title: 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.
authors.affiliation: Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra, 400076 (India).
authors.forename: Chinmoy
authors.initials: C
authors.lastname: Das
authors.affiliation: Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra, 400076 (India).
authors.forename: Shefali
authors.initials: S
authors.lastname: Vaidya
authors.affiliation: Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra, 400076 (India).
authors.forename: Tulika
authors.initials: T
authors.lastname: Gupta
authors.affiliation: EaStCHEM School of Chemistry, The University of Edinburgh, David Brewster Road, Edinburgh, EH9 3FJ (UK).
authors.forename: Jamie M
authors.initials: JM
authors.lastname: Frost
authors.affiliation: 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).
authors.forename: Mattia
authors.initials: M
authors.lastname: Righi
authors.affiliation: EaStCHEM School of Chemistry, The University of Edinburgh, David Brewster Road, Edinburgh, EH9 3FJ (UK).
authors.forename: Euan K
authors.initials: EK
authors.lastname: Brechin
authors.affiliation: 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). marco.affronte@unimore.it.
authors.forename: Marco
authors.initials: M
authors.lastname: Affronte
authors.affiliation: Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra, 400076 (India). eswar@chem.iitb.ac.in.
authors.forename: Gopalan
authors.initials: G
authors.lastname: Rajaraman
authors.affiliation: Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, Maharashtra, 400076 (India). rajaraman@chem.iitb.ac.in.
authors.forename: Maheswaran
authors.initials: M
authors.lastname: Shanmugam
keywords: ab initio calculations; clusters; dysprosium; magnetism; magnetocaloric effect