Transition metal complexes of 2,2'-bipyridine are coordination complexes containing one or more 2,2'-bipyridine ligands. Complexes have been described for all of the transition metals. Although few have any practical value, these complexes have been influential.[1] 2,2'-Bipyridine is classified as a diimine ligand. Unlike the structures of pyridine complexes, the two rings in bipy are coplanar, which facilitates electron delocalization. As a consequence of this delocalization, bipy complexes often exhibit distinctive optical and redox properties.

Complexes

Bipy forms a wide variety of complexes. Almost always, it is a bidentate ligand, binding metal centers with the two nitrogen atoms. Examples:

Tris-bipy complexes

Three-dimensional view of the [Fe(bipy)3]2+ complex.

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). These complexes have six-coordinated, octahedral structures and exists as enantiomeric pairs:

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, some tris(bipy) complexes can be reduced to neutral derivatives containing bipy− ligands. Examples include M(bipy)3, where M = Al, Cr, Si.[3]

Square planar complexes

Structure of [Pt(bipy)2]2+ as determined by X-ray crystallography.[4]

Square planar complexes of the type [Pt(bipy)2]2+ react with nucleophiles because of the steric clash between the 6,6' positions between the pair of bipy ligands. This clash is indicated by the bowing of the pyridyl rings out of the plane defined by PtN4.[4]

Many ring-substituted variants of bipy have been described, especially dimethyl-2,2'-bipyridines.[5][6] Alkyl substituents enhance the solubility of the complexes in organic solvents. 6,6'-Substituents tend to protect the metal center.[7]

The related N,N-heterocyclic ligand phenanthroline forms similar complexes. With respective pKa's of 4.86 and 4.3 for their conjugate acids, phenanthroline and bipy are of comparable basicity.[8]

2,2'-Biquinoline is closely related to bipy as a ligand.

References

  1. 1 2 Constable; Housecroft (2019). "The Early Years of 2,2'-Bipyridine—A Ligand in its Own Lifetime". Molecules. 24 (21): 3951. doi:10.3390/molecules24213951. PMC 6864536. PMID 31683694.
  2. ↑ Lay, P. A.; Sargeson, A. M.; Taube, H.; Chou, M. H.; Creutz, C. (1986). "cis-Bis(2,2′-bipyridine-N,N′) complexes of ruthenium(III)/(II) and osmium(III)/(II)". Inorganic Syntheses. 24: 291–299. doi:10.1002/9780470132555.ch78. ISBN 9780470132555.
  3. ↑ Scarborough, Christopher C.; Wieghardt, Karl (2011). "Electronic Structure of 2,2′-Bipyridine Organotransition-Metal Complexes. Establishing the Ligand Oxidation Level by Density Functional Theoretical Calculations". Inorganic Chemistry. 50 (20): 9773–9793. doi:10.1021/ic2005419. PMID 21678919.
  4. 1 2 Clare, Bronya R.; McInnes, Claire S.; Blackman, Allan G. (2005). "Bis(2,2′-bipyridine-κ2N,N′)platinum(II) Bis(perchlorate)". Acta Crystallographica Section E. 61 (10): m2042–m2043. doi:10.1107/S1600536805029089.
  5. ↑ Smith, A. P.; Lamba, J. J. S.; Fraser, C. L. (2002). "Efficient Synthesis of Halomethyl-2,2′-Bipyridines: 4,4′-Bis(chloromethyl)-2,2′-Bipyridine". Organic Syntheses. 78: 82. doi:10.15227/orgsyn.078.0082.
  6. ↑ Smith, A. P.; Savage, S. A.; Love, J.; Fraser, C. L. (2002). "Synthesis of 4-, 5-, and 6-Methyl-2,2′-Bipyridine by a Negishi Cross-Coupling Strategy". Organic Syntheses. 78: 51. doi:10.15227/orgsyn.078.0051.
  7. ↑ Bhattacharya, Moumita; Sebghati, Sepehr; Vanderlinden, Ryan T.; Saouma, Caroline T. (2020). "Toward Combined Carbon Capture and Recycling: Addition of an Amine Alters Product Selectivity from CO to Formic Acid in Manganese Catalyzed Reduction of CO2". Journal of the American Chemical Society. 142 (41): 17589–17597. doi:10.1021/jacs.0c07763. PMC 7584391. PMID 32955864.
  8. ↑ J. G. Leipoldt; G. J. Lamprecht; E. C.Steynberg (1991). "Kinetics of the Substitution of Acetylacetone in Acetylactonato-1,5-cyclooctadienerhodium(I) by Derivatives of 1,10-Phenantrholine and 2,2′-Dipyridyl". Journal of Organometallic Chemistry. 402 (2): 259–263. doi:10.1016/0022-328X(91)83069-G.
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