ISSN 1063-7745, Crystallography Reports, 2015, Vol. 60, No. 6, pp. 853–859. © Pleiades Publishing, Inc., 2015. Original Russian Text © N.V. Somov, F.F. Chausov, R.M. Zakirova, M.A. Shumilova, V.A. Aleksandrov, V.G. Petrov, 2015, published in Kristallografiya, 2015, Vol. 60, No. 6, pp. 915–921.
STRUCTURE OF ORGANIC COMPOUNDS
Synthesis, Structure, and Properties of Nitrilo-tris(methylenephosphonato)-triaquairon(II) {Fe[μ-NH(CH2PO3H)3](H2O)3}, as an Ingredient of Anticorrosive Protective Coatings on the Steel Surface N. V. Somova*, F. F. Chausovb**, R. M. Zakirovab, M. A. Shumilovac, V. A. Aleksandrovc, and V. G. Petrovc** a
Lobachevsky State University of Nizhny Novgorod, pr. Gagarina 23, Nizhny Novgorod, 603950 Russia b Udmurt State University, ul. Universitetskaya 1, Izhevsk, 426034 Russia c Institute of Mechanics, Ural Branch, Russian Academy of Sciences, ul. Tat’yany Baramzinoi 34, Izhevsk, 426067 Russia *e-mail:
[email protected] **e-mail:
[email protected] ***e-mail:
[email protected] Received January 30, 2015
Abstract—A new non-electrolyte complex of iron(II), {Fe[μ-NH(CH2 PO3H)3](H2O)3}, has been synthesized and investigated. The crystallographic characteristics of the complex are as follows: sp. gr. P21/c, Z = 4, a = 9.2619(3) Å, b = 16.0548(3) Å, c = 9.7570(3) Å, and β = 115.685(4)°. The iron atom is octahedrally coordinated by the three phosphonate oxygen atoms and three water molecules in the meridional configuration. The complex has a coordination polymer structure; each Fe atom closes the eight-membered chelating cycle Fe–O–P–C–N–C–P–O, and one of the phosphorus atoms of this cycle is bound with an iron atom of a neighboring structural unit. DOI: 10.1134/S1063774515060334
INTRODUCTION The stabilization of low-stable oxidation states of transition metals is a fundamental problem of chemistry and materials science [1, 2]. This problem is of great practical importance for iron compounds, because annual corrosion loss is about 10–15% of the total amount of iron in circulation [3, 4]. The main corrosion loss for iron and steel is due to the reactions 2Fe0
+ 2H2O + O2 →
2Fe2+
+
4OH–
and 4Fe2+ + 2H2O + O2 → 4Fe3+ + 4OH–. The latter reaction leads to the formation of a loose layer of iron(III) compounds, which cannot protect the steel surface from corrosion. Therefore, it is an urgent problem to stabilize iron in the low-stable intermediate oxidation state (+2) in order to protect steelworks from corrosion. Soon after the development of production technologies of organic phosphonic acids [5], their derivatives were used as corrosion inhibitors [6, 7], although the data on the mechanism of their action had been
empirical for a long time. The mechanism of interaction of organophosphonate zinc complexes with steel surface in corrosive media was comprehensively investigated in [8, 9]. It was established that organophosphonate zinc complexes react with iron ions: ZnL + Fe2+ → Zn2+ + FeL, where L is an organophosphonate ligand. The main corrosion inhibition factors are (i) the formation of a protective layer of zinc hydroxo complexes, which shield the steel surface; (ii) coating of steel surface by a layer of phosphonate iron complexes; and (iii) modification of the phase structure of magnetite protective film by phosphonate complexes. Studies by potentiometry [10], optical spectroscopy and X-ray diffraction [11], and Mössbauer and X-ray photoelectron spectroscopies [12–15] confirmed the data of [8]. Some organic phosphonic acids and their derivatives are commercially produced as corrosion inhibitors. Along with being used as corrosion inhibitors, organic phosphonic acids and their derivatives are applied, in particular, in food industry [16, 17] as
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agents for precipitating iron by aggregating Fe2+ ions present in a solution into poorly soluble complexes. Complexes of iron(II) with methyl phosphonic and ethylene diphosphonic acids are of interest as lowdimensional (linear and layered) ferromagnetic structures [18, 19]. Despite the fact that organophosphonate iron complexes are of great practical importance, they have not been comprehensively investigated. In particular, the fundamental monograph on coordination chemistry [20] contains 1524 references to studies of coordination compounds of iron, and only two of them are related to compounds with organic phosphonic acids. The coordination compounds of iron in the most stable oxidation state (+3) have been studied more thoroughly. In particular, researchers described complexes with methyl phosphonic and amino methyl phosphonic acids [21]; the mixed-igand complex with methyl phosphonic and oxalic acids [22]; and polynuclear oxo-hydroxo complexes of iron(III), stabilized by pyridine and cyclohexene phosphonic [23] or tertbutyl phosphonic [24–26] acids. The structure of the complex of Fe3+ ion with ethylene diphosphonic acid is known [27]. This compound is a three-dimensional coordination polymer; the ligand in its structure is monoprotonated and pentadentate, and the sixth position in the coordination octahedron of iron(III) atom is occupied by a water molecule. Fe(III) forms a two-dimensional coordination polymer with methylene diphosphonic acid; some water molecules in its structure coordinate Fe3+ and the rest of the water is crystallization [28]. Phenyl phosphonic acid forms a one-dimensional coordination polymer with Fe3+ [29]. Complexes with Fe2+ and Fe3+, stabilized with, respectively, ethylenediammonium and diethylenetriammonium, were obtained for 1-hydroxy ethylidene diphosphonic acid [30]. Coordination compounds of iron(II) with 3-amino-1-hydroxy-propylidene-1,1diphosphonic and phosphono hydroacetic acids were described in [31] and [32, 33], respectively. A complex of iron(II) with phosphonomethyl iminodiacetic acid is a linear coordination polymer [34]. A complex of iron(II) with ethylphosphonic acid was obtained as a product of interaction of the latter (taken with double excess) with FeOCl [35]. A widespread corrosion inhibitor for steel and some other metals is nitrilo tris (methylene phosphonic) acid Н6N(CH2PO3)3 (NTP); therefore, the study of the structure and properties of coordination compounds of iron with NTP is an urgent problem. In this paper we describe the synthesis, crystal structure, thermal stability, and mechanism of thermal decomposition of nitrilo-tris-methylene-phosphonato-triaquairon(II) {Fe[μ-NH(CH2 PO3H)3] (H2O)3} (FeNTP).
EXPERIMENTAL Synthesis was performed using iron(II) sulfate hexahydrate of chemically pure grade (GOST (State Standard) 4148-78) and import NTP, purified by recrystallization to a content of phosphate ions of no more than 0.3%. A 0.025-mol aqueous solution of FeSO4 was added drop by drop to a solution of 0.025-mol NTP in 250 mL of water under constant intense stirring. White FeNTP precipitate was separated by filtration, washed on a filter, and dried at room temperature. FeNTP crystals for X-ray diffraction were grown in a gel of silicic acid [36, 37]. A mixture of a solution of 2.5 cm3 70% acetic acid in 10 cm3 water with an aqueous solution containing 5 cm3 Na2SiO3 solution with a density of 1.24 g/cm3 was placed in a U-shaped tube. After the formation of a silicic acid gel, 5 cm3 solution of 0.2 mol/dm3 FeSO4 were introduced into one knee of the tube and 5 cm3 solution of 0.2 mol/ dm3 NTP were introduced into the other knee. Colorless transparent FeNTP crystals from 0.1 to 2 mm in size were formed after 2 months. The crystals were mechanically separated, washed in water and then in ethanol, and dried at room temperature. The crystallographic characteristics, details of the X-ray experiment, and parameters of the structure refinement are listed in Table 1. The primary fragments of the FeNTP structure were found by the direct method; the positions of non-hydrogen atoms were determined from difference electron-density maps and refined in the anisotropic approximation. The positions of hydrogen atoms were determined from difference electron-density maps and refined independently in the isotropic approximation. The crystallographic data on FeNTP have been deposited with the Cambridge Structural Database (CCDC no. 1035265). IR absorption spectra of FeNTP and products of its decomposition were recorded with an FSM-1201 Fourier spectrometer in the range of 450–5000 cm–1 on pressed pellets containing 1 mg material under study per 250 mg KBr. Raman spectra of FeNTP crystals were recorded in the range of 470–570 nm with a Centaur U-HR microscope–microspectrometer using laser excitation at a wavelength of 473 nm. Thermogravimetric analysis of FeNTP in air and argon was performed on a Shimadzu DTG-60H automatic derivatograph in the temperature range of 30– 500°C at a heating rate of 3°C/min. RESULTS AND DISCUSSION Figure 1 shows a fragment of the FeNTP structure. The interatomic distances and bond angles are listed in Table 2.
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Table 1. Crystallographic characteristics, experimental details, and parameters of the {Fe[μ-NH(CH2 PO3H)3](H2O)3} structure refinement Monoclinic, P21/c, 4 9.2619(3), 16.0548(3), 9.7570(3) 115.685(4) 1307.49(7)
System, space group, Z a, b, c, Å β, deg V, Å3
2.067
D x, g/cm3 Radiation, λ, Å, monochromator
MoKα, 0.71073, graphite 1.583
μ, T, K mple size, mm Diffractometer; Scan mode Absorption correction, Tmin/Tmax θmax, deg Ranges of indices h, k, l Number of reflections: measured/unique (N1), R int/with I > 2σ(I) (N2) Refinement technique mm–1
293(2) 0.210 × 0.1244 × 0.065 Xcalibur, Sapphire3, Gemini Ω [38], 0.920/1 30.508 –13 ≤ h ≤ 13, –22 ≤ k ≤ 22, –13 ≤ l ≤ 13 27012/3963, 0.0162/3864 Full-matrix least-squares method on F2 245 0.0198/0.0514 0.0191/0.0511 1.103 –0.279/0.409
Number of parameters R1/wR 2 for N 1 R1/wR 2 for N 2 S Δρmin /Δρmax, e/ Å 3 Program packages
SHELX97 [39], WinGX [40]
The packing of formula units in a FeNTP crystal is a set of polymer chains parallel to the [001] direction, the intermolecular bonding of which is implemented via van der Waals interaction and hydrogen bonds. The ligand molecule in the complex retains the structure of zwitterion, which is characteristic of free NTP [41]. Due to the splitting off of two protons during FeNTP formation, each phosphonate ligand group contains two deprotonated oxygen atoms and one protonated atom; the nitrogen atom is also protonated. The P–O distance is 1.4971(8)–1.5175(8) Å for deprotonated oxygen atoms and 1.5627(9)– 1.5667(9) Å for protonated atoms. The O–P–C angle for deprotonated oxygen atoms (107.46(5)–110.30(5) °) deviates from the tetrahedral angle by no more than 2.01°. On the contrary, for protonated oxygen atoms, the HO–P–C angle (98.68(5)–104.47(5)°) is much smaller than tetrahedral. This is explained by the higher angular mobility of protonated oxygen atom in comparison with deprotonated ones and mutual repulsion of oxygen atoms of the PO3 group, which leads to deviation of the more mobile protonated O atom. All oxygen atoms in each phosphonate group are symmetrically nonequivalent. CRYSTALLOGRAPHY REPORTS
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O4
O6
O5
Fe1 Fe1*
O3
O9 O8
O4 P2 O3
O7
C3 C1
O2 P1
C2 N1 O10
O1
P3
O11
O12 Fig. 1. Fragment of the nitrilo-tris-methylene phosphonate-triaquairon(II) structure.
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Table 2. Interatomic distances d (Å) and bond angles ω (deg) in the {Fe[μ-NH(CH2 PO3H)3](H2O)3} structure Bond Fe1*–O3 Fe1*–O4 Fe1–O5 Fe1–O6 Fe1–O7 Fe1–O8 N1–C1 Angle O3–Fe1*–O4 O4–Fe1–O5 O4–Fe1–O6 O4–Fe1–O8 O7–Fe1–O8 O7–Fe1–O6 O7–Fe1–O5 O7–Fe1–O3 O3–Fe1–O5 O5–Fe1–O6 O6–Fe1–O8 O8–Fe1–O3 O3–Fe1–O6
d 2.1622(9) 2.0584(8) 2.1271(10) 2.1519(9) 2.0810(8) 2.2048(10) 1.5025(13) ω 88.08(3) 89.05(4) 87.12(4) 86.44(4) 91.03(4) 93.03(4) 93.51(4) 92.02(3) 89.91(5) 84.57(5) 85.56(4) 99.58(4) 172.74(4)
Bond
d
N1–C2 N1–C3 C1–P1 C2–P3 C3–P2 P1–O1 P1–O2 Angle O5–Fe1–O8 O4–Fe1–O7 C1–N1–C2 C1–N1–C3 C2–N1–C3 N1–C1–P1 N1–C2–P3 N1–C3–P2 O1–P1–C1 O2–P1–C1 O3–P1–C1 O4–P2–C3 O7–P2–C3
1.5074(13) 1.5117(13) 1.8273(10) 1.8315(11) 1.8291(11) 1.5020(8) 1.5627(9) ω 169.34(4) 177.44(4) 111.58(8) 114.16(8) 110.57(8) 115.20(7) 112.80(7) 117.99(7) 108.90(5) 98.68(5) 110.18(5) 109.36(5) 110.30(5)
Bond
d
P1–O3 P2–O4 P2–O7 P2–O9 P3–O10 P3–O11 P3–O12 Angle O9–P2–C3 O10–P3–C2 O11–P3–C2 O12–P3–C2 O1–P1–O3 O1–P1–O2 O2–P1–O3 O4–P2–O7 O4–P2–O9 O7–P2–O9 O10–P3–O11 O10–P3–O12 O11–P3–O12
1.5041(9) 1.4971(8) 1.5027(8) 1.5667(9) 1.5175(8) 1.5639(9) 1.5008(8) ω 100.24(5) 109.14(5) 104.47(5) 107.46(5) 115.05(5) 112.23(5) 110.55(6) 115.55(5) 110.97(5) 109.34(5) 107.04 (5) 115.32 (5) 112.84 (5)
*Symmetrically equivalent position: x, –y + 3/2, z + 1/2.
The FeNTP structure is somewhat similar to the previously studied structures of protonated nitrilotris-methylene phosphonate complexes of Cu(II) [42, 43] and Zn [43, 44]. The iron atom is coordinated by six oxygen atoms in the configuration of distorted octahedron (Fe–O = 2.0584(9)–2.2048(10) Å; O–Fe–O = 169.34(4)°– 177.44(4)° and 84.57(5)°–99.58(4)°). Coordination of Fe by deprotonated oxygen atoms of two phosphonate groups leads to the closing of the Fe–O–P–C–N–C–P–O chelating cycle. The second deprotonated oxygen atom of one of phosphonate groups involved in building the chelating cycle forms a bridge with the neighboring structural unit of the complex. Thus, FeNTP is a linear coordination polymer. The third phosphonate group of the ligand molecule is not involved in the coordination of metal atom. Thus, all three phosphonate groups in the ligand molecule are nonequivalent. The N–C–P (115.20(7)°– 117.99(7)°) and C–N–C (114.16(8)°) bond angles at the atoms entering the chelating cycle are much larger than the tetrahedral angle and the corresponding angles in methyl phosphonate groups that are not involved in the coordination of iron (N–C–P = 112.80(7)°, C–N–C = 110.57(8)°–111.58(8)°).
The three meridional positions in the coordination sphere of Fe atom are occupied by water molecules. The Fe–O(H2O) distance (2.1271(10)–2.2048(10) Å) is on average somewhat larger than the Fe–O(phosphonate) distance (2.0584(8)–2.1622(9) Å). The low symmetry of the FeNTP structure is confirmed by molecular vibrational spectra (Fig. 2). Most spectral lines disobey alternative forbiddenness. The nonequivalence of oxygen atoms of water molecules and phosphonate groups in the coordination environment of Fe manifests itself in splitting of the ν(Fe–O) band into a doublet with peaks at 578 and 590 cm–1. The ν(C–P) (728 cm–1) and ν(C–N) (768 cm–1) bands also disobey alternative forbiddenness. The nonequivalence of all phosphonate groups and oxygen atoms in these groups leads to the occurrence of a wide unresolved band at 1000–1250 cm–1 with a shoulder at 950 cm–1 in the IR spectrum. The Raman spectrum contains asymmetric vibrations with frequencies νas((Fe)O–P–O(Fe)) (950 cm–1), νas(O=P–O(Fe)) (1021 cm–1), ν(P–O) (1100 cm–1), and ν(P=O) (1206 cm–1). The band at 1430 cm–1 corresponds to δas(CH2) vibrations. There are also stretching vibration bands of hydrogen atoms at 2780 (N–H), 2860 (O–H), 2930 (νsCH2), 2960 (νasCH2), and 3020 (N–H) cm–1.
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T, % 100
I, counts/min 1000 1
80
800
60
600
40
400
2
20
0
500
200
1000 1500 2000 2500 3000 ν, cm1
0
Fig. 2. Molecular vibrational spectra of FeNTP: (1) IR absorption spectrum and (2) Raman spectrum.
Figure 3 shows a difference electron density map in the O5–O7–O8 cross section of the FeNTP crystal structure. One can see that the electron density distribution in the vicinity of the Fe1–O7 and Fe1–O4 bonds is approximately the same as near the Fe1–O5 and Fe1–O8 bonds; hence, it can be assumed that all bonds in the coordination environment of iron have approximately equal strengths. y, Å 4
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The equality of bond strengths in the coordination polyhedron of Fe significantly differentiates the FeNTP complex from the previously investigated CuNTP complex [42, 43]. In the NTP complex with Cu(II), the coordination copper polyhedron is extended due to the Jahn–Teller effect; as a result, the Cu–O bonds are weakened along the extension direction. For this reason, CuNTP is soluble in water. The FeNTP complex, the object of our study, is poorly soluble in water. Due to this, a protective anticorrosive layer is formed on the surface of iron and steel alloys when inhibiting corrosion by NTP and its derivatives. The thermogravimetric study of the mechanism of thermal decomposition of FeNTP (Fig. 4) also revealed a significant difference for this complex from the previously studied copper and zinc complexes [43], in which splitting off of the three water molecules coordinating the metal atom occurs in three separate stages in a temperature range of 50–150°C. Calcination of the FeNTP complex expands the splitting-off interval to 50–200°C. Dehydration occurs endothermally at 50–100°C and exothermally at 130–200°C. Above 100°C, water removal is accompanied by simultaneous iron oxidation, which manifests itself in the different weights of the samples calcinated in argon and in air. At 200–220°C, nitrogen is split off with a weak endothermic effect. At higher temperatures, iron oxidation is accompanied by heat release. The thermal effects are more pronounced when the process is carried out in argon, which can be explained by the lower thermal conductivity of this gas. Above 350–370°C, the thermal effect of the process becomes negative, because heat is spent on the evaporation of reaction products. m, % 100
O4
Q, rel. units 20
2 H8
0
H4
10
1
O8 H5
Fe1
H9
95
90 O7
2
0
4
10
85
2 P2
80 4 4
3 2
0
2
4 x, Å
75
Fig. 3. Difference electron density map of FeNTP crystal structure in the O5–O7–O8 plane. The solid and dashed curves are, respectively, isolines of positive and negative values of difference between the observed and calculated electron densities; the isoline step is 0.05 e/Å3. CRYSTALLOGRAPHY REPORTS
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100
200
300 t, °C
400
20 500
Fig. 4. FeNTP thermograms. Dependences of (1, 2) heat release Q and (3, 4) mass m on temperature t upon heating in (1, 3) argon and (2, 4) air. 2015
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The IR spectrum of FeNTP thermal decomposition products contains characteristic bands of iron(III) phosphates and phosphites [45, 46]: ν(Fe–O) (500– 550 cm–1), δ(P–O) (730–770 cm–1), ν(P–O) (947 см–1), δ(P–H) (1080 cm–1), ν(P=O) (1280 cm–1), and ν(P–H) (2340 cm–1). CONCLUSIONS Nitrilo-tris-(methylenephosphonato)-triaquairon(II) {Fe[μ-NH(CH2 PO3H)3](H2O)3}, which is an ingredient of anticorrosive protective coatings on the surface of iron and steel under corrosion inhibition by nitrilo tris(methylene phosphonic) acid, was synthesized, selected, and investigated for the first time. The ligand in this compound carries out two functions: it builds a Fe–O–P–C–N–C–P–O chelating cycle and simultaneously forms a coordination bridge with the Fe atom of neighboring structural unit. Due to this, FeNTP has the structure of coordination polymer. The structure of methylenephosphonato complex has low symmetry: all phosphonate groups in the ligand molecule and all oxygen atoms in each phosphonate group are nonequivalent. The oxygen atoms of water molecules and phosphonic groups in the environment of Fe atom are also nonequivalent. These differences manifest themselves in molecular vibrational spectra as the absence of alternative forbiddenness and splitting of the fundamental characteristic lines. Despite the low symmetry, the oxygen coordination environment of the Fe atom is sufficiently strong. Due to this, the complex is poorly soluble in water and sufficiently thermally stable to be used as an ingredient of protective anticorrosive layers. ACKNOWLEDGMENTS This study was supported by the Russian Foundation for Basic Research and the Government of the Udmurt Republic, project no. 13-02-96007, and carried out within the basic part of government contract for research no. 2014/134 (project code 2312). REFERENCES 1. J. Kleinberg, Unfamiliar Oxidation States and Their Stabilization (University of Kansas Press, Lawrence, 1950). 2. Yu. M. Kiselev and Yu. D. Tret’yakov, Usp. Khim. 68 (5), 401 (1999). 3. L. H. Bennett, J. Kruger, R. L. Parker, et al., Economic Effects of Metallic Corrosion in the United States. A report to the Congress by the National Bureau of Standards (Government Printing Office, Washington, DC, 1978). 4. Ya. M. Kolotyrkin, Metal and Corrosion (Metallurgiya, Moscow, 1985) [in Russian]. 5. B. Blaser and K.-H. Worms, Germany Patent No. 1082235 (1960).
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Translated by Yu. Sin’kov
