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 (MOP) 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 MOP 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 MOP (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.