2,2′-Bipyridine


2,2′-Bipyridine is an organic compound with the formula C10H8N2. This colorless solid is an important isomer of the bipyridine family. It is a bidentate chelating ligand, forming complexes with many transition metals. Ruthenium and platinum complexes of bipy exhibit intense luminescence, which may have practical applications.

Preparation, structure, and general properties

It is prepared by the dehydrogenation of pyridine using Raney nickel:
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 complexation, the related N,N-heterocyclic ligand phenanthroline does not incur an enthalpic and entropic penalty, and thus its complexes tend to be more stable. With respective pKa's of 4.86 and 4.3 for their conjugate acids, phenanthroline and bipy are of comparable basicity.
Reflecting the popularity of this ligand design, many substituted variants of bipy have been described.

Coordination chemistry

Illustrative complexes

Bipyridine complexes absorb intensely in the visible part of the spectrum. The electronic transitions are attributed to metal-to-ligand charge transfer. In the "tris complexes" three bipyridine molecules coordinate to a metal ion, written as n+. These complexes have six-coordinated, octahedral structures and two enantiomers as follows:
Image:Fe-Bipy-3.png|thumb|right|Three-dimensional view of the 2+ complex.
These and other homoleptic tris-2,2′-bipy complexes of many transition metals are electroactive. Often, both the metal centred and ligand centred electrochemical reactions are reversible one-electron reactions that can be observed by cyclic voltammetry. Under strongly reducing conditions, most tris complexes can be reduced to neutral derivatives containing bipy− ligands. Examples include M3, where M = Al, Cr, Si.