Tris(imidazolin-2-ylidene-1-yl)borate Complexes of the Heavier Alkaline Earths: Synthesis and Structural Studies

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• 作者：Merle Arrowsmith ; Alex Heath ; Michael S. Hill ; Peter B. Hitchcock ; Gabriele Kociok-Khn
• 刊名：Organometallics
• 出版年：2009
• 出版时间：August 10, 2009
• 年：2009
• 卷：28
• 期：15
• 页码：4550-4559
• 全文大小：1010K
• 年卷期：v.28,no.15(August 10, 2009)
• ISSN：1520-6041

文摘

Heteroleptic tris(imidazolin-2-ylidene-1-yl)borate complexes of the heavier alkaline earth elements calcium, strontium, and barium have been synthesized by deprotonation of boronium salt ligand precursors with [KN(SiMe₃)₂] in the presence of CaI₂, SrI₂, or BaI₂. Complex formation invariably involved partial B−N bond cleavage of the ligand precursors, leading to the formation of the silylamide complexes [{HB(Im^tBu)₃}M{N(SiMe₃)₂}(N-Im^tBu)_n] (M = Ca, n = 0; Sr, n = 1; Ba, n = 1.5). All three silylamide complexes are stable toward Schlenk-type ligand redistribution in solution and show catalytic activity in the intramolecular hydroamination of aminoalkenes. In the case of M = Ca attempts to synthesize heteroleptic halide-containing species led to a 1:9 mixture of monomeric [{HB(Im^tBu)₃}CaI(THF)] and [{HB(Im^tBu)₃}CaI(N-Im^tBu)] following deprotonation of the boronium salt ligand precursor with [KN(SiMe₃)₂] in the presence of CaI₂ or to dimeric [{HB(Im^tBu)₃}CaBr]₂ when using [Ca{N(SiMe₃)₂}(THF)₂] as both base and calcium source. However, similar reactions with M = Sr resulted in the formation of only homoleptic [{HB(Im^tBu)₃}₂Sr] probably due to the larger ionic radius of the strontium center combined with a less sterically demanding halide co-ligand. X-ray diffraction analyses of all compounds demonstrated in each case that the monoanionic borate ligand coordinates to the alkaline earth metal center in a C₃-symmetric facial κ³-binding mode via the three N-heterocyclic carbene (NHC) σ-donors.