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

ABSTRACT

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 ZnII-LnIII heterometallic species. Herein, three types of ZnII-LnIII heterometallic compounds (LnIII = GdIII, TbIII) 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 ZnII-LnIII heterometallic species but also provides an understanding of how common inorganic anions tune coordination-driven self-assemblies as well as the subsequent luminescence properties.