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. 2021 Aug 23;60(17):12719–12723. doi: 10.1021/acs.inorgchem.1c01905

Holmium(III) Single-Ion Magnet for Cryomagnetic Refrigeration Based on an MRI Contrast Agent Derivative

Borja Rodríguez-Barea ‡, Júlia Mayans ‡, Renato Rabelo ‡, Adrián Sanchis-Perucho ‡, Nicolás Moliner ‡, José Martínez-Lillo ‡,*, Miguel Julve ‡, Francesc Lloret ‡, Rafael Ruiz-García ‡, Joan Cano ‡,*
PMCID: PMC8424628  PMID: 34424680

Abstract

graphic file with name ic1c01905_0004.jpg

The coexistence of field-induced blockage of the magnetization and significant magnetocaloric effects in the low-temperature region occurs in a mononuclear holmium(III) diethylenetriamine-N,N,N′,N″,N″-pentaacetate complex, whose gadolinium(III) analogue is a commercial MRI contrast agent. Both properties make it a suitable candidate for cryogenic magnetic refrigeration, thus enlarging the variety of applications of this simple class of multifunctional molecular nanomagnets.

Short abstract

A mononuclear holmium(III) complex is proposed as a new prototype of a multifunctional molecular nanomagnet for the physical implementation of cryomagnetic refrigeration near the strategically relevant hydrogen liquefaction temperature. Significant magnetocaloric effects occur in a wide temperature range just above the field-induced magnetization blockage.

Mononuclear complexes of lanthanide(III) ions exhibiting slow relaxation of the magnetization, so-called lanthanide single-ion magnets (SIMs), have attracted much attention in diverse areas of nanoscience and nanotechnology.1−7 Because of their well-known magnetothermal and quantum coherence properties, mononuclear gadolinium(III), dysprosium(III), and holmium(III) derivatives are illustrative examples of the promising applications of Ln SIMs as both molecular cryomagnetic coolants and qubits for cryogenic magnetic refrigeration (CMR) and quantum information processing technologies.8−11 In the search for molecular materials that exhibit various physical properties linked to technological applications, choosing a starting molecule with one of these properties should be a good strategy.

In this context, we have turned our eyes toward the well-known family of mononuclear complexes with LnIII ions (Ln = Gd, Dy, and Ho) and either linear or cyclic polyaminocarboxylate ligands, widely investigated as magnetic resonance imaging (MRI) contrast agents (Scheme S1).12,13 Herein we report our first results along this line covering the synthesis, structural, and general physicochemical characterization of the novel holmium(III) diethylenetriamine-N,N,N′,N″,N″-pentaacetate (DTPA) complex of the formula Na2[HoIII(DTPA)(H2O)]·8H2O (1), as well as a preliminary investigation of the magnetic field dependence of its magnetic and magnetothermal properties through direct-current (dc) and alternating-current (ac) magnetic measurements. Compound 1 constitutes a unique example of HoIII SIM that would operate as a cryomagnetic coolant close enough to the strategically relevant hydrogen liquefaction temperature.

1 was prepared from the stoichiometric reaction of H5DTPA and holmium(III) oxide in water under reflux for several hours after neutralization with NaHCO3 until pH ca. 6.5 (see the experimental section and Figures S1–S3). X-ray-suitable pale-pink prisms were obtained after several weeks of slow evaporation at room temperature. It crystallizes in the P21/n space group of the monoclinic system (Table S1). The asymmetric unit consists of two sodium(I)-bridged dianionic mononuclear [HoIII(DTPA)(H2O)]2– units, as shown in Figure 1. The two crystallographically independent nine-coordinate Ho1 and Ho2 atoms exhibit similar spherically distorted geometries intermediate between a tricapped trigonal prism (TCTPR) and a monocapped square antiprism (CSAPR) of D3h and C4v symmetry (Scheme S2 and Table S2).14,15 The values of the shape measures for the Ho1/Ho2 atoms are S(TCTPR) = 1.150/1.000 and S(CSAPR) = 1.083/1.089 (S = 0 for a perfect match with the ideal polyhedron), supporting the previous conclusion.16

Figure 1.

Figure 1

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Perspective view of the diholmium(III) entity in 1 (a) showing the double sodium(I)-bridged mononuclear units with the atom numbering scheme. The remaining Na and O atoms from the sodium(I)-coordinated and hydrogen-bonded crystallization water molecules are omitted for clarity. Solid lines represent the Na–O bonds. Views of the metal coordination polyhedra for the two crystallographically independent Ho1 (b) and Ho2 (c) atoms.

The diholmium(III) entities are further connected through the six-coordinate (Na1 and Na2) and five-coordinate (Na3) Na atoms to give a carboxylate/aqua-bridged sodium(I)–holmium(III) layer of mixed square-decagonal topology growing within the crystallographic ab plane (Figure S4a). The remaining six-coordinate Na4 atoms act as additional bridges between adjacent heterobimetallic NaIHoIII layers to give a pillared-layer three-dimensional (3D) network (Figure S4b), featuring small rectangular pores filled by most of the hydrogen-bonded crystallization water molecules (Figure S5). The Ho1···Ho2 separation (r) through the Na1 and Na2 bridges is 6.414 Å, while the shortest intra- and interlayer Ho1···Ho1I and Ho2···Ho2II separations are 8.190 and 10.074 Å (symmetry codes: I, −x + 1/2, −y + 1/2, −z + 1/2; II, −x, −y, −z).

The static magnetic properties of 1 have been investigated by variable-temperature (2–300 K) molar dc magnetic susceptibility (χM) measurements and variable-temperature (2–20 K) and variable-field (0–8 T) molar magnetization (M) measurements (see the experimental section). The χMT versus T and M versus H/T plots (Figure S6) are typical of a highly anisotropic 5I8 ground state (J = 8 with S = 2 and L = 6) resulting from the large first-order spin–orbit coupling in HoIII ions.17

At room temperature, χMT for 1 is 14.40 cm3 mol–1 K. This value is close to that calculated for one HoIII ion (14.07 cm3 mol–1 K).15 Upon cooling, χMT continuously decreases to 5.84 cm3 mol–1 K at 2.0 K (Figure S6a). Ground 5I8 and first excited 5I7 states are well-separated (ΔE = −8λ with λ0 = −541 cm–1) so that the former is the only populated below room temperature.17 The large deviations of the experimental magnetic susceptibility data from the Curie law are then attributed to the action of the ligand-field (LF) potential on the 5I8 ground state (see the Computational details).18,19 This LF effect causes the splitting of their 17 components into one singlet (mJ = 0) and eight Kramers doublets (mJ from ±1 to ±8), which is ultimately responsible for the high local magnetic anisotropy of the HoIII ion. This conclusion is further supported by the isothermal magnetization curves in the temperature range 2–20 K (Figure S6b). Hence, the M value at T = 2 K for H = 8 T (6.08 Nβ) is well below the calculated value of the magnetization saturation for one HoIII ion (10 Nβ).17 Moreover, the isothermal magnetization curves do not superpose and largely deviate from the corresponding Brillouin function for a heptadecet state (solid line in Figure S6b).17

The dynamic magnetic properties of 1 have been investigated by variable-temperature (2–7 K) and variable-field (0–1 T) in-phase (χM′) and out-of-phase (χM″) molar ac magnetic susceptibility measurements in the frequency range 1–10 kHz (see the experimental section). The χM′ and χM″ versus T and H plots reveal a large temperature and field dependence of the spin dynamics (Figures 2 and S7 and S8).

Figure 2.

Figure 2

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3D contour color map for the temperature and field dependence of χM″ for 1 at 10 kHz of the ±5 Oe oscillating field in the magnetic field and temperature ranges 0–1 T and 2–7 K.

In the absence of a dc magnetic field (H = 0), there is an incipient χM″ signal with no χM′ maximum at 10 kHz (Figure S7). However, when a relatively small dc magnetic field is applied, χM″ shows maxima in the temperature range 2–4 K, with 0.25 T being the optimal working magnetic field (Figure 2). Likewise, a maximum appears in the χM″ versus H plots that broadens below 4 K (Figure S8b), showing the appearance of a second competing relaxation process that is clearly noticeable only at 2 K (H ≈ 0.2 T). These field-induced slow magnetic relaxation (SMR) effects are typical of Ln SIMs, as was earlier reported for the related complexes of the formulas Na[GdIII(EDTA)(H2O)3]·5H2O and Na[DyIII(DOTA)(H2O)]·4H2O (Scheme S1).20−22

To confirm the occurrence of field-induced SIM behavior, the influence of the frequency on the spin dynamics was then investigated at the optimum applied dc magnetic field. The χM′ and χM″ versus T and ν plots in the frequency range 1–10 kHz at H = 0.25 T are compared in Figures S9 and S10. The magnetic relaxation time (τ) values for 1 in the temperature range 2.25–4 K at H = 0.25 T can be calculated from a joint analysis of the χM and χM″ versus ν plots through the generalized Debye equations, where α describes the distribution of the magnetic relaxation times (solid lines in Figures S9 and S10). The moderate α values in the range 0.36–0.44 support a wide distribution of magnetic relaxation processes (0 and 1 for single and infinite magnetic relaxation processes).

The estimated τ values at H = 0.25 T are represented in the form of the ln τ versus 1/T (so-called Arrhenius) plots in Figure S11. The experimental data follow a linear Arrhenius law characteristic of a single thermally activated magnetic relaxation process that was satisfactorily simulated through a two-phonon Orbach mechanism [τ–1 = τ0–1 exp(−Ueff/kBT)]. The ln τ versus ln T plot does not show any temperature region in which direct relaxation prevails (data not shown). The values of τ0 and Ueff [3.2(4) × 10–8 s and 6.8(2) cm–1] are within the wide range found for the other few examples of Ho SIMs (Table S3).23−31

The magnetothermal properties of 1 have been investigated by variable-temperature (2–20 K) and variable-field (0–8 T) magnetization measurements (Figure S12). The resulting magnetic entropy change (−ΔSM) versus T and ΔH plots obtained through a numerical approach described elsewhere according to the Maxwell equation [ΔSM = ∫(∂M/∂T) dH] are depicted in Figures 3 and S13.32

Figure 3.

Figure 3

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Contour color map for the temperature and field dependence of the −ΔSM for 1 in the field and temperature ranges 0–8 T and 2–20 K.

The contour map in Figure 3 indicates that the optimal working temperature is 9.5 K, reaching a CMR performance higher than 75% for ΔH > 4 T or 50% at 2 K for ΔH = 2 T because of the rapid saturation of the magnetic entropy. Indeed, high magnetic entropy global changes and large slopes of the isothermal magnetic entropy curves are mandatory for potential applications as molecular cryomagnetic coolants.

Blockage of the magnetization at low temperatures under a weak dc magnetic field (TB < 2.5 K for H = 0.25 T) should preclude the observation of large magnetocaloric effects (MCEs), as suggested by the lack of a −ΔSM maximum for ΔH < 1 T. The increase of the magnetic field induces a progressive shift of this maximum covering a wide temperature range above the liquid-helium temperature, from 3.2 (ΔH = 2 T) to 9.5 K (ΔH = 8 T) (Figure S13a). This feature is also related to the MCE efficiency and eventually to the practical applications of 1 as a cryomagnetic coolant in the low temperature range. By comparison, the mononuclear gadolinium(III) polyoxometalates of the formulas Na9[Gd(W5O18)2] and K12[Gd(P5W30O110)(H2O)], abbreviated as GdW10 and GdW30, exhibit SMR effects at very low temperatures (TB < 0.1 K) in the absence of a magnetic field, with −ΔSM maxima just below the liquid-helium temperature.8,9

The magnetic entropy increases with the magnetic field, and it tends asymptotically to a thermally dependent value reached more quickly at lower temperatures (Figure S13b). Hence, the maximum −ΔSM value in gravimetric units for ΔH = 8 T increases from 4.5 (T = 20 K) up to 6.2 J kg–1 K–1 (T = 10 K), and then it further decreases to 3.8 J kg–1 K–1 (T = 2 K). These values are higher than those found for GdW10 and GdW30 (−ΔSM = 1.9 and 4.7 J kg–1 K–1 at T = 1.3 and 1.8 K, respectively, for ΔH = 7 T),9 comparing well with those of metallic holmium and related intermetallic alloys (HoB2 and HoAl2), recently proposed as cryomagnetic coolants just above the hydrogen liquefaction temperature.33−35

Yet the maximum −ΔSM value in molar units for 1 (−ΔSM = 4.7 J mol–1 K–1 at T = 10 K for ΔH = 8 T) is rather lower than the limiting value for one HoIII ion with no zero-field splitting (zfs) [ΔSM = R ln(2J + 1) = 23.6 J mol–1 K–1 with J = 8]. Although the HoIII ion possesses the highest total angular momentum along the lanthanide series, this almost 5-fold reduction of the MCE efficiency is expected because of its rather high magnetic anisotropy, as discussed above. In fact, the large zfs of the ground heptadecet state strongly minimizes the magnetic entropy at zero field, thereby reducing the maximal magnetic entropy change (in absolute value) under the application of a magnetic field.

In summary, 1 behaves as a novel Ln SIM with a significant field dependence of the SMR at relatively low blocking temperatures. Likewise, it exhibits moderate MCE in a wide temperature range just above the occurrence of the field-induced magnetization blockage, with strong field-dependent magnetic entropy maxima between the liquid helium and hydrogen temperatures. Despite their limited CMR performance, the results reported herein illustrate the potential of magnetically anisotropic holmium(III)-based SIMs as prototypes of molecular cryomagnetic coolants operating near the strategically relevant hydrogen liquefaction temperature.

Acknowledgments

A.S.-P. and R.R. thank the MICIU and the Generalitat Valenciana for doctoral grants (FPU/2017/02638 and GRISOLIA/2017/057). J.M. acknowledges the MICIU for a Juan de la Cierva contract.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01905.

  • Synthetic, structural, magnetic, and magnetocaloric data (Tables S1–S3, Schemes S1 and S2, and Figures S1–S13) (PDF)

Accession Codes

CCDC 2069070 contains 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 data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Author Present Address

† Weizmann Institute of Science, Rehovot, Israel

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

This work was supported by the Spanish Ministry of Science, Innovation and Universities (MICIU; Project PID2019-109735GB-I00 and Unidad de Excelencia María de Maeztu CEX2019-000919-M) and the Generalitat Valenciana (AICO/2020/183).

The authors declare no competing financial interest.

Supplementary Material

ic1c01905_si_002.pdf (10.4MB, pdf)

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Supplementary Materials

ic1c01905_si_002.pdf (10.4MB, pdf)

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