Phosphorus Centers of Different Hybridization in Phosphaalkene-Substituted Phospholes

Phosphole-substituted phosphaalkenes (PPAs) of the general formula Mes*P=C(CH(3))–(C(4)H(2)P(Ph))–R 5 a–c (Mes*=2,4,6-tBu(3)Ph; R=2-pyridyl (a), 2-thienyl (b), phenyl (c)) have been prepared from octa-1,7-diyne-substituted phosphaalkenes by utilizing the Fagan–Nugent route. The presence of two differently hybridized phosphorus centers (σ(2),λ(3) and σ(3),λ(3)) in 5 offers the possibility to selectively tune the HOMO–LUMO gap of the compounds by utilizing the different reactivity of the two phosphorus heteroatoms. Oxidation of 5 a–c by sulfur proceeds exclusively at the σ(3),λ(3)-phosphorus atom, thus giving rise to the corresponding thioxophospholes 6 a–c. Similarly, 5 a is selectively coordinated by AuCl at the σ(3),λ(3)-phosphorus atom. Subsequent second AuCl coordination at the σ(2),λ(3)-phosphorus heteroatom results in a dimetallic species that is characterized by a gold–gold interaction that provokes a change in π conjugation. Spectroscopic, electrochemical, and theoretical investigations show that the phosphaalkene and the phosphole both have a sizable impact on the electronic properties of the compounds. The presence of the phosphaalkene unit induces a decrease of the HOMO–LUMO gap relative to reference phosphole-containing π systems that lack a P=C substituent.