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.