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Synthesis, characterization and catalytic activity of nickel NHC complexes
Molecular Catalysis 440 (2017) 25–35
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Synthesis, characterization and catalytic activity of nickel NHC complexes Agata Włodarska ∗ , Jowita Gołaszewska-Gajda, Maciej Dranka, Antoni Pietrzykowski Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw, Poland
a r t i c l e
i n f o
Article history: Received 27 March 2017 Received in revised form 5 June 2017 Accepted 26 June 2017 Keywords: Nickel NHC carbenes Suzuki–Miyaura coupling Oligomerization Ethyl diazoacetate
a b s t r a c t Several new cyclopentadienylnickel complexes were synthesized and characterized. All the complexes exhibited catalytic activity in Suzuki–Miyaura cross-coupling reaction with conversion rates from 10 to 80% and very high selectivity. The catalytic activity of the complexes strongly depended on their composition and structure. It was shown that ionic complexes are better catalysts than their covalent analogues; increased electron density in the cyclopentadienyl ligand improved their catalytic activity; bromide complexes provided better results than chloride ones. Two CpNi(NHC)Cl complexes were tested as initiators in the oligomerization of ethyl acetate carbene confirming our earlier observation that N-heterocyclic carbene from the catalyst precursor was incorporated into the oligomer chain. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The isolation of the first stable crystalline carbene by Arduengo in 1991 [1] initiated a rapid development of the chemistry of Nheterocyclic carbenes, and they have become an important class of ligands in organometallic chemistry [2–17]. A part of this success is due to the electron donor ability of NHC ligands, which allows them to form very strong bonds with most metals (stronger than phosphines), and also due to electronic and structural diversity. Current research is not only focused on the chemistry of metal complexes with NHC ligands but also on their applications in transition-metalbased catalyst systems. Among many transition-metal NHC complexes, nickel ones are currently gaining interest as they exhibit interesting properties and good catalytic activity. A number of NHC nickel complexes, both covalent [18–33] and ionic [23,32,34–42], have been synthesized and characterized. Significant interest has been drawn to their application as catalysts in various organic reactions, such as olefin polymerization [22,25,29,31], carbene polymerization and oligomerization [43–49], dehalogenation and aryl amination [20,28], carbonyl hydrosilylation [50,51], acyclic ketone arylation [52], and carbon-carbon bond formation [41,53–63]. In this paper, we report the synthesis and characterization of several NHC cyclopentadienylnickel complexes unsymmetrically
∗ Corresponding author. E-mail address: [email protected] (A. Włodarska). http://dx.doi.org/10.1016/j.mcat.2017.06.033 2468-8231/© 2017 Elsevier B.V. All rights reserved.
substituted at nitrogen atoms of the NHC ring. The allyl group was the substituent at one carbene nitrogen atom, and various oxygen-containing substituents (ester, ether, ketone, and alcohol) were attached to another nitrogen. The idea was to enable hemilabile coordination of an oxygen atom to nickel, to prepare ionic complexes analogous to those with coordination via an allyl group double bond (e.g. [Cp’Ni(NHCallyl )]+ Br− [32]). Unfortunately, we did not observe such coordination. These complexes, and also those, previously described, symmetrically allyl-substituted at nitrogen [29,32], were applied as catalysts in Suzuki–Miyaura cross-coupling reactions. The syntheses of other unsymmetrical nickel NHC complexes and their applications in Suzuki–Miyaura reactions have been recently published [63]. The second part of this paper is devoted to the oligomerization of ethyl acetate carbene derived from ethyl diazoacetate. We have previously shown that N-heterocyclic carbene from the catalyst precursor is incorporated into the oligomer chain [29]. In this paper, we prove that this is a more general phenomenon, and even NHC carbenes with bulky substituents at nitrogen atoms can be inserted into oligomer chains. 2. Experimental 2.1. General information All manipulations were carried out using standard Schlenk techniques under argon atmosphere. Toluene, THF, hexane, acetonitrile, and dichloromethane were dried over potassium, KOH, CaH2 , and
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Fig. 1. Imidazolium salts used for synthesis of nickel NHC complexes.
P2 O5 , respectively, and distilled under argon prior to use. Nickelocene [64], 1-allylimidazole [65], 1-(4-vinylbenzyl)imidazole [66], CpNi(IMes)Cl [67], and cyclopentadienylnickel complexes with N-allyl-substituted NHC ligands [29,32,68] were synthesized according to the literature procedures. All other chemicals were purchased from commercial suppliers and used without further purification. NMR spectra were recorded at ambient temperature on a Mercury-400BB spectrometer (400 MHz for 1 H and 101 MHz for 13 C) and on a 500 MHz Varian NMR System. EI (70 eV) mass spectra were recorded on an AMD-604 spectrometer. ESI mass spectra were recorded on a Waters Maldi SYNAPT G2-S HDMS. Average molecular weights were measured on a LabAlliance liquid chromatograph on a Jordi Gel u VB Mixed Bed column (250 mm × 10 m) using CH2 Cl2 as the mobile phase at 30 ◦ C and calibrated with standard PMMAs. MALDI-TOF MS spectra were acquired with a Bruker Daltonics UltrafleXtremeTM mass spectrometer (matrix HABA). Elemental analyses were recorded on an Elementar Vario EL III analyser. 2.2. Crystal structure determination A selected single crystal was mounted in inert oil and transferred to the cold gas stream of the diffractometer. Diffraction data were measured at 120.0(1) K with graphite-monochromated MoK␣ radiation ( = 0.71073) on an Oxford Diffraction -CCD Gemini A Ultra diffractometer. Cell refinement and data collection as well as data reduction and analysis were performed with the crysalispro software [69]. The structure was solved by direct methods using the shelxT [70] structure solution program and refined by full-matrix least squares against F2 with the shelxl-2015 [71] and olex2 [72] programs. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were added to the structure model at geometrically idealized coordinates and refined as riding atoms. Crystals of 17 were found to be non-merohedrically twinned, and the hklf 5 procedure was used for refinement. Two domains with the ratio of 0.7017(8):0.2983(8) in the final refine-
ment were related by a twofold rotation around the [001] direction. The crystal data and experimental parameters are summarized in Table S1. CCDC1518513-CCDC1518516 (17, 10, 7, 8) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data request/cif. 2.3. Syntheses of imidazolium salts Synthesised and characterized imidazolium salts are depicted on Fig. 1: 2.3.1. Synthesis of 1 Ethyl chloroacetate (2.3 mL, 18.5 mmol) was added to 1allylimidazole (2.0 g, 18.5 mmol) in acetonitrile (20 mL). The solution was refluxed for 12 h. The solvent was evaporated under reduced pressure. The resulting light yellow oil was washed three times with THF (20 mL) and dried under reduced pressure. Yield 73% (3.125 g). 1 H NMR (400 MHz, CDCl3 ) ␦ = 10.54 (s, 1H, NCHN), 7.72 (s, 1H,NCHCHN), 7.43 (s, 1H, NCHCHN), 5.98 (m, 1H, CH ), 5.51 (s, 2H,NCH2 O), 5.44 (d, 1H, CHHtrans ), 5.40 (d, 1H, CHHcis 3 JH,H = 9.8 Hz), 4.95 (d, 2H, NCH2 , 2 JH,H = 6.3 Hz,), 4.20 (q, 2H, CH2 CH3 , 3 JH,H = 7.1 Hz), 1.24 (m, 3H, CH2 CH3 ).13 C NMR (101 MHz, CDCl3 ) ␦ = 181.3 (C O), 138.6 (NCN), 132.9 (NCCN), 129.5 (NCCN), 120.9 ( CH ), 118.3 ( CH2 ), 66.1 (CH3 CH2 O ), 64.2 (C(O) CH2 N), 53.6 (NCH2 ), 16.8 (CH3 CH2 O ). ESI–MS (m/z): 195.24 [M−Cl]+ . ESI-HRMS: calcd for [C10 H15 N2 O2 ]+ 195.2384; found 195.2379. 2.3.2. Synthesis of 2 Chloroacetone (2.5 mL, 18.5 mmol) was added to 1allylimidazole (2.0 g, 18.5 mmol) in acetonitrile (20 mL). The solution was refluxed for 20 h. The solvent was evaporated under reduced pressure. The resulting light yellow oil was washed three times with THF (20 mL) and dried under reduced pressure. Yield 84% (3.125 g). 1 H NMR (400 MHz, CDCl3 ) ␦ = 9.84 (s, 1H, NCHN),
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7.75 (s, 1H, NCHCHN), 7.33 (s, 1H, NCHCHN), 5.85 (m, 1H, CH ), 5.65 (s, 2H, NCH2 O), 5.29 (d, 1H, CHHcis 3 JH,H = 6.1 Hz), 5.26 (d, 1H, CHHtrans , 3 JH,H = 13.1 Hz), 4.80 (d, 2H, NCH2 , 2 JH,H = 6.3 Hz), 2.19 (m, 3H, CH3 ).13 C NMR (101 MHz, CDCl3 ) ␦ = 198.4 (C O), 136.6 (NCN), 128.9 (NCCN), 123.5 (NCCN), 121.2 ( CH ), 120.3 ( CH2 ), 57.2 (C(O) CH2 N), 51.3 (NCH2 ), 26.3 (CH3 C(O)). ESI–MS (m/z): 165.2 [M−Cl]+ . ESI-HRMS: calcd for [C9 H13 N2 O]+ 165.1028; found 165.1047. 2.3.3. Synthesis of 3 Chloromethyl ethyl ether (2.16 mL, 22.2 mmol) was added in portions to a suspension of 1-allylimidazole (2.0 g, 18.5 mmol) in THF (2 mL). The reaction was exothermic, so the reaction flask was cooled with ice. The product appeared immediately as dark brown oil, immiscible with THF. The solvent was evaporated under reduced pressure. The resulting oil was washed three times with THF (20 mL) and dried under reduced pressure. Yield 68% (2.545 g). 1 H NMR (400 MHz, CDCl3 ): ␦ = 10.97 (s, 1H, NCHN), 7.54 (s, 1H, NCHCHN), 7.42 (s, 1H, NCHCHN), 6.03 (m, 1H, CH ), 5.82 (s, 2H, NCH2 O), 5.48 (d, 1H, CHHtrans ), 5.32 (d, 1H, CHHcis , 3J 2 H,H = 9.8 Hz), 4.82 (d, 2H, NCH2 , JH,H = 6.3 Hz), 3.69 (q, 2H, −OCH2 CH3 , J = 7.0 Hz), 1.22 (t, 3H, OCH2 CH3 , J = 7.0 Hz). 13 C NMR (101 MHz, CDCl3 ): ␦ = 165.5 (NCN), 126.9 (NCCN), 123.1 (NCCN), 121.7 ( CH ), 118.1 ( CH2 ), 92.0 (NCH2 O), 61.3 (CH3 CH2 O-), 52.4 (NCH2 CH ), 14.4 (CH3 CH2 O ). ESI–MS (m/z): 167.1 [M−Cl]+ . ESIHRMS: calcd for [C9 H15 N2 O]+ 167.1184; found 167.1193. 2.3.4. Synthesis of 4 1-Allylimidazole (4.2 g, 38.9 mmol) was added to 2-chloroethyl methyl ether 18.1 mL (194.4 mmol), which was used also as a solvent. The solution was refluxed for 20 h. The solvent was evaporated under reduced pressure. The resulting light brown oil was washed three times with THF (20 mL) and dried under reduced pressure. Yield 26% (2.049 g). 1 H NMR (400 MHz, CDCl3 ): ␦ = 10.54 (s, 1H, NCHN), 7.72 (s, 1H, NCHCHN), 7.43 (s, 1H, NCHCHN), 5.98 (ddt, 1H, CH , J = 16.6, 10.2, 6.3 Hz), 5.52 (d, 1H, CHHtrans ), 5.38 (d, 1H, CHHcis , 3 JH,H = 8.8 Hz), 4.78 (d, 2H, NCH2 -, 2 JH,H = 6.3 Hz), 4.20 (q, 2H, NCH2 CH2 -, J = 7.1 Hz), 3.60 (q, 2H, NCH2 CH2 , J = 6.6 Hz), 1.24 (m, 3H, −OCH3 ). 13 C NMR (101 MHz, CDCl3 ): ␦ = 164.5 (NCN), 125.8 (NCCN), 124.2 (NCCN), 120.7 ( CH = ), 118.6 ( CH2 ), 79.2 (NCH2 CH2 ), 68.1 (NCH2 CH2 ), 52.4 (NCH2 CH ), 17.4 (CH3 O ). ESI–MS (m/z): 167.1 [M−Cl]+ . ESI-HRMS: calcd for [C9 H15 N2 O]+ 167.1184; found 167.1199. 2.3.5. Synthesis of 5 2-chloroethanol (1.25 mL, 22.2 mmol) was added to 1allylimidazole (2.0 g, 18.5 mmol) in ethanol (1.5 mL). The solution was refluxed for 72 h. The solvent was evaporated under reduced pressure. The resulting brown oil was washed three times with THF (20 mL) and dried under reduced pressure. Yield 95.7% (3.331 g). 1 H NMR (400 MHz, DMSO): ␦ = 9.73 (s, 1H, NCHN), 8.25 (t, 1H, NCHCHN, J = 1.7 Hz), 8.19 (t, 1H, NCHCHN, J = 1.8 Hz), 6.48 (m, 1H, CH ), 5.86 (s, 1H, OH-), 5.77 (ddd, 1H, CHHtrans , J = 5.0, 3.1, 1.9 Hz), 5.68 (m, 1H, CHHcis ), 5.30 (t, NCH2 CH2 OH, J = 9.9 Hz, 2H), 4.67 (dd, 2H, NCH2 CH2 OH, J = 15.1, 9.9 Hz). 13 C NMR (101 MHz, CDCl3 ): ␦ = 169.1 (NCN), 125.1 (NCCN), 123.9 (NCCN), 120.4 ( CH = ), 116.1 ( CH2 ), 63.3 ( CH2 CH2 OH), 53.1 (NCH2 CH ), 17.1 ( CH2 CH2 OH). ESI-HRMS: calcd for [C8 H13 N2 O]+ 153.1028; found 153.1041. 2.3.6. Synthesis of 6 4-vinylbenzylchloride (1.7 mL, 10.8 mmol) was added to 1-(4vinylbenzyl)imidazol (2.0 g, 10.8 mmol) in acetonitrile (20 mL). The solution was stirred for 24 h at 70 ◦ C. The solvent was evaporated under reduced pressure. The resulting yellow solid was washed three times with THF (20 mL) and dried under reduced pressure.
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Yield 70% (2.561 g). 1 H NMR (400 MHz, CDCl3 ) ␦ = 10.69 (s, 1H, NCHN), 7.37 (s, 2H, NCHCHN), 7.29 (m, aromatic, 4H), 7.25–7.07 (m, aromatic, 4H), 6.53 (ddd, 4H, CH CH2 , J = 28.5, 18.2, 9.5 Hz), 5.56 (d, 2H, CHHtrans , 3 JH,H = 17.6 Hz), 5.40 (s, 4H, NCH2 ), 5.12 (d, 2H, CHHcis , 3 JH,H = 17.6 Hz).13 C NMR (101 MHz, CDCl3 ) ␦ = 138.2 (C CH CH2 ), 136.7 ( CH CH2 ), 135.5 (NCN), 132.4 ( NCH2 C ), 129.0 (aromatic), 126.5 (aromatic), 121.9 (NCCN), 115.1 ( CH CH2 ), 52.6 (NCH2 ). ESI–MS (m/z): 301.4 [M−Cl]+ . ESI−HRMS: calcd for [C21 H21 N2 ]+ 302.4128; found 302.4115. 2.4. Syntheses of nickel complexes 2.4.1. Synthesis of 7 A solution of nickelocene (1.005 g, 5.32 mmol) in 20 mL of THF was added to 6 (1.793 g, 5.32 mmol). The mixture was stirred for 5 days at ambient temperature. The solution slowly turned colour from dark green to dark red. The solvent was evaporated from the red solution, and the remaining solid was washed with hexane (3 × 20 mL) in order to remove unreacted nickelocene. The resulting residue was extracted with toluene (3 × 10 mL), and the solution was reduced in volume to about 5 mL. After standing overnight at 4 ◦ C red crystals suitable for X-ray measurements, were obtained (Fig. 2). Yield: 40.5% (0.987 g). Elemental analysis: calcd for C26 H25 N2 ClNi: C 67.94% H 5.48% N 6.09% found: C 67.75; H, 5.68; N, 6.23. EI-MS (70 eV) m/z (relative intensity) (58 Ni, 35 Cl): 458 (M+ , 59%), 423 ([M−Cl]+ , 51%), 157 ([CpNiCl]+ , 81%), (99 [CpCl]+ , 93%). EI-HRMS: calcd for C26 H25 35 ClN2 58 Ni 458.1060 found 458.1066. 1 H NMR (500 MHz, toluene-d8 , 25 ◦ C): ␦ = 7.11 (m, 4H, aromatic), 6.98 (m, 4H, aromatic), 6.53 (m, 2H, CH CH2 ), 6.15 (s, 1H NCHH ), 5.79 (bs, 2H), 5.57 (d, 2H, CH CHHtrans, 1 3J H,H = 18.5 Hz), 5.07 (s, 5H, Cp). H NMR (500 MHz, toluene-d8 , ◦ −50 C): ␦ = 7.10 (m, 8H, aromatic), 6.50 (dd, 2H, CH CH2 , J = 16.9, 11.2 Hz), 6.10 (bs, 2H, NCHH), 5.90 (d, 2H, −NCHH, J = 19.8 Hz), 5.61 (d, 2H, CH CHHtrans , 3 J H,H = 17.5 Hz), 5.53 (d, 2H, CH CHHcis , 3J 13 C NMR (101 MHz, toluene): H,H = 15.0 Hz), 5.09 (s, 5H, Cp). ␦ = 175.3 (NCN), 136.6 ( C CH CH2 ), 134.1 ( CCH CH2 ), 130.7 ( NCH2 C), 127.6 (aromatic), 126.8 (aromatic), 122.1 (NCCN), 116.4 ( CCH CH2 ), 91.7 (Cp), 55.3 (N CH2 C). 2.4.2. Synthesis of 8 and 9 A solution of nickelocene (0.891 g, 4.72 mmol) in 20 mL of THF was added to 1 (1.633 g, 7.08 mmol). The mixture was stirred for 3 days at ambient temperature. The solution slowly turned colour from dark green to dark red. After decanting the red solution, a dark green solid remained as a residue. The solvent was evaporated from the red solution, and the remaining solid was washed with hexane (3 × 20 mL) in order to remove unreacted nickelocene. The resulting residue was extracted with toluene (3 × 10 mL), and the solution was reduced in volume to about 5 mL. After standing overnight at 4 ◦ C red crystals of 8, suitable for X-ray measurements, were obtained (yield: 30.6%; 0.511 g, 1.45 mmol) (Fig. 3). The green solid was washed with THF (3 × 20 mL), dissolved in acetonitrile (20 mL) and filtered. After concentrating to about 5 mL and keeping overnight at 4 ◦ C green crystals of 9 were obtained (yield 56.9%; 1.103 g, 2.01 mmol). The crystals were not suitable for single-crystal X-ray measurements. 8: 1 H NMR (400 MHz, benzen): ␦ = 6.29 (s, 1H, NCHCHN), 6.18 (d, 1H, NCHCHN, J = 1.7 Hz), 5.72 (ddd, J = 16.1, 10.4, 5.6 Hz, 1H, CH = ), 5.24 (s, 1H, NCH2 C O), 5.21 (s, 5H, Cp), 5.04 (s, 2H, NCH2 CH = ), 4.87 (d, J = 10.3 Hz, 1H, CHHcis ), 4.76 (d, J = 17.3 Hz, 1H, CHHtrans ), 3.91 (d, J = 6.8 Hz, 2H, −OCH2 CH3 ), 0.95 (t, J = 7.1 Hz, 3H, OCH2 CH3 ). 1 H NMR (500 MHz, toluene-d , −49 ◦ C, all lines were signifi8 cantly broadened): ␦ = 6.43 (s, 1H, NCHCHN), 6.27 (d, J = 1.7 Hz, 1H, NCHCHN), 5.91 (m, 1H, CH ), 5.63 (bs, 2H, NCH2 COOCH2 CH3 ), 5.31 (dd, 1H, NCHHCH CH2 ), 5.15 (bs, 1H, NCHH CH CH2 ),
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Fig. 2. Ortep plot of 7 with atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths [Å]: Ni1−Cl1 2.2060(7), Ni1−C1 1.874(2), Ni−Cpcent 1.7553(17).
Fig. 3. Ortep plot of 8 with atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths [Å]: Ni1−Cl1 2.1976(5), Ni1−C1 1.8761(19), Ni−Cpcent 1.7545(9).
5.08 (s, 5H, Cp), 4.76 (d, J = 9.1 Hz, 1H, CHHcis ), 4.60 (d, J = 16.3 Hz, 1H, CHHtrans ), 3.75 (d, J = 6.3 Hz, 2H, OCH2 CH3 ), 0.97 (t, J = 7.8 Hz, 3H, OCH2 CH3 ).13 C NMR (101 MHz, benzen): ␦ = 169.1 (C O), 165.6 (NCN), 134.2 ( CH ), 123.9 (NCCN), 122.2 (NCCN), 118.4 ( CH2 ), 91.8 (Cp), 62.1 (NCH2 COOCH2 CH3 ), 54.5 (NCH2 ), 52.7 (NCH2 COOCH2 CH3 ), 14.2 (NCH2 COOCH2 CH3 ). Elemental analysis: calcd for C15 H19 ClN2 NiO2 C, 50.97, H, 5.42, N, 7.93. found C, 51.23, H, 5.51, N, 7.98. EI-MS (70 eV) m/z (relative intensity) (58 Ni, 35 Cl) 352
(M+ , 52%), 318 ([M−Cl]+ , 43%), 157 ([CpNiCl]+ , 76%), 99 ([CpCl]+ , 89%). EI-HRMS: calcd. for C15 H19 35 ClN2 58 NiO2 352.0488, found 352.0498. 9: 1 H NMR (400 MHz, CDCl3 , all lines significantly broadened): ␦ = 7.19 (s, 2H, NCHCHN), 6.92 (s, 2H, NCHCHN), 5.81 (bs, 2H, CH ), 5.17 (s, 5H, Cp), 5.13 (bs, 4H, -NCH2 C O), 4.92 (s, 4H, NCH2 CH ), 4.72 (bs, 2H, CHHcis ), 4.65 (bs, 2H, CHHtrans ), 3.84 (bs, 4H, OCH2 CH3 ), 0.91 (bs, 6H, −OCH2 CH3 ). 13 C NMR (101 MHz,
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Fig. 4. Ortep plot of 10 with atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths [Å]: Ni1−Cl1 2.1971(4), Ni1−C1 1.8743(13), Ni−Cpcent 1.7644(6).
Scheme 1. Synthesis of covalent and ionic nickel NHC complexes 8–16.
CDCl3 ): ␦ = 168.8 (C O), 165.1 (NCN), 134.4 ( CH ), 123.2 (NCCN), 121.4 (NCCN), 118.6 ( CH2 ), 91.2 (Cp), 61.4 (NCH2 COOCH2 CH3 ), 54.4 (NCH2 ), 52.1 (NCH2 COOCH2 CH3 ), 14.6 (NCH2 COOCH2 CH3 ). EI-HRMS: calcd for C25 H33 N4 58 NiO4 511.1855, found 511.1873. 2.4.3. Synthesis of 10 and 11 The reaction was carried out analogously to the previous one using a solution of nickelocene (1.294 g, 6.85 mmol) in 20 mL of THF and 3 (2.083 g, 10.27 mmol). Red crystals of 10 suitable for Xray measurements were obtained. Yield: 18.6% (0.415 g, 1.28 mmol) (Fig. 4). The green crystals of 11 (yield 43.4%; 1.097 g, 2.23 mmol) were not suitable for single-crystal X-ray measurements. 10: 1 H NMR (500 MHz, toluene-d8 , 25 ◦ C): ␦ = 6.51 (d, J = 1.8 Hz, 1H, NCHNCH), 6.23 (d, J = 1.7 Hz, 1H, NCHNCH), 5.84 (s, 2H, O CH2 N), 5.79 (m, 1H, NCH2 CH CH2 ), 5.07 (s, 5H, Cp), 4.90 (dd, J = 10.2, 1.0 Hz, 1H, NCH2 CH CHHcis ), 4.79 (d, J = 17.1 Hz, 1H, NCH2 CH CHHtrans ), 3.53 (dd, J = 13.6, 6.7 Hz, 2H, O CH2 CH3 ), 1.05 (t, J = 7.0 Hz, 3H, O CH2 CH3 ). 1 H NMR (500 MHz, toluened8 , −50 ◦ C): ␦ = 6.63 (d, J = 1.8 Hz, 1H CHCH), 6.26 (d, J = 1.5 Hz, 1H, CHCH), 5.78 (dd, J = 17.3, 7.1 Hz, 2H, O CH2 -N), 5.73 (m, 1H, NCH2 CH CH2 ), 5.50 (dd, J = 15.3, 5.0 Hz, 1H, NCHH CH CH2 ),
5.14 (s, 5H, Cp), 4.89 (bs, 1H, NCHH CH CH2 ), 4.88 (d, J = 8.4 Hz, 1H, NCH2 CH CHHcis ), 4.71 (d, J = 17.0 Hz, 1H, NCH2 CH CHHtrans ), 3.55 (ddd, J = 22.4, 15.0, 7.9 Hz, 2H, O CH2 CH3 ), 0.99 (m, 3H, O CH2 CH3 ). 13 C NMR (100 MHz, toluene-d8 , 25 ◦ C): ␦ = 168.2 (NCN), 134.7 ( CH ), 125.1 (NCCN), 124.2 (NCCN), 117.2 ( CH2 ), 92.5 (Cp), 89.1 (CH3 CH2 OCH2 -), 64.1 (CH3 CH2 OCH2 -), 52.2 (NCH2 -), 17.5 (CH3 CH2 OCH2 ). Elemental analysis: calcd for C14 H19 ClN2 NiO: C, 51.67; H, 5.88; N, 8.61. found: C, 51.89; H, 6.06; N, 8.76. EI-MS (70 eV) m/z (relative intensity) (58 Ni, 35 Cl) 324 (M+ , 59%), 289 ([M−Cl]+ , 37%), 157 ([CpNiCl]+ , 82%), 99([CpCl]+ , 91%). EI-HRMS: calcd for C14 H19 35 ClN2 58 NiO 324.0540, found 324.0529. 11: 1 H NMR (400 MHz, CDCl3 , all lines significantly broadened): ı 6.81 (bs, 2H, NCHNCH), 6.52 (bs, 2H, NCHNCH), 5.62 (bs, 4H, O CH2 N), 5.51 (m, 2H, NCH2 CH CH2 ), 5.19 (s, 5H, Cp), 4.82 (bs, 2H, NCH2 CH CHHcis ), 4.69 (bs, 2H, NCH2 CH CHHtrans ), 3.32 (bs, 4H, O CH2 CH3 ), 1.18 (bs, 6H, O CH2 CH3 ). 13 C NMR (100 MHz, CDCl3 ): ␦ = 166.7 (NCN), 139.8 ( CH ), 124.5 (NCCN), 123.6 (NCCN), 118.3 ( CH2 ), 91.6 (Cp), 88.3 (CH3 CH2 OCH2 -), 62.5 (CH3 CH2 OCH2 ), 56.1 (NCH2 ), 16.1 (CH3 CH2 OCH2 ). ESI-HRMS: C23 H33 N4 58 NiO2 455.1957; found 455.1948.
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2.4.4. Synthesis of 12–16 The complexes 12–16 were synthesized in reactions of nickelocene with an appropriate imidazolium salt (2 for 12 and 13, 4 for 14 and 15, and 5 for 16) (Scheme 3). The reactions were carried out analogously to those for synthesis of 8, 9, 10 and 11. The reactions of 2 and 4 led in each case to the formation of two complexes: the covalent, red solids 12 and 14, and the ionic, green ones 13 and 15 respectively. The reaction of nickelocene with 5 produced only one product, the ionic, green complex 16. Although separation of the complexes was easy due to differences in their solubility, crystallization from various solvents gave no crystals suitable for single-crystal X-ray measurements. Also quality of NMR spectra was not fully satisfactory. The main method of the complexes identification was high resolution mass spectrometry. 2.4.5. Synthesis of 12 and 13 The reaction was carried out analogously to the previous one using a solution of nickelocene (1.123 g, 5.95 mmol) in 20 mL of THF and 2 (1.789 g, 8.92 mmol). Yield of 12 18.9% (0.365 g, 1.13 mmol), yield of 13 43.1% (0.937 g, 1.92 mmol). Crystals of 12 and 13 were not suitable for single-crystal X-ray measurements.12: 1 H NMR (400 MHz, benzene) ␦ = 6.72 (s, 1H, NCHCHN), 6.42 (s, 1H, NCHCHN), 5.43 (m, 1H, CH ), 5.30 (s, 5H, Cp), 5.23 (s, 2H, NCH2 O), 5.15 (d, 1H, CHHcis 3 JH,H = 5.9 Hz), 4.92 (d, 1H, CHHtrans , 3J H,H = 14.7 Hz), 4.62 (d, 2H, NCH2 ), 1.89 (m, 3H, />CH3 ).EI-HRMS: calcd for C14 H17 35 ClN2 58 NiO 322.0383, found 322.0365. 13: 1 H NMR (400 MHz, CDCl3 , all lines significantly broadened) ␦ = 6.54 (bs, 1H, NCHCHN), 6.38 (bs, 2H, NCHCHN), 5.52 (m, 2H, CH ), 5.14 (s, 5H, Cp), 5.06 (bs, 2H, NCH2 O), 4.87 (bs, 2H, CHHcis ), 4.72 (d, 2H, CHHtrans ), 4.43 (d, 2H, NCH2 ), 1.71 (m, 3H, −CH3 ).ESI-HRMS: calcd. for (C23 H29 N4 58 NiO2 )+ 451.1644; found 451.1629. 2.4.6. Synthesis of 14 and 15 The reaction was carried out analogously to the previous one using a solution of nickelocene (0.951 g, 5.04 mmol) in 20 mL of THF and 4 (1.530 g, 7.55 mmol). Yield of 14 37.5% (0.615 g, 1.89 mmol), yield of 15 43.3%; (0.804 g, 1.64 mmol). Crystals of 14 and 15 were not suitable for single-crystal X-ray measurements.14: 1 H NMR (400 MHz, benzen): ␦ = 6.76 (s, 1H, NCHCHN), 6.33 (d, 1H, NCHCHN, J = 1.7 Hz), 5.91 (ddd, J = 16.0, 10.2, 5.8 Hz, 1H, −CH = ), 5.53 (bs, 2H, NCH2 -CH = ),5.26 (s, 5H, Cp), 4.94 (d, J = 10.3 Hz, 1H, CHHcis ), 4.68 (d, J = 17.3 Hz, 1H, CHHtrans ), 3.67 (bs, 2H, NCH2 CH2 O CH3 ), 3.07 (t, J = 6.1 Hz, 3H, OCH3 ), 1.62 (q, 2H, NCH2 CH2 O CH3 ). EIHRMS: calcd for C14 H19 35 ClN2 58 NiO 324.0540, found 324.0529. 15: 1 H NMR (400 MHz, CDCl , all lines significantly broadened): ␦ = 6.92 3 (bs, 2H, NCHCHN), 6.52 (2H, NCHCHN), 5.87 (ddd, 2H, CH), 5.62 (bs, 4H, NCH2 -CH), 5.19 (s, 5H, Cp), 4.82 (bs, 2H, CHHcis ), 4.60 (bs, 2H, CHHtrans ), 3.92 (bs, 4H, −NCH2 CH2 O CH3 ), 3.28 (bs, 6H, OCH3 ), 1.62 (bs, 4H, NCH2 CH2 O CH3 ). ESI-HRMS: calcd for (C23 H33 N4 58 NiO2 )+ 455.1957; found 455.1949. 2.4.7. Synthesis of 16 A solution of nickelocene (1.221 g, 6.47 mmol) in 20 mL of THF was added to 5 (1.830 g, 9.70 mmol). In this case the only one product was formed, the green solid. Yield 79,0% (1.776 g, 3.83 mmol). 16: 1 H NMR (400 MHz, CDCl3 , lines significantly broadened): ␦ = 6.82 (bs, 2H, NCHCHN), 6.59 (bs, 2H, NCHCHN), 5.72 (m, 2H, CH), 5.38 (bs, 2H, OH ), 5.31 (s, 5H, Cp), 5.23 (bs, 2H, CHHtrans ), 5.02 (m, 2H, CHHcis ), 4.80 (bs, NCH2 CH2 OH, 4H), 4.61 (bs, 4H, NCH2 CH2 OH). ESI-HRMS: calcd for (C21 H29 N4 58 NiO2 )+ 427.1644; found 427.1628. 2.4.8. Synthesis of 17 55 mg (0.3 mmol) of potassium hexafluorophosphate was added to a solution of the complex CpNi(NHCallyl )Br [29] (74 mg, 0.21 mmol) in dichloromethane (5 mL). The suspension was stirred
overnight and then filtered. The solvent was evaporated under reduced pressure and the complex 17 was obtained as a dark red solid which was recrystallized from a mixture of diethyl ether and dichloromethane. Yield: 49 mg (56%) (Fig. 5). 17: 1 H NMR (400 MHz, CD3 CN): ␦ = 7.21 (bd, 1H, NCHCHN), 7.02 (bd, 1H, NCHCHN), 6.09 (m,1H, NCH2 CH CH2 , not coordinated), 5.92 (m, 1H, NCH2 CH CH2 , coordinated), 5.82 (s, 5H,C5 H5 ), 5.31 (m, 1H, NCH2 CH CHHcis , not coordinated, 2 JHH = 8.4), 5.04 (m, 1H, NCH2 CH CHHtrans , not coordinated, 2 JHH = 15.2), 4.87 (d, 1H, NCHHCH CH2 , coordinated, 3 JHH = 8.2), 4.54 (d, 1H, NCHHCH CH2 , not coordinated), 4.42 (m, 1H, NCHHCH CH2 , not coordinated), 4.35 (m, 1H, NCHHCH CH2 , not coordinated), 3.81 (m, 1H, NCH2 CH CHHcis , coordinated, 2 JHH = 8.5), 3.34 (m, 1H, NCH2 CH CHHtrans , coordinated, 2 JHH = 16.4).13 C NMR (101 MHz, CD3 CN): ␦ = 163.1 (NCN), 136.8 (-CH = not coordinated), 124.8 (NCCN), 124.3 (NCCN), 123.7 ( CH2 not coordinated), 95.3 (C5 H5 ), 83.4 (-CH = coordinated), 55.1 ( CH2 coordinated), 52.9 (NCH2 not coordinated), 50.2 (NCH2 not coordinated). EI-HRMS: calcd. for (C14 H17 N2 Ni)+ 271.0745, found 271.0727.
2.5. Procedure for Suzuki-Miyaura coupling A Schlenk tube was charged with 4-bromoacetophenone (0.337 g, 1.69 mmol, 1 eq.), phenylboronic acid (0.227 g, 1.87 mmol, 1.1 eq.), K3 PO4 (0.791 g, 3.73 g, 2.2 eq.) and the catalyst (3 mol%). Toluene was injected (7 mL) and the mixture immediately heated with vigorous stirring in an oil bath at 90 ◦ C. After 2 h, the reaction was stopped by cooling the flask to room temperature. The reaction mixture was washed with water and the toluene layer was separated. The water layer was extracted with diethyl ether. The organic layers were then combined together, dried over Na2 SO4 and solvents were evaporated under reduced pressure. Raw product was purified by silica gel column chromatography (hexane/EtOAc,10:1) to give the isolated coupling product. Conversion and selectivity were determined by gas chromatography using tetradecane as an internal standard. 4-acetylbiphenyl: 1 H NMR 400 MHz, CDCl3): ␦(ppm) 8.03(d, J = 9.6 Hz, 2H),7.68 (dt, J = 9.0, 1.8 Hz, 2H), 7.65(d, J = 8.0, 2H), 7.48(t, J = 7.4 Hz, 2H), 7.44(tt, J = 7.5, 1.5 Hz, 1H), 2.68 (s, 3H). 13 C NMR (125 MHz, CDCl3): ␦(ppm) 197.4, 145.3, 139.1, 135.7, 128.4, 128.5, 128.6, 127.5, 127.2, 26.3.
2.6. Procedure for carbene polymerization A solution of MAO (17.5 mL 10 wt.% in toluene, 27.8 mmol) was added to a solution of the catalyst (7 or CpNi(IMes)Cl) (0.09 mmol) in toluene (14 mL). The colour of the reaction mixture turned from red to light brown. The resulting catalytic system was activated by stirring at room temperature for 30 min. The solution of EDA (ethyl diazoacetate) (0.32 g, 2.8 mmol, 2.4 mL 15% solution in toluene) was then added, and the mixture was stirred for 3 h at room temperature. The reaction was terminated by treatment with 9% aqueous hydrochloric acid solution. The organic layers were separated and poured into an excess of methanol. The resulting dark brown oils were separated, washed with cold toluene and dried under reduced pressure. Yields: 47% – cat. 7; 58% – cat. [CpNi(IMes)Cl]. Oligomers obtained with 7 as the catalyst: elemental analysis: found N 6.14/6.16, C 70.23/70.38, H 10.55/10.68. 1 H NMR (400 MHz, CDCl3 ): ␦ = 7.04 (m, 4H, CH, aromatic), 6.35 (s, 2H, CHCH), 3.15 (m, −CH2 CH3 ), 2.29 (m, CHCOOCH2 CH3 ), 1.28 (bs, 3H, CH3 ), 0.98 (m, CH2 CH3 ). 13 C NMR (100 MHz; CDCl3 ): ␦ (ppm) = 11.3–13.7 (–OCH2 CH3 ), 39.1–48.6 (CH), 59.9–61.1 (–OCH2 CH3 ), 166.3–174.9 (C O). Oligomers obtained with CpNi(IMes)Cl as the catalyst: elemental analysis: found N 5.61/5.51, C 67.22/67.01, H 10.39/10.34.
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Fig. 5. Ortep plot of 17 with atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths [Å]: Ni1−C1 1.8793(19), Ni1−C11 2.024(3), Ni1−C12 2.027(2), Ni−Cpcent 1.7246(10), C11–C12 1.380(4).
3. Results and discussion 3.1. Synthesis of nickel complexes Imidazolium chloride, with allyl substituents attached to nitrogen atoms in the NHC ring, reacts with nickelocene in a different manner than imidazolium bromide. We have noticed before [32] that two products, covalent CpNi(NHCallyl )Cl and biscarbene ionic [CpNi(NHCallyl )2 ]+ Cl− , were formed in the reaction of 1,3-diallyl imidazololium chloride with nikelocene. On the other hand, only one product, covalent CpNi(NHCallyl )Br, was formed in the reaction of 1,3-diallyl imidazolium bromide with nickelocene [29]. Unsymmetrically substituted imidazolium chlorides (1–4) reacted with nickelocene analogously to symmetrical diallyl imidazolium chloride. One carbene nitrogen atom was substituted with the allyl group, and another one with various oxygen-containing substituents. The idea was to enable the hemilabile coordination of the oxygen atom to nickel, to prepare ionic complexes analogous to those with coordination via the allyl group double bond (e.g. 17 and [Cp’Ni(NHCallyl )]+ Br− [32]). Unfortunately, we did not observe such coordination. Reactions proceeded according to the equation shown in Scheme 1. Two types of complexes were formed in these reactions: covalent, red solids, soluble in toluene (type A, 8, 10, 12, 14) and ionic, green solids, soluble in acetonitrile (type B, 9, 11, 13, 15). Only one product (type B, 16) was formed for a CH2 CH2 OH substituent at one nitrogen atom. The separation of the complexes from the reaction mixtures was easy, due to differences in their solubility. However, complicated NMR spectra made their characterization difficult. Only complexes 8–11 were fully characterized by elemental analysis, 1 H, 13 C NMR, and high resolution mass spectrometry. 1 H NMR spectra of covalent complexes 8 and 10 exhibited the same dynamic behaviour in solution as before. The signals of N−CH2 − protons were extremely broad and their non-equivalence was not visible even at low temperatures. This dynamic behaviour has been explained previously with the help of DFT calculations [32].
Complexes 7, 8, and 10 are structurally similar to those we have described previously [29,32]. The molecular structures of 7, 8, and 10 do not differ significantly from each other with regard to the coordination mode and geometry. The geometry of these complexes is trigonal planar at the nickel centre considering Cpcent. , the carbene carbon, and the chlorine atoms. The Ni Ccarbene and Ni Cpcent distances measure 1.874(3) and 1.754(1) Å, 1.877(2) and 1.755(1) Å, 1.874(2) and 1.765(1) Å for 7, 8, and 10, respectively, and are similar to the corresponding distances described in the literature for other [NiCp(NHC)X] complexes.
3.2. Suzuki–Miyaura cross-coupling reactions The catalytic activity of cyclopentadienylnickel NHC complexes in the Suzuki cross-coupling was tested in a model reaction of 4bromoacetophenone with phenylboronic acid (Scheme 2). K3 PO4 was used as the base. Cyclopentadienylnickel complexes with NHC ligands having allyl substituents at both the N atoms (NHCallyl ) were tested as catalysts in the first part of the studies. The syntheses and structures of most of these complexes have been published previously [29,32,66]. The complexes used as catalysts are depicted in Scheme 3. The reactions were carried out in toluene, using 3 mol% of the catalyst and the substrates (4-bromoacetophenone: phenylboronic acid: K3 PO4 ) at a 1:1.1:2.2 molar ratio. Small amounts of other solvents (1.5 mL) were added to improve catalyst solubility while using ionic complexes (Table 1, entry 3–acetonitrile; entry 7–THF; entry 8–CH2 Cl2 ). The results are shown in Table 1. The main product, 4-acetylbiphenyl, was formed in all experiments with high selectivity (in most cases between 96 and 97%). The formation of small amounts of 4,4 -diacetylbiphenyl, the product of 4-bromoacetophenone homocoupling, was observed in all reactions. It can be concluded that the catalyst has no substantial influence on the selectivity of the reaction. On the contrary, the con-
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Scheme 2. Suzuki–Miyaura cross-coupling of phenylboronic acid with 4-bromoacetophenone.
Scheme 3. Complexes used as catalysts in Suzuki–Miyaura reactions.
Table 1 Conversion, yield, and selectivity in the Suzuki–Miyaura coupling reaction catalysed by nickel NHCallyl complexes.
1 2 3 4 5 6 7 8 a
Catalyst
Time [min]
Conversion [%]
Yield [%]a
Selectivity [%]
CpNi(NHCallyl )Br [29] CpNi(NHCallyl )Br [29] [CpNi(NHCallyl )2 ]+ Cl− [32] CpNi(NHCallyl )Cl [32] Cp’Ni(NHCallyl )Cl [68] Cp’Ni(NHCallyl )Br [32] [Cp’Ni(NHCallyl )]+ Br− [32] [CpNi(NHCallyl )]+ PF6 − 17
60 120 120 120 120 120 120 120
40 51 30 27 38 65 79 54
32 42 22 19 30 55 70 48
97.0 96.9 89.8 96.6 96.4 97.2 96.9 92.5
Isolated yield of 4-acetylbiphenyl.
version strongly depends on the kind of catalyst used. The analysis of the results shown in Table 1 led to the following conclusions: – ionic complexes are more active catalysts than their covalent analogues (entry 7 vs. 6); – increasing electron density in the cyclopentadienyl ligand by substituting it with the methyl group increases the catalytic activity of complexes (entries 5 vs. 4 and 6 vs. 2); – bromide complexes are better catalysts than their chloride analogues (entries 2 vs. 4 and 6 vs. 5). This shows that bromide is a better leaving group than chloride, which facilitates ionization. most active catalyst, the ionic complex The [Cp’Ni(NHCallyl )]+ Br− , was then used to determine the dependence of conversion and selectivity on the reaction time. Samples were taken at specified intervals and then analysed (Fig. 6).
Table 2 Conversion and selectivity dependence on temperature and solvents in the Suzuki–Miyaura coupling reaction. Time [min]
Temperature [◦ C]
Solvent
Conversion [%]
Selectivity [%]
120 120 120 120
20 90 reflux 90
toluene/MeCN toluene/MeCN toluene/MeCN H2 O/DMF
9 79 22 –
93.5 96.9 94.1 –
Catalyst Cp’Ni(NHCallyl )]+ Br&minus
The reaction was carried out in toluene/THF mixture. The reaction reached the desired temperature (90 ◦ C) after 10 min. At that time, conversion was 11%, which may indicate that the catalyst was already active below that temperature. After 2 h, conversion reached 78%, and then its increase was very slow, reaching 85% after 24 h. From the economic point of view, it seems that 2 h is the optimal reaction time. On the other hand, selectivity does not depend on the reaction time. The same catalyst was used to study the influence of temperature on conversion and selectivity (Table 2). Temperature increase from 90 ◦ C to about 115 ◦ C (reflux) substantially reduced the level of conversion. The reason for it can be the decomposition of the catalyst or its deactivation. There were no coupling temperature
Table 3 Conversion, yield, and selectivity in the Suzuki–Miyaura coupling reaction catalysed by complexes 8–16.
1 2 3 4 5 6 7 8 9 a
Catalyst
Conversion.[%]
Yield[%]a
Selectivity[%]
8 9 10 11 12 13 14 15 16
10.5 22.8 22.6 27.3 29.3 21.2 11.5 30.4 36.7
7.3 15.0 15.7 20.9 21.0 17.1 7.9 23.0 29.5
99.4 99.7 99.7 99.9 99.8 99.7 99.9 99.9 99.8
Isolated yield of 4-acetylbiphenyl.
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Fig. 6. Conversion and selectivity dependence on the reaction time in the Suzuki–Miyaura coupling reaction.
Scheme 4. Carbene oligomerization catalysed by CpNi(NHC)Cl/MAO.
and solvents products in the reaction carried out in a mixture of water and DMF. The only product detected was the cyclic trimer of phenylboronic acid. In a reaction carried out at ambient temperature for 120 min the conversion reached only 9%. In the second part of this work, complexes 8–16 were tested in the Suzuki–Miyaura coupling reaction, the same as above (Scheme 1). The reactions were carried out at conditions described above (toluene at 90 ◦ C, 2 h, 3 mol% of the catalyst, 1:1.1:2.2 molar ratio of the substrates). The results are shown in Table 3. Although the conversion rates were rather disappointing, selectivity was almost 100%. Only traces of the homocoupling products (4,4 -diacetylbiphenyl or biphenyl) were detected in some reactions. The results shown in Table 3 indicate that ionic complexes are more active than covalent ones containing the same NHC ligands (9 vs. 8, 11 vs. 10, 13 vs. 12, 15 vs. 14). A comparison of the results from Table 3 with those from Table 1 (entries 3 and 4) made it possible to conclude that substituents at the nitrogen atoms of NHC ligands had no substantial influence on the catalytic activity of the studied complexes in the Suzuki–Miyaura coupling reaction. A recently published comprehensive paper [63] indicates that some unsymmetrical NCN pincertype Ni(II) complexes with NHC-triazole arms showed a higher catalytic activity than our unsymmetrical complexes 8–16. The authors noticed a strong influ-
ence of the base used, and the best results were obtained with K2 CO3 (Table 2, entry 10). On the other hand, with K3 PO4 as the base, at conditions quite similar to ours (4-bromoacetophenone: phenylboronic acid: K3 PO4 = 1:1.2:2.4; Ni complex – 2 mol%; 110 ◦ C in toluene, reaction time 2 h), the best catalyst described in that paper (compound 4, Table 2, entry 3) exhibited the same activity (78%) as our best catalyst (compound [Cp’Ni(NHCallyl )]+ Br− , Table 1, entry 7, conversion 79%).
3.3. Carbene oligomerization In our previous paper, we showed that cyclopentadienylnickel(NHC) complexes could be used as initiators in the oligomerization of ethyl acetate carbene derived from ethyl diazoacetate [29]. In that paper we noted, for the first time, that N-heterocyclic carbene from the catalyst precursor was inserted into the oligomer chain. That was a new and unexpected observation. Here, we show that this is a more general phenomenon, and even NHC carbenes bearing bulky substituents at nitrogen atoms can be incorporated into oligomer chains. We carried out the oligomerization of ethyl acetate carbene using CpNi(NHC)Cl complexes activated with methylalumoxane (MAO) as initiators (Scheme 4).
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Table 4 Carbene oligomerization with 7 and CpNi(IMes)Cl as catalysts.
1 2 3 4 5 6 7 8
Catalyst
Solvent
Temperature[◦ C]
Molar ratioNi:EDA:MAO
Yield [%]
Mw
Mw /Mn
7 7 7 7 CpNi(IMes)Cl CpNi(IMes)Cl CpNi(IMes)Cl CpNi(IMes)Cl
Toluene Toluene Toluene THF Toluene Toluene Toluene THF
RT 60 RT RT RT 60 RT RT
1:30:300 1:30:300 1:30:50 1:30:300 1:30:300 1:30:300 1:30:50 1:30:300
47 43 32 56 58 48 39 54
700 654 457 510 710 632 490 610
1.27 1.34 1.87 1.53 1.29 1.76 1.64 1.42
Two complexes, with sterically demanding substituents at the nitrogen atoms of the NHC ligand, were used as initiators: the well-known CpNi(IMes)Cl [65] and complex 7 synthesized and characterized in this work. The complexes were activated with an excess of MAO for 30 min at room temperature, and then EDA was added. The reactions were quenched after 3 h of vigorous stirring. Oligomers were formed in moderate yields, with the average molecular weight of about 700 Da and low polydispersity (about 1.3) (Table 4). MALDI TOF MS, 1H NMR, and elemental analysis proved the presence of NHC carbene in the oligomer chain, but it was difficult to define where exactly it was located. We studied the effects of temperature, the solvent, and the Ni:EDA:MAO molar ratio. The best results were obtained in toluene as solvent, at room temperature, and at a Ni:EDA:MAO molar ratio of 1:30:300. Decreasing the MAO:Ni molar ratio from 300:1 to 50:1 resulted in the non-insertion of NHC carbene into the oligomer chain. A temperature increase to 60 ◦ C resulted in a slight decrease in the yield and the molecular weight of oligomers.
4. Conclusions
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
Several new cyclopentadienylnickel complexes with symmetrically and unsymmetrically substituted N-heterocyclic carbenes were synthesized and characterized. These complexes were applied as catalysts in the Suzuki–Miyaura cross-coupling reaction of 4bromoacetphenone with phenylboronic acid. All complexes were active catalysts with reactant conversion rates from 10 to 80% and very high selectivity, but the best results were obtained using a bromide catalyst, on which we published previously [32]. The catalytic activity of the complexes strongly depends on their composition: ionic complexes are more active than their covalent analogues; complexes with increased electron density in the cyclopentadienyl ligand are more active catalysts; bromide complexes are better catalysts than their chloride analogues. Two CpNi(NHC)Cl complexes were tested as initiators in the oligomerization of ethyl acetate carbene to confirm our previous observation that N-heterocyclic carbene from the catalyst precursor was inserted into the oligomer chain.
Acknowledgements This work was financially supported by the National Science Centre (grant no. UMO-2014/15/N/ST5/02006).
[22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mcat.2017.06. 033. The crystal data and experimental parameters, bond lengths and bond angles for the compounds 7, 8, 10 and 17; MALDI-TOF spectra of obtained oligomers.
[40] [41] [42] [43] [44] [45]
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Synthesis, characterization and catalytic activity of nickel NHC complexes Agata Włodarska ∗ , Jowita Gołaszewska-Gajda, Maciej Dranka, Antoni Pietrzykowski Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664 Warsaw, Poland
a r t i c l e
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Article history: Received 27 March 2017 Received in revised form 5 June 2017 Accepted 26 June 2017 Keywords: Nickel NHC carbenes Suzuki–Miyaura coupling Oligomerization Ethyl diazoacetate
a b s t r a c t Several new cyclopentadienylnickel complexes were synthesized and characterized. All the complexes exhibited catalytic activity in Suzuki–Miyaura cross-coupling reaction with conversion rates from 10 to 80% and very high selectivity. The catalytic activity of the complexes strongly depended on their composition and structure. It was shown that ionic complexes are better catalysts than their covalent analogues; increased electron density in the cyclopentadienyl ligand improved their catalytic activity; bromide complexes provided better results than chloride ones. Two CpNi(NHC)Cl complexes were tested as initiators in the oligomerization of ethyl acetate carbene confirming our earlier observation that N-heterocyclic carbene from the catalyst precursor was incorporated into the oligomer chain. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The isolation of the first stable crystalline carbene by Arduengo in 1991 [1] initiated a rapid development of the chemistry of Nheterocyclic carbenes, and they have become an important class of ligands in organometallic chemistry [2–17]. A part of this success is due to the electron donor ability of NHC ligands, which allows them to form very strong bonds with most metals (stronger than phosphines), and also due to electronic and structural diversity. Current research is not only focused on the chemistry of metal complexes with NHC ligands but also on their applications in transition-metalbased catalyst systems. Among many transition-metal NHC complexes, nickel ones are currently gaining interest as they exhibit interesting properties and good catalytic activity. A number of NHC nickel complexes, both covalent [18–33] and ionic [23,32,34–42], have been synthesized and characterized. Significant interest has been drawn to their application as catalysts in various organic reactions, such as olefin polymerization [22,25,29,31], carbene polymerization and oligomerization [43–49], dehalogenation and aryl amination [20,28], carbonyl hydrosilylation [50,51], acyclic ketone arylation [52], and carbon-carbon bond formation [41,53–63]. In this paper, we report the synthesis and characterization of several NHC cyclopentadienylnickel complexes unsymmetrically
∗ Corresponding author. E-mail address: [email protected] (A. Włodarska). http://dx.doi.org/10.1016/j.mcat.2017.06.033 2468-8231/© 2017 Elsevier B.V. All rights reserved.
substituted at nitrogen atoms of the NHC ring. The allyl group was the substituent at one carbene nitrogen atom, and various oxygen-containing substituents (ester, ether, ketone, and alcohol) were attached to another nitrogen. The idea was to enable hemilabile coordination of an oxygen atom to nickel, to prepare ionic complexes analogous to those with coordination via an allyl group double bond (e.g. [Cp’Ni(NHCallyl )]+ Br− [32]). Unfortunately, we did not observe such coordination. These complexes, and also those, previously described, symmetrically allyl-substituted at nitrogen [29,32], were applied as catalysts in Suzuki–Miyaura cross-coupling reactions. The syntheses of other unsymmetrical nickel NHC complexes and their applications in Suzuki–Miyaura reactions have been recently published [63]. The second part of this paper is devoted to the oligomerization of ethyl acetate carbene derived from ethyl diazoacetate. We have previously shown that N-heterocyclic carbene from the catalyst precursor is incorporated into the oligomer chain [29]. In this paper, we prove that this is a more general phenomenon, and even NHC carbenes with bulky substituents at nitrogen atoms can be inserted into oligomer chains. 2. Experimental 2.1. General information All manipulations were carried out using standard Schlenk techniques under argon atmosphere. Toluene, THF, hexane, acetonitrile, and dichloromethane were dried over potassium, KOH, CaH2 , and
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Fig. 1. Imidazolium salts used for synthesis of nickel NHC complexes.
P2 O5 , respectively, and distilled under argon prior to use. Nickelocene [64], 1-allylimidazole [65], 1-(4-vinylbenzyl)imidazole [66], CpNi(IMes)Cl [67], and cyclopentadienylnickel complexes with N-allyl-substituted NHC ligands [29,32,68] were synthesized according to the literature procedures. All other chemicals were purchased from commercial suppliers and used without further purification. NMR spectra were recorded at ambient temperature on a Mercury-400BB spectrometer (400 MHz for 1 H and 101 MHz for 13 C) and on a 500 MHz Varian NMR System. EI (70 eV) mass spectra were recorded on an AMD-604 spectrometer. ESI mass spectra were recorded on a Waters Maldi SYNAPT G2-S HDMS. Average molecular weights were measured on a LabAlliance liquid chromatograph on a Jordi Gel u VB Mixed Bed column (250 mm × 10 m) using CH2 Cl2 as the mobile phase at 30 ◦ C and calibrated with standard PMMAs. MALDI-TOF MS spectra were acquired with a Bruker Daltonics UltrafleXtremeTM mass spectrometer (matrix HABA). Elemental analyses were recorded on an Elementar Vario EL III analyser. 2.2. Crystal structure determination A selected single crystal was mounted in inert oil and transferred to the cold gas stream of the diffractometer. Diffraction data were measured at 120.0(1) K with graphite-monochromated MoK␣ radiation ( = 0.71073) on an Oxford Diffraction -CCD Gemini A Ultra diffractometer. Cell refinement and data collection as well as data reduction and analysis were performed with the crysalispro software [69]. The structure was solved by direct methods using the shelxT [70] structure solution program and refined by full-matrix least squares against F2 with the shelxl-2015 [71] and olex2 [72] programs. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were added to the structure model at geometrically idealized coordinates and refined as riding atoms. Crystals of 17 were found to be non-merohedrically twinned, and the hklf 5 procedure was used for refinement. Two domains with the ratio of 0.7017(8):0.2983(8) in the final refine-
ment were related by a twofold rotation around the [001] direction. The crystal data and experimental parameters are summarized in Table S1. CCDC1518513-CCDC1518516 (17, 10, 7, 8) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data request/cif. 2.3. Syntheses of imidazolium salts Synthesised and characterized imidazolium salts are depicted on Fig. 1: 2.3.1. Synthesis of 1 Ethyl chloroacetate (2.3 mL, 18.5 mmol) was added to 1allylimidazole (2.0 g, 18.5 mmol) in acetonitrile (20 mL). The solution was refluxed for 12 h. The solvent was evaporated under reduced pressure. The resulting light yellow oil was washed three times with THF (20 mL) and dried under reduced pressure. Yield 73% (3.125 g). 1 H NMR (400 MHz, CDCl3 ) ␦ = 10.54 (s, 1H, NCHN), 7.72 (s, 1H,NCHCHN), 7.43 (s, 1H, NCHCHN), 5.98 (m, 1H, CH ), 5.51 (s, 2H,NCH2 O), 5.44 (d, 1H, CHHtrans ), 5.40 (d, 1H, CHHcis 3 JH,H = 9.8 Hz), 4.95 (d, 2H, NCH2 , 2 JH,H = 6.3 Hz,), 4.20 (q, 2H, CH2 CH3 , 3 JH,H = 7.1 Hz), 1.24 (m, 3H, CH2 CH3 ).13 C NMR (101 MHz, CDCl3 ) ␦ = 181.3 (C O), 138.6 (NCN), 132.9 (NCCN), 129.5 (NCCN), 120.9 ( CH ), 118.3 ( CH2 ), 66.1 (CH3 CH2 O ), 64.2 (C(O) CH2 N), 53.6 (NCH2 ), 16.8 (CH3 CH2 O ). ESI–MS (m/z): 195.24 [M−Cl]+ . ESI-HRMS: calcd for [C10 H15 N2 O2 ]+ 195.2384; found 195.2379. 2.3.2. Synthesis of 2 Chloroacetone (2.5 mL, 18.5 mmol) was added to 1allylimidazole (2.0 g, 18.5 mmol) in acetonitrile (20 mL). The solution was refluxed for 20 h. The solvent was evaporated under reduced pressure. The resulting light yellow oil was washed three times with THF (20 mL) and dried under reduced pressure. Yield 84% (3.125 g). 1 H NMR (400 MHz, CDCl3 ) ␦ = 9.84 (s, 1H, NCHN),
A. Włodarska et al. / Molecular Catalysis 440 (2017) 25–35
7.75 (s, 1H, NCHCHN), 7.33 (s, 1H, NCHCHN), 5.85 (m, 1H, CH ), 5.65 (s, 2H, NCH2 O), 5.29 (d, 1H, CHHcis 3 JH,H = 6.1 Hz), 5.26 (d, 1H, CHHtrans , 3 JH,H = 13.1 Hz), 4.80 (d, 2H, NCH2 , 2 JH,H = 6.3 Hz), 2.19 (m, 3H, CH3 ).13 C NMR (101 MHz, CDCl3 ) ␦ = 198.4 (C O), 136.6 (NCN), 128.9 (NCCN), 123.5 (NCCN), 121.2 ( CH ), 120.3 ( CH2 ), 57.2 (C(O) CH2 N), 51.3 (NCH2 ), 26.3 (CH3 C(O)). ESI–MS (m/z): 165.2 [M−Cl]+ . ESI-HRMS: calcd for [C9 H13 N2 O]+ 165.1028; found 165.1047. 2.3.3. Synthesis of 3 Chloromethyl ethyl ether (2.16 mL, 22.2 mmol) was added in portions to a suspension of 1-allylimidazole (2.0 g, 18.5 mmol) in THF (2 mL). The reaction was exothermic, so the reaction flask was cooled with ice. The product appeared immediately as dark brown oil, immiscible with THF. The solvent was evaporated under reduced pressure. The resulting oil was washed three times with THF (20 mL) and dried under reduced pressure. Yield 68% (2.545 g). 1 H NMR (400 MHz, CDCl3 ): ␦ = 10.97 (s, 1H, NCHN), 7.54 (s, 1H, NCHCHN), 7.42 (s, 1H, NCHCHN), 6.03 (m, 1H, CH ), 5.82 (s, 2H, NCH2 O), 5.48 (d, 1H, CHHtrans ), 5.32 (d, 1H, CHHcis , 3J 2 H,H = 9.8 Hz), 4.82 (d, 2H, NCH2 , JH,H = 6.3 Hz), 3.69 (q, 2H, −OCH2 CH3 , J = 7.0 Hz), 1.22 (t, 3H, OCH2 CH3 , J = 7.0 Hz). 13 C NMR (101 MHz, CDCl3 ): ␦ = 165.5 (NCN), 126.9 (NCCN), 123.1 (NCCN), 121.7 ( CH ), 118.1 ( CH2 ), 92.0 (NCH2 O), 61.3 (CH3 CH2 O-), 52.4 (NCH2 CH ), 14.4 (CH3 CH2 O ). ESI–MS (m/z): 167.1 [M−Cl]+ . ESIHRMS: calcd for [C9 H15 N2 O]+ 167.1184; found 167.1193. 2.3.4. Synthesis of 4 1-Allylimidazole (4.2 g, 38.9 mmol) was added to 2-chloroethyl methyl ether 18.1 mL (194.4 mmol), which was used also as a solvent. The solution was refluxed for 20 h. The solvent was evaporated under reduced pressure. The resulting light brown oil was washed three times with THF (20 mL) and dried under reduced pressure. Yield 26% (2.049 g). 1 H NMR (400 MHz, CDCl3 ): ␦ = 10.54 (s, 1H, NCHN), 7.72 (s, 1H, NCHCHN), 7.43 (s, 1H, NCHCHN), 5.98 (ddt, 1H, CH , J = 16.6, 10.2, 6.3 Hz), 5.52 (d, 1H, CHHtrans ), 5.38 (d, 1H, CHHcis , 3 JH,H = 8.8 Hz), 4.78 (d, 2H, NCH2 -, 2 JH,H = 6.3 Hz), 4.20 (q, 2H, NCH2 CH2 -, J = 7.1 Hz), 3.60 (q, 2H, NCH2 CH2 , J = 6.6 Hz), 1.24 (m, 3H, −OCH3 ). 13 C NMR (101 MHz, CDCl3 ): ␦ = 164.5 (NCN), 125.8 (NCCN), 124.2 (NCCN), 120.7 ( CH = ), 118.6 ( CH2 ), 79.2 (NCH2 CH2 ), 68.1 (NCH2 CH2 ), 52.4 (NCH2 CH ), 17.4 (CH3 O ). ESI–MS (m/z): 167.1 [M−Cl]+ . ESI-HRMS: calcd for [C9 H15 N2 O]+ 167.1184; found 167.1199. 2.3.5. Synthesis of 5 2-chloroethanol (1.25 mL, 22.2 mmol) was added to 1allylimidazole (2.0 g, 18.5 mmol) in ethanol (1.5 mL). The solution was refluxed for 72 h. The solvent was evaporated under reduced pressure. The resulting brown oil was washed three times with THF (20 mL) and dried under reduced pressure. Yield 95.7% (3.331 g). 1 H NMR (400 MHz, DMSO): ␦ = 9.73 (s, 1H, NCHN), 8.25 (t, 1H, NCHCHN, J = 1.7 Hz), 8.19 (t, 1H, NCHCHN, J = 1.8 Hz), 6.48 (m, 1H, CH ), 5.86 (s, 1H, OH-), 5.77 (ddd, 1H, CHHtrans , J = 5.0, 3.1, 1.9 Hz), 5.68 (m, 1H, CHHcis ), 5.30 (t, NCH2 CH2 OH, J = 9.9 Hz, 2H), 4.67 (dd, 2H, NCH2 CH2 OH, J = 15.1, 9.9 Hz). 13 C NMR (101 MHz, CDCl3 ): ␦ = 169.1 (NCN), 125.1 (NCCN), 123.9 (NCCN), 120.4 ( CH = ), 116.1 ( CH2 ), 63.3 ( CH2 CH2 OH), 53.1 (NCH2 CH ), 17.1 ( CH2 CH2 OH). ESI-HRMS: calcd for [C8 H13 N2 O]+ 153.1028; found 153.1041. 2.3.6. Synthesis of 6 4-vinylbenzylchloride (1.7 mL, 10.8 mmol) was added to 1-(4vinylbenzyl)imidazol (2.0 g, 10.8 mmol) in acetonitrile (20 mL). The solution was stirred for 24 h at 70 ◦ C. The solvent was evaporated under reduced pressure. The resulting yellow solid was washed three times with THF (20 mL) and dried under reduced pressure.
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Yield 70% (2.561 g). 1 H NMR (400 MHz, CDCl3 ) ␦ = 10.69 (s, 1H, NCHN), 7.37 (s, 2H, NCHCHN), 7.29 (m, aromatic, 4H), 7.25–7.07 (m, aromatic, 4H), 6.53 (ddd, 4H, CH CH2 , J = 28.5, 18.2, 9.5 Hz), 5.56 (d, 2H, CHHtrans , 3 JH,H = 17.6 Hz), 5.40 (s, 4H, NCH2 ), 5.12 (d, 2H, CHHcis , 3 JH,H = 17.6 Hz).13 C NMR (101 MHz, CDCl3 ) ␦ = 138.2 (C CH CH2 ), 136.7 ( CH CH2 ), 135.5 (NCN), 132.4 ( NCH2 C ), 129.0 (aromatic), 126.5 (aromatic), 121.9 (NCCN), 115.1 ( CH CH2 ), 52.6 (NCH2 ). ESI–MS (m/z): 301.4 [M−Cl]+ . ESI−HRMS: calcd for [C21 H21 N2 ]+ 302.4128; found 302.4115. 2.4. Syntheses of nickel complexes 2.4.1. Synthesis of 7 A solution of nickelocene (1.005 g, 5.32 mmol) in 20 mL of THF was added to 6 (1.793 g, 5.32 mmol). The mixture was stirred for 5 days at ambient temperature. The solution slowly turned colour from dark green to dark red. The solvent was evaporated from the red solution, and the remaining solid was washed with hexane (3 × 20 mL) in order to remove unreacted nickelocene. The resulting residue was extracted with toluene (3 × 10 mL), and the solution was reduced in volume to about 5 mL. After standing overnight at 4 ◦ C red crystals suitable for X-ray measurements, were obtained (Fig. 2). Yield: 40.5% (0.987 g). Elemental analysis: calcd for C26 H25 N2 ClNi: C 67.94% H 5.48% N 6.09% found: C 67.75; H, 5.68; N, 6.23. EI-MS (70 eV) m/z (relative intensity) (58 Ni, 35 Cl): 458 (M+ , 59%), 423 ([M−Cl]+ , 51%), 157 ([CpNiCl]+ , 81%), (99 [CpCl]+ , 93%). EI-HRMS: calcd for C26 H25 35 ClN2 58 Ni 458.1060 found 458.1066. 1 H NMR (500 MHz, toluene-d8 , 25 ◦ C): ␦ = 7.11 (m, 4H, aromatic), 6.98 (m, 4H, aromatic), 6.53 (m, 2H, CH CH2 ), 6.15 (s, 1H NCHH ), 5.79 (bs, 2H), 5.57 (d, 2H, CH CHHtrans, 1 3J H,H = 18.5 Hz), 5.07 (s, 5H, Cp). H NMR (500 MHz, toluene-d8 , ◦ −50 C): ␦ = 7.10 (m, 8H, aromatic), 6.50 (dd, 2H, CH CH2 , J = 16.9, 11.2 Hz), 6.10 (bs, 2H, NCHH), 5.90 (d, 2H, −NCHH, J = 19.8 Hz), 5.61 (d, 2H, CH CHHtrans , 3 J H,H = 17.5 Hz), 5.53 (d, 2H, CH CHHcis , 3J 13 C NMR (101 MHz, toluene): H,H = 15.0 Hz), 5.09 (s, 5H, Cp). ␦ = 175.3 (NCN), 136.6 ( C CH CH2 ), 134.1 ( CCH CH2 ), 130.7 ( NCH2 C), 127.6 (aromatic), 126.8 (aromatic), 122.1 (NCCN), 116.4 ( CCH CH2 ), 91.7 (Cp), 55.3 (N CH2 C). 2.4.2. Synthesis of 8 and 9 A solution of nickelocene (0.891 g, 4.72 mmol) in 20 mL of THF was added to 1 (1.633 g, 7.08 mmol). The mixture was stirred for 3 days at ambient temperature. The solution slowly turned colour from dark green to dark red. After decanting the red solution, a dark green solid remained as a residue. The solvent was evaporated from the red solution, and the remaining solid was washed with hexane (3 × 20 mL) in order to remove unreacted nickelocene. The resulting residue was extracted with toluene (3 × 10 mL), and the solution was reduced in volume to about 5 mL. After standing overnight at 4 ◦ C red crystals of 8, suitable for X-ray measurements, were obtained (yield: 30.6%; 0.511 g, 1.45 mmol) (Fig. 3). The green solid was washed with THF (3 × 20 mL), dissolved in acetonitrile (20 mL) and filtered. After concentrating to about 5 mL and keeping overnight at 4 ◦ C green crystals of 9 were obtained (yield 56.9%; 1.103 g, 2.01 mmol). The crystals were not suitable for single-crystal X-ray measurements. 8: 1 H NMR (400 MHz, benzen): ␦ = 6.29 (s, 1H, NCHCHN), 6.18 (d, 1H, NCHCHN, J = 1.7 Hz), 5.72 (ddd, J = 16.1, 10.4, 5.6 Hz, 1H, CH = ), 5.24 (s, 1H, NCH2 C O), 5.21 (s, 5H, Cp), 5.04 (s, 2H, NCH2 CH = ), 4.87 (d, J = 10.3 Hz, 1H, CHHcis ), 4.76 (d, J = 17.3 Hz, 1H, CHHtrans ), 3.91 (d, J = 6.8 Hz, 2H, −OCH2 CH3 ), 0.95 (t, J = 7.1 Hz, 3H, OCH2 CH3 ). 1 H NMR (500 MHz, toluene-d , −49 ◦ C, all lines were signifi8 cantly broadened): ␦ = 6.43 (s, 1H, NCHCHN), 6.27 (d, J = 1.7 Hz, 1H, NCHCHN), 5.91 (m, 1H, CH ), 5.63 (bs, 2H, NCH2 COOCH2 CH3 ), 5.31 (dd, 1H, NCHHCH CH2 ), 5.15 (bs, 1H, NCHH CH CH2 ),
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Fig. 2. Ortep plot of 7 with atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths [Å]: Ni1−Cl1 2.2060(7), Ni1−C1 1.874(2), Ni−Cpcent 1.7553(17).
Fig. 3. Ortep plot of 8 with atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths [Å]: Ni1−Cl1 2.1976(5), Ni1−C1 1.8761(19), Ni−Cpcent 1.7545(9).
5.08 (s, 5H, Cp), 4.76 (d, J = 9.1 Hz, 1H, CHHcis ), 4.60 (d, J = 16.3 Hz, 1H, CHHtrans ), 3.75 (d, J = 6.3 Hz, 2H, OCH2 CH3 ), 0.97 (t, J = 7.8 Hz, 3H, OCH2 CH3 ).13 C NMR (101 MHz, benzen): ␦ = 169.1 (C O), 165.6 (NCN), 134.2 ( CH ), 123.9 (NCCN), 122.2 (NCCN), 118.4 ( CH2 ), 91.8 (Cp), 62.1 (NCH2 COOCH2 CH3 ), 54.5 (NCH2 ), 52.7 (NCH2 COOCH2 CH3 ), 14.2 (NCH2 COOCH2 CH3 ). Elemental analysis: calcd for C15 H19 ClN2 NiO2 C, 50.97, H, 5.42, N, 7.93. found C, 51.23, H, 5.51, N, 7.98. EI-MS (70 eV) m/z (relative intensity) (58 Ni, 35 Cl) 352
(M+ , 52%), 318 ([M−Cl]+ , 43%), 157 ([CpNiCl]+ , 76%), 99 ([CpCl]+ , 89%). EI-HRMS: calcd. for C15 H19 35 ClN2 58 NiO2 352.0488, found 352.0498. 9: 1 H NMR (400 MHz, CDCl3 , all lines significantly broadened): ␦ = 7.19 (s, 2H, NCHCHN), 6.92 (s, 2H, NCHCHN), 5.81 (bs, 2H, CH ), 5.17 (s, 5H, Cp), 5.13 (bs, 4H, -NCH2 C O), 4.92 (s, 4H, NCH2 CH ), 4.72 (bs, 2H, CHHcis ), 4.65 (bs, 2H, CHHtrans ), 3.84 (bs, 4H, OCH2 CH3 ), 0.91 (bs, 6H, −OCH2 CH3 ). 13 C NMR (101 MHz,
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Fig. 4. Ortep plot of 10 with atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths [Å]: Ni1−Cl1 2.1971(4), Ni1−C1 1.8743(13), Ni−Cpcent 1.7644(6).
Scheme 1. Synthesis of covalent and ionic nickel NHC complexes 8–16.
CDCl3 ): ␦ = 168.8 (C O), 165.1 (NCN), 134.4 ( CH ), 123.2 (NCCN), 121.4 (NCCN), 118.6 ( CH2 ), 91.2 (Cp), 61.4 (NCH2 COOCH2 CH3 ), 54.4 (NCH2 ), 52.1 (NCH2 COOCH2 CH3 ), 14.6 (NCH2 COOCH2 CH3 ). EI-HRMS: calcd for C25 H33 N4 58 NiO4 511.1855, found 511.1873. 2.4.3. Synthesis of 10 and 11 The reaction was carried out analogously to the previous one using a solution of nickelocene (1.294 g, 6.85 mmol) in 20 mL of THF and 3 (2.083 g, 10.27 mmol). Red crystals of 10 suitable for Xray measurements were obtained. Yield: 18.6% (0.415 g, 1.28 mmol) (Fig. 4). The green crystals of 11 (yield 43.4%; 1.097 g, 2.23 mmol) were not suitable for single-crystal X-ray measurements. 10: 1 H NMR (500 MHz, toluene-d8 , 25 ◦ C): ␦ = 6.51 (d, J = 1.8 Hz, 1H, NCHNCH), 6.23 (d, J = 1.7 Hz, 1H, NCHNCH), 5.84 (s, 2H, O CH2 N), 5.79 (m, 1H, NCH2 CH CH2 ), 5.07 (s, 5H, Cp), 4.90 (dd, J = 10.2, 1.0 Hz, 1H, NCH2 CH CHHcis ), 4.79 (d, J = 17.1 Hz, 1H, NCH2 CH CHHtrans ), 3.53 (dd, J = 13.6, 6.7 Hz, 2H, O CH2 CH3 ), 1.05 (t, J = 7.0 Hz, 3H, O CH2 CH3 ). 1 H NMR (500 MHz, toluened8 , −50 ◦ C): ␦ = 6.63 (d, J = 1.8 Hz, 1H CHCH), 6.26 (d, J = 1.5 Hz, 1H, CHCH), 5.78 (dd, J = 17.3, 7.1 Hz, 2H, O CH2 -N), 5.73 (m, 1H, NCH2 CH CH2 ), 5.50 (dd, J = 15.3, 5.0 Hz, 1H, NCHH CH CH2 ),
5.14 (s, 5H, Cp), 4.89 (bs, 1H, NCHH CH CH2 ), 4.88 (d, J = 8.4 Hz, 1H, NCH2 CH CHHcis ), 4.71 (d, J = 17.0 Hz, 1H, NCH2 CH CHHtrans ), 3.55 (ddd, J = 22.4, 15.0, 7.9 Hz, 2H, O CH2 CH3 ), 0.99 (m, 3H, O CH2 CH3 ). 13 C NMR (100 MHz, toluene-d8 , 25 ◦ C): ␦ = 168.2 (NCN), 134.7 ( CH ), 125.1 (NCCN), 124.2 (NCCN), 117.2 ( CH2 ), 92.5 (Cp), 89.1 (CH3 CH2 OCH2 -), 64.1 (CH3 CH2 OCH2 -), 52.2 (NCH2 -), 17.5 (CH3 CH2 OCH2 ). Elemental analysis: calcd for C14 H19 ClN2 NiO: C, 51.67; H, 5.88; N, 8.61. found: C, 51.89; H, 6.06; N, 8.76. EI-MS (70 eV) m/z (relative intensity) (58 Ni, 35 Cl) 324 (M+ , 59%), 289 ([M−Cl]+ , 37%), 157 ([CpNiCl]+ , 82%), 99([CpCl]+ , 91%). EI-HRMS: calcd for C14 H19 35 ClN2 58 NiO 324.0540, found 324.0529. 11: 1 H NMR (400 MHz, CDCl3 , all lines significantly broadened): ı 6.81 (bs, 2H, NCHNCH), 6.52 (bs, 2H, NCHNCH), 5.62 (bs, 4H, O CH2 N), 5.51 (m, 2H, NCH2 CH CH2 ), 5.19 (s, 5H, Cp), 4.82 (bs, 2H, NCH2 CH CHHcis ), 4.69 (bs, 2H, NCH2 CH CHHtrans ), 3.32 (bs, 4H, O CH2 CH3 ), 1.18 (bs, 6H, O CH2 CH3 ). 13 C NMR (100 MHz, CDCl3 ): ␦ = 166.7 (NCN), 139.8 ( CH ), 124.5 (NCCN), 123.6 (NCCN), 118.3 ( CH2 ), 91.6 (Cp), 88.3 (CH3 CH2 OCH2 -), 62.5 (CH3 CH2 OCH2 ), 56.1 (NCH2 ), 16.1 (CH3 CH2 OCH2 ). ESI-HRMS: C23 H33 N4 58 NiO2 455.1957; found 455.1948.
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2.4.4. Synthesis of 12–16 The complexes 12–16 were synthesized in reactions of nickelocene with an appropriate imidazolium salt (2 for 12 and 13, 4 for 14 and 15, and 5 for 16) (Scheme 3). The reactions were carried out analogously to those for synthesis of 8, 9, 10 and 11. The reactions of 2 and 4 led in each case to the formation of two complexes: the covalent, red solids 12 and 14, and the ionic, green ones 13 and 15 respectively. The reaction of nickelocene with 5 produced only one product, the ionic, green complex 16. Although separation of the complexes was easy due to differences in their solubility, crystallization from various solvents gave no crystals suitable for single-crystal X-ray measurements. Also quality of NMR spectra was not fully satisfactory. The main method of the complexes identification was high resolution mass spectrometry. 2.4.5. Synthesis of 12 and 13 The reaction was carried out analogously to the previous one using a solution of nickelocene (1.123 g, 5.95 mmol) in 20 mL of THF and 2 (1.789 g, 8.92 mmol). Yield of 12 18.9% (0.365 g, 1.13 mmol), yield of 13 43.1% (0.937 g, 1.92 mmol). Crystals of 12 and 13 were not suitable for single-crystal X-ray measurements.12: 1 H NMR (400 MHz, benzene) ␦ = 6.72 (s, 1H, NCHCHN), 6.42 (s, 1H, NCHCHN), 5.43 (m, 1H, CH ), 5.30 (s, 5H, Cp), 5.23 (s, 2H, NCH2 O), 5.15 (d, 1H, CHHcis 3 JH,H = 5.9 Hz), 4.92 (d, 1H, CHHtrans , 3J H,H = 14.7 Hz), 4.62 (d, 2H, NCH2 ), 1.89 (m, 3H, />CH3 ).EI-HRMS: calcd for C14 H17 35 ClN2 58 NiO 322.0383, found 322.0365. 13: 1 H NMR (400 MHz, CDCl3 , all lines significantly broadened) ␦ = 6.54 (bs, 1H, NCHCHN), 6.38 (bs, 2H, NCHCHN), 5.52 (m, 2H, CH ), 5.14 (s, 5H, Cp), 5.06 (bs, 2H, NCH2 O), 4.87 (bs, 2H, CHHcis ), 4.72 (d, 2H, CHHtrans ), 4.43 (d, 2H, NCH2 ), 1.71 (m, 3H, −CH3 ).ESI-HRMS: calcd. for (C23 H29 N4 58 NiO2 )+ 451.1644; found 451.1629. 2.4.6. Synthesis of 14 and 15 The reaction was carried out analogously to the previous one using a solution of nickelocene (0.951 g, 5.04 mmol) in 20 mL of THF and 4 (1.530 g, 7.55 mmol). Yield of 14 37.5% (0.615 g, 1.89 mmol), yield of 15 43.3%; (0.804 g, 1.64 mmol). Crystals of 14 and 15 were not suitable for single-crystal X-ray measurements.14: 1 H NMR (400 MHz, benzen): ␦ = 6.76 (s, 1H, NCHCHN), 6.33 (d, 1H, NCHCHN, J = 1.7 Hz), 5.91 (ddd, J = 16.0, 10.2, 5.8 Hz, 1H, −CH = ), 5.53 (bs, 2H, NCH2 -CH = ),5.26 (s, 5H, Cp), 4.94 (d, J = 10.3 Hz, 1H, CHHcis ), 4.68 (d, J = 17.3 Hz, 1H, CHHtrans ), 3.67 (bs, 2H, NCH2 CH2 O CH3 ), 3.07 (t, J = 6.1 Hz, 3H, OCH3 ), 1.62 (q, 2H, NCH2 CH2 O CH3 ). EIHRMS: calcd for C14 H19 35 ClN2 58 NiO 324.0540, found 324.0529. 15: 1 H NMR (400 MHz, CDCl , all lines significantly broadened): ␦ = 6.92 3 (bs, 2H, NCHCHN), 6.52 (2H, NCHCHN), 5.87 (ddd, 2H, CH), 5.62 (bs, 4H, NCH2 -CH), 5.19 (s, 5H, Cp), 4.82 (bs, 2H, CHHcis ), 4.60 (bs, 2H, CHHtrans ), 3.92 (bs, 4H, −NCH2 CH2 O CH3 ), 3.28 (bs, 6H, OCH3 ), 1.62 (bs, 4H, NCH2 CH2 O CH3 ). ESI-HRMS: calcd for (C23 H33 N4 58 NiO2 )+ 455.1957; found 455.1949. 2.4.7. Synthesis of 16 A solution of nickelocene (1.221 g, 6.47 mmol) in 20 mL of THF was added to 5 (1.830 g, 9.70 mmol). In this case the only one product was formed, the green solid. Yield 79,0% (1.776 g, 3.83 mmol). 16: 1 H NMR (400 MHz, CDCl3 , lines significantly broadened): ␦ = 6.82 (bs, 2H, NCHCHN), 6.59 (bs, 2H, NCHCHN), 5.72 (m, 2H, CH), 5.38 (bs, 2H, OH ), 5.31 (s, 5H, Cp), 5.23 (bs, 2H, CHHtrans ), 5.02 (m, 2H, CHHcis ), 4.80 (bs, NCH2 CH2 OH, 4H), 4.61 (bs, 4H, NCH2 CH2 OH). ESI-HRMS: calcd for (C21 H29 N4 58 NiO2 )+ 427.1644; found 427.1628. 2.4.8. Synthesis of 17 55 mg (0.3 mmol) of potassium hexafluorophosphate was added to a solution of the complex CpNi(NHCallyl )Br [29] (74 mg, 0.21 mmol) in dichloromethane (5 mL). The suspension was stirred
overnight and then filtered. The solvent was evaporated under reduced pressure and the complex 17 was obtained as a dark red solid which was recrystallized from a mixture of diethyl ether and dichloromethane. Yield: 49 mg (56%) (Fig. 5). 17: 1 H NMR (400 MHz, CD3 CN): ␦ = 7.21 (bd, 1H, NCHCHN), 7.02 (bd, 1H, NCHCHN), 6.09 (m,1H, NCH2 CH CH2 , not coordinated), 5.92 (m, 1H, NCH2 CH CH2 , coordinated), 5.82 (s, 5H,C5 H5 ), 5.31 (m, 1H, NCH2 CH CHHcis , not coordinated, 2 JHH = 8.4), 5.04 (m, 1H, NCH2 CH CHHtrans , not coordinated, 2 JHH = 15.2), 4.87 (d, 1H, NCHHCH CH2 , coordinated, 3 JHH = 8.2), 4.54 (d, 1H, NCHHCH CH2 , not coordinated), 4.42 (m, 1H, NCHHCH CH2 , not coordinated), 4.35 (m, 1H, NCHHCH CH2 , not coordinated), 3.81 (m, 1H, NCH2 CH CHHcis , coordinated, 2 JHH = 8.5), 3.34 (m, 1H, NCH2 CH CHHtrans , coordinated, 2 JHH = 16.4).13 C NMR (101 MHz, CD3 CN): ␦ = 163.1 (NCN), 136.8 (-CH = not coordinated), 124.8 (NCCN), 124.3 (NCCN), 123.7 ( CH2 not coordinated), 95.3 (C5 H5 ), 83.4 (-CH = coordinated), 55.1 ( CH2 coordinated), 52.9 (NCH2 not coordinated), 50.2 (NCH2 not coordinated). EI-HRMS: calcd. for (C14 H17 N2 Ni)+ 271.0745, found 271.0727.
2.5. Procedure for Suzuki-Miyaura coupling A Schlenk tube was charged with 4-bromoacetophenone (0.337 g, 1.69 mmol, 1 eq.), phenylboronic acid (0.227 g, 1.87 mmol, 1.1 eq.), K3 PO4 (0.791 g, 3.73 g, 2.2 eq.) and the catalyst (3 mol%). Toluene was injected (7 mL) and the mixture immediately heated with vigorous stirring in an oil bath at 90 ◦ C. After 2 h, the reaction was stopped by cooling the flask to room temperature. The reaction mixture was washed with water and the toluene layer was separated. The water layer was extracted with diethyl ether. The organic layers were then combined together, dried over Na2 SO4 and solvents were evaporated under reduced pressure. Raw product was purified by silica gel column chromatography (hexane/EtOAc,10:1) to give the isolated coupling product. Conversion and selectivity were determined by gas chromatography using tetradecane as an internal standard. 4-acetylbiphenyl: 1 H NMR 400 MHz, CDCl3): ␦(ppm) 8.03(d, J = 9.6 Hz, 2H),7.68 (dt, J = 9.0, 1.8 Hz, 2H), 7.65(d, J = 8.0, 2H), 7.48(t, J = 7.4 Hz, 2H), 7.44(tt, J = 7.5, 1.5 Hz, 1H), 2.68 (s, 3H). 13 C NMR (125 MHz, CDCl3): ␦(ppm) 197.4, 145.3, 139.1, 135.7, 128.4, 128.5, 128.6, 127.5, 127.2, 26.3.
2.6. Procedure for carbene polymerization A solution of MAO (17.5 mL 10 wt.% in toluene, 27.8 mmol) was added to a solution of the catalyst (7 or CpNi(IMes)Cl) (0.09 mmol) in toluene (14 mL). The colour of the reaction mixture turned from red to light brown. The resulting catalytic system was activated by stirring at room temperature for 30 min. The solution of EDA (ethyl diazoacetate) (0.32 g, 2.8 mmol, 2.4 mL 15% solution in toluene) was then added, and the mixture was stirred for 3 h at room temperature. The reaction was terminated by treatment with 9% aqueous hydrochloric acid solution. The organic layers were separated and poured into an excess of methanol. The resulting dark brown oils were separated, washed with cold toluene and dried under reduced pressure. Yields: 47% – cat. 7; 58% – cat. [CpNi(IMes)Cl]. Oligomers obtained with 7 as the catalyst: elemental analysis: found N 6.14/6.16, C 70.23/70.38, H 10.55/10.68. 1 H NMR (400 MHz, CDCl3 ): ␦ = 7.04 (m, 4H, CH, aromatic), 6.35 (s, 2H, CHCH), 3.15 (m, −CH2 CH3 ), 2.29 (m, CHCOOCH2 CH3 ), 1.28 (bs, 3H, CH3 ), 0.98 (m, CH2 CH3 ). 13 C NMR (100 MHz; CDCl3 ): ␦ (ppm) = 11.3–13.7 (–OCH2 CH3 ), 39.1–48.6 (CH), 59.9–61.1 (–OCH2 CH3 ), 166.3–174.9 (C O). Oligomers obtained with CpNi(IMes)Cl as the catalyst: elemental analysis: found N 5.61/5.51, C 67.22/67.01, H 10.39/10.34.
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Fig. 5. Ortep plot of 17 with atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths [Å]: Ni1−C1 1.8793(19), Ni1−C11 2.024(3), Ni1−C12 2.027(2), Ni−Cpcent 1.7246(10), C11–C12 1.380(4).
3. Results and discussion 3.1. Synthesis of nickel complexes Imidazolium chloride, with allyl substituents attached to nitrogen atoms in the NHC ring, reacts with nickelocene in a different manner than imidazolium bromide. We have noticed before [32] that two products, covalent CpNi(NHCallyl )Cl and biscarbene ionic [CpNi(NHCallyl )2 ]+ Cl− , were formed in the reaction of 1,3-diallyl imidazololium chloride with nikelocene. On the other hand, only one product, covalent CpNi(NHCallyl )Br, was formed in the reaction of 1,3-diallyl imidazolium bromide with nickelocene [29]. Unsymmetrically substituted imidazolium chlorides (1–4) reacted with nickelocene analogously to symmetrical diallyl imidazolium chloride. One carbene nitrogen atom was substituted with the allyl group, and another one with various oxygen-containing substituents. The idea was to enable the hemilabile coordination of the oxygen atom to nickel, to prepare ionic complexes analogous to those with coordination via the allyl group double bond (e.g. 17 and [Cp’Ni(NHCallyl )]+ Br− [32]). Unfortunately, we did not observe such coordination. Reactions proceeded according to the equation shown in Scheme 1. Two types of complexes were formed in these reactions: covalent, red solids, soluble in toluene (type A, 8, 10, 12, 14) and ionic, green solids, soluble in acetonitrile (type B, 9, 11, 13, 15). Only one product (type B, 16) was formed for a CH2 CH2 OH substituent at one nitrogen atom. The separation of the complexes from the reaction mixtures was easy, due to differences in their solubility. However, complicated NMR spectra made their characterization difficult. Only complexes 8–11 were fully characterized by elemental analysis, 1 H, 13 C NMR, and high resolution mass spectrometry. 1 H NMR spectra of covalent complexes 8 and 10 exhibited the same dynamic behaviour in solution as before. The signals of N−CH2 − protons were extremely broad and their non-equivalence was not visible even at low temperatures. This dynamic behaviour has been explained previously with the help of DFT calculations [32].
Complexes 7, 8, and 10 are structurally similar to those we have described previously [29,32]. The molecular structures of 7, 8, and 10 do not differ significantly from each other with regard to the coordination mode and geometry. The geometry of these complexes is trigonal planar at the nickel centre considering Cpcent. , the carbene carbon, and the chlorine atoms. The Ni Ccarbene and Ni Cpcent distances measure 1.874(3) and 1.754(1) Å, 1.877(2) and 1.755(1) Å, 1.874(2) and 1.765(1) Å for 7, 8, and 10, respectively, and are similar to the corresponding distances described in the literature for other [NiCp(NHC)X] complexes.
3.2. Suzuki–Miyaura cross-coupling reactions The catalytic activity of cyclopentadienylnickel NHC complexes in the Suzuki cross-coupling was tested in a model reaction of 4bromoacetophenone with phenylboronic acid (Scheme 2). K3 PO4 was used as the base. Cyclopentadienylnickel complexes with NHC ligands having allyl substituents at both the N atoms (NHCallyl ) were tested as catalysts in the first part of the studies. The syntheses and structures of most of these complexes have been published previously [29,32,66]. The complexes used as catalysts are depicted in Scheme 3. The reactions were carried out in toluene, using 3 mol% of the catalyst and the substrates (4-bromoacetophenone: phenylboronic acid: K3 PO4 ) at a 1:1.1:2.2 molar ratio. Small amounts of other solvents (1.5 mL) were added to improve catalyst solubility while using ionic complexes (Table 1, entry 3–acetonitrile; entry 7–THF; entry 8–CH2 Cl2 ). The results are shown in Table 1. The main product, 4-acetylbiphenyl, was formed in all experiments with high selectivity (in most cases between 96 and 97%). The formation of small amounts of 4,4 -diacetylbiphenyl, the product of 4-bromoacetophenone homocoupling, was observed in all reactions. It can be concluded that the catalyst has no substantial influence on the selectivity of the reaction. On the contrary, the con-
32
A. Włodarska et al. / Molecular Catalysis 440 (2017) 25–35
Scheme 2. Suzuki–Miyaura cross-coupling of phenylboronic acid with 4-bromoacetophenone.
Scheme 3. Complexes used as catalysts in Suzuki–Miyaura reactions.
Table 1 Conversion, yield, and selectivity in the Suzuki–Miyaura coupling reaction catalysed by nickel NHCallyl complexes.
1 2 3 4 5 6 7 8 a
Catalyst
Time [min]
Conversion [%]
Yield [%]a
Selectivity [%]
CpNi(NHCallyl )Br [29] CpNi(NHCallyl )Br [29] [CpNi(NHCallyl )2 ]+ Cl− [32] CpNi(NHCallyl )Cl [32] Cp’Ni(NHCallyl )Cl [68] Cp’Ni(NHCallyl )Br [32] [Cp’Ni(NHCallyl )]+ Br− [32] [CpNi(NHCallyl )]+ PF6 − 17
60 120 120 120 120 120 120 120
40 51 30 27 38 65 79 54
32 42 22 19 30 55 70 48
97.0 96.9 89.8 96.6 96.4 97.2 96.9 92.5
Isolated yield of 4-acetylbiphenyl.
version strongly depends on the kind of catalyst used. The analysis of the results shown in Table 1 led to the following conclusions: – ionic complexes are more active catalysts than their covalent analogues (entry 7 vs. 6); – increasing electron density in the cyclopentadienyl ligand by substituting it with the methyl group increases the catalytic activity of complexes (entries 5 vs. 4 and 6 vs. 2); – bromide complexes are better catalysts than their chloride analogues (entries 2 vs. 4 and 6 vs. 5). This shows that bromide is a better leaving group than chloride, which facilitates ionization. most active catalyst, the ionic complex The [Cp’Ni(NHCallyl )]+ Br− , was then used to determine the dependence of conversion and selectivity on the reaction time. Samples were taken at specified intervals and then analysed (Fig. 6).
Table 2 Conversion and selectivity dependence on temperature and solvents in the Suzuki–Miyaura coupling reaction. Time [min]
Temperature [◦ C]
Solvent
Conversion [%]
Selectivity [%]
120 120 120 120
20 90 reflux 90
toluene/MeCN toluene/MeCN toluene/MeCN H2 O/DMF
9 79 22 –
93.5 96.9 94.1 –
Catalyst Cp’Ni(NHCallyl )]+ Br&minus
The reaction was carried out in toluene/THF mixture. The reaction reached the desired temperature (90 ◦ C) after 10 min. At that time, conversion was 11%, which may indicate that the catalyst was already active below that temperature. After 2 h, conversion reached 78%, and then its increase was very slow, reaching 85% after 24 h. From the economic point of view, it seems that 2 h is the optimal reaction time. On the other hand, selectivity does not depend on the reaction time. The same catalyst was used to study the influence of temperature on conversion and selectivity (Table 2). Temperature increase from 90 ◦ C to about 115 ◦ C (reflux) substantially reduced the level of conversion. The reason for it can be the decomposition of the catalyst or its deactivation. There were no coupling temperature
Table 3 Conversion, yield, and selectivity in the Suzuki–Miyaura coupling reaction catalysed by complexes 8–16.
1 2 3 4 5 6 7 8 9 a
Catalyst
Conversion.[%]
Yield[%]a
Selectivity[%]
8 9 10 11 12 13 14 15 16
10.5 22.8 22.6 27.3 29.3 21.2 11.5 30.4 36.7
7.3 15.0 15.7 20.9 21.0 17.1 7.9 23.0 29.5
99.4 99.7 99.7 99.9 99.8 99.7 99.9 99.9 99.8
Isolated yield of 4-acetylbiphenyl.
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Fig. 6. Conversion and selectivity dependence on the reaction time in the Suzuki–Miyaura coupling reaction.
Scheme 4. Carbene oligomerization catalysed by CpNi(NHC)Cl/MAO.
and solvents products in the reaction carried out in a mixture of water and DMF. The only product detected was the cyclic trimer of phenylboronic acid. In a reaction carried out at ambient temperature for 120 min the conversion reached only 9%. In the second part of this work, complexes 8–16 were tested in the Suzuki–Miyaura coupling reaction, the same as above (Scheme 1). The reactions were carried out at conditions described above (toluene at 90 ◦ C, 2 h, 3 mol% of the catalyst, 1:1.1:2.2 molar ratio of the substrates). The results are shown in Table 3. Although the conversion rates were rather disappointing, selectivity was almost 100%. Only traces of the homocoupling products (4,4 -diacetylbiphenyl or biphenyl) were detected in some reactions. The results shown in Table 3 indicate that ionic complexes are more active than covalent ones containing the same NHC ligands (9 vs. 8, 11 vs. 10, 13 vs. 12, 15 vs. 14). A comparison of the results from Table 3 with those from Table 1 (entries 3 and 4) made it possible to conclude that substituents at the nitrogen atoms of NHC ligands had no substantial influence on the catalytic activity of the studied complexes in the Suzuki–Miyaura coupling reaction. A recently published comprehensive paper [63] indicates that some unsymmetrical NCN pincertype Ni(II) complexes with NHC-triazole arms showed a higher catalytic activity than our unsymmetrical complexes 8–16. The authors noticed a strong influ-
ence of the base used, and the best results were obtained with K2 CO3 (Table 2, entry 10). On the other hand, with K3 PO4 as the base, at conditions quite similar to ours (4-bromoacetophenone: phenylboronic acid: K3 PO4 = 1:1.2:2.4; Ni complex – 2 mol%; 110 ◦ C in toluene, reaction time 2 h), the best catalyst described in that paper (compound 4, Table 2, entry 3) exhibited the same activity (78%) as our best catalyst (compound [Cp’Ni(NHCallyl )]+ Br− , Table 1, entry 7, conversion 79%).
3.3. Carbene oligomerization In our previous paper, we showed that cyclopentadienylnickel(NHC) complexes could be used as initiators in the oligomerization of ethyl acetate carbene derived from ethyl diazoacetate [29]. In that paper we noted, for the first time, that N-heterocyclic carbene from the catalyst precursor was inserted into the oligomer chain. That was a new and unexpected observation. Here, we show that this is a more general phenomenon, and even NHC carbenes bearing bulky substituents at nitrogen atoms can be incorporated into oligomer chains. We carried out the oligomerization of ethyl acetate carbene using CpNi(NHC)Cl complexes activated with methylalumoxane (MAO) as initiators (Scheme 4).
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Table 4 Carbene oligomerization with 7 and CpNi(IMes)Cl as catalysts.
1 2 3 4 5 6 7 8
Catalyst
Solvent
Temperature[◦ C]
Molar ratioNi:EDA:MAO
Yield [%]
Mw
Mw /Mn
7 7 7 7 CpNi(IMes)Cl CpNi(IMes)Cl CpNi(IMes)Cl CpNi(IMes)Cl
Toluene Toluene Toluene THF Toluene Toluene Toluene THF
RT 60 RT RT RT 60 RT RT
1:30:300 1:30:300 1:30:50 1:30:300 1:30:300 1:30:300 1:30:50 1:30:300
47 43 32 56 58 48 39 54
700 654 457 510 710 632 490 610
1.27 1.34 1.87 1.53 1.29 1.76 1.64 1.42
Two complexes, with sterically demanding substituents at the nitrogen atoms of the NHC ligand, were used as initiators: the well-known CpNi(IMes)Cl [65] and complex 7 synthesized and characterized in this work. The complexes were activated with an excess of MAO for 30 min at room temperature, and then EDA was added. The reactions were quenched after 3 h of vigorous stirring. Oligomers were formed in moderate yields, with the average molecular weight of about 700 Da and low polydispersity (about 1.3) (Table 4). MALDI TOF MS, 1H NMR, and elemental analysis proved the presence of NHC carbene in the oligomer chain, but it was difficult to define where exactly it was located. We studied the effects of temperature, the solvent, and the Ni:EDA:MAO molar ratio. The best results were obtained in toluene as solvent, at room temperature, and at a Ni:EDA:MAO molar ratio of 1:30:300. Decreasing the MAO:Ni molar ratio from 300:1 to 50:1 resulted in the non-insertion of NHC carbene into the oligomer chain. A temperature increase to 60 ◦ C resulted in a slight decrease in the yield and the molecular weight of oligomers.
4. Conclusions
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Several new cyclopentadienylnickel complexes with symmetrically and unsymmetrically substituted N-heterocyclic carbenes were synthesized and characterized. These complexes were applied as catalysts in the Suzuki–Miyaura cross-coupling reaction of 4bromoacetphenone with phenylboronic acid. All complexes were active catalysts with reactant conversion rates from 10 to 80% and very high selectivity, but the best results were obtained using a bromide catalyst, on which we published previously [32]. The catalytic activity of the complexes strongly depends on their composition: ionic complexes are more active than their covalent analogues; complexes with increased electron density in the cyclopentadienyl ligand are more active catalysts; bromide complexes are better catalysts than their chloride analogues. Two CpNi(NHC)Cl complexes were tested as initiators in the oligomerization of ethyl acetate carbene to confirm our previous observation that N-heterocyclic carbene from the catalyst precursor was inserted into the oligomer chain.
Acknowledgements This work was financially supported by the National Science Centre (grant no. UMO-2014/15/N/ST5/02006).
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mcat.2017.06. 033. The crystal data and experimental parameters, bond lengths and bond angles for the compounds 7, 8, 10 and 17; MALDI-TOF spectra of obtained oligomers.
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