Mechanism of Alkyl Migration in Diorganomagnesium 2,6-Bis(imino)pyridine Complexes: Formation of Grignard-Type Complexes with Square-Planar Mg(II) Centers

Dialkylmagnesium compounds [Mg­R2L2] (R = n-Bu, L = none or R = Bn, L = THF) react with 2,6-bis­(imino)­pyridines (BIP) to afford different types of Mg­(II) alkyl complexes, depending on the nature of R. For R = n-Bu, thermally stable products resulting from selective alkyl transfer to the pyridine nitrogen (N1) atom are obtained. However, NMR studies showed that the reaction of [Mg­(Bn)2­THF2] with iPrBIP at −65 °C leads to a thermally unstable product arising from benzyl migration to position C2 in the pyridine ring. Above +5 °C, this compound rearranges, cleanly yielding a mixture of two isomeric complexes, in which the benzyl group has migrated to positions C3 or C4 of the central ring, respectively. Similar isomeric mixtures were obtained when [Mg­(Bn)2­THF2] was reacted with iPrBIP or MesBIP at room temperature. Such mixtures are thermally stable below 80 °C, but at this temperature, the 3-benzyl isomer converts into the thermodynamically favored 4-benzyl product, albeit not quantitatively. An alternate route was devised for the selective syntheses of the latter type of compounds. The X-ray diffraction structure of one of them provided an unusual example of a square-planar alkyl­magnesium­(II) center.

二烷基镁化合物(Dialkylmagnesium compounds) [MgR₂L₂]（其中R为正丁基(n-Bu)时L无配体，R为苄基(Bn)时L为四氢呋喃(THF)）可与2,6-双(亚胺基)吡啶(BIP)反应，生成不同类型的镁(II)烷基配合物，具体产物种类取决于R基团的性质。当R为正丁基时，可得到烷基选择性转移至吡啶氮原子(N1)的热稳定产物。然而核磁共振(NMR)研究表明，[Mg(Bn)₂(THF)₂]与异丙基取代BIP(iPrBIP)在-65℃下反应时，会生成热不稳定产物，该产物由苄基迁移至吡啶环的C2位而来。当温度高于+5℃时，该化合物发生专一性重排，得到两种异构配合物的混合物，其中苄基分别迁移至中心吡啶环的C3或C4位。当[Mg(Bn)₂(THF)₂]与iPrBIP或均三甲苯基取代BIP(MesBIP)在室温下反应时，也可得到类似的异构混合物。此类混合物在80℃以下具有热稳定性，但当温度升至80℃时，3-苄基异构体可转化为热力学更稳定的4-苄基产物，不过该转化并非定量进行。研究人员设计了一条替代合成路线，以选择性制备此类4-苄基取代的配合物。其中一种产物的X射线衍射(XRD)晶体结构，为平面四配位烷基镁(II)中心提供了罕见的实例。