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Inorganica Chimica Acta 360 (2007) 2513–2517

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Preparation, crystal structure and magnetic property of abinuclear oxalate-bridged iron(III) compound:

Fe2(C2O4)Cl4(DMF)4 (DMF = dimethylformamide)

Bin Zhang a,*, Yan Zhang b, Mohamedally Kurmoo c

a Organic Solid Laboratory, BNLMS, CMS and Institute of Chemistry, The Chinese Academy of Science, Beijing 100080, PR Chinab Department of Physics, Peking University, Beijing 100871, PR China

c Laboratoire de Chimie de Coordination Organique, CNRS-UMR7140, Universite Louis Pasteur, 4 Rue Blaise Pascal, 67000 Strasbourg Cedex 1, France

Received 1 August 2006; received in revised form 19 October 2006; accepted 29 October 2006Available online 10 November 2006

Abstract

An oxalato-bridged binuclear iron(III) compound, Fe2(C2O4)Cl4(DMF)4 (DMF = dimethylformamide), was obtained by electrocrys-tallization for three weeks at 3.4 V and it displays a strong antiferromagnetic interaction of J = �6.74(4) cm�1.� 2006 Elsevier B.V. All rights reserved.

Keywords: Iron compound; Binuclear; Oxalate; X-ray crystal structure; Magnetic property

1. Introduction

The oxalate ion (C2O42�) is one among the most com-

mon organic connectors, which include cyanide, azide, dic-yanamide, hydroxide, carboxylates, etc., used to mediateefficient magnetic interaction between paramagnetic cen-ters in the search for molecular magnets. This is due toits range of versatile connection modes, and for its abilityto connect several metal centers as well as its delocalizedelectronic configuration. The ground state can be antiferro-magnetic, ferrimagnetic, ferromagnetic and canted antifer-romagnetic depending on the metal, the mode of bridgingand the presence of other ligands involved in the first coor-dination sphere of the metal centers [1]. It has also beenemployed in developing building blocks in the search formolecular magnetic superconductors and has so farresulted in some exceptional examples which include aparamagnetic organic superconductor [2], a ferromagneticmolecular conductor [3] and an antiferromagnetic insulator[4]. The latter employment is due to the fact that these lay-ers or linear polymeric anions act as templates to dictate

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

doi:10.1016/j.ica.2006.10.028

* Corresponding author. Tel.: +86 10 62558982; fax: +86 10 62569373.E-mail address: zhangbin@iccas.ac.cn (B. Zhang).

the packing of the organic donors within the crystals whichconsequently result in the different ground states. It is to benoted that in most cases, magnetic exchange interactionbetween the conduction electrons and the moment on theparamagnetic centers is either very weak or negligible.Coordination compounds of paramagnetic transition met-als combining oxalate-bridges and chlorine have produceda series of molecular magnets with ordering temperaturesas high as 70 K [5], and linear building blocks for weak-ferrromagnetic insulator and conductor having TN rangingfrom 4.5 to 20 K [6]. Consequently, searching for new com-pounds of Fe(III) with oxalate and chlorine can only helpus to understand the interactions between Fe(III) in bis-bidentate mode [7], and can also provide counterions aspotential building blocks for magnetic molecular conduc-tor [2–4]. Here, we report a binuclear Fe(III) compoundcontaining oxalate-bridge and chlorine, which is synthe-sized by electrocrystallization as well as its crystal structureand magnetic properties.

2. Experimental

BETS (bis(ethylenedithio)tetraselenafulvalene, 5.0 mg)and 50 mg of crystals of (Me4N)[Fe(C2O4)Cl2], which were

Table 1Crystallographic data of title compound

Empirical formula C14H28Cl4Fe2N4O8

Formula weight 633.90T (K) 293(2)Wavelength (A) 0.71073Crystal system triclinicSpace group P�1a (A) 8.3919(4)b (A) 8.7834(4)c (A) 11.2881(7)a (�) 70.248(2)b (�) 71.993(2)c (�) 64.423(2)V (A3) 693.17(6)Z 1Dc (g cm�3) 1.519l (Mo Ka) (mm�1) 1.473Number of reflections 3138Number of parameters 153Rint 0.0972wR2 0.0959R1 0.0441Goodness-of-fit 0.873Residual (e/A3) (minimum and maximum) 0.224, �0.357

Fig. 1. ORTEP drawing of a Fe2(C2O4)Cl4(DMF)4 dimer and the atomsnumbering scheme.

2514 B. Zhang et al. / Inorganica Chimica Acta 360 (2007) 2513–2517

quantitatively synthesized by the reaction of (Me4N)3-[Fe(C2O4)3] and FeCl3 Æ 6H2O in ethanol, were dissolvedin 25.0 ml C6H5Cl and 5.0 ml DMF (DMF = dimethyl-formamide) under an argon atmosphere. After three weeksunder an applied voltage of 3.4 V, yellow block crystals of1 were obtained on the cathode (Anal. Calc. forC14H24N4O8Cl4Fe2: C, 26.70; H, 3.84; N, 8.89; Cl, 22.51.Found: C, 26.36; H, 4.39; N, 8.90; Cl, 22.57%. IR mmax

(cm�1): 1682(s), 1609(s), 1367(m), 2928(s)).A single crystal of dimension 0.05 · 0.10 · 0.15 mm [3]

was mounted for X-ray data collection at room tempera-ture on a Nonius Kappa CCD diffractometer equippedwith monochromated Mo Ka (k = 0.71073 A) radiation.The intensity data were corrected for Lorentz and polariza-tion and finally for absorption using an empirical method[8]. The structure was solved by the direct method and dif-ference-Fourier map. Hydrogen atoms were found by cal-culation and treated with isotropic thermal factors. Allnon-hydrogen atoms were refined anisotropically by thefull-matrix method [9]. The crystallographic data are col-lected in Table 1.

A polycrystalline sample weighing 3.12 mg was used forthe magnetic susceptibility measurement using a QuantumDesign MPMS 7XL SQUID in an applied field of 1000 Oefrom 2 to 300 K. The isothermal magnetization was mea-sured at 2 K. The data were corrected for diamagnetismusing Pascal constants after subtraction of the contributionof the container [10].

3. Results and discussion

We have for several years been developing charge-trans-fer complexes of organic donors such as TTF and itsderivatives with paramagnetic building blocks such as½FeðC2O4Þ33�� and most recently, ½FeðC2O4ÞCl2

��n as coun-

terion in the search for organic–inorganic hybrids that maybehave both as a magnet and a superconductor. Severalcomplexes have been isolated with a gamut of electricaland magnetic ground states. We and others have found thatelectrical and magnetic ground states of the crystalsproduced by electrocrystallization depend on the solvents,current and voltage in addition to the organic donors andinorganic anions; for example, [TTF]7[Fe(C2O4)3]2 Æ 4H2Owas obtained from CH2Cl2, while [TTF]5[Fe2-(C2O4)5] Æ 2C6H5CN Æ 2H2O and [TMTTF]4[Fe2(C2O4)5] ÆC6H5CN Æ 4H2O were obtained from a mixture ofC6H5CN–CH3CN [4b], and (BEDT-TTF)4Fe2(C2O4)5 wasobtained from a mixture of CH2Cl2–CH3CN [4]. On theother hand, (BETS)4Fe2(C2O4)5 was obtained from a mix-ture of C6H5Cl and C2H5OH under 3.0 V and j-BETS2-FeCl4 and j-(BETS)2Fe(C2O4)Cl2 were obtained on thecathode of the cell operating at 3.2 V [4,6]. When wereplaced the C2H5OH, which is commonly used to increasethe solubility of the anion complexes in the electrocrystalli-zation process for charge-transfer complexes of BETS, byDMF and set the voltage to 3.4 V, a binuclear coordinationcompound Fe2(C2O4)Cl4(DMF)4 was obtained but not acharge-transfer complex [11]. It is rare that a coordinationcompound is obtained and not a charge-transfer salt.

In the crystal structure of 1, each Fe of a dimer is coor-dinated in a distorted octahedron to two oxygen atomsfrom oxalate, two oxygen atoms from DMF and two chlo-rine atoms. The asymmetric unit contains half of a dimer,which is one Fe, half oxalate, two chlorine and twoDMF molecules, and there is one neutral molecule per unitcell as shown in the perspective drawing of Fig. 1. Selected

Table 2Selected bond lengths and angles

Fe(1)–O(1) 2.017(2) Fe(1)–O(2) 2.034(2) Fe(1)–O(4) 2.085(2)Fe(1)–O(3) 2.088(2) Fe(1)–Cl(1) 2.272(1) Fe(1)–Cl(2) 2.276(1)O(1)–C(1) 1.237(4) O(2)–C(4) 1.254(4) O(3)–C(7)#1 1.253(4)O(4)–C(7) 1.250(4) C(1)–N(1) 1.307(4) C(4)–N(2) 1.293(4)N(1)–C(2) 1.445(4) N(1)–C(3) 1.462(4) N(2)–C(5) 1.444(5)N(2)–C(6) 1.471(4) C(7)–O(3)#1 1.253(4) C(7)–C(7)#1 1.536(7)

O(1)–Fe(1)–O(2) 166.27(10) O(1)–Fe(1)–O(4) 85.73(10)O(2)–Fe(1)–O(4) 84.10(9) O(1)–Fe(1)–O(3) 83.76(10)O(2)–Fe(1)–O(3) 85.22(9) O(4)–Fe(1)–O(3) 78.71(9)O(1)–Fe(1)–Cl(1) 95.12(8) O(2)–Fe(1)–Cl(1) 93.52(8)O(4)–Fe(1)–Cl(1) 171.10(8) O(3)–Fe(1)–Cl(1) 92.56(8)O(1)–Fe(1)–Cl(2) 94.11(8) O(2)–Fe(1)–Cl(2) 95.34(7)O(4)–Fe(1)–Cl(2) 91.38(7) O(3)–Fe(1)–Cl(2) 169.97(7)Cl(1)–Fe(1)–Cl(2) 97.39(4) C(1)–O(1)–Fe(1) 129.4(2)C(4)–O(2)–Fe(1) 125.3(3) C(7)–O(3)–Fe(1)#1 113.9(2)C(7)–O(4)–Fe(1) 114.7(2) O(1)–C(1)–N(1) 123.7(4)C(1)–N(1)–C(2) 120.8(3) C(1)–N(1)–C(3) 122.3(3)C(2)–N(1)–C(3) 116.9(3) C(4)–N(2)–C(5) 120.7(3)C(4)–N(2)–C(6) 121.7(3) C(5)–N(2)–C(6) 117.6(3)O(4)–C(7)–O(3)#1 127.3(3) O(4)–C(7)–C(7)#1 115.9(4)O(3)#1–C(7)–C(7)#1 116.8(4)

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

Table 3Hydrogen bonds of title compound

D–H H. . .A D. . .A \(DHA)

0.96 2.90 3.780(5) 153.0 C(5)–H(5C). . .Cl(1)_$10.96 2.69 3.594(4) 156.7 C(5)–H(5B). . .O(3)_$20.96 2.46 3.383(4) 160.4 C(6)–H(6C). . .O(4)_$30.96 2.95 3.824(4) 152.4 C(3)–H(3D). . .Cl(1)_$40.96 2.86 3.724(4) 149.5 C(3)–H(3C). . .Cl(2)_$5

Symmetric operation: $1 �1 + X,Y,Z; $2 1 � X, 1 �Y, 2 � Z; $3X,�1 + Y,Z; $4 X, 1 + Y,Z; $5 1 + X,Y,Z.

B. Zhang et al. / Inorganica Chimica Acta 360 (2007) 2513–2517 2515

bond lengths and angles are listed in Table 2. In one mol-ecule, four chlorine, two Fe(III) and the oxalate bridge arecoplanar with a maximum deviation of 0.02 A. The bondlengths within this plane are Fe–O 2.085(2), 2.088(2) Aand Fe–Cl 2.272(1), 2.276(1) A. These are in the rangeobserved for related compounds [2,4,5]. The bond anglesof O–Fe–O are less than 90� and those of Cl–Fe–Cl aremore than 90�. The two DMF molecules bind to the Fefrom the direction nearly perpendicular to the FeOCl planewith the bond angles O–Fe–O (DMF) being less than 90�and Cl–Fe–O (DMF) more than 90�. Fe–O bond lengths

Fig. 2. Packing of the dimers

2.017(2) and 2.034(2) A to DMF are shorter than Fe–Oin oxalate-bridge Fe(III) anion. The DMF molecule isdeparted from center of molecule with a N–C@O bondangles 123.7(4)� and 124.6(4)�. The two Fe atoms andatoms of DMF, except for methyl groups, in one moleculeexist in another plane with deviation of 0.13 A and thedihedral angle of the two planes in one molecule is85.75(9)�. The distance between two Fe(III) within the mol-ecule is 5.471(1) A, which is in the range reported for oxa-late-bridged Fe(III) atoms: 5.472 A [4], 5.449 A, 5.437(1)and 5.471(2) A [7]. In the crystal, the molecules are packedin a face-to-face mode along the a-axis as shown in Fig. 2.There are hydrogen bonds between molecules in the crystalwith C–H. . .O along the b-axis and C–H. . .Cl along the a-axis (Table 3), and a two-dimensional hydrogen-bondingnetwork within the ab-plane (Fig. 3). There is no interac-tion between molecules along the c-axis.

The magnetic susceptibility of a polycrystalline sampleof 1 in an applied field of 1000 Oe increases from 300 Kto a maximum at 30 K with a value three times that at300 K. The susceptibility then decreases rapidly to a valueat 2 K less than that at 300 K (Fig. 4). Above 100 K, thecompound behaves as a Curie–Weiss paramagnet (C =

viewed along the a-axis.

0

0.01

0.02

0.03

0.04

0.05

1 10 100

Warming

Cooling

Sus

cept

ibili

ty (

emu/

mol

)

Temperature (K)

Fig. 4. Temperature dependence of the magnetic susceptibility onwarming and cooling in a magnetic field of 1000 Oe; symbols areexperimental data and solid lines are theoretical fits.

Fig. 3. Inter-molecular hydrogen bonds forming the layer connections in the ab-plane.

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

-100 -50 0 50 100 150 200 250 300

Expt.

Fit

M (

Nμ B

)

H (kOe)

g = 2.0469Δ = 5.21 K

Fig. 5. Experimental isothermal magnetization at 2 K (symbols) andtheoretical fit (solid line, see text for details).

2516 B. Zhang et al. / Inorganica Chimica Acta 360 (2007) 2513–2517

4.60(1) emu K mol�1 and h = �55(1) K ). The Curie valueis found to be slightly higher than 4.375 emu K mol�1 forFeIII (S = 5/2) with g = 2.00, and the large Weiss tempera-ture means there is a considerable antiferromagneticinteraction between the two Fe(III) atoms through the oxa-late bridge within the molecule [11]. This suggests a favor-able coordination mode for strong antiferromagneticinteraction as observed for other oxalate-bridge Fe(III)compounds.

Considering an isotropic exchange interaction betweenthe two Fe(III) centers having a distance close to 5 A, theHamiltonian can be written as bH ¼ �JS1S2 with S1 = S2

= 5/2. The susceptibility data were fitted with this isotropicexchanged dimer model, for both cooling and warming,and including a small contribution for temperature-inde-pendent-paramagnetism [12]:

vD ¼2Ng2b2

kTex þ 5e3x þ 14e6x þ 30e10x þ 55e15x

1þ 3ex þ 5e3x þ 7e6x þ 9e10x þ 11e15x

where x = J/kT.

vM ¼ vD þ vTIP

The best fit with an agreement for the data from 2 to 300 Kgave vTIP = 0.003(1) emu mol�1 and J = �6.74(4) cm�1

[4a]. The anti-ferromagnetic exchange interaction in 1 isstronger than that reported for oxalate-bridged dimers suchas [Fe2(acac)4(C2O4)] Æ 0.5H2O �3.61 cm�1 [7], (NEt4)4-[Fe2(NCS)8(C2O4)] �3.84 cm�1 [13], charge-transfer salts�3.44 to �6.12 cm�1 [4], and charge-transfer complex withoxalate-bridged one-dimensional magnetic chain�5.10(3) cm�1 [6]. The isothermal field-dependence magne-tization at 2 K (Fig. 5) suggests an activated like behaviordue to a very low magnetization and the start of what ap-

B. Zhang et al. / Inorganica Chimica Acta 360 (2007) 2513–2517 2517

pears to be an energy levels crossing or spin-flop. We modelit as a level crossing assuming degenerate levels, which areZeeman splitted in the applied field. The fit correspondsto a gap of 5.21 K.

In conclusion, we have obtained an unexpected oxalate-bridged binuclear Fe(III) compound by electrolysis tech-nique and it displays a strong antiferromagnetic interactionbetween Fe within the molecule and hydrogen-bondsbetween molecules.

Acknowledgement

This work was supported by NSFC under Grant Nos.20473095, 20673120 and CMS-CX200510.

Appendix A. Supplementary material

CCDC 606712 contains the supplementary crystallo-graphic data for this paper. These data can be obtained freeof charge via http://www.ccdc.cam.ac.uk/conts/retriev-ing.html, or from the Cambridge Crystallographic DataCentre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.ica.2006.10.028.

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