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

Abstract

The first bis(N-heterocyclic silylene)-stabilized zero-valent silicon compound, [Si^II(Xant)Si^II]Si⁰ (<b>4</b>, 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^- (<b>2</b>). The electronic structure of silylone <b>4</b>, 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 <i>sp</i>^0.41-type lone-pair and a delocalized <i>p</i> lone-pair. With the electron-rich and oxophilic Si⁰ center, silylone <b>4</b> exhibits a striking reactivity toward small gaseous molecules. Remarkably, the oxidation of silylone <b>4</b> by N₂O can be controlled to generate distinct products by regulating the amount of added N₂O. Exposing <b>4</b> to an excess or two molar equivalents of N₂O yields the unexpected oxidation product <b>5</b>, bearing a central six-membered Si₄O₂ ring. When <b>4</b> is mixed with one molar equivalent of N₂O, the unique compound <b>6</b> 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 <b>4</b>, representing the first example of NH₃ activation in silylone chemistry. In the presence of the Lewis acid BPh₃, silylone <b>4</b> achieves heterolytic dihydrogen cleavage and ethylene addition to form the corresponding hydridosilyliumylidene hydroborate salt <b>8</b> and the zwitterionic compound <b>9</b>, respectively, which represent a new type of frustrated Lewis pair based on an electron-rich Si⁰ donor and a borane acceptor.