Structural studies on a novel 2D polymeric barium complex of keto tautomeric form of 2-hydroxynicotinic acid –poly[aqua(μ 3 -chlorido)(μ 3 -2-oxo-1,2-dihydropyridine-3-carboxylato)barium(II)]

Abstract

A 2D barium complex formulated as [Ba(C₆H₄NO₃)(Cl)(H₂O)]_n was successfully synthesized by gel diffusion technique using hydrosilica gel medium at room temperature. Single crystal X-ray diffraction studies confirm that the complex is triclinic with space group P $\bar{1}$ and exhibits a 2D framework by the interconnection of oxobridge and chlorobridge in alternative layers. The unit cell dimensions are a = 5.7737(2) Å, b = 6.8503(2) Å, c = 11.5378(4) Å, α= 106.432(2)°, β= 100.644(2)°, γ= 93.049(2)°. The crystal structure is further stabilized by a 3D one through a pair of N–H ⋯O intermolecular hydrogen bonds to form R₂²(8) ring motifs. Each barium cation is nine coordinated to form distorted tricapped trigonal prism geometry. The gel grown crystals were characterized by several physico-chemical techniques including elemental analysis, FT-IR, UV-Visible spectral studies, powder X-ray diffraction and thermogravimetry.

Keywords

Gel diffusion technique single crystal X-ray diffraction 2D framework 2-hydroxynicotinic acid

1 Introduction

The class of metal–organic frameworks (MOFs) organized by the combination of inorganic metal nodes and organic linker units has become as an enticing part of research because of the manifold applications in the field of gas storage, toxic gas adsorption, selective separation, catalysis and luminescent probes [1 –4]. The design and tuning of the novel structures are still challenging due to the unpredictability of the structures formed. The structural diversity of MOFs plays a significant role in the concept of reticular chemistry and is dependent on several dynamic factors such as the coordination geometry of the metal nodes, nature of the organic ligand, pH of the reaction medium, solvent, temperature, reaction route, metal-ligand ratio etc. [5, 6]. Here, the hydroxyl derivative of nicotinic acid, 2-hydroxynicotinic acid (H₂pyco) has been selected as organic linker for the construction of the title compound. It is a rigid acid containing salicylate moiety along with N and O heteroatoms which is able to bind with metals in various types such as monodentate, bridging, N,O-chelating (to form a 4 membered chelate ring by pyridine-N and hydroxyl-O group) and O,O-chelating (to form a 6 membered chelate ring by hydroxyl-O and carboxylate-O group) [7 –9]. From a biological point of view, H₂pyco has a lot of significance in the treatment of atherosclerosis, hypoglycemia and also acts as a metabolic inhibitor of nicotinamide adenine dinucleotide (NADH) in human blood platelets [10, 11]. From the structure, it is clear that acidic and labile hydrogen atom of –OH group is adjacent to the basic N atom of pyridine ring and can be easily attached to it. As a result, the ligand structure would be stabilized by enol-ketone tautomerism. The ketone form of the free ligand is stable in the solid state because of the presence of intramolecular hydrogen bonding between the –COOH and –C = O group [12, 13]. The tautomeric representation is shown in Scheme 1.

Scheme 1

Enol-keto tautomerism in H₂pyco.

The crystal structure and characterization studies on a novel 2D polymeric framework of barium complex BAPYC by gel diffusion technique at room temperature is reported here. Alkaline earth MOFs have earned a lot of attention due to the large ionic radii, high affinity towards N and O donor atoms, higher coordination number and various coordinating abilities [14]. To the best of our knowledge, this is the first report of the title compound containing Ba-Cl-Ba bridges and carboxylate group of ketonic tautomer, Hpyco showing μ₃-η²:η² coordination mode for structural continuity. The gel grown crystals were further subjected to several characterization techniques which include elemental analysis, FT-IR, single crystal X-ray diffraction, powder X-ray diffraction, UV-Visible spectral studies and thermal analysis.

2 Experimental

2.1 Reagents

Sodium metasilicate (CDH), glacial acetic acid (CDH), 2-hydroxypyridine-3-carboxylic acid (Sigma Aldrich) and barium chloride dihydrate (CDH) were commercially purchased and used without further purification.

2.2 Characterization

The carbon, hydrogen and nitrogen content in the crystals were determined by using Elementar Vario-EL III CHNS analyzer. The FT-IR spectrum was recorded from potassium bromide pellets on a Thermo Nicolet, Avatar 370 spectrometer in the range 4000-400 cm^–1. The UV-Vis-NIR spectrum was recorded with a Varian Cary 5000 UV-vis-NIR spectrometer in the range 200–1200 nm. The TG & DTG experiments were carried out on a Perkin Elmer Diamond TG/DTG analyzer instrument with a heating rate of 10°C/min in nitrogen atmosphere. Powder X-ray diffraction studies were carried out using a Bruker AXS D8 advance XRD with Cu Kα radiation (λ= 1.54056Å).

2.3 Growth procedure of poly[aqua(μ₃-chlorido)(μ₃-2-oxo-1,2-dihydropyridine-3- carboxylato)barium(II)] (BAPYC)

The growth of single crystals of the title compound BAPYC was carried out by gel diffusion technique. It is an effective and easy way to perform reactions in simple boiling tubes at room temperature [15 –21]. Here, sodium metasilicate dissolved in distilled water was used as the gel medium. It was then filtered to obtain clear solution in order to be free from impurities. The density of the gel medium (1.03–1.06 g cm^–3) was measured by using a sensitive hydrometer and pH (5–7.5) of the solution was varied by adding acetic acid drop wise. Several batches of experiments were conducted at different percentage composition, gel density and pH of the solution. The optimum condition for the growth of crystals was obtained by dissolving 0.75 g of H₂pyco in 25 mL of 1.04 g cm^–3 dense gel medium and consequently the solution was filtered to remove any impurities present. From the above solution 5 mL was poured into the boiling tubes and the pH of the solution was adjusted to 6.5 by adding acetic acid. Once the gel medium was properly set, 3 ml of 0.5 M barium chloride solution was carefully added to the top of the gel medium. The boiling tubes were then covered well with plastic sheets and kept undisturbed at room temperature. IR (KBr,cm^–1): 3520 (b),3440 (b),3020 (w),1640 (s),1523(s),779 (m),579 (w). Elemental Analysis Calculated for C₆H₆BaClNO₄: C, 21.89; H, 1.82; N, 4.25. Found: C, 21.49; H, 1.48; N, 4.20. The crystallization process can be explained by the expected chemical reaction taking place by the diffusion of ions in gel medium and is shown below. $\begin{matrix} C_{6} H_{5} {NO}_{3} + {BaCl}_{2} {. 2 H}_{2} O \to [{Ba (C}_{6} H_{4} {NO}_{3} {) (Cl) (H}_{2} {O)] + HCl + H}_{2} O \\ (BAPYC) \end{matrix}$

2.4 Single crystal X-ray diffraction studies

The crystal structure of the complex was confirmed by single crystal X-ray diffraction carried out using a Bruker AXS Kappa Apex2 CCD diffractometer at room temperature with graphite monochromated Mo Kα (λ= 0.71073 Å) radiation. The unit cell dimensions and intensity data were recorded at 293 K. The programs SAINT/XPREP and APEX2/SAINT were used for data reduction and cell refinement respectively [22]. The structure was solved by direct methods and was refined by full-matrix least squares on F² using SHELXL-97 computer program [23]. All non-hydrogen atoms were refined with anisotropic thermal parameters. Carbon bound hydrogen atoms were placed in calculated positions and included in the refinement in the riding model approximation. Molecular graphics were plotted by using the IUCr software Mercury (Version 3.8) and ToposPro software [24].

3 Results and discussion

3.1 Crystal growth

Crystals of BAPYC were seen at the gel interface within one week after the completing the procedures for crystal growth. The grown crystals were then separated, washed with distilled water and dried. The photographs of BAPYC crystals at gel interface is shown in Fig. 1.

Fig.1

Photographs of BAPYC crystals at gel interface.

3.2 FT-IR spectral studies

The FT-IR spectrum was recorded with KBr discs in the range 4000-400 cm^–1 and is shown in Fig. 2. The appearance of bands at 3520 and 3440 cm^–1 may be attributed to the hydrogen bonded O-H and N-H stretching frequency. This verifies that the proton of the hydroxyl group is transferred to the nitrogen atom of imine pyridine ring by enol-keto tautomerism. The bands observed at 3069 and 3020 cm^–1 indicates sp² hybridized –C–H stretching modes of aromatic ring. Comparing with the free H₂pyco ligand, the band assigned for the carbonyl group at 1743 cm^–1 was absent in the complex. It assures the O,O-chelating mode of the ligand through the ketonic group and the carboxylate group after deprotonation [12, 25]. The asymmetric stretching vibration, (υ_asCOO^–), and symmetric stretching vibration, (υ_sCOO^–), of carboxylate groups were found at 1640 and 1523 cm^–1 respectively. The Δυ= 117 cm^–1 confirms that the carboxylate group has coordinated in a chelating mode [26]. The presence of medium band at 779 cm^–1 may be due to the out-of-plane bending vibrations of –C–H protons of the aromatic ring. Band assigned at 575 cm^–1 could be due to Ba–O stretching vibrations.

Fig.2

FT-IR spectrum of BAPYC.

3.3 Single crystal X-ray diffraction studies

From the single crystal X-ray diffraction studies, the compound BAPYC crystallizes in triclinic with space group, P $\bar{1}$ . As depicted in Fig. 3, one crystallographically independent Ba(II) cation, one pyco ligand, one chlorine atom and one coordinated water molecule constitute the fundamental unit. Relevant details concerned with the crystal data and structure refinement parameters of complex are detailed in Table 1.

Fig.3

Fundamental unit of BAPYC.

Table 1

Crystal data and structure refinement for [Ba(C₆H₄NO₃)(Cl)(H₂O)]

Empirical formula	C₆H₆BaClNO₄
Formula weight	328.91
Temperature	293(2) K
Wavelength	0.71073 Å
Crystal system	Triclinic
Space group	P $\bar{1}$
Unit cell dimensions	a = 5.7737(2) Å, α= 106.432(2)°
	b = 6.8503(2) Å, β= 100.644(2)°
	c = 11.5378(4) Å, γ= 93.049(2)°
Volume	427.47(2) Å³
Z	2
Density (calculated)	2.555 Mg/m³
Absorption coefficient	4.936 mm^–1
F(000)	308
Crystal size	0.100×0.100×0.050 mm³
Theta range for data collection	3.120 to 24.991°
Index ranges	–6< = h< = 6, –8< = k< = 8, –13< = l< = 13
Reflections collected	8174
Independent reflections	1502 [R(_int) = 0.0292]
Completeness to theta = 24.991°	99.9%
Absorption correction	Semi-empirical from equivalents
Max. and min. transmission	0.7498 and 0.5034
Refinement method	Full-matrix least-squares on F²
Data/restraints/parameters	1502/4/130
Goodness-of-fit on F²	1.115
Final R indices [I > 2sigma(I)]	R₁ = 0.0102, wR₂ = 0.0262
R indices (all data)	R₁ = 0.0107, wR₂ = 0.0265
Extinction coefficient	n/a
Largest diff. peak and hole	0.264 and –0.387 e.Å^–3

R₁ =∑||Fo| – |Fc||/∑|Fo|, wR₂ = [∑w(Fo²–Fc²)²/∑w(Fo²)²]^1/2.

From the coordination environment of the complex BAPYC shown in Fig. 4, it is clear that each barium cation is nona coordinated with five oxygen atoms from three completely deprotonated pyco ligands (O1,O2,O3,O1’,O2’), one oxygen atom from coordinated water molecule (O4) and three chloride ions (Cl1,Cl1’,Cl1”) to form [BaO₆Cl₃] core unit. Here, three pyco ligand coordinates to the metal centre in three different ways such as monodentate, bidentate and O,O-chelating to form a 6 membered chelate ring by ketonic-O and carboxylate-O group. The coordination polyhedron around Ba(II) ion could be described as a distorted tricapped trigonal prism arrangement as shown in Fig. 5. The corresponding Ba–O and Ba–Cl bond distances are in the range 2.709(12) –2.916(13) Å and 3.1259(4) – 3.2246(5) Å respectively. These values are found to be consistent with the reported structures [27 –29]. Similarly, O–Ba–O and Cl–Ba–Cl bond angles vary from 45.34(3) – 149.17(4)° and 70.61(13) – 131.47(16)°. The high range of bond angles observed may be due to the large size of the Ba atom which usually prefers high coordination numbers 8–10 resulting in increased structural flexibility [29]. Selected bond lengths and bond angles are listed in Table 2.

Fig.4

Coordination environment of BAPYC.

Fig.5

Distorted tricapped trigonal prism geometry of BAPYC.

Table 2

Selected bond lengths (Å) and bond angles (°) for BAPYC

O(1)-Ba(1)	2.8293(13)
O(1)-Ba(1)#1	2.9162(13)
O(2)-Ba(1)#2	2.7095(12)
O(2)-Ba(1)#1	2.8368(13)
O(3)-Ba(1)#2	2.8033(14)
O(4)-Ba(1)	2.8683(15)
Ba(1)-Cl(1)	3.1259(4)
Ba(1)-Cl(1)#3	3.2070(5)
Ba(1)-Cl(1)#4	3.2246(5)
O(2)#3-Ba(1)-O(3)#3	63.31(4)
O(2)#3-Ba(1)-O(1)	79.85(4)
O(3)#3-Ba(1)-O(1)	82.65(4)
O(2)#3-Ba(1)-O(2)#1	69.79(5)
O(3)#3-Ba(1)-O(2)#1	132.34(4)
O(1)-Ba(1)-O(2)#1	97.43(4)
O(2)#3-Ba(1)-O(4)	122.06(4)
O(3)#3-Ba(1)-O(4)	69.69(4)
O(1)-Ba(1)-O(4)	125.87(4)
O(2)#1-Ba(1)-O(4)	135.64(4)
O(2)#3-Ba(1)-O(1)#1	98.92(4)
O(3)#3-Ba(1)-O(1)#1	149.17(4)
O(1)-Ba(1)-O(1)#1	68.93(4)
O(2)#1-Ba(1)-O(1)#1	45.34(3)
O(4)-Ba(1)-O(1)#1	137.25(4)
O(3)#3-Ba(1)-Cl(1)	104.66(3)
O(1)-Ba(1)-Cl(1)	72.74(3)
O(2)#1-Ba(1)-Cl(1)	120.97(3)
O(4)-Ba(1)-Cl(1)	70.85(3)
O(1)#1-Ba(1)-Cl(1)	78.72(3)
O(2)#3-Ba(1)-Cl(1)#3	73.76(3)
O(3)#3-Ba(1)-Cl(1)#3	78.17(3)
O(1)-Ba(1)-Cl(1)#3	152.31(3)
O(2)#1-Ba(1)-Cl(1)#3	81.38(3)
O(4)-Ba(1)-Cl(1)#3	64.94(3)
O(1)#1-Ba(1)-Cl(1)#3	123.03(3)
Cl(1)-Ba(1)-Cl(1)#3	131.478(16)
O(2)#3-Ba(1)-Cl(1)#4	131.44(3)
O(3)#3-Ba(1)-Cl(1)#4	135.78(3)
O(1)-Ba(1)-Cl(1)#4	135.78(3)
O(2)#1-Ba(1)-Cl(1)#4	73.16(3)
O(4)-Ba(1)-Cl(1)#4	69.03(3)
O(1)#1-Ba(1)-Cl(1)#4	75.03(3)
Cl(1)-Ba(1)-Cl(1)#4	75.894(13)
Cl(1)#3-Ba(1)-Cl(1)#4	70.612(13)
Cl(1)-Ba(1)-Ba(1)#5	147.568(10)

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

Here, the organic linker pyco ligand adopts μ₃-η²:η² bonding mode (each oxygen atom connects two metal atoms) there by linking three metal centers as represented in Fig. 6. The ketonic-O and carboxylate-O group at position-3 chelated to metal forms a 6 membered ring by O,O-chelation. This bridging μ₃-η²:η² extends the polymeric chain formation of 1D oxobridge [Ba₂O₂] between two metal atoms in a zig-zag fashion. The coordinated chloride group adopts bridging μ₃-η³ mode leading to four membered 1D [Ba₂Cl₂] ring formation within the crystal structure. Within these 1D chains, Ba…Ba nonbonding distance was found to be 4.737 and 5.008 Å. These two 1D chains are connected in alternative layers and results in the construction of 2D framework as shown in Fig. 7. The perspective view of packing diagram along ‘b’ axis shown in Fig. 8 represents ladder shaped [Ba₂Cl₂] ring running within the structure. From the references, a similar structure was observed in chloride bridged 3D polymeric barium complex of isonicotinic acid, [Ba(in)(Cl)]_n. Here the barium centre is seven coordinated by four oxygen atoms from two isonicotinate ligands, a nitrogen atom from the pyridyl ring of another isonicotinate ligand and two chlorine atoms [27].

Fig.6

Coordination mode of pyco ligand.

Fig.7

2D polymeric construction of alternative [Ba₂O₂] and [Ba₂Cl₂] rings.

Fig.8

View of packing diagram along ‘b’ axis.

Presence of supramolecular interactions like intermolecular hydrogen bonding plays an important role for the overall stability of the 2D polymeric arrangement of the complex. Hydrogen bonding interactions are tabulated in Table 3 and is depicted in Fig. 9. There are three strong O(4)–H(4A) …O(2), O(4)–H(4B)…O(1) and N(1)–H(1)…O(3) hydrogen bonds existing in the structure. The oxygen atom O(4) of coordinated water molecule acts as the donor for forming hydrogen bond with the carboxylate oxygen atoms O(1) and O(2) of pyco ligand. In addition to this, the nitrogen atom N(1) atom of the imine pyridine ring is hydrogen bonded with ketonic oxygen O(3) atom to form eight-membered rings represented by graphical set notation R₂²(8) motifs which enhances the stabilization and formation of 3D structure.

Table 3

Hydrogen bonds for BAPYC [Å and °]

D-H…A	d(D-H)	d(H…A)	d(D…A)	< (DHA)
C(3)-H(3)…Cl(1)#6	0.93	2.61	3.512(2)	164.6
N(1)-H(1)…O(3)#7	0.827(16)	2.178(17)	2.994(2)	170(2)
O(4)-H(4B)…O(1)#8	0.822(17)	2.123(18)	2.9324(19)	168(3)
O(4)-H(4A)…O(2)#9	0.824(16)	2.48(2)	3.122(2)	135(2)
O(4)-H(4A)…Cl(1)#3	0.824(16)	2.79(3)	3.2739(15)	119(3)

Symmetry transformations used to generate equivalent atoms: #6 -x+1,-y,-z+1; #7 -x,-y,-z+1; #8 x,y-1,z; #9 x+1,y-1,z.

Fig.9

Hydrogen bonding interactions in BAPYC.

3.4 Powder X-ray diffraction studies

The homogeneity and the purity of the sample in the solid state have been detected by the powder X-ray diffraction. The diffraction pattern plots of intensity against the angle of detector, 2θ is shown in Fig. 10. The measured PXRD patterns closely match with the simulated pattern which is obtained from the single crystal X-ray data using Mercury software [30]. The intensity difference may be arising due to the preferred orientation of the powder samples [31, 32].

Fig.10

Powder X-ray diffractogram of BAPYC.

3.5 UV-Visible spectral studies

UV-Visible absorption studies were carried out using Varian Cary 5000 UV-Visible-NIR spectrometer in the range 200–1200 nm and the absorption spectrum is shown in Fig. 11. The complex is transparent to the entire visible region (400–800 nm) as expected for Ba(II) complexes.

Fig.11

UV-Visible absorption spectrum of BAPYC.

3.6 Thermal analysis

In order to evaluate the thermal stability of compound, TG analysis was performed under N₂ atmosphere at a heating rate of 10°C/min in the temperature range of 39–850°C. The TG-DTG curve shown in Fig. 12 indicates the first weight loss of 5.34% in the temperature range 200–255°C corresponding to the loss of coordinated water molecule (calcd: 5.47%). The next three weight losses observed at a temperature range 270–557°C correlate with the collapse of the framework due to the decomposition of the organic moiety (obs: 33.90%, calcd: 34.50%). The final residue of 59.99% left at 617°C is BaCO₃ (calcd: 60.01%).

Fig.12

TG-DTG curve of BAPYC.

4 Conclusion

To summarize, a novel alkaline earth complex poly[aqua(μ₃-chlorido)(μ₃-2-oxo-1,2-dihydropyridine-3-carboxylato)barium(II)] was constructed from keto form of rigid linker 2-hydroxynicotinic acid by gel diffusion technique at room temperature. The crystal structure of the compound was determined by single crystal X-ray diffraction studies and it confirms the compound exhibit triclinic with centrosymmetric space group, P $\bar{1}$ . The most striking feature of the compound is that it consists of 4 membered [Ba₂O₂] and [Ba₂Cl₂] rings present in the alternative layers responsible for the construction of 2D structure. Moreover, the two-dimensional layers extend into three-dimensional supramolecular architecture via intermolecular O-H-O and N-H-O hydrogen bonds. The elemental composition of the compound was determined and it also matches with the single crystal X-ray data. The presence of certain functional groups present in the compound was established from FT-IR spectral studies. UV-Visible spectrum indicates the compound is transparent to all visible radiations. The bulk purity of the sample in solid state was confirmed by powder X-ray diffraction. The thermo-chemical property of the sample was analyzed from TG-DTG curve.

Footnotes

Acknowledgments

RD is thankful to the University of Kerala, Trivandrum, India for the University Research Fellowship. The authors are thankful to the authorities of SAIF, Kochi for instrumental facilities. We extremely thank Dr. M.R. Prathachandra Kurup, Department of Applied Chemistry, Cochin University of Science and Technology, Kochi, for his help and valuable suggestions for doing the research.

CCDC 1552594 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge at (or Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44 1223 336 033; E-mail: deposit@ccdc.cam.ac.uk).

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Structural studies on a novel 2D polymeric barium complex of keto tautomeric form of 2-hydroxynicotinic acid –poly[aqua(μ 3 -chlorido)(μ 3 -2-oxo-1,2-dihydropyridine-3-carboxylato)barium(II)]

Abstract

Keywords

1 Introduction

2.1 Reagents

2.2 Characterization

2.3 Growth procedure of poly[aqua(μ3-chlorido)(μ3-2-oxo-1,2-dihydropyridine-3- carboxylato)barium(II)] (BAPYC)

2.4 Single crystal X-ray diffraction studies

3 Results and discussion

3.1 Crystal growth

Footnotes

Acknowledgments

References

2.3 Growth procedure of poly[aqua(μ₃-chlorido)(μ₃-2-oxo-1,2-dihydropyridine-3- carboxylato)barium(II)] (BAPYC)