Synthesis, properties, and functions of bowl-shaped pi-conjugated molecules including a hetero atom-doped system are reviewed.

Although the concept of aromaticity has long played an important role in carbon chemistry, it has been unclear how applicable the stabilizing framework is to the heaviest elements. Here we report the synthesis of dilithiotetraphenylplumbole by reduction of hexaphenylplumbole. X-ray crystallography revealed a planar structure with no alternation of carbon-carbon bond lengths in the five-membered ring core. Nuclear magnetic resonance spectra and relativistic theoretical calculations show considerable aromatic character in the molecule, thus extending aromaticity to carbon's heaviest congener.

Plumbacyclopentadienylidenes, in which the lead atoms have divalent states and are coordinated by THF, pyridine and N-heterocyclic carbene, were synthesized and characterized. The THF- and pyridine-stabilized compounds can be regarded as rare examples of hypervalent 10-X-4 species. The equilibrium between the THF adduct and the free plumbacyclopentadienylidene was evidenced by spectroscopic analysis and theoretical calculations. The THF adduct in benzene converted into a plumbylene dimer, where one of the lead centers is coordinated by THF and the other lead atom is coordinated by a divalent lead atom, the dimer gradually decomposing into spiroplumbole. The THF adduct unexpectedly reacted with trifluoroborane and trichlorogallane to afford fluoroborole and chlorogallole, which are the first examples of non-annulated fluoroborole and gallole, respectively.

In the last decade, much attention has been focused on the anions and dianions of siloles [1] and germoles, [1f, 2] which are heavier congeners of the cyclopentadienyl anion.[3] The degree of aromaticity of silolyl anions depends on the substituent, [1b,f] while the germolyl anions do not show aromaticity because the negative charge is localized on the germanium atom. [1f, 2a,2c] In contrast, the negative charges in the dianions of siloles and germoles are significantly delocalized in the C 4 M (M = Si, Ge) ring and they were concluded to be aromatic. [1d-f, 2b,2d,2e, 4] The aromatic delocalization in these dianions was evidenced by NMR studies, calculations, and Xray crystal structural analyses, which showed no alternation of the CÀC bonds within the ring. In contrast to the wellinvestigated mono-and dianions of siloles and germoles, neither mono-nor dianions of stannoles had been reported before we undertook a study of such species a few years ago.