N-Heterocyclic Carbene-Mediated Ring Opening of Reduced Diazamagnesacycles,Organometallics

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N-Heterocyclic Carbene-Mediated Ring Opening of Reduced Diazamagnesacycles
Organometallics ( IF 2.5 ) Pub Date : 2020-11-20 , DOI: 10.1021/acs.organomet.0c00620
Akachukwu D. Obi ₁ , Lucas A. Freeman ₁ , Diane A. Dickie ₁ , Robert J. Gilliard ₁

Affiliation

Using a bis(N-heterocyclic carbene) ligand system, we have synthesized magnesium complexes bearing redox-active α-diimines and observed structural rearrangements promoted by dynamic N-heterocyclic carbene (NHC) dissociation. The reduction of a bis(NHC)-stabilized magnesium dihalide (^iPrNHC)₂MgBr₂ (1; ^iPrNHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) with KC₈ in the presence of the respective diimine, affords the doubly reduced compounds (^iPrNHC)₂Mg(^MesDAB^Me) (2), (^iPrNHC)₂Mg(^MesDAB^H) (3), and (^iPrNHC)₂Mg(^DippDAB^Me) (4) (^MesDAB^Me = N,N′-bis(2,4,6-trimethylphenyl)-2,3-dimethyl-1,4-diaza-1,3-diene, ^MesDAB^H = N,N′-bis(2,4,6-trimethylphenyl)-1,4-diazabutadiene, ^DippDAB^Me = N,N′-bis(2,6-diisopropylphenyl)-2,3-dimethyl-1,4-diaza-1,3-diene) as mononuclear five-membered magnesacycles. In contrast to the κ²-diamide coordination in 2–4, (^iPrNHC)₂Mg(^DippDAB^H) (5; ^DippDAB^H = N,N′-bis(2,6-diisopropylphenyl)-1,4-diazabutadiene), prepared under similar conditions, crystallizes as the dinuclear 10-membered magnesacycle [(^iPrNHC)Mg(μ-^DippDAB^H)]₂ (6), where the bridging η¹:η¹-enediamide ligands are involved in cooperative bonding interactions with the NHC ligands. The diradical complex Mg(^DippDAB^H)₂ (7) was also obtained from a solution of 5, which supports an equilibrium between 5 and 6. The rearrangement of 6 to 5 results in an Mg(DAB)^2– species that is not stabilized by a Lewis base, which can undergo a disproportionation reaction to form the stable Mg(DAB^•–)₂ diradical (7). The mechanism for the formation of 6 was evaluated, and a comparative mono(NHC) stabilization of the methylated DAB analogue Mg(^DippDAB^Me) afforded the solid-state coordination polymer [(^iPrNHC)Mg(^DippDAB^Me)·KBr]_n (8). The observation of a KBr interaction with the magnesacycle highlights the accessibility to a more Lewis acidic magnesium center upon carbene dissociation from bis(NHC)-stabilized species.

中文翻译：

N-杂环卡宾介导的还原二氮杂镁环的开环

使用双（N-杂环卡宾）配体系统，我们合成了带有氧化还原活性α-二亚胺的镁配合物，并观察到了动态N-杂环卡宾（NHC）分解促进的结构重排。在各自存在下用KC ₈还原双（NHC）稳定的二卤化镁（^iPr NHC）₂ MgBr ₂（1 ; ^iPr NHC = 1,3-二异丙基-4,5-二甲基咪唑-2-亚丙基）二亚胺，提供双还原的化合物（^iPr NHC）₂ Mg（^Mes DAB ^Me）（2），（^iPr NHC）₂ Mg（^MesDAB ^H）（3）和（^iPr NHC）₂ Mg（^Dipp DAB ^Me）（4）（^Mes DAB ^Me = N，N'-双（2,4,6-三甲基苯基）-2,3-二甲基-1 ，4-diaza-1,3-diene，^Mes DAB ^H = N，N'-双（2,4,6-三甲基苯基）-1,4-diazabutadiene，^Dipp DAB ^Me = N，^N' -bis （2， 6-二异丙基苯基）-2,3-二甲基-1,4-二氮杂-1,3-二烯）为单核五元磁环。与此相反的κ ²中-diamide协调2 - 4（^iPr NHC）₂ Mg（^Dipp DAB ^H）（5 ; ^Dipp DAB ^H = N，N'-双（2,6-二异丙基苯基）-1,4-二氮杂丁二烯），在双核条件下结晶为10 -元magnesacycle [（^的iPr NHC）的Mg（μ-^卜先生DAB ^ħ）] ₂（6），其中桥连η ¹：η ^1个-enediamide配体参与与配体的NHC合作合相互作用。双^自由基复合物Mg（^Dipp DAB ^H）₂（7）也是从5的溶液中获得的，该溶液支持5和6之间的平衡。从6到5的重排会导致Mg（DAB）^2–物种无法被Lewis碱稳定，它可能会发生歧化反应以形成稳定的Mg（DAB ^•–）₂双自由基（7）。评价了形成6的机理，并比较了甲基化DAB类似物Mg（^Dipp DAB ^Me）的单（NHC）稳定化，得到了固态配位聚合物[（^iPr NHC）Mg（^DippDAB ^Me）·KBr] _n（8）。KBr与大磁环的相互作用的观察结果突出显示，从卡宾从双（NHC）稳定的物种解离后，可接近更路易斯的酸性镁中心。

更新日期：2020-12-28

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