Reaction of Hydrogen or Ammonia with Unsaturated Germanium or Tin Molecules under Ambient Conditions: Oxidative Addition versus Arene Elimination

摘要

The reactions of hydrogen or ammonia with germylenes and stannylenes were investigated experimentally and theoretically. Treatment of the germylene GeAr_2~# (Ar~# = C_6H_3-2,6-(C_6H_2-2,4,6-Me_3)_2) with H_2 or NH_3 afforded the tetravalent products Ar_2~#GeH_2 (1) or Ar_2~#Ge(H)NH_2 (2) in high yield. The reaction of the more crowded GeAr'_2 (Ar' = C_6H_3-2,6-(C_6H_3-2,6-'Pr_2)_2) with NH_3 also afforded a tetravalent amide Ar'_2Ge(H)NH_2 (3), whereas with H_2 the tetravalent hydride Ar'GeH_3 (4) was obtained with Ar'H elimination. In contrast, the reactions with the divalent Sn(ll) aryls did not lead to Sn(IV) products. Instead, arene eliminated Sn(ll) species were obtained. SnAr_2~# reacted with NH_3 to give the Sn(ll) amide {Ar~#Sn(μ-NH_2)}_2 (5) and Ar~#H elimination, whereas no reaction with H_2 could be observed up to 70 ℃. The more crowded SnAr'_2 reacted readily with H_2, D_2, or NH_3 to give {Ar'Sn(μ-H)}_2 (6), {Ar'Sn(μ-D)}_2 (7), or {Ar'Sn(μ-NH_2)}_2 (8) all with arene elimination. The compounds were characterized by ~1H, ~(13)C, and ~(119)Sn NMR spectroscopy and by X-ray crystallography. DFT calculations revealed that the reactions of H_2 with EAr_2 (E = Ge or Sn; Ar = Ar~# or Ar') initially proceed via interaction of the σ orbital of H_2 with the 4p(Ge) or 5p(Sn) orbital, with back-donation from the Ge or Sn lone pair to the H_2 σ~* orbital. The subsequent reaction proceeds by either an oxidative addition or a concerted pathway. The experimental and computational results showed that bond strength differences between germanium and tin, as well as greater nonbonded electron pair stabilization for tin, are more important than steric factors in determining the product obtained. In the reactions of NH_3 with EAr_2 (E = Ge or Sn; Ar = Ar~# or Ar'), the divalent ArENH_2 products were calculated to be the most stable for both Ge and Sn. However the tetravalent amido species Ar_2Ge(H)NH_2 were obtained for kinetic reasons. The reactions with NH_3 proceed by a different pathway from the hydrogenation process and involve two ammonia molecules in which the lone pair of one NH_3 becomes associated with the empty 4p(Ge) or 5p(Sn) orbital while a second NH_3 solvates the complexed NH_3 via intermolecular N-H…N interactions.