7-NEt 2 -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), [PdCl 2 (Cl-py)(NHC)], [PdC(allyl)] 2 , and [RuCl 2 -(ind)(NHC)(SIMes)] -sorted according to the respective fluorescence quantum yields of between = 0.70 [Au] and = 0.005 [Ru]. The fluorescence intensity of the complexes depends on the electron density at the metal and can be increased by elec- [a] Nonetheless, Bodipy fluorophores also possess a few less favorable properties: [24] a) often small Stokes shifts are observed, as little as 10 nm are not unusual; b) treatment with strong bases causes problems with acidic protons, nucleophiles can react with the BF 2 unit; and c) typically excitation requires > 500 nm light, excitation maxima approaching 400 nm are rare. [25] It therefore appears rewarding, to look for alternatives, to complement the weaknesses of Bodipy. Coumarin is an established lipophilic fluorophore whose fluorescence properties can be designed and which has found use in numerous applications and it can be easily modified to allow UV-light excitation of the fluorescence. [26] We therefore set out to synthesize, coumarintagged organometallic complexes and to study the fluorescence properties during ligand-substitution reactions.
Results and Discussion
Synthesis of Coumarin-Tagged Azolium Salts and NHC-Metal ComplexesIt was decided to use 7-NEt 2 coumarins since these fluorophores are characterized by blue to ultraviolet excitation Scheme 1. Synthesis of a coumarin-substituted azolium salt (Mes = 2,4,6trimethylphenyl). Reagents and conditions: a) coumarin acid chloride 1, azolium salt 2, Et 3 N, CH 2 Cl 2 , room temp., 24 h; yield 78 %; b) 3·HBF 4 and Amberlite IRA910 Cl resin, 1:1 MeOH/CH 2 Cl 2 , room temp., overnight, yield 99 %.Scheme 2. Synthesis of coumarin-tagged NHC-metal complexes. Reagents and conditions: a) [AuCl(Me 2 S)], 3·HCl, K 2 CO 3 , 60°C, acetone; b) 3·HCl, [PdCl(allyl)] 2 and K 2 CO 3 acetone, 60°C, 4 h; c) PdCl 2 , 3·HCl, 3-Cl-pyridine solvent, K 2 CO 3 , 80°C, 24 h; d) 3·HCl, Ag 2 O, 40°C, CH 2 Cl 2 then [(SIMes)RuCl 2 (ind)(py)], (ind = 2phenylindene-1-ylidene), 60°C, toluene; e) [RhCl(cod)] 2 , acetone, 60°C, 3·HCl, K 2 CO 3 or [IrCl(cod)] 2 , CH 2 Cl 2 , 40°C, 3·HCl, Ag 2 O; f) CO, 30 min, room temp., CH 2 Cl 2 .
