Amine-Functionalized Indazolin-3-ylidene Complexes of Palladium(II

Article pubs.acs.org/Organometallics

Amine-Functionalized Indazolin-3-ylidene Complexes of Palladium(II) by Postmodification of a Single Precursor Jan C. Bernhammer, Harvenjit Singh, and Han Vinh Huynh* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore S Supporting Information *

ABSTRACT: A series of five trans-[PdBr2(amine)(indy)] complexes (amine = diethylamine, dipropylamine, dibutylamine, diisobutylamine, morpholine; indy = indazolin-3-ylidene) with pendant teriary amine functionalities in the side chain of the NHC ligand has been prepared by postcoordinative modification of a single bromoalkyl-functionalized precursor complex. This approach allows for a synthesis of functionalized N-heterocyclic carbene complexes more efficient than the metalation of prefunctionalized azolium salts. All complexes have been fully characterized, and the molecular structures of three complexes are reported. A correlation exists between the 13C NMR shift of Ccarbene and the pKb values of the coordinated amines. Furthermore, all complexes were found to be active catalysts for the direct arylation of 1-methylpyrrole with good to excellent yields.

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INTRODUCTION From humble beginnings two decades ago, after Arduengo’s seminal discovery of the first stable free N-heterocyclic carbene (NHC),1 these compounds have established themselves as one of the most important classes of ligands in transition-metal catalysis,2 rivaled only by phosphine ligands, with which they share some characteristics.3 The reasons for this success story are the trouble-free syntheses of a wide variety of suitable azolium salts to serve as carbene precursors and their easy subsequent metalation,4 the high stability of NHC complexes,5 their strong σ donation,6 and the fact that sterics and electronics can be readilyand almost independently of each otherfine-tuned by modifications of wingtip substituents and NHC backbones.7 Beyond the most commonly studied imidazolin-2-ylidenes and imidazolidin-2-ylidenes, in recent years an increasing amount of attention has been given to NHCs with reduced heteroatom stabilization, triggered by the discovery of the abnormally binding imidazolin-4-ylidenes and the superior performance of their complexes in catalysis.8 These NHCs, in which one or both nitrogen atoms adjacent to Ccarbene are replaced by a less electronegative element, are stronger electron donors than their classical counterparts, due to a higher-lying σHOMO orbital.9 Complexes with NHCs derived from a diverse array of heterocycles such as pyrazole and pyridine and their benzannelated derivatives have been reported.10,11 Other notable examples of such strongly donating NHCs are Bertrand’s cyclic alkyl amino carbenes (CAACs), which possess a quaternary carbon adjacent to the ylidene carbon.12 Indazolin-3-ylidenes (indy) are prime examples of such strongly donating NHCs. The organic chemistry of the in situ generated free ligands has been studied, and a number of © XXXX American Chemical Society

complexes with various late transition metals have been described as well.13,14 However, reports on their catalytic activities are very scarce.15 Furthermore, no derivatives of this class of ligands incorporating additional donor functionalities have been synthesized so far, although pendant donors hold the potential to enhance catalyst lifetimes by providing additional stabilization.16 To probe the coordination chemistry of such compounds and the potential their complexes have as catalysts, we decided to synthesize the first examples of aminefunctionalized indy complexes by means of postmodification of a single bromoalkyl-functionalized indy complex. This approach is more efficient than the synthesis of appropriately functionalized azolium salts and their subsequent metalation, because a diverse array of complexes can be generated with less experimental effort.17 Additionally, it avoids functional group incompatibilities occasionally associated with the preparation of amine-functionalized NHC precursors.16c,18

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RESULTS AND DISCUSSION Synthesis of the Precursor Complex. Several procedures for the N-methylation of indazole (1) to yield 1-methylindazole (3) have been described.19 However, none of them proceed regioselectively, and mixtures of the two possible methylation products 3 and 4 are always obtained. A very convenient process using cheap and environmentally benign dimethyl carbonate as N-methylating agent for indoles has been described by Jiang et al.19b We have adapted this method for the N-methylation of indazole. Although a long reaction time of 48 h was required to achieve full conversion, the desired Received: June 13, 2014

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dx.doi.org/10.1021/om5006275 | Organometallics XXXX, XXX, XXX−XXX

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Postmodification. Treatment of the bromopropyl-functionalized complex 7 with a large excess of an aliphatic secondary amine at 60 °C for 40 h gives complexes 8−12 as yellow to pale orange solids (Scheme 4). The bromo function in the side chain is readily substituted via an SN2 reaction by the amine, and the weakly coordinating acetonitrile ligand is lost via ligand replacement by the same compound. Both acyclic and cyclic secondary amines reacted readily, with generally good yields ranging from 75% for 8 to 85% for 11. The complexes were found to be readily soluble in polar organic solvents such as acetone, acetonitrile, and DMSO as well as in chlorinated solvent and insoluble in hydrocarbons. The solubility in ethereal solvents and ethyl acetate increases with the length of the aliphatic amine chains. Elemental analyses and ESI-MS suggest the formula trans-[PdBr2(amine)(indy)] for these complexes, ruling out the formation of a κ2C,N coordinated cis-[PdBr2(indy)] species. The secondary amines, which served both as reactants and bases to neutralize the hydrobromic acid formed as a byproduct in the reaction, also coordinate to the metal center. This was further supported by 1H NMR spectroscopy. In the spectra of 8−12, two distinct sets of signals corresponding to the amines are observed. In complex 11, which incorporates bulky isobutyl chains, one of these signal sets shows substantial peak broadening and the methyl groups resonate as three distinct doublets, suggesting that steric encumbrance leads to hindered rotation. The signals for 8−11 are within less than 0.2 ppm from their counterparts in the starting material 7, with the exception of the methylene group adjacent to the newly introduced secondary amine. The change from a bromo to a tertiary amine substituent leads to an average upfield shift of 0.60 ppm. The 13C NMR spectra show a downfield shift of Ccarbene by ∼8 ppm, reflecting the more strongly donating nature of the secondary amine ligands in comparison to acetonitrile.24 The chemical shifts are close to those reported for structurally related complexes incorporating amine ligands and an imidazolin-2-ylidene in a trans arrangement.25 On close examination, a strong correlation between the chemical shifts of Ccarbene and the pKb values26 of the coordinated secondary amine emerges (Figure 1 and Table 1). An influence of the tertiary amines introduced in the side chain on Ccarbene can be ruled out, since the modification takes place in a position too remote for any electronic communication with the ylidene carbon.17 Higher basicities of the amines are indicative of energetically higher HOMOs and thus better σ-donating properties, with a direct bearing on complex properties.27 The chemical shift of Ccarbene is known to show a strong dependence on the net donor strength of the trans ligand, explaining the observed trend.24 Various attempts to remove the metal-bound amine and to obtain the κ2C,N chelating complex were futile. Navarro and co-workers have shown that secondary amines bind more strongly to the metal center than their tertiary congeners in trans-[PdCl2(amine)(NHC)] complexes.25b Intramolecular hydrogen bonds between the amine proton and one of the chlorido ligands resulted in a shorter bond distance, a higher trans influence, and a stronger σ donation of the secondary amines. Similar hydrogen bonds are also present in the molecular structures of 11 and 12, offering an explanation for the unusual preference for the nonchelating forms of the complexes (vide infra).

product 3 could be separated easily by column chromatography from its regioisomer in 51% yield (Scheme 1). Scheme 1. Alkylation of Indazole using Dimethyl Carbonate

Due to the low nucleophilicity of 1-methylindazole, harsh reaction conditions and a long reaction time were required for the introduction of the bromoalkyl chain. Heating 3 with neat 1,3-dibromopropane at 90 °C in a closed vessel lead to the slow formation of the desired product, which precipitated as a white solid over the course of the reaction (Scheme 2).20 When the Scheme 2. Synthesis of 2-(3-Bromopropyl)-1methylindazolium Bromide

reaction temperature was raised in an attempt to speed up the alkylation, the formation of the alicyclic byproduct 6 was observed,21 arising from the (formal) elimination of methyl bromide from salt 5 and ring closure by intramolecular alkylation. However, when the reaction mixture was reheated after isolating a first crop of product after 48 h, a good overall yield of 69% could be reached. A palladium(II) complex could be readily synthesized starting from 5 using a silver carbene transfer protocol.22 However, initial attempts showed that the bromo substituent in the side chain was replaced by acetate in the product. To avoid the formation of acetate anions by the basic hydrolysis of wet acetonitrile in the presence of metal ions,23 the reaction was run in freshly distilled, anhydrous acetonitrile under an inert atmosphere, giving complex 7 in 63% yield as an orange solid (Scheme 3). Scheme 3. Synthesis of a Bromopropyl-Functionalized Indazolin-3-ylidene Palladium Complex

The complex is readily soluble in polar, coordinating organic solvents such as acetone, acetonitrile, and DMSO, poorly soluble in chlorinated organic solvents, and insoluble in hydrocarbons, ethereal solvents, and ethyl acetate. In the 1H NMR spectrum, the absence of the signal attributable to the proton on C3 corroborated the successful complex formation, in addition to further evidence from elemental analysis and ESIMS. The 13C NMR resonance for the ylidene carbon was observed at 152.8 ppm, a value close to those reported for a structurally related complex.10g B


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Scheme 4. Introduction of Amine Groups by Postmodification

Figure 2. Molecular structures of 10·HBr·2CHCl3 and 11. Thermal ellipsoids are shown at the 50% probability level; hydrogen atoms, solvent molecules, counteranions, and disordered atoms are omitted for clarity. Figure 1. Correlation between the 13C chemical shifts of Ccarbene in 8− 12 and the pKb values of the coordinated amines (R2 = 0.9967).

Table 1. Chemical Shifts of Ccarbene and pKb Values of the Coordinated Amines complex

δ(Ccarbene) (ppm)

pKb

8 9 10 11 12

161.0 160.9 161.2 160.6 158.9

3.02 3.00 2.75 3.50 5.64

Molecular Structures. Single crystals of 10−12 suitable for X-ray diffraction studies were obtained by slow evaporation of concentrated chloroform or acetonitrile solutions. All complexes show a square-planar geometry around the palladium center, with a trans arrangement of the indy and amine ligands (Figures 2 and 3). The NHC ring planes are almost perpendicular to the coordination plane. The aliphatic substituents of the amine ligands are oriented parallel to the NHC ring planes, and one of the butyl chains in 10 shows crystallographic disorder. Interestingly, single crystals thought to be of 10 were found to be of 10·HBr instead, with the protonation taking place at N4. In all probability the protonated form crystallizes more easily than the neutral complex. Elemental analysis of this

Figure 3. Molecular structure of 12. Thermal ellipsoids are shown at the 50% probability level; hydrogen atoms are omitted for clarity.

compound supports the assumption that the bulk of the isolated material is made up of the nonprotonated form 10, however. The Pd1−C1 bond lengths fall in the range from 1.959(2) Å in 12 to 1.964(8) Å in 10 (Table 2). These values do not deviate significantly from the Pd−C bond distances in trans[PdCl2(diethylamine)(SIPr)].25b The Pd1−N3 bonds vary unsystematically between 2.128(8) Å in 10 and 2.144(3) Å in 11 and are longer than the Pd−N bonds in Navarro’s C


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Table 2. Bond Distances (Å) and Angles (deg) in Complexes 10−12 bond param

10·HBr·2CHCl3

11

12

Pd1−C1 Pd1−N3 Pd1−Br1 Pd1−Br2 C1−Pd1−Br1 C1−Pd1−Br2 N3−Pd1−Br1 N3−Pd1−Br2 C1−Pd1−N3 Br1−Pd1−Br2

1.964(8) 2.128(8) 2.4237(9) 2.4249(9) 88.7(2) 90.8(2) 91.5(2) 89.0(2) 176.8(3) 178.87(4)

1.961(4) 2.144(3) 2.4260(8) 2.4356(8) 87.1(1) 91.4(1) 92.23(8) 89.34(8) 178.9(1) 178.42(2)

1.959(2) 2.142(2) 2.4186(5) 2.4225(5) 87.31(6) 90.74(6) 92.87(5) 89.25(5) 172.44(8) 177.65(2)

Scheme 5. Direct Arylation of 1-Methylpyrrole

Table 3. Catalyst Screening for the Direct Arylation of 1Methylpyrrolea

complexes due to the stronger trans influence of the indy ligand. Pd−Br bonds fall in the range from 2.4186 to 2.4356 Å, which is typical for these bonds.10g In the molecular structures of 11 and 12, it was found that the distance between the proton on N3 and Br2 was shorter than the sum of their van der Waals radii (3.05 Å),28 suggesting the presence of hydrogen bonds similar to those observed in structurally related complexes (Figure 4).25b Due to the

entry

precatalyst

yield (%)b

1 2 3 4 5 6 7

7 8 9 10 11 12

0 85 97 88 87 84 88

a

Reaction conditions: 1 mol % of precatalyst, 1-methylpyrrole (3.0 equiv), 4-bromoacetophenone (1.0 mmol), KOAc (2.0 equiv), DMA (3 mL), 150 °C, 20 h. bYields determined by GC-MS with decane as internal standard; average of two runs.

While no product was observed when the reaction was run without adding a precatalyst, 1 mol % of all tested complexes catalyzed the reaction with excellent yields ranging from 84 to 97%. Due to their closely related structures, all complexes perform with similar levels of activity. While the least bulky diethylamine complex 8 gave an almost quantitative yield (entry 3), yields ranging from 84 to 88% were obtained with the other precatalysts. The underlying reason for this observation, which does not match the order of donor strength of the secondary amine ligand established above, seems to be the difference in steric bulk of the pendant tertiary amine moiety and subsequently the ease with which they can coordinate to the metal center. By providing additional support to the catalytically active species, the lifetime of the catalyst is prolonged under the harsh reaction conditions, resulting in increased yields with less bulky amine substituents.2i,32 This is further corroborated by the fact that the bromo-functionalized complex 7 was among the least active precatalysts tested. All tested indy complexes were found to be precatalysts comparable to or better than benzimidazolin-2-ylidene complexes tested previously under identical conditions.31c

Figure 4. Hydrogen bonding in 11 and 12 as a contribution to the amine−palladium interaction. Thermal ellipsoids are shown at the 50% probability level, hydrogen atoms are omitted, and ligands are simplified for clarity.

disorder of the amine ligand, no such bond could be observed for complex 10. Additionally, the IR spectra of all complexes 8−12 exhibited broad absorption bands in the range from 3433 to 3490 cm−1, indicating the presence of hydrogen bonds. These interactions provide additional stabilization to the complexes and hamper the formation of the κ2C,N-coordinated complexes. Direct Arylation of 1-Methylpyrrole. Cross-coupling reactions are a fast and efficient way of synthesizing biaryls, but they often require the use of organometallic reagents, which can be difficult to handle, cumbersome to prepare, and generate potentially toxic side products in the coupling reaction. An alternative are direct arylations, based on the C−H activation of polyfluoroarenes or electron-poor heteroaromatics and the subsequent coupling with halidoarenes.10g,29,30 One example of this type of reaction is the direct arylation of 1-methylpyrrole, which is known to be catalyzed by several palladium(II) NHC complexes.31 Therefore, complexes 7−12 were tested in a preliminary study as precatalysts for the reaction between 1methylpyrrole (18) and 4-bromoacetophenone (19) (Scheme 5 and Table 3).

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CONCLUSION Postcoordinative modification of a readily available palladium(II) complex with a bromopropyl-functionalized indazolin-3ylidene ligand has allowed the rapid synthesis of a series of five amine-functionalized complexes, which would have required a considerably larger effort using conventional prefunctionalization strategies. All complexes, which are the first examples of indy complexes to incorporate additional donor functionalities, have been fully characterized. The molecular structures of trans[PdBr2(amine)(indy)] (amine = diisobutylamine (11), morpholine (12)) revealed an unusual intramolecular hydrogen bond contributing to the interaction between the PdBr2 fragment and the amine ligand, strengthening the Pd−N bond. All complexes were found to be useful precatalysts for the direct arylation of 1-methylpyrrole, and a correlation between the pendant tertiary amine group’s steric bulk and catalyst activity was found. Future research will focus on expanding the substrate scope of the reaction to other aromatic heterocycles D


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volatiles were distilled off. The residue was taken up in dichloromethane (80 mL), and the solution was washed with water (3 × 50 mL). The organic layer was dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was washed with a mixture of hexane and diethyl ether (1/1, 50 mL). An orange solid was obtained (438 mg, 0.75 mmol, 75%). 1H NMR (300 MHz, CDCl3): δ 8.33 (d, 1 H, 3JH−H = 8 Hz, Ar-H), 7.61−7.53 (m, 1 H, ArH), 7.30−7.24 (m, 1 H, Ar-H), 7.18 (d, 3JH−H = 8 Hz, 1 H, Ar-H), 5.04 (t, 3JH−H = 8 Hz, 2 H, NCH2), 3.96 (s, 3 H, NCH3), 3.23−3.10 (m, 2 H, NCH2), 2.98−2.89 (m, 2 H, CH2), 2.83 (q, 3JH−H = 7 Hz, 4 H, NCH2), 2.74−2.53 (m, 4 H, NCH2, CH2), 1.64 (t, 3JH−H = 7 Hz, 6 H, CH3), 1.17 (t, 3JH−H = 7 Hz, 6 H, CH3). 13C{1H} NMR (75 MHz, CDCl3): δ 161.0 (Ccarbene), 141.5, 132.7, 130.7, 129.5, 123.2, 109.8 (Ar-C), 50.5 (NCH2), 50.3 (NCH2), 47.5 (NCH2), 47.2 (NCH2), 34.7 (NCH3), 26.8 (CH2), 16.7 (CH3), 10.7 (CH3). Anal. Calcd for C19H34Br2N4Pd·CH2Cl2: C, 35.87; H, 5.42; N, 8.37. Found: C, 35.76; H, 5.28; N, 8.14. MS (ESI): m/z 584 [M + H]+. trans-Dibromido(dipropylamine)(2-(3-dipropylaminopropyl)-1-methylindazolin-3-ylidene)palladium(II) (9). Complex 7 (93.0 mg, 0.17 mmol, 1.00 equiv) was dissolved in acetonitrile (20 mL), and dipropylamine (370 μL, 2.70 mmol, 15.9 equiv) was added. The resulting solution was heated to 60 °C for 40 h. Then all volatiles were distilled off, and the residue was taken up in dichloromethane (5 mL). The solution was passed over a short plug of silica (dichloromethane/diethyl ether 1/1 to elute impurities, then ethanol + 1% dipropylamine to elute the product). The solvents were removed under reduced pressure, and the residue was redissolved in dichloromethane (20 mL). The solution was washed with water (3 × 20 mL), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was washed with hexane (20 mL). An orange solid was obtained (86.0 mg, 0.13 mmol, 76%). 1H NMR (300 MHz, CDCl3): δ 8.38−8.30 (m, 1 H, Ar-H), 7.58 (t, 3JH−H = 7 Hz, 1 H, Ar-H), 7.32−7.26 (m, 1 H, Ar-H), 7.18 (d, 3JH−H = 9 Hz, 1 H, ArH), 5.02 (t, 3JH−H = 7 Hz, 2 H, NCH2), 3.95 (s, 3 H, NCH3), 3.22− 3.09 (m, 2 H, NCH2), 2.87 (t, 3JH−H = 7 Hz, 2 H, CH2), 2.69−2.32 (m, 10 H, NCH2, CH2), 2.08−1.94 (m, 2 H, CH2), 1.67−1.53 (m, 4 H, CH2), 1.07 (t, 3JH−H = 7 Hz, 6 H, CH3), 0.92 (t, 3JH−H = 7 Hz, 6 H, CH3). 13C{1H} NMR (75 MHz, CDCl3): δ 160.9 (Ccarbene), 141.4, 132.7, 130.7, 129.6, 123.3, 109.7 (Ar-C), 55.8 (NCH2), 55.0 (NCH2), 51.9 (NCH2), 50.6 (NCH2), 34.6 (NCH3), 27.3 (CH2), 23.7 (CH2), 19.4 (CH2), 12.5 (CH3), 12.3 (CH3). Anal. Calcd for C23H42Br2N4Pd: C, 43.11; H, 6.61; N, 8.74. Found: C, 43.47; H, 6.65; N, 8.81. MS (ESI): m/z 641 [M + H]+. trans-Dibromido(dibutylamine)(2-(3-dibutylaminopropyl)1-methylindazolin-3-ylidene)palladium(II) (10). Complex 7 (68.0 mg, 0.12 mmol, 1.00 equiv) was dissolved in acetonitrile (20 mL), and dibutylamine (329 μL, 1.95 mmol, 16.3 equiv) was added. The resulting solution was heated to 60 °C for 40 h. Then all volatiles were distilled off and the residue was taken up in dichloromethane (5 mL). The solution was passed over a short plug of silica (hexane/ethyl acetate 1/1 to elute impurities, then ethanol + 1% dibutylamine to elute the product). The solvents were removed under reduced pressure, and the residue was redissolved in dichloromethane (20 mL). The solution was washed with water (3 × 20 mL), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was washed with hexane (20 mL). An orange solid was obtained (73.0 mg, 0.10 mmol, 83%). 1H NMR (300 MHz, CDCl3): δ 8.39−8.30 (m, 1 H, Ar-H), 7.57 (t, 3JH−H = 8 Hz, 1 H, Ar-H), 7.32−7.26 (m, 1 H, Ar-H), 7.17 (d, 3JH−H = 8 Hz, 1 H, Ar-H), 5.00 (t, 3JH−H = 8 Hz, 2 H, NCH2), 3.94 (s, 3 H, NCH3), 3.25−3.10 (m, 2 H, NCH2), 2.82−2.72 (m, 2 H, CH2), 2.64−2.45 (m, 8 H, NCH2, CH2), 2.41−2.29 (m, 2 H, CH2), 2.05−1.89 (m, 2 H, CH2), 1.60−1.41 (m, 8 H, CH2), 1.38−1.24 (m, 4 H, CH2), 1.03 (t, 3JH−H = 7 Hz, 6 H, CH3), 0.91 (t, 3JH−H = 7 Hz, 6 H, CH3). 13C{1H} NMR (75 MHz, CDCl3): δ 161.2 (Ccarbene), 141.5, 132.8, 130.2, 129.6, 123.3, 109.7 (Ar-C), 53.6 (NCH2), 53.0 (NCH2), 51.8 (NCH2), 50.4 (NCH2), 34.7 (NCH3), 32.6 (CH2), 27.5 (CH2), 27.0 (CH2), 21.1 (CH2), 21.1 (CH2), 14.8 (CH3), 14.5 (CH3). Anal. Calcd for C27H50Br2N4Pd: C, 46.53; H, 7.23; N, 8.04. Found: C, 46.20; H, 7.20; N, 8.01. MS (ESI): m/z 697 [M + H]+.

and exploring the possibility of introducing functional groups other than tertiary amines on the side chains of the indy ligand.

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EXPERIMENTAL SECTION

General Considerations. All reactions were carried out without precautions to exclude air and moisture, unless stated otherwise. All solvents and chemicals were used as received or dried using standard procedures when appropriate. NMR chemical shifts (δ) were internally referenced to the residual solvent signals relative to tetramethylsilane (1H, 13C). Elemental analyses were performed at the Department of Chemistry, National University of Singapore. 1-Methylindazole (3). Indazole (1; 3.54 g, 30.0 mmol, 1.00 equiv), potassium carbonate (2.07 g, 15.0 mmol, 0.50 equiv) and dimethyl carbonate (2; 3.8 mL, 45.0 mmol, 1.50 equiv) were heated to 130 °C in DMF (250 mL) for 48 h. Then the reaction mixture was cooled to room temperature, followed by slow addition of deionized water (600 mL). The resulting mixture was extracted with dichloromethane (2 × 200 mL). The combined organic layers were dried over Na2SO4 and filtered. The solvent was evaporated under reduced pressure, and the residue was purified by column chromatography (silica gel, hexane/ethyl acetate 6:1). A yellow solid was obtained (2.03 g, 15.4 mmol, 51%). 1H NMR (300 MHz, CDCl3): δ 7.99 (s, 1 H, ArH), 7.76−7.71 (m, 1 H, Ar-H), 7.42−7.38 (m, 2 H, Ar-H), 7.19−7.11 (m, 1 H, Ar-H), 4.08 (s, 3 H, NCH3). 13C{1H} NMR (75 MHz, CDCl3): δ 140.5, 133.2, 127.1, 124.5, 121.8, 121.2, 109.6 (Ar-C), 36.1 (NCH3). ESI (MS): m/z 133 [M + H]+. The analytical data were in accordance with reported values.19 2-(3-Bromopropyl)-1-methylindazolium Bromide (5). A mixture of 1,3-dibromopropane (2.00 mL, 19.6 mmol, 9.80 equiv) and 3 (264 mg, 2.00 mmol, 1.00 equiv) was heated to 90 °C for 48 h. The precipitated product was isolated by filtration, washed with ethyl acetate (2 × 10 mL), and dried in vacuo. The remaining filtrate was heated to 90 °C for an additional 48 h, and a second crop of the product was isolated as before. The product was obtained as a white solid (461 mg, 1.38 mmol, 69%). 1H NMR (300 MHz, CDCl3): δ 9.81 (s, 1 H, NCH), 7.98 (d, 3JH−H = 9 Hz, 1 H, Ar-H), 7.83−7.69 (m, 2 H, Ar-H), 7.47−7.39 (m, 1 H, Ar-H), 5.43 (t, 3JH−H = 7 Hz, 2 H, NCH2), 4.49 (s, 3 H, NCH3), 3.59 (t, 3JH−H = 6 Hz, 2 H, CH2Br), 2.65 (quint, 3 JH−H = 7 Hz, 2 H, CH2). 13C{1H} NMR (75 MHz, CDCl3): δ 141.6 (NCH), 134.7, 134.6, 126.3, 124.1, 120.2, 111.3 (Ar-C), 51.0 (NCH2), 35.5 (CH 2 Br), 33.0 (NCH 3 ), 29.6 (CH2). Anal. Calcd for C11H14Br2N2: C, 39.55; H, 4.22; N, 8.39. Found: C, 39.72; H, 4.07; N, 8.37. ESI (MS): m/z 255 [M − Br]+. trans-(Acetonitrile)dibromido(2-(3-bromopropyl)-1-methylindazolin-3-ylidene)palladium(II) (7). Salt 5 (1.00 g, 3.00 mmol, 1.00 equiv), silver oxide (556 mg, 2.40 mmol, 0.80 equiv), and palladium bromide (800 mg, 3.00 mmol, 1.00 equiv) were suspended in anhydrous acetonitrile (30 mL) under an inert atmosphere and heated to 60 °C for 40 h shielded from light. After it was cooled to ambient temperature, the reaction mixture was filtered over a short plug of Celite and concentrated under reduced pressure. The solution was passed over a short plug of silica. After the solvent was removed under reduced pressure, the crude product was suspended in diethyl ether and stirred for 15 h. The product was obtained as an orange solid (1.06 g, 1.89 mmol, 63%). 1H NMR (300 MHz, CD3CN): δ 8.20 (d, 3 JH−H = 8 Hz, 1 H, Ar-H), 7.71−7.64 (m, 1 H, Ar-H), 7.42 (d, 3JH−H = 9 Hz, 1 H, Ar-H), 7.34−7.28 (m, 1 H, Ar-H), 5.14−5.07 (m, 2 H, NCH2), 3.99 (s, 3 H, NCH3), 3.72 (t, 2 H, 3JH−H = 6 Hz, CH2Br), 2.81−2.71 (m, 2 H, CH2), MeCN not observed. 13C{1H} NMR (75 MHz, CD3CN): δ 152.8 (Ccarbene), 141.1, 132.9, 129.6, 128.8, 123.2, 110.9 (Ar-C), 51.3 (NCH2), 34.7 (CH2Br), 32.8 (NCH3), 31.4 (CH2), MeCN not observed. Anal. Calcd for C13H16Br3N3Pd: C, 27.86; H, 2.88; N, 7.50. Found: C, 28.34; H, 3.04; N, 7.31. MS (ESI): m/z 480 [M − Br]+. trans-Dibromido(diethylamine)(2-(3-diethylaminopropyl)-1methylindazolin-3-ylidene)palladium(II) (8). Complex 7 (560 mg, 1.00 mmol, 1.00 equiv) was dissolved in acetonitrile (20 mL), and diethylamine (1.57 mL, 15.0 mmol, 15.0 equiv) was added. The resulting solution was heated to 60 °C for 40 h. After this time, all E


Organometallics

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trans-Dibromido(diisobutylamine)(2-(3-diisobutylaminopropyl)-1-methylindazolin-3-ylidene)palladium(II) (11). Complex 7 (112 mg, 0.20 mmol, 1.00 equiv) was dissolved in acetonitrile (20 mL), and diisobutylamine (524 μL, 3.00 mmol, 15.0 equiv) was added. The resulting solution was heated to 60 °C for 40 h. After this time, all volatiles were distilled off. The residue was taken up in dichloromethane (80 mL), and the solution was washed with water (3 × 50 mL). The organic layer was dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was washed with hexane (20 mL). A yellow solid was obtained (116 mg, 0.17 mmol, 85%). 1H NMR (300 MHz, CDCl3): δ 8.38−8.27 (m, 1 H, Ar-H), 7.56 (t, 3JH−H = 8 Hz, 1 H, Ar-H), 7.28 (t, 3JH−H = 8 Hz, 1 H, Ar-H), 7.16 (d, 3JH−H = 8 Hz, 1 H, Ar-H), 4.99 (t, 3JH−H = 8 Hz, 2 H, NCH2), 3.90 (s, 3 H, NCH3), 3.22−2.99 (m, 2 H, NCH2), 2.93−2.58 (m, 6 H, NCH2), 2.46−2.05 (m, 6 H, NCH2, CH2, CH), 1.96−1.67 (m, 2 H, CH), 1.13 (d, 3JH−H = 6 Hz, 6 H, CH3), 1.06 (d, 3JH−H = 7 Hz, 6 H, CH3), 0.95 (d, 3JH−H = 6 Hz, 12 H, CH3). 13C{1H} NMR (75 MHz, CDCl3): δ 160.6 (Ccarbene), 141.3, 132.6, 130.6, 129.6, 123.2, 109.6 (Ar-C), 63.9 (NCH2), 60.4 (NCH2), 60.1 (NCH2), 53.8 (NCH2), 51.0 (NCH2), 34.6 (NCH3), 27.9 (CH), 27.8 (CH), 26.4 (CH2), 21.9 (CH3), 21.5 (CH3), 21.2 (CH3). Anal. Calcd for C27H50Br2N4Pd·0.5 H2O: C, 45.94; H, 7.28; N, 7.94. Found: C, 45.98; H, 6.97; N, 8.07. MS (ESI): m/z 697 [M + H]+. trans-Dibromido(morpholine)(2-(3-morpholinepropyl)-1methylindazolin-3-ylidene)palladium(II) (12). Complex 7 (93.0 mg, 0.17 mmol, 1.00 equiv) was dissolved in acetonitrile (20 mL), and morpholine (156 μL, 1.80 mmol, 10.6 equiv) was added. The resulting solution was heated to 60 °C for 40 h. Then all volatiles were distilled off, and the residue was taken up in dichloromethane (5 mL). The solution was passed over a short plug of silica (dichloromethane/ diethyl ether 2/3 to elute impurities, then ethanol + 1% morpholine to elute the product). The solvents were removed under reduced pressure, and the residue was redissolved in dichloromethane (20 mL). The solution was washed with water (3 × 20 mL), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was washed with hexane (20 mL). A yellow solid was obtained (85.0 mg, 0.14 mmol, 82%). 1H NMR (300 MHz, CDCl3): δ 8.33 (d, 3JH−H = 8 Hz, 1 H, Ar-H), 7.59 (t, 3JH−H = 8 Hz, 1 H, Ar-H), 7.32−7.26 (m, 1 H, Ar-H), 7.17 (d, 3JH−H = 9 Hz, 1 H, Ar-H), 4.99 (t, 3JH−H = 7 Hz, 2 H, NCH2), 3.94 (s, 3 H, NCH3), 3.90−3.83 (m, 2 H, CH2O), 3.80−2.72 (m, 4 H, CH2O), 3.57−3.44 (m, 4 H, NCH2, CH2O), 3.06−2.97 (m, 2 H, NCH2), 2.67−2.56 (m, 6 H, NCH2), 2.55−2.45 (m, 2 H, CH2). 13 C{1H} NMR (75 MHz, CDCl3): δ 158.9 (Ccarbene), 141.4, 132.8, 130.6, 129.8, 123.3, 109.6 (Ar-C), 68.3 (CH2O), 66.5 (CH2O), 55.4 (NCH2), 53.6 (NCH2), 50.0 (NCH2), 48.2 (NCH2), 33.9 (NCH3), 26.3 (CH2). Anal. Calcd for C19H30Br2N4O2Pd·1/2H2O: C, 36.71; H, 5.03; N, 9.01. Found: C, 36.83; H, 4.93; N, 9.08. MS (ESI): m/z 613 [M + H]+. General Procedure for the Direct Arylation of 1-Methylpyrrole. A Schlenk tube was charged with precatalyst (10 μmol, 1.0 mol %), 4-bromoacetophenone (199 mg, 1.00 mmol, 1.00 equiv), and potassium acetate (196 mg, 2.00 mmol, 2.00 equiv) under an atmosphere of dry nitrogen. Degassed dimethylacetamide (3 mL) was added, followed by 1-methylpyrrole (266 μL, 3.00 mmol, 3.00 equiv). The tube was immersed in an oil bath preheated to 150 °C, and the mixture was allowed to react for 20 h. After this time, the Schlenk tube was taken out of the oil bath, the suspension was diluted with diethyl ether (10 mL), and decane as an internal standard was added. Samples were analyzed by GC-MS. X-ray Diffraction Studies. X-ray data were collected with a Bruker AXS SMART APEX diffractometer, using Mo Kα radiation at 100(2) K, with the SMART suite of programs.33 Data were processed and corrected for Lorentz and polarization effects with SAINT34 and for absorption effects with SADABS.35 Structural solution and refinement were carried out with the SHELXTL suite of programs.36 The structure was solved by direct methods to locate the heavy atoms, followed by difference maps for the light, non-hydrogen atoms. All hydrogen atoms were placed at calculated positions. All non-hydrogen atoms were generally given anisotropic displacement parameters in the

final model. A summary of crystallographic data is given in Table 2 and the Supporting Information.

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ASSOCIATED CONTENT

* Supporting Information S

Figures, CIF files, and a table giving 1H and 13C NMR spectra for all new molecules and crystallographic data for 10−12. 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 for H.V.H.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National University of Singapore and the Singapore Ministry of Education for financial support (WBS R 143-000-483-112 and SINGA scholarship) and the CMMAC staff of the department of chemistry for technical assistance.

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REFERENCES

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Amine-Functionalized Indazolin-3-ylidene Complexes of Palladium(II

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