Synthesis, Structure and Electronic Properties of a Stable π‐Type 3‐Electron‐2‐Center‐Bonded Species: A Silicon Analogue of a Bicyclo[1.1.0]butane Radical Anion

Abstract

σ-Type 3-electron-2-center (3e-2c) bonds have been extensively studied as one of the key bonding motifs in radical chemistry and some biological systems. "π-Type 3e-2c-bonded species" that contain a 3e-2c πbond without an underlying σ-bond framework, however, have been unexplored so far both theoretically and experimentally. Herein, we report the synthesis of the first stable π-type 3e-2c-bonded species, a silicon analogue of a bicyclo[1.1.0]butane radical anion. This compound exhibits an extremely long bridgehead SiÀ Si bond (3.0638(8) Å) and a strong near-IR absorption at 922 nm in solution which arises from a HOMO!SOMO [π(SiÀ Si)!π*(SiÀ Si)] transition. DFT calculations revealed a π-type bonding interaction between the two bridgehead silicon atoms with an unpaired electron mainly delocalized across the corresponding π*-type orbital, which introduces a novel bonding motif for constructing π-electron systems.

Heteroatom-Based Diradical(oid)s

“…Alkynyl substituents on the planarized Si atoms enabled intermediate trans -annular Si–Si distances of 2.7112(7) and 2.6992(11) Å . In 2022, Iwamoto also prepared another derivative with trimethylsilyl substituents on the tricoordinated Si atom ( 125 ), and in that case, the trans -annular Si–Si distance was found to be 2.844(1) Å …”

“…408 In 2022, Iwamoto also prepared another derivative with trimethylsilyl substituents on the tricoordinated Si atom (125), and in that case, the transannular Si−Si distance was found to be 2.844(1) Å. 409 It is interesting to note that, already in 1985, Masamune et al investigated the related system [(Dep 2 Si)(μ-Si t Bu)] 2 (126). They observed reactivity toward O 2 , H 2 O, and Cl 2 , but the lack of structural data of 126 impeded a detailed analysis and a rapidly inverting butterfly type structure was presumed.…”

Hybrid Bicyclo[1.1.0]butanes [(R4Si3)CH2] and [(R4Si2Ge)CH2] (R = SiMetBu2): Synthesis and Thermal Isomerization to Alkyl-Substituted Cyclopropenes [R3Si3]CH2R and [R3Si2Ge]CH2R

“…In contrast to the well-studied disila- and tetra­sila­bicyclo[1.1.0]butanes with the standard , or long Si–Si bridging bonds, their analogues manifesting very short Si–Si bridges were unknown prior to our studies. − In the preliminary communication, we reported the first examples of this novel structural motif in the chemistry of Si- and Ge-containing bicyclo[1.1.0]butanes, featuring very short Si–Si bridging bonds. In this paper, we present a full account of the chemistry of such compounds, including their synthesis, structural studies, and thermal isomerization to alkyl-substituted heavy cyclopropene analogues, with a particular focus on the mechanism of this unusual isomerization.…”

“…Experimentally, valence interplay of the isolable silicon-containing bicyclo[1.1.0]butanes with isomeric cyclobutenes, 1,3-butadienes, and cyclopropenes was documented, and several examples of such electrocyclic interconversions were reported for disila-, trisila-, − and tetrasila- (bicyclo[1.1.0]butanes). From the viewpoint of the bond-stretch isomerism, silicon-containing bicyclo[1.1.0]butanes with both normal Si–Si bridging bond (SB-isomers) , and long Si–Si bridging bond (LB-isomers) were isolated and structurally characterized. Among the LB-isomers, tetrasilabicyclo[1.1.0]butanes with an exceptionally long Si–Si bridge of 2.5822(11)–2.871(1) Å and planar Si 4 rings are of interest, being interpreted as possessing either a bridging Si–Si π-single bond ,, or a 1,3-biradicaloid structure…”

π-Aromaticity Dominating in a Saturated Ring: Neutral Aromatic Silicon Analogues of Cyclobutane-1,3-diyls

“…There are only a few silicon analogues of 1,3-cyclobutanedilyls that have been isolated. A couple of tetrasilabicyclo[1.1.0]­butanes A (Char 1) with a 1,3-Si–Si σ bond had been reported, whereas the existence of a Si–Si π bond in tetrasilabicyclo[1.1.0]­butanes B and C has been reported by Iwamoto and Kyushin groups. , The closed-shell 1,3-cyclotetrasilanediyl D (Chart ) with a long 1,3-Si–Si bond distance was isolated by the Scheschkewitz group . These results indicated pronounced substituent effects on the electronic structures of such Si 4 rings.…”