Electrochemical reduction, radical anions, and dehalogenation of fluorinated/chlorinated 2,1,3-benzothia/selenadiazoles

At the first stage of electrochemical reduction in DMF, fluorinated/chlorinated 2,1,3-benzothia/selenadiazoles formed long-lived radical anions characterized by EPR and DFT. Gas-phase electron affinities (EA 1 ) from DFT correlated well with the first-peak potentials separately for S and Se derivatives

Hydrogen replacement by fluorine affects many properties of (hetero) aromatics including (hetero) aromaticity itself. 28Particularly, it enlarges EA1 of 2,1,3-benzothia/selenadiazoles, 2,3 i.e. their electron-acceptor ability, which can be used in the design of functional materials.Recently, our group suggested unified synthetic approach to fluorinated 2,1,3-benzothia/selenadiazoles. 29,30Since redox properties of compounds are of general significance for organic chemistry and its applications, 31 in this work we report on electrochemical reduction (ECR) of new fluorinated 2,1,3-benzothia/selenadiazoles bearing also chlorine and some other substituents (1-20, Figure 1), studied by cyclic voltammetry (CV), as well as on their persistent RAs characterized by EPR spectroscopy and DFT calculations.Compounds 3 and 12 together with their RAs have been characterized earlier 2 as well as related fluorinated 1,4-benzodiazines (quinoxalines) and their RAs. 32For some derivatives, chalcogen-and halogen-dependent dehalogenation was observed being of special chemical interest.

Results and Discussion
Electrochemical reduction and dehalogenation.Most of the studied compounds are known ones.S1).The ECR of 2 (Figure 2) is characterized by two peaks 1С and 2С at the first cycle of the CV, 0 > E > −2.2 V, the first of which is one-electron and reversible; no additional peaks were observed in the range 0 > E > −1.5 V covering the first step of the ECR only.The irreversible one-electron peak 2C corresponds to the formation of unstable dianion (DA) which can undergo an addition of two H + to N atoms (cf.ref. 33) or one H + to the carbocycle with subsequent hydrodechlorination.For 2, the latter process seems to be slow because the reversible peaks 3C and 3A ( C 3 p E = −1.12,3A p E = −1.07V) corresponding to the formation of the RA of hydrodechlorinated product 4 (detected by EPR) were observed at the second and subsequent potential sweep cycles only with  > 0.3 Vs −1 in the range 0 > E > −2.2 V (Figure 2).The ECR of 1 is similar except that an additional peak 3С was observed (Table 1; ESI: Figure S2).Overall, in interesting contrast to related quinoxalines whose © ARKAT USA, Inc hydrodechlorination proceeds via RAs, 32 that of studied thiadiazoles involves DAs and their protonation (Scheme 1).
Similarly, peak 1C of selenadiazoles 10-20 (Table 1) is one-electron, diffusion-controlled and reversible, i.e. corresponding to the formation of long-lived RAs (for 11 and 12, see Figure 3; for the other, ESI: Figures S1 and  S2, Table S1).Peaks 2C and 3C, however, are essentially irreversible for all 10-20, and peak 2C is more than oneelectron ( C 2 p I / C 1 p I > 1, Figure 3; ESI, Figure S2).For 12 in the range 0 > E > −2.2 V, reversible one-electron peaks 4C and 4A ( were observed at the second cycle of potential sweep, whereas no additional peaks were detected in the range 0 > E > − 1.7 V (Figure 3).The peaks were attributed to one-electron ECR of the product resulted from defluorination of 12. Importantly, no additional peaks in the second sweep cycle in the range were detected for 14-17 with non-halogen substituents in the positions 5 and 6, whereas weak ones were observed in anode branches of the CVs of 13, 18 and 19 with F atoms in the position 6 (ESI: Figure S2).Altogether, these suggest that defluorination of 12 occurs regioselectively at equivalent positions 5 / 6 not involving the positions 4 / 7 (note that in fluorinated selenadiazoles the positions 5 and 6 are much more active in nucleophilic substitution than the positions 4 and 7). 29This is, however, only minor process because peaks 4C and 4A are characterized by substantially lower currents as compared with the first peaks in CVs (Figure 3).© ARKAT USA, Inc -2 .5 -2 .0 -1 .5 -1 .0 -0 .5 0 .0 E values of selenadiazoles are ca.0.1  0.03 V less negative, and EA1 ca.0.17 eV more positive, than those of thiadiazoles with the same substitution patterns (Table 1, pairs 1/10, 2/11, 3/12, 5/13, 6/14, 8/16 and 9/17).They form two independent linear regressions EA1 = a C 1 p E +b (Figure 4) with a, b and r 2 equal to 1.79 eVV −1 , 3.50 eV and 0.972, respectively, for thiadiazoles 1-9; and to 1.61 eVV −1 , 3.30 eV and 0.948 for selenadiazoles 10-19; r is correlation coefficient.The values of a and b for both regressions are comparable with those for related compounds whose C 1 p E were measured in MeCN. 2 Altogether, these findings indicate that, in spite of lesser atomic EA1 and Allen electronegativity of Se (2.02 and 2.42) vs. S (2.08 and 2.59), selenadiazoles are better electron acceptors than their S congeners.Earlier, this property was pointed with B3LYP calculations, and with MP2 ones it was shown that the result is not an artifact of the DFT approach. 3Now this non-trivial trend (covering also Te congeners of compounds under discussion) 3 received experimental electrochemical confirmation.Tentatively, it might be explained by better charge/spin delocalization in the RAs of Se derivatives caused by more diffuse 4p-AO of Se as compared with 3p-AO of S.
For compounds with electron-donating substituents MeO and Me2N, C 1 p E and EA1 values reveal additivity under their accumulation.Thus, for MeO-substituted thiadiazoles 5 and 6, the negative shifts of the C 1 p E relative to that of the parent 3 are −0.09(one MeO) and −0.19 V (two MeO), respectively; for Me2N-substituted thiadiazoles 8 and 9 the corresponding shifts are −0.15 and −0.28 V (Table 1).For selenadiazoles, the additivity of such shifts is more exact to be −0.10,−0.20, −0.40 V for MeO-substituted 13, 14 and 15 in relation to C 1 p E of the parent 12; and −0.15 and −0.30V for Me2N-substituted 16 and 17 (Table 1).Previously, the additivity of C 1 p E was observed for benzenes and naphthalenes on accumulation of electron-withdrawing substituents CF3. 34The present work, therefore, generalizes the trend.
EPR of radical anions.The EPR and DFT data of ECR generated (DMF, 295 K) RAs 1-21 are represented in Table 2 and Figs. 5 and 6; the EPR spectrum of product of ECR of 3 in MeCN was reported earlier. 2 Experimental and DFT-calculated isotropic hyperfine coupling (hfc) constants are in reasonable agreement.a Numbers of RAs correspond to those of their neutral precursors; numbers of atoms H and F are the same as for C atoms they are bound with (Figure 1).C nuclei (the positions 3a, 4, 7 and 7a) at their natural abundance.EPR spectrum of RA 21 was obtained under ECR of 12 at the potential of peak 2C (Figure 3); that of RA 3 in MeCN was reported.Except for 2 and 12, EPR spectra were measured under conventional conditions and attributed to primary RAs of the compounds.EPR spectrum of RA 2 (Figure 5, Table 2) was obtained with stationary electrolysis in the potential range C 1 p E > E > −1.8 V (Figure 2); at the electrolysis potential decreased to 2C p E or more, the spectrum was assigned as superposition of spectra of RAs 2 (90%) and 4 (10%).This proves the suggested two-electron mechanism of the hydrodechlorination of 2 in DMF with the participation of corresponding DA (Scheme 1).RAs of the other chlorine containing compounds 1, 10 and 11 (Table 2), possessing more positive EA1 than 2 (Table 1), were much stable; in any way, their possible transformations associated with the dechlorination were not detected.Stationary electrolysis of 12 at the potential C p 1 E (Figure 3) resulted in its RA whose identity was confirmed by EPR and DFT (Figure 6, Table 2).The decrease of the potential to C 2 p E afforded RA 21 (Scheme 2; Figure 6, Table 2).
In some cases, the (U)B3LYP calculations overestimated Fermi-contact spin densities at 19 F nuclei but practically quantitatively reproduced hfc constants with 14 N nuclei (Table 2).According to the calculations, all RAs are planar as expected for the π-species (Figure 7, examples for RAs 2 and 11).Due to this, the hfc constants with 35,37 Cl nuclei are determined by spin-polarization mechanism of hyperfine interaction and, therefore, small in magnitude (Table 2, RAs 1, 2, 10 and 11).For RA 6, the most long-lived among the studied RAs, the hfs from 13 C nuclei was observed at their natural abundance (Figure 6).The resolved hfc constants with 13 C nuclei in the positions 3a, 4, 7 and 7a are practically equal, whereas those with nuclei in the positions 5 and 6 are small and not resolved.DFT suggests negative hfc constants with 13 C nuclei for the positions 3a and 7a and positive ones for the positions 4 and 7 for all studied RAs (for typical examples, see Figure 7).It should be noted that the hybrid functional B3LYP ca.2.5 times overestimates Fermi-contact spin densities at 13 C nuclei, and that the calculated hfc constants with 13 C nuclei are almost equal which agrees with the experimental data (Table 2).

Conclusions
In DMF, the first stage of ECR of fluorinated/chlorinated 2,1,3-benzothia/selenadiazoles 1-20 (bearing also substituents MeO or R2N) is one-electron reversible process giving long-lived RAs whose authenticity is confirmed by EPR spectroscopy and DFT calculations at the (U)B3LYP/6-31+G(d) level of theory.The ECR firstpeak potentials correlate well with gas-phase calculated EAs forming independent linear regressions for S and Se compounds.The potentials of selenadiazoles are less negative, and EAs more positive, than those of thiadiazoles; in contrast to the atomic EAs and Allen electronegativities, this suggests better electron-acceptor ability of Se derivatives which may be used in the design and synthesis of molecular functional materials.At the second stage of ECR, hydrodechlorination of thia/selenadiazoles proceeds via corresponding DAs and their protonation.Non-hydrodefluorination of selenadiazoles at the same stage, involves reductive activation by two or more transferred electrons.These dehalogenations differ from those of related aromatics (benzenes, naphthalenes) 35,36 and aza-aromatics (quinoxalines) 32 controlled by instability of their RAs, and therefore are of interest to organic chemistry.

Experimental Section
General. 1 H (300.13 MHz) and 19 F (282.36 MHz) NMR spectra were measured with Bruker AV-300 spectrometer for solutions in CDCl3; standards were TMS and C6F6 (δ 19 F = −162.9with respect to CFCl3).High-resolution MS spectra (EI, 70 eV) were obtained with DFS Thermo Electron instrument.UV-Vis spectra were collected with Varian Cary 5000, and fluorescence (FL) spectra with Varian Cary Eclipse, spectrophotometers, respectively, for solutions in heptane.Elemental analyses for C, H and N were performed with Carlo Erba Model 1106 instrument, and those for F by standard spectrophotometric method with Ln complex of alizarin complexone.Studied compounds 1-17 and 20 were synthesized by known methods (Scheme 3) 29,30,37 and compounds 18 and 19 in a similar way (below).diaminobenzenes with SOCl2 or SeO2. 29,30,37clic voltammetry.The CV measurements on compounds 1-20 in DMF (1-2.6 mM solutions) were performed at 295 K in an argon atmosphere.The supporting electrolyte was 0.1 M Et4NClO4.A PG 310 USB potentiostat (HEKA Elektronik GmbH, Germany) was used for the measurements.A standard electrochemical cell with solution volume of 5 ml connected to the potentiostat with a three-electrode scheme was employed.A stationary Pt electrode (S = 0.064 cm 2 ) was used as a working electrode, and Pt helix as an auxiliary electrode.Peak potentials were quoted with reference to a saturated calomel electrode (SCE).he Simplex algorithm was used for optimization of hfc constants and line widths.DFT calculations.The DFT calculations on compounds 1-20 and their RAs were performed with full geometry optimization at the (U)B3LYP/6-31+G(d) level of theory using the GAMESS program. 39For all studied RAs the value S 2 did not exceed 0.76.

Figure 2 .
Figure 2. CV of 2 in DMF in the potential ranges 0 > E > −2.2 V (left) and 0 > E > −1.5 V (right) at different sweep rates indicated by color.

Figure 3 .Scheme 2 .
Figure 3. Left: CVs of 11 in the potential ranges 0 > E > −1.5 V (solid lines) and 0 > E > −2.2 V (dotted lines) at different sweep rates indicated by color.Right: CV of 12 in the potential ranges 0 > E > −1.7 V at different sweep rates, and 0 > E > −2.2 V at 100 mVs −1 (black solid and dotted lines correspond to the first and second cycles, respectively).

Figure 5 .Figure 6 .
Figure 5. (a) The EPR spectrum of RA 2 in DMF and (b) its transformation when the stationary electrolysis potential is decreased to C 2 p E .

2
Figure 6.EPR spectra of RAs 1, 2 and 4-21 in DMF, experiment (black) and simulation (blue).For RA 6, green arrows indicate hfs from13 C nuclei (the positions 3a, 4, 7 and 7a) at their natural abundance.EPR spectrum of RA 21 was obtained under ECR of 12 at the potential of peak 2C (Figure3); that of RA 3 in MeCN was reported.2