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 alpha -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 letrakis-(1-pyrazolyl)borate anion (B(pz)(4)(-); n=1). The complexes each exhibit strong visible absorption assigned as a pi*(L)<-- d<pi>(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 pi -acceptor abilities of the coordinated Z are: Z = (1H-pyrazolyl)(2) similar to (pz)(2)B(pz)(2) < (pyridine)(2) < (pz)(2)CH(pz). Uncoordinated pz groups of cis-(bpy)(2)Ru(pz)(2)B(pz)(2)(+) can be reacted to form a sterically hindered, localized-valence (K-com similar to 33 1 mol(-1)) cis,cis-(bpy)(2)Ru-II(pz)(2)B(pz)(2)Ru-II(bpy)(2)(3+) dimer. The dimer properties are interpreted by comparison to the known cis-(bpy)(2)Ru-II(pz)(2)Ru-II(bpy)(2)(2+) analog. The dimer is photoreactive and undergoes an asymmetrical photocleavage in CH3CN (yielding cis-(bpy)(2)Ru-III(pz)(2)B(pz)(2)(2+) and cis-(bpy)(2)R-II(CH3CN)(2)(2+)), similar to the corresponding thermal reaction observed for the mixed-valence cis-(bpy)(2)Ru-II(pz)(2)Ru-III(bpy)(2)(3+) system. (C) 2000 Elsevier Science B.V. All rights reserved.