Closely-Related ZnII2LnIII2 Complexes (LnIII = Gd, Yb) with Either Magnetic Refrigerant or Luminescent Single-Molecule Magnet Properties

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• 作者：Jos茅 Ruiz ; Giulia Lorusso ; Marco Evangelisti ; Euan K. Brechin ; Simon J. A. Pope ; Enrique Colacio
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：April 7, 2014
• 年：2014
• 卷：53
• 期：7
• 页码：3586-3594
• 全文大小：490K
• 年卷期：v.53,no.7(April 7, 2014)
• ISSN：1520-510X

文摘

The reaction of the compartmental ligand N,N鈥?N鈥?trimethyl-N,N鈥?bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H₂L) with Zn(NO₃)₂路6H₂O and subsequently with Ln(NO₃)₃路5H₂O (Ln^III = Gd and Yb) and triethylamine in MeOH using a 1:1:1:1 molar ratio leads to the formation of the tetranuclear complexes {(渭₃-CO₃)₂[Zn(渭-L)Gd(NO₃)]₂}路4CH₃OH (1) and{(渭₃-CO₃)₂[Zn(渭-L)Yb(H₂O)]₂}(NO₃)₂路4CH₃OH (2). When the reaction was performed in the absence of triethylamine, the dinuclear compound [Zn(渭-L)(渭-NO₃)Yb(NO₃)₂] (3) is obtained. The structures of 1 and 2 consist of two diphenoxo-bridged Zn^II鈥揕n^III units connected by two carbonate bridging ligands. Within the dinuclear units, Zn^II and Ln^III ions occupy the N₃O₂ inner and the O₄ outer sites of the compartmental ligand, respectively. The remaining positions on the Ln^III 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鈥揹iphenoxo triply bridged Zn^IIYb^III dinuclear units, where the Yb^III exhibits a YbO₉ coordination environment. Variable-temperature magnetization measurements and heat capacity data demonstrate that 1 has a significant magneto鈥揷aloric effect, with a maximum value of 鈭捨?i>S_m = 18.5 J kg^鈥? K^鈥? 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 蟿_o = 3.1 脳 10^鈥? s and 27.0(9) K with 蟿_o = 8.8 脳 10^鈥? 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 L^{2鈥?/sup> ligand is able to act as an 鈥渁ntenna鈥?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鈥?050 nm region, corresponding to 2F_5/2鈫?sup>2}F_7/2 transitions arising from the ligand field splitting of both multiplets. The observed luminescence lifetimes 蟿_obs are 0.515 and 10 渭s for 2 and 3, respectively. The shorter lifetime for 2 is due to the presence of one coordinated water molecule on the Yb^III center (and to a lesser extent noncoordinated water molecules), facilitating vibrational quenching via O鈥揌 oscillators. Therefore, complexes 2 and 3, combining field-induced SMM behavior and NIR luminescence, can be considered to be dual magneto鈥搇uminescent materials.