Synthesis of a Novel Porphyrin-Based Metal–Organic Framework (Co-Por MOF)

: In this study, a novel porphyrin-based metal–organic framework (Co-Por MOF) was successfully synthesized by a simple one-step solvothermal method. We report a metal–organic framework based on the covalent interaction of an organic linker, 5,10,15,20-tetrakis(4-carboxyphenyl) porphyrin (TCPP) with cobalt clusters. The properties of this material were analyzed by powder X-ray diffraction (XRD), ultraviolet visible adsorption spectroscopy (UV–Vis), Fourier-transform differential reflectance spectroscopy (DRS). USA) X-ray diffractometer, with Cu K α radiation. The accelerating voltage and the applied current were 40 KV and 40 mA, respectively. Data were recorded in the 2 θ range of 5° to 40°. UV-visible absorption spectra of TCPP ligands were determined by a double beam UV–Vis spectrophotometer (Shimadzu UV-1700, Tokyo, Japan). The solid-state UV/Vis spectra (DRS) of the MOF was obtained for the dry-pressed disk samples using a Shimadzu (MPC-2200) spectrophotometer. The IR experiments were carried out on a Shimadzu FTIR-8400S spectrophotometer, over a scan range from 4000 to 400 cm − 1 .


Introduction
Metal-organic frameworks (MOFs) have attracted significant attention as a new class of porous materials, developed in the last two decades. They have unique properties as permanently microporous materials. The ability to utilize a virtually unlimited range of organic struts [1][2][3], together with recent advances in purification and post synthesis manipulation, have facilitated the rational design of materials for many applications, such as gas storage [4], chemical separations [5], catalysis [6], drug delivery [7], biomedical imaging, photosensitizers [8], sensors [9], etc.
Metal-organic frameworks (MOFs) are a class of hybrid materials assembled from organic linkers and metal ions or clusters [10,11]. In other words, MOFs are comprised of a central metal or metal complex that binds to organic ligands, creating patterned layers, which can be stacked with organic struts to create a three-dimensional porous structure.
Linker modification is one of the most direct methods for MOF functionalization. Porphyrinic ligands are such a category of versatile linkers that have been extensively explored [12,13].
Porphyrins and other pyrrolic systems are fairly widespread in nature and play vital roles in biological systems by contributing to the catalytic activity of many enzymes as cofactors in photosynthesis and respiration. Porphyrin and metalloporphyrin molecules also possess unique biological and chemical functionalities, so they can be used as anticancer drugs, catalysts, sensors, nonlinear optical materials, DNA-binding or cleavage agents, photomedicine, and energy/electrontransfer systems [14]. Therefore, there are many metal-organic coordination networks assembled from porphyrin or metalloporphyrin building blocks, which present interesting topological network structures, as well as great thermal and chemical stabilities [12].
Herein, we report the synthesis, and characterization of a new porphyrin-based MOF obtained from the reaction of 5,10,15,20-tetrakis(4-Carboxyphenyl) porphyrin with cobalt nitrate under solvothermal conditions. (Scheme 1) This novel porous material can be applied as a heterogeneous photocatalyst that exhibits efficient CO2 photochemical reduction under visible-light irradiation, by cooperating with the porphyrin moiety photosensitizer.

General Information
The commercial chemicals that were used were purchased unless mentioned otherwise. Pyrrole and 4-carboxybenzaldehyde, ethanol, methanol, propionic acid, DMF, and Co(NO3)2.6H2O were purchased from Sigma-Aldrich (St. Louis, MO, USA) or Merck (Kenilworth, NJ, USA) and were of analytical grade. All purchased materials were used without further purification.

Synthesis of TCPP
Porphyrin TCPP was prepared and purified by the methods reported previously [15]. First, 70 mL of propionic acid as solvent and catalyst was heated to refluxing temperature and 0.58 g of 4-carboxybenzaldehyde was added. Then 5 mL of pyrrole was dropwise added to the solution and refluxed for 2 h. The obtained solid was filtered, and then washed with distilled water and ethanol in order to remove the propionic acid and by products from porphyrin. The resulted purple porphyrin was dried at room temperature.

Synthesis of Co-Por MOF
Cobalt nitrate hexahydrate, TCPP, and mixed solvents of DMA (dimethylacetamide): MeOH:H2O (v:v:v = 4:1:1) were ultrasonically dissolved in a 150 mL high-pressure vessel. The mixture was sealed and heated at 115 °C for 24 h. After the vial cooled down to room temperature, dark green crystalline powder of Co-Por MOF was collected by filtration. Subsequently, they were washed using DMF and dried in air at room temperature.

Characterization
X-ray diffraction (XRD) patterns were collected on Bruker (AXS D8 Advance, Billerica, MA, USA) X-ray diffractometer, with Cu Kα radiation. The accelerating voltage and the applied current were 40 KV and 40 mA, respectively. Data were recorded in the 2θ range of 5° to 40°. UV-visible absorption spectra of TCPP ligands were determined by a double beam UV-Vis spectrophotometer (Shimadzu UV-1700, Tokyo, Japan). The solid-state UV/Vis spectra (DRS) of the MOF was obtained for the dry-pressed disk samples using a Shimadzu (MPC-2200) spectrophotometer. The IR experiments were carried out on a Shimadzu FTIR-8400S spectrophotometer, over a scan range from 4000 to 400 cm −1 .

Result and Discussion
The UV-Vis absorption spectrum of TCPP porphyrin is shown in Figure 1. As can be seen, the TCPP spectrum exhibited a Soret band at 420 nm and four Q bands in the range of 500 to 700 nm.  As can be seen in Figure 3, in the FT-IR spectrum of the Co-Por MOF the stretching vibration of the =C-N and -C=N bands (pyrrole) appeared at 1373 cm −1 and 1720 cm −1 , respectively, and the stretching vibration of C=O at 1600 cm −1 . The stretching asymmetric and symmetric vibration bands attributed to the C-H (CH2) band are discernible in 2846, 2918, and 2939 cm −1 . The powder X-ray diffraction (PXRD) pattern of Co-Por MOF is in good agreement with the similar materials pattern (Figure 4), and the phase purity of the bulk Co-Por MOF verified by these studies shows little contamination. It also indicates that the particles are well crystallized.

Conclusions
In conclusion, guided by a solvothermal condition, a porphyrin-based MOF was constructed with TCPP porphyrin and Co(NO3)2.6H2O. Then it was characterized by XRD, FT-IR, and UV-Vis techniques. This MOF material can be used in catalytic reaction for photocatalytic reduction of CO2 under visible light irradiation.