Growth, structural, vibrational, DFT and thermal studies of bis(β-alanine) nickel(II) dihydrate crystals

Journal Pre-proof Growth, structural, vibrational, DFT and thermal studies of bis(β-alanine) nickel(II) dihydrate crystals J.G. Oliveira Neto, J.G. da Silva Filho, N.S. Cruz, F.F. de Sousa, P.F. Façanha Filho, A.O. Santos PII:

S0022-3697(19)32197-3

DOI:

https://doi.org/10.1016/j.jpcs.2020.109435

Reference:

PCS 109435

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 30 September 2019 Revised Date:

24 February 2020

Accepted Date: 25 February 2020

Please cite this article as: J.G.O. Neto, J.G. da Silva Filho, N.S. Cruz, F.F. de Sousa, P.F.Faç. Filho, A.O. Santos, Growth, structural, vibrational, DFT and thermal studies of bis(β-alanine) nickel(II) dihydrate crystals, Journal of Physics and Chemistry of Solids (2020), doi: https://doi.org/10.1016/ j.jpcs.2020.109435. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

J. G. Oliveira Neto: Formal analysis, Visualization and Writing-

Original draft preparation. J.G. da Silva Filho: Formal analysis, Writing- Original draft preparation and Writing- Reviewing and Editing. N. S. Cruz: Investigation and Data curation. F. F. de Sousa: Investigation and Writing - Review & Editing. P. F. Façanha Filho: Writing - Review & Editing. A. O. Santos: Project administration, Funding acquisition and Supervision.

Growth, structural, vibrational, DFT and thermal studies of bis(βalanine) nickel(II) dihydrate crystals J.G. Oliveira Netoa, J.G. da Silva Filhoa, N.S. Cruza, F.F. de Sousab, P.F. Façanha Filhoa, A.O. Santosa a

Centro de Ciências Sociais, Saúde e Tecnologia, Universidade Federal do Maranhão, Imperatriz, MA 65900-410, Brazil b Instituto de Ciências Exatas e Naturais, Universidade Federal do Pará, CEP 66075-110 Belém, PA, Brazil

Abstract Bis(β-alanine) nickel(II) dihydrate single crystals were grown from an aqueous solution through the slow evaporation method, in 2:1 molar ratio of amino acid:metal. The structural, vibrational and thermal properties were studied by X-ray powder diffraction, Raman and Fourier-transform infrared spectroscopy and thermal analyses. Additionally, all the Raman and infrared bands were assigned by means of density functional theory calculations to aid a better interpretation of the intramolecular vibrational modes. The results showed that the complex belonged to a triclinic system with P1-space group and had the following lattice parameters: a = 8.362 (7), b = 6.723 (7), c = 4.909 (4) Å, α = 103.72 (6)°, β = 94.66 (6)° and γ = 102.16 (4)°. The thermal, X-ray diffraction and Raman analyses indicated a phase transformation from the triclinic crystalline phase to anhydrous phase due to a loss of water of crystallization. Keywords: X-ray diffraction; Raman spectroscopy; Growth of crystals; Bis(β-alanine) nickel(II) dihydrate crystals. * Corresponding author. e-mail: [email protected] (Corresponding author).

1. Introduction Materials with non-linear optical (NLO) properties have attracted considerable interest in recent years due to their applications in a wide range of applied fields, such as telecommunications, optical data storage and optical information processing [1,2]. Several materials including inorganic, organic and polymeric have been investigated due to their NLO activity. However, limitations on the maximum attainable nonlinearity in inorganic compounds, and difficulties encountered in the growth process of device-grade organic crystals, have stimulated the development of alternative materials [3,4]. Complexes of amino acids with inorganic components are particularly interesting, since they show desirable aspects of organic and inorganic materials, such as high nonlinearity and excellent mechanical and thermal properties [5]. In addition, a variety of amino acids are found in zwitterionic form in the solid state, which makes them particularly useful for optical applications. Although β-amino acids are less abundant than α-amino acids, they can also be found among peptides and in other types of natural compounds [6]. For instance, β-alanine (a positional isomer of L-α-alanine) is one of the most commonly found residues in bioactive peptides containing β-amino acids [7]. This compound and its derivatives have been widely employed in the synthesis of different semi-organic crystals [8,9]. In particular, β-alanine exhibits an excellent chelating ability to coordinate transition metal ions, forming stable complex hybrid compounds [10,11]. From the biological point of view, β-amino acids have been used for the synthesis of several drugs including enzyme inhibitors and antiviral and antibacterial drugs [12,13], as well as for the treatment of inflammatory disease and pain [14]. The nickel(II) complexes of the Schiff base of glycine and β-alanine have been applied in lowcost synthesis of R- and β-amino acids, for instance, the preparation of R-aryl-β-amino acids and R,R-disubstituted β-amino acids via nickel(II) complex [15]. Therefore, it is important to investigate the thermal stability of this kind of crystal, taking detailed information on its structural behavior [16–18]. The studies of the temperature-induced phase transformations on metal–amino acids complexes are of fundamental importance to investigate the hydrogen-bonding arrangements and the origin of lattice instabilities [16,19]. Moreover, such investigations could contribute to

a better understanding of the relevant physical properties of these materials. To the best of our knowledge, however, no published report has evaluated the thermal behavior of bis(β-alanine) nickel(II) dihydrate crystals. To fill this gap, we present an investigation of the structural, vibrational and thermal characterization of bis(β-alanine) nickel(II) dihydrate crystals [20], emphasizing the comprehension of the influence of coordinate water on the thermal stability of this compound. The high-temperature effects on the structural and vibrational properties of bis(β-alanine) nickel(II) dihydrate crystal are investigated by X-ray powder diffraction, Fourier-transform infrared (FT-IR) and Raman spectroscopy, and differential thermal analysis/thermal gravimetry (DTA-TG). In addition, the Raman and infrared vibrational modes assignments were performed through density functional theory (DFT) calculations.

2. Experimental 2.1. Single crystal growth Single crystals of bis(β-alanine) nickel(II) dihydrate, named Ni(β-alan)2·2H2O, were grown from an aqueous solution by the slow evaporation technique. The β-alanine (Sigma-Aldrich, 99%) and nickel(II) chloride hexahydrate (Sigma-Aldrich, 99%) were taken in the molar ratio of 2:1. The obtained crystals were blue rhomb-shaped platelets of typical size of about 0.690 mm × 0.330 mm × 0.260 mm. The prepared solution was stirred on a magnetic stirrer for 5 h at 45 °C until reaching a homogeneous mixture. The pH of the mixture was adjusted to 8.5 (with 2 mol L−1 NaOH). The final solution was filtered off and allowed to evaporate. Green single crystals were obtained after a month. 2.2. Characterizations Temperature-dependent X-ray diffraction (XRD) patterns of Ni(β-alan)2·2H2O crystal were collected using a PANalytical Empyrean powder diffractometer equipped with an Anton-Paar TTK450 temperature chamber measuring with Cu Kα (λ = 1.5418 Å) radiation operated at 45 kV and 40 mA. Diffraction patterns were obtained in the 2θ range of 5–45° with a step size of 0.02° and a counting time of 2 s. Analysis temperature was in the range of 303–473 K. In

addition, the diffraction patterns were refined through the Rietveld method with EXPIGUIGSAS software [21], using the structural parameters reported elsewhere [10]. The FT-IR spectrum was obtained using a Bruker Vertex 70V spectrophotometer by KBr pastille method, in the 400–4000 cm−1 spectral region. The Raman spectra were recorded in the 100–3400 cm−1 region using a triple-grating spectrometer (Jobin-Yvon, T64000) coupled to a CCD detection system. An argon ion laser operating at 514.5 nm was used as an excitation source. The slits were adjusted for a spectral resolution of 2 cm−1. The high-temperature Raman measurements were performed in a Linkam furnace CCR1000 (10 K·min−1 step), attached to the microscope, controlled by a PID temperature controller with an accuracy of 1 K. Simultaneous TG and DTA analyses were performed in nitrogen on a Shimadzu DTG-60 thermal analysis system. Approximately 5 mg of the sample was heated from 293 to 773 K in alumina crucibles under a synthetic air flow of 100 mL·min−1 and a heating rate of 10 K·min−1.

3. Calculations Our calculations were performed using the quantum-chemical package GAMESS (US) [22]. The initial atomic structure of bis(β-alanine) nickel(II), taken from XRD data [20], was fully optimized with unrestricted DFT method [23] considering the hybrid B3LYP (Becke exchange with Lee, Yang, and Parr correlation) functional [23,24], including the D3 version of Grimme’s dispersion correction [25]. The large triple-ζ basis sets with polarization functions def2-TZVP were used on all atoms [26]. The conductor-like polarizable continuum model was included to account for the water solvation effect (ε = 78.39) [27]. After optimization, the vibrational frequencies were evaluated at the same level of theory and then scaled to 0.960 [28]. The Raman activities (Si) obtained from GAMESS (US) were converted to relative Raman intensities ( ) using the following equation [29,30]: =

( − ) . [1 − exp(−ℎ /

)]

,

where v0 is the laser exciting wavenumber in cm−1, vi is the vibrational wavenumber of the ith normal mode, Si is the Raman scattering activity of the normal mode vi, f is a suitable

(1)

normalization factor for all peak intensities (10−13) and h, k, c and T are Plank and Boltzmann constants, speed of light and temperature in Kelvin, respectively. The simulated spectra were plotted using a Lorentzian line shape with a full width at half maximum of 10 cm−1. 4. Results and discussion 4.1. Rietveld refinement at room temperature The Rietveld refinement of the Ni(β-alan)2·2H2O XRD pattern at room temperature is depicted in Fig. 1a. The sample crystallizes in a triclinic phase (P1-space group), with four molecules per unit cell (Z = 4) and refined cell parameters a = 8.362 (7), b = 6.723 (7), c = 4.909 (4) Å, α = 103.72 (6)°, β = 94.66 (6)° and γ = 102.16 (4)° (Fig. 1b). The R-factors are obtained: Rwp = 12.95%, Rp = 8.91% and goodness of fit indicator S = 2.94. The molecular structure of Ni(β-alan)2·2H2O and its numbering scheme is shown in Fig. 1c. The complex anion comprises the nickel atom surrounded by two water molecules at axial positions and two zwitterionic β-alanine molecules, at equatorial sites. The Ni(β-alan)2·2H2O compound crystallizes in a similar way as for amino metal complexes, such as nickel glycine dihydrate, bis-(fl-aminobutyrato) copper(II) dihydrate and copper proline dihydrate [10].

Fig. 1. (a) XRD Rietveld refinement of bis(β-alanine) nickel(II) dihydrate crystal at 298 K, (b) triclinic unit cell of Ni(β-alan)2·2H2O along the c axis showing the hydrogen bonds formed with water molecules within the ab plane and (c) molecular structure of Ni(β-alan)2·2H2O.

4.2. Thermal analyses The DTA-TG measure of Ni(β-alan)2·2H2O crystal is shown in Fig. 2. The TG curve indicates that the thermal decomposition proceeds by two steps. Thus, between 408 and 483 K, there is a weight loss that can be attributed to evaporation of the water molecules coordinated to the metal which corresponds to the first decomposition stage. Initially, the mass loss is 14.22%, equivalent to 0.830 mg. The evaporation of these water molecules is associated with the two endothermic peaks at around 429 and 451 K, as seen on the DTA curve. These two endothermic peaks may represent the difference in intensity of interaction to which each of the two water molecules are subjected. The peaks that appear

in these events are attributed to phase transformation (dehydration) from the dihydrate phase to the anhydrous (water-free) phase.

Fig. 2. DTA-TG curve of Ni(β-alan)2·2H2O crystal.

These results are in good agreement with the previous work of Markovic et al. [18], in which the loss of water molecules in the coordination environment in a copper–valine complex was analyzed. After that, a large decomposition step is observed in the TG curve from 503 to 700 K with weight loss of 58.57%, equivalent to 3.41 mg. This weight loss corresponds to the final decomposition step. The strong exothermic peak in the DTA curve, located at about 633.70 K, along with a significant change in weight in the TG profile, indicates a release of gaseous substances during the sample decomposition process, similar to previously reported behavior of bis(L-threonine) copper(II) monohydrate crystal [31]. In Table 1 is given an events summary and mass loss in regarding to the heating of the crystal. Table 1 Fragmentation events observed for Ni(β-alan)2·2H2O in DTA-TG analysis.

TG Molecule fragment 2·H2O

Organic compounds

T. DTA (K) 429.0 (endo) 451.0 (endo) 529.5 (exo) 578.5 (endo) 609.4 (exo) 633.7 (exo)

Weight loss (%) 14.22

Ni2+ + organic compounds Total molecular weight

58.57

Weight loss (mg) 0.83

3.41

Molar mass (g mol−1) “Exp” *Calc*

“38.22” *36.00*

“157.03” *174.00*

“73.22” *58.71* “268.47” *268.71*

4.3. XRD study at high temperatures The in-situ high-temperature XRD patterns of Ni(β-alan)2·2H2O crystal obtained upon heating from 303 to 473 K are shown in Fig. 3. The XRD peaks shift to lower angles as the temperature increases from 303 to 433 K, indicating a crystal lattice expansion. Above this temperature and up to 453 K, the peak linewidths increase and their relative intensities decrease; however, their positions remain unaltered. Considering the thermogram of the compound (Fig. 2), this structural behavior can be related to the progressive evaporation of water molecules between 408 and 483 K. For temperatures above 453 K, the intensities of many reflections decrease drastically, revealing the transformation from the dihydrate to anhydrous phase and then beginning the amorphization process, in accordance with the TG data of the sample at high temperatures.

Fig. 3. Representative XRD patterns of Ni(β-alan)2·2H2O crystal recorded at high temperatures.

From the refined unit cell parameters of Ni(β-alan)2·2H2O crystal taken before the transformation, it is possible to determine the thermal expansion coefficients. The behavior of the linear thermal expansion (∆L/L0) and the values of thermal expansion coefficients (αL) from 300 to 380 K for the Ni(β-alan)2·2H2O crystal axes are shown in Fig. 4. The thermal expansion curves show a gradual decrease of axis a with the estimated coefficient of α[100] = −91.18(1) × 10−6 K−1. As temperature increases, the axis b is nearly unchanged with value of α[010] = 12.24(2) × 10−6 K−1, while axis c increases with α[001] = 119.33(2) × 10−6 K−1. Among these results, it is important to highlight the remarkable anisotropic behavior which can be related to the large differences in distances and angles of the hydrogen-bond lattice [32]. Although there is a gradual decrease of the a axis and the loss of water molecules from the crystal, the liquid volume variation of the crystal is positive. As temperature increases, the crystal becomes more disordered. The calculated volumes at selected temperatures are presented in Fig. S1. The XRD results show that the peak linewidths increase and their relative intensities decrease, indicating a gradual amorphization of the material. In fact, using the Rietveld refinement, the XRD pattern from the triclinic cell is still observed for temperatures around 463 K. Above this value, the pattern diffraction lines are drastically suppressed and

application of Rietveld refinement becomes rather complicated (Fig. S1). Therefore, our data indicate a direct phase transition from the triclinic to the amorphous structure.

Fig. 4. Thermal expansion of the three Ni(β-alan)2·2H2O crystal axes. The slopes of the fitting curves were used to calculate the linear thermal expansion coefficients.

4.4. Quantum-chemical computations The relaxed intramolecular bond lengths, angles and dihedral angles are listed in Table 2 along with the reported experimental values [10]. The calculated C–C bond lengths are slightly shorter than the experimental values; however, the calculated C=O bonds are somewhat larger than the corresponding experimental ones. This can be explained by the absence of intermolecular interactions in our simulation, which are prevalent in the condensed phase. In the crystalline environment, specific hydrogen bonds are observed between the carboxyl and water molecules (O7···H2O) that influence not only the carboxyl bonds (C=O) but also the backbone bond lengths of β-alanine. Similarly, there is a decrease in the calculated equatorial bond lengths of the complex. In our calculation, the Ni–N bonds are greater than Ni–O bond lengths, in contrast to the experimental data. However, the

optimized Ni–N and Ni–O bond lengths are in the normal range of octahedral nickel(II) complexes with chelating ligands [33]. As noted here, the optimized bond angles slightly differ from the experimental ones. For the dihedral angles, greater differences are observed between the experimental and calculated values, particularly in those related to the β-alanine backbone. For example, the dihedral angle O6–C3–C2–C1 presents an experimental value of 26.8°, while the relaxed value is around 41.0°. However, the optimized dihedral angle O9–Ni4–C3–C2 has a lower value than that observed in the crystalline system. The calculated geometrical parameters indicate a slightly distorted coordination around the nickel atom, which is in good correlation with the experimental observations and, therefore, can be used as the basis for the vibrational calculations. Therefore, even without considering the intermolecular interactions, we obtain a good correlation with the experimental observations for almost all the geometrical parameters Table 2 The geometrical parameters of bis(β-alanine) nickel(II): bond lengths (Å), bond angles (°) and dihedral angles determined from X-ray diffraction data [1] and computed with DFT-B3LYP-D3/def2-TZVP. Bond lengths (Å) X-ray Calc.

Bond angle (°)

X-ray Calc.

Dihedral angle (°)

X-ray

Calc.

C1–C2

1.57

1.52

C2–C1–N5

109.7 110.3

Ni4–C3–C2–C1

43.7

47.8

C1–C5

1.50

1.48

C1–C2–C3

113.1 112.9

N5–C1–C2–C3

−73.8

−71.6

C2–C3

1.54

1.52

C1–C5–Ni4

115.2 117.5

O6–C3–C2–C1

26.8

41.0

C3–O6

1.22

1.30

C2–C3–O6

122.9 117.4

O7–C3–C2–C1

−146.8 −138.6

C3–O7

1.28

1.23

C2–C3–O7

114.1 120.6

N8–Ni4–C3–C2

167.3

160.6

Ni4–N5

2.10

2.08

O6–C3–07

122.7 122.0

O9–Ni4–C3–C2

24.0

11.8

Ni4–O6

2.14

2.01

C3–O6–Ni4

123.0 128.1

C10–N8–Ni4–C3

−158.4 −167.1

Ni4–N8

2.10

2.08

N5–Ni4–O6

91.3

C11–O9–Ni4–N8

35.3

30.53

Ni4–O9

2.14

2.01

N5–NI4–N8

180.0 180.0

C12–C11–O9–Ni4

−29.0

−13.3

N8–C10

1.50

1.48

N5–NI4–O9

88.7

84.4

O13–C11–O9–Ni4

157.9

167.1

O9–C11

1.22

1.30

O6–Ni4–N8

88.7

84.4

C10–C12

1.57

1.52

O6–Ni4–O9

180.0 180.0

C11–C12

1.54

1.52

N8–Ni4–O9

91.3

C11–O13

1.28

1.23

Ni4–N8–C10

115.2 117.5

Ni4–O9–C11

123.0 128.1

95.6

95.6

N8–C10–C12

109.7 110.3

O9–C11–C12

122.9 117.4

O9–C11–O13

122.7 122.0

C10–C12–C11

113.1 112.9

C12–C11–O13

114.1 120.6

4.5. Vibrational analysis at room temperature The FT-IR and Raman (measured and simulated) spectra are shown in Figs. S2 and S3. The peak positions of observed IR and Raman bands are given in Table 3, as well as their corresponding vibrational assignments. There is a strong and broad infrared absorption between 3150 and 3500 cm−1, very likely due to the O–H stretching vibrations. In general, these vibrations are located at higher frequencies, but the presence of strong hydrogen bonding (O7···H2O and O13···H2O) in the Ni(β-alan)2·2H2O crystal probably shifts and broadens these vibrations. The experimental Raman bands observed at about 3313 and 3263 cm−1 (IR modes centered at 3317 and 3269 cm−1) are assigned as asymmetric and symmetric stretching of the NH2 group. These bands are in good agreement with the Bellamy–Williams relationship [34] for primary amines ν(a′) = 345.5 + 0.876ν(a″). In addition, a small peak located around 3170 cm−1 is observed in the Raman spectrum and is attributed to a symmetric stretching of NH2. The vibrational modes observed between 2800 and 3000 cm−1 are assigned as CH2 stretching vibrations. The Raman bands at 1561 and 1596 cm−1 are assigned as a scissoring of NH2 groups and stretching vibration of C=O, respectively. In the infrared spectrum there is a broad band consisting of three peaks near 1626, 1600 and 1564 cm−1, which are associated with water bending motion, C=O stretching vibrations and NH2 deformations, respectively. These vibrational assignments are in agreement with previous assignments of other metal–amino complexes, such as the nickel(II) complex of L-histidine [35] and the copper(II) complex of Laspartic acid [16,36]

Note that the complementary FT-Raman and FT-IR data are particularly useful to probe the nature of the high-frequency bands since polar O–H unit vibrations present strong intensities in the infrared spectra but have weak scattering in the Raman spectra. In addition, the absorption bands related to H-bonds of NH, NH2 and C=O units appear more pronounced in the infrared spectrum. These observations combined with the DFT calculation allow us to appropriately assign even the low intensity Raman bands, whose thermal behavior will be discussed later. The vibrational bands lying within 1200–1500 cm−1 are mainly associated with CH2 deformations, combined with stretching vibrations of C=O bonded to the metal. In the Raman spectrum, three bands associated with scissoring of CH2 are observed at about 1464, 1426 and 1421 cm−1. For lower frequencies, potential energy distribution (PED) analysis indicates that there is a combination of CH2 twisting and C=O stretching vibrations related to the vibrational bands at 1383 and 1323 cm−1. The bands at approximately 1308 and 1268 cm−1 in the Raman spectrum, as well as the infrared band at 1263 cm−1, are assigned as deformations of CH2 along the β-alanine backbone. Beyond that, the Raman bands located around 1162 and 1131 cm−1 are assigned as wagging (out-of-plane) deformations of the amino groups (N5H2 and N8H2). The correspondent IR modes are centered at 1168 and 1126 cm−1. The modes observed within 600–1000 cm−1 are attributed to a coupling of deformations of C−C, CH2, CO2 and NH2 groups of β-alanine ligands. Furthermore, the narrow peak in the IR spectrum at about 977 cm−1 corresponds to coupled C−C stretching vibrations and CO2 scissoring deformations. A similar assignment is made for the very intense Raman band centered at around 870 cm−1. In the spectral range of 600–750 cm−1 is found a contribution from several rocking deformations of NH2 coupled to out-of-plane vibrations of the carboxyl group of β-alanine molecules [37]. The wavenumber range of 600–200 cm−1 is mainly related to metal–ligand vibrations. Two peaks in the infrared spectrum at nearly 509 and 434 cm−1 are ascribed as vibrational motions involving the N−Ni−O complex bonding. The narrow Raman band located at 511 cm−1 is also

associated with this motion. The NiN2 symmetric stretching coupled to the CO2 rocking vibrations are assigned to the band near 413 cm−1 in the Raman spectrum. Moreover, the band at 370 cm−1 is associated with the symmetrical motion of NiO2. The Raman band at 279 cm−1 is related to in-plane scissoring vibration of N−Ni−O bonds, whereas the symmetric stretching of N−Ni−N and O−Ni−O fragments is associated with the band located at around 279 cm−1. The bands centered at about 241 and 211 cm−1 are also assigned as torsions and out-of-plane motions involving the coordinated nickel atom. Finally, the five low-wavenumber modes (<200 cm−1) are assigned as lattice modes, i.e. they are associated with collective motions of all atoms in the Ni(β-alan)2·2H2O crystal and their corresponding hydrogen bonds [36,38,39]. Table 3. Selected mode analysis: calculated vibrational wavenumbers (ωcal in cm−1), experimental Raman band positions (ωR in cm−1) and experimental infrared band position (ωIR in cm−1) assignment of the vibrational modes of bis(β-alanine) nickel(II) with PED ≥ 10%. a

ωcal (cm−1)

ωR (cm−1)

ωIR (cm−1)

Assignments

－

116

－

lattice mode

－

129

－

lattice mode

－

158

－

lattice mode

－

189

－

lattice mode

232

205

－

τ(C12C10N8Ni4)(35.87) + τ(C2C1N5Ni4)(36.37)

237

211

－

τ(C11O9Ni4O6)(86.68)

256

241

－

φ(Ni4N8N5O9)(48) + ρ(N5H2)(12) + ρ(N8H2)(10)

302

279

－

νs(N5Ni4N8)(32) + νs(O6Ni4O9)(34)

324

292

－

sc(O9Ni4N8)(25) + sc(N5Ni4O6)(25)

372

370

－

νa(O6Ni4O9)(63)

451

413

－

ρ(O13C11O9)(14) + ρ(O7C3O6)(14) + νs(N5Ni4N8)(42)

400

－

434

νa(N5Ni4N8)(58)

507

511

509

ρ(O13C11O9)(16) + ρ(O7C3O6)(16) + νs(N5Ni4N8)(24)

627

612

－

ν(C2C3)(14) + ν(C11C12)(14) + νs(O6Ni4O9)(24)

647

－

617

ν(C2C3)(12) + ν(C11C12)(11) + νs(O6Ni4O9)(22)

657

620

－

ρ(N5H2)(27) + ρ(N8H2)(27)

695

－

652

ρ(N5H2)(10) + ρ(N8H2)(10) + φ(C11C12O9O13)(15) + φ(C3C2O6O7)(15)

724

712

－

ρ(N5H2)(11) + ρ(N8H2)(11) + φ(C11C12O9O13)(12) + φ(C3C2O6O7)(13)

741

720

723

ρ(N5H2)(11) + ρ(N8H2)(11) + φ(C11C12O9O13)(14) + φ(C3C2O6O7)(14)

855

－

839

sc(O13C11O9)(16) + sc(O7C3O6)(12) + ν(C2C3)(15) + ν(C12C11)(13)

861

870

876

sc(O13C11O9)(18) + sc(O7C3O6)(18) + ν(C2C3)(13) + ν(C12C11)(13)

877

894

－

ν(C10N8)(13) + ν(C1N5)(13) + sc(C12C11O9)(11) + sc(C2C3O6)(11)

883

－

897

ν(C10N8)(12) + ν(C1N5)(11) + sc(C12C11O9)(10) + sc(C2C3O6)(10)

951

－

977

ν(C2C3)(14) + ν(C11C12) (12) + sc(C11O13O9)(15) + sc(C3O7O6)(15)

948

982

－

ν(C2C3)(13) + ν(C11C12) (13) + sc(C11O13O9)(15) + sc(C3O7O6)(15)

976

1004

－

tw(N5H2)(11) + tw(N8H2)(11) + ρ(C1H2)(11) + ρ(C12H2)(11)

981

－

1006

tw(N5H2)(10) + tw(N8H2)(10) + ρ(C1H2)(12) + ρ(C12H2)(12)

1029

1076

1080

ν(C1C2)(14) + ν(C10C12)(14) + ν(C1N5H2)(21) ν(C10N8H2)(21)

1101

－

1126

wag(N5H2)(13) + wag(N8H2)(13)

1098

1131

－

wag(N5H2)(13) + wag(N8H2)(13)

1112

1162

1168

wag(N5H2)(29) + wag(N8H2)(29)

1219

－

1263

tw(C2H2)(16) + δ(C12H2)(16)

1225

1268

－

tw(C2H2)(14) + δ(C12H2)(14)

1272

1308

－

tw(C1H2)(13) + tw(C10H2)(15)+ wag(C2H2)(11) + wag(C12H2)(15)

1298

1323

1323

ν(C3O6)(17) + ν(O9C11)(17) + tw(C1H2)(12) + wag(C12H2)(12)

1363

1382

1383

ν(C3O6)(11) + ν(O9C11)(11) + tw(C1H2)(10) + wag(C12H2)(10)

1355

－

1413

wag(C10H2)(11) + wag(C1H2)(11)

1416

1421

－

sc(C2H2)(15) + sc(C12H2)(15)

1417

1426

－

sc(C2H2)(15) + sc(C12H2)(15)

1436

1464

1460

sc(C1H2)(26) + sc(C10H2)(30)

1567

1561

－

sc(N5H2)(19) + sc(N8H2)(27)

1594

1596

1600

ν(C3O7)(39) + ν(C11O13)(39)

2908

2878

2879

νs(C2H2)(49) + νs(C12H2)(49)

2934

2917

2928

νs(C1H2)(50) + νs(C10H2)(48)

2935

2931

2953

νa(C1H2)(49) + νa(C10H2)(48)

2980

2954

－

νa(C1H2)(49) + νa(C10H2)(48)

2989

2977

2976

νa(C2H2)(53) + νa(C12H2)(39)

3303

－

3170

νs(N5H2)(44) + νs(N8H2)(41)

3304

3263

3269

νs(N5H2)(54) + νs(N8H2)(45)

3365

3313

3317

νa(N5H2)(50) + νa(N8H2)(49)

aCalculated

wavenumbers with DFT-B3LYP-D3/def2-TZVP approach are scaled by 0.960.

bNomenclature:

τ = torsion; sc = scissoring; tw = twisting; φ = out-of-plane; wag = wagging; ν = stretching; ρ = rocking; νa = asymmetric stretching; νs = symmetric stretching.

4.6. Raman spectroscopy under high temperatures The Raman spectra of Ni(β-alan)2·2H2O crystal recorded at high temperatures of 300–430 K in the spectral region of 90–300 and 300–700 cm−1 are shown in Fig. 5a and b, respectively. As the temperature increases from 300 to 360 K, the bands associated with the lattice modes (<200 cm−1) and the metal–ligand vibrations remain almost unchanged, with a slight shift in their peak positions to lower frequencies (Fig. 5a). When the temperature reaches 390 K, these bands become broader and less pronounced. For temperatures above 410 K, however, these bands become unresolved and their frequencies can no longer be reliably measured, indicating a highly disordered structure. It is important to note that those changes are observed at somewhat lower apparent temperatures in Raman spectroscopy than in the thermal analysis. This could be due to additional heating of the sample promoted by the laser, despite using a low laser power. Additionally, the different heat rates exchanged with the sample for each technique may also contribute to the different commencement of the transformation process [40]. Similar behavior is found for the bands within 300–700 cm−1 (Fig. 7b). Although the Raman intensities of intramolecular bands decrease, the internal modes associated with metal– ligand vibrations are still present even at 430 K. Indeed, these bands become even more visible in the spectrum recorded after the crystal is cooled to ambient temperature (300 K*). Those spectral changes can be explained in terms of the complete water loss seen in the TG curve (above 483 K), i.e. the breaking of hydrogen bonds between the water molecules and the carboxyl group of β-alanine, which maintain the crystalline structure, as well as those that link the water molecules to the nickel atom. In fact, since the spectra at 420 and 300 K (after heating) present similar patterns, all water molecules have likely exited the crystal lattice at 420 K. As will be shown, this behavior is evident for all spectral regions.

Fig. 5. Raman spectra of Ni(β-alan)2·2H2O crystal at selected temperatures in the wavenumber ranges (a) 90–300 −1 and (b) 300–700 cm .

The temperature evolution of the Raman spectra of Ni(β-alan)2·2H2O crystal in the wavenumber ranges of 710–1200 and 1300–1500 cm−1 are shown in Fig. 6a and b, respectively. As the temperature increases, there are significant changes in the Raman spectra in Fig. 6a. The intensity of the Raman mode located at 870 cm−1, assigned as a scissoring vibration of CO2 group, decreases but is still visible at 430 K. At this temperature, however, the other four bands in this spectral region practically disappear. In Fig. 6b the band at 1323 cm−1 originates from stretching vibration of the carboxyl oxygen bonded to the nickel(II) atom, while the bands at 1426 and 1465 cm−1 relate to CH2 scissoring vibrations. The intensities of these bands decrease and almost disappear in the temperature range of 300–430 K. The two small bands located at 1564 and 1596 cm−1, assigned as C=O stretching vibrations and NH2 scissoring vibrations, respectively, have a similar behavior when the temperature

reaches 410 K. When the crystal is cooled to 300 K*, the Raman spectrum profile in this region is mainly characterized by bands at 870, 1322, 1416 and 1464 cm−1, while the other bands become highly broadened.

Fig. 6. Raman spectra of Ni(β-alan)2·2H2O crystal at selected temperatures in the wavenumber ranges (a) 700– 1200 and (b) 1300–1900 cm−1.

The broadness and relative intensity changes on the bands at 1564 and 1596 cm−1 suggest that the hydrogen bonds associated with the amine and carboxyl groups are highly affected by temperature. These changes may be attributed to a structural rearrangement of the β-alanine molecules to compensate for the water loss due to thermal dehydration. Selected Raman spectra of Ni(β-alan)2·2H2O crystal in the ranges of 2850–2975 and 3150– 3400 cm−1 are shown in Fig. 7a and b, respectively. The broadening of the bands within 2850– 3000 cm−1 (ascribed to symmetric and asymmetric stretching vibrations of CH2 groups) and a drastic decrease in their intensities occur with the increase of temperature up to 410 K (Fig. 9a). Above this temperature, the intensities of these bands decrease even more and reach a

shallow minimum. The same behavior is found for the bands at 3169, 3263 and 3313 cm−1, attributed to the symmetric and asymmetric stretching vibrations of NH2 groups.

Fig. 7. Raman spectra of Ni(β-alan)2·2H2O crystal at selected temperatures in the wavenumbers (a) 2850–3050 and (b) 3150–3400 cm−1.

It is important to note that when the crystal returns to the ambient temperature (300 K*), these bands become highly broadened, and those close to each other overlap and even merge. This is indicative of a wide distribution of CH2 and NH2 bond lengths and angles in the anhydrous phase. These modifications suggest that the β-alanine backbone, with its corresponding CH2 and NH2 groups, presents a disordered conformation within the anhydrous phase. At this point, it is useful to compare the most important structural features of the temperature-dependent study of bis(β-alanine) nickel(II) dihydrate with similar compounds. For instance, the monoclinic structure of bis(L-glutaminato) copper(II) presented a stable

phase that remains ordered until the temperature of decomposition (near 500 K) [16]. Similar behavior was also observed for the tetragonal phase of bis(histidinato) nickel(II), but the structure had higher thermal stability (>620 K) [41]. However, the related material [bis(L‐ alaninato)diaqua] nickel(II) presented a monoclinic structure at room temperature and ambient pressure, and underwent a first phase transformation at 333 K due to the loss of water of crystallization molecules [17]. In addition, a second phase transformation was also observed around 368 K, which was associated with the loss of the remaining coordinated water molecules. Interestingly, this second phase transformation is similar to that reported in this paper for bis(β-alanine) nickel(II) dihydrate; however, the loss of these water molecules occurs at higher temperatures, between 453 and 463 K.

5.

Conclusions In this work, the thermal stability of Ni(β-alan)2·2H2O crystal as a function of temperature

was investigated using XRD, DTA-TG and Raman spectroscopy techniques. In addition, the vibrational (FT-IR and Raman) bands were duly ascribed by quantum chemistry calculations using the DFT method. The changes in the XRD patterns for 453–463 K were in good agreement with the loss of water molecules evidenced by the DTA-TG curves. The thermal expansion coefficients revealed anisotropic behavior of the crystalline axes. The phase transformation associated with the anhydrous phase was observed through two endothermic peaks in the DTA curve at about 429 and 451 K. Furthermore, the high-temperature Raman and XRD measurements suggested that the bis(β-alanine) nickel complex underwent a structural rearrangement in order to compensate for the water loss due to thermal dehydration. This transformation was duly confirmed by the spectral changes in the Raman bands of Ni(βalan)2·2H2O crystal at high temperatures. In particular, the disappearance of the Raman bands assigned as lattice vibrations and the broadening of bands attributed to vibrations of CH2, amine and carboxyl groups indicated that the β-alanine backbone presented a disordered conformation in the anhydrous phase. This work contributes to a better understanding of the structural, vibrational and thermal properties of Ni(β-alan)2·2H2O crystal, potentially leading to various industrial applications.

Acknowledgments J.G. Oliveira Neto acknowledges the Brazilian agency CAPES for a fellowship. J.G. da Silva Filho acknowledges computational support from the CENAPAD-SP (proj780). A.O. Santos acknowledges the MCT/CNPq and CAPES for financial support. F.F. Sousa, PhD, also acknowledges the CAPES and MCT/CNPq (Grants #: 23038.010322/2013; 309080/2016-9; 438753/2018-6). The authors also wish to thank the FAPEMA (Research Support Foundation of Maranhão). References [1]

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•

The obtained Ni(β-alan)2·2H2O crystals were blue rhomb-shaped platelets.

•

The TG curve indicates that the thermal decomposition proceeds by two steps.

•

The XRD results show a gradual amorphization of the material.

•

Raman spectra suggest that hydrogen bonds are highly affected by temperature.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: