Four chromophores in one building block: Synthesis, structure and characterization of trans -[Ru(MQ)4Cl2]4+ and trans -[Ru(4,4-bpy)4Cl2] (MQ+ = N-methyl-4,4-bipyridinium, bpy = bipyridine)

Abstract

We report the synthesis, crystal structure, electrochemical and spectroscopic properties of trans-[Ru(MQ)4Cl2]4+ (14+), where MQ+ = N-methyl-4,4′-bipyridinium. The crystal structure of 14+ shows the pyridinic rings bound in a trans fashion to the Ru ion. In 14+ and in its analog trans-[Ru(4,4′-bpy)4Cl2] (2, bpy = bipyridine), the electrochemical reduction waves of the four iminic ligands occur at very closely spaced potentials. In comparison with previously reported analogues, 14+ and 2 present high molar extinction coefficients (ε = 20,000-50,000 M-1cm-1), and red-shifted absorptions (up to 800 nm for 14+ and up to 600 nm for 2) in acetonitrile. Upon protonation of the four exposed nitrogens of 2 and electrochemical reduction of the ligands in 14+ and 2, spectral changes occur with retention of isosbestic points, revealing the presence of four independent chromophores. These results suggest poor electronic communication between the aromatic ligands within each compound, in contrast to related complexes such as cis-[Ru(2,2′-bpy)2Cl2] (3).