Organothallium I Compounds

In contrast to other organothallium(I) compounds, cyclopentadienyl-thallium(I) is a remarkably stable compound. Samples can be stored in sealed bottles for months without appreciable decomposition occurring it is unaffected by water and dilute alkali and it is only slowly oxidized by air at room temperature. Cyclopentadienyltballium(I) was first prepared by Meister in 1956 by addition of freshly distilled cyclopentadiene to a suspension of thallium(I) sulfate in dilute potassium hydroxide solution 101, 102). A number of variations of this procedure have been described (5, 25, 34, 56), and the compound has been made in other ways 35, 56,110, 164), but Meister s preparation, in which the yield of crude product is greater than 90%, remains the method of choice. Purification of crude cyclopenta-dienylthallium(I) is best accomplished by vacuum sublimation, and purity of samples can readily be assessed by gas-liquid chromatography on silicone oil at 170° C using hydrogen as carrier gas (7). 

The salt is a colorless crystalline solid which is virtually insoluble in all common organic solvents. It reacts slowly with chloroform and carbon tetrachloride to give thallium(I) chloride 25), gives a characteristic red coloration with carbon disulfide, and undergoes the Diels-Alder reaction with maleic anhydride 110). It is rapidly decomposed by acids, but is stable to water this latter fact has been interpreted (55) in terms of the small free energy change for the reaction 

The structure of cyclopentadienylthallium(I) has been the subject of controversy and while the arguments have not been entirely satisfactorily settled, the evidence now available indicates that the compound is primarily ionic in the solid state but possibly mainly covalent in the gaseous phase. The former conclusion at least is reasonable in view of the well-known stability of the cyclopentadienyl anion. Cyclopentadienylthallium(I) has 

The NMR spectrum (114) of the salt in tetrahydrofuran at 34° C consists of a singlet at 5.2 t, again apparently indicating an ionic bond and suggesting. 

A number of alkylated and ring-annelated derivatives of cyclopentadienyl-thallium(I) have been reported. All were prepared by the same procedure used for the parent compound, and relevant experimental data are listed in Table I. None of these compounds is as stable as cyclopentadienylthallium-(I) the methyl-substituted derivative, for example, undergoes essentially spontaneous oxidation on exposure to the atmosphere (25), and, qualitatively, the order of stability has been assessed (105) as 

---

Very little is known as yet of the chemistry of cyclopentadienylthallium(I) and the related compounds listed in Table I. The parent compound gives tribromocyclopentane on treatment with bromine and the hexabromo derivative with potassium hypobromite 112). By far the most important use discovered so far for these organothallium(I) compounds is the preparation of metallocenes and cyclopentadiene-transition metal complexes. These preparations are, in general, characterized by manipulative simplicity and high yields, and details of the reactions reported thus far are summarized in Tables II-IV. 

The 205JJ chemical shifts of organothallium(III) compounds cover a range of over 2000 ppm and they display solvent dependence, although not as striking as that for Tl(I). Solvent basicity and ion pairing contribute to the solvent dependence. The... 

The recently reported (757) conversion of 5-pyrazolones directly to a,j8-acetylenic esters by treatment with TTN in methanol appears to be an example of thallation of a heterocyclic enamine the suggested mechanism involves initial electrophilic thallation of the 3-pyrazolin-5-one tautomer of the 5-pyrazolone to give an intermediate organothallium compound which undergoes a subsequent oxidation by a second equivalent of TTN to give a diazacyclopentadienone. Solvolysis by methanol, with concomitant elimination of nitrogen and thallium(I), yields the a,)S-acetylenic ester in excellent (78-95%) yield (Scheme 35). Since 5-pyrazolones may be prepared in quantitative yield by the reaction of /3-keto esters with hydrazine (168), this conversion represents in a formal sense the dehydration of /3-keto esters. In fact, the direct conversion of /3-keto esters to a,jS-acetylenic esters without isolation of the intermediate 5-pyrazolones can be achieved by treatment in methanol solution first with hydrazine and then with TTN. 

Thallium(I) halides can be converted into organothallium compounds by treatment with a highly reactive reagent. Thus, MesTl is formed from Til and methyllithium in the presence of methyl iodide, and this reaction is believed to go via initial formation of MeTl followed by oxidative addition of Mel, and reaction with LiMe . 

A general consensus is that hardness increases (or softness decreases) with increasing positive oxidation state. For example, Ni(0) is soft, Ni(II) is borderline, but Ni(IV) is hard the sulfur atom in RS is soft, medium soft in RSO , and it is hard in RSOf is softer than SO 3 . There are a few exceptions to the rule, however. For instance, Tl(III), Sn(IV), and Pb(IV) are softer than their respective lower valent ions. Because T1(I), Sn(II), and Pb(II) ions have electrons in their outermost shells, the shielding of the d electrons decreases the so/tness of the lower valent species (12). One factual demonstration of this reverse hardness/valence relationship is that inorganic thallium compounds are generally more stable in the -1-1 valence state, while covalent organothallium derivatives are stable only in the -i-3 state. 

I. Morishkima, T. Inubushi, S. Uemura, and H. Miyoshi. Nitroxide as a nuclear spin decoupling reagent. Application to C n.m.r. studies of organothallium compounds. J. Amer. Chem. Soc., 1978,100, 354. 

---