Silicon Compounds in Low Oxidation States Supported by a Cyclic (Alkyl)(Amino)Carbene

Abstract

<strong>Abstract:</strong> Silicon compounds in low oxidation states, especially dihalosilylenes (SiX₂), are highly unstable molecules, which are involved as fleeting intermediates in the industrial production of chlorosilanes and semiconductor grade silicon. Their high reactivity, which ultimately lead to either disproportionation or polymerization, could be tamed by the use of strong σ-donating N-heterocyclic carbene (NHC) ligands, which sequestered their electrophilicity through the reversed polarized ylidic Si^δ−−NHC^δ+ bond. This concept enabled the isolation of dihalosilylenes (SiX₂) as bottleable SiX₂(NHC) adducts, which had in recent years a major impact on the development of molecular silicon chemistry. It was anticipated that the ylidic character could be tuned by using a strong σ-donor and strong π-acceptor cyclic (alkyl)(amino)carbene (CAAC) ligand leading to an unique reactivity pattern of the resulting adduct SiX₂(CAAC). For this sake, the CAAC stabilized dibromosilylene SiBr₂(CAAC) (<strong>1</strong>) was prepared and a detailed synthetic and physiochemical study was planned.<br /><strong>Highlights:</strong> In this work the high synthetic potential of compound <strong>1</strong> could be shown in a plethora of novel unsaturated compounds with intriguing synthetic potential. Among them CAAC-supported silicon(II) bromides SiBr(R)(CAAC) (R = SiTMS₃ (<strong>2-Si</strong>), PMes₂ (<strong>2-P</strong>), Mes (<strong>2-Mes</strong>), NTMS₂ (<strong>2-N</strong>), OMes* (<strong>2-O</strong>)) (Mes* = C₆H₂-2,4,6-<em>t</em>Bu₃), neutral two-coordinated silicon(I) radicals Si(R')(CAAC) (R' = SiTMS₃ (<strong>3-Si</strong>), NTMS₂ (<strong>3-N</strong>), OMes* (<strong>3-O</strong>)), 1,4-diamino-2,3-disila-butadienes Si₂(R'')₂(CAAC)₂ (R'' = Br (<strong>9-Br</strong>), Cp'(<strong>9-Cp'</strong>), Mes (<strong>9-Mes</strong>), C≡CTMS (<strong>9-C₂TMS</strong>), C≡CMes (<strong>9-C₂Mes</strong>)), potassium silenides SiK(R''')(CAAC) (R''' = SiTMS₃ (<strong>4-Si</strong>), C≡CTMS (<strong>10-C₂TMS</strong>)) and CAAC-supported heavier tetrelavinylidenes (CAAC)E=SiBr(Tbb) (E = Si (<strong>14</strong>), Ge (<strong>14-Ge</strong>)) (Tbb = C₆H₂-2,6-Dsi₂-4-<em>t</em>Bu, Dsi = CH(SiMe₃)₂) are particularly instructive allowing to probe the electronic structure via a combination of structural, spectroscopic and theoretical studies. Detailed kinetic studies revealed an irreversible <em>E→</em><em>Z</em> isomerization of the Si=C^CAAC double bond in SiBr(Eind)(CAAC) (<strong>2-Eind</strong>) and an irreversible (<em>Z</em>,<em>Z</em>)<em>→</em>(<em>E</em>,<em>E</em>) isomerization of the two Si=C^CAAC bonds in disilabutadiene Si₂(Mes)₂(CAAC)₂ (<strong>9-Mes</strong>), as well as a reversible <em>E→</em><em>Z</em> isomerization of the Si=C^CAAC bond in the silenyl-germylene (Ar^Mes)Ge–Si(C₂TMS)(CAAC) (<strong>12</strong>) (Ar^Mes = C₆H₃-2,6-Mes₂).