Fluorescent Dyes in Organometallic Chemistry: Coumarin‐Tagged NHC–Metal Complexes

Abstract

7‐NEt2‐ and julolidene‐coumarin‐substituted azolium salts were synthesized and used to prepare the respective coumarin‐substituted NHC–metal complexes: [AuCl(NHC)], [PdCl(allyl)(NHC)], [MCl(CO)2(NHC)] (M = Rh, Ir), [MCl(cod)(NHC)] (M = Rh, Ir), [PdCl2(Cl‐py)(NHC)], [PdC(allyl)]2, and [RuCl2(ind)(NHC)(SIMes)] – sorted according to the respective fluorescence quantum yields of between φ = 0.70 [Au] and φ = 0.005 [Ru]. The fluorescence intensity of the complexes depends on the electron density at the metal and can be increased by electron‐withdrawing groups and decreased by electron‐donating groups. In this vein, the substitution of cod (1,5‐cyclooctadiene) by two CO in [MCl(cod)(NHC)] (M = Rh, Ir) generates [MCl(CO)2(NHC)] and an up to 18‐fold increase in the fluorescence. Solutions of [MCl(cod)(NHC)] or soaked filter paper strips were treated with CO and excited with 360 nm UV light, resulting in a bright‐blue fluorescence. The use of a julolidine‐derived coumarin NHC ligand leads to a stable, acid‐stable fluorescence signal even in a highly polar environment.