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Inorganica Chimica Acta 361 (2008) 161–172

Influence of counter anions on the structures and magneticproperties of trinuclear Cu(II) complexes containing a l3-OH core

and Schiff base ligands

Pampa Mukherjee a, Michael G.B. Drew b, Marta Estrader c,Carmen Diaz c,*, Ashutosh Ghosh a,*

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

c Departament de Quımica Inorganica, Universitat de Barcelona, Marti i Franques 1-11, 08028 Barcelona, Spain

Received 11 May 2007; received in revised form 25 June 2007; accepted 25 June 2007Available online 1 July 2007

Abstract

Four trinuclear Cu(II) complexes, [(CuL1)3(l3-OH)](NO3)2 (1), [(CuL2)3(l3-OH)](I)2ÆH2O (2), [(CuL3)3(l3-OH)](I)2 (3) and[(CuL1)3(l3-OH)][CuII3] (4), where HL1 (8-amino-4-methyl-5-azaoct-3-en-2-one), HL2 [7-amino-4-methyl-5-azaoct-3-en-2-one] andHL3 [7-amino-4-methyl-5-azahept-3-en-2-one] are the three tridentate Schiff bases, have been synthesized and structurally characterizedby X-ray crystallography. All four complexes contain a partial cubane core, [(CuL)3(l3-OH)]2+ in which the three [CuL] subunits areinterconnected through two types of oxygen bridges afforded by the oxygen atoms of the ligands and the central OH� group. Thecopper(II) ions are in a distorted square-pyramidal environment. The equatorial plane consists of the bridging oxygen of the centralOH� group together with three atoms (N,N,O) from the Schiff base. The oxygen atom of the Schiff base also coordinates to the axialposition of Cu(II) of another subunit to form the cyclic trimer. Magnetic susceptibilities have been determined for these complexes overthe temperature range of 2–300 K. The isotropic Hamiltonian, H = �J12S1S2 � J13S1S3 � J23S2S3 has been used to interpret the mag-netic data. The best fit parameters obtained are: J = �54.98 cm�1, g = 2.24 for 1; J = �56.66 cm�1, g = 2.19 for 2; J = �44.39 cm�1,g = 2.16 for 3; J = �89.92 cm�1, g = 2.25 for 4. The EPR data at low temperature indicate that the phenomenon of spin frustrationoccurs for complexes 1–3.� 2007 Elsevier B.V. All rights reserved.

Keywords: Trinuclear copper; Schiff base; X-ray structure; Magnetic properties; Spin frustration

1. Introduction

The studies of magnetic exchange in simple polynuclearparamagnetic clusters are a topic of current focus in magne-tochemistry [1–10]. A particularly important issue is to findwell-characterized compounds for which one could obtainprecise information on both the ground state and theexcited states of a strongly coupled paramagnetic system.

0020-1693/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2007.06.037

* Corresponding authors.E-mail addresses: carme.diaz@qi.ub.es (C. Diaz), ghosh_59@yahoo.

com (A. Ghosh).

In this regards, cyclic trinuclear complexes of [Cu3O] core,being a simple three electron spin system, are an interestingclass of compounds which offer the opportunity to test themagnetic exchange model. The three unpaired electronson this core interact magnetically via super exchangeinvolving Cu2+–O–Cu2+ pathways. The core could exist ineither a spin-frustrated (Stotal = 1/2, doubly degenerate)state or in a Stotal = 3/2, quartet state. The existence of anM3O core held by bridging ligands is well documented inthe chemistry of iron(III) and chromium(III) [11–16]. Thereare only a few reports with a [Cu3OH] core of which themajority contain oxime-oximate- [17–22], N,N-pyrazole-[23–25], or N,N-triazole [26–28] type peripheral ligands.

162 P. Mukherjee et al. / Inorganica Chimica Acta 361 (2008) 161–172

Examples of cyclic trinuclear copper(II) complexes withketonic oxygen bridges are rather scanty [29–33].

Recently, it has been found that the group of N, N, Odonor tridentate Schiff base ligands, derived from 1:1 con-densation of a b-diketone and a diamine, can readily beinvolved in basal–apical type peripheral oxo-bridging toresult in the formation of trinuclear complexes with a[Cu3OH] core [29–33]. The steric factors of the Schiff basecan be changed systematically by introducing substituentsin the diamine or b-diketone part to bring about struc-tural variations that is required to deduce a relationbetween magnetic coupling and structural features. Allsuch complexes reported so far are antiferromagneticand the coupling constant (J) varies from �2.4 to�66.7 cm�1. For their magneto-structural correlations, itis proposed that the greatest coplanarity of the three prin-cipal ligand planes, the lowest distance of the O(H) abovethe Cu3 plane and the greatest Cu–O–Cu 0 angles providethe strongest antiferromagnetic coupling. However, itshould be noted that the electronic factors related to the

CuX2.6H2O / NEt3

[(CuL)3(OH)](CZ=B

KI in MeOH/H2O

Z=A[(CuL2)3(OH)](I)2

.H2O

2

[(CuL1)3

N OCu

NH2

OO

C

N

(C

n(H2C)

R

R

Scheme

nature of the ligands which affords the bridge are alsoimportant for magnetic coupling. For a better under-standing of the magneto-structural correlation, it is neces-sary to prepare some complexes that should have thesame ligand but at the same time have some variationsin structures. We noticed that the counter anion in allreported [(CuL)3(l3-OH)]2+ cores is perchlorate. There-fore, we would like to synthesize some complexes, withthe same core that the previously described by Costes[29], Kawiatkowski [30], and by us [32] varying the coun-ter anion with the hope that requirements of crystal pack-ing may cause changes in structural features withoutchanging the electronic factors of the ligands which areimportant for magnetic coupling.

This paper reports the synthesis, crystal structure, mag-netic properties and EPR studies of four cyclic trinuclearcopper(II) compounds, [(CuL1)3(l3-OH)](NO3)2 (1), [(CuL2)3-(l3-OH)](I)2ÆH2O (2), [(CuL3)3(l3-OH)](I)2 (3) and[(CuL1)3(l3-OH)][CuII3] (4) where HL1 = 8-amino-4-methyl-5-azaoct-3-en-2-one, HL2 = 7-amino-4-methyl-5-

n = 1, R = H, HL1

n = 0, R = CH3, HL2

n = 0, R = H, HL3.

HL=HL1, X=NO3-; 1

HL=HL1, X=ClO4-; A

HL=HL2, X=ClO4-; B

HL=HL3, X=ClO4-; C

ZlO4)2

KI in MeOH/H2O

Z=C

KI in MeOH/H2O

[(CuL3)3(OH)](I)2

(OH)](CuI3)

X2

3

4

HCu

H2N

O

N

u

NH2

H2)n

(CH2)n

R

=

1.

P. Mukherjee et al. / Inorganica Chimica Acta 361 (2008) 161–172 163

azaoct-3-en-2-one and HL3 = 7-amino-4-methyl-5-azah-ept-3-en-2-one (Scheme 1). The structural and magneticproperties of the complexes are compared with the data ofthe other similar compounds reported in the literature forclearer understanding of magneto-structural correlation.

2. Experimental

2.1. Materials

The diamines (1,3-diaminopropane, 1,2-diaminopro-pane, and 1,2-diaminoethane) and triethylamine were driedover potassium hydroxide and distilled prior to use. Pen-tane-2,4-dione was dried over anhydrous sodium sulphateand distilled; the fraction boiling at 138–139 �C was col-lected. All the chemicals were of reagent grade and usedwithout further purification.

2.2. Preparations

2.2.1. Preparation of the ligands HL1[8-amino-4-methyl-5-

azaoct-3-en-2-one], HL2 [7-amino-4-methyl-5-azaoct-3-en-2-one] and HL3[7-amino-4-methyl-5-azahept-3-en-2-one]

All three ligands are the half condensation products of1,3-propanediamine, 1,2-propanediamine and 1,2-ethane-diamine, respectively, with 2,4-pentanedione.

They were prepared by the condensation of appropriatediamines (1,3-propanediamine, 1,2-propanediamine and1,2-ethanediamine) with pentane-2,4-dione under the con-dition of high dilution as reported earlier [29–52].

2.2.2. Synthesis of [(CuL1)3(l3-OH)](NO3)2 (1)

A solution of Cu(NO3)2 Æ 6H2O (1.208 g, 5 mmol) inmethanol (10 cm3) was added to a stirred solution of theligand HL1 (0.78 g, 5 mmol) in methanol (10 cm3). Trieth-ylamine (0.51 g, 5 mmol) in 5 cm3 methanol was addeddrop wise to this solution with constant stirring. Theresultant dark green solution on keeping overnight at roomtemperature yielded dark green single crystals suitable forX-ray analysis.

Complex 1: (Yield: 1.59 g; 60%) Anal. Calc. for C24H46-Cu3N8O10 (797.31) C, 36.15; H, 5.82; N, 14.05. Found: C,36.02; H, 5.17; N, 14.19%. kmax/nm (emax/dm3 mol�1 cm�1)(methanol), 612 (289); IR: m(N–H), 3232 and 3148 cm�1,m(C@N), 1589 cm�1, m(O–H), 3402 cm�1.

2.2.3. Synthesis of complexes [(CuL2)3(l3-OH)](I)2 ÆH2O (2), [(CuL3)3(l3-OH)](I)2 (3) and

[(CuL1)3(l3-OH)](CuII3) (4)

Complexes [(CuL1)3(OH)](ClO4)2 (A), [(CuL2)3(OH)](ClO4)2 (B) and [(CuL3)3(OH)](ClO4)2 (C) were preparedby following methods as reported earlier [30,32,29] andused as precursor for the preparation of the complexes 2–4, respectively.

A solution of KI (0.996 g, 6 mmol) in 10 cm3 methanol–water mixture (9:1 v/v) was added to stirred solutions ofeach of the complexes A (0.87 g, 1 mmol), B (0.87 g,

1 mmol) and C (0.83 g, 1 mmol) in 10 cm3 methanol. Themixtures were stirred for 1 h at room temperature. Theresulting solutions were allowed to undergo slow evapora-tion in open atmosphere. Dark blue single crystals suitablefor X-ray analysis of complex 2 (resulted from the solutioncontaining complex B) and dark green single crystals of 3and 4 (obtained from the solutions containing complexesC and A, respectively) were separated in 2 days.

Complex 2: (Yield: 1.89 g; 60%). Anal. Calc. for C24H48-Cu3N6O5I2 (945.10): C, 30.50; H, 5.12; N, 8.89. Found: C,30.42; H, 4.97; N, 8.76%. kMax/nm (emax/dm3 mol�1 cm�1)(methanol), 608 (201); IR: m(N–H), 3232 and 3127 cm�1,m(C@N), 1602 cm�1, m(O–H), 3348 cm�1.

Complex 3: (Yield: 1.92 g; 65%). Anal. Calc. for C21H40-Cu3N6O4I2 (885.01): C, 28.50; H, 4.56; N, 9.50. Found: C,28.42, H 4.22, N 9.14%. kMax/nm (emax/dm3 mol�1 cm�1)(methanol), 608 (214); IR: m(N–H), 3233 and 3125 cm�1,m(C@N), 1601 cm�1, m(O–H), 3377 cm�1.

Complex 4: (Yield: 1.48 g; 60%) Anal. Calc. for C24H46-Cu4I3N6O4 (1117.57): C, 25.79; H, 4.15; N, 7.52. Found: C,25.42; H, 3.97; N, 7.19%. kMax/nm (emax/dm3 mol�1 cm�1)(methanol), 615 (163); IR: m(N–H), 3213 and 2930 cm�1,m(C@N), 1584 cm�1, m(O–H), 3422 cm�1.

2.3. Physical measurements

Elemental analyses (carbon, hydrogen and nitrogen)were performed using a Perkin–Elmer 240 C elemental ana-lyzer. IR spectra in KBr (4500–500 cm�1) were recordedusing a Perkin–Elmer RXI FT-IR spectrophotometer.Electronic spectra in methanol (1200–350 nm) wererecorded in a Hitachi U-3501 spectrophotometer. Themagnetic measurements were carried out in the ‘‘Serveide Magnetoquimica (Universitat de Barcelona)’’ on poly-crystalline samples (20 mg) with a Quantum DesignSQUID MPMSXL magnetometer in the temperature rangeof 2–300 K and the magnetic field was 0.1 T. The diamag-netic corrections were evaluated from Pascal’s constants.EPR spectra were recorded on powder samples at X-bandfrequency with a Bruker 300E automatic spectrometer,varying the temperature between 4 and 300 K.

2.4. Crystallographic studies

Intensity data were measured using the Oxford X-Cal-ibur CCD System using Mo Ka radiation. The crystalswere positioned at 50 mm from the CCD. Three hundredand twenty one frames were measured with a countingtime of 3 s. Data analysis was carried out with the CRY-

SALIS program [65]. The structures were solved usingdirect methods with the SHELX97 program [66]. Thenon-hydrogen atoms were refined with anisotropic ther-mal parameters. The hydrogen atoms bonded to carbonwere included in geometric positions and given thermalparameters equivalent to 1.2 times those of the atom towhich they were attached. In 2 one of the iodide anionswas disordered over two sites with population factors of

Table 1Crystal data and refinement details of complexes 1–4

Compound 1 2 3 4

Formula C24H46Cu3N8O10 C24H48Cu3N6O5I2 C21H40Cu3N6O4I2 C24H46Cu4I3N6O4

M 797.31 945.10 885.01 1117.57Space group triclinic, P�1 triclinic, P�1 triclinic, P�1 triclinic, P�1

Cell dimensions

a (A) 11.347(1) 11.845(1) 11.701(1) 11.444(1)b (A) 12.020(1) 12.132(1) 11.856(1) 12.441(2)c (A) 14.232(1) 15.141(1) 12.724(1) 13.633(1)a (�) 73.06(1) 67.39(1) 99.72(1) 67.88(1)b (�) 82.52(1) 76.93(1) 114.08(1) 77.65(1)c (�) 60.96(1) 60.70(1) 90.10(1) 88.12(1)U (A3) 1623.5(3) 1749.8(3) 1583.6(2) 1754.1(3)Z, dm(gm cm�3) 2, 1.631 2, 1.794 2, 1.856 2, 2.116Number of reflections measured 11194 12273 11268 12540Reflections/constraints/parameters 8854/0/406 9761/0/365 8901/0/319 9856/0/360R1, wR2 (I > 2r(I)) 0.0617, 0.1679 0.0536, 0.1220 0.0355, 0.0911 0.0408, 0.0816R1, wR2 (all data) 0.1022, 0.1790 0.1021, 0.1278 0.0530, 0.0940 0.0870, 0.0861Largest peak/hole (e A�3) 1.540, �1.145 1.668, �1.357 1.645, �1.043 1.351, �1.064

164 P. Mukherjee et al. / Inorganica Chimica Acta 361 (2008) 161–172

0.889(1) and 0.111(1). In 3 one of the carbon atoms isdisordered over two sites each refined with populationparameters of 0.50. Hydrogen atoms of water moleculescould not be located. Absorption corrections were carriedout using the ABSPACK program [67]. The structures wererefined on F2 using SHELX97 [66]. Crystal data are givenin Table 1.

3. Results and discussion

We report here four trinuclear complexes having generalformula [(CuL)3(l3-OH)]X2 (X ¼ NO3

� for 1; X = I� for 2

and 3; X ¼ 0:5CuI32� for 4). Complex 1 has been prepared

by the reaction of 8-amino-4-methyl-5-azaoct-3-en-2-one(HL1) with copper nitrate hexahydrate and triethylaminein 1:1:1 ratio in methanol solution. HL2 and HL3 unlikeHL1 do not yield any such crystalline compounds by simi-lar reaction; instead a hygroscopic, sticky mass results. Wecould neither purify nor crystallize the products using com-mon organic solvents. However, the counter perchlorateions of [(CuL)3(l3-OH)](ClO4)2 (L = L2 and L3) can bereplaced readily by the iodide ion when treated with amethanol–water solution of KI to result in the formationof pure crystalline compounds 2 and 3. On the contrary,treatment of KI solution with the methanol solution of[(CuL1)3(l3-OH)](ClO4)2 yielded complex 4 in which thecounter ion is CuI3

2� indicating the occurrence of a redoxreaction during anion exchange

4½ðCuIIL1Þ3ðOHÞ�ðCIO4Þ2 þ 12 KI

! 3½ðCuIIL1Þ3ðOHÞ�½CuII3� þ 3=2I2 þ 8KCIO4

þOH� þ 4Kþ þ 3L1� ð1Þ

It is well known that iodide ions reduce Cu(II)! Cu(I) butwhen Cu(II) forms stable complex with a chelating ligand,such reduction is prevented. Thus, the reduction of part ofthe Cu(II) by iodide in the complexes of L1 but not of L2 or

L3 suggests that the Schiff base containing six-membereddiamine chelate rings forms less stable trinuclear complexcompare to those containing five-membered rings. Thelonger bond distances around hydroxyl group (av.2.045 A) in [(CuL1)3(l3-OH)](ClO4)2 compare to that in[(CuL)3(l3-OH)](ClO4)2 (av. 2.012 and 2.017 A forL = L2 and L3, respectively) as found in their crystal struc-tures, are in agreement with this observation. However, inall cases the hydroxo-bridged trinuclear core remains intacton replacement of the perchlorate ion showing its sufficientstability with other counter ions also. On the other hand,exchange of perchlorate with azide, a stronger coordinatinganion, leads to the rupture of trinuclear entity to form anazide-bridged dinuclear Cu(II) complex [32,38].

3.1. IR and electronic spectra

The IR spectra of complex 1 shows an absorption bandat 3402 cm�1 which is assigned to m(OH) of the triply bridg-ing hydroxy group. The absorption band corresponding toazomethine (C@N) group is distinct and appears at1589 cm�1. The bands at 3232 and 3148 cm�1 may beassigned to m(N–H) stretching vibrations of NH2 group.

Similarly, complexes 3 and 4 show broad absorptionbands at 3377 and 3422 cm�1, respectively for m(OH) ofthe triply bridging hydroxy group. Compound 2 containsa non-coordinating water molecule, which makes theassignment of the O–H stretching somewhat ambiguous.A broad band at 3348 cm�1 may be assigned to both OHand H2O. The absorption bands at 1602, 1601 and1584 cm�1 is attributed to the m(C@N) for the complexes2–4, respectively. The bands at 3232, 3127 cm�1 for 2,3233, 3125 cm�1 for 3 and 3213, 2930 cm�1 for 4 may beassigned to m(N–H) stretching vibrations.

The electronic spectra of these compounds are recordedin methanol solution. The electronic spectra show a singleabsorption bands at 612, 608, 608 and 615 nm for com-

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

Compounds 1 (x = 9) 2 (x = 8) 3 (x = 8) 4 (x = 9)

Cu(1)–O(11) 1.913(3) 1.928(3) 1.908(2) 1.926(3)Cu(1)–N(15) 1.967(4) 1.949(5) 1.942(3) 1.947(5)Cu(1)–N(1x) 1.990(4) 1.988(4) 1.991(3) 2.008(5)Cu(1)–O(21) 2.409(3) 2.375(4) 2.347(2) 2.363(4)Cu(1)–O(100) 2.027(3) 2.002(3) 2.011(2) 2.029(4)Cu(2)–O(21) 1.919(3) 1.916(4) 1.911(3) 1.929(4)Cu(2)–N(25) 1.966(4) 1.930(4) 1.937(3) 1.963(4)Cu(2)–N(2x) 1.987(4) 2.001(5) 1.994(3) 1.997(5)Cu(2)–O(31) 2.318(3) 2.410(4) 2.453(3) 2.329(4)Cu(2)–O(100) 2.034(3) 2.009(3) 2.015(3) 2.030(4)Cu(3)–O(31) 1.932(3) 1.902(4) 1.913(2) 1.914(4)Cu(3)–N(35) 1.957(4) 1.926(4) 1.945(3) 1.959(4)Cu(3)–N(3x) 1.980(5) 1.974(5) 2.002(3) 2.106(4)Cu(3)–O(11) 2.415(3) 2.453(5) 2.438(3) 2.375(4)Cu(3)–O(100) 2.016(3) 1.982(3) 2.014(2) 2.050(4)

O(11)–Cu(1)–N(15) 94.9(1) 94.1(2) 94.3(1) 95.4(2)O(11)–Cu(1)–N(1x) 164.6(1) 165.3(2) 170.7(1) 151.1(2)O(11)–Cu(1)–O(21) 96.3(1) 95.7(1) 94.4(1) 98.0(1)O(11)–Cu(1)–O(100) 82.4(1) 87.1(1) 84.7(1) 84.0(1)N(15)–Cu(1)–N(1x) 96.3(1) 85.6(2) 84.5(1) 96.7(2)N(15)–Cu(1)–O(21) 104.7(1) 101.8(2) 107.6(1) 98.7(2)N(15)–Cu(1)–O(100) 174.5(1) 175.9(2) 177.9(1) 170.7(2)N(1x)–Cu(1)–O(21) 90.9(1) 98.6(2) 94.7(1) 105.9(2)N(1x)–Cu(1)–O(100) 87.2(1) 94.2(2) 96.2(1) 88.1(2)O(21)–Cu(1)–O(100) 81.8(1) 74.2(1) 84.7(1) 82.3(2)

O(21)–Cu(2)–N(25) 95.5(1) 93.6(2) 94.7(1) 91.6(2)O(21)–Cu(2)–N(2x) 156.8(2) 170.7(2) 174.0(1) 175.8(2)O(21)–Cu(2)–O(31) 95.0(1) 92.9(2) 97.5(1) 93.1(1)O(21)–Cu(2)–O(100) 81.8(1) 85.2(2) 84.7(1) 82.3(2)N(25)–Cu(2)–N(2x) 97.1(2) 85.0(2) 84.9(2) 91.1(2)N(25)–Cu(2)–O(31) 104.6(1) 109.9(2) 104.1(2) 121.8(2)N(25)–Cu(2)–O(100) 176.3(1) 177.0(2) 176.4(1) 165.0(2)N(2x)–Cu(2)–O(31) 100.5(2) 96.2(2) 88.4(1) 82.8(2)N(2x)–Cu(2)–O(100) 86.3(1) 95.8(2) 96.1(1) 96.0(2)

O(31)–Cu(2)–O(100) 73.2(1) 72.9(1) 72.5(1) 72.4(2)O(31)–Cu(3)–N(35) 94.6(2) 94.8(2) 94.5(1) 95.9(2)O(31)–Cu(3)–N(3x) 163.1(2) 169.3(2) 175.2(1) 158.1(2)O(31)–Cu(3)–O(11) 93.1(1) 99.6(2) 92.4(1) 94.8(2)

P. Mukherjee et al. / Inorganica Chimica Acta 361 (2008) 161–172 165

pounds 1–4, respectively. The positions of these bands areof typical d–d transitions in the square-pyramidal Cu(II)surroundings [29–33].

3.2. Description of structures of complexes 1–4

The four compounds have similar structures that arebuilt up from discrete trinuclear cations [(CuL)3(l3-OH)]2+ and discrete anions required to balance the charge,two nitrates in 1, two iodides in 2 and 3, and [CuI3]2� in 4.In the [CuI3]2� counteranion, the Cu–I distances are2.497(1), 2.538(1), and 2.606(1) A respectively, in anapproximate trigonal arrangement. In complex 2, in addi-tion there is one lattice water molecule. The trinuclearstructure of 1 is shown in Fig. 1 together with the commonnumbering scheme [complexes 2–4 have very similar molec-ular structures, see Figs. S1–S3 respectively in Supplemen-tary material]. Selected bond lengths and bond angles aresummarized in Table 2.

The trinuclear cations of each complex comprise of three[CuL] subunits (where L represents, deprotonated, triden-tate, monoanionic ligand), which are roughly perpendicu-lar to each other and are interconnected through twotypes of oxygen bridges afforded by the carbonyl oxygenatoms of the ligands and the central OH� group. Thereare only small discrepancies among the correspondingbond lengths and angles of the three CuL subunits andthe overall structure lacks any element of threefold symme-try in all complexes. Each copper atom adopts a (4 + 1)square-pyramidal (NNOO and O) geometry. In complex1 there is a close contact of one of the copper atoms tothe oxygen of nitrate ion but the distance is too long(Cu1–O5 = 2.783 A) to be considered as a coordinatebond. The four coordinating atoms making up the basalplane are the carbonyl oxygen atom, one imine nitrogenatom and one amino nitrogen atom from the tridentate

Fig. 1. ORTEP-3 view of the asymmetry unit of 1 with ellipsoids at 25%probability.

O(31)–Cu(3)–O(100) 82.6(1) 85.8(2) 85.5(1) 72.4(1)N(35)–Cu(3)–N(3x) 96.7(2) 85.1(2) 84.7(1) 98.2(2)N(35)–Cu(3)–O(11) 105.1(2) 98.7(2) 101.4(1) 100.9(2)N(35)–Cu(3)–O(100) 175.1(2) 173.1(2) 173.5(1) 173.0(2)N(3x)–Cu(3)–O(11) 96.1(2) 91.0(2) 92.4(1) 98.8(2)N(3x)–Cu(3)–O(100) 87.0(2) 95.5(2) 95.9(1) 86.4(2)O(11)–Cu(3)–O(100) 71.1(1) 74.4(2) 72.0(1) 84.0(1)I(1)–Cu(4)–I(2) 123.2(1)I(1)–Cu(4)–I(3) 115.6(1)I(2)–Cu(4)–I(3) 121.1(1)

Schiff base ligand (L), and the hydroxo group OH�, whilethe apical site is occupied by one oxygen atom of anothertridentate Schiff base ligand i.e. each carbonyl oxygen atomwithin the basal plane of one CuL is in turn apical to a cop-per atom in another subunit. The square-pyramidal geom-etry within each subunit is distorted in the usual way, i.e,the copper ion is displaced from the basal plane towardsthe apical oxygen atom by 0.06–0.15 A, 0.032–0.103 A,0.021–0.095 A and 0.08–0.17 A for complexes 1–4, respec-tively. The lengths of the bonds between the copper atomsand the donor sites (O, N, N) of the tridentate ligands are

Fig. 2. Hydrogen-bonded dimeric structure of complex 1.

166 P. Mukherjee et al. / Inorganica Chimica Acta 361 (2008) 161–172

within the range of values normally found for such systems[29–33]. The three copper(II) ions and the bridging hydro-xyl group form a flattened trigonal pyramid with the cop-per sites roughly defining an equatorial triangle. Thedeviations of the copper atoms from the mean planes ineach subunit, Cu–Cu distances and Cu–Cu–Cu angles oftriangular arrangement in the complexes are shown inTable S1. The distortions of the coordination polyhedronfrom the square pyramid to the trigonal bipyramid havebeen calculated by the Addison parameter (s) [53] as anindex of the degree of trigonality. The s values of the com-plexes are presented in Table 4.

In four complexes all the six-membered chelate ringsincorporating the b-diketone core is essentially planer, sim-ilar to the analogous complexes as reported earlier [29–33].In complex 1, the second six-membered chelate ring withineach subunit that incorporates the trimethylene fragmentof the starting diamine, assumes a half-chair conformationaround Cu1 and an intermediate conformation betweenscrew-boat and boat around Cu2 and Cu3. In complex 4,the same ring assumes intermediate conformation betweenboat and twist-boat around Cu1, boat around Cu2 andintermediate between screw-boat and boat around Cu3.In complex 2, the five-membered rings with Cu1 and Cu2have half-chair conformations twisted on N18� � �C17 andN38� � �C37, respectively, whereas, the ring with Cu3 is inenvelope conformation. In all three subunits of complex3, the five-membered chelate rings assume envelopeconformation.

All four complexes show hydrogen-bonding interactionsamong the trinuclear units and their corresponding anionsto link them in pairs (Table 3). In 2 the water of crystalli-

Table 3Hydrogen bonding distances (A) and angles (�) for the complexes 1–4

Compound D–H� � �A D–H

1 N19–H19A� � �O44p 0.899N19–H19B� � �O42q 0.900N29–H29A� � �O51q 0.899N29–H29A� � �O52q 0.900N29–H29B� � �O44p 0.899N39–H39A� � �O41q 0.900N39–H39B� � �O51q 0.900O100–H100� � �O42p 0.980

2 N18–H18A� � �I1j 0.901N38–H38A� � �I2j 0.901N38–H38B� � �O200k 0.899O100–H100� � �O200j 0.981

3 N18–H18B� � �I2 0.900N28–H28A� � �I2l 0.900N28–H28B� � �I1 0.899N38–H38A� � �I2 0.900

4 N19–H19B� � �I3m 0.900N29–H29B� � �I1 0.900N39–H39A� � �I3m 0.900N39–H39B� � �I1 0.900O100–H100� � �I3 0.980

Symmetry elements: (j) 1 + x, y, z; (k) 1 � x, 1 � y, �z; (l) �x, �y, �z; (m) �

zation also participates in H-bonding network. In complex1, two centrosymmetrically related trinuclear units arelinked by N–H� � �O hydrogen bond involving hydrogenatoms H19b, H19a(N19), H29b(N29), H39a(N39) andH100(O100) and all six oxygen atoms of the two centro-symmetrically related nitrate anions (O41, O42, O44).The hydrogen atom H29a(N29) is involved in bifurcatedhydrogen bond with two oxygen atoms O51 and O52 ofanother nitrate ion (Fig. 2). In complex 2, two centrosym-metrically related trinuclear units are linked together by N–H� � �O hydrogen bond involving hydrogen atomsH38b(N38) and H100(O100) and water of crystallization(O200) (Fig. 3). Other hydrogen atoms, H18b(N18) andH38a(38) form H-bonds with the iodide ions I(1) andI(2), respectively. In complex 3, two trinuclear units are

D� � �A H � � �A � � �\ D–H� � �A3.211(7) 2.460 141.313.011(6) 2.150 159.683.332(10) 2.494 155.223.088(13) 2.252 154.403.033(6) 2.160 163.723.011(7) 2.112 177.932.848(12) 2.113 138.282.829(5) 1.856 171.733.650(5) 2.764 167.933.767(6) 2.941 153.202.965(7) 2.066 178.452.831(6) 1.883 161.873.636(4) 2.941 135.223.640(4) 2.758 166.963.563(4) 2.880 133.893.776(4) 3.012 143.723.774(6) 2.917 159.613.710(6) 2.906 149.513.681(5) 2.851 154.013.541(5) 2.892 130.313.598(4) 2.653 162.19

x, 1 � y, 1 � z; (p) 1 � x, �y, 1 � z; (q) 1 + x, y, z.

Fig. 3. Hydrogen-bonded dimeric structure of complex 2.

Fig. 4. Hydrogen-bonded dimeric structure of complex 3.

Fig. 5. Hydrogen-bonded dimeric structure of complex 4.

0 15000 30000 450000.0

0.2

0.4

0.6

0.8

1.0

M /

Nμ B

H / G0 50 100 150 200 250 300

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

χ MT

/ cm

3 mol

-1 K

T / K

Fig. 7. Plot of the vMT vs. T in the range 2–300 K for 2. Inset:magnetization vs. H at 2 K for 2.

0 50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

χ MT

/ cm

3 mol

-1K

T / K

0 15000 30000 45000

0.0

0.2

0.4

0.6

0.8

1.0

M /

Nμ B

H / G

Fig. 6. Plot of the vMT vs. T in the range 2–300 K for 1. Inset:magnetization vs. H at 2 K for 1.

P. Mukherjee et al. / Inorganica Chimica Acta 361 (2008) 161–172 167

linked through iodide ion (I2) which acts as acceptor forthe three H-bonds involving hydrogen atoms H18b(N18),H28a(N28) and H38a(N38). The other iodide ion (I1)forms a weak hydrogen bond with H28b(N28) (Fig. 4).In complex 4, one of the iodine atoms (I3) of [CuI3]2� beingthe acceptor of three H-bonds involving hydrogen atomsH19b(N19), H39a(N39) and H100(O100) links two centro-symmetrically related trinuclear units. Another iodineatom (I1) of [CuI3]2� is engaged in intra-molecular H-bonds with H29b(N29) and H39b(N39) (Fig. 5).

3.3. Magnetic properties

In complexes 1–4 the three Cu(II) ions, the l3-hydroxyion and the three l2-carbonyl oxygen atom of the corre-sponding Schiff base ligands form an incomplete cube, withone apex lacking from the cube. Magnetic susceptibilitieswere determined for these complexes over the temperaturerange of 2–300 K in an applied field of 0.1 T. A plot of vMT

versus T for complexes 1–4 are shown in Figs. 6–9 respec-tively, in all cases vMT being the magnetic susceptibility per[Cu3O] entity. At room temperature the vMT values are

closed to 1.2 cm3 mol�1 K as expected for three indepen-dent Cu(II) ions (1.27 cm3 mol�1 K for 1, 1.21 cm3 mol�1

K for 2, 1.22 cm3 mol�1 K for 3 and 1.23 for 4). As T is

0 50 100 150 200 250 3000.2

0.4

0.6

0.8

1.0

1.2

χ MT

/ cm

3 mol

-1K

T / K

0 15000 30000 450000.00.20.40.60.81.0

M /

Nμ B

H / G

Fig. 8. Plot of the vMT vs. T in the range 2–300 K for 3. Inset:magnetization vs. H at 2 K for 3.

0 50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8

1.0

1.2

χ MT

/ cm

3 mol

-1 K

T / K

0 15000 30000 450000.00.20.40.60.81.0

M/N

μ B

H / G

Fig. 9. Plot of the vMT vs. T in the range 2–300 K for 4. Inset:magnetization vs. H at 2 K for 4.

168 P. Mukherjee et al. / Inorganica Chimica Acta 361 (2008) 161–172

lowered vMT decreases reaching values of 0.32, 0.30, 0.32and 0.43 cm3 mol�1 K for complexes 1–4 respectively. Onlycomplex 4 shows the typical plateau of 0.45 cm3 mol�1 Kfrom 20 K to 5 K, before reaching the final value. The for-mation of this plateau (S = 1/2) is dependent upon the cor-responding existence of noticeable intermolecular inter-trinuclear exchange interactions or in the case of frustratedsystems when the antisymmetric exchange is operating. Theisotropic Heisenberg Dirac van Vleck (HDvV) Hamilto-nian (Eq. (1)) is the main term used to interpret the mag-netic data.

H ¼ �J 12S1S2 � J 13S1S3 � J 23S2S3 ð1ÞSince the three copper atoms of the [Cu3O] unit define aquasi equilateral triangle, the three Cu(II) ions can be con-sidered equivalent, and so the three Jij constants are equal.

The magnetic susceptibility deduced from the Hamilto-nian is given below, where N, g, v and k have their usualmeanings [26–28,55].

vM ¼ ðNb2g2=4kT Þ½1þ 5 expð3J=2kT Þ�=½1þ expð3J=2kT Þ�ð2Þ

To take into account the interaction between the trinuclearentities, the susceptibility can be corrected by the molecularfield approximation [54] (Eq. (3)) where zJ 0 denotes the in-tra-molecular exchange parameter.

v0M ¼ vM=½1� 2zJ 0vM=Ng2b2� ð3ÞThe experimental data for 1–4, corrected from diamagneticcontributions and TIP (0.3 · 10�3 cm3 mol�1), were ana-lyzed by Eqs. (2) and (3) on vMT. The best-fitted curvesare plotted in Figs. 6–9 along with the experimental data.The best fit parameters obtained are: J = �54.98 cm�1,g = 2.24 and zJ 0 = �2.72 with R = 1.8 · 10�4 for 1; J =�56.66 cm�1, g = 2.19 and zJ 0 = �2.85 with R = 2 · 10�4

for 2; J = �44.39 cm�1, g = 2.16 and zJ 0 = �2.59 withR = 1.2 · 10�4 for 3; J = �89.92 cm�1, g = 2.25 andzJ 0 = �0.78 with R = 8 · 10�5 for 4.

On the light of the structures, it is difficult to explain therather large zJ 0 values calculated for complexes 1–3 takinginto account that the inter-trimeric Cu–Cu contact are inall cases larger than 6.7 A, so the decrease of vMT valuesat low temperatures could be also result of the antisymmet-ric exchange (see Section 3.4). The copper(II) ions are in asquare-pyramidal environment in all the complexes. Theaxial bond of each copper(II), which involves the oxygenatom of the carbonyl group of the ligand, is elongated(above 2.3–2.4 A), so a very weak or no contribution tothe magnetic exchange between each pair of copper(II) ionsthrough this oxygen atom is expected, and the J value isdue mainly to the interaction through the l3-OH groupof the [Cu3O] core. The J values and significant structuralparameters are gathered in Table 4, together with theresults of trinuclear complexes of [Cu3O] core with analo-gous peripheral ligands. The reduced molar magnetizationof complexes 1–4, at 2 K, tends to 1 N lB when the fieldreaches 5 T (Figs. 6–9 inset) indicating that the groundstate at this temperature is 1/2.

Trinuclear oxo-centered metal complexes have been ofgreat interest as systems to test current theories of mag-netic and electronic coupling between metal atoms. A fea-ture that has attracted attention is that when the threemetal atoms are identical, and of the same oxidationstate, the complex nearly always fails to adopt the three-fold symmetry that might be expected. One interestingphenomenon is spin frustration, which occurs when allthe interactions between spin pairs cannot simultaneouslyhave the optimal value. The isotropic Hamiltonian for theinteraction between any two spins can be written as thescalar product of the spin operators: H ¼ �

PijJ ijSiSj;

therefore the energy is minimized for collinear parallelor antiparallel spin alignments. If Jij is negative, (antifer-romagnetic interaction) and is equal for all nearest neigh-bor pairs, then only two of the three spins constraints canbe satisfied simultaneously and the system is geometricallyfrustrated. For these systems, the available experimental

Table 4Selected structural parameters for complexes 1–4 and other reported complexes [29,30,32] related to their magnetic properties

Ref. Cu(1)–O(H)–Cu(2) (�)

Cu(1)–O(H)–Cu(3) (�)

Cu(2)–O(H)–Cu(3) (�)

Cu–O(H)–Cu0 (�) (av)

O(H)� � �Cu3plane (A)a

Cu/Cu0

(�) (av)bsc �J

(cm�1)d

1e 108.36 108.08 104.51 106.98 0.754(3) 88.5 0.33, 0.20 �54.984e 106.63 105.27 105.46 105.79 0.794(4) 83.4 0.33, 0.18, 0.25 �89.92[30] 105.4(2) 106.5(2) 108.3(2) 106.7(2) 0.796(5) 83.7(3) 0.26, 0.09, 0.20 �30

2e 105.93 104.19 106.20 105.44 0.789(3) 85.7 0.17, 0.07, 0.10 �56.66[32] 107.3(2) 102.9(2) 105.8(2) 105.3 0.797(4) 85.8 0.19, 0.08, 0.10 �36.6

3e 104.31 106.60 106.61 105.84 0.784(2) 84.4 0.12, 0.04, 0.03 �44.39[29] 104.8(1) 104.9(1) 105.5(1) 105.1 0.804(2) 82.9(1) 0.01, 0.11, 0.09 �24.0

a Deviation of the oxygen atom of triply bridging hydroxy group above the plane of the three copper atoms.b Coplanarity between the least-squares planes defined by [O(H), N, N, O]; average value calculated from the Cu1/Cu2, Cu1/Cu3, Cu2/Cu3 dihedral

angles.c Tau-descriptor for 5-coordination.d The J values from the literature have been modified according to the Hamiltonian used in this paper.e This work.

P. Mukherjee et al. / Inorganica Chimica Acta 361 (2008) 161–172 169

data are in good agreement with the isotropic HDvVmodel in the region of relatively high temperatures. How-ever, there are discrepancies between the predictions ofthe theory and the experimental data at low temperatures.In order to interpret the magnetic properties of these frus-trated systems it is necessary to go beyond the frameworkof the above isotropic exchange model and introduce anantisymmetric exchange interaction GAB [26,55–57]. Infor-mation about GAB values for isolated complexes is veryscarce. Experimental applications of this exchange termhave been reported for various Cu3(II) complexes [58–60]. The low temperature magnetic behavior of complexes1–3 could be attributed to additional antiferromagneticinteractions and to the antisymmetric exchange phenome-non. From the literature data it seems appropriated tostudy the possibility of the presence of spin frustrationfor this kind of complexes by means of EPR spectra atvery low temperatures when only the degenerate S = 1/2states are populated.

3.4. EPR spectra

The X-band EPR spectra of complexes 1–4 wererecorded on powder samples varying the temperaturebetween 4 and 298 K (Fig. S4 in Supplementary material).The EPR spectra of trinuclear S = 1/2 compounds hasbeen analyzed in terms of three active states, one quartetand two doublets, with a temperature dependent popula-tion [61–64]. In the light of the super-exchange parameterfrom the magnetic measurements, at low temperature onlythe doublets should be populated, whereas at room temper-ature signals due to the transition inside the quartet couldalso be observed. This transition may show noticeablezero-field splitting (ZFS) (S = 3/2). Complexes 1–3 showat 298 K broad bands at low fields. This band is welldefined for complexes 1 and 2 at g values of 3.8 and 4.55,respectively. For complex 3 the band is very broad andfor complex 4, the spectra are not well solved at room tem-perature (Fig. 10). For exchange coupled trimers, the largeanisotropy in the g-tensor for the S = 1/2 ground state with

a rather small g^ value, is usually attributed to the effects ofantisymmetric exchange interaction [68–70]. At 4 K thespectra of complexes 1–4 are anisotropic, the fitting resultsare the following (Fig. 10): gi = 2.14 and g^ = 1.80 for 1,gi = 2.15 and g^ = 1.90 for 2, g1 = 2.15, g2 = 2 andg3 = 1.89 for 3 and g1 = 2.21, g2 = 2.11 and g3 = 2.01.The low values of g^ for 1 and 2, and g3 for 3 are indicativeof a spin-frustrate system. The EPR spectra of complex 3

clearly show the hyperfine splitting.

3.5. Discussion

Complexes with a [Cu3O4] core, with an incompletecubane structure, are scarce [29–34]. From the magneticpoint of view complexes 1–4 are trinuclear in which themagnetic interaction takes place fundamentally, throughthe l3-OH group in the [Cu3O] core. The general trendsare: the shortest distance of (OH) group above the planeformed by the three copper ions in the triangular copper(II)complexes, the greatest Cu–O–Cu 0 angle and the morecoplanarity between the three least-square planes definedby [O(H),N,N,O] provide the strongest antiferromagneticcoupling [55,17,23]. In a previous paper we discussed therelationships for this kind of complexes with analogousperipheral bridges [33]. In Table 4, we correlate the magni-tude of the magnetic coupling and some characteristicstructural parameters of complexes 1–4, together withsome previously described analogous species, with the samecore and perchlorate as counter anion. Due to the fact thatthe dihedral angles of around 85� between the three least-square planes defined by [O(H),N,N,O] show that there isno coplanarity, the most important interaction seems tobe the central l3-O bridge. Comparing [(CuL2)3(l3-OH)3](ClO4)2 [32] with [(CuL2)3(l3-OH)3]I2 Æ H2O (2), thehighest absolute value of J of the complex with I� couldbe justified by the shortest distance of the (OH) groupabove the plane formed by the three copper ions. In thecase of [(CuL1)3(l3-OH)]I2 (3) and the analogous one withClO4

� as counteranion [29] the highest value of J of com-plex 3 is consistent with the greatest Cu–O–Cu 0 angle and

0 1000 2000 3000 4000 5000

298 K

H / Gauss

4 K

complex 1

0 1000 2000 3000 4000 5000

298 K

4 K

H / Gauss

complex 2

0 1000 2000 3000 4000 5000

4 K

H / Gauss

298 K

complex 3

0 1000 2000 3000 4000 5000 6000H/ Gauss

4 K

298 K

complex 4

Fig. 10. Powder spectra at 2 K and 298 K, and simulated EPR spectra (dotted lines) at 4 K for complexes 1–4.

170 P. Mukherjee et al. / Inorganica Chimica Acta 361 (2008) 161–172

the shortest distance of the (OH) group above the planeformed by the three Cu(II) ions. The comparison of[(CuL3)3(l3-OH)](NO3)2 (1), [(CuL3)3(l3-OH)][CuI3] (4),and their analogue with ClO4

� as counteranion [30] it isnot easy. Complex 1 shows an intermediate J value andthe shortest distance of the O(H) group above the [Cu3]plane; complex 4 shows the high J value and the low Cu–O(H)–Cu 0 angle value.

In triangular arrangements of symmetric antiferromag-netically coupled metal ions, the magnetic properties atlow temperatures can become complicated by the possiblepresence of spin frustration effects, because of the presenceof the antisymmetric exchange interaction (ASE), describedby the GAB term. The experimental EPR data indicate thatfor complexes 1–3 this term is non-zero and the phenome-non of spin frustration occurs but for complex 4, no notice-able spin frustration takes place.

4. Conclusion

Synthesis of the four complexes (1–4) shows that the tri-nuclear cationic core, [(CuL)3(l3-OH)]2+ has sufficient sta-

bility and can be isolated in solid state not only withperchlorate counter ion but also with several otherweakly-/non-coordinating counter anions. For this kindof complexes, it is very difficult to make correlationsbetween magnetic properties and structural features as sev-eral factors must be taken into account. The complexesobtained in this work show highest antiferromagnetic cou-pling than their analogues with perchlorate counteranion.The results obtained seem to indicate a correlation betweenthe sizes of the counteranion, which forces different kind ofpacking in each case and produce small changes in somestructural factors of the [Cu3O] core. The interesting spinfrustration phenomenon is observed in three complexes(1–3).

Acknowledgements

This work was supported by CSIR, Government of India[Junior Research Fellowship to P.M, Sanction no.09/028(0663)/2006-EMR-I] and the Spanish Government (GrantBQU2003-00539). We also thank EPSRC and theUniversity of Reading for funds for the X-Calibur system.

P. Mukherjee et al. / Inorganica Chimica Acta 361 (2008) 161–172 171

Appendix A. Supplementary material

The ORTEP diagram for complexes 2–4 and tempera-ture dependence of the powder EPR spectra of complexes1–4.

CCDC 637023, 637024, 637025, 637026 contain the sup-plementary crystallographic data for compounds 1, 2, 3, 4.These data can be obtained free of charge via http://www.ccdc.ac.uk/conts/retrieving.html, or from the Cam-bridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; ore-mail: deposit@ccdc.cam.ac.uk. Supplementary data asso-ciated with this article can be found, in the online version,at doi:10.1016/j.ica.2007.06.037.

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