Badly behaving bipyridine: the surprising coordination behaviour of 5,5′-substituted-2,2′-bipyridine towards iron(II) and ruthenium(II) ions

A new tetrapyridine ligand, 5,5′-(3-pyridyl)-2,2′-bipyridine (2) was prepared from 5,5′-dibromo-2,2′-bipyridine (1) via Pd(0)-mediated Suzuki coupling. The reaction was performed either directly upon the free ligand, or upon its Ru(II) complex. The structures of [Ru(1)₃](PF₆)₂ and [Ru(2)₃](PF₆)₂ were studied in solution by NMR, and in the solid state by single crystal X-ray diffraction. Solution NMR data suggests distorted structures, consistent with the observed slow reactivity of the ligands towards Ru(II) ions, and their reluctance to coordinate to Fe(II) ions. However, solid-state X-ray data indicated little distortion from ideal geometries, suggesting the effect may be more electronic than steric in nature.