Structural and Thermal Investigations of L-CuC4H4O6·3H2O and DL-CuC4H4O6·2H2O Single Crystals

Copper(II) L-tartrate trihydrate, L-CuC4H4O6·3H2O, and copper(II) DL-tartrate dihydrate, DL-CuC4H4O6·2H2O, crystals were grown at room temperature by the gel method using silica gels as the growth medium. Differential scanning calorimetry, thermogravimetric-differential thermal analysis, and X-ray diffraction measurements were performed on both crystals. The space group symmetries (monoclinic P21 and P21/c) and structural parameters of the crystals were determined at room temperature and at 114 K. Both structures consisted of slightly distorted CuO6 octahedra, C4H4O6 and H2O molecules, C4H4O6–Cu–C4H4O6 chains linked by Cu–O bonds, and O–H–O hydrogen-bonding frameworks between adjacent molecules. Weight losses due to thermal decomposition of the crystals were found to occur in the temperature range of 300–1250 K. We inferred that the weight losses were caused by the evaporation of bound water molecules and the evolution of H2CO, CO, and O2 gases from C4H4O6 molecules, and that the residual reddish-brown substance left in the vessels after decomposition was copper(I) oxide (Cu2O).

As mentioned above, it is expected that a copper(II) DL-tartrate compound can be synthesized using DL-tartaric acid and Cu(II) 2+ ions as the divalent cations. The crystal structure of the resulting compound, CuC 4 H 4 O 6 ·3H 2 O, containing three water molecules has not been determined yet, except for its crystal system and lattice constants. In this paper, we describe the synthesis of copper(II) L-tartrate trihydrate (L-CuC 4 H 4 O 6 ·3H 2 O) and copper(II) DL-tartrate dihydrate (DL-CuC 4 H 4 O 6 ·2H 2 O) crystals by the gel method, and determine their crystal structures using single-crystal X-ray diffraction. Moreover, the thermal properties of these crystals are studied by differential scanning calorimetry (DSC) and thermogravimetric-differential thermal analysis (TG-DTA).

Crystal Growth
The L-CuC 4 H 4 O 6 ·3H 2 O and DL-CuC 4 H 4 O 6 ·2H 2 O crystals were grown in silica gel medium at room temperature using single test tube diffusion method. The gels for the growth were prepared in test tubes (with length of 200 mm, and diameter of 30 mm) using aqueous solutions of Na 2 SiO 3 (20 ml of 1 M), L-C 4 H 6 O 6 (or DL-C 4 H 6 O 6 ) (25 ml of 1 M), and CH 3 COOH (25 ml of 1 M for the L-compound and 25 ml of 1.5 M for the DL-compound). The gels were aged for seven days, and solutions of CuSO 4 ·5H 2 O (30 ml of 0.5 M) were then gently poured on top of the gels. The L-CuC 4 H 4 O 6 ·3H 2 O and DL-CuC 4 H 4 O 6 ·2H 2 O crystals were harvested after about two and four months, respectively. Figure 1 shows the photograph of (a) L-CuC 4 H 4 O 6 ·3H 2 O and (b) DL-CuC 4 H 4 O 6 ·2H 2 O single crystals grown in the gel medium. The L-CuC 4 H 4 O 6 ·3H 2 O crystals, which are prism in shape and blue in color, are very similar to those reported in the previous paper (Jethva, Dabhi, & Joshi, 2016).

Structure Determination
The X-ray diffraction measurements were performed using a Rigaku Saturn CCD X-ray diffractometer with graphite-monochromated Mo K α radiation (λ = 0.71073 Å). The diffraction data for the L-CuC 4 H 4 O 6 ·3H 2 O and DL-CuC 4 H 4 O 6 ·2H 2 O crystals were collected at 299 and 114 K, respectively, using an ω scan mode with a crystal-to-detector distance of 40 mm, and processed using the CrystalClear software package. The intensity data were corrected for Lorentz polarization and absorption effects. The crystal structures were solved by direct methods using the SIR2014 program and refined on F 2 by full-matrix least-squares methods using the SHELXL-2017 program in the WinGX package (Burla et al., 2015;Farrugia, 2012;Sheldrick, 2015).

Thermal Measurements
DSC and TG-DTA measurements were carried out in the temperature ranges of 100-310 and 300-1250 K, respectively, using DSC7020 and TG-DTA7300 systems from Seiko Instruments Inc. Aluminium (for DSC) and platinum (for TG-DTA) open pans were used as measuring vessels and reference pans. Fine powder samples of L-CuC 4 H 4 O 6 ·3H 2 O and DL-CuC 4 H 4 O 6 ·2H 2 O for the thermal measurements were obtained by grinding several pieces of single crystals. The sample amount varied between 5.29 and 7.81 mg, and the heating rates were 10 K min -1 under the flow of nitrogen gas (40 ml min -1 for DSC and 300 ml min -1 for TG-DTA).

Crystal Structures
The crystal structures of L-CuC 4 H 4 O 6 ·3H 2 O and DL-CuC 4 H 4 O 6 ·2H 2 O were determined at room temperature and at 114 K, respectively. The lattice parameters calculated from all the observed X-ray reflections showed that both crystals belong to a monoclinic system. The systematic extinctions of the reflections from L-CuC 4 H 4 O 6 ·3H 2 O revealed that the space group is P2 1 or P2 1 /m, and the examination of the intensities of symmetry-equivalent reflections indicated that the crystal belongs to a non-centric space group. Therefore, the structure of L-CuC 4 H 4 O 6 ·3H 2 O was determined to be monoclinic with space group P2 1 . On the other hand, the systematic extinctions from DL-CuC 4 H 4 O 6 ·2H 2 O revealed that the space group is P2 1 /c. Consequently, the structure of DL-CuC 4 H 4 O 6 ·2H 2 O was found to be monoclinic with space group P2 1 /c. Some isotropic thermal parameters for hydrogen atoms belonging to water molecules in L-CuC 4 H 4 O 6 ·3H 2 O were fixed during the structural refinement because the parameters did not converge to reasonable values. Final R-factors of 2.56% and 4.61% for the L-CuC 4 H 4 O 6 ·3H 2 O and DL-CuC 4 H 4 O 6 ·2H 2 O crystals, respectively, were calculated for 7938 and 5283 unique observed reflections. The largest residual electron density peak and hole in the final difference Fourier map of DL-CuC 4 H 4 O 6 ·2H 2 O were 5.813 and -1.310 eÅ -3 , respectively, as shown in Table 1. From the following experimental results, we believe that the presence of the large values is mainly caused by radiation damage to the sample during the X-ray data collection at 114 K.
The relevant crystal data, and a summary of intensity data collection and structure refinement are given in Table 1. Figure 2 shows the projections of the (a) L-CuC 4 H 4 O 6 ·3H 2 O and (b) DL-CuC 4 H 4 O 6 ·2H 2 O crystal structures along the a-axis. The positional parameters in fractions of the unit cell and the thermal parameters are listed in Table 2. Selected bond lengths and angles are given in Table 3, and hydrogen-bond geometries are presented in Table 4.

Thermal Analysis
Two large and two small weight losses are seen at around 350, 500, 600, and above 640 K in the TG curves for both crystals, as shown in Fig. 3(a) and (b). The experimental weight losses for L-CuC 4 H 4 O 6 ·3H 2 O in the temperature ranges of 300-460, 460-584, 584-640, and 640-1250 K were found to be 15.9, 52.1, 2.7 and 4.2%, respectively, and those for DL-CuC 4 H 4 O 6 ·2H 2 O in the ranges of 300-460, 460-546, 546-640, and 640-1250 K were 13.1, 45.5, 8.7 and 6.1%, respectively. Table 5 shows the experimental and theoretical weight losses in each temperature range. The theoretical weight losses were calculated based on the following considerations. The weight losses in the TG curves for both crystals may have been caused by the evolution of gases from the samples, similarly to our previous studies (Fukami, Hiyajyo, Tahara, & Yasuda, 2017;Fukami & Tahara, 2018;Fukami & Tahara, 2020). There are two crystallographically independent formula units in the unit cells of both crystals, as described in Table 2(a) and (b). Therefore, the theoretical weight loss rates due to the thermal decomposition are calculated using twice the formula weight of L-CuC 4 H 4 O 6 ·3H 2 O (2M L = 531.33 g mol -1 ) and of DL-CuC 4 H 4 O 6 ·2H 2 O (2M DL = 495.30 g mol -1 ). The elimination of bound water molecules from the crystals presumably occurs with increasing temperature in the temperature range from 300 to 460 K. Above the temperature of 460 K, the evolutions of gases and copper compounds occur through chemical reactions described by the following chemical equations: , and the generation of copper oxides (CuO 2 and CuO) take place in the range of 460-584 K. Thus, the theoretical weight loss is calculated to be 50.1% (=(18.02+4×30.03+4×28.01+16.00)/531.33). Similarly, the evolution of gases (4H 2 CO and 3CO) and the generation of copper compounds (CuO 2 and CuCO 3 ) also take place in the range of 460-546 K for DL-CuC 4 H 4 O 6 ·2H 2 O, and the theoretical weight loss is calculated to be 41.2% (=(4×30.03+3×28.01)/495.30). With increased temperature, the theoretical weight losses in the temperature ranges of 584-640 and 640-1250 K for L-CuC 4 H 4 O 6 ·3H 2 O are calculated to be 3.0% (=16.00/531.33) because of the evolution of (1/2)O 2 gases, which is represented in the third and fourth equations corresponding to the ranges. According to the equations for DL-CuC 4 H 4 O 6 ·2H 2 O in the temperature ranges of 546-640 K and 640-1250 K, the theoretical weight losses due to the evolutions of CO and (1/2)O 2 gases, and of O 2 gas are calculated to be 8.9% (=(28.01+16.00)/495.30) and 6.5% (=32.00/495.30), respectively. Weight losses based on the chemical reactions for L-CuC 4 H 4 O 6 ·3H 2 O and DL-CuC 4 H 4 O 6 ·2H 2 O are listed in Table 5, and are almost close to the experimental loss values at each temperature range, respectively. The total theoretical weight losses of the L-CuC 4 H 4 O 6 ·3H 2 O and DL-CuC 4 H 4 O 6 ·2H 2 O crystals are found to be 73.1% (=17.0+50.1+3.0+3.0) and 71.1% (=14.5+41.2+8.9+6.5), respectively. These values are almost close to the total experimental weight losses in the range of 300-1250 K: 74.9% (=15.9+52.1+2.7+4.2) and 73.4% (=13.1+45.5+8.7+6.1), respectively, as given in Table 5 and Fig. 3. After heating up to 1250 K for the TG-DTA measurements of both crystals, we found that reddish-brown materials were present in the respective vessel. These residual materials are presumed to be copper(I) oxide Cu 2 O based on the chemical reactions highlighted above.

Summary
Single crystals of L-CuC 4 H 4 O 6 ·3H 2 O and DL-CuC 4 H 4 O 6 ·2H 2 O were grown in silica gel medium using gel technique at room temperature. The structures and thermal properties of these crystals were studied by means of X-ray diffraction, DSC, and TG-DTA. The crystal structures of L-CuC 4 H 4 O 6 ·3H 2 O at room temperature and DL-CuC 4 H 4 O 6 ·2H 2 O at 114 K were determined to be monoclinic with space groups P2 1 and P2 1 /c, respectively. The structures consisted of slightly distorted CuO 6 octahedra, C 4 H 4 O 6 and H 2 O molecules, C 4 H 4 O 6 -Cu-C 4 H 4 O 6 chains linked by Cu-O bonds, and O-H-O hydrogen-bonding frameworks between adjacent molecules. In both crystals, no phase transition was observed in the temperature range of 100-310 K, and the weight losses due to thermal decomposition were found to occur in the temperature range of 300-1250 K. The chemical equations illustrating the decomposition reaction of the crystals were presented, with corresponding temperature ranges. We suggested that the weight losses are caused by the evaporation of bound H 2 O molecules, and the evolution of H 2 CO, CO, and O 2 gases from C 4 H 4 O 6 molecules. The residual reddish-brown substances left in the vessel after decomposition were identified as copper(I) oxide Cu 2 O.