Correlation between bonding, philicity and substituent effects in cyclopropenylidenes

Abstract

The electronic structure and bonding of cyclopropenylidenes with different substituents at the olefinic position viz., –H (1), electron-withdrawing groups (–CHO (2) and –COOH (3)) and electron-donating groups (–N(CH3)2 (4) and –N(iPr)2 (5)), have been explored at the M06/def2-TZVPP//BP86/def2-TZVPP level of theory. The ground state of cyclopropenylidenes 1 – 5 is a singlet 2π-aromatic system with a considerable singlet–triplet energy gap. The geometrical and electronic structure analyses indicate that the nucleophilicity of the carbene lone pair increases when substituents change from electron-withdrawing to electron-donating groups. The energy decomposition analysis combined with natural orbitals for chemical valence (EDA-NOCV) analysis indicates that the bonding interaction between the carbene carbon and the –C2R2 fragment in the parent cyclopropenylidene (1) can be best represented by two electron-sharing σ-bonds and a donor–acceptor π-bond from –C2R2 fragment to the carbene carbon. The pz-orbital on the carbene carbon atom is susceptible to nucleophilic attack if the incoming nucleophile is stronger than the π -MO of the acetylenic fragment. When the substituents change to electron-withdrawing groups, −CHO (2) and –COOH (3), the σ-bonds turn into donor–acceptor interactions. The py and pz-orbitals on the carbene carbon atom are accepting electrons from the acetylenic fragment. Hence these orbitals are susceptible to nucleophilic attack. On the other hand, when the substituents change to electron-donating groups, −N(CH3)2 (4) and −N(iPr)2 (5), the best bonding situation changes into one electron-sharing σ-bond and π-bond along with one donor–acceptor σ-bond from −C2R2 fragment to the carbene carbon. The py-orbital is accepting electrons from the acetylenic fragment. Hence, py-orbital is susceptible to nucleophilic attack, which in turn leads to Ccarbene–C bond cleavage. These bonding descriptions are in agreement with the experimental isolation of several derivatives of cyclopropenylidene.