Communication pubs.acs.org/Organometallics
C−CN Bond Activation of Acetonitrile using Cobalt(I) Hongwei Xu, Paul G. Williard, and Wesley H. Bernskoetter* Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States S Supporting Information *
ABSTRACT: A cobalt(I) methyl species, (PMe3)4CoCH3, was found to promote C−CN bond oxidative addition of acetonitrile at ambient temperature. The isolated product of acetonitrile activation, cis,mer-(PMe3)3Co(CH3)2CN, was characterized by NMR, IR, and single-crystal X-ray diffraction studies and presents a higher valent metal in comparison to those previously observed for base-metal-mediated nitrile activations. A short-lived reaction intermediate was detected during nitrile cleavage and identified as fac-(PMe3)3Co(CH3)2CN, the kinetic product of C−CN oxidative addition. Conversion of the kinetic product to cis,mer(PMe3)3Co(CH3)2CN proceeds with a rate constant of [1.0(1)] × 10−3 s−1 at 27 °C. 4 h to afford cis,mer-tris(trimethylphosphine)cobalt(III) dimethyl cyanide (3) as a yellow-green solid in excellent yield. This diamagnetic dimethyl cobalt cyanide species may alternatively be prepared via substitution from 1 with sodium cyanide (Figure 1).9
The cleavage of C−C bonds by organometallic complexes has remained an area of challenge and intrigue since the advent of transition-metal-mediated synthetic methods. Success in this field has largely been limited to substrates which contain strained cyclic C−C bonds or other attributes such as chelation and aromatization, which bias the thermodynamic profile.1 However, the scission of strong C−C bonds in nitriles has seen a moderate degree of progress,2 owing in part to the favorable enthalpy of forming a metal−cyano bond.3 This feature has been widely explored by Garcia and Jones using nickel(0) complexes,4 and their efforts have fostered a few examples of catalytic nickel(0)/Lewis acid mediated nitrile activations.5 Expanding the role of nitrile activation in synthetic organic methodology could be facilitated by targeting C−CN bond oxidative additions which produce first-row transition-metal species in valencies higher than those typically obtained from nitrile cleavage.2,4 Higher valent organometallic complexes may obviate the need for Lewis acid or silane additives and might prove better disposed toward reductive elimination of functionalized nitrile products in catalytic processes.6 Directed by these motivations, we have undertaken investigations into cobalt(I)-mediated C−CN bond scission to afford twoelectron-oxidized products. We report herein the preparation of a cobalt(III) dimethyl cyanide species derived from acetonitrile cleavage and the identification of the kinetic and thermodynamic products of nitrile activation. Strong-field trialkylphosphine ligands are potent σ-donors which have frequently been leveraged to stabilize transition metals in high oxidation states.7 Our laboratory has recently used this feature to moderate the reactivity of cis,mertris(trimethylphosphine)cobalt(III) dimethyl iodide (1) in order to study its mechanism for reductive C−C bond formation.8 During the course of extending these mechanistic studies, we have discovered that a related microscopic reverse process, C−C bond oxidative addition of acetonitrile, can be promoted by tetrakis(trimethylphosphine)cobalt(I) methyl (2). Dissolving complex 2 in acetonitrile at ambient temperature resulted in C−CN bond oxidative cleavage over approximately © 2012 American Chemical Society
Figure 1. Preparation of cis,mer-tris(trimethylphosphine)cobalt(III) dimethyl cyanide by C−CN cleavage and sodium cyanide substitution.
Complex 3 was characterized by a combination of multinuclear NMR and IR spectroscopy as well as combustion analysis and X-ray diffraction. The 1H NMR spectrum of 3 in benzene-d6 displays the number of signals expected for this Cssymmetric complex, with two distinct cobalt−methyl resonances at 0.24 and −0.75 ppm. These signals appear with triplet of doublets and doublet of triplets multiplicities, consistent with coupling to three 31P nuclei. The 1H−13C HSQC NMR spectrum displays correlations between the two 1H NMR resonances and 13C NMR chemical shifts of 4.19 and −5.02 ppm, respectively. Confirmation for the assignment of the cobalt−methyl resonances was achieved by employing acetonitrile-d3 in the synthetic procedure to generate the isotopologue cis,mer- (PMe3)3Co(CN)(CD3)(CH3) (3-d3) (eq 1).The 1H NMR spectrum of 3-d3 retains both cobalt−methyl resonances, but each with approximately half the integration observed for 3. This feature indicates that the acetonitrilederived methyl ligand is likely installed via a symmetric intermediate (vide infra) and/or the two methyl groups undergo site exchange. Rapid intramolecular methyl−methyl Received: December 22, 2011 Published: February 9, 2012 1588
dx.doi.org/10.1021/om201270n | Organometallics 2012, 31, 1588−1590
Organometallics
Communication
Their experimental and computational work has identified η2nitrile intermediates to oxidative addition which transition through a higher energy η3-nitrile structure en route to alkyl− CN cleavage.4 In an effort to identify similarities in the reaction to form the higher oxidation state cobalt(III) alkyl cyanide species, the synthesis of 3 was monitored at short reaction times. Addition of acetonitrile to 2, rapid removal of the solvent after 10 min, and redissolving of the residue in benzene-d6 revealed three new 1H NMR signals along with the product 3 and residual starting material. A resonance with the appearance of a broad triplet was observed at 0.55 ppm in addition to two doublet Co−PMe3 signals at 1.05 and 0.69 ppm. The lifetime of this intermediate species was limited at ambient temperature, and the new resonances decayed from the 1H NMR spectrum with a first-order rate constant of [1.0(1)] × 10−3 s−1 at 27 °C (t1/2 = ca. 10 min). This decay corresponded with a growth in the signals for 3 and did not afford any observable free PMe3.9 The broad resonance at 0.55 ppm in the 1H NMR spectrum of the intermediate could originate from either a Co−CH3 fragment or an η2-acetonitrile adduct. However, several observations suggest that this resonance is better ascribed to a metal−alkyl. For example, 1H−13C HSQC NMR spectra correlate this 1H NMR peak to a 13C NMR resonance at 3.22 ppm. Both 1H and 13C NMR chemical shifts are slightly upfield of those reported for related nickel η2-acetonitrile adducts but are analogous to the Co−CH3 shifts observed for 3.2h,4c Additionally, in situ monitoring of the reaction of 2 with acetonitrile-d3 revealed that the broad “triplet” signal persisted upon labeling, but with half the relative integration observed in perprotio samples. These observations are more consistent with an intermediate species bearing chemically equivalent cobalt− methyl ligands than with an η2-nitrile complex (Figure 3).
site exchange has been observed in closely related cobalt(III) species,8 although 2D EXSY spectra of 3 acquired under analogous conditions (mixing time 175 ms; 27 °C) failed to detect site exchange. Additional spectroscopic features of 3 include two broad resonances in the 31P NMR spectrum at 4.94 and 17.33 ppm with a 1:2 integration ratio as well as a broad feature at 146.63 ppm in the 13C NMR spectrum corresponding to the quaternary Co−CN resonance. The presence of the metal cyanide was also evidenced by a strong band at 2081 cm−1 in the solid-state infrared spectrum.10 Further confirmation of the molecular structure of 3 was achieved by single-crystal X-ray diffraction experiments on a sample obtained from a chilled diethyl ether solution (Figure 2). The data reveal a generalized
Figure 2. Molecular structure of (PMe3)3Co(CH3)2CN (3) with 30% probability ellipsoids. All hydrogen atoms are omitted for clarity. Select bond distances (Å) and angles (deg): Co(1)−C(1) = 2.054(5), Co(1)−C(2) = 2.062(5), Co(1)−C(3) = 1.911(5), Co(1)−P(1) = 2.207(2), Co(1)−P(2) = 2.229(1), Co(1)−P(3) = 2.205(1), C(3)− N(1) = 1.160(6); C(1)−Co(1)−C(3) = 179.0(2), P(1)−Co(1)−P(3) = 162.07(5), P(2)−Co(1)−C(2) = 167.4(1).
Figure 3. Plausible pathway for formation of 3 via acetonitrile activation.
octahedral geometry about the metal center with the cyanide ligand bound trans to one of the cobalt−methyl bonds. The two cobalt−methyl bonds, Co(1)−C(1) and Co(1)−C(2), have similar bond lengths of 2.054(5) and 2.062(5) Å, respectively, and are slightly elongated compared to the analogous Co−C distances of 2.038(7) and 2.033(10) Å in 1.11 For the methyl bound opposite the anionic ligand, Co(1)− C(1), the elongation is likely a reflection of the strong trans influence of cyanide compared to iodide. The cyanide carbon− nitrogen and metal−carbon bond lengths of 1.160(6) and 1.911(5) Å in 3 are comparable to those reported for cobalt− cyanide species of varying oxidation states, including cobalt(II) complexes prepared by arylnitrile activation.12 The mechanism of nitrile activation at related nickel complexes has been extensively studied by Garcia and Jones.
Further information regarding the geometry of the intermediate species was gleaned from the NMR signals attributed to the phosphine ligands. The 2:1 ratio of the CoPMe3 signals in the 1H and 31P NMR spectra suggest that the intermediate retains three bound phosphines. More significantly, the doublet multiplicity of these signals in the 1H NMR spectrum is inconsistent with a trans orientation of the PMe3 ligands, as this arrangement typically produces a virtual triplet coupling pattern.13 These data all support a tentative assignment of the reaction intermediate as fac-tris(trimethylphosphine)cobalt(III) dimethyl cyanide, an isomer of complex 3 and likely the kinetic product of nitrile activation. 1589
dx.doi.org/10.1021/om201270n | Organometallics 2012, 31, 1588−1590
Organometallics
Communication
(2) (a) Parshall, G. W. J. Am. Chem. Soc. 1974, 96, 2360. (b) Ozawa, F.; Iri, K.; Yamamoto, A. Chem. Lett. 1982, 1707. (c) Churchill, D.; Shin, J. H.; Hascall, T.; Hahn, J. M.; Bridgewater, B. M.; Parkin, G. Organometallics 1999, 18, 2403 and references therein.. (d) Marlin, D. S.; Olmstead, M. M.; Mascharak, P. K. Angew. Chem., Int. Ed. 2001, 40, 4752. (e) Taw, F. L.; White, P. S.; Bergman, R. B; Brookhart, M. J. Am. Chem. Soc. 2002, 124, 4192. (f) Taw, F. L.; Mueller, A. H.; Bergman, R. B.; Brookhart, M. J. Am. Chem. Soc. 2003, 125, 9808. (g) Lu, T.; Zhang, X.; Li, Y.; Chen, S. J. Am. Chem. Soc. 2004, 126, 4760. (h) Schaub, T.; Doring, C.; Radius, U. Dalton Trans. 2007, 1993. (i) Yang, L.-Z.; Zhuang, X.-M.; Jiang, L.; Chen, J.-M.; Luck, R. L.; Lu, T.-B. Chem. Eur. J. 2009, 15, 12399. (j) Swartz, B. D.; Brennessel, W. W.; Jones, W. D. Organometallics 2011, 30, 1523. (k) Evan, M. E.; Jones, W. D. Organometallics 2011, 30, 3371. (l) Tanabe, T.; Evans, M. E.; Brennessel, W. W.; Jones, W. D. Organometallics 2011, 30, 834. (m) Grochowski, M. R.; Morris, J.; Brennessel, W. W.; Jones, W. D. Organometallics 2011, 30, 5604. (3) Dietz, O.; Rayon, V. M.; Frenking, G. Inorg. Chem. 2003, 42, 4977 and references therein. (4) (a) Garcia, J. J.; Jones, W. D. Organometallics 2000, 19, 5544. (b) Garcia, J. J.; Brunkan, N. M.; Jones, W. D. J. Am. Chem. Soc. 2002, 124, 9547. (c) Garcia, J. J.; Arevalo, A.; Brunkan, N. M.; Jones, W. D. Organometallics 2004, 23, 3997. (d) Atesin, T. A.; Li, T.; Lachaize, S.; Brennessel, W. W.; Garcia, J. J.; Jones, W. D. J. Am. Chem. Soc. 2007, 129, 7562. (e) Atesin, T. A.; Li, T.; Lachaize, S.; Garcia, J. J.; Jones, W. D. Organometallics 2008, 27, 3811. (5) (a) Hirata, Y.; Yada, A.; Morita, E.; Nakao, Y.; Hiyama, T.; Ohashi, M.; Ogoshi, S. J. Am. Chem. Soc. 2010, 132, 10070. (b) Yada, A.; Yukaw, T.; Idea, H.; Nakao, Y.; Hiyama, T. Bull. Chem. Soc. Jpn. 2010, 83, 619. (c) Arevalo, A.; Garcia, J. J. Eur. J. Inorg. Chem. 2010, 4063. (6) (a) Tobisu, M.; Chatani, N. Chem. Soc. Rev. 2008, 37, 300. (b) Tobisu, M.; Kita, Y.; Ano, Y.; Chatani, N. J. Am. Chem. Soc. 2008, 130, 15982. (7) Hartwig, J. In Organotransition Metal Chemistry: From Bonding to Catalysis; University Science: Sausalito, CA, 2010. (8) Xu, H.; Bernskoetter, W. H. J. Am. Chem. Soc. 2011, 133, 14956. (9) See the Supporting Information for further details. (10) For examples of IR bands of Co(III)−CN complexes see: (a) McKinney, R. J. Inorg. Chem. 1982, 21, 2051. (b) Metz, J.; Hanack, M. J. Am. Chem. Soc. 1983, 105, 828. (c) Ranasami, T.; Endicott, J. F. J. Am. Chem. Soc. 1985, 107, 389. (11) Beck, R.; Klein, H. F. Z. Anorg. Allg. Chem. 2008, 634, 1971. (12) (a) Li, X.; Sun, H.; Yu, F.; Florke, U.; Klein, H.-F. Organometallics 2006, 25, 4695. (b) Fengli, Y.; Qibao, W.; Li, X. Acta Crystallogr., Sect. E 2008, 64, M112. (c) Xu, X.; Feng, L.; Li, X. Acta Crystallogr., Sect. E 2011, 67, M503. (13) (a) Musher, J. I.; Corey, E. J. Tetrahedron 1962, 18, 791. (b) Macomber, R. S. In A Complete Introduction to Modern NMR Spectroscopy; Wiley-Interscience: New York, 1998.
Still, a pair of minor inconsistencies offer some pause for the assignment of the intermediate as the fac isomer of 3. First, the apparent triplet multiplicity of the 1H NMR resonance for the cobalt−methyl groups does not display the full second-order coupling pattern expected for the magnetically inequivalent pair.13b This is likely a result of unresolved coupling within the broad line shape of the signal, but attempts to improve the resolution by acquiring NMR spectra at −60 °C did not afford a clear observation of the full expected coupling. Additionally, attempts to detect a second Co−CN vibration in the infrared spectrum for the intermediate isomer were unsuccessful. It is possible that the band for the intermediate is obscured by the corresponding band in the product 3, which was present in all samples. Notably, no strong band expected for an η2-nitrile (1650−1900 cm−1) was observed.4a While the limited lifetime of the intermediate obviated additional structural examination, the spectroscopic evidence generally supports characterization as fac-tris(trimethylphosphine)cobalt(III) dimethyl cyanide and suggests that C−CN bond oxidative addition is rapid compared to nitrile coordination under conditions of excess acetonitrile. Significantly, this feature of the cobalt reaction contrasts with observations of multiple nickel(0) C−CN cleavage reactions, where oxidative addition after nitrile coordination is a relatively slow process.2h,4 In summary, we have observed acetonitrile activation by the organometallic complex (PMe3)4CoCH3 to produce a d6 cobalt(III) dimethyl cyanide species. This C−CN bond cleavage process affords a higher valent first-row transitionmetal complex in comparison to those previously described for this class of activation and should be advantageous for further functionalization of the nitrile substrates as well as development of catalytic synthetic organic applications. Careful monitoring of the cleavage reaction revealed the presence of a short-lived intermediate species which was identified as the kinetic product of C−CN activation, fac-(PMe3)3Co(CH3)2CN, suggesting that, once nitrile binding occurs, bond rupture at cobalt is relatively swift. Ongoing investigations in our laboratory are directed toward exploring the substrate scope of this intriguing nitrile activation and leveraging the cobalt(III) alkyl cyanide products in catalytic coupling reactions of nitriles with organic unsaturates and other substrates.
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ASSOCIATED CONTENT
S Supporting Information *
Text, figures, and a CIF file giving experimental details, select NMR spectra, and kinetic plots. 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 gratefully acknowledge Brown University and its Richard B. Salomon Faculty Research Award for support of this work.
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REFERENCES
(1) (a) Jun, C. H. Chem. Soc. Rev. 2004, 33, 610. (b) Necas, D.; Kotora, M. Curr. Org. Chem. 2007, 11, 1566. 1590
dx.doi.org/10.1021/om201270n | Organometallics 2012, 31, 1588−1590
