Binuclear metal ruthenium complexes bridged by isomeric bis(ethynyl)pyridine: Syntheses, characterization and electronic coupling properties

Abstract

Isomeric diruthenium complexes with bis(ethynyl)pyridine as bridge core [2,6-bis(ethynyl)pyridine (1), 3,5-bis(ethynyl)pyridine (2), 2,5-bis(ethynyl)pyridine (3)] were successfully prepared and characterized by 1H NMR, 13C NMR and elemental analysis. The electrochemical properties of complexes 1–3 were investigated by cyclic voltammetry (CV) and square-wave voltammetry (SWV), and different potential difference values were revealed and affected by N atom position and distance between redox metal centers. Electron absorption spectra showed that the absorption of the linear molecule 3 displayed a red shift relative to that of complexes 1 and 2. The broad near-infrared absorption characteristic of the singly-oxidized complex 3+ revealed the existence of a mixed-valence state, which can be classified as a class II mixed-valence system according to the intervalence charge transfer (IVCT) absorption parameter Hab and Г. These results indicated that 3 has higher conjugation and charge delocalization ability than isomers 1 and 2. Density functional theory (DFT) calculations spin density distribution analysis further confirmed the above results, and time-dependent DFT showed the NIR multi-absorption of complex 3+, and effectively attributed to Ru–CC→Ru IVCT absorption characteristics.