Building Boron-Containing Polyphenylenes

Thanks to a spontaneous ring-opening reaction, there’s now a better route to boron-containing building blocks for making optoelectronic materials (J. Am. Chem. Soc., DOI: 10.1021/ja110947k). Incorporating electron-deficient boron into the extended π-electron system of polyphenylenes is a powerful tool chemists can use to tweak the properties of these materials for use in organic electronics and photonics. But only a few limited, two-component condensation polymerization pathways to boron-containing polyphenylenes have been developed. A team led by Matthias Wagner and Max C. Holthausen of Goethe University, in Frankfurt, Germany, has now found a versatile single-component approach: the in situ synthesis of the unstable antiaromatic compound borafluorene, which undergoes a spontaneous ring-opening reaction leading to single-crystalline boron-containing phenylene oligomers. The oligomer backbone (pentamer shown) is reinforced by three-center, two-electron B–H–B bridging bonds, Wagner explains. These bonds reversibly break and re-form, enabling them to react with alkynes. This treatment establishes a π-delocalized system via three-coordinate boron bridges, offering new options to influence the properties of polyphenylenes, he notes.