Synthesis, structures, optical and electrochemical properties, and complexation of 2,5-bis(pyrrol-2-yl)phospholes

2,5-Bis(pyrrol-2-yl)phosphole derivatives were prepared using the reaction of titanacycles, generated in situ from a TiII reagent and pyrrole-capped 1,6-heptadiynes, with dichloro(phenyl)phosphine. The 2,5-bis(pyrrol-2-yl)phosphole derivatives were found to possess narrower optical HOMO-LUMO gaps and less positive oxidation potentials than those of previously reported 2,5-diarylphosphole analogs. This demonstrates that the intrinsic nature of the electron excessive pyrrole subunits as well as the effective π-conjugation over the coplanar heterole rings. The σ3-P type 2,5-bis(5-phenylpyrrol-2-yl)phosphole underwent complexation with AuCl(SMe2) and PtCl2 to yield the respective AuCl-monophosphine and PtCl2-bisphosphine complexes. In the square planar PtII complex, the pyrrolic NH protons were found to form intramolecular hydrogen bonds with the chlorine atoms that gave rise to symmetrically split, parallel π-chromophores linked by two Pt-P bonds. Density functional theory calculations on a PtII model complex suggested that this cooperative interaction induces a significant split of the original LUMOs of the symmetrical π-conjugated ligands.