Electrochemical studies of verdazyl radicals

Article abstract of DOI:10.1021/ol702163a

(Chemical Equation Presented) The redox properties of verdazyl radicals are presented using cyclic voltammetry techniques. These radicals can be reversibly reduced as well as oxidized. Electron-donating and -withdrawing substituents have significant effects on the oxidation and reduction potentials as well as the cell potential (E_cell = |E_ox° - E _red°|) for these radicals; a correlation between the electron spin distribution and redox properties is developed.

Full text of DOI:10.1021/ol702163a

ORGANIC
LETTERS
2007
Vol. 9, No. 23
4837-4840
Electrochemical Studies of Verdazyl
Radicals
Joe B. Gilroy, Stephen D. J. McKinnon, Bryan D. Koivisto, and Robin G. Hicks*
Department of Chemistry, UniVersity of Victoria, P.O. Box 3065, STN CSC, Victoria,
British Columbia V8W 3V6, Canada
rhicks@uVic.ca
Received September 3, 2007
ABSTRACT
The redox properties of verdazyl radicals are presented using cyclic voltammetry techniques. These radicals can be reversibly reduced as well
as oxidized. Electron-donating and -withdrawing substituents have significant effects on the oxidation and reduction potentials as well as the
cell potential (Ecell ) |
Eox° −
Ered°|) for these radicals; a correlation between the electron spin distribution and redox properties is developed.
Stable radicals1 have long been of fundamental interest and
find a broad array of uses including as ligands for transition
metals,2 as spin labels,3 as components of magnetic or
conducting materials,4 and as mediators of living radical
polymerization processes5 Much of the interest in, and utility
of, stable radicals stems directly from the unpaired electron
inherent to these species. However, stable radicals are also
often redox-active, and electrochemical studies of many kinds
of stable radicals have been reported.6 The redox properties
of stable radicals are central to, e.g., their intramolecular
electron-transfer chemistry,7 their efficacy as building blocks
for single-component molecular conductors,8 and their poten-
tial use as active components of organic-based batteries.9
Verdazyls (1, 2) are the only family of neutral radicals
whose stability rivals that of the well-known nitroxides
(including nitronyl nitroxides); both verdazyls and nitroxides
are sterically unprotected radicals, air- and water-stable, and
resistant toward dimerization.10 The main foci of research
(6) (a) Boere´, R. T.; Moock, K. H.; Parvez, M. Z. Anorg. Allg. Chem.
1994, 620, 1589. (c) Ziessel, R.; Ulrich, G.; Lawson, R. C.; Echegoyen, L.
J. Mater. Chem. 1999, 9, 1435. (c) Boere´, R. T.; Roemmele, T. L. Coord.
Chem. ReV. 2000, 210, 369. (d) Ionita, P.; Whitwood, A. C.; Gilbert, B. C.
J. Chem. Soc., Perkin Trans. 2 2001, 1453. (e) Coronado, E.; Gimenez-
Saiz, C.; Nicolas, M.; Romero, F. M.; Rusanov, E.; Stoeckli-Evans, H. New
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Malinski, T. J. Phys. Chem. 1988, 92, 3745. (g) Malinski, T.; Fish, J. R. J.
Electrochem. Soc. 1988, 135, C149. (h) Krzyczmonik, P.; Scholl, H. J.
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L.; Schollhorn, B. New J. Chem. 2006, 30, 430.
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Rovira, C.; Gutlich, P.; Veciana, J. J. Am. Chem. Soc. 2003, 125, 1462. (b)
Sporer, C.; Ratera, I.; Ruiz-Molina, D.; Zhao, Y. X.; Vidal-Gancedo, J.;
Wurst, K.; Jaitner, P.; Clays, K.; Persoons, A.; Rovira, C.; Veciana, J.
Angew. Chem., Int. Ed. 2004, 43, 5266. (c) Heckmann, A.; Lambert, C. J.
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(1) (a) Hicks, R. G. Org. Biomol. Chem. 2007, 5, 1321. (b) Power, P. P.
Chem. ReV. 2003, 103, 789-809.
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39, 331. (b) Chaudhuri, P.; Wieghardt, K. Prog. Inorg. Chem. 2001, 50,
151. (c) Preuss, K. E. J. Chem. Soc., Dalton Trans. 2007, 2357.
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10.1021/ol702163a CCC: $37.00
© 2007 American Chemical Society
Published on Web 10/10/2007

on verdazyls have been their magnetic properties11 and
coordination chemistry.12 The redox properties of specific
verdazyl derivatives have been reported,13 but there have been
no systematic studies. Herein we present electrochemical
studies on a range of verdazyl radicals with a view to
correlating redox properties with molecular structure.
Table 1. List of Substituents for Verdazyls 1 and Precursors
3-5
derivative
R1
R2
R3
derivative R1
R2
R3
a
b
c
d
e
fa
g
H
H
H
h
Me Me
H
OMe OMe Me
Me
H
Cl
CN
Me
i
Me Me Cl
Me Me CN
Me Me NO2
Me Me CF3
CF3 CF3 Me
NO2 NO2 Me
Me
H
Cl
CN
Me
Me
Me
Me
Me
ja
ka
la
ma
nb
OMe
a Formazan 4 prepared, but verdazyl 1 synthesis was unsuccessful.
b Formazan synthesis unsuccessful.
Methylene-bridged triaryl verdazyls 1 were prepared using
established procedures.10a Formazans 4 (prepared by reactions
of hydrazones 3 with diazonium salts14) react with form-
aldehyde in the presence of base to produce tetrahydro-
tetrazines 5 which are oxidized in air to give the verdazyls
1 (Scheme 1). This is a broadly applicable procedure for
A smaller series of 6-oxoverdazyls 2 was also prepared.
1,3,5-Triphenyl-6-oxoverdazyl 2a was prepared according
to Milcent’s procedure15 with some modifications (see
Supporting Information). 1,5-Dimethyl-3-phenyl-6-oxo-
verdazyl 2b was the lone N,N′-dimethylverdazyl employed
in this study; the stability of N-N-dimethyl-substituted
verdazyls varies widely depending on R3 in unpredictable
ways. The remaining oxoverdazyls 2c-2f have isopropyl
groups on the nitrogens, which are much more robust.16
Derivatives 2c,17 2d, and 2e16 were prepared as reported,
while 2f is a new verdazyl radical derivative.
Scheme 1
The electrochemical properties of radicals 1a-e, g-i, and
2a-2f were studied using cyclic voltammetry; data are
summarized in Table 2. With few exceptions, verdazyls
display fully reversible oxidation and reduction processes.
Reversibility of the redox processes were confirmed as
(12) (a) Brook, D. J. R.; Lynch, V.; Conklin, B.; Fox, M. A. J. Am.
Chem. Soc. 1997, 119, 5155. (b) Brook, D. J. R.; Fornell, S.; Stevens, J.
E.; Noll, B.; Koch, T. H.; Eisfeld, W. Inorg. Chem. 2000, 39, 562. (c) Hicks,
R. G.; Lemaire, M. T.; Thompson, L. K.; Barclay, T. M. J. Am. Chem.
Soc. 2000, 122, 8077. (d) Wu, J. Z.; Bouwman, E.; Reedijk, J.; Mills, A.
M.; Spek, A. L. Inorg. Chim. Acta 2003, 351, 326. (e) Gilroy, J. B.; Koivisto,
B. D.; McDonald, R.; Ferguson, M. J.; Hicks, R. G. J. Mater. Chem. 2006,
16, 2618. (f) Pointillart, F.; Train, C.; Herson, P.; Marrot, J.; Verdaguer,
M. New J. Chem. 2007, 31, 1001.
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Kawamura, H.; Anzai, H. Z. Naturforsch. B 1998, 53, 495. (d) Barr, C. L.;
Chase, P. A.; Hicks, R. G.; Lemaire, M. T.; Stevens, C. L. J. Org. Chem.
1999, 64, 8893. (e) Fico, R. M.; Hay, M. F.; Reese, S.; Hammond, S.;
Lambert, E.; Fox, M. A. J. Org. Chem. 1999, 64, 9386. (f) Keeney, L.;
Hynes, M. J. Dalton Trans. 2005, 133. (g) Chahma, M.; Wang, X. S.; van
der Est, A.; Pilkington, M. J Org. Chem. 2006, 71, 2750.
the synthesis of verdazyls, although with some limitations
when strong electron-withdrawing groups are present. In one
case (R1 ) R2 ) NO2, R3 ) Me) the formazan 4n could not
be made, while other formazans with electron-withdrawing
groups (4f, j, k, l, m) could be successfully prepared but
could not be converted to the corresponding verdazyl radicals
(Table 1).
(9) (a) Nakahara, K.; Iwasa, S.; Satoh, M.; Morioka, Y.; Iriyama, J.;
Suguro, M.; Hasegawa, E. Chem. Phys. Lett. 2002, 359, 351. (b) Nishide,
H.; Iwasa, S.; Pu, Y. J.; Suga, T.; Nakahara, K.; Satoh, M. Electrochim.
Acta 2004, 50, 827. (c) Suga, T.; Pu, Y. J.; Oyaizu, K.; Nishide, H. Bull.
Chem. Soc. Jpn. 2004, 77, 2203. (d) Buhrmester, C.; Moshurchak, L. M.;
Wang, R. L.; Dahn, J. R. J. Electrochem. Soc. 2006, 153, A1800. (e)
Katsumata, T.; Satoh, M.; Wada, J.; Shiotsuki, M.; Sanda, F.; Masuda, T.
Macromol. Rapid Commun. 2006, 27, 1206. (f) Nakahara, K.; Iwasa, S.;
Iriyama, J.; Morioka, Y.; Suguro, M.; Satoh, M.; Cairns, E. J. Electrochim.
Acta 2006, 52, 921. (g) Li, H. Q.; Zou, Y.; Xia, Y. Y. Electrochim. Acta
2007, 52, 2153.
(14) Nineham, A. W. Chem. ReV. 1955, 55, 355.
(15) Milcent, R.; Barbier, G.; Capelle, S.; Catteau, J. P. J. Heterocycl.
Chem. 1994, 31, 319.
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Org. Biomol. Chem. 2005, 3, 4258.
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(b) Neugebauer, F. A.; Fischer, H.; Siegel, R. Chem. Ber. 1988, 121, 815.
(11) Koivisto, B. D.; Hicks, R. G. Coord. Chem. ReV. 2005, 249, 2612.
(17) Gilroy, J. B.; McKinnon, S. D. J.; Kennepohl, P.; Zsombor, M. S.;
Ferguson, M. J.; Thompson, L. K.; Hicks, R. G. J. Org. Chem. 2007, 72,
8062.
4838
Org. Lett., Vol. 9, No. 23, 2007

discussed above for R1/R2 but smaller in magnitude. To some
extent this may reflect the fact that the effects of R1 and R2
are being considered collectiVely (i.e., there are two substit-
uents which can influence the redox properties). This can
be deduced from the data for the series of verdazyls 1a-
1d-1h-1c, for which each member has one more p-methyl
group than the one before it. Inspection of the oxidation and
reduction potentials for these compounds suggests that each
p-Me group lowers the oxidation potentials by ∼0.02 V
(although the difference between 1d and 1h is larger at 0.05
V) and lowers reduction potentials by ∼0.02 V as well.
Overall the substituent effects appear to be mainly induc-
tive in nature. This can be gleaned by inspection of the
verdazyl singly occupied molecular orbital (SOMO)sthe
orbital involved for both oxidation and reduction processess
which is known to be a π* orbital spanning the four nitrogen
atoms (7). There are two nodal planes, one of which passes
through the bond connecting the R3 phenyl group to the
verdazyl ring itself. This nodal plane should therefore
preclude direct conjugative effects from this substituent. Para
substituents R1 and R2 can, in principle, also affect the
SOMO directly because the phenyl groups connect these
substituents to the verdazyl at ring positions containing
substantial contributions to the SOMO. This is borne out by
comparison; for example, 1c has p-OMe substituents which
are inductively withdrawing but donating by resonance; the
lower oxidation potential of 1c compared to 1b suggests that
resonance effects are also important here.
Table 2. Electrochemical Parametersa for Verdazyl Radicals
cmpd
Eox°
Ered
°
Ecell
cmpd
Eox°
Ered
°
Ecell
1a
1b
1c
1d
1e
1g
1h
-0.22 -1.23
-0.39 -1.33
-0.31 -1.29
-0.24 -1.26
-0.15 -1.14
1.01
0.94
0.99
1.02
0.98
1i
-0.26 -1.26
+0.44 -0.94
+0.27 -1.28
+0.18 -1.38
+0.20 -1.36
1.00
1.38
1.55
1.56
1.55
2a
2b
2c
2d
2e
2f
-0.30 -1.26b n.a.
-0.29 -1.27 0.98
+0.24 -1.25b n.a.
+0.23 -1.31 1.54
a Potentials are reported in V vs Fc/Fc+. CVs were performed in MeCN
solution with 0.1 M Bu4NBF4 as electrolyte and at a scan rate of 100 mV/
s. b Irreversible process, cathodic peak potential only given.
follows: (1) the ratio of anodic:cathodic peak currents was
approximately 1; (2) peak potentials were independent of
scan rate; and (3) the peak-to-peak separations were com-
parable to that of the ferrocene/ferrocenium redox couple
run under the same conditions.
The oxidation potentials of the methylene-bridged
verdazyls 1 occur between -0.39 to -0.15 V vs Fc/Fc+,
rendering them good electron donors (a few verdazyls have
in fact been used as electron donors in charge-transfer
salts13c). The reductions of these radicals occur at rather
negative potentials (-1.14 to -1.33 V). Substituent effects
are qualitatively predictable. For example, the series 1b-
1c-1d-1e consists of verdazyls with the same R3 group
(Me) and progressively more electron-withdrawing R2 and
R2 substituents (OMe-Me-H-Cl). The oxidation potentials
in this foursome rise from -0.39 V for 1b to -0.15 V for
1e, a range of nearly 250 mV. In comparison, electrochemical
studies on 4-(p-phenyl substituted)-1,2,3,5-dithiadiazolyl
radicals 6 showed a range of under 100 mV for a wider range
of donating/withdrawing substituents, although in these
compounds the substituents are attenuated because the point
of attachment to the radical ring lies on a nodal plane (see
below).8a,18
The redox properties of the 6-oxoverdazyls 2 differ from
those of the methylene-bridged verdazyls 1. For comparative
purposes the CVs of 1a, 2a, and 2b are depicted in Figure
The reduction potentials also follow the expected trend
and are affected qualitatively by the same magnitude,
spanning from a low of -1.33 V for 1b to a high of -1.14
V for 1e. Substituent effects arising from changes in R3 are
qualitatively smaller than the effects of R1 and R2: The series
1g-1c-1h-1i now have R1 and R2 fixed (Me) with R3
following the same substituent progression (OMe-Me-H-
Cl) and a smaller range of oxidation potentials (-0.30 for
1g to -0.26 for 1i). The irreversible nature of the reduction
of 1g precludes its inclusion in R3 substituent effects, but
the trend in reduction potentials for the remaining members
1c (-1.29V)-1h (-1.27V)-1i (-1.26V) is similar to that
Figure 1. Cyclic voltammograms of (a) 1a, (b) 2a, and (c) 2b.
The redox event centered at 0 mV in (c) corresponds to the
ferrocene/ferrocenium redox couple (ferrocene added as an internal
reference). All CVs performed in MeCN solution at scan rates of
100 mV/s with 0.1 M Bu4NBF4 electrolyte.
(18) Aherne, C. M.; Banister, A. J.; Gorrell, I. B.; Hansford, M. I.;
Hauptman, Z. V.; Luke, A. W.; Rawson, J. M. J. Chem. Soc., Dalton Trans.
1993, 967.
Org. Lett., Vol. 9, No. 23, 2007
4839

1 as representative triarylverdazyl, triaryl-6-oxoverdazyl, and
N,N′-dialkyl-3-aryl-6-oxoverdazyl radicals respectively. In
general the oxoverdazyl radicals are considerably more
difficult to oxidize; for example, the oxidation potential of
2a is 660 mV more positive than that of 1a. This is most
likely due to the inductive electron-withdrawing effect of
the carbonyl group in derivatives of 2. Within the limited
series of derivatives of 2a, the effects of the C3 group on
the redox properties are similar to those previously described
for derivatives of 1, i.e. electron-withdrawing groups (e.g.,
2d-f) are slightly more easily reduced and slightly more
difficult to oxidize.
Table 2 also presents the cell potentials, Ecell for verdazyl
radicals (where Ecell ) |Eox° - Ered°|). This parameter is
believed to correlate with gas-phase IP-EA and/or dispro-
portionation energies (the energy associated with the process
2R• f R+ + R-) and is an important consideration in the
design of neutral radical- based conductors.8 The Ecell values
for verdazyls 1 are all very close to 1.0 V and do not appear
to be affected significantlysor systematicallysby any of the
para substituents. N,N-Dialkyl-6-oxoverdazyls have even
larger Ecell values of ∼1.5 V, while the lone triaryl-6-
oxoverdazyl 2a has a cell potential of 1.38 V, intermediate
between the other two classes of verdazyl.
Kaszynski has noted that “larger” (more delocalized)
radicals tend to have lower disproportionation energies and
smaller cell potentials.19 In this respect, it is not surprising
that the N,N-dialkyl-6-oxoverdazyls have larger cell poten-
tials compared to those of verdazyls 1 or 2a; triarylverdazyls
have N-aryl groups onto which spin can delocalize. The cell
potentials of 1,3,5-triphenyloxoverdazyl 2a and its methyl-
ene-bridged counterpart 1a (and by extension, other deriva-
tives of triarylverdazyls 1) bears some scrutiny. The cell
potential of 2a is much larger than that of 1a despite the
fact that the two radicals have essentially the same skeleton.
The carbonyl group present in 2a does not contribute to the
radical SOMO and so should not add to the conjugation/
delocalization in this radical.
stantially smaller than the aN’s for the two-coordinate
nitrogens.10b Thus, the amide-like nitrogens have compara-
tively more spin density in verdazyls of type 1 compared to
that for 2a. One consequence of this is that spin delocaliza-
tion onto the N-aryl groups is greater for derivatives of 1.
This is indeed the case. A variety of EPR, ENDOR, and
NMR studies confirm that the magnitude of hyperfine
coupling constants to phenyl protons on N-aromatic groups
is significantly larger (up to a factor of 2) for methylene-
bridged vedazyls 1 compared to that for 2a.20 This suggests
that there is a correlation between the spin distribution within
the verdazyl heterocycle and the redox properties (in
particular cell potential): methylene-bridged verdazyls 1 have
a somewhat larger spin density on the amide-like nitrogens,
facilitating more effective spin delocalization onto the N-aryl
groups and leading to smaller cell potentials compared to
that for 2a.
In conclusion, we have described the first systematic study
of the redox properties of verdazyls. Several features are
worthy of comment here. (1) These radicals can be reversibly
oxidized and reduced, and ring substituent effects as well as
spin distribution contribute to the overall redox and cell
potentials. (2) The methylene-bridged radicals 1 are oxidized
at quite low potentials, highlighting their potential use as
electron donors13c or as alternatives to nitroxide radicals as
oxidation catalysts.21 (3) The cell potentials for all verdazyls
are rather larger than those found for other kinds of neutral
radicals such as polycyclic thiazyl radicals or spirocyclic
phenalenyls,8 two classes of radicals which have been
targeted as building blocks for conducting materials specif-
ically because of their small cell potentials.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada, the Petroleum
Research Fund of the American Chemical Society, and the
University of Victoria for financial support.
Supporting Information Available: Experimental pro-
cedures, spectroscopic and electrochemical data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
The spin distributions in verdazyls 1 and 2 differ from
one another: EPR hyperfine coupling constants (aN) to the
two different kinds of nitrogens are nearly the same in
derivatives of 1,10a whereas for the oxoverdazyls 2 the aN
values for the substituent-bearing nitrogen atoms are sub-
OL702163A
(20) Neugebauer, F. A.; Fischer, H.; Krieger, C. J. Chem. Soc., Perkin
Trans. 2 1993, 535.
(21) (a) Bobbitt, J. M.; Flores, M. C. L. Heterocycles 1988, 27, 509. (b)
Sheldon, R. A.; Arends, I. W. C. E. AdV. Synth. Catal. 2004, 346, 1051.
(19) Kaszynski, P. J. Phys. Chem. A 2001, 105, 7626.
4840
Org. Lett., Vol. 9, No. 23, 2007