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Polyhedron 27 (2008) 105–112

A pyrazole-based orthogonal ferromagnetically coupled [2 · 2]homoleptic square Cu4 grid: Magnetostructural correlations

Somnath Roy a, Tarak Nath Mandal a, Anil Kumar Barik b, Samik Gupta a,Ray J. Butcher c, Mohamed Salah El Fallah d,*, Javier Tercero d, Susanta Kumar Kar a,*

a Department of Chemistry, University College of Science, 92, A.P.C. Road, Kolkata 700 009, Indiab Department of Chemistry, St. Paul’s C.M. College, 33/1, Raja Rammohan Roy Sarani, Kolkata 700 009, India

c Department of Chemistry, Howard University, 2400 Sixth Street, N.W., Washington, DC 200 59, USAd Departament de Quımica Inorganica, Facultat de Quımica, Universitat de Barcelona, Martı i Franques 1-11, 08028 Barcelona, Spain

Received 4 July 2007; accepted 23 August 2007Available online 23 October 2007

Abstract

The synthesis and structure of a pyrazole-based orthogonal ferromagnetically coupled tetracopper(II) 2 · 2 homoleptic grid complex[Cu4(PzOAPyz)4(ClO4)2](ClO4)2 Æ 6H2O (1), formed by the reaction between the ditopic ligand PzOAPyz and Cu(ClO4)2 Æ 6H2O, aredescribed. The ligand contains terminal pyrazole and pyrazine residues bound to a central flexible diazine subunit (N–N) as well asone potentially bridging alkoxo group. The two adjacent metal centers are linked by an alkoxo oxygen forming essentially a squareCu4(l-O4) cluster. In the Cu4(l-O4) core, out of the four copper centers, two copper centers are penta-coordinated and the remainingtwo are hexa-coordinated. In each case of hexa-coordination, the sixth position is occupied by one of the oxygen atoms of a coordinatedperchlorate ion. Complex 1 has been characterized structurally and magnetically. Although the large Cu–O–Cu bridge angles (137–138�)and short Cu–Cu distances (3.964–3.970 A) are suitable for the transmission of the expected antiferromagnetic coupling, the square-based Cu4(l-O4) cluster exhibits an intramolecular ferromagnetic exchange (J = 7.47 cm�1) between the metal centers with an S = 2magnetic ground state associated with the quasi orthogonal arrangement of the magnetic orbitals (dx2�y2 ). The exchange pathway param-eters have been evaluated from density functional calculations.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Tetranuclear copper 2 · 2 grid; Schiff base; X-ray structure; Magnetic property

1. Introduction

Self-assembly processes involving carefully designedmultidentate ligands and metal ions with appropriatestereoelectronic preferences can lead to the efficient andspecific formation of aesthetically looking sophisticatedpolynuclear grids [1,2]. Although at the beginning, manyof these structures arose accidentally, recently worldwidesystematic attempts are in line to determine the relationshipbetween the number, type and spatial disposition of the

0277-5387/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2007.08.040

* Corresponding authors. Tel.: +91 033 24322936; fax: +91 03323519755.

E-mail addresses: salah.elfallah@qi.ub.es (M.S. El Fallah), skkar_cu@yahoo.co.in (S.K. Kar).

binding sites of the ligand for a proper metal ion. Thisnewly found relationship allows a considerable degree ofcontrol over the nature of the structure [3]. A significantamount of control can be exerted over the formation ofrelatively low nuclearity molecular clusters (<M6) by theuse of poly-functional and polytopic ligands with welldefined and appropriately positioned coordination pockets.Hence, with the proper choice of paramagnetic metal ionsto form complexes with these ligands, magnetic couplingcan be fine-tuned in order to achieve the result of desiredand interesting supramolecular magnetic properties. Themajor interest so far in these types of grid complexes hasemerged from their remarkable and fantastic structures.The specific self-assembly processes leading to these grid-like complexes find immense application in information

106 S. Roy et al. / Polyhedron 27 (2008) 105–112

storage and processing technology [4–6]. However, there isalso considerable scope for the study of magnetic interac-tions in these complexes where several metal ions areassembled in a well-defined spatial array linked by suitablebridging ligands to transmit the interaction. Physicochem-ical properties and magnetic exchange interactions havebeen extensively studied between the pairs of metal centersin tetranuclear complexes. Recently, the magnetic proper-ties of high-nuclearity clusters have also created immenseinterest [2] for the development of new magnetic materials.Transition metal template cyclisations have successfullyproduced many tetranuclear Mn4, Ni4, Cu4 and Zn4 com-plexes, [7–13] in which the bridging arrays among the metalions involve oxygen donor centers. Most of the so farreported alkoxo-bridged poly-nuclear grids [Mn(II), Co(II)and Ni(II)] show anti-ferromagnetic coupling [9–13], whilea few Cu(II) grids [14–18] exhibit a ferromagnetic interac-tion. The origin of this ferromagnetic exchange emergesfrom the orthogonal orientation of the magnetic orbitals(dx2�y2 ).

In continuation of our very recent report [15] of a squareCu4-grid, in this paper we describe the syntheses, structuralcharacterisation and magnetic properties of a tetranuclearcopper(II) complex prepared from the simple multidentatechelating ligand PzOAPyz, containing pyrazine and pyra-zole as heterocyclic rings. The assembly of this structureis driven by the stereoelectronic requirement of the cop-per(II) ion for an elongated tetragonal geometry. The resul-tant grid-like copper(II) complex contains arrays of fourcopper(II) ions linked by alkoxide bridges, which areknown to be effective mediators of magnetic exchangeinteractions; accordingly, detailed magnetic susceptibilitystudies on this complex are described. A brief discussionof its DFT study has also been made.

2. Experimental

2.1. Materials

2-Cyanopyrazine was purchased from Aldrich ChemicalCompany. Other commercially available chemicals and sol-vents were used and purified by standard procedures [19].

N

N

CN

Na

MeOH

NH

N

CH3

O

NHNH2

N

N

NH

OM

AcOH

Scheme

2.2. Physical measurements

Elemental analyses (carbon, hydrogen and nitrogen) ofthe ligand PzOAPyz and the metal complex 1 were carriedout with a Perkin-Elmer CHNS/O analyzer 2400 at theIndian Association for the Cultivation of Science, Kolkata.The electronic spectra of 1 in purified CH3CN solutionwere recorded on a Hitachi model U-3501 spectrophotom-eter. IR spectra (KBr pellet, 4000–400 cm�1) were recordedon a Perkin Elmer model 883 infrared spectrophotometer.The mass spectrum of the ligand was done at theIndian Institute for Chemical Biology, Kolkata with aJEOLJMS-AX 500 mass spectrometer. Room temperaturemagnetic susceptibility was measured with a PAR 155vibrating sample magnetometer. Magnetic susceptibilitymeasurements for the compound were carried out on poly-crystalline samples at the Servei de Magnetoquımica of theUniversitat de Barcelona, with a Quantum Design SQUIDMPMS-XL susceptometer apparatus working in the range2–300 K under magnetic field of approximately 500 G(2–30 K) and 1000 G (35–300 K). Diamagnetic correctionswere estimated from Pascal Tables.

2.3. Synthesis and characterization of the ligand

2.3.1. Synthesis of 5-methylpyrazole-3-carbohydrazide

5-Methylpyrazole-3-carbohydrazide was prepared fol-lowing an established method [20].

2.3.2. Synthesis of PzOAPyzThe methyl ester of iminopyrazine-2-carboxylic acid,

isolated as a white precipitate, was prepared in situ(Scheme 1) by the direct reaction of 2-cyanopyrazine(1.75 g, 0.0166 mol) with sodium methoxide solution, pre-produced by dissolving sodium metal (2.63 g, 0.114 mol)in dry methanol (10 mL) when required. 5-Methylpyraz-ole-3-carbohydrazide (3.5 g, 0.025 mol) in 15–20 mL drymethanol was added to the above precipitate with stirring.Within a few minutes, the white precipitate was found to bedissolved. The stirring was continued for an additional 1 h.Then the mixture was refluxed for 5 h and cooled to roomtemperature. Excess methanol was removed in a rotary

N

N

NH

OMe

e

N

N

N

NH2

NHN

N

CH3

O

PzOAPyz

1.

Table 1Crystallographic data for 1

Compound 1

Empirical formula C40H52Cl4Cu4N28O26

Formula weight 1737.06Temperature (K) 100(2)Wavelength (A) 0.71073Crystal system monoclinicSpace group P2/n

Unit cell dimensions

a (A) 14.8346(19)b (A) 10.3371(13)c (A) 21.155(3)b (�) 96.392(2)

Volume, A3 3223.9(7)Z 2Dcalc (Mg/m3) 1.789Absorption coefficient (mm�1) 1.571F(000) 1760Crystal size (mm) 0.50 · 0.35 · 0.25h range for data collection (�) 1.60–30.61Index ranges �21 6 h 6 21, �14 6 k 6 14,

�30 6 l 6 29Reflections collected 37159Independent reflections [Rint] 9884 [0.0821]Completeness to h = 25.00� 99.6%Absorption correction multiscanMaximum and minimum

transmission1.000 and 0.617

Refinement method full-matrix least-squares on F2

Data/restraints/parameters 9884/37/534Goodness-of-fit on F2 1.037Final R indices [I > 2r(I)] R1 = 0.0631, wR2 = 0.1589R indices (all data) R1 = 0.0889, wR2 = 0.1780Largest difference in peak and hole

(e A�3)1.969 and �1.871

S. Roy et al. / Polyhedron 27 (2008) 105–112 107

evaporator, leaving a red coloured oily sticky liquid. Thiswas then mixed with water (30 mL) and neutralized withAcOH (pH �5), which afforded a yellow shining powder.The solid was filtered off, washed thoroughly with metha-nol and dried in vacuo over fused CaCl2. Yield (3.32 g,60%). M.p. >250 �C. MS (m/z) 245 (M+, 100%). IR/cm�1: mNH 3350; mCO/CN 1659(s), 1548(s); mN-Npz 1065(s);mpy 1020(s). Anal. Calc. for C11H12N6O: C, 48.97; H,4.49; N, 40.00. Found: C, 48.91; H, 4.51; N, 39.93%.

2.4. Preparation of the complex

2.4.1. [Cu4(PzOAPyz)4(ClO4)2](ClO4)2 Æ 6H2O (1)

The ligand PzOAPyz (0.122 g, 0.5 mmol) was added to ahot solution of Cu(ClO4)2 Æ 6H2O (0.185 g, 0.5 mmol) inH2O–CH3OH (30 mL) (60:40 v/v). The resulting suspensionwas stirred at 60 �C until complete dissolution of the ligandoccurred. The resulting deep green solution was filtered toremove any undissolved ligand and left at room tempera-ture. Dark green crystals suitable for X-ray diffraction wereisolated from the filtrate after standing for several days(yield 0.108 g, 40%). IR/cm�1: mNH/H2O 3371, 3458; mCO/CN

1669(s), 1536(s); mN-Npz 1084(s); mpy 1032(s). UV–Vis (kmax):316(sh), 400(sh), 730(br) nm. lRT 3.42 B.M. Anal. Calc. for[Cu4(PzOAPyz)4(ClO4)2](ClO4)2 Æ 6H2O (1): C, 27.63; H,2.99; N, 22.56. Found: C, 27.49; H, 3.12; N, 22.51%.

2.5. Crystallographic data collection and refinement of the

structure

Selected crystal data for 1 are given in Table 1. Data col-lections for 1 were made using a Bruker SMART CCDarea detector diffractometer equipped with a Mo Ka radi-ation (k = 0.71073 A) source in the / and x scan modes at100(2) K. Cell parameters were determined and refinedusing SMART software [21]. Data reductions were carriedout using the SAINT program [22]. The structure was solvedby conventional direct methods and refined by full-matrixleast square methods using F2 data. SHELXS-97 and SHEL-

XL-97 programs [23] were used for structure solution andrefinement, respectively. H-atoms bonded to water oxygenshad their coordinates refined. Refinement of F2 was doneagainst all reflections. The weighted R-factor wR and good-ness-of-fit S are based on F2, conventional R-factors R arebased on F, with F set to zero for negative F2. The thresh-old expression of F2 > 2r (F2) is used only for calculatingR-factors (gt), etc. and is not relevant to the choice ofreflections for refinement. R-factors based on F2 are statis-tically about twice as large as those based on F, and R-fac-tors based on all data will be even larger.

3. Result and discussion

3.1. Syntheses

The ditopic ligand PzOAPyz (Scheme 1) on treatmentwith a water–methanol solution (60:40 v/v) of copper per-

chlorate hexahydrate in a 1:1 ratio yielded an interestingorthogonal ferromagnetically coupled tetracopper(II)2 · 2 homoleptic square grid complex. In methanol, thecomplex readily precipitated out from the metal–ligandreaction mixture, hence a water–methanol (60:40 v/v)mixed solvent system was chosen as the reaction mediumto prevent the precipitation. The formation and stabilityof this self-assembled square tetrameric complex may bevisualized in the following way (Scheme 2).

Each of the four chelating ditopic ligands in the tetra-meric Cu4(l-O4) core coordinates to any two neighbouringCu(II) centers, occupying three and two coordination sitesrespectively, using the alkoxide fragment as a bridger. Therepetition of this process leads to the formation of the self-assembled square Cu4(l-O4) unit, where the successive fourligand molecules are orthogonal in orientation.

3.2. Description of the crystal structure

[Cu4(PzOAPyz)4(ClO4)2](ClO4)2 Æ 6H2O (1)

The structure of the tetranuclear cluster cation complexis illustrated in Fig. 1. An ORTEP view of the asymmetricunit is shown in Fig. 2. Selected crystal data for 1 are sum-marized in Table 1. Important bond distances and angles

N

N

N

NH2

NHN

N

CH3

OH

Cu(ClO4 )2 . 6H2O

H2 O CH3OH

NN

N

NH2

NHN

N

CH3

O

N

N

N

NH2

NH

N

N

CH3

O

NN

N

NH2

NHN

N

CH3

O

N

N

N

NH2

NH

N

N

CH3

O

Cu

Cu

Cu

Cu

ClO4

ClO

-

4

Scheme 2.

Fig. 1. Structural representation of the cationic complex 1.

Fig. 2. Ortep diagram of the asymmetric unit and atom numberingscheme of the cationic complex 1.

Table 2Selected bond distances (A) and angles (�) in 1

Cu(1)–N(1A) 2.052(3) Cu(2)–N(6A)#1 1.966(3)Cu(1)–N(4A) 1.906(3) Cu(2)–N(1B) 2.048(3)Cu(1)–N(6B) 1.967(3) Cu(2)–N(4B) 1.908(3)Cu(1)–O(1A) 1.965(2) Cu(2)–O(1A)#1 2.279(2)Cu(1)–O(1B) 2.285(2) Cu(2)–O(1B) 1.970(2)Cu(1)–O(13) 2.663(2)

N(4A)–Cu(1)–O(1A) 79.92(11) N(4B)–Cu(2)–N(6A)#1 175.20(12)N(4A)–Cu(1)–N(6B) 174.19(12) N(4B)–Cu(2)–O(1B) 79.22(11)O(1A)–Cu(1)–N(6B) 99.43(10) N(6A)#1–Cu(2)–O(1B) 101.35(11)N(4A)–Cu(1)–N(1A) 79.79(12) N(4B)–Cu(2)–N(1B) 79.82(12)O(1A)–Cu(1)–N(1A) 159.70(11) N(6A)#1–Cu(2)–N(1B) 98.62(12)N(6B)–Cu(1)–N(1A) 100.81(12) O(1B)–Cu(2)–N(1B) 156.24(11)N(4A)–Cu(1)–O(1B) 110.46(11) N(4B)–Cu(2)–O(1A)#1 108.91(10)O(1A)–Cu(1)–O(1B) 92.06(9) N(6A)#1–Cu(2)–O(1A)#1 75.72(10)N(6B)–Cu(1)–O(1B) 75.30(10) O(1B)–Cu(2)–O(1A)#1 100.72(9)N(1A)–Cu(1)–O(1B) 94.55(10) N(1B)–Cu(2)–O(1A)#1 96.56(10)N(4A)–Cu(1)–O(13) 87.61(10) Cu(1)–O(1A)–Cu(2)#1 138.10(12)O(1A)–Cu(1)–O(13) 100.89(8) Cu(2)–O(1B)–Cu(1) 137.69(12)N(6B)–Cu(1)–O(13) 86.85(10)N(1A)–Cu(1)–O(13) 78.74(9)O(1B)–Cu(1)–O(13) 159.50(8)

108 S. Roy et al. / Polyhedron 27 (2008) 105–112

are listed in Table 2. A view of the tetranuclear core,depicting the immediate donor atoms only, is shown inFig. 3. The cationic cluster fragment consists of a squarearrangement of Cu(II) centers bridged just by alkoxideoxygen atoms. The two bridging oxygen atoms (O1A) are

located below the Cu4 plane and other two bridging oxygenatoms (O1B) are located above the plane in boat-like con-formation. The two five coordinated and two six coordi-nated copper(II) centers of the Cu4 (l-O4) core arearranged in a slightly distorted square (displacement ofmetals from the Cu4 least square plane; two Cu(1)+0.122 A, two Cu(2) �0.122 A) and are bridged by fouralkoxide oxygens (two O1A and two O1B) from the fourdeprotonated ligands, arranged in pairs above and belowthe Cu4 pseudo plane in a boat-like conformation. TheO–Cu–O angles fall in the range 92–100� and the Cu–Cuseparations are close to 4 A (Cu1–Cu2 3.97 A, Cu2–Cu13.96 A). The ligands allow an orientational distinctionamong its four moieties and are arranged in two groupsof parallel pairs, with each ligand in a roughly eclipsedarrangement compared with its partner. The total coordi-nation sites requirement of the four metals (22 bonds) iscompleted by four ligands, which fill 20 coordination sitesand two ClO4

� ions, binding to each of the diagonallyrelated Cu1 centers. The square pyramidal Cu2 centers

Fig. 3. Structural representation and atom numbering scheme of thetetranuclear core in 1.

Table 3Hydrogen bonding parameters in 1 (A) and (�)

D–H. . .A d(D–H) d(H. . .A) d(D. . .A) \(DHA)

N(3A)–H(3AB). . .O(3W) 0.88 2.11 2.980(6) 171.9O(3W)–H(3W1). . .O(11)#6 0.82 2.10 2.892(8) 160.3

Symmetry transformations used to generate equivalent atoms: #6 �x + 1,�y, �z + 2.

0 50 100 150 200 250 3001.5

1.8

2.1

2.4

2.7

3.0

0 10000 20000 30000 40000 500000

1

2

3

4

M /

Nβ

H / G

χ MT

/ cm

3 mol

-1m

ol-1

T / K

Fig. 4. Plots of vmT vs. T and M/Nb vs. H at T = 2 K (inset) for 1. Solidlines correspond to the best fit (see text).

S. Roy et al. / Polyhedron 27 (2008) 105–112 109

have a N4O chromophore (all the donor centers are fromligand moieties), while the axially distorted octahedralCu1 centers have a N4O2 chromophore (the additionalsixth donor center is occupied by the perchlorate oxygen).

The copper–oxygen (alkoxide) bond distances are quitevariable, but an interesting trend is observed based onthe copper chromophore. The two Cu1 centres have longand short contacts to O1B and O1A, respectively (Cu1–O1B 2.285(2) A, Cu1–O1A 1.965(2) A). Similarly the othertwo Cu2 centres also have alternate long and short contactsto O1A and O1B respectively (Cu2–O1A 2.279(2) A, Cu2–O1B 1.970(2) A). This establishes an asymmetric, alternat-ing long–short bridging arrangement within the Cu4 squareplane. The Cu–O–Cu angles fall in the range 137.69–138.10�. Both the separations and angle values are quitetypical for these grid systems [14–16].

The most prominent feature of the structure involves theencapsulation of the two five-coordinate and two six-coor-dinate Cu(II) centers within the grid, an unusual feature inrelated non-homoleptic Cu4 (2 · 2) square grid complexes[17,18]. Hexa-coordinated Cu(II) undergoes Jahn-Tellerdistortion and in this case each of two six-coordinateCu(II) ions undergo a tetragonal elongation. The axis ofelongation of the copper center includes two bridging oxy-gen connections (O1A and O1B), leading to a situationwhere the copper equatorial planes are tilted at 86.45� tothe neighbouring penta-coordinated metal center. Thisorbital orthogonality situation leads to significant intramo-lecular ferromagnetic interaction. In the asymmetric unit,the two independent perchlorate groups are disordered,one about a twofold axis and the other about an inversioncenter. Systematic and regular solvent and perchloratechannels alternate in between the layer structures of thecluster molecules. This stabilizes the lattice structurethrough hydrogen bonding. In the tetrameric complex,there are also weak hydrogen bonds (Table 3) betweenH3AD and O3W and between H3W1 and O3W. Thismakes an infinite straight chain throughout the crystal.

The lattice structure involves a number of water mole-cules (here six) as is typical for grids in this class [14–18].In this crystal structure, one of the water molecules

(O1W) is present with unit-occupancy and there are alsotwo half-occupancy water molecules (O2W, O3W) in theasymmetric unit.

4. Magnetic behaviour

The variable temperature magnetic susceptibility datafor the compound were recorded between 300 and 2 K.Plot of vmT versus T in Fig. 4 shows a ferromagneticbehavior. The vmT value is 1.568 cm3 mol�1 K at 300 K,being close to that expected for four uncoupled S = 1/2spins (with g = 2.00). vmT increases slightly when tempera-ture is lowered and reached a maximum value of2.885 cm3 mol�1 K at 3 K. This maximum is consistentwith that expected for ferromagnetically coupled copper(II)ions.

The structure of 1 consists of copper ions linked by theligand (PzOAPyz), giving a tetranuclear compound. Tak-ing into account the compound topology (slight distortionfrom square array, see crystallographic data), three cou-pling parameters J1, J2 and J3 can be considered to inter-pret the magnetic interactions in the complex, using theHamiltonian:

H ¼ �J 1ðS1 � S2 þ S3 � S4Þ � J 2ðS1 � S4 þ S2 � S3Þ� J 3ðS1 � S3 þ S2 � S4Þ

Based on the structural data of the Cu4(l-O4) core, we haveconsidered that J1 = J2 = J and J3 = 0 (no cross-couplingconnection). For this square disposition, the En value can

2.20 2.22 2.24 2.26 2.28 2.30 2.32 2.34 2.36 2.385

6

7

8

9

10

J/ c

m-1

d(Cu-O)/ Å

Fig. 5. Plot of the J values vs. longer d(Cu–O) distances (5% error barswere added) for the ferromagnetic [2 · 2] square Cu4 complexes charac-terized to date (see Table 4).

110 S. Roy et al. / Polyhedron 27 (2008) 105–112

be obtained by using the Kambe method [24] from theHamiltonian:

H ¼ �J 1ðS1 � S2 þ S2 � S3 þ S3 � S4 þ S1 � S4Þ ð1ÞThe analysis of the experimental susceptibility data havebeen performed by the use of the expression:

vmT ¼ Ng2b2T ðf ðJ ; T Þ=3KÞ ð2ÞHere

f ðJ ; T Þ ¼ 6 expðJ=KT Þ þ 12 expð2J=KT Þ þ 30 expð3J=KT Þ1þ 3 expðJ=KT Þ þ 7 expð2J=KT Þ þ 5 expð3J=KT Þ

ð3ÞThe best fit parameters found are: J = +7.47 cm�1 and g =2.03 with an error R = 4.2 · 10�5, where R = v[(vmT)obs �(vmT)calc]

2/v(vmT)obs2. The field dependence of magnetiza-

tion (0–5 T) measured at 2 K (Fig. 4, inset) suggests thatthe magnetization tends to 4.0 electrons, which is close tothe expected value for the ferromagnetically coupled fourCuII ions (S = 1/2 and g = 2.03).

4.1. Magneto-structural correlations

The value of the superexchange parameter J = +7.47cm�1 for compound 1, should be considered as normal,taking into account the structural and magnetic datareported in the literature [12,14,15,25]. In Table 4, wereport significant structural data and the calculated J

values for the published ferromagnetic [2 · 2] square Cu4

complexes, along with those of complex 1. These fit nicelyinto the list. It might be interesting to compare the ferro-magnetic behavior of these complexes, which increaseswhen the Cu–O distance decreases, as can be observed inFig. 5. The variation observed can be rationalized-basedon the Hay–Thibeault–Hoffmann model [26] where the glo-bal coupling constant is given by

J ¼ 2Kab �ðe1 � e2Þ2

J aa � J ab¼ J F þ J AF ð4Þ

The poor overlap between the nearly orthogonal dx2�y2

orbitals (Fig. 6) approaches to zero or reduces considerably

Table 4Significant average structural and magnetic data for ferromagnetic [2 · 2] squ

Compounda d(Cu� � �O) (A) Cu

[Cu4(POAP-H)4(H2O)2](ClO4) Æ 4H2O[Cu4(POAPZ-H)4(H2O)](NO3)4 Æ 3H2O 2.31 14[Cu4(6POAP-H)4](ClO4)4 2.24 13[Cu4(PZOAP-H)4](NO3)4 Æ 3H2O 2.28 14[Cu4(MPOAP-H)4](ClO4)4 Æ 8H2O 2.22 13[Cu4(IOAPM–H)4](ClO4)4 Æ 3CH3CN 2.33 13[Cu4(PzOAP)4(NO3)2](NO3)2 Æ 4H2O 2.37 14[Cu4(pcoap-H)2(pcaop-2H)2](NO3)2 Æ 15H2O 2.27 13[Cu4(PzOAPyz)4(ClO4)2](ClO4)2 Æ 6H2O 2.28 13

a Abbreviations used: POAP = N3-(2-pyridyl)-2-pyridinecarboxamidrazonhydroxy methyl-2-pyridine carboxylate; MPOAP = 6-methyl-2-picolinic acidmethyl-2-pyridine carboxylate; Pcoap = 6-methyloxycarbonyl-2-pyridinecarbamidrazone.

the antiferromagnetic contributions (second term). In thissituation, we would expect the first term, 2Kab, to dominategiving overall positive and large J values, which diminishwith longer Cu–O distances as we can observe as a generaltrend (Fig. 5).

On the other hand, the DFT calculation carried outon the real structure of complex 1 confirms with accu-racy the value of the magnetic interaction reportedbefore, being 7.2 cm�1. The computational methodologywas reported recently in a similar case [15]. The analysisof the spin density distribution shows the predominanceof a delocalization mechanism in the coordinated atomsto the copper ions (Fig. 6) [27]. The spin population onthe copper atoms is around 0.63e�, and the missing spindensity, relative to one unpaired electron, appearsmainly to be delocalized over the bridging ligands(�0.13e�). The topology of the SOMOs and the spindensity map in this case indicates that adjacent copper(II)centres are quasi orthogonal, leading a ferromagneticinteraction.

are Cu4 complexes

–O–Cu (�) d(Cu� � �Cu) (A) Jexp (cm�1) Reference

+7.3 [12]0.6 4.04 +9.89.3 3.97 +9.40.6 4.05 +8.29.2 3.95 +7.5 [20]9.7 4.05 +7.20.1 4.09 +5.4 [15]9.7 4.02 +7.2 [14]7.9 3.97 +7.7 this work

e; POAPZ = N3-(2-pyridyl)-2-pyrazinecarboxamidrazone, 6POAP = 6-; IOAPM = 4-methyl-5-imidazole carboxylic acid hydrazide; PzOAP =oxylic acid hydrazide; PzOAPyz = N3-(3-pyrazolyl)-2-pyrazinecarbox-

Fig. 6. Spin density distribution for 1, corresponding to the S = 2 groundstate, showing a nearly orthogonality between the adjacent copper(II)atoms. White regions indicate positive spin populations.

S. Roy et al. / Polyhedron 27 (2008) 105–112 111

5. Concluding remarks

The mixed pyrazole-pyrazine-based ditopic ligand PzO-APyz forms a homoleptic 2 · 2 square grid-complex in thepresence of the Cu(II) ion, in which the pyrazole ringsremain protonated. The structural backbone of the ligandcontains two diazine nitrogens linking two peripheral het-erocyclic rings. The ligand is made by tracing the same syn-thetic route as before [15] in which a hydrazide was reactedwith a methyl imino ester. This leads to the desired ligandwith a pyrazole and a pyrazine fragment at the two ends.The alkoxide fragment (–OH group) in the ligand PzOA-Pyz plays an important role towards the self-assembly pro-cess, forming a grid by bridging two adjacent coppercenters simultaneously. The –OH group is deprotonatedduring the course of the reaction and serves as a ferromag-netic mediator between the two adjacent paramagneticcopper ions. The –NH2 groups are not involved in thecoordination. The ligand donor set combination providesa template for binding the two adjacent metals which leadsto the formation of the cluster by a self-assembly process.The four metal centers in the tetrameric complex arearranged in a distorted square shape with two pairs ofroughly parallel ligands. The self-assembly mechanism isdifficult to predict but one could rationalize an initial stepof coordination of the ligand at three sites, one pyrazole orpyrazine nitrogen, one diazine nitrogen and the adjacentdeprotonated –OH, in a meridional fashion.

In the present grid complex, the two copper centers aresquare pyramidal and other two are axially distorted octa-hedral. In the Cu4(l-O4) core, alternative short and longcontacts lead to an unsymmetrical orientation from a per-fect square grid arrangement. However, in the Cu4(l-O4)unit, the main backbone of the complex arises through a

self-assembly process. This is controlled by employing thesense that a predetermined structural arrangement is logi-cally based on the strategic positions of the donor groupswithin the ligand moiety. Variable temperature magneticsusceptibility studies show that the square copper clusterexhibits an intramolecular ferromagnetic spin exchange,associated with the orthogonal alkoxide bridging arrange-ment and the close proximity of the copper centers. Theorigin of this ferromagnetic coupling lies in the approxi-mately orthogonal orientation of the magnetic orbitalsdx2�y2 .

Acknowledgements

Financial support [01(1916)/04/EMR-II] from theCouncil of Scientific and Industrial Research (CSIR),New Delhi, India is gratefully acknowledged. M. S. El Fal-lah is grateful for the financial support given by the Span-ish (CTQ2006-01759 and CTQ2005-08123-C02-02/BQU)and Catalan (2005SGR-00593 and 2005SGR-00036) gov-ernments. J. Tercero is grateful to the Centre de Computac-io de Catalunya (CESCA) with a grant provided byFundacio Catalana per a la Recerca (FCR) and the Uni-versitat de Barcelona.

Appendix A. Supplementary material

CCDC 649860 contains the supplementary crystallo-graphic data for 1. These data can be obtained free of chargevia http://www.ccdc.cam.ac.uk/conts/retrieving.html, orfrom the Cambridge Crystallographic Data Centre, 12Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplemen-tary data associated with this article can be found, in theonline version, at doi:10.1016/j.poly.2007.08.040.

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