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Spectrochimica Acta Part A 76 (2010) 174–181

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular andBiomolecular Spectroscopy

journa l homepage: www.e lsev ier .com/ locate /saa

ynthesis, structural characterization, catalytic, thermal and electrochemicalnvestigations of bidentate Schiff base ligand and its metal complexes

ustafa Dolaza, Vickie McKeeb, Serhan Urus c, Necmettin Demird,li E. Sabikd, Aysegül Gölcüd, Mehmet Tümerd,∗

K.Maras Sütcü Imam University, Environmental Engineering Department, Faculty of Engineering and Architecture, 46100 K.Maras, TurkeyUniv Loughborough, Dept Chem, Loughborough LE11 3TU, Leics, England, United KingdomK.Maras Sütcü Imam University, University-Industry-Public Collaboration, Research-Development-Application Centre, 46100 K.Maras, TurkeyK.Maras Sütcü Imam University, Chemistry Department, Faculty of Science and Arts, 46100 K.Maras, Turkey

r t i c l e i n f o

rticle history:eceived 20 January 2010eceived in revised form 25 February 2010ccepted 11 March 2010

eywords:

a b s t r a c t

In this study, we prepared the Schiff base ligand (L) and its Cu(II), Co(II) and Ni(II) complexes. The com-pounds were characterized by the analytical and spectroscopic methods. The ligand (L) behaves as abidentate ligand and coordinates to the metal ions via the nitrogen atoms. The complexes have themononuclear structures. The analytical and spectroscopic results indicated that the chloride ions coordi-nate to the metal ions. The complexes have the general formulae [M(L)(Cl)2] (M: Cu(II), Co(II) and Ni(II)metal ions). Electrochemical properties were investigated as ligand and metal centres in the different

ingle crystal X-ray diffractionchiff baseatalysislectrochemicallkane-oxidation

solvents and at the scan rates, respectively. The thermal properties of the metal complexes were stud-ied in the N2 atmosphere. We investigated the improved catalytic activity of the Cu(II), Co(II) and Ni(II)complexes on the cyclohexane as a substrate. Obtained data showed that the best catalyst is the Cu(II)complex. The single crystal of the ligand (L) was obtained from CH3CN solution. There is a C–H. . .N H-bond linking the molecules into chains (C6). . .N(2) 3.4415(18) Å under symmetry operation (x + 1,y,z) as

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well as �–� stacking on t

. Introduction

Through the years, Schiff bases have played a special role ashelating ligands in main group and transition metal coordinationhemistry, due to their stability under a variety of oxidative andeductive conditions, and to the fact that imine ligands are bor-erline between hard and soft Lewis bases [1,2]. Transition metalomplexes of tetra dentate Schiff base ligands find applicationsn catalysis [3] and as biomimetic enzyme models [4]. Although aarge number of compounds of the type M(N2O2), M(N2S2), M(N2)nd M(N4) (using the binding atom representation), have beenynthesized and characterized, such complexes have been inves-igated because of their catalytic activity in oxygenation reactions5], carrying out, for instance, the selective oxidation of organicubstrates, such as olefins, alcohols, alkanes and aldehydes, underild conditions. This property may be very attractive [6], since the

xidation products can provide important starting materials for theroduction of fine chemicals and polymers [7–8]. In addition, thexidation of organic substrates mediated by high valent rutheniu-oxo species evokes much interest in the modeling of cytochrome

∗ Corresponding author. Tel.: +90 344 219 10 93; fax: +90 344 219 10 42.E-mail address: mtumer@ksu.edu.tr (M. Tümer).

386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2010.03.008

tside of the “V” shape – nothing on the inside.© 2010 Elsevier B.V. All rights reserved.

P-450 [9]. Several researches have proposed that the redox poten-tial in Schiff-base complexes is directly related to many of thebiologically relevant chemical characteristics of the entire com-plex, e.g. dioxygen binding ability and nucleophilicity [10]. Thus,there has been a strong interest in determining thermodynamicallymeaningful redox potentials of copper Schiff-base complexes andin understanding the relationship between these potentials and thedetailed structure of the Schiff-base ligand [11]. Numerous elec-trochemical studies have been made for a fairly large number ofacyclic and macrocyclic copper(II) complexes derived from Schiff-bases. These investigations revealed that the redox properties ofcopper(II) complexes are markedly influenced by structural andelectronic factors [12].

Cyclohexane (CyH) oxidation into cyclohexanol (Cy–OH) andcyclohexanone (Cy O) has a great importance in the industry.Cy O and Cy–OH are produced over one billion tones each yearwhich are generally used for the synthesis of Nylon-6 and Nylon-6,6[13]. Microwave technology using green oxidant hydrogen perox-ide can reduce the reaction times and energy consumption with an

increase cyclohexane conversion and Cy–OH and Cy O selectivity[14].

In this study, we prepared the ligand trans-N,N′-bis[(3,4-dichlorophenyl)methylidene] cyclohexane-1,2-diamine and itsCu(II), Co(II) and Ni(II) metal complexes. The ligand and its metal

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omplexes were characterized by the spectroscopic and analyti-al methods. We obtained the suitable single crystals of the ligandor the X-ray analysis. Ligand and its metal complexes show theeversible or irreversible peak potentials in DMF solution. The cat-lytic properties of the Cu(II), Co(II) and Ni(II) metal complexesere investigated using the cyclohexane as a substrate. The results

howed that the ligand has enhanced the catalytic properties of theomplexes under microwave conditions using H2O2 as oxidant.

. Physical measurement

.1. General

The metal salts CuCl2·2H2O, CoCl2·6H2O and NiCl2·6H2Ond organic solvents were purchased from commercialources and used as received, unless noted otherwise. 3,4-ichlorobenzaldehyde, (±)trans-1,2-cyclohexanediamine and

[CH3(CH2)3]4NBF4) were obtained from Fluka. Elemental analysesC, H, N) were performed using a LECO CHNS 932 elemental anal-ser. IR spectra were obtained using KBr discs (4000–400 cm−1)n a Shimadzu 8300 FT-IR spectrophotometer. The electronicpectra in the 200–900 nm range were obtained on a PerkinElmerambda 45 spectrophotometer. Magnetic measurements werearried out by the Gouy method using Hg[Co(SCN)4] as calibrant.olar conductances of the Schiff base ligands and their transitionetal complexes were determined in DMF (∼10−3 M) at room

emperature using a Jenway Model 4070 conductivity meter. Masspectra of the ligands were recorded on a LC/MS APCI AGILENT100 MSD spectrophotometer. 1H and 13C NMR spectra wereecorded on a Varian XL-200 instrument. TMS was used as internaltandard and DMSO-d6 as solvent. The metal and chloride contentsf the complexes were determined as gravimetrically accordingo the known procedure [15]. The thermal analysis studies ofhe complexes were performed on a PerkinElmer Pyris DiamondTA/TG Thermal System under nitrogen atmosphere at a heating

ate of 10 ◦C/min.Cyclic voltammograms were recorded on an Iviumstat Electro-

hemical workstation equipped with a low current module (BASA–1) recorder. The electrochemical cell was equipped with aAS glassy carbon working electrode (area 4.6 mm2), a platinumoil auxiliary electrode and an Ag/AgCl reference electrode filledith tetrabutylammonium tetrafluoroborate (0.1 M) in DMSO andMF solvents and adjusted to 0.00 V vs SCE. Cyclic voltammetriceasurements were made at room temperature in an undivided

ell (BAS model C–3 cell stand) with a platinum counter elec-rode and an Ag/AgCl reference electrode (BAS). All potentials areeported with respect to Ag/AgCl. The solutions were deoxygenatedy passing dry nitrogen through the solution for 30 min prior tohe experiments, and during the experiments the flow was main-ained over the solution. Digital simulations were performed usingigiSim 3.0 for windows (BAS, Inc.). Experimental cyclic voltammo-rams used for the fitting process had the background subtractednd were corrected electronically for ohmic drop.

The microwave experiments were carried out using a BergofWS3+ (Germany) equipped with pressure and temperature con-

rol. Microwave experiments were done in closed DAP60 vessels.he reaction products were characterized and analyzed by usingerkinElmer Clarus 600 GC (USA) equipped with MS detector fit-ed with Elite-5 MS and FID detector fitted with BPX5 capillary

olumns.

Preparation of the ligand (L), preparation of the complexes,yclohexane oxidation under microwave irradiation and X-raytructure solution and refinement for the Schiff base ligand (L) wereiven in the Supporting information.

a Part A 76 (2010) 174–181 175

3. Results and discussions

The ligand trans-N,N′-bis[(3,4-dichlorophenyl)methylidene]cy-clohexane-1,2-diamine (L) and its metal complexes are microcrys-talline, non-hygroscopic, partially soluble in most common organicsolvents but entirely soluble in DMF and DMSO. The analytical datawere showed that the metal to ligand ratio is 1:1 stoichiometry (inthe Supporting information). The compounds are very stable solidsat room temperature without decomposition for a long time. Themolar conductance values are in the 6.5–8.2 �−1 mol−1 cm2 range,indicating the non-electrolytic nature of the complexes [16]. Fromthe CH3CN solution, we obtained suitable single crystals for X-raydiffraction analysis.

The 1H(13C) NMR spectra of the ligand and its nickel(II) com-plex were recorded using DMSO-d6 as a solvent, and the spectraldata are given in the Supporting information. The ligand has a sym-metric nature. Therefore, the hydrogen atoms in the benzene andcyclohexane rings are shown as one signal for each other. For exam-ple, two hydrogen atoms of the azomethine groups have one signalin the 1H NMR spectrum. The azomethine groups were shown at ı8.27 ppm as a singlet. In the nickel(II) complex, these groups shiftedto the lower regions (ı 8.34 ppm) and this shift shows that thenitrogen atom of the CH N group coordinated to the nickel ion.Aromatic rings protons are shown in the range ı 7.26–8.08 ppmrange as a multiplet. In the 1H NMR spectrum of the complex, thearomatic ring protons shifted to the lower regions. In the ligand, thehydrogen atoms of the CH2/CH groups in the cyclohexane ring areobserved in the ı 3.40–1.21 ppm range, and, in the complex, thesesignals slightly shifted to the lower regions.

In order to further information about the ligand and its com-plexes, the 13C NMR spectra were investigated. In the ligand, theazomethine carbon atom is shown at ı 160.17 ppm. In the nickel(II)complex, this signal shifted to the lower region. This situation con-firms that the nitrogen atom of the CH N group coordinated tothe nickel(II) ion. The aromatic ring C atoms were shown in theı 138.25–129.01 ppm range. The C atoms of the cyclohexane ringwere shown in the ı 26.31–79.56 ppm range. In the spectrum ofthe Ni(II) complex, the signals of the aromatic and aliphatic carbonatoms shifted to the lower regions.

The mass spectral studies for the ligand and its metal com-plexes were done and obtained data are given in the Supportinginformation. The mass spectra for the ligand and its metal com-plexes were characterized by a peak corresponding to Schiff basefragmentation. The mass spectrum of the ligand (L) shows molecu-lar ion peak ([M]+, 100%) at m/z 428. Moreover, the fragmentationpeaks at m/z 427 (82%) and 430 (30%) can be attributed to the[M−1]+ and [M+2]+2 ions, respectively. The mass spectra of thecomplexes show the similar molecular ion and corresponding frag-mentation peaks. In the complexes, the peaks at m/z 563 and 558can be attributed to the molecular ion peaks ([M]+) of the copper(II),cobalt(II) and nickel(II) complexes, respectively.

The infrared spectral data of all compounds were given in theSupporting information. In the spectrum of the ligand, the �(CH N)vibration of the azomethine group is shown at the 1643 cm−1. Inthe complexes, this vibration shifted to the lower regions and canbe attributed to the complexation of the metal ion and nitrogenatoms of the azomethine groups. In the spectra of the complexes,the vibrations �(CH2) of the cyclohexane ring are seen in the2933–2854 cm−1 range. The observed bands in the 437–432 cm−1

range are �(M–N) vibration.The electronic spectra of the ligand and their complexes were

recorded in DMF as a solvent. The spectral data are given in theSupporting information. In the spectra of the ligand, the band atthe 342 nm may be assigned to the n–�* transitions. The observedbands in the 310–290 nm range can be attributed to the �–�* and�–�* transitions. The complexes exhibit multiple absorptions in

176 M. Dolaz et al. / Spectrochimica Acta Part A 76 (2010) 174–181

Table 1Crystal data and structure refinement for the Schiff base ligand (L).

Empirical formula C20H18Cl4N2

Formula weight 428.16Temperature 150(2) KWavelength 0.71073 ÅCrystal system TriclinicSpace group P-1

Unit cell dimensions a = 6.1924(2) Å ˛ = 95.3270(10)◦

b = 10.2882(3) Å � = 95.2370(10)◦

c = 16.3868(5) Å � = 103.8790(10)◦

Volume 1002.14(5) Å3

Z 2Density (calculated) 1.419 mg/m3

Absorption coefficient 0.597 mm−1

F(0 0 0) 440Crystal size 0.42 mm × 0.30 mm × 0.25 mmCrystal description Colourless blockTheta range for data collection 2.05–27.50◦ .Index ranges −8 ≤ h ≤ 8, −13 ≤ k ≤ 13, −21 ≤ l ≤ 21Reflections collected 9859Independent reflections 4575 [R(int) = 0.0175]Completeness to theta = 27.50◦ 99.5%Absorption correction Semi-empirical from equivalentsMax. and min. transmission 1.0000 and 0.878557Refinement method Full-matrix least-squares on F2

Data/restraints/parameters 4575/0/235Goodness-of-fit on F2 1.037

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Final R indices [I > 2sigma(I)] R1 = 0.03R indices (all data) R1 = 0.03Largest diff. peak and hole 0.355 andCCDC deposition number 685057

he UV–visible region. In the visible region, the copper(II), cobalt(II)nd nickel(II) complexes show the d–d transitions at 633, 662 and10 nm, respectively. Moreover, the bands at 346, 327 and 325 nman be assigned to the d�(M) → �*(ligand) metal to ligand chargeransfer transitions. The intra-ligand transitions n–�*, �–�* and–�* in the metal(II) complexes are seen in the range 260–302 nm.he bands with higher energy in the UV region are of intra-ligand–�* type or charge-transfer transitions involving energy levels

hich are higher in energy than the ligand lowest unoccupiedolecular orbital (LUMO).The magnetic moment data of the Cu(II), Co(II) and Ni(II)

etal complexes determined at room temperature are given

Fig. 1. ORTEP diagram of the ligand (L) with

2 = 0.07982 = 0.082752 eÅ−3

in the the Supporting information. The magnetic moment ofCo(II) complex is 4.19�B, which compares favorably with that of4.1–4.8�B expected for the Co(II) complexes with three unpairedelectrons [17,18]. Thus, in agreement with the electronic spec-tral studies, Co(II) complex has tetrahedral geometry. The Ni(II)complex has the diamagnetic character and its geometry issquare-planar geometry. The magnetic moment of Cu(II) com-plex is 1.91�B indicating the presence of one unpaired electron

[19,20]. The electronic spectral results clearly indicated tetra-hedral geometry for the Cu(II) system, which should containone unpaired electron, where �eff value would be in the range1.8–2.1�B.

thermal ellipsoids at 50% probability.

M. Dolaz et al. / Spectrochimica Acta Part A 76 (2010) 174–181 177

romatic hydrogen and imine nitrogen atoms of the crystallized ligand.

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Table 2Hydrogen bonds for the Schiff base ligand (L) [Å and◦]a.

D-H. . .A d(D-H) d(H. . .A) d(D. . .A) <(DHA)

Fn

Fig. 2. The dashed lines depict the hydrogen bonds between the a

.1. Molecular geometry of the ligand (L)

The structure of the Schiff base ligand (L) has been newly solvedy X-ray diffraction. Suitable crystals for X-ray crystal diffractionnalyses of the ligand (L) were grown by re-crystallization fromcetonitrile solution for a long time. The data were collectedt 150(2) K on a Bruker APEX II diffractometer. The structureas solved using direct methods and refined on F2 using all the

eflections [21]. All the non-hydrogen atoms were refined usingnisotropic atomic displacement parameters and hydrogen atomsere inserted at calculated positions using a riding model. Param-

ters for data collection and refinement are summarised in Table 1.

rans-N,N′-Bis[(3,4-dichlorophenyl)methylidene] cyclohexane-,4-diamine crystallises in triclinic P-1 space group, the molecule

ies on a centre of symmetry, in an extended conformation. Ahermal ellipsoid diagram [22,23] of the molecule indicating atomumbering scheme with thermal ellipsoids at 50% probability is

ig. 3. Packing diagram of the ligand (L) displaying the unit cell viewed down the c axis.itrogen atoms. Cl-atoms have been omitted for clarity.

C(6)–H(6). . .N(2)#1 0.95 2.65 3.4415(18) 140.9

a Symmetry transformations used to generate equivalent atoms: #1 x + 1,y,z.

illustrated in Fig. 1, hydrogen bonding interactions between theC6(H). . .N(2) and the crystal packing in the unit cell are shownin Figs. 2 and 3, respectively. As can be seen from the ppt file andFig. 1, one of the most interesting features of the ligand (L), thereis a C–H. . .N H-bond linking the molecules into chains (C6). . .N(2)

3.4415(18) Å under symmetry operation (x + 1,y,z) as well as �–�stacking on the outside of the “V” shape – nothing on the inside(Table 2). There are no solvent molecules in the crystal structure.

Selected bond lengths and bond angles are listed in Table 3.The space group is P–1 with a = 6.1924(2) Å, b = 10.2882(3) Å,

The dashed lines depict the hydrogen bonds between the hydrogen and the imine

178 M. Dolaz et al. / Spectrochimica Acta Part A 76 (2010) 174–181

Table 3Bond lengths [Å] and angles [◦] for the Schiff base ligand (L).

C(1)–C(6) 1.3951(19) C(10)–C(11) 1.525(2)C(1)–C(2) 1.3953(19) C(11)–C(12) 1.525(2)C(1)–C(7) 1.4741(19) C(12)–C(13) 1.5271(19)C(2)–C(3) 1.3860(19) C(13)–N(2) 1.4626(17)C(3)–C(4) 1.389(2) N(2)–C(14) 1.2644(18)C(3)–Cl(1) 1.7280(14) C(14)–C(15) 1.4772(18)C(4)–C(5) 1.393(2) C(15)–C(16) 1.3937(19)C(4)–Cl(2) 1.7298(14) C(15)–C(20) 1.396(2)C(5)–C(6) 1.382(2) C(16)–C(17) 1.3893(19)C(7)–N(1) 1.2638(18) C(17)–C(18) 1.388(2)N(1)–C(8) 1.4604(17) C(17)–Cl(3) 1.7282(14)C(8)–C(9) 1.5290(19) C(18)–C(19) 1.391(2)C(8)–C(13) 1.5377(19) C(18)–Cl(4) 1.7295(13)C(9)–C(10) 1.527(2) C(19)–C(20) 1.382(2)C(6)–C(1)–C(2) 119.21(12) C(10)–C(11)–C(12) 111.62(12)C(6)–C(1)–C(7) 121.81(12) C(11)–C(12)–C(13) 111.09(12)C(2)–C(1)–C(7) 118.96(12) N(2)–C(13)–C(12) 109.13(11)C(3)–C(2)–C(1) 120.66(13) N(2)–C(13)–C(8) 109.56(11)C(2)–C(3)–C(4) 119.74(13) C(12)–C(13)–C(8) 109.75(11)C(2)–C(3)–Cl(1) 119.27(11) C(14)–N(2)–C(13) 117.19(12)C(4)–C(3)–Cl(1) 120.99(11) N(2)–C(14)–C(15) 121.97(12)C(3)–C(4)–C(5) 119.86(13) C(16)–C(15)–C(20) 119.27(12)C(3)–C(4)–Cl(2) 120.78(11) C(16)–C(15)–C(14) 119.03(12)C(5)–C(4)–Cl(2) 119.35(11) C(20)–C(15)–C(14) 121.67(12)C(6)–C(5)–C(4) 120.37(13) C(17)–C(16)–C(15) 120.33(13)C(5)–C(6)–C(1) 120.16(13) C(18)–C(17)–C(16) 120.11(13)N(1)–C(7)–C(1) 122.84(13) C(18)–C(17)–Cl(3) 121.02(10)C(7)–N(1)–C(8) 116.74(12) C(16)–C(17)–Cl(3) 118.86(11)N(1)–C(8)–C(9) 110.18(12) C(17)–C(18)–C(19) 119.65(12)N(1)–C(8)–C(13) 108.89(11) C(17)–C(18)–Cl(4) 120.89(11)

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C(9)–C(8)–C(13) 109.39(11) C(19)–C(18)–Cl(4) 119.46(11)C(10)–C(9)–C(8) 110.89(12) C(20)–C(19)–C(18) 120.45(13)C(11)–C(10)–C(9) 111.47(13) C(19)–C(20)–C(15) 120.18(13)

= 16.3868(5) Å, ˛ = 95.3270(10)◦, ˇ = 95.2370(10)◦,= 103.8790(10)◦. According to the single crystal X-ray study,

he colourless block form consists of a molecule with a dihedralngle of 109.56◦ [N(2)–C(13)–C(8)].

Six-membered cyclohexane ring of the symmetric Schiff baseigand has an ideal chair conformation, and all the geometricarameters in the ring are quite normal. Each phenyl moiety inhe molecule is planar. There are also deviations from the over-ll planarity of the phenyl ring C1, C2, C3, C4, C5, C6 atoms buthis time describing an envelope. The deviations from the mainlane through C2, C3, C4 and C5 do not exceed 0.12 Å. All bond

engths and angles in the phenyl rings have normal values. The newtructure shows considerable flexibility. Exact conformation, as aalance between packing and molecular bonding forces, is com-licated by charge transfer interactions which is also a balanceetween electronic and steric effects. As the cyclohexyl ring hastetrahedral geometry around the C atoms. �-Electron delocaliza-

ion does not continue all over the molecule. This property onlyecomes between the phenyl rings and azomethine groups.

Cyclic voltammograms of the ligand and complexes were runn DMF–0.1 M NBu4PF6 and DMSO–0.1 M NBu4PF6 as supportinglectrolyte at 293 K. Unless otherwise stated, all potentials quotedefer to measurements run at a scan rates (‘�’) of 50, 75, 100 and50 mV s−1 and against an internal ferrocene–ferrocenium stan-ard. The electrochemical data of the ligand and its complexesre summarised in Table 4. The cv curves of the ligand at the 50nd 250 mV s−1 scan rates in DMF solution show the reversiblerocess (Ipa: Ipc ∼= 1.0). Its potential range changes from −1.46o −1.15 V (Epc) and 1.17 to 1.40 V (Epa). This process is shownn Fig. 4. The electrochemical curves of the ligand (L) at 50, 75

nd 100 mV s−1 are shown in Fig. 5a. As seen from Fig. 5a, theeduction–oxidation process of the ligand (L) at 50 and 250 mV s−1

can rates in DMF solution is reversible. In DMSO solution and atll scan rates, the ligand behaves the irreversible processes (Ipa: Ipc

1.0). Oxidation–reduction peaks of the ligand in solvents shifted Tab

le4

Elec

troc

hem

ica

Com

pou

nd

L [Cu

2L(

Cl)

4(H

2

[Co 2

L(C

l)4(H

2

[Ni 2

L(C

l)4(H

2

ai E

pa(E

pc):

sco

nce

ntr

atio

no

M. Dolaz et al. / Spectrochimica Act

F

tccaitsp|I

Fi(ic

ig. 4. Reversible reduction–oxidation processes of the ligand in dmf solution.

o the lower or higher potentials by the scan rates. The electro-hemical curves of the [Cu(L)(Cl)2], [Co(L)(Cl)2] and [Ni(L)(Cl)2]omplexes at 100 mV s−1 scan rate are shown in the Fig. 5b. Inddition to the ligand-based processes, all complexes show anrreversible or quasi-reversible two-electron reduction. The elec-rochemical process (in DMSO solution) at 75 mV s−1 scan rate

howed the similar process as a scan rate 50 mV s−1. All the com-lexes have quasi-reversible processes at Epc/Epa: |−0.87| V/0.64 V,

−0.39| V/0.51 V, |−0.42| V/0.54 V and Epc/Epa: |−0.26| V/0.35 V (Ipa:pc ≥ 0.7), respectively.

ig. 5. (a) Cyclic voltammograms of the ligand (L) in the presence of 0.1 M NBu4PF6

n DMF, : 100 mV s−1, : 75 mV s−1 and : 50 mV s−1 scan rates.b) Cyclic voltammograms of the metal complexes in the presence of 0.1 M NBu4PF6

n DMF (sr: 100 mV s−1). : [Ni(L)Cl2], : [Co(L)Cl2] and : [Cu(L)Cl2]omplexes.

a Part A 76 (2010) 174–181 179

The electrochemical results indicate that the electron transfermay occur as follows for all complexes:

M(II) + e− → M(I)[M : Cu(II), Co(II)andNi(II)]

Although the forward peaks in reduction process in DMF solu-tion and reverse peaks in oxidation process in DMSO solution arealmost invariant, the position and broadness of the return peakvaries markedly depending on the anion present. This suggests thatclose association of the anions with the metal(II) centres may occurfollowing reduction at the electrode surface. No MI–M0 (M: Cu(II),Co(II) and Ni(II)) reduction was detected for the metal complexes.Of all of the complexes were sufficiently soluble in DMF and DMSOto allow a cyclic voltammogram to be run. In DMSO solvent, thecomplexes show an identical pattern of peaks. However, in gener-ally, the potentials of all these processes are slightly more positivein DMSO than their positions in DMF. As the ligand has the elec-tron attractive chloride atoms, in all complexes, the reduction andoxidation potentials were shifted to the higher positive regions.

Thermal properties of the metal complexes were investigated inthe 20–1000 ◦C temperature range under nitrogen atmosphere ata heating rate of 10 ◦C/min. Thermal curves of the Cu(II) and Co(II)complexes were given in Fig. 6a and b. All complexes have the chlo-ride ions coordinated to the metal ions. As seen from also thermalcurves, in the first step, in the 150–250 ◦C temperature range, thecoordinated chloride ions have been move away from the complex.In the second step, the organic parts (L) of the complexes decom-posed and finally, the metal oxides (CuO, NiO and Co3O4) occurredat the higher temperatures. The Co(II) complex has the higher ther-mal stability. Order of thermal stability is Co(II) > Cu(II) > Ni(II).

3.2. Cyclohexane oxidation under microwave irradiation

The optimum conditions were obtained as cata-lyst:substrate:oxidant ratio of 1:100:200 in acetonitrile under300 W microwave power for 30 min. The temperature and pres-sure were controlled at about 100 ◦C and 30 bar by the instrument.A blank has been run under the same conditions without anycatalyst. According to the possible oxidation mechanism, firstand slow step is the oxidation of CyH to Cy–OH and then Cy Oand other further oxidized products. If the first step is controlled,the ratio of desired products Cy–OH and Cy O selectivity can beincreased. The possible reaction products are shown in Fig. 7 usingH2O2 and catalyst under microwave. It has been shown that themicrowave power and the novel catalysts could affect the selectiveoxidation of CyH to Cy–OH and Cy O. The Cu(II) complex showedmore good catalytic activity than the Co(II) and Ni(II) complexes.The Co(II) complexes have been explained as a good catalyst in alot of literature [13,14]. In our study, the Co(II) complex showedlow catalytic activity on the cyclohexane. This situation may bedue to the structure of the ligand. In the complexes, the �–�electron delocalization phenomena does not occur throughoutthe ligand for the cyclohexane. This situation may be effect theoxidation reaction. The best selectivities of desired productsCy–OH and Cy O were obtained in the Cu(II) complex (Table 5,Fig. 8). The catalytic performance of Cu(II) complex is comparable

to the activity exhibited by the complexes reported earlier [13,14].The key point in the conversion of cyclohexane to the oxidizedproducts is the reduction of Cu(II)–L to Cu(I)–L (L:ligand). Thisreduction to Cu(I)–L is facilitated the ligands available aroundthe metal cation. The formation of the oxidation products Cy–OHand Cy O show the preferential attack of the activated bonds.Tetrahedral geometry of Cu(II) complex may be the reason of thehaving higher activity rather than the other complexes.

180 M. Dolaz et al. / Spectrochimica Acta Part A 76 (2010) 174–181

Fig. 6. Thermal curves of the [Co(L)(Cl)2] complex (a) and the [Cu(L)(Cl)2] complex (b) in the 10–1000 ◦C temperature range. Green line is TGA, blue line is DTA and red lineis DTG. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Table 5Catalytic oxidation of cyclohexane (CyH) with H2O2 under microwave irradiation.

Entry Catalysta CyH Conversion (mol %) Desired products (mol %) ol:one By products (mol %)

1b Blank 0.86 Cy–OH (0.54); Cy O (0.32) 1.72 –2c [Cu(L)(Cl)2] 94.65 Cy–OH (33.34); Cy O (35.14) 0.95 25.873c [Co(L)(Cl)2] 99.18 Cy–OH (10.46); Cy O (4.39) 2.38 84.334c [Ni(L)(Cl)2] 99.76 Cy–OH (13.59); Cy O (22.94) 0.59 63.23

a 300 W power were applied for 30 min. The reaction temperature and pressure were hb 2 mmol cyclohexane:4 mmol hydrogene peroxide and 5 mL acetonitrile were used inc 0.02 mmol catalyst:2 mmol cyclohexane:4 mmol hydrogene peroxide (1:100:200) an

Fig. 7. Catalytic oxidation of cyclohexa

eld at around 100 ◦C and 30 bar in closed DAP60 vessels.experiment without catalyst.d 5 mL acetonitrile were used for each reaction.

ne under microwave irradiation.

M. Dolaz et al. / Spectrochimica Act

Fig. 8. Influence of the complexes in cyclohexane oxidation under microwaveirradiation. 0.02 mmol catalyst:2 mmol cyclohexane:4 mmol hydrogene peroxide(1:100:200) and 5 mL acetonitrile were used for each reaction. 300 W power wasapplied for 30 min. The reaction temperature and pressure were held at around100 ◦C and 30 bar in closed DAP60 vessels.

[[[[

[

[

[[[[[[[[

a Part A 76 (2010) 174–181 181

Supplementary material

Full crystallographic data for the reported ligand (L) has beendeposited with the Cambridge Crystallographic Data Center, CCDCNos. 685057. Copies of this information may be obtained by writingyour request to: The Director, CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: +44 1223 336 033; email: deposit@ccdc.cam.ac.uk orwww.http://www.ccdc.cam.ac.uk).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.saa.2010.03.008.

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