Mononuclear and dinuclear ruthenium complexes of 2,3-Di-2-pyridyl-5,6- diphenylpyrazine: Synthesis and spectroscopic and electrochemical studies

Reported here are a new bridging ligand, 2,3-di-2-pyridyl-5,6- diphenylpyrazine (dpdpz), and its complexation with one or two ruthenium atoms. This ligand was designed so that it could bind to metal species in either a N^∧N bidentate fashion or a C^∧N^∧N tridentate mode to form a metallacycle. The reaction between dpdpz and (tpy)RuCl₃ (tpy = 2,2′:6′,2″-terpyridine) afforded C^∧N^∧N-type mono- and dinuclear cyclometalated complexes in moderate yields. On the other hand, N^∧N-type mono- and dinuclear noncyclometalated complexes could be isolated from the reaction of dpdpz with (bpy)₂RuCl₂ (bpy = 2,2′-bipyridine). An asymmetric diruthenium complex, bridged by dpdpz, was prepared with one ruthenium atom cyclometalated and another one noncyclometalated. The electronic properties of these complexes were probed by electrochemical and spectroscopic techniques. They exhibited multiple reversible redox processes. However, the formal potentials and electrochemical energy gap are greatly dependent on the binding nature and number of ruthenium atoms. As indicated by electrochemical and spectroelectrochemical studies, diruthenium complexes bridged by dpdpz exhibited electronic coupling between the two metal centers. A comparison of the electronic absorption and emission properties of these complexes is also presented.