Carbenaporphyrins: No Longer Missing Ligands in N-Heterocyclic Carbene Chemistry

The synthesis of an NHC-containing porphyrinoid ligand is presented. The formally antiaromatic 20 pi e(-) macrocyclic framework can be obtained via a 1,3-dipolar cycloaddition (click-reaction) to form two triazole moieties which were alkylated to the respective triazolium macrocycle. Deprotonation of the ligand precursor with lithium bases to the respective dilithio carbenaporphyrin complex and transmetallation to scandium lead to complexes that exhibit orange fluorescence. Optical property combined with TD-DFT studies verify an aromatic character for each heterocyclic moiety rather than an antiaromatic macrocycle in the ligand precursor as well as in the complexes. While the geometric features of the carbenaporphyrin ligand strongly resemble those of porphyrin, DFT calculations reveal a stronger electron-donating ability of the new ligand.

Automated summary

The synthesis of a novel NHC-containing porphyrinoid ligand with good fluorescence properties was successfully achieved. Experimental results demonstrated that the heterocyclic moieties in both the ligand and its metal complexes exhibit aromatic character instead of antiaromatic macrocycles.