Department of Chemistry, Tongji University, Shanghai 200092, People's Republic of China.
Inorg Chem. 2012 Feb 20;51(4):2212-21. doi: 10.1021/ic2021947. Epub 2012 Feb 1.
Incorporating two quadruply bonded dimolybdenum units Mo(2)(DAniF)((3)) (ancillary ligand DAniF = N,N'-di-p-anisylformamidinate) with two hydroselenides (SeH(-)) gave rise to Mo(2)(DAniF)(3)(μ-SeH)(2) (1). With the molecular scaffold remaining unchanged, aerobic oxidation of 1, followed by autodeprotonation, generated Mo(2)(DAniF)(3)(μ-Se)(2) (2). The two complexes share a common cyclic six-membered Mo(2)/Se core, but compound 2 is distinct from 1 by having structural, electronic, and magnetic properties that correspond with aromaticity. Importantly, the aromatic behaviors for this non-carbon system are ascribable to the bonding analogy between the δ component in a Mo-Mo quadruple bond and the π component in a C-C double bond. Cyclic π delocalization via d(δ)-p(π) conjugation within the central unit, which involves six π electrons with one electron from each of the Mo(2) units and two electrons from each of the bridging atoms, has been confirmed in a previous work on the oxygen- and sulfur-bridged analogues (Fang, W.; et al. Chem.-Eur. J.2011, 17, 10288). Of the three members in this family, compound 2 exhibits an enhanced aromaticity because of the selenium bridges. The remote in-plane and out-of-plane methine (ArNCHNAr) protons resonate at chemical shifts (δ) 9.42 and 7.84 ppm, respectively. This NMR displacement, Δδ = 1.58 ppm, is larger than that for the oxygen-bridged (1.30 ppm) and sulfur-bridged (1.49 ppm) derivatives. The abnormally long-range shielding effects and the large diamagnetic anisotropy for this complex system can be rationalized by the induced ring currents circulating the Mo(2)/chalcogen core. By employment of the McConnell equation {Δσ = Δχ[(l - 3 cos 2θ)/3R(3)N]}, the magnetic anisotropy (Δχ = χ(⊥) - χ(||)) is estimated to be -414 ppm cgs, which is dramatically larger than -62.9 ppm cgs for benzene, the paradigm of aromaticity. In addition, it is found that the magnitude of Δχ is linearly related to the radius of the bridging atoms, with the selenium analogue having the largest value. This aromaticity sequence is in agreement with that for the chalcogen-containing aromatic family, e.g., furan < thiophene < selenophene.
将两个四配位二钼单元Mo(2)(DAniF)((3))（辅助配体 DAniF = N,N'-二对-茴香基甲脒）与两个氢硒化物（SeH(-)）结合，得到Mo(2)(DAniF)(3)(μ-SeH)(2)（1）。在分子支架保持不变的情况下，1 的有氧氧化，随后自动去质子化，生成Mo(2)(DAniF)(3)(μ-Se)(2)（2）。这两个配合物具有共同的循环六元 Mo(2)/Se 核，但化合物 2 与 1 不同，具有与之对应的芳香性的结构、电子和磁性性质。重要的是，对于这个非碳系统，芳香性归因于 Mo-Mo 四重键的δ分量和 C-C 双键的π分量之间的键合类比。在以前关于氧和硫桥接类似物的工作中，已经证实了通过中央单元内的 d(δ)-p(π)共轭进行的循环π离域，其中涉及来自每个 Mo(2)单元的一个电子和来自每个桥接原子的两个电子的六个π电子（Fang, W.; 等人。Chem.-Eur. J.2011, 17, 10288）。在这个家族的三个成员中，由于硒桥的存在，化合物 2 表现出增强的芳香性。处于平面内和平面外的次甲基（ArNCHNAr）质子的化学位移（δ）分别为 9.42 和 7.84 ppm。这种 NMR 位移，Δδ = 1.58 ppm，大于氧桥接（1.30 ppm）和硫桥接（1.49 ppm）衍生物。对于这个复杂的系统，可以通过围绕 Mo(2)/杂原子核心循环的感应环电流来合理化这种异常的远程屏蔽效应和大的抗磁性各向异性。通过使用 McConnell 方程{Δσ = Δχ[(l - 3 cos 2θ)/3R(3)N]}，可以估算出磁各向异性（Δχ = χ(⊥) - χ(||)）为 -414 ppm cgs，这大大大于芳香性的典范苯的 -62.9 ppm cgs。此外，还发现Δχ的大小与桥接原子的半径线性相关，硒类似物具有最大的值。这种芳香性序列与含硫族元素的芳香族家族一致，例如呋喃 < 噻吩 < 硒吩。
