Flattening a puckered cyclohexasilane ring by suppression of the pseudo-Jahn-Teller effect

We report the experimental and theoretical characterization of neutral Si₆X₁₂ (X = Cl, Br) molecules that contain D_3d distorted six-member silicon rings due to a pseudo-Jahn-Teller (PJT) effect. Calculations show that filling the intervenient molecular orbitals with electron pairs of adduct suppresses the PJT effect in Si₆X₁₂, with the Si₆ ring becoming planar (D_6h) upon complex formation. The stabilizing role of electrostatic and covalent interactions between positively charged silicon atoms and chlorine atoms of the subject [Si₆Cl₁₄]^2- dianionic complexes is discussed. The reaction of Si₆Cl₁₂ with a Lewis base (e.g., Cl^-) to give planar [Si₆Cl₁₄]^2- dianionic complexes presents an experimental proof that suppression of the PJT effect is an effective strategy in restoring high Si₆ ring symmetry. Additionally, the proposed pathway for the PJT suppression has been proved by the synthesis and characterization of novel compounds containing planar Si₆ ring, namely, [ⁿBu₄N]₂[Si₆Cl₁₂I₂], [ⁿBu₄N]₂[Si₆Br₁₄], and [ⁿBu₄N]₂[Si₆Br₁₂I₂]. This work represents the first demonstration that PJT effect suppression is useful in the rational design of materials with novel properties.