Convenient synthetic access to fluorescent rhodacyclopentadienes via ligand exchange reactions

We have previously reported the formation of fluorescent 2,5-bis(arylethynyl) rhodacyclopentadienes from the reaction of [Rh(k(2)-O, O-acac)(PMe3)(2)] (acac = acetylacetonato) with alpha,omega-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(k(2)-O, O-acac)(P(p-tolyl(3))(2))] with alpha,omega-bis(arylbutadiynyl) alkanes, which gives exclusively 2,5-bis(arylethynyl) rhodacyclopentadienes, and subsequent phosphine ligand exchange. The rhodacyclopentadienes bearing P(p-tolyl(3)) 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 S-1, while no phosphorescence from T-1 is observed despite the presence of the heavy rhodium atom. One of the P(p-tolyl(3)) ligands can also be exchanged for an N-heterocyclic carbene (NHC), leading to unsymmetrically coordinated rhodacyclopentadienes. (C) 2017 Elsevier B. V. All rights reserved.