A topologically unique alternating { CoIII 3 Gd III 3 } magnetocaloric ring

The magnetocaloric effect (MCE) is described as the reversible adiabatic temperature change (DTad) and magnetic entropy change (DSm) of a material, following the application or removal of a magnetic field. Promising magnetic refrigerants benefit from the large MCE displayed by certain molecular materials. This is where coordination chemistry and molecular magnetism become powerful tools for optimising design towards the ideal magnetic refrigerant. A large MCE at low temperatures is favoured by a negligible magnetic anisotropy. Hence, several molecule-based refrigerants are complexes of isotropic Gd(III) ions. Another common trend is to maximise the magnetic : nonmagnetic ratio, so as to increase values of the refrigeration power and DSm, when reported per unit mass or unit volume. The inherent drawback of this approach is that spin–spin correlations increase unavoidably, which ultimately limit the lower bound of DTad and the lowest temperature that can be attained in an adiabatic demagnetisation process. Therefore, a compromise becomes necessary. Some recent studies explore the combined use of 3d/4f ions in search of the enhancement of magnetocaloric properties, such as Co/Gd. As expected, Co(II) ions influence negatively the MCE, because of the characteristic large magnetic anisotropy, which makes reorientation of the magnetic moment more difficult. Herein, we explore an attractive solution to this problem, namely tuning the oxidation state, by changing anisotropic Co(II) to diamagnetic Co(III), concomitantly with an effective dilution of the Gd(III) ions, in order to favour DTad. We recently reported large heterometallic {Mn18Cu6} complexes, obtained by following a directed synthesis approach, based on the use of the metallo–organic precursor [Cu(H6L)Cl]Cl (H6L = bis–tris propane). This prompted us to investigate the reactivity of bis–tris propane with 4f ions in the presence of 3d ions. Similar aminopolyol-type ligands seem to promote the oxidation of different Co(II) starting materials. Therefore, the exploration of Co(II) precursors containing H6L in the design of new magnetic refrigerants becomes highly attractive. Our approach is to use {Co(H6L)} precursors that can undergo facile oxidation to diamagnetic Co(III), whilst encapsulating the cobalt centres and directing/separating the Ln(III) ions. Using this strategy, we present the magnetocaloric properties of a new {Co 3 Gd III 3 } star-shaped ring, showing that the Co(III) ions have a significant impact on the adiabatic temperature change in this system, by separating the Gd(III) ions and weakening the Gd(III) Gd(III) interaction. In terms of DTad, this complex is among the best gadolinium-based molecular refrigerants reported so far (vide infra). By combining the metalloligand [Co(H6L)(CH3COO)2] (1) and Gd(acac)3 H2O we are able to obtain a new hexametallic complex [Co 3 Gd III 3 (H2L)3(acac)2(CH3COO)4(H2O)2] (2) with a unique alternating wheel-like structure (Fig. 1 and Fig. S4, S12, Table S1, ESI†). The pre-formation of the metalloligand (see Fig. S1–S3 and S10, ESI†) seems to be essential for the assembly of 2, as previously seen for the {Mn18Cu6} complexes. During the reaction, under aerobic conditions in the presence of bis–tris propane, Co(II) is oxidised to Co(III) and hence, the magnetic properties of 2 are defined exclusively by the paramagnetic Gd(III) ions. The structure of 2 contains three octahedral Co(III) ions, each one encapsulated by one tetra-deprotonated H2L 4 ligand through four O and two N atoms. The two remaining ligand arms are uncoordinated. Each {Co(H2L)} unit is linked to two octacoordinated Gd(III) ions through four m-O bridging atoms a WestCHEM, School of Chemistry, University of Glasgow, University Avenue, Glasgow, G12 8QQ, UK. E-mail: mark.murrie@glasgow.ac.uk b Instituto de Ciencia de Materiales de Aragón (ICMA), CSIC and Universidad de Zaragoza, 50009 Zaragoza, Spain. E-mail: evange@unizar.es † Electronic supplementary information (ESI) available: Experimental sections, spectroscopic studies, magnetic studies, crystallographic details. CCDC 1533722. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7cc02243c ‡ Current address: Institute for Integrated Cell-Material Science (WPI-iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan. Received 24th March 2017, Accepted 5th April 2017


Materials and physical measurements
All reagents and solvents were obtained from commercial suppliers and used without further The position of the frequency bands related to the acetate groups in the spectrum of 1 (υ (C=O) = 1561 cm −1 , υ (C−O) = 1406 cm −1 ) suggests that the counterions are coordinated to the Co(II) ions in a monodentate mode. 1,2 he similarity between both spectra, the tendency of H 6 L to encapsulate 3d metal ions in the central {N 2 O 2 }-pocket previously shown by other complexes, [3][4][5] and given that the crystal structure of [Ni(H 6 L)(CH 3 COO) 2 ] is known, 6 we believe that the Co(II) ion in 1 presents an analogous coordination environment to that displayed for the nickel monomer.
Considering that all the values are comprised in the range of 10 -100

Crystallographic details
Crystallographic data was collected for 2 at 100 K using Mo-Kα radiation (λ = 0.71073 Å), using a Bruker-Nonius Kappa CCD diffractometer with an Oxford Cryosystems cryostream device mounted on a sealed tube generator.The structure was solved using SUPERFLIP 8 and refined using full-matrix least squares refinement on F 2 using SHELX2014 9, 10 within OLEX2. 11mplex 2 crystallises in the tetragonal space group I-42d (see Table S1).The asymmetric Consequently, SQUEEZE (in PLATON) 12,13 was used to identify the solvent voids and account for the electron density within them, calculated to contain 2124 electrons per unit cell, corresponding to approximately 265 electrons per complex.(regions B and C).Co is displayed in red, while Gd is in green. x

Magnetic and magnetocaloric studies
Magnetic measurements were performed on polycrystalline samples of 1 (in eicosane) and 2 using a Quantum Design MPMS-XL SQUID magnetometer.Data were corrected for the diamagnetic contribution of the sample holder (and eicosane for 1) by measurements, and for the diamagnetism of the compounds ( (dia) for 2 = 9.78•10 −4 cm 3 •mol −1 ).

𝜒 𝑀
Heat capacity measurements were carried out for temperatures down to ca.

𝜒 𝑀 𝑇
The room temperature susceptibility value (2.86 cm 3 •mol −1 •K) is in good agreement with that expected for an anisotropic Co(II) mononuclear complex (2.81 cm 3 •mol −1 •K, considering S = 3/2, g = 2.45). 16The experimental values decrease gradually down to 150 K, before reaching a minimum of 1.50 cm 3 •mol −1 •K at 2 K.This behaviour is consistent with an octahedral Co(II) centre subject to 1 st order spin-orbit coupling.

1 iii
Fig S2 UV-Vis spectrum for 1.The studies were performed on methanolic solutions of 1 at c = 1 mM, 5 mM, 50 mM.

𝜐 3 Fig S3 ESI + mass spectrum for 1 .
Fig S3 ESI + mass spectrum for 1.The experiments were carried out using methanol as a solvent.
Fig S4 Detail of the metal alkoxide core of 2. Co, fuchsia; Gd, green; N, blue; O, red.Polyhedra are shown in fuchsia (Co) and green (Gd).

Fig S6− 8 Fig
Fig S6−8 EDX spectra of 2. The inset displays the area of the sample used for the analysis; the Atomic% is shown for each area.
Fig S10 Temperature dependence of for 1 in an applied field of 1000 Oe.
contains 1/2 molecule of [Co III 3 Gd III 3 (H 2 L) 3 (acac) 2 (CH 3 COO) 4 (H 2 O) 2 ].The structure contains also significant solvent accessible voids.Difference Fourier maps of the solvent regions suggest the presence of several CH 3 CN, CH 3 OH, and H 2 O molecules in the crystal lattice.However, they are poorly defined, and it was not possible to obtain a good model.

Table S1
Crystal Data and Structure Refinement Parameters of 2.

Table S2
14,15 measures of 2, {Co III 3 Gd III 3 } relative to ideal 8-vertex polyhedra.The lowest CShMs value, and thus most coincident geometry is highlighted in pink.14,15Thesymmetry analyses around the Gd(III) ion reveal a triangular dodecahedron (D 2d ) as the closest ideal geometry for both Gd1, and Gd2 centres.