Preparation, crystal structures, electrochemical and spectroscopic properties of bis(2,2'-bipyridine)ruthenium(II) complexes containing 8-(diphenylphosphino)quinoline or 2-(diphenylphosphino)pyridine

Mixed-ligand bis(2,2'-bipyridine)ruthenium(II) complexes containing a hybrid donor-type P-N ligand of 8-(diphenylphosphino)quinoline (Ph2Pqn) or 2-(diphenylphosphino)pyridine (Ph₂Ppy), [Ru(bpy)₂(Ph₂Pqn or Ph₂Ppy)](PF₆)₂ (1 or 2; bpy = 2,2'-bipyridine), have been prepared. The X-ray crystal structure analyses of 1 and 2 revealed that Ph₂Pqn and Ph₂Ppy act as a bidentate ligand to form a five- and four-membered chelate ring, respectively. The four-membered chelate ring formed by Ph₂Ppy has a very small bite angle (P(1)-Ru-N(1): 68.4(1)°), but the Ru^II(bpy)₂ moiety in 2 has a typically strain-free structure. On the other hand, in the Ph₂Pqn complex 1, one of the bpy planes is skewed remarkably from the Ru coordination plane due to an intramolecular steric interaction with a phenyl ring of Ph₂Pqn. The redox potentials and the ¹MLCT transition energies of the Ph₂Pqn and Ph₂Ppy complexes (1 and 2) were not so different from the averaged values of those of the corresponding 1,2-bis(diphenylphosphino)benzene (dppb) and 1,10-phenanthroline (phen) complexes, [Ru(bpy)₂(dppb or phen)](PF₆)₂ (3 or 4). These facts indicate that the electronic asymmetry inherently present in Ph₂Pqn and Ph₂Ppy does not affect remarkably the ground state properties of the complexes. However, unlike complexes 2-4, the Ph₂Pqn complex 1 in EtOH/MeOH (4:1) glass at 77 K exhibited a novel dual emission, giving a biexponential emission decay. The shorter-lived (τ = 10.8 μs) emission is attributable to the bpy-based ³MLCT emission, similar to the other Ru^II-polypyridine complexes, while the longer-lived (τ = 62.1 μs) emission can probably be assigned as arising from the quinoline-based ³(π-π*) excited state.