Introduction

Nontrivial complexing ability towards metal cations [1], especially alkaline and alkali earth metals (AEM), makes macrocyclic polyethers or crown ethers (CE) the promising key components in compositions applicable in the various fields of modern chemistry [2, 3]. Research activity in the thermochemical analysis of free CE and their complexes with metal salts have been stimulated by using this class of compounds in design of crown-ether containing ionic liquids [4,5,6], liquid crystals [7] and as metal oxide precursors in CVD (chemical vapor deposition) method [8,9,10,11,12]. The functionality of such materials strongly depends on the thermal stability of the macrocyclic complexes as a whole and, above all, on the ability of the macrocycle to withstand the ring opening under exposure of a relatively high temperature.

Macrocyclic complexes CE·Men+m·Am−n, in which Men+ and Am− presents mono- or multivalent metal cations and anions, respectively, are ternary systems. Their thermal stability should depend on the nature of constituents, namely, ligand, cation and anion, and a strength of CE⋅⋅⋅Men+ and CE·Men+⋅⋅⋅Am− interactions. Previously, thermolysis of macrocyclic complexes was studied quite intensively [10, 13,14,15]. However, the influence of cation and anion on the mechanism of their destruction remains unclear and difficult to predict. Thus, the effect of metal cation on the stability of the complexes of lithium, sodium and potassium salts with benzo-15-crown-5 (B15C5) and its nitro- and amino-substituted derivatives has been investigated [15]. Melting points (Tm) of the nitro-B15C5 complexes increased with a decrease in cation size: K+ (168 °C) < Na+ (188 °C) < Li+ (238 °C), but the temperatures of their thermal destruction (Tdestr) were practically the same: Li+ (272 °C) < Na+ = K+ (273 °C) [15]. It should be noted that the nature of anions (SCN− and Br−) in these compounds was varied. The decomposition temperature and Tm of 18C6 complexes with potassium (A = Br−, NO3−, OH−, ClO3−, BrO3−, OAc− etc.) [4, 6] and barium (A = Cl−, Br−, I−, NO3−) [8] salts have been determined, but the effect of the anion on these parameters has not been analyzed. The structure and thermal stability of macrocyclic complexes with AEM diketonates have been systematically investigated [8, 11, 12]. In particular, a series of 18C6 complexes with hexafluoroacetylacetonates of Ca2+, Sr2+, Ba2+ and Pb2+ has been synthesized and a relationship between their thermochemical behavior and chemical structure has been stated [11]. The melting temperatures of these compounds increased as follows: Ca2+ (173 °C) < Sr2+ (243 °C) < Ba2+ (256 °C), while the dependence of Tdestr on the size of the cation was less pronounced: Ca2+ (244 °C) ≈ Sr2+ (243 °C) < Ba2+ (261 °C). The authors have explained these trends by the effect of cation nature on a formation of “additional intermolecular contacts” under heating and conformational rearrangement of the 18C6 macrocycle, respectively. At the same time, the diketonate-anion involves in the coordination of the metal cation that can significantly influence the energy of the CE⋯Men+ interaction and contribute to the mechanism of the thermal destruction and the stability of the complexes.

In contrast to the “solid-state” studies, the effect of the cation nature on the stability of macrocyclic complexes in the gas phase and in solutions has been investigated in more details (energy of CE⋯Men+ interaction [16,17,18,19,20]; stability constants [21]). As a rule, the complexes with the best conformity of the cation diameter to the size of the macrocyclic cavity are most stable in polar solvents [1]. For example, the stability constants (logK) of 18C6 complexes with AEM salts in water changes as follows: Ca2+ (0.5) < Sr2+ (2.72) < Ba2+ (3.87) [21]. On the contrary, the calculated and experimental data obtained for "isolated" macrocyclic complexes in gas phase indicates the opposite trend—their stability increases with a decrease in the cation size [17,18,19, 22,23,24]. Such a difference relates to the competition for the metal cation between the macrocycle and the solvent at complexing in solution that is lacking in the case of "isolated", gas-phase complexes. Based on the above, the study of the thermal stability of macrocyclic complexes in the solid state is of particular interest, because of lacking the solvation effects similarly to gas-phase conditions. On the other hand, the anion included in the crystal lattice of the complex may impact on the energy of the macrocycle/cation interaction.

The complexes of 18C6 and dicyclohexano-18-crown-6 (DCH18C6) with AEM halides seem to be very suitable systems for investigation. The simplified structural formulas of 18C6 and stereoisomers of DCH18C6 are given in Supplementary Information. First, it is possible to “tune” CE⋯Men+ and CE·Men+⋯Am− interactions in these macrocyclic compounds. Halide anions are thermally inert, which avoids the contribution of anion destruction to the common mechanism of the thermolysis. These crown ethers have the equal size of the polyether ring; however, the presence of cyclohexane substituents in the structure of DCH18C6 and its stereochemical variety may have an impact on their thermal behavior. Accordingly, the aim of the present study was to synthesize the complexes of 18C6 and DCH18C6 with AEM halides and to study the effect of DCH18C6 stereoisomerism, metal cations and halide anions nature on their thermochemical behavior.

Experimental

Synthesis and characterization

Crystallohydrate salts CaCl2·2H2O (99%, Ecolan, Russia), SrCl2·6H2O (99.8%, Ecolan, Russia) and BaCl2·2H2O (> 99.7%, Ecolan, Russia) were preliminary dehydrated at 150 °C [25], 320 °C [26] and 116 °C [27], respectively, until constant mass. 18C6 (99%) and DCH18C6 (97%, mixture of stereoisomers) were purchased in Alfa Aesar company. Ethanol, chloroform and diethyl ether (reagent grade, Chimmed, Russia) were used as received.

The complexes of 18C6 with MeCl2 (Me = Ca2+, Sr2+, Ba2+) and SrHal2 (Hal = Br−, I−) were synthesized by dissolving equimolar amounts of crown ether and the corresponding dehydrated salt in ethanol. An illustrative example is the synthesis of the 18C6·CaCl2 complex. Dehydrated CaCl2 (0.21 g, 1.9 mmol) was added to a solution of 0.58 g (2.2 mmol) 18C6 in 9 mL of ethanol and the reaction mixture was stirred at ambient condition until complete dissolution of the salt. The resulting solution was filtered out and the solvent was evaporated at room temperature under stirring. The solid residue was washed three times with diethyl ether and after removing traces of solvent at reduced pressure (by a vacuum pump), a white polycrystalline powder was obtained (0.69 g, 98% yield relative to the initial amount of CaCl2).

Individual cis-syn-cis- and cis-anti-cis-isomers of DCH18C6 were isolated from a commercial stereoisomeric mixture using Izatt’s method [28]. Macrocyclic complexes of two individual stereoisomers DCH18C6 with MeCl2 (Me = Ca2+, Sr2+, Ba2+) were synthesized by addition of ten-fold molar excess of AEM chloride to CE solution in chloroform [29]. The detailed descriptions of the synthesis were given elsewhere [30].

The “free” crown ethers and the synthesized macrocyclic complexes were characterized by the 1H- and 13C-NMR, IR-spectroscopy and TGA/DSC (Supplementary Information, Fig. S1, S2, Table S1, S2). 1H- and 13C-NMR spectra were recorded on Bruker “Avance 600” spectrometer. The values of chemical shifts and the details of the experiment are given in Supplementary Information. FTIR spectra of 18C6, DCH18C6 and their complexes with AEM halides were measured by using a Nicolet iS50 FT-IR spectrometer with a resolution of 2 cm−1. The CE and their complexes (mass of 5–10 mg) were suspended in 2–3 drops of mineral oil in agate mortar to prepare a suspension for FTIR spectroscopy. This suspension was placed between KBr plates, and FTIR spectra were measured. The FTIR spectrum of mineral oil was preliminary measured and used as a background. The sample spectrum and that of mineral oil were normalized by using the intensity of the absorption band at 722 cm−1 (characteristic absorption band of exclusively mineral oil) and background spectrum was subtracted. The spectra of DCH18C6⋅BaC12 complexes were recorded in compressed CsI tablets.

Simultaneous thermogravimetric analysis and differential scanning calorimetry (STA/TG-DSC) were carried out on a NETZSCH STA 449 F3 apparatus. Samples (5–8 mg) were heated in Al2O3 pan with a vented cap or in Al closed pan in the range from room temperature to 500 °C at the heating rate of 10 °C min−1 under argon flow of 30 mL min−1. The temperature of the thermobalance was calibrated using indium (melting point 156.6 °C), Sn (231.9 °C), Bi (271.4 °C), Zn (419.5 °C) (for Al pan) and Al (660.3 °C) (for Al2O3 pans) standards from Netzsch DTA/DSC Calibration Set. The relative error of temperature measurement in the temperature range from 30 to 500 °C used in the present study did not exceed 1.5%. The balance resolution was 0.1 μg; the relative error of mass measurement was ± 1%. The thermobalance was calibrated by using mass standards from Netzsch DTA/DSC Calibration Set. All complexes were stored under vacuum before the TGA/DSC measurement.

Results and discussions

Effect of cation size on thermal stability of 18C6 and DCH18C6 complexes with AEM chlorides

To compare the thermal stability of the CE⋅MeCl2 complexes, the temperature of 5% mass loss of the samples (T5%) was taken as a quantitative criterion. The free macrocycles, 18C6 and the stereoisomers of DCH18C6, are measurably different in thermal behavior (Supplementary Information, Fig. S1), whereas their complexes with AEM chlorides demonstrate both the distinct and common features (Fig. 1). Relying on TGA and FTIR data, only the complex of 18C6 with BaCl2 was hydrated, while the other synthesized compounds were essentially anhydrous (Supplementary Information, Table S1, Fig. S2). The thermolysis of “barium” complex started with a mass loss by 7% in the range of 100–130 °C that, in combination with the appearance of a wide endothermic peak with a maximum at 128 °C on the DSC curve (Fig. 1, c, peak I; Supplementary Information, Table S1), allowed us to ascribe this stage to the dehydration of the complex. The presence of water in the molecule of 18C6⋅BaCl2⋅2H2O was supported by the absorption bands of OH-group stretching vibrations (3473 and 3325 cm−1) and H2O bending vibrations (1610 cm−1) in the FTIR spectra (Supplementary Information, Fig. S2). All other transformations illustrated by TGA/DSC curves on Fig. 1 were associated with thermochemical destruction of the anhydrous complexes. As a common feature, the main mass loss in all samples occurred in the temperature range from 200 to 430 °C and resulted from the cleavage of the Me⋅⋅⋅OCE coordination bonds, and the decomposition of the complexes into salt and CE followed by evaporation of the macrocycle. T5% for complexes of 18C6 and DCH18C6 stereoisomers with the same metal cation is noteworthy to be comparable (Fig. 1). Hence, one can conclude that the structure of the macrocycle, specifically, the presence of cyclohexyl groups in the 18-member polyether ring and the syn/anti stereoisomerism in the case of DCH18C6, has no noticeable effect on the thermal behavior of the complexes. Conversely, the AEM cation size impacts on the mechanism of destruction significantly. Thus, the T5% temperature was 242 °C (Ba2+) < 300 °C (Sr2+) < 314 °C (Ca2+) (Supplementary Information, Fig. S3) and 252 °C (Ba2+) < 302 °C (Sr2+) < 313 °C (Ca2+) for complexes of 18C6 and DCH18C6 (cis-syn-cis), respectively. This data implies that the stronger deviation of the cation diameter (Dcation) from the size of the macrocycle cavity increased the thermal stability of the complex. These geometric parameters are given in Supplementary Information, Table S3. More important, in our opinion, is that the single-stage destruction of the CE⋅SrCl2 and CE⋅BaCl2 complexes transforms to two-stage for CE⋅CaCl2 (Fig. 1a). The reason for this will be discussed below.

Fig. 1
figure 1

TGA/DSC analysis of CaCl2 (a), SrCl2 (b) and BaCl2 (c) complexes with 18C6 (orange lines), cis-syn-cis-DCH18C6 (black lines) and cis-anti-cis-DCH18C6 (navy blue lines). TGA results are given as solid lines, that of DSC—as dash lines. Endothermic peaks in the figure: I—dehydration, II—melting of the complex, III—destruction of the complex and evaporation of CE. The detailed interpretation of the peaks on the DSC curves is given in Table S1 of Supplementary Information

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From a formal point of view, the positive charge density of the metal cation and its polarizing effect on the polyether oxygen atoms increase in the Ba → Sr → Ca series. As a consequence, this should enhance the Me2+⋅⋅⋅OCE interaction. Such situation has been discussed in detail for gas phase isolated complexes [17, 18]. The calculated and experimental data of those studies showed that the energy of the Me2+⋅⋅⋅OCE interaction was higher for the small size cations. This was reflected in shortening Me2+⋅⋅⋅OCE bonds [17], a distortion in the symmetry of the CE and an increase in the probability of the coexistence of several stable conformers of the macrocycle [18] (Supplementary Information, Table S3). Similarly, the conformation of the macrocycle changes from flat, highly symmetrical one in Ba2+ complex to the folded, distorted one in that of Ca2+ in solid-state DCH18C6 and 18C6 complexes [30, 31] (Supplementary Information, Table S3). It can be expected that the conformational rearrangement of the macrocycle, by analogy with gas-phase conditions, was also induced by an increase of Me2+⋅⋅⋅OCE interaction with a decrease in the cation size in the solid state. Such changes in the structure of the macrocycle should be reflected in conformation-sensitive ranges of the FTIR spectra [32].

Actually, despite the general features in the spectra of 18C6⋅MeCl2 complexes, they have specific patterns at vibration frequencies of 1200–1000 cm−1 and 1000–900 cm−1 (Fig. 2a, b, e). In the first range, the most intense absorption bands with maxima at 1101 cm−1 (Ca2+), 1094 cm−1 (Sr2+) and 1095 cm−1 (Ba2+) relate to the asymmetric stretching vibrations of the COC unit (νas(COC)) [33]. The width of these bands on their half-height increases measurably from Ba2+ to Ca2+ that indicates a conformational nonequivalence of the COC fragments of the macrocycle for the latter cation. This finding is consistent with the published data [18].

Fig. 2
figure 2

FTIR spectra of 18C6·CaCl2 (a), 18C6·SrCl2 (b), 18C6·SrBr2 (c), 18C6·SrI2·0.5H2O (d), 18C6·BaCl2 ·2H2O (e). FTIR spectrum of 18C6 (f) is given as a comparison

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The second intensive band related to the rocking ρ(CH2) vibration [33,34,35] is observed in the spectra at 980 (Ca2+), 973 (Sr2+) and 967 (Ba2+) cm−1 (Supplementary Information, Table S2). A distinctive feature of 18C6·CaCl2 is the shoulder at 962 cm−1. Specifically, the singlet pattern of the absorption band in the range of the rocking vibration may be a marker of conformational equivalence of all six OCCO units of the 18-membered macrocycle [35] and its D3d symmetry, when all OCCO units ideally have a trans-gauche-trans conformation with torsion angles being close to 180° (trans) and 60° (gauche) [35, 36]. A classic example is the macrocycle with D3d symmetry in 18C6·KSCN complex, in which FTIR spectrum a sharp absorption band at 963 cm−1 has been observed [35]. In the spectra of the 18C6⋅MeCl2, the shift of the absorption band of ρ(CH2) vibrations to the high-frequency region against this value increases as follows: Ba2+ (4 cm−1) < Sr2+ (10 cm−1) < Ca2+ (17 cm−1). The frequency of the ρ(CH2) vibrations is extremely sensitive to changes in the –O–C–C–O– dihedral angle [36] and its shift to the high-frequency region can be caused by a decrease in the value of this angle for the smaller size Ca2+ cation. Meanwhile, the shoulder at 962 cm−1 in the spectrum of 18C6·CaCl2 suggests the nonequivalence of OCCO units with a pronounced difference in dihedral angles. Summing up, it can be concluded that a decrease in the diameter of the cation in 18C6⋅MeCl2 complexes leads to a distortion of the symmetry of the macrocycle as a result of reducing the dihedral angles (Ca2+, Sr2+) and increasing the nonequivalence of the OCCO units of the macrocycle (Ca2+). Moreover, the FTIR spectrum of 18C6⋅CaCl2 may indicate the occurrence of two macrocycle conformers in this compound, as has been predicted for isolated complexes [18].

A detailed analysis of the conformation-sensitive vibrational frequencies of the stereoisomeric complexes of DCH18C6 with CaCl2, SrCl2 and BaCl2 has been carried out previously [30]. For these compounds, as well as for 18C6 complexes, the same trend of narrowing the most intense band of asymmetric stretching vibrations νas(COC) in the order Ca2+  > Sr2+  > Ba2+ has been found [30] that is indicative of decrease in the conformational nonequivalence of the OCCO units of the macrocycle for larger cations. It has been assumed that the conformation of the macrocycle in the DCH18C6⋅MeCl2 complexes also changes from a rather disordered C2 to a symmetric D3d with an increase in the cation size from Ca2+ to Ba2+ (Supplementary Information, Table S1).

Thus, the TGA/DSC data in combination with the analysis of conformation-sensitive ranges of the FTIR spectra suggest that the increase in thermal stability of CE⋅CaCl2 complexes in comparison with CE⋅BaCl2 occurs due to enhanced interaction of unshared electron pairs of polyether oxygen atoms with a smaller size cation having a higher positive charge density. This result is quite unexpected, since one would expect the appearance of torsion stresses in the distorted macrocycle that will degrade its thermal stability. Apparently, the problem of the torsion stresses in the macrocycle may be counterbalance by the mobility of –O–CH2–CH2– units at elevated temperature.

Effect of anion size on thermal stability of 18C6 complexes with strontium halides

As far as we know, systematic studies of the effect of halide anions on the thermal stability of macrocyclic complexes have not been conducted to date. Meanwhile, it can be expected that the nature of the anion in the salt component is an important parameter. Actually, in the case of 18C6 complexes with SrHal2, where Hal−  = Cl−, Br−, I−, two main effects were found. First, a rather sharp increase in T5% occurred when chloride in strontium salt was substituted for bromide and iodide: Cl− (300 °C) < Br− (424 °C) and I− (402 °C) (Fig. 3; Supplementary Information, Fig. S4 and Table S1)). Second, the mechanism of the 18C6⋅SrHal2 decomposition changes from single-stage (for Cl−) to two-stage (for Br− and I−). In our opinion, this is direct evidence for the influence of the anion nature on the energy of the Men+⋅⋅⋅OCE interaction.

Fig. 3
figure 3

TGA/DSC analysis of 18C6·SrCl2 (orange lines), 18C6·SrBr2 (black lines), 18C6·SrI2·0.5H2O (navy blue lines) complexes. TGA results are given as solid lines, that of DSC—as dash lines. The detailed interpretation of the peaks on the DSC curves is given in Table S1 of Supplementary Information. (Color figure online)

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Indeed, an increase in the effective radius of the anion in the series of Cl− (181 pm) < Br− (196 pm) < I− (220 pm) [37] is accompanied by a decrease in charge density and a growth in polarizability [38]. This leads to more covalent nature of the bond between the cation and the anion in halides of alkaline and alkaline earth metals. As for the macrocyclic complexes, in which both crown ether and anions (OCE⋅⋅⋅Men+⋅⋅⋅nA−) are involved in cation coordination, only Knochel and coauthors [39] noted the higher stability of the complexes with highly polarizable anions, to the best of our knowledge. However, the reasons for this trend have not been discussed in details. On the other hand, the thermal stability of the crystallohydrates of AEM halides MeHal2·nH2O (n = 1 and 6) is known to depend on the polarizability of the anion and the nature of the cation [40,41,42]. The dehydration temperature of MeHal2·nH2O crystallohydrates grows with an increase in the anion size (Cl−  < Br−  < I−) and a decrease in the cation radius (Sr2+  < Ca2+) [40, 41]. According to the published X-ray and FTIR data, the anion in MeHal2·H2O monohydrates is bound to the cation through hydrogen bonds of the water molecule Me2+⋅⋅⋅ OH2 ⋅⋅⋅Hal−, and the hydrogen Hal⋅⋅⋅HOH bonds weaken with an increase in the size of the anion [43, 44]. Then, taking into account the dependence of the thermal stability of MeHal2·H2O on the anion size, it can be assumed that the elongation of Hal⋅⋅⋅HOH bonds would strengthen those of Me2+⋅⋅⋅OH2O. A similar situation may take place at thermal destruction of macrocyclic 18C6⋅SrHal2 complexes. With an increase in the size and polarizability of Hal−, the [Sr18C6]2+⋅⋅⋅Hal− bonds will elongate that may result in growth of the Sr2+⋅⋅⋅OCE interaction.

In a similar way to CE⋅MeCl2 complexes, a change in the energy of the cation interaction with the macrocycle in 18C6⋅SrHal2 should be reflected in the conformationally sensitive regions of FTIR spectra (1100–900 cm−1). Nevertheless, despite the difference in their decomposition temperatures by more than 100 °C (cf 18C6⋅SrCl2 and 18C6⋅SrBr2), the FTIR spectra appears to be almost identical in this range (Fig. 2b and c). The shifts of the maxima of the two intensive absorption bands at 1093 and 973 cm−1 (Cl−), 1094 and 974 cm−1 (Br−), 1097 and 969 cm−1 (I−) were very small (about 1–3 cm−1). The similarity of the FTIR spectra of 18C6⋅SrCl2 and 18C6⋅SrBr2 complexes in this range suggests the absence of a significant change in the conformation of the macrocycle, while the broadening of the absorption bands at 1097 and 969 cm−1 in the case of the 18C6 complex with SrI2 may be due to its hydration (Supplementary Information, Fig. S6, Sect. 8). The occurrence of only two intensive bands in the range from 1100 to 900 cm−1 and their position allow us to suppose that the conformation of the macrocycle in 18C6⋅SrCl2 and 18C6⋅SrBr2 was close to the slightly distorted D3d symmetry [35]. A less pronounced change in the FTIR spectra of 18C6⋅SrHal2 complexes, compared with those of 18C6⋅MeCl2, may be caused by a better matching between the macrocycle cavity size and the diameter of the Sr2+ cation as compared with Ca2+. So, a significant change in the conformation of the macrocycle in the 18C6⋅SrHal2 complexes is scarcely probable. On the other hand, similarly to the FTIR spectra of 18C6⋅MeCl2 complexes, the shift of the ρ(CH2) vibration band to the high-frequency region by 1 cm−1, when Cl− substituted for Br−, may imply a decrease in the OCCO dihedral angles due to an increase in Men+⋅⋅⋅OCE interaction.

Therefore, the anion size, by analogy with hexa- and monohydrates of AEM halides, may be a key factor of the thermal stability of the 18C6⋅SrHal2 complexes, since it defines the energies of [MeCE]2+⋅⋅⋅Hal−2 and Me2+⋅⋅⋅OCE interactions. At the same time, an increase in the polarizability of the anion from Br− to I− reduces this effect, which leads to a comparable T5% for the macrocyclic complexes with SrBr2 and SrI2 (Supplementary Information, Table S1).

Two- and one-step thermal degradation of 18C6 and DCH18C6 complexes

In accordance with TGA data, the ratio of CE:Me2+ in all studied complexes is 1:1 (Supplementary Information, Table S1). In light of this, one could expect the identity of the mechanism of their thermal destruction. However, macrocyclic complexes with CaCl2, SrBr2 and SrI2 decompose in two-stages process. In other words, a decrease in cation diameter (Ca2+) and an increase in the anion size (Br− and I−) resulted in a more complicated decomposition pattern.

Generally speaking, the examples of multistage decomposition of macrocyclic complexes under heating are known from the literature [3, 9, 14, 15, 45,46,47]. The reason was due to gradual dehydration and hydrolysis of the metal salt included in the complex [45,46,47], thermal destruction of the anion [9] or the decomposition of macrocycle [3, 14, 15]. In our opinion, none of the above reasons can explain the two-stage thermolysis of CE complexes with CaCl2, SrBr2 and SrI2. In particular, halide anions are thermally inert, while dehydration of 18C6⋅SrI2⋅0.5H2O occurs long before the main mass loss of the sample as a result of its decomposition into salt and CE (Fig. 3). Moreover, complexes of 18C6 and DCH18C6 with CaCl2 do not contain crystallohydrate water, which follows from the lacking the absorption bands of stretching (3400–3500 cm−1) and bending (1610–1650 cm−1) vibrations of water molecules in their FTIR spectra (Supplementary Information, Fig. S2). Previously, it has been also assumed that the second stage of the thermolysis of macrocyclic complexes was associated with the destruction of the macrocycle at high temperatures [3, 14, 15]. We found that this is not the case and another reason is responsible for the two-stage destruction of some complexes with AEM halides (CE⋅CaCl2, 18C6⋅SrBr2 and 18C6⋅SrI2⋅0.5H2O). This follows from the results of two TGA/DSC tests (Figs. 4, 5). In the first one, the 18C6⋅CaCl2 was isothermally annealed at 305 °C (Fig. 4). This temperature is significantly lower than that of the second stage of its decomposition (360–430 °C) caused, as previously thought [3, 15], by the destruction of the macrocycle itself. Accordingly, the probability of thermal degradation of 18C6 was negligible at 305 °C and complete decomposition of the complex in a single-stage process should have occurred. However, the curve of mass loss reached a plateau with a value significantly exceeding the mass fraction of CaCl2 (~ 40% instead of 29%) (Fig. 4, arrow B), indicating the significant part of the complex remained in undamaged state. An increase in the temperature (above 305 °C), after 120 min of thermal treatment, initiated a further mass loss to reproduce the two-stage character of 18C6⋅CaCl2 thermolysis.

Fig. 4
figure 4

The modes of thermal treatment (solid lines) and TGA data (dash lines) of 18C6·CaCl2 (1, 1') and 18C6·SrBr2 (2, 2'). Sampling the partially thermalized complexes for measuring of FTIR spectra was carried out at the time points indicated by A-D arrows

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Fig. 5
figure 5

TGA of 18C6·CaCl2: heating from 33 to 362 °C (1), cooling to 137 °C (2); reheating from 137 to 500 °C (3). TGA curve of the complex in the range from 33 to 500 °C is given for comparison (dash line)

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The second test included heating the 18C6⋅CaCl2 complex to 362 °C, cooling to 137 °C and reheating to 500 °C. It is illustrated by Fig. 5. The first heating (curve 1) was carried out at temperature below the expected moment of the “macrocycle destruction” (the start of the second stage, see Fig. 1a). Upon cooling the sample (curve 2) the thermolysis of the complex stopped, and reheating (curve 3) again initiates the destruction of 18C6⋅CaCl2. In this respect, if the second stage of the decomposition of the complex would be related to the destruction of 18C6, then a two-stage TGA curve should be reproduced during reheating. However, the curve 3 demonstrates a single-stage mass loss, in fact, identical to the TGA curve at the second stage of decomposition of the complex in the temperature range from 350 to 500 °C (Fig. 5, dash line). This suggests that the complicated destruction pattern in this case was associated with the decay of two form of 18C6⋅CaCl2 being identical in chemical composition, but markedly different in their thermal stability (forms I and II). The same reasons, in our opinion, were responsible for the two-stage decomposition of other complexes (18C6⋅SrBr2 and 18C6⋅SrI2), rather than the contribution of the macrocycle destruction.

Most probably, the difference in thermal stability of two forms of these complexes is caused by the conformational inhomogeneity of the 18C6 macrocycle resulted from variation in their energy of interaction with the metal cation. This should be reflected in FTIR spectra measured before and after isothermal annealing the samples (the sampling times are shown by arrows A-D in Fig. 4). The related spectra of the 18C6⋅CaCl2 and 18C6·SrBr2 are shown in Figs. 6 and 7 (the full-scale FTIR spectra are given in Supplementary Information, Fig. S10, S11). Actually, depending on the annealing time the spectra of the 18C6⋅CaCl2 differ markedly in the conformationally sensitive range (Fig. 6). As it was discussed above, the initial spectrum characterized by the broadened absorption band with maxima at 1101 cm−1 and slightly asymmetric one at 979 cm−1 with a shoulder at 962 cm−1, which supposes nonequivalence of the OCCO units in the macrocycle. In the FTIR spectrum of the more thermally stable residue (form II), a narrow absorption band at 1089 cm−1 (νas(COC)) and a single band at 969 cm −1 (r(CH2)) were observed. This can be interpreted as an increase in structural homogeneity of the OCCO units and, in particular, reducing the scatter of the torsion angle values in form II of the 18C6 macrocycle as compared to the initial, non-annealed state. Thus, the shoulder at 962 cm−1 and noticeable broadening the band of stretching C–O–C vibrations (~ 1100 cm−1) confirms the idea on the superposition of the signals from two forms with various conformations of the macrocycle in the FTIR spectrum of the initial 18C6⋅CaCl2 complex.

Fig. 6
figure 6

FTIR spectra of initial 18C6·CaCl2 complex (solid line), after 45 (dash line) and 70 (dash dot line) min of its thermal treatment. Sampling was done at time points shown with arrow A and B in Fig. 4

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Fig. 7
figure 7

FTIR spectra of initial 18C6·SrBr2 complex (solid line), after heating to 375 °C (dash line) and after 3 min of its isothermal annealing at 420 °C (dash dot line). Sampling was done at time points shown with arrow C and D in Fig. 4

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Isothermal annealing induced less pronounced changes in the FTIR spectra of the 18C6⋅SrBr2 complex (Fig. 7). The spectra before (initial complex) and after (form II) the thermal treatment are almost identical in the range from 1100 to 800 cm−1 (Fig. 7). As it was shown above, initially the 18C6 macrocycle has a conformation with the symmetry being close to D3d. Annealing of the sample at 420 °C resulted in the shift of the absorption bands in the FTIR spectrum to the low-frequency range by 1–2 cm−1, and the band at 1092 cm−1 became narrow. In other words, the differences in the spectral patterns of the initial 18C6⋅SrBr2 complex and its more thermally stable form were not so significant as in the case of the calcium complex. Nevertheless, the trends were similar. In our opinion, the narrowing and shift to the low-frequency region of the absorption band νas(COC) implies an increase in the structural equivalence of the –O–CH2–CH2– units and the stronger binding of the metal cation.

An additional support of the conformational inhomogeneity of the complexes follows from the complicated DSC curves (Figs. 1, 3) being typical for the polymorphic behavior [48, 49]. Indeed, it is possible to almost unambiguously interpret only relatively simple DSC patterns of the 18C6 and DCH18C6 complexes with BaCl2, where the endothermic peaks I, II and III correspond to dehydration, melting of the complex and evaporation of the macrocycle, respectively (Fig. 1). For all other studied compounds, several endothermic peaks were observed (Figs. 1 and 3), the most intense of which may be associated with the melting of the corresponding conformational form. The polymorphs differing in the conformation of the macrocycle [50, 51] or the orientation of the macrocycle in the crystal lattice [52, 53] (conformational polymorphism and packing polymorphism, respectively), in general, is known in macrocyclic chemistry. It has been also shown the occurrence of more than one 18C6 conformers in isolated, gas phase 18K6⋅⋅⋅Me2+ complexes with the strong discrepancy between the CE cavity size and the diameter of metal cation [18]. Intriguingly, that in case of 18C6⋅CaCl2 and 18C6⋅SrBr2 complexes in solid state, the form with a more ordered macrocyclic structure had a greater thermal stability, while in according with gas-phase studies, when diameter of the metal cation mismatches the size of the macrocycle cavity, the conformation of CE with a greater equivalence of –O–CH2–CH2– units is usually less energetically advantageous [16, 18, 19]. Apparently, in 18C6⋅CaCl2 and 18C6⋅SrBr2 complexes, the stabilization of form II occurs as a result of more favorable intermolecular interaction and better crystal packing, which is typical for most cases of conformational polymorphism [49].

Conclusions

The results of the present study highlighted the relationship between thermochemistry of the macrocyclic complexes and the energy of the interaction of the donor atoms with the metal cation coordinated in polyether cavity. An increase in this interaction, caused by varying the size of the AEM cation or halide anion, led to the growth in the temperature of complex destruction. Thus, the thermal stability of macrocyclic complexes (T5%) can serve to assess qualitatively the influence of the component nature on the binding energy in ternary OCE⋅⋅⋅Men+⋅⋅⋅nA− system. This is an important result for the development of the fundamental chemistry of macrocyclic compounds.

The thermal behavior of the macrocyclic complexes is also of interest from an applied point of view. On the one hand, the compounds studied in this work can be considered the models useful in design of chemical vapor deposition processes. The substitution of the sufficiently large organic anions, such as diketonates and acetonates, for that of halides allowed us, first of all, to obtain information on the effect of the negative charge density of the counterion on the thermal stability of the complex as a whole. This opens up the possibility of targeted development and synthesis of new precursors promising in the CVD processes, including for the preparation of multilayer coatings. We can assume that the revealed relationship between cation size/macrocycle conformation/thermal resistance will remain actual when using metal cations other than AEM. On the other hand, the data on the thermochemistry of macrocyclic complexes is highly demanded in separation science, particularly, in the development of new ligands/complexing agents selective with respect to metal cations. This is especially important, for example, in the separation and extraction of Sr and Cs radionuclides, which radioactive decay is accompanied with an intensive heat release. Accordingly, the macrocyclic ligand and its complex must be resistant not only to the effects of ionizing radiation, but also to elevated temperatures. The data presented in this paper convincingly indicates the absence of a macrocycle cleavage during thermal destruction of complexes in the studied temperature range, which is extremely important for their reuse in CVD processes and ensuring the necessary stability of macrocyclic systems in separation science.