Single‐Molecule Magnetism,Enhanced Magnetocaloric Effect,and Toroidal Magnetic Moments in a Family of Ln4 Squares |
| |
| Authors: | Chinmoy Das Shefali Vaidya Tulika Gupta Jamie M. Frost Mattia Righi Prof. Dr. Euan K. Brechin Prof. Dr. Marco Affronte Prof. Dr. Gopalan Rajaraman Prof. Dr. Maheswaran Shanmugam |
| |
| Affiliation: | 1. Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra, 400076 (India);2. EaStCHEM School of Chemistry, The University of Edinburgh, David Brewster Road, Edinburgh, EH9 3FJ (UK);3. CNR Institute of Nanosciences S3 and Dipartimento di Scienze Fiscihe, Infomatiche e Matematiche, Università di Modena e Reggio Emilia via G. Campi213?A, 41125 Modena (Italy) |
| |
| Abstract: | Three cationic [Ln4] squares (Ln=lanthanide) were isolated as single crystals and their structures solved as [Dy4(μ4‐OH)(HL)(H2L)3(H2O)4]Cl2?(CH3OH)4?(H2O)8 ( 1 ), [Tb4(μ4‐OH)(HL)(H2L)3(MeOH)4]Cl2?(CH3OH)4?(H2O)4 ( 2 ) and [Gd4(μ4‐OH)(HL)(H2L)3(H2O)2(MeOH)2]Br2?(CH3OH)4?(H2O)3 ( 3 ). The structures are described as hydroxo‐centered squares of lanthanide ions, with each edge of the square bridged by a doubly deprotonated H2L2? 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 DyIII 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. |
| |
| Keywords: | ab initio calculations clusters dysprosium magnetism magnetocaloric effect |
|
|
