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 ß-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.

Diverse reactivity of a magnesium silanide toward ketones.

The reactivity of a molecular magnesium silanide toward ketones displays a significant variability of outcome, resulting in adduct formation, deprotonation, dearomatisation or deoxygenation, that is dependent on the structure and electronic character of the carbonyl-containing reagent.

Isocyanate deoxygenation by a molecular magnesium silanide.

The stoichiometric reactivity of a ß-diketiminato (BDI) magnesium silanide towards a variety of organic isocyanates has been assessed. While the primary outcome of reactions of t-BuNCO, DippNCO and CyNCO was the production of ß-diketiminato magnesium siloxide adducts of the isonitrile resulting from isocyanate deoxygenation, analogous treatment with i-PrNCO led to multiple products, four of which have been positively identified. Although the specificity of this latter reaction was hampered by competitive isocycanate addition at the γ-methine carbon of the BDI supporting ligand, the identification of [{i-PrNCO}CH{(Me)CNDipp}Mg{Me2PhSi}C(O)Ni-Pr]6 provided corroborative evidence for the likely generation of sila-amidate intermediates in all four reactions under study. The formation of [{Me2PhSi}C(O)NR]- anions as the most likely initial species formed en route to isonitrile and siloxide formation was, therefore, validated by a computational density functional theory (DFT) study.