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.