New Magnetic and Luminescent Dy(III) and Dy(III)/Y(III) Based Tetranuclear Silsesquioxane Cages

We report here the synthesis, structure, luminescence, and magnetic properties of three new cage-like tetranuclear silsesquioxanes (Et4N)2[(PhSiO1..5)8(Y0.75Dy0.25O1.5)4(O)(NO2.5)6(EtOH)2(MeCN)2] and (Cat)2 [(PhSiO1.5)8(DyO1.5)4(O)(NO2.5)6(EtOH)2(MeCN)2] (Cat=Et4N or Ph4P). They present a characteristic greenyellow emission. The Y 3+ /Dy 3+ compound displays in addition a field-induced Single Molecule Magnet (SMM) behavior making it the first reported bifunctional magneto-luminescent silsesquioxane. Lanthanide ions-based coordination compounds have attracted a great deal of attention for several decades due to their tremendous optical and magnetic properties. On one hand, they exhibit narrow emission bands in the visible and Infra-Red spectral regions due to the 4f–4f transitions suitable for different applications in displays, sensors, imagery, luminescent immunoassays and others. 1 On the other hand, the recent discovery of lanthanide-based Single Molecule Magnets (SMMs) exhibiting a slow relaxation of the magnetization associated with magnetic bistability at the molecular scale at temperatures surpassing the temperature of liquid nitrogen renders the lanthanide complexes extremely interesting as memory units for upcoming applications in spintronic. 2 For these reasons, the coordination chemistry of lanthanide ions employed in association with numerous ligands in the aim to design mono and polynuclear compounds has been the subject of a huge development during recent decades. 3 In this line of thought, a particular attention has been done to design of multifunctional magneto-luminescent SMMs because they not only can exhibit a slow relaxation of the magnetization and a characteristic Ln 3+ emission, 4 but, in some cases involving mononuclear Dy 3+ based compounds, the correlation of the energy gap between the ground and excited states determined from the luminescence and the energy barrier obtained by magnetic measurements was established. 5 Surprisingly, the introduction of lanthanides into cage-like metallasilsesquioxanes (CLMSs) has received much less attention. In this connection, only two families of lanthanide-based CLMSs have been reported up to now. 6 The first one consists in cubane-like complexes including Yb 3+ , Nd 3+ , Er 3+ , Ce 3+ , Eu 3+ , Sm 3+ or Pr 3+ ions. 7 The second one is sandwich-like silsesquioxane cages containing a tetranuclear oxo-lanthanide core (including La 3+ , Nd 3+ , Gd 3+ and Dy 3+ ). 8 It should be pointed out that these CLMSs have mainly been designed and investigated regarding their potential catalytic activity, 9 whereas magnetic and optical properties have been considered only scarcely. In this connection, only one study reported the luminescence of a mononuclear Eu 3+ -based CLMS, but its crystal structure has not been elucidated. 10 Very recently, we reported on the first example of Tb 3+ and Eu 3+ based cage silsesquioxanes with a full characterization of their X-ray structures and bi-functional (magnetic/luminescent) properties. 11 Yet, the lack of magnetic anisotropy precludes the observation of a slow relaxation of the magnetization. In the present communication we report on the synthesis, crystal structure and investigation of optical and magnetic properties of three new tetranuclear Dy 3+ and mixed Y 3+ /Dy 3+ based CLMSs. All of them present Dy 3+ characteristic emission, while the mixed Y 3+ /Dy 3+ compound exhibits in addition a field induced slow relaxation of the magnetization making it the first multifunctional SMM CLMS. Moreover, we performed the correlation between the photo-emission and the magnetic properties. The synthesis of anionic lanthanide-based CLMSs has been performed by adapting the previously described 11 procedure consisting in two-step reaction involving firstly the in-situ formation of phenylsiloxanolate [PhSi(O)ONa]x species with their following self-assembling reaction with the Dy 3+ or the Dy 3+ /Y 3+ salts (1/3 stoichiometry) and Et4NCl or Ph4PCl (Figure S1, see Supporting Information for details). The crystallization from an acetonitrile/ethanol mixture affords the formation of single crystals suitable for X-Ray diffraction with chemical formula (Et4N)2[(PhSiO1.5)8(Y0.75Dy0.25O1.5)4(O)(NO2.5)6(EtOH)2(MeCN)2] (1) (Cat)2[(PhSiO1.5)8(DyO1,5)4(O)(NO2.5)6(EtOH)2(MeCN)2] (where Cat=Et4N (2), Cat=Ph4P (3)). X-Ray diffraction analysis indicates that the compounds 1–3 crystallize in the P-1 space group and present very similar crystal structures (CCDC 2062487 (1), CCDC 2002354 (2) and CCDC 2002355 (3)). The relevant parameters for them are summarized in Table S1, Supporting Information. The molecular structure of 1 contains Et4N + cation and an anionic CLMS moiety. The latter may be viewed as a prism-like polyhedron, which looks like a new year paper lantern. It consists of a (Y0.75Dy0.25O1.5)4 core situated in the middle inserted between two cyclic tetraphenylcyclotetrasiloxanolate ligands (Figure 1a). It should be noted that ligands of such particular type is a rare feature for CLMSs, reported earlier for titanium-, 12 iron-, 13 cobalt-, 14 zinc-based 15 silsesquioxanes. The core of 1 has a shape of a distorted square in which the Ln 3+ ions are linked through bridging oxygen atoms (Figure 1b). A statistical distribution of Y/Dy was considered. There are two different Dy/Y sites, being both eight coordinated with a distorted square antiprism geometry. The first one is coordinated by four bridging oxygens and four oxygens from two nitrates, while the second is surrounded by four bridging oxygens, one acetonitrile and one ethanol molecules. The Ln−O distances involving bridging Figure 1: (a) Molecular structure of 1. Hydrogen atoms and crystallized ethanol and acetonitrile molecules have been omitted for clarity. A statistical distribution of Dy and Y was considered. (b) Representation of the core fragment of 1 showing the coordination geometry of Dy/Y sites. (c) Representation of the cyclic tetraphenylcyclotetrasiloxanolate fragment. Color code: purple, Dy/Y; yellow, Si; red, O; blue, N, grey, C. oxygens are in the range 2.2451(1)–2.3340(1) Å, while those involving terminal nitrates and ethanol molecule are in the range 2.3900(2)–2.5784(2) Å. The Ln−O angles in the square are in the range 70.731(3)–82.060(3)°. The Dy/Y−N distances are equal to 2.5358(2) and 2.4933(1) Å. The crystal packing shown in the bc plane (Figure S2, Supporting Information) indicates that the anionic molecular moieties are almost aligned along the b axis and alternated with Et4N + cations. The shortest intermolecular Dy...Dy distance is equal to 7.9580(2) Å. The molecular structures of 2 and 3 are quite similar to 1 (Figure S3–S6, Supporting Information). The relevant distances and angles are summarized in Table S2, Supporting Information. The magnetic properties have been performed by using a SQUID MPMS-XL magnetometer working in 1.8–300 K temperature range up to 7 T in the direct current (dc) and alternating current (ac) modes. The temperature dependence of the magnetic susceptibility performed for 1–3 under an applied magnetic field of 1000 Oe is shown in Figure 2a. The obtained room temperature χT values of 14.2, 56.9 and 53.7 cm K mol for 1, 2 and 3, respectively, correspond well to the values expected for one (14.2 cm K mol) and four (56.7 cm K mol) Dy ions in the free-ion approximation (J=15/2, g=4/3). 16 Upon cooling, the compounds exhibit the classical decrease of χT caused by the thermal depopulation of the Kramers doublets and by the possible presence of Figure 2: (a) Temperature dependence of χT under a 1000 Oe dc magnetic field for 1–3. (b) Field dependence of the magnetization at 1.8 K for 1–3. The solid lines represent theoretical fits with the PHI software. Inset: a schematic representation of the considered magnetic exchange interactions between Dy 3+ ions in 2 and 3. antiferromagnetic interactions between the Dy 3+ centers for 2 and 3 (Figure 2a). The field dependences of the magnetization performed at 1.8 K for 1–3 show a S-shape curve for 2 and 3 suggesting the presence of a spin flip of magnetic moment under an applied magnetic field related to antiferromagnetic interaction, 17 while a linear increase of the magnetization at low fields is observed for 1. The three curves never reach saturation and the magnetization values of 6.3, 24.6 and 21.9 Nβ under 5 Tesla field for 1, 2 and 3, respectively, indicate the presence of a significant magnetic anisotropy (see Figure 2b). In the case of 1 containing a statistically distributed dysprosium ion in the tetranuclear core, χT vs T and M vs H curves were fitted with the PHI software by using the Hamiltonian taking into account the Zeeman and the Stevens operator order terms and with mJ=15/2 and g=4/3 for Dy 3+ in 1. 18 For CLMSs 2 and 3 containing four Dy 3+ ions, their contribution was approximated as 4 pseudospins ( ̃ ), associated with a diagonal g-tensor with the diagonal components gx, gy and gz, allowing to take into account the magnetic anisotropy. Magnetic interactions (J) between the four pseudospins were also included in the model (see insert of Figure 2b). The obtained parameters (Tables S3, Supporting Information) confirm the expected important magnetic anisotropy for Dy in these compounds and the presence of antiferromagnetic interactions for 2 and 3, which may explain the observed S-shape M vs H curves. The dynamic measurements in the ac mode have been performed for three CLMSs, but only sample 1 demonstrated a slow relaxation of the magnetization. This fact may be explained by the presence of magnetic interactions between Dy 3+ , which negatively affect the relaxation dynamics. 19 For CLMS 1, the in-phase, χ’, and the out-of-phase, χ’’, components of the ac susceptibility did not present an important signal under a zero-dc field, which may be ascribed to the presence of the fast Quantum Tunneling of the Magnetization (QTM). In order to avoid its influence, the frequency dependence of the ac