An Efficient and Recyclable Catalytic System Comprising Nano-Iridium

ARTICLE pubs.acs.org/Organometallics

An Efficient and Recyclable Catalytic System Comprising Nano-Iridium(0) and a Pyridinium Salt of nido-Carboranyldiphosphine for the Synthesis of One-Dimensional Boronate Esters via Hydroboration Reaction Yinghuai Zhu,*,† Shi Hui Agnes Jang,† Yue Hao Tham,† Oh Biying Algin,† John A. Maguire,‡ and Narayan S. Hosmane§ †

Institute of Chemical and Engineering Sciences, No. 1 Pesek Road, Jurong Island, Singapore 627833 Department of Chemistry, Southern Methodist University, Dallas, Texas 75275-0314, United States § Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115-2862, United States ‡

bS Supporting Information ABSTRACT: The reaction of N-pentyl-4-methylpyridinium bromide with Na[7,8-bis(dicyclohexylphosphino)-7,8-dicarbanido-C2B9H10] led to the formation of a pyridinium salt of the carboranyldiphosphine ligand, [N-pentyl-4-methylpyridinium]+ [7,8-bis(dicyclohexylphosphino)-7,8-dicarba-nido-C2B9H10]
(1), in 86% yield. [7,8-bis(dicyclohexylphosphino)-7,8-dicarba-nidoC2B9H10]
was produced in situ from the decapitation of 1,2bis(dicyclohexylphosphino)-1,2-dicarba-closo-dodecaborane with sodium hydroxide. Ligand 1 is highly soluble in ionic liquids such as trihexyltetradecylphosphonium dodecylbenzene sulfonate ([THTdP][DBS]) and 1-hexyl-3-methylimidazolium trifluoromethansulfonate ([HMIM][CF3SO3]) and was used as a bulky catalyst composite with the ionic liquids [THTdP][DBS] and [HMIM][CF3SO3], with stabilized iridium(0) nanoparticles produced by thermolysis of Ir4(CO)12, or with the iridium complex [Ir(coe)2Cl]2 (coe = cyclooctene) and in the ionic liquid [THTdP][DBS] and [HMIM][CF3SO3] for the hydroboration of methyl oleate (MO), methyl 10-undecenoate, and 1-hexene. The new Ir(0) catalytic system was found to be efficient, robust, and recyclable so that a number of repetitious hydroborations of MO with high product selectivity could be accomplished.

’ INTRODUCTION Organoboronates are one of the most extensively used intermediates in organic synthesis and pharmaceutical chemistry.1
9 Two methods have been popularly used to synthesize organoboronates: (1) metal-catalyzed C
H bond borylation, which is a well-documented, direct, and selective strategy,9
13 and (2) catalytic hydroboration, which is another important method to prepare alkyl and alkenyl boronates from the corresponding alkenes and alkynes.14
24 Transition metal complex derived catalysts are commonly used in both methods.14
24 However, similar to the case for other homogeneous catalytic processes, the recovery and recycling of the metal complex catalyst composites from the product mixture remains a sizable challenge that may hinder their further applications on industry scale. To avoid the contamination of heavy metals in the products and reuse of the high-cost catalysts and ligands, the recycling of both ligands and metals is highly desirable for reducing raw material costs and engineering a greener process. Therefore, rhodium complexes such as [Rh(1,5-cod)((R)-BINAP)]BF4 (1,5-cod = 1,5-cyclooctadiene, (R)-BINAP = 2,20 -bis(diphenylphosphino)-1,10 -binaphthyl), which are commonly used homogeneous catalysts for the hydroboration reactions to produce branching boronates, have r 2011 American Chemical Society

been successfully heterogenized by immobilization on different solid supports, resulting in high catalytic activity.25,26 Recently, the use of metal nanoparticle based catalysts has attracted much attention, in both academia and industry.27
32 In addition, because it has been recognized that ionic liquids combine the advantages of a solid for supporting the catalyst and the advantages of a liquid reaction media for allowing the catalyst to move freely, many catalytic organic reactions have been examined in ionic liquids.33
35 The catalyst composites comprising of ionic liquids and metal nanoparticles can be conveniently used and easily recovered.34,35 In the hydroboration reaction, an impressive example is that gold(0) nanoparticles have been found to be a highly active catalyst for diboration reactions.36 In a previous study, we found that iridium(0) nanoparticle catalysts were effective for arylborylation reactions via the activation of C
H bonds.35 Encouraged by these promising preliminary results and in an attempt to achieve efficient, economic and green hydroboration Special Issue: F. Gordon A. Stone Commemorative Issue Received: May 7, 2011 Published: August 17, 2011 2589

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Scheme 1. Synthesis of Linear (One-Dimensional) Products Derived from Methyl Oleate (MO)

reactions, we have investigated the potential of using ionic liquid stabilized Ir(0) nanoparticles as catalysts for the hydroboration reaction to synthesize linear terminal boronates. The catalytic transformations of methyl oleate (MO) into linear (one-dimensional) compounds are of growing research interest in biomass research.37
39 Thus far, the linear products of MO, including aldehydes, diesters, and boronates, have been obtained through isomerization processes, such as hydroformylation,40 carbonylation,41 and hydroboration42 reactions like those depicted in Scheme 1. Catalytic hydroboration of MO would be an attractive endeavor, as the resulting linear boronate of MO can be further converted into high-value-added chemicals such as alcohol, amine, aldehyde, and alkyl halide.1,4,7,9,10 However, the isomerization of MO to produce the corresponding linear products with high selectivity and conversion remains a sizable challenge. In the study, the hydroboration reaction of MO was selected to examine our catalyst system comprising ionic liquid stabilized iridium(0) nanoparticles and phosphine auxiliaries. In the presence of a novel ligand, the pyridinium salt of nidocarboranyldiphosphine (1), the iridium nanoparticles demonstrated their use in the hydroboration of both internal (MO) and terminal (10-undecenoate and 1-hexene) alkenes with pinacolborane to generate the corresponding linearly terminal boronates. The homogeneous catalyst [Ir(coe)2Cl]2 (coe = cyclooctene) was also examined as a benchmark catalyst.13,42 To improve the product selectivity, phosphine ligands of varying sizes were studied as auxiliaries. Herein, we report the preliminary results of the nanoscaled iridium catalyzed hydroboration reactions.

’ RESULTS AND DISCUSSION Various precursors have been used to generate iridium nanoparticles.43
46 In our work, the iridium nanoparticles were conveniently synthesized by the thermolysis reaction of commercially available Ir4(CO)12 at 210 °C in a water-stable and environmentally benign ionic liquid, trihexyltetradecylphosphonium dodecylbenzene sulfonate ([THTdP][DBS]). In the thermolytic procedure, after complete release of CO, no other components were left except iridium particles. The resulting Ir nanoparticles were analyzed with TEM and XPS to determine their particle size and oxidation states. TEM images demonstrated that the nanoparticles were small, with an average size of ∼4.0 nm, and had a narrow size distribution, as shown in Figure 1. The XPS spectrum shows typical Ir(0) absorptions at 60.8 and 63.7 eV for 4f7/2 and 4f5/2, respectively, with Δ = 2.9 eV, which is consistent with the literature results.47 All preparative work was carried out under an argon atmosphere to guard against oxide

formation; no evidence of Ir
O contamination was found in the XPS results.46,48 It is well-known that phosphine ligands play an important role in the product selectivity for reactions through the CdC migration procedure.40,42,49 Bulky phosphine ligands induce the formation of intermediates with less steric strain in their molecular structure and thus help in producing more linear isomers.40,42,49 To improve linear product selectivity in the reaction, we chose to synthesize a pyridinium salt of the bulky carboranyldiphosphine ligand [N-pentyl-4methylpyridinium]+[7,8-bis(dicyclohexylphosphino)-7,8-nidodicarba-C2B9H10]
(1), as shown in Scheme 2. N-Pentyl-4-methylpyridinium bromide was synthesized in 96% yield by refluxing 1-bromopentane and 4-methylpyridine. The pyridinium salt of 1 was synthesized in 86% yield by the reaction of N-pentyl-4-methylpyridinium bromide with Na[7,8-bis(dicyclohexylphosphino)7,8-dicarba-nido-C2B9H10], which was generated in situ from the decapitation of 1,2-bis(dicyclohexylphosphino)-1,2-dicarba-closododecaborane with sodium hydroxide.50 This pyridinium salt of bulky ligand 1 is miscible with ionic liquids such as [THTdP] [DBS], 1-n-butyl-3-methylimidazolium chloride, and [HMIM] [CF3SO3]. Accordingly, it can be separated conveniently, together with the nano-Ir catalyst, by extracting the products from the reaction mixture with diethyl ether. Ligand 1 was subsequently used for the isomerization
hydroboration of MO (see Scheme 3) with iridium catalysts and compared with other bulky phosphines, such as 1,2-bis(diphenylphosphino)ethane (dppe), 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos), 1,10 -bis(diphenylphoshino)ferrocene (dppf), 1,3-bis(phosphaadamantyl) propane (pap), and 1,2-bis(9-phosphabicyclo[3,3,1]nonyl);ethane (pcne), as shown in Figure 2. For comparison with the [THTdP] [DBS]-stabilized nano-Ir catalyst, the complex [Ir(coe)2Cl]2 has also been examined as a catalyst under the same reaction conditions. The catalyzed hydroboration of MO was performed in a solvent mixture of dichloromethane (DCM) and ionic liquid in a 1/1 volume ratio. In this work, both phosphonium- and imidazolium-based ionic liquids, [THTdP][DBS] and [HMIM] [CF3SO3], were used as reaction media. After reaction, the volatile dichloromethane was removed under reduced pressure, and the resulting sticky residue was repeatedly extracted with diethyl ether to separate the catalyst and the products. Since catalyst composites are insoluble in diethyl ether, the catalyst could be easily recovered and reused for further runs. The ether solution was concentrated and separated by TLC (SiO2), and pure linear ω-boronate was isolated and identified on the basis of its 1H and 13C NMR spectra. The hydroboration results of MO are summarized in Table 1. 2590

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Figure 1. TEM images (a, b), particle histograms (c, 160 particles counted), and XPS (d) of [THTdP][DBS]-stabilized nano-Ir(0) catalyst.

From Table 1, it can be seen that iridium nanoparticles are active catalysts for the hydroboration of MO in both [THTdP] [DBS] and [HMIM][CF3SO3]. There was not much difference in the catalytic activity of phosphonium- and imidazolium-based ionic liquids. The yields of obtained linear hydroborylation product are partially influenced by the ligand used, which is consistent with the literature.40,42,49 With common phosphine ligands and the same conditions, the activity of the nano-Ir(0) catalyst is comparable with that of the homogeneous catalyst [Ir(coe)2Cl]2 for all three examined substrates. However, with the pyridinium salt of the carboranyldiphosphine ligand 1, the nano-Ir(0) catalyst demonstrates a significant improvement in selectivity for the linear

ω-boronate in both [THTdP][DBS] and [HMIM][CF3SO3]. Considering the active centers are surface atoms of the iridium(0) nanopartilces, we partially attribute this result to the high solubility of 1 in both dichloromethane and ionic liquid that might enable 1 to reach the catalyst centers freely in comparison with other ligands, thus leading to the formation of the sterically favored linear products with improved yields. On the other hand, the nanoIr(0) catalyst may work through a different mechanism from the homogeneous catalyst [Ir(coe)2Cl]2, which usually goes through a six-coordinate intermediate.13,17 For the bulky ligand 1, it could be difficult to form the crowded intermediate. The contrasting experiments in which no ligands were added were also performed 2591

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Organometallics under the same conditions. After removing catalyst and purifying similarly, we analyzed the product by 1H and 13C NMR spectroscopy. The conversions of MO are given in Table 1 (column 2). Many undesired peaks caused by byproduct were observed in the NMR spectra (see the Supporting Information). The results confirmed that the phosphine auxiliary is crucial for the hydroboration reaction. For the catalysts [Ir(coe)2Cl]2 and nanoIr(0), bulky phosphine ligands favor the formation of the linearly terminal product. Importantly, the catalyst mixture of [THTdP][DBS]-stabilized nano-Ir(0) and 1 can be conveniently recycled and reused at least eight times with consistent yields of 78
74% (see the Supporting Information), for the hydroboration of MO in the [THTdP][DBS]/DCM mixed solvent. The particle size distribution of the nano-Ir(0) remains unchanged after three runs (see the Supporting Information). The concentrations of iridium and the carborane cage, leached into the reaction media, were found to be less than 2.7 and 4.0 ppm, respectively, on the basis of ICP results. Under the current conditions, no hydrogenation products were isolated, which is consistent with the literature.51 In addition, 1 was also found to catalyze the hydroboration of methyl 10-undecenoate and 1-hexene to produce the corresponding linearly terminal boronate esters, as shown in Scheme 3. Both the homogeneous catalyst [Ir(coe)2Cl]2 and nano-Ir(0) were efficient for the reaction, with high selectivity for the linear product. With the catalyst composite nano-Ir(0)/1, yields of 96% and 94% were achieved for 10-undecenoate and 1-hexene, respectively, in [THTdP][DBS]/DCM media. In [HMIM] [CF3SO3]/DCM media, slightly lower yields of 93% and 89% were obtained from 10-undecenoate and 1-hexene, respectively. Under the same conditions, the combination [Ir(coe)2Cl]2/ dppe resulted in a yield of 88% for the hydroboration of methyl 10-undecenoate, which is comparable with literature results.42 Although the exact mechanism is not known, according to previous studies on the catalytic hydroboration reaction and

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our current observations,13,16,35,36 the catalytic cycle shown in Scheme 4 can be proposed. The phosphine ligands coordinate with the [THTdP][DBS]-stabilized nano-Ir(0) to form the true catalyst composite A. A reacts with MO and pinacolborane via path I (coordination and B
H activation), generating the active intermediate B. B further conducts the migratory insertion (path II) reaction to form C. After an eliminatation reaction (path III), linear boronates are produced. The proposed mechanism is based on the following observations. (1) To exclude the possibility of a residual homogeneous iridium catalyst initiating the hydroboration reaction, mercury poisoning studies were conducted. For the hydroboration of MO with the catalyst combination of the [THTdP][DBS]-stabilized nano-Ir(0)/1, in the presence of mercury, no products could be

Figure 2. Phosphine and diphosphine ligands of varying sizes and shapes.

Table 1. Summary of Isomerization
Hydroboration of Methyl Oleatea (Yield (%)b) ligand cat.

Scheme 2. Synthesis Method of Ligand 1

none

dppe

dppf

xantphos

pap

pcne

1

A

c

37

45

47

51

54

51

55

B

24c

41

44

46

53

46

78

C

30c

45

38

42

58

41

69

42

a

All reactions were conducted at room temperature for 24 h in a 4 mL mixture of DCM and [THTdP][DBS] or [HMIM][CF3SO3] in a ratio of 1/1 (v/v); [MO]mol/[Ir]mol = 40. b Average isolated yield of linear hydroboration product from three runs with catalyst [Ir(coe)2Cl]2 (A), nano-Ir(0)/[THTdP][DBS] (B) and nano-Ir(0)/[HMIM][CF3SO3] (C) respectively. c Conversion (%) of MO.

Scheme 3. Hydroboration Reactions

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Scheme 4. Cycle of Nano-Ir-Catalyzed Hydroboration

Table 2.

31

P NMR Data of Phosphine Ligandsa ligand nano-Ir/

δ31P (ppm)

dppe

xantphos

xantphos

1

nano-Ir/1


16.0


15.9


17.0


17.1

47.03

47


2.8 Δδ (ppm)

nano-Ir/

dppe

+13.1


10 +7.1

30.1
16.9

a

All reactions were performed at room temperature for 24 h in a 2 mL mixture of DCM-d2 and [THTdP][DBS] (1/1 v/v); [ligand]mol/[Ir]mol = 2.

obtained under the same conditions. This result confirmed that the ionic liquid stabilized nano-Ir(0) is the true catalyst.35 (2) The interactions of the [THTdP][DBS]-stabilized nano-Ir(0) with the phosphine ligands dppe, xantphos, and 1 were examined by 31P NMR spectroscopy. In the absence of pinacolborane and MO, ligands were mixed with the [THTdP][DBS]-stabilized nano-Ir(0) in [THTdP][DBS]/dichloromethane-d2 (CD2Cl2) and further stirred for 24 h at room temperature; new peaks emerged for all the tested phosphines in comparison with pristine ligands in the 31P NMR spectra, as given in Table 2 (see spectra in the Supporting Information). Contrasting experiments were also performed by mixing the ionic liquid [THTdP][DBS] and dppe in CD2Cl2; after the same operation, no significant δ31P shifts were found for dppe. Obviously, the new peaks were caused by interaction of the nano-Ir with ligands. Unlike the interaction of dppe and xantphos with nano-Ir, which caused a downshift in δ31P, mixing 1 with nano-Ir led to upshifted δ31P of 1. This could suggest that 1 reacts with the nano-Ir diffeently or perhaps more strongly. (3) Interestingly, stirring a mixture of pinacolborane and the [THTdP][DBS]-stabilized nano-Ir(0) in CD2Cl2, resulted in the emergence of a new peak at δ11B 34.3 ppm in the 11B NMR spectrum (Figure3a). Although the peak is weak and broad, it is readily detectable. This δ11B value is consistent with the δ11B absorption of a well-characterized iridium complex, [(dtbpy)Ir(1,5-cod)(Bpin)2]OTf (dtbpy = 4,40 -di-tert-butyl-2,

20 -bipyridine, 1,5-cod = 1,5-cyclooctadiene, Bpin = pinacolborate, OTf = trifluoromethanesulfonate) (δ11B 34 ppm).52 This could suggest the formation of a weak Ir
B bond by activating the H
B bond in pinacolborane. Different from the case for the nano-Au catalyst, nano-Ir was able to activate H
B bonds in the absence of base.36 (4) On the other hand, mixing MO with the [THTdP] [DBS]-stabilized nano-Ir(0) in CDCl3 and stirring for 24 h led to new peaks in the 1H NMR spectrum resulting from absorption of the
CHdCH
group in MO (Figure 3b). We attributed the appearance to the coordination of the CdC bond in MO with nano-Ir particles. The activated CdC bond could further migrate to the chain terminus. Therefore, the hydroboration of MO could be conducted through an isomerization
hydroboration procedure, which is consistent with the literature.16,23,24 When all the results are combined, it is reasonable to propose the catalyst cycle as described in Scheme 4.

’ CONCLUSIONS The pyridinium salt of the nido-carboranyldiphosphine ligand (1) and [THTdP][DBS]-stabilized iridium(0) nanoparticles have been synthesized and characterized. These composites are highly effective catalysts, with high selectivity, in hydroboration reactions to produce the ω-boronated linear species of the biodiesel component methyl oleate. The composites also show high selectivity for the linear products in the hydroboration of methyl 10-undecenoate and 1-hexene. While these catalyst composites are well dispersed in organic reaction mixtures to produce a “mock” homogeneous catalytic system, they can also be easily separated by solvent extraction of the product mixture with sustained activity. We expect the catalyst composite to find broad applications in both academia and the materials industry. ’ EXPERIMENTAL SECTION General Considerations. All operations were conducted under an argon atmosphere with glovebox or standard Schlenk lines. Iridium and boron atomic absorption standard solutions and other reagents were 2593

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Figure 3.

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11

B NMR spectrum of nano-Ir/HBPin (a) and 1H NMR spectrum of nano-Ir/MO (b).

purchased from Sigma-Aldrich Pte. Ltd.; 1,2-bis(dicyclohexylphosphino)1,2-dicarba-closo-dodecaborane was prepared according to the literature.50 NMR spectra were recorded on a Bruker 400 analyzer at 400.13 and 100.62 MHz relative to SiMe4 (1H and 13C) and 128.38 MHz (11B) relative to BF3 3 OEt2. Near-infrared (near-IR) spectra were measured using a BIORAD spectrophotometer as KBr pellets. Inductively coupled plasma
optical emission spectroscopy (ICP-OES) analysis was determined with a VISTA-MPX CCD simultaneous ICP-OES analyzer. Transmission electron microscopy (TEM) measurements were carried out on a JEOL Tecnai-G2 FEI analyzer at 200 kV. MS measurements were carried out on a Thermo Finnigan MAT XP95 analyzer using the EI model.

Synthesis of N-Pentyl-4-methylpyridinium Bromide. The compound was synthesized according to a literature method.53 A 4.0 mL sample of 1-bromopentane and 12 mL of dry 4-methylpyridine were added to a 50 mL round-bottom flask. The resulting mixture was refluxed overnight and dried under reduced pressure. The crude product was purified by washing with ether. After drying under high vacuum, N-pentyl-4-methylpyridinium bromide was obtained in 96% yield. 1H NMR (CD2Cl2, ppm): δ 9.31 (d, C5H4N, 3JHH = 6.6 Hz, 2H), 7.87 (d, 3JHH = 6.0 Hz, C5H4N, 2H), 4.85 (t, 3JHH = 7.6 Hz, NCH2
, 2H), 2.64 (s, 4-CH3C5H4N, 3H), 2.09 (m, NCH2CH2
, 2H), 1.34 (m,
CH2CH2CH3, 4H), 0.59 (t, 3JHH = 6.8 Hz,
CH2CH3, 3H). 2594

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Organometallics C NMR (CD2Cl2, ppm): δ 159.51 (C5H4N, 1C), 144.59 (C5H4N, 2C), 129.21 (C5H4N, 2C), 61.37 (NCH2
), 31.83 (4-CH3C5H4N, 1C), 28.41 (NCH2CH2
, 1C), 22.55 (
CH2CH2CH3, 1C), 22.49 (
CH2CH3, 1C), 14.03 (
CH2CH3, 1C). Anal. Calcd for C11H18BrN: C, 54.11; H, 7.43; N, 5.74. Found: C, 53.74; H, 7.15; N, 5.33. Mp: 92
94 °C. IR (KBr pellet, cm
1): ν 3430 (vs, br), 3034 (m, s), 2957 (s, s), 2933 (s, s), 2862 (m, s), 2057 (w, br), 1642 (vs, s), 1572 (w, s), 1519 (m, s), 1472 (s, s), 1379 (w, s), 1310 (w, s), 1209 (w, s), 1173 (s, s), 1040 (w, s), 831 (m, s), 706 (m, s), 552 (m, s), 490 (m, s). Synthesis of Compound 1. A 2.0 g (3.73 mmol) sample of 1,2bis(dicyclohexylphosphino)-1,2-dicarba-closo-dodecaborane was added to a prepared solution of 3.0 g of sodium hydroxide dissolved in 100 mL of 95% ethanol with constant stirring. The resulting mixture was heated to reflux for 14 h, cooled to 0 °C, and bubbled with CO2 to neutralize excess sodium hydroxide. After filtration, the filtrate was dried under reduced pressure. The resulting residue was redissolved in a minimum quantity of deionized water and precipitated with a saturated solution of N-pentyl-4-methylpyridinium bromide in deionized water. The resulting mixture was filtered, washed with cold deionized water (5 mL), and then dried in vacuo to a constant weight to isolate 1 as a waxy solid (2.21 g, 86% yield). 1H NMR (DMSO-d6, ppm): δ 8.94 (d, 3JHH = 6.4 Hz, C5H4N, 2H), 7.98 (d, 3JHH = 6.0 Hz, C5H4N, 2H), 4.51 (t, 3JHH = 6.8 Hz, NCH2
, 2H), 3.34 (s, 4-CH3C5H4N, 3H), 1.05
1.97 (m, br, N(CH2)3
, 2(P
C6H11)2, C2B9H9, 59H), 0.86 (t, 3JHH = 6.4 Hz,
CH2CH3, 3H),
2.40 to
3.28 (br, BHbridge, 1H). 13C NMR (DMSO-d6, ppm): δ 158.77 (C5H4N, 1C), 143.68 (C5H4N, 2C), 128.35 (C5H4N, 2C), 66.34 (C2B9, 2C), 59.90 (NCH2
, 1C), 34.05, 33.41, 30.68, 30.21, 27.48, 25.54, 25.40, 24.53, 24.50, 21.51, 21.34 (
(CH2)3
, 2  (PC6H11)2, 4-CH3C5H4N, 28C), 13.70 (
CH2CH3, 1C). 11B NMR (DMSO-d6, ppm): δ
10.96 (2B, 1JBH = 136 Hz),
16.95 (2B, 1JBH = 127 Hz),
17.72 (1B, 1JBH = 116 Hz),
22.30 (2B, 1JBH = 148 Hz),
33.35 (1B, 1JBH = 135 Hz),
37.86 (1B, 1JBH = 139 Hz). 31P NMR (DMSO-d6, ppm): δ 47.03. Anal. Calcd for C37H72B9NP2: C, 64.38; H, 10.52; N, 2.03. Found: C, 63.92; H, 10.87; N, 1.71. Mp: 47
49 °C. IR (KBr pellet, cm
1): ν 3437 (vs, br), 3055 (m, s), 2930 (s, s), 2854 (s, s), 2515 (vs, s, νB
H), 2313 (w, s), 1724 (m, s), 1641 (s, s), 1519 (w, s), 1449 (s, s), 1379 (m, s), 1290 (m, s), 1211 (w, s), 1165 (m, br), 1076 (w, s), 1029 (w, br), 940 (w, s), 826 (m, s), 752 (w, br), 531 (m, s), 492 (w,s). Synthesis of the Iridium(0) Nanoparticles. Nanoscale iridium was prepared by the thermolytic dissociation reduction of 20.0 mg of Ir4(CO)12 in 5.0 mL of [THTdP][DBS] at 210 °C under an argon atmosphere until the mixture turned black, and the system was cooled to room temperature. To isolate Ir nanoparticles, the black residue was dissolved in 10.0 mL of dichloromethane and subjected to centrifugation (5000 rpm, 40 min) followed by washing with dichloromethane (2  10.0 mL) and drying under reduced pressure. The resulting iridium nanoparticles were subjected to analysis by XRD, XPS, and TEM. Hydroboration of Methyl Oleate. The reaction of methyl oleate (0.34 mL, 1.0 mmol) with pinacolborane (0.16 mL, 1.1 mmol) is catalyzed by [THTdP][DBS]-stabilized nano-Ir(0) (25 μmol of [Ir]) or [Ir(coe)2Cl]2 (33 μmol) with 66 μmol of the pyridinium salt of ligand 1 or other bulky phosphine ligands given in Figure 2, and 2 mL each of the ionic liquid [THTdP][DBS] or [HMIM][CF3SO3] and dichloromethane. The reaction was conducted at room temperature for 24 h with continuous stirring. The reaction mixture was dried to remove dichloromethane, and the resulting residue was extracted with moistureand oxygen-free diethyl ether (10 mL  3). The ether solutions were then combined and concentrated in vacuo, followed by purification with chromatography (SiO2, eluted with a hexane/diethyl ether solvent mixture (5/1 v/v), to isolate the pure product as summarized in Table 1 (see the Supporting Information for NMR spectra). Anal. Calcd for C25H49BO4: C, 70.74; H, 11.64. Found: C, 70.52; H, 10.27. 1H NMR (CDCl3, ppm): δ 3.66 (s,
CO2CH3, 3H), 2.31 (t, 3JHH = 7.2 Hz,
CH2CO2
, 2H), 1.60 (m,
CH2CH2B, 2H), 1.40
1.24 (m, br, 13

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(CH2)13
, 4  CCH3, 38H), 0.88 (t, 3JHH = 7.2 Hz,
CH2B, 2H). C NMR (CDCl3, ppm): δ 174.30 (
CO2
, 1C), 82.71 (2x
C(CH3)2OB, 2C), 51.38 (
CO2CH3, 1C), 34.09, 31.91, 31.46, 29.67, 29.57, 29.34, 29.27, 29.24, 29.14, 24.94, 24.77, 22.67 (
(CH2)17
, 4  CH3, 21C, 10 C overlapped). 11B NMR (CDCl3, ppm): δ 34.40 (s, br). Hydroboration of Methyl 10-Undecenoate. The reaction of methyl 10-undecenoate (0.22 mL, 1.0 mmol) with pinacolborane (0.16 mL, 1.1 mmol) was catalyzed by [THTdP][DBS]-stabilized nano-Ir(0) (25 μmol [Ir]) or [Ir(coe)2Cl]2 (33 μmol) with 66 μmol of the pyridinium salt of ligand 1, 2 mL each of the ionic liquid [THTdP][DBS] and dichloromethane. The reaction was conducted at room temperature for 12 h with continuous stirring. The reaction mixture was dried to remove dichloromethane, and the resulting residue was extracted with moisture and oxygen-free diethyl ether (10 mL  3). The ether solutions were then combined and concentrated in vacuo, followed by purification with chromatography (SiO2, eluted with a hexane/diethyl ether solvent mixture (8/1 v/v), to isolate the pure product as a colorless solid in yields of 96% and 88% for nano-Ir(0) and [Ir(coe)2Cl]2, respectively. Under the above conditions, in 2 mL each of [HMIM][CF3SO3] and dichloromethane, a yield of 93% was obtained for catalysis by nano-Ir(0) (see the Supporting Information for NMR spectra). Anal. Calcd for C18H35BO4: C, 66.26; H, 10.81. Found: C, 65.80; H, 10.49. 1H NMR (CDCl3, ppm): δ 3.66 (s,
CO2CH3, 3H), 2.30 (t, 3JHH = 7.6 Hz,
CH2CO2
, 2H), 1.60 (m,
CH2CH2B, 2H), 1.40
1.20 (m, br,
(CH2)7
, 4  CCH3, 26H), 0.76 (t, 3JHH = 7.6 Hz,
CH2B, 2H). 13C NMR (CDCl3, ppm): δ 173.29 (
CO2
, 1C), 81.79 (2 
C(CH3)2OB, 2C), 50.39 (
CO2CH3, 1C), 33.09, 31.39, 28.47, 28.41, 28.35, 28.23, 28.14, 23.95, 23.79, 22.97 (
(CH2)10
, 4  CH3, 14C, four C overlapped). 11B NMR (CDCl3, ppm): δ 34.26 (s, br). Hydroboration of 1-Hexene. The reaction of 1-hexene (0.13 mL, 1.0 mmol) with pinacolborane (0.16 mL, 1.1 mmol) was also catalyzed by [THTdP][DBS]-stabilized nano-Ir(0) (25 μmol of [Ir]) with 66 μmol of the pyridinium salt of ligand 1, and 2 mL each of the ionic liquid [THTdP][DBS] or [HMIM][CF3SO3] and dichloromethane. The reaction was conducted at room temperature for 12 h with continuous stirring. The reaction mixture was dried to remove dichloromethane, and the resulting residue was extracted with moistureand oxygen-free diethyl ether (10 mL  3). The ether solutions were then combined and concentrated in vacuo, followed by purification with chromatography (SiO2, eluted with a hexane/diethyl ether solvent mixture (6/1 v/v), to isolate the pure product as a waxy solid in yields of 94% and 89% for [THTdP][DBS]/DCM and [HMIM][CF3SO3]/ DCM, respectively. Anal. Calcd for C12H23BO2: C, 68.59; H, 11.03. Found: C, 68.30; H, 10.87. 1H NMR (CDCl3, ppm): δ 1.34
1.17 (m, 4  CCH3,
(CH2)4
, 20H), 0.80 (t, 3JHH = 6.4 Hz, CH3
, 3H), 0.69 (t, 3 JHH = 7.6 Hz,
CH2B, 2H). 13C NMR (CDCl3, ppm): δ 82.76 (2 
C(CH3)2OB, 2C), 32.07, 31.61, 24.77, 23.93, 22.55, 14.05 (
(CH2)5
, 5  CH3, 10C, three C overlapped). 11B NMR (CDCl3, ppm): δ 34.33 (s, br). Mercury Poisoning Tests. Two experiments were designed and conducted to investigate the effects of Hg on the precatalyst and the preformed nano-Ir(0) catalysts for the hydroboration of MO with nanoIr(0)/1 catalyst in [THTdP][DBS] and dichloromethane. First, after the formation of nano-Ir(0), a large excess (370 equiv) of Hg was added to the ionic liquid solution.44 After it was stirred for 2 h at room temperature under argon, the mixture containing Hg was used in the isomerization
hydroboration reaction. After the same operation to collect product as described above, no isolable products were obtained in this experiment. In the second experiment, excess Hg was added to an active catalyst solution. In the experiment, after a 5 h reaction process, the reaction was interrupted and 370 equiv of Hg was carefully added under an argon atmosphere. After stirring for about 19 h at room temperature followed by similar purification, 0.166 g of the linear boronate ester product was obtained in 39% yield. 13

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’ ASSOCIATED CONTENT

bS

Supporting Information. Figures giving additional TEM images and NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +6567963801. Fax: +6563166182. E-mail: zhu_yinghuai@ ices.a-star.edu.sg.

’ ACKNOWLEDGMENT This work was supported by the Institute of Chemical and Engineering Sciences (ICES), Singapore. We are grateful for the donation of the phosphine ligands pcp and pcne by Professor Eite Drent at Leiden Institute of Chemistry, Leiden, The Netherlands, Dr. Rex Ren at IL-TECH Inc. for providing the ionic liquid, and Professor Leong Weng Kee at the Department of Chemistry, National University of Singapore, for text editing. N.S.H. acknowledges a grant from the National Science Foundation (No. CHE0906179), and J.A.M. gratefully acknowledges support from the Robert A. Welch Foundation (No. N-1322). ’ REFERENCES (1) Miyaura, N.; Suzuki, A. Chem. Commun. 1979, 866–867. (2) Matteson, D. S.; Majumdar, D. Organometallics 1983, 2, 1529–1535. (3) Brown, H. C.; Singh, S. M. Organometallics 1986, 5, 994–997. (4) Dyker, G. Angew. Chem., Int. Ed. 1999, 38, 1698–1712. (5) Jia, D.; Kitamura, T.; Fujiwara, Y. Acc. Chem. Res. 2001, 34, 633–639. (6) Zapf, A.; Beller, M. Top. Catal. 2002, 19, 101–109. (7) Ishiyama, T.; Takagi, J.; Ishida, K.; Miyaura, N.; Anastasi, N. R.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 390–391. (8) Blaser, H.; Indoless, A.; Naud, F.; Nettekoven, U.; Schnyder, A. Adv. Synth. Catal. 2004, 346, 1583–1598. (9) Burkhardt, E. R.; Matos, K. Chem. Rev. 2006, 106, 2617–2650. (10) Corbet, J.-P.; Mignani, G. Chem. Rev. 2006, 106, 2651–2710. (11) Zhu, Y.; Koh, C.; Luo, J.; Chong, S. H.; Yong, C. H.; Emi, A.; Su, Z.; Winata, M.; Hosmane, N. S.; Maguire, J. A. J. Organomet. Chem. 2007, 692, 4244–4250. (12) Mkhalid, I. A.; Coapes, R. B.; Edes, S. N.; Coventry, D. N.; Souza, F. E. S.; Thomas, R. Ll.; Hall, J. J.; Bi, S.-W.; Lin, Z.; Marder, T. B. Dalton Trans. 2008, 1055–1064. (13) Mkhalid, I. A.; Barnard, J. H.; Marder, T. B.; Murphy, J. M.; Hartwig, J. F. Chem. Rev. 2010, 110, 890–931 and references therein. (14) Burgess, M. J.; Ohlmeyer, M. J. J. Org. Chem. 1988, 53, 5178–5179. (15) Pelter, A.; Smith, K.; Brown, H. C. Borane Reagents; Academic: London, 1988. (16) Burgess, K.; Ohlmeyer, M. J. Chem. Rev. 1991, 91, 1179–1191. (17) Beletskaya, I.; Pelter, A. Tetrahedron 1997, 53, 4957–5026. (18) Brown, J. M.; Doucet, H.; Fern
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