Synthesis and characterization of dinuclear ruthenium complexes with tetra-2-pyridylpyrazine as a bridge

The syntheses and characterization of a series of complexes of the general formula [Cl(bpy')Ru(tppz)Ru(bpy')Cl](PF6)(2), where tppz is tetra-2-pyridylpyrazine and bpy' is 2,2'-bipyridine or substituted 2,2'-bipyridine, are described. Preparations with 2,2'-bipyridine or 4,4'-dimethyl-2,2'-bipyridine (1 and 2, respectively) give mixtures of two isomers. The isomers of 2 were separated chromatographically and the X-ray crystal structure of the cis isomer was solved. Significant distortion is observed in both the coordination geometry around each ruthenium atom and in the tppz ligand resulting in short intramolecular Ru ... Ru and Cl ... Cl separations (6.558(1) and 5.880(2) Angstrom, respectively). Ligands containing two bipyridine groups linked through eight- and nine-atom bridges have also been synthesized. The length of the bridge between bipyridine groups allows for the formation of cis complexes 3-5 having the general formula shown above but prevents formation of a trans isomer.