Chapter-6 Complexes of Nickel(II), (3dshodhganga.inflibnet.ac.in/bitstream/10603/25210/10/10_chapter 6.pdfA mononuclear complex formed with inert Co(III) was characterized by a crystal

Chapter-6

Complexes of Nickel(II), (3d8) 6.1: INTRODUCTION

Nickel usually occurs in +2 oxidation state. It is an important metal both

industrially and biologically. It is one among the essential trace elements along

with cobalt, copper, zinc and manganese in the human diet [1]. According to

Bertand and Nakamure [2], nickel and cobalt play a direct role in nutritional

phenomenon. The maximum recommended concentration of nickel(II) ion in

drinking water for livestock is 2.5 mg/l [3]. Nickel, which is bound to

ribonucleic acid, has a special affinity for bone and skin and has been

suggested to play an important role in pigmentation [4]. However, nickel can

cause allergic reactions and certain nickel compounds are carcinogenic [5-7].

Nickel is an essential trace element involved in the metabolism of several

species of bacteria, archea and plants. Most bacteria and archea that can live

under anaerobic conditions (including beneficial bacteria in the human gut)

produce several enzymes that require nickel [8]. In these organisms, nickel is

involved in enzymes that catalyze both non-redox (e.g. urease and glyoxalase I)

and redox (e.g. hydrogenase, carbon monoxide dehydrogenase, superoxide

dismutase) reactions and proteins involved in the transport, storage,

metallocenter assembly and regulation of nickel concentration. Studies of

structure/function relationships of nickel biochemistry reveal that cystein

ligands are used to stabilize Ni(III/II) redox couple. Urease is a nickel metallo-

enzyme and is an essential micronutrient for plants [9-10].

As with most transition metals, the uptake, transport, storage, intracellular

concentration and biosynthesis of nickel metalloenzymes are tightly regulated

by proteins that specially bind Ni(II). These proteins include examples of Ni-

specific permeases (e.g. NixA, HoxN, NikA-NikE) [11], metallochaperones

Complexes of Nickel(II), (3d8)

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(e.g. UreE, HypB, CooJ) [12-14] and proteins involved in the regulation of

biosynthesis (e.g. NikR and H2-sensor) [15-17]. An interesting example of Ni-

specific uptake and the regulation is provided by the Nik system in Escherichia

coli [18].

Further, nickel is one of the important alloying elements for steel and cast iron.

Nickel in the form of pure metal and binary or ternary alloys as well as nano-

structured metal matrix composites have found widespread uses in industries as

corrosion protection, stress corrosion, cracking resistance, wear resistance,

catalysts, soft magnet, hydrogen-storage and purification, electrodes for

hydrogen evolution reactions and fuel cells [19-22]. Notably, the Shell Higher

Olefins Process (SHOP) is one of the most prominent applications of nickel in

homogeneous catalysis. The SHOP catalysts contain a bidentate [P-O]- ligand

coordinated to nickel(II) and show exceptionally high selectivities for linear

olefins [23-25].

6.2: REVIEW OF THE LITERATURE

M. A. Ali et al. [26] synthesized new nickel(II) and copper(II) complexes of

general formulae [M(Ap-SR)] and [M(Ap-SR)B] ( Ap-SR = dianionic forms of

the Schiff’s bases of 2-hydroxyacetophenone and S-alkyl esters of

dithiocarbazic acid; M = NiII or CuII; R = Me or CH2Ph; B = py, phen or dipy)

and characterized by a variety of physicochemical techniques. Magnetic and

spectroscopic data support an oxygen-bridged binuclear structure for the

[M(Ap-SR)] complexes. The [M(Ap-SR)py] complexes are four-coordinate

and square planar, whereas the [M(Ap-SR)B] complexes (B = phen or dipy) are

five-coordinate and probably trigonal bipyramidal.

T. Kawamoto et al. [27] reported the synthesis and characterization of

nickel(II) and cooper(II) complexes of 2,6-Bis[(2-acetylphenyl)carbamoyl]-

pyridine. The ligand and complexes are characterized on the basis of elemental


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analyses, FTIR, 1H NMR, 13C NMR and X-ray crystallography. The effects of

molecular structure on crystal lattice are also discussed.

The new complexes of bivalent Ni, Pd, Zn, Cd and Hg with 3-

thionicotinoylaminodibenzofuran (TNADBF) are described by A. Kriza et al.

[28]. The complexes are characterized by elemental analysis, molar

conductance measurements, IR and electronic spectral studies. The complexes

are found to be of [MLCl2] type. The thioamide acts as bidentate ligand using

both sulphur and nitrogen atoms as donors in the formation of complexes.

The asymmetric tripodal 4.2.2’-tetraamine N{(CH2)4NH2}{(CH2)2NH2}2 bee,

3.2.2’-tetraamine N{(CH2)3NH2}{(CH2)2NH2}2 pee and 3.3’.2-tetraamine

N{(CH2)3NH2}2{(CH2)2NH2} ppe ligands have been prepared by H. Keypour

et al. [29]. In the presence of CuII or NiII ions, these ligands condense with 2,6-

diacetylpyridine. In some cases cyclization occurs and new isomers of CR-type

with a pendant primary amine group are formed. In other cases ring closure

does not occur and coordinated acyclic hexadentate ligands have been isolated.

At room temperature and in absolute methanol the latter acyclic complexes are

the only products. I.r., u.v.-vis., reflectance spectra and magnetic susceptibility

for the complexes were recorded.

Several new complexes of a tridentate ONS Schiff’s base derived from the

condensation of S-benzyldithiocarbazate with salicylaldehyde have been

characterized on the basis elemental analyses, molar conductivity

measurements, i.r. and electronic spectra by M. T. H. Tarafder et al. [30]. The

Schiff’s base (HONSH) forms mono-ligand complexes: [M(ONS)X], [M =NiII,

CuII, CrIII, SbIII, ZnII, ZrIV, or UVI with X = H2O, Cl]. The ligand produced a

bis-chelated complex of composition [Th(ONS)2] with ThIV. Square planar

structures are proposed for the NiII and CuII complexes.

W. G. Hanna et al. [31] reported that the asymmetric 7-formyanil-substituted-

imino-4-(4-methyl-2-butanone)-8-hydroxyquinoline-5-sulphonic acid (Schiff’s


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bases), react with CoII, NiII and CuII ions to give 1:2, 1:1 and 2:1 complexes.

The complexes were investigated by elemental analyses, molecular weight

determinations, molar conductance, magnetic susceptibility measurements and

thermal analysis and i.r., u.v.-vis. and e.s.r. spectra. The complexes have an

octahedral crystal structure and general formula [ML.(OH2)2], where MII = Co,

Ni and Cu and L = Na[7-X-HL], (-X- = (CH2)2, (CH2)3, p-C6H4, o-C6H4).

The new electron transfer complexes of [M-N2S2] type formed by NiII, PdII and

PtII with naphthoquinonic ligands have been synthesized by N. Muresan et al.

[32]. Sulphur and nitrogen atoms of the ligands are involved in the

coordination to the metal. Their participation in the electron-transfer processes

has been investigated by polarography.

The X-band e.s.r. and optical absorption spectra of the imidazolate bridged

heterobimetallic complexes [(tren)Cu-E-Im-Zn-(tren)](ClO4)3 and [(tren)Cu-E-

Im-Ni-(tren)](ClO4)3, where tren = tris(2-aminoethyl)amine, E-Im = 2-

ethylimidazolate ion and related mononuclear complexes [Cu(tren)](ClO4)2 and

[(tren)Cu-E-ImH)](ClO4)2 have been described by N. K. Singh et al. [33].

Biological activities (superoxide dismutase and antimicrobial) have been

screened and compared with reported complexes.

2,6-diformylpyridine bis(N4-methylthiosemicarbazone) and bis(N4-dimethyl-

thiosemicarbazone), H22,6Fo4M and H22,6Fo4DM, respectively and 2,6-

diacetylpyridine bis(N4-methylthiosemicarbazone), 2,6-diacetylpyridine-

bis(N4-dimethylthiosemicarbazone), H22,6Ac4M and H22,6Ac4DM and their

copper(II) and nickel(II) complexes have been synthesized by C. A. Brown et

al. [34]. The 1H NMR spectra of the free bis(thiosemicarbazone) show that,

most often, one of the thiosemicarbazone moieties is hydrogen bonded to the

pyridine nitrogen and in [2H6]-DMSO there is interaction with solvent oxygen.

D. Kumar et al. [35] reported the reaction of aldehydropolystyrene with 2-

aminoethanethiol or 2-aminothiophenol in 1:4.68 molar ratio to form


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polystyrene-anchored Schiff’s bases, PS-LH (1) or PS-L’H (2), respectively.

PS-LH (1) or PS-L’H (2) reacts with metal salts in DMF to form the

polystyrene-anchored coordination compounds, [PS-LM(CH3COO).3DMF]

(where M = Cu, Zn, Cd, UO2), [PS-LM’(CH3COO).3DMF] (where M’ = Co,

Ni), [PS-LFeCl2.2DMF) and [PS-LMoO2(acac)] (where acacH = acetyl

acetone). PS-L’H also forms the similar type of compounds. The polystyrene-

anchored coordination compounds have been characterized by elemental

analyses, IR, reflectance spectral and magnetic susceptibility measurements.

The CdII and ZnII compounds are tetrahedral, CuII compounds are square

planar, CoII, NiII, FeII, MoO2VI and UO2

VI compounds are octahedral.

A series of complexes of CuII, NiII, CoII, MnII, FeIII, VOII and ZnII with the s-

triazine, 4,6-diamino-1,3,5-triazine-2-thiol known as thioammeline (HTA) and

its sodium salt (NaTA) have been prepared and characterized through

elemental analyses, magnetic susceptibility measurements, electronic, IR and

ESR spectroscopic techniques by C. P. Jousua et al. [36]. The electrochemical

behavior of the complexes was explored using cyclic voltammetry. It is

observed that the metal-ligand linkage has high covalent nature. The CuII and

NiII complexes are square planar, CoII, MnII and ZnII complexes are tetrahedral,

VOII complex is square pyramidal and FeIII complex is octahedral.

Some new complexes of FeIII, CoII, NiII, CuII, ZnII, CdII and HgII have been

synthesized with bidentate heterocyclic ligand (N, S) of the dibenzofuran series

by L. S. Sbîrnă et al. [37]. The complexes were characterized by elemental

analyses, UV/Vis., IR, EPR, NMR spectral studies, conductivity and magnetic

susceptibility measurements. A sulphure and nitrogen atoms of the ligand are

involved in the coordination to the metal.

The complexing process proceeding in the NiII-thiocarbohydrazide (H2N-

HNC(=S)-NH-NH2)-propanone triple system in EtOH solution and

nickel(II)hexacyanoferrate(II) gelatin-immobilized matrix has been studied by


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O. V. Mikhailov et al. [38]. It has been found that in the first case, template

synthesis leading to formation of three coordination compounds of NiII with

(N,N,S,S)-donor tetradentate ligands having NiL1, NiL2 and NiL3 compositions

[where L1 is 4,6,6-trimethyl-2,3,7,8-tetraazanonen-3-di(thiohydrazide)-1,9, L2

is 4,6,6-trimethyl-1,9-dithio-2,3,7,8,10,11-hexaazatridecadien-3,11-hydrazide-1

and L3 is 2,8,10,10,16-pentamethyl-5-13-dithio-3,4,6,7,11,12,14,15-octaaza-

heptadecatrien-2,7,15], whereas in the gelatin-immobilized matrix, a

complexing process does not occur.

N. W. Alcock et al. [39] prepared the potentially pentadentate lignad 2,6-

bis[N-(2’-pyridylmethyl)carbamyl]pyridine (H2L1) from reaction of a diester of

pyridine-2,6-dicarboxylic acid (H2dipic) and 2-aminomethylpyridine (ampy)

which shows limited tendency to form 1:1 M:L complexes with labile metal

ions. Although [CuL1] and [NiL1] were observed as minor species, the later

characterized by crystal structure analysis. A mononuclear complex formed

with inert Co(III) was characterized by a crystal structure as the neutral

complex [Co(L1)(HL1)]. Fe(III) is known to form a symmetrical 1:2 complex.

However, the most remarkable tendency of HL1 is towards the formation of

robust double helical complexes: a dinuclear Cu(II) complex [CuL12] forms, as

well as a trinuclear Ni(II) complex [Ni3(L1)2(OAc)2(MeOH)2]. Moreover, in the

presence of added H2dipic, the tetranuclear complex [Cu4(L1)2(dipic)2(OH2)2] is

obtained. Using the chelating ligand pentane-2,4-dione (acac), each of the two

pairs of adjacent monodentate ligands in [Ni(L1)2(OAc)2(OH2)2] have been

shown to be available for substitution with out destroying the helical structure,

to form [Ni(L1)2(acac)2].

M. A. Zahed et al. [40] studied some transition metals (M) and amoxicillin

trihydrate (ACT) ligand complexes (M-ACT) that formed in solution involved

in spectrophotometric determination of stoichiometric ratios and their stability

constants. The separated solid complexes were studied using elemental


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analyses, IR, reflectance spectra, magnetic susceptibility measurements, mass

spectra and thermal analyses (TGA and DTA). The proposed general formulae

of these complexes were found to be ML(H2O)ω(H2O)x(OH)y(Cl)z, where M =

Fe(II) and Co(III), ω = 0, x = 2, y = 0, z = 1; M = Co(II), ω = 0, x = 1, y = 0, z

= 1; M = Fe(III), ω = 0, x = 1, y = 2, z = 0; M = Ni(II), Cu(II) and Zn(II), ω =

2, x = 0, y =1, z = 0, where ω = water of crystallization, x = coordinated water,

y = coordinated OH- and z = Cl- in the outer sphere of the complex.

New neutral Schiff’s base complexes of Cu(II), Co(II), Ni(II) and Zn(II)

derived from 4-aminoantipyrine and N-(1-piperidinobenzyl) acetamide

(Mannich base) have been synthesized by N. Raman et al. [41]. The structural

features of the complexes have been characterized by microanalytical data, IR,

UV-Vis, 1H NMR, ESR, CV, TGA and powder XRD techniques. Electronic

absorption spectra of the complexes indicate an octahedral geometry around the

metal ion. The powder XRD pattern indicates the crystalline nature of the

complexes.

S. Chandra et al. [42] synthesized the Mn(II), Co(II) and Ni(II) complexes of

2-methylcyclohexanonethiosemicarbazone (MCHTSCL1) and 2-methylcyclo-

hexanone-4N-methyl-3-thiosemicarbazone (MCHMTSCL2) with composition

[M(L)2X2] (where M = Mn(II), Co(II), Ni(II), L = L1 or L2 and X = Cl-, NO3-

and 1/2SO42-) and characterized by elemental analyses, magnetic susceptibility

measurements, UV-Vis., IR, EPR and mass spectral studies. Various

physicochemical techniques suggest an octahedral geometry for the complexes.

Y. Harek et al. [43] synthesized a new NiII complex of 5,6-dihydro-2H-pyran-

3-aldehydethiosemicarbezone (HDPTSC) and characterized by microanalyses,

magnetic susceptibility measurements, molar conductance measurements and

by spectral methods (i.r., u.v.-vis.,1H-.nm.r.). The structure of [Ni(DPTSC)2].DMF

has been solved using X-ray diffraction and found to be highly symmetrical with

a trans-arrangement of the two bidentate ligands.


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Mn(II), Co(II), Ni(II) and Cu(II) complexes with thiosemicarbazone (L)

derived from pyrrole-2-carboxyaldehyde also synthesized by S. Chandra et al.

[44]. These complexes are characterized by elemental analyses, molar

conductance, magnetic susceptibility measurements, mass, IR, electronic and

EPR spectral studies. The molar conductance measurements of the complexes

in DMSO indicates that the complexes are non-electrolyte except Co(L)2(NO3)2

and Ni(L)2(NO3)2 complexes which are 1:2 electrolyte. All the complexes are

of high-spin type. On the basis of spectral studies an octahedral geometry may

be assigned for Mn(II), Co(II) and Ni(II) complexes except Co(L)2(NO3)2 and

Ni(L)2(NO3)2 complexes which are of tetrahedral geometry. A tetragonal

geometry may be suggested for Cu(II) complexes.

N,N,N’,N’-Tetrakis(2-pyridylmethyl)benzene-1,3-diamine (1,3-tpbd), a bis-

(tridentate)ligand, its protonated derivative, [1,3-tpbdH2]2+ and a series of new

dinuclear complexes [Fe2(1,3-tpbd)(CH3CN)6](ClO4)4.(CH3CN)2.(H2O)0.5,

[Fe2(1,3-tpbd)(DMF)6](ClO4)4, [Ni2((1,3-tpbd)(DMF)6](ClO4)4, [Zn2(1,3-tpbd)-

(CH3CN)2(SO3CF3)2(H2O)](SO3CF3)2, [Zn2(1,3-tpbd)Cl4].H2O, [CuZn(1,3-

tpbd)Cl4], and [Cu2(1,3-tpbd)2Cl4](ClO4)4.2H2O have been prepared and

structurally characterized by S. Foxon et al. [45]. Complex [Cu2(1,3-

tpbd)2Cl4](ClO4)4.2H2O called “dimetallocyclophane” only formed in the

presence of zinc ions. A possible reaction product, the dinuclear copper

complex [Cu2(2,6-tpcd)(H2O)(Cl)](ClO4)2.2H2O of a phenolate derivative of

1,3-tpbd was synthesized and structurally characterized independently from

oxidation reactions from copper(II) perchlorate and the ligand 2,6-bis[bis(2-

pyridylmethyl)amino]-p-cresol.

T. Irrgang et al. [46] prepared two sterically demanding iminopyridine

ligands, (2,6-diisopropylphenyl)[6-(2,4,6-triisopropylphenyl)pyridine-2-

ylmethylene]amine and (2,6-diisopropylphenyl)[6-(2,6-dimethylphenyl)-

pyridine-2-ylmethylene]amine by a two step process: first, condensation of 6-

bromopyridine-2-carbaldehyde with an equimolecular amount of 2,6-


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diisopropylaniline and second, kumada-type coupling of in-situ-formed

Grignard compounds of 1-bromo-2,6-dimethylphenyl and 1-bromo-2,4,6-

triisopropylphenyl. Dichloride complexes of the ligands were synthesized

starting from FeCl2, [PdCl2(cod)], [NiCl2(dme)] and CoCl2 (cod = 1,5-

cyclooctadiene, dme = dimethoxyethane). X-ray crystal structure analyses of

Fe, Pd and Co complexes were determined.

B. K. Singh et al. [47] synthesized nickel(II) complexes of reduced glutathione

(GSH) of general composition Na[Ni(L)(X)]H2O, where H2L = GSH; X = NO3-,

SCN-, CH3COO- and Cl- and characterized by elemental analysis, infrared

spectra, electronic spectra, magnetic susceptibility measurements, thermal and

X-ray powder diffraction studies. Kinetic and thermodynamic parameters were

computed from the thermal decomposition data.

The nickel(II) complexes of isatin-3,2’-quinolyl-hydrazones of the type

[Ni(L)X] (where X = Cl-, Br-, NO3-, CH3COO- and ClO4

-] and their adducts

Ni(L)X.2Y [where Y = pyridine or dioxime and X = Cl-, Br-, NO3- and ClO4

-]

have also been synthesized under controlled experimental conditions by S.

Chandra et al. [48]. The complexes were characterized by using the modern

spectroscopic and physicochemical techniques viz. mass, 1H NMR, IR,

electronic, elemental analyses, magnetic susceptibility measurements and

molar conductance. On the basis of spectral studies a four coordinated

tetrahedral geometry is assigned for [Ni(L)X] type complexes, whereas the

adducts Ni(L)X.2Y were found to have six coordinated distorted octahedral

geometry.

A. D. Azaz et al. [49] prepared a complex of NiII with 2,6,diacetylpyridine-

dihydrazone (L) and characterized by means of elemental analyses, IR,

electronic spectra and single crystal X-ray analyses. [NiL2](NO3)2 was

crystallized in the tetragonal space group P-4 21 c. The complex exhibits the

expected coordination sphere with six nitrogen atoms coordinated to the central

NiII with a deformation from pseudo-octahedral geometry.


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Four heterocyclic Schiff’s base ligands from condensation of 4-amino-1,3-

dimethyl-2,6-pyrimidine-dione with 2-hydroxybenzaldehyde, 2-methoxy-

benzaldehyde, 4-hydroxy-3-methoxy- benzaldehyde and 4-(dimethylamino)

benzaldehyde (HL1, L2, HL3 and HL4), respectively and their Co(II) and Ni(II)

complexes have been prepared and characterized via elemental analyses, molar

conductance, magnetic susceptibility measurements, thermal and XRPD

analysis as well as spectral data (IR, 1H NMR, mass and solid reflectance) by

Z. H. A. El-Wahab et al. [50]. The solid reflectance spectral data and

magnetic moment measurements suggest octahedral, tetrahedral and square

planar geometrical structures for the metal complexes.

J. Sanmartín et al. [51] synthesized N-[2-(Tosylamino)benzylidene]-2-

[(tosylamino)methyl]aniline (H2L) ligand and its complexes MII2(L)2.xMeCN.-

yH2O (M = Ni, Pd, Cu, Zn and Cd; x = 0 or 2; y = 0, 4, 6 or 8).

Physicochemical characterization data are indicative of the dinuclear nature of

the complexes, where the potential N,N,N-donor L behaves as a dianionic and

tridentate ligand. The crystal structure of Cu2(L)2.2MeCN was solved. The

planar geometries around the copper ions undergo a seesaw-shaped distortion,

which is probably related to secondary Cu….O interactions with neighbouring

tosyl groups. This spatial arrangement seems to the ferromagnetic behavior

shown by the complex.

6.3: PRESENT WORK

The present work is pertaining to the synthesis and characterization of Ni(II)

complexes with five Schiff’s base ligands. The complexes are characterized on

the basis of various physicochemical studies such as elemental analyses, molar

conductance, magnetic susceptibility measurements and spectral studies such

as IR, electronic and thermogravimetric analyses.


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EXPERIMENTAL

The complexes with Ni(II) metal ions are synthesized by using the following

methods:

1. Synthesis of complexes with ligands L1, L2 and L4:

A hot ethanolic (15 mL) solution of the corresponding ligand (1 mmol) was

added slowly to the hot ethanolic (10 mL) solution of the corresponding metal

salts (nitrate, chloride or acetate) (1 mmol) with continuous stirring. The

resulting solution was refluxed for 14-18 h at 80-90C. On cooling the coloured

product was precipitated out, which was filtered, washed with cold ethanol and

dried under vacuum over P4O10.

2. Synthesis of complexes with ligands L3 and L5:

To a hot solution of corresponding ligand (1 mmol) in acetonitrile (15 mL), a

hot solution of corresponding metal salts (nitrate, chloride or acetate) (1 mmol)

in acetonitrile (10 mL) was added slowly with constant stirring. The mixture

was refluxed for 15-16 h at 75-80C. On cooling the resulting content

overnight at 0C, the coloured microcrystalline product was separated out,

which was filtered, washed with acetonitrile and dried under vacuum over

P4O10.

6.4: RESULTS AND DISCUSSION

The interaction of Schiff’s base ligands with Ni(II) metal ions produces the

solid complexes which are fairly stable in air and non-hygroscopic in nature.

The complexes are soluble in DMF and DMSO. The characterization of

complexes is discussed in detail as follows:


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6.4.1: Physical Properties

Physical characterization, microanalytical data and molar conductance data of

the complexes are summarized in Table 6.1. The elemental analyses of the

complexes are consistent with 1:1 and 2:1 metal:ligand ratio having the general

stoichiometry Ni(HL1)X, Ni(L2)X2, Ni(L3)X2 and Ni(L5)X2 (where X= NO3,

Cl and CH3COO) and Ni2(H2L4)X2 (where X = NO3 and CH3COO). The

molar conductance measurements of the complexes in DMSO indicate that the

[Ni(HL1)X], [Ni(L3)X2] and [Ni2(H2L4)X2] complexes are non-electrolytes

whereas the [Ni(L2)X]X and [Ni(L5)X]X complexes are 1:1 electrolytes [52].

The value of magnetic moments calculated with the help of measured value of

magnetic susceptibility indicates that the complexes with ligands L1, L2, L3 and

L5 are paramagnetic due to the presence of two unpaired electrons except the

complexes with ligand L4 which are diamagnetic. The magnetic moments for

the complexes with ligands L1, L2, L3 and L5 lie in the range 2.81-2.99 B.M.

The value of magnetic moments suggests the spin-free octahedral configuration

around the metal ions for complexes of ligands L1, L2, L3 and L5 and square

planar coordination environment for the complexes of ligand L4 [53].

6.4.2: IR Spectra

The IR spectra of the complexes are depicted in Figs. 6.1-6.4. The IR bands of

ligands and complexes most useful for determining the mode of coordination of

ligands with metal ions are presented in Tables 6.2 and 6.3. The key IR bands

of ligands L1, L2 and L4 are amide I/thioamide I, amide II/thioamide II and

amide III/thioamide III. These bands are found to undergo hypsochromic shift

(blue shift) or bathochromic shift (red shift) upon coordination to central metal

atom. This is due to the effect of electron density drift from these groups. These

findings suggest that the oxygen atoms of amide groups or sulphur atoms of

thioamide groups and nitrogen atoms of azomethine are coordinated to the


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central Ni(II) metal ion [54-60]. Moreover, the ligands L1 and L2 also show IR

bands due to pyridine ring-stretching, in-plane-ring-bending and out-of-plane-

ring-bending vibrations. In the IR spectra of complexes, these bands show

substantial positive shift with fairly low intensity indicating the involvement of

nitrogen atom of pyridine entity present in the ligands to central metal atom

[61]. This discussion reveals that the ligands L1 and L2 possess five potential

coordination sites, whereas the ligand L4 possesses four donor sites.

An important feature of IR spectrum ligand L3 is the occurrence of two strong

bands centered at 1667 cm1 and 1606 cm1 which may be attributed to the

(C=O) and (C=N) stretching vibrations. The bonding of the ligand through

oxygen of carbonyl groups and nitrogen of azomethine groups is shown by the

appearance of these bands at lower wavenumbers in the IR spectra of

complexes. These spectral features suggest the tetradentate coordinating nature

of ligand with ONNO donor sites [62-67].

The key IR bands of ligand L5 are corresponding to the C=O (1647 cm1)

stretching, C=N (1621 cm1) stretching, δNH (in-plane-bending) (1532 cm1) and

CS (768 cm1) stretching vibrations. When the ligand is coordinated to the

metal ion, the most notable change in the ligand spectral features is the

hypsochromic shift (blue shift) of these key IR bands. These findings may be

taken as an evidence for the participation of the nitrogen atoms of azomethine

and NH-groups and sulphur atom of CS group in coordination to central

Ni(II) metal ion. However, the IR band due to C=O remains unaffected on

complexation indicating the noninvolvement of carbonyl oxygen in

coordination [68-74]. These spectral features indicate the pentadentate

coordinating nature of ligand with NNSNN donor atoms.

Further, the coordination of ligands through nitrogen or oxygen or sulphur

atoms is manifests by the appearance of extra bands in the IR spectra of


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complexes. The complexes show the new bands centered in the range 342-479

cm1, 502-573 cm1 and 326-359 cm1 which may be attributed (MN),

(MO) and (MS) stretching vibrations [75-78], respectively.

IR bands due to anions

The nitrato complexes with ligands L1, L2, L3 and L5 show the IR bands in the

range 1407-1458 cm1 (5), 1313-1336 cm1 (1) and 1023-1075 cm1 (2) due

to NO stretching vibrations of NO3 groups. The value of Δ(ν5-ν1) i.e. 94-130

cm-1 suggests the monodentate coordination of NO3- ion [79-82]. The nitrato

complex with ligand L4 shows these IR bands at 1494 cm1, 1320 cm1 and

1088 cm1 and value of Δ(ν5-ν1) i.e. 174 cm1 indicates the bidentate

coordination of NO3- ion [83-85]. The acetato complexes with ligands L1, L2,

L3 and L5 show the IR bands in the region 1406-1461 cm1 and 1207-1292

cm1 due to νas(OAc) and νs(OAc) stretching vibrations. The Δν i.e. 160-199

cm1 suggests the unidentate coordination of the OAc anion, whereas that of

the ligand L4 shows these bands at 1481 cm1 and 1381 cm1 and the Δν i.e. 100

cm1 suggests the bidentate coordination of the OAc anion [86]. The chloro

complexes show the IR bands in the range 318-333 cm1 due to ν(M-Cl) [87].

6.4.3: Electronic Spectra

The electronic spectra of complexes were recorded in DMF/DMSO solution and

are depicted in Figs. 6.5-6.8. The absorption bands displayed by the complexes are

listed in Table 6.4. The complexes exhibit the high energy bands in the range

34129-41322 cm1 which are attributed to the LM charge transfer bands.

The absorption spectra of complexes with ligands L1, L2, L3 and L5 display

three d-d transition bands in the range 1037311318 cm1, 1313218688 cm1

and 2141328248 cm1. These bands may be assigned to the 3A2g (F) 3T2g

(F) 1, 3A2g (F) 3T1g (F) 2 and 3A2g (F) 3T1g (P) 3 transitions, respectively


155

[88-93]. In addition, the complexes also display the absorption bands in the

range 13812-14102 cm1 which may be due to 3A2g 1Eg forbidden transition

and occur frequently in the electronic spectra of Ni(II) octahedral complexes

[94]. These transitions reveal that the nickel complexes with ligands L1, L2, L3

and L5 possess octahedral geometry.

The absorption spectra of complexes with ligand L4 display three d-d transition

bands in the range 1312315479 cm1, 1858718867 cm1 and 2415427247

cm1. The lowest energy transition corresponds to the transition 1A1g 1A2g

( 2 2xy x yd d

) 1 which suggest the energy levels order 2z

d < dxy [95]. The other

transitions correspond to 1A1g 1B1g ( 2 2 2z x yd d

) 2 and 1A1g 1Eg

( 2 2xy yz x yd ,d d

) 3, respectively. These transitions reveal that the nickel

complexes with ligand L4 possess square planar geometry and D4h symmetry [94].

The square complexes display 1 band at higher energy in comparison to other

octahedral complexes. This is due to the fact that the crystal field energy for

square planar complexes is large and the energy separation between 2 2x yd

orbital and the next lowest orbital is invariably greater as compared to the

octahedral complexes. In the case of square planar complexes, the first band

arises from the dxy orbital to the 2 2x yd

orbital, whose one-electron energy

separation is 10 Dq [94].

For Ni(II) in an octahedral field, the energies, E, of the states to the spherical

field are given by the following equations.

For 3T2g: E = –2Dq (6.1)

For 3A2g: E = –12Dq (6.2)

For 3T1g (F) and 3T1g (P):


156

[6Dqp – 16(Dq)2] + [–6Dq – p]E + E2 = 0 (6.3)

where p is the energy of the 3P state. There are two roots to the last equation

corresponding to the energies of the states 3T1g (F) and 3T1g (P).

From the equation it is observed that the energies of both 3T2g and 3A2g are

linear functions of Dq. For any ligand that produces a spin-free octahedral

nickel complex, the difference in the energy between the 3T2g state and 3A2g

state in the complex is 10Dq. Since the lowest transition 3A2g (F) 3T2g (F) is

a direct measure of the energy difference of these states, 10 Dq can be equated

to the transition energy i.e. the frequency of this band [96].

Equation (6.3) can be solved for the energies of the other states. However, the

above equations have been derived by assuming that the ligands are point

charges or point dipoles and that there is no covalence in the metal-ligand

bond. If this is true, the value for Dq just determined could be substituted in to

the equation (6.3), the energy of 3P obtained from the atomic spectrum of the

gaseous ion and the energy of the other two levels in the complex calculated

from the equation (6.3).The frequencies of the expected spectral transitions are

calculated for one band corresponding to the difference between the energies of

the levels 3A2g (F) 3T1g (F) and for the other band from the energy difference 3A2g (F) 3T1g (P). The experimental energies obtained from the spectra are

almost always lower than the values calculated in this way. The deviation is

attributed to covalency in the bonding.

The effect of covalency is to reduce the positive charge on the metal ion, as a

consequence of electron donation by the ligand. With reduced positive charge

the radial extension of the d orbital increases: this decreases the electron-

electro repulsions, lowering the energy of the 3P state. Covalency is foreign to

the crystal field approach and is incorporated into the ligand field approach.


157

The difference in energy between the 3P and 3F state in the complex relative to

that in the gaseous ion is decreased by covalency and as a result, the gas phase

value can not be used for p (6.3); rather, p must be experimentally evaluated for

each complex. Equation (6.3) can be employed for this calculation by using the

Dq value from the 3A2g (F) 3T2g (F) transition. The only unknown quantity in

the equation (6.3) is p. The lowering of 3P is measure of covalency, among

other effects. It is referred to as the nephelauxetic effect and sometimes

expressed by a parameter β0, a percentage lowering of the 3P state in the

complex compared to the energy of 3P state in the free gaseous ion. The

nephelauxetic parameter β has been calculated by using the expression:

β = Bcomplex/Bfree ion

where, the value of B i.e. Racah parameter, for Ni(II) free ion is 1041 cm1. It

should be noted that p is proportional to B. In the case of Ni(II) , the energy of

the 3P state in the complex can be substituted along with Dq into equation (6.3)

and the other root calculated. The difference in the energy between this root

and the energy of 3A2g gives the frequency of the middle band 3A2g (F) 3T1g (F). The agreement of the calculated and experimental values

for this band is a good evidence for octahedral symmetry.

The other ligand field parameters like Racah inter-electronic repulsion

parameter B, ligand field splitting stabilization energy 10 Dq and ligand field

stabilization energy (LFSE) have been calculated for the complexes. The

complexes in the present study show the lower value of B than that of the free

ion which indicates the orbital overlapping and delocalization of d-orbital. The

complexes show the value of β less than unity which suggests the covalent

nature of metal-ligand bond [97-103].


158

6.4.5: Thermal analyses

The thermal decomposition studies of the complexes with ligand L4 were

carried out in the temperature range 30-600°C with a sample heating rate

5°C/min. The complexes do not show any decomposition upto 300°C which

indicates the absence of lattice water and coordinated water molecules. The

complexes show the first step decomposition exp. 39.21-39.88% (ca. 39.28-

39.97%) between 330-370°C due to removal of coordinated anions and half

part of the ligand. Finally the complexes undergo second step decomposition

exp. 31.48-31.87% (ca. 31.59-31.96%) between 430-470°C due to the loss of

remaining part of ligand. The final residue was analyzed by IR spectra and

identified as NiO which corresponds to the calculated value. These features

support the proposed formulae of the complexes.


159

Table 6.1: Analytical data and physical properties of complexes

S.No. Complex Color m.p. (C)

Molar conductance (1cm2mol1)

Yield (%)

Elemental analyses data (%) calculated(found)

M C H N

1 [Ni(HL1)NO3] NiC11H16N10O5

Green >300 18 70 13.75 (13.72)

30.93 (30.89)

3.75 (3.71)

32.80 (32.83)

2 [Ni(HL1)Cl]

NiC11H16N9O2Cl Light Green

290 24 72 14.66 (14.62)

32.98 (32.92)

3.99 (3.92)

31.48 (31.41)

3 [Ni(HL1)OAc] NiC13H19N9O4

Green 292 10 68 13.85 (13.80)

36.82 (36.77)

4.48 (4.45)

29.73 (29.69)

4 [Ni(L2)NO3]NO3 NiC11H17N11S2O6

Brown >300 121 50 11.25 (11.19)

25.30 (25.22)

3.26 (3.21)

29.52 (29.46)

5 [Ni(L2)Cl]Cl NiC11H17N9S2Cl2

Brown 296 117 53 12.52 (12.43)

28.16 (28.10)

3.63 (3.56)

26.88 (26.83)

6 [Ni(L2)OAc]OAc NiC15H23N9S2O4

Brown >300 129 56 11.38 (11.31)

34.90 (34.83)

4.46 (4.39)

24.43 (24.37)

7 [Ni(L3)(NO3)2]

NiC28H32N10O8 Light Green

220 24 60 8.45 (8.37)

48.37 (48.30)

4.61 (4.55)

20.15 (20.10)


160

Table 6.1: Contd.....

S.No. Complex Color m.p. (C)

Molar conductance (1cm2mol1)

Yield (%)

Elemental analyses data (%) calculated(found)

M C H N

8 [Ni(L3)Cl2] NiC28H32N8O2Cl2

Light Green

215 16 53 9.15 (9.09)

52.36 (52.43)

4.99 (4.92)

17.45 (17.37)

9 [Ni(L3)(OAc)2] NiC32H38N8O6

Shiny Green

>300 14 56 8.52 (8.46)

55.76 (55.67)

5.52 (5.45)

16.26 (16.19)

10 [Ni2(H2L4)(NO3)2] Ni2C8H16N12O8

Greyish Blue

>300 27 63 22.35 (22.26)

18.27 (18.19)

3.05 (2.99)

31.98 (31.92)

11 [Ni2(H2L4)(OAc)2] Ni2C12H22N10O6

Greyish Blue

>300 29 65 22.60 (22.52)

27.72 (27.67)

4.24 (4.18)

26.95 (26.89)

12 [Ni(L5)NO3]NO3

NiC32H42N10SO8 Green 280 88 61 7.48

(7.43) 48.94

(48.84) 5.35

(5.30) 17.84

(17.78)

13 [Ni(L5)Cl]Cl

NiC32H42N8SO2Cl 2

Light Green

268 96 63 8.02 (7.96)

52.48 (52.41)

5.74 (5.67)

15.31 (15.26)

14 [Ni(L5)OAc]OAc

NiC36H48N8SO6 Light Green

276 106 60 7.54 (7.47)

55.48 (55.42)

6.16 (6.10)

14.38 (14.33)


161

Table 6.2: Important IR bands and assignments of Schiff’s base ligands and Ni(II) complexes

Compound Amide I/ Thioamide I

Amide II/ Thioamide II

Amide III/ Thioamide III

Pyridine ring

(MN) (MO)/ (MS)

Ligand (L1) 1685vs 1567ms 1444s 1508vs, 597mw, 478mw _ _

[Ni(HL1)NO3] 1630s 1581m 1430m 1541mw, 621mw, 517w 342sh 532w

[Ni (HL1)Cl] 1647m 1593m 1496mw 1528mw, 620sh, 502sh 343sh 560w

[Ni(HL1)OAc] 1632s 1571m 1462m 1542mw, 648w, 516w 380sh 562sh

Ligand (L2) 1695m 1357ms 1288m 1569ms, 593w, 430w _ _

[Ni(L2)NO3]NO3 1666m 1378m 1262m 1598mw, 599w, 521w 383sh 352sh

[Ni(L2)Cl]Cl 1656mw 1335m 1260m 1596mw, 589w, 513w 465w 341sh

[Ni(L2)OAc]OAc 1682m 1336m 1261m 1598mw, 617mw, 521w 433w 359sh

Ligand(L4) 1675vs 1624s 1551s – – –

[Ni2(H2L4)(NO3)2] 1658s 1598m 1573m – 477br 510br

[Ni2(H2L4)(OAc)2] 1636s 1570m 1530m – 404mw 502mw Abbreviations: vs=very strong, s=strong, ms=medium strong, m=medium, mw=medium weak, w=weak, br=broad, sh=sharp


162

Table 6.3: Important IR bands and assignments of Schiff’s base ligands and Ni(II) complexes

Compound (C=N) (C=O) (NH) (CS) (MN) (MO)/(MS)

Ligand(L3) 1606s 1667vs _ _ _ _

[Ni(L3)(NO3)2] 1601m 1616m 452mw 566mw

[Ni(L3)Cl2] 1601s 1630m _ _ 454m 506m

[Ni(L3)(OAc)2] 1630m 1685m _ _ 387mw 573m

Ligand (L5) 1621s 1647vs 1532s 768ms _ _

[Ni(L5)NO3]NO3 1571ms 1651s 1493m 652br 479m 326m

[Ni(L5)Cl]Cl 1570m 1640m 1405m 673br 384sh 343sh

[Ni(L5)OAc]OAc 1591m 1648ms 1406m 651br 475mw 329m

Abbreviations: vs=very strong, s=strong, ms=medium strong, m=medium, mw=medium weak, w=weak, br=broad, sh=sharp


163

Table 6.4: Magnetic moment values, electronic spectral data and ligand field parameters of complexes

Complex eff (B.M.) max (cm1) Dq (cm1) B(cm1) β LFSE (kJmol1)

[Ni(HL1)NO3] 2.83 11235, 15856, 26239, 34129 1123.5 559.33 0.54 161.08

[Ni(HL1)Cl] 2.86 11185, 14285, 28011, 34129 1118.5 582.73 0.56 160.36

[Ni(HL1)OAc] 2.91 10373, 13132, 28248, 37593 1037.3 684.06 0.66 148.72

[Ni(L2)NO3]NO3 2.88 11318, 14102, 17891, 21569, 36130 1131.8 367.07 0.35 162.27

[Ni(L2)Cl]Cl 2.94 10894, 13936, 18110, 21868, 34143 1089.4 486.40 0.47 156.87 [Ni(L2)OAc]OAc 2.81 11135, 18621, 25510, 36900 1113.5 715.07 0.69 159.64

[Ni(L3)(NO3)2] 2.86 10834, 18621, 22471, 41322 1083.4 372.66 0.36 155.33 [Ni(L3)Cl2] 2.97 10526, 13812, 18621, 25773, 36496 1052.6 854.40 0.82 150.91

[Ni(L3)(OAc)2] 2.99 10395, 18621, 22371, 36630 1039.5 653.81 0.63 149.03 [Ni 2(H2L4)(NO3)2] Dia. 13123, 18587, 27247, 38910 1312.3 – – –

[Ni2(H2L4)(OAc)2] Dia. 15479, 18867, 24154, 38759 1547.9 – – – [Ni(L5)NO3]NO3 2.84 11248, 18621, 21413, 36101 1124.8 419.33 0.40 161.26

[Ni(L5)Cl]Cl 2.87 11185, 18688, 27322, 38022 1118.5 830.33 0.79 160.36

[Ni(L5)OAc]OAc 2.93 11135, 18621, 25510, 36900 1113.5 715.07 0.69 159.64


164

6.5: STRUCTURE OF THE COMPLEXES

The above physiochemical, spectral and thermal studies reveal that in the

present study the Ni(II) complexes possess the hexacoordinated octahedral

geometry except the complexes with ligand L4 whose geometry is four

coordinated square planar. The suggested structures of complexes are given in

Figs. 6.9-6.14.

N

N

N

CH3CH3

HN

CO

NH2H

C

N

N

O

NH2

X

NHNi

Fig. 6.9: Structure of [Ni(HL1)X] complexes, where X= NO3, Cl and CH3COO

N CC

N

CH3CH3

N

NH

C

NH

CNHS

NH2

NH S

NH2

Ni

X

X

Fig. 6.10: Structure of [Ni(L2)X]X complexes, where X= NO3, Cl and CH3COO


165

NN

CH3

CH3ONN

CH3

CH3O

C

N

H N NC

N

H

Ni

X

X

Fig. 6.11: Structure of [Ni(L3)X2] complexes, where X= NO3, Cl and CH3COO

O

O

Ni NO

N NC

H

N

C

N

H

NN

CC

ONHNH2

H2NHNO O

ONiON

Fig. 6.12: Structure of [Ni2(H2L4)(NO3)2] complex


166

O

O

Ni CMe

N NC

H

N

C

N

H

NN

CC

ONHNH2

H2NHNO O

ONiMeC

Fig. 6.13: Structure of [Ni2(H2L4)(OAc)2] complex

NN

Me

MeN

Et

S CCO O

NHNH

NN

Me

MeN

EtX

Ni

X

Fig. 6.14: Structure of [Ni(L5)X]X complexes, where X= NO3, Cl and CH3COO


167

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