Liu Fang-Lin, Guo Jin-Chang, Zhai Hua-Jin
Nanocluster Laboratory, Institute of Molecular Science, Shanxi University, Taiyuan 030006, China.
Phys Chem Chem Phys. 2024 Feb 14;26(7):6049-6057. doi: 10.1039/d3cp05826c.
Boron-based complex clusters are a fertile ground for the exploration of exotic chemical bonding and dynamic structural fluxionality. Here we report on the computational design of a ternary MgTaB cluster global structural searches and quantum chemical calculations. The cluster turns out to be a new member of the molecular rotor family, closely mimicking a turning clock at the subnanoscale. It is composed of a hexagonal B ring with a capping Ta atom at the top and bottom, whereas the Mg atom is linked to one Ta site as a radial Ta-Mg dimer. These components serve as the dial, axis, and hand of a nanoclock, respectively. Chemical bonding analyses reveal that the inverse sandwich TaB motif in the cluster features 6π/6σ double aromaticity, whose electron counting conforms to the (4 + 2) Hückel rule. The Ta-Mg dimer has a Lewis-type σ bond, and the Mg site has negligible bonding with B ring. The ternary cluster can be formulated as an [Mg][TaB] complex. Molecular dynamics simulations suggest that the cluster is structurally fluxional analogous to a nanoclock, even at a low temperature of 100 K. The Ta-Mg hand turns almost freely around the Ta axis and along the B dial. The tiny intramolecular rotation barrier is less than 0.3 kcal mol, being dictated by the bonding nature of double 6π/6σ aromaticity. The present system offers a new type of molecular rotor in physical chemistry.
硼基复合团簇是探索奇异化学键和动态结构流动性的肥沃土壤。在此，我们报告了一种三元MgTaB团簇的计算设计——全局结构搜索和量子化学计算。该团簇原来是分子转子家族的新成员，在亚纳米尺度上酷似一个转动的时钟。它由一个六边形B环组成，顶部和底部各有一个盖帽Ta原子，而Mg原子作为径向Ta-Mg二聚体与一个Ta位点相连。这些组分分别充当纳米钟的表盘、轴和指针。化学键分析表明，团簇中的反夹心TaB基序具有6π/6σ双芳香性，其电子计数符合(4 + 2)休克尔规则。Ta-Mg二聚体具有路易斯型σ键，Mg位点与B环的键合可忽略不计。该三元团簇可表示为[Mg][TaB]配合物。分子动力学模拟表明，即使在100 K的低温下，该团簇在结构上也类似于纳米钟那样具有流动性。Ta-Mg指针几乎可以围绕Ta轴并沿着B表盘自由转动。微小的分子内旋转势垒小于0.3 kcal mol，这由双6π/6σ芳香性的键合性质决定。本体系在物理化学中提供了一种新型的分子转子。
