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.

金属有机框架（Metal–organic framework, MOF）材料已被制备得到，其以机械互锁分子（mechanically interlocked molecule, MIM）作为柱撑配体，连接两个周期性羧酸锌层。该MIM连接基为[2]轮烷，包含一个24冠6（24C6）大环与带有末端吡啶给体基团的苯胺基轴。对UWDM-2（采用1,4-重氮苯基二羧酸盐配体）与UWDM-3（采用1,4-联苯二羧酸盐配体）的单晶X射线结构分析显示，两种框架均拥有足够大的自由体积，可满足互锁的24C6大环旋转所需的空间需求，但二者的框架均存在互穿现象：UWDM-2为三重互穿结构，UWDM-3为二重互穿结构。具体而言，UWDM-3中的柱撑MIM的24C6环恰好位于相邻羧酸锌层的方形孔道内部。变温（Variable-temperature, VT）氘固体核磁共振（2H SSNMR）测试表明，UWDM-2与UWDM-3中的24C6大环仅能发生与环柔性或部分旋转相关的受限运动，无法实现完全自由旋转。变温粉末X射线衍射研究显示，对UWDM-3进行脱除溶剂的活化处理后，其发生了相变。所得UWDM-3的新型β相仍保持结晶性，且2H SSNMR测试证实，此时柱撑MIM的24C6大环已具备足够自由度，可以实现完全旋转。最为关键的是，该相变具有可逆性：通过再溶剂化可将UWDM-3的β相还原为初始的α相，这首次证实了固态材料内部机械互锁分子的动力学行为可通过可逆相变进行调控。