Abstract
Three cationic [Ln4 ] squares (Ln=lanthanide) were isolated as single crystals and their structures solved as [Dy4 (μ4 -OH)(HL)(H2 L)3 (H2 O)4 ]Cl2 ⋅(CH3 OH)4 ⋅(H2 O)8 (1), [Tb4 (μ4 -OH)(HL)(H2 L)3 (MeOH)4 ]Cl2 ⋅(CH3 OH)4 ⋅(H2 O)4 (2) and [Gd4 (μ4 -OH)(HL)(H2 L)3 (H2 O)2 (MeOH)2 ]Br2 ⋅(CH3 OH)4 ⋅(H2 O)3 (3). The structures are described as hydroxo-centered squares of lanthanide ions, with each edge of the square bridged by a doubly deprotonated H2 L(2-) ligand. Alternating current magnetic susceptibility measurements show frequency-dependent out-of-phase signals with two different thermally assisted relaxation processes for 1, whereas no maxima in χM " appears above 2.0 K for complex 2. For 1, the estimated effective energy barrier for these two relaxation processes is 29 and 100 K. Detailed ab initio studies reveal that complex 1 possesses a toroidal magnetic moment. The ab initio calculated anisotropies of the metal ions in complex 1 were employed to simulate the magnetic susceptibility by using the Lines model (POLY_ANISO) and this procedure yields J1 =+0.01 and J2 =-0.01 cm(-1) for 1 as the two distinct exchange interactions between the Dy(III) ions. Similar parameters are also obtained for complex 1 (and 2) from specific heat measurements. A very weak antiferromagnetic super-exchange interaction (J1 =-0.043 cm(-1) and g=1.99) is observed between the metal centers in 3. The magnetocaloric effect (MCE) was estimated by using field-dependent magnetization and temperature-dependent heat-capacity measurements. An excellent agreement is found for the -ΔSm values extracted from these two measurements for all three complexes. As expected, 3 shows the largest -ΔSm variation (23 J Kg(-1) K(-1) ) among the three complexes. The negligible magnetic anisotropy of Gd indeed ensures near degeneracy in the (2S+1) ground state microstates, and the weak super-exchange interaction facilitates dense population of low-lying excited states, all of which are likely to contribute to the MCE, making complex 3 an attractive candidate for cryogenic refrigeration.
Highlights
The large magnetic moments and unquenched first order orbital angular momenta associated with certain lanthanide ions makes their coordination complexes potentially useful in various applications such as information storage,1 spintronics,2 molecular switches,3 spin valves 4 and qubits5
In this article we have reported an unusual family of O-centred Ln4 squares whose structures were determined by single crystal X-ray diffraction
Unambiguous evidence for the existence of antiferromagnetic interaction between the anisotropic lanthanide ions is confirmed through ab initio calculations and heat capacity measurements
Summary
The large magnetic moments and unquenched first order orbital angular momenta associated with certain lanthanide ions makes their coordination complexes potentially useful in various applications such as information storage, spintronics, molecular switches, spin valves 4 and qubits. Several lanthanide based SMMs, in particular dysprosium based clusters ranging from monomers to tetracosanuclear species, have been reported.1h, 7 Among them more often, multinuclear clusters show unusual magnetic properties, for fascinating recent examples include Dy3 triangles in which SMM behaviour was observed to originate from the excited states, Dy5 and Dy4K2 clusters exhibiting enormous Ueff values1b, and the presence of magnetic bistability in symmetric oligomeric lanthanide complexes due to a toroidal arrangement of anisotropic axes.7b various promising synthetic approaches have been reported to increase the effective energy barrier to the reorientation of the magnetization of SMMs, the blocking temperature (TB) associated with the onset of hysteresis in molecular systems still sits below T = 14 K,9 hampering any realistic development of molecular devices for practical application This is partly due to the lack of understanding of the combined roles of orbital (L) and spin (S) angular momenta to the resultant magnetic moment of the molecule. This detailed experimental and theoretical investigation reveals that complex 1 exhibit a rare toroidal magnetic moment which is evident from the magnetization measurements (vide infra) and complex 3 shows large variation in - Sm, making it a promising candidate for cryogenic refrigeration
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