J O U R N A L O F C H E M I S T R Y Materials Hybrid molecular materials based on organic molecules and the inorganic magnetic cluster [M4(H2O)2(PW9O34)2]10- (M2+=Co, Mn)† Miguel Clemente-Leo�n,a Eugenio Coronado,*a Jose-Ramo�n Gala�n-Mascaro� s,a Carlos Gime�nez-Saiz,a Carlos J. Go�mez-Garcý�aa and Toribio Ferna�ndez-Oterob aDepartamento de Quý�mica Inorga�nica. Universidad de Valencia, 46100 Burjasot, Spain bL ab.Electroquý�mica, Fac. de Quý�mica, UPV, PO Box 1072, 20080 San Sebastian, Spain The synthesis and physical characterization of new organic–inorganic hybrids formed by conducting and magnetic networks are reported. The crystalline radical salts are formed by BEDT-TTF type donors as the organic part, and by large metal–oxide clusters of the type [M4(H2O)2(PW9O34)2]10- (M2+=Co, Mn) as the inorganic part.We also show how these magnetic clusters can be incorporated in conducting organic polymers to give hybrid organic–inorganic films.The search for new molecule-based materials combining con- of definite sizes and shapes which can accommodate one or more magnetic centers in their structures. The solubility of ducting and magnetic properties constitutes a current challenge in materials science, which only very recently has begun to be such clusters in both aqueous and non-aqueous solvents makes it possible to electrochemically oxidize the organic donor in explored.A convenient chemical approach to obtain such multiproperty materials is the so-called organic–inorganic the presence of these counter-ions so as to obtain crystalline salts of these hybrids.8 In this context, the most successful hybrid approach.1 It consists of using as building blocks organic molecules or polymers possessing itinerant electrons results have been obtained from the BEDT-TTF molecule (1 in Scheme 1) which has allowed the growth of crystals contain- and inorganic metal complexes possessing localized magnetic moments.ing polyoxometalates9 with metal nuclearities comprised between 6 and 18 [Fig. 1(a)–(c)]. Owing to the diYculty in As the organic part one can use p-electron donor molecules of the TTF type which are the basic ingredient for most of the crystallizing these salts as the complexity of the cluster is increased, obtaining crystalline radical salts containing larger molecular conductors and superconductors.2 Another interesting possibility is to use conducting polymers of the type clusters is a chemical challenge.polypyrrole or polyaniline. These polymers are relatively simple to obtain by chemical or electrochemical oxidative polymerization of the monomer molecules and they have considerable potential technological applications.3 As the inorganic part, a variety of anionic metal complexes of various nuclearities and dimensionalities can be chosen.Starting with the most simple magnetic anions one can find the mononuclear metal halides (FeCl4-, CuCl42-,…). These anions are being successfully combined with several organic donors to Scheme 1 give radical ion salts with coexistence of localized magnetic moments and itinerant electrons.4 Another important mono- With the aim of introducing clusters of higher nuclearities nuclear anion is the iron(III) tris(oxalato) complex, Fe(ox)33-, we have tried the reaction of BEDT-TTF with the magnetic which combined with the BEDT-TTF donor resulted in the polyoxoanions [M4(PW9O34)2]10- (M2+=Co, Mn) which preparation of the first molecular material with coexistence of have a metal nuclearity of 22 [Fig. 1(d)]. We present here the paramagnetic centers and superconductivity.5 Themost complex synthesis and physical characterization of these crystalline class of inorganic complexes we can use are polymeric. Anion hybrids. We also report how these magnetic clusters can be magnetic chains or layers are known that on paper can be incorporated in conducting organic polymers to give hybrid combined with cation radicals. From the magnetic point of organic–inorganic films.view, polymeric layered complexes such as, for example, the bimetallic oxalato-bridged complexes [MIIMIII(ox)3]- (MII= Mn, Fe, Ni, Co, Cu; MIII=Cr, Fe), are the most interesting since they can confer to the hybrid material cooperative magnetic properties such as ferromagnetism.6 However, these polymeric anions are not easy to handle in terms of the chemistry (extremely insoluble and only stable in the solid state).In fact, only discrete bimetallic oxalato complexes but not polymeric ones have been combined so far with organic donors.7 In between these two extreme cases one can find the polyoxometalate complexes. These are big metal–oxide clusters Fig. 1 Polyhedral representation of the: (a) Lindquist, [M2O19]2 (M= Mo, W), (b) Keggin [XM12O40]n- (M=Mo, W), (c) Dawson–Wells, † Presented at the 58th Okazaki Conference, Recent Development and Future Prospects of Molecular Based Conductors, Okazaki, Japan, [X2M18O62]n- (M=Mo, W) and (d) [M4 (H2O)2(PW9O34)2]10- (M2+=Co, Cu, Mn, Fe, Cr, Ni and Zn) polyanions 7–9 March 1997. J. Mater. Chem., 1998, 8(2), 309–312 309Fig. 3 Plot of xmT vs. T for the [Mn4(H2O)2(PW9O34)2]10- polyanion Fig. 2 Plot of xmT vs. T for the [Co4(H2O)2(PW9O34)2]10- polyanion in its K+ (×), BEDT-TTF+ (#) salts in its K+ (×), BEDT-TTF+ (#) and polypyrrole ($) salts No influence coming from the organic component on the Organic–inorganic crystalline materials based on magnetic coupling within or among the clusters is detected BEDT-TTF electron donors down to 2 K.Thus, in the Co derivative the product xmT shows a sharp increase below 50 K upon cooling and a The polyoxoanions [M4(PW9O34)2]10- are of magnetic intermaximum at ca. 6 K. Such a behavior is analogous to that est since they contain the tetranuclear magnetic clusters Co4 observed in the K+ salt (see Fig. 2) and unambiguously and Mn4 encapsulated in between two polyoxotungstate moietdemonstrates that the ferromagnetic cluster is maintained when ies [PW9O34] [see Fig. 1(d)]. In the cobalt case the ions are we change K+ to BEDT-TTF+. Furthermore, it is indicative ferromagnetically coupled giving rise to a magnetic ground of a lack of interactions between the two components as a state comprising 12 unpaired electrons,10 while in the mangaconsequence of the good insulation of the Co4 cluster provided nese one this exchange coupling is antiferromagnetic and by the polyoxotungstate framework.Low temperature EPR results in a non-magnetic ground state (S=0).11 Furthermore, spectra support such a conclusion (Fig. 4). We observe that at the type of exchange coupling is diVerent in both clusters. 4.2 K the two samples show the same spectrum: a very broad Thus, while high-spin octahedral Mn2+ has a fully isotropic and anisotropic signal mainly coming from the ground 6A1 ground state, high-spin octahedral Co2+ has an orbitally Kramers doublet of the cluster, centered around 1620 G (g= degenerate 4T1 ground state which is split into six anisotropic 4.1), which extends from 1000 to 4000 G.This signal has to Kramers doublets by the eVect of spin–orbit coupling and be attributed to the Co4 cluster. No signal coming from the distortions of the octahedron. As a consequence the exchange organic radical is observed at this temperature. This should interaction between Co2+ ions is highly anisotropic resulting indicate that the unpaired electrons located at the BEDT- in a complete splitting of the highly degenerate ground state TTF+ cations are strongly coupled in the solid so that they in spin doublets.In fact the ground state of this cluster is an are magnetically silent at low temperature. anisotropic Kramers doublet which is well separated in energy The similarity between the BEDT-TTF+ and K+ derivatives from the other excited doublets (the closest energy level is at is also evident in the Mn case (Fig. 3). Thus, the two magnetic ca. 14 cm-1).12 curves are coincident in the whole temperature range, within Black crystals of composition BEDT-TTF6H4[M4(H2O)2- the experimental error. This result proves that the antiferro- (PW9O34)2] (M 2 +=Co, Mn) have been obtainedcoupled Mn4 cluster is maintained intact in the crystallization.Although they are still not of suYcient quality radical salt. The close coincidence between the two magnetic to be studied by X-ray diVraction, a preliminary study of their unit cells indicates that they are isostructural.‡ In view of the stoichiometry of these salts (651), four protons had to be introduced in order to compensate the charges.With this assumption the six organic molecules should be completely charged (+1). Accordingly, the compounds are insulators (the electrical conductivity has been measured on pressed pellets). The magnetic properties of the BEDT-TTF salts are shown in Fig. 2 and 3 and compared to those of the potassium salts of the two polyanions. In both cases the low temperature magnetic behavior is dominated by the inorganic component.§ ‡ Unit cell of the cobalt(II) derivative (from indexation of 17 reflections): a=11.85(2), b=13.23(1), c=27.741(5) A ° , a=83.15(5), b= 87.17(4), c=73.85(9)°; unit cell of the manganese(II) derivative (from indexation of 16 reflections): a=11.79(1), b=13.23(1), c=27.48(1) A °, a=88.95(6), b=89.96(4), c=73.79(8)°.§ At high temperatures the magnetic moments of both the BEDT-TTF salt and the polypyrrole film are higher than that observed in the potassium salt (see Fig. 2). The origin of such diVerences may come from the large anisotropy of the magnetic cluster. Slightly diVerent octahedral distortions within the clusters can give rise to significant variations in the local Lande� tensors and therefore in the magnetic properties.In fact, if one compares the magnetic properties of the Co4 cluster encapsulated by the [PW9O34] ligands or by the [P2W15O56] ones, we observe that in the former case xmT varies from 14 emu K mol-1 at high T to 24 emu K mol-1 at 6 K, while in the latter this Fig. 4 EPR spectra of the [Co4(H2O)2(PW9O34)2]10- polyanion in variation is only from 10 to 15 emu K mol-1. However, the position of the characteristic maximum in xmT stays constant in both cases.its K+ and BEDT-TTF salts 310 J. Mater. Chem., 1998, 8(2), 309–312biggest ever used in the synthesis of radical cation salts, and (ii ) the salts constitute the first known examples of hybrid materials containing a magnetic cluster and an organic donor. In the second case we have shown that these magnetic clusters can be incorporated in polypyrrole films to give magnetic films having semiconducting properties.More hybrid films of this kind can now be prepared in which new functional properties can be introduced by playing for example with the electrochromic character of the polyoxometalate component. Another aspect that is being developed in this context in order to improve the properties of the film is that of creating well organized hybrid films by using the Langmuir–Blodgett technique.13 Experimental Fig. 5 Plot of the conductivity vs. T of a [Co4(H2O)2(PW9O34)2]10-– Single crystals of the radical salts BEDT-TTF6H4[M4- polypyrrole film (H2O)2(PW9O34)2] (M2+=Co, Mn) were obtained on a platinum wire electrode by anodic oxidation of the organic donor curves also shows that the presence of BEDT-TTF+ radicals ET (2×10-3 M in a 152 mixture of CH3CN–CHCl2CH2Cl) in does not aVect the magnetic coupling within or between the a U-shaped electrocrystallization cell under low constant cur- Mn4 clusters.As for the EPR spectra, these are dominated by rent (I=1.2 mA) in the presence of a solution of the polyanion a broad signal centered at g=2 arising from the Mn4 cluster.in toluene. After two weeks very small hexagonal plate-like A sharp signal of very weak intensity is superimposed onto single crystals were observed in the anode. They were collected, the cluster signal. This may be associated with paramagnetic washed with CH3CN and air dried. Found: C, 10.37; radical impurities. The above crystalline materials demonstrate H, 0.96; N, 0.16; S, 22.01.BEDT-TTF6H4[Co4(H2O)2- the ability of this type of large magnetic clusters to form (PW9O34)2]·(CH3CN)·5(H2O) requires C, 10.36; H, 0.90; N, crystalline organic–inorganic radical salts with the BEDT- 0.19; S, 21.40%. Found: C, 9.91; H, 1.05; N, 0.0; S, 20.85. TTF donor, despite its big size and charge. BEDT-TTF6H4[Mn4(H2O)2(PW9O34)2]·9H2O requires C, 10.02; H, 1.04; N, 0.0; S, 21.41%.The IR spectra of both salts are very similar and show all the characteristic bands of the Organic–inorganic films based on polypyrrole polyanion and BEDT-TTF molecules. In view of the stability of the above clusters in both aqueous The films of polypyrrole with the cobalt-containing polyand non-aqueous solutions we have examined the possibility of anion were prepared by electrochemical oxidation in a N2 obtaining conducting polymers incorporating this kind of magatmosphere of an aqueous solution of pyrrole (0.5 M) in the netic polyoxometalates.Preliminary results with the Co4 cluster presence of the polyanion (3.6×10-3 M). The intensity of the show that by aqueous electrochemical polymerization of pyrrole current was fixed at 5 mA and after several minutes the (2 in Scheme 1) in the presence of this polyoxometalate, organic– polypyrrole–polyanion films were collected from the anode.inorganic films containing ca. 80 pyrrole units per cobalt cluster can be obtained.¶ The magnetic properties are very close to We thank the Spanish DGICYT for founding this work (Grant those observed in the K+ salt (Fig. 2) indicating that the PB94–0998). M. C. L. and J. R. G. M. thank the Generalitat structure of the ferromagnetic cluster is maintained in the film. Valenciana for a predoctoral Grant. The electrical properties show a semiconducting behavior with an electrical conductivity at room temperature of ca. 0.1 S cm-1 (Fig. 5). The material constitutes a clear example of the ability References of large polyoxometalate clusters to be incorporated in polymer 1 E.Coronado, J. R. Gala�n Mascaro� s, C. Gime�nez-Saiz and films. It represents the first hybrid film formed by a high spin C. J. Go�mez-Garcý�a in Magnetism: A Supramolecular Function, ed. cluster embedded in a polypyrrole polymer in which the large O. Kahn, NATO ASI Ser. C, Kluwer Academic Publishers, magnetic moments localized on the polyoxometalate coexist Dordrecht, 1996, 484, 281 and references therein. 2 J.M.Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson, U. Geiser, with a delocalized electron framework. The strategy presented H. H. Wang, A. M. Kini and M. H. 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