Triiron dodecacarbonyl

Iron dodecarbonyl is the chemical compound Fe₃(CO)₁₂. This species was one of the first metal carbonyl clusters prepared.^[1] It is a more reactive form of Fe(0) than Fe(CO)₅.

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General properties

Fe₃(CO)₁₂ is a dark green solid, which vacuum-sublimes with significant decomposition at elevated temperatures. It is soluble in nonpolar organic solvents to give intensely green solutions. Most low nuclearity clusters are pale yellow or orange. Heating solutions of Fe₃(CO)₁₂ affords iron mirrors, which can be pyrophoric in air. The solid decomposes slowly in air, and thus samples are typically stored cold under an inert atmosphere.

Synthesis

It was occasionally obtained from the thermolysis of Fe(CO)₅:

3 Fe(CO)₅ → Fe₃(CO)₁₂ + 3 CO

Traces of the compound are easily detected because of its characteristically deep green color. UV-photolysis of Fe(CO)₅ produces Fe₂(CO)₉, not Fe₃(CO)₁₂.

An efficient synthesis of Fe₃(CO)₁₂ proceeds via the reaction of Fe(CO)₅ with base:^[2]

3 Fe(CO)₅ + [(C₂H₅)₃N] + H₂O → [(C₂H₅)₃NH][HFe₃(CO)₁₁] + 3 CO + CO₂

followed by oxidation of the resulting hydride with acid:

[(C₂H₅)₃NH][HFe₃(CO)₁₁] + HCl + CO → Fe₃(CO)₁₂ + H₂ + [(C₂H₅)₃NH]Cl

The original synthesis, by Walter Hieber, entailed the reaction of H₂Fe(CO)₄ with MnO₂. The cluster was formulated merely as "Fe(CO)₄".^[3]

Structure

Fe₃(CO)₁₂ features a triangle of Fe atoms surrounded by 12 CO ligands. Ten of the CO ligands are terminal and two span an Fe---Fe edge, resulting in an overall C_2v point group symmetry. In contrast, Ru₃(CO)₁₂ and Os₃(CO)₁₂ adopt D_3h-symmetric structures, wherein all 12 CO ligands are terminally bound to the metals. Overall, it can be appreciated that these three clusters formally arise from condensation of three 16-electron M(CO)₄ fragments, akin to the condensation of CH₂ into cyclopropane.

Elucidation of the structure of Fe₃(CO)₁₂ proved to be challenging because the CO ligands are disordered in the crystals. Early evidence for its distinctive C_2v structure came from Mößbauer spectroscopic measurements that revealed two quadrupole doublets with similar isomer shifts but different (1.13 and 0.13 mm/s) quadrupole coupling constants.

The anion [HFe₃(CO)₁₁]^- is structurally related to Fe₃(CO)₁₂, with the hydride replacing one bridging CO ligand. The bonding in the Fe-H-Fe subunit is described using concepts developed for diborane.

Reactions

Like most metal carbonyl, Fe₃(CO)₁₂ undergoes substitution reactions, making, for example, Fe₃(CO)₁₁(P(C₆H₅)₃.

Heating Fe₃(CO)₁₂ gives a low-yield of the carbido cluster Fe₅(CO)₁₅C. Such reactions proceed via disproportionation of CO to give CO₂ and carbon.

Fe₃(CO)₁₂ reacts with 1,3-propanedithiol to air-stable µ-(1,3-Propanedithiolato)-hexacarbonyldiiron in which both thiol sulfur atoms form a bridge between two iron atoms. This compound is a model compound for certain all-iron dehydrogenases ^[4].

Safety

Fe₃(CO)₁₂, like all metal carbonyls is hazardous as a source of volatile iron and as a source of carbon monoxide. Solid samples, especially when finely divided, and residues from reactions can be pyrophoric, which can ignite the organic solvents used for such reactions.

References

1. ^ Elschenbroich, C.; Salzer, A. ”Organometallics : A Concise Introduction” (2nd Ed) (1992) from Wiley-VCH: Weinheim. ISBN 3-527-28165-7
2. ^ McFarlane, W.; Wilkinson, G. W. Inorganic Syntheses 1966, volume 8, page 181-3.
3. ^ Hieber, W.; Leutert, F. (1932). "Über Metallcarbonyle. XII. Die Basenreaktion des Eisenpentacarbonyls und die Bildung des Eisencarbonylwasserstoffs (Metal carbonyls. XII. The Reaction of Iron Pentacarbonyl with Bases and the Formation of Iron Hydrocarbonyl)". Zeitschrift für anorganische und allgemeine Chemie 204: 145-64. doi:10.1002/zaac.19322040115.
4. ^ Synthesis, Purification, and Characterization of a µ-(1,3-Propanedithiolato)-hexacarbonyldiiron Laboratory Experiment or Mini-Project for Inorganic Chemistry or Integrated Laboratory Carmen F. Works 836 Journal of Chemical Education Vol. 84 No. 5 May 2007 Abstract