Synthesis, Characterization, and Solution Lability of N-Heterocyclic Carbene Adducts of the Heavier Group 2 Bis(trimethylsilyl)amides
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- 作者：Anthony G. M. Barrett ; Mark R. Crimmin ; Michael S. Hill ; Gabriele Kociok-Kö ; hn ; Dugald J. MacDougall ; Mary F. Mahon ; Panayiotis A. Procopiou
- 刊名：Organometallics
- 出版年：2008
- 出版时间：August 11, 2008
- 年：2008
- 卷：27
- 期：15
- 页码：3939-3946
- 全文大小：249K
- 年卷期：v.27,no.15(August 11, 2008)
- ISSN：1520-6041
文摘
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. 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 Ph3PO > NHC ∼ THF > PPh3. In the case of triphenylphosphine oxide the structure of the reaction product, [(Ph3PO)2Ca{N(SiMe3)2}2], was confirmed by independent synthesis from addition of Ph3PO to [Ca{N(SiMe3)2}2].