Transition metal complexes of pyridine-N-oxides and amine oxides

Transition metal complexes of pyridine-N-oxides encompass coordination complexes that contain pyridine-N-oxides as ligands. Transition metal complexes of amine-N-oxides encompass coordination complexes that contain amine oxides as ligands. The two classes of complexes are similar.

Bonding

Pyridine-N-oxides and amine oxides are classified as L ligands in the covalent bond classification method. In the usual electron counting method, they are two-electron ligands. With respect to HSAB theory, they are classified aa hard, as is typical for oxygen-based ligands.

Amine oxides are moderate bases. For (CH3)3NOH+, the pKa is around 9.7.[2] Pyridine-N-oxides are significantly weaker bases.

Pyridine-N-oxides

Particularly common are the octahedral homoleptic complexes of the type [M(ONC5H5)6]2+ where M = Mn(II), Fe(II), Co(II), Ni(II).[3] Many variations of pyridine N-oxide are known, such as the dioxides of 2,2'- and 4,4'-2,2'-bipyridine.[4] Complexes derived from the trioxide of terpyridine have been crystallized as well.[5]

Pyridine-N-oxides bind to metals through the oxygen. According to X-ray crystallography, the M-O-N angle is approximately 130° in many of these complexes. As reflected by the pKa of 0.79 for C5H5NOH+, pyridine N-oxides are weakly basic ligands. Their complexes are generally high spin, hence they are kinetically labile.

Amine oxides

Oxides of tertiary amines, which are more basic than pyridine-N-oxides, form a variety of metal complexes. One such complex is derived from N-methylmorpholine-N-oxide and osmium tetroxide. This adduct is a probable intermediate in the Upjohn dihydroxylation.[6]

Applications

Zinc pyrithione is a coordination complex of a sulfur-substituted pyridine-N-oxide. This zinc complex has useful fungistatic and bacteriostatic properties.[7]

Further reading

  • Terpyridine-N-oxide: Waldmann, T.; et al. (2012). "Oxidation of an Organic Adlayer: A Bird's Eye View". Journal of the American Chemical Society. 134 (21): 8817–22. doi:10.1021/ja302593v. PMID 22571820.

References

  1. ^ Van Ingen Schenau, A. D.; Verschoor, C. G.; Romers, C. (1974). "The Crystal and Molecular Structure of Hexakis(pyridine-N-oxide)nickel(II) Bis(tetrafluoroborate)". Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry. 30 (7): 1686–1694. Bibcode:1974AcCrB..30.1686V. doi:10.1107/S0567740874005632.
  2. ^ Maisonneuve, B. (2003). "Amine Oxides". Kirk-Othmer Encyclopedia of Chemical Technology. doi:10.1002/0471238961.0113091413010919.a01.pub2. ISBN 978-0-471-48494-3.
  3. ^ Carlin, Richard L.; De Jongh, L. J. (1986). "Structural and Magnetic Properties of Transition Metal complexes of Pyridine N-Oxide". Chemical Reviews. 86 (4): 659–680. doi:10.1021/cr00074a001.
  4. ^ Jia, Junhua; Hubberstey, Peter; Champness, Neil R.; Schröder, Martin (2009). "Supramolecular Chemistry of 4,4′-Bipyridine-N,N′-dioxide in Transition Metal Complexes: A Rich Diversity of Co-ordinate, Hydrogen-Bond and Aromatic Stacking Interactions". Molecular Networks. Structure and Bonding. Vol. 132. pp. 135–161. doi:10.1007/430_2008_9. ISBN 978-3-642-01366-9.
  5. ^ Amoroso, Angelo J.; Burrows, Miles W.; Dickinson, Anthony A.; Jones, Cameron; Willock, David J.; Wong, Wing-Tak (2001). "Geometrical preferences of complexes of terpyridine N-oxide ligands: Synthesis and crystal structures of nickel(II) with terpyridine 1,1′,1″-trioxide, terpyridine 1,1″-dioxide and terpyridine 1-oxide". Journal of the Chemical Society, Dalton Transactions (3): 225–227. doi:10.1039/b008993l.
  6. ^ Bailey, Alan J.; Bhowon, Minu G.; Griffith, William P.; Shoair, Abdel G. F.; White, Andrew J. P.; Williams, David J. (1997). "Oxo osmium(VIII) complexes in oxidation: Crystal structures of OsO4·nmo (Nmo = N-methylmorpholine N-oxide) and OsO4·NMM (NMM = N-methylmorpholine), and use of cis-[OsO4(OH)2]2− as an oxidation catalyst". Journal of the Chemical Society, Dalton Transactions (18): 3245–3250. doi:10.1039/A702965I.
  7. ^ Barnett, B. L.; Kretschmar, H. C.; Hartman, F. A. (1977). "Structural characterization of bis(N-oxopyridine-2-thionato)zinc(II)". Inorg. Chem. 16 (8): 1834–8. doi:10.1021/ic50174a002.
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