Bidentate ligands that contain pyrrole in place of pyridine

A series of ligands are prepared that are analogues of benzo-fused derivatives of 2,2'-bipyridine (bpy), in which pyrrole has been substituted for a pyridine ring. These ligands include 2-(2'-pyridyl)-indole, 2-(2'-pyrrolyl)-quinoline, and pyrrolo[3,2-h]quinoline. A novel reductive cyclization approach to the latter species is presented. All these ligands react with [Ru(bpy-d8)2Cl2], undergoing cyclometalation with concurrent deprotonation, to form complexes of the type [Ru(L)(bpy-d8)2]+ where L binds as a monoanionic ligand. The complexes are readily characterized by their 1H NMR spectra. Changes in the redox potentials and the electronic absorption spectra of both the ligands and the complexes are interpreted in terms of charge delocalization on the ligand.