Diiron nonacarbonyl

Diiron nonacarbonyl (referred to as diiron enneacarbonyl in the older literature) is the chemical compound with the formula Fe₂(CO)₉. This metal carbonyl is an important reagent in organometallic chemistry and of occasional use in organic synthesis.^[1] It is a more reactive source of Fe(0) than Fe(CO)₅ and less dangerous to handle because it is nonvolatile. One of the strangest properties of this micaceous, orange solid is that it is virtually insoluble in all known solvents.

Additional recommended knowledge

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Synthesis

Photolysis of Fe(CO)₅, typically in acetic acid solution, produces Fe₂(CO)₉ in good yield:^[2]

2 Fe(CO)₅ → Fe₂(CO)₉ + CO

Structure

Fe₂(CO)₉ consists of a pair of Fe(CO)₃ centers linked by three bridging CO ligands. The iron atoms are equivalent and octahedral. Elucidation of the structure of Fe₂(CO)₉ has proven to be challenging because of its very low solubility, which prevents growth of crystals. The Mößbauer spectrum reveals one quadrupole doublet, consistent with the D_3h-symmetric structure.

Reactions

Fe₂(CO)₉ is primarily employed as a precursor to compounds of the type Fe(CO)₄L and Fe(CO)₃(diene). Such syntheses are typically conducted in THF solution. In these conversions, it is proposed that small amounts of Fe₂(CO)₉ dissolve according the following reaction:

Fe₂(CO)₉ → Fe(CO)₅ + Fe(CO)₄(THF)

Cyclobutadieneiron tricarbonyl is prepared using Fe₂(CO)₉. Fe₂(CO)₉ has also been employed in the synthesis of cyclopentadienones via a net [2+3]-cycloaddition from dibromoketones.^[3]

Safety

Fe₂(CO)₉ is a source of CO although it is nonvolatile and insoluble.

References

1. ^ Elschenbroich, C.; Salzer, A. ”Organometallics : A Concise Introduction” (2nd Ed) (1992) Wiley-VCH: Weinheim. ISBN 3-527-28165-7
2. ^ King, R. B. Organometallic Syntheses. Volume 1 Transition-Metal Compounds; Academic Press: New York, 1965. ISBN 0-444-42607-8
3. ^ R. Noyori, R.; Yokoyama, K.; Hayakawa, Y. (1988). "Cyclopentanones from α, α'-Dibromoketones and Enamines: 2,5-Dimethyl-3-Phenyl-2-Cyclopenten-1-one". Org. Synth.; Coll. Vol. 6: 520.