Bis(imidazolin-2-ylidene-1-yl)borate Complexes of the Heavier Alkaline Earths: Synthesis and Studies of Catalytic Hydroamination
Bis(imidazolin-2-ylidene-1-yl)borate Complexes of the Heavier Alkaline Earths: Synthesis and Studies of Catalytic Hydroamination
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DOI：
10.1021/om8010933
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发表时间：
2009-03-23
期刊：
影响因子：
2.8
通讯作者：
Kociok-Koehn, Gabriele
中科院分区：
文献类型：
--
作者：
Arrowsmith, Merle;Hill, Michael S.;Kociok-Koehn, Gabriele
含有双（咪唑啉-2-亚基-1-基）硼酸盐配体的较重碱土元素钙和锶的杂配配合物可以通过在CaI 2或SrI 2存在下用[KN（SiMe 3）（2）]使硼鎓盐配体前体去质子化来合成。含有N-叔丁基咪唑啉-2-亚基片段的硅酰胺配合物[{H_2B（Im'Bu）（2）}M[N（SiMe_3）（2）}（THF）（n）]（M = Ca，n = 1; M = Sr，n = 2）对分子间溶液再分配是稳定的，尽管在M = Ca的情况下有一些溶液流动性的证据。合成异配位的含碘化物物质的尝试导致均配位化合物[{H2 B（Im（t）Bu）（2）}（2）MTHF）（n）]（M = Ca，n = 1; M = Sr，n = 2）的分离，这很可能是因为卤化物共配体的空间需求降低。尽管将“一锅法”合成方法扩展到含有N-均三甲苯取代的咪唑的前体会导致配体大量降解，但使用[Ca{N（SiMe 3）（2）}（2）（THF）（2）]作为碱和第2族元素来源提供了目标杂配位硅烷基酰胺配合物。X-射线衍射分析表明，硼酸盐配体采用面配位模式，配位是通过两个氮杂环卡宾（NHC）供体和BH 2残基上一个氢的Agostic型相互作用提供的。这种配体结合的DFT方法的分析表明，虽然从碳中心的键是类似的性质，由中性NHC配体提供的，键合是由一个静电分量，这是表现为一个协调的一个candidic BH键补充。总相互作用的离子性质足以在胺供体的存在下维持金属-配体结合，并且允许评价Ca和Sr络合物[{H2 B（Im（t）Bu）（2）}M{N（SiMe 3）（2）}（THF）（n）]作为氨基烯烃的分子内氢胺化的预催化剂。虽然钙催化剂提供了与我们先前报道的β-二酮亚胺络合物[CH{C（Me）N（2，6-（Pr 2C 6 H3）-Pr 1））Ca{N（SiMe 3）（2））（THF）]相当的活性，但锶衍生物在所有情况下都是上级的。
Heteroleptic complexes of the heavier alkaline earth elements, calcium and strontium, containing bis(imidazolin-2-ylidene-1-yl)borate ligands may be synthesized by deprotonation of boronium salt ligand precursors with [KN(SiMe3)(2)] in the presence of CaI2 or SrI2. The silylamide complexes [{H2B(Im'Bu)(2)}M[N(SiMe3)(2)}(THF)(n)] (M = Ca, n = 1; M = Sr, n = 2), containing N-tert-butyl-substituted imidazolin-2-ylidene fragments, are stable to intermolecular solution redistribution despite some evidence of solution fluxionality in the case of M = Ca. Attempts to synthesize heteroleptic iodide-containing species led to the isolation of the homoleptic compounds [{H2B(Im(t)Bu)(2)}(2)MTHF)(n)] (M = Ca, n = 1; M = Sr, n = 2) most likely because of the reduced steric demands of the halide coligand. Although extension of the "one pot" synthetic method to precursors containing N-mesityl-substituted imidazoles resulted in extensive ligand degradation, use of [Ca{N(SiMe3)(2)}(2)(THF)(2)] as both base and group 2 element source provided the target heteroleptic silylamide complex. X-ray diffraction analyses demonstrated that the borate ligands adopt a facial eta(3)-binding mode in which coordination is provided through the two N-heterocyclic carbene (NHC) donors and an additional agostic-type interacton from one hydrogen of the BH2 residue. Analysis of this ligand binding by DFT methods indicates that, although the bonds from the carbon centers are similar in character to those provided by neutral NHC ligands, the bonding is supplemented by an electrostatic component which is manifested as a coordination of a hydridic BH bond. The ionic nature of the overall interaction is sufficient to maintain the metal-ligand binding in the presence of amine donors and has allowed the evaluation of the Ca and Sr complexes [{H2B(Im(t)Bu)(2)}M{N(SiMe3)(2)}(THF)(n)] as precatalysts for the intramolecular hydroamination of aminoalkenes. While the calcium catalyst provides activities commensurate with our previously reported beta-diketiminato complex, [CH{C(Me)N(2,6-(Pr2C6H3)-Pr-i))Ca{N(SiMe3)(2))(THF)], the strontium derivative is superior in all cases.