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

A series of N-heterocyclic carbene (NHQ adducts of the heavier group 2 bis(trii-nethylsilyl)amides, of the general formulas [(L')MjN(SiMe3)2j21, RL 2)CajN(SiMe3)2)21, and [(L1)CajN(SiMe3)2)C1l (L' = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene and L 2 = 1,3-bis(2,6-diisopropylphenyl)imidazol-2ylidene, 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 (7-donation of the lone pair to the clectrophilic metal center. Although 'H, C, 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 catalytic a] I y-rel ev ant conditions. Furthermore, a series of reactions of [L'Ca[N(SiMe3)2)21 with Lewis bases suggest that the strength of the interaction between the metal and neutral ligand decreases across the series Ph3P=O > NHC - THF > PPh3. In the case of triphenylphosphine oxide the structure of the reaction product, [(Ph3P=0)2CajN(SiMe3)2121, was confirmed by independent synthesis from addition of Ph3P=O to [CafN(SiMe3)2)21,