Redox potential inversion by ionic hydrogen bonding between phenylenediamines and pyridines

Article abstract of DOI:10.1021/ol2007764

In electrochemical oxidations, the second oxidation potential of phenylenediamines (PD) varies because of hydrogen-bonding formation for PD ^+· with pyridines. A linear relationship was obtained for the potential shift as a function of pK_a<

Full text of DOI:10.1021/ol2007764

ORGANIC
LETTERS
2011
Vol. 13, No. 11
2826–2829
Redox Potential Inversion by Ionic
Hydrogen Bonding between
Phenylenediamines and Pyridines
Yi-Chun Chung,† Yi-Jung Tu,† Shih-Hua Lu,† Wan-Chi Hsu,† Kuo Yuan Chiu,† and
Yuhlong Oliver Su*,†,‡
Department of Applied Chemistry, National Chi Nan University, 1 University Road,
Puli, Nantou, Taiwan 545, and Department of Materials Science and Engineering,
National Chung Hsing University, 250, Kuo Kuang Road, Taichung, Taiwan 402
yosu@nchu.edu.tw
Received March 24, 2011
ABSTRACT
In electrochemical oxidations, the second oxidation potential of phenylenediamines (PD) varies because of hydrogen-bonding formation for PDþ•
with pyridines. A linear relationship was obtained for the potential shift as a function of pKa of the protonated pyridines and potential inversion
could be observed. The oxidized PDþ• could also form hydrogen bonding with alcohols and the shift of potential exhibits a different pattern.
Many organic compounds may undergo reduction reac-
tions in two steps to form the anion radical first and then
the dianion.
when there is a significant structural change1 or large
solvation2 or ion-pairing3 that is coupled with one or two
electron-transfer steps.
0
A þ eꢀ ¼ Aꢀ• E0
ð1Þ
ð2Þ
Electrochemically induced hydrogen bonding had
been mainly on the reductive aspect of carbonyl4 and
nitro5ꢀ9 compounds. Research groups of Linschitz4
and Smith5ꢀ9 made important contributions in this
area. Their work elucidates the interaction between
the reduced form of electron-affinitive compounds with
hydrogen donors such as alcohol, amides, ureas, etc.
The interaction could be monitored by cyclic voltam-
metry (CV), and digital simulation was used to estimate
the formation constants. In the oxidative aspect, me-
tallocenes had been used to form a complex with
anions.10 The hydrogen-bond strength was predicted
1
00
A-• þ eꢀ ¼ A2 ꢀ
E
2
Usually the second reduction occurs at a potential more
negative than the first0one due to electrostatic repulsion.
The situation where E0 1 > E0 2 is called “normal ordering
0
of reduction potential”. On the other hand, a process when
the second reduction takes place easier than the first one
(i.e., E0 1 < E0 2) is referred to as “potential inversion”1ꢀ3
and characterized as a single two-electron wave. Generally,
such an “inverted” order is very rare and occurs mostly
0
0
† National Chi Nan University.
(4) Gupta, N.; Linschitz, H. J. J. Am. Chem. Soc. 1997, 119, 6384–
6391.
‡ National Chung Hsing University.
(5) Ge, Y.; Lilienthal, R. R.; Smith, D. K. J. Am. Chem. Soc. 1996,
118, 3976–3977.
(1) (a) Evans, D. H.; Hu, K. J. Chem. Soc., Faraday Trans. 1996, 92,
3983–3990. (b) Macıas-Ruvalcaba, N. A.; Evans, D. H. J. Phys. Chem. B
´
(6) Ge, Y.; Smith, D. K. Anal. Chem. 2000, 72, 1860–1865.
(7) Ge, Y.; Miller, L.; Ouimet, T.; Smith, D. K. J. Org. Chem. 2000,
65, 8831–8838.
2006, 110, 5155–5160. (c) Lord, R. L.; Schultz, F. A.; Baik, M.-H. Inorg.
Chem. 2010, 49, 4611–4619.
ꢀ
(2) Hapiot, P.; Kispert, L. D.; Konovalov, V. V.; Saveant, J.-M.
J. Am. Chem. Soc. 2001, 123, 6669–6677.
(3) (a) Nafady, A.; Chin, T. T.; Geiger, W. E. Organometallics 2006,
25, 1654–1663. (b) Macıas-Ruvalcaba, N. A.; Evans, D. H. J. Phys.
´
(8) Bu, J.; Lilienthal, N. D.; Woods, J. E.; Nohrden, C. E.; Hoang,
K. T.; Truong, D.; Smith, D. K. J. Am. Chem. Soc. 2005, 127, 6423–6429.
(9) Chan-Leonor, C.; Martin, S. L.; Smith, D. K. J. Org. Chem. 2005,
70, 10817–10822.
Chem. B 2005, 109, 14642–14647.
r
10.1021/ol2007764
Published on Web 05/02/2011
2011 American Chemical Society

from acidꢀbase molecular properties by using the pKa
wave also exhibits less reversibility than two original
waves. Interestingly, when the same experiment is mea-
sured with PD-Ph, there is no significant difference in wave
potential and shape (Figure S1, Supporting Information).
This strongly suggeststhatthese differences are caused by a
H-bonding interaction between PD-1þ• and 3,5-Cl2Py.
The presence of H-bonding is also supported by UVꢀvis
spectra that exhibit the decrease in the characteristic IVCT
band of PD-1þ• after addition of 3,5-Cl2Py (Figure S9,
Supporting Information).
slide rule.11
Several pyridines with different substituents, and hence
of different pKa’s, had been investigated for their effect on
the electrochemical waves. The new redox potential is
closer to the first one as the pyridine derivative has a higher
pKa (Table S1, Supporting Information).
Addition of pyridine produces no potential shift in the
first oxidation wave of PD-1, indicating no significant
interaction between the neutral form and pyridine. The
second oxidation in the presence of pyridines, however,
results in a significant negative shift in potential and
broadening of wave shape. The new redox couple, of which
the shift of potential depends on the pKa of pyridines,
gradually moves to a more negative potential and then
merges with the first one (Figures S2ꢀS6, Supporting
Information)
Figure 1. Structure of substituted phenylenediamines.
To our knowledge, however, there is no example found
toinvertthe potential order through changes inH-bonding
strength. In this paper, we report the ionic hydrogen
bonding12,13 formation through electrooxidation of a ser-
ies of p-phenylenediamines (Figure 1) and their interaction
with pyridines and alcohols through different patterns.
The ionic hydrogen bonding plays a crucial role in stabiliz-
ing the oxidation product and hence favors the second
oxidation.
When 2,4,6-trimethylpyridine (2,4,6-Me3Py, pKa
=
7.48)14 is added to the solution of PD-1, the CV exhibits
a different pattern with those for 3,5-Cl2Py. The new redox
couple appears at a potential less positive than that of the
first one (Figure 3A). The final CV in the presence of more
than 1 equiv of 2,4,6-Me3Py exhibits only one redox couple
ox
ox
0
at E2 = þ0.53 V, which is more cathodic than E1
.
Also, the final peak current is about 1.6 times in magnitude
than the first one.
Figure 2. CVs of 1 mM PD-1 in CH3CN containing 0.1 M TBAP
and various concentration of 3,5-Cl2Py. Scan rate: 0.1 V/s.
Working electrode: glassy carbon disk (area = 0.07 cm2).
All phenylenediamines (PD) exhibit two reversible oxi-
dation waves in acetonitrile (CH3CN), corresponding to
the formation of radical cation, PDþ•, and dication, PD2þ
As shown in Figure 2(a), the CV of PD-1 shows two
.
ox
ox
reversible redox couples at E1 = þ0.61 V and E2
=
=
þ1.03 V. As 3,5-dichloropyridine (3,5-Cl2Py, pKa
ox
0.67)14 is added to the solution of PD-1, a new oxidation
wave (E2 0) grows between the two original oxidation
Figure 3. (A) Experimental and (B) simulated CVs of 1 mM PD-
1 in CH3CN containing 0.1 M TBAP and various concentra-
tions of 2,4,6-Me3Py. Scan rate: 0.1 V/s. Working electrode:
glassy carbon disk (area = 0.07 cm2).
waves. The first oxidation remains unchanged, and the
current of second oxidation at E2ox = þ1.03 V gradually
decreases as the 3,5-Cl2Py concentration increases. When
the 3,5-Cl2Py concentration reaches 1 equiv of PD-1, the
second oxidation wave completely disappears and the
appearance of the new wave on the negative side of the
second oxidation wave is thus assigned as the new second
oxidation associated with 3,5-Cl2Py. The new oxidation
A plot of ΔEox as a function of pKa is thus shown in
Figure 4. A linearity with a slope of 57 mV per pKa
indicates the nernstian relationship15a between the poten-
tial shift and the ratio of formation constant for PDþ•(py)
and PD2þ(py), i.e., log(K2/K1) and thus significant inter-
action between the strength of ionic hydrogen bonding can
(11) Gilli, P.; Pretto, L.; Bertolasi, V.; Gilli, G. Acc. Chem. Res. 2008,
42, 33–44.
(12) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford
University, 1997.
(13) Mautner, M. Chem. Rev. 2005, 105, 213–284.
(14) The pKa of protonated pyridines in water.
(15) See the Supporting Information:(a) pp S9ꢀS10. (b) pp S16ꢀS20.
(c) pp S21ꢀS25.
Org. Lett., Vol. 13, No. 11, 2011
2827

order to observe negative shift in the second oxidation
potential. Becausetheshift inpotential of secondoxidation
ox
(E2 ꢀ E2ox) depends on the ratio of K2 to K1, K2 is
0
obtained by following eq 3.16
ox0
K2 ¼ K1 exp[ ꢀ nF(E2 ꢀ E2ox)=RT]
ð3Þ
In the PD/3,5-Cl2Py system, we set K1 to be 1000 and
then K2 was calculated to be 1.5 ꢁ 105. Other sets of
equilibrium constants in the presence of various pyridines
are displayed in Supporting Information. In general, the
values of K1 are in the range of 103ꢀ104 Mꢀ1 15b
.
Figure 4. Plots of oxidation potentials difference for PD-1 as a
function of pyridines pKa.
In addition, the forward rates of H-bonding should be
very fast so that the shift in potential is effective. In a
previous work of Smith, a very rapid value for kf’s of
H-bonding between nitroaniline and 1,3-diphenylurea was
be measured by oxidation potential thermodynamically.
As the basicity of the pyridine derivative is higher than
in the range of 108ꢀ109 Mꢀ1
s
ꢀ1.8 In our PD/3,5-Cl2Py
case, the rate constant for PDþ• and pyridine (kf,1) is in the
range of 105ꢀ106 Mꢀ1 sꢀ1 and that for PD2þ and pyridine
(kf,2) is approximately the value of 108 Mꢀ1 sꢀ1 (Figures
S11 and S12, Supporting Information). As for the broad-
ening of the new redox waves and decrease in the current
magnitude, it is attributed mainly to a rather slow electron-
transfer rate (k0s,2) for the reduction of PD2þ(py) to PDþ•-
(py) (Figure S13, Supporting Information), which is ham-
pered by a rapid H-bonding equilibrium between PD2þ
and pyridine.
ox
5.62, the E2 0 merged with E1ox. The new oxidation keeps
shifting cathodically in potential as the basicity of pyridine
derivatives increases.
We carried out digital simulation of experimental CV
using the Digisim 3.03 Program16 to further understand
the redox-dependent H-bonding systems. The reaction
mechanism consisting of only electron-transfer and hydro-
gen bonding equilibria is represented by a six-membered
square scheme in Scheme 1.
With the above consideration, the simulations are able
toreproducethe waveshapeformostPD/pyridine-binding
systems (Figure 2B).15b However, when switching pyridine
from 3,5-Cl2Py to py or 2,4,6-Me3Py, the second oxidation
wave occurs at a more negative potential than the first.
Two 1eꢀ oxidation waves of PD coalesce into a single 2eꢀ
oxidation wave upon addition of these strongly basic
pyridines so that the determination of oxidation potential
(E1 and E2 0) and equilibrium constant (K1 and K2)
becomes quite difficult. Since PD2þ is acidic, the proton
transfer from PD2þ to pyridine or other reactions might
happen as the more basic pyridine is present. We tenta-
tively ignore the additional reactions and continue to
simulate the CVs of PD in the present of py or 2,4,6-Me3Py
with the same reaction mechanism (i.e., Scheme 1). Satis-
factory simulation results are still obtained for potential
inversion (Figure 3A,B).
The extremely strong H-bonding between PD2þ and
2,4,6-Me3Py has the effect of neutralizing the positive
charge on PD2þ, which expedites the oxidation of
PDþ•(py) to PD2þ(py). Once PDþ•(py) is formed, it is
simultaneously oxidized to PD2þ(py). Thus, a 2eꢀ oxi-
dation wave prior to the first original wave grows at the
expense of two original waves of unbounded PD.
A different type of behavior in second oxidation is
observed upon changing the H-bonding acceptor from
pyridine to alcohol (Figure 5A). Stepwise addition of
excess ethanol (EtOH) can cause the gradual potential
shift to the cathodic direction for the second oxidation
rather than a new oxidation wave formation while the first
oxidation has little change.
Scheme 1a
ox
0
ox
a ks,1, ks,2, k0s,1, k0s,2: the electron-transfer rate constant (reduction).
K1, K2: formation constants for PDþ•(py) and PD2þ(py) complexes,
respectively. kf,1, kf,2: the forward rate constants for PDþ•(py) and
PD2þ(py) complex formation.
The H-bonding equilibrium between neutral PD and
pyridines can be eliminated from the scheme due to no
significant interaction between them. Two equilibrium
constants (K1 and K2) and their related kinetic constants
(kf,1 and kf,2) need to be determined. In particular, the
determination of the K1 is quite important because the
value of K1 largely rules the behaviors of the second
oxidation which would exhibit two resolved waves or a
shifted wave in the presence of pyridines. Thanks to
previous models well supported by simulation,16 some
reasonable starting values for different parameters can be
used to simulate the reactions suggested in Scheme 1.
It is clear from a qualitative analysis of CV shapes that
K1 cannot be too small, or a shifted wave rather than two
separate waves in a second oxidation will be observed.16
On the other hand, K2 must be much larger than K1 in
To investigate the substituent effect on the H-bonding,
the substituted phenylenediamines (Figure 1) are also
(16) Miller, S. R.; Gustowski, D. A.; Chen, Z.-H.; Gokel, G. W.;
Echegoyen, L.; Kaifer, A. E. Anal. Chem. 1988, 60, 2021–2024.
2828
Org. Lett., Vol. 13, No. 11, 2011

Table 1. Values of the H-Bonded Ethanol Number (m),
H-Binding Constant (K2) to PD2þ, and the Redox Potential for
ox
0
Second Oxidation (E2
)
ox
0
ꢀR2
m
K2
E2
log K2
ꢀOCH3
ꢀH
2.13
2.58
1.95
2.40
2.26 ꢁ 10
1.50 ꢁ 102
2.63 ꢁ 102
4.21 ꢁ 102
þ0.91
þ1.01
þ1.03
þ1.06
1.35
2.18
2.42
2.62
ꢀBr
ꢀCN
Figure 5. (A) Experimental and (B) simulated CV of 1 mM PD-1
in CH3CN containing 0.1 M TBAP and various concentrations
of ethanol. [ethanol] = 0ꢀ2.85 M overlay. Scan rate: 0.1 V/s.
Working electrode: glassy carbon disk (area = 0.07 cm2).
the range of 10ꢀ3ꢀ10ꢀ4 Mꢀ1 because there is no significant
change in the first wave and a single shifted wave in second
oxidation is observed.16 Although K1 is very small, we
expect its corresponding forward rate constant (kf,1) to
measured by CV. The CVs appear similar in shape except
the difference in redox potential. From these potential
shifts for E2, we estimate the number (m) of ethanol
H-bonded to PD2þ and their respective formation con-
stants (K2) according to eq 4.4
be the large values of 108ꢀ109 Mꢀ1 sꢀ1 15c
.
An excellent
agreement between experimental and simulated CV has
been obtained (Figure 5).
ox0
ox
m
E2 ¼ E2 þ 0:059 log(1 þ K2[EtOH] )
ð4Þ
Scheme 2a
In this way, it is clear from Figure 6 that estimates of K2
can reflect the substituent effect on H-bonding of PD2þ to
a ks,1, ks,2, k0s,1, k0s,2: the electron-transfer rate constant (reduction).
kf,1, kf,2: the forward rate constant for H-bonding formation.
In conclusion, we report that the second oxidation wave
of PDs varies due to H-bonding interaction with pyridines
or alcohol. As 2,4,6-Me3Py is used as a hydrogen acceptor,
the redox potential inversion of PD-1 occurs. It is possible
that the potential inversion by ionic H-bonding would also
take place if we change the hydrogen donor and/or accep-
tor and/or the size of electrolytes.
ox0
Figure 6. Change of E2 for PD-1, -2, -3, and -4 vs log [EtOH].
ethanol. The value of K2 increases as the electron-with-
drawing power of the substituent increases. A linearity for
ox
0
E2 and log K2 is also observed (Figure S23, Supporting
Information). Since ethanol is a very weak base, it takes
several ethanol molecules to neutralize the positive charge
in PD2þ. The slopes from Figure 6 give the number (m) of
ethanol associated with the dications to be in the range of
Acknowledgment. This work was supported by the
National Science Council of Republic of China.
ox
1.95ꢀ2.58. The values of m,K2,andE2 0 are listed in Table 1.
Supporting Information Available. Syntheses, experi-
mental and simulated procedures for PD-1, -2, -3, and -4,
and additional titration data. This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 5B shows the results of simulation for PD-1 in the
presence of ethanol according to the mechanism in Scheme
2 and the parametersin Table 1. We estimate the K1 tobe in
Org. Lett., Vol. 13, No. 11, 2011
2829