Nickel(II)- and Palladium(II)-NHC Complexes from Hydroxypyridine

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Nickel(II)- and Palladium(II)-NHC Complexes from Hydroxypyridine Functionalized C,O Chelate Type Ligands: Synthesis, Structure, and Catalytic Activity toward Kumada−Tamao−Corriu Reaction Irshad Ahmad Bhat,†,‡ Iruthayaraj Avinash,† and Ganapathi Anantharaman*,† †

Department of Chemistry, Indian Institute of Technology, Kanpur-208016, India S. A. M. Degree College, Budgam, Jammu and Kashmir, 191111, India

‡

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S Supporting Information *

ABSTRACT: Hydroxypyridine functionalized imidazolium salts (2a−c) have been prepared in a one pot neat reaction between alkyl/aryl imidazoles and 2-chloro-3-hydroxypyridine. The imidazolium salts were used as proligands for the synthesis of new Ni(II) (3a, 3c, and 3c′) and Pd(II) NHC (4a−4c) complexes. Complexes, 3a, 3c, and 4a−4c are four coordinated with square planar geometry around the metal center and feature the C,O chelation of the heterobidentate NHC ligands, using carbene atoms and the O atom of the hydroxypyridine arm. Depending upon the steric bulk of the alkyl/aryl substituents on the ligand, either cis (3a and 4a) or trans (3c, 4b, and 4c) complexes are obtained. For the nickel complex with the 2c ligand, two different isomeric forms (3c and 3c′) were observed. In 3c, both the NHC ligands are bound via C2 and O atoms of the hydroxypyridine arm, whereas, in 3c′, one of the two ligands binds via C4 and N atoms of the hydroxypyridine side arm. 3c′ represents a unique square planar complex with the central metal atom bound simultaneously to both normal and abnormal carbenes. All the five complexes, except 3c′, were evaluated as catalysts for the Kumada−Tamao−Corriu cross-coupling reaction between phenylmagnesium bromide and different aryl chlorides at room temperature.

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INTRODUCTION The design of new sustainable and efficient catalytic systems employing cheaper and earth abundant metals is an important area of ongoing research. In particular, an increasing interest has been drawn toward the development of 3d-transition metal based catalytic transformations.1 Besides the cost effectiveness, a set of accessible oxidation states of nickel (0−IV), facile oxidative addition of organic substrates on the nickel center, and the low Ni−C bond strength are some of the key advantages for the studies on nickel based catalytic transformations.2 However, improving and controlling the reactivity on the nickel center is one of the challenging tasks. Catalysis using nickel based complexes is known for several decades now, and the first set of reports on nickel catalyzed crosscoupling reactions between aryl halides and Grignard reagents also known as the Kumada−Tamao−Corriu (KTC) coupling appeared in 1972.3 Some of the most important advancements with regard to the application of nickel complexes for KTC reaction in the recent past include the use of readily available phenolic salts or aryl sulfonates and triflates as electrophiles.4 Compared to the other cross-coupling reactions involving aryl boronic acids and similar type of organometallic reagents which are often themselves prepared from Grignard reagents,5 the direct use of Grignard reagents as the transmetalating agent © XXXX American Chemical Society

is more advantageous in terms of atom economy and its involvement in the enhancement of the rate of the transmetalation step which sometimes tends to be the rate determining for other reactions like Suzuki coupling involving milder reagents.6 The disadvantages such as limited substrate scope due to the greater reactivity of Grignard reagents have been recently circumvented either by carrying out the reaction at sufficiently lower temperatures7 or by the slow addition of Grignard reagent into the reaction mixture.8 Buchwald and others have shown that the use of special phosphine ligands could enhance the catalytic activity of palladium based catalysts toward KTC reaction and the overall KTC coupling reactions could be carried out even at lower temperature.7a In pursuit of this, much of the time was devoted toward finding the right combination of ligand which not only stabilizes the nickel but also activates the nickel center for the coupling reactions.7b,c In the last three decades, N-heterocyclic carbenes (NHCs) have emerged as an excellent alternative to the phosphines due to their enhanced sigma donation capacity to the metal center that increases the activity of the transition metal based catalysts used in cross-coupling reactions. In situ generated or wellReceived: December 5, 2018

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DOI: 10.1021/acs.organomet.8b00878 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

nickel and palladium complexes as well as catalytic applications of metal complexes in KTC reaction.

defined nickel complexes of NHCs have been successfully employed as catalysts for the KTC coupling reaction since the first report by Hermann who utilized in situ generated NiNHC complexes to carry out the challenging cross-coupling reaction between aryl chlorides/fluorides with Grignard reagents.9 In contrast to the monodentate NHC ligands, nickel complexes having bidentate or pincer type ligands perform better in KTC cross-coupling reactions.10,11 Compared to the neutral NHC ligands, the development of anionic tethered NHCs and utilization of their metal complexes for different catalysis have been widely explored.12,13 Use of such anion tethered NHC-metal complexes in coupling reaction has been reported by Bouwman’s group with the premise that the presence of an anionic group accelerates the reaction by binding to the Ni atom of the catalyst and the Mg atom of the Grignard reagent together.14a Such a design was also reported with a phosphine ligand by Nakamura’s group (Chart 1).15 Compared to the anionic nitrogen center, the anionic oxygen atoms are preferable due to better coordinating ability of oxygen atom toward both nickel and magnesium centers.

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RESULTS AND DISCUSSION Synthesis of Imidazolium Salts HIMepyOH·Cl (2a), HIMespyOH·Cl (2b), HIMespyOH·PF6 (2b·PF6), and HIDipppyOH·Cl (2c). 2a−c were prepared in good yield by heating the 1-alkyl/aryl imidazoles (1a−c) and 2-chloro-3hydroxypyridine (Scheme 1) under neat conditions. They were Scheme 1. Synthesis of 2a−c

characterized by spectroscopic, spectrometric, and elemental analysis. The imidazolium proton (NCHN) of 2a−c in 1H NMR resonates around 9.77 ppm, authenticating the formation of the expected products. Also the characterization of 2b by single crystal X-ray diffraction confirms the product formation and further reveals that the OH group in pyridine is pointed away from the imidazolium proton (Figure 1).

Chart 1. Design of Various Ligands with an Anionic Arm for KTC Reaction

Functionalization of an NHC with hydroxyl, aryloxy, amido, and enolate tether are some of the main ways for achieving anionic NHCs.16 The metal complexes of alcohol and phenolate functionalized NHCs in particular have been of interest in the past few years, and this area of work was recently reviewed by Braunstein and co-workers.12e Synthesis of aryloxy functionalized NHCs has been a challenge due to the reason that alkyl/aryl imidazoles or imidazolidine derivatives are too weak a nucleophile to react with simple aryl halides. Therefore, it often necessitates the use of metal catalyzed coupling reactions or tedious cyclization procedure.17 To overcome such complications, the hydroxypyridine moiety could be embedded onto the nitrogen of the imidazole using commercially available 2-chloro-3-hydroxypyridine via quarternization under neat reaction condition. Moreover, the presence of such hydroxypyridine adjacent to C2/C4 of imidazole is advantageous in terms of variable coordination of the ligand to the metal center by means of heterobidentate C,O and/or C,N chelation in which the carbene carbon atom could be of either normal or abnormal type (Chart 2). Herein, we describe a straightforward and convenient synthesis of a series of new hydroxypyridine functionalized imidazolium salts and their

Figure 1. ORTEP diagram of 2b with ellipsoids in 30% probability. Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): N(1)−C(1) 1.338(4), N(2)−C(1) 1.333(4), C(3)−C(2) 1.343(5), O(1)−C(5) 1.354(4), C(1)−N(1)−C(3) 108.7(3).

Interestingly, the solid state structure shows that the bulky mesityl group is orthogonal to the imidazolium ring, whereas the hydroxypyridine is twisted with respect to the imidazolium ring (dihedral angle between the planes of imidazolium and hydroxypyridine: 29.6(1)°). The twisting of these two rings is important for a short intramolecular contact between the OH group and C−H (C3) of the imidazolium unit (2.857(4) Å) and hydrogen bonding between the O-H···Cl (3.015 Å) and CH···Cl of the imidazolium C1-H atom, leading to a supramolecular network. From 2b, anion exchange with KPF6 in aqueous medium produced the 2b·PF6 in excellent yield. Preparation of Ni(II)- and Pd(II)-NHC Complexes. Complexes of nickel (3a, 3c, and 3c′) were prepared by treating the imidazolium salts 2a and 2c with either Ni(PPh3)2Cl2 (for 3a) or NiCl2·6H2O (for 3c and 3c′) under basic condition in acetonitrile (Scheme 2). The reaction with the methyl substituted salt (2a) yielded the complex 3a as a bright yellow-orange solid in near quantitative yield. However, the reaction with Dipp substituted salt (2c) led to

Chart 2. Possible Binding Modes of Carbenes Derived from 2a−c

B


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Organometallics Scheme 2. Synthesis of Metal-NHC Complexes

Information, Figure S1b). After deducting the signals corresponding to 3c, the remaining peaks were assigned to the product 3c′. The presence of a highly downfield signal at 9.85 ppm for the C2-H proton, four doublets for methyl (∼1.0−1.50 ppm), and two septets for CH (2.02 and 3.66 ppm) of isopropyl group protons validates the formation of an unsymmetrically substituted mesoionic nickel carbene (C4bound nickel-NHC) complex in 3c′. Such type of complexity, especially in the alkyl region in 1H NMR, was also observed in a closely related mesoionic carbene palladium complex in the literature.18a Besides, ignoring the signals due to 3c, the remaining peaks could be accounted for all the protons of 3c′ with proper integral values (Supporting Information, Figure S1c). Structures of 3a, 3c, 3c′, and 4a−4c. The molecular structures of 3a, 3c, 3c′, and 4a−c were determined by the single crystal X-ray diffraction (Figures 2−4). All the

the formation of two different nickel complexes (3c and 3c′). The pink-red colored complex (3c) was exclusively obtained by washing the crude compound with methanol, whereas the removal of methanolic filtrate and further workup with a mixture of diethyl ether and pentane (refer to Experimental Section) afforded the mixture of pink-red crystals (3c) and yellow crystals (3c′) which could not be separated out completely from 3c due to their microcrystalline nature. However, the crystals of 3c′ were analyzed through single crystal X-ray diffraction method. The reaction of the mesityl substituted imidazolium salt (2b) with the nickel precursor under basic medium resulted in an insoluble precipitate (3b). In contrast to the above, the reaction of palladium dichloride with 2a−c under basic medium afforded only two types of palladium complexes, the cis complex 4a and the trans complexes 4b and 4c depending upon the steric bulk of the organic group in the ligand (Scheme 2). All the compounds, except 3c′, were thoroughly characterized by spectroscopic, spectrometric, and elemental analysis, besides single crystal Xray diffraction studies. The 1H NMR spectra of 3a and 3c as well as 4a−c were void of the C2-H and OH proton peaks, and an appearance of a peak around 160 ppm in 13C NMR indicates the formation of the metal-carbene complexes. The formation of complexes with 1:2 metal to ligand ratios were confirmed by the presence of mass (M + H)+ in the respective ESI-MS and also with elemental analysis. In contrast, it was difficult to obtain 3c′ in pure form for its separate characterization either by NMR or by elemental analysis. However, a careful analysis of the 1H NMR spectrum of the crude product obtained from diethyl ether/pentane extractions of the residue from methanolic filtrate (see Experimental Section) allowed us to distinctly identify the peaks of 3c′ along with 3c (see Supporting Information, Figure S1a). The signals of 3c were marked with an asterisk (*) and can be easily identified by comparing it with the 1H NMR of the pure sample of 3c (Supporting

Figure 2. Molecular structures of 3a (left) and 3c (right) with 35% probability of the ellipsoids. Hydrogen atoms and the solvent molecules of crystallization have been omitted for clarity.

Figure 3. Molecular structure of 3c′ with 35% probability of the ellipsoids. Hydrogen atoms and the solvent molecules of crystallization have been omitted for clarity. Selected bond lengths (Å) and bond angles (deg) for 3c′: M1−C1 1.863(4), M1−O1 1.874(2), M1−C23 1.865(4), M1−N6 1.952(3), C1−M1−O1 90.90(13), C1− M1−C23 97.28(15), C23−M1−O1166.67(13), C23−M1−O1 166.67(13), C23−M1−N6 83.69(13), O(1)−Ni(1)−N(6) 89.56(11).

complexes possess square planar structures with either cis or trans geometry around the central metal atom, depending upon the steric hindrance of the alkyl or aryl substituents. Among them, 3a and 4a exhibit a distorted square planar geometry around the metal center surrounded by ligands forming the MC2O2 unit. Here, the chelating bidentate ligands bind to the metal centers in a cis fashion. The other nitrogen donors of the ligands remain free. C


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Figure 4. Molecular structures of 4a (left), 4b (middle), and 4c (right) with 35% probability of the ellipsoids. Hydrogen atoms have been omitted for clarity.

Table 1. Selected Bond Distances (Å) and Angles (deg) for 3a, 3c, and 4a−c

M1−C1 M1−O1 M1−C10 M1−O2 C1−M1−O1 C1−M1−C10 C10−M1−O2 O1−M1−O2 C1−M1−O2 C10−M1−O1

3a

4a

3c

4b

4c

M = Ni

M = Pd

M = Ni

M = Pd

M = Pd

1.821(7) 1.897(5) 1.859(6) 1.862(5) 91.1(3) 96.0(3) 91.6(2) 85.9(2) 163.0(3) 163.0(3)

1.969(3) 2.032(2) 1.950(3) 2.054(2) 90.26(11) 96.89(13) 87.33(12) 87.12(9) 166.69(11) 170.42(11)

1.929(2) 1.8492(17) 1.929(2) 1.8492(17) 91.29(9) 180.0 91.29(9) 180.0 88.71(9) 88.71(9)

2.006(3) 1.9925(19) 2.006(3) 1.9925(19) 88.99(10) 180.000(14) 88.99(10) 180.000 91.01(10) 91.01(10)

2.058(7) 1.976(5) 2.058(7) 1.976(5) 89.1(2) 180.0 89.1(2) 180.0 90.9(2) 90.9(2)

M1−C1 M1−O1 M1−C1i M1−O1i C1−M1−O1 C1−M1−C1i C1i−M1−O1i O1−M1−O1i C1−M1−O1i C1i−M1−O1

attributed to the higher trans influence of NHC in the latter complexes. For all the complexes except 3c′, the ligands bind to the metal centers through the C2 of the imidazolylidene and the O atom of the hydroxypyridine arm in a C,O chelating fashion (Chart 2, binding mode-I). Interestingly, in the case of 3c′, one of the two ligands was coordinated through C4 of the imidazolylidene (abnormal carbene) and the N atom of the hydroxypyridine arm by C,N chelating mode, whereas the other ligand was bound through C,O chelating mode as observed for other cases (Chart 2, binding mode-III). Out of the four different binding modes for this ligand system outlined in Chart 2, the binding mode-I is the more prevalent, but in 3c′, the same ligand exhibits two different binding modes, suggesting that the ligand can be regarded as the chelating ambidentate as it can act as a bidentate ligand in four different ways. Similar to the other complexes, the nickel atom in 3c′ adopts a square planar geometry. It is interesting to note that, despite different binding modes, the two metal−carbon bonding distances in 3c′ are almost similar. Besides, the molecule has intra- and intermolecular C-H···O interaction (Table S2, Supporting Information) in the crystal structure of 3c′. It should be pointed out that, though the examples of metal centers having both normal and abnormal binding modes are already known in the literature from the single NHC ligand,20 the formation of the bidentate unit containing two different chelating heteroatoms (C2Ô, C4^N) to a single metal center is not reported so far. Kumada−Tamao−Corriu Coupling Reaction. All the complexes obtained in this study, except 3c′, were evaluated as catalysts for the Kumada−Tamao−Corriu (KTC) crosscoupling reaction between phenylmagnesium bromide and 4chloroanisole. Preliminary control reactions in THF as the solvent at room temperature using nickel (3a and 3c) and

The average M−C (NHC) bond distances in the bidentate cis complexes (av. Ni−C1, 1.840(7) Å, 3a and av. Pd−C: 1.959(3)Å, 4a) fall within the range of reported values, wherein one of the values of Ni1−C1 (1.821(7) Å) and Pd1− C1 (1.969(3) Å) in 3a and 4a become the shortest (Ni−C: 1.838(2)−1.873(2) Å) and longest (1.946(2)−1.953(3) Å) distances, respectively, known for these type of complexes (Table 1).12e,18 The M−O bond distances are 1.8625(5) and 1.897(5) Å for nickel (3a) and 2.032(2) and 2.054(2) Å for palladium (4a) complexes, respectively. It is noteworthy that the lowest and highest values of Ni−O and Pd−O bond distances, respectively, define the new shorter Ni−O and longer Pd−O distances known for similar type of complexes in the literature (Ni−O: 1.872(8)−1.915(2) Å; Pd−O 2.011(4)/ 2.023(4) Å).18 On the other hand, 3c, 4b, and 4c display a perfect square planar geometry around the metal centers. Compared to the smaller methyl substituent in 3a and 4a, the sterically more demanding mesityl (4b) and diisopropylphenyl substituents (3c and 4c) preferably direct the chelating bidentate ligands to bind the metal center in a trans fashion. This enhances the symmetry within the complexes, featuring only half of the molecule in the asymmetric unit of the crystal structure. The M−C distances [Ni−C1: 1.929(2) Å (3c); Pd−C1: 2.006(3) Å (4b); Pd−C1: 2.058(7) Å (4c)] in the trans complexes are longer as expected than the corresponding cis complexes (3a and 4a). The Ni−C bond distances in 3c are longer than the similarly known trans complex (1.908(5) Å), whereas Pd−C bond distances fall in the range of the known examples (2.007(2)−2.023(4) Å).19,18a Further, the M−O bond distances in 3c (Ni−O: 1.8492(17) Å), 4b (1.9925(19) Å), and 4c (1.976(5) Å) are shorter, like it is known for such types of complexes in the literature (Ni−O: 1.835(2) Å; Pd−O: 2.058(1) Å)16i,18a and also to either 3a or 4a which can be D


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the cases which stands out as a distinct advantage over other commonly used catalysts for KTC reaction, such as Ni(dppe)2Cl2, Ni(acac)2, and Ni(PPh3)2Cl2, under the similar experimental conditions (Table 2, entries 7, 8, and 9). Using the above optimized condition (Table 2, entry 4), different unsubstituted, substituted, and the heteroaromatic chlorosubstrates were screened for the KTC coupling in the presence of phenylmagnesium bromide, and the isolated yield of cross-coupled products are provided in Table 3. Substrates

palladium (4a−c) complexes were evaluated. For a direct comparison of the catalytic activity between the above synthesized catalysts and the well-known catalysts like Ni(dppe) 2Cl2, Ni(acac)2, and Ni(PPh3) 2Cl2, the same reactions were carried out under similar experimental conditions, and the results obtained are given in Table 2. Table 2. KTC Coupling of 4-Chloroanisole with Phenylmagnesium Bromidea

Table 3. KTC Coupling Results with 3a as the Catalysta conversion (%)b entry

catalyst (mol %)

PhMgBr (equiv)

1

3a (1)

1

2

3a (1)

2

3

3a (2)

1.5

4

3a (1)

1.5

5

3a (0.5)

1.5

6

3c (1)

1.5

7

Ni(dppe)Cl2 (1)

1.5

8

Ni(PPh3)2Cl2 (1)

1.5

9

Ni(acac)2 (1)

1.5

time (h)

C−C coupled

homocoupled

1 2 4 1 2 4 1 2 4 1 2 4 1 3 6 2 4 6 2 4 8 2 4 8 2 4 11

7 15 51 33 80 95 43 75 93 4 61 93 1 13 33 56 62 66 22 33 33 15 50 62 0 3 5

1 1 7 2 3 5 1 2 2 1 4 5 1 2 4 4 4 9 14 42 43 13 21 22 5 9 9

a

Reaction conditions: Substrate 1 mmol, phenylmagnesium bromide 1.5 mmol, catalyst 0.01 mmol, THF 4 mL, 30 °C. bIsolated yield.

30 °C, 4 mL of THF, 4-chloroanisole 1 mmol. GC-MS yield.

a

b

containing methoxy or methyl at the para position gave excellent yield. Similarly, the yields of monocoupled products using chloronaphthalene and chloropyrimidine as well as biscoupled product of 2,6-dichloropyridine are quantitative, except in few cases like 2-chloropyridine, 2,5-dichloropyridine, and 4-trifluoromethyl chlorobenzene. There are very few nickel complexes containing NHC or non-NHC ligands in the literature that were found to be efficient catalysts for KTC cross-coupling of similar substrates used in Table 3.7b,c,9c,10a,e,11e Each of them are advantageous in terms of either of these factors such as low catalyst loading, wider substrate scope, higher yield, and room temperature. Among them, 3a is promising in terms of (a) shorter reaction time, (b) room temperature, and (c) low catalyst loading. Moreover, similar to the observation noted by Bouwman and co-workers, the anionic oxygen atom in the nickel complex not only serves as an anchor for bringing the Grignard reagent in close proximity to the metal center but also might be involved in increasing the rate of oxidative addition as proposed by

Only nickel complexes 3a and 3c gave the cross-coupling products, whereas the corresponding palladium complexes 4a− c were inactive. Generally, for KTC cross-coupling reaction which involves the use of the reactive Grignard reagents, palladium based catalysts tend to be less effective as compared to the corresponding nickel based catalysts.21 Apart from this, the performances of the catalysts Ni(dppe)2Cl2, Ni(PPh3)2Cl2, and Ni(acac)2 were found to be sluggish as compared to other catalysts with less selectivity for the cross-coupled product (Table 2, entries 7, 8, and 9). Further screening of the reaction revealed that the best yields were obtained when 1 mol % of catalyst 3a and 1.5 equiv of the Grignard reagent are used for the reaction (Table 2, entry 4). As reported by Newman and co-workers, a slow addition of Grignard reagent over the period of 20−30 min gave a better yield than the bulk addition of the reagent at once.8 Traces of homocoupled product in the form of biphenyl were also observed in almost every reaction; however, the cross-coupled product was obtained as the major product in all E


Article

Organometallics Yoshikai and co-workers.15 Further understanding of mechanism of the coupling reaction and wider substrate scopes are underway.

8.05 (dd, J = 4, 1 Hz, 1H, py), 8.28 (s, 1H, Im), 9.79 (s, 1H, NCHN), 11.96 (br s, 1H, OH); 13C NMR (125 MHz, DMSO-d6, δ ppm) 36.7, 121.1, 123.9, 126.6, 127.1, 134.9, 137.1, 139.2, 146.3. IR (KBr): ṽ 3422 (br, m), 3159 (m), 3063 (s), 2949 (s), 2903 (s), 2739 (s), 2607 (s), 2565 (s), 2516 (s), 1769 (w), 1650 (w), 1609 (s), 1578 (s), 1535 (s), 1468 (s), 1408 (w), 1360 (s), 1313 (s), 1297 (s), 1272 (s), 1223 (s), 1169 (m), 1113 (s), 1069 (m), 1057 (s), 969 (m), 826 (s), 798 (s), 763 (s), 746 (s), 692 (m), 645 (m), 633 (w), 610 (s), 559 (w) cm−1. ESI-MS m/z calcd. for [C9H10N3O]+, 176.0818 [M − Cl]+; found, 176.0827. Anal. Calcd for C9H10ClN3O: C, 51.07; H, 4.76; N, 19.85. Found: C, 51.34; H, 4.73; N, 19.82. 2b. 1-(2,4,6-Trimethylphenyl)-1H-imidazole (1b) (1.86 g, 10 mmol) and 2-chloro-3-hydroxy pyridine (1.55 g, 12 mmol) were taken in a Schlenk tube and heated at 130 °C for 72 h to yield a dull brown residue. The residue was scratched out of the Schlenk tube and washed with ethyl acetate three times (by stirring the residue in 100 mL of ethyl acetate for 10−12 h), and the purity was checked by recording a 1H NMR after every wash with the ethyl acetate. The final suspension was filtered through an open frit to yield 2b as an off-white powder that was dried under vacuum. Yield 2.25 g (71.4%). Mp: 282−285 °C. 1H NMR (500 MHz, CDCl3, δ ppm): 2.10 (s, 6H, oCH3), 2.39 (s, 3H, p-CH3), 7.09 (s, 2H, m-CH), 7.31 (s, 1H, Im-4,5CH), 7.36 (dd, J = 8.5, 4 Hz, 1H, py-CH), 8.02 (dd, J = 5, 2 Hz, 1H, py-H), 8.41 (dd, J = 8.5, 1.5 Hz, 1H, py-H), 8.64 (s, 1H, Im-CH), 9.77 (s, 1H, Im-NCHN). The O-H proton was not observed in CDCl3 which may be due to the exchange of proton with the solvent. However, the sample recorded in DMSO-d6 shows the presence of OH proton (see Supporting Information, Figure S4).13C NMR (125 MHz, CDCl3, δ ppm): 17.5, 21.2, 120.8, 122.4, 126.8, 129.0, 130.1, 130.4, 133.3, 134.1, 135.8, 138.7, 142.1, 146.6. IR (KBr): ṽ 3432 (br, m), 3195 (m), 3167 (s), 3064 (m), 3008 (s), 2863 (s), 2766 (s), 2628 (s), 2506 (w), 1617 (s), 1579 (s), 1553 (w), 1526 (s), 1466 (s), 1388 (w), 1346 (m), 1331 (s), 1302 (s), 1270 (s), 1242 (s), 1184 (w), 1165 (w), 1123 (s), 1100 (w), 1084 (m), 1053 (m), 964 (w), 934 (w), 888 (w), 863 (m), 839 (w), 822 (s), 768 (m), 757 (s), 670 (s), 589 (w), 565 (w), 535 (w), 434 (w) cm−1. ESI-MS m/z calcd. for [C17H18N3O]+, 280.1449 [M − Cl]+; found, 280.1455. Anal. Calcd for C17H18ClN3O: C, 64.66; H, 5.75; N, 13.31. Found: C, 64.53; H, 5.82; N, 12.96. 2b·PF6. 2b (2.23g, 7.1 mmol) and KPF6 (1.31g, 7.1 mmol) were dissolved separately in 70 and 50 mL of distilled water, respectively. The solution of 2b in water was warmed to completely dissolve the imidazolium salt and filtered through Whatman paper. The aqueous KPF6 solution was then added to the aqueous solution of 2b in one go; a white precipitate was immediately thrown out of the solution. After properly mixing, the contents were filtered through an open frit. The white precipitate was washed with water and dried in a 50 °C oven and then further dried under high vacuum. Yield 2.76 g (92%). Mp: > 270 °C. 1H NMR (400 MHz, CDCl3, δ ppm): 2.11 (s, 6H, oCH3), 2.38 (s, 3H, p-CH3), 7.08 (s, 2H, m-CH), 7.37 (s, 2H, Im-H +py-H), 7.91 (d, J = 8.4 Hz, 1H, py-CH), 8.04 (d, J = 3.6 Hz, 1H, pyH), 8.62 (s, 1H, Im-H), 9.64 (s, 1H, Im-NCHN). The O-H proton was not observed in CDCl3 which may be due to the exchange of proton with the solvent. 13C NMR (100 MHz, CDCl3, δ ppm):17.5, 21.2, 121.2, 122.7, 127.0, 128.3, 130.1, 133.4, 134.1, 135.7, 135.8, 139.2, 139.3, 142.1. IR (KBr): ṽ 3481 (m), 3195 (w), 3148 (m), 2926 (w), 1619 (w), 1582(w), 1552 (w), 1535 (m), 1479 (m), 1463 (w), 1383 (w), 1331 (w), 1308 (m), 1266 (m), 1250 (m), 1228 (m), 1179 (w), 1166 (w), 1121 (w), 1097 (w), 1057 (w), 968 (w), 903 (w), 852 (s), 813 (s), 747 (m), 662 (w), 644 (w), 583 (w), 556 (s), 462 (w). ESI-MS m/z calcd. for [C17H18N3O]+, 280.1449 [M − Cl]+; found, 280.1455. Anal. Calcd for C17H18F6N3OP: C, 48.01; H, 4.27; N, 9.88. Found: C, 48.39; H, 4.09; N, 9.91. 2c. The synthetic procedure of 2c was similar to that of 2b except that 1-(2,6-diisopropylphenyl)-1H-imidazole (1c) (2.28 g, 10 mmol) and 2-chloro-3-hydroxy pyridine (1.55 g, 12 mmol) were used. Yield 2.10 g (59%). Mp: 269−275 °C. 1H NMR (400 MHz, CDCl3, δ ppm): 1.18 (m, 12H, CH(CH3)2), 2.33 (sep., J = 6.4 Hz, 2H, CH(CH3)2), 7.32−7.39 (m, 4H, 2Ar-CH + Im-H + py-H), 7.59 (m,1H, Ar-H), 8.00 (d, J = 4 Hz, 1H, py-H), 8.41 (d, J = 8 Hz, 1H,

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CONCLUSION In conclusion, a convenient synthesis of new hydroxypyridine functionalized imidazolium salts has been reported which were then used as proligands to prepare nickel and palladium NHC complexes, demonstrating that hydroxypyridine functionalization can be one of the efficient ways to prepare heterobidentate C,O chelate type NHC ligands. The solid state structures of the complexes reveal that the ligand with a smaller methyl substituent at the nitrogen atom of the imidazole ring tends to produce cis complexes with distorted square planar geometry, whereas, with bulkier substituents on the nitrogen atom of the ligand, trans complexes with perfect square planar geometries are produced. For one of the nickel complexes, two different constitutional isomers were observed (3c and 3c′) and characterized by 1H NMR spectroscopy and single crystal Xray diffraction method. 3c has both the NHC ligands bound to the metal in a normal C2 fashion like all the other complexes, whereas 3c′ has one NHC binding in an abnormal C4 fashion. All the complexes were evaluated as catalysts for Kumada− Tamao−Corriu cross-coupling reaction between phenylmagnesium bromide and less reactive aryl chlorides under ambient conditions, and out of all the complexes, 3a gave the best yields.

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EXPERIMENTAL SECTION

Materials and Methods. Glassware was dried in an oven at 100 °C prior to use. The chemicals PdCl2, 2-chloro-3-hydroxypyridine, and 1-methylimidazole (1a) (Sigma-Aldrich) were purchased and used as received. 1-(2,4,6-Trimethylphenyl)-1H-imidazole (1b) and 1-(2,6-diisopropylphenyl)-1H-imidazol (1c) were prepared by using the literature procedure.22 Melting points were taken in a capillary (sealed tube for ligands), and the values reported are uncorrected. IR spectra were recorded in KBr pellets using a PerkinElmer FTIR model 1320 spectrophotometer operating from 4000 to 400 cm−1. 1H NMR and 13C NMR spectra were recorded on JEOL-DELTA 400 and 500 MHz spectrometers, and the chemical shifts are referenced with respect to tetramethylsilane (TMS). ESI-MS (positive mode) spectra were recorded on a Waters-Q-TOF Premier-HAB213 instrument. Elemental analyses for the compounds were done using a PerkinElmer series-II CHNS/O analyzer 2400. For single crystal X-ray crystallography, the crystal data were collected on a Bruker D8 QUEST CCD diffractometer using graphite-monochromated MoKα radiation (λ = 0.71073 Å) at 293 K. The data integration and reduction were processed with SAINT software,23 and the data were corrected for absorption by using the SADABS program.24 All the structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined by full-matrix least-squares methods with SHELXL-2014.25 All of the hydrogen atoms were included in idealized positions and a riding model was used. Nonhydrogen atoms were refined with anisotropic displacement parameters. Preparation of Imidazolium Salts HIMepyOH·Cl (2a), HIMespyOH·Cl (2b), HIMespyOH·PF6 (2b·PF6), and HIDipppyOH· Cl (2c). 2a. 1-Methyl imidazole (1a) (0.82 g, 10 mmol) and 2-chloro3-hydroxy pyridine (1.55 g, 12 mmol) were taken in a Schlenk tube and heated at 130 °C for 72 h to yield a brown residue. The residue was dissolved in a minimum quantity of methanol (20 mL) and precipitated out by adding diethyl ether (100 mL). This process was repeated three times to get an off-white solid that was filtered out and dried under vacuum. Yield 1.2 g (57%). Mp: 218−225 °C. 1H NMR (500 MHz, DMSO-d6, δ ppm) 3.96 (s, 3H, CH3), 7.45 (dd, J = 8.5, 4.5 Hz, 1H, py), 7.73 (dd, J = 8.5, 1.5 Hz, 1H, py), 7.86 (s, 1H, Im), F


Article

Organometallics

699.2952 [M + H]+; found, 699.2952. Anal. Calcd for C40H44N6NiO2: C, 68.68; H, 6.34; N, 12.01. Found: C, 68.37; H, 6.22; N, 11.89. Synthesis of Palladium NHC Complexes Pd(IMepyO)2 (4a), Pd(IMespyO)2 (4b), and Pd(IDipppyO)2 (4c). [Pd(IMepyO)2] 4a. 2a (0.212 g 1 mmol), K2CO3 (0.414 g 3 mmol), and PdCl2 (0.89 g 0.5 mmol) were taken in a Schlenk tube and kept under high vacuum for 30 min before purging with nitrogen gas. Then 3 mL of dimethyl sulfoxide (DMSO) was added and the contents were heated at 100 °C for 15 h. A yellow colored suspension was obtained. The reaction was stopped, and the solvent was removed under vacuum and the residue left was isolated with DCM (3 × 20 mL) and filtered through Whatman paper. Toluene (5 drops) was added to the collected filtrate. Upon slow evaporation, yellow colored crystals of 4a were obtained. Yield 0.080 g, 25%. Mp: > 270 °C. 1H NMR (400 MHz, DMSO-d6, δ ppm):δ 3.25 (s, 6H, N-CH3), 7.08 (m, 2H, py-H), 7.20 (d, J = 8.4 Hz, 2H, Im-H), 7.58 (m, 4H, 2py-H+2Im-H), 7.98 (s, 2H, py-H).13C NMR (100 MHz, DMSO-d6, δ ppm):37.5, 119.3, 124.7, 125.3, 127.9, 132.4, 141.8, 154.7, 156.2. IR (KBr): ṽ 3121 (w), 3062 (w), 1566 (m), 1459 (s), 1428 (s), 1397 (m), 1378 (m), 1346 (m), 1300 (s), 1266 (m), 1241 (s), 1200 (m), 1117 (m), 1078 (m), 966 (m), 852 (w), 797 (w), 786 (m), 744 (m), 732 (m), 651 (w), 618 (w), 604 (m), 582 (w), 526 (w) cm−1. ESI-MS m/z calcd. for [C18H17N6O2Pd]+, 455.0448 [M + H]+; found, 455.0459 Anal. Calcd for C18H16N6O2Pd: C, 47.54; H, 3.55; N, 18.48. Found: C, 47.28; H, 3.32; N, 18.15. [Pd(IMespyO)2] 4b. 2b·PF6 (0.425 g, 1 mmol), K2CO3 (0.414 g, 3 mmol), and PdCl2 (0.89 g 0.5 mmol) were taken in a Schlenk flask and kept under vacuum for 30 min before purging with nitrogen gas. Acetonitrile (25 mL) was added, and the contents were refluxed for 12 h. A bright yellow colored suspension was obtained. Upon cooling to room temperature, the solution was filtered and a pale yellow precipitate was obtained. The precipitate was washed thoroughly with water and dried first in a low temperature oven and then under vacuum. The residue was then extracted with DCM (3 × 40 mL) and filtered. The yellow DCM filtrates were combined and solvent removed to give a yellow powder. Suitable crystals were obtained by slow evaporation of CHCl3 solution at room temperature. Yield (0.050 g, 15%). 1H NMR (500 MHz, CDCl3; δ ppm): 2.33 (m, 18H, Meso-CH3+p-CH3), 6.25 (d, J = 8 Hz, 2H, Im-H), 6.79 (s, 4H, MesmCH), 7.00 (s, 4H, 2Im-H+2py-H), 7.57 (s, 2H, py-H), 8.18 (s, 2H, py-H).13C NMR (125 MHz, CDCl3; δ ppm): 18.5, 21.2, 108.5, 116.9, 119.3, 122.9, 123.8, 128.4, 129.0, 131.3, 132.9, 134.9, 138.2, 153.3. IR (KBr): ṽ 3158 (w), 3073 (w), 2961 (m), 2923 (s), 2851 (m), 1740 (w), 1575 (w), 1466 (m), 1433 (m), 1376 (w), 1338 (m), 1316 (m), 1292 (m), 1261 (s), 109 (s), 1021 (s), 960 (w), 931 (w), 854 (w), 801 (s), 691 (w), 644 (w), 592 (w), 517 (w) cm−1. ESI-MS m/z calcd. for [C34H33N6O2Pd]+, 663.1700 [M + H]+; found, 663.1703. Anal. Calcd for C34H32N6O2Pd: C, 61.59; H, 4.86; N, 12.67. Found: C, 61.28; H, 4.71; N, 12.38. [Pd(IDipppyO)2] 4c. 2c (0.358 g, 1 mmol), K2CO3 (0.414 g, 3 mmol), KPF6 (0.372 g 2 mmol), and PdCl2, (0.89 g, 0.5 mmol) were taken in a dry Schlenk flask and kept subjected to vacuum for 30 min before purging with nitrogen gas. Acetonitrile (25 mL) was added and the contents were refluxed for 12 h. A bright yellow suspension was obtained. After filtration through a G-4 frit, a dull yellow precipitate was obtained which was thoroughly washed with water and dried first in a low temperature oven and then under vacuum. The precipitate was then isolated with DCM (25 mL) and filtered to give a clear yellow DCM filtrate which upon slow evaporation yielded bright yellow crystals of 4c. Yield 0.140 g (38%). Mp: > 270 °C. 1H NMR (400 MHz, CDCl3; δ, ppm): 1.10 (d, J = 6.8 Hz, 12H, CH(CH3)2), 1.24 (d, J = 6.8 Hz, 12H, CH(CH3)2), 2.93 (sep, J = 6.8 Hz, 4H, CH(CH3)2), 5.50 (dd, J = 8.4, 1.6 Hz, 2H,Im-H), 6.69 (dd, J = 8.4, 4 Hz, 2H, py-H), 6.75 (d, J = 2 Hz, 2H, Im-H), 7.26 (d, J = 7.6 Hz, 4H, Dippm-CH), 7.46 (t, J = 8 Hz, 2H, Dippp-CH), 7.52 (dd, J = 4, 1.6 Hz, 2H, py), 8.21 (d, J = 2 Hz, 2H, Py-H).13C NMR (100 MHz, CDCl3,δ, ppm): 24.0, 24.4, 28.6, 116.5, 122.8, 123.9, 125.5, 128.9, 129.3, 132.5, 137.3, 140.0, 145.4, 152.7, 164.9. IR (KBr): ṽ 3182 (w), 3102 (w), 3135 (w), 3064 (w),2928 (m), 2869 (m), 2965 (m),1688

py-H), 8.68 (s, 1H, Im-H), 9.77 (s, 1H, Im-NCHN). The O-H proton was not observed in CDCl3 which may be due to the exchange of proton with the solvent. However, the sample recorded in DMSO-d6 shows the presence of O-H proton (see Supporting Information, Figure S7). 13C NMR (100 MHz, CDCl3, δ ppm): 24.2, 24.4, 28.9, 120.7, 123.6, 125.0, 126.8, 129.0, 129.8, 132.5, 133.1, 135.9, 138.7, 145.2, 146.7. IR (KBr): ṽ 3416 (br), 3179 (w), 3054 (w), 2961 (s), 2868 (m), 2761 (w), 2623 (w), 1617 (s), 1578 (m), 1524 (s), 1478 (s), 1385 (w), 1325 (m), 1301 (m), 1270 (s), 1222 (m), 1178 (m), 1106 (w), 1056 (m), 895 (w), 824 (m), 809 (m), 774 (m), 760 (m), 667 (m), 558 (w), 532 (w) cm−1. ESI-MS m/z calcd. for [C20H24N3O]+, 322.1919 [M − Cl]+; found, 322.1913. Anal. Calcd for C20H24ClN3O: C, 67.12; H, 6.76; N, 11.74. Found: C, 66.87; H, 6.56; N, 11.88. Synthesis of Nickel NHC Complexes, Ni(IMepyO)2 (3a) and Ni(IDipppyO)2 (3c). 3a. 2a (0.386 g, 1.82 mmol), Ni(PPh3)2Cl2 (0.595 g, 0.0.91 mmol), and K2CO3 (0.753 g, 5.46 mmol) were taken in a dry Schlenk flask and subjected to vacuum for 30 min. Acetonitrile (50 mL) was added, and the suspension was stirred at room temperature for 20 h to obtain a yellow suspension. All the volatiles were removed under vacuum, and the resultant yellow colored residue was washed with pentane. The residue was then dissolved in DCM and filtered. Block shaped crystals of the desired complex were obtained upon slow evaporation. Yield (0.345 g, 93%). Mp: > 270 °C. 1H NMR (CDCl3, 400 MHz; δ, ppm): 3.20 (s, 6H, NCH3), 6.95 (d, J = 2.4 Hz, 2H, Im-H), 7.07 (dd, J = 8.4, 4.8 Hz, 2H, py-H), 7.55 (dd, J = 8.4, 1.2 Hz, 2H, py-H), 7.67 (dd, J = 4.8, 1.6 Hz, 2H, py-H), 7.96 (d, J = 2 Hz, 2H, Im-H).13C NMR (CDCl3, 100 MHz; δ, ppm): 37.0, 118.8, 123.7, 123.9, 129.1, 132.7, 140.1, 152.6, 157.0. IR (KBr): ṽ 3170 (w), 3135 (w), 2951 (w), 1635 (w), 1573 (m), 1565 (m), 1463 (s), 1447 (s), 1433 (s), 1401 (m), 1375 (w), 1349 (m), 1310 (s), 1270 (m), 1238 (m), 1205 (w), 1120 (w), 1079 (w), 965 (w), 856 (w), 818 (w), 806 (w), 791 (w), 747 (m), 729 (m), 686 (w), 662 (w), 618 (w), 583 (w), 521 (w) cm−1. ESI-MS m/ z calcd. for [C18H17N6NiO2]+, 407.0766 [M + H]+; found, 407.0774. Anal. Calcd for C18H16N6NiO2·CH2Cl2: C, 46.38; H, 3.69; N, 17.08. Found: C, 46.12; H, 3.58; N, 16.81. 3c. 2c (0.089 g, 0.25 mmol), NiCl2·6H2O (0.030 g, 0.125 mmol), and K2CO3 (0.104 g, 0.75 mmol) were taken in a Schlenk flask and subjected to vacuum for 30 min. Acetonitrile (20 mL) was added, and then the reaction mixture was refluxed for 12 h to obtain a yellow colored suspension. All the volatiles were removed under vacuum, yielding a pale yellow colored residue. The residue was thoroughly washed with water to remove the leftover base and inorganic byproducts. The residue was then washed with methanol (15 mL) and filtered through an open frit ($). The leftover residue was dissolved in DCM and filtered. Pink-red colored crystals of 3c were obtained upon slow evaporation at room temperature. Yield of 3c 0.060 g (69%). Preparation of 3c′: The methanol filtrate obtained from the above procedure ($) was evacuated to dryness to afford a yellow colored residue which was dissolved in a mixture of diethyl ether and pentane (1:2 ratio) and subsequently kept for crystallization. A mixture of pink red (3c) and yellow colored crystals of 3c′ were obtained. Even attempts to separate the yellow crystals by repeated crystallization of the mixture were not successful as the pink colored crystals (3c) always form along. 3c: Mp: > 270 °C. 1H NMR (CDCl3, 400 MHz; δ, ppm): 1.13 (d, J = 6.8 Hz,12H, CH(CH3)2), 1.28 (d, J = 6.8 Hz, 12H, CH(CH3)2), 3.32 (sep, J = 6.8 Hz, 4H, CH(CH3)2), 5.19 (dd, J = 8, 1.2 Hz, 2H, Im-H), 6.53 (dd, J = 8.4, 4.8 Hz, 2H, py-H), 6.69 (d, J = 2 Hz, 2H, Im-H), 7.18 (d, J = 8 Hz, 4H, Dipp-H), 7.32 (m, 2H, py-H), 7.44 (dd, J = 4.8, 2 Hz, 2H, Dipp-H), 8.25 (d, J = 2 Hz, 2H, py-H). 13C NMR (CDCl3, 100 MHz; δ, ppm): 23.7, 24.5, 28.7, 115.5, 122.0, 123.8, 125.7, 128.2, 128.7, 132.0, 138.1, 138.8, 144.3, 149.7, 160.3. IR (KBr): ṽ 3183 (w), 3136 (w), 3105 (w), 3068 (w), 2966 (s), 2928 (m), 2869 (m), 1692 (w), 1593 (w), 1580 (m), 1567 (m), 1468 (s), 1435 (s), 1400 (m), 1360 (m), 1336 (m), 1310 (s), 1281 (m), 1267 (m), 1222 (m), 1182 (m), 1119 (m), 1103 (m), 1072 (m), 1056 (m), 976 (w), 947 (m), 907 (w), 864 (w), 798 (s), 766 (m), 757 (m), 748 (s), 688 (m), 647 (w), 599 (w), 566 (w), 515 (w), 483 (w) cm−1. ESI-MS m/z calcd. for [C40H45N6NiO2]+, G


Article

Organometallics (w), 1637 (w), 1591 (w), 1575 (m), 1565 (m), 1463 (s), 1427 (s), 1404 (m), 1360 (m), 1336 (m), 1286 (s), 1262 (m), 1222 (m), 1199 (m), 1118 (m), 1103 (m), 1073 (m),1056 (m), 975 (w), 947 (m), 859 (w), 799 (s), 746 (s), 685 (m), 597 (w) cm−1. ESI-MS m/z calcd. for [C40H45N6O2Pd]+, 747.2639 [M + H]+; found, 747.2644. Anal. Calcd for C40H44N6O2Pd: C, 64.29; H, 5.94; N, 11.25. Found: C, 64.01; H, 5.73; N, 11.05. Procedure for the Kumada−Tamao−Corriu (KTC) Coupling Reaction. Catalyst, the solid substrates, and a magnetic stirring bar were charged into a Schlenk flask. The contents were kept under high vacuum for 30 min before purging with dry nitrogen gas. Then 4 mL of the dry THF was syringed into the flask and the contents were stirred for 10 min (the liquid substrates were injected at the time of adding solvent). Then the required amounts of Grignard reagent (1.5 equiv) were added slowly over the period of 30 min with continuous stirring. In the majority of the cases, addition of the Grignard reagent changes the color of the reaction mixture from yellow to deep red. The reaction was monitored through GC-MS by taking out 0.2 mL of the reaction mixture after regular intervals of time. Before the GC-MS measurements, the sample was filtered through a pad of silica gel and eluted by 3 mL of ethyl acetate.

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K.; Yoshikai, N. Low-Valent Cobalt Catalysis: New Opportunities for C − H Functionalization. Acc. Chem. Res. 2014, 47, 1208−1219. (f) Gandeepan, P.; Cheng, C. H. Cobalt Catalysis Involving π Components in Organic Synthesis. Acc. Chem. Res. 2015, 48, 1194− 1206. (g) Standley, E. A.; Tasker, S. Z.; Jensen, K. L.; Jamison, T. F. Nickel Catalysis: Synergy between Method Development and Total Synthesis. Acc. Chem. Res. 2015, 48, 1503−1514. (h) Tobisu, M.; Chatani, N. Cross-Couplings Using Aryl Ethers via C − O Bond Activation Enabled by Nickel Catalysts. Acc. Chem. Res. 2015, 48, 1717−1726. (i) Tasker, S. Z.; Standley, E. A.; Jamison, T. F. Recent advances in homogeneous nickel catalysis. Nature 2014, 509, 299− 309. (2) (a) Rosen, B. M.; Quasdorf, K. W.; Wilson, D. A.; Zhang, N.; Resmerita, A. M.; Garg, N. K.; Percec, V. Nickel-Catalyzed CrossCouplings Involving Carbon - Oxygen Bonds. Chem. Rev. 2011, 111, 1346−1416. (b) Tollefson, E. J.; Hanna, L. E.; Jarvo, E. R. Stereospecific Nickel-Catalyzed Cross-Coupling Reactions of Benzylic Ethers and Esters. Acc. Chem. Res. 2015, 48, 2344−2353. (c) Kurahashi, T.; Matsubara, S. Nickel-Catalyzed Reactions Directed toward the Formation of Heterocycles. Acc. Chem. Res. 2015, 48, 1703−1716. (d) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.; Grubbs, R. H.; Bansleben, D. A. Neutral, SingleComponent Nickel (II) Polyolefin Catalysts That Tolerate Heteroatoms. Science 2000, 287, 460−462. (e) Ananikov, V. P. Nickel: The “Spirited Horse” of Transition Metal Catalysis. ACS Catal. 2015, 5, 1964−1971. (f) Henrion, M.; Ritleng, V.; Chetcuti, M. J. Nickel N Heterocyclic Carbene-Catalyzed C − C Bond Formation: Reactions and Mechanistic Aspects. ACS Catal. 2015, 5, 1283−1302. (g) Ritleng, V.; Henrion, M.; Chetcuti, M. J. Nickel N - Heterocyclic CarbeneCatalyzed C − Heteroatom Bond Formation, Reduction, and Oxidation: Reactions and Mechanistic Aspects. ACS Catal. 2016, 6, 890−906. (h) Fiorito, D.; Folliet, S.; Liu, Y.; Mazet, C. A General Nickel-Catalyzed Kumada Vinylation for the Preparation of 2Substituted 1,3-Dienes. ACS Catal. 2018, 8, 1392−1398. (3) (a) Tamao, K.; Sumitani, K.; Kumada, M. Selective CarbonCarbon Bond Formation by Cross-Coupling of Grignard Reagents with Organic Halides. Catalysis by Nickel-Phosphine Complexes. J. Am. Chem. Soc. 1972, 94, 4374−4376. (b) Corriu, R. J. P.; Masse, J. P. Activation of Grignard Reagents by Transition-metal Complexes. A New and Simple Synthesis of trans-Stilbenes and Polyphenyls. J. Chem. Soc., Chem. Commun. 1972, 144a. (4) (a) Kim, C.-B.; Jo, H.; Ahn, B.-K.; Kim, C. K.; Park, K. Nickel NHeterocyclic Carbene Catalyst for Cross-Coupling of Neopentyl Arenesulfonates with Methyl and Primary Alkyl Grignard Reagents. J. Org. Chem. 2009, 74, 9566−9569. (b) Yu, D.-G.; Li, B.-J.; Zheng, S.F.; Guan, B.-T.; Wang, B.-Q.; Shi, Z.-J. Direct Application of Phenolic Salts to Nickel-Catalyzed Cross-Coupling Reactions with Aryl Grignard Reagents. Angew. Chem., Int. Ed. 2010, 49, 4566−4570. (c) Joshi-Pangu, A.; Wang, C.-Y.; Biscoe, M. R. Nickel-Catalyzed Kumada Cross-Coupling Reactions of Tertiary Alkylmagnesium Halides and Aryl Bromides/Triflates. J. Am. Chem. Soc. 2011, 133, 8478−8481. (5) (a) Van Veen, R.; Bickelhaupt, F. Attempted Synthesis of Dibenzo[b,e]borin (9-Boraanthracene) and Its Formation under Electron IMPACT. J. Organomet. Chem. 1970, 24, 589−594. (b) Bailey, C. L.; Murphy, C. L.; Clary, J. W.; Eagon, S.; Gould, N.; Singaram, B. Reaction of Grignard Reagents with Diisopropylaminoborane. Synthesis of Alkyl, Aryl, Heteroaryl and Allyl Boronic Acids From Organo(diisopropyl)aminoborane by a Simple Hydrolysis. Heterocycles 2012, 86, 331−341. (c) Hartwig, J. F. Borylation and Silylation of C-H Bonds: A Platform for Diverse C-H Bond Functionalizations. Acc. Chem. Res. 2012, 45, 864−873. (6) (a) Zim, D.; Lando, V. R.; Dupont, J.; Monteiro, A. L. NiCl2(PCy3)2 : A Simple and Efficient Catalyst Precursor for the Suzuki Cross-Coupling of Aryl Tosylates and Arylboronic Acids. Org. Lett. 2001, 3, 3049−3051. (b) Braga, A. A. C.; Ujaque, G.; Maseras, F. A DFT Study of the Full Catalytic Cycle of the Suzuki − Miyaura Cross-Coupling on a Model System. Organometallics 2006, 25, 3647− 3658. (c) Li, Z.; Zhang, S. L.; Fu, Y.; Guo, Q. X.; Liu, L. Mechanism

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00878. Crystallographic data of reported complexes, NMR and ESI-MS spectra of all the new compounds, and hydrogen bonding interactions in 3c′ (PDF) Accession Codes

CCDC 1454595 and 1543389−1543394 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (+91)-0512-259-7517. ORCID

Ganapathi Anantharaman: 0000-0001-7740-5581 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank SERB India for funding. I.A.B. and I.A. thank their respective funding agencies (UGC and IITK) for their doctoral fellowship. The contribution of our group member Sabeeha Parveen in the preparation of this manuscript is gratefully acknowledged. Dedicated to Professor V. K. Singh on the occasion of his 60th birthday.

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REFERENCES

(1) (a) Nakamura, E.; Sato, K. Managing the scarcity of chemical elements. Nat. Mater. 2011, 10, 158−161. (b) Plietker, B., Ed. Iron Catalysis in Organic Chemistry, Reactions and Applications; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2008. DOI: 10.1002/9783527623273. (c) Bauer, I.; Knolker, H. J. Iron Catalysis in Organic Synthesis. Chem. Rev. 2015, 115, 3170−3387. (d) Furstner, A. Iron Catalysis in Organic Synthesis: A Critical Assessment of What It Takes To Make This Base Metal a Multitasking Champion. ACS Cent. Sci. 2016, 2, 778−789. (e) Gao, H


Article

Organometallics of Ni-Catalyzed Selective C−O Bond Activation in Cross-Coupling of Aryl Esters. J. Am. Chem. Soc. 2009, 131, 8815−8823. (d) Jover, J.; Fey, N.; Purdie, M.; Lloyd-Jones, G. C.; Harvey, J. N. A computational study of phosphine ligand effects in Suzuki−Miyaura coupling. J. Mol. Catal. A: Chem. 2010, 324, 39−47. (e) Li, Z.; Jiang, Y. Y.; Fu, Y. Theoretical Study on the Mechanism of Ni-Catalyzed Alkyl−Alkyl Suzuki Cross-Coupling. Chem. - Eur. J. 2012, 18, 4345− 4357. (f) Amatore, C.; Le Duc, G.; Jutand, A. Mechanism of Palladium-Catalyzed Suzuki−Miyaura Reactions: Multiple and Antagonistic Roles of Anionic “Bases” and Their Countercations. Chem. - Eur. J. 2013, 19, 10082−10093. (g) Zhang, T. X.; Li, Z. A DFT study on Pd-catalyzed Suzuki cross-coupling polycondensation of aryl bromide monomers. Comput. Theor. Chem. 2013, 1016, 28−35. (h) Liu, L.; Chen, P.; Sun, Y.; Wu, Y.; Chen, S.; Zhu, J.; Zhao, Y. Mechanism of Nickel-Catalyzed Selective C−N Bond Activation in Suzuki-Miyaura Cross-Coupling of Amides: A Theoretical Investigation. J. Org. Chem. 2016, 81, 11686−11696. (7) (a) Martin, R.; Buchwald, S. L. Pd-Catalyzed Kumada-Corriu Cross-Coupling Reactions at Low Temperatures Allow the Use of Knochel-type Grignard Reagents. J. Am. Chem. Soc. 2007, 129, 3844− 3845. (b) Wang, Z.-X.; Wang, L. Amido pincer complex of nickelcatalysed Kumada cross-coupling reactions. Chem. Commun. 2007, 2423−2425. (c) Asano, E.; Hatayama, Y.; Kurisu, N.; Ohtani, A.; Hashimoto, T.; Kurihara, Y.; Ueda, K.; Ishihara, S.; Nagao, H.; Yamaguchi, Y. Acetylacetonato-based pincer-type nickel(ii) complexes: synthesis and catalysis in cross-couplings of aryl chlorides with aryl Grignard reagents. Dalton Trans 2018, 47, 8003−8012. (8) Hua, X.; Masson-Makdissi, J.; Sullivan, R. J.; Newman, S. G. Inherent vs Apparent Chemoselectivity in the Kumada−Corriu CrossCoupling Reaction. Org. Lett. 2016, 18, 5312−5315. (9) (a) Bohm, V. P. W.; Weskamp, T.; Gstöttmayr, C. W. K.; Herrmann, W. A. Nickel-Catalysed Cross-Coupling of Aryl Chlorides with Aryl Grignard Reagents. Angew. Chem., Int. Ed. 2000, 39, 1602− 1604. (b) Bohm, V. P. W.; Gstöttmayr, C. W. K.; Weskamp, T.; Herrmann, W. A. Catalytic C-C Bond Formation through Selective Activation of C-F Bonds. Angew. Chem., Int. Ed. 2001, 40, 3387− 3389. (c) Iglesias, M. J.; Prieto, A.; Nicasio, M. C. Kumada−Tamao− Corriu Coupling of Heteroaromatic Chlorides and Aryl Ethers Catalyzed by (IPr)Ni(allyl)Cl. Org. Lett. 2012, 14, 4318−4321. (10) (a) Huynh, H. V.; Jothibasu, R. Formation of Homoleptic Tetracarbene versus cis-Chelating Dicarbene Complexes of Nickel(II) and Applications in Kumada−Corriu Couplings. Eur. J. Inorg. Chem. 2009, 2009, 1926−1931. (b) Berding, J.; Lutz, M.; Spek, A. L.; Bouwman, E. Synthesis of Novel Chelating Benzimidazole-Based Carbenes and Their Nickel(II) Complexes: Activity in the Kumada Coupling. Organometallics 2009, 28, 1845−1854. (c) Guo, J.; Lv, L.; Wang, X.; Cao, C.; Pang, G.; Shi, Y. Synthesis, structure and reactivity of NHC−nickel complexes containing carbonato ligands. Inorg. Chem. Commun. 2013, 31, 74−78. (d) Vechorkin, O.; Hu, X. NickelCatalyzed Cross-Coupling of Non-activated and Functionalized Alkyl Halides with Alkyl Grignard Reagents. Angew. Chem., Int. Ed. 2009, 48, 2937−2940. (e) Xi, Z.; Liu, B.; Chen, W. Room-Temperature Kumada Cross-Coupling of Unactivated Aryl Chlorides Catalyzed by N-Heterocylic Carbene-Based Nickel(II) Complexes. J. Org. Chem. 2008, 73, 3954−3957. (11) (a) Liu, A.; Zhang, X.; Chen, W. New Pincer CĆ C Complexes of Nickel(II) via Chloronickelation of Alkyne-Bearing N-Heterocyclic Carbenes. Organometallics 2009, 28, 4868−4871. (b) Zhang, K.; Conda-Sheridan, M.; Cooke, S. R.; Louie, J. N-Heterocyclic Carbene Bound Nickel(I) Complexes and Their Roles in Catalysis. Organometallics 2011, 30, 2546−2552. (c) Guo, W. J.; Wang, Z. X. CrossCoupling of ArX with ArMgBr Catalyzed by N - Heterocyclic Carbene-Based Nickel Complexes. J. Org. Chem. 2013, 78, 1054− 1061. (d) Zhang, K.; Conda-Sheridan, M.; Cooke, S. R.; Louie, J. NHeterocyclic Carbene Bound Nickel(I) Complexes and Their Roles in Catalysis. Organometallics 2011, 30, 2546−2552. (e) Sun, Y.; Li, X.; Sun, H. [CNN]-pincer nickel(II) complexes of N-heterocyclic carbene (NHC): synthesis and catalysis of the Kumada reaction of unactivated C−Cl bonds. Dalton Trans 2014, 43, 9410−9413.

(f) Zhang, J.; Xu, J.; Xu, Y.; Sun, H.; Shen, Q.; Zhang, Y. Mixed NHC/Phosphine Ni(II) Complexes: Synthesis and Their Applications as Versatile Catalysts for Selective Cross-Couplings of ArMgX with Aryl Chlorides, Fluorides, and Methyl Ethers. Organometallics 2015, 34, 5792−5800. (g) Banach, L.; Guńka, P. A.; Buchowicz, W. Half-sandwich nickel complexes with ring-expanded NHC ligands − synthesis, structure and catalytic activity in Kumada − Tamao − Corriu coupling. Dalton Trans 2016, 45, 8688−8692. (h) Matsubara, K.; Yamamoto, H.; Miyazaki, S.; Inatomi, T.; Nonaka, K.; Koga, Y.; Yamada, Y.; Veiros, L. F.; Kirchner, K. Dinuclear Systems in the Efficient Nickel-Catalyzed Kumada − Tamao − Corriu CrossCoupling of Aryl Halides. Organometallics 2017, 36, 255−265. (12) (a) Liddle, S. T.; Edworthy, I. S.; Arnold, P. L. Anionic tethered N-heterocyclic carbene chemistry. Chem. Soc. Rev. 2007, 36, 1732− 1744. (b) Liu, M.; Nieger, M.; Schmidt, A. Mesomeric betaine − Nheterocyclic carbene interconversions of 1,2,4-triazolium-phenolates. Sulfur, selenium, and borane adduct formation. Chem. Commun. 2015, 51, 477−479. (c) Bastin, S.; Barthes, C.; Lugan, N.; Lavigne, G.; César, V. Anionic N-Heterocyclic Carbene Complexes of Gold(I) as Precatalysts for Silver-Free Cycloisomerization of Enynes. Eur. J. Inorg. Chem. 2015, 2015, 2216−2221. (d) Yang, D.; Tang, Y.; Song, H.; Wang, B. Synthesis, Structures, and Norbornene Polymerization Behavior of Palladium Complexes Bearing Tridentate o-Aryloxide-Nheterocyclic Carbene Ligands. Organometallics 2016, 35, 1392−1398. (e) Hameury, S.; de Frémont, P.; Braunstein, P. Metal complexes with oxygen-functionalized NHC ligands: synthesis and applications. Chem. Soc. Rev. 2017, 46, 632−733. (13) (a) Patel, D.; Liddle, S. T.; Mungur, S. A.; Rodden, M.; Blake, A. J.; Arnold, P. L. Bifunctional yttrium(III) and titanium(IV) NHC catalysts for lactide Polymerization. Chem. Commun. 2006, 1124− 1126. (b) Jones, N. A.; Liddle, S. T.; Wilson, C.; Arnold, P. L. Titanium(III) Alkoxy-N-heterocyclic Carbenes and a Safe, Low-Cost Route to TiCl3(THF)3. Organometallics 2007, 26, 755−757. (c) Arnold, P. L.; Turner, Z. R.; Bellabarba, R.; Tooze, R. P. Carbon - Silicon and Carbon - Carbon Bond Formation by Elimination Reactions at Metal N-Heterocyclic Carbene Complexes. J. Am. Chem. Soc. 2011, 133, 11744−11756. (d) Arnold, P. L.; Marr, I. A.; Zlatogorsky, S.; Bellabarba, R.; Tooze, R. P. Activation of carbon dioxide and carbon disulfide by a scandium N-heterocyclic carbene complex. Dalton Trans 2014, 43, 34−37. (e) Zhang, D.; Zi, G. Nheterocyclic carbene (NHC) complexes of group 4 transition metals. Chem. Soc. Rev. 2015, 44, 1898−1921. (14) (a) Berding, J.; van Dijkman, T. F.; Lutz, M.; Spek, A. L.; Bouwman, E. N-donor functionalized N-heterocyclic carbene nickel(II) complexes in the Kumada coupling. Dalton Trans. 2009, 6948− 6955. (b) Breitenfeld, J.; Ruiz, J.; Wodrich, M. D.; Hu, X. Bimetallic Oxidative Addition Involving Radical Intermediates in NickelCatalyzed Alkyl − Alkyl Kumada Coupling Reactions. J. Am. Chem. Soc. 2013, 135, 12004−12012. (15) Yoshikai, N.; Mashima, H.; Nakamura, E. Nickel-Catalyzed Cross-Coupling Reaction of Aryl Fluorides and Chlorides with Grignard Reagents under Nickel/Magnesium Bimetallic Cooperation. J. Am. Chem. Soc. 2005, 127, 17978−17979. (16) (a) Arnold, P. L.; Mungur, S. A.; Blake, A. J.; Wilson, C. Anionic Amido N-Heterocyclic Carbenes: Synthesis of Covalently Tethered Lanthanide−Carbene Complexes. Angew. Chem., Int. Ed. 2003, 42, 5981−5984. (b) Arnold, P. L.; Rodden, M.; Davis, K. M.; Scarisbrick, A. C.; Blake, A. J.; Wilson, C. Asymmetric lithium(I) and copper(II) alkoxy-N-heterocyclic carbene complexes; crystallographic characterisation and Lewis acid catalysis. Chem. Commun. 2004, 1612−1613. (c) Ketz, B. E.; Ottenwaelder, X. G.; Waymouth, R. M. Synthesis, structure, and olefin polymerization with nickel(II) Nheterocyclic carbene enolates. Chem. Commun. 2005, 5693−5695. (d) Yagyu, T.; Oya, S.; Maeda, M.; Jitsukawa, K. Syntheses and Characterization of Palladium(II) Complexes with Tridentate NHeterocyclic Carbene Ligands Containing Aryloxy Groups and Their Application to Heck Reaction. Chem. Lett. 2006, 35, 154−155. (e) Wang, Z. G.; Sun, H. M.; Yao, H. S.; Yao, Y. M.; Shen, Q.; Zhang, Y. Bis-aryloxo-functionalized NHC complexes of ytterbium(III): I


Article

Organometallics Syntheses and structures of Yb[O-4,6-tBu2-C6H2 -2-CH2 {C(RNCHCHN)}]2 N(iPr)2 (R=iPr, Me). J. Organomet. Chem. 2006, 691, 3383−3390. (f) Hameury, S.; de Frémont, P.; Breuil, P.-A. R.; Olivier-Bourbigou, H.; Braunstein, P. Synthesis and characterization of oxygen-functionalised-NHC silver(I) complexes and NHC transmetallation to nickel(II). Dalton Trans. 2014, 43, 4700−4710. (g) Jacques, B.; Hueber, D.; Hameury, S.; Braunstein, P.; Pale, P.; Blanc, A.; de Frémont, P. Synthesis, Characterization, and Catalytic Activity of Alcohol-Functionalized NHC Gold(I/III) Complexes. Organometallics 2014, 33, 2326−2335. (h) Dang, L.; Song, H.; Wang, B. Synthesis, Structures, and Norbornene Polymerization Behavior of o- Aryloxide-Substituted NHC-Ligated σ, π- Cycloalkenyl Palladium Complexes. Organometallics 2014, 33, 6812−6818. (i) Borré, E.; Dahm, G.; Aliprandi, A.; Mauro, M.; Dagorne, S.; Bellemin-Laponnaz, S. Tridentate Complexes of Group 10 Bearing Bis-Aryloxide NHeterocyclic Carbene Ligands: Synthesis, Structural, Spectroscopic, and Computational Characterization. Organometallics 2014, 33, 4374−4384. (17) (a) Waltman, A. W.; Grubbs, R. H. A New Class of Chelating N-Heterocyclic Carbene Ligands and Their Complexes with Palladium. Organometallics 2004, 23, 3105−3107. (b) Liu, M.; Nieger, M.; Hübner, E. G.; Schmidt, A. Formation of N-Heterocyclic Carbenes by Tautomerization of Mesomeric Betaines: Cyclic Boron Adducts and Palladium Complexes From 2-(Imidazolium-1-yl)phenolates. Chem. - Eur. J. 2016, 22, 5416−5424. (18) (a) Kong, Y.; Ren, H.; Xu, S.; Song, H.; Liu, B.; Wang, B. Synthesis, Structures, and Norbornene Polymerization Behavior of Bis (aryloxide-N-heterocyclic carbene) Palladium Complexes. Organometallics 2009, 28, 5934−5940. (b) Kong, Y.; Cheng, M.; Ren, H.; Xu, S.; Song, H.; Yang, M.; Liu, B.; Wang, B. Synthesis, Structures, and Norbornene Polymerization Behavior of Bis(aryloxide-N-heterocyclic carbene) Nickel Complexes. Organometallics 2011, 30, 1677−1681. (19) Liao, C.-Y.; Chan, K.-T.; Chang, Y.-C.; Chen, C.-Y.; Tu, C.-Y.; Hu, C.-H.; Lee, H. M. Unexpected Solvent-Induced Cis/Trans Isomerization and Catalytic Application of a Bis-bidentate Nickel(II) Complex with N-Heterocyclic Carbene and Amido Functionalities. Organometallics 2007, 26, 5826−5833. (20) (a) Scheele, U. J.; Dechert, S.; Meyer, F. Non-Innocence of NHeterocyclic Carbene Ligands: Intermolecular C-H Activation in Allyl Palladium NHC Complexes. Chem. - Eur. J. 2008, 14, 5112−5115. (b) Wang, Y.; Xie, Y.; Abraham, M. Y.; Wei, P.; Schaefer, H. F., III; Schleyer, P. v. R.; Robinson, G. H. A Viable Anionic N-Heterocyclic Dicarbene. J. Am. Chem. Soc. 2010, 132, 14370−14372. (c) Krüger, A.; Kluser, E.; Müller-Bunz, H.; Neels, A.; Albrecht, M. Chelating C4Bound Imidazolylidene Complexes through Oxidative Addition of Imidazolium Salts to Palladium(0). Eur. J. Inorg. Chem. 2012, 2012, 1394−1402. (d) Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. An overview of N-heterocyclic carbenes. Nature 2014, 510, 485−496. (e) Bitzer, M. J.; Pöthig, A.; Jandl, C.; Kühn, F. E.; Baratta, W. Ru−Ag and Ru−Au dicarbene complexes from an abnormal carbene ruthenium system. Dalton Trans 2015, 44, 11686−11689. (f) Tan, K. V.; Dutton, J. L.; Skelton, B. W.; Wilson, D. J. D.; Barnard, P. J. Nickel(II) and Palladium(II) Complexes with Chelating N Heterocyclic Carbene Amidate Ligands: Interplay between Normal and Abnormal Coordination Modes. Organometallics 2013, 32, 1913− 1923. (g) Vivancos, Á .; Segarra, C.; Albrecht, M. Mesoionic and Related Less Heteroatom-Stabilized N-Heterocyclic Carbene Complexes: Synthesis, Catalysis, and Other Applications. Chem. Rev. 2018, 118, 9493−9586. (21) (a) Yamamura, M.; Moritani, I.; Murahashi, S. I. The Reaction of α-Vinylpalladium Complexes with Alkyl-lithiums. Stereospecific Syntheses of Olefins from Vinyl Halides and Alkyllithiums. J. Organomet. Chem. 1975, 91, C39−C42. (b) Negishi, E. I.; Baba, S. Novel Stereoselective Alkenyl-Aryl Coupling via Nickel -catalysed Reaction of Alkenylalanes with Aryl Halides. J. Chem. Soc., Chem. Commun. 1976, 596b−597b. (c) Negishi, E. I.; Van Horn, D. E. Selective Carbon-Carbon Bond Formation via Transition Metal Catalysis. 4. A Novel Approach to Cross-Coupling Exemplified by the Nickel-Catalyzed Reaction of Alkenylzirconium Derivatives with Aryl

Halides. J. Am. Chem. Soc. 1977, 99, 3168−3170. (d) Huang, J.; Nolan, S. P. Efficient Cross-Coupling of Aryl Chlorides with Aryl Grignard Reagents (Kumada Reaction) Mediated by a Palladium/ Imidazolium Chloride System. J. Am. Chem. Soc. 1999, 121, 9889− 9890. (22) Liu, J.; Chen, J.; Zhao, J.; Zhao, Y.; Li, L.; Zhang, H. A Modified Procedure for the Synthesis of 1-Arylimidazoles. Synthesis 2003, 17, 2661−2666. (23) SAINT+, version 6.02; Bruker AXS: Madison, WI, 1999. (24) Sheldrick, G. M. SADABS 2.0; University of Göttingen: Göttingen, Germany, 2000. (25) (a) Sheldrick, G. M. SHELXL-2014: Program for Crystal Structure Refinement; University of Göttingen: Göttingen, Germany, 2014. (b) Sheldrick, G. M. Acta Crystallogr. 2015, C71, 3−8.

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