Mechanically Interlocked Linkers inside Metal–Organic Frameworks: Effect of Ring Size on Rotational Dynamics

A series
of metal–organic framework (MOF) materials has
been prepared, each containing a mechanically interlocked molecule
(MIM) as the linker and a copper­(II) paddlewheel as the secondary
building unit (SBU). The MIM linkers are [2]­rotaxanes with varying
sizes of crown ether macrocycles ([22]­crown-6, 22C6;
[24]­crown-6, 24C6; [26]­crown-6, 26C6; benzo[24]­crown-6, B24C6) and an anilinium-based axle containing four carboxylate
donor groups. Herein, the X-ray structures of MOFs UWCM-1 (no crown) and UWDM-1(22) are
compared and demonstrate the effect of including a macrocycle around
the axle of the linker. The rotaxane linkers are linear and result
in nbo-type MOFs with void space that allows for motion of the interlocked
macrocycle inside the MOF pores, while the macrocycle-free linker
is bent and yields a MOF with a novel 12-connected bcc structure.
Variable temperature 2H solid-state nuclear magnetic resonance
showed that the macrocycles in UWDM-1(22), UWDM-1(24), and UWDM-1(B24) undergo different degrees
and rates of rotation depending on the size and shape of the macrocycle.