Carbene Rearrangements in Three-Coordinate N-Heterocyclic Carbene Complexes of Cobalt(II) Bis(trimethylsilyl)amide
Carbene Rearrangements in Three-Coordinate N-Heterocyclic Carbene Complexes of Cobalt(II) Bis(trimethylsilyl)amide
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DOI：
10.1021/ic501677k
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发表时间：
2014-10-06
影响因子：
4.6
通讯作者：
Layfield, Richard A.
中科院分区：
文献类型：
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作者：
Day, Benjamin M.;Pal, Kuntal;Layfield, Richard A.
钴 (II) N-杂环卡宾 (NHC) 配合物 [(NHC)-Co{N(SiMe3)(2)}(2)] 的合成和分子结构，其中 NHC = 1,3-双(二异丙基苯基)-亚咪唑基 (IPr) (6)、1,3-双(异三叉基)亚咪唑基 (IMes) (7)，和报道了 1,3-双(叔丁基)咪唑-2-亚基 ((IBu)-Bu-t) (8)。配合物 6-8 是三配位钴 NHC 配合物的罕见例子。 6和7中钴(II)中心的配位环境内的空间拥挤导致了目前已知的最长的Co-C(NHC)距离。研究6-8的热稳定性，我们发现6中的空间拥挤使得将配合物在甲苯中加热回流会促使IPr配体从正常的C2键合模式重排到相应的“异常”或介离子键合模式。重排导致形成 [(aIPr)Co{N(SiMe3)(2)}(2)] (9)，这是异常 NHC 配体的第一个钴配合物。 9中的Co C键比6中的类似键短0.06埃，这表明虽然重排是由于IPr配体的空间需求而发生的，但Co C键也存在热力学驱动力。与 6 的情况相反，复合物 7 在正常到异常重排方面是稳定的。在甲苯中回流配合物 8 会激活叔丁基取代基，并以异丁烯形式消除，随后形成 1-叔丁基咪唑配合物 [((t)BuIm)Co{N-(SiMe3)(2)}(2)] (10)。
The synthesis and molecular structures of the cobalt (II) N-heterocyclic carbene (NHC) complexes [(NHC)-Co{N(SiMe3)(2)}(2)], where NHC = 1,3-bis(diisopropylphenyl)-imidazolylidene (IPr) (6), 1,3-bis(mesityl)imidazolylidene (IMes) (7), and 1,3-bis(tert-butyl)imidazol-2-ylidene ((IBu)-Bu-t) (8), are reported. Complexes 6-8 are rare examples of three-coordinate cobalt NHC complexes. The steric congestion within the coordination environments of the cobalt(II) centers in 6 and 7 results in the longest Co-C(NHC) distances currently known. Investigating the thermal stability of 6-8, we have found that the steric congestion in 6 is such that heating the complex to reflux in toluene prompts a rearrangement from the normal, C2-bonding mode of the IPr ligand to the corresponding "abnormal" or mesoionic bonding mode. The rearrangement results in formation of [(aIPr)Co{N(SiMe3)(2)}(2)] (9), which is the first cobalt complex of an abnormal NHC ligand. The Co C bond in 9 is 0.06 angstrom shorter than the analogous bond in 6, suggesting that, although the rearrangement occurs due to the spatial demands of the IPr ligand, there is also a thermodynamic driving force in terms of the Co C bond. In contrast to the case for 6, complex 7 is stable with respect to the normal-to-abnormal rearrangement. Refluxing complex 8 in toluene results in activation of a tert-butyl substituent, which is eliminated as isobutene, followed by formation of the 1-tert-butylimidazole complex [((t)BuIm)Co{N-(SiMe3)(2)}(2)] (10).