Bipyridine

Bipyridines also known as bipyridyls, dipyridyls, and dipyridines, are a family of chemical compounds with the formula (C₅H₄N)₂, which are formed by the coupling of two pyridine rings. They are aromatic nitrogen heterocycles that form complexes with most transition metals. They interact with metals via both σ-donating nitrogen atoms and π-accepting molecular orbitals. Bipyridines figure prominently in studies of electron and energy transfer, supramolecular and materials chemistry, and catalysis.

Six isomers of bipyridine exist, but two isomers are prominent: 2,2'-bipyridine is a popular ligand in coordination chemistry and 4,4'-bipyridine is a precursor to the herbicide paraquat. The bipyridines are colourless solids, which are soluble in organic solvents and slightly soluble in water.

The 3,4'-Bipyridine derivatives inamrinone and milrinone are used occasionally for short term treatment of congestive heart failure. They inhibit phosphodiesterase and thus increasing cAMP, exerting positive inotropy and causing vasodilation. Inamrinone causes thrombocytopenia. Milrinone decreases survival in heart failure.

2,2'-Bipyridine

2,2'-Bipyridine (bipy or bpy (pronounced "bip-ee")) 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 complex and platinum complexes of bipy exhibit intense luminescence, which may have practical applications.

Preparation 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 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.

4,4'-Bipyridine

4,4'-Bipyridine (4,4'-bipy) is a bipyridine which is mainly used as a precursor to N,N'-dimethyl-4,4'-bipyridinium [(C₅H₄NCH₃)₂]²⁺, known as paraquat. This species is electroactive, and its toxicity arises from the ability of this dication to interrupt biological electron transfer. Because of its structure, 4,4'-bipyridine can bridge between metal centres to give coordination polymers. 4,4'-bpy can also mediate electronic effects between two paramagnetic metal centers.

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