Closely-related Zn^II2Ln^III2 complexes (Ln^III = 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 (H L) with Zn(NO)·6HO and subsequently with Ln(NO)·5HO (Ln = Gd and Yb) and triethylamine in MeOH using a 1:1:1:1 molar ratio leads to the formation of the tetranuclear complexes {(?- CO)[Zn(?-L)Gd(NO)]} ·4CHOH (1) and{(?-CO) [Zn(?-L)Yb(HO)]}(NO) ·4CHOH (2). When the reaction was performed in the absence of triethylamine, the dinuclear compound [Zn(?-L)(?-NO )Yb(NO)] (3) is obtained. The structures of 1 and 2 consist of two diphenoxo-bridged Zn-Ln units connected by two carbonate bridging ligands. Within the dinuclear units, Zn and Ln ions occupy the NO inner and the O outer sites of the compartmental ligand, respectively. The remaining positions on the Ln 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 GdO and trigonal dodecahedron YbO coordination spheres, respectively. Complex 3 is made of acetate-diphenoxo triply bridged ZnYb dinuclear units, where the Yb exhibits a YbO coordination environment. Variable-temperature magnetization measurements and heat capacity data demonstrate that 1 has a significant magneto-caloric effect, with a maximum value of -?S = 18.5 J kg K 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 . = 3.1 × 10 s and 27.0(9) K with . = 8.8 × 10 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 ligand is able to act as an antenna group, sensitizing the near-infrared (NIR) Yb -based luminescence in complexes 2 and 3 through an intramolecular energy transfer to the excited states of the accepting Yb ion. These complexes show several bands in the 945-1050 nm region, corresponding to F->F transitions arising from the ligand field splitting of both multiplets. The observed luminescence lifetimes . are 0.515 and 10 ?s for 2 and 3, respectively. The shorter lifetime for 2 is due to the presence of one coordinated water molecule on the Yb 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. © 2014 American Chemical Society.