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Palladacycles of Thioethers Catalyzing Suzuki−Miyaura C−C Coupling: Generation and Catalytic Activity of Nanoparticles Gyandshwar Kumar Rao, Arun Kumar, Satyendra Kumar, Umesh B. Dupare, and Ajai K. Singh* Department of Chemistry, Indian Institute of Technology Delhi, New Delhi-110016, India. S Supporting Information *

ABSTRACT: Tridentate thioether ligands, 2−HO−4−R−C6H3− (C6H4)CHNH(CH2)3SPh [R = H (L1) or −OMe (L2)] react with Na2PdCl4, giving palladacycles [PdCl(C−,N,S)] (1: (C−,N,S) = L1−H; 2: (C−,N,S) = L2−H). The 1H and 13C{1H} NMR spectra of ligands and their palladacycles have been found to be characteristic. Complexes 1 and 2 have also been characterized with HR-MS. The crystal structure of 2 has been solved. The Pd−S bond length is 2.428(2) Å, and palladium has a nearly square planar geometry. During the course of catalysis of Suzuki−Miyaura C−C coupling using 1 and 2 as catalysts, unexpected formation of Pd16S7 nanoparticles (NPs) has been observed with both complexes. This is the first time that such an observation has been made with palladacycles of thioethers used in this coupling reaction. The efficiency of 2 in carrying out the coupling is significantly lower than that of 1. Complex 2 has an additional −OMe group in the ligand structure, and the size of Pd16S7 NPs formed from this complex are larger (6 nm) than those obtained from 1 (2 nm).

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INTRODUCTION Transition metal-catalyzed C−C bond forming reactions are powerful synthetic tools in organic chemistry.1,2 Palladium species constitute a class of versatile and useful catalysts for such organic transformations. The facile interchange between Pd(0) and Pd(II) or Pd(II) and Pd(IV) and the tolerance of palladium compounds to many functional groups present on substrate are mainly responsible for their versatility.2 Not only Suzuki−Miyaura coupling but other C−C bond forming reactions such as ethylene oligo/polymerization, Heck and Negishi coupling, etc., also rely on this facile interconversion of oxidation states. The most important Suzuki−Miyaura catalysts include complexes of Pd(II) with bulky and electron-rich phosphines3,4 and carbenes,5 and palladacycles,2,6,7 owing to their high efficiency and the ease with which they can be modified. In the last few decades interest in palladium complexes of ligands containing a combination of various donor groups, such as Suzuki−Miyaura catalysts, has grown significantly as the distinct features of each donor atom can confer unique properties to the complex.2,6−8 Sulfur has been incorporated in framework of many ligands, and Pd(II) complexes of sulfated Schiff bases,9 pincer type S ligands,7 and some other Scontaining ligands10 have emerged as a family of air-stable, moisture-insensitive, and efficient catalysts. The sulfur-ligated palladacycles are important among this family of Suzuki catalysts, and in recent years an increase in research on them has been noticed. They have shown good promise for the coupling of aryl bromides and iodides, but most of them7c−g,8,9a,c−e have not been reported to transform effectively © 2013 American Chemical Society

aryl chlorides, which are the cheapest and most readily available among the aryl halides. The L1 and L2 synthesized as a part of our research program on chalcogenated Schiff bases11 and their reduced forms have been found worth studying to design thioether palladacycles 1 and 2, which may show potential as Suzuki catalysts. The in situ formation of Pd NPs has been reported in Suzuki−Miyaura coupling reactions catalyzed with palladium complexes of organosulfur ligands.7b These Pd NPs are reported to be stabilized in the presence of additive nBu4NBr.7g,11d However, in some cases the absence of this additive gives a better yield.7g There is enough evidence that suggests that Pd(0) species leached from the surface of such NPs are the true catalysts during the course of reaction, and the role of the ligand (including its architecture) is limited to affecting size, dispersion, and the chemical nature of the NPs. However, formation and involvement of NPs of a palladium sulfide phase in the catalytic Suzuki coupling reaction have never been reported. Suzuki−Miyaura coupling carried out in the presence of palladacycles 1 and 2 results in the formation of nanosized particles of composition Pd16S7, which appear to play a role in the catalysis of the coupling via generation of Pd(0) species. The formation of the Pd16S7 phase in Suzuki coupling has been noticed for the first time. The unexpectedly high difference in activities of 1 and 2 has been observed. The complex 2, having an additional −OMe (with respect to 1), has been made a part of this study, as its single crystal structure Received: March 11, 2013 Published: April 4, 2013 2452

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through Celite. The solvent was removed with a rotary evaporator, and ligands L1 and L2 were obtained as a light yellow liquid. L1: Yield: 0.272 g (78%). 1H NMR (300 MHz, CDCl3): δ (ppm) 1.212 (s, 2H, NH + OH), 1.867−1.883 (m, 2H, H6), 2.741−2.971 (m, 4H, H5, H7), 4.873 (s, 1H, H8), 6.720 (t, J = 7.2 Hz, 1H, H13), 6.814− 6.868 (m, 2H, H11, H14), 7.118−7.315 (m, 11H, H1, H2, H3, H12, H16, H17, H18). 13C{1H} NMR (75 MHz, CDCl3): δ (ppm) 28.58 (C6), 30.07 (C5), 46.23 (C7), 63.93 (C8), 115.93 (C11), 118.75 (C13), 125.58 (C9), 127.06 (C1), 127.43 (C17), 127.70 (C18), 127.92 (C12), 128.07 (C2), 128.30 (C14), 128.39 (C16), 129.06 (C3), 136.36 (C4), 142.81 (C15), 156.68 (C10). L2: Yield: 0.325 g (86%). 1H NMR (300 MHz, CDCl3): δ (ppm) 1.258 (s, 2H, NH + OH), 1.830−1.911 (m, 2H, H6), 2.749−2.959 (m, 4H, H5 + H7), 3.723 (s, 1H, H19), 4.822 (s, 1H, H8), 6.285 (dd, J = 2.1 Hz, 8.1 Hz, 1H, H13), 6.434 (d, J = 2.1 Hz, 1H, H11), 6.686 (d, J = 8.4 Hz, 1H, H14), 7.158−7.296 (m, 10H, H1, H2, H3, H16, H17, H18), 7.499−7.622 (m, 4H,). 13C{1H} NMR (75 MHz, CDCl3): δ (ppm) 28.82 (C6), 31.46 (C5), 46.64 (C7), 55.07 (C8), 67.20 (C19), 102.10 (C11), 105.26 (C13), 116.90 (C9), 126.11 (C1), 127.20 (C16), 127.73 (C18), 128.86 (C2), 128.96 (C17), 129.43 (C3), 129.60 (C14), 135.77 (C4), 141.79 (C15), 158.78 (C10), 160.26 (C12). Synthesis of 1 and 2. The Na2PdCl4 (0.205 g, 0.7 mmol) was dissolved in 5.0 mL of water. A solution of ligand L1 (0.175 g, 0.5 mmol)/L2 (0.191 g, 0.5 mmol) made in 10 mL of acetone was added to it with vigorous stirring. The mixture was further stirred for 2 h. The orange-red solution was extracted with chloroform. The chloroform layer was washed with 100 mL of water, dried with anhydrous Na2SO4, and evaporated to dryness on rotary evaporator to obtain 1 and 2 as an orange-colored powder. The single crystals of 2 were grown from 8:2 mixtures of CHCl3 and hexane. 1: Yield: 0.158 g (65%); mp 153 °C (dec). Anal. Found: C, 53.81; H, 4.46; N, 2.91. Calcd for C22H22ClNOPdS: C, 53.89; H, 4.52; N, 2.86. 1H NMR (300 MHz, DMSO-d6): δ (ppm) 2.182−2.285 (m, 2H, H6), 2.882−2.905 (m, 4H, H5 + H7), 4.354 (s, 1H, H8), 6.275−6.382 (m, 1H), 6.814−7.505 (m, 9H), 7.747 (d, J = 7.2 Hz, 1H), 8.244− 8.345 (m, 2H). 13C{1H} NMR (75 MHz, DMSO-d6): δ (ppm) 28.70 (C6), 29.00 (C5), 51.48 (C7), 65.90 (C8), 114.06, 118.97, 126.96, 128.48, 128.64, 129.34, 129.57, 129.89, 120.25, 132.71, 139.20, 157.97, 160.99. HR-MS [M − Cl] (m/z) = 454.0456; calcd value for C22H22NOPdS = 454.0459 (ppm error δ: 0.7). 2: Yield: 0.185 g (71%); mp 159 °C (d). Anal. Found: C, 53.08; H, 4.66; N, 2.67. Calcd for C23H24ClNO2PdS: C, 53.09; H, 4.65; N, 2.69. 1 H NMR (300 MHz, CDCl3): δ (ppm) 1.958−2.976 (m, 1H), 2.171− 2.238 (m, 1H), 3.142−3.225 (m, 2H), 3.360−3.429 (m, 2H), 3.624 (s, 3H, OMe), 4.051 (s, 1H, H8), 6.214 (s, 1H), 6.366−6.421 (m, 1H), 7.155−7.460 (m, 8H), 8.101−8.128 (m, 1H), 8.180−8.200 (m, 1H). 13 C{1H} NMR (75 MHz, CDCl3): δ (ppm) 26.24 (C6), 35.86 (C5), 53.17 (C7), 55.04 (C8), 70.81 (H19), 102.17, 103.46, 106.63, 117.82, 128.67, 129.07, 129.75, 130.71, 132.46, 135.45, 136.43, 140.13, 160.38, 165.38, 169.97. HR-MS [M − Cl] (m/z) = 484.0554; calcd value for C23H24NO2PdS = 484.0565 (ppm error δ: 2.4). General Procedure for Suzuki Reaction of Aryl Halides with Phenylboronic Acid. An oven-dried flask was charged with aryl halide (1.0 mmol), phenylboronic acid (1.3 mmol), K2CO3 (2.0 mmol), and DMF/H2O (4.0 mL). A solution of catalyst 1 or 2 in DMF was then added via syringe. The flask was placed on an oil bath at 100 °C under aerobic conditions, and the reaction mixture stirred until maximum conversion of aryl halide to coupled product occurred. The mixture was extracted with diethyl ether (100 mL). The extract was washed with water (100 mL) and dried over anhydrous Na2SO4. The solvent of the extract was removed with a rotary evaporator, and the resulting residue purified by column chromatography on silica gel using an ethylacetate/hexane mixture (5:95% to 15:85%) as eluent. Isolation of Nanoparticles Formed from Palladacycles 1 and 2 in Suzuki−Miyaura C−C Coupling. A mixture of palladacycle 1/2 (0.50 mmol), phenylboronic acid (1.3 mmol), 1-bromo-4-nitrobenzene (1 mmol), and K2CO3 (2 mmol) in a DMF (4 mL)/water (4 mL) mixture was heated at 100 °C in an oil bath for 2.5 h and then cooled to room temperature. The solvent was decanted off, and the black residue was mixed with 10 mL of acetone and 30 mL of water.

determination is possible, which throws light on the structure of analogous 1. The Pd16S7 NPs resulting from 2 are larger in size than those obtained from 1. This may cause a slow release of Pd(0) from NPs of 2 and consequently low catalytic efficiency. These results are described in this paper.

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EXPERIMENTAL SECTION

General Experiments. Thiophenol, NaBH4, 3-chloropropylamine hydrochloride, 2-hydroxybenzophenone, 2,4-dihydroxybenzophenone, sodium tetrachloropalladate, potassium carbonate, and aryl halides were procured from Sigma-Aldrich (USA). Reagents (commercially available from local sources) were used as received without further purification. The progress of every coupling reaction was monitored by NMR spectroscopy. Yields refer to isolated yields of compounds that have a purity of ≥95% [established by 1H NMR]. The products of Suzuki reactions were authenticated by matching spectroscopic data of the products obtained by us with those reported in the literature. 1 H and 13C{1H} NMR spectra were recorded on a Bruker Spectrospin DPX 300 NMR spectrometer at 300.13 and 75.47 MHz, respectively, with chemical shifts reported in ppm relative to the residual deuterated solvent or the internal standard tetramethylsilane. Carbon-13 DEPT NMR experiments were used routinely to determine the number of hydrogen atoms linked to carbon atoms. Elemental analyses were carried out with a Perkin-Elmer 2400 Series II C, H, N analyzer. Melting points were determined in an electrically heated apparatus by taking the sample in a glass capillary sealed at one end. High-resolution mass spectral (HR-MS) measurements were performed with electron spray ionization (10 eV, 180 °C source temperature, and sodium formate as calibrent) on a Bruker Micro TOF-Q II, taking the sample in CH3CN. Suitable crystals of palladacycle 2 were obtained by slow evaporation of its solution made in a chloroform−hexane (8:2) mixture. X-ray diffraction data for crystals of 2 were collected on a Bruker AXS SMART-APEX diffractometer equipped with a CCD area detector (Kα = 0.71073 Å; graphite monochromator). Frames were collected at T = 298 K by ω, φ, and 2θ rotations with a full quadrant data collection strategy (four domains each with 600 frames) at 10 s per frame with SMART apparatus. The measured intensities were reduced to F2 and corrected for absorption with SADABS. Structure solution, refinement, and data output were carried out with the SHELXTL package by direct methods. Non-hydrogen atoms were refined anisotropically. All hydrogen atoms were included in idealized positions, and a riding model was used for the refinement. Images were created with the program Diamond. TEM studies were carried out with a Technai G2 20 electron microscope operated at 200 kV. The specimens for TEM were prepared by dispersing the powder in chloroform by ultrasonic treatment, dropping onto a porous carbon film supported on a copper grid, and then drying in air. The nanostructural phase morphology of the sample was observed by using a Carl Zeiss EVO5O scanning electron microscope (SEM). Nanostructures observed in the SEM, for its elemental composition, were analyzed by an EDX system model Quan Tax 200, which is based on the SDD technology and provides an energy resolution of 127 eV at Mn Kα. The samples were scanned in different regions in order to minimize the error in the analysis made to evaluate the morphological parameters. Samples were mounted on a circular metallic sample holder with a sticky carbon tape. Powder X-ray diffraction (PXRD) studies were carried out on a Bruker D8 Advance diffractometer with Ni-filtered Cu Kα radiation using a scan speed of 1 s and scan step of 0.05°. Thermogravimetric analysis (TGA) was carried out using a Perkin-Elmer system in flowing nitrogen atmosphere with a heating rate of 10 °C/min. Synthesis of L1 and L2. The C6H5S−(CH2)3−NC(Ph)C6H4− 2−OH (0.347 g, 1 mmol)/C6H5S−(CH2)3−NC(Ph)−4−OMe− C6H3−2−OH (0.377 g, 1 mmol) prepared by reported methods9e and NaBH4 (0.380 g, 1.0 mmol) were stirred for 10 h in 100 mL of dry ethanol. The solvent was removed on a rotary evaporator. The ligand L1/L2 was dissolved into 20 mL of dry chloroform and filtered 2453

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Scheme 1. Synthesis of Ligands and Their Pd(II) Complexes

Figure 1. ORTEP diagram of 2 with 30% probability ellipsoids. Selected bond lengths (Å): Pd(1)−C(23) 1.970(7); Pd(1)−N(1) 2.059(6); Pd(1)− Cl(1) 2.319(2); Pd(1)−S(1) 2.428(2). Selected bond angles (deg): C(23)−Pd(1)−N(1) 81.5(3); C(23)−Pd(1)−Cl(1) 93.3(2); N(1)−Pd(1)− Cl(1) 173.73(19); N(1)−Pd(1)−S(1) 98.7(2); Cl(1)−Pd(1)−S(1) 86.47(8), C(6)−S(1)−C(7) 101.1(4); C(6)−S(1)−Pd(1) 106.9(3). The mixture was centrifuged. The black residue was further washed with a mixture of acetone and water (3:1, v/v) and dried. The residue thus obtained was separated and subjected to powder-XRD. It was found to be amorphous in the case of both complexes. These powders were annealed in an argon atmosphere at 380 °C for 4 h, which resulted in crystalline Pd16S7 nanoparticles in the case of both complexes. Hg Poisoning Test. The Hg (Hg:Pd::500:1) was taken in a reaction flask before the addition of coupling reactants. Thereafter 4bromoanisole (1.0 mmol), phenylboronic acid (1.3 mmol), and 1/2 (0.1 mol %) or nanoparticles (1 mol % of Pd) isolated during decomposition of 1/2 as catalyst were added, and the reaction under optimum conditions was carried out. The expected product 4methoxybiphenyl was not obtained in significant yield after 24 h. PPh3 Poisoning Test. The coupling reaction of 4-bromoanisole with phenylboronic acid using 1/2 (0.1 mol %) or nanoparticles (1 mol % of Pd) isolated from decomposition of 1/2 in Suzuki−Miyaura coupling, as a catalyst, was carried out in the presence of PPh3 (10 mol %) under optimum conditions. After 24 h the expected cross-coupled product 4-methoxybiphenyl was not obtained. Two-Phase Test. First, 4-bromobenzoic acid was heterogenized by immobilizing it on silica gel using a standard procedure (details in the Supporting Information).12 Thereafter, a mixture of the immobilized species (0.20 g), phenylboronic acid (0.36 g, 3 mmol), 4bromoacetophenone (0.20 g, 1 mmol), and K2CO3 (0.56 g, 4 mmol) was heated at 90 °C for 12 h in a DMF/water (6/3 mL) mixture. After cooling to room temperature, the resulting mixture was filtered through a G-4 crucible and the residue was washed with 20 mL of H2O followed by diethyl ether (50 mL). Water (50 mL) was added to the mixture of filtrate and washings, which were collected together. The resulting mixture was extracted with diethyl ether (50 mL). The

solvent of the extract was removed under vacuum, and the residue was analyzed using 1H NMR spectroscopy. The hydrolysis of the solid residue (obtained in a G-4 crucible after washing) was carried out with KOH (1.68 g dissolved in 10 mL of EtOH + 5 mL of H2O) at 90 °C, and after 3 days, the resulting solution was neutralized with aqueous 20% (v/v) HCl. The neutralized solution was extracted with dichloromethane followed by ethyl acetate. The solvent of the mixture of extracts was evaporated under vacuum, and the resulting residue was analyzed with 1H NMR.

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RESULTS AND DISCUSSION L1, L2, 1, and 2, prepared as given in Scheme 1, are stable under ambient conditions and may be stored for several months. The composition of complexes of L1 and L2 formed with Pd(II) does not change on varying the metal:ligand ratio during their synthesis. The ligands have higher solubility in CHCl3, CH2Cl2, CH3CN, CH3OH, C2H5OH, and acetone than those of their complexes. Pd complex 1 has good solubility in DMSO and DMF, whereas in CHCl3, CH2Cl2, and CH3CN it is sparingly soluble. Complex 2 shows good solubility in DMSO, DMF, and CHCl3 but is sparingly soluble in CH2Cl2 and CH3CN. These ligands and their diamagnetic complexes 1 and 2 have been characterized by 1H and 13C{1H} NMR spectroscopy and HR-MS (see Supporting Information for scans of original spectra). These data are consistent with the structures depicted for them in Scheme 1. The 13C{1H} NMR spectra of complexes 1 and 2 have a new quaternary carbon signal between 160 and 161 ppm, showing palladation of L in both cases. The structure of 2 has been elucidated by X-ray 2454

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diffraction on its single crystals obtained by slow evaporation of its solution made in a chloroform/hexane mixture (8:2). However attempts to crystallize complex 1 were unsuccessful. The crystal data and structure refinement for complex 2 are given in Table S1. Table S2 contains selected bond length and bond angles for complex 2. Figure 1 shows an ORTEP diagram of 2 with some selected bond lengths and angles. There is a distorted square planar geometry at Pd in complex 2 with S and C(aryl) disposed trans to each other. The Schiff base9e precursor of L1 binds in an (O−, N, S) mode with Pd(II). The (C−, N, S) bonding mode of L1/L2 in 1 and 2 leaving the OH group uncoordinated occurs because ortho-palladation probably precedes Pd−O bond formation. The Pd−N bond length of 2.060(6) Å in 2 is almost the same as the reported value, 2.010(4) Å, for Pd(II) complexes of tridentate selenated Schiff bases9e and is longer than the 1.986(6) Å reported9e for [PdCl{(C6H5)(C6H4-2-O−)CN(CH2)3SePh}]. The Pd−C distance [1.970(7) Å] in 2 is consistent with the value of 1.971(16) Å7d reported for the Pd−C bond of the palladium(II)-(S,C,S) pincer complex. The Pd−S bond distance [2.428(2) Å] of 2 has been found to be longer than that of a tridentate sulfated Schiff base−Pd(II) complex,9e whereas the length of the Pd−Cl bond of 2 [2.319(2) Å] is consistent with the value [2.3159(7) Å] reported for the same Pd(II) complex of a tridentate sulfated Schiff base.9e In the light of the structure of 2 and the presence of a quaternary carbon signal in the 13 C{1H} NMR spectra of 1, the ligand L1 presumably coordinates with Pd in 1 in a monoanionic tridentate (S, N, C−) mode, forming one six-membered and one five-membered chelate ring. The geometry around Pd in 1 is also presumably square planar. The Suzuki reactions (Scheme 2) were carried

Table 1. Suzuki−Miyaura Coupling Reactions Catalyzed by 1a Ar–X + PhB(OH)2 → Ar–Ph (X; Cl, Br) entry no.

aryl halide

mol % Pd

yield (%)b

TON

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

1-bromo-4-nitrobenzene 4-bromobenzonitrile 4-chlorobenzaldehyde 4-chlorobenzaldehyde 4-bromobenzaldehyde 2-bromobenzaldehyde 4-bromobenzoic acid 4-Bromotoluene 4-bromotoluene 4-chlorotoluene 4-bromoanisole 4-bromoaniline 2-Bromopyridine 3-bromopyridine 4-chloropyridine 4-bromopyridine 3-bromoquinoline

0.1 0.001 1 0.01 0.001 0.1 0.1 0.001 0.1 0.05 0.001 0.5 0.001 0.001 0.1 0.002 0.001

84 95 94 28 95 86 94 84 95 93 94 95 50 40 50 53 72

840 95 000 94 2800 95 000 860 940 84000 950 1860 94 000 190 50000 40 000 500 26 500 72 000

a

Reaction conditions: 1.0 equiv of aryl halide, 1.3 equiv of phenylboronic acid, 2 equiv of K2CO3, aquous DMF as solvent, reaction time 15 h, and temperature of bath 100 °C. bIsolated yield after column chromatography.

Table 2. Suzuki−Miyaura Coupling Reactions Catalyzed by 2a Ar–X + PhB(OH)2 → Ar–Ph (X; Cl, Br)

Scheme 2. Suzuki−Miyaura Coupling Reaction

out in the presence of 1 and 2, and the results are given in Tables 1 and 2. For the reaction between 4-bromobenzaldehyde and phenylboronic acid under aerobic conditions at 100 °C for 15 h (catalyzed with 1), the different bases and solvents were studied, and the best results were obtained with K2CO3 and DMF/water. The catalytic efficiency of 1 is higher than that of 2, which is moderately active, as 0.05 to 1 mol % are required for significant conversions (Table 2). For example coupling between 4-bromobenzaldehyde and phenylboronic acid in the presence of 0.001 mol % of 1 and for a reaction time of 15 h at 100 °C, the product biphenyl-4-caboxyldehyde in 95% yield with TON 95000 (Table 1, entry 5) was obtained. When catalyst 2 was used for the same coupling under similar reaction conditions, a higher catalyst loading (0.05 mol %) gave a TON of ∼1820 (Table 2, entry 4). Further the results of Suzuki− Miyaura coupling given in Tables 1 and 2 indicate that the coupling of aryl chlorides can be successfully achieved when 1 is used as catalyst, even in the case of deactivated ones. For example 4-chlorotoluene is converted to 4-methylbiphenyl in 93% yield (TON ∼1860) within 15 h using 0.05 mol % of 1 as catalyst (Table 1, entry 10). The deactivated species 4bromoaniline has also been successfully coupled (yield ∼95%) using 0.5 mol % of 1 (Table 1, entry 12). The crosscoupling reactions of heteroaryl bromides and chlorides have also been successfully catalyzed with 1. The coupling of 3-

entry no.

aryl halide

mol % Pd

t (h)

1 2 3 4 5 6 7 8 9 10 11

1-bromo-4-nitrobenzene 4-bromobenzonitrile 4-chlorobenzaldehyde 4-bromobenzaldehyde bromobenzene 4-bromotoluene 4-bromoanisole 4-chloroanisole 4-bromoaniline 3-bromopyridine 4-bromopyridine

0.05 0.05 1.0 0.05 0.1 0.5 1.0 1.0 1.0 0.5 0.5

2 2 24 5 5 12 2 24 15 12 12

yield (%)b

TON

93 95

1860 1900

91 87 86 93

1820 870 172 93

87 86 90

87 172 180

a

Reaction conditions: 1.0 equiv of aryl halide, 1.3 equiv of phenylboronic acid, 2 equiv of K2CO3, aquous DMF as solvent, and temperature of bath 100 °C. b Isolated yield after column chromatography.

bromoquinoline with phenylboronic acid (catalyzed by 0.001 mol % of 1) gives 3-phenylquinoline in ∼72% yield (TON on the order 72 000). Similarly Suzuki−Miyaura coupling of 4chloropyridine giving 4-phenylpyridine in 50% yield has occurred in the presence of 0.1 mol % of 1. The palladacyclic complex 2 does not show any significant catalytic activity toward aryl chlorides even at its 1 mol % loading (Table 2, entries 3 and 8). Complexes 1 and 2 differ in structure by one −OMe group, which is present in the case of 2. Such a large variation in catalytic efficiency due to just the −OMe group is unexpected. Therefore an attempt is made to gain insight into the catalytic process by characterizing and analyzing the role of, black particles that appear in the reactions during the course of catalysis. It has already been reported on the basis of various 2455

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spherical (size 6 nm) when originated from 1 and rice-shaped (size 100 nm) in the case of 2 (Figure 4). It has already been

experimental results that sulfur-containing palladacycles decompose to Pd(0), and an equilibrium between palladium NPs and the actual active species [Pd(0)−Pd(II)]7b exists during the catalytic reaction.7b In view of such reports and the appearance of black particles during the course of the catalytic reactions, it is possible that the complexes (1 and 2) are not true catalysts but dispensers of real catalysts either directly or via some intermediate nanoparticle formulation. Thus a reaction between 1-bromo-4-nitrobenzene and PhB(OH)2 under optimum conditions catalyzed with 1 as well as 2 has been examined in detail with the hope that it may lead to some understanding of correlation (if any) between the ligand’s framework, the catalytic activity, and the nature of the NPs. The black residues formed during a representative reaction catalyzed with 1 and 2 were separated and characterized by HR-TEM, EDX, and powder X-ray diffraction (which reveal their amorphous nature). The HR-TEM (Figure 2) indicates that the black

Figure 4. TEM image of palladium sulfide nanoparticles obtained from palladacycles 1 and 2, respectively, after annealing at 380 °C.

reported that the phase Pd16S7 has a body-centered cubic structure with 46 atoms in the cubic cell and is structurally related to the γ-brass structure.13 The average formal oxidation state for palladium is +0.875, the majority of Pd−S contacts are ∼2.4 or 2.8 Å, and the Pd−Pd bond exists; therefore some Pd most probably exists in the (0) oxidation state. To check whether the catalytic activity is also associated with these NPs, the aggregated Pd16S7 NPs obtained from 1 and 2 were examined for Suzuki−Miyaura coupling reaction (Table 3) of 4-bromonitrobenzene, bromobenzene, and 4-bromoaniTable 3. Suzuki−Miyaura Coupling Reactions Catalyzed by Pd16S7 NPsa

Figure 2. TEM image of nanoparticles obtained from complexes 1 and 2.

NPs obtained from 1

powder consists of highly uniform and monodisperse spherical shaped NPs (size: ∼2 nm for 1 and 6 nm for 2) (Figure 3). The

entry no. 1 2 3

aryl halide 1-bromo-4nitrobenzene bromobenzene 4-bromoanisole

NPs obtained from 2

mol %

yield (%)b

mol %

yield (%)b

0.1

88

1.0

81

0.1 0.1

81 84

1.0 1.0

69 80

a

Reaction conditions: 1.0 equiv of aryl halide, 1.3 equiv of phenylboronic acid, 2 equiv of K2CO3, aquous DMF as solvent, reaction time 24 h, and temperature of bath 100 °C. bIsolated yield after column chromatography.

sole with PhB(OH)2 under optimum conditions. The reaction was smooth in the presence of 0.1 mol % of NPs obtained from 1 and 1 mol % of NPs obtained from 2. The reaction for 24 h at 100 °C resulted in the products 4-nitrobiphenyl, biphenyl, and 4-methoxybiphenyl, respectively, in good yields (Table 3). However these NPs failed to couple ArCl. Overall these results support the proposition that the palladacycles 1 and 2 act as dispensers of nanosized Pd16S7, which seems to play a role in catalysis of Suzuki coupling. In the recent past various pathways for the involvement of in situ generated Pd NPs in catalysis have been suggested. The mechanism involving Pd atom escape from the palladium nanoparticles by an oxidative addition generating discrete Pd(II) species in solution has been proposed for the Suzuki reaction.14 Alternatively, the Pd(II) complexes can be formed by oxidative addition to Pd(0) atoms that have already leached into the solution.15 Recently XAS (Xray absorption spectroscopy) has been utilized to track quantitatively the local structure of monodispersed Pd nanoparticles during a Suzuki coupling reaction. By following

Figure 3. Size distribution of NPs obtained from 1 and 2.

large variation in catalytic activities of 1 and 2 may be partly ascribed to the difference in size of NPs generated in situ from them. These NPs were annealed at 380 °C for 4 h in argon, and the resulting crystalline forms were characterized by HR-TEM, EDX, and powder X-ray diffraction. The EDX studies (Figures S26, S28, S30, and S32; Supporting Information) suggest that the Pd:S ratio in NPs obtained from 1 and 2 both before and after annealing is almost the same. After annealing, formation of crystalline Pd16S7 nanoparticles from both 1 and 2 is supported by their powder X-ray diffraction patterns (Figures S17 and S18; Supporting Information). The HR-TEM of crystalline Pd16S7 NPs generated on annealing has suggested that they are 2456

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Scheme 3. Two-Phase Test on Suzuki−Miyaura Coupling with Catalyst 1

the local coordination environment in an operando mode (realtime measurement under working conditions), strong evidence has been found indicating that their reactivity is indeed heterogeneous in origin and associated with the presence of stable surface defect atoms.16 It has been hypothesized that the pathways involving soluble metal complexes and insoluble metal particles may take place in the same reaction vessel, and both can contribute to the product formation. Thus all the approaches, boundary cases of purely homogeneous or purely heterogeneous systems as well as “cocktail”-like mixtures of the catalysts, are possible.17 In view of these reports, the possibility of molecular catalysis cannot be ruled out despite positive Hg poisoning18 and PPh3 tests18 carried out during catalysis by 1, 2, or Pd16S7 NPs. Further NPs may contribute in a homogeneous as well as heterogeneous fashion. The possibility of homogeneous catalysis may arise due to leaching of surface atoms of heterogeneous nanoparticles of Pd16S7 in solution (to form a soluble Pd(II) intermediate11 such as Ar−Pd−Br by oxidative addition). A two-phase test (Scheme 3) was performed to understand further catalytic processes;19,20 of course it cannot conclusively indicate the colloidal or molecular nature of the catalyst. This test (called a three-phase test when the catalyst is a solid phase) was developed by Rebek and coworkers.19 If the activity of the catalyst is fully heterogeneous in nature, the supported aryl halide is not expected to be converted to a coupled product. However, the homogeneous activity of the catalyst due to leaching of Pd(0) in solution will lead to the conversion of the supported substrate to product. Further, the colloidal catalyst being in solution can always cause some conversion. A two-phase test (shown in Scheme 3) was conducted utilizing the reaction of a mixture of 4bromoacetophenone and immobilized 4-bromobenzoic acid (as amide) with phenylboronic acid under optimum reaction conditions. The soluble part was separated by filtration and analyzed after workup with 1H NMR to determine the yield of the cross-coupled product (4-acetylbiphenyl), which was found to be ∼99%. The solid phase was hydrolyzed, and analysis of the resulting products (after workup) with 1H NMR indicates that ∼58% of the immobilized 4-bromobenzoic acid (as amide) was converted to the cross-coupled product (biphenyl-4carboxylic acid), whereas ∼42% remained unreacted. These results indicate that there is a significant contribution of homogeneous Pd species (molecular or colloidal) to catalysis. Either the palladium species (responsible for homogeneous catalysis) is the palladium atoms leached from the in situ generated nanoparticles of Pd16S7, or the molecular complex or both are contributing to C−C coupling.17 Thus, it appears that in the case of 1 and 2 coupling is catalyzed with a contribution of nanosized Pd species homogeneously as well as heterogeneously, and such a possibility has been recently described as a “cocktail”-like mixture of the catalysts.17

An intuitive proposition can be made that secondary interactions due to the −OMe group in 2 make somewhat large molecular aggregates, which in turn in the reaction medium result in bigger and less efficient NPs, whereas in the case of 1 such a possibility is less and NPs formed are small and more efficient. The high efficiency may be assumed to some extent to be related to the higher surface area of NPs obtained from 1. The high surface area facilitates the reaction in every case whether reactivity is due to Pd atoms escaped from the rim of NPs or is associated with the presence of stable surface defect atoms.

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CONCLUSION The newly synthesized palladacycle 1 of organosulfur ligand L1 is an efficient precatalyst for Suzuki−Miyaura C−C coupling reactions. The catalytic activities of both 1 and 2 appear to be via Pd16S7 NPs, generated in situ unexpectedly during the course of catalysis, and mainly the Pd species leached from them contribute to catalysis of Suzuki coupling probably via a homogeneous process. The overall catalytic process has been indicated as “cocktail” type by the two-phase test. This is the first time that NPs of a palladium−sulfur phase have been found to be involved in Suzuki−Miyaura coupling catalyzed with palladacycles of thioethers. The size of Pd16S7 NPs and the catalytic efficiency of precursor Pd complexes vary significantly upon incorporation of a small substituent, an −OMe group, in the ligand’s backbone. These results are of importance for tailoring new efficient catalysts for Suzuki−Miyaura C−C coupling reactions.

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ASSOCIATED CONTENT

S Supporting Information *

The NMR spectral data of L1, L2, 1, and 2; HR-MS of 1 and 2; structural and refinement data of 2 (CCDC no. 891159); SEM, EDX, PXRD of NPs; and NMR data of Suzuki coupling products. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Fax: +91 11 26581102. Tel: +91 11 26591379. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Council of Scientific and Industrial Research and Department of Science and Technology India supported the work through the award of SRF/RA and projects [CSIR: 01(2421)10/EMRII; DST: SR/S1/IC-40/2010]. The authors thank Professor A. 2457

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K. Ganguli of IIT Delhi for providing the HR-TEM and powder XRD facility.

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