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 π-backdonation (C_cAAC←E) of the bonded element (E) to the carbene carbon atom and C_cAAC→E σ-donation. The C_cAAC←E π-backdonation has a significant effect on the electronic environments of the ¹⁵N nucleus. The estimation of the π-backdonation has been achieved by recording the chemical shift values of the ¹⁵N nuclei via two-dimensional heteronuclear multiple-bond correlation spectroscopy. The chemical shift values of the ¹⁵N nuclei of several cAAC-containing compounds and/or complexes were recorded. The ¹⁵N nuclear magnetic resonance chemical shift values are in the range from −130 to −315 ppm. When the cAAC forms a coordinate σ-bond (C_cAAC→E), the chemical shift values of the ¹⁵N nuclei are around −160 ppm. In case the cAAC is bound to a cationic species, the numerical chemical shift value of the ¹⁵N nucleus is downfield-shifted (−130 to −148 ppm). The numerical values of the ¹⁵N nuclei fall in the range from −170 to −200 ppm when σ-donation (C_cAAC→E) of cAAC is stronger than C_cAAC←E π-backacceptance. The π-backacceptance of cAAC is stronger than σ-donation, when the chemical shift values of the ¹⁵N 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.