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 (5) based on anionic dicarbene (ADC) scaffolds (ADC = PhC{N(Dipp)C}₂; Dipp = 2,6-iPr₂C₆H₃) as a green-yellow crystalline solid. Compound 5 is prepared by KC₈ reduction of the Si(IV) chloride [(ADC)SiCl₃]₂ (3) or the cyclic bis-chlorosilylene [(ADC)SiCl]₂ (4), which are also prepared for the first time. 5 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 5. The diradical character (y) for 5 amounts to 15%. At room temperature, 5 readily reacts with dihydrogen (H₂) to form the elusive bis-hydridosilylenes [(ADC)SiH]₂ (Z)-6 and (E)-6. The [4 + 2]-cycloaddition of 5 and PhC≡CPh in yielding the barrelene-type bis-silylene [(ADC)SiCPh]₂ (7) emphasizes the diradical reactivity of 5. With elemental sulfur, 5 results in the S₂- and S₃-bridged silathione derivatives [(ADC)Si(S)]₂(μ-S₂) (8a) and [(ADC)Si(S)]₂(μ-S₃) (8b). Moreover, the treatment of 5 with Fe₂(CO)₉ affords the Fe(0) complex [(ADC)Si(Fe(CO)₄)]₂(μ-CO) (9), 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.