Steric and Electronic Properties of Indole‐Derived CAAC Ligands

The electronic properties of two indole‐derived CAAC ligands, featuring N−Et (II) or N−H (IV) substituents, have been investigated by DFT calculations. The calculations revealed frontier orbitals (HOMO and LUMO) with energies closely related to those of the conventional N‐DIPP substituted CAAC I. The selenium adduct of the N−H substituted CAAC 1 has been prepared and characterized by X‐ray diffraction methods and 77Se NMR spectroscopy revealing π‐acceptor properties of the CAAC similar to classical CAACs of type I. While the electronic properties of the indole‐derived CAACs II and IV are similar to those of classical CAACs, their % Vbur is significantly smaller due to the smaller N‐substituents falling in the range of % Vbur for sterically less demanding N‐heterocyclic carbenes. The indole‐derived CAAC derivatives bearing sterically less demanding N−Et (II) or N−H substituents (IV) exhibit electronic properties closely related to those of the classical CAAC I. However, their steric demand is significantly reduced in comparison to I as illustrated by rather small % Vbur values.