Communication pubs.acs.org/Organometallics
Intramolecular C(sp3)−H Bond Activation Reactions of Low-Valent Cobalt Complexes with Coordination Unsaturation Zhenbo Mo,† Dake Chen,‡ Xuebin Leng,† and Liang Deng*,† †
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai, People's Republic of China 200032 ‡ College of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai, People's Republic of China 200237 S Supporting Information *
ABSTRACT: Intramolecular C(sp3)−H bond activation reactions mediated by low-valent cobalt, both Co(I) and Co(0), have been observed in the reactions of the three-coordinate cobalt complex [Co(IMes)2Cl] (IMes = 1,3-dimesitylimidazol-2-ylidene) with alkylation reagents and sodium amalgam. The reactions with alkylation reagents gave [Co(IMes)(IMes′)(N2)], featuring a metalated IMes′ anion, whereas the oneelectron-reduction reaction afforded [Co(IMes′)2]. The Co(II) complex can react with CO, isocyanide, and a diazo compound to furnish interesting cobalt complexes bearing functionalized N-heterocyclic carbene ligands. The establishment of these conversions demonstrates the capability of low-valent cobalt with coordination unsaturation to mediate C(sp3)−H bond activation and functionalization. coordination unsaturation to mediate C(sp 3)−H bond activation. Furthermore, our studies have also revealed that the C(sp3)−H bond activation product [Co(IMes′)2] can react with carbon monoxide, isocyanide, and a diazo compound to furnish functionalized NHC ligands, by which cobalt-mediated net C(sp3)−H bond functionalization reactions have been achieved. The three-coordinate compound [Co(IMes)2Cl] (1) was obtained in good yield from the reaction of [Co(PPh3)3Cl] with 2 equiv of IMes in THF.10 Similar to a handful of reported three-coordinate Co(I) complexes,11 1 is paramagnetic and has a solution magnetic moment of 4.4(1) μB.12 X-ray diffraction studies (Figure 1) have revealed a distorted-trigonal-planar geometry for its cobalt center, having nearly identical C(1)− Co(1)−Cl(1) and C(2)−Co(1)−Cl(1) angles of 114.9(2) and 115.5(2)°, respectively. Its Co−C(carbene) bond distances (1.953(5) and 1.955(5) Å) are longer than those of the lowspin Co(I) species [CpCo(NHC)(CO)] (NHC = IPr2, 1.90 Å; NHC = IPr, 1.89 Å)13 and [Co(IEt2Me2)4]1+ (1.93 Å)14 but shorter than those in the high-spin compound [(TIMEN)Co(CO)]Cl (2.04 Å).15 No short contact between the cobalt center and the benzylic C(sp3)−H bonds was noticed. The isolation of 1 implies the distinct reactivity of Co(I) in comparison to that of its heavy group 9 congeners. Nolan, Aldridge, and Sola studied the reactions of some Rh(I) and Ir(I) compounds with IMes, which generally yield intramolecular C(sp3)−H activation products.16 In contrast, under an N2 atmosphere, 1 was found to be quite stable in THF-d8, as heating the solution at 70 °C overnight did not lead to a
C(sp3)−H bond activation by low-valent cobalt is a challenging subject of great interest in modern organometallic chemistry.1 Efforts in this area can be traced back to the gas-phase studies of simple cobalt ions with alkanes in the 1980s.2 However, since then, little progress has been made until recently. In 2007, Brookhart et al. reported that the Co(I) complex Cp*Co(CH2CHTMS)2 can catalyze the conversion of vinylsilylprotected amines to enamines, which represents the first example of a Co(I)-catalyzed C(sp3)−H bond activation reaction. 3 Later, Bradley et al. found Cp*Co(μ-η 4 :η 4C6H5CH3)CoCp* can also facilitate similar transfer-hydrogenation reactions.4 In addition to these, Klein and co-workers’ studies on the methane-elimination reactions of the low-spin complex Co(PMe3)4Me with certain phosphines added several other rare examples to this chemistry.5 The relative scarcity of these known examples forms a sharp contrast to the plethora of Rh- and Ir-mediated C(sp3)−H bond activations6 and has spurred extensive theoretical investigations7 to disclose the cause for the lack of reactivity of cobalt,8 which, however, has not yet been fully understood.9 This status quo, in addition to growing interest in catalysis with nonprecious metals,1 creates an opportunity to explore the largely unknown C(sp3)−H bond activation chemistry of lowvalent cobalt complexes, especially those with coordination unsaturation. With regard to this, we report herein our observations of the intramolecular C(sp3)−H bond activation reactions of a three-coordinate cobalt complex, [Co(IMes)2Cl], which can undergo cyclometalation reactions when treated with alkylation reagents or be reduced with sodium amalgam to produce [Co(IMes)(IMes′)(N2)] or [Co(IMes′)2], respectively. The establishment of these conversions demonstrates the capability of low-valent cobalt, both Co(I) and Co(0), with © 2012 American Chemical Society
Received: August 20, 2012 Published: October 9, 2012 7040
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reagents, C(sp3)−H bond activation reaction could occur. Addition of 1 equiv of CH3Li to a solution of 1 in THF at −78 °C under an N2 atmosphere resulted in the formation of a deep green solution. The color of the solution persisted on warmed to room temperature, from which the cyclometalation product [Co(IMes)(IMes′)(N2)] (3) was isolated as deep brown crystals in 62% yield (Scheme 1).10 In addition to CH3Li, the reactions using TMSCH2Li and p-MePhMgBr could also afford 3 in comparable yields. In the case using p-MePhMgBr, the concomitant formation of toluene has been confirmed by control experiments.10 Complex 3 has been characterized by 1H and 13C NMR, UV−vis, elemental analysis, IR, and single-crystal X-ray diffraction studies. As shown in Figure 1, in accord with its diamagnetic nature the molecular structure of 3 displays a square-planar geometry, in which the cobalt center is bonded with one bidentate [IMes′]− anion, one intact IMes ligand, and one terminally bonded N2 molecule. The chelation of [IMes′]− to the cobalt center forms a six-membered metallacycle having Co−C(carbene) and Co−C(benzyl) bond distances of 1.905(2) and 1.991(2) Å. The Co−N2 moiety has a Co−N separation of 1.760(2) Å and a N−N distance of 1.092(3) Å. These bond lengths, in addition to the high νN−N stretching frequency (2006 cm−1) in the IR spectrum of 3, indicate the weak coordination of N2 to the Co(I) center.18 The formation of 3 is proposed to proceed by alkylation of 1 with CH3Li to afford Co(IMes)2(CH3) (A). This intermediate could undergo an intramolecular C(sp3)−H activation reaction, by either σ-bond metathesis or oxidative addition/reductive elimination mechanism,19 to eliminate methane and yield the cyclometalation product B. B might then coordinate with a dinitrogen molecule to give 3 (Scheme 2).11d Noting the
Figure 1. Molecular structures of [Co(IMes)2Cl] (1), [Co(IMes)2]+ in [Co(IMes)2][BPh4] (2), [Co(IMes)(IMes′)(N2)] (3), and [Co(IMes′)2] (4), showing 30% probability ellipsoids and the partial atom numbering scheme.
noticeable change in its 1H NMR spectrum. To examine whether intramolecular C(sp3)−H activation could take place on a two-coordinate Co(I) center, 1 was further treated with 1 equiv of Na[BPh4] in THF, which, however, resulted in the isolation of the two-coordinate complex [Co(IMes)2][BPh4] (2) in 70% yield (Scheme 1).10 Scheme 1. Synthesis and Reactivity of the Three-Coordinate Co(I) Complex
Scheme 2. Possible Routes for the Formations of 3 and 4
different spin states of 1 (triplet) and 3 (singlet), spin crossover from a triplet energy surface to a singlet surface should have happened during the reaction course.7b,c,20 Treatment of 1 with 1 equiv of sodium amalgam in THF, after 2 days, gave a red-brown solution, from which the Co(II) compound [Co(IMes′) 2] (4) was obtained in 60% yield as red crystals (Scheme 1).10 Complex 4 is paramagnetic and has a solution magnetic moment of 2.6(2) μB.21 X-ray diffraction studies disclosed a distorted-square-planar geometry for its cobalt center that is bonded with two bidentate [IMes′]− ligands with Co−C(carbene) and Co−C(benzyl) bond distances of 1.887(2) and 2.025(2) Å and a C(benzyl)−Co− C(benzyl) angle of 146.0(1)°. The deviation from planarity of the coordination plane of the cobalt center in 4 induces Co− C(benzyl) separations longer than those of the low-spin Co(II) alkyl complexes.22 A mechanism involving the sequential Co(0)- and Co(II)mediated C(sp3)−H bond activation steps shown in Scheme 2 is proposed for the formation of 4. Single-electron reduction of
Complex 2 has a solution magnetic moment of 4.1(1) μB, which again suggests the significant contribution of orbital angular momentum of the high-spin d8 cobalt center.12 In accord with its paramagnetic nature, the 1H NMR spectrum of 2 in THF-d8 features four sets of isotropically shifted signals between 86.11 and −22.20 ppm. Complex 2 represents the first structurally characterized two-coordinate Co(I) complex.17 As shown in Figure 1, the univalent cation [Co(IMes)2]1+ possesses an idealized D2h symmetry with a linear C(1)− Co(1)−C(2) angle (178.57(7)°). Both of the Co−C(carbene) bonds lengths are 1.937(2) Å, which lies between those of the Co−C(carbene) bonds in the high-spin complex 1 and the lowspin Co(I)−NHC complexes.13,14 Again, no noticeable short contact between the cobalt center and the flanking mesityl moieties was seen. While removal of the chloride in 1 cannot induce intramolecular C(sp3)−H bond activation reactions, we found that when 1 was reacted with organolithium or Grignard 7041
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step of α-metalation of the ketone moiety could proceed by oxidative addition of the α-C−H bond of the ketone moiety to the cobalt(0) center to give a Co(II) hydride, which might then convert to 5 upon coordination with CO. Relevant conversion of cobalt(II)−H to Co(I) by loss of H2 is known for [(nacnac)Co(μ-H)2Co(nacnac)].28 In conclusion, we have shown in this paper that the threecoordinate cobalt complex [Co(IMes)2Cl] provides an excellent platform for investigating the C(sp3)−H bond activation chemistry of low-valent cobalt with coordination unsaturation. This Co(I) complex can be converted to the corresponding cyclometalation products [Co(IMes)(IMes′)(N2)] and [Co(IMes′)2], on treatment with alkylation reagents or sodium amalgam. Further reactivity studies on the cyclometalation product [Co(IMes′)2] have led to the preparation of interesting cobalt complexes bearing functionalized carbene ligands. The establishment of these conversions demonstrates the potential usage of low-valent cobalt, both Co(I) and Co(0), with coordination unsaturation for C(sp3)− H bond activation and functionalization reactions.
1 with Na could give the two-coordinate Co(0) intermediate Co(IMes)2 (C). The Co(0) center in C might interact with one of its peripheral benzylic C(sp3)−H bonds, possibly via oxidative addition, to yield the Co(II) hydride HCo(IMes)(IMes′) (D). D could then eliminate one H2 molecule by a second C(sp3)−H activation step, probably via σ-bond metathesis, to furnish 4.23 This proposed mechanism is supported by the NMR experiments of two reactions, 1 with sodium amalgam and 4 with H2, in which the presence of two paramagnetic intermediates (presumably C and D) and the equilibriums among the two intermediates, 4, and H2 were observed (Figures s5 and s6, Supporting Information).10 Attempts to isolate these two intermediates were unsuccessful. Notably, while a precedent example of oxidative addition of C(sp3)−H bonds with Co(0) is lacking, we think the coordination unsaturation and lower oxidation state of the cobalt center in C and the proximity of benzylic C(sp3)−H bonds to the cobalt center, in addition to the spin-allowed transformation on the doublet energy surface (both C and D might have a doublet electronic configuration),7b,c are the key factors enabling the reaction to occur. After achieving these low-valent cobalt-mediated C(sp3)−H bond activation reactions, we then studied the migratory insertion reactivity of the cyclometalation products with the aim of exploring the feasibility of cobalt-mediated C(sp3)−H bond functionalization.24 Our preliminary study has shown that 4 can react with CO, 2,6-Me2-PhNC, and 2,6-Me2-PhOCOCHN2 to give the novel cobalt complexes 5−7, featuring functionalized NHC ligands (Scheme 3).10 Interestingly,
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ASSOCIATED CONTENT
* Supporting Information S
Text, a table, figures, and CIF files giving X-ray crystallographic data for complexes 1−7, experimental procedures, and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
Scheme 3. Reactions of 4 with CO, Isocyanide, and Diazo Compounds
*Tel: 86-21-54925460. Fax: 86-21-54925460. E-mail: deng@ sioc.ac.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for financial supports from the National Basic Research Program of China (973 Program, No. 2011CB808705) and the National Natural Science Foundation of China (Nos. 21002114, 20872168, and 2112106210).
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ABBREVIATIONS NHC, N-heterocyclic carbene; IMes, 1,3-dimesitylimidazol-2ylidene; Mes, 2,4,6-trimethylphenyl; IPr2, 2,5-diisopropylimidazol-1-ylidene; IEt2Me2, 2,5-diethyl-3,4-dimethylimidazol-1-ylidene; TMS, trimethylsilyl; Cp*, pentamethylcyclopentadienyl
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possibly due to steric congestion, no reaction took place when 6 and 7 were further treated with the isocyanide and the diazo compound. Complexes 5−7 have been characterized by NMR, IR, UV−vis, and elemental analyses, and their molecular structures have been established by X-ray diffraction studies (Figure s2, Supporting Information).10 Complexes 6 and 7 apparently come from the migratory insertion reactions of Co(II)−C(benzyl) with the corresponding unsaturated substrates,25,26 whereas the reaction course to 5 seems more complicated. As proposed in Scheme s1 (Supporting Information), although the detailed mechanism is not clear at this stage, the formation of the ketone moiety in 5 might entail the migratory insertion of CO into one Co(II)−C(benzyl) bond followed by reductive elimination, as in a recent report on the reactions of Co(II) terphenyl compounds with CO.27 The
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(b) Everett, G. W.; Holm, R. H. J. Am. Chem. Soc. 1965, 87, 5266− 5267. (22) For examples, please see: (a) Gibson, V. C.; Tellmann, K. P.; Humphries, M. J.; Wass, D. F. Chem. Commun. 2002, 2316−2317. (b) Kleigrewe, N.; Steffen, W.; Blömker, T.; Kehr, G.; Fröhlich, R.; Wibbeling, B.; Erker, G.; Wasilke, J.-C.; Wu, G.; Bazan, G. C. J. Am. Chem. Soc. 2005, 127, 13955−13968. (c) Bowman, A. C.; Milsmann, C.; Bill, E.; Lobkovsky, E.; Weyhermüller, T.; Wieghardt, K.; Chirik, P. J. Inorg. Chem. 2010, 49, 6110−6123. (23) Co(II)-hydride-mediated intramolecular C(sp3)−H bond activation might be similar to that of iron(II)-NHC-alkyl complexes: (a) Ohki, Y.; Hatanaka, T.; Tatsumi, K. J. Am. Chem. Soc. 2008, 130, 17174−17186. (b) Hatanaka, T.; Ohki, Y.; Tatsumi, K. Chem. Asian J. 2010, 5, 1657−1666. (24) Known cobalt-mediated C(sp3)−H bond functionalization reactions are restricted to non-organometallic type reactions of cobalt oxides and imides with C(sp3)−H bonds. For reviews, see: (a) Gunay, A.; Theopold, K. H. Chem. Rev. 2010, 110, 1060−1081. (b) Lu, H.; Zhang, X. P. Chem. Soc. Rev. 2011, 40, 1899−1909. (25) Nuel, D.; Dahan, F.; Mathleu, R. Organometallics 1986, 5, 1278−1279. (26) Yamamoto, Y.; Tanase, T.; Sugano, K. J. Organomet. Chem. 1995, 486, 21−29. (27) Gridley, B. M.; Blake, A. J.; Davis, A. L.; Lewis, W.; Moxey, G. J.; Kays, D. L. Chem. Commun. 2012, 8910−8912. (28) Ding, K.; Brennessel, W. W.; Holland, P. L. J. Am. Chem. Soc. 2009, 131, 10804−10805.
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