Hydrogen Bonded 3D Molecular Self Assembly Constructed from [(Ni(nicotinamide)2(thiocyanate)2(H2O)2] Complex Showing Spin Canted Anti-ferromagnetic Character

Deepanjali Pandeya①Shahid S. NarviaGopal K. MehrotraaRaymond J. Butcherba（Department of Chemistry， Motilal Nehru National Institute of Technology Allahabad-211004， India）

b（Department of Chemistry， Howard University， 2400 Sixth Street， N.W， Washington， DC 20059， USA）

A new three-dimensional nickel （II） hydrogen-bonded molecular self assembly containing ［（Ni（nicotinamide）2（thiocyanate）2（H2O）2］ complex has been synthesized and characterized by single-crystal X-ray diffraction， FTIR spectroscopy， thermal analysis and magnetic measurements. Structural analysis reveals that the complex crystallizes in triclinic space group P 1 （crystal data a = 7.5574， b = 8.2683， c = 9.0056 Å， α = 73.010， β = 69.698， γ = 66.51°） and exhibits a distorted octahedral coordination sphere. Most interesting point in its structure is the involvement of sulphur atom of thiocyanate moiety in the trifurcated hydrogen bonding to build up the hydrogen-bonded self assembly. The magnetic behavior as determined by squid magnetometer （2～300 K temp. range）reveals dominating antiferromagnetic interaction followed by spin canting behavior below 20 K.

hydrogen bonded 3D structure, intermolecular magnetic interaction, spin canting;

1 INTRODUCTION

Over the last decades enormous efforts were made in the investigations on the synthesis， structures， and properties of coordination polymers or metal-organic frameworks［1］. Especially， compounds with welldefined magnetic properties mediated by e.g. small anionic linker ligands are of considerable interest［2］. In this context among others， thiocyanates are promising candidates because they can mediate magnetic exchange interactions and exhibit distinct structural coordination diversity［3］.

It is well known that thiocyanate ligand can exhibit two typical linking modes: the end-to-end （EE） and the end-on［4］.

For the construction of high dimensional thiocyanate complexes， multidentate ligands such as di and triamines［5］， and bpy = 2，2-bipyridine［6］etc have been used as co-ligand. Nicotinamide pos-sesses three potential donor atoms， from which significant number of possibilities to coordinate itself in different manners should arise［7］.

Intermolecular magnetic interaction only involves coupled spin-carries that are linked by intermolecular forces such as π-π stacking， hydrogen bonds，etc. and not by chemical bonds. Compared with intramolecular magnetic study， not much work has been published on the intermolecular magnetism［8］. Similar to chemical bonds， intermolecular interactions also play an important role in deciding thenature of magnetic coupling. For example， some authors found that the π-π stacking led to a strong anti-ferromagnetic interaction between spin carriers. Strong anti-ferromagnetic coupling between Cu（II）ions through O-H··O hydrogen bonding has also been reported［9］. Therefore， intermolecular forces should be one of the key factors that dominate the magnetic coupling properties.

In this paper， we report the synthesis， structural characterization， magnetic and thermal properties of a new 3D Ni（II） hydrogen-bonded molecular self assembly containing mononuclear complex， ［（Ni（nicotinamide）2（thiocynate）2（H2O）2］.

2 EXPERIMENTAL

2. 1 Materials and physical measurements

Nickel（II） acetate tetra hydrate （Sigma Aldrich），potassium thiocyanate （Sigma Aldrich） and 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-1region. Variable-temperature magnetic susceptibility data were collected using a Quantum Design MPMS SQUID magnetometer. Magnetic susceptibility of the complex was corrected for diamagnetism of the ligand molecules using Pascal’s constant （-302 × 10-6cgs.）.

2. 2 Synthesis of the complex

An aqueous solution （10 mL） of nickel acetate tetrahydrate （0.2460 g， 1 mmol） and potassium thiocyanate （0.196 g， 2 mmol） was slowly added dropwise to hot aqueous solution （10 mL） of nicotinamide （0.244 g， 2 mmol） with stirring. Greenish blue color solution was obtained. After filtration， the final clear solution left undisturbed at room temperature for slow evaporation. On the next day， needle shaped greenish blue crystals were collected and dried in vacuo over silica gel. Crystals suitable for single-crystal X-ray diffraction was manually selected and immersed in silicon oil （Yield 87%）. C14H16NiN6O4S2: C， 36.9； H， 3.5； N， 18.4； S，14.1 Found: C， 36.8； H， 3.5； N， 18.6； S， 14.0%.

2. 3 Crystal structure determination

The selected greenish blue single crystal of the title compound with approximate dimensions of 0.48mm × 0.32mm × 0.26mm was mounted on a glass fiber. 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 multiscan 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［10］. All 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. The final R = 0.0462， wR = 0.1105 （w = 1/［σ2（Fo2） + （0.0466P）2+ 0.6377P］， where P = （Fo+ 2Fc2）/3）. S = 1.031，（Δρ）max= 0.501， （Δρ）min= -0.710 e/Å3and （Δ/σ）max= 1.00000. Selected bond lengths and bond angles are listed in Tables 1 and 2， respectively.

3 RESULTS AND DISCUSSION

3. 1 IR spectrum

The IR spectrum （Fig. 1） shows broad absorption bands in the range of 3378～3178 cm-1， corresponding to the hydrogen bonding between aqua molecule & CONH2group of nicotinamide［11］. Strong IR bands at 2100 & 832 cm-1correspond toν（CN） & ν（CS） stretching frequencies respectively，which indicates N-coordinated terminal thiocyanate moiety［12］. In the bridging mode， the thiocyanate moiety is expected to show absorbance in the region of ＞ 2100 cm-1， which confirms non bridging mode for the ligand..IR spectrum shows two bands for theamide group of nicotinamide （νC=O and NH2） at 1667 and 1609 cm-1， respectively. Pyridine ring vibration of free nicotinamide at 1565 cm-1shifts to 1485 cm-1in the complexes， indicating the pyridine ring is in coordination. The ring skeletal vibrations are observed at 1197， 1107， 782 and 703 cm-1. Nicotinamide showed a broad band in this region （1418～1318 cm-1） for ring stretching and ν（CN） of the primary amide［13］.

Table 1. Selected Bond Lengths (Å) and Bond Angles (°)

Table 2. Hydrogen Bond Lengths (Å) and Bond Angles (°)

Fig. 1. IR spectrum

3. 2 Crystal structure determination of the complex

Single-crystal X-ray analysis reveals that the complex crystallizes in triclinic， space group P 1. The data related to the crystal structure analysis are listed in Table 1. The crystal structure consists of single monomeric complexes. The Ni ions are located at the centers of inversion. Centro-symmetrically related pairs of thiocyanate moieties， nicotinamide moieties and water molecules are octahedrally coordinated to the Ni ions via the N atoms of thiocyanate and nicotinamide moieties， and the O atoms of water molecules.

Nickel（II） ion being a hard acid prefers N-bonding，whereas in the case of platinum thio-cyanate complex the coordination of S donor site is preferred［14］. Each nickel centre is located at an inversion centre consisting of two equatorial N atoms from nicotinamide and two equatorial O atoms from water together with two axial N donors from the thiocyanate moiety.

The bond distance of Ni-O1（W） is 2.088（2） Å，very close to the value for Ni-N（thiocyanate） of 2.090（2） Å. The Ni-N（1） bond in 2.178（2） Å is longer because the nicotinamide moiety is a bulkier ligand having more steric hindrance in binding to the metal ion.

The O1（W）-Ni-O1（W）， N-Ni-N， and N1-Ni-N1 angles are constrained by symmetry to 180°. The N（thiocyanate）-Ni-N（nicotinamide）， O1（W）-Ni-N（thiocyanate）， and O1（W）-Ni-N1 angles are 87.48（6）， 88.85（7）， and 89.75（6）°， respectively， indicating a slightly distorted octahedral coordination for the Ni（II） ion.

The thiocyanate moiety is almost linear with the N-C-S bond angle being 178.3（2）°. It coordinates in a little bent fashion to Ni， exhibiting a Ni-N-C angle of 159.56°. As a result of the trans orientation of two terminal N-bonded thiocyanate groups around the Ni（II） atom， the bond angle N（1）-Ni（1）-N（1） is 180°. The S-C and C-N bond distances of 1.638（2） and 1.158（2） Å are in accordance with the values observed in other thiocyanate complexes［15］. Both the nicotinamides are trans to each other. The nicotinamide ligand generally acts as a bidentate chelating ligand， coordinating to the metal ion through the carbonyl O and pyridine N atoms. However， in this structure it acts as a unidentate ligand， in which the pyridine N coordinates to the Ni ion while the carbonyl O is involved in hydrogen bonding with another water molecule.

The coordination of nitrogen atoms of each thiocyanate ion and nicotinamide molecules results in the formation of two symmetrical axes N（1）··Ni··N（1） and N（2）··Ni··N（2）， respectively. The oxygen atoms of water molecules describe the third axis O（1） WA··Ni··O （1）WB. The Ni-Ni distance spaced by the thiocyanate ligand is 7.548 Å.

As can be seen from the packing diagram， the nickel atoms are located at the centre of the axis of the unit cell and the monomer molecules are linked by intermolecular hydrogen bonds O-H··O， O-H··S and N-H··S hydrogen bonds， forming a supramolecular structure. Dipole-dipole and van der Waals interactions are also effective in the molecular packing.

The crystal packing of the complexes is a composite of intermolecular hydrogen-bonding interactions. The discrete units are connected by bifurcated hydrogen bonds between the coordinated water molecules and terminal thiocynate sulfur atoms （O（1）-S = 3.224 Å）， giving 1D chains （Fig. 3a）which form a ladder like structure. The terminal uncoordinated oxygen atom of nicotinamide （H（1）-O = 2.86 Å） connects with the hydrogen atom of water molecule， making a 2D chain. This type of hydrogen bonding allows the 2D sheets to build parallel to the （110） plane， which extends along the （110） and （100） directions， as shown in Fig. 4a and 4b. The hydrogen-bonding interactions are listed in Table 2.

Fig. 2. An ORTEP view of complex [Ni(nicotinamide)2(thiocynate)2(H2O)2] with atom labeling

Fig. 3. Unit cell packing diagram of the complex. Hydrogen bonds are shown as dashed lines

Fig. 4. (a) 1D chain running along the b axis (110 plane). (b) 2D chain running along the b axis

All the available hydrogen atoms take part in hydrogen bonding. The water oxygen， O（1W），donates its two hydrogen atoms to the thiocyanate sulphur， S（1）， and the carbonyl oxygen， O（1）， of two different complex moieties. The amide nitrogen，N（1）， is the donor in N-H··S bonds to the thiocyanate sulphur atoms of two different moieties.

Thus， while each carbonyl oxygen atom accepts only one hydrogen bond from the coordinated water molecule， each thiocyanate sulphur accepts three hydrogen bonds， with one from a water molecule and the other two from two different amide nitrogen atoms. These intermolecular hydrogen bonds give an overall 3D structure to the assembly （Fig. 5）.

Fig. 5. Perspective view of the three-dimensional structure as viewed along the b-axis

The supramolecular solid-state architecture is further stabilized by π-π intermolecular stacking interactions established between the pyridine rings of nicotinamide ligands belonging to neighbouringchains ranging from 3.865 Å［16］， thereby giving additional strength to a 3D structure.

3. 3 Thermal analysis

In order to reveal the thermal stability of compound TGA， measurement has been carried out on a Perkin-Elmer Diamond TGA/DTA instrument with a flow of dry air at a heating rate of 5 ℃/min from room temperature to 1000℃. As shown in Fig. 7，TGA curve shows three steps of weight loss. In the first step， the weight loss in the temperature range of 160～170 ℃ is 8.2% （calculated 7.9%） which is attributed to the removal of two coordinated water molecules as supported by endothermic peak in DTA curve. Then the product undergoes decomposition of organic groups under a wide temperature range， with the second weight loss of 53.4% （calculated 53.60） observed in the temperature from 270 to 320 ℃. Further， in the temperature range of 324～370 ℃， molecules of nitrogen were liberated with a weight loss of 6.6% （calculated 6.8%）， which is supported by the endothermic peak in the DTA curve. The end product was nickel oxide as confirmed by powder X-ray of the residue.

Fig. 6. Stacking of two pyridine rings of the neighboring complexes through π-π interactions running along the c axis

Fig. 7. Thermo-gram of the complex

3. 4 Magnetic measurement

A variable temperature magnetic susceptibility study was performed on the polycrystalline sample. The magnetic susceptibility （χm） of this compound was examined at 1000 Oe field in the 2.0～300 K temperature range. The effective magnetic moment calculated at 300 K is 3.44 BM for the Ni（II） ion which is slightly higher than the expected spin only value. The ground state for nickel（II） ion in the octahedral ligand field is3A2gand expected to have orbital contribution completely quenched. However，the reason for departure from the spin only value for Ni（II）， d8ion lies in the existence of the second order Zeeman effect between the ground and the higher ligand field terms.

The molar magnetic susceptibility 1/χmvs. T and χmT vs. T of the complex is plotted in Figs. 8a and 8b，respectively. The curve （Fig. 8a） is fitted to the Curie-Weiss law in the temperature range of 20～300 K， which gave the value of C = 1.66 emu mol-1and θ = -14.8. Below this temperature the susceptibility declines sharply.

Fig. 8. (a) shows 1/χmversus T plot (the red line curie Weiss fit). (b) Shows plot between χmT versus T.(c) Hysteresis loop at 300 K. (d) χmT versus T plot at various magnetic fields

The large negative value for θ suggests a predominantly anti-ferromagnetic coupling between the Ni2+ions. The value of χmT is slightly dependent of temperature at 300 K， 1.65061 cm3·mol-1·K， as expected for two magnetically isolated nickel（II）ions［17］. The χmT versus T plot （Fig. 8b） shows a continuous decrease in the value of χmT from 1.65061 cm3·mol-1·K at 300 K to 0.887 cm3·mol-1·K at 20.12 K， which is indicative of the presence of anti-ferromagnetic intermolecular interaction through intermolecular forces， such as π-π stacking and hydrogen bonding. The weak π-π stacking interac- tion （3.86 Å） seems to be the minor and hydrogen bonding interaction is the major contributor of magnetic interaction［18］. Justifying therefore， strongly hydrogen bonded entities as obtained by the X-ray structure.

The variable temperature magnetic susceptibility data in the temperature range of 300～20 K （Below this temperature nature of magnetic interaction changes owing to spin canting） were fitted to the formula （Eq. 2）， which was obtained based on isotropic Hamiltonian for two coupled Ni（II） ions as presented in Eq. 1. Equation 2 gave a good fit to the experimental data and the relevant fitting parameters are: g = 2.1 and 2J = -1.8 cm-1and the agreement factor R = 5.4 × 10-3. The value 2J = -1.8 cm-1further indicates an antiferromagnetic interaction between the adjacent Ni（II） ions.

At 300 K， the field dependence of the magnetization shows a small hysteresis loop at lowfield strength （Fig. 8c）. The magnetization value is 0.81 Nβ at 50.2 kOe， far from the saturation value of 2 Nβ for a Ni（II） ion， which is also in agreement with an anti-ferromagnetic state.

On further cooling， the χmT increases rapidly to 1.26 at temp 10.03 K， suggesting a change in magnetic behavior. It is likely that the magnetic behavior at low temperature may be either ferromagnetic or canted antiferromagnetic， in which a predominantly antiferromagnetic phase possesses a spontaneous magnetization originating from a small deviation between the antiparallel alignment of the spins.

The field-cooled χmT values at different applied fields as a function of temperature shows a spontaneous increase around 23 K， which is more noticeable at lower field strength （Fig. 8d）. This feature suggests the possibility of a spin-canted antiferromagnetic behaviour［19］， in which a predominantly antiferromagnetic phase possesses a spontaneous magnetization originating from a small deviation between the antiparallel alignments of the spins.

4 CONCLUSION

In summary， new ［（Ni（nicotinamide）2（thiocynate）2（H2O）2］ complex having 3D hydrogenbonded molecular self assembly was synthesized & characterized by IR， TGA-DTA， and low temperature magnetic measurement. IR spectrum shows 2100 cm-1peak which indicates N-coor-dinated terminal thiocyanate ion. The crystal structure determined by X-ray diffraction shows a distorted octahedral geometry. The nickel ion coordinates 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. Water is used as solvent， but it acts as the ligand. Most appealing point in this structure is the sulphur atom of thiocyanate molecule，showing trifurcated hydrogen bonding. Magnetic measurements indicate the existence of antiferromagnetic interaction with spin canting behaviour between the nickel ions at lower temperature.

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10.14102/j.cnki.0254-5861.2011-0259