The catalytic applicability of two kinds of Rh/MelCOF-1 or Rh/MelCOF-2 was studied by investigating the single N-methylation of nitrobenzene as a model reaction (Table ). In the initial condition screening process, we used Rh/MelCOF-2 with a relatively high content of Rh load, and the substrates were 2 mmol nitrobenzene, 2 ml methanol and 1.2 eq base. With the increase of reaction time, the conversion of reactant 1a increased, while the selectivity of by-product 1e and 1f decreased, while the selectivity of main product 1c increased, so it was inferred that 1e and 1f were possible intermediates. The reaction temperature has a great influence on catalytic performance. When the reaction temperature increases from 90 to 130 degrees Celsius, the conversion rate of 1c is also greatly increased to 98%. At the same time, it is observed that only 1e and 1f are formed at 90 degrees Celsius, indicating that these two substances are preferentially formed under the catalysis of Rh/MelCOF, which provides an important idea for the kinetic analysis in the following article. In order to confirm whether the reaction was catalyzed by Rh or MelCOF in Rh/MelCOF, other comparision experiments were conducted and found that 1c was not formed in the reaction without catalyst, or only using RhCl3 or MelCOF2 as the catalyst. Then the effects of different bases on the reaction were evaluated at 130 °C for 16 h. Different from the effects of different kinds of bases in the existing literature, almost all inorganic bases had good selectivity, such as Cs2CO3, NaOH, KOH, KO t Bu, Na2CO3, K2CO3, and there was no methylation. The formation of 1d was mainly the selective competition between 1b and 1c. However, the conversion of organic alkali Et3N is low due to its weak basicity. For this reason, under the optimum reaction conditions of 130 °C, 16 h and Cs2CO3, the 1c selectivity is lower when using the Rh/MelCOF-1 than that of use Rh/MelCOF-2. Then, based on the optimized conditions of methylation and hydrogenation of nitroaromatic hydrocarbons using methanol as the source of methylation and hydrogenation, it is proved that Rh/MelCOF is generally applicable in the synthesis of N-monomethylamine substrates. As shown in Table , 2 mmol substrates, 3 mg Rh/MelCOF-2 catalyst and 2.4 mmol Cs2CO3 (1.2eq) are used for COF supported Rh catalyzed tandem hydrogenation and mono-methylation in 2 ml MeOH at 130 °C for 16 h. It can be seen that many nitro compounds with different functional groups have good tolerance, which proves their universal application in mono-N methylation. Among them, the substrates with para-electron donor groups have good reaction selectivity, and all of them are quantitatively converted into monomethylated derivatives (2c, 5c, 5c, 6c, 7c). However, the electron donor groups on the m and ortho-position will lead to the decrease of selectivity and even incomplete conversion, and the main competitive product is aniline aromatics, which may be attributed to the increase of spatial shielding of nitro functional groups. As a result, the transfer hydrogenation step is inhibited (3c). However, the conversion and selectivity of electron-withdrawing halogensubstituted derivatives at the para-position are poor, and the main competitive products are aniline aromatics, azobenzene, and azobenzene oxide homologs (8c, 9c, 10c). On the other hand, the reaction effect of the meta-electron-absorbing group is better than that of the para-substituent, the conversion can reach more than 99%, and the selectivity is also higher, which may be attributed to the synergistic effect of electron synergy and space effect (11c, 12c, 13c). On the other hand, the spatial effect of ortho-electron-absorbing halogen-substituted derivatives is stronger than that of electron synergism, which leads to a decrease in the yield of N-monomethylamine (14c, 15, 16c). However, heterocycles, biphenyl rings, and substrates with steric hindrance all have good reactivity (17c, 18c, 19c, 20c). Moreover, according to the reaction law, the effect of electronic synergy on the methylation and hydrogenation of nitroaromatic hydrocarbons using methanol as a methylation source is higher than the spatial effect, and heterocyclic compounds can counteract the effect of electronic synergy. These prove that the prepared Rh/MelCOF heterogeneous catalyst can obtain hydrogen directly from methanol resources and carry out the mono-N methylation, and highly selective synthesize the challenging N-mono-methylamine products via the tandem transfer hydrogenation and mono-metylation of amine. Then the reusability of Rh/MelCOF-2 catalyst was tested for N-monomethylation reaction of 1a (Figure ). The catalyst recycle ability test was carried out under the optimized reaction conditions, and when every single reaction was finished, the reaction samples were analyzed using GC. Then the residue was centrifugally precipitated and the reaction solution was extracted, afterward, methanol is added to the remaining solid-liquid mixture, and again the centrifugal precipitation is carried out which is repeated until no color showed according to TLC. In this case, the next catalyst recycle test could be carried out according to the standard procedure, the reaction mixture is loaded into the tube and carried out for the next run. As shown in Figure , the catalyst can be successfully recycled 4 times without losing significant catalytic reactivity and product selectivity. In the fifth cycle, although the conversion still kept over 99%, the selectivity of 1c decreased to 49%, which is probably due to the decrease of catalyst performance and the decrease in the reaction rate from 1b to 1c.