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Inorganica Chimica Acta 361 (2008) 1291–1297

Comparative investigations on a series of [hexakis(1-(tetrazol-1-yl)alkane-N4)iron(II)] bis(tetrafluoroborate)

spin crossover complexes: Methyl- to butyl-substituted species

Nader Hassan a, Peter Weinberger a,*, Kurt Mereiter b, Franz Werner a, Gabor Molnar c,Azzedine Bousseksou c, Markus Valtiner a,1, Wolfgang Linert a,*

a Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9/163-AC, A-1060 Vienna, Austriab Institute for Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/164-SC, A-1060 Vienna, Austria

c Laboratoire de Chimie de Coordination, LCC-CNRS UPR8241, 205 route de Narbonne, F-31077 Toulouse, France

Received 16 May 2007; received in revised form 13 August 2007; accepted 17 August 2007Available online 30 August 2007

Abstract

A series of 1-(tetrazol-1-yl)alkanes [ntz] with n = 1–4 were synthesised as ligands for iron(II) spin crossover complexes. Within thisseries 1-(tetrazol-1-yl)butane [4tz] was prepared for the first time, whereas 1-(tetrazol-1-yl)methane [1tz], 1-(tetrazol-1-yl)ethane [2tz],1-(tetrazol-1-yl)propane [3tz] and the [hexakis(ntz)iron(II)]bis(tetrafluoroborate) complexes were prepared according to the literature.Aiming for a comparative study we characterized all four compounds by XRPD, magnetic susceptibility measurements, 57Fe-Moess-bauer spectroscopy and IR spectroscopy. [Fe(4tz)6](BF4)2 yielded appropriate single crystals and an X-ray structure of the new com-pound [Fe(4tz)6](BF4)2 is presented. The magnetic and structural properties of all [Fe(ntz)6](BF4)2 are compared and discussed.� 2007 Elsevier B.V. All rights reserved.

Keywords: Iron(II); Tetrazole; Spin crossover; Magnetic measurements; Vibrational spectroscopy

1. Introduction

The behaviour of spin crossover compounds is one ofthe most fascinating ones shown by relatively simple mole-cules. The outstanding properties of some of these com-plexes were discussed extensively in the literature andwere reviewed among others recently in a book series [1,and references therein]. For many years 1-substituted tet-razoles have been investigated as versatile ligands for Fe(II)spin crossover complexes [2–8]. Especially, 1-(tetrazol-1-yl)alkane compounds are known to form spin-transition

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

doi:10.1016/j.ica.2007.08.023

* Corresponding authors.E-mail addresses: weinberg@mail.zserv.tuwien.ac.at (P. Weinberger),

wlinert@mail.zserv.tuwien.ac.at (W. Linert).1 Current address: Max-Planck-Institut fur Eisenforschung, Christian

Doppler Laboratory for Polymer/Metal Interfaces, Max-Planck Str. 1, D-40239 Dusseldorf, Germany.

complexes with several iron(II) salts with so-called non-coordinating anions (e.g. Fe(BF4)2, Fe(ClO4)2, etc.). Thesynthesis of the 1tz, 2tz and 3tz as well as their iron(II) spincrossover complexes, their magnetic properties and crystalstructures, has already been published [9]. The considerablydifferent spin-transition behaviour of these three com-pounds raised the question of the factors being responsible.Aiming to elucidate the factors governing this quite a dif-ferent behaviour we extended the series of known 1-(tetra-zol-1-yl)alkanes and its complexes to a further homologue,which will be presented in this paper. Going this systematicway of slightly modifying the spin-transition compounds(e.g. elongating the alkyl-chain, variation of anions, . . .)and analyzing the resulting properties seems to be verypromising to understand the different factors that governthe spin-transition behaviour.

In this paper we present the synthesis and characteriza-tion of the higher homologue [Fe(4tz)6](BF4)2. We report

1292 N. Hassan et al. / Inorganica Chimica Acta 361 (2008) 1291–1297

the characterization of the new compound with powder X-ray diffraction, magnetic susceptibility measurements, vibra-tional spectroscopy and 57Fe-Mossbauer spectroscopy. AnX-ray structural analysis for [Fe(4tz)6](BF4)2 is presentedas well. The different spin-transition behaviour within theseries of compounds [Fe(ntz)6](BF4)2with n = 1–4 is com-pared and discussed.

2. Results and discussion

2.1. X-ray powder diffraction

The four ligands ntz with n = 1–4 have been synthe-sized according to the procedure published by Frankeet al. [2] and characterized via 1H NMR (for detailsrefer to Section 3). The ligands have been complexedwith Fe(BF4)2 Æ6H2O and the obtained solids are charac-terized via elemental analysis and X-ray powderdiffraction.

According to the complexes’ patterns (Fig. 1) the com-pounds are well crystallized with some minor amorphouscontribution and are single phase as seen from Rietveldrefinements [10]. The following crystallographic planesare affected by texture: 1tz {h00}, 2tz {0k�k}, 3tz {00 l}

Fig. 1. Powder patterns of the [Fe(ntz)6](BF4)2 complexes (k. . .CuKa1,2 = 1.54184 A). The y-axis is given in

pI units to produce a more

even intensity distribution over the angular range.

and 4tz {00 l} (see Fig. 1). These are the planes that formthe hexagonal sheets.

2.2. XRD-measurements – crystal structure of [Fe(4tz)6]-

(BF4)2

The crystal system of the high-spin (HS) species[Fe(4tz)6](BF4)2 is trigonal rhombohedral, space group R�3(No. 148). The dimensions of the unit cell are a = b =10.959(4) A, c = 37.89(2) A and V = 3940(3) A3. The unitcell contains three formula units and has a calculated den-sity of 1.247 g/cm3. Atomic coordinates are given in theSupplementary material. The structure is built up from acationic [Fe(4tz)6]2+ complex with symmetry �3 and froma BF4

� anion with symmetry 3, the latter is showing essen-tially an apex up–down-orientation disorder (Fig. 2).

Iron displays a fairly regular octahedral coordinationby the six symmetry equivalent N(1) ligand atoms. Thebond length Fe–N(1) = 2.191(2) A corresponds well to aHS state of Fe2+. The N(1)–Fe–N(1) bond angles are veryclose to an ideal octahedron. Bond lengths and angles inthe tetrazole ligand display the usual values, except forthe C(3)–C(4) bond (see Table 1) in the bent alkyl sidechain, which is systematically short due to an apparentdisorder of C(3) and C(4), both of which show stronglyanisotropic displacement ellipsoids elongated perpendicu-lar to the n-butyl chain. Attempts to model this disorderby introducing split C(3)/C(4) positions were done, butbecause of parameter correlations these attempts did notprovide advantages compared with a non-split modelhence, these attempts have been discarded. The structureconsists of trigonal layers parallel to (001) of [Fe(4tz)6]2+

complexes in a hexagonal close packed arrangement withthe disordered BF4

� tetrahedra in the interstices. One of

Fig. 2. Thermal ellipsoid plot (20% ellipsoids) of constituents of HS[Fe(4tz)6](BF4)2.

Table 1Selected bond lengths and bond angles of HS [Fe(4tz)6](BF4)2

Fe(1)–N(1) 2.191(2) N(1)–Fe(1)–N(1) (3·) 180.00N(1)–C(1) 1.302(3) N(1)–Fe(1)–N(1) (3·) 89.1(1)N(1)–N(2) 1.349(3) N(1)–Fe(1)–N(1) (3·) 90.9(1)N(2)–N(3) 1.290(3) N(1)–C(1)–N(4) 109.4(2)N(3)–N(4) 1.324(3) C(1)–N(1)–N(2) 105.8(2)N(4)–C(1) 1.309(3) N(1)–N(2)–N(3) 109.4(2)N(4)–C(2) 1.474(4) N(2)–N(3)–N(4) 107.3(2)C(2)–C(3) 1.414(8) N(3)–N(4)–C(1) 108.1(2)C(3)–C(4)a 1.185(10) C(1)–N(4)–C(2) 129.5(3)C(4)–C(5) 1.510(11) N(3)–N(4)–C(2) 122.3(2)

C(2)–C(3)–C(4)a 137.7(7)C(3)–C(4)–C(5)a 137.1(11)

a Values affected by disorder.

Fig. 3. A layer-like part of HS [Fe(4tz)6](BF4)2 around z = 0 viewed downthe c-axis.

Fig. 4. Packing diagrams of HS [Fe(4tz)6](BF4)2 viewed perpendicular to (1(vertical) – left – and only those [Fe(4tz)6]2+ and BF4

� groups are shown the

N. Hassan et al. / Inorganica Chimica Acta 361 (2008) 1291–1297 1293

these layers, located at about z = 0 is shown in Fig. 3.According to the trigonal rhombohedral lattice, spacegroup R�3, these layers are stacked in cubic closest packedfashion, i.e. in the stacking sequence � � �ABCABC� � � withiron centred at xyz = 0,0,0; 2/3,1/3,1/3, and 1/3,2/3,2/3,etc. A view of the three-dimensional structure is shown inFig. 4. This building principle is already known from HS

[Fe(3tz)6](BF4)2, HS [Fe(3tz)6](ClO4)2, and related com-pounds such as the corresponding Fe, Ni and Zn saltswith cyclopropyltetrazole ligands [11,12]. This means thatHS [Fe(4tz)6](BF4)2 can be considered as in principle iso-structural with HS [Fe(3tz)6](BF4)2, and the only majordifference is that the alkyl chain in HS [Fe(4tz)6](BF4)2

is longer than in the propyl-analogue extending alongthe c-axis, whereas the a and b axes are very similarfor both, namely a = b = 10.959(4) A, c = 37.89(2) Aand V = 3940(3) A3 for HS [Fe(4tz)6](BF4)2 at 297 K,and a = b = 10.833(3) A, c = 33.704(6) A and V =3425(1) A3 for HS [Fe(3tz)6](BF4)2 at 295 K [9]. Thoughoutside the scope of this paper to make further structuralcomparisons with other relatives, like [Fe(1tz)6](BF4)2,[Fe(2tz)6](BF4)2, etc., it shall be mentioned that up to tet-razole side chain lengths of three atoms (methyl, ethyl,propyl, cyclopropyl, haloethyl) their structures are allbased on hexagonal closest packed layers such as theone shown in Fig. 3. Gradually differing among them isstacking of the layers, anion ordering, and anion orienta-tion, which leads to space group symmetries of the typesP21/n, P21/c, C2/c, and P�1 apart from R�3 as in HS

[Fe(4tz)6](BF4)2.

10) showing the stacking of the [Fe(4tz)6](BF4)2 layers along the c-axiscentroids of which are arranged on the (110) plane – right.

Velocity (mm/s)

293K

180K

120K

Rel

. Tra

nsm

issi

on (%

)

-4 4-2 20

100

100

100

99.4

97.5

1294 N. Hassan et al. / Inorganica Chimica Acta 361 (2008) 1291–1297

2.3. Magnetic susceptibility measurements

Temperature dependent magnetic susceptibility mea-surements of the complexes were performed in externalfields of 0.3 T for [Fe(ntz)6](BF4)2 (n = 1,2) and of 0.5 Tfor [Fe(ntz)6](BF4)2 (n = 3,4) within the temperature rangefrom 10 to 250 K. The magnetic properties of[Fe(ntz)6](BF4)2 (n = 1–3) match the literature data [2]. Acomparative plot of the spin-transition curves is shown inFig. 5. The methyl- and the ethyl-substituted speciesshowed an incomplete spin transition. The other homo-logues showed a complete spin transition. Within thesehomologues only [Fe(3tz)6](BF4)2 showed an abrupt spintransition with a thermal hysteresis of 9 K suggestingstrong cooperativity. However, it was shown that the hys-teresis in the case of [Fe(3tz)6](BF4)2 can be attributed tophase transitions [9]. In contrary, [Fe(4tz)6](BF4)2 showsa more gradual spin transition within a temperature inter-val of 60 K without hysteresis.

80K100

92

87

Fig. 6. 57Fe-Mossbauer spectra of [Fe(4tz)6](BF4)2 at selected tempera-tures 80, 120, 180 and 293 K (the solid lines represent fitted curves).

2.4. 57Fe-Mossbauer spectroscopy

The temperature dependent 57Fe-Mossbauer spectra of[Fe(4tz)6](BF4)2] were recorded between 80 and 293 K.Fig. 6 shows four representative spectra obtained in theheating mode. The values of the Mossbauer parametersare listed in Table 2. At 80 K the spectrum consists of aunique singlet with parameters typical for iron(II) in theLS state (d = 0.579(1) mm/s, half-line width = 0.155(2)mm/s). Upon increasing the temperature, a high-spin quad-rupole doublet appears gradually, in agreement with themagnetic curve, evidencing clearly a gradual spin crossoverin the material. No hysteresis loop is observed as a functionof temperature. It is worth to notice some line-broadeningon the LS component observed at 180 K characteristic of

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

0 50 100T

mol

*T [c

m3 K/

mol

]X

Fig. 5. Magnetic behaviour (vmol Æ T vs. T) of

some dynamical processes usually observed on the minoritycomponent at the end of the transition. As there is a tem-perature dependence of the Debye–Waller factor [13], theambient temperature spectrum (refer to the non-fitted data– top spectrum – in Fig. 6) showing a peculiar form

150 200 250 [K]

1tz2tz3tz4tz

the series of complexes of [Fe(ntz)6](BF4)2.

Table 2Least-squares-fitted Mossbauer data obtained for [Fe(4tz)6](BF4)2 at selected temperatures (d: isomer shift with reference to metallic iron at 293 K; D:quadrupole splitting; C: half-line width, the error bars are given in parentheses)

T (K) Low spin High spin Ratio high spin (%)

d C d D C

180 0.56(3) 0.42(6) 1.129(4) 1.825(8) 0.150(6) 71(3)120 0.580(4) 0.182(5) 1.16(2) 2.06(4) 0.16(3) 17(3)80 0.579(1) 0.155(4) – – – 0

N. Hassan et al. / Inorganica Chimica Acta 361 (2008) 1291–1297 1295

suggesting perhaps a considerable change in the Debye–Waller factor for traces of remnant low spin species in ahigh-spin phase, but this feature is subject of further inves-tigations beyond the scope of this paper.

2.5. Discussion and conclusions

Extending the series of [Fe(ntz)6](BF4)2 to a higherhomologue showed that a longer alkyl-chain leads to ahigher spin-transition temperature T1/2 featuring a very dif-ferent spin-transition behaviour – e.g. from incomplete(n = 1, 2), cooperative with a hysteresis (n = 3) to moregradual without a hysteresis (n = 4). Arguing with the idealmodel for spin transitions (i.e. Boltzmann distribution), thetendency within this series may be influenced by enthalpyand/or entropy differences within this series. Obtainingthese thermodynamic parameters, experimentally or theo-retically, is therefore an important point for the furtherunderstanding of the factors governing the tendency withinthis series. The deviations from ideal spin-transition behav-iour may be seen in context with the crystal structures ofthese compounds [14–16]. The comparatively large thermaldisplacement ellipsoids of the butyl chain indicate a signif-icant mobility of these chains in contrary to the well knownother homologues. Therefore, we believe that longer alkylchains will lead to much more ill-defined structures. As aconsequence the higher homologues in this series areexpected to show more gradual spin-transition behaviouras a result of more and more independent iron-centresand less defined crystal structures. This subject is currentlyunder further investigation.

3. Experimental

3.1. Chemicals

NaN3 (99%), triethylorthoformate (99%), glacial aceticacid (99%), 1-alkylamine (>98%) and NaOH (97%) wereobtained from Aldrich. All other chemicals were standardreagent grade and used as supplied.

3.2. Elemental analysis

Elemental analyses (C, H, and N) were performed by theMikroanalytisches Laboratorium, Institute for PhysicalChemistry, Vienna University, Wahringerstrasse 42,A-1090 Vienna, Austria.

3.3. NMR and IR spectroscopy

1H NMR and 13C NMR in deuterated CDCl3-d1 weremeasured using a Bruker 250 FS FT NMR spectrometer.Proton NMR chemical shifts are reported in ppm versusTMS. Mid-range FTIR spectra of the compounds wererecorded as KBr-pellets within the range 4400–450 cm�1

using a Perkin–Elmer 16PC FTIR spectrometer. Pellets wereobtained by pressing the powdered mixture of the samples inKBr in vacuo using a hydraulic press applying a pressure ofapprox. 10000 kg/cm2 for 5 min. Far FTIR spectra wererecorded within the range 600–200 cm�1 on a Perkin–ElmerSystem 2000 far FTIR spectrometer as polyethylene pellets.The variable temperature IR spectra have been recordedusing a Graseby Specac thermostatable sample holder.

3.4. Ligands synthesis

The synthesis of the series of 1-alkyltetrazoles wasperformed according to the literature [2] with slight modi-fications. Equimolar amounts of alkylamine, sodium azideand triethyl orthoformate were dissolved in approximately40 M equiv. of 99.5% acetic acid and stirred at 95 �C for24 h. After removing the solvent under vacuum 80M equiv. of 2 N HCl were added. The resulting solutionwas extracted three times with ethyl acetate. The organicphase was neutralized with saturated NaHCO3-solution.Then, the organic phase was dried with saturated NaClsolution and finally with Na2SO4. Afterwards, the solventwas evaporated in vacuo. The resulting oil was distilled ina Kugelrohr apparatus at 150 �C and a pressure of 1 hPayielding colourless oil. The obtained yields after distillationin vacuo were between 30% and 40%.

3.5. NMR and IR data of tetrazol-1-yl-alkanes (for

assignment of 13C data refer to Fig. 7)

3.5.1. Tetrazol-1-yl-methane (1tz)1H NMR (250.13 MHz, CDCl3) d [ppm]: 8.71 (1, 1H,

Ha); 3.91 (1, 3H, Hb). 13C NMR (62.86 MHz, CDCl3) d[ppm]: 143.8 (d, Ca); 34.4 (q, Cb). IR: mC–H(tz) = 3137 cm�1.

3.5.2. Tetrazol-1-yl-ethane1H NMR (250.13 MHz, CDCl3-d1) d [ppm]: 8.71 (1, 1H,

Ha); 4.42 (4, 3J = 7.3 Hz, 2H, Hb); 1.47 (3, 3J = 7.3 Hz, 2H,Hc).

13C NMR (62.86 MHz, CDCl3-d1) d [ppm]: 142.8 (d,Ca); 44.0 (t, Cb); 15.4 (q, Cc). IR: mC–H(tz) = 3133 cm�1.

N

N

CH

N

N

C

C

C

CH3

a b

c

d

e

Fig. 7. Assignments of 13C NMR data for the tetrazole-1-yl-alkanes.

1296 N. Hassan et al. / Inorganica Chimica Acta 361 (2008) 1291–1297

3.5.3. Tetrazol-1-yl-propane1H NMR (250.13 MHz, CDCl3-d1) d [ppm]: 8.67 (1, 1H,

Ha); 4.38 (3, 3J = 7.3 Hz, 2H, Hb); 1.92 (6, 3J = 7.3 Hz, 2H,Hc); 0.90 (3, 3J = 7.3 Hz, 3H, Hd). 13C NMR (62.86 MHz,CDCl3-d1) d [ppm]: 143.3 (d, Ca); 50.2 (t, Cb); 23.4 (t, Cc);11.0 (q, Cd). IR: mC–H(tz) = 3132 cm�1.

3.5.4. Tetrazol-1-yl-butane1H NMR (250.13 MHz, CDCl3-d1) d [ppm]: 8.63 (1, 1H,

Ha); 4.43 (3, 3J = 9.1 Hz, 2H, Hb); 1.91 (5, 3J = 9.3 Hz, 2H,Hc); 1.35 (6, 3J = 9.3 Hz, 2H, Hd); 0.94 (3, 3J = 9.0 Hz, 3H,He).

13C NMR (62.86 MHz, CDCl3-d1) d [ppm]: 143.2 (d,Ca); 48.2 (t, Cb); 32.1 (t, Cc); 19.9 (t, Cd); 13.7 (q, Ce).IR: mC–H(tz) = 3130 cm�1.

3.6. Synthesis of the complexes

The synthesis of [hexakis(1-(tetrazol-1-yl)alkane-N4)-iron(II)]bis(tetrafluoroborate) [Fe(ntz)6](BF4)2 with n = 1–4 was performed under argon according to the literatureusing the Schlenk technique following the general procedure:

Fe(BF4)2 Æ 6 H2O (1.00 mmol) was dissolved in ethanol(5 mL), the ligand (6.00 mmol) was dissolved in ethanol(7 mL). The ethanolic solution was added drop-wise to theiron solution at 35 �C. To avoid oxidation it is of para-mount importance to keep the iron(II) containing solutionbelow 40 �C. The solution was stirred for 3 h at room tem-perature. After keeping the solution for 2 days at 4 �C dieth-ylether was added drop-wise until precipitate was formed.The white precipitate was filtered and dried under vacuum.Yield: [Fe(4tz)6](BF4)2 = 70% of theory. Single crystals of[Fe(4tz)6](BF4)2 have been grown by re-crystallizing in eth-anol over several weeks. The obtained hexagonal shapedsingle crystals were colourless and very soft.

Elemental analysis of [Fe(4tz)6](BF4)2. [986.4 g/mol] –found (calc.) for C30H60N24FeB2F8: C: 35.6% (35.2%); H:5.9% (6.1%); N: 34.2% (34.1%), IR: mC–H(tz) (300 K) =3142 cm�1, IR: mC–H(tz) (100 K) = 3152 cm�1.

The 10 cm�1 upshift of the tetrazolic C–H vibrationupon cooling from ambient temperature (HS species) to100 K (LS species) is due to the change of the C–H bondstrength induced by the changes of the Fe–N bond length(and strength) according to the literature [17,18].

3.7. Crystallographic study

A platy crystal of [Fe(4tz)6](BF4)2 was glued to a glassfibre, transferred to a Bruker SMART diffractometer(graphite monochromated Mo Ka radiation from a sealed

X-ray tube, k = 0.71073 A, platform 3-axis goniometer,CCD area detector) and intensity data were collected at298 K. After raw data extraction with program SAINT,absorption and related effects were corrected with programSADABS (multi-scan method) and data analysis was carriedout with the program XPREP [19]. The structure was thensolved with program SHELXS97 followed by structurerefinement on F2 with program SHELXL97 [20]. Non-hydro-gen atoms were refined anisotropically. Hydrogen atomswere inserted in calculated positions and refined with theriding model. Orientation disorder in the BF4 anion wasdealt with a set of five partially occupied fluorine positions.At this stage the close structural relationship to[Fe(3tz)6](BF4)2 was recognized and atom designations aswell as setting of the asymmetric unit were adopted fromWiehl [9] and a concluding refinement was carried out.Salient crystal data are: [Fe(4tz)6](BF4)2, C30H60B2F8-FeN24, FW = 986.49, trigonal rhombohedral, space groupR�3 (no. 148), a = b = 10.959(4) A, c = 37.89(2) A, V =3940(3) A3, Z = 3, l = 0.363 mm�1, dx = 1.247, T = 297K; 13262 reflections collected (hmax = 25�), 1524 indepen-dent (Rint = 0.030); final R-values (all data): R1 = 0.064,wR2 = 0.167, 126 parameters.

3.8. Magnetic susceptibility measurements

The powder samples of [Fe(ntz)6](BF4)2 were placed in aplastic sample holder. Variable temperature magnetic sus-ceptibility measurements of the complex were performedwith an MPMS magnetometer (Quantum Design) in exter-nal fields up to 0.5 T within the temperature range of 10and 250 K with heating and cooling rates of 1 K min�1.The data were corrected for temperature independent dia-magnetism using Pascal’s constants [21].

3.9. 57Fe-Mossbauer spectroscopy

The variable temperature 57Fe-Mossbauer measure-ments were obtained on a constant-acceleration spectro-meter with a 1.8 GBq source of 57Co in a Rh-matrix. Theisomer shift values (d) are given with respect to metalliciron at room temperature. The absorber was a sample ofmicrocrystalline powder enclosed in a 2 cm diameter cylin-drical plastic holder, the size of which had been determinedto optimize the absorption. The variable temperature spec-tra were obtained in the range of 77–300 K using a nitrogenbath cryostat (Oxford). Fitting parameters for all spectrawere obtained by using a least squares computer program(the Recoil least-squares fitting program package underthe assumption of a discrete superposition of Lorentzianlines). The standard deviations of statistical origin aregiven in parentheses.

3.10. X-ray powder diffraction (XRPD)

The complexes were gently ground and filled into circu-lar sample holders. The XRPD measurements were carried

N. Hassan et al. / Inorganica Chimica Acta 361 (2008) 1291–1297 1297

out on a Philips X’Pert diffractometer in Bragg-Brentanogeometry using Cu Ka1,2 radiation.

Acknowledgements

Lionel Rechignat (LCC/CNRS, Toulouse) is warmlyacknowledged for his technical support. We thank Matth-ias Grunert (TU Vienna) for preliminary experimentalinvestigations within his Ph.D. thesis and Markus Valtiner(Max-Planck Institut fur Eisenforschung, Dusseldorf) foruseful discussions. We want to thank COST Project D35/0014/05 for granting a short-term scientific mission(COST-STSM-D35-02231) of Nader Hassan at the LCC-CNRS (Toulouse, France). Financial support of the Aus-trian Science Fund (FWF) Project 19335-N17 is gratefullyacknowledged.

Appendix A. Supplementary material

CCDC 612.757 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.2007.08.023.

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