Synthesis,structural,spectral and photoluminescent studies of new polymorphs of Ca(II) and Ba(II) complexes of diglycolic acid

2.1 Materials and methods

Diglycolic acid (CDH), calcium nitrate (CDH), barium chloride (CDH) and acetic acid (CDH) of AR grade (99%) and were used without further purification. The FT-IR spectra were recorded in the range 4000-400 cm^–1 using Thermo Nicolet, Avatar 370 Spectrometer in KBr pellets. The compositions of carbon, hydrogen, nitrogen in the compound were determined by using Elementar Vario-EL 111 CHN analyzer. The UV-Visible spectral studies were carried out using Varian Cary 5000 UV-Vis-NIR spectrometer in the range 200–1200 nm. Single crystal X-ray diffraction studies were carried out using Bruker AXS Kappa Apex 2 CCD diffractometer, with graphite monochromated Mo Kα radiation (λ= 0.71073 Å). The powder X-ray diffraction studies were conducted using a Bruker AXS D8 advance XRD with Cu Kα radiation (λ= 1.54056 Å). The thermogravimetric analysis of the grown crystals was carried out on a Perkin Elmer Diamond TG/DTG analyzer instrument with a heating rate of 10°C/min in nitrogen atmosphere. The photoluminescence of the samples are recorded using Fluoromax-3 spectroflurometer consisting of 150 W Xenon arc lamp, monochromator and a detector.

2.2 Growth Procedure of [Ca(dga)(H₂O)₅].H₂O (1) and [Ba(dga)(H₂O)] (2)

Gel diffusion method was employed for the crystal growth of the compounds 1 and 2 [20]. Sodium metasilicate of density 1.03–1.06 gcm^–3 was employed in gel preparation with double distilled water. The gel was prepared in several glass tubes by adding the ligand, diglycolic acid (0.5–1 M) to sodium metasilicate and the pH was adjusted by using acetic acid. After setting the gel, calcium nitrate or barium nitrate solution of desired molarity (0.5–1 M) was carefully added along the sides of the test tubes without disturbing the gel interface. The tubes are covered inorder to avoid the reaction of foreign particles from the atmosphere.

Complex (1): IR (KBr, υ cm^–1): 1592 s υ_asym(COO^–), 1441 s (COO^–), 1135 s υ(C–O–C), 572 m υ(M–O), 3405υ(H₂O). From the elemental analysis, the experimental and calculated percentages of carbon and hydrogen are % C = 17.21(calc. = 17.02) and % H = 5.92 (calc. = 5.71). The experimentally found value and calculated values are in good agreement with each other and agrees with the formulae [Ca(C₄H₄O₅)(H₂O)₅].H₂O.

Complex (2): IR (KBr, υ cm^–1): 1592 s υ_asym(COO^–), 1436 s υ_sym(COO^–), 1120 s υ(C–O–C), 556 m υ(M–O), 3449υ(H₂O). From the elemental analysis, the experimental and calculated percentages of carbon and hydrogen are % C = 16.65 (calc. = 16.70) and % H = 1.69 (calc. = 2.09). The experimental and calculated values are in good agreement with each other and agrees with the formulae [Ba(C₄H₄O₅)(H₂O)].

2.3 X-ray Crystallography

Single crystal X-ray diffraction studies of the complex were collected using Bruker AXS Kappa Apex2 CCD diffractometer (Bruker, AXS GmbH, Karlsruhe, Germany) with graphite monochromated Mo Kα radiation (λ= 0.71073 Å). The unit cell dimensions and intensity data were recorded at 296 K. The programme SAINT/XPREP was used for data reduction and APEXT2/SAINT for cell refinement [21]. The structures were solved by direct methods using SIR92 and refinement were carried out by full-matrix least squares on F² using SHELXL-97 [22, 23]. All the non-hydrogen atoms were refined anisotropically and the hydrogen atoms were refined isotropically. The structures were plotted by DIAMOND software version 3.1f [24]. The crystal data and structure refinement parameters for the complexes are given in Table 1.

3.1 Crystal growth

Transparent, needle shaped crystals appear at the gel interface within 1 week for 1 M diglycolic acid, 0.5 M CaNO₃ solution, pH 6.5 and 1 M diglycolic acid, 0.5 M BaNO₃ solution, pH 5.5 as its optimum crystal growth condition for compound 1 and 2 respectively. The crystals were separated, washed with doubly distilled water and dried. The photographs of crystalline complexes 1 and 2 are shown in Fig. 1(a) and (b).

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Fig.1

(a) Photograph of the crystalline complex 1. (b) Photograph of the crystalline complex 2.

In the present study, we report the new polymorphs of the complexes 1 and 2. The crystal structure of complex 1 is a new polymorph, which crystallizes in monoclinic space group, P2₁/n. In the reported structure, the space group is P2₁/a and the β value is 118.03(5)°, but in the present structure, the space group is P2₁/n and β value is 90.804(7)°. The asymmetric unit with atom numbering scheme, the molecular packing diagram viewed along ‘a’ axis are shown in Fig. 2(a) and (b) respectively. The selected bond lengths (Å) and bond angles (°) are listed in Table 2.

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Fig.2

(a) Asymmetric unit of complex 1 with atom numbering scheme. (b) Packing diagram viewed along ‘a’ axis.

The asymmetric unit consists of one diglycolate and 5 water molecules coordinated to calcium metal ion. A lattice water molecule is also present in it. The diglycolate ligand in complex 1 act as a chelating unit of two end carboxylate oxygen atoms and one etherial oxygen atom and the other carboxylate oxygen atoms remains uncoordinated. So it possesses a monomeric structure unlike to that of complex 2. Here, the calcium ion is coordinated to 9 oxygen atoms (O1, O3, O5 from the diglycolate and O6, O7, O8, O9, O10 from 5 water molecules) and a lattice water molecule. The Ca–O bond lengths ranging from 2.3743(17) to 2.557(2) Å. The CaO₈ polyhedra exist as monomeric in nature and these monomeric units interacts each other via lattice and coordinated water molecule to form 2D supramolecular sheets and the hydrogen bonding interaction are shown in Table 3.

The crystal structure reveals that compound 2 crystallizes in orthorhombic space group, Pnma. The molecular structure is shown in Fig. 3. The selected bond lengths (Å) and bond angles (°) are listed in Table 4.

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Fig.3

The molecular structure of complex 2.

In the asymmetric unit of 2, three oxygen atoms from one diglycolate and one oxygen atom from water molecule co-ordinates to the central Ba(II) metal atom. Considering the coordination environment, barium is coordinated to 8 oxygen atoms, (1 oxygen atom from one water molecule, 3 oxygen atoms from one of the diglycolate ligand, 4 oxygen atoms from 4 other diglycolate unit). In the reported paper, barium cation is surrounded by O₉ donor sets in monocapped square antiprism environment and the capping site occupied by second coordinating water molecule. The carboxylate ends of diglycolate coordinates to barium centres in a bridging μ _{2  -}  η ¹ - η ² mode (one of the carboxylate oxygen atoms connect one Ba(II) and other oxygen atoms connected to two different Ba(II) centers). The Ba–O bond distance are in the range of 2.7473(18)–3.295(4) Å, which is close nearer to the values of bond lengths 2.744(10)–3.316(3) Å in the reported structure. The O–Ba–O bond angles are in the range of 56.74(4)°–147.96(6)° and are higher than that of reported paper [56.7(3)°–95.88(9)°]. The highest bond angle O(3)#4–Ba(1)–O(1) = 147.96(6)° is observed in 2. The non-bonded distance between two Ba(II) atoms is 4.3572(3) Å and is slightly lower than that of the reported one [Ba…Ba(#5,#6) = 4.374(1)] Å. Packing diagram viewed along ‘c’ axis is shown in Fig. 5(a). The adjacent barium centres are connected by diglycolate oxygen atoms to form Ba₂O₂ rings, which are extended to create 3D packing. The oxygen atom from water molecule is hydrogen bonded to O(1) and O(3) of carboxylate group and similar to the reported one. Hydrogen bonding interactions stabilizes the unit cell packing. The H-bonding interactions are shown in Table 5. H-bonded packing diagram of the molecule is shown in Fig. 4. The absence of water molecules in the pores, even though the crystal structure is having water molecule, shows that the channels are hydrophobic.

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Fig.4

H-bonded packing diagram of complex 2 viewed along ‘a’ axis.

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Fig.5

(a) FT-IR spectrum of complex 1. (b) FT-IR spectrum of complex 2.

To determine the coordination modes of ligand to metal atom FT-IR spectral studies are carried out. The FT-IR spectrum of the complex 1 and 2 are interpreted on the basis of the various metal diglycolates [25–27]. The infrared spectrum exhibits several bands due to various coordination modes present in the complex. The broad band observed in the region 3400 to 3500 cm^–1 in the complex 1 and 2 is attributed to O–H stretching vibration of water molecule. A strong band at 1706 cm^–1 attributed to the ν(C=O) vibration of ligand is absent in the complexes and indicates that all carboxylic acid groups are deprotonated and carboxylate oxygen atoms are coordinated to the central metal atom. The corresponding asymmetric and symmetric stretching vibrations of COO^– group appear at 1592 and 1441 cm^–1 in 1 and 1592 and 1436 cm^–1 in 2 support these observations. Δυ= 151, 156 cm^–1 in complex 1 and 2 indicates that the coordinating carboxylic groups are bridging type. The very strong band at 1146 cm^–1 in the free diglycolic acid is attributed to C–O–C stretching, which is shifted to lower wave number 1135, 1120 cm^–1 in 1, 2 respectively. This suggests the coordination of etherial oxygen with the central metal ion. The M–O stretching vibrations are observed at 572, 556 cm^–1 in 1 and 2. The FT-IR spectra of 1 and 2 are shown in Fig. 5(a, b).

3.4 Powder X-ray diffraction studies

The crystalline nature of the title compound was revealed from the well defined Braggs peak at specific 2θ angles. Inorder to check the bulk purity of the compounds, powder X-ray diffraction pattern was experimentally measured and compared with X-ray diffraction pattern obtained from mercury software using single crystal XRD data. These two results are in good agreement with each other which indicates that the compounds possess bulk purity. The powder X-ray diffraction pattern of compounds 1 and 2 are shown in Fig. 6(a, b).

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Fig.6

(a) Experimental and simulated X-ray diffractograms of [Ca(dga)(H₂O)₅].H₂O. (b) Experimental and simulated X-ray diffractograms of [Ba(dga)(H₂O)].

The solid state absorption spectrum of complex 1 and 2 were recorded in a region of 200–1200 nm. The absorbance at 325 and 363 nm in 1 and 2 can be attributed to the intraligand charge transitions of the ligand. There is no absorption band is observed in a region of 400–800 nm in both the complexes, which shows the compounds are highly transparent in the entire visible region. The absorbance spectrum of 1 and 2 are shown in Fig. 7(a, b).

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Fig.7

(a) Absorption spectrum of [Ca(dga)(H₂O)₅].H₂O. (b) Absorption spectrum of [Ba(dga)(H₂O)]

The thermal stability of the compound 1 and 2 has been studied by thermogravimetry experiments conducted at a heating rate of 10°C/min in nitrogen atmosphere. The compound 1 decomposes in 4 stages. The first stage of decomposition corresponds to the removal of one lattice and two coordinated water molecules with an observed weight loss of 19.0% (19.2% calc.). The second stage of decomposition occurs within a high temperature range of 137 to 397°C with an observed weight loss of 18.3% (19.2% calc.), which cooresponds to the removal of the remaining three strongly coordinated water molecule. The third and fourth stage corresponds to the removal of one diglycollate ligand with a weight loss of 38.5% (40.67% calc.) and finally the residual weight is obtained as CaO with a weight loss of 20.0% (19.9% calc.).

The compound 2 decomposes in three stages. The first stage of decomposition corresponds to the sharp peak at 243°C in DTG curve, with an observed weight loss of 6.8% (6.2% calc.) and corresponds to the removal of one water molecule. The second stage of decomposition at 416°C with an observed weight loss of 24.1% (25.0% calc.) corresponds to the pyrolysis of the ligand moieties and after the third step, the percentage of residual weight obtained from the graph (67.3%) is in well agreement with the calculated value of 68.5%, indicating that the end product obtained is barium carbonate (BaCO₃). TG/DTG curves of compounds 1 and 2 are shown in Fig. 8(a, b).

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Fig.8

(a) TG/DTG curve of [Ca(dga)(H₂O)₅].H₂O. (b) TG/DTG curve of [Ba(dga)(H₂O)].

The solid state photoluminescent spectrum of the complexes 1 and 2 and the ligand, diglycolic acid was recorded at room temperature. The photoluminescent spectrum of complex 1 shows broad emission with a maximum wavelength of 410 nm upon excitation of 280 nm. The free diglycolic acid shows emissions at 384 nm, 387 nm upon excitation of 280 nm. The luminescent property of 2 has been assigned to the ligand centered transfer (n-π* or π-π*) and intensity of the complex 2 is higher than that of the ligand, which reveals the complex is red shifted.

The complex 2 shows broad emission with a maximum wavelength of 415 nm, 430 nm upon excitation of 280 nm. The free diglycolic acid shows a small emission at 384 nm, 387 nm upon excitation of 280 nm, which reveals the complex, is red shifted. The spectral pattern is due to the complexation of ligand to metal with a ligand centred charge transfer (n-π* or π-π*) [28]. The luminescent spectra of complexes 1 and 2 are shown in Fig. 9(a, b).

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Fig.9

(a) The solid state photoluminescent spectrum of diglycolic acid and [Ca(dga)(H₂O)₅].H₂O. (b) The solid state photoluminescent spectrum of diglycolic acid and [Ba(dga)(H₂O)]