2,9-Di-(2‘-pyridyl)-1,10-phenanthroline:  A Tetradentate Ligand for Ru(II)

The tetradentate ligand 2,9-di-(2‘-pyridyl)-1,10-phenanthroline is synthesized in 62% yield by the Stille coupling of 2,9-dichloro-1,10-phenanthroline and 2-(tri-n-butylstannyl)pyridine. Treatment of this ligand with RuCl₃·3H₂O and a 4-substituted pyridine results in the formation of a complex in which the tetradentate ligand occupies the equatorial plane and two pyridines are bound axially. The interior N−Ru−N angles vary from 76.1° to 125.6°, showing considerable distortion from the 90° ideal. The lowest energy electronic transition is sensitive to the electronegativity of the 4-substituent on the axial pyridines, varying from 516 nm for the CF₃ group to 580 nm for the NMe₂. The oxidation potentials mirror this trend, spanning a range of 1.36−1.03 V, while the reduction potentials show less variation (−0.97 to −1.08 V). The complexes are nonemissive, presumably due to competitive nonradiative processes caused by distortion of the system.