Efficient Photocatalytic Selective Oxidation of Benzylamines over Cobalt Molecular Catalyst Covalently Bonded to Carboxyl Functionalized Carbon Nitride

Photocatalysis is accepted as a promising method for selective oxidative coupling of amine to imine. However, owing to the rapid dehydrogenation of the generated imine intermediate of α‐amino hydrogen to butyronitrile, it remains a huge challenge to fabricate an exquisite catalyst with enhanced conversion and selectivity. Herein, a novel heterogeneous catalyst (Bpy‐CoCl2/CNCOOH) is developed, where carboxyl functionalized carbon nitride (CNCOOH) serves as a substrate material and couples with highly efficient cobalt‐aminopyridine (Bpy‐CoCl2) molecular catalyst through covalent bonding. The catalyst not only effectively facilitates the transfer of photogenerated charges, but also generates mildly oxidatively active species of superoxide radicals, which can effectively oxidize various functionalized benzylamines to corresponding imines in the presence of oxygen with at least 78.5% conversion and >99% selectivity using O2 as oxygen source, especially for the unmodified benzylamine with >99% conversion and >99% selectivity. This unique combination of a selective molecular catalyst and a simple and robust semiconducting material open new avenues for the selective oxidative coupling of amines.


Introduction
[3] Imines can be obtained through various synthetic methods, such as hydrocoupling reactions of nitro compounds and [4] oxidative condensation of amines with alcohols. [5]Among them, the oxidative coupling reaction of benzylamine is one of the effective methods to prepare the corresponding imines. [6,7]However, the self-coupling of primary amines is still challenging, as the imine intermediate of the generated α-amino hydrogen experiences rapid dehydrogenation to butyronitrile with reduced conversion and selectivity to the desired imine product. [8]Among various advanced oxidation technologies, photocatalysis is a potential method to achieve high selectivity using O 2 as a green and mild oxidant.Some semiconductor photocatalysts have great potential in the synthesis of imines, such as TiO 2 , [9] In 2 S 3 , [10] BiOX (X = Cl, I, Br), [11] g-C 3 N 4 , [12] triazine photocatalysts. [13,14]n order to ensure the selectivity of imine products through coupling oxidation synthesis, homogeneous molecular catalysts have attracted extensive attention due to their advantages such as high activity and selectivity, tunable structure, and fine control of the active site environment.However, the stability, recyclability, and scale-up of homogeneous molecular catalysts over time are limiting factors for their development. [15]Therefore, solid materials with high stability, high recovery rate, and recyclability are suitable supports for the construction of hybrid catalysts with highly reactive molecular catalysts.At present, a large number of studies have reported the combination of noble metalcontaining molecular catalysts and supports to form heterogeneous catalysts. [16]However, there are few studies on the heterogeneity of non-noble metals and carbon-based support materials. [17]In order to establish an efficient electronic coupling between the support and the molecular catalyst with the maintained catalytic function of the catalyst, commonly used coupling strategies include noncovalent bonding interactions (π-π stacking, [18] hydrogen bonding interaction, [19] or electrostatic interaction [20] ) and covalent links. [21]Coupling molecular catalysts through covalent bonds can more effectively promote mutual electron transport, thereby achieving efficient photocatalytic performance.Although many researchers are interested in the design of metal complex/semiconductor hybrid catalysts, there are few reports on the photocatalytic efficient oxidation of benzylamines to imines using such catalysts.

DOI: 10.1002/aesr.202300114
Photocatalysis is accepted as a promising method for selective oxidative coupling of amine to imine.However, owing to the rapid dehydrogenation of the generated imine intermediate of α-amino hydrogen to butyronitrile, it remains a huge challenge to fabricate an exquisite catalyst with enhanced conversion and selectivity.Herein, a novel heterogeneous catalyst (Bpy-CoCl 2 /CN─COOH) is developed, where carboxyl functionalized carbon nitride (CN─COOH) serves as a substrate material and couples with highly efficient cobalt-aminopyridine (Bpy-CoCl 2 ) molecular catalyst through covalent bonding.The catalyst not only effectively facilitates the transfer of photogenerated charges, but also generates mildly oxidatively active species of superoxide radicals, which can effectively oxidize various functionalized benzylamines to corresponding imines in the presence of oxygen with at least 78.5% conversion and >99% selectivity using O 2 as oxygen source, especially for the unmodified benzylamine with >99% conversion and >99% selectivity.This unique combination of a selective molecular catalyst and a simple and robust semiconducting material open new avenues for the selective oxidative coupling of amines.
Metal-free carbon nitride (g-C 3 N 4 ) has the characteristics of high stability and thermal stability, easy synthesis, sustainability, and moderate bandgap.After being modified on the surface, it can provide specific anchoring centers. [22]The introduction of carboxyl groups (─COOH) on the surface of carbon nitride can not only use carboxyl groups to immobilize molecular catalysts in a covalent manner, but also effectively improve the rapid separation of photogenerated carriers.Therefore, the conversion rate and selectivity of benzylamine oxidative coupling to imine can be improved, and the turnover frequency (TOF) of the reaction can be further improved.
Overall, the carboxylic carbon nitride (CN─COOH) was coupled with the highly efficient cobalt-aminopyridine molecular catalyst (Bpy-CoCl 2 ) by covalent bonding, which facilitated the electron leaving domain.In addition, the method effectively inhibits the recombination of photogenerated electron-hole pairs, improves the separation of charge carriers, and enhances the photocatalytic activity toward selective oxidation of amines to imines.Mechanism analysis proves that holes (h þ ) and superoxide radicals (•O 2 À ) serve as the main active species.Under simulated sunlight irradiation and normal pressure conditions, using oxygen as the oxidant and acetonitrile as the solvent, both conversion and selectivity of benzylamine to imine exceed 99% in 5 h, and the TOF can reach as high as about 985.1 μmol g À1 h À1 .

Characterization of Photocatalysts
The morphological characteristics of the prepared catalyst were determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).As shown in Figure 1a and Figure S4, Supporting Information, covalent coupling, Bpy-CoCl 2 /CN─COOH composite catalyst with irregular blocky surface, suggests that carbon nitrides modified with -COOH groups may provide favorable sites for the grafting of molecular catalysts.In addition, it can be seen from the TEM images of Figure 1b,c that Bpy-CoCl 2 /CN─COOH retains the similar 2D layered structure with the original carbon nitride (Figure S5, Supporting Information).
In order to further confirm the crystal structure and functional group composition of the Bpy-CoCl 2 /CN─COOH composite catalyst, the catalyst was tested by X-ray diffraction (XRD) and fourier transform infrared spectrometer (FTIR).The XRD pattern of Bpy-CoCl 2 /CN─COOH looks quite similar to the pristine CN─COOH, where two main diffraction peaks at 13.1°and 27.9°indexing to the (100) and (002) lattice planes are attributed to the in-plane tri-s-triazine motifs and periodic stacking of conjugated aromatic systems in CN─COOH, respectively.This result indicates that the introduction of cobalt-aminopyridine molecular does not influence the phase structure of CN─COOH (Figure 1d). [23]The coupling of these two units through the formation of an amide bond was confirmed by FTIR spectroscopy results, as shown in Figure 1e.The presence of ─COOH groups in the pure CN─COOH is proved by the bands at 1651 and 3421 cm À1 , related to the C═O and O─H stretching vibration in the carboxylic groups, respectively. [24]hile for the Bpy-CoCl 2 /CN─COOH composite catalyst, the characteristic peak at 3241 cm À1 is weakened, which is attributed to the reaction between the ─COOH group in CN─COOH and the ─NH 2 group of the molecular catalyst, thereby forming a covalent bond. [25]This is confirmed by the shift of the C═O stretching band from 1651 cm À1 (C═O in carboxylic group) to 1530 cm À1 (C═O in amide group) and the vibration at 1530 cm À1 , corresponding to the superimposition of the C═C vibration from CN─COOH and in-plane N─H deformation coupled with C─N stretching, validating the formation of the ─CONH─ motif. [26]In the FTIR spectrum of Bpy-CoCl 2 / CN─COOH, the characteristic absorption peak of Bpy-CoCl 2 molecule can also be observed, and there is a characteristic peak of C─N─Co bond at 811 cm À1 .These results indicate that the molecular unit hybridizes with CN─COOH by covalent bonding and that the molecular structure of CN─COOH is retained while being functionalized.In addition, it can be seen that the thermal stability of the Bpy-CoCl 2 /CN─COOH catalyst is higher than that of the CN─COOH catalyst by thermal gravimetric analyzer (TGA) analysis (Figure 1f ).
The elemental composition, surface chemistry, and valence states of the Bpy-CoCl 2 /CN─COOH composite catalysts were mainly verified by the X-ray photoelectron spectroscopy (XPS) test, as shown in Figure 2.For the Bpy-CoCl 2 /CN─COOH, the N 1s peaks centered at 395.8 and 397.6 eV belong to C─N═C and N─(C) 3 of the triazine motif, respectively.The peak centered at 400.7 eV is consistent with C─N─H from covalent amide linkage.Compared with the N 1s spectrum of pure CN─COOH, the characteristic peaks of the Bpy-CoCl 2 / CN─COOH composite catalyst are slightly blueshifted, which is mainly due to the electronic effect of the carbon nitride adjacent to the molecular catalyst. [27]The C 1s spectrum is composed of three components: a major one at 284.8 eV, corresponding to sp 2 -hybridized carbon atoms in CN─COOH, a second one at 284.8 eV in CN─COOH coupled with Bpy-CoCl 2, and a third one at 288.0 eV from C─N═C attributed to the coupling of ─COOH.In addition, the C 1s spectrum is consistent with the binding energy shift trend in the N 1s spectrum, and the C─N═C binding energy also has a slight blue shift in the C 1s spectrum.This shift indicates that there is an electron transfer between Bpy-CoCl 2 and CN─COOH, resulting in the formation of an electron-rich π-conjugated heptazine structure on CN─COOH, which further indicates that the molecular catalyst and CN─COOH are covalently bonded.The O 1s spectrum of the Bpy-CoCl 2 /CN─COOH composite catalyst can be fit into two characteristic peaks at 530.8 and 529.4 eV, which can be individually attributed to the surface-adsorbed water and C═O in the amide bond.The two characteristic peaks at 531.2 and 529.6 eV of the O 1s spectrum of the CN─COOH catalyst are assigned to the surface-adsorbed water and the C═O in the ─COOH group, respectively. [28]On the other hand, a weak Co 2p signature at %782 eV can be observed, proving the presence of cobalt inside the hybrid material.The above results further confirmed the coupling between Bpy-CoCl 2 unit and CN─COOH support.

Evaluation of Photocatalytic Performance
The photocatalytic performance of the prepared catalyst was evaluated by selective oxidation of benzylamine.The oxidation experiment was carried out under the irradiation of simulated sunlight (AM 1.5 G) with acetonitrile solvent and O 2 oxidant.In order to explore the optimal conditions in the reaction, the photocatalytic activity of different reaction atmospheres was first explored (Figure 3a).In an argon atmosphere, the conversion of benzylamine was only 3.1%, and the selectivity to the imine product was 67.6%.When reacted in air, the conversion of benzylamine increased to 53.4%, and the selectivity of imine was 78.2%.When oxygen was used as the oxidant in the reaction, benzylamine was completely converted to the corresponding imine (benzylamine conversion >99%, imine selectivity >99%, TOF reached 985.1 μmol g À1 h À1 ).This result indicates that oxygen is an indispensable oxidant in the reaction.Subsequently, in order to avoid excess catalyst, the effect of catalyst dosage on photocatalytic activity was explored.As shown in Figure 3b, with the increase of catalyst dosage, both the conversion rate and selectivity of photocatalytic selective oxidation of benzylamine increased.When the amount of catalyst was 20 mg, benzylamine was completely converted, while when the amount of catalyst was more than 20 mg, the photocatalytic activity decreased greatly.At the same time, as the reaction time continued to prolong, the conversion rate of benzylamine continued to increase, and the selectivity of the product imine remained >99% (Figure 3c).When the reaction time was extended to 5 h, benzylamine was completely converted into imine.These results demonstrated that the prepared catalyst possessed excellent photocatalytic activity and stability.Based on above optimized parameters, the photocatalytic activity of Bpy-CoCl 2 /CN─COOH was further compared with that of pure CN─COOH and g-C 3 N 4 photocatalysts, as shown in Figure 3d.Apparently, CN─COOH grafted with Bpy-CoCl 2 molecular catalyst through covalent bonds exhibiting the highest photocatalytic activity, while the conversion of pure CN─COOH and pure g-C 3 N 4 could only achieve 57.6% and 28.8%, respectively.Moreover, comparing the activity of Bpy/CN─COOH with that of Bpy-CoCl 2 /CN─COOH, the activity increased from 70% to 99%.These results indicate that the Bpy-CoCl 2 molecular catalysts can significantly improve their photocatalytic activities after grafting through stable covalent bonds.Meanwhile, we speculate that the introduction of cobalt ions can conduct fast and efficient extraction of photogenerated holes and electrons in reactions to facilitate the separation of photogenerated carriers.According to this, it is to some extent confirmed that the covalent attachment of efficient and highly selective molecular catalysts on polymer materials is a promising method to improve the catalyst activity.
In order to demonstrate the universal applicability of Bpy-CoCl 2 /CN─COOH, the photocatalytic coupling reactions of benzylamine derivatives with different substituents were examined (Table 1).All benzylamine derivatives gave an excellent selectivity (>99%).However, substrate conversion varies for different substitutions of electron-donating groups (─CH 3 and ─OCH 3 ) and electron-withdrawing groups (─F, ─Cl, and ─CF 3 ).An electrondonating group function exhibits higher catalytic activity than the situation of electron-withdrawing group modification.Furthermore, ─F substituent has a stronger electronwithdrawing effect than ─Cl substituent; thus, a lower TOF is achieved.Based on the above activity and universality tests of the Bpy-CoCl 2 /CN─COOH composite catalyst, it is concluded that the prepared catalyst has excellent photocatalytic activity for selective oxidation of benzylamines.

Optical Properties and Photoelectrochemical Behaviors
In order to explore the effect of Bpy-CoCl 2 /CN─COOH catalyst on the photocatalytic selective oxidation toward benzylamide, the optical properties of the catalyst were characterized by UV/vis diffuse reflection spectroscopy (DRS) (Figure 4a).The hybrid catalyst is a dark brown solid, which is different from the originally  4b), Bpy-CoCl 2 /CN─COOH exhibits a slightly narrower bandgap (2.42 eV) than that of CN─COOH (2.50 eV).Mott-Schottky plots were measured to explore the band structure of the materials and the positive slope manifests the n-type characteristic of the samples (Figure 4c,d).The flat-band potentials (E fb ) of CN─COOH and Bpy-CoCl 2 / CN─COOH are calculated to be À0.38 and À0.21 V versus Ag/AgCl, respectively.A typical three-electrode configuration was used for the measurements.E CB (V vs. RHE) and E v are calculated according to the formula (E CB = E fb -X, and , where X is from 0.1-0.3V.For Bpy-CoCl 2 / CN─COOH semiconductors, we used 0.3 V. Thus, the conduction band positions of Bpy-CoCl 2 /CN─COOH and CN─COOH can be calculated as À0.68 and À0.51 V, and the valence band position is 1.74 V and 1.99 V respectively.After grafting with the Bpy-CoCl 2 molecular catalyst, the valence band position of the catalyst is more consistent with that for benzylamide oxide (Figure S7, Supporting Information).Simultaneously, the conduction band position is more likely to activate oxygen.
To obtain more insights into the role of the covalent linkage on the photoinduced charge transfer dynamics, additional spectroscopy analysis and photoelectrochemical measurements were conducted.The electrochemical impedance spectroscopy (EIS) Nyquist plots of various samples are depicted in Figure 5a.Due to its smaller arc radius, Bpy-CoCl 2 /CN─COOH displays a notably decreased charge transfer resistance relative to pure CN─COOH.Although the photoluminescence (PL) spectra (Figure 5b) of Bpy-CoCl 2 /CN─COOH and CN─COOH exhibit similar emission peak at about 463 nm with exciting irradiation of 375 nm, the emission intensity of Bpy-CoCl 2 /CN─COOH exhibits obvious decline compared with pure CN─COOH.This result illustrates that Bpy-CoCl 2 /CN─COOH exhibits higher charge separation efficiency and slower e À -h þ pair recombination.The time-resolved PL spectra further explore the lifetime of photoinduced charge carriers (Figure 5c).The photoborne carrier lifetimes of Bpy-CoCl 2 /CN─COOH and pure CN─COOH were determined to be 9.83 and 10.68 ns, respectively.In contrast, the photoborne carrier lifetime of Bpy-CoCl 2 /CN─COOH is shortened to 0.85 ns, which demonstrates the electron transfer between the Bpy-CoCl 2 molecular catalyst and CN─COOH as a result of the accelerated separation and migration efficiency of photogenerated electron-hole pairs.In addition, the photocurrent response curves of the samples were measured with repeated on/off illumination cycles with simulated solar light, and the result is shown in Figure 5d.At the moment of light on, the transient photocurrent response value of Bpy-CoCl 2 /CN─COOH is significantly higher than that of CN─COOH, implying the repaid electron transfer for   Bpy-CoCl 2 /CN─COOH.All the above results verified that Bpy-CoCl 2 /CN─COOH can effectively isolate the photoactive carriers, thus greatly improving the photocatalytic activity.

Photocatalytic Mechanism
To elucidate the mechanism of Bpy-CoCl 2 /CN─COOH for the photocatalytic oxidative coupling of benzylamines, the roles of possible active species in photocatalysis were investigated by a series of trapping experiments.As shown in Figure 6a, p-benzoquinone (BQ), L-Tryptophan, AgNO 3 , KI, and tertiary butanol (TBA) were added to quench superoxideradical (•O 2 À ), singlet oxygen ( 1 O 2 ), e À , h þ , and hydroxyl radical (•OH) during the photocatalytic reaction, respectively.The electron scavenger AgNO 3 or hole scavenger KI added into the mixture resulted in dramatically decreased conversions, reflecting the importance of photogenerated electrons and holes in the photocatalysis.When BQ was added, a decrease in conversion was also observed, which proved the role of •O 2 À as main reactive oxygen species in this reaction pathway.Additionally, when L-tryptophan or TBA was added, the conversion of benzylamine nearly maintained unchanged, indicating that Moreover, with the prolongation of irradiation time, the intensities of above characteristic peaks constantly increase, indicating that with the increase of reaction time, benzylamine is gradually decomposed to form the benzaldehyde intermediate for the production of imine. [29]ased on the observations from the aforementioned experiments, a plausible mechanism for the photocatalytic oxidative coupling of amines was proposed (Figure 7).Under visible light irradiation, Bpy-CoCl 2 /CN─COOH is first photoexcited to produce electrons and holes.The photogenerated electrons are rapidly transferred and diffused to the catalyst surface, thus reducing O 2 adsorbed on the catalyst surface to •O 2 À .Similarly, the photogenerated holes oxidize benzylamine into its cationic radical.These processes can be supported by the electronic band structure of Bpy-CoCl 2 /CN─COOH.Subsequently, •O 2 À reacts with the benzylamine cation radical to form the benzaldehyde intermediate that is further hydrolyzed with benzylamine to form the final imine product.

Conclusion
In summary, we developed a novel heterogeneous photocatalyst (Bpy-CoCl 2 /CN─COOH) for the photocatalytic benzylamine conversion under mild conditions.The photocatalyst uses carbon nitride as the substrate material, and after the introduction of carboxyl groups (─COOH) on the surface, it is attached to the highly efficient cobalt-aminopyridine molecular catalyst through covalent bonding.It combines the high selectivity of the Bpy-CoCl 2 molecular catalyst with the excellent stability of carbon nitride as a substrate material and exhibits up to >99% conversion and >99% selectivity in the benzyamine-to-imine reaction.This work provides an effective strategy for the highly selective conversion of benzylamine to N-benzylbenzaldimine.

1 O 2
and •OH had little effect on the reaction system.To further confirm the reactive oxygen species (ROS), in situ electron spin resonance (ESR) spectroscopy was conducted using corresponding spin-trapping agents in the presence of Bpy-CoCl 2 /CN─COOH and CN─COOH.5,5-dimethyl-pyridine-N-oxide (DMPO) was selected as trapping agents to probe •O 2 À .As shown in Figure 6b, noticeable ESR signals corresponding to •O 2 À were observed in the presence of Bpy-CoCl 2 /CN─COOH and CN─COOH by light illumination for 5 min while no signals appeared in the dark.Among them, the signal of Bpy-CoCl 2 / CN─COOH photocatalyst was significantly higher than that of pure CN─COOH, indicating that Bpy-CoCl 2 /CN─COOH produced more•O 2 À , consistent with its improved photocatalytic activity.In order to clarify the oxidative coupling of amines of the Bpy-CoCl 2 /CN─COOH photocatalyst, in situ FTIR was utilized in this work and the result is displayed in Figure 6c,d.After 5 min in simulated sunlight, the weak peak at 1718 cm À1 was ascribed to the C═O stretching vibrations of benzaldehyde and the peaks at 1686, 1673, and 664 cm À1 belonged to the stretching vibration of aldehyde group in the structure of benzaldehyde.