Isolable compounds that contain π bonding without an underlying σ-bond framework (π-type single bonding) remain scarce. Herein we report the synthesis and properties of 1,3-diiodotetrasilabicyclo[1.1.0]butane 2 via 1,2-diiodination of the corresponding bridgehead disilene 1. Single-crystal X-ray diffraction analysis of 2 revealed a planar bicyclic silicon ring with a planar geometry around the bridgehead silicon atoms and a long bridgehead Si−Si distance. DFT calculations suggested a π-type single bond between the bridgehead Si atoms in 2 and indicated that the high planarity of 2 should most likely be attributed to the bulky substituents at the bridge positions.

Complexes that could be switched between two electronic states by external stimuli have attracted much attention for their potential application in molecular devices. However, a realization of such a phenomenon with low-valent main-group element-centered complexes remains challenging. Herein, we report the synthesis of cyclic (alkyl)(amino)silylene (CAASi)ligated monatomic silicon(0) complexes (silylones). The bis-(CAASi)-ligated silylone adopts a π-localized ylidene structure (greenish-black color) in the solid state and a π-delocalized ylidene structure (dark-purple color) in solution that could be reversibly switched upon phase transfer (ylidene [L: → :Si = L ↔ L = Si: ← :L]). The observed remarkable difference in the physical properties of the two isomers is attributed to the balanced steric demand and redox noninnocent character of the CAASi ligand which are altered by the orientation of the two terminal ligands with respect to the Si−Si−Si plane: twisted structure (π-localized ylidene) and planar structure (π-delocalized ylidene). Conversely, the CAASi/CDASi-ligated heteroleptic silylone (CDASi = cyclic dialkylsilylene) only exhibited the twisted π-localized ylidene structure regardless of the phase. The synthesized silylones also proved themselves as monatomic silicon surrogates. Thermolysis of the silylones in the presence of an ethane-1,2-diimine afforded the corresponding diaminosilylenes. Analyses of the products suggested a stepwise mechanism that proceeds via a disilavinylidene intermediate.

Heavy element analogues of bicyclo[1.1.0]butane have attracted much attention as they often exhibit skeletal isomerizations that are not observed for all-carbon bicyclo[1.1.0]butanes. The reaction of a bicyclo[1.1.0]tetrasil-1(3)-ene with CCl4 at...