Tetranuclear homo- (ZnII4and CdII4) and hetero-metal (ZnII2TbIII2and CdII2TbIII2) complexes with a pair of carboxylate ligands in a rare η2:η2:μ4-bridging mode: syntheses, structures and emission properties

Abstract

Four tetranuclear complexes involving both homo- and hetero-metal combinations, viz. [Zn(II)(2)L(2)(μ(4)-PhCOO)(2)Zn(II)(2)(hfac)(2)] (1), [Cd(II)(2)L(2)(μ(4)-PhCOO)(2)Cd(II)(2)(hfac)(2)] (2), [Zn(II)(2)L(2)(μ(4)-PhCOO)(2)Tb(III)(2)(hfac)(4)] (3), and [Cd(II)(2)L(2)(μ(4)-PhCOO)(2)Tb(III)(2)(hfac)(4)] (4) have been prepared following a single-pot synthesis protocol using N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5-dimethylbenzyl)ethylenediamine (H(2)L) as a primary ligand. Both benzoate and hexafluoroacetylacetonate (hfac(-)), used here as ancillary ligands, play crucial roles in generating a tetranuclear core with high thermodynamic stability. Oxygen atoms of each carboxylate moiety bind all the four metal centers together in a rare η(2):η(2):μ(4)-bridging mode as confirmed by X-ray crystallography. In the homo-metallic complexes (1 and 2), the metal centers are all lying in a square plane, each occupying a corner, and remain connected together by oxygen bridges forming octagonal metallacrowns. These structures remain intact in solution as confirmed by (1)H NMR spectroscopy and photoluminescent studies. In the hetero-metal complexes (3 and 4), the metal centers are arrayed in alternate positions of the tetranuclear core. The Tb(III) centers have eight coordinate bi-capped trigonal prismatic coordination environments with different degrees of distortions. The all oxygen O(8) core surrounding each Tb(III) center is devoid of solvent molecules that make fluorescent emission from these molecules (3 and 4) quite interesting. The hfac(-)-based (1)(π-π*) emissions observed in 1 and 2 are quenched in 3 and 4. These sensitized Tb(III) emissions [(5)D(4)→(7)F(j); j = 6, 5, 4, and 3] are influenced by the local environments surrounding the 4f-metal center. The lifetime for the luminescence decay of 3 ((5)D(4)→(7)F(5) transition) is about 1.5 times longer than that of 4 in all the solvents studied at 298 K.