Nucleophilic Magnesium Silanide and Silaamidinate Derivatives

Density functional theory (DFT) calculations demonstrate that the previously reported reaction of [(BDI)Mg-n-Bu] (BDI = HC{(Me)CN-Dipp}₂; Dipp = 2,6-diisopropylphenyl) with the silaborane Me₂PhSi-Bpin provides the magnesium silanide derivative [(BDI)MgSiMe₂Ph], through the intermediacy of a short-lived silyl-pinacolato-organoborate species. The nucleophilic character of the resultant silanide anion is assayed through a series of reactions with RN═C═NR (R = i-Pr, Cy, t-Bu) and p-tolN═C═N-p-tol. When they are performed in a strict 1:1 stoichiometry, all four reactions result in silyl addition to the carbodiimide carbon center and formation of the corresponding β-diketiminato magnesium silaamidinate complexes. Although the performance of the reaction of [(BDI)MgSiMe₂Ph] with 2 equiv of p-tolylcarbodiimide also results in the formation of a silaamidinate anion, the second equivalent is observed to engage with the nucleophilic γ-methine carbon of the BDI ligand to provide a tripodal diimino-iminoamidate ligand. This behavior is judged to be a consequence of the enhanced electrophilicity of the N-aryl-substituted carbodiimide reagent, a viewpoint supported by a further reaction with the N-isopropyl silaamidinate complex [(BDI)Mg(i-PrN)₂CSiMe₂Ph]. This latter reaction not only provides an identical diimino-iminoamidate ligand but also results in 2-fold insertion of p-tolN═C═N-p-tol into a Mg-N bond between the magnesium center and the silaamidinate anion.