Palladium(II) Complexes Bearing an Indazole-Derived N-Heterocyclic

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Palladium(II) Complexes Bearing an Indazole-Derived N‑Heterocyclic Carbene and Phosphine Coligands as Catalysts for the Sonogashira Coupling and the Hydroamination of Alkynes Jan C. Bernhammer, Ning Xi Chong, Ramasamy Jothibasu, Binbin Zhou, and Han Vinh Huynh* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 S Supporting Information *

ABSTRACT: Indazolin-3-ylidenes (indy) are among the most strongly donating N-heterocyclic carbenes, but the structural diversity of their complexes is still limited. Two dimeric palladium(II) complexes, [PdBr 2 (indy-5)] 2 (2a) and [PdBr2(indy-6)]2 (2b) (indy-5 = 2,3-dihydro-1H-pyrazolo[1,2-a]indazolin-3-ylidene, indy-6 = 6,7,8,9tetrahydropyridazino[1,2-a]indazolin-3-ylidene], bearing indazolin-3-ylidene ligands with different sizes of the fused aliphatic ring can be obtained by silver carbene transfer. The reaction of these dimers with pyridine yielded trans-[PdBr2(indy)(pyridine)] complexes (3a,b), while the poorly soluble monophosphine complexes cis-[PdBr2(indy)(PPh3)] (4a,b) were obtained by reaction with triphenylphosphine. Ligand substitution of the latter with silver trifluoroacetate afforded cis-[Pd(O2CCF3)2(indy)(PPh3)] complexes (5a,b) with improved solubilities, allowing for their detailed characterizations. In the presence of sodium tetrafluoroborate, cationic bis(phosphine) complexes trans[PdBr(PPh3)2][BF4] (6a,b) could be obtained. Similarly, cis-[PdBr(dppe)][BF4] (7a,b) and cis-[PdBr(dppp)][BF4] (8a,b) were obtained (dppe = bis(diphenylphosphino)ethane; dppp = bis(diphenylphosphino)propane) with the respective chelating diphosphines. A preliminary catalytic study revealed that the complexes incorporating monodentate phosphine ligands are good catalysts for the Sonogashira cross-coupling, while moderate to good yields were achieved with all complexes for the hydroamination of carbon−carbon triple bonds.

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have been described.17 Since the initial report, complexes of copper, gold, palladium, and rhodium have been synthesized.18 Among these, the structural diversity is largest for the gold complexes, with examples of gold(I) and gold(III) species incorporating various coligands such as other NHCs, phosphines, or halides. For palladium, fewer species have been reported. These include dimeric complexes as well as complexes with triphenylphosphine and allyl coligands. Copper and rhodium complexes have been even less thoroughly explored than their gold and palladium counterparts. Accounts of the catalytic activities of indy complexes are even scarcer, with preliminary studies described only for some gold and copper complexes.18f,g The systematic exploration of the coordination chemistry of indy complexes of palladium(II) and their application in catalysis is thus highly attractive.

INTRODUCTION A dazzling array of N-heterocyclic carbenes (NHCs) have been reported since Arduengo’s first isolation of a stable, crystalline, free NHC two decades ago.1,2 In their complexes, NHCs exhibit strongly σ-donating properties and highly stable metal bonding,3,4 while being easily tunable in both steric and electronic terms.5 These characteristics, and the ease with which a wide variety of suitable carbene precursors can be synthesized6 make them one of the most popular classes of ligands next to phosphines. Most research efforts have been based on imidazole and imdazoline backbones, but virtually any heterocyclic imine that can be N-alkylated can serve as a starting point for the development of new carbenes. Benzannelated and ringexpanded systems have been described,7,8 as well as NHCs based on oxygen- and sulfur-containing azoles.9,10 NHCs with reduced heteroatom stabilization, i.e., with only one heteroatom adjacent to the carbene center, are particularly interesting systems to study. The removal of one or both heteroatoms from the positions next to the ylidene carbon increases the energy of the σ-HOMO orbital and leads to a more strongly donating NHC ligand.11 Mesoionic imidazolin-4-ylidenes,12 Bertrand’s cyclic alkyl amino carbenes,13 pyrazolin-3-ylidenes,14 pyrazolin-4-ylidenes,15 and benzo[c]quinolin-6-ylidenes16 are just a few examples of this concept. Recently, complexes of a new class of NHCs with reduced heteroatom stabilization based on the indazole backbone (indy) © XXXX American Chemical Society

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RESULTS AND DISCUSSION Complex Synthesis. A small number of synthetic approaches to fused-ring indazolium salts have been described previously, but they require either starting materials that are not readily available or inert reaction conditions.19 However, a more straightforward approach involving the deprotonation of indazole by sodium hydroxide, followed by a nonregioselective Received: May 27, 2014

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dx.doi.org/10.1021/om500566n | Organometallics XXXX, XXX, XXX−XXX

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Scheme 1. Two-Step, One-Pot Synthesis of Indazolium Salts

formation of the complexes was confirmed by NMR spectroscopy, mass spectrometry, and elemental analysis. The characteristic downfield signals at 9.55 and 9.89 ppm, which are assigned to the acidic proton on C3 in the salts 1a,b, respectively, are not observed in the 1H NMR spectra of 2a,b. This supports the notion of a successful complex synthesis. An upfield shift of 0.2−0.6 ppm is observed for the aliphatic protons in both complexes, although a direct comparison to the salts is hampered by the change of solvent used for NMR spectroscopy. In the 13C NMR spectra of the complexes, Ccarbene resonates at 141.0 and 142.0 ppm, respectively. These chemical shifts are slightly upfield compared to those reported for Ccarbene in structurally related complexes.14f,17 In the acetonitrile solutions used for NMR spectroscopy, it is reasonable to assume that the complexes exist in fact as solvent adducts, and indeed, single crystals of such solvent adducts were obtained by slow evaporation of concentrated acetonitrile solutions of 2a,b (vide inf ra). However, peaks corresponding to [M − Br + CH3CN]+, [M + NH4]+, and [M + Na]+ were observed in their ESI-MS spectra, and elemental analyses were in line with a dimeric species as well. The easy formation of acetonitrile adducts underscores that bromido-bridged dimeric NHC complexes are readily cleaved by even weakly coordinating donor ligands. In order to exploit this, reactions with pyridine and triphenylphosphine as examples of simple nitrogen- and phosphorus-based donors were attempted (Scheme 3). The complexes were suspended in dichloromethane and treated with either 2 equiv or an excess of pyridine. A color change from orange to yellow indicated successful coordination, and either way, the desired pyridine adducts trans[PdBr2(indy)(pyridine)]2 (3a,b) were obtained in good to excellent yields as pale yellow solids. With an excess of pyridine, the reaction was complete rapidly, while it took considerably longer when only a stoichiometric amount of pyridine was used. The pyridine adducts show an improved solubility in most common organic solvents, with the exception of ethyl acetate, hydrocarbons, and ethereal solvents, in which they remain insoluble. Although 3a,b were readily soluble in chlorinated solvents, upon prolonged standing, the formation of an orange precipitate was observed, and NMR spectroscopy revealed the presence of free pyridine besides the desired complex. Two reasons contribute to this ligand dissociation. The strong trans-

alkylation by a suitable dibromoalkane, and subsequent ringclosure under more forcing reaction conditions to yield the desired salts is a feasible alternative (Scheme 1).18c Both 2,3-dihydro-1H-pyrazolo[1,2-a]indazolium bromide (indy-5·HBr, 1a) and the previously described compound 6,7,8,9-tetrahydropyridazino[1,2-a]indazolium bromide (indy6·HBr, 1b) were obtained in high yields as white to off-white, hygroscopic solids. The successful formation of the salts was corroborated by NMR spectroscopy and ESI-MS. While attempts at synthesizing homo(bis)-NHC complexes were unsuccessful, dimeric mono-NHC complexes [PdBr2(indy-5)]2 (2a) and [PdBr2(indy-6)]2 (2b) could be synthesized either by reacting the salts with palladium(II) acetate using a protocol optimized for similar complexes bearing benzimidazolin-2-ylidene ligands7d or by the silver carbene transfer method developed by Lin et al.20 Both approaches yielded the desired complexes in good to excellent yields, and the reactions were equally convenient to perform. However, the palladium(II) acetate pathway was more suitable for salt 1a, while 1b performed better using the silver carbene transfer protocol (Scheme 2). Scheme 2. Synthetic Routes to the Dimeric Indy Complexes [PdBr2(indy)]2 (2a,b)

The complexes were obtained as orange solids, which are insoluble in hydrocarbons, ethyl acetate, and ethereal solvents, poorly soluble in chlorinated solvents, and soluble in polar, coordinating solvents such as acetonitrile and DMSO. The

Scheme 3. Reaction between Dimeric Indy Complexes and an Additional Donor Ligand

B


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such as DMSO, acetonitrile, acetone, dichloromethane, and chloroform, while remaining insoluble in hydrocarbons, ethereal solvents, and ethyl acetate. The successful ligand exchange was confirmed by NMR spectroscopy, ESI-MS, and elemental analyses. Their 1H NMR spectra differ significantly from the acetonitrile and pyridine adducts. Besides the presence of the additional aromatic signals due to the triphenylphosphine group, the peak patterns and chemical shifts of the alicyclic protons changed drastically. Instead of the well-resolved triplets in the spectra of 2a,b and 3a,b, the NCH2 groups resonate as complicated multiplets. Three of these signals are significantly shifted upfield and one signal is shifted downfield in 5a, and in 5b two signals are shifted downfield, while the other two are shifted upfield. The resonance due to the CH2 group in 5a is split into two distinct multiplets, found at more upfield chemical shifts than the corresponding resonance in 2a and 3a. In 5b, three of the methylene protons are observed as multiplets at slightly more upfield chemical shifts, while one proton is considerably more downfield. The reason for all these changes is the magnetic anisotropy induced by the aromatic substituents of the triphenylphosphine in close proximity to the alicyclic protons. The ylidene atoms resonate as doublets at 149.2 and 160.8 ppm, respectively. The coupling constants are typical for 2JC−P coupling in cis-configured palladium(II) NHC complexes. It is noteworthy that Ccarbene in indy-6 resonates more downfield than the ylidene carbon in indy-5. This trend, which might be due to some subtle steric or electronic difference in the complexes, is observed for all subsequently synthesized mixed NHC phosphine complexes as well (vide inf ra). The triphenylphosphine groups are observed as singlets at 27.9 and 27.5 ppm, respectively, in the 31P NMR spectra, and the trifluoroacetato groups resonate as singlets at 1.3 and 0.5 ppm in 5a and 0.7 ppm and −0.1 ppm in 5b in their 19F NMR spectra. All these values corroborate the notion of a cisarrangement as a result of transphobia,15e,g,17,21,23 and chemical shifts for carbon, fluorine, and phosphorus are within the typical ranges found for structurally related compounds.17 During the syntheses of the neutral monophosphine complexes 4a,b, traces of a cationic bis(phosphine) complex were observed as a side product. To obtain sufficient material for study, the reaction was optimized for the cationic complexes by increasing the amount of triphenylphosphine used and adding NaBF4 to provide a noncoordinating counteranion (Scheme 5). NMR spectroscopy, ESI-MS, and elemental analyses confirmed the success of the reactions, and the desired bis(phosphine) complexes trans-[PdBr(indy)(PPh3)2]BF4 (6a,b) were obtained as pale yellow solids in moderate to good yields.

effect of the unusually electron-rich indy ligands leads to a weaker coordination than usually observed for palladium(II) complexes bearing a classical NHC and a pyridine ligand, and the precipitation of the poorly soluble dimeric complexes 2a,b from the solution shifts the equilibrium of the reaction further to the product side. This phenomenon explains why a considerable excess of pyridine was often needed to achieve complete conversion. The NMR spectra of complexes 3a,b hold no surprises. In the 1H NMR spectra, minor changes in chemical shift occur when compared to the respective starting materials, but these might as well be attributed to the change in deuterated solvent from MeCN-d3 to CDCl3. Additionally, signals for five additional aromatic protons are observed, which can be attributed to the presence of the pyridine ligand. In the 13C NMR spectra, the resonance of Ccarbene is observed at 153.3 ppm for both complexes. This value is more downfield than in the starting materials, reflecting the higher donor strength of the pyridine ligand when compared to acetonitrile.21 It is noteworthy that the chemical shifts of the ylidene carbon are within 1 ppm of each other in the starting materials and identical in the products, indicating a very similar electronic situation in the two different ligands in these complexes. The chemical shifts are also remarkably similar to the shifts reported for a structurally related pyrazolin-3-ylidene complex.14f Similarly to the synthesis of 3a,b, suspensions of 2a,b in dichloromethane were treated with 2 equiv of triphenylphosphine, leading to a similar color change to that observed for the reactions with pyridine. After 2 h, the mixed indy/phosphine complexes cis-[PdBr2(indy)(PPh3)] (4a,b) were obtained as pale yellow to off-white solids in good to excellent yields. The properties of these mixed NHC phosphine complexes differ significantly from those of the pyridine adducts 3a,b, and they were found to be virtually insoluble in any solvent tested. The poor solubility of these compounds precluded the characterization by NMR spectroscopy, and for 4b, it was even impossible to obtain an ESI-MS spectrum. To enhance the solubility and resolve the associated problems with characterizing 4a,b, the bromido were replaced by trifluoroacetato coligands (Scheme 4).17,22 Due to the more Scheme 4. Preparation of the Carboxylato Complexes cis[Pd(O2CCF3)2(indy)(PPh3)] (5a,b)

Scheme 5. Syntheses of Cationic Bis(phosphine) Complexes trans-[PdBr(indy)(PPh3)2]BF4 (6a,b)

weakly coordinating nature of carboxylato ligands, the resulting complexes cis-[Pd(O2CCF3)2(indy)(PPh3)] (5a,b) possess more readily available coordination sites, which might be beneficial in catalytic applications. To carry out the ligand exchange, suspensions of 4a,b in acetonitrile were treated with 2.2 equiv of silver(I) trifluoroacetate while shielding the solution from light. After 15 h at ambient temperature, the bis(trifluoroacetato) complexes 5a,b were obtained in good to excellent yields as pale yellow solids. Compared to the starting materials, the complexes were better soluble in common organic solvents C


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similar to those reported for other NHC complexes bearing dppe and dppp ligands.24 Molecular Structures. Single crystals suitable for X-ray diffraction analysis were obtained by slow evaporation of concentrated solutions (2a′ and 2b′ from acetonitrile, 3a from dichloromethane/acetonitrile, 3b from CHCl3/hexane, 4a from dichloromethane/toluene, 5a and 5b from dichloromethane/ diethyl ether, and 8a from acetonitrile/hexane). The slow diffusion of diethyl ether into a concentrated solution of 7b yielded single crystals of sufficient quality as well. Despite clear evidence for a dimeric structure of 2a,b, single crystals of the acetonitrile adducts 2a′ and 2b′ were obtained, a result of growing the crystals from acetonitrile solutions (Figure 1 and Table 1). The coordination geometry at the palladium center is square planar, with a trans-arrangement of the bromido ligands. The planes defined by the NHC ring and the coordination plane around the metal center are twisted by ∼70° with respect to each other. In the case of 3a and 3b, the pyridine and NHC ring planes are essentially parallel to each other. The five-membered aliphatic rings in 2a and 3a adopt a distorted envelope conformation, and the six-membered rings in 2b and 3b adopt a distorted armchair configuration. Pd−C bond lengths for these complexes range from 1.945 to 1.961 Å, and Pd−N bonds are found to be between 2.083 and 2.113 Å long. The bonds in 3a and 3b are slightly elongated compared to the acetonitrile complexes, reflecting the higher trans-influence of the stronger pyridine ligand. The Pd−Br bonds vary from 2.4212 to 2.4394 Å. All bond lengths are in good agreement with values reported for corresponding pyrazolin-3-ylidene complexes.14f Palladium(II) complexes bearing both NHC and phosphine ligands usually adopt a square planar cis-configuration due to transphobia, and indeed, molecular structures confirm this structural assignment for the monophosphine complexes 4a and 5a,b (Figure 2). No single crystals of 4b could be obtained, but it is highly likely that the complex adopts a similar geometry. Again, the NHC ring plane is almost perpendicular to the coordination plane, with a twist angle of ∼75° in the three complexes. The aliphatic ring in 4a adopts the same distorted envelope conformation as in 2a and 3a, but in 5a, the ring is found to be almost planar. In 5b, the six-membered ring adopts a distorted armchair configuration. Bond lengths in complex 4a were 1.965(5) Å for the Pd−C bond, 2.268(1) Å for the Pd−P bond, and 2.4808(6) and 2.4928(6) Å for the Pd−Br bonds (Table 2). These values are in good agreement with values reported previously for a related compound.14f The exchange of the anionic ligands leads to a slight contraction of the Pd−C bond, which is found to be 1.949(4) and 1.954(3) Å long in 5a,b, respectively. Similarly, the Pd−P bond contracts to ∼2.23 Å. Pd−O bond lengths fall between 2.087 and 2.099 Å. These values do not deviate significantly from previously reported data.17 The crystallization of the cationic complexes was plagued by a tendency of these complexes to undergo slow decomposition by losing the indy ligands. However, it was possible to crystallize 7b and 8a, although the latter was obtained with a bromide counteranion instead of the expected tetrafluoroborate (Figure 3). The aliphatic rings of the indy ligands adopt the expected distorted envelope and armchair conformations, although there was disorder in both cases. The NHC ring planes are almost perpendicular to the coordination planes, with torsion angles of

In contrast to the insoluble monophosphine complexes, complexes 6a,b were readily soluble in polar organic solvents and sparingly soluble in chlorinated organic solvents. Their 1H NMR spectra show aromatic signals corresponding to the presence of two triphenylphosphine groups in addition to the four aromatic protons of the indy ligands. The methylene resonances of the aliphatic cycles are found upfield from the resonances in the starting materials 2a,b due to the shielding effect of the phenyl groups, but in contrast to the monophosphine complexes 5a,b, no signal splitting is observed, suggesting a more uniform magnetic environment. The 13C NMR spectra show the resonances for Ccarbene as a doublet at 156.4 ppm and as a triplet at 165.3 ppm, respectively. The former signal is possibly a doublet of doublets with near identical 2JC−P values, caused by restricted rotation in the complex and therefore a slight magnetic inequality of the phosphorus atoms. The triphenylphosphine groups resonate as singlets at 23.2 ppm in 6a and 22.8 ppm in 6b, which is indicative of a trans-arrangement of these groups. The observed chemical shifts are close to those described for a cationic bis(phosphine) complex bearing a pyrazolin-3-ylidene ligand.14f In order to expand the scope of cationic complexes, bis(diphenylphosphino)ethane (dppe) and bis(diphenylphosphino)propane (dppp) were examined as potential ligands as well (Scheme 6). Complexes cis-[PdBrScheme 6. Synthesis of Cationic Complexes Bearing Chelating Diphosphine Ligands

(indy)(dppe)]BF4 (7a,b) and cis-[PdBr(indy-6)(dppp)]BF4 (8b) were obtained in excellent yields of >90%, and a 72% yield was obtained for complex cis-[PdBr(indy-5)(dppp)]BF4 (8a). The pale yellow to orange solids showed similar solubilities to those of 6a,b, and complex formation was supported by a full characterization in all cases. The NMR spectra of all complexes are relatively similar, while differing greatly from those of the bis(triphenylphosphine) species. In the 1H NMR spectra of all complexes, the 24 aromatic protons resonate as multiplets between 8.05 and 6.19 ppm, the signals of the NCH2 groups are observed between 4.54 and 3.23 ppm, and the remaining methylene groups resonate as multiplets between 3.04 and 1.23 ppm. The 13C NMR spectra are very complex due to hindered rotation of the phenyl group and C−P coupling. The resonances for Ccarbene are found at 163.5 ppm for 7a, 173.6 ppm for 7b, 161.8 ppm for 8a, and 172.2 ppm for 8b. With the exception of 7a, for which the cis-P−C coupling was not resolved, all signals were doublets of doublets, confirming the expected cis-geometry. 31P resonances for the dppe ligand were observed as two doublets at 61.1 and 50.7 ppm in 7a and 60.4 and 50.3 ppm in 7b. The dppp ligand resonates at 13.3 and −4.0 ppm in 8a and 12.7 and −4.0 ppm in 8b. These shifts are D


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Figure 1. Molecular structures of 2a,b and 3a,b. Thermal ellipsoids are shown at 50% probability; hydrogen atoms and solvent molecules in 3b have been omitted for clarity.

Table 1. Bond Distances (Å) and Angles (deg) in Complexes 2a, 2b, 3a, and 3b

Table 2. Bond Distances (Å) and Angles (deg) in Complexes 4a, 5a, and 5b

bond parameter

2a

2b·MeCN

3a·MeCN

3b·CHCl3

bond parameter

4a

5a

5b

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

1.945(3) 2.095(3) 2.4341(5) 2.4394(5) 88.53(8) 87.51(8) 90.43(7) 93.52(7) 177.9(1) 176.02(1)

1.943(3) 2.083(3) 2.4350(5) 2.4350(5) 88.31(9) 88.04(9) 92.46(9) 91.28(9) 176.9(1) 175.88(2)

1.956(3) 2.113(2) 2.4235(4) 2.4297(4) 87.47(8) 89.36(8) 91.37(7) 91.84(7) 177.5(1) 176.62(2)

1.961(2) 2.100(2) 2.4212(6) 2.4384(6) 88.90(7) 90.28(7) 89.91(6) 90.87(6) 178.60(9) 176.84(2)

Pd1−C1 Pd1−P1 Pd1−Br1/O1 Pd1−Br2/O3 C1−Pd1−Br1/O1 C1−Pd1−P1 Br2/O3−Pd1−P1 Br2/O3−Pd1−Br1/O1 C1−Pd1−Br2/O3 P1−Pd1−Br1/O1

1.965(5) 2.268(1) 2.4808(6) 2.4928(6) 86.3(1) 89.3(1) 93.25(3) 92.33(2) 170.4(1) 171.18(4)

1.949(4) 2.230(1) 2.088(3) 2.087(3) 89.6(1) 91.4(1) 87.95(9) 91.0(1) 172.1(1) 178.90(9)

1.954(3) 2.231(2) 2.088(3) 2.099(3) 89.3(1) 92.1(1) 86.92(7) 91.4(1) 172.5(1) 177.21(7)

Figure 2. Molecular structures of 4a and 5b. Thermal ellipsoids are shown at 50% probability; hydrogen atoms and disordered atoms in 5b have been omitted for clarity. E


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Figure 3. Molecular structures of 7b and 8a. Thermal ellipsoids are shown at 50% probability; hydrogen atoms, counteranions, solvent molecules, and disordered atoms have been omitted for clarity.

93.5(2)° in 7b and 80.4(4)° in 8a. The diphosphine ligands bind in a cis-chelating manner, with a bite angle of 84.78(2)° for dppe and a wider angle of 92.72(4)° for dppp (Table 3).

Scheme 7. Hydroamination of Phenylacetylene with 2,6Dimethylaniline

Table 3. Bond Distances (Å) and Angles (deg) in Complexes 7b and 8a bond parameter

7b

8a

Pd1−C1 Pd1−P1 Pd1−P2 Pd1−Br1 C1−Pd1−P1 C1−Pd1−Br1 P2−Pd1−P1 P2−Pd1−Br1 C1−Pd1−P2 P1−Pd1−Br1

2.037(2) 2.2433(8) 2.3058(7) 2.4832(6) 89.11(7) 91.72(7) 84.78(2) 94.37(2) 173.02(7) 179.06(2)

2.018(4) 2.254(1) 2.329(1) 2.4803(8) 87.6(1) 87.0(1) 92.72(4) 92.64(4) 179.1(1) 174.64(4)

Table 4. Catalyst Performance in the Hydroamination of Phenylacetylenea entry

precatalyst

yield (%)b

entry

precatalyst

yield (%)b

1 2 3 4 5 6 7 8

2a 3a 4a 5a 6a 7a 8a

0 37 44 43 56 13 >99 75

9 10 11 12 13 14 15 16

2b 3b 4b 5b 6b 7b 8b

0 30 43 17 50 6 92 72

a

Reaction conditions: precatalyst (1 mol %), phenylacetylene (2.0 equiv), 2,6-dimethylaniline (1.0 mmol), toluene (3 mL), 100 °C, 15 h. b Yields determined by GC-MS with decane as internal standard; average of two runs.

Notably, the Pd−C bonds at 2.037(2) Å in 7b and 2.018(4) Å in 8a are longer than those in the neutral complexes as a result of internal electron density redistribution upon coordination of the more strongly donating second phosphine moiety instead of a bromido coligand.25 Pd−P bonds range from 2.2433 to 2.329 Å, with the bond trans to the indy ligand being consistently longer due to trans-influence. Bond lengths and angles are within the range expected for this kind of compound.24,26 Hydroamination of Phenylacetylene. The synthesis of carbon−nitrogen bonds can be achieved with complete atom economy by addition of primary or secondary amines to an alkene or alkyne. The usefulness of this reaction makes it an interesting target for research.27 Since this important reaction does not proceed uncatalyzed despite being thermodynamically favored, a suitable catalyst is required. We and others have shown the possibility of catalyzing hydroaminations efficiently by using palladium(II) NHC complexes as precatalysts,22b,28,29 and hence we decided to examine the catalytic performance of indy complexes in this reaction (Scheme 7 and Table 4). All complexes were found to be active catalysts for this reaction, although the bis(triphenylphosphine) complexes 6a,b gave only very low yields (entries 6 and 14). A small amount of triflic acid was present in order to accelerate the protolytic cleavage of the Pd−C bond in the final step of the catalytic cycle.27b,30 In the absence of precatalyst, no product was detected.

Comparison between the indy-5 and indy-6 series of complexes revealed a slightly higher performance of complexes bearing the indy-5 ligand. Whether this is an electronic or a steric effect cannot be said with certainty, since the differences between the ligands are small with respect to both donor strength and sterical bulk. The neutral complexes gave moderate yields in all cases, with little variation between themselves (entries 2−5 and 10−13), with the exception of 5a,b, which showed a better performance. The introduction of less tightly bound trifluoroacetato ligands into the complex renders coordination sites more readily accessible for incoming substrates and enhances catalytic activity.21b Plausible reasons for the poor performance of 4a and 4b are the poor solubility of these compounds, especially in nonpolar solvents such as toluene, the propensity to decompose, which was considerably higher than for the other complexes, and a sterically very saturated coordination sphere, which might hamper substrate coordination. By contrast, excellent yields were obtained using the cationic complexes bearing more robust, bidentate phosphine ligands. Given the propensity of 7a,b and 8a,b to decompose by loss of the indy ligand, and the F


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counterparts, due to the higher kinetic stability of the chelating phosphine ligands and the associated inability to efficiently form a catalytically active species.

known catalytic activity of palladium(II) complexes bearing chelating diphosphines and labile coligands,31 the possibility exists that the active species does not incorporate an indy ligand at all. The differences between dppe and dppp complexes (entries 7/15 and 8/16) can be accounted for by bite angle effects, which are well known to influence catalytic performance.26,32 Sonogashira Coupling. The Sonogashira cross-coupling is a powerful and widely used tool to couple aryl halides and terminal alkynes.33 Palladium NHC complexes are known to catalyze this reaction efficiently.34 Hence we selected this reaction as a mechanistically different benchmark reaction to probe the catalytic activity of the indy complexes (Scheme 8).

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CONCLUSION Two series of palladium(II) complexes bearing 2,3-dihydro-1Hpyrazolo[1,2-a]indazolin-3-ylidene (indy-5, 2a−8a) or 6,7,8,9tetrahydropyridazino[1,2-a]indazolin-3-ylidene (indy-6, 2b− 8b) ligands have been synthesized. The coligands in these series include mono- and bidentate phosphines as well as pyridine as a nitrogen donor, and they comprise both neutral and cationic complexes. With the exception of the insoluble monophosphine complexes 4a,b, all new compounds have been fully characterized spectroscopically. With the exception of the cationic bis(triphenylphosphine) complexes 6, an example for each complex from at least one series has been crystallized and studied by X-ray diffraction. The catalytic activities in two mechanistically distinct reactions have been studied, furnishing the first data on the catalytic activity of palladium(II) indy complexes. In the hydroamination of phenylacetylene, moderate yields were obtained with neutral complexes, low reactivity was observed for the cationic bis(triphenylphosphine) complexes 6a and 6b, and excellent yields were obtained with cationic complexes featuring chelating diphosphines. By contrast, the Sonogashira cross-coupling was catalyzed efficiently when precatalysts incorporating monodentate phosphine ligands were used. An effort at further expanding the coordination chemistry of indazolin-3-ylidene ligands is currently in progress in our laboratory.

Scheme 8. Sonogashira Coupling of Phenylacetylene 4Bromoacetophenone

In the absence of precatalyst, no product formation was observed. With phosphine-free complexes as precatalysts, only minor traces of cross-coupled product were obtained or no reaction occurred at all (Table 5, entries 2, 3, 10, and 11). On Table 5. Catalyst Performance for the Sonogashira Couplinga entry

catalyst

yield (%)b

entry

catalyst

yield (%)b

1 2 3 4 5 6 7 8

2a 3a 4a 5a 6a 7a 8a

0 2 3 84 82 >99 18 30

9 10 11 12 13 14 15 16

2b 3b 4b 5b 6b 7b 8b

0 0 2 93 96 >99 17 17

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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) or externally to CF3CO2H (19F) or 85% H3PO4 (31P). Elemental analyses were performed at the Department of Chemistry, National University of Singapore. Salt 1b was prepared following an established procedure.18c 2,3-Dihydro-1H-pyrazolo[1,2-a]indazolium Bromide (1a). Sodium hydroxide (1.20 g, 30.0 mmol, 1.50 equiv) was added to a solution of indazole (2.36 g, 20.0 mmol, 1.00 equiv) in acetonitrile (250 mL). After stirring for 30 min at ambient temperature, 1,3dibromopropane (2.84 mL, 28.0 mmol, 1.40 equiv) was added. The reaction mixture was stirred at ambient temperature for 6 h, and then the temperature was increased to 90 °C for 15 h. The mixture was allowed to cool to ambient temperature, and the solvent was distilled under reduced pressure. The residue was taken up in dichloromethane (150 mL), and the resulting suspension was filtered. The solvent was removed in vacuo to yield an off-white solid (4.18 g, 17.5 mmol, 87%). 1 H NMR (300 MHz, CDCl3): δ 9.55 (s, 1 H, NCH), 7.96 (d, 3JH−H = 9 Hz, 1 H, Ar−H), 7.78−7.71 (m, 1 H, Ar−H), 7.69−7.62 (m, 1 H, Ar−H), 7.47−7.38 (m, 1 H, Ar−H), 5.27 (t, 3JH−H = 8 Hz, 2 H, NCH2), 4.84 (t, 3JH−H = 7 Hz, 2 H, NCH2), 3.22 (quint, 3JH−H = 8 Hz, 2 H, CH2). 13C{1H} NMR (75 MHz, CDCl3): δ 136.4 (Ar−C), 133.5 (NCH), 128.5, 125.8, 125.0, 124.2,.111.6 (Ar−C), 51.1 (NCH2), 47.7 (NCH2), 28.3 (CH2). Because of the hygroscopic nature of 1a, an elemental analysis could not be obtained. ESI (MS): m/z 159 [M − Br]+. Di-μ-Bromidobis(indy-5)dibromidodipalladium(II) (2a). Salt 1a (475 mg, 2.00 mmol, 1.10 equiv), palladium(II) acetate (405 mg, 1.80 mmol, 1.00 equiv), and sodium bromide (740 mg, 7.20 mmol, 4.00 equiv) were dissolved in DMSO (30 mL), and the mixture was heated to 70 °C for 24 h. Then the solvent was distilled off under

a

Reaction conditions: CuI (5 mol %), precatalyst (1 mol %), 4bromoacetophenone (1.0 equiv), DMF (degassed, 2 mL), phenylacetylene (2.0 equiv), NEt3 (1.2 equiv), 80 °C, 3 h. bYields determined by NMR spectroscopy; average of two runs.

their own, the indy ligands are sterically unassuming, and in the absence of sufficient steric bulk, the reductive elimination of the desired product does not occur efficiently.4a,35 High to quantitative yields were obtained with complexes bearing triphenylphosphine as a coligand. The trends observed for the hydroamination do not translate to this (mechanistically vastly different) reaction: Instead of the indy-5 complexes, the indy-6 complexes were the better catalysts, and the introduction of trifluoroacetato groups showed no significant increase in yield (entries 12 and 13) or even led to a decrease (entries 4 and 5). The best performance was observed with the cationic bis(triphenylphosphine) complexes 6a,b, which performed worst in the hydroamination reaction. It seems likely that the same catalytically active species is formed from complexes 4−6. The higher yields obtained with 6a,b can be attributed to a better stabilization of the precatalyst, serving as a reservoir to replenish the catalytically active species over the course of the reaction. The dppe and dppp complexes performed considerably worse (entries 7, 8, 15, and 16) than their monophosphine G


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reduced pressure. The residue was taken up in dichloromethane (100 mL), and the resulting suspension was extracted with water (3 × 50 mL), dried over Na2SO4, filtered, and concentrated in vacuo. The solid residue was washed with diethyl ether (100 mL) and dried in vacuo. The product was obtained as an orange solid (764 mg, 0.90 mmol, >99%). 1H NMR (500 MHz, CD3CN): δ 8.17 (d, 3JH−H = 8 Hz, 2 H, Ar−H), 7.67−7.63 (m, 2 H, Ar−H), 7.43 (d, 3JH−H = 9 Hz, 2 H, Ar− H), 7.30 (t, 3JH−H = 8 Hz, 2 H, Ar−H), 4.77 (t, 3JH−H = 7 Hz, 4 H, NCH2), 4.45 (t, 3JH−H = 7 Hz, 4 H, NCH2), 2.96 (quint, 3JH−H = 7 Hz, 4 H, CH2). 13C{1H} NMR (75 MHz, CD3CN): δ 141.0 (CCarbene), 136.3, 133.8, 131.8, 128.6, 122.4, 110.6 (Ar−C), 50.2 (NCH2), 46.2 (NCH2), 27.6 (CH2). Anal. Calcd for C20H20N4Br4Pd2: C, 28.30; H, 2.37; N, 6.60. Found: C, 28.02; H, 2.22; N, 6.82. ESI (MS): m/z 427 [0.5 M + H]+, 812 [M − Br + CH3CN]+. Di-μ-Bromidobis(indy-6)dibromidodipalladium(II) (2b). Salt 1b (2.31 g, 9.10 mmol, 2.20 equiv) and silver(I) oxide (1.14 g, 4.90 mmol, 1.20 equiv) were stirred in dichloromethane (100 mL) at ambient temperature for 15 h shielded from light. The resulting solution was filtered into a freshly prepared solution of palladium(II) bromide (1.09 g, 4.10 mmol, 1.00 equiv) in acetonitrile (80 mL). The combined solutions were allowed to react for 1 h at ambient temperature. Then the mixture was filtered and concentrated under reduced pressure, and the residue was purified by column chromatography (silica gel, dichloromethane/acetonitrile, 1:2). The product was obtained as an orange solid (1.49 g, 1.70 mmol, 83%). 1H NMR (500 MHz, CD3CN): δ 8.20 (d, 3JH−H = 8 Hz, 2 H, Ar−H), 7.69−7.65 (m, 2 H, Ar−H), 7.43 (d, 3JH−H = 8 Hz, 2 H, Ar−H), 7.34 (t, 3JH−H = 8 Hz, 2 H, Ar−H), 4.90 (t, 3JH−H = 6 Hz, 4 H, NCH2), 4.12 (t, 3JH−H = 6 Hz, 4 H, NCH2), 2.23−2.17 (m, 4 H, CH2), 2.16−2.12 (m, 4 H, CH2). 13C{1H} NMR (125 MHz, CD3CN): δ 142.0 (CCarbene), 132.6, 129.8, 128.6, 123.7, 110.8 (Ar−C), 53.8 (NCH2), 48.5 (NCH 2 ), 22.3 (CH 2 ), 21.2 (CH 2 ). Anal. Calcd for C22H24N4Br4Pd2: C, 30.13; H, 2.76; N, 6.39. Found: C, 30.01; H, 3.36; N, 6.54. ESI (MS): m/z 899 [M + NH4]+, 903 [M + Na]+. trans-Dibromido(indy-5)(pyridine)palladium(II) (3a). Pyridine (0.25 mL) was added to dimer 2a (100 mg, 0.12 mmol) in dichloromethane (25 mL), and the resulting solution was stirred at ambient temperature for 1 h. The reaction mixture was filtered, and the filtrate was dried in vacuo. The solid residue was washed repeatedly with diethyl ether (2 × 10 mL) and dried in vacuo to afford a pale yellow solid (115 mg, 0.23 mmol, 95%). 1H NMR (300 MHz, CDCl3): δ 9.15−9.05 (m, 2 H, Ar−H), 8.44 (d, 1 H, 3JH−H = 8 Hz, Ar−H), 7.79−7.18 (m, 1 H, Ar−H), 7.60−7.53 (m, 1 H, Ar−H), 7.40−7.28 (m, 3 H, Ar−H), 7.24−7.18 (m, 1 H, Ar−H) 4.95 (t, 2 H, 3 JH−H = 8 Hz, NCH2), 4.36 (t, 2 H, 3JH−H = 7 Hz, NCH2), 3.00 (quin, 3 JH−H = 7 Hz, 2 H, CH2). 13C{1H} NMR (75 MHz, CDCl3): δ 153.3 (Ar−C), 145.7 (Ccarbene), 138.4, 136.4, 135.0, 132.0, 130.0, 125.1, 122.8, 109.5 (Ar−C), 49.6 (NCH2), 45.6 (NCH2), 27.7 (CH2). Anal. Calcd for C15H15Br2N3Pd: C, 35.78; H, 3.00; N, 8.35. Found: C, 35.50; H, 3.06; N, 7.99. ESI (MS): m/z 456 [M − Br + MeOH]+, 503 [M − Br + py]+. trans-Dibromido(indy-6)(pyridine)palladium(II) (3b). Pyridine (16 μL, 0.20 mmol, 1.00 equiv) was added to dimer 2b (88 mg, 0.10 mmol, 0.50 equiv) in dichloromethane (20 mL), and the resulting solution was stirred at ambient temperature for 15 h. The solvent was distilled off under reduced pressure. The solid residue was washed with hexane (3 × 10 mL) and diethyl ether (3 × 10 mL) and dried in vacuo to afford a pale yellow powder (76 mg, 0.15 mmol, 73%). 1H NMR (300 MHz, CDCl3): δ 9.14−9.09 (m, 2 H, Ar−H), 8.49 (d, 1 H, JH−H = 8 Hz, Ar−H), 7.79−7.72 (m, 1 H, Ar−H), 7.64−7.56 (m, 1 H, Ar− H), 7.39−7.31 (m, 3 H, Ar−H) 7.26−7.21 (m, 1 H, Ar−H), 5.11 (t, 2 H, 3JH−H = 6 Hz, NCH2), 4.02 (t, 2 H, 3JH−H = 5 Hz, NCH2), 2.45− 2.23 (m, 4 H, CH2). 13C{1H} NMR (75 MHz, CDCl3): δ 153.3 (Ar− C), 142.5 (Ccarbene), 138.4, 132.6, 130.9, 129.9, 125.1, 120.0, 109.8 (Ar−C), 53.5 (NCH2), 48.5 (NCH2), 22.8 (CH2), 21.9 (CH2). Anal. Calcd for C16H17Br2N3Pd: C, 37.13; H, 3.31; N, 8.12. Found: C, 37.43; H, 3.65; N, 7.90. Despite our best efforts, ESI (MS) data could not be obtained. cis-Dibromido(indy-5)(triphenylphosphine)palladium(II) (4a). Triphenylphosphine (37 mg, 0.14 mmol, 1.00 equiv) was added

to 2a (60 mg, 0.07 mmol, 0.50 equiv) in dichloromethane (15 mL), and the reaction mixture was stirred at ambient temperature for 2 h. The resulting suspension was filtered, and the filtrate was dried under reduced pressure. The solid residue was washed with hexane (10 mL) and diethyl ether (10 mL) and dried in vacuo to give a pale yellow solid (92 mg, 0.13 mmol, 96%). 1H and 13C{1H} NMR spectra could not be measured due to poor solubility. Anal. Calcd for C28H25Br2N2PPd: C, 48.97; H, 3.67; N, 4.08. Found: C, 48.62; H, 3.99; N, 4.00. ESI (MS): m/z 607 [M − Br]+. cis-Dibromido(indy-6)(triphenylphosphine)palladium(II) (4b). Triphenylphosphine (37 mg, 0.14 mmol, 1.00 equiv) was added to 2b (62 mg, 0.07 mmol, 0.50 equiv) in dichloromethane (15 mL), and the reaction mixture was stirred at ambient temperature for 2 h. The resulting suspension was filtered, and the filtrate was dried under reduced pressure. The solid residue was washed with hexane (10 mL) and diethyl ether (10 mL) and dried in vacuo to give an off-white solid (82 mg, 0.12 mmol, 84%). 1H and 13C{1H} NMR spectra and ESI (MS) data could not be measured due to poor solubility. Anal. Calcd for C29H27Br2N2PPd: C, 49.71; H, 3.88; N, 4.00. Found: C, 49.50; H, 4.14; N, 4.18. cis-Ditrifluoroacetato(indy-5)(triphenylphosphine)palladium(II) (5a). Complex 4a (44 mg, 0.06 mmol, 1.00 equiv) and silver(I) trifluoroacetate (31 mg, 0.14 mmol, 2.30 equiv) were suspended in acetonitrile (15 mL) and stirred at ambient temperature for 15 h shielded from light. The resulting suspension was filtered, and the filtrate was concentrated under reduced pressure. The yellow residue was redissolved in dichloromethane (5 mL), and the crude product was precipitated out by addition of diethyl ether (20 mL). The solid was isolated, washed with diethyl ether (3 × 20 mL), and dried in vacuo to afford a pale yellow solid (36 mg, 0.05 mmol, 80%). 1H NMR (300 MHz, CD2Cl2): δ 8.12 (d, 1 H, Ar−H), 7.60−7.41 (m, 10 H, Ar−H), 7.39−7.28 (m, 6 H, Ar−H), 7.24−7.13 (m, 2 H, Ar−H), 5.17−5.04 (m, 1 H, NCH2), 4.26−4.09 (m, 1 H, NCH2), 3.99−3.84 (m, 1 H, NCH2), 3.75−3.58 (m, 1 H, NCH2), 2.85−2.67 (m, 1 H, CH2), 2.38−2.19 (m, 1 H, CH2). 13C{1H} NMR (75 MHz, CD2Cl2): δ 149.2 (d, 2JP−C = 10 Hz, Ccarbene), 135.9 (Ar−C), 134.3 (d, 2JP−C = 11 Hz, Ar−C), 132.7 (d, 1JP−C = 2 Hz, Ar−C), 132.1 (Ar−C), 132.0 (d, 4JC−P = 3 Hz, Ar−C), 128.9 (d, 3JP−C = 12 Hz, Ar−C), 128.2, 127.4, 127.1, 123.0, 109.5 (Ar−C), 48.9 (NCH2), 45.5 (NCH2), 27.0 (CH2), trifluoroacetate not observed. 19F{1H} NMR (282 MHz, CD2Cl2): δ 1.3 (CF3), 0.5 (CF3). 31P{1H} NMR (122 MHz, CD2Cl2): δ 27.9 (PPh3). Anal. Calcd for C32H25F6N2O4PPd·1/3CH2Cl2: C, 49.71; H, 3.31; N, 3.59. Found: C, 49.79; H, 3.50; N, 3.36. ESI (MS): m/z 639 [M − O2CCF3]+. cis-Ditrifluoroacetato(indy-6)(triphenylphosphine)palladium(II) (5b). Complex 4b (42 mg, 0.06 mmol, 1.00 equiv) and silver(I) trifluoroacetate (31 mg, 0.14 mmol, 2.30 equiv) were suspended in acetonitrile (15 mL) and stirred at ambient temperature for 15 h shielded from light. The resulting suspension was filtered, and the filtrate was concentrated under reduced pressure. The yellow residue was redissolved in dichloromethane (5 mL), and the crude product was precipitated out by addition of diethyl ether (20 mL). The solid was isolated, washed with diethyl ether (3 × 20 mL), and dried in vacuo to afford a pale yellow powder (46 mg, 0.06 mmol, >99%). 1H NMR (300 MHz, CD2Cl2): δ 8.20 (d, 1 H, JH−H = 8.4 Ar−H), 7.67− 7.47 (m, 10 H, Ar−H), 7.42−7.31 (m, 6 H, Ar−H), 7.30−7.22 (m, 2 H, Ar−H), 5.39−5.33 (m, 2 H, NCH2), 4.07−3.92 (m, 2 H, NCH2), 3.75−3.65 (m, 1 H, CH2), 2.19−2.00 (m, 2 H, CH2), 1.97−1.82 (m, 1 H, CH2). 13C{1H} NMR (75 MHz, CD2Cl2): δ 160.8 (d, 2JC−P = 8 Hz, Ccarbene), 141.3 (Ar−C), 134.3 (d, JC−P = 11 Hz, Ar−C), 132.6 (Ar− C), 132.0 (d, JC−P = 3 Hz, Ar−C), 129.0 (d, JC−P = 11 Hz, Ar−C), 128.4, 128.2, 127.4, 127.2, 123.9, 109.6 (Ar−C), 52.6 (NCH2), 47.8 (NCH2), 21.4 (CH2), 20.8 (CH2), trifluoroacetate not observed. 19 1 F{ H} NMR (282 MHz, CD2Cl2): δ 0.7 (CF3), −0.1 (CF3). 31P{1H} NMR (122 MHz, CD2Cl2): δ 27.5 (PPh3). Anal. Calcd for C33H27F6N2O4PPd·2/3CH2Cl2: C, 49.10; H, 3.47; N, 3.40. Found: C, 48.96; H, 3.78; N, 3.52. MS (ESI): m/z 653 [M − O2CCF3]+. trans-Bromido(indy-5)bis(triphenylphosphine)palladium(II) Tetrafluoroborate (6a). Triphenylphosphine (139 mg, 0.53 mmol, 2.20 equiv) and sodium tetrafluoroborate (29 mg, 0.26 mmol, 1.10 H


Organometallics

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equiv) were added to 2a (100 mg, 0.12 mmol, 0.50 equiv) in dichloromethane (25 mL). The reaction mixture was stirred at ambient temperature for 15 h. The resulting suspension was filtered over a short plug of Celite, and the filtrate was dried under reduced pressure. The solid residue was washed with hexane (2 × 10 mL) and dried in vacuo to afford a pale yellow solid (168 mg, 0.17 mmol, 70%). 1 H NMR (500 MHz, CD3CN): δ 7.74 (d, JH−H = 8 Hz, 1 H, Ar−H), 7.56−7.50 (m, 12 H, Ar−H), 7.49−7.44 (m, 7 H, Ar−H), 7.38−7.32 (m, 12 H, Ar−H), 7.10−7.02 (m, 2 H, Ar−H), 3.80−3.72 (m, 4 H, NCH2), 2.29 (quint, 3JH−H = 7 Hz, 2 H, CH2). 13C{1H} NMR (125 MHz, CD3CN): δ 156.4 (d, 2JP−C = 10 Hz, Ccarbene), 136.6 (Ar−C), 135.2 (t, JC−P = 6 Hz, Ar−C), 132.8, 132.7, 132.4 (Ar−C), 130.4 (t, JC−P = 25 Hz, Ar−C), 129.6 (t, JC−P = 5 Hz, Ar−C), 126.2, 123.6, 111.1 (Ar−C), 49.2 (NCH2), 46.2 (NCH2), 27.0 (CH2). 19F{1H} NMR (282 MHz, CD3CN): δ −75.5 (BF4), −75.6 (BF4). 31P{1H} NMR (202 MHz, CD3CN): δ 23.2 (PPh3). Anal. Calcd for C46H40BBrF4N2P2Pd·0.5CH2Cl2: C, 55.94; H, 4.14; N, 2.81. Found: C, 56.10; H, 4.54; N, 3.14. ESI (MS): m/z 869 [M − BF4]+. trans-Bromido(indy-6)bis(triphenylphospine)palladium(II) Tetrafluoroborate (6b). Triphenylphosphine (113 mg, 0.43 mmol, 2.15 equiv) and sodium tetrafluoroborate (24 mg, 0.22 mmol, 1.10 equiv) were added to 2b (88 mg, 0.10 mmol, 0.50 equiv) in dichloromethane (15 mL). The reaction mixture was stirred at ambient temperature for 15 h. The resulting suspension was filtered, and the filtrate was concentrated under reduced pressure. The solid residue was washed with diethyl ether (3 × 20 mL) and dried in vacuo to afford a pale yellow solid (158 mg, 0.16 mmol, 81%). 1H NMR (300 MHz, CD3CN): δ 7.78 (d, 1 H, JH−H = 8 Hz, Ar−H), 7.57−7.43 (m, 20 H, Ar−H), 7.40−7.31 (m, 11 H, Ar−H), 7.17−7.06 (m, 2 H, Ar− H), 3.96 (t, 2 H, 3JH−H = 6 Hz, NCH2), 3.55 (t, 2 H, 3JH−H = 6 Hz, NCH2), 1.70−1.60 (m, 2 H, CH2), 1.51−1.41 (m, 2 H, CH2). 13 C{1H} NMR (75 MHz, CD3CN): δ 165.3 (t, 2JC−P = 9 Hz, Ccarbene), 140.9 (Ar−C), 135.1 (t, JC−P = 6 Hz, Ar−C), 133.1, 132.2 (Ar−C), 130.1 (t, JC−P = 25 Hz, Ar−C), 129.4 (t, JC−P = 5 Hz, Ar−C), 127.4, 125.7, 124.2, 110.7 (Ar−C), 52.6 (NCH2), 47.7 (NCH2), 21.0 (CH2), 20.1 (CH2). 19F{1H} NMR (282 MHz, CD3CN): δ −75.4 (BF4), −75.5 (BF4). 31P{1H} NMR (121 MHz, CD3CN): δ 22.8 (PPh3). MS (ESI) m/z 883 [M − BF4]+. Anal. Calcd for C47H42BBrF4N2P2Pd: C, 58.20; H, 4.36; N, 2.89. Found: C, 58.00; H, 4.06; N, 2.81. cis-Bromido(indy-5)(dppe)palladium(II) Tetrafluoroborate (7a). 1,2-Bis(diphenylphosphino)ethane (104 mg, 0.26 mmol, 1.1 equiv) and sodium tetrafluoroborate (29 mg, 0.26 mmol, 1.10 equiv) were added to 2a (100 mg, 0.12 mmol, 0.50 equiv) in dichloromethane (25 mL). The reaction mixture was stirred at ambient temperature for 15 h. The resulting suspension was filtered over a short plug of Celite, and the filtrate was dried under reduced pressure. The solid residue was washed with hexane (2 × 10 mL) and dried in vacuo to afford a yellow solid (199 mg, 0.24 mmol, >99%). 1H NMR (500 MHz, CD3CN): δ 8.02−7.89 (m, 4 H, Ar−H), 7.81 (d, JH−H = 8 Hz, 1 H, Ar−H), 7.74−7.45 (m, 11 H, Ar−H), 7.43−7.33 (m, 5 H, Ar−H), 7.15 (dd, JH−H = 8 H, JH−H = 12 Hz, 2 H, Ar−H), 7.08 (t, JH−H = 1 H, 8 Hz, Ar−H), 4.43−4.35 (m, 1 H, NCH2), 0.4.31−4.25 (m, 1 H, NCH2), 4.21−4.14 (m, 1 H, NCH2), 3.58−3.50 (m, 1 H, NCH2), 2.84−2.67 (m, 4 H, CH2), 2.43−2.26 (m, 2 H, CH2). 13 C{1H} NMR (125 MHz, CD3CN): δ 163.5 (d, 2JC−P = 145 Hz, Ccarbene), 136.9 (d, JC−P = 4 Hz, Ar−C), 135.0 (d, JC−P = 11 Hz, Ar− C), 134.9 (d, JC−P = 12 Hz, Ar−C), 134.6 (d, JC−P = 11 Hz, Ar−C), 133.8 (d, JC−P = 3 Hz, Ar−C), 133.4 (d, JC−P = 3 Hz, Ar−C), 133.0 (d, JC−P = 11 Hz, Ar−C), 132.1 (Ar−C), 130.4, 130.4 (d, JC−P = 3 Hz, Ar−C), 130.3 (d, JC−P = 4 Hz, Ar−C), 130.2 (Ar−C), 130.2 (d, JC−P = 3 Hz, Ar−C), 130.1, 129.7, 129.6, 129.1, 129.0, 128.5, 127.8, 122.9, 111.0 (Ar−C), 49.6 (NCH2), 46.0 (NCH2), 29.9 (dd, 2JC−P = 17 Hz, 1 JC−P = 34 Hz, PCH2), 27.6 (CH2), 25.3 (dd, 2JC−P = 12 Hz, 1JC−P = 31 Hz, PCH2). 19F{1H} NMR (282 MHz, CD3CN): δ −75.5 (BF4), −75.5 (BF4). 31P{1H} NMR (202 MHz, CD3CN): δ 61.1 (d, 2JP−P = 15 Hz), 50.7 (d, 2JP−P = 15 Hz). Anal. Calcd for C36H34BBrF4N2P2Pd: C, 52.11; H, 4.13; N, 3.38. Found: C, 52.05; H, 4.13; N, 3.00. ESI (MS): m/z 743 [M − BF4]+. cis-Bromido(indy-6)(dppe)palladium(II) Tetrafluoroborate (7b). 1,2-Bis(diphenylphosphino)ethane (88 mg, 0.22 mmol, 1.10

equiv) and sodium tetrafluoroborate (24 mg, 0.22 mmol, 1.10 equiv) were added to 2b (88 mg, 0.10 mmol, 0.50 equiv) in dichloromethane (20 mL). The reaction mixture was stirred at ambient temperature for 15 h. The resulting suspension was filtered, and the filtrate was concentrated under reduced pressure. The residue was dissolved in a small amount of dichloromethane and precipitated out by addition of diethyl ether. The precipitate was dried in vacuo to afford a pale yellow solid (156 mg, 0.18 mmol, 92%). 1H NMR (300 MHz, CD3CN): δ 7.99−7.91 (m, 5 H, Ar−H), 7.73−7.34 (m, 17 H, Ar−H), 7.17−7.04 (m, 2 H, Ar−H), 4.39−4.19 (m, 2 H, NCH2), 3.94−3.87 (m, 1 H, NCH2), 3.75−3.69 (m, 1 H, NCH2), 2.90−2.64 (m, 3 H, CH2), 2.19− 1.80 (m, 4 H, CH2), 1.31−1.23 (m, 1 H, CH2). 13C{1H} NMR (75 MHz, CD3CN): δ 173.6 (dd, 2JC−P = 7 Hz, 2JC−P = 145 Hz, Ccarbene), 142.0 (d, JC−P = 4 Hz, Ar−C), 135.0 (d, JC−P = 6 Hz, Ar−C), 134.9 (d, JC−P = 5 Hz, Ar−C), 134.5 (d, JC−P = 11 Hz, Ar−C), 133.9 (d, JC−P = 3 Hz, Ar−C), 133.4 (d, JC−P = 3 Hz, Ar−C), 132.9 (d, JC−P = 10 Hz, Ar−C), 132.8 (Ar−C), 130.6, 130.4 (d, JC−P = 6 Hz, Ar−C), 130.2 (d, JC−P = 6 Hz, Ar−C), 130.2, 130.2 (Ar−C), 130.1 (d, JC−P = 5 Hz, Ar− C), 129.5, 129.4, 129.1, 128.8, 128.4, 127.7, 123.8, 111.0 (Ar−C), 53.6 (NCH2), 48.1 (NCH2), 30.1 (dd, 2JC−P = 17 Hz, 1JC−P = 34 Hz, PCH2), 25.0 (dd, 2JC−P = 11 Hz, 1JC−P = 31 Hz, PCH2), 21.8 (CH2), 20.9 (CH2). 19F{1H} NMR (282 MHz, CD3CN): δ −75.4 (BF4), −75.5 (BF4). 31P{1H} NMR (121 MHz, CD3CN): δ 60.4 (d, 2JP−P = 15 Hz), 50.3 (d, 2JP−P = 15 Hz). MS (ESI): m/z 757 [M − BF4]+. Anal. Calcd for C37H36BBrF4N2P2Pd·2/3CH2Cl2: C, 51.42; H, 4.24; N, 3.21. Found: C, 51.28; H, 4.06; N, 3.24. cis-Bromido(indy-5)(dppp)palladium(II) Tetrafluoroborate (8a). 1,3-Bis(diphenylphosphino)propane (107 mg, 0.26 mmol, 1.10 equiv) and sodium tetrafluoroborate (29 mg, 0.26 mmol, 1.10 equiv) were added to 2a (100 mg, 0.12 mmol, 0.50 equiv) in dichloromethane (25 mL). The reaction mixture was stirred at ambient temperature for 15 h. The resulting suspension was filtered over a short plug of Celite, and the filtrate was dried under reduced pressure. The solid residue was washed with hexane (2 × 20 mL) and dried in vacuo to afford a pale orange solid (145 mg, 0.17 mmol, 72%). 1H NMR (500 MHz, CD3CN): δ 7.98−7.92 (m, 2 H, Ar−H), 7.88 (d, 3 JH−H = 9 Hz, 1 H, Ar−H), 7.75−7.69 (m, 2 H, Ar−H), 7.66−7.56 (m, 3 H, Ar−H), 7.55−7.45 (m, 6 H, Ar−H), 7.39−7.34 (m, 1 H, Ar−H), 7.32−7.27 (m, 1 H, Ar−H), 7.24 (d, 3JH−H = 9 Hz, 1 H, Ar−H), 7.16− 7.03 (m, 7 H, Ar−H), 4.48−4.41 (m, 1 H, NCH2), 4.24−4.18 (m, 1 H, NCH2), 4.06−4.00 (m, 1 H, NCH2), 3.99−3.93 (m, 1 H, NCH2), 2.95−2.87 (m, 1 H, CH2), 2.81−2.60 (m, 4 H, CH2), 2.54−2.44 (m, 2 H, CH2), 1.79−1.67 (m, 1 H, CH2). 13C{1H} NMR (125 MHz, CD3CN): δ 161.8 (dd, 2JC−P = 4 Hz, 2JC−P = 148 Hz, Ccarbene), 136.7 (d, JC−P = 4 Hz, Ar−C), 134.7 (d, JC−P = 10 Hz, Ar−C), 134.6 (d, JC−P = 12 Hz, Ar−C), 134.3 (d, JC−P = 11 Hz, Ar−C), 132.9 (d, JC−P = 3 Hz, Ar−C), 132.8 (d, JC−P = 10 Hz, Ar−C), 132.4 (d, JC−P = 2 Hz, Ar−C), 132.3 (d, JC−P = 3 Hz, Ar−C), 132.2, 132.1 (Ar−C), 131.9 (d, JC−P = 2 Hz, Ar−C), 131.8, 131.5, 131.1, 130.3, 130.1, 130.0, 129.9 129.7, 129.7 (Ar−C), 129.6 (d, JC−P = 1 Hz, Ar−C), 129.5 (d, JC−P = 3 Hz, Ar−C), 129.4, 127.7, 122.8, 110.7 (Ar−C), 49.6 (NCH2), 45.9 (NCH2), 27.5 (CH2), 25.8 (dd, 3JC−P = 7 Hz, 1JC−P = 30 Hz, PCH2), 24.9 (dd, 3JC−P = 7 Hz, 1JC−P = 28 Hz, PCH2), 19.2 (CH2). 19F{1H} NMR (282 MHz, CD3CN): δ −75.5 (BF4), −75.5 (BF4). 31P{1H} NMR (202 MHz, CD3CN): δ 13.3 (d, 2JP−P = 42 Hz), −4.0 (d, 2JP−P = 42 Hz). Anal. Calcd for C37H36BBrF4N2P2Pd: C, 52.67; H, 4.30; N, 3.32. Found: C, 52.58; H, 4.31; N, 3.00. ESI (MS): m/z 757 [M − BF4]+. cis-Bromido(indy-6)(dppp)palladium(II) Tetrafluoroborate (8b). 1,3-Bis(diphenylphosphino)propane (91 mg, 0.22 mmol, 1.10 equiv) and sodium tetrafluoroborate (24 mg, 0.22 mmol, 1.10 equiv) were added to 2b (88 mg, 0.10 mmol, 0.50 equiv) in dichloromethane (20 mL). The reaction mixture was stirred at ambient temperature for 15 h. The resulting suspension was filtered, and the filtrate was dried under reduced pressure. The residue was dissolved in a small amount of dichloromethane and precipitated out by addition of diethyl ether. The precipitate was dried in vacuo to afford an off-white powder (170 mg, 0.19 mmol, 98%). 1H NMR (300 MHz, CD3CN): δ 8.05−7.99 (m, 2 H, Ar−H), 7.93 (d, JH−H = 8 Hz, 1 H, Ar−H), 7.66−7.52 (m, 8 H, Ar−H), 7.47−7.44 (m, 2 H, Ar−H), 7.37−7.25 (m, 4 H, Ar−H), I


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7.19−7.12 (m, 3 H, Ar−H), 7.09−7.03 (m, 2 H, Ar−H), 6.98−6.19 (m, 2 H, Ar−H), 4.54−4.48 (m, 1 H, NCH2), 4.36−4.28 (m, 1 H, NCH2), 4.21−4.17 (m, 1 H, NCH2), 3.31−3.23 (m, 1 H, NCH2), 3.04−2.99 (m, 1 H, CH2), 2.70−2.63 (m, 2 H, CH2), 2.49−2.41 (m, 1 H, CH2) 2.26−2.12 (m, 2 H, CH2), 2.01−1.96 (m, 2 H, CH2), 1.61− 1.53 (m, 2 H, CH2). 13C{1H} NMR (75 MHz, CD3CN): δ 172.2 (dd, 2 JC−P = 4 Hz, 2JC−P = 147 Hz, Ccarbene), 142.0 (d, JC−P = 5 Hz, Ar−C), 135.2 (d, JC−P = 12 Hz, Ar−C), 134.8 (d, JC−P = 10 Hz, Ar−C), 134.1 (d, JC−P = 10 Hz, Ar−C), 133.2, (Ar−C), 133.1 (d, JC−P = 3 Hz, Ar− C), 132.8 (Ar−C), 132.6 (d, JC−P = 3 Hz, Ar−C), 132.5 (Ar−C), 132.4 (d, JC−P = 10 Hz, Ar−C), 132.1 (d, JC−P = 3 Hz, Ar−C), 131.7 (d, JC−P = 3 Hz, Ar−C), 131.6, 130.3, 130.2, 129.8, 129.8 129.7 (Ar− C), 129.5 (d, JC−P = 2 Hz, Ar−C), 129.4 (Ar−C), 129.3 (d, JC−P = 2 Hz, Ar−C), 129.1, 128.8 127.5, 123.7, 110.7 (Ar−C), 53.7 (NCH2), 48.2 (NCH2), 25.8 (dd, 3JC−P = 6 Hz, 1JC−P = 31 Hz, PCH2), 24.8 (dd, 3 JC−P = 7 Hz, 1JC−P = 28 Hz, PCH2), 22.0 (CH2), 20.9 (CH2), 19.2 (CH2). 19F{1H} NMR (282 MHz, CD3CN): δ −75.5 (BF4), −75.5 (BF4). 31P{1H} NMR (121 MHz, CD3CN): δ 12.7 (d, 2JP−P = 43 Hz), −4.0 (d, 2JP−P = 43 Hz). MS (ESI): m/z 771 [M − BF4]+. Anal. Calcd for C38H38BBrF4N2P2Pd: C, 53.21; H, 4.47; N, 3.27. Found: C, 53.10; H, 4.47; N, 2.91. General Procedure for Catalytic Hydroaminations. A Schlenk tube was charged with precatalyst (10 μmol, 1.0 mol %) under an atmosphere of dry nitrogen. Anhydrous toluene (3 mL) and triflic acid (2.0 μL, 20 μmol, 2.0 mol %) were added, and the resulting suspension was stirred for 5 min at ambient temperature. Then phenylacetylene (219 μL, 2.00 mmol, 2.00 equiv) and 2,6-dimethylaniline (123 μL, 1.00 mmol, 1.00 equiv) were added, and the Schlenk tube was immersed in an oil bath preheated to 100 °C. The mixture was allowed to react for 15 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. General Procedure for Sonogashira Cross-Couplings. Copper(I) iodide (10 mg, 50 μmol, 5.0 mol %), 4-bromoacetophenone (199 mg, 1.00 mmol, 1.00 equiv), and precatalyst (10 μmol, 1.0 mol %) were placed in a Schlenk flask under an atmosphere of dry nitrogen. Degassed DMF (2 mL) was added, followed by phenylacteylene (219 μL, 2.00 mmol, 2.00 equiv) and triethylamine (166 μL, 1.20 mmol, 1.20 equiv). The Schlenk tube was immersed in an oil bath preheated to 80 °C, and the mixture was allowed to react for 3 h. After this time, the mixture was allowed to cool to ambient temperature, and dichloromethane (25 mL) was added. The solution was extracted with water (3 × 25 mL), dried over Na2SO4, filtered, and concentrated under reduced pressure. The residue was analyzed by 1H NMR spectroscopy. 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.36 Data were processed and corrected for Lorentz and polarization effects with SAINT37 and for absorption effect with SADABS.38 Structural solution and refinement were carried out with the SHELXTL suite of programs.39 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 put 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 Tables 1−3 and the Supporting Information.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [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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ASSOCIATED CONTENT

* Supporting Information S

Figures of 1H and 13C NMR spectra for all new molecules, CIF files, plots of molecular structures, and crystallographic data for 2a, 2b, 3a, 3b, 4a, 5a, 5b, 7b, and 8a. This material is available free of charge via the Internet at http://pubs.acs.org. J


Organometallics

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