Authentic Sn═B-Double Bonds in Polar Stannaborene Derivatives

Abstract

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).

Germaborene reactivity study – addition of carbon nucleophiles, cycloaddition reactions, coordination chemistry

“…In the case of the stannaborene, we recently presented a magnesium substituted [SnB–MgBr] stannaborene derivative. 20 The change of polarity of the GeB–X bond in germaborene chemistry from halide ( 1a , 1b ) or phenyl ( 1c ) to magnesium or another electropositive substituent would make a new reactivity pattern at the boron atom of the GeB unit possible. Therefore, we studied the synthesis of the homologous magnesium derivative.…”

“…The bonding situation in Me NHC-adduct 2 can be compared with the homologous Me NHC-supported stannaborenium cation, featuring a Sn B double bond. 20 The CO-cation 6 however, features a B–C π-bond, which is polarized to the boron atom. Obviously the π-accepting character of the CO-ligand dominates the delocalisation of the electron pair.…”

“…1–3 In addition to the development of a synthesis strategy, studies on the reactivity of the [BE] unit and the investigation of the electronic structures of the unsaturated molecules are rewarding challenges in molecular chemistry. Research in this area has led to the presentation of many examples of boron-element double bonds: BB, 4–8 BC, 9–11 BSi, 12–16 BGe, 17–19 BSn, 20 BN, 21–29 BP, 30–34 BAs, 33,35 BO, 36–41 BS, 41–43 BSe, 36,41,43,44 and BTe. 41,44 Our research is focused on the chemistry of heavy elements of the group 14.…”

“… 65 In analogy to the synthesis of 1, we applied an intramolecular stannylene Lewis pair to this procedure to give the first examples of stannaborenes realized in a stannaborenyl anion and a stannaborenium cation. 20 The stannaborenium cation adds ammonia at the Sn B double bond to give a B–H and Sn–NH 2 unit. 20 …”

“… 20 The stannaborenium cation adds ammonia at the Sn B double bond to give a B–H and Sn–NH 2 unit. 20 …”