Synthesis and X-Ray Crystal Structure Determination of Pyridine-2,6-Dicarboxylate Ligand-Based 2D Complex with Heterometallic Ions

This title compound, [C14H20N2NaO15Sm]n, is synthesized by reacting Sm(NO3)3 and 2,6-pdca in the presence of 1M NaOH. The 2D sheet-based complex is formed, where Sm(III) ion occupied nine coordinating sites and Na (I) ion occupied six coordination sites. The Sm(III) atom is coordinated by two pyridine N atoms and four carboxylate O atoms from two doubly deprotonated pyridine-2,6-dicarboxylate ligands in a distorted octahedral geometry. There are three water molecules coordinated with Sm(III) ions. One Na(I) cation is coordinated by three carboxylate O atoms and two water molecules and the other is coordinated by five carboxylate O atoms and two water molecules in an irregular geometry. In this complex, Na(I) cations are connected with Sm(III) ions through bridging coordinated water molecules O1W and O3W. Topologically, structure consists of layers (1 0 0) with point symbol for Na(I) ion is (32.4.5.62.74) and for net is (3.4.5)(32.4.5.62.74) and 3,5-c 2-nodal net with topological type is gek1.


Introduction
The rational design and synthesis of coordination polymers (CPs) with predictable structures has acquired great interest in fields of catalysis, chromatographic media, magnets, and nonlinear optics [1,2]. They have intrinsic structural aspects and variety of coordination system with interesting compositions, geometries, nature of ligands (neutral and charged organic ligands) as well as coordinating organic and inorganic anions [3,4]. In the past, most of the work regarding CPs has been focused on CPs containing transition-metal ions [5,6] and few lanthanide ions [7]. This is because of predictability of the coordination geometry of the transition metals compared to the lanthanide ions. Most of times lanthanide metal ions containing CPs acquire flexibility as compare to transition metal ions [8,9]. The flexibility of the lanthanides based CPs combined with the rigidity of the transition metal ions based CPs can be beneficial to give variety of structures These CPs are dictated by the different size of the lanthanide ions and can result in fascinating and interesting structures with specific properties. More recently, fascinating coordination networks have been achieved by the using of pyridine carboxylate based ligands and followed and lately by choosing isonicotinate N-oxide, where both carboxylate and N-oxide units can bind to construct lanthanide CPs. Our original aim was to combine lanthanide ions and alkali metals to construct specifically stable CPs where alkali metals can play the role of bridging units as well as increase the dimensionality. The CPs can be denoted as heterometallic CP where it can have properties of both lanthanides as well as alkali metal ions. It is evident that alkali metal ions, incorporated as bridging component, do not act simply as a  passive constituent but rather contribute to the increased complexity and hence functionality. Hydrogen-bonding involving water molecules play an important role in self-assembly processes for building metal organic framework (MOF) architectures. The structure of the title compound has already been reported [10,11], role of lattice solvent molecules was not fully understood. We here report the X-ray crystal structure analysis of the compound, and demonstrate a unique hydrogen-bonding cluster of water molecules located in interlayer spaces. It has been investigated from single-crystal data that the coordinated and bridging water molecules play important role to generate higher topological structures. There are similar structures present in literature with Co(II) metal ions by Liu et al. [12] and Alexander et al. [13] and with Ni(II) metal based ions by Xiang et al. [14]. Complex (I) has been solved afresh and included for the sake of explanation of its crystal structure which is different from previously discussed structures.

Materials and Physical Measurements
All the starting reagents of analytical grade were used without further purification. The melting points were determined with an electrically heated apparatus. C, H, N elemental analyses were obtained with a CHNS-O analyzer flesh-EA-1112 series. Single-crystal structural X-ray diffraction was carried out on a Bruker's Apex-II CCD diffractometer using Mo Kα (λ = 0.71069) at room temperature.

Synthesis of Complex (I)
Sm(NO 3 ) 3 (0.338 g, 1.0 mmol) and 2,6-pdca (0.167 g. 1.0 mmol) were dissolved in 10 mL mixture of distilled water and methanol (1:1) and stirred well at room temperature and then to obtain clear solution, solution sodium hydroxide (1 M, 1 mL) was added dropwise. The mixture was stirred for 30 min, filtered and set aside for crystallization by slow evaporation.

X-Ray Crystal Structure
The X-ray crystal structure revealed that this complex is solved in monoclinic space group P2 1 /n. The asymmetric unit of this complex is having one Sm(III) ion, one Na(I) ion, two completely deprotonated 2,6-pdca ligands, four coordinated and three lattice water molecules (Fig. 2). The Sm(III) metal is having nine-coordination sites (Fig. 3a) surrounded by two 2,6-pdc ligands through four oxygen atoms of carboxylate groups and two pyridine nitrogen atoms with Sm-O distances are in the range of 2.4099 (17)-2.5451(16) Å and average Sm-N distances are 2.5265(16) Å, occupied distorted octahedral geometry. Three coordinated water molecules are coordinated to complete its other three coordination places. The Na(I) ion is six-coordinated with distorted octahedral geometry (Fig. 3b), where three coordinated water molecules O1W, O3W, O4W, and three carboxylate oxygen atoms O4, O5 and O7 have coordinated with Na-O distances are in the range of 2.388 (2)-2.4537 (17) Å. There are two coordinated water molecules O1W, O3W, and carboxylate oxygen atoms O4 and O5 are bridging between Sm(III) and Na(I) ions. Due presence of bridging water molecules in this bimetallic complex 2D-polymeric sheet is formed (Fig. 4). The 2D sheets or layers are formed in this complex from six-coordinated NaO 6 polyhedra and ninecoordinated SmN 2 O 7 polyhedra. These two polyhedra share edges with each other along the c-axis by bridging O atoms of water and carboxylate groups forming 1D chain (Fig. 5). The octameric chair shaped H-bonded rings (Fig. 6) 4 ) and 3,5-c 2-nodal net with topological type is gek1 (Fig. 7). The structure refinement data and H-bonding interactions are present in Tables 1 and 2.

Conclusion
In summary, we have synthesized a 2D sheet-based complex, where Sm(III) ion-occupied nine coordinating sites and Na(I) ion-occupied six coordination sites. The Sm(III) atom is coordinated by two pyridine N atoms and four carboxylate O atoms from two doubly deprotonated pyridine-2,6-dicarboxylate ligands in a distorted octahedral geometry. There are three water molecules coordinated with Sm(III) ions. One Na(I) cation is coordinated by three carboxylate O atoms and two water molecules and the other is coordinated by five carboxylate O atoms and two water molecules in an irregular geometry. In this complex, Na(I) cations are connected with Sm(III) ions through bridging coordinated water molecules O1W and O3W. Topologically, structure consists of layers (1 0 0) with point symbol for Na(I) ion is (3 2 .4.5.6 2 .7 4 ) and for net is (3.4.5)(3 2 .4.5.6 2 .7 4 ) and 3,5-c 2-nodal net with topological type is gek1.

Supporting Information
Tables of selected bond distances, bond angles, and torsion angles; The cif and structure factor data are available from Cambridge structure data base, free of cost, CCDC number = 1010192