Conformationally Switchable Silylone: Electron Redistribution Accompanied by Ligand Reorientation around a Monatomic Silicon

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.