Communication pubs.acs.org/Organometallics
A New Stepwise Mechanism for Formation of a Metallacyclobutane via a Singlet Diradical Intermediate Jack T. Fuller,† Daniel J. Harrison,‡ Matthew C. Leclerc,‡ R. Tom Baker,‡ Daniel H. Ess,*,† and Russell P. Hughes*,§ †
Department of Chemistry, Brigham Young University, Provo, Utah 84602, United States Center for Catalysis Research and Innovation, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada § Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, United States ‡
S Supporting Information *
ABSTRACT: Density functional calculations (M06/def2-TZVP//M06/LACVP** in THF solvent) reveal that the formation of a perfluorometallacyclobutane from tetrafluoroethylene, C2F4 (TFE), and the difluorocarbene complex CoCp(PPh2Me)(CF2) is highly exothermic and involves a stepwise [2 + 2] cycloaddition mechanism with a 1,4-singlet diradical intermediate. This pathway is significantly lower in energy than pathways involving initial η1 or η2 binding of TFE to cobalt. The stability of the singlet-diradical intermediate results from the formation of a strong CF2−CF2 bond coupled with the radical stabilizing effect of a difluoromethylene group. A concerted [2 + 2] transition state between the 18-electron complex CoCp(PPh2Me)(CF2) and TFE is very high in energy and essentially forbidden.
A
productive in any event: previous studies on an iridium analogue 4, containing an ethylene ligand, showed that the carbene and alkene ligands were rotationally locked and effectively unable to acquire a conformation from which a metallacyclobutane could be formed.7
lkene metathesis is one of the most useful reactions in organometallic chemistry.1 The mechanism, involving facile and reversible [2 + 2] cycloaddition between a metal carbene and a cis-ligated alkene to give a metallacyclobutane, is one of the most thoroughly studied in the field, due to the importance of this reaction in organic and polymer synthesis.1b,2 In contrast to the many successful metathesis reactions for alkenes, catalytic metathesis reactions involving tetrafluoroethylene (TFE) have been unsuccessful, although TFE has recently been used as a cross-metathesis reagent in a process involving alternating MCF2 and MCH(OCH3) intermediates.3 Recent observations that the difluorocarbene cobalt complex 1 (L = PPh2Me)4 reacted slowly with excess TFE at room temperature in THF solution to give perfluorometallacyclobutane 2 illustrated that a key step of the metathesis sequence was viable for perfluorinated analogues.5 Perfluorometallacyclobutanes are rare: only two other examples have been reported, each made by methods that do not involve C−C bond formation.6 This cobalt reaction is exceptional because the starting metal complex 1 is coordinatively saturated, yet the normally accepted mechanism for metallacycle formation requires prior coordination of the alkene cis to the carbene ligand.1d,e The rate of product formation was not inhibited by a large excess of free phosphine, indicating that dissociative phosphine substitution by alkene to produce 3 was very unlikely.5 Formation of 3 is unlikely to be © 2015 American Chemical Society
Given the decidedly unusual nature of this reaction, we used density functional calculations at the M06/def2-TZVP//M06/ LACVP** level with an implicit THF Poisson−Boltzmann solvent model to comprehensively examine this mechanism. This combination of functional8 and basis set9 performs well in reproducing metric parameters previously determined crystallographically for 1 and 2 (see the Supporting Information for full details).4b,5 Here we report that calculations have identified a unique stepwise [2 + 2] cycloaddition mechanism via a singletdiradical intermediate that leads to perfluorometallacyclobutane formation. Received: October 15, 2015 Published: October 26, 2015 5210
DOI: 10.1021/acs.organomet.5b00863 Organometallics 2015, 34, 5210−5213
Communication
Organometallics Four pathways for product formation were envisaged and explored computationally (Scheme 1). We first located the Scheme 1
INT1C (ΔG = 37.9 kcal/mol), supporting the concept that the carbene carbon, rather than Co, is the better nucleophile. However, a more detailed analysis of INT1B showed a triplet and unrestricted wave function instability. Reoptimization of INT1B as a triplet reveals that INT1Bt (ΔG = 1.4 kcal/mol) is 25.4 kcal/mol more stable than zwitterionic INT1B (Figure 1).
transition state TScon for a concerted [2 + 2] cycloaddition. Intrinsic reaction coordinate (IRC) calculations indicate that TScon directly connects 2 and separated 1/TFE and does not require prior TFE−Co coordination. However, the activation barrier for this pathway is very high (ΔG⧧ > 50 kcal/mol) despite the very favorable thermodynamics (ΔG = −25.3 kcal/ mol), indicative of a forbidden, yet irreversible, reaction. The strong thermodynamic favorability of this cobalt reaction contrasts with models of the Grubbs metathesis catalysts (eq 1), for which calculated values of ΔG for perfluorometallacycle formation are significantly endergonic for M = Fe, Ru and are only slightly exergonic for M = Os:10
Figure 1. Calculated free energy profile (M06/def2-TZVP/THF) for the reaction of 1 with TFE via singlet (black) and triplet pathways (red).
[M(Im)Cl 2(CF2)] + CF2CF2 → [M(Im)Cl 2(CF2CF2CF2)]
The optimized open-shell singlet INT1Bs was obtained with S2 = 1.0,15 and the spin-projected singlet energy16 lies slightly lower in energy (ΔG = 1.1 kcal/mol) than the triplet state. A comparison of spin-projected energies with non-spin-projected values is provided in the Supporting Information. The 1,4diradical description of these open-shell intermediates is consistent with the calculated excess spin densities: INT1Bs, 0.98(α)Co and 0.80(β)C; INT1At, 1.04(α)Co and 0.81(α)C. The singlet TS1Bs and triplet TS1Bt transition states leading to these open-shell intermediates were located at ΔG⧧ = 27.8 kcal/mol (spin-projected value) and 29.3 kcal/mol, respectively (Figure 1). This suggests that the majority of the reaction proceeds via the open-shell singlet transition state TS1Bs. The relative ordering of singlet and triplet transition states is also consistent with the excited state triplet 1 being 10.9 kcal/mol higher in energy than the ground state singlet 1, and therefore spin intersystem crossing via the minimum energy crossing point (MECP)17 (ΔG = 11.4 kcal/mol) is likely not important prior to C−C bond formation. However, the nearly degenerate singlet and triplet energies of INT1Bs and INT1Bt suggest the possibility of rapid spin intersystem crossing at the 1,4-diradical intermediate. Closure of the diradical intermediate occurs via low-energy rotation about the new C−C bond to give INT2Bs and rapid radical combination to give the final product 2 via TS2Bs. The calculated overall reaction barrier (ΔG⧧ = 27.8 kcal/mol) is consistent with experimental observations of a slow reaction over several days at room temperature with an excess of TFE. Finally, the possibility of reaction pathways initiated by electron transfer to produce traces of either the radical cation 2•+ or radical anion 2•− were examined in case a radicaladdition or radical-catalyzed pathway via such species might be
(1)
The prohibitively high barrier to concerted cycloaddition prompted us to explore the possibility of well-precedented η5 → η3 cyclopentadienyl ring slippage11 to generate INT1A with η2-TFE coordination (Scheme 1). Inner-sphere cycloaddition of carbene and η2-bound alkene could then yield perfluorometallacycle product 2 by the familiar inner-sphere alkene metathesis mechanism. However, INT1A lies 35.6 kcal/mol higher in energy than the reactants. This relatively high energy intermediate and significantly lower energy alternative pathways (see below) indicate that the Co metal center remains coordinatively saturated during the cycloaddition reaction process. As alternatives to concerted and ring-slipping pathways we considered the possibility of polar-stepwise mechanisms involving either initial C−C or Co−C bond formation to generate zwitterionic intermediates INT1B and INT1C (Scheme 1). Formation of INT1B involves the carbene carbon atom serving as a formal nucleophile, while INT1C is formed if the d8 Co(I) center12 acts nucleophilically, as previously demonstrated by reactions of CoCpL2 complexes with electrophilic carbon centers.13 Whether difluorocarbene ligands contain nucleophilic or electrophilic carbon is still unclear.14 Experimental observations of reactions of proton and carbon electrophiles with 1 to yield observable complexes 5 (R = H, CH3) seem to favor a nucleophilic (and basic) description of the carbene, although the kinetic site of nucleophilicity is not clearly established.4b,14b Location of the zwitterionic intermediates shows that INT1B (ΔG = 26.8 kcal/mol) is ∼10 kcal/mol more stable than 5211
DOI: 10.1021/acs.organomet.5b00863 Organometallics 2015, 34, 5210−5213
Communication
Organometallics competitive. Disproportionation of 2 (eq 2), electron transfer from 2 to TFE (eq 3), and oxidation of 2 with traces of O2 (eq 4) are all calculated to be significantly endergonic, leaving the diradical pathway as by far the lowest energy kinetic path to the experimentally observed product. 2 → 2•+ + 2•−
ΔG = 53.2 kcal/mol
2 + TFE → 2•+ + TFE•− 2 + O2 → 2•+ + O2•−
ΔG = 59.6 kcal/mol
ΔG = 43.7 kcal/mol
(2) (3) (4)
The reasons for this unusual chemistry originate in the different effects of fluorination on the energies of CC π bonds and C−C σ bonds. In comparison to ethylene, TFE is more susceptible to attack by carbon radicals.18 This reactivity results from two key factors: the unusually weak π bond in TFE (52.5 kcal/mol) relative to that in ethylene (59 kcal/mol),19 due to the strong preference for terminal •CF2 radicals to be pyramidal,20 and the unusually strong σ bonds formed from CF2 groups to other carbon atoms.18 Consequently, many organic reactions of TFE involve radical or diradical intermediates. For example, an open-shell 1,4-diradical, calculated to lie lower in energy than the reactants, is a proposed intermediate in the thermal dimerization of TFE to give perfluorocyclobutane.21 Even more remarkable is the reaction of TFE with butadiene, where the initially formed 1,4diradical intermediate 6 closes to give to give the [2 + 2] vinylcyclobutane 7 rather than a [4 + 2] Diels−Alder adduct.22 Early computational studies by Borden at the Hartree−Fock level indicated that perfluorination of the alkene does not destabilize the concerted Diels−Alder pathway but, instead, strongly stabilizes the diradical intermediate 6.22 We note that, in contrast to reaction of TFE with 1, which occurs at room temperature,5 its cycloaddition with butadiene to give 7 requires temperatures as high as 170 °C.23
Figure 2. Calculated (M06/LACVP**/THF) structures for 1, 2, and the open-shell singlet species TS1Bs, INT1Bs, INT2Bs, and TS2Bs, with bond lengths (Å) and angles (deg). Hydrogens are omitted for clarity. Values in red are from crystallographic studies.
Calculated structures for key species are presented in Figure 2. INT1Bs possesses an essentially fully formed Cα−Cβ σ bond (1.536 Å) and a strongly pyramidal CF2• radical (∑CF2 angles 341°). However, in the early TS1Bs the incipient Cα−Cβ bond (2.284 Å) is much less developed and there is significantly less pyramidalization at the CF2 carbon centers. Consequently TS1Bs gains relatively little from the factors stabilizing INT1Bs. Factors contributing to the remarkable stability of the openshell intermediate INT1Bs were explored by examination of the corresponding reaction of 1 with CH2CH2 and its two possible regioisomeric additions to CF2CH2. While TFE has a weaker π bond (52.5 kcal/mol) than ethylene (59 kcal/ mol),19 that in the more polar CF2CH2 is actually stronger (63 kcal/mol).24 Calculated energetics are illustrated in Figure 3 and are color coded. Comparison of the TFE (black) and CH2CH2 (red) reactions illustrates that, relative to starting materials, metallacycle 2B is considerably less stable than fully fluorinated 2A. This results from formation of two stronger CF2−CF2 bonds and one stronger Co−CF2 bond in 2A coupled with the lower energy cost of breaking the weaker π bond in CF2CF2. The same trend is seen in intermediates 1A and 1B and, to a lesser extent, in their preceding transition
Figure 3. Calculated energetics (M06/def2-TZVP/THF) for reactions of 1 with CH2CH2 and CF2CH2 (two regioisomers) in comparison with CF2CF2. The initially formed C−C bond is shown as a boldface line, and the final Co−C bond is shown as a dashed line.
states. Data for the two regioisomers arising from CF2CH2 provide more illumination. Metallacycle 2C (green) is more stable than 2D (blue), consistent with the proposal that fluorinated carbons have a larger stabilizing effect when bound to a transition metal such as Co in comparison to that to another carbon.25 However, intermediate 1C is less stable than 1D, demonstrating that formation of the strong CF2−CF2 σ bond contributes more to stabilizing the intermediate diradical than does pyramidalization of a terminal •CF2 radical center; a 5212
DOI: 10.1021/acs.organomet.5b00863 Organometallics 2015, 34, 5210−5213
Communication
Organometallics
(6) (a) Karel, K. J.; Tulip, T. H.; Ittel, S. D. Organometallics 1990, 9, 1276−1282. (b) Xu, L.; Solowey, D. P.; Vicic, D. A. Organometallics 2015, 34, 3474−3479. (7) Yuan, J.; Hughes, R. P.; Golen, J. A.; Rheingold, A. L. Organometallics 2010, 29, 1942−1947. (8) (a) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215− 241. (b) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157−167. (9) (a) Schaefer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571−2577. (b) Schaefer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829−5835. (10) Vasiliu, M.; Arduengo, A. J.; Dixon, D. A. J. Phys. Chem. C 2014, 118, 13563−13577. (11) (a) Orian, L.; Swart, M.; Bickelhaupt, F. M. ChemPhysChem 2014, 15, 219−228. (b) O’Connor, J. M.; Casey, C. P. Chem. Rev. 1987, 87, 307−318. (c) Schuster-Woldan, H. G.; Basolo, F. J. Am. Chem. Soc. 1966, 88, 1657−1663. (12) DFT analysis of CoCp(PPh3)(CF2) supports a formal Co(I) oxidation state and a neutral carbene ligand (see ref 4b). (13) Werner, H. Angew. Chem., Int. Ed. Engl. 1983, 22, 927−949. (14) (a) Brothers, P. J.; Roper, W. R. Chem. Rev. 1988, 88, 1293− 1326. (b) Yuan, J.; Bourgeois, C. J.; Rheingold, A. L.; Hughes, R. P. Dalton Trans. 2015, DOI: 10.1039/C5DT02275D. (15) (a) Grafenstein, J.; Cremer, D. Mol. Phys. 2001, 99, 981−989. (b) Cramer, C. J. Essentials of Computational Chemistry, 2nd ed.; Wiley: Hoboken, NJ, 2004. (16) (a) Ess, D. H.; Cook, T. C. J. Phys. Chem. A 2012, 116, 4922− 4929. (b) Wittbrodt, J. M.; Schlegel, H. B. J. Chem. Phys. 1996, 105, 6574−6577. (c) Yamanaka, S.; Kawakami, T.; Nagao, H.; Yamaguchi, K. Chem. Phys. Lett. 1994, 231, 25−33. (d) Yamaguchi, K.; Takahara, Y.; Fueno, T.; Houk, K. N. Theor. Chim. Acta 1988, 73, 337−364. (e) Yamaguchi, K.; Yoshioka, Y.; Takatsuka, T.; Fueno, T. Theor. Chim. Acta 1978, 48, 185−206. (f) Ziegler, T.; Rauk, A.; Baerends, E. J. Theor. Chim. Acta 1977, 43, 261−271. (g) Yamaguchi, K.; Yoshioka, Y.; Fueno, T. Chem. Phys. Lett. 1977, 46, 360−365. (17) Harvey, J. N.; Aschi, M.; Schwarz, H.; Koch, W. Theor. Chem. Acc. 1998, 99, 95−99. (18) (a) Uneyama, K. Organofluorine Chemistry; Blackwell Publishing: Oxford, U.K., 2006. (b) Smart, B. E. Mol. Struct. Eng. 1986, 3, 141−191. (c) Smart, B. E. Fluorocarbons. In Chemistry of Functional Groups; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1983; Vol. 1, Supplement D, pp 603−655. (19) Wu, E.-C.; Rodgers, A. S. J. Am. Chem. Soc. 1976, 98, 6112− 6115. (20) (a) Wang, S. Y.; Borden, W. T. J. Am. Chem. Soc. 1989, 111, 7282−7283. (b) Borden, W. T. Chem. Commun. 1998, 1919−1925. (21) (a) Kalnin’sh, K. K. Russ. J. Appl. Chem. 2002, 75, 589−597. (b) Buravtsev, N. N.; Kolbanovsky, Y. A. J. Fluorine Chem. 1999, 96, 35−42. (22) Getty, S. J.; Borden, W. T. J. Am. Chem. Soc. 1991, 113, 4334− 4335. (23) Coffman, D. D.; Barrick, P. L.; Cramer, R. D.; Raasch, M. S. J. Am. Chem. Soc. 1949, 71, 490−496. (24) (a) Pickard, J. M.; Rodgers, A. S. J. Am. Chem. Soc. 1976, 98, 6115−6118. (b) Pickard, J. M.; Rodgers, A. S. J. Am. Chem. Soc. 1977, 99, 695−696. (25) (a) Taw, F. L.; Clark, A. E.; Mueller, A. H.; Janicke, M. T.; Cantat, T.; Scott, B. L.; Hay, P. J.; Hughes, R. P.; Kiplinger, J. L. Organometallics 2012, 31, 1484−1499. (b) Algarra, A. G.; Grushin, V. V.; Macgregor, S. A. Organometallics 2012, 31, 1467−1476.
similar conclusion was reached in the case of diradicals formed in organic reactions of TFE.20b Notably, when 1C collapses to product 2C the additional stability of the newly formed Co− CF2 bond results in 2C being more stable than its isomer 2D. The weaker C−C bond in 1C coupled with the additional cost of breaking the stronger π bond in CF2CH2 places 1C higher than 1B, formed from CH2CH2. An identical trend is observed in the preceding transition states. In conclusion, it appears that TFE is an unusual alkene in its reactivity with these difluorocarbene complexes. As in its organic chemistry, its propensity to form unusually stable 1,4diradical intermediates generates a novel organometallic pathway for formation of a metallacyclobutane. However, the considerable stability of the resultant perfluorometallacyclobutane, resulting from the formation of unusually strong, fluorinated C−C18b,c and M−C25 σ bonds from relatively weak π bonds in the starting materials does not bode well for the future of perfluoroalkene metathesis with these particular systems.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00863. Full details on all computational methods, coordinates, and energies for all calculated structures, non-spinprojected energies for singlet species, and large views of the figures (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for D.H.E.:
[email protected]. *E-mail for R.P.H.:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS R.P.H. is grateful to Professor Robert Ditchfield (Dartmouth) for several illuminating conversations. R.P.H. and D.H.E are grateful to Professor Jeremy Harvey (KU Leuven) for a copy of his MECP Program. D.H.E thanks BYU and the Fulton Supercomputing Lab. M.C.L. gratefully acknowledges support from NSERC.
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REFERENCES
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DOI: 10.1021/acs.organomet.5b00863 Organometallics 2015, 34, 5210−5213
