Rational design of triple-bridged dinuclear ZnIILnIII-based complexes: a structural, magnetic and luminescence study

Abstract

Twelve novel dinuclear Zn^II–Ln^III complexes of general formula [Zn(μ-L)(μ-OAc)Ln(NO₃)₂] (Ln^III = Y (1), Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), Tb (7), Dy (8), Ho (9), Er (10), Tm (11), Yb (12)) have been prepared from the symmetric compartmental Mannich base ligand N,N′-dimethyl-N,N′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)-ethylenediamine (H₂L). Structural determinations indicate that in all these complexes, Zn^II ions are pentacoordinated in the inner position of the ligand, linked to two nitrogen and two oxygen atoms from L²⁻ and a bridging syn–syn acetate group. The Ln^III ions are nonacoordinated in the outer position, linked to four oxygen atoms from L²⁻, two bidentate nitrate anions and the bridging acetate. Complexes 3, 6, 7, 8, 10 and 12 exhibit field induced slow relaxation of the magnetization. It should be remarked that this behaviour is very infrequent in Gd^III and Nd^III coordination compounds. For compounds 8 and 12, it was possible to extract the effective energy barrier (U_eff) for the reversal of the magnetization, with a value of 27.5 K (52.6 K considering the Raman relaxation process in addition to the Orbach mode) and 13.2 K for Dy^III and Yb^III compounds, respectively. Moreover, the photophysical properties of these complexes have been studied at different temperatures and show that L²⁻ is able to sensitize luminescence emission of many lanthanide ions. Sm^III, Eu^III, Tb^III, Dy^III and Tm^III-based compounds show fluorescence in the visible region, whereas the Yb^III-based one is sensitized in the near-infrared (NIR) region. Therefore, complexes 7, 8 and 12 may be considered as dual magneto-luminescent materials.