Edinburgh Explorer Magnetic and magnetocaloric properties of an unusual family of carbonate-panelled [LnIII6ZnII2] cages

The reaction of the pro-ligand H 4 L, which combines the complementary phenolic oxime and diethanolamine moieties within the same organic framework, with Zn(NO 3 ) 2 ·6H 2 O and Ln(NO 3 ) 3 ·6H 2 O in a basic methanolic solution generates a family of isostructural heterometallic coordination compounds of general formula [Ln 6 Zn 2 (CO 3 ) 5 (OH)(H 2 L) 4 (H 3 L) 2 (H 4 L)]NO 3 · x MeOH [Ln = Gd, x = 30 ( 1 ), Ln = Dy, x 10 = 32 ( 2 ), Ln = Sm, x = 31 ( 3 ), Ln = Eu, x = 29 ( 4 ), Ln = Tb, x = 30 ( 5 )]. The octametallic skeleton of the cage describes a heavily distorted [Gd III6 ] octahedron capped on two faces by Zn II ions. The metal core is stabilised by a series of µ 3 - and µ 4 -CO 32- ions, originating from the serendipitous fixation of atmospheric CO 2 . The magnetic properties of all family members were examined via SQUID magnetometry, with the χ M T product and VTVB data of the Gd analogue ( 1 ) being independently fitted by numerical 15 diagonalisation to afford the same best-fit parameter J Gd-Gd = -0.004 cm -1 . The MCE of complex 1 was elucidated from specific heat data, with the magnetic entropy change reaching a value of 22.6 J kg -1 K -1 at T = 1.7 K, close to the maximum entropy value per mole expected from six Gd III spins ( S Gd = 7/2), 23.7 J kg -1 K -1 .


Introduction
The large value of their total angular momentum, their often strong magnetic anisotropy and the inherently weak magnetic exchange mediated via their contracted f-orbitals engender Lnbased molecular cages with some fascinating and potentially useful low temperature physics. [1][2][3][4][5] In academia these have been 25 much exploited for the construction of Single-Molecule Magnets (SMMs) 6 and Molecular Coolers. 7 The prospect of employing molecular cages in low temperature cooling applications is based upon the compounds magneto-caloric effect (MCE), as derived from the change in magnetic entropy upon application of a 30 magnetic field. 8 The design of such molecular materials therefore requires the control and optimisation of quantum properties at the molecular level (spin ground state, magnetic anisotropy, the presence of low-lying excited spin states), which in turn requires the synthetic chemist to follow a particular recipe that includes 35 high spin, anisotropic metal ions and lightweight organic bridging ligands. 9 When a magnetic field is applied to a polynuclear molecular magnetic material in which the magnetic exchange interaction between constitutive metal centres and the local magnetic 40 anisotropies are small, the magnetic moments of the constitutive paramagnetic centres become polarised by the magnetic field. When this magnetisation process is performed at constant temperature, the total magnetic entropy of the material is reduced. In a subsequent adiabatic demagnetisation process, the 45 temperature of the material decreases, thereby cooling the material. 10 This is a particularly attractive, and potentially Scheme 1. The structure of the ligand H4L which contains both phenolic oxime and diethanolamine moieties. 50 technologically important phenomenon, since recent studies have shown that the MCE of some molecular clusters can be much larger than that found in the best intermetallic and lanthanide alloys, and magnetic nanoparticles employed commercially. [11][12] The obvious metal ion of choice is Gd III since it possesses an 55 isotropic S = 7/2, and its clusters will exhibit weak magnetic exchange courtesy of the contracted f-orbitals, resulting in the presence of field-accessible, low-lying excited states. Indeed the vast majority of clusters reported recently to display an enhanced MCE have contained multiple Gd III centres. [13][14][15][16][17][18] We continue this We have previously shown that this ligand is highly effective in forming transition metal cages with aesthetically pleasing structures and fascinating magnetic properties, and we now extend its coordination chemistry to the 4f elements. 19 Experimental 5

Materials and physical measurements
All manipulations were performed under aerobic conditions, using materials as received (reagent grade). (Z)-1-(3-((bis(2hydroxyethyl)amino)methyl)-2-hydroxy-5-methylphenyl)ethan-1one oxime) [H 4 L] was synthesised as described in the literature. 19 10 Magnetisation data were acquired on a MPMS-XL SQUID magnetometer equipped with a 5 T dc magnet. Freshly isolated crystalline material was covered immediately with hexadecane (MPt = 18 °C) in order to suppress loss of co-crystallized solvent. Diffraction data were collected on a Bruker Smart Apex CCD diffractometer equipped with an Oxford Cryosystems LT device, using Mo radiation. Data collection parameters and structure solution and refinement details are listed in Table S1. Full details can be found in the CIF files provided in the supporting 45 information and CCDC 1055091-1055095.

Results and Discussion
Compounds 1-5 are isostructural, and so for the sake of brevity we limit discussion to complex 1, [Gd III 6 Zn II 2 (CO 3 ) 5  approximately ~25% are 4f complexes and ~5% are heterometallic 3d-4f complexes. 20 While the majority have been 35 formed serendipitously, this observation has led some researchers to deliberately employ Na 2 CO 3 , NaHCO 3 and CO 2 as reaction ingredients. 21 The CSD search also highlights the extraordinary coordinative flexibility of the CO 3 2ion demonstrating bridging modes ranging from bidentate to nonadentate -with the majority 40 (65%) being tridentate and forming M 3 triangles, a topology of inherent interest to the magnetochemist. 22

Magnetic properties
The d.c molar magnetic susceptibility, χ M , of polycrystalline samples of complexes 1 -5 were measured in an applied These are in good agreement with the sum of Curie constants for a [Gd III 6 ] unit (47.3 cm 3 K mol -1 , g Gd = 2.0) for 1, a [Dy III 6 ] unit (85.0 cm 3 K mol -1 , g Dy = 4/3) for 2, a [Sm III 6 ] unit (0.5 cm 3 K mol -1 , g Sm = 2/7) for 3, and a [Tb III 6 ] unit (70.9 cm 3 K mol -1 , g Tb = 3/2) for 5. In the case of 4, although the 7 F 0 ground state of Eu III 55 possesses no magnetic moment and thus, the [Eu III 6 ] unit should be diamagnetic at low temperatures, a finite magnetic moment is observed at room temperature, due to the low-lying 7 F 1 first excited state that is partly populated at room temperature. Upon cooling, the χ M T product of 1 remains essentially constant down 60 to approximately 20 K, wherefrom it begins to decrease upon further cooling to reach 42.3 cm 3 K mol -1 at 2 K. Given that the anisotropy of Gd III is negligible, this behaviour is consistent with the presence of weak intramolecular antiferromagnetic exchange interactions. The χ M T product of 2 and 5 decreases continuously 65 upon cooling, reaching 64.3 and 53.5 cm 3 K mol -1 , respectively, at 2 K. This behaviour can be ascribed to the large magnetic anisotropy of Dy III and Tb III and potentially to the presence of weak intramolecular magnetic exchange interactions. The χ M T product of 3, remains essentially constant in the investigated 70 temperature range, at the low, but finite, value of 0.3 cm 3 K mol -1 , a consequence of the low Landé g-factor of the ground 6 H 5/2 term, indicating a splitting between the ground and first excited Kramers doublets of the 6 H 5/2 term larger than the thermal energy at 280 K. Finally, the χ M T product of 4 decreases continuously 75 upon cooling and reaches virtually zero at 2 K, reflecting the thermal depopulation of the 7 F 1 first excited state upon cooling, likely indicating mixing of the 7 F 0 ground state with excited states possessing a magnetic moment. To better define the lowtemperature magnetic properties of complexes 1 -5, low 80 temperature variable-temperature-and-variable-field (VTVB) magnetisation data were measured in the temperature and magnetic field ranges 2 to 12 K and 0 to 5 T for 1 and 2 to 8 K and 0 to 5 T for the remaining complexes. The VTVB magnetisation data of 1 are shown in and Tb III (9.0 µ B ) centres, for which the m J = -15/2 projection of the 6 H 15/2 ground term or the m J = -6 projection of the 7 F 6 ground term, respectively, is the lowest energy state. Furthermore, the VTVB data of 2 ( Figure S5) and 5 ( Figure S8) present nesting when plotted against µ B B/kT. These observations indicate that the 5 energy spectrum of 2 and 5 presents significant splittings with respect to the temperature of measurement, at zero magnetic field. The VTVB magnetisation data of 3 ( Figure S6) present no nesting when plotted against the reduced quantity µ B B / kT and are also linear with magnetic field. This behaviour is consistent 10 with the presence of a thermally isolated Kramers doublet as the ground state of the 6 H 5/2 ground term, in agreement with the analysis of the temperature dependence of the χ M T product. Finally, the VTVB magnetisation data of 4 ( Figure S7) responds in a linear fashion with magnetic field and constant temperature, 15 and are temperature independent at a constant field. This behaviour indicates a field-induced mixing of the 7 F 0 ground state with excited states possessing a magnetic moment, consistent with the analysis of the temperature dependence of the χ M T product. The hexanuclear nature of complexes 2 -5, combined with the low symmetry of the local coordination sphere of the Ln III centres and the ensuing large number (twenty-seven) of associated ligand field parameters per Ln III ion, precludes any quantitative 30 interpretation of the magnetic properties of these complexes. However, in the case of 1, given that the orbital angular moment for Gd III is quenched, a quantitative analysis is possible through the use of a spin-Hamiltonian parameterisation. Thus, we employed the general form of the isotropic spin-Hamiltonian (1) where the summation indexes i, j run through the constitutive Gd III centres, Ŝ is a spin operator and J is the isotropic exchange interaction parameter. In our spin-Hamiltonian model we include the following isotropic exchange parameters: J 12 , J 13  Levenberg-Marquardt algorithm. 23 Both fits result in the same best-fit parameter: J Gd-Gd = -0.004 cm -1 . The best-fit curves are shown as solid lines in Figure 3. Next, we report the specific heat (C) data collected for a polycrystalline sample of 1, in the temperature range 0.3 to 30 K 50 and in applied magnetic fields, B, of 0, 1, 3 and 7 T (Figure 4). At the higher temperatures, the specific heat is dominated by a nonmagnetic contribution arising from thermal vibrations of the lattice, which can be modelled by the Debye-Einstein model (dotted line). 8 The phonon specific heat simplifies to a C/R = aT 3 55 dependence at the lowest temperatures, where R is the gas constant and a = 1.1 × 10 −2 K −3 . For B ≥ 1 T, we model the fielddependent specific heat as the sum of the Schottky curves arising from the field-split levels of Gd III independent spins (solid lines). Note the overall nice agreement with the experimental data, 60 suggesting that applied fields of B ≥ 1 T are nearly sufficient for fully decoupling the spin centres. The zero-applied-field specific heat can be described by the Schottky curve depicted in Figure 4 as a dashed line. This curve is calculated by assuming that every spin centre is experiencing an effective field B eff = 0.25 T, as the 65 result of the magnetic interactions involved. By making use of the specific heat data, we calculate the entropy (S) according to the expression S/R = ∫C/TdT, which we plot in Figure S10 as a function of temperature and for the corresponding applied field values. The final step in the evaluation of the MCE of 1 consists 70 in obtaining the magnetic entropy change -∆S m (T), for selected applied field changes ∆B. The result is shown in Figure 4. This calculation is straightforwardly obtained from the S(T) curves in Figure S10 and also from the magnetisation data in Figure 3 by employing the Maxwell relation, ∆S m = ∫∂M/∂TdB. As can be 75 seen in Figure 4, the nice agreement between the results obtained via both methods is validation of the approaches employed. For the largest applied field change (∆B = 7 T), the magnetic entropy change, -∆S m , reaches 22.6 J kg -1 K -1 at T = 1.7 K. Because of the very weak strength of the magnetic exchange interactions, this 80 value of -∆S m is close to the maximum entropy value per mole involved, corresponding to six Gd III spins (S Gd = 7/2), calculated as 6Rln(2 S Gd +1) = 103.7 J mol -1 K -1 , that is, 23.7 J kg -1 K -1 . Thus, in 1, nearly the full magnetocaloric potential of Gd III is achieved.

10
A highly unusual family of Ln6Zn2 cages whose structures are based on highly distorted bicapped octahedra can be constructed from the simple one-pot self-assembly reaction between the two metal salts and the ligand H 4 L in basic methanolic solutions. The ligand has been previously used in Mn coordination chemistry to 15 produce dodecametallic wheels and truncated tetrahedra. Magnetic exchange between the Gd III ions in the octahedron is shown to be vanishingly small by independent fits of both susceptibility and magnetisation data. The MCE of complex 1 was elucidated from specific heat data, with the magnetic entropy 20 change reaching a value of 22.6 J kg -1 K -1 at T = 1.7 K, close to the maximum entropy value per mole expected from six Gd III spins, 23.7 J kg -1 K -1 .