Two Carbenes versus One in Magnesium Chemistry: Synthesis of Terminal Dihalide, Dialkyl, and Grignard Reagents

Substantial progress
has been made in the coordination chemistry
of main-group elements with neutral donor ligands, largely ushered
in by the development of stable N-heterocyclic carbenes (NHCs). There
is growing interest in the synthesis of well-defined coordination
compounds containing s-block metals; however, examples of molecular
compounds containing “normal” NHCs bound to magnesium
remain relatively understudied. We report that NHCs react with magnesium
halides, MgX2 (X = Cl, Br, I), to afford (IPr)­MgCl2(THF) (1), [(IPr)­MgCl2]2 (2), (sIPr)2MgCl2 (3), (sIPr)2MgBr2 (4), and (sIPr)2MgI2 (5), where IPr is 1,3-bis­(2,6-di­iso­propyl­phenyl)­imidazole-2-ylidine
and sIPr is 1,3-di­isopropyl-4,5-di­methyl­imidizol-2-ylidine.
Using the IPr ligand, weak NHC coordination and dynamic interaction
with THF are observed in solution. In contrast, the coordination of
two sIPr ligands results in higher purity, enhanced stability, and
no observation of THF coordination. Dual carbene complexation with
commercially available MgnBu2 produced
terminal dialkyl (sIPr)2Mg­(nBu)2 (6). The reaction of methylmagnesium bromide with 2
equiv of sIPr afforded the first structurally characterized example
of a terminal Grignard reagent which is stabilized by NHCs, (sIPr)2Mg­(Me)Br (7). Notably, compounds 3–7 represent the first members of a new class
of compounds where two untethered NHCs are bound to a single Mg center.
These experiments suggest that two less sterically demanding NHCs
can have superior stabilizing properties compared to one bulky NHC.
The structural identity of each compound was confirmed using single-crystal
X-ray diffraction studies, and the bonding in these complexes was
investigated by density functional theory.

稳定氮杂环卡宾（N-heterocyclic carbenes, NHCs）的发展极大推动了主族元素与中性给体配体配位化学领域的长足进步。当前学界对合成含s区金属的结构明确配位化合物的关注度与日俱增，但针对结合了常规NHCs的镁基分子化合物的研究仍相对匮乏。本研究发现，NHCs可与卤化镁MgX₂（X=Cl、Br、I）发生反应，合成得到(IPr)MgCl₂(THF)（化合物1）、[(IPr)MgCl₂]₂（化合物2）、(sIPr)₂MgCl₂（化合物3）、(sIPr)₂MgBr₂（化合物4）以及(sIPr)₂MgI₂（化合物5）；其中IPr为1,3-双(2,6-二异丙基苯基)咪唑-2-卡宾，sIPr为1,3-二异丙基-4,5-二甲基咪唑-2-卡宾。当使用IPr作为配体时，溶液中可观测到NHCs的弱配位作用以及与四氢呋喃（THF）的动态相互作用。与之相反，采用两个sIPr配体进行配位时，产物纯度更高、稳定性更强，且未观测到THF配位现象。通过与市售二正丁基镁进行双卡宾配位反应，可得到末端二烷基配合物(sIPr)₂Mg(nBu)₂（化合物6）。溴化甲基镁与2当量的sIPr发生反应，得到了首例由NHCs稳定的末端格氏试剂的结构表征产物(sIPr)₂Mg(Me)Br（化合物7）。值得注意的是，化合物3至7代表了一类全新化合物的首个成员：两个未桥连的NHC配体与单个镁中心配位的配合物。本实验结果表明，相较于单个大位阻NHC，两个空间位阻更小的NHCs具备更优异的稳定性能。所有化合物的结构均通过单晶X射线衍射分析得以确证，配合物的成键性质则通过密度泛函理论（DFT）进行了研究。