Thallium pseudohalide complexes

Thallium(I) forms salts with cyanide (CN ), cyanamide (NCN ), azide (Ns ), cyanate (OCN ), isocyanate (CNO ), thiocyanate (SCN ), and selenocyanate (SeCN ) however, complexes with these ligands, like the Tl -halide complexes, are very weak. In contrast, the neutral Tl X3 species are not well known, although the Tl -pseudohalide complexes are more or less stable. 

The pseudohalides and related compounds have been investigated by a number of workers. Thallium(I) cyanate and thiocyanate are both known, and the latter has been shown to be ionic in the solid state 288 differing values have been reported for the stability constants in the T1/NCS system in aqueous or mixed water/non-aqueous systems, but the overall evidence is that the complexes are extremely weak.289 The ionic selenocyanate has been reported, but the chemistry has not been investigated.1... 

Relatively little work has been done on the redox reaction between thallium and halide/pseudohalide ions (75, 93, 97,110, 328-330). Let us consider the qualitative order of stability of thallium(III) in the form of TlXp " complexes, where X = Cl, Br, I, SCN, CN, Thallium(III) forms strong complexes with all these ligands on the other hand, it can oxidize the X ions to X2. It is well known that the thallium(III) chloride complexes are perfectly stable for an indefinite period of time. The corresponding bromide complexes are usually stable, but at low Br/Tl ratios Tl(III) can be reduced by Br the reduction is easily prevented by adding excess of bromine. The iodo and thiocyanato complexes are approximately equally unstable toward redox reaction Tl(III) is rapidly reduced by the anion.Finally, the cyano complexes... 

We reasoned that such a decarboxylation step could also be employed in a redox-neutral cross-coupling reaction with carbon electrophiles. On this basis, we drew up a catalytic cycle that starts with an oxidative addition of aryl halides or pseudohalides to a coordinatively unsaturated palladium(O) species f (Scheme 5). The more weakly coordinating the leaving group X, the easier should be its subsequent replacement by a carboxylate. At least for X = OTf, the palladium(ll) carboxylate h should form quantitatively, whereas for X = halide, it should be possible to enforce this step by employing silver or thallium salts as species g. The ensuing thermal decarboxylation of the palladium(ll) intermediate i represents the most critical step. Myers results indicated that certain palladium(ll) carboxylates liberate carbon dioxide on heating. However, it remained unclear whether arylpalladium (II) carboxylate complexes such as i would display a similar reactivity. If this were to be the case, they would form Ar-Pd-Ar intermediates k, which in turn are... 

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