Nature of the Arsonium-Ylide Ph3As=CH2and a Uranium(IV) Arsonium-Carbene Complex

Treatment of [Ph3EMe][I] with [Na{N(SiMe3)(2)}] affords the ylides [Ph3E=CH2] (E=As,1As; P,1P). For1Asthis overcomes prior difficulties in the synthesis of this classical arsonium-ylide that have historically impeded its wider study. The structure of1Ashas now been determined, 45 years after it was first convincingly isolated, and compared to1P, confirming the long-proposed hypothesis of increasing pyramidalisation of the ylide-carbon, highlighting the increasing dominance of E+-C(-)dipolar resonance form (sp(3)-C) over the E=C ene pi-bonded form (sp(2)-C), as group 15 is descended. The uranium(IV)-cyclometallate complex [U{N(CH2CH2NSiPr3i)(2)((CH2CH2SiPr2CH)-C-i(Me)CH2)}] reacts with1Asand1Pby alpha-proton abstraction to give [U(Tren(TIPS))(CHEPh3)] (Tren(TIPS)=N(CH2CH2NSiPr3i)(3); E=As,2As; P,2P), where2Asis an unprecedented structurally characterised arsonium-carbene complex. The short U-C distances and obtuse U-C-E angles suggest significant U=C double bond character. A shorter U-C distance is found for2Asthan2P, consistent with increased uranium- and reduced pnictonium-stabilisation of the carbene as group 15 is descended, which is supported by quantum chemical calculations.