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Synthesis and structure of indenylnickel(II) chlorides bearing free N-heterocyclic carbene ligands and their catalysis for styrene polymerization
Polyhedron 28 (2009) 2585–2590
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Synthesis and structure of indenylnickel(II) chlorides bearing free N-heterocyclic carbene ligands and their catalysis for styrene polymerization Ling-Zhi Xie, Hong-Mei Sun *, Dong-Mei Hu, Zhi-Hong Liu, Qi Shen, Yong Zhang The Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
a r t i c l e
i n f o
Article history: Received 20 March 2009 Accepted 18 May 2009 Available online 16 June 2009 Keywords: N-heterocyclic carbene Nickel(II) complexes Indenyl Styrene Polymerization
a b s t r a c t The reaction of (R-Ind)2Ni (Ind = C9H7, indenyl) with an equivalent of a bulky aryl-substituted imidazolium salt in CH2Cl2/THF at 45 °C results in the corresponding N-heterocyclic carbene (NHC) indenylnickel(II) chloride of the type (R-Ind)Ni(L)Cl [R = 1-H, L = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), 1; R = 1-Me, L = IPr, 2] in high yield. Complexes 1 and 2 were characterized by elemental analysis, NMR spectroscopy and X-ray crystallography. The resulted NHC indenylnickel(II) complexes are capable of polymerizing styrene in the presence of NaBPh4 to give atactic polystyrene with Mn values in the range of 104. The present studies show a close relationship between the structure and catalytic activity of the NHC indenylnickel(II) halides (including the previously reported indenylnickel(II) halides bearing alkylsubstituted NHC ligands), and complex 2 shows the highest catalytic activity. In comparison with its phosphine analogue (1-Me-Ind)Ni(PPh3)Cl, complex 2 shows significant improvements in stability and catalytic performance. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction During the past decade there has been increasingly interesting in the application of N-heterocyclic carbenes (NHCs) as a structure principle for developing late transition metal-based homogeneous catalysts [1–4]. In many types of catalytic processes, NHCs have been used as a valid alternative to the traditional phosphine ligands due to their unique and tunable steric and electronic properties [5–7]. The replacement of phosphine ligands with electronrich NHC ligands could represent significant catalytic activity in most cases. For example, benzannulated NHC Ni(II) complexes of the general formula Ni(NHC)2Br2 exhibited much higher catalytic activity than their phosphine analogues, Ni(PPh3)2Br2, in reductive Ullmann coupling reactions of aryl bromides [8]. However, in comparison with the intensively studied NHC complexes of palladium, a relatively small number of well-defined NHC nickel complexes have been reported to date. Only recently have NHC complexes of nickel garnered more attention and emerged as effective catalysts for a series of reactions, such as C–F bond [9] or C–H bond activation [10], C–C or C–N cross-coupling reactions [8,11–14], olefin dimerization [15,16], polymerizations [17– 24], and so on. The (NHC)nNi(CO)m system has been used as a model for the understanding of electronic and steric effects of NHCs [25,26], nevertheless, the principles regulating their structures
* Corresponding author. Tel.: +86 512 65880330; fax: +86 512 65880305. E-mail address: [email protected] (H.-M. Sun). 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2009.05.080
and reactivities remain unclear. This prompted us to develop the free NHC-based indenylnickel(II) complexes. In fact, although tertiary phosphine analogues of the type (R-Ind)Ni(PR03 )X (X = halide, alkyl, alkynyl, imidate, thiolate) have been reported, and include examples that are effective precatalysts for the oligomerization or polymerization of ethylene, styrene and alkynes, the dehydrogenative oligomerization or polymerization of phenylsilane [27], NHC indenylnickel(II) complexes have received little attention until our recent report on a one-step facile synthetic method, i.e. the direct reaction of the bis-indenyl Ni(II) complex of (1-H-Ind)2Ni with an alkyl-substituted imidazolium salt, for the synthesis of alkyl-substituted NHC indenylnickel(II) complexes (1-H-Ind)Ni(iPr)X (iPr = 1,3-diisopropylimidazole-2-ylidene) in high yields [28,29]. This procedure needs no extra steps for the isolation and purification of the corresponding N-heterocyclic carbene, and provides simple access to NHC Ni(II) complexes with polydentate co-ligands. Despite our investigation, there has only been one example reported so far that such NHC complexes could be synthesized by the reaction of (R-Ind)Ni(PPh3)X with free NHC, mentioned briefly in one review [27]. To explore a simple route to a large number of complexes in this family would be desirable. Herein we described the extension of our methodology to the synthesis of indenylnickel(II) chlorides bearing sterically bulky aryl-substituted NHC ligands, (1-H-Ind)Ni(IPr)Cl [IPr = 1,3bis(2,6-diisopropylphenyl)imidazol-2-ylidene, 1) and its alkylsubstituted indenyl derivative (1-Me-Ind)Ni(IPr)Cl (2). The present investigation enlarges the list of analogous complexes already described [28,29], to such an extent that comparative studies of
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the relationship between their structures and catalytic activities in styrene polymerization may be possible. Their catalytic activities for styrene polymerization have also been compared to their tertiary phosphine analogues.
2. Experimental 2.1. General procedures All manipulations were performed under pure argon with rigorous exclusion of air and moisture using standard Schlenk techniques. Solvents were distilled from Na/benzophenone ketyl under pure argon prior to use. Indene (Fluka) (dried over 4 Å molecular sieves) was distilled immediately before use. (1-H-Ind)2Ni [28], (1-Me-Ind)2Ni [30], 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (IPrHCl [31], (1-H-Ind)Ni(iPr)X [iPr = 1,3diisopropylimidazole-2-ylidene, X = Cl (3) and Br (4)] [28], (1Me-Ind)Ni(PPh3)Cl (5) [32] and CpNi(IPr)Cl (Cp = cyclopentadienyl, 6) [33] were prepared by published methods. Elemental analysis was performed by direct combustion on a Carlo-Erba EA-1110 instrument. NMR (CDCl3) spectra were measured on a Unity Inova-400 spectrometer at 25 °C, and were assigned by the assistance of 2D techniques (H–H correlation spectroscopy and C–H heteronuclear correlation spectroscopy). Molecular weight and molecular weight distribution were determined by gel-permeation chromatography (GPC) measurements calibrated to commercial polystyrene standards and performed on a PE PL-GPC50 apparatus with two PLgel 10 lm MIXED-B columns in THF (1.0 mL/min) at 40 °C. 2.2. Synthesis of (1-H-Ind)Ni(IPr)Cl (1) A Schlenk flask was charged with 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (0.85 g, 2.00 mmol), CH2Cl2 (5 mL) and a stir bar. To this solution was added (1-H-Ind)2Ni (0.57 g, 2.00 mmol) in 20 mL of THF via syringe at room temperature. The solution was stirred overnight at 45 °C, filtered and then evaporated to dryness. After being washed with hexane, the residue was extracted with hot toluene and recrystallized from concentrated toluene at 10 °C, yielding bright red crystals (0.32 g, 63%) suitable for elemental analysis and X-ray diffraction studies. M.p. 176–178 °C. Anal. Calc. for C36H43ClN2Ni: C, 72.32; H, 7.25; N, 4.69. Found: C, 72.24; H, 7.43; N, 4.45%. 1H NMR (400 MHz, CDCl3, 25 °C): d 1.05 (d, 12H, Me), 1.34 (d, 12H, Me), 2.65 (br, 1H, H3), 3.07 (br, 2H, CHMe2), 4.44 (br, 2H, CHMe2), 6.38 (s, 1H, H1), 6.62 (br, 2H, H5/H8 and H2), 7.01 (s, 2H, CH@CH), 7.35-7.57 (m, 9H, H5/H8, H6, H7 and Ph). 13C NMR (100 MHz, CDCl3, 25 °C): d 23.1 (Me), 26.6 (Me), 29.0 (CHMe2), 103.3 (C1), 118.1 and 118.2 (C2 and C5/C8), 124.7 and 125.5 (m-Ph, C6 and C7), 125.8 (CH@CH), 130.4 (p-Ph), 132.73 (C4 and C9), 136.8 (o-Ph), 146.7 (ipso-Ph), 175.4 (Ccarbene); however, the assignment of some of these signals remains ambiguous, such as the resonance for C3 which is missing and should be presented at ca. 50. 2.3. Synthesis of (1-Me-Ind)Ni(IPr)Cl (2) Following a procedure similar to the synthesis of 1, (1-Me-Ind)2Ni (0.62 g, 2.00 mmol) in 20 mL of THF was added to a solution of 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (0.85 g, 2.00 mmol) in CH2Cl2 (5 mL) at room temperature. After workup, dark red crystals (0.86 g, 70%) suitable for elemental analysis and X-ray diffraction studies were obtained from concentrated toluene at 10 °C. M.p. 206–207 °C. Anal. Calc. for C37H45ClN2Ni: C, 72.62; H, 7.41; N, 4.58. Found: C, 72.31; H, 7.07; N, 4.47%. 1H NMR (400 MHz, CDCl3, 25 °C): d 0.95 (m, 9H, Me and 1-Me-Ind), 1.15 (d, 6H, Me), 1.23 (d, 6H, Me), 1.39 (d, 6H, Me), 2.55 (br, 2H, CHMe2),
3.17 (br, 2H, CHMe2), 4.16 (s, 1H, H3), 5.65 (d, 1H, H5), 6.01 (s, 1H, H2), 6.29 (m, 1H, H6), 6.57 (d, 1H, H8), 6.77 (d, 1H, H7), 7.01 (s, 2H, CH@CH), 7.32-7.53 (m, 6H, Ph). 13C NMR (100 MHz, CDCl3, 25 °C): d 12.2 (1-Me-Ind), 22.7 (Me), 23.6 (Me), 26.1 (Me), 27.0 (Me), 29.0 (CHMe2), 58.1 (C3), 93.9 (C1), 103.8 (C2), 116.7 (C8), 117.0 (C5), 124.5 (C6), 124.6 (m-Ph), 125.5 (C7), 125.8 (CH@CH), 130.2 (pPh), 132.0 (C4), 134.0 (C9), 136.9 (o-Ph), 146.7 (ipso-Ph), 177.1 (Ccarbene). 2.4. Structure determination Suitable single crystals of 1 and 2 were sealed in a thin-walled glass capillary for X-ray structural analysis. Diffraction data were collected on a Rigaku Mercury CCD area detector at 193(2) K. The structures were solved by direct methods and refined by full-ma-
Table 1 X-ray crystallographic data for 1 and 2. 1
2
Empirical formula Formula weight Temperature (K) k (Mo Ka) (Å) Crystal system Space group
C36H43ClN2Ni 597.88 193(2) 0.7107 monoclinic p21/n
C37H45ClN2Ni 611.91 153(2) 0.7107 monoclinic p21/n
Unit cell dimensions a (Å) b (Å) c (Å) b (°) V (Å3) Z Dcalc (g cm3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm3) 2hmax (°) Reflections collections Independent reflections Goodness-of-fit on F2 Final R indices [I > 2r(I)] Rw
9.8993(9) 19.239(2) 16.883(1) 106.072(2) 3089.7(5) 4 1.285 0.742 1272 0.30 0.20 0.15 25.34 30454 5642 (Rint = 0.0421) 1.103 0.0415 0.0816
10.3994(11) 19.057(2) 16.861(2) 105.843(2) 3214.9(6) 4 1.264 0.714 1304 0.25 0.25 0.07 25.3 31317 5869 (Rint = 0.0717) 1.156 0.0598 0.0985
Table 2 Select bond lengths (Å), angles (°) and structural parameters for 1 and 2. 1
2
Ni–Cl Ni–C(10) Ni–C1 Ni–C(2) Ni–C(3) Ni–C(4) Ni–C(9) C(1)–C(2) C(1)–C(9) C(2)–C(3) C(3)–C(4) C(4)–C(9) C1–Ni–Cl C1–Ni–C3 C3–Ni–C10 C10–Ni–Cl DM-C (Å)a HA (°)b FA (°)c a
2.1759(7) 1.896(2) 2.119(2) 2.019(3) 2.028(2) 2.412(2) 2.437(2) 1.394(4) 1.442(3) 1.421(4) 1.465(4) 1.421(3) 96.5(8) 66.3(1) 102.6(1) 95.4(1) 0.35 11.4 12.5
Ni–Cl Ni–C(11) Ni–C1 Ni–C(2) Ni–C(3) Ni–C(4) Ni–C(9) C(1)–C(2) C(1)–C(9) C(2)–C(3) C(3)–C(4) C(4)–C(9) C1–Ni–Cl C1–Ni–C3 C3–Ni–C11 C11–Ni–Cl DM-C (Å)a HA (°)b FA (°)c
2.1829(9) 1.895(3) 2.132(3) 2.041(3) 2.025(3) 2.385(3) 2.415(3) 1.407(5) 1.454(5) 1.423(5) 1.458(5) 1.419(4) 95.0(1) 66.6(1) 103.4(1) 95.5(1) 0.32 10.1 11.4
DM-C = 1/2[M-Cav(for C4, C5)] 1/2[M-Cav(for C1, C3)]. HA is the angle formed between the planes formed by the atoms C(1), C(2), C(3) and C(1), C(3), C(4), C(5). c FA is the angle formed between the planes formed by the atoms C(1), C(2), C(3) and C(4), C(5), C(6), C(7), C(8), C(9). b
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trix least-squares procedures based on F2. All non-hydrogen atoms were refined with anisotropic displacement coefficients. Hydrogen atoms were treated as idealized contributions. The structures were solved and refined using SHELXS-97 and SHELXL-97 programs, respectively. Crystal data, collection and main refinement parameters are given in Table 1. Selected bond lengths (Å) and angles (°) for 1 and 2 are given in Table 2. 2.5. A typical procedure for the polymerization of styrene A typical procedure for the polymerization of styrene is given as follows. Under dry argon, complex 1 (12.0 mg, 0.02 mmol), NaBPh4 (47.9 mg, 0.14 mmol), toluene (0.2 mL), and styrene (0.7 mL, 6.0 mmol) were added into a dry glass ampule in turn. Then, the sealed ampule was placed in a water bath held at 80 °C. After a definite reaction time, the polymerization was stopped by adding 1 mL of 5% HCl/ethanol. After evaporation of the solvent and unreacted monomer, the resultant polymer was dissolved in THF, followed by precipitation in 95% ethanol. After filtration, the white polymer was dried in vacuo at room temperature overnight. The polymer yield was determined gravimetrically. 3. Results and discussion 3.1. Synthesis and structural characterization of 1 and 2 Focusing on the nickel-based system, the bulky aryl-substituted NHC complexes exhibit unusual stability or reactivity in certain instances [8,25,34]. So, as a continuation of our previous work on NHC indenylnickel(II) complexes, it was considered to test whether the reported imidazolium salts metathesis and indene elimination reaction [28–29] could also occur between 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (IPrHCl) and (1-H-Ind)2Ni, to yield the corresponding bulky aryl-substituted NHC indenylnickel(II) complex. Thus, a 1:1 (in molar ratio) mixture of (1-H-Ind)2Ni and IPrHCl was stirred at 45 °C in CH2Cl2/THF overnight. This reaction worked smoothly from the change of color from red to dark red. After workup, the designed NHC complex 1 was isolated as dark red crystals in ca. 63% yield (Scheme 1). The proposed structure for 1 was first supported by elemental analysis and NMR spectroscopy. The 1H NMR and 13C NMR data indicated the presence of one indenyl group and one N-heterocyclic carbene group, respectively. Moreover, in the 13C NMR spectrum, a singlet Ccarbene atom signal appears at d 175.4 ppm, which resembles a clear downfield shift compared to the reported Ni(II) complexes of Dd 9 ppm for (1-H-Ind)Ni(iPr)Cl (3, d = 166.3 ppm) [28] or Dd 6 ppm for CpNi(IPr)Cl (6, d = 169.3 ppm) [33]. The identity of this complex is further confirmed by X-ray analysis. To further ascertain the versatility of this synthetic approach, the similar reaction of the alkylsubstituted bis-indenyl Ni(II) complex of (1-CH3-Ind)2Ni with the same imidazolium salt was tested in turn. The analogous chloride 2 was isolated as dark red crystals in ca. 70% yield by a similar procedure (Scheme 1). Elemental analysis, NMR spectroscopy and X-ray analysis confirmed the expected product composition. In
its 13C NMR spectrum, the singlet Ccarbene atom signal appears at d 177.1 ppm, which is very close to that of 1. These results demonstrated undoubtedly that the imidazolium salt metathesis and indene elimination is a valid synthetic route to a variety of NHC indenylnickel(II) complexes. Compared with the alkyl-substituted NHC indenylnickel(II) complexes (1-H-Ind)Ni(iPr)Cl (iPr = 1,3-diisopropylimidazole-2ylidene, 3) [28], both 1 and 2 are much more thermally stable and melt at ca. 177 °C for 1 and ca. 206 °C for 2 (the melting point of 3 is 147 °C). Besides, they are also much less sensitive to air and moisture than complex 3, and can even be kept unchanged in the open air for several hours in the solid state, compared to only a few minutes for complex 3 [28]. Additionally, it is worth mentioning that the tertiary phosphine analogues, (1-H-Ind)Ni(PPh3)Cl [35] or (1-Me-Ind)Ni(PPh3)Cl (5) [32], decompose immediately in open air. 3.2. Crystal structures of 1 and 2 Crystals of 1 and 2 suitable for X-ray structure determination were grown from cold toluene solution. The crystallographic and measurement data are shown in Table 1, and selective bond lengths and angles are listed in Table 2. The molecular structures of these complexes are shown as ORTEP drawings in Figs. 1 (1) and 2 (2). The molecular structures of 1 and 2 are similar, and both are monomeric in the solid state. The nickel atom is bonded to the indenyl group in a highly unsymmetric mode. The bond lengths of Ni–C1, Ni–C2 and Ni–C3 equal 2.119(2), 2.019(3) and 2.028(2) Å in 1 and 2.132(3), 2.041(3) and 2.025(3) Å in 2, respectively, which are within the normal bonding distances, while the other two carbon atoms of the five-ring of the indenyl group (C4 and C9) are considerably further away (Ni–C4 = 2.412(2), Ni– C9 = 2.437(2) Å in 1; Ni–C4 = 2.385(3), Ni–C9 = 2.415(3) Å in 2). Such a highly unsymmetric mode of the indenyl ligand is not only very similar to the reported NHC indenylnickel(II) complexes [28– 29] but also common in the family of the corresponding tertiary phosphine indenylnickel(II) complexes [27]. The Ni–Ind interaction could be quantitatively calculated by the main geometrical parameters such as (a) the slip value represented by DM-C and (b) the hinge and fold angles (HA and FA) representing the bending of the indenyl ligand at C1/C3 and C4/C9, respectively [28]. For example, DM-C values close to zero indicate little distortion from g5 hapticity, whereas values above ca. 0.5 Å indicate nearly g3 hapticity; HA values of ca. 14° or greater are associated with nearly trihapto cases. Here, in complex 1, DM-C = 0.35 Å, HA = 11.4°, FA = 12.74°; and in complex 2, DM-C = 0.32 Å, HA = 10.1°, FA = 11.4° (Table 2). These values led us to propose that the Ni–Ind bonding fashion here can be described as an intermediate hapticity between the g5- and g3-modes, which is somewhat similar to that found in (1-H-Ind)2Ni (DM-C = 0.42 Å, HA = 13.9°, FA = 13.1°) [27]. However, the larger parameter values (DM-C, HA, and FA) for 1 indicated a more unsymmetric Ni–Ind interaction is present in 1. This phenomenon that the methyl substituent on
R Ind Cl i
i
Pr
(R-Ind)2Ni +
N i
Pr
Cl-
i
Pr
Pr
-indene N
N i
Pr
THF/CH2Cl2, 45°C
i
Pr
Ni
i
Pr
N i
Pr
R = 1-H (1), Yield: 63% R = 1-Me (2), Yield: 70% Scheme 1. Synthesis of NHC indenylnickel(II) complexes of 1 and 2.
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Fig. 1. The crystal structure of complex 1.
Fig. 2. The crystal structure of complex 2.
the indenyl ligand of 2 leads to a less unsymmetric Ni-Ind interaction, i.e. represents a stronger Ni–Ind interaction, is very similar to the one found in its PPh3-based analogous [32]. Furthermore, since the bond lengths of Ni–C1 and C2–C3 are longer than those of Ni– C3 and C1–C2 in 1 and 2, respectively, it can be argued that the distortion in the hapticity of the indenyl ligand is not quite g5 M g3 but rather g5 M (g1, g2), i.e. the C1 = C2 occupy a single coordination site. In addition, the NMR data are also valuable in assessing the indenyl hapticity in solution [27]. For example, Zargarian has suggest that large chemical shift differences between H1 and H3 (>2 ppm) or C1 and C3 (>20 ppm) correlated well with a partial localization of bonding in the allyl moiety of the indenyl ligand in (I-H-Ind)Ni(PPh3)Cl, since such a bonding type requires that C1 and C3 be quite different in terms of hybridization [35]. Herein
we note that even if the assignment of these signals remains somewhat ambiguous, the H1 and H3 signals for 1 appear at 2.65 and 6.38 ppm, respectively, meanwhile the C1 and C3 signals for 2 appear at 58.1 and 93.3 ppm, respectively. The pairwise d values are more than 3 ppm (H1/H3 in 1) or 35 ppm (C1/C3 in 2) apart, attesting to the greater sp2 character of C1 in 1 and 2, respectively [27]. Thus, the nickel atom is bonded to a Cl, Ccarbene, C1, C2 and C3 of the indenyl group, to form a highly distorted square plane with C1 = C2 occupying a single coordination site (i.e. C1–C2 < C2–C3, Table 2). Such a coordinated fashion can be mainly attributed to the tendency of Ni(II) to avoid forming 18-electron complexes [27–28] and the unequal trans influences of the N-heterocyclic carbene and the halogen group [28]. Besides, as compared with 0.31 Å (DM-C), 12.1° (HA) and 9.1° (FA) reported for (1-H-Ind)Ni(iPr)Cl (3)
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and 0.30 Å (DM-C), 12.3° (HA) and 9.5° (FA) reported for (1-H-Ind)Ni(iPr)Br (4) [28], the differences in these parameters among 1, 2, 3 and 4 demonstrate that the NHC ligand has a great influence on the Ni–Ind interaction presented in (R-Ind)Ni(NHC)X. The bulky IPr ligand possesses a more trans influence than the iPr ligand, which leads to a more unsymmetric Ni–Ind interaction. In addition, in comparison with the previously measured parameters for (I-HInd)Ni(PPh3)Cl (i.e. DM-C = 0.26 Å) [35] and 5 (i.e. DM-C = 0.25 Å) [32], it is found that there is a more unsymmetric Ni–Ind interaction for NHC-based systems, which could be attributed to the stronger donor ability of the NHC ligand than that of PPh3 [25] and a more unequal trans influences between the NHC ligand and the halogen group. The Ni–Ccarbene bond lengths are almost the same for the two complexes (1.896(2) Å in 1 and 1.895(3) Å in 2), and both are longer than that of 1.886(3) Å in 3 or 1.888(4) Å in 4 [28]. These data indicate that the ancillary ligand of (R-Ind)Ni(NHC)X, i.e. the indenyl moiety or the halogen atom has subtle effect on the Ni–Ccarbene interaction, meanwhile steric factor differences of the NHC ligands should be responsible for the weaker interaction between the nickel atom and the carbene carbon. Nevertheless, since alkyl-substituted NHCs are marginally more electron-donating than their aryl-substituted counterparts [25], it should be taken into account that small differences in donor capacities of NHC ligands may also play an important role in the Ni–Ccarbene interaction. The Ni–Cl bond length of 2.1759(7) Å in 1 is slightly shorter than 2.1829(9) Å in 2, and obviously shorter than that of 2.2025(7) Å in 3 [28]. The strongest Ni–Cl interaction in 1 might be a compensation for the weakening trend of the Ind–Ni interaction on going from 1 to 2 and then to 3. Alternatively, a weakening trend of the Ni–Cl interaction is probably caused by the Cl–Ni pdonation [32]. However, it is well known that the X–M p-donation would destabilize d8 complexes. In addition, the Ni–Cl distances in 1 or 2 are slightly shorter than that of 2.1822(7) Å in (I-H-Ind)Ni(PPh3)Cl [35] or 2.1865(10) Å in 5 [32], respectively. 3.3. Catalytic activity of the free NHC indenylnickel(II) complexes for styrene polymerization Previous studies had indicated that the neutral tertiary phosphine indenylnickel(II) complexes of (R-Ind)Ni(PR3)X (X = halide) alone were inert toward the insertion of styrene, whereas the addition of proper amounts of AgBPh4 (or NaBPh4) and phosphine ligands led to the polymerization of styrene [36–41]. The role of AgBPh4 (or NaBPh4) was to abstract the halide ligand from these complexes, with the in situ formation of catalytically active cationic species, while addition of the phosphine ligand was needed to suppress the decomposition of the cationic species [27,39–41] unless the indenyl ligand tethered an amine moiety [36–38]. In the present paper, the catalytic activity of a series of free NHC indenylnickel(II) complexes for styrene polymerization has been investigated for the first time.1 The preliminary results are listed in Table 3. The results show that the addition of a proper amount of NaBPh4 is also necessary in the present case, which indicates that the present styrene polymerization also is catalyzed by the in situ generated electronically and coordinatively unsaturated species. For example, complex 1 alone shows no catalytic activity for styrene polymerization in toluene at 80 °C (Table 3, run 1), however, together with NaBPh4, it shows considerable catalytic activity, i.e. 60% yield of polystyrene was obtained after 48 h polymerization (Table 3, run 2).
1 In one of our previous reports, i.e. Ref. [24], the catalytic activity of 4 for the polymerization of styrene had been mentioned briefly in comparison with the corresponding Ni(II) bromide containing an indenyl ligand tethered NHC moiety. However, we should point out that the reaction time of the styrene polymerization by 4 was incorrect, and it should be 48 h instead of the mentioned time of 24 h.
Table 3 Ni(II)-catalyzed homopolymerization of styrene. Run
Initiator
NaBPh4/[Ni]
t (h)
Yield (%)
Mn 104
Mw/Mn
1 2 3 4 5 6a 7 8a 9
1 1 2 3 4 2 5 5 6
0 7 7 7 7 7 7 7 7
48 48 48 48 48 48 48 24 48
0 60 75 26 36 33 24 78 40
3.6 5.7 2.6 4.4 3.3 0.4 0.6 2.3
2.0 2.4 3.6 3.1 2.6 2.4 4.7 1.9
Conditions: styrene/[Ni] = 300:1 (molar ratio), toluene: 0.2 mL, T = 80 °C. a [NaBPh4]/[Ni]/[PPh3] = 7:1:1.
The influences of the indenyl ligand, the NHC ligand and the halide ligand on the catalytic activity of the corresponding Ni(II) complexes are investigated in turn. On the basis of the polymer yield, complex 2 shows the highest catalytic activity while complex 4 possess the lowest one (Table 3, runs 2–5). Meanwhile, the polystyrene with the highest number-average molecular weight (Mn = 57 000) was obtained with 2. The diverse activities observed for these complexes may simply indicate that the catalytic active species remain ligated by the indenyl and carbene ligands, and the aryl-substituted NHC ligand together with the alkyl-substituted indenyl ligand are a benefit to enhance the catalytic activity of the NHC indenylnickel(II) complexes for the styrene polymerization. The difference in activity between 3 and 4 are probably due to their different thermal stabilities during the polymerization process [28,40]. All of the polymers obtained here are well soluble in THF and have very similar microstructures. Since a typical 13C NMR spectrum (CDCl3) of the obtained polystyrene shows characteristically broad signals of atactic polystyrene in the range of ca. 144– 148 ppm [42], it is clear that the polystyrene obtained is an atactic polymer, which is very similar to those obtained by PPh3-based indenylnickel(II) complexes [39–41] or by NHC-based cationic allylnickel(II) complexes [22]. It is worth noticing that the addition of a catalytic amount of other additives, i.e. PPh3, is not necessary in the present case. For example, in the presence of one equivalent of PPh3, the yield of polystyrene decreased greatly from 75% to 33% with a nearly half decrease of the number-average molecular weight (Table 3, runs 3 and 6). On the contrary, the addition of a catalytic amount of phosphine ligands is very necessary to styrene polymerization initiated by the phosphine analogues [39–41]. For example, in the absence of PPh3, complex 5 shows a much lower catalytic activity, i.e. 24% yield of polystyrene with a relatively low number-average molecular weight (Mn = 4000) was obtained in the present conditions (Table 3, run 7). Notably, black powders appeared quickly in the glass ampule during this polymerization process, which indicated the decomposition of 5. Otherwise, the addition of one equivalent of PPh3 could efficiently suppress this decomposition and provide a much higher polymer yield (Table 3, run 8). Nevertheless, the polymer obtained still possesses a lower number-average molecular weight and a broadened molecular weight distribution. A similar phenomenon of decomposition was not observed in the styrene polymerization by NHC indenylnickel(II) complexes in the absence of PPh3. The great difference of catalytic behaviors between the NHC indenylnickel(II) complexes and their tertiary phosphine analogues also indicates that the Ni center remains ligated by carbenes and the NHC ligands could provide a better coordination environment around the catalytic active Ni(II) species than the tertiary phophine ligands do in the indenylnickel(II) complexes. Finally, the catalytic activity of CpNi(IPr)Cl (6) for the same polymerization has been investigated. As expect, its catalytic activity is lower than that of 1, i.e. 40% yield of polystyrene was
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obtained after a 48 h polymerization (Table 3, run 9), which should be due to the so-called ‘‘indenyl effect” [32]. 4. Conclusion We have prepared two indenylnickel(II) complexes bearing a free bulky aryl-substituted NHC ligand via the reaction of the easily available metal precursor (R-Ind)2Ni and imidazolium salts. This procedure needs no extra step for the isolation and purification of the corresponding N-heterocyclic carbene, and could provide a valid synthetic route to a large number of NHC Ni(II) complexes containing polydentate ligands. The present work shows that the free NHC indenylnickel(II) halides are capable of polymerizing styrene in the presence of NaBPh4, and a close relationship between their structures and catalytic activities is observed for the first time. The obtained results suggest that the presence of the bulky aryl-substituent on the NHC ligand, together with the alkyl-substituent on the indenyl ligand in these complexes, is of benefit not only to enhance their thermal/air stabilities but also to improve their catalytic activities. In comparison with the catalytic behaviors of PPh3-based systems under the same conditions, the NHC-based ones show significant improvements in stability and catalytic performance. Further investigations focusing on developing the reactivity of NHC indenylnickel(II) complexes are in progress. Supplementary data CCDC 294978 and 665469 contain the supplementary crystallographic data for compounds 1 and 2. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Acknowledgements We thank the Chinese National Natural Science Foundation (Grant 20772089), the Department of Education of Jiangsu Province and the Key Laboratory of Organic Chemistry of Jiangsu Province for financial support. References [1] E.A.B. Kantchev, C.J. O’Brien, M.G. Organ, Angew. Chem., Int. Ed. 46 (2007) 2768. [2] F. Glorius, N-Heterocyclic Carbenes in Transition Metal Catalysis, SpringerVerlag, Berlin, Germany, 2007. [3] H. Clavier, S.P. Nolan, Annu. Rep. Prog. Chem., Sect. B 103 (2007) 193.
[4] F. Boeda, S.P. Nolan, Annu. Rep. Prog. Chem., Sect. B 104 (2008) 184. [5] L. Cavallo, A. Correa, C. Costabile, H. Jacobsen, J. Organomet. Chem. 690 (2005) 5407. [6] F.E. Hahn, Angew. Chem., Int. Ed. 45 (2006) 1348. [7] N.M. Scott, H. Clavier, P. Mahjoor, E.D. Stevens, S.P. Nolan, Organometallics 27 (2008) 3181. and the references cited. [8] H.V. Huynh, L.R. Wong, P.S. Ng, Organometallics 27 (2008) 2231. and the references cited. [9] S. Kuhl, R. Schneider, Y. Fort, Adv. Synth. Catal. 345 (2003) 341. [10] S. Caddick, F.G.N. Cloke, P.B. Hitchcock, A.K.de K. Lewis, Angew. Chem., Int. Ed. 43 (2004) 5824. [11] K.. Matsubara, K. Ueno, Y. Koga, K. Hara, J. Org. Chem. 72 (2007) 5069, and the references cited. [12] C.-H. Lee, W.-C. Ke, K.-T. Chan, C.-L. Lai, C.-H. Hu, H.M. Lee, Chem. Eur. J. 13 (2007) 582. [13] Z.X. Xi, X.M. Zhang, W.Z. Chen, S.Z. Fu, D.Q. Wang, Organometallics 26 (2007) 6636. [14] M.A. Schade, A. Metzger, S. Hug, P. Knochel, Chem. Commun. (2008) 3046. [15] D.S. McGuinness, W. Mueller, P. Wasserscheid, K.J. Cavell, B.W. Skelton, A.H. White, U. Englert, Organometallics 21 (2002) 175. [16] A.L. Mackinnon, M.C. Baird, J. Organomet. Chem. 683 (2003) 114. [17] W. Buchowicz, A. Koziol, L.B. Jerzykiewicz, T. Lis, S. Pasynkiewicz, A. Pecherzewska, A. Pietrzykowski, J. Mol. Catal. A: Chem. 257 (2006) 118. [18] J. Cámpora, L.O. De la Tabla, P. Palma, E. Álvarez, F. Lahoz, K. Mereiter, Organometallics 25 (2006) 3314. [19] W.F. Li, H.M. Sun, M.Z. Chen, Q. Shen, Y. Zhang, J. Organomet. Chem. 693 (2008) 2047. [20] X. Wang, S. Liu, G.X. Jin, Organometallics 23 (2004) 6002. [21] B.E. Ketz, X.G. Ottenwaelder, R.M. Waymouth, Chem. Commun. (2005) 5693. [22] S. Benson, B. Payne, R.M. Waymouth, J. Polym. Sci. A: Polym. Chem. 45 (2007) 3637. [23] W.F. Li, H.M. Sun, M.Z. Chen, Z.G. Wang, D.M. Hu, Q. Shen, Y. Zhang, Organometallics 24 (2005) 5925. [24] H.M. Sun, D.M. Hu, Y.S. Wang, Q. Shen, Y. Zhang, J. Organomet. Chem. 692 (2007) 903. [25] S. Díez-González, S.P. Nolan, Coord. Chem. Rev. 251 (2007) 874. [26] N.M. Scott, H. Clavier, P. Mahjoor, E.D. Stevens, S.P. Nolan, Organometallics 27 (2008) 3181. [27] D. Zargarian, R. Giasson, Coord. Chem. Rev. 233–234 (2002) 157. [28] H.M. Sun, Q. Shao, D.M. Hu, W.F. Li, Q. Shen, Y. Zhang, Organometallics 24 (2005) 331. [29] D.M. Hu, H.M. Sun, Y.S. Wang, Q. Shen, F. Xue, Chin. J. Struct. Chem. 25 (2006) 1255. [30] F.H. Köhler, Chem. Ber. 107 (1974) 570. [31] L. Jafarpour, E.D. Stevens, S.P. Nolan, J. Organomet. Chem. 606 (2000) 49. [32] T.A. Huber, M. Bayrakdarian, S. Dion, I. Dubuc, F. Bélanger-Gariépy, D. Zargarian, Organometallics 16 (1997) 5811. [33] R.A. Kelly III, N.M. Scott, S. Díez-González, E.D. Stevens, S.P. Nolan, Organometallics 24 (2005) 3442. [34] V. Ritleng, C. Barth, E. Brenner, S. Milosevic, M.J. Chetcuti, Organometallics 27 (2008) 4223. [35] T.A. Huber, F. Bélanger-Gariépy, D. Zargarian, Organometallics 14 (1995) 4997. [36] L.F. Groux, D. Zargarian, Organometallics 20 (2001) 3811. [37] L.F. Groux, D. Zargarian, Organometallics 22 (2003) 3124. [38] L.F. Groux, D. Zargarian, Organometallics 22 (2003) 4759. [39] H.M. Sun, W.F. Li, X.Y. Han, Q. Shen, Y. Zhang, J. Organomet. Chem. 688 (2003) 132. [40] W.F. Li, H.M. Sun, Q. Shen, Y. Zhang, K.B. Yu, Polyhedron 23 (2004) 1473. [41] M. Jiménez-Tenorio, M.C. Puerta, I. Salcedo, P. Valerga, S.I. Costa, L.C. Silva, P.T. Gomes, Organometallics 23 (2004) 3139. [42] N. Ishihara, T. Seimiya, M. Kuramoto, M. Uoi, Macromolecules 19 (1986) 2464.
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Synthesis and structure of indenylnickel(II) chlorides bearing free N-heterocyclic carbene ligands and their catalysis for styrene polymerization Ling-Zhi Xie, Hong-Mei Sun *, Dong-Mei Hu, Zhi-Hong Liu, Qi Shen, Yong Zhang The Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
a r t i c l e
i n f o
Article history: Received 20 March 2009 Accepted 18 May 2009 Available online 16 June 2009 Keywords: N-heterocyclic carbene Nickel(II) complexes Indenyl Styrene Polymerization
a b s t r a c t The reaction of (R-Ind)2Ni (Ind = C9H7, indenyl) with an equivalent of a bulky aryl-substituted imidazolium salt in CH2Cl2/THF at 45 °C results in the corresponding N-heterocyclic carbene (NHC) indenylnickel(II) chloride of the type (R-Ind)Ni(L)Cl [R = 1-H, L = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), 1; R = 1-Me, L = IPr, 2] in high yield. Complexes 1 and 2 were characterized by elemental analysis, NMR spectroscopy and X-ray crystallography. The resulted NHC indenylnickel(II) complexes are capable of polymerizing styrene in the presence of NaBPh4 to give atactic polystyrene with Mn values in the range of 104. The present studies show a close relationship between the structure and catalytic activity of the NHC indenylnickel(II) halides (including the previously reported indenylnickel(II) halides bearing alkylsubstituted NHC ligands), and complex 2 shows the highest catalytic activity. In comparison with its phosphine analogue (1-Me-Ind)Ni(PPh3)Cl, complex 2 shows significant improvements in stability and catalytic performance. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction During the past decade there has been increasingly interesting in the application of N-heterocyclic carbenes (NHCs) as a structure principle for developing late transition metal-based homogeneous catalysts [1–4]. In many types of catalytic processes, NHCs have been used as a valid alternative to the traditional phosphine ligands due to their unique and tunable steric and electronic properties [5–7]. The replacement of phosphine ligands with electronrich NHC ligands could represent significant catalytic activity in most cases. For example, benzannulated NHC Ni(II) complexes of the general formula Ni(NHC)2Br2 exhibited much higher catalytic activity than their phosphine analogues, Ni(PPh3)2Br2, in reductive Ullmann coupling reactions of aryl bromides [8]. However, in comparison with the intensively studied NHC complexes of palladium, a relatively small number of well-defined NHC nickel complexes have been reported to date. Only recently have NHC complexes of nickel garnered more attention and emerged as effective catalysts for a series of reactions, such as C–F bond [9] or C–H bond activation [10], C–C or C–N cross-coupling reactions [8,11–14], olefin dimerization [15,16], polymerizations [17– 24], and so on. The (NHC)nNi(CO)m system has been used as a model for the understanding of electronic and steric effects of NHCs [25,26], nevertheless, the principles regulating their structures
* Corresponding author. Tel.: +86 512 65880330; fax: +86 512 65880305. E-mail address: [email protected] (H.-M. Sun). 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2009.05.080
and reactivities remain unclear. This prompted us to develop the free NHC-based indenylnickel(II) complexes. In fact, although tertiary phosphine analogues of the type (R-Ind)Ni(PR03 )X (X = halide, alkyl, alkynyl, imidate, thiolate) have been reported, and include examples that are effective precatalysts for the oligomerization or polymerization of ethylene, styrene and alkynes, the dehydrogenative oligomerization or polymerization of phenylsilane [27], NHC indenylnickel(II) complexes have received little attention until our recent report on a one-step facile synthetic method, i.e. the direct reaction of the bis-indenyl Ni(II) complex of (1-H-Ind)2Ni with an alkyl-substituted imidazolium salt, for the synthesis of alkyl-substituted NHC indenylnickel(II) complexes (1-H-Ind)Ni(iPr)X (iPr = 1,3-diisopropylimidazole-2-ylidene) in high yields [28,29]. This procedure needs no extra steps for the isolation and purification of the corresponding N-heterocyclic carbene, and provides simple access to NHC Ni(II) complexes with polydentate co-ligands. Despite our investigation, there has only been one example reported so far that such NHC complexes could be synthesized by the reaction of (R-Ind)Ni(PPh3)X with free NHC, mentioned briefly in one review [27]. To explore a simple route to a large number of complexes in this family would be desirable. Herein we described the extension of our methodology to the synthesis of indenylnickel(II) chlorides bearing sterically bulky aryl-substituted NHC ligands, (1-H-Ind)Ni(IPr)Cl [IPr = 1,3bis(2,6-diisopropylphenyl)imidazol-2-ylidene, 1) and its alkylsubstituted indenyl derivative (1-Me-Ind)Ni(IPr)Cl (2). The present investigation enlarges the list of analogous complexes already described [28,29], to such an extent that comparative studies of
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the relationship between their structures and catalytic activities in styrene polymerization may be possible. Their catalytic activities for styrene polymerization have also been compared to their tertiary phosphine analogues.
2. Experimental 2.1. General procedures All manipulations were performed under pure argon with rigorous exclusion of air and moisture using standard Schlenk techniques. Solvents were distilled from Na/benzophenone ketyl under pure argon prior to use. Indene (Fluka) (dried over 4 Å molecular sieves) was distilled immediately before use. (1-H-Ind)2Ni [28], (1-Me-Ind)2Ni [30], 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (IPrHCl [31], (1-H-Ind)Ni(iPr)X [iPr = 1,3diisopropylimidazole-2-ylidene, X = Cl (3) and Br (4)] [28], (1Me-Ind)Ni(PPh3)Cl (5) [32] and CpNi(IPr)Cl (Cp = cyclopentadienyl, 6) [33] were prepared by published methods. Elemental analysis was performed by direct combustion on a Carlo-Erba EA-1110 instrument. NMR (CDCl3) spectra were measured on a Unity Inova-400 spectrometer at 25 °C, and were assigned by the assistance of 2D techniques (H–H correlation spectroscopy and C–H heteronuclear correlation spectroscopy). Molecular weight and molecular weight distribution were determined by gel-permeation chromatography (GPC) measurements calibrated to commercial polystyrene standards and performed on a PE PL-GPC50 apparatus with two PLgel 10 lm MIXED-B columns in THF (1.0 mL/min) at 40 °C. 2.2. Synthesis of (1-H-Ind)Ni(IPr)Cl (1) A Schlenk flask was charged with 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (0.85 g, 2.00 mmol), CH2Cl2 (5 mL) and a stir bar. To this solution was added (1-H-Ind)2Ni (0.57 g, 2.00 mmol) in 20 mL of THF via syringe at room temperature. The solution was stirred overnight at 45 °C, filtered and then evaporated to dryness. After being washed with hexane, the residue was extracted with hot toluene and recrystallized from concentrated toluene at 10 °C, yielding bright red crystals (0.32 g, 63%) suitable for elemental analysis and X-ray diffraction studies. M.p. 176–178 °C. Anal. Calc. for C36H43ClN2Ni: C, 72.32; H, 7.25; N, 4.69. Found: C, 72.24; H, 7.43; N, 4.45%. 1H NMR (400 MHz, CDCl3, 25 °C): d 1.05 (d, 12H, Me), 1.34 (d, 12H, Me), 2.65 (br, 1H, H3), 3.07 (br, 2H, CHMe2), 4.44 (br, 2H, CHMe2), 6.38 (s, 1H, H1), 6.62 (br, 2H, H5/H8 and H2), 7.01 (s, 2H, CH@CH), 7.35-7.57 (m, 9H, H5/H8, H6, H7 and Ph). 13C NMR (100 MHz, CDCl3, 25 °C): d 23.1 (Me), 26.6 (Me), 29.0 (CHMe2), 103.3 (C1), 118.1 and 118.2 (C2 and C5/C8), 124.7 and 125.5 (m-Ph, C6 and C7), 125.8 (CH@CH), 130.4 (p-Ph), 132.73 (C4 and C9), 136.8 (o-Ph), 146.7 (ipso-Ph), 175.4 (Ccarbene); however, the assignment of some of these signals remains ambiguous, such as the resonance for C3 which is missing and should be presented at ca. 50. 2.3. Synthesis of (1-Me-Ind)Ni(IPr)Cl (2) Following a procedure similar to the synthesis of 1, (1-Me-Ind)2Ni (0.62 g, 2.00 mmol) in 20 mL of THF was added to a solution of 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (0.85 g, 2.00 mmol) in CH2Cl2 (5 mL) at room temperature. After workup, dark red crystals (0.86 g, 70%) suitable for elemental analysis and X-ray diffraction studies were obtained from concentrated toluene at 10 °C. M.p. 206–207 °C. Anal. Calc. for C37H45ClN2Ni: C, 72.62; H, 7.41; N, 4.58. Found: C, 72.31; H, 7.07; N, 4.47%. 1H NMR (400 MHz, CDCl3, 25 °C): d 0.95 (m, 9H, Me and 1-Me-Ind), 1.15 (d, 6H, Me), 1.23 (d, 6H, Me), 1.39 (d, 6H, Me), 2.55 (br, 2H, CHMe2),
3.17 (br, 2H, CHMe2), 4.16 (s, 1H, H3), 5.65 (d, 1H, H5), 6.01 (s, 1H, H2), 6.29 (m, 1H, H6), 6.57 (d, 1H, H8), 6.77 (d, 1H, H7), 7.01 (s, 2H, CH@CH), 7.32-7.53 (m, 6H, Ph). 13C NMR (100 MHz, CDCl3, 25 °C): d 12.2 (1-Me-Ind), 22.7 (Me), 23.6 (Me), 26.1 (Me), 27.0 (Me), 29.0 (CHMe2), 58.1 (C3), 93.9 (C1), 103.8 (C2), 116.7 (C8), 117.0 (C5), 124.5 (C6), 124.6 (m-Ph), 125.5 (C7), 125.8 (CH@CH), 130.2 (pPh), 132.0 (C4), 134.0 (C9), 136.9 (o-Ph), 146.7 (ipso-Ph), 177.1 (Ccarbene). 2.4. Structure determination Suitable single crystals of 1 and 2 were sealed in a thin-walled glass capillary for X-ray structural analysis. Diffraction data were collected on a Rigaku Mercury CCD area detector at 193(2) K. The structures were solved by direct methods and refined by full-ma-
Table 1 X-ray crystallographic data for 1 and 2. 1
2
Empirical formula Formula weight Temperature (K) k (Mo Ka) (Å) Crystal system Space group
C36H43ClN2Ni 597.88 193(2) 0.7107 monoclinic p21/n
C37H45ClN2Ni 611.91 153(2) 0.7107 monoclinic p21/n
Unit cell dimensions a (Å) b (Å) c (Å) b (°) V (Å3) Z Dcalc (g cm3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm3) 2hmax (°) Reflections collections Independent reflections Goodness-of-fit on F2 Final R indices [I > 2r(I)] Rw
9.8993(9) 19.239(2) 16.883(1) 106.072(2) 3089.7(5) 4 1.285 0.742 1272 0.30 0.20 0.15 25.34 30454 5642 (Rint = 0.0421) 1.103 0.0415 0.0816
10.3994(11) 19.057(2) 16.861(2) 105.843(2) 3214.9(6) 4 1.264 0.714 1304 0.25 0.25 0.07 25.3 31317 5869 (Rint = 0.0717) 1.156 0.0598 0.0985
Table 2 Select bond lengths (Å), angles (°) and structural parameters for 1 and 2. 1
2
Ni–Cl Ni–C(10) Ni–C1 Ni–C(2) Ni–C(3) Ni–C(4) Ni–C(9) C(1)–C(2) C(1)–C(9) C(2)–C(3) C(3)–C(4) C(4)–C(9) C1–Ni–Cl C1–Ni–C3 C3–Ni–C10 C10–Ni–Cl DM-C (Å)a HA (°)b FA (°)c a
2.1759(7) 1.896(2) 2.119(2) 2.019(3) 2.028(2) 2.412(2) 2.437(2) 1.394(4) 1.442(3) 1.421(4) 1.465(4) 1.421(3) 96.5(8) 66.3(1) 102.6(1) 95.4(1) 0.35 11.4 12.5
Ni–Cl Ni–C(11) Ni–C1 Ni–C(2) Ni–C(3) Ni–C(4) Ni–C(9) C(1)–C(2) C(1)–C(9) C(2)–C(3) C(3)–C(4) C(4)–C(9) C1–Ni–Cl C1–Ni–C3 C3–Ni–C11 C11–Ni–Cl DM-C (Å)a HA (°)b FA (°)c
2.1829(9) 1.895(3) 2.132(3) 2.041(3) 2.025(3) 2.385(3) 2.415(3) 1.407(5) 1.454(5) 1.423(5) 1.458(5) 1.419(4) 95.0(1) 66.6(1) 103.4(1) 95.5(1) 0.32 10.1 11.4
DM-C = 1/2[M-Cav(for C4, C5)] 1/2[M-Cav(for C1, C3)]. HA is the angle formed between the planes formed by the atoms C(1), C(2), C(3) and C(1), C(3), C(4), C(5). c FA is the angle formed between the planes formed by the atoms C(1), C(2), C(3) and C(4), C(5), C(6), C(7), C(8), C(9). b
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trix least-squares procedures based on F2. All non-hydrogen atoms were refined with anisotropic displacement coefficients. Hydrogen atoms were treated as idealized contributions. The structures were solved and refined using SHELXS-97 and SHELXL-97 programs, respectively. Crystal data, collection and main refinement parameters are given in Table 1. Selected bond lengths (Å) and angles (°) for 1 and 2 are given in Table 2. 2.5. A typical procedure for the polymerization of styrene A typical procedure for the polymerization of styrene is given as follows. Under dry argon, complex 1 (12.0 mg, 0.02 mmol), NaBPh4 (47.9 mg, 0.14 mmol), toluene (0.2 mL), and styrene (0.7 mL, 6.0 mmol) were added into a dry glass ampule in turn. Then, the sealed ampule was placed in a water bath held at 80 °C. After a definite reaction time, the polymerization was stopped by adding 1 mL of 5% HCl/ethanol. After evaporation of the solvent and unreacted monomer, the resultant polymer was dissolved in THF, followed by precipitation in 95% ethanol. After filtration, the white polymer was dried in vacuo at room temperature overnight. The polymer yield was determined gravimetrically. 3. Results and discussion 3.1. Synthesis and structural characterization of 1 and 2 Focusing on the nickel-based system, the bulky aryl-substituted NHC complexes exhibit unusual stability or reactivity in certain instances [8,25,34]. So, as a continuation of our previous work on NHC indenylnickel(II) complexes, it was considered to test whether the reported imidazolium salts metathesis and indene elimination reaction [28–29] could also occur between 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (IPrHCl) and (1-H-Ind)2Ni, to yield the corresponding bulky aryl-substituted NHC indenylnickel(II) complex. Thus, a 1:1 (in molar ratio) mixture of (1-H-Ind)2Ni and IPrHCl was stirred at 45 °C in CH2Cl2/THF overnight. This reaction worked smoothly from the change of color from red to dark red. After workup, the designed NHC complex 1 was isolated as dark red crystals in ca. 63% yield (Scheme 1). The proposed structure for 1 was first supported by elemental analysis and NMR spectroscopy. The 1H NMR and 13C NMR data indicated the presence of one indenyl group and one N-heterocyclic carbene group, respectively. Moreover, in the 13C NMR spectrum, a singlet Ccarbene atom signal appears at d 175.4 ppm, which resembles a clear downfield shift compared to the reported Ni(II) complexes of Dd 9 ppm for (1-H-Ind)Ni(iPr)Cl (3, d = 166.3 ppm) [28] or Dd 6 ppm for CpNi(IPr)Cl (6, d = 169.3 ppm) [33]. The identity of this complex is further confirmed by X-ray analysis. To further ascertain the versatility of this synthetic approach, the similar reaction of the alkylsubstituted bis-indenyl Ni(II) complex of (1-CH3-Ind)2Ni with the same imidazolium salt was tested in turn. The analogous chloride 2 was isolated as dark red crystals in ca. 70% yield by a similar procedure (Scheme 1). Elemental analysis, NMR spectroscopy and X-ray analysis confirmed the expected product composition. In
its 13C NMR spectrum, the singlet Ccarbene atom signal appears at d 177.1 ppm, which is very close to that of 1. These results demonstrated undoubtedly that the imidazolium salt metathesis and indene elimination is a valid synthetic route to a variety of NHC indenylnickel(II) complexes. Compared with the alkyl-substituted NHC indenylnickel(II) complexes (1-H-Ind)Ni(iPr)Cl (iPr = 1,3-diisopropylimidazole-2ylidene, 3) [28], both 1 and 2 are much more thermally stable and melt at ca. 177 °C for 1 and ca. 206 °C for 2 (the melting point of 3 is 147 °C). Besides, they are also much less sensitive to air and moisture than complex 3, and can even be kept unchanged in the open air for several hours in the solid state, compared to only a few minutes for complex 3 [28]. Additionally, it is worth mentioning that the tertiary phosphine analogues, (1-H-Ind)Ni(PPh3)Cl [35] or (1-Me-Ind)Ni(PPh3)Cl (5) [32], decompose immediately in open air. 3.2. Crystal structures of 1 and 2 Crystals of 1 and 2 suitable for X-ray structure determination were grown from cold toluene solution. The crystallographic and measurement data are shown in Table 1, and selective bond lengths and angles are listed in Table 2. The molecular structures of these complexes are shown as ORTEP drawings in Figs. 1 (1) and 2 (2). The molecular structures of 1 and 2 are similar, and both are monomeric in the solid state. The nickel atom is bonded to the indenyl group in a highly unsymmetric mode. The bond lengths of Ni–C1, Ni–C2 and Ni–C3 equal 2.119(2), 2.019(3) and 2.028(2) Å in 1 and 2.132(3), 2.041(3) and 2.025(3) Å in 2, respectively, which are within the normal bonding distances, while the other two carbon atoms of the five-ring of the indenyl group (C4 and C9) are considerably further away (Ni–C4 = 2.412(2), Ni– C9 = 2.437(2) Å in 1; Ni–C4 = 2.385(3), Ni–C9 = 2.415(3) Å in 2). Such a highly unsymmetric mode of the indenyl ligand is not only very similar to the reported NHC indenylnickel(II) complexes [28– 29] but also common in the family of the corresponding tertiary phosphine indenylnickel(II) complexes [27]. The Ni–Ind interaction could be quantitatively calculated by the main geometrical parameters such as (a) the slip value represented by DM-C and (b) the hinge and fold angles (HA and FA) representing the bending of the indenyl ligand at C1/C3 and C4/C9, respectively [28]. For example, DM-C values close to zero indicate little distortion from g5 hapticity, whereas values above ca. 0.5 Å indicate nearly g3 hapticity; HA values of ca. 14° or greater are associated with nearly trihapto cases. Here, in complex 1, DM-C = 0.35 Å, HA = 11.4°, FA = 12.74°; and in complex 2, DM-C = 0.32 Å, HA = 10.1°, FA = 11.4° (Table 2). These values led us to propose that the Ni–Ind bonding fashion here can be described as an intermediate hapticity between the g5- and g3-modes, which is somewhat similar to that found in (1-H-Ind)2Ni (DM-C = 0.42 Å, HA = 13.9°, FA = 13.1°) [27]. However, the larger parameter values (DM-C, HA, and FA) for 1 indicated a more unsymmetric Ni–Ind interaction is present in 1. This phenomenon that the methyl substituent on
R Ind Cl i
i
Pr
(R-Ind)2Ni +
N i
Pr
Cl-
i
Pr
Pr
-indene N
N i
Pr
THF/CH2Cl2, 45°C
i
Pr
Ni
i
Pr
N i
Pr
R = 1-H (1), Yield: 63% R = 1-Me (2), Yield: 70% Scheme 1. Synthesis of NHC indenylnickel(II) complexes of 1 and 2.
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Fig. 1. The crystal structure of complex 1.
Fig. 2. The crystal structure of complex 2.
the indenyl ligand of 2 leads to a less unsymmetric Ni-Ind interaction, i.e. represents a stronger Ni–Ind interaction, is very similar to the one found in its PPh3-based analogous [32]. Furthermore, since the bond lengths of Ni–C1 and C2–C3 are longer than those of Ni– C3 and C1–C2 in 1 and 2, respectively, it can be argued that the distortion in the hapticity of the indenyl ligand is not quite g5 M g3 but rather g5 M (g1, g2), i.e. the C1 = C2 occupy a single coordination site. In addition, the NMR data are also valuable in assessing the indenyl hapticity in solution [27]. For example, Zargarian has suggest that large chemical shift differences between H1 and H3 (>2 ppm) or C1 and C3 (>20 ppm) correlated well with a partial localization of bonding in the allyl moiety of the indenyl ligand in (I-H-Ind)Ni(PPh3)Cl, since such a bonding type requires that C1 and C3 be quite different in terms of hybridization [35]. Herein
we note that even if the assignment of these signals remains somewhat ambiguous, the H1 and H3 signals for 1 appear at 2.65 and 6.38 ppm, respectively, meanwhile the C1 and C3 signals for 2 appear at 58.1 and 93.3 ppm, respectively. The pairwise d values are more than 3 ppm (H1/H3 in 1) or 35 ppm (C1/C3 in 2) apart, attesting to the greater sp2 character of C1 in 1 and 2, respectively [27]. Thus, the nickel atom is bonded to a Cl, Ccarbene, C1, C2 and C3 of the indenyl group, to form a highly distorted square plane with C1 = C2 occupying a single coordination site (i.e. C1–C2 < C2–C3, Table 2). Such a coordinated fashion can be mainly attributed to the tendency of Ni(II) to avoid forming 18-electron complexes [27–28] and the unequal trans influences of the N-heterocyclic carbene and the halogen group [28]. Besides, as compared with 0.31 Å (DM-C), 12.1° (HA) and 9.1° (FA) reported for (1-H-Ind)Ni(iPr)Cl (3)
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and 0.30 Å (DM-C), 12.3° (HA) and 9.5° (FA) reported for (1-H-Ind)Ni(iPr)Br (4) [28], the differences in these parameters among 1, 2, 3 and 4 demonstrate that the NHC ligand has a great influence on the Ni–Ind interaction presented in (R-Ind)Ni(NHC)X. The bulky IPr ligand possesses a more trans influence than the iPr ligand, which leads to a more unsymmetric Ni–Ind interaction. In addition, in comparison with the previously measured parameters for (I-HInd)Ni(PPh3)Cl (i.e. DM-C = 0.26 Å) [35] and 5 (i.e. DM-C = 0.25 Å) [32], it is found that there is a more unsymmetric Ni–Ind interaction for NHC-based systems, which could be attributed to the stronger donor ability of the NHC ligand than that of PPh3 [25] and a more unequal trans influences between the NHC ligand and the halogen group. The Ni–Ccarbene bond lengths are almost the same for the two complexes (1.896(2) Å in 1 and 1.895(3) Å in 2), and both are longer than that of 1.886(3) Å in 3 or 1.888(4) Å in 4 [28]. These data indicate that the ancillary ligand of (R-Ind)Ni(NHC)X, i.e. the indenyl moiety or the halogen atom has subtle effect on the Ni–Ccarbene interaction, meanwhile steric factor differences of the NHC ligands should be responsible for the weaker interaction between the nickel atom and the carbene carbon. Nevertheless, since alkyl-substituted NHCs are marginally more electron-donating than their aryl-substituted counterparts [25], it should be taken into account that small differences in donor capacities of NHC ligands may also play an important role in the Ni–Ccarbene interaction. The Ni–Cl bond length of 2.1759(7) Å in 1 is slightly shorter than 2.1829(9) Å in 2, and obviously shorter than that of 2.2025(7) Å in 3 [28]. The strongest Ni–Cl interaction in 1 might be a compensation for the weakening trend of the Ind–Ni interaction on going from 1 to 2 and then to 3. Alternatively, a weakening trend of the Ni–Cl interaction is probably caused by the Cl–Ni pdonation [32]. However, it is well known that the X–M p-donation would destabilize d8 complexes. In addition, the Ni–Cl distances in 1 or 2 are slightly shorter than that of 2.1822(7) Å in (I-H-Ind)Ni(PPh3)Cl [35] or 2.1865(10) Å in 5 [32], respectively. 3.3. Catalytic activity of the free NHC indenylnickel(II) complexes for styrene polymerization Previous studies had indicated that the neutral tertiary phosphine indenylnickel(II) complexes of (R-Ind)Ni(PR3)X (X = halide) alone were inert toward the insertion of styrene, whereas the addition of proper amounts of AgBPh4 (or NaBPh4) and phosphine ligands led to the polymerization of styrene [36–41]. The role of AgBPh4 (or NaBPh4) was to abstract the halide ligand from these complexes, with the in situ formation of catalytically active cationic species, while addition of the phosphine ligand was needed to suppress the decomposition of the cationic species [27,39–41] unless the indenyl ligand tethered an amine moiety [36–38]. In the present paper, the catalytic activity of a series of free NHC indenylnickel(II) complexes for styrene polymerization has been investigated for the first time.1 The preliminary results are listed in Table 3. The results show that the addition of a proper amount of NaBPh4 is also necessary in the present case, which indicates that the present styrene polymerization also is catalyzed by the in situ generated electronically and coordinatively unsaturated species. For example, complex 1 alone shows no catalytic activity for styrene polymerization in toluene at 80 °C (Table 3, run 1), however, together with NaBPh4, it shows considerable catalytic activity, i.e. 60% yield of polystyrene was obtained after 48 h polymerization (Table 3, run 2).
1 In one of our previous reports, i.e. Ref. [24], the catalytic activity of 4 for the polymerization of styrene had been mentioned briefly in comparison with the corresponding Ni(II) bromide containing an indenyl ligand tethered NHC moiety. However, we should point out that the reaction time of the styrene polymerization by 4 was incorrect, and it should be 48 h instead of the mentioned time of 24 h.
Table 3 Ni(II)-catalyzed homopolymerization of styrene. Run
Initiator
NaBPh4/[Ni]
t (h)
Yield (%)
Mn 104
Mw/Mn
1 2 3 4 5 6a 7 8a 9
1 1 2 3 4 2 5 5 6
0 7 7 7 7 7 7 7 7
48 48 48 48 48 48 48 24 48
0 60 75 26 36 33 24 78 40
3.6 5.7 2.6 4.4 3.3 0.4 0.6 2.3
2.0 2.4 3.6 3.1 2.6 2.4 4.7 1.9
Conditions: styrene/[Ni] = 300:1 (molar ratio), toluene: 0.2 mL, T = 80 °C. a [NaBPh4]/[Ni]/[PPh3] = 7:1:1.
The influences of the indenyl ligand, the NHC ligand and the halide ligand on the catalytic activity of the corresponding Ni(II) complexes are investigated in turn. On the basis of the polymer yield, complex 2 shows the highest catalytic activity while complex 4 possess the lowest one (Table 3, runs 2–5). Meanwhile, the polystyrene with the highest number-average molecular weight (Mn = 57 000) was obtained with 2. The diverse activities observed for these complexes may simply indicate that the catalytic active species remain ligated by the indenyl and carbene ligands, and the aryl-substituted NHC ligand together with the alkyl-substituted indenyl ligand are a benefit to enhance the catalytic activity of the NHC indenylnickel(II) complexes for the styrene polymerization. The difference in activity between 3 and 4 are probably due to their different thermal stabilities during the polymerization process [28,40]. All of the polymers obtained here are well soluble in THF and have very similar microstructures. Since a typical 13C NMR spectrum (CDCl3) of the obtained polystyrene shows characteristically broad signals of atactic polystyrene in the range of ca. 144– 148 ppm [42], it is clear that the polystyrene obtained is an atactic polymer, which is very similar to those obtained by PPh3-based indenylnickel(II) complexes [39–41] or by NHC-based cationic allylnickel(II) complexes [22]. It is worth noticing that the addition of a catalytic amount of other additives, i.e. PPh3, is not necessary in the present case. For example, in the presence of one equivalent of PPh3, the yield of polystyrene decreased greatly from 75% to 33% with a nearly half decrease of the number-average molecular weight (Table 3, runs 3 and 6). On the contrary, the addition of a catalytic amount of phosphine ligands is very necessary to styrene polymerization initiated by the phosphine analogues [39–41]. For example, in the absence of PPh3, complex 5 shows a much lower catalytic activity, i.e. 24% yield of polystyrene with a relatively low number-average molecular weight (Mn = 4000) was obtained in the present conditions (Table 3, run 7). Notably, black powders appeared quickly in the glass ampule during this polymerization process, which indicated the decomposition of 5. Otherwise, the addition of one equivalent of PPh3 could efficiently suppress this decomposition and provide a much higher polymer yield (Table 3, run 8). Nevertheless, the polymer obtained still possesses a lower number-average molecular weight and a broadened molecular weight distribution. A similar phenomenon of decomposition was not observed in the styrene polymerization by NHC indenylnickel(II) complexes in the absence of PPh3. The great difference of catalytic behaviors between the NHC indenylnickel(II) complexes and their tertiary phosphine analogues also indicates that the Ni center remains ligated by carbenes and the NHC ligands could provide a better coordination environment around the catalytic active Ni(II) species than the tertiary phophine ligands do in the indenylnickel(II) complexes. Finally, the catalytic activity of CpNi(IPr)Cl (6) for the same polymerization has been investigated. As expect, its catalytic activity is lower than that of 1, i.e. 40% yield of polystyrene was
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obtained after a 48 h polymerization (Table 3, run 9), which should be due to the so-called ‘‘indenyl effect” [32]. 4. Conclusion We have prepared two indenylnickel(II) complexes bearing a free bulky aryl-substituted NHC ligand via the reaction of the easily available metal precursor (R-Ind)2Ni and imidazolium salts. This procedure needs no extra step for the isolation and purification of the corresponding N-heterocyclic carbene, and could provide a valid synthetic route to a large number of NHC Ni(II) complexes containing polydentate ligands. The present work shows that the free NHC indenylnickel(II) halides are capable of polymerizing styrene in the presence of NaBPh4, and a close relationship between their structures and catalytic activities is observed for the first time. The obtained results suggest that the presence of the bulky aryl-substituent on the NHC ligand, together with the alkyl-substituent on the indenyl ligand in these complexes, is of benefit not only to enhance their thermal/air stabilities but also to improve their catalytic activities. In comparison with the catalytic behaviors of PPh3-based systems under the same conditions, the NHC-based ones show significant improvements in stability and catalytic performance. Further investigations focusing on developing the reactivity of NHC indenylnickel(II) complexes are in progress. Supplementary data CCDC 294978 and 665469 contain the supplementary crystallographic data for compounds 1 and 2. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Acknowledgements We thank the Chinese National Natural Science Foundation (Grant 20772089), the Department of Education of Jiangsu Province and the Key Laboratory of Organic Chemistry of Jiangsu Province for financial support. References [1] E.A.B. Kantchev, C.J. O’Brien, M.G. Organ, Angew. Chem., Int. Ed. 46 (2007) 2768. [2] F. Glorius, N-Heterocyclic Carbenes in Transition Metal Catalysis, SpringerVerlag, Berlin, Germany, 2007. [3] H. Clavier, S.P. Nolan, Annu. Rep. Prog. Chem., Sect. B 103 (2007) 193.
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