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Journal of Physics and Chemistry of Solids 68 (2007) 1540–1548

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Synthesis, crystal structure, Cu2+ doped EPR, thermal andvoltammetric studies of [Ni(isonicotinamide)2(H2O)4] � (sac)2

single crystal

Ibrahim Uc-ar�, Necmi Dege, Bunyamin Karabulut, Ahmet Bulut

Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayıs University, Kurupelit, 55139 Samsun, Turkey

Received 13 October 2006; received in revised form 1 March 2007; accepted 16 March 2007

Abstract

The single crystal of [Ni(ina)2(H2O)4] � (sac)2, (NINS), (ina is isonicotinamide and sac is saccharinate) complex has been prepared and

its structural, spectroscopic and thermal properties have been determined. The title complex crystallizes in monoclinic system with space

group P21/c, Z ¼ 2. The octahedral Ni(II) ion, which rides on a crystallographic centre of symmetry, is coordinated by two monodentate

ina ligands through the ring nitrogen and four aqua ligands to form discrete [Ni(ina)2(H2O)4] unit, which captures two saccharinate ions

in up and down positions, each through intermolecular hydrogen bands. The magnetic environment of copper(II) doped NINS crystal

has also been identified by electron paramagnetic resonance (EPR) technique. The g and A values of Cu2+ doped NINS single crystal

were calculated from the EPR spectra recorded in three mutually perpendicular planes. These values indicated that the paramagnetic

centre has a rhombic symmetry with the Cu2+ ion having distorted octahedral environment. The complex exhibits only metal centred

electroactivity in the potential range of �2.00, 1.25V versus Ag/AgCl reference electrode.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Organometallic compounds; C. Thermogravimetric analysis; C. X-ray diffraction; D. Crystal structure; D. Electron paramagnetic resonance

1. Introduction

Isonicotinamide, a pyridine derivate with an amidogroup (–CONH2) in g-position possesses strong anti-tubercular, anti-pyretic, fibrinolytic and anti-bacterialproperties. Because of their strong pharmacological effect,mixed salts of isonicotinamide find extensive use as drugsin various biological and medicinal processes [1]. Theisonicotinamide is also interesting agent for inorganicchemistry since it has three donor sites: (i) pyridine ringnitrogen, as the title complexes, (ii) amino nitrogen,(iii) carbonyl oxygen, acting as a monodentate ligand [2].

In the present study, we have aimed to synthesize mixedligand complex of isonicotinamide with saccharin, which iswidely used as an artificial sweetener. Owing to its possibleuse in pharmacology and evaluated as a pharmacologicalagent, the detailed knowledge of its physical properties

front matter r 2007 Elsevier Ltd. All rights reserved.

s.2007.03.033

ng author. Tel.: +90362 3121919.

ss: iucar@omu.edu.tr (I. Uc-ar).

should be known. In this context, we have determined bothstructural and magnetic properties of NINS. In order toobtain EPR data, transition metal ions should be doped inthe host lattice of NINS as an impurity. It is now wellknown that the transition metal ions as a probe can be usedto determine the symmetry environments of the complexesin host lattices by EPR technique [3–7]. When these ionsform paramagnetic centres, then one can get informationabout the local symmetry. Since Cu2+ ions are generallyused as probes to enter the lattice substitutionally in placeof the divalent cation in the lattices containing divalentcations in the literature, we have used Cu2+ ions in NINSand obtained the EPR data.

2. Experimental

2.1. General method

All chemical reagents were analytical grade commercialproducts. Solvents were purified by conventional methods.

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Table 1

Crystal data and structure refinement for NINS

Formula C26H28N6NiO12S2Formula weight 739.37

Temperature (K) 297(2)

Wavelength (Mo Ka) 0.71073

Crystal system Monoclinic

Space group P21/c

Unit cell dimensions

a, b, c (A) 6.993(5), 16.206(5), 13.708(5)

b (1) 98.464(5)

Volume (A3) 1536.6(13)

Z 2

Calculated density (g cm�3) 1.598(14)

m (mm�1) 0.841

F(0 0 0) 764.0

Crystal size (mm) 0.21� 0.35� 0.40

y range (1) 2.25–27.45

Index ranges �8php8

�21pkp21

�18plp18

Reflections collected 24226

Independent reflections 3008 [Rint ¼ 0.108]

Reflections observed (42s) 2769

Absorption correction Integration

Refinement method Full-matrix least-squares on

F2

Data/restrains/parameters 3008/0/239

Goodness-of-fit on F2 1.065

Final R indices [I42s(I)] 0.0394

R indices (all data) 0.0427

Largest diffraction peak and

hole (A�3)

0.38, �0.59

I. Uc-ar et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1540–1548 1541

The EPR spectra were recorded using a Varian E-109Cmodel X-band spectrometer. The magnetic field modula-tion frequency was 100 kHz and the microwave power wasaround 10mW. The single crystals were mounted on agoniometry and the spectra were recorded in threemutually perpendicular planes at 101 intervals at 298K.The g values were obtained by comparison with adiphenylpicrylhydrazyl sample of g ¼ 2.0036. The thermaldecomposition of �10mg of the prepared complex wasmeasured under the nitrogen atmosphere with PRISDiamond TG/DTA thermal analyser at a heating rate of10 1C/min in the temperature range 35–1000 1C for NINSusing platinum crucibles. The IR spectra were recorded ona Jasco 430 FT/IR spectrometer using KBr pellets andoperating in 4000–200 cm�1 range.

An EcoChemie Autolab-30 potentiostat with the elec-trochemical software package GPES 4.9 (Utrecht, TheNetherlands) was used for voltammetric measurements.A three-electrode system was used: a Pt counter electrode,an Ag/AgCl reference electrode and a Pt wire electrode asworking electrode. The potentiostat/galvonastat have anIR-compensation option. Therefore, the resistance due tothe electrode surface was compensated throughout themeasurements. Oxygen-free nitrogen was bubbled throughthe solution before each experiment. All experiments werecarried out at room temperature.

2.2. Synthesis of [Ni(ina)2(H2O)4] � (sac)2

Into aqueous solution of the corresponding Ni(II)acetate, [Ni(OAc)2] (2mmol, 20mL) was added to anaqueous solution of sodium saccharinate (4mmol, 20mL).After stirring for 30min, precipitates were filtered andwashed with acetone to yield the compounds [Ni(sacchar-inato)2(H2O)4] � 2H2O. An aqueous solution of isonicoti-namide (4mmol, 20mL) was added into aqueous solutionsof these compounds (2mmol, 20mL), under stirring, andthe mixtures were allowed to stand at the room tempera-ture. After a few days, well-formed crystals were selectedfor X-ray studies. The single crystals of Cu2+ doped NINSwere also grown by slow evaporation of the saturatedaqueous solutions admixtured in stochiometric ratios withabout 0.05% CuCl2 salt. The well-developed single crystalsof suitable sizes were selected for EPR investigation afterabout a week.

2.3. X-ray crystallography

A suitable single crystal was mounted on a glassfiber and data collection was performed on a STOEIPDSII image plate detector using Mo Ka radiation(l ¼ 0.71019 A). Details of crystal structures are given inTable 1. Data collection: Stoe X-AREA [8]. Cell refine-ment: Stoe X-AREA [8]. Data reduction: Stoe X-RED [8].The structure was solved by direct methods using SIR-97[9] and anisotropic displacement parameters were appliedto non-hydrogen atoms in a full-matrix least-squares

refinement based on F2 using SHELXL-97 [10]. All carbonhydrogens were positioned geometrically and refined by ariding model with Uiso 1.2 times that of attached atoms andremaining H atoms were located from the Fourierdifference map. Molecular drawings were obtained usingORTEP-III [11].

3. Results and discussion

3.1. Crystal structure of NINS

The metal cation is located on a crystallographicinversion centre and the compound consists of a complexcation together with two sac anions (Fig. 1). The Ni(II) ionis hexa-coordinated by four oxygens of aqua ligandscomposing the basal plane, and two trans nitrogen atomsfrom the monodentate ina ligand occupying the axial sites,adopting a distorted octahedral sphere. The observedNi–Nina lengths are similar to those found in [Ni(na)2(H2O)4] � (sac)2 (na: nicotinamide) [12], [Ni(tea)2] � (sac)2(tea: triethanolamine) [13], [Ni(pyet)2(H2O)2] � (sac)2(2-pyridylethanole) [14], [Ni(sac)2(py)4] � 2py (py: pyridine)[15] but slightly longer than those found in [Ni(pia)2(H2O)2] � (sac)4 �H2O (pia: picolinamide) [16], [NiX2(py)4][X ¼ Cl, Br] [17] and [Ni(im)2(sac)2(H2O)2] (im: imidazole)

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Fig. 1. The molecular structure of NINS, showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 40% probability level and H

atoms are shown as small spheres of arbitrary radii (dashed lines indicate the hydrogen bonds, symmetry code (i) 1�x, �y, 1�z).

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[18]. The Ni–Oaqua distances are similar to those found in[Ni(na)2(H2O)4] � (sac)2 and [Ni(pyet)2(H2O)2] � (sac)2 whilethose distances are slightly shorter than those found in[Ni(pia)2(H2O)2] � (sac)2 � 4H2O most probable due to thehydrogen bonding interactions between the aqua andacceptor groups. The ring plane of ina nearly bisects theadjacent coordination planes containing the octahedronaxis. The dihedral angle between the NCQO plane andthe pyridine ring plane is 20.63(15)1 in NINS. As expected,the molecular skeletons of the sac ions nearly planar inNINS (rms deviation of atoms from the mean plane of0.023 A) and the N3–S1 [1.620(2) A] and N3–C7[1.342(3) A] bond distances are close to those found insodium saccharinate [19] and in the related saccharinatecomplexes [20]. The bond angles of the metal-freesaccharinate ions in which the ring nitrogen is deproto-nated as in the title complex are in good agreement withthose of the metal-bonded saccharinato ligands such as[Cu(H2O)(py)2(sac)2] (I) [21] and [Cu(H2O)(na)2(sac)2] �H2O (II) [22], showing that the metal bonding to thering nitrogen exerts little effect on the molecular dimen-sions; for instance, the bond angle S–N–C ¼ 110.68(17)1in NINS (Table 2) and the corresponding angles of111.8(2)1 and 112.1(2)1 in (I) and (II). Similarly, acomparison of the CQO bond distance [1.236(3) A] ofthe amide groups in (II) and the title complex shows thatthe effect of the metal bonding to amide oxygen on theCQO bond length in (II) is negligible [C6QO3 distance is1.231(3) A in NINS].

Analysis of the crystal packing indicates that there arethree types of intermolecular hydrogen bond interactions(O–H?O, N–H?O and O–H?N) in the complex,involving the oxygen atoms of aqua ligands, carboxyamidegroup of ina ligand and saccharinate carbonyl oxygenand nitrogen atoms (Fig. 2). Apart from these, thereis also symmetry-unrelated weak slipping face toface p–p stacking interaction between the saccharinate(S1–N3–C7–C8–C13, ring A) and pyridine ring of ina(ring B). The centroid-to-centroid and centroid-to-planedistances between the rings A and B are 3.811(3) and3.467 A, respectively. The closest interatomic distance is(C5?C7) 3.561(4) A.

3.2. EPR investigation

The EPR spectra of Cu2+ doped [Ni(ina)2(H2O)4] � (sac)2taken at all orientations of the magnetic field at roomtemperature show two sets of four hyperfine lines originat-ing from two magnetically inequivalent sites. Since the linewidth is relatively broad, the 63Cu and 65Cu hyperfine linesdo not appear to be resolvable at all orientations as can beseen in Fig. 3. The EPR spectrum of Cu2+ ion is recordedwhen the magnetic field is 501 and 1401 away from theb-axis (Fig. 3). Spectra consist of two sets of four hyperfinelines due to the interaction of the unpaired electron(S ¼ 1/2) with the copper nucleus (I ¼ 3/2). The angularvariations of the EPR spectra of Cu2+ doped NINS singlecrystal are shown in Fig. 4. It is known for single crystals

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Fig. 2. Three-dimensional structure of NINS. Dashed lines indicate the hydrogen bonds.

Table 2

Interatomic bond distances (A) and angles (1) around the Ni(II) ion

(a) Bond lengths, bond angles

Bond lengths (A)

O1–Ni1: 2.056(2) O2–Ni1: 2.056(2) N1–Ni1: 2.106(2)

N3–S1: 1.620(2) C7–N3: 1.342(3) C6–O3: 1.231(3)

Bond angles (1)

O2–Ni1–O1: 91.64(9) O2–Ni1–N1: 89.62(8) O1–Ni1–N1: 91.26(8)

O1–Ni1–O1i: 180.00(8) O2–Ni1–N1i: 90.38(8) O1–Ni1–N1i: 88.71(7)

C7–N3–S1: 110.68(17) O5–S1–N3: 110.66(13) O6–C7–N3: 123.4(2)

(b) Hydrogen-bonding interactions (A, 1)

D–H?A D–A H?A D?A D–H?A

O1–H1A?O6 0.81(3) 1.89(4) 2.694(3) 169(4)

O1–H1B?O6ii 0.81(5) 2.09(5) 2.884(4) 165(4)

O2–H2A?O3iii 0.80(4) 1.84(4) 2.639(3) 179(3)

O2–H2B?N3i 0.83(4) 2.03(3) 2.867(4) 177(2)

N2–H2C?O5iv 0.83(3) 2.40(3) 3.217(4) 168(3)

N2–H2D?O4v 0.83(4) 2.14(4) 2.961(4) 168(4)

Symmetry codes: (i) 1�x, �y, 1�z ; (ii) �x, �y, 1�z; (iii) x, 1/2�y, 1/2+z; (iv) 1�x, 1/2+y, 1/2�z ; (v) 1+x, 1/2�y, 1/2+z

I. Uc-ar et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1540–1548 1543

that when the magnetic field is along the crystallographicb-axis or in the a*c plane the spectra consist of a single setof four hyperfine lines, but when the magnetic field is in thea*b or bc planes, the spectra consist of two sets of fourhyperfine lines of the Cu2+ ions. This behaviour isconsistent with the monoclinic symmetry. The wholespectra were fitted with a rhombic spin-Hamiltonian,

H ¼ bðgxxBxSx þ gyyBySy þ gzzBzSzÞ

þ A I S þ A I S þ A I S . ð1Þ

zz z z xx x x yy y y

The principal values of the g and A tensors and theirdirection cosines were found by a diagonalization proce-dure [23] and the results are given in Table 3. From theseresults, it is inferred that there is magnetically inequivalentbut chemically equivalent two Cu2+ ions in the unit cells ofthe NINS single crystals. These results are consistent withthe monoclinic symmetry properties. Hence, we concludethat the Cu2+ ion has entered the Ni2+ site in this complexby considering that the ionic radius of Ni2+ (0.69 A) isappropriate for substitution of Cu2+ (0.72 A). From theEPR parameters (Table 3), it can be deduced that Cu2+

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Fig. 3. EPR spectrum of Cu2+ doped NINS single crystal when the magnetic field is in the bc plane and is away (a) 501 and (b) 1401 from the b-axis.

Fig. 4. Angular variations of the g2 values of all lines in three mutually perpendicular planes of Cu2+ doped NINS single crystal.

Table 3

Spin-Hamiltonian parameters of Cu2+ doped NINS (Dg ¼70.002, DA ¼73G)

Site g Direction cosines A (G) Direction cosines

a* b c a* b c

I gxx ¼ 2.205 �0.325 0.664 0.672 Axx ¼ 59.2 0.866 0.307 0.333

gyy ¼ 2.046 0.945 0.214 0.245 Ayy ¼ 75.0 �0.489 0.501 0.713

gzz ¼ 2.369 �0.019 �0.715 0.698 Azz ¼ 98.7 �0.096 0.781 �0.616

II gxx ¼ 2.202 0.275 0.680 0.679 Axx ¼ 48.0 0.728 �0.474 �0.493

gyy ¼ 2.045 0.961 �0.181 �0.207 Ayy ¼ 78.8 0.667 0.332 0.665

gzz ¼ 2.370 0.017 �0.701 0.703 Azz ¼ 95.5 0.151 0.814 �0.559

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ions have an octahedral environment with a rhombicdistortion.

Fig. 5 shows powder and the simulated EPR spectra ofCu2+ ion in NINS. The values measured from the powderspectrum agree with these of the single crystal data. Thesimulation made by using experimental values gives nearlythe same spectrum. It can be clearly seen from the powderspectrum that the symmetry of the complex in the crystal isnot axial. The powder EPR spectrum was partially resolvedinto three components. These are consistent with theresults for the single crystal. For the powder spectrum, themeasured values are gxx ¼ 2.205, gyy ¼ 2.059, gzz ¼ 2.371;Axx ¼ 60G, Ayy ¼ 76G and Azz ¼ 100G.

The unaxial behaviour of the hyperfine and g valuevariations show that the octahedron is distorted asindicated by the spin Hamiltonian parameters. It is knownthat when R ¼ ðgx � gyÞ=ðgz � gxÞ is less than unity, theunpaired electron is dominantly in the dx2�y2 state, and for

Fig. 5. The powder EPR spectrum of Cu2+ doped NINS single crystal

(solid line) and its computer simulation (dashed line).

Table 4

Ground state wave function parameters of Cu2+ ions, observed in different e

Environment Temperature (K) Site k

[Ni(ina)2(H2O)4] � (sac)2 300 I 0.362

II 0.392

CdK2(SO4)2 � 6H2O 77 0.315

DL-Aspartic acid 300 I 0.137

II 0.136

[Zn(sac)2(H2O)4] � 2H2O 300 I 0.317

II 0.322

Na2Zn(SO4)2 � 4H2O 30 0.21

K2Zn(SO4)2 � 6H2O 20 0.28

Rb2Zn(SO4)2 � 6D2O 20 0.29

K2Zn(ZrF6)2 � 6H2O 4.2 0.38

R greater than unity, it is in the d3z2�r2 state. The observedR values for two sites, RI ¼ 0.94 and RII ¼ 0.96, are lessthan unity and therefore the ground state of the electron isdominantly in dx2�y2 state [24–26]. In fact, when thecoordination around the Cu2+ constitutes an octahedronand shortened along the C4u-axis, the ground state is3dx2�y2 , and in the lengthened case it is in 3dz2 . In therhombic lower symmetry cases, the ground state is anadmixture of the 3dx2�y2 and 3dz2 orbitals. Bhaskar andNarayana [25] developed the most general expressions forthe ground state wave function in which the covalencyparameter and mixing coefficients are distinguished.They expressed the wave function as

C ¼ ða02Þ1=2½ajx2 � y2i þ bj3z2 � r2i�, (2)

where a02 is the covalency parameter which indicates theprobability of finding electron-spin density on the metalCu2+ d orbital, a and b are the mixing coefficients forx2 � y2�� �

and 3z2 � r2�� �

orbitals. They assumed a � 1, b �0 and analysed the spectra only for the compressedoctahedral symmetry with a rhombic distortion super-imposed on it. Using Eq. (2) together with the experimentalvalues (Table 3), a02, a, b and k parameters were calculatedand are given in Table 4. The ground state wave functionsof the sites by using the parameters in Table 4 are found tobe as

CI ¼ ð0:895Þ1=2 0:941 x2 � y2

�� �þ 0:337 3z2 � r2

�� �� �;

CII ¼ ð0:855Þ1=2 0:942 x2 � y2

�� �þ 0:336 3z2 � r2

�� �� �;

where the covalency parameter a02 is 0.895 for site I and0.855 for site II, obviously explains that the unpairedelectron spends 10.5% of its time for site I and 14.5% of itstime for site II on ligand orbitals, whereas the rest is spenton the Cu2+ d orbitals. Since the coefficient of dx2�y2 issignificantly greater than that of d3z2�r2 , one can concludethat the rhombic distortion results dominantly from d3z2�r2

orbital of the Cu2+ ion.

nvironments

a02 a b dgexp dgcal Ref.

0.895 0.941 0.337 0.162 0.2090 This work

0.855 0.942 0.336 0.157 0.206

0.769 0.996 0.077 0.07 0.084 [27]

0.868 0.994 0.111 0.041 0.052 [28]

0.814 0.992 0.125 0.057 0.036

0.900 0.987 0.163 0.087 0.100 [29]

0.892 0.986 0.169 0.082 0.091

0.99 0.310 0.940 0.253 0.263 [25]

0.90 0.977 0.213 0.117 0.118 [25]

0.95 0.981 0.194 0.110 0.114 [25]

0.84 0.990 0.118 0.066 0.065 [25]

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Fig. 6. Thermal analysis curves for NINS.

Table 5

The most characteristic IR bands of Na(sac) � (H2O), ina ligand and NINS

Assignment (cm�1) Na(sac) �H2O [19] ina [1] NINS

n(OH) 3486s — 3424s

nas(NH2) — 3370s 3328s

ns(NH2) — 3187s 3195s

n(CH)py — 3060w 3095w

n(CO)amid — 1668vs 1681vs

n(CO)sac 1642vs — 1608vs

n(CN)+n(CC)py — 1552vs 1563vs

n(CC)py — 1425w 1417w

n(CN)amid — 1391m 1394m

nas(SO2) 1258vs — 1265vs

ns(SO2) 1150vs — 1141vs

d(CH)py — 1063w 1047w

Ring breathpy — 993m 1016w

nas(CNS) 950s — 943s

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3.3. Thermal investigation

The simultaneously recorded thermal analysis curves(TG, DTG and DTA) of [Ni(ina)2(H2O)4] � (sac)2 complexare given in Fig. 6. As can be seen from Fig. 6, thedecomposition process of the complex has four stages. Thefirst decomposition stage of the complex is deaquation,which occurs in one step and has a decomposition enthalpyof 499.5 kJ/mol found by converting DTA signals toDSC data. Four moles of aqua ligands were releasedin the 83–229 1C temperature intervals. In the second(229–349 1C) and the third stages (349–578 1C), neutral inaligand together with saccharinate ion start to decompose.In these consecutive stages, a number of undefinedprocesses take place in this range and eventually the finalproduct is NiO.

o.p. ring defpy — 757w 759w

NH2 wagging — 658s 678m

OQCN bend — 594 597

py: pyridine, sac: saccharinate, as: asymmetric, s: symmetric, d: in plane

deformation, o.p.: out of plane, def: deformation, vs: very strong, s:

strong, m. medium, w: weak, sh: shoulder.

3.4. FT-IR investigation

The most characteristic bands of NINS together withthese respective bands of isonicotiamide ligand and sodiumsahharinate complexes [1,19] are given in Table 5 andshown in Fig. 7.

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Fig. 7. The FT-IR spectra of complex NINS.

Fig. 8. The cyclic voltammogram of 5� 10�3M NINS at a Pt wire

electrode in 0.1M DMF–Bu4NClO4, potential scan rate 100mV s�1.

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The n(CQO) carbonyl vibrations of ina shift to higherfrequencies as compared with that of its free molecule. Thisis most probably due to increasing double bond characterof the carbonyl group. When the pyridine nitrogen of ina isinvolved in coordination, the electron density in the ligandshift toward the pyridine nitrogen. This shift results fromeither an inductive effect or a combination of inductiveeffects and resonance, depending on the amide, and leadsto increase in double bond character of carbonyl group ofina. The n(NH) vibration bands due to the amide group inina appear at 3370 and 3187 cm�1. In NINS complex, theregion of the NH2 stretching vibrations is relativelybroadened by partial superposition of the OH vibrationsof the aqua ligands. The antisymmetric and symmetricstretching vibrations of the NH2 group together with OHstretching vibrations are listed in Table 5. The OH/NHfrequency range implies that neither the water moleculesnor the ina NH groups in the NINS participate in verystrong hydrogen bonds. Moreover, the existence of n(OH)bands close to 3500 cm�1 might be taken as an indicationof that some of the water OH groups are very weaklyhydrogen bonded. In the CN stretching vibrations of theamide group of NINS complex, a slight shift has beenobserved. On the other hand, pyridine ring vibrations offree ina and its derivatives are observed to shift to higherfrequencies in NINS complex. This is expected because thedonor power of pyridine ring nitrogen is quite strong [30].

Regarding with the vibration modes of saccharinate, then(CQO) stretching vibration shifts to lower energy thanthat of its sodium salt indicating that this group is involvedin strong hydrogen bonding. No considerable changes areobserved for n(SO2) and n(CNS) vibrations indicating thatsaccharinate is not coordinated to metal.

3.5. Voltammetric behaviour of NINS

The redox behaviour of complex is also studied usingcyclic voltammetry at a platinum working electrode at scanrate of 100mV s�1. Cyclic voltammetric study of NINS(5� 10�3M) was carried out in dimethylsulfoxide (DMSO)solution containing 0.05M n-Bu4NClO4 supporting elec-trolyte in a potential range of �2.0 to 1.25V versus

Ag/AgCl. The representative cyclic voltammogram isdisplayed in Fig. 8. No oxidation or reduction peaks wereobserved for sodium saccharinate and ina in the selectedpotential range [31]. The cyclic voltammogram of complexshows only one irreversible reduction wave in the cathodicpotential region at Epc ¼ �1.63V corresponding to theNi(II)/Ni(I) couple. Upon a reverse scan, one irreversibleoxidation process is observed at anodic potential(Epa ¼ 0.71V) for the NINS complex and assigned toNi(II)/Ni(III) couple.

4. Supplementary data

Crystallographic data (excluding structure factors) forthe structure in this paper have been deposited with theCambridge Crystallographic Data Centre as the supple-mentary publication no. CCDC 621565. Copies of the datacan be obtained, free of charge, on application to CCDC,12 Union Road, Cambridge, CB12 1EZ, UK, fax:+44 1223 366 033, e-mail: deposit@ccdc.ac.uk or on theweb: http://www.ccdc.cam.ac.uk

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