Uzelac Marina, Hernán-Gómez Alberto, Armstrong David R, Kennedy Alan R, Hevia Eva
WestCHEM , Department of Pure and Applied Chemistry , University of Strathclyde , Glasgow , G1 1XL , UK . Email:
Chem Sci. 2015 Oct 1;6(10):5719-5728. doi: 10.1039/c5sc02086g. Epub 2015 Jul 3.
Advancing the rational design of main-group N-heterocyclic carbene complexes, this study reports the synthesis, X-ray crystallographic and NMR spectroscopic characterisation of a novel series of Ga complexes containing neutral or anionic NHC ligands using the unsaturated carbene IPr (IPr = 1,3-bis-(2,6-di-isopropylphenyl)imidazol-2-ylidene). Starting from normal adduct GaR·IPr () (R = CHSiMe), the addition of polar LiR led to the formation of NHC-stabilised gallate species IPr·LiGaR (), resulting from co-complexation of the single-metal species. Contrastingly, reversing the order of addition of these organometallic reagents, by treating unsaturated free IPr, first with LiR followed by GaR, furnished novel heteroleptic gallate (THF)Li[:C{[N(2,6-iPrCH)]CHCGa(CHSiMe)}] (), which contains an anionic NHC ligand acting as an unsymmetrical bridge between the two metals, coordinating through its abnormal C4 position to Ga and through its normal C2 position to Li. Electrophilic interception studies of using methyl triflate (MeOTf), methanol and imidazolium salt (IMes·HCl) led to the isolation and structural elucidation of the two novel neutral abnormal NHC (aNHC) complexes [CHC{[N(2,6-iPrCH)]CHCGa(CHSiMe)}] () and aIPr·GaR () (aIPr = HC{[N(2,6-iPrCH)]CHC}). These studies disclose the preference of the anionic IPr ligand present in to react with electrophiles its C2 position, leaving its Ga-C4 bond intact. Abnormal complex can also be accessed by a thermally induced rearrangement of its normal isomer . Combining NMR spectroscopic and kinetic studies with DFT calculations, new light has been shed on this intriguing transformation, which suggests that it occurs a dissociative mechanism, highlighting the importance of the donor ability of the solvent used in these thermal isomerizations as well as the steric bulk of the substituents on the NHC and the Ga reagent. These findings intimate that relief of the steric hindrance around Ga by forming an abnormal complex is a key driving force behind these rearrangements.
为推动主族氮杂环卡宾配合物的合理设计，本研究报告了一系列新型含中性或阴离子型NHC配体的镓配合物的合成、X射线晶体学和核磁共振光谱表征，这些配合物使用不饱和卡宾IPr（IPr = 1,3 - 双 -（2,6 - 二异丙基苯基）咪唑 - 2 - 亚基）。从正常加合物GaR·IPr（）（R = CHSiMe）出发，加入极性LiR导致形成NHC稳定的镓酸盐物种IPr·LiGaR（），这是由单金属物种的共配位作用产生的。相反，颠倒这些有机金属试剂的添加顺序，即先将不饱和游离IPr用LiR处理，然后再用GaR处理，得到了新型杂配镓酸盐（THF）Li[:C{[N(2,6 - iPrCH)]CHCGa(CHSiMe)}]（），它含有一个阴离子型NHC配体，作为两种金属之间的不对称桥，通过其异常的C4位置与Ga配位，通过其正常的C2位置与Li配位。使用三氟甲磺酸甲酯（MeOTf）、甲醇和咪唑盐（IMes·HCl）对进行亲电截获研究，导致分离并阐明了两种新型中性异常NHC（aNHC）配合物[CHC{[N(2,6 - iPrCH)]CHCGa(CHSiMe)}]（）和aIPr·GaR（）（aIPr = HC{[N(2,6 - iPrCH)]CHC}）的结构。这些研究揭示了中存在的阴离子型IPr配体倾向于在其C2位置与亲电试剂反应，而其Ga - C4键保持完整。异常配合物也可以通过其正常异构体的热诱导重排得到。将核磁共振光谱和动力学研究与密度泛函理论计算相结合，为这一有趣的转变提供了新的线索，这表明它通过解离机制发生，突出了这些热异构化中所用溶剂的给体能力以及NHC和镓试剂上取代基的空间位阻的重要性。这些发现表明，通过形成异常配合物来缓解镓周围的空间位阻是这些重排背后的关键驱动力。
