Anion-Dependent Structure and Luminescence Diversity in ZnII-LnIII Heterometallic Architectures Supported by a Salicylamide-Imine Ligand

To advance the structural development and fully explore the application potential, it is highly desirable but challenging to elucidate the relationship between the structures and properties of Zn^II-Ln^III heterometallic species. Herein, three types of Zn^II-Ln^III heterometallic compounds (Ln^III = Gd^III, Tb^III) formulated as [Zn₁₆Ln₄L₁₂(μ₃-O)₄(NO₃)₁₂]·8CH₃CN (ZnLn-1), [Zn₂Ln₂L₂(NO₃)₆(H₂O)₂]·3CH₃CN (ZnLn-2), and [Zn₄Ln₂L₈(OAc)₁₂]·xCH₃CN (ZnLn-3: for Ln = Gd, x = 5; for Ln = Tb, x = 4) were dictated by common inorganic anions, NO₃^- and OAc^-, with the aid of the multidentate ligand H₂L with propane as the central skeleton and 3-methoxysalicylamide and 3-methoxysalicylaldimine as terminal groups. ZnLn-1 features cubic cages with four {Zn₄L₃} tetrahedral subunits and four Ln³⁺ centers positioned at the eight vertices alternately when NO₃^- was introduced into the reaction system exclusively. An attempt to replace NO₃^- in ZnLn-1 with OAc^- partially led to the formation of {Zn₂Ln₂L₂} heterometallic wheels. Meanwhile, ZnLn-3 featuring double-hairpin-like {Zn₄Ln₂L₄} hemicycles that are orthogonal to each other assisted by intermolecular hydrogen bonds was constructed when NO₃^- in ZnLn-1 was completely replaced by OAc^-. Their structural integrity in solution were ascertained by both emission and ¹H NMR spectroscopy. Ascribed to the different Zn²⁺-containing antenna, ZnTb-2 possesses a relatively strong emission characteristic of Tb³⁺; ZnTb-1 has moderate Tb³⁺ luminescence, yet an absence of Tb³⁺ emission is found in ZnTb-3. Such an emission difference could be mainly attributed to the antenna effect directed by distinct structural characteristics induced by anions. The anion-dictated self-assembly strategy presented herein not only offers a facile approach to regulate the coordination mode of H₂L to such an extent to obtain diverse structures of Zn^II-Ln^III heterometallic species but also provides an understanding of how common inorganic anions tune coordination-driven self-assemblies as well as the subsequent luminescence properties.