Tetranuclear homo- (ZnII4 and CdII4) and hetero-metal (ZnII2TbIII2 and 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^II₂L₂(μ₄-PhCOO)₂Zn^II₂(hfac)₂] (1), [Cd^II₂L₂(μ₄-PhCOO)₂Cd^II₂(hfac)₂] (2), [Zn^II₂L₂(μ₄-PhCOO)₂Tb^III₂(hfac)₄] (3), and [Cd^II₂L₂(μ₄-PhCOO)₂Tb^III₂(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 (H₂L) 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 η²:η²:μ₄-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 ¹H 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 O₈ 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 [⁵D₄ → ⁷F_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 (⁵D₄ → ⁷F₅ transition) is about 1.5 times longer than that of 4 in all the solvents studied at 298 K.