Convenient synthetic access to fluorescent rhodacyclopentadienes via ligand exchange reactions

Abstract We have previously reported the formation of fluorescent 2,5-bis(arylethynyl)rhodacyclopentadienes from the reaction of [Rh(κ2-O,O-acac)(PMe3)2] (acac = acetylacetonato) with α,ω-bis(arylbutadiynyl)alkanes. However, a second isomer series, namely phosphorescent rhodium biphenyl complexes, was also obtained from the same reaction mixture, which made purification of the 2,5-bis(arylethynyl)rhodacyclopentadienes challenging and led to low isolated yields. Herein, we describe a synthetic protocol to access the desired fluorescent rhodium complexes by reaction of [Rh(κ2-O,O-acac)(P(p-tolyl3)2)] with α,ω-bis(arylbutadiynyl)alkanes, which gives exclusively 2,5-bis(arylethynyl)rhodacyclopentadienes, and subsequent phosphine ligand exchange. The rhodacyclopentadienes bearing P(p-tolyl3) ligands have been investigated and compared to their PMe3 analogs with regard to their photophysical properties, showing that the aromatic phosphine ligands enhance non-radiative decay from the singlet excited state S1, while no phosphorescence from T1 is observed despite the presence of the heavy rhodium atom. One of the P(p-tolyl3) ligands can also be exchanged for an N-heterocyclic carbene (NHC), leading to unsymmetrically coordinated rhodacyclopentadienes.