Closely-Related Zn2IILn2III Complexes (LnIII = Gd, Yb) with Either Magnetic Refrigerant or Luminescent Single-Molecule Magnet Properties

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 center dot)6H(2)O and subsequently with Ln(NO3)(3)center dot 5H(2)O (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 {(mu 3-CO3)(2)[Zn(mu-L)Gd(NO3)](2)}center dot 4CH(3)OH (1) and{(mu(3)-CO3)(2)[Zn(mu-L)Yb(H2O)](2)}(NO3)(2)center dot 4CH(3)OH (2). When the reaction was performed in the absence of triethylamine, the dinuclear compound [Zn(mu-L)(mu-NO3)Yb(NO3)(2)] (3) is obtained. The structures of 1 and 2 consist of two diphenoxo-bridged Zn-II-Ln(III) units connected by two carbonate bridging ligands. Within the dinuclear units, Zn-II and Ln(III) ions occupy the N3O2 inner and the O-4 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-diphenoxo triply bridged (ZnYbIII)-Yb-II 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-caloric effect, with a maximum value of -Delta S-m = 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 tau(o) = 3.1 X 10(-6) s and 27.0(9) K with tau(o) = 8.8 X 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) Yb-III-based luminescence in complexes 2 and 3 through an intramolecular energy transfer to the excited states of the accepting Yb-III ion. These complexes show several bands in the 945-1050 nm region, corresponding to F-2(5/2)-> F-2(7/2) transitions arising from the ligand field splitting of both multiplets. The observed luminescence lifetimes tau(obs) are 0.515 and 10 mu 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-H oscillators. Therefore, complexes 2 and 3, combining field-induced SMM behavior and NIR luminescence, can be considered to be dual magneto-luminescent materials.