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, [Si^II(Xant)Si^II]Si⁰ (4, Xant = 9,9-dimethyl-xanthene-4,5-diyl), has been synthesized via the reduction of the corresponding chlorosilyliumylidene chloride precursor {[Si^II(Xant)Si^II]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 sp^0.41-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 Si^II 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.