Accepted Manuscript

Synthesis, characterization, crystal structure determination, computational

study, and thermal decomposition into NiO nano-particles of a new NiIIL2 Schiff

base complex (L=2-{(E)-[2-chloroethyl)imino]methylphenolate)

Gholamhossein Grivani, Mohammad Vakili, Aliakbar Dehno Khalaji, Giuseppe

Bruno, Hadi Amiri Rudbari, Maedeh Taghavi, Vida Tahmasebi

PII: S0022-2860(14)00426-8

DOI: http://dx.doi.org/10.1016/j.molstruc.2014.04.059

Reference: MOLSTR 20566

To appear in: Journal of Molecular Structure

Received Date: 15 March 2014

Revised Date: 16 April 2014

Accepted Date: 17 April 2014

Please cite this article as: G. Grivani, M. Vakili, A.D. Khalaji, G. Bruno, H.A. Rudbari, M. Taghavi, V. Tahmasebi,

Synthesis, characterization, crystal structure determination, computational study, and thermal decomposition into

NiO nano-particles of a new NiIIL2 Schiff base complex (L=2-{(E)-[2-chloroethyl)imino]methylphenolate), Journal

of Molecular Structure (2014), doi: http://dx.doi.org/10.1016/j.molstruc.2014.04.059

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Synthesis, characterization, crystal structure determination, computational

study, and thermal decomposition into NiO nano-particles of a new NiIIL2

Schiff base complex (L=2-{(E)-[2-chloroethyl)imino]methylphenolate)

Gholamhossein Grivania,*, Mohammad Vakilib, Aliakbar Dehno Khalajic, Giuseppe

Brunod, Hadi Amiri Rudbarid, Maedeh Taghavia, Vida Tahmasebia

aSchool of Chemistry, Damghan University, Damghan, P. O. Box 36715-364, Iran

bDepartment of Chemistry,Ferdowsi University of Mashhad, Mashhad 91775-1436, Iran

cDepartment of Chemistry, Faculty of Science, Golestan University, Gorgan, Iran

dDipartimento di Chimica Inorganica, Vill. S. Agata, Salita Sperone 31, Università di

Messina, 98166 Messina, Italy

*Corresponding author. E-mail address:

Tel/fax.: +98 2325235431

E-mail address: grivani@du.ac.ir

Abstract

The Nickel (II) Schiff base complex of NiL2, (L= 2-{(E)-[2-chloroethyl)

imino]methylphenolate) have been synthesized and characterized by elemental (CHN)

analysis, UV-Vis and FT-IR spectroscopy. The molecular structure of [NiL2] was determined

by single crystal X-ray diffraction technique. The Schiff base ligand HL acts as a chelating

ligand and coordinates via one nitrogen atom and one oxygen atom to the metal center. The

nickel (II) center is coordinated by two nitrogen and two oxygen atoms from two Schiff base

ligands in an approximately square planar trans-[MN2O2] coordination geometry.

2

Thermogravimetric analysis of NiL2 showed that it decomposed in three stages. In addition,

complex NiL2 thermally decomposed in air at 660 ◦C and the XRD pattern of the obtained

solid showed the formation of NiO nanoparticles with an average size of 43 nm. In addition,

the conformational analysis and molecular structures of NiL2 were investigated by means of

density functional theory (DFT) calculations at B3LYP/6-311G* level and the calculated

geometrical parameters were compared with the experimental results.

Keywords: Nickel(II) · Complex · Single-crystal · Thermal decomposition . Nano particle.

DFT.

1. Introduction

Schiff base complexes derived from substituted salicylaldehydes and various amines

have been widely investigated because of versatility of their properties and functionalities, by

choosing the appropriate amine precursors and ring substituent and their wide applicability

[1-4]. Recently a large number of publications and studies, describing the synthesis of nickel

(II) complexes with salicylaldehyde substituted Schiff-base ligands, are reported for their

interesting structural applications and properties [5-10]. Schiff base ligands and complexes

derived from substituted salicylaldehydes and amines containing alkyl halide pendant group

are very rare. Recently we described the synthesis, characterization, crystal structure

determination of some of them and their metal complexes [11-15]. In those researches we

described the structural features of some of these ligands and their vanadyl complexes and the

catalytic activity of complexes. The other activities such as medicinal and biological

activities of the ligands and their complexes can be investigated. In this research we describe

the synthesis, characterization, crystal structure determination, thermal study of a new

nickel(II) Schiff base complex of [NiL2] (Scheme 1), and improve understanding of

conformational and structural information of the mentioned complex by means of density

3

functional theory (DFT) studies. The calculated conformational analysis and geometrical

parameters are compared with those observed experimentally.

<Scheme 1>

2. Experimental

2.1. Physical Techniques and Materials

All reagents and solvents for synthesis and analysis were commercially available and

used as received without further purifications. Elemental analyses were carried out using a

Heraeus CHN-O-Rapid analyzer, and the results agreed with calculated values. UV-vis

spectra were recorded by Perkin Elmer Spectrometer Lambda 25. The TG analysis was

performed on a Perkin Elmer TG/DTA lab system 1 (Technology by SII) in air atmosphere

with a heating rate of 20 ◦C/min in the temperature range 30–660 ◦C.

2.2. Method of analysis

In this study, the conformational analysis and molecular structure of NiL2 complex

were computed with the Gaussian 09W software system [16]. The stability of four possible

conformers, their relative stability, the optimized geometrical parameters of the stable

conformers (with Ci and C2 symmetry) were calculated at the B3LYP level [17, 18] of theory

with the 6-311G* basis set. The zero point vibrational energy, ZPE, corrections were

obtained at the B3LYP/6-311G* level, without applying any scaling.

Acetonitrile and carbon tetrachloride, as solvents with different polarities, were

selected for studying the relative energy studying of two Ci and C2 stable conformers of NiL2

complex in solution following the SCRF/PCM method [19]. In this method the solute is

embedded in the dielectric medium surrounded by a cavity shaped in the form of the solute

[20, 21]. The Van der Waals radii suggested by Bondi were adopted for atoms [22]. In this

4

level of calculation the specific solute–solvent effects are not taken into account and the

obtained solvation energies correspond to the electrostatic contributions, which, still, play a

dominant role in tautomerization reactions [23].

Since, very good results have been obtained for similar compounds by using the

B3LYP/6-311G* level [24, 25], and existence of a good agreement between calculated

geometrical optimization and X-ray data, it was used in this work.

2.3. Preparation of Schiff base ligand (HL)

The ligand 2-{(E)-[2-chloroethyl)imino]methyl}phenol (HL), was prepared and

characterized as described earlier [26]. Anal. Calc. for C9H10ClNO: C, 58.85; H, 5.45; N,

7.63%. Found: C, 58.51; H, 5.37; N, 7.48%. FT- IR (KBr pellet, cm-1): v (C=N) 1641 cm-1.

2.4. Preparation of NiL2

To a stirred solution of HL (2 mmol) in 60 ml of methanol was added 1 mmol of

Ni(NO3)·6H2O and the mixture was refluxed for 4 h. After cooling to room temperature, the

content filtered off and washed with 50 ml of methanol about five times and the obtained

green precipitate was dried in air. The green crystals were obtained in 1:1 mixture of

methanol and chloroform by recrystallization. Anal. Calc. for C18H18Cl2N2NiO2: C, 50.97; H,

4.24; N, 6.60. Found, %: C, 50.28; H, 4.15; N, 7.23. FT-IR (KBr pellet, cm-1): v (C=N) 1615

cm-1.

2.5. X-ray crystallography

A Single crystal of the dimension 0.24 mm × 0.22 mm × 0.08 mm of NiL2 was chosen

for X-ray diffraction study. Diffraction data were measured on a Bruker–Nonius X8 ApexII

diffractometer equipped with a CCD area detector by using graphite-monochromated Mo Ka

5

radiation (k = 0.71073 Å) generated from a sealed tube source. Data were collected and

reduced by smart and saint software [27] in the Bruker package. The structure was solved by

direct methods [28] and then developed by least squares refinement on F2 [29, 30]. All non-H

atoms were placed in calculated positions and refined as isotropic with the ‘‘riding-model

technique’’. Crystallographic data and details of the data collection, structure solution and

refinements are listed in Table 1.

<Table 1>

3. Results and Discussion

3.1. Synthesis and characterization

By the reaction of equimolar of 2-chloroethyl ammonium hydrochloride with

salicylaldehyde in the presence of the NaOH in methanol, the Schiff base ligand 2-{(E)-[2-

chloroethyl)imino]methyl}phenol was prepared. The NiL2 Schiff base complex was also

synthesized in simple preparation procedure by the reaction of HL with its corresponding

nitrate salt (in 2:1 molar ratio, respectively), in methanol as a solvent in the reflux conditions

for 4 h. The Ni(II) complex was characterized by elemental (CHN) and single crystal X-ray

analysis, UV-Vis and FT-IR spectroscopy.

3.2. FT-IR spectra

The sharp band appearing at 1641 cm−1 in the FT-IR spectrum of the Schiff base

ligand of HL is attributed to ν(C=N) (azomethine). This band is shifted to the lower wave

numbers and appeared at 1615 cm−1 in the FT-IR spectrum of NiL2, indicating the binding of

the azomethine nitrogen to the metal center (Fig. 1).

<Fig. 1>

3.3. UV-vis spectra

6

The UV-vis spectra of HL, and NiL2 are given in the Fig. 2. The ligand HL shows two

bands in 253 and 312 nm due to the π→π* and n→π* transitions, respectively. For the Schiff

base complex of NiL2 each of these bands splits into two and three bands, respectively. The

bands in the higher energy region for the Schiff base complex attributed to the π→π*

transitions split into two bands and appear at 250 and 266 nm for the Schiff base complex.

The n→π* transition for the Schiff base complex appears approximately in the same region

for the Schiff base ligand of HL. In addition, some new bands in the low energies are

appeared in the UV-vis spectra of the Schiff base complex. These bands attributed to the

LMCT transitions (O→M2+ and N→M2+) and placed at the 382 and 411 nm for the Schiff

base complex [31].

<Fig. 2>

3.4. Crystal structures of NiL2

An ORTEP view of NiL2 with the atom-numbering scheme is presented in Fig. 3. The

crystallographic data reveal that the metal center are four-coordinated by two phenolate

oxygen and two imine nitrogen atoms of two Schiff base ligands. The ligands coordinate to

the Ni(II) center in trans geometry with respect to each other. The geometry around the metal

center is a distorted square-planar, with P21/c space group, as indicated by the unequal metal-

ligand bond distances and angles (see Table 2).

<Fig. 3 >

<Table 2>

Two non-classical intremolecular hydrogen bonds of the type C-H···O and C-H···Cl

are formed between the phenyl H-atoms of the bidentate Schiff base ligand and the un-

7

coordinated Cl and coordinated O atoms (Fig. 4, Table 3), that the NiL2 molecules are

eventually linked together via these hydrogen bonds.

<Fig. 4>

<Table 3>

3.5. Thermal analysis

Thermogravimetric analysis of the NiL2 under air atmosphere was examined and its

TG profile is given in the Fig .5. The NiL2 complex is stable up to 120 ˚C. During furthers

heating, the NiL2 undergoes decomposition in three stages and shows mass losses of about

12.44 %, 13.31% and 53.88% in the temperature range of 120–170 ˚C, 350–400 ˚C and 450–

520 ˚C, respectively. The 12.44% mass loss in the temperature range of 120–170 ˚C ˚C, can

be related to the elimination of one CH3Cl unit from the Schiff base ligand (calcd. 11.91 %).

In the second stageunit of the C2H4Cl loss with 13.31 % mass loss ( calcd. 14.85%). The

terminal stage, in the temperature range of 450–520 ˚C, is observed with a mass loss of

53.88% corresponding to the elimination of organic residual ( calcd. 52.12%), resulting in the

formation of NiO.

<Fig. 5>

We also examined the preparation of NiO nano-particle via simple thermal

decomposition. The NiL2 Schiff base complex was thermally decomposed in air in an oven at

660 ˚C. After cooling to the room temperature, the residual powder was analyzed by X-ray

powder diffraction. Based on the resulting XRD pattern (Fig .6) the average crystallite size

calculated by using Scherrer’s formula was found to be around 43 nm.

<Fig. 6>

3.6. Conformational analysis and Molecular geometry

8

From the theoretical point of view, by considering the conformations of ethyl chloride

groups, as pendant groups, in NiL2, with respect to the plane of the complex, four conformers

can be drawn for it (Fig. 7). According to the theoretical calculations, only two a and b

conformers with Ci and C2 symmetries, respectively, are stable for NiL2 complex (Fig. 7)

whereas the c and d structures are not stable and under full optimization turn to the C2 form.

The Fig.7 shows the optimized conformers of the NiL2 complex. The calculated total

electronic energies (Hartree), the relative stabilities of Ci respect to C2 conformer (kcal/mol)

in gas phase and solution with different polarities (CH3CN and CCl4), along with their

calculated dipole moments in the gas and solution phases calculated at the B3LYP level with

6-311G* basis set are summarized in Table 4. Predictably, a conformer with a greater dipole

moment has a greater stability in more polar solvent. Theoretical calculations in the gas and

solution phase indicate that the energy difference between Ci and C2 conformers is negligible

(0.10 kcal/mol in the gas phase and 0.2 to 0.4 kcal/mol in CCl4, and CH3CN solvents,

respectively). Zero point energy (ZPE) corrections in the gas phase small changes this energy

difference for example 0.14 kcal mol-1. Therefore, from the theoretical point of view, the

coexistence of both C2 and Ci conformers of NiL2 complex in the sample is suggested.

However, the X-ray results suggest that the Ci form in the sample is predominant and rule out

the presence of the C2 conformer, which is given in the section 3.4. This is most likely due to

our theoretical calculations which DFT calculations have been done at the gas phase whereas

the experimental data are obtained from the solid complex.

<Fig. 7>

<Table 4>

The optimized geometrical parameters of the C2 and Ci conformers with the selected

X-ray diffraction results are summarized in Table 2. As it is shown in this Table, the

9

geometrical parameters of C2 and Ci forms are in excellent agreement with the results of the

experimental structures nearly in the experimental error range. Table 2 shows that the

calculated bond lengths of C4-C9/C18-C13 are longer than that in the residual C-C bond

lengths in phenyl ring. This difference confirms existing of significant resonance between

phenyl group with the N(1,2)-C(3,12)-O(1,2) atoms in NiL2 complex.

Conclusion

A new nickel (II) Schiff base complex of NiL2 was synthesized and characterized by

the elemental analyses, UV-Vis and FT-IR spectroscopy. Single crystals of NiL2 were

successfully grown from solution by slow evaporation technique. The single crystal X-ray

diffraction revealed monoclinic structures with space group P21/c and one symmetry

independent molecule C18H18Cl2N2MO2 in NiL2. The geometries of C2 and Ci stable

conformers of NiL2 complex are fully optimized at B3LYP (DFT) level of theory using 6-

311G* basis set. The resulted ground state energies suggest a small difference between the

stability of these two forms in gas phase and solution.

Acknowledgements

We acknowledge the Damghan University (DU) for partial support of this work.

Appendix A. Supplementary data

Crystallographic data (excluding structure factors) for the structure reported in this

paper has been deposited with the Cambridge Crystallographic Center, CCDC Nos. 839121

NiL2. Copies of the data can be obtained free of charge on application to The Director,

CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, fax: +44 1223 336 033, e-mail:

deposit@ccdc.cam.ac.uk or http:www.ccdc.cam.ac.uk.

10

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12

Ni

O

OH

Cl

HCl.H2N

CH3OH/ NaOH

Reflux

N

OH

Cl

Ni(NO3)2 .6H2O CH3OH/Reflux

N

Cl

ON

O

Cl

HL

NiL2

Scheme 1. Synthesis of NiL2.

13

Fig. 1. The FT-IR spectra of the HL and NiL2 (10-4 M in CHCl3).

14

Fig. 2. The UV-vis spectra of HL and NiL2 (10-4 M in CHCl3).

Fig. 3. An ORTEP view of NiL2, with the atom numbering scheme.

15

Fig. 4. Crystal packing of NiL2. Dashed lines represent the hydrogen bonds

16

Fig. 5. The TG profiles of NiL2

17

Fig. 6. The XRD patterns of the residual powder obtained from thermal decompositions of

the NiL2

18

(a)

(b)

(c) (d)

Fig.7. The possible conformers of NiL2 complex: the stable conformers of a with Ci and b with

C2 point group and unstable conformers of c and d.

19

Table 1

Crystallographic data and experimental details for NiL2

1

Chemical formula

Formula weight

C18H18Cl2NiN2O2

423.95

Crystal system

Space group

T, K

Monoclinic

P21/c

296

a, Å 9.2030(12)

b, Å 10.6192(13)

c, Å 18.680(2)

β, deg 96.017(7)

V, Å3 1815.5(4)

Z 4

μ, mm–1

1.377

Measured reflections

Independent reflections

64359

3966

Rint 0.010

GOF on F2 1.048

Number of parameters 226

F(000) 872

Theta range for data collection 2.19 to 27.00 deg

Limiting indices -11 ≤ h ≤ 11

-13 ≤ k ≤ 13

-23 ≤ l ≤ 23

Goodness-of-fit on F2 1.048

R[F2 > 2σ(F2)]

wR(F2)

0.0301

0.0846

Largest diff. peak and hole 0.298 and -0.383 e.A-3

20

Table 2

Selected experimental (X-ray) and calculated geometrical parameters of NiL2

complex (bond lengths in Å, bond angles in º)a

Theoretical X-ray(experimental)

C2 Ci

Bond lengths

Ni1-O1/ Ni1-O2 1.8443 1.8432 1.8308(13), 1.8324(13) Ni1-N1/ Ni1-N2 1.9325 1.9337 1.9233(13), 1.9209(13), N2-C12/N1-C3 1.2999 1.3005 1.289(2)/1.294(2) N2-C11/N1-C2 1.4740 1.4764 1.479(2)/1.474(2) O1-C9/O2-C18 1.3004 1.3000 1.303(2)/1.310(2)

C18-C13/ C4-C9 1.4234 1.4231 1.400(2)/1.399(2) C13-C14/C4-C5 1.4152 1.4156 1.407(2)/1.415(2) C14-C15/C5-C6 1.3762 1.3759 1.372(3)/1.362(3) C15-C16/C6-C7 1.4093 1.4096 1.381(3)/1.386(3) C16-C17/C7-C8 1.3789 1.3788 1.388(3)/1.372(3) C17-C18/C8-C9 1.4187 1.4192 1.406(2)/1.414(2)

Bond angles

O1-Ni1-O2 179.3 180.0 175.41(6) O1-Ni1-N2/O2-Ni1-N1 87.8 87.6 86.98(6)/92.97(5) O2-Ni1-N2/O1-Ni1-N1 92.2 92.4 93.06(5)/92.97(5)

N2-Ni1-N1 178.8 180.0 177.98(5) C12-N2-C11/C3-N1-C2 115.3 114.7 114.53(14)/114.58(14) C12-N2-Ni1/C3-N1-Ni1 124.3 124.6 124.94(11)/124.75(11) C11-N2-Ni1/C2-N1-Ni1 120.5 120.7 120.50(11)/120.67(10) C18-O2-Ni1/C9-O1-Ni1 130.7 131.6 130.19(11)/130.92(11)

a Calculated at B3LYP/6-311G

Table 3

Intermolecular hydrogen bonds geometries (Å, °) in the crystal packing of NiL2

D-H···A D-H H···A D···A D-H···A

C14-H14···O2 0.930 2.841 3.710(2) 155.99

21

C6-H6···Cl2 0.930 2.929 3.769(2) 151.01

Table 4

The calculated total electronic energies (Hartree), the relative stability energies for Ci and C2

conformers of NiL2 complex (in kcal/mol), and their dipole moment (Debye) in gas phase

and two solvent at B3LYP/6-311G* level of theory

Total electronic energy ( E) Dipole moment Gas CCl4 CH3CN Gas CCl4 CH3CN Ci

-3385.759

868

-3385.765

368

-3385.773

674 0.0000 0.0000 0.0000 C2

-3385.760

005

-3385.765

764

-3385.774

299 1.6124 2.1757 2.3655 ∆Ea 0.1(0.14) 0.2 0.4 a ΔE = E(C2) – E(Ci) (the corrected values for ZPE are given in parenthesis)

22

Graphical abstract

Synthesis, characterization, crystal structure determination, computational study, and thermal decomposition into NiO nano-particles of a new NiIIL2 Schiff base complex (L=2-{(E)-[2-chloroethyl)imino]methylphenolate)

O

OH

CiC2

Stable conformers

NiL2

NiL2 NiO

Thermal decomposition

23

Highlights • A new NiL2 Schiff base complex was prepared • Nickel (II) was centered in an approximately square planar coordination geometry • It was characterized by X-ray crystallography, CHN analysis and FT-IR spectra • The conformational analysis and molecular structures of NiL2 were investigated • The results showed a small difference between the stability of them • Thermal decomposition of NiL2 resulted in formation of the NiO nano-particles