Evaluation of the extent of conjugation in symmetrical and asymmetrical aryl-substituted acetophenone azines using electrochemical methods

Article abstract of DOI:10.1021/jo0056287

The electrochemical behavior of a series of symmetrical and unsymmetrical aryl-substituted acetophenone azines (1-X/Y, where X and Y are 4-NO₂, 4-CN, H, 3-OCH₃, 4-OCH₃, 4-CH₃, and 4-N(CH₃)₂) was studied in acetonitrile and N,N-dimethylformamide (DMF) solution using cyclic voltammetry (CV). Compounds 1-X/Y, where neither X or Y are nitro substituents, undergo successive reduction to their radical anion (1-X/Y^·-) and then dianion (1-X/Y^2-), respectively. In all cases, the formation of the radical anion is completely reversible and the standard reduction potentials, E°_1-X/Y/1-X/Y^·- could be determined. The reversibility of the second electron transfer is substituent dependent with certain dianions sufficiently basic to be protonated under our conditions. Standard reduction potentials (E°_1-X/Y/1-X/Y^·-) for the formation of radical anions exhibit a large substituent effect with values differing by more than 0.66 V throughout the series going from 1-4-CN/4-CN to 1-4-OCH₃/4-OCH₃; similar substituent effects were determined for the formation of the dianion. The nitro-containing azines deviate from the above-mentioned behavior. With the exception of 1-4-NO₂/4-NO₂, they exhibit single electron waves that have values of E°_1-X/Y/1-X/Y^·- within 40 mV of each other and thus the reduction is not subject to the same substituent effect as the other azines. 1-4-NO₂/4-NO₂ exhibits an E° at a similar potential, but is a two-electron reversible wave with features indicative of a reduction system containing two localized, nonconjugated redox centers. The reduction potentials of all the aryl azines were correlated with Hammett σ parameters to look at variations in E°_1-X/Y/1-X/Y^·- vs SCE as a function of substituent. The small ρ values in combination with the other electrochemical data provide support for single bond character of the N-N bond and evidence for a lack of strong electronic communication between the two aryl centers through the azomethine bonds, especially for those systems with electron-withdrawing groups.

Full text of DOI:10.1021/jo0056287

J . Org. Chem. 2001, 66, 831-838
831
Eva lu a tion of th e Exten t of Con ju ga tion in Sym m etr ica l a n d
Asym m etr ica l Ar yl-Su bstitu ted Acetop h en on e Azin es Usin g
Electr och em ica l Meth od s
Vittorio A. Sauro and Mark S. Workentin*
Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada
mworkent@julian.uwo.ca
Received August 29, 2000
The electrochemical behavior of a series of symmetrical and unsymmetrical aryl-substituted
acetophenone azines (1-X/Y, where X and Y are 4-NO2, 4-CN, H, 3-OCH3, 4-OCH3, 4-CH3, and
4-N(CH3)2) was studied in acetonitrile and N,N-dimethylformamide (DMF) solution using cyclic
voltammetry (CV). Compounds 1-X/Y, where neither X or Y are nitro substituents, undergo
successive reduction to their radical anion (1-X/Y•-) and then dianion (1-X/Y2-), respectively. In all
cases, the formation of the radical anion is completely reversible and the standard reduction
•-
potentials, E°1-X/Y/1-X/Y could be determined. The reversibility of the second electron transfer is
substituent dependent with certain dianions sufficiently basic to be protonated under our conditions.
•-
Standard reduction potentials (E°1-X/Y/1-X/Y ) for the formation of radical anions exhibit a large
substituent effect with values differing by more than 0.66 V throughout the series going from 1-4-
CN/4-CN to 1-4-OCH3/4-OCH3; similar substituent effects were determined for the formation of
the dianion. The nitro-containing azines deviate from the above-mentioned behavior. With the
•-
exception of 1-4-NO2/4-NO2, they exhibit single electron waves that have values of E°1-X/Y/1-X/Y
within 40 mV of each other and thus the reduction is not subject to the same substituent effect as
the other azines. 1-4-NO2/4-NO2 exhibits an E° at a similar potential, but is a two-electron reversible
wave with features indicative of a reduction system containing two localized, nonconjugated redox
centers. The reduction potentials of all the aryl azines were correlated with Hammett σ parameters
•-
to look at variations in E°1-X/Y/1-X/Y vs SCE as a function of substituent. The small F values in
combination with the other electrochemical data provide support for single bond character of the
N-N bond and evidence for a lack of strong electronic communication between the two aryl centers
through the azomethine bonds, especially for those systems with electron-withdrawing groups.
In tr od u ction
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10.1021/jo0056287 CCC: $20.00 © 2001 American Chemical Society
Published on Web 01/09/2001

832 J . Org. Chem., Vol. 66, No. 3, 2001
Sauro and Workentin
prisingly, despite the synthetic utility and potential
material applications very little is known about their
electronic properties.
assumes a trans conformation about the azine moiety
having the expected 180° torsion angle. Theoretical
calculations have shown that the potential energy surface
is quite shallow in the region of 120° < τ < 180°, and
thus, the energy difference for conversion between the
gauche and trans conformations is relatively small.35,36
The data also showed that there is significant twisting
of the aryl groups out of the best plane of the azine that
also suggests poor conjugative interactions. Deviation in
the observed torsion angles and the twisting of the phenyl
rings is possibly the result of crystal packing effects. As
well as the extensive investigation of the orientation of
the azine molecules, Glaser also has examined the bond
lengths in an attempt to find evidence for conjugation.
By examining possible resonance structures of the sym-
metric and asymmetric acetophenone azines, he has
identified selected bonds, which should be affected if any
significant conjugation were present. However, the con-
clusion from their extensive X-ray analyses, including the
conformational properties about the N-N and Ar-C
bonds, bond length analysis and theoretical studies
showed that there is no conjugation apparent. In total,
although the CdN-NdC spacer appears to have the
structural elements necessary to make it a good conjuga-
tion bridge, Glaser concludes that the azine unit is in
fact a “conjugation stopper” at least in the solid state.35,37
Our interest lies in whether the lack of conjugation
observed in the solid state is also present in solution,
where the molecules have much more conformational
flexibility. While the redox properties of azines have been
Azines are 2,3-diaza derivatives of butadienes (>Cd
N-NdC<). The two imine bonds that make up the azine
moiety can be considered polar acceptor groups oriented
in opposite directions since they are joined by an N-N
bond. This characteristic of azines, when bonded to two
aryl rings that contain a donor and an acceptor group,
respectively, suggested that they would make ideal
candidates for NLO materials. Over the past decade,
Glaser and co-workers have been interested in the
possible linear and nonlinear optical properties of these
systems.30-39 To this end, they have surveyed a large
number of symmetric and asymmetrical acetophenone
azines (1) with donor and acceptor groups in the para
positions of the aryl rings and have been examining their
molecular structure and crystal packing features. In the
course of their survey, they have recently reported a
number of prototypical aryl azines that exhibit some of
the features that may provide the NLO response.35,37
The structure of aryl azines appears to be ideally suited
for complete conjugation throughout the entire molecule
because of the extended π system. This question of azine
conjugation is being addressed both experimentally and
theoretically.8,9,30-40 One of the conditions for complete
conjugation is the planarity of the molecule both in the
azine spacer as well as the aryl rings. Glaser and
co-workers reported the solid-state structures of a wide
variety of azines, including the acetophenone azines,
paying particular attention to the their extent of conjuga-
tion and to the geometry around the CdN-NdC bonds.
Their work showed that in the solid state there is
significant structural variation in the geometry around
the CdN-NdC bonds as a function of both the steric and
electronic properties of the substituents on the azine
moiety. In general, the CdN-NdC bonds assume gauche
conformations resulting in a lack of planarity, and
suggesting poor conjugative interactions. For some sys-
tems, particularly symmetric aryl azines with strong
electron donating or withdrawing groups, that molecule
studied in the past,19,41-45 to date there has been no
systematic examination of the electrochemical properties
of a series of related azines. In an early electrochemical
study, the reduction potential of the azine system C6H5-
CHdN-NdCH-C6H5 was reported to be identical to that
of the corresponding hydrazone, C6H5-CH2-NH-NdCH-
C6H5, suggesting a lack of conjugation in the former.45
Zuman and co-workers more recently reported the aque-
ous redox properties of a number of triazine derivatives
that contain a cyclic azine moiety.46-48 Of their important
results, which examine the irreversible reduction of the
protonated forms of the azine in protic solvents, one
pertinent to this question is illustrated by the reduction
of the herbicide metamitron.46 The first reduction wave
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Soc., Perkin Trans. 2 1995, 2311-2317.
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Chem. 1982, 139, 395-411.
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7446-7455.
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Chem. Perkin Trans. 2 1995, 1449-1458.
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1998, 457, 177-190.
(47) (a) Riedl, F.; Ludv´ık, J .; Liska, F.; Zuman, P. J . Heterocycl.
Chem. 1996, 33, 2063-2064. (b) Privman, M.; Zuman, P. J . Electroanal.
Chem. 1998, 455, 235-246.
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of Aryl Azines: Evidence for Weak Conjugation. In Organic Electro-
chemistry: Manuel L. Baizer Award Symposium in Honor of J . Simonet
and J . W. P. Utley (4th); Fry, A., Steckhan, E., Matsumura, Y., Eds.;
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1371-1378.
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11, 336-340.

Conjugation in Aryl-Substituted Acetophenone Azines
J . Org. Chem., Vol. 66, No. 3, 2001 833
reversibility depends on the scan rate. Examples of the
reduction of other 1-X/Y appear in Figure 1b-d. The first
two reduction waves represent the successive reduction
of the corresponding azine to its radical anion (1-X/Y•-)
and then its dianion (1-X/Y2-), respectively. In all cases
the formation of 1-X/Y•- is completely chemically revers-
ible.
For this series, standard reduction potentials,
•-
E°1-X/Y/1-X/Y , vary as a function of substituent and are
summarized in Table 1. On the other hand, the revers-
ibility of the formation of the dianion is substituent and
scan rate dependent. The formation of the dianions of
1-3-OCH3/3-OCH3, 1-H/H, 1-H/4-CH3, 1-H/4-OCH3, 1-4-
CH3/4-CH3, 1-3-OCH3/4-OCH3, 1-4-CN/4-N(CH3)2, and
1-4-OCH3/4-OCH3 were not reversible on the CV time
scale up to 50 V s-1 in either solvent. In the cases of 1-4-
CN/4-CN, 1-4-OCH3/4-NO2, 1-3-OCH3/4-NO2, 1-4-CN/3-
OCH3, 1-H/3-OCH3, and 1-H/4-NO2, the dianions are suf-
•-/1-X/Y2-
ficiently stable and provide a value for E°1-X/Y
. The
F igu r e 1. Cyclic voltammograms of selected azines measured
in 0.1 M TEAP/acetonitrile at 0.1 V s-1: (a) 1-3-OCH3/3-OCH3
showing the complete CV; (b) 1-4-OCH3/4-OCH3; (c) 1-4-OCH3/
4-CN; (d) 1-4-CN/4-CN showing only the first two reduction
waves. In the CV for 1-3-OCH3/3-OCH3 (a), the dashed lines
are the CVs when the potential is switched between A, B and
B, C.
dianions of 1-H/4-CN and 1-4-OCH3/4-CN are sufficiently
stable only at faster scan rates in acetonitrile, but at all
scan rates in DMF while the dianion of 1-4-CN/4-N(CH3)2
is sufficiently stable only at faster scan rates in both
•-/1-X/Y2-
solvents. Table 1 also provides E°1-X/Y
or Ep values
depending on the reversibility described above. We
believe that the lack of reversibility of certain dianions
indicates that they are sufficiently basic to abstract a
proton from solvent molecules or, more likely, residual
water present in the “dry” solvent ([H2O] estimated to
be <0.1 mM in acetonitrile). The cathodic wave labeled
C (Figure 1a) is a multielectron wave (∼2e-) caused by
the subsequent reduction of the products from the pro-
tonated dianion (see below) presumably to form amines
as the ultimate products.50 In some cases, the multielec-
tron wave is not observed, since it occurs at too negative
a potential and occurs into the solvent discharge.
of metamitron is coincident with the reduction of 2,3-
dihydrometamitron and is thought to involve the reduc-
tion of the 1,6-imine bond. Since the reduction of the
azine bond in metamitron is coincident with its dihydro
counterpart, this indicates a lack of conjugation and thus
a lack of electronic communication, between the two
azomethine bonds of the azine moiety in metamitron in
aqueous solution.49
To properly exploit the chemistry of azines, an under-
standing of the electronic factors that affect their stability
and reactivity is required. Central to this issue is to
understand the extent of conjugation through the CdN-
NdC. We now report the electrochemical redox properties
of a series of structurally related substituted acetophe-
none azines ((1-X/Y, where X and Y are 4-NO2, 4-CN, H,
3-OCH3, 4-OCH3, 4-CH3, and 4-N(CH3)2) in aprotic sol-
vents. Both symmetrical (X ) Y) and asymmetrical (X *
Y) aryl azines were examined, including many examined
in the solid state by Glaser and co-workers, to determine
if the lack of conjugation found in the solid-state mani-
fests itself in the solution-phase redox properties.
Resu lts a n d Discu ssion
The acetophenone azines 1-X/Y required for this study
were prepared by published procedures, or modifications
of published procedures; full details are in the Experi-
mental Section or the Supporting Information. The
electrochemical behavior of the series of symmetrical and
unsymmetrical aryl-substituted acetophenone azines (1-
X/Y) was studied in acetonitrile (MeCN) and N,N-
dimethylformamide (DMF), using cyclic voltammetry
(CV). All the nonnitro containing azines showed similar
cyclic voltammetric features. Figure 1a shows a repre-
sentative cyclic voltammogram for 1-3-OCH3/3-OCH3.
The characteristic voltammetric features include a single
electron cathodic wave that is reversible at all scan rates
investigated (A), followed at more negative potentials by
a second single electron cathodic wave (B) where the
CVs showing the reversible reduction of 1-X/Y to 1-X/
Y
•- at 100 mV s-1 are shown in Figure 2, which illustrates
the large substituent effect on the single-electron reduc-
tion (values differ by more than 1.1 V); the more electron-
withdrawing the substituent the easier the azine is
to reduce and the more positive the E° value. Each
1-X/Y shows a single electron, reversible wave, based
on the ip/xν value in comparison with ferrocene as a
standard.
•-/1-X/Y2-
The potentials (E°1-X/Y
or Ep) for the second
reduction follow the same trend with substituent donat-
ing and withdrawing ability (see Table 1). The substitu-
ent effect is slightly larger for the secondwave with
(50) Soucaze-Guillous, B.; Lund, H. Acta Chem. Scand. 1997, 51,
354-358.
(49) Zuman, P.; Ludvik, J . Tetrahedron Lett. 2000, 41, 7851-7853.

834 J . Org. Chem., Vol. 66, No. 3, 2001
Sauro and Workentin
Ta ble 1. Electr och em ica l Da ta for th e Red u ction of Azin es in 0.1 M TEAP /Aceton itr ile Solu tion (Da ta in P a r en th eses
Ar e Va lu es in 0.1 M TEAP /DMF Solu tion )
1-X/Y
•-a
a,g
•-/12-
X
Y
E°1/1 (V)
E°/1
a,c (V)
Ep(C) (V)
4-NO2
4-NO2
4-NO2
4-NO2
4-CN
4-CN
4-CN
4-CN
4-CN
H
4-NO2
3-OCH3
H
4-OCH3
4-CN
3-OCH3
H
4-OCH3
4-N(CH3)2
H
4-CH3
4-OCH3
4-N(CH3)2
4-CH3
3-OCH3
H
-0.952b (-0.934)b
-0.977 (-0.970)
-0.983 (-0.973)
-0.999 (-0.978)
-1.426 (-1.367)
-1.593 (-1.544)
-1.602 (-1.550)
-1.648 (-1.595)
-1.669 (-1.632)
-1.932 (-1.891)
-1.965 (-1.929)
-2.004 (-1.969)
-2.054 (-2.026)
-2.005 (-1.972)
-1.898 (-1.867)
-1.912 (-1.876)
-1.983 (-1.957)
-2.088 (-2.059)
-1.482d (-1.502)d
-1.494d (-1.513)d
-1.545d (-1.565)d
-1.602d (-1.597)d
-1.893d (-1.912)d
-1.915e (-1.926)d
-1.973e (-2.002)d
-2.011e (-2.035)e
-2.252f (-2.290)f
-2.331f (-2.343)f
-2.320f (-2.355)f
-2.353f (-2.416)f
-2.371f (-2.402)f
-2.231f (-2.268)f
-2.264f (-2.329)e
-2.313f (-2.370)f
-2.430f (-2.462)f
-2.665 (-2.775)
-2.450 (-2.589)
-2.441 (-2.623)
-2.455 (-2.735)
-2.431 (-2.446)
-2.680 (-2.772)
-2.743 (-)h
H
H
H
-h (-2.781)
-h (-)h
4-CH3
3-OCH3
3-OCH3
3-OCH3
4-OCH3
-2.723 (-)h
-2.692 (-)h
-2.684 (-)h
4-OCH3
4-OCH3
-h (-)h
-2.753 (-2.774)
a
b
d
Reported in V versus SCE. Not E° for 1-X/Y/1-X/Y•-, see text. c E° unless noted. Reversible at all scan rates. e Reversible only at
faster scan rates. f Reported as peak potentials at 0.1 V s-1 scan rate. Irreversible up to 50 V s-1 scan rate. Peak potential for the
g
reduction of the protonated dianion at 0.1 V s-1 scan rate; see text. The Ep(C) for these compounds was obscured by the solvent cutoff.
h
F igu r e 3. Cyclic voltammograms of the nitro-containing
azines measured in 0.1 M TEAP/acetonitrile at 0.1 V s-1: (a)
1-4-NO2/4-NO2; (b) 1-3-OCH3/4-NO2; (c) 1-H/4-NO2; (d) 1-4-
OCH3/4-NO2. Also shown is a second reversible reduction wave
that is typical for all of the nitro containing azines except for
1-4-NO2/4-NO2.
The nitro-containing azines deviate from the above-
mentioned behavior. The nitro-substituted azines, with
the exception of 1-4-NO2/4-NO2, exhibit single electron
waves that have standard potentials within 40 mV of
each other (see Figure 3). 1-4-NO2/4-NO2 exhibits a
standard potential at a similar potential but is a two-
electron reversible wave. The reduction wave for the 1-4-
NO2/4-NO2 has the same peak parameters (∆Ep ) 59 mV
at 100 mV s-1) as a reversible one-electron-transfer
reduction with the exception that the peak current is
twice the height. The characteristics exhibited by 1-4-
NO2/4-NO2 are indicative of a reduction system contain-
ing two localized, nonconjugated redox centers.51 This
suggests that reduction occurs at two independent cen-
ters roughly simultaneously and the observed reduction
F igu r e 2. Cyclic voltammograms of the first reversible
reduction waves for selected azines in 0.1 M TEAP/acetonitrile
at 0.1 V s-1 scan rate: (a) 1-4-CN/4-CN; (b) 1-3-OCH3/3-OCH3;
(c) 1-H/H; (d) 1-4-CH3/4-CH3; (e) 1-4-OCH3/4-OCH3; (f) 1-3-
OCH3/4-CN; (g) 1-H/4-CN; (h) 1-4-OCH3/4-CN; (i) 1-4-CN/4-
N(CH3)2; (j) 1-H/3-OCH3; (k) 1-H/4-CH3; (l) 1-3-OCH3/4-OCH3;
(m) 1-H/4-OCH3; (n) 1-H/4-N(CH3)2.
potentials differing by over 0.8 V through the series from
1-4-CN/4-CN to 1-4-OCH3/4-OCH3. Typical CVs are il-
lustrated in Figure 1.
(51) Bott, A. W. Current Separations 1997, 16, 61-66.

Conjugation in Aryl-Substituted Acetophenone Azines
J . Org. Chem., Vol. 66, No. 3, 2001 835
is reduced, whereas in the latter reduction occurs on the
C6H5-(CH3)CdN-X unit. Taking this into account (e.g.,
4-CN substituents are not included in correlation for X
) H, or 3-OCH3), the F values are smaller (F3-OCH /Y 0.33
3
V and FH/Y ) 0.29 V, respectively), indicating again only
a modest substituent effect on the reduction.
•-/1-X/Y2-
Similar plots for E°1-X/Y
(reduction of the radical
anion to the dianion) have higher F values for the electron
withdrawing substituents, suggesting a greater substitu-
ent effect (F24-NO /Y ) 0.17; F24-CN/Y ) 0.39) but are similar
2
H/Y
for the electron donating series (F2
) 0.30 V and
3-OCH3
F2
/Y ) 0.29 V). This aspect will be addressed below.
The preferential site for reduction has been confirmed
by constant potential electrolysis product studies from
the reduction of 1-X/Y in the presence of a proton donor,
namely the nonnucleophilic acid 2,2,2-trifluoroethanol
(TFE). Upon the addition of TFE the cathodic peak
current of the first reduction wave (A) increases with
increasing acid concentration with a concomitant de-
crease in the current of the second wave (B). The
reversibility of the first wave also decreased. These
voltammetric features are indicative of an ECE mecha-
nism where 1-X/Y•- is protonated by TFE to yield what
we assume to be the benzylic type radical, which is
quickly reduced since it has a reduction potential much
more positive than the radical anion (Scheme 1). The
resulting anion is then protonated to yield the corre-
sponding hydrazone, 2. This series of events causes the
two original single electron reduction waves to merge into
a single irreversible wave, twice its original current
height, at high enough acid concentration. Accompanying
the increase in peak current with increasing acid con-
centration, there is also a shift in peak potential to more
positive values. This shift is related to the rate constant
(kH+) for protonation of the radical anion. Constant
current and constant potential coulometric experiments
performed in the presence of a large excess of acid,
indicate that the number of electrons involved in the
reduction of the azines is always 2 F mol-1. Hydrogena-
tion of 1-X/Y in the presence of Pd catalysts adds one
equivalent of H2 to yield the corresponding hydrazone.
The electrolyses produced only a single product for each
azine, verified by HPLC analysis and co-injection of
authentic samples prepared by the hydrogenation of the
1-X/Y. The products from the bulk electrolysis or hydro-
genation were not stable to hydrolysis and were used
immediately for HPLC analysis with the minimum
amount of exposure to the atmosphere. The hydrogena-
tion products were sufficiently stable to allow for NMR
and UV-vis spectral characterization. Spectra are con-
sistent with products where a single azomethine unit is
reduced. In the case of the asymmetrical azines, the
reduction (either by H2/Pd or electrochemical) occurs
across the imine bond closest to the more electron
withdrawing of the two aryl substituents (verified by
NMR and HPLC co-injection). The hydrogenated product
of 1-4-CN/4-CN was stable enough to hydrolysis to
perform an electrochemical experiment. The reduction
observed for this compound matched those observed for
the parent azine, in the presence of excess acid, except
for the disappearance of the initial two electron irrevers-
ible wave.
•-
F igu r e 4. Hammett-type plot of σ versus E°1-XY/1-XY mea-
sured in 0.1 M TEAP/acetonitrile solution at 0.1 V s-1. The 9,
[, 1, and b are the data for the 1-4-NO2/Y, 1-4-CN/Y, 1-H/Y,
and 1-3-OCH3/Y azines, respectively. Hammett σ values: 4-CN
) 0.7, 4-NO2 ) 0.81, H ) 0, 4-OCH3 ) -0.28, 4-CH3 ) -0.14,
3-OCH3 ) 0.1, 4-N(CH3)2 ) -0.32.
wave is the product of two independent single-electron
transfers which occur at the same reduction potential
(∆E° ca. 35 mV). The nitro-containing azines also show
a second one-electron reduction wave except for 1-4-NO2/
4-NO2 that does not show a second reduction wave under
the conditions of our experiment (vide supra). A summary
of the electrochemical data appears in Table 1.
The reduction potentials of the aryl azines can be
correlated with Hammett σ parameters to examine
•-
variations in E°1-X/Y/1-X/Y as a function of substituent;
this is shown in Figure 4.52 The plot shows distinct
correlations for series of 1-X/Y where one of the substit-
uents is held constant and the other is varied: 1-4-NO2/
Y, 1-4-CN/Y, 1-H/Y, and 1-3-OCH3/Y. For 1-4-NO2/Y, the
F4-NO /Y value equals 0.04 V, essentially indicating no
2
•-
substituent effect. In addition, the E°1-X/Y/1-X/Y values
are all approximately equal to -1 V vs SCE, roughly the
same potential as nitrobenzene in the same solvents
(-1.1 V vs SCE). These observations suggest that the
reduction is localized on the NO2 group and not at one of
the azine CdN imine double bonds. In other words, the
reduction of the nitro-substituted azines is behaving like
substituted aryl nitro groups, as opposed to nitro-
substituted aryl azines.
The second correlation is for the series 1-4-CN/Y. Here,
F4-CN/Y ) 0.23 V, suggesting a modest variation as a
function of substituent. The last correlations shown are
for the remaining azines. For these systems the F values
are larger (FH/Y ) 0.41 V and F3-OCH /Y ) 0.39 V) and would
3
suggest a greater substituent effect. In fact, we believe
that these values are unnaturally high since the position
of reduction changes as the Y substituent is changed.
Product studies (see below) indicate that reduction takes
place preferentially on the imine unit (Ar(CH3)CdNX)
closest to the most electron-withdrawing aryl substituent
and this acts to attenuate the F value. For example, in
the 1-H/Y series the position of reduction on the molecule
changes as we proceed from 1-4-CN/H to 1-4-N(CH3)2/H;
in the former it is the NC-C6H4-(CH3)CdN-X unit that
The data for the reduction of neutral 1-4-NO2/Y, in
particular 1-4-NO2/4-NO2 provides compelling evidence
for the lack of conjugation through the azine unit. What
about for other substituents? We believe the small F
(52) Zuman, P. Substituent Effects in Organic Polarography; Ple-
num: New York, 1967.

836 J . Org. Chem., Vol. 66, No. 3, 2001
Sauro and Workentin
F igu r e 5. LUMO and HOMO of selected 1-X/Y and 1-X/Y•-, respectively, determined by semiempirical PM3 calculations.
Sch em e 1
values from the data presented in Figure 4 provide
qualitative evidence for the lack of strong electronic
communication between the two aryl centers through the
CdN-NdC bonds. A complete lack of any substituent
effect would, of course, lead to reduction potentials for a
particular series (e.g., 1-4-CN/Y) being practically identi-
cal and a F value near zero, which is true only for 1-4-
NO2/Y. The magnitudes of the substituent effects (and
the resulting F values) for the other series are nonzero
indicating some effect of the substituent. The magnitude
is reasonable for transmission of the electronic effects
either by inductive effects alone or with some conjugative
effects; at present there is no clear way to determine the
relative contribution from each. Of course, the extent of
the latter may vary as a function of substituent if the
geometry around the Ar-RCdN-NdCR-Ar or the nature
of the LUMO varies.
that the geometry optimal for conjugation throughout the
entire molecule is not preferred. Furthermore, the cal-
culations also provide qualitative indication that the
LUMO of the azines containing electron-withdrawing
groups resides on the aryl moiety with no significant
contribution on the azine >CdN-NdC< unit. In the case
of the 1-4-CN/Y and 1-4-NO2/Y, the LUMO is mainly
localized on the NC-aryl or O2N-aryl fragment; this is
illustrated for 1-H/4-NO2 and 1-H/4-CN in Figure 5. This
provides further support for the conclusions from the
electrochemical studies. For 1-X/Y containing more elec-
tron-donating substituents the LUMO is more delocal-
ized, but still has substantial contributions on the aryl
moiety (for example see 1-H/4-OCH3 in Figure 5). In these
cases, there is more likelihood for some extended conju-
gation, as is also suggested in the more substantial
substituent effect on the redox properties.
The lack of strong conjugative interaction is consistent
with the conclusions reached from X-ray crystallographic
structural data for this series of azines, summarized in
the Supporting Information, from our own data and that
of Glaser’s. The newer data is analogous to Glaser’s and
show that in the solid-state structure the majority of the
1-X/Y have gauche geometries around the >CdN-Nd
C< moiety and that the bond lengths are not consistent
with any extended conjugation (for example, no signifi-
cant -NdN- character). Semiempirical calculations
were also performed to qualitatively examine the struc-
ture of 1-X/Y.53 Calculated structures show a definite
torsion angle about the N-N bond that is also apparent
in the X-ray crystal structures (even when minimizations
were started with the all planar structure), indicating
In contrast to 1-X/Y, the semiempirical calculations of
1-X/Y•- indicate that the molecules adopt a more planar
configuration and that the HOMO is more highly delo-
calized with a significant contribution on the CdN-NdC
fragment. Qualitatively, this provides support that 1-X/
Y•- assume a geometry that is more capable of conjuga-
tion than the parent azines, which would provide a larger
substituent effect on the second reduction, as is observed.
For the reduction of the radical anions the effects of the
substituents appear to be additive, with the Ep values
correlating with the sum of the substituent parameters.
This is expected for complete conjugation.52 To further
examine the characteristics of the 1-X/Y•-, the UV-vis
spectra of each of the radical anions were measured. Each
radical anion is highly colored. Representative UV-vis
spectra of 1-X/Y•- are shown in Figure 6 and the λmax are
summarized in Table 2. These results coupled with the
(53) Hyperchem; Hypercube, Inc.: Gainsville, FL, 1996.

Conjugation in Aryl-Substituted Acetophenone Azines
J . Org. Chem., Vol. 66, No. 3, 2001 837
in >CdN-NdC< units observed in the solid state mani-
fests itself in solution. This undoubtedly plays a role in
the chemistry and application of this functional group.
In contrast, the corresponding azine radical anions
exhibit a larger substituent effect on their reduction.
This, along with the qualititative semiempirical calcula-
tions and absorption spectroscopy, may reflect a stronger
conjugative interaction. This aspect is currently being
explored.
Exp er im en ta l Section
Ma ter ia ls. All chemicals used were purchased from Aldrich
and used as received unless otherwise stated. Acetonitrile was
distilled over calcium hydride and then immediately cycled
through activated alumina just prior to use. N,N-Dimethyl-
formamide (DMF) was stored over NaHCO3 and then distilled
under reduced pressure over calcium hydride just prior to use.
Argon was flushed over the acetonitrile and DMF during the
transfers into the electrochemical and absorption spectroscopy
cells. Tetraethylammonium perchlorate (Kodak) was recrystal-
lized three times and dried under vacuum at 60 °C prior to
use and stored in a vacuum desiccator. Ferrocene was sub-
limed once before use. NMR spectra were recorded on a Varian
Gemini 300 MHz or Varian Mercury 400 MHz instrument. All
chemical shifts are reported in parts per million downfield from
tetramethylsilane. Melting points were recorded using a
Gallenkamp melting point apparatus and are uncorrected. IR
data was recorded on a Perkin-Elmer System 2000 FT-IR.
Mass spectra analysis was performed with a Finnigan MAT
8200 mass-data system. Gas chromatography analysis was
performed on a Hewlett-Packard 5890 Series II gas chromato-
graph equipped with a 10 m HP5 column and an FID detector.
UV-vis spectra were recorded on a Varian Cary 100 spec-
trometer.
F igu r e 6. UV-vis spectra of selected 1-X/Y•- in 0.1 M TEAP/
DMF solution: (‚ - ‚) are for 1-4-OCH3/4-OCH3, 1-H/4-N(CH3)2,
and 1-4-CN/4-OCH3, (solid line) are for 1-3-OCH3/3-OCH3, 1-H/
H, and 1-4-CN/4-N(CH3)2, (dash) are for 1-4-CN/3-OCH3, 1-H/
4-CN, and 1-4-CN/4-CN in (i), (ii), and (iii), respectively.
Ta ble 2. UV-Vis Sp ectr a l Da ta for 1-X/Y•- in 0.1 M
TEAP /DMF Solu tion
Electr och em istr y: Cyclic Volta m m etr y. Experiments
were performed using either a Princeton Applied Research
(PAR) 283 or 263 potentiostat interfaced to a personal com-
puter. PAR M270 software was used to conduct the experi-
ments as well as analyze the derived data. Measurements were
conducted in an all-glass cell kept at 25 °C. All glassware was
dried in an oven and assembled hot while flushing with argon.
The working electrode was glassy carbon (Tokai), and the
counter electrode was a platinum flag. The reference electrode
was a silver wire housed in a glass tube sealed with a porous
ceramic tip and filled with a 0.1 M solution of tetraethylam-
monium perchlorate (TEAP). The same reference electrode was
used in all experiments. Concentrations of 2 mM of azine in
0.1 M TEAP in acetonitrile or DMF were used. Ferrocene was
used in all experiments as an internal reference, and potentials
are referenced in turn to the saturated calomel electrode (E°
) 0.449 V and 0.470 V vs SCE in MeCN and DMF, respec-
tively). Positive feedback IR compensation was used in all
experiments to minimize the effects of uncompensated solution
resistance. Con sta n t Cu r r en t Cou lom etr y. 2 mM of azine
was dissolved in 25 mL of 0.1 M TEAP/acetonitrile solution
at 25 °C. A flow of argon was bubbled into the solution
continuously to prevent oxygen from entering the cell. The
solution was vigorously stirred throughout the experiment.
The working electrode was a mercury pool. The counter
electrode was a platinum coil separated from the bulk solution
by a 2 mm thick layer of activated alumina and a sintered
glass disk. A constant current (25 mA) was applied to the cell
using a Hewlett-Packard 6212B power supply, monitored by
a Fluke 8000A digital multimeter, for various amounts of time.
The experiment was monitored by CV employing a glassy
carbon working electrode. Con sta n t P oten tia l Cou lom etr y.
Typically, 30 mg of azine was dissolved in an appropriate
amount of 0.1 M TEAP/acetonitrile solution at 25 °C. Argon
was continuously bubbled to eliminate oxygen and the solution
was vigorously stirred. The working electrode was a mercury
pool. The counter electrode was a platinum coil separated from
the bulk solution by a 2 mm thick layer of activated alumina
1-X/Y
X
Y
λ
max/nm
λ/nm
468
4-CN
4-CN
4-CN
4-CN
4-CN
H
4-CN
3-OCH3
H
564
547
548
538
547
515
514
510
522
514
522
518
515
504
504
4-OCH3
4-N(CH3)2
H
4-CH3
4-OCH3
4-N(CH3)2
4-CH3
3-OCH3
H
497, 467
509, 475
478
482, 447
481, 447
487
479, 447
482, 447
478, 447
482, 447
471
H
H
H
4-CH3
3-OCH3
3-OCH3
3-OCH3
4-OCH3
4-OCH3
4-OCH3
•-/1-X/Y2-
larger substituent effect on E°1-X/Y
there is more extensive conjugation in the radical anion
than in the neutral species.
suggest that
Based on Glaser and co-worker’s studies of the solid-
state structures of 1-X/Y they have coined the term
“conjugation-stopper” to describe the azine unit; our
electrochemical data supports this analogy. Of course, the
dihedral angle around the CdN-NdC and thus the extent
of conjugation may vary somewhat as a function of
substituent; this is currently difficult to interpret since
the geometry in solution is much more fluxional, espe-
cially around the N-N bond. Limited delocalization may
also occur by interactions through the nitrogen lone pairs
and not directly through the π system. Since the elec-
trochemical studies do show a small substituent effect
on the reduction potentials, the azine unit may not be a
complete “conjugation stopper” in solution, but the extent
of conjugation is limited. Thus, the limited conjugation

838 J . Org. Chem., Vol. 66, No. 3, 2001
Sauro and Workentin
and a sintered glass disk. The appropriate constant potential
was applied using either a Princeton Applied Research (PAR)
283 or 263 potentiostat.
6.96 (d, 1H), 7.32 (t, 1H), 7.42 (m, 4H), 7.50 (m, 1H), 7.89 (m,
2H); mp ) 74-76 °C.
4-Meth yla cetop h en on e a zin e (1-H/4-CH3): 1H NMR (300
MHz) δH (CDCl3) 2.29 (s, 3H) 2.30 (s, 3H), 2.38 (s, 3H), 7.21
(d, 2H), 7.40 (m, 3H), 7.79 (d, 2H), 7.88 (m, 2H); mp ) 90-91
°C.
Bu lk Electr olysis a n d HP LC An a lysis. Bulk electrolysis
was performed as per constant potential coulometry using
approximately 10-12 mg of 1-X/Y and a large excess of 2,2,2-
trifluoroethanol (TFE) in 25 mL of 0.1 M TEAP/acetonitrile
solution at 25 °C. Once electrolysis was complete, 100 µL was
removed from the electrochemical cell and diluted to 2 mL with
acetonitrile. The resultant solution was analyzed using a
Waters Millipore HPLC on a C18 reversed-phase column with
20:80 water, acetonitrile mixture as eluent. The results were
compared to hydrogenated samples of selected 1-X/Y.
Hyd r ogen a tion a n d HP LC An a lysis.54 A 10-12 mg
sample of azine was dissolved in a solution of DMF containing
a catalytic amount of palladium black. One equivalent of
hydrogen gas was added in a sealed hydrogenation apparatus
at atmospheric pressure. A 100 µL portion of the solution was
filtered and diluted to 2 mL with acetonitrile. The resultant
solution was analyzed using a Waters Millipore HPLC on a
C18 reverse phase column with 20:80 water, acetonitrile
mixture as eluent. The results were compared to the bulk
electrolysis samples of selected 1-X/Y.
Syn th esis of Acetop h en on e Azin es. The symmetric
azines 1-4-CN/4-CN, 1-3-OCH3/3-OCH3, 1-4-OCH3/4-OCH3,
1-4-CH3/4-CH3, 1-4-NO2/4-NO2, and 1-H/H were synthesized
according to a published procedure and the spectral charac-
terization and other physical data matched that previously
reported.55 The asymmetrical azines were synthesized by
adding the appropriate substituted acetophenone to the cor-
responding substituted acetophenone(diethoxyphosphinyl)-
hydrazone in benzene/NaH. Both the preparation of the
substituted-acetophenone(diethoxyphosphinyl)hydrazone and
1-X/Y were done according to the procedure of Chen et al. with
some modification:32
3,4′-Dim eth oxyacetoph en on e azin e (1-3-OCH3/4-OCH3):
1H NMR (400 MHz) δH (CDCl3) 2.28 (s, 3H), 2.29 (s, 3H), 3.84
(s, 3H), 3.85 (s, 3H), 6.94 (m, 3H), 7.31 (t, 1H), 7.43 (m, 1H),
7.50 (m, 1H), 7.86 (d, 2H); mp ) 79-81 °C.
3-Meth oxy-4′-n itr oa cetop h en on e a zin e (1-3-OCH3/4-
NO2): 1H NMR (300 MHz) δH (CDCl3) 2.29 (s, 3H), 2.33 (s,
3H), 3.86 (s, 3H), 6.98 (m, 1H), 7.33 (t, 1H), 7.47 (m, 2H), 8.05
(d, 2H), 8.26 (d, 2H); mp ) 88-90 °C.
3-Meth oxy-4′-cya n oa cetop h en on e a zin e (1-3-OCH3/4-
CN): 1H NMR (300 MHz) δH (CDCl3) 2.27 (s, 3H), 2.30 (s, 3H),
3.85 (s, 3H), 6.99 (m, 1H), 7.33 (t, 1H), 7.46 (m, 2H), 7.69 (d,
2H), 7.99 (d, 2H); mp ) 85-87 °C.
3,3′-Dim eth oxyacetoph en on e azin e (1-3-OCH3/3-OCH3):
1H NMR (300 MHz) δH (CDCl3) 2.27 s, 6H), 3.85 (s, 6H), 6.95
(m, 2H), 7.32 (t, 2H), 7.45 (m, 6H); mp ) 97-99 °C.
4-Cya n o-4′-d im eth yla m in oa cetop h en on e a zin e (1-4-
1
CN/4-N(CH3)2): H NMR (300 MHz) δH (CDCl3) 2.29 (s, 3H),
2.35 (s, 3H), 3.01 (s, 6H), 6.71 (d, 2H), 7.67 (d, 2H), 7.84 (d,
2H), 7.99 (d, 2H); mp ) 202-204 °C.
X-r a y Cr ysta llogr a p h y. Single crystals of 1-H/4-CN, 1-H/
4-OCH3, 1-H/4-NO2, 1-3-OCH3/3-OCH3, 1-3-OCH3/4-CN, and
1-3-OCH3/4-NO2 were grown by slow evaporation of a concen-
trated solution of 1-X/Y in an appropriate solvent. Crystals
were mounted on a glass fiber. Data were collected on a Nonius
Kappa-CCD diffractometer using COLLECT (Nonius, 1998)
software. The unit cell parameters were calculated and refined
from the full data sets. Crystal cell refinement and data
reduction were carried out using the Nonius DENZO package.
The data were scaled using SCALEPACK (Nonius, 1998), and
no other adsorption corrections were applied. The SHELXTL
5.1 (Sheldrick, G. M., Madison, WI) program package was used
to solve the structures by direct methods, followed by succes-
sive difference Fouriers. All non-hydrogen atoms were refined
with anisotropic thermal parameters. The hydrogen atoms
were calculated geometrically and were riding on their respec-
tive carbon atoms. X-ray data are given in the Supporting
Information.
The following 1-X/Y are, to the best of our knowledge, either
new compounds or compounds where the complete spectral
data has not yet been reported. X-ray crystallographic data is
also available for new compounds and is summarized in the
Supporting Information.
1
4-Nitr oa cetop h en on e a zin e (1-H/4-NO2): H NMR (300
MHz) δH (CDCl3) 2.31 (s, 3H), 2.34 (s, 3H), 7.42 (m, 3H), 7.90
(m, 2H), 8.05 (d, 2H), 8.25 (d, 2H); mp ) 113-114 °C.
1
4-Cya n oa cetop h en on e a zin e (1-H/4-CN): H NMR (300
MHz) δH(CDCl3) 2.30 (s, 3H), 2.31 (s, 3H), 7.42 (m, 3H), 7.70
(d, 2H), 7.90 (m, 2H), 8.00 (d, 2H); mp ) 132-134 °C.
4-Meth oxya cetop h en on e a zin e (1-H/4-OCH3): 1H NMR
(300 MHz) δH (CDCl3) 2.32 (s, 3H), 2.34 (s, 3H), 3.88 (s, 3H),
6.94 (d, 2H), 7.40 (m, 3H), 7.90 (m, 4H); mp ) 134-136 °C.
4-Dim eth yla m in oa cetop h en on e a zin e (1-H/4-N(CH 3)2):
1H NMR (400 MHz) δH (CDCl3) 2.29 (s, 3H), 2.33 (s, 3H), 3.00
(s, 6H), 6.71 (d, 2H), 7.39 (m, 3H), 7.83 (d, 2H), 7.89 (m, 2H);
mp ) 146-147 °C.
Ack n ow led gm en t. The financial support of the
Natural Science and Engineering Research Council of
Canada, Canadian Foundation for Innovation, ORCDF,
and the University of Western Ontario (ADF) is ap-
preciated. V.A.S. thanks the OGS program for a schol-
arship. Discussions with Professor P. Zuman about the
relationship between our results and his examples of
cyclic azines in aprotic solvents were beneficial.
3-Meth oxya cetop h en on e a zin e (1-H/3-OCH3): 1H NMR
(400 MHz) δH (CDCl3) 2.28 (s, 3H), 2.29 (s, 3H), 3.89 (s, 3H),
Su p p or tin g In for m a tion Ava ila ble: A complete Experi-
mental Section, including spectral characterization of all new
compounds and tables of crystallographic data. This material
is available free of charge via the Internet at http://pubs.acs.org.
(54) Yoshino, K.; Ohkatsu, Y.; Tsuruta, T. Bull. Chem. Soc. J pn.
1979, 52, 1738-1742.
(55) Cohen, S. G.; Groszos, S. J .; Sparrow, D. B. J . Am. Chem. Soc.
1950, 72, 3947-3951.
J O0056287