Rotating Phenyl Rings as a Guest-Dependent Switch in Two-Dimensional Metal–Organic Frameworks

A semirigid bis­(1,2,4-triazole) ligand
binds in a syn conformation
between copper­(I) chains to form a series of two-dimensional metal–organic
frameworks that display a topology of fused one-dimensional metal–organic
nanotubes. These anisotropic frameworks undergo two different transformations
in the solid state as a function of solvation. The 2D sheet layers
can expand or contract, or, more remarkably, the phenyl rings can
rotate between two distinct positions. Rotation of the phenyl rings
allows for the adjustment of the tube size, depending on the guest
molecules present. This “gate” effect along the 1D tubes
has been characterized through single-crystal X-ray diffraction. The
transformations can also be followed by powder X-ray diffraction (PXRD)
and solid-state 13C cross-polarization magic-angle-spinning
(CP-MAS) NMR. Whereas PXRD cannot differentiate between transformations,
solid-state 13C CP-MAS NMR can be employed to directly
monitor phenyl rotation as a function of solvation, suggesting that
this spectroscopic method is a powerful approach for monitoring breathing
in this novel class of frameworks. Finally, simulations show that
rotation of the phenyl ring from a parallel orientation to a perpendicular
orientation occurs at the cost of framework–framework energy
and that this energetic cost is offset by stronger framework–solvent
interactions.