A tetrazole-containing triphenylamine-based metal–organic framework: Synthesis and photocatalytic oxidative CC coupling reaction

Inorganic Chemistry Communications 105 (2019) 9–12

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Short communication

A tetrazole-containing triphenylamine-based metal–organic framework: Synthesis and photocatalytic oxidative CeC coupling reaction

T

Dongying Shia, , Xiangyang Guob, Tianhua Laia, Kaijun Zhenga, Qiuyu Wua, Chenyi Suna, ⁎ ⁎ Cheng Heb, , Junwei Zhaoc, ⁎

a

Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry, Zhengzhou 450002, PR China State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, PR China c Henan Key Laboratory of Polyoxometalate Chemistry, Henan University, Kaifeng 475004, PR China b

GRAPHICAL ABSTRACT

A novel tetrazole-containing triphenylamine-based MOF {TBA[Cu2(TPA)]}·CH3CN has been prepared, which exhibits remarkable photocatalytic activities for the oxidative CeC coupling reaction under mild conditions.

ARTICLE INFO

ABSTRACT

Keywords: Photocatalysis Coupling reaction Metal–organic framework Triphenylamine-based ligand

A novel tetrazole-containing triphenylamine-based MOF {TBA[Cu2(TPA)]}·CH3CN (Cu–TPA; TBA = tetrabutylammonium cation) has been solvothermally synthesized by the reaction of CuCl2·2H2O, tris(4(2H-tetrazol-5-yl)phenyl)amine (H3TPA) and (TBA)4[W10O32] and structurally characterized by IR spectrum, UV–Vis spectrum, fluorescence analysis, powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction. More interestingly, Cu–TPA exhibits remarkable heterogeneous photocatalytic property and promotes the oxidative CeC coupling reaction under mild conditions. The high catalytic efficiency, high stability and good recyclability of the photocatalyst Cu–TPA demonstrate the superiority of the tetrazole-containing triphenylaminebased MOF over homogeneous systems and other noble-metal-catalyzed methods.

Transition-metal-catalyzed cross-coupling reactions through the CeH activation and subsequent CeC formation exhibit a highly efficient and atom-economic organic transformation, which does not require pre-functionalization of the substrates and does not produce undesired byproducts [1,2]. Recent investigations also reveal that the

⁎

visible-light-mediated photoredox catalysis is a promising approach to such reaction sequences with respect to the development of new sustainable and green synthetic methods [3]. The photocatalytic oxidation of sp3 CeH bonds adjacent to nitrogen atoms in tertiary amines represents the valuable and highly reactive iminium ion intermediates

Corresponding authors. E-mail addresses: [email protected] (D. Shi), [email protected] (C. He), [email protected] (J. Zhao).

https://doi.org/10.1016/j.inoche.2019.04.021 Received 26 February 2019; Received in revised form 12 April 2019; Accepted 14 April 2019 Available online 18 April 2019 1387-7003/ © 2019 Elsevier B.V. All rights reserved.

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D. Shi, et al.

Fig. 1. (a) The coordinate environment of Cu1 ions in Cu–TPA [Symmetry operation: (A): 1 – x, y, 1.5 – z]. (b) View of the 3D anionic framework of [Cu2(TPA)]− along the c direction. (c) View of the 3D framework of Cu–TPA (TBA drawn as green). (d) The 2-nodal (3,5)-connected topological network for Cu–TPA. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

that can be used for further functionalization, which is a powerful strategy for CeC bond formation [4,5]. Bearing in mind the use of toxic, expensive, and irrecoverable noble-metal polypyridyl complexes as photocatalysts in the reported procedures [6–8], it is highly desirable to develop the CeC coupling reactions in the presence of heterogeneous photocatalysts constructed from earth-abundant elements and less harmful organic ligands. Metal–organic frameworks (MOFs) are hybrid solids with infinite network structures built from inorganic connecting nodes and organic bridging ligands [9]. The tenability and flexibility of MOFs granted by the diversity of their building blocks allow the incorporation of photoactive organic ligands and the necessary adducts into one single framework, which represents a new approach to heterogenizing photocatalytic transformations [10,11]. The regular distribution of catalytic sites within the confined micro-environment benefits the fixation and stabilization of the active intermediates formed under irradiation to overcome restrictions of homogeneous processes [12]. To achieve precise control of the shifting energy levels of the ground and excited states and the enhancement of the visible-light harvesting ability of MOFs, continuous structural modulation of a dye-based ligand scaffold is highly desirable, however, it is still in its infancy [13]. Recently, we have launched the study on the reaction of tetrazolecontaining triphenylamine-based ligand tris(4-(2H-tetrazol-5-yl) phenyl)amine (H3TPA) with copper metal cations to synthesize the tetrazole-containing triphenylamine-based MOFs for the photocatalytic CeC coupling reaction based on the following aspects: (1) MOFs containing the tetrazole ring as a multiple-dentate N-donor ligand have significantly higher structural and thermal stabilities than pyridyl analogues [14–16]. The increased stability of tetrazolate-containing frameworks partly stems from the present of multiple-metal-coordinated sites, where the anionic group formed by the deprotonation of tetrazole ring results in an optimal N-donor ligand that generates strong coordination bonds with metal cations [17]. (2) A well-known photo-responsive triphenylamine-based ligand has been successfully incorporated into MOFs for photocatalytic CeC coupling reactions [18], and its three aryl moieties with their C3-symmetry provide potential sites for iterative decoration to achieve continuous modulation of the photoelectronic properties [19]. (3) Copper(I) complexes as

earth-abundant and relatively low-cost materials have been characterized by the d10 electronic configuration and show the relatively intense luminescent behavior [20]. Herein, we report a tetrazole-containing triphenylamine-based MOF, {TBA[Cu2(TPA)]}·CH3CN (Cu–TPA; TBA = tetrabutylammonium cation), which exhibits a remarkable photocatalytic activity for the CeC coupling reaction of N-aryl-tetrahydroisoquinoline and nitromethane with excellent yield and outstanding durability. The orange block crystals of Cu–TPA were synthesized by a solvothermal reaction of CuCl2·2H2O, H3TPA and (TBA)4[W10O32] at pH 2.5 with a yield of 46% [21]. Elemental analyses and powder X-ray diffraction (PXRD) reveal the pure phase of bulk sample. Single-crystal structural analysis indicates that Cu–TPA crystallizes in a monoclinic space group C2/c [22]. The crystallographically independent Cu(I) cation adopts a four-coordinate tetrahedral geometry with four nitrogen atoms from four TPA ligands. Two symmetry-equivalent Cu(I) ions are linked by three TPA ligands to form a binuclear {Cu2} cluster (Fig. 1a). Moreover, these symmetry-equivalent Cu(I) ions are connected by three nitrogen atoms (N2, N3 and N4) from TPA ligands to produce a 2D sheet (Fig. S1); adjacent sheets are further coupled together by N6 atoms of TPA ligands to generate the 3D anionic framework of [Cu2(TPA)]− with embedded TBA cations and CH3CN molecules (Fig. 1b and c). The exchanged experiment of TBA and the assessment of guest-accessible volume in MOFs can be reliably done by using confocal fluorescence microscopy with a tool-box of basic orange 14 dye (Fig. S8) [23]. In addition, inorganic Na+- and Li+-exchanged studies indicate that TBA organic cations in the cavities can be fully exchanged at room temperature after 48 h, as proven by elemental analyses and atomic absorption experiments [24]. From the topological viewpoint, each {Cu2} cluster can be regarded as a 5-connected node and each TPA ligand as a 3-connected node. Thus, the framework of Cu–TPA can be simplified as a 2-nodal (3,5)-connected network (Fig. 1d). The IR spectrum of Cu–TPA has been recorded between 4000 and 400 cm−1 with KBr pellets (Fig. S4). In the high-wavenumber region, three vibration bands observed at 3600–3275 cm−1, 3096–2990 cm−1 and 2986–2670 cm−1 are attributed to the ν(NeH) and ν(CeH) stretching vibration of tetrazole groups, benzene rings and TBA cations, 10

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Table 1 Recycle experiments and control experiments for the photocatalytic CeC coupling reaction of N-phenyltetrahydroisoquinoline and nitromethane.

Entry

Photocatalyst

State

Yield (%)

1

Cu–TPA (Round 1)

Heterogeneous

79

2

Cu–TPA (Round 2)

Heterogeneous

78

3

Cu–TPA (Round 3)

Heterogeneous

76

4

Cu–TPA (No Light)

Heterogeneous

<5

5

No Catalyst

Homogeneous

17

6

H3TPA, CuCl

Homogeneous

38

Reaction conditions: N-phenyl-tetrahydroisoquinoline (0.2 mmol), 3.0 mol% catalyst, 2.0 mL nitromethane, irradiation by 26 W fluorescent lamp for 30 h. Yield after column chromatography.

respectively [25]. In the low-wavenumber region, the combined vibration modes of benzene rings and tetrazole groups deformation appear. The strong characteristic bands at 1705–1190 cm−1 can be assigned to the ν(C=C), ν(C=N), and ν(N=N) stretching vibration of tetrazole groups and benzene rings [26]. Due to the coexistence of benzene groups and tetrazole groups, it is difficult to differentiate the contribution of benzene groups from that of tetrazole groups. In addition, the signals at 1190–500 cm−1 respond to the ν(CeC) and ν(CeN) stretching vibration as well as δ(NeH) and δ(CeH) bending vibration [27]. In short, these results of IR spectrum of Cu–TPA are well consistent with the single-crystal structural analysis. Compared with the band of the free ligand H3TPA, the UV–Vis absorption spectrum of Cu–TPA in the solid state displays a wide absorption band centered at 350 nm, which is typically assigned to the overlapping π to π* transitions of the aromatic triarylamine core (Fig. S5) [17]. In addition, the absorption at 710 nm corresponds to the effect of coordinated copper(I) cations on the excited state of the ligand, d-d transitions of copper(I) cations, and π-stacking of TPA ligands [28]. The

3D framework of Cu–TPA exhibits the absorption spectrum in the UV–Vis region that partially overlaps the solar emission spectrum, opening the possibility to carry out benign solar-photoassisted applications [29]. The photoluminescence property of Cu–TPA was studied at room temperature. Upon excitation at the absorption band (Ex = 310 nm, and Ex = 390 nm), Cu–TPA shows an intense luminescence band at about 480 nm, which should be ascribed to the intra-ligand emission (Figs. S6 and S7) [30]. The observed red shifts of the emission maximum between Cu–TPA and H3TPA are considered to mainly originate from the coordination interactions between the metal atoms and the ligands [31]. N-aryl-tetrahydroisoquinoline, one of the most common substrate in CeC coupling reactions, represents a significant contribution in many important chemical syntheses and potential biological applications [32]. The photocatalytic transformation was initially examined by using N-phenyl-tetrahydroisoquinoline and nitromethane as the CeC coupling partners, Cu–TPA as the excellent photocatalyst, oxygen as the oxidizing agent, and a common fluorescent lamp (26 W) as the light source. The result reveals the successful execution of our tetrazolecontaining triphenylamine-based MOF design, showing a high reaction efficiency (Table 1, entry 1). Control experiments show that only a small amount of product is generated in the dark, and low yield of the background reaction is observed in the absence of Cu–TPA catalyst under the measured conditions (Table 1, entries 4–5). When the reaction system contained the mixture of metal salt with the ligand, low yield is also observed under the same conditions (Table 1, entry 6), which confirm that Cu–TPA is a true heterogeneous catalyst in the photocatalytic CeC coupling reaction. Furthermore, the stability of Cu–TPA for the photocatalytic transformation could be confirmed by recycling experiments. Solids of Cu–TPA were isolated from the reaction suspension by simple filtration alone and reused at least three times with insignificant loss of activity (Table 1, entries 1–3). The PXRD patterns of Cu–TPA filtrated off from the reaction mixture suggest a well maintenance of the crystallinity (Fig. 2). Additionally, the scope of photocatalytic CeC coupling reaction has been expanded by using more starting materials with different substituents (Table S1, entries 1–3). The high catalytic efficiency of Cu–TPA for photocatalytic CeC coupling reaction reveals the potential advantage of tetrazole-containing triphenylamine-based MOF. Based on

Fig. 2. PXRD patterns of Cu–TPA (the simulated, as-synthesized and recovered from the photocatalytic reaction). 11

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previous related reports, a possible reaction mechanism is proposed as outlined (Scheme S1). First, a photoinduced electron transfer from Naryl-tetrahydroisoquinoline to the excited state of Cu–TPA* gives an aminyl cation radical, through H-abstraction of the sp3 CeH adjacent to nitrogen catalyzed by Cu–TPA, which then losts a hydrogen atom by a radical anion to generate an iminium ion intermediate [33]. Subsequently, trapping of iminium ions with pronucleophiles gives the corresponding CeC coupling product. In summary, we have developed a promising tetrazole-containing triphenylamine-based MOF of Cu–TPA via in situ synthesis. Under visible-light irradiation, Cu–TPA exhibits remarkable heterogeneous photocatalytic property and promotes the oxidative CeC coupling of Naryl-tetrahydroisoquinoline with nitromethane under mild conditions, which can offer an environmentally-friendly route for widening the scope of CeC formation in both laboratory and industry. Further studies to extend the scope of organic transformations using Cu–TPA as the photocatalyst is ongoing in our laboratory.

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Acknowledgments We are grateful to the support from the National Natural Science Foundation of China (21701147), State Key Laboratory of Fine Chemicals (KF1701), Open Research Fund of Henan Key Laboratory of Polyoxometalate Chemistry (HNPOMKF1604), and the Startup Fund for PhDs of Natural Scientific Research of Zhengzhou University of Light Industry (2016BSJJ026).

[20] [21]

Appendix A. Supplementary material [22]

CCDC 1892290 for Cu–TPA contains the supplementary crystallographic data for this paper. This data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam. ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j. inoche.2019.04.021 References

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