Estimation of σ-Donation and π-Backdonation of Cyclic Alkyl(amino) Carbene-Containing Compounds

Herein, we present a general method for a reliable estimation of the extent of pi-backdonation of the bonded element (E) to the carbene carbon atom and C-cAAC -> E, sigma-donation. The C-cAAC <- E pi-backdonation has a significant effect on the electronic environments of the N-15 nucleus. The estimation of the pi-backdonation has been achieved by recording the chemical shift values of the N-15 nuclei via two-dimensional heteronuclear multiple-bond correlation spectroscopy. The chemical shift values of the N-15 nuclei of several cAAC-containing compounds and/or complexes were recorded. The N-15 nuclear magnetic resonance chemical shift values are in the range from 130 to 315 ppm. When the cAAC forms a coordinate sigma-bond (C-cAAC -> E), the chemical shift values of the N-15 nuclei are around 160 ppm. In case the cAAC is bound to a cationic species, the numerical chemical shift value of the N-15 nucleus is downfield-shifted (-130 to 148 ppm). The numerical values of the N-15 nuclei fall in the range from 170 to 200 ppm when a-donation (C-cAAC -> E) of cAAC is stronger than C-cAAC <- E pi-backacceptance. The pi-backacceptance of cAAC is stronger than a-donation, when the chemical shift values of the N-15 nuclei are observed below 220 ppm. Electron density and charge transfer between C-cAAC and E are quantified using natural bonding orbital analysis and charge decomposition analysis techniques. The experimental results have been correlated with the theoretical calculations. They are in good agreement.