Stabilization of Cyclic C₄ by Four Donor Ligands: A Theoretical Study of (L)₄C₄ (L = Carbene)

Quantum chemical studies using density functional theory were carried out for the (L)₄C₄ 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)₄C₄ → C₄ + 4L. However, their stability with respect to (L)₄C₄ → 2(L)₂C₂ is crucial for the assessment of their experimental viability. The (L)₄C₄ complexes with L = cAAC and DAC dissociate exergonically at room temperature into two (L)₂C₂ units. In contrast, the other (L)₄C₄ complexes with L = NHC, SNHC, MIC1, and MIC2 are thermochemically stable with respect to dissociation, (L)₄C₄ → 2(L)₂C₂. The computed adiabatic ionization potentials of (L)₄C₄ complexes with L = NHC, MIC1, and MIC2 are lower than those for the cesium atom. Particularly, (MIC1)₄C₄ and (MIC2)₄C₄ will very easily lose electrons to form cationic complexes. The SNHC ligand is the best for the experimental realization of (L)₄C₄ complexes, followed by NHC. The bonding analysis using charge and energy decomposition methods suggests that the (L)₃C₄-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)₄C₄ against dissociation.