Breathing and Twisting: An Investigation of Framework Deformation and Guest Packing in Single Crystals of a Microporous Vanadium Benzenedicarboxylate

The structural details of the compounds vanadium benzenedicarboxylate VO(bdc)·Guest, where the Guests are the absorbed six-ring molecules: benzene, 1,4-cyclohexadiene, 1,3-cyclohexadiene, cyclohexene and cyclohexane, have been determined from single crystal X-ray data. All of the six-ring guest molecules show a high degree of ordering inside the channels of VO(bdc). The interactions between the guests and the host framework are dominated by van der Waals bonding. The six-ring molecules are all packed in two columns in the channels, either in herringbone or close to parallel patterns. The packing changes the space group symmetry of VO(bdc) from Pnma to the noncentrosymmetric space group P212121. The VO(bdc) framework deforms to closely adapt to the shape and thickness changes of the double columns of the guest molecules. In addition to the well studied breathing deformation, a twisting deformation mechanism that involves a cooperative rotation of the octahedral chains accompanied by bending of the bdc ligand is apparent in the detailed structural data. More quantitative information on the remarkable flexibility of the VO(bdc) framework was obtained from ab initio calculations.

本研究通过单晶X射线衍射数据（single crystal X-ray data）解析得到苯二甲酸钒（vanadium benzenedicarboxylate，VO(bdc)·Guest）的结构细节，其中客体（Guest）为吸附的六元环分子：苯、1,4-环己二烯、1,3-环己二烯、环己烯与环己烷。所有六元环客体分子在VO(bdc)的孔道内均呈现高度有序排布。客体与主体骨架间的相互作用以范德华键（van der Waals bonding）为主导。六元环客体分子在孔道内均以双列形式堆积，排布模式为人字形（herringbone）或近似平行排列。该堆积方式使VO(bdc)的空间群（space group）对称性从Pnma转变为非中心对称空间群（noncentrosymmetric space group）P2₁2₁2₁。VO(bdc)骨架发生形变，以精准适配客体分子双列结构的形状与厚度变化。除了已被广泛研究的呼吸形变（breathing deformation）外，详细的结构数据中还显现出一种扭转形变机制：该机制涉及八面体链（octahedral chains）的协同旋转，并伴随bdc配体（bdc ligand）的弯曲。通过从头算计算（ab initio calculations），我们获取了更多关于VO(bdc)骨架卓越柔性的定量信息。