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

Abstract

Herein, we present a general method for a reliable estimation of the extent of π-backdonation (CcAAC←E) of the bonded element (E) to the carbene carbon atom and CcAAC→E σ-donation. The CcAAC←E π-backdonation has a significant effect on the electronic environments of the (15)N nucleus. The estimation of the π-backdonation has been achieved by recording the chemical shift values of the (15)N nuclei via two-dimensional heteronuclear multiple-bond correlation spectroscopy. The chemical shift values of the (15)N nuclei of several cAAC-containing compounds and/or complexes were recorded. The (15)N nuclear magnetic resonance chemical shift values are in the range from -130 to -315 ppm. When the cAAC forms a coordinate σ-bond (CcAAC→E), the chemical shift values of the (15)N nuclei are around -160 ppm. In case the cAAC is bound to a cationic species, the numerical chemical shift value of the (15)N nucleus is downfield-shifted (-130 to -148 ppm). The numerical values of the (15)N nuclei fall in the range from -170 to -200 ppm when σ-donation (CcAAC→E) of cAAC is stronger than CcAAC←E π-backacceptance. The π-backacceptance of cAAC is stronger than σ-donation, when the chemical shift values of the (15)N nuclei are observed below -220 ppm. Electron density and charge transfer between CcAAC 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.