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

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.

, sodium(I) bicarbonate, and holmium(III) oxide were of reagent grade; they were purchased from commercial sources and used as received.

Preparation and general physicochemical characterization
Na2[Ho III (DTPA)(H2O)]·8H2O (1): Powdered Ho2O3 (0.95 g, 2.5 mmol) was added in three small portions to a hot aqueous solution (50 mL) of H5DTPA (1.97 g, 5.0 mmol) under stirring. The mixture was heated under reflux for ca. 5 h until the solution became transparent (pH = 1.0). The pH of the solution was adjusted to 6.5 by dropwise addition of an aqueous solution of NaHCO3 (0.84 g, 10.0 mmol). The resulting pink solution was concentrated to 25 mL and then filtered on paper. After several weeks of slow evaporation of the filtered solution, an oil was formed and then slowly transformed into a crystalline solid by standing at room temperature under atmospheric conditions after several days ( Figure S1). A small amount of X-ray quality light pink prims of 1 were grown. The chemical identity and purity of the bulk crystalline material were further confirmed by X-ray powder diffraction (XRPD) ( Figure S2), whose pattern is identical to the calculated one from the single-crystal XRD analysis. It shows a whitish pink color under white light illumination, while a weak purple luminescence, typical of holmium(III) complexes, appears under UV-A light irradiation at 365 nm ( Figure S3). Yield: 3.4 g (90%). Anal. Calcd: C,22.09;H,4.77;N,5.52%. Found: C,21.65;H,4.

Physical techniques
S3 Elemental (C, H, N) analyses were performed by the Servei Central de Suport a la Investigació Experimental (SCSIE) de la Universitat de València. Fourier-transform infrared (FTIR) spectra were recorded on a Nicolet-5700 spectrophotometer as KBr pellets. Powder X-ray diffraction (XRD) patterns of powdered crystalline samples were collected at room temperature on a D8 Advance A25 Bruker diffractometer by using graphite-monochromated Cu-Ka radiation (λ = 1.54056 Å). Optical microscopy images were carried out at 80 amplification using a Nikon SMZ1000 optical microscope equipped with a Nikon Digital Sight DS-Fi1 camera. Images were taken with the NIS-Elements D software under illumination with a UV-A (lexc = 365 nm) lamp.

Magnetic measurements
Variable-temperature (2-300 K) direct current (dc) magnetic susceptibility measurements under applied magnetic field of 0.5 T and variable-temperature (2-20 K) and variable-field (0-8 T) magnetization measurements on powdered crystalline samples were carried out using Quantum Design Superconducting Quantum Interference Devices (SQUID) magnetometer and Physical Property Measurement System (PPMS). The samples were embedded in n-eicosane to prevent any crystal reorientation.
Variable-temperature (2-7 K) and variable-field (0-1 T) alternating current (ac) magnetic susceptibility measurements under ±5.0 Oe oscillating field at frequencies in the range 1-10 kHz were performed with a Quantum Design PPMS. The magnetic susceptibility data were corrected for the diamagnetism of the constituent atoms and the sample holder.

Computational details
The magnetic susceptibility data of 1 were simulated from a ligand field (LF) model.
The application of this Hamiltonian on the 5 I8 ground state allowed the simulation of the experimental cMT vs. T data ( Figure S6a) with the following best-fit parameters: # ) 〈 # 〉 = 284.1, * ) 〈 * 〉 = 758.6, + ) 〈 + 〉 = 2.8, and + + 〈 + 〉 = 5.0, which are of the same order of magnitude than those in the literature. 19 A temperature-independent paramagnetism of 1590 ´ 10 -6 cm 3 mol -1 was considered. The agreement factor defined was 5.8 ´ 10 -5 . However, several sets of values can be found that correctly simulate the experimental data, even without the last two terms of the Hamiltonian and particularly the last one.

X-ray crystallographic data collection and structure refinement
Single-crystal X-ray diffraction data of 1 were collected on a Bruker D8 Venture diffractometer with a PHOTON II detector using monochromatic Mo-K α radiation (λ =  18.0903 (7) 120 (2)