Generation of lanthanide coordination polymers with dicarboxylate ligands: synthesis, structure, thermal decomposition and magnetic properties of the two-dimensional triaquatris(malonato)dipraseodymium(III) dihydrate {[Pr₂(C₃H₂O₄)₃(H₂O)₃]·2H₂O}

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Abstract

The malonate complex of formula [Pr₂(C₃H₂O₄)₃(H₂O)₃]·2H₂O (1) was prepared and his crystal structure determined by X-ray diffraction methods. 1 crystallizes in the monoclinic space group P2₁, Z = 4, with unit cell parameters a = 7.631(2), b = 12.899(4), c = 8.923(2) Å and β = 101.11(3)°. 1 is a polymer which grows in the (110) plane. The hydrogen bond stabilizes the crystal structure forming a three-dimensional network. The two non-equivalent praseodymium(III) ions have different environments. Finally, the thermal behaviour and magnetic properties were investigated.

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Introduction

The design of new solid-state architectures has recently driven much attention and increasing interest.¹ Advanced crystal engineering by selecting structural ligands and the coordination geometry of lanthanide(III) metal centers as a building block can give a series of novel inorganic frameworks with an interesting variety of crystal-packing motifs. This is in accordance with the experience that the crystal structures of the lanthanide compounds frequently change along the series as a result of the lanthanide contraction.

Among a number of ligands used as building blocks, the two end functional groups of dicarboxylic acids could yield a variety of crystal structures of coordination compounds even with close chemical formulae, depending on the conformation of the carbon chains and the end functional groups. Those compounds are characterized as organic/inorganic hybrids with pillared sheet-like structures assembled from the molecular precursor in solution.

In order to check the ability of rare earth ions to lead to high dimensional materials, we have thus undertaken a study of the lanthanide malonate complexes (where malonate stands for the dianion of the propanedioic acid, H₂mal). This ligand has, indeed, a great ability to form infinite connections with metal ions^2,3 and a remarkable versatility in adopting several different modes of bonding, ranging from unidentate, chelating and bridging, sometimes in more than one way in the same compound. Seven bonding modes have been observed according to the coordination environment of COO⁻ groups as shown in Scheme 1.

Scheme 1

Also, malonic acid is widely used in many important fields as a result of its excellent coordination ability. The complexes of malonic acid with lanthanide and transition elements have been used in the fluorescent probe technique, solid fluorescent materials, biological active materials and modifiers of polyethylene.^4,5 Therefore, studies of the new complexes with malonic acid are quite valuable in theory and application.

In this paper we describe the synthesis, IR spectrum, thermal behaviour, magnetic properties and crystal structure of [Pr₂(mal)₃(H₂O)₃]·2H₂O, (1).

Experimental

Materials

Malonic acid, praseodymium(III) nitrate hexahydrate Pr(NO₃)₃·6H₂O and sodium metasilicate nonahydrate Na₂Si O₃·9H₂O were purchased from commercial sources and used as received. Elemental analysis (C,H) was performed with an EA 1108 CHNS-0 automatic analyzer.

Physical techniques

Thermogravimetric (TG) and calorimetric analyses were performed on a Netzsch STA 409 EP simultaneous thermobalance and a differential scanning calorimeter (DSC), respectively, at 5 °C min⁻¹ to a maximum temperature of 400 °C in dynamic dinitrogen atmosphere of ca. 70 cm³ min⁻¹. Variable-temperature (1.9–290 K) magnetic susceptibility measurement on a polycrystalline sample was carried out with a Quantum Design SQUID magnetometer operating at 100 G (T < 50 K) and 1000 G (over the whole range).

Synthesis

Single crystals have been grown in silica gel medium, using the techniques described by Henisch.⁶ An approximately 1 M solution of sodium metasilicate nonahydrate was added to a volume of 2 M malonic acid with continuos stirring. The mixture was introduced into test tubes, covered, and allowed to set for one day at room temperature. A 0.5 M solution of Pr(NO₃)₃·6H₂O was introduced above the gel, care being taken to avoid damaging the surface of the gel, and the tubes were stored at 40 °C. Three weeks later, green crystals appeared suitable for X-ray analysis. Anal. Calc. for C₉H₁₆O₁₇Pr₂: C, 15.94; H, 2.38. Found: C, 17.49; H, 2.56%.

Crystallographic data collection and structure determination

A crystal of dimensions 0.45 × 0.60 × 0.24 mm³ was used for data collection on an Enraf–Nonius MACH3 four-circle diffractometer. Data were collected at 293(2) K using graphite-monochromatized MoKα radiation (λ = 0.71069 Å) and the ω-scan technique. Details of the crystal structure are given in Table 1. The data were corrected for Lorentz, polarization and absorption.⁹ The maximum and minimum transmission factors were 1.693 and 0.476. The structure was solved using SIR97¹⁰ and refined using SHELXL97.¹¹ In the final least squares cycles all non-H atoms were allowed to vibrate anisotropically. Hydrogen atoms of the malonate ligand were included at calculated positions on carbon centres. Hydrogen atoms attached to oxygen atoms of water molecules were not found. The final geometrical calculations and the graphical manipulations were carried out with the PARST95¹² and PLATON¹³ programs, respectively.

Table 1 Crystallographic data for [Pr₂(mal)₃(H₂O)₃]·2H₂O] (1)^a

Chemical formula	C9H16O17Pr2
a Click here for full crystallographic data (CCDC no. 1350/37).
Formula weight	678.03
T/K	293(2)
Crystal system	Monoclinic
Space group	P21
a/Å	7.631(2)
b/Å	12.899(4)
c/Å	8.923(2)
β/°	101.11(3)
V/Å^3	861.9(4)
Dc/g cm^−3	2.613
Z	4
µ/mm^−1	5.678
Measured/independent reflections R(int)	5372/4962 0.012
Final R indices [I > 2σ(I)]	R1 = 0.031, wR2 = 0.081
R indices (all data)	R1 = 0.032, wR2 = 0.082

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Results and discussion

Description of the structure

The structure of the compound can be visualised as chains of praseodymium(III) ions linked through two crystallographically independent malonate ligands, L1 and L2 (see Fig. 1a), which run parallel to the a axis. Their conformation is the same: L1 and L2 are three-times chelating including the six-membered rings, which adopts boot conformation. The O(1), O(3), O(6) and O(7) are the oxygen–carboxylate atoms chelate praseodymium ions and they also coordinate to adjacent praseodymium ions in the form of μ₂–O. The distances Pr⋯Pr through this bridging pathway is 4.2301(10) Å [Pr2⋯Pr1(1 + x, y, z)] for O(1) 4.4605(9) Å (Pr1⋯Pr2) for O(3) and O(6) and 4.2301(10) Å [Pr1⋯Pr2(−1 + x, y, z)] for O(7).

Fig. 1 (a) Perspective view of the chain. (b) A view of a fragment of the layer in the (110) plane.

The chains are linked in the perpendicular direction (along the crystallographic b axis) through a new independent malonate ligand, L3 (Fig. 1b), forming a two-dimensional structure. L3 adopts simultaneously bidentate [at Pr(2)] and monodentate [at Pr(1)] coordination modes and exhibits a boat conformation (see Fig. 2). Two carboxylate bridges O(9)C(7)O(10) and O(11)C(9)O(12) exhibit the anti–anti conformation. A hydrogen bond between one coordinated water molecule [O(2w)] and a carboxylate–oxygen atom O(9), O(2w)⋯O(9)(−1 + x, y, z) 3.151(7) Å occurs within each layer. Bidimensional networks can be found in different malonate complexes containing lanthanides¹⁴ and metal transitions ions.^15–17

Fig. 2 Malonate L3 conformation: the thermal ellipsoids are drawn at the 50% probability level and the hydrogen atoms have been omitted. Symmetry codes a  = −1 − x, −½  + y, −1 − z.

The crystal structure is stabilized through extensive hydrogen bonding involving carboxylate groups and water molecules, forming a three-dimensional network (see Fig. 3 and Table 2). In all hydrogen bonds, the water oxygens act as donor atoms, and the carboxylate or other water oxygens act as acceptor atoms. Furthermore, additional hydrogen bonds between a carboxyl group and a water molecule occur.

Fig. 3 Projection of the structure down the a axis. Dashed lines correspond to hydrogen bridging.

Table 2 Donor⋯acceptor distances D⋯A (Å) for compound 1

D⋯A		D⋯A
Symmetry codes: a = x, y, z − 1; b = x, y, z + 1; c = x − 1, y, z − 1; d = −2 − x, y + ½ , −1 − z; e = −2 − x, −½ + y, −1 − z.
O1w⋯O2c	2.820(7)	O4w⋯O4e	2.858(7)
O1w⋯O11d	2.976(7)	O4w⋯O8	2.729(7)
O1w⋯O12d	3.006(8)	O4w⋯O2wa	2.880(9)
O1w⋯O2wd	2.852(8)	O4w⋯O3wc	2.815(7)
O3w⋯O5b	2.724(6)	O4w⋯H8A-C8c	2.603(5)

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The two crystallographically independent praseodymium(III) ions [Pr(1) and Pr(2)] have different coordination numbers. Pr1 is coordinated by three water molecules and five malonate ligands. The coordination number of Pr1 is ten and the coordination geometry is a distorted bi-capped dodecahedron, while Pr2 is surrounded by nine oxygen atoms forming a distorted monocapped square antiprism. Both geometries are observed in other lanthanide–malonate complexes reported in the literature.^18–20 The oxygens around Pr2 are provided by five malonate ligands. Both polyhedrons share two corners O(3) and O(6). The praseodymium ions environments are shown in Fig. 4a and the two polyhedrons are depicted in Fig. 4b. The Pr1–O bond distances are in the range 2.433(4)–2.719(5) Å; their average value is 2.573(4) Å. The bond Pr1–O1(−1 + x, y, z) is considerably longer than the average value. The Pr2–O bond distances are smaller and they lie in the range 2.393(5)–2.684(4) Å, with an average value of 2.526(4) Å. This value is similar to that found for nine-coordinated praseodymium cations.

Fig. 4 (a) Perspective drawing of praseodymium ions environments. (b) Coordination geometry of both praseodymium cations.

The average C–O bond distances and O–C–O bond angles are 1.254(7) Å and 120.7(5)° for L1 and L2, and 123.3(5)° for L3, respectively. These values agree well with that of other previously reported malonate-containing lanthanide complexes.^15–17

Thermal analyses

TG-DSC of [Pr₂(mal)₃(H₂O)₃]·2H₂O 1 (Fig. 1) shows a first step with two mass loss. The first gradual mass loss is between 29 and 75 °C with an endothermic process centered at 48 °C (5.14%). The second mass loss occurs between 76 and 193 °C with two endothermic processes—one centered at 85 °C and the other at 152 °C (8.02%). The total observed weight loss in the latter two stages is 13.16% and it corresponds to the release of five water molecules per formula unit (calculated value is 13.23%). This process seems to take place in two steps, suggesting that all water molecules are not similar, as deduced from the crystallographic data (vide supra). The temperatures for weight loss of hydration and coordinated water molecules are in agreement with those found for other compounds described in the literature.^3,21–25

The second step of weight loss occurs between 193 and 300 °C as shown in Fig. 1, and it is a strong exothermic process [T_peak]_DSC = 291 °C, which is caused by the combustion of part of organic matter of the organic ligand.^3,21)

Fig. 5 TG and DSC curves at 5 °C min⁻¹ in dynamic dinitrogen atmosphere at 70 cm³ min⁻¹ for [Pr₂ (mal)₃ (H₂O)₃]·2H₂O (1).

Magnetic properties

The magnetic properties of 1 under the form of the χ_MT product vs.T [χ_M being the magnetic susceptibility per two Pr(III) cations] are shown in Fig. 6. The value of χ_MT at room temperature is approximately 2.80 cm³ mol⁻¹ K and it continuously decreases upon cooling. An incipient maximum in the susceptibility curve (see insert of Fig. 6) is observed at ca. 20 K. These features are indicative of an overall antiferromagnetic coupling. The examination of the crystal structure of 1 reveals the presence of chains of Pr(III) cations which are bridged by two malonate–oxygen atoms. Consequently, the exchange pathway for the antiferromagnetic coupling observed most likely involves these oxygen atoms. The ground state for a Pr(III) single ion is ³H₄ and the first excited state, ³H₅, is 2000 cm⁻¹ above the low-lying one. So, the ground state is the only thermally populated at room temperature and the magnetic properties of 1 are due to this term. Keeping in mind that the angular momentum J = 4 is large enough as to be considered a classical angular momentum and neglecting the ligand field effects, we can use the Fisher equation describing a Heisenberg classical magnetic chain. For that, we use S = 4 in the Fisher expression, the Hamiltonian being H = −j Σ_iS_i·S_i + 1. The best-fit results are: j = −1.8 cm⁻¹, g = 0.79, ρ = 0.03%, and R = 9.0 × 10⁻⁵ρ and R are the percentage of monomeric impurities and the agreement factor defined as Σ_i[(χ_M)_obs(i) − (χ_M)_calc(i)]²/Σ_i[(χ_M)_obs(i)]², respectively. The computed value of g is in good agreement with that expected for the ground state ³H₄ (g = 0.80). The calculated curve (solid line in Fig. 6) matches very well the experimental magnetic data except in the very low temperature region. It is important to note that ligand field effects break the degeneration of the levels of the ground state multiplet. So, the calculated value for the exchange coupling parameter must be considered as an upper limit for the intrachain antiferromagnetic interaction. In fact, the influence of the ligand field effects can be seen in the very low temperature region where the theoretical model is unable to reproduce the experimental susceptibility data.

Fig. 6 Thermal dependence of the χ_MT product. The inset shows the thermal dependence of the molar magnetic susceptibility.

Acknowledgements

Financial support from Spanish Dirección General de Investigación Científica y Técnica (DGICYT) through Projects PB97-1479-C02-02 and PB97-1397, from the Gobierno Autónomo de Canarias through Project PI1999/061 and from the European Union (TMR Programme) through Contract ERBFM-RXCT980181 is gratefully acknowledged.

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