Polyoxometalates as advanced-performance anions for ∼D_5h Dy(III) single-ion magnets

Abstract

We enhance single-ion magnet (SIM) magnetisation reversal barriers by engineering the second coordination sphere, substituting conventional small monoanions with a bulky polyoxometalate (POM) trianion. Importantly, our approach serves as a model for creating new high-performance multifunctional hybrid materials.

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Single-molecule magnets (SMMs) exhibit slow relaxation and retention of their magnetisation in the absence of a magnetic field.^1–3 Monometallic SMMs (or Single-Ion Magnets (SIMs)) can be designed by controlling the metal ion coordination environment to maximise the axial magnetic anisotropy.^4–6 The complexes with the highest energy barriers to magnetisation reversal (U_eff) and blocking temperatures contain lanthanide ions, in particular Dy(III).^7–10 Metallocene-based complexes have the highest blocking temperatures, although from a synthetic viewpoint the need for dry and inert atmospheres imposes more challenges on their crystal field engineering.^9,11–13 Therefore, when looking to enhance the performance of SIMs through synthetic modification, complex stability and synthetic flexibility are also highly desirable properties. The functionalisation and substitution of coordinating ligands have been shown to drastically change the magnetic properties of SIMs.^14,15 However, due to the fundamental importance of the lanthanide crystal field, changes to peripheral ligands, anions and non-coordinating solvents/ligands have a significant effect on the magnetic properties of SIMs.^16–19

Previous work, in collaboration with Rajaraman's group, highlighted the potential to target the second coordination sphere to increase SIM magnetisation reversal barriers (U_eff).¹⁹ This was done by using in silico models to remove the chloride anions, non-coordinating ligands and solvent from the crystal lattice of the pseudo-D_5h SIM [Dy(H₂O^eq)₅(HMPA^ax)₂]Cl₃·HMPA·H₂O {eq = equatorial; ax = axial; HMPA = hexamethylphosphoramide}.¹⁹ The chloride anions are located in the ∼D_5h equatorial plane, hydrogen-bonded to the equatorial H₂O ligands and hence, they contribute to the transverse crystal field. In other words, they have a detrimental effect on the magnetic properties. Clearly, complete removal of the anions is impossible synthetically due to the disruption of the charge balance. Therefore, our idea was to substitute the three small chloride anions for a large, bulky trianionic polyoxometalate (POM) anion, to target (i) distancing the negative charge from the lanthanide centre; (ii) moving the negative charge out of the equatorial plane; (iii) increasing the distance between lanthanide centres and (iv) exploring the connection between the SIM properties and the surrounding environment.²⁰ Importantly, although the incorporation of polyoxometalates as anions has been proposed previously, all published compounds exclusively exhibit no appreciable U_eff barrier under zero applied magnetic field.²¹

Our first efforts to do this using our [Dy(H₂O)₅(HMPA)₂]³⁺ complexes and phosphotungstic acid were hampered by synthetic issues, where we obtained either structural data that we couldn't fully refine or complexes with coordination of more than two HMPA ligands. Undeterred, we investigated a similar pseudo-D_5h complex containing axial P=O ligands [Dy(H₂O^eq)₅(Cy₃PO^ax)₂]³⁺ {Cy₃PO = tricyclohexylphosphine oxide}.²² We found that this complex, which contains a bulkier axial ligand, was stable in the presence of a POM anion and herein we report, for the first time, the synthesis, structural and magnetic characterisation of [Dy(H₂O)₅(Cy₃PO)₂][W₁₂PO₄₀]·2(Cy₃PO)·5THF·H₂O (1). Importantly, we show that this new air-stable compound, containing a trianionic α-Keggin polyoxometalate, not only has the largest U_eff value in the [Dy(H₂O)₅(Cy₃PO)₂]³⁺ family, but our approach opens the door to creating high-performance multifunctional hybrid compounds.

Compound 1 was synthesised using [Dy(H₂O)₅(Cy₃PO)₂](CF₃SO₃)₃·2(Cy₃PO) (P1) as the precursor in an anion substitution reaction.¹⁸ A THF solution of P1 along with H₃[W₁₂PO₄₀], dissolved in a small amount of water, was heated and after workup, vapour diffusion using Et₂O yielded crystals of [Dy(H₂O)₅(Cy₃PO)₂][W₁₂PO₄₀]·2(Cy₃PO)·5THF·H₂O (1) where the three triflate anions in P1 are replaced by a bulkier trianionic polyoxometalate (POM) anion (see ESI for synthesis and characterisation (Fig. S1)†). Importantly, we are the first to co-crystallise a high performance ∼D_5h Dy(III) SIM with a POM anion whilst keeping the {Dy(H₂O^eq)₅(L^ax)₂} unit intact. Compound 1 crystallises in the triclinic space group P1̄ (Fig. 1). The Dy(III) centre exhibits a pentagonal bipyramidal geometry (∼D_5h) as confirmed by continuous shape measures analysis (CShM = 0.204).²³ The axial Dy–O bond lengths are 2.208(9) and 2.210(10) Å, with the equatorial Dy–O bonds averaging 2.362(10) Å. The axial O–Dy–O angle of 176.3° and average equatorial O–Dy–O angle of 72.05° are close to the ideal values of 180° and 72°, respectively. The equatorial plane of [Dy(H₂O)₅(Cy₃PO)₂]³⁺ is hydrogen bonded to four THF and one H₂O lattice solvents, two co-crystallised Cy₃PO molecules and two [W₁₂PO₄₀]³⁻ anions (Fig. S2†). By replacing three small anions in P1 with the large bulky POM trianion, the shortest intermolecular Dy⋯Dy distance increases from 12.6 Å in P1 to 14.9 Å in 1, leading to a better isolation of Dy(III) centres. The centre of the closest POM trianion lies 27.87° out of the {Dy(H₂O)₅} equatorial plane and 9.83 Å away from the Dy(III) centre (see Fig. 1).

Fig. 1 The relative position of the POM anions to the Dy complex in [Dy(H₂O)₅(Cy₃PO)₂][W₁₂PO₄₀] with solvent, co-crystalised Cy₃PO and H atoms omitted for clarity (only one POM anion is present within the asymmetric unit). C, grey; Dy, cyan; O, red; P, orange; W, blue.

The magnetic properties of 1 constrained in eicosane were measured in an applied dc field of 1000 Oe from 280–2 K (Fig. S3†). Upon cooling χ_MT decreases steadily from 13.8 cm³ mol⁻¹ K to 12.4 cm³ mol⁻¹ K at 7 K, which precedes a sharp drop to 7.8 cm³ mol⁻¹ K at 2 K. The χ_MT value at 280 K is slightly lower than expected for a free Dy(III) ion (14.17 cm³ mol⁻¹ K), which can be attributed to ligand field effects.^24,25 The sharp decrease in χ_MT at low temperatures is characteristic of a large magnetic anisotropy.¹⁹ The zero-field cooled and field cooled magnetic susceptibility of 1 diverges at 8 K, with a clear maximum observed at 4.5 K (Fig. S5†). Hysteresis measurements at a sweep rate of 200 Oe s⁻¹ show open, waist-restricted, loops at zero field. This is a common feature amongst ∼D_5h SIMs due to the presence of unsuppressed quantum tunnelling of the magnetisation (QTM), which arises due to distortions from D_5h symmetry, introducing non-axial crystal field terms.^26–28 Compound 1 exhibits a blocking temperature (T_B(Hyst)) of 12 K (Fig. 2).

Fig. 2 Magnetic hysteresis measurements for 1 with a sweep rate of 200 Oe s⁻¹.

Alternating current (ac) magnetic susceptibility measurements show well-defined maxima in vs. T up to 35 K for 1 (Fig. S6†). Large increases in are observed at low temperatures, further indicating efficient QTM, which has been observed for a number of pseudo-D_5h complexes of general formula [Dy(H₂O^eq)₅(L^ax)₂]³⁺ (Fig. 3).^18,19,27 The relaxation times τ were determined using the Cole–Cole plots of vs. via CCFIT2, using a generalized Debye model^29,30 (see Fig. S8 and S11†). In plotting τ⁻¹vs. temperature, relaxation parameters were obtained by fitting the data using the equation for 1, at zero applied field (Fig. 4) and under a 1000 Oe applied field (see Fig. S11†).

Fig. 3 Frequency dependence of the out-of-phase magnetic susceptibility, under zero dc field, for 1 from 35–3 K.

Fig. 4 Temperature dependence of the relaxation rate for 1 without an applied dc field. The solid line represents the best fit to Orbach, Raman and QTM relaxation (see text for details). Black vertical bars are estimated standard deviations in the relaxation times derived from Debye fits according to ref. 29.

Under zero applied dc field, the τ values for 1 were fitted over the temperature range 3–35 K giving U_eff = 625(1) K, τ₀ = 4.6(5) × 10⁻¹² s, C = 2.9(13) × 10⁻⁴ K⁻ⁿ s⁻¹, n = 3.4(1) and τ_QTM⁻¹ = 8.9(6) s⁻¹. The Orbach parameters obtained are consistent with those obtained under a 1000 Oe applied field where U_eff = 621(6) K, τ₀ = 5.0(8) × 10⁻¹² s (Fig. S11†). The U_eff observed for 1 (625 K) is the highest magnetisation reversal barrier observed within the [Dy(H₂O)₅(Cy₃PO)₂]³⁺ family (see Table 1) indicating that the incorporation of the polyoxometalate has a significant effect on the magnetic behaviour. At this time, we are unable to carry out CASSCF calculations on 1 due to the large size and nature of this system, as also noted recently by Kong et al.²¹

Table 1 A comparison of U_eff, τ₀ and T_B(Hyst) for 1 and other [Dy(H₂O)₅(Cy₃PO)₂]³⁺ SIMs

Complex	U eff (K)	τ 0 (s)	T B(Hyst) (K) (200 Oe s^−1)
[Dy(Cy3PO)2(H2O)5]Cl3 (Cy3PO)·H2O·EtOH C1 ^21	472(7)	8.7 × 10^−12	11
[Dy(Cy3PO)2(H2O)5]Br3 ·2(Cy3PO)·2H2O·2EtOH C2 ^21	543(2)	2.0 × 10^−11	20
[Dy(H2O)5(Cy3PO)2](CF3SO3)3 ·2(Cy3PO) P1 ^18	562(7)	1.7(5) × 10^−11	n/a
1 This work	625(1)	4.6(5) × 10^−12	12

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In order to investigate the effects of incorporating the POM anion, 1 has been compared to other [Dy(H₂O)₅(Cy₃PO)₂]X₃ compounds, with different axial O––O angles and pentagonal bipyramidal CShM values (see Table 2). Between all complexes, the axial and equatorial Dy–ligand bonds vary by only 0.032 Å and 0.010 Å, respectively, allowing us to disregard this as a reason for the increase in U_eff. A comparison of 1 to compound C1, which has similar axial O––O angles (176.3° vs. 175.8°) and CShM values (0.204 vs. 0.239), allows us to focus on the large increase in the Dy–anion distance (4.59 vs. 9.83 Å) and the increased {Dy(H₂O)₅} plane–anion angle (4.22 vs. 27.87°). These increases will result in a decrease in the transverse field contribution¹⁹ by the POM anion and are accompanied by a large increase in U_eff (of 153 K). Whilst a change in hydrogen bonding is expected to alter the relaxation dynamics, rationalisation of the effects is not trivial. A full comparison with the other compounds offers no correlation with U_eff or T_B(Hyst) and can be found in Table S2.†^19,31 We note that an increase in U_eff has also been observed upon diamagnetic dilution in another ∼D_5h system.³² Remarkably, the large U_eff observed in 1 exceeds that of C2 (543 K) which has near linear axial O––O angle of 179° and a CShM of 0.142. However, the large U_eff associated with 1 does not translate to the persistence of hysteresis loops above temperatures reported for C2, highlighting the necessity to consider through the barrier mechanisms such as Raman and QTM, noting that the L^ax–Dy–L^ax linearity is an integral factor in designing high performance SIMs.^6,33

Table 2 A comparison of selected bond lengths, angles and CshMs values for 1 and other [Dy(H₂O)₅(Cy₃PO)₂]³⁺ SIMs in the literature

Compound	Dy–Oax (Å)	Average Dy–Oeq (Å)	CShM PBPY-7 (D5h)	O– –O angle (°)	Dy⋯Dy intermolecular distance (Å)	Dy–anion distance (Å)	{Dy(H2O)5} plane–anion angle (°)
a S atoms of the triflate anion used. b Centre of the polyoxometalate anion used.
[Dy(Cy3PO)2(H2O)5]Cl3·(Cy3PO)·H2O·EtOH C1 ^21	2.217(4), 2.221(4)	2.359(5)	0.239	175.8(1)	8.420	4.59	4.22
[Dy(Cy3PO)2(H2O)5]Br3·2(Cy3PO)·2H2O·2EtOH C2 ^21	2.189(3), 2.210(3)	2.352(3)	0.142	179.0(1)	11.23	4.77	4.42
[Dy(H2O)5(Cy3PO)2](CF3SO3)3·2(Cy3PO) P1 ^18	2.202(2), 2.203(2)	2.362(3)	0.639	173.4(1)	12.64	5.38^a	1.48^a
1 This work	2.208(9), 2.210(10)	2.362(10)	0.204	176.3(4)	14.89	9.83^b	27.87^b

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When measuring slow magnetic relaxation, the primary method for the separation of Ln(III) SIMs focusses on doping the paramagnetic complex into a diamagnetic host. Here we demonstrate an alternative strategy for designing better separated Dy(III) systems by focussing on the anions present. Importantly, this approach may be especially relevant when doping into a diamagnetic host is too complicated or even impossible. By engineering the secondary coordination sphere of [Dy(H₂O)₅(Cy₃PO)₂]³⁺, replacing three small anionic ligands with a large bulky trianion, we have successfully implemented the design criteria previously laid out,¹⁹ to improve the U_eff energy barrier in this ∼D_5h system. Specifically, by incorporating a polyoxometalate anion we have designed a new hybrid compound achieving both the largest U_eff barrier and T_B(Hyst) of any POM containing SIM. Several compounds have been synthesised in recent years with POMs as anions (see Table S3†) or ligands (see Table S4†). However, the landmark compound 1 exhibits a U_eff over six times larger than any compound containing a POM anion, which opens up the possibility of new high-performance hybrid compounds. Considering the numerous applications and fascinating properties of POMs, this research also opens the door to a vast array of new multifunctional materials.

Data availability

Crystallographic data for 1 have been deposited under CCDC 2378028† and can be obtained from https://www.ccdc.cam.ac.uk/structures/.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank EPSRC UK (EP/T517896/1) for funding and the EPSRC UK National Crystallography Service at the University of Southampton for the collection of the crystallographic data.³⁴ We thank Professor Gopalan Rajaraman for helpful discussions. For the purpose of open access, the authors have applied a Creative Commons Attribution (CC BY) licence to any Author Accepted Manuscript version arising from this submission.

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, crystallographic details and magnetic studies. CCDC 2378028. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt02877e

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