Single‐Molecule Magnetism, Enhanced Magnetocaloric Effect, and Toroidal Magnetic Moments in a Family of Ln4 Squares. Issue 44 (18th September 2015)

• Record Type: Journal Article
• Authors: Das, Chinmoy; Vaidya, Shefali; Gupta, Tulika; Frost, Jamie M.; Righi, Mattia; Brechin, Euan K.; Affronte, Marco; Rajaraman, Gopalan; Shanmugam, Maheswaran
• Is Part Of: Chemistry. Volume 21:Issue 44(2015)
• Page Start: 15639
• Page End: 15650
• Publication Date: 2015-09-18
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201502720
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>Three cationic [Ln<sub>4</sub>] squares (Ln=lanthanide) were isolated as single crystals and their structures solved as [Dy<sub>4</sub>(μ<sub>4</sub>‐OH)(HL)(H<sub>2</sub>L)<sub>3</sub>(H<sub>2</sub>O)<sub>4</sub>]Cl<sub>2</sub>⋅(CH<sub>3</sub>OH)<sub>4</sub>⋅(H<sub>2</sub>O)<sub>8</sub> (<bold>1</bold>), [Tb<sub>4</sub>(μ<sub>4</sub>‐OH)(HL)(H<sub>2</sub>L)<sub>3</sub>(MeOH)<sub>4</sub>]Cl<sub>2</sub>⋅(CH<sub>3</sub>OH)<sub>4</sub>⋅(H<sub>2</sub>O)<sub>4</sub> (<bold>2</bold>) and [Gd<sub>4</sub>(μ<sub>4</sub>‐OH)(HL)(H<sub>2</sub>L)<sub>3</sub>(H<sub>2</sub>O)<sub>2</sub>(MeOH)<sub>2</sub>]Br<sub>2</sub>⋅(CH<sub>3</sub>OH)<sub>4</sub>⋅(H<sub>2</sub>O)<sub>3</sub> (<bold>3</bold>). The structures are described as hydroxo‐centered squares of lanthanide ions, with each edge of the square bridged by a doubly deprotonated H<sub>2</sub>L<sup>2−</sup> ligand. Alternating current magnetic susceptibility measurements show frequency‐dependent out‐of‐phase signals with two different thermally assisted relaxation processes for <bold>1</bold>, whereas no maxima in <italic>χ<sub>M</sub></italic>" appears above 2.0 K for complex <bold>2</bold>. For <bold>1</bold>, the estimated effective energy barrier for these two relaxation processes is 29 and 100 K. Detailed ab initio studies reveal that complex <bold>1</bold> possesses a toroidal magnetic moment. The ab initio calculated anisotropies of the metal ions in complex <bold>1</bold> were employed to simulate the magnetic susceptibility by using the Lines model (POLY_ANISO) and this procedure yields <italic>J</italic><sub>1</sub>=+0.01 and <italic>J</italic><sub>2</sub>=−0.01 cm<sup>−1</sup> for <bold>1</bold> as the two distinct exchange interactions between the Dy<sup>III</sup> ions. Similar parameters are also obtained for complex <bold>1</bold> (and <bold>2</bold>) from specific heat measurements. A very weak antiferromagnetic super‐exchange interaction (<italic>J</italic><sub>1</sub>=−0.043 cm<sup>−1</sup> and <italic>g</italic>=1.99) is observed between the metal centers in <bold>3</bold>. The magnetocaloric effect (MCE) was estimated by using field‐dependent magnetization and temperature‐dependent heat‐capacity measurements. An excellent agreement is found for the −Δ<italic>S<sub>m</sub></italic> values extracted from these two measurements for all three complexes. As expected, <bold>3</bold> shows the largest −Δ<italic>S<sub>m</sub></italic> variation (23 J Kg<sup>−1</sup> K<sup>−1</sup>) 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 <bold>3</bold> an attractive candidate for cryogenic refrigeration.</p> </abstract>