Facile Synthesis of Bistridentate Ru^II Complexes Based on 2,6-Di(quinolin-8-yl)pyridyl Ligands: Sensitizers with Microsecond ³MLCT Excited State Lifetimes

Abstract

Synthetic routes to meridional bistridentate ruthenium(II) complexes based on 2,6-di(quinolin-8-yl)pyridyl (dqp) ligands have been investigated. Microwave-assisted synthesis at 200 degrees C allowed the high yield (49-87%) preparation of homoleptic meridional [Ru(dqp)(2)](2+)-based complexes containing inert functional groups. Applying this protocol for the synthesis of mer-[Ru(dqp)(2)](2+) (mer-1) but lowering the temperature to 180 degrees C and shorter reaction times revealed the formation of the facial isomers cis,fac-1 and trans,fac-1 (56% and 12% yields, respectively). The facial isomers were characterized by NMR spectroscopy and X-ray diffraction analysis. In a stepwise protocol, the reaction of Ru(dqp)Cl(3) or Ru(dqp)(L)Cl(2) (L = MeCN or DMSO) and a second equivalent dqp gave mer-1 in 12-26% yields and N(5)Cl-coordinated [Ru(dqp)(2)Cl](+) (28-46%). [Ru(dqp(2))Cl](+) was photochemically, or thermally in the presence of Ag(I), converted to mer-1. By using mer-[Ru(dqp)(MeCN)(3)] (2+), which was crystallographically characterized, a wide range of homo- and heteroleptic meridional [Ru(dqp)(2)](2+)-based complexes was synthesized in up to 77% yield. The synthetic utility of meridional [Ru(dqp)(2)](2+)-based complexes as building blocks was demonstrated by palladium-catalyzed homocoupling of mer-[Ru(dqp)(dqpPhBr)](2+) to form a dinuclear complex. The redox and photophysical properties of the meridional complexes are discussed.