Supramolecular Hydrogen Bonded 3 D Molecular Self Assembly Constructed from [ ( Co ( nicotinamide ) 2 ( thiocyanate ) 2 ( H 2 O ) 2 ] Complex Showing Antiferromagnetic Character

* Corresponding author: deepanjalipandey@gmail.com ABSTRACT A new three dimensional hydrogen bonded cobalt frame work from [Co(nicotinamide)2(thiocyanate)2(H2O)2] was synthesized and characterized by X-ray diffraction, magnetism ,TGA and IR spectroscopy. The compound crystallizes in Triclinic space group P-1 with a = 7.5475(19), b = 8.054(2), c =8.932(2). Alpha=73.347(4), beta=70.067(4), gamma=66.559(4) with space group P-1Z = 1, 'C14 H16 N6 Co O4 S2', Mr =455.38, F(000) = 233 and μ(MoKα) =0.71073 mm -1 . The final R = 0.0497 and wR = 0.1461 for 4185 observed reflections with I > 2σ(I) and R = 0.0721 and wR = 0.1619 for all data. X-ray diffraction analyses revealed that Co(1) is linked by the nicotinamide ligands to form the ladder shape along the c axis, which is further extended into two-dimensional networks via the joint of Co(2) along the a axis. Moreover, these two dimensional motifs are interconnected by the thiocyanate S...H bridges to form a complicated 3-D polymeric framework. The magnetic susceptibility data at 1000Oe external field in the temperature range


INTRODUCTION
Metal-organic frameworks (MOF) have received much attention in recent years due to their potential applications in nonlinear optics, magnetism, molecular recognition and gas storage [1,2] Based on current research, to select a multifunctional organic ligand is crucial in an assembly of a MOF with a desired framework because the multi-functional organic ligands can provide not only versatile binding sites to connect metal ions but also hydrogen bond donor or acceptor to form supramolecular interactions. [3,4] As is known, hydrogen bonds and p-p interaction are powerful supramolecular interactions and they can connect the low-dimensional frameworks into high dimensional supramolecular architectures. Pseudo halide anions are excellent ligands for obtaining discrete, one-dimensional, two-dimensional or three-dimensional systems. [5] As one of the most important pseudo halide anions, SCN presents multiform coordination modes connecting the metal ions with terminal and/or bridging fashions. . The thiocyanate ligand, with the ambidentate ability, seems to play an important role in the design of extended one, two, and three-dimensional compounds with specific magnetic properties. According to the concept of hard-soft acid-base [6,7,8], the SCN_ ion prefers to bind to Cd(II) center in both N-and S-bonded fashion, whereas in the only N-terminal mode to Co(II) ion. Many thiocyanate complexes have been reported in combination with different ligand via-{[Cu(4,4_-bpy)_(BF4)2(H2O)2]_(4,4_-bpy)}n [9], {[Mn(4,4_bpy)(NCS)2(H2O)2]_(4,4_-bpy)}n [10], and {[Cu(4,4_-bpy)(pyz)(H2O)2]_(PF6)}nH2O [11].
Nicotinamide is also well played as coligand documented in literature. Nicotinamide is known as a component of the vitamin B complex as well as a component of the coenzyme, nicotinamide adenine dinucleotide (NAD). These are more important for transfer of hydrogen in the cell breath. The presence of pyridine ring in numerous naturally abundant compounds, adducts of nicotinamide are also scientific interest. Therefore, the structure of nicotinamide has been the subject of many studies [12][13][14][15].
Our current interest is the construction of thiocyanate complexes containing anionic SCN ligand and nicotinamide as coligand with d 6 , d 7 , d 8 ,d 9 , d 10 metal ions. This strategy is anticipated to affect the crystal structure and obtain complexes with a new type of stacking structure. Previously we report the crystal structure with d 8 metal ions and their characterization and magnetic properties (16). As a continuance of our research work in the assemblies and properties of coordination polymers, here we report the synthesis, crystal structure and magnetic properties of the new thiocyanato S.... H bridged 3D coordination motifs, built by complex, [Co (SCN)2(nicotinamide) 2 (H2O)2].

Materials and Physical Measurements
Nickel(II) acetate tetra hydrate (Sigma Aldrich), Potassium thiocyanate (Sigma Aldrich), Nicotinamide( Sigma Aldrich), were purchased and used without further purification. IR spectra were recorded in KBr pellets with a Nicolet 170 SXFT-IR spectrophotometer in the 4000～400 cm -1 region. Variable-temperature magnetic susceptibility data were collected using a Quantum Design MPMS SQUID magnetometer. Magnetic susceptibility of complex was corrected for diamagnetism of the ligand molecules using Pascal's constant (-302×10 -6 cgs.).

Crystal structure determination
A selected greenish blue single crystal of the title compound with approximate dimensions of 0.48 x 0.32 x 0.26 mm 3 was mounted on a glass fibre. Crystal data of complex were collected on a SCXmini CCD diffractometer equipped with a graphite-monochromatic MoKα radiation (λ = 0.71073 Å) using an ω-2θ scan mode in the range of 5.05 ≤θ≤37.78° (-12<=h<=11, -13<=k<=14, -15<=l<=15) at 123(2) K. A total of 8114 reflections including 4752 unique ones were collected. Out of which 3477 with I >2σ (I) were considered as observed and used for the succeeding refinement. The data sets were corrected for absorption by multi-scan technique. The structure was solved by direct methods using SHELXS-97 program and refined with SHELXL-97 by full-matrix least-squares techniques on F2 [17][18][19][20]. All of the non-hydrogen atoms were refined anisotropically. The hydrogen atoms of lattice water molecules were found in the electron density map and refined by riding, and the other hydrogen atoms were generated with idealized geometries and treated as riding.

IR Spectrum
The IR spectrum of the complex as can be seen in fig. (1) exhibits the characteristic bands of the thiocyanate and nicotinamide ligands. The strong and broad absorption bands occurred in the frequency range 3318-3132 cm −1 are assigned to the antisymmetric and symmetric stretching vibrations of the NH2 group of nicotinamide molecules involved in hydrogen bonding, while the relatively weak bands in the frequency range 3000-3100 cm −1 are due to the absorption of the CH groups (21,22). The broad bands observed in the range 3400 and 1675 cm −1 are attributable to νstr (O-H) stretching and δ (O-H) bending frequency of the coordinated water molecule. A characteristic peak for νstr(C-N) stretching frequency was found at 2000 and 2100 cm−1, corresponding to N-bonded terminal thiocyanate ligand [23,24].

Crystal structure
The structural unit of hydrogen bonded framework of cobalt is depicted in Figure (2). Co (II) is at a slightly distorted octahedral coordination environment. The equatorial positions are occupied by two nitrogen atoms from two nicotinamide ligands, the Co-N(nicotinamide) bond length is 2.117(3) Å, and two oxygen atoms from two water molecules, the Co-O(water) bond length is 2.075(2)Å and two nitrogen atoms from NCS-groups occupy the axial positions with bond length of 2.049(3) Å for Co-N(thiocyanate).
Both ligands generally acts as bidentate, however in this polymer plays as unidentate. The thiocyanate SCN presents multiform coordination modes connecting the metal ions with terminal and/or bridging fashions. According to the concept of hard-soft/ acid-base the SCN ion prefers to bind to Cd(II) center in both N-and S-bonded fashion, whereas in the only N-terminal mode to Co(II )ion.
As expected, the SCN anion is almost linear angle: 178° and coordinates in a little bent fashion to Co, exhibiting a Co-N-C angle of 159.56°. These structural features have already been observed in other thiocyanato-containing metal complexes. [25] As a result of the trans orientation of two terminal N-bonded thiocyanate groups around the Co(II) atom, the bond angle N(1)-Co(1)-N(1) is 180°. The S-C and C-N distances of 1.638(2) Å and 1.158(2)Å in the SCN-moiety show the normal structure of the thiocyanate in the complex which is also observed in other thiocyanate complexes. The N--C bond length is little larger than the corresponding value [1.140(11)A] found in the [Mn(H-quin)(quin)(NCS)(H20)2] complex (H-quin is quinaldic acid) with terminal nitrogen-bonded thiocyanato ligands [r].The nicotinamides molecules are trans to each other with angle N(2)-Co-N(2) is 180°. The nicotinamide ligand generally acts as a bidentate chelating ligand, coordinating to the metal ion through the carbonyl O and pyridine N atoms , but in this structure it acts as a unidentate ligand in which the pyridine N is coordinated to the Co ion while the carbonyl O is involved in hydrogen bonding with another water molecule. Water used as solvent whereas it involved in coordination with metal ions and act as ligand.
The coordination of nitrogen atoms of each thiocynate molecules and nicotinamide molecules results in the formation of two symmetrical axis N1···Co···N1, N2···Co···N2 respectively. The oxygen atoms of water molecules describe the third axis O(1)WA···Co···O(1)WB. The Co-Co distance spaced by the thiocyanate ligand is 7.548Å.   Additionally the structure stabilized through pi-pi interaction exists between parallel edges.
The shortest distance between two parallel benzene rings from the parallel nicotinamide ligands of the ladder structure is 3.865 Å show in fig. (7). which is in good with the literature, indicating a significant pi-pi interaction. [27] O c t o b e r 0 5 , 2 0 1 4 As can be seen in fig. (8) the intermolecular hydrogen bonding gives the molecular units R 2 2 (12) and R 2 2 (28) appears to have reliable utility for construction of high dimensions of polymer.

Tharmogravimetric analysis
In order to reveal the thermal stability of complex, TGA measurement has been carried out on a Perkin-Elmer Diamond TG/DTA instrument with a flow of dry air at a heating rate of 5 o C/min from room temperature to 1000 o . As shown in Fig.  (

Magnetism
The variable temperature magnetic susceptibility data of the title complexes were measured in the range of 2-300 K under a magnetic field of 1 T. The plots of Xm and XmT versus T are shown in Figs. 10(a) and (b), respectively. It can be seen from Fig.10 Fig.10

(b)Shows plot betweenXmT versusT
The value of XmT gradually decreases on cooling, indicating the overall antiferromagnetic exchange between the Co(II) ions. The XmT value at room temperature is 2.38 cm 3 mol _1 K, greater than the spin-only value expected for an isolated Co(II) ion. The magnetic susceptibility data at 1000Oe external field in the temperature range 2-300 K obeys the Curie-Weiss law, giving θ = -24.12 K and C =2.43 thus indicating a dominant strong antiferromagnetic interaction and/or spin orbital coupling between the Co (II) ions.
Magnetic susceptibility of complex was corrected for diamagnetism of the constituent atoms, which was calculated using Pascal's constant [28] and the temperature-independent paramagnetism of Co (II) ion. The variable temperature magnetic susceptibility data in the temperature range 300K to 2K was fitted to the formula (Equation 2), which was obtained based on isotropic Hamiltonian for two coupled Ni(II) ions as presented in Equation 1.

Conclusion
In summary new [(Co (nicotinamide)2 (thiocynate)2(H2O)2] complex having 3D structure was synthesized & characterized by IR, TGA-DTA, Low temperature magnetic measurement . IR spectrum shows 2110cm -1 peak which indicates N-coordinated terminal thiocyanate ion. The crystal structure determined by X-ray diffraction shows a distorted octahedral geometry. Cobalt ion coordinated with four nitrogen atoms and two oxygen atoms, two nitrogen atoms from nicotinamide, two nitrogen atoms from thiocyanate and two oxygen atoms from water molecules respectively. Water used as solvent, however, it act as ligand. Most appealing point in this structure is sulphur atom of thiocyanate molecule showing trifurcated hydrogen bonding. Magnetic measurements indicate the existence of antiferromagnetic interaction with θ = -24.12 K and C =2.43 value.