Stabilizing antiaromatic C₁₃ through metal encapsulation: a dual stabilization mechanism achieving closed-shell...

Cyclo[13]carbon (C₁₃) is inherently antiaromatic and unstable, with an open-shell triplet ground state. Here we show, via first-principles calculations, that encapsulating an alkaline earth atom (M = Ca, Sr, and Ba) within the ring transforms C₁₃ into a closed-shell MC₁₃ complex with significantly enhanced stability. The bonding is predominantly ionic, with the metal atom donating its two valence s-electrons individually to the in-plane and out-of-plane π-systems of the carbon ring. This electron transfer simultaneously achieves two stabilizing effects. It grants the metal center a closed-shell noble-gas-like electronic configuration while providing two additional π-electrons that confer dual aromatic character on the carbon ring. This dual stabilization mechanism, arising from both the closed-shell metal center and the aromatic carbon ring, converts an inherently unstable antiaromatic molecule into a robust aromatic system. Furthermore, MC₁₃ exhibits exceptional nonlinear optical responsiveness under external electric fields, offering a design strategy for field-controllable nonlinear optical materials.