Luminescent α-diimine complexes of ruthenium(II) containing scorpionate ligands

We describe the synthesis, characterization, and reactivity of several Ru(II) complexes of the type cis-L2Ru(Z)n+, where L is an α-diimine [e.g. 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen)] ligand and Z is a bis-coordinated scorpionate ligand such as tris-(1-pyrazolyl)methane (HC(pz)3, pz=1-pyrazolyl; n=2) or tetrakis-(1-pyrazolyl)borate anion (B(pz)4 −; n=1). The complexes each exhibit strong visible absorption assigned as a π*(L)←dπ(Ru) metal-to-ligand charge-transfer (MLCT) transition characteristic of the cis-L2Ru2+ kernel. A corresponding MLCT excited state emission is observed in room temperature CH3CN solution, although emission energies, lifetimes, and quantum yields are reduced relative to Ru(bpy)3 2+. Electronic spectra and cyclic voltammetry measurements indicate that the relative π-acceptor abilities of the coordinated Z are: Z=(1H-pyrazolyl)2∼(pz)2B(pz)2<(pyridine)2<(pz)2CH(pz). Uncoordinated pz groups of cis-(bpy)2Ru(pz)2B(pz)2 + can be reacted to form a sterically hindered, localized-valence (Kcom∼33 l mol−1) cis,cis-(bpy)2RuII(pz)2B(pz)2RuII(bpy)2 3+ dimer. The dimer properties are interpreted by comparison to the known cis-(bpy)2RuII(pz)2RuII(bpy)2 2+ analog. The dimer is photoreactive and undergoes an asymmetrical photocleavage in CH3CN (yielding cis-(bpy)2RuIII(pz)2B(pz)2 2+ and cis-(bpy)2RuII(CH3CN)2 2+), similar to the corresponding thermal reaction observed for the mixed-valence cis-(bpy)2RuII(pz)2RuIII(bpy)2 3+ system.