Reactions of Cyclopentadienylidenes with CF3I: Electron Bond

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Reactions of Cyclopentadienylidenes with CF3I: Electron Bond Donation versus Halogen Bond Donation of the Iodine Atom Stefan Henkel,†,‡ Iris Trosien,† Joel Mieres-Peŕ ez,†,# Thomas Lohmiller,§,⊥ Anton Savitsky,§,∥ Elsa Sanchez-Garcia,‡,# and Wolfram Sander*,† †

Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, 44801 Bochum, Germany Max-Planck-Institut für Kohlenforschung, 45470 Mülheim an der Ruhr, Germany § Max-Planck-Institut für Chemische Energiekonversion, 45470 Mülheim an der Ruhr, Germany Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on August 3, 2018 at 18:31:52 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: The interaction of cyclopentadienylidene and tetrachlorocyclopentadienylidene with the halogen bond donor CF3I has been studied by matrix isolation spectroscopy. The carbenes were produced by photolysis of the corresponding diazo compounds, matrix-isolated in argon doped with 1% CF3I at 3 K. Bimolecular reactions between the carbenes and CF3I were induced by annealing these matrices to 25−30 K to allow for the diffusion of trapped species. Instead of classical halogen-bonded complexes, these carbenes form complexes in which the iodine atom is shared between the carbene center and the CF3 group. Photolysis of the complexes at 3 K yields radical pairs, which reversibly react back to the complexes when the matrices are warmed to 25−30 K.

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INTRODUCTION Halogen bonding between a halogen bond donor such as iodopentafluorobenzene C6F5I or iodotrifluoromethane CF3I and a halogen bond acceptor has been recognized as being the hydrophobic equivalent of hydrogen bonding with comparable bond strengths.1−6 Typical hydrogen bond acceptors, such as carbonyl groups or amines, in general, also form strong halogen bonds with good halogen bond donors. For decades, halogen bonding was an important motif in crystal engineering,2−6 and more recently, it was used to construct molecular capsules.7−9 Halogen bonding has been recognized as a novel principle for the design of organocatalysts.10−12 Closed-shell singlet carbenes have a lone electron pair at the carbene center and are therefore highly basic molecules which should be able to act as strong halogen bond acceptors.

Scheme 1. Reaction of Diphenylcarbene 1 with CF3I

of NHCs considerably.18 Consequently, T-1 is a poor hydrogen or halogen bond donor, but S-1 forms both strong hydrogen19,20 and halogen bonds.17 The stabilization of S-1 with H2O or CH3OH as hydrogen bond donors or with CF3I or CF3Br as halogen bond donors is large enough to result in singlet ground-state complexes, thus resulting in inversion of the spin state of 1 from T to S. The halogen-bonded complex 2a formed upon interaction of 1 with the strong halogen bond donor CF3I was found to be in equilibrium with a second structure 2b (Scheme 1).17 In 2b, the iodine atom is transferred to the carbene center, interacting with the CF3 fragment through a “reversed” halogen bond. This binding situation was first predicted by calculations and termed an “ion-paired” halogen bond.15 The unconventional halogen bond is only observed when the halogen bond is strong relative to the C−X bond dissociation energy (BDE) of the halogen bond donor. Because the C−Br bond is stronger

Although this was demonstrated for stable N-heterocyclic carbenes (NHCs) by Arduengo et al. 25 years ago13,14 and later confirmed in theoretical studies,15,16 halogen bonding toward reactive carbenes is much less investigated. We recently studied halogen bonding of the highly reactive diphenylcarbene 1 under the conditions of matrix isolation at cryogenic temperatures (Scheme 1).17 The triplet ground state T-1 of this carbene is nonpolar and exhibits typical radical reactivity. The closed-shell singlet state S-1 lies roughly 5 kcal/mol (gas phase) above the triplet and exhibits a reactivity that is described best as that of a 1,1-zwitterion: the charge distribution is highly unsymmetrical, and it is both a strong electrophile and nucleophile. The basicity of S-1 exceeds that © 2018 American Chemical Society

Received: May 24, 2018 Published: July 18, 2018 7586

DOI: 10.1021/acs.joc.8b01328 J. Org. Chem. 2018, 83, 7586−7592

The Journal of Organic Chemistry

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spectrum of 9 in SbF5 is in better agreement with a singlet ground state.28 Cyclopentadienylidenes as electron acceptors were studied by Janulis and Arduengo, who found that photolysis of diazo precursor 10 in p-chloro- or pbromotoluene (or other Lewis bases) leads to the formation of ylides 11 (Scheme 3).29 These ylides show 19F NMR resonances similar to that of the corresponding cyclopentadienyl anions. In contrast to 7 with a partial positive charge in the five-membered ring, leading to a pronounced antiaromatic character, ylide 11 shows a reversed charge distribution and thus exhibits an aromatic character. Here, we report the reaction of cyclopentadienylidene 5 and tetrachlorocyclopentadienylidene 6 with the halogen bond donor CF3I in low-temperature argon matrices.

than the C−I bond, only the classical complex was observed for the interaction of 1 with CF3Br under otherwise identical conditions.17 These experimental findings were complemented by high-level ab initio calculations, establishing a barrierless pathway leading from 1 in its triplet state to the products on the singlet surface. The formal insertion product 4 was observed in small quantities only, whereas photolysis of complexes 2a and 2b produced radical 3 and CF3 in high yields. Cyclopentadienylidene 5 and tetrachlorocyclopentadienylidene 6 are interesting cyclic carbenes because hydrogen as well as halogen bonding with the closed-shell singlet states as acceptors is expected to induce a partial transfer of positive charge toward the five-membered ring systems. This, in turn, should result in antiaromatic destabilization of the complexes. Therefore, we expect that the stabilizing complex formation competes with the destabilizing charge separation. Carbenes 5 and 6 had been previously isolated and spectroscopically characterized in inert gas matrices.21−23 Several bimolecular reactions of these carbenes were studied under the conditions of matrix isolation21−23 or by laser flash photolysis.24

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RESULTS AND DISCUSSION Matrix Isolation and Characterization. Carbenes 5 and 6 were generated by 450 nm photolysis of their matrix-isolated diazo precursors 12 and 16, respectively (Scheme 4). The IR

Scheme 4. Reaction of Cyclopentadienylidenes 5 and 6 with CF3I

Recently, we reported that the reaction of 6 with BF3 in argon matrices yields zwitterion 7, consisting of a cyclopentadienyl cation bound to a negatively charged BF3 fragment (Scheme 2).25 With a positively charged five-membered ring, 7 Scheme 2. Reaction of Tetrachlorocyclopentadienylidene 6 with BF325

spectrum of 5 is in accordance with IR data reported by Bell and Dunkin.30 Wasserman et al. had characterized the triplet state of 5 in organic glasses at 4 and 77 K by EPR spectroscopy and reported zero-field-splitting parameters D = 0.4089 cm−1 and E = 0.00160 cm−1, typical of triplet carbenes.31 Hoffmann and Borden et al. calculated the triplet state T-5 to lie 3.9 kcal/ mol below the lowest singlet state at the CASPT2(6,6)/ccpVTZ//CASSCF(6,6)/6-31G* level of theory.32 According to these calculations, the lowest lying singlet state of 5 is the open-shell singlet (1A2) state osS-5, which is 7 kcal/mol more stable than the nonplanar closed-shell singlet csS-5.32 Only the highly polar csS-5 is expected to act as strong acceptor in hydrogen or halogen bonds, whereas the much less polar osS-5, similar to T-5, is predicted to undergo only weak van der Waals interactions with hydrogen or halogen bond donors. As csS-5 is reported to lie approximately 11 kcal/mol above T-5,32 very strong hydrogen or halogen bonds are required to stabilize this state below the triplet state and thus switch the spin state of this carbene from triplet to singlet. The IR spectrum of carbene 6 is in good agreement with the IR spectrum previously reported by Bell and Dunkin.30 However, only a moderate agreement is found with an IR spectrum calculated at the B3LYP-D3/def2-TZVP level of

classifies as an antiaromatic molecule. Calculations of the geometry and of the magnetic properties of 7 confirmed the pronounced antiaromatic character of the zwitterion. Noteworthy, most cyclopentadienyl cations, including the tetrachloro derivative 9 (Scheme 3), were found by Breslow et al. to exhibit triplet ground states.26,27 In contrast, the IR Scheme 3. Cyclopentadienyl Cation 9 and Formation of Ylides 11

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theory (see below). In particular, an intense band at 1350 cm−1 is not reproduced by the calculation. The triplet state T-6 is calculated to be 7.4 kcal/mol more stable than the singlet S-6. In a theoretical study, Frenking et al. reported a smaller singlet−triplet gap of 2.2 kcal/mol at the BP86/def2-TZVPP level of theory,33 which agrees with our recent coupled cluster calculations giving 3.7 kcal mol−1.25 Nevertheless, because the IR spectra of carbene 5 and of the proposed reaction products of both carbenes are nicely reproduced with B3LYP-D3/def2TZVP, all calculations were performed at this level of theory to keep the calculated data consistent. Attempts to record a continuous wave (cw) X-band EPR spectrum of matrix-isolated 6 failed, which is attributed to a large broadening of the signals. A Q-band pulse EPR spectrum obtained upon photolysis of precursor 16 in solid MTHF at 5 K shows the characteristic signals of a triplet carbene with zerofield-splitting parameters of D = 0.542 cm−1 and E = 0.01 cm−1 (see the Supporting Information). Both carbenes 5 and 6 were also synthesized in argon matrices doped with 0.5−1% water at 3 K. These matrices were subsequently annealed at temperatures between 20 and 30 K to allow the water molecules to diffuse in the solid argon matrices. In similar experiments, diphenylcarbene T-1 readily reacts with water or methanol to give hydrogen-bonded complexes of the singlet carbene S-1.19,20 With 5 and 6, interaction with water results in only small shifts of some of the IR bands, characteristic of the formation of weak complexes of the triplet carbenes. Apparently, hydrogen bonding with water molecules does not suffice to stabilize the closed-shell singlet states of carbenes 5 and 6 below the corresponding triplet states. Reactions with CF3I. Annealing of an argon matrix containing T-5 and 1% of CF3I at 25 K allows the diffusion of CF3I, which results in a rapid decrease of all IR bands assigned to the carbene. Several new bands are formed with the most intense absorption at 998 cm−1 (Figure 1a). By comparison with the calculated spectra of various possible products, the main product was identified as singlet species 13 (Scheme 4 and Figure 1b). In 13, the iodine atom is located closer to the carbene center (2.0 Å) than to the CF3 fragment (2.4 Å) with a C−I−C angle of 108° (B3LYP-D3/def2-TZVP, Figure 2). Thus, the structure of this species is very different from a halogen-bonded complex between carbene 5 and CF3I. The CF stretching vibrations of the CF3 fragment are sensitive to the pyramidalization and thus to the charge of the CF3 fragment.17 In CF3I, the A1 symmetrical CF stretching vibration is found at 1064 cm−1, and the degenerate E symmetrical vibration is found at 1175 cm−1. In 13, these vibrations are only slightly shifted to 998 and 1157 cm−1, respectively, indicating little buildup of positive charge in the CF3 fragment. Carbene 6 shows an analogous behavior when reacting with CF3I (Figure 1e). Annealing an argon matrix containing carbene 6 and doped with 1% of CF3I results in the formation of singlet complex 17. The calculated structure of 17 is very similar to that of 13, with the CF3 fragment oriented toward the iodine at an angle of 105°. In the UV−vis spectrum, a broad feature at 460 nm is formed upon annealing and attributed to 17 (see the Supporting Information). The singlet complexes 13 and 17 are rather photolabile, and irradiation with red light (650 nm) rapidly bleaches all bands assigned to the complexes, whereas two intense bands at 1244 and 1249 cm−1 of the CF3 radical are formed (Figure 1d,g).

Figure 1. IR difference spectra showing the reaction of cyclopentadienylidenes 5 and 6 with CF3I in argon. (a) Difference spectrum obtained after annealing of a matrix containing 5 and 1% of CF3I to 25 K for 10 min. (b) Calculated spectra of 5 and 13. (c) Difference spectrum obtained after 650 nm photolysis of a matrix containing 13 for 5 min. (d) Calculated spectra of 13 and 14. (e) Difference spectrum obtained after annealing of a matrix containing 6 and 1% of CF3I to 25 K for 10 min. (f) Calculated spectra of 6 and 17. (g) Difference spectrum obtained after 650 nm photolysis of a matrix containing 17 for 5 min. (h) Calculated spectra of 17 and 18. Calculations were carried out at the B3LYP-D3/def2-TZVP level of theory. In the difference spectra, bands decreasing in intensity are going downward and bands increasing upward.

Figure 2. Optimized geometries of complexes 13 and 14 at the B3LYP-D3/def2-TZVP level of theory. Energies (in kcal/mol) are given relative to noninteracting 5 + CF3I.

The formation of the CF3 radical was found previously in the reaction of 1 with CF3I.17,34 The photoproducts of complexes 13 and 17 are therefore assigned to the triplet radical pairs 14 and 18, respectively (Scheme 4), which was confirmed by comparison with computed IR spectra (Figure 1c,f). Interestingly, subsequent annealing of the matrix restores the signals of the singlet complexes 13 and 17 (see the Supporting Information), whereas the radical recombination to give the formal CI insertion products 15 and 19, which is calculated to be highly exothermic by 65 and 58 kcal/mol, respectively, is not observed. Presumably, the recombination requires larger movements of the radical species, which are suppressed by matrix cage effects. In contrast, the singlet complexes 13 and 7588



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cyclopentadiene (cp) rings being −0.57 and −0.68 and the CF3 fragments being −0.13 and −0.08 (see the Supporting Information). Thus, the CF3 fragments are almost neutral, whereas the iodine atoms are strongly positively polarized and the cp rings are strongly negatively polarized. This asymmetric charge distribution between the two carbon atoms adjacent to the iodine causes the I−C(cp) bond to be much shorter than the I−C(CF3) bond. On the triplet PES of the system 5 + CF3I, the radical pair 14 is found to be the minimum structure with a C−I distance of 2.1 Å (Figure 4). The energy of this radical pair does not

17 and the corresponding triplet radical pairs 14 and 18 are calculated to be energetically similar. These species interconvert by sequential irradiation and annealing steps of the matrix (Figure 2). The reaction of 5 and 6 with CF3I was also investigated by EPR spectroscopy (Figure 3). Photolysis of precursor 12 in

Figure 3. EPR spectra showing the reaction of 5 with CF3I. Red line: Spectrum of 5 in argon doped with 1% of CF3I at 4 K. Solid black line: Spectrum of the same matrix after annealing to 25 K for 10 min. Dashed black line: Spectrum obtained after 650 nm irradiation for 10 min. Inset shows the radical region together with a simulated spectrum of CF3 radicals.35

CF3I-doped argon gives rise to the characteristic triplet signals of 5 with zero-field-splitting parameters of D = 0.420 cm−1 and E = 0.012 cm−1.31 Additionally, signals around 3400 G are assigned to the formation of CF3 radicals by their characteristic hyperfine couple splitting.17,35 Annealing of the matrix diminishes the signals of the triplet, as expected for the formation of the EPR-silent singlet complex 13. Concomitantly, the CF3 signals decrease, indicating the thermal recombination to give a singlet species. Subsequent photolysis with 650 nm restores the signals of CF3, in agreement with the formation of radical pair 14 as observed by IR spectroscopy. Whereas carbene 6 does not show observable signals in the cw EPR spectrum, the characteristic EPR signals of the CF3 radicals are observed after annealing/irradiation sequences analogous to those which resulted in the formation of radical pair 18 in the IR experiments. DFT Calculations. The reaction pathways of carbenes 5 and 6 with CF3I on the lowest energy singlet and triplet potential energy surfaces (PES) were explored at the B3LYPD3/def2-TZVP level of theory. The first step is the formation of weakly bound complexes between the triplet carbenes and CF3I. Bond formation between the iodine atom and the carbene center and intersystem crossing to the singlet PES results in the formation of the singlet species 13 and 17. These reactions are considerably exothermic by 20 and 23 kcal/mol, respectively. The CF3−I bond distance in 13 and 17 is 2.4 Å longer than that in CF3I (2.2 Å), indicating a substantially weakened C−I bond in the complexes. However, the orientation of the CF3 group suggests that bond character remains, making the iodine formally hypervalent. At the same time, the calculated C−I bond distances of 2.0 Å of the C5H4I and C5Cl4I fragments are shorter than that of CF3I and C5X4I− anions (2.1 Å for both X = H and Cl). The NBO charges of the iodine atoms in 13 and 17 are calculated to be +0.70 and +0.75, respectively, with the

Figure 4. Relaxed potential energy scans along the C−I distance and the C−I−C angle of 5 and CF3I. The triplet (red-yellow) and singlet (purple-blue) surfaces were calculated at the B3LYP-D3/def2-TZVP level of theory. Energies are relative to the minimum on the singlet PES.

strongly depend on the C−I−C angle. For large C−I−C bond angles, the triplet surface lies well below the singlet surface; however, with decreasing bond angles, the singlet and triplet PES are closer in energy. Thus, intersystem crossing to the singlet surface becomes feasible at small C−I−C bond angles. On the singlet PES, 13 corresponds to a minimum energy structure, which is predicted to be formed after ISC from the triplet surface. The difference in energy of structures 13 and 14 is only 1.4 kcal/mol, and their interconversion requires relatively small geometric changes, in agreement with the experimental findings. For carbene 6, a halogen-bonded singlet complex 20 could be located on the singlet PES, in addition to complex 17 (Figure 5). Complex 20, which is not observed experimentally, is predicted to be higher in energy than the noninteracting triplet 6 + CF3I by 0.6 kcal/mol. In contrast to 17, 20 shows a linear C−I−C angle and C−I bond distances of 2.7 and 2.2 Å and thus can be classified as a “conventional” halogen-bonded complex.17 It is interesting to compare the binding situation in the two singlet species 17 and 20: In the halogen-bonded 7589



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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

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General Procedures. Diazocyclopentadiene36 12 and diazotetrachlorocyclopentadiene37 16 were prepared by published methods.25 Matrix isolation experiments were performed by standard techniques using two-staged closed-cycle helium cryostats (cooling power 1 W at 4 K) to obtain temperatures around 3 K. The matrices were generated by co-deposition of 12 or 16 with a large excess of argon containing 1% of CF3I on top of a cold CsI window at 3 K. The corresponding carbenes 5 and 6 were generated by photolysis with a LED source at λ = 450 nm. Computational Details. All calculations were performed using Becke’s three-parameter hybrid functional,38 the correlation functional of Lee, Yang, and Parr,39 and Grimme’s dispersion correction40 (B3LYP-D3) and the def2-TZVP basis set41 as implemented in Gaussian 09.42 Geometries were visualized using CYLview.43 Experimental EPR spectra were analyzed using the Easyspin program packet.44

Figure 5. Optimized geometries of singlet complexes 17 and 20 at the B3LYP-D3/def2-TZVP level of theory with selected structural parameters. The electrostatic potentials (top view, 0.003 a −3 isodensity surface, potential from −12 (red) to 31 (blue) kcal/mol) are shown. Energies (kcal/mol) are relative to the noninteracting 6 + CF3I.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b01328.

complex 20, the iodine interacts with the lone pair of the closed-shell singlet carbene csS-5 through its electron-deficient region, leading to a positive partial charge in the cp ring and large deviations from planarity (Figure 5). This is in contrast to the binding situation in ylide 17, where the CF3 fragment is interacting with the electronegative region of the iodine atom, indicating a negative partial charge in the cyclopentadiene ring, in line with the planar geometry. Plots of the electrostatic potential of 17 and 20 underline the differences in partial charge between these two structures (Figure 5).

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Spectroscopic and computational data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Stefan Henkel: 0000-0003-3362-8825 Thomas Lohmiller: 0000-0003-0373-1506 Anton Savitsky: 0000-0002-6505-9412 Elsa Sanchez-Garcia: 0000-0002-9211-5803 Wolfram Sander: 0000-0002-1640-7505

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CONCLUSION Closed-shell singlet carbenes can be considered as 1,2zwitterions, and their reactivity is governed by the presence of both an electrophilic and a nucleophilic center at the same carbon atom. For the cyclopentadienylidenes 5 and 6, the zwitterionic closed-shell singlet states csS-5 and csS-6 are destabilized due to the antiaromatic character of the positively polarized five-membered ring. This destabilization prevents the formation of the halogen-bonded singlet carbenes upon interaction with CF3I. Instead, electron bond donation to the carbene center is observed, resulting in the formation of the ylides 13 and 17 with more favorably negatively polarized ring systems. The aromatic stabilization associated with the formation of a negatively polarized cyclopentadiene ring in 13 and 17 balances the buildup of a positive charge at the iodine atom, which substantially elongates the bond between the iodine and the CF3 fragment. The ylides 13 and 17 are stable at low temperatures but rearrange photochemically to the corresponding triplet radical pairs 14 and 18. This rearrangement is mildly endothermic by 0.7 and 1.4 kcal/mol−1, respectively. Although the radical pairs are stable as long as the matrix temperature is kept below 10 K, annealing at higher temperatures results in radical recombination and intersystem crossing back to 13 and 17. The extremely weak I−C(CF3) bonds in the ylides should make these species capable CF3 transfer reagents. In summary, whereas the cyclopentadienylidenes 5 and 6 strongly interact with CF3I, the expected stabilization of the closed-shell singlet carbenes via halogen bonding is not observed. Instead of acting as nucleophiles, the carbenes act as electrophiles in their reaction with CF3I.

Present Addresses #

Computational Biochemistry, Universität Duisburg Essen, 45141 Essen, Germany. ⊥ Berlin Joint EPR Lab, Institute for Nanospectroscopy, Helmholtz-Zentrum Berlin fü r Materialien and Energie, 12489 Berlin, Germany. ∥ Lehrstuhl für Experimentelle Physik III, TU Dortmund, 44221 Dortmund, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Cluster of Excellence RESOLV (EXC 1069) funded by the Deutsche Forschungsgemeinschaft (DFG). S.H. thanks the DFG for a Postdoctoral Research Return Fellowship. E.S.-G. acknowledges a Plus-3 grant by the Boehringer Ingelheim Foundation.

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REFERENCES

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