Partially Fluoride-Substituted Hydroxyapatite as a Suitable Support for

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Partially Fluoride-Substituted Hydroxyapatite as a Suitable Support for the Gold-Catalyzed Homocoupling of Phenylboronic Acid: An Example of Interface Modification Setsiri Haesuwannakij, Yumi Yakiyama, and Hidehiro Sakurai ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03524 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 16, 2017

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Partially Fluoride-Substituted Hydroxyapatite as a Suitable Support for the Gold-Catalyzed Homocoupling of Phenylboronic Acid: An Example of Interface Modification Setsiri Haesuwannakij, † Yumi Yakiyama, ‡ Hidehiro Sakurai* ‡ †

Department of Functional Molecular Science, SOKENDAI (The Graduate University for

Advanced Studies) Myodaiji, Okazaki 444-8787, Japan ‡

Division of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1

Yamada-oka, Suita, Osaka 565-0871, Japan

ABSTRACT

Interface modification of hydroxyapatite-supported nanogold by fluoride ion improved the activity of the catalyst toward aerobic homocoupling of phenylboronic acid. In the aerobic homocoupling reaction of PhBF3K catalyzed by hydroxyapatite-supported gold nanoclusters (Au:HAP), a study on the reactivity and reusability of the Au:HAP catalyst showed an unusual yield profile for the reaction. Intensive characterization of the catalyst by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopic analysis, exhibited

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that the structure of the metal oxide support undergoes transformation from HAP to fluorapatite (FAP) and finally to calcium fluoride (CaF2) during the course of these cycles of homocoupling of PhBF3K. Investigations of the interactions of Au:HAP with PhBF3K and of the fluoride ioninduced structural changes in both the support and the AuNCs, induced that partially fluoridesubstituted HAP (F-HAP) is the optimal support for the homocoupling of PhB(OH)2. This is effective both as a stabilizer for AuNCs through the phosphate moiety and in activation of C–B bond transmetalation through B–F interactions. The results strongly suggest that fine tuning of the structure of the interface between metal clusters and their support (namely, surface modification) might be important in developing chemo-selective catalysts. KEYWORDS Interface modification, Gold nanoclusters, Homocoupling reaction, Apatite, C-B bond activation

INTRODUCTION C-C bond formation reaction is one of the most important synthetic approach in organic synthesis.1 In terms of that, although still Pd-based catalysts have been the dominant especially in Suzuki-Miyaura type reactions in which homo- and cross-coupling products competitively form, Au-based catalyst has been the attractive target in terms of higher selectivity to obtain homo-coupling product in the case of oxidative coupling reaction such as generation of biaryls from aryl boronic acids ArB(OH)2.2 The colloidal gold nanoclusters (AuNCs) protected by poly(N-vinylpyrrolidone) (PVP) (Au:PVP) is an effective catalyst for the aerobic homocoupling of arylboronic acids at room temperature under basic conditions,3 and its properties and mechanism were elucidated by experimentally and theoretically in detail.4 Whilst, heterogeneous AuNCs has also been studied which enables more practical catalysts that show high recyclability


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and greater ease of operations.5 However, rational approaches to heterogeneous catalysis are less established due mainly to the lack of information about the structure of the interface. In addition, its reactivity is generally lower than that of quasi-homogeneous systems so that, in general, higher temperatures are necessary to perform the reaction.2a,5 We recently developed a new method for transdeposition of AuNCs from a PVP matrix onto hydroxyapatite (HAP), which possesses a good adsorption capacity and a non-porous structure, without a significant change in cluster size.6 This method permits the precise study of various aspects of the catalytic activity of heterogeneous AuNCs. Here we show the a unique size dependency in the Au:HAP-catalyzed aerobic oxidation of 1indanol as well as the fluoride ion induced a structural change in the support from HAP to CaF2, through partial formation of fluorapatite (FAP), during recycling, which also markedly changed the catalytic activity of the gold. Finally, by altering the quantity of fluoride present on the solid support, we were also able to tune its properties to promote selective homocoupling of PhB(OH)2 under neutral conditions.

EXPERIMENTAL SECTION Preparation of Poly(N-vinyl-2-pyrrolidone) (K-15; Mw = 10 kDa) (PVP(K-15))-stabilized Gold Nanoclusters (Au:PVP (K-15)). The PVP (K-15)-stabilized gold nanoclusters (Au:PVP (K-15) was followed the reported procedure.7 The rapid reduction of HAuCl4 by NaBH4 was performed in aqueous solution of PVP (K-15, 10 kDa) solution under 0 °C to yield a brown hydrosol of Au:PVP (K-15). The molar ratio of HAuCl4, monomer unit of PVP (K-15) and NaBH4 was kept at 1:50:10. The resulting solution mixture was concentrated to 10 mL by centrifugal ultrafiltration (MWCO 3kDa), and


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washed with pure water three times. The obtained brownish solution was further used in the next step.

General Procedure for Trans-deposition of Au:PVP to HAP. The trans-deposition process was followed the reported procedure.6 To reaction tube (Φ = 42 mm), 0.025 mmol of Au:PVP and 250 mg of HAP was dispersed in 30 mL of ethanol/water (1:1 (v/v)), and refluxed under argon atmosphere at 1700 rpm for 90 min. The solid part was separated by centrifugation at 7500 rpm and washed with ethanol. The resulting solid was dried under vacuum at 45 °C overnight and calcined at 450 °C for 3 h under vacuum to obtain Au:HAP.

General Procedure for Aerobic Homocoupling Reaction of PhBF3K. The aerobic homocoupling reaction was performed in a temperature-controlled personal organic synthesizer (EYELA PPS-1510, Tokyo Rikakikai Co.Ltd., Tokyo) under aerobic conditions. PhBF3K (13.8 mg, 0.075 mmol) and Au:HAP (0.00375 mmol, 5 atom%) were placed in a 15-mm-diameter test tube. The lower Au/substrate ratio resulted in the termination of the reaction without completion (ex. 1 atom% afforded only 86% yield even after 24 h). Buffer solution (pH 6.86, 0.9 mL) and ultrapure water (1.5 mL) were added, and the mixture was stirred at 1300 rpm at 300 K for five hours. The catalyst was separated by using membrane filter and washed with ethyl acetate (10 mL). The filtrate was extracted with ethyl acetate (3 × 10 mL), and the organic layer were combined, diluted to 50 mL, and subjected to quantitative analysis by gas chromatography (GC; GC-2014, Shimadzu, Kyoto) on an Rtx-5MS (30 m × 0.25 mm × 0.25 µm)


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capillary column. Hexadecane was used as an internal standard. The GC yield was calculated by using a calibration curve. The used Au:HAP catalyst was dried under a vacuum at 45 °C for three hours, and a portion was collected for structural determination by scanning transmission electron microscopy–energydispersive x-ray spectral analysis (STEM-EDX), X-ray diffraction (XRD), and inductively coupled plasma–atomic emission spectroscopy (ICP-AES). The remaining Au:HAP was used in a subsequent reaction cycle without further treatment.

Aerobic Homocoupling of PhB(OH)2; General Procedure. The aerobic homocoupling was performed in a temperature-controlled personal organic synthesizer (EYELA PPS-1510) under aerobic conditions. PhB(OH)2 (9.14 mg, 0.075 mmol) and Au:HAP (0.00375 mmol, 5 atom%) were placed in a 15-mm-diameter test tube. Buffer solution (pH 6.86, 0.9 mL) and ultrapure water (1.5 mL) were added, and the mixture was stirred at 1300 rpm at 300 K for five hours. The catalyst was separated by using a membrane filter and washed with ethyl acetate. The filtrate was extracted with ethyl acetate (3 × 10 mL), and the organic layers were combined, diluted to 50 mL, and subjected to quantitative analysis by GC (GC-2014, Shimadzu) on an Rtx-5MS (30 m × 0.25 mm × 0.25 µm) capillary column. Hexadecane was used as internal standard. The GC yield was calculated by using a calibration curve. The used Au:HAP catalyst above was dried under a vacuum at 45 °C for three hours and a portion was collected for structural determination by STEM-EDX, XRD, and ICP-AES. The remaining Au:HAP was used in a subsequent reaction cycle without further treatment.

Preparation of Partially Fluoride-Substituted HAP (F-HAP).


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A 30-mm test tube was charged with HAP (500 mg) and PhBF3K (184 mg, 1 mmol). Buffer solution (pH 6.86, 12 mL) and ultrapure water (20 mL) were added, and the mixture was stirred at 1300 rpm at 300 K for five hours. The resulting white solid was collected by membrane filtration, washed with ultrapure water, and dried under a vacuum.

Preparation of Fluoroapatite. Fluorapatite (FAP) was prepared by following the reported procedure.8 A 0.03 M aqueous solution of diammonium hydrogen phosphate [(NH4)2HPO4], a 0.05 M aqueous solution of calcium nitrate tetrahydrate [Ca(NO3)2·4H2O], and a mixed solution containing 0.03 M aqueous ammonia (NH3) and 0.01 M aqueous ammonium fluoride (NH4F) were prepared. The mixed solution was used as a buffer solution to adjust the pH of the aqueous solutions of (NH4)2HPO4 and Ca(NO3)2·4H2O to a value of 9.72. The (NH4)2HPO4 solution was added dropwise to the Ca(NO3)2.4H2O solution by using syringe pump at a flow rate of 400 mL/h. The resulting white precipitate was collected by centrifugation and washing twice. The resulting white powder was dried overnight under a vacuum at 45 °C.

Deposition–Precipitation Method; General Procedure. Gold nanoclusters were deposited onto the support by using urea as a precipitating agent by following the reported procedure.9 Typically, 125 mg of solid support was added to an aqueous solution containing 0.0125 mmol of HAuCl4 and 1.25 mmol of urea. The mixture was heated to 90 °C and stirred for four hours at 1000 rpm. The resulting solid product was collected by filtration and washed several times with water. The solid was dried overnight and then calcined at 300 °C for four hours to give a purple solid product.


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RESULTS AND DISCUSSION Homocoupling Reaction of PhBF3K Catalyzed by Au:HAP. Au:HAP was prepared according to our recently reported transdeposition method from Au:PVP (Supplementary Information; Fig. S1).6 The catalyst, which had a mean size of 1.5 ± 0.4 nm, was used in subsequent investigations. When PhBF3K was treated with 5 atom% Au:HAP in pH 6.86 buffer solution under ambient conditions, the reaction was complete within five hours and gave biphenyl (1) in 92% yield, accompanied by less than 1% of phenol (2, Scheme 1). The activity and selectivity of Au:HAP were comparable to those of the parent Au:PVP.

Scheme 1. Oxidative homocoupling reaction of PhBF3K catalyzed by AuNCs.

Because Au:HAP showed an excellent catalytic activity toward aerobic homocoupling of PhBF3K and because reusability was one of our prime motivations in developing this catalyst, we conducted a reusability test on Au:HAP by using 5 atom% AuHAP under neutral conditions. After five hours of reaction time, the catalyst was collected by membrane filtration and dried under vacuum for reuse in a subsequent cycle. This second cycle always gave better results than the first cycle, and 1 was obtained in quantitative yield without any byproduct. A third cycle also


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resulted in a quantitative reaction, but the reactivity suddenly decreased from the fourth cycle onward, although the yield of 1 remained satisfactory (76%) after the fifth cycle (Fig. 1). To rationalize this trend, we characterized the structural changes in the catalyst during the recycling process by means of transmission electron microscopy (TEM), energy-dispersive X-ray spectral analysis (EDX), and powder X-ray diffraction (PXRD) studies.

Figure 1. The yield change of the coupling product 1 in five catalytic cycle using Au:HAP. Phenol was found in trace amount in all cases.

Characterizations of Au:HAP TEM provided direct information about the size and morphology of the AuNCs, and it showed both the size and the surface structure of the particles changed significantly as the number of reaction cycles increased (Fig. 2). Importantly, EDX analysis revealed the presence of fluoride ion (F–) on the surface of the catalyst, even after the first cycle with PhBF3K (Fig. S2–S4). This result suggests that hydroxy groups on HAP are partially replaced by fluoride ions resulting in


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the partial formation of FAP.10,11,12 The molar ratio of Ca to F approached 1:2 after the fifth cycle, corresponding to formation of CaF2 (Table S2). These results were consistent with the PXRD results (Fig. 3). The PXRD pattern of the original Au:HAP almost corresponded to that of HAP. After repeated use of Au:HAP for more than three cycles, the main peaks in the PXRD patterns at 2θ = 28°, 47°, 56°, and 69° could all be assigned to CaF2 and the peaks for the HAP skeleton had disappeared. A PXRD pattern for the Au (111) plane at 2θ = 39° became visible after several cycles of reaction. To monitor when the structure of the surface began to change, we stopped the first cycle of reaction after 30 minutes of reaction time and we investigated the PXRD pattern of the collected Au:HAP. The observed PXRD pattern was slightly different from that of the parent Au:HAP especially at 2θ = 28° (Fig. S5). In a comparison with the PXRD of Au:HAP collected after 5 h of reaction time, new peak at 2θ = 28° became clear and there was a slight decrease in the intensity of the peaks corresponding to the initial HAP skeleton. It is impossible to distinguish the PXRD pattern of HAP from that of FAP; nevertheless, the EDX data (Fig. S2–S4) suggest that the surface of the HAP were partially converted into FAP, and then finally covered with CaF2 during repeated catalytic cycles. This structural transformation might arise from generation of fluoride ion from PhBF3K, which induced partial exchange of hydroxy ions for fluoride ions on the HAP support during the catalytic reaction, finally giving calcium fluoride (CaF2) as supported by FT-IR data of Au:HAP using ATR attachment after each reaction cycle (Fig. S6).10,11,12 Similar observations were made Masuyama in the case of a Suzuki–Miyaura-type cross-coupling reaction of aryl halides with potassium aryltrifluoroborates catalyzed by palladium(II)-exchanged hydroxyapatite (Pd:HAP) under basic condition.12 A loss of the structural properties of the HAP surface was observed when the Pd:HAP was repeatedly used in several cycles of the reaction.


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Figure 2. TEM images and core-size distribution of Au:HAP after each catalytic cycle a)-f) in oxidative homocoupling of PhBF3K.


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Figure 3. PXRD pattern changes after the repeating use of Au:HAP, HAP, FAP and CaF2. The pattern of FAP was obtained from the database.13 The peak intensities are normalized.

To determine the source of the fluoride ions that effected the transformation of the HAP, we examined the effect of fluoride salts. Time-dependent PXRD measurements revealed that Au:HAP was unchanged when it was kept in a pH 6.86 buffer solution containing an equimolar amount of a fluoride source such as potassium fluoride (KF) (Fig. S7). In contrast, after the addition of PhBF3K, the PXRD patter of Au:HAP began to change with forming CaF2, indicating the shorter distance between the fluoride ion and the support than in case of KF. This confirmed that adsorption of PhBF3− onto the surface of Au:HAP is essential for the structural transformation of HAP into CaF2.


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The Dependence of the Catalytic Activity of Au:HAP on its Structure Because the nature of their solid support strongly affects the catalytic properties of metal nanoclusters,2a,5a-b,14,15,16 it is reasonable to assume that changes in the structure of the support from HAP to CaF2 were responsible for the differences in the yields after each reuse of the catalyst. The surface of HAP might play important roles in stabilizing the AuNCs to agglomeration via the phosphate group17 and in activating C–B bond transmetalation through the B–O coordination through the activation of a surface hydroxy group.18 After partial substitution of OH by F on the HAP surface to generate FAP, the stabilization effect AuNCs was maintained while the activation effect to transmetalation was enhanced because of the strong interactions associated with B–F coordination.19 On the other hand, on further reaction to form CaF2, no more phosphate surface remains and leaching and aggregation of AuNCs starts, causing a decrease in the catalytic activity (Scheme 2 and Table S3).

Scheme 2. Structural change affect properties of AuNCs.

Our analysis strongly suggested that FAP or partially F-substituted HAP (F-HAP) might be more suitable as a support for AuNCs in the catalyzed homocoupling of organoboron


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compounds. However, because PhBF3K causes a structural change by releasing fluoride ions during the reaction, we decided to investigate the use of PhB(OH)2 as a substrate for the following two reasons. The first was to exclude the effects of F-induced structural change. The second was that phosphate and/or fluoride on the surface might be a sufficiently good activator of C–B bond transmetalation, even under neutral conditions. We therefore prepared three kinds of AuNCs: AuNCs supported on partially fluoride-substituted HAP (Au:F-HAP), on FAP (Au:FAP), or on CaF2 (Au:CaF2). The four different types AuNCs; AuNCs supported on HAP (Au:HAP), partially fluoride-substituted HAP (Au:F-HAP), on FAP (Au:FAP), or on CaF2 (Au:CaF2), were characterized by TEM, STEM-EDX, XPS and FT-IR (Fig. S8-S11, Table S4). The partially fluoride-substituted HAP (F-HAP) was prepared by anion exchange of HAP with PhBF3K under neutral conditions. The Au:F-HAP catalyst was prepared by the same transdeposition method as used for Au:HAP;6 it had a core size of 1.5 ± 0.3 nm (Fig. S8a). Au:FAP and Au:CaF2 were also prepared by the conventional deposition–precipitation method, resulting in a mean size of 4.0 nm in both cases (Fig. S8b and S8c). To compare the activities of these catalysts under neutral conditions (Scheme 3), reactions were carried out in pH 6.86 buffer solution at 300 K and quenched after 2 h. The results are shown in Table 1. As expected, all the catalysts exhibited activity toward the oxidative homocoupling reaction though the size effect of the catalysts cannot be ignored. The reaction proceeded even under neutral conditions in which the formation of 2 was suppressed. The yields of biphenyl 1 after two hours with Au:HAP, Au:F-HAP, Au:FAP, and Au:CaF2 were 76%, 89%, 89%, and 83%, respectively. In comparison with other heterogeneous catalysts previously reported, Au:HAP and the related catalysts gave the excellent yields.5


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Scheme 3. Oxidative homocoupling reaction of PhB(OH)2 catalyzed by AuNCs.

Table 1. The catalyst-dependent reaction yield of the oxidative homocoupling after 2 h.

Entry

AuNCs

1

%Yield 1

2

Au:HAP

76

Partially Fluoride-Substituted Hydroxyapatite as a Suitable Support for

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