Cationic Polycarbazole Networks as Visible-Light Heterogeneous

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Cationic Polycarbazole Networks as Visible-Light Heterogeneous Photocatalysts for Oxidative Organic Transformations Haipeng Liang, Qi Chen, and Bao-Hang Han ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04494 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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ACS Catalysis

Cationic Polycarbazole Networks as Visible-Light Heterogeneous Photocatalysts for Oxidative Organic Transformations Hai-Peng Liang,a,b Qi Chen,*,a Bao-Hang Han*,a,b

a

CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS

Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China b

Sino-Danish Center for Education and Research, University of Chinese

Academy of Sciences, Beijing 100190, China

Tel: +86 10 8254 5576; Email: [email protected] Tel: +86 10 8254 5708; Email: [email protected]

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ABSTRACT: Photoredox catalysis has aroused great interest of chemists as it offers a powerful tool to organic synthesis. Cationic polycarbazole networks (CPOP-28 and CPOP-29) were prepared via simple oxidative coupling reactions and applied as heterogeneous photocatalysts for a wide range of oxidative organic transformations including

oxidation

of

sulfides,

hydroxylation

of

arylboronic

acids,

and

cross-dehydrogenative coupling reactions in the presence of visible light and air. Remarkably, photocatalytic activities are enhanced by ingenious introduction of trifluoromethyl group to the polymeric network. The effects of trifluoromethyl group on photocatalytic activities were elucidated in terms of photophysical and electrochemical properties. The appealing photocatalytic performance of trifluoromethylated polymer is ascribed to superior light-absorption capability, longer fluorescence lifetime, and more oxidative photoexcited state. In addition, the photocatalysts showed good recyclability and could be reused after simple separation workup.

KEYWORDS: cationic, polycarbazole, trifluoromethyl group, heterogeneous catalyst, photocatalyst

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INTRODUCTION In face of ever-increasing global demands on fossil fuels, solar energy as an abundant and renewable resource from nature has been exploited for supply of chemical energies

1

and to actuate artificial photosynthesis.2,3 The use of light to promote organic

transformations has attracted great attention over the past decades. As yet, a diversity of photocatalysts including inorganic semiconductors,4 organic dyes,5 and transition-metal complexes

6

have been developed. Semiconductor-type photocatalysts such as TiO2 and

ZnO typically require UV-irradiation owing to their large band gaps. Element-doping, dye sensitization, or construction of heterojunction is frequently adopted to raise the visible-light absorption ability and photocatalytic activity, however such intricate modifications limit their practical use.7 With respect to organic dyes, they have been employed as photoredox catalysts for organic transformations since seminal contributions by Neumann and coworkers,8 whereas the issue of photobleaching is known to have detrimental impacts on photocatalytic system. Recently, transition-metal-based photocatalysts such as ruthenium and iridium complexes have been well investigated and developed on grounds of their wide absorption range in visible light region and superior excited-state lifetimes. 9 In comparison with ruthenium complexes, a library of ligands with disparate electronic properties are available for iridium to tailor the chemical structures and redox properties, which renders iridium-based photocatalysts designability and tunability. Iridium complexes have been exploited as catalysts for water photolysis

10

and CO2 3


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photoreduction. 11 Outstanding efficiencies of iridium-based photocatalysts were also demonstrated in a wide range of useful chemical transformations, for example, cycloaddition reactions,12,13 trifluoromethylation of arenes,14 and α-C–H arylation of amine.15 The photoredox nature of iridium complexes renders them dual power for oxidation or reduction of substrates. Given the versatility and good efficiency for photocatalysis, iridium-based photocatalysts are emerging as promising candidates to achieve the goals of green chemistry and atom economy. Albeit remarkable achievements in developing novel iridium-based photocatalysts have been made, effective approach for heterogenization is scarcely reported.16,17,18 From a sustainable and environmental-friendly standpoint, immobilization of precious metal catalysts on heterogeneous supports for the purpose of facile separation and easy recycling drastically lowers the cost. Historically, mesoporous materials and polymeric resins were frequently employed as solid supports for immobilization of organometallic catalyst via covalent linkage or physical adsorption.19,20,21 The post-modification strategy was also found applicable to the state-of-the-art porous materials, as exemplified by porous

organic

polymers

22

and

metal–organic

frameworks.

23

Alternatively,

copolymerization strategy by which catalytic functionalities are introduced as integral parts of polymer backbone has been considered as a potent tool against leaching of active species.17,

24

Polycarbazole

networks,

prepared

by

simple

and

cost-effective

FeCl3-promoted coupling reactions, have been explored as heterogeneous photocatalysts for various organic transformations, in which cases they functioned as organic 4 ACS Paragon Plus Environment

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ACS Catalysis

semiconductors.25,26,27 With enhanced visible-light absorption and reduced likelihood of exciton recombination, these polymers proved highly active in catalyzing both oxidative and reductive reactions. Very recently, our group devised a novel photocatalytic system based on polycarbazole containing fac-Ir(ppy)3 (ppy = phenylpyridine) moieties, which showed outstanding catalytic performance toward aza-Henry reaction. 28 With the aim of developing versatile catalysts coping with multiple tasks, herein we present cationic polycarbazole networks (CPOP-28 and CPOP-29, where CPOPs denote a series of carbazole-based porous organic polymers developed by our group

28,29,30,31,32,33,34

) based

on heteroleptic iridium complexes containing carbazolyl moieties, and apply them as heterogeneous photocatalysts for a wide range of aerobic organic transformations, i.e. selective

oxidation

of

sulfides,

hydroxylation

of

arylboronic

acids,

and

cross-dehydrogenative reactions. The polymeric networks function well in benign conditions and can be recycled after simple separation workup. The influence of trifluoromethyl substituent on electronic structure, photophysical property, and catalytic performance

was

investigated.

Enhanced

photocatalytic

efficiencies

for

the

trifluoromethylated polymer were demonstrated in oxidative organic transformations. Furthermore, the mechanisms for photocatalysis were studied.

EXPERIMENTAL SECTION Materials

and

Characterization.

Iridium(III)

chloride

hydrate

and 5


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tetrakis(triphenylphosphine) palladium(0) were purchased from Energy Chemical Ltd. 2-Bromo-5-(trifluoromethyl)pyridine,

1,4-dibromobenzene,

tetrahydroisoquinoline,

sulfides, arylboronic acids, phosphite ester, nitromethane, and nitroethane were purchased from J&K Scientific Ltd. All other chemicals and reagents were purchased from Aladdin reagent Co., Ltd. (4-(9H-Carbazol-9-yl)phenyl)boronic acid and cyclometalating ligand 9-(4-pyridin-2-yl-phenyl)-9H-carbazole (PPC) were synthesized following the literature procedures.28 Unless otherwise stated, all chemicals and reagents were used as received without further purification. The characterization methods are available in the Supporting Information. Synthesis

of

Cyclometalating

9-(4-(5-(Trifluoromethyl)pyridine-2-yl)-phenyl)-9H-carbazole

Ligand (TfPPC).

An

oven-dried Schlenk flask was charged with 4-(9-carbazolyl)phenylboronic acid (3.0 g, 10.5 mmol), 2-bromo-5-(trifluoromethyl)pyridine (5.92 g, 26.2 mmol), and DMF (50 mL). After two freeze–pump–thaw degassing cycles, aqueous solution of potassium carbonate (2.0 M, 20 mL) and tetrakis(triphenylphosphine) palladium(0) (190 mg, 164 µmol) were added. Then another two degassing cycles were applied and the reaction mixture was heated at 120 °C for 2 d under magnetic stirring. After cooling to room temperature, the resulting mixture was poured into 200 mL water. The organic layer was collected by separatory funnel, and the aqueous layer was extracted by ethyl acetate (30 mL × 3). The combined organic phase was dried over Na2SO4, and subsequently separated from the insoluble inorganic impurities with a Büchner funnel. The solution 6 ACS Paragon Plus Environment

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was removed under reduced pressure and the crude product was rinsed with a large quantity of hexane to offer pure product as white solid (2.77 g, 78%). 1H NMR (400 MHz, CDCl3): δ 9.00 (s, 1H), 8.26 (d, J = 8.4 Hz, 2H), 8.15 (d, J = 7.7 Hz, 2H), 8.02 (dd, J = 8.3, 2.0, 1H), 7.90 (d, J = 8.3 Hz, 1H), 7.72 (d, J = 8.4 Hz, 4H), 7.52−7.39 (m, 4H), 7.31 (t, J = 4 Hz, 3H). 13C NMR (100 MHz, CDCl3) δ 159.7, 146.8, 140.6, 139.5, 136.7, 134.2, 128.8, 127.3, 126.1, 124.9, 123.7, 120.4, 120.3, 119.9, 109.8. MS (MALDI-TOF) m/z: calculated for C24H15F3N2: 388.4 [M]; found: 388.2 [M]. Synthesis of Chloro-bridged Iridium Dimers [Ir(PPC)2Cl]2 and [Ir(TfPPC)2Cl]2. A mixture of PPC (1.0 g, 3.12 mmol), iridium(III) chloride hydrate (396 mg, 1.25 mmol), 2-ethoxyethanol (21 mL), and water (7 mL) was heated at 120 °C for 1 d under nitrogen. After cooling to room temperature, the reaction mixture was poured into 100 mL water to precipitate the crude product that was then collected by filtration and washed with water (100 mL), methanol (100 mL), and diethyl ether (100 mL), and dried in vacuo. [Ir(TfPPC)2Cl]2 was synthesized according to a similar synthetic procedure for [Ir(PPC)2Cl]2 except for replacing PPC with TfPPC. [Ir(PPC)2Cl]2 (1.01 g, 75%) and [Ir(TfPPC)2Cl]2 (1.09 g, 70%) were recrystallized in hexane to afford pure products. For [Ir(PPC)2Cl]2: 1H NMR (400 MHz, DMSO-d6) δ 9.78 (d, J = 5.9 Hz, 2H), 9.57 (d, J = 5.8 Hz, 2H), 8.46 (d, J = 8.2 Hz, 2H), 8.38 (d, J = 8.2 Hz, 2H), 8.26 (d, J = 8.3 Hz, 2H), 8.17 (d, J = 7.9 Hz, 10H), 7.95 (ddd, J = 9.9, 5.4, 2.5 Hz, 4H), 7.57−7.04 (m, 36H). 13C NMR (100 MHz, DMSO-d6) δ 166.9, 166.5, 154.4, 152.7, 151.1, 147.6, 142.9, 142.4, 139.8, 139.5, 138.9, 138.2, 137.4, 128.2, 126.9, 126.5, 125.7, 124.2, 123.6, 123.3, 121.1, 120.9, 7 ACS Paragon Plus Environment

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120.8, 120.5, 120.0, 119.6, 110.4. For [Ir(TfPPC)2Cl]2: 1H NMR (400 MHz, DMSO-d6) δ 10.52 (s, 2H), 9.93 (s, 2H), 8.60 (dd, J = 22.4, 8.7 Hz, 4H), 8.38 (dd, J = 16.5, 8.5 Hz, 6H), 8.30 (d, J = 8.4 Hz, 2H), 8.19 (d, J = 7.8 Hz, 8H), 7.50−7.17 (m, 32H).

13

C NMR

(100 MHz, DMSO-d6) δ 170.7, 170.5, 155.0, 149.4, 148.7, 147.5, 141.1, 140.6, 139.8, 139.3, 138.9, 137.4, 136.4, 128.8, 128.0, 127.8, 126.6, 125.2, 124.8, 124.7, 124.4, 124.1, 123.6, 123.5, 121.9, 121.7, 121.0, 120.9, 120.6, 120.1, 110.4, 110.2. Synthesis

of

Heteroleptic

Iridium

Complexes

[Ir(PPC)2(bpy)]Cl

and

[Ir(TfPPC)2(bpy)]Cl. A mixture of [Ir(PPC)2Cl]2 (1.0 g, 0.58 mmol) and 2,2′-bipyridine (225 mg, 1.44 mmol) in DCM/MeOH (30 mL, V:V = 1:1) was refluxed at 60 °C for 24 h. After cooling to room temperature, the solution was removed under reduced pressure. The resulting crude product was redissolved in CH3CN (20 mL) and the leftover iridium dimer was removed by filtration. The filtrate was concentrated to 5 mL under reduced pressure and the desired product was precipitated by addition of hexane (100 mL). The yellow solid [Ir(PPC)2(bpy)]Cl (973 mg, 82%) was collected by filtration after cooling the suspension at 0 °C for 3 h. Similar synthetic procedure except for replacing [Ir(PPC)2Cl]2 with [Ir(TfPPC)2Cl]2 was performed to obtain [Ir(TfPPC)2(bpy)]Cl as yellow solid (1.06 g, 79%). For [Ir(PPC)2(bpy)]Cl: 1H NMR (400 MHz, DMSO-d6) δ 8.98 (d, J = 8.2 Hz, 2H), 8.46 (d, J = 8.1 Hz, 2H), 8.41−8.30 (m, 4H), 8.27−8.14 (m, 6H), 7.86 (t, J = 7.8 Hz, 2H), 7.79 (t, J = 6.8 Hz, 2H), 7.71 (d, J = 5.6 Hz, 2H), 7.49−7.37 (m, 6H), 7.36−7.20 (m, 8H), 7.07 (t, J = 6.6 Hz, 2H), 6.45 (d, J = 1.8 Hz, 2H). 13C NMR (100 MHz, DMSO-d6) δ 166.4, 155.9, 152.5, 151.0, 149.6, 143.0, 140.4, 139.6, 138.5, 129.5, 8 ACS Paragon Plus Environment

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ACS Catalysis

127.9, 126.9, 126.5, 125.8, 124.6, 123.4, 121.1, 121.0, 120.8, 119.9, 110.5. MS (MALDI-TOF) m/z: calculated for [C56H38N6Ir]+ [(M−Cl)+]: 987.2; found: 987.6. For [Ir(TfPPC)2(bpy)]Cl: 1H NMR (400 MHz, DMSO-d6) δ 9.01 (d, J = 8.2 Hz, 2H), 8.66 (d, J = 8.7 Hz, 2H), 8.49 (d, J = 8.5 Hz, 2H), 8.42 (t, J = 7.9 Hz, 2H), 8.37−8.25 (m, 4H), 8.20 (d, J = 7.7 Hz, 4H), 7.87 (t, J = 6.8, 2H), 7.70 (s, 2H), 7.51 (dd, J = 8.4, 1.8 Hz, 2H), 7.45−7.20 (m, 12H), 6.50 (d, J = 1.8 Hz, 2H).

13

C NMR (100 MHz, CD3OD) δ 170.4,

156.0, 152.5, 150.8, 145.0, 140.6, 140.5, 140.4, 139.6, 136.0, 128.9, 128.0, 127.9, 125.6, 125.3, 123.7, 120.4, 120.2, 119.9, 109.5. MS (MALDI-TOF) m/z: calculated for [C58H36F6N6Ir]+ [(M−Cl)+]: 1123.2; found: 1123.4. Synthesis of CPOP-28 and CPOP-29. To a suspension of FeCl3 (0.54 g, 3.3 mmol) in anhydrous dichloromethane (20 mL), a dichloromethane solution (30 mL) of [Ir(PPC)2(bpy)]Cl (200 mg, 0.195 mmol) or [Ir(TfPPC)2(bpy)]Cl (200 mg, 0.173 mmol) was added dropwise via a dropping funnel under nitrogen. The reaction mixture was kept stirring at room temperature for 24 h to complete the polymerization. Then, the mixture was stirred in methanol (60 mL) for 1 h. The precipitate was collected by filtration and washed with methanol, tetrahydrofuran, and dichloromethane, respectively. The as-prepared polymers were further purified with methanol for 24 h and tetrahydrofuran for another 24 h in a Soxhlet extractor, and dried in vacuo at 70 °C for 12 h. CPOP-28 (yield: 93%) and CPOP-29 (yield: 90%) were obtained as yellow and orange solid, respectively.


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RESULTS AND DISCUSSION Scheme 1. Schematic Synthesis of Heteroleptic Iridium Complexes and Their Corresponding Polymers

N

120 C

N

R

R=H R=CF3

Ir Cl

N

PPC TfPPC

Ir+

DCM/MeOH, 55 °C

N

2

R

Cl-

bipyridine

Cl Ir

N

N

N

N

IrCl 3H2O 2-ethoxyethanol/H 2O

R

2

R

N

N N

N

R

[Ir(PPC) 2(bpy)]Cl [Ir(TfPPC) 2(bpy)]Cl

[Ir(PPC) 2Cl]2 [Ir(TfPPC) 2Cl]2 FeCl3, CH2Cl2 R.T.

N

R

R

R

N

R

N

N

N

N

N

CPOP-28 (R=H)

N Ir+ Cl-

+

Ir Cl-

N

N

N

N

CPOP-29 (R=CF3)

N

N ClIr+ N

N

R

N

N

R

The electronic effects of substituents have been studied extensively in light-emitting materials containing iridium phosphors. 35 , 36 , 37 Electron-withdrawing fluorous substitutes are of substantial use to construct blue emitters inasmuch as fluorination can effectively shift the frontier orbital energies.38,39 This prompts us to investigate the photophysical properties and photocatalytic activities of iridium catalyst bearing trifluoromethyl group. As shown in Scheme 1, for comparison, two distinct iridium complexes and their corresponding polymeric networks were designed and synthesized. To ingeniously introduce trifluoromethyl group, cyclometalating ligand 10 ACS Paragon Plus Environment

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ACS Catalysis

TfPPC

was

synthesized

using

4-(9-carbazolyl)phenylboronic

acid

and

2-bromo-5-(trifluoromethyl)pyridine via a Suzuki reaction. The chloro-bridged iridium dimer [Ir(TfPPC)2Cl]2 was synthesized through a Nonoyama reaction in which TfPPC and iridium trichloride hydrate were refluxing in a mixture of water and 2-ethoxyethane. Heteroleptic iridium complex [Ir(TfPPC)2(bpy)]Cl was obtained by substituting the dimer with ancillary ligand 2,2′-bipyridine. [Ir(PPC)2(bpy)]Cl was synthesized following a similar synthetic procedure for [Ir(TfPPC)2(bpy)]Cl. CPOP-28 and CPOP-29 were prepared in high yield through FeCl3-promoted coupling polymerization at room temperature under nitrogen. As probed by scanning electron microscopy (SEM) (Figure S1, Supporting Information) and transmission electron microscopy (TEM) (Figure S2, Supporting Information),

both

CPOP-28

and

CPOP-29

feature

aggregates

comprising

interconnected nanoparticles with sizes of 100−200 nm. The as-prepared cross-linked polymers are chemically stable and insoluble in water and common organic solvents such as alcohol, DCM, THF, and DMF. The thermal stabilities of the polymers were examined by thermogravimetric analysis (TGA) under nitrogen (Figure S3, Supporting Information). Approximatively 20% weight loss was observed when the temperature was raised to 450 °C, which was ascribed to the dissociation of ancillary ligands. Structural details of the polymers were elaborated by solid-state 13C and 19F NMR spectroscopy. As shown in Figure 1a, the two polymers show similar resonance peaks ranging from 175−105 ppm. The peaks centered at 170 ppm for CPOP-29 and 164 ppm 11 ACS Paragon Plus Environment

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for CPOP-28 are assigned to the carbons bonding to pyridinyl nitrogen in cyclometalating ligands. The broad peak at 159−146 ppm belongs to carbons neighboring to nitrogen in bipyridine ligand. The peak at about 140 ppm corresponds to the carbons bonding to nitrogen in carbazole moiety. The peak at about 125 ppm results from the residual carbons in pyridinyl ring and substituted phenyl carbons. The signals for unsubstituted phenyl carbons are found at 119 and 110 ppm. The successful introduction of trifluoromethyl group is substantiated by the fluorine signal at −63.3 ppm (Figure 1b).

a

125 140

119 153

170

110

CPOP-29

124

139 119 110

164 152 CPOP-28 220

200

180

160

140

120

100

80

60

Chemical shift (ppm)

b

-63.3

*

*

-30

-40

-50

-60

-70

-80

-90

-100

Chemical shift (ppm)

Figure 1. (a) Solid-state

13

C CP/MAS NMR spectra of CPOP-28 and CPOP-29. (b)

Solid-state 19F NMR of CPOP-29 (asterisks denote spinning sidebands). 12 ACS Paragon Plus Environment

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a

C1s

Intensity (a.u.)

F1s N1s

O1s

Ir4f

Cl2p

CPOP-29

CPOP-28

800

700

600

500

400

300

200

100

0

Binding evergy (eV)

b

Ir 4f7/2

III

Ir 62.24

Intensity (a.u.)

Ir 4f5/2

Ir-O 61.00

69

68

67

66

65

64

63

62

61

60

59

58

59

58

Binding energy (eV)

c

Ir 4f7/2 III

Ir 62.54

Ir 4f5/2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Ir-O 61.63

69

68

67

66

65

64

63

62

61

60

Binding energy (eV)

Figure 2. (a) XPS survey spectra. (b) Ir 4f of CPOP-28. (c) Ir 4f of CPOP-29. 13 ACS Paragon Plus Environment

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Elemental compositions of the polymers were surveyed by X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDS). The overall elements (688.6 eV for F 1s, 400.6 eV for N 1s, 285.6 eV for C 1s, 198.6 eV for Cl 2p, and 62.6 eV for Ir 4f) were unambiguously confirmed in XPS spectra (Figure 2a) and EDS spectra (Figure S4, Supporting Information). The oxygen peak is ascribed to partial oxidation of iridium during oxidative polymerization and the coordinated solvent molecules.40 An in-depth analysis of Ir 4f region (Figures 2b and 2c) indicates +3 oxidation state of Ir species.41 The binding energy of IrIII species in CPOP-29 was found 0.3 eV higher than that of CPOP-28 due to the electronic effect induced by trifluoromethyl group. As disclosed by EDS, the Ir contents of CPOP-28 and CPOP-29 are 19.7 wt% and 15.2 wt%, respectively, which are consistent with the theoretical values (18.8 wt% for CPOP-28 and 16.6 wt% for CPOP-29). The permanent porosities of CPOPs were studied by nitrogen sorption analysis at 77 K (Figure S5, Supporting Information). The Brunauer−Emmett−Teller (BET) surface area of CPOP-28 and CPOP-29 are 97 and 76 m2 g−1, respectively, which are significantly

lower

than

the

values

of

previously-reported

polycarbazole

networks.28,29,30,31,32 The lack of rigidity in coordination structures, inclusion of counterions, and the stoichiometric amounts of heavy iridium atoms in the polymeric network may account for the low surface area.42,43,44,45 A close inspection on optoelectronic properties of iridium complexes and their corresponding polymers was carried out by UV–Vis and steady/transient fluorescence 14 ACS Paragon Plus Environment

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spectroscopy (Figure 3). As recorded in CH3CN at 298 K, fine electronic structures are reflected in the absorption spectra of [Ir(PPC)2(bpy)]Cl and [Ir(TfPPC)2(bpy)]Cl. The predominant bands with higher energies in the range of 200−300 nm arise from spin-allowed singlet ligand-centered (LC) 1π−π transitions of cyclometalating and ancillary ligands. The lower-energy bands with weaker intensity between 300−450 nm are assigned to spin-allowed singlet 1MLCT, ligand−ligand charge transfer (1LLCT) and spin-forbidden triplet transitions such as 3MLCT, 3LLCT and LC 3π−π. It’s noticeable that introduction of trifluoromethyl group results in distinct bathochromic shift of about 30 nm compared to the unsubstituted counterpart. Similar phenomena were also observed in some other fluorinated iridium complexes, mainly due to the lower LUMO level.38,46 The absorption spectra of polymers span the whole UV−Vis region as a consequence of extended π-conjugated structures. Specifically, UV−Vis diffuse reflectance spectra of CPOP-29 are significantly red shifted for ca. 50 nm with respect to CPOP-28, which is in good conformity with the color change of solid samples, from yellow to orange for CPOP-28 and CPOP-29, respectively. Upon excitation at 420 nm, [Ir(PPC)2(bpy)]Cl and [Ir(TfPPC)2(bpy)]Cl emit greenish-yellow and bluish-green color, respectively, in CH3CN at 298 K. A broad and featureless emission peak centered at 568 nm is observed for [Ir(PPC)2(bpy)]Cl indicating the main contribution from 3MLCT. [Ir(TfPPC)2(bpy)]Cl manifests vibrational structure with an emission maximum at 507 nm and a shoulder peak at 543 nm, which suggests a larger portion of LC 3π−π character than 3MLCT in excited state.47 Broad peaks with similar emission maxima were recorded 15 ACS Paragon Plus Environment

ACS Catalysis

when the solid samples of polymers were excited at 430 nm.

a

1.0

1.2 [Ir(TfPPC)2(bpy)]Cl

[Ir(PPC)2(bpy)]Cl

1.0

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

Normalized PL intensity (a.u.)

Normalized absorbance (a.u.)

1.2

0.0 0.0 200 250 300 350 400 450 500 550 600 650 700 750

Wavelength (nm)

1.0

b

1.0 0.8

0.8 0.6

CPOP-28

CPOP-29

0.6

0.4

0.4

0.2

0.2

Normalized PL intensity (a.u.)

1.2

1.2

Normalized absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 40

0.0 0.0 300 350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

Figure 3. Normalized UV−Vis absorption and emission spectra of iridium complexes in acetonitrile (a) and polymers in solid state (b) at 298 K. The excited-state lifetimes of complexes in solution and polymers in solid state were measured by time-resolved fluorescence spectroscopy in air at 298 K (Figure S6, Supporting Information). The polymers have shorter lifetimes than their corresponding monomers owing to the interaction between excited and non-excited molecules in high concentration. Moreover, [Ir(TfPPC)2(bpy)]Cl and CPOP-29 that bear trifluoromethyl group have much longer decay lifetimes than their non-substituted counterparts, i.e. 16 ACS Paragon Plus Environment

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[Ir(PPC)2(bpy)]Cl and CPOP-28, which can be ascribed to the greater LC 3π−π character.48,49 Table 1. Photocatalytic Selective Oxidation of Sulfides Using CPOPs a

Time (h)

Conv. (%) b

Select. (A:B) c

CPOP-28

10

79

98:2

Ph

CPOP-29

10

>99

99:1

3

4-CH3−Ph

CPOP-28

10

71

98:2

4

4-CH3−Ph

CPOP-29

10

>99

97:3

5

4-Br−Ph

CPOP-28

15

74

98:2

6

4-Br−Ph

CPOP-29

15

>99

96:4

7

4-Cl−Ph

CPOP-29

16

>99

98:2

8

4-F−Ph

CPOP-29

13

>99

98:2

9

4-MeO−Ph

CPOP-29

10

96

96:4

Entry

a

Aryl

Catalyst

1

Ph

2

As standard conditions for oxidation of sulfides were sulfides (0.5 mmol), CPOPs (5

µmol, 1 mol% based on monomer), MeOH (0.5 mL), air, 23 W white LED lamp and room temperature.

b

Conversions were determined by 1H NMR.

c

Selectivities of A:B

were determined by 1H NMR.


ACS Catalysis

100

1st

2nd

3rd

4th

5th

80

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 40 20 0 0

10

20

30

40

50

Time (h)

Figure 4. Recyclability of CPOP-29 in oxidation of methyl phenyl sulfide within five cycles. The catalyst was recycled from reaction mixture by centrifugation, washed with MeOH and DCM, dried, and reused in fresh reaction solution. Given the remarkable light absorption ability in visible region and long excited-state lifetimes, the photocatalytic activities of CPOPs as heterogeneous catalysts for aerobic oxidation reactions were systematically investigated. As a model reaction, the selective oxidation of sulfide was performed (Table 1). The oxidation reactions of sulfide to sulfoxide were catalyzed by CPOPs in air with irradiation of a 23 W white LED lamp. Light irradiation, air, and photocatalyst are indispensable to promote the oxidation reactions (Table S1, Entries 1−3, Supporting Information). All sulfides were transformed to corresponding sulfoxides in moderate to high conversions. Superior efficiencies are more readily to achieve with CPOP-29 than CPOP-28 under the same conditions (Table 1, Entries 1−6). Furthermore, various substrates bearing electron-donating groups (EDG, Table 1, Entries 4 and 9) and electron-withdrawing groups (EWG, Table 1, Entries 6−8) were converted quantitatively with the aid of CPOP-29. In consistence with previous 18 ACS Paragon Plus Environment

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literatures,25,26 sulfides with electron-withdrawing substituents (−F, −Cl and −Br) were found having much slower reaction kinetics than those with electron-donating substituents (−CH3 and −OCH3). It should be noted that high selectivities of sulfides against sulfones were manifested in all cases. CPOP-29 was recycled from reaction mixture of oxidation of methyl phenyl sulfide, 90% conversion remained after five cycles (Figure 4). Photooxygenation of sulfides involves either electron-transfer process in which superoxide radical (O2•−) plays a crucial role as electron mediator in photoredox cycle or energy-transfer process in which singlet oxygen reacts with sulfides to afford the sulfoxides. 50 , 51 To get a better understanding of the reaction mechanism, electron paramagnetic resonance (EPR) measurements were performed to provide direct evidence. No EPR signal of 1O2 was detected when 2,2,6,6-tetramethyl-1-piperidine (TEMP) was used as trapping agent. Instead, characteristic quartet peak of O2•− was unequivocally detected when a solution of CPOP-29 and 5,5′-dimethyl-1-pyrroline N-oxide (DMPO) in MeOH was irradiated (Figure S7, Supporting Information). When hole scavenger KI was added to the reaction mixture, the conversion declined to 16%. Using p-benzoquinone as O2•− scavenger, only 7% conversion was observed (Table S1, Entries 4−7, Supporting Information). Based on these findings, a plausible reaction mechanism for oxidation of sulfides was proposed in Scheme S1 (Supporting Information). The excited photocatalyst CPOP* undergoes oxidative quenching process by transferring electrons to oxygen molecules. Through a redox reaction, the substrate sulfides are oxidized by CPOP+ with 19 ACS Paragon Plus Environment

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simultaneous regeneration of neutral CPOP and formation of sulfide radical cation which is further oxidized by O2•− to afford the desired products. Meanwhile, sulfides are oxidized to sulfoxides by oxygen intermediates that are generated by hole-induced water oxidation. Table 2. Photocatalytic Hydroxylation of Arylboronic Acids Using CPOPs a

Entry

a

Aryl

Catalyst

Yield (%) b

1

Ph

CPOP-28

76

2

Ph

CPOP-29

98

3

4-CHO−Ph

CPOP-28

86

4

4-CHO−Ph

CPOP-29

99

5

4-COOMe−Ph

CPOP-28

89

6

4-COOMe−Ph

CPOP-29

96

7

4-OMe−Ph

CPOP-29

82

8

4-CN−Ph

CPOP-29

99

9

3-CN−Ph

CPOP-29

83

10

2-CN−Ph

CPOP-29

67

11

4-COOH−Ph

CPOP-29

99

12

4-Me−Ph

CPOP-29

85

As standard conditions for hydroxylation of arylboronic acids were arylboronic acids

(0.5 mmol), CPOPs (10 µmol, 2 mol% based on monomer), iPr2NEt (1 mmol), DMF (0.5 mL), air, 48 h, 23 W white LED lamp and room temperature. b Yields were determined by 1

H NMR. Phenols as important intermediates in pharmaceutical and chemical industries have 20 ACS Paragon Plus Environment

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ACS Catalysis

been conventionally prepared through hydroxylation of arylboronic acids.52 Nevertheless, strong oxidants such as H2O2 and m-chloroperoxybenzoic acid are usually required.53,54 In view of environmental concerns, synthesis of arylboronic acids in mildly oxidizing manners has been the aim of chemists.55,56 Next, we applied CPOPs to photocatalytic hydroxylation of arylboronic acids (Table 2). The catalysis reactions were performed using oxygen air as oxidizing agent and N,N-diisopropylethylamine (iPr2NEt) as sacrificial electron donor. Under the same reaction condition, higher yields are achieved in the reactions catalyzed by CPOP-29 (Table 2, Entries 1−6,). A wide scope of substrates bearing electron-neutral substitute (Table 2, Entry 1), EWG (Table 2, Entries 3−6 and 8−11), and EDG (Table 2, Entries 7 and 12,) were studied. Most of the substrates were quantitatively depleted and transformed into corresponding phenols within 48 h. In general, substrates with EWG (−COOMe, −COOH, −CHO, and −CN) have better reaction efficiencies than those with EDG (−Me and −OMe). In addition, the reaction efficiencies are correlated with the positions of substitute (4-CN > 3-CN > 2-CN). Employment of KI as hole scavenger, the conversion decreased significantly to 23%. By using p-benzoquinone as O2•− scavenger, a low conversion of 12% was obtained (Table S2, Entries 4−7, Supporting Information). C–H bond activation provides powerful synthetic tools for synthesis of natural products and meets the demands of green chemistry.57 Regarded as one of the most promising C−H activation methodologies, cross-dehydrogenative coupling (CDC) reactions have come into focus for the reason that chemical bonds (e.g., C−C, C−P, C−O) 21 ACS Paragon Plus Environment

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can be constructed directly without pre-activation of substrate molecules. 58,59,60 To demonstrate the versatility, CPOPs were employed to catalyze CDC reactions under aerobic conditions (Table 3). Prior to the test, CPOPs were subject to a solution of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) under aerobic and irradiation conditions to verify the ability to mediate an electron-transfer process.61,62 The color change of solution from colorless to blue and new absorption bands at 564 and 613 nm clearly indicate the generation of cationic radical of TMPD, which in turn proves that CPOPs has mediated the electron transfer from TMPD to dioxygen (Figure 5). The more intense absorption band for the solution containing CPOP-29 suggests stronger electron mediation ability. The substrates were converted smoothly in both phosphonation reactions (Table 3, Entries 1−5) and aza-Henry reaction (Table 3, Entries 6−8) in good yield (80−95%). As in the foregoing examples, CPOP-29 was found to be a more effective catalyst. The scope of substrates (N-aryl-tetrahydroisoquinoline) and nucleophiles (nitroalkanes and phosphite esters) were examined to establish applicability of the catalyst. It was shown that EWG on N-aryl ring gave higher yield than EDG. It is noted that the species of phosphite and nitroalkane do not exhibit marked difference on the reaction efficiencies. The charged polycarbazole networks (CPOP-28 and CPOP-29) proved highly active for oxidative organic transformations despite their low surface area. Indeed, high catalytic efficiencies have been observed for polymers with low-surface area

42,43,44

or

even non-porousity.45,63 This can be reasonably explained by the swelling of cross-linked 22 ACS Paragon Plus Environment

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ACS Catalysis

polymers in solvents, in which cases the distended skeletons enrich accessible catalytic sites and expedite the mass transport process. Table 3. Photocatalytic Cross-Dehydrogenative Coupling Reactions Using CPOPs a

NuH

Catalyst

Yield (%) b

Entry

R

Product

1

H

CPOP-28

83

2

H

CPOP-29

92

3

Br

CPOP-29

N EtO P O EtO

89 Br

4

OMe

CPOP-29

80

5

H

CPOP-29

94

6

H

CPOP-28

81

CPOP-29

96

CPOP-29

95

CH3NO2

a

7

H

8

H

As

standard

C2H5NO2 condition

for

phosphonylation

of

amines

were

N-aryl-tetrahydroisoquinolines (0.5 mmol), phosphite ester (0.5 mmol), CPOPs (5 µmol, 1 mol% based on monomer), MeOH (0.5 mL), air, 24 h, 23 W white LED, room temperature. Aza-Henry reactions were conducted in similar conditions except 1 mL nitromethane or nitroethane was used instead of phosphite ester and MeOH. b Yields were determined by 1H NMR.


ACS Catalysis

2.0

Absorbance (a.u.)

No cat. CPOP-29

1.5

1.0 CPOP-29 0.5

CPOP-28 No catalyst

0.0 400

450

500

550

600

650

700

Wavelength (nm)

Figure 5. UV−Vis absorption spectra and photograph of the cationic radical species of N,N,N′,N′-tetramethyl-p-phenylenediamine generated by CPOPs in the presence of oxygen and irradiation of white LED for 10 min. -2.0

CPOP-28

Redox potential (V vs. SCE)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 40

-1.5 -1.0

E1/2(M/M‾) +

*

E1/2(M /M )

CPOP-29 –1.40

–1.28

–0.95

–0.88

-0.5 0.0 0.5 *

1.0

E1/2( M/M‾)

0.85

+

1.5

E1/2(M /M)

1.30

1 1.40

2.0

Figure 6. Redox potentials of CPOP-28 and CPOP-29. M = photocatalyst, * = excited state.

To rationalize the superior efficiency of trifluoromethylated polymer CPOP-29 for photocatalytic hydroxylation of arylboronic acids and CDC reactions, electrochemical measurements and fluorescence quenching experiments were performed. Reversible oxidation waves that are attributed to oxidation of IrIII/IrIV redox couple are shown in 24 ACS Paragon Plus Environment

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ACS Catalysis

anodic scans for both polymers. Additionally, an irreversible and a quasi-reversible reduction wave are observed for CPOP-28 and CPOP-29, respectively, upon cathodic scans. The reduction waves are assigned to the reduction of ancillary ligand bipyridine as well as cyclometalating ligand (Figure S8, Supporting Information). The excited-state potentials that are essential for photocatalytic chemical transformations can be simply approximated on the basis of ground-state potentials and E0–0.64,65 The ground-state and excited-state potentials of the polymers are summarized in Table S3 (Supporting Information). In keeping with previous observations,38,

66 , 67

introduction of

trifluoromethyl group markedly shifts both oxidation and reduction potential to more positive values (Figure 6). Indeed, the excited photocatalyst can be quenched by both reductive quencher iPr2EtN and oxidative quencher O2 considering the potentials of excited state. More effective fluorescence quenching was observed when iPr2EtN and N-phenyl-tetrahydroisoquinoline (N-phenyl-THIQ) were used as quencher than oxygen, which feasibly supported a reductive quenching process in the catalysis reactions (Figures S9−11, Supporting Information). This is reasonable considering the fact that the oxidation potential of iPr2EtN (E1/2(M+/M) = 0.90 V vs SCE) (E1/2(M+/M) = 0.83 V vs SCE)

68

65

and N-phenyl-THIQ

is more negative than the reduction potential of excited

photocatalyst. The more positive value of E1/2(*M/M−) for CPOP-29 that contributes to thermodynamic driving force in reductive quenching, stronger visible-light absorption ability, and longer excited-state lifetime are probably the main reasons for better photocatalytic efficiencies. 25 ACS Paragon Plus Environment

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The proposed mechanisms for photocatalytic hydroxylation of arylboronic acids and CDC reactions are illustrated in Schemes S2 and S3 (Supporting Information). The excited photocatalyst CPOP* is first reductively quenched by amines (iPr2EtN or N-aryl-THIQ) to produce amine radical cations and CPOP−. Some amines are oxidized by photogenerated holes to give radical cations. O2•− radicals are generated through electron transfer processes with CPOP− and photogenerated electrons. The arylboronic acids react with O2•− to produce anion intermediates that further transform into phenols via the electron-mediating and hydrolysis processes. As for CDC reactions, O2•− attracts a hydrogen from the amine radical cation to form iminium ion that is subsequently attacked by nucleophile to provide the desired product. The direct evidence for generation of O2•− was supported by EPR experiments (Figures S12−13, Supporting Information). The EPR signals of O2•− was evidently detected when CPOP-29 was irradiated in DMF and MeOH solution of DMPO, yet no 1O2 species was captured by TEMP. The intensity of O2•− signal was significantly increased upon addition of iPr2EtN, which was ascribed to the rapid formation of reduced photocatalyst (CPOP−) and thus generation of superoxide radicals. Upon addition of N-phenyl-THIQ, the EPR signal attenuates due to the consumption of O2•− by imine radical cation.

CONCLUSIONS Cationic polycarbazole networks (CPOP-28 and CPOP-29) were prepared via simple FeCl3-promoted oxidative coupling reactions. By introducing trifluoromethyl 26 ACS Paragon Plus Environment

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ACS Catalysis

groups (–CF3) into the polymeric backbone, CPOP-29 displayed greater visible-light absorption ability, longer excited-state lifetime, and more oxidative photoexcited state as compared to CPOP-28. The polymers were employed as heterogeneous photocatalysts in far-ranging aerobic organic transformations including selective oxidation of sulfides, hydroxylation of arylboronic acids, and cross-dehydrogenative coupling reactions under aerobic and white LED irradiation conditions. In spite of low surface area, CPOP-28 and CPOP-29 are highly active in photocatalytic synthesis. Taking advantages of the superior photophysical and oxidative properties, CPOP-29 manifests greater catalytic efficiency in oxidative transformation reactions. Moreover, good catalytic efficiency was retained after multi-round use. This work highlights the potential of obtaining highly efficient photocatalyst through simply modulating the substitute of skeleton. More research studies concerning tuning the photocatalytic properties by regulating the electronic structures of CPOPs and expanding the scope of organic transformations are underway.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Phone: +86 10 8254 5576. *E-mail: [email protected]. Phone: +86 10 8254 5708. ORCID Bao-Hang Han: 0000-0003-1116-1259 Notes 27 ACS Paragon Plus Environment

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Page 28 of 40

The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information Instrumentations and characterization methods, general procedures for photocatalysis, additional tables, catalysis mechanisms, additional figures, 1H NMR, 13C NMR and mass spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS The financial support of the National Natural Science Foundation of China (Grants 21574031, 21574032, and 21302150) and the Sino-German Center for Research Promotion (Grant GZ1286) is acknowledged.

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Graphic TOC


Cationic Polycarbazole Networks as Visible-Light Heterogeneous

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