Visible-Light-Mediated Photocatalytic Selective N-Methylation of Amines using CO2 under mild reaction condition via NH2-MIL-125 (Ti) MOF catalyst

We herein describe a facile synthesis method of novel NH2-MIL-125 (Ti) metal organic framework as a photocatalyst, and the photochemical system was firstly developed for the direct N-methylation of amines under very mild reaction conditions of one bar CO2 and NaBH4. According to the optimization of reaction conditions, the selective synthesis of methylamines is achieved in good to excellent conversion and selectivity using visible light mediated photocatalytic system.


Introduction
In recent decades, carbon dioxide (CO2) have become a serious threat in environment inspect, which was known as the main factor for the green-house effect. However, conventional methods of the utilization of CO2, for example, decreasing the CO2 level in the atmosphere through using chemical synthesis to produce functional chemicals or fuels concurrently. 1 Traditionally, the effective routes to harvest CO2 is thermal energy activation via using higher reaction temperature or gas pressure, which can make this reaction difficult to industrialize. More attractive and green strategies for the utilization of CO2 is photocatalytic or electrocatalysis. About the above aspects, photocatalysis is one of the most effective and clean routes, wide application and perspective of pollution degradation and hydrogen evolutions. Remarkable efforts have been directed towards achieving efficient CO2 activating with photocatalysts by using wide-bandgap semiconductors, such as TiO2, SrTiO3, ZnO, SiC, ZnS, etc 2, 3 . Besides, the heterogeneous include metal-organic frameworks (MOFs) in addition to the oxide semiconductors such as MIL-101, ZIF-8 and UiO-66 4,5 have been recently employed as photocatalysts [6][7][8][9][10][11] , since the modifcation of the coordination chemistry, energy bandgap, organic ligands, and composition of metal clusters can regulate light absorption and charge separation. 12 It is known that MIL(125)-NH2 (Ti) has a preferable photocatalytic effect upon light excitation, which was consisted of titanium clusters coordinated to the organic linkers, i.e., 2-aminoterephthalic acid. Since previous studies have revealed that the applications of MIL(125)-NH2 (Ti) in photocatalytic CO2 reduction, while utilizing CO2 as a feedstock to produce small molecule chemicals, such as CO, CH4, HCOOH and CH3OH. [13][14][15][16] However, there is still no report on the CO2 associated selective photocatalytic N-methylation of amines over MOFs.
As shown in Scheme 1, the reduction approaches of CO2 have been developed using stable catalysts and renewable reductants (H2) while requiring hazard conditions with corresponding catalyst. 17,18 Here, recent advancements on catalytic methylations of amines with CO2 in presence of mild hydrides such as hydrosilanes or hydroboranes, which feature CO2 inserted Si-H or B-H bond to form CH3-B or CH3-Si species. 19 In the past decade, tremendous progress was made in the synthesis of formamide by using CO2 as C1 source in the presence of silanes. 20 Meanwhile, related catalytic system can also reduce amides to form amines with hydrosilanes. Therefore, it would be ideal if using CO2 to synthesis the methylated amines can be realized by means of combining these two processes since it is a more economical one-pot process. Actually, recent studies have revealed that reductive methylation of amines only used one bar of CO2. 18 However, the mechanism of the N-methylation of amines with CO2 and hydroboranes is different with hydrosilanes since it is difficult to transform amides to amines by using most hydroboranes, such as BH3NH3 and NaBH4. Taking reductant sodium borohydride (NaBH4) as an example, the N-methylation of amines over all examined CO2 atmosphere can occur only when heated. 21 Herein, we developed a photocatalysis strategy to realize this process at room temperature with a balloon CO2.

Result and discussion
To get the optimization of reaction conditions, we used N-methylaniline (1a) as a model substrate to explore photocatalytic selective N-methylation of amines using carbon dioxide ( Table 1). Rare product can be obtained in the absence of catalyst or in the presence of catalyst but in a dark atmosphere (entry 1, 10, 13). Interestingly, commercial TiO2 gives a half conversion under the identical conditions (in CH3CN at 450nm and 12 h, entry 12), which unambiguously indicates that photocatalytic is suitable for for this process but TiO2 behave as the best catalytic activity in UV region. Besides, trace product is surprisingly detectable in other solvents with commonly used methylation, such as DMF and dioxane (entry 7-8). It is delighted to find that the enough NaBH4 is excellent for high selective formation of N, N-dimethyl aniline (3a) (entry 1, 14). It is noteworthy that the temperature of the reaction system increased on light irradiation because of the photothermal effect. 22 In sharp contrast experiments show that less 3a formation in thermocatalytic system was observed with the 30  yield of 3a and 10  yield of 2a at refluxed temperature 80 C (entry 15), indicating that the N-methylation of amines using carbon dioxide is not driven by heat. The NH2-MIL-125 (Ti), obtained by a simple solvothermal reaction, was synthesized following our previous reported procedure. 6 In brief, 2-aminoterephthalic acid (H2ATA) (2.178 g, 12 mmol), 18 mL of N,Ndimethyformamide (DMF) and 2 mL of MeOH wwere added to a Teflon liner with a magneton, and Ti(OC4H9)4 (1 ml, 3 mmol) was added drop by drop. Then, the above mixture was stirred for 30 min, and magneton was removed.
Finnally, the whole liner packed stainless steel autoclave and heated at 150 C for 48 h. The reactor was cooled to room temperature to separate the solid and washed with DMF and methanol, respectively, and the yellow solid was obtained (NH2-MIL-125 (Ti)). X-ray diffraction (XRD) patterns indicates that a narrow and sharp diffraction peak indicates that the precursor has a good crystallinity (Figure 1a, black line), which is consistent with the literature. 23 Delightly, XRD characterizations of the reused catalyst still remained same diffraction peak and demonstrated high structural stability (Figure 1a, red line). As expected, the little spectral changes were observed from UV/Vis ab-sorption upon fresh and recycled catalyst (Figure 1b). As shown in Figure 1b, a significant absorption was observed in the wavelength band of 400-500 nm, that is why the catalyst in 450 nm light exhibit the best catalytic effect. Meanwhile, Transmission electron microscopy (TEM) observation for NH2-MIL-125 (Ti) as a representative suggests that it has a good crystal structure and rule shape (Figure 2b). Therefore, this is further approved by scanning electron microscope (SEM) (Figure 2a). The elemental energy dispersive X-ray (EDX) mapping indicates uniform distribution of various elements (Figure 2c). With the optimized reaction conditions in hand, we evaluated a range of substrates for N-methylation of amines with CO2. As we know, for the amines, we studied fatty amines and aromatic amines with various substituents, including electron withdrawing group and electron donating group, which react with CO2 to give the corresponding desired methylamine in excellent yields (Table 2). For instance, methyl substituted anilines, including para-, ortho-, meta-, all were obtained in excellent conversion above 80  and good selectivity above 95  ( Table 2, entry 3-5). Aromatic amines with electron-withdrawing groups such as ;N-methyl-4-fluoroaniline was well tolerated , giving N,N-Dimethyl-4-fluorobenzeneamine conversion of up to 75 without by-product (Table 2, entry 7). Besides, aliphatic amines and large spatial displacement chain aromatic also converted to the corresponding methylamine in high selective conversions ( Table 2, entry 9-10).

Conclusion
In conclusion, we have firstly developed visible light mediated photocatalytic selective N-methylation of amines with a bar of CO2 as a sustainable C1 source in presence of NaBH4 as a reductant. According to the optimization of reaction conditions, affording the high selective products of methylamines in excellent yields. MOF as a good crystallinity can maintain shape and structure stable after reaction of N-methylation of amines, which has the potential to be recycled.