(560gy) Characterization of Cyclodextrin-Based Metal-Organic Frameworks with Catalyst Molecules

Cyclodextrin-based metal organic framework (CD-MOF) consisting of g-cyclodextrin (g-CD) and potassium hydroxide has a porous cubic crystal structure in which the primary units of (g-CD)₆ were packed into a body-centered cubic array. CD-MOF is easily prepared by the methanol vapor diffusion method at room temperature. The spherical nanopores with a diameter of 1.7 nm are formed at the center of (g-CD)₆ units and show a hydrophilic nature by the free primary 6-OH groups in the (g-CD)₆ unit. Meanwhile, a pair of g-CD tori in CD-MOF forms hydrophobic nanopores (size of 1 nm). Thus the channel in CD-MOF is a pearlnecklace-like channel connected by hydrophilic and hydrophobic nanopores. Once larger guest molecules compared to the aperture size of g-CD tori are encapsulated into the CD-MOF, they cannot escape from CD-MOF without the collapse of the crystal. CD-MOF has high crystallinity and is a stable crystal at temperatures below 473 K in organic solvents. CD-MOF encapsulating catalytically active molecules would be expected as an advanced catalyst having a high specific surface area.

In the previous paper (Nagai et al., 2018), water soluble porphyrin (TCPP) was found to be introduced into the spherical hydrophilic nanopores during the crystallization process. Fullerene C60 was successfully encapsulated in the hydrophobic nanopores of CD-MOF by the methanol vapor diffusion using C60/g-CD complex in KOH solution.

In this research, Co(II)TCPP was synthesized from cobalt(II) acetate tetrahydrate and encapsulated into CD-MOF (Co(II)TCPP/CD-MOF). The catalytic activity of Co(II)TCPP/CD-MOF was investigated in the coupling reaction of 2-methoxy-4-methylphenol. In addition, we investigated hydroxylation of phenylboronic acid over C60/CD-MOF under visible light irradiation to confirm the generation of singlet oxygen from C60/CD-MOF.

The crystallization yield of CD-MOF was 74% and decreased to 28% for C60/CD-MOF and 57% for Co(II)TCPP/CD-MOF, respectively. However, an XRD analysis proved that there was no change in crystal structure of CD-MOF after introducing the C60 and Co(II)TCPP molecule.

The oxidative coupling of 2-methoxy-4-methylphenol (cresol) was carried out in methanol solution of Co(II)TCPP and Na₂CO₃ under oxygen atmosphere at 333 K for 15 hours. Conversion of cresol was as high as 88%. Meanwhile, Co(II)TCPP/CD-MOF behaves as a heterogeneous catalyst in methanol. Under the same conditions as Co(II)TCPP, the conversion of 90% for Co (II) TCPP/CD-MOF revealed the almost same catalytic activity as the homogeneous catalyst. These results indicate that CD-MOF is effective as a catalyst support of organic complex catalysts and organocatalysts.

Fullerene C60 is known to generate singlet oxygen under visible-light irradiation and therefore attracting attention as a photodynamic therapy drug that attacks cancer cells. In addition C60 capable of generating singlet oxygen can be used as a metal-free photocatalyst. In this study, photocatalytic hydroxylation of phenylboronic acid in chloroform was performed using C60/CD-MOF. After 6-h reaction, the conversion for the agglomerated C60 catalyst was 65.3%, while the conversion for the C60/CD-MOF catalyst greatly decreased to 6.0 %. This low conversion is considered to be influenced by pore diffusion rate of phenylboronic acid in CD-MOF. Effect of the pore diffusion rate on the reaction rate in this system will be investigated in the future.