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

Abstract

The reactions of hydrogen or ammonia with germylenes and stannylenes were investigated experimentally and theoretically. Treatment of the germylene GeAr(#)(2) (Ar(#) = C(6)H(3)-2,6-(C(6)H(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(6)H(3)-2,6-(C(6)H(3)-2,6-(i)Pr(2))(2)) with NH(3) also afforded a tetravalent amide Ar'(2)Ge(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(II) aryls did not lead to Sn(IV) products. Instead, arene eliminated Sn(II) species were obtained. SnAr(#)(2) reacted with NH(3) to give the Sn(II) amide {Ar(#)Sn(mu-NH(2))}(2) (5) and Ar(#)H elimination, whereas no reaction with H(2) could be observed up to 70 degrees C. The more crowded SnAr'(2) reacted readily with H(2), D(2), or NH(3) to give {Ar'Sn(mu-H)}(2) (6), {Ar'Sn(mu-D)}(2) (7), or {Ar'Sn(mu-NH(2))}(2) (8) all with arene elimination. The compounds were characterized by (1)H, (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 sigma 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) sigma* 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(2)Ge(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.