I. Heterogeneous Tandem Catalysis with a Metal-Organic Framework II. Porphyrin Boxes: Rationally Designed Porous Organic Cages

DoctorThis thesis describes (1) the study of a metal-organic framework (MOF) (which can be readily constructed from metal ions/clusters and organic ligands) for the application of heterogeneous tandem catalysis, and (2) a new general approach to the synthesis of shape-persistent organic cages through rational design, and their applications as a porous material. In light of growing demand for efficient heterogeneous catalysts for chemical processes, the synthesis of MOFs has become immensely important because of their potential applications in heterogeneous catalysis. Especially, MOFs provide unique platform for incorporation of various catalytic moieties at both metal centers and ligands, which may show extraordinary selectivity in catalytic reactions because of the confinement effect of pores. In Chapter 2, a MOF accommodating both acid and base catalytic moieties in the same framework, NH2-MIL-101(Al), by site isolation of the catalytic moieties, which is difficult to achieve in a homogeneous catalyst system is presented. This MOF, without any post-synthetic modification, functions as a bifunctional site-isolated Lewis acid-Brønsted base heterogeneous catalyst for tandem Meinwald rearrangement–Knoevenagel condensation reactions. The Lewis acidic Al(III) sites catalyze Meinwald rearrangement of epoxides with remarkable selectivity for substrates, which generate aldehydes via benzylic 3° carbocation, and the Brønsted basic amino groups in organic linkers catalyze Knoevenagel condensation of resulting aldehyde with malononitrile in tandem. Porous organic cages have attracted considerable attention in recent years owing to their potential applications in gas storage, separation and many other areas. Because high structural and chemical stability and easy processability is essential for many practical applications, the quest for porous organic cages with high thermal and chemical stability has been a subject of intense research. However, to date, most of the porous organic cages have been synthesized with limited synthetic approaches, mainly based on the combination of 2-connected linear (or bent) and 3-connected triangular shaped building units. Furthermore, there are only a few organic cages with a large cavity; most of them become nonporous organic solids during the activation process mainly due to the lack of structural rigidity. In Chapter 3, the synthesis and characterization of porphyrin boxes (PB-1 and PB-2), which are rationally designed porous organic cages with a large cavity using well-defined and rigid 3-connected triangular and 4-connected square shaped building units are presented. PB-1 has a cavity as large as 1.95 nm in diameter and shows high chemical stability in a broad pH range (4.8 to 13) in aqueous media. The crystalline nature as well as cavity structure of the shape-persistent organic cage crystals were intact even after complete removal of guest molecules, leading to one of the highest surface areas (1370 m2g 1) among the known porous organic molecular solids. The size of the cavities and windows of the porous organic cages can be modulated using different sized building units while maintaining the topology of the cages, as illustrated with PB-2. Interestingly, PB-2 crystals showed unusual N2 sorption isotherms as well as high selectivity for CO2 over N2 and CH4 (201 and 47.9, respectively at 273 K at 1 bar). The general approach presented here may thus be applicable to the synthesis of ultra-large, shape-persistent organic cages leading to highly porous molecular solids