Nickel-Catalyzed Coupling of Carbon Dioxide with Cyclohexene

Jan 11, 2017 - Ting-Yu Lee† , Yi-Jen Lin†, Yuan-Zhen Chang†, Li-Shin Huang‡, Bao-Tsan Ko‡, and Jui-Hsien Huang§. † Department of Applied ...
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Nickel-Catalyzed Coupling of Carbon Dioxide with Cyclohexene Oxide by Well-Characterized Bis(N-Heterocyclic Carbene) Carbazolide Complexes Ting-Yu Lee,*,† Yi-Jen Lin,† Yuan-Zhen Chang,† Li-Shin Huang,‡ Bao-Tsan Ko,‡ and Jui-Hsien Huang§ †

Department of Applied Chemistry, National University of Kaohsiung, Kaohsiung 81148, Taiwan Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan § Department of Chemistry, National Changhua University of Education, Changhua 500, Taiwan ‡

S Supporting Information *

ABSTRACT: The facile synthesis, structural characterization, and catalytic studies for CO2/epoxide coupling of nickel acetates based on carbazolide-bis(NHC) (NHC = N-heterocyclic carbene) were reported. Treatment of 3,6-di-tert-butyl-1,8-bis(3-alkylimidazolium1-yl)carbazole salt proligands (bis(R-Im-X)Cz, R = benzyl (Bn), nbutyl (nBu), methyl (Me), X = Br or I) with nickel acetate tetrahydrate in the presence of excess triethylamine generated monomeric four-coordinate nickel complexes [(bis(R-Im)Cz)Ni(OAc)] (R = Bn (3), R = nBu (4), and R = Me (5)). Single-crystal X-ray diffraction of Ni complexes 4 and 5 indicates that the bis(NHC)-carbazolide fragment behaves as a CNC-tridentate pincer ligand to coordinate the metal center, and the ancillary acetate group assumes a terminal acetate bonding mode. Catalysis for coupling of carbon dioxide with cyclohexene oxide (CHO) by these carbazolide-bis(NHC)-ligated Ni complexes was systematically examined. Experimental results displayed that cycloaddition of CHO and CO2 catalyzed with complex 4 could give cyclohexene carbonate (CHC) with >99% cis-isomer selectivity on using low catalyst concentrations and high reaction temperature, whereas catalyst 3 was able to copolymerize CHO and CO2 to afford a narrowly dispersed and perfectly alternating poly(cyclohexene carbonate) (PCHC) as the major product at the higher catalyst loadings and lower copolymerization temperature. This is the first time that the air-stable bis(NHC)-carbazolide nickel(II) acetate is an effective and versatile catalyst for the formation of either biodegradable PCHCs or cis-CHCs.

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merization of epoxides and CO2.3 Comparing with discrete catalysts using the aforementioned metal center, structurally well-characterized nickel complexes for copolymerizing CO2/ epoxides were less explored,4 and only a few di- or trinuclear nickel complexes as single-component homogeneous catalysts for such copolymerization are recorded in the literature.4c−f For instance, Ko and co-workers have shown a new class of bimetallic Ni complexes containing bis(benzotriazole iminophenolate) or diamine-bis(benzotriazole phenolate) derivatives as high-performance catalysts for copolymerization of CO2 with cyclohexene oxide (CHO); the latter system exhibited not only high turnover frequencies (TOFs) but also excellent copolymerization behaviors including >99% poly(cyclohexene carbonate) selectivity and >99% carbonate repeat units.4e Most recently, Lin et al. reported a series of bi- and trinickel catalysts based on NNO-tridentate Schiff-base derivatives for CO2/ CHO copolymerization, and the trimetallic acetate complexes were found to effectively catalyze this alternating copolymerization with a moderate TOF in a controlled manner.4f

INTRODUCTION Considerable attention has been focused recently on the technological development of metal-catalyzed coupling of epoxide derivatives with other reactants due to the efficient synthesis of useful fine chemicals or polymeric materials.1 Particularly, carbon dioxide (CO2) as the second substrate source for such coupling/copolymerization reactions is beneficial for not only CO2 removal and reuse but also the production of well-defined biodegradable polycarbonates with a broad architecture in the polymer backbone (Scheme 1).2 Extensive efforts have shown that diverse catalytic systems mainly consisting of aluminum, chromium, cobalt, and zinc complexes incorporating various ancillary ligands were demonstrated to effectively catalyze the alternating copolyScheme 1. Alternating Copolymerization of Epoxides with CO2

Received: September 26, 2016

© XXXX American Chemical Society

A

DOI: 10.1021/acs.organomet.6b00756 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 2. Synthetic Routes for Nickel Complexes 3−5

the presence of organic bases without the harsh reaction conditions. A series of different substituted 3,6-di-tert-butyl-1,8bis(3-alkylimidazolium-1-yl)carbazole proligand salts, bis(R-ImX)Cz (R = benzyl (Bn), X = Br (1); R = n-butyl (nBu), X = Br (2); R = methyl (Me), X = I (A)6) were prepared in good yields (83−86%) by reaction of the corresponding alkyl halides and 3,6-di-tert-butyl-1,8-bis(imidazole)carbazole, which was obtained through coupling reaction of 3,6-di-tert-butyl-1,8dibromocarbazole 9 and imidazole by using CuI/tmeda (tetramethylethylenediamine) as the catalyst system rather than (CuOTf)2·C6H6/1,10-phenanthroline/dba (dibenzylideneacetone).6 1H NMR spectra revealed that the resonances of the methylene groups adjacent to the nitrogen atoms in the imidazole moieties for 1 and 2 appear at δ 6.0 and 4.6 ppm, along with those of N−H in the carbazole fragments at δ 12.3 and 12.0 ppm, respectively. In addition, the proton resonances of the N2C−H in the imidazolium motifs of 1 and 2 were shown at δ 11.6 and 11.1 ppm, respectively. These spectral characteristics clearly indicated the successful synthesis of the expected imidazolium salts. As shown in Scheme 2, the bis(R-Im-X)Cz salts reacted with nickel acetate tetrahydrate and excess triethylamine (NEt3) through the deprotonation process to give the air- and moisture-stable nickel(II) acetates [(bis(R-Im)Cz)Ni(OAc)] (R = Bn (3), R = nBu (4), and R = Me (5)) in fair yields (44− 76%). The addition of an excess amount (>7 equiv) of NEt3 was necessary; otherwise a mixture would be obtained because the nickel halide analogue was formed. The disappearance of the proton signals of the N−H and N2C−H in ligand precursors indicates the formation of the desired carbene nickel carbazolide species. The NHC groups in Ni complexes 3−5 are also reconfirmed by the resonances of the carbene C in the 13C NMR spectra at about δ 160 ppm. The 1H NMR spectrum of 3−5 exhibits the proton resonances of the methyl groups of the acetate (OAc) groups at δ 1.68, 1.91, and 2.09 ppm, respectively. The trend of downfield shifting of the acetato groups in the 1H NMR spectra from 3 to 5 implies that the R groups somewhat influence the electronic atmosphere in Ni compounds. It is worthy to note that the proton resonances of methylene groups next to the nitrogen atoms of NHC moieties in 4 showed a triplet signal at δ 4.5 ppm and those of methyl groups in 5 presented a singlet signal at δ 4.1 ppm, but two humps at δ 5.2 and 5.3 ppm were assigned as the resonances of the methylene groups of the benzyl motifs in 3 at ambient temperature (290 K) according to the 1H−13C HSQC 2D NMR spectrum (Figure S1 in the Supporting Information (SI)). Interestingly, the coalescence of two signals was detected at 300 K by employing the variable-temperature NMR method,

Since the CO2/CHO copolymerization process is believed to involve a coordination−insertion step, the performance of the catalysts would highly depend on their electronic and steric features from ancillary ligands. We study nickel catalysts based on tridentate ligands comprising strong electron-donating functional groups for CO2/CHO copolymerization. The anionic CNC pincer-type ligands with bis-N-heterocyclic carbene (bis-NHC) moieties have drawn the attention of our group because NHCs provide strong σ-donicity and sterically hindered diversities.5,6 Kunz et al. have reported that the rhodium complex supported by the bis(3-methylimidazolin-2ylidene)carbazolide (bimca) ligand, which could be described as a monoanionic carbazolide donor combined with two NHC motifs, possesses a greatly nucleophilic nature.6 Fascinatingly, this rhodium carbonyl complex assisted by Lewis acids was able to catalyze Meinwald rearrangement of monoalkylated epoxides into methyl ketones efficiently and regioselectively.6d Cyclohexene oxide can therefore be transformed into cyclohexenone. The bimca ligand was also put into use for synthesis of anionic zerovalent group 10 complexes by reduction of bimca metal chloride compounds, which were prepared from their corresponding hydrido complexes.6c Nevertheless, the (bimca)Ni−H complex was extremely sensitive and only isolated in a small-scale experiment.6c Very recently, Grotjahn et al. have demonstrated that direct metalation could synthesize pincer protic N-heterocyclic carbene (PNHC) group 10 chloride complexes, which could be converted to the acetate analogues.7 While the yield of the PNHC-nickel chloride was not high and chromatographic purification was executed, this direct metalation method for synthesis of an NHC-nickel complex would be very inspiring. However, no air-stable carbene complex of Ni(II) acetates was isolated for the coupling of CO2 with epoxides to date, although bis(phenolate) NHC titanium(IV) complexes were demonstrated to be active catalysts for the copolymerization of this kind.8 Herein, we present a convenient synthetic route and molecular structures of a series of nickel acetate complexes containing 1,8-bis(3-alkylimidazolin-2-ylidene) carbazolide derivatives. Moreover, the catalytic studies of CO2/CHO coupling by these carbazolide-bis(NHC)-ligated Ni complexes are systematic investigated.

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RESULTS AND DISCUSSION Synthesis and Spectroscopic Characterization of 1,8Bis(3-alkylimidazolin-2-ylidene) Carbazolyl Nickel Acetates (bis(R-Im)Cz-NiOAc, 3−5). In comparison to the preparation of the previously reported bis(NHC)-carbazolide metal complexes,6,7 acetato nickel complexes bearing similar ligands could be easily synthesized via salt elimination of nickel acetate salts with carbazolide-bis(NHC) ligand precursors in B

DOI: 10.1021/acs.organomet.6b00756 Organometallics XXXX, XXX, XXX−XXX

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Organometallics and the AB pattern was observed at low temperature (Figure S2 in the SI). All nickel complexes were isolated as crystalline solids and were also characterized on the basis of MS and elemental analysis as well as IR spectroscopy. Molecular structures of nickel complexes 4 and 5 were further verified by single-crystal X-ray crystallography. Molecular Structure of Complexes 4 and 5. Single crystals of 4 and 5 suitable for X-ray diffraction were obtained by recrystallization from a mixture of CH2Cl2/ethanol/hexane and dichloromethane/Et2O solution, respectively. Their molecular structures, selective bond distances, and bond angles are shown in Figures 1 and 2. Complex 4 is a monomeric four-

Figure 2. ORTEP drawing of Ni complex 5 with probability ellipsoids drawn at the 50% level. Selected bond lengths (Å) and angles (deg): Ni1−O1, 1.912(4); Ni1−N1, 1.847(5); Ni1−C13, 1.933(7); Ni1− C17, 1.936(7); O1−C29, 1.226(8); O2−C29, 1.271(8); O1−Ni1− N1, 161.9(2); O1−Ni1−C13, 93.0(2); O1−Ni1−C17, 90.0(2); N1− Ni1−C13, 90.5(2); N1−Ni1−C17, 91.1(2); C13−Ni1−C17, 165.4(2).

1.936(4) Å, which is slightly longer than those in the earlier reports for the mononuclear NHC-nickel complexes (in the range of 1.87−1.92 Å).7,10 In comparison, the Ni−N(1) distance of 4 is 1.834(3) Å, which is ∼0.06 Å shorter than that (1.898(5) Å) of the CpNNi(carbazolato) complex,11 but is very similar to that (1.8259(5) Å) of the PNHC-featuring Ni complex.7 Compound 5 crystallizes in triclinic space group P1̅ with two independent molecules of [(bis(Me-Im)Cz)Ni(OAc)] in an asymmetric unit, and one representative molecular structure is illustrated in Figure 2. The similar geometric coordination is recognized in complex 5, as evidenced by the bond angles of O(1)−Ni(1)−N(1) = 161.9(2)°, C(13)−Ni(1)−C(17) = 165.4(2)°, N(1)−Ni(1)− C(13) = 90.5(2)°, and N(1)−Ni(1)−C(17) = 91.1(2)°, respectively. It is worthy of note that the ancillary acetate group in crystal structures 4 and 5 adopts a terminal acetate bonding mode to coordinate the metal center. The average bond distance between the Ni center and Ccarbene atoms in NHC fragments of 5 is 1.932(7) Å, almost the same as that in 4. The average Ni−N distance (1.848(5) Å) and Ni−O distance (1.909(4) Å) in 5 are also similar to those (1.834(3), 1.902(2) Å) in 4. The Ni−O bond length of 1.9067(11) Å in [(PNHC)Ni(OAc)]7 is similar to that in 5. However, the discriminative bond distance differences between 4 and 5 are observed in the acetate groups. The average distance between the C(carbonyl carbon) atom and O(bonded) atom that is connected to a nickel atom is 1.239(8) Å, shorter than the C(carbonyl carbon)−O(nonbonded) average distance of 1.260(8) Å in 5, which is the reverse for 4, with a C(35)− O(1) distance of 1.258(4) Å and a C(35)−O(2) distance of 1.197(4) Å, respectively.

Figure 1. ORTEP drawing of Ni complex 4 with probability ellipsoids drawn at the 50% level. All hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ni−O1, 1.902(2); Ni− N1, 1.834(3); Ni−C13, 1.936(4); Ni−C20, 1.935(4); O1−C35, 1.258(4); O2−C35, 1.197(4); O1−Ni−N1, 162.90(12); N1−Ni− C13, 90.89(13); N1−Ni−C20, 90.78(13); O1−Ni−C13, 90.36(13); O1−Ni−C20, 93.29(13); C13−Ni−C20, 161.94(15).

coordinated species, and the Ni(II) center assumes a distorted square planar geometry with the chelation of two C-donor atoms originating from the NHC moieties, one nitrogen atom from the carbazole moiety, and an oxygen atom from the acetate group. The bond angles of O(1)−Ni−N(1), C(13)− Ni−C(20), N(1)−Ni−C(13), and N(1)−Ni−C(20) are 162.90(12)°, 161.94(15)°, 90.89(13)°, and 90.78(13)°, respectively. The distortion might be attributed to the steric encumbrance resulting from the alkyl substituents on the N atoms, since the analogous angles of O−Ni−N and C−Ni−C in complex [(PNHC)Ni(OAc)] are 174.84(5)° and 175.95(7)°, respectively.7 The molecular structure is consistent with the diamagnetic feature of the d8 Ni(II) square planar complex. The nickel atom departs from the C(13)−N(1)− C(20) by about 0.29 Å. The average bond length of the Ni atom and carbene carbon (Ni−C(13) and Ni−C(20)) for 4 is C

DOI: 10.1021/acs.organomet.6b00756 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Table 1. Coupling of CHO and CO2 Catalyzed by Using Carbene Nickel(II) Complexes 1−3a

entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17h

cat. (mol %) 3 4 5 3 4 5 3 4 5 3 3 3 3

(0.10) (0.10) (0.10) (0.10) (0.10) (0.10) (0.50) (0.50) (0.50) (0.50) (0.50) (0.25) (0.25)

f

3 (1.0) 3 (1.0) 3 (0.50)

pCO20 (psi)

temp (°C)

time (h)

% CHO convb

% CHC [% trans]b

TONc

TOF/ h−1d

% copolymer [% carbonate]b

500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500 500

130 130 130 170 170 170 130 130 130 130 110 110 110 170 110 95 130

24 24 24 24 24 24 24 24 24 48 48 72 144 24 48 48 24

9 28 41 71 99 73 54 95 84 90 81 39 70

>99 [18] >99 [8] >99 [8] >99 [4] >99 [99 [2] 39 [9] 98 [3] 65 [10] 50 [19] 38 [7] 36 [11] 32 [10]

90 280 410 710 990 730 108 190 168 180 162 156 280

3 12 17 30 41 30 5 8 7 4 3 2 2

93 74 99

33 [ 6000 g/mol) as depicted in entry 13 of Table 1. Attempts to obtain a better copolymer selectivity through the adjustments of different catalyst concentrations and coupling temperatures were D

DOI: 10.1021/acs.organomet.6b00756 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 3. 1H NMR spectrum of the purified copolymer obtained by using Ni complex 3 (Table 1, entry 13) in CDCl3. Peak at δ = 4.65 ppm is assigned to the methine protons in PCHC, and no significant signal at 3.2−3.5 ppm confirms >99% carbonate linkages in PCHC. Peaks at 4.42 and 3.58 ppm are assigned to the methine protons on the end group (OCH(CH2)4CHOH). The resonances at 0.86 and 1.25 ppm are attributed to the residual n-hexane in the polymeric product.

moiety acts as a CNC-tridentate ligand to coordinate the metal center. All Ni complexes were demonstrated to be active catalysts for coupling of CHO and CO2 without cocatalysts. Experimental results indicated that complex 4, with the better electron-donating substituents (nBu groups) in the pincer ligand, was able to efficiently catalyze the CHO/CO 2 cycloaddition to produce cyclohexene carbonate with >99% cis-isomer selectivity at the lower catalyst concentrations and higher coupling temperature (0.1 mol % catalyst loading, 170 °C). Particularly, Ni catalyst 3 could copolymerize CHO with CO2 to give a narrowly dispersed poly(cyclohexene carbonate) with >99% carbonate-linkage content as the major product under the conditions of 0.25 mol % catalyst loadings and 110 °C. These results provide a successful example of using a switchable bis(NHC)-carbazolide-ligated Ni(II) acetate catalyst for either cycloaddition or copolymerization via adjustments of catalyst concentrations and coupling temperature.

unsuccessful, as displayed in entries 15 and 16 of Table 1. On the basis of the 1H NMR spectroscopic studies of the purified copolymer obtained by 3 (Figure 3), a characteristic resonance at 4.65 ppm could be assigned to the methine protons in the carbonate repeating units, and no significant signal at 3.2−3.5 ppm related to the ether linkages was observed, suggesting >99% carbonate linkages in PCHC. The MALDI-TOF (matrixassisted laser desorption/ionization time-of-flight) spectrum from the obtained PCHC sample produced by Ni complex 3 (Table 1, entry 13) has been studied to understand the microstructure and composition of the PCHC as depicted in Figure S4. The mass spectrum shows that a series of peaks are divided into fixed intervals with a molecular mass about 142 Da, indicative of a repeated unit of PCHC. On the basis of the MALDI-TOF spectrum in Figure S4b, these main peaks corresponding to [OH + (C7H10O3)n + C6H10O + C2H3O2 + Na]+ ((C7H10O3)n = (−C6H10−O−C(O)−O−)n, C6H10O = −C6H10O−, and C2H3O2 = OAc) were observed, suggesting that the CHO/CO2 copolymerization is alternating and the chain propagation was initiated by the acetato group. As illustrated in Figure S5, all the yielded polycarbonates from Ni catalyst 3 possess a bimodal molecular weight distribution. Moreover, trace quantities of protic species in the CHO monomer might act as the chain transfer reagent and as a bifunctional initiator to result in lower molecular weight polymers compared to the theoretical molecular weights.13 In comparison with catalysis for CO2/CHO copolymerization, NHC-Ni complex 3 displays a lower catalytic performance including activity and copolymer selectivity than that of the bimetallic nickel catalyst containing the bis(benzotriazole iminophenolate) ancillary ligand under similar conditions.4c This is a successful example of switching catalysis14 for cycloaddition or copolymerization of CHO with CO2 via adjustments of the effects of catalyst concentrations or coupling temperature by using one well-defined bis(NHC)-carbazolide nickel(II) complex as a catalyst.

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EXPERIMENTAL SECTION

General Considerations. All air- and moisture-sensitive reactions were performed under a N2 atmosphere using standard Schlenk techniques. All reagents were purchased from commercial sources and used as received. 1,8-Dibromo-3,6-tert-butyl-9H-carbazole was synthesized according to the literature.9 3,6-Di-tert-butyl-1,8-bis(imidazole)carbazole was synthesized using CuI/TMEDA instead of (CuOTf)2· C6H6/1,10-phenanthroline/dba as catalyst by a modified procedure of the literature. 6a 3,6-Di-tert-butyl-1,8-bis(3-methylimidazolium)carbazole diiodide (A) was prepared according to the literature.6a 1H and 13C NMR spectra were obtained on a Varian Mercury 300 Plus spectrometer. Chemical shifts for 1H and 13C spectra were recorded in ppm relative to the residual protons and 13C of CDCl3 (δ 7.26, 77.7). Mass spectra were recorded on a Thermo Electron Corporation PolarisQ. Elemental analyses were performed on a Heraeus CHN-OS Rapid at the Instrument Center, NCHU. Gel permeation chromatography (GPC) measurements were performed on a Jasco PU-2080 Plus system equipped with an RI-2031 detector using THF (HPLC grade) as an eluent. The chromatographic column was a Phenomenex Phenogel 5 μm, 103 Å, and the calibration curve used to calculate Mn(GPC) was produced from polystyrene standards. The GPC results were calculated using the Scientific Information Service Corporation (SISC) Chromatography Data Solution 3.1 edition. Synthesis of 3,6-Di-tert-butyl-1,8-bis(3-benzylimidazolium)carbazole Dibromide (1). A mixture of 3,6-di-tert-butyl-1,8-bis(imidazole)carbazole (3.00 g, 7.29 mmol) and benzyl bromide (2.7 mL, 22.7 mmol) in toluene (10.0 mL) was stirred and refluxed for 24 h. The precipitate was filtered and washed with toluene and acetonitrile to give a white solid, 1 (2.27 g, 83%). 1H NMR (300 MHz, CDCl3, 298 K, ppm): δ 1.45 (s, 18H, C(CH3)3), 6.01 (s, 2H, NCH2), 7.17 (s, 2H, Hcarb-4/5), 7.38 (d, 3J(HH) = 1.5 Hz, 2H, Himi-5),

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CONCLUSION Two new bis(NHC)-carbazolide salt proligands 1 and 2 and three novel nickel acetate complexes 3−5 containing such biscarbene ligands have been synthesized and fully characterized by spectroscopic methods as well as elemental analysis. The molecular structures of complexes 4 and 5 were determined by single-crystal X-ray crystallography: both complexes possess a tetracoordinated Ni center that adopts a distorted square planar geometry in the solid state, and the bis(NHC)-carbazolide E

DOI: 10.1021/acs.organomet.6b00756 Organometallics XXXX, XXX, XXX−XXX

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Organometallics 7.41 (m, 10H, N-C-C6H5), 7.67 (d, 3J(HH) = 1.5 Hz, 2H, Himi-4), 8.20 (s, 2H, Hcarb-2/7), 11.60 (s, 2H, Himi-2), 12.32 (s, 1H, NH). 13C{1H} NMR (75 MHz, CDCl3, 298 K, ppm): δ 31.9 (C(CH3)3), 35.0 (C(CH3)3), 54.2 (N-CH2), 118.7, 119.7, 126.4, 129.2, 129.9, 133.1, 133.3, (carb, Ph), 121.4 (imi-C5), 121.9, 138.6 (imi-C2), 144.3 (carbC3/C6). Anal. Calcd for C40H43N5Br2·H2O: C, 62.26; H, 5.88; N, 9.08. Found: C, 62.65; H, 6.12; N, 9.12. Synthesis of 3,6-Di-tert-butyl-1,8-bis(3-n-butylimidazolium)carbazole Dibromide (2). To a stirred suspension of 3,6-di-tertbutyl-1,8-bis(imidazole)carbazole (0.50 g, 1.21 mmol) in acetonitrile (10.0 mL) in a Schlenk flask was added 1.0 mL (1.27 g, 9.27 mmol) of 1-bromobutane. The mixture was stirred and refluxed for 24 h and then filtered after cooling to give an off-white solid. After washing with toluene (15 mL × 2), the product 2 was obtained (0.72 g, 86%). 1H NMR (300 MHz, CDCl3, 298 K, ppm): δ 0.90 (t, 3J(HH) = 7.2 Hz, 6H, N-C-C-C-CH3), 1.47 (s, 18H, C(CH3)3), 4.57 (t, 3J(HH) = 7.2 Hz, 4H, N-CH2), 7.15 (s, 2H, Hcarb-4/5), 7.26 (d, 3J(HH) = 1.8 Hz, 2H, Himi-5), 7.37 (d, 3J(HH) = 1.8 Hz, 2H, Himi-4), 8.07 (s, 2H, Hcarb2/7), 11.09 (s, 2H, Himi-2), 11.98 (s, 1H, NH). 13C{1H} NMR (75 MHz, CDCl3, 298 K, ppm): δ 13.6 (N-CCC-CH3), 19.7 (N-CC-CH2), 31.5 (C(CH3)3), 34.9 (C(CH3)3), 31.8 (N-C-CH2), 50.3 (N-CH2), 118.6 (carb-C2/C7), 118.7 (carb-C4/C5), 121.6 (imi-C5), 121.9 (imiC4), 118.7 (carb-1/8), 126.3 (carb-C4a/C5a), 133.2 (carb C1a/C8a), 138.8 (imi-C2),144.0 (carb-C3/C6). Anal. Calcd for C34H47N5Br2· 3H2O: C, 55.21; H, 7.22; N, 9.47. Found: C, 55.08; H, 7.39; N, 9.36. Synthesis of 3,6-Di-tert-butyl-1,8-bis(3-n-butylimidazolin-2ylidene)carbazolyl Nickel Acetate ([(bis(Bn-Im)Cz)Ni(OAc)], 3). A 1.00 g amount of 1 (1.33 mmol) was suspended in acetonitrile (5 mL) at 60 °C, sequentially followed by addition of 1.85 mL of triethylamine (1.34 g, 13.2 mmol) and 0.34 g (1.40 mmol) of Ni(OAc)2·4H2O. The mixture was stirred and refluxed overnight, and then the solvent was removed in vacuo. The residue was redissolved by CH2Cl2 and filtered through Celite. After removal of the solvent in vacuo and recrystallization by hexane/dichloromethane, a golden yellow solid, 1, was obtained (0.75 g, 76%). 1H NMR (300 MHz, CDCl3, 290 K, ppm): δ 1.49 (s, 18H, C(CH3)3), 1.68 (br, 3H, C(O)CH3), 5.2, 5.4 (br, 4H,N-CH2), 6.93 (s, 2H, Hcarb-4/5), 7.33 (m, 10H, N-C-C6H5), 7.49 (s, 2H, Hcarb-2/7), 7.76 (s, 3J(HH) = 2 Hz, 2H, Himi-4), 8.04 (d, 3J(HH) = 2 Hz, 2H, Himi-5). 13C{1H} NMR (75 MHz, CDCl3, 298 K, ppm): δ 24.1 (C(O)CH3), 32.2 (C(CH3)3), 34.8 (C(CH3)3), 53.1 (N-CH2), 109.1 (carb-C2/C7), 114.5 (imi-C5), 115.6 (imi-C4), 123.7 (carb-C4/C5), 127.1, 127.6, 128.8 (N-C-C6H5), 123.2 (carb-C1/C8), 127.5 (carb-C4a/C5a), 137.7 (N-C-C6H5), 135.2 (carb-C1a/C8a), 139.7 (carb-C3/C6), 160.9 (carbene), 177.7 (C(O) CH3). EI (12 eV) mass spectrum (m/z): 706.96. IR absorptions (cm−1, neat): 2961 (m), 1629 (m), 1596 (m), 1496 (w), 1442 (m), 1420 (m), 1362 (m), 1305 (s), 1266 (s), 1243 (s), 1202 (m), 1165 (w), 1111 (w), 1029 (w), 893 (w), 842 (s), 781 (m), 747 (s), 709 (s), 683 (s), 616 (w). Mp: >250 °C. Anal. Calcd for C42H43N5NiO2: C, 71.20; H, 6.12; N 9.88. Found: C, 70.60; H, 6.01; N, 9.43. Synthesis of 3,6-Di-tert-butyl-1,8-bis(3-n-butylimidazolin-2ylidene)carbazolyl Nickel Acetate ([(bis(nBu-Im)Cz)Ni(OAc)], 4). A 1.00 g amount of 2 (1.45 mmol) was suspended in acetonitrile (5 mL) at 60 °C, and then 1.48 mL of triethylamine (1.07 g, 10.6 mmol) was added, followed by addition of 0.40 g (1.61 mmol) of Ni(OAc)2· 4H2O. The mixture was stirred and refluxed overnight and then filtered through Celite. A brown solution was obtained, and the solvent was removed in vacuo, followed by washing with ethanol. Recrystallization from hexane/dichloromethane yielded a brown solid, 4 (0.40 g, 44%). 1H NMR (300 MHz, CDCl3, 298 K, ppm): δ 1.06 (t, 6H, 3J(HH) = 9 Hz N-CCC-CH3), 1.54 (m, 4H, N-CCCH2), 1.48 (s, 18H, C(CH3)3), 1.91 (br, 3H, C(O)CH3), 2.11 (m, 4H, N-C-CH2), 4.51 (br, 4H, N-CH2), 7.09 (d, 3J(HH) = 2 Hz, 2H, Hcarb4/5), 7.60 (d, 3J(HH) = 3 Hz, 2H, Hcarb-2/7), 7.78 (d, 3J(HH) = 2 Hz, 2H, Himi-4), 7.99 (d, 3J(HH) = 2 Hz, 2H, Himi-5). 13C{1H} NMR (75 MHz, CDCl3, 298 K, ppm): δ 14.2 (N-CCC-CH3), 20.5 (N-CC-CH2), 24.8 (C(O)CH3), 32.5 (C(CH3)3), 35.0 (C(CH3)3), 34.5 (N-C-CH2), 50.1 (N-CH2), 109.2 (carb-C2/C7), 114.5 (carb-C4/C5), 115.3 (Himi-C5), 123.3 (Himi-C4), 123.6 (carb-1/8), 127.8 (carb-C4a/ C5a), 135.5 (carb-C1a/C8a), 139.8 (carb-C3/C6), 160.2 (carbene),

177.7 (C(O)CH3). EI (12 eV) mass spectrum (m/z): 639.12. IR absorptions (cm−1, neat): 3089 (w), 2955 (s), 2868 (m), 1594 (s), 1442 (s), 1421 (s), 1385 (s), 1360 (s), 1336 (s), 1291 (w), 1269 (m), 1244 (m), 1210 (w), 1175 (w), 1107 (w), 1008 (w), 936 (w), 902 (w), 857 (w), 842 (w), 782 (w), 749 (s), 684 (s), 669 (w), 646 (w), 618 (w). Mp: 132 °C. Anal. Calcd for C36H47N5NiO2: C, 67.51; H, 7.40; N, 10.93. Found: C, 67.12; H, 7.66; N, 10.93. Synthesis of 3,6-Di-tert-butyl-1,8-bis(3-methylimidazolin-2ylidene)carbazolyl Nickel Acetate ([(bis(Me-Im)Cz)Ni(OAc)], 5). To a stirred suspension of A (1.00 g, 1.44 mmol) in acetonitrile (5 mL) at 60 °C was added 1.48 mL of triethylamine (1.07 g, 10.6 mmol), followed by addition of 0.40 g (1.61 mmol) of Ni(OAc)2· 4H2O. The mixture was stirred and refluxed overnight and then filtered through Celite to give a brown solution. The solvent was removed in vacuo, and the residue was washed with water. Recrystallization from hexane/dichloromethane gave 5 (0.50 g, 63%) as a brown solid. 1H NMR (300 MHz, CDCl3, 298 K, ppm): δ 1.50 (s, 18H, C(CH3)3), 2.09 (s, 3H, C(O)CH3), 4.09 (s, 6H, N−CH3), 7.02 (s, 2H, Hcarb-4/5), 7.51 (s, 2H, Hcarb-2/7), 7.77 (d, 3J(HH) = 2 Hz, 2H, Himi-4), 8.03 (d, 3J(HH) = 2 Hz, 2H, Himi-5). 13C{1H} NMR (75 MHz, CDCl3, 298 K, ppm): δ 25.1 (C(O)CH3), 32.3 (C(CH3)3), 34.7 (C(CH3)3), 38.6 (N-CH3), 108.9 (carb-C2/C7), 114.2 (carb-C4/C5), 114.7 (Himi-C5), 124.9 (Himi-C4), 123.4 (carb-C1/C8), 127.6 (carbC4a/C5a), 134.9 (car-C1a/C8a), 139.9 (carb-C3/C6), 160.5 (carbene), 177.4 (C(O)CH3). EI (12 eV) mass spectrum (m/z): 555.08 (M+). IR absorptions (cm−1, neat): 2951 (s), 2865 (w), 1594 (s), 1448 (s), 1392 (s), 1360 (s), 1329 (s), 1304 (s), 1269 (s), 1244 (s), 1200 (w), 1178 (w), 1100 (w), 1077 (w), 1023 (w), 859 (w), 842 (s), 793 (w), 747 (s), 719 (s), 707 (s), 679 (s), 661 (m), 643 (m), 614 (m). Mp: >250 °C. Anal. Calcd for C30H35N5NiO2·0.5CH2Cl2: C, 61.18; H, 6.06; N, 11.70. Found: C, 61.31; H, 6.39; N, 11.75. Coupling of CO2 and CHO Catalyzed by Ni Complexes 3−5. A representative procedure for the cycloaddition of cyclohexene oxide with CO2 was exemplified by complex 4. A mixture of Ni catalyst [(bis(nBu-Im)Cz)Ni(OAc)] (4) (32.0 mg, 0.05 mmol) was dissolved in 5.0 mL of neat cyclohexene oxide under a dry nitrogen atmosphere. The mixture solution was then added to the 100 mL autoclave with a magnetic stirrer under a CO2 atmosphere. CO2 was then charged into the reactor until the pressure of 500 psi was reached, and the stirrer was started. The reaction was carried out at 170 °C for 24 h. Then the reactor was placed into ice water, and excess CO2 was released. The CHO conversion (99%) was analyzed by 1H NMR spectroscopic studies. Spectral characteristics of cyclohexene carbonate: mutiplets, δ = 3.9 ppm (trans-CHC) or 4.63 ppm (cis-CHC).4d A representative procedure for the copolymerization of cyclohexene oxide with CO2 was exemplified by complex 3. Nickel catalyst [(bis(Bn-Im)Cz)Ni(OAc)] (3) (88.6 mg, 0.125 mmol) was dissolved in 5.0 mL of neat cyclohexene oxide under a dry nitrogen atmosphere. A mixing solution was added to the 100 mL autoclave with a magnetic stirrer under a CO2 atmosphere. CO2 was then charged into the reactor until the pressure of 500 psi was reached, and the stirrer was started. The reaction was performed at 110 °C for 144 h. Then the reactor was placed into ice water, and excess CO2 was released. After the reaction was dissolved in CH2Cl2 (5 mL), the polymer was precipitated into MeOH. The CHO conversion (70%) was analyzed by 1H NMR spectroscopic studies. The copolymer was characterized by the methine proton resonances in the 1H NMR spectra (d6benzene/CDCl3, v/v = 3/1), including the copolymer carbonate linkages (br, δ = 4.90−4.60 ppm), copolymer ether linkages (br, δ = 3.70−3.30 ppm), and a cyclic carbonate (multiples, δ = 3.85−3.75 ppm (cis-CHC), 3.25−3.15 ppm (trans-CHC)).12b Considering removal of the metal salt, the mixture was diluted with CH2Cl2 (20 mL) and washed with aqueous HCl (4 N, 100 mL). The organic layer was extracted and concentrated to 5 mL by vacuum evaporation. The copolymer was then precipitated into n-hexane (150 mL) to give the polymeric solids. X-ray Crystallographic Studies. Suitable crystals of 4 were mounted on a Bruker APEX2 diffractometer to collect diffraction data at 298 K. Suitable crystals of complexes 5 were mounted onto a glass fiber using perfluoropolyether oil and cooled rapidly in a stream of F

DOI: 10.1021/acs.organomet.6b00756 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

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cold nitrogen gas to collect diffraction data at 150 K using a Bruker APEX2 diffractometer. Intensity data were collected in 1350 frames with increasing w (width of 0.5° per frame). The absorption correction was based on the symmetry-equivalent reflections using the SADABS program.15 The space group determination was based on a check of the Laue symmetry and systematic absence and was confirmed by the structure solution. The structures were solved with direct methods using the SHELXTL package.15 All non-H atoms were located from successive Fourier maps, and hydrogen atoms were treated as a riding model on their parent C atoms. Anisotropic thermal parameters were used for all non-H atoms, and fixed isotropic parameters were used for H atoms. Drawing of the molecules was done using Oak Ridge Thermal Ellipsoid Plots (ORTEP).16 Crystallographic data of complexes 4 and 5 are summarized in Table S1 of the SI. CCDC files 729459 and 1503395 also contain the supplementary crystallographic data for complexes 4 and 5.

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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.6b00756. Figures giving additional NMR spectra for 4 and 5 (PDF) Crystallographic data for complexes 4 and 5 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.-Y. Lee). Tel: 886-7-5919465. Fax: 886-7-591-9348. ORCID

Ting-Yu Lee: 0000-0002-8672-5875 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Ministry of Science and Technology, Taiwan (MOST 1052113-M-390-003 to T.-Y.L. and MOST 105-2119-M-005-005 to B.-T.K.).

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REFERENCES

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DOI: 10.1021/acs.organomet.6b00756 Organometallics XXXX, XXX, XXX−XXX