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Closely-related ZnII2LnIII2 complexes (LnIII = Gd, Yb) with either magnetic refrigerant or luminescent single-molecule magnet properties

Ruiz, José, Lorusso, Giulia, Evangelisti, Marco, Brechin, Euan K., Pope, Simon J. A. and Colacio, Enrique 2014. Closely-related ZnII2LnIII2 complexes (LnIII = Gd, Yb) with either magnetic refrigerant or luminescent single-molecule magnet properties. Inorganic Chemistry 53 (7) , pp. 3586-3594. 10.1021/ic403097s

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The reaction of the compartmental ligand N,N′,N″-trimethyl-N,N″-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H2L) with Zn(NO3)2·6H2O and subsequently with Ln(NO3)3·5H2O (LnIII = Gd and Yb) and triethylamine in MeOH using a 1:1:1:1 molar ratio leads to the formation of the tetranuclear complexes {(μ3-CO3)2[Zn(μ-L)Gd(NO3)]2}·4CH3OH (1) and{(μ3-CO3)2[Zn(μ-L)Yb(H2O)]2}(NO3)2·4CH3OH (2). When the reaction was performed in the absence of triethylamine, the dinuclear compound [Zn(μ-L)(μ-NO3)Yb(NO3)2] (3) is obtained. The structures of 1 and 2 consist of two diphenoxo-bridged ZnII–LnIII units connected by two carbonate bridging ligands. Within the dinuclear units, ZnII and LnIII ions occupy the N3O2 inner and the O4 outer sites of the compartmental ligand, respectively. The remaining positions on the LnIII ions are occupied by oxygen atoms belonging to the carbonate bridging groups, by a bidentate nitrate ion in 1, and by a coordinated water molecule in 2, leading to rather asymmetric GdO9 and trigonal dodecahedron YbO8 coordination spheres, respectively. Complex 3 is made of acetate–diphenoxo triply bridged ZnIIYbIII dinuclear units, where the YbIII exhibits a YbO9 coordination environment. Variable-temperature magnetization measurements and heat capacity data demonstrate that 1 has a significant magneto–caloric effect, with a maximum value of −ΔSm = 18.5 J kg–1 K–1 at T = 1.9 K and B = 7 T. Complexes 2 and 3 show slow relaxation of the magnetization and single-molecule magnet (SMM) behavior under an applied direct-current field of 1000 Oe. The fit of the high-temperature data to the Arrhenius equation affords an effective energy barrier for the reversal of the magnetization of 19.4(7) K with τo = 3.1 × 10–6 s and 27.0(9) K with τo = 8.8 × 10–7 s for 2 and 3, respectively. However, the fit of the full range of temperature data indicates that the relaxation process could take place through a Raman-like process rather than through an activated Orbach process. The chromophoric L2– ligand is able to act as an “antenna” group, sensitizing the near-infrared (NIR) YbIII-based luminescence in complexes 2 and 3 through an intramolecular energy transfer to the excited states of the accepting YbIII ion. These complexes show several bands in the 945–1050 nm region, corresponding to 2F5/2→2F7/2 transitions arising from the ligand field splitting of both multiplets. The observed luminescence lifetimes τobs are 0.515 and 10 μs for 2 and 3, respectively. The shorter lifetime for 2 is due to the presence of one coordinated water molecule on the YbIII center (and to a lesser extent noncoordinated water molecules), facilitating vibrational quenching via O–H oscillators. Therefore, complexes 2 and 3, combining field-induced SMM behavior and NIR luminescence, can be considered to be dual magneto–luminescent materials.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Publisher: American Chemical Society
ISSN: 0020-1669
Last Modified: 21 Sep 2020 11:30

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