pH-Controlled Assembly of Organophosphonate-Bridged Dysprosium(III)

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pH-Controlled Assembly of Organophosphonate-Bridged Dysprosium(III) Single-Molecule Magnets Based on Polyoxometalates Yu Huo, Yan-Cong Chen, Si-Guo Wu, Jian-Hua Jia, Wen-Bin Chen, Jun-Liang Liu,* and Ming-Liang Tong* Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, P. R. China S Supporting Information *

Herein, we report the synthesis of a sandwich-type POM, [Dy6(ampH)4(H2O)23(ampH2)(PW11O39)2]·21H2O (1), and a propeller-shaped POM, Cs9K13Na13[Dy9(CO3)3(ampH)2(H2O)12(PW10O37)6]·66H2O (2), from subtle alteration of the reaction conditions. Interestingly, 1 is a neutral molecule, while 2 is a mixed cesium−potassium−sodium salt. Magnetic studies reveal that 2 exhibits single-molecule magnet (SMM) behavior without an appreciable quantum tunneling, whereas 1 hardly shows SMM behavior above 2 K. Both complexes are synthesized in water by the reaction of [P2W19O69]14−, (aminomethyl)phosphonic acid (ampH2), with DyCl3, under different pH conditions: pH = 2.9 for 1 and pH = 7.4 for 2. Notably, additional CO32− is found in 2, which may come from CO2 in air under higher pH conditions, thus facilitating the formation of a larger cluster.11 Single-crystal X-ray diffraction indicates that 1 consists of a hexanuclear dysprosium(III) core, [Dy6(ampH)4(ampH2)(H2O)23]14+, that is sandwiched between two [PW11O39]7− units oriented at 180° with respect to each other (Figure 1a). To better understand the arrangement, the core has been simplified as the backbone of Dy atoms with their bridging ligands. As shown in Figure 1b, the core can be viewed as an octahedral structure, where the Dy(III) ions are located on the vertices. The octahedron contains three different types of triangles with Dy1···Dy2/Dy2A, Dy1···Dy2B/

ABSTRACT: Two structurally intriguing dysprosium(III)-substituted polyoxometalates, [Dy 6 (ampH) 4 (H 2O) 23 (ampH 2 )(PW 11 O 39 ) 2 ] (1) and [Dy9(CO3)3(ampH)2(H2O)12(PW10O37)6]35− (2), are assembled by the same precursor under different pH conditions. The structure of 1 contains an octahedral {Dy 6 (ampH) 4 } core, and a unique windmill-type {Dy9(CO3)3(ampH)2} for 2. Single-molecule magnet behavior is observed for 2 with a thermally activated energy barrier of 56 K and no appreciable quantum tunneling of magnetization under zero field.

P

olyoxometalates (POMs) are anionic, nanosized metal− oxygen clusters of the early transition metals (TMs) with enormous structural versatility and properties relevant to a wide variety of applications.1 In particular, the highly negatively charged lacunary POMs are useful multidentate building blocks allowing for the construction of various high-nuclearity TMs or lanthanide (Ln) clusters.2 The study of POMs featuring highnuclearity TM clusters has been a field of rapid growth because of their wide variety of applications in catalysis, medicine, and magnetism,3 whereas the construction of POMs incorporating high-nuclearity Ln clusters of greater than 5 remains relatively underexplored because of the versatile coordination geometries and higher coordination numbers of Ln ions.4 To date, only Yb6containing {Yb6P4W30O112(μ6-O)(μ3-OH)6(H2O)6},5 Ce10-containing {[Ce10P6W48O183(OH)6(CO3)(H2O)11},6 and Ln26containing {Ln27Ge10W106O406(OH)4(H2O)24}7 have been characterized, and all of these systems are purely inorganic. A commonly employed strategy to achieve polynuclear clusters is to utilize flexible and multidentate ligands, such as amino acids, carboxylates, and organophosphonates, which can limit Ln ions hydrolysis and aggregation of the hydroxo intermediates to some extent.8 In particular, organophosphonates as multidentate ligands can provide appropriate electrondonor characteristics and symmetry for the construction of highnuclearity Ln-containing POMs, offering additional stability and opportunity to fine-tune the properties.9 Furthermore, targeted POM structures can be significantly affected by the variable synthesis conditions, and especially delicately controlling the pH of the reaction system has been exploited to achieve the new nanoscale functional architectures.10 © XXXX American Chemical Society

Figure 1. (a) Combined polyhedral/ball-and-stick representation of 1. (b) Structure of the {Dy6} cluster. (c) Coordination environments for Dy centers in 1. Color code: W, light blue; O, red; Dy, green; P, yellow; C, gray 50%; N, blue; WO6 octahedral, red; PO4, yellow. Received: March 25, 2018

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DOI: 10.1021/acs.inorgchem.8b00795 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 2. (a) Combined polyhedral/ball-and-stick representation of polyanion 2 (the −CH2NH3 groups are omitted for clarity). (b) The sandwich-type building block. (c) {A-α-PW10O37} fragments. (d) Side view of the {Dy9} cluster, where two P atoms from ampH− reside on the pseudo-C3 axis. (e) Top view of the {Dy9} cluster. H atoms are omitted for clarity. Color code: {WO6} octahedral, red; PO4, yellow.

ligands (Figure 2d). The central ampH− ligands are located on the opposite sides of the {Dy9} cluster, which direct the arrangement of polyanion 2 and stabilize the inorganic core structure. The P···P distance of the ampH− ligands is 5.6 Å, with associated O−P−O bond angles ranging between 110.0(12)° and 112.3(13)°. Interestingly, the dihedral angle of each planar triangle approximates to 60° (Figure 2e), which represents a unique windmill-type cluster. Strictly speaking, the cluster is asymmetric but has an approximate D3h point symmetry with a pseudo-C3 axis passing through the organophosphonate C and P atoms. Notably, five crystallographically independent Dy ions show two different coordination geometries: Dy(3,5) ions located on the terminal vertices display distorted squareantiprism geometry, and Dy(1,2,4) ions residing on the sharing vertices of the bottom edge have capped trigonal-prismatic geometry (Table S4). The {Dy9} and six {PW10O37} fragments assembled into a propeller-like structure, enwrapping a windmilltype cluster with a side length of ca. 2.63 nm (Figure S2). BVS analysis indicates that no bridging O atoms of the subunits are protonated, and the charge is balanced by countercations. Direct-current (dc) magnetic susceptibility measurements were performed on 1 and 2 under a 1 kOe applied field in the temperature range 2−300 K (Figure S8). The room temperature χMT product of 13.39 cm3 K mol−1 per Dy for 1 is smaller than the theoretical value (6H15/2, 14.17 cm3 K mol−1), which might be ascribed to the large energy splitting of the crystal-field sublevels; thus, it cannot be equally populated even at room temperature.15 The χMT value of 14.04 cm3 K mol−1 per Dy for 2 is close to the value expected for an isolated Dy(III). Upon cooling, the χMT value of 1 decreases, gradually reaching a value of 66.86 cm3 K mol−1, and the χMT product of 2 gradually decreases to ca. 10 K, where it sharply drops to 75.61 cm3 K mol−1, which is attributed to depopulation of the crystal-field sublevels and antiferromagnetic coupling. The molar magnetization M(H) values show a rapid increase at low fields and then gradually increase to 31.42 for 1 and 48.5 Nβ for 2 at 70 kOe (Figure S8, inset), respectively. In addition, the nonsuperposition M vs H/T for 1 and 2 suggest the presence of low-lying magnetic states arising from the crystalfield splitting or weak magnetic couplings (Figure S9). Alternating-current (ac) susceptibility measurements were carried out under zero field to explore the magnetic dynamics for 1 and 2. For 1, these data show no frequency-dependent peak, and slow relaxation of magnetization was hardly observed above

Dy2C, Dy2···Dy2A, and Dy2···Dy2C distances of 6.2016(7), 5.9113(5), 8.1863(6), and 5.5587(6) Å, respectively. Three vertices of ΔDy1Dy2Dy2C are linked by an ampH− ligand, which is located above the triangular plane. Both crystallographically independent Dy ions show square-antiprism coordination geometry: the Dy1 ion is surrounded by two bridging oxido ligands from the ampH− ligands, four terminal oxido ligands from the lacunary faces of {PW11O39}, and two terminal oxido ligands from water with Dy1−O distances of 2.302(3)−2.553(6) Å; the coordination environment of Dy2 ion is similar except that three bridging oxido ligands are replaced by aqua ligands with Dy2−O bond lengths in the range of 2.217(4)−2.551(5) Å. Notably, the aqua ligand O33 is disordered over two positions with an occupancy factor of 0.75/0.25 (O33/ampH2; Figure 1c). On the basis of bond-valence-sum (BVS) calculations,12 the oxidation states of W, Dy, and the heteroatom P are 6+, 3+, and 5+, respectively, and no bridging O atoms of the {PW11O39} subunits are protonated. Interestingly, the charge of the two [PW11O39]7− units is exactly balanced by the {Dy6(ampH)4(ampH2)(H2O)23}14+ core, making 1 a rare neutral POM without countercations.13 Polyanion 2 is composed of three anionic sandwich-type building blocks [DyIII3(CO3)(H2O)4(PW10O37)2]11− (2a) linked together by two ampH− ligands (Figure 2a). In subunit 2a (Figure 2b), the planar triangular [DyIII3(CO3)(H2O)4]7+ ({Dy3}) unit bridged by a μ3-η2:η2:η2-carbonate ligand is sandwiched by two dilacunary Keggin {A-α-PW10O37} fragments (Figure 2c), and the dihedral angles between the {Dy3} triangle and equatorial planes of the {A-α-PW10O37} fragments are 20.662(34)° and 23.057(29)°, respectively (Figure S3). Similar trinuclear Ln motifs are found in other sandwiched structures14 but encapsulated by two trilacunary Keggin anions, where the Ln···Ln distances are almost equal to 4.8 Å. The planar triangular unit in 2a is approximate to an isosceles triangle, with the vertex angle and basic angles approximating to 73° and 53° and the lengths of the “waist” and base of the triangle close to 4.6 and 5.5 Å, respectively (Figure S4). The Dy−Ocarbonate bonds range between 2.298(19) and 2.80(3) Å, and the Ocarbonate−Dy− Ocarbonate bond angles lie in the range of 51.6(11)−55.3(7)°. Alternatively, polyanion 2 can be viewed as six {PW10O37} fragments assembled around an inorganic−organic hybrid [Dy9(CO3)3(ampH)2(H2O)12]19+ ({Dy9}) cluster. The {Dy9} cluster consists of three {Dy3} units capped around two ampH− B

DOI: 10.1021/acs.inorgchem.8b00795 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

is observed at 2 K with the coercive field of ca. 500 Oe (Figure 4, inset), which further strengthens suppression of the QTM under zero field. This behavior is extremely rare in POM-based SMMs3b,18 because the broken axial symmetry will induce mixing of the mJ states, thus leading to fast QTM. Even though the symmetry of each Dy ion here is not perfect,19 it is suspicious that the QTM is suppressed by magnetic interactions between neighboring Dy ions, which highlight the advantages of assembling individual spin carriers into a cluster.20 In summary, two novel Dy-substituted POM clusters were successfully isolated by delicately controlling the pH of the reaction mixtures. 1 containing an octahedral [Dy6(ampH)4(ampH2)(H2O)23]14+ core represents a rare example of a neutral POM molecule. 2 is composed of a unique windmill-type [Dy9(CO3)3(ampH)2(H2O)9]19+ cluster with pseudo-D3h symmetry stabilized by six dilacunary [PW10O37]9− fragments, which represents the highest-nuclearity {Dy9} cluster in all Dy-containing POM chemistry. The synthesis of 1 and 2 demonstrates that conventional synthetic conditions combined with the coligand strategy can favor the formation of POMs with high-nuclearity Ln clusters, and the resulting structures can be tuned by the pH of the reactants. Additionally, 2 is a SMM without appreciable zero-field QTM, leading to an open magnetic hysteresis with a coercive field of ca. 500 Oe.

2 K (Figure S10). For 2, the frequency dependence of both the in-phase (χM′) and out-of-phase (χM′′) components indicates the presence of slow magnetic relaxation and is characteristic of SMM behavior (Figure 3).16 The relaxation times (τ) were

Figure 3. (a and b) Temperature and frequency dependencies of the out-of-phase (χM′′) products under 0 Oe for 2.

extracted by a generalized Debye model with α values of 0.17− 0.63 (Figure S14), revealing broad distributions of τ, which may due to the different local coordination environments of the crystallographically independent Dy(III) centers and magnetic interactions in 2.17 For the high-temperature regime, τ obeys the Arrhenius law as τ = τ0 exp(Ueff/kBT), corresponding to the Orbach process, and the best fit gives an effective energy barrier of Ueff/kB = 56(2) K with τ0 = 2.06 × 10−6 s (Figure 4). Upon

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00795. Syntheses, crystallographic details, structures, characterization details, and magnetic characterization (PDF) Accession Codes

CCDC 1590468−1590469 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Figure 4. Plots of ln(τ) versus T−1 for 2 under zero dc field. The red and black lines show the results of Arrhenius and dual-process fittings for the data, respectively. Inset: Expanded view clearly revealing an open magnetic hysteresis for 2 at 2 K.

ORCID

Yan-Cong Chen: 0000-0001-5047-3445 Jun-Liang Liu: 0000-0002-5811-6300 Ming-Liang Tong: 0000-0003-4725-0798

lowering of the temperature, τ gradually diverge from the Arrhenius law, which cannot be analyzed with only the Raman/ Orbach process (Figure S15). The best fit is obtained with the sum of the Orbach and Raman processes, typically as τ−1 = τ0−1 exp(−Ueff/kBT) + CTn, where C and n are the parameters for the Raman process. The fit gives Ueff/kB = 56(3) K and τ0 = 2.43 × 10−6 s, with the Raman parameters of C = 0.04 s−1 K−n and n = 3.9(2). Moreover, the frequency dependence of the ac susceptibility was also measured at 2 K in different dc fields. Interestingly, the longest relaxation time for 2 is found at zero field, in contrast to those under applied fields (Figure S13), suggesting that the quantum tunneling of magnetization (QTM) is largely blocked at zero field. Additionally, an observable magnetization hysteresis

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Project (Grant 2014CB845602) and National Science Foundation of China (Grants 91422302 and 21620102002).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b00795 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

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DOI: 10.1021/acs.inorgchem.8b00795 Inorg. Chem. XXXX, XXX, XXX−XXX