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)Si/Pr-3](2) (1) precipitates as colorless cuboids (C24H60MgN4Si2, a = 2269.6(2), b = 1029.58(5), c = 1593.2(l) pm, beta = 120.826(8)degrees, monoclinic, C2/c, Z = 4). The amide nitrogen atoms are coordinated planarily with strongly widened Mg-N-Si bond angles of 139.2(1)degrees. The metalation of triisopropylsilylamine with dimethylmagnesium in THF yields quantitatively heteroleptic [(thf)MeMg-N(H)SiJPr(3)](2) (2) which crystallizes as colorless needles (C28H66Mg2N2O2Si2, a = 1982.4(2), b = 2034.1 (1), c = 907.22(6) pm, beta = 95,021(9), monoclinic, P2(1)/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).