Polyoxometalate salts of cationic nitronyl nitroxide free radicals

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Solid State Sciences 10 (2008) 1794e1799 www.elsevier.com/locate/ssscie

Polyoxometalate salts of cationic nitronyl nitroxide free radicals Eugenio Coronado, Carlos Gime´nez-Saiz1, Carlos J. Go´mez-Garcı´a, Francisco M. Romero*,1 Institut de Cie`ncia Molecular, Universitat de Vale`ncia, Polı´gon La Coma s/n, E-46980 Paterna, Spain Received 6 November 2007; received in revised form 25 January 2008; accepted 28 January 2008 Available online 6 February 2008

Abstract The cationic nitronyl nitroxide free radical of the N-methylpyridinium type p-MepyNNþ has been combined with [Mo8O26]4 and Keggin [SiW12O40]4 polyanions to afford salts ( p-MepyNN)4[Mo8O26]$DMSO (DMSO ¼ dimethylsulfoxide) (1) and ( p-MepyNN)4[SiW12O40]$6DMF (DMF ¼ dimethylformamide) (2). Herein, their structural and magnetic properties are described. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Nitroxide radicals; Polyoxometalates; Magnetism

1. Introduction Purely organic crystalline materials showing ferromagnetism have attracted the interest of synthetic chemists for a long time [1]. These materials consist in a crystalline arrangement of open-shell molecules that propagate magnetic interactions in such a way that bulk ferromagnetism becomes possible at very low temperatures. Among the readily available paramagnetic molecules, the family of nitronyl nitroxide (NN) free radicals has been thoroughly studied due to its persistence and ease of functionalization [2]. Also, single crystals can be grown by simple techniques and the molecules can be organized in the solid state using the concepts of crystal engineering [3]. For instance, hydrogen bonding and other types of non-covalent interactions have been used to create specific crystal packings by design [4]. The challenge is now to control the sign and strength of magnetic interactions between neighboring molecules, a problem that is also being addressed from a theoretical point of view. For many years, the main guideline for the design of magnetic interactions in organic ferromagnets came from McConnell first proposal, establishing that ferromagnetic coupling arises from intermolecular interactions * Corresponding author. Tel.: þ34 96 354 44 05; fax: þ34 96 354 32 73. E-mail address: [email protected] (F.M. Romero). 1 Fundacio´ General de la Universitat de Vale`ncia (FGUV). 1293-2558/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2008.01.031

between atoms carrying spin densities of opposite signs [5]. The model was convenient for experimentalists because spin densities of solid materials can be mapped efficiently by NMR or polarized neutron diffraction methods [6,7]. However, it has been shown that McConnell I model fails to predict the magnetic behavior of numerous crystalline assemblies of NN radicals [8]. Theoretical investigations are now oriented to the calculation of the exchange interaction for pairs of rad˚ for the intericals by ab initio methods. An upper limit of 7 A molecular O/O distance inside a pair is generally considered. Cluster Heisenberg models comprising all the interactions within this limit are proposed and the corresponding Hamiltonian is then solved with the calculated exchange coupling parameters. Once the thermodynamic properties are obtained, they can be compared with the experimental values to check the validity of the model [9]. Still, the presence of many different magnetic interaction pathways hinders the establishment of simple rules to predict the magnetic behavior of these solids from an inspection of their crystal packing. Only in a few cases a simple magnetostructural correlation could be derived, for instance, by comparing isostructural radical-based solids that show different magnetic properties [10]. We have recently explored the possibility of restricting magnetic interactions between the spin carriers via the synthesis of crystalline salts that combine large anions, like the Lindqvist polyoxometalate (POM) [W6O19]2, with nitronyl nitroxide

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free radicals derived from N-alkylpyridinium or tetraalkylammonium cations [11e13]. This strategy affords simpler exchange interaction patterns that could, in principle, be predicted and compared with the results of theoretical calculations. In this paper, we extend our analysis to other POM salts based on [Mo8O26]4 and [SiW12O40]4 anions. 2. Experimental 2.1. Synthesis and characterization p-MepyNNClO4, (TBA)4[Mo8O26] and (TBA)4[SiW12O40] (TBA ¼ tetrabutylammonium) were prepared by previously reported procedures [14,15]. All other chemicals were used as received unless otherwise stated. Infrared spectra were recorded at room temperature on a Thermo Nicolet Nexus FTIR spectrophotometer in the 4000e400 cm1 range. CHN elemental analyses of freshly precipitated compounds were carried out in a CE instruments EA 1110 CHNS analyzer. Variable temperature susceptibility measurements were carried out on polycrystalline samples in the 2e300 K temperature range at magnetic fields of 0.1 T (Quantum Design MPMSXL-5 magnetometer equipped with a SQUID sensor). Diamagnetic contributions were corrected with Pascal constants. 2.1.1. Synthesis of 1-methyl-4-(1-oxyl-3-oxide-4,4,5,5-tetra methylimidazolin-2-yl)pyridinium octamolybdate(VI) (1) p-MepyNNClO4 (53 mg, 0.15 mmol) was dissolved in acetonitrile (10 mL). (TBA)4[Mo8O26] (82 mg, 0.038 mmol) was dissolved in 10 mL CH3CN and added to the radical solution. The mixture was stirred and the green product that precipitates was collected by centrifugation and washed with diethyl ether (65 mg, 78% yield). Green prismatic single crystals suitable for X-ray studies were obtained by evaporation of a DMSO solution. IR (KBr pellets, cm1): 2964, 2936, 2876, 1469, 1372 (NeO), 922, 655. Elem. Anal. for (C13H19N3O2)4 [Mo8O26] (Mr: 2180.76) Calcd: C, 28.64; H, 3.51; N, 7.71; found: C, 28.95; H, 3.63; N, 7.53. 2.1.2. Synthesis of 1-methyl-4-(1-oxyl-3-oxide-4,4,5,5-tetra methylimidazolin-2-yl)pyridinium dodecatungstosilicate (2) p-MepyNNClO4 (35 mg, 0.1 mmol) was dissolved in acetonitrile (10 mL). (TBA)4[SiW12O40] (96 mg, 0.025 mmol) was dissolved in 10 mL CH3CN and added to the radical solution. The mixture was stirred and the green product that precipitates was collected by centrifugation and washed with diethyl ether (73 mg, 75% yield). Green needle-like single crystals suitable for X-ray studies were obtained by evaporation of a DMF solution. IR (KBr pellets, cm1): 2962, 2928, 2874, 1455, 1374 (NeO), 982, 867. Elem. Anal. for (C13H19N3O2)4 [SiW12O40] (Mr: 3871.51) Calcd: C, 16.13; H, 1.98; N, 4.34; found: C, 16.49; H, 2.21; N, 4.02. 2.2. Crystal structure determination Crystallographic data were collected on Nonius Kappa CCD (for 1) and Bruker Smart CCD (for 2) diffractometers

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equipped with a graphite monochromated Mo Ka radiation source. Data collection was performed at room temperature. Intensities were corrected for Lorentzian and polarization effects. Empirical absorption corrections were applied by using the SADABS program based on the Laue symmetry of the reciprocal space. The structure was solved by direct methods (SIR97) [16] and refined against F2 with a full-matrix least-squares algorithm using SHELX-97 [17] and the WinGX (1.64) software package [18]. All non-hydrogen atoms were refined anisotropically. The positions of the hydrogen atoms bonded to carbon atoms were included in calculated positions and treated as riding atoms. Crystal data for compounds 1 and 2 are gathered in Table 1. The occupancy factor of the dimethylsulfoxide molecule of 1 was refined adopting a value close to 0.5 and, in the next refinements, it was fixed to this value. In 2, the polyanion displays the well-known Keggin structure containing a Si(IV) center in its central tetrahedral cavity. However, due to disorder, the Si(IV) unit appears in a centrosymmetric cubic environment surrounded by eight ˚ with half occupancy. This disorder O atoms at 1.57e1.68 A causes high U(eq) values of the O atoms which are linking W atoms (WeOeW) and also a clear elongation of their thermal ellipsoids in the parallel direction to the molecular threefold symmetry axes [19]. The high degree of thermal motion observed for the dimethylformamide molecules is attributed to librational disorder. The slightly short bond lengths in these molecules are an expected consequence of libration. CCDC 665726 and CCDC 665727 contain the supplementary crystallographic data for 1 and 2, respectively. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax: þ44 1223 336033; e-mail: [email protected]). Table 1 Summary of crystallographic data for 1 and 2

Empirical formula Formula weight Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a (deg) b (deg) g (deg) ˚ 3) V (A Z Dcalcd (Mg m3) m (mm1) T (K) ˚) l (A Data/restraints/parameters Final R indices [I > 2s(I )] R indices (all data)

1

2

C54H82Mo8N12O35S 2258.90 Triclinic P-1 10.8630(3) 12.1660(4) 15.3960(6) 77.550(5) 89.308(5) 79.482(5) 1952.70(11) 1 1.921 1.361 293(2) 0.71073 8771/24/527 R1 ¼ 0.0585 wR2 ¼ 0.1510 R1 ¼ 0.1375 wR2 ¼ 0.1927

C70H118N18O54SiW12 4310.11 Triclinic P-1 12.206(2) 14.864(3) 15.494(3) 86.538(5) 84.812(5) 85.180(5) 2785.7(9) 1 2.569 12.438 298(2) 0.71069 15,763/54/734 R1 ¼ 0.0567 wR2 ¼ 0.1160 R1 ¼ 0.1449 wR2 ¼ 0.1444

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3. Results and discussion The family of cationic NN free radicals of the N-alkylpyridinium type is well described [20e23]. Their iodide salts can be easily prepared by N-alkylation of the corresponding pyridine NN derivative with methyl iodide. Salts with different anions, like perchlorate, can be prepared by metathesis. Also, the anionic counterpart can be an extended magnetically-ordered network [24e26]. In this context, we have reported on salts combining p-MepyNN cations (Scheme 1) and bimetallic oxalato-bridged lattices [27,28].

O

N

N

O

N p-MepyNN 4 p-MepyNNClO4

+

TBA4[Mo8O26]

(p-MepyNN)4[Mo8O26] · DMSO (1)

4 p-MepyNNClO4

+

TBA4[SiW12O40]

(p-MepyNN)4[SiW12O40] · 6 DMF (2)

Scheme 1.

It has previously been shown that metathesis is also a convenient method for the synthesis of POM salts of cationic free radicals [11e13]. The same strategy has been applied here. Thus, addition of TBA4[Mo8O26] (TBA ¼ tetrabutylammonium) to

a CH3CN solution of perchlorate salt p-MepyNNClO4 (Scheme 1) affords a green precipitate of the cationic free radical as its [Mo8O26]4 salt. Single crystals of 1 were then obtained from a DMSO solution. Compound 2 was obtained by the same method using DMF as solvent of crystallization. Compound 1 crystallizes in the P1 space group. The [bMo8O26]4 anions lie on a center of symmetry and there are two independent radicals per asymmetric unit. The structure can be considered as formed by two types of two-dimensional motives: a polar region that contains the polyoxoanions (Fig. 1) and the pyridinium cations held together by electrostatic interactions, and a less polar region formed by the imidazolyl fragments packed together by van der Waals interactions. The layered motives alternate in the crystal along the y direction. This means that the relevant intermolecular contacts between nitroxide radicals are extended in two dimensions. In contrast with previously reported structures [11], pep stacking interactions between the pyridinium subunits are not observed. The most relevant contact between nitroxide moieties ˚) within each layer (O16/O16 (x, 1  y, z) ¼ 3.052(12) A forms pairs of radicals around an inversion center. Another ˚ (O15/O17 strong interaction at distances lower than 4 A ˚ (1 þ x, y, z) ¼ 3.703(10) A) links the dimer along the x direction in a tetrameric unit (Fig. 2). Two additional interactions ˚ range. The shortest one (O14/ appear in the 4e5 A ˚ ) connects the tetramers in ladder chains O17 ¼ 4.371(10) A running along the [100] direction. The chains are finally linked together in the plane by weaker interactions (O14/O14 ˚ ). Several additional in(1  x, 1  y, 1  z) ¼ 4.892(14) A ˚. plane contacts between NeO groups were found above 6 A

Fig. 1. View of the crystal structure of 1 along the a axis. Solvent molecules are omitted for clarity.

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Fig. 2. View of the crystal structure of 1 along the y direction showing the radical connectivity. The tetramers are shown by red rods. Dashed lines refer to intertetramer contacts.

They correspond mainly to next-nearest-neighbors’ interactions and were not taken into account in the description of the magnetic properties of the compound. The shortest distance between nitroxide functionalities belonging to different ˚. layers is 5.835 A Magnetic susceptibility measurements of 1 have been measured in the 2e300 K temperature range (Fig. 3). At 300 K,

1.6

T (emu.K/mol)

1.4

1.2

1

0.8

0.6

0

50

100

150

200

250

300

T (K) Fig. 3. Temperature dependence of cT for 1. The solid line corresponds to the best-fit data using a tetramer model.

the product of molar magnetic susceptibility times temperature (cT ) equals 1.41 emu K mol1, slightly lower than the expected value for four independent S ¼ 1/2 spins per formula unit (1.5 emu K mol1), indicating that some radicals (circa 7%) are in a reduced form. The cT value remains constant on decreasing the temperature down to 50 K. Further cooling leads to a fast decrease of cT to a value of 0.62 emu K mol1 at 2.0 K. This behavior is characteristic of dominant antiferromagnetic interactions. In a first approach, only the two shortest interactions (O16/O16, denoted as J1, and O15/O17, denoted as J2) were considered in the analysis of the magnetic properties. A tetrameric model results (see Fig. 2) that can be described by the following exchange Hamiltonian: H ¼ 2J1 S2 S3  2J2 ðS1 S2 þ S3 S4 Þ, with S1 ¼ S2¼ S3¼ S4¼ 1/2. The total Hamiltonian was solved for different values of the exchange parameters using the MAGPACK code [29]. Simulations where J2 > J1 reproduced the experimental data better. This is not surprising since the OeNeCeNeO planes (where most of the spin density is found) of molecules 2 and 3 are parallel while the corresponding planes of molecules 1 and 2 intersect with a dihedral angle of 125 . Thus, J2 could be stronger than J1, even if the distance between adjacent radicals is somewhat higher. The best simulation yielded the following values of the exchange coupling constants: J1 ¼ 0.5 K, J2 ¼ 1.5 K. Compound 2 crystallizes also in the P1 space group with the Keggin polyanions lying on an inversion center. This location is frequently observed in centrosymmetric structures and leads

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Fig. 4. View of the crystal structure of 2 along the a axis. Solvent molecules are omitted for clarity.

necessarily to disorder of the central tetrahedral SiO4 unit. Consequently, the Si atom appears in a cubic arrangement surrounded by eight oxygen atoms with half occupancy. The lower charge density of [SiW12O40]4 as compared to [Mo8O26]4 anions affords a less compact structure in 2. This is translated in an increase of the number of solvent molecules (6 DMF molecules versus 1 DMSO molecule in 1) and, hence, in longer distances between radical centers. As in 1, there are two independent cationic free radicals per asymmetric unit. One of the non-equivalent molecules packs itself as dimers (Fig. 4) in a head-to-tail arrangement driven by electrostatic interactions between the nitroxide function and the pyridinium ˚ ). This brings unit (O25/N4 (1  x, 1  y, z) ¼ 3.345(15) A the nitroxide moieties to a relatively close distance (O25/ ˚ ). The second molecule O25 (1  x, 1  y, z) ¼ 5.48(2) A packs itself in a similar way as that observed in 1, with the pyridinium cations pointing towards the polyoxometalates and the imidazolyl fragments interacting strongly. The two independent molecules are linked by short contacts between ˚ ). This is the the nitroxide radicals (O23/O26 ¼ 3.521(17) A

˚ only intermolecular Orad/Orad contact shorter than 5 A present in the structure and organizes dimers (termed A in ˚ Fig. 5). Other relevant contacts below the upper limit of 7 A ˚, are the following: O24/O24 (2  x, y, 1  z) ¼ 5.00(3) A and the already discussed O25/O25 (B and C interactions in Fig. 5, respectively). The three interactions alternate in the structure to form chains with ABAC topology that run parallel to the ½111 direction. The shortest interchain distance between nitroxide units is O23/O25 (1  x, 1  y, 1  z) ¼ ˚. 7.419(17) A The cT product of compound 2 (Fig. 6a) equals 1.49 emu K mol1 at 300 K, close to the calculated value for four non-interacting S ¼ 1/2 spins per formula unit. Upon cooling to 50 K, a constant cT value is observed. Below this temperature cT decreases sharply on further cooling and a value of 0.64 emu K mol1 is observed at 2.0 K. This behavior is also characteristic of dominant antiferromagnetic interactions. Again, in a first approach, a dimer model that takes into account the strongest A interaction can be discussed [30]. This model affords a coupling constant JA ¼ 1.8 K.

Fig. 5. View of the chains of radicals in 2 along its y direction. Dimeric units are labeled and denoted by red rods.

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1.6

T (emu.K/mol)

1.4

1.2

1

0.8

0.6 0

50

100

150

200

250

300

T (K) Fig. 6. Temperature dependence of cT for 2. The solid line corresponds to the best-fit data to the BleaneyeBowers equation with J ¼ 1.8 K.

4. Conclusion The study of crystalline salts that combine cationic free radicals and polyoxometalate anions is interesting for several reasons. First, a palette of compounds in which the same organic free radical can be found in different crystal arrangements can be easily obtained. Also, by tuning the charge and size of the polyoxoanion, it is possible to control the distance between the organic spin carriers. This is important in the establishment of magnetostructural correlations that can help to understand the magnetic behavior of these exciting materials. Acknowledgments This work was financially supported by the Spanish Ministerio de Educacio´n y Ciencia (MAT2004-3849) and Generalitat Valenciana. References [1] For an overview of purely organic magnetism see: The Proceedings of the 8th International Conference on Molecule-based Magnets Polyhedron 14e17 (2003) 1725. [2] E.F. Ullman, J.H. Osiecki, D.G.B. Boocock, J. Am. Chem. Soc. 94 (1972) 7049.

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