Synthesis of NHC Pincer Hydrido Nickel Complexes and Their

Feb 7, 2018 - Institut für Nanotechnologie (INT) und Karlsruher Nano-Micro-Facility (KNMF), Karlsruher Institut für Technologie (KIT), Hermann-von-H...
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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Synthesis of NHC Pincer Hydrido Nickel Complexes and Their Catalytic Applications in Hydrodehalogenation Zijing Wang,† Xiaoyan Li,† Hongjian Sun,*,† Olaf Fuhr,‡ and Dieter Fenske‡ †

School of Chemistry and Chemical Engineering, Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Shandong University, Shanda Nanlu 27, 250199 Jinan, People’s Republic of China ‡ Institut für Nanotechnologie (INT) und Karlsruher Nano-Micro-Facility (KNMF), Karlsruher Institut für Technologie (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany S Supporting Information *

ABSTRACT: The C(carbene)N(amino)N(amine)-pincer nickel(II) bromides 1a−c were hydrogenated to the corresponding nickel(II) hydrides 2a−c by (EtO)3SiH/NaOtBu or NaBH4. These nickel(II) hydrides 2a−c were characterized by NMR and IR spectroscopy as well as X-ray diffraction. The catalytic performance of complex 2b for hydrodehalogenation reactions was explored. With a combination of 3 mol % catalyst loading, (EtO)3SiH/NaOtBu/toluene/80 °C and different reaction times, organic halides were successfully reduced to the related alkanes. A catalytic radical mechanism is proposed and partially verified by experiments.

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INTRODUCTION Great progress has been made in the chemistry of transitionmetal pincer complexes since the pioneering work by Shaw1 and van Koten.2 These unique structures of pincer complexes provide a variety of applications, including homogeneous catalysis, activation of strong chemical bonds, selective formation of C−C, C−N, and C−metal bonds, and transfer hydrogenation reactions.3,4 In recent years, N-heterocyclic carbenes (NHCs), instead of the traditional phosphines, have been used by many research groups in the pincer complexes because the NHC is an electron-rich, stable (to air and water), and strong Lewis base with excellent coordination ability to transition metals.5−10 Hydrido transition-metal complexes are some of the most important and efficient catalysts and intermediates for many organometallic reactions. Studies have proved that hydrido iron, cobalt, and nickel complexes have excellent catalytic effects in alkyne polymerization,11 olefin addition,12,13 unsaturated complex reduction,14−16 formic acid dehydrogenation,17 carbon dioxide reduction,18,19 hydrodehalogenation,20 etc. Catalytic hydrodehalogenation is of significance to both treatment of pollutants and utilization of chemical waste. However, the catalytic hydrodehalogenation catalyzed by nickel complexes is still rare. Gade reported the catalytic enantioselective hydrodehalogenation with chiral [NNN]-pincer nickel hydrides.21 He also found that nickel fluoride complexes bearing [NNN]-pincer ligands could catalyze the hydrodefluorination of germinal difluorocyclopropanes with high Z selectivities.22 Meanwhile, related reaction pathways have been investigated both experimentally and theoretically.23 Tashiro24 © XXXX American Chemical Society

reduced chlorobipenyls to biphenyl with Raney Ni-Al alloy. Hu and co-workers reduced halides in high yields through a hydrido [NNN] pincer nickel complex. 20 A type of unsymmetric pincer ligand with NHC and amine as donors has been reported by our laboratory.25 The complex [CNN-NiBr] (1) could catalyze the Kumada coupling of aryl chlorides or aryl dichlorides under mild conditions (Scheme 1). Scheme 1. Synthesis of Hydrido Nickel Complex 2

As a continuation of our research work in this field, in this paper we synthesized the hydrido nickel complexes [CNN-NiH] (2) from [CNN-Ni-Br] (1) (Scheme 1). These hydrido CNN pincer nickel complexes 2 could efficiently catalyze hydrodehalogenation reactions, especially the reduction of Received: November 26, 2017

A

DOI: 10.1021/acs.organomet.7b00848 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

2a−c as catalysts to optimize the reaction conditions (Table 1). The catalytic performances of 2a−c were compared under the conditions of 5 mol % catalyst loading and (EtO)3SiH/ NaOtBu/THF/25 °C. The results showed that 2b gave the highest catalytic activity among them (entries 1−3). According to the results of mass spectrometry, no octene was generated in this catalytic system. The reaction temperature played an important role in the catalytic process. Although the reaction could also have a high yield at a lower temperature (50 °C) (entries 4 and 6), at a higher temperature (80 °C) the catalyst loading and reaction time were greatly reduced and gave a high yield (99%) (entry 8). Through the screening of four types of bases (NaOtBu, NaOMe, K2CO3, and CH3COONa), the highest yield could be obtained with NaOtBu (entries 7−10). Different hydrogen sources were also tested. It was found that the suitable hydrogen source was (EtO)3SiH in comparison with Et3SiH, (Me)2PhSiH, and Ph3SiH (entries 8 and 11−13). 2b had the highest catalytic activity with a catalyst loading of 3 mol % at 80 °C (entry 8). The control experiments showed that there were no conversions without silane (entries 14 and 15). Under the optimized conditions, nickel compounds Ni(acac)2, NiBr2(DME), NiBr2, and Ni(OAC)2 without pincer ligands were also tested as catalysts and the results were poor (entries 16−19). Complex 1b could also catalyze this hydrodebromonation reaction, but 5 mol % catalyst loading was needed (entry 20). Under the optimized conditions, we expanded the substrate scope using 2b as a catalyst (3 mol %) in toluene at 80 °C for different reaction times (Table 2). With the aliphatic halides, including the secondary alkyl halide chlorocyclohexane, excellent yields were reached (entries 1−4). For the aliphatic halogenated hydrocarbons with phenyl groups, the reduction reactions ran in high yields (entries 5−7). Phenyl halides could hardly be reduced under these conditions (entries 8 and 9). Interestingly, phenyl halides with electron-withdrawing groups (−NO2, −CF3, −Br, etc.) gave rise to yields of between 63% and 93% (entries 10−20) regardless of whether the electronwithdrawing group was at an ortho, meta, or para position. For 1-bromo-2-iodobenzene the C−I bond was preferentially activated (entry 16). 2b could also catalyze the hydrodehalogenation of 1-bromonaphthalene and heterocyclic halide in moderate yields (entries 21 and 22). Bromobenzenes with electron-donating groups could not be hydrodebrominated even when the catalytic reaction was carried out at a higher temperature (entries 23−25). With 2-chlorobenzaldehyde as the substrate, no benzaldehyde hydrodehalogenation product was detected because of the nucleophilic reaction between the −CHO group and the hydrido hydrogen of the catalyst (entry 26). Hydrodehalogenation Mechanism. According to Gade’s21−23 and Hu’s25 reports, we were encouraged to propose a mechanism for this hydrodehalogenation process in Scheme 2. The hydrodehalogenation reaction is initiated from hydrido catalyst 2b. The hydrogen radical H• is produced from 2b with the formation of nickel(I) intermediate A. H• reacts with RX to give rise to radical X• and the expected product RH. The combination of A with X• affords intermediate B, a nickel(II) halide. The nucleophilic substitution by OtBu− at the nickel(II) center of B delivers intermediate C with the formation of NaX. Catalyst 2b is recovered through the interaction of C with (EtO)3SiH. To adminiculate the feasibility of the reaction mechanism, the following experiments were performed.

aliphatic halides and aromatic halides with electron-withdrawing groups.

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RESULTS AND DISCUSSION Synthesis and Characterization of the Catalysts. As we know, the direct hydrogenation of nickel halides to nickel hydrides is a common method to synthesize nickel hydrides.26 Therefore, we tried to carry out the reaction of [nBu(CNN)-NiBr] (1a) with (EtO)3SiH as a hydrogen source, but the reaction did not work even when the reaction solution was heated. When NaOtBu as a base was added to the reaction mixture, the reaction solution color changed from green to purple and then to orange. After workup, the product [nBu(CNN)-Ni-H] (2a) could be isolated. The IR spectrum of 2a shows the signal for the Ni−H bond at 1894 cm−1. As an alternative, the reaction of [nBu(CNN)-Ni-Br] (1a) with NaBH4 could also generate [nBu(CNN)-Ni-H] (2a) with higher yield. According to this route, we synthesized three hydrido nickel complexes with three different substituents: [ nBu (CNN)-Ni-H] (2a), [iPr(CNN)-Ni-H] (2b), and [Bn(CNN)-Ni-H] (2c) in yields of 47%, 64%, and 55%, respectively. Complexes 2a−c are orange solids obtained through precipitation from Et2O. They are stable in air for 2 h and are air-sensitive in solution. The characteristic hydrido signals were observed in the 1H NMR spectra at −20.94 (2a), −21.02 (2b) and −20.69 (2c) ppm, respectively. Single-crystal X-ray diffraction analysis confirmed the molecular structure of 2a (Figure 1). The central nickel atom is in a distorted-square-

Figure 1. Molecular structure of 2a at the 50% probability level. Hydrogen atoms except for the hydrido atom are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ni1−C15 1.828(3), Ni1−N3 1.885(2), Ni1−N4 1.975(2), Ni1−H50 1.79(4); C15−Ni1− N3 91.8(10); N3−Ni1−N4 86.77(9), C15−Ni1−H50 93.0(12), N4− Ni1−H50 90.3(12), C15−Ni1−N4 172.34(9), N3−Ni1−H50 165.2(11).

planar coordination geometry (τ4 = 0.159).27 The Ni1−N4 bond distance (1.975(2) Å) is remarkably longer than the Ni1− N3 bond distance (1.885(2) Å) due to the strong trans influence of the carbon atom of the NHC coordination moiety. The Ni1−C15 bond distance (1.828(3) Å) is comparable with the analogous bond distance (1.865(2) Å)28 and shorter than those of the normal single Ni−C bonds29 because Ni1−C15 is a metal−carbene double bond. The Ni1−H50 bond distance (1.79(4) Å) is a slightly longer than those in the literature.30 Catalytic Hydrodehalogenation of Alkyl and Aryl Halides. The hydrodebromination of n-octyl bromide was selected as a model reaction with the hydrido nickel complexes B

DOI: 10.1021/acs.organomet.7b00848 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Optimization of Reaction Conditions

entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 a

silane (1.2 equiv) (EtO)3SiH (EtO)3SiH (EtO)3SiH (EtO)3SiH (EtO)3SiH (EtO)3SiH (EtO)3SiH (EtO)3SiH (EtO)3SiH (EtO)3SiH Et3SiH (Me)2PhSiH Ph3SiH

(EtO)3SiH (EtO)3SiH (EtO)3SiH (EtO)3SiH (EtO)3SiH

base (1.2 equiv) t

NaO Bu NaOtBu NaOtBu NaOtBu NaOMe NaOtBu NaOMe NaOtBu K2CO3 CH3COONa NaOtBu NaOtBu NaOtBu NaOtBu KOtBu NaOtBu NaOtBu NaOtBu NaOtBu NaOtBu

solvent

temp (°C)

cat. loading (mol %)

time (h)

conversn/yielda (%)

THF THF THF toluene toluene THF THF toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene toluene

25 25 25 50 50 50 50 80 80 80 80 80 80 80 80 80 80 80 80 80

5 (2a) 5 (2b) 5 (2c) 3 (2b) 3 (2b) 3 (2b) 3 (2b) 3 (2b) 5 (2b) 5 (2b) 5 (2b) 5 (2b) 5 (2b) 5 (2b) 5 (2b) Ni(acac)2 NiBr2(DME) NiBr2 Ni(OAC)2 5 (1b)

3 3 3 5 5 5 5 2 8 8 8 8 8 8 8 2 2 2 2 5

64/− 69/− 47/− 82/80 50/48 98/95 60/57 100/>99 0/0 0/0 0/0 0/0 0/0 0/0 0/0 45/− 0/0 20/− 0/0 99/98

GC yields.

3 mol % catalyst loading, 100% conversion and 99% GC yield could be achieved. A catalytic radical mechanism was proposed and partially verified by experiments.

Complex 2b was treated with iodomethane in THF at room temperature. The orange reaction solution gradually turned green with the release of methane gas. Iodo nickel complex 3b was isolated from Et2O after workup in a yield of 58%. A similar reaction occurred with bromoethane in a yield of 47% (Scheme 3). Even though intermediate C could not be isolated, its homologous complex 4b could be obtained from the reaction of 1b with NaOMe (Scheme 4). The methoxy Ni(II) complex 4b was isolated as purple crystals from its n-pentane solution at 0 °C in a yield of 60%. Complex 4b was stable at room temperature and could be handled in the air in the solid state for hours without noticeable decomposition. In addition, it was experimentally confirmed that 4b could transform to 2b in the presence of (EtO)3SiH in toluene (Scheme 5). This reaction is a collateral evidence for the transformation of intermediate C to 2b in the proposed mechanism. In the presence of tempo (2,2,6,6-tetramethylpiperidine-1oxyl, a radical scavenger), no catalytic reaction occurred under the optimized catalytic conditions (Scheme 6). It is conjectured that tempo reacted with catalyst 2b to impede the catalytic reaction. These experimental results (Schemes 3−6) provide favorable support for the mechanism in Scheme 2.

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EXPERIMENTAL SECTION

General Procedures. All experiments were carried out under an N2 atmosphere, and all solvents were freshly distilled before use under an argon atmosphere. Aryl chlorides were purchased from commercial sources. Infrared spectra (4000−400 cm−1) were recorded on a Bruker ALPHA FT-IR instrument from Nujol mulls between KBr disks. NMR data were obtained on a Bruker Avance 300 or Bruker Avance 500 spectrometer. Elemental analyses were carried out on an Elementar Vario ELIII instrument. GC-MS measurements were carried out with a TRACE-DSQ instrument. Synthesis of 2a. A 100 mL reaction vessel was charged with 1a (0.95 g, 2.0 mmol), NaBH4 (0.12 g, 3.0 mmol), and THF (60 mL) under an N2 atmosphere. The reaction mixture was stirred for 5 h until the green reaction mixture turned yellow. After removal of the solvent under vacuum, the yellow residue was washed with n-pentane (30 mL) and then extracted with Et2O (3 × 20 mL) and filtered through Celite. 2a as orange crystals was obtained from diethyl ether at 0 °C. Yield: 47% (0.37 g). Dec pt: >135 °C. IR (Nujol, cm−1): 1878.44 ν(Ni−H); 1578.43 ν(CC). Anal. Calcd for C21H26N4Ni (393.15 g mol−1): C, 64.15; H, 6.67; N, 14.25. Found: C, 63.98; H, 6.76; N, 14.30. 1H NMR (300 MHz, C6D6): δ 8.30 (dd, J = 8.6, 1.1 Hz, 1H, Harom), 7.80 (dd, J = 8.2, 1.2 Hz, 1H, Harom), 7.20 (dt, J = 8.1, 1.5 Hz, 1H, Harom), 7.04 (td, J = 15.4, 1.4 Hz, 1H, Harom), 6.92 (d, J = 1.1 Hz, 1H, Harom), 6.86 (dd, J = 7.9, 1.3 Hz, 1H, Harom), 6.79 (dt, J = 15.0, 1.3 Hz, 2H, Harom), 6.63−6.57 (m, 1H, Harom), 6.23 (d, J = 2.0 Hz, 1H, Harom), 4.02 (t, J = 7.3 Hz, 2H, CH2CH2CH2), 2.94 (s, 6H, N(CH3)2), 1.74 (dt, J = 15.0, 7.5 Hz, 2H, CH2CH2CH2), 1.32−1.22 (m, 2H, CH2CH2CH2), 0.92 (t, J = 7.4 Hz, 3H, CH2CH3), −20.84 (s, 1H, NiH). 13C NMR (75 MHz, C6D6): 172.9, 151.5, 147.0, 143.1, 138.1, 128.7, 127.2, 126.4, 120.8, 120.4, 119.4, 117.7, 117.6, 116.3, 116.0, 114.8, 67.8, 54.5, 51.5, 30.2, 25.8. Synthesis of 2b. A 100 mL reaction vessel was charged with 1b (0.92 g, 2.0 mmol), NaBH4 (0.12 g, 3.0 mmol), and THF (60 mL) under an N2 atmosphere. The reaction mixture was stirred for 5 h until the green reaction mixture turned yellow. After removal of the solvent

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CONCLUSION In summary, the hydrido nickel complexes 2a−c bearing unsymmetric pincer ligands with NHCs and amines as donors have been successfully synthesized by the hydrogenation of nickel halides 1a−c and fully characterized. Complex 2a was characterized by single-crystal X-ray diffraction. In complex 2a both five-membered and six-membered chelate rings were formed in a distorted-square-planar coordination geometry with the nickel atom in the center. Complex 2b exhibited high catalytic activity to the hydrodehalogenation of organic halides. With a combination of (EtO)3SiH/NaOtBu/toluene/80 °C and C

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Organometallics Table 2. continued

Table 2. Scope of Catalytic Hydrodehalogenation of Halidesa

a

Conditions: 2b (3 mol %) was used as the catalyst, and the (EtO)3SiH/NaOtBu/toluene/80 °C combination was applied. Isolated yields are given unless noted otherwise. bGC yields.

Scheme 2. Proposed Mechanism

Scheme 3. Reaction between 2b and Iodomethane/ Bromoethane

Scheme 4. Synthesis of complex 4b

Scheme 5. Reaction between 4b and (EtO)3SiH

under vacuum, the yellow residue was washed with n-pentane (30 mL) and then extracted with Et2O (3 × 20 mL) and filtered through Celite. 2b as orange crystals was obtained from diethyl ether at 0 °C. Yield: D

DOI: 10.1021/acs.organomet.7b00848 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

3H, OCH3). 13C NMR (75 MHz, C6D6): δ 150.7, 148.5, 143.6, 129.8, 126.8, 126.6, 125.4, 120.4, 119.9, 119.6, 118.0, 117.5, 117.2, 117.1, 115.1, 49.8, 32.4, 29.9, 26.7, 23.5, 22.9. X-ray Crystal Structure Determination. Intensity data were collected on a Bruker P4 diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Crystallographic data for complex 2a are summarized in the Supporting Information. The structure was solved by direct methods and refined with full-matrix least squares on all F2 (SHELXL-97) with non-hydrogen atoms anisotropic. CCDC1546801 (2a) contains supplementary crystallographic data for this paper. General Procedure for Catalytic Hydrodehalogenation. A Schlenk tube was charged with NaOtBu (0.15 g, 1.5 mmol). The substrate (1.0 mmol), internal standard (1.0 mmol), toluene (2 mL), and a toluene solution of 2b (0.5 mL/0.03 mmol, 0.06 M) were added in sequence. A solution of (EtO)3SiH (0.2 g, 1.2 mmol) in 0.5 mL of toluene was added dropwise to this mixture. After it was stirred at 80 °C for 2−8 h, the reaction mixture was quenched with HCl (2 mL, 1 M) and extracted with Et2O (3 mL). The organic layer containing the product was separated. Conversion was determined by GC.

Scheme 6. Reaction with Radical Scavenger

64% (0.49 g). Dec pt: >137 °C. IR (Nujol, cm−1):1894.33 ν(Ni− H);1580.46 ν(CC). Anal. Calcd for C20H24N4Ni (379.12 g mol−1): C, 63.36; H, 6.38; N, 14.78. Found: C, 63.53; H,6.49; N, 14.91. 1H NMR (300 MHz, C6D6): δ 8.29 (dd, J = 9.0, 1.2 Hz, 1H, Harom), 7.78 (dd, J = 9.0, 1.2 Hz, 1H, Harom), 7.22 (s, 1H, Harom), 7.10 (s, 1H, Harom), 7.07−7.01 (m, 1H, Harom), 6.97 (d, J = 2.1 Hz, 1H, Harom), 6.85 (dd, J = 7.9, 1.3 Hz, 1H, Harom), 6.82−6.75 (m, 1H, Harom), 6.67−6.54 (m, 1H, Harom), 6.37 (d, J = 2.1 Hz, 1H, Harom), 5.61−5.52 (m, 1H, CH(CH3)2), 2.93 (s, 6H, N(CH3)2), 1.15 (d, J = 6.8 Hz, 6H, CH(CH3)2), −20.92 (s, 1H, NiH). 13C NMR (75 MHz, C6D6): δ 151.5, 147.0, 143.2, 127.2, 126.3, 120.8, 120.5, 119.4, 117.6, 116.8, 116.2, 115.8, 114.7, 51.6, 50.9, 33.3, 30.2, 20.0, 14.0, 8.5. Synthesis of 2c. A 100 mL reaction vessel was charged with 1c (0.92 g, 2.0 mmol), NaBH4 (0.12 g, 3.0 mmol), and THF (60 mL) under an N2 atmosphere. The reaction mixture was stirred for 5 h until the green reaction mixture turned yellow. After removal of the solvent under vacuum, the yellow residue was washed with n-pentane (30 mL) and then extracted with Et2O (3 × 20 mL) and filtered through Celite. 2c as orange crystals was obtained from diethyl ether at 0 °C. Dec pt: >141 °C. Yield: 55% (0.42 g). IR (Nujol, cm−1): 1841.47 ν(Ni−H); 1580.63 ν(CC). Anal. Calcd for C24H24N4Ni (427.17 g mol−1): C, 67.48; H, 5.66; N, 13.12. Found: C, 67.21; H, 5.82; N, 13.01. 1H NMR (300 MHz, C6D6): δ 8.28 (dd, J = 8.4, 1.5 Hz, 1H, Harom), 7.78 (dd, J = 8.1, 1.2 Hz, 1H, Harom), 7.25 (d, J = 1.8 Hz, 2H, Harom), 7.19−7.14 (m, 2H, Harom), 7.12−7.07 (m, 3H, Harom), 7.03−6.97 (m, 1H, Harom), 6.84 (d, J = 2.1 Hz, 1H, Harom), 6.80 (dd, J = 8.1, 1.5 Hz, 1H, Harom), 6.77−6.72 (m, 1H, Harom), 6.60−6.53 (m, 1H, Harom), 6.19 (d, J = 2.1 Hz, 1H, Harom), 5.36 (s, 2H, NCH2Ph), 2.84 (s, 6H, N(CH3)2), −20.63 (s, 1H, NiH). 13C NMR (150 MHz, C6D6): δ 151.1, 146.5, 142.8, 137.8, 128.4, 127.9, 127.4, 127.3, 126.8, 126.0, 120.4, 120.1, 119.0, 117.3, 117.2, 115.9, 115.6, 114.4, 54.1, 51.4, 32.1, 30.1, 22.9, 13.8. Synthesis of 3b. MeI (0.54 g, 2.0 mmol) in 20 mL of THF was added to 2b (0.76 g, 2.0 mmol) in 30 mL of THF. The mixture was stirred at room temperature overnight, and then the solvent was removed by vacuum. The residue was extracted with Et2O. 3b as green crystals was obtained from diethyl ether at 0 °C. Yield: 58% (0.59 g). Dec pt: >165 °C. IR (Nujol, cm−1): 1582.87 ν(CC). Anal. Calcd for C20H23IN4Ni (505.02 g mol−1): C, 47.57; H, 4.59; N, 11.09. Found: C, 47.41; H, 4.42; N, 11.01. 1H NMR (300 MHz, CDCl3): δ 7.77 (dd, J = 8.4, 1.2 Hz, 1H, Harom), 7.25 (d, J = 1.2 Hz, 1H, Harom), 7.11−7.05 (m, 1H, Harom), 7.03 (d, J = 7.9 Hz, 1H, Harom), 6.87−6.80 (m, 1H, Harom), 6.80−6.74 (m, 2H, Harom), 6.60 (s, 1H, Harom), 6.59−6.52 (m, 1H, Harom), 6.21 (s, 1H, Harom), 6.13 (m, 1H, CH(CH3)2), 3.22 (s, 3H, N(CH3)2), 2.65 (s, 3H, N(CH3)2), 1.30−1.25 (m, 6H, CH(CH3)2). 13 C NMR (75 MHz, C6D6): 149.9, 147.6, 142.2, 130.7, 127.2, 126.8, 120.6, 120.5, 120.1, 119.6, 118.6, 118.4, 116.8, 116.1, 53.9, 52.4, 49.4, 29.7, 24.8, 22.2. Synthesis of 4b. A 100 mL reaction vessel was charged with 1b (0.92 g, 2.0 mmol), NaOMe (0.11 g, 3.0 mmol), and THF (60 mL) under an N2 atmosphere. The reaction mixture was stirred overnight. After removal of the solvent under vacuum, the residue was washed with n-pentane and filtered through Celite. 4b as purple crystals was obtained from n-pentane/water at 0 °C. Dec pt: >148 °C. Yield: 60% (0.49 g). IR (Nujol, cm−1): 1573.01 ν(CC). Anal. Calcd for C21H26N4NiO (409.15 g mol−1): C, 61.65; H, 6.41; N, 13.69. Found: C, 61.51; H, 6.60; N, 13.79. 1H NMR (300 MHz, CDCl3): δ 7.98 (dd, J = 8.4, 1.2 Hz, 1H, Harom), 7.41 (dd, J = 8.1, 1.2 Hz, 1H, Harom), 7.17− 7.06 (m, 2H, Harom), 7.00 (dd, J = 7.8, 1.2 Hz, 1H, Harom), 6.94−6.87 (m, 1H, Harom), 6.83−6.75 (m, 2H, Harom), 6.63−6.56 (m, 1H, Harom), 6.28 (d, J = 2.1 Hz, 1H, Harom), 6.20 (m, 1H, CH(CH3)2), 3.01 (s, 3H, N(CH3)2), 2.76 (s, 3H, N(CH3)2), 1.29 (s, 6H, CH(CH3)2), 0.98 (s,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00848. Selected crystallographic data and original IR, 1H NMR, and 13C NMR spectra of the compounds (PDF) Accession Codes

CCDC 1546801 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for H.S.: [email protected]. ORCID

Xiaoyan Li: 0000-0003-0997-0380 Hongjian Sun: 0000-0003-1237-3771 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC Nos. 21572119/21372143. REFERENCES

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DOI: 10.1021/acs.organomet.7b00848 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.7b00848 Organometallics XXXX, XXX, XXX−XXX