Thermal behavior of glycolic acid, sodium glycolate and its compounds with some bivalent transition metal ions in the solid state A. L. C. S. do Nascimento1 • J. A. Teixeira1 • W. D. G. Nunes1 • D. J. C. Gomes2 • C. Gaglieri3 • O. Treu-Filho1 • M. Pivatto4 • F. J. Caires3 • M. Ionashiro1 Received: 25 June 2016 / Accepted: 6 February 2017 / Published online: 20 February 2017 � Akadémiai Kiadó, Budapest, Hungary 2017 Abstract Synthesis, characterization and thermal behavior of some transition metal glycolates, M(C2H3O3)2 [M = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II)], as well as the thermal behavior of glycolic acid and its sodium salt (NaC2H3O3) were investigated employing simultaneous thermogravimetry and differential thermal analysis, infra- red spectroscopy (FTIR), simultaneous thermogravimetry– differential scanning calorimetry coupled to FTIR (evolved gas analysis), complexometry and high-resolution electro- spray ionization mass spectrometry. The lowest energy model structure of each complex has been proposed by using the density functional theory at the B3LYP/6- 311??g(d) level of theory. The results provided infor- mation concerning the composition, dehydration, thermal stability, thermal decomposition and identification of the gaseous products evolved during the thermal decomposi- tion of these compounds in dynamic dry air atmosphere. Keywords Bivalent transition metals � Glycolate � Thermal analysis � DFT Introduction Glycolic acid is a carboxylic acid and lower structurally representative of the class of alpha hydroxy acids (compounds that present a carboxyl and a hydroxyl group in the alpha position), molecular formula C2H4O3. As it has two functional groups in the same structure, – OH and –COOH, the molecule can act as a versatile ligand in coordination chemistry. In the study of coor- dination compounds, thermal analysis techniques are powerful characterization tools, making it possible to understand their thermal behavior, hydrate formation, thermal stability, existence of polymorphs, composition, intermediates and so on [1, 2]. Therefore, conducting the thermal study of metal complexes is a fundamental step before proposing technological applications of these compounds. Several studies involving the synthesis, crystal struc- tures, thermal decomposition of metal glycolate (C2H3O3 -), as well as the use of these compounds as precursors in other syntheses have been described. These researches reported a structural study of the orthorhombic tris-glycolates of lanthanum (III) and gadolinium (III) [3]; complexing the trivalent Am, Cm and Bk ions by glycolate [4]; manganese (II)/(III) glycolates; prepara- tion, X-ray crystallographic study and application in radical cycloaddition reactions [5]; synthesis and struc- ture of two cerium complexes with mixed-ligands oxalate and glycolate [6]; zinc glycolate: a precursor to ZnO [7]; synthesis of MWCNT/nickel glycolate polymer core–shell nanostructures and their nonenzymatic Electronic supplementary material The online version of this article (doi:10.1007/s10973-017-6161-3) contains supplementary material, which is available to authorized users. & F. J. Caires caires.flavio@fc.unesp.br 1 Departamento de Quı́mica Analı́tica, Instituto de Quı́mica, UNESP- Univ Estadual Paulista, Campus de Araraquara, Araraquara, SP, Brazil 2 Instituto Federal do Piauı́, Campus Paulistana, Paulistana, PI, Brazil 3 Departamento de Quı́mica, FACULDADE DE CIÊNCIAS, UNESP—Univ. Estadual Paulista, Campus Bauru, Bauru, SP, Brazil 4 Instituto de Quı́mica, Universidade Federal de Uberlândia, Uberlândia, MG, Brazil 123 J Therm Anal Calorim (2017) 130:1463–1472 DOI 10.1007/s10973-017-6161-3 http://dx.doi.org/10.1007/s10973-017-6161-3 http://crossmark.crossref.org/dialog/?doi=10.1007/s10973-017-6161-3&domain=pdf http://crossmark.crossref.org/dialog/?doi=10.1007/s10973-017-6161-3&domain=pdf electrocatalytic activity toward glucose [8]; thermal decomposition and spectroscopic investigation of a new aqueous glycolato(-peroxo) Ti(IV) solution–gel precursor [9]; and assembly of three coordination polymers from glycolate ligands: synthesis, crystal structures and ther- mal properties [10]. This paper presents the synthesis of compounds of glycolic acid with bivalent transition metal ions (Mn, Co, Ni, Cu and Zn) and its characterization by using different analytical techniques in order to study its thermal behavior. Experimental Synthesis The glycolic acid (C2H4O3, HL) with 99% purity was obtained from Sigma–Aldrich and used without any additional purification. Bivalent manganese, cobalt, nickel, copper and zinc carbonates were prepared by adding slowly with continuous stirring saturated sodium hydrogen carbonate solution to the corresponding metal chlorides (or sulfate for copper solutions) (50.0 mL, 0.1 mol L-1) until quantitative precipitation of the metal ions. The precipitates were washed with distilled water for elimination of chloride or sulfate ions (qualitative test with AgNO3/HNO3 solution for chloride and BaCl2 solution for sulfate) and maintained in aqueous suspension. Solid-state Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) compounds were prepared by mixing the corresponding metal carbonate aqueous suspension with a slight excess of glycolic acid solution (0.15 mol L-1) and slowly heated to near ebullition until total neutralization of the metal car- bonate. The volume of resulting solutions was reduced by evaporation to 20 mL (no precipitate was observed), and then, ethanol was added, which caused the precipitation of the compounds. The formed precipitates were filtered, washed five times with ethanol to eliminate the excess of glycolic acid, dried at 50 �C in a forced air circulation oven during 24 h and kept in a desiccator over anhydrous calcium chloride until constant mass. Scheme of the synthesis and reactions involved is presented below: Step 1 MCl2(aq) ? 2NaHCO3 (aq) ? M(CO3)(s) ? 2NaCl(aq) ? CO2(g) ? H2O(l) (the carbonates were washed to remove the chloride or sulfate anions) Step 2 M(CO3)(s) ? 2HL(aq) (slight excess) ? [M(L)2](aq) ? 3CO2(g) ? 3H2O(l) ? HL (excess) Step 3 [M(L)2](aq) ? HL (excess) �!Ethanol [M(L)2] (s) ? HL(ethanol) (Filtration, washing the precipitate to remove excess glycolic acid and drying the compounds) Characterization The solid-state metal ions, hydration water and glycolate contents were determined from TG curves. The metal ions were also determined by complexometry with standard EDTA solution after igniting the compounds to the respective oxides and their dissolution in hydrochloric acid solution [11, 12]. Carbon and hydrogen contents were determined by calculations based on the mass losses of the TG curves, since the thermal decomposition occurs with the formation of their respective oxides as the final residue, with known stoichiometry. High-resolution electrospray ionization mass spectrom- etry (HRESIMS) was performed in an ultrOTOF (Bruker Daltonics) spectrometer, operating in the negative mode. Methanol/water (1:1, v/v) was used as the solvent system, and the samples were infused into the ESI source at a flow rate of 5 mL min-1. The calculated values for the charged complex ions were made by using ChemDraw Ultra 14.0. The X-ray powder diffraction patterns were obtained using a Siemens D-5000 X-ray diffractometer, employing CuKa radiation (k = 1.541 Å) and setting of 40 kV and 20 mA. The attenuate total reflectance infrared spectra for gly- colic acid, sodium glycolate and its metal ion compounds were run on a Nicolet iS10 FTIR spectrophotometer, using an ATR accessory with Ge window. The FTIR spectra were recorded with 32 scans per spectrum at a resolution of 4 cm-1. Simultaneous TG–DTA curves were obtained with a thermal analysis system, model SDT 2960, from TA Instruments. The purge gas was air, with a flow rate of 100 mL min-1, a heating rate of 10 �C min-1 and samples weighing about 7 mg. Alumina crucibles were used for recording the TG–DTA curves. The identification of the evolved gaseous products was carried out using a Mettler Toledo Thermogravimetric Analyzer (TG–DSC) coupled to the Nicolet FTIR spec- trometer equipped with a gas cell and DTGS KBr detector. The furnace and heated gas cell (250 �C) were coupled through a heated (T = 200 �C) 120-cm stainless steel transfer line with 3.0 mm diameter, both purged with dry air (50 mL min-1). The FTIR spectra were recorded with 16 scans per spectrum at a resolution of 4 cm-1. Computational strategy In this study, the employed quantum chemical approach to determine the molecular structures was Beck’s three parameter hybrid theory [13] using the Lee–Yang–Parr correlation functional (B3LYP) [14], and the basis sets used for calculations were 6-311??g (d) [15, 16]. The 1464 A. L. C. S. do Nascimento et al. 123 performed molecular calculations in this study were done using the Gaussian 09 routine [17]. The theoretical infrared spectrum was calculated using a harmonic field [18] based on C1 symmetry (electronic state 1A). Frequency values (not scaled), relative intensities, assignments and descrip- tion of vibrational modes are presented. The geometry optimization was computed using the optimized algorithm of Berny [19], and the calculations of vibrational fre- quencies were also implemented to determine an optimized geometry that constitutes minimum or saddle points. The principal infrared-active fundamental modes assignments and descriptions were made by the GaussView 5.0.2 W graphics routine [20]. Results and discussion Glycolic acid The TG–DTA curves of glycolic acid are shown in Fig. 1. These curves show that the anhydrous compound is stable up to 79 �C and above this temperature the mass losses occur in two consecutive steps between 79 and 400 �C (TG) with thermal events corresponding to these losses or due to physical phenomenon (DTA). The endothermic peak at 79 �C is due to the melting, and the first mass loss between 79 and 200 �C (50.48%) corre- sponding to the endothermic event between 125 and 200 �C is attributed to the partial evaporation of the molten compound with the formation of carbonaceous residue, and this was confirmed from visual inspection on a melting point equipment. The mass loss between 200 and 400 �C (49.52%) corresponding to an exothermic peak at 300 �C is attributed to the oxidation of the organic matter and/or of the gaseous products evolved in this step. Sodium glycolate The TG–DTA curves of anhydrous sodium glycolate are shown in Fig. 2. These curves show mass losses in two steps and thermal events corresponding to these losses or due to physical phenomenon. The sharp endothermic peak at 217 �C is attributed to the melting of the compound, which was confirmed in a melting point apparatus. The small sudden apparent mass loss observed during the melting of the compound undoubtedly is due to the trans- formation of the pulverized solid compound to liquid state. When the sample was heated up to 217 �C, cooled to ambient temperature and heated again, no mass loss was observed during the fusion, as shown in Fig. 2b. The first mass loss between 217 and 355 �C (27.44%) corresponds to the thermal decomposition and oxidation of the organic matter with the formation of carbonaceous residue, confirmed from visual inspection of the sample heated to that temperature. No thermal event is observed in the beginning of the mass loss, probably because up to 300 �C, endothermic and exothermic reactions might be occurring simultaneously and the net heat is not sufficient to produce a thermal event. The second mass loss between 355 and 477 �C (18.22%) corresponding to the sharp exothermic peak at 476 �C is attributed to the oxidation of the carbonaceous residue and/ or of the gaseous products evolved during the thermal decomposition. The total mass loss up to 477 �C is in agreement with the formation of sodium carbonate (The- or. = 45.95%; TG = 45.66%). Tests with hydrochloric acid solution on sample heated up to the temperature 100 0 30 60 90 Exo up 200 300 400 500 –6 –3 0 DTA TG Temperature/°C M as s/ % ΔT /° C Fig. 1 Simultaneous TG–DTA curves (9.2532 mg) of the glycolic acid 100 50 75 100 50 75 100 Exo up 200 300 400 500 600 700 0 (b) (a) 4 0 5 10 Temperature/°C DTA TG M as s/ % ΔT /° C Fig. 2 Simultaneous TG–DTA curves of the sodium glycolate (a) 3.2770 mg and (b) 3.0570 mg Thermal behavior of glycolic acid, sodium glycolate and its compounds with some bivalent… 1465 123 indicated by the TG–DTA curves (477 �C) confirmed the formation of sodium carbonate. Transition metal complexes Analytical results To the synthesized compounds, the analytical and ther- moanalytical (TG) data are shown in Table 1. These results permitted to establish the stoichiometry of these com- pounds, which are in agreement with the general formula [M(L)2] where M represents Mn(II), Co(II), Cu(II) and Zn(II), L is glycolate, except for Ni (II) presenting the general formula [Ni(L2)]�0.25H2O. Mass spectrometric analyses High-resolution mass spectra of the glycolate complexes are presented in supplementary material (Figs S1 to S5). These spectra show peaks at m/z 203.9475, 207.9423, 206.2441, 212.9472 and 212.9383 correspond to the molecular ions [M–H]– ([Mn(C2H3O3)2–H]–, [Co(C2H3- O3)2–H], [Ni(C2H3O3)2–H]–, [Cu(C2H3O3)2–H]– and [Zn(C2H3O3)2–H]–), which confirms the 1:2 metal/glyco- late composition and it is in accordance with the minimum formula determined by other analytical methods (TG and EDTA). Mass spectra of the complexes are typical for their isotopic pattern, as can be exemplified in the mass spec- trum of the zinc complex, where the signals (m/z) corre- spond to the isotopes 64Zn (48.6%), 66Zn (27.9%), 67Zn (4.1%), 68Zn (18.8%) and 70Zn (0.6%) isotopes, with the nuclide abundance in parentheses. XRD The X-ray patterns (supplementary material, Fig S6) show that all compounds have crystalline structure and that cobalt, nickel and copper compounds show evidence for formation of isomorphous series. The crystallinity of these compounds follows the order: Cu[Ni[Zn[Mn � Co: The difference in the crystallinity of these compounds may be due to the conditions of precipitation, since the solid compounds are obtained by heating the solution to near its ebullition temperature and the heating, as well as the time of ebullition was not controlled. Infrared vibrational spectroscopy and theoretical calculations To determine the probable structures of the complex, the- oretical calculations have been performed, since numerous attempts to obtain single crystals were not successful. The optimized geometries of the lower energy isomer are shown in Fig. 3, and the other structures of the higher energy isomers and further details around the coordinating sphere (angles and bonding lengths) can be found in the supplementary material. Three isomers are proposed for each complex: the first with ligands coordinated by hydroxyl group and by one oxygen atom of the carboxylate group forming a five-membered ring in trans configuration (isomer A); the second as isomer A but in the cis config- uration (isomer B); and the third presenting ligands coor- dinated only by the carboxylate group (isomer C). Isomer A (trans) is the most stable for Mn, Co and Ni complexes, while for Cu and Zn complexes, isomer C is the most stable. The structures of isomer B (cis) to the complex of Mn and Zn were not optimized. From the structures of the isomers A and C, theoretical infrared vibrational spectra were generated and then com- pared with the experimental data (Fig. 4). These results show that the theoretical vibrational spectra of the isomer C for all compounds are very similar to the experimental spectra even though for the Mn, Co and Ni complexes the theoretical results show that the most probable structure is the isomer A, since it has the lowest energy. These dif- ferences are probably associated to the fact that calcula- tions were performed considering isolated individual molecules of the complex, while the experimental data were obtained in the solid state, in which there are many intermolecular interactions. Table 1 Analytical and thermoanalytical (TG) data for the [M(L)2] compounds Complexes Metal oxide/% L (lost)/% Water/% C/% H/% Final residue Calc. EDTA TG Calc. TG Calc. TG Calc. E.A. Calc. E.A. [Mn(L)2] 37.21 37.07 37.21 62.79 62.79 23.43 23.43 2.96 2.96 Mn3O4 [Co(L)2] 35.83 35.42 35.64 64.17 64.36 22.97 23.04 2.90 2.91 CoO [Ni(L)2]�0.25H2O 35.03 35.05 35.03 64.97 64.97 2.13 2.13 22.52 22.52 3.08 3.08 NiO [Cu(L)2] 37.24 37.06 36.89 62.76 63.11 22.49 22.61 2.84 2.85 CuO [Zn(L)2] 37.77 37.89 37.51 62.23 62.49 22.30 22.39 2.81 2.82 ZnO L glycolate 1466 A. L. C. S. do Nascimento et al. 123 The main frequencies of the IR spectra of glycolic acid, sodium salt and complex were assigned based on the cal- culated spectra for the isomer C and are presented in Table 2. For sodium glycolate, the bands centered at 1594 cm-1 and 1383 cm-1 were assigned to antisymmetric (masCOO-) and symmetric (msCOO-) carboxyl stretching frequencies, respectively [21, 22]. The values of Dm (masCOO-–msCOO-, carboxylate vibrations) for the syn- thesized compounds are smaller than those ones calculated for the sodium salt (Table 2), suggesting that the coordi- nation is carried out through the carboxylate group of the glycolate in a bidentate mode (chelating and/or bridging Isomer A trans-[Mn(L)2] –1.10353674 × 106 kcal mol–1 Isomer A trans-[Co(L)2] Isomer A trans-[Ni(L)2] Isomer C [Cu(L)2] Isomer C [Zn(L)2] –1.24899513 × 106 kcal mol–1 –1.32777092 × 106 kcal mol–1 –1.41071753 × 106 kcal mol–1 –1.49786066 × 106 kcal mol–1 Fig. 3 Theoretical 3D structure of lower energy of the complexes Thermal behavior of glycolic acid, sodium glycolate and its compounds with some bivalent… 1467 123 ligand) [23], in accordance with the results of the theo- retical calculation. On the other hand, the results of Table 2 show that the m(C–OH) is shifted toward lower energy than those observed in the glycolic acid, suggesting that metal ion can be coordinated by the hydroxyl group which is also in agreement with the literature [24, 25] and the results of the theoretical calculation for the compounds of Mn, Co and Ni. Thus, these results reinforce the conclusion that the coordination of ligand is carried out through the oxygen atoms of the carboxylate and the hydroxyl groups. Thermal analysis (TG–DTA and EGA) Manganese compound The simultaneous TG–DTA curves are shown in Fig. 5a. The thermal decomposition of the anhydrous compound occurs in three overlapping steps, between 100–270, 270–317 and 317–400 �C with losses of 9.30, 11.60 and 41.89%, respectively, corresponding to an endothermic peak at 270 �C and an exothermic one at 400 �C, which are attributed to thermal decomposition and oxidation of the organic matter, respectively. No thermal event corresponding to second step is observed in the DTA curve, probably because endothermic and exothermic reactions may be occurring simultaneously and the net heat is not sufficient to produce a thermal event. The mass loss up to 400 �C is in agreement with formation of Mn3O4 (Theor. = 62.79%, TG = 62.79%). The mass gain (TG = 0.56%) that occurs between 475 and 544 �C is attributed to the oxidation of Mn3O4 to Mn2O3 (The- or. = 61.50%, TG = 62.30%), which was confirmed by X-ray powder diffractometry. The last mass loss observed between 930 and 970 �C (1.81%), corresponding to an endothermic peak at 930 �C, is attributed to the partial reduction of Mn2O3 to MnO (Theor. = 65.40%, TG = 64.11%) and in agreement with the literature [26]. Cobalt compound The simultaneous TG–DTA curves are shown in Fig. 5b. The thermal decomposition of the anhydrous compound occurs in two consecutive steps between 310–354 and 354–383 �C, with losses of 20.12 4000 3500 Theoretical: isomer 3 Theoretical: isomer 3 Theoretical Theoretical Experimental Experimental Experimental Experimental Theoretical: isomer 3 Theoretical: isomer 3 Theoretical: isomer 3 Experimental Experimental Experimental (d) (d*) (c) (c*) (b) (b*) (a) (a*) (g) (g*) (f) (f*) (e) (e*) 3000 2500 2000 1500 1000 Wavenumber/cm–1 Wavenumber/cm–1 4000 3500 3000 2500 2000 1500 1000 T ra ns m itt an ce /% T ra ns m itt an ce /% Fig. 4 Experimental and theoretical* infrared spectra of (a) glycolic acid (b) sodium glycolate and (c, d, e, f and g) complexes of bivalent transition metals 1468 A. L. C. S. do Nascimento et al. 123 and 43.35%, respectively, corresponding to endothermic and exothermic peaks at 350 and 385 �C, respectively. These peaks are attributed to the beginning of the thermal decomposition of organic ligand (endo) and oxidation of the organic matter and/or of the gaseous products evolved in this step (exo). The mass loss up to 385 �C suggests the formation of CoO (Theor. = 64.17%; TG = 63.47%). The mass gain (TG = 1.53%) that occurs between 427 and 584 �C is attributed to the oxidation of CoO to Co3O4 (Theor. = 61.62%, TG = 61.94%), which was confirmed by X-ray powder diffractometry. The last mass loss between 908 and 924 �C is attributed to reduction reaction of Co3O4 to CoO (Theor. = 2.55%; TG = 2.42%) [27]. Nickel compound The simultaneous TG–DTA curves are shown in Fig. 5c. The first mass loss that occurs between 30 and 200 �C is due to dehydration with loss of 0.25 H2O (Theor. = 2.11%, TG = 2.13%). In this step, no thermal event is observed in the DTA curve, probably because the small mass loss (water adsorbed) occurs so slowly that the heat involved is not sufficient to produce a thermal event. After the dehydration, the thermal decomposition ofT a b le 2 M ai n v ib ra ti o n al d at a (M IR /F T IR ) fo r th e g ly co li c ac id , so d iu m g ly co la te an d th is co m p o u n d w it h so m e b iv al en t io n s A ss ig n m en t H L * N a( L ) [M n (L ) 2 ] [C o (L ) 2 ] [N i( L ) 2 ]0 .2 5 H 2 O [C u (L ) 2 ] [Z n (L ) 2 ] W av en u m b er /c m - 1 C al cd . E x p . C al cd . E x p . C al cd . E x p . C al cd . E x p . C al cd . E x p . C al cd . E x p . C al cd . E x p . mO – H 3 7 5 4 /2 8 9 0 * 3 2 8 6 * m 3 7 4 7 3 0 5 4 3 7 3 4 3 0 5 5 3 7 4 8 3 0 6 3 3 7 9 6 3 0 3 3 3 7 4 1 – 3 7 4 1 3 0 1 9 mC – H 2 9 3 8 m 3 0 6 1 /3 0 1 2 2 9 4 5 /2 9 2 0 2 9 9 7 2 9 1 2 2 9 8 7 2 9 1 6 3 0 2 7 2 9 7 0 2 9 9 2 2 9 7 2 2 9 9 4 2 9 1 6 O – H ��� �O 2 7 6 0 – 2 5 6 3 m – 2 8 0 0 – 2 5 7 0 – 2 8 0 0 – 2 5 8 0 – 2 8 0 0 – 2 5 9 0 – 2 8 0 0 – 2 6 0 0 – 2 8 0 0 – 2 2 0 0 – 2 8 3 0 – 2 5 8 8 mC = O 1 7 2 4 1 7 2 7 /1 7 0 4 v s – – – – – – – – – – ma sC O O - 1 5 7 0 1 5 9 4 1 5 6 8 1 5 8 9 1 5 4 4 1 5 8 2 1 5 5 4 1 5 7 9 1 5 4 7 1 5 6 0 1 5 5 8 1 5 8 3 dC H 2 ? tC H 2 ? b O – H 1 4 2 7 /1 4 0 7 1 4 3 1 s 1 4 9 8 /1 4 6 5 1 4 8 8 /1 4 3 7 1 4 8 6 /1 4 7 6 1 4 7 4 /1 4 2 3 1 4 9 1 /1 4 7 8 1 4 7 4 1 5 2 7 /1 4 9 1 1 4 7 3 /1 4 3 9 1 5 0 8 /1 5 7 6 1 4 8 0 /1 4 4 3 1 4 7 1 1 4 7 3 /1 4 3 8 ms C O O - 1 3 7 9 1 3 8 3 1 3 8 8 1 3 8 7 1 4 0 5 1 3 9 8 1 4 1 0 1 4 0 1 1 4 0 5 1 3 8 3 1 4 0 1 1 3 9 9 cO – H ? dC H 2 ? tC H 2 1 3 4 7 /1 3 3 4 1 3 5 8 m – 1 2 9 0 1 2 6 9 1 2 9 4 1 2 9 4 1 2 9 7 1 2 1 9 1 2 9 9 1 3 1 3 1 3 0 1 1 3 1 1 1 2 9 8 cO – H ? sC H 2 ? tC H 2 1 2 2 6 /1 1 9 4 1 2 4 0 v s 1 2 2 6 /1 2 0 4 – – – 1 2 1 0 1 2 3 6 – 1 2 3 6 1 2 3 9 1 2 2 5 1 2 4 4 1 2 3 7 mC – O H ? cC H 2 1 0 8 7 /1 0 7 7 1 0 8 9 v s 1 0 2 1 1 0 6 3 1 1 0 4 1 0 6 4 1 0 1 4 1 0 6 5 1 1 0 9 1 0 6 5 1 1 0 3 1 0 6 7 1 1 0 5 1 0 6 8 q C H 2 ? tC H 2 ? cO – H 1 0 2 7 1 0 0 3 w 1 0 2 5 1 0 1 4 – – 9 0 9 9 4 9 9 4 8 9 5 4 1 0 3 2 1 0 2 6 1 0 3 3 1 0 2 0 q C H 2 ? cO – H ? mC – C 8 7 8 9 3 1 m 9 1 8 9 0 9 9 4 1 9 2 0 – – – – 9 3 9 9 3 9 9 5 2 9 4 7 cO – H ? b C O O - ? tC H 2 – – 7 9 8 8 4 2 7 2 5 7 7 7 7 4 8 7 7 4 7 0 1 7 8 6 7 2 3 8 4 7 /7 1 9 7 3 2 7 8 6 m, st re tc h in g ; b, in p la n e; c, o u t o f p la n e d ef o rm at io n s; q, ro ck in g ; s, tw is ti n g ; d, sc is so ri n g ; t, w ag g in g ; s, st ro n g ; v s, v er y st ro n g ; m , m ed iu m ; sh , sh o u ld er ; w , w ea k , H L * , g ly co li c ac id (d im er ); L , g ly co la te 200 50 50 50 50 50 100 100 100 100 100 Exo up 400 600 800 1000 Temperature/°C M as s/ % ΔT /° C (b) (c) (d) (e) (a) DTA TG Fig. 5 Simultaneous TG–DTA curves of the compounds: (a) [Mn(L)2] (7.0351 mg), (b) [Co(L)2] (7.0775 mg), (c) [Ni(L)2]0.25- H2O (7.0135 mg), (d) [Cu(L)2] (7.0314 mg) and (e) [Zn(L)2] (7.0520 mg) Thermal behavior of glycolic acid, sodium glycolate and its compounds with some bivalent… 1469 123 anhydrous compound occurs in a single step between 280 and 346 �C with loss of 65.16%, corresponding to the exothermic peak at 345 �C in DTA curve. This peak is attributed to the oxidation of the organic matter and/or of the gaseous products evolved during the thermal decom- position, with the formation of a mixture of Ni� and NiO in no simple stoichiometric relation (Theor. = 72.48% (Ni), Theor. = 64.22% (NiO), TG = 67.30%). The mass gain (TG = 2.32%) that occurs between 362 and 506 �C is attributed to the oxidation reaction of Ni� to Ni (II) with formation of NiO as the final residue (Theor. = 64.97%, TG = 64.97%), which was confirmed by X-ray powder diffractometry. Copper compound The simultaneous TG–DTA curves are shown in Fig. 5d. These curves show that the compound is anhydrous and stable up to 205 �C. Above this temperature, the thermal decomposition occurs in a single step between 205 and 283 �C corresponding to the exothermic peak at 280 �C attributed to the oxidation of the organic matter and/ or of the gaseous products evolved during the thermal decomposition. The mass loss up to 283 �C is in agreement with the formation of Cu2O (Theor. = 66.20%, TG = 65.96%). The mass gain observed between 283 and 750 �C is attributed to the oxidation reaction of Cu2O to CuO (Theor. = 3.74%, TG = 2.95%). The total mass loss up to 750 �C is in agreement with the formation of CuO as the final residue (Theor. = 62.52%; TG = 63.11%), which was confirmed by X-ray powder diffractometry. Zinc compound The simultaneous TG–DTA curves are shown in Fig. 5e. These curves show that the compound is anhydrous and stable up to 250 �C. The thermal decom- position of the anhydrous compound occurs in three steps between 250–273, 273–304 and 304–430 �C. The first two are overlapping ones, with losses of 8.78, 10.52 and 43.19%, respectively, corresponding to endothermic peaks at 269 �C and 287 �C (shoulder) ascribed to the thermal decomposition. The exothermic peak at 407 �C was attributed to the oxidation of the organic matter and/or of the gaseous products evolved during the thermal decom- position. The total mass loss up to 430 �C is in agreement with the formation of ZnO as the final residue (The- or. = 62.23%; TG = 62.49%), which was confirmed by X-ray powder diffractometry. Evolved gas analysis (EGA) The gaseous products evolved during the thermal decomposition of the sodium 4000 3500 3000 3000 –0.00 100 50 0.50 1.00 2500 2000 2000 1500 1000 1000 500 Ti m e/ m in 100 50 Ti m e/ m in Wavenumber/cm–1 3000 –0.00 0.10 0.20 0.30 0.40 0.50 2000 1000Wavenumber/cm–1 4000 3500 Spectrum at 200 °C Spectrum at 300 °C CH2O2 CO CO CO2 3000 2500 2000 1500 1000 500 Wavenumber/cm–1 Wavenumber/cm–1 T ra ns m itt an ce /% In te ns ity In te ns ity T ra ns m itt an ce /% (a) (b) CH2O2 CH2O2 CO2 CO2 CO2 CO2 Fig. 6 IR spectra and 3D FTIR plot of the gaseous products evolved during the decomposition of the a manganese and b cobalt compounds 1470 A. L. C. S. do Nascimento et al. 123 and bivalent transition metal glycolates were monitored by FTIR, and the spectra of the gaseous products evolved during the thermal decomposition of manganese and cobalt compounds are shown in Fig. 6, as representative of all compounds. Based on the reference spectra database available in the equipment software, the main gaseous products identified were CO2, CO and formic acid (Mn and Cu) or CO and CO2 (Na, Co, Ni, Zn). Conclusions This paper presents a successful route for synthesis of compounds of glycolic acid with bivalent transition metal ions (Mn, Co, Ni, Cu and Zn), the quantum chemical approach calculations to help suggest the molecular struc- tures and its characterization by using different analytical techniques. From TG, high-resolution mass spectrometry, com- plexometry and elemental analysis results, a general for- mula could be established for the synthesized compounds. The infrared spectroscopic data are in good agreement with the theoretical calculations, which permitted to sug- gest that both hydroxyl and carboxyl groups act as coor- dination sites, as well as to determine the probable three- dimensional structure of the compounds. 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S. do Nascimento et al. 123 Thermal behavior of glycolic acid, sodium glycolate and its compounds with some bivalent transition metal ions in the solid state Abstract Introduction Experimental Synthesis Characterization Computational strategy Results and discussion Glycolic acid Sodium glycolate Transition metal complexes Analytical results Mass spectrometric analyses XRD Infrared vibrational spectroscopy and theoretical calculations Thermal analysis (TG--DTA and EGA) Manganese compound Cobalt compound Nickel compound Copper compound Zinc compound Evolved gas analysis (EGA) Conclusions Acknowledgements References