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).

Keywords:

• Ruthenium tetrapyridine
• N-methyl-4,4′-bipyridinium
• Supramolecular building block
• Red sensitization

Acknowledgments

A.C. is a postdoctoral fellow and J.H.H. a member of the scientific staff of CONICET. The authors thank Dr Pablo Alborés for crystal structure refinement, Dr Fabio Cukiernik for fruitful discussions about halogen bonding, and Dr Leonardo Slep for revising this manuscript. A.C. acknowledges ALN for the inspiring brainstorming sessions.

Additional information

Funding

This work was supported by the University of Buenos Aires (UBA) [grant number UBACYT 01-w971]; the Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT) [grant number PICT-2012-2041], [grant number PICT-2009-107].