Reduced Graphene Oxide-Immobilized Tris(bipyridine)ruthenium(II

Research Article www.acsami.org

Reduced Graphene Oxide-Immobilized Tris(bipyridine)ruthenium(II) Complex for Efficient Visible-Light-Driven Reductive Dehalogenation Reaction Xiaoyan Li, Zhongkai Hao, Fang Zhang,* and Hexing Li* The Education Ministry Key Lab of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234, People’s Republic of China S Supporting Information *

ABSTRACT: A sodium benzenesulfonate (PhSO3Na)-functionalized reduced graphene oxide was synthesized via a two-step aryl diazonium coupling and subsequent NaCl ion-exchange procedure, which was used as a support to immobilize tris(bipyridine)ruthenium(II) complex (Ru(bpy)3Cl2) by coordination reaction. This elaborated Ru(bpy)3-rGO catalyst exhibited excellent catalytic efficiency in visible-light-driven reductive dehalogenation reactions under mild conditions, even for ary chloride. Meanwhile, it showed the comparable reactivity with the corresponding homogeneous Ru(bpy)3Cl2 catalyst. This high catalytic performance could be attributed to the unique two-dimensional sheet-like structure of Ru(bpy)3-rGO, which efficiently diminished diffusion resistance of the reactants. Meanwhile, the nonconjugated PhSO3Na-linkage between Ru(II) complex and the support and the very low electrical conductivity of the catalyst inhibited energy/electron transfer from Ru(II) complex to rGO support, resulting in the decreased support-induced quenching effect. Furthermore, it could be easily recycled at least five times without significant loss of catalytic reactivity. KEYWORDS: visible-light photocatalysis, reduced graphene oxide, Ru(bpy)3Cl2, diazonium coupling, reductive dehalogenation reaction reactions.8−11 However, most of them often suffer from laborious synthetic processes, unsatisfactory catalytic performances and substrate scope due to the enhanced diffusion resistance, and the altered coordinated microenvironment around ruthenium cation.12 Graphene is a highly attractive two-dimensional nanostructured support owing to the high specific surface area and the minimal mass transfer resistance.13 Until now, many attempts have been made to integrate graphene with inorganic semiconductor photocatalysts to enhance these composites performances in various photocatalytic applications, such as water splitting, pollutant degradation and organic synthesis.14−16 Nevertheless, only limited examples of the preparation of graphene-based photoactive transition metal complex for visible-light-driven organic synthesis have been reported thus far.17 The main difficulty is the lack of a feasible approach to install Ru(bpy)32+ complex on the graphene surface. Furthermore, the immobilization method could inhibit the negative quenching effect came from the photoinduced electron transfer from Ru(bpy)32+ complex to the sheets of the graphene-based support.18,19 In this context, reduced graphene oxide was believed to be a good candidate to immobilize the

1. INTRODUCTION Natural photosynthetic processes that can efficiently capture sunlight and convert it into organic molecules have inspired scientists to construct artificial photosynthetic systems.1 Recently, visible-light-driven organic synthesis received much attention due to its abundance, ease of use, environmental benignity, and energy savings.2 In general, the utilization of photosensitizer catalysts is a viable tool to promote visible-light photosynthesis because it can overcome the limited visible light absorption of organic reactants.3 Among various photosensitizer catalysts, Ru(bpy)32+ complex (bpy = 2,2′-bipyridine) and its derivatives are the most extensively studied owing to high quantum efficiency and photoredox activity.4 Accordingly, a vast range of synthetically useful organic transformations have been realized by using Ru(bpy)32+ photocatalyst.5 However, the high cost and potential hazard of metal contamination in the final product inevitably hamper the industrial large-scale applications in the future.6 In the context, the development of efficient and reusable heterogeneous Ru(bpy)32+ photocatalysts is of considerable current interest owing to their obvious advantages of simple workup, reduced cost and pollution, and continuous operation.7 To date, several groups have reported the use of polymer or metal−organic framework as the supports for the preparation of immobilized Ru(bpy)32+ photocatalyst in a wide range of organic transformations such as carbon−carbon coupling, oxyamination, and dehalogenation © 2016 American Chemical Society

Received: January 27, 2016 Accepted: April 22, 2016 Published: April 22, 2016 12141

DOI: 10.1021/acsami.6b01100 ACS Appl. Mater. Interfaces 2016, 8, 12141−12148

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Schematic Illustration of the Preparation of Reduced Graphene Oxide-Immobilized Tris(bipyridine)ruthenium(II) Complex

and then sonicated for 1.0 h. After stirring for 24 h at 80 °C under nitrogen atmosphere, the powder product was filtered and washed thoroughly with freshly absolute ethanol and deionized water to remove physically absorbed Ru(II) complex and followed by vacuum drying at 80 °C for 12 h. 2.2. Catalyst Characterization. The ruthenium loading was calculated by inductively coupled plasma optical emission spectrometer (ICP, Varian VISTA-MPX). The contents of carbon, hydrogen and sulfur were determined by elemental analyzer (Vario EL III). Fourier transform infrared spectra (FTIR) were collected on a Nicolet Magna 550 spectrometer. The electronic state of ruthenium element was analyzed by X-ray photoelectron spectroscopy (XPS, PerkinElmer PHI 5000C ESCA). The binding energy value was calibrated by using C1S = 284.6 eV as a reference. X-ray powder diffraction (XRD) data was collected on a Rigaku D/maxr B diffractometer using Cu Kα radiation. Raman spectroscopy measurement was performed on a Jobin Yvon micro-Raman spectroscope (Super LabRam II). Transmission electron microscopy (TEM) image and energy dispersive Xray analysis were carried out on a JEOL JEM-2011 electron microscope. UV−vis spectrum was observed by using a Cary 500 -NIR spectrophotometer. Fluorescence spectrum was recorded on a FLS920 fluorescence spectrophotometer. Specific surface areas (SBET) were analyzed on a Quantachrome NOVA 4000e analyzer and the values were calculated by using BET model. 2.3. Activity Test. In a typical run of visible-light-driven reductive dehalogenation reactions, 2.0 mmol 2-Bromoacetophenone, 4.4 mmol Hantzsch ester, 6.0 mmmol iPr2NEt, 2.0 mmol nitrobenzene as an internal standard and a Ru(bpy)3-rGO catalyst with 0.05 mmol Ru(II) were added into 1.5 mL DMF under nitrogen atmosphere via a syringe needle. The mixture was allowed to stir at 25 °C for 6.0 h at a distance of around 5.0 cm from a 50 W fluorescent lamp. The product was analyzed on a high performance liquid chromatography analyzer (HPLC, Agilent 1260). The reaction conversion was calculated based on 2-Bromoacetophenone. The reproducibility was checked by repeating the experiment at least three times and was found to be within acceptable limits (±5%). To test the recyclability of Ru(bpy)3-rGO catalyst, the mixture was centrifuged for 10 min at high speed after each run of the reactions and then the supernatant was decanted slowly. The residual solid catalyst in the bottom of the centrifuge tube was washed thoroughly with dichloromethane and acetone, followed by vacuum drying at 60 °C for 12 h. Subsequently, the catalytic performance of the reused Ru(bpy)3rGO catalyst was determined by using a fresh charge of the reactants and the solvent under the same reaction conditions.

photosensitizer catalysts for visible-light-driven chemical transformation because the low electrical conductivity probably decreased fluorescence quenching while it still remain the unique a two-dimensional sheet-like structure. In this study, we report the fabrication of reduced graphene oxide-immobilized tris(bipyridine)ruthenium(II) catalyst through ion-pair association of the Ru(bpy)3Cl2 complex with sodium benzenesulfonate (PhSO3Na-) functionalized reduced graphene oxide. The obtained Ru(bpy)3-rGO catalyst displayed comparable reactivity with the corresponding homogeneous catalyst in visiblelight-mediated reductive dehalogenation reactions owing to the diminished support-induced quenching effect and the decreased mass transfer limitation. Moreover, it could be used repetitively at least five times without a remarkable decrease of catalytic reactivity.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. 2.1.1. Synthesis of Reduced Graphene Oxide (rGO). Graphene oxide (GO) was prepared and purified by the reported Hummers approach.20 Then, 0.60 g sodium borohydride (NaBH4) was slowly added into 200 mL GO aqueous suspension (1.0 mg/mL) at 90 °C and the mixture was allowed to continuous stirring for 8.0 h. The obtained black powder was washed by deionized water for several times and subsequently purified by dialysis in distilled water for 48 h, resulting in reduced graphene oxide. 2.1.2. Preparation of NaSO3-Functionalized Reduced Graphene Oxide (NaSO3-rGO). First, 4-benzenediazoniumsulfonate was synthesized for ary diazonium coupling reagent.21−23 In a typical procedure, 5.2 g sulfanilic acid (0.030 mol) was dispersed in 300 mL HCl aqueous solution (1.0 M) in a three-necked ground flask. The flask was moved into an ice−water bath, and the mixture temperature was controlled at 3−5 °C with magnetic stirring. Next, a certain amount of 1.0 M NaNO2 aqueous solution was added dropwise in the mixture until a clear solution was obtained. After stirring for another 30 min, the white precipitate was filtered and washed with deionized water. The obtained 4-benzenediazoniumsulfonate was dissolved in 120 mL water−ethanol mixture. Then, 180 mg rGO was added into the mixture at 5 °C and subsequently 120 mL 50 wt % H3PO2 aqueous solution was slowly introduced within 90 min under continuous stirring. Finally, the product was intensively washed with deionized water and vacuum-dried for 12 h. Next, the obtained product (100 mg) was immersed in 10 mL saturated NaCl solution and allowed to ultrasonic treat for 30 min. The mixture was stirred for 24 h at 60 °C and then washed by distilled water and vacuum-dried for 12 h, leading to PhSO3Na-functionalized reduced graphene oxide. 2.1.3. Fabrication of Reduced Graphene Oxide-Immobilized Tris(bipyridine)ruthenium(II) Catalyst (Ru(bpy)3-rGO). First, 100 mg SO3Na-rGO sample was added in 10 mL Ru(bpy)3Cl2·6H2O aqueous solution (20 mg/mL) in a glass tube and then sonicated for 1.0 h. After stirring for 24 h at 80 °C under nitrogen atmosphere, the powder product was filtered and washed thoroughly with freshly absolute ethanol and deionized water to remove physically absorbed Ru(II) complex and followed by vacuum drying at 80 °C for 12 h. 2.1.4. Fabrication of Amberlyst 15 Resin-Immobilized Tris(bipyridine)ruthenium(II) Catalyst (Ru(bpy)3-Resin). First, 100 mg of the commercial Amberlyst 15 resin was added in 10 mL Ru(bpy)3Cl2·6H2O aqueous solution (20 mg/mL) in a glass tube

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. Scheme 1 briefly showed the synthetic route of Ru(bpy)3-rGO. We first modified reduced graphene oxide with sodium benzenesulfonate groups through a two-step aryl diazonium coupling24 and NaCl ionexchange procedure. Elemental analysis of NaSO3-rGO (Table S1) revealed that the amount of sulfur was 0.92 mmol/g, demonstrating the successful modification of PhSO3Namoieties on the rGO surface. Then, tris(bipyridine)ruthenium(II) complex (Ru(bpy)3Cl2) can be immobilized on the rGO surface via coordination reaction. After optimizing the immobilization conditions including reaction solvent and 12142


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Figure 1. (a) FTIR spectra of GO, rGO and Ru(bpy)3-rGO, and (b) XPS spectra of and Ru(bpy)3-rGO samples.

Figure 2. (a) XRD spectra of GO, rGO and Ru(bpy)3-rGO samples, and (b) Raman spectra of rGO and Ru(bpy)3-rGO samples.

oxide (GO) showed the remarkable decrease peak intensities at 3420, 1358, 1732, and 1049 cm−1, which were assigned to C− OH stretching, CO bond stretching, and C−O bond stretching vibrations, respectively.26 This result clearly confirmed the carbonyl, hydroxyl, and epoxy groups were mostly removed after NaBH4 reduction. Moreover, Ru(bpy)3-rGO sample displayed two additional peaks around 1255 cm−1 corresponding to SO bond stretching vibration and 1598 cm−1 indicative of CN bond stretching vibration (Figure 1a), implying the presence of Ru(bpy)32+ complex on the rGO surface.27 As shown in Figure 1b, XPS spectrum of Ru(bpy)3rGO revealed all the ruthenium species were present in the bivalent state, corresponding to the binding energy of Ru 3p3/2 at 462.8 eV.28 In comparison with Ru(bpy)3Cl2 complex (462.2 eV), the Ru element binding energy in the Ru(bpy)3-rGO sample was shifted positively by 0.60 eV, which was probably due to that the PhSO3Na-functional groups substituted the chlorine ions, resulting in Ru electron deficiency. On the basis of these results, we can therefore be sure that the immobilization of Ru(bpy)32+ complex on rGO was through chelating with the PhSO3Na- functional groups. As shown in Figure 2a, XRD pattern of GO displayed a sharp diffraction peak at 2θ = 10.8° indicative of the (001) plane of the pristine GO.29 However, this typical peak has almost

temperature, 0.21 mmol/g Ru(II) loading was obtained in Ru(bpy)3-rGO sample. Even using twice the amount of Ru(bpy)3Cl2·6H2O in the catalyst preparation, the Ru(II) content in Ru(bpy)3-rGO increased to 0.25 mmol/g (Table S2). The slight increase in the Ru(bpy)3 loading was propably owing to the steric hindance of Ru(II) complex in the basal planes of reduce graphene oxide, which reduced the efficient interactions between PhSO3Na- functional groups and Ru(bpy)3Cl2·6H2O compound. The chemical state of the sulfur species in the PhO3Nafunctional reduced graphene oxide (PhSO3Na-rGO) could be analyzed by XPS spectrum. As shown in Figure S1, PhSO3NarGO displayed the S2p peak at 168 eV, suggesting the successful modification of PhSO3Na-functional groups on the rGO surface.25 To prove the successful fabrication of such reduced oxide graphene-immobilized Ru(bpy)3 complex, we first analyzed the filtrate of Ru(bpy)3-rGO sample after the washing workup by ICP analysis. No ruthenium species can be detected in the filtrate, indicating that the existence of the physically absorbed Ru(bpy)3 complex on the rGO support could be excluded. Furthermore, we characterized its chemical composition using Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). Compared with graphene oxide (GO), FTIR spectrum of reduced graphene 12143


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Figure 3. (a) TEM picture and (b) ruthenium element EDX image of Ru(bpy)3-rGO sample.

completely disappeared and turned into a new broad peak at 2θ = 24° for rGO. It revealed that, during the reduction treatment, the aggregate GO sheets were efficiently exfoliated to the separate sheets. After immobilizing Ru(bpy)32+ complex on rGO, Ru(bpy)3-rGO exhibited an additional weak peak at 2θ = 13° and remained the representative peaks of rGO, which was maybe attributed to the existence of amorphous Ru(II) complex in rGO surface. Raman spectra of rGO and Ru(bpy)3-rGO samples were shown in Figure 2b. As expected, rGO displayed the strong and broad D and G bands at 1345 and 1584 cm−1, respectively. The intensity of the D band was higher than the G band due to the surface defects and partially disordered structure of the sheets.30 Meanwhile, Ru(bpy)3-rGO showed the similar Raman spectrum with rGO. Furthermore, the intensity ratio of ID/IG for rGO and Ru(bpy)3-rGO was 1.16 and 1.20, respectively, implying that the structure of graphene sheets could be preserved after aryl diazonium coupling, ion-exchange and immobilization processes. The direct observation of the structural property of Ru(bpy)3-rGO could be obtained by transmission electron microscopy (TEM). TEM image of Ru(bpy)3-rGO (Figure 3a) confirmed that the immobilization process caused no significant damage to the GO sheet structure, and it still remained a unique two-dimensional plane structure. The corresponding energy-dispersive spectrometry (EDS) mapping (Figure 3b) further demonstrated the distribution of Ru element was uniform throughout the graphene sheet. 3.2. Photocatalysis. Visible-light-mediated reductive dehalogention reactions have recently attracted considerable attention by virtue of their environmentally benign and excellent selectivity.31,32 We then chosen reductive dehalogenation of 2-Bromoacetophenone under visible light irradiation as a probe to examine the photocatalytic efficiency of Ru(bpy)3rGO catalyst. Initial success was achieved with 50.5% yield of acetophenone product upon subjecting 2-Bromoacetophenone to irradiation by a 50 W fluorescent lamp with Ru(bpy)3-rGO (2.5 mol %) as a catalyst in the presence of Hantzsch ester (1.1 equiv) and iPr2NEt (2.0 equiv) in DMF for 6.0 h (Table 1). Furthermore, we used different monochromatic lights to activate Ru(bpy)3-rGO catalyst and found 450 nm wavenumber light was more efficient (Figure S2). It was due to that the maximum absorption wavelength of Ru(bpy)3Cl2 complex centered around 455 nm that evidenced by UV−vis spectrum (Figure S3). Next, we carefully optimized the reaction condition including Ru(bpy)3-rGO content, the amounts of Hantzsch ester and iPr2NEt (Table 1), and solvent amount (Figure S4). As shown in Table 1, under the optimal reaction condition, 2-Bromoacetophenone was efficiently transferred to

Table 1. Catalytic Performances of Visible Light-Mediated Reductive Dehalogenation Reactionsa

catalyst 2.5 mol % Ru(bpy)3rGO 2.5 mol % Ru(bpy)3rGO 2.5 mol % Ru(bpy)3rGO 2.5 mol % Ru(bpy)3rGO 1.5 mol % Ru(bpy)3rGO 3.5 mol % Ru(bpy)3rGO 2.5 mol % Ru(bpy)3rGO / PhSO3Na-rGO 2.5 mol % Ru(bpy)3resin 2.5 mol % Ru(bpy)3Cl2

Hantzsch ester (equiv)

iPr2NEt (equiv)

1.1

2

6

50.5

1.1

3

6

68.1

2.2

2

6

78.0

2.2

3

6

81.3

2.2

3

6

56.8

2.2

3

6

78.9

2.2

3

37.2

2.2 2.2 2.2

3 3 3

6 (no light) 6 6 6

2.2

3

6

91.0

time (h)

yield (%)

36.0 36.4 71.5

a

Reaction conditions: a certain amount of Ru(II) catalyst, 2.0 mmol 2Bromoacetophenone, 1.5 mL DMF, a 50 W fluorescent lamp (450 nm), T = 25 °C.

the dehalogented product with 81.3% yield after 6.0 h. Also, we tested the blank reaction and the control experiment by using PhSO3Na-functionalized reduced graphene oxide support (PhSO3Na-rGO) as the catalyst. Under the optimized reaction conditions, the slow background reaction can lead to the detectable amounts of the debrominated product with 36.0% yield under light irradiation while PhSO3Na-rGO gave the similar yield (36.4%) with the blank reaction. Meanwhile, without visible light irradiation, the similar yield (37.2%) with the blank reaction was obtained. These results clearly indicated that Ru(II) complex was essential for this transformation.32 Next, we also prepared Ru(bpy)32+-containing Amberlyst 15 resin (Ru(bpy)3-resin) by a two-step ion-exchange and coordination procedure that similar to the synthetic process of Ru(bpy)3-rGO. ICP analysis revealed that 0.16 mmol/g Ru(II) content was obtained in Ru(bpy)3-resin catalyst. In the same reaction conditions, Ru(bpy)3-resin gave the yield of 71.5%. Next, we investigated the amounts of Hantzsch ester 12144


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Figure 4. (a) UV−vis spectra of Ru(bpy)3Cl2 and Ru(bpy)3-rGO. (B) Fluorescence emission spectra of Ru(bpy)3Cl2·6H2O and Ru(bpy)3-rGO upon excitation at 465 nm.

465 nm corresponds to metal-to-ligand charge-transfer while Ru(bpy)3-rGO showed the similar absorption peak.33 Furthermore, the fluorescence spectra of the Ru(bpy)3Cl2·6H2O and Ru(bpy)3-rGO samples at 465 nm excitation were shown in Figure 4b. Ru(bpy)3Cl2 solution exhibited a metal-to-ligand charge-transfer emission at 615 nm. Interestingly, Ru(bpy)3rGO showed almost the same peak and meanwhile the fluorescence emission intensity did not have the significant decrease, indicating the negligible energy/electron transfer from Ru(II) complex to rGO support. Furthermore, we tested the electrical conductivity of Ru(bpy)3-rGO sample and it very had low conductivity with 0.30 s/cm. Thus, the negligible emission quenching was maybe due to the nonconductive PhSO3Na-linkage and the very low electrical conductivity of the catalyst.34,35 With the aim of exploring the possibility of the wide utilization of Ru(bpy)3-rGO catalyst, we furthermore tested a series of visibl-light-driven reductive dehalogention reactions and the results were shown in Table 2. Different derivatives with the electron donor or withdrawing functional groups in the benzene rings were investigated (entries 1 and 2). As expected, a 2-Bromoacetophenone derivative with NO2electron withdrawing substituent was conducive to the dehalogention reaction and 85.6% yield of the product was obtained. On the contrary, CH3- electron donor substituent led to a slight decrease yield of 72.8% after 7.0 h. Moreover, the change of halogen position or the increase of reactant size did not affect the yields and both of them gave the good catalytic activities (entries 3 and 4). Ever for acetophenone chloride with strong bound leaving group (entry 5), Ru(bpy)3-rGO catalyst also showed the 75.2% photoreduction yield. Ru(bpy)3-resin could catalyze different reactants with the electron donor or withdrawing functional groups as well as chlorinated substances. However, all the obtained yields were lower than those of Ru(bpy)3-rGO catalyst in the same reaction conditions, further demonstrating the advantage of Ru(bpy)3-rGO catalyst. On the basis of the catalytic results of Ru(bpy)3-rGO, the reaction mechanism was consistent with the homogeneous Ru(bpy)3C12 catalyzed visible-light-driven dehalogenation reaction that proposed by Stephenson group.31 Specifically, Ru(II) complex was first activated by visible light to form the excited Ru(II)* species (Scheme 2). Then, the single-electron transfer process that from iPr2NEt to Ru(II)* generated Ru(I)

and iPr2NEt and the catalyst loading in the Ru(bpy)3-resin catalyzed 2-Bromoacetophenone dehalogenation reaction. As shown in Table S3, even in the optimized reaction conditions, Ru(bpy)3-resin still displayed the inferior catalytic efficiency in comparison with Ru(bpy)3-rGO catalyst. Furthermore, we increased the amount of iPr2NEt base to 4.0 equiv, and the yield of acetophenone product was 73.8%. Moreover, we used NaOH aqueous solution (1.0 mol/L) to neutralize the sulfonic acid groups in the Ru(bpy)3-resin sample. The basified Ru(bpy)3-resin obtained the yield with 78.0%, confirming the surface sulfonic acids groups have the negative effect on the outcome of this reaction. Next, we measured the surface areas of Ru(bpy)3-resin and Ru(bpy)3-rGO samples by using N2 sorption analysis. Ru(bpy)3-resin and Ru(bpy)3-rGO had 40 and 125 m2/g, respectively, which confirmed the easier accessibility of Ru(II) complex in the Ru(bpy)3-rGO catalyst for the reaction. Moreover, Figure S5 showed that Ru(bpy)3resin readily settled down from the reaction solvent while Ru(bpy)3-rGO remained high dispersion in solution within 1.0 h. This semihomogeneous catalytic behavior was probably related to the enhanced reactivity of Ru(bpy)3-rGO catalyst. These results demonstrated that the advantage of the twodimensional sheet-like structure of Ru(bpy)3-rGO catalyst. Furthermore, we tested the catalytic efficiency of homogeneous catalyst Ru(bpy)3Cl2 in the same reaction conditions and it obtained the yield of 91.0%. This similar yields between Ru(bpy)3Cl2 and Ru(bpy)3-rGO revealed that the reduced graphene oxide support did not generative the significant negative effects on the catalytic efficiency. To explain this phenomenon, we first measured the absorbance of Ru(bpy)3Cl2·6H2O, rGO and PhSO3Na-rGO samples at the same concentration (0.1 mg/mL) in DMF solution. As shown in Figure S6, Ru(bpy)3Cl2·6H2O displayed the significantly stronger absorption intensity than those of rGO and PhSO3NarGO samples in the UV−vis range from 300 to 800 nm. Moreover, PhSO3Na-rGO support exhibited the decreased absorption intensity due to the increased nonconjugated PhSO3Na-linkage on the surface of rGO, which indicated that the Ru(II) complex in the reduced graphene oxide could efficient adsorb the light. Then, we compared the photoluminescence spectra of Ru(bpy)3Cl2 complex and Ru(bpy)3rGO. As shown in Figure 4a, the UV−vis spectrum of Ru(bpy)3Cl2 solution displayed a peak broad visible band at 12145


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ACS Applied Materials & Interfaces Table 2. Substrate Scope for Ru(bpy)3-rGO and Ru(bpy)3Resin Catalyzed Visible Light-Mediated Reductive Dehalogenation Reactionsa

1.5 ppm existed in the reaction solution. Therefore, the present photocatalytic system was indeed heterogeneous in nature. An attractive advantage of the Ru(bpy)3-rGO catalyst is its easy recycle and reusability, which reduces the cost and heavy metal pollution.37 As shown in Figure 5, Ru(bpy)3-rGO catalyst could

a Reaction conditions: 0.05 mmol Ru(bpy)3-rGO catalyst, 2.0 mmol carbonyl halogen derivative, 4.4 mmol Hantzsch ester, 6.0 mmmol iPr2NEt, 1.5 mL DMF, a 50 W fluorescent lamp (450 nm), T = 25 °C.

Figure 5. Recycling tests of Ru(bpy)3-rGO and Ru(bpy)3-resin catalysts during visible light-mediated reductive dehalogenation reaction. Reaction condition was shown in Table 1.

be repeated at least five times without significant loss of catalytic activity. However, Ru(bpy)3-resin exhibited the remarkable loss in catalytic reactivity (30%) after being reused for four recycles (Figure 2). We first analyzed ruthenium leaching both in the reused Ru(bpy)3-rGO and Ru(bpy)3-resin catalysts by ICP analysis. The result revealed that around 8% ruthenium lost after the fifth recycles (Table S2). But, the significant Ru(II) leaching (27.5%) was found in the Ru(bpy)3resin catalyst, which was maybe due to the easier leaching of the sulfonic acid groups that connected with Ru(bpy)3 complex in the Ru(bpy)3-resin.38 TEM image (Figure S7) showed that the slight aggregation of Ru(bpy)3-rGO catalyst could be found after the recycled test. Meanwhile, the deactivation pathway through radical addition reported by Stephenson group was also probably existed in our system.39 However, the molecular level analysis of Ru(II) complex in the heterogeneous Ru(bpy)3-rGO catalyst was hard to observe based on the current phyco-chemical characterization techniques. Therefore,

complex. Finally, reduction of the carbon−halogen bond by the electron-rich Ru(I) generated the alkyl radical, which abstracts a hydrogen atom from Hantzsch ester to yield the reduction product. To confirm whether the heterogeneous or the leaching Ru(bpy)32+ complex was the real active species, the following procedure was executed.36 After the conversion exceeded 50% in the reductive dehalogention reaction using 2-Bromoacetophenone as the reactant (1.0 h), the mixture was centrifuged to remove the solid catalyst and then the remaining liquor continued to stir for another 6.0 h. No significant change in the 2-Bromoacetophenone conversion or acetophenone yield was observed, suggesting that the catalytic activity by the leaching Ru(bpy)32+ complex could be approximately excluded in our system. Furthermore, the liquid phase of the reaction mixture was analyzed by ICP-AES after each run. The result demonstrated that a very low amount of ruthenium around

Scheme 2. Plausible Mechanism for the Ru(bpy)3-rGO Catalyzed Visible-Light-Driven Reductive Dehalogenation Reaction

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we supposed that the deactivation of Ru(bpy)3-rGO catalyst was mainly attributed to the ruthenium leaching and the catalyst aggregation.

4. CONCLUSIONS In summary, we developed a unique approach for the construction of reduced graphene oxide-immobilized tris(bipyridine)ruthenium(II) catalyst by using aryl diazonium coupling and ion-exchange approaches associated with postgrafting treatment. This heterogeneous visible-light photocatalyst showed high catalytic efficiency in the reductive dehalogenation reactions and could be readily recycled by filtration and reused without significant loss of its catalytic reactivity in a test run five times. The novel synthetic route is of great potential in the control synthesis of highly active and stable artificial photocatalytic systems for various visible-lightdriven organic syntheses.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01100. Elemental analysis of rGO and NaSO3-rGO; catalyst loading and reaction condition optimization; catalytic performance of Ru(bpy)3-resin; S 2p XPS spectrum of PhNaSO3-rGO, UV−vis spectrum of Ru(bpy)3Cl2·6H2O, rGO, and PhNaSO3-rGO. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-21-64322272. Fax: +86-21-64322272. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (51273112), PCSIRT (IRT1269) and RFDP (20123127120007).

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REFERENCES

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