π-conjugated phosphasilenes stabilized by fused-ring bulky “Rind” groups

Recent progress in the chemistry of π-conjugated phosphasilenes stabilized by bulky protecting groups based on a fused-ring octa-R-substituted s-hydrindacene skeleton (Rind groups) is reviewed. The phosphasilenes with a variety of aryl substituents at the silicon atom are covered in detail. The X-ray crystallography analysis showed the highly coplanar π-framework reinforced by the perpendicularly-fixed Rind groups. Strong π-π* absorptions have been observed, demonstrating the extension of π-conjugation over the skeleton. The DFT calculations indicated that the LUMO involves significant contribution by the 3pπ*(Si-P)-2pπ*(carbon π-electron system) conjugation. The preparation of the gold(I) complexes of the π-conjugated phosphasilenes is also included.