Microwave induced intercalation synthesis of chiral tailed porphyrin into layered antimony hydrogen phosphate host material

Hu Ximing, Liu Haiyang, Liu Yi, Bai Dejun
( Department of Applied Chemistry, South China University of Technology, Guangzhou 510640, China)

AbstractSuccess is achieved in the microwave-induced intercalation of the larger guest molecule, chiral tailed porphyrin, into an antimony hydrogen phosphate layered host material. The dielectric constant of reaction medium was found to play an important role in the microwave-induced intercalation synthesis and thus an organic acid solution with high dielectric constant was selected for the intercalation system. The microwave inducement in the acid solution makes possible the direct intercalation of protonated guest porphyrin into the layered host material. Our experiments demonstrated an effective intercalation synthetic method for the preparation of immobilized composite materials and better spacing channels are one of the advantages of layered intercalated structure over guest molecule encapsulated zeolite structure.

One of the techniques for the preparation of advanced materials is intercalation synthesis that incorporates or generates target guest molecules in the space available in pillared lamellar structure [1-5] . These chemically synthesized composite materials have been found with numerous applications, including size/shape/chiral selective separation or purification, selective catalysis, artificial photosynthesis, long effective drugs and so on. In our earlier work, we reported the in-situ intercalation [2-4] and the microwave-induced intercalation [5] of tetraphenylporphyrin into layered hosts to prepare layered host supported complexes. Recently we succeeded in the microwave induced intercalation synthesis of chiral tailed porphyrin, 5[ p-( 1-L-phenylalanyloxy) ethoxylphenyl-10,15,20-triphenylporphyrin ( CTTPPH2) , into an antimony hydrogen phosphate layered host material ( SbP) . As far as we know, it is the first instance that so large a molecule, chiral tailed porphyrin, was directly intercalated to an inorganic layered host SbP.

CTTPPH was prepared and characterized by our previously published method [6] . The product was purified by means of chromatography on silica gel with chloroform as an eluent. Final purple solid product CTTPPH, as shown in Fig.1, was obtained with the yield of 55%.
Layered host material SbP was prepared by previously published method [1,2] , using solid state synthesis and then ionic exchanging. In an aqueous solution of acetic acid, SbP and required amount of chiral tailed porphyrin were stirred for about 30min at room temperature and then refluxed in MI-9700 microwave oven for the intercalation synthesis. The microwave frequency is 2450GHz. Crude product was then washed by water, ethanol and acetone respectively to exclude acetic acid solvent and surface absorbed porphyrin. The incorporation and crystalline structure of the tailed porphyrin in the intercalated materials depend on not only the  molar ratio of SbP to chiral tailed porphyrin but also microwave inducement power and microwave inducement time. UV-Vis spectra were recorded with a DV-7( HS) spectrophotometer. The XRD data were recorded on a XD-3A X-ray diffractometer and element analyses were performed with a Perkin-Elmer 240 element analyzer.

Fig.1 Chiral tailed porphyrin ( CTTPPH2)

The molecular configuration of layered SbP is similar to the well-known layered phosphatozirconic acid, ZrP, to a certain extent but the different metal atoms in their structures result in obvious difference between their properties [1] . In SbP, the metal, antimony, is in the+5 oxidation state rather than the +4state. It is the charge density that makes the interlamellar space of SbP more able to be separated than those of ZrP for the intercalation of large guest molecules.
Our experiments showed that although the layered material SbP and porphyrin was refluxed for long time using various organic solvents such as acetone and methanol etc., porphyrin could not be intercalated directly into the interlamellar space of SbP, only a small amount of porphyrin was surface absorbed to the host. Using acetone as a reaction solvent, Cady [7] also failed in the direct intercalation of porphyrin into mica-type silicates. However, we found that in a proper concentration of aqueous acetic acid solution, the guest molecule, chiral tailed porphyrin CTTPPH, even though it is so large a molecule, could be directly intercalated into antimony hydrogen phosphate layered host with the help of microwave inducement. Element analysis, X-ray diffraction, UV-Vis spectrum were used for the investigation of the intercalation synthesis.
A chloroform solution containing ( C5) 4 ion was used to partially exchange the guest chiral tailed porphyrin intercalated in the purified product and the UV-Vis spectrum of the solution displayed the typical bands of chiral tailed porphyrin at 419, 516, 551, 592 and 649 nm, as reported in our previous work [6] . It demonstrated the retention of the target guest molecule chiral tailed porphyrin CTTPPH within antimony hydrogen phosphate host. The elemental analysis of the purified solid product showed it could be incorporated up to 21.4% ( w: w) of chiral tailed porphyrin CTTPPH in SbP host, depending on the reaction conditions. Also, X-ray diffraction technique was used to monitor the process of intercalation synthesis. We found that the d002 spacing of SbP in connection with interlamellar distance was increased with the intercalation of the porphyrin. In a series of our experiments, we used the fixed microwave inducement power but adjusted microwave inducement time to control the intercalation reaction. The X-ray powder pattern exhibited the fine crystalline structure of the intercalated product and the increased d002 spacing of purified and dried porphyrin-intercalated product was 1.32 nm with a reflux intercalation reaction time of 30 min, as shown in the X-ray diffraction pattern ( a) of Fig.2, indicating a gallery height about 0.54 nm. In comparison with the thickness of porphyrin, the guest molecule should be somewhat slanted to the interlayer. Further uptake of chiral tail porphyrin under longer time of microwave inducement can result in a d002 spacing of 1.51 nm but the layered crystalline structure of host material SbP was found to become ruinous to a certain extent, shown in Fig.2, X-ray diffraction pattern ( b) .

Fig.2 X-ray diffraction patterns of the intercalated product:( a) 30 min reflux time; ( b) 4 hrs reflux time

The overall reactions in the intercalation can be written as the following:

In our earlier research we found that the microwave inducement was in direct proportion to dielectric constant of reaction medium [8] . In this microwave induced reaction system, acetic acid is a kind of reaction medium that possesses higher dielectric constant in aqueous solution than acetone or methanol as the reaction medium. On the other hand,the tailed porphyrin can form a kind of protonated ions in the aqueous solution of acetic acid,the protonated form of porphyrin is much easier than porphyrin itself to be microwave-induced and be intercalated into the layered phosphate host to replace the active hydrogen ion in the interlamellar space of SbP host. Layered intercalated structures are usually more easily fine tuned than 3 dimensional fixed structures such as zeolites. One of the advantages of layered intercalated structure of the chiral porphyrin over zeolite-encapsulated structure for the preparation of composite materials is that layered intercalated structure can provide better spacing channel for reaction components or eluates to diffuse in it.
In our experiments, we found the microwave induced intercalation synthesis is not only to give rise to good reproducibility but also better to control the chiral porphyrin content in layered phosphate host material than our previous in-situ synthesis of porphyrin. More important, it provides an effective intercalation synthetic method for some larger molecule encapsulated composite materials.