Ligand Exchange Reaction Between an Alkylzinc Primary Amide and a Dialkylmagnesium Compound:  Crystal Structures of [(thf)MeMg-μ-N(H)SiiPr₃]₂ and (tmeda)Mg[N(H)SiiPr₃]₂

The reaction of alkylzinc triisopropylsilylamide with dialkylmagnesium leads to a ligand exchange. Besides the starting materials, heteroleptic alkylmagnesium triisopropylsilylamide and homoleptic magnesium bis(triisopropylsilylamide) are detected by NMR spectroscopy. After the addition of 1,2-bis(dimethylamino)ethane (TMEDA) to the reaction mixture, (tmeda)Mg[N(H)SiiPr₃]₂ (1) precipitates as colorless cuboids (C₂₄H₆₀MgN₄Si₂, a = 2269.6(2), b = 1029.58(5), c = 1593.2(1) pm, β = 120.826(8)°, monoclinic, C2/c, Z = 4). The amide nitrogen atoms are coordinated planarily with strongly widened Mg−N−Si bond angles of 139.2(1)°. The metalation of triisopropylsilylamine with dimethylmagnesium in THF yields quantitatively heteroleptic [(thf)MeMg−N(H)SiiPr₃]₂ (2) which crystallizes as colorless needles (C₂₈H₆₆Mg₂N₂O₂Si₂, a = 1982.4(2), b = 2034.1(1), c = 907.22(6) pm, β = 95.021(9), monoclinic, P2₁/n, Z = 4). Because of the bridging position of the triisopropylsilylamide anion, the tetracoordinate nitrogen atoms show rather long Mg−N bond lengths of 210.7 pm (average value).