Synthesis, crystal structure, magnetic and

Inorganica Chimica Acta 360 (2007) 3047–3054 www.elsevier.com/locate/ica

Synthesis, crystal structure, magnetic and luminescence investigations of new 2Ln3+–Sr2+ heteronuclear polymers with 2-furoic acid Constantin Turta a,*, Silvia Melnic a, Marco Bettinelli d, Sergiu Shova c, Cristiano Benelli b, Adolfo Speghini d, Andrea Caneschi b, Maria Gdaniec e, Yurii Simonov c, Denis Prodius a, Valeriu Mereacre a a Institute of Chemistry of ASM, Academiei Street 3, Chisinau, Republic of Moldova Department of Chemistry and INSTM Research Unit, University of Florence, Florence, Italy c Institute of Applied Physics of ASM, Academiei Street 1, Chisinau, Republic of Moldova Dipartimento Scientifico e Tecnologico, Universita` di Verona, and INSTM, UdR Verona, Ca’ Vignal, Strada Le Grazie 15, I-37134 Verona, Italy e Faculty of Chemistry, A. Mickiewicz University, Poznan´, Poland b

d

Received 31 October 2006; received in revised form 13 February 2007; accepted 24 February 2007 Available online 13 March 2007

Abstract New first examples of complexes with the general formula {[Ln2Sr(C4H3OCOO)8(H2O)4]}n, where Ln = La3+ (1), Pr3+ (2), Nd3+ (3), Sm3+ (4), Eu3+ (5), Gd3+ (6), Tb3+ (7), Ho3+ (8), Yb3+ (9) and Er3+ (10) have been prepared and investigated by photoluminescence spectroscopy and magnetic susceptibility measurements. The X-ray crystal structure has been determined for the {[Er2Sr(C4H3OCOO)8(H2O)4]}n (10) complex, indicating that this complex is built from two crystallographic independent coordination polymers {[Er2Sr(C4H3OCOO)8(H2O)4]}n in the triclinic crystal system and P1 space group. The X-ray diffraction (XRD) pattern of the samples shows that all lanthanide compounds are isostructural to 10. The luminescence spectrum of a microcrystalline sample of ‘‘2Eu–Sr’’ compound displays the characteristic Eu3+ (5D0 ! 7FJ (J = 0–4)) metal centred transitions; also ‘‘2Nd–Sr’’ proved to be luminescent in the near IR. Measurements of the magnetic susceptibility for 2, 3, 5 and 10 were described using Crystal Field approach. 2007 Elsevier B.V. All rights reserved. Keywords: Lanthanide–strontium coordination polymer; Furoic acid; Luminescence spectra; Magnetic properties

1. Introduction The 2-furane-carboxyl acid (FCA) has shown to be a very efficient complexing agent. Recently Bismondo et al. [1] investigated the thorium(IV) complexes containing this ligand. An interesting characteristic of the FCA and their derivate ligands is the presence of three possible sites of coordination (two carboxylic oxygen atoms and one heterocyclic oxygen), which work in all ‘‘spectrums’’ of their potential of coordination: as monodentate [2], bidentate *

Corresponding author. Tel.: +373 22 739955; fax: +373 22 739954. E-mail addresses: [email protected], [email protected] (C. Turta), [email protected] (M. Bettinelli), [email protected] (S. Shova), cristiano.benelli@unifi.it (C. Benelli). 0020-1693/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2007.02.045

[3], tridentate [4,5], tetradentate [3,5] and finally pentadentate [6] ligands. Other aspects of the use of FCA and its derivatives were intensively studied in many bacteria strains systems [7], oxidation processes [8], as antitumour and microbial activity compounds [9] and in polymer chemistry [10]. The study of the coordination compounds of the trivalent lanthanide ions (Ln3+) continues to be an active research area, which may be attributed to the luminescent properties of these compounds and their applications as optical and electroluminescent devices and luminescent probes in biological systems [11–13]. Some organic ligands in coordination compounds can act as an ‘‘antenna’’, absorbing and transferring energy efficiently to the rare earth ion and consequently increasing the luminescence intensity [14]. Luminescent Ln3+-complexes con-

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taining the carboxylate anion as ligands have been extensively reported [15–20]. The special attention given to the Eu3+ ion (4f 6) lies in the fact that its compounds generally present intense luminescence in the red region (615 nm), due to the large energy gap between the 5D0 emitting level and the lower lying states; this minimizes the non-radiative relaxation processes. Besides, from the emission and excitation spectra of the compounds containing europium ions useful information about ligand field splitting of the 4f 6 levels can be obtained. This is caused by the fact that the main emitting level (5D0) is non-degenerate, therefore is not split in any site symmetry [21]. On the other hand, the complexes containing lanthanide metals are of great interest due to their unique physico-chemical properties, especially the presence of the antiferromagnetic exchange interactions in clusters [22,23]. Here we present synthesis, crystalline structure, photoluminescence and magnetic properties of the new heterotrinuclear complexes of strontium and lanthanides with the 2-furoic acid ligand. 2. Experimental 2.1. General considerations 2-Furoic acid (Aldrich, >98%) was purified by recrystallization from water/methanol [24]. Starting materials were Sr(C4H3OCOO)2 Æ 4H2O and Ln(ClO4)3 Æ 6H2O. Sr(C4H3OCOO)2 Æ 4H2O was synthesized by reaction between SrCO3 and 2-furoic acid. Ln(ClO4)3 Æ 6H2O (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Yb and Er) was prepared by dissolving the respective lanthanide oxides (99.9% pure) in ca. 50% HClO4 and then recrystallizing the resulting salt by evaporating the aqueous solution on a steam bath. Caution: Perchlorate salts are potentially explosive and were handled with great care. The carbon and hydrogen content of complex was determined by standard micro-methods in the group of microanalysis of the Institute of Chemistry and strontium determination was carried out using the Atomic Absorption Spectroscopy (Spectrophotometer AAS-3N Karl Zeiss Jena DDR) in the Automatic and Metrology Centre of the Academy of Sciences of Moldova. IR spectrum of polycrystalline sample was recorded as oil mulls on a Specord M-75 spectrophotometer. 2.2. Preparations 2.2.1. [La2Sr(C4H3OCOO)8(H2O)3,5] Æ 2H2O (1) Solutions of Sr(C4H3OCOO)2 Æ 4H2O (0.20 g, 0.52 mmol) in 10 mL of water and a La(ClO4)3 Æ 6H2O (0.30 g, 0.52 mmol) in 15 mL of ethanol were stirred until formation of white gel-mass. After 10 days a colourless microcrystalline product was filtered off, quickly washed by water and air dried. Yield: 23% (on La basis). Anal. Calc. for C40H35O29.5La2Sr: C, 35.51; H, 2.61; Sr, 6.47. Found: C, 35.22; H, 2.40; Sr, 6.19%. IR (cm1): 3500b, 1625m,

1580vs, 1230m, 1200s, 1140w, 1070s, 1010s, 940m, 880m, 820w, 790, 785, 765s, 720w, 615w, 595m, 470s. The IR spectra of other complexes presented below have the same characteristic bands. 2.2.2. [Pr2Sr(C4H3OCOO)8(H2O)3,5] Æ 2H2O (2) This complex and all other complexes of this publication were obtained by the method similar to that for 1. The crystals of 2 have a pale green colour. Yield: 38% (on Pr basis). Anal. Calc. for C40H35O29.5Pr2Sr: C, 35.40; H, 2.60; Sr, 6.46. Found: C, 35.72; H, 2.39; Sr, 6.52%. 2.2.3. [Nd2Sr(C4H3OCOO)8(H2O)3,5] Æ 2H2O (3) This complex was obtained as lilac crystals. Good single crystals for X-ray study were collected from the aqueous solution after 10 days. Yield: 33% (on Nd basis). Anal. Calc. for C40H35O29.5Nd2Sr: C, 35.23; H, 2.59; Sr, 6.42. Found: C, 35.47; H, 2.33; Sr, 6.34%. 2.2.4. [Sm2Sr(C4H3OCOO)8(H2O)3,5] Æ 2H2O (4) This complex was obtained as pale yellow crystals. Good crystals for X-ray study were collected from the aqueous solution after 12 days. Yield: 42 % (on Sm basis). Anal. Calc. for C40H35O29.5Sm2Sr: C, 34.91; H, 2.56; Sr, 6.37. Found: C, 35.26; H, 2.28; Sr, 6.20%. 2.2.5. [Eu2Sr(C4H3OCOO)8(H2O)3,5] Æ 2H2O (5) This complex was obtained as colourless crystals. Good crystals for X-ray study were collected from the aqueous solution after 1 week. Yield: 57% (on Eu basis). Anal. Calc. for C40H35O29.5Eu2Sr: C, 34.83; H, 2.56; Sr, 6.35. Found: C, 34.63; H, 2.21; Sr, 6.31%. 2.2.6. [Gd2Sr(C4H3OCOO)8(H2O)3,5] Æ 2H2O (6) This complex was obtained as colourless crystals. Yield: 61% (on Gd basis). Anal. Calc. for C40H35O29.5Gd2Sr: C, 34.59; H, 2.54; Sr, 6.30. Found: C, 34.67; H, 2.51; Sr, 6.10%. 2.2.7. [Tb2Sr(C4H3OCOO)8(H2O)3,5] Æ 2H2O (7) This complex was obtained as colourless crystals. Yield: 32% (on Tb basis). Anal. Calc. for C40H35O29.5Tb2Sr: C, 34.48; H, 2.53; Sr, 6.29. Found: C, 34.35; H, 2.48; Sr, 6.13%. 2.2.8. [Ho2Sr(C4H3OCOO)8(H2O)3,5] Æ 2H2O (8) This complex was obtained as pale yellow crystals. Good crystals for X-ray study were collected from the aqueous solution after 1 week. Yield: 47% (on Ho basis). Anal. Calc. for C40H35O29.5Ho2Sr: C, 34.19; H, 2.51; Sr, 6.23. Found: C, 34.45; H, 2.59; Sr, 6.12%. 2.2.9. [Yb2Sr(C4H3OCOO)8(H2O)3,5] Æ 2H2O (9) This complex was obtained as white crystals. Yield: 17% (on Yb basis). Anal. Calc. for C40H35O29.5Yb2Sr: C, 33.80; H, 2.48; Sr, 6.16. Found: C, 34.02; H, 2.31; Sr, 6.09%.


2.2.10. [Er2Sr(C4H3OCOO)8(H2O)3,5] Æ 2H2O (10) This complex was obtained as light pink crystals. Good single crystals for X-ray study were collected by recrystallization of product from the aqueous solution after 1 week. Yield: 35% (on Er basis). Anal. Calc. for C40H35O29.5Er2Sr: C, 34.08; H, 2.50; Sr, 6.21. Found: C, 33.95; H, 2.59; Sr, 6.02%. 2.3. Physical measurements Magnetic susceptibilities of the powdered samples were measured in the temperature range 1.8–300 K using a Quantum Design Squid magnetometer, equipped with a helium continuous cryostat. The experimental susceptibilities were corrected for the diamagnetism of the constituent atoms (Pascal tables [25]). 2.4. Luminescence spectra The 488.0 nm line of a Spectra-Physics Stabilite 2017 Argon Laser was used to excite the luminescence spectra. The signal was dispersed with a 0.46 m monochromator with a 150 or 1200 lines/mm grating and detected with an air cooled CCD device. The luminescence decay curves were measured exciting with the second harmonic (532 nm) or third harmonic (355 nm) of a pulsed Nd– YAG laser. A fiber optic probe was employed to collect the emission. The signal was analyzed by means of a monochromator mentioned above. A GaAs photomultiplier and a digital oscilloscope were used to measure the decay curves. The decay times were obtained from the experimental curves using a deconvolution procedure, which takes into account the shape and the duration of the excitation pulse. All the spectroscopic measurements were performed at room temperature.

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Table 1 Summary of crystal data and refinement details for [Er2Sr(C4H3OCOO)8(H2O)3,5] Æ 2H2O (10) Empirical formula M ˚) Wavelength (A T (K) Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) V (A Z q(calc) (Mg/m3) lMo (mm1) Crystal size (mm) h Range () Number of reflections: Measured Unique [Rint] Number of refined parameters Goodness-of-fit for F2 Ra wRb ˚ 3) Dqmax and Dqmin (e A P P a R ¼ kF o j jF c k= jF o j: P P b 2 wR ¼ ½ wðjF o j jF 2c jÞ2 = wjF 2o j2 1=2 .

C40H35Er2SrO29.5 1410.36 0.71073 130 triclinic P 1 10.364(2) 12.702(3) 18.248(4) 79.88(3) 79.67(3) 77.86(3) 2286.5(9) 2 2.048 4.902 0.40 · 0.30 · 0.20 2.37–29.23 10,391 8024 [0.0377] 639 1.012 0.0703 0.1735 3.213 and 4.039

tion in a riding model with isotropic temperature factor fixed at 1.2 · Ueq of the relevant carbon atom. Positional parameters of H-atoms of the water molecules were verified by the geometric parameters of the corresponding hydrogen bonds. Crystallographic data and refinement details are summarised in Table 1. 3. Results and discussion

2.5. Crystal structure determination 3.1. Structural analysis X-ray crystallographic measurements for 10 were performed at 130 K using a KUMA-4CCD diffractometer, fitted with an Mo Ka radiation source and a graphite monochromator (k = 0.71073). The crystal was placed 60 mm from the CCD detector chamber. More than a hemisphere of reciprocal space was covered by combination of three sets of exposures; each set had a different u-angle (0, 90, 270) and exposure of 30 s covered 0.75 in x. The unit cell determination and data integration were carried out using the CrysAlis package of Oxford Diffraction [26]. Intensity data were corrected for the Lorentz and polarization effects. The absorption correction was introduced by a semi-empirical method from symmetrically equivalent reflections [27]. Maximum and minimum transmission factors were 0.457 and 0.312, respectively. The structure was solved by direct methods [28] and refined by full-matrix least-squares on F2 with anisotropic displacement parameters for non-H atoms [29]. The hydrogen atoms attached to carbon were included in idealized posi-

Compound 10 crystallizes in centrosymmetric primitive group P 1 where the two crystallographic independent Sr atoms reside on the inversion centres (positions 1a and 1h) [30], while Er atoms occupy two general positions [31]. In the crystal the metal ions and coordinated water molecules are linked into two independent, 1D coordination polymers, running along a direction in the crystal. Perspective view of one of the chain is depicted in Fig. 1. The role of bridging ligands is fulfilled by 2-furoic anions, which exhibit a large diversity of structural function, as it is shown in Scheme 1. The asymmetric units for two independent chains along with the numbering scheme are shown in Fig. 2a and b, while the main interatomic distances are summarised in Table 2. In the chains (1 or 2) the metal ions alternate in the order: Er Er Sr Er Er with the Er1 Er1 0 ˚ , Er1 Sr1 3.980(2) A ˚ and Er2 Er2 0 4.280(2) 4.443(2) A ˚ ˚ A, Er2 Sr2 4.190 (2) A separations. It is noteworthy, that

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Fig. 1. The fragment of coordination polymer in [Er2Sr(C4H3OCOO)8(H2O)3,5] Æ 2H2O (10).

Er

Er

O

O

O

Er

Er

Er

O

O

O Sr

Sr O

O

O

1c

1b

1a Er

Sr

O

O

Sr

Er O

O

O

1d

O

1e

Scheme 1. The coordination functions of the 2-furoic acid in structure of 10.

one of the residues of the 2-furoic acid, from the polymeric chain denoted by Er2 and Sr2 atoms, was found to be disordered over two resolvable positions with equal probability. Indeed, it lies in different orientations as well as structural functions which are fulfilled by the respective carboxylate ligand, as shown in Fig. 2b. Actually, it results into two independent polymeric chains in the crystal due to the different mode of joining of the metal ions via bridging systems. Although 1a–1c types (Scheme 1) of coordination for three ligands are maintained, the essential differences are observed for the disordered carboxylate ligand defined by O23 and O24 atoms (O23* and O24*), respectively. Its coordination modes are represented by Scheme 1d and e. The coordination number of Er1 atom is equal to 8, the Er1–O bond distances being in the range between 2.29(1) ˚ . In the second chain, different coordination and 2.49(1) A

polyhedra of Er2 with coordination number (CN) equal ˚ ) and 9 (the same to 8 (Er2–O in the range 2.27(1)–2.51(1) A range of bond distances) alternate with the equal probabilities. The increase of the Er2 CN results in the decrease of CN of Sr2. Both atoms Sr1 and Sr2 reside on inversion centres. In the first polymeric chain the CN of Sr1 is equal to 12 (Table 2), in the second polymeric chain for two positions of disordering the CN of Sr2 is equal to 10. The diminishing of the Sr2 CN results in the decrease of Sr2– ˚ (Table 2). O23 distance up to 2.28(1) A It is important to note that while the coordination environment of the rare-earth metal is completed by the oxygen atoms of exclusively carboxyl groups and water molecules, then the coordination core of the Sr atoms (CN 12, Table 2) is formed by the carboxylic groups’ oxygen atoms as well as by the oxygen atom of the heterocycle (see Scheme 1). Coordinated and solvated water molecules via O–H O hydrogen bonds additionally consolidate the structural units both inside the polymeric chains and between them. These hydrogen bonds are responsible for the formation of the 3D structure. Water molecule O5w is characterized by the partial occupancy (0.5) that is dictated by the steric constrains connected with the orientation of the disordered fragments of 2-furoic acid. 3.2. Luminescence spectra 3.2.1. Europium The room temperature laser-excited luminescence spectrum of powders of {[Eu2Sr(C4H3OCOO)8(H2O)4]}n measured with excitation wavelength 488.0 nm, consists of f ! f emission transitions from the 5D0 excited state to the 7FJ (J = 0–4) multiplets of the Eu3+ ion (see Fig. 3). The 5D0 ! 7F0 transition cannot be split by the crystal field and its profile gives information on the number of different coordination sites accommodating the Eu3+ ion. The presence of a single 5D0 ! 7F0 band, characterized by a


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Table 2 ˚ ) in the coordination polyhedra of Er and Sr Interatomic distances (A atoms for 1 2.799(10) 2.767(10) 2.899(10) 2.604(7) 2.841(8) 2.695(8) 2.35(1) 2.44(1) 2.43(1) 2.31(1) 2.37(1) 2.32(1) 2.49(1) 2.29(1) 2.937(13) 2.559(7) 2.732(11) 2.841(12) 2.276(10) 2.94(2) 2.32(1) 2.41(1) 2.27(1) 2.36(1) 2.51(1) 2.33(1) 2.27(1) 2.27(1) 2.36(2) 2.44(2)

Symmetry operation: 1 x, 2 y, z; x, 1 y, 1 z.

Lorentzian shape, with FWHM 7.0 ± 0.2 cm1, in the high resolution luminescence spectrum of the Eu complex indicates that the Eu3+ ion occupies effectively a single site (see inset of Fig. 3). The peak position of the 5D0 ! 7F0 transition is related to the nephelauxetic effect and to the covalency of the

D0

7

F2

Intensity (a.u.)

Sr1–O1 Sr1–O3 Sr1–O4 Sr1–O6 Sr1–O7 Sr1–O9 Er1–O1w Er1–O2w Er1–O3 Er1–O5 0 Er1–O6 Er1–O11 Er1–O10 Er1–O12 0 Sr2–O13 Sr2–O14 Sr2–O17 Sr2–O16 Sr2–O23 Sr2–O24* Er2–O3w Er2–O4w Er2–O1500 Er2–O14 Er2–O17 Er2–O20 Er2–O2100 Er2–O24 Er2–O23* Er2–O24*

5

Intensity (a.u.)

Fig. 2. Perspective view of asymmetric units along with the labelling scheme for two independent coordination chains. The thermal ellipsoids are drawn at the 40% probability level. One position of the disordered 2-furoic acid is shown with clean-dash double lines.

2000

7

F0

1000

0

578.8

579.2

579.6

Wavelength (nm) 7

7

F1

7

F4

7

F0

575

600

625

F3

650

675

700

Wavelength (nm) Fig. 3. Room temperature emission spectra for the {[Eu2Sr(C4H3OCOO)8(H2O)4]}n sample (kexc = 488.0 nm). Inset: Lorentzian fit of the 5 D0 ! 7F0 band (see text) [34].

Eu3+–ligand bond [32]. The 5D0 ! 7F0 maximum (579.21 ± 0.03 nm) is located in the ‘‘covalent’’ region of the nephelauxetic scale [33]. The ratio R of the integrated intensities of the 5D0 ! 7F2 and 5D0 ! 7F1 transitions can be considered indicative of the asymmetry of the coordination polyhedron of the Eu3+ ion [34] (Fig. 3, Table 2). The observed R-value (2.6 ± 0.1) points toward a slightly distorted first coordination sphere around the Eu3+ ion, in agreement with the crystal structure described above for the compound 1, in which the Er3+ ion occupies a site of C1 symmetry, but whose geometry is reminiscent of a distorted square antiprism with D4d symmetry (Fig. 4). However, the fact that the 5D0 ! 7F2 transition is observed indicates that the symmetry is lower than D4d [35], so that a D4 symmetry can be considered as a good approximation for the effective geometry around Eu3+. The room temperature decay curve of the 5D0 emission shows an exponential behaviour. From a fit of the emission

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Fig. 4. The symmetry of nearest environment of Er ion in {[Er2Sr(C4H3OCOO)8(H2O)4]}n.

decay, a lifetime of 0.47 ± 0.01 ms has been obtained for the 5D0 level. Despite the presence of quenching centres in the proximity of the Eu3+ ion, such as two water molecules directly coordinated, the emission seems to be efficient. In fact, a value close to 0.5 ms for the decay time of the 5D0 level is relatively high and indicates that nonradiative transitions are not dominating the decay properties [36]. 3.2.2. Neodymium The room temperature laser-excited luminescence spectrum of powders of {[Nd2Sr(C4H3OCOO)8(H2O)4]}n measured with excitation wavelength at 355 nm, shows a structured broad band in the 860–920 nm range (see Fig. 5). This band is due to f ! f emission transitions involving the crystal field components of the 4F3/2 excited level and 4I9/2 ground state of the Nd3+ ion. The luminescence in this spectral range is relatively intense. From a fit of the laser-excited emission decay (kexc = 355 nm, kem = 890 nm) with a single exponential curve, a lifetime of 90 ± 5 ns has been estimated for the 4F3/2 level. This value is relatively high, as both multiphonon relaxation and cross-relaxation could contribute to the quenching of the 4F3/2 state. In fact, the energy gap separating this level from the lower energy 4I15/2 state is only about 5500 cm1, 4

4

840

860

I9/2

Intensity (a.u.)

F3/2

880

900

920

940

Wavelength (nm) Fig. 5. Room temperature emission spectrum for the {[Nd2Sr(C4H3OCOO)8(H2O)4]}n sample (kexc = 355.0 nm).

and can be bridged by only two O–H vibrational quanta, deriving from the two coordinated water molecules. In fact, it is well known that strong Nd3+ emission can only be obtained by minimising the multiphonon relaxation, carefully shielding the metal centre from water molecules and O–H vibrations [37–40]. This is illustrated by the fact that the lifetime of the 4F3/2 state of hydrated Nd3+ ions in aqueous solution is 29 ± 3 ns [38]. Moreover, a cross-relaxation process of the type (4F3/2, 4I9/2) ! (4I15/2, 4I15/2) could also contribute to the emission quenching, as the distance between neighbouring Nd3+ ions in the present compound ˚ , a value coincident with the is estimated to be about 4 A typical critical distance of the cross-relaxation process. The observation of an easily measurable and relatively long-lived emission from the 4F3/2 level in the coordination compound under investigation clearly indicates that the quenching mechanisms are not very efficient. 3.3. Magnetic properties of [Ln2Sr] [Ln = Pr3+, Nd3+, Er3+ and Eu3+] complexes As the rare earth ions different from Gd3+ are characterized by orbitally degenerate ground state, the analysis of their magnetic behavior is not straightforward. The starting point of our analysis was an attempt to reproduce the temperature dependence of the dimers containing Nd3+, Pr3+, Er3+ and Eu3+ considering the 4f ions as isolated ones. The magnetic properties were calculated in the framework of a classical Crystal Field approach for Pr3+, Nd3+, Er3+ derivatives assuming a pseudo D4 symmetry with the aim to reduce the number of parameters. This assumption agrees with the results deriving from the luminescence spectroscopy of the Eu3+ complex. In the fit the B20 , B40 , B44 , B60 , B64 , parameters were used [25]. The Eu3+ magnetic susceptibility was treated with a different model according to its specific electronic ground state. As the systems are in any case overparameterized, the fitting sets are not discussed in detail: the CF model was used only to determine the presence of sizeable magnetic exchange. The temperature dependence of the magnetic susceptibility for the Nd, Pr and Er derivatives was reproduced by using the Van Vleck’s equation based on the energy levels derived from a complete diagonalization of the CF and Zeeman matrixes. For all these compounds the experimental data were nicely reproduced in the whole temperature range (Fig. 6a – for Nd, b – for Pr, c – for Er) without introducing any kind of magnetic interaction. The Bkq parameters used in the best fit are presented in the supplementary material. The ground 7F multiplet of Eu3+ is split by the spin orbit-coupling in seven states living the non magnetic 7F0 level as ground state. The magnetic behavior of this ion is due to the thermal population of the excited states with energies E(J) = kJ(J + 1)/2: its magnetic susceptibility was reproduced as described in the literature [23,41] relatively to the parameter k. The temperature dependence


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Fig. 6. vT vs. T plot for complexes: (a) Nd2Sr; (b) Pr2Sr; (c) Er2Sr; (d) Eu2Sr.

for the Eu derivative is shown in Fig. 6d: the curve was calculated by using a value of 370.1 cm1. The analysis of the low temperature range is consistent with the hypothesis that dipolar magnetic interactions are probably active but their effects are less effective than Crystal Field ones [41].

transitions; also ‘‘2Nd–Sr’’ proved to be luminescent in the near IR. Magnetic susceptibility data for ErIII, PrIII, NdIII and EuIIIcontaining complexes were described using Crystal Field approach.

4. Conclusions

The research described in this publication was made possible in part by SCOPES Award No. IB7320-110823. V. Mereacre thanks also the AvH Foundation. The authors gratefully thank Erica Viviani (University of Verona) for expert technical assistance.

New examples of heterotrinuclear coordination polymers complexes with the general formula {[Ln2Sr(C4H3OCOO)8(H2O)4]}n, where Ln = La3+ (1), Pr3+ (2), Nd3+ (3), Sm3+ (4), Eu3+ (5), Gd3+ (6), Tb3+ (7), Ho3+ (8), Yb3+ (9) and Er3+ (10) have been prepared using simple synthetic procedure. The X-ray crystal structure has been determined for the {[Er2Sr(C4H3OCOO)8(H2O)4]}n (10) complex, indicating that this complex is built from two crystallographic independent coordination polymers {[Er2Sr(C4H3OCOO)8(H2O)4]}n in the triclinic crystal system and P1 space group. The X-ray diffraction (XRD) pattern of the samples shows that all lanthanide compounds are isostructural to 10. The luminescence spectrum of a microcrystalline sample of ‘‘2Eu–Sr’’ compound displays the characteristic Eu3+ (5D0 ! 7FJ (J = 0–4)) metal centred

Acknowledgements

Appendix A. Supplementary material CCDC 623858 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica. 2007.02.045.

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