Transition metal complexes of aldehydes and ketones

Transition metal complexes of aldehydes and ketones describes coordination complexes with aldehyde (RCHO) and ketone (R₂CO) ligands. Because aldehydes and ketones are common, the area is of fundamental interest. Some reactions that are useful in organic chemistry involve such complexes.

Structure of an η²-formaldehyde complex.

Structure and bonding

Structure of [ZnBr₃(η¹-acetone)]⁻.

In monometallic complexes, aldehydes and ketones can bind to metals in either of two modes, η¹-O-bonded and η²-C,O-bonded. These bonding modes are sometimes referred to sigma- and pi-bonded. These forms may sometimes interconvert.

The sigma bonding mode is more common for higher valence, Lewis-acidic metal centers (e.g., Zn²⁺).^[1] The pi-bonded mode is observed for low valence, electron-rich metal centers (e.g., Fe(0) and Os(0)).^[2]

For the purpose of electron-counting, O-bonded ligands count as 2-electron "L ligands": they are Lewis bases. η²-C,O ligands are described as analogues of alkene ligands, i.e. the Dewar-Chatt-Duncanson model.^[3]

η²-C,O ketones and aldehydes can function as bridging ligands, utilizing a lone pair of electrons on oxygen. One such complex is [(C₅H₅)₂Zr(CH₂O)]₃, which features a Zr₃O₃ ring.^[4]

Related ligands

Related to η¹-O-bonded complexes of aldehydes and ketones are metal acetylacetonates and related species, which can be viewed as a combination of ketone and enolate ligands.

Reactions

(Benzylideneacetone)iron tricarbonyl is an organoiron compound with an η² ketone ligand.

Some η²-aldehyde complexes insert alkenes to give five-membered metallacycles.^[5]

η¹-Complexes of alpha-beta unsaturated carbonyls exhibit enhanced reactivity toward dienes. This interaction is the basis of Lewis-acid catalyzed Diels-Alder reactions.

References

1. Andreev, V. P.; Sobolev, P. S.; Tafeenko, V. A. (2017). "Coordination of Zinc Tetraphenylporphyrin with Pyridine Derivatives in Chloroform Solution and in the Solid Phase". Russian Journal of General Chemistry. 87 (7): 1572–1579. doi:10.1134/S1070363217070210. S2CID 102539549.
2. Berke, Heinz; Huttner, Gottfried; Weiler, Gertrud; Zsolnai, Laszlo (1981). "Struktur und Reaktivität eines Formaldehydeisen-Komplexes". Journal of Organometallic Chemistry. 219 (3): 353–362. doi:10.1016/S0022-328X(00)90020-2.
3. Huang, Yo Hsin; Gladysz, J. A. (1988). "Aldehyde and Ketone Ligands in organometallic complexes and catalysis". Journal of Chemical Education. 65 (4): 298. Bibcode:1988JChEd..65..298H. doi:10.1021/ed065p298.
4. Kropp, Kurt; Skibbe, Volker; Erker, Gerhard; Krueger, Carl (1983). "Fischer-Tropsch intermediates: Tris[(.eta.2-formaldehyde)zirconocene] from the carbonylation of a zirconium hydride". Journal of the American Chemical Society. 105 (10): 3353–3354. doi:10.1021/ja00348a075.
5. Hoshimoto, Yoichi; Ohashi, Masato; Ogoshi, Sensuke (2015). "Catalytic Transformation of Aldehydes with Nickel Complexes through η²-Coordination and Oxidative Cyclization". Accounts of Chemical Research. 48 (6): 1746–1755. doi:10.1021/acs.accounts.5b00061. PMID 25955708.