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Polyhedron 25 (2006) 2859–2868

Synthesis of new ‘end-off’ l-phenoxo and bis-l-acetatotri-bridged copper(II), nickel(II) and zinc(II) complexes:Spectral, magnetic, electrochemical and catalytic studies

K. Shanmuga Bharathi a, A. Kalilur Rahiman a, K. Rajesh a, S. Sreedaran a,P.G. Aravindan b, D. Velmurugan b, V. Narayanan a,*

a Department of Inorganic Chemistry, School of Chemical Sciences, University of Madras, Guindy Campus, Chennai, Tamil Nadu 600 025, Indiab Department of Crystallography and Biophysics, University of Madras, Guindy Campus, Chennai, Tamil Nadu 600 025, India

Received 23 December 2005; accepted 13 April 2006Available online 8 May 2006

Abstract

A new end-off type acyclic ligand with two N-methyl piperazine arms, 2,6-bis[(4-methyl piperazin-1-yl)]-4-formyl phenol [L], has beensynthesized by a simple Mannich reaction. The mono and binuclear complexes of Cu(II), Ni(II) and Zn(II) have been prepared. Thecomplexes were characterized by elemental and spectral analysis. The EPR spectrum of the mononuclear copper complex shows fourhyperfine splittings and the binuclear complex shows a broad signal due to an anti-ferromagnetic interaction. The room temperaturemagnetic moment of the mono and binuclear copper complexes are found to be 1.72 lB (leff) and 1.58 lB (leff), respectively. The mono-nuclear Ni(II) complex is square planar and diamagnetic, whereas the six-coordinated binuclear Ni(II) complex shows a magneticmoment value of 2.98 lB (leff). Cyclic voltammetric studies evidenced one irreversible reduction wave for all the mononuclear complexesand two irreversible one-electron reduction waves for the binuclear complexes are obtained in the cathodic region. In the anodic region,one irreversible oxidation wave for the mononuclear nickel(II) complex and two irreversible oxidation waves for the binuclear nickel(II)complexes are obtained. Kinetic studies on the oxidation of pyrocatechol to o-quinone using the mono and binuclear copper(II) com-plexes as catalysts and on the hydrolysis of 4-nitrophenylphosphate using the mono and binuclear copper(II), nickel(II) and zinc(II) com-plexes as catalysts were carried out. The binuclear complexes have higher rate constant values than those of the correspondingmononuclear complexes. The rate constant values for the complexes for the hydrolysis are in the order nickel(II) > copper(II) > zinc(II).� 2006 Elsevier Ltd. All rights reserved.

Keywords: Crystal structure; ‘End-off’ ligand; Electrochemistry; Kinetic studies; Mono and binuclear metal complexes

1. Introduction

Transition metal complexes of multidentate ligands havebeen widely used in modern chemical applications includ-ing catalyst development [1–5], supramolecular self-assem-bly [6–10], anion recognition [11–14] and molecularelectronics [15,16]. Transition metal ions provide structuralrigidity to complexes of multidentate ligands through var-ious coordination geometries [9,17], and they can also serve

0277-5387/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2006.04.022

* Corresponding author. Tel./fax: +91 44 2230 0488.E-mail address: vnnara@yahoo.co.in (V. Narayanan).

as centers for non-covalent interactions [18,19,13] andchemical transformations [20,21].

Bimetallic cores are versatile at the active sites of manymetalloenzymes and model studies using simple binuclearmetal complexes are becoming increasingly important tounderstand the biological significance of such bimetalliccores. For this purpose binucleating compartmentalligands, especially of the ‘end-off’ type having a phenolicor alcoholic oxygen as an endogenous bridge, have oftenbeen used for modelling bimetallic biosites, because theyprovide l-phenoxo-l-carboxylato- and l-phenoxo-di(l-carboxylato)dimetal cores relevant to the active sites of

Table 1Summary of crystallographic data for ligand (L)

Empirical formula C19H33Cl3N4O17

Formula weight (g mol�1) 695.84Temperature (K) 293(2)Wavelength (A) 0.71073Crystal system triclinicSpace group P�1Unit cell dimensions

a (A) 9.848(3)b (A) 10.679(5)c (A) 15.645(8)a (�) 93.50(4)b (�) 99.96(4)c (�) 104.55(3)

V (A3) 1559.1(12)Z 2Dcalc (Mg m�3) 1.482Absorption coefficient (mm�1) 0.373F(000) 724Crystal size (mm) 0.60 · 0.32 · 0.25h Range for data collection (�) 1.33–28.97Index ranges �13 6 h 6 13,

�14 6 k 6 14,0 6 l 6 21

Reflections collected/unique (Rint) 8758/8257 (0.075)Refinement method full-matrix least-squares on F2

Data/restraints/parameters 8257/0/404Goodness-of-fit on F2 1.438Final R indices [I > 2r(I)] R1 = 0.119, wR2 = 0.345R indices (all data) R1 = 0.162, wR2 = 0.376Largest difference in peak

and hole (e A�3)1.442 and �0.683

2860 K.S. Bharathi et al. / Polyhedron 25 (2006) 2859–2868

some bimetallic enzymes [22–24]. The catecholase activityof binuclear Cu(II) complexes was studied to identify func-tional as well as structural models for type III copperenzymes like tyrosinase and catechol oxidase [25–27].Hydrolysis is found to be the key process in the active sitesof a number of enzymes, namely urease, carbon monoxidedehydrogenase, superoxide dismutase, etc., mediated bynickel ions [28] and carboxy peptidase, alkaline phospha-tase and b-lactanase mediated by zinc ions [29,30]. Thehydrolysis of phosphate mono-, di- and tri-esters by metalcomplexes such as Cu(II), Zn(II), Cd(II), etc., are used inthe development of artificial nucleases for use in moleculargenetics and genetic engineering [31–33]. A variety of ‘end-off’ binuclear complexes containing an endogenous pheno-lato bridge with either one or two exogenous bridgingunits, such as acetato, hydroxo, azido and cyano groups,have already been reported [34–38].

Thus, in the present work, we have synthesized a new‘end-off’ binucleating ligand (L) by the Mannich base reac-tion. The ligand contains a highly reactive aldehyde group,which may lead to the extension of its structure into asupramolecular moiety. Synthesis and electrochemicalstudies of such supramolecular moieties are also in pro-gress. The structure of ligand (L) has been solved by theX-ray crystallographic method. The ligand has been usedto prepare Cu(II), Ni(II) and Zn(II) mononuclear com-plexes and l-phenoxo endogenous and bis-l-acetato exog-enous tribridged binuclear complexes. The spectral,electrochemical, magnetic and catalytic studies of thesecomplexes are also discussed.

2. Experimental

2.1. Physical measurements

Elemental analysis of the complexes was obtained usinga Haereus C, H, N rapid analyzer. 1H NMR spectra wererecorded using a JEOL GSX 400 MHz NMR spectrome-ter. Electronic spectral studies were carried out on a Hit-achi 320 spectrophotometer in the range 200–800 nm. IRspectra were recorded on a Shimadzu FT-IR 8300 seriesspectrophotometer on KBr disks in the range 4000–400 cm�1. Mass spectra were obtained on a JEOL DX-303 Mass spectrometer. Molar conductivity was measuredby using an Elico digital conductivity bridge model CM-88using a freshly prepared solution of the complex indimethyl formamide. Cyclic voltammograms wereobtained on CHI-600A electrochemical analyzer. The mea-surements were carried out under oxygen free conditionsusing a three-electrode cell in which a glassy carbon elec-trode was the working electrode, a saturated Ag/AgCl elec-trode was the reference electrode and a platinum wire wasused as the auxiliary electrode. A ferrocene/ferrocenium(1+) couple was used as an internal standard and E1/2 ofthe ferrocene/ferrocenium (Fc/Fc+) couple under theexperimental conditions is 470 mV. Tetra(n-butyl)ammo-nium perchlorate (TBAP) was used as the supporting elec-

trolyte. The room temperature magnetic moment wasmeasured on a PAR vibrating sample magnetometerModel-155. The variable-temperature magnetic momentwas measured on an EG&G Princeton applied researchVSM model 4500. X-band ESR spectra were recorded at25 �C on a Varian EPR-E 112 spectrometer using diph-enylpicrylhydrazine (DPPH) as the reference. The atomicabsorption spectral data were recorded using a Varianspectra AA-200 model atomic absorption spectrophotome-ter. The catalytic oxidation of catechol to o-quinone by thecopper complexes and the hydrolysis of 4-nitrophenylphos-phate by the copper, nickel and zinc complexes were stud-ied in a 10�3 M dimethylformamide solution. The reactionwas followed spectrophotometrically by choosing thestrongest absorbance of o-quinone at 390 nm and monitor-ing the increase in the absorbance, and the hydrolysis ofp-nitrophenyl phosphate was monitored by following theUV absorbance change at 390 nm (assigned to the 4-nitro-phenolate anion) as a function of time. A plot of log(Aa/Aa � At) versus time was made for each complex and therate constant for the catalytic oxidation and the hydrolysisof 4-nitrophenyl phosphate was calculated.

The crystal data of ligand L were measured on a CAD4diffractometer using the x/2h scan mode. The stability ofthe crystal was checked by monitoring three standardreflections after every 200 reflections. Unit cell parameterswere determined by least-squares refinement of 18 reflec-

Table 2Selected bond lengths (A) and angles (�)

Atoms Bond length (A) Atoms Angle (�)

C1–O1 1.347(4) O2–C7–C4 124.5(4)C7–O2 1.219(6) C12–N1–C9 109.4(3)C8–N1 1.466(5) C10–N2–C11 109.8(3)C9–N1 1.488(5) C18–N3–C15 109.1(3)C10–N2 1.491(6) C16–N4–C17 109.7(3)C11–N2 1.494(6)C12–N1 1.456(5)C13–N2 1.492(6)C14–N3 1.499(5)C15–N3 1.509(5)C16–N4 1.492(5)C17–N4 1.504(6)C18–N3 1.501(4)C19–N4 1.545(6)

CHO

OH

CHO

OH

+ 2 HCHO + 2

Reflux, 24hCH3OH

N

N

N

N

CH3H3C

HN

N

CH3

Scheme 1.

K.S. Bharathi et al. / Polyhedron 25 (2006) 2859–2868 2861

tions in the range 7.29–13.52�. The ranges of h, k and l are�13–13, �14–14 and 0–21, respectively.

The structure was solved by direct methods using theprogram SHELXS-97 [39]. All non-hydrogen atoms wererefined anisotropically [39] by full-matrix least-squares pro-cedures with the weight w ¼ 1=½r2ðF 2

oÞ þ ð0:2000P Þ2�,where P ¼ ðF 2

o þ 2F 2cÞ=3. All hydrogen atoms were fixed

geometrically and allowed to ride along with the corre-sponding non-hydrogen atoms during the refinement. Thecrystal data and structure refinement details for ligand Lare given in Table 1, whereas selected bond lengths andangles are listed in Table 2.

2.2. Materials and safety note

Formaldehyde solution, N-methyl piperazine andp-hydroxybenzaldehyde were purchased from Qualigensand used as such. Methanol, acetonitrile and dimethylformamide were purchased from Qualigens and distilledbefore usage. TBAP, used as the supporting electrolyte inelectrochemical measurements, was purchased from Flukaand recrystallised from hot methanol. (Caution! TBAP ispotentially explosive; hence care should be taken in han-dling the compound). All other chemicals and solventswere of analytical grade and were used as received withoutany further purification.

2.3. Synthesis of the ligand, 2,6-bis[(N-methyl piperazine-1-

yl)methyl]-4-formyl phenol (L)

The ligand (L) was synthesized by taking p-hydroxy-benzaldehyde (4.27 g, 0.035 mol) in methanol (100 ml)mixed with N-methyl piperazine (7.76 ml, 0.07 mol) andthe solution was refluxed for 1 h. Then formaldehyde solu-tion (37%) (5.28 ml, 0.07 mol) was added slowly with stir-ring and the resulting solution was kept at thistemperature for 24 h. Two 5 ml portions of formaldehydewere added at approximately 8 h intervals. After that time,methanol was evaporated under vacuum and the resultingoil was extracted with chloroform and recrystallised with

diethylether (Scheme 1). A colourless compound wasobtained. Yield: 8.1 g (67%), m.p.: 96 �C. Mass (EI) m/z:346.47. Anal. Calc. for C19H30N4O2: C, 65.87; H, 8.73;N, 16.17. Found: C, 65.97; H, 8.62; N, 16.29%. SelectedIR data (KBr disc, m/cm�1): 1680 (C@O) s, 3398 (OH)br, 1456 (N–CH3) br. 1H NMR (d ppm in CDCl3) �2.22(s, 6H, N–CH3), �2.45 (broad S, 16H, methylene protons),�3.64 (s, 4H, benzylic protons), �7.3 (s, 2H, Ar–H), 9.8 (s,1H, –CHO proton). UV–Vis [kmax (nm) (e, M�1 cm�1)] inMeOH: 286 (21900).

2.4. Synthesis of the mononuclear complexes

2.4.1. Synthesis of the [CuL(OAc)] Æ H2O complexTo a 40 ml methanol solution of the ligand (L) (1.039 g,

0.003 mol) was added Cu(OAc)2 Æ H2O (0.598 g, 0.003 mol)dissolved in 20 ml of methanol, and the resulting solutionwas refluxed for 4 h. After the reaction was over, the solu-tion was filtered when hot and allowed to stand at roomtemperature. After slow evaporation of the solvent at25 �C, a dark green compound was obtained. The com-pound was recrystallised from acetonitrile. Yield: 1.32 g(75%). Anal. Calc. for [C21H32N4O4Cu] Æ H2O: C, 51.89;H, 7.05; N, 11.53; Cu, 13.07. Found: C, 51.81; H, 7.12;N, 11.47; Cu, 13.13%. Selected IR data (KBr disc, m/cm�1): 3449, 1680, 1450, 1103, 651. UV–Vis [kmax (nm)(e, M�1 cm�1)] in MeOH: 590 (125), 352 (17200), 288(23500), gi = 2.21, g^ = 2.09 and Ai = 180. leff = 1.72 BM.

2.4.2. Synthesis of the [NiL(OAc)] Æ H2O complexThe Ni(II) complex was prepared by following the

above procedure using the ligand (L) (1.039 g, 0.003 mol)

2862 K.S. Bharathi et al. / Polyhedron 25 (2006) 2859–2868

and Ni(OAc)2 Æ 4H2O (0.746 g, 0.003 mol). A light greencompound was obtained on recrystallisation from acetoni-trile. Yield: 1.38 g (79%). Anal. Calc. for [C21H32N4O4-

Ni] Æ H2O: C, 52.41; H, 7.12; N, 11.64; Ni, 12.20, Found:C, 52.44; H, 7.06; N, 11.67; Ni, 12.16%. Selected IR data(KBr disc, m/cm�1): 3451, 1680, 1453, 1106, 653. UV–Vis[kmax (nm) (e, M�1 cm�1)] in MeOH: 574 (143), 356(16100), 289 (21700).

2.4.3. Synthesis of the [ZnL(OAc)(H2O)2] complex

The Zn(II) complex was prepared by following the aboveprocedure using the ligand (L) (1.039 g, 0.003 mol) andZn(OAc)2 Æ 2H2O (0.658 g, 0.003 mol). A colourless com-pound was obtained on recrystallisation from acetonitrile.Yield: 1.25 g (71%). Anal. Calc. for [C21H32N4O4Zn-(H2O)2]: C, 49.85; H, 7.17; N, 11.07; Zn, 12.92. Found: C,49.81; H, 7.21; N, 11.15; Zn, 12.83%. Selected IR data(KBr disc, m/cm�1): 3451, 1680, 1455, 1108, 649. UV–Vis[kmax (nm) (e, M�1 cm�1)] in MeOH: 286 (21600).

2.5. Synthesis of the binuclear complexes

2.5.1. Synthesis of the [Cu2L(OAc)2](ClO4) Æ 2H2O

complex

A solution of the ligand (L) (0.693 g, 0.002 mol) inMeOH (40 ml) was added to Cu(OAc)2 Æ H2O (0.798 g,0.004 mol) in MeOH (20 ml), and the resulting solutionwas refluxed for 7 h. The mixture was then reacted withone equivalent of NaClO4 dissolved in 10 ml of methanoland refluxed for a further hour. A dark green precipitatewas collected on evaporation of the resulting solution tohalf the volume and then cooling by standing at room tem-perature (25 �C). A dark green coloured compound wasobtained on recrystallisation from acetonitrile. Yield:1.13 g (78%). Anal. Calc. for [C23H35N4O6Cu2](ClO4) Æ2H2O: C, 38.04; H, 5.41; N, 7.72; Cu, 17.50. Found: C,38.09; H, 5.36; N, 7.62, Cu; 17.38%. Selected IR data(KBr disc, m/cm�1): 3447, 1680, 1485, 1105, 654. Conduc-

M(OAc)2.xH2O

C

ON

N

CH3

M

OA

MeOH, 4hreflux

Complexes 1 2 3

M Cu N Z

y

y

CHO

OHN

N

N

N

CH3H3C

Scheme

tance (Km/S cm2 mol�1) in DMF 84. UV–Vis [kmax (nm)(e, M�1 cm�1)] in MeOH: 662 (118), 358 (15800), 284(24600). g = 2.1. leff = 1.58 BM.

2.5.2. Synthesis of the [Ni2L(OAc)2(H2O)2]ClO4 complex

The Ni(II) complex was prepared by following theabove procedure by using the ligand (L) (0.693 g,0.002 mol) and Ni(OAc)2 Æ 4H2O (0.995 g, 0.004 mol). Alight green compound was obtained on recrystallisationfrom acetonitrile. Yield: 1.04 g (73%). Anal. Calc. for[C23H35N4O6Ni2(H2O)2](ClO4): C, 38.56; H, 5.49; N,7.82; Ni, 16.38. Found: C, 38.43; H, 5.38; N, 7.97; Ni,16.33%. Selected IR data (KBr disc, m/cm�1): 3449, 1680,1467, 1104, 653. Conductance (Km/S cm2 mol�1) in DMF89. UV–Vis [kmax (nm) (e, M�1 cm�1)] in MeOH: 800(105), 618 (140), 561 (225), 363 (16700), 286 (23100).

2.5.3. Synthesis of the [Zn2L(OAc)2(H2O)2]ClO4 complex

The Zn(II) complex was prepared by following theabove procedure using the ligand (L) (0.693 g, 0.002 mol)and Zn(OAc)2 Æ 2H2O (0.878 g, 0.004 mol). A colourlesscompound was obtained on recrystallisation from acetoni-trile. Yield: 1.02 g (70%). Anal. Calc. for [C23H35N4O6-Zn2(H2O)2](ClO4): C, 36.07; H, 5.66; N, 7.31; Zn, 17.08.Found: C, 36.18; H, 5.54; N, 7.42; Zn, 17.05%. SelectedIR data (KBr disc, m/cm�1): 3451, 1680, 1475, 1108, 652.Conductance (Km/S cm2 mol�1) in DMF 81. UV–Vis [kmax

(nm) (e, M�1 cm�1)] in MeOH: 286 (22500).

3. Results and discussion

A new end-off type ligand has been synthesized by theusual procedure of the Mannich reaction (Scheme 1) andcharacterized. Using this end-off ligand, mono and binu-clear Cu(II), Ni(II) and Zn(II) complexes were preparedas shown in Scheme 2. The complexes were characterizedby spectral studies. Their magnetic, electrochemical andcatalytic behaviors are discussed.

HO

c

CHO

ON

N N

N

H3C CH3

M MOO

CH3

O O

CH3

+

(ii) NaClO4

(i) M(OAc)2.xH2O,

in

x 142

Complexes 4 5 6

MCu Ni Zn

x 142

y y

y--H2O

y-H2OH2O

N

N

CH3

MeOH, 4hreflux

2.

K.S. Bharathi et al. / Polyhedron 25 (2006) 2859–2868 2863

3.1. The structure of ligand (L)

Crystals of the ligand L of X-ray quality were obtainedfrom 1% perchloric acid in methanol by slow evaporationat room temperature. The ORTEP diagram of the ligandis shown in Fig. 1. The average Csp2 –Csp2 [1.395(2) A] inthe phenyl ring and Csp3 –N [1.481(2) A] bond distancesare comparable with the corresponding values reportedby Allen et al., in 1987 [40] [1.397(9) A and 1.499(18) A].The piperazine rings adopt a chair conformation, whichis confirmed by puckering parameters [41] and asymmetryparameters [42] [QT = 0.579(8) and h = 179.1(7)�]. The mir-ror and twofold symmetry pass through N1(DCs = 0.006(4)), C9 (DCs = 0.002(4)) and C10(DCs = 0.007(4)) atoms and N1–C12 (DCs = 0.006(3)),C9–N1 (DC2 = 0.004(3)) and C10–C9 (DC2 = 0.004(3))bonds, respectively. N–H� � �O, O–H� � �O and C–H� � �Ohydrogen bonds stabilize the crystal packing. The O8 atomof the perchlorate is involved in C–H� � �O hydrogen bonds[H11A� � �O8i = 2.53 A; C11� � �O8i = 3.380(1) A and C11–H11A� � �O8i = 134�; i = �1 + x, y, z] and the O9 atom isinvolved in bifurcated hydrogen bonds C14–H14A� � �O9� � �H15B–C15 = 42.7(1)�. Neighboring mole-cules linked through N–H� � �O hydrogen bonds run alongthe ‘ac’ plane, stabilizing the unit cell packing.

3.2. Spectral studies

The IR spectrum of the ligand shows a broad band at3398 cm�1 due to the presence of the phenolic OH group.A sharp band at 1680 cm�1 shows the presence of an aro-

Fig. 1. ORTEP diagram of the ligand L with atom labels and numbering schemlevel for the ORTEP plot is 35%.

matic aldehyde in the ligand and the fact it remainsunchanged in all the complexes indicates that the aromaticaldehyde group does not change on complexation. Theabsence of a peak around 3398 cm�1 in all the complexesindicates the absence of m(OH) due to deprotonation fol-lowed by complexation [43a]. The strong m(CO2) bands ataround 1450 cm�1 in the mononuclear complexes indicatesthe presence of an acetate linkage. The biacetato bridgedbinuclear complexes showed strong m(CO2) bands at1460–1490 cm�1 [43b]. A broad band in the region around3450 cm�1 indicates the presence of m(OH) of water mole-cules for all the complexes. The presence of uncoordinatedperchlorate anions in all complexes 1–6 are inferred from asingle broad band around 1100 cm�1 (m3-antisymmetricstretching), which is not split, and a band around650 cm�1 (m4-antisymmetric bending). The band around930 cm�1 (m2-symmetric stretching) due to coordinated per-chlorate is not observed and this vividly indicates that noperchlorate ions are coordinated in the complexes [44,45].

The electronic spectra of the mono and binuclear Cu(II)complexes and the mononuclear Ni(II) complex exhibitthree main transitions. The moderately intense peakobserved in the range 280–300 nm is probably due to anintra-ligand charge transfer transition [46]. A peak orshoulder in the range 350–365 nm is due to phenolato tocopper(II) charge transfer transitions [47]. The mononu-clear Cu(II) complex shows a kmax at 590 nm due to a d–dtransition, which is characteristic of a square planar geom-etry, and the d–d transition for the binuclear Cu(II) com-plex, observed at 662 nm, is indicative of a distortedsquare-pyramidal geometry [37,48]. The binuclear

e (perchlorate and water molecules are omitted for clarity). The probability

2864 K.S. Bharathi et al. / Polyhedron 25 (2006) 2859–2868

copper(II) complexes with exogenous diacetato bridgessuggest that the endogenous bridging phenolate oxygenadopts the more strained pyramidal geometry. The bridg-ing acetate groups in binuclear complexes present in asemi-bridging mode is primarily the result of the overallsteric strain induced by the ligand, perhaps combined withthe presence of two bridging acetate groups rather than one[37]. The mononuclear Ni(II) complex shows a single weakd–d band observed at 574 nm due to an 1A1g! 1A2g tran-sition and this indicates that the complex has a square pla-nar geometry [49,50]. The binuclear Ni(II) complex showsthree d–d transitions at 800 nm, 618 nm and 561 nmassigned to 3A2g! 3T2g, 3A2g! 3T1g, 3A2g! 3T1g(P) spinallowed electronic transitions, which are characteristic ofNi2+ in an octahedral site [51,52]. The mono and binuclearZn(II) complexes display only ligand centered charge trans-fer transitions in the UV region [53].

3.3. ESR spectra

The solid state ESR spectra of the copper(II) complexeswere recorded at room temperature in the X-band region.The ESR spectra of the mononuclear complex [CuL-(OAc)] Æ H2O and the binuclear complex [Cu2L(OAc)2]-ClO4 Æ 2H2O are shown in Fig. 2. The ESR spectra of themononuclear copper (II) complex shows four lines [54]with nuclear hyperfine spin 3/2 due to hyperfine splitting.The observed gi value is 2.21, g^ = 2.09 and Ai is 180. Abroad [55] spectrum with no hyperfine splitting wasobtained for the dicopper complex with g = 2.1, indicatingthe presence of an anti-ferromagnetic interaction betweenthe two copper ions. Owing to the favorable relative orien-tation of the magnetic orbitals and in spite of the Cu–Cuseparation, an anti-ferromagnetic interaction is observed.

3.4. Magnetic studies

The magnetic moment values of the mono and binuclearcomplexes were calculated at room temperature. The mag-

Fig. 2. ESR spectra of mononuclear: (a) [CuL(OAc)] Æ H2O (1) andbinuclear: (b) [Cu2L(OAc)2]ClO4 Æ 2H2O (4) complexes.

netic moment value for the mononuclear copper(II) com-plex [CuL(OAc)] Æ H2O is 1.72 BM, which is very close tothat of the spin only value (1.73 BM) expected for com-plexes having one copper(II) ion [56] with a single unpairedelectron sited in an essentially dx2�y2 orbital. The value ofthe magnetic moment observed at room temperature forthe binuclear copper (II) complex [Cu2L(OAc)2]-ClO4 Æ 2H2O is 1.58 BM. This confirms the presence of ananti-ferromagnetic interaction between the two copper(II)ions [57]. To evaluate the singlet–triplet energy separation(�2J), a variable-temperature magnetic study of the binu-clear copper complex (Fig. 3) was carried out in the tem-perature range 77–300 K and the experimental magneticsusceptibility values were fitted to the modified Bleaney–Bowers equation [58]

vm ¼ ðNg2b2=3kT Þ½3þ expð�2J=kT Þ��1ð1� P Þþ ð0:45P=T Þ þ N a;

where vm is the paramagnetic susceptibility per metal atomafter the correction for diamagnetism, N is Avogadro’snumber, b is the Boltzmann constant, g is the average gyro-magnetic ratio and P is the fraction of monomeric cop-per(II) impurities. �2J is the singlet–triplet energyseparation, Na is the temperature independent fraction,usually assumed to be 120 · 10�6 cgs units for copper (II)compounds. The value of the exchange integral �2J forthe binuclear copper complex is 134 cm�1, indicative ofweak antiferromagnetic coupling between the copper (II)ions. With few exceptions, magneto-structural correlations[59] for several phenoxo bridged dicopper complexes showthat the major factor controlling the exchange interactionsis the Cu–O–Cu bridge angle, however, other factors suchas degree of distortion from planarity [60] and the dihedralangle [61] between the two copper planes also have aninfluence on the extent of the spin–spin interaction.

The position of the maximum in vCu for the bis(l-ace-tato) complex occurs at much lower temperatures becauseof weak exchange coupling. The acetate groups in the binu-clear copper complex may connect the basal plane ðdx2�y2Þ

Fig. 3. Temperature-dependence magnetic properties for complex[Cu2L(OAc)2]ClO4 Æ 2H2O, where s is leff/lB, r is v.

Fig. 4. Cyclic voltammograms of the mono and binuclear nickel(II)complexes: (a) [NiL(OAc)] Æ H2O (2); (b) [Ni2L(OAc)2(H2O)2]ClO4 (5)(reduction process).

Fig. 5. Cyclic voltammograms of the mono and binuclear zinc(II)complexes: (a) [ZnL(OAc)(H2O)2] (3); (b) [Zn2L(OAc)2(H2O)2](ClO4) (6)(reduction process).

K.S. Bharathi et al. / Polyhedron 25 (2006) 2859–2868 2865

positions on one Cu with the apical ðdz2Þ positions on theother, which will lead to a low overall J value, probablybecause of Jferro contributions from these pathways [37].Any contributions from the counter complementary effectsof phenolate versus acetate bridging orbitals, proposed byReed et al. [62] and Nishida and Kida [63], will be small inthe complex because of the relative disposition of the Cuðdx2�y2Þ orbitals. The mononuclear Ni(II) complex has asquare planar geometry and is diamagnetic [56]. The binu-clear Ni(II) complex has a magnetic moment value of2.98 BM, characteristic of Ni2+ in an octahedral site[52,64].

3.5. Electrochemistry

3.5.1. Reduction process at negative potential

Conductivity measurements for the binuclear complexeswere carried out in DMF, which showed conductance val-ues in the range 80–90 (Km/S cm2 mol�1), indicating thatthey are of 1:1 electrolyte type [65]. The electrochemicalproperties of the complexes were studied by cyclic voltam-metry in the potential range 0 to �1.7 V in DMF, contain-ing 0.1 M TBAP as the supporting electrolyte, and the dataare summarized in Table 3. The electronegativity and hardnature of the phenoxide ligands influence the electrochem-ical properties of the complexes [66–68]. In general, thistype of end-off complexes shows irreversible reductionwaves in the cathodic potential region [37]. The cyclic vol-tammograms for mono and binuclear nickel(II) and zin-c(II) complexes are depicted in Figs. 4 and 5,respectively. Cyclic voltammograms with different scanrates show no corresponding anodic peak in the cathodicdirection, which indicates the electrode process is irrevers-ible in nature. The mononuclear copper(II), nickel(II)and zinc(II) complexes 1–3 show an irreversible one-elec-tron transfer reduction in the cathodic region. The binu-clear copper(II), nickel(II) and zinc(II) complexes 4–6

show two-step irreversible one-electron transfer reductions.Controlled potential electrolysis was also carried out forthe complexes at 100 mV s�1 more negative to the cathodicpeak, consuming one electron per molecule (n = 0.95),which reports that each wave corresponds to a one-electrontransfer process. So the two reduction processes areassigned generally as follows:

Table 3Electrochemical dataa for the complexes (reduction at cathodic potential)

Number Complexes E1pc ðVÞ E2

pc ðVÞ1 [CuL(OAc)] Æ H2O �0.742 [NiL(OAc)] Æ H2O �0.833 [ZnL(OAc)(H2O)2] �0.844 [Cu2L(OAc)2](ClO4) Æ 2H2O �0.76 �1.215 [Ni2L(OAc)2(H2O)2](ClO4) �0.73 �1.356 [Zn2L(OAc)2(H2O)2](ClO4) �0.74 �1.61

a Measured by CV at 50 mV/s. E vs. Ag/AgCl conditions: GC workingand Ag/AgCl reference electrodes; supporting electrolyte TBAP; concen-tration of complex 1 · 10�3 M, concentration of TBAP 1 · 10�1 M.

MIIMII!MIIMI!MIMI.

3.5.2. Oxidation process at anodic potential

The cyclic voltammogram of the nickel(II) complexes 2

and 4 in the anodic potential region is given in Fig. 6. Themononuclear Ni(II) complex shows an irreversible oxida-tion wave in the positive potential at +0.76 V. The binu-clear Ni(II) complex shows a two-step one-electrontransfer processes in the potential range +0.82 V and+1.40 V and both are found to be irreversible in nature.The two oxidation processes are assigned generally asfollows:

MIIMII!MIIIMII!MIIIMIII.

3.6. Kinetic studies

3.6.1. Oxidation of pyrocatechol (catecholase activity)

Measurement of the catecholase activity of both monoand binuclear copper(II) complexes synthesized in the pres-ent work was carried out using pyrocatechol as the conve-nient model substrate for the identification of functionalmodels for the metalloenzymes [25,26]. For this purpose,

Fig. 6. Cyclic voltammograms of the mono and binuclear nickel(II)complexes: (a) [NiL(OAc)] Æ H2O (2); (b) [Ni2L(OAc)2(H2O)2](ClO4) (5)(oxidation process).

Table 4Hydrolysis of 4-nitrophenylphosphatea and catecholase activitya

Number Complexes Rate constant, k

(·10�3 min�1)

NPP Catecholase

1 [CuL(OAc)] Æ H2O 4.60 6.402 [NiL(OAc)] Æ H2O 4.903 [ZnL(OAc)(H2O)2] 4.304 [Cu2L(OAc)2](ClO4) Æ 2H2O 11.10 12.805 [Ni2L(OAc)2(H2O)2](ClO4) 12.56 [Zn2L(OAc)2(H2O)2](ClO4) 9.60

a Measured spectrophotometrically in DMF. Concentration of thecomplexes: 1 · 10�3 M; concentration of 4-nitrophenylphosphate:1 · 10�1 M; concentration of pyrocatechol: 1 · 10�1 M.

2866 K.S. Bharathi et al. / Polyhedron 25 (2006) 2859–2868

10�3 mol dm�3 solutions of the complexes in dimethyl-formamide were treated with 100 equivalents of pyrocate-chol in the presence of air. The course of the reactionwas followed spectrophotometrically at 390 nm for nearly45 min at regular time intervals of 5 min. The slope wasdetermined by the method of initial rates by monitoringthe growth of the 390 nm band of the product o-quinone.A linear relationship for initial rate and the complex con-centration obtained for both the copper(II) complexesshows a first-order dependence on the complex concentra-tion for the systems.

Plots of log(Aa/Aa � At) versus time for catecholaseactivity of the mono and binuclear complexes are obtainedand shown in Fig. 7. The observed initial rate constantvalue for the mononuclear and binuclear copper(II) com-plexes is given in Table 4. It is found that the binuclear cop-per(II) complex show higher reactivity than itsmononuclear complex. This is due to the fact that the oxi-dation of pyrocatechol to o-quinone requires two metalions in close proximity [27]; hence the rate constant forthe binuclear copper complex is high.

0

0.2

0.

log

(Aα/

Aα–

At)

4

0.6

0.8

0 10 20 30 40 50

a

b

Time (min-1)

Fig. 7. Catecholase activity of the copper (II) complexes: (a) [CuL(OAc)]Æ H2O (1); (b) [Cu2L(OAc)2](ClO4) Æ 2H2O (4).

3.6.2. Kinetic studies of hydrolysis of 4-nitrophenylphosphate

The catalytic activity of the mono and binuclearcopper(II), nickel(II) and zinc(II) complexes on the hydroly-sis of 4-nitrophenylphosphate was determined spectrophoto-metrically by monitoring the increase of the 4-nitrophenolateanion characteristic absorption at 420 nm over the time inDMF at 25 �C. For this purpose, 10�3 mol dm�3 solutionsof the complexes in dimethylformamide and a 2.5 · 10�3 Msolution of tetramethylammonium hydroxide were treatedwith 100 equivalents of nitrophenylphosphate in the pres-ence of air.

The course of the reaction was followed at 420 nm fornearly 45 min at regular time intervals. The slope wasdetermined by the method of initial rates by monitoringthe growth of the 420 nm band of the product, the 4-nitro-phenolate anion. A linear relationship for all the complexesshows a first-order dependence on the complex concentra-tion for the systems. Plots of log(Aa/Aa � At) versus timefor the hydrolysis of 4-nitro phenyl phosphate activity ofthe complexes are obtained and shown in Fig. 8. Theobserved initial rate constant value for the mononuclearand binuclear copper(II) complexes is given in Table 4. It

0

0.2

0.4

0.6

0.8

0 10 20 30 40 50

ab

c

d

e

f

Time (min-1)

log

(Aα/

Aα–

At)

Fig. 8. Hydrolysis of 4-nitrophenylphosphate by copper(II), nickel(II) andzinc(II) complexes. (a) [CuL(OAc)] Æ H2O (1); (b) [NiL(OAc)] Æ H2O (2); (c)[ZnL(OAc)(H2O)2] (3); (d) [Cu2L(OAc)2](ClO4) Æ 2H2O (4); (e) [Ni2L-(OAc)2(H2O)2](ClO4) (5); and (f) [Zn2L(OAc)2(H2O)2](ClO4) (6).

K.S. Bharathi et al. / Polyhedron 25 (2006) 2859–2868 2867

is found that the binuclear complexes show higher reactiv-ity than their corresponding mononuclear complexes dueto the fact that this kind of mechanism requires two metalions in close proximity. The mechanism of action hasalready been explained with the help of model complexes[69,70].

On the comparison of the hydrolysis of 4-nitrophenyl-phosphate by the copper(II), nickel(II) and zinc(II) com-plexes, the nickel(II) complexes have a higher rateconstant value than the copper(II) and zinc(II) complexes.It is similar to the result of higher catalytic activity of theNi(II)(tren) complex than the copper(II)(tren) and zin-c(II)(tren) complexes reported by De Rosch and Trogler[32]. From the results, it is found that the catalytic activityof copper(II) complexes towards oxidation of pyrocatecholis more than that of the phosphate hydrolysis.

4. Supplementary material

Full crystal data and structure refinement details forligand L have been deposited with the Cambridge Crystal-lographic Data Centre, CCDC No. 272757. Copies of thisinformation may be obtained free of charge from TheDirector, CCDC, 12 Union Road, Cambridge CB2 1EZ,UK (fax: +44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk or www.ccdc.cam.ac.uk).

Acknowledgement

Financial support from the University Grants Commis-sion, New Delhi is gratefully acknowledged.

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