Stable Mononuclear Lead(III) Compound: A Lead-Centered Radical

摘要

Stable compounds with unpaired electrons are predominantly found within the realm of the transition metals. Stable or at least persistent[1] molecular radicals of main-group elements are still rare. Most of the known examples are either formed by certain combinations of electron-rich atoms of high electronegativity, such as oxygen, nitrogen, or fluorine, or stabilized by bulky substituents or extensive delocalization of the unpaired electron. Radicals of the heavier elements of Groups 13-15 were not structurally characterized until 1993, even though persistent species such as E[Y(SiMe3)2]3 (E=Ge, Sn, Y=N, CH) and E[CH(SiMe3)2]2 (E=P, As) were synthesized before 1980 in the pioneering work of Lappert and co-workers.[2] Stable mononuclear radicals centered at Si, Ge, and Sn were synthesized by oxidation of corresponding alkali-metal salts and structurally characterized by Sekiguchi and co-workers only very recently.[3] All efforts at synthesizing stable mononuclear radicals of Tl, Pb, and Bi have so far failed. The radical cluster anion Pb93- could be isolated in salts.[4] Its relative stability against disproportionation or oligomerization results from delocalization of the unpaired electron over the whole cluster skeleton as well as from intermolecular coulomb repulsion.