Electrostatic Properties of N-Heterocyclic Carbenes Obtained by Experimental Charge-Density Analysis of Two Selenium Adducts

High-resolution X-ray diffraction data and Hansen-Coppens multipole formalism have been used to obtain the charge density distribution in two selenium adducts with the N-heterocyclic carbenes 1,3-dimesitylimidazolidin-2-ylidene (1) and 1,3-dimesityl-4,5-dioxoimidazolidin-2-ylidene (2). Application of Bader's quantum theory of atoms in molecules and calculation of the atomic charges by integration of the experimental electron density over the atomic basins showed that the main electrostatic difference between the Se adducts is in the significantly different accumulation of electron density in their C-(carbene)-Se fragments (-0.10 vs. + 0.30 e in 1 and 2). Deformation electron density features along the C-Se bond and topological values at the bond critical point indicate that this bond has pi character in both compounds but it is more pronounced in the case of 2. This is in agreement with stronger C-N bonds and a larger N-C-N angle in 1. The experimental charge density distribution also clearly indicates the important role of the N-mesityl substituent as an electron-donating group, which increases the electron density in the imidazole ring thus contributing to the electronic stabilization of the carbene C atom. Because the N-mesityl substituents donate similar amounts of electron density in both molecules (0.75 and 0.64 e in 1 and 2), it was concluded that the decisively different electronic properties of the two NHCs have to be attributed to their different backbone structures.