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Papers by Chinmoy Das
Chemistry - A European Journal, 2015
Three cationic [Ln 4 ] squares (Ln = lanthanide) were isolated as single crystals and their struc... more Three cationic [Ln 4 ] squares (Ln = lanthanide) were isolated as single crystals and their structures solved as [Dy 4 (m 4-OH)(HL)(H 2 L) 3 (H 2 O) 4 ]Cl 2 •(CH 3 OH) 4 •(H 2 O) 8 (1), [Tb 4 (m 4-OH)(HL)(H 2 L) 3 (MeOH) 4 ]Cl 2 •(CH 3 OH) 4 •(H 2 O) 4 (2) and [Gd 4 (m 4-OH)(HL)(H 2 L) 3 (H 2 O) 2 (MeOH) 2 ]Br 2 •(CH 3 OH) 4 •(H 2 O) 3 (3)&&L = ? &&. The structures are described as hydroxo-centered squares of lanthanide ions, with each edge of the square bridged by a doubly deprotonated H 2 L 2À ligand. Alternating current magnetic susceptibility (c M)&&?&&measurements show frequency-dependent out-of-phase signals with two different thermally assisted relaxation processes for 1, whereas no maxima in c M " 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 J 1 = + 0.01 and J 2 = À0.01 cm À1 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 (J 1 = À0.043 cm À1 and g = 1.99) is observed between the metal centers in 3. The magnetocaloric effect (MCE) was estimated by using field-dependent magnetization and temperaturedependent heat-capacity measurements. An excellent agreement is found for the ÀDS m values extracted from these two measurements for all three complexes. As expected, 3 shows the largest ÀDS m variation (23 J Kg À1 K À1) 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.
Chemistry - A European Journal, 2015
Three cationic [Ln 4 ] squares (Ln = lanthanide) were isolated as single crystals and their struc... more Three cationic [Ln 4 ] squares (Ln = lanthanide) were isolated as single crystals and their structures solved as [Dy 4 (m 4-OH)(HL)(H 2 L) 3 (H 2 O) 4 ]Cl 2 •(CH 3 OH) 4 •(H 2 O) 8 (1), [Tb 4 (m 4-OH)(HL)(H 2 L) 3 (MeOH) 4 ]Cl 2 •(CH 3 OH) 4 •(H 2 O) 4 (2) and [Gd 4 (m 4-OH)(HL)(H 2 L) 3 (H 2 O) 2 (MeOH) 2 ]Br 2 •(CH 3 OH) 4 •(H 2 O) 3 (3)&&L = ? &&. The structures are described as hydroxo-centered squares of lanthanide ions, with each edge of the square bridged by a doubly deprotonated H 2 L 2À ligand. Alternating current magnetic susceptibility (c M)&&?&&measurements show frequency-dependent out-of-phase signals with two different thermally assisted relaxation processes for 1, whereas no maxima in c M " 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 J 1 = + 0.01 and J 2 = À0.01 cm À1 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 (J 1 = À0.043 cm À1 and g = 1.99) is observed between the metal centers in 3. The magnetocaloric effect (MCE) was estimated by using field-dependent magnetization and temperaturedependent heat-capacity measurements. An excellent agreement is found for the ÀDS m values extracted from these two measurements for all three complexes. As expected, 3 shows the largest ÀDS m variation (23 J Kg À1 K À1) 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.