Organothallium chemistry

Organothallium compounds are compounds that contain the carbon-thallium bond. The area is not well developed because of the lack of applications and the high toxicity of thallium. The behavior of organothallium compounds can be inferred from that of organogallium and organoindium compounds. Organothallium(III) compounds are more numerous than organothallium(I) compounds.

Organothallium(I) chemistry

Organothallium(I) compounds remain obscure and limited in scope. Attempts to generate the simple compound phenylthallium result in disproportionation, giving thallium(III) derivatives:

3 TlCl + 3 LiPh → TlPh3 + 3 LiCl + 2 Tl Contrariwise, bulky tris(trimethylsilyl)methylthallium forms a stable tetramer, and bulky arylthallium(I) compounds also exist. A well-known organothallium(I) species is thallium cyclopentadienide. It arises by treatment of thallium(I) salts, such as thallium sulfate, with cyclopentadiene. Thallium(I) cyclopentadienide adopts a zig-zag chain structure of cyclopentadienide and thallium.

Organothallium(III) chemistry

Trialkyl compounds In the gas and liquid phase, trialkyl organothallium compounds are monomeric and planar. In the solid phase, there is significant intermolecular interactions between the monomers. Trialkyl thallium compounds, like those of indium and gallium, can be prepared from thallium trihalides and Grignard reagents or organolithium reagents, though the former may yield the ether complex of the product. Trimethyl thallium can be prepared from methyl iodide, methyl lithium, and thallium(I) iodide. Triethyl thallium can be similarly prepared. Trialkyl thallium compounds can undergo alkyl exchange with itself and some acidic hydrocarbons like alkynes and cyclopentadiene. Some trialkyl thallium compounds are photosensitive.

Dialkyl compounds Dialkylthallium(III) compounds are ionic salts. They are water soluble and the hydroxide is strongly basic. The cations are linear, resembling isoelectronic dialkylmercury compounds. However, dimeric or polymeric structures may exist in apolar solvents or crystals. R2TlX can be prepared from stoichiometric proportions of Grignard reagents and thallium trihalides, or trialkylthalliums and a hydrohalic acid. Alternatively, dialkylmercurials react with thallium trihalides to give dialkylthallium salts, instead of trialkylthalliums, and certain aryldiazonium tetrafluoroborate salts oxidize thallium metal. Dialkylthallium compounds are mostly stable to air and moisture. The halide atom can be substituted by nucleophiles, and the alkyl group can be abstracted by mercury acetate. (C5F6)2TlBr oxidizes low-valent metal halides, donating both alkyl ligands to the metal and reducing itself to thallium(I) bromide.

Aryl derivatives

The most prominent contribution of organothallium chemistry to organic synthesis is the reaction of thallium(III) trifluoroacetate with arenes and (to a lesser extent) alkenes. Thallium(III) is a more potent electrophile than Hg(II), delivering arylthallium(III) derivatives:

ArH + Tl(O2CCF3)3 → ArTl(O2CCF3)2 + HO2CCF3 Arylthallium compounds can also be prepared directly from TlCl3 through transmetalation (ipso subsitution) on a boronic acid. Excess boronic acid will result in diarylthallium chloride formation. Phenylthallium dihalides are Lewis acidic in nature, and tend to eliminating the corresponding halobenzene. They grow less stable from down the period, and the diiodide is unknown. Nucleophiles can displace the halide atom, and disproportionation to thallium(III) halides and diphenylthallium halides is also possible. The dichloride transmetallates onto mercuric chloride.

History The first organothallium compound, diethylthallium chloride, was prepared in 1870, shortly after the discovery of the element thallium.

See also Organogallium chemistry Organoindium chemistry Thallium Organometallic chemistry

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