2,2'-Bipyridine

2,2'-Bipyridine is a chemical compound with the formula (C₅H₄N)₂. This colorless solid, commonly abbreviated bipy (pronounced "bip-ee"), is a widely used isomer of bipyridine. 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.

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Preparation and general properties

It is prepared by the dehydrogenation of pyridine using Raney nickel.^[1] Reflecting the popularity of this ligand design, many substituted variants of bipy have been described.^[2][3]

Bipyridine complexes absorb intensely in the visible part of the spectrum. The electronic transitions are attributed to metal-to-ligand charge transfer (MLCT).

Illustrative bipy complexes

• Mo(CO)₄(bipy), derived from Mo(CO)₆.
• RuCl₂(bipy)₂,^[4] a useful precursor to mixed ligand complexes.
• [Ru(bipy)₃]Cl₂, a well known lumiphore.
• [Fe(bipy)₃]²⁺ is used for the colorimetric analysis of iron ions.

Tris-bipy complexes

Tris(bipy) complexes have three bipyridine molecules are coordinated to a metal ion, written as [M(bipy)₃]ⁿ⁺ (M = metal ion; Cr, Fe, Co, Ru, Rh and so on; bpy = 2,2'-bipyridine). These complexes have six-coordinated, octahedral structures and two enantiomers as follows:

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(bipy) complexes can be reduced to neutral derivatives containing bipy^- ligands. Examples include M(bipy)₃, where M = Al, Cr, Si.

References

1. ^ Sasse, W. H. F. “2,2’-Bipyridine” Organic Syntheses, Collected Volume 5, p.102 (1973).
2. ^ Smith, A. P.;Lamba, J. J. S.; Fraser, C. L. “Efficient Synthesis of Halomethyl-2,2'-Bipyridines: 4,4'-Bis(chloromethyl)-2,2'-Bipyridine” Organic Syntheses, Collected Volume 10, p.107 (2004).
3. ^ Smith, A. P.; Savage, S. A.; Love, J.; Fraser, C. L. “Synthesis of 4-, 5-, and 6-Methyl-2,2'-Bipyridine by a Negishi Cross-Coupling Strategy” Organic Syntheses, Collected Volume 10, p.517 (2004).
4. ^ Lay, P. A.; Sargeson, A. M.; Taube, H.”cisBis(2,2’-Bipyridine-N,N’) Complexes of Ruthenium(III)/(II) and Osmium(III)/(II)” Inorganic Syntheses, 1986, volume 24, 291-299. ISBN 0-471-83441-6