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Organometallics 2009, 28, 652–655
Notes Temperature-Dependent Nuclearity in Bis(benzimidazoyl) Nickel Complexes and Their Catalysis toward Conjugate Addition of Thiophenols to r,β-Enones Way-Zen Lee,* Tzu-Li Wang, Huan-Sheng Tsang, Cheng-Yuan Liu, and Chien-Tien Chen* Department of Chemistry, National Taiwan Normal UniVersity, Taipei 11650, Taiwan
Ting-Shen Kuo Instrumentation Center, Department of Chemistry, National Taiwan Normal UniVersity, Taipei 11650, Taiwan ReceiVed September 30, 2008 Summary: The nuclearity of two preViously prepared nickel complexes, [LNiCl(µ-Cl)]2 · 2CH3OH (1) and L′NiCl2 (2) (L ) bis(1-methylbenzimidazoyl-2-methyl)amine and L′ ) bis(1-methylbenzimidazoyl-2-methyl)-10-camphorsulfonamide), was found to be temperature-dependent. Mononuclear [LNi(CH3OH)2Cl]Cl · 2H2O (3) and dinuclear [L′NiCl(µ-Cl)]2 (4) were obtained as crystals at -20 °C. It is noteworthy that complex 2 can catalyze the conjugate addition of thiophenols to R,β-enones in high yields; in contrast, the same reaction catalyzed by NiCl2 · THF or complex 3 was far less effectiVe. The nuclearity of a metal complex can be influenced by many factors, such as counterion,1 solvent,2 pH value,3 and ligand.4 For instance, a dinuclear nickel chloride complex bridged by a hexadentate poly(oxime) ligand may be converted to a mononuclear nickel complex, when the bonded chlorides are switched to innocent nitrate anions.1a Recently, we observed that the nuclearity of nickel complexes could be controlled by employing a bis(benzimidazole) ligand.4a Moreover, we discovered that the nuclearity of two previously prepared nickel complexes, [LNiCl(µCl)]2 · 2CH3OH (1) and L′NiCl2 (2) (L ) bis(1-methylbenzimidazoyl-2-methyl)amine and L′ ) bis(1-methylbenzimidazoyl2-methyl)-10-camphorsulfonamide; Chart 1), was also dependent on the temperature during their crystallization. The number of open sites on the nickel centers for the four bis(benzimidazoyl) nickel complexes was found to depend upon their nuclearity. It is well documented that nickel complexes with open site(s) can
* To whom correspondence should be addressed. E-mail: wzlee@ ntnu.edu.tw (W.-Z.L.);
[email protected] (C.-T.C.). (1) (a) Bera, J. K.; Nethaji, M.; Samuelson, A. G. Inorg. Chem. 1999, 38, 218–228. (b) Deters, E. A.; Goldcamp, M. J.; Bauer, J. A. K.; Baldwin, M. J. Inorg. Chem. 2005, 44, 5222–5228. (2) Kawata, S.; Breeze, S. R.; Wang, S.; Greedan, J. E.; Raju, N. P. Chem. Commun. 1997, 717–718. (3) (a) Kim, G.-S.; Zeng, H.; Neiwert, W. A.; Cowan, J. J.; VanDerveer, D.; Hill, C. L.; Weinstock, I. A. Inorg. Chem. 2003, 42, 5537–5544. (b) Liu, H.-J.; Hung, Y.-H.; Chou, C.-C.; Su, C.-C. Chem. Commun. 2007, 495–497. (4) (a) Lee, W.-Z.; Tseng, H.-S.; Kuo, T.-S. Dalton Trans. 2007, 2563– 2570. (b) Chen, C.-T.; Bettigeri, S.; Weng, S.-S.; Pawar, V. D.; Lin, Y.H.; Liu, C.-Y.; Lee, W.-Z. J. Org. Chem. 2007, 72, 8175–8185.
Chart 1
be used as catalysts in organic synthesis.5 However, only a few nickel complexes have been reported to possess catalytic ability toward the conjugate addition of protic nucleophiles to R,βunsaturated carbonyl compounds (i.e., Michael addition).6 Herein we wish to demonstrate that complex 2 can facilitate such a catalytic process in high yields. As reported previously, complex 1 was prepared by the reaction of NiCl2 · 6H2O with the tripodal ligand L in methanol and was recrystallized at room temperature.4a Interestingly, the mononuclear [LNi(CH3OH)2Cl]Cl · 2H2O (3) was obtained when the product isolated from the reaction was recrystallized at -20 °C. The X-ray crystal structure of complex 3 revealed that the tripodal L was facially coordinated to the nickel center in 3, and the complete coordination sphere of the nickel center was fulfilled by one chloride trans to the amine nitrogen of L and two methanol molecules trans to the imidazole nitrogens of L (Figure 1). The Ni-Namine bond is 0.127 Å longer than the average Ni-Nimidazole bond, implying that the bonding by chloride is much stronger than that of the coordinated methanol molecules (Table 1). Although the nickel centers of 1 and 3 (5) (a) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friendrich, S. K.; Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460–462. (b) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283–315. (c) Meinhard, D.; Wegner, M.; Kipiani, G.; Hearley, A.; Reuter, P.; Fischer, S.; Marti, O.; Rieger, B. J. Am. Chem. Soc. 2007, 129, 9182–9191. (6) (a) Kanemasa, S.; Oderaotoshi, Y.; Wada, E. J. Am. Chem. Soc. 1999, 121, 8675–8676. (b) Evans, D. A.; Thomson, R. J.; Franco, F. J. Am. Chem. Soc. 2005, 127, 10816–10817. (c) Soloshonok, V. A.; Cai, C.; Yamada, T.; Ueki, H.; Ohfune, Y.; Hruby, V. J. J. Am. Chem. Soc. 2005, 127, 15296–15303.
10.1021/om8009427 CCC: $40.75 2009 American Chemical Society Publication on Web 12/30/2008
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Figure 1. Thermal ellipsoid representation of [LNiCl(CH3OH)2]Cl · 2H2O (3) at the 50% probability level. Hydrogen atoms and solvent molecules of 3 are omitted for clarity.
Figure 3. Thermal ellipsoid representation of [L′NiCl(µ-Cl)]2 (4) at the 50% probability level. Hydrogen atoms of 4 are omitted for clarity.
from the nickel center. It was suspected that the dinuclear 1 was dissociated to form two [LNiCl2] fragments in methanol solution and crystallized out as 1 at room temperature or as 3 at - 20 °C (Scheme 1). Notably, the coordination geometry of the species in solution was found to be different from those of complexes 1 and 3 in the solid state, as evidenced by their UV/ vis spectra (Figure 2).
Figure 2. UV/vis spectra of [LNiCl(µ-Cl)]2 · 2CH3OH (1) and [LNiCl(CH3OH)2]Cl · 2H2O (3) in methanol and the solid state. Table 1. Selected Bond Lengths (Å) for Complexes 3 and 4 Ni1-N1 Ni1-N3 Ni1-N5 Ni1-Cl1 Ni1-Cl1A Ni1-Cl2 Ni1-O1 Ni1-O2
3
4
2.056 2.189 2.068 2.367
2.023 2.060 2.378 2.423 2.309
2.108 2.095
were independently coordinated with somewhat different ligands (two bridging chloride ions in 1 and two methanol molecules in 3), the geometries of both complexes were octahedral in the solid state. The same geometries in 1 and 3 led to similar absorption profiles in their solid-state UV/vis spectra (Figure 2). It was quite obvious that mononuclear 3 was converted from dinuclear 1, since 1 was composed of two identical [LNiCl(µCl)] fragments. Interestingly, one chloride remained coordinated to the Ni(II) ion of 3 and the other chloride was dissociated
Similarly, complex 2 was prepared by the reaction of NiCl2 · 6H2O with the tripodal ligand L′ in methanol and was recrystallized at room temperature.4a When the same solution was stored at -20 °C, red crystals of dinuclear [L′NiCl(µ-Cl)]2 (4) suitable for X-ray crystallographic analysis were obtained. The molecular structure of complex 4 revealed a dinickel core bridged by two chloride ligands, and each nickel was coordinated with a bidentate L′ and a terminal chloride to construct a distorted-square-pyramidal geometry with a τ value of 0.26 (Figure 3).7 The two imidazole nitrogens of L′ were coordinated at different positions on the nickel center: one at the axial position with a Ni-N bond length at 2.02 Å and the other at the equatorial plane with a longer bond length of 2.06 Å. Meanwhile, the length of the Ni-Clterminal bond is about 0.09 Å shorter than that of the average Ni-Clbridged bond (Table 1). The Ni2Cl2 core was flat with a Ni · · · Ni distance of 3.62 Å. According to the UV/vis spectra of complexes 2 and 4 taken both in acetonitrile and in the solid state (Figure 4), 2 possessed the same tetrahedral geometry both in solution and in the solid state, whereas the dinuclear 4 was dissociated in solution to form 2 equiv of 2, as depicted in Scheme 2. Recently, Shimazaki et al. reported a similar phenomenon.8 Their dinuclear nickel complex bearing a tripodal N3O ligand and two bridging chlorides on each nickel ion was dissociated to form a fivecoordinated mononuclear species in methanol. Since complex 2 is only four-coordinated, we suspect that nucleophiles, such as carbonyl compounds, are capable of
Scheme 1
654 Organometallics, Vol. 28, No. 2, 2009
Notes Table 2. Thia-Michael Additions of Thiophenols to r,β-Enones
Figure 4. UV/vis spectra of L′NiCl2 (2) and [L′NiCl(µ-Cl)]2 (4) in acetonitrile and the solid state. Inset: the intensity of the absorption band of 4 is double of that of 2 at the same concentration, illustrating that 4 was dissociated into 2 equiv of 2 in acetonitrile. Scheme 2
coordinating to the nickel center of 2 in solution. Bearing this idea in mind, we extended our investigation toward the conjugate addition of thiophenols to R,β-enones catalyzed by complex 2 with an incomplete coordination sphere.9 2-Cyclohexenone (5; 2 equiv) was first selected as a test Michael acceptor, and 4-methoxythiophenol (9) was employed as a test Michael donor in the presence of catalyst 2. An almost quantitative amount of 3-(4-methoxyphenylsulfanyl)cyclohexanone was produced in 2 h by using 2 mol % of 2 in acetonitrile (0.2 M of 9) at room temperature (entry 1 in Table 2). Under the same reaction conditions, NiCl2 · THF and complex 3 were employed as control catalysts for comparison to evaluate the catalytic ability of complex 2. The product yields catalyzed by NiCl2 · THF and by 3 are 65% and 75%, respectively. Apparently, complex 2 behaves as a more effective catalyst for the conjugate addition of thiolphenols to R,β-enones. With this preliminary success, thia-Michael additions to other Michael acceptor classes such as but-2-enoylbenzene (6), 5-methyl-3-hexenone (7), and N-but2-enoyl-1,3,2-oxazolidinone (8) by three 4-methoxythiophenol derivatives, 4-tert-butylthiophenol (10), 2-naphthylthiol (11), and 4-chlorothiophenol (12), were further explored. As the nucleophilicity of thiophenols is decreased by installing an electronwithdrawing substituent at the para position of thiophenols, the (7) Selected bond angles (deg) of 4: N1-Ni1-N5 ) 105.72, N1-Ni1Cl2 ) 97.06, N5-Ni1-Cl2 ) 91.46, N1-Ni1-Cl1 ) 102.40, N5-Ni1Cl1 ) 150.90, Cl2-Ni1-Cl1 ) 92.42, N1-Ni1-Cl1A ) 96.45, N5Ni1-Cl1A ) 87.25, Cl2-Ni1-Cl1A ) 166.27, Cl1-Ni1-Cl1A ) 82.26. (8) Shimazaki, Y.; Huth, S.; Karasawa, S.; Hirota, S.; Naruta, Y.; Yamauchi, O. Inorg. Chem. 2004, 43, 7816–7822. (9) For conjugate additions of week nucleophiles to R,β-enones or enoates catalyzed by vanadyl triflate, see: Lin, Y.-D.; Kao, J.-Q.; Chen, C.-T. Org. Lett. 2007, 9, 5195.
a Isolated yield. b Reaction time: 12 h. c The yield of entry 15 was determined by 1H NMR spectroscopy. d The yield for entry 1 catalyzed by NiCl2 · THF is 65%, and that by complex 3 is 75%.
yields for the conjugate additions are decreased (entries 4, 8, 12, and 16 in Table 2). In addition, if the β-position of the alkene moiety in the enones is sterically more hindered, the yields of the conjugate additions are also decreased (entries 9-12 in Table 2). As represented in the thia-Michael additions to substrate 7, the reaction rates drop by 2-3 times. To the best of our knowledge, this is the first successful example of conjugate additions to a sterically hindered R,β-unsaturated carbonyl compound by thiophenols catalyzed by nickel complexes. Another less reactive Michael acceptor class such as N-but-2enoyl-1,3,2-oxazolidinone (8) was also successfully converted to the corresponding products 3-(4-methoxyphenylsulfanyl)-2oxazolidinone and 3-(4-tert-butylphenylsulfanyl)-2-oxazolidinone in about 50% yield with prolonged reaction time (12 h, entries 13 and 14 in Table 2). In conclusion, we have revealed that the nuclearity of bis(benzimidazoyl) nickel complexes was governed not only
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Organometallics, Vol. 28, No. 2, 2009 655
Table 3. Selected X-ray Crystallographic Data for Complexes 3 and 4 empirical formula fw T (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (g cm-3) µ (mm-1) cryst size (mm) goodness of fit R1 wR2
3
4
C20H27Cl2N5NiO2 499.08 200(2) 0.71073 monoclinic P2/m 13.5208(19) 8.1518(10) 26.366(4) 90 104.592(3) 90 2812.3(6) 4 1.179 0.901 0.36 × 0.17 × 0.04 1.028 0.0978 0.2603
C60H74Cl12N10Ni2O6S2 1638.23 100(2) 0.71073 triclinic P1j 12.0810(2) 12.3990(2) 13.7750(3) 115.2590(10) 92.7920(13) 106.5790(13) 1753.60(6) 1 1.551 1.110 0.40 × 0.26 × 0.12 1.182 0.0608 0.1641
Table 4. UV/vis Data of Complexes 1-4 complex
UV/vis λmax/nm (ε/M-1 cm-1)
1
394 (15), 660 (9), 744 (8, sh), 952 (15) (in MeOH) 412, 674, 816, 1138 (solid) 551 (sh, 99), 587 (118), 997 (51) (in CH3CN) 556, 624, 828, 1018 (solid) 394 (16), 660 (10), 740 (9, sh), 952 (16) (in MeOH) 409, 675, 817, 1147 (solid) 551 (sh, 98), 587 (115), 997 (50) (in CH3CN) 455, 522, 800 (solid)
2 3 4
by the employed ligand but also by the crystallization temperature. Among NiCl2 · THF and the four bis(benzimidazoyl) nickel complexes, the coordinatively unsaturated complex 2, L′NiCl2, possessed effective catalytic ability toward the conjugate additions of thiophenols to R,β-enones, even for 5-methyl3-hexenone (a hindered Michael acceptor) and N-but-2-enoyl1,3,2-oxazolidinone (a sluggish Michael acceptor).
Experimental Section Methods and Materials. All manipulations were performed under nitrogen using standard Schlenk and glovebox techniques. The synthesis of complexes 1 and 2 was described previously.4a Methanol was dried in magnesium/iodine prior to use. Acetonitrile was distilled once from P2O5 and freshly distilled before use from CaH2. Diethyl ether was dried in a sodium benzophenone still prior to use. 2-Cyclohexenone (5), 5-methyl-3-hexenone (7), 4-methoxythiophenol (9), 4-tert-butylthiophenol (10), 2-naphthalenethiol (11), and 4-chlorothiophenol (12) are commercially available and were used as received. But-2-enoylbenzene (6) was purchased and purified by column chromatography on silica gel (hexane/AcOEt, 9/1) before use. N-But-2-enoyl-1,3,2-oxazolidinone (8) was prepared according to the reported procedure from crotonyl chloride and 2-oxazolidinone.10 Electronic spectra were recorded on Hitachi (10) (a) Evans, D. A.; Chapman, K. T.; Bisaha, J. J. Am. Chem. Soc. 1988, 110, 1238–1256. (b) Evans, D. A.; Morrissey, M. M.; Dorow, R. L. J. Am. Chem. Soc. 1985, 107, 4346–4348.
U-3501 and Hitachi U-4100 spectrophotometers equipped with an integrating sphere for reflectance measurements. IR spectra were recorded as Nujol mulls on a Perkin-Elmer Paragon 500 spectrometer. Elemental analyses of C, H, and N were performed on a PerkinElmer 2400 analyzer at the NSC Regional Instrumental Center at National Taiwan University, Taipei, Taiwan. X-ray Structure Determination. Crystals of suitable size were selected under a microscope and mounted on the tip of a glass fiber, which was positioned on a copper pin. The X-ray data for complexes 3 and 4 were collected on a Bruker-Nonius Kappa CCD diffractometer employing graphite-monochromated Mo KR radiation at 200 K and the θ-2θ scan mode. The space groups for all complexes were determined on the basis of systematic absences and intensity statistics, and the structures of 3 and 4 were solved by direct methods using SIR92 or SIR97 and refined with SHELXL97. An empirical absorption correction by multiscans was applied to all structures. All non-hydrogen atoms were refined with anisotropic displacement factors. Hydrogen atoms were placed in ideal positions and fixed with relative isotropic displacement parameters. Selected crystallographic data for 3 and 4 are given in Table 3. Detailed crystallographic information for complexes 3 and 4 is provided in the Supporting Information (CIF). [LNi(CH3OH)2Cl]Cl · 2H2O (3). Complex 3 was prepared by the same manner reported previously for complex 14a but was recrystallized at -20 °C by the slow diffusion of diethyl ether into a methanol solution of 2 to form light green crystals (yield 0.342 g, 64%). Anal. Calcd for NiC20H31N5Cl2O4: C, 44.89; H, 5.84; N, 13.09. Found: C, 44.48; H, 5.16; N, 13.38. IR (Nujol): 3406 cm-1 (νOH), 3227 cm-1 (νNH). UV/vis data are provided in Table 4. [L′Ni(µ-Cl)Cl]2 (4). Complex 4 was synthesized by the same reaction procedures reported previously for complex 2,4a and the isolated blue solution was kept at -20 °C for crystallization. Red crystals were obtained in a yield of 80.3% (0.522 g) from the acetonitrile solution of the purified product. Anal. Calcd for Ni2C56H66N10Cl4S2O6: C, 51.80; H, 5.12; N, 10.79. Found: C, 52.35; H, 4.72; N, 11.00. IR (Nujol): 1743 cm-1 (νCdO). UV/vis data are provided in Table 4. General Procedure for the Conjugate Addition Catalyzed by Complex 2. To a blue acetonitrile solution (4 mL) of complex 2 (16.4 mg, 0.02 mmol) was added an R,β-enone (2.0 mmol) under a nitrogen atmosphere with stirring for 5 min. As a thiol (1.0 mmol) was added to the reaction mixture, the color of the solution became brick red. The resulting solution was stirred at room temperature for 2 h, and the solvent was evaporated under vacuum. The residue was then purified by column chromatography on silica gel (hexane/ AcOEt 9/1) to give the conjugate adduct. Characterization of the conjugate addition products are listed in the Supporting Information.
Acknowledgment. We gratefully acknowledge financial support from the National Science Council of Taiwan (NSC Grant No. 97-2113-M-003-004 to W.-Z.L. and NSC Grant No. 97-2667-M-003-001 to C.-T.C.). Supporting Information Available: CIF files giving X-ray crystallographic data for complexes 3 and 4 text and figures giving characterization data and 1H and 13C NMR spectra of the conjugate addition products. This material is available free of charge via the Internet at http://pubs.acs.org. OM8009427
