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Substitution Effects and Linear Free Energy Relationships during Reduction of 4- 
Benzoyl-A'-(4-substituted benzyl)pyridinium Cations 

Nicholas Leventis*'''^, Guohui Zhang^, Abdel-Monem M. Rawashdeh** and Chariklia 

Sotiriou-Leventis '^ 

* Materials Division, NASA Glenn Research Center, 21000 Brookpark Road, M. S. 49-1, 

Cleveland, OH 44135 

^Department of Chemistry, University of Missouri-Rolla, RoUa, MO 65409 

Nicholas.Leventis@grc.nasa.gov and cslevent@umr.edu 

tel.: (216) 433-3202 (N.L.); (573) 341-4353 (C. S.-L.) 




In analogy to 4-(p-substituted benzoyl)-#-methylpyridinium cations (1-X's), the title 
species (2-X's, -X = -OCH3, -CH3, -H, -Br, -COCH3, -NO2) undergo two reversible, 
well-separated (£'//2>650 mV) one-electron reductions. The effect of substitution on the 
reduction potentials of 2-X's is much weaker than the effect of the same substituents on 
1-X's: the Hammett p-values are 0.80 and 0.93 for the 1''- and 2"''-e reduction of 2-X's 
vs. 2.3 and 3.3 for the same reductions of 1-X's, respectively. Importantly, the nitro 
group of 2-NO2 undergoes reduction before the 2"'*-e reduction of the 4- 
benzoylpyridinium system. These results suggest that the redox potentials of the 4- 
benzoylpyridinium system can be course-tuned via /^-benzoyl substitution and fme-tuned 
via /7-benzyl substitution. Introducing the recently derived substituent constant of the 

-NO2" group ((T = -0.97) yields an excellent correlation for the 3'''-e reduction of 2- 

P-NO2 

NO2 (corresponding to the reduction of the carbonyl group) with the 2"''-e reduction of 
the other 2-X's, and confirms the electron donating properties of-N02'". 

his report is a preprint of an article submitted to 

journal for publication. Because of changes that 

lay be made before formal publication, this , 

reprint is made available with the understanding ^ 

lat it will not be cited or reproduced without the 

ermission of the author 



We reported recently that 4-benzoyl-A^-methylpyridinium cations, 1-X's, undergo 
two successive, well-separated (-0.6 V) one-electron (1-e) reductions where the l^'-e 
neutralizes the pyridinium and the 2"** one yields the enolate, [1-X]' (eq 1). ' 

0" 




The 1-e reduced ketones [1-X] are stable towards pinacolization,' while the 2-e 
reduced forms [1-X]" (eq 1) are chemically stable only in the absence of proton donors. 
In the presence of protic acids [1-X]''s develop hydrogen (H)-bonding through the 
enolate-0, shifting the second cyclic voltammetric reduction wave to more positive 
potentials; the first reduction wave remains completely unaffected.''^ H-bonded adducts 
with stronger acids (e.g., CH3CO2H) undergo irreversible proton transfer to the 
corresponding quaternized carbinols within the time scale (20-30 s) of cyclic 
voltammetry at 0.1 V s''.' H-bonded adducts with weak protic acids (e.g., water, 
alcohols) undergo proton transfer on longer time scales (~400 s) to the corresponding 
non-quaternized carbinols. These findings suggest that this class of compounds could be 
used as main-stream redox catalysts or electron-transfer quenchers, while in aprotic 
environments they could be used as electrochromic materials (turning from colorless to 
red upon 1-e reduction), for which the basic requirement is chemical reversibility.^ In 
view of those applications, the synthetic accessibility of the />arfl-position of the benzoyl 
group provides an additional degree of freedom, by allowing tuning of the redox 
properties of the 4-benzoylpyridinium system (1-X) through the electronic properties of 
the substituent X (eq 1). As expected, both reductions are facilitated by electron- 
withdrawing substituents, and it was found experimentally that both Ems correlate well 
with the substituent constants, o ^ • In that context, the 2"''-e reduction is more sensitive 

p-X ^ 

to the electronic properties of the substituent (p2.e=3.3) than the first one (p/.e=2.3), 
reflecting the relative distance of the substituent from the point of reduction.^ Along 
those studies, it was also observed that the nitro group of I-NO2 undergoes reduction 
before the carbonyl. So, when the carbonyl group of I-NO2 is reduced, -NO2 has been 
already transformed into -NO2'. Reasoning that the redox potential of the carbonyl 



reduction of I-NO2 (third wave) should correlate with the corresponding redox potentials 
of the other 1-X's (second waves), has allowed calculation of the substituent constant for 
-N02"". It was found that G _^^^. = -0.97, suggesting that -N02'' is a strong electron 

donor.^ 

Hammett Linear Free Energy correlations are used generally in order to quantify 
experimental observations and deduce stereoelectronic communication effects (e.g., 
resonance vs. inductive) between substituents and reaction sites. By the same token, 
Hammett correlation studies with compounds undergoing successive e-transfer reactions 
are rather uncommon.^ So, motivated by the results summarized above, we decided to 
explore how reduction of the two redox centers would be affected by placing the 
substituents closer to the pyridinium rather than to the carbonyl, and thus perhaps get a 
glimpse of how the redox properties of the 4-benzoylpyridinium system could be adjusted 
by varying the two sets of substitutents independently. These issues were investigated 
with compounds 2-X, where X= -OCH3, -CH3, -H, -Br, -COCH3 and -NO2. 

o 

BF4- 




All six 2-X's were prepared by straigthforward quaternization of 4- 
benzoylpyridine with the corresponding jt7-substituted benzyl halides. With the exception 
of the chloride salt of 2-H, which was first reported in 1959 in conjunction with its 
potential properties as a local anesthetic,"* all other 2-X's are new compounds. 

Generally, the redox properties of 2-X's were determined with 3 mM solutions by 
cyclic voltammetry at 0.1 V s'' in anhydrous CH3CN/O.I M tetrabutylammonium 
perchlorate (TBAP) using ferrocene as internal standard. All electrochemical data are 
summarized in Table 1 and representative voltammograms are shown in Fig. 1 . With the 
exception of 2-NO2, all other 2-X's show two chemically reversible redox waves 
(cathodic-to-anodic peak current ratios, ip,c/ip,a~l)- By analogy to 1-X's (eq 1), all first 
reduction waves correspond to the reduction of the pyridinium moieties, and with the 
exception of 2-NO2, all second reduction waves correspond to the reduction of the 
carbonyl. In most cases, the cathodic to anodic peak-to-peak potential separations, AEp.p, 



3 



are comparable to the values given by the ferrocenium/ferrocene (Fc VFc) couple (73 ± 2 
mV),^ and are close to the theoretical value of 60 mV for 1-e kinetically reversible 
systems. As expected, Fig. lA shows that electron- withdrawing substituents facilitate 
both reductions, as both waves move to less negative potentials. However, the effect of 
substitution does not appear to be strong. Meanwhile, the behavior of the nitro- 
derivative, 2-NO2, is remarkably different: at 0.1 V s"' 2-NO2 shows four irreversible 
waves (Fig. IB dotted line). A potential sweep at the same sweep rate through the first 
wave only shows that the l^'-e reduction of 2-NO2 is chemically reversible (see Table 1 
and Fig. S.l in Supporting Information). On the other hand, successive potential sweeps 
through the 2"''- and 3'^''-e reduction waves of 2-NO2 produce irreversible behavior (see 
Fig.s S.2 and S.3 in Supporting Information). By decreasing the concentration of 2-NO2 
to -0.3 mM and by increasing simultaneously the sweep rate to > 5 V s'', 2-NO2 yields 
three apparently reversible voltammetric waves (Fig. IB solid line). The first wave 
correlates with the first reduction wave of all other 2-X's (vide infra) and corresponds to 
the reduction of the pyridinium ring (in analogy to eq 1); as expected, it is positively- 
shifted in comparison to the first waves of the other 2-X's (Table 1), reflecting the strong 
electron withdrawing properties of-N02. The second (middle) wave of 2-NO2 is close to 
the reduction of nitrobenzene (see Fig. IB-inset), and is associated with the reduction of 
the /?-nitrobenzyl substituent (eq 2). The third wave is due to the reduction of the 





[2-NO2] [2-N02-"] 

carbonyl and it has been shifted to more negative potentials, in contrast to the 2"'' waves 
of all other 2-X's, which move in the positive direction as the electron- withdrawing 
ability of the substituents increases (Table 1). As it will be discussed below, this is 
attributed to the electron donating properties of -NO2". Meanwhile, the fact that the three 
expected reversible waves are observed only by simultaneously decreasing the 
concentration of 2-NO2 and increasing the potential sweep rate, indicates that the 2-e 
reduced form, [2-N02"]", participates in irreversible bimolecular reactions. This 
deduction, taken together with the fact that the 1 -e reduced form of nitrobenzene does not 



seem to interfere with the redox processes of 1-H (Fig. IB-inset), suggests that the 
reduced nitrobenzyl group is not involved, at least initially, in any bimolecular reaction. 
Therefore, it is suggested that the bimolecular reaction responsible for the irreversibility 
introduced as early as during the 0.1 V s'^ sweep through the second reduction wave of 2- 
NO2, should be related to the electrostatic perturbation imposed by -N02'" upon the 
reduced pyridinium group of the same [2-N02'']" species. 

The relative efficiency by which substituents X influence the reduction of the two 
redox centers of the 2-X's is expressed via Hammett Linear Free Energy relationships. 
Fig. 2 shows good correlations between substituent constants Gp-x and the E1/2 of both 
waves of 2-X's. The slopes are 0.047 V and 0.055 V for the reduction of the pyridinium 
and the carbonyl, corresponding to reaction constants p/.e=0.80 and p2-e=0.93, 
respectively. It should be noted here that by using the o f^^-, value (0.78), neither the 

second nor the third wave of 2-NO2 correlate with the second waves (reduction of the 
carbonyl) of the other 2-X's. However, using the g _ (= -0.97) value,^ the reduction 

of the carbonyl group of 2-NO2 (third voltammetric wave. Fig. 2B solid line), correlates 
well with the reduction of the same group in all other 2-X's. That finding confirms both 
the stereoelectronic communication between the para-henzyl substituents and the 
reduction sites, and the predictive power of the g . constant. Now, the good 

correlation of all Ej/2's with the substituent constants Gp.x indicates a purely inductive 
interaction, and the fact that p2-e is somewhat larger than p/.^ was rather unexpected. On 
the other hand, both p-values are <1.00, reflecting a rather poor electronic 
communication of the substituents with the redox sites. (Note that p-values less than 
"one" are well established with thermodynamic parameters -equilibrium constants- in 
systems containing methylene spacers.^) The weak electronic communication of the 
substituents even with the point of the 2"''-e reduction (where the p-value is somewhat 
higher) is reflected upon the lack of any significant ability of the p-benzyl sustituents to 
affect H-bonding developing between the enolate oxygen and methanol in solution. In 
analogy to 1-X's, such H-bonding shifts the wave of the second electron reduction to 
more positive potentials leaving the first wave unaffected (Fig. 3). The slope of the 
potential separation between the two waves vs. log([MeOH]) (Fig. 3-inset) yields the 



average (apparent) number of methanol molecules, <m>, participating in the H-bonded 
adducts, [2-X]'...m(CH30H) (see Table 2).^ The intercepts of the same curves are related 
to the equilibrium constants (.^h), and therefore the free energy (AG°'^) of the H-bonded 
adduct formation.''^ While in the case of 1-X's substitution played a significant role 
both on the strength of H-bonding and the number of MeOH molecules associating with 
the enolate-0, in the case of 2-X's there is little variation in either one of those two 
properties as a function of substitution. 

In conclusion, the effect of /7ara-benzyl substituents on the reduction of the 
quatemized 4-benzoylpyridinium system is significantly weaker than the effect of para- 
benzoyl substitution. This finding implies that the redox potential of the 4- 
benzoylpyridinium system can be tuned first via /)flra-benzoyl substitution and then fine- 
tuned via /»ara-benzyl substitution. For example, based on our previous results with 1- 
X's, it is expected that a change of the /?ara-benzoyl substituent from -OCH3 to -CH3 
(A(7p.x= 0.3) will change the redox potential of the l^'-e reduction of the 4-benzoyl-A^- 
benzylpyridinium system by -40 mV. Then a change of the para-benzyl substituent from 
-OCH3 to -NO2 (A(7p.x= 1.05) is expected to change the redox potential of the same 
reduction by -50 mV, and therefore by judicious choice of the para-henzyl substituents 
the potential range between the l^'-e reductions of the 4-(/7-CH30-benzoyl)- and the 4-(p- 
CH3-benzoyl)-A^-benzylpyridinium cations can be covered almost continuously. 

Acknowledgement. We gratefully acknowledge support by the Petroleum Research 
Fund, administered by the ACS (Grant No. 35154-ACS), and from the National Cancer 
Institute (Grant No. 1 R15 CA82141-01A2 to C. S.-L.). 

Supporting Information Available: (a) Experimental section with preparation and 
characterization of 2-X's. (b) Cyclic voltammetry of 2-NO2 at 0.1 V s'' through the first 
only (Fig. l.S), the second (Fig. 2.S) and the first three (Fig. 3.S) reduction waves. 



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References 

1. Leventis, N.; Elder, I. A.; Gao, X.; Bohannan, E. W.; Sotiriou-Leventis, C; 
Rawashdeh, A.-M. M.; Overschmidt, T. J.; Gaston, K. R. J. Phys. Chem. B 2001, 105, 
3663-3674. 

2. Leventis, N.; Rawashdeh, A.-M. M.; Zhang, G.; Elder, I. A.; Sotiriou-Leventis, C. J. 
Org. Chem. 2002, 67, 7501-7510. 

3. See for example: (a) Sauro, V. A.; Workentin, M. S. J. Org. Chem. 2001, 66, 831- 
838. (b) Connelly, N. G.; Davis, P. R. G.; Harry, E. E.; Klangsinsirikul, P.; Venter, M. J. 
Chem. Soc, Dalton Trans. 2000, 2273-2277. (c) Zagal, J. H.; Gulppi, M. A.; Cardenas- 
Jiron, G. Polyherdon 2000, 22-23, 2255-2260. (d) Aguilar-Martinez, M.; Cuevas, G.; 
Jimerez-Estrada, M.; Gonzalez, I.; Lotina-Hennsen, B.; Macias-Ruvalcaba, N. J. Org. 
Chem. 1999, 64, 3684-3694. (e) Mclnnes, E. J. L.; Farley, R. D.; Rowlands, C. C; 
Welch, A. J.; Rovatti, L.; Yellowlees, L. J. J. Chem. Soc, Dalton Trans., 1999, 4203- 
4208. (f) Andersen, M. L.; Nielsen, M. F.; Hammerrich, O. Acta Chem. Scand. 1997, 51, 
94-107. (g) Darensbourg, M. Y.; Bischoff, C. J.; Houliston, S. A.; Pala, M.; Reibenspies, 
J. J. Am. Chem. Soc. 1990, 112, 6905-6912.; (h) Queiros, M. A. M.; Simao, J. E. J.; Dias, 
A. R. J. Organomet. Chem. 1987, 329, 85-97. 

4. Lyle, R. E.; Troscianiec, H. J.; Warner, G. H. J. Org Chem. 1959, 24, 338-42. 

5. Isaacs, N. Physical Organic Chemistry, 2nd ed.; Longman Scientific and Technical: 
Essex, U.K., 1995: p 152. 

6. Daschbach, J.; Blackwood, D.; Pons, J. W.; Pons, S. J. Electroanal. Chem. 1987, 237, 
269-273. 

7. Once the -NO2 group is reduced to the strongly electron donating -NOa'" group, the 
reduced pyridinium group of the same species becomes a reducing agent for the 
pyridinium of 2-NO2. So, [2-N02"]" diffusing away from the electrode can react in a 
comproportionation fashion with 2-NO2 diffusing towards the electrode and yield not 
only 2 moles of [2-N02]", but also [2-N02"'].^ The latter is a new species that is generated 
only transiently because, upon reduction, the -NO2 group changes from an electron 
acceptor into an electron donor. [2-NO2"] finds itself in two concentration gradients, one 
towards the electrode and one towards the bulk solution. [2-N02''] diffusing towards the 
electrode is reduced into [2-N02""]". [2-N02"]" lost by diffusion towards the bulk becomes 
responsible for the chemical irreversibility observed past the second wave. 



8. (a) Leventis, N.; Gao, X. J. Electroanal. Chem. 2001, 500, 78. (b) Rongfeng, Z.; 
Evans, D. H. J. Electroanal. Chem. 1995, 385, 201. 

9. For example, the p values for the ionization (in water) of ArCOiH, ArCH2C02H and 
ArCH2CH2C02H are 1.00, 0.56 and 0.24 respectively (Ar: substituted phenyl group). '° 

10. Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry, Third Edition, Part A: 
Structure and Mechanisms; Plenum Press: New York, N.Y., 1993: p 202. 



10 



Figure Captions 

Figure 1. (A) Cyclic Voltammetry of 2-OCH3 ( ; 3.04 mM), 2-H ( ; 3.32 mM, 

and 2-COCH3 (....; 3.27 mM) in CH3CN/O.I M TBAP with a Au-disk electrode 
(0.0201 cm^) at 0.1 V s"'. Current bar: 10 ^lA (B) Cyclic Voltammetry of 2-NO2 0.33 

mM at 0.1 V s'' ( ; current bar: 2.7 |iA) and at 5 V s"' ( ; current bar: 16 \xA). 

Inset : Voltammetry in the same electrolytic solution at 0.1 V s"' of a mixture containing 
1-H (3.08 mM) and nitrobenzene (PhN02, 1.90 mM). The middle wave is produced by 
the reduction of PhN02. Similar results have been obtained with 2-H and PhN02. 

Figure 2. Hammett plots for the l^'-e reduction (dark circles; slope = 0.047 V; intercept = 
-1.023 V; correlation (r^) =0.981) and the 2"''-e reduction (open circles; slope = 0.055 V; 
intercept = -1.678 V; correlation (r^) = 0.908) for the 4-benzoyl-A/^-(p-substituted 
benzyl)pyridinium cations (2-X's) of Table 1. The open triangle corresponds to the 1-e 
reduction of the/»-nitrobenzyl moiety of 2-NO2 (refer to the middle wave in Figure IB). 

Figure 3. The effect of methanol on the cyclic voltammetry of 2-CH3 (3.44 mM) in 
CH3CN/O.I M TBAP at 0.1 V s"', using a Au-disk electrode (0.0201 cm^). [CH30H[: (a) 
0.0 M; (b) 0.098 M; (c) 0.196 M; (d) 0.484 M; (e) 0.95 M; (f) 2.24 M. Inset: \AE,/2\ of 
the two waves vs. log[CH30H]; slope = 140 mV; intercept = 448 mV; correlation (r^) = 
0.983. 



11 



Figure 1 



1 1 1 1 1 1 r 








O 



H \ \ h 



H H 



- B 




1.8 -1.5 -1.2 -0.£ 
vo(ts vs. ferrocene 



JL 



_!_ 



-2.0 -1.8 -1.6 



-1.4 -1.2 -1.0 

volts vs. ferrocene 



-0.8 -0.6 



-0.4 



12 



Figure 2 



-0.8 



-1.0 



0) 

§ -1.2 

o 

a 

03 

> -1.4 

■4— » 

o 
> 



Uj' -16 



-1.8 





' 1 ' 


1 ' 1 ' 1 


' 1 


? 


1 ' 1 








-OMe -H 




-AC 


♦ 


■ 


; 




-Me 


-Br 




-NO^ 


■ 


■ 












■ 


- 










-N0j(2"''wave) 


- 


■ 










A 


■ 










-AC 








-N07 


-OMe -H 
-Me 


-Br 


. — e — 


▲ 




- 


1 


1 1 1 1 1 


1 




-N0j(3"'wave) 

1 . F 


- 



-0.9 



-0.6 



-0.3 0.0 

a 

P-x 



0.3 



0.6 



0.9 



13 



Figure 3 



-20 



-10 - 



1 



c 


o 



10 



20 



30 




■1.4 -0.7 0.0 0.7 

log ([MeOH], M)- 



-2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 

volts VS. ferrocene 



-0.6 -0.4 



14 



Substitution Effects and Linear Free Energy Relationships during Reduction of 
4-Benzoyl-A'-(4-substituted benzyl)pyridinium Cations 

Nicholas Leventis*^ Guohui Zhang^ Abdel-Monem M. Rawashdeh^ and Chariklia 

Sotiriou-Leventis*'^ 

^ Materials Division, NASA Glenn Research Center, 21000 Brookpark Road, M. S. 

49-1, Cleveland, OH 44135 

^Department of Chemistry, University of Missouri-Rolla, Rolla, MO 65409 

Nicholas.Leventis@grc.nasa.gov and csieventrajumr.edu 

SUPPORTING INFORMATION 

Experimental Section 

Methods. All electrochemical experiments were carried out with an Au disk 
working electrode (1.6 mm in diameter, from Bioanalytical Systems, Inc., West 
Lafayette, IN), an aqueous Ag/AgCl reference electrode (also from BAS), and an Au 
foil (2.5 cm ) as a counter electrode, in Ar-degassed anhydrous CH3CN/O.I M TBAP 
solutions at room temperature (23 ±1 °C). All voltammograms have been 80% 
compensated for solution resistance. All reactions were carried out under N2. 
Melting points are uncorrected. Elemental analyses were performed by Prevalere Life 
Sciences, Inc. (Former Oneida Research Services, Inc.), Whiteboro, NY. 

Materials. All starting materials, reagents and solvents were commercially 
available and were used as received. Ferrocene and 4-benzoylpyridine were 
sublimed. Tertrabutylammonium perchlorate (TBAP) was prepared as described 
before. All 2-X's were prepared by quaternization of 4-benzoylpyridine according to 
the following general procedure. 4-Benzoylpyridine (1 equiv) and the corresponding 
/?- substituted benzyl halide (2-3 equiv bromide or chloride) were dissolved in CH3CN 
and refluxed for 24 h. At the end of the period, the solvent was removed and the 



' Leventis, N.; Elder, I. A.; Gao, X.; Bohannan, E. W.; Sotiriou-Leventis, C; Rawashdeh, A.-M. M.; 
Overschmidt, T. J.; Gaston, K. R. J. Phys. Chem. B 2001, 105, 3663. 




residue was dissolved in water. An aqueous solution of ammonium tertrafuoroborate 

(-10 equiv) was added and the mixture was refluxed until all solids were dissolved. 

The solution was cooled to ~ 5 "C, and the precipitate was collected, dissolved in 

CH3CN and precipitated with diethylether. The precipitate was collected and 

recrystallized once again from water. All 2-X's of this study are white solids. 'H 

NMR assignments are based on the following numbering convention: 

O 

3 

'I ■ BF4 

12 20\^18 
19 

2-X 

4-Benzoyl-j'V-benzylpyridinium tetrafluoroborate (2-H) was prepared from 4- 
benzoylpyridine (5g, 0.027mol) and benzyl bromide (14g, 0.08 Imol). Yield 5.2 g 
(53%); mp 151-153°C; 'H NMR (DMS0-c?6, 400 MHz) <5 5.93(2H, s, -CHiN^), 7.45- 
7.50 (3H, m, H-1,2,6), 7.58-7.63 (4H, m, H-3,5, 17,19), 7.77-7.81 (IH, m, H-18), 
7.83-7.86 (2H, m, H-16,20), 8.38 (2H, d, J<),m= J\2,u= 6.6 Hz, H-9,13), 9.36 (2H, d, 
Aio= ^12,13= 6.8 Hz„ H-10,12); '^C NMR (DMSO-Jg, 100 MHz) 5 63.5 (-CHzN^"), 
127.5, 129.2, 129.4, 129.6, 130.4, 133.9, 134.2, 134.9, 145.8, 151.9, 192.1 (C=0). 
Anal Calcd for Ci9H,6NOBF4: C, 63.19; H, 4.47; N, 3.88. Found: C, 62.92; H, 4.12; 
N, 3.91. 

4-Benzoyl-A'-(4-methylbenzyl)pyridinium tetrafluoroborate (2-CH3) was 
prepared from 4-benzoylpyridine (5g, 0.027mol) and 4-methylbenzyl bromide (lOg, 
0.054mol). Yield 4.6 g (45%); mp 190-192°C; 'H NMR (DMSO-c/^, 400 MHz) 5 
2.32 (3H, s, -CH3) 5.88 (2H, s, -CHzN^), 7.29 (2H, d, ^2,3=^5,6=8.0 Hz, H-2,6), 7.50 
(2H, d, ^2,3=^6=8.0 Hz, H-3,5), 7.59-7.66 (2H, m, H-17,19), 7.77-7.82 (IH, m, H- 
18), 7.83-7.86 (2H, m, H-16,20), 8.37 (2H, d,J9, 10 = ^12, 13= 6.8 Hz, H-9,13), 9.34 (2H, 
d, Aio = ^12,13= 6.8 Hz, H-10,12); '^C NMR (DMSO-Jg, 100 MHz) 5 20.8 (-CH3), 
63.5 (-CHzN^), 127.5, 129.0, 129.2, 129.7, 130.0, 130.4, 130.9, 134.1, 139.3, 145.6, 
151.8, 192.1 (C=0). Anal Calcd for C20H18NOBF4: C, 64.03; H, 4.84; N, 3.73. 
Found: C, 63.95; H, 4.55; N, 3.80. 

4-Benzoyl-A^-(4-nitrobenzyl)pyridinium tetrafluoroborate (2-NO2) was 
prepared from 4-benzoylpyridine (5g, 0.027mol) and 4-nitro-benzyl bromide (lOg, 



0.047mol). Yield 7.8 g (71%); mp 145-147°C; 'H NMR (DMSO-^6, 400 MHz) d 
6.09 (2H, s, -CH2N^), 7.58-7.64 (2H, m, H-17,19), 7.70 (2H, d, J2,3 = ^5,6= 9.0 Hz, H- 
3,5), 7.75-7.80 (IH, m, H-18), 7.82-7.85 (2H, m, H-20,26), 8.22 (2H, d, ^9,10 = ^12,13= 
6.6 Hz, H-10,12), 8.30 (2H, d, 72,3 = ^6= 8.8 Hz, H-2,6), 8.93 (2H, d, J9,io = ^12,13= 
6.8 Hz, H-9,13); "C NMR (CD3CN, 100 MHz) 5 64.4 (-CH2N^), 125.3, 128.9, 130.1, 
131.3, 131.6, 135.2, 135.9, 140.3, 146.9, 149.8, 153.9, 192.1 (C=0). Anal Calcd for 
C19H15N2O3BF4: C, 56.19; H, 3.72; N, 6.90. Found: C, 56.25; H, 3.36; N, 6.84. 

4-Benzoyl-A^-(4-methoxylbenzyl)pyridinium tetrafluoroborate (2-OCH3) was 
prepared from 4-benzoylpyridine (5g, 0.027mol) and 4-methoxylbenzyl chloride (9g, 
0.058mol). Yield 3.3 g (31%); mp 130-132°C; 'H NMR (DMSO-de, 400 MHz) 5 
3.77 (3H, s, -OCH3), 5.85 (2H, s, -CHzN^), 7.03 (2H, d, J2,3=^5,6=8.8 Hz, H-3,5), 7.59 
(2H, d, J2,3=^5,6=8.6 Hz, H-2,6), 7.61-7.65 (2H, m, H-17,19), 7.77-7.82 (IH, m, H- 
18), 7.83-7.86 (2H, m, H-16,20), 8.36 (2H, d, 79,10 = ^12,13= 6.8 Hz, H-9,13), 9.34 (2H, 
d, J9,io = ^12,13= 7.0 Hz, H-10,12); '^C NMR (DMSO-c/e, 100 MHz) S 55.3 (-OCH3), 
63.3 (-CHzN^), 114.7, 125.7, 127.4, 129.1, 130.4, 131.0, 134.1, 134.8, 145.5, 151.7, 
160.2, 192.1 (C=0). Anal Calcd for C2oH,8N02BF4: C, 61.41; H, 4.64; N, 3.58. 
Found: C, 61.10; H, 3.94; N, 3.68. 

4-Benzoyl-A^-(4-bromobenzyl)pyridinium tetrafluoroborate (2 -Br) was 
prepared from 4-benzoylpyridine (5g, 0.027mol) and 4-bromobenzyl bromide (12g, 
0.049mol). Yield 5.9 g (51%); mp 188-190°C; 'H NMR (DMSO-Jg, 400 MHz) d 
5.90 (2H, s, -CHaN^), 7.58 (2H, d, ^2,3=^5,6=8.6 Hz, H-3,4), 7.59-7.65 (2H, m, H- 
16,20), 7.70 (2H, d, 72,3-^6=8.6 Hz, H-2,6), 7.77-7.82 (IH, m, H-18), 7.83-7.87 (2H, 
m, H-16,20), 8.38 (2H, d, J9,io^Jn,u^ 6.8 Hz, H-9,13), 9.35 (2H, d, J9,io = 7i2,i3= 7.0 
Hz, H-10,12); "C NMR (DMSO-^^, 100 MHz) 5 62.8 (-CH2N^), 127.5, 129.2, 130.4, 
132.2, 133.2, 134.2, 134.9, 145.8, 151.9, 192.1 (C=0). Anal Calcd for 
Ci9Hi5NOBrBF4: C, 51.86; H, 3.44; N, 3.18. Found: C, 51.80; H, 2.55; N, 3.18. 

4-Benzoyl-7V-(4-acetyIbenzyl)pyridinium tetrafluoroborate (2-COCH3) was 
prepared from 4-benzoylpyridine (5g, 0.027mol) and 4-bromomethylacetophenone. 
The latter was prepared from 4-methylacetophenone (lOg, 0.075mol), N- 
bromosuccinimide (15g, 0.084mol) and l,l-azobis(cyclohexanecarbonitrile) (3.6g, 
0.01 5mol) in 150 ml CCI4 (reflux, 24 h) and was used as received (light yellow solid), 
without further purification. Yield 7.6 g (70%); mp 175-177°C; 'H NMR (DMSO-ds, 



400 MHz) 52.59 (3H, s, -COCH3) 6.02 (2H, s, -CHiN^), 7.60-7.65 (2H, m, H-17,19), 
7.72 (2H, d, J2,3=^5,6=8.6 Hz, H-3,5), 7.77-7.83 (IH, m, H-18), 7.84-7.88 (2H, m, H- 
16,20), 8.04 (2H, d, ^2,3=^6=8.6 Hz, H-2,6), 8.41 (2H, d, ^9,10 = ^12,13= 6.8 Hz, H- 
9,13), 9.39 (2H, d, J9,\o = Jn,n= 7.0 Hz, H-10,12); '^C NMR (DMSO-Jg, 100 MHz) 5 
26.9 (-CH3) 63.0 (-CHiN^), 127.5, 129.0, 129.3, 129.3, 130.4, 134.1, 134.9, 137.4, 
138.6, 146.0, 152.0, 192.1 (C=0), 197.7 (acetyl C=0). Anal Calcd for 
C2iH,8N02BF4: C, 62.56; H, 4.50; N, 3.47. Found: C, 62.47; H, 3.78; N, 3.50. 



Figure S.l Cyclic Voltammogram across the first reduction wave of 2-NO2 (-0.6 
mM) in CH3CN/O.I M TBAP at 0.1 V s\ 



O 




-800 



-600 



volts 



-400 



Figure S.2 Cyclic Voltammogram across the first two reduction waves of 2-NO2 
(-0.6 mM) in CH3CN/O.I M TBAP at 0.1 V s"'. 




Figure S.3 Cyclic Voltammogram across the first three reduction waves of 2-NO2 
(-0.6 mM) in CH3CN/O.I M TBAP at 0.1 V s\ 




-1400 -1200 



-1000 -800 

volts 



-600 -400