Synthesis of an Isolable Bis(silylene)-Stabilized Silylone and Its Reactivity Toward Small Gaseous Molecules

The first bis­(N-heterocyclic
silylene)-stabilized zero-valent silicon
compound, [SiII(Xant)­SiII]­Si0 (4, Xant = 9,9-dimethyl-xanthene-4,5-diyl), has been synthesized
via the reduction of the corresponding chloro­silylium­ylidene
chloride precursor {[SiII(Xant)­SiII]­SiCl}+Cl– (2). The electronic structure
of silylone 4, whose molecular structure is confirmed
spectroscopically and crystallographically, is investigated by DFT
calculations and Natural Bond Orbital analysis, showing two perpendicular
lone-pairs of electrons on the central Si0 atom, i.e.,
an sp0.41-type lone-pair and a delocalized p lone-pair. With the electron-rich and oxophilic Si0 center, silylone 4 exhibits a striking reactivity
toward small gaseous molecules. Remarkably, the oxidation of silylone 4 by N2O can be controlled to generate distinct
products by regulating the amount of added N2O. Exposing 4 to an excess or two molar equivalents of N2O
yields the unexpected oxidation product 5, bearing a
central six-membered Si4O2 ring. When 4 is mixed with one molar equivalent of N2O, the
unique compound 6 is obtained, resulting from a rare
1,4-addition of two central silicon atoms to a phenyl ring of an amidinate
ligand coordinated to the SiII atom. In addition, cleavage
of the strong N–H bond in ammonia is also readily accomplished
by silylone 4, representing the first example of NH3 activation in silylone chemistry. In the presence of the
Lewis acid BPh3, silylone 4 achieves heterolytic
dihydrogen cleavage and ethylene addition to form the corresponding
hydrido­silylium­ylidene hydroborate salt 8 and the zwitterionic compound 9, respectively, which
represent a new type of frustrated Lewis pair based on an electron-rich
Si0 donor and a borane acceptor.