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Polyhedron 25 (2006) 2175–2182

Synthesis, structure and properties of heterotrinuclearcarboxylate complexes [Fe2M(Ca, Sr, Ba)O(CCl3COO)6(THF)n]

Denis Prodius a, Constantin Turta a,*, Valeriu Mereacre a, Sergiu Shova b, Maria Gdaniec c,Yurii Simonov d, Janusz Lipkowski e, Victor Kuncser f, George Filoti f, Andrea Caneschi g

a Institute of Chemistry of the Academy of Sciences of Moldova, Department of Inorganic Chemistry Laboratory of Bioinorganic Chemistry,

Academiei Street 3, Box 232, MD-2028 Chisinau, Republic of Moldovab National University of Moldova, MD-2009 Chisinau, Republic of Moldova

c Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Polandd Institute of Applied Physics of the Academy of Sciences of Moldova, MD-2028 Chisinau, Republic of Moldova

e Institute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Polandf National Institute of Physics of Materials, Bucharest-Magurele, Romania

g Department of Chemistry and INSTM Research Unit, University of Florence, Florence, Italy

Received 16 September 2005; accepted 7 January 2006Available online 7 March 2006

Abstract

Three novel l3-oxo trinuclear carboxylates [Fe2CaO(CCl3COO)6(THF)4] Æ THF (1), [Fe2SrO(CCl3COO)6(THF)6] Æ 0.5H2O Æ 0.5THF(2) and [Fe2BaO(CCl3COO)6(THF)6] Æ 0.5H2O Æ 0.5THF (3) have been synthesised and subsequently characterised by X-ray structureanalysis, thermal gravimetry (TG), infra red (IR) spectroscopy and magnetic measurements. Compounds 1 and 3 are molecular com-plexes with s- and d-metals displayed in the apexes of isosceles triangles with a triply bridging oxygen atom at its centre. The Ca� � �Feand Ba� � �Fe separations have the values 3.750(1) and 4.177(1) A, respectively. In comparison with these, Fe� � �Fe separations, 3.229(2) Ain 1 and 3.201(1) A in 3, appear much shorter. The molecular complexes obey C2 symmetry with the twofold axis passing through s metaland l3-oxo atom. The coordination numbers are 6, 7 and 9 for iron, calcium and barium correspondingly.

Magnetic studies reveal the presence of an antiferromagnetic exchange in the isosceles triangular skeletons of the trinuclear species.Using the spin Hamiltonian H = �2J12[S1S2], the fitting parameters gFe = 2.00, JFe–Fe = �58.90 cm�1 for 1 and �60.40 cm�1 for 3;gFe = 2.14, JFe–Fe = �75.40 cm�1 for 2 have been estimated.� 2006 Elsevier Ltd. All rights reserved.

Keywords: {Fe2MO} core; Heterotrinuclear carboxylates; Alkaline-earth metals; Crystal structure; Antiferromagnetic exchange

1. Introduction

Carboxylate-bridged iron centres are the essential partof the active sites in various metal-proteins: ribonucleotide[1], hemerytherin [2] and methane monooxygenase [3].Among them, one commonly occurring carboxylate-bridged moiety is the trinuclear oxo-centred metal com-

0277-5387/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2006.01.019

* Corresponding author. Tel.: +373 22 72 54 90/73 97 55; fax: +373 2273 99 54.

E-mail addresses: turtalcba@yahoo.com, cturta@cc.acad.md(C. Turta).

plex, belong to a class known as ‘‘basic acetates’’, whichhave generated a lot of investigations [4–15].

The formation of l3-oxo homotrinuclear carboxylate-bridged complexes of the general formula [M3(l3-O)-(RCOO)6(L)3]+ is characteristic for a number of transitionmetals. The vast bibliography in this area was extensivelyreviewed in [4,16,17]. Complexes of this structure containa triangular arrangement of metal ions bridged by a centrall3-oxo atom. Each carboxylate anion spans two metalcentres at the periphery of the [M3(l3-O)]6+ core, whilethe neutral monodentate ligands occupy the remainingcoordination sites on each metal centre, and as a result

2176 D. Prodius et al. / Polyhedron 25 (2006) 2175–2182

the coordination around the metal centre is approximatelyoctahedral.

The molecular heterometallic iron-containing trinuclearcarboxylates [Fe2MO(RCOO)6(L)3], where M is a bivalent3d transition metal, attract special attention due to beingable to provide an opportunity to study many spin-cou-pling interactions in the system with the same configuration[18]. A considerable number of such complexes containingeither heterometallic [Fe2MnO]6+ [18,19], [Fe2CoO]6+

[20,21], [Fe2NiO]6+ [18,22], [Fe2ZnO]6+ [23,24] or homo-metallic mixed-valence [Fe2FeO]6+ [4,25–34] cores, hasbeen investigated by various methods.

All of the above-mentioned complexes contain iron(III) and bivalent 3d metals: Mn, Fe, Co, Ni, Zn. The dif-ference in the atomic radii of iron and the other 3d ele-ments is negligible. For example, the atomic radius ofhigh-spin iron (III) is equal to 0.65 A, while the atomicradii for 3d bivalent elements Mn(II), Fe(II), Co(II),Ni(II) range from 0.75 to 0.80 A. This difference doesnot essentially influence the stability of the trinuclear[Fe2MO] core. A survey of the CCDC [35] revealed thatin neutral complexes which include a fFe2

IIIFeIIOg core,the average Fe� � �Fe separation is equal to 3.280(2) A(Fe–O(l3) 1.875(3) A, Fe–Ocarb 2.040(3) A). In ionic com-pounds with a fFe3

IIIOg core, the corresponding distancesadopt the values 3.290(2), 1.890(2) and 2.025 A, respec-tively. These values suggest a skeleton of the molecularstructure with a symmetry close to D3h.

Previously reported was the preparation and the studyof complexes containing the [Fe2MgO]6+ core [22]. In thecompound [Fe2MgO(CCl3COO)6(Py)3] Æ CH3C6H5, oneiron atom occupies a special position on the mirror plane,while the other iron and the magnesium atom of theFe2MgO core occupy the known general position withequal probability (50%). This arrangement occurs in spiteof a different mode of interaction of the coordination envi-ronment with the d(Fe) and s(Mg) metals. The rather dif-ferent ionic radii of Fe(III) 0.65 A and Mg(II) 0.72 Aconserved a practically equilateral triangle of the threemetal atoms. The geometric rather than the electronic fac-tor, probably, avoid any dramatic rearrangements in thetrinuclear cluster.

The present study describes the results obtained with thealkaline-earth elements Ca (ionic radius 1.00 A), Sr(1.18 A) and Ba (1.35 A) and the way to synthesize newiron-containing molecular l3-oxo clusters.

2. Experimental

2.1. General

The carbon and hydrogen content was determined bystandard micro-methods. Atomic absorption spectroscopywas used to determine the Fe(III), Ca(II), Sr(II) and Ba(II)content.

IR spectra of polycrystalline samples were recorded asoil mulls on a Specord M-75 spectrophotometer.

TG studies were performed on a Paulik–Paulik–Erdeyderivatograph in air, with Al2O3 as the standard calibra-tion. DTG – 1/5; DTA – 1/10; TG – 100/100. Tmax =1000 �C for 1 and 3, and 500 �C for 2.

Magnetic measurements of 1 and 3 were performedusing the Gouy method in the temperature range 110.0–293.0 K. The temperature of the sample was determinedwith ±1.5 K accuracy using the Cu–Constantan thermo-couple. The variable temperature susceptibility of 2 wasmeasured with an Oxford Instruments Vibrating SampleMagnetometer (VSM) working between 0 and 12 T andin the 1.5–350.0 K temperature range. DiamagneticPascal’s constants were used to correct the magnetic values[36].

The Mossbauer spectrum for 1 was acquired using aconstant acceleration system with a symmetrical waveform.A 57Co (1.0 MBq) source in a Rh matrix was used, but theisomer shift refers to a-Fe.

2.2. Synthesis

Most of the precursors, except Ca(CCl3COO)2 Æ 4H2O,Sr(CCl3COO)2 Æ 4H2O and Ba(CCl3COO)2 Æ 4H2O, whichwere synthesized by the reaction between CaCO3, SrCO3orBaCO3 with trichloroacetic acid, were commerciallyobtained and used without any further purification.

All the products are air stable and also soluble in com-mon organic solvents such as MeOH, EtOH, THF,CH3CN and DMSO. Single crystals suitable for X-raycrystallography for 1 and 3 were obtained by slow evapo-ration of their THF solutions, at room temperature.

2.2.1. Synthesis of [Fe2CaO(CCl3COO)6(THF)4] Æ THF

(1)

A mixture of Fe(NO3)3 Æ 9H2O (0.808 g, 2.00 mmol) andCa(CCl3COO)2 Æ 4H2O (1.748 g, 4.00 mmol) in 30 ml ofmethanol was heated on a water bath (�70 �C) with per-manent stirring until dryness. The solid product wasextracted with three small portions (10 ml) of THF. After2 days a light red microcrystalline material precipitatedfrom the THF solution, which was filtered off, washed withhexane and finally dried in air. Yield: 65% (on iron basis).Anal. Calc. for C32H40Cl18O18CaFe2: C, 25.58; H, 2.68; Fe,7.43; Ca, 2.67. Found: C, 25.27; H, 2.54; Fe, 7.32; Ca,2.48%. IR data (oil mulls, cm�1): 3650 m, 3470 mb, 1700m, 1673 msh, 1645 s, 1593 s, 1590 m, 1380 s, 1340 s,1010 w, 940 w, 832 s, 826 s, 748 sh, 720 m, 675 m, 670 s,469 m, 440 m.

2.2.2. Synthesis of [Fe2SrO(CCl3COO)6(THF)6] Æ0.5THF Æ 0.5H2O (2)

The light red coloured complex 2 was synthesized iden-tically to 1, but using Sr(CCl3COO)2 Æ 4H2O instead ofCa(CCl3COO)2 Æ 4H2O. Yield: � 52% (on iron basis). Anal.Calc. for C38H53Cl18O20Fe2Sr: C, 27.37; H, 3.20; Fe, 6.70;Sr, 5.25. Found: C, 27.11; H, 3.17; Fe, 6.31; Sr, 5.01%. IRdata (oil mulls, cm�1): 3650 m, 3540 mb, 3180 wb, 1690 s,

D. Prodius et al. / Polyhedron 25 (2006) 2175–2182 2177

1675 ssh, 1660 s, 1380 s, 1340 ssh, 1030 w, 1015 w, 950 w,910 w, 830 ssh, 820 s, 740 s, 730 msh, 690 msh, 675 s, 460m, 430 w.

2.2.3. Synthesis of [Fe2BaO(CCl3COO)6(THF)6] Æ0.5THF Æ 0.5H2O (3)

A mixture of Fe(NO3)3 Æ 9H2O (0.808 g, 2.00 mmol) andBa(CCl3COO)2 Æ 4H2O (2.136 g, 4.00 mmol) in 50 ml ofmethanol was heated on a water bath (�70 �C) with stir-ring for 3 h. The solution was filtered to remove insolubleBa(NO3)2 and evaporated at room temperature. The solidproduct was dissolved in 50 ml THF. After 1 week, as aresult of very slow evaporation, a dark red crystalline mate-rial was filtered off, washed with hexane and air dried. It isnecessary to mention that using this procedure crystals upto �0.5 cm3 were obtained. Yield: �83% (on iron basis).Anal. Calc. for C38H53Cl18O20BaFe2: C, 26.58; H, 3.11;Fe, 6.50; Ba, 8.00. Found: C, 25.88; H, 2.91; Fe, 6.60;Ba, 7.68%. IR data (oil mulls, cm�1): 3460 mb, 1675 s,1630 s, 1380 s, 1340s, 1120 vw, 1020 m, 940 w, 890 vw,860 sh, 832 s, 820 s, 720 m, 690 m, 670 s, 465 m, 445 w.

2.3. Crystallographic data

The structural parameters together with details of datacollection and refinement for compounds 1 and 3 are givenin Table 1.

The X-ray intensity data for 1 were collected at 150 Kon a Nonius Kappa CCD diffractometer equipped withgraphite monochromated Mo Ka radiation using x rota-

Table 1Summary of crystal data for 1 and 3

Compound 1

Empirical formula C32H40CaCl18Fe2O18

M 1502.52T (K) 150k (A) 0.71073Space group Pbcn

a (A) 17.1653(2)b (A) 19.3427(3)c (A) 17.5660(3)V (A3) 5832.32(15)Z 4qcalc (Mg/m3) 1.711lMo (mm�1) 1.471F(000) 3016Crystal size (mm) 0.20 · 0.20 · 0.15h Range (�) 2.32 to 25.35Index ranges �20 6 h 6 20, �23 6 k 6

Number of measured reflections 10148Number of unique reflections (Rint) 5342 (0.0206)Number of refined parameters 351Maximum and minimum transmission 0.894 and 0.801Goodness-of-fit for F2 1.029Ra 0.0388wRb 0.0933Dqmax and Dqmin (e A3) 0.662 and �0.728

a R =P

iFo| � |Fci/P

|Fo|.b wR ¼ ½

PwðjF 2

oj � jF 2c jÞ

2=P

wjF 2oj

2�1=2.

tion with a sample-to-detector distance of 40 mm. Unit cellparameters were obtained and refined using the whole dataset. Frames were integrated and corrected for Lorenz andpolarization effects using DENZO [37]. The scaling, as wellas the global refinement of the crystal parameters, was per-formed by SCALEPACK [38]. Reflections, which were partlymeasured on previous and subsequent frames, were usedto scale these frames on each other. The absorption correc-tion for 1 and 3 were introduced by a semi-empiricalmethod based on symmetry equivalent reflections [37].

Data collection for 3 was performed on a KUMA KM-4CCD diffractometer equipped with graphite monochro-mated Mo Ka radiation at 100 K. The unit cell determina-tion and data integration were carried out using theCrysAlis package of Oxford Diffraction [39]. Intensity datawere corrected for Lorentz and polarization effects. Themean value of 0.693 for |E2 � 1| suggested a non-centro-symmetric space group, but taking into consideration theRint 0.071 value for the Laue group 4/m and systematicabsences, the structure settlement with SHELXS-97 [40] wasinitially envisaged by using the P42/nmc space group. TheM3O(COO)6 core was easily recognized on the Fouriermap though further calculations of the DF maps did notallow a better value of R than 0.12 to be obtained. In sub-sequent attempts, the structure was successfully solved viaSHELXL-97 [40] using a twinning approach by introducingthe instruction TWIN 0 1 0 1 0 0 0 0 �1. The fractional con-tributions of the two twin domains in structure 3 are in a1:1 ratio. Refinement of the structures revealed that THF(coordinated to Fe) in compound 1, one of the two THF

3

C38H53BaCl18Fe2O20

1716.941000.71073P42/n16.0020(7)16.0020(7)27.0470(12)6925.6(5)41.6471.73034200.50 · 0.40 · 0.3252.84 to 24.97

23, �21 6 l 6 21 �19 6 h 6 16, �18 6 k 6 19, �32 6 l 6 32480896110 (0.0453)3710.306 and 0.2551.0860.07380.08601.310 and �0.831

2178 D. Prodius et al. / Polyhedron 25 (2006) 2175–2182

(coordinated to Ba), as well as solvated THF molecules in 3

occupy two positions each with 50% probability. Only oneof the three asymmetrical –CCl3 groups was found to bedisordered into two resolute positions, in both complexes1 and 3.

3. Results and discussion

To date, various heterotrinuclear complexes with chlorocontaining acetic acids have been synthesized and structur-ally characterized [19,20,22,23]. Most of these complexeswere synthesized through a step concerning treatment ofbasic acetates of d-elements with the corresponding acidsin aqueous solutions. It is worth mentioning that com-plexes 1–3 were prepared by a self-assembly reaction ofFe(NO3)3 Æ 9H2O with trichloroacetates of alkaline earthmetals in methanol.

3.1. Molecular structures of 1 and 3

The X-ray study revealed that the compounds 1 and 3,containing Ca and, respectively, Ba, belong to molecularcomplexes with THF solvated molecules placed in theexternal coordination sphere. It has to be mentioned thatcompound 3 includes solvated water molecules too. As inthe case of the complex [Fe2MnO(CHCl2COO)6(THF)2-H2O] [19], the character of distribution of the metal cationsin the various M3–l3-O cores was unambiguously demon-strated. It appeared possible even in the case of an essentialdifference in the values of ionic radii of the metals used,M2+ (Ca, Ba) and Fe3+. This difference produced the for-mation of an isosceles triangle of C2 symmetry (Fig. 1aand b), with the lateral equivalent distances Ca� � �Fe

a b

Fig. 1. Structures of the molecular complexes: (a) [Fe2CaO(CCl3COO)6(THF)4

clarity.

3.750(1) A for 1 (Ba� � �Fe 4.177(1) A for 3) and the basaldistance Fe� � �Fe 3.229(2) A for 1 and 3.201(1) A for 3.The data show that this difference in the length of the sidesof the triangle could be dictated by the stability of themolecular core, [M3O(R–COO)6]. The character of thebridges forming this core was not essentially changed incomparison with the classic l3-oxo-iron complexes. Thestability of the trinuclear core is dictated by the l3-bridgeoxygen atom O2� and by six carboxylate anions joint ina syn–syn mode along the sides of the triangle. Althoughthe general architecture of the core remains the same, thelocations in the first coordination sphere of the metal differessentially.

3.1.1. Compound [Fe2CaO(CCl3COO)6(THF)4] Æ THF

(1)

Fig. 1a presents the molecular trinuclear complex struc-ture. The geometric characteristics of the coordination core(Table 2) denoted by iron(III) metals in compound 1 donot differ essentially from those reported in [35]. The coor-dination number of Fe(1) atoms equals 6, and represent aweakly distorted octahedron. Four oxygen atoms of fourcarboxylic groups occupy the equatorial plane with Fe–Odistances ranging from 1.996(2) to 2.065(2) A. The coordi-nation is completed by oxygen atom O(1) and oxygen atomO(9) of THF molecules at the distances of 1.827(1) A and,respectively, 2.155(2) A. Atom Fe(1) is out of the plane ofthe four carboxyl oxygen atoms in the direction of the l3-oxygen at a distance of 0.286(2) A.

The Ca2+ local arrangement differs essentially from thesurrounds of the Fe(1) atom: the coordination number ofcalcium is 7; the coordination sphere includes four carbox-ylic oxygen atoms at distances in the range 2.336(2)–

] (1); (b) [Fe2BaO(CCl3COO)6(THF)6] (3). Chlorine atoms are omitted for

Table 2Interatomic distances (A) in coordination polyhedra of Ca, Ba and Fe in 1

and 3

Compound 1 Compound 3

Ca(1)–O(1) 2.530(3) Ba(1)–O(1) 3.000(7)Ca(1)–O(5) 2.353(2) · 2 Ba(1)–O(5) 2.697(6) · 2Ca(1)–O(7) 2.336(2) · 2 Ba(1)–O(7) 2.688(7) · 2Ca(1)–O(8) 2.424(2) · 2 Ba(1)–O(8) 2.850(7) · 2

Ba(1)–O(9) 2.803(7) · 2

Fe(1)–O(1) 1.827(1) Fe(1)–O(1) 1.816(3)Fe(1)–O(2) 2.065(2) Fe(1)–O(2) 2.078(6)Fe(1)–O(3)(1) 2.060(2) Fe(1)–O(3)(2) 2.065(6)Fe(1)–O(4) 1.996(2) Fe(1)–O(4) 1.997(7)Fe(1)–O(6) 2.003(2) Fe(1)–O(6) 1.993(6)Fe(1)–O(9) 2.155(2) Fe(1)–O(10) 2.178(6)

Symmetry transformations: (1)�x, y,�z + 0.5; (2)�x + 0.5,�y + 0.75, z.

Fig. 2. Structure of the 2D layer in 1.

Fig. 3. Projection of the crystal structure 3 along the c-axis.

D. Prodius et al. / Polyhedron 25 (2006) 2175–2182 2179

2.353(2) A, two oxygen atoms of THF ligands (Ca(1)–O(8)2.424(2) A) and l3-O at 2.530(3) A. The difference in theM–O(1) distances (Ca–O and Fe–O) is 0.703(2) A, whichexceeds the difference in the ionic radii of Ca2+ (1.00 A)and Fe3+ (0.65 A). The Ca atom deviates by 0.443 A fromthe plane of the four oxygen atoms in the direction of theneutral ligands. Concluding, the main result of the X-raystudy is related to reliable information about the magneticmoiety that includes two iron(III) atoms interacting viaFe–O–Fe and two syn–syn carboxylic groups bridges.

The X-ray study of the compounds [Fe2SrO(CCl3-COO)6(THF)6] Æ 0.5H2O Æ 0.5THF (2) and [Fe2BaO(CCl3-COO)6(THF)6] Æ 0.5H2O Æ 0.5THF (3) has shown that theyare isostructural. In the following, as an example, isdescribed in detail the structure of 3, for which a completeX-ray single crystal investigation was carried out.

The main distinction between compound 3 and com-pound 1 consists of a different coordination geometry forthe s elements. There is a 9-coordination number forBa2+ and its environment includes four THF ligands(Fig. 1b). The Ba atom deviates considerably, at1.003(6) A, from the plane of four coordinated oxygenatoms of carboxylate groups towards neutral ligands. Thecoordination polyhedron of Fe3+ is similar to thatobserved in 1. The deviation of the Fe(1) atom from themain plane of four carboxylate oxygen atoms was foundto be 0.304(7) A in the direction of O(1) oxygen.

The crystal packing for both 1 and 3 compounds wasderived from the arrangement of the corresponding neutralcomplexes [Fe2CaO(CCl3COO)6(THF)4] and [Fe2BaO-(CCl3COO)6(THF)6] according to the Cl� � �Cl intermolecu-lar contacts, which adopt the shortest values of 3.566 A for1 and 3.522 A for 3. All other intermolecular separationsare not smaller than the sum of Van der Waals radii. Thedisordered solvate THF and water molecules are accom-modated within the free space formed by the packing ofthe bulky trinuclear complexes (Figs. 2 and 3). Thesespaces represent closed cavities formed by staggered two-dimensional layers in structure 1 (Fig. 2) and the channelspropagated along the c-crystallographic axes in structure 3(Fig. 3).

3.2. Infrared spectra

The solid state IR spectra of complexes 1–3 indicate thepresence of carboxylate, THF and {Fe2MO} groups. Thecharacteristic vibration frequencies for THF groups appearat �3450–3600 and 2800–3000 cm�1. The IR data for com-pounds 1–3 show a single pair of carboxylate stretchingvibrations at 1645 and 1380 cm�1 for 1, 1670 and1380 cm�1 for 2 and 1675 and 1380 cm�1 for 3 assignedto mas(COO) and ms(COO), respectively, suggesting the exis-tence of only one type of carboxylate group in their crystalstructures. These bands are shifted to lower energy by 45–75 cm�1 relative to the trinuclear basic acetates [4] due tothe replacement of the CH3 radical by the much more pow-erfully electron-accepting CCl3 moieties which weaken theM–Ocarb force constant.

In Table 3, some band assignments for (COO), (CCl),(CH) and (CC) groups for l3-oxo heterotrinuclear clusters

Fig. 4. Fe57 Mossbauer spectrum of 1 at room temperature; the solid lineis the least-squares fit; isomer shifts are referred to a-iron at roomtemperature.

Table 3Some band assignments of IR spectra of l3-oxo heterotrinuclear clusters [Fe2M(Ca, Sr, Ba)O(CCl3COO)6(THF)n] Æ mSolv

Ca(TCA)2a {Fe2CaO} Sr(TCA)2

a {Fe2SrO} Ba(TCA)2a {Fe2BaO} Assignments

1649 vs 1645 s 1660 s 1670 s 1670 s 1675 s mas(COO)1596 vs 1593 s 1660 s 1630 s1372 s 1380 s 1373 s 1380 s 1379 s 1380 s ms(COO)

1370 s947 w 940 w 942 w 950 w 940 w 940 w m(CC)

832 s 830 ssh 832 s mas(C–Cl); A00

826 s 820 s 820 s mas(C–Cl); A0

720 m 730 msh 720 m d(COO)675 m 690 msh 690 m mas(Fe2MO); A1

670 s 675 s 670 s q(OCO)460 m 460 m 465 m mas(Fe2MO); B2

440 m 430 w 445 w ms(C–Cl)

CCl3COO ” TCA; s – strong; v – very; m – medium; w – weak; sh – shoulder; m – stretching.a Data from [42].

2180 D. Prodius et al. / Polyhedron 25 (2006) 2175–2182

according to [41,42] are given. It is worth mentioning thatthe band corresponding to mas(Fe3O) = 595 cm�1 fromhomotrinuclear basic acetates is missing in IR spectra ofcompounds 1–3. This band is replaced by another twobands at �690–675 cm�1, corresponding to mas(Fe2MO)(A1), and at 465–460 cm�1, corresponding to mas(Fe2MO)(B2) according to [4]. Cannon and co-workers [43–45] haveexamined the Fe2M–O vibration modes and such bandshave been found for a number of [Fe2MII(l3-O)(RCOO)6-(L)3] complexes. Such behaviour was expected because ofsymmetry lowering of the complexes from D3h to C2v [4,46].

3.3. TG study

A specific feature of the investigated complexes is relatedto their low temperature stability. Already at temperatureshigher than 50–70 �C exo and endo effects accompanied byweight loss have been registered.

On TG and DTG curves of the investigated compounds,it is possible to notice that the basic processes of decompo-sition finishes in the range 300–360 �C.

The first endothermic stage of decomposition corre-sponds to removal of solvent molecules (THF, or THFand H2O) and some quantity of intrasphere THF (1.5 for1, and 0.5 for 2 and 3).

It is very probably the removal of intrasphere moleculesof THF is accompanied by reorganisation of the cluster insuch a way to assure all coordination places of the s-metalion by changing the syn–syn mode of coordination of thecarboxylic ligands to a cyclo-bridged one [8,9].

The subsequent second process for all three clusters ischaracterised by several superpositioned processes: removalof coordinated molecules of THF, decomposition of tri-chloroacetic acid anions and reorganisation of the com-plexes with the formation of the final product, Fe2MO4,with a small quantity of impurities.

3.4. Mossbauer spectrum

The Mossbauer spectrum of compound 1 at room tem-perature presents only one doublet with slightly asymmet-

ric lines (Fig. 4). The value of the isomer shift parameterdFe is 0.46 mm/s and that of the quadrupole splittingparameter DEQ is 1.11 mm/s. These parameters indicatethe presence of iron(III) in a high spin state (S = 5/2).There are rather insignificant changes in dFe and DEQ val-ues related to the substitution of one trivalent Fe3+ cation[47] by a bivalent Ca2+.

3.5. Magnetic properties

The magnetic properties of compounds 1–3 have beenmeasured in different ranges of temperatures: 300–110 Kfor 1 and 3, and 300–2.9 K for 2. All three substances arecharacterized by a gradual reduction of the vMT value withthe lowering of temperature, thus indicating an antiferro-magnetic interaction between the iron (III) ions. It is evidentthat ions of s-metals do not contribute to the magneticexchange. In order to describe the magnetic properties ofthe investigated complexes the following spin–spin interac-tion Hamiltonian was used [36]:

Hexch ¼ �2J12S1 � S2. ð1ÞThe energy levels of Hamiltonian (1) are given by the fol-lowing expression:

0.0000

0.0013

0.0026

0.0039

0.0052

0 80 160 240 320

T, oK

χMT

, cm

3 K

mo

l-1

Fig. 5. Temperature dependence of the magnetic susceptibility of 2,plotted as vMT vs. T; the solid line is the least-squares fit.

D. Prodius et al. / Polyhedron 25 (2006) 2175–2182 2181

EðS0Þ ¼ �J 12 S0ðS0 þ 1Þ � S1ðS1 þ 1Þ � S2ðS2 þ 1Þ½ �; ð2Þwhere S 0 is the total spin of the system. S 0 = (S1 + S2),(S1 + S2 � 1), . . . |S1 � S2|; S1 = S2 is the iron(III) ion spin,equal to 5/2. For the investigated systems the values of S 0

are: 5, 4, 3, 2, 1, 0. The expression for molar magnetic sus-ceptibility vM could be presented as

vM ¼3g2

4T55 expð30xÞ þ 30 expð20xÞ þ 14 expð12xÞ þ 5 expð6xÞ þ expð2xÞ½ �

11 expð30xÞ þ 9 expð20xÞ þ 7 expð12xÞ þ 5 expð6xÞ þ 3 expð2xÞ þ 1½ � ; ð3Þ

where g is the Lande factor of splitting, x = J12/(kT).Using the fitting program MAGMUN [48], the parameters

of magnetic exchange J12 have been determined, givingthe following values: J12 = JFe–Fe = �58.9 ± 0.5 cm�1 for1; �75.4 ± 0.8 cm�1 for 2; �60.4 ± 0.4 cm�1 for 3; g isequal to 2.0 for 1 and 3, and 2.14 for 2 (see Fig. 5).

From the acquired data, it is obvious that the absolutevalue of the magnetic exchange parameter J12 changesfrom complex to complex. At the same time it is usefulto note that the values of the parameter of exchangebetween iron (III) ions in the discussed trinuclear clustersare significantly larger in their absolute values comparedto similar homonuclear carboxylates of iron (III)[16,36,49], but they are practically the same for heteronu-clear acetates with {Fe2MO} cores, where M is a 3d-ele-ment [18]. From the comparison of L(Fe–Fe) and |JFe–Fe|values for 1 and 3, one can see that they have a certainmutual correlation. The fact that the value |JFe–Fe| for 2

is considerably different from those of 1 and 3 probably

is the result of the different temperature ranges in whichthe susceptibility curves have been measured and the differ-ent values of the fitted g factor.

4. Conclusions

Three new l3-oxo heterotrinuclear iron and s-metal con-taining carboxylates of the composition [Fe2M(Ca, Sr,Ba)O(RCOO)6(L)n], where R = CCl3, L = tetrahydrofuran(THF), have been synthesized.

For the first time the possibility to obtain a trinuclearcore [Fe2MO(R–COO)6] with M differing in both electronicproperty (Fe – alkaline-earth metals) and dimension (Fe/Ba, Fe/Sr, Fe/Ca) was demonstrated. The positions ofparamagnetic and diamagnetic centres in the core of the tri-nuclear molecule have been assigned unambiguously. Thisallows an analysis of the magnetic interactions in the Fe–Fefragment where the iron atoms are connected by two differ-ent bridges.

The absolute value of the antiferromagnetic interactionbetween iron(III) ions (magnetic exchange parameter�58.9 ± 0.5 cm�1 for 1, �75.4 ± 0.8 cm�1 for 2,�60.4 ± 0.4 cm�1 for 3) is higher than the analogousparameter in homo-iron carboxylates, but they are practi-cally the same for heteronuclear acetates with {Fe2MO}cores, where M is a 3d-element.

5. Supplementary material

Crystallographic data (excluding structure factors) for 1and 3 have been deposited with the Cambridge Crystallo-graphic Data Centre as a supplementary publication,

CCDC reference number 247520 for 1 and 247519 for 3.Copies of the data can be obtained, free of charge, onapplication to CCDC, 12 Union Road, Cambridge CB21EZ, UK [fax: +44 1223 336 033 or e-mail: deposit@ccdc.cam.ac.uk].

Acknowledgements

The research described in this publication was madepossible in part by Award No. BGP III #MOB-2-3058-CS-03 of the Moldovan Research and DevelopmentAssociation (MRDA) and the US Civilian Research &Development Foundation (CRDF) and by INTAS-00565Grant. V. Mereacre thanks also the Alexander von Hum-boldt Foundation for financial support.

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