Single‐Molecule Magnetism, Enhanced Magnetocaloric Effect, and Toroidal Magnetic Moments in a Family of Ln₄ Squares

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.

Role of the Diamagnetic Zinc(II) Ion in Determining the Electronic Structure of Lanthanide Single‐Ion Magnets

“…To understand the fivefold increase in the U eff barrier for the complex containing the diamagnetic Zn II ion, the influence that the Zn II ions have on the electronic structure and therefore the magnetic anisotropy of the Ln III ions was probed, using state‐of‐the‐art ab initio calculations. Detailed post‐Hartree–Fock ab initio calculations were undertaken for all complexes, 1 a , 1 b , 2 , 3 , and 4 to validate the experimental observations, using the MOLCAS 7.8 code, as this has proved its aptness on several occasions . In this multi‐configurational approach, relativistic effects are treated using the Douglas–Kroll Hamiltonian.…”

“…[3] In addition other novel magneticp henomena such as single-molecule toroidal behavior have been detailed. [4] Numerous lanthanidebased coordination complexes have subsequently flooded the literature, [5] with examples revealingr ecord high anisotropy barriers (U eff ), the energy required to flip the orientation of the magnetization vector, with values as large as 1261cm À1 . [6] Althought he magnitudeo ft he anisotropy barrier is significantly largert han the average thermale nergy at room temperature, in many SMM complexes the blockingt emperature (T B ), given as am agnetization relaxationt ime of 100 s, lies at extremely low temperatures, usually < 2K,d ue to quantum tunneling of the magnetization (QTM).A tt he present time there is no straightforward routet owardst ackling the problemo ft his quantum behavior;h owever, one method of subduing QTM can be achieved by enhancing the exchange interaction betweenl anthanide ions.…”

“…Am eanst om odulate the electrostatic charge aroundt he trivalentL nion has been developed by incorporating the diamagnetic Zn II ion into variousc omplexes. [11] The factors that influence the magnetizationr elaxation dynamics using this approacha re reported from the study of four complexes of molecular formulae:[ Ln III (HL) 2 (NO 3 ) 3 ]w here Ln III = Dy (1)o rP r ( 2), [Zn II Dy III (L) 2 (CH 3 CO 2 )(NO 3 ) 2 ]( 3)a nd [Zn II 2 Pr III (L) 2 (CH 3 CO 2 ) 4 NO 3 ]( 4), where HL = 2-methoxy-6-[(E)p henyliminomethyl]phenol.…”

Observation of Slow Relaxation and Single‐Molecule Toroidal Behavior in a Family of Butterfly‐Shaped Ln4 Complexes

“…Upon describing the individual metal ion properties, we attempted to elucidate the exchange coupling between the Ln III centers within the Lines model as implemented in the POLY_ANISO routine and validated earlier . This uses a single‐parameter exchange Hamiltonian to explain the isotropic interaction between spin terms in absence of spin‐orbit coupling.…”

Coordination‐Cluster‐Based Molecular Magnetic Refrigerants

“…In 2015, Shanmugam et al isolated a family of O‐centered {Ln 4 } squares (Figure ),46 35a , 35b , and 35c . The structures can be depicted as hydroxo‐centered squares of Ln III ions, with each edge of the square linked by a doubly deprotonated H 2 mbzpd 2– ligand.…”