Synthesis, characterization, and solution lability of N-heterocyclic carbene adducts of the heavier group 2 bis(trimethylsilyl)amides

A series of N-heterocyclic carbene (NHC) adducts of the heavier group 2 bis(trimethylsilyl)amides, of the general formulas [(L¹)M{N(SiMe ₃)₂}₂], [(L²)Ca{N(SiMe ₃)₂}₂], and [(L¹)Ca{N(SiMe ₃)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 monomelic 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. Furthermore, a series of reactions of [L¹Ca{N(SiMe₃)₂} ₂] with Lewis bases suggest that the strength of the interaction between the metal and neutral ligand decreases across the series Ph ₃P=O > NHC ∼ THF > PPh₃. In the case of triphenylphosphine oxide the structure of the reaction product, [(Ph ₃P=O)₂CaJ N(SiMe₃)₂}₂], was confirmed by independent synthesis from addition of Ph₃P=O to [Ca{N(SiMe₃)₂}₂].