There is nothing wrong with being soft: using sulfur ligands to increase axiality in a Dy(III) single-ion magnet

Abstract

A new air-stable sulfur-ligated Dy(III) single-ion magnet has been successfully isolated with U_eff = 638 K and hysteresis loops open up to 7 K. In silico studies show that the S co-ligands significantly boost the axiality and that Te- and Se-donors have the potential to further enhance the magnetic properties.

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The interest in Single-Molecule Magnets (SMMs), i.e. molecular systems, which display the ability to block magnetisation, resulting in the appearance of magnetic hysteresis of molecular origin,¹ is because these systems could revolutionise electron spin-based technologies.² Lanthanide SMMs are often associated with large magnetic moments and large magnetic anisotropy, with Dy(III) being a key component of single-ion magnets (SIMs).³ Through the combination of theory⁴ and experiment, an exciting era has emerged with a new generation of SMMs/SIMs showing impressive energy barriers⁵ and high blocking temperatures, T_B,⁶ reaching 80 K.⁷ In fact, complexes with D_3h,⁸D_4d⁹ and D_5h,¹⁰ axial point group symmetries are very promising. Our group recently reported the blueprint for generating strong uniaxial magnetic anisotropy for Dy(III) in ∼D_6h symmetry (hexagonal bipyramidal), by combining a strong linear axial ligand field with a weak equatorial ligand field, by using a polydentate ligand L^N6 (L^N6 = N6-hexagonal plane from the neutral Schiff base ligand formed from 2,6-diacetylpyridine and ethylenediamine).¹¹

Most of the reported 4f-SIMs incorporate traditional ligands with O-, N-, C- and halogen-donor atoms.³ Rarer are the examples of SMMs exploring ligands with more “exotic” donor atoms from the main group, as was reviewed recently by Guo et al.¹² Recently we have explored how ligand electronics can tune SIM properties¹³ and herein, we sought to explore how different donor groups originating from the p block affect the magnetisation dynamics. We report for the first time the synthesis, structure, magnetic characterisation and ab initio studies of [Dy^IIIL^ON3(C₅H₁₀NS₂)₂]·0.5THF (1) (Fig. 1) which is a S-ligated single-ion magnet with hysteresis loops open up to 7 K and a magnetisation reversal barrier of U_eff = 638 K, which is unprecedented in the very small family of S-ligated Dy(III) single-ion magnets (see Table 1).^14–20 Compound 1 was isolated by using the cage-like ligand N-(3,5-di-tert-butyl-2-hydroxybenzyl)-N,N-bis(2-pyridylmethyl)amine (HL^ON3) (see ESI†).²¹

Fig. 1 Synthesis and structure of 1. Dy, gold; O, red; N, blue; S, brown; C, grey. H atoms and solvent are omitted for clarity.

Table 1 The small family of Dy(III) SIMs/SMMs with S-based ligands

Complex	U eff (K)	τ 0 (s)	H dc (Oe)	Ref.
[Dy^III(Pc)(STBPP)]	194	4.7 × 10^−8	0	14
[(dtc)3Dy(dmbipy)]	9.8	1.9 × 10^−6	1000	15
[(dtc)3Dy(phen)]	20	1.80 × 10^−8	400	16
[(dbm)2Dy(dtc)(phen)]	45	2.8 × 10^−8	1000	16
	40	4.1 × 10^−8	0
[Dy((−)-pbipy)(pdtc)3]	56.7	4.2 × 10^−7	3000	17
Dy2S@C82-C3v	6.5	3.6		18
	48	6.2 × 10^−4
	1232	6.0 × 10^−13
[Li(thf)4][Dy4{N(SiMe3)2}4(μ-SEt)8(μ4-SEt)]	66	4.3 × 10^−6	0	19
	71	2.1 × 10^−6	2000
[{Cp2′Dy(μ-SSiPh3)}2]	192	2.38 × 10^−7	0	20
[Dy ^III L ^ON3 (C 5 H 10 NS 2 ) 2 ]	638	2.99 × 10 ^−12	0	This work

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Our synthetic strategy generates one short Dy–O bond, to direct the magnetic anisotropy,²² and three longer Dy–N bonds.²³ The rest of the coordination sphere is completed with soft S-donor groups, by using diethyldithiocarbamate co-ligands, (Fig. 1) giving longer Dy–S bonds (Fig. S1 and Table S2, ESI†). Importantly, through a detailed in silico study we also examine how O-, Se- and Te-based co-ligands affect the calculated magnetisation reversal barrier, U_cal, of 1.

Complex 1 (Fig. 1) was isolated from a dry THF solution (see ESI†) with the phase purity of the bulk sample confirmed by powder X-ray diffraction (Fig. S2, ESI†). Continuous shape measures²⁴ analysis gives a value of 2.7 for a biaugmented trigonal prism geometry (C_2v symmetry) (Fig. S3 and Table S3, ESI†). The only strong oxygen donor group in 1 has a relatively short Dy–O bond length (2.1591(16) Å) while the Dy–N and Dy–S bonds fall in the range of 2.5237(18)–2.5711(17) Å and 2.8133(5)–2.9647(6) Å, respectively (Table S2 and Fig. S1, ESI†). Additionally, there is no intermolecular hydrogen bonding in 1, while the shortest Dy⋯Dy distance is 8.39 Å (Fig. S4, ESI†).

The dc magnetic susceptibility and magnetisation measurements for 1 are shown in Fig. S5 (ESI†). The field cooled (FC) and zero-field cooled (ZFC) magnetic susceptibility show divergence at 5 K (Fig. S6, ESI†) with the magnetic hysteresis measurements, M(H) loops, performed on a powder sample of 1, remaining open up to 7 K (average sweep rate of 20 mT s⁻¹) (Fig. 2 upper and Fig. S7, ESI†).

Fig. 2 (upper) Powder magnetic hysteresis measurements for 1 with an average sweep rate of 20 mT s⁻¹. (upper inset) M(H) loops open up to 7 K for 1. (lower) Plots of χ_M′′(ν) in zero applied dc field in the temperature range of 2–40 K for 1.

Measurements of the variable temperature alternating current (ac) susceptibility between 1–940 Hz were performed in order to investigate the dynamics of the magnetisation for 1 (Fig. 2 lower and Fig. S8–S16, ESI†). Under zero external dc field, the out-of-phase, χ_M′′ magnetic susceptibility data exhibit well-defined maxima with χ_M′′ peaks clearly observable up to ∼35 K (Fig. S8, ESI†), indicating a high magnetisation reversal barrier. The relaxation times, τ, were extracted from the fits of the Argand plots of χ_M′′ vs. χ_M′ using the generalized Debye model (Fig. S11, ESI†).²⁵ The α parameters found are in the range of 0–0.3 (2–40 K) for 1. The τ⁻¹vs. T data were fitted using the equation τ⁻¹ = τ_QTM⁻¹ + CTⁿ + τ₀⁻¹ exp(−U_eff/T), in which C and n are parameters of the Raman process and τ_QTM is the rate of the quantum tunnelling of magnetisation (QTM).²⁶ The best fit gives a magnetisation reversal barrier, U_eff of 638 K, τ₀ = 2.99 × 10⁻¹² s, n = 3.24, C = 0.02 K⁻ⁿ s⁻¹, τ_Qτ_M = 0.017 s, under zero dc field (Fig. S17, ESI†) and U_eff = 656 K, τ₀ = 1.94 × 10⁻¹² s, n = 3.96 and C = 3.95 × 10⁻⁵ K⁻ⁿ s⁻¹ under an optimum field of 1200 Oe (Fig. S18, ESI†). The observed values of the pre-factor τ₀,¹⁰C and n are within the commonly observed range for Dy(III) SMMs.³ The exponent n of the Raman process has a smaller value than expected for a Kramers ion (n = 9) suggesting the presence of Raman processes involving optical acoustic phonons.²⁶ To the best of our knowledge, this is the largest magnetisation reversal barrier observed for a Dy(III) single-ion magnet that has S-donor ligands (see Table 1).

In order to gain insight into the mechanism that governs the magnetic relaxation of 1, we have performed ab initio calculations using the CASSCF/RASSI-SO/SINGLE_ANISO approach implemented in MOLCAS 8.2²⁷ (see ESI†). The eight Kramers Doublets (KDs) in 1 span an energy range of 964 K (Table S4, ESI†). The ground state (m_J = ±15/2) of the Dy(III) ion in 1 is highly anisotropic with near-perfect axiality (g_zz = 19.859, g_xx = g_yy = 0.001, Table S4, ESI†). The main anisotropy axis is nearly collinear with the relatively short Dy–O bond (Fig. S19, ESI†) resulting from our synthetic strategy. Using the CASSCF wavefunction, the computed Loprop²⁸ charge on the oxygen atom is found to be nearly three times larger than the nitrogen atoms of the L^ON3 ligand and twice as large as the sulfur atoms of the diethyldithiocarbamate ligands (Fig. S20, ESI†). The axial nature is also observed for the first and second excited states (m_J = ±13/2, g_xx = 0.023, g_yy = 0.028, g_zz = 17.359 and m_J = ± 11/2, g_xx = 0.281 g_yy = 0.380, g_zz = 14.372, Table S4, ESI†), with the higher KDs showing relatively stronger admixtures (Fig. 3). The maximum calculated relaxation barrier, U_cal, is estimated at 651 K (Fig. 3), which is in excellent agreement with the experimentally determined magnetisation reversal barrier (U_eff) of 638 K found in zero applied dc field. A relatively small transverse magnetic moment is calculated for the first three KDs (0.35 × 10⁻³, 0.88 × 10⁻², 1.2 × 10⁻¹μ_B, respectively), which indicates relaxation via the third excited state (Fig. 3). In addition, the Orbach processes for the m_J and m_J + 1 excited states of opposite magnetisation between the first four KDs are found to be very small (≤0.43 μ_B, Fig. 3).

Fig. 3 Ab initio calculated relaxation dynamics for 1. The arrows show the connected energy states with the number representing the matrix element of the transverse moment (see text for details). The black line indicates the KDs as a function of magnetic moments. The red dashed arrow represents QTM (QTM = quantum tunnelling of the magnetisation) via the ground state and TA-QTM (TA-QTM = thermally assisted QTM) via excited states. The blue dashed arrow indicates possible Orbach processes. The pink thick arrow indicates the mechanism of magnetic relaxation. The numbers above each arrow represent corresponding transverse matrix elements for the transition magnetic moments.²⁹

To investigate the importance of the coordination environment and the ligand electronics on the magnetisation dynamics of 1, we have changed the co-ligand coordination environment in silico. We have created a family of three different model systems and used ab initio calculations to examine how O-, Te- and Se-based co-ligands affect the calculated magnetisation reversal barrier of 1 (Fig. 4 and Fig. S21, S22, ESI†). Importantly, replacing the S-atoms of the diethyldithiocarbamate co-ligands with more commonly used oxygen donors (i.e. common carboxylate ligands, model 1-O, Fig. S22 upper, ESI†) gives stronger transverse components, with larger g_xx/g_yy values obtained for the ground and excited states (see Table S5 (ESI†) for model 1-O). Specifically, the QTM probabilities are calculated to be larger for the first three KDs of model 1-O (0.63 × 10⁻³, 0.31 × 10⁻¹ and 0.85 μ_B see Fig. S21 upper, ESI†) compared to 1 (Fig. 3), leading to a smaller calculated barrier of U_cal = 528 K (see Fig. 4 and Fig. S21, Table S5, ESI†). These results are similar to earlier observations for Dy–O vs. Dy–S substitution¹⁴ suggesting a likely generality of such behaviour in Dy(III) complexes.

Fig. 4 The effect of the O-, S-, Te- and Se-donor co-ligands on the U_cal barrier using in silico models based on 1.

In contrast, the g_xx/g_yy values obtained for model systems 1-Te and 1-Se, where the S-atoms in 1 are replaced with Te- and Se-atoms (Fig. S22 lower, ESI†), suggest that the magnetisation relaxes via the fourth KD, giving higher calculated barriers of U_cal = 718 K for 1-Te and 752 K for 1-Se (see Fig. 4 and Fig. S21, Table S5, ESI†). Importantly, our results suggest that substitution of the S-atoms in 1 with O-atoms favours a stronger transverse anisotropy, while substitution with Te- and/or Se-atoms stabilises stronger axiality, with smaller transverse magnetic moments calculated for the first four KDs and smaller g_xx/g_yy values (see Fig. 4 and Fig. S21, Table S5, ESI†). This is in excellent agreement with a study performed on pnictogen-ligated compounds.³⁰ The ratio between B⁰₂ and the corresponding non-axial crystal field parameters increases in the following order 1-O < 1 < 1-Te < 1-Se (Table S7, ESI†) in line with the increasing U_cal barrier (Fig. 4).

In conclusion, [Dy^IIIL^ON3(C₅H₁₀NS₂)₂]·0.5THF (1) is the first S-ligated single-ion magnet with hysteresis loops open up to 7 K and a magnetisation reversal barrier of 638 K, which is significantly higher than any reported Dy(III) SIM that has S-donor ligands (Table 1). This novel complex was isolated by a carefully designed synthetic strategy that generates one short Dy–O bond,²² which directs the magnetic anisotropy, combined with three longer Dy–N bonds, with the remainder of the coordination sphere completed with soft S-donor groups, giving longer Dy–S bonds. Furthermore, through detailed in silico studies we examine how O-, Se- and Te-based co-ligands affect the calculated magnetisation reversal barrier and magnetisation dynamics in 1, finding higher U_cal values for Te- and Se-based co-ligands. We hope that this study will generate further interest in the investigation of S-ligated SIMs and prompt the study of new Te- and Se-ligated SIMs.

We thank EPSRC UK (EP/N01331X/1, EP/M000923/1); UGC (SRF fellowship); UGC-UKIERI (184-1/2018(lC)); UGC and SERB (CRG/2018/000430); CSIR (01(2980)/19/EMR-II) and IIT Bombay (CRAY supercomputing facility) for funding.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, magnetic studies, crystallographic details, ab initio studies. CCDC 1950766. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9cc07292f

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