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. 2019 Dec 23;23(1):100793. doi: 10.1016/j.isci.2019.100793

Synergistic Effect over Sub-nm Pt Nanocluster@MOFs Significantly Boosts Photo-oxidation of N-alkyl(iso)quinolinium Salts

Shan-Shan Fu 1, Xiu-Ying Ren 2, Song Guo 1,∗, Guangxu Lan 3, Zhi-Ming Zhang 1,2,4,∗∗, Tong-Bu Lu 1, Wenbin Lin 3
PMCID: PMC6992937  PMID: 31958757

Summary

Quinolones and isoquinolones are of interest to pharmaceutical industry owing to their potent biological activities. Herein, we first encapsulated sub-nm Pt nanoclusters into Zr-porphyrin frameworks to afford an efficient photocatalyst Pt0.9@PCN-221. This catalyst can dramatically promote electron-hole separation and 1O2 generation to achieve synergistic effect first in the metal-organic framework (MOF) system, leading to the highest activity in photosynthesis of (iso)quinolones in >90.0% yields without any electronic sacrificial agents. Impressively, Pt0.9@PCN-221 was reused 10 times without loss of activity and can catalyze gram-scale synthesis of 1-methyl-5-nitroisoquinolinone at an activity of 175.8 g·gcat−1, 22 times higher than that of PCN-221. Systematic investigations reveal the contribution of synergistic effect of photogenerated electron, photogenerated hole, and 1O2 generation for efficient photo-oxidation, thus highlighting a new strategy to integrate multiple functional components into MOFs to synergistically catalyze complex photoreactions for exploring biologically active heterocyclic molecules.

Subject Areas: Catalysis, Inorganic Materials, Organic Chemistry

Graphical Abstract

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Highlights

  • •

    A state-of-the-art photocatalyst for preparation of bioactive (iso)quinolones

  • •

    Synergistic catalysis of photogenerated e−/h+ and 1O2

  • •

    Sub-nm Pt0.9@PCN-221 with a high efficiency of e−-h+ separation and 1O2 generation

Catalysis; Inorganic Materials; Organic Chemistry

Introduction

Quinolones and isoquinolones are important drug candidates due to their potent biological effects (Glushkov and Shklyaev, 2001, Carey et al., 2006, Claassen et al., 2009, Forbis and Rinehart, 1973). For instance, quinolones exhibited strong anticancer and antibiotic activities (Forbis and Rinehart, 1973, Ni et al., 2006), whereas isoquinolones have been used to treat stomach cancer and brain tumors (Glushkov and Shklyaev, 2001). As a result, significant efforts have been devoted to the synthesis of (iso)quinolones from readily available N-alkyl(iso)quinolinium salts (Fan-Chiang et al., 2016, Ishi-i et al., 2010, Khalil et al., 2016, Wang et al., 2016, Luo et al., 2016, Jin et al., 2017). A number of catalysts, including K3Fe(CN)6 (Fan-Chiang et al., 2016, Ishi-i et al., 2010, Khalil et al., 2016), copper (Wang et al., 2016), iodine (Luo et al., 2016), and eosin Y (Jin et al., 2017), have been used to convert N-alkyl(iso)quinolinium salts to (iso)quinolones. However, the practical application of these catalysts is limited by their potential toxicity, high energy consumption, poor functional group tolerance, and the difficulty in catalyst recycle and reuse. More environment-friendly heterogeneous catalysts with a broad substrate scope are needed for the facile synthesis of (iso)quinolones.

Metal-organic frameworks (MOFs) have shown great potential in photocatalytic reactions by virtue of their well-defined structures, porosity, and the ability to incorporate multiple functionalities (Zhang et al., 2017a, Zhang et al., 2017b, Wu et al., 2012, Chen et al., 2019, Liu et al., 2014, Wu and Zhao, 2017, Ji et al., 2017, Dhakshinamoorthy et al., 2018, Xia et al., 2017, Aijaz and Xu, 2014, Lee et al., 2009, Pascanu et al., 2019, Yang and Wang, 2018, Zhou et al., 2012, Saha et al., 2014, Luo et al., 2019). In particular, MOFs have provided a versatile platform to introduce multiple components, such as photosensitizers (PSs) and catalysts, for efficient conversion of solar energy via synergistic catalysis (Deng et al., 2017, Dhakshinamoorthy et al., 2018, Muzzio et al., 2019, Sun and Li, 2016, Wang and Li, 2016, Wu and Zhao, 2017, Xia et al., 2017, Yang et al., 2017). With their excellent stability and tailorability, Zr-based MOFs have been widely studied for artificial photosynthesis (Abdel-Mageed et al., 2019, Bai et al., 2016a, Chen et al., 2017, Howarth et al., 2016, Kandiah et al., 2010, Liu et al., 2018, Wang et al., 2012) and photocatalytic organic reactions (Wang et al., 2012, Paille et al., 2018, Sun et al., 2018, Xu et al., 2015) by hierarchical integration of PSs and catalysts to accelerate electron transfer and promote the separation of photogenerated electron-hole pairs (Choi et al., 2017, Paille et al., 2018, Pullen et al., 2013, Xiao et al., 2016, Yang et al., 2017, Zhang et al., 2015). However, most studies to date have focused on oxidation or reduction half-reactions in the presence of sacrificial agents (Wang et al., 2012, Xiao et al., 2016). New and exciting approaches are merging with the goal of exploiting cooperative effects between photogenerated electrons and holes to further enhance the efficacy of photocatalytic organic reactions (Liu et al., 2018, Paille et al., 2018, Wu et al., 2019).

Noble metal nanoparticles are known to efficiently trap electrons migrated from an n-type semiconductor. This Schottky effect effectively separates photogenerated electrons and holes. Electron-rich Pt nanoparticles can also activate O2 to form singlet oxygen (1O2), which exhibits high activity in oxidation reactions (Chen et al., 2017). We hypothesize that the encapsulation of Pt nanoparticles into Zr-based MOFs can not only enhance the separation of photoexcited electron-hole pairs but also boost 1O2 generation, leading to synergistic activation of organic substrates for difficult transformations.

Herein, we demonstrate for the first-time synergistic combination of photoexcited electron-hole pairs with 1O2 generated on sub-nm Pt nanoclusters (NCs) in an ultrafine Pt NC-encapsulated Zr-porphyrin MOF for photocatalytic oxidation of N-alkyl(iso)quinolinium salts to (iso)quinolones (Figures S1–S26). The sizes of Pt NCs in the MOF matrix were precisely controlled from sub-nm to 3 nm by in situ photoreduction of K2PtCl4 to afford a family of heterogeneous photocatalysts Ptx@PCN-221 (where x = 0.9, 1.6, 2.2, and 3.0 nm) with Pt NC size-dependent photocatalytic activities. For photocatalytic oxidation of N-alkylquinolinium salts, Pt0.9@PCN-221 shows the best catalytic activity among all these composite catalysts to produce quinolones in >90.0% yield after 2 h of visible light irradiation. Pt0.9@PCN-221 was reused 10 times without loss of activity and catalyzed photo-oxidation of 1-methyl-5-nitroisoquinolinium iodide in gram scale in 6 h to afford 1-methyl-5-nitroisoquinolinone at an activity of 175.8 g·gcat−1, which is 22 times higher than that of PCN-221. A series of spectroscopic and quenching studies revealed the role of synergistic electron-hole separation and 1O2 generation in achieving the extremely high activity of photocatalytic N-alkyl (iso)quinolinium by the sub-nm Pt NC@MOF composite.

Results

Synthesis and Characterization

Ultrafine Pt NCs were loaded into Zr-based porphyrin MOF (PCN-221) by in situ photoreduction of K2PtCl4 via an intermittent illumination process (Figure 1A). In this process, the sizes of MOF-encapsulated Pt NCs were readily tuned from sub-nm to 3.0 nm by controlling the loading amount of K2PtCl4 and the irradiation time. Sub-nm Pt NCs were obtained with a low K2PtCl4 loading of 9.2 mg in 14.0 mg PCN-221 and a short irradiation time of 6 s (interval of dark time of 54 s) for 20 cycles with a total irradiation time of 120 s. By increasing the total irradiation time to 360 s, 1.6-nm Pt NCs were obtained with the same K2PtCl4 loading; 3.0-nm Pt NCs were synthesized by increasing the K2PtCl4 loading to 18.3 mg with a total irradiation time to 360 s (Figure 1).

Figure 1.

Figure 1

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Preparation and Structure Characterization of Ptx@PCN-221 and Ptx@N-C

(A) Schematic illustration of the preparation of Ptx@PCN-221 composites with different sizes of Pt NCs.

(B) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Ptx@N-C with different sizes of Pt NCs; hundreds of NCs were counted to get the size distribution.

(C) Elemental mapping images of Pt0.9@N-C corresponding to Pt, C, N, and O elements, respectively.

(D) Simulated and experimental PXRD patterns of Pt nanoparticles, PCN-221, and Ptx@PCN-221 composites.

(E) N2 adsorption-desorption isotherms of Ptx@PCN-221 obtained at 77 K.

High-resolution scanning electron microscopic images of both PCN-221 and Ptx@PCN-221 showed an irregular octahedral shape (Figure S27). Powder X-ray diffraction of Ptx@PCN-221 samples exhibited identical patterns to that simulated for PCN-221, indicating that the structure and crystallinity of PCN-221 are maintained upon encapsulating Pt NCs (Figure 1D). It is worth noting that no diffraction peaks from Pt NCs were detected, likely as a result of ultrafine Pt NCs inside the PCN-221 framework. Taking Pt1.6@PCN-221 as an example, high-resolution transmission electron microscopic studies revealed that all NCs uniformly distribute in the PCN-221 skeleton with an average size of 1.6 nm (Figure S28E). To further clarify the structure of Pt NCs, Ptx@PCN-221 samples were calcined at 500°C to transform organic components in the MOF into N-doping porous carbon (N-C) (Cao et al., 2017), and Zr was removed from the support with HF acid etching to eliminate the interference of Zr in high-angle annular dark-field imaging scanning transmission electron microscopic (HAADF-STEM) imaging studies (Figures 1B and S28–S30). The content of Pt was determined as 2.83% and 2.81% before and after the removal of Zr, respectively. Pt NCs were clearly visible on the N-C support, and their average sizes are determined as 0.9, 1.6, 2.2, and 3.0 nm, respectively, with a narrow size distribution (Figures 1B and S29). Energy-dispersive X-ray spectroscopic elemental mapping showed that N, C, and O were homogeneously distributed around N-C, whereas Pt clusters were clearly observed on the N-C support (Figure 1C). In addition, the diameter of polyhedral cages of PCN-221 was ∼2.0 nm, which was smaller than that of 2.2 and 3.0 nm Pt NCs. This can be attributed to partial MOF framework distortion during Pt NC formation. Moreover, the diameter of the cage opening was around 0.8 nm, manifesting that 0.9-nm Pt NCs can be trapped into PCN-221.

In addition, the loadings of Pt were determined as 2.59% in Pt0.9@PCN-221, 2.79% in Pt1.6@PCN-221, 1.15% in Pt2.2@PCN-221, and 8.43% in Pt3.0@PCN-221, by inductively coupled plasma-mass spectrometry. N2 sorption isotherms at 77 K showed that the pore diameter and pore volumes of Ptx@PCN-221 composites are much smaller than those of pristine PCN-221, consistent with the encapsulation of Pt NCs in the cages of the MOFs (Figure 1E) (Chen et al., 2019). Among these composites, Pt0.9@PCN-221 exhibits larger Brunauer-Emmett-Teller surface area and pore volume than other composites with larger particle size Pt NCs (Table S1).

Separation of Photogenerated Electron-Hole Pairs

Efficient electron-hole separation is key to realizing photocatalytic reactions (Xiao and Jiang, 2019, Kong et al., 2018). The charge separation efficiency of different Ptx@PCN-221 samples was studied in detail by absorption spectroscopy and photoluminescence (PL) spectroscopy (Figures 2 and S31). UV-visible spectra of Ptx@PCN-221 composites showed essentially the same absorption features in the 300–700 nm range as PCN-221. With decreasing size of Pt NCs, the scattering at >700 nm becomes weaker, likely due to spatial separation between the MOF skeleton and small NCs (Xiao et al., 2016, Sarina et al., 2014). PL spectroscopy provided important insights into the photo-induced charge transfer processes in Ptx@PCN-221 (Sun et al., 2018). The steady-state PL intensity decreased in the order PCN-221 > Pt3.0@PCN-221 > Pt1.6@PCN-221 > Pt0.9@PCN-221, indicating that the manipulation of Pt-MOF interactions by reducing the size of NCs can efficiently suppress the radiative electron-hole recombination process. Pt0.9@PCN-221 showed the highest efficiency of charge separation among all the samples. Furthermore, the photogenerated charge separation dynamics was investigated by time-resolved PL (Figure 2C). The luminescence lifetime of Ptx@PCN-221 became shorter than that of PCN-221 composite, indicating an efficient electron transfer from PCN-221 to Pt NCs. As the size of Pt NCs decreased, the decay became faster, manifesting that ultrafine Pt NCs greatly contributed to the separation of photogenerated electron and hole. These observations were supported by photocurrent measurements (Figure 2D). The photocurrent of the composite becomes larger with the decreasing size of Pt NCs, indicating that ultrafine NCs, especially for sub-nm NCs, can efficiently extract electrons from photoexcited PCN-221. In addition, electrochemical impedance spectroscopy results revealed that the radius of capacity impedance correlates with the size of NCs. Pt0.9@PCN-221 exhibits the smallest radius, implying its lowest charge transfer resistance when compared with other composites. All these results indicate that Pt0.9@PCN-221 possesses the highest efficiency of photo-induced electron-hole separation among the Ptx@PCN-221 samples.

Figure 2.

Figure 2

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Photogenerated Electron-Hole Separation of Composites

(A) UV-visible diffuse reflectance spectra.

(B) PL emission spectra (excited at 410 nm).

(C) Time-resolved photoluminescence decays of PCN-221 and Ptx@PCN-221.

(D and E) (D) Photocurrent responses and (E) electrochemical impedance spectroscopy Nyquist plots for PCN-221 (black), Pt0.9@PCN-221 (blue), Pt1.6@PCN-221 (magenta), and Pt3.0@PCN-221 (olive).

(F) Oxidation state distribution of Pt determined by X-ray photoelectron spectroscopy measurements of Pt NCs with various sizes.

X-ray photoelectron spectroscopy was used to determine the valence states of Pt NCs with different sizes (Figure S32) (Wang et al., 2013). As shown Figure 2F, Pt NCs show a combination of three valence states of Pt0, Pt2+, and Pt4+. The ratio of high-valence Pt centers increased with decreasing Pt NC particle size (Bai et al., 2016b; Wang et al., 2013). The smaller Pt NCs with more high-valence Pt centers possess better ability to accept electron from exited PCN-221, which can promote photogenerated electron-hole separation of PCN-221. The electron-rich Pt NCs will facilitate 1O2 generation (Chen et al., 2017). As a result, the enhanced efficiency of charge separation for small Pt NCs can thus be attributed to two reasons. (1) Proximity between ultrafine Pt NCs and the MOF framework facilitates electron injection from photoexcited PCN-221 to Pt NCs. (2) Higher-valence Pt centers in smaller Pt NCs endow them with stronger ability to extract electrons from photoexcited PCN-221, which can promote the photocatalytic activity.

Photo-oxidation of N-Alkyl(iso)quinolinium Salts

We next examined the photocatalytic activity of Ptx@MOF composites with different cluster sizes. In a typical photocatalytic reaction, 2 mg Ptx@MOF was used to catalyze the oxidation of 0.4 mmol N-methylquinolinium iodide or N-alkylisoquinoline salts in tetrahydrofuran (THF) (4 mL) in the presence of 0.6 mmol Cs2CO3 with irradiation by a 425-nm light-emitting diode. As shown in Table 1, the decreasing size of Pt NCs increased the photocatalytic performance; the yield for the quinolinone product increased from 55.3% for Pt3.0@PCN-221 to 92.8% for Pt0.9@PCN-221 in 2 h. This photocatalytic activity is much enhanced compared with that of PCN-221 (25.6%). To optimize the condition, the catalytic performance using Cs2CO3, K2CO3, K3PO4, and Na2CO3 as bases were compared under similar condition (Table 1). These results indicate that the photocatalytic activity with Cs2CO3 is much better than that with other bases. It should be noted that the lower yield of Pt2.2@PCN-221 compared with Pt0.9@PCN-221 could be attributed to its low content of Pt. Furthermore, no quinolinone was detected without light irradiation or in the absence of air, indicating that both of them are necessary for the photo-oxidation of N-methylquinolinium iodide. Consistent with this, photo-oxidation reaction in oxygen atmosphere further increased the quinolinone yield to 95.8%. As Pt0.9@PCN-221 was the most efficient catalyst among the composites, it was used for subsequent studies. In addition, the photocatalytic experiments were also performed with the same amount of Pt for comparing the mass-specific activity of Pt in these composites (Table S2). Their photocatalytic activities were in the order of Pt0.9@PCN-221 > Pt1.6@PCN-221 > Pt2.2@PCN-221 > Pt3.0@PCN-221, highlighting that Pt0.9@PCN-221 represents the most efficient photocatalyst for photo-oxidation of N-alkyl(iso)quinolinium salts among these composites.

Table 1.

Visible-Light-Driven Aerobic Oxidation of 1-Methylquinoline Iodide (1a) to 1-Methyl-2-quinolinone (2a) with Ptx@PCN-221 as Catalysts

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Entry Catalyst t/h Yield/%a
1 PCN-221 2 25.6
2b Pt0.9@PCN-221 2 92.8
3c Pt0.9@PCN-221 2 91.5
4d Pt0.9@PCN-221 2 64.3
5e Pt0.9@PCN-221 2 41.5
3b Pt1.6@PCN-221 2 83.6
4b Pt2.2@PCN-221 2 30.1
5b Pt3.0@PCN-221 2 55.3
6b Pt0.9@PCN-221 6 (14) 54.5 (86.3)f
7b PCN-221 6 (14) 2.4 (5.3)f
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Reaction conditions: 1a (0.4 mmol); photocatalysts (2 mg) with 0.068 mol %, 0.073 mol %, 0.030 mol %, and 0.22 mol % Pt versus 1a for Pt0.9@PCN-221, Pt1.6@PCN-221, Pt2.2@PCN-221, and Pt3.0@PCN-221, respectively; Cs2CO3 (0.6 mmol); THF (4 mL); air; room temperature; irradiation with 425-nm LED in a 5-mL glass tube.

a

Isolated yield.

b

Cs2CO3.

c

K2CO3.

d

K3PO4.

e

Na2CO3.

f

Reaction conditions: 1b (1 g), 2 mg Pt0.9@PCN-221 (0.0084 mol% Pt versus 1b), Cs2CO3 (0.9 mmol), THF (6 mL), air, room temperature, irradiation with 425-nm LED.

We further showed that Pt0.9@PCN-221 could be reused by simple centrifugation without further activation and its photocatalytic activity remained constant for 10 cycles (Figures 3 and S33A) (Liu et al., 2019). HAADF-STEM imaging further revealed that the sizes of Pt NCs remained around 0.9 nm after five cycles of photocatalytic reactions (Figures 3C and 3D). PXRD pattern and shape of Pt0.9@PCN-221 also remained unchanged after photocatalytic reactions (Figures 3B and S27). These results indicate that Pt0.9@PCN-221 is a robust and recyclable photocatalyst for photo-oxidation of N-alkylquinolinium iodide. We also demonstrated gram-scale synthesis of 1-methyl-5-nitroisoquinolinone using Pt0.9@PCN-221 photocatalyst under visible-light irradiation (Table 1). A catalytic activity of 175.8 g·gcat−1 was achieved within 6 h toward 1-methyl-5-nitroisoquinolinone, over 22 times higher than that of PCN-221 (7.8 g·gcat−1). As the reaction prolonged to 14 h, the yield of 1-methyl-5-nitroisoquinolinone reached 86.3%, over 16 times higher than that of PCN-221 (5.3%). Furthermore, we performed the recycle experiments in gram-scale reaction with 5 mg catalyst. As shown in Figure S33B, the yield of 1-methyl-5-nitroisoquinolinone remained over 90% after being reused three times, highlighting the excellent stability of Pt0.9@PCN-221 for photosynthesis. The excellent photocatalytic stability of Pt0.9@PCN-221 could be attributed to the encapsulation of Pt NCs into the cages of PCN-221, which can be used to well isolate Pt NCs to prevent the aggregation of entrapped NCs in the photocatalytic process.

Figure 3.

Figure 3

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Recyclability of Pt0.9@PCN-221

(A) Recyclability of Pt0.9@PCN-221 for photo-oxidation of 1a.

(B) Predicted and experimental PXRD patterns of Pt0.9@PCN-221 before and after the catalytic reaction.

(C and D) (C) HAADF-STEM image and (D) size distributions of Pt NCs after catalytic reaction; hundreds of NCs were counted to get the distribution.

Photocatalytic Mechanism

The mechanism of photo-oxidation by Pt0.9@PCN-221 was studied in detail using 1a as substrate (Figures 4 and S34–S38). As shown in Figure 4A, the electron spin resonance (ESR) signals of PCN-221 detected in situ showed a light-dark difference that is consistent with the generation of porphyrin π-cation radical (Xu et al., 2018). Pt0.9@PCN-221 showed similar ESR signals as PCN-221 but with a much larger light-dark difference (Figure 4A), suggesting much enhanced production of porphyrin π-cation radical in Pt0.9@PCN-221 over PCN-221 due to electron transfer from photoexcited porphyrin to Pt NCs. To confirm this proposal, triethanolamine (TEOA), as a hole scavenger, was intentionally added to the photocatalytic reaction to quench porphyrin π-cation radicals (Xu et al., 2015, Yuan et al., 2017). As shown in Figure S35, the yield of 2a drastically decreased from 92.8% to 13.5% when the TEOA concentration increased from 0 M to 1.8 M. This result further indicates the important role of photogenerated holes in reacting with intermediates to facilitate complex catalytic reactions.

Figure 4.

Figure 4

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Photocatalytic Mechanism

(A) ESR detection of porphyrin π-cation radical in PCN-221 and Pt0.9@PCN-221.

(B) ESR spectra of the solution containing 4-oxo-TMP and PCN-221/Pt0.9@PCN-221 with/without carotene under visible-light irradiation or in dark.

(C) Proposed mechanism for visible-light-driven photo-oxidation of N-methylquinolinium iodide. Ef stands for the Fermi level of Pt NC; FL is the fluorescence.

The increased yield of 2a in the O2 atmosphere suggests that the oxidative species comes from O2. To ascertain the key oxidant in photocatalytic process, 4-oxo-2,2,6,6-tetramethyl-4-piperidone (4-oxo-TMP) and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were employed to identify the production of 1O2 and other reactive oxygen species (Figures 4, S37, and S38) (Konaka et al., 1999). In the presence of 4-oxo-TMP with Pt0.9@PCN-221 as the catalyst, the ESR spectra display a 1:1:1 triplet signal that is consistent with the generation of 1O2 (Figure 4B) (Zhou et al., 2012). This triplet signal was almost completely quenched by carotene, a well-known scavenger for 1O2 via triplet-triplet energy transfer mechanism, confirming that the ESR signal originates from 1O2 (Badwey and Karnovsky, 1980). This ESR signal was absent under dark condition, indicating that light was a prerequisite for 1O2 generation. In comparison, PCN-221 just showed a weak ESR signal for 1O2 generation due to its poor ability to produce 1O2.

The generation of 1O2 was supported by PL spectroscopy. As shown in Figure S36, all the samples exhibited an emission peak at 1,265 nm corresponding to characteristic emission of 1O2 and the emission intensity decreased in the order of Pt0.9@PCN-221 > Pt1.6@PCN-221 > Pt3.0@PCN-221 > PCN-221, which correlates with their photocatalytic activity (Table 1) (Wang et al., 2015). Notably, the signal of 1O2 significantly decreased after the addition of 1a to the above solution, suggesting the consumption of 1O2 by the substrate (Figure S37A). Furthermore, the yield of 2a significantly decreased from 92.8% to 28.9% with the addition of carotene, supporting the role of 1O2 as the oxidant in the photo-oxidation process (Figure S37B). In addition, the potential generation of O2·- and ·OH by Pt0.9@PCN-221 was ruled out as no ESR signal was observed upon light irradiation when DMPO was used as a radical-trapping agent (Figure S38). These results thus demonstrate that 1O2 is efficiently generated in the Pt0.9@PCN-221 photocatalytic system and acts as a predominant oxidant to promote the photo-oxidation reaction (Ogilby, 2010, Montagnon et al., 2008).

Finally, an intermediate radical in the photocatalytic reaction was detected by introducing DMPO into the mixture of Pt0.9@PCN-221 and Cs2CO3 in the presence of substrate 1a. As shown in Figure S38, a sextet signal was generated upon excitation at 425 nm, which can be assigned to an alkyldioxyl DMPO radical adduct (Jin et al., 2017, Qian et al., 2017). Thus, this alkyldioxyl radical is proposed as intermediate II in this photocatalytic system (Figure 4C).

On the basis of the above-mentioned results, we propose the photocatalytic mechanism for Pt0.9@PCN-221 as shown in Figure 4C. Upon excitation with visible light, electron-hole separation occurs on the porphyrin ligand. The electron is quickly transferred to Pt NCs to promote 1O2 generation with the subsequent single electron transfer (SET) to 1a to afford radical I, which is further oxidized by 1O2 to afford alkyldioxyl radical intermediate II. Intermediate II is transformed into intermediate III via a hydrogen rearrangement process. The porphyrin π-cation has strong oxidation capacity to receive an electron from III via another SET process to produce cationic intermediate IV. IV can be easily transformed into the final product V by reacting with Cs2CO3 and I−. The Pt NC and PCN-221 framework thus synergistically promote electron-hole separation and 1O2 generation to greatly boost photo-oxidation of N-alkyl(iso)quinolinium salts to biologically active (iso)quinolinones.

Substrate Scope

Encouraged by the high activity of Pt0.9@PCN-221 in photocatalytic oxidation of 1a, we examined the substrate scope with various N-methylquinolinium and N-alkylisoquinolinium salts (Table 2). Most of these reactions were highly efficient with the isolation of target products in >80.0% yields within 3 h. The substrates with electron-donating groups appear to exhibit a lower reactivity than those with electron-withdrawing groups. For example, relatively electron-rich substrate 2f with a large size shows a moderate reactivity to produce the corresponding quinolinone in 69.5% yields (Wu et al., 2012, Saito et al., 2013). The broad substrate scope and good functional group tolerance indicate that Pt0.9@PCN-221 is indeed a state-of-the-art and robust catalyst with a unique mechanism for the photocatalytic oxidation of N-alkyl(iso)quinolinium salts.

Table 2.

Visible Light-Driven Photo-oxidation of N-alkyl(iso)quinolinium Salts

Inline graphic
Entry Product t/ha Yield/%b
1 graphic file with name fx4.gif 2 92.8
2 graphic file with name fx5.gif 1.5 93.4
3 graphic file with name fx6.gif 2 91.6
4 graphic file with name fx7.gif 3 80.6
5 graphic file with name fx8.gif 2 88.3
6 graphic file with name fx9.gif 2.5 69.5
7 graphic file with name fx10.gif 2 95.3
8 graphic file with name fx11.gif 1.5 91.2
9 graphic file with name fx12.gif 1.5 94.7
10 graphic file with name fx13.gif 2 81.2
11 graphic file with name fx14.gif 2 86.2
12 graphic file with name fx15.gif 2 82.1
13 graphic file with name fx16.gif 3 90.5
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Reaction conditions: 1a–1f or 3a–3g (0.4 mmol); Pt0.9@PCN-221 (0.068 mol%); Cs2CO3 (0.6 mmol); THF (4 mL); the mixture was irradiated with 425-nm LED in the air at room temperature.

a

Reaction time.

b

Yield of isolated products. X = I for 1a–f, 3a, 3c, and 3g; X = Br for 3b, 3d, and 3e.

Discussion

In this work, we encapsulated ultrafine Pt NCs into PCN-221 by in situ photoreduction of K2PtCl4 to afford a family of heterogeneous photocatalysts Ptx@PCN-221 (where x = 0.9, 1.6, 2.2, and 3.0 nm) with prominent Pt NC size-dependent catalytic activities in photo-oxidation of N-alkyl(iso)quinolinium salts. Synergistic combination of photoexcited electron-hole separation with 1O2 generation in Ptx@PCN-221 greatly contributes to the excellent performance in (iso)quinolone synthesis from N-alkyl(iso)quinolinium salts. Among these composites, Pt0.9@PCN-221 with sub-nm Pt NCs exhibits the best synergy of electron-hole separation and 1O2 generation, leading to the highest activity of N-alkyl(iso)quinolinium photo-oxidation to produce (iso)quinolones in >90% yields. The heterogeneous photocatalyst was readily reused 10 times without a loss of catalytic activity. Thus, the photo-oxidation reaction can be easily extended to gram-scale synthesis of 1-methyl-5-nitroisoquinolinone at a catalytic activity of 175.8 g·gcat−1, 22 times higher than that of PCN-221. Spectroscopic evidences and quenching results support the role of synergistic electron-hole separation and 1O2 generation in photocatalytic N-alkyl (iso)quinolinium oxidation by the Ptx@MOF composite. This work thus presents a new strategy to integrate multiple functional components into MOF-based composite materials to synergistically catalyze complex photoreactions for the synthesis of biologically active heterocyclic molecules.

Limitations of the Study

In this article, we use sub-nm Pt nanocluster@MOFs as photocatalyst for efficient preparation of (iso)quinolones. However, the photocatalytic reaction was performed in THF, which is against the concept of green chemistry. In future work, the green solvent will be investigated.

Methods

All methods can be found in the accompanying Transparent Methods supplemental file.

Acknowledgments

This work was supported by National Key R&D Program of China (2017YFA0700104) and the National Natural Science Foundation of China (No. 21722104, 21671032, 21703155), Natural Science Foundation of Tianjin City of China (18JCQNJC76500, 18JCJQJC47700, 17JCQNJC05100), and 111 Project of China (No. D17003).

Author Contributions

S.G. and Z.-M.Z. conceived and designed this project; S.-S.F. and X.-Y.R. performed the experiments; S.-S.F., S.G., and Z.-M.Z. analyzed the data; S.-S.F., S.G., G.L., Z.-M.Z., T.-B.L., and W.L. wrote and revised the article.

Declaration of Interests

The authors declare no competing interests.

Published: January 24, 2020

Footnotes

Supplemental Information can be found online at https://doi.org/10.1016/j.isci.2019.100793.

Contributor Information

Song Guo, Email: guosong@email.tjut.edu.cn.

Zhi-Ming Zhang, Email: zmzhang@email.tjut.edu.cn.

Supplemental Information

Document S1. Transparent Methods, Figures S1–S38, and Tables S1 and S2
mmc1.pdf (3.3MB, pdf)

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Associated Data

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Supplementary Materials

Document S1. Transparent Methods, Figures S1–S38, and Tables S1 and S2
mmc1.pdf (3.3MB, pdf)

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