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

Abstract

We describe the synthesis, characterization, and reactivity of several Ru(II) complexes of the type cis-L 2Ru(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-L 2Ru 2+ kernel. A corresponding MLCT excited state emission is observed in room temperature CH 3CN 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 ( K com∼33 l mol −1) cis, cis-(bpy) 2Ru II(pz) 2B(pz) 2Ru II(bpy) 2 3+ dimer. The dimer properties are interpreted by comparison to the known cis-(bpy) 2Ru II(pz) 2Ru II(bpy) 2 2+ analog. The dimer is photoreactive and undergoes an asymmetrical photocleavage in CH 3CN (yielding cis-(bpy) 2Ru III(pz) 2B(pz) 2 2+ and cis-(bpy) 2Ru II(CH 3CN) 2 2+ ), similar to the corresponding thermal reaction observed for the mixed-valence cis-(bpy) 2Ru II(pz) 2Ru III(bpy) 2 3+ system.