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.