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Journal of Physics and Chemistry of Solids 68 (2007) 45–52

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Synthesis, crystal structure, Cu2+ doped EPR and voltammetric studiesof bis[N-(2-hydroxyethyl)ethylenediamine]zinc(II) squarate

monohydrate

Ibrahim Uc-ar�, Bunyamin Karabulut, Ahmet Bulut, Orhan Buyukgungor

Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayıs University, Kurupelit, 55139, Samsun, Turkey

Received 17 May 2006; received in revised form 4 August 2006; accepted 12 September 2006

Abstract

Crystal structure of [Zn(hydet-en)2] �C4O4 �H2O (ZHES) (hydet-en is N-(2-hydroxyethyl)ethylenediamine) complex has been

synthesized and characterized by analytical, spectroscopic (IR, UV/Vis) and voltammetric techniques. After doping Cu2+ ion, its

magnetic environment has been identified by electron paramagnetic resonance (EPR) technique. The title complex crystalizes in

monoclinic system with space group P21/c and with Z ¼ 4. Each hydet-en ligand acts as a tridentate ligand through the two N atoms and

the hydroxyl O atom, resulting in a six coordinate Zn(II) ion. The EPR spectra were recorded in three perpendicular planes of Cu2+

doped ZHES single crystal. The calculated g and A values indicated that the paramagnetic center is rhombic symmetry with the Cu2+ ion

having distorted octahedral environment. The molecular orbital bond coefficients of the Cu(II) ion in d9 state is also calculated by using

EPR and optical absorption parameters. The dianion SQ2� is oxidized reversibly in two consecutive steps to the corresponding radical

monoanion and neutral form.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: D. Electron paramagnetic resonance (EPR)

1. Introduction

Increasing attention has been devoted to the study of thecoordination chemistry of the squarate ligand, C4O4

2�, byboth inorganic and bioinorganic chemists during the pastfew years [1]. Squarate acts as a bridge between two ormore metal atoms in mono or polydentate coordinationmodes when acting as a ligand towards first row transitionmetal ions [2,3]. It coordinates to Fe(II), Fe(III), Ni(II),and Cu(II) complexes in a m-1,3 fashion, giving binuclear[4,5] and chain structures [6,7], whereas the m-1,2 coordina-tion mode has been reported for binuclear complex ofCu(II) [8]. It is also observed that the squarate anion, withCu(II) and Ni(II), acts as a tetramonodentate ligand andforms polynuclear compounds [9]. The chelating and bis-chelating coordination modes are only possible in com-plexes with larger metal ions, such as alkaline and rare

e front matter r 2006 Elsevier Ltd. All rights reserved.

cs.2006.09.008

ng author. Tel.: +90362 3121919

ss: iucar@omu.edu.tr (I. Uc-ar).

earth cations [10–12]. In all the cases reported so far, metal-squarate complexes have been found interesting in terms ofthe structural relationships between their respective solid-state architectures [13]. In our ongoing research on squaricacid, we have synthesized some mixed-ligand metal(II)complexes of squaric acid, and their structures have beenreported [14–17]. In these compounds, squaric acid behavesas a monodentate ligand [18,19] or acts as both amonodentate and a bidentate ligand [20], while in ZHES,reported in the study, it has not coordinated to Zn(II) ionand acts as a counter anion.It is also aimed in this study to see the magnetic

properties of the compound by electron paramagneticresonance (EPR) technique. In order to achieve these,transition metal ions should be doped in the diamagnetichost lattice of ZHES as an impurity. It is now well knownthat the transition metal ions as a probe can be used todetermine the symmetry environments of the complexes inhost lattices by EPR technique [21,22]. When these ionsform paramagnetic centers then one can get information

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Table 1

Crystal data and structure refinement for ZHES

Formula [Zn(C4H12N2O)2] � (C4O4) �H2O

Molecular weight (g) 403.76

Temperature (K) 297(2)

Wavelength (A) 0.71069

Crystal system Monoclinic

Space group P21/c

Unit cell dimensions (A), (1) a ¼ 11.8537(7)

b ¼ 8.9114(4)

c ¼ 17.0014(10)

b ¼ 97.259(5)

Volume (A3) 1781.52(17)

Z 4

Calculated density (Mg m�3) 1.505

m (mm�1) 1.419

F(0 0 0) 849

Crystal size (mm) 0.25� 0.35� 0.42

y range 2.42–27.82

Index ranges �15php15

�11pkp11

�22plp22

Reflections collected 25365

Independent reflections 4193 (R(int) ¼ 0.050)

Reflections observed (42s) 3642

Absorption correction Integration

Max. and min. transmission 0.3938 and 0.5973

Refinement method Full-matrix least-squares on F2

Data/restrains/parameters 4193/4/241

Goodness-of-fit on F2 1.052

Final R indices (I42s(I)) R1 ¼ 0.0282, wR2 ¼ 0.0690

R indices (all data) R1 ¼ 0.0340, wR2 ¼ 0.0713

Largest diff. peak and hole (A�3) 0.810 and �0.350

I. Uc-ar et al. / Journal of Physics and Chemistry of Solids 68 (2007) 45–5246

about the local symmetry. Since Cu2+ ions are generallyused as probes to enter the lattice substitutionally in placeof the divalent cation in the lattices containing divalentcations in the literature, we have used Cu2+ ions in ZHESand recorded EPR data.

2. Experimental

2.1. Chemical preparation

The squaric acid (0.57 g, 5mmol) dissolved in 25mlwater was neutralized with NaOH (0.40 g, 10mmol) andwas added to a hot solution of the ZnCl2 � 2H2O (0.86 g,5mmol) dissolved in 100ml water. The mixture wasrefluxed at 353K for 12 h and then cooled to roomtemperature. The blue colorless crystals formed werefiltered and washed with water and alcohol and dried invacuum. A solution of N-(2-hydroxyethyl)ethylenediamine(0.208 g, 2mmol) ethanole (50ml) was added drop wiseupon stirring to a suspension of ZnC4O4 � 2H2O (0.21 g,1mmol) in water (50ml). The resulting white solution wasrefluxed for about 2 h and than cooled to room tempera-ture. A few days later, well-formed white crystals wereselected for X-ray studies. The Cu2+ doped ZHES singlecrystals were also grown by slow evaporation of thesaturated aqueous solution admixtured in stochiometricratios with about 0.05% CuCl2 salt. The well-developedsingle crystals of suitable sizes were selected for EPRinvestigation after about a week.

2.2. Spectroscopic details

A suitable single crystal was mounted on a glass fiberand data collections were performed on a STOE IPDSIIimage plate detector using Mo Ka radiation(l ¼ 0.71019 A). Details of crystal structure are given inTable 1. Data collection: Stoe X-AREA [23]. Cell refine-ment: Stoe X-AREA [23]. Data reduction: Stoe X-RED[23]. The structure was solved by direct-methods usingSIR-97 [24] and anisotropic displacement parameters wereapplied to non-hydrogen atoms in a full-matrix least-squares refinement based on F2 using SHELXL-97 [25]. Allhydrogens were positioned geometrically and refined by ariding model with Uiso 1.2 times that of attached atoms.Molecular drawings were obtained using ORTEP-III [26].

The EPR spectra were recorded using a Varian E-109Cmodel X-band spectrometer. The magnetic field modula-tion frequency was 100 kHz and the microwave power wasaround 10mW. The single crystals were mounted on agoniometer and the spectra were recorded in threemutually perpendicular planes at 101 intervals at 298K.The g values were obtained by comparison with adiphenylpicrylhydrazyl sample of g ¼ 2.0036. The opticalabsorption spectra of ZHES were recorded at roomtemperature on a CINTRA 20 UV–VIS spectrometer withdiffuse reflectance accessory working between 300 and900 nm. The IR spectra were recorded on a Jasco 430

FT/IR spectrometer using KBr pellets and operating in4000–200 cm�1 range.An EcoChemie Autolab-30 potantiostat with the elec-

trochemical software package GPES 4.9 (Utreccht, Nether-lands) was used for voltammetric measurements. A three-electrode system was used: a Pt counter electrode, anAg/AgCl reference electrode, and a Pt wire electrode asworking electrode. The potantiostat/galvonastat has an IR-compensation option. Therefore, the resistance due to theelectrode surface was compensated throughout the mea-surements. Oxygen-free nitrogen was bubbled through thesolution before each experiment. All experiments werecarried out at room temperature.

3. Results and discussion

3.1. Description of the structure of ZHES

A view of ZHES and its atom numbering schemes areshown in Fig. 1. The crystal structure consists of a complexcation, one squarate counter anion, and one solvent watermolecule. In the complex cation, the Zn(II) ion issandwiched by two bulky hydet-en ligands. Each hydet-en ligand acts as a tridentate ligand through the two Natoms and the hydroxyl O atom, resulting in a six

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Fig. 1. The molecular structure of ZHES, showing the atom-numbering

scheme. Displacement ellipsoids are drawn at the 40% probability level

and H atoms are shown as small spheres of arbitrary radii.

Table 2

Interatomic bond distances (A) and angles (1) around metal ion (ZHES)

Bond distances (A)

Zn1–O1 2.2584(14)

Zn1–O2 2.4158(16)

Zn1–N1 2.0820(14)

Zn1–N2 2.1273(14)

Zn1–N3 2.1041(15)

Zn1–N4 2.0725(14)

N1–C2 1.475(2)

N1–C3 1.469(2)

O3–C9 1.245(2)

O4–C10 1.242(2)

O5–C11 1.240(2)

O6–C12 1.255(2)

C9–C10 1.465(2)

C10–C11 1.472(2)

C11–C12 1.456(2)

Bond angles (1)

O1–Zn1–O2 81.11(6)

O1–Zn1–N1 76.78(5)

O1–Zn1–N2 156.84(6)

O1–Zn1–N3 92.88(6)

O1–Zn1–N4 94.00(5)

O2–Zn1–N1 94.20(6)

O2–Zn1–N2 88.70(6)

O2–Zn1–N3 157.75(5)

O2–Zn1–N4 74.78(5)

N1–Zn1–N2 83.34(6)

N1–Zn1–N3 105.32(6)

N1–Zn1–N4 166.76(6)

N2–Zn1–N3 103.85(6)

N2–Zn1–N4 103.37(6)

N3–Zn1–N4 84.37(6)

Fig. 2. Three-dimensional structure of ZHES. Dashed lines indicate the

hydrogen bonds.

I. Uc-ar et al. / Journal of Physics and Chemistry of Solids 68 (2007) 45–52 47

coordinate Zn(II) ion. The coordination geometry aroundthe Zn(II) ion is irregular and indicates a distortedoctahedral geometry. The equatorial plane is constructedby the coordination of the two primary amines [Zn–N ¼ 2.1042(14)–2.1271(14) A] and two hydroxyl O atoms[Zn–O ¼ 2.2591(13)–0.4163(15) A] of hydet-en ligands,whereas the secondary amines occupy the axial position

[Zn–N ¼ 2.0722(14)–0.0819(14) A]. The Zn–N bonds ofprimary amines are slightly longer than the Zn–N bonds ofsecondary amines and Zn–O distances are significantlydifferent from each other; this may be a consequence ofsteric constraints arising from the shape of the hydet-enligands. There is also a significant tetragonal distortion ofthe equatorial plane [maximum atomic deviation of�0.3945(8) A for atom O2], in which the Zn atom is0.0149(6) A out of this mean plane. The squarate counter-anion is planar with the r.m.s. deviation of 0.0137 A [themaximum deviation from the mean plane is 0.0218(12) Afor atom O5] and intramolecular bond distances are foundto be almost the same as is in 8-aminoquinoliniumhemisquarate [27] and picolinamidium squarate [28]. Thedihedral angle between the squarate plane and theequatorial plane is 81.71(4)1. The O–C bond distances arein the range 1.240(2)–1.254(2) A in squarate anion, whilethe unique C–C bond distances are in the range1.455(2)–1.472(2) A. These bond lengths agree well withthe fact that the squarate anion, which possesses apronounced degree of delocalization, is considered to bearomatic (Table 2).The crystal packing of ZHES is stabilized by inter-

molecular O–H?O and N–H?O hydrogen bondinginteractions (Fig. 2). The solvent water molecule, hydroxylO and amine N atoms link the complex cation to thesquarate dianion via hydrogen bonding interactions (seeTable 3 for details).

3.2. EPR investigation

The EPR spectra of Cu2+ doped ZHES single crystalsexhibit four lines when the magnetic field is along the a*c

plane and perpendicular to c-axis as in Fig. 3a. For other

ARTICLE IN PRESSI. Uc-ar et al. / Journal of Physics and Chemistry of Solids 68 (2007) 45–5248

orientations two sets of four hyperfine lines are observed inthe EPR spectrum (Fig. 3b). The 63Cu lines are notdistinguishable in a*c and bc planes and cannot be clearlyobservable in all spectra due to overlapping. The g2

variation of a specific line with respect to rotation anglein each plane must fit to the expression

g2kðyÞ ¼ g2

ii cos2 yi þ g2

jj sin2 yj þ 2g2

ij sin yi cos yj, (1)

where i; j; k ¼ x; y; z cyclically laboratory coordinates and yis the rotation angle. g2

ii; g2jj ; and g2

ij are the g tensorelements which will be found by fitting [29].

The spectra, obviously, belong to Cu2+, for which S ¼ 12

and I ¼ 32. The Cu2+ spectra can be fitted to the spin

Table 3

Hydrogen bonding interactions for ZHES (A,1)

D—H?A d(D–H) d(H?A) d(D?A) +(DHA)

N2–H2C?O7i 0.900 2.211 3.061(2) 157.33

N2–H2D?O4ii 0.900 2.073 2.966(2) 171.66

N3–H3C?O5iii 0.900 2.171 2.955(2) 145.24

N3–H3D?O4iv 0.900 2.062 2.957(2) 173.32

O1–H1?O6iii 0.72(3) 1.90(3) 2.616(2) 168(4)

N1–H9?O5iv 0.84(2) 2.01(2) 2.829(2) 166.3(17)

O2–H2?O7i 0.77(3) 1.97(3) 2.715(3) 164(3)

N4–H10?O3ii 0.82(2) 2.04(2) 2.835(2) 164.(3)

O7–H7C?O3i 1.04(4) 1.72(4) 2.701(2) 156(3)

O7–H7D?O6 1.03(4) 1.82(4) 2.704(3) 142(3)

Simetri kodları: (i) �x+1, �y+2, (ii) �z+1; �x+1, �y+1, �z+1; (iii)

�x+1, y�1/2, �z+1/2; (iv) x+1, y, z.

Fig. 3. (a) EPR spectrum of Cu2+ doped ZHES single crystal when the magne

and is away 1501 from the a*-axis.

Hamiltonian

H ¼ b gxxHxSx þ gyyHySy þ gzzHzSz

� �þ AzzIzzSz þ AyIySyAxIxSx ð2Þ

which includes only electron Zeeman and hyperfineinteraction. Nuclear Zeeman, nuclear quadruple andspin–orbit interactions are neglected.In order to find the g and A values, we have used an

iterative numerical technique [30]. After the calculation, g

and A tensors were constructed and diagonalized to findprincipal g and A values. The results for Cu2+ ioncomplexes are given in Table 4. Since Cu2+ has a 3d9

configuration and gz4gx4gy, the electron must be inx2 � y2�� �

orbital.The variation of g2 when the magnetic field is rotated in

the bc, a*b, and a*c planes for every 101 orientation of themagnetic field is shown in Fig. 4. It can be apparently seenfrom the variation that when the magnetic field is in the a*c

plane, the spectrum of two complex sites coincides which isconsistent with monoclinic symmetry. The complex sitesare easily resolved from the variation of lines in crystallineplanes shown in Fig. 4.Fig. 5 shows the powder and simulated EPR spectrum of

Cu2þ doped ZHES. The values measured from the powderspectrum agree with the single crystal data (Table 4). Thesimulation made using experimental values gives nearly thesame spectrum. The observed rhombic symmetry of the g

and A tensors can be explained in terms of the distortedoctahedral coordination around Cu2+ ion.

tic field is in the a*c plane and is away 1501 from the c-axis, (b) a*b plane

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Table 4

Principal g and A values of Cu2þdoped in ZHES single crystal at room temperature (Dg ¼ � 0:002; DA ¼ � 3G)

Site g Direction cosinesHyperfine A(G)

Direction cosines

a* b c a* b c

I gxx ¼ 2.118 �0.655 0.594 0.465 Axx ¼ 72.1 0.076 �0.627 0.774

gyy ¼ 2.060 �0.067 �0.660 0.748 Ayy ¼ 44.0 �0.384 0.698 0.603

gzz ¼ 2.252 0.752 0.458 0.472 Azz ¼ 97.7 0.919 0.344 0.187

II gxx ¼ 2.093 0.679 0.482 0.552 Axx ¼ 72.6 �0.011 0.422 �0.906

gyy ¼ 2.058 �0.042 �0.725 0.686 Ayy ¼ 46.1 0.180 0.892 0.113

gzz ¼ 2.253 0.732 �0.489 �0.472 Azz ¼ 96.3 0.983 �0.158 0.085

Powder gxx ¼ 2.116, gyy ¼ 2.062, gzz ¼ 2.251 Axx ¼ 72.1, Ayy ¼ 45.7, Azz ¼ 98.0

Fig. 4. Angular variations of the g2 values of all lines in three mutually perpendicular planes of Cu2+ doped ZHES single crystal.

I. Uc-ar et al. / Journal of Physics and Chemistry of Solids 68 (2007) 45–52 49

These results suggest that the Cu2+ enters into the Zn2+

site in ZHES single crystals assuming a monoclinicsymmetry. The ionic radius of Zn2+ (74 pm) is enoughfor the admission of Cu2+ (72 pm). Referring to theprincipal values of g in Table 4, gzz4gxx4gyy and R ¼

ðgxx � gyyÞ�ðgzz � gxxÞ is smaller than unity for the two

sites. Therefore, a 3dx2�y2 ground state for Cu2+ seems tobe predominant. Formulation of the admixture of thex2 � y2�� �

and 3z2 � r2�� �

orbitals in the ground statewavefunction of the Cu2+ in a rhombic symmetry hasbeen established and applied by many authors. The groundstate wavefunction, including also the covalency effect ofthe metal ion, is written [31] as

C ¼ a02h i1=2

a x2 � y2�� �

þ b 3z2 � r2�� �� �

, (3)

where a02 is the probability of finding the unpaired electronin the metal d orbital and it is a measure of the covalency.The normalization condition for mixing coefficient a and b is

a2 þ b2 ¼ 1. (4)

Using the experimental values for ZHES, the ground statewave function can be constructed for two complex sites as

CI ¼ ð0:430Þ1=2 0:984 x2 � y2

�� �þ 0:172 3z2 � r2

�� �� �, 5(a)

CII ¼ ð0:391Þ1=2 0:992 x2 � y2

�� �þ 0:119 3z2 � r2

�� �� �. 5(b)

The value of the covalency parameter a02(0.430)(Table 5) indicates that the unpaired electron spends57% of its time on the ligand orbitals, whereas therest is spent on the d orbitals of the Cu2+ ion for site I.Also, the coefficients of the dx2�y2 are greater than thatof thed3z2�r2 . This means that the unpaired electronspends most of its time on the dx2�y2 orbital of Cu2+ ion.The results of the site II are very close to site I.

3.3. Optical absorption

The room temperature optical absorption spectrum ofCu2+ doped ZHES single crystal in the wavelength range300–900 nm exhibited two bands in the visible range at320 nm (32000 cm�1) and at 656.8 nm (15234 cm�1). Thebroad band at 320 nm is assigned to (2B1g !

2B2g)dxy2dx2�y2 transition of Cu2+ ion in a distortedoctahedral site. The other band is assigned to a chargetransfer band between metal and ligands. By correlatingEPR and optical absorption data, the molecular orbitalcoefficients g2 and b21 can be expressed by using theformulae as follows:

g2 ¼7

12

Axx þ Ayy � 2Azz

P

�2ðgzz � geÞ �5

14ððgxx � geÞ þ ðgyygeÞ

, ð6Þ

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Fig. 5. (a) The powder EPR spectrum of Cu2+ doped ZHES single crystal

and (b) its computer simulation.

Table 5

Ground-state bond parameters for Cu2+ ion in ZHES crystal

Site a02 a b k

1 0.430 0.984 0.172 0.789

2 0.410 0.989 0.143 0.792

Powder 0.428 0.986 0.165 0.794

Table 6

The molecular orbital coefficients of Cu2+ doped ZHES single crystals at

room temperature. Some other results of related system are also included

for comparison

Lattice g2 b21 Reference

Zn site1 0.725 0.790 Present work

Zn site 2 0.723 0.795 Present work

Zn Powder 0.727 0.788 Present work

Sodium citrate 0.89 1.00 [22a]

Cadmium(II) formate diydrate 1.00 0.63 [32]

Cu/ZnNi(CN)4 0.99 0.76 [33]

Li Ba B 0.65 0.81 [34]

Na Ba B 0.65 0.74 [34]

K Ba B 0.65 0.73 [34]

½CdðCH3NH2CH2COOÞCl2� 0.94 0.79 [35]

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gzz ¼ 2:0023 14lg2b21

B1g2B2g

� �, (7)

where P is a constant (0.036 cm�1), l is the spin–orbitcoupling constant (�828 cm�1 for Cu2+ ion) and ge is thefree electron g value, (ge ¼ 2.0023). The parameters g2 andb21 can be taken as a measure of the in-plane s bonding andin-plane p bonding between d orbital of the central metalion and p orbitals of the ligands. Using differentpermutations of the signs of Ax, Ay, Az we have foundthat only Ax40, Ay40 and Azo0 give acceptable g2 value,We have then used the above equations to calculate g2, and

b21 values. The calculated values are given in Table 6. Theresults of some other works are also included in the tablefor comparison.Both g2 and b21 are smaller than unity and indicate the

covalent or ionic character of bonding between metal andligand orbitals. If their values are in the order b21 h g

2 itmeans that in-plane p bonding is more covalent than in-plane s bonding, otherwise s bonding is more covalent andp bonding becomes significantly ionic [32–35]. ReferringTable 6, it will be seen that the molecular orbitalcoefficients obtained by correlating EPR and optical dataindicate that in plane s and p bonding are nearly ioniccharacter.

3.4. FT IR results

Infrared spectra of squarate-containing complexes arequite characteristic of the mode of coordination. Theinfrared spectrum shows a very intense (Table 7) anddefined band at 1533 cm�1 assigned to a coupled mode COand CC stretching. The vibrational band analysis aroundthis region could be related to an extended electronicdelocalization over the oxocarbon ring, because thequantities and the shape of the bands depend on thesymmetry of the chemical species. In ZHES, this band isvery broad contrary to that in (NH4)2C4O4 [36], thus,suggesting the squarate ring has less symmetric. The crystalstructure data show that CC bonds of the anion isapproximately equal, as well as CO bands (as it can beseen from Table 2), which support the extended electronicdelocalization. The weak-intensity IR bands at 1708 and1643 cm�1 were taken as evidence for the presence oflocalized C¼O bands and C¼C bands, respectively.Characteristic IR absorption bands of the hydet-en ligand

were observed at 3325–3218, (2949–2868), 1616, 1469, and1067 cm�1 which originate from n(OH), n(NH), n(CH),d(NH), d(CH), and n(CO), respectively. Comparing the IRspectrum of the complex with the pure hydet-en ligand(Table 7), OH stretching vibration at 3627 cm�1 and NHstretching vibrations involved in the hydrogen bond at 3262,

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Table 7

The most characteristic IR bands of hydet-en ligand, squaric acid, ammonium squarate, and ZHES single crystal

Tentative assignment (cm�1) Hydet-en ligand Squaric acid Ammonium squarate (NH4)C4O4[36] Complex (ZHES)

n(MN) — — — 520 m

n(MO) — — — 544 m

n(CO) 1067 s — — 1031 s

n(CC) — 1057 m 1092 m 1099 m

d(CH) 1469 s — — —

d(NH) 1616 s — 1445 m 1448 m

n(CC)+n(CO) — 1530, 1516 sb 1533 s 1533 sb

n(C¼C) — 1618 m — 1643 m

n(C¼O) — 1818 w 1707 w 1708 m

n(CH) 2949 w — — 2952,2942,2919,2877 m

n(NH) 3218 m — 2887,3053,3119 m 3262,3214,3137 s

n(OH) 3325 b 3462 m — 3627 w

m: medium; s: strong; w: weak; b: broad; sb: strong and broad.

Fig. 6. The cyclic voltammogram of 5� 10�3M ZHES at a Pt wire

electrode in 0.1M DMF-Bu4NClO4, potential scan rate 100mV s�1.

I. Uc-ar et al. / Journal of Physics and Chemistry of Solids 68 (2007) 45–52 51

3214, and 3137cm�1 are seen. The n(OH) bond shifts tohigher frequency in the complex, probably due to weakerhydrogen bonding, and coordinating to metal atom. Inaddition, the NH stretching peaks in the spectrum split intothree separate bonds originating from non-equivalentcoordination positions of the ligand to central Zinc atom.The n(CH) bonds also show four splitting in the spectrum(2952, 2942, 2919, and 2877 cm�1), which might be attributedto the changing of vibration frequencies of the C1, C2, C3,and C4 (Fig. 1). The peak observed at 1031 cm�1 corre-sponds to n(CO). Also weak bands in the region 400–50 cm�1

are due to Zn–N and Zn–O stretching vibrations.

3.5. Voltammetric behavior

Studies on the electrochemical behavior of SQH2

(squaric acid) in acidic medium have already been reportedin the literature [37]. Furthermore, the dianion SQ2�

(squarate) and its monosubstituted derivatives can beoxidized reversibly in two consecutive steps to thecorresponding radical monoanions and neutral formsðE0

1 ¼ 0:120V and E02 ¼ 0:810V vs: Pt for SQ2�

Þ [38–40].The cyclic voltammogram of 5mM ZHES complex at a

platin wire electrode with dimethylformamide (DMF) asthe solvent is depicted in Fig. 6. No evidences of Zn2+

metal ion and hydet-en ligand oxidation or reduction in theselected potential range (from �1 to 1V) are observed.ZHES complex yields two oxidation peaks at 0.183 and0.589V in the anodic branch, and two reduction peaks at0.102 and 0.461V on the reverse scan (Fig. 6). As alreadymentioned, these redox couples correspond to two succes-sive monoelectronic oxidations and reductions of thedianion SQ2�. The first step is relating to oxidation ofthe dianion into the radical-anion,

SQ2�3SQ�� þ e� ðE1=2 ¼ 0:142V vs: Ag=AgClÞ: (8)

The second step corresponds to oxidation of the radicalanion into the neutral form,

SQ��3SQþ e� ðE1=2 ¼ 0:525V vs: Ag=AgClÞ. (9)

The second electron transfer on the ligand induces a lossof p delocalization as in the electrooxidation of thesquarate dianion SQ2� (or C4O4

2�) into the instabletetraacetone C4O4.As can be seen in Fig. 6, the ratio of peak current

(Ipc/Ipa) is 0.560 for Eq. (8) and 0.371 for Eq. (9). However,the peak current increases with the increase of the squareroot of the scan rate (50–500mV s�1). The Ip/v

1/2 value isalmost constant for all scan rates. This establishes theelectrode process as diffusion controlled. DEp( ¼ Epa�Epc)can provide a rough evaluation of the degree of thereversibility of electron transfer reaction. DEp of the peaksis greater than 59/nmV, where n is the number of theelectrons transferred, and increases with increasing scanrate. Therefore, the electron transfer process is not inequilibrium and shown to be slow. From all the voltam-metric data, the redox couples Eq. (8) and (9) can beattributed to the electrode reaction of the dianion SQ2�

within a quasi-reversible 1e-transfer process.

ARTICLE IN PRESSI. Uc-ar et al. / Journal of Physics and Chemistry of Solids 68 (2007) 45–5252

4. Supplementary data

Crystallographic data (excluding structure factors) forthe structure in this paper have been deposited with theCambridge Crystallographic Data Center as the supple-mentary publication no. CCDC 607370. Copies of the datacan be obtained, free of charge, on application to CCDC,12 Union Road, Cambridge, CB12 1EZ, UK, fax: +441223 366 033, e-mail: deposit@ccdc.ac.uk or on the worldwide web: /http://www.ccdc.cam.ac.ukS.

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