Stabilization of Cyclic C 4 by Four Donor Ligands: A Theoretical Study of (L) 4 C 4 (L = Carbene).

Quantum chemical studies using density functional theory were carried out for the (L) 4 C 4 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) 4 C 4 → C 4 + 4L. However, their stability with respect to (L) 4 C 4 → 2(L) 2 C 2 is crucial for the assessment of their experimental viability. The (L) 4 C 4 complexes with L = cAAC and DAC dissociate exergonically at room temperature into two (L) 2 C 2 units. In contrast, the other (L) 4 C 4 complexes with L = NHC, SNHC, MIC1, and MIC2 are thermochemically stable with respect to dissociation, (L) 4 C 4 → 2(L) 2 C 2 . The computed adiabatic ionization potentials of (L) 4 C 4 complexes with L = NHC, MIC1, and MIC2 are lower than those for the cesium atom. Particularly, (MIC1) 4 C 4 and (MIC2) 4 C 4 will very easily lose electrons to form cationic complexes. The SNHC ligand is the best for the experimental realization of (L) 4 C 4 complexes, followed by NHC. The bonding analysis using charge and energy decomposition methods suggests that the (L) 3 C 4 -C L 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) 4 C 4 against dissociation.