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.

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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.