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.