Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA.
Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA.
Nature. 2018 Feb 21;554(7693):505-510. doi: 10.1038/nature25765.
Mechanical stimuli can modify the energy landscape of chemical reactions and enable reaction pathways, offering a synthetic strategy that complements conventional chemistry. These mechanochemical mechanisms have been studied extensively in one-dimensional polymers under tensile stress using ring-opening and reorganization, polymer unzipping and disulfide reduction as model reactions. In these systems, the pulling force stretches chemical bonds, initiating the reaction. Additionally, it has been shown that forces orthogonal to the chemical bonds can alter the rate of bond dissociation. However, these bond activation mechanisms have not been possible under isotropic, compressive stress (that is, hydrostatic pressure). Here we show that mechanochemistry through isotropic compression is possible by molecularly engineering structures that can translate macroscopic isotropic stress into molecular-level anisotropic strain. We engineer molecules with mechanically heterogeneous components-a compressible ('soft') mechanophore and incompressible ('hard') ligands. In these 'molecular anvils', isotropic stress leads to relative motions of the rigid ligands, anisotropically deforming the compressible mechanophore and activating bonds. Conversely, rigid ligands in steric contact impede relative motion, blocking reactivity. We combine experiments and computations to demonstrate hydrostatic-pressure-driven redox reactions in metal-organic chalcogenides that incorporate molecular elements that have heterogeneous compressibility, in which bending of bond angles or shearing of adjacent chains activates the metal-chalcogen bonds, leading to the formation of the elemental metal. These results reveal an unexplored reaction mechanism and suggest possible strategies for high-specificity mechanosynthesis.
机械刺激可以改变化学反应的能量景观，并开启反应途径，提供一种与传统化学互补的合成策略。这些机械化学机制在一维聚合物中受到拉伸应力的影响，通过开环和重排、聚合物解拉链和二硫键还原等模型反应得到了广泛研究。在这些系统中，拉力拉伸化学键，引发反应。此外，已经表明垂直于化学键的力可以改变键离解的速率。然而，在各向同性的压缩应力（即静水压力）下，这些键激活机制是不可能的。在这里，我们通过分子工程结构来展示通过各向同性压缩进行机械化学的可能性，这些结构可以将宏观各向同性压力转化为分子水平的各向异性应变。我们设计了具有机械异质成分的分子——可压缩的（“软”）机械力化学基团和不可压缩的（“硬”）配体。在这些“分子铁砧”中，各向同性压力导致刚性配体的相对运动，各向异性地变形可压缩的机械力化学基团并激活键。相反，刚性配体在空间上的接触阻碍了相对运动，阻止了反应性。我们结合实验和计算，证明了在包含具有异质可压缩性的分子元素的金属有机硫属化合物中，静水压力驱动的氧化还原反应，其中键角的弯曲或相邻链的剪切激活金属-硫属键，导致元素金属的形成。这些结果揭示了一种未被探索的反应机制，并为高特异性机械合成提供了可能的策略。
