Metal–Organic Frameworks with Mechanically Interlocked Pillars: Controlling Ring Dynamics in the Solid-State via a Reversible Phase Change

Metal–organic
framework (MOF) materials have been prepared
that contain a mechanically interlocked molecule (MIM) as the pillaring
strut between two periodic Zn-carboxylate layers. The MIM linker is
a [2]­rotaxane with a [24]­crown-6 (24C6) macrocycle and
an aniline-based axle with terminal pyridine donor groups. The single-crystal
X-ray structures of MOFs UWDM-2 (1,4-diazophenyl-dicarboxylate)
and UWDM-3 (1,4-biphenyl-dicarboxylate) show that both
frameworks are large enough to contain the free volume required for
rotation of the interlocked 24C6 macrocycle, but the
frameworks are interpenetrated (UWDM-2, three-fold, and UWDM-3, two-fold). In particular, for UWDM-3
the 24C6 rings of the pillaring MIM are
positioned directly inside the square openings of neighboring zinc
dicarboxylate layers. Variable-temperature (VT) 2H SSNMR
demonstrated that the 24C6 macrocycles in UWDM-2 and UWDM-3 can only undergo restricted motions related
to ring flexibility or partial rotation but are incapable of undergoing
free rotation. VT-powder X-ray diffraction studies showed that upon
activation of UWDM-3, by removing solvent, a phase change
occurs. The new β-phase of UWDM-3 retained crystallinity,
and 2H SSNMR demonstrated that the 24C6 macrocyclic
ring of the pillared MIM strut is now free enough to undergo full
rotation. Most importantly, the phase change is reversible; the β
version of the MOF can be reverted to the original α state by
resolvation, thus demonstrating, for the first time, that the dynamics
of a MIM inside a solid material can be controlled by a reversible
phase change.