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

Abstract

Metal carboxylates possess wide range of intriguing structural topologies due to their diverse ligational behavior of carboxylate group. Two new polymorphs of monomeric complex, [Ca(dga)(H₂O)₅].H₂O (1) and the three-dimensional metal-organic framework, [Ba(dga)(H₂O)] (2) (dgaH₂ = diglycolic acid) have been grown by gel diffusion technique at room temperature. Colourless transparent crystals were obtained within one week. Single crystal X-ray diffraction study reveals that the title compounds crystallized in monoclinic P2₁/n in 1 and orthorhombic system with space group Pnma in 2. The monomeric units, CaO₈ interact each other via extensive hydrogen bonding among the coordinated and the lattice water molecules with the carboxylate oxygen atoms to form a 2D supramolecular sheet like structure in 1. The adjacent BaO₈ polyhedra are connected by diglycolate ligand forming the 3D framework with porous architecture in 2. Elemental analysis, FT-IR, UV-Visible, thermogravimetric, powder X-ray diffraction and solid state photoluminescence studies were also carried out.

Keywords

Crystal structure gel diffusion technique single crystal X-ray diffraction 3D and 2D structure photoluminescence

1 Introduction

Over the past decades, metal complexes of carboxylic acid having N, O-donors with polymeric or network type fascinating structures have received much attention. An extended array of M-L-M connectivity formed of isolated metal atoms or clusters with polyfunctional organic ligands having porous architectures are termed as Metal Organic Frameworks (MOFs). The studies of MOFs have increasing interest in research fields due to their structural diversities, porous nature and the versatile coordination modes of MOFs create one, two or three dimensional structural topologies [1 –3].

MOFs show many potential applications due to their large open spaces in its structures. MOFs show wide range of applications in the field of gas storage, gas adsorption, catalysis, molecular magnetism and photoluminescence [4 –6]. The ligands are usually selected as polyfunctional ligands, which exhibit diverse coordination modes and variety of structural features [7 –9]. Diglycolic acid is otherwise known as oxydiacetic acid (HOOCCH₂OCH₂COOH), is a polydentate ligand having four carboxylate oxygen atoms and one etherial oxygen atom, which act as bridging chelate coordination to central metal atom possessing increasing network topologies [10]. Metal complexes of diglycolic acid are employed for pharmaceuticals and have biological applications, which are further enhanced by some N-heterocyclic derivatives. Literature studies indicate that transition, rare earth or main group metals are used for generating the various metal carboxylates [11 –13]. Among them,

The Cu(II) complexes of oxydiacetate ligand with 4-picolinic acid, [Cu(oda)(4-pic)H₂O]2H₂O and with 2,2’-bipyridine ligand, [Cu(oda)(bipy)(H₂O)].4H₂O exhibited considerable biocatalytic activity towards the dismutation of superoxide anion. The biological activity of metal oxydiacetate was further enhanced by the presence of auxiliary ligand especially N-heterocyclic derivatives such as picolinic acid. The copper, cobalt and iron complexes of oxydiacetic acid with picolinic acid: [M(oda)(4-pic)H₂O].xH₂O {M = Cu(II), x = 2; Co(II), x = 4} and [Fe(oda)(4-pic)].Cl shows remarkable antimicrobial activities against Escherichia coli, Bacillus subtilis, Staphylococcus aureus, Salmonella typhimurium, Candida albicans, Aspergillus fumigatus and Penicillium marneffei. The biological activity of the vanadium oxydiacetate, [VO(oda)(H₂O)₂] suggests that the compound is an inhibitory agent of osteoblast differentiation [11 –13].

Alkaline earth metal carboxylates possess larger radii, high affinity for oxygen atom and show applications in material science [14, 15]. Majority of Ca(II) complexes exhibit higher coordination number due to the lack of d electron in Ca(II) ion. The higher ionic radii and oxophilic nature of Ba(II) ion results in the formation of inorganic-organic hybrid frameworks. The alkaline earth metal oxydiacetates, monomeric [Ca(oda)(H₂O)₅].H₂O and polymeric [Ba(oda).H₂O]_n, [{Mg(oda)(H₂O)₂}.H₂O]_n, [Sr(oda)(H₂O)₃]_n.nH₂O (oda = –O₂CCH₂OCH₂CO₂–) have been reported by slow evaporation method [16 –19]. Slow evaporation method is a technique in which a saturated solution of metal salts along with the ligand and suitable solvent is kept at a particular temperature and made for evaporation.

Taking these observations, herein we have grown two metal diglycolates of Ca(II) and Ba(II), [Ca(dga)(H₂O)₅].H₂O (1) and [Ba(dga)(H₂O)] (2) which are polymorphs of the complexes reported earlier. To the best of our knowledge, this is the first time that these complexes are grown by gel diffusion technique. The compound (1) is a monomer and (2) is MOF which explores the structural diversity of the ligand. The title compounds grown by slow evaporation technique provides the same chemical formula as that of the reported one, while the gel method provides new polymorphs belonging to monoclinic, P2₁/n and the angle β is of 90.804(7)° in 1 and orthorhombic system, Pnma with central Ba(II) is eight coordinated in 2. The polymorph in the sense that in the reported structure the Ca(II) complex crystallizes in monoclinic space group P2₁/a with angle β is of 118.03°, while Ba(II) complex crystallizes in orthorhombic space group Pna2(1) and central barium is nine coordinated to form monocapped square antiprism geometry. The grown crystals were characterized by elemental analysis, FT-IR, UV-Visible, thermogravimetric and powder X-ray diffraction studies. The photoluminescent properties of the compounds and the ligand were also investigated.

2 Experimental

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.

Table 1
Crystal data and structure refinement for [Ca(dga)(H₂O)₅].H₂O (1) and [Ba(dga)(H₂O)] (2)

1 2

Empirical formula C₄ H₁₆ Ca O₁₁ C₄ H₆ Ba O₆

Formula weight 280.25 287.43

Temperature 571(2) K 296(2) K

Wavelength 0.71073 Å 0.71073 Å

Crystal system Monoclinic Orthorhombic

Space group P2₁/n Pnma

Unit cell dimensions a = 6.186(9) Å, α= 90° a = 7.2259(3) Å, α= 90°.

b = 15.892(9) Å, β= 90.804(7)° b = 10.0453(4) Å, β= 90°.

c = 11.332(8) Å, γ= 90° c = 9.7860(4) Å, γ= 90°.

Volume 1114.0(19) Å³ 710.33(5) Å³

Z 4 4

Density (calculated) 1.671 Mg/m³ 2.688 Mg/m³

Absorption coefficient 0.612 mm^–1 5.572 mm^–1

F(000) 592 536

Crystal size 0.400×0.300×0.200 mm³ 0.350×0.300×0.250 mm³

Theta range for data collection 3.131 to 27.437° 2.906 to 24.995°

Index ranges –7≤h≤7, –20≤k≤17, –10≤l≤14 –6≤h≤8, –11≤k≤11, –11≤l≤11

Reflections collected 8203 4544

Independent reflections 2450 [R(int) = 0.0656] 668 [R(int) = 0.0265]

Completeness to theta = 25.242 97.5% = 24.995°,100.0%

Absorption correction Empirical Semi-empirical from equivalents

Max. and min. transmission 0.803 and 0.693 0.339 and 0.245

Refinement method Full-matrix least-squares on F² Full-matrix least-squares on F²

Data/restraints/parameters 2450/18/193 668/1/60

Goodness-of-fit on F² 0.993 1.217

Final R indices [I>2sigma(I)] R1 = 0.0394, wR2 = 0.1025 R1 = 0.0140, wR2 = 0.0335

R indices (all data) R1 = 0.0409, wR2 = 0.1042 R1 = 0.0152, wR2 = 0.0344

Extinction coefficient n/a 0.0064(5)

Largest diff. peak and hole 0.350 and -0.285 e.Å^–3 0.312 and –0.359 e.Å^–3

R₁ =Σ||F_o| – |F_c|| /Σ|F_o|, wR₂ = [Σw(F_o²-F_c²)²/Σw(F_o²)²]^1/2.

3 Results and discussion

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).

Fig.1

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

3.2 Crystal structure of [Ca(dga)(H₂O)₅].H₂O (1) and [Ba(dga)(H₂O)] (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.

Fig.2

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

Table 2

Bond lengths (Å) and bond angles (°) for [Ca(dga)(H₂O)₅].H₂O

Ca(1)-O(6)	2.3743(17)
Ca(1)-O(7)	2.425(3)
Ca(1)-O(3)	2.4310(17)
Ca(1)-O(10)	2.4585(19)
Ca(1)-O(1)	2.459(2)
Ca(1)-O(5)	2.472(3)
Ca(1)-O(8)	2.505(2)
Ca(1)-O(9)	2.557(2)
O(6)-Ca(1)-O(7)	81.71(8)
O(6)-Ca(1)-O(3)	138.66(6)
O(7)-Ca(1)-O(3)	127.18(5)
O(6)-Ca(1)-O(10)	80.16(6)
O(7)-Ca(1)-O(10)	70.71(5)
O(3)-Ca(1)-O(10)	133.28(5)
O(6)-Ca(1)-O(1)	155.24(5)
O(7)-Ca(1)-O(1)	74.88(9)
O(3)-Ca(1)-O(1)	64.87(6)
O(10)-Ca(1)-O(1)	84.56(6)
O(6)-Ca(1)-O(5)	75.67(6)
O(7)-Ca(1)-O(5)	140.92(6)
O(3)-Ca(1)-O(5)	63.45(4)
O(10)-Ca(1)-O(5)	133.88(5)
O(1)-Ca(1)-O(5)	128.32(6)
O(6)-Ca(1)-O(8)	86.03(5)
O(7)-Ca(1)-O(8)	71.82(5)
O(3)-Ca(1)-O(8)	78.26(5)
O(10)-Ca(1)-O(8)	141.49(6)
O(1)-Ca(1)-O(8)	94.11(6)
O(5)-Ca(1)-O(8)	75.17(8)
O(6)-Ca(1)-O(9)	94.82(7)
O(7)-Ca(1)-O(9)	140.72(6)
O(3)-Ca(1)-O(9)	79.80(5)
O(10)-Ca(1)-O(9)	70.15(7)
O(1)-Ca(1)-O(9)	98.17(9)
O(5)-Ca(1)-O(9)	73.43(9)
O(8)-Ca(1)-O(9)	147.30(5)

Symmetry transformations used to generate equivalent atoms: #1 x+1,y,z #2 x-1/2,-y+1/2,z-1/2 #3 x+1/2,-y+1/2,z-1/2. #4 -x-1/2,y+1/2,-z+1/2 #5 -x+1/2,y+1/2,-z+1/2. #6 -x+1,-y+1,-z+1 #7 x-1,y,z #8 -x,-y+1,-z+1. #9 x+1/2,-y+1/2,z+1/2 #10 -x+1/2,y-1/2,-z+1/2.

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.

Table 3

Hydrogen bonds for [Ca(dga)(H₂O)₅].H₂O [Å and deg.]

D–H⋯A	d(D–H)	d(H⋯A)	d(D⋯A)	< (DHA)
O(9)–H(9A)⋯O(1)#1	0.863(10)	2.173(12)	3.025(4)	169(3)
O(9)–H(9B)⋯O(4)#2	0.860(10)	1.956(12)	2.800(2)	167(3)
O(10)–H(10A)⋯O(11)#3	0.853(10)	2.039(13)	2.868(3)	164(2)
O(10)–H(10B)⋯O(11)#4	0.858(10)	2.130(12)	2.974(4)	168(3)
O(6)–H(6A)⋯O(2)#5	0.862(10)	2.002(13)	2.836(2)	163(2)
O(6)–H(6B)⋯O(5)#6	0.857(10)	1.967(12)	2.797(2)	163(2)
O(7)–H(7A)⋯O(5)#7	0.852(10)	1.958(10)	2.807(3)	175(3)
O(7)–H(7B)⋯O(8)#8	0.851(10)	1.983(10)	2.832(2)	175(2)
O(8)–H(8A)⋯O(4)#7	0.854(10)	1.836(11)	2.677(3)	168(2)
O(8)–H(8B)⋯O(2)#9	0.850(10)	1.901(11)	2.735(3)	167(3)
O(11)–H(11A)⋯O(9)#10	0.856(10)	2.154(11)	3.003(3)	171(3)
O(11)–H(11B)⋯O(2)	0.855(10)	2.052(10)	2.903(2)	173(2)

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.

Fig.3

The molecular structure of complex 2.

Table 4

Bond lengths (Å) and bond angles (°) for [Ba(dga)(H₂O)]

O(1)-Ba(1)	2.798(2)
O(2)-Ba(1)	2.761(2)
O(3)-Ba(1)#3	2.7473(18)
O(3)-Ba(1)	2.7724(19)
O(4)-Ba(1)	2.769(3)
O(4)-Ba(1)#4	3.295(4)
Ba(1)-O(3)#5	2.7473(18)
Ba(1)-O(3)#4	2.7473(18)
Ba(1)-O(3)#2	2.7724(19)
Ba(1)-O(1)#2	2.798(2)
Ba(1)-O(4)#3	3.295(4)
Ba(1)-Ba(1)#4	4.3571(3)
Ba(1)-Ba(1)#3	4.3572(3)
Ba(1)#3-O(3)-Ba(1)	104.25(6)
Ba(1)-O(4)-Ba(1)#4	91.45(9)
Ba(1)-O(4)-H(4A)	120(3)
Ba(1)#4-O(4)-H(4A)	85(3)
O(3)#5-Ba(1)-O(3)#4	68.24(8)
O(3)#5-Ba(1)-O(2)	142.41(5)
O(3)#4-Ba(1)-O(2)	142.41(5)
O(3)#5-Ba(1)-O(4)	71.24(7)
O(3)#4-Ba(1)-O(4)	71.24(7)
O(2)-Ba(1)-O(4)	96.04(10)
O(3)#5-Ba(1)-O(3)	86.77(5)
O(3)#4-Ba(1)-O(3)	124.56(3)
O(2)-Ba(1)-O(3)	85.87(7)
O(4)-Ba(1)-O(3)	146.21(4)
O(3)#5-Ba(1)-O(3)#2	124.56(3)
O(3)#4-Ba(1)-O(3)#2	86.77(5)
O(2)-Ba(1)-O(3)#2	85.87(7)
O(4)-Ba(1)-O(3)#2	146.21(4)
O(3)-Ba(1)-O(3)#2	67.54(8)
O(3)#5-Ba(1)-O(1)#2	147.96(6)
O(3)#4-Ba(1)-O(1)#2	86.15(6)
O(2)-Ba(1)-O(1)#2	56.74(4)
O(4)-Ba(1)-O(1)#2	82.75(6)
O(3)-Ba(1)-O(1)#2	124.54(6)
O(3)#2-Ba(1)-O(1)#2	70.15(5)
O(3)#5-Ba(1)-O(1)	86.15(6)
O(3)#4-Ba(1)-O(1)	147.96(6)
O(2)-Ba(1)-O(1)	56.74(4)
O(4)-Ba(1)-O(1)	82.75(6)
O(3)-Ba(1)-O(1)	70.15(5)
O(3)#2-Ba(1)-O(1)	124.54(6)
O(1)#2-Ba(1)-O(1)	109.28(8)
O(3)#5-Ba(1)-O(4)#3	61.31(6)
O(3)#4-Ba(1)-O(4)#3	61.31(6)
O(2)-Ba(1)-O(4)#3	142.27(9)
O(4)-Ba(1)-O(4)#3	121.69(9)
O(3)-Ba(1)-O(4)#3	63.26(6)
O(3)#2-Ba(1)-O(4)#3	63.26(6)
O(1)#2-Ba(1)-O(4)#3	123.15(4)
O(1)-Ba(1)-O(4)#3	123.15(4)

Symmetry transformations used to generate equivalent atoms: #1 -x,-y+1,-z #2 x,-y+1/2,z #3 x-1/2,-y+1/2,-z+1/2. #4 x+1/2,-y+1/2,-z+1/2 #5 x+1/2,y,-z+1/2.

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.

Fig.4

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

Fig.5

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

Table 5

Hydrogen bonds for [Ba(dga)(H₂O)] [Å and deg.]

D–H⋯A	d(D–H) (Å)	d(H⋯A) (Å)	d(D⋯A) (Å)	< (DHA) (Å)
O(4)–H(4A)⋯O(1)#6	0.834(18)	2.17(2)	2.994(2)	171(4)
O(4)–H(4A)⋯O(3)#7	0.834(18)	2.52(4)	3.116(4)	129(4)

Symmetry transformations used to generate equivalent atoms: #1 -x,-y+1,-z #2 x,-y+1/2,z #3 x-1/2,-y+1/2,-z+1/2. #4 x+1/2,-y+1/2,-z+1/2 #5 x+1/2,y,-z+1/2 #6 -x+1,y-1/2,-z #7 x+1,-y+1/2,z.

3.3 FT-IR spectroscopy

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).

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)].

3.5 UV-Vis spectral studies

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).

Fig.7

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

3.6 Thermal studies

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).

Fig.8

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

3.7 Photoluminescent studies

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).

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)]

4 Conclusion

Single crystals of two new polymorphs of monomeric complex of 1 and three-dimensional metal-organic framework of complex 2 have been successfully grown by gel diffusion technique at room temperature. Single crystal X-ray diffraction analysis reveals that the complex 1 belongs to monoclinic P2₁/n, having a monomeric structure and converted in to 2D supramolecular sheets by extensive hydrogen bonding interaction. The complex 2 belongs to orthorhombic space group Pnma, having a 3D framework consists of hydrophobic channels. The composition of the complexes was obtained from the elemental analysis. The FT-IR spectral analysis confirms that the ligand coordinate to the metal atom through carboxylate and etherial oxygen atoms. UV-Visible spectrum shows that that the compounds are highly transparent in the entire visible region. The thermal decomposition behavior of the compounds was provided by TG studies. The crystallinity of the compounds was confirmed by powder X-ray diffraction studies. The luminescent property of the compounds can be used as potential photoactive material.

Footnotes

Acknowledgments

One of the authors, USSM is thankful to the UGC for providing the financial assistance in the form of Junior Research Fellowship. The authors are thankful to SAIF, CUSAT, Kochi, India and Dr. A. Santhosh Kumar, School of Pure and Applied Physics, M. G University, Kottayam, India for providing analytical facilities. We are also thankful XRD Lab, SAIF, IIT-Madras, Chennai, India for single crystal X-ray diffraction measurements and crystal structure refinement.

CCDC no 1501100, 1501101, contains the supplementary crystallographic data for the compounds [Ca(C₄H₄O₅)(H₂O)₅].H₂O, [Ba(C₄H₄O₅)(H₂O)] respectively. These data can be obtained free of charge via or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2, 1EZ, UK; Fax: (+44)1223–336-033; E-mail: deposit@ccdc.cam.ac.uk.

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	1	2
Empirical formula	C₄ H₁₆ Ca O₁₁	C₄ H₆ Ba O₆
Formula weight	280.25	287.43
Temperature	571(2) K	296(2) K
Wavelength	0.71073 Å	0.71073 Å
Crystal system	Monoclinic	Orthorhombic
Space group	P2₁/n	Pnma
Unit cell dimensions	a = 6.186(9) Å, α= 90°	a = 7.2259(3) Å, α= 90°.
	b = 15.892(9) Å, β= 90.804(7)°	b = 10.0453(4) Å, β= 90°.
	c = 11.332(8) Å, γ= 90°	c = 9.7860(4) Å, γ= 90°.
Volume	1114.0(19) Å³	710.33(5) Å³
Z	4	4
Density (calculated)	1.671 Mg/m³	2.688 Mg/m³
Absorption coefficient	0.612 mm^–1	5.572 mm^–1
F(000)	592	536
Crystal size	0.400×0.300×0.200 mm³	0.350×0.300×0.250 mm³
Theta range for data collection	3.131 to 27.437°	2.906 to 24.995°
Index ranges	–7≤h≤7, –20≤k≤17, –10≤l≤14	–6≤h≤8, –11≤k≤11, –11≤l≤11
Reflections collected	8203	4544
Independent reflections	2450 [R(int) = 0.0656]	668 [R(int) = 0.0265]
Completeness to theta	= 25.242 97.5%	= 24.995°,100.0%
Absorption correction	Empirical	Semi-empirical from equivalents
Max. and min. transmission	0.803 and 0.693	0.339 and 0.245
Refinement method	Full-matrix least-squares on F²	Full-matrix least-squares on F²
Data/restraints/parameters	2450/18/193	668/1/60
Goodness-of-fit on F²	0.993	1.217
Final R indices [I>2sigma(I)]	R1 = 0.0394, wR2 = 0.1025	R1 = 0.0140, wR2 = 0.0335
R indices (all data)	R1 = 0.0409, wR2 = 0.1042	R1 = 0.0152, wR2 = 0.0344
Extinction coefficient	n/a	0.0064(5)
Largest diff. peak and hole	0.350 and -0.285 e.Å^–3	0.312 and –0.359 e.Å^–3

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

Abstract

Keywords

1 Introduction

2 Experimental

2.1 Materials and methods

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

2.3 X-ray Crystallography

3.1 Crystal growth

3.4 Powder X-ray diffraction studies

Footnotes

Acknowledgments

References

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