Syntheses and Properties of Tin‐Containing Conjugated Heterocycles

Heterocycles that contain tin atoms can be aromatic in a similar sense to well‐known aromatic compounds such as benzene or thiophene, but such examples are rare. However, due to the low‐lying σ*‐orbitals of the tin‐substituent bond in stannoles, they are capable of σ*–π* conjugation in a way that is exclusive to heavier element containing heterocycles. This makes stannoles very interesting alternatives for purely organic heterocycles in material applications, in which optoelectronic properties are of interest. This Concept article will highlight the synthesis, reactivity and physical properties of stannoles and related fluorenostannoles. At first, a brief introduction to different types of tin‐containing heterocycles is presented, followed by a discussion on different approaches to prepare stannoles, their reactivity and their physical properties. In addition, the first stannole‐containing polymer will be reviewed. Stannoles can be prepared with different substitution patterns and even certain polymeric species have been reported. Although they are formally merely cyclopentadienes in which the sp3 carbon atom has been replaced by a tin atom, their optoelectronic properties are unusual: low lying σ*‐orbitals can overlap with the π*‐orbitals of the cyclopentadienyl moiety, leading to an overall lowering of the LUMO of the system.