ORIGINAL PAPER

Synthesis, spectroscopy, electrochemistry and thermal studyof uranyl N3O2 Schiff base complexes

Ali Hossein Kianfar • Mostafa Kazemi Boudani •

Mahmoud Roushani • Mojtaba Shamsipur

Received: 19 August 2011 / Accepted: 11 October 2011 / Published online: 3 January 2012

� Iranian Chemical Society 2011

Abstract The new [UO2L(CH3OH)] [where L = bis

(salicylaldehyde)2,6-diiminopyridine (L1), bis(5-methoxy-

salicylaldehyde)2,6-diiminopyridine (L2), bis(5-bromosalicyl-

aldehyde)2,6-diiminopyridine (L3), bis(5-nitrosalicylaldehyde)

2,6-diiminopyridine (L4)] complexes were synthesized and

characterized by IR, UV–vis and elemental analysis.

Methanol solvent is coordinated to uranyl complexes. The

electrochemical properties of the uranyl complexes were

investigated by cyclic voltammetry in DMF solvent.

Thermogravimetry and differential thermoanalysis of the

uranyl complexes were carried out in the range of

20–700 �C. The UO2L4 complex was decomposed in two

and the others were decomposed in three stages. Up to

85 �C, the coordinated solvent was released then the Schiff

base ligands were decomposed in one or two steps.

Decomposition of synthesized complexes is related to the

Schiff base characteristics. The thermal decomposition

reaction is first order for the studied complexes.

Keywords Schiff base complexes � Uranyl complexes �Electrochemistry � Thermogravimetry

Introduction

The uranyl complexes are interesting because of their

reactivity, coordination property and their applications in

recent years. Salen and salophen Schiff base ligands were

applied to separate of actinides from lanthanides in nuclear

waste [1–6]. Presence of synthesized solvent on the solid

structure of salen and the other tetradentate Schiff base

uranyl compounds was studied well in these compounds

[7, 8]. The solvents containing oxygen can bind to uranyl

as seventh ligands in these complexes. Thermogravimetry

(TG) and differential thermoanalysis (DTA) are valuable

techniques for studying the thermal behavior of these

compounds [9–13]. The present study is an extension of

our work [14, 15], in which the Schiff bases were prepared

by the condensation of salicylaldehyde derivatives as

starting materials within 1,6-diaminopyridine. The uranyl

complexes of synthesized ligands were prepared in meth-

anol solvent. The newly prepared complexes were identi-

fied by IR, UV–vis spectroscopy and elemental analysis.

Thermal and electrochemical properties of the synthesized

complexes were reported. From thermal decomposition

data kinetics parameters were calculated using Coats and

Redfern [16] method.

Experimental

Chemicals and apparatus

All of the chemicals and solvents used for synthesis were of

commercially available reagent grade and they were used

without purification. Infrared spectra were recorded as KBr

discs on a FT-IR JASCO-680 spectrophotometer in the

4,000–400 cm-1. The elemental analysis was determined

A. H. Kianfar (&) � M. K. Boudani

Department of Chemistry, Yasouj University, Yasouj, Iran

e-mail: akianfar@mail.yu.ac.ir; asarvestani@yahoo.com

M. Roushani

Department of Chemistry, Ilam University, Ilam, Iran

M. Shamsipur

Department of Chemistry, Razi University, Kermanshah, Iran

123

J IRAN CHEM SOC (2012) 9:449–453

DOI 10.1007/s13738-011-0055-2

on a CHN-O-Heraeus elemental analyzer. UV–vis spectra

were recorded on a JASCO V-570 spectrophotometer in

the 190–900 nm. The 1HNMR spectra were recorded in

DMSO-d6 on DPX-400 MHz FT-NMR. Thermogravimetry

(TG) and differential thermoanalysis (DTA) were carried

out on a PL-1500. The measurements were performed in air

atmosphere. The heating rate (/) was kept at 10 �C min-1.

Cyclic voltammograms were performed using an autolab

modelar electrochemical system (ECO Chemie, Utrecht,

The Netherlands) equipped with a PSTA 20 module and

driven by GPES (ECO Chemie) in conjunction with a three-

electrode system and a personal computer for data storage

and processing. An Ag/AgCl (saturated KCl)/3 M KCl ref-

erence electrode, a Pt wire as counter electrode and a glassy

carbon electrode as working electrode (Metrohm glassy

carbon, 0.0314 cm2) were employed for the electrochemical

studies. Voltammetric measurements were performed at

room temperature in acetonitrile solution with 0.1 M tetra-

butylammonium perchlorate as the supporting electrolyte.

Synthesis of ligands and complexes

The Schiff base ligands used in this study were prepared

according to the previously published methods [17], by

condensation of 1,6-diaminopyridine with salicylaldehyde

derivatives (1:2 mol ratio) in methanol and recrystallized

by dichloromethane/methanol mixed solvent through par-

tial evaporation of dichloromethane.

The uranyl complexes were synthesized by refluxing a

methanolic solution of the pentadentate Schiff base ligands

and uranylacetate monohydrate. The reaction was contin-

ued for 2 h. During the reaction red precipitate appeared.

The precipitate was filtered, washed with methanol and

dried in vacuum.

Results and discussion

Elemental analysis

The elemental analysis (Table 1) is in good agreement with

those calculated for the proposed formula.

IR characteristics

The IR spectra of Schiff base ligands and uranyl Schiff

base complexes synthesized are listed in Table 2. The IR

spectra of Schiff bases and complexes exhibit several

bands in the 400–4,000 cm-1 region.

The characteristic C=N bond appears in 1,607–1,608 cm-1

region for the synthesized complexes, respectively. The C=N

stretching in the complexes is generally shifted to a lower

frequency relative to the free Schiff base ligands, indicating

a decrease in the C=N bond order due to the coordinate

bond formation between the metal and the imine nitrogen

lone pair [14, 15, 18].

The band at 903–912 cm-1 is assigned to m (O=U=O).

This band is observed as a new peak for the complexes and

is not observed in the spectrum of free ligands [14, 15, 19].

Electronic spectra

The spectral data of the synthesized ligands and complexes

are listed in Table 3. In all ligands and complexes, the band

in the 271–278 nm region, involves p ? p* transition

related to aromatic ring. The band in the 377–396 nm

region, involves p ? p* transition related to azomethine

group.

Table 1 Elemental analysis data for the uranyl complexes

Compounds Found (calc) (%)

C H N

UO2L1[CH3OH] 40.1 (38.8) 3.2 (2.6) 6.5 (6.8)

UO2L2[CH3OH] 39.2 (38.9) 3.5 (3.1) 6.7 (6.2)

UO2L3[CH3OH] 31.2 (30.9) 2.1 (1.9) 6.2 (5.4)

UO2L4[CH3OH] 34.7 (33.9) 2.4 (2.1) 10.1 (9.9)

Table 2 Characteristic IR bands (cm-1) of the uranyl complexes

Compound m O–H

(cm-1)

m C=N

(cm-1)

m C=C

(cm-1)

m C–O

(cm-1)

m O=U=O

(cm-1)

HL1 2,700–3,600 1,609 1,484 1,192 –

HL2 2,800–3,600 1,613 1,490 1,156 –

HL3 2,900–3,600 1,612 1,474 1,174 –

HL4 2,500–3,500 1,615 1,481 1,229 –

UO2L1 3,350 1,607 1,487 1,198 903

UO2L2 3,400 1,607 1,489 1,154 906

UO2L3 3,350 1,607 1,466 1,167 906

UO2L4 3,400 1,608 1,476 1,130 912

Table 3 UV–vis spectral data (nm) for the uranyl complexes in DMF

Compound (e, L mol-1 cm-1) 1k (e, L mol-1 cm-1) 3k

HL1 298 (8,000) 398 (7,200)

HL2 285 (5,500) 396 (3,800)

HL3 278 (11,000) 382 (7,500)

HL4 271 (7,600) 408 (4,500)

UO2L1 272 (5,500) 385 (3,000)

UO2L2 278 (26,000) 396 (16,300)

UO2L3 271 (18,700) 380 (1,800)

UO2L4 271 (12,800) 377 (12,400)

450 J IRAN CHEM SOC (2012) 9:449–453

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Electrochemical study of uranyl complexes

The cyclic voltammetry of uranyl complexes were carried out

in DMF solution at room temperature under nitrogen atmo-

sphere. A typical cyclic voltammogram of UO2L1 complex

in the potential range from 0.0 to -1.7 V (vs. Ag/AgCl)

is shown in Fig. 1. An oxidation peak is observed at about

Ca. -0.827 V. UO2L1 is reduced to the mono anion

[UO2L1]- in a reversible one-electron step [20]. Upon

reversal of the scan direction, the U(V) complex is oxidized to

U(VI) at higher potentials. Multiple scans resulted in nearly

superposable cyclic voltammograms, thereby showing that

the complex is stable in both oxidation states, at least on the

cyclic voltammetry time scale. These results revealed that the

redox process of all uranyl Schiff base complexes under

study is the one-electron transfer reaction. The reduction and

oxidation potentials for the different complexes are set out in

Table 4. The formal potentials (E1/2 (VI $ V)) for the

U(VI/V) redox couple were calculated as the average of the

cathodic (Epc) and anodic (Epa) peak potentials of this process.

Thermal analysis

As can be seen from the TG/DTA curves in Fig. 2, the

thermal decomposition of the uranyl complexes presented

characteristic pathways, depending on the nature of the

ligands. The absence of weight loss up to 80 �C indicates that

there is no water molecule in the crystalline solid. In these

complexes, the TG showed wight loss up to 85 �C indicating

the presence of solvent molecule coordinated with complexes

[11, 12]. All the complexes, except [UO2L4(CH3OH)], were

decomposed in three steps. In all complexes the first step of

decomposition is related to release of methanol solvent. The

Schiff base ligand is decomposed in one step for L4 and two

steps for other ligands [18].

Kinetics aspects

All the well-defined stages were selected to study the kinetics

of decomposition of the complexes. The kinetics parameters

(the activation energy E and the pre-exponential factor A)

were calculated using the Coats–Redfern equation [16],

loggðaÞT2

� �¼ log

AR

/E1� 2RT

Ea

� �� Ea

2:303RTð1Þ

where g(a) = [Wf)/(Wf - W)]. In the present case, a plot of

LHS (left hand side) of this equation against 1/T gives

Fig. 1 Cyclic voltammogram of UO2L1, in DMF at room tempera-

ture. Scan rate: 100 mV s-1

Table 4 Electrochemical data for the uranyl complexes in DMF

Compound Ea Ec E1/2

UO2L1 -0.808 -0.827 -0.817

UO2L2 -0.632 -0.913 -0.772

UO2L3 -0.783 -1.035 -0.90

UO2L4 -0.549 -1.237 -0.893

Fig. 2 The TG and DTA of UO2L1 complex

Fig. 3 Coats–Redfern plots of UO2L1 complex, step 2, A =log

WfWf �W

T2 ,

r2 = 0.991

J IRAN CHEM SOC (2012) 9:449–453 451

123

straight line (Fig. 3) whose slope and intercept are used to

calculate the kinetic parameters by the least square method.

The goodness of fit was checked by calculating the corre-

lation coefficient. The other systems and their steps show

the same trend.

The entropy of activation S= was calculated using the

equation

A ¼ kTs

he

S 6¼R ð2Þ

where k, h and Ts are the Boltzmann constant, the Planck’s

constant and the peak temperature, respectively. The

enthalpy and free energy of activation were calculated

using equations:

Ea ¼ H 6¼ þ RT ð3Þ

G 6¼ ¼ H 6¼ � TS 6¼ ð4Þ

The various kinetics parameters calculated are given in

Table 5. The activation energy (Ea) in the different stages are

in the range of 14.59–162.45 kJ mol-1. The respective

values of the pre-exponential factor (A) vary from 2.75 9 10

to 3.34 9 1013 s-1. The corresponding values of the entropy

of activation (S=) are in the range of -224 to -33 J mol-1.

The corresponding values of the enthalpy of activation (H=)

are in the range of 9.21–157.18 kJ mol-1. The corresponding

values of the free energy of activation (G=) are in the range

of 130–208 kJ mol-1. There is no definite trend in the values

of activation parameters among the different stages in the

present series. The negative values of entropy of activation

indicate that the activated complex has a more ordered

structure than the reactants [21, 22].

Conclusions

Considering the spectroscopy and thermogravimetrical

properties of uranyl complexes the following conclusions

have been made.

1. Coordinated methanol was confirmed by elemental

analysis and thermogravimetry of complexes.

2. The complex containing simple ligand (L4) is decom-

posed in two steps while the other complexes are

decomposed in three steps.

3. The thermal decomposition pathways of the complexes

are related to the Schiff base characteristics.

4. The thermal decomposition reactions are first order in

all steps for the studied complexes.

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Table 5 Thermal and kinetics parameters for uranyl complexes

Compounds DT (�C)a E* (kJ mol-1) A* (s-1) S* (kJ mol-1 K-1) H* (kJ mol-1) G* (kJ mol-1)

UO2L1 70–280 8.59 1.44 9 10-3 -3.02 9 102 5.91 1.27 9 102

290–450 54.91 2.91 9 10 -2.24 9 102 49.27 2.02 9 102

450–590 64.56 2.72 9 10 -2.26 9 102 57.86 2.41 9 102

UO2L2 200–250 58.19 1.78 9 103 -1.78 9 102 53.86 1.51 9 102

300–490 52.59 3.77 9 100 -2.40 9 102 47.25 2.02 9 102

520–630 161.88 4.15 9 107 -1.08 9 102 154.93 2.45 9 102

UO2L3 80–220 6.32 1.32 9 105 -1.49 9 102 3.18 5.91 9 10

360–480 91.42 1.78 9 104 -1.68 9 102 87.09 1.47 9 102

515–585 69.31 1.52 9 104 -1.71 9 102 63.97 1.74 9 102

UO2L4 50–140 26.99 0.441 -2.53 9 102 44.01 1.53 9 102

290–450 162.45 3.34 9 1011 -3.35 9 10 157.18 1.78 9 102

a The temperature range of decomposition pathways

452 J IRAN CHEM SOC (2012) 9:449–453

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