Vinylogous Tetrathiafulvalene Based Podands - American Chemical

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Vinylogous Tetrathiafulvalene Based Podands: Complexation Interferences on the Molecular Movements Triggered by Electron Transfer Dominique Lorcy,* Michel Guerro, Jean-François Bergamini, and Philippe Hapiot* Institut des Sciences Chimiques de Rennes − UMR N° 6226 CNRS-Université de Rennes 1 (Equipe MaCSE) Campus de Beaulieu, Bât 10C, 35042 Rennes Cedex, France ABSTRACT: Substituted vinylogous tetrathiafulvalenes (TTFVs) containing two freely moving polyoxyethyl chains were prepared. Investigations of their redox behaviors in organic solvent show that these TTFV could efficiently complex metallic dications such as Pb2+ or Ba2+, leading to considerable modifications of their electrochemical response. As main feature, the molecule senses the association between the TTFV core and the metallic dication through a modification of the molecular motion triggered by the electron transfer. The complexation creates a link between the two parts of the TTFV core, causing considerable changes in the nature of the molecular motion. The resulting behavior is totally unusual as the 2-positively charged TTFV2+ appears to present the highest association constants with the metallic dication.

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INTRODUCTION Fascinating switchable processes have been built on electronrich tetrathiafulvalene (TTF) derivatives.1 Among the various redox-active receptors designed for that purpose, TTFs bearing crown ether binding sites display effective cation complexation properties.2 Thanks to the redox active TTF core, the binding occurrence is detected by modifications of the redox behavior of the TTF ligand.2 Actually, a cyclic binding site is not an essential condition for the elaboration of electroactive receptors as it was shown that TTFs bearing acyclic attachment sites show also good selectivity for metal ions3 and could be used in TTF assembly for this purpose.4 In this field of host−guest chemistry, we have previously examined a series of crown ether substituted vinylogous tetrathiafulvalenes (TTFV) 1 as potential ligands toward various cations (see scheme 1).5 The electrochemical detection of their binding ability is based on

the modification brought by the complexation of the molecular motions triggered by the electron transfer. For instance, in a series of cyclic TTFV such as 1a−c, with the same substituents but with links of various sizes between the two phenyl rings, the different redox responses are correlated to molecular movements of opposite nature associated with the electron transfer.5 For TTFV with a long link, such as for 1c, a stretch movement occurs upon electron transfer and two monoelectronic processes are observed.6 On the contrary, for the cyclic vinylogous TTF with shorter links (for example 1a,b), a fast molecular clip movement is induced upon oxidation and a single bielectronic system is observed. We found that the complexation of a metal ion by TTFV 1c alters the structural changes upon oxidation and therefore the redox properties of the TTFV. As noncyclic binding sites could be as effective in the complexation and are easier to synthesize, we prepared TTFV-podands 2 bearing two polyethyleneoxy arms (eight O atoms) that display preorganized structures for cations binding due to the conformational constraints imposed by the TTFV (Scheme 1).7 We could expect considerable influence of metallic complexation on the molecular conformation and thus resulting effects on the redox behaviors. In this paper, we report on the synthesis of a new class of TTFV-podands 2. We focus on the influence of the binding on their redox behaviors with the example of two metal dications (Pb2+ and Ba2+), which are investigated by cyclic voltammetry and simulations of the electrochemical responses.

Scheme 1. Structure of the Crown Ether TTFV 1a−c and TTFV Podands 2a,b

Received: February 12, 2013 Revised: March 27, 2013 Published: March 27, 2013 © 2013 American Chemical Society

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dx.doi.org/10.1021/jp401537a | J. Phys. Chem. B 2013, 117, 5188−5194

The Journal of Physical Chemistry B

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MHz) δ 18.9, 19.0, 59.1, 69.2, 69.7, 70.6, 70.7, 70.8, 71.9, 114.6, 114.9, 123.9, 126.9, 128.1, 128.9, 129.8, 156.9; HRMS (ESI) calcd for C19H26NaO4S4+ 469.0611, found 469.0610. Synthesis of Vinylogous Tetrathiafulvalenes TTFV 2a,b. To a solution of DTF 6 (1 mmol, 382 mg for 6a and 450 mg for 6b) in 20 mL of dry degassed CH2Cl2 under inert atmosphere was added I2 (1.27 g, 3 mmol), and the mixture was heated to reflux for 5 h. A large excess of Na2S2O4 (2 g) was added to the solution to reduce the dicationic TTFV (22+) formed in the medium, and the reaction mixture was stirred for 2 h at room temperature. After filtration of the suspension, the filtrate was washed with water (2 × 50 mL) and dried over MgSO4, and the solvent was removed under vacuum. TTFVs 2a,b were purified by column chromatography on silica gel using diethyl ether as eluent and were obtained as viscous oils in 45% yield. Data for 2a: 1H NMR (CDCl3, 300 MHz) δ 1.86 (s, 6H, CH3), 1.93 (s, 6H, CH3), 3.38 (s, 6H, OCH3), 3.50−3.70 (m, 16H, CH2),3.81 (t, 4H, CH2, 3J = 4.6 Hz), 4.10 (t, 4H, CH2, 3J = 4.6 Hz), 6.85 (d, 4H, CH Ar, 3J = 9 Hz), 7.37 (d, 4H, CH Ar, 3 J = 9 Hz); 13C NMR (CDCl3, 75 MHz) δ 13.1, 13.7, 59.1, 67.2, 69.7, 70.5, 70.6, 70.8, 71.9, 114.4, 121.1, 122.4, 122.8, 127.7, 130.7, 134.9, 156.8; HRMS (ESI) calcd for C38H50NaO8S4+ 785.2286, found 785.2280. Data for 2b: 1H NMR (CDCl3, 300 MHz) δ 2.41 (s, 6H, CH3), 2.42 (s, 6H, CH3), 3.38 (s, 6H, OCH3), 3.54−3.75 (m, 16H, CH2), 3.84 (t, 4H, CH2, 3J = 4.6 Hz), 4.11 (t, 4H, CH2, 3J = 4.6 Hz), 6.84 (d, 4H, CH Ar, 3J = 9 Hz), 7.31 (d, 4H, CH Ar, 3 J = 9 Hz); 13C NMR (CDCl3, 75 MHz) δ 18.2, 59.0, 67.3, 69.7, 70.5, 70.6, 70.8, 71.9, 114.6, 124.5, 124.8, 127.8, 128.1, 130.1, 133.4, 157.5; HRMS (ESI) calcd for C38H50NaO8S8+ 913.1169, found 913.1151. Electrochemical Experiments. All cyclic voltammetry experiments were carried out at 20 ± 0.1 °C using a cell equipped with a jacket allowing circulation of water from the thermostat. The counter electrode was a Pt wire, and the reference electrode an aqueous saturated calomel electrode (E°/SCE = E°/NHE − 0.2412 V) with a salt bridge containing the supporting electrode (n-Bu4NPF6, Fluka puriss electrochemical grade). The SCE electrode was checked against the ferrocene/ferrocenium couple in CH2Cl2 (E° = +0.528 V/ SCE) before each set of experiments. The working electrode was a 1 mm diameter glassy carbon disk, which was carefully polished before each set of voltammograms with a 1 μm diamond paste and ultrasonically rinsed in absolute ethanol. Electrochemical instrumentation consisted of a PAR Model 175 Universal programmer and home-built potentiostat equipped with a positive feedback compensation device.9 The data were acquired with a 310 Nicolet oscilloscope. Special care was taken to ensure that the residual ohmic drop remained negligible in the peak potential measurements. Pb2+ or Ba2+ aliquots were added from concentrated stock solutions of Pb(ClO4)2 or Ba(ClO4)2 dissolved in CH3CN. Numerical simulations of the voltammograms were performed with the KISSA 1D program10 using the default numerical options with the assumption of planar diffusion. Butler−Volmer law was considered for the electron transfer kinetics.11 The transfer coefficient, α, was taken as 0.5, and the diffusion coefficients equal for all of the species (D = 10−5 cm2 s−1).11b

EXPERIMENTAL SECTION All syntheses were performed under an argon atmosphere. The solvents were purified and dried by standard methods. Dithiafulvene 4a5 and 4-formylphenyl benzoate8 were prepared according to literature procedures. All other reagents were commercially available and used without further purification. NMR spectra were recorded on a Bruker AV300III spectrometer. Chemical shifts are reported in ppm referenced to TMS for 1H NMR and 13C NMR. Melting points were measured on a Kofler hot-stage apparatus and are uncorrected. Mass spectra were recorded with a Varian MAT 311 instrument by the Centre Régional de Mesures Physiques de l′Ouest, Rennes. Column chromatography was performed using silica gel Merck 60 (70−260 mesh). Synthesis of Dithiafulvenes (DTF). Dithiafulvene 4b. 4,5-Bisthiomethyl-1,3-dithiole-2-thione (1g, 4.42 mmol) and 4formylphenyl benzoate (1g, 4.42 mmol) were added under inert atmosphere to 5 mL of P(OEt)3. The reaction mixture was heated to 100 °C for 5h and then allowed to reach room temperature. The precipitate was filtered off and washed with EtOH to afford DTF 4b in 55% yield: mp 122 °C; 1H NMR (CDCl3, 300 MHz) δ 2.44 (s, 3H, CH3), 2.46 (s, 3H, CH3), 6.51 (s, 1H), 7.20−7.30 (m, 4H, CH Ar), 7.51−7.68 (m, 3H, CH Ar), 8.25 (d, 2H, CH Ar, 3J = 7.4 Hz); 13C NMR (CDCl3, 75 MHz) δ 18.9, 19.0, 113.9, 121.8, 124.1, 127.2, 127.8, 128.6, 129.5, 130.1, 131.2, 133.6, 134.1, 148.6, 165.1; HRMS (ESI) calcd for C19H16O2S4 404.0033, found 404.0029. Dithiafulvene 5b. Cs2CO3 (1.24g, 3,82 mmol) was added to a stirred solution of dithiafulvene 4b (1g, 2.47 mmol) in 40 mL of dimethoxyethane. The reaction mixture was refluxed for 24 h, and the solvent was removed under vacuum. The residue was dissolved in 60 mL of CH2Cl2, washed with water (3 × 50 mL), and dried over MgSO4. After removal of the solvents, dithiafulvene 5b was obtained as a beige powder in 90% yield and used in the next step without further purification: 1H NMR (CDCl3) δ 2.40 (s, 6H, CH3), 5.24 (s, 1H, OH), 6.40 (s, 1H, CH), 6.86 (d, 2H, CH Ar, 3J = 8.3 Hz) 7.18 (d, 2H, CH Ar, 3J = 8.3 Hz); 13C NMR (CDCl3) δ 18.2, 18.4, 111.5, 115.8, 128.1, 128.4, 130.4, 130.7, 134.2, 155.4 Dithiafulvenes 6a,b. NaH (60 mg, 2,5 mmol) was added to a solution of dithiafulvene 5 (1.5 mmol, 350 mg for 5a and 450 mg for 5b) in 20 mL of distilled THF. The reaction mixture was refluxed for 1 h under inert atmosphere, and tri(ethylene glycol) methylether tosylate was added (1.5 mmol, 480 mg) in 5 mL of THF. The solution was refluxed for an additional 48 h. After removal of the solvent, 30 mL of CH2Cl2 was added. The organic phase was washed with water (3 × 50 mL) and dried over MgSO4. After column chromatography on silica gel using Et2O as eluent the DTFs 6a,b were obtained as viscous oils. Data for 6a: yield 65% ; 1H NMR (CDCl3, 300 MHz) δ 1.91 (s, 3H, CH3), 1.93 (s, 3H, CH3), 3.36 (s, 3H, OCH3), 3.50− 3.75 (m, 8H, CH2), 3.85 (t, 2H, CH2, 3J = 5.1 Hz), 4.09 (t, 2H, CH2, 3J = 5.1 Hz), 6.35(s, 1H, CH), 6.87 (d, 2H, CH Ar), 7.15 (d, 2H, CH Ar); 13C NMR (CDCl3, 75 MHz) δ 12.9, 13.7, 59.0, 67.4, 69.7, 70.5, 70.5, 70.7, 71.3, 111.3, 114.5, 120.6, 120.7, 127.9, 130.2, 131.1, 156.3; HRMS (ESI) calcd for C19H26NaO4S2+ 405.1170, found 405.1163. Data for 6b: yield 70% ; 1H NMR (CDCl3, 300 MHz) δ 2.35 (s, 3H, CH3), 2.37 (s, 3H, CH3), 3.32 (s, 3H, OCH3), 3.40− 3.65 (m, 8H, CH2), 3.79(t, 2H, CH2, 3J = 5.1 Hz), 4.06 (t, 2H, CH2, 3J = 5.1 Hz), 6.36 (s, 1H, CH), 6.87 (d, 2H, CH Ar 3J = 9 Hz), 7.09 (d, 2H, CH Ar 3J = 9 Hz); 13C NMR (CDCl3, 75 5189



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Scheme 2. Chemical Pathways for the Synthesis of the Dithiafulvalenes

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RESULTS AND DISCUSSION

The oxidation potentials of DTFs 6a,b are influenced by the nature of the substituents on the dithiole ring, and as for 6b the Epa value is (+110 mV) positively shifted compared to 6a due to the presence of the electron-withdrawing effect of the two thiomethyl substituents (Epa = 0.50 V vs SCE for 6a and Epa = 0.61 V vs SCE for 6b). Oxidative coupling of DTFs 6a,b was realized using iodine as oxidizing agent, and the solution was then treated with a large excess of Na2S2O4 to reduce the TTFV dication formed in the medium (Scheme 3). Vinylogous TTFV 2a,b were isolated after column chromatography as viscous oils in 45% yield.

An efficient and simple access to TTFV consists of the preparation of the corresponding dithiafulvenes DTF followed by an oxidative coupling of two DTF derivatives.6,7,12 The DTFs derivatives were synthesized according to the chemical pathway outlined in Scheme 2. Two main approaches were used for the synthesis of DTFs 4a,b: for 4a, the reaction of the 4,5-dimethyl-1,3-dithiolylphosphonate anion5 with 4-formylphenyl benzoate, or for 4b, the heterocoupling of 4,5bisthiomethyl-1,3-dithiole-2-thione13 and 4-formylphenyl benzoate in neat triethyl phosphite. Deprotection of the phenol group is realized by treating 4a,b with cesium carbonate in refluxing dimethoxyethane (DME) to afford DTFs 5a,b in excellent yields.14 The grafting of the binding site is realized by first reacting dithiafulvenylphenol 5a,b with sodium hydride in dry THF followed by the addition of p-toluenesulfonate of tri(ethylene glycol) methylether (Scheme 2).15 Corresponding DTFs 6a,b were isolated in good yields after column chromatography as viscous oils. Cyclic voltammetry experiments were first carried out in CH2Cl2 for both DTFs 6a,b in order to determine if upon oxidation they lead to the formation of the vinylogous TTF cores through oxidative coupling. Both derivatives exhibit on the first anodic scan one irreversible oxidation wave and upon recurrent scans the appearance of novel reversible processes at lower oxidation potential. Upon recurrent scans, the intensity of the irreversible oxidation system decreases at the expense of the reversible oxidation one (Figure 1). The shape of these voltammograms indicates that after the oxidation of DTFs, the generated DTF cation radicals 6+• couple into protonated dimers, which after deprotonation leads to TTFVs 2a,b.7

Scheme 3. Oxidative Coupling of DTFs to TTFVs 2a,b

The redox behavior of these derivatives has been analyzed by cyclic voltammetry in dichloromethane using n-Bu4NPF6 as supporting electrolyte with a glassy carbon working electrode. Two close monoelectronic reversible oxidation processes were observed. They successively correspond to the reversible oxidation of the TTFV into the cation radical and then to the dication. As noticed for DTF, the introduction of electronwithdrawing substituent significantly influences the oxidation potentials: E°1 = 0.22 V and E°2 = 0.37 V for 2a; E°1 = 0.38 V and E°2 = 0.470 V for 2b. The redox behavior of TTFV 2a,b is similar to those of TTFV 1c (E°1 = 0.19 V and E°2 = 0.34 V vs SCE) for which two close monoelectronic waves were observed. On the contrary, for 1a,b the presence of shorter polyoxyethyl chains induces steric hindrance, and only one bielectronic system was observed. As previously demonstrated for different series of cyclic TTFV, a different redox behavior could be correlated to molecular movements of opposite nature associated with the electron transfer.5,6 For the noncyclic vinylogous TTFV 2a,b, a stretch movement occurs upon electron transfer as illustrated in Scheme 4. Variations of the peak potentials with the scan rates were studied for both TTFV 2a,b in order to determine the electrochemical standard rates for the first ks1 and the second ks2 electron transfers (Figure 2), taking a value of α = 0.5 for the transfer coefficient and D = 10−5 cm s−1 for the diffusion coefficients.11b

Figure 1. Cyclic voltammogram of DTF 6a in CH2Cl2 with 0.1 mol L−1 n-Bu4NPF6 at a scan rate of 0.1 V s−1. 5190



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In the framework of the mechanism depicted in Scheme 5 in which electron transfer steps and association equilibria of TTFV with dications (M2+) are considered as fast, this leads to the following relations between the different equilibrium constants of Ba2+: KTTFV+/KTTFV = 0.4 and KTTFV2+/KTTFV+ = 3.5. Simulations of the voltammograms using estimated equilibrium constants are shown in Figure 3c and show a good agreement between experiments and simulated curves obtained with the proposed mechanism.11 Similar general patterns were observed concerning the complexation of 2b with Pb2+ (see Figure 4). The two monoelectronic oxidation waves coalesce after addition of 2 × 10−3 mol L−1 of Pb(ClO4)2 into one single reversible system with E1/2 = 0.376 V and ΔEp = 68 mV. Unfortunately, the low solubility of Pb2+ in CH2Cl2 impedes the use of higher concentrations of dications. Using the same relation between E1/2, ΔEp, and the individual standard potentials E°1 and E°2, we derived that upon addition of 2 × 10−3 mol L−1 of Pb(ClO4)2, E°1 is almost unchanged as for the second process, and the corresponding E°2 is negatively shifted by almost −60 mV. Figure 4c shows a good agreement between simulations and experimental curves considering such values. From these simulations, we could estimate that association constants of Pb2+ with the different redox forms of 2b: KTTFV+/KTTFV ≈ 1 and KTTFV2+/KTTFV+ = 55. As a first result, our studies show that TTFV 2b could efficiently complex Pb2+ and Ba2+ and that both polyoxyethyl chains on the same TTFV presumably ligate the dication (Scheme 5). However, the observed redox behaviors are rather unusual in the sense that the dicationic form of the vinylogous TTFV presents the highest association constants. Indeed, it is generally observed that the complexation of a metallic cation by a redox active sensitive molecule results in a positive shift of the redox potential due to the binding of a positively charged ion in the close vicinity of the redox core.2,3 Thus, associations are larger with the neutral form of the molecule than with the other species that are produced after oxidation, which explains the sense of the potential variation.2,3 The situation is different in 2b because the localization of the polyoxyethyl chain on the phenyl rings impedes the electronic communication between the electroactive moiety and the binding part. Therefore, the electrostatic interaction between the associated metal dication and the TTFV core could not account for the observed effect. The effects of complexation observed for TTFV podand 2b are totally different from those observed with the cyclic crown ether TTFV 1c.5 For 1c upon addition of Pb2+, the peak-to-

Scheme 4. Schematic Representation of the Stretch Movement of TTFV 2a,b upon Oxidation

In both cases, ks2 is found to be larger than ks1 (for 2a, ks1 = 0.10 cm s−1, ks2 = 0.17 cm s−1 and for 2b, ks1 = 0.04 cm s−1, ks2 = 0.10 cm s−1). Actually, similar values were obtained for the cyclic derivatives 1c with ks1 = 0.035 cm s−1 and ks2 = 0.12 cm s−1, indicating that the first electron transfer, which is associated with the conformational changes, is slower than the second one in the free TTFV.5,11 Complexation properties of TTFV in dichloromethane were examined by cyclic voltammetry with the example of compound 2b. Evolutions of the voltammograms upon addition of Ba(ClO4)2 or Pb(ClO4)2 are followed and compared with the voltammogram of 2b alone in the same media. As shown in Figure 3, addition of aliquots of Ba(ClO4)2 to a solution of TTFV 2b induces a positive shift of the first oxidation process and a negative shift of second redox systems. This leads to the coalescence of the two processes into a single reversible one after addition of 5 × 10−3 mol L−1 Ba2+. This peak is characterized by its E1/2 = 0.406 V (E1/2 = (Epa + Epc)/2 where Epa and Epc are the anodic and cathodic peak potentials) with a peak-to-peak potential separation ΔEp = Epa − Epc = 72 mV.16 We observed that there is no considerable influence of the scan rate on the shape of the voltammograms in the 0.1−1 V s−1 range. This observation indicates that redox processes and associated reactions are fast at the time scale of the cyclic voltammetry and that measured potentials could be considered for estimating the thermodynamic values.6 Under these conditions, individual standard potentials E°1 and E°2 in the presence of the cation could be directly derived from the values of E1/2 and ΔEp using previously published procedure.5,6 We found that the standard potential of the first wave E°1 is shifted by +23 mV and E°2 for the second oxidation process by −31 mV upon addition of 5 × 10−3 mol L−1 of Ba(ClO4)2.

Figure 2. Cyclic voltammograms of TTFVs 2a (left) and 2b (right) (10−3 mol L−1), at different increasing scan rates in n-Bu4PF6 (0.2 mol L−1) in CH2Cl2 on a glassy carbon electrode. Scan rates = 0.2, 1, 10 (left); 0.2, 1, 30 (right) V s−1. 5191



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Figure 3. Voltammograms of TTFV 2b (10−3 mol L−1) on glassy carbon working electrode in CH2Cl2 (+0.2 mol L−1 n-Bu4NPF6) at a scan rate of 0.2 V s−1 on 2b alone (black curves) and in solutions containing 2 × 10−3 (green curves) and 5 × 10−3 mol L−1 (blue curves) of Ba2+(Ba(ClO4)2). Panel b shows the simulations of panel a considering the association constants of Ba2+ with the redox forms of 2b: KTTFV+/KTTFV = 0.4 and KTTFV2+/ KTTFV+ = 3.5 (see text).

To understand the origin and specificity of the redox behavior of 2b upon the metal dication addition, we could consider some general results concerning hindered TTV vinylogues. Notably, the introduction of bulky substituents on the TTF core does not modify the donor ability of the molecules but prevents the donor from being planar due to steric interactions, inducing large molecular movements associated with the electron transfer.6 Without additional constraints, the induced conformational changes correspond to a stretch of the molecule, as the TTF core tries to be planar in TTF2+ species. The amplitude of the movement depends on the energetic stabilities of each redox states that are mainly related to the steric hindrance. In previous studies, we have examined the effects of additional steric strain that are introduced when the two dithiole rings are linked with an alkyl or polyalkoxyl chain.5,6 Depending on the length of the linking chain, either a stretch or a clip movement could be obtained. Interestingly, in a defined vinylogous TTF, the passage to a clip movement upon oxidation is characterized by the coalescence of both oxidation processes.6 The fact that both oxidation waves of 2b also coalesce into one single process after addition of the metallic dication suggests a similar strain as that introduced by the alkyl chain link. The nature of the motion triggered by the electron transfer is modified, and due to the binding of the dication, the TTFV(M2+) 2b undergoes a clip movement upon oxidation instead of having a stretch motion of

Scheme 5. Considered Mechanism for the Oxidation of TTFV in the Presence of a Metallic Dication M2+

peak separation ΔEp that characterizes the kinetics of the first electron transfer considerably increases contrariwise to the redox potential E°1 (derived as the half sum potential) that remains almost unchanged (less than 10 mV). In this system, the second oxidation process is totally unaffected by the addition of the dication. Such variations correspond to larger activation energy of the first oxidation of 1c (in the order of 0.3 eV) after addition of Pb2+ that was ascribed to a stronger rigidity created by the presence of the dication in the crown ether.5 Therefore for 1c, complexation alters the structural changes occurring during the first electron transfer but does not affect the thermodynamics of the contrariwise to TTFV-podand 2b.

Figure 4. Voltammograms of the oxidation of TTFV 2b (10−3 mol L−1) on glassy carbon working electrode in CH2Cl2 (+0.2 mol L−1 n-Bu4NPF6) at a scan rate of 0.2 V s−1 before (black curve) and after addition of 2 × 10−3 mol L−1 of Pb(ClO4)2 (red curve). Panel b shows the simulations of panel a considering the association constants of Pb2+ with the redox forms of TTFV 2b: KTTFV+/KTTFV ≈ 1 and KTTFV2+/KTTFV+ = 55 (see text). 5192



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ACKNOWLEDGMENTS Profs. I Svir and C. Amatore are warmly thanked for providing to us an evaluation version of the KISSA software package, which was used for the mechanistic simulations of the voltammograms.

the free ligand, explaining the coalescence of the two waves (Scheme 6). Scheme 6. Clip Motion upon Oxidation of TTFV Podand 2b when Complexation of M2+ Occurs

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REFERENCES

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To explain differences between a TTFV bearing a cyclic crown ether with a TTFV-podand 2, we could highlight that the complexation creates a new link in 2b contrarily to a crown ether TTFV in which the link already exists, with the result that the modification of the motion triggered by the electron transfer is more substantial for a TTFV podand 2 than for a redox molecule bearing a cyclic crown ether. Additionally, the ligating part incorporates additional binding sites, 8 oxygen atoms for 2 compared to the 6 oxygen atoms present in 1c. This allows an efficient wrapping around the cation and a better screening of the electrostatic interactions. All of these special characteristics of TTFV podand 2 versus “classical” cyclic crown ether TTF explain the observed counterintuitive behavior in which the complexation of a metallic dication appears to be the strongest with the most charged form of the redox core.

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CONCLUSION Our studies show that TTFV substituted with two freely moving polyoxyethyl chains could efficiently complex metallic dications as Pb2+ or Ba2+. This type of redox sensitive molecule detects the dication not by an electronic interaction between the binding site and the redox center as generally observed but through a subtle modification of the molecular motion. It results that the nature of the observed redox behaviors is totally unusual in the sense that the 2-positively charged TTFV2+ displays the highest association constants with a metallic dication. This behavior is explained by the fact that the complexation creates a link between the two parts of the TTFV core causing a change of the motion triggered by the electron transfer inducing large modifications of the redox responses. It is likely that this principle could be extended to other sensitive redox systems where large conformational movements accompany the electron transfer.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.L.); philippe. [email protected] (P.H.). Notes

The authors declare no competing financial interest. 5193



Article

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Vinylogous Tetrathiafulvalene Based Podands - American Chemical

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