X-ray diffraction studies, thermal, electrical and optical properties of oxovanadium(IV) complexes with quadridentate schiff bases

$Page 1: X-ray diffraction studies, thermal, electrical and optical properties of oxovanadium(IV) complexes with quadridentate schiff bases$
Materials Chemistry and Physics 88 (2004) 357–363

X-ray diffraction studies, thermal, electrical and optical properties ofoxovanadium(IV) complexes with quadridentate schiff bases

Saikat Sarkara, Yildirim Aydogdub,∗, Fethi Dagdelenb, Bijali Bikash Bhaumikc,Kamalendu Deyc

a Department of Chemistry, Santipur College, Santipur, Nadia, Indiab Department of Physics, Faculty of Arts and Sciences, Firat University, 23169 Elazig, Turkey

c Department of Chemistry, University of Kalyani, Kalyani 741235, West Bengal, India

Received 5 May 2004; accepted 3 August 2004

Abstract

The oxovanadium(IV) complexes with quadridentate schiff bases have been prepared and were characterized by means of X-ray diffraction( 1 2 3

h he thermo-g rements.T ts. Opticalaa , however,t ducting be-h affords ane©

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fceopamscc

loen-ningality

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XRD), electrical conductivity and optical absorption techniques. X-ray analysis shows that the [VO(L)], [VO(L )] and [VO(L )] complexesave monoclinic structure. The thermal properties of complexes were examined by the method of thermogravimetric analysis. In travimetric analysis studies 20◦C min−1 heating rate was used. Electrical transport properties were studied by dc conductivity measuhe electrical activation energies of the complexes which were in the range of 0.48–1.18 eV were calculated from Arrhenius plobsorption studies in the wavelength range of 190–1100 nm at room temperature showed that the optical band gapEg of [VO(L1)], [VO(L 2)]nd [VO(L3)] complexes were 3.45, 2.65 and 2.80 eV, respectively. The complexes were electrically insulator at room temperature

heir conductivities increased as the temperature increases from 330 K, indicating their semiconducting behaviour. The semiconaviour of the oxovanadium(IV) complexes with quadridentate schiff bases was determined by their chemical structure, whichxtended conjugation.2004 Elsevier B.V. All rights reserved.

eywords:Oxovanadium(IV); Quadridentate schiff base; Semiconducting behaviour

. Introduction

The biological importance of vanadium compounds in dif-erent oxidation states are many faceted[1–8]. Vanadiumompounds are also of major concern because of their adverseffect on the hydroprocessing catalysts used in the refiningf crude oil [9,10]. The revelation that coordination com-ounds of vanadium in different oxidation states can playvital role in nitrogen activation and nitrogen fixation andany other biologically important reactions[1–8,11–14], has

timulated interest in the synthesis, characterization, stereo-hemistry and reactivity of its coordination compounds thatontain diazo-, hydrazido- and imine-group because they can

∗ Corresponding author. Tel.: +90 424 2370000; fax: +90 424 2330062.E-mail address:[email protected] (Y. Aydogdu).

provide some understanding of the mechanism of metalzymatic reduction of dinitrogen and are capable of confimetal atoms and controlling their properties and function[14].

Despite extensive work done on the preparation, proties, magnetic properties and structural aspects of oxodium(IV) complexes with quadridentate schiff bases, cparatively little work is available on the studies of XRelectrical and optical properties of oxovanadium(IV) copounds with the said types of ligands. The interest instudies is, the increasing being shown by different Scof researchers as an interdisciplinary field of study[15].The present paper records the results of such investigawith oxovanadium(IV) complexes with schiff basesN,N′-(2-hydroxy)propylenebis{(2-imino-3-oximino)butane}, N,N′-(2-hydroxy)propylenebis(salicylidimine) andN,N′-(2-hyd-

254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.matchemphys.2004.08.001

358 S. Sarkar et al. / Materials Chemistry and Physics 88 (2004) 357–363

roxy)propylenebis(7-methylsalicylaldimine) synthesized bythe condensation of 1,3-diaminopropane-2-ol with diacetyl-monoxime (H2L1), salicylaldehyde (H2L2) and orthohydrox-yacetophenone (H2L3), the structural formulations of whichare shown below. The ligands were synthesized by our pre-viously published methods[16–19].

2. Experimental

2.1. Synthesis of the oxovanadium(IV) complexes withquadridentate schiff bases[17]

The greenish–grey complexes [VO(L1)], [VO(L 2)] and[VO(L3)] were synthesized by the reactions ofH2L1, H2L2

andH2L3 with equimolar quantities of vanadyl acetate inm rizedb mag-n opicd iveni idalglf dylo

2

om-p tion( me-t .

PyrisD ra-t anget urvesw2

lexesi -s edb con-t es oft 6514s hely2 werec

Fig. 1. The chemical structure of complexes: (a) VO(L1); (b) VO(L2); and(c) VO(L3).

where (I) is the current in ampere,Vc the potential drop acrossthe sample of cross-sectional area (a) and thickness (d).

Optical absorption spectra was taken by using aUV–vis spectrophotometer (Perkin-Elmer Lambda 2S Dou-ble Beam), in the wavelength range 190–1100 nm.

3. Results and discussion

3.1. X-ray diffraction studies

X-ray powder patterns of the complexes were givenin Figs. 2–4. The crystal structure of the complexes was

ethanol under reflux. The compounds were charactey the help of elemental analyses, molar conductances,etic susceptibilities, molecular weights and spectroscata (UV–vis, IR) and the proposed structures were g

n Fig. 1, where vanadium(IV) ion attains square pyrameometry. The ligandH2L1 functions as a dibasic N4 donor

igand, whileH2L2 andH2L3 behave as dibasic N2O2 donorashion. The fifth position being occupied by the vanaxygen.

.2. Measurements

The structural characterization of oxovanadium(IV) clexes were carried out by analysis of the X-ray diffracXRD) pattern, obtained using an X-ray powder diffractoer (Rigaku Geigerflex) with Cu K�1 (λ = 1.54056A) source

The measurements were carried out by Perkin-Elmeriamond TG/DTA to determine the composition tempe

ures of the complexes and to determine in which heat rhe electrical measurements can be made. The TG cere recorded in the temperature range of 20–400◦C for0◦C min−1 heating rate.

Experimental samples were prepared from the compn the form of tablets their thickness was∼0.1 cm at a presure of approximately 1× 108 Pa. These tablets were placetween two copper electrodes with silver paste and

acts were tested to be ohmic. The electrical conductivitihe prepared complexes were measured with a Keithleyystem electrometer, by applying dc voltage using Keit30 programmable voltage source. The conductivitiesalculated by using the general equation ofσ = (I/Vc)(d/a),
Fig. 2. The pattern of X-ray diffraction for VO(L1).

S. Sarkar et al. / Materials Chemistry and Physics 88 (2004) 357–363 359

Fig. 3. The pattern of X-ray diffraction for VO(L2) complex.

Fig. 4. The pattern of X-ray diffraction for VO(L3) complex.

determined according to trial and error method. The param-eters of unit cell parameters for each complexes were de-termined and results of the analysis and powder diffractionpatterns of the [VO(L1)], [VO(L 2)] and [VO(L3)] complexeswere given inTables 1–3andFigs. 2–4, respectively. It wasfound that the [VO(L1)], [VO(L 2)] and [VO(L3)] complexeshave monoclinic structure.

Table 1The parameters of unit cell, and observed and calculated X-ray diffraction data of VO(L1) complex

System: monoclinica = 16.0619A, b = 11.5517A, c = 12.7262A β = 122.427◦, V = 1993.05A3

Peak no. d (Obs.) d (Cal.) I/Io 2θ (Obs.) 2θ (Cal.) �2θ (h k l)

1 6.7630 6.7787 36 13.080 13.050 0.030 2 0 02 5.7629 5.7758 243 5.3610 5.3709 284 4.6954 4.7058 355 4.5163 4.5191 1006 4.2189 4.2085 417 4.0624 4.0593 968 3.9448 3.9332 519 3.6685 3.6714 98

10 3.3884 3.3893 6811 3.3091 3.3091 3912 2.9209 2.9232 3113 2.5870 2.5850 2314 2.5447 2.5442 2215 2.3999 2.4003 3811

Fig. 5. TG curves of VO(L1), VO(L2) and VO(L3) samples in air at20◦C/min.

3.2. Thermal properties

Fig. 5 shows the thermogravimetric curves of the com-plexes in air; it can be seen that the weight loss of VO(L2)complex takes place in single-step. The VO(L2) complexshows good thermal stability and the decomposition tem-perature starts above∼274◦C. The weight loss of VO(L1)and VO(L3) complexes takes place in a gradual and contin-uous form. The decomposition temperatures of VO(L1) andVO(L3) starts∼234 and∼92◦C, respectively. The mass lossof the VO(L3) complex in the range of 92–100◦C is due tothe water molecules in the structure. According to this thereal mass loss is at about 230◦C.

3.3. Electrical conductivity

According to thermal decomposition temperatures, elec-trical conductivities of the complexes were measured at therange of 300–500 K temperature.Fig. 6(a–c) illustrates a

6 2.2827 2.2785 237 2.1074 2.1071 17

15.363 15.328 0.035 0 2 016.522 16.491 0.031 0 0 218.884 18.842 0.042 2 0 119.640 19.628 0.012 3 0 021.040 21.092 −0.052 3 1 021.861 21.878 −0.017 −3 0 322.521 22.588 −0.067 0 2 224.241 24.222 0.019 −1 3 126.280 26.273 0.007 4 0 026.921 26.921 0.000 −1 2 330.581 30.556 0.025 4 2 034.645 34.673 −0.028 −6 1 235.240 35.248 −0.008 4 3 037.442 37.435 0.007 1 0 439.442 39.518 −0.076 4 3 142.879 42.885 −0.006 −6 3 4



System: monoclinica = 24.2814A, b = 5.2970A, c = 10.1056A β = 103.360◦, V = 1264.60A 3


1 11.8400 11.8121 75 7.460 7.478 −0.018 2 0 02 9.8619 9.8321 58 8.959 8.986 −0.027 0 0 13 8.4502 8.4138 100 10.460 10.505 −0.045 1 0 14 4.6718 4.6734 49 18.980 18.974 0.006 −1 1 15 4.2024 4.2069 10 21.124 21.101 0.023 2 0 26 3.9378 3.9374 29 22.561 22.563 −0.002 6 0 07 3.7792 3.7948 25 23.521 23.423 0.098 3 0 28 3.6215 3.6220 21 24.561 24.558 0.003 −2 1 29 3.4875 3.4920 17 25.520 25.486 0.034 −6 0 2

10 3.1930 3.1961 23 27.919 27.891 0.028 −4 0 311 2.9549 2.9530 17 30.220 30.240 −0.020 8 0 012 2.8117 2.8065 75 31.800 31.860 −0.060 −3 1 313 2.6391 2.6381 42 33.940 33.954 −0.014 −5 1 314 2.3805 2.3798 24 37.759 37.771 −0.012 8 1 115 2.2392 2.2413 10 40.241 40.201 0.040 2 2 216 1.9665 1.9664 14 46.121 46.123 −0.002 0 0 517 1.7471 1.7472 17 52.322 52.320 0.002 9 1 318 1.5864 1.5866 13 58.099 58.091 0.008 −1 2 0 519 1.5657 1.5662 10 58.941 58.920 0.021 −1 0 2 4

typical temperature dependence of the electrical conductivityduring the heat treatment. By analyzing the shape of lnσ =f(103/T) graphs inFig. 6(a–c), useful information regardedthe processes occurring in investigated complexes during theheat treatment can be obtained. The dc conductivity of semi-conductors has the general form,

σ = σo exp

(−Ea

kT

)(1)

whereEa is the thermal activation energy for the electricalconduction,σo the parameter depending on the semiconduc-


System: monoclinica = 14.3724A, b = 7.7961A, c = 11.1708A β = 116.115◦, V = 1123.90A3


1 7.7957 7.7961 23 11.341 11.340 0.001 0 1 02 6.4481 6.4526 20 13.722 13.712 0.010 2 0 03 5.0063 5.0152 34 17.702 17.670 0.032 0 0 24 4.5807 4.5855 100 19.362 19.342 0.020 2 0 15 4.2267 4.2178 56 21.001 21.046 −0.045 0 1 26 3.7079 3.7096 35 23.980 23.969 0.011 −2 0 37 3.5640 3.5622 32 24.963 24.976 −0.013 −4 0 18 3.4036 3.4067 38 26.160 26.136 0.024 −2 2 19 3.3020 3.3057 55 26.981 26.950 0.031 −1 1 3

10 3.1928 3.1948 25 27.921 27.903 0.018 −1 2 21111111112

tor nature, andk is the Boltzmann constant. A plot of lnσversus1000/T yields a straight line whose slope can be usedto determine the thermal activation energies of the complexes[20,21]. As can be seen fromFig. 6(a–c), during the heating,the electrical conductivity may increase with the increasingin temperature, at the range of 300–500 K. The curves aregenerally characterized by two portion with different slopes[22]. The calculated activation energies may correspond todifferent levels. These two activation energies are associatedwith the intramolecular and the intermolecular conductivityprocess. Particularly, the lower values ofEa are associated

1 3.1317 3.1252 242 3.0818 3.0777 203 2.8643 2.8647 344 2.7943 2.7881 315 2.6524 2.6491 276 2.5813 2.5810 197 2.5143 2.5156 208 2.3340 2.3364 189 2.1893 2.1890 140 1.8674 1.8689 14

28.477 28.537 −0.060 −2 2 228.958 28.988 −0.030 0 2 231.200 31.196 0.004 −5 0 232.003 32.076 −0.073 −2 0 433.765 33.808 −0.043 −5 1 134.724 34.727 −0.003 5 0 035.680 35.660 0.020 0 3 138.540 38.499 0.041 −6 0 341.200 41.206 −0.006 1 1 448.722 48.680 0.042 −1 2 5


Fig. 6. Temperature dependence of the electrical conductivity of complexes: (a) VO(L1); (b) VO(L2); (c) VO(L3).

with the intermolecular conduction process, while the highervalues are related to the intramolecular conduction process.In these semiconductor complexes, there are two stages inthe movement of carrier motion within the sample, which arethe intramolecular and intermolecular transfer of the currentcarrier. In the intramolecular transfer of electrons, electronscan hop from one atomic site to another if orbitals exist atthese sites with the same energy levels. In the case of inter-molecular orbital overlap, electrons or holes can travel fromone kind of macromolecule to another. Therefore,�-electronscan also move from one type of macromolecule to anotherby hopping if orbitals with the same energy levels exist be-tween the complex molecules. If we assume excited carrierswithin the molecules, the carriers are retarded by the barrier

macromolecules. The activation energy of the intramolecu-lar conduction process is higher. Therefore, the first step ofconduction starts between molecules since the lower activa-tion energy corresponds to intermolecular transfer, while thehigher activation energy corresponds to intramolecular trans-fer. But, basically both processes should be considered. Theintramolecular conduction process occurs between the metalatom and the ligands in the complex and the intermolecularconduction process between two macromolecular complexes[20,21].

The electrical conductivity of the complexes have positivetemperature coefficient. That is, with the increase in tempera-ture, conductivity increases exponentially. The increase startswhen charge carriers have enough activation energy. Also,


Fig. 7. The optical absorption spectra for the complexes: (a) VO(L1); (b)VO(L2); (c) VO(L3).

during the increase of temperature the mobility of these car-riers increases. This is a property of typical semiconductivity.But in the heats above 423 K, the electrical conductivities ofthe complexes have negative heat coefficients. Thermal probemeasurements showed that these complexes have n-type electrical conductivity, that is, most of carriers are electrons.

Table 4Electronic parameters of samples.Ea; activation energy of electrical con-duction,Egd; energy gap for allowed direct transitions

Complex Ea1 (eV) Ea2 (eV) Egd (eV)

VO(L1) 0.48 1.18 3.45VO(L2) 1.01 0.64 2.65VO(L3) 0.64 1.14 2.80

3.4. Optical properties

The nature of the optical transition involved in the com-plexes can be determined on the basis of the dependence ofabsorption coefficient (α) on photon energy (hν). For transi-tion, α is given by[23],

αhν = A(hν − Eg)n (4)

whereEg is the energy gap between the bottom of the conduc-tion band and the top of the valence band at the same valueof wave number (k). The energy gap for allowed direct (Egd)and indirect (Egi) transitions, can be determined by means ofrelations, respectively:

αhν = Aa(hν − Egd)1/2 (5)

and

αhν = Ai (hν − Egi)2 (6)

where,α is the absorption coefficient,hν is the photon energy,andAa andAi are the characteristic parameters for respectivetransitions, independent onν. On the basis of theseEqs. (5)and (6), Eg can be determined, from extrapolation to zero oflinear parts of (αhν)2 = f(hν) and (αhν)1/2 = f(hν) curves.

Optical absorption spectra of complexes are given inF d outt fac-tto noTF ob-t

4

ando ithq crys-t wderX a-d avem emi-c uctor.T ncedb
-
ig. 7(a–c) and analysis of the absorption data was carrieo determine the predominant optical transition. A satisory fit was obtained for (αhν)2 as a function ofhν, showinghe existence of a direct gap. The plots of (αhν)2 versus h�f complexes were shown inFig. 7(a–c). The extrapolatiof the graphs to (αhν)2 = 0 yields the optical band gapEgd.he values of the optical band gapEgd were determined fromig. 7(a–c) by the least squares fitting of the data. The

ained values were given inTable 4.

. Conclusion

The crystal structure, electrical conductivity, thermalptical properties of the oxovanadium(IV) complexes wuadridentate schiff bases have been investigated. The

al structure of the complexes was determined by po-ray diffraction (XRD). XRD showed that the oxovanium(IV) complexes with quadridentate schiff bases honoclinic structure. These complexes show typical s

onducting characteristics. They act as n-type semicondhe electron transport in investigated complexes is influey their molecular structures.


Acknowledgements

Investigation of electrical, optical and thermal propertiesof this study was made in the research of semiconductorphysics laboratory that is supported by T.R. Prime Min-istry State Planning Organization (Project no.: DPT 2003K120440-1). One of us (S.S.) is grateful to the University ofKalyani for JRF.

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X-ray diffraction studies, thermal, electrical and optical properties of oxovanadium(IV) complexes with quadridentate schiff bases