Guest Exchange through Single Crystal–Single Crystal Transformations in a Flexible Hydrogen-Bonded Framework

A molecular framework based on guanidinium
cations and 1,2,4,5-tetra­(4-sulfonatophenyl)­benzene
(TSPB), an aromatic tetrasulfonate with nominal 2-fold and mirror
symmetry, exhibits three crystallographically unique one-dimensional
channels as a consequence of molecular symmetry and complementary
hydrogen bonding between the guanidinium (G) ions and the sulfonate
(S) groups of TSPB. Unlike previous GS frameworks, this new topology
is sufficiently flexible to permit reversible release and adsorption
of guest molecules in large single crystals through a cyclic shrinkage
and expansion of the channels with retention of single crystallinity,
as verified by single crystal X-ray diffraction. Moreover, the G4TSPB framework permits guest exchange between various guest
molecules through SCSCTs as well as exchange discrimination based
on the size and character of the three different channels. The exchange
of guest molecules during single crystal–single crystal transformations
(SCSCT), a rare occurrence for hydrogen-bonded frameworks, is rather
fast, with diffusivities of approximately 10–6 cm2 s–1. Rapid diffusion in the two channels
having cross sections sufficient to accommodate two guest molecules
can be explained by two-way or ring diffusion, most likely vacancy
assisted. Surprisingly, rapid guest exchange also is observed in a
smaller channel having a cross-section that accommodates only one
guest molecule, which can only be explained by guest-assisted single-file
unidirectional diffusion. Several single crystals of inclusion compounds
can be realized only through guest exchange in the intact framework,
suggesting an approach to the synthesis of single crystalline inclusion
compounds that otherwise cannot be attained through direct crystallization
methods.

以胍阳离子与1,2,4,5-四(4-磺酰苯基)苯（1,2,4,5-tetra(4-sulfonatophenyl)benzene, TSPB）——一种具备名义二重旋转对称性与镜面对称性的芳香族四磺酸盐——为构筑基元的分子骨架，因分子固有对称性以及胍（G）离子与TSPB的磺酰（S）基团间的互补氢键相互作用，呈现出三个晶体学上独立的一维孔道。与既往报道的胍-磺酰骨架不同，该新型拓扑结构具备足够柔性，可通过孔道的周期性收缩与扩张实现客体分子的可逆脱附与吸附，且过程中单晶完整性得以保持，该结论已通过单晶X射线衍射得到验证。此外，G4TSPB骨架可通过单晶-单晶转变（single crystal–single crystal transformations, SCSCT）实现多种客体分子间的交换，并能基于三种不同孔道的尺寸与特性对客体交换进行选择性区分。在氢键骨架体系中极为罕见的单晶-单晶转变过程中，客体分子的交换速率极快，扩散系数约为10^-6 cm²·s^-1。对于截面尺寸足以容纳两个客体分子的两条孔道，其快速扩散行为可通过双向扩散或环形扩散加以解释，其机制极有可能为空位辅助型。令人意外的是，在仅能容纳单个客体分子的较小孔道中同样观测到了快速客体交换，这一现象仅能通过客体辅助的单链单向扩散来阐释。多种仅能通过完整骨架内客体交换过程得以制备的包合物单晶，为合成直接结晶法无法获得的单晶包合物提供了可行策略。