2-(2-Pyridyl)pyridine is colorless to off-white crystal powder, an important isomer of the bipyridine family. It is a bidentate chelating ligand, forming complexes with many transition metals. For example, the complex of Fe(II) ion with 2,2’-bipyridine CAS: 366-18-7 is used for colorimetric analysis; N719 dye CAS:207347-46-4 is complex of Ru(II) with 2,2’-bipyridine-4,4’-dicarboxylic acid CAS: 6813-38-3, used for dye-sensitized solar cell (DSSC) with very high light to electricity conversion efficiency over 10%.
2-(2-Pyridyl)pyridine is prepared by the dehydrogenation of pyridine using Raney nickel
2C5H5N → (C5H4N)2 + H2
Although uncoordinated bipyridine is often drawn with its nitrogen atoms in cis conformation, the lowest energy conformation both in solid state and in solution is in fact coplanar, with nitrogen atoms in trans position. Protonated bipyridine adopts a cis conformation. Upon binding to metal ions the related N,N-heterocyclic ligand phenanthroline does not incur an enthalpic and entropic penalty, and thus its complexes tend to be more stable.
Reflecting the popularity of this ligand design, many substituted variants of bipy have been described.
Bipyridine complexes absorb intensely in the visible part of the spectrum. The electronic transitions are attributed to metal-to-ligand charge transfer (MLCT). In the "tris(bipy) complexes" three bipyridine molecules coordinate to a metal ion, written as [M(bipy)3]n+ (M = metal ion; Cr, Fe, Co, Ru, Rh and so on; bipy = 2,2′-bipyridine). These complexes have six-coordinated, octahedral structures and two enantiomers as follows: