Through-Space Communication Effects on the Electrochemical

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Through-Space Communication Effects on the Electrochemical Reduction of Partially Oxidized Pillar[5]arenes Containing Variable Numbers of Quinone Units Mehdi Rashvand Avei, and Angel E. Kaifer J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b01366 • Publication Date (Web): 19 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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The Journal of Organic Chemistry

Through-Space Communication Effects on the Electrochemical Reduction of Partially Oxidized Pillar[5]arenes Containing Variable Numbers of Quinone Units Mehdi Rashvand Avei and Angel E. Kaifer* Department of Chemistry, University of Miami, Coral Gables, FL 33124-0431 KEYWORDS (Word Style “BG_Keywords”). If you are submitting your paper to a journal that requires keywords, provide significant keywords to aid the reader in literature retrieval.

ABSTRACT: The electrochemical reduction of five partially oxidized pillar[5]arenes containing variable numbers (1!5) of quinone units was investigated in acetonitrile and dichloromethane solutions. The cathodic voltammetric behavior of all the macrocyclic compounds containing two or more quinone units indicates the existence of detectable levels of electronic communication between them. We have also investigated these compounds and their reduced forms using DFT computations. Our computational results indicate the presence of through-space communication between quinone units.

Introduction Pillar[n]arenes (n = 5-10) are a family of macrocyclic hosts composed of 1,4-dialkoxybenzene (or hydroquinone) units connected by methylene bridges.1,2 These compounds have recently attracted considerable attention as supramolecular hosts due to their rigid, “pillar”-like structures and their ease of functionalization.3-5 Most of the attention has focused on the binding properties and derivatization of pentameric (pillar[5]arene) and hexameric (pillar[6]arene) analogs. In the particular case of pillar[5]arene hosts,6 functionalization of the macrocyclic core by oxidation of one or more of the constituent hydroquinone units has been independently demonstrated by the groups of Stoikov,7 Xue,8 Huang,9 Wen10 and Ogoshi.11 We recently reported the voltammetric behavior of pillar[5]quinone,12 the fully oxidized macrocycle containing five equivalent 1,4-benzoquinone units connected by methylene bridges. In dichloromethane solution, our voltammetric data revealed that the uptake of the first five electrons follows a 21-2 pattern dictated by the pentagonal symmetry of the molecule and the minimization of electrostatic repulsions among the electrogenerated charges. Therefore, we were interested in expanding our voltammetric studies to other redox-active, quinone-containing pillar[n]arenes. In this work, we report the electrochemical behavior of 1,4-decamethoxypillar[5]arenes in which one to five of the aromatic units are oxidized to 1,4benzoquinones (see Figure 1 for structures).

Figure 1. Structures of the pillar[5-n]arene[n]quinones (PnQ) used in this work.

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Results and Discussion The various pillar[5-n]arene[n]quinones (n=1-4) were prepared by oxidation of 1,4-decamethoxypillar[5]arene using modifications of the methods reported by Stoikov and coworkers.7 Full synthetic details are given in the Experimental Section. Pillar[5]quinone (P5Q) was prepared by the method of Shivakumar and Sanjayan.13 For simplicity, we elected to name the compounds according to the number of oxidized quinone units present in the macrocycle (PnQ). We recognize that both P2Q and P3Q have constitutional isomers, which are not represented in our work, and will address the behavior of those specific macrocycles in future work. Electrochemistry. While our previous electrochemical work on P5Q was carried out in dichloromethane solution,12 we decided to investigate the voltammetric behavior of macrocycles P1Q-P5Q in both acetonitrile (ACN) and dichloromethane (DCM) solvents in order to address the solvent effect on the redox potentials of all compounds.

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corresponding dianion [P1Q]2-, respectively. Not surprisingly, the voltammetric behavior of this compound is similar to that of p-xyloquinone because the electron transfers take place on the same quinone unit in the molecule. This electrochemical behavior is consistent with the expectation that two wellresolved waves will be observed for two sequential electron transfers involving the same molecular orbital without any dramatic structural changes happening during the electron transfer process. Host P2Q shows voltammetric behavior (Figure S1) characterized by the presence of two close waves corresponding to the one-electron reduction of each of its two quinone units. The fact that these two waves are resolved, suggests that there is some degree of electronic communication between the two quinone units in P2Q (see below), even if they are separated by a dimethoxybenzene residue. Only a single, broader wave was observed for the second electron reduction of the quinone residues. If we assume that the two identical quinone units in P2Q are fully independent from one another, the theoretical difference between the two consecutive reduction potentials would be 35.6 mV (at 25 oC).14 The experimental difference observed between the first two reduction potentials of P2Q is 77 mV, which suggests a moderate level of communication between the two redox centers. In other words, the one-electron reduction of one of the quinones, hinders the electron uptake by the second quinone residue to a larger extent than anticipated by statistical considerations.

Figure 2. Cathodic voltammetric behavior on glassy carbon (0.07 cm2) of 1.0 mM p-xyloquinone (red) and P1Q (blue) in acetonitrile solution also containing 0.1 M TBAPF6 as the supporting electrolyte. (A) Square wave voltammetry. (B) Normal pulse voltammetry.

The voltammetric behavior of the single-quinone macrocyle P1Q is shown in Figure 2. Both square wave voltammetry (SWV) and normal pulse voltammetry (NPV) show that P1Q undergoes two consecutive one-electron reductions at halfwave potentials (E1/2’s) of -0.368 V and -0.960 V vs Ag/AgCl. These two reduction processes are assigned to the sequential formation of the semiquinone anion radical [P1Q]-• and the

Figure 3. Cathodic voltammetric behavior on glassy carbon (0.07 cm2) of 1.0 mM P4Q in acetonitrile solution also containing 0.1 M TBAPF6 as the supporting electrolyte. (A) Square wave voltammetry. (B) Normal pulse voltammetry.

The electrochemical behavior of P3Q and P4Q follows similar patterns. Host P3Q shows three resolved waves in the potential region corresponding to the quinone-to-semiquinone


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reduction with potential differences of 119 mV between the first and the second reductions and 134 mV between the second and third reductions (Tables 1, S1 and S2), which confirm the repulsive interactions between the quinone units. For a molecule with three non-interacting, identical redox centers, one expects the spacing between the first and the third redox potentials to be around 57 mV,14 while for P3Q, this value is much greater, strongly suggesting communication among the quinone units. Again, further reduction yields a broad wave around -1.1 V (Figure S2). Tetraquinone host P4Q shows two barely resolved waves, at -0.184 and -0.233 V, followed by another couple of resolved waves, at -0.436 and -0.583 V (Figure 3). These waves were all assigned to one-electron reductions for each of the four benzoquinone units. The difference between successive reduction potentials (50, 202, and 147 mV), as well as the total potential range covered between the first and the fourth reduction wave (399 mV, as tabulated in Tables 1, S1 and S2) provides additional support to the presence of repulsive interactions among the quinone units. A final broad wave was observed at -0.840 V, presumably related to the formation of quinone dianions in the macrocycle. Finally, host P5Q shows cathodic voltammetric behavior (Figure S3) similar to that previously reported in dichloromethane solution,12 in which voltammetric waves for the sequential uptake of two, one and two electrons are observed in the potential region corresponding to the quinone-tosemiquinone reduction region. Even though two separate waves, each involving two-electron transfers, were observed for the first and the third reduction waves of P5Q, the potential differences between the three waves (114 and 86 mV) confirm the existence of communication between the quinone units accepting the electrons.

Figure 4. Dependence of the half-wave potential for the first reduction wave observed in the cathodic voltammetric behavior of compounds P1Q to P5Q on the number of benzoquinone units present in the macrocycle.

The reduction potential values observed with the P1Q-P5Q series of compounds are affected by the electron withdrawing or donating character of neighboring units in the macrocycle. For example, the first reduction potential of P2Q is less negative than that observed for P1Q, reflecting the presence of a second quinone unit in the former host. More generally, a plot of the first reduction potential of PnQ as a function of the number of quinone units (n) in the macrocycle is illustrative (see Figure 4). Clearly, the gradual replacement of 1,4dimethoxybenzene units by benzoquinone units has a pronounced effect on the first reduction potential, which shifts to less negative values as the number of benzoquinone units in-

creases. We attribute this trend to the increased electron affinity of the macrocyclic compounds resulting from the replacement of electron-rich 1,4-dimethoxybenzene units by electronpoor benzoquinone units. This matter is one of the main subjects of computational work that we describe later in the manuscript. It is interesting to note that the smallest potential change is observed in going from P4Q to P5Q, probably reflecting that P4Q already has a substantial electron-poor character, which the last replacement of dimethoxybenzene by benzoquinone does not significantly alter. As mentioned before, we investigated the cathodic voltammetric behavior of P1Q-P5Q in two solvents: acetonitrile (ACN) and dichloromethane (DCM), which differ substantially in the value of their dielectric constants, 37.5 and 9.08, respectively, at 20 oC. We anticipate that the larger dielectric constant of ACN will result in better solvation and deeper stabilization of charged species in this solvent compared to the lower polarity DCM. Figure 5 presents the correlation between the half-wave potentials for the quinone-to-semiquinone conversions of P3Q and P5Q determined in DCM against those measured in ACN. The correlation coefficients and unity slopes (within a few percent error) support the idea that the Gibbs energy associated with the transfer of the redox system from DCM to ACN is essentially independent of the nature of the compound. P2Q and P4Q showed similar behavior (not shown).

Figure 5. Half-wave potentials for the quinone-to-semiquinone conversions in DCM measured with compounds P3Q and P5Q against the same values measured in ACN.

These findings can be rationalized using the Born equation, which is commonly employed to estimate the solvation free energies of charged species: ∆𝐺 = −𝑁& 𝑧( 𝑒 * 2𝑟( ∙ 1 − 1 𝜖0 (1) Assuming that the radii of all charged species are the same in both solvents, the free energy of solvation is fully controlled by the dielectric constant, which leads to a constant ratio between the free energies in the two solvents, consistent with the observed linear relationship between the measured half-wave potentials. Among the surveyed pillarquinone macrocycles, P5Q is the only one whose behavior is poorly explained by this hypothesis, probably reflecting the fact that two of the



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three quinone-to-semiquinone electrochemical waves result from the convolution of two electron transfer processes. Table 1 shows the key experimental data collected from the voltammetric experiments, as well as the diffusion coefficients (Do) measured with NMR spectroscopic techniques,12 which were combined to determine the number of electrons associated with each voltammetric wave. Table 1. Diffusion coefficients and voltammetric parameters for the reduction of compounds P1Q-P5Q at 22 oC.

Compound

D×106, cm2/sa

XQ

17.46

P1Q

6.68

P2Q

P3Q

P4Q

P5Q

6.62

6.75

6.96

5.40

E1/2,V vs. Ag/AgClb

i d, µAc

ncumd

ne

-0.392

10.96

1.0

1.0

-0.928

20.82

2.0

1.0

-0.368

6.668

1.0

1.0

-0.960

12.28

1.84

0.84

-0.336

7.096

1.07

1.07

-0.413

13.98

2.12

1.05

-0.948

25.21

3.82

1.70

-0.244

6.943

1.03

1.03

-0.363

13.94

2.07

1.04

-0.497

20.56

3.05

0.98

-1.044

29.24

4.34

1.29

-0.184

7.009

1.03

1.03

-0.234

13.07

1.92

0.89

-0.436

19.34

2.84

0.92

-0.583

24.48

3.60

0.76

-0.840

32.25

4.74

1.14

-0.180

12.60

2.10

2.10

-0.336

18.99

3.16

1.06

-0.564

29.33

4.88

1.72

-0.920

40.64

6.77

1.89

a

Obtained directly from 1H PGSE NMR measurements in CDCl3. bObtained directly from peak potential values measured in SWV experiments. cLimiting current values measured from the initial baseline, giving rise to cumulative numbers of electrons (ncum), which were used to obtain the number of electrons (n) associated with each wave. dCumulative number of electrons taken from the beginning of the reduction process. eNumber of electrons corresponding to the particular wave.

Computational Studies. The potential values obtained in electrochemical experiments allow the detection of electronic communication between otherwise equivalent redox units.

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However, in order to understand the origin of the communication effects, we decided to carry out density functional theory (DFT) computational studies with hosts P1Q-P5Q and their various charged states during the course of their multielectronic reduction processes. The theoretical characterization of PnQ macrocycles in different radical anion states is complicated due to the possibility of different spin multiplicities and the existence of dianion units with a biradicaloid character. To explore the electronic and structural properties of the PnQ compounds, we used both closed-shell and open-shell state calculations with 6-31G+(d,p) basis set15 at different global hybrid (B3LYP, BHHLYP),16,17 global hybrid meta (M062X),18 and range-separated hybrid (CAM-B3LYP,19 wB97X20 and wB97XD21) levels. In addition to the assessment of various functionals to find the most suited to describe the behavior or these compounds, we wanted to model these compounds in the solution phase, where a more polar environment tends to favor the stabilization of localized charges. Ideally, dynamic simulations are necessary to describe solvation at the microscopic level, including not only short-range specific solvation, but also longer-range dielectric effects. These simulations are computationally demanding and out of our reach for the compounds of interest in this work. Therefore, we elected to consider solvent effects using the integral equation formalism22 of the polarizable continuum model (IEF-PCM) method. The results of wavefunction stability calculations of neutral PnQ compounds show that they are stable under closed singlet state as the most stable configuration in their ground states. For the reduced states of PnQs, we performed a lot of optimizations at all possible multiplicities for each reduced compound. The biradicaloid configuration is the most stable state for [P1Q]-2 at all levels. [P2Q]-1, [P2Q]-2, [P3Q]-1, [P3Q]-2, [P3Q]-3, [P4Q]-1, [P4Q]-2 and [P4Q]-4 are stable at their highest spin open shell wavefunctions as tabulated in Table S4. For other reduced compounds, we obtained inconsistent results at different levels arising from different exact-exchange admixture of different functionals. The most stable structure was chosen as that having the lowest value of corrected Gibbs free energy. In Kohn-Sham density functional theory, the selfinteraction error is the main problem.23,24 Most approximate exchange functionals fail to cancel accurately the interaction of an electron with its own charge cloud, which results from the classical coulomb term in Kohn–Sham theory. This problem leads to the so-called “self-interaction error,” a significant problem of numerous contemporary functionals, and results in density or spin-density distributions with an excessive degree of delocalization. In other words, the energy of different spin states is very dependent on the approximations used for the exchange-correlation functional and, by increasing the emphasis on exact exchange, a higher spin state (i.e., more stabilizing exchange interactions) is favored with respect to the lower spin state. A way to lower the self-interaction error is to use exchange functionals that include a certain amount of exact Hartree-Fock (HF) exchange, replacing some of the local or semi-local exchange. HF exchange cancels out the coulomb self-interaction error exactly. However, if we use 100% exact exchange the local or semi-local exchange would be eliminated. Therefore, a trade-off between a reduction of the selfinteraction error and a partial conservation of non-dynamical correlation contributions is required. As an example, the B3LYP functional uses 20% HF exchange and 80% semi-local exchange (with some level of semi-empirical scaling of the


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gradient corrections to exchange and correlation). This compromise provides reasonable thermochemical accuracy for many systems, but the relatively low amount of exact exchange is not enough to properly correct the excessive delocalization produced by local or semi-local functionals. As mentioned above, since we are modeling various radical anionic states, using an unrestricted approach is inevitable. Therefore, spin contamination is another intervening artifact that results from the artificial mixing of multiple electronic states within the wavefunction. The deviation in the eigenvalue of the spin-squared operator from the exact values (e.g., 0.75 for a doublet, 2 for a triplet and so forth) was utilized as a quantitative measure of spin contamination. Table S4 shows that the correlation coefficient for WB97X is close to unity and no spin contamination was seen for the most stable structures obtained at this level. For other levels, some extent of spin contamination is observed. As a component of our computational work with macrocycles P1Q to P5Q using DFT methods, we studied the correlation between the eigenvalues of the lowest unoccupied molecular orbital (LUMO) and experimental reduction potentials, since the standard potentials for reduction processes are related to the energy of the pertinent LUMO. Following Sasaki,25 Saeva26 and Cramer,27 the functional relationship between the measured reduction electrode potentials and the LUMO energies must be of the form: 𝐸 = 𝑎 + 𝑏𝐸5678 (2)

replacement of electron-rich 1,4-dimethoxybenzene units by electron-poor benzoquinone units. Fig. 7 shows the first halfwave potential of the compounds P1Q to P5Q versus the LUMO energy of their neutral forms. The resulting linear plot has a correlation coefficient of 0.99 at the wB97X level. We obtained acceptable values at other levels except for B3LYP one (Table S5). The poor descriptive power of the B3LYP functional is attributed to the self-interaction error, as discussed above. In contrast, the hybrid BHHLYP (50% HF exchange) and the meta-hybrid M06-2X (54% HF exchange) functionals with a large portion of HF exchange, as well as the range-separated CAM-B3LYP, wB97X (15.77% short range HF exchange, 100% long range HF exchange) and wB97XD (22.2% short range HF exchange, 100% long range HF exchange) functionals are capable of producing more consistent results. The results obtained at the wB97X level show the highest consistency with the experimental results compared to other functionals. It is noteworthy that wB97XD and wB97X functionals do not only differ in having the dispersion term, which is expected to play no significant role in the estimation of more accurate energies, but also the w parameter, which defines the short and long range separation, has a different value of 0.2 and 0.3 bohr-1 for wB97XD and wB97X, respectively.

When dealing with radical species, in an open-shell approach we have both alpha and beta electrons. Here we considered the beta LUMO which has the lowest energy. With the only exception of the values based on B3LYP functionals, the computed LUMO energies were consistent with the above equation as shown in Figure 6 and Table S2.

Figure 7. The first half-wave potentials for compounds P1Q-P5Q measured in acetonitrile vs the corresponding LUMO energies obtained at the wB97X level. Figure 6. Half-wave potentials for compounds P1Q-P5Q measured in acetonitrile vs LUMO energies obtained at the wB97X level.

As mentioned at the beginning of this section, we speculated that the electron affinity of compounds P1Q-P5Q increases with addition of more electron-deficient quinone units. Fortunately, Koopman’s theorem allows us to utilize the negative value of the LUMO energy as the electron affinity parameter. Therefore, a reasonable decrease in the energy of the LUMO of the studied compound should be seen as the number of electron-poor quinone units increases in the surveyed structures. Our computational results nicely confirm the electron affinity increase of the macrocyclic compounds resulting from the

Figure 8 shows the isodensity plots of the b-LUMO of the studied molecules. Also, figures S5 to S9 depict the isodensity plots of other frontier molecular orbitals of the same compounds. The frontier molecular orbitals of P3Q, P4Q and P5Q compounds clearly show direct p-orbital overlap between each of the two neighboring quinone units, which confirms the fact that the electron communication between them takes place by a through-space mechanism. Also, the existence of many conformations for each compound, resulting from the rotation of each quinone unit around itself, increases the probability of spatial communication every time that both quinone units are in the same plane and the electron could hop from one quinone unit onto the other one. However, all our computations were done on the most stable host conformations, in which all aro-



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matic units are roughly parallel, and the effect of these rotamers is not captured in our computational work.

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Since the LUMO orbitals (in P4Q and P5Q) are distributed over several quinone units, we cannot predict in which LUMO orbital the electron would reside. Therefore, after calculation of the spin densities of both alpha and beta electrons, the spin densities of the beta electrons were subtracted from those of the alpha electrons. As shown in Figure 9, the distribution of the electron after each step is obvious. In some cases, resonance sharing of one electron between two quinone groups is observed, which supports the proposed through-space communication between quinone residues. If spatial communication exists between the quinone units, we could claim that there should be a spatial conjugation between the units. As it is well-known, for any conjugated system, the larger the degree of conjugation, the lower the HOMO-LUMO energy gap is expected to be. Figures S10 to S14 show nicely the HOMOLUMO energy gap for each of the PnQ compounds. As shown in figure S10, since no conjugation takes place in P1Q after taking the first and even the second electrons, no significant change in the HOMO-LUMO energy gap is expected in this case. For the remaining PnQ compounds, significant changes are observed, which evidence the existence of spatial communication between quinone units. Figure 10 shows the HOMOLUMO energy gaps for all the oxidation states of P5Q.

Figure 8. Isodensity plots of the b-LUMO for compounds P1QP5Q. The red circles enclose regions of though-space orbital overlap between quinone units connected by bridging methylenes.

Figure 10. Frontier orbital energies of P5Q and its reduced states at the wB97X level. (0 = [P5Q], -1 = [P5Q]-1, -2 = [P5Q]-2, -3 = [P5Q]-3, -4 = [P5Q]-4, -5 = [P5Q]-5).

Figure 9. Isodensity plots of difference densities between a (lavender) and b (turquoise) spins for compounds P1Q-P5Q. bLUMO for compounds P1Q-P5Q. The red circles enclose regions of through-space orbital overlap between quinone units connected by bridging methylenes.

It is interesting to briefly compare the experimental results reported here with previous work done with quinonecontaining calixarenes.28,29 Although there are considerable structural similarities between calix[n]quinones and pillar[n]quinones, the electrochemical behavior of the former is more complex, reflecting the presence of equilibria between various conformers. In 1993, Liberko et al. investigated the electronic structure of helicene-bisquinone anion radicals30 and concluded that they had delocalized electronic structures owing to the spatial proximity between the two quinone units. In 1998, Hünig and co-workers reported the electrochemical behavior of a series of bisquinones,31 in which the two quinoid groups are tethered by 2-6 methylenes. They found that the communication between the two quinone units faded quickly with the length of the tethering chain. More recently, Long and co-workers surveyed the electrochemical behavior of three bis(ubiquinone) compounds with methylene, phenylene and biphenylene connectors.32 Their findings indicated that the methylene-bridged compound exhibits more efficient electronic communication than the other two compounds, in spite of the fact that the latter have aromatic tethers. The experimental and computational results reported here are consistent with previous experimental work. Our results


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indicate that the macrocyclic structures of P1Q-P5Q and the well-defined pentagonal shape of these molecules leads to through-space electronic communication among the quinone residues. The regions encircled in red in Figure 8 and 9 provide support for direct orbital contact between adjacent quinones. Furthermore, since each aromatic residue in PnQ macrocycles can rotate freely around the axis connecting its two adjacent methylenes, through-space communication between nonadjacent quinone units is also possible. As pointed out before, our computational studies were conducted on minimum energy conformations, in which all the aromatic groups are essentially parallel, but the straightforward inspection of molecular models provides strong support for this hypothesis. Also, Ogoshi and co-workers have also reported through-space p-delocalization in pillar[5]arenes derivatized with phenylethynyl groups.33 We conclude that the macrocyclic nature of these hosts determines to a large extent their properties as electron acceptors. Another interesting point that we will address in further work is that the electrochemical waves corresponding to the second electron uptake on each of the quinone units seem to be underdeveloped in terms of total current uptake. Finally, our future work will explore the changes in the electrochemical behavior of hosts P1Q to P5Q brought about by the presence of cationic guests bound inside their cavities. Experimental Materials. All the chemicals, solvents and reagents for synthesis and analysis were analytical grade, obtained from commercial suppliers without further purification. Electrochemical studies were carried out with a CH Instruments, model 900 Scanning Electrochemical Microscope, which could be used as a potentiostat/galvanostat. We used a single-compartment electrochemical cell, fitted with a glassy carbon working electrode, a platinum wire serving as the auxiliary electrode, and a Ag/AgCl/KCl (3.5 M) reference electrode. All potentials are reported against this reference electrode. Voltammetric data were recorded after thorough purging of the test solution with purified nitrogen gas. All 1H-NMR spectra were acquired in CDCl3 solution using a 400 MHz NMR spectrometer. Mass spectra were obtained on a MicroQ-TOF ESI mass spectrometer. All measurements were carried out at 295 ± 2 K. Synthesis of the pillar[n]quinones followed published procedures7,10 with suitable modifications to improve on reported yields. Synthesis of 1,4-decamethoxypillar[5]arene. 1,4decamethoxypillar[5]arene was synthesized by mixing a 1:1 ratio of 1,4-dimethoxybenzene (13.82 g) and finely grinded paraformaldehyde (3.00 g) in 950 ml of 1,2-dichloroethane. After addition of the starting materials, 50 ml of triflouroacetic acid (as a catalyst) was added and the reaction mixture refluxed for 2 hours at 85 ℃. While the reaction is running, the color of the reaction mixture changes from pale yellow to dark green color. The reaction mixture was allowed to cool down and then added to 950 ml of methanol to precipitate the crude product. This precipitate was dissolved in a minimum amount of chloroform and placed in a beaker. (It should be noted that it does not dissolve completely and gives a murky solution.) The same volume of acetone was added to drive precipitation of the product for 15 min, which was filtered off and washed with excess acetone. After further washing of the product with 20 ml of cold chloroform, an off-white precipitate was obtained (10.67 g, 70 ± 2 % yield). 1H NMR (400 MHz, CDCl3)

d: 6.80 (s, 10H); 3.77 (s, 10H); 3.67 (s, 30H). MS (ESI) m/z: calcd for [C45H50O10Na]+ 773.84, found 773.3. Synthesis of P1Q. A 2.0-g sample (2.66 mmol) of 1,4decamethoxypillar[5]arene was dissolved in 200 mL of CH2Cl2/THF (1:1). Separately, 3.21 g (5.86 mmol) of (NH4)2[Ce(NO3)6] was dissolved in a minimum volume of water and added dropwise to the first solution during a period of 1 hour. The reaction mixture was stirred under an argon atmosphere for 24 hours at room temperature, washed with water (200 mL ´ 3), dried with anhydrous sodium sulfate and concentrated by rotary evaporation. The product was isolated by column chromatography (silica gel 60-70 size; EtOAC:hexane = 30:1 V/V, 1.17 g, yield: 60 ± 2 %). 1H NMR (400 MHz, CDCl3) δ: 6.86 (s, 2H), 6.82(4) (s, 2H), 6.81(7) (s, 2H), 6.68 (s, 2H), 6.67 (s, 2H), 3.77 (s, 6H), 3.74 (s, 6H), 3.73 (s, 6H), 3.71 (s, 6H), 3.63 (s, 6H), 3.59 (s, 4H). MS (ESI) m/z: calcd for [C43H44O10Na+]: 743.772; found 743.2. Synthesis of P2Q. A 2.0-g sample (2.66 mmol) of 1,4decamethoxypillar[5]arene was dissolved in 200 ml of CHCl3/THF (1:1). Separately, 6.43 g (11.72 mmol) of (NH4)2[Ce(NO3)6] was dissolved in a minimum volume of water and added dropwise to the first solution during a period of 1 hour. The reaction mixture was worked up as in the procedure to prepare P1Q. The product was isolated by column chromatography (silica gel 60-70 size; EtOAC:hexane = 30:1 V/V, 0.92 g, yield: 50 ± 2 %). 1H NMR (400 MHz, CDCl3) δ: 6.88 (s, 2H), 6.72 (s, 2H), 6.70 (s, 6H), 3.80 (s, 2H), 3.79 (s, 6H), 3.74 (s, 6H), 3.71 (s, 6H), 3.60 (s, 8H). MS (ESI) m/z: calcd for [C41H38O10Na+]: 713.704; found 714.1. Synthesis of P3Q. Identical procedure to that used in the preparation of P1Q. In this case, 9.64 g (17.58 mmol) of (NH4)2[Ce(NO3)6] was dissolved in a minimum volume of water and added dropwise to the first solution during a period of 1 hour. The reaction mixture was worked up as in the procedure to prepare P1Q. The product was isolated by column chromatography (silica gel 60-70 size; EtOAC:hexane = 30:1 V/V, 1.14 g, yield: 65 ± 2 %). 1H NMR (400 MHz, CDCl3) δ: 6.81 (s, 2H), 6.76 (s, 4H), 6.74 (s, 2H), 6.63 (s, 2H), 3.78 (s, 6H), 3.76 (s, 6H), 3.62 (s, 8H), 3.47 (s, 2H). MS (ESI) m/z: calcd for [C39H32O10Na+]: 683.636; found 683.2. Synthesis of P4Q. A 2.0-g sample (2.66 mmol) of 1,4decamethoxypillar[5]arene was dissolved in 400 mL of CH2Cl2/THF (1:1). Separately, 16 g (29.18 mmol) of (NH4)2[Ce(NO3)6] was dissolved in the minimum amount of water and added dropwise to the first solution during a period of 1 hour. The reaction mixture was stirred under an argon atmosphere for 60 hours at room temperature, washed with water (400 mL ´ 3), dried with anhydrous sodium sulfate and concentrated by rotary evaporation. The product was isolated by column chromatography (silica gel 60-70 size; EtOAC:hexane = 20:1 V/V, 0.92 g, yield: 55 ± 2 %). 1H NMR (400 MHz, CDCl3) δ: 6.82 (s, 2H), 6.78 (s, 2H), 6.76 (s, 2H), 6.72 (s, 2H), 6.67 (s, 2H), 3.79 (s, 6H), 3.64 (s, 4H), 3.48 (s, 6H). MS (ESI) m/z: calcd for [C37H26O10Na+]: 653.568; found 653.1. Computational details. All computations have been carried out by using the ultrafine (99,590) pruned integration grid which guarantees the accuracy and stability of all numerical computation processes, with the Gaussian 09 program package (version e.01). The optimization of structures and corresponding energies has been done at the DFT level with the global



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hybrid (B3LYP, BHHLYP) global hybrid meta (M062X) and range-separated hybrid (CAM-B3LYP, wB97X and wB97XD) generalized gradient approximation (CGA) functionals and with the 6-31+G(d,p) basis set, because it includes diffuse functions, which are more appropriate for the treatment of anionic moieties. In order to take into account solvent interactions, all structures were optimized in the presence of solvent with the self-consistent reaction field (SCRF) theory using the integral equation formalism model (IEFPCM). The IEFPCM model incorporates the solvent as a continuous, uniform dielectric medium, characterized by its dielectric constant e. The solute is represented by means of a cavity built with a number of interlocked spheres. Frequency calculations carried out at the same level of theory ensure that the local minima possess all real frequencies. The molecular orbitals and spin densities have been computed at isovalues of 0.02 and 0.0004 electron/bohr3.

ASSOCIATED CONTENT Supporting Information. Additional voltammetric, 1H NMR spectroscopic and computational data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Angel E. Kaifer, Department of Chemistry, University of Miami, Coral Gables, FL 33124-0431, USA.

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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors are grateful to the National Science Foundation for its generous support of this work (to AEK, CHE-1412455). MRA acknowledges partial support from a Summer Graduate Award from the University of Miami College of Arts and Sciences.

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TOC Graph


Through-Space Communication Effects on the Electrochemical

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