Communication pubs.acs.org/Organometallics
Stoichiometric and Catalytic Reactions of Thermally Stable Nickel(0) NHC Complexes Jianguo Wu, John W. Faller, Nilay Hazari,* and Timothy J. Schmeier Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States S Supporting Information *
ABSTRACT: Although there are many organic reactions that are catalyzed by either Ni0 or Pd0 complexes, in comparison with the case for Pd0 there has been significantly less work studying coordinatively unsaturated Ni0 complexes. Here, we develop a simple synthetic route for preparing a number of thermally stable NHC-supported Ni0 hexadiene complexes in good yield. We examine the stoichiometric reactivity of one of these species and demonstrate that the coordinated hexadiene moiety is labile and can be replaced with a variety of different ligands, including CO, phosphines, isonitriles, and olefins. In addition, we show that the Ni0 hexadiene complexes are relatively rare examples of homogeneous first-row transition-metal catalysts for the hydrogenation of olefins.
T
Scheme 1
here are numerous organic reactions which are catalyzed by either zerovalent Ni or Pd complexes.1 However, although Ni0 is more accessible than Pd0, there have been many more investigations into the properties of coordinatively unsaturated Pd0 species.2 One of the major reasons for this difference is the relative paucity of suitable Ni0 precursors. The most frequently used Ni0 source for both stoichiometric and catalytic reactions is almost certainly commercially available Ni(COD)2, which is relatively difficult to prepare3 and is both air and thermally unstable. Although Ni(COD)2 is an effective Ni0 source, it would be useful to have more potential Ni0 synthons. Pörschke and co-workers have demonstrated that direct reduction of NiII complexes in the presence of a ligand of interest can generate Ni0 complexes, but the resulting Ni0 complexes have rarely been used as synthons.4 Similarly, precursors of the type Ni(t,t,t-cdt) (t,t,t-cdt = trans,trans,trans-1,5, 9-cyclododecatriene),3a Ni(c,c,c-cdt) (c,c,c-cdt = cis,cis,cis-1,5, 9-cyclododecatriene),5 and Ni(C2H4)36 are not commercially available and are difficult to handle, which limits their utility. In contrast, complexes of the type Ni(NHC)2 have been described and are quite stable, but ligand substitution in these species is difficult, which also limits their use as Ni0 precursors.7 In Pd0 chemistry, coordinated chelating linear dienes are easy to substitute and Pörschke and co-workers have prepared related Ni0 complexes of the type Ni2(1,6-diene)3,8 as well as Ni0 phosphine complexes supported by nonchelating olefin9 and alkyne ligands.10 Although these complexes show promise as precursors for a range of Ni0 complexes, they are not easy to access.8,11 Recently, we described the unusually stable IPrsupported Ni0 1,5-hexadiene complex 1, as a decomposition product of thermally unstable (η1-allyl)(η3-allyl)Ni(IPr).12 Previously, related phosphine and bis(imine) supported complexes were prepared following a similar route (Scheme 1), but these are highly unstable at room temperature and are unsuitable as Ni0 synthons.9a,13 Here, we report a new and straightforward route to prepare a variety of NHC-supported Ni0 hexadiene complexes, which does not involve any thermally © 2012 American Chemical Society
unstable intermediates. We describe the stoichiometric reactivity of one of these thermally stable Ni0 species and show that the hexadiene ligand can be easily replaced, so that our new complexes can act as synthons for a variety of other Ni0 complexes containing a single NHC ligand. Furthermore, we demonstrate that the Ni0 hexadiene complexes are rare examples of homogeneous Ni catalysts for olefin hydrogenation. Our new synthesis of NHC-supported Ni0 hexadiene complexes (1−4) starts from commercially available anhydrous NiCl2 (eq 1). Reaction of NiCl2 with the appropriate NHC in Received: January 19, 2012 Published: January 27, 2012 806
dx.doi.org/10.1021/om300045t | Organometallics 2012, 31, 806−809
Organometallics
Communication
labile and can be replaced with a number of different ligands, including phosphines, CO, isonitriles, and olefins (Scheme 2).
THF, followed by treatment with allyl Grignard, generates compounds 1−4 in yields greater than 65% in all cases. Presumably, the reaction proceeds through an intermediate of the type (η1-allyl)(η3-allyl)Ni(NHC). Importantly, our synthesis does not involve the isolation of any thermally unstable intermediates and requires only two steps, which can be performed in a single pot. The new complexes 2−4 were fully characterized and are stable for more than 24 h in refluxing toluene. Using our synthetic method, it was also possible to prepare the PiPr3-supported Ni complex (hexadiene)Ni(PiPr3) (5). In contrast to the case for other phosphine-supported species,9a,13 5 could be heated to 60 °C before decomposition occurred. X-ray-quality crystals of 2 and 3 were grown from saturated solutions of pentane at −35 °C (Figure 1). In the case of
Scheme 2
Reaction of 1 with CO gives the coordinatively saturated Ni0 complex Ni(CO)3(IPr) (6). In contrast, reaction of 1 with t BuNC or 1 equiv of the bidentate phosphine dppe (dppe = 1,2-bis(diphenylphosphino)ethane) gives the three-coordinate Ni0 complexes Ni(tBuNC)2(IPr) (7) and Ni(dppe)(IPr) (8), respectively, although surprisingly no reaction is observed between 1 and 2,2′-bipyridine. When 1 is treated with 1 equiv of more electron rich bidentate phosphines, such as dmpe (dmpe = 1,2-bis(dimethylphosphino)ethane) and dppp (dppp = 1,2-bis(diphenylphosphino)propane), both the hexadiene and IPr ligands are substituted and the Ni0 complexes Ni(dmpe)2 and Ni(dppp)2 are formed, along with 0.5 equiv of starting material. Addition of excess dmpe or dppe results only in the formation of Ni(dmpe)2 or Ni(dppp)2. The monodentate phosphine PEt3 gives similar reactivity, and only Ni(PEt3)4 was isolated from reactions between 1 and excess PEt3. The hexadiene ligand can also be replaced with olefins. Reaction between 1 and 1 atm of ethylene or 2 equiv of electronpoor dimethyl fumarate gives the unusual three-coordinate bis(olefin) species 9 or 10, respectively. In the case of the reaction with ethylene, there is no evidence to suggest that the tris(ethylene) complex Ni(C2H4)3 was formed.6 When 1 is treated with the chelating olefin allyl ether or dimethylvinyl silyl ether (dvse), the coordinatively unsaturated Ni species 11 or 12 is formed. Compounds 11 and 12 were characterized by X-ray crystallography (Figure 2). Although crystallographically characterized Pd0 species with allyl ether ligands are known,15 to the best of our knowledge 11 is the first crystallographically characterized example of a Ni0 species with an allyl ether ligand and 12 is a rare example with a dvse ligand.16 The overall geometries of 11 and 12 are similar to those described for 2 and 3. The Ni−C bond lengths in 11 and 12 are slightly shorter than those in 2 and 3, consistent with allyl ether and dvse being better ligands than 1,5-hexadiene. All of the above substitution reactions demonstrate that 1 can be used as an easily accessible precursor for the preparation of a range of different Ni0 complexes, including several species
Figure 1. (a) X-ray structure of 2 (hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Ni1−C1 = 2.051(5), Ni1− C2 = 2.071(9), Ni1−C5 = 2.065(9), Ni1−C6 = 2.039(5), Ni1−C7 = 1.938(3), C1−C2 = 1.472(11), C2−C3 = 1.479(14), C3−C4 = 1.527(15), C4−C5 = 1.412(14), C5−C6 = 1.357(9); C1−Ni1−C2 = 41.8(3), C1−Ni1−C5 = 116.9(3), C1−Ni1−C6 = 155.33(19), C1− Ni1−C7 = 106.78(18). (b) X-ray structure of 3 (hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (deg) for one independent molecule in the unit cell: Ni1−C1 = 2.010(4), Ni1−C2 = 1.998(4), Ni1−C5 = 2.003(4), Ni1−C6 = 2.027(4), Ni1−C7 = 1.912(4), C1−C2 = 1.399(5), C2−C3 = 1.518(5), C3−C4 = 1.528(5), C4−C5 = 1.493(6), C5−C6 = 1.393(5); C1−Ni1−C2 = 40.84(15), C1−Ni1−C5 = 117.69(15), C1−Ni1−C6 = 155.64(15), C1−Ni1− C7 = 99.03(14).
compound 2, there is significant disorder in the hexadiene ligand, due to up and down conformations of the methylene groups, relative to the Ni coordination plane. Nevertheless, it is clear that complexes 2 and 3 have similar geometries, and the solid-state structures confirm a pseudo-three-coordinate trigonal-planar geometry around Ni, with moderate back-donation from the coordinated olefins to the metal center.12 The new structures are analogous to the previously reported structure of 1 and represent rare examples of structurally characterized transition-metal complexes containing 1,5-hexadiene ligands.12 The imidazole rings of the NHC ligands are perpendicular to the metal coordination plane; therefore, the angles between the planes are 62.1° in 2 and 57.9° in 3. The Ni−NHC bond lengths are significantly longer (Ni1−C7 = 1.938(3) Å in 2 and Ni1−C7 = 1.912(4) in 3) than those reported by Arduengo and co-workers7a in a complex of the type Ni(NHC)2 and comparable with those reported by Hillhouse et al. in a two-coordinate Ni(II) complex containing a single NHC ligand.14 In order to probe the reactivity of the hexadiene complexes, compound 1 was used as a model. The hexadiene ligand is 807
dx.doi.org/10.1021/om300045t | Organometallics 2012, 31, 806−809
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Ni benzene adduct, similar to that observed by Pörschke and co-workers.9c It also indicated that it may be possible to use 1 as a catalyst for olefin hydrogenation. We screened complexes 1−4 as catalysts for the hydrogenation of cyclopentene, along with the related PiPr3 (5)- and PPh3-supported Ni hexadiene complexes (Table 1). Although Table 1. Catalyst Screening for the Hydrogenation of Cyclopentene Figure 2. (a) X-ray structure of 11, one of three independent structures in the unit cell (hydrogen atoms and isopropyl groups of the IPr ligand omitted for clarity). Selected bond lengths (Å) and angles (deg) for one independent molecule in the unit cell: Ni1−C1 = 1.964(10), Ni1−C2 = 1.989(10), Ni1−C5 = 1.982(9), Ni1− C6 = 1.962(10), Ni1−C19 = 1.910(8), C1−C2 = 1.418(13), C2− C3 = 1.464(11), C4−C5 = 1.487(12), C5−C6 = 1.413(13); C1− Ni1−C2 = 42.0(4), C1−Ni1−C5 = 125.0(4), C1−Ni1−C6 = 166.4(4), C1−Ni1−C19 = 94.0(4). (b) X-ray structure of 12 (hydrogen atoms and isopropyl groups of IPr ligand omitted for clarity). Selected bond lengths (Å) and angles (deg) for one independent molecule in the unit cell: Ni1−C1 = 1.987(4), Ni1−C2 = 2.013(3), Ni1−C3 = 2.009(3), Ni1−C4 = 1.989(4), Ni1−C9 = 1.906(3), C1−C2 = 1.392(5), C2−Si1 = 1.833(3), Si1−O1 = 1.636(3), O1−Si2 = 1.635(2), Si2−C3 = 1.838(4), C3−C4 = 1.390(5); C1−Ni1−C2 = 40.72(13), C1−Ni1−C3 = 132.04(13), C1−Ni1−C4 = 172.68(14), C1−Ni1−C9 = 90.42(12).
cat.
time (h)
conversn (%)a
(hexadiene)Ni(IPr) (1) (hexadiene)Ni(SIPr) (2) (hexadiene)Ni(IMes) (3) (hexadiene)Ni(SIPrMes) (4) (hexadiene)Ni(PiPr3) (5) (hexadiene)Ni(PPh3) (C2H4)2Ni(IPr) (9) Ni(COD)2b
23 23 23 23 23 23 23 23
62 98 95 76 54 0 35 41
a Average of two runs measured by 1H NMR spectroscopy using an internal standard. bA heterogeneous catalyst is formed when Ni(COD)2 is used as the catalyst. The conditions for catalytic reactions were cyclopentene (8.0 mg, 0.118 mmol), catalyst (0.012 mmol), ferrocene (0.059 mmol, internal standard), and 1 atm H2 in 0.4 mL of C6D6 at 50 °C.
which are coordinatively unsaturated. Prior to this work some of these coordinatively unsaturated species could only be prepared using metal-vapor synthesis, and most required the generation of thermally unstable intermediates, which makes our route a significant improvement.16a Furthermore, compound 1 also undergoes facile oxidative addition with allyl chloride or 2-methylallyl chloride to form compounds 13 and 14, respectively. This suggests that it may be possible to utilize hexadiene-supported species as precatalysts for reactions which utilize Ni0/NiII catalytic cycles. Interestingly, reaction of 1 with propyne did not give the expected bis(propyne) complex. Instead, the propyne was selectively converted into 1,2,4-trimethylbenzene at room temperature. In fact, 1 could catalytically perform this transformation (eq 2). Although it has been previously demonstrated that Ni0
the activity was low, and a high catalyst loading was required, 1−5 were able to hydrogenate cyclopentene using only 1 atm of H2. The PPh3-supported complex was completely inactive, presumably due to its thermal instability, while the bis(ethylene) complex 9, which is relatively thermally stable, gave limited activity. Although it is difficult to rigorously exclude the possibility that Ni nanoparticles are responsible for catalysis,18 no decrease in catalytic activity was observed when the reaction was run in the presence of elemental Hg. In addition, treatment of the reaction mixture at the end of a catalytic run with ethylene resulted in the clean formation of 9 (>95% yield based on starting catalyst), suggesting that a significant amount of Ni was still present in solution. After filtration of the reaction mixture, the solution containing 9 could then be used for an additional catalytic reaction. In contrast to the apparent homogeneous hydrogenation performed by 1−5, when Ni(COD)2 was used as a catalyst, under the same reaction conditions, a black precipitate immediately formed. This black precipitate was active for the hydrogenation of cyclopentene, but no catalytic activity was obtained when the solution was filtered, suggesting that Ni(COD)2 forms a heterogeneous catalyst. The SIPr-supported species 2 was the most active catalyst for hydrogenation and was utilized to investigate the substrate scope (Table 2). Compound 2 can hydrogenate a variety of cyclic olefins, although the catalyst is very slow for substrates other than highly activated cyclopentene. Interestingly, the linear olefin 1-octene can be hydrogenated, with no evidence for isomerization products, which are common products from the reaction of Ni species with alkenes.19 The disubstituted alkenes isobutene and methylstyrene can also be hydrogenated after extended reaction times. Homogeneous Ni catalysts for hydrogenation are relatively rare, and there have only been a small number of previous reports.20 In particular, Bouwman
catalysts can cyclotrimerize alkynes to form benzenes, this is a rare example which starts from a well-defined Ni0 starting material.17 Notably, no catalytic activity was observed when Ni(COD)2 was used as the catalyst. Treatment of 1 with H2 resulted in the elimination of hexane and the formation of an unidentified Ni complex, which could not be isolated (eq 3). Reaction of the unidentified Ni complex
with ethylene resulted in the formation of 9, which suggests that the unidentified Ni complex may be an unstable NHC 808
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(4) Bach, I.; Pörschke, K.-R.; Goddard, R.; Kopiske, C.; Krueger, C.; Rufinska, A.; Seevogel, K. Organometallics 1996, 15, 4959. (5) Jonas, K.; Heimbach, P.; Wilke, G. Angew. Chem., Int. Ed. 1968, 7, 949. (6) Fischer, K.; Jonas, K.; Wilke, G. Angew. Chem., Int. Ed. 1973, 12, 565. (7) (a) Arduengo, A. J. III; Gamper, S. F.; Calabrese, J. C.; Davidson, F. J. Am. Chem. Soc. 1994, 116, 4391. (b) Sato, Y.; Sawaki, R.; Mori, M. Organometallics 2001, 20, 5510. (c) Schaub, T.; Backes, M.; Radius, U. Organometallics 2006, 25, 4196. (d) Schaub, T.; Backes, M.; Radius, U. J. Am. Chem. Soc. 2006, 128, 15964. (e) Matsubara, K.; Miyazaki, S.; Koga, Y.; Nibu, Y.; Hashimura, T.; Matsumoto, T. Organometallics 2008, 27, 6020. (f) Böhm, V. P. W; Gstöttmayr, C. W. K.; Weskamp, T.; Herrmann, W. A. Angew. Chem., Int. Ed. 2001, 40, 3387. (8) Proft, B.; Pörschke, K.-R.; Lutz, F.; Krueger, C. Chem. Ber. 1991, 124, 2667. (9) (a) Bonrath, W.; Pörschke, K.-R.; Michaelis, S. Angew. Chem., Int. Ed. 1990, 29, 298. (b) Kaschube, W.; Schröder, W.; Pörschke, K.-R.; Angermund, K.; Krüger, C. J. Organomet. Chem. 1990, 389, 399. (c) Nickel, T.; Goddard, R.; Krüger, C.; Pörschke, K.-R. Angew. Chem., Int. Ed. 1994, 33, 879. (10) (a) Rosenthal, U.; Pulst, S.; Kempe, R.; Pörschke, K.-R. Tetrahedron 1998, 54, 1277. (b) Proft, B.; Pörschke, K.-R.; Lutz, F.; Krueger, C. Chem. Ber. 1994, 127, 653. (c) Pörschke, K.-R. J. Am. Chem. Soc. 1989, 111, 5691. (11) (a) Haack, K.-J.; Goddard, R.; Pörschke, K.-R. J. Am. Chem. Soc 1997, 119, 7992. (b) Pörschke, K.-R.; Pluta, C.; Proft, B.; Lutz, F.; Krueger, C. K. Z. Naturforsch., B: J. Chem. Sci. 1993, 48, 608. (12) Wu, J.; Hazari, N.; Incarvito, C. D. Organometallics 2011, 30, 3142. (13) Henc, B.; Jolly, P. W.; Salz, R.; Stobbe, S.; Wilke, G.; Benn, R.; Mynott, R.; Seevogel, K.; Goddard, R.; Krüger, C. J. Organomet. Chem. 1980, 191, 449. (14) Laskowski, C. A.; Miller, A. J. M.; Hillhouse, G. L.; Cundari, T. R. J. Am. Chem. Soc. 2011, 133, 771. (15) Krause, J.; Haack, K.-J.; Cestaric, G.; Goddard, R.; Pörschke, K.-R. Chem. Commun. 1998, 1291. (16) (a) Cloke, F. G. N.; Hitchcock, P. B.; Lappert, M. F.; MacBeath, C.; Mepsted, G. O. J. Chem. Soc., Chem. Commun. 1995, 87. (b) Maciejewski, H.; Sydor, A.; Kubicki, M. J. Organomet. Chem. 2004, 689, 3075. (17) Saito, S.; Yamamoto, Y. Chem. Rev. 2000, 100, 2901. (18) (a) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A 2003, 198, 317. (b) Crabtree, R. H. Chem. Rev. 2012, 112, DOI: 10.1021/cr2002905. (19) Lim, H. J.; Smith, C. R.; RajanBabu, T. V. J. Org. Chem. 2009, 74, 4565. (20) (a) Hamada, Y.; Koseki, Y.; Fujii, T.; Maeda, T.; Hibino, T.; Makino, K. Chem. Commun. 2008, 6206. (b) Hibino, T.; Makino, K.; Sugiyama, T.; Hamada, Y. ChemCatChem 2009, 1, 237. (c) Angulo, I. M.; Kluwer, A. M.; Bouwman, E. Chem. Commun. 1998, 2689. (d) Angulo, I. M.; Lok, S. M.; Quiroga Norambuena, V. F.; Lutz, M.; Spek, A. L.; Bouwman, E. J. Mol. Catal. A: Chem. 2002, 187, 55. (e) Angulo, I. M.; Bouwman, E.; van Gorkum, R.; Lok, S. M.; Lutz, M.; Spek, A. L. J. Mol. Catal. A: Chem. 2003, 202, 97. (21) Enthaler, S.; Junge, K.; Beller, M. Angew. Chem., Int. Ed. 2008, 47, 3317. (22) (a) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc 2004, 126, 13794. (b) Daida, E. J.; Peters, J. C. Inorg. Chem. 2004, 43, 7474.
Table 2. Substrate Scope for Hydrogenation Using Precatalyst 2 substrate
time (h)
conversn (%)a
cyclooctene cyclohexene cyclopentene isobutene methylstyrene norbornylene 1-octene
92 68 23 64 57 27 44
88 93 98 58 94 93 98
a
Average of two runs measured by 1H NMR spectroscopy using an internal standard. Reaction conditions: substrate (0.118 mmol), catalyst 2 (6.2 mg, 0.012 mmol), ferrocene (0.059 mmol, internal standard), and 1 atm H2 in 0.4 mL of C6D6 at 50 °C.
and co-workers have reported that NiII complexes supported by phosphine ligands can hydrogenate 1-octene.20d,e Although Bouwman’s catalysts give higher turnover numbers than our systems, catalysis was performed using 49 atm of H2, which makes direct comparison of catalytic activity difficult. However, it should be noted that more efficient first row transition metal hydrogenation catalysts using Fe are known.21,22 In conclusion, we have developed a straightforward synthesis of a series of thermally stable Ni0 hexadiene complexes supported by NHC ligands. We have demonstrated that one of these complexes, (hexadiene)Ni(IPr) (1), can be used as a synthon to prepare a variety of coordinatively unsaturated Ni0 species supported by phosphines, isonitriles, and olefins and undergoes oxidative addition to form well-defined Ni II products. Furthermore, our Ni0 complexes are active catalysts for the cyclotrimerization of alkynes and the hydrogenation of olefins.
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ASSOCIATED CONTENT
S Supporting Information *
Text, tables, figures, and CIF files giving experimental details, characterization data, and X-ray crystallographic information. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Chris Incarvito for assistance with X-ray crystallography. REFERENCES
(1) (a) Tsuji, J. Perspectives in Organopalladium Chemistry for the 21st Century; Elsevier: Amsterdam, 1999. (b) Tsuji, J. Palladium Reagents and Catalysis: Innovations in Organic Synthesis; Wiley: Chichester, U.K., 1995. (c) Modern Organonickel Chemistry; Tamaru, Y., Ed.; Wiley-VCH: Weinheim, Germany, 2005. (2) See for example: Krause, J.; Cestaric, G.; Haack, K.-J.; Seevogel, K.; Storm, W.; Pörschke, K.-R. J. Am. Chem. Soc. 1999, 121, 9807. Hartwig, J. F. Angew. Chem., Int. Ed. 1998, 37, 2046 and references therein. (3) (a) Wilke, G. Angew. Chem., Int. Ed. 1963, 2, 105. (b) Wender, P. A.; Smith, T. E.; Zuo, G.; Duong, H. A.; Louie, J. Bis(1,5-cyclooctadiene) nickel(0). In e-EROS Encyclopedia of Reagents for Organic Synthesis; Wiley: New York, 2001. 809
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