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Inorganica Chimica Acta 359 (2006) 4519–4525

Synthesis, characterization, and anion selectivity ofcopper(II) complexes with a tetradentate Schiff base ligand

Shouvik Chattopadhyay a, Michael G.B. Drew b, Ashutosh Ghosh a,*

a Department of Chemistry, University College of Science, University of Calcutta, 92, A.P.C. Road, Kolkata 700 009, West Bengal, Indiab School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6 6AD, UK

Received 25 March 2006; received in revised form 24 May 2006; accepted 26 May 2006Available online 9 June 2006

Abstract

A new mononuclear Cu(II) complex, [CuL(ClO4)2] (1) has been derived from symmetrical tetradentate di-Schiff base, N,N 0-bis-(1-pyridin-2-yl-ethylidene)-propane-1,3-diamine (L) and characterized by X-ray crystallography.The copper atom assumes a tetragonallydistorted octahedral geometry with two perchlorate oxygens coordinated very weakly in the axial positions.Reactions of 1 with sodiumazide, ammonium thiocyanate or sodium nitrite solution yielded compounds [CuL(N3)]ClO4 (2), [CuL(SCN)]ClO4 (3) or [CuL(NO2)]-ClO4 (4), respectively, all of which have been characterized by X-ray analysis.The geometries of the penta-coordinated copper(II) incomplexes 2–4 are intermediate between square pyramid and trigonal bipyramid (tbp) having the Addition parameters (s) 0.47, 0.45and 0.58, respectively.In complex 4, the nitrite ion is coordinated as a chelating ligand and essentially both the O atoms of the nitriteoccupy one axial site.Complex 1 shows distinct preference for the anion in the order SCN� > N3

� > NO2� in forming the complexes 2–

4 when treated with a SCN�=N3�=NO2

� mixture. Electrochemical electron transfer study reveals CuIICuI reduction in acetonitrilesolution.� 2006 Elsevier B.V. All rights reserved.

Keywords: Copper(II); Di-Schiff base; Crystal structure; Anion selection

1. Introduction

Encouragement for the syntheses of the symmetricalSchiff bases derived from 2-pyridyl carbonyl compoundsand for the study of their physical and chemical propertiesis provided by their potential abilities in biological modelingapplication, e.g. to mimic the N4 donor set of copper(II) insuperoxide dismutase (SOD) [1–7], a copper(II)–Zn(II)enzyme which destroys the toxic superoxide by catalyzingits disproportionation into peroxide and oxygen [8–12].The copper atom in SOD is located in the centre of a dis-torted square pyramid of five coordinating atoms, fournitrogen atoms of four histidylimidazolate residues fromthe base and the water molecule occupies the apical site.

0020-1693/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2006.05.029

* Corresponding author.E-mail address: ghosh_59@yahoo.com (A. Ghosh).

The hexa-coordinated Cu(II) ion with d9 configurationprefers distorted octahedral geometry which is a direct con-sequence of Jahn–Teller effect [13]. Thus, with a set of fourstrongly and two weekly coordinating ligands, the later twoalways occupy the axial positions. We would like to inves-tigate if any change in geometry around Cu(II) occur whenone of these weekly coordinating groups (e.g. perchlorateion) is replaced by another anion of stronger coordinatingability. We are also interested to study whether suchreplacement reaction shows any preference for one anionover others when treated with a mixture of anions. Toour knowledge, no study of such anion selection has beenreported in the literature till date.

In the present paper, we report the synthesis, character-ization and X-ray crystal structure analysis of four cop-per(II) complexes with a tetradentate symmetrical di-Schiffbase ligand, N,N 0-bis-(1-pyridin-2-yl-ethylidene)-propane-

(ClO4)2]

(1)

NH4SCN

NaN3 NaNO2

N

H3C N N

N

CH3

(L)

Cu(ClO4)2·6H2O

Methanol

Methanol

MethanolMethanol

[CuL

ClO4[CuL(N3)] ClO4[CuL(SCN)] ClO4[CuL(NO2)](3) (4)(2)

Scheme 1.

4520 S. Chattopadhyay et al. / Inorganica Chimica Acta 359 (2006) 4519–4525

1,3-diamine (L) (Scheme 1) and examine the preference andeffect of substitution of one of the perchlorate groups of[CuL(ClO4)2] by a SCN�; N3

� or NO2� ion.

2. Experimental

All chemicals were of reagent grade and were used with-out further purification.

Caution!!! Although no problems were encountered inthis work, perchlorate salts containing organic ligands arepotentially explosive. Only a small amount of the materialshould be prepared and they should be handled with care.

2.1. Preparations

2.1.1. Synthesis of the ligand L

The ligand (L) was prepared by condensation of 1,3-dia-minopropane (10 mmol, 0.84 cm3) and 2-acetylpyridine(20 mmol, 2.4 cm3) in methanol (25 cm3) under reflux for3 h. The Schiff base ligand was not isolated and the yellowcoloured methanolic solution was used directly for the syn-thesis of complex 1 [14].

2.1.2. Synthesis of complex 1The methanolic solution of the ligand L was cooled and a

methanolic solution of Cu(ClO4)2 Æ 6H2O (10 mmol, 3.7 g)was added into it and refluxed again for an additional 1 h.A bright blue crystalline compound was separated out oncooling. It was recrystallized from acetonitrile to obtainneedle shaped single crystals suitable for X-ray diffraction(yield: 3.80 g, 70%). Anal. Calc. for C17H20Cl2CuN4O8: C,37.62; H, 3.71; N, 10.32; Cu, 11.71. Found: C, 37.5; H,3.8; N, 10.4; Cu, 11.6%. UV–Vis: kmax (nm), (emax

(dm3 mol�1 cm�1)) (acetonitrile), 609 (109), 284 (8588).Magnetic moment, l = 1.76 BM.

2.1.3. Syntheses of complexes 2, 3 and 4Complexes 2, 3 and 4 were prepared by adding methanol

solution (10 cm3) of sodium azide (5 mmol, 325 mg),ammonium thiocyanate (5 mmol, 380 mg) or sodium nitrite(5 mmol, 345 mg), respectively, to the methanol solution(20 cm3) of 1 (5 mmol, 2.7 g) and the mixture was putunder reflux for 30 min. In all cases, green crystalline com-pounds started to separate within a few hours. Deep-greenneedle-shaped single crystals of 2, 3 and 4 suitable for X-ray diffraction were obtained by slow evaporation ofCH3CN solutions of the respective compounds in air.

Complex 2 (yield: 2.06 g, 85%). Anal. Calc. forC17H20ClCuN7O4: C, 42.07; H, 4.15; N, 20.20; Cu,13.09%; Found: C, 42.2; H, 4.0; N, 20.4; Cu, 13.0%. UV–Vis: kmax (nm), (emax (dm3 mol�1 cm�1)) (acetonitrile),827 (240), 286 (44168). Magnetic moment, l = 1.71 BM.

Complex 3 (yield: 1.88 g, 75%). Anal. Calc. forC18H20ClCuN5O4S: C, 43.11; H, 4.02; N, 13.97; Cu,12.67. Found: C, 43.2; H, 4.2; N, 13.9; Cu, 12.7%. UV–Vis: kmax (nm), (emax (dm3 mol�1 cm�1)) (acetonitrile),750 (211), 281 (14552). Magnetic moment, l = 1.78 M.

Complex 4 (yield: 1.54 g, 63%). Anal. Calc. forC17H20ClCuN5O6: C, 41.72; H, 4.12; N, 14.31; Cu, 12.99.Found: C, 41.8; H, 4.1; N, 14.5; Cu, 13.1%. UV–Vis: kmax

(nm), (emax (dm3 mol�1 cm�1)) (acetonitrile), 676 (132), 277(12912). Magnetic moment, l = 1.75 M.

2.2. Selectivity for the anions

Syntheses of the complexes 2–4 from 1 clearly indicatethat one of the weekly coordinated perchlorate ions canbe replaced easily by stronger coordinating anionic ligandsðN3

�; SCN� or NO2�Þ.

To investigate if there is any selectivity for this anionreplacement reaction, we did the following experiments: amethanolic solution (10 ml) of 1 (540 mg, 1 mmol) is mixedwith a methanolic solution (10 ml) of NaN3 (1 mmol,65 mg) and NH4SCN (76 mg, 1 mmol) and the resultingmixture was put under reflux for half an hour. The solutionwas evaporated in a water bath to half of its original vol-ume. On cooling, a green crystalline compound was sepa-rated out. The product was analyzed by elementalanalysis and IR spectra and found to be pure compound3 (yield: 350 mg, 72%). Thus, azide ligand remains as spec-tator while thiocyanate ion preferentially replaces the per-chlorate ion to produce compound 3 exclusively.Similarly, when a methanol solution of a mixture ofazide/nitrite (1 mmol of each) and thiocyanate/nitrite(1 mmol of each) was added separately to the methanolsolution of 1 (540 mg, 1 mmol) compounds 2 and 3, respec-tively, were the exclusive products (yield for 2: 371 mg,74%, yield for 3: 294 mg, 60%).

It is also observed that when methanol solution of com-pound 2 (1 mmol) is mixed with NH4SCN (76 mg,1 mmol), refluxed and cooled, it transforms into compound3 (yield 335 mg, 69%). Similarly, both thiocyanate andazide ions can replace nitrite from 4. On the other hand,

Table 1Crystal data and refinement details of complexes 1, 2, 3 and 4

Compound 1 2 3 4

Formula C17H20Cl2CuN4O8 C17H20ClCuN7O4 C18H20ClCuN5O4S C17H20ClCuN5O6

Formula weight 542.82 485.40 501.46 489.37Crystal system monoclinic triclinic monoclinic triclinicSpace group P21/c P�1 P21/c P�1Crystal colour black black black blackCrystal description needle needle needle needleCrystal dimensions (mm) 0.03 · 0.03 · 0.35 0.05 · 0.05 · 0.30 0.03 · 0.03 · 0.25 0.03 · 0.03 · 0.25Unit cell dimensions

a (A) 9.118(12) 8.649(9) 9.610(12) 8.367(9)b (A) 17.376(19) 8.651(9) 13.869(15) 9.258(11)c (A) 13.848(15) 13.636(15) 16.268(17) 13.257(14)a (�) 90 97.997(10) 90 90.282(10)b (�) 99.041(10) 92.668(10) 101.434(10) 93.516(10)c (�) 90 94.587(10) 90 96.721(10)

Volume (A) 2167(4) 1005.4 (18) 2125(4) 1018(2)Z 4 2 1 2Dcalc (g cm�1) 1.664 1.603 1.542 1.597Temperature (K) 293 293 293 293Radiation (A) 0.71073 0.71073 0.71073 0.71073l (Mo Ka) 1.3 1.3 1.2 1.2F(000) 1108 498 1013 502Number of unique data 3714 3561 3824 3557Number of data with [I > 2r(I)] 2892 3111 3488 3101R1, wR2 [I > 2r(I)] 0.1467, 0.2523 0.0484, 0.1275 0.0622, 0.1528 0.0542, 0.1333

S. Chattopadhyay et al. / Inorganica Chimica Acta 359 (2006) 4519–4525 4521

azide or nitrite cannot replace the thiocyanate ion from 2,and nitrite cannot replace azide from 3.

It is therefore clear that complex 1 shows distinct prefer-ence for the anion assimilation into its coordination spherein the order SCN� > N3

� > NO2� > ClO4

�. The order is inagreement (at least for NCS�ðor N3

�Þ > NO2� > ClO4

�)with the ligand field strength of the anions towards cop-per(II) [15]. The replacement of azide by thiocyanate, how-ever, indicates that other factors such as solubility of thecomplexes (3 < 2 < 4 in methanol) may also be responsiblefor such order.

2.3. Physical measurements

Elemental analysis (carbon, hydrogen and nitrogen) wasperformed using a Perkin–Elmer 240C elemental analyzer.IR spectra in KBr (4500–500 cm�1) were recorded using aPerkin–Elmer RXI FT-IR spectrophotometer. Electronicspectra in acetonitrile (1200–350 nm) were recorded in aHitachi U-3501 spectrophotometer. The magnetic suscepti-bility measurements were done with an EG & PAR vibrat-ing sample magnetometer, model 155 at room temperatureand diamagnetic corrections were made using Pascal’s con-stants. Electrochemical measurements were performedunder a dry nitrogen atmosphere in a three-electrode con-figuration using Pt disc working electrode, Pt auxiliaryelectrode and Ag/AgCl reference electrode on a PAR Vers-astat-2 electrochemistry system in this work and wereuncorrected for junction contribution. The value for theferrocenium–ferrocene couple under our conditions is0.39 V.

2.4. X-ray crystallography

Crystal data for the four crystals are given in Table 1.Data were measured with Mo Ka radiation using theMAR research Image Plate System. The crystals were posi-tioned at 70 mm from the Image Plate. 100 frames weremeasured at 2� intervals with a counting time of 2 min. Dataanalyses were carried out with the XDS program [16]. Thestructures were solved using direct methods with theSHELX-97 program [17]. Non-hydrogen atoms were refinedwith anisotropic thermal parameters. The hydrogen atomsbonded to carbon were included in geometric positionsand given thermal parameters equivalent to 1.2 times thoseof the atom to which they were attached. An empiricalabsorption correction was applied using DIFABS [18]. Thestructures were refined on F2 using SHELXL [17]. The crystalstructure illustrations were generated using ORTEP-3 [19].

3. Results and discussion

3.1. Syntheses

Facile condensation of 2-acetylpyridine with 1,3-diami-nopropane in a 2:1 molar ratio furnished the neutral tetrad-entate ligand N,N 0-bis-(1-pyridin-2-yl-ethylidene)-propane-1,3-diamine (L). Reaction of L with Cu(ClO4)2 Æ 6H2O indry methanol yielded neutral copper complex 1. The com-plex was further made to undergo reaction separately withanionic ligands: azide, thiocyanate, and nitrite to form pen-tacoordinated copper(II) complexes (2, 3 and 4, respec-tively) (Scheme 1).

4522 S. Chattopadhyay et al. / Inorganica Chimica Acta 359 (2006) 4519–4525

It is to be noted that the same ligand has been used pre-viously to synthesize the complex with copper(II) perchlo-rate, adopting a different procedure [5]. Interestingly, theresulting hydrated complex, [CuL(H2O)](ClO4)2 was a dis-torted square pyramid with the coordinated water moleculein the apical site. Thus a small change in the reaction con-dition leads to the formation of Cu(II) complexes havingdifferent geometry.

3.2. Description of the structures

3.2.1. Complex 1An ORTEP view of compound 1 together with the atom-

numbering scheme is shown in Fig. 1(a). The co-ordinationpolyhedron around the copper atom is best described as adistorted octahedron. The four nitrogen atoms [two iminenitrogen N(2), N(6) and two pyridine nitrogen, N(10),N(15)] of the tetradentate Schiff base ligand define the equa-torial plane. Two oxygen atoms, O(24) and O(30), of two

Fig. 1. ORTEP-3 diagrams of (a) complex 1 and (b) complex 2 with 20%thermal ellipsoid probability.

perchlorate groups are semi-coordinated in the trans axialpositions at distances of 2.587(11) and 2.603(10) A, respec-tively (Table 2). There is a slight distortion to the squareplane and the deviations of the coordinating atoms N(2),N(6), N(10), N(15) from the least-square mean planethrough them are �0.24(1), 0.24(1), �0.23(1) and0.22(1) A, respectively, and that of copper atom from thesame plane is �0.015(1) A. The conformation of the satu-rated six-membered ring is intermediate between boat andtwist boat with the puckering parameters [20,21]q3 = 0.069(17) A, q2 = Q = 0.281(18) A, h = 75.7(35)�, / =235(4)�. The N(2)–Cu(1)–N(6) angle is 97.4(5)� and is typi-cal of six-membered chelate [22–30].

It is to be noted that C(4), the middle carbon atom ofthe diamine chelate ring, has a high thermal parameter per-pendicular to the ring. We tried refining C(4) in two posi-tions with reduced occupancy but while this modelconverged satisfactorily, there was no decrease in R valueand therefore we consider that our original refinement isthe best that can be achieved and should be reported.The thermal parameters of that carbon atom in the otherreported complex are even higher and consequently thesix-membered diamine chelate ring is flatter and the twovicinal C–C diamine bonds are as short as 1.288 A [29].

3.2.2. Complexes 2 and 3The coordination polyhedra around the metal centres in

2 and in 3 are best described as elongated (4 + 1) square

Table 2Selected bond lengths (A) and bond angles (�) for complexes 1, 2, 3 and 4

Compound 1 2 3 4

Bond lengths (A)

Cu(1)–N(2) 1.967(11) 1.988(4) 2.013(5) 1.993(5)Cu(1)–N(6) 1.970(11) 2.020(4) 1.993(4) 1.993(4)Cu(1)–N(10) 1.980(10) 2.019(4) 2.077(5) 2.099(4)Cu(1)–N(15) 1.994(10) 2.104(4) 2.029(4) 2.018(4)Cu(1)–N(23) 2.089(5) 2.093(6)Cu(1)–O(24) 2.588(13) 2.351(6)Cu(1)–O(30) 2.603(12)Cu(1)–O(25) 2.498(8)

Bond angles (�)

N(2)–Cu(1)–N(6) 97.4(5) 92.4(1) 91.8(2) 91.8(2)N(6)–Cu(1)–N(10) 81.1(4) 79.7(1) 80.2(2) 79.9(1)N(10)–Cu(1)–N(15) 103.5(4) 99.0(1) 100.0(2) 99.2(1)N(15)–Cu(1)–N(2) 81.3(4) 79.6(1) 79.6(2) 79.3(1)N(2)–Cu(1)–N(10) 166.7(4) 138.3(2) 139.9(2) 132.9(2)N(6)–Cu(1)–N(15) 165.1(4) 166.4(1) 166.9(2) 167.0(2)N(23)–Cu(1)–N(2) 100.8(2) 114.9(2)N(23)–Cu(1)–N(6) 113.5(2) 94.8(2)N(23)–Cu(1)–N(10) 92.5(2) 105.0(2)N(23)–Cu(1)–N(15) 108.29(2) 97.7(2)O(24)–Cu(1)–O(25) 48.1(2)N(2)–Cu(1)–O(24) 95.3(2)N(6)–Cu(1)–O(24) 97.3(2)N(10)–Cu(1)–O(24) 131.7(2)N(15)–Cu(1)–O(24) 93.0(2)N(2)–Cu(1)–O(25) 142.6(2)N(6)–Cu(1)–O(25) 99.6(2)N(10)–Cu(1)–O(25) 84.4(2)N(15)–Cu(1)–O(25) 93.2(2)

Fig. 2. ORTEP-3 diagrams of (a) complex 3 and (b) complex 4 with 20%thermal ellipsoid probability. Perchlorate ion is not shown for clarity.

S. Chattopadhyay et al. / Inorganica Chimica Acta 359 (2006) 4519–4525 4523

pyramid with CuN5 chromophores, furnished by a tetrad-entate symmetrical di-Schiff base ligated by two iminenitrogen [N(2), N(6)] and two pyridine nitrogen [N(10),N(15)] forming the basal plane and the apical position iscoordinated by a nitrogen [N(23)] from azide ion in 2

[Fig. 1(b)] or thiocyanate ion in 3 [Fig. 2(a)]. Deviationsof the coordinating atoms N(2), N(6), N(10), N(15) fromthe least-square mean planes through them are �0.26,0.26, �0.24, 0.24 A, respectively, in complex 2 and �0.26,0.25, �0.23 and 0.23, respectively, in complex 3. This devi-ation from planarity of the equatorial plane is also manifestitself in the trans angles [N(2)–Cu(1)–N(10), N(2)–Cu(1)–N(10)], which are 138.3(2)� and 166.4(1)� in 2 and139.9(2)� and 166.9(2)� in 3, respectively. As is usual forsquare pyramid structures, the copper(II) ion is displaced0.46 A from the plane towards the apically coordinatednitrogen (N23) both in 2 and 3.

The s (Addition parameter) values [31] are 0.47 and 0.45in complexes 2 and 3, respectively, indicating the actualgeometries are in between trigonal bipyramid (tbp) andsquare pyramid. The puckering analysis [18] of Cu(1)–N(2)–C(3)–C(4)–C(5)–N(6) ring in 2 with q3 = 0.060(4) A,q2 = Q = 0.777(4) A, h = 85.6(3)�, / = 196.8(3)� and in 3

with q3 = 0.064(5) A, q2 = Q = 0.772(5) A, h = 85.3(4)(3)�,/ = 162.8(4)�, indicates the six-membered chelate ringsassume a boat conformation in 2 and a twist boat in 3.The N(2)–Cu(1)–N(6) angles are 92.4(2)� and 91.8(2)� in 2

and 3, respectively.The axial bond length Cu(1)–N(23) is 2.089(5) A in 2

and 2.093(6) in 3. The azide ion in 2 and thiocyanate ionin 3 are practically linear with N(23)–N(24)–N(25) angleof 176.92� and N(23)–C(24)–S(25) angle of 178.09�, respec-tively. The Cu(1)–N(23)–N(24) angle in 2 [Fig. 1(b)] andCu(1)–N(23)–C(24) angle in 3 [Fig. 2(a)] are 126.78� and172.45�, respectively, as usual for the coordinated azideand N-bonded thiocyanate ions.

3.2.3. Complex 4The structure of 4 shows a bonded nitrite anion in addi-

tion to the tetradentate ligand [Fig. 2(b)]. As is apparentfrom the figure, the oxygen atoms in this anion having highthermal motion and this may be an indication of disorder.The four nitrogen atoms N(2), N(6), N(10), N(15) deviate0.32, �0.31, 0.29 and �0.29 A, respectively, from the meanplane with the Cu(1) atom is 0.52 A away from the sameplane towards the apically coordinated nitrite. The two oxy-gen atoms of the nitrite ligand are at a distance of 2.351(7)and 2.498(8) A from the metal. The small value (48.12) ofthe angle [O(24)–Cu(1)O–(25)], subtended by the two Oatoms at the metal, traces on the point that the two oxygenatoms are sharing one axial site, i.e. the nitrite group is bet-ter thought of as a single entity occupying just one stereo-chemical site in the coordination sphere of the metalinstead of as a bidentate ligand. Thus the structure of thecomplex 4 is described as a distorted tbp with the value ofAddition parameter, s = 0.58 [31]. The saturated six-mem-bered ring exhibits twist boat conformation with the puck-

ering amplitude (Q) = 0.770(5) A, h = 94.3(4)�, / =339.4(4)� [20]. The N(2)–Cu(1)–N(6) angle is 91.8(2)�.

There is also an additional peak in the electron densitymap close to the metal atom at ca. 0.5 A. This was inter-preted as emanating from a second site for the metal atom.The two sites were refined with occupancy factors of x and1 � x, with x refining to 0.90(1).

The nitrite ion is known to coordinate metal ions in avariety of ways [32]. As a monodentate ligand, it may bindeither through N, or through one of the oxygen atoms (andthus can function as an ambidentate ligand, giving rise tothe possibility of linkage isomerism) or may function as abridging group with N or one of the oxygen atoms as thedonor atoms. In the present complex, it functions as a che-lating bi-dentate group. Such coordination mode is scantyand known only in few complexes [32].

3.3. Discussions

Complex 1 possesses elongated octahedral geometrywith the two weakly coordinating perchlorate ions in theaxial positions as is usual for a d9 metal ion. TheN3�; SCN� or NO2

� ion, when added to the solution,

Table 3Photophysical data for complexes 1–4

Sample Absorption Emission

1 284 3422 286 343

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replaces one of the perchlorate ions and due to its strongercoordinating ability, comes closer to the metal ion and con-sequently increases the non-bonded interactions. To getrelief of this strain two of the trans atoms in the basal planego away from it and push the other weakly coordinatingperchlorate ion out of the coordination sphere and resultin a penta-coordinated complex having the geometry inter-mediate between trigonal bi-pyramid and square pyramid(Scheme 2). The closer the anion comes to the metal ion,higher is the distortion towards tbp geometry. Among thethree, the steric demand of the nitrite ion is the most as itacts as a chelating ligand (both the oxygen atoms occupiesthe same axial site) and consequently distortion towardstbp geometry is the highest.

In this context, it would be worthy to compare the struc-tures of some earlier prepared complexes with the presentones. There are two reported complexes of Cu(II) perchlo-rate with the similar N4 donor di-Schiff base of the samediamine. In one, the two methyl groups of the ketone partof the present ligand are replaced by H atoms [29] and inother by phenyl groups [3]. The structure of the former issimilar to that of 1; only difference is that the perchlorateO is slightly closer to Cu(II) (2.51 A). The later, on theother hand, contains tetra coordinated Cu(II) with uncoor-dinated perchlorate ion. The difference can conveniently beexplained considering the steric requirement of the tetrad-entate ligand. We would also like to compare the structuralchanges (i.e. distortion in the penta-coordinated species) ongoing from the present complexes to [CuL(H2O)](ClO4)2 [5]and [CuL 0(N3)]ClO4 {L 0 = N,N 0-(bis(pyridine-2-yl)phenyli-dine)propane-1,3-diamine} [30]. The water molecule beingthe week field ligand cannot approach to Cu(II) as closeto as the azide/thiocyanate/nitrite ions. As a result, the dis-tortion of the square pyramid towards trigonal-bi-pyramidis less pronounced as is reflected in its s value (0.23). Thevery high s (0.62) of [CuL 0(N3)]ClO4 can also be attributedto the higher steric interaction of the ligand (due to thepresence of phenyl group) with the azide.

3.4. IR and electronic spectra and magnetic moments

In the IR spectra of all four complexes the sharp singlepeak at around 1100 cm�1 indicating the presence of per-chlorate group and distinct band due to azomethine(C@N) group within 1649–1573 cm�1 are routinely noticed.The lowering of the positions of the bands due to C@Nstretching vibration indicates their coordination to the

Cu

N1 N2

N3N4

OClO3

OClO3

X-

N1

X

N3

Cu

N4

Cu

N1 N2

N3N4

OClO3

X N2

ClO4--

Elongated Octahedral Trigonal bipyramidal

Scheme 2.

metal centres [29]. In the spectra of 1, the splitting of theperchlorate band at 1098 cm�1 and 1120 cm�1 is indicativeof the presence of coordinated perchlorate [33].

The presence as well as coordination mode of azide to atransition metal can be detected by the intense IR band dueto mas(N3) that usually appears within 2000–2055 cm�1 forterminal azide [34] In the IR spectra of 2, the appearance ofstrong band at 2055 cm�1 indicates the presence of monod-entate azide [33]. Similarly, a band at 2079 cm�1 in com-plex 3 is indicative of the presence of the N-coordinatedthiocyanate where as twin bands at 1262 and 1324 cm�1

indicates unsymmetrical chelating nitrite group in complex4 [20,33].

The electronic spectra consist of one d-d transition bandof lower intensity and one intense intramolecular chargetransfer band for all the four complexes. Room tempera-ture magnetic susceptibility measurements show that allthe complexes have magnetic moments close to 1.73 BMas expected for discrete magnetically non-coupled spin-only value for copper(II) ion, as was observed in similarsystems [29,35].

3.5. Luminescence studies

The spectroscopic data of the four complexes in acetoni-trile solution are listed in Table 3. These are assigned tointraligand 1(p–p*) fluorescence.

3.6. Electrochemistry

The cyclic voltammogram of the four complexes weremeasured in acetonitrile solution in the range from�1.0 V to 1.0 V with no trace of decomposition as reflectedin smooth curves. The redox of CuIIL/CuIL in complex 1

(tetraethylammonium perchlorate as supporting electro-lyte) under nitrogen atmosphere is quasi-reversible withDEp = 80 mV (SCE is standard). The cyclovoltammogramof complex 2 displays an electrochemically irreversible(DEp = 120 mV) redox process (Table 4). The similar pat-tern in electron transfer behavior of 3 and 4 at about samepotential establishes the fact that they have closely similar

3 281 3574 277 345

Table 4Electrochemistry data of 1, 2, 3 and 4

Complex Epa (V) Epc (V) DEp (V)

1 0.57 0.49 802 0.55 0.43 1203 0.54 0.41 1304 0.36 0.21 150

S. Chattopadhyay et al. / Inorganica Chimica Acta 359 (2006) 4519–4525 4525

structures. The results of cyclic voltammetry also closelyresemble that of the similar reported compounds, whichserve as further evidences for similar structural and elec-tronic properties [30].

4. Conclusion

The symmetrical tetradentate Schiff base ligand (L)yielded elongated octahedral complex 1 with two perchlo-rate ions in the axial positions. On replacement of one ofthe perchlorate by SCN�; N3

� or NO2� the geometry

around Cu(II) changes into an intermediate between trigo-nal bi-pyramid and square pyramid. The anions show clearpreference for this replacement reaction in the order:SCN� > N3

� > NO2�.

Acknowledgements

We thank the CSIR, India for awarding a junior Re-search Fellowship (Sanction No. 9/28(614)/2003-EMR-I)to one of the authors (S.C.). We also thank EPSRC andthe University of Reading for funds for the Image Platesystem. We are thankful to Dr. Shyamal Chattopadhyay,Department of Chemistry, Bengal Engineering and ScienceUniversity, Shibpur, Howrah, India for providing facilitiesfor electrochemical studies.

Appendix A. Supplementary data

Crystallographic data for the analysis have been depos-ited with the Cambridge Crystallographic data Centre,CCDC Nos. 601406 (complex 1), 601407 (complex 2),601408 (complex 3) and 601409 (complex 4). Copies of thisinformation may be obtained free of charge from CCDC,12 Union Road, Cambridge CB2 1EZ, UK (fax: +441223 336 033; e-mail: deposit@ccdc.cam.ac.uk or www:http://www.ccdc.cam.ac.uk). Supplementary data associ-ated with this article can be found, in the online version,at doi:10.1016/j.ica.2006.05.029.

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