Annulated\n1,4-Disilabenzene-1,4-diide and Dihydrogen\nSplitting

The\nisolation of silicon analogues of phenyl anions such as (C<sub>6</sub>H<sub>5</sub>)<sup>−</sup> and (C<sub>6</sub>H<sub>4</sub>)<sup>2–</sup> is challenging owing to their extremely\nhigh reactivity associated with their silylene character and weak\nC–Si π-interaction. Herein, we report the first annulated\n1,4-disilabenzene-1,4-diide compound [(ADC)Si]<sub>2</sub> (<b>5</b>) based on anionic dicarbene (ADC) scaffolds (ADC = PhC{N(Dipp)C}<sub>2</sub>; Dipp = 2,6-<i>i</i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>) as a green-yellow crystalline solid. Compound <b>5</b> is prepared by KC<sub>8</sub> reduction of the Si(IV) chloride\n[(ADC)SiCl<sub>3</sub>]<sub>2</sub> (<b>3</b>) or the cyclic\nbis-chlorosilylene [(ADC)SiCl]<sub>2</sub> (<b>4</b>), which\nare also prepared for the first time. <b>5</b> is a neutral\nmolecule, and each of the two-coordinated Si(I) atoms has a lone pair\nand an unpaired electron. Experimental and theoretical data indicate\ndelocalization of the silicon unpaired electrons, resulting in a 6π-electron\nC<sub>4</sub>Si<sub>2</sub> ring in <b>5</b>. The diradical\ncharacter (<i>y</i>) for <b>5</b> amounts to 15%.\nAt room temperature, <b>5</b> readily reacts with dihydrogen\n(H<sub>2</sub>) to form the elusive bis-hydridosilylenes [(ADC)SiH]<sub>2</sub> (<i>Z</i>)-<b>6</b> and (<i>E</i>)-<b>6</b>. The [4 + 2]-cycloaddition of <b>5</b> and\nPhCCPh in yielding the barrelene-type bis-silylene [(ADC)SiCPh]<sub>2</sub> (<b>7</b>) emphasizes the diradical reactivity of <b>5</b>. With elemental sulfur, <b>5</b> results in the S<sub>2</sub>- and S<sub>3</sub>-bridged silathione derivatives [(ADC)Si(S)]<sub>2</sub>(μ-S<sub>2</sub>) (<b>8a</b>) and [(ADC)Si(S)]<sub>2</sub>(μ-S<sub>3</sub>) (<b>8b</b>). Moreover, the treatment\nof <b>5</b> with Fe<sub>2</sub>(CO)<sub>9</sub> affords the\nFe(0) complex [(ADC)Si(Fe(CO)<sub>4</sub>)]<sub>2</sub>(μ-CO)\n(<b>9</b>), in which each silicon atom serves as a two-electron\nσ-donor ligand and shares one electron with the bridging CO\nunit to form two Si–C bonds. The molecular structures of all\ncompounds have been established by X-ray diffraction, and representative\ncompounds have been analyzed by quantum chemical calculations.