Organoactinide chemistry

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Tetrakis(cyclopentadienyl)thorium(IV) an organoactinide compound Tetrakis(cyclopentadienyl)thorium(IV)-3D-balls.png
Tetrakis(cyclopentadienyl)thorium(IV) an organoactinide compound

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.

Contents

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

.mw-parser-output .template-chem2-su{display:inline-block;font-size:80%;line-height:1;vertical-align:-0.35em}.mw-parser-output .template-chem2-su>span{display:block;text-align:left}.mw-parser-output sub.template-chem2-sub{font-size:80%;vertical-align:-0.35em}.mw-parser-output sup.template-chem2-sup{font-size:80%;vertical-align:0.65em} U[CH(SiMe3)2]3, the first uranium alkyl compound to be synthesized Uranium alkyl.png
U[CH(SiMe3)2]3, the first uranium alkyl compound to be synthesized

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. [1]

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. [1]

On the other hand, only one homoleptic thorium alkyl is known. [2] 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). [3]

Metallacycles

Uranium and thorium both form metallacycles with a diverse chemistry. [4] 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).

a uranium-containing metallacycle Uranium metallacycle.png
a uranium-containing metallacycle

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 pi orbitals.

Cyclooctatetraene complexes

Actinocenes

A Sandwich compound with two Cyclooctatetraene ligands Uranocene-3D-vdW.png
A Sandwich compound with two Cyclooctatetraene ligands

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 K(COT)2 with UCl4 in tetrahydrofuran at 0 °C. [5] The compound itself is a pyrophoric green solid that is otherwise quite unreactive. [6]

The original synthesis of uranocene Uranocene synthesis.svg
The original synthesis of uranocene

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. [7]

:

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.

This compound is present in solution as the THF adduct.

Complexes of substituted cyclooctatetraenes

Many substituted uranocenes have been synthesized. [8] [9] The methodology followed was the same as for simple U(COT)2, but the properties of some of the compounds were found to be different.

The tetraphenylcyclooctatetraene complex was found to be completely air stable by Streitwieser. This high stability is probably due to the hindering effects of the phenyl groups, protecting the U4+ center from an attack by oxygen. [9]

All these derivatives are much more soluble in organic solvents such as benzene, in which they form green solutions that are more air sensitive than the crystalline solids.

Substituted cyclooctatetraene ligands COT derivatives.png
Substituted cyclooctatetraene ligands

Plutonium also forms a sandwich complex with 1,4-bis(trimethylsilyl)cyclooctatetraenyl (1,4-COT’’) and its 1,3 isomer. This compound is prepared by the oxidation of the anionic green Pu(III) complex Li(THF)4[Pu(1,4-COT’’)2] with cobalt(II) chloride which leads to the formation of Pu(1,4-COT’’)(1,3-COT’’). The reaction is easily noticeable by the THF solution changing to a dark red colour, characteristic of Pu(IV). [10]

The structures of Pu(3-COT'')(4-COT'') and Np(COT''')2 Silyl substituted COT Np and Pu complexes.png
The structures of Pu(3-COT’’)(4-COT’’) and Np(COT’’’)2

The neptunium equivalent with the trisubstituted COT’’’ has also been reported [11] and the complexes of both the tri- and di- substituted ligands with thorium and uranium are well known. [12] They were synthesized according to the following reaction schemes:

An = Th, U Synthesis of uranium and thorium sandwich complexes with substituted COT.png
An = Th, U

Cyclopentadiene complexes

Tris(cyclopentadienyl)actinide complexes

The general structure of tris(cyclopentadiene)actinide complexes AnCp3.png
The general structure of tris(cyclopentadiene)actinide complexes

Most trivalent f block elements form compounds with cyclopentadiene with the formula M(Cp)3. These complexes have been isolated up to californium, with the einsteinium equivalent having been observed in the gas phase. [13]

An = Th, U, Np, Pu, Am, Cm, Bk, Cf AnCp3 synthesis.png
An = Th, U, Np, Pu, Am, Cm, Bk, Cf

The synthesis of the AnCp3 usually follows the reaction scheme shown above [4] [14] with a few more added steps that are sometimes needed to synthesize the trichlorides from the commercially supplied oxides. [13] Nevertheless, other syntheses are also used by some authors: alkali metal cyclopentadienides can be used instead of the beryllium complex, and An(IV) complexes can also be used via a reductive elimination reaction.

The colours of AnCp3 complexes: [15]
ThUNpPuAmCmBkCf
greenbrownpale greengreenfleshcolourlessamberred

These compounds have been known since the sixties, however until 2018 only the neptunium compound was structurally characterised. Kovàcs and coworkers were able to analyse the plutonium and uranium complexes, finding that all three structures were similar, with an asymmetrical distribution of cylopentadienide ligands and a higher covalent character to the carbon-actinide bond than in organolanthanide compounds. [16]

Tetrakis(cyclopentadienyl)actinide complexes

Tetravalent thorium, uranium and neptunium easily form MCp4 compounds by a metathesis reaction from potassium cyclopentadienide using benzene as a solvent. [4]

An= Th, U, Np AnCp4 synthesis.png
An= Th, U, Np

See also

Related Research Articles

The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide.

<span class="mw-page-title-main">Metallocene</span>

A metallocene is a compound typically consisting of two cyclopentadienyl anions (C
5H−
5, abbreviated Cp) bound to a metal center (M) in the oxidation state II, with the resulting general formula (C5H5)2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride or vanadocene dichloride. Certain metallocenes and their derivatives exhibit catalytic properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to [Cp2ZrCH3]+ catalyze olefin polymerization.

<span class="mw-page-title-main">Protactinium</span> Chemical element, symbol Pa and atomic number 91

Protactinium is a radioactive chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray actinide metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds in which protactinium is usually present in the oxidation state +5, but it can also assume +4 and even +3 or +2 states. Concentrations of protactinium in the Earth's crust are typically a few parts per trillion, but may reach up to a few parts per million in some uraninite ore deposits. Because of its scarcity, high radioactivity and high toxicity, there are currently no uses for protactinium outside scientific research, and for this purpose, protactinium is mostly extracted from spent nuclear fuel.

<span class="mw-page-title-main">Cyclooctatetraene</span> Chemical compound

1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because of its stoichiometric relationship to benzene, COT has been the subject of much research and some controversy.

<span class="mw-page-title-main">Titanocene dichloride</span> Chemical compound

Titanocene dichloride is the organotitanium compound with the formula (η5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowly hydrolyzes in air. It shows antitumour activity and was the first non-platinum complex to undergo clinical trials as a chemotherapy drug.

<span class="mw-page-title-main">Hapticity</span> Number of contiguous atoms in a ligand that bond to the central atom in a coordination complex

In coordination chemistry, hapticity is the coordination of a ligand to a metal center via an uninterrupted and contiguous series of atoms. The hapticity of a ligand is described with the Greek letter η ('eta'). For example, η2 describes a ligand that coordinates through 2 contiguous atoms. In general the η-notation only applies when multiple atoms are coordinated. In addition, if the ligand coordinates through multiple atoms that are not contiguous then this is considered denticity, and the κ-notation is used once again. When naming complexes care should be taken not to confuse η with μ ('mu'), which relates to bridging ligands.

Uranocene, U(C8H8)2, is an organouranium compound composed of a uranium atom sandwiched between two cyclooctatetraenide rings. It was one of the first organoactinide compounds to be synthesized. It is a green air-sensitive solid that dissolves in organic solvents. Uranocene, a member of the "actinocenes," a group of metallocenes incorporating elements from the actinide series. It is the most studied bis[8]annulene-metal system, although it has no known practical applications.

Transmetalation (alt. spelling: transmetallation) is a type of organometallic reaction that involves the transfer of ligands from one metal to another. It has the general form:

<span class="mw-page-title-main">Organozirconium chemistry</span>

Organozirconium chemistry is the science of exploring the properties, structure, and reactivity of organozirconium compounds, which are organometallic compounds containing chemical bonds between carbon and zirconium. Organozirconium compounds have been widely studied, in part because they are useful catalysts in Ziegler-Natta polymerization.

<span class="mw-page-title-main">Organouranium chemistry</span> Area of chemistry

Organouranium chemistry is the science exploring the properties, structure, and reactivity of organouranium compounds, which are organometallic compounds containing a carbon to uranium chemical bond. The field is of some importance to the nuclear industry and of theoretical interest in organometallic chemistry.

Organoiron chemistry is the chemistry of iron compounds containing a carbon-to-iron chemical bond. Organoiron compounds are relevant in organic synthesis as reagents such as iron pentacarbonyl, diiron nonacarbonyl and disodium tetracarbonylferrate. While iron adopts oxidation states from Fe(−II) through to Fe(VII), Fe(IV) is the highest established oxidation state for organoiron species. Although iron is generally less active in many catalytic applications, it is less expensive and "greener" than other metals. Organoiron compounds feature a wide range of ligands that support the Fe-C bond; as with other organometals, these supporting ligands prominently include phosphines, carbon monoxide, and cyclopentadienyl, but hard ligands such as amines are employed as well.

Ligand bond number (LBN) represents the effective total number of ligands or ligand attachment points surrounding a metal center, labeled M. More simply, it represents the number of coordination sites occupied on the metal. Based on the covalent bond classification method, the equation for determining ligand bond number is as follows:

<span class="mw-page-title-main">Actinide chemistry</span> Branch of nuclear chemistry

Actinide chemistry is one of the main branches of nuclear chemistry that investigates the processes and molecular systems of the actinides. The actinides derive their name from the group 3 element actinium. The informal chemical symbol An is used in general discussions of actinide chemistry to refer to any actinide. All but one of the actinides are f-block elements, corresponding to the filling of the 5f electron shell; lawrencium, a d-block element, is also generally considered an actinide. In comparison with the lanthanides, also mostly f-block elements, the actinides show much more variable valence. The actinide series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.

<span class="mw-page-title-main">Actinocene</span> Class of chemical compounds

Actinocenes are a family of organoactinide compounds consisting of metallocenes containing elements from the actinide series. They typically have a sandwich structure with two dianionic cyclooctatetraenyl ligands (COT2-, which is C
8H2−
8) bound to an actinide-metal center (An) in the oxidation state IV, resulting in the general formula An(C8H8)2.

<span class="mw-page-title-main">Thorium compounds</span> Any chemical compound having at least one atom of thorium

Many compounds of thorium are known: this is because thorium and uranium are the most stable and accessible actinides and are the only actinides that can be studied safely and legally in bulk in a normal laboratory. As such, they have the best-known chemistry of the actinides, along with that of plutonium, as the self-heating and radiation from them is not enough to cause radiolysis of chemical bonds as it is for the other actinides. While the later actinides from americium onwards are predominantly trivalent and behave more similarly to the corresponding lanthanides, as one would expect from periodic trends, the early actinides up to plutonium have relativistically destabilised and hence delocalised 5f and 6d electrons that participate in chemistry in a similar way to the early transition metals of group 3 through 8: thus, all their valence electrons can participate in chemical reactions, although this is not common for neptunium and plutonium.

<span class="mw-page-title-main">Neptunocene</span> Chemical compound

Neptunocene, Np(C8H8)2, is an organoneptunium compound composed of a neptunium atom sandwiched between two cyclooctatetraenide (COT2-) rings. As a solid it has a dark brown/red colour but it appears yellow when dissolved in chlorocarbons, in which it is sparingly soluble. The compound is quite air-sensitive.

<span class="mw-page-title-main">Plutonocene</span> Chemical compound

Plutonocene, Pu(C8H8)2, is an organoplutonium compound composed of a plutonium atom sandwiched between two cyclooctatetraenide (COT2-) rings. It is a dark red, very air-sensitive solid that is sparingly soluble in toluene and chlorocarbons. Plutonocene is a member of the actinocene family of metallocenes incorporating actinide elements in the +4 oxidation state.

<span class="mw-page-title-main">Jaqueline Kiplinger</span> American inorganic chemist

Jaqueline Kiplinger is an American inorganic chemist who specializes in organometallic actinide chemistry. Over the course of her career, she has done extensive work with fluorocarbons and actinides. She is currently a Fellow of the Materials Synthesis and Integrated Devices group in the Materials Physics and Applications Division of Los Alamos National Laboratory (LANL). Her current research interests are focused on the development of chemistry for the United States’ national defense and energy needs.

Protactinium compounds are compounds containing the element protactinium. These compounds usually have protactinium in the +5 oxidation state, although these compounds can also exist in the +2, +3 and +4 oxidation states.

<span class="mw-page-title-main">Organothorium chemistry</span> Study of the carbon-thorium bond

Organothorium chemistry describes the synthesis and properties of chemical compounds containing a carbon to thorium chemical bond.

References

  1. 1 2 Seaman LA, Walensky JR, Wu G, Hayton TW (April 2013). "In pursuit of homoleptic actinide alkyl complexes". Inorganic Chemistry. 52 (7): 3556–64. doi:10.1021/ic300867m. PMID   22716022.
  2. ↑ Lauke H, Swepston PJ, Marks TJ (October 1984). "Synthesis and characterization of a homoleptic actinide alkyl. The heptamethylthorate(IV) ion: a complex with seven metal-carbon .sigma. bonds". Journal of the American Chemical Society. 106 (22): 6841–6843. doi:10.1021/ja00334a062.
  3. ↑ Edwards PG, Andersen RA, Zalkin A (February 1984). "Preparation of tetraalkyl phosphine complexes of the f-block metals. Crystal structure of Th(CH2Ph)4(Me2PCH2CH2PMe2) and U(CH2Ph)3Me(Me2PCH2CH2PMe2)". Organometallics. 3 (2): 293–298. doi:10.1021/om00080a023.
  4. 1 2 3 Behrle AC, Walensky JR (2015), "Actinides: Organometallic Chemistry", Encyclopedia of Inorganic and Bioinorganic Chemistry, American Cancer Society, pp. 1–41, doi:10.1002/9781119951438.eibc0002.pub2, ISBN   9781119951438
  5. ↑ Streitwieser Jr A, Mueller-Westerhoff U (1968-12-01). "Bis(cyclooctatetraenyl)uranium (uranocene). A new class of sandwich complexes that utilize atomic f orbitals". Journal of the American Chemical Society. 90 (26): 7364. doi:10.1021/ja01028a044.
  6. ↑ Cotton, Simon. (2013). Lanthanide and Actinide Chemistry. John Wiley & Sons. ISBN   9781118681367. OCLC   897575995.
  7. ↑ Starks DF, Parsons TC, Streitwieser A, Edelstein N (June 1974). "Bis(.pi.-cyclooctatetraene)protactinium". Inorganic Chemistry. 13 (6): 1307–1308. doi:10.1021/ic50136a011.
  8. ↑ Streitwieser A, Harmon CA (1973-05-01). "Uranocenes with hydrocarbon substituents". Inorganic Chemistry. 12 (5): 1102–1104. doi:10.1021/ic50123a024.
  9. 1 2 Streitwieser A, Walker R (September 1975). "Bis-π-(1,3,5,7-tetraphenylcyclooctatetraene)uranium, an air stable uranocene". Journal of Organometallic Chemistry. 97 (3): C41–C42. doi:10.1016/S0022-328X(00)89316-X.
  10. ↑ Apostolidis C, Walter O, Vogt J, Liebing P, Maron L, Edelmann FT (April 2017). "A Structurally Characterized Organometallic Plutonium(IV) Complex". Angewandte Chemie. 56 (18): 5066–5070. doi:10.1002/anie.201701858. PMC   5485009 . PMID   28371148.
  11. ↑ Tassell MJ, Kaltsoyannis N (August 2010). "Covalency in AnCp4 (An = Th-Cm): a comparison of molecular orbital, natural population and atoms-in-molecules analyses". Dalton Transactions. 39 (29): 6719–25. doi:10.1039/c000704h. PMID   20631951.
  12. ↑ Rausch J, Apostolidis C, Walter O, Lorenz V, Hrib CG, Hilfert L, Kühling M, Busse S, Edelmann FT (2015). "One ligand fits all: lanthanide and actinide sandwich complexes comprising the 1, 4-bis (trimethylsilyl) cyclooctatetraenyl (= COT′′) ligand". New Journal of Chemistry. 39 (10): 7656–66. doi: 10.1039/c5nj00991j .
  13. 1 2 Laubereau PG, Burns JH (May 1970). "Microchemical preparation of tricyclopentadienyl compounds of berkelium, californium, and some lanthanide elements". Inorganic Chemistry. 9 (5): 1091–1095. doi:10.1021/ic50087a018.
  14. ↑ Baumgärtner, F.; Fischer, E. O.; Kanellakopulos, B.; Laubereau, P. (1966-01-07). "Tri-cyclopentadienyl-americium(III)". Angewandte Chemie (in German). 78 (1): 112–113. doi:10.1002/ange.19660780113.
  15. ↑ Marks TJ (September 1982). "Actinide organometallic chemistry". Science. 217 (4564): 989–97. Bibcode:1982Sci...217..989M. doi:10.1126/science.217.4564.989. PMID   17839316.
  16. ↑ Apostolidis C, Dutkiewicz MS, Kovács A, Walter O (February 2018). "Solid-State Structure of Tris-Cyclopentadienide Uranium(III) and Plutonium(III)". Chemistry. 24 (12): 2841–2844. doi:10.1002/chem.201704845. PMC   5861669 . PMID   29193373.
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