Molybdate templated assembly of Ln₁₂Mo₄-type clusters (Ln = Sm, Eu, Gd) containing a truncated tetrahedron core

Abstract

Three heterometallic cluster complexes {Ln₁₂Mo₄} featuring an Ln₁₂ core of a distorted truncated tetrahedron were synthesized with the assistance of four MoO₄²⁻ anions as ancillary ligands. Magnetic studies of the {Gd₁₂Mo₄} cluster revealed a large magnetocaloric effect due to the presence of the large number of weakly coupled Gd(III) ions.

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Polynuclear lanthanide hydroxide complexes are of interdisciplinary interest.^1–8 They are synthetically challenging as the oxophilicity of the lanthanide ions favors extensive interactions with hydroxo groups, leading frequently to intractable precipitates of lanthanide hydroxides. However, an increasing number of structurally pleasing polynuclear lanthanide hydroxide complexes with polyhedral cluster cores, though obtained serendipitously, have appeared in the literature and inspired the exploratory rational syntheses of these species.^1–8 Such efforts have further been stimulated by their exciting applications as molecule-based magnetic materials,⁴ artificial nucleases for the hydrolytic cleavage of DNA and RNA,⁹ contrast agents for biomedical imaging,¹⁰ and fixation media for atmospheric CO₂.⁷

The most successful synthetic approach to these compounds is the ligand-controlled hydrolysis of lanthanide ions in which preoccupation of the coordination sphere by certain ancillary ligands, such as α-amino acids⁵ and β-diketonates,¹¹ limits the number of hydroxo ligands resulting from the deprotonation of the lanthanide-bound and activated aqua ligands, and thus preventing the precipitation of lanthanide hydroxides.

An interesting observation is the incorporation of small anionic species in many lanthanide hydroxide cluster complexes so-obtained. These include halides,^5a–c carbonate,^5d and oxalate,¹² either present in the original reaction mixture or generated in situ due to ligand decomposition. Studies have established their indispensible role in templating the assembly of the cluster species. Their significance has also been demonstrated in the recent syntheses of a number of 3d–4f heterometallic clusters with giant magnetocaloric effects (MCEs) that are critical for the development of sub-Kelvin magnetic refrigeration technology.¹³

Due to the variation in composition, charge, and/or geometry, the use of different anions is expected to lead to clusters with distinct structures and interesting properties. Based on the templating role of the oxoanions mentioned above,^5d,12 we set out to explore the use of simple oxometalates with the hope of producing novel lanthanide hydroxide clusters. The key objectives are to study the possible templating effects of these less commonly encountered anions, to obtain new heterometallic cluster species, and ultimately, to create new functional materials. In this communication, we report our results of utilizing molybdate anions (MoO₄²⁻) in the assembly of three novel lanthanide–Mo hydroxide clusters.

Compounds 1–3 of the general formula [Ln₁₂Mo₄O₁₆(HL)₆(μ₃-OH)₄(OOCCH₃)₁₂]·12MeOH·8H₂O [Ln = Gd(1), Eu(2), Sm(3); H₃L = (E)-2-(2,3-dihydroxypropylimino)methyl)-phenol] were synthesized under solvothermal conditions using a mixture containing the racemic Schiff-base ligand H₃L (Fig. S1, ESI†), Ln(CH₃CO₂)₃·3H₂O, (n-Bu₄N)₄Mo₈O₂₆, and triethylamine and methanol. The formulation was established by single-crystal X-ray diffraction in conjunction with satisfactory elemental analyses (ESI†).

The crystal structure of 1 is discussed as the representative of the isomorphous compounds. The asymmetric unit contains one complete molecule (Fig. 1a), 12 methanol, and 8 water molecules. The Gd₁₂ core, analogous to that of a previously reported cluster of nickel phosphonate,¹⁴ can be conveniently viewed as a distorted truncated tetrahedron (Fig. 1b) with each of its 12 vertices occupied by one Gd atom (the truncated tetrahedron is the simplest Archimedean polyhedron consisting of four triangles and four hexagons). Each of the triangular faces in 1 is capped by a triply bridging OH⁻ group while each of the hexagonal faces is capped by a MoO₄²⁻ anion (Fig. 2; average Mo = O distance 1.72 Å) with one of them (Mo4) showing a unique highly symmetrical coordination to the six Gd atoms within the related hexagon in a 6.2220 (Harris notation)¹⁵ bridging mode (Fig. S2a, ESI†) using three of its four O atoms; each of the remaining MoO₄²⁻ (Mo1, Mo2, and Mo3) anions acts as a ligand using three of its four O atoms to coordinate to five Gd atoms in a 5.2210 bridging mode (Fig. 2 and Fig. S2b, ESI†).

Fig. 1 (a) Ball-and-stick plot of Gd₁₂Mo₄O₁₆(HL²⁻)₆(μ₃-OH)₄(CH₃CO₂)₁₂; (b) the {Gd₁₂} unit corresponds to a distorted truncated tetrahedron.

Fig. 2 Demonstration of the {Gd₁₂} unit capped by four MoO₄²⁻ ions. The long Mo–O(HL) bond distances of 2.475(8)–2.548(11) Å are comparable to those of 2.482(11)–2.539(12) Å (dotted lines) are in a previously reported Mo complex but contain a different coordination type of the metal centers.¹⁶

The Gd atoms are further coordinated by six HL²⁻ ligands in 2.2111 and 3.2211 bridging modes (Fig. S2c and S2d, ESI†) and by 12 acetate ligands in 2.21 and 3.21 modes. The Gd atoms are either octa- or nona-coordinated. Gd1, Gd2, and Gd3 are octacoordinated, featuring the coordination of the (N,O,O) moiety of one HL²⁻ ligand, of two alkoxido O atoms from another HL²⁻ ligand, of two molybdate O atoms, and of one μ₃-OH group. Gd4, Gd5, and Gd6 are also octacoordinated by the (N,O,O) moiety of one HL²⁻ ligand, furthermore by the phenolate O atom from another HL²⁻ ligand, two acetate O atoms, one molybdate O atom, and one μ₃-OH group. The remaining Gd atoms are all nonacoordinated with a tri-capped trigonal prismatic environment. Gd7, Gd9, and Gd11 are each coordinated by four acetate O, two alkoxido O, two molybdate O atoms, and one μ₃-OH group, while the other three (Gd8, Gd10 and Gd12) are each coordinated by six acetate O, and two molybdate O atoms, and one μ₃-OH group. The shortest Gd⋯Gd separation is 3.647(1) Å, comparable to the values found in similar μ₃-OH-bridged Gd clusters.¹⁷ The shortest Gd⋯Mo distance is 3.610(2) Å, slightly shorter than 3.652 Å of a previously reported Gd(III)–Mo(VI) complex.¹⁸

The large number of magnetically interesting Gd(III) in 1 prompted us to investigate the magnetic properties of this unusual cluster compound. Its direct-current (DC) magnetic susceptibility was measured on a polycrystalline sample in the temperature range of 2.0 to 300 K under an applied magnetic field of 1000 Oe. As shown in Fig. S3 (ESI†), the χ_MT product at 300 K is 95.2 cm³ mol⁻¹ K, in good agreement with the expected value of 94.5 cm³ mol⁻¹ K for 12 non-interacting Gd(III) ions (⁸S_7/2, g = 2.0).¹⁹ Upon lowering the temperature, the χ_MT value remains nearly constant above 75 K, below which it drops rapidly, to 50.6 cm³ mol⁻¹ K at 2 K. The decrease of the χ_MT value at lower temperature, together with the easy saturation of magnetization at ca. 6 T (Fig. S4, ESI†), indicates weak yet dominant antiferromagnetic coupling between adjacent Gd(III) ions.

In order to estimate the magnitude of the antiferromagnetic exchange between the Gd(III) ions, a simulation of the magnetization was performed. The huge size of the Hilbert space of 12 spins s = 7/2 (68,719,476,736) precludes an exact quantum treatment even if symmetries are applied.²⁰ Two approximations were adopted instead: (1) classical Monte-Carlo simulations and (2) the Finite Temperature Lanczos Method (FTLM).²¹ The former is expected to be rather accurate for the treatment of a large spin such as s = 7/2 and for thermal energies that are bigger than the exchange, while the latter is a very accurate quantum mechanical approximation for Hilbert space dimensions up to about 10⁹. It was therefore applied to an analogous structure with single spins of 5/2. The Hilbert space dimension of 2.176.782.336 remains huge, but the magnetization can be obtained using the National Supercomputer at LRZ Garching. A single exchange constant J between nearest neighbor Gd spins was assumed, and a good agreement is reached with both methods for J = −0.04 K using a 2J-Heisenberg Hamiltonian. This number is somewhat smaller than in other Gd compounds,^13e,22 but can be rationalized as some of the exchange pathways are long due to the involvement of a molybdenum atom.

The MCE of 1 is estimated from the magnetization data obtained at various fields and temperatures (Fig. S4, ESI†). Values of ΔS_m are calculated by fitting the magnetization data with the equation ΔS_m(T, ΔH) = ∫[∂M(T,H)/∂T]_HdH (ΔS_m: the change of magnetic entropy following a change of the applied magnetic field).²³ As depicted in Fig. 3, the largest −ΔS_m = 35.3 J kg⁻¹ K⁻¹ was obtained at 3 K for ΔH = 7 T, lower than the calculated value of 41.6 J kg⁻¹ K⁻¹ per mole for 12 uncorrelated Gd(III) spins (S = 7/2),^23c possibly due to the presence of weaker intra-cluster antiferromagnetic interactions that contribute negatively to its MCE. Though not as large as the highest value of 47.7 J kg⁻¹ K⁻¹ recently reported for a Gd(III)-containing coordination polymer,²⁴ the experimentally obtained −ΔS_m value reported here is one of only a handful that are larger than 35.0 J kg⁻¹ K⁻¹.^13b,d,24,25 That the observed −ΔS_m value is smaller than that of the ferromagnetic coupled {Gd₂} cluster²⁵ but significantly larger than that of the antiferromagnetically coupled {Gd₇} cluster²⁶ suggests that the existence of the relatively large number of Gd(III) ions with weaker coupling is important in enhancing the field dependence of MCE due to an increase of the number of low-lying excited spin states.²⁷

Fig. 3 −ΔS_m obtained from magnetization data of 1 at various fields and temperatures.

How the material behaves in an adiabatic cooling process can be understood by evaluating the magnetic isentropes. In a perfect adiabatic demagnetization process, the system would follow its curve of constant entropy down to the respective temperature at B = 0. We suspect that sub-Kelvin temperatures should indeed be achievable, as can be deduced from Fig. S5 (ESI†).

In summary, three new heterometallic lanthanide–Mo cluster complexes have been prepared by lanthanide hydrolysis under solvothermal conditions and in the presence of molybdate anions. The four oxometalate ions serve as templates for the assembly of 12 lanthanide ions into a fascinating truncated tetrahedron. The unique templating role and reactivity of MoO₄²⁻ and possibly other oxometalates merit further studies.²⁸ For one of these clusters, the Gd-containing compound, magnetic studies revealed a large MCE, suggesting that the existence of a large number of high-spin Gd(III) ions and weak antiferromagnetic interactions²⁹ are critical in enhancing the field dependence of the MCE. Thus, the present work not only offers a distinctly different approach to realizing structurally interesting heterometallic lanthanide-transition metal clusters, but also provides new insights into the design of materials with large MCEs for potential magnetic cooling applications.^4c,29

We thank the 973 Project from MSTC (Grant No. 2012CB821704) and the NNSFC (Grant No. 20825103, 90922031, and 21021061) for the financial support. A.M. and J.S. thank the Deutsche Forschungsgemeinschaft for continuous support. Supercomputing time at the LRZ Garching is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Syntheses, crystallographic data and characterization details of 1–3, CCDC 861086–861088, Fig. S1–S5. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2cc36530h

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