Unsymmetric diruthenium complexes

The new diruthenium complexes trans-[(NH3)5Ru(L–L)Ru(NH3)4(bpy-Me)](PF6)5 (L–L are the bridging ligands pyrazine, 2a; 4,4′dipyridyl, 2b; and trans-1,2-bis(4-pyridyl)-ethylene, 2c; bpy-Me is N-methyl-pyridyl-pyridinium) are generated from the new complexes (L–L)Ru(NH3)4(bpy-Me)](PF6)3 (1a–c) and [(NH3)5Ru(H2O)](PF6)2. Cyclic voltammetry on the new compounds in acetonitrile electrolyte reveals two quasi-reversible oxidation steps corresponding to the two RuII/III couples in 2b and 2c and a single oxidations for 1a–c. In addition, two reduction waves are observed for the bpy-Me ligands of 1a–c and 2a–c. All of the new compounds exhibit multiple metal-to-ligand charge transfer (MLCT) bands in the visible region of the spectrum. For compounds 1a–c the most intense absorption in the visible region decreases in energy as the length of the aromatic bridging ligands increases. By contrast the strongest absorption band of 2a is lower in energy than the most intense bands of both 2b and 2c. Single crystal X-ray analysis of 2a reveals that the bridging pyridine is coplanar with the pyridyl ring of the bpy-Me ligand that is attached to the Ru center. DFT calculations on 2a indicate that the HOMO is localized predominately on the {(NH3)5Ru-pyz-Ru(NH3)4} portion of the complex and the LUMO has slightly more contribution from the bpy-Me ligand.

Graphical abstractCyclic voltammogram of trans-[(NH3)5Ru-bpy-RuII(NH3)4(bpy-Me)](PF6)6.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Synthesis and characterization of unsymmetric diruthenium complexes. ► Terminal pentaaminoruthenium center is readily oxidized while distal methyl-pyridinium ligand is most easily reduced. ► The metal-to-ligand charge transfer bands arise from a delocalized HOMO and LUMO.