ISSN 1070-4272, Russian Journal of Applied Chemistry, 2014, Vol. 87, No. 8, pp. 1031−1037. © Pleiades Publishing, Ltd., 2014. Original Russian Text © F.F. Chausov, R.M. Zakirova, N.V. Somov, V.G. Petrov, V.A. Aleksandrov, M.A. Shumilova, E.A. Naimushina, I.N. Shabanova, 2014, published in Zhurnal Prikladnoi Khimii, 2014, Vol. 87, No. 8, pp. 1046−1052.
INORGANIC SYNTHESIS AND INDUSTRIAL INORGANIC CHEMISTRY
Thermal Stability and Thermal Decomposition Mechanism of Nitrilotris(Methylene Phosphonate) Complexes of Copper and Zinc with Varied Coordination F. F. Chausova, R. M. Zakirovaa, N. V. Somovb, V. G. Petrovc, V. A. Aleksandrovc, M. A. Shumilovac, E. A. Naimushinaa, and I. N. Shabanovad Udmurt State University, Izhevsk, Udmurtia, Russia Lobachevsky State University of Nizhni Novgorod, Nizhni Novgorod, Russia c Institute of Mechanics, Ural Branch, Russian Academy of Sciences, Izhevsk, Udmurtia, Russia d Physical-Technical Institute, Ural Branch, Russian Academy of Sciences, Izhevsk, Udmurtia, Russia e-mail:
[email protected] a
b
Received June 24, 2014
Abstract—Thermogravimetry, XPA, and ESCA were used to study the thermal stability and decomposition mechanism of [Cu(H2O)3μ-N(CH2PO3)3H4], [Zn(H2O)3μ-N(CH2PO3)3H4], Na8[CuN(CH2PO3)3]2·19H2O, and Na4[ZnN(CH2PO3)3]·13H2O in the atmosphere of air and argon. It was shown that the decomposition point, decomposition mechanism, and composition of the products being formed depend on the composition and structure of coordination compounds, and for Na8[CuN(CH2PO3)3]2·19H2O и Na4[ZnN(CH2PO3)3]·13H2O, also on the composition of the atmosphere. The stability of the complexes is affected by the configuration of the coordination polyhedron and by the electron density distribution in the coordination environment of a metal. The complex Na4[ZnN(CH2PO3)3] has the highest thermal stability in both air and argon (onset of decomposition at about 400°C). DOI: 10.1134/S1070427214080047
Complexes of d-elements with nitrilotris(methylenephosphonic acid) N(CH2PO3)3H6 (NTP) are widely industrially used as corrosion inhibitors for ferrous [1–3] and nonferrous [4, 5] metals, bactericides [6], and salt deposition inhibitors [7, 8]. Introduction of these metalcomplex inhibitors into aqueous media makes it possible to many times reduce the corrosion rate and prolong the service life of metallic technological apparatus in oil-andgas extracting industries, power engineering, metallurgy, and chemical industry. Until quite recently, only mixtures of products formed in the interaction of metal ions with NTP, which include complexes with various degrees of protonation and varied coordination, have been obtained and studied in laboratory and industrial conditions. The complex of zinc with a protonated NTP, [Zn(H2O)3μ-N(CH2PO3)3H4], was isolated in the pure form in [9]. This complex is a
coordination polymer with bridging NTP anions. Tests of this compound as a corrosion inhibitor have shown that the oxygen corrosion inhibition factor for carbon steel in an aqueous medium is 2.7. One of authors of the present study has synthesized a chelate complex Na4[ZnN(CH2PO3)3]·13H2O that demonstrated in similar tests a higher corrosion inhibition factor of 14.3. Thus, the structure-selective synthesis of nitrilotris(methylene phosphonate) complexes opened up ways to improve the efficiency of metal-complex corrosion inhibitors based on compounds of both zinc and other d-metals and, in particular, copper. However, the practical use of the inhibitors is only possible when these substances sustain a prolonged keeping at high temperatures, e.g., in power boilers at 200°C and more. In these conditions, oxygen is frequently present in the aqueous medium. Therefore, it is topical for practical purposes not only to
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raise the corrosion inhibition factor, but also to improve the thermal stability of compounds of the given class. The goal of our study was to examine the thermal stability and the thermal decomposition mechanism of copper and zinc complexes with NTP. EXPERIMENTAL The coordination compounds under study were obtained with twice-recrystallized NTP free of admixtures of phosphoric and phosphorous acids, zinc oxide [GOST (State Standard) 10262–73, analytically pure grade], basic copper carbonate (GOST 8927–79, analytically pure grade), and sodium hydroxide (GOST 4328–77, chemically pure grade). [Zn(H 2 O) 3 μ-N(CH 2 PO 3 ) 3 H 4 ] and Na 4 [ZnN– (CH2PO3)3]·13H2O were synthesized by the methods described in [9, 10], respectively. To obtain [Cu(H2O)3μ-N(CH2PO3)3H4], an aqueous solution of 0.1 M of NTP was added dropwise under continuous stirring to an aqueous suspension of 0.1 mol of finely ground basic copper carbonate, heated to 70–80°C, and the mixture was stirred at the same temperature for 2 h. The resulting transparent greenish-blue solution was filtered, cooled to room temperature, and crystallized at a gradual evaporation of water. The resulting crystals were washed with a mixture of water with ethanol (11 : 9 by mass), cooled to 10°C. The product had the form of light green monoclinic crystals. To obtain Na8[CuN(CH2PO3)3]2·19H2O, an aqueous solution of 0.4 mol of NaOH was added dropwise at a temperature of 30–35°C under continuous stirring to an aqueous solution of [Cu(H2O)3μ-N(CH2PO3)3H4], prepared as described above, and the mixture was stirred at the same temperature for 2 h. The resulting transparent bluish-green solution was filtered, cooled to room temperature, and crystallized at a slow evaporation of water. The resulting crystals were washed with a mixture of water with ethanol (11 : 9 by mass), cooled to 10°C. The product had the form of dark green monoclinic crystals. The structure of the resulting compounds was determined by X-ray diffraction analysis on an Oxford Diffraction Gemini S automated four-circle diffractometer with a Sapphire III CCD detector and MoKα radiation and a graphite monochromator in the Ω scanning mode. The results were processed using CrysAlisPro 1.171.36.21 software (Agilent Technologies), with the absorption taken into account by the Multi-scan empirical method.
The primary structural fragments were found by SHELX 97-2 direct method. The positions of nonhydrogen atoms were found by the differential electron-density synthesis and refined by the least-squares method for F2 in the SHELX 97-2 software package with WinGX program [11]. The final refinement was made using the computational weighting scheme w = [σ2(Fо2) + (0.0199P)2 + 0.2181P]–1, where P = (Fо2 + 2Fc2)/3. To study [Cu(H2O)3μ-N(CH2PO3)3H4], we chose a visually transparent 0.57 × 0.33 × 0.26 mm single crystal. We measured 25 872 reflections (–13 ≤ h ≤ 13, –22 ≤ k ≤ 22, –13 ≤ l ≤ 13), of which 3919 were independent and 3851 significant (with I > 2σI). The factor RInt for equivalent reflections was 0.0185. The positions of hydrogen atoms were found from the differential electrondensity synthesis. The final values of the factors were R1 = 0.0197, wR1 = 0.051 for the significant reflections and R1 = 0.0202, wR1 = 0.0513 for all the reflections. The residual electron density was –0.321–0.414 e/Å3. The CIF file containing the structural information was deposited in CCDC 977616. To study Na8[CuN(CH2PO3)3]2·19H2O, we chose a visually transparent 0.55 × 0.23 × 0.17 mm single crystal. We measured 40 954 reflections (–15 ≤ h ≤ 15, –24 ≤ k ≤ 24, –16 ≤ l ≤ 16), of which 6373 were independent and 5861 significant (with I > 2σI). The factor RInt for equivalent reflections was 0.0297. The positions of only six hydrogen atoms bonded to carbons could be determined from the differential electron-density synthesis. The coordinates of hydrogen atoms in water molecules were found by the Nardelli method [12]; it was impossible to determine the hydrogen atom positions for two crystallization water molecules. The final values of the factors were R1 = 0.0361, wR1 = 0.0931 for the significant reflections and R1 = 0.0402, wR1 = 0.0955 for all the reflections. The residual electron density was –0.852–1.417 e/Å3. The CIF file containing the structural information was deposited in CCDC 981005. A thermogravimetric analysis of the compounds we synthesized was made in the atmosphere of air and argon with a Shimadzu DTG-60H automated derivatograph in the temperature range 30–500°C at a heating rate of 3 deg min–1. An X-ray phase analysis (XPA) of the thermal decomposition products was made on an a DRON-6 general-purpose diffractometer with CoKα radiation. To obtain X-tray photoelectron spectra, the crystalline products ground to a particle size of 1–3 μm were sprayed
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as a thin solid layer onto a substrate wetted with a mixture of water with ethanol. The spectra were measured with an electron spectrometer with double focusing by magnetic field [13] under excitation with AlKα radiation. This device made it possible to obtain electron spectra in situ for a sample subjected to a temperature treatment. RESULTS AND DISCUSSION The structure of the compounds we obtained is shown in Fig. 1. It can be seen that the complexes [Cu(H 2 O) 3 μ-N(CH 2 PO 3 ) 3 H 4 ] and [Zn(H 2 O) 3 μN(CH 2PO 3) 3H 4] are isostructural. The same refers to the complexes Na8[CuN(CH2PO3)3]2·19H2O and
Cu1*
7
P2
12 7 1 С1
С3 N2 С2 6 P2
С1
3
С3
10
9
O8
Na4[ZnN–(CH2PO3)3]·13H2O. The geometric configurations of the coordination sphere of the metal atoms in the complexes under study are different. In the complexes [Cu(H2O)3μ-N(CH2PO3)3H4] and [Zn(H2O)3μ-N(CH2– PO3)3H4], the metal atom is coordinated with six oxygen atoms at vertices of a distorted octahedron. In this case, the difference between the distances to oxygen atoms in the environment of zinc does not exceed 3%, and the coordination octahedron of copper is elongated by 16% due to the Jahn–Teller effect [14]. In the complexes Na8[CuN(CH2PO3)3]2·19H2O and Na4[ZnN(CH2– PO3)3]·13H2O, the coordination environment of the metal atom has the configuration of a distorted trigonal bipyramid, with the nitrogen atom of the ligand at one P1 2 С2
11
P2
5
Cu1
7*
1
4
2
5
2
9 7
Cu1 Cu1*
3
O8
P3
(b)
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7 Zn1
P2
N1 С2
P1
3 O8 С
P3 11*
10
С3
7
1
С1 3
P3
4
11 7*
O8
P1
9
Cu1
N1
6
7*
(a)
Zn1*
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5
С2
N1
С1 2
1
P1
9 3
Zn1
6 5 P2
4
Zn1*
4*
6 1 (c)
(d)
Fig. 1. Structure of the copper and zinc complexes under study. (a) [Cu(H2O)3μ-N(CH2PO3)3H4], (b) Na8[CuN(CH2PO3)3]2·19H2O, (c) [Zn(H2O)3μ-N(CH2PO3)3H4], and (d) Na4[ZnN(CH2PO3)3]·13H2O. The ions of sodium and molecules of crystallization water are dropped for clarity. RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 87 No. 8 2014
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m, %
m, %
Q, rel. units
T, °C
Q, rel. units
T, °C
Fig. 2. Thermograms of the complexes (1) [Zn(H 2O)3μN(CH2PO3)3H4] and (2) Na4[ZnN(CH2PO3)3]·13H2O in the atmosphere of argon. (m) Sample mass, (Q) heat effect, and (T) temperature; the same for Figs. 3 and 4.
Fig. 3. Ther mograms of the complex [Cu(H 2 O) 3 μN(CH2PO3)3H4] in (1) argon and (2) air.
of its vertices and oxygen atoms at another vertex and at base. The difference of the distances between the zinc atom and oxygen atoms is within the range 3.6%, and that between the copper atom and oxygen atoms reaches a value of 16% because of the axial compression of the trigonal bipyramid due to the Jahn–Teller effect. For the complexes [Zn(H2O)3μ-N(CH2PO3)3H4] and Na4[ZnN(CH2PO3)3]·13H2O, the thermograms obtained in air hardly differ from those recorded in the atmosphere of argon (Fig. 2). This is indicative of a high resistance of nitrilotris(methylene phosphonate) complexes of zinc to the effect of atmospheric oxygen. The thermogram of the compound [Zn(H2O) 3μN(CH2–PO3)3H4] shows three discernible endothermic effect at 130, 140, and 150°C. The total loss of mass in this temperature range corresponds to abstraction of three molecules of coordination water. The weakly pronounced endothermic effect in the temperature range accompanies the loss of a nitrogen atom. The exothermic in a wide temperature range 250–280°C is not accompanied by loss of mass and presumably corresponds to an internal rearrangement and possibly to an internal redox process. The XPA of thermal decomposition products reveals phases of zinc peroxide ZnO 2, zinc metaphosphate Zn(PO3)2, and graphite. Endothermic processes above 360°C, accompanied by the loss of mass, presumably
include the oxidation of graphite by zinc peroxide to give CO and CO2. The thermogram of the complex Na4[ZnN(CH2PO3)3]·13H2O contains a strong endothermic effect at 40–110°C, accompanied by the loss of 11 molecules of crystallization water; the loss of the remaining two water molecules occurs in a wide temperature range up to 330°C. An exothermic effect accompanied by the loss of a nitrogen atom is observed in the interval 410–430°C. The release of heat continues with a lower intensity and without loss of mass up to a temperature of 470°C, which corresponds to internal redox reactions. ZnO2, zinc orthophosphate Zn3(PO4)2·4H2O, zinc diphosphate Zn2P2O7·5H2O, and zinc pyrophosphate ZnH2P2O5, as well as sodium carbonate Na2CO3 and trace amounts of graphite are found in the thermal decomposition products. The copper complexes under study are isostructural with the above-described complexes of zinc; despite that, the manners of their thermal decomposition strongly differ. The thermograms of nitrilotris(methylene phosphonate) complexes of copper, recorded in air, differ from those recorded in the atmosphere of argon, which points to the interaction of copper complexes with atmospheric oxygen. Figure 3 shows the thermograms of the complex [Cu(H2O)3μ-N(CH2PO3)3H4]. The loss of three molecules
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of coordination water in both air and atmosphere of argon is accompanied by three separate endothermic effects at 80, 120, and 140°C. The abstraction of the first water molecule (O11 in Fig. 1a) occurs at a lower temperature than that for the complex of zinc because of the increase in the interatomic distance and the weakening of the Cu1–O11 bond due to the manifestation of the Jahn–Teller effect. The abstraction of the nitrogen atom at around 320°C entails a strong exothermic effect, rather than a weak endothermic effect (as in the case of decomposition of the zinc complex). A possible reason is that the decomposition of NTP with loss of a nitrogen atom leads to an increase in the number of species and, according to the entropy theory [15], makes lower the free energy of the dissociated state. For the zinc complex, this change in the entropy is compensated for by the enthalpy of ligand binding with the zinc atom, whereas in the copper complex, the metal–ligand bonding is weakened by the Jahn–Teller effect, and just this circumstance entails a fast exothermic decomposition of the complex. A strong exothermic effect corresponding to the oxidation of the organic constituents is manifested in air at a temperature of 380°C. In the case of heating in the atmosphere of argon, this effect does not occur, and a weak exothermic effect is manifested at around 400°C. The products of thermal decomposition of [Cu(H2O)3μ-N(CH2–PO3)3H4] in both air and atmosphere of argon are X-ray amorphous. The thermograms of the complex Na 8 [CuN(CH 2 PO 3 ) 3 ] 2 ·19H 2 O, recorded in air and argon, are shown in Fig. 4. In both air and argon, the crystallization water is fully eliminated in the range 40– 80°C. At a temperature of 260°C, a nitrogen molecule is abstracted, which is accompanied by a minor exothermic effect, presumably because internal stresses are relieved in the coordination sphere of the copper atom. Heating in air in the range 290–320°C leads to a strong exothermic effect due to the decomposition of the coordination sphere of the copper atom. A less intense release of heat in the temperature range up to 360°C is presumably indicative of the oxidation of organic products formed in the destruction of the ligand by atmospheric oxygen. In the atmosphere of argon at 380°C, there occurs “explosive” decomposition of the coordination compounds, accompanied by a strong exothermic effect. A small fluctuation of mass in the exothermic decomposition of a sample is presumably caused by the influence of ascending and descending flows of argon. The XPA of the products formed in the thermal decomposition of the complex demonstrates the presence of copper dihydrophosphate CuHPO4·H2O, as
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m, % Q, rel. units
20
10
10 500 T, °C Fig. 4. Thermograms of the complex Na8[CuN(CH2PO3)3]2·19H2O in (1) argon and (2) air.
well as that of sodium carbonate and pyrophosphate and trace amounts of graphite. It can be concluded that the highest thermal stability is observed for the complex Na4[ZnN(CH2–PO3)3]·13H2O, which sustains without being decomposed (aside from the loss of crystallization water) heating to 410°C in air and argon. According to Tschugaeff’s rule of cycles [16], this is accounted for by the structure of the complex, which includes three closed five-member rings with a common Zn–N bond. The higher stability of the zinc complexes, compared with the copper complex of a similar structure, is accounted for by the electronic configuration d10 inherent in Zn 2+. This configuration has a weaker dependence on the valence angle in comparison with the electronic configuration d9, inherent in Cu2+, for which the configuration of the trigonal bipyramid is untypical and unstable. Of interest in this context is a study of the thermal decomposition mechanism of the complex ion [ZnN(CH2PO3)3]4–. The C1s X-ray photoelectron spectra of the complex Na4[ZnN(CH2PO3)3]·13H2O on a Cu substrate, obtained at various temperatures, are shown in Fig. 5. Broadened asymmetric peaks corresponding to values of the binding energy Eb of C1s can be observed in the spectra. Gaussian decomposition of these curves can distinguish four components whose intensity varies with temperature. The component peaked at a binding energy of 283.6 eV is related to the C–C bond of carbon atoms
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thermogram. The pronounced contribution of C–N and C–P bonds at Eb = 286.8 eV evidences that the nitrogencarbon-phosphorus skeleton of the molecule is preserved. Upon further heating to 400–450°C, the contribution of C–N and C–P bonds decreases, which demonstrates that nitrogen is abstracted and the skeleton of the molecule decomposes. The remaining contribution of C–O bonds (Eb = 288.7 eV) presumably corresponds to the formation of sodium carbonate. CONCLUSIO NS (1) Among the compounds studied, Na4[ZnN(CH2PO3)3]·13H2O has the highest thermal stability. Its decomposition occurs above 400°C. The disintegration of the complex [Cu(H2O)3μ-N(CH2PO3)3H4] occurs at 320°C, the complexes [Zn(H 2 O) 3 μN(CH 2 PO 3 ) 3 H 4 ] and Na 8 [CuN(CH 2 PO 3 ) 3 ] 2 ·19H 2 O decompose at about 280°C. (2) Water and nitrogen are gaseous products formed in decomposition of the compounds studied; the solid residue formed in decomposition of zinc complexes contains zinc peroxide and phosphates, sodium carbonate, and graphite; copper complexes decompose to copper hydrophosphate, sodium pyrophosphate and carbonate, and graphite. ACKNOWLEDGMENTS
Eb, eV Fig. 5. C1s X-ray photoelectron spectra of the complex Na4[ZnN(CH2PO3)3]·13H2O on a Cu-substrate at temperatures of (1) 30, (2) 170, (3) 250, (4) 400, and (5) 450°C. (Eb) Electron binding energy.
The study was financially supported by the Russian Foundation for Basic Research and the Government of the Udmurt Republic (joint project no. 13-02-96007). REFERENCES
and in all probability belongs to insignificant impurities contained in a preparation or to those introduced when fabricating a sample. On heating the sample to 170°C, this component disappears. The main components of the C1s spectrum of the complex under study are related to C–H bonds (Eb = 285 eV) and also to C–N and C–P bonds (Eb = 286.6 eV). The component peaked at a binding energy of 288.7 eV is associated with C–O bonds, which are absent in the structure of the pure inhibitor; their presence can be attributed to an admixture of oxidation products. Upon heating to 250°C, the contribution from C–H bonds disappears, which indicates that the inhibitor starts to decompose with abstraction of hydrogen. This structural transformation is not reflected on the
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