Organoactinide chemistry

Organoactinide chemistry is the science exploring the properties, structure, and reactivity of organoactinide compounds, which are organometallic compounds containing a carbon to actinide chemical bond. Like most organometallic compounds, the organoactinides are air sensitive and need to be handled using the appropriate methods.

Organometallic complexes with σ-bonding Most common organoactinide complexes involve π-bonding with ligands such as cyclopentadienyl, but there are a few exceptions with σ-bonding, namely in thorium and uranium chemistry as these are the most easily handleable elements of this group.

Alkyl and aryl compounds

Attempts to synthesize uranium alkyls were first made during the Manhattan project by Henry Gilman, inspired by the volatility of main group organometallics. However he noticed that these compounds tend to be highly unstable. Marks and Seyam attempted to synthesize them from UCl4 using organolithium reagents, but these decomposed quickly. In 1989, a group finally synthesized a homoleptic complex with trimethylsilyl groups: U[CH(SiMe3)2]3. Since then, variants of higher coordination numbers such as [Li(TMEDA)]2[UMe6] have also been synthesized. On the other hand, only one homoleptic thorium alkyl is known. The seven coordinate heptamethylthorate(IV) anion was synthesized in 1984 using a similar procedure to the equivalent uranium complex. Mixed phosphine containing complexes of thorium and uranium tetramethyls have also been made, using dmpe as the organophosphorus ligand stabilising the structure (amides can also assume this role).

Metallacycles Uranium and thorium both form metallacycles with a diverse chemistry. These complexes are very labile so trimethylsilyl groups are again present for protection. These compounds are formed by reacting weaker alkylating agents (LiCH3 and Mg(CH3)2 are too strong and lead to the formation of simple alkyls) with ClAn[N(Si(CH3)2]3 (An = Th, U).

Organometallic complexes with π-bonding A large majority of the organoactinides involve Cyclopentadienyl (Cp) or Cyclooctatetraene (COT) and their derivatives as ligands. These usually take part in η5- and η8-bonding, donating electron density through their π orbitals.

Cyclooctatetraene complexes

Actinocenes

Actinides form sandwich complexes with cyclooctatetraene analogously to how transition metals react with cyclopentadienyl ligands. Actinide ions have atomic radii that are too large to form MCp2 compounds, so that they prefer to react with C8H82- ions instead. The first example of this type of chemical species was discovered in 1968 by Andrew Streitwieser, who prepared uranocene by reacting K2(COT) with UCl4 in tetrahydrofuran at 0 °C. The compound itself is a pyrophoric green solid that is otherwise quite unreactive.

Most tetravalent actinides react similarly to form actinocenes: Bis(cyclooctatetraene)protactinium was first prepared in 1973 by turning protactinium(V) oxide into the pentachloride and reducing it with aluminium powder before reacting it with potassium cyclooctatetraenide.

Pa 2 O 5 + SOCl 2 → 400 C PaCl 5 {\displaystyle {\ce {Pa2O5 + SOCl2 ->[400C] PaCl5}}}

3 PaCl 5 + Al ⟶ 3 PaCl 4 + AlCl 3 {\displaystyle {\ce {3PaCl5 + Al -> 3PaCl4 + AlCl3}}}

PaCl 4 + 2 K 2 ( COT ) ⟶ Pa ( COT ) 2 + 4 KCl {\displaystyle {\ce {PaCl4 + 2K2(COT) -> Pa(COT)2 + 4KCl}}}

Neptunocene and thorocene were made similarly using the tetrachlorides. Plutonocene is the exception here: as there is no stable plutonium(IV) chloride known, (Hpy)2PuCl6 had to be used. The later actinides also form complexes with COT but these don't usually assume the classic neutral sandwich structure. Trivalent actinides form ionic compounds with COT ligands, this can be exemplified by the reaction of americium triiodide with K2COT.

AmI 3 + K 2 ( COT ) ⟶ KAm ( COT ) 2 {\displaystyle {\ce {AmI3 + K2(COT) -> KAm(COT)2}}}

This compound is present in solution as the THF adduct.

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