Urea Metal–Organic Frameworks as Effective and Size-Selective Hydrogen-Bond Catalysts

A new urea-containing metal–organic framework
(MOF) was
synthesized to act as a heterogeneous catalyst. Ureas are well-known
for self-recognition and aggregation behavior, resulting in loss of
catalytic competency. The catalyst spatial isolation achievable in
a porous MOF environment suggests a potentially general solution.
The combination of a symmetrical urea tetracarboxylate strut, 4,4′-bipyridine,
and Zn­(NO3)2·6H2O under solvothermal
conditions afforded a new microporous MOF (NU-601). This
material is indeed an effective hydrogen-bond-donor catalyst for Friedel–Crafts
reactions between pyrroles and nitroalkenes, whereas a homogeneous
urea is much less competent. The higher rates of reaction of small
substrates relative to larger ones with NU-601 strongly
suggest that catalysis primarily occurs within the pores of this new
material rather than on its exterior. To the best of our knowledge,
this approach is the first example of specific engineering of successful
hydrogen-bonding catalysis into a MOF material.