Tetranuclear and dinuclear phenoxido bridged copper complexes based on unsymmetrical thiosemicarbazone ligands

Abstract

We report on the synthesis of new dinucleating phenol-based “end-off” compartmental ligands HL^MeH and HL^Me2 bearing two different binding sites, one bis(2-methylpyridyl)aminomethyl (BPA) and one thiosemicarbazone (TSC) site, and their corresponding copper(II) complexes 1t and 2d. With the ligand HL^MeH, a tetranuclear entity (1t) has been isolated in the solid state, whereas with HL^Me2, which differs from HL^MeH by a methyl substituent on the N-terminal amino group of the TSC arm, a dinuclear form (2d) is obtained. X-ray crystallography analysis shows that the nuclearity di vs. tetra is modulated by interactions between copper atoms and hydroxido bridges along with the sulphur atoms of TSC arms. From a magnetic point of view, 1t can be considered as an association of two dinuclear forms leading for both complexes to overall antiferromagnetic coupling. Analysis in acetonitrile solution of structure–property relationships has been carried out by comparing their UV/Vis, electrochemistry, ESI-MS, and NMR (variable temperature and DOSY = diffusion ordered spectroscopy) properties with trends from computational calculations (DFT). HRMAS-DOSY (High Resolution Magic Angle Spinning) NMR spectroscopy has been performed to evaluate the presence of different species in solution at room temperature.