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Role of Surface Basic Sites on Sonogashira Coupling Reaction over Ca5(PO4)3OH Supported Pd Catalyst: Investigation by Diffuse Reflectance Infrared Fourier Transform Spectroscopy Vishali Bilakanti, Venu Boosa, Vijay Kumar Velisoju, Naresh Gutta, Sudhakar Medak, and Venugopal Akula J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07620 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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The Journal of Physical Chemistry

Role of Surface Basic Sites on Sonogashira Coupling Reaction over Ca5(PO4)3OH Supported Pd Catalyst: Investigation by Diffuse Reflectance Infrared Fourier Transform Spectroscopy Vishali Bilakanti, Venu Boosa, Vijay Kumar Velisoju, Naresh Gutta, Sudhakar Medak and Venugopal Akula* Catalysis Laboratory, Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad-500 007, Telangana, India *Corresponding author: E-mail: [email protected] Tel.: +91-40-27193165; Fax: +91 40 27160921

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ABSTRACT: Hydroxyapatite [Ca5(PO4)3(OH): HAP] supported Pd catalyst has been identified as an efficient and reusable heterogeneous catalyst for Sonogashira coupling reaction under copper and phosphine free conditions. The 2wt%Pd/HAP offered excellent yields (96%) compared to 2wt%Pd/MgO and 2wt%Pd/Al2O3 catalysts. The formic acid adsorbed diffuse reflectance infrared Fourier transform spectra (DRIFTS) revealed that moderate basic sites on 2wt%Pd/HAP are found to be selective for the Sonogashira coupling reaction. Strong basic sites present on Pd/MgO demonstrated homo-coupling of the substrate in parenthesis to Sonogashira reaction. The physicochemical characteristics of the catalysts are rationalized by temperature programmed desorption (TPD) of CO2, H2-temperature programmed reduction (TPR), formic acid adsorbed DRIFTS, (Brunnauer-Emmet-TellerSurface Area) BET-SA, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and CO chemisorption techniques. The 2wt%Pd/HAP catalyst was recovered and reused for four recycles that showed consistent activity and selectivity. Nature and strength of the surface basic sites influence the coupling reaction through oxidative addition and reductive elimination on surface Pd sites.


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1. INTRODUCTION The palladium catalyzed cross-coupling reaction is one of the significant and widely used methods for the synthesis of aryl alkynes and enynes.1-3 The Sonogashira cross-coupling is an industrially important method due to extensive use of compounds derived from this process being used in the preparation of pharmaceutical ingredients, electronic materials, agrochemicals and liquid crystal materials.4-7 Sonogashira coupling reactions are routinely performed in the presence of palladium and a copper catalyst in an amine solvent. However, use of copper halides (CuX; X = Cl and I) as a co-catalyst has some drawbacks such as formation of homo coupling products and non eco-friendly nature of these salts.8-12 Therefore, efforts have been made to develop catalytic systems that work in the absence of copper for Sonogashira cross-coupling reactions. Generally, copper free Sonogashira reactions were carried out in the presence of excess amine, either as solvent or as a base which could be a hurdle from the environmental and economical perspectives. Application of homogeneous Pd based catalysts for the Sonogashira coupling reaction requires separation and catalyst recovery which again is a tedious task and might result in unacceptable palladium contamination of the products.13-15 Hence, development of heterogeneous catalyst systems particularly supported metal catalysts have become a topic of interest, since it enables an easy recovery and repeated use of catalyst. The Pd based heterogeneous catalysts have broad scope owing to an advantage of avoiding both copper and amine solvent. A large variety of organic and inorganic materials have been explored for Sonogashira coupling over Pd based catalysts e.g. silica, charcoal, synthetic polystyrene and alumina as supports.16 The Pd/MgO catalyst displayed good performance in the Sonogashira reaction of haloarenes using various terminal acetylenes with the exception of the coupling of chlorobenzene with phenylacetylene.17 Earlier studies are reported on the polymer supported Pd catalysts under aerobic conditions in water in the absence of Cu and phosphine.18,19 3 ACS Paragon Plus Environment


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Recently, hydroxyapatite (HAP) has been identified as a choice of support material for several metal and/or metal oxide catalysts owing to porous nature and also its acid-base characteristics. For example, HAP supported gold and ruthenium catalysts were used in the water gas shift reaction as well as a stand-alone catalysts for the dehydrogenation and dehydration of alcohols.20-22 HAP has been used as a support for Ni, Pd, Ru and Cu catalysts for the vapour phase hydrogenation of biomass derived levulinic acid into alkyl levulinates.23, 24

Scheme 1. Cross coupling reaction of iodobenzene and phenyl acetylene.

To date there are no reports on Pd/HAP catalyst for Sonogashira coupling reaction in the absence of Cu and phosphine. In this investigation the Pd supported on HAP [Ca5(PO4)3OH], Al2O3 and MgO were examined for Sonogashira coupling reaction (Scheme 1). The aim of this study is four fold: first, to examine the catalytic activity of Pd supported on HAP (a naturally occurring mineral) for Sonogashira coupling, second, comparative analysis of Pd supported on HAP, MgO (a strong basic nature material) and an amphoteric oxide i.e. Al2O3; third, to avoid the use of both amine and CuI in the coupling process. And finally, to understand the nature and strength of the surface active sites for Sonogashira coupling reaction is rationalized by formic acid adsorbed diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and CO pulse chemisorption techniques. The catalysts were also characterized by Brunnauer-Emmet-Teller-Surface Area (BET-surface area), transmission electron microscope (TEM), temperature programmed desorption (TPD) of CO2, powder X-ray diffraction (XRD), H2 - temperature programmed reduction (TPR), X-ray 4 ACS Paragon Plus Environment

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photoelectron spectroscopy (XPS) and carbon-hydrogen-nitrogen-sulphur (CHNS) analyses. The Sonogashira coupling reaction was carried out using aryl iodide and phenylacetylene as model substrates using different solvents such as H2O, CH3OH, DMF, DMF:H2O (mixture), in solvent free conditions and also in the absence of CuI and phosphine.

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. Hydroxyapatite [Ca5(PO4)3OH] was prepared by precipitation method and the conditions were maintained according to an earlier report.19 The precipitate was washed thoroughly, filtered and dried at 100 °C overnight and then calcined in static air at 500 °C for 5 h. These calcined HAPs are designated as HAP-7, HAP-10 and HAP-12 corresponding to their pH maintained during the gelation/preparation. The MgO (BET-surface area: 39.8 m2/g) support was prepared by a precipitation method using Mg(NO3)2.6H2O as a precursor and K2CO3 as a precipitating agent. The 2wt%Pd supported on HAP, MgO and Al2O3 (Harshaw Al-3945, BET surface area: 191.4 m2/g) catalysts were prepared by a wet impregnation method. In a typical method, required amount of aqueous Pd(NO3)2.xH2O precursor was taken to give a metal loading of 2wt% over support. The solvent is then evaporated with constant stirring. The samples were dried at 120 °C for 12 h and subsequently calcined in air at 450 °C for 5 h at a ramping rate of 5 °C/min. The in-situ reduction of Pd(II)/support was performed using hydrazine hydrate in ethanol at room temperature for 3 h to give an air-stable black Pd(0)/support powder. 2.2. Characterization of Catalysts. Purification of reaction products was carried out by flash chromatography using 100-200 mesh silica gel and a mixture of ethyl acetate and petroleum ether as the eluting agent. 1H NMR spectra were recorded by using Bruker VX NMR FT-300 or Varian Unity 500 and 13C NMR spectra were recorded by using Bruker VX NMR FT-75 MHz spectrometers instrument in CDCl3. Chemical shifts (δ) are reported in 5 ACS Paragon Plus Environment


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ppm, using TMS as an internal standard. All the catalysts were extensively characterized by various techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H2-temperature programmed reduction (H2-TPR), temperature programmed desorption of CO2 (TPD of CO2), transmission electron microscopy (TEM), atomic absorption spectroscopy (AAS) and CO-pulse chemisorption. All the details pertaining to characterization details and instrumentation are given in ESI. The nature of basic sites is investigated using formic acid adsorption in conjunction with FT-IR spectroscopy. The spectra were collected in the range of 2000-1200 cm-1 with a resolution of 4 cm-1 and 64 no. of scans. The experiments were carried out using DRIFT (Harrick) cell connected to a vacuum-adsorption unit. The self supported samples were then pre-heated at 300 °C for 30 min. After cooling down to 125 °C, the spectrum was collected and used as a blank spectrum. Then the samples were exposed to formic acid (98%; Sigma-Aldrich) followed by vacuum for 30 minutes. The spectra were obtained after formic acid adsorption subtracted from the blank spectrum. Finally, the resultant spectra were quantified using the Kubelka-Munk function. 2.3. General Catalytic Procedure for the Sonogashira Coupling Reaction. Aryl iodide (0.5 mmol), phenyl acetylene (0.5 mmol), K2CO3 (0.5 mmol), catalyst (20 mg) were taken in an oven dried round bottom flask was placed on a magnetic stirrer cum hot plate. The mixture was dissolved in CH3OH:H2O (1:1) and stirred at 95 °C for 150 min. The reaction mixture was then cooled down to room temperature and 5 mL of CH3COOC2H5 was added to reaction mixture and centrifuged to recover the catalyst. The aqueous layer was extracted with CH3COOC2H5 and the combined organic layers were dried in Na2SO4. The crude reaction mixture was then purified by column chromatography (silica gel, eluent, hexane/ethyl acetate mixtures) to afford corresponding alkynes. All the products were identified on the basis of 1H and 13C NMR spectral data.


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3. RESULTS The catalytic activities over 2wt%Pd supported on HAP were examined for the Sonogashira coupling reaction (Scheme 1). In order to optimize the reaction conditions such as solvent, reaction temperature and catalyst loading; a series of experiments were carried out using iodobenzene (Table 1). The Pd/HAP gave 79% yield in CH3OH solvent at 95 °C and the yield is further improved to 96% using CH3OH:H2O mixture, whereas the catalytic activity significantly decreased to 22% when H2O alone was used as a solvent. The decrease in yield is probably due to the insolubility of the substrate in H2O. Considering above results CH3OH:H2O mixture was used as a solvent and compared to DMF:H2O under the conditions employed (Table 1). Table 1 Screening of Reaction Parameters for Sonogashira Coupling Reaction over Pd Supported on HAP, MgO and Al2O3 Reaction temperature: 95 °C. 2wt%Pd Time Solvent Base Yield By supported on (h) (%) product HAP-12 8.0 CH3OH K2CO3 79.0 0.0 HAP-12 2.5 CH3OH+ H2O K2CO3 96.0 0.0 HAP-12 24.0 H2 O K2CO3 22.0 0.0 HAP-12 8.0 DMF K2CO3 64.6 0.0 HAP-12 3.0 DMF+ H2O K2CO3 89.2 0.0 HAP-7 3.0 CH3OH+ H2O K2CO3 52.3 0.0 HAP-10 3.0 CH3OH+ H2O K2CO3 68.9 0.0 HAP-12 8.0 CH3OH Et3N 46.0 0.0 HAP-12 8.0 DMF Et3N 24.0 0.0 MgO 3.0 CH3OH+ H2O K2CO3 64.5 35.5a Al2O3 8.0 CH3OH+ H2O K2CO3 42.6 0.0

The product yields were low when the reaction temperature is less than 95 °C. Using triethylamine the product yields were (46 and 24%) low compared to K2CO3. With this preliminary data, we choose to optimize Pd-catalyst for the standard reaction, using K2CO3 as a base and CH3OH:H2O mixture as solvent. In the comparative analysis, the Pd/HAP-12 exhibited higher yields than the Pd/HAP-10 and Pd/HAP-7 catalysts (Table 1).



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Figure 1. The CO2-TPD patterns of a) HAP-7 b) HAP-10 c) HAP-12 samples.

The high catalytic activity can be explained by the higher number of basic sites present on (Figure 1) Pd/HAP-12 compared to Pd/HAP-7 and Pd/HAP-10, which are in good agreement with the literature reports.18 Under optimized conditions, the 2wt%Pd/Al2O3 showed 42.6% yield. Using a strong basic support i.e. MgO; the 2wt%Pd/MgO exhibited 64.5% desired product with a by-product formation of about 35.5%. These results emphasized that the coupling reaction rate is also dependent upon the basicity of the catalyst. It is also envisaged that the nature of basic sites play a key role on the coupling reaction. 3.1. Substrate Scope. Under the optimized conditions, we then proceed to assess the scope of the reaction in the coupling of various activated and inactivated aryl halides and terminal acetylenes over 2wt%Pd/HAP. The reactions with various electron-deficient and electron-rich aryl halides demonstrated the desired product in good to excellent yields. As expected, the aryl iodides having electron-withdrawing groups produced higher yields (9597%) than the substrate contained electron-donating groups, due to an increase in the electron 8 ACS Paragon Plus Environment

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density over aromatic ring leading to a decrease in the elimination possibility of iodide group from the substrate (Table 2, entry 1-8, 12, 13). The reaction is sensitive to nitro substitution on the ortho position of the iodobenzene due to steric effect that eventually afforded 80% yield of the product (Table 2, entry 8). The iodobenzene containing electron donating groups gave marginally lower yields (75-80%; Table 2, entry 9-11). Finally, to extend the scope of this protocol, compatibility of the reaction was investigated for aryl bromide. The substituted bromobenzenes as substrates gave corresponding cross-coupling products of relatively lower yields than iodobenzene substrates (Table 2, entry 14-20). When 2-Bromopyridine and 2Bromothiophene were subjected to the same procedure, about 65% and 62% yields of products were isolated (Table 2, entry 18-19) respectively. To our delight, this reaction is not limited to simple phenyl acetylene and its derivatives, it was more challenging with heterocyclic alkynes such as 10-(prop-2-yn-1-yl)-10H-phenothiazine, 10-(prop-2-yn-1-yl)10H-phenoxazine that were also reactive towards 4-iodo acetophenone and the desired products were obtained in 60 and 62% yields respectively (Table 2, entry 21-22).

Table 2 Sonogashira 2wt%Pd(0)/HAP S.No. Aryl halide

Reaction

Alkyne

with

Different

Substrate

Time Product (h)

Compounds

Yield %

1

2.5

96

2

2.5

97

3

2.5

97

4

2.5

94

5

2.5

96

6

2.5

96

7

2.5

95

O


over


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8

2.5

80

9

2.5

80

10

2.5

75

11

2.5

78

12

2.5

95

13

2.5

96

14

4

75

15

4

80

16

4

72

17

4

76

18

4

65

19

4

62

20

4

60

21

5

62

22

5

60

Reaction conditions: 20 mg of 2 wt% Pd/HAP catalyst, halo benzenes (0.5mmol), acetylenes (0.5mmol), K2CO3 (0.5mmol), CH3OH:H2O (1:1), 95 °C, 3 h.


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Using the activity data and the surface basic sites on Pd/HAP; a plausible mechanism is proposed for the Cu-free Sonogashira reaction. The Pd catalyzed Cu-free Sonogashira coupling reaction mechanism proceeds through two most common routes: (a) carbopalladation26,27 and (b) deprotonation28 explained by DFT calculations and kinetic studies29,30 Earlier reports show that the carbopalladation mechanism is a high energy pathway which reveals that the mechanism does not occur under the reaction conditions. From the Gibbs free energy profiles Ujaque et al. stated that alternative deprotonation mechanism was more feasible in the Cu-free Sonogashira reaction through cationic and anionic pathways.30, 31 The reaction mechanism is dependent upon on the nature of substituent attached to the alkyne and with highly electron donating groups attached to alkyne favoring the cationic mechanism, while both strong and moderate electron withdrawing groups attached to the alkyne and unsubstituted alkyne proceeds through anionic mechanism. Thus, it is reasonable to expect that the catalytic cycle for our choice of substrates follows the anionic route. This mechanism initially proceeds through oxidative addition of alkyl halide to Pd (0) giving intermediate-I in which Pd oxidized from Pd (0) to Pd (II). Subsequently alkyne is coordinated to intermediate-I forms π-coordinated complex, with Pd remaining as Pd (II). In the next step the complex undergoes deprotonation in the presence of a base, where the Pd attached with alkyne and alkyl group (R-X) forms intermediate-II. In the final step, the intermediate undergoes reductive elimination from Pd (II) to Pd (0) gives in the substituted alkyne product (Scheme 2).



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Scheme 2. Plausible mechanism for Sonogashira reaction on Pd/HAP catalyst.

3.2. Recyclability. Among the Pd supported on HAP, MgO and Al2O3; the Pd/HAP-12 demonstrated better yields hence the recyclability of Pd/HAP-12 has been examined using iodobenzene and phenylacetylene at 95 °C and K2CO3 as a base using CH3OH:H2O mixture as solvent. After completion of the reaction, catalyst was recovered by a simple centrifugation and washed with water several times, oven-dried and directly used for the next successive cycle. The AAS analysis of the 2wt%Pd/HAP fresh and used (catalyst recovered after 5th cycle) showed 1.98% and 1.96% of Pd respectively. The elemental analysis data revealed that there is no leaching of Pd from the HAP surface suggesting the robust nature of Pd/HAP for the Sonogashira coupling reaction. From these results, it can be concluded that 2wt%Pd/HAP demonstrated sustained activity and product selectivity in the absence of Cu and phosphine at moderate reaction conditions for up to 4 recycles (Figure S1).

4. DISCUSSION


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The CO2-TPD patterns of hydroxyapatite (HAP) derived at various pHs (7, 10, and 12) reported in Figure 1 and their corresponding CO2 uptakes are reported in Table S1. The elemental analyses of the HAPs prepared at different pH (7, 10 and 12) showed an increase in Ca/P ratio (Table S1) with increase in pH of the gel during the precipitation. The CO2 desorption profiles showed a sharp peak centred around 200 °C corresponding to weak basic sites. A shoulder peak around 290 °C is assigned to moderate basic sites and its intensity increased with increase in pH. HAP-10 and HAP-12 samples showed multiple peaks of CO2 desorption at different temperatures. A sharp peak centred at Tmax of 210 °C with a shoulder around 264 °C; and several peaks due to strong basic sites have emanated in samples synthesized at higher pH. The CO2 uptakes on these HAP samples demonstrated stronger basic sites on HAP-12 as compared to HAP-10 and HAP-7 samples. Presence of strong base sites is presumably due to an increase in Ca/P ratio from 1.3 (HAP-7) to 1.85 (HAP-12) (Table S1). A decrease in Ca/P ratio leads to a decrease in the number of both PO43- and OHgroups, in such case the overall charge is balanced by the formation of HPO42-.32 The probe adsorbed IR technique is commonly used to identify the surface base properties of solids at gas-solid interface.33-35 The adsorption of CO2 can be expected to take place on two types of surface basic sites such as O2- (Lewis) and/or species OH- (Brønsted). Contribution of these sites has been deduced from the formic acid adsorbed DRIFT spectroscopy and the activities are discussed in relation to the basic sites present on the catalyst surface. The influence of supports such as MgO and Al2O3 supported Pd catalysts were also examined under similar experimental conditions. However, both the Pd/MgO and Pd/Al2O3 catalysts afforded lower yields of the desired product in comparison with Pd/HAP-12 catalyst. Interestingly, the Pd/MgO exhibited other by product formation due to homo coupling of the substrate. Although no byproducts were found over Pd/Al2O3, the yield was only 42.6% after 8 h. These results prompted us to investigate further on the surface



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characteristics of Pd supported on MgO, Al2O3 and HAP-12 samples by BET surface area, powder XRD, H2-TPR and CO pulse chemisorption measurements to rationalize the nature of active sites (Table 3). Table 3. Physicochemical Characteristics of Catalysts 2wt%Pd BETH2 CO uptake Dispersion Metal Particle b area Size supported Surface uptake µmol/gcat µmol/m2 ( % ) (µmol/g)a (m2/g)c (nm)d on area (m2/g) HAP-12 47.9 147.2 28.4 0.59 13.4 1.34 7.85 MgO 39.3 137.5 22.4 0.57 11.9 1.06 9.93 Al2O3 189.4 149.4 45.7 0.24 24.4 2.16 4.87 a H2 uptakes measured by TPR analysis calibrated with TPR of Ag2O. b %Dispersion = [CO uptake (µmol/gcat) x 100]/[total metal (µmol/gcat)]. c Metal area = metal cross sectional area x No. of metal atoms on surface; Pd cross sectional area = 0.0787 nm2, d Particle size (nm) = 6000/[metal area (m2 g M-1) x ρ]; ρ: Pd density = 11.4 g/cc

The physicochemical properties of Pd supported on HAP-12, MgO and Al2O3 catalysts are illustrated in Table 3. A decrease in BET-surface area is observed over supported Pd catalysts compared to their respective supports, which is explained due to pore blocking. The CO chemisorption studies revealed that the CO uptakes are found to be in the range of 22.4 45.7 micromoles per gram of catalyst. The normalized CO uptake is more or less similar on both the Pd/HAP-12 (0.59 × 10-6 mol m-2) and Pd/MgO (0.57 × 10-6 mol m-2) whereas the Pd/Al2O3 showed a lower CO uptake of ca. 0.24 × 10-6 mol m-2 (Table 3). The lower coupling activity of Pd/Al2O3 can be understood from the lower number of surface Pd sites per unit surface area of the catalyst. In contrast, the Pd/MgO showed 65.4% and Pd/HAP-12 demonstrated 96.0% yields of the product although these samples have displayed more or less similar CO uptake per unit area. It can be seen that the homo-coupled by-product is observed only in case of Pd/MgO which could be explained by the strong basic sites present on the catalyst surface.36,37 On the other hand selective formation of desired coupling product is observed on both Al2O3 and HAP-12 supported Pd samples. From these results it is inferred that a metal site i.e. Pd in conjunction with a base site are crucial in the selective Sonogashira 14 ACS Paragon Plus Environment

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coupling reaction and the nature of basic site seems to influence the coupling reaction rate. Hence, the role of surface basic sites on these samples is rationalized by formic acid adsorbed DRIFT spectroscopy and the spectra are reported in Figure 2.

Figure 2. (A) IR spectra of (a) HAP-7; (b) HAP-10; (c) HAP-12; and formic acid adsorbed on (a') HAP-7; (b') HAP-10; (c') HAP-12 samples collected at 200 °C; (B) IR spectra of reduced (a) 2wt%Pd/HAP-7; (b) 2wt%Pd/HAP-10; (c) 2wt%Pd/HAP-12; and formic acid adsorbed on (a') 2wt%Pd/HAP-7; (b') 2wt%Pd/HAP-10; (c') 2wt%Pd/HAP-12 samples collected at 200 °C; (C) IR spectra of reduced (a) 2wt%Pd/Al2O3; (b) 2wt%Pd/HAP12; (c) 2wt%Pd/MgO;

and

formic

acid

adsorbed on

reduced

(a') 2wt%Pd/Al2O3; (b')

2wt%Pd/HAP12; (c') 2wt%Pd/MgO samples collected at 200 °C.

The nature and strength of surface basic sites are further evaluated by formic acid adsorbed DRIFTS of HAP-7, HAP-10 and HAP-12 samples (Figure 2A). It shows that a strong band at 1590 cm-1 appeared only in the formic acid adsorbed IR spectra of HAPs which is not observed in the absence of formic acid. This band is ascribed to C=O stretching mode of formic acid adsorbed on the surface, which is sharp at pH ~7 and has become broad in samples at both pH 10 and 12 with a shoulder peak emerged at 1647 cm-1 due to



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associative adsorption of formic acid and 1695 cm-1 signal probably assigned to formic acid ligated to surface Ca2+ species.38 These results thus confirm the enhancement in the number of basic sites on the surface of samples prepared at higher pH, which was in accordance with the TPD of CO2 analysis (Figure 1). A prominent shoulder peak at 1625 cm-1 was also observed with pH ~ 12 sample, which was characteristic of physisorbed water on HAP.38 The broad multi-component contributions observed in the region 1550-1300 cm-1 is assignable to the presence of carbonate groups.39 Upon Pd impregnation over HAP (prepared at different pHs ~ 7, 10 and 12) no significant difference in the peak position is seen (Figure 2B). However, relatively a decrease in peak intensities is observed. The formic acid adsorbed spectra (DRIFTS) over Pd/MgO and Pd/Al2O3 were recorded and compared with Pd/HAP sample. The strong band centred around 1598 cm-1 is probably due to Ca2+ - formate and the signal around 1720 cm-1 region is due to physisorbed formic acid. The band intensity of this signal gradually decreased upon increasing the temperature from 125 to 200 °C (Figure S2 and Figure S3). Signals in the region 1700 to 1300 cm-1 arise from the νas (OCO) and νs (OCO) stretching modes of the carboxyl groups on the surface.40 It is unlikely that formic acid decomposition occurs at 200 °C over Pd surface. The mono dentate formate species contribute to the bands at ca. 1690 and 1340 cm-1 which are very strong over Pd/MgO compared to Pd/HAP-12 and Pd/Al2O3 samples. The vibrational bands ascribed to the bridged bidentate formates with νas (OCO) signal around 1600 and 1560 cm-1 and νs (OCO) signals around 1410 and 1341 cm-1 are found to be strong on Pd/MgO sample.41 The envisaged basic sites on the catalyst surface are O2- and OH- and the adsorption of HCOOH on these sites are illustrated as follows: HCOOH + Mn+ OH- → HCOO- + H2O → (1st case) HCOOH + Mn+O2-

→ HCOO- + OH- → (2nd case)


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The normalized IR spectra of HCOOH adsorbed on Pd/Al2O3 revealed that very weak basic sites (due to the band near 1600 cm-1) are present on it (Figure 3C(a′)) whereas the Pd/HAP showed moderate basic sites (Figure 3C(b′)). On the other hand Pd/MgO sample (Figure 3C(c′)) demonstrated very strong basic sites ~ 5 times higher than on Pd/HAP sample. The better performance of Sonogashira coupling manifested over Pd/HAP is due to moderate basicity of the catalyst towards the desired compounds. Quite contrast to this, homo-coupling reaction occurred in the presence of strong basic sites present on the Pd/MgO surface. It should be mentioned here that acid sites did not play major role in the reaction sequence and the product distribution was mainly dependant on the surface basic and metal sites which was clearly observed from the surface characteristics of the catalysts and from the activity data. XRD analysis of these samples showed their relevant support phases (Figure 3). The diffraction peaks due to Pd are probably in smaller size or may be in micro-crystalline form. The diffraction lines at 2θ = 66.9, 45.8, 36.18ο can be attributed to γ-Al2O3 phase and their corresponding ‘d’ values are observed at 0.139, 0.197 and 0.228 nm (ICDD #: 10-0425). Similarly both MgO and Ca5(PO4)3(OH) phases are found in the Pd/MgO and Pd/Ca5(PO4)3(OH) samples respectively, which are in good correlation with ICDD # 4-829 for MgO and (ICDD # 86-0740) for Ca5(PO4)3(OH).23



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Figure 3. XRD patterns of reduced 2wt%Pd supported on (a) γ-Al2O3, (b) MgO and (c) HAP catalysts.

TEM images of the reduced and used Pd/HAP (Figure 5) samples showed an average Pd particle size of ~18.4 nm and ~20.6 nm respectively. The shapes of palladium particles are spherical in both reduced and used samples. From the TEM images it is concluded that the catalyst morphology and the average Pd particle size have not changed much after the use; indicating the stability of the Pd/HAP catalyst.


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Figure 5. TEM images of (a) reduced and (b) used Pd/HAP samples (Scale bar = 435 nm).

The active palladium particles undergo oxidation in the first step of the Sonogashira coupling followed by reduction in the last step. To gain an insight on the reduction behaviour of the palladium particles the H2-TPR is performed over 2wt%Pd supported on Al2O3, MgO and HAP catalysts (Figure 6) and the corresponding H2 uptakes are reported in Table 3. The signals that appeared in the TPR patterns are associated with PdO reduction to Pd(0). The reduction maxima at Tmax ~ 290 °C is more or less same for all the samples and the H2 uptakes are in the range of 137 - 169 µmol (gcat)-1. The H2 uptakes measured by the H2-TPR are found to be in close range indicating the bulk property of the technique and the degree of PdO reduction is found to be ~85%.



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Figure 6. The H2-TPR profiles of 2wt%Pd supported on (a) γ-Al2O3, (b) MgO and (c) HAP samples.

The XPS spectra of Ca 2p, P 2p and O 1s for 2wt%Pd/HAP sample are also collected and the data is given in ESI (Figure S4 - S6). The binding energies of these elements in the catalyst are in good agreement with the reported results.42 The XPS analysis of the reduced and used 2wt% Pd/HAP samples are reported in Figure 7. The metallic Pd species are present in the near surface region which is confirmed by the signal at binding energies (BE) of 334.95 eV and 340.15 eV for Pd 3d5/2 and Pd 3d3/2 over reduced catalyst.25 The used catalyst showed a shift in BE ~ 0.24 eV when compared to reduced form.


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Figure 7. The XPS spectra of (Pd 3d5/2, 3d3/2) a) reduced and b) used Pd(0)/HAP catalysts.

Absence of diffraction lines due to palladium crystals exemplifying the fine dispersion of Pd on these supports. H2-TPR results revealed that reduction of ionic Pd particles occurred below 600 °C; showing the ease of reduction although the PdO particles are interacted with different supports and the Tmax was 290 °C. These results substantiate the ionic palladium species at the interface were reduced at a similar temperature. Therefore, it can be inferred that the reactivity of surface palladium site would be influenced by its neighbouring basic site, particularly the moderate basic sites. DRIFT spectroscopic data explained the role of moderate basic sites and the surface metal concentration (measured by CO pulse chemisorption data; Table 3) wherein a metallic Pd in conjunction with a basic site is responsible for the coupling reaction. TEM images of the fresh and used Pd/HAP-12 catalyst clearly demonstrated the stability of the catalyst as there was not much change in Pd particle size and its morphology even after 4 recycles. On the other hand a small shift in BE of Pd 3d signal towards higher energy region is observed with the used catalyst. This is presumably 21 ACS Paragon Plus Environment


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due to the presence of slightly a high ratio of ionic Pd than the metallic Pd in the near surface region compared to its reduced form.

4.0 CONCLUSIONS In conclusion, hydroxyapatite a naturally occurring mineral is explored as a stable support for Sonogashira coupling reaction. A simple and efficient method is developed for copper and phosphine free Sonogashira reaction using a 2wt%Pd/HAP with 96% yield of desired compound under moderate reaction conditions. Base sites on the HAP were found to promote the activity of palladium to get desired products. When compared the Sonogashira coupling activity over an amphoteric support i.e. γ-Al2O3 and a strong basic support MgO for Pd; the Pd/HAP demonstrated better yields. Although a similar surface Pd sites are present on both Pd/HAP and Pd/MgO; the Pd/HAP displayed better performance which was summarised due to a combination of Pd site in conjunction with moderate basic sites that are desired for the coupling reaction. On the other hand, presence of strong basic sites on Pd/MgO led to formation of homo-coupling by-product. The HCOOH adsorbed DRIFT spectra revealed the role of moderate and strong basic sites on the Sonogashira coupling reaction. The catalyst can be readily recovered and reused for five cycles.

ASSOCIATED CONTENT Supporting Information Supporting information is available free of charge on the ACS Publications website at DOI: Experimental details on preparation, characterization and activity evolution of the catalysts. Characterization results of XPS, BET-SA, TPD of CO2, HCOOH-DRIFTS and Elemental analysis.


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AUTHOR INFORMATION Corresponding Author * Tel.: +91-40-27193165, E-mail: [email protected], Fax: +91 40 27160921. Notes: The authors declare no competing financial interest.

ACKNOWLEDGEMENTS B. Vishali thanks the UGC, New Delhi for fellowship.

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Influence of surface basic sites on the Pd supported on hydroxyapatite for Sonogashira coupling reaction was examined by DRIFT spectroscopy 206x89mm (96 x 96 DPI)

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