Redox-Gated Tristable Molecular Brakes of Geared Rotation - The

Article pubs.acs.org/joc

Redox-Gated Tristable Molecular Brakes of Geared Rotation Ting Tseng,† Hsiu-Feng Lu,‡ Chen-Yi Kao,† Chun-Wei Chiu,† Ito Chao,*,‡ Chetti Prabhakar,∥ and Jye-Shane Yang*,† †

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan ∥ Department of Chemistry, National Institute of Technology, Kurukshetra 136119, India ‡

S Supporting Information *

ABSTRACT: p-Bis(arylcarbonyl)pentiptycenes 2 (aryl = 4(trifluoromethyl)phenyl) and 3 (aryl = mesityl) have been prepared and investigated as redox-gated molecular rotors. For 2, rotations about the pentiptycene−carbonyl bond (the α rotation) and about the aryl−carbonyl bond (the β rotation) are independent, and the rotation barriers are 11.3 and 9.5 kcal mol−1, respectively, at 298 K. In contrast, the α and β rotations in 3 are correlated (geared) in a 2-fold cogwheel pathway between the aryl and the pentiptycene groups with a much lower rotation barrier of 6.5 kcal mol−1 at 298 K in spite of the bulkier aryl groups. Electrochemical reduction of the neutral forms led first to radical anions (2•− and 3•−) and then to a bis(radical anion) for 22− but a dianion for 32−. The redox operations switch the independent α and β rotations in 2 into a geared rotation in both 2•− and 22− and result in a slow−fast−stop rotation mode for 2−2•−−22−. The two redox states 3•− and 32− retain the geared α and β rotations and follow a fast−slow−stop mode for 3−3•−−32−. Both molecular systems mimic tristable molecular brakes and display 8−9 orders of magnitude difference in rotation rate through the redox switching.

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INTRODUCTION Molecular switches are (supra)molecular systems that undergo reversible changes in mechanical, electronic, catalytic, or magnetic properties in response to external stimuli such as temperature, pressure, chemicals, light, and voltage. 1−3 Molecular mechanical switches, including molecular rotors, molecular shuttles, and conformational switches, focus on the control of the rotational or translational motions of one or more subunits.2−10 Electrochemically driven systems are particularly interesting because they could be integrated with other molecular electronics and readily connected to the macroscopic world.3,6−9 Moreover, those having multiple redox states could perform motional modes beyond the common on− off switching.7,9 While electrochemical molecular shuttles and conformational switches have been well demonstrated,3,5−7 the corresponding electrochemical molecular rotors are relatively rare.8,9 We recently reported an electrochemical molecular rotor (1) based on the redox activity of p-phenylenediamine (PDA), in which the two N-aryl-N-methylamino groups serve as the rotators and the rigid H-shaped pentiptycene scaffold11 serves as the stator (Figure 1).9 Because of the steric and electronic interplay between the rotator and the stator, the rotation rate strongly depends on the redox states of the PDA moiety. The resulting slow−fast−stop sequence on going from the neutral to the radical cation and to the double radical cation form mimics a tristable molecular brake. In conjunction with our © 2017 American Chemical Society

continued interests in pentiptycene-derived molecular switches,9,10 the intriguing behavior of 1 has prompted us to search for novel redox-gated multistate molecular rotors/brakes that contain the pentiptycene scaffold. In this context, the tristable redox activity and stability of pdibenzoylbenzene (DBB)12 and the potential aryl−aryl correlated (geared) rotations13,14 of its derivatives have attracted our attention. The parent DBB system displays excellent redox stability in both radical anionic and dianionic states under applied electrical potentials (Figure 2a). The redox potentials of DBB could be tuned by substituents: namely, electron-withdrawing groups lower the redox potentials, but the opposite is true with electron-donating substituents. When the substitutions are sterically demanding such as the case of 1,4bis(mesitoyl)durene (BMD), the rotation about the durenyl− carbonyl Cdu−CCO (α) bond is no longer independent of that about the mesityl−carbonyl Cme−CCO (β) bond; instead, the central and terminal aryl rings undergo a 2-fold disrotatory geared rotation (Figure 2b). Despite these intriguing properties, the concept of redox-gated tristable molecular brakes or gears with DBB or its derivatives has not been demonstrated. Herein we report the synthesis, redox properties, syn−anti conformational switching, and α-/β-rotation behavior of two pentiptycene-derived DBB analogues 2 and 3 (Chart 1). Our Received: March 29, 2017 Published: April 25, 2017 5354

DOI: 10.1021/acs.joc.7b00729 J. Org. Chem. 2017, 82, 5354−5366

Article

The Journal of Organic Chemistry

Figure 1. p-Phenylenediamine (PDA)-based redox-gated slow−fast−stop tristable molecular brake (1), in which the central pentiptycene scaffold is the stator and the two anilino groups are the rotators.

Figure 2. (a) Structures of redox states of p-dibenzoylbenzene (DBB) in the neutral and reduced forms and (b) the anti → syn interconversion of 1,4-bis(mesitoyl)durene (BMD) through a correlated rotation about durenyl−carbonyl (the α rotation) and carbonyl−mesityl (the β rotation) bonds. The two ortho-methyl groups are colored for clarity of the geared rotation.

compounds has failed because the reactions led to a mixture of unidentified products. Alternatively, we used an excess amount of DIBAL to convert 5 to the diformylpentiptycene 6 followed by reaction with the ArLi reagents to provide the dihydroxyl intermediates 2a and 3a. Three possible stereoisomers (RR, SS, and RS) are expected for these intermediates, but no attempt was made to isolate the stereoisomers, as the next step of oxidation reactions would remove the chirality. The intermediate 2a can be readily oxidized by Jones reagent to afford the target system 2 with 81% yield, but decomposition occurred for 3a under the same conditions. This problem was resolved by using the relatively milder oxidizing reagent DMP, although the yield of 3 was unsatisfactory (34%). Syn−Anti Conformation. The bulky pentiptycene scaffold in 2 and 3 would twist the DBB π-backbone and result in two possible orientations, syn and anti, for the two arylcarbonyl groups with respect to the pentiptycene central ring (Figure 3). DFT calculations at the BMK/6-311+G(d,p)//B3LYP/631G(d) level17 with the consideration of CH2Cl2 solvation using the solvation model based on density (SMD)18 predicted that the optimized anti conformer is slightly more stable by 1.19 and 0.15 kcal mol−1 than the corresponding optimized syn conformer for 2 and 3, respectively. The anti conformer of 2 is indeed observed in the crystals obtained from mixed dichloromethane and hexane solutions (Figure S1 and Tables S1 and S2) and dominated in the CD2Cl2 solutions at low temperatures (vide infra). For comparison, the anti form of BMD was

Chart 1

results show that the α and β rotations are independent in 2 but correlated in 3. In addition, the α rotation follows the slow− fast−stop sequence for 2 on going from the neutral state to the first cathodic and to the second cathodic redox states, but it is in a fast−slow−stop mode in the case of 3. The steric and electronic origins of these observations are also discussed.

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RESULTS AND DISCUSSION Synthesis. The previously reported15 pentiptycene building block 4 provides a good starting point for the synthesis of the target molecular switches 2 and 3 (Scheme 1). The Rosenmund−von Braun reaction16 of 4 with CuCN afforded the dicyano-substituted pentiptycene 5 with 96% yield. An attempt to react 5 with the ArLi reagents toward the target 5355


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The Journal of Organic Chemistry Scheme 1. Synthesis of 2 and 3

of the more polar syn form, but the crystals from chloroform solutions adopted the anti form.14 The syn−anti conformational exchange in 2 and 3 is evidenced by variable-temperature (VT) 1H NMR spectroscopy. The presence of only one set of signals in the 1H NMR spectra of both 2 and 3 at ambient temperature indicates a fast interconversion of these two conformers (Figure 4). However, at 183 K some of the signals for 3 are significantly broadened, indicating a syn−anti interconversion rate approaching the NMR time scale (milliseconds), and the proton signals are split into two sets in the case of 2, revealing a syn−anti interconversion slower than the NMR time scale. The syn-toanti ratio is 1:2.5 for 2 at 183 K according to the intensity of the bridgehead proton signals (2 and 2′ in Figure 4a). This corresponds to an energy difference of 0.33 kcal mol−1 for the two conformers, which is somewhat lower than the prediction of DFT calculations (vide supra). The peak assignments were carried out with the support of 2D NMR spectroscopies, including ROESY, NOESY, COSY, HSQC, and HMBC (Figures S2−S6), and the corresponding atom labels are shown in Figure 3.

Figure 3. Anti and syn conformers of 2 and 3 interconverted by rotation about the carbonyl−peniptycenyl (α) bond. The OC−C C dihedral angles φPp (red) and φAr (blue) and the CPp−C(O)−CAr bond angle θ (green) are for the discussion of molecular conformations, and the numerical labels are for the discussion of VT 1 H and 13C NMR spectra.

also reported to be more stable by 0.36 kcal mol−1 than the syn isomer, but the solid-state conformation depends on the solvent used for crystallization: ethanol solutions led to crystals

Figure 4. VT 1H NMR (500 MHz, CD2Cl2) spectra of the selected region for (a) 2 and (b) 3 from 283 to 183 K with an interval of 20 K. 5356


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The Journal of Organic Chemistry DFT structure optimizations have been carried out both in the gas phase and in solution (CH2Cl2) using the SMD at the B3LYP/6-31G(d) level. The structural details defined by the CPp−C(O) (dPp) and CAr−C(O) (dAr) bond lengths, the pentiptycene−carbonyl CPp−CPp−C−O (φPp) and the arene− carbonyl CAr−CAr−C−O (φAr) dihedral angles, and the CPp− C(O)−CAr bond angle θ (Figure 3) for the DFT-optimized anti- and syn-conformers of 2 and 3 are listed in Table 1. For Table 1. Structural Parameters for 2 and 3 in the Gas Phasea, CH2Cl2b and Crystalc compd 2

conformation a

syn-DFT anti-DFTa syn-DFTb anti-DFTb anticrystal 1 anticrystal 2

3

syn-DFTa anit-DFTa syn-DFTb anti-DFTb

dPpd

dArd

φPpe

φArf

Θg

1.509 1.509 1.511 1.514 1.500 1.500 1.510 1.510 1.510 1.510 1.512 1.512

1.501 1.500 1.498 1.497 1.499 1.489 1.489 1.489 1.504 1.504 1.501 1.502

−57.8 117.6 −65.2 74.9 111.7 100.9 92.3 92.3 −129.7 130.3 −126.2 48.9

165.2 −13.0 166.3 8.1 9.7 3.6 15.0 15.0 −133.4 −42.4 −134.8 41.5

119.7 119.6 119.6 119.5 119.4 117.9 118.3 118.3 121.7 121.6 122.1 122.2

a

From DFT calculations in the B3LYP/6-31G(d) level in the gas phase. bFrom DFT calculations at the SMD/B3LYP/6-31G(d) level in CH2Cl2. cFrom X-ray crystal structure. dCPp−C(O) and CAr−C(O) bond lengths (in Å). eThe CPp−CPp−C−O dihedral angle (in degree). f The CAr−CAr−C−O dihedral angle (in degree). gThe CAr−C(O)− CPp bond angle (in degree).

Figure 5. Cyclic (black) and differential pulse (red) voltammograms of (a) DBB, (b) 2, and (c) 3 in DMF with 0.1 M Bu4NPF6 at the scan rate of 100 mV s−1.

comparison, Table 1 also includes the corresponding data for the anti form of 2 in the X-ray crystal structure, which consists of two crystallographically independent noncentrosymmetrical conformers. The calculated gas-phase structural parameters for 2 resemble those of one of the X-ray conformers (labeled as anticrystal 1 in Table 1), and the solution ones are closer to the other conformer (anticrystal 2). A comparison of 2 and 3 reveals that the d and θ values are similar, but the φPp and φAr differ to a significant extent. For 2, the aryl groups are almost in the same plane of the carbonyl group (φAr = 8.1°), and the carbonyl groups are twisted by ∼75° (φPp = 74.9°) from the pentiptycene central ring, indicating that the carbonyl groups have stronger electronic coupling (conjugation) interactions with the aryl vs pentiptycene groups. Such a preference in arene−carbonyl conjugation interactions might reflect the steric hindrance between the carbonyl and the bulky pentiptycene scaffold. In contrast, the pentiptycene−carbonyl and arene− carbonyl interplanar angles in 3 are similar, which are ∼49° and ∼42° (φPp = 48.9°; φAr = 41.5°), respectively. This could be attributed to the presence of o-methyl substituents in the mesityl groups, which raises steric hindrance to an extent similar to that exerted by the pentiptycene unit. Redox Properties. The redox behavior of 2 and 3 has been investigated with cyclic voltammetry (CV), differential pulse voltammetry (DPV), and spectroelectrochemistry. For comparison, the corresponding studies on DBB were also performed under the same experimental conditions. Figure 5 shows the CV and DPV of DBB, 2, and 3 in DMF (1 mM) with the redox potential relative to the ferrocene/ ferrocenium (Fc/Fc+) redox couple. Each voltammogram displays two reversible cathodic waves of the same current

density in the potential scan up to −2.5 V. The first reduction potential is negatively shifted on going from DBB (−1.41 V) to 2 (−1.83 V) and to 3 (−1.99 V), which correlates well with the relative planarity of their DBB backbone: namely, the more planar is the DBB backbone, and the lower is the reduction potential. The electronic and steric nature of the pentiptycene group might also play a role in increasing the reduction potential for 2 and 3 relative to DBB because the π-electron polarizability is decreased for the central ring of pentiptycene relative to an unsubstituted phenylene,19 and the bulky pentiptycene group could lengthen the distance and thus retard the heterogeneous electron transfer between the electrode and the substrate.20 The electron-withdrawing CF3 groups in 2 and the electron-donating CH3 groups in 3 should also contribute to the positive shift of the first reduction potential for 2 vs 3. Another feature of the voltammograms is the difference between the first and the second reduction potentials (ΔE), which is much larger for DBB (0.48 V) than for 2 (0.18 V) and 3 (0.17 V). This is again attributable to the reduced planarity of the π-backbone in 2 and 3 such that the electronic coupling between the two carbonyl redox centers is diminished. All three systems display high redox stability as evidenced by the unchanged CV profiles after multiple scans, which secures the robustness of the redox gating on the rotation kinetics. The spectroelectrochemistry provides insights into the electronic character of the redox states of 2 and 3. The twostage changes in absorption spectra for the parent DBB at an incremental increase of applied potentials from −1.38 to −2.04 V are shown in Figure 6a. In the first stage of potential scan from −1.38 to −1.44 V, which corresponds to the first 5357


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generated 22− and 32− with an excess amount of sodium metal in d8-THF for 1H NMR characterization (Figure 7). Whereas

Figure 7. 1H NMR (400 MHz, d8-THF) spectra of (a) 22− and (b) 32− generated by sodium metal reduction of 2 and 3, respectively, at 298 K.

the proton spectrum of 22− displays sharp solvent peaks with nearly flattened spectra in the expected region of compound signals, sharp signals of 32− attributable to a mixture of the syn and anti conformers were observed. This is consistent with the conclusion of spectroelectrochemistry that 22− is dominated by a di(radical anion) character, but it is mainly a quinoidal dianion character for 32−. It is also noted that the signals for the methyl groups at 2.2−2.4 ppm for 32− are split into two peaks, albeit poorly resolved, which is consistent with a “stop” state for the internal rotations predicted by the DFT calculations (vide infra). SMD/B3LYP/6-31G(d) calculations also support the assignment of a di(radical anion) character for 22− and a simple dianion character for 32−. As shown in Figure 8, the HOMO of

Figure 6. Spectroelectrochemistry of (a) DBB, (b) 2, and (c) 3 in DMF, in which the labels are potential values relative to the redox couple of Fc/Fc+. Black and red curves are for the first and the second stages of potential scans, respectively.

reduction process, the intensity of peaks at 264, 372, and >800 nm was increased (black curves). When the applied potential continued to increase from −1.44 to −2.04 V, at which the second reduction process occurs, the 372 and >800 nm bands faded away, and a new broad band at 684 nm was developed (the second stage, red curves). It is known that the first and the second reduction processes of DBB correspond to the formation of radical anion (DBB•−) and dianion (DBB2−), respectively.12 This observation is consistent with that a dianion species absorbs at shorter wavelength than the corresponding radical anion.21 The spectroelectrochemistry of 2 behaves differently (Figure 6b), in which the first stage of absorption spectra (black curves) recorded under a potential scan from −1.82 to −1.98 V underwent an intensity growth at 258, 347, 463, and 674 nm, and at the second stage of potential scan from −1.98 to −2.18 V the spectral intensity continued to increase at 258 and 674 nm but became decreased at 347 and 463 nm (red curves). Interestingly, the spectroscopic behavior of 3 resembles that of DBB more than that of 2; the spectra in the first stage of electrochemical reduction from −1.90 to −2.04 V led to new absorption bands at 259, 386, and >800 nm. In the second stage from −2.04 to −2.24 V the 386 nm band was broadened, and the near IR (>800 nm) band was bleached (Figure 6c). We thus concluded that the dianion 32− resembles DBB2− by having a quinoidal character (Figure 2a) for the pentiptycene central ring, but it is a di(radical anion) for 22−, in which the pentiptycene central ring retains largely a benzenoid character. Such a difference between 22− and 32− could be attributed to a larger carbonyl−pentiptycene steric interaction for 2 vs 3 (vide supra) that prevents the two carbonyl groups from being coplanar which is required for the quinoidal form. With the same dianion character for 3 and DBB, a blue shift of the spectra for 32− vs DBB2− again reflects the relative planarity of the π-system. Another evidence for a different electronic character of 22− vs 2− 3 is from the line shape of 1H NMR spectra. We chemically

Figure 8. SMD/B3LYP/6-31G(d)-derived frontier molecular orbitals and energy levels of 2 and 3 that show a nearly degenerate LUMO and LUMO+1 for 2 but not for 3.

both 2 and 3 is localized on the pentiptycene moiety, but their LUMO and LUMO+1 are concentrated in the molecular longaxis bis(arylcarbonyl)phenylene π-backbone, consistent with the location of the redox centers for reduction. However, a difference in LUMO and LUMO+1 between 2 and 3 is noted: contribution of the central phenylene relative to the two arylcarbonyl groups is increased in 3 vs 2. Consequently, the LUMO and LUMO+1 are nearly degenerate for 2 but have a significant gap for 3. This is consistent with a triplet di(radical anion) state of 22− but a singlet dianion state of 32− upon twoelectron reduction. 5358


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The Journal of Organic Chemistry Rotation Profiles of the Neutral State. The internal rotational behavior of 2 and 3 involves the rotations about the CPp−C(O) bond (the α rotation) and the CAr−C(O) bond (the β rotation) that are both of 2-fold symmetry. Knowing the relative kinetics of the α and β rotations would allow one to make conclusions on the relationship (correlated or noncorrelated) of these two rotation modes. In this context, the rotation kinetics of 2 has been investigated with VT NMR spectroscopy because the rotation kinetics in 2 falls in the time scale of NMR at 183−283 K (Figure 4). In principle, the NMR signal exchange associated with the syn and anti interconversion (Figure 9a) corresponds to the kinetics of α rotation and

analysis on the signals from the pentiptycene and aryl groups. The line-shape analyses for the α and β rotations were performed on the VT 13C NMR spectra by monitoring the quaternary carbon pair C1 and C1′ in the central pentiptycene ring and the carbon signals C9/C9′ and C11/C11′ in the trifluoromethylphenyl groups, respectively.22 In contrast, the method of NMR line-shape analysis on rotation kinetics could not be applied to 3 because the exchanging signals could not be resolved even at 173 K (Figure S7). As such, the rotation kinetics as well as the related structural details of 3 were investigated by DFT calculations. To justify the computational methods, the same calculations were also performed for 2, and the calculated and experimental data were compared. Figure 10 shows the experimental and simulated spectra of the C1/C1′, C9/C9′, and C11/C11′ signals of 2 in the range 183− 243 K with an interval of 10 K. The rate constant (k) shown for each trace corresponds to the signal exchange rate constant for the α (Figure 10a) or β (Figure 10b) rotations. Note that the unequal energy of the syn and anti conformers (e.g., ΔG ∼ 0.33 kcal mol−1 at 183 K) would lead to different barriers for the α (syn → anti) and α (anti → syn) rotations, and the k values reported in Figure 10a are for the α (anti → syn) rotation. The activation parameters for the α (anti → syn) and β rotations derived from Arrhenius and Eyring plots (Figure S8) are summarized in Table 2, and a full table that also contains the data for α (syn → anti) is provided as Table S2. The results show that at the same temperature the β rotation is always faster than the α rotation, revealing a noncorrelated rotation behavior. The free energy of activation at 298 K (ΔG‡(298 K)) is 11.3 ± 0.6 kcal mol−1 for the α (anti → syn) rotation and 9.5 ± 0.6 kcal mol−1 for the β rotation. The smaller preexponential value (log A = 10.8 vs 13.0) and more negative entropy of activation (ΔS‡ = −10.3 vs −0.2 cal mol−1 K−1) for the α vs β rotation indicates a larger constraint in the transition states of

Figure 9. Schematic representation of (a) the α rotation and (b) the β rotation that shows potential NMR signal exchange due to the syn (S)−anti (A) conformational exchange in the former and to the aryl group flip in the latter. The two faces of the aryl groups in (b) are shown in different color for clarity of aryl group flip.

that with the flip of the aryl group (Figure 9b) to the β rotation. Therefore, the rotation rates could be retrieved via line-shape

Figure 10. VT 13C NMR (125 MHz, CD2Cl2) spectra (left) and line-shape fitting spectra (right) of 2 in the region of (a) the C1, C1′ pair from 193 to 243 K and (b) the C9, C9′ and C11, C11′ pairs from 183 to 243 K; the corresponding 2-fold exchange rate constants are given for each trace. 5359


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(Figure 11a). Likewise, a decremental variation of one φAr followed by structural optimization provides the β-rotational profile. It should be noted that the β-rotation could occur independently in the syn and anti conformers, and thus each case has been evaluated separately. As expected, the results show similar rotation profiles and energetics for the β rotation in either the syn or the anti form, and the case of the latter is shown in Figure 12a for representation. The relative energies of the rotational ground and transition states in CH2Cl2 were further refined at the SMD/BMK/6-311+G(d,p)//SMD/ B3LYP/6-31G(d) level of theory, and the results are depicted in Figure 11b and 12b. Shown in Figures 11c and 12c are the corresponding molecular structures and refined energies. A different structural view of the lowest ground state (GS) and the highest transition states (TSα, TSβ(anti), and TSβ(syn)) for inspection is provided in Figure S9. Note that the transition states D‡ (12.09 kcal mol−1) and H‡ (12.30 kcal mol−1) in Figure 11c are supposed to be mirror images (enantiomers) of the same energy. However, minor energy variations caused by different dispositions of the trifluoromethyl groups were not explored in the scan, so the energy of equivalent structures may be somewhat different. As such, the lower energy species D‡ is adopted as the TSα. Likewise, the ground states C (0.00 kcal mol−1) and Gb (0.05 kcal mol−1) in Figure 11c should be equivalent, and the lower energy species C is adopted as the GS. The agreement between the experimental and computational rotation barriers (ΔG‡) for both the α-rotation (11.3 ± 0.6 vs 12.1 kcal mol−1) and the β-rotation (9.5 ± 0.6 vs 9.8 (TSβ(syn)) kcal mol−1) justifies the functional and basis set adopted for the current study. Selected structural data defined by dPp, dAr, φPp, φAr, and θ for the GS, TSα, TSβ(anti), and TSβ(syn) of 2 are listed in Table 3. In GS (conformer C in Figure 11c), the carbonyl has better

Table 2. Kinetic Parameters for the α and β Rotations in 2 from Line-Shape Analysis of the VT 13C NMR Spectra in CD2Cl2 type of rotation

α (anti → syn)

β

nuclei of inspection Ea (kcal mol−1) log A ΔH‡ (kcal mol−1) ΔS‡ (cal mol−1 K−1) ΔG‡(298 K) (kcal mol−1) k(298 K) (s−1)

C(1,1′) 8.6 ± 0.3 10.8 ± 0.3 8.2 ± 0.3 −10.3 ± 1.6 11.3 ± 0.6 34666

C(9,11; 9′,11′) 9.8 ± 0.3 13.0 ± 0.3 9.4 ± 0.3 −0.2 ± 1.6 9.5 ± 0.6 715454

the former rotation. It is also noted that the NMR signals undergo a downfield shift upon increasing the temperature, indicating the presence of either intramolecular or intermolecular structural dependence on the temperature. Regarding the bulky pentiptycene scaffold and the lack of explicit directional intermolecular interactions, the shift is less likely a consequence of changes in intermolecular interactions. Instead, intramolecular conformational variation, particularly the spatial alignment between the sterically constrained aryl and pentiptycene groups, in response to temperature, could account for the shift. DFT calculations were performed to gain insights into the α and β rotations in 2. As the two arylcarbonyl rotators undergo independent rotations, the rotation profile was constructed based only on one of the two rotators to simplify the calculations. More specifically, the relaxed scan calculations at the SMD/B3LYP/6-31G(d) level started with conformer C, one of the local minimum, and were carried out by decrementally varying the dihedral angle φPp by 5° of one rotator followed by optimization of the system for each decrement to construct the α-rotational profile in CH2Cl2

Figure 11. (a) SMD/B3LYP/6-31G(d)-derived α-rotational potential energy surface diagram of 2 in CH2Cl2 by changing the φPp of one arylcarbonyl rotator for a 360° rotation process and (b) refined free energy diagram and (c) conformations and relative free energy values for the ground and transition states in (a) calculated at the SMD/BMK/6-311+G(d,p)//SMD/B3LYP/6-31G(d) level of theory. 5360


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Figure 12. (a) SMD/B3LYP/6-31G(d)-derived β-rotational potential energy surface diagram of 2 in CH2Cl2 by changing the φAr of one arylcarbonyl rotator for a 360° rotation process and (b) refined free energy diagram and (c) conformations and relative free energy values for the ground and transition states in (a) calculated at the SMD/BMK/6-311+G(d,p)//SMD/B3LYP/6-31G(d) level of theory.

Table 3. DFT-Derived Structural Parameters for the α- and β-Rotational Ground State (GS) and Transition States (TSα and TSβ) for 2 and 3 in the Neutral and Redox States compd 2

3

2−•

22−

3−•

32−

state

dPpa

dAra

φPpb

φArc

θd

ΔGe

GS TSα TSβ(anti) TSβ(syn) GS TSα TSβ(anti) TSβ(syn) GS(syn) TSα TSβ(anti) TSβ(syn) GS TSα TSβ(anti) TSβ(syn) GS TSα TSβ(anti) TSβ(syn) GS(syn) TSα TSβ(anti) TSβ(syn)

1.514 1.505 1.518 1.517 1.512 1.513 1.562 1.546 1.484 1.462 1.510 1.505 1.515 1.442 1.530 1.482 1.463 1.511 1.531 1.502 1.410 1.521 1.470 1.470

1.497 1.515 1.519 1.518 1.502 1.516 1.534 1.548 1.488 1.518 1.521 1.521 1.438 1.514 1.470 1.505 1.517 1.506 1.538 1.541 1.523 1.445 1.531 1.531

74.9 1.2 91.2 −82.4 48.9 1.1 82.5 −80.7 −42.3 0.7 113.8 −60.4 61.4 1.4 39.2 −27.8 150.5 −84.0 107.5 −58.1 2.4 −82.5 120.9 −46.8

8.1 84.1 85.5 84.9 41.5 83.4 92.0 81.3 −17.0 82.7 70.5 69.6 4.3 84.1 −23.3 55.2 −56.5 2.3 58.7 57.7 70.6 −0.6 38.0 36.2

119.5 124.5 118.9 118.5 122.2 124.6 123.0 123.0 121.2 124.9 120.6 119.8 120.9 125.4 130.5 124.6 123.2 123.9 123.5 122.4 122.8 124.9 123.6 123.6

0.00 12.09 10.41 9.81 0.00 6.49 24.58 23.04 0.00 7.99 8.28 8.21 0.00 19.02 23.67 24.52 0.00 13.39 34.23 26.64 0.00 18.71 34.81 35.84

CPp−C(O) and CAr−C(O) bond length (in Å). bThe CPp−CPp−C−O dihedral angle (in degree). cThe CAr−CAr−C−O dihedral angle (in degree). The CAr−C(O)−CPp bond angle (in degree). eThe free energy difference is relative to the GS of the same redox state at the SMD/BMK/6311+G(d,p)//SMD/B3LYP/6-31G(d) level. a

d

interactions with the pentiptycene stator. In TSα (conformer D‡ in Figure 11c), the carbonyl becomes coplanar with the

conjugation with the arene (φAr ≈ 8°) than with the pentiptycene (φPp ≈ 75°), and the arene has no explicit steric 5361


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Figure 13. Correlation plots of the dihedral angles φPp (black) and φAr (red) of 2 (a) when φPp is the varying group (VG) and φAr is the response group (RG) and (b) when φAr is the VG and φPp is the RG.

specifically, varying the φPp concomitantly changes the φAr to a similar extent in an opposite direction (i.e., disrotatory geared motion) and vice versa. To further investigate the cost for an independent β (β-ind) rotation (i.e., gear slippage), we located transition structures for both TSβind(syn) and TSβind(anti) (resembling transition states (e.g., Db‡) for the noncorrelated β rotation of 2 in Figure 12c) for 3. The resulting structures and energies are shown in Figure S13. The rotation barrier estimated in such a manner is as high as 23.0 kcal/mol for TSβind(syn) and 24.58 kcal mol−1 for TSβind(anti). A loss of carbonyl−peniptycene (φPp = −80.7°) and carbonyl−arene (φAr = 81.3°) conjugation stabilization and the steric hindrance between arene and the H-shaped pentiptycene scaffold are likely responsible for the high rotation barrier. For comparison, the geared transition state TSα (6.5 kcal/mol) features carbonyl−pentiptycene conjugation interactions (φPp = 1.1°). Rotation Profiles of the Redox States. Regarding that NMR spectroscopy could not be applied to the radical species 2•−, 22− (a triplet state), and 3•− and that the dianion species 32− has poorly resolved NMR signals (Figure 7), the rotation behavior of the redox states of 2 and 3 was investigated by DFT calculations with the same method and strategy as for the neutral forms. A summary of the calculated rotation barrier and the α-rotation rate under geared and independent rotation modes is shown in Table 4, and the φPp−φAr correlation plots for 2•−, 22−, 3•−, and 32− are shown in Figures S11 and S12. The data of Table 4 led to two important conclusions: first, the α and β rotations are geared for all four ionic species 2•−, 22−, 3•−, and 32−, because performing an independent enforced β rotation would encounter a much larger rotation barrier than performing a geared rotation. Consequently, redox switching

pentiptycene central ring (φPp ∼ 1.2°) and nearly perpendicular to the arene plane (φAr ≈ 84°), and the arene carbon has close contact (2.256 Å) with the bridgehead hydrogen of the Vshaped iptycenyl group. In TSβ (conformer Db‡ in Figure 12c), the carbonyl has poor conjugation with either pentiptycene (φPp ≈ 91°) or arene (φAr ≈ 86°), and there is no explicit steric hindrance between the arene plane and the pentiptycene stator. Therefore, it appears that the destabilization of TSα vs GS has a strong steric origin by the assumption that the carbonyl− pentiptycene conjugation is comparable to the carbonyl−arene conjugation; however, the destabilization of TSβ vs GS is mainly from loss of the carbonyl−arene conjugation interactions. Figure 13 shows the correlation plots between the dihedral angles φPp and φAr for 2. When φPp is decrementally variated (varying group, VG) by 5° for each step to accomplish a 360° variation of φPp (i.e., one cycle of α rotation), the response of the dihedral angle φAr (response group, RG) also undergoes a full 360° variation corresponding to one cycle of disrotatory β rotation (Figure 13a). However, when φAr is the VG (β rotation), the RG φPp (α rotation) is no longer correlated (Figure 13b). Since the β rotation has a lower barrier than the α rotation (vide supra), the noncorrelated mode would dominate the rotation behavior of 2, which is consistent with the experimental observations. A smaller rotation barrier for 3 vs 2 as indicated by VT NMR experiments (Figure 4) is counterintuitive, as the bulkier mesityl vs trifluorotolyl group was expected to encounter a larger steric hindrance during the α and β rotations. To evaluate the size of rotation barrier and to understand the origin of the low rotation barriers in 3, DFT calculations as described above for the case of 2 were also performed for 3. The resulting rotational profile and ground- and transition-state conformations and energies in CH2Cl2 for the α-rotation are shown in Figure S10, and φPp−φAr correlation plots for the αand β-rotation are provided in Figures S11 and S12. Selected structural data for the corresponding GS, TSα, and TSβ states are shown in Table 3. The calculated ΔG‡ value of 6.5 kcal mol−1 for the α rotation of 3 is indeed lower than that for 2 (12.1 kcal mol−1). However, there is only a subtle structural difference in the TSα of 2 and 3, indicating that the lower rotation barrier for 3 results from a less stable ground state rather than a more stable transition state. Since the GS of 2 and 3 differ mainly in φAr (8.1° vs 41.5°), the decrease in the carbonyl−arene conjugation interactions is likely to be responsible for destabilization of the GS state of 3 vs 2. The φPp−φAr correlation plots (Figures S11 and S12) show that the α rotation is correlated with the β rotation in 3. More

Table 4. DFT-Derived Rotational Free Energy Differencesa under α/β-Geared (ΔG‡gear) and β-Independent (ΔG‡βind)b Rotation Modes and the Corresponding α-Rotation Rate for 2 and 3 in the Neutral and Redox States at 298 K (kα298 K) compound

ΔG‡gear (kcal/mol)

ΔG‡βind (kcal/mol)

kα298 K (s−1)

2 2−• 22− 3 3−• 32−

12.1 8.0 19.0 6.5 13.4 18.7

9.8 8.2b 23.7b 23.0b 26.6b 34.8b

104 107 10−1 108 103 10−1

a

Using SMD/BMK/6-311+G(d,p)//SMD/B3LYP/6-31G(d) at 298 K. bThe independent β rotation barrier was estimated with transition structures resembling the conformations in Figure 12c. 5362


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Figure 14. DFT-derived conformations and relative free energies of the GS and TSα of 2, 2•−, 22−, 3, 3•−, and 32−.

Figure 15. DFT-derived rotational behavior of 32− that shows the “meet and follow” mechanism (A → E′‡ → C′ → D‡ → C and A′ → E‡ → C→ D‡ → C′) of the rear resting rotator. Blue and red arrows denote clockwise and counterclockwise rotation of the front working rotator (please follow the front carbonyl group). Black arrows denote the small concurrent oscillation of the front and rear rotators.

among 3, 3•−, and 32− retains gears for the α and β rotations, but switching between 2 (independent) and 2•− (geared) follows a clutch−declutch mechanism.23 Second, the α-rotation rate is highly dependent on the redox states for both 2 and 3. In the case of 2, the rotation barriers for 2, 2•−, and 22− are 12.1, 8.0, and 19.0 kcal mol−1, respectively, which corresponds to a relative rotation rate of 1:103:10−5 at room temperature. With the rotation rate on the order of 104 s−1 for 2 at room temperature according to VT 13C NMR (Table 2), the rotation rate would increase to 107 s−1 for 2•− and drop to 10−1 s−1 for 22−, corresponding to a slow−fast−stop sequence of rotary mechanical states in 2−2•−−22−. The decrease in rotation rate on going from 2•− to 22− is as large as 8 orders of magnitude. Unlike the slow−fast−stop sequence of rotation behavior for 2, the system 3 undergoes a fast−slow−stop sequence with a relative rotation rate of 1:10−5:10−9 for 3, 3•−, and 32−, respectively, on the basis of rotation barriers of 6.5, 13.4, and 18.7 kcal mol−1 in the redox state series. The estimated rotation rates are 108, 103, and 10−1 s−1 for 3: 3•−:32−, respectively, at 298 K. Molecular brakes 2 and 3 have a similar braking effect of 8−9 orders of magnitude from the highest to the lowest rotation rate, but the mode (slow−fast−stop vs fast−slow− stop) and rate constants (104−107−10−1 vs 108−103−10−1 s−1) are quite different.

To understand the relative rotation barriers of the two redox series, the conformations (Figure 14) and structural parameters (Table 3) of GS and TSα are analyzed. For the series 2−2•−− 22−, all three species display common features in GS and TSα, in which the carbonyl is conjugated with the arene in GS but with the pentiptycene in TSα. As judged by the bond lengths dPp and dAr, a larger stabilization of TSα (dPp: 1.505 → 1.462 Å) than GS (dAr: 1.497 → 1.488 Å) on going from 2 to 2•− might account for the decrease of the energy barrier. In contrast, a larger stabilization of GS (dAr: 1.488 → 1.438 Å) than TSα (dPp: 1.462 → 1.442 Å) on going from 2•− to 22− might be responsible for the increase of the energy barrier. For the series 3−3•−−32−, the GS of 3 has poor carbonyl−arene and carbonyl−pentiptycene conjugation interactions, but the carbonyl−pentiptycene conjugation interactions are present in the TSα. Along the redox series 3 → 3•− → 32−, the GS has gained more carbonyl−pentiptycene conjugation interactions (φPp: 48.9° → −29.5° → 2.4°), but the opposite is true for the TSα (φPp: 1.1° → −84.0° → −82.5°). Since a quinoidal electron delocalization (Figure 2a) is important for the ionic species of 3 according to the redox properties (vide supra), the conformational changes in GS and TSα correspond to a continued stabilization of GS but destabilization of TSα along the redox series, which conforms to the observed order of 5363



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rotational barrier 3 < 3•− < 32−. Figure 15 shows the rotation mode with related conformations and free energies for 32− for further discussion. An intriguing rotation behavior observed particularly for 32− has been uncovered: that is, the two arylcarbonyl rotators are not completely independent. As shown in Figure 15, when the front “working” rotator is passing the Tα state and approaching to the site antiparallel to the rear “resting” rotator (i.e., A → E′‡ → C′ or A′ → E‡ → C), the resting rotator is enforced to pass the V-shaped iptycenyl substituent to become a synparallel geometry for the two rotators. The working rotator then couples with the relocated rear rotator and passes the V-shaped iptycenyl substituent in a concerted manner to form a mirror image (i.e., C′ → D‡ → C or C → D‡ → C′; see the black arrows in Figure 15). Such a correlated process renders the resting rotator to mimic a “meet and follow” action on the working rotator. In view of the distorted pentiptycene scaffold near the rotators in conformers C and C′, the driving force for the meet and follow action seems to minimize the steric interactions between the rotators and the iptycenyl substituent. Provided that the meet action was not performed, the V-shaped iptycenyl group would be squeezed by the two rotators and cause a large strain. More structural information about the distorted conformer C of 32− is provided in Figure S14. For comparison, no such distortion of the pentiptycene scaffold is present in the other species in all stages of the rotations. Note that the meet process (E′‡ → C′ or E‡ → C) is unidirectional. Moreover, the interconversion of conformers C and C′ has a relatively low barrier of 4.2 kcal mol−1, which could be attributed to the favorable quinoidal geometry of the central phenylene ring in the transition state (D‡). Since 32− is in a “stop” state due to the high energy (18.7 kcal mol−1) of transition states for a 360° α-rotation, the torsional motion of the rotators would be dominated by the concerted oscillation among conformations C, D‡, and C′ that pass over the Vshaped iptycenyl cavity. For comparison, the rotator in the stop state of 22− is oscillating within the U-shaped cavity of the pentiptycene stator (resembling conformations A, B‡, and C for the noncorrelated β rotation of 2 in Figure 12c).

Article

EXPERIMENTAL SECTION

General Methods. 1D and 2D 1H NMR spectra were determined with a 500 or 400 MHz spectrometer using 5 mm gradient TBI and TBO probes, respectively. The chemical shifts for 1H and 13C spectra were referenced to the signals of CDCl3 (δ(1H) = 7.26 and δ(13C) = 77.00), CD2Cl2 (δ(1H) = 5.32 and δ(13C) = 54.00), or THF-d8 (δ(1H) = 1.73, 3.58 and δ(13C) = 25.37, 67.57). In the case of variabletemperature measurements, the sample temperature was calibrated by 1 H signals of ethylene glycol and methanol to ensure a temperature error within ±1 K, and a sufficient temperature equilibration time (10−15 min) was allowed before signal acquisition. The NMR line shape analysis was performed with a commercially available NMR program. Infrared spectra were recorded by using a KBr plate at room temperature. The cyclic voltammetry (CV) data were recorded on an electrochemical analyzer at room temperature, and a glassy carbon electrode served as the working electrode in DMF. The potentials were calibrated relative to the redox couple of ferrocene (E0(Fc/Fc+) = 0.45 V versus SCE).25 The spectroelectrochemistry was carried out at room temperature in a 1 mm quartz cell with N2-bubbled DMF solutions containing 0.5 mM substrate and 0.1 M Bu4NPF6 electrolyte. A Pt grid was used as the working electrode, a Pt wire as the counter electrode, and a Ag wire as the reference electrode. Single crystals of 2 were obtained from mixed solvent DCM and hexane and determined with a CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 295 K by the Instrumentation Center of National Taiwan University (NTU). DFT Calculations. Potential energy surface (PES) scans of α or β rotation were performed for 2, 3, and their anionic states at the SMD/ B3LYP/6-31G(d) level (see text) by changing the φPp or φAr of conformation A or C as the initial starting structure of the scan. Full local minimum and transition state optimizations were performed to locate the stationary points on the PES. Independent β rotational profile (without correlated α rotation) was successfully obtained only for 2 with the scan. To estimate the ease/difficulty of independent β rotation for 2•−, 22−, 3, 3•−, and 32−, conformations of stationary points on the PES of β-rotation of 2 were used as the starting geometries for locating transition structures of β rotation for these species. The relative free energies in CH2Cl2 at the local minima and transition states during the rotation process were computed at the theory level of SMD/BMK/6-311+G(d,p)//SMD/B3LYP/6-31G(d). All calculations were carried out using the Gaussian 09 program.26 Materials. Anhydrous THF and MeCN were obtained from a solvent purifier equipped with Al2O3 columns. The moisture content was less than 10 ppm. All the other solvents and materials for synthesis were reagent grade. DBB was prepared according to the literature.12 The synthesis of compound 4 has been reported.15 Detailed synthetic procedures and structural characterization data for new compounds 2, 3, 5, and 6 are provided below, and their 1H and 13C NMR spectra are shown in Figures S15−S18. Synthesis of 2. Under an atmosphere of nitrogen, 5.0 mL of nbutyllithium in hexane (1.6 M, 6.30 mmol) was added dropwise to a solution of 4-bromobenzotrifluoride (1.20 mL, 6.30 mmol) in THF (20 mL) at −78 °C. The mixture was kept at −78 °C for 1 h, and the lithium reagent was then transferred with cannula into a solution of 6 (1.00 g, 2.06 mmol) in THF (20 mL). The mixture was warmed to room temperature slowly and stirred for another 16 h. The reaction was quenched by adding 1.0 M HCl until the solution was at pH ∼ 7. The solvent was removed by reduced pressure. The residue was dissolved in acetone (20 mL), and Jones reagent was added to the solution at 0 °C. The mixture was stirred at room temperature for 3 h. The reaction was quenched with isopropanol (50 mL) and then stirred for 30 min. The solvent was removed under reduced pressure, and the residue was dissolved in CH2Cl2 (30 mL) and water (30 mL). The organic layer was dried over with anhydrous MgSO4, and the filtrate was concentrated under reduced pressure. Purification was carried out by column chromatography with CH2Cl2/hexane (1:1) as eluent, and white solid product 2 (1.28 g, 1.66 mmol) was obtained with a yield of 79%: mp > 300 °C; 1H NMR (400 MHz, DMF-d7) δ 5.48 (s, 4H), 6.92−6.94 (m, 8H), 7.15−7.17 (m, 8H), 7.87−7.89 (d, J = 6.6 Hz,

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CONCLUSION The redox-dependent α(CPp−C(O) bond) and β (CAr−C(O) bond) rotational behaviors of p-bis(arylcarbonyl)pentiptycenes 2 and 3 have been experimentally and/or computationally characterized. Our results reveal that the steric and electronic nature of the aryl groups plays a critical role in the rotation and redox properties. While both 2 and 3 mimic tristable molecular brakes gated by redox potentials in the sequence of neutral, radical anionic, and dianionic forms, it is a slow−fast−stop mode for 2−2•−−22− but a fast−slow−stop mode for 3−3•−− 32− with a braking power of 8−9 orders of magnitude. In addition, the dianion state possesses a bis(radical anion) character for 22− but a radical-free dianion for 32−. Moreover, the α and β rotations are independent in 2 but correlated (geared) in 3 and all four redox states 2•−, 22−, 3•−, and 32−. The rotation behavior of 3 represents, to the best of our knowledge, the first example of a tristable molecular gear24 with brake function or a molecular brake with geared rotation and that of 2 a new example of molecular gear of clutch−declutch function.23 All these features demonstrate the merit of redox gating for the operation of molecular mechanical switches. 5364


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The Journal of Organic Chemistry 4H), 8.05−8.07 (d, J = 6.6 Hz, 4H); 13C{1H}NMR (100 MHz, CDCl3) δ 51.2, 119.5, 122.2, 123.7, 124.9, 125.6, 126.3, 127.6, 130.5, 131.8, 134.9, 135.3, 135.6, 135.9,140.5, 140.9, 143.9, 196.2; IR (KBr) 3070, 3022, 2971, 1675, 1580, 1508, 1460, 1324, 1177, 1133 cm−1; HRMS (FAB-TOF) calcd for C50H28F6O2, 774.1993; found, 774.1998. Synthesis of 3. Under an atmosphere of nitrogen, 5.0 mL of nbutyllithium in hexane (1.6 M, 6.30 mmol) was added dropwise to a solution of 2-bromomesitylene (0.80 mL, 6.30 mmol) in THF (20 mL) at −78 °C. The mixture was kept at −78 °C for 1 h, and the lithium reagent was then transferred with cannula into a solution of 6 (1.00 g, 2.06 mmol) in THF (20 mL). The mixture was warmed to room temperature slowly and stirred for another 16 h. The reaction was quenched by adding 1.0 M HCl until the solution was at pH ∼ 7. The solvent was removed by reduced pressure. The residue was dissolved in CH2Cl2 (20 mL), and Dess−Martin periodinane (1.78 g, 4.20 mmol) was added to the solution at room temperature and stirred for 2 h. The mixture was washed with distilled water (30 mL). The organic layer was dried over with anhydrous MgSO4, and the filtrate was concentrated under reduced pressure. Purification was carried out by column chromatography with CH2Cl2/hexane (1:1) as eluent, and white solid product 3 (0.52 g, 0.71 mmol) was obtained with a yield of 34%: mp >300 °C; 1H NMR (400 MHz, CDCl3) δ 2.00 (s, 12H), 2.46 (s, 6H), 5.51 (s, 4H), 6.84−6.86 (m, 8H), 6.93−6.95 (m, 8H), 7.01 (s, 4H); 13C{1H}NMR (100 MHz, CDCl3) δ 21.1 21.3, 50.6, 123.6, 125.2 130.4, 135.6, 137.0, 138.5, 141.1, 141.9, 144.3, 200.4; IR (KBr) 3066, 3021, 2957, 2823, 2858, 1694, 1655, 1609, 1460 cm−1; HRMS (FABTOF) calcd for C54H42O2, 722.3185; found, 722.3193. Synthesis of 5. The mixture of compound 4 (1.54 g, 2.65 mmol), CuCN (1.15 g, 13.2 mmol), and 30 mL of dried DMF in a Schlenk flask was refluxed for 12 h. After cooling to room temperature, the mixture was extracted with CH2Cl2 and H2O. The organic layer was dried with MgSO4 and concentrated under reduced pressure. Purification was carried out by column chromatography with CH2Cl2/hexane (1:1) as eluent, and white solid product 5 (1.22 g, 2.54 mmol) was obtained with 96%: mp >300 °C; 1H NMR (400 MHz, CDCl3) δ: 7.44−7.42 (m, 8H), 7.04−7.02 (m, 8H), 5.77 (s, 4H); 13C{1H}NMR (100 MHz, CDCl3) δ 52.1, 107.8, 114.9, 124.3, 126.2, 142.8, 146.6; IR (KBr) 3068, 3044, 3021, 2974, 2359, 2343, 1460, 1390, 1324, 1190 cm−1; HRMS (FAB-TOF) calcd for C36H19N2 (M+), 479.1548; found, 479.1561. Synthesis of 6. Under an atmosphere of nitrogen, 1.0 mL of DIBAL in CH2Cl2 (1.2 M, 1.26 mmol) was added dropwise to a solution of 5 (200 mg, 0.416 mmol) in CH2Cl2 (10 mL) at 0 °C. The mixture was then stirred at room temperature for 16 h. The reaction was quenched by adding 2.0 mL of water, and the solvent was removed under reduced pressure. The residue was extracted by CH2Cl2 (10 mL) and water (10 mL). The organic layer was dried over with anhydrous MgSO4, and the filtrate was concentrated under reduced pressure. Purification was carried out by column chromatography with CH2Cl2/ hexane (1:1) as eluent, and yellow solid product 6 (0.18 g, 0.374 mmol) was obtained with a yield of 90%: mp >300 °C; 1H NMR (400 MHz, CDCl3) δ 6.48 (s, 4H), 6.97−7.01 (m, 8H), 7.36−7.41 (m, 8H), 11.08 (s, 2H); 13C{1H}NMR (100 MHz, CDCl3) δ 48.1, 124.1, 125.8, 129.1, 144.0, 145.6, 191.5; IR (KBr) 3014, 2924, 2873, 2768, 1686 cm−1; HRMS (FAB-TOF) calcd for C36H22O2, 486.1620; found, 486.1625.

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Crystallographic data of 2 (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chetti Prabhakar: 0000-0003-1773-9429 Jye-Shane Yang: 0000-0003-4022-2989 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this research was provided by the Ministry of Science and Technology (MOST 104-2113-M-002-004MY3) of Taiwan and by National Taiwan University (104R8956-2 and 105R8956-2) and Academia Sinica. The computing time granted by the National Center for HighPerformance Computing and the Computing Centers of National Taiwan University and Academia Sinica is acknowledged. We also thank Prof. Shie-Ming Peng and Mr. Yi-Hung Liu for determining the X-ray crystal structure of 2, Prof. YingChih Lin, and Miss Shou-Ling Huang for the support of dynamic NMR measurements, and Mr. Wei-Hsiang Tan for assistance in compound characterization. Dr. Chetti Prabhakar thanks SERB-New Delhi, India, for financial support under Young Scientist Start-Up Research Grant.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b00729. Thermal ellipsoid, Eyring, and Arrhenius plots of 2, DFT-derived rotational ground and transition state structures of 2 and 3, VT 1H NMR spectra and DFTderived rotational profile of 3, φPp−φA correlation plots for 2 and 3 and their redox states, a full table of activation parameters, and 1H and 13C NMR spectra of new compounds (PDF) 5365


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