Influence of alkyl substituents in 1,3-diethyl-2-thiobarbituric acid on the coordination environment in M(H2O)2(1,3-diethyl-2-thiobarbiturate)2 M = Ca2+, Sr2+

Abstract Two new isostructural complexes, [Ca(H2O)2(μ2-Detba-O,O′)2]n (1) and [Sr(H2O)2(μ2-Detba-O,O′)2]n (2) (HDetba = 1,3-diethyl-2-thiobarbituric acid), were synthesized and characterized by single-crystal and powder X-ray diffraction analysis, TG-DSC, FT-IR, and emission spectroscopy. The single-crystal X-ray diffraction data revealed that 1 and 2 are polymeric where M2+ (M = Ca, Sr) is a six-coordinate octahedral binding four Detba− ions and two water molecules. The octahedra are linked through bridging Detba− ions forming a 2-D layer. Two intermolecular hydrogen bonds O–H…S in the crystal form a 3-D net. The comparison of M(Detba)2 and M(Htba)2 (H2tba = 2-thiobarbituric acid) structures showed that the coordination number of metals in M(Detba)2 does not exceed six and there are no π–π interactions, unlike compounds with Htba−; Detba− ions are only bridges in HDetba coordination compounds. Thermal decomposition of 1 and 2 includes dehydration, which mainly ends at 200 °C, and organic ligand oxidation at 300–350 °C with a release of CO2, SO2, H2O, NH3, and isocyanate. Upon excitation at 220 nm, 1 and 2 exhibit an intense emission maximum at 557 nm.


Introduction
The s-block element coordination polymers have attracted attention because of their intriguing topology and promising applications in catalysis, adsorption (gas storage), and separation technologies [1][2][3][4][5][6]. The chemistry of coordination polymers based on group 2 metals with organic linkers is relatively less explored in comparison with the chemistry of d-elements. Mg(II) and Ca(II) compounds play a vital role in the human body [7], and Sr(II) reduces the risk of a bone fracture from osteoporosis [8]. These factors make the synthesis and evaluation of coordination polymer networks with alkaline earth metal cations a strategic challenge [5].
Anal. Calcd for C 16

X-ray diffraction analysis
The intensity patterns were collected from single crystals of 1 and 2 at 25 °C using the SMART APeX II X-ray single-crystal diffractometer (Bruker AXS) equipped with a CCD-detector, graphite monochromator, and Mo Kα radiation. The absorption corrections were applied using SADABS. The structures were solved by direct methods using SHelXS and refined in the anisotropic approach for non-hydrogen atoms using the SHelXl program [34]. All hydrogens of Detba − ligands in 1 and 2 were positioned geometrically as riding on their parent atoms with d(C-H) = 0.93 Å for the C5-H5 bond, d(C-H) = 0.97 Å for all other C-H bonds, and U iso (H) = 1.2U eq (C). All hydrogens of H 2 O molecules were found via Fourier difference maps and refined with bond length restraints only. The structural tests for the presence of missing symmetry elements and possible voids were produced using the PlATON program [35]. DIAMOND was used for plotting the crystal structure [36].
Powder X-ray diffraction data were obtained using a D8 ADVANCe diffractometer (Bruker) equipped by a VANTeC detector with a Ni filter. The measurements were carried out using Cu Kα radiation. The structural parameters defined by single-crystal analysis were used as a base for the powder pattern Rietveld refinement. The refinement was performed using TOPAS 4.2 [37]. The low R-factors and good refinement results shown in figures 1S and 2S indicate the crystal structure of the powder samples to be representative of 1 and 2 bulk structures, respectively.

Physical measurements
TGA characteristics were recorded on the simultaneous SDT-Q600 thermal analyzer (TA Instruments, USA) under dynamic air atmosphere at 50 ml min −1 flow rate from 25 to 800 °C at 10 °C min −1 . The sample weight was 7.268 mg for 1 and 6.348 mg for 2. Platinum crucibles with perforated lids were used as the sample containers. The qualitative composition of the evolved gas was determined by FT-IR spectrometer Nicolet380 (Thermo Scientific, USA) combined with a thermal analyzer and with the TGA/ FT-IR interface (homemade attachment for the gas phase analysis). This setup allows one to receive the DTA and TG data simultaneously, and the released gas phase composition. The temporal dependence of the optical density for each of the released gasses was obtained from IR spectra.
The IR absorption spectra of the compounds were recorded from 400 to 4000 cm −1 at room temperature on a VeCTOR 22 Fourier spectrometer (Bruker, Germany). The spectral resolution during the measurements was 5 cm −1 . The photoluminescence spectra (Pl) of air-dried samples were taken using a spectrofluorimeter SDl-2 (lOMO ltd, Russia) at room temperature, under identical conditions, and the intensity of the bands can be directly compared.

Crystal structures of 1 and 2
The main structural characteristics of 1 and 2 are shown in table 1. The main defined bond lengths of only one structurally resolved enol form of HDetba [38,39]

Comparison of HDetba complex structures
All HDetba coordination compounds with metals are polymers, and bridging Detba − ions exhibit various coordination types (table 2) (table 2). Previously, it was shown that the Detba − in coordination compounds can exist in two different conformational states: conformer (A) with a large value of torsion angle C8-C7-C9-C10 (150°); conformer (B) with a small value of this angle (0…12°) (table 1S). Compounds of MDetba with big M + ions, ion radii are equal to or bigger than K + , tend to form conformers (B) because they can be packed closer [29,30]. The present investigation shows that 1 and 2 have conformational state (A), in agreement with the assumption.

Comparison of M(Detba) 2 and M(Htba) 2 structures
The coordination number of metals in M(Detba) 2 does not exceed six and there are no π-π interactions between the Detba − rings unlike compounds with Htba − . Changing Htba − to the more volumetric Detba − ligand leads to a decrease of coordination number from 7 to 6 for Ca(II) and from 8 or 9 to 6 for Sr(II) [22]. Thus, the difference between the central ion size becomes less significant.

IR spectroscopy
The similarity of IR spectra of 1 and 2 confirms that these compounds are isostructural. The following bands were found in the IR absorption spectrum of 1, as shown in figure , 1622 vs, 1406 vs, 1319 s, 1309 s, 1260 m, 1204 m, 1158w, 1108 m, 1086 w, 1052 w, 964 w, 913  w, 835 w, 802 m, 773 w, 728 w, 698 w, 662 w, 638 w, 511 m, 475 m, 436 m, 422 m, and 405 m. IR spectra of 1 and 2 drastically differ from IR spectra of HDetba ( figure 4S, curve 1). The broad band at 3600-3300 cm −1 with its maximum at 3365-3360 cm −1 in IR spectra of 1 and 2, which is absent in the spectrum of HDetba, corresponds to ν(OH) of coordinated water. IR spectra of 1 and 2 contain two very strong bands at 1622-1621 and 1406-1401 cm −1 , as shown in figure 4S, from C-O stretches. For pure Hdetba, the ν(CO) bands are located at 1646 and 1521 cm −1 . The difference is consistent with ligand coordination through oxygens. The strong band at 1158 cm −1 in the IR spectrum of HDetba, which, by analogy with H 2 tba [38,39], can be attributed to ν(C=S), is comparatively weak in IR spectra of 1 and 2, affected by incorporation of sulfur in intermolecular H-bonds O-H…S. Therefore, the results of IR spectroscopy are consistent with the structural data.

Thermal decomposition
In 1, mass loss occurs at 150 °C ( Figure 5S) and 150-200 °C with mass loss (Δm) of 7.80%. This value is close to the theoretical value Δm = 7.59% calculated under the supposition that two water molecules are deleted from the compound. According to IR analysis of flue gases, this compound loses water vapor. The dehydration corresponds to the endoeffect at 197.0 °C. The sample weight remains practically unchanged from 200 to 350 °C. Oxidation of the organic ligand from 350 to 450 °C leads to a drastic reduction in the sample mass (Δm = 56.3%) accompanied by release of CO 2 , SO 2 , H 2 O, NH 3 , and isocyanate ( Figure 6S). Strong exothermic peak is measured on the DSC curve at 426.2 °C. On further heating, the weight changes become constant (Δm = 83.4%) at 750 °C. Two endoeffects at 547.3 and 712.7 °C with a CO 2 gas release are observed during transformation from 500 to 750 °C. As in the case of 2-thiobarbituric acid with Ca(II) [22], CaSO 4 and CaO are likely to be the final products of the thermolysis of 1, that does not contradict to the measured value Δm.
In 2, a small weight loss (Δm = 0.4%)) was observed at 100 °C. At higher temperatures, the endoeffect is found at 162.4 °C ( Figure 7S). The total weight loss at 162.4 °C was 8.80%, exceeding the theoretically calculated Δm for removal of two water molecules from the compound (Δm = 6.90%); water vapor is the only gaseous product released according to IR spectroscopic data ( Figure 8S). excessive weight loss during the thermal decomposition of 2 at 162.4 °C may be associated with the substance hygroscopicity and, also, with a possible HDetba sublimation and impurity decomposition accompanied by releasing gases which were not identified by IR spectroscopy. The sample weight decrease is only 2.4% under heating from 200 to 300 °C. The oxidation of the organic part occurred at 300-450 °C, and the sample weight drastically decreases (Δm = 55.3%). Two exothermic effects at 402.8 and 420.7 °C and a release of CO 2 , SO 2 , H 2 O, NH 3 , and isocyanate gases are associated with this decomposition stage. The weakly expressed exoeffect was observed at 450-795 °C, and the weight decreased by 5.0% with a small amount of CO 2 and H 2 O gases released. The total sample weight loss under the heating to 795 °C was 75%; similar to 1, this can be explained by formation of SrSO 4 and SrO mixture [22].

Luminescence study
The luminescence observation of HDetba, 1 and 2, was performed in the solid state at room temperature. The highest intensity of the excitation spectrum was from 200 to 220 nm. Free HDetba exhibits an intense emission maximum at 557 nm and relatively strong bands at 449, 448, 421, and 392 nm upon excitation at 220 nm. The luminescence spectra of 1 and 2 excited at 220 nm are almost the same ( Figure 9S). The absence of noticeable luminescence band shift on transition from HDetba to 1 and 2 is the result of electron density isolation of the ligand, which participates in the luminescence emission. The emission behavior of the ligand may be assigned as intraligand π*-π and π*-n charge transfer transitions [41].  (table 2). In 1 and 2, coordination of Detba − through sulfur is absent and two intermolecular H-bonds (O-H…S) appeared. Comparatively, in coordination compounds of singly charged metal ions with Detba − , there were only weak intramolecular hydrogen bonds C-H…O and C-H…S [30][31][32][33]. The crystal structures of most of the metal complexes with H 2 tba are stabilized by π-π interaction between Htba − ions. Among all metal compounds with HDetba, only the Ag(I) compound [33] has π-π interaction with Detba − . Regardless of the metal ion nature, coordination numbers in compounds with the larger Detba − are 4 or 6 forming tetrahedral, octahedral, or trigonal prisms, unlike compounds like Htba − . For other derivatives of 2-thiobarbituric acid, the molecular and supramolecular structures of metal complexes will be different. A diversity of topological nets in coordination compounds of Detba − with different metals makes it possible to assume that some outstanding topological nets can be found in the future.

Conclusion
Thermal decomposition of 1 and 2 starts at a higher temperature than that in HDetba (T > 112 °C) [30,42] and comprises the dehydration and oxidation steps of the organic ligand with CO 2 , SO 2 и H 2 O, NH 3 , and isocyanate release. Coordination of Detba − through oxygen is proved by IR spectroscopy.