Syntheses and structural characterizations of 24-membered dimetal (Mn, Ni, Fe) macrocyclic complexes and the C–S bond formation between acetylacetone and a mercapto N-heterocycle

Abstract

An organic ligand 2,5-di(3-pentanedionylthio)-1,3,4-thiadiazole (H₂L) reacts with metal (Mn, Ni, Fe) salts, resulting in 24-membered dimetal macrocyclic complexes [MnL(H₂O)(dmso)]₂·2dmso (1), [NiL(H₂O)(dmf)]₂·2dmf (2), [MnL(dmf)₂]₂ (3) and [Fe₂L₂(solvent)₂(SO₄)] (solvent = dmso (6); H₂O (7); dmf (8)). Di-manganese macrocyclic complexes [MnL(dmf)(dmso)]₂ (4) and [MnL(H₂O)₂]₂·6H₂O (5) can also be obtained directly by aerobic assembly reaction of MnCl₂, dipotassium 1,3,4-thiadiazole-2,5-dithiolate (K₂tdadt) and acetylacetone (H₂acac) in various solvents, accompanying a C–S bond formation between acetylacetone and the mercapto N-heterocycle. Disulfide has been considered as the intermediate in the assembly reaction. Meanwhile an assembly reaction including MnCl₂, 2-mercaptobenzimidazole and H₂acac has produced an organic compound 2-(3-pentanedionylthio)benzimidazole with a new C–S bond. These dimetal complexes have similar macrocyclic structures, in which solvent molecules and sulfate coordinate to the octahedral metal in trans-configuration, whereas a pair of water molecules are located in octahedral cis-positions for 5 owing to a small steric effect. A host cavity of sufficiently large size exists in the macrocyclic structure to trap the solvent molecules and the sulfate anion. The IR spectra have been used to assign the solvent molecules trapped and the sulfate anion which is shown as a bridged bidentate ligand. Thermal analyses show the stability of the macrocyclic backbone below 200 °C and gradual release processes of the trapped solvent molecules. Decomposition and oxidation of the dimetal macrocycle backbone occur at 300–500 °C, resulting in a metal sulfate. Further decomposition led to metal oxide at 500–600 °C.