Communication pubs.acs.org/Organometallics
Cobalt Fluorocarbene Complexes Daniel J. Harrison, Serge I. Gorelsky, Graham M. Lee, Ilia Korobkov, and R. Tom Baker* Department of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, 30 Marie Curie, Ottawa, Ontario K1N 6N5, Canada S Supporting Information *
ABSTRACT: Unusual nucleophilic cobalt fluorocarbene complexes ([CoI]CFR; R = F, CF3) are described. Only a handful of fluorocarbenes of first-row transition metals have been reported, and these have exhibited electrophilic reactivity at the metal−carbene bonds. The title compounds are the first persistent/isolable cobalt fluorocarbenes and also rare examples of metal fluorocarbenes with relatively nucleophilic MC bonds (vs other metal fluorocarbene complexes), demonstrated by their reactions with H+ and Me+ and their unusual stability toward water. X-ray crystal structures reveal the shortest terminal cobalt−carbene bonds reported to date, and DFT studies provide details into the nature of these bonds.
T
Scheme 1
ransition-metal carbene complexes feature divalent carbon centers that play important roles in organometallic chemistry and organic synthesis,1 such as annulations,2 catalyzed alkene metathesis,3 and cyclopropanation.4 While much effort has focused on the reactivity of metal carbene complexes with electronic structures ranging from early-metal alkylidenes5 to Fischer carbenes6 and even to metal Nheterocyclic carbene complexes,7 less attention has been paid to metal fluorocarbenes. The first transition-metal difluorocarbenes and fluoro(perfluoroalkyl)carbene ([M]CFR; R = F, perfluoroalkyl)8 were synthesized, but not isolated, in 1978,9 and surprisingly few (50% cobalt character and smaller coefficients on the carbene atoms (Supporting Information), suggesting that proton attack at cobalt may be kinetically preferred. However, our efforts to observe an intermediate by 1 H and 19F NMR spectroscopy in the reaction between 1a and triflic acid (−80 °C, in CD2Cl2) were unsuccessful (Supporting Information). Thus, it is unclear whether the cobalt or the carbon atom is protonated first, but the CoCFR bonds are clearly nucleophilic sites, especially in relation to all other known first-row metal fluorocarbenes. The protonation of complexes 2a,b gave mixtures of diastereomers containing the CF(CF3)H ligand in 8:1 and 2:1 ratios, respectively. Attempts to methylate the cobalt fluorocarbene complexes using MeOTf gave complex reaction mixtures; however, for the reaction of 1b, the [Co]−CF2Me substructure was clearly visible in the major product by 1H and 19F NMR spectroscopy (Supporting Information). Attempts to isolate and fully characterize the methylated species failed, as decomposition occurred in solution and upon solvent removal. Remarkably, the cobalt fluorocarbenes reported here (1a,b, 2a,b) do not react with water ([cobalt fluorcarbene] = 0.1 mM, [H2O] = 0.5 mM in CD3CN, ≥12 h at room temperature),
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ACKNOWLEDGMENTS We thank the NSERC and the Canada Research Chairs program for generous financial support and the University of Ottawa, Canada Foundation for Innovation and Ontario Ministry of Economic Development and Innovation for essential infrastructure. D.J.H. and G.M.L. gratefully acknowledge support from the province of Ontario and University of Ottawa (Vision 2020 and OGS scholarships).
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REFERENCES
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(7) N-Heterocyclic Carbenes: From Laboratory Curiosities to Efficient Synthetic Tools; Dı ́ez-González, S., Ed.; Royal Society of Chemistry: London, 2010. (8) Regarding nomenclature: here, “carbene” describes any CRR′ fragment where R and/or R′ is a heteroatom substituent, including fluorine. (9) Reger, D. L.; Dukes, M. D. J. Organomet. Chem. 1978, 153, 67−72 ([CpMo(CFR)(CO)3](SbF6)). (10) Brothers, P. J.; Roper, W. R. Chem. Rev. 1988, 88, 1293−1326. See refs 14 and 15 for more examples of precious-metal [M]CFR complexes. (11) Difluorocarbene ligands in bridging modes ([M]2(μ-CF2) or [M](μ-CF2)[M′]): (a) Schulze, W.; Hartl, H.; Seppelt, K. Angew. Chem. 1986, 98, 189−190; Angew. Chem., Int. Ed. 1986, 25, 185−187. (b) Bonnet, J. J.; Mathieu, R.; Poilblanc, R.; Ibers, J. A. J. Am. Chem. Soc. 1979, 101, 7487−7496. (c) Crespi, A. M.; Sabat, M.; Shriver, D. F. Inorg. Chem. 1988, 27, 812−816. See also ref 10. (12) (a) Richmond, T. G.; Crespi, A. M.; Shriver, D. F. Organometallics 1984, 3, 314−319 ([CpFe(CF2)(CO)2]+ and [F2CMn(CO)5]+; the latter compound was not isolable). (b) Crespi, A. M.; Shriver, D. F. Organometallics 1985, 4, 1830−1835 ([CpFe( CF2)(CO)(PPh3)]+). (13) All carbene ligands are formulated as neutral two-electron donors and d-electron counts are calculated accordingly. The DFT analysis of the electronic structure indicates donor-acceptor bonding between the carbene and metal fragments, small net charge on the carbene ligands in the complexes and, thus, supports a formal CoI oxidation state and a neutral, π-accepting carbene ligand for the new complexes reported here. (14) For electrophilic precious metal difluorocarbenes, see, for example: (a) Huang, D.; Koren, P. R.; Folting, K.; Davidson, E. R.; Caulton, K. G. J. Am. Chem. Soc. 2000, 122, 8916−8931 ([F2C M(CO)(F)(H)L2]; M = Ru, Os, L = phosphine). (b) Clark, G. R.; Hoskins, S. V.; Roper, W. R. J. Organomet. Chem. 1982, 234, C9−C12 ([F2CRu(CO)(PPh3)2Cl2]). (15) The reaction proceeds through a [Cd]−[Ru]−CF3 intermediate, and spontaneous elimination of [Cd]−F furnishes the product: (a) Clark, G. R.; Hoskins, S. V.; Jones, T. C.; Roper, W. R. J. Chem. Soc., Chem. Commun. 1983, 719−721. For weakly nucleophilic Ir(I) difluorocarbenes: (b) Brothers, P. J.; Burrell, A. K.; Clark, G. R.; Rickard, C. E. F.; Roper, W. R. J. Organomet. Chem. 1990, 394, 615− 642. For a weakly nucleophilic Os(0) difluorocarbene, see ref 10. (16) Hughes, R. P.; Laritchev, R. B.; Yuan, J.; Golen, J. A.; Rucker, A. N.; Rheingold, A. L. J. Am. Chem. Soc. 2005, 127, 15020−15021. (17) Hughes, R. P. J. Fluorine Chem. 2010, 131, 1059−1070. (18) Derivatives of Hughes’ [Ir]CFR complexes: (a) Bourgeois, C. J.; Hughes, R. P.; Yuan, J.; DiPasquale, A. G.; Rheingold, A. L. Organometallics 2006, 25, 2908−2910 ([Cp*Ir(CFR)(CO)]; R = F, CF3, CF2CF3). (b) Yuan, J.; Hughes, R. P.; Rheingold, A. L. Eur. J. Inorg. Chem. 2007, 4723−4725 ([Cp*Ir(C(CF 3 ) 2 )(CO)]). (c) Yuan, J.; Hughes, R. P.; Golen, J. A.; Rheingold, A. L. Organometallics 2010, 29, 1942−1947 ([Cp*Ir(CF(CF3))(η2C2H4)]). See also ref 17. (19) For Co(II) difluorocarbenes observed at low temperatures: Cho, H.-G.; Andrews, L. J. Phys. Chem. A 2010, 114, 8056−8068. (20) We used slightly modified (see the Supporting Information for details) procedures from the literature: (a) King, R. B.; Treichel, P. M.; Stone, F. G. A. J. Am. Chem. Soc. 1961, 83, 3593−3597 (syntheses of [CpCo(CF2R)(CO)I]). (b) Burns, R. J.; Bulkowski, P. B.; Stevens, S. C. V.; Baird, M. C. J. Chem. Soc., Dalton Trans. 1974, 415−420 (syntheses of [CpCo(CF2R)(L)I]; L = phosphine, phosphite). (21) Reported yields are for the second two steps outlined in Scheme 2. (22) Attempts to make complexes with L = 2,6-dimethylphenyl isocyanide, 1,3-bis(isopropyl)imidazol-2-ylidene or PMe2Ph were unsuccessful. (23) For 1a, one of two crystallographically independent molecules in the unit cell is shown. Single orientations are depicted for the rotationally disordered P−Ph and Cp groups in the structure of 2a. A
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