Comparative Study of In Situ and Presynthesized X‑Pillar Ligand in Self-Assembly of Homochiral Porous Frameworks

In
situ ligand formation plays a unique role in the synthesis of
metal–organic framework (MOF) materials. However, for a given
metal–ligand system, in situ ligand formation can be limiting,
especially for the synthesis of kinetically stable porous phases,
because reaction conditions must be chosen to meet the requirements
of both in situ ligand formation and MOF crystal growth. Such requirements
may be mutually incompatible, because ligand formation involving the
breaking of inert bonds may require conditions more harsh than those
needed for the growth of porous materials. For homochiral MOFs, less
harsh reaction conditions are also desirable because of the reduced
risk for ligand racemization. In this study, we propose and demonstrate
the concept that the use of presynthesized ligand (in place of in
situ-formed ligand) to decouple the ligand synthesis reaction from
MOF crystal growth allows access to more porous and kinetically stable
phases that may not be possible through the in situ process. A homochiral
Zn-d-camphorate-TPB system (TPB = 1,2,4,5-tetra­(4-pyridyl)­benzene)
is chosen to illustrate this concept. A new homochiral MOF (CPM-322)
has been synthesized that contains infinite homochiral sheets of zinc
paddlewheel dimers and d-camphorate ligands. An exceptional
feature is the novel X-pillaring mechanism, in distinct contrast with
simple pillaring by rod-like ditopic ligands (here called the I-pillaring
mechanism). The perfect geometry matching in both pillaring distances
and angles between TPB X-pillars and wavy homochiral sheets allows
the TPB ligand to act as X-shaped pillars that prop up homochiral
layers with two pyridyl groups on each side, leading to a highly open
and rigid three-dimensional homochiral porous framework.