Phosphorus-Based Substituents in Two-Coordinate Silicon Chemistry: Diphosphanosilylenes and [1.1.1]Propellanes

Abstract

Low oxidation-state silicon compounds are fundamentally interesting for both academic research and industry. Phosphorus-substituted, two-coordinate congeners thereof are especially appealing, mostly for electronic reasons. However, they remain rare, with only the NHC-stabilized phosphasilenylidene (Im-Dipp₂)Si=PMes* (<strong>1</strong>) reported in 2015 (<nobr>Im-Dipp₂ =</nobr> 1,3-Dipp₂-imidazol-2-ylidene, <nobr>Dipp =</nobr> 2,6-<em>i</em>-Pr₂C₆H₃, <nobr>Mes* =</nobr> 2,4,6-<em>t</em>-Bu₃C₆H₂). As a hybrid of the seminal NHC-stabilized disilicon(0) (Im-Dipp₂)Si=Si(Im-Dipp₂) and diphosphene Mes*P=PMes*, compound <strong>1</strong> is a promising starting material in low-valent main group element chemistry. Results of prior reactivity studies on compound <strong>1</strong> include the formation of the &eta;¹ transition metal complexes [Fe(CO)₄(<strong>1</strong>)] and [CuCl(<strong>1</strong>)] as well as the NHC-stabilized 2-silaheteroallenes (Im-Dipp₂)(X)SiPMes* (<strong>1-X</strong>; X = O, NMes; <nobr>Mes =</nobr> 2,4,6-Me₃C₆H₂). These inspiring findings prompted the additional reactivity studies on <strong>1</strong> featured in this work. For instance, [Ni(CO)₃(<strong>1</strong>)] was afforded by reaction of [Ni(CO)₄] with <strong>1</strong> and served to determine its Tolman electronic parameter. In terms of NHC-stabilized 2-silaheteroallenes, chalcogenation of <strong>1</strong> with SePCy₃ and TePCy₃ (<nobr>Cy =</nobr> cyclohexyl) gave <strong>1-Se</strong> and <strong>1-Te</strong>, while <strong>1-NTripp</strong> was obtained by treatment of <strong>1</strong> with the organic azide N₃Tripp (<nobr>Tripp =</nobr> 2,4,6-<em>i</em>-Pr₃C₆H₂). Unexpectedly, reaction of <strong>1</strong> with the phosphinidene transfer reagent Mes*PPMe₃ afforded the disilatriphospha[1.1.1]propellane Si₂(PMes*)₃ (<strong>2-Si</strong>). It constitutes the lighter congener of Ge₂(PMes*)₃ (<strong>2-Ge</strong>), which has been previously isolated in our laboratory. Protonation of <strong>2-Si</strong> and <strong>2-Ge</strong> with <nobr>[H(OEt₂)₂][BR^F₄]</nobr> (<nobr>R^F =</nobr> 3,5-(CF₃)₂C₆H₃) yielded the respective phosphonium salts <nobr>[H(<strong>2-Si,Ge</strong>)][BR^F₄]</nobr> under conservation of the [1.1.1]propellane functionalities. Furthermore, reaction of <strong>1</strong> with the NHC Im-<em>i</em>-Pr₂-Me₂ (Im-<em>i</em>-Pr₂<nobr>Me₂ =</nobr> 1,3-<em>i</em>-Pr₂-4,5-Me₂-imidazol-2-ylidene) afforded Si₂(Si(Im-<em>i</em>-Pr₂-Me₂)PMes*)(PMes*)₂ (<strong>3</strong>), which may be described as an NHC-stabilized trimer of a phosphasilenylidene, an NHC-stabilized phosphasilene, and a [1.1.1]propellane. <br> Although stable diphosphanosilylenes are highly desired compounds, they remain elusive despite several literature-reported synthetic attempts. In this work, a novel synthetic approach is followed featuring a Hückel-aromatic cyclic tetrylene design, similar to Arduengo carbenes, and a key late-stage salt metathesis at the oxidation state of Si(II). Thus, the first stable P-heterocyclic silylenes (PHSis) Si(PR(CH))₂ (<nobr>R =</nobr> Mes* (<strong>4a-Si</strong>), Eind (<strong>4b-Si</strong>, <nobr>Eind =</nobr> 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl)) could be isolated. Both PHSis were afforded from the corresponding 1,4-bisphosphanides Li₂(PR(CH))₂ (R = Mes* (<strong>7a</strong>), Eind (<strong>7b</strong>)) by reaction with suitable NHC-stabilized silicon dihalogenides, respectively. The precursors <strong>7a,b</strong> were obtained from the respective 1,4-diphospha-1,3-butadienes RP(CH)₂PR (<strong>6a,b</strong>) via direct lithiation reactions. Initial reactivity studies on the PHSi <strong>4a-Si</strong> towards Im-Me₄ (Im-Me_<nobr>4 =</nobr> 1,3-<em>i</em>-Pr₂-4,5-Me₂-imidazol-2-ylidene), B(C₆F₅)₃, TePCy₃, and HCl yielded the corresponding NHC and borane adducts, a 1,3-ditelluradisiletane, and a chlorosilane. Remarkably, <strong>4a,b-Si</strong> underwent unprecedented transmetalation reactions to the respective germylenes Ge(PR(CH))₂ (<strong>4a,b-Ge</strong>) upon reaction with GeCl₂(1,4-dioxane). The PHSis <strong>4a,b-Si</strong> reacted with transition metal reagents to predominantly form complexes of the type [L_nM(<strong>4a,b-Si</strong>)] (<nobr>L_nM =</nobr> 16 valence electron transition metal fragment). Also, complex [Cp₂Ti(PMe₃)(<strong>4a-Si</strong>)] decomposed in a stepwise, partially selective manner to the s-<em>trans</em>-1,4-diphospha-1,3-butadiene complex [Cp₂Ti(<strong>6a</strong>)]. <br> This work also features the synthesis of the <em>ß</em>-phosphaenone Ter^MesPC(Me)C(Me)O (<nobr>Ter^Mes =</nobr> 2,6-Mes₂C₆H₃) starting from diacetyl using the phospha-Wittig reagent Ter^MesPPMe₃. Moreover, the 1,4-diphospha-1,3-butadiene <strong>6a</strong> was shown to rearrange to the 1,3-diphospha-1,4-pentadiene (Im-Me₄)C(H)PMes*C(H)PMes* upon reaction with the NHC Im-Me₄. Lastly, the synthesis of the 16 valence electron titanocene NHC complex [Cp₂Ti(Im-Me₄)] was revisited.