Planar hexasilylbenzene dianion with thermally accessible triplet state
Planar hexasilylbenzene dianion with thermally accessible triplet state
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DOI：
10.1021/ja9739648
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发表时间：
1998-02-18
影响因子：
15
通讯作者：
Sakurai, H
中科院分区：
文献类型：
--
作者：
Ebata, K;Setaka, W;Sakurai, H
由于具有D3 h或更高对称性的苯衍生物具有2重简并最高成键和最低反键π轨道能级，因此相应的二价阴离子和二价阳离子可能是三重态。虽然到目前为止已经知道了许多具有基态或热可及三重态的苯衍生物的二价阴离子和二价阳离子1，但它们中的大多数具有更扩展的π系统，如苯并三苯、苯并菲和1，3，5-三苯基苯。最近对六（三甲基硅基）苯并[1，2]噻吩的实验研究以及对苯并[1，3]噻吩的理论研究表明，它们是具有非平面环体系的单重态，即具有双桥锂阳离子的船形。作为一种不受任何额外π系统稳定的三重态苯二价阴离子的方法，我们研究了六硅基苯，1，3，4，6，7，9-六硅杂-1，1，3，3，4，4，6，6，7，7，9，9-十二甲基三氢茚（1），4的锂金属还原，该六硅基苯被设计为抗平面环几何形状变形的全硅基苯。实际上，相应的苯二锂配合物在固态下几乎是平面的，并显示出热可及的三重态ESR信号。当在室温下将1与过量的锂金属在干燥的无氧四氢呋喃中混合时，在几分钟内获得12-的深棕色溶液（当量1）。真空除去溶剂后，分离出12-[Li+（thf）n] 2（2a），为无定形黑色固体。苯二价阴离子盐2a对空气和水分非常敏感。在奎宁环（C7 H13 N，2摩尔）作为锂阳离子的配体的存在下进行类似的还原，然后从甲苯中重结晶，得到12-[Li+（C7 H13 N）] 2（2b）的单晶。一个锂原子位于苯环上方，另一个位于苯环下方;每个锂原子与一个奎宁环配位。锂原子与苯环平面之间的距离为1.667（Li 1）和1.717 <$（Li 2），并且苯碳与锂原子之间的所有距离近似相等（2.196-2.322 <$，av 2.245 <$）。正如预期的那样，2b的苯环大部分是平面的，与六（三甲基硅基）苯二锂相反; 2 2b的所有四个连接的芳族碳的扭转角仅为0.96-3.78（平均值为2.22）。苯环的所有内角几乎都是120（118.7-121.6，总共720.0）。然而，2b的苯环从D 6 h对称性显著扭曲为醌式D2 h结构; C1-C2（1.470 Ω）、C2-C3（1.511 Ω）、C4-C5（1.494 Ω）和C5-C6（1.486 Ω）的键长大于C1-C6（1.448 Ω）和C3-C4（1.443 Ω）的键长。Si原子的位置从苯环平面上下交替地略微偏离，Si-Car-Car-Si的扭转角为2.0-12.9（av 6.5）。C1-C6-Si 2-C7-Si 1的五元环是近似平坦的，而其他的是褶皱的。2b的苯平面在晶体中沿轴沿着规则地堆叠。晶体中两个苯环中心之间的最短距离为10.7 π，表明在固态下没有显著的分子间自旋-自旋相互作用。
It is well recognized that, since benzene derivatives with D3h or higher symmetry have 2-fold degenerate highest bonding and lowest antibonding π-orbital levels, the corresponding dianions and dications are potentially triplet states. Although a number of dianions and dications of benzene derivatives with ground or thermally accessible triplet states have been known so far, 1 most of them have more extended π systems such as coronene, triphenylene, and 1, 3, 5-triphenylbenzene. A recent experimental study of hexakis (trimethylsilyl) benzene dilithium2 as well as a theoretical study of benzene dilithium3 has revealed that they are singlet states with a nonplanar ring system, ie, a boat form with doubly bridged lithium cations. As an approach to a triplet benzene dianion without stabilization by any extra π systems, we have investigated lithium metal reduction of a hexasilylbenzene, 1, 3, 4, 6, 7, 9-hexasila-1, 1, 3, 3, 4, 4, 6, 6, 7, 7, 9, 9-dodecamethyltrindane (1), 4 which is designed as a persilylbenzene that resists deformation from planar ring geometry. Actually, the corresponding benzene dilithium complex is almost planar in the solid state and shows thermally accessible triplet ESR signals. When 1 was mixed with excess lithium metal in dry oxygenfree tetrahydrofuran at room temperature, a dark-brown solution of 12-was obtained within a few minutes (eq 1). After the solvent was removed in vacuo, 12-[Li+(thf) n] 2 (2a) was isolated as amorphous black solid. Benzene dianion salt 2a was extremely air-and moisture-sensitive. A similar reduction in the presence of quinuclidine (C7H13N, 2 mol) as a ligand for lithium cation followed by recrystallization from toluene gave single crystals of 12-[Li+(C7H13N)] 2 (2b).The molecular structure of 2b determined by X-ray crystallography5 is shown in Figure 1. One lithium atom is located above the benzene ring and the other below; each lithium atom is coordinated by one quinuclidine. The distances between a lithium atom and the benzene ring plane are 1.667 (Li1) and 1.717 Å (Li2), and all distances between benzene carbons and lithium atoms are approximately equal (2.196-2.322 Å, av 2.245 Å). As expected, the benzene ring of 2b is mostly planar in contrast to that of hexakis (trimethylsilyl) benzene dilithium; 2 the torsion angles of all sets of four connected aromatic carbons of 2b are only 0.96-3.78 (the average value, 2.22). All interior angles of the benzene ring are almost 120 (118.7-121.6, total 720.0). However, the benzene ring of 2b is significantly distorted from D6h symmetry to quinoid D2h structure; the bond lengths of C1-C2 (1.470 Å), C2-C3 (1.511 Å), C4-C5 (1.494 Å), and C5-C6 (1.486 Å) are longer than those of C1-C6 (1.448 Å) and C3-C4 (1.443 Å). Positions of the Si atoms slightly deviate up and down alternately from the benzene ring plane; torsion angles of Si-Car-Car-Si are 2.0-12.9 (av 6.5). A five-membered ring of C1-C6-Si2-C7-Si1 is approximately flat, while the others are puckered. The benzene planes of 2b are regularly stacking along the axis in the crystal. The shortest distance between centers of the two benzene rings in the crystal is 10.7 Å, being indicative of no significant intermolecular spin-spin interaction in the solid state.