Coordination-Driven Self-Assembled Zn^II₆-Ln^III₃ Metallocycles Based on a Salicylamide Imine Ligand: Synthesis, Structure, and Selective Luminescence Enhancement Induced by OAc^–

Abstract

Five heterometallic ZnII6-LnIII3 macrocycles based on a salicylamide imine multidentate unsymmetrical ligand H2L [1-(2-hydroxy-3-methoxy-benzamido)-2-(2-hydroxy-3-methoxy-benzylideneamino)-ethane] have been prepared via a coordination-driven self-assembly strategy. Single-crystal X-ray diffraction analysis reveals that the five metallocycles are isomorphic with a formula of [Zn6Ln3L6(OH)2(NO3)4(H2O)]·3NO3·nCH3CN (ZnLn-1, where Ln = Pr, Nd, Eu, Tb, or Yb; for ZnPr-1, n = 4; for ZnNd-1, ZnEu-1, and ZnTb-1, n = 2; for ZnYb-1, n = 3), where six octadentate ligands L2– and two in situ-formed μ2-OH– ions bridged the alternating ZnII-LnIII-ZnII subunits into a macrocycle. Along with the structural novelty, ZnNd-1 shows distinctive enhanced emission in the visible and near-infrared range upon addition of OAc–. On the basis of ultraviolet–visible spectroscopy, 1H nuclear magnetic resonance, single-crystal X-ray diffraction, and high-resolution electrospray ionization mass spectrometry, we deduced that this emission enhancement could be attributed to the synergistic effect of TICT and the absent nonradiative transition of μ2-OH– induced distinctively by OAc– bridging. Our results demonstrate that the NdIII-containing heterometallic macrocycle can act as a host for anion exchanging and provide a nice example of heterometallic macrocycles with interesting properties and potential applications.