Structure-property relationships of stable free radicals: Verdazyls with electron-rich aryl substituents

Article abstract of DOI:10.1021/jo8019829

Substitution of the 3 position of 6-oxoverdazyl free radicals with electron-rich arylamines, phenols, and aryl ethers elicits changes in the UV-vis spectra and in the pK_a of the aryl substituents consistent with the verdazyl being electron with

Full text of DOI:10.1021/jo8019829

Structure-Property Relationships of Stable Free Radicals:
Verdazyls with Electron-Rich Aryl Substituents
Victoria Chemistruck, Dallas Chambers, and David J. R. Brook*
Department of Chemistry, San Jose´ State UniVersity, One Washington Square, San Jose´, California 95192
dbrook@science.sjsu.edu
ReceiVed September 8, 2008
Substitution of the 3 position of 6-oxoverdazyl free radicals with electron-rich arylamines, phenols, and
aryl ethers elicits changes in the UV-vis spectra and in the pKa of the aryl substituents consistent with
the verdazyl being electron withdrawing. The pKa of substituents is decreased: in 80% methanol phenols
3a and 3b have pKa of 10.4 and 10.9, respectively, while the ammonium ion from protonation of 3j has
pKa ) 2.4. On the basis of these measurements, Hammett parameters for the verdazyl have been estimated:
-
σp ) +0.48 and σm ) +0.27. The longest wavelength band in the visible spectrum is red-shifted with
increasingly electron-rich aromatic rings and with increasingly polar solvents, consistent with a transition
from the highest fully occupied orbital to the radical SOMO. Exceptions occur when additional interactions
occur between verdazyl and substituent; hydrogen bonding in the case of 3c and steric interference for
3f. Measurements such as ESR and electrochemistry that are dependent largely on the SOMO are relatively
insensitive to changes in substituent.
Introduction
and combined with other chromophores they provide the basis
for a variety of novel molecular probes and switchable organic
materials.8,9 Control and understanding of the role free radicals
play in these applications requires a knowledge of the electronic
structure of the radicals and how they interact with neighboring
groups and coordinated metal ions. In particular, we are
interested in understanding how free radicals may interact with
and perturb other functional groups. With a half-filled non-
Stable free radicals have intrinsic fundamental importance
in organic chemistry as a result of their unusual electronic
structure. The presence of an unpaired electron also facilitates
their application as spin labels and components of magnetic
materials both on their own and coordinated with metal ions.1
Furthermore their propensity for one-electron redox chemistry
enables their use as charge storage components in batteries,2-7
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Mater. 2007, 19, 2910–2914.
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C. E. Solid State Ionics 2007, 178, 1546–1551.
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Ogata, N.; Masuda, T. Chem. Eur. J. 2008, 14, 3250–3259.
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Mizoguchi, K.; Masuda, T. Macromolecules 2008, 41, 1175–1183.
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1850 J. Org. Chem. 2009, 74, 1850–1857
10.1021/jo8019829 CCC: $40.75 2009 American Chemical Society
Published on Web 02/05/2009

Structure-Property Relationships of Free Radicals
SCHEME 1. Synthesis of Oxophenyl and
Aminophenyl-Substituted Verdazyl Radicals
FIGURE 1. Structure and numbering of verdazyl radicals, and cartoon
representation of the highest occupied orbitals.
bonding orbital, a particular free radical may act as an electron
donor, electron acceptor, or possibly even both. Substituents
on a free radical may interact with both the singly occupied
orbital and HOMO and LUMO orbitals, potentially stabilizing
or destabilizing the radical. Predicting the exact nature and ex-
tent of these interactions depends on understanding the precise
radical structure, which may in turn be probed by examining
the physical and spectroscopic properties of both radical and
substituent.
Among stable free radicals, 6-oxoverdazyls 1 (Figure 1) have
gained interest because of their ability to coordinate metal ions
through nitrogen giving strong magnetic interactions,10-14 their
potential structural variability, and also because of the possibility
of radical mediating interactions between the multiple substit-
uents on the verdazyl ring. Synthetically, the 3 position in the
6-oxoverdazyl structure is the easiest to vary. The 3 position of
the verdazyl is also the site of a node in the SOMO (Figure 1),
so substituents in this position have little direct effect on the
radical itself. This is reflected in the ESR spectra15 and
electrochemical properties.16 Nevertheless, effects of the ver-
dazyl on the substituent itself may still be apparentsfor example,
we may see changes in substituent pKa, or in the UV-vis
spectrum of the verdazyls. Understanding the UV-visible
spectra is especially important since it provides a direct window
into the electronic structure of these species and may facilitate
the understanding of verdazyl coordination compounds and the
design of radical-based organic devices.
To address some of these questions we have synthesized a
series of new donor-substituted verdazyls. We report the
characterization of these and some previously reported species,
particularly in terms of pKa, electrochemistry, and UV-visible
spectroscopy, and discuss their physical properties in terms of
the radical-substituent interaction. We also discuss the implica-
tions for further development and application of verdazyl
chemistry.
tetrazanes 2a-j with benzoquinone (Scheme 1)15 The 4-meth-
oxyphenylverdazyl 3d was synthesized by methylating the
corresponding phenol by using cesium carbonate and methyl
iodide in dimethyl formamide.
All of the verdazyls studied are yellow to red crystalline solids
and are stable in air at ambient temperatures. ESR spectra of
all free radicals were almost identical with those reported for
other 1,5-diisopropyl verdazyls.15 Hyperfine parameters are
listed in the Supporting Information. The verdazyl structure was
also confirmed by single-crystal X-ray diffraction on the
4-hydroxyphenyl verdazyl 3a and the 2-methoxyphenyl verdazyl
3f. Bond lengths and angles within the verdazyl were found to
be consistent with earlier studies17 and are also provided in the
Supporting Information. Of particular importance is the angle
between the planes of the verdazyl and phenyl rings. For 3a
the angle is 26°, but in 3f steric interaction with the methoxy
substituent increases the angle to 61°. Thermal ellipsoid plots
of both structures are shown in Figure 2.
In solution the neutral verdazyls are yellow to red depending
upon the type and position of the substituents. Similar to other
reported 3-aryl-6-oxoverdayls, most of the UV-visible spectra
show two overlapping bands with maxima typically near 420
and 500 nm. The position of the latter band varies significantly
with changes in substituent (including deprotonation of phenols
3a-c or protonation of amine 3j). Maxima and extinction
coefficients are listed in Table 1 while representative UV-visible
spectra of both are shown in Figure 3a-d.
To gain further insight into the UV-vis spectra, the energy
of the two visible bands is plotted against Hammett constant
(σp or σm values from ref 27) in Figure 4. In addition UV-vis
spectra of 3j and the anion of 3a (for which solvent effects are
most pronounced) were recorded in a variety of protic and
aprotic solvents. Plots of the energy of the longest wavelength
band vs solvent polarity (as measured by the solvent parameter
Results
Ten different verdazyls 3a-j were studied (Scheme 1). The
2′- and 4′-hydroxyphenylverdazyls (3a and 3c) have been
reported previously.15 The remaining verdazyls with the excep-
tion of 3d were synthesized by oxidation of the corresponding
(9) Hicks, R. G. Org. Biomol. Chem. 2007, 5, 1321–1338.
(10) Barclay, T. M.; Hicks, R. G.; Lemaire, M. T.; Thompson, L. K. Inorg.
Chem. 2001, 40, 5581–5584.
(11) Barclay, T. M.; Hicks, R. G.; Lemaire, M. T.; Thompson, L. K. Chem.
Commun. 2000, 2141.
(12) Barclay, T. M.; Hicks, R. G.; Lemaire, M. T.; Thompson, L. K.; Xu,
Z. Q. Chem. Commun. 2002, 1688–1689.
(13) Hicks, R. G.; Lemaire, M. T.; Thompson, L. K.; Barclay, T. M. J. Am.
Chem. Soc. 2000, 122, 8077–8079.
(14) Norel, L.; Pointillart, F.; Train, C.; Chamoreau, L.; Boubekeur, K.;
Journaux, Y.; Brieger, A.; Brook, D. J. R. Inorg. Chem. 2008, 47, 2396–2403.
(15) Pare´, E. C.; Brook, D. J. R.; Brieger, A.; Badik, M.; Schinke, M. Org.
Biomol. Chem. 2005, 3, 4258–4261.
(16) Gilroy, J. B.; McKinnon, S. D. J.; Koivisto, B. D.; Hicks, R. G. Org.
Lett. 2007, 9, 4837–4840.
(17) Brook, D. J. R.; Yee, G. T. J. Org. Chem. 2006, 71, 4889–4895.
J. Org. Chem. Vol. 74, No. 5, 2009 1851

Chemistruck et al.
FIGURE 2. Thermal ellipsoid plots of 3a (a) and 3f (b). Only one of the two crystallographically independent molecules of 3f is shown. Ellipsoids
are drawn at the 50% level.
TABLE 1. UV-Vis Maxima for Radicals 3a-j in Acetonitrile
Discussion
λ
1,max/nm
λ2,max/nm
As noted in the introduction, the verdazyl SOMO has a node
at the 3 position of the ring. This means that the SOMO is
relatively insensitive to substituents at the 3 position. Further-
more, properties largely dependent on the SOMO, such as ESR
spectra and oxidation potentials, show the same insensitivity.
Consistent with this, the ESR spectra of all the radicals studied
are very similar to those reported for other diisopropyl verdazyl
free radicals.17 A similar phenomenon was noted for a series
of nitronyl nitroxides, where even very small changes in
hyperfine coupling to hydrogens on the phenyl ring could not
be attributed to resonance effects such as the position of the
substituent.24
radical/substituent
(ε1,max/L mol-1 cm-1
)
(ε2,max/L mol-1 cm-1
)
3a/4′-OH
3b/3′-OH
3c/2′-OH
3d/4′-OCH3
3e/3′-OCH3
3f/2′-OCH3
419 (1020)
414 (1010)
425 (1300)
418 (700)
414 (1530)
492 (1150)
420 (970)
423 (1000)
422 (900)
425 (470)
412 (1880)
515 (520)
490 (470)
525 (600)
496, 515 (305)
491 (520)
445 (830)
520 (460)
528 (530)
523 (450)
569 (370)
466 (506)
3g/3′,4′-(OH)2
3h/3′-(OCH3)-4′-OH
3i/3′,4′-(OCH3)2
3j/4′-N(CH3)2
+
3j-H+/4′-NH(CH3)2
Similarly, a recent article by Hicks and co-workers discussed
the electrochemistry of verdazyls16 and noted that substituent
effects at the 3 position of the verdazyl were small. As reference
species, both 1,5-diisopropyl-3-phenylverdazyl and 1,5-diiso-
propyl-3-methyl verdazyls have oxidation potentials E°(V•/V+)
of +0.6 V vs SCE (Table 3).16,17,25 Electron-deficient pyridyl
and imidazolyl heterocycles at the 3 position increase E°(V•/
V+) by 0.04-0.06V.16 The verdazyls in this study, with electron-
rich phenyl groups at the 3 position, generally give slightly lower
E°(V•/V+) (by up to 0.08 V) though within this group it is hard
to find meaningful correlations. The 2′-methoxyphenyl verdazyl
shows no conjugation between the phenyl ring and the verdazyl,
so it is not surprising that E°(V•/V+) is unchanged, but only
small (e0.01V) changes in E°(V•/V+) are observed for the 3′-
hydroxyphenyl and 4′-hydroxyphenyl radicals 3a,b and the
dimethoxyphenyl radical 3i. The 3′-substituted radicals 3b and
3e have slightly higher E°(V•/V+) than their 4′-counterparts
which may be consistent with a loss of resonance interaction
between the substituent and verdazyl. Strangely, however, the
3′-methoxyphenyl and 4′-methoxyphenyl radicals 3d,e are easier
to oxidize than their hydroxy counterparts 3a,b and the
dimethylaminophenyl radical 3j actually has a higher E°(V•/
V+) than the 4-methoxyphenyl 3d.
π*) and hydrogen bond donor ability R (for solvents with
nonzero R) are shown in Figure 5.
Solutions of the phenolic verdazyls 3a-c and the amino
verdazyl 3j in buffered 80% (w/w) methanol/water were titrated
with NaOH and/or HCl. Analysis of the UV-vis spectra as a
function of corrected pH18 provided the pKa values listed in
Table 2. Full sets of titration data are shown in the Supporting
Information.
Cyclic voltammetry was recorded on all verdazyls in aceto-
nitrile. An internal ferrocene standard was used and potentials
were corrected to SCE. All species showed reversible one-
electron oxidation waves near +0.6 V vs SCE. In addition
verdazyls without acidic hydrogens showed reversible one-
electron reduction, and in some cases, a second reversible wave
corresponding to oxidation of the donor substituted benzene ring.
For most of the phenols (3a-c,g but not 3h) an irreversible
reduction wave was generally associated with a poorly defined
return oxidation near +0.2 V vs SCE. At positive potentials an
irreversible wave associated with oxidation of the phenol was
observed. Details are provided in Table 3.
(18) Canals, I.; Oumada, F. Z.; Rose´s, M.; Bosch, E. J. Chromatogr. A 2001,
911, 191–202.
(19) Parsons, G. H.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1 1975,
71, 1058–1068.
(20) Bacarella, A. L.; Grunwald, E.; Marshall, H. P.; Purlee, E. L. J. Org.
Chem. 1955, 20, 747–762.
(21) Porcal, G.; Bertolotti, S. G.; Previtah, C. M.; Encinas, M. V. Phys. Chem.
Chem. Phys. 2003, 5, 4123–4128.
These weak and irregular effects cannot be easily explained
with a simple model, though their small size is consistent with
the orbital model described above. Nevertheless, some variations
(22) Williams, L. L.; Webster, R. D. J. Am. Chem. Soc. 2004, 126, 12441–
12450.
(23) Fico, R. M., Jr.; Hay, M. F.; Reese, S.; Hammond, S.; Lambert, E.;
Fox, M. A. J. Org. Chem. 1999, 64, 9386–9392.
(24) Cirujeda, J.; Vidal-Gancedo, J.; Juergens, O.; Mota, F.; Novoa, J. J.;
Rovira, C.; Veciana, J. J. Am. Chem. Soc. 2000, 122, 11393–11405.
(25) Barr, C. L.; Chase, P. A.; Hicks, R. G.; Lemaire, M. T.; Stevens, C. L.
J. Org. Chem. 1999, 64, 8893.
1852 J. Org. Chem. Vol. 74, No. 5, 2009

Structure-Property Relationships of Free Radicals
FIGURE 3. (a) UV spectra of 3a (s) and the corresponding anion in 80% aqueous methanol (---) and acetonitrile ( · · ·). (b) UV-vis spectra of
radical 3b (s) and the corresponding anion (---) in 80% aqueous methanol. (c) UV-vis spectra of radical 3c (s) and the corresponding anion (---)
in 80% aqueous methanol and radical 3f in acetonitrile ( · · ·). (d) UV-vis spectra of radical 3j (s) and the corresponding cation (---) in 80%
aqueous methanol.
in E°(V•/V+) are more easily rationalized. Protonation of the
dimethylaminophenyl radical 3j to form an ammonium cation
increases E°(V•/V+) by 0.15 V, as a result of both the electron-
withdrawing cation and the overal increase in charge. E°(V•/
V+) for the 2′-hydroxy phenyl radical 3c is 0.1 V greater than
any of the other hydroxyphenyl verdazyls, probably as a result
of an intramolecular hydrogen bond that is broken upon
oxidation. The presence of this hydrogen bond was confirmed
by comparison of the IR spectra of 3a, 3b, and 3c in chloroform
solution. Both the 4′- substituted phenol 3a and the 3′-substituted
phenol 3b give well-resolved, non-hydrogen bonded OH
stretches at 3595 cm-1 whereas the OH stretch in the 2-substi-
3j occurs 0.1-0.2 V higher than in veratrole and N,N-
dimethylaniline respectively.
The electron-withdrawing effects of the verdazyl ring are
more evident in the pKa measurements (Table 2) and electronic
spectra (Figure 3-5 and Table 1). Phenol 3a has a pKa between
that of p-bromophenol and 4-hydroxybenzaldehyde.19 A plot
of phenol pKa in 80% methanol (where pKa values in 80%
methanol were not available they were calculated26 from the
-
measured pKa in water) vs Hammett σm/σp is linear, with
correlation coefficient R ) -0.99 and F ) -2.43. Consequently
we can use the pKa of 3a,b to estimate the Hammett constants
-
σm/σp for a verdazyl substituent. This gives σp-(verdazyl) )
+0.48 and σm(verdazyl) ) +0.27. This compares with other
tuted species 3c is at 3413 cm-1
.
nitrogen heterocycles such as 2-benzoimidazolyl (σp- ) +0.48)
Comparison of our electrochemical results with those reported
earlier for directly linked and methylene bridged verdazyls show
that the verdazyl itself is electron withdrawing. This results in
increases in E°(V•/V+) from 0.05 to 0.1 V. While this also might
be revealed in the oxidation potentials of groups attached to
the verdazyl (E°(Ar/Ar•+)), none of the substituents show a clear
reversible oxidation before the oxidation of the verdazyl itself.
Not surprisingly the verdazylium ion has a stronger electron
withdrawing effect than the verdazylsE°(Ar/Ar•+) for 3i and
and 2-indolyl (σp ) 0.39)27 and suggests that the change in
-
pKa is a result of the heterocyclic structure in general rather
than an effect of the unpaired electron. The 2-hydroxyphenol
3c is less acidic than phenol itself, presumably as a result of
the internal hydrogen bond; a similar effect is seen in comparing
4-hydroxybenzaldehyde (pKa ) 9.08 in 80% MeOH) with
salicylaldehyde (pKa ) 10.00 in 80% MeOH).26 Similar to the
(26) Rose´s, M.; Rived, F.; Bosch, E. J. Chromatogr. A 2000, 867, 45–56.
J. Org. Chem. Vol. 74, No. 5, 2009 1853

Chemistruck et al.
FIGURE 4. Plot of the energy of visible band maxima vs Hammett constant σp/σm.
FIGURE 5. (a) Plot of energy of the long wavelength visible band of the anion of 3a vs hydrogen bond donation ability, R. (b) Plot of energy of
the longest wavelength visible band vs π* for 3j (squares) and the anion of 3a (triangles).
TABLE 2. Measured pKa of Radicals 3a-c, 3j and Comparable
Phenols and Ammonium Ions in 80% Methanol/Water
wavelength band, the negative correlation of energy with solvent
polarity measured by the π* solvent scale (Figure 5a) suggests
that this is a π-π* transition. Furthermore the correlation of
energy with Hammett constants σp and σm (Figure 4) suggests
a degree of charge transfer interaction between the aromatic
ring and the verdazyl in the excited state; that is, resonance
structures such as that depicted in Figure 6 make a contribution
to the excited state.
radical/substituent
pKa
3a/4′-OH
3b/3′-OH
10.4(1)
10.9(1)
12.6(1)
2.4(1)
11.63a
10.86a
9.08a
3c/2′-OH
3j-H+/4′-NH(CH3)2
phenol
+
4-bromophenol
4-hydroxybenzaldehyde
N,N-dimethylanilinium
This is further supported by the effect of hydrogen bonding
on the anion of 3a, which stabilizes the phenoxide, reducing
its ability to donate electrons and increasing the energy of the
transition (Figure 5b). A similar observation was made with
Dimroth’s dye (4) where the absorbance results from a charge
transfer from phenolate to pyridinium and the energy of this
transition is directly related to the solvation of the phenolate.29
3.82b
a Values from ref 19. b Value from ref 20.
phenol 3a, the ion resulting from protonation of 3j is more acidic
than dimethylanilinium by ∼1.5 pKa units.
In one of our initial reports on coordination chemistry of
6-oxoverdazyls we assigned the two bands in the visible region
to n-π* and π-π* transitions respectively based on compu-
tational data.28 On the basis of the current data, these assign-
ments may be incorrect, or at the least, simplistic. For the longest
These observations collectively suggest a description of the
low energy transition in 3-aryl verdazyls as a transition from a
π orbital into the verdazyl SOMO. The donor π orbital has an
(28) Brook, D. J. R.; Fornell, S.; Stevens, J. E.; Noll, B.; Koch, T. H.; Eisfeld,
W. Inorg. Chem. 2000, 39, 562.
(29) Coleman, C. A.; Murray, C. J. J. Org. Chem. 1992, 57, 3578–3582.
(27) Exner, O. In Correlation Analysis in Chemistry: Recent AdVances;
Chapman, N. B., Shorter, J., Eds.; Plenum: New York, 1978; p 439.
1854 J. Org. Chem. Vol. 74, No. 5, 2009

Structure-Property Relationships of Free Radicals
TABLE 3. Electrochemistry of Radicals 3a-j and Related Speciesf
a From ref 21. b From ref 22. c From ref 17. d From ref 16. e From ref 23. f Potentials were calibrated with internal ferrocene and reported vs SCE.
FIGURE 6. Depiction of the charge transfer contribution to the first
excited state of the anion of 3a.
increasing contribution from the aryl ring as electron density
on the ring is increased.
conjugation between phenyl and verdazyl and a larger degree
of structural flexibility (Figure 3c, dotted line). Deprotonation
of 3c results in a loss of hydrogen bonding, changing the
conformation from a planar to a nonplanar geometry and
resulting in a blue shift in the spectrum (Figure 3c, dashed line)
rather than the red shift observed upon deprotonation of the
4′-hydroxyphenyl radical 3a (Figure 3a).
Several further specific observations are also consistent with
this model. For the 2′-hydroxyphenyl verdazyl 3c, hydrogen
bonding between verdazyl and phenol enhances the vibronic
structure by increasing the rigidity of the system (Figure 3c solid
line). Contrasting this, the 2′-methoxyphenyl-substituted species
3f absorbs at significantly shorter wavelength and the UV-vis
spectrum shows no vibronic structure, consistent with reduced
This model of the electronic spectra of oxoverdazyls is also
consistent with changes in UV-vis spectra associated with metal
J. Org. Chem. Vol. 74, No. 5, 2009 1855

Chemistruck et al.
m), 7.33 (1H, t, 7.5); 13C NMR (75 MHz, DMSO) 18.4, 19.5, 47.8,
55.3, 70.9, 112.5, 113.6, 118.2, 129.8, 137.2, 154.3, 159.8; EI-MS
m/z (rel intensity) 292 (M+, 74), 177 (84), 134 (100).
1,5-Diisopropyl-3-(3′-methoxyphenyl)-6-oxoverdazyl (3e). 2,4-
Diisopropyl-6-(3′-methoxyphenyl)-1,2,4,5-tetrazane-3-one (63.8 mg)
gave 46.2 mg of verdazyl 5b (72%) with mp 87.0-88.2 °C; IR
(NaCl) 2980, 2933 (C-H), 1680 (CdO); EI-MS m/z (rel intensity)
289 (M+,66), 205 (100), 134 (32). Anal. Calcd for
C15H21N4O2 ·0.25CH3OH: C 61.62, H 7.41, N 18.86. Found: C
61.69, H 7.78, N 19.02.
coordination. That the longest wavelength (lowest energy) band
is a charge transfer to the verdazyl SOMO fits with the observed
ligand dependence in Cu(I) phosphine coordination com-
pounds.30 The smaller changes in the UV-visible spectrum
observed with other reported verdazyl transition metal coordina-
tion compounds11,13,30 point to weaker metal-ligand overlap
with these systems.
Conclusion
2,4-Diisopropyl-6-(2′-methoxy)-1,2,4,5-tetrazane-3-one (2f).
2-Methoxybenzaldehyde (62 mg) gave 120 mg of tetrazane 4a
(82%) with mp 176.4-179.9 °C; IR (NaCl) 3251 (N-H), 2968,
2933 (C-H), 1609 (CdO); 1H NMR (300 MHz, CDCl3) 1.08 (6H,
d, J ) 6.9 Hz), 1.10 (6H, d, J ) 6.9 Hz), 3.89 (3H, s), 4.38 (1H,
t, J ) 12.0 Hz), 4.67 (2H, d, J ) 12.0 Hz), 4.69 (1H, septet, J )
6.9 Hz), 6.96(1H, dd, J ) 1, 8.3 Hz), 7.01 (1H, td, J ) 1, 7.5 Hz),
7.25 (1H, dd, J ) 1.8, 7.5 Hz), 7.35 (1H, ddd, J ) 1.8, 7.5, 8.3
Hz); 13C NMR (75 MHz, CDCl3) 18.3, 19.4, 47.7, 55.5, 74.0, 111.4,
121.6, 123.4, 130.2, 130.3, 153.9, 157.0; EI-MS m/z (rel intensity)
292 (M+, 53), 177 (74), 134 (100).
Because of the node in the SOMO at C3, properties of
verdazyls that are dependent largely on the shape and energy
of the SOMO (electrochemistry, ESR) are for the most part
insensitive to changes in substituents at the 3 position. Never-
theless, properties that also depend upon other orbitals (UV-vis,
substituent pKa) do show some variation. In particular these
properties reveal that the verdazyl group is electron withdrawing
to an extent comparable with other nitrogen heterocycles and
can act as an electron acceptor with sufficiently strong donors.
1,5-Diisopropyl-3-(2′-methoxy)-6-oxoverdazyl (3f). 2,4-Diiso-
propyl-6-(2′-methoxyphenyl)-1,2,4,5-tetrazane-3-one (99 mg) gave
68.9 mg of verdazyl 4b (70%) with mp 100.3-102.4 °C; IR (NaCl)
2994, 2974 (C-H), 1676 (CdO); EI-MS m/z (rel intensity) 289
(M+, 100), 205 (89). Anal. Calcd for C15H21N4O2: C 62.28, H 7.27,
N 19.38. Found: C 62.03, H 7.32, N 19.05.
Experimental Section
General. Tetrazanes and verdazyls were synthesized by using
the procedure described by Pare´ and co-workers.15 Verdazyls 3a
and 3c have been reported previously. Other general experimental
details are provided in the Supporting Information.
2,4-Diisopropyl-6-(3′,4′-dihydroxyphenyl)-1,2,4,5-tetrazane-
3-one (2g). 3,4-Dihydroxybenzaldehyde (138 mg) gave 195 mg of
tetrazane 7a (66%) with mp 218.2-226.9 °C; IR (NaCl) 3237
2,4-Diisopropyl-6-(3′-hydroxyphenyl)-1,2,4,5-tetrazane-3-
one (2b). 3-Hydroxybenzaldehyde (0.244 g) gave 0.126 g of
tetrazane 2a (23%) with mp 197 °C dec; IR (NaCl) 3244 (N-H),
1
(N-H), 2968, 2932 (C-H), 1584 (CdO); H NMR (300 MHz,
1
dmso-d6) 0.97 (6H, d, J ) 6.7 Hz), 1.01 (6H, d, J ) 6.7 Hz), 4.11
(1H, t, J ) 12.1 Hz), 4.43 (2H, septet, J ) 6.0 Hz), 4.76 (2H, d,
J ) 11.7 Hz), 6.69 (1H, d, J ) 8.1 Hz), 6.76 (1H, dd, J ) 8.4, 1.8
Hz), 6.90 (1H, ds, J ) 1.8 Hz); 13C NMR (75 MHz, dmso-d6) 18.8,
19.9, 47.2, 71.8, 114.7, 115.7, 117.8, 127.8, 145.4, 145.8, 154.0;
EI-MS m/z (rel intensity) 294 (M+, 50), 179 (40), 136 (100).
1,5-Diisopropyl-3-(3′,4′-dihydroxyphenyl)-6-oxoverdazyl (3g).
2,4-Diisopropyl-6-(3′,4′-dihydroxyphenyl)-1,2,4,5-tetrazane-3-
one (170 mg) gave 85 g of verdazyl 7b (50%) recrystallized from
methanol/water to give a red solid with mp 190 °C dec; IR (NaCl)
3330 (O-H), 2979, 2932 (C-H), 1648 (CdO); EI-MS m/z (rel
intensity) 291 (M+, 65), 207 (100), 136 (67), 135 (64). Anal. Calcd.
for C14H19N4O3 ·0.3CH3OH: C 57.02, H 6.74, N 18.57. Found: C
57.18, H 6.59, N 18.26.
2967, 2924 (C-H), 1578 (CdO); H NMR (300 MHz, DMSO)
1.02 (6H, d, J ) 6.6 Hz), 1.05 (6H, d, J ) 6.6 Hz). 4.25 (1H, t, J
) 11.6 Hz), 4.49 (1H, septet, J ) 6.6 Hz), 4.88 (2H, d, J ) 11.6
Hz), 6.74 (1H, d, J ) 8.1 Hz), 6.98 (2H, m,), 7.18 (1H, t, J ) 8.1
Hz), 9.49 (1H, s); 13C NMR (75 MHz, DMSO) 18.3, 19.4, 46.6,
71.4, 113.6, 115.0, 117.0, 129.2, 137.7, 153.4, 157.2; EI-MS m/z
(rel intensity) 278 (M+, 55), 163 (100), 120 (95).
1,5-Diisopropyl-3-(3′-hydroxyphenyl)-6-oxoverdazyl (3b). 2,4-
Diisopropyl-6-(3′-hydroxyphenyl)-1,2,4,5-tetrazane-3-one (69.5 mg)
gave 5.7 mg of verdazyl 2b (8.3%) with mp 166 °C dec; IR (NaCl)
3291 (O-H), 2978, 2936 (C-H), 1653 (CdO); EI-MS m/z (rel
intensity) 275 (M+,63), 191 (100), 120 (37). Anal. Calcd for
C14H19N4O2: C 60.96, H 7.04, N 19.96. Found: C 61.09, H 6.91, N
20.36.
2,4-Diisopropyl-6-(3′-methoxy-4′-hydroxyphenyl)-1,2,4,5-tet-
razane-3-one (2h). 3-Methoxy-4-hydroxybenzaldehyde (77 mg)
gave 100.2 mg of tetrazane 8a (65%) with mp 173.6-175.7 °C;
IR (NaCl) 3247 (N-H), 2970, 2935 (C-H), 1607 (CdO); 1H NMR
(300 MHz, CDCl3) 1.15 (6H, d, J ) 6.5 Hz), 1.16 (6H, d, J ) 6.5
Hz), 3.70 (2H, d, J ) 12.1 Hz), 3.93 (3H, s), 4.54 (1H, t, J ) 12.1
Hz), 4.69 (2H, septet, J ) 6.5 Hz), 5.85 (1H, ds, J ) 9.1 Hz), 6.93
(1H, d, J ) 9.0 Hz), 7.07 (2H, m); 13C NMR (75 MHz, CDCl3)
18.4, 19.6, 47.7, 56.0, 70.9, 109.0, 114.4, 118.8, 127.6, 146.1, 146.6,
154.4; EI-MS m/z (rel intensity) 308 (M+, 32), 193 (37), 150 (100).
1,5-Diisopropyl-3-(3′-methoxy-4′-hydroxyphenyl)-6-oxover-
dazyl (3h). 2,4-Diisopropyl-6-(3′-methoxy-4′-hydroxyoxyphenyl)-
1,2,4,5-tetrazane-3-one (77.8 mg) gave 57.4 mg of verdazyl 8b
(73%) with mp 129.5-131.4 °C; IR (NaCl) 2282 (O-H), 2976,
2937 (C-H), 1769 3(CdO); EI-MS m/z (rel intensity) 305 (M+,
58), 221 (100), 150 (47). Anal. Calcd for C15H21N4O3: C 59.02, H
6.89, N 18.36. Found: C 58.75, H 7.00, N 18.12.
1,5-Diisopropyl-3-(4′-methoxyphenyl)-6-oxoverdazyl (3d). 1,5-
Diisopropyl-3-(4′-hydroxyphenyl)-6-oxoverdazyl (41 mg) was dis-
solved in minimal DMF (ca. 1 mL). Excess (0.2 g) Cs2CO3 was
added to the solution resulting in a deep purple-blue color. Addition
of excess (0.2 mL) CH3I turned the solution red over a period of
∼5 min. Stirring was continued at room temperature for 30 min
after which the solution was diluted with 5 mL of water and
extracted with 1 mL of dichloromethane. Evaporation of the
dichloromethane gave 39 mg of verdazyl 3d (90%) as a red
crystalline solid with mp 59.6-63.8 °C; IR (NaCl) 2979, 2932,
2875, 2839 (C-H), 1676 (CdO); EI-MS m/z (rel intensity) 289
(M+,40), 205(100), 134(53); EI-MS m/z (rel intensity) 289 (M+,40),
205 (100), 134 (53). Anal. Calcd for C15H21N4O2 ·0.3CH3OH: C
61.41, H 7.45, N 18.69. Found: C 61.31, H 7.34, N 18.73.
2,4-Diisopropyl-6-(3′-methoxyphenyl)-1,2,4,5-tetrazane-3-
one (2e). 3-Methoxybenzaldehyde (68 mg) gave 86.5 mg of
tetrazane 5a (60%) with mp 128.2-128.9 °C; IR (NaCl) 3240
1
2,4-Diisopropyl-6-(3′,4′-dimethoxyphenyl)-1,2,4,5-tetrazane-
3-one (2i). 3,4-Dimethoxybenzaldehyde (83 mg) gave 91.6 mg of
tetrazane 9a (57%) with mp 117.3-119.8 °C; IR (NaCl) 3244
(N-H), 2967, 2932 (C-H), 1602 (CdO); H NMR (300 MHz,
CDCl3) 1.14 (6H, d, J ) 6.6 Hz), 1.16 (6H, d, J ) 6.6 Hz), 3.74
(2H, d, J ) 12 Hz), 3.84 (3H, s), 4.57 (1H, t, J ) 12 Hz), 4.68
(1H, septet, J ) 6.6 Hz), 6.90 (1H, dd, J ) 8.1, 2.4 Hz), 7.18 (2H,
1
(N-H), 2963, 2917, 2848 (C-H), 1609 (CdO); H NMR (300
MHz, DMSO) 1.15 (6H, d, J ) 6.3 Hz), 1.16 (6H, d, J ) 6.3 Hz),
3.70 (2H, d, J ) 12.0 Hz), 3.891 (3H, s), 3.913 (3H, s), 4.56 (1H,
t, J ) 12.0 Hz), 4.69 (2H, septet, J ) 6.3 Hz), 6.88 (1H, d, J ) 8.4
(30) Brook, D. J. R.; Abeyta, V. J. Chem. Soc., Dalton Trans. 2002, 4219–
4223.
1856 J. Org. Chem. Vol. 74, No. 5, 2009

Structure-Property Relationships of Free Radicals
Hz), 7.13 (2H, m); 13C NMR (75 MHz, CDCl3) 154.4, 149.4, 149.0,
128.2, 118.1, 111.0, 109.6, 70.8, 55.9, 47.7, 19.5, 18.4. Note that
the two methoxy carbons have fortuitously identical chemical shifts.
This was demonstrated by using HETCOR (see the Supporting
Information); EI-MS m/z (rel intensity) 322 (M+, 42), 207 (39),
164 (100).
verdazyl as a greenish-black solid after recrystallization from
methanol-water: mp 123-124 °C; IR (NaCl plate) 2975, 2931,
2873, 1677 (CdO); EI-MS m/z (rel intensity) 302 (M+, 50), 246
(30), 218 (80), 147 (90), 146(100), 145 (95). Anal. Calcd for
C15H24N5O: C 63.58, H 7.95, N 23.18. Found: C 63.25, H 8.02, N
22.84.
1,5-Diisopropyl-3-(3′,4′-dimethoxyphenyl)-6-oxoverdazyl (3i).
2,4-Diisopropyl-6-(3′,4′-dimethoxyphenyl)-1,2,4,5-tetrazane-3-
one (80 mg) gave 68.9 mg of verdazyl 9b (70%) with mp 119 °C;
IR (NaCl) 2963, 2917 (C-H), 1668 (CdO); EI-MS m/z (rel
intensity) 319 (M+, 68), 235 (100), 164 (47). Anal. Calcd for
C16H23N4O3: C 60.19, H 7.21, N 17.55. Found: C 59.99, H 7.37, N
17.40.
Acknowledgment. This research was supported in part by
San Jose State University. We also thank the Santa Clara
University Chemistry Department for access to their X-ray
diffractometer and Dr. Michael Ruf for additional help with
collection of single-crystal X-ray data.
2,4-Diisopropyl-6-(4′-dimethylaminophenyl)-1,2,4,5-tetrazane-
3-one (2j). 4-Dimethylaminobenzaldehyde (175 mg) gave the
corresponding tetrazane recrystallized from heptane (161 mg, 45%):
mp 207-208 °C; 1H NMR (300 MHz CDCl3) 7.41 (d, 2H, J ) 8.1
Hz), 6.73 (d, 2H, J ) 8.1 Hz), 4.70 (septet, 7H, J ) 6.6 Hz), 4.511
(t, 1H, J ) 12 Hz), 3.65 (d, 2H, J ) 12 Hz), 2.97 (s, 6H), 1.16 (d,
6H, J ) 6.6 Hz), 1.14 (d, 6H, J ) 6.6 Hz); 13C NMR (75 MHz
CDCl3) 154.3, 150.7, 126.8, 123.1, 112.2, 70.8, 47.7, 40.4, 19.5,
18.4; IR (NaCl plate) 3239 (NH), 2967, 2928, 1613 (CdO); EI-
MS m/z (rel intensity) 305 (M+, 20), 190 (19), 148 (50), 147 (100).
1,5-Diisopropyl-3-(4′-dimethylaminophenyl)-6-oxoverdazyl (3j).
Oxidation of tetrazane 10 (161 mg) with benzoquinone (85 mg,
1.5 equiv) in hot toluene for 15 min gave 97 mg (61%) of the
Supporting Information Available: Thermal ellipsoid plots
and full details of crystallographic data collection for
verdazyls 3a,f in cif format, UV-visible spectra for radicals
1
3d,e,g-i, tables of ESR data for 3b,d-j, H NMR and 13C
NMR spectra for all new tetrazanes (2b,d-j), GCMS traces
for verdazyls 3b,d-f,h-j, HPLC traces for verdazyl 3b,d-j,
and plot of phenol pKa in 80% methanol vs Hammett σ
constant used to determine Hammett constants for the
verdazyl substituent. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO8019829
J. Org. Chem. Vol. 74, No. 5, 2009 1857