Assembly of a Series of Trinuclear Zinc(II) Compounds with N₂O₂ Donor Tetradentate Symmetrical Schiff Base Ligand

Abstract

Self-assembly of the symmetrical tetradentate N 2 O 2 donor Schiff base ligand, that is, bis(salicylidene)trimethylenediamine (sy-H 2 L 3 ), with different zinc salts has led to six anion-dependent complexes, namely, [Zn 3 (sy-L 3 ) 2 (μ 2 -Cl)Cl(C 2 H 5 OH)]·C 2 H 5 OH ( 1 ), Zn[Zn(sy-L 3 )(μ 2 -NO 2 )(C 2 H 5 OH)] 2 ·2H 2 O ( 2 ), Zn[Zn(sy-L 3 )(μ 2 -CH 3 COO)] 2 ( 3 ), Zn(DMF) 2 [Zn(sy-L 3 )(H 2 O)(SCN)] 2 ( 4 ), Zn[Zn(sy-L 3 )(μ 2 -HCOO)(DMF)] 2 ·H 2 O ( 5 ) and Zn[Zn(sy-L 3 )(μ 2 -HCOO)] 2 ( 6 ), respectively. Six compounds were characterized by elemental analyses, IR spectra, mass spectrometry and thermogravimetric analyses, of which five structures 1, 2, 4, 5, and 6 were further determined by single-crystal X-ray diffraction analyses, and compound 3 was previously reported in the literature. It was observed that μ 2 -bridges in these trinuclear complexes were formed through the phenolic oxygens of the ligand or the anions in the medium. In the presence of nitrite (for 2 ), acetate (for 3 ), thiocyanate (for 4 ), or formate (for 5 − 6 ), μ 2 -bridges formed between a pair of adjacent zinc atoms resulting in linear trinuclear complexes with one symmetrical center. However, μ 2 -bridges involving chloride and the phenolic oxygens of ligand sy-H 2 L 3 led to the formation of a V-shaped trinuclear complex 1 . It is noteworthy that in the assembling process of complexes 4 − 6, when the appropriate amount of DMF was added to the reaction mixture, thiocyanate anion coordinated to the terminal Zn 2+ ion in a η 1 -N coordination manner and gave compound 4 with a coordination of water molecules and DMF molecules to the terminal and the middle zinc ions in a range from room temperature to 70 °C. Above ∼70 °C, C−N bond cleavage of part of the solvent DMF molecules induced the formation of compound 5 with coordination of formate and DMF to Zn 2+ . Upon increasing the temperature higher than 110 °C, because of the decomposition of a large number of DMF molecules, the same reaction produced trinuclear complex 6, in which only formate coordinated to a pair of the adjacent zinc atoms in a μ 2 -bridging coordination fashion. In addition, photoluminescent properties and biological activities of ligand sy-H 2 L 3 and six corresponding compounds were also studied.