Sophie Haberland

Aaron D Finke

Nicolas Kerisit

Claudine Katan

Yann Trolez

Przemyslaw Gawel

Ivo Leito

Mart Lokov

Robert Jarviste

Märt Lõkov

Robert Järviste

Karl Kaupmees

Nils Trapp

Laurent Ruhlmann

Corinne Boudon

Daniel Himmel

François Diederich

Enhancement of Push-Pull Properties of Pentafulvene and Pentafulvalene Derivatives by Protonation at Carbon

Keywords: Fulvenes, Fulvalenes, Cumulenes, Carbocations, Push-Pull Chromophores

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Introduction

Fulvenes are π-conjugated hydrocarbons composed of an oddmembered ring with one exocyclic, cross-conjugated C=C double bond. If this double bond is directly cross-conjugated to another odd-membered π-conjugated ring, the resulting compounds are called fulvalenes. [1] The first to mention color changes of fulvene derivatives induced by the addition of acids were Prinzbach et al. in 1966. [2] They showed that protonation of a sesquifulvalene (five and seven-membered π-conjugated rings connected by a crossconjugated C=C bond) at carbon gave a stable carbocation with consistent NMR signals. In the early 1980s, Neuenschwander and coworkers described the cationic polymerization of 6alkyl(penta)fulvenes. [3] They suggested that protonation takes place at one of the endocyclic carbon atoms C2 or C5 (for numbering, see Figure 1) based on the obtained polymer structures, mechanistic studies, as well as theoretical calculations. However, Olah et al. were the first to report systematic studies of a variety of fulvenium ions that were prepared by protonation in super-acidic media. [4] Based mainly on 1 H and [START_REF]repulsion between the phenyl ring and the tert-butyl group at C2[END_REF] C NMR studies, they showed that protonation occurs exclusively at the C2 and C5 positions. Protonated fulvenium ions have also been the subject of theoretical calculations, [START_REF] Jasper | Proc. Combust. Inst[END_REF] and electronic absorption spectroscopy in neon matrices at 6 K, paired with calculations, suggested isomerization between protonated fulvenium ions and protonated benzene. [START_REF] Garkusha | [END_REF] Kende and Izzo proposed C-protonation of sesquifulvalenes with a three-and a five-membered ring. [7] It seems that over time, after Prinzbach's early findings, changes in optical properties upon protonation of fulvenes and fulvalenes, have largely been forgotten. It is known that protonation of certain donor-π-bridge-acceptor (D-π -A) molecules can reinforce their push-pull character. [8] • However, this is only the case if protonation takes place at the electron accepting moiety of the molecule, thus, making it an even stronger acceptor. At the same time, it is rare that the acceptor moiety is more basic than the donor moiety due to its intrinsic lack of electrons.

Our study demonstrates the enhancement of π-conjugated pushpull effects by protonation at a carbon atom. It is a wellestablished practice for push-pull molecules with basic amines as electron-donors to subject them to acidic conditions to test whether the longest-wavelength electronic absorption band belongs to the intramolecular charge-transfer (ICT) transition. [9] If this is the case, this band is quenched and a higher-energy band representing the protonated ammonium chromophore appears. Accordingly, we expected a solution of compound 1 (Figure 1) to lose its color upon protonation. To our great surprise, addition of trifluoroacetic acid (TFA) to an initially orange solution of 1 in CH2Cl2 did not lead to color quenching but instead the solution turned deep blue. This effect is fully reversible upon neutralization with NEt3. The same effect was observed in other chlorinated solvents, such as CHCl3 and 1,2-dichloroethane (1,2-DCE), as well as in acetonitrile.

With the objective of understanding this phenomenon, we synthesized a series of 6-phenylfulvenes 1-7 with different parasubstituents on the phenyl rings (Figure 1) and investigated their protonation behavior. We installed tert-butyl groups at the endocyclic positions in order to simplify NMR analysis, prevent polymerization, and enhance solubility. Interestingly, we also found that fulvalene 8 and the complex pentafulvene-derived chromophore (±)-9 (Figure 1) show an even stronger bathochromic shift of their main electronic absorption band upon acidification. This paper reports the synthesis of 1 to (±)-9, the experimental pKa values of these chromophores in acetonitrile and 1,2-DCE, as well as NMR, UV/Vis, and electrochemical studies on their protonation behavior. The experimental findings are rationalized with the help of DFT and TD-DFT calculations, by comparing Gibbs free energies of species protonated at different sites and calculated and measured optical properties.

Results and Discussion

Synthesis

The synthesis of 6-phenylfulvenes 1-7 takes advantage of the unusually high acidity of cyclopentadienes (CPDs, pKa ≈ 16) compared to other hydrocarbons. [10] It is known that 1,2,3,4tetraphenylcyclopenta-1,3-diene undergoes condensation with arylaldehydes in the presence of sodium methoxide in methanol at reflux. [11] The same method was used herein with 1,3-di-tertbutylcyclopentadiene 10 [12] and para-substituted benzaldehydes 11-17 (Scheme 1A). (E)-Configured fulvenes 2-7 were isolated in 40-78% yields as bright orange solids. [START_REF]repulsion between the phenyl ring and the tert-butyl group at C2[END_REF] Reaction times increase from 7 hours (for R = NO2) to five days (for R = OMe) with increasing electron-donating strength of the para-substituent on the benzaldehyde. This general protocol did not work well with 4-(N,N-dimethylamino)benzaldehyde 11, presumably due to its much lower electrophilicity. We instead treated a solution of 10 with n-butyl lithium at -78 °C followed by addition of aldehyde 11, which afforded 1 as a bright orange solid in 27% yield. The synthesis of both 8 and (±)-9 (Scheme 1B) started from alkyne 18, which was prepared from 19 [START_REF] Johnson | [END_REF] by addition of lithiated trimethylsilylacetylene and successive methylation with dimethyl sulfate according to literature procedures (Section S2 in the Supporting Information (SI)). In both cases, the in situ formed acetylide of 18 reacts with electrophiles, cyclopentadienone 19 or tropylium tetrafluoroborate, to give 20 and 21, respectively, in good yields. The structure of compound 21 was confirmed by single-crystal X-ray diffraction (Section S10 in the SI). Alcohol 20 was subsequently subjected to reductive elimination with tin(II) chloride in acidic ethereal solution, which is a commonly used method for cumulene synthesis. [15] After 2.5 h, pure cumulene 8 was filtered from the reaction mixture as a dark brown solid. Treatment of 21 with an excess of TFA led to the unexpected formation of fulvene derivative (±)-9 in 33% yield (for a discussion of the possible mechanism, see Section S4 in the SI).

X-ray Crystal Structures

The structures of 6-phenylfulvenes 1 and 6, intermediate 21, extended fulvalene 8, and fulvene (±)-9 were determined by single-crystal X-ray diffraction (Figure 2, Section S10 in the SI). Monoclinic crystals (space group C1c1 (no. 9)) of 1 were obtained by slow evaporation of CH2Cl2. The two rings of the molecule are twisted by 21°. Monoclinic crystals (space group P21/c (no. 14)) of 8 were grown by layering MeOH onto a fluorobenzene solution. The unit cell accommodates one half of centrosymmetric 8. At 100 K, bond length alternation (BLA) [START_REF] Januszewski | BLA in cumulenes is reported as the (average) difference in the bond length between the two central-most[END_REF] of the cumulenic bonds was determined to be 0.099 Å. This value is a little larger than expected, based on comparison with other symmetric

[3]cumulenes. [17] This could be attributed to the strong πconjugation between the terminal cyclopenta1,3-diene rings and the cumulene moiety of 8. [18] Conjugation to the peripheral phenyl rings is hindered since they are twisted by 45° to 61° relative to the planar extended fulvalene core. The intramolecular center-tocenter distance between phenyl rings attached to different cyclopenta-1,3-diene rings (Figure 2) was measured to be 4.7 Å, reflecting only weak aromatic π-π-stacking interactions. [19] Brown crystals (monoclinic, space group P21/n (no. 14)) suitable for X-ray diffraction were obtained by slow diffusion of pentane into an ethyl acetate solution of compound (±)-9 at 5 °C. Only the displayed stereoisomer (2R,3S,6aR,7R)-9 was present in the measured crystal. Similar to 8, phenyl groups are twisted by 38° to 64° out of the plane of the cyclopenta-1,3-diene rings to which they are attached.

Discovery of Unusal Protonations by Electronic Absorption Spectroscopy

We recorded the UV/Vis spectra of compounds 1-8 in both CH2Cl2 and 1,2-DCE and of compound (±)-9 in CH2Cl2. The spectra were also measured upon addition of acid (TFA). For all compounds 1-(±)-9, the spectra of the neutral compound could be fully restored by addition of base (NEt3) to the acidified solution.

The spectra of 1 are shown in Figure 3. In 1,2-DCE, the longestwavelength absorption band of a yellow-colored, neutral solution of 1 appears at λmax = 398 nm (3.12 eV), which we attribute to an ICT between the amine donor and the fulvene acceptor. Upon acidification with TFA, the solution color changes in a remarkable way (Figure 3, top) to deep blue and in presence of around 20 equiv. of the acid, when protonation is complete the spectrum features a broad, longest-wavelength band at λmax = 593 nm (2.09 eV) (Figure 3, middle). [START_REF]The different amount of TFA needed to fully form the protonated 6phenylfulvenes qualitatively correlates with the pKa values; the more basic, the less excess of TFA is needed. We also note that in the NMR titrations, less[END_REF] The initial strong band of neutral 1 at 398 nm has nearly disappeared, and a new, higher-energy band arises at 309 nm (4.01 eV), which is very similar to that of compound 4 with an unsubstituted phenyl ring (experimental: λmax = 307 nm, 4.04 eV). As shown in detail in the following sections, the longest-wavelength band at 593 nm reflects a species protonated at carbon, whereas the band at 309 nm can be assigned to an N-protonated species. Upon addition of triethylamine, the spectrum of the neutral solution is fully restored apart from a small decrease in intensity attributed to the diluting effect of acid and base addition (see Figure 3, middle). Upon addition of an excess of even stronger acids to 1, such as HBF4 or MeSO3H, the solution turns yellow. In the presence of HBF4, a . Bottom: UV/Vis spectra of (±)-9 in CH2Cl2 neutral, acidified, [START_REF]The different amount of TFA needed to fully form the protonated 6phenylfulvenes qualitatively correlates with the pKa values; the more basic, the less excess of TFA is needed. We also note that in the NMR titrations, less[END_REF] and reneutralized with excess NEt3.

i iii ii iv v 11 → 1 R = NMe 2 (27%) 12 → 2 R = OMe (60%) 13 → 3 R = Me (74%) 14 → 4 R = H (78%) 15 → 5 R = Cl (74%) 16 → 6 R = CN (76%) 17 → 7 R = NO 2 (40%)
Neutral (±)-9 has limited stability in solution. After a few hours in CH2Cl2, the solutions turn red, which also occurs immediately upon heating to 40 °C. The stability of the protonated forms of the 6-phenylfulvenes in CDCl3 at room temperature was monitored for compounds 2 and 3 by 1 H NMR spectroscopy. After addition of 100 equiv. of TFA, 1 H NMR spectra were measured every 24 h over the course of 3 days showing no decomposition for both cations (see Figures S26 and S29 in the SI). [START_REF]The different amount of TFA needed to fully form the protonated 6phenylfulvenes qualitatively correlates with the pKa values; the more basic, the less excess of TFA is needed. We also note that in the NMR titrations, less[END_REF]

Plausible Protonation Sites

We expected the nitrogen atom to be the most basic site of 6phenylfulvene 1. Protonation at nitrogen, leading to 1a (Figure 5), diminishes the push-pull effect by eliminating the ability of the amine lone pair to take part in π-conjugation. Hence, it does not explain the bathochromic shift of the longest-wavelength absorption band that leads to the intense blue color. Based on previous reports on fulvene protonation, we suggest protonation to occur at C5 leading to 1b. [4,7] As a result of the positive charge at C4, the push-pull character of 1b is even more pronounced than in 1. This can be illustrated by analysis of its resonance structure 1e, which is an iminium ion, with a highly delocalized charge. However, protonation at both sites, nitrogen and C5 leads to a structure (1d) where the push-pull effect is almost canceled out. Thus, we propose, that upon addition of excess TFA, there is tautomerism only between 1a and 1b. Similar considerations can be made for compounds 2-7.

For extended fulvalene 8, we suggest protonation at C2 (for numbering, see Figure 1), which produces the species with the largest number of reasonable resonance structures.

For compound (±)-9, the situation is more complex (Figure 6). Protonation is most plausible to take place at (i) the ester ((±)-9a),

(ii) one end of the π-conjugated system ((±)-9b and (±)-9c), or at (iii) the other end ((±)-9d). When protonation takes place at carbon, cations with highly delocalized charges are formed. Protonated (±)-9b and (±)-9c can be presented as four resonance structures where the positive charge could to some extent (due to nonplanarity) be delocalized into a phenyl group. In contrast, (±)-9d has only three reasonable resonance structures with charges adjacent to a phenyl group. Additionally, for (±)-9d one of the resonance structures is formally antiaromatic. Figure 6 shows the three diastereoisomers (±)-9b, (±)-9c, and (±)-9d that are believed to be sterically favored. These hypotheses are cacked in the following based on extensive theoretical and NMR studies.

NMR and EPR Studies

In order to understand the equilibrium between 1 and its protonated species 1a and 1b, we conducted 1 H NMR titrations of 1 with TFA in CH2Cl2 (Figure 7). Addition of one and two equiv. of TFA mainly leads to spectra at fast exchange between species with shifts and integrals that fit the structure of the N-protonated species 1a. The aromatic signals at 6.7 ppm and 7.5 ppm move closer together, indicating protonation at nitrogen. This is because upon protonation of the amine, the π-donor effect vanishes. Thus, the ortho-(3',5') and meta-positions (2',6') relative to the amino group become electronically less distinguished. However, upon addition of more TFA, we see slow exchange between 1a and another species. In the presence of five equiv. of TFA, the spectrum fits the structure of the C5-protonated form 1b. The signal of the methylene group for 1b can be found at 3.8 ppm as a singlet (Figure 7, yellow background). This result is comparable to earlier findings about fulvene protonation where methylene groups give signals at 4.2 to 4.8 ppm. [4,7] Upon addition of deuterated TFA, the methylene singlet vanishes completely, which shows that both C5 protons are exchanged by deuterium. Thus, we propose, that in the presence of four equiv. of TFA, Nprotonated 1a and C5-protonated 1b coexist. Addition of an excess of TFA leads to a shift of the equilibrium towards the Cprotonated form 1b. [30] Figure At the presence of four equiv. of TFA, selective 1D 1 H EXSY NMR measurements show in fact an exchange of the signals at 3.8 and 6.3 ppm with exchange rates of k1 = 7.4 s -1 and k2 = -5.1 s -1 (Figure 5). [22] In the presence of very strong acids, such as HBF4, the solution turns from intense blue to yellow and we were able to observe the di-protonated species 1d (Figure S22 in the SI). We also recorded electron paramagnetic resonance (EPR) spectra of 1 in CH2Cl2 in the presence of an excess of TFA. The EPR signal was completely silent and formation of any single electron species can be excluded. Similarly, acidified solutions of fulvenes 2 and 3 were analyzed by 1 H NMR (see Section S3.2 in the SI). For both, in the presence of 100 equiv. of TFA the spectra nicely fit the structures of 2 and 3 protonated at C5 with the new methylene singlet coming in at 4.14 ppm for 2 and 4.25 ppm for 3 (Figures S25 and S28 in the SI).

Theoretical Studies

In order to better understand the optical spectra as well as the most plausible protonation site(s), we computed optical properties and Gibbs free energies in CH2Cl2, for the neutral forms of 1, 4, 6, 8, and (±)-9 and several protonated species. Computations were performed using the Gaussian09 program package, [START_REF] Frisch | Gaussian 09, Revision D.01[END_REF] and details are found in the Experimental and in Section S8 in the SI. For fulvene 1, we compared protonation at C2, C3, C4, and C5 (1b), nitrogen (1a), as well as di-protonation (C5 and nitrogen; 1d). Protonation at C5 led to the lowest Gibbs free energy species compared to protonation at the other carbon atoms. (E) to (Z) isomerization of the exocyclic double bond of 1 led to higherenergy species, regardless of protonation state. In C-protonated 1b, the fulvene and phenyl rings are almost in plane with a torsion angle of 3° whereas for the neutral form 1 the rings are twisted by 21° (crystal structure; calc: 33°) and even more so for N-protonated 1b (torsion angle of 40°). As the diprotonated form 1d again is more planar (torsion angle of 6°), protonation at C5 seems to help keeping the torsion angle small. This effect should facilitate π-conjugation throughout the entire molecule and could explain the experimentally found lower absorption energy of the main transition of dication 1d (2.79 eV) compared to neutral 1 (3.13 eV).

Regarding optical spectra, for all protonation sites of 1, except nitrogen, the main absorption band is clearly bathochromically shifted as compared to the neutral form. Protonation at C5 (1b) leads to an intense band at 470 nm (2.64 eV, f = 1.23, f being the oscillator strength). This is significantly lower than the experimental value (593 nm; 2.09 eV). However, the bathochromic shift of the longest-wavelength absorption band upon protonation is in good agreement with the experiment (calculated: 0.95 eV; experimental: 1.03 eV). Besides, the hypsochromic shift of the longest-wavelength absorption band computed for the protonation at nitrogen (1a) is also in fair agreement with experiment (calc. 0.77eV versus exp. 0.89 eV, respectively) supporting the hypothesis of coexistence of these two protonated species upon acidification. For all protonated species, the lowest energy transitions have a HOMO-to-LUMO character. Figure 8 compares frontier orbital lobes of 1, 1b, and 1d. For 1a, the corresponding oscillator strength is low and the lowest-wavelength absorption band mainly involves HOMO-1 and LUMO. On the other hand, for all other computed protonated species, this HOMO-to-LUMO transition contributes most to the longest-wavelength absorption peak. The ranking of the main absorption band is λmax: cation (1a) < neutral (1) < dication (1d) < cation (1b). This is in full agreement with the experimental findings. The dication 1d retains a sizeable push-pull character. This can be attributed to the increased planarity of the system as discussed above. The calculated optical data for compound 4 are consistent with the experimental findings (Figure S35 in the SI). Compared with compound 1, HOMO and HOMO-1 are inverted concomitantly with the character of the first two excitations of compound 4. Thus, the lowest-energy transition remains weak and the second-lowest corresponds to the main absorption band. Upon protonation, the nature of the HOMO changes (HOMO-1 of the neutral species becomes HOMO of the cation), which leads to increased oscillator strength of the first excitation and concomitant bathochromic shift. Similarly, computational results for compound 6 are consistent with experimental findings (Figure S37 in the SI) and show a comparable behavior upon protonation. The HOMO-1 of the neutral species becomes the HOMO of the species protonated at C5. In the neutral form, the highest absorption band corresponds to the second excitation that involves mainly an electron redistribution from HOMO-1 to LUMO and in the protonated form it is the first transition (HOMO-to-LUMO). Neutral [3]cumulene 8 has a calculated BLA of 0.072 Å, which is smaller than the experimental one determined by single-crystal Xray analysis (0.099 Å). For 8, we compared protonation at C2 and C3 (see Figure 1). Protonation at C3 leads to the lowest Gibbs free energy. Additionally, it leads to an increased BLA of 0.088 Å (compared to the neutral calculated one) indicating an intensified push-pull character. Noteworthy, both protonation sites lead to significant bathochromic shifts of the longest-wavelength absorption band compared to the neutral compound. This shift between the neutral compound and the one protonated at C2 is in fair agreement with experimental data (calculated: 0.64 eV; experimental: 0.75 eV (see above and Figure S39 in the SI)), whereas it is significantly larger (1.06eV) for protonation at C3. This is also reflected in BLA = 0.11 Å, which is the largest for the C3-protonated form. For the neutral form, the longest-wavelength absorption band corresponds to the third excitation that involves HOMO-2 and LUMO, the lower-intensity transitions having almost vanishing oscillator strength. Upon protonation at C2(C3), HOMO-2 gets destabilized and becomes HOMO-1 (HOMO) of the C2(C3)-protonated form. Thus, the transitions of sizeable oscillator strength keep a similar character and are significantly bathochromically shifted. The level of theory used herein leads to fair qualitative agreement with experimental data and corroborates that the changes in optical spectra upon acidification are a result of the protonation at C5 (C2 for compound 8) of the fulvene moiety. It should be possible to reach quantitative agreement by implementing state specific corrections for solvent effects, vibronic contributions, and/or considering explicit solvent molecules as well as the dynamic conformational diversity present in solution at room temperature.

Electrochemistry

Electrochemical investigations of compounds 1 to (±)-9 were carried out by cyclic voltammetry (CV) and rotating disk voltammetry (RDV) in CH2Cl2 containing 0.1 M n-Bu4NPF6 (Table 1) with and without the addition of TFA. Table S1 in the SI shows all collected data. For all compounds without an amino group, one oxidation step is observed and is usually irreversible. Only compound 5 presents one reversible wave at +0.96 V. In the case of compound 1, one reversible one-electron oxidation is detected at +0.34 V, which is attributed to the oxidation of the amino group, accomplished prior to the oxidation of the fulvene moiety. Furthermore, one or two successive reduction steps are detected. For instance, two successive reversible one-electron reductions are detected for 7. The first reduction corresponds to the reduction of the NO2 group as previously observed. [START_REF] Hilger | [END_REF] Furthermore, 6 also presents two successive reductions, however, the second reduction is not reversible. In contrast, only one reduction (one electron exchanged and not reversible) is observed for compounds 4 and 5. In contrast, for compounds 1 and 2, the first reductions are still irreversible but exchanged two electrons. In the presence of 6 equiv. of TFA, the potentials of the oxidation step do not change much. One exception is 1, where the first oxidation at +0.34 V (oxidation of the amino group) disappeared due to protonation of the amino group (Me2NH + ). In the cathodic part, in acidic media, compounds 1, 2, 3, and 8 are more easily reduced in one new reduction step (between -0.4 and -0.8 V).

The other compounds present one large and irreversible wave at around -1.9 to -2.2 V. On the basis of the experimental values, HOMO and LUMO energy levels were estimated. These energy levels lead to HOMO-LUMO gaps in agreement with the optical gaps calculated from the lowest-energy absorption in the UV/Vis spectra. In the presence of TFA, only the four compounds 1, 6, 7, and 8 show experimental values for LUMO and HOMO energy levels comparable to those obtained with the optical spectra. For the four other compounds, one new reduction wave near -0.4 to -0.8 V is present. In these cases, the calculation of EOX1 -ERED1 (with E being the redox potentials) seems to be inaccurate using the potential of this peak. Finally, the plot of the correlations between the ligand electrochemical parameter (first oxidation and first reduction) and the Hammett substituent parameter σpara, as well as the plot of (EOX1 -ERED1) versus σpara show a linear trend as observed from optical measurement (see Figure S46 in the SI). In acidic media, the same behavior is observed for the first oxidation step under exclusion of compound 1 because of the presence of the protonated Me2NH + group. It must be noted that, for 7, the second reduction was used for the linear fit, due to the first reduction being the reduction of the NO2 group. For 1, the first oxidation potential corresponds to the oxidation of the amino group. Thus, we have used the second potential attributed to the oxidation of the fulvene unit. In this case, a better linear fit is obtained. Figure 9 shows a plot of the first oxidation potential of compounds 1-7 in the presence of TFA versus their Hammett substituent parameter σpara. Compound 1 is clearly off since it is protonated at the amino group first. In the case of compound 8, two successive one-electron oxidation steps and one-electron reduction steps are detected. In the presence of an excess of TFA, the oxidation waves are observed nearly at the same potential, but in the cathodic region, only one irreversible process is still present at -0.84 V versus two successive reduction steps at -0.90 and -1.17 V respectively in the absence of TFA. The redox properties of compound (±)-9 are reported in Section S8 in the SI.

Basicity

In order to get an insight of the actual quantitative basicity of the investigated fulvenes, pKa values of the protonated forms were determined making use of the spectrophotometric method of relative basicity measurements (for details, see Section S6 in the SI). Solubilities of compounds 1 to (±)-9 are limited to polar organic solvents such as dichloromethane, 1,2-dichloroethane, and acetonitrile. None of the compounds investigated herein are soluble in water or DMSO, which are more popular solvents for pKa measurements. [25] We decided to apply the technique of relative measurements which circumvents the need of measuring the acidity of the medium. [26] Where possible, we determined pKa values acetonitrile and pKip (ion-pair basicity) values in 1,2-DCE. The results are shown in Table 1.

Self-consistent basicity scales including weak bases are available in acetonitrile [27a] in in 1,2-DCE [27b] and can be used for comparison. Here for measurements in acetonitrile, acceptorsubstituted anilines were used as reference bases. These are very weak bases in acetonitrile. [27,[START_REF]Examples for reference bases: 2-NO2-aniline (pKa,MeCN = 4.80), 5-Cl-2-NO2-aniline (pKa,MeCN = 3.22), 4-NO2-aniline (pKa,MeCN = 6.22), 2[END_REF] For 6 and 7, we estimated pKa values in acetonitrile from those in 1,2-DCE since their basicity is too low to be measured in acetonitrile. [29] For 6-phenylfulvenes 1-7, experimental pKa values correlate well with pKa values calculated based on Hammett parameters of the para-substituents at the phenyl rings (Figure 10). We were not able to determine the pKa value of 8 and the second pKa value of 1 since they both are too low to be measured in the solvents used herein. Due to its instability in solution, pKa measurements of (±)-9 could not be meaningfully performed.

1.21 [b] 2.15 7 0.92 [b] 1.77 R where aF and aR are the substituent constants for the resonance effect and the field effect for para-substituents taken from [21].

Conclusions

Compounds 1-(±)-9 were successfully synthesized from commercially available starting materials. They showed longerwavelength absorptions upon protonation, as compared to their neutral forms accompanied by visible color changes. The effect is reversed by addition of base. In all cases, NMR studies, UV/Vis spectra, electrochemistry, and theoretical calculations corroborate that protonation takes place at C5 (C2 for compound The effect of this protonation on optical spectra can be rationalized by (i) effective delocalization of the cationic charge all over the π-system, (ii) for those molecules that carry electron-donating groups, reinforcement of the push-pull character by enhancement of the electron-withdrawing ability of the protonated fulvene moiety, and (iii) planarization of the 6phenylfulvenes.

The protonation behavior of compound 1 is particularly interesting.

Only at low acid concentration, compound 1 is effectively protonated at its dimethyl amino group to form 1a. However, when acid concentration is increased, this form is in equilibrium with the tautomer 1b, which is protonated at C5 and can be drawn as the iminium ion 1e. Additionally, pKa values confirm that we are dealing with weak bases with basicities comparable to those of acceptor-substituted anilines. The second pKa value of 1 is too low to be measured in 1,2-DCE. However, upon addition of much stronger acids such as HBF4, the di-protonated species 1d can be observed by 1 H NMR and UV/Vis spectroscopies.

In this paper, we show that it is possible to protonate the acceptor moiety of a push-pull system when the resulting positive charge is efficiently conjugated throughout the molecule. The study shows that care is advised when assigning charge-transfer bands of push-pull compounds to a specific electronic absorption band based on changes in optical spectra upon acidification. This particular approach only works if the donor is effectively the most basic part of the molecule. Altogether, we presented a new group of pH sensitive chromophores, whose scope and suitability for future applications in advanced functional materials, pH sensing, and imaging will be further elaborated in our laboratories.

Experimental Section

Theoretical Calculations

Computations were performed using the Gaussian09 program package. [START_REF] Frisch | Gaussian 09, Revision D.01[END_REF] All computed structures were characterized through frequency calculations for the verification of minima on the potential energy surface. Geometries were optimized using the ω B97XD exchange-correlation functional and the 6-31+G* basis set in DCM (PCM model). [START_REF] Frisch | Gaussian 09, Revision D.01[END_REF] Gibbs free energies were computed at the same level of theory. Optical properties were computed at the TD-ωB97XD/6-31+G* level of theory in DCM. [30] The bandwidths have been fixed to 0.25 eV. For compounds 8 and (±)-9, we used the 6-31G* basis set for geometries, Gibbs free energies, and optical properties due to convergence issues. Gibbs free energies, frontier orbital representations, and optical properties of all computed structures can be found in Section S8 in the SI.

Synthesis

4-[(E)-(2,4-Di-tert-butylcyclopenta-2,4-dien-1-ylidene)methyl]-N,Ndimethylaniline (1).

A solution of 1,3-di-tert-butylcyclopenta-1,3-diene 10 [12] (1.78 g, 10 mmol) in THF (50 mL) under nitrogen atmosphere was cooled to -78 °C, treated with BuLi, 1. General procedure for 6-phenylfulvenes (2-7). Adapted from [11]. A solution of 1,3-di-tert-butylcyclopenta-1,3-diene 10 [12] (500 mg, 2.8 mmol, 1 eq) and the aldehyde (4 eq) in MeOH (15 mL) was heated to reflux and dropwise treated with a solution of 5 wt% NaOMe (35 mL, 30.8 mmol) in MeOH over 30 min. The mixture was stirred at reflux until the starting material 1,3-di-tert-butylcyclopenta-1,3-diene was consumed (monitored by TLC). After evaporation, a solution of the residue in CH2Cl2 was washed with water two times, dried over MgSO4, and evaporated. Flash column chromatography (hexane/CH2Cl2) gave the 6-phenylfulvenes 2-7. Their characterization is in Section S2.

1,1',1",1"',1"",1""',1""",1"""'-Ethene- A flask was charged with 21 (1.13 g, 2.2 mmol) and a stir bar. The flask was evacuated and filled with nitrogen three times. CH2Cl2 (50 mL) and then TFA (0.84 mL, 11 mmol) were added. The mixture was stirred at room temperature for 1 h (monitored by TLC hexane/EtOAc 9:1) and poured onto a sat. NaHCO3 solution. After separation of the layers, the aqueous layer was extracted with CH2Cl2 three times. The combined organic layers were washed with H2O and brine, dried over MgSO4, and evaporated. Flash column chromatography (hexane/EtOAc) gave (±)-9 (391 mg, 33%) as a dark pink solid. M.p. 210 °C;

Figure 1 .

 1 Figure 1. Structures of 6-phenylfulvenes 1-7, [3]cumulene-expanded fulvalene 8, and fulvene derivative (±)-9. The shown arbitrary numbering of carbon atoms

Scheme 1 .

 1 Scheme 1. A) Synthesis of 6-phenylfulvenes 1-7. i) n-BuLi, THF for 1; NaOMe, MeOH for 2-7; B) Synthesis of compounds 8 and (±)-9 from alkyne 18. ii) n-BuLi, THF, -78 °C to 25 °C, 20 h; iii) SnCl2, HCl, Et2O, 25 °C, 2.5 h; iv) EtMgBr, THF, -78 °C to 25 °C, 4 h; v) TFA, CH2Cl2, 25 °C, 1 h. THF = tetrahydrofuran; TFA = trifluoroacetic acid.

1 - 7 (

 17 longest-wavelength band at λmax = 444 nm (2.79 eV) appears,indicative of yet a new species formation (see FigureS32in the SI) Below, we provide evidence for the formation of a dicationic species, protonated at N and C. In the absence of acid, compounds 2-7 show absorption bands with longest-wavelength maxima between λmax = 307-333 nm (3.72-4.04 eV) (see Figures S33-S38 in the SI). These values do not correlate well with the Hammett parameter para of the parasubstituents at the phenyl rings (r 2 = 0.45). Upon protonation at carbon (see below), 6-phenylfulvenes 2-7 all show a significant bathochromic shift of the longest-wavelength absorption band which appears between λmax = 535 (2, 2.32 eV) and 449 nm (7, 2.76 eV; see FiguresS33-S38in the SI). After protonation, a moderately good correlation (r 2 = 0.87) of the absorption maxima with the Hammett para-parameter is now observed for fulvenes Figure4). Neutral solutions of expanded fulvalene 8 in CH2Cl2 are deep red and turn dark grey after addition of TFA (FigureS39in the SI). In the absence of acid, the longest-wavelength absorption of 8 appears at λmax = 505 nm (2.45 eV), whereas in the presence of acid it shifts bathochromically to 731 nm (1.70 eV), reflecting protonation at carbon.

Figure 2 .

 2 Figure 2. Crystal structures of A) 1; B) 8, and C) (2R,3S,6aR,7R)-9. T = 100 K. Thermal ellipsoids at 50 % probability, for 9, hydrogen atoms are omitted for clarity.

Figure 3 .

 3 Figure 3. Top: Acidified solutions of 1 in CH2Cl2. Middle: UV/Vis spectra of 1 inCH2Cl2, neutral, acidified, and reneutralized with excess NEt3. Bottom: UV/Vis spectra of (±)-9 in CH2Cl2 neutral, acidified,[START_REF]The different amount of TFA needed to fully form the protonated 6phenylfulvenes qualitatively correlates with the pKa values; the more basic, the less excess of TFA is needed. We also note that in the NMR titrations, less[END_REF] and reneutralized with excess NEt3.

Figure 4 .

 4 Figure 4. Maximum absorption wavelengths λmax of 6-phenylfulvenes 1-7 inCH2Cl2 after addition of TFA, plotted against the corresponding Hammett parameters σpara (taken from[21]).

5 .

 5 Figure 5. Postulated Lewis structures of protonated 1.

Figure 6 .

 6 Figure 6. Postulated Lewis structures of protonated (±)-9. The new proton is marked in red.

Figure 7 . 1 H

 71 Figure 7.1 H NMR spectra (500 MHz, T = 298 K of 1 in pure CH2Cl2 and after addition of 1, 2, 3, 4, and 5 equiv. of TFA. Highlighted in yellow is the appearance of the methylene singlet of C-protonated 1b upon addition of excess TFA.

Figure 8 .

 8 Figure 8. Frontier orbital representations of neutral 1, C5-protonated 1b, and di-protonated 1d.

Figure 9 .

 9 Figure 9. First oxidation potential of compounds 1-7 in CH2Cl2 + 6 equiv. of TFA, plotted against the corresponding Hammett parameters σpara. Compound 1 is excluded from linear regression due to protonation at the amine.

  [a] Protonates at NMe2. [b] acetonitrile values estimated from 1,2-DCE values (see Section S6 in the SI).

Figure 10 .

 10 Figure 10. Experimental pKa values of 2-7 in MeCN, plotted against predicted pKas (y axis) by using the multiple correlation formula pKa = pKa0 + aF σF + aR σ

8 )

 8 of the fulvene ring.

  -7.00 ppm (m, 12 H; H-C(2,3,5,6) of Ph); 13 C NMR (101 MHz, CDCl3): 127.18 (C(4')), 127.21 (C(4')), 127.27 (C(2',6')), 127.64 (C(2',6')), 130.13 (C(3',5')), 130.28 (C(3',5')), 132.62 (C(1')), 132.67 (C(2,5)), 135.04 (C(1')), 136.57 (C(1')), 146.87 (C(3,4)), 161.51 ppm (C=C=C=C); IR (ATR): 𝜈 ̃ = 3057 (w), 2835 (very w), 1596 (w), 1573 (very w), 1483 (w), 1439 (w), 1338 (w), 1280 (very w), 1230 (m), 1180 (w), 1110 (w), 1070 (w), 1028 (w), 1004 (w), 977 (w), 917 (w), 907 (w), 842 (w), 803 (w), 760 (m), 747 (m), 725 (very w), 691 (s), 669 (m), 641 cm -1 (w); UV/Vis (CH2Cl2) λmax (ε) = 505 nm (53700 dm 3 mol -1 cm -1 ); MALDI-MS: m/z (%): 762.3191 (20), 761.3156 (65), 760.3123 (100, [M] + calcd for C60H40 + : 760.3125); elemental analysis calcd (%) for C60H40: C 94.70, H 5.30; found (%): C 78.37, H 4.86.

Table 1 .

 1 pKa values of 6-phenylfulvenes 1-7 measured in acetonitrile and pKa values in 1,2-DCE.

	Compound	pKa in MeCN	pKa in 1,2-DCE
	1	10.29	-[a]
	2	4.57	6.46
	3	3.52	5.37
	4	2.93	4.50
	5	2.58	3.83

  MHz, CDCl3): 56.18 and 60.35 (C(2,7)), 80.46 and 82.27 (C(3,6a), 125.89 and 126.06 (CF3), 126.69 (C-H of Ph), 126.71 (C-H of Ph), 126.78 (C-H of Ph), 126.87 (C-H of Ph), 126.92 (C-H of Ph), 127.09 (C-H of Ph), 127.13 (C-H of Ph), 127.14 (C-H of Ph), 127.31 (C-H of Ph), 127.32 (C-H of Ph), 127.94 (C-H of Ph), 128.04 (C-H of Ph), 128.10 (C-H of Ph), 128.12 (C-H of Ph), 128.17 (C-H of Ph), 128.18 (C-H of Ph), 128.22 (C-H of Ph), 128.28 (C-H of Ph), 128.65 (C-H of Ph), 128.67 (C-H of Ph), 129.83 (C-H of Ph), 130.16 (C-H of Ph), 130.25 (C-H of Ph), 130.55 (C-H of Ph), 131.30 (Cquart), 131.60 (Cquart), 131.69 (Cquart), 133.16 (Cquart), 134.53 (Cquart), 135.23 (Cquart), 135.59 (Cquart), 136.10 (Cquart), 136.13 (Cquart), 138.25 (Cquart), 138.51 (Cquart), 139.91 (Cquart), 141.32 (Cquart), 141.81 (C(1) of Ph), 142.31 (C(1) of Ph), 144.15 (C(1) of 1 Ph), 144.58 (C(1) of 1 Ph), 144.85 (C(1) of 1 Ph), 148.28 (C(1) of 1 Ph), 150.08 (C(1) of 1 Ph), 151.71 (C(1) of 1 Ph), 151.78 (C(1) of 1 Ph), 152.13 (CO2CF3), 169.49 ppm (C(1) of 1 Ph); 19 F NMR (282 MHz, CDCl3): -74.18 ppm; IR (ATR): 𝜈 ̃ = 3056 (w), 3026 (w), 1779 (m, C=O st), 1713 (w) 1598 (w), 1492 (m), 1441 (m), 1402 (w), 1316 (w); 1260 (w), 1220 (m), 1170 (m), 1142 (m), 1072 (m), 1027 (m), 1002 (w), 920 (w), 840 (w), 803 (w), 751 (m) 751 (m), 693 (s) cm -1 ; UV/Vis (CH2Cl2) λmax (ε) = 540 (13300), 358 (8970) nm (dm 3 mol -1 cm -1); MALDI-MS: m/z (%): 1080.4058 (34), 1079.4024 (83, [M + H] + , calcd for C78H54F3O2 + : 1079.4070), 1078.3990 (100, [M] + , calcd for C78H53F3O2 + : 1078.3992), 965.4127 (26, [M -C2F3O2] + , calcd for C76H53 + : 965.4142), elemental analysis calcd (%) for C78H53O2F3: C 86.80, H 4.95, O 2.96, F 5.28; found (%): C 86.85, H 5.10, F 3.00.

1 H NMR (600 MHz, CDCl3): 3.64 (d, J = 1.4 Hz 1 H, H-C(2)), 4.30 (s, 1 H, H-C(7)), 5.47 (d, J = 1,4 Hz, 1 H, H-C(3)), 7.24-6.34 ppm (m, 50 H, H-C(2,3,4,5,6) of 10 Ph); 13 C NMR (151

Acknowledgements

This work was supported by the Swiss National Science Foundation (SNF 200020_159802). The authors thank Dr. Bruno Bernet for assistance in the spectral evaluations as well as René Arnold and Dr. Marc-Olivier Ebert for recording selective 1D EXSY NMR spectra.

Acid-induced protonation of novel fulvenes and fulvalenes under ambient conditions leads to the formation of stable carbocations with unusual absorption spectra having bathochromcially shifted longestwavelength absorptions as compared to their neutral forms.


Jovana Milić

Thomas Schneeberger

Michal Zalibera

Karolina Z Milowska

Nils Trapp

Laurent Ruhlmann

Corinne Boudon

Carlo Thilgen

François Diederich

Thioether-Functionalized Resorcin[4]arene Cavitands: Electroswitchable Molecular Actuators

The utility of molecular actuators in nanoelectronics requires responsiveness to electric charge by mechanical motion at the interface with conductive surfaces. We hence functionalized redox-active resorcin[4]arene cavitands with thioether groups and investigated their propensity to act as electroswitchable actuators that can adopt two conformations in response to changes in external potential. Molecular design was probed by Density Functional Theory (DFT) calculations that suggest covalent binding of the cavitands to the Au surface in a preferentially upward orientation. The electronic properties were experimentally studied by cyclic voltammetry on Au electrodes, while redox interconversion between the oxidized (quinone, Q) and reduced (semiquinone radical anion, SQ) state was monitored by UV-Vis spectroelectrochemistry and EPR spectroscopy. It was shown that the reduction to the SQ dianions induces a conformational change to a closed form, providing the basis for voltage-controlled molecular actuating devices in the future.

Introduction

Molecular actuators can convert external energy inputs into mechanical motion by performing a switch between two conformations upon exposure to various chemical [1][2][3][4] or physical [5][6][7][8] stimuli, such as chemical agents, pH, temperature, or electrochemical processes. [1] Resorcin [4]arene cavitands were shown to be potential candidates in the development of molecular actuating devices, primarily owing to their conformational switch between the open (kite) and closed (vase) conformation. [2,5] These nanosystems are relevant in molecular electronics as switches, [2] sensors, [9][10][11][12] or elements in nanorobotics, [5,13] while the magnetic representatives are of interest as components of memory devices. [14,15] The applications of such systems however mainly require control by physical stimuli, such as voltage or light. [14,16,17] Furthermore, functionalization of molecular actuators for interfacing with conducting surfaces is a prerequisite to their integration into functional devices. [18][19][20] While progress has been made in developing strategies for controlled actuation of resorcin [4]arenes, the functionalization of redox-active analogues remains underexplored. In this regard, the lower rim of resorcin [4]arenes was shown to be functionalizable for assembling onto metallic surfaces, [21] and thioether chains of medium length (5-10 carbon atoms) were identified as suitable for covalent binding to Au. [22] In our recent work, we have demonstrated the first electrochemically switchable resorcin [4]arene cavitands. [14] These systems were based on quinone (Q) moieties and were shown to act as paramagnetic molecular switches upon reduction to the semiquinone (SQ) radical anion state (Figure 1a). In order to facilitate their installation into electroswitchable nanodevices, their functionalization remains unexplored. Herein, we present a new generation of thioether-functionalized resorcin [4]arene cavitands comprising naphthoquinone (NQ) and triptycene-quinone (TQ) redox-active walls (Figure 1b).

Their optoelectronic and conformational properties were studied in solution on Au electrodes by means of cyclic (CV) and rotating disc voltammetry (RDV), UV-Vis spectroelectrochemistry, and EPR spectroscopy, which was complemented by ab initio calculations, with the aim of evaluating their behavior as electroswitchable actuators. [a] Dr. J. Milić, T. Schneeberger, Dr. N. Trapp, Prof. C. Thilgen, Prof. F. Diederich, Laboratory of Organic Chemistry, ETH Zurich, Vladimir-Prelog-Weg 3, 8093 Zurich, Switzerland. Email: diederich@org.chem.ethz.ch.

[b] Dr. M. Zalibera, Institute of Physical Chemistry and Chemical Physics, FCHPT STU, Radlinského 9, 81237 Bratislava, Slovakia. Email: michal.zalibera@stuba.sk.

[c] Dr. K. Z. Milowska, Department of Physics and Center for Nanoscience (CeNS), Ludwig-Maximilians-Universität (LMU), Amalienstaße 54, 80799 Munich Germany. Email: kzm21@cam.ac.uk.

Results and Discussion

Functionalization of resorcin [4]arene cavitands 1 and 2 was performed through an attachment of the corresponding quinone walls 3-4 onto a quinoxaline tetrol platform 5 equipped by thioether chains in the lower rim (Scheme 1, bottom). The installation of the walls followed a common nucleophilic substitution approach [2,21] under basic reaction conditions employing Cs 2 CO 3 as a base, which afforded both cavitands in very good yields. Thioether-functionalized quinoxaline platform 5 was prepared via removal of the two opposite quinoxaline walls in the reaction of the tetraquinoxaline 6 with pyrocatechol as a nucleophile (Scheme 1, top). The intermediate 6 was accessed via substitution reaction of the quinoxaline wall precursor 7 with resorcin [4]arene octol 8 (crystal structure presented in Figure 2a). The octol 8 was synthesized through condensation of the commercially available resorcinol and the corresponding aldehyde 9, which was freshly prepared before the reaction by oxidation via pyridinium chlorochromate (PCC) of the alcohol 10. Preparation of the alcohol involved a synthetic procedure [22,23] based on the addition of butanethiol onto the terminal olefin bond in presence of 9-borabicyclo(3. DFT calculations of the prospective actuators 1 and 2 on Au(111) surface (Figure 2) indicated that they bind covalently in both redox states, preferentially in an upward orientation, with the thioether groups directly interacting with the surface (for details on the calculations on the surface refer to Section S8 of the Supporting Information, SI). Moreover, it was shown that the closed conformation in the SQ state is energetically more preferred for interaction with the Au(111) surface than the Q state. The electronic properties of 1 and 2 were experimentally analyzed by cyclic (CV) and rotating disc (RDV) voltammetry at ambient temperatures. Electrochemical measurements were carried out in CH2Cl2 by employing a gold wire (Au) as working electrodes in the presence of ferrocene (Fc + /Fc) as internal reference. CV of 1 demonstrated three successive reduction waves that were detected at -1.07 V, -1.68 V and -2.21 V (Figure 3a). RDV confirmed that each of the reduction steps occurred as a 2e - processes (Figure 3b). The first reversible 2e -reduction corresponded to the reduction of the two quinone groups and formation of the SQ - radical anions (2SQ - ) whereas the following quasi-reversible reductions could be attributed to the reduction of the semi-quinone radical anions and the formation of the quinone dianions (2Q 2-) and the quinoxaline reduction, respectively. [14] The redox properties of the triptycene-quinone 2 were comparable to those of the naphthoquinone 1 (Figure 3c-d), with reduction potentials shifted to more positive values (for more details refer to Section S4 in the SI).

This redox interconversion of thioether-functionalized cavitands 1-2 between the oxidized quinone (2Q) and reduced semiquinone (2SQ - ) radical anion states was anticipated to induce a change of geometry owing to stronger noncovalent interactions between the neighboring walls in the SQ state. [14] Such conformational properties can be monitored by NMR spectroscopy [24] and UV-Vis-NIR spectroelectrochemistry. [4,14] The geometry of resorcin [4]arene cavitands in the oxidized Q state can be probed by 1 H NMR spectroscopy through observing the chemical shifts of the methine protons, which indicate the open (kite) form by the appearance in the area between 3.5-4.5 ppm, whereas its occurrence at 5.5-6.5 ppm was shown to be indicative of the closed (vase) conformation. [5,[9][10][11]24] Both cavitands 1 and 2 in the Q state were shown to adopt kite conformations in CDCl3 at ambient temperatures (for details see Section S2 in the SI). Paramagnetic properties of the cavitands in the reduced SQ radical dianion state were shown to prevent using NMR to evaluate the properties due to fast relaxations in presence of free electrons that lead to disappearance of proton resonances. [14,15] UV-Vis spectroscopy was thus demonstrated as an alternative method to evaluate conformational properties of quinoxaline-based cavitands. [4,14,18,20] Changes in the absorbance in the domain between 300-350 nm, which arise from the absorption of the quinoxaline wall, were shown to be indicators of conformational changes. This absorption band undergoes a hypo-hypochromic shift upon the conformational change from the open to the closed form, which can be used to monitor the actuating motion. [14,21,22] We thereby relied on UV-Vis-NIR spectroelectrochemistry to monitor the changes in geometry of the cavitands 1 and 2 in the SQ radical anion state (Figure 4). 1 and (d) 2 recorded during the reduction in CH2Cl2. Disappearance of the absorption at 250 nm corresponds to the Q consumption (blue), whereas appearance of the band with the maximum at 440 nm reflects SQ radical anions formation (red). The changes of the Qx absorption at 300-350 nm indicates conformational changes of the cavitand from the open (blue) to the closed (red) conformation. [14] UV-Vis-NIR spectra at the first reduction wave of 1 and 2 confirmed the formation of the SQ radical anions (2SQ - ) (Figure 4) through the appearance the absorption in the visible 400-500 nm domain that can be attributed to the SOMO-LUMO transitions. [14,21,[25][26][27] Furthermore, inspection of the spectroscopic changes in the quinoxaline (Qx) absorption domain at 300-350 nm revealed a hypo-hypsochromic shift of the absorption at 300-350 nm for both cavitands 1 and 2 (Figure 4a-b), which suggested that the conformational change to the closed (vase) form took place, consistent with the behavior of quinone-based cavitands. [14,23] The complementary information on the geometry in the SQ state can be deduced from its paramagnetic properties owing to zero field splitting (ZFS) arising from the interaction of the two unpaired electrons that is a function of the distance between them. [23] In accordance with the previous reports, the continuous wave (CW) EPR spectrum of the reduced quinonebased cavitands (Figure 5a; for details see Section S6 in the SI) represents a composite signal corresponding to the presence of two paramagnetic species in solution. [23] The central singlet line can be assigned to the quinone-semiquinone radical monoanion (Q -SQ - ; where only one of the two quinone groups reduced; spin state S = ½), while the signals at the edges of the spectrum to the triplet 2SQ - ; semiquinone radical anions where the two quinone were reduced (S = 1). Accordingly, the signal of the 2 1(-) was simulated using the rhombic g tensor (gx 2.0073, gy 2.0053, gz 2.0019) with principal values determined in accordance with our previous study (green line in Figure 5a; for further details see Section S6, SI). The spectrum of the 2 2(-) semiquinone radical anions can be well reproduced using a rhombic g tensor (gx 2.0070, gy 2.0052, gz 2.0020) and a slightly rhombic, almost axial ZFS tensor characterized by the parameter D = 79.7 MHz and E = 3.9 MHz (blue line in Figure 5a), which are compatible with analogous systems reported previously. Assuming a dominantly dipolar character of the ZFS tensor and using the point-dipole approximation, [23] the distance of 9.9 ± 0.5 Å between the two unpaired electrons can be estimated. This value is in good agreement with the distance between the two quinone rings in the closed conformation of the functionalized cavitand, confirming the closed geometry in the triplet dianion state. This finding suggested that the actuating motion took place upon redox interconversion to the SQ radical anions state (2SQ - ). Reversibility of this process was assessed by monitoring the evolution of the SQ absorption in the UV-Vis spectrum (Figure 5b). Reversible changes of the absorption at 456 nm, 340 nm, and 247 nm over several reduction-oxidation cycles confirmed that the redox interconversion between the Q and SQ states ensued with remarkable reversibility, particularly in the case of triptycene-quinone 2. Cavitand 1 demonstrated two stepwise changes of the absorption at 340 nm and 247 nm (for details see Section S5 in the SI), which was likely to indicate that the conformational change took place on a slower timescale compared to the redox interconversion. This was not evident for the triptycene-quinone 2 (Figure 5b), presumably as the conformational switch occurred at a faster rate. This finding further suggests the potential of such host molecules to act as molecular grippers, [5,14,[28][29][30] whose assembly on Au surfaces is a subject of an ongoing research in our group. These studies promise to provide the basis for designing molecular actuators for conductive surfaces towards development of future nanoactuators in molecular electronics.

Conclusions

In summary, this study presents an investigation of thioetherfunctionalized cavitands 1 and 2, whose electronic and actuating properties were probed by DFT calculations on Au surfaces and evaluated via electrochemical and UV-Vis-NIR spectroelectrochemical studies on Au electrodes complemented by EPR spectroscopy. Furthermore, high reversibility of the redox interconversion between the Q and SQ state was shown to be accompanied by a conformational change from an open to a closed conformation upon electrochemical reduction for both 1 and 2 in the SQ state. These findings revealed the capacity of these functionalized systems to act as electroswitchable molecular actuators at the interface with conductive surfaces, which sets the stage for the development of molecular actuating devices in the future.

Experimental Section

General Information. Synthesis of compounds 1-2 was performed by using precursors 3-9 that were prepared according to modified literature procedures by using commercially available resorcinol, 4-penten-1-ol, 1butanthiol, 2,3-dichloroquinoxaline, and 2,3-dichloronaphthoquinone. The procedures and characterization of the compounds are reported in the Supporting Information, which also contains the information on NMR, EPR, spectroscopy, crystal structure data, electrochemistry, UV-Vis spectroelectrochemistry, and DFT calculations.

Electroswitching Studies. Electronic properties were analysed by cyclic (CV) and rotating disc voltammetry (RDV) in conjugation with UV-Vis spectroelectrochemistry (SEC) and EPR spectroscopy. Electrochemical measurements were carried out in CH2Cl2 containing 0.1 M n-Bu4NPF6 in a classical three-electrode cell by employing either a glassy carbon disk (GC, 3 mm in diameter) or a gold wire (Au) electrode as a working electrode, Pt wire as an auxiliary, and another Pt wire as a pseudo reference electrode. Ferrocene (Fc + /Fc) was used as internal reference. UV-Vis spectroelectrochemical experiments were performed in situ with 1-2 mM sample solution in degassed CH2Cl2 and 0.2 M n-Bu4NBF4 as the supporting electrolyte. A three-electrode arrangement with Pt mesh as WE, glassy carbon CE and Ag wire as pseudoreference electrode was used in a thin layer optical cell with KBr windows. Further details can be found in the Supporting Information.

Figure 1 .

 1 Figure 1. Top: Schematic representation of a binary switch between an open (kite) form in the oxidized Q state, and the closed (vase) conformation in the reduced SQ radical dianion state of quinone-based cavitands. Bottom: Structure of thioether-functionalized resorcin[4]arene cavitands 1-2 employed in this study. NQ = naphthoquinone; TQ = triptycene-quinone.

[

  Scheme 1. Synthesis of resorcin[4]arene cavitands 1 and 2. Top: (a) 9-BBN, THF, 20 °C, 3 h, 65% 10; (b) PCC, CH2Cl2, 20 °C, 5 h, 81% 9; (c) HCl, EtOH, 0-70 °C, 16 h, 98% 8; (d) Cs2CO3, DMSO, 50 °C, 16 h, 40% 6; (e) CsF, DMF, 80 °C, 1.5 h, 55% 3. Bottom: (a) Cs2CO3, DMF, 70 °C, 2 h, 78% 1; (b) Cs2CO3, DMF, 50 °C, 72 % 2. 9-BBN = 9-borabicyclo[3.3.1]nonane; PCC = pyridinium chlorochromate.

Figure 2 .

 2 Figure 2. (a) Crystal structure of quinoxaline cavitand 6 (P-1 space group, for details see Section S3 in the SI) and optimized geometries (DFT B3LYP/6-31G(d)) of the closed conformations of (b) naphtoquinone cavitand 1 in the Q (left) and the SQ (right) states, as well as (c) triptycene-quinone cavitand 2 in the Q (left) and the SQ (right) states. The thioether groups of the lower rim of cavitands 1 and 2 are omitted for clarity.

Figure 3 .

 3 Figure 3. CV of cavitands (a) 1 and (c) 2 in 0.1 M n-Bu 4 NPF 6 /CH 2 Cl 2 at scan rates of 0.1 Vs -1 , along with RDV of (b) 1 and (d) 2 on Au electrodes. Ferrocene (Fc + /Fc) is used as internal standard.

Figure 4 .

 4 Figure 4. UV-Vis-NIR spectroelectrochemical study of (a) 1 and (c) 2 at ambient temperature upon electrochemical reduction in 0.1M n-Bu4NPF6/CH2Cl2 in the potential region of the first reduction step at scan rate of 10 mVs -1 . Inset shows the corresponding CV at scan rate of 0.01 Vs -1 . Differential UV-Vis spectra of (b) 1 and (d) 2 recorded during the reduction in CH2Cl2. Disappearance of the absorption at 250 nm corresponds to the Q consumption (blue), whereas appearance of

Figure 5 .

 5 Figure 5. (a) X-band (9.4 GHz) cw EPR spectrum of 0.5 mM 2 in CH2Cl2 reduced with two-fold excess of CoCp2 at 100K (black). Red dotted lines show the simulation. The green and the blue lines show the contributions of the monoanionic (S = ½) and dianionic (S = 1) species in the simulated spectra, respectively. (b) Evolution of the UV-Vis-NIR spectra of 2 over several switching cycles upon electrochemical reduction in the potential region of the first reduction step illustrating the reversibility of the Q-SQ interconversion. For the corresponding details on cavitand 1, please refer to the Supporting Information.

  COMMUNICATIONThioether-functionalized quinone-based resorcin[4]arene cavitands act as electroswitchable molecular actuators at the interface of conductive surfaces by reversibly contracting upon electrochemical reduction. Their conformational and optoelectronic properties reveal a promising platform for molecular actuating devices.Jovana Milić, Thomas Schneeberger, Michal Zalibera, Karolina Z. Milowska, Nils Trapp, Laurent Ruhlmann, Corinne Boudon, Carlo Thilgen, François Diederich *

Acknowledgements

The authors are grateful to Dr. Bruno Bernet (ETH Zurich) for proofreading the manuscript. K. Z. M. acknowledges the Interdisciplinary Centre for Mathematical and Computational Modeling at the University of Warsaw (Grant No. G54-8) for providing computing resources. J. M. is grateful to the Dositeja Fellowship for Young Talents.

Keywords: molecular actuators • electroswitch • supramolecular chemistry • host-guest systems • molecular devices


Moufida Merzougui

Kamel Ouari

Jean Weiss

ULTRASOUND ASSISTED SYNTHESIS, CHARACTERIZATION AND ELECTROCHEMICAL STUDY OF A TETRADENTATE OXOVANADIUM DIAZOMETHINE COMPLEX

Keywords: Schiff base, Oxovanadium, Ultrasound Irradiation, X-ray Diffraction, Cyclic Voltammetry

Ultrasound assisted synthesis, characterization and electrochemical study of a tetradentate oxovanadium diazomethine complex

Moufida Merzougui, Kamel Ouari, Jean

Introduction

Schiff base ligands (SB) derived from diamines and phenolic aldehydes have proven to be a class of versatile ligands for many transition metals including vanadium(IV) [1][2][3].

Recently, the coordination chemistry of vanadyl complexes has extraordinary developed in various directions due to their interesting structural features [4][5][6], catalytic applications [7] potential roles in a variety of biorelated processes, such as insulin mimics [8][9], as well as antimicrobial or anti-leukemia effects [10][11]. The presence of different heterocyclic moieties in some oxovanadium SB complexes has been linked to their antitumoral propertiesconfering them a promising future in treatment of cancers [12].

The majority of oxovanadium (IV) SB complexes exhibiting N2O2 coordination and reported in the literature are green. Their structure is generally well established as monomers with five-coordinate square-pyramidal geometry [5,6] but in rare cases, trigonal bipyramidal or distorted octahedral geometries have been observed [13][14][15]. Heterogeneous oxovanadium SB complexes catalysts, immobilized in microspheres, have been successfully used in the aerobic selective oxidation of ethyl benzene to acetophenone and exhibit high catalytic activity and excellent selectivity [16].

Electrochemicalstudy and structural proprieties of oxovanadium complexes with tetradentate SB ligands attract particular attention because of their reversible one-electron redox behavior [17][18]. However, oxovanadium complexes with a large number of tetradentate SB ligands have been widely reported [19][20].

Electrochemical study and structural properties of oxovanadium complexes bearing tetradentate SB ligands have attracted particular attention owing to their two successive one electron redox couple [17][18] and oxovanadium complexes with a large number of tetradentate SB ligands have been widely reported [19][20]. Over the last decades considerable efforts have been aiming at the development of new synthetic procedures including the solvent-free method [21], microwave assisted synthesis approach [22], electrosynthesis [23] and ultrasound assisted synthesis [24]. Ultrasound irradiation is considered as a green and efficient technique for the activation of reagents in the synthesis of organic [25][26] and inorganic compounds [27]. The formation of several metal complexes bearing azo ligands, like Schiff bases [28][29], porphyrins [30]and quinolones [31], have been enhanced under ultrasound. The significant features of the ultrasound approach are the formation of pure products in prominent yields [32], the improved rate of reactions and the easier handling of reagents and products. The ultrasound synthesis technique is thus highly adapted to green chemistry approaches in coordination chemistry [33].

Considering the importance of oxovanadium SB complexes, we wish to report in this work a safe procedure without solvent under ultrasound irradiation for the synthesis of an oxovanadium complex by template method, previously prepared by F.C. Anson [34] using a conventional procedure. The complex has been characterized by single crystal X-ray diffraction and the electrochemical properties of this complex have been investigated by cyclic voltammetry at a glassy carbon (GC) electrode in DMF solvent. The diffusion coefficient has been determined using the Levich plot on a GC rotating disk electrode.

Experimental

Materials and measurements

All chemical reagents and solvents were purchased from Merck or Aldrich and used without further purification.Sonication was performed on anUS-bathwith frequency of 50-60 Hz and a nominal power (720 W).Elemental analyses were performed on an Elementar-Vario EL III CHNS analyzer. Infrared spectra were obtained using potassium bromide (KBr) pellets (4000-400 cm -1 ) on a Shimadzu FTIR spectrophotometer.Electronic spectra in the 200-900 nm range were recorded using a Shimadzu UV-1800 spectrophotometer using DMF as solvent. Molar conductance of VOL complex was determined in DMF (10 -3 M) at room temperature using MeterLab CDM-210 conductivity meter.Melting point of the complex was determined on a Kofler Bank 7779 apparatus.

Electrochemical experiments were performed on a PGZ 301 potentiostat, cyclic voltammograms (CV) and rotating disk Electrode (RDE) measurements were recorded at room temperature in DMF solutions containing 0.1 M LiClO4 as supporting electrolyte, using a glassy carbon (GC) as working electrode, platinum wire and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. Prior to the experiments the working electrode was polished and rinsed thoroughly with distilled water and acetone.

The polished electrode was placed in 0.5 M H2SO4 solution and the electrochemical activation of the electrodes were performed by continuous potential cycling from -500 to 1800mV at a sweep rate of 100 mV s -1 until a stable voltammogram is obtained [35]. Before each electrochemical investigation, the electrolyte was purged with nitrogen during 15 minutes.

Cyclic voltammogram measurements were performed in therange -2200 to 1600mV

versus SCE in which the electrochemical redox couple of V IV /V V isobserved in the positive window of the scan.Under identical conditions, the ferrocene/ferricinium(Fc/Fc + ) couple is observed at 455mV.

Synthesis procedures 1.2.1. Ultrasonication method

A reaction flask containing 0.344 g (2 mmol) of 2-hydroxy-1-naphthaldehyde, 66.8 µl (1 mmol) of 1,2-diaminoethane and 0.265 g (1 mmol) of bis (acetylacetonato) oxovanadium(Scheme.1), mixed in a mortar, was immersed in an ultrasonic bath at a temperature of 50 °C. The reaction mixture was exposed to ultrasound irradiation for 40 min.

Upon reaction completion shown by TLC analysis (silica gel, CH2Cl2/MeOH, 9.5/0.5, V/V).

The obtained solid was filtered and washed several times with methanol and diethylether, successively and dried over night in vacuum.Single crystals, suitable for X-ray diffraction, were obtained after 2 days of crystallizationfrom DMSO/MeOH.Color: green, Yield: 95.4 %, mp: >270°.

O OH H 2 N NH 2 N O N O H H V O 2 VO(acac) 2 +
Solvent-free

))) + 50°C + Scheme 1. Synthesis of the oxovanadium complex (VOL).

Conventional method

To a solution of 0.344 g (2 mmol) of 2-hydroxy-1-naphthaldehyde in 8 ml of methanol was added 66.8 µl (1mmol) of 1,2-diaminoethane in 5ml of methanol.0.265g (1mmol) of bis (acetylacetonato) oxovanadiumdissolved in 15 ml of methanol was added drop wise. The reaction was stirred and refluxed for 3 hours under nitrogen atmosphere. Reaction complete, based on TLC analysis, the resulting compound was filtered and washed with methanol and diethylether to afford pure desired product yielding 77.6%.S.A. Amer [36], N. Choudhary [37] and F.C. Anson [34][35][36][37][38] have reported the synthesis of this complex yielding70-90%.

Both oxovanadium complexes gave satisfactory elemental analysis, in good agreement with calculated values. Analysis calculated for C24H18N2O3V, prepared under ultrasound irradiation: C: 66.52%, H: 4.19%, N: 6.46% ; found: C: 66.83% ; H:4.35 %; N: 6.

X-ray crystallography

Suitable single crystals of the oxovanadium complex (VOL)were grown by slow layer diffusion of DMSO into a MeOH solution at room temperature. A green plate single crystal of dimensions0.25x 0.18x 0.12 mm 3 suitable for X-ray analysis was used for data collection at 173(2) K on a Bruker APEX II DUO Kappa-CCD diffractometer equipped with an Oxford Cryosystem liquid device, using Mo/Kα radiation (α = 0.71073Å). The crystal-detector distance was 38mm.

The cell parameters were determined (APEX2 software) [START_REF]M86-E01078 APEX2 User Manual[END_REF] from reflections taken from three sets of 12 frames, each at 10s exposure. The structure was solved by direct methods using the program SHELXS-97 [START_REF] Sheldrick | SHELXS-97. Program for Crystal Structure Determination[END_REF]. The refinement and all further calculations were carried out using SHELXL-2013 [START_REF] Sheldrick | A short history of SHELX[END_REF]. The H-atoms were included in calculated positions and treated as riding atoms using SHELXL default parameters. The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on F 2 . A semi-empirical absorption correction was applied using SADABS in APEX2 [START_REF]M86-E01078 APEX2 User Manual[END_REF]; transmission factors: Tmin/Tmax = 0.6157/0.7456.

Results and discussion

Molar Conductance

The molar conductance of 10 -3 M of VOLcomplex in DMF as solvent is Λ=1.74 Ω -1 cm² mol -1 , this very low value indicating that the title complex is electritically neutral [42][43].

Crystal structure of VOLcomplex

The crystal structure of the complex, depicted in Fig. 1, consists of one molecule in the asymmetric unit. The compound crystallizes in the monoclinic system with space group P21/c and the cell dimensions a=8.3960 (5); b= 12.5533 (8); c= 18.7804 (11) Parameters for data collection and refinement are summarized in Table 1. Selected bond lengths and angles are listed in Table 2.

Å; α = γ = 90°; β =104.843° (2).

Table 1 Table 2

The structure of the VOL complex has a slightly distorted square pyramidal coordination geometry.The basal square plane is formed by the N,N'-bis (2-hydroxy-naphthalidenato)diaminoethane molecule, which acts as a tetradentate ligand through its two N atoms and two deprotonated phenol O atoms. The two naphthalene rings are nearly coplanar with a dihedral angle of 2.315 [START_REF] Sheldrick | A short history of SHELX[END_REF]° between the two rings. Typical values of V-N bond distances of 2.0367(16) and 2.0401(16) Å, and V-O bond distances of 1.9491(14) and 1.9324(14) Å, are found in agreement with those reported for other oxovanadium (IV) complex [44]. The V1-O3 bond distance is 1.5951( 14) Å and the V1 atom is located 0.596 Å above the mean plane defined by atoms N1/N2/O1/O2. In the cell, the crystal is stabilized by a network of intermolecular hydrogen bonds, which are shown in Fig. 2. In these circumstances, there are no other intramolecular interactions in the structure.

FTIR spectra

The IR spectrum of the VOL complex exhibits several prominent bands in the 4000-400 cm -1 region. Among these absorptions, the VOL complex displayed strong C=N stretch (around 1618 cm -1 ) which indicates the C=N group of the coordinated SB ligand [45]. It can be noticed that ν C=N band in the complex is shifted to the lower energy region of approximately 20 cm -1 in the corresponding free ligand reported in the literature [46][47].

Another broad band around 1540 cm -1 is related to the aromatic C=C vibration [1]. In addition, the stretching vibration of C-O, in the naphtholatemoities, at 1340-1360 cm -1 suggests the coordination of the naphtholate oxygen to the vanadium ion as reported in the literature [46], this absorption frequency is higher than that observed in the spectrum of corresponding ligand (1327 cm -1 ) [48][49].

The complex exhibits characteristic sharp and medium ν(V=O) at 987 cm -1 indicating that there is no intermolecular interaction with the participation of the oxovanadium moiety [2][3][4][5][6][7][8][9][10].

Electronic spectroscopy

The electronic spectrum of the oxovanadium complex was recorded using DMF solution at room temperature in the range 200-800 nm.The complex showed two intense bands at 323 and 380 nm that can be assigned to π→π* transitions of the aromatic rings of naphthalene and n→π* transitions of the azomethine chromophores respectively [50]. A broad shoulder is observed near 400 nm corresponding to the ligand-to-metal charge-transfer transition (LMCT), from thelone pair system of both O-atoms in naphthalene rings to the d orbital of the vanadium atom [19].The vanadium (IV) complex exhibits an additional low intensity broad band in the range of 550-800 nm due to d→d transitions [5].The UV-vis absorption spectrum of VOL complex is shown in Fig. 3.

Electrochemical

The redox properties of the SB ligand and its oxovanadium complex were investigated by cyclic voltammetry under nitrogen atmosphere, using DMF solution containing 0.1 M LiClO4 as supporting electrolyte, at a glassy carbon as a working electrode, the redox potentials are expressed with reference to SCE. The cyclic voltammograms are given in Fig. 4. = -1704mV/ SCE are assigned to the VO V /VO IV and VO IV /VO III redox processes, similar compounds are widely reported in the literature. [17,[53][54].The reduction wave occurred at E pc = -1537 mV/SCE is due to the reduction of the azomethine groups. [55] In the free ligand, two irreversible waves are observed on its cyclic voltammogram, an anodic wave appearing at +1116 mV/SCE and a cathodic one at -1653 mV/SCE attributed respectively to the oxidation of the hydroxyl functionsand the reduction of the azomethine groups as reported in the literature [56,57]. The oxidation and the reduction peak potentials of the ligand are shifted to a slightly lower potential values, reveals that the hydroxyl(-OH) and the azomethine(C=N) groups are involved in the binding of the vanadyl core. [58] Table 3 Cyclic voltammogram measurements at different scan rates, between 50 and 400 mVs -1 (Fig. 5) indicate, in the potential range of 150 -600 mV/SCE, that E p values are slightly dependent upon the scan rate. By increasing the scan rate values, the anodic and the cathodic peak potentials shift respectively to a more positive andamore negativevalues. The ratios of the anodic to cathodic peakipa/ipccurrents are close to unity(Table 3). The peak separation between the cathodic and the anodic peak potentials, ΔE p , have beendetermined, and reach a value of82 mV at 100 mVs -1 scan rate that is greater than that expected for reversible systems, which indicates a quasi-reversible electrode process. Similar results have been reported for several oxovanadium SB complexes [12,17].A linear relationship between the anodic peak current and the square root of the scan rates(ipa = f(v 1/2 )) is observed (inset of Fig. 5), this fact implies that this electrochemical process is mainly diffusion-controlled [6,59].

Fig. 5.

In order to determine the diffusion coefficient, a hydrodynamic voltammetry experiment is carried outusinga glassy carbon rotating disk electrode (RDE) at various rotation rates, from 250 to 3000 rpm.The experiment is realized in DMF solution containing 1 mmol of VOL complex and 0.1 M LiClO4. The current-potential curves and the Levich plotsare shownare in Fig. 6. Under the studied conditions, a linear correlation between the limiting current and the square root of the rotating rate is observed which leads to the determination of the value of the proportionality constant in the Levich equation for a one electron VO V /VO IV reduction [60].

Diffusion coefficient was calculated to be (2.7.10 -6 cm²/s) from the slopes of the ilim vs. ω 1/2 plots, using Levich equation [START_REF] Bard | Electrochemical Methods: Fundamentals and Applications[END_REF]:

i lim = 0.620 n F A D 2 3 ω 1 2 ν -1 6 C
where ilim is the Levich current (A/cm²), n is the number of electrons transferred, F is the faraday constant (C/mol), A is the electrode area (cm²), υ is the kinematic viscosity (cm²/s), D and C are the diffusion coefficient (m²/s) and the analyte concentration (mol/cm 3 ) respectively,ω is the angular rotation rate of the electrode (rad/s).

Fig. 6.

Conclusion

In this paper, we describe the sonochemical and conventional synthesis ofa symmetrical oxovanadium Schiff basecomplex. In free solvent conditions, the ultrasonic irradiation procedure reduces the reaction time to 40 minutes instead of 3 hours compared with the conventional method and affords the desired product in a higher yield and purity. The non-electrolytic nature of the VOL complex was confirmed on the basis of their molar conductance value.The crystallographic data showed that the vanadium (IV) ion in VOL Schiff base complex is five coordinated with distorted square-pyramid geometry.

The electrochemical behavior of the complex exhibitsa quasi-reversible one electron redox process at E1/2=374 mV/SCE, with current ratio ipa/ipc equals to unity and the peak-topeak separation value, ΔEp, is of 82 mV. The linear relationship observed between the anodic peak current (ipa) and the square root of the scan rates (v The inset shows the anodic currents vs v 1/2 .

  32 % ; IR (KBr pellets  cm -1 ) 1618 (C=N),1340-1360 (C-O),1542 (C=C), 983 (V=O); UV-Vis: DMF, λ nm,[ε M -1 cm -1 ]: 323[7722], 380[4166], 610 [83].

Fig. 1 .

 1 Fig. 1.

Fig. 2 .

 2 Fig. 2.

Fig. 3 .

 3 Fig.3.

Fig. 4 .

 4 Fig. 4.

Fig. 2 .Fig. 3 .

 23 Fig. 2. Crystal packing of VOL complex showing intermolecular hydrogen bonds as dashed lines.

Fig. 4 .Fig. 5 .

 45 Fig. 4. Cyclic voltammograms of VOL complex (a), SB ligand (b) and Fc/Fc + (c) in 0.1 M LiClO4/DMF solution at scan rate of 100 mVs -1 .

Fig. 6 .

 6 Fig. 6. Voltammograms of VOL complex at RDE in 0.1 M LiClO4/DMF solution. Rotation rates: from 250 to 3000 rpm. The inset shows the Levich plot.

  1/2 ) indicates a diffusion-controlled nature of the electrode process. The diffusion coefficient is 2.5.10 -6 cm²/s from the Levich plots, Ilim = f(ω 1/2 ). ORTEP plot of the X-ray crystal structure of VOL complex with atom labeling scheme. Displacement ellipsoids are drawn at the 50% probability level except for the H atoms, which are shown as circles of arbitrary radius.

	Captions
	Fig. 1.
	Supplementary data
	CCDC 1003714contains the supplementary crystallographic data for VOL. These data
	can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from
	the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
	(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.

Table 2

 2 Selected bond lenghts (Å), and bond angles (°) for VOL et complex.

	Bond lenghts (Å)	Bond angles (°)
	V1-N1 2.0367 (16)	O1-V1-O2 89.21 (6)	O1-V1-O3 107.09 (7)
	V1-O1 1.9491 (14)	O1-V1-N1 85.41 (6)	O1-V1-N2 148.88 (6)
	V1-O3 1.5951 (14)	O2-V1-O3 111.75 (7)	O2-V1-N1 140.29(6)
	V1-N2 2.0401 (16)	O2-V1-N2 85.83 (6)	O3-V1-N1 107.42 (7)
	V1-O2 1.9324 (14)	O3-V1-N2 103.27 (7)	N1-V1-N2 79.12 (6)

Table 3

 3 Cyclic voltammetric parameters for the oxovanadium SB complex in different scan rates

	v	𝐄 𝐩𝐚	𝐄 𝐩𝐜	𝚫𝐄 𝐩	𝐄 𝟏/𝟐	𝐢 𝐩𝐚	𝐢 𝐩𝐜	ipa/ipc
	(mVs -1 )	(mV/SCE)	(mV/SCE)	(mV)	(mV/SCE)	(mA)	(mA)	/
	50	415	335	80	375	10.33	10.13	1.01
	100	415	333	82	374	14.66	14.35	1.02
	150	417	335	82	376	18.11	17.2	1.05
	200	418	334	84	376	20.54	19.56	1.05
	250	420	334	86	377	22.79	22.49	1.01
	300	422	332	90	377	25.35	25.22	1.01
	350	423	331	92	377	26.56	25.79	1.03
	400	424	330	94	377	28.24	27.76	1.02

Acknowledgements

The Authors gratefully acknowledge the financial support from The Algerian Ministry of Higher Education and Scientific Research. They also acknowledge the help to access the NMR Facility and Microanalysis Service at the Chemistry Institute of the University of Strasbourg -France.

] 1/2 𝑤 = 1/[𝜎 2 (𝐹 0 2 ) + (0.0430𝑃) 2 + 0.4910𝑃]where𝑃 = (𝐹 0 2 + 2𝐹 𝑐 2 )/3