January 04, 2021
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Ever since the discovery of ferrocene in the 1940s, hundreds of “sandwich” compounds, in which a metal atom is “sandwiched” between two arene rings, have been synthesized. In 1968, Andrew Streitweiser, Jr.,* and Ulrich Mueller-Westerhoff at the University of California, Berkeley, advanced the field by preparing uranocene, an actinide metal sandwiched between two cyclic polyolefins.

In contrast to ferrocene with its cyclopentadienide (Cp) rings, the coordinating molecule in uranocene is the dianion of cyclooctatetraene (COT). Streitweiser and Mueller-Westerhoff treated COT with elemental potassium to make the dianion and then added uranium tetrachloride at 0 ºC to produce uranocene, a green crystalline solid. COT has a folded structure; but its dianion is planar, making it a suitable “bread” for the sandwich.

Uranocene is stable to water, acids, bases, and moderate heating—but it ignites spontaneously in air. In ferrocene, the Cp rings’ six π electrons interact with the 3d orbitals in iron. In uranocene, uranium’s 6d orbitals similarly combine with the COT’s 10 π electrons; but uranium 5f orbitals are also involved, contributing to the molecule’s stability.

In 2004, Ditmar Seyferth at MIT wrote a comprehensive review of organometallic compounds that contain f-orbital elements.

Uranocene hazard information

Hazard class* Hazard statement
Acute toxicity, oral, category 1 H300—Fatal if swallowed Chemical Safety Warning
Acute toxicity, inhalation, category 1 H330—Fatal if inhaled Chemical Safety Warning
Specific target organ toxicity, repeated exposure, category 2 H373—Causes damage to organs through prolonged or repeated exposure Chemical Safety Warning
Hazardous to the aquatic environment, long-term hazard, category 2 H411—Hazardous to the aquatic environment, long-term hazard Chemical Safety Warning

*Globally Harmonized System of Classification and Labeling of Chemicals.  
Explanation of pictograms.

MOTW update

Ibogaine was the Molecule of the Week for June 16, 2014. It is a hallucinogenic, neurotoxic indole alkaloid that is successful in reducing withdrawal effects from abused drugs such as alcohol, nicotine, and methamphetamine. But ibogaine’s toxicity, especially its tendency to induce cardiac arrhythmia, led researchers to investigate safer efficacious analogues. David E. Olson and co-workers at the University of California, Davis, have synthesized the simpler molecule tabernanthalog that does not cause hallucinations in mice, nor does it inhibit the potassium channels linked to ibogaine’s cardiotoxicity. In rodent experiments, tabernanthalog reduced alcohol- and heroin-seeking behavior and had antidepressant effects.

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Uranocene fast facts

CAS Reg. No. 11079-26-8
Uranium, bis(η8-1,3,5,7-cyclooctatetraene) coordination compound
Empirical formula C16H16U
Molar mass 446.33 g/mol
Appearance Green crystals
Melting point 180 ºC (subl) (4 Pa)
Water solubility Insoluble
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