Transition metal complexes of aldehydes and ketones

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Structure of an η2-formaldehyde complex.[1]

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 and bonding

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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+).[2] The pi-bonded mode is observed for low valence, electron-rich metal centers (e.g., Fe(0) and Os(0)).[3]

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.[4]

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.[5]

Formaldehyde complexes

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Formaldehyde, being the simplest organic carbonyl and being an important industrial chemical, holds a special position as a ligand. Commonly it binds as η2-CH2O, i.e. "side-on bonded", comparable to ethylene. The first example was Os(η2-CH2O)(CO)2(PPh3)2 (Ph = phenyl, C6H5).[6] By virtue of the lone pairs of electrons on oxygen, M(η2-CH2O) entity can bridge to other metals.

Complexes are also known for vanadium,[7] rhenium,[8] zirconium (Zr),[9] ruthenium (Ru),[10] and niobium (Nb).[11]

Synthesis

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Usually formaldehyde complexes are prepared by treating low-valence metal complexes with formaldehyde or one of its oligomers such as paraformaldehyde:

Os(CO)2(PPh3)3 + CH2O → Os(η2-CH2O)(CO)2(PPh3)2 + PPh3

More exotic routes have been demonstrated such as the addition of CO to metal hydride complexes.[12] Such reactions are proposed to resemble steps in Fischer-Tropsch hydrogenation of CO.[13]

The complex W(PMe3)4(η2-CH2O)H2results from the addition of methanol to W(PMe3)4(η2-CH2PMe2)H.

Compounds in which metals replace the aldehydic hydrogen, instead of enolizing the carbonyl, are transition metal acyl complexes.

Reactions

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The reactivity of metal-formaldehyde complexes has been well investigated. W(PMe3)4(η2-CH2O)H2 can be hydrogenated to give W(PMe3)4(CH3O)H3 and then methanol to generate W(PMe3)4H4.[14]

In some cases, alkenes can inert into the M-C bond of the M(η2-CH2O) entity.[15]

Reported reactions of W(PMe3)4(η2-CH2O)H2.
(Benzylideneacetone)iron tricarbonyl is an organoiron compound with an η2 ketone ligand.

Some η2-aldehyde complexes insert alkenes to give five-membered metallacycles.[16]

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

The hydroformylation of formaldehyde gives glycolaldehyde:[17]

CH2O + H2 + CO → HOCH2CHO

References

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  1. ^ Brown, Kevin L.; Clark, George R.; Headford, Christine E. L.; Marsden, Karen; Roper, Warren R. (1979). "Organometallic models for possible Fischer-Tropsch intermediates. Synthesis, structure, and reactions of a formaldehyde complex of osmium". Journal of the American Chemical Society. 101 (2): 503–505. Bibcode:1979JAChS.101..503B. doi:10.1021/ja00496a050.
  2. ^ 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.
  3. ^ 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.
  4. ^ 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.
  5. ^ 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. Bibcode:1983JAChS.105.3353K. doi:10.1021/ja00348a075.
  6. ^ Clark, G.R.; Headford, C.E.L.; Marsden, K.; Roper, W.R. (June 1982). "Synthesis, structure and reactions of a dihapto-formaldehyde complex, Os(η2-CH2O)(CO)2(PPh3)2". Journal of Organometallic Chemistry. 231 (4): 335–360. doi:10.1016/s0022-328x(00)81212-7. ISSN 0022-328X.
  7. ^ Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. (April 1982). "Metal-formaldehyde chemistry: coordination, disproportionation and Lewis acid-promoted transformation to oxymethylene derivatives". Journal of the American Chemical Society. 104 (7): 2019–2020. doi:10.1021/ja00371a038. ISSN 0002-7863.
  8. ^ Buhro, William E.; Patton, Alan T.; Strouse, Charles E.; Gladysz, J. A.; McCormick, Fred B.; Etter, Margaret C. (February 1983). "Syntheses, chemical properties, and x-ray crystal structures of rhenium formaldehyde and thioformaldehyde complexes". Journal of the American Chemical Society. 105 (4): 1056–1058. doi:10.1021/ja00342a070. ISSN 0002-7863.
  9. ^ Kropp, Kurt; Skibbe, Volker; Erker, Gerhard; Krueger, Carl (May 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. ISSN 0002-7863.
  10. ^ Chaudret, Bruno N.; Cole-Hamilton, David J.; Nohr, Ronald S.; Wilkinson, Geoffrey (1977-01-01). "The reactions of chlorohydrido- and dichloro-tris(triphenylphosphine)ruthenium(II) with alkali hydroxides and alkoxides. Hydridohydroxobis(triphenylphosphine)ruthenium(II) monosolvates, their reactions and related compounds". Journal of the Chemical Society, Dalton Transactions (16): 1546–1557. doi:10.1039/DT9770001546. ISSN 1364-5447.
  11. ^ Wolczanski, Peter T.; Threlkel, Richard S.; Bercaw, John E. (January 1979). "Reduction of coordinated carbon monoxide to "zirconoxy" carbenes with permethylzirconocene dihydride". Journal of the American Chemical Society. 101 (1): 218–220. doi:10.1021/ja00495a037. ISSN 0002-7863.
  12. ^ Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. (March 1983). "Genesis, bonding mode and reaction with carbon monoxide of an oxymethylene unit bridging two metal atoms". Journal of the American Chemical Society. 105 (6): 1690–1691. doi:10.1021/ja00344a066. ISSN 0002-7863.
  13. ^ Fachinetti, Giuseppe; Floriani, Carlo; Pucci, Sergio (1978-01-01). "Stoicheiometric reduction of CO and CO2 to methanol: evidence for carbon monoxide insertion into zirconium–hydrogen bond". Journal of the Chemical Society, Chemical Communications (6): 269–270. doi:10.1039/C39780000269. ISSN 0022-4936.
  14. ^ Green, Malcolm L. H.; Parkin, Gerard; Moynihan, Kelly J.; Prout, Keith (1984-01-01). "Formation of an η2-formaldehyde compound from methanol and its hydrogenation giving methanol". Journal of the Chemical Society, Chemical Communications (22): 1540. doi:10.1039/C39840001540. ISSN 0022-4936.
  15. ^ Green, Malcolm L. H.; Parkin, Gerard (1986-01-01). "Ethylene insertion into the W–C bond of the η2-formaldehyde ligand system of W(PMe3)4(η2-CH2O)H2 giving the oxometallacyclopentane derivative W(OCH2CH2CH2)(PMe3)2(C2H4)2 and related studies". Journal of the Chemical Society, Chemical Communications (1): 90–91. doi:10.1039/C39860000090. ISSN 0022-4936.
  16. ^ 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.
  17. ^ Wei, Jie; Li, Maoshuai; Wang, Meiyan; Feng, Shixiang; Dai, Weikang; Yang, Qi; Feng, Yi; Yang, Wanxin; Yang, Cheng; Ma, Xinbin (2021). "Review Hydroformylation of formaldehyde to glycolaldehyde: An alternative synthetic route for ethylene glycol". Chinese Journal of Chemical Engineering. 35: 3–16. doi:10.1016/j.cjche.2020.08.056.