Mesoporous Cages in Chemically Robust MOFs Created by a Large Number of Vertices with Reduced Connectivity

We report the design
and synthesis of two metal–organic
frameworks (MOFs) with permanent porosity, MOF-818 and MOF-919, using
a small ditopic organic linker, 1H-pyrazole-4-carboxylic acid (H2PyC), 0.4 nm in length. Three mesoporous cages of unprecedented
polyhedra are identified in these MOFs, a wuh cage in
MOF-818 and yys and liu cages in MOF-919,
with diameters of 3.8, 4.9, and 6.0 nm, respectively. The ditopic
H2PyC linker functions as the edge in the structure, while
two types of metal-containing second building units (SBUs) function
as the vertices. 28 vertices are present in the wuh cage;
50 in the yys cage; and 70 in the liu cage.
Systematic analysis of these cages along with other mesoporous cages
in supramolecules and MOFs constructed by ditopic linkers reveals
that the extension of cage size is dictated by both the number and
connectivity of the vertices. The increase in cage size is proportional
to the number of vertices, while the growth rate is determined by
their connectivity. The reduction in connectivity is found to be an
effective way to create large cages. All three cages in this report
are constructed by three-connecting (3-c) vertices and two-connecting
(2-c) vertices. This [2-c, 3-c] connectivity represents the least
connectivity required for the construction of cages and the most effective
one for cage size expansion. The largest cage, liu, exhibits
a cage size to linker size ratio of 15, outstanding in supramolecules
and MOFs. MOF-818 is stable in water with a wide pH range (pH = 2–12),
and the wuh cage is big enough for the inclusion of biomolecules
such as vitamin B12 and insulin.