Synthesis and characterization of new unsymmetrical ‘side-off’ tetra and hexa coordinate homobinuclear Cu(II) and heterobinuclear Cu(II)–Zn(II) complexes: Magnetic, electrochemical and kinetic studies

Spectrochimica Acta Part A 94 (2012) 334–339

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Synthesis and characterization of new unsymmetrical ‘side-off’ tetra and hexa coordinate homobinuclear Cu(II) and heterobinuclear Cu(II)–Zn(II) complexes: Magnetic, electrochemical and kinetic studies K. Shanmuga Bharathi a,b , S. Sreedaran c , A. Kalilur Rahiman d , V. Narayanan a,∗ a

Department of Inorganic Chemistry, School of Chemical Sciences, University of Madras, Guindy Campus, Chennai 600 025, India Mahendra Institute of Technology, Mallasamudram 637 503, Namakkal, Tamilnadu, India c Department of Chemistry, Government Arts College, Udhagamandalam 643 002, India d PG and Research Department of Chemistry, The New College (Autonomous), Chennai 600 014, India b

a r t i c l e

i n f o

Article history: Received 18 August 2011 Received in revised form 1 March 2012 Accepted 9 March 2012 Keywords: Unsymmetrical ‘Side-off’ complexes Homo and hetero binuclear Magnetic properties Electrochemistry Catalytic activity

a b s t r a c t A new class of phenol based unsymmetrical side-off tetra and hexa coordinate homobinuclear Cu(II) and heterobinuclear Cu(II)–Zn(II) complexes have been synthesized and characterized by elemental and spectral analysis. The electronic spectra of all the complexes show “Red shift” in LMCT band, for the ligand H2 L2 compared to that of the ligand H2 L1 due to the relatively higher electron donating nature of their substitutents. The homobinuclear Cu(II) complexes (1 and 2) illustrate an antiferromagnetic interaction (eff : 1.58 and 1.60 BM) at 298 K with a broad EPR signal. Variable temperature magnetic moment study of the binuclear copper (II) complexes shows that the extent of antiferromagnetic coupling is greater in the case of H2 L2 complexes than H2 L1 complexes (−2 J values: 192 cm−1 and 184 cm−1 respectively). The heterobinuclear Cu(II)–Zn(II) complexes (3 and 4) have a magnetic moment value close to the spin only value with four hyperfine EPR signals. Electrochemical studies of the complexes reveal that all the binuclear complexes show two irreversible one-electron transfer reduction waves in the cathodic region. There is an “anodic shift” in the first reduction potential of the complexes, of the ligand H2 L1 when compared to that of the ligand H2 L2 due to the presence of relatively higher electron donating N-substituents in the later case than in the former case. The catecholase activity of the complexes reveals that the homobinuclear Cu(II) complexes show higher catalytic activity than the corresponding heterobinuclear Cu(II)–Zn(II) complexes. In the hydrolysis of 4-nitrophenylphosphate, the heterobinuclear Cu(II)–Zn(II) complexes show better catalytic activity than the corresponding homobinuclear Cu(II) complexes. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Designing phenol based unsymmetrical dicompartmental ligands makes much interest in the recent years because of their potential applications as models for dinuclear metallobiosites and also in catalytic oxidation and hydrolysis reactions [1–4]. They also posses two different coordination environments and their capability to bind two similar or dissimilar metal centers in close proximity which may leads to magnetic interaction between the two metal centers [3,5,6]. Most of the metallobiosites present in nature are having binuclear entities and they are also showing chemical or geometrical or metal asymmetry. Copper containing proteins are involved in distinct process in living systems. The catecholase activity of the binuclear Cu(II) complexes was studied to identify functional as

∗ Corresponding author. Tel.: +91 44 2230 0488; fax: +91 44 2230 0488. E-mail address: [email protected] (V. Narayanan). 1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2012.03.022

well as structural models for type III copper enzymes, having antiferromagnetically coupled binuclear active sites, like tyrosinase (hydroxylation of monophenols and oxidation of catechols) and catechol oxidase (oxidation of catechols) [7]. Hydrolysis of phosphorous esters has received considerable attention due to its significant role in many crucial enzymatic reactions [8] and due to the detoxification of pesticides (such as acetyl-cholinesterase) and chemical warfare agents. Moreover, most of the enzymes responsible for the metabolism of phosphate compounds require divalent metal ions for activation [9] and they have been shown to have the metal ions present in the active site during the catalytic cycle [10]. Since dicompartmental complexes such as end-off, side-off and macrocycles that possess endogenous phenoxide bridge have been proved as more suitable to mimic the activities of metallobiosites [3,11–13], synthesis of such compartmental complexes is much more essential. At present, though variety of symmetrical systems [14–18] is well established there are only very limited number of unsymmetrical systems reported in the literature [3,11–13,19]. Moreover, the synthesis of few distinct types of tetra and hexa

K. Shanmuga Bharathi et al. / Spectrochimica Acta Part A 94 (2012) 334–339

coordinated side-off binuclear complexes were reported [20,21] but they are not used much for such enzyme mimic activities. In this study, we have reported the synthesis of a new class of dicompartmental tetra and hexa coordinated side-off homo binuclear Cu(II) and hetero binuclear Cu(II)–Zn(II) complexes through template method. Among the two compartments, the amine compartment which provides four coordinating sites, consists of two tertiary nitrogens and two phenolic oxygens (N2 O2 ), whilst the imine compartment which provides six coordinating sites, comprises of two imine nitrogens, two tertiary nitrogens and two phenolic oxygens (N4 O2 ). All the complexes were characterized by spectral, electrochemical and magnetic studies and they were also subjected to catacholase activity and hydrolysis of pnitrophenylphosphate. The effect of N-substitution at the side arms of the complexes on spectral, electrochemical, magnetic and catalytic studies was also discussed. 2. Experimental 2.1. Analytical and physical measurements Elemental analysis of the complexes was obtained using Haereus C, H, N; rapid analyzer. The atomic absorption spectral data were recorded using Varian spectra AA-200 model atomic absorption spectrophotometer. FAB mass spectra were obtained on a JEOL SX-102 Mass spectrometer. IR spectra were recorded on a Shimadzu FT-IR 8300 series spectrophotometer on KBr disks in the range 4000–400 cm−1 . Electronic spectral studies were carried out on a Hitachi 320 spectrophotometer in the range 200–1100 nm. X-band ESR spectra were recorded at 25 ◦ C on a Varian EPR-E 112 spectrometer using diphenylpicrylhydrazine (DPPH) as the reference. Room temperature magnetic moment was measured on a PAR vibrating sample magnetometer Model-155. Variable temperature magnetic moment was measured on an EG & G Princeton applied research VSM model 4500. Molar conductivity was measured by using an Elico digital conductivity bridge model CM-88 using freshly prepared solution of the complex in dimethylformamide. Cyclic voltammograms were obtained on CHI600A electrochemical analyzer. The measurements were carried out under oxygen free condition using a three-electrode cell in which a glassy carbon electrode was the working electrode, a saturated Ag/AgCl electrode was the reference electrode and platinum wire was used as the auxiliary electrode. A ferrocene/ferrocenium (1+) couple was used as an internal standard and E1/2 of the ferrocene/ferrocenium (Fc/Fc+ ) couple under the experimental condition is 470 mV. Tetra (n-butyl)ammonium perchlorate (TBAP) was used as the supporting electrolyte. The catalytic oxidation of catechol to quinone by the copper complexes and the hydrolysis of 4-nitrophenylphosphate by the copper and zinc complexes were studied in a 10−3 M dimethylformamide solution. The reaction was followed spectrophotometrically by choosing the strongest absorbance of o-quinone at 390 nm and monitoring the increase in the absorbance and the hydrolysis of p-nitrophenylphosphate was monitored by following the UV absorbance change at 420 nm (assigned to the 4-nitrophenolate anion) as a function of time. A plot of log(A␣ /A␣ − At ) vs time was made for each complexes and the rate constant for the catalytic oxidation and the hydrolysis of 4-nitrophenyl phosphate was calculated. 2.2. Materials Formaldehyde solution, piperazine, N,N-dimethylpropane-1,3diamine, N,N-diethylpropane-1,3-diamine and p-cresol were purchased from Qualigens and used as such. Methanol, acetonitrile and dimethylformamide were purchased from Qualigens and

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distilled before the usage. TBAP used as supporting electrolyte in electrochemical measurement was purchased from Fluka and recrystallized from hot methanol. (Caution! All perchlorate salts are explosive and hence care should be taken while handling.) All other chemicals and solvents were of analytical grade and were used as received without any further purification. 2.3. Synthesis of precursor compounds The precursor compounds PC-1 (5-methyl salicylaldehyde) [22] and PC-2 (2,2 -diformyl-4,4 -dimethyl-6,6 -[piperazine-1,4-diylbis-(methylene)]bis phenol) [23] were synthesized by following the literature procedure. 2.4. Synthesis of side-off mononuclear complexes The mononuclear copper(II) complexes were synthesized according to the reported procedure [23] by refluxing the mixture of precursor compound PC-2 and copper(II) acetate (1:1 molar ratio) in chloroform-methanol (30:70, v/v). 2.5. Synthesis of side-off binuclear complexes: 2.5.1. Synthesis of side-off homobinuclear complexes 2.5.1.1. [Cu2 L1 ](ClO4 )2 ·H2 O. A solution of the side-off mononuclear copper(II) complex (0.89 g, 0.002 mol) in chloroform (20 ml) was added to Cu(OAc)2 ·H2 O (0.40 g, 0.002 mol) in MeOH (40 ml) and then N,N-dimethylpropane-1,3-diamine (0.50 ml, 0.004 mol). The mixture was refluxed for 11 h. Then the mixture was reacted with two equivalents of NaClO4 dissolved in 10 ml of methanol and refluxed further for an hour. A dark green precipitate was collected on evaporation of the resulting solution to half the volume and then allowed to cool by standing at room temperature (25 ◦ C). A dark green colored compound was obtained on recrystallization from acetonitrile. Yield: 1.32 g (74%). FAB mass (m/z): 891 (M+ ). Analytical data for C32 H50 N6 O11 Cl2 Cu2 : Calculated: C, 43.05; H, 5.64; N, 9.41, Cu, 14.23; Found: C, 43.12; H, 5.75; N, 9.36, Cu, 14.27 (%). Selected IR data (KBr): 3455 cm−1 ␯(OH), 1620 cm−1 ␯(C N), 1100 cm−1 [␯(ClO4 − ) uncoordinated], 651 cm−1 [␯(ClO4 − ) uncoordinated], conductance (m /S cm2 mol−1 ) in DMF: 162, g: 2.10, eff : 1.58 BM. 2.5.1.2. [Cu2 L2 ](ClO4 )2 ·H2 O. Complex [Cu2 L2 ](ClO4 )2 ·H2 O was synthesized by following the above procedure by using mononuclear copper(II) complex, N,N-diethylpropane-1,3-diamine and Cu(OAc)2 ·H2 O. A dark green compound was obtained on recrystallization from acetonitrile. Yield: 1.46 g (77%). Anal. Calc. for C36 H58 N6 O11 Cl2 Cu2 : C, 45.57; H, 6.16; N, 8.86, Cu, 13.39. Found: C, 45.64; H, 6.25; N, 8.81, Cu, 13.45 (%). Selected IR data (KBr): 3460 cm−1 ␯(OH), 1624 cm−1 ␯(C N), 1105 cm−1 [␯(ClO4 − ) uncoordinated], 653 cm−1 [␯(ClO4 − ) uncoordinated], conductance (m /S cm2 mol−1 ) in DMF: 165, g: 2.10, eff : 1.60 BM. 2.5.2. Synthesis of side-off hetero binuclear complexes 2.5.2.1. [CuZnL1 ](ClO4 )2 ·2H2 O. A solution of the side-off mononuclear copper(II) complex (0.89 g, 0.002 mol) in chloroform (20 ml) was added to Zn(OAc)2 ·4H2 O (0.44 g, 0.002 mol) in MeOH (40 ml) and then N,N-dimethylpropane-1,3-diamine (0.50 ml, 0.004 mol). The mixture was refluxed for 11 h. Then it was reacted with two equivalents of NaClO4 dissolved in 10 ml of methanol and refluxed further for an hour. A dark green precipitate was collected on evaporation of the resulting solution to half the volume and then allowed to cool by standing at room temperature (25 ◦ C). A dark green colored compound was obtained on recrystallization from acetonitrile. Yield: 1.29 g (71%). FAB mass (m/z): 911 (M+ ). Anal. Calc. for C32 H52 N6 O12 Cl2 CuZn: C, 42.11; H, 5.74; N, 9.21;

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2+

N N

OH O

OH O

Cu(ClO4)2.6H2O CH3OH, 12h, reflux

O

N

(i) M(ClO4)2.6H2O

O

(ii) H2N(CH2)3NR2

Cu N

O

CH3OH, 12h, reflux

O

where, M = Cu, Zn R = -CH3 -C2H5 H2L1 H2L2

O

N M1 N

N M

O

2

N

R N R N R R

Complexes M1

M2

1, 2

Cu

Cu

3, 4

Cu

Zn

Scheme 1. Synthesis and characterization of new unsymmetrical ‘side-off’ tetra and hexa coordinate binuclear complexes.

Cu, 6.96; Zn, 7.16. Found: C, 42.18; H, 5.79; N, 9.15; Cu, 6.92; Zn, 7.19(%). Selected IR data (KBr): 3465 cm−1 ␯(OH), 1621 cm−1 ␯(C N), 1101 cm−1 [␯(ClO4 − ) uncoordinated], 652 cm−1 [␯(ClO4 − ) uncoordinated], conductance (m /S cm2 mol−1 ) in DMF: 155. 2.5.2.2. [CuZnL2 ](ClO4 )2 ·2H2 O. Complex [CuZnL2 ](ClO4 )2 ·2H2 O was synthesized by following the synthetic procedure of [CuZnL1 ](ClO4 )2 ·2H2 O by using N,N-diethylpropane-1,3-diamine instead of N,N-dimethylpropane-1,3-diamine. A green compound was obtained on recrystallization from acetonitrile. Yield: 1.43 g (74%). Anal. Calc. for C36 H60 N6 O12 Cl2 CuZn: C, 44.63; H, 6.24; N, 8.67; Cu, 6.56; Zn, 6.75. Found: C, 44.68; H, 6.30; N, 8.65; Cu, 6.60; Zn, 6.78(%). Selected IR data (KBr): 3445 cm−1 ␯(OH), 1621 cm−1 ␯(C N), 1100 cm−1 [␯(ClO4 − ) uncoordinated], 651 cm−1 [␯(ClO4 − ) uncoordinated], conductance (m /S cm2 mol−1 ) in DMF: 159. 3. Results and discussion A new series of phenol based unsymmetrical side-off type tetra and hexa coordinated homobinuclear Cu(II) and heterobinuclear Cu(II)–Zn(II) complexes have been synthesized through template method (Scheme 1) and characterized. Their spectral, magnetic, electrochemical and catalytic behaviors are discussed and compared on the basis of the electron donating nature of N-substituents at the side arms of the phenolic ring. 3.1. Spectral analysis The FT IR spectra of all the side-off homo and hetero binuclear complexes show a broad band in the range 3445–3465 cm−1 which indicates the presence of ␯(OH) of water molecules. The peak due to coordinated water molecules was not observed at 1650 cm−1 . This indicates that the water molecules present in the complexes are all uncoordinated to the metal centers [24]. The absence of a peak around 3395 cm−1 in all the complexes indicates the absence of phenolic ␯(OH) due to deprotonation followed by complexation [25]. The peak due to C N is observed in the range 1620–1640 cm−1 . The presence of uncoordinated perchlorate anions in all the binuclear complexes are inferred from single broad band around 1100 cm−1 (␯3 -antisymmetric stretching) which are not split and a band around 650 cm−1 (␯4 -antisymmetric bending). The band around 930 cm−1 (␯2 -symmetric stretching) due to

coordinated perchlorate is not observed and this clearly indicates that no perchlorate ions are coordinated in the complexes [26]. The electronic spectra of the side-off binuclear complexes were recorded in methanol and the data are shown in Table 1. The absorption spectra of the complexes exhibit three main features: one intense peak below 300 nm assigned for the intraligand charge transfer transition, a peak or shoulder in the range 350–375 nm due to phenolate to metal charge transfer transitions [27] and the d–d transition for homo binuclear Cu(II) complexes is observed around 705 nm. The d–d band in the higher wavelength region is due to the six coordination, possibly octahedral geometry [28,29], for the Cu(II) center present in the imine compartment, that is, N4 O2 compartment. The d–d transition for hetero binuclear Cu(II)–Zn(II) complexes observed around 605 nm is due to 2 Eg → 2 T2g transition [30,31] of Cu(II) present in N2 O2 compartment. It seems interesting to compare the LMCT band for the complexes of the ligands H2 L1 and H2 L2 . A red shift is observed in the LMCT band for the complexes of the ligand H2 L2 (354–375 cm−1 ) when compared to that of the H2 L1 (350–369 cm−1 ). This is due to the difference in the electron donating nature of the N-substituents present in the side arms of the complexes. The higher electron donating N-substituents, C2 H5 groups, in the complexes of H2 L2 , compared to the relatively lower electron donating N-substituents, CH3 groups, in the complexes of H2 L1 , increases the electron density around the metal center present in the imine compartment. This decreases the Lewis acidity and causes “Red shift” in the LMCT band i.e., to the lower energy region [32–34]. The same trend was observed in the LMCT band for the hetero binuclear Cu(II)–Zn(II) complexes of the ligand H2 L2 when compared to that of the ligand H2 L1 .

Table 1 Electronic spectral data of side-off homo and heterobinuclear complexes. No.

Complexes

1 2 3 4

[Cu2 L1 ](ClO4 )2 ·H2 O [Cu2 L2 ](ClO4 )2 ·H2 O [CuZnL1 ](ClO4 )2 ·2H2 O [CuZnL2 ](ClO4 )2 ·2H2 O

max , nm (ε, M−1 cm−1 ) d–d

LMCT

Intraligand

702 (126) 708 (131) 602 (129) 608 (135)

355 (18 900) 361 (20 400) 369 (18 900) 375 (17 300)

268 (32 400) 272 (30 500) 273 (27 800) 276 (26 500)

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Table 2 Electrochemical dataa of side-off homo and hetero binuclear complexes (reduction process). No.

Complexes

1 Epc (V)

2 Epc (V)

1 2 3 4

[Cu2 L ](ClO4 )2 ·H2 O [Cu2 L2 ](ClO4 )2 ·H2 O [CuZnL1 ](ClO4 )2 ·2H2 O [CuZnL2 ](ClO4 )2 ·2H2 O

−0.72 −0.86 −0.93 −1.06

−1.10 −1.14 −1.31 −1.35

1

a Measured by CV at 50 mV s−1 . E vs Ag/AgCl conditions: GC working and Ag/AgCl reference electrodes; supporting electrolyte TBAP; concentration of complex 1 × 10−3 M, concentration of TBAP 1 × 10−1 M.

Fig. 1. Temperature dependence magnetic properties for homobinuclear Cu(II) complex [Cu2 L1 ](ClO4 )2 ·H2 O (1).

3.2. ESR spectra The solid state ESR spectra of the homo binuclear copper(II) complexes [Cu2 L1 ](ClO4 )2 ·H2 O, [Cu2 L2 ](ClO4 )2 ·H2 O and hetero binuclear [CuZnL1 ](ClO4 )2 ·2H2 O, [CuZnL2 ](ClO4 )2 ·2H2 O complexes were recorded in the X-band region at room temperature (25 ◦ C). A broad spectrum with no hyperfine splitting was obtained for both the homo binuclear copper(II) complexes 1 and 2 with g = 2.1 centered at 3200 G, indicating the presence of an antiferromagnetic interaction between the two copper ions and the ESR spectra of the hetero binuclear complexes [CuZnL1 ](ClO4 )2 ·2H2 O (3) and [CuZnL2 ](ClO4 )2 ·2H2 O (4) resemble the mononuclear copper(II) complexes which show four lines with a nuclear hyperfine spin 3/2 having gll values of 2.21 and 2.24, g⊥ values of 2.05 and 2.08, and All values of 147 and 150 cm−1 respectively [35]. 3.3. Magnetic studies The magnetic moment studies of the side-off homo binuclear copper(II) complexes were carried out at room temperature (25 ◦ C). The room temperature magnetic moment values of homo binuclear copper(II) complexes 1 and 2 are, eff : 1.58 and 1.60 BM respectively and they are found to be less than the spin only value (1.73 BM). This low magnetic moment values at room temperature suggest the presence of exchange interaction between the two copper(II) ions in the complex. In order to find out the magnetic exchange interaction, the magnetic moment of the binuclear copper(II) complexes were measured in the temperature range 77–300 K and it is shown in Fig. 1. The temperature dependence of the magnetic property of the complex was interpreted using the modified Bleany–Bowers equation [36]. m =



Ng 2 ˇ2 3 + exp 3kT

 −2J −1 kT

(1 − P) +

side arms of the complexes. The binuclear copper(II) complexes said above show max at 708 and 702 nm respectively. This small difference shows that the N-substituent does not have much influence on the structure of the complexes which may have the same geometry. So the change in the magnetic exchange interaction is mainly influenced by the “electron donating nature” of the N-substituents of the side arms of the complexes. The higher electron donating C2 H5 N-substituent in the complexes of the ligand H2 L2 , compared to the relatively lower electron donating CH3 N-substituent in the complexes of H2 L1 , increases the electron density on the copper centers [32,33] in the former complexes and hence the antiferromagnetic coupling becomes more predominant. Increase in the electron density strengthen the Cu O bond and increases the spin exchange interaction [37–39]. It shows that the antiferromagnetic interaction increases with increasing electron donating nature of the N-substituent. Hence, the −2 J value of the complex [Cu2 L2 ](ClO4 )2 ·H2 O is higher than that of the complex [Cu2 L1 ](ClO4 )2 ·H2 O (192 and 184 cm−1 respectively). 3.4. Electrochemical studies The conductivity measurement for all the side-off homo and hetero binuclear complexes was carried out using freshly prepared solution of the complexes in DMF. Conductance values of the binuclear complexes are in the range 150–170 (m /S cm2 mol−1 ) indicating that they are of 1:2 electrolyte type [40]. Electrochemical properties of the complexes were studied by cyclic voltammetry in the potential range 0 to −1.8 V in DMF containing 0.1 M TBAP as supporting electrolyte and the data are summarized in Table 2. The electronegativity and hard nature of the phenoxide ligands influence the electrochemical properties of the complexes [41,42]. The cyclic voltammograms for the homo and hetero binuclear complexes show two step irreversible one electron transfer reductions (Figs. 2 and 3). Controlled potential electrolysis was also carried out for all the complexes at 100 mV s−1 more negative to the cathodic peak, consumed one electron per molecule (n = 0.94), which represents that

0.45P + N˛ , T

where, m = molar paramagnetic susceptibility per mole corrected for diamagnetism using Pascal’s constant and all other symbols have their usual meanings. The effective magnetic moment was calculated using the relationship,  = 2.828 (m × T)1/2 . The antiferromagnetic exchange interaction (−2 J) values observed for the binuclear copper complexes are comparable. A higher −2 J value is observed for the complex [Cu2 L2 ](ClO4 )2 ·H2 O (192 cm−1 ) than the complex [Cu2 L1 ](ClO4 )2 ·H2 O (184 cm−1 ). This higher −2 J value may be due to the change in geometry around Cu(II) ions and also due to the effect of N-substitution in the

Fig. 2. Cyclic voltammograms of side-off homobinuclear Cu(II) complexes: (a) [Cu2 L1 ](ClO4 )2 ·H2 O (1) and (b) [Cu2 L2 ](ClO4 )2 ·H2 O (2) (reduction process).

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Fig. 3. Cyclic voltammograms of side-off heterobinuclear Cu(II)–Zn(II) complexes: (a) [CuZnL1 ](ClO4 )2 ·2H2 O (3) and (b) [CuZnL2 ](ClO4 )2 ·2H2 O (4) (reduction process).

each wave corresponds to one electron transfer process. So, the two reduction processes are assigned generally as follows, MII MII → MII MI → MI MI The electrochemical studies of similar type of complexes reveal that the metal(II) ion present in the imine compartment reduces at a lesser potential (i.e., first reduction potential) compared to the other metal(II) ion present in the piperazine compartment [6,25,43]. In the complexes reported here, the M(II) ion present in the imine compartment (N4 O2 compartment) reduces at −0.72 to −1.10 V and −0.93 to −1.05 V in homo binuclear and hetero binuclear complexes respectively. The M(II) ion present in the piperazine compartment (N2 O2 compartment) reduces at −1.10 to −1.58 V and −1.31 to −1.55 V in homo binuclear and hetero binuclear complexes respectively. The higher reduction potential is due to the influence of the electron density on the M(I) ion in the imine compartment through exchange interaction between the two metal ions through bridging sites [25]. It is remarkable to compare the reduction potentials for the complexes of the ligands H2 L1 with H2 L2 . The first reduction potential for the complexes of the ligand H2 L1 (−0.72 to −0.98 V) is lower than that of the complexes of the ligand H2 L2 (−0.86 to −1.10 V). This “anodic shift” is due to the presence of relatively higher electron donating C2 H5 groups as N-substituents in the complexes of the ligand H2 L2 when compared to the lower electron donating CH3 groups as N-substituents in the complexes of the ligand H2 L1 . This is because, the higher electron donating N-substituent, C2 H5 , in the complexes of the ligand H2 L2 increases the electron density around the metal ion and make difficult the reduction [32,33,44,45]. The same trend was observed in the reduction potentials of the hetero binuclear complexes.

Fig. 4. Catecholase activities of side-off homobinuclear Cu(II) and heterobinuclear Cu(II)–Zn(II) complexes: (a) [Cu2 L1 ](ClO4 )2 ·H2 O (1), (b) [Cu2 L2 ](ClO4 )2 ·H2 O (2), (c) [CuZnL1 ](ClO4 )2 ·2H2 O (3) and (d) [CuZnL2 ](ClO4 )2 ·2H2 O (4). Table 3 Catecholase activitya and hydrolysis of 4-nitrophenylphosphate.a No.

Complexes

1 2 3 4

[Cu2 L1 ](ClO4 )2 ·H2 O [Cu2 L2 ](ClO4 )2 ·H2 O [CuZnL1 ](ClO4 )2 ·2H2 O [CuZnL2 ](ClO4 )2 ·2H2 O

Rate constant (k) (×10−3 min−1 ) Catecholase

Hydrolysis of NPP

12.40 11.70 5.80 5.20

11.40 10.60 10.70 10.20

Concentration of the complexes: 1 × 10−3 M. Concentration of 4-nitrophenylphosphate: 1 × 10−1 M. Concentration of pyrocatechol: 1 × 10−1 M. a Measured spectrophotometrically in DMF.

3.5.2. Hydrolysis of 4-nitrophenylphosphate All the homo and hetero binuclear complexes synthesized so for were subjected for hydrolysis activity. The course of the reaction was followed by monitoring the growth at 420 nm band of the product 4-nitrophenolate anion for nearly 45 min at regular time intervals and the slope was determined by the initial rate method. Plots of log(A␣ /A␣ − At ) vs time for hydrolysis of 4nitrophenylphosphate activity of the complexes are shown in Fig. 5. A linear relationship for all the complexes shows a first-order dependence on the complex concentration for the systems and the observed initial rate constant value for the binuclear complexes is given in Table 3.

3.5. Kinetic studies 3.5.1. Oxidation of pyrocatechol (catecholase oxidation) All the homobinuclear Cu(II) complexes and the heterobinuclear Cu(II)–Zn(II) complexes were subjected for catecholase activity. The course of the reaction was followed spectrophotometrically at 390 nm for nearly 45 min at regular time intervals of 5 min and the slope was determined by the method of initial rates by monitoring the growth of 390 nm band of the product o-quinone. A linear relationship for initial rate and the complex concentration obtained for the copper(II) complexes shows a first-order dependence on the complex concentration for the systems. A Plot of log(A␣ /A␣ − At ) vs time for catecholase activity of the homo binuclear Cu(II) and hetero binuclear Cu(II)–Zn(II) complexes are obtained and shown in Fig. 4. The observed initial rate constant values are also reported in Table 3.

Fig. 5. Hydrolysis of 4-nitrophenylphosphate by side-off homobinuclear Cu(II) and heterobinuclear Cu(II)–Zn(II) complexes: (a) [Cu2 L1 ](ClO4 )2 ·H2 O (1), (b) [Cu2 L2 ](ClO4 )2 ·H2 O (2), (c) [CuZnL1 ](ClO4 )2 ·2H2 O (3) and (d) [CuZnL2 ](ClO4 )2 ·2H2 O (4).

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The structural features and electrochemical properties are important factors in determining the catalytic activity of the complexes. It is certainly noteworthy to say that the reactivity of the complexes differs significantly by varying the N-substituents of the side arms of the phenolic ring. The catalytic activity of the complexes of the ligand H2 L1 (M2 L1 ) is higher than that of the ligand H2 L2 (M2 L2 ). This is due to the higher electron donating Nsubstituent, C2 H5 , which causes the reduction of the metal ion into more harder [32,33,43,44] and binds weakly with the substrate than the relatively lower electron donating N-substituent, CH3 . It is seen that if the reduction potential is too negative, the complex has a decreased catalytic activity due to a more difficult reduction to metal(I), and a more positive reduction potential of the complex gives a higher catalytic activity since the donor atoms stabilize metal(I) at the expense of metal(II) [46]. From the results obtained, it is found that the homobinuclear Cu(II) complexes show higher activity than their corresponding heterobinuclear Cu(II)–Zn(II) complexes in both catecholase activity and in the hydrolysis of 4-nitrophenylphosphate. It seems interesting that the catalytic activity of homobinuclear copper(II) complexes towards oxidation of pyrocatechol is more than that of the phosphate hydrolysis and the hetero binuclear Cu(II)-Zn(II) complexes show higher catalytic activity in the phosphate hydrolysis than that of the oxidation of pyrocatechol. 4. Summary A new class of phenol based unsymmetrical “side-off” tetra and hexa coordinated homobinuclear Cu(II) and heterobinuclear Cu(II)–Zn(II) complexes have been synthesized and characterized. Both the variable temperature magnetic and ESR spectral studies of the binuclear Cu(II) complexes exhibit the presence of magnetic exchange interaction between the two copper atoms. The complexes show a significant change in their characteristics by varying the nature of N-substituent at the free side arms of the phenolic ring. Higher electron donating N-substituent when compared to the relatively lower electron donating N-substituent at the side arms of the phenolic ring causes, (a) a “Red shift” in LMCT-charge transfer band in electronic spectra of the complexes and (b) an increase in antiferromagnetic interaction between the two copper(II) centers. And while decreasing the electron donating nature of the N-substituent causes anodic shift in the reduction potential of the metal. Acknowledgments Financial support from University Grants Commission, New Delhi and Central Drug Research Institute, Lucknow for recording FAB mass spectra are gratefully acknowledged. References [1] D.E. Fenton, H. Okawa, in: R.W. Hay, J.R. Dilworth, K.B. Nolan (Eds.), Perspectives on Bioinorganic Chemistry, vol. 2, Jai Press, London, 1993, pp. 81–137. [2] T.N. Sorrell, Tetrahedron 45 (1989) 3–68. [3] (a) M. Lubben, R. Hage, A. Meetsma, K. Byma, B.L. Feringa, Inorg. Chem. 34 (1995) 2217–2224; (b) S. Parimala, M. Kandaswamy, Inorg. Chem. Commun. 6 (2003) 1252–1254. [4] (a) W. Kaim, Rall, J. Angew. Chem 108 (1996) 47–64; (b) N. Kitajima, Y. Moro-Oka, Chem. Rev. 94 (1994) 737–757. [5] S. Uozumi, M. Ohba, H. Okawa, D.E. Fenton, Chem. Lett. 26 (1997) 673–674. [6] S. Karunakaran, M. Kandaswamy, J. Chem. Soc., Dalton Trans. (1995) 1851–1855. [7] (a) K. Moore, G.S. Vigee, Inorg. Chim. Acta 66 (1982) 125–130; (b) D. Bolus, G.S. Vigee, Inorg. Chim. Acta 67 (1982) 19–25. [8] (a) T. Hall, J.A. Porter, P.A. Blachy, D.J. Leahy, Nature 378 (1995) 212–216; (b) G.J. King, B. Zerner, Inorg. Chim. Acta 225 (1997) 381–388.

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