Strategic MOF Linker Methylation to Direct Pore Structure and Local Dynamics

The marvelous substrate recognition of enzyme hosts is attributed to induced-fit binding, in which the enzyme host maintains a delicate balance between preorganization and flexibility. For synthetic porous hosts such as MOFs, programmable control over the local and global flexibility modes is thus crucial for the optimization of host-guest interactions. Here we show that methylation adjacent to the pyrene-arene junction of the linker permits the synthesis of a derivative of NU-1000 in which aryl rotation is thermally inaccessible but small angle librations remain facile. Mechanochemically promoted Suzuki cross-coupling was able to overcome the twin challenges of poor solubility and steric congestion to access a linker that could not be prepared in solution. The incorporation of methyl groups in the linker controls the local dynamics of the MOF pore and permit a window that interconnects channels in the unsubstituted MOF to evolve into a separate micropore environment. In addition, we show how the different chemical shifts observed for nuclei that extend into distinct MOF pores permits experimental determination of rotational barriers via variable-temperature 13C solid-state NMR.