Synthesis, Characterization, and Solution Lability of N-Heterocyclic Carbene Adducts of the Heavier Group 2 Bis(trimethylsilyl)amides

Abstract

A series of N-heterocyclic carbene (NHC) adducts of the heavier group 2 bis(trimethylsilyl)amides, of the general formulas [(L1)M{N(SiMe3)2}2], [(L2)Ca{N(SiMe3)2}2], and [(L1)Ca{N(SiMe3)2}Cl] (L1 = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene and L2 = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, M = Ca, Sr, and Ba), have been synthesized by either the addition of the group 2 amide to the appropriate imidazolium salt or direct addition of the carbene to the solvent-free, homoleptic, metal amide. X-ray diffraction studies of the reaction products revealed the formation of monomeric three-coordinate alkaline earth metal species in which the NHC binds via conventional σ-donation of the lone pair to the electrophilic metal center. Although 1H, 13C, and NOESY NMR experiments suggest this coordination is retained in solution, reactions with protic substrates such as 2-methoxyethylamine, diphenylamine, and di-p-tolylphosphine demonstrate the lability of the NHC under catalytically-relevant conditions. Fu...