YUAN Wei-Guan TANG Wei ZHANG Hong-Ling

ZHAO Bo XIONG Fang JING Lin-Hai QIN Da-Bin②

(Key Laboratory of Chemical Synthesis and Pollution Control of Sichuan Province, School of Chemistry and Chemical Engineering,China West Normal University, Nanchong 637002, China)

1 INTRODUCTION

As is known, NHC-metal complexes of Ag[1],Cu[2], Pd[3]and some lanthanide metals[4]as metal center with NHC have played important roles in the development of N-heterocyclic carbene chemistry,forming organometallic catalysts used to C–C, C–H,C–O, and C–N bond formation in organic syntheses.Some of the nickel catalysts[5-6], in fact, have been shown more effective than their corresponding palladium systems. In addition, nickel-NHC complexes have advantages of cheaper cost, higher activities toward unreactive aryl halides[7], and easier removal from the final products. At present,carbene-hybrid complexes of nickel still an intense research direction.

As an extension of the previous work, in this paper, we present two amide-functionalized imidazolium NHC complexes of nickel, Ni(L1)2(1)and Ni(L2)2·CH3CN·H2O (2)(Shown in Scheme 1).Although, similar carbene ligands of nickel(II)complexes have been reported by Lee and coworkers[8-9], our study still have some innovative points and practical significance. There is a new carbene hybrid ligand (L1Cl)containing an amide,an aryl and a pyridine N-donor group, and another(L2Cl)has been reported mediates palladium-based Suzuki cross-coupling reactions[8], which contain an amide group and a naphthalin group. Therefore, two ligands generate two types of different space groups and system of crystals. Crucially, the chelating amine ligand makes the catalyst more resistant to Ni–N bond cleavage[10], providing an opportunity to control the stability and reactivity of active centers more efficiently in homogeneous catalysis of the Suzuki coupling reaction. Complex 1 was applied in Suzuki coupling reaction. Through the investigation of reaction conditions, the optimal choice conditions of 80 ℃, K3PO4, 3% mol catalysts, 6% mol PPh3and toluene as solvent system could afford 96%yield in 5 h.

Scheme 1. Two NHC precursors L1Cl and L2Cl and the synthetic routes of complexes 1 and 2

2 EXPERIMENTAL

2. 1 General

Two ligands of L1Cl and L2Cl are presented in Scheme 1, which can be easily obtained according to the same procedure from N-substituted α-chloroacetamide and N-substituted imidazoles in high yields in two steps[9]. All manipulations were performed using Schlenk techniques, and solvents were purified by standard procedures. All the reagents for syntheses and analyses were of analytical grade and used without further purifycation. NMR spectra were recorded on Bruker Avance Ш 400 (1H NMR,400 MHz;13C NMR, 100 MHz, respectively).

2. 2 Preparation of compound 1

A mixture of L1Cl (0.107 g, 0.324 mmol), K2CO3(0.134 g, 0.972 mmol)and NiCl2.6H2O (0.0210 g,0.162 mmol)in acetonitrile (20 mL)was heated at 80 ℃ for 20 h. After cooling, the solvent was completely removed under vacuum. The residue was redissolved in dichloromethane (20 mL)and the organic layer was washed twice with water and dried with anhydrous MgSO4. The volume of the solvent was reduced to ca. 3 mL under vacuum, which was filtered, washed with diethyl ether and dried under vacuum, giving a yellow solid.1H NMR (DMSO-d6,400 MHz), δ (ppm): 8.49～7.01 (m, 18H, Ar-H,Py-H), 7.06 (s, 2H, imi-H), 6.78 (s, 2H, imi-H), 5.94(d, J = 13.9 Hz, 2H, CH2-Py), 4.84 (d, J = 14.4 Hz,2H, CH2-C=O), 4.37 (d, J = 13.9 Hz, 2H, CH2-Py),4.20 (d, J = 14.4 Hz, 2H, CH2-C=O).13C NMR(DMSO-d6, 100 MHz), δ (ppm): 164.1, 153.9, 149.9,138.8, 122.9, 119.4, 53.4, 51.7.

2. 3 Preparation of compound 2

Compound 2 was prepared by a similar procedure for 1 by replacing L1Cl with L2Cl. A yellow solid was obtained.1H NMR (DMSO-d6, 400 MHz),δ(ppm): 8.07～6.26 (m, 18H, Ar-H, Ny-H), 6.21(s,2H, imi-H), 6.18 (s, 2H, imi-H), 5.44 (d, J = 15.1,2H, CH2-C=O), 4.95 (d, J = 14.4 Hz, 2H, CH2-Ny),4.26 (d, J = 14.4 Hz, 2H, CH2-Ny), 4.01 (d, J = 15.1 Hz, 2H, CH2-C=O).13C NMR (DMSO-d6, 100 MHz), δ (ppm): 166.3, 165.9, 146.7, 135.6, 124.1,122.9, 121.1, 57.4, 53.3.

2. 4 General procedure for the Suzuki coupling reactions

In a typical reaction, a mixture of aryl halide (1.0 mmol), phenyboronic acid (1.3 mmol), base (2.6 mmol), catalyst (1～3 mol%), and triphenylphosphane (0.6 mol%)in toluene (3 mL)was stirred at 80 ℃ for an appropriate period of time (0.5～24 h)under nitrogen atmosphere in air. The solution was allowed to cool to ambient temperature and the solvent was removed completely under vacuum. A 1:1 mixture of diethyl ether/water (20 mL)was added. Then the organic layer was washed and separated, further washed with another 10 mL of diethyl ether, then dried with anhydrous MgSO4and filtered. The solvent and some volatile substances were removed completely under high vacuum to give a crude product which was subjected by flash chromatography and analyzed by1H NMR spectroscopy.

2. 5 X-ray crystal structure determination

Yellow crystals of 1 and 2 with dimensions of 0.27mm × 0.27mm × 0.27mm and 0.30mm ×0.27mm × 0.09mm were separately chosen and mounted on a Rigaku SPIDER diffractometer equipped with a graphite-monochromator with a Mo-Kα (λ = 0.71073 ?)by using an ω scan mode at 93(2)K. The structures were solved by direct methods and refined with full-matrix least-squares procedures on F2with the SHELX-97[11-12]. All non-hydrogen atoms were refined with anisotropic displacement parameters except little disordered solvent molecules, and all the hydrogen atom positions were generated geometrically at the idealized positions and refined by using the riding model. The crystal used for diffraction study showed no decomposition during data collection. The data for crystal and refinement are listed in Table 1, the selected bond lengths, bond angles and torsion angles in Table 2, and hydrogen bond data in Table 3.

Table 1. Experimental Crystallographic Data for Complexes 1 and 2

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Table 2. Selected Bond Lengths (?), Bond Angles (°)and Torsion Angles (°)for Complexes 1 and 2

Table 3. Hydrogen Bonding Data of 1 and 2a

3 RESULTS AND DISCUSSION

3. 1 Description of the structure

Crystals of 1 and 2 suitable for X-ray diffraction were grown by slow diffusion of diethyl ether into their acetonitrile solution. A well-formed crystal 1 of Ni (L1)2(Fig. 1)crystallizes in the P21/c space group with monoclinic crystal system and has two amineimidazolium NHC ligands in the unite cell. The four-coordinated Ni(II)center displays distorted quadrilateral coordination geometry, and the distorted angle is about 9.179(7)°. The pyridine and benzene arms lie in the same side of imidazol plane to adopt a cis-conformation. Two pyridine rings form the dihedral angles of 66.324(5)and 74.949(6)°with the centric imidazol plane. The benzene rings and the centric imidazol plane form the dihedral angles of 69.115(7)° and 81.607(7)° separately. Two C–Ni–N bonds deviate from near linearity with the bond angles of 172.48(7)° and 173.79(8)°. The N(2)–Ni(1)–N(6)and C(10)–Ni(1)–C(27)angles are 91.39(7)° and 96.91(8)°, respectively, reflecting the distorted square coordination geometry of the nickel center. The Ni–C bond (1.857(2)～1.865(2)?)is shorter than Ni–N (1.949(16)～ 1.949(17)?),comparable to those of nickel(II)complexes with NHC moieties in the cis-disposition[9]. The 3D supramolecule is formed via C–H··O hydrogen bonds (the hydrogen atom being from imidazole rings, pyridine rings and methane, and the oxygen atoms from ketonic oxygen)and C–H··π stacking interactions (the hydrogen atom being from imidazole rings, benzene rings, pyridine rings and methane group)are observed in Table 3 and Fig. 2.

Fig. 1. A view of complex 1, showing 37% probability displacement ellipsoids.Hydrogen atoms and solvent molecules are omitted for clarity

Fig. 2. 3D supramolecule by hydrogen bonds and C–H··π contacts (indicated by dashed lines)in 1

Interestingly, unlike complex 1, the crystal 2 of Ni(L2)2·CH3CN·H2O (Fig. 3)crystallizes in the Pbca space group presenting orthorhombic crystal system.Noteworthily, guest solvent molecules of CH3CN and H2O in their symmetric units participant in intermolecular weak interactions. Similar to complex 1, the four-coordinated Ni(II)center displays distorted square coordination geometry, but the distorted angle is about 5.379(11)°. Simultaneously,two naphthalene rings form the dihedral angles of 75.137(8)and 82.795(9)° with the centric imidazol plane, and two benzene rings with the centric imidazol plane form the dihedral angles being 61.187(1)and 63.745(1)° separately. Two C–Ni–N bonds drift off linearity with the bond angles of 174.56(13)and 175.17(13)°, and the bond angles N(1)–Ni(1)–N(4)and C(11)–Ni(1)–C(33)are 89.11(11)and 95.86(14)°, respectively. The Ni–C bond (1.871(5)～1.877(3)?)is shorter than Ni–N(1.935(3)～1.953(3)?)and comparable with those in the related analogues with amide donors[9]. In contrast to complex 1, substituents of naphthalene,pyridine and benzene exert a strong influence on the molecular structure of its Ni complex. Besides, the formation of the 3D supramolecule is via C–H··O,C–H··N and O–H··O hydrogen bonds with the hydrogen atom from imidazole rings, nitrogen atoms from acetonitrile, and oxygen atoms from ketonic oxygen, water molecules)and C–H··π contacts (the hydrogen atom being from imidazole rings, benzene rings, naphthalene rings, methane group and acetonitrile molecule)are observed in Table 3 and Fig.4.

3. 2 Catalytic studies

Metal complexes of Ni(II)-NHCs show enhanced catalytic activities in various organic processes including C–C and C–N coupling reactions[13,9]. We tested the catalytic performance of 1 in the Suzuki cross-coupling between phenylboronic acid and bromobenzene. Initially, the coupling reaction between phenylboronic acid, bromobenzene and 3 mol% of 1 as catalyst in the presence of 6 mol% of PPh3(1:2 Ni/PPh3)over a period of 24 h was employed as the standard reaction for screening the solvent, base and temperature. Following this preliminary screening, the coupling of aryl halides or N-heteroaryl bromides with phenylboronic acid at 80 ℃ in toluene with a 3 mol% catalyst 1 and K3PO4·H2O as base in 2eq PPh3was selected as a standard test reaction. The results summarized in Table 4 show that catalyst 1 is very active in the coupling of both activated and inactivated aryl halides that are iodobenzenes (entries 1～3),bromobenzenes (entries 4～14), chlorobenzenes(entries 15, 16), as well as heteroaryl compounds(entries 17～19). The effect of steric factors plays an important role in the reaction yield (entries 5～10).The least active aryl chloride affords satisfactory yield of 73% (entry 15). The coupling reaction of heteroaryl bromides, which is also efficient in the catalyst system, can be conveniently coupled with moderate yields of 45～64%. 3-bromopyridine can successfully give a relative high yield with 6 h (entry 17).

Fig. 3. A view of complex 2, showing 37% probability displacement ellipsoids. Hydrogen atoms and solvent molecules are omitted for clarity

Fig. 4. 3D supramolecule by the hydrogen bonds and C–H··π contacts (indicated by dashed lines)in 2

The mechanism for the nickel catalyzed Suzuki cross-coupling reactions has been proposed to comprise the steps depicted in Scheme 2[14]. A reaction between the starting Ni(II)complex (1)and PPh3yields a Ni(0)active species (A). Then oxidative addition of aryl halides to a nickel(0)complex affords a trans-a-nickel(II)complex (B). Substitution of halide on the nickel ion with the aryl group of phenylboronic acid affords a biaryl product (C). Reductive elimination of the biaryl product furnishes the coupling product and the starting Ni(0)species.

Scheme 2. Proposed mechanism for the nickel catalyzed Suzuki cross-coupling reaction

Table 4. Suzuki Coupling of Phenylboronic Acid with Aryl Halides and N-heteroaryl Bromides

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