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

Abstract

We report the experimental and theoretical characterization of neutral Si(6)X(12) (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 Si(6)X(12), with the Si(6) 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 [Si(6)Cl(14)](2-) dianionic complexes is discussed. The reaction of Si(6)Cl(12) with a Lewis base (e.g., Cl(-)) to give planar [Si(6)Cl(14)](2-) dianionic complexes presents an experimental proof that suppression of the PJT effect is an effective strategy in restoring high Si(6) ring symmetry. Additionally, the proposed pathway for the PJT suppression has been proved by the synthesis and characterization of novel compounds containing planar Si(6) ring, namely, [(n)Bu(4)N](2)[Si(6)Cl(12)I(2)], [(n)Bu(4)N](2)[Si(6)Br(14)], and [(n)Bu(4)N](2)[Si(6)Br(12)I(2)]. This work represents the first demonstration that PJT effect suppression is useful in the rational design of materials with novel properties.

Chloride‐Induced Aufbau of Perchlorinated Cyclohexasilanes from Si2Cl6: A Mechanistic Scenario

“…[4] In the same work we also made the baffling observation that dissolution of Si 2 [5] Structural and binding properties of this and related systems have subsequently been explored experimentally [6][7][8] and theoretically. [9,10] Further elaboration of our previous findings led us to consider the chloride ion itself as a Lewis base to trigger a disproportionation reactivity of perchlorinated silanes (for related examples of silane disproportionation reactions in the presence of chloride ions see Refs. [1b,d,e,f,k]).…”

Pseudo Jahn-Teller origin of instability of planar configurations of hexa-heterocycles. Application to compounds with 1,2- and 1,4-C4X2 skeletons (X = O, S, Se, Te)

“…For example of the realization of the mechanism (1), the driven by the PJTE out-of-plane displacement of the iron atom from the porphyrin ring in hemoglobin was shown to return into the plane by oxygenation due to the increase of the energy gap to the active excited state, thus suppressing the PJTE [22]. Examples of realization of the mechanism (2) were demonstrated recently [25]. For instance, by coordination of two chloride anions to nonplanar (distorted by the PJTE) chair structures of Si 6 X 12 (X = Cl, Br) to form dianions of the type [Si 6 Cl 14 ] 2À , the latter were proved to have a planar Si 6 -ring structure, in which the PJTE is suppressed because the orbitals of the active excited states of the ring become occupied by the Cl À electrons [25].…”

“…Examples of realization of the mechanism (2) were demonstrated recently [25]. For instance, by coordination of two chloride anions to nonplanar (distorted by the PJTE) chair structures of Si 6 X 12 (X = Cl, Br) to form dianions of the type [Si 6 Cl 14 ] 2À , the latter were proved to have a planar Si 6 -ring structure, in which the PJTE is suppressed because the orbitals of the active excited states of the ring become occupied by the Cl À electrons [25]. These two possibilities to quench the PJTE by means of chemical influence, increasing the energy gap to the excited state or populating the excited state, may be achieved also by other external perturbations.…”

“…[23], and then employed to explain the origin of this structural property of a variety of molecular systems (see, e.g., [20,21,[23][24][25][26][27][28] and references therein). In particular, based on the understanding of the origin of the driving force of puckering, chemical substitutions that quench the PJTE and restore the planar configuration were suggested and realized [25,26].…”

“…[23], and then employed to explain the origin of this structural property of a variety of molecular systems (see, e.g., [20,21,[23][24][25][26][27][28] and references therein). In particular, based on the understanding of the origin of the driving force of puckering, chemical substitutions that quench the PJTE and restore the planar configuration were suggested and realized [25,26]. There are two ways to suppress the PJTE instability in order to restore the high-symmetry configuration [20,21]: (1) to increase the energy gap between the ground state and the active excited state that produces the instability of the ground states, and (2) to populate that excited state with electrons.…”