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Inorganica Chimica Acta 318 (2001) 8–14

Synthesis, crystal structure, spectroscopic characterisation andmagnetic properties of [Cu2(BIBM)2(C2O4)2]·4H2O

(BIBM=bis(2-imidazolyl)bis(methoxycarbonyl)methylmethane)

Hugo Nunez, Juan-Jesus Timor, Juan Server-Carrio, Lucıa Soto, Emilio Escriva *Departament de Quımica Inorganica, Uni�ersitat de Valencia, c/ Vicent Andres Estelles, s/n, 46100 Burjassot, Valencia, Spain

Received 23 October 2000; accepted 14 February 2001

Abstract

The structure and spectroscopic and magnetic properties of bis(�-1,2,3-oxalato)bis[bis(2-imidazolyl)bis(methoxycarbonyl)-methylmethane]dicopper(II) tetrahydrate are described. The compound is built of centrosymmetric neutral dimeric[Cu2(BIBM)2(C2O4)2] entities linked through hydrogen bonds involving water molecules and oxalate groups. In the dimeric unitthe two centrosymmetrically related copper — which are involved in CuN2O2O2� chromophores lying in an elongated octahedralenvironment — are bridged through the oxalate group which acts in a bidentate–monodentate (�-1,2,3) fashion. Both electronicand EPR spectra are indicative of an essentially dx 2−y 2 ground state for the copper(II) ions. Magnetic susceptibility measurementsin the range 1.8–200 K show very weak antiferromagnetic exchange between the copper(II) ions (2J= −0.35 cm−1). This featureis discussed on the basis of the relative orientation of the coordination polyhedron around the metal atom and the bridgingnetwork. © 2001 Published by Elsevier Science B.V.

Keywords: Crystal structures; Magnetism; Copper complexes; Oxalato complexes; Dinuclear complexes

1. Introduction

Over the last two decades increasing interest has beendevoted to study of exchange interactions betweenmetal ions through extended bridging groups in dini-clear and polynuclear metal complexes [1–3]. In thiscontext, owing to the proved ability of the oxalatoligand to propagate magnetic interactions, the oxalato-bridged systems have been one of the most activelystudied [4–11]. In particular, the magnetic interactionsin dimeric copper(II) oxalato-bridged compounds havebeen thoroughly analysed and several papers have beendevoted to explore magneto-structural correlations[12,13].

Four different bridging modes have been reported forthe oxalato group (Scheme 1) the first one (1a) being,by far, the more frequently exhibited. The number of

compounds structurally characterized in which the ox-alate group acts as tridentate ligand (1b, 1c) is scarce.

Scheme 1.* Corresponding author. Tel.: +34-6-386 4534; fax: +34-6-386

4960.

0020-1693/01/$ - see front matter © 2001 Published by Elsevier Science B.V.

PII: S 0 0 20 -1693 (01 )00390 -5

H. Nunez et al. / Inorganica Chimica Acta 318 (2001) 8–14 9

Scheme 2.

2.3. Physical measurements

The IR spectra (KBr pellets) were recorded on a PyeUnicam SP 2000 spectrophotometer. Diffuse reflectancespectra were obtained using a Perkin–Elmer Lambda 9UV/VIS/IR spectrophotometer. Polycrystalline powderEPR spectra were recorded at room temperature (295K) on a Bruker ESP-300 working at X-band (9.45GHz). Magnetic susceptibility was measured by meansof a commercial SQUID magnetometer, Quantum De-sign model MPMS7 down to 1.8 K and using a mag-netic field of 0.5 T. Mercury(II) tetrakis(thiocyanato)-cobaltate(II) was used as a susceptibility standard. Theexperimental susceptibilities were corrected both for thediamagnetic contributions using the Pascal constants[22] and for the temperature independent paramag-netism of the Cu(II) ion estimated to be 6·10−5

cm3.mol−1 per copper(II) ion.

2.4. X-ray structure determination

The selected blue prismatic crystal, with approximatesize 0.15×0.20×0.25 mm3, was mounted on an Enraf-Nonius CAD-4 single-crystal diffractometer and inten-sity measurements were carried out at 293 K usinggraphite monochromated MoK� radiation (�=0.71069A� ). The unit cell dimensions were determined from theangular settings of 25 reflections with 8���22°. Theintensity data of 3833 reflections were measured be-tween the limits 1���25° using �–2� scan technique,variable speed, width (0.80+0.35 tan �)° in �, andmaximum scan time of 70 s per reflection in the hklrange 0 to 9, −11 to 11, −16 to 16. Three reflectionswere measured every hour as intensity controls withoutsignificant decay. Data reduction was done with theprogram XRAY-76 [23]. The average of some doublemeasured reflections gave an Rint=0.0087. From the3398 independent reflections 2817 were considered ob-served with the Fo�4�(Fo) criterion. An empiricalabsorption correction, following the procedure DIFABS

[24] with minimum and maximum correction coeffi-cients of 0.74 and 1.00 was applied.

The structure was solved by direct methods using theprogram SIR-92 [25]. All non-hydrogen atoms wereanisotropically refined by least-squares on F2 usingSHELXL-97 [26]. The hydrogen atoms of the ligandmolecule were geometrically calculated and only thehydrogen atoms of one of the two water molecules perasymmetric unit were located by difference synthesis.Geometrical calculations were made with PARST [27].Graphical manipulations with ORTEP3 for Windows[28]. Other relevant parameters of the crystal structuredetermination are listed in Table 1.

In general, these tridentate coordination modes havebeen observed in systems containing the [Cu(C2O4)2]2−

entity with long Cu�–O distances and exhibiting weakantiferromagnetic coupling between the copper(II) ions[14–20]. The aim of this paper is to present a newcopper(II) oxalato-bridged dimer with the relativelyunusual �-1,2,3 coordination mode exhibited for theoxalato group. In this novel dimeric compound, theBIBM (BIBM=bis(2-imidazolyl)bis(methoxycarbonyl)-methylmethane) a bis(imidazole) ligand (Scheme 2) hasbeen used as peripheral ligand.

2. Experimental

2.1. Synthesis of the ligand

Bis(2-imidazolyl)bis(methoxycarbonyl)methyl-methane (BIBM) was prepared according to Joseph etal. [21] and characterised by 1H NMR, 13C NMR andIR spectroscopies and powder X-ray diffraction. Allother reagents were used as supplied. Elemental analy-ses (C; H; N; Cl) were performed by Servei de Mi-croanalisi, Consell Superior d’InvestigacionsCientifiques, Barcelona, Spain.

2.2. Preparation of the complex

A water+ethanolic (1:2) solution of BIBM (0.25mmol, 25 ml) and an aqueous solution of CuCl2·2H2O(0.25 mmol, 5 ml) were mixed together. To the resultedsolution, 2 ml of an aqueous solution of Na2C2O4

(0.125 mmol) was added and immediately yielded ablue–greenish powder precipitate, which was separatedby filtration. The filtrate was allowed to stand at roomtemperature for about several days to produce singleblue crystals of the title compound. The resulting crys-tals were separated by filtration and washed with awater+ethanol mixture. Anal. Calc. for CuC14H18N4-O10: C, 36.09; H, 3.90; Cu, 13.64; N, 12.03. Found: C,35.93; H, 3.96; Cu, 13.53; N, 11.95%.

H. Nunez et al. / Inorganica Chimica Acta 318 (2001) 8–1410

3. Results and discussion

3.1. Crystal structure description

The structure of the title compound is built up ofcentrosymmetric dimeric [Cu2(BIBM)2(C2O4)2] entitieswhich are linked through hydrogen bonds involvingwater molecules and imidazole/oxalato moieties. Fig. 1shows a perspective view of the dimeric unit with theatomic numbering scheme.

In the dimeric entity the two metal atoms are con-nected through the oxalate groups which act in a

monodentante–bidentate mode, leading to a metal–metal separation of 5.08(3) A� . The coordination aboutthe copper atom can be described as a very elongatedoctahedron. The equatorial plane is comprised of twooxygen atoms belonging to an oxalate group and twonitrogen atoms from a BIBM molecule. This approxi-mately square-planar coordination is octahedrally ex-tended by weak apical contacts to two oxygen atoms,O5 (from a carboxymethyl group) and O3� from theoxalate ion which is coordinated to the centrosymmetri-cally related copper(II) ion (O5–Cu–O3� 168.1(1)°).The coordination geometry about the copper atom canthus be viewed as 4+2* (CuN2O2O2� chromophores).The Cu–N and Cu–Oeq bond distances (average valuesof 1.942(2) and 1.958(2) A� , respectively) are similar tothose reported for related systems [8]. The four basalatoms are not coplanar, showing a slight but significanttetrahedral distortion. In fact, the N2 and O1 atoms aredisplaced by 0.133(3) A� and 0.139(3) A� on one side ofthe CuN2O2 least-squares plane while the two remain-ing atoms are 0.141(3) A� (N3) and 0.190(3) A� (O2) onthe other side, with the copper atom lying nearly in thatplane (it is displaced (0.001(1) A� towards O3�). TheCu–O3� distances exceeds the Cu–Oeq ones (see Table2) by ca. 0.94 A� , and are similar to those reported inrelated oxalato–copper(II) complexes where theC2O4

2− groups act in a bidentate–monodentate fashion(see Table 4) as well as in several 4+2* and 4+1+1*CuO6 octahedra [29]. According to Hathaway’s crite-rion [30] the axial oxygen atoms are in the borderline tobe considered as semicoordinated. In fact, the tetrago-nality parameter [31] (T=0.67) is significantly low,indicating a pronounced tetragonal distortion for thecoordination polyhedron of the copper atom with avery weak Cu–Oax interaction.

All the relevant parameters for the BIBM ligand arein good agreement with the data obtained for therelated HBIP (HBIP is the 3,3-bis(2-imida-zolyl)propionic acid) molecule in different HBIP-con-taining complexes [8,32–34]. Each imidazole ring isplanar (largest deviation 0.003 A� ). The imidazole moi-eties of each ligand however are not coplanar (dihedralangle of 35.3(2)°) owing to the coordination to themetal.

The oxalato group is not planar, with deviations upto 0.124 A� from the least-squares plane. This distortionis associated with a twisting about the C1–C2 bondwhich leads to a dihedral angle of 9.2(4)° between theO1–C1–O3 and O2–C2–O4 groups. A similar distor-tion is frequently observed in terminal oxalato ligands[8,35]. The C–O bond distances satisfy the trendC–Ocoord�C–Osemi-coord�C–Ouncoord, as expectedfrom the polarisation of the charge density towards themetal-bonded oxygen atoms. Furthermore, both unco-ordinated and semicoordinated oxygen atoms (O4 andO3 respectively) are also affected by H-bonding (seebelow).

Table 1Crystal data for the complex

Formula C28H36N8O20Cu2

M 465.86Symmetry triclinic, P1�Unit cell dimensions

a (A� ) 7.858(1)b (A� ) 9.916(1)c (A� ) 13.668(1)� (°) 69.69(1)� (°) 75.65(1)� (°) 80.02(1)

U (A� 3) 963.2(2)Z 1Dcalc (g cm−3) 1.61F(000) 478

11.9� (cm−1)Goodness-of-fit 1.095�max, �min (e A� −3) 0.46, −0.28R1 0.041

0.113wR2a

a w=1/[�2(Fo2)+(0.0624P)2+0.52P ], where P= (Fo

2+2Fc2)/3.

Fig. 1. Perspective view and atomic numbering of the[Cu2(BIBM)2(C2O4)2] dimeric units. C symbol has been omitted forclarity.

H. Nunez et al. / Inorganica Chimica Acta 318 (2001) 8–14 11

Table 2Bond lengths (A� ) and bond angles (°) for the title complex a

Copper coordination sphereBond lengths

Cu–N(2) 1.960(2)Cu–O(1) 1.954(2)Cu–N(3)1.929(2) 1.956(2)Cu–O(2)

2.892(2)Cu–O(5) Cu–O(3I) 2.876(2)

Bond anglesO(1)–Cu–O(2) O(2)–Cu–O(3I)84.5(1) 89.2(1)

N(2)–Cu–N(3)O(1)–Cu–N(2) 91.3(1)171.6(1)N(2)–Cu–O(5)92.8(1) 83.2(1)O(1)–Cu–N(3)

104.7(1)O(1)–Cu–O(5) N(2)–Cu–O(3I) 85.0(1)87.1(1)O(1)–Cu–O(3I) N(3)–Cu–O(5) 80.2(1)

N(3)–Cu–O(3I)92.8(1) 100.7(1)O(2)–Cu–N(2)169.5(1)O(2)–Cu–N(3) O(5)–Cu–O(3I) 168.1(1)

90.8(1)O(2)–Cu–O(5)

BIBM moleculeBond lengths

N(3)–C(8)1.202(4) 1.389(4)O(5)–C(11)O(6)–C(11) 1.309(4) N(4)–C(7) 1.337(4)

N(4)–C(9)1.438(5) 1.366(4)O(6)–C(12)1.207(5)O(7)–C(13) C(3)–C(7) 1.498(4)1.307(5)O(8)–C(13) C(3)–C(4) 1.500(4)

C(3)–C(10)1.475(6) 1.558(5)O(8)–C(14)1.343(4)N(1)–C(4) C(5)–C(6) 1.338(5)

C(8)–C(9)1.374(4) 1.335(4)N(1)–C(5)C(10)–C(13)N(2)–C(4) 1.509(5)1.313(4)C(10)–C(11)1.379(4) 1.524(5)N(2)–C(6)

N(3)–C(7) 1.322(4)

Bond anglesN(3)–C(7)–N(4) 110.2(3)C(11)–O(6)–C(12) 116.7(3)N(3)–C(7)–C(3)117.3(5) 126.9(3)C(13)–O(8)–C(14)N(4)–C(7)–C(3) 122.6(3)C(4)–N(1)–C(5) 107.5(3)C(9)–C(8)–N(3)106.3(3) 108.8(3)C(4)–N(2)–C(6)C(8)–C(9)–N(4)C(7)–N(3)–C(8) 106.9(3)106.1(2)C(13)–C(10)–C(11)107.9(3) 110.4(3)C(7)–N(4)–C(9)

C(7)–C(3)–C(4) C(13)–C(10)–C(3)110.5(2) 112.7(3)C(11)–C(10)–C(3)109.5(3) 110.4(3)C(7)–C(3)–C(10)

113.1(3)C(4)–C(3)–C(10) O(5)–C(11)–O(6) 125.3(4)O(5)–C(11)–C(10)N(2)–C(4)–N(1) 123.0(3)110.5(3)O(6)–C(11)–C(10)125.0(3) 111.5(3)N(2)–C(4)–C(3)

124.5(3)N(1)–C(4)–C(3) O(7)–C(13)–O(8) 124.9(4)106.3(3)C(6)–C(5)–N(1) O(7)–C(13)–C(10) 124.4(4)

O(8)–C(13)–C(10) 110.7(4)109.4(3)C(5)–C(6)–N(2)

Oxalate groupBond lengths

1.272(4)O(1)–C(1) O(4)–C(2) 1.217(4)C(1)–C(2)1.276(4) 1.528(5)O(2)–C(2)

O(3)–C(1) 1.234(4)

Bond anglesO(3)–C(1)–O(1) O(4)–C(2)–O(2)125.4(4) 126.2(4)

O(4)–C(2)–C(1) 118.9(3)O(3)–C(1)–C(2) 119.5(3)O(2)–C(2)–C(1) 115.0(3)115.1(3)O(1)–C(1)–C(2)

a Symmetry operator: I −x, −y, −z+1.

chains running along this axis with a Cu–Cu(III) dis-tance of 9.916(1) A� (for symmetry operators see Table3). In addition, these chains are cross-linked by anarray of hydrogen bonds involving both watermolecules (O9 and O10): the N1 acts as hydrogendonor towards O9, which is also acceptor of O10. Atthe same time, O10 acts as hydrogen donor towards anO3 atom belonging to the neighbouring [Cu(II)(BIBM)-(C2O4)] unit (Cu–Cu(II) distance of 13.68(3) A� ) connect-ing the dimers in a layered structure along the [1,1,0]direction. Therefore, the bulk structure of the titlecompound also might be viewed as a two-dimensionalarray in the xy plane of [Cu(BIBM)(C2O4)] units —through N–H···O interactions — linked in pairsthrough the axial and weak Cu···Ooxalate interactions.

3.2. Vibrational and electronic spectra

The IR spectrum shows a band in the 3600–3300cm−1 range assignable to (OH) stretching vibrationsof lattice water molecules [36]. The N–H stretchingvibrations of complex appear in the 3165–3020 cm−1

region and their frequencies are consistent with theexistence of hydrogen bonds between imidazole N–Hand other groups [36]. In the 1600–800 cm−1 regionthe spectrum displays a large number of absorptionswhich correspond to the coexistence of the imidazole,oxalate and methoxycarbonyl moieties, hindering a

Table 3Hydrogen bonds in the [Cu2(BIBM)2(C2O4)2]·4H2O crystal a

X–H···Y H···YX···Y �X–H···YX–H(A� ) (A� )(A� ) (A� ) (°)

0.89 2.766(4)O(10)–H(10A)···O(3II) 1.96 1490.91 2.851(7)O(10)–H(10B)···O(9) 2.07 143

1690.86 1.96N(1)–H(1)···O(9) 2.804(4)2.323.009(4) 1370.86N(4)–H(4)···O(4III)

N(4)–H(4)···O(3III) 0.86 2.973(4) 2.25 142

a Symmetry operators: II x+1, y+1, z ; III x, y+1, z.

Fig. 2. Crystal packing of the title compound.

The hydrogen bonds listed in Table 3 mainly deter-mine the crystal packing shown in Fig. 2. The two N4atoms of each dimer act as hydrogen donors towardsthe O3 and O4 atoms of the corresponding neighbour-ings dimers in the y direction, thus affording infinite

H. Nunez et al. / Inorganica Chimica Acta 318 (2001) 8–1412

Fig. 3. Magnetic behaviour of [Cu2(BIBM)2(C2O4)2]·4H2O.

Table 4Structural and magnetic parameters for bidentate–monodentate oxalato bridges between copper(II) ions

J (cm−1)Compound Bridge Chromophore Ref.Cu�–O (A� )

−1.2 [16]Na2[Cu(C2O4)2]·2H2O �-1,1,2 CuO4O2� 2.803(2)�-1,1,2 CuO4O2�(pyH)2[Cu(C2O4)2]·H2C2O4 2.893(3) [17]�-1,1,2 CuO4O2�(ImH)2[Cu(C2O4)2] 2.875(3) −1.09 [19]�-1,1,2 CuN2O2O�[Cu(Him)2(C2O4)]2[Cu(Him)2(C2O4)(OH2)]2 2.837(2) −2.64 [20]�-1,2,3(NH4)2[Cu(C2O4)2]·2H2O [14,15]−0.62.74(1)CuO4O2��-1,2,3 CuN4O2� /CuO4[Cu(en)2][Cu(C2O4)2] 2.539(2) −1.95 [18]�-1,2,3[Cu2(BIBM)2(C2O4)2]·4H2O CuN2O2O2� 2.892(2) −0.35 this work

definitive assignment in some cases. Notwithstanding,by contrasting with those reported for several oxalatocomplexes [8] as well as with the IR spectrum of thefree BIBM molecule, some absorptions were tentativelyassigned. Therefore, the two absorptions observed at1725 and 1680 cm−1 were assigned to as(COO) of themethoxycarbonyl and oxalato groups, respectively.With regard to the s(COO) absorptions a broad bandis observed centered at ca 1435 cm−1 and the distinc-tion between the absorptions corresponding to theCO2Me and C2O4 groups is not possible. On the otherhand the medium-intensity band centered at ca. 1280cm−1 is consistent with the presence of pseudo-biden-tate oxalato groups [37–39].

The diffuse reflectance spectrum of the title com-pound exhibits a very broad absorption in the visibleregion with a maximum centered at ca. 16 900 cm−1,which is consistent with the presence of very elongatedCuN2O2O2� chromophores. Besides the d–d band, thespectrum displays two more intense bands centered atca. 31 000 and 36 000 cm−1, which can be assigned to�(imidazole)�Cu(II) (dx 2−y 2) LMCT and internal lig-and ���* transitions, respectively.

3.3. Electron spin resonance spectroscopy and magneticproperties

The room temperature polycrystalline X-band EPRspectrum of the title compound shows a wide axialsignal giving g��=2.285 and g�=2.045 (gav=2.13).Lowering the temperature to 100 K produces no effecton the shape and the position of the signal which isdevoid of any hyperfine structure. The g-values arethose expected for CuN2O2O2� chromophores with thecopper atom exhibiting a very elongated octahedralenvironment, and indicate a basically dx 2−y 2 groundstate for the Cu(II) ion. Moreover, increasing the spec-trometer gain, a weak-field signal of �Ms= �2 atg�4.2 can be clearly observed, even at room tempera-ture, indicating the existence of weak low-dimensionalmagnetic interactions.

The observed magnetic behaviour of the title com-pound agrees with the EPR interpretation. The mag-netic susceptibility exhibits a Curie–Weiss dependence.So, the whole susceptibility data were fitted to theexpression �=C/(T−�), affording C=0.429 cm3

mol−1 K and �= −0.12 K. From the Curie constantC=N�2g2S(S+1)/3k with S=1/2 a g value of 2.14

H. Nunez et al. / Inorganica Chimica Acta 318 (2001) 8–14 13

can be obtained, in excellent agreement with that ob-tained from the EPR spectrum. A plot of �MT versusthe temperature (Fig. 3) exhibits a slight decrease atT�10 K, corresponding to a variation of �eff between1.86�B (at 10 K) and 1.80�B (at 1.8 K) per copperatom, indicating very weak antiferromagnetic interac-tions between the copper(II) ions.

Taking into account the crystal structure of the titlecompound and in light of the very large interdimerCu···Cu distance, the only non-negligible interactionsbetween the paramagnetic centers must be the in-tradimer ones. Therefore, the studied compound mightbe considered, from the magnetic point of view as anassembly of non-interacting dimers. The magnetic be-haviour of [Cu2(BIBM)2(C2O4)2]·4H2O may describedby means of a dimer exchange equation. Since theexchange coupling constant is expected to be small(�J ��g�BH�1 cm−1), the application of the Bleaney-Bowers [40] expression would be inappropriate. There-fore, the data were fitted to the expression (1) derivedfrom the exchange Hamiltonian for a pair of exchange-coupled S=1/2 ions, which considers the external field[41].

�= [Ng�B sinh(g�BH/kT)]{H [exp(−2J/kT)

+2 cosh(g�BH/kT)+1]}−1+N� (1)

Magnetic data were least-squares-fitted to this equa-tion (solid line in Fig. 3), and with the g value fixed at2.14 (obtained from the Curie constant), a 2J value of−0.35 cm−1 was obtained, with the agreement factorR=4.31·10−5 (R is defined as �[(�M)obsd− (�M)calcd]2/�[(�M)obsd]2).

This behaviour may be understood in terms of thenature of the orbitals involved in the exchange interac-tions together with structural considerations on thebridging network. As discussed above, in the presentcompound the unpaired electron of the copper(II) ion isessentially described by a magnetic orbital built fromthe dx 2−y 2 metallic orbital with little contribution ofthe dz 2 orbital, and being localized basically in the basalplanes, which are nearly normal to the direction of theexchange propagation. Hence, as it is usual for axial–equatorial (parallel-planar) copper(II) dimers, the ob-served weak coupling must be attributed to the smallz2-type character acquired by the magnetic orbitals. InTable 4 are gathered the more relevant data for mag-netically and sctructural characterised copper(II) com-plexes with bidentate–monodentate oxalato bridges.The whole of the observed single-triplet gap energies inthese systems span a very cramped range (from −2.64to +0.35 cm−1) and only little variations are observedwith the different structural parameters. Thus, anymagneto-structural correlation may be outlined.

Acknowledgements

We thank Drs. F. Lloret and J.-V. Folgado for theirassistance with susceptibility measurements and EPRexperiences, respectively.

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