes Al(III) and Fe(III) can be recast as follows: 2[Al(OH2)6]3+ + [Fe(OH2)6]3+ + 4Cl– + 5OH– = [FeAl2(OH)5Cl4(H2O)5] + 13H2O
(1)
2[Al(OH2)5OH]2+ + [Fe(OH2)5OH]2+ + 4Cl– + 2OH– = [FeAl2(OH)5Cl4(H2O)5] + 10H2O
(2)
2[Al(OH2)4(OH)2]+ + [Fe(OH2)4(OH)2]+ + 4Cl– = [FeAl2(OH)5Cl4(H2O)5] + OH– + 7H2O
(3)
9
Table 1 Heat effect of model reactions (1)-(3) calculated for gas phase (∆Hgas ) and solution (∆Hsolv) ∆Hgas, kcal/mol
∆Hsolv, kcal/mol
Complex form
Complex form
Model reaction i
ii
iii
iv
i
ii
iii
iv
(1)
-2183
-2172
-2173
-2165
-359
-345
-343
-337
(2)
-1065
-1055
-1055
-1047
-148
-134
-132
-126
(3)
-254
-243
-244
-236
+35
+50
+51
+57
The processes (1)-(3) liberate very high energies mostly due to the Coulomb effects. The reactants exhibit many positive and negative charges and lead to uncharged (eq.1, 2) of less charged (eq.3) products. It can be seen from Table 1 that the formation of the complex forms from mono-aqua complexes in aqueous medium is the most feasible. However, at experimental conditions (pH 4) the existence of the Fe(III) and Al(III) bihydroxo complexes is most likely not thermodynamically probable ( see eq. 3). The calculated IR spectrum for [FeAl2(OH)5Cl4(H2O)5] (form (i)) is shown in Fig. 5 and agrees qualitatively with the experimental data (Fig.3, see three groups of peaks at 700, 1700, 3400 cm-1).
250 0
IR Intencities
200 0
150 0
100 0
50 0
0 40 00
3 500
30 00
2500
200 0
15 00
F req uen cies, cm
100 0
500
0
-1
Fig. 5 Model IR spectrum calculated for the [FeAl2(OH)5Cl4(H2O)5] complex (form (i))
10
(ii) [Fe2Al4(OН)15Cl3] In DFT calculations of different structures with an assumed [Fe2Al4(OН)15Cl3] composition no water molecules were directly included; solvent effects were addressed explicitly using the PCM. Finally we are selected five different forms which are displayed in Fig. 6. According to our results the cyclic forms are more favourable than linear ones. The highest possible spin state (the spin number is 5) corresponds to the ground state for all the polynuclear species considered which is in agreement with the well-known feature of Fe(III) aqua- and aquahydrocomplexes to form high spin structures. The existence of structure (v) in gas phase and in water solution seems to be the most probable (see Table 2). It should be mentioned that both entropic terms and hydration effects do not change the energy row of the complex forms under study. There is a correlation between the observed differences in hydration energies of the complexes and their dipole moments. For example, the dipole moment of structure (iv) was calculated to be the smallest (1.4 D) among the other complex forms. At the same time the dipole moment of structure (v) amounts to 4.7 D which is less as compared with the value found for structure (iii), 11 D. The calculated quadrupole moments of complexes correlate in a more straightforward way with the row of hydration energies presented in Table 2.
form (i)
form (ii)
11
form (iii)
form (iv)
form (v) Fig. 6 Optimized geometry of different forms of the [Fe2Al4(OН)15Cl3] complex Table 2. Total (E) and free (F) energy difference (kcal mol-1) between the five forms of the [Fe2Al4(OН)15Cl3] complex (see Fig. 6) in gas phase and in water; the energy of the most favourable form was taken as zero Complex form
i
ii
iii
iv
v
∆E(gas)
19.2
8.1
34.8
69.1
0
∆F(gas)
17.5
7.7
31.8
64.3
0
∆E(solv)
16.9
8.0
30.0
66.2
0
12
Table 3. Coordination number of aluminum (n Al) and iron (nFe) and average interatomic distances (Å) in different structures of the complex [Fe2Al4(OH)15Cl3] (Fig. 6) Complex form
nAl
nFe
r(Fe-Al), Å
r(Al-Al), Å
r(Al-Cl), Å
(i)
4; 5
5
2.980÷3.394
2.886÷2.943
2.259÷2.270
(ii)
4; 5
5
3.041÷3.420
2.915÷2.940
2.247÷2.255 *
(2.326)
*
(iii)
4; 5; 6
4
2.915÷4.449
2.888÷3.446
2.210÷2.343
(iv)
4; 5
4; 5
2.810÷3.677
2.901÷3.222
2.210÷2.296
(v)
4; 5
5
3.007÷3.410
2.912÷2.930
2.246 (2.309, 2.326)*
r(Fe-Cl) is given in parentheses.
Table 4. NPA charges of the iron, aluminium, chlorine atoms for different complex forms (Fig. 6) Complex form
(i)
(ii)
(iii)
(iv)
(v)
Fe
1.136
1.124
1.101
1.413
1.239
Al
2.018
2.205
2.013
2.008
2.090
Cl
-0.615
-0.537
-0.605
-0.597
-0.469
It follows from the analysis of the geometry of complex forms that the iron and aluminum atoms are bound through double hydroxyl bridges; the chloride-ion does not seem to form bridges. The more stable is the structure, the shorter are the Fe-Al and Al-Al distances. In the most favourable structure (v) the chlorine atom is bond to the Fe atoms. The Fe and Al atomic charges reveal some tendency to be lower at the decreasing of thermodynamic structure stability. We considered three model processes (4-6) describing the possible formation of polyheteronuclear complexes [Fe2Al4(OН)15Cl3] from Al(III) and Fe(III) aqua- and aquahydroxocomplexes:
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4[Al(OH2)6]3+ + 2[Fe(OH2)6]3+ + 3Cl– + 15OH– = [Fe2Al4(OН)15Cl3 ] + 36H2O
(4)
4[Al(OH2)5OH ]2+ + 2[Fe(OH2) 5OH]2+ + 3Cl– + 9OH– = [Fe2Al4(OН)15Cl3] + 30H2O
(5)
4[Al(OH2)4(OH)2]+ + 2[Fe(OH2)4(OH)2]+ + 3Cl– + 3OH– = [Fe2Al4(OН)15Cl3] + 24H2O
(6)
The heat of these reactions (structure (v) is assumed) in aqueous solution was calculated to be -941, -1100 and -484 kcal mol-1, for (4), (5), and (6) respectively. The hydration of all reactants and products was considered in terms of the PCM model. The reactions (4-6), therefore, are most likely feasible. From the viewpoint of experimental conditions, model processes (5), (6) are more relevant in the present work (pH =9), while process (4) relates to acid solutions. Although according to the results compiled in Table 2, structure (6) is the most probable from the view point of thermodynamics, we have to take into consideration the approximate character of model estimations. Another insight into the problem of the most probable complex forms can be gained form the DFT modelling of IR spectra. As can be seen from Table 5, the calculated spectra for certain forms generally agree with the experimental results. The IR intensities can be assigned to the O-H valence, OH- deformation, and Fe-O (Al-O) valence vibrations. It should be noted that the calculated frequencies are harmonic and are, therefore, plagued by the neglect of an anharmonicity treatment. This neglect induces a systematic upshift error of more than 200 cm−1 as compared to experiment. This fact favours, for example, a better agreement with the experiment when considering structures (i) and (v). Therefore, structures (i) and (v) might co-exist in the aqueous solutions under study.
14
3 00 0
(v) ( i) ( ii ) ( ii i) ( iv )
IR intencity
2 50 0
2 00 0
1 50 0
1 00 0
50 0
0 4 000
30 0 0
20 0 0
10 00
F r e q u e n c y, c m
0
-1
Fig. 7 Model IR spectra calculated for five different structures of the [Fe2Al4(OН)15Cl3] complex (Fig. 6)
Table 5. Selected band wave numbers (cm-1) of the model IR spectra (Fig. 7) structure
structure
structure
structure
structure
(v)
(i)
(ii)
(iii)
(iv)
3331-4054
3554-4061
3165-4048
2027-4070
2877-4057
2920-3500
907-1277
859-1211
831-1308
774-1754
1003-1642
890-1600
626-901
636-746
628-753
625-762
660-928
690-890
vibr. mode
exp.(Fig.3)
O-H valence O-H deformation Al-O, Fe-O valence 4. Conclusions It has been found by the potentiometry, XRF analysis, IR spectroscopy and quantum chemical modeling that redox processes involving elemental aluminum and Fe(III) in chloride solutions result in the formation of heteronuclear coordination compounds. Such compounds contain Fe(III)
15
and Al(III); their composition is characterized by a general formula [FemAlnClk(OH)q]2m+3n-k-q, where the m, n, k, and q values depend on the concentration of components and solution pH. With the help of DFT calculations we have determined the most probable forms of the heteronuclear complexes. These findings were justified by the estimation of the model reaction heats and comparing the calculated IR spectra with experimental ones. Thus, a combined experimental and computational study makes it possible to gain a deeper insight into the structure of Fe(III)-Al(III) heteronuclear complexes. According to results reported in work [16] some heteropoly anions can reveal challenging barrier properties in interfacial electron transfer reactions. It would be promising to model the redox behaviour of Fe(III) and Al(III) containing heteronuclear coordination compounds in different reaction layers on the basis of quantum mechanical theory and molecular simulations (see, for example, review [17]). 5. Acknowledgment We are indebted to Prof. A.B. Remizov for the help with IR measurements. This work was supported in part by the RF Ministry of education and science under State contract no. 4.1584.2014/К . References [1] Y. Zhong, Q. Yang, X. Wu et al. Fe(II)-Al(III) layered double hydroxides prepared by ultrasound-assisted coprecipitation method for the reduction of bromate, J. Hazard. Mater. 250-251 (2013) 345-353. [2] A.F. Dresvyannikov, M.E. Kolpakov, Chemical synthesis of alpha-iron in aqueous FeCl3, Mat. Res. Bull. 37 (2002) 291-296. [3] C. Ruby, M. Usman, S. Naille, K. Hanna, C. Carteret, M. Mullet, M. François, M. Abdelmoula, Synthesis and transformation of iron-based layered double hydroxides, Appl. Clay Sci. 48 (2010) 195-202.
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[4] Y. Zhu, E.J. Elzinga, Formation of Layered Fe(II)-Hydroxides during Fe(II) Sorption onto Clay and Metal-Oxide Substrates, Environ. Sci. Technol. 48 (2014) 4937-4945. [5] G. Zhang, F. Yang, L. Liu, Comparative study of Fe2+/H2O2 and Fe3+/H2O2 electro-oxidation systems in the degradation of amaranth using anthraquinone/polypyrrole composite film modified graphite cathode, J. Electroanal. Chem. 632 (2009) 154-161. [6] R.A. Yusupov, R.F. Abzalov, N.I. Movchan, S.G. Smerdova, 2000, Complicated equilibria in the Pb(II) − H2O − OH– System, Russ. J. Phys. Chem. 74 (2000) 535-539. [7] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery, Jr.T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill; B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, revision B.04;Gaussian, Inc.: Pittsburgh, PA, 2003. [8] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270-283. [9] http://www.chemcraftprog.com. [10] P.J. Dagdigian, H.W. Cruse, R.N. Zare, Laser fluorescence study of AlO formed in the reaction Al + O2: Product state distribution, dissociation energy, and radiative lifetime, J. Chem. Phys. 61 (1975) 1824-1834. http://dx.doi.org/10.1063/1.430710
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[11] N. Sreedhara Murthy, S.P. Bagare, B. Narasimha Murthy, Dissociation energy for the ground state of AlO from true potential energy curve, J. Quant. Spectrosc. Radiat. Trans. 19 (1978) 455459. [12] J.K. McDonald, K.K. Innes A low-lying excited electronic state of the AlO molecule and the ground-state dissociation energy, J. Mol. Spectrosc. 32 (1969) 501-510. [13] D.L. Hildenbrand, Dissociation energies of the molecules AlO and Al2O, Chem. Phys. Lett. 20 (1973) 127-129. [14] G. Orlova, J.D. Goddard, Density functional theory predictions for small redicals containing boron and aluminium: broken symmetry problems and solutions, Mol. Phys. 98 (2000) 961-966. [15] I.D. Sorokina, A.F. Dresvyannikov, R.A. Yusupov, F.V. Devyatov, Heteronuclear compounds formed in the systems based on Fe(II), Fe(III), Al(III), SO42−, Cl− − H2O − OH−, and NH3, Russ. J. Appl. Chem. 83 (2010) 593-597. [16] M.I. Borzenko, R.R. Nazmutddinov, D.V. Glukhov, G.A. Tsirlina, M. Probst, Self-inhibition phenomena in the electroreduction of hexamolybdocobaltate(III): a combined experimental and computational study, Chem. Phys. 319 (2005) 200-209. [17] R.R. Nazmutdinov, M.D. Bronshtein, T.T. Zinkicheva, D.V. Glukhov, Modelling of electron transfer across electrochemical interfaces: State-of-the-art., Int. J. Quant. Chem., 116 (2016), 189201.
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Graphical abstract
[Fe2Al4(OН)15Cl3]
19
S0277-5387(16)00107-8 http://dx.doi.org/10.1016/j.poly.2016.02.021 POLY 11837
To appear in:
Polyhedron
Received Date: Accepted Date:
10 November 2015 10 February 2016
Please cite this article as: I.D. Sorokina, T.T. Zinkicheva, R.R. Nazmutdinov, A.F. Dresvyannikov, Fe(III) and Al(III) Heteronuclear Coordination Compounds: a Combined Experimental and Quantum Chemical Study, Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.02.021
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Fe(III) and Al(III) Heteronuclear Coordination Compounds: a Combined Experimental and Quantum Chemical Study
Irina D. Sorokina#, Tamara T. Zinkicheva, Renat R. Nazmutdinov and Alexander F. Dresvyannikov
Kazan National Research Technological University, K. Marx Str. 68 420015 Kazan, Republic of Tatarstan, Russian Federation
Abstract A possibility to synthesize heteronuclear compounds from aqueous solutions containing the Fe(II), Fe(III), Al(III), Cl− and OH− ions is explored. The XRF analysis, IR-spectroscopy, a model based on the potentiometric titration data, as well as molecular modelling and density functional theory are employed to determine the elemental and phase composition and the structure of synthesized compounds. The most probable forms of the heteronuclear complexes are proposed on the basis of a combined quantum chemical and spectroscopic analysis.
Keywords: aluminum, iron, poly- and heteronuclear coordination compounds, synthesis, IRspectroscopy, density functional theory.
#
Corresponding author, E-mail: [email protected]
1. Introduction Nowadays, there is a growing interest in chemistry of polynuclear, heteroligand, and heteronuclear coordination structures formed from metal hydroxo- and oxocompounds in aqueous multicomponent solutions. Systems based on iron and aluminium oxocompounds attract special attention because they can be used as coagulants in water decolouration and as catalyst precursors. The presence of aluminium(III) is known to stabilize the necessary Fe(II)/Fe(III) concentration ratio. Solutions with such well-defined ratios are used, for example, for preparing ironing electrolytes for electroplating. Experimental data on potentiometric titration can be used for the development of models of equilibrium processes in order to predict the composition of compounds in solutions and deposits, to estimate equilibrium constants, and to set up the synthesis of target products. Polynuclear and heteronuclear compounds formed in Fe(II), Fe(III), Al(III), Cl− − H2O−OH− systems, however, are not properly identified so far, and the description of their properties is still contradictive [1-5]. The aim of this work is to investigate and to determine the structure of the polynuclear and heteronuclear coordination compounds formed in the Fe(II), Fe(III), Al(III), Cl− − H2O −OH− systems. This paper is organized as follows: Some details of synthesis and DFT calculations are given in Section II. The main results are reported in Section III. Concluding remarks can be found in Section IV. 2. Material and Methods 2.1 Materials The experiments were performed on the compact and narrowly fractionated samples of dispersed aluminum (purity not less than 99.0%), surface areas range from 63.5 cm2/g (fraction of particles 2
350 ± 55 microns) to 1000 cm /g (25 ± 15 microns). FeCl3•6H 6H2O (iron(III) concentration 0.1-2.0 M) is used as the main reagent without further purification.
2
2.2 Experimental Procedure Iron(III) chloride hexahydrate, aluminum foil (А 999 with the Al content 99.9%) and aluminum powder with particle size of 10-405 microns were used to obtain iron-aluminum poly- and heteronuclear compounds. An excess of metallic aluminum was dissolved at room temperature in the 100 cm3 of Fe(III) chloride solution (the FeCl3 concentration amounts to 0.1-1.5 M) and stirred for 10-15 minutes until the termination of intensive gas generation. The process kinetics was studied by sampling at fixed intervals and subsequent potentiometric titration to determine the iron(II) and iron(III) with the use of complexometric methods. The supplementary control of the total content of iron in solution was performed by the X-ray fluorescence analysis using VRA-20L (Carl Zeiss). The hydrogen amount produced at the oxidation of Al by Fe(III) was determined by the pH measurement which gives full information on the gas evolution. 2.3 Potentiometric studies Ionic equilibria in the M − H2O − OH− systems (M = Fe(II), Fe(III), Al(III)) were addressed by potentiometric titration using the methods described in work [6]. We employed a mathematical model resulting from the material balance equation, which is suitable for the most probable species formed in the system under consideration. Their stoichiometry and the equilibrium constants were determined from the experimental curve nOH- = f (pH), nOH- = CNaOH · VNaOH / CM · VM (CM and VM are the concentration and the initial volume of salt solution, respectively). The solution pH was measured using high-resistance millivoltmeter (pH Seven Multi S80-K, Mettler Toledo, Switzerland). The pH value was adjusted by adding a stock solution of an appropriate quantity of hydrochloric acid. The iron(III) chloride solution was passed through purified argon to remove dissolved oxygen. The pH value corresponding to the precipitation onset was recorded during titration. The beginning of precipitation was determined visually at the time of the solution turbidity. Then we built the potentiometric titration curves of iron(III) and aluminium(III) in the coordinates nOH- vs pH.
3
2.4 Spectral measurements The deposits of the synthesized compounds were removed from the mother solution into a Schott funnel under vacuum and dried at 80°С for for 22 h. h. A A weighed weighed portion of poorly soluble compound was stirred with a prescribed weighed portion of boric acid in an agate mortar for 10 min. Then a mixture was placed in a mold of 12 mm in diameter, pressed into tablets under the pressure 2000 MPa. The content of Fe, Al and Cl elements was determined by the X-ray fluorescence method in vacuum (Energy-dispersive X-ray spectrometer TXRF S2 PICOFOX with High Efficiency Module; X-ray tube voltage 15 kV, current 800 mA, time 50 s). The tablets were additionally analyzed by using an infrared spectrometer Shimadzu FTIR-8400S (Japan). To obtain reliable experimental data each experiment was performed at least three times. 2.5 Quantum chemical modelling The quantum chemical calculations were performed at the DFT level with the hybrid exchangecorrelation functional B3LYP as implemented in the Gaussian 03 program suite [7]. The valence orbitals of the Fe and Al atoms were described on a basis set of double-zeta (DZ) quality, while the effect of inner electrons was included in the effective core potential developed by Hay and Wadt (LanL2) [8]. The standard basis set D95V was used to describe the electrons in the Cl, O, and H atoms. The open shell systems were treated in terms of unrestricted formalism. Anti-ferromagnetic states of the model molecules may compete in general with paramagnetic states. For simplicity we did not consider the anti-ferromagnetic state which takes a special analysis based on the broken symmetry technique. The geometry of species was fully optimized without symmetry restrictions. The natural population analysis (NPA) was employed to calculate the atomic charges. Water molecules were not included directly into the first coordination sphere; aqueous medium was addressed by the PCM (Polarized Continuum Model) as implemented into the Gaussian 03 package [7]. A value of 78 was used for the static dielectric constant of electrolyte solutions. Nonelectrostatic (cavitation, dispersion and repulsion) contributions to the solvation free energy were
4
calculated as well [7]. The solvation effects were found in further calculations to be important. The program ChemCraft [9] was used for the visualization of model IR spectra; the computed frequencies were not scaled. The stability of Kohn-Sham orbitals was checked with the help of a standard routine in the Gaussian 09 package. To demonstrate the accuracy of computational level we have performed the test calculations of AlO and Fe-O molecules using the standard basis sets of double- and triple-zeta (including diffuse and polarization functions) quality: LanL2DZ, 6-311++G(d, p), TZVP and the B3LYP functional [7]. Spin multiplicity (s) of Fe-O molecule was varied from 1 to 11, the value s = 5 was found to characterise the ground state (i.e. gives the lowest energy)1. Vibration frequencies were calculated using harmonic approximation. As can be seen from the data compiled in Table 1S, the results obtained with the less CPU time consuming Lanl2DZ basis set show a reasonable agreement with experimental data and some theoretical predictions from literature. 3. Results and discussion 3.1 Chemical equilibrium in Water Solutions of Fe(II), Fe(III), Al(III) Cations. IR spectra. Oxocompounds of iron and aluminum attract interest, because they are used as coagulants for water purification, as well as precursors for the preparation of sorbents. It is known that the presence of aluminum favours to keep a certain Fe(II)/Fe(III) concentration ratio. Such systems are employed, for example, in the preparation of iron plating electrolytes in galvanotechnics. Fig. 1 demonstrates the typical kinetics curves of the Fe(III) reduction by disperse aluminum at room temperature. The process takes place at the Fe(III) excess in the solution which is reduced to Fe(0) through a multistep electron transfer. Due to the high reaction heat and the small size of Al particles (and, therefore, high reaction rates) a self-heating usually occurs, which leads to a significant increasing the solution temperature. The graphic and Ostwald-Noyes methods showed that for aluminum surplus, a decrease of the Fe(III) concentration in the solution is described by the 1
Information on the total spin number of molecules can be extracted, for example from the Electronic Paramagnetic Resonance (EPR) measurements. We dit not find, however, relevant experimental data.
5
first order equation (or pseudo-first order). Temperature was found to affect not only the phase separation rate, but also the mechanism of the process. At low temperature (278 K) the first step Fe(III) → Fe(II) dominates. The polyheteronuclear structures were formed at the redox processes with subsequent aging, i.e. molecular oxygen oxidizes Fe(II) up to Fe(III) again. The presence of aluminum(III) makes the iron(II) oxidation with the air oxygen noticeably slower.
1.0
2
C, M
0.8
0.6
0.4
0.2
1 0.0 0
200
400
600
800
1000
1200
τ, s Fig. 1 Kinetic curves describing the reduction of iron(III) ions from water solution on the aluminium particles of 70-100 µm size at 278 К; 1 − Fe(III); 2 − Fe(II).
Fig. 2 exhibits the curves of the potentiometric titration of mother liquor after the interaction between FeCl3 solution and elemental aluminum of relative concentration CAl = 1.64 M, CFe(II) = 0.21 M. We have concluded previously using the X-ray fluorescence (XRF), Nuclear Magnetic Resonance (NMR) and potentiometric measurements that it is possible to distinguish two heteronuclear compounds with presumable stoichiometric composition [FeAl2(OH)5Cl4] (pH = 4) and [Fe2Al4(OH)15Cl3] (pH = 9) [15]. The both complexes contain Fe(III), the presence of Fe(II) is less
6
favourable from the view point of thermodynamics. This can be explained in terms of the Lewis acid concept: Fe(III) is a stronger Lewis acid than Fe(II) and, therefore, favours the formation of “bridges” which stabilizes the heteronuclear complex forms. It is interesting to note that these compounds with above-mentioned stoichiometric composition cannot be obtained by simple mixing of the salt solutions (Al(III), Fe(II) and Fe(III)). Now we are focused on determining the molecular structure of such species resting on the ex situ IR spectroscopy (see Section 2.4) and a quantum chemical approach.
7 6
nOH
-
5 4 3 2 1 0
2
4
6
8
10
12
14
pH Fig. 2 The solution pH as a function of hydroxyl ion (nOH−) during the titration of mother solution after the reaction between FeCl3 solution and elemental aluminium
The experimental IR spectra of the [FeAl2(OH)5Cl4 ] and [Fe2Al4(OН ))15Cl3] complexes species are shown in Fig. 3. The second specie was additionally investigated in a more detail using the Fourier Transform IR spectrospopy (FTIR). There are two main groups of peaks: in high- (3000 – 3500 cm-1) and low frequency (1000 – 1500 cm-1) regions. To interpret the data, we calculated IR spectra for several model forms of the hereronuclear complexes (see below).
7
Fig. 3 Experimental FTIR spectra, obtained for the [FeAl2(OH)5Cl4] (blue) and [Fe2Al4(OН)15Cl3] (red) complexes 3.2 Quantum chemical modelling of Fe(III) and Al(III) heteronuclear compounds (i) [FeAl2(OH)5Cl4 ] When performing DFT calculations we assumed that the polyheteronuclear complexes in aqueous solutions can be modelled as small clusters. As a starting point the most energy favourable monoatomic Fe(III) and Al(III) aqua-chlorocomplexes were examined. Then a set of several complexes of chained structure (also linked through one or two bridges) was investigated as well. The solvent effects were addressed on the basis of the PCM and five water molecules was directly included in the model heteronuclear structures. The optimized geometry of polyheteronuclear complexes and the calculated heats of some model processes are collected in Table 1 and shown in Fig. 4. As showen the geometry analysis the Fe and Al atoms are linked either by double chloride or hydroxyl bridges (with an additional stabilization due to hydrogen bonding between the hydroxyl ion and the water molecules, forms (i) – (iii)) or single chloride bridge and hydrogen bonds (form (iv)). In the complexes (i) – (iii) the Al-Fe separation was found to be smaller as compared with the form (iv); moreover, these complexes are more stable as well.
8
form (i): c.n. Fe = 6, Al = 5; r(Fe-Al1 ) = 3.32Å; r(Fe-Al2) = 3.586Å; raverage(Fe-Al) = 3.45Å
form (ii): c.n. Fe = 6, Al = 5 and 6; r(Fe-Al1)=3.35Å; r(Fe-Al2 )=3.96Å; raverage(Fe-Al)=3.65Å
form (iii): c.n. Fe = 6, Al = 5; r(Fe-Al1) = 3.34Å; r(Fe-Al2) = 3.58Å; raverage(Fe-Al) = 3.46Å
form (iv): c.n. Fe = 5, Al = 5; r(Fe-Al1)=4.17Å; r(Fe-Al2 )=3.81Å; raverage(Fe-Al)=3.99Å
Fig. 4 The optimized geometry of several forms of the [FeAl2(OH)5Cl4] complex obtained using the results of DFT calculations.
The model processes of the formation of [FeAl2(OH2)5Cl4(H2O)5] polyheteronuclear complexes from aqua and aqua hydroxo complexes Al(III) and Fe(III) can be recast as follows: 2[Al(OH2)6]3+ + [Fe(OH2)6]3+ + 4Cl– + 5OH– = [FeAl2(OH)5Cl4(H2O)5] + 13H2O
(1)
2[Al(OH2)5OH]2+ + [Fe(OH2)5OH]2+ + 4Cl– + 2OH– = [FeAl2(OH)5Cl4(H2O)5] + 10H2O
(2)
2[Al(OH2)4(OH)2]+ + [Fe(OH2)4(OH)2]+ + 4Cl– = [FeAl2(OH)5Cl4(H2O)5] + OH– + 7H2O
(3)
9
Table 1 Heat effect of model reactions (1)-(3) calculated for gas phase (∆Hgas ) and solution (∆Hsolv) ∆Hgas, kcal/mol
∆Hsolv, kcal/mol
Complex form
Complex form
Model reaction i
ii
iii
iv
i
ii
iii
iv
(1)
-2183
-2172
-2173
-2165
-359
-345
-343
-337
(2)
-1065
-1055
-1055
-1047
-148
-134
-132
-126
(3)
-254
-243
-244
-236
+35
+50
+51
+57
The processes (1)-(3) liberate very high energies mostly due to the Coulomb effects. The reactants exhibit many positive and negative charges and lead to uncharged (eq.1, 2) of less charged (eq.3) products. It can be seen from Table 1 that the formation of the complex forms from mono-aqua complexes in aqueous medium is the most feasible. However, at experimental conditions (pH 4) the existence of the Fe(III) and Al(III) bihydroxo complexes is most likely not thermodynamically probable ( see eq. 3). The calculated IR spectrum for [FeAl2(OH)5Cl4(H2O)5] (form (i)) is shown in Fig. 5 and agrees qualitatively with the experimental data (Fig.3, see three groups of peaks at 700, 1700, 3400 cm-1).
250 0
IR Intencities
200 0
150 0
100 0
50 0
0 40 00
3 500
30 00
2500
200 0
15 00
F req uen cies, cm
100 0
500
0
-1
Fig. 5 Model IR spectrum calculated for the [FeAl2(OH)5Cl4(H2O)5] complex (form (i))
10
(ii) [Fe2Al4(OН)15Cl3] In DFT calculations of different structures with an assumed [Fe2Al4(OН)15Cl3] composition no water molecules were directly included; solvent effects were addressed explicitly using the PCM. Finally we are selected five different forms which are displayed in Fig. 6. According to our results the cyclic forms are more favourable than linear ones. The highest possible spin state (the spin number is 5) corresponds to the ground state for all the polynuclear species considered which is in agreement with the well-known feature of Fe(III) aqua- and aquahydrocomplexes to form high spin structures. The existence of structure (v) in gas phase and in water solution seems to be the most probable (see Table 2). It should be mentioned that both entropic terms and hydration effects do not change the energy row of the complex forms under study. There is a correlation between the observed differences in hydration energies of the complexes and their dipole moments. For example, the dipole moment of structure (iv) was calculated to be the smallest (1.4 D) among the other complex forms. At the same time the dipole moment of structure (v) amounts to 4.7 D which is less as compared with the value found for structure (iii), 11 D. The calculated quadrupole moments of complexes correlate in a more straightforward way with the row of hydration energies presented in Table 2.
form (i)
form (ii)
11
form (iii)
form (iv)
form (v) Fig. 6 Optimized geometry of different forms of the [Fe2Al4(OН)15Cl3] complex Table 2. Total (E) and free (F) energy difference (kcal mol-1) between the five forms of the [Fe2Al4(OН)15Cl3] complex (see Fig. 6) in gas phase and in water; the energy of the most favourable form was taken as zero Complex form
i
ii
iii
iv
v
∆E(gas)
19.2
8.1
34.8
69.1
0
∆F(gas)
17.5
7.7
31.8
64.3
0
∆E(solv)
16.9
8.0
30.0
66.2
0
12
Table 3. Coordination number of aluminum (n Al) and iron (nFe) and average interatomic distances (Å) in different structures of the complex [Fe2Al4(OH)15Cl3] (Fig. 6) Complex form
nAl
nFe
r(Fe-Al), Å
r(Al-Al), Å
r(Al-Cl), Å
(i)
4; 5
5
2.980÷3.394
2.886÷2.943
2.259÷2.270
(ii)
4; 5
5
3.041÷3.420
2.915÷2.940
2.247÷2.255 *
(2.326)
*
(iii)
4; 5; 6
4
2.915÷4.449
2.888÷3.446
2.210÷2.343
(iv)
4; 5
4; 5
2.810÷3.677
2.901÷3.222
2.210÷2.296
(v)
4; 5
5
3.007÷3.410
2.912÷2.930
2.246 (2.309, 2.326)*
r(Fe-Cl) is given in parentheses.
Table 4. NPA charges of the iron, aluminium, chlorine atoms for different complex forms (Fig. 6) Complex form
(i)
(ii)
(iii)
(iv)
(v)
Fe
1.136
1.124
1.101
1.413
1.239
Al
2.018
2.205
2.013
2.008
2.090
Cl
-0.615
-0.537
-0.605
-0.597
-0.469
It follows from the analysis of the geometry of complex forms that the iron and aluminum atoms are bound through double hydroxyl bridges; the chloride-ion does not seem to form bridges. The more stable is the structure, the shorter are the Fe-Al and Al-Al distances. In the most favourable structure (v) the chlorine atom is bond to the Fe atoms. The Fe and Al atomic charges reveal some tendency to be lower at the decreasing of thermodynamic structure stability. We considered three model processes (4-6) describing the possible formation of polyheteronuclear complexes [Fe2Al4(OН)15Cl3] from Al(III) and Fe(III) aqua- and aquahydroxocomplexes:
13
4[Al(OH2)6]3+ + 2[Fe(OH2)6]3+ + 3Cl– + 15OH– = [Fe2Al4(OН)15Cl3 ] + 36H2O
(4)
4[Al(OH2)5OH ]2+ + 2[Fe(OH2) 5OH]2+ + 3Cl– + 9OH– = [Fe2Al4(OН)15Cl3] + 30H2O
(5)
4[Al(OH2)4(OH)2]+ + 2[Fe(OH2)4(OH)2]+ + 3Cl– + 3OH– = [Fe2Al4(OН)15Cl3] + 24H2O
(6)
The heat of these reactions (structure (v) is assumed) in aqueous solution was calculated to be -941, -1100 and -484 kcal mol-1, for (4), (5), and (6) respectively. The hydration of all reactants and products was considered in terms of the PCM model. The reactions (4-6), therefore, are most likely feasible. From the viewpoint of experimental conditions, model processes (5), (6) are more relevant in the present work (pH =9), while process (4) relates to acid solutions. Although according to the results compiled in Table 2, structure (6) is the most probable from the view point of thermodynamics, we have to take into consideration the approximate character of model estimations. Another insight into the problem of the most probable complex forms can be gained form the DFT modelling of IR spectra. As can be seen from Table 5, the calculated spectra for certain forms generally agree with the experimental results. The IR intensities can be assigned to the O-H valence, OH- deformation, and Fe-O (Al-O) valence vibrations. It should be noted that the calculated frequencies are harmonic and are, therefore, plagued by the neglect of an anharmonicity treatment. This neglect induces a systematic upshift error of more than 200 cm−1 as compared to experiment. This fact favours, for example, a better agreement with the experiment when considering structures (i) and (v). Therefore, structures (i) and (v) might co-exist in the aqueous solutions under study.
14
3 00 0
(v) ( i) ( ii ) ( ii i) ( iv )
IR intencity
2 50 0
2 00 0
1 50 0
1 00 0
50 0
0 4 000
30 0 0
20 0 0
10 00
F r e q u e n c y, c m
0
-1
Fig. 7 Model IR spectra calculated for five different structures of the [Fe2Al4(OН)15Cl3] complex (Fig. 6)
Table 5. Selected band wave numbers (cm-1) of the model IR spectra (Fig. 7) structure
structure
structure
structure
structure
(v)
(i)
(ii)
(iii)
(iv)
3331-4054
3554-4061
3165-4048
2027-4070
2877-4057
2920-3500
907-1277
859-1211
831-1308
774-1754
1003-1642
890-1600
626-901
636-746
628-753
625-762
660-928
690-890
vibr. mode
exp.(Fig.3)
O-H valence O-H deformation Al-O, Fe-O valence 4. Conclusions It has been found by the potentiometry, XRF analysis, IR spectroscopy and quantum chemical modeling that redox processes involving elemental aluminum and Fe(III) in chloride solutions result in the formation of heteronuclear coordination compounds. Such compounds contain Fe(III)
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and Al(III); their composition is characterized by a general formula [FemAlnClk(OH)q]2m+3n-k-q, where the m, n, k, and q values depend on the concentration of components and solution pH. With the help of DFT calculations we have determined the most probable forms of the heteronuclear complexes. These findings were justified by the estimation of the model reaction heats and comparing the calculated IR spectra with experimental ones. Thus, a combined experimental and computational study makes it possible to gain a deeper insight into the structure of Fe(III)-Al(III) heteronuclear complexes. According to results reported in work [16] some heteropoly anions can reveal challenging barrier properties in interfacial electron transfer reactions. It would be promising to model the redox behaviour of Fe(III) and Al(III) containing heteronuclear coordination compounds in different reaction layers on the basis of quantum mechanical theory and molecular simulations (see, for example, review [17]). 5. Acknowledgment We are indebted to Prof. A.B. Remizov for the help with IR measurements. This work was supported in part by the RF Ministry of education and science under State contract no. 4.1584.2014/К . References [1] Y. Zhong, Q. Yang, X. Wu et al. Fe(II)-Al(III) layered double hydroxides prepared by ultrasound-assisted coprecipitation method for the reduction of bromate, J. Hazard. Mater. 250-251 (2013) 345-353. [2] A.F. Dresvyannikov, M.E. Kolpakov, Chemical synthesis of alpha-iron in aqueous FeCl3, Mat. Res. Bull. 37 (2002) 291-296. [3] C. Ruby, M. Usman, S. Naille, K. Hanna, C. Carteret, M. Mullet, M. François, M. Abdelmoula, Synthesis and transformation of iron-based layered double hydroxides, Appl. Clay Sci. 48 (2010) 195-202.
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[11] N. Sreedhara Murthy, S.P. Bagare, B. Narasimha Murthy, Dissociation energy for the ground state of AlO from true potential energy curve, J. Quant. Spectrosc. Radiat. Trans. 19 (1978) 455459. [12] J.K. McDonald, K.K. Innes A low-lying excited electronic state of the AlO molecule and the ground-state dissociation energy, J. Mol. Spectrosc. 32 (1969) 501-510. [13] D.L. Hildenbrand, Dissociation energies of the molecules AlO and Al2O, Chem. Phys. Lett. 20 (1973) 127-129. [14] G. Orlova, J.D. Goddard, Density functional theory predictions for small redicals containing boron and aluminium: broken symmetry problems and solutions, Mol. Phys. 98 (2000) 961-966. [15] I.D. Sorokina, A.F. Dresvyannikov, R.A. Yusupov, F.V. Devyatov, Heteronuclear compounds formed in the systems based on Fe(II), Fe(III), Al(III), SO42−, Cl− − H2O − OH−, and NH3, Russ. J. Appl. Chem. 83 (2010) 593-597. [16] M.I. Borzenko, R.R. Nazmutddinov, D.V. Glukhov, G.A. Tsirlina, M. Probst, Self-inhibition phenomena in the electroreduction of hexamolybdocobaltate(III): a combined experimental and computational study, Chem. Phys. 319 (2005) 200-209. [17] R.R. Nazmutdinov, M.D. Bronshtein, T.T. Zinkicheva, D.V. Glukhov, Modelling of electron transfer across electrochemical interfaces: State-of-the-art., Int. J. Quant. Chem., 116 (2016), 189201.
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Graphical abstract
[Fe2Al4(OН)15Cl3]
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