Stabilization of Cyclic C by Four Donor Ligands: A Theoretical Study of (L)C (L = Carbene).

Quantum chemical studies using density functional theory were carried out for the (L)C complexes with L = cAAC, DAC, NHC, SNHC, MIC1, and MIC2. The results show that the title complexes are highly stable with respect to dissociation, (L)C → C + 4L. However, their stability with respect to (L)C → 2(L)C is crucial for the assessment of their experimental viability. The (L)C complexes with L = cAAC and DAC dissociate exergonically at room temperature into two (L)C units. In contrast, the other (L)C complexes with L = NHC, SNHC, MIC1, and MIC2 are thermochemically stable with respect to dissociation, (L)C → 2(L)C. The computed adiabatic ionization potentials of (L)C complexes with L = NHC, MIC1, and MIC2 are lower than those for the cesium atom. Particularly, (MIC1)C and (MIC2)C will very easily lose electrons to form cationic complexes. The SNHC ligand is the best for the experimental realization of (L)C complexes, followed by NHC. The bonding analysis using charge and energy decomposition methods suggests that the (L)C-C bond can be best described as a typical electron-sharing double bond with a strong σ-bond and a weaker π-bond. Therefore, the core bonding pictures in the title complexes resemble a [4]radialene. Larger substituents at the carbene ligands enhance the stability of the complexes (L)C against dissociation.