Annulated 1,4-Disilabenzene-1,4-diide and Dihydrogen Splitting

The isolation of silicon analogues of phenyl anions such as (C₆H₅)^- and (C₆H₄)^2- is challenging owing to their extremely high reactivity associated with their silylene character and weak C-Si π-interaction. Herein, we report the first annulated 1,4-disilabenzene-1,4-diide compound [(ADC)Si]₂ (<b>5</b>) based on anionic dicarbene (ADC) scaffolds (ADC = PhC{N(Dipp)C}₂; Dipp = 2,6-<i>i</i>Pr₂C₆H₃) as a green-yellow crystalline solid. Compound <b>5</b> is prepared by KC₈ reduction of the Si(IV) chloride [(ADC)SiCl₃]₂ (<b>3</b>) or the cyclic bis-chlorosilylene [(ADC)SiCl]₂ (<b>4</b>), which are also prepared for the first time. <b>5</b> is a neutral molecule, and each of the two-coordinated Si(I) atoms has a lone pair and an unpaired electron. Experimental and theoretical data indicate delocalization of the silicon unpaired electrons, resulting in a 6π-electron C₄Si₂ ring in <b>5</b>. The diradical character (<i>y</i>) for <b>5</b> amounts to 15%. At room temperature, <b>5</b> readily reacts with dihydrogen (H₂) to form the elusive bis-hydridosilylenes [(ADC)SiH]₂ (<i>Z</i>)-<b>6</b> and (<i>E</i>)-<b>6</b>. The [4 + 2]-cycloaddition of <b>5</b> and PhC≡CPh in yielding the barrelene-type bis-silylene [(ADC)SiCPh]₂ (<b>7</b>) emphasizes the diradical reactivity of <b>5</b>. With elemental sulfur, <b>5</b> results in the S₂- and S₃-bridged silathione derivatives [(ADC)Si(S)]₂(μ-S₂) (<b>8a</b>) and [(ADC)Si(S)]₂(μ-S₃) (<b>8b</b>). Moreover, the treatment of <b>5</b> with Fe₂(CO)₉ affords the Fe(0) complex [(ADC)Si(Fe(CO)₄)]₂(μ-CO) (<b>9</b>), in which each silicon atom serves as a two-electron σ-donor ligand and shares one electron with the bridging CO unit to form two Si-C bonds. The molecular structures of all compounds have been established by X-ray diffraction, and representative compounds have been analyzed by quantum chemical calculations.