ð cat Þ ½ M M ð C O Þ Š Structuralandmagneticpropertiesoftwo-andthree-dimensionalmolecule-basedmagnets ARTICLEINPRESS

Journal of Magnetism and Magnetic Materials 272–276 (2004) 1089–1090

Structural and magnetic properties of two-and three- dimensional molecule-based magnets ðcatÞ ^þ ½M ^II M ^III ðC ₂ O ₄ Þ ₃

Nikolai S. Ovanesyan

*, Gena V. Shilov

, Alex A. Pyalling

, Cyrille Train

, Patrick Gredin

, Michel Gruselle

, L! aszl o F. Kiss !

, L aszl ! o Botty! ! an

aInstitute of Problems of Chemical Physics, Ras Moscow Region, Chernogolovka 142432, Russia

bLab. de Chimie Inorg. et Mater. Mol! ecularies, Universit! e Pierre et Marie Curie, 4 Place Jussieu, Case 42, 75252 Paris C! edex 05, France!

cResearch Institute for Solid State Physics and Optics, P.O.B. 49, H-1525 Budapest, Hungary

dKFKI Research Institute for Particle and Nuclear Physics, P.O.B. 49, H-1525 Budapest, Hungary

Abstract

We discuss the different structural arrangements of NBu₄½Fe^IICr^IIIðC2O₄Þ₃layered compounds in their racemic and enantiomeric forms and related magnetic properties. For ½Mn^IIFe^IIIðC₂O₄Þ₃ networks of dimensionalities 2 and 3 Mossbauer spectroscopy was applied to study the Fe. ^III sublattice magnetization. Unusual magnetic relaxation phenomena belowTNwere observed for both 2D and 3D networks.

r2004 Elsevier B.V. All rights reserved.

PACS: 11.30.R; 33.15.K; 68.55; 33.45

Keywords: Chirality; Enantioselective self-assembly; Molecular magnets; Mossbauer spectroscopy.

The ability of oxalate anion ðC2O₄Þ² to form bridging bonds between transition metal ions has allowed to design extended bimetallic networks of the general formulaðcatÞ^þ½M^IIM^IIIðC₂O₄Þ₃ as new mole- cular materials exhibiting bulk ferro- ðM^III¼Cr^IIIÞ or ferrimagnetic ðM^III¼Fe^IIIÞproperties. These materials may be prepared from the combination of tris-oxalato metalates½M^IIIðoxÞ₃³ with other di-cationic transition metal ions (M^II¼Mn^II; Ni^II; Fe^II:..) [1]. Such tris- bidentate complexes display a propeller-like chirality.

The relative configuration of the connected hexacoordi- nated centers determines the 2D or 3D architecture of the network. A hetero-chiral arrangement½M1ðDÞM2ðLÞ or½M1ðLÞM2ðDÞleads to a 2D layer structure[2]. The anionic sub-lattice displays a stack of such layers while the cationic moiety, ðcatÞ^þ; a tetra-alkyl ammonium ðNR^þ₄Þion (R is Pr¼n-C₃H₇;Bu¼n-C₄H₉), is located between the anionic layers. On the other hand, a homo-

chiral arrangement ½M1ðDÞM2ðDÞ or ½M1ðLÞM2ðLÞ leads to a helical organization of the connected metallic ions. Therefore a three-dimensional anionic network is obtained with the associated cationic counter-part fitting in the vacancies of the anionic network[3].

Exceptional is the X-ray structure of NPn₄

½MnFeðoxÞ₃ ðPn¼n-C₅H₁₁Þ: In that case, the reaction leads to single crystals from the racemic materials to occur with a spontaneous chiral resolution of NPn₄½MnðDÞFeðLÞðoxÞ₃ and NPn₄½MnðLÞFeðDÞðoxÞ₃ enantiomers (space group C222₁) [4]. These results proved theexistence of2Dnetworks in a chiral form.

Pursuing our investigations in the field of the synthesis of optically active molecular magnets, based on 2D or 3D bimetallic oxalate networks, we report herein the structural and optical experimental evidences supporting the chiral character of the 2D networks in NBu₄½FeCrðoxÞ₃obtained from resolved anionic D or L½CrðoxÞ₃³ bricks.

The circular dichroism (CD) curves recorded from 200 to 700 nm in KBr dispersion reveal the enantiomeric purity of the compounds NBu4½FeðDÞCrðLÞðoxÞ₃ and

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*Corresponding author. Tel.: +7-096-522 3119; fax: +7-096- 514-3244.

E-mail address:ovanesyan@icp.ac.ru (N.S. Ovanesyan).

0304-8853/$ - see front matterr2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmmm.2003.12.029

NBu₄½FeðLÞCrðDÞðoxÞ₃: Powder X-ray diffraction pat- terns ofDandLisomer structures can be assigned to the chiral space groupP6₃:The two-layerP6₃and six-layer achiral R3c structure of NBu₄½FeCrðoxÞ₃ differ in the packing of layers which is of importance from the viewpoint of magnetism of 2D compounds. If interlayer magnetic interactions (at no less than B9A) are( feasible, these different kinds of packing may influence the magnetic ground state. However, magnetization versus temperature curves at the lowest applied field of 3 Oe reveal identical ferromagnetic transition tempera- turesðT_C¼12 KÞfor the chiral and the racemic form.

Since the principal electric field gradient tenzor compo- nent,V_zz is out of plane by crystal structure considera- tions, Mossbauer. data in both enantiomers are indicative of a magnetization confined into the crystal’s basal plane. Consequently, such compounds show an ideal 2DXY magnetic structure.

Unexpected properties of both 2D and 3D fMnFeg networks were studied by Mossbauer spectroscopy. Two. racemicNBu₄½MnFeðoxÞ₃powder samples (S1 and S2) were prepared by slightly different synthetic procedures.

Paramagnetic Mossbauer spectra of S1 and S2 are. identical. However, atT¼4:2 K the magnetic easy axis apparently lies in the crystal’s basal plane (XY magnet) for S1, while it declines out of the basal plane byB35 for S2. Temperature dependence of⁵⁷Fe hyperfine field and spectral line width are displayed inFig. 1.

A prominent feature in Mossbauer spectra of. NBu4½MnFeðoxÞ₃ is an unusual magnetic relaxation well below T_N: With increasing temperature, lines broaden followed by a development of a ‘paramagnetic’

fraction in the ordered state. The broadening is the largest for S1, i.e. forXY magnet (Fig. 1).

A sample of 3DfMnFegoptically active network was obtained in a synthesis when the templating monocation possess D₃ symmetry, a representative example being the fRuðbpyÞ₂ppyg^þ ½MnFeðoxÞ₃ compound (S3).

Indeed, the Fe^III magnetization versus temperature follows the Brillouin function forS¼⁵₂which is typical for a 3D magnet. For comparison, typical temperature dependence for a 2D Ising magnet from a previous study is shown inFig. 1 [5]. Non-typical for the 3DfMnFeg compound is the lowerT_Nand the strong increase of the line width with temperature. We suggest that this is due to the specific molecular structure, causing a competi- tion between anisotropy and exchange energy.

X-ray powder diffraction spectra of NBu₄

½MnFeðoxÞ₃; in view of chiral structural data, are indicative of stacking faults in the polycrystalline samples (P6₃layer sequence faults withinR3c sequence and vice versa). However, the ratio ofR3c toP6₃phase for all samples is practically the same. The only difference found is the profile width being the largest for S2.

Electron microscopy photographs of S1 and S2 display a mean particle size of 2.5 and 2:7mm;

respectively. However, each particle represents an agglomerate of small hexagonally shaped single crystals of about 0.3–0:5mm:

In summary, in view of the magnetic two-dimension- ality of the layered oxalates, the magnetic particle size seems to be scaled by the size of the individual layers within each small single crystal of the powder.

Apparently, with increasing mean particle size the magnetic structure changes from an easy plane config- uration to a partially out of plane structure (some data not shown here). The unusual magnetic relaxation behavior is related to the low magnetic dimensionality in these systems. The threefold site symmetry suggests a six-fold magneto-crystalline symmetry in the basal plane, which, in turn, implies a low in-plane anisotropy as well.

Supported in part by RFBR Grant # 02-03-33283.

References

[1] H. Tamaki, Chem. Lett. (1992) 1975.

[2] N.S. Ovanesyan, G.V. Shilov, L.O. Atovmyan, R.N.

Lyubovskaya, A.A. Pyalling, Y.G. Morozov, Mol. Cryst.

Liq. Cryst. 273 (1995) 175.

[3] R. Andr!es, M. Brissard, M. Gruselle, C. Train, J. Vaissermann, B. Mal!ezieux, J.P. Jamet, M. Verdaguer, Inorg. Chem. 40 (2001) 4633.

[4] G.V. Shilov, N.S. Ovanesyan, N.A. Sanina, L.O. Atovm- yan, M. Gruselle, Russian J. Coord. Chem. 27 (2001) 605.

[5] L. Bottyan, L. Kiss, N.S. Ovanesyan, A.A. Pyalling, N.A.! Sanina, A.B. Kashuba, JETP Lett. 70 (1999) 697.

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Fig. 1. Magnetic hyperfine field,H_hf (solid symbols), and line width, G(open symbols) versus temperature for½MnFeðoxÞ₃ networks (sample S1: squares, S2: circles, S3: diamonds described in the text). For S3, magnetization follows a B₅₌₂ Brillouin function. For comparison, data (triangles) for a 2D Ising magnet NPn4½FeFeðoxÞ₃are shown with model functions [5]. Other lines are guides to the eye.

N.S. Ovanesyan et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 1089–1090 1090