Article pubs.acs.org/JPCA
Electroreduction and Acid−Base Properties of Dipyrrolylquinoxalines Zhen Fu,† Min Zhang,† Weihua Zhu,† Elizabeth Karnas,‡ Kentaro Mase,§ Kei Ohkubo,§ Jonathan L. Sessler,*,‡,⊥ Shunichi Fukuzumi,*,§,# and Karl M. Kadish*,† †
Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States Department of Chemistry and Biochemistry, The University of Texas, Austin, Texas 78712-0165, United States § Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan ⊥ Department of Chemistry, Yonsei University, 262 Seonsanno, Sinchon-dong, Seodaemun-gu, Seoul 120-749, Korea # Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea ‡
S Supporting Information *
ABSTRACT: The electroreduction and acid−base properties of dipyrrolylquinoxalines of the form H2DPQ, H2DPQ(NO2), and H2DPQ(NO2)2 were investigated in benzonitrile (PhCN) containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). This study focuses on elucidating the complete electrochemistry, spectroelectrochemistry, and acid−base properties of H2DPQ(NO2)n (n = 0, 1, or 2) in PhCN before and after the addition of trifluoroacetic acid (TFA), tetra-nbutylammonium hydroxide (TBAOH), tetra-n-butylammonium fluoride (TBAF), or tetra-n-butylammonium acetate (TBAOAc) to solution. Electrochemical and spectroelectrochemical data provide support for the formation of a monodeprotonated anion after disproportionation of a dipyrrolylquinoxaline radical anion produced initially. The generated monoanion is then further reduced in two reversible one-electron-transfer steps at more negative potentials in the case of H2DPQ(NO2) and H2DPQ(NO2)2. Electrochemically monitored titrations of H2DPQ(NO2)n with OH−, F−, or OAc− (in the form of TBA+X− salts) give rise to the same monodeprotonated H2DPQ(NO2)n produced during electroreduction in PhCN. This latter anion can then be reduced in two additional one-electron-transfer steps in the case of H2DPQ(NO2) and H2DPQ(NO2)2. Spectroscopically monitored titrations of H2DPQ(NO2)n with X− show a 1:2 stoichiometry and provide evidence for the production of both [H2DPQ(NO2)n]− and XHX−. The spectroscopically measured equilibrium constants range from log β2 = 5.3 for the reaction of H2DPQ with TBAOAc to log β2 = 8.8 for the reaction of H2DPQ(NO2)2 with TBAOH. These results are consistent with a combined deprotonation and anion binding process. Equilibrium constants for the addition of one H+ to each quinoxaline nitrogen of H2DPQ, H2DPQ(NO2), and H2DPQ(NO2)2 in PhCN containing 0.1 M TBAP were also determined via electrochemical and spectroscopic means; this gave rise to log β2 values ranging from 0.7 to 4.6, depending upon the number of nitro substituents present on the H2DPQ core. The redox behavior of the H2DPQ(NO2)n compounds of the present study were further analyzed through comparisons with simple quinoxalines that lack the two linked pyrrole groups, i.e., Q(NO2)n where n = 0, 1, or 2. It is concluded that the pyrrolic substituents play a critical role in regulating the electrochemical and spectroscopic features of DPQs.
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refined by Behr et al. in 1973.22 The DPQ entity was not considered as a possible colorimetric anion sensor until three decades later when Sessler and co-workers reported the synthesis and anion binding properties of two related compounds, 2,3-dipyrrol-2′-yl-6-nitroquinoxaline, H2DPQ(NO2)20 and 2,3-dipyrrol-2′-yl-6,7-dinitroquinoxaline, H2DPQ(NO2)2.2
INTRODUCTION
Over the past decades, considerable effort has been devoted to exploring receptors capable of binding neutral and anionic species.1−14 This effort has been stimulated by the fact that these types of receptors are capable of recognizing, sensing, and transporting their targeted substrates not only in supramolecular constructs15−19 but also in systems that may have potential clinical applications.20 One attractive anion receptor is 2,3-dipyrrol-2′-ylquinoxaline H2DPQ, which has the advantage of possessing a built-in chromophore and being readily synthesized in two steps from commercially available materials,2,20 as first reported by Oddo in 191121 and later © 2012 American Chemical Society
Received: July 27, 2012 Revised: September 17, 2012 Published: September 18, 2012 10063
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In a recent communication23 we reported the preparation and chemical fate of dipyrrolylquinoxaline-derived radical anions of H2DPQ(NO2)2 and showed that the internal pyrrole protons were involved in a disproportionation of the product generated after a one-electron reduction in acetonitrile (CH3CN). The relevant electrochemical and chemical reactions are shown in Scheme 1. The initial one-electron reduction of
Chart 1. Structures of the Dipyrrolylquinoxalines Considered in This Studya
Scheme 1. Published Mechanism for the First Reduction of H2DPQ(NO2)2 in CH3CN23
a
These systems are shown in their respective neutral, acidic, and basic forms.
simple quinoxalines that lack the two covalently bound pyrrole groups.
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EXPERIMENTAL SECTION Materials. H2DPQ(NO2)n, where n = 0, 1, or 2, were synthesized according to literature procedures.1,2,20 Q(NO2)n where Q represents quinoxaline and n = 0, 1, or 2 were purchased from Sigma Aldrich Co. and used without further purification. Benzonitrile (PhCN) was purchased from Sigma Aldrich Co. and freshly distilled over P2O5 before use. Tetra-nbutylammonium perchlorate (TBAP), tetra-n-butylammonium hydroxide (TBAOH), tetra-n-butylammonium fluoride (TBAF), tetra-n-butylammonium acetate (TBAOAc), and trifluoroacetic acid (TFA) were also purchased from Sigma Aldrich Co. and used without further purification. Instrumentation. Cyclic voltammetry was carried out at 298 K using an EG&G Princeton Applied Research (PAR) 173 potentiostat/galvanostat. A homemade three-electrode cell was used for cyclic voltammetric measurements. It consists of a glassy carbon working electrode, a platinum counter electrode, and a homemade saturated calomel reference electrode (SCE). The SCE is separated from the bulk solution by a fritted glass bridge of low porosity, which contained the solvent/supporting electrolyte mixture. UV−visible spectra were measured using a Hewlett-Packard 8453 diode array spectrophotometer. UV−visible spectroelectrochemical experiments were performed using a home-built thin-layer cell, which has a transparent platinum net working electrode. Potentials were applied and monitored with an EG&G PAR Model 173 potentiostat. High purity N2 from Trigas was used to deoxygenate the solution. Solutions were kept under N2 during each electrochemical and spectroelectrochemical experiment.
H2DPQ(NO2)2 is followed by a disproportionation reaction that leads to two products, one an unreduced monodeprotonated species having a single proton shared by the two pyrrole groups,24 [HDPQ(NO2)2]− (labeled as 1H− in Scheme 1) and the other, a doubly reduced protonated species having one proton on each of the two pyrroles and a third proton bound to one of the two quinoxaline nitrogens. The latter compound is represented as [H3DPQ(NO2)2]− (1H3−) in Scheme 1. This was the first example where disproportionation of radical anions proved to be mediated by internal protons without the involvement of external protons. The present study expands upon our initial communication of the dinitro-substituted dipyrrolylquinoxaline radical anion behavior of H2DPQ(NO2)2 (1H2)23 and focuses on elucidating the complete electrochemistry, spectroelectrochemistry and acid−base properties of not just H2DPQ(NO2)2 but also H2DPQ and H2DPQ(NO2) in benzonitrile (PhCN) before and after the addition of acid in the form of trifluoroacetic acid (TFA), or base in the form of tetra-n-butylammonium hydroxide (TBAOH), tetra-n-butylammonium fluoride (TBAF), or tetra-n-butylammonium acetate (TBAOAc). The structures of the investigated dipyrrolylquinoxalines are shown in Chart 1 along with their abbreviated notations, given as 1H2, 2H2, and 3H2 for the neutral compounds, and 1H42+, 2H42+, and 3H42+ or 1H−, 2H−, and 3H− for the totally protonated and monodeprotonated forms of [H2DPQ(NO2)n], respectively. Cyclic voltammetry was used to measure the redox potentials of each electrode reaction, and thin-layer spectroelectrochemistry was employed to record the UV−visible spectra of each electrogenerated product. Equilibrium constants were calculated from the results of UV−visible spectroscopic titrations of H2DPQ(NO2)n in PhCN with trifluoracetic acid (TFA) or base (TBAX where X = OH−, F−, or OAc−). To aid in interpreting the findings from these and other experiments, comparisons are made between the redox behavior of H2DPQ(NO2)n and
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RESULTS AND DISCUSSION Electroreduction of Q(NO2)n, n = 0, 1, or 2. To understand the chemical reactions following electron addition to H2DPQ(NO2)n, measurements were first carried out under the same solution conditions on two groups of quinoxalines 10064
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leads to a 770 mV positive shift in E1/2 for the first one-electron reduction (−1.67 to −0.90 V). A smaller substituent effect is seen upon addition of the second nitro group where E1/2 shifts from −0.90 V for reduction of Q(NO2) to −0.64 V for reduction of Q(NO2)2 (Figure 1a). The absolute difference in E1/2 values between formation of the radical anion and dianion of 5 and 6 amounts to 640 and 560 mV, respectively, as shown in Figure 1a. Reversible one-electron reductions have been reported for the structurally related dimethylquinoxaline (7), dithienylquinoxaline (8), and difurylquinoxaline (9), with E1/2 values ranging between −1.94 and −2.28 V vs Fc/Fc+ being recorded in CH3CN.28 These E1/2 values, when corrected so as to be relative to SCE, would range from about −1.54 to −1.78 V vs the Fc/Fc+ couple, which is located at 0.40 V vs SCE in CH3CN. One key point that emerges from an analysis of the six quinoxaline comparison compounds (4−9) shown in Chart 2 is that all of the in situ generated radical anions are stable on the cyclic voltammetric time scale. This is ascribed to the lack of a radical anion disproportionation reaction, a process that requires the presence of protons. The requisite protons are present in the form of the pyrrole NH protons in the case of the dipyrrolylquinoxalines H2DPQ(NO2)2, H2DPQ(NO2), and H2DPQ (1H2−3H2). However, such an internal source of protons is not available in the case of the control compounds. Another trend underscored by the data is that the mono- and dinitro-substituted quinoxaline moieties, 4 and 5, are much more easily reduced than the methyl, thienyl and furyl substituted compounds 7−9 that lack an electron-withdrawing nitro group (see structures in Chart 2). As indicated above, a second reversible one-electron reduction occurs for 5 and 4 (at −1.54 or −1.20 V, respectively). Additional reductions beyond the monoanion radical are also seen for the nitro-substituted dipyrrolylquinoxalines, 2H2 and 3H2, whose electrochemistry is described below. Electroreduction of H2DPQ(NO2)n, n = 0, 1, or 2. As can be seen from an inspection of Figure 1b, dipyrrolylquinoxaline, H2DPQ (3H2), exhibits a single irreversible reduction at Epc = −1.70 V which is coupled with an irreversible oxidation peak at Ep = −0.80 V at a scan rate of 0.1 V s−1. The return oxidation peak is not present if the scan is terminated before the first reduction and the overall shape of the current−voltage curve in Figure 1b is consistent with a chemical reaction following electron transfer (an ED mechanism where D is the disproportionation step).29 It should be noted that the cathodic peak potential for the irreversible one-electron reduction of H2DPQ (3H2) at Epc = −1.70 V (Figure 1b) is almost identical to the cathodic peak potential Ep for the reversible one-electron reduction of Q located at E1/2 = −1.67 V (Figure 1a). The reversible first reductions of 2H2 (E1/2 = −0.97 V) and 1H2 (E1/2 = −0.73 V) give E1/2 values that are quite similar to those for the first reduction of 5 (E1/2 = −0.90 V) and 4 (E1/2 = −0.64 V). However, there is a 70−90 mV negative shift in the potential corresponding to the reduction of the H 2 DPQ(NO 2 ) n compounds as compared to Q(NO2)n, where n = 1 or 2. Such a finding is consistent with the known electron-donating effect of the two pyrrole groups on 1H2 and 2H2. This leads to an increased negative charge within the conjugated π system of the quinoxaline groups and a more difficult reduction. The effect of NO2 substituents on the redox reactions of compounds 1H2−3H2 and compounds 4−6 is also similar. For
lacking a pyrrole unit. One group of comparison molecules is represented as Q(NO2)n where n = 2, 1, or 0 (compounds 4− 6) and the other as R2Q (compounds 7−9) whose structures are shown in Chart 2. Chart 2. Substituted Quinoxalines Used as Comparison Compounds
The quinoxaline moiety without added substituents, Q 6, has previously been characterized as undergoing a one-electron reduction in nonaqueous media25−27 with potentials ranging from E1/2 = −1.62 vs SCE in CH3CN26 to −1.80 in DMF.27 The reversible one electron reduction of Q in PhCN containing 0.1 M TBAP is located at E1/2 = −1.67 V (Figure 1a) and
Figure 1. Cyclic voltammograms of (a) Q(NO2)n (4−6) and (b) H2DPQ(NO2)n (1H2−3H2) where n = 2, 1, or 0 in PhCN, 0.1 M TBAP. Scan rate = 0.1 V s−1.
generates the quinoxaline radical anion, Q•−, which is stable on the electrochemical and spectroelectrochemical time scales. Stable radical anion and dianions are also generated during the stepwise reversible one-electron reductions of Q(NO2) 5 (E1/2 = −0.90 and −1.54 V) and Q(NO2)2 4 (E1/2 = −0.64 and −1.20 V). Cyclic voltammograms illustrating these processes are shown in Figure 1a and UV−visible spectral changes monitored during the formation of [Q(NO2)n]•− under the same solution conditions are shown in Figure S1, Supporting Information. As seen in Figure 1a, the reversible half-wave potentials for conversion of Q(NO2)n to Q(NO2)n•− are shifted positively with each added electron-withdrawing NO2 substituent on going from n = 0 to n = 2. The addition of one NO2 group to Q 10065
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example, the difference in E1/2 associated with the reduction of Q and Q(NO2) is 770 mV. Moreover, almost the same difference in potential (∼700 mV) is seen for the related dipyrrolequinoxalines where H2DPQ is reduced at Epc = −1.70 V as compared to Epc = −1.00 V for H2DPQ(NO2) at a scan rate of 0.1 V s−1 (Figure 1b). As in the case of Q(NO2) and Q(NO2)2, a smaller substituent effect is seen upon appending a second nitro group to the dipyrrolylquinoxaline core. For instance, E1/2 shifts from −0.97 V for the first reduction of H2DPQ(NO2 ) to −0.73 V for the first reduction of H2DPQ(NO2)2. This similarity in reduction potentials and substituent effects for compounds in the series 1H2−3H2 and the control systems 4−6 leads us to propose that the processes in question involve a chemically analogous site of electron transfer, namely the conjugated quinoxaline π system in all cases. As can be seen from an inspection of Figure 1b, the initial one-electron additions to H2DPQ(NO2) and H2DPQ(NO2)2 are reversible for both the mono- and dinitro compounds. These reduction processes, at E1/2 = −0.97 V (2H2) and −0.73 V (1H2), are followed at more negative potentials by a second irreversible one-electron addition for which values of Ep = −1.23 V (2H2) and −1.07 V (1H2) are recorded at a scan rate of 0.1 V s−1. It should also be noted that the first one-electron reduction process for both H2DPQ(NO2) (2H2) and H2DPQ(NO2)2 (1H2) proved irreversible at all accessible scan rates and temperatures when the scan was extended to potentials beyond the second reduction, i.e., to values greater than −1.23 V in the case of 2H2 and greater than −1.07 V in the case of 1H2. In contrast, the first reduction was always reversible when the negative potential sweep was reversed at a value positive of (prior to) the second electron addition. This is consistent with the initial formation of an electroactive species in solution, which is then further reduced in two one-electron-transfer steps, the first of which is reversible and located at E1/2 = −1.13 V (2H2) or −0.90 V (1H2). This second reductions of H2DPQ(NO2) and H2DPQ(NO2)2, at Epc = −1.23 and −1.07 V, respectively, in Figure 1 are both coupled to well-defined reoxidation peaks which appear at Epa = −1.11 V (2H2) or −0.88 V (1H2) on the first and all subsequent sweeps at a scan rate of 0.1 V s−1 (Figure 2). However, as discussed below, the second cathodic (reduction) and first anodic (reoxidation) peaks are not associated with the same electrochemical process and actually correspond to two separate redox couples separated by one or more intervening chemical reactions. The electroactive product generated after the first reduction and disproportionation of the mono- and dinitro DPQ derivatives, respectively, were previously assigned to the monodeprotonated species, [HDPQ(NO2)n]− and the protonated two-electron reduced species, [H3DPQ(NO2)n]−, on the basis of the electrochemical and 1H NMR spectral data.23,24 The monodeprotonated product, [HDPQ(NO2)n]−, has an unreduced π ring system and undergoes two one-electron additions as shown in Figure 2 (dashed line) and Scheme 2. The protonated two-electron reduced species of H2DPQ(NO2)n, [H3DPQ(NO2)n]−, display no further electrochemical reactivity upon reduction. The second negative potential scans for reduction of 2H2 and 1H2 in Figure 2 involve the stepwise one-electron reduction of electrogenerated 2H− and 1H− as shown in Scheme 2. The first electron addition occurs at −1.13 V for 2H− (as compared to
Figure 2. Cyclic voltammograms of (a) H2DPQ(NO2) (2H2) and (b) H2DPQ(NO2)2 (1H2) as recorded in PhCN containing 0.1 M TBAP: first scan (solid line); second scan (dotted line). Here, 2H− and 1H− are the electroactive species generated at the electrode surface. The same current−voltage curves were obtained for the second, third, and fourth potential sweeps. Scan rate = 0.1 V s−1.
−0.97 V for 2H2) and −0.90 V for 1H− (as compared to −0.73 V for 1H2). This 160−170 mV negative shift in E1/2 for reduction of the monodeprotonated species is consistent with the single negative charge being localized on the two pyrrole rings. Additional evidence for the conversion of H2DPQ(NO2)n to [HDPQ(NO2)n]− prior to the redox reactions shown in Scheme 2 came from thin-layer electrochemical and spectroelectrochemical analyses. Scanning the potential from 0.00 to −1.50 V in the thin-layer cell reveals two initial reduction processes involving H2DPQ(NO2) (Figure 3). This behavior resembles what is observed in the first two electron-transfer steps under conditions of routine cyclic voltammetry at a scan rate of 0.1 V s−1 (Figure 1). The initial reversible reduction in the thin-layer cell at E1/2 = −0.96 V (dashed line in Figure 3a) is followed by a second irreversible reduction at Epc = −1.22 V. On the other hand, only one return anodic peak is seen on the reverse scan. This process is located at Epa = −1.09 V when the analysis is carried out at a scan rate of 10 mV s−1 (Figure 3a). The peak current for reduction of H2DPQ(NO2) at Epc = −1.22 V is less than that for the first reduction of the same compound at E1/2 = −0.96 V (Figure 3a). Again, this consistent with what is seen in the “regular” cyclic voltammogram shown in Figure 2. Furthermore, the reduction peak at Ep = −1.22 V in the thin-layer cyclic voltammogram (Figure 3) totally disappears when the potential is scanned from 0.00 to −1.05 V and then held for three minutes before continuing to −1.40 V and being reversed (Figure 3b). Under these conditions, the disproportionation reaction shown in Scheme 1 proceeds to completion and a new cathodic peak is seen at Epc = −1.15 V (Figure 3b). The new reduction process observed at Epc = −1.15 V is coupled to the original anodic peak at Epa = −1.09 V allowing an E1/2 of −1.12 V to be calculated for the processes observed in the thin-layer cell (Figure 3b). This half-wave potential is virtually identical to the E1/2 = −1.13 V measured from the “regular” cyclic voltammogram shown in Figure 2a. Finally, it should be noted that the current for reoxidation of H2DPQ(NO2) in Figure 3b (23 μA) is about half of that observed for reduction on the initial negative scan (41 μA). Such a finding is fully consistent with the mechanism shown in Scheme 1 for H2DPQ(NO2)2. 10066
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Scheme 2
1H2 gives rise to similar bands at 351 and 469 nm (Figure 4b-i) in the UV−vis spectrum, whereas the corresponding radical anion generated after reduction at −1.20 V (1H•2−) is characterized by bands at 342 and 466 nm in addition to a broad near-IR feature that covers the 700−1000 nm spectral range (Figure 4b-iii). The first reduction product of H2DPQ(NO2), produced in PhCN containing 0.1 M TBAP when the solution is subject to an applied potential of −1.05 V for 335 s, is characterized by absorption bands at 350, 430, and 566 nm (Figure 4a-ii). The same set of absorbance bands are seen upon addition of TBAOH to a neutral 10−3 M solution of H2DPQ(NO2) (Figure 5). The ratio of the 566 nm band of the reduction product over the 458 nm band of the initial compound, A566(final)/A458(initial), is 0.80 under the electrochemical conditions shown in Figure 4 where [2H2] = 0.71 × 10−3 M. A similar ratio of 0.72 is obtained for a 10−3 M solution of H2DPQ(NO2) in PhCN containing 1 equiv of TBAOH (Figure 5). This result leads us to suggest that the same monodeprotonated form of the compound exists in these solutions after the addition of 1.0 equiv TBAOH as is produced after the first reduction at an applied potential of −1.05 V. This conclusion is further supported by electrochemical experiments carried out in PhCN containing the organic soluble bases TBAOH, TBAF and TBAOAc. These results are described in the following section for solutions containing TBAOH. Electrochemistry and Spectroelectrochemistry of H2DPQ(NO2)n in PhCN Containing TBAOH. Further evidence for the conclusion that one proton is lost during initial electroreduction of H2DPQ(NO2)n came from cyclic voltammograms recorded in PhCN while titrating the sample with increasing quantities of TBAOH. An example is shown in Figure 6b, where a plot of the maximum peak current vs the [TBAOH]/[H2DPQ(NO2)] ratio consists of two straight line segments with an intersection at 1.0, a finding consistent with the addition of 1.0 equiv of TBAOH to solutions of H2DPQ(NO2) being sufficient to change the electroactive species being reduced at E1/2 = −0.97 V. The UV−visible spectrum of the electroreduced H2DPQ(NO2) was also measured in PhCN containing between 2.0 and 5.0 equiv of added TBAOH (Figure 5). At 2 equiv of TBAOH, the ratio of peak absorbances for the starting species to that produced following TBAOH addition, Afinal,566/Ainitial,459, was found to equal 1.01. This ratio increases slightly to 1.07 when 5.0 equiv of TBAOH is added to solution. The UV−visible spectrum of H2DPQ(NO2) in PhCN containing 0.2 M TBAP recorded after controlled potential reduction at Eapp = −1.35 V (Figures 4a-iii) is characterized by bands at 320, 461, and 950
Figure 3. Thin-layer cyclic voltammograms of H2DPQ(NO2) (2H2) recorded in PhCN, 0.2 M TBAP under different conditions as indicated in panels (a) and (b). Scan rate = 10 mV s−1. The first reduction is reversible when the scan is reversed prior to the second electron addition at Ep = −1.22 V (dashed line in Figure 3a).
Spectroelectrochemistry of H2DPQ(NO2)n where n = 0, 1, or 2. The electrochemically initiated conversion of H2DPQ(NO2)n to [HDPQ(NO2)n]− proceeds in two steps, both of which could be followed as a function of time by measuring changes in the UV−visible spectra during controlled potential reduction in the thin-layer cell. These spectral changes are shown in Figure 4 for H2DPQ(NO2) (2H2) and H2DPQ(NO2)2 (1H2) under the application of a controlled reducing potential. For both the mono- and dinitro derivatives, only small differences in the overall morphology of the UV− visible spectra are observed after the initial application of an applied potential (up to 94 s in the case of 2H2 and up to 45 s in the case of 1H2; cf. top two spectra in Figure 4). However, quite substantial changes then occur at longer times (up to 173 and 335 s for 1H2 and 2H2, respectively), as can be seen from an inspection of the middle two spectra shown in Figure 4. The products obtained after controlled potential reduction of 2H2 at −1.35 V (Figure 4a-iii) or 1H2 at −1.20 V (Figure 4biii) involve the conversion of 2H− to 2H•2− and 1H− to 1H•2−, as shown in Scheme 2. The neutral compound 2H2 is characterized by absorption bands at 328 and 458 nm in the UV−vis spectrum (Figure 4a-i), whereas the radical anion generated after reduction at −1.35 V (2H•2−) displays similar bands at 320 and 461 nm in addition to a broad near-IR band between 700 and 1000 nm (Figure 4a-iii). The neutral species 10067
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Figure 4. UV−vis spectral changes observed during stepwise controlled potential reduction of (a) H2DPQ(NO2) (2H2) at −1.05 and −1.35 V and (b) H2DPQ(NO2)2 (1H2) at −0.90 and −1.20 V in PhCN, 0.2 M TBAP. See Figure 3 for the corresponding cyclic voltammograms.
this report, it is now possible to elaborate a more detailed mechanism for the multielectron reduction of H2DPQ(NO2) and H2DPQ(NO2)2 in PhCN containing 0.1 M TBAP or 0.1 M TBAP plus 1.0 equiv of TBAOH as presented in Scheme 3. Titrations of H2DPQ(NO2)n with TBA+X− Salts. UV− visible spectral changes were monitored during the titration of dilute (10−5 M) solutions of H2DPQ(NO2)n in PhCN containing various anions (X−) added in the form of their corresponding TBA+ salts. One representative set of spectra is provided in Figure 8, which shows the changes in the spectra seen upon the addition of OH− (as TBAOH) to solutions of H2DPQ, H2DPQ(NO2), or H2DPQ(NO2)2. Likewise, the spectral changes seen upon the addition of TBAOH, TBAF, or TBAOAc to PhCN solutions of H2DPQ(NO2) are given in Figure 9. Similar results are observed in each titration as can be seen from an inspection of Figures 8 and 9. A log−log plot analysis of the data provides a straight line in all cases with a slope of 1.9−2.0 (see figure insets). The overall reaction can be interpreted in terms of eq 1 for all three bases.30 The formation of HF2−, as proposed in eq 1, was confirmed via the reaction of H2DPQ(NO2)2
Figure 5. UV−vis spectra for a 10−3 M solution of H2DPQ(NO2) in PhCN recorded in the presence of increasing quantities of TBAOH.
nm. Virtually the same spectrum is obtained when the reduction is carried out in PhCN containing 1.2 or 5 equiv TBAOH (Figure 7). On this basis, we suggest that the same electroreductive product, 2H•2−, is formed under both sets of conditions. Taken in concert, the electrochemistry and spectroelectrochemistry features of H2DPQ(NO2)n as recorded in PhCN and PhCN containing TBAOH are consistent with the conclusion that a monodeprotonated anion with an unreduced π ring system is obtained after disproportionation of a dipyrrolylquinoxaline radical anion that is formed as the initial reduction product. This is in accord with what was proposed in our initial communication.23 However, based on the findings of
β2
H 2DPQ(NO2 )n + 2X− ⇄ [HDPQ(NO2 )n ]− + HX 2− (1)
with TBAF. Such treatment leads to the appearance of a triplet at 16.2 ppm in 1H NMR spectrum that is readily ascribable to the FHF− anion.31,32 Signals corresponding to the HDPQ10068
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(NO2)2− anion are also seen (Figure S8a−d in the Supporting Information). In the case where H2DPQ(NO2)2 is reacted with TBAOH, the same 1H NMR signals ascribable to HDPQ(NO2)2− are seen as are observed in the case of treatment with TBAF (Figure S8e in the Supporting Information). The experimentally calculated log β2 value for the reaction in eq 1 was determined from the zero intercept of the Hill plot in Figures 8 and 9 and varied as a function of both the base strength (TBAOH > TBAF > TBAOAc) and acidity of the dipyrrolylquinoxaline, i.e., H2DPQ(NO2)2 > H2DPQ(NO2) > H2DPQ. The measured deprotonation constants are summarized in Table 1 and range from a low log β2 of 5.3 for the titration of H2DPQ with TBAOAc to a high log β2 of 8.8 for the reaction between H2DPQ(NO2) and TBAOH. As shown below, the magnitude of the equilibrium constant is directly related to the ease of the reduction in question. Moreover, a plot of log β2 vs E1/2 for compounds 1H2, 2H2, and 3H2 proved to be linear with a correlation coefficient of 0.98. No spectral changes occurred when tetra-n-alkylammonium salts of Cl−, Br−, I−, N3−, or SCN− are added to solutions of H2DPQ(NO2)n. On the other hand, the addition of either CN− (as the TBA+ salt) or piperidine to PhCN solutions of H2DPQ(NO2)n produced UV−visible spectral changes that are identical to those seen in the case of titration with base (OH−, F−, or OAc−; all as the TBA+ salts) (Figure 9). These results are consistent with a monodeprotonation process as opposed to just anion binding, as well as the overall reduction and deprotonation reactions shown in Scheme 3. Protonation of H2DPQ(NO2)n with TFA. The protonation properties of the three dipyrrolylquinoxalines of this study were studied in PhCN. Toward this end, they were titrated with trifluoroacetic acid (TFA), with a representative set of spectral
Figure 6. (a) Cyclic voltammograms of H2DPQ(NO2) (2 × 10−3 M) in PhCN, 0.1 M TBAP with added 0 to 1.2 equiv of added TBAOH and (b) peak current for the first reduction as a function of the [TBAOH]/[H2DPQ(NO2)] ratio in solution.
Figure 7. (a) UV−vis spectral changes observed during the controlled potential reduction at −1.35 V of a 7.1 × 10−4 M solution of H2DPQ(NO2) in PhCN containing 0.2 M TBAP and reduction of the same solution (b) after addition of 1.2 equiv of TBAOH or (c) 5.0 equiv of TBAOH. The applied potential is shown by an arrow in the cyclic voltammogram inset. 10069
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Scheme 3
Figure 8. UV−vis spectral changes of H2DPQ, H2DPQ(NO2), and H2DPQ(NO2)2 (10−5 M) upon titrating with TBAOH in PhCN. Figure 9. UV−vis spectral changes seen when H2DPQ(NO2) is titrated with TBAOH, TBAF, and TBAOAc.
changes being shown in Figure 10. A Hill-plot analysis was carried out with the results being shown in the figure insets. On this basis, it was concluded that two protons are simultaneously added to the two nitrogen atoms of the 10070
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Table 1. Equilibrium Constants (logβ2) for Deprotonation and Protonation of H2DPQ(NO2)n Compounds in PhCN deprotonationa
a
−
compound
OH
H2DPQ H2DPQ(NO2) H2DPQ(NO2)2
7.1 8.3 8.8
F
−
7.0 8.1 8.7
protonationb −
OAc
TFA
5.3 6.4 6.9
4.6 2.2 0.7
Equation 1. bEquation 2.
Figure 11. Cyclic voltammograms of H2DPQ(NO2) titrated by TFA using the indicated equivalents of acid in PhCN containing 0.1 M TBAP.
protons from TFA in PhCN, because this process occurs at Epc = −1.66 and −1.86 V under the same solution conditions (Figure S9, Supporting Information). The cathodic peak current for reduction of unprotonated H2DPQ(NO2) (at E1/2 = −0.97 V) decreases as more acid is added to the solution. Moreover, a plot of ip vs the [TFA]/ [H2DPQ(NO2)] molar ratio reveals a distinct break at 2.0 equiv (Figure 11b), consistent with two protons being added in a single step. As indicated above, the calculated values of log β2 for deprotonation of a pyrrole proton correlate in a linear manner with the E1/2 corresponding to the first reduction of H2DPQ(NO2)n. The same linear relationship between log β2 and E1/2 holds for protonation of the two quinoxaline nitrogen atoms, as shown in Figure 12. Moreover, a plot of log β2 for protonation of the two quinoxaline nitrogen atoms vs log β2 for deprotonation of the two pyrrolic nitrogen atoms is also linear with a correlation coefficient of 0.99 (Figure 13). The protonation reaction is more affected by the redox potentials (which reflect the charge density of the molecules) than is the deprotonation. Such a finding is fully consistent with the structures of the compounds.
Figure 10. UV−vis spectral changes seen when PhCN solutions of H2DPQ(NO2)n (n = 0−2) were titrated with TFA.
quinoxaline, leading to the formation of [H4DPQ(NO2)n]2+, as shown in eq 2. Protonation constants were measured from data of the type shown in Figure 10, with log β2 ranging from 0.7 in the case of H2DPQ(NO2)2 to 4.6 in the case of H2DPQ. The values are listed in Table 1. β2
H 2DPQ(NO2 )n + 2H+ ⇄ [H4DPQ(NO2 )n ]2 +
(2)
Further evidence for two protons being added came from electrochemically monitored titrations of H2DPQ(NO2)n with TFA as shown in Figure 11 for the specific case of H2DPQ(NO2). As indicated earlier, the first one-electron reduction of H2DPQ(NO2) is located at E1/2 = −0.97 V in PhCN. However, the current for this process decreases upon addition of acid to the solution. In addition, two new reduction peaks appear. These new peaks are located at Epc = −0.42 and −0.72 V and cannot be attributed to the reduction of free 10071
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AUTHOR INFORMATION
Corresponding Author
*E-mail: S.F.,
[email protected]; J.L.S.,
[email protected]; K.M.K.,
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support by the Robert A. Welch Foundation (Grants E-680 and F-1018 to K.M.K. and J.L.S., respectively), the Grant-in-Aid (awards nos. 20108010 and 23750014 to S.F. and K.O., respectively) and Global Center of Excellence (COE) programs from the Ministry of Education, Culture, Sports, Science and Technology, Japan (to S.F.), the U.S. National Science Foundation (grant CHE 1057904 to J.L.S.), and the World Class University (WCU) program (projects R31-2008-00010010-0 and R32-2010-000-10217-0 for S.F. and J.L.S., respectively) administered through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (MEST) Korea.
Figure 12. Plots of log β2 for (a) deprotonation and (b) protonation of H2DPQ(NO2)n vs the reduction potential (E1/2) for the same molecules as determined in PhCN containing 0.1 M TBAP.
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(1) Dietrich, B.; Hosseini, M. W. In Supramolecular Chemistry of Anions; Bianchi, A., Bowman-James, K., Garcís-España, E., Eds.; WileyVCH: New York, 1997; pp 45−55. (2) Mizuno, T.; Wei, W.; Eller, L. R.; Sessler, J. L. J. Am. Chem. Soc. 2002, 124, 1134−1135. (3) Rivadehi, S.; Reid, E. F.; Hogan, C. F.; Bhosale, S. V.; Langford, S. J. Org. Biomol. Chem. 2012, 10, 705−709. (4) Bhosale, A. V.; Kalyankar, B.; Langford, S. J. Org. Lett. 2009, 11, 5418−5421. (5) Caltagirone, C.; Gale, P. A.; Hiscock, J. R.; Brooks, S. J.; Hursthouse, M. B.; Light, M. E. Chem. Commun. 2008, 3007−3009. (6) Caltagirone, C.; Hiscock, J. R.; Hursthouse, M. B.; Light, M. E.; Gale, P. A. Chem.−Eur. J. 2008, 14, 10236−10243. (7) Chmielewski, M. J.; Zhao, L. Y.; Brown, A.; Curiel, D.; Sambrook, M. R.; Thompson, A. L.; Santo, S. M.; Flex, V.; Davis, J. J.; Beer, P. D. Chem. Commun. 2008, 3154−3156. (8) Das, A.; Ganguly, B.; Kumar, D. K.; Jose, D. A. Org. Lett. 2004, 6, 3445−3448. (9) Hiscock, J. R.; Caltagirone, C.; Light, M. E.; Hursthouse, M. B.; Gale, P. A. Org. Biomol. Chem. 2009, 7, 1781−1783. (10) Jimenez, R.; Martinez-Manez, R.; Soto, J. Tetrahedron Lett. 2002, 43, 2823−2825. (11) Miyaji, H.; Sessler, J. L. Angew. Chem., Int. Ed. 2001, 40, 154− 157. (12) Pfeffer, F. M.; Lim, K. F.; Sedgwick, K. J. Org. Biomol. Chem. 2007, 5, 1795−1799. (13) Piatek, P. V.; Lynch, M.; Sessler, J. L. J. Am. Chem. Soc. 2004, 126, 16073−16076. (14) Yu, J. O.; Browning, C. S.; Farrar, D. H. Chem. Commun. 2009, 1020−1022. (15) Fukuzumi, S.; Ohkubo, K.; D’Souza, F.; Sessler, J. L. Chem. Commun. 2012, 48, 9801−9815. (16) Sessler, J. L.; Karnas, E.; Kim, S. K.; Ou, Z.; Zhang, M.; Kadish, K. M.; Ohkubo, K.; Fukuzumi, S. J. Am. Chem. Soc. 2008, 130, 15256− 15257. (17) Park, J. S.; Karnas, E.; Ohkubo, K.; Chen, P.; Kadish, K. M.; Fukuzumi, S.; Bielawski, C.; Hudnall, T. W.; Lynch, V. M.; Sessler, J. L. Science 2010, 329, 1324−1326. (18) Fukuzumi, S.; Ohkubo, K.; Kawashima, Y.; Kim, D. S.; Park, J. S.; Jana, A.; Lynch, V. M.; Kim, D.; Sessler, J. L. J. Am. Chem. Soc. 2011, 133, 15938−15941. (19) Karnas, E.; Kima, S. K.; Johnson, K. A.; Sessler, J. L.; Ohkubo, K.; Fukuzumi, S. J. Am. Chem. Soc. 2010, 132, 16617−16622.
Figure 13. Plot of log β2 for deprotonation vs log β2 for protonation for the compounds of this study. Note the linear relationship.
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SUMMARY Taken in concert, the electrochemical and spectroelectrochemical data obtained from an analysis of the three dipyrrolylquinoxalines (H2DPQ(NO2)n) of this study provide support for the conclusion that a monodeprotonated anion with an unreduced π system is obtained as a first reduction product in PhCN. The same monodeprotonated product is formed during a titration with OH−, F−, or OAc− (studied in the form of their TBA+ salts) under these solution phase conditions (eq 1). The first one-electron reduction occurs at E1/2 = −0.97 V for H2DPQ(NO2) and −0.73 V for H2DPQ(NO2)2. These E1/2 values are shifted relative to the half-wave potentials for reduction of two unsubstituted quinoxaline control species, namely Q(NO2) and Q(NO2)2, i.e., E1/2 = −0.90 and −0.64 V, respectively (Figure 1). On this basis we conclude that the 2,3dipyrrol-2′-ylquinoxaline derivatives of this study (H2DPQ(NO2)n, n = 0, 1, or 2) display redox properties that are reflective of not only the quinoxaline core but also specific deprotonation of one of the pyrrole protons.
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REFERENCES
ASSOCIATED CONTENT
S Supporting Information *
Figures S1−S9, showing UV−vis spectra, 1H NMR spectra, and a cyclic voltammogram. This material is available free of charge via the Internet at http://pubs.acs.org. 10072
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(20) Black, C. B.; Andrioletti, B.; Try, A. C.; Ruiperez, C.; Sessler, J. L. J. Am. Chem. Soc. 1999, 121, 10438−10439. (21) Oddo, B. G. Chim. Ital. 1911, 41, 248−255. (22) Behr, D.; Brandange, S.; Lindstrom, B. Acta Chem. Scand. 1973, 27, 2411−2414. (23) Fukuzumi, S.; Mase, K.; Ohkubo, K.; Fu, Z.; Karnas, E.; Sessler, J. L.; Kadish, K. M. J. Am. Chem. Soc. 2011, 133, 7284−7287. (24) Pietrzak, M.; Try, A. C.; Andrioletti, B.; Sessler, J. L.; Anzenbacher, P. J.; Limbach, H.-H. Angew. Chem., Int. Ed. 2008, 47, 1123−1126. (25) Kadish, K. M.; E, W.; Sintic, P. J.; Ou, Z.; Shao, J.; Ohkubo, K.; Fukuzumi, S.; Govenlock, L. J.; McDonald, J. A.; Try, A. C.; Cai, Z.; Reimers, J. R.; Crossley, M. J. J. Phys. Chem. B 2007, 111, 8762−8774. (26) Barqawi, K. R.; Atfah, M. A. Electrochim. Acta 1987, 32, 597− 599. (27) Ames, J. R.; Houghtaling, M. A.; Terrian, D. L. Electrochim. Acta 1992, 37, 1433−1436. (28) Angulo, G.; Dobkowski, J.; Kapturkiewicz, A.; Maciolek, K. J. Photochem. Photobiol. A: Chem. 2010, 213, 101−106. (29) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 2000; pp 471−528. (30) The titration of H2DPQ(NO2)2 with TBAOH (as a 37% methanol solution) in acetonitrile gave a different stoichiometry from that obtained in PhCN as shown in Figure S2 (Supporting Information). In the presence of methanol, the absorption band due to [HDPQ(NO2)2]− was observed after addition of 1 equiv of TBAOH. The further addition of TBAOH resulted in further deprotonation from [HDPQ(NO2)2]−. It should be noted that no further deprotonation of [HDPQ(NO2)2]− occurred with TBAOH in benzonitrile. The different stoichiometry may result from the use of TBAOH in methanol. However, the solvent effect on the deprotonation of H2DPQ(NO2)2 has yet to be studied in detail. (31) (a) Shenderovich, I. G.; Tolstoy, P. M.; Golubev, N. S.; Smirnov, S. N.; Denisov, G. S.; Limbach, H.-H. J. Am. Chem. Soc. 2003, 125, 11710−11720. (b) Kang, S. O.; Nguyen, Q. P. B.; Kim, T. H. Bull. Korean Chem. Soc. 2009, 30, 2735−2738. (32) For hydrogen diacetate (XHX−: X = OAc), see: (a) Tolstoy, P. M.; Schah-Mohammedi, P.; Smirnov, S. N.; Golubev, N. S.; Denisov, G. S.; Hans-Heinrich Limbach, H.-H. J. Am. Chem. Soc. 2004, 126, 5621−5634. (b) Schah-Mohammedi, P.; Shenderovich, I. G.; Detering, C.; Limbach, H.-H.; Tolstoy, P. M.; Smirnov, S. N.; Denisov, G. S.; Golubev, N. S. J. Am. Chem. Soc. 2000, 122, 12878−12879.
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