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Synthesis, structure, and catalytic activity of nickel complexes with new chiral binaphthyl-based NHC-ligands
Journal of Organometallic Chemistry 729 (2013) 40e45
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Synthesis, structure, and catalytic activity of nickel complexes with new chiral binaphthyl-based NHC-ligands Haibin Song a, *, Dongna Fan a, b, Yuqiao Liu a, b, Guohua Hou b, Guofu Zi b, * a b
State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China Department of Chemistry, Beijing Normal University, Beijing 100875, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 December 2012 Received in revised form 23 January 2013 Accepted 24 January 2013
Two new chiral NHC-nickel complexes have been prepared from the reactions between Ni(OAc)2, K2CO3 or K2CO3/NaBPh4, and imidazolium salts 3 or 4, which are derived from (R)-2,20 -diamino-1,10 -binaphthyl. Treatment of mono-imidazolium salt 3 with 1 equiv of Ni(OAc)2 in the presence of K2CO3 in 1,4-dioxane at 105 C gives, after recrystallization from a CH2Cl2 solution, the chiral bis-ligated nickel complex (3)2Ni (5). Under similar reaction conditions, bis-imidazolium salt 4 affords a tetranuclear nickel complex [{(4) Ni}3Ni](BPh4)2 (6). All compounds have been characterized by various spectroscopic techniques, and elemental analyses. The solid-state structures of compounds 3, 5, 6 have been further confirmed by X-ray diffraction analyses. Nickel complexes 5 and 6 show a good (106 g/mol h) norbornene polymerization activity upon activation with methylaluminoxane (MAO), leading to thermally very stable polynorbornenes. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Carbene complex Nickel Binaphthyl ligand Polymerization of norbornene
1. Introduction Chiral transition-metal complexes based on N-Heterocyclic carbenes (NHCs) have received growing attention in the past two decades [1e7]. Besides their stability to air and moisture and their strong s-donor but poor p-acceptor abilities, another driving force for this work is the longstanding interest in the development of catalysts for enantioselective reactions such as olefin metathesis [8], conjugate addition of enones [9e11], allylic alkylations [12e15], olefin hydrogenations [16] and hydrosilylations [17e20], in which excellent enantioselectivities have been achieved. While many chiral transition-metal NHC-complexes have been studied extensively, only a small number of chiral NHC-nickel complexes, which are potential catalysts for various chemical transformations, have been reported [21e32]. Even within this class, few of them show significant reactivity [21e32]. Thus, the development of new chiral nickel NHC-complexes is still a desirable goal. Recently, we have developed a series of chiral binaphthyl-based multi-dentate ligands, and their Ti(IV), Zr(IV), V(IV), Ta(V), Rh(III) and lanthanide complexes that are useful catalysts for a wide range of transformations [33e44]. In our endeavor to further explore the chemistry of 2,20 -diamino-1,10 -binaphthyl, we recently have extended our research work to new multi-dentate pro-ligands (R)-3 and (R)-4, which contain one or two imidazolium salts and two
* Corresponding authors. Tel.: þ86 10 5880 6051; fax: þ86 10 5880 2075. E-mail address: [email protected] (G. Zi). 0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2013.01.020
s-donating atoms that can act in a tridentate or tetradentate fashion. In this paper, we report on some observations concerning the pro-ligands (R)-3 and (R)-4, and their use in nickel chemistry. 2. Experimental section 2.1. General methods All chemicals were purchased from Aldrich Chemical Co. and Beijing Chemical Co., and used as received unless otherwise noted. (R)-1 [45] and (R)-2 [45] were prepared according to literature methods. Infrared spectra were obtained from KBr pellets on an Avatar 360 Fourier transform spectrometer. Molecular weights of the polymer were estimated by gel permeation chromatography (GPC) using a PL-GPC 220 apparatus at 150 C. 1H NMR spectra were recorded on a Bruker AV 400 spectrometer. All chemical shifts are reported in d units with reference to the residual protons of the deuterated solvents. Melting points were measured on an X-6 melting point apparatus and were uncorrected. Elemental analyses were performed on a Vario EL elemental analyzer. Thermal gravimetric analyses (TGA) were performed on a NETZSCH TG-209 thermal analyzer. 2.2. Preparation of 3 Compound (R)-1 (1.00 g, 3.0 mmol) and 2-(ClCH2)C5H4N (0.51 g, 4.0 mmol) were stirred in CH3CN (10 mL) at 80 C for two
H. Song et al. / Journal of Organometallic Chemistry 729 (2013) 40e45
days. After cooling to room temperature, the white precipitate was filtered and washed with diethyl ether to give 3 as a white solid. Yield: 1.24 g (89%). M.p.: 240e243 C. 1H NMR (DMSO-d6): d 9.37 (br s, 1H, OH), 8.42 (d, J ¼ 4.08 Hz, 1H, aryl), 8.37 (d, J ¼ 8.8 Hz, 1H, aryl), 8.22 (d, J ¼ 8.2 Hz, 1H, aryl), 7.89 (m, 4H, aryl), 7.75 (t, J ¼ 6.7 Hz, 1H, aryl), 7.70 (m, 1H, aryl), 7.60 (s, 1H, NCH]N), 7.49 (t, J ¼ 7.7 Hz, 1H, aryl), 7.37 (m, 2H, aryl), 7.25 (m, 4H, aryl), 6.82 (d, J ¼ 8.2 Hz, 1H, aryl), 6.74 (d, J ¼ 7.6 Hz, 1H, aryl), 5.49 (d, J ¼ 15.6 Hz, 1H, CH2), 5.43 (d, J ¼ 15.6 Hz, 1H, CH2). IR (KBr, cm1): n 3440 (s), 3061 (s), 1623 (w), 1591 (w), 1551 (s), 1485 (s), 1384 (s), 1265 (s), 1187 (s), 996 (s), 833 (s), 745 (s). Anal. Calcd for C29H22N3ClO: C, 75.07; H, 4.78; N, 9.06. Found: C, 75.19; H, 4.75; N, 8.97%. Colorless crystals 3$0.5H2O suitable for X-ray structural analysis were grown from an CH3CN solution at room temperature.
41
2.6. General procedure for polymerization of norbornene In a nitrogen-filled glove box, norbornene (1.00 g), the nickel complex (0.40 mmol, based on nickel metal ion) and chlorobenzene (14.34 mL) were added into a flask (100 mL). After the mixture was stirred at the desired temperature for 10 min, 0.66 mL of MAO (1.5 M in chlorobenzene, 1.0 mmol) was added into the flask via syringe, and the reaction was started. Ten minutes later, the polymerization was quenched by the addition of acidified methanol (Vmethanol:Vconcd HCl ¼ 10:1). The resulting precipitated polymer was collected, washed with methanol and water several times, and dried in vacuum at 80 C overnight. IR (KBr, cm1): n 2946 (vs), 2866 (vs), 1472 (m), 1453 (s), 1384 (m), 1294 (m), 1259 (m), 1107 (m), 1039 (w), 942 (w), 890 (m). 2.7. X-ray crystallography
2.3. Preparation of 4 This compound was prepared as a white solid from the reaction of (R)-2 (1.00 g, 2.6 mmol) with BrCH2CH2OH (0.75 g, 6.0 mmol) in CH3CN (10 mL) by a similar procedure as in the synthesis of 3. Yield: 0.71 g (71%). M.p.: 157e158 C. 1H NMR (DMSO-d6): d 9.05 (s, 2H, NCH]N), 8.44 (d, J ¼ 8.8 Hz, 2H, aryl), 8.24 (d, J ¼ 8.4 Hz, 2H, aryl), 7.86 (d, J ¼ 8.8 Hz, 2H, aryl), 7.75 (m, 2H, aryl), 7.60 (m, 4H, aryl), 7.26 (d, J ¼ 8.8 Hz, 2H, aryl), 7.01 (s, 2H, aryl), 5.15 (br s, 2H, OH), 4.11 (m, 4H, CH2), 3.61 (m, 4H, CH2). IR (KBr, cm1): n 3442 (s), 1569 (s), 1541 (s), 1388 (s), 1206 (s), 1070 (s), 878 (s), 758 (s). Anal. Calcd for C30H28N4Br2O2: C, 56.62; H, 4.43; N, 8.80. Found: C, 56.72; H, 4.63; N, 8.78%.
Single-crystal X-ray diffraction measurements were carried out on a Bruker Smart APEX II CCD diffractometer or on a Rigaku Saturn CCD diffractometer at 113(2) K using graphite monochromated Mo; A). An empirical absorption correction Ka radiation (l ¼ 0.71070 was applied using the SADABS program [46]. All structures were solved by direct methods and refined by full-matrix least squares on F2 using the SHELXL-97 program package [47]. All the hydrogen atoms were geometrically fixed using the riding model. The crystal data and experimental data for 3, 5 and 6 are summarized in Table 1. Selected bond lengths and angles for 5 and 6 are listed in Table 2. 3. Results and discussion
2.4. Preparation of 5 3.1. Synthesis and characterization of pro-ligands Under a nitrogen atmosphere, a mixture of 3 (232 mg, 0.50 mmol), Ni(OAc)2∙4H2O (124 mg, 0.50 mmol), K2CO3 (207 mg, 1.50 mmol) was stirred in 1,4-dioxane (10 mL) for two days at 105 C. After cooling to room temperature, removal of the solvent and the residue was extracted with CH2Cl2 (15 mL 3) and filtered. The volume of the combined filtrate was reduced to approximately 5 mL. Orange crystals of 5 were isolated when this solution was allowed to stand at room temperature overnight. Yield: 105 mg (46%). M.p.: >300 C. 1H NMR (DMSO-d6): d 8.68 (m, 1H, aryl), 8.39 (m, 1H, aryl), 8.17 (m, 1H, aryl), 8.03 (m, 1H, aryl), 7.93 (m, 1H, aryl), 7.83 (m, 1H, aryl), 7.63 (m, 8H, aryl), 7.50 (m, 4H, aryl), 7.26 (m, 8H, aryl), 6.76 (m, 4H, aryl), 6.35 (m, 6H, aryl), 5.85 (m, 1H, CH2), 5.73 (m, 1H, CH2), 5.51 (m, 1H, CH2), 4.92 (m, 1H, CH2). IR (KBr, cm1): n 2964 (m), 1607 (s), 1425 (s), 1258 (s), 1094 (s), 1022 (s), 803 (s), 752 (s). Anal. Calcd. for C58H40N6NiO2: C, 76.41; H, 4.42; N, 9.22. Found: C, 76.38; H, 4.41; N, 9.26%. 2.5. Preparation of 6 This compound was prepared as yellow crystals from the reaction of 4 (318 mg, 0.50 mmol) with Ni(OAc)2∙4H2O (124 mg, 0.50 mmol), K2CO3 (414 mg, 3.00 mmol), and NaBPh4 (343 mg, 1.00 mmol) in 1,4-dioxane (10 mL) at 105 C, followed by recrystallization from a CH2Cl2 solution by a similar procedure as in the synthesis of 5. Yield: 149 mg (52%, based on Ni). M.p.: >300 C. 1H NMR (DMSO-d6): d 8.11 (m, 6H, aryl), 7.98 (m, 6H, aryl), 7.53 (m, 12H, aryl), 7.32 (m, 6H, aryl), 7.15 (m, 24H, Ph), 7.04 (m, 6H, aryl), 6.89 (m, 16H, Ph), 6.76 (m, 12H, aryl), 6.22 (m, 12H, OCH2), 3.21 (m, 12H, CH2). IR (KBr, cm1): n 3049 (s), 2929 (m), 1579 (s), 1419 (s), 1121 (s), 1098 (s), 818 (s), 731 (s). Anal. Calcd for C138H112N12B2Ni4O6: C, 72.35; H, 4.93; N, 7.34. Found: C, 72.33; H, 4.96; N, 7.32.
Imidazolium salts can be easily prepared by quaternization of the imidazole rings with alkyl halides [43,44]. For example, Table 1 Crystal data and experimental parameters for compounds 3, 5 and 6. Compound
3$0.5H2O
5
6
Formula Formula weight Crystal system Space group a ( A) b ( A) c ( A) a ( ) b ( ) g ( ) V ( A3) Z Dcalc (g/cm3) m(Mo/Ka)calc (mm1) Size (mm) F(000) 2q range ( ) No. of reflns, collected No. of unique reflns No. of obsd reflns Abscorr (Tmax, Tmin) R Rw Rall gof
C29H23N3ClO1.5 472.95 Orthorhombic P212121 12.298(1) 20.556(2) 9.485(1) 90 90 90 2397.7(4) 4 1.310 0.189
C58H40N6NiO2 911.67 Orthorhombic P212121 10.048(2) 16.088(3) 32.937(5) 90 90 90 5324.6(15) 4 1.137 0.409
C138H112N12B2Ni4O6 2290.86 Triclinic P1 11.382(1) 14.933(1) 20.600(2) 99.50(1) 102.86(1) 100.94(1) 3271.7(5) 1 1.163 0.623
0.24 0.20 x 0.18 988 3.86e55.82 18,916
0.20 0.20 x 0.20 1896 2.82e55.72 53,186
0.32 0.14 x 0.06 1194 1.89e50.50 13,814
5720 (Rint ¼ 0.1069) 4052
12,360 (Rint ¼ 0.0532) 10,937
13,814 (Rint ¼ 0.0000) 9996
0.97, 0.96
0.92, 0.92
0.96, 0.83
0.049 0.103 0.065 0.94
0.053 0.143 0.060 1.08
0.066 0.152 0.089 0.96
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H. Song et al. / Journal of Organometallic Chemistry 729 (2013) 40e45
Table 2 Selected bond distances ( A) and bond angles (deg) for compounds 5 and 6. Compound 5 Ni(1)eC(21) Ni(1)eO(2) C(21)eNi(1)eC(50) C(21)eNi(1)eN(3) Torsion (arylearyl) Compound 6 Ni(1)eC(13) Ni(1)eO(1) Ni(2)eO(1) Ni(2)eO(3) Ni(2)eO(5) Ni(3)eC(73) Ni(3)eO(5) Ni(4)eC(43) Ni(4)eO(3) C(13)eNi(1)eC(28) O(2)eNi(1)eC(28) C(73)eNi(3)eC(88) O(6)eNi(3)eC(88) C(43)eNi(4)eC(58) C(58)eNi(4)eO(4) Torsion (arylearyl)
N
N 1.870(3) 1.882(2) 90.6(1) 87.6(1) 79.9(1), 88.3(1)
Ni(1)eC(50) Ni(1)eN(3) O(2)eNi(1)eC(50) O(2)eNi(1)eN(3) e
1.883(3) 1.952(3) 95.6(1) 86.2(1) e
1.882(9) 1.860(6) 1.994(5) 2.059(5) 2.074(6) 1.894(10) 1.877(6) 1.855(8) 1.882(6) 96.3(4) 92.1(3) 98.8(4) 91.7(3) 98.1(4) 92.7(3) 84.8(3), 86.7(3), 86.8(3)
Ni(1)eC(28) Ni(1)eO(2) Ni(2)eO(2) Ni(2)eO(4) Ni(2)eO(6) Ni(3)eC(88) Ni(3)eO(6) Ni(4)eC(58) Ni(4)eO(4) O(1)eNi(1)eC(13) O(1)eNi(1)eO(2) O(5)eNi(3)eC(73) O(5)eNi(3)eO(6) C(43)eNi(4)eO(3) O(3)eNi(4)eO(4)
1.879(9) 1.889(6) 2.118(6) 2.051(6) 2.024(6) 1.842(9) 1.888(6) 1.876(8) 1.869(5) 93.6(3) 79.2(3) 91.7(3) 78.5(2) 91.7(3) 80.4(2)
treatment of imidazoles (R)-1 or (R)-2, prepared from (R)-2,20 diamino-1,10 -binaphthyl [45], with an excess of 2-(chloromethyl) pyridine or 2-bromoethanol in CH3CN at 80 C gives the monoimidazolium salt (R)-3 and bis-imidazolium salt (R)-4, respectively, in good yields (Scheme 1). Compounds 3 and 4 are air-stable, soluble in DMSO, acetone and methanol but insoluble in CH2Cl2 and CHCl3. They have been characterized by various spectroscopic techniques and elemental analyses. The solid-state structure of compound 3 has been further confirmed by X-ray diffraction analysis. The X-ray diffraction analysis shows that there are two molecules of 4 and one H2O molecule in the asymmetric unit. In the molecular structure the binaphthyl groups adopt a staggered geometry (Fig. 1). The twisting between the binaphthyl rings of torsion angle is 84.8(1) , indicating that they are almost perpendicular with respect to each other. 3.2. Synthesis and characterization of complexes Treatment of mono-imidazolium salt 3 with 1 equiv of Ni(OAc)2 in the presence of K2CO3 in 1,4-dioxane at 105 C gives, after recrystallization from a CH2Cl2 solution, the chiral bis-ligated biscarbene nickel complex (3)2Ni (5) in 46% yield (Scheme 1), and no mono-ligated/mono-carbene complex was isolated. However, under similar reaction conditions, treatment of bis-imidazolium salt 4 with 1 equiv of Ni(OAc)2 in the presence of K2CO3 and NaBPh4 in 1,4-dioxane at 105 C gives a tetranuclear nickel complex [{(4) Ni}3Ni](BPh4)2 (6) which has been isolated in 52% yield (based on Ni) (Scheme 1). This observation is presumably due to the steric and electronic effects of the ligand. Complexes 5 and 6 are air-stable, and they are soluble in organic solvents such as DMSO, acetone, and CH3CN but partial soluble in toluene, and benzene. They have been characterized by various spectroscopic techniques, elemental analyses, and X-ray diffraction analyses. The molecular structure of 5 shows that the ion Ni2þ is four coordinate and s-bound to two carbon atoms (carbenes) and one nitrogen atom and one oxygen atom from the two ligand 3, the other oxygen (O(1)) and nitrogen (N(6)) atoms are far away from the metal center (Fig. 2). The sum of the angles in the equatorial plane containing the nitrogen atom and oxygen atom and carbene carbon atoms is 360.0(1) , indicating a coplanar arrangement, in
N O Ni
O N
Ni(OAc)2
N OH
N
N
N
K2CO3 1,4-dioxane
5
N N
Cl
CH2Cl
CH3CN
3
N NH2 NH2
HCHO(aq,10 eq.) OHCCHO(aq,10 eq.) NH4Cl(10 eq.)
OH + 1
H2O, 1,4-dioxane H3PO4
2 HOCH2CH2Br N
N
N
N
CH3CN
OH OH
N
N
N
N
N
2
2Br 4
K2CO3 Ni(OAc)2 1,4-dioxane NaBPh4 N
N Ni O O N N
Ni
N O O N Ni N N
N N O Ni O N N
2BPh4
6 Scheme 1. Synthesis of complexes 5 and 6.
which the two carbenes adopt a cis arrangement. The NieO distance is 1.882(2) A, and the NieN distance is 1.952 (3) A. The average NieC(carbene) distance is 1.877(3) A, which is comparable to those found in bis(aryloxide-NHC)Ni complexes (1.84e1.87 A) [48]. The twisting between the binaphthyl rings of torsion angles are 79.9(1) and 88.3(1) , which are comparable to that found in 3 (84.8(1) ). The molecular structure of 6 shows that it consists of well-separated, alternating layers of the anions [BPh4] and the complex cations [{(4)Ni}3Ni]2þ. Each cation [{(4)Ni}3Ni]2þ adopts
H. Song et al. / Journal of Organometallic Chemistry 729 (2013) 40e45
43
a
Fig. 1. Molecular structure of the cation in 3 (thermal ellipsoids drawn at the 35% probability level).
a C3 quasi-symmetric structure (Fig. 3a and b). The coordination environments around the three end nickel atoms ((Ni(1), Ni(3) and Ni(4)) are similar, each ion Ni2þ is four coordinate and sbound to two carbon atoms (carbenes) and two oxygen atoms from one ligand 4. The bis(carbene) moiety acts as a chelate with a bite angle of CeNieC of 96.3(4) for Ni(1), 98.8(4) for Ni(3), and 98.1(4) for Ni(4), respectively. The sum of the angles in the equatorial plane containing the oxygen atoms and carbene carbon atoms is 361.2(3) for Ni(1), 360.7(4) for Ni(3), and 362.9(4) for Ni(4), respectively, indicating that these Ni atoms take a distorted square-planar coordination geometry. The average NieC(carbene) distance is 1.881(9) A for Ni(1), 1.868(10) A for Ni(3), and 1.862(2) A for Ni(4), respectively, and the average NieO distance is 1.875(6) A for Ni(1), 1.883(6) A for Ni(3), and 1.876(6) A for Ni(4), respectively. These structural data are close to those found in bis(aryloxide-NHC)Ni complexes [48].
b
Fig. 3. (a) Molecular structure of the cation in 6 (thermal ellipsoids drawn at the 35% probability level). (b) Core structure of 6 (thermal ellipsoids drawn at the 35% probability level).
The central Ni (Ni(2)) is six coordinate and s-bound to six oxygen atoms from three ligands 4 in a distorted-octahedral geometry with the average NieO distance of 2.053(6) A. The twisting between the naphthyl rings of torsion angles are 84.8(3), 86.7(3) and 86.8(3), which are comparable to those found in 3 (84.8(1) ) and 5 (79.9(1) and 88.3(1) ). 3.3. Catalytic activity of complexes
Fig. 2. Molecular structure of 5 (thermal ellipsoids drawn at the 35% probability level).
To examine the catalytic ability of the nickel NHC-complexes toward the asymmetric reactions, several common reactions such as the allylation of benzaldehyde with allyltributyltin, the Henry reaction (nitroaldol reaction) of benzaldehyde with nitromethane, the cyclopropanation of styrene with ethyl diazoacetate, the amination of oxindole with di-tert-butyl azodicarboxylate, and the coupling reaction of benzaldehyde, triethylsilane with phenylmethylacetylene or 1-cyclohexylallene have been test. Unfortunately, these complexes exhibited either no activity or rather poor activity for these transformations, in which only racemic products with low conversions (<5%) were detected even when heated in
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H. Song et al. / Journal of Organometallic Chemistry 729 (2013) 40e45
solvents such as THF, methanol, CH2Cl2, CH3CN or toluene and with 20% precatalysts loading. Since the nickel complexes do not show effective catalytic activity in asymmetric reactions, a different transformation was investigated. In recent years, vinyl polynorbornene (PNB) has received considerable attention due to its dielectric and mechanical properties for technical application as an interlevel dielectric in microelectronics applications [49]. Recently, some nickel NHC-complexes have shown excellent catalytic activity in the polymerization of norbornene [48,50e52]. Therefore it is rational to propose that our chiral nickel NHC-complexes would have a potential use as catalysts in this transformation. In fact, the polymerization data show that the nickel complexes 5 and 6 can initiate the polymerization of norbornene in the presence of MAO under mild conditions (Table 3). For complex 5, the activity decreases with increasing temperature from 20 to 80 C (entries 1e4). The activity increases with increasing the Al/Ni ratio from 1500 to 2500 (entries 1, 5, 6). But the activity decreases when the Al/Ni ratio further increases (entries 7e9). Under similar reaction conditions, complex 6 exhibits a lower catalytic activity (only up to 1.34 106 g/mol h) in the polymerization of norbornene (entries 10 and 11), presumably due to the steric hindrance around the metal center and the firm bonding between the alkoxide-NHC ligands and Ni atom, which obstruct the generation of the active species. In the absence of nickel complex or MAO no polymer has been obtained. No typical double bond absorption at 1600e1700 cm1 is detected in the IR spectra of the obtained polymers, indicating that the polymerization initiated by our nickel complexes/MAO system proceeds as a vinyl-addition polymerization [50]. TGA studies on the obtained polymers reveal that these polymers are thermally very stable (up to 450 C). Under the conditions examined, molecular weights of the obtained polymers range from 184 to 1025 kg mol1. Complex 5 produces a narrow molecular weight distribution (ca. 2.5), indicating a single-site catalyst, while complex 6 gives a much broader molecular distribution (ca. 11.0), due to its two different mediated sites, or consistent with its C3 quasi-symmetric structure. Our results show that the catalytic activity of 5 resembles that of the bis(aryloxide-NHC)Ni complex [48], while it is more active than that of 6, presumably because of less steric hindrance around the metal center.
Table 3 Polymerization of norbornene catalyzed by Ni-complexes.a
Ni NHC-complex n
Yieldb Entry Complex Temp. MAO (Al/Ni) (g) ( C)
Activity Mwc Mw/Mnc Tdecd (106 g/ (kg/mol) ( C) mol h)
1 2 3 4 5 6 7 8 9 10 11
6.57 3.35 1.54 1.12 1.71 4.34 5.48 5.19 5.15 1.34 0.81
20 40 60 80 20 20 20 20 20 20 40
2500 2500 2500 2500 1500 2000 3000 3500 4000 2500 2500
0.4382 0.2234 0.1028 0.0746 0.1142 0.2895 0.3654 0.3462 0.3432 0.0895 0.0543
1025 572 278 224 274 725 907 874 865 251 184
2.4 2.3 2.7 2.8 2.5 2.6 2.3 2.4 2.3 10.3 11.5
445 440 442 445 446 445 443 446 445 448 450
Conditions: norbornene (1.0 g); precatalyst (0.40 mmol, based on nickel metal ion); MAO (1.5 M) in toluene, solvent, chlorobenzene (15 mL), time, 10 min. b Isolated yields. c Determined by GPC in 1,2,4-trichlorobenzene at 150 C using polystyrene standards. d Decomposition temperature was determined by TG (heating rate: 10 C/min). a
In conclusion, two new chiral NHC-nickel complexes 5 and 6 have been prepared from the reactions between Ni(OAc)2, K2CO3 or K2CO3/NaBPh4, and imidazolium salts 3 or 4, which are derived from (R)-2,20 -diamino-1,10 -binaphthyl. The steric and electronic effects of the ligand play an important role in the NHC-complex formation. For example, treatment of mono-imidazolium salt 3 with 1 equiv of Ni(OAc)2 in the presence of K2CO3 in 1,4-dioxane at 105 C gives the bis-ligated mononuclear nickel complex (3)2Ni (5). However, under similar reaction conditions, bis-imidazolium salt 4 affords a tetranuclear nickel complex [{(4)Ni}3Ni](BPh4)2 (6). Nickel complexes 5 and 6 show a good (106 g/mol h) norbornene polymerization activity upon activation with methylaluminoxane (MAO), leading to thermally very stable polynorbornenes. Further development of new chiral nickel NHC-complexes and exploration of these catalysts toward other types of transformations are still underway. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 20872062, 21074013, 21172022, 21272026), the Program for New Century Excellent Talents in University (NCET-10e0253), and Beijing Municipal Commission of Education. Supplementary materials CCDC 914692, 914694 and 914694 contain the supplementary crystallographic data for 3, 5 and 6. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (þ44) 1223-336-033; or e-mail: deposit@ ccdc.cam.ac.uk. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
MAO
5 5 5 5 5 5 5 5 5 6 6
4. Conclusions
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
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H. Song et al. / Journal of Organometallic Chemistry 729 (2013) 40e45 [27] Y.N. Belokon, A.V. Grachev, V.I. Maleev, V.N. Khrustalev, A.S. Peregudov, M. North, Tetrahedron: Asymmetry 19 (2008) 756e760. [28] W.W.N. O, A.J. Lough, R.H. Morris, Organometallics 28 (2009) 6755e6761. [29] F. Gao, K.P. McGrath, Y. Lee, A.H. Hoveyda, J. Am. Chem. Soc. 132 (2010) 14315e14320. [30] P.D. Newman, K.J. Cavell, B.M. Kariuki, Organometallics 29 (2010) 2724e2734. [31] K. Yoshida, S. Horiuchi, T. Takeichi, H. Shida, T. Imamoto, A. Yanagisawa, Org. Lett. 12 (2010) 1764e1767. [32] T. Shibata, S. Ito, M. Doe, R. Tanaka, H. Hashimoto, I. Kinoshita, S. Yano, T. Nishioka, Dalton Trans. 40 (2011) 6778e6784. [33] L. Xiang, Q. Wang, H. Song, G. Zi, Organometallics 26 (2007) 5323e5329. [34] G. Zi, L. Xiang, H. Song, Organometallics 27 (2008) 1242e1246. [35] Q. Wang, L. Xiang, H. Song, G. Zi, Inorg. Chem. 47 (2008) 4319e4328. [36] G. Zi, Q. Wang, L. Xiang, H. Song, Dalton Trans. (2008) 5930e5944. [37] G. Zi, X. Liu, L. Xiang, H. Song, Organometallics 28 (2009) 1127e1137. [38] G. Zi, Dalton Trans. (2009) 9101e9109. [39] G. Zi, F. Zhang, L. Xiang, Y. Chen, W. Fang, H. Song, Dalton Trans. 39 (2010) 4048e4061.
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Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, structure, and catalytic activity of nickel complexes with new chiral binaphthyl-based NHC-ligands Haibin Song a, *, Dongna Fan a, b, Yuqiao Liu a, b, Guohua Hou b, Guofu Zi b, * a b
State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China Department of Chemistry, Beijing Normal University, Beijing 100875, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 December 2012 Received in revised form 23 January 2013 Accepted 24 January 2013
Two new chiral NHC-nickel complexes have been prepared from the reactions between Ni(OAc)2, K2CO3 or K2CO3/NaBPh4, and imidazolium salts 3 or 4, which are derived from (R)-2,20 -diamino-1,10 -binaphthyl. Treatment of mono-imidazolium salt 3 with 1 equiv of Ni(OAc)2 in the presence of K2CO3 in 1,4-dioxane at 105 C gives, after recrystallization from a CH2Cl2 solution, the chiral bis-ligated nickel complex (3)2Ni (5). Under similar reaction conditions, bis-imidazolium salt 4 affords a tetranuclear nickel complex [{(4) Ni}3Ni](BPh4)2 (6). All compounds have been characterized by various spectroscopic techniques, and elemental analyses. The solid-state structures of compounds 3, 5, 6 have been further confirmed by X-ray diffraction analyses. Nickel complexes 5 and 6 show a good (106 g/mol h) norbornene polymerization activity upon activation with methylaluminoxane (MAO), leading to thermally very stable polynorbornenes. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Carbene complex Nickel Binaphthyl ligand Polymerization of norbornene
1. Introduction Chiral transition-metal complexes based on N-Heterocyclic carbenes (NHCs) have received growing attention in the past two decades [1e7]. Besides their stability to air and moisture and their strong s-donor but poor p-acceptor abilities, another driving force for this work is the longstanding interest in the development of catalysts for enantioselective reactions such as olefin metathesis [8], conjugate addition of enones [9e11], allylic alkylations [12e15], olefin hydrogenations [16] and hydrosilylations [17e20], in which excellent enantioselectivities have been achieved. While many chiral transition-metal NHC-complexes have been studied extensively, only a small number of chiral NHC-nickel complexes, which are potential catalysts for various chemical transformations, have been reported [21e32]. Even within this class, few of them show significant reactivity [21e32]. Thus, the development of new chiral nickel NHC-complexes is still a desirable goal. Recently, we have developed a series of chiral binaphthyl-based multi-dentate ligands, and their Ti(IV), Zr(IV), V(IV), Ta(V), Rh(III) and lanthanide complexes that are useful catalysts for a wide range of transformations [33e44]. In our endeavor to further explore the chemistry of 2,20 -diamino-1,10 -binaphthyl, we recently have extended our research work to new multi-dentate pro-ligands (R)-3 and (R)-4, which contain one or two imidazolium salts and two
* Corresponding authors. Tel.: þ86 10 5880 6051; fax: þ86 10 5880 2075. E-mail address: [email protected] (G. Zi). 0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2013.01.020
s-donating atoms that can act in a tridentate or tetradentate fashion. In this paper, we report on some observations concerning the pro-ligands (R)-3 and (R)-4, and their use in nickel chemistry. 2. Experimental section 2.1. General methods All chemicals were purchased from Aldrich Chemical Co. and Beijing Chemical Co., and used as received unless otherwise noted. (R)-1 [45] and (R)-2 [45] were prepared according to literature methods. Infrared spectra were obtained from KBr pellets on an Avatar 360 Fourier transform spectrometer. Molecular weights of the polymer were estimated by gel permeation chromatography (GPC) using a PL-GPC 220 apparatus at 150 C. 1H NMR spectra were recorded on a Bruker AV 400 spectrometer. All chemical shifts are reported in d units with reference to the residual protons of the deuterated solvents. Melting points were measured on an X-6 melting point apparatus and were uncorrected. Elemental analyses were performed on a Vario EL elemental analyzer. Thermal gravimetric analyses (TGA) were performed on a NETZSCH TG-209 thermal analyzer. 2.2. Preparation of 3 Compound (R)-1 (1.00 g, 3.0 mmol) and 2-(ClCH2)C5H4N (0.51 g, 4.0 mmol) were stirred in CH3CN (10 mL) at 80 C for two
H. Song et al. / Journal of Organometallic Chemistry 729 (2013) 40e45
days. After cooling to room temperature, the white precipitate was filtered and washed with diethyl ether to give 3 as a white solid. Yield: 1.24 g (89%). M.p.: 240e243 C. 1H NMR (DMSO-d6): d 9.37 (br s, 1H, OH), 8.42 (d, J ¼ 4.08 Hz, 1H, aryl), 8.37 (d, J ¼ 8.8 Hz, 1H, aryl), 8.22 (d, J ¼ 8.2 Hz, 1H, aryl), 7.89 (m, 4H, aryl), 7.75 (t, J ¼ 6.7 Hz, 1H, aryl), 7.70 (m, 1H, aryl), 7.60 (s, 1H, NCH]N), 7.49 (t, J ¼ 7.7 Hz, 1H, aryl), 7.37 (m, 2H, aryl), 7.25 (m, 4H, aryl), 6.82 (d, J ¼ 8.2 Hz, 1H, aryl), 6.74 (d, J ¼ 7.6 Hz, 1H, aryl), 5.49 (d, J ¼ 15.6 Hz, 1H, CH2), 5.43 (d, J ¼ 15.6 Hz, 1H, CH2). IR (KBr, cm1): n 3440 (s), 3061 (s), 1623 (w), 1591 (w), 1551 (s), 1485 (s), 1384 (s), 1265 (s), 1187 (s), 996 (s), 833 (s), 745 (s). Anal. Calcd for C29H22N3ClO: C, 75.07; H, 4.78; N, 9.06. Found: C, 75.19; H, 4.75; N, 8.97%. Colorless crystals 3$0.5H2O suitable for X-ray structural analysis were grown from an CH3CN solution at room temperature.
41
2.6. General procedure for polymerization of norbornene In a nitrogen-filled glove box, norbornene (1.00 g), the nickel complex (0.40 mmol, based on nickel metal ion) and chlorobenzene (14.34 mL) were added into a flask (100 mL). After the mixture was stirred at the desired temperature for 10 min, 0.66 mL of MAO (1.5 M in chlorobenzene, 1.0 mmol) was added into the flask via syringe, and the reaction was started. Ten minutes later, the polymerization was quenched by the addition of acidified methanol (Vmethanol:Vconcd HCl ¼ 10:1). The resulting precipitated polymer was collected, washed with methanol and water several times, and dried in vacuum at 80 C overnight. IR (KBr, cm1): n 2946 (vs), 2866 (vs), 1472 (m), 1453 (s), 1384 (m), 1294 (m), 1259 (m), 1107 (m), 1039 (w), 942 (w), 890 (m). 2.7. X-ray crystallography
2.3. Preparation of 4 This compound was prepared as a white solid from the reaction of (R)-2 (1.00 g, 2.6 mmol) with BrCH2CH2OH (0.75 g, 6.0 mmol) in CH3CN (10 mL) by a similar procedure as in the synthesis of 3. Yield: 0.71 g (71%). M.p.: 157e158 C. 1H NMR (DMSO-d6): d 9.05 (s, 2H, NCH]N), 8.44 (d, J ¼ 8.8 Hz, 2H, aryl), 8.24 (d, J ¼ 8.4 Hz, 2H, aryl), 7.86 (d, J ¼ 8.8 Hz, 2H, aryl), 7.75 (m, 2H, aryl), 7.60 (m, 4H, aryl), 7.26 (d, J ¼ 8.8 Hz, 2H, aryl), 7.01 (s, 2H, aryl), 5.15 (br s, 2H, OH), 4.11 (m, 4H, CH2), 3.61 (m, 4H, CH2). IR (KBr, cm1): n 3442 (s), 1569 (s), 1541 (s), 1388 (s), 1206 (s), 1070 (s), 878 (s), 758 (s). Anal. Calcd for C30H28N4Br2O2: C, 56.62; H, 4.43; N, 8.80. Found: C, 56.72; H, 4.63; N, 8.78%.
Single-crystal X-ray diffraction measurements were carried out on a Bruker Smart APEX II CCD diffractometer or on a Rigaku Saturn CCD diffractometer at 113(2) K using graphite monochromated Mo; A). An empirical absorption correction Ka radiation (l ¼ 0.71070 was applied using the SADABS program [46]. All structures were solved by direct methods and refined by full-matrix least squares on F2 using the SHELXL-97 program package [47]. All the hydrogen atoms were geometrically fixed using the riding model. The crystal data and experimental data for 3, 5 and 6 are summarized in Table 1. Selected bond lengths and angles for 5 and 6 are listed in Table 2. 3. Results and discussion
2.4. Preparation of 5 3.1. Synthesis and characterization of pro-ligands Under a nitrogen atmosphere, a mixture of 3 (232 mg, 0.50 mmol), Ni(OAc)2∙4H2O (124 mg, 0.50 mmol), K2CO3 (207 mg, 1.50 mmol) was stirred in 1,4-dioxane (10 mL) for two days at 105 C. After cooling to room temperature, removal of the solvent and the residue was extracted with CH2Cl2 (15 mL 3) and filtered. The volume of the combined filtrate was reduced to approximately 5 mL. Orange crystals of 5 were isolated when this solution was allowed to stand at room temperature overnight. Yield: 105 mg (46%). M.p.: >300 C. 1H NMR (DMSO-d6): d 8.68 (m, 1H, aryl), 8.39 (m, 1H, aryl), 8.17 (m, 1H, aryl), 8.03 (m, 1H, aryl), 7.93 (m, 1H, aryl), 7.83 (m, 1H, aryl), 7.63 (m, 8H, aryl), 7.50 (m, 4H, aryl), 7.26 (m, 8H, aryl), 6.76 (m, 4H, aryl), 6.35 (m, 6H, aryl), 5.85 (m, 1H, CH2), 5.73 (m, 1H, CH2), 5.51 (m, 1H, CH2), 4.92 (m, 1H, CH2). IR (KBr, cm1): n 2964 (m), 1607 (s), 1425 (s), 1258 (s), 1094 (s), 1022 (s), 803 (s), 752 (s). Anal. Calcd. for C58H40N6NiO2: C, 76.41; H, 4.42; N, 9.22. Found: C, 76.38; H, 4.41; N, 9.26%. 2.5. Preparation of 6 This compound was prepared as yellow crystals from the reaction of 4 (318 mg, 0.50 mmol) with Ni(OAc)2∙4H2O (124 mg, 0.50 mmol), K2CO3 (414 mg, 3.00 mmol), and NaBPh4 (343 mg, 1.00 mmol) in 1,4-dioxane (10 mL) at 105 C, followed by recrystallization from a CH2Cl2 solution by a similar procedure as in the synthesis of 5. Yield: 149 mg (52%, based on Ni). M.p.: >300 C. 1H NMR (DMSO-d6): d 8.11 (m, 6H, aryl), 7.98 (m, 6H, aryl), 7.53 (m, 12H, aryl), 7.32 (m, 6H, aryl), 7.15 (m, 24H, Ph), 7.04 (m, 6H, aryl), 6.89 (m, 16H, Ph), 6.76 (m, 12H, aryl), 6.22 (m, 12H, OCH2), 3.21 (m, 12H, CH2). IR (KBr, cm1): n 3049 (s), 2929 (m), 1579 (s), 1419 (s), 1121 (s), 1098 (s), 818 (s), 731 (s). Anal. Calcd for C138H112N12B2Ni4O6: C, 72.35; H, 4.93; N, 7.34. Found: C, 72.33; H, 4.96; N, 7.32.
Imidazolium salts can be easily prepared by quaternization of the imidazole rings with alkyl halides [43,44]. For example, Table 1 Crystal data and experimental parameters for compounds 3, 5 and 6. Compound
3$0.5H2O
5
6
Formula Formula weight Crystal system Space group a ( A) b ( A) c ( A) a ( ) b ( ) g ( ) V ( A3) Z Dcalc (g/cm3) m(Mo/Ka)calc (mm1) Size (mm) F(000) 2q range ( ) No. of reflns, collected No. of unique reflns No. of obsd reflns Abscorr (Tmax, Tmin) R Rw Rall gof
C29H23N3ClO1.5 472.95 Orthorhombic P212121 12.298(1) 20.556(2) 9.485(1) 90 90 90 2397.7(4) 4 1.310 0.189
C58H40N6NiO2 911.67 Orthorhombic P212121 10.048(2) 16.088(3) 32.937(5) 90 90 90 5324.6(15) 4 1.137 0.409
C138H112N12B2Ni4O6 2290.86 Triclinic P1 11.382(1) 14.933(1) 20.600(2) 99.50(1) 102.86(1) 100.94(1) 3271.7(5) 1 1.163 0.623
0.24 0.20 x 0.18 988 3.86e55.82 18,916
0.20 0.20 x 0.20 1896 2.82e55.72 53,186
0.32 0.14 x 0.06 1194 1.89e50.50 13,814
5720 (Rint ¼ 0.1069) 4052
12,360 (Rint ¼ 0.0532) 10,937
13,814 (Rint ¼ 0.0000) 9996
0.97, 0.96
0.92, 0.92
0.96, 0.83
0.049 0.103 0.065 0.94
0.053 0.143 0.060 1.08
0.066 0.152 0.089 0.96
42
H. Song et al. / Journal of Organometallic Chemistry 729 (2013) 40e45
Table 2 Selected bond distances ( A) and bond angles (deg) for compounds 5 and 6. Compound 5 Ni(1)eC(21) Ni(1)eO(2) C(21)eNi(1)eC(50) C(21)eNi(1)eN(3) Torsion (arylearyl) Compound 6 Ni(1)eC(13) Ni(1)eO(1) Ni(2)eO(1) Ni(2)eO(3) Ni(2)eO(5) Ni(3)eC(73) Ni(3)eO(5) Ni(4)eC(43) Ni(4)eO(3) C(13)eNi(1)eC(28) O(2)eNi(1)eC(28) C(73)eNi(3)eC(88) O(6)eNi(3)eC(88) C(43)eNi(4)eC(58) C(58)eNi(4)eO(4) Torsion (arylearyl)
N
N 1.870(3) 1.882(2) 90.6(1) 87.6(1) 79.9(1), 88.3(1)
Ni(1)eC(50) Ni(1)eN(3) O(2)eNi(1)eC(50) O(2)eNi(1)eN(3) e
1.883(3) 1.952(3) 95.6(1) 86.2(1) e
1.882(9) 1.860(6) 1.994(5) 2.059(5) 2.074(6) 1.894(10) 1.877(6) 1.855(8) 1.882(6) 96.3(4) 92.1(3) 98.8(4) 91.7(3) 98.1(4) 92.7(3) 84.8(3), 86.7(3), 86.8(3)
Ni(1)eC(28) Ni(1)eO(2) Ni(2)eO(2) Ni(2)eO(4) Ni(2)eO(6) Ni(3)eC(88) Ni(3)eO(6) Ni(4)eC(58) Ni(4)eO(4) O(1)eNi(1)eC(13) O(1)eNi(1)eO(2) O(5)eNi(3)eC(73) O(5)eNi(3)eO(6) C(43)eNi(4)eO(3) O(3)eNi(4)eO(4)
1.879(9) 1.889(6) 2.118(6) 2.051(6) 2.024(6) 1.842(9) 1.888(6) 1.876(8) 1.869(5) 93.6(3) 79.2(3) 91.7(3) 78.5(2) 91.7(3) 80.4(2)
treatment of imidazoles (R)-1 or (R)-2, prepared from (R)-2,20 diamino-1,10 -binaphthyl [45], with an excess of 2-(chloromethyl) pyridine or 2-bromoethanol in CH3CN at 80 C gives the monoimidazolium salt (R)-3 and bis-imidazolium salt (R)-4, respectively, in good yields (Scheme 1). Compounds 3 and 4 are air-stable, soluble in DMSO, acetone and methanol but insoluble in CH2Cl2 and CHCl3. They have been characterized by various spectroscopic techniques and elemental analyses. The solid-state structure of compound 3 has been further confirmed by X-ray diffraction analysis. The X-ray diffraction analysis shows that there are two molecules of 4 and one H2O molecule in the asymmetric unit. In the molecular structure the binaphthyl groups adopt a staggered geometry (Fig. 1). The twisting between the binaphthyl rings of torsion angle is 84.8(1) , indicating that they are almost perpendicular with respect to each other. 3.2. Synthesis and characterization of complexes Treatment of mono-imidazolium salt 3 with 1 equiv of Ni(OAc)2 in the presence of K2CO3 in 1,4-dioxane at 105 C gives, after recrystallization from a CH2Cl2 solution, the chiral bis-ligated biscarbene nickel complex (3)2Ni (5) in 46% yield (Scheme 1), and no mono-ligated/mono-carbene complex was isolated. However, under similar reaction conditions, treatment of bis-imidazolium salt 4 with 1 equiv of Ni(OAc)2 in the presence of K2CO3 and NaBPh4 in 1,4-dioxane at 105 C gives a tetranuclear nickel complex [{(4) Ni}3Ni](BPh4)2 (6) which has been isolated in 52% yield (based on Ni) (Scheme 1). This observation is presumably due to the steric and electronic effects of the ligand. Complexes 5 and 6 are air-stable, and they are soluble in organic solvents such as DMSO, acetone, and CH3CN but partial soluble in toluene, and benzene. They have been characterized by various spectroscopic techniques, elemental analyses, and X-ray diffraction analyses. The molecular structure of 5 shows that the ion Ni2þ is four coordinate and s-bound to two carbon atoms (carbenes) and one nitrogen atom and one oxygen atom from the two ligand 3, the other oxygen (O(1)) and nitrogen (N(6)) atoms are far away from the metal center (Fig. 2). The sum of the angles in the equatorial plane containing the nitrogen atom and oxygen atom and carbene carbon atoms is 360.0(1) , indicating a coplanar arrangement, in
N O Ni
O N
Ni(OAc)2
N OH
N
N
N
K2CO3 1,4-dioxane
5
N N
Cl
CH2Cl
CH3CN
3
N NH2 NH2
HCHO(aq,10 eq.) OHCCHO(aq,10 eq.) NH4Cl(10 eq.)
OH + 1
H2O, 1,4-dioxane H3PO4
2 HOCH2CH2Br N
N
N
N
CH3CN
OH OH
N
N
N
N
N
2
2Br 4
K2CO3 Ni(OAc)2 1,4-dioxane NaBPh4 N
N Ni O O N N
Ni
N O O N Ni N N
N N O Ni O N N
2BPh4
6 Scheme 1. Synthesis of complexes 5 and 6.
which the two carbenes adopt a cis arrangement. The NieO distance is 1.882(2) A, and the NieN distance is 1.952 (3) A. The average NieC(carbene) distance is 1.877(3) A, which is comparable to those found in bis(aryloxide-NHC)Ni complexes (1.84e1.87 A) [48]. The twisting between the binaphthyl rings of torsion angles are 79.9(1) and 88.3(1) , which are comparable to that found in 3 (84.8(1) ). The molecular structure of 6 shows that it consists of well-separated, alternating layers of the anions [BPh4] and the complex cations [{(4)Ni}3Ni]2þ. Each cation [{(4)Ni}3Ni]2þ adopts
H. Song et al. / Journal of Organometallic Chemistry 729 (2013) 40e45
43
a
Fig. 1. Molecular structure of the cation in 3 (thermal ellipsoids drawn at the 35% probability level).
a C3 quasi-symmetric structure (Fig. 3a and b). The coordination environments around the three end nickel atoms ((Ni(1), Ni(3) and Ni(4)) are similar, each ion Ni2þ is four coordinate and sbound to two carbon atoms (carbenes) and two oxygen atoms from one ligand 4. The bis(carbene) moiety acts as a chelate with a bite angle of CeNieC of 96.3(4) for Ni(1), 98.8(4) for Ni(3), and 98.1(4) for Ni(4), respectively. The sum of the angles in the equatorial plane containing the oxygen atoms and carbene carbon atoms is 361.2(3) for Ni(1), 360.7(4) for Ni(3), and 362.9(4) for Ni(4), respectively, indicating that these Ni atoms take a distorted square-planar coordination geometry. The average NieC(carbene) distance is 1.881(9) A for Ni(1), 1.868(10) A for Ni(3), and 1.862(2) A for Ni(4), respectively, and the average NieO distance is 1.875(6) A for Ni(1), 1.883(6) A for Ni(3), and 1.876(6) A for Ni(4), respectively. These structural data are close to those found in bis(aryloxide-NHC)Ni complexes [48].
b
Fig. 3. (a) Molecular structure of the cation in 6 (thermal ellipsoids drawn at the 35% probability level). (b) Core structure of 6 (thermal ellipsoids drawn at the 35% probability level).
The central Ni (Ni(2)) is six coordinate and s-bound to six oxygen atoms from three ligands 4 in a distorted-octahedral geometry with the average NieO distance of 2.053(6) A. The twisting between the naphthyl rings of torsion angles are 84.8(3), 86.7(3) and 86.8(3), which are comparable to those found in 3 (84.8(1) ) and 5 (79.9(1) and 88.3(1) ). 3.3. Catalytic activity of complexes
Fig. 2. Molecular structure of 5 (thermal ellipsoids drawn at the 35% probability level).
To examine the catalytic ability of the nickel NHC-complexes toward the asymmetric reactions, several common reactions such as the allylation of benzaldehyde with allyltributyltin, the Henry reaction (nitroaldol reaction) of benzaldehyde with nitromethane, the cyclopropanation of styrene with ethyl diazoacetate, the amination of oxindole with di-tert-butyl azodicarboxylate, and the coupling reaction of benzaldehyde, triethylsilane with phenylmethylacetylene or 1-cyclohexylallene have been test. Unfortunately, these complexes exhibited either no activity or rather poor activity for these transformations, in which only racemic products with low conversions (<5%) were detected even when heated in
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H. Song et al. / Journal of Organometallic Chemistry 729 (2013) 40e45
solvents such as THF, methanol, CH2Cl2, CH3CN or toluene and with 20% precatalysts loading. Since the nickel complexes do not show effective catalytic activity in asymmetric reactions, a different transformation was investigated. In recent years, vinyl polynorbornene (PNB) has received considerable attention due to its dielectric and mechanical properties for technical application as an interlevel dielectric in microelectronics applications [49]. Recently, some nickel NHC-complexes have shown excellent catalytic activity in the polymerization of norbornene [48,50e52]. Therefore it is rational to propose that our chiral nickel NHC-complexes would have a potential use as catalysts in this transformation. In fact, the polymerization data show that the nickel complexes 5 and 6 can initiate the polymerization of norbornene in the presence of MAO under mild conditions (Table 3). For complex 5, the activity decreases with increasing temperature from 20 to 80 C (entries 1e4). The activity increases with increasing the Al/Ni ratio from 1500 to 2500 (entries 1, 5, 6). But the activity decreases when the Al/Ni ratio further increases (entries 7e9). Under similar reaction conditions, complex 6 exhibits a lower catalytic activity (only up to 1.34 106 g/mol h) in the polymerization of norbornene (entries 10 and 11), presumably due to the steric hindrance around the metal center and the firm bonding between the alkoxide-NHC ligands and Ni atom, which obstruct the generation of the active species. In the absence of nickel complex or MAO no polymer has been obtained. No typical double bond absorption at 1600e1700 cm1 is detected in the IR spectra of the obtained polymers, indicating that the polymerization initiated by our nickel complexes/MAO system proceeds as a vinyl-addition polymerization [50]. TGA studies on the obtained polymers reveal that these polymers are thermally very stable (up to 450 C). Under the conditions examined, molecular weights of the obtained polymers range from 184 to 1025 kg mol1. Complex 5 produces a narrow molecular weight distribution (ca. 2.5), indicating a single-site catalyst, while complex 6 gives a much broader molecular distribution (ca. 11.0), due to its two different mediated sites, or consistent with its C3 quasi-symmetric structure. Our results show that the catalytic activity of 5 resembles that of the bis(aryloxide-NHC)Ni complex [48], while it is more active than that of 6, presumably because of less steric hindrance around the metal center.
Table 3 Polymerization of norbornene catalyzed by Ni-complexes.a
Ni NHC-complex n
Yieldb Entry Complex Temp. MAO (Al/Ni) (g) ( C)
Activity Mwc Mw/Mnc Tdecd (106 g/ (kg/mol) ( C) mol h)
1 2 3 4 5 6 7 8 9 10 11
6.57 3.35 1.54 1.12 1.71 4.34 5.48 5.19 5.15 1.34 0.81
20 40 60 80 20 20 20 20 20 20 40
2500 2500 2500 2500 1500 2000 3000 3500 4000 2500 2500
0.4382 0.2234 0.1028 0.0746 0.1142 0.2895 0.3654 0.3462 0.3432 0.0895 0.0543
1025 572 278 224 274 725 907 874 865 251 184
2.4 2.3 2.7 2.8 2.5 2.6 2.3 2.4 2.3 10.3 11.5
445 440 442 445 446 445 443 446 445 448 450
Conditions: norbornene (1.0 g); precatalyst (0.40 mmol, based on nickel metal ion); MAO (1.5 M) in toluene, solvent, chlorobenzene (15 mL), time, 10 min. b Isolated yields. c Determined by GPC in 1,2,4-trichlorobenzene at 150 C using polystyrene standards. d Decomposition temperature was determined by TG (heating rate: 10 C/min). a
In conclusion, two new chiral NHC-nickel complexes 5 and 6 have been prepared from the reactions between Ni(OAc)2, K2CO3 or K2CO3/NaBPh4, and imidazolium salts 3 or 4, which are derived from (R)-2,20 -diamino-1,10 -binaphthyl. The steric and electronic effects of the ligand play an important role in the NHC-complex formation. For example, treatment of mono-imidazolium salt 3 with 1 equiv of Ni(OAc)2 in the presence of K2CO3 in 1,4-dioxane at 105 C gives the bis-ligated mononuclear nickel complex (3)2Ni (5). However, under similar reaction conditions, bis-imidazolium salt 4 affords a tetranuclear nickel complex [{(4)Ni}3Ni](BPh4)2 (6). Nickel complexes 5 and 6 show a good (106 g/mol h) norbornene polymerization activity upon activation with methylaluminoxane (MAO), leading to thermally very stable polynorbornenes. Further development of new chiral nickel NHC-complexes and exploration of these catalysts toward other types of transformations are still underway. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 20872062, 21074013, 21172022, 21272026), the Program for New Century Excellent Talents in University (NCET-10e0253), and Beijing Municipal Commission of Education. Supplementary materials CCDC 914692, 914694 and 914694 contain the supplementary crystallographic data for 3, 5 and 6. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (þ44) 1223-336-033; or e-mail: deposit@ ccdc.cam.ac.uk. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
MAO
5 5 5 5 5 5 5 5 5 6 6
4. Conclusions
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