Authentic SnB-Double Bonds in Polar Stannaborene Derivatives

We report on the synthesis of an authentic SnB-moiety realized in a stannaborenyl anion and stannaborenium cation. Starting with an oxidative addition of boron tribromide to a stannylene-phosphine Lewis pair [o-C6H4(Ar*BrSn-BBr2–PPh2)] (2a) [Ar* = C6H3(2,6-Trip)2, Trip = 2,4,6-C6H2 iPr3] was synthesized. Reduction of 2a with magnesium yields the Grignard-type stannaborene [o-C6H4(Ar*SnB­{PPh2}­MgBr­{thf})]2 (3)2 featuring a SnB double bond and a B–Mg interaction. Following an alternative protocol, hydride substitution at 2a yields the tinhydride [o-C6H4(Ar*HSn-BBr2–PPh2)] (4a). HBr elimination of 4a in reaction with MeNHC (MeNHC = 1,3,4,5-tetramethylimidazol-2-ylidene) gives the carbene and phosphine stabilized stannyl-borylene [o-C6H4(Ar*BrSn-B­{PPh2}­{MeNHC})] (5) after simultaneous bromide transfer from boron to tin. In reaction of 5 with Li­[Al­(OC­{CF3}3)4] or Na­[BArF 4] in a mixture of o-DFB/benzene a stannaborene [o-C6H4(Ar*SnB­{PPh2}­{MeNHC})]+ [6] stabilized by the respective weakly coordinating anion was isolated (ArF = C6H3-3,5-(CF3)2, o-DFB = o-difluorobenzene). The phosphine and NHC-supported stannaborenium cation 6 adds ammonia at room temperature under splitting of a N–H bond and formation of Sn-NH2 and B–H bonds to give [o-C6H4(Ar*­{H2N}­Sn-BH­{PPh2}­{MeNHC})]+ (7).