Zinc complexes with nitrilotris(methylenephosphonic) acid N(CH2PO3)3H6 (NTP) are effective corrosion inhibitors in neutral aqueous media [1, 2] and are widely used in that capacity in industry. Zinc nitrilotris(methylenephosphonic) acid complexes of different structure differ in the anticorrosion activity under equal other conditions. The most effective corrosion inhibitor is the complex Na4[ZnN(CH2PO3)3]·13H2O with the chelate structure of the inner coordination sphere [3]. Copper nitrilotris(methylenephosphonic) acid complexes, including the chelate complex Na8[CuN(CH2PO3)3]2·19H2O [4], are known as bactericides acting, in particular, against sulfate-reducing bacteria.

Because Cu(II) and Zn(II) can form in many cases complexes of similar structure, it is interesting to prepare heterometallic complexes Na4[(Cu,Zn)N(CH2PO3)3] and study complexes in the concentration substitution series Na4[CuxZn1–xN(CH2PO3)3] (0 < x < 1). It has been shown that heterometallic copper–zinc complexes form a fully isomorphous series of mixed crystals Na4[CuxZn1–xN(CH2PO3)3]·13H2O (0 < x < 1), including isomorphous end members Na4[ZnN(CH2PO3)3]·13H2O and Na4[CuN(CH2PO3)3]·13H2O [5]. The copper content of the crystalline product is virtually proportional to the copper concentration in the solution, suggesting no significant energy barriers to mutual substitution of metal atoms in the crystal [5]. This fact opens prospects for the commercial preparation of uniform batches of crystalline copper–zinc nitrilotris(methylenephosphonic) acid complexes with the preset Cu : Zn ratio and structure ensuring the best anticorrosion properties.

Favorable combination of anticorrosion and bactericidal properties can be expected for such crystalline compounds, which can give significant economic benefit when using them in heat engineering and water recycling systems, cooling systems with cooling towers, and systems for collecting stratal water and maintaining stratal pressure at oil and gas fields. The use of coordination compounds as corrosion inhibitors in such systems is associated with different temperature conditions of storage and dosage. Hence, to reach the maximum possible performance of these inhibitors, it is necessary to know the features of their thermal behavior, including the decomposition onset temperature and the mechanism and products of thermal decomposition of complexes of different composition.

The decomposition temperature of monometallic copper and zinc nitrilotris(methylenephosphonic) acid complexes depends on the geometric structure of the coordination polyhedron of the metal atom [6]. Monometallic copper and zinc chelates with fully deprotonated nitrilotris(methylenephosphonic) acid are dimeric in the crystal. The coordination polyhedron of the metal atom is a distorted trigonal bipyramid with the metal atom in the center, three oxygen atoms of different PO3 groups of the nitrilotris(methylenephosphonic) acid molecule in the basal positions, the nitrogen atom of the same ligand molecule in one of the apical positions, and the oxygen atom of one of PO3 groups of the adjacent nitrilotris(methylenephosphonic) acid molecule in the dimer in the other apical position [2]. The distortion pattern of the trigonal bipyramid in the zinc and copper complexes is different: In the zinc complex, the coordination polyhedron is elongated along the Zn–N bond [2], and in the copper complex, along one of the Cu–O bonds in the base of the trigonal bipyramid [4].

Because the thermal stability and thermal decomposition mechanism of nitrilotris(methylenephosphonic) acid complexes depend on the structure of the coordination polyhedron, the thermochemical behavior of heterometallic complexes Na4[CuxZn1–xN(CH2PO3)3]·13H2O will apparently differ from that of the end members of the isostructural series, Na4[ZnN(CH2PO3)3]·13H2O and Na4[CuN(CH2PO3)3]·13H2O. In the isomorphous substitution series Na4[CuxZn1–xN(CH2PO3)3]·13H2O, the distortion pattern varies gradually as x is increased from 0 to 1. As a result, the distortions compensate each other to certain extent, and the coordination polyhedron becomes the closest to the regular trigonal bipyramid at a certain intermediate value of x [5] (Fig. 1). An interesting structural feature of heterometallic complexes Na4[CuxZn1–xN(CH2PO3)3]·13H2O is that the oxygen atoms O7, O8, and O9 of one of the phosphonate groups in the ligand molecule are disordered over sites А and B, whereas in the position of the oxygen atoms of the other two phosphonate groups there is no disordering. In the structure of the monometallic zinc and copper complexes, none of the phosphonate groups are disordered. The main distances in the coordination surrounding of the metal atom are given in Table 1.

Fig. 1.
figure 1

Dimeric structure of the complex anion [CuxZn1–xN(CH2PO3)3]28– in the structure of heterometallic zinc and copper complexes with nitrilotris(methylenephosphonic) acid, according to data of single crystal X-ray diffraction analysis [5]. The atoms occupying the symmetrically equivalent position –x, –y, –z are marked with an asterisk.

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This study deals with the thermochemical behavior of heterometallic complexes in the concentration substitution series Na4[CuxZn1–xN(CH2PO3)3]·13H2O.

Table 1. Selected interatomic distances d, Å, in compounds of the series Na4[CuxZn1–xN(CH2PO3)3]·13H2O at x = 0–1, according to data of single crystal X-ray diffraction analysis [5]
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EXPERIMENTAL

Nitrilotris(methylenephosphonic) acid (pure grade, Wuhan Mulei New Material Co., China) was recrystallized twice before use to reduce the PO43– content below 0.3%. ZnO (analytically pure grade, Vekton, Russia), Cu2CO3(OH)2 (analytically pure grade, Vekton), NaOH (chemically pure grade, Bashkir Soda Company, Russia), dimethyl sulfoxide (chemically pure grade, Kupavnareaktiv, Russia), ethylenediaminetetraacetic acid disodium salt (Na2EDTA, chemically pure grade, Reakhim, Russia), 4-(2-pyridylazo)resorcinol (PAR) indicator (analytically pure grade, Tatkhimprodukt, Russia), Eriochrome Black T indicator (analytically pure grade, Tatkhimprodukt), Na2S2O4 (EKOS-Ural, Russia), and gaseous argon (supreme grade, Technical Gases, Russia) were used without additional purification.

The complex Na4[ZnN(CH2PO3)3]·13H2O was synthesized as described previously [2].

The complex Na4[CuN(CH2PO3)3]·13H2O was prepared by the reaction of Cu2CO3(OH)2 with nitrilotris(methylenephosphonic) acid and NaOH according to [5].

Na4[CuxZn1–xN(CH2PO3)3]·13H2O heterometallic complexes were prepared by dissolving weighed portions of the complexes Na4[ZnN(CH2PO3)3]·13H2O and Na4[CuN(CH2PO3)3]·13H2O, taken in the ratio corresponding to the chosen value of x, in a small amount of water. The resulting solution was allowed to stand for 24 h, equal volume of dimethyl sulfoxide was added, and crystals of the mixed complex were grown by slow evaporation of the solvent at room temperature. Heterometallic complexes Na4[CuxZn1–xN(CH2PO3)3]·13H2O are transparent triclinic crystals of the color from light yellowish-green (at low values of x) to bright green (at x close to unity).

Elemental analysis of the products was performed by the complexometric method similar to that described previously for quantitative determination of copper and nickel in heterometallic complexes with nitrilotris(methylenephosphonic) acid [7]. The crystalline samples were broken down by boiling with an acidified (NH4)2S2O8 solution. The Cu2+ + Zn2+ sum was determined by titration with Na2EDTA at pH 5.0–6.0 in the presence of 4-(2-pyridylazo)resorcinol indicator. The Zn2+ content was determined after complete precipitation of copper with excess Na2S2O4 [8, 9] by titration with Na2EDTA at pH 8.0–9.0 in the presence of Eriochrome Black T indicator. The Cu2+ content was determined by calculation. The elemental analysis results are given in Table 2.

Table 2. Results of elemental analysis of crystalline compounds Na4[ZnN(CH2PO3)3]·13H2O (ZnNTP), Na4[CuxZn1–xN(CH2PO3)3]·13H2O (CuxZn1–xNTP), and Na4[CuN(CH2PO3)3]·13H2O (CuNTP)
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The results of single crystal X-ray diffraction analysis have been filed at the Cambridge Crystallographic Data Centre (CCDC) [10]. VESTA 3.5.7 program was used for imaging of molecular structures determined by X-ray diffraction analysis.

Thermal gravimetric and differential thermal analysis of the crystalline products was performed on a Shimadzu DTG-60H automatic derivatograph in an argon atmosphere in the temperature interval 30–500°С at a heating rate of 3 deg min–1.

The X-ray photoelectron spectra were recorded with an EMS-3 automatic X-ray photoelectron spectrometer (Udmurt Federal Research Center, Ural Branch, Russian Academy of Sciences) using AlKα radiation (hν = 1486.6 eV). The samples were rubbed in the moist surface of a pyrolytic graphite support and immediately placed into the working chamber of the spectrometer. The residual pressure in the working chamber of the spectrometer did not exceed 10–5 Pa. The energy scale was calibrated with respect to the C1s peak assuming EB(C1s) = 285 eV. We recorded the core level Cu2p, Cu3s, P2p, Zn3s, O1s, and N1s spectra and the valence band (EB = 0–30 eV) spectra. The samples were heated in situ (in the working chamber of the spectrometer) in the temperature interval 100–450°C using the built-in heating attachment.

Statistical processing of the experimental data, including determination of the measurement uncertainty, Shirley subtraction of the background from inelastically scattered electrons [11], and determination of the intensity of separate spectrum lines, was performed with Fityk 0.9.8 program.

RESULTS AND DISCUSSION

Thermal gravimetric and differential thermal analysis of all the complexes (Fig. 2) reveals a strong endothermic effect in a wide temperature interval 40–200°С with the heat absorption maximum at 64°С, corresponding to the loss of 11 water molecules. For the complex Na4[Cu¼Zn¾N(CH2PO3)3]·13H2O in the interval 280–340°С, the weight loss corresponding to ½NH3 without thermal effect is observed. The temperature dependence of the integral intensity of the N1s X-ray photoelectron spectrum (Fig. 3) confirms the loss of approximately ½N (half of nitrilotris(methylenephosphonic) acid molecules) at approximately 300°С.

Fig. 2.
figure 2

Thermograms of the heterometallic complexes Na4[CuxZn1–xN(CH2PO3)3]·13H2O (CuxZn1–xNTP) at x = 1/4, 1/2, and 3/4 in comparison with those of the complexes Na4[ZnN(CH2PO3)3]·13H2O (ZnNTP) and Na4[CuN(CH2PO3)3]·13H2O (CuNTP) [6].

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Fig. 3.
figure 3

Integral intensity of the N1s X-ray photoelectron spectrum of the complexes Na4[CuxZn1–xN(CH2PO3)3]·13H2O at x = 1/4, 1/2, and 3/4 as a function of temperature.

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In the interval 390–440°C, there is an exothermic effect with the maximum at 420°C and the weight loss corresponding to a methanol molecule. The exothermic effect in the interval 440–490°C, manifested as a shoulder in the Q(T) curve, corresponds to the loss of ½NH3 and is accompanied by a decrease in the integral intensity of the N1s X-ray photoelectron spectrum almost to zero.

Thermal decomposition of the complex Na4[Cu½Zn½N(CH2PO3)3]·13H2O in the interval 250–290°С is characterized by an exothermic effect with the maximum at 266°С without weight loss. The integral intensity of the N1s X-ray photoelectron spectrum does not noticeably decrease in this temperature interval. The exothermic effect in the interval 300–408°С with the maximum at 385°С corresponds to the loss of nitrogen in the form of NH3 and agrees with a sharp decrease in the integral intensity of the N1s X-ray photoelectron spectrum in this temperature interval. The exothermic effect in the interval 408–420°С with the maximum at 413°С is accompanied by a slight increase in the sample weight. This may be due to the uptake of a small amount of water from air.

Thermal decomposition of the complex Na4[Cu¾Zn¼N(CH2PO3)3]·13H2O is accompanied by two pronounced exothermic effects with the maxima at 230 and 260°С and no weight loss. They correspond to the rearrangement of the internal molecular structure. In the interval 280–350°С, there is an exothermic effect with the maximum at 335°С and loss of (H2O + ½NH3), and in the interval 350–375°С, an exothermic effect with the maximum at 365°С and loss of ½NH3. The integral intensity of the N1s X-ray photoelectron spectrum correspondingly decreases in these intervals. The exothermic effect in the interval 375–390°С with the maximum at 378°С and small (about ¼H2O) weight gain is due to the water uptake from air.

Thus, the thermal stability of the compounds studied can be characterized both by the temperature at which the molecular structure starts to change and by the temperature at which the sample starts to lose weight through elimination of gaseous products (Fig. 4).

Fig. 4.
figure 4

Temperatures of the (1, 2) onset of changes in the molecular structure of the complexes Na4[CuxZn1–xN(CH2PO3)3]·13H2O and (3, 4) weight loss onset as functions of the Cu mole fraction x.

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Analysis of the X-ray photoelectron spectra of the initial complexes Na4[CuxZn1–xN(CH2PO3)3]·13H2O at x = ¼, ½, and ¾ (Fig. 5) shows that all the phosphorus atoms are in the equivalent chemical state (are bound to the metal atom via oxygen atom of the PO3 group); this follows from the presence of only a single peak in the P2p photoelectron spectrum with the maximum at the binding energy EB = 133.5–133.7 eV. In the Zn3s photoelectron spectrum, there is also only one peak with EB = 139.7–139.8 eV. The Cu2p photoelectron spectrum has a complex structure characteristic of transition metals with the incompletely filled 3d shell. It contains a typical Cu2p³⁄²–Cu2p½ spin–orbit doublet with the binding energies of the constituents EB = 932.5–933.0 and 952.8–953.2 eV, respectively. Each constituent of the Cu2p doublet has several strong satellites suggesting differences in the electronic structure of the coordination surrounding of the copper atom in the complexes with x = ¼, ½, and ¾. The intensity of the spectrum lines corresponding to the atoms of the complex-forming metals varies in proportion with the copper fraction x in the complexes.

Fig. 5.
figure 5

Fragments of the X-ray photoelectron spectra [(a) P2p and Zn3s; (b) Cu2p] of the complexes Na4[CuxZn1–xN(CH2PO3)3]·13H2O at x = (1) 1/4, (2) 1/2, (3) 1/4 at 120°C and (4) of decomposition products of the complex Na4[Cu½Zn½N(CH2PO3)3]·13H2O at 475°C.

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Thermal decomposition leads to changes in the structure of the complexes Na4[CuxZn1–xN(CH2PO3)3]·13H2O (Fig. 5, curve 4). The P2p spectrum becomes noticeably broadened, which indicates that the nearest surrounding of the phosphorus atoms in the decomposition products is nonequivalent. The intensity maximum in the Zn3s spectrum is shifted toward higher binding energies (EB = 140.7–139.8 eV), which indicates that the electron density is shifted from the zinc atom toward oxygen atoms in its surrounding, i.e., that the Zn–O bond becomes more ionic. The constituents of the Cu2p spin–orbit doublet also become broadened, and the intensity of their satellites appreciably increases.

The observed differences in the decomposition onset temperatures and mechanisms of thermal decomposition of heterometallic complexes Na4[CuxZn1–xN(CH2PO3)3]·13H2O and monometallic Na4[ZnN(CH2PO3)3]·13H2O and Na4[CuN(CH2PO3)3]·13H2O, shown most clearly in Fig. 4, can be rationalized with the following assumptions:

– the nitrilotris(methylenephosphonic) acid molecule coordinated by the Cu atom decomposes more readily (at a lower temperature) than that coordinated by the Zn atom;

– the inner coordination sphere of the metals in all the examined crystalline complexes contains two nitrilotris(methylenephosphonic) acid molecules, each of which is coordinated to either Cu or Zn atom.

Thus, the inner coordination sphere of each structural unit in the crystalline complexes Na4[CuxZn1–xN(CH2PO3)3]·13H2O can be presented by one of the following structures (Scheme 1).

Scheme
scheme 1

1.

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The crystal structure of the monometallic complexes Na4[ZnN(CH2PO3)3]·13H2O and Na4[CuN(CH2PO3)3]·13H2O, studied previously [8], is built of clusters of only one type, (1) and (3), respectively. Therefore, the thermal decomposition mechanism is relatively simple, and the characteristic temperature points in Fig. 4 coincide.

The replacement of ¼ of the zinc atoms in the structure of Na4[ZnN(CH2PO3)3]·13H2O leads to the structure described by the formula Na4[Cu¼Zn¾N(CH2PO3)3]·13H2O. This formula can correspond both to a combination of ¾ structural unit (1) with ¼ structural unit (3) and to a combination of ½ structural unit (1) with ½ structural unit (2); more complex structures indiscernible by X-ray diffraction analysis are also possible. The fact that the thermal decomposition of Na4[Cu¼Zn¾N(CH2PO3)3]·13H2O in the interval 280–340°С is accompanied by the loss of ½NH3 counts in favor of a combination of ½ structural unit (1) with ½ structural unit (2). Clusters (2) with the more strained structure are probably the constituents that decompose in the interval 280–340°С.

The bond dissociation energy determined by electron impact mass spectrometry, according to the published data, is 226 kJ mol–1 for the C–N bond in the triethylamine (N(CH3)3) molecule [12] and 477 kJ mol–1 for the C–P bond in the methylphosphine (H2PCH3) molecule [13]. Hence, in thermal decomposition of the ligand molecule, the C–N bond should be expected to be cleaved first. This agrees with the fact that, when heated to 100°C for 9 days in the presence of excess Cu2+ ions, nitrilotris(methylenephosphonic) acid decomposed with the formation of iminobis(methylenephosphonic) acid zwitterion H2N+(CH2PO3H2)(CH2PO3H–), which was confirmed by the X-ray diffraction analysis of the copper complex [14, 15].

We believe that the C–N bonds in the nitrilotris(methylenephosphonic) acid molecule are cleaved by the hydrolytic mechanism Scheme 2.

Scheme
scheme 2

2.

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The methanol elimination in the interval 390–440°C probably occurs in accordance with the scheme

HOCH2P(O)O2M + H2O → MHPO4 + CH3OH↑.

The exothermic effect in the interval 440–490°C corresponds to decomposition of clusters (1) in accordance with Scheme 3. The transformation of the complex Na4[Cu½Zn½N(CH2PO3)3]·13H2O in the temperature interval 250–290°С without weight changes can be described, in our opinion, by the following scheme involving formation of a heteroligand complex (Scheme 3), which agrees with the C–N bond cleavage and formation of iminobis(methylenephosphonic) acid according to [14, 15]. The nitrogen loss in the interval 300–408°С is probably due to thermal decomposition of the heteroligand complex (Scheme 4).

Scheme
scheme 3

3.

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Scheme
scheme 4

4.

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Scheme
scheme 5

5.

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The thermal stability of all the complexes studied (Fig. 4) is the lower, the larger is the difference between the interatomic distances (Table 1) in the base of the coordination polyhedron (trigonal bipyramid) of the metal atom, i.e., between the longest distance d(M1–O1) and the shortest distances d(M1–O9А) and d(M1–O5). This fact suggests that the main cause of cleavage of the ligand molecule is the Baeyer strain in chelate rings.

Differences in the interatomic distances in the surrounding of the copper atom (Table 1) agree also with differences in the satellite structure observed in the Cu2p electron spectra of the corresponding complexes (Fig. 5b) [16, 17]. However, the mechanism of the formation of the corresponding satellites in the spectra is not fully understood [18], which complicates structural-chemical interpretation of the spectrum features and requires further studies. Thermal decomposition leads to changes in the satellite structure, which becomes close to that in the spectrum of Cu3(PO4)2 [16, 17]. As we showed previously [6], the monometallic complex Na4[ZnN(CH2PO3)3]·13H2O decomposes on heating to zinc peroxides and phosphates, and the complex Na4[CuN(CH2PO3)3]·13H2O, to copper hydrophosphate and sodium pyrophosphate. The same compounds are probably formed in different ratios upon decomposition of heterometallic complexes Na4[CuxZn1–xN(CH2PO3)3]·13H2O.

CONCLUSION

Combination of thermal gravimetric and differential thermal analysis and X-ray photoelectron spectroscopy with the thermal action in situ furnishes new information on the thermochemical behavior and mechanism of thermal decomposition of heterometallic complexes Na4[CuxZn1–xN(CH2PO3)3]·13H2O (0 < x < 1) compared to the results of single crystal X-ray diffraction analysis.

In particular, water of crystallization is eliminated from all the complexes studied in a wide temperature interval, 40–200°С. The complexes Na4[Cu¼Zn¾N(CH2PO3)3]·13H2O and Na4[Cu¾Zn¼N(CH2PO3)3]·13H2O decompose at approximately 280°С in two steps in accordance with differences between the complex anions containing monometallic and heterometallic M–O–M–O rings. The complex Na4[Cu½Zn½N(CH2PO3)3]·13H2O starts to decompose in the interval 250–290°С with an exothermic transformation without elimination of volatile products; the loss of nitrogen in the form of ammonia occurs at approximately 300°С. The mechanism of the thermal decomposition of Na4[Cu¾Zn¼N(CH2PO3)3]·13H2O is the most complex. It involves two exothermic transformations and two steps of the decomposition of intermediates with the loss of nitrogen in the form of ammonia. The complex Na4[Cu¼Zn¾N(CH2PO3)3]·13H2O is characterized by the highest decomposition onset temperature (280°С), and the complex Na4[Cu½Zn½N(CH2PO3)3]·13H2O, by the highest weight loss onset temperature (300°С).