Electronic interactions in verdazyl biradicals

Article abstract of DOI:10.1021/jo990962s

A series of bisverdazyl biradicals in which the atoms bearing the greatest spin density are separated by several different aromatic spacer groups has been prepared in order to compare their properties to those of the nonspaced 1,1',5,5'-tetramethyl-6,6'-dioxo-3,3'-bisverdazyl (1). In addition, the 6,6'-dithio derivative (1,1',5,5'-tetramethyl-6,6'-dithio-3,3'- bisverdazyl (7)) was prepared. Cyclic voltammograms of 1,4-bis(1,5-dimethyl- 6-oxo-3-verdazyl)benzene (5) and 2,5-bis(1,5-dimethyl-6-oxo-3- verdazyl)thiophene (6) both show a single two-electron oxidation near 700 mV. In contrast 1, 7, and 1,3-bis(1,5-dimethyl-6-oxo-3-verdazyl)benzene (4) display two one-electron oxidation waves, with the first oxidative wave appearing also near 700 mV. The absorption spectrum of each of these biradicals was red-shifted from the maximum observed for 1. Biradicals 4, 5, and 6 exhibited linear Curie plots, although a curved Curie plot was observed for 7 with J = 560 cm^-1.

Full text of DOI:10.1021/jo990962s

9386
J . Org. Chem. 1999, 64, 9386-9392
Electr on ic In ter a ction s in Ver d a zyl Bir a d ica ls
Rosario M. Fico J r., Michael F. Hay, Scott Reese, Steve Hammond, Elizabeth Lambert, and
Marye Anne Fox*
Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712, and
Department of Chemistry North Carolina State University, Raleigh, North Carolina 27675
Received J une 15, 1999
A series of bisverdazyl biradicals in which the atoms bearing the greatest spin density are separated
by several different aromatic spacer groups has been prepared in order to compare their properties
to those of the nonspaced 1,1′,5,5′-tetramethyl-6,6′-dioxo-3,3′-bisverdazyl (1). In addition, the 6,6′-
dithio derivative (1,1′,5,5′-tetramethyl-6,6′-dithio-3,3′-bisverdazyl (7)) was prepared. Cyclic voltam-
mograms of 1,4-bis(1,5-dimethyl-6-oxo-3-verdazyl)benzene (5) and 2,5-bis(1,5-dimethyl-6-oxo-3-
verdazyl)thiophene (6) both show a single two-electron oxidation near 700 mV. In contrast 1, 7,
and 1,3-bis(1,5-dimethyl-6-oxo-3-verdazyl)benzene (4) display two one-electron oxidation waves,
with the first oxidative wave appearing also near 700 mV. The absorption spectrum of each of
these biradicals was red-shifted from the maximum observed for 1. Biradicals 4, 5, and 6 exhibited
linear Curie plots, although a curved Curie plot was observed for 7 with J ) 560 cm-1
.
In tr od u ction
There has been recent interest in the development and
study of the magnetic properties of stable organic radicals
and polyradicals.1-7 Radicals can act as electron donors
or acceptors, can ligate to metals, and can act as redox-
active centers.8-13 These are desirable properties for the
development of molecular wires and molecular based
magnetic materials. Organic materials are anticipated
to be easily processible, lightweight, soluble in organic
solvents, and/or optically transparent. It is hoped that
by employing common organic synthetic techniques, these
molecules may be “tuned” to have the desired properties.
To this end we have investigated a stable biradical,
1,1′,5,5′-tetramethyl-6,6′-dioxo-3,3′-bisverdazyl (1), a com-
pound first prepared by Neugebauer and Fischer and
closely related to the monoverdazyl radicals studied by
Kuhn and Trischman.14-16
Bisverdazyl biradical 1, a stable biradical whose HOMO
and LUMO are very similar to those of tetramethyl-
eneethane (TME),10,17 can be thought of as a heterocyclic
analogue to TME. Previous Hu¨ckel MO-LCAO calcula-
tions of verdazyl biradicals have indicated that the
unpaired electrons reside in degenerate molecular orbit-
als and the electron density at C3 is very near zero.18
Verdazyls are generally stable compounds that can be
easily handled in the lab and are synthetically adaptable
to a wide variety of substituents.
The electronic interactions in 1 were studied previously
by this group.10 Bisverdazyl 1 is a non-Kekule´ disjoint
biradical, existing as a ground state singlet with small
contributions of a thermally populated triplet. The spaced
compounds 2 and 3 exist as ground-state triplets with a
zero field splitting parameter, D, of 0.0050 and 0.0040
cm-1, respectively.19,20
(1) Miller, J . S.; Epstein, A. J . Angew. Chem., Int. Ed. Engl. 1994,
33, 385.
(2) Borden, W. T.; Iwamura, H.; Berson, J . A. Acc. Chem. Res. 1994,
27, 109.
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201.
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1989, 22, 392.
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R. G.; MacKinnon, C. D.; Mawhinney, R. C.; Oakley, R. T.; Palstra, T.
T. M.; Perel, A. S. J . Am. Chem. Soc. 1996, 118, 330.
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Haddon, R. C.; J eter, D. Y.; Mawhinney, R. C.; Oakley, R. T.; Palstra,
T. T. M.; Patenaude, G. W.; Reed, R. W.; Westwood, N. P. C. J . Am.
Chem. Soc. 1997, 119, 2633.
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1996, 100, 2066.
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Soc. 1996, 119, 5155.
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927.
(16) Kuhn, R.; Neugebauer, F. A.; Trischmann, H. Angew. Chem.,
Int. Ed. Engl. 1964, 3, 232.
(13) Luneau, D.; Laugier, J .; Rey, P.; Ulrich, B.; Ziessel, R.; Legoll,
P.; Drillon, M. J . Chem. Soc., Chem. Commun. 1994, 741.
(14) Neugebauer, F. A.; Fischer, H. Angew. Chem., Int. Ed. Engl.
1980, 19, 724.
(15) Kuhn, R.; Trischmann, H. Angew. Chem., Int. Ed. Engl. 1963,
2, 155.
(17) Borden, W. T.; Davidson, E. R. J . Am. Chem. Soc. 1977, 99,
4587.
(18) Fischer, H. H. Tetrahedron 1967, 23, 1939.
(19) Mukai, K.; Azuma, N.; Shikata, H.; Ishizu, K. Bull. Chem. Soc.
J pn. 1970, 43, 3958.
(20) Azuma, N.; Ishizu, K.; Mukai, K. J . Chem. Phys. 1974, 61, 2294.
10.1021/jo990962s CCC: $18.00 © 1999 American Chemical Society
Published on Web 12/07/1999

Electronic Interactions in Verdazyl Biradicals
J . Org. Chem., Vol. 64, No. 26, 1999 9387
This difference in multiplicity prompted us to study
the effects of the intervening spacer group on the
electronic characteristics of a series of bisverdazyl bi-
radicals. To this end, with 1 as the parent compound,
three bisverdazyls with aromatic spacer units were
synthesized: 1,3-bis(1,5-dimethyl-6-oxo-3-verdazyl)ben-
zene (4), 1,4-bis(1,5-dimethyl-6-oxo-3-verdazyl)benzene
(5), and 2,5-bis(1,5 dimethyl-6-oxo-3-verdazyl)thiophene
(6).
F igu r e 1. Cyclic voltammetric scan on Pt wire of 1.0 mM 1
in degassed CH3CN containing 0.1 M NBu4PF6. Scan rate: 250
mV/s.
The room-temperature EPR spectrum of 7 is also
almost identical to that of 1. The observation of a triplet
spectrum in frozen chloroform, however, reveals zero-field
splitting parameters (zfp) and substantial changes in
electron density and molecular symmetry. The absorption
spectrum of 7 is also red-shifted, and electrochemical
measurements indicate the thio analogue to be less stable
and more easily oxidized and reduced than 1.
Furthermore, we were able to observe the variation in
electron density in the π framework by substituting the
oxygen in 1 for the more electronegative sulfur as in
1,1′,5,5′-tetramethyl-6,6′-dithio-3,3′-bisverdazyl (7).
Electr och em ica l Beh a vior . Despite the many EPR
studies of verdazyls, very few substituted verdazyls have
been studied electrochemically.10,14,15,22-29 The available
literature reports have focused on the electrochemistry
of 1,3,5-triphenylverdazyl in determining the effect of
solvent on reaction entropies and disproportionation
equilibria.30,31 Oxidation of 1,3,5-triphenylverdazyl to the
cation was reported to be reversible, but the reduction
to the anion is only quasireversible.30,31 E1/2 potentials
were not stated, but the peak separation between the
To fully understand the intramolecular interactions
within this series, we have investigated the electrochemi-
cal properties and the absorption and electron spin
resonance spectra of these biradicals.
ox
anodic and cathodic peaks was given as ∆EP ) 60 mV
red
and ∆EP ) 110 mV, respectively.30,31 It is likely that
the stability of the mono and dications of 1 and of 4-7
are related to the stability of the analogous cations.
Resu lts a n d Discu ssion
Cyclic voltammetry of 1, 7, and 4 in degassed aceto-
nitrile solutions shows substantially different electro-
chemical behavior. In 1, two quasireversible oxidation
waves are seen as the neutral biradical is first converted
to a radical cation (803 mV) and then to a dication (1.05
V) (Figure 1). In the reduction, one two-electron wave is
observed as the molecule goes from the neutral biradical
to the dianion. In 7, two one-electron oxidation waves are
also seen (778 mV, 902 mV), only one of which is
reversible (Figure 2). The return reduction wave of 7 is
suppressed, indicating that the two-electron reduction is
Syn th esis a n d P r op er ties of 4-7. Biradicals 4-6
were synthesized by condensing isophthalaldehyde, tereph-
thaldicarboxaldehyde, or 2,5-thiophenedicarboxaldehyde
with 2,4-dimethyl carbonohydrazide. Oxidations to the
neutral biradical were carried out with potassium ferri-
cyanide for the electrochemical and absorption experi-
ments and with lead oxide or tetraphenylhydrazine for
the EPR experiments. The thio analogue 7 was similarly
prepared from the thiohydrazide. Compounds 4-6 were
sufficiently stable to be characterized by chemical ioniza-
tion high-resolution mass spectra analysis, but compound
7 proved to be too air sensitive for MS analysis and was
characterized by other in situ spectral methods.
Room-temperature electron paramagnetic resonance
(EPR) spectra of the spaced biradicals 4-7 are similar
to that of 1 within our experimental determination.
Linear Curie plots were observed for 4-7, indicating
either a ground state triplet or nearly degenerate singlet/
triplet energy levels.21 However, the absorption spectra
of 4-7 are red-shifted compared with 1. Electrochemical
measurements show that the stability of the dications
and dianions derived from of these biradicals are also
sensitive to structural variation.
(22) Katritzky, A. R.; Belyakov, S. A.; Durst, H. D.; Xu, R.; Dalal,
N. S. Can. J . Chem. 1994, 72, 1849.
(23) Neugebauer, F. A. Angew. Chem., Internat. Ed. Engl. 1973, 12,
455.
(24) Kuhn, R.; Neugebaur, F. A.; Trischmann, H. Monatsch. 1966,
97, 525.
(25) Neugebauer, F. A. Tetrahedron 1970, 26, 4853.
(26) Dormann, E.; Winter, H.; Dyakonow, W.; Gotschy, B.; Lang,
A.; Naarmann, H.; Walker, N. Ber. Bunsen-Ges. Phys. Chem. 1992,
96, 922.
(27) Neugebauer, F. A.; Fischer, H.; Krieger, C. J . Chem. Soc., Perkin
Trans. 2 1993, 535.
(28) Neugebauer, F. A.; Fischer, H.; Siegel, R. Chem. Ber. 1988, 121,
815.
(29) Neugebauer, F. A.; Fischer, H.; Krieger, C. Chem. Ber. 1983,
116, 3461.
(21) Berson, J . A. The Chemistry of the Quinonoid Compounds;
Wiley: New York, 1988; Vol. 2, Chapter 10.
(30) J aworski, J . S. Electroanal. Chem. 1991, 300, 167.
(31) J aworski, J . S.; Krawczyk, I. Monatsh. Chem. 1992, 123, 43.

9388 J . Org. Chem., Vol. 64, No. 26, 1999
Fico et al.
F igu r e 2. Cyclic voltammetric scan on Pt wire of 1.0 mM 7
in degassed CH3CN containing 0.1 M NBu4PF6. Scan rate:
1000 mV/s.
F igu r e 4. Cyclic voltammetric scan on Pt wire of 1.0 mM 5
in degassed CH3CN containing 0.1 M NBu4PF6. Scan rate:
1000 mV/s.
F igu r e 5. Cyclic voltammetric scan on Pt wire of 1.0 mM 6
in degassed CH3CN containing 0.1 M NBu4PF6. Scan rate:
1000 mV/s.
F igu r e 3. Cyclic voltammetric scan on Pt wire of 1.0 mM 4
in degassed CH3CN containing 0.1 M NBu4PF6. Scan rate:
1000 mV/s.
indicating significant irreversibility in the dianion. Di-
anion instability is also shown by the suppression of the
return reduction wave in 5, indicating either unimolecu-
lar decomposition of the reduced product or its participa-
tion in a complicating side reaction.
Thus, cyclic voltammetry shows little coupling between
radical centers in 5 and 6, but substantial coupling in 1,
4, and 7. As with the verdazyl monoradicals, the stability
of the cations is greater than that of the anions, as
revealed by the larger peak separation in the reduction
and the suppression of the return reduction waves in 5
and 6.
Ta ble 1. Electr och em ica l Da ta for Ver d a zyl Bir a d ica ls
(m V vs SCE)
E1/2
(0/•+)
E1/2
(•+/2+)
E1/2
(0/•-)
∆
ox
(mV)
∆Ep
∆Ep
∆Ep
1
7
4
755
701
706
93
152
144
1013
763
814
75
275
360
-728
-304
849
105
irr.
79
248
123
216
E1/2
E1/2
(0/2+)
∆Ep
(0/2-)
∆Ep
5
6
685
749
134
155
-800
-815
>200
130
Absor p tion Sp ectr oscop y. Each of the biradicals (1,
4-7) is deeply colored, ranging from yellow to black.
Simple MO calculations suggest that in the SOMO the
bridging carbons are at nodal positions or that the
electron density is close to zero at C3.10,32 However, the
absorption spectra of the spaced biradicals are all red-
shifted from 1 (Figure 6), suggesting an increase in
conjugation. In the LUMO, the bridging carbons are not
at nodal positions and there would be considerable
interaction between the verdazyl radical units and the
spacer group. Indeed, from the spacer groups examined
here, the degree of red shift follows the expected path of
increased conjugation in the LUMO.
not reversible. As with 1 and 7, 4 also displays two one-
electron oxidation waves (778, 994 mV) (Figure 3). In the
reduction, however, only one two-electron wave is ob-
served. As indicated in Table 1, the difference between
the first one-electron oxidation wave and the second one-
electron oxidation wave, ∆ox, is twice as large in 1 as in
7. This suggests that there is greater interaction between
the two unpaired electrons in 1 than in 7.
In the spaced biradicals 5 (Figure 4) and 6 (Figure 5),
cyclic voltammetry shows one two-electron reduction
wave and one two-electron oxidation wave, rather than
the two one-electron oxidation waves as observed in 1,
4, and 7. These waves are found at approximately the
same potentials in these biradicals (Table 1). This
indicates that there is less communication between the
two unpaired electrons in 5 and 6 than in 1.
The meta-spaced bisverdazyl 4 is only red-shifted from
1 by 6 nm. m-Phenylene permits the least through-π
interaction of the spacer groups examined. In 5, whose
connectivity is para through the phenyl spacer, the
In all of these systems, the peak separation between
the forward and reverse scans of the reduction waves is
larger than that for the oxidation waves (Table 1), further
(32) Forrester, A. R.; Hay, J . M.; Thomson, R. H. Organic Chemistry
of Stable Free Radicals; Academic Press: London, 1968.

Electronic Interactions in Verdazyl Biradicals
J . Org. Chem., Vol. 64, No. 26, 1999 9389
bonds in 15 causes an even larger red shift of the long
wavelength (by 50 nm).27
Previous experimental and computational studies have
shown that the substitution at N1 and N5 positions has
little effect on the electronic structure of the verdazyl
monoradical, unless the nitrogens participate in an
extension of the conjugated π-system.18 Manipulation of
substituents at N1 and N5 similarly results in only slight
shifts of either the absorption spectrum or the chemical
stability of the radical.23 The similarity in spectral
properties observed in biradicals 5 and 7 is therefore
quite reasonable.
F igu r e 6. Absorption spectra of 1, 4-7 in degassed acetoni-
trile solution.
absorption spectrum is red-shifted by 36 nm from 1. The
thiophene group in 6 shifts the absorption maximum by
66 nm from 1. The thiophene group also broadens the
spectrum of 6 considerably, giving rise to several shoul-
ders on a broad peak, with a tail extending into the
visible region. The long wavelength bands in 5 and 6 are
particularly weak, indicating that although the addition
of a thiophene or phenyl group provides some extension
of the conjugated system, electronic transitions to this
state are forbidden. The absorption spectrum of 7 (Figure
6) shows that the substitution of oxygen by sulfur at C6
induces a red shift in the absorption maximum by 46
nm.27 Noticeably absent are strong absorptions in the
visible region.
EP R Sp ectr a . EPR spectra of chloroform solutions of
4-7 are similar to spectra reported previously for the
phenyl-substituted monoverdazyl 16.14
Solution spectra of 4-7 show at least nine lines with
a 1:4:10:16:19:16:10:4:1 splitting pattern consistent with
hyperfine coupling constants A(N2,4) ≈ 6-5 Hz and
A(N1,5) ≈ 5 Hz.23 Smaller hyperfine coupling constants
could not be determined due to large line widths and
underlying monoradical impurities. These splitting pat-
terns indicate the presence of four nitrogen atoms with
nearly the same spin densities.18 The resolution of these
spectra is considerably poorer than that of 16, as is
consistent with previous studies that show decreased
resolution upon increasing the number of radical centers
linked through a phenyl group.18,24 The observed line
broadening was assigned to intermolecular hyperfine
interactions and the intermolecular magnetic dipole-
dipole interactions.18,26
These results can be best interpreted in relationship
to the spectrum of the monoverdazyl radical. The absorp-
tion spectrum of monoverdazyls show two bands between
275 and 325 nm, two bands in the visible region from
400 to 450 nm, and one long wavelength band near 700
nm characteristic of π-π* transitions in highly conju-
gated chromophoric chains, as seen in 1,3,5-triphenylver-
dazyl.22 Bands in the visible region have previously been
assigned to Φh (SOMO) f Φl (LUMO) and Φh-1 f Φh
(SOMO) transitions.27
Using an analogous numbering system as in the
bisverdazyls, substitution at C3 greatly affects electronic
structure, as reflected by changes in the absorption
spectrum. A comparison of the absorption spectra of 1,5-
dimethyl-6-thioverdazyl (11) with that of 1,5-dimethyl-
3-phenyl-6-thioverdazyl (12) shows that a phenyl group
at C3 induces a red shift of approximately 30 nm from
276 to 303 nm and from 495 to 526 nm.28
EP R F r ozen Solu tion s. The spectra for compounds
4-7 in frozen solutions are consistent with those of
randomly oriented triplets. In all compounds, a trace of
a doublet impurity could be found. However, zero field
splitting parameters (zfp) were still observed and simu-
lated (Table 2).33
The EPR spectra of frozen solutions of 5 and 6 are very
Substitution at C6 also results in a significant shift,
as seen in the substantial red shift induced by replace-
ment of oxygen with sulfur at the C6 position in 1,3,5-
triphenyl-6-oxoverdazyl (13) and 1,3,5-triphenyl-6-thio-
verdazyl (14). Replacement of the sulfur with two C-H
similar. The simulated zfp33 for 5 were |D/hc| ) 0.0043
(33) Powder EPR spectra were simulated using WINEPR SimFonia,
Shareware Version 1.25, Bru¨ker Analytische Messtechnik GmbH,
Copyright © 1994-1996.

9390 J . Org. Chem., Vol. 64, No. 26, 1999
Fico et al.
Ta ble 2. Zer o F ield Sp littin g P a r a m eter s a n d Cu r ie P lot
Resu lts for Ver d a zyl Bir a d ica ls
|D/hc| (cm-1
)
|E/hc| (cm-1
)
Curie plot
1
7
4
5
6
0.038
0.0016
0.0012
0.0037
0.00042
0.00047
J ) 887 cm-110
J ) 560 cm-1
linear
linear
linear
0.018
0.0063
0.0043
0.0051
F igu r e 8. EPR spectrum of 6 in a frozen, degassed THF
solution.
F igu r e 7. EPR spectrum of 5 in a frozen, degassed THF
solution.
cm-1, |E/hc| ) 0.00042 cm-1 (Figure 7). The simulated
zfp for 6 were |D/hc| ) 0.0051 cm-1 and |E/hc| ) 0.00047
cm-1 (Figure 8), indicating very little difference in the
degree of electronic interaction between radical centers
in compounds containing the p-phenylene and 2,5-
thiophene spacers. Both compounds also gave linear
Curie plots, indicating either a ground state triplet or a
nearly degenerate ground state.21
The similarities in 5 and 6 are expected. Recall that
cyclic voltammetry revealed one two-electron-oxidation
wave, showing little interaction between the two un-
paired electrons. Since the D value is inversely proportion
to the average distance between the two electrons, the
sites bearing electron density in these compounds must
be farther apart. As such, lower levels of coupling would
be expected than if they were more strongly interacting
as in 4, which had two one-electron oxidation waves. The
zfp of 4 are larger than those of 5 and 6. For 4, |D/hc| )
0.0063 cm-1, |E/hc| ) 0.00037 cm-1 (Figure 9). However,
4 also gave a linear Curie plot.
The spectrum of 7 in frozen chloroform is consistent
with the hyperfine structure of a randomly oriented
triplet34 (Figure 7). The large featureless absorption in
the middle of the spectrum is identical to that previously
attributed to a doublet impurity.10 Zero field parameters
F igu r e 9. EPR spectrum of 4 in a frozen, degassed THF
solution.
|D/hc| and |E/hc| were determined to be 0.018 cm-1 and
0.0012 cm-1, respectively. The zero field parameters are
smaller than those of the previously reported bisverdazyl
1 of |D/hc| ) 0.038 cm-1 and |E/hc| ) 0.0016 cm-1 10
.
The
D value is smaller in 7 than in 1, indicating a higher
degree of delocalization of the unpaired electrons into the
orbitals of the thione. Because the D parameter is a
measure of the distance between the two unpaired
electrons, and because the value for 7 is approximately
half that of 1, the unpaired electrons are likely localized
further apart onto the two sulfur atoms, which is
(34) Wasserman, E.; Snyder, L. C.; Yager, W. A. J . Chem. Phys.
1964, 41, 1763.

Electronic Interactions in Verdazyl Biradicals
J . Org. Chem., Vol. 64, No. 26, 1999 9391
dication. Room-temperature EPR spectra of 7 are nearly
identical to those for 1, but |D/hc| and |E/hc| parameters
derived from spectra of frozen solution suggest a higher
degree of electronic delocalization in 7 than in 1. Al-
though biradical 7 is highly air sensitive, it displays a
smaller singlet-triplet energy gap than the oxygen
analogue 1.
Exp er im en ta l Section
Gen er a l Meth od s. Phosgene (20% from Fluka) and meth-
ylhydrazine were used without further purification, producing
2,4-dimethylhydrazide.35 This in turn was condensed with the
dialdehyde of the spacer of interest to produce the correspond-
ing tetrahydrazine. The tetrahydrazine was then oxidized with
either potassium ferricyanide, tetraphenylhydrazine, or lead
oxide to produce the desired bisverdazyl biradical.14
F igu r e 10. Dependence of the product of susceptibility and
temperature (øNT) on temperature (T) for 7. The solid line is a
ø
T)
best fit to the function NT ) P + ke(-∆E/k with ∆E ) 0.069
B
1H and 13C NMR spectra were recorded on an NMR
spectrometer at 300 and 75.5 MHz, respectively. X-band EPR
spectra were recorded on a spectrometer fitted with a liquid
helium cryostat. Powder EPR spectra were simulated using
WINEPR SimFonia, Shareware version 1.25. Absorption spec-
tra were recorded on a scanning spectrophotometer equipped
with a near-infrared (IR) detector. Electrochemical measure-
ments were carried out on a potentiostat using a Pt wire
working electrode, a Pt coil auxiliary electrode, and a Ag/AgCl
reference electrode. For each electrochemical experiment, the
supporting electrolyte was 0.1 M NBu4PF6 in acetonitrile. The
electrolyte had been recrystallized three times from absolute
ethanol and dried under vacuum, and the solvent was purified
according to previously described methods.36 All other reagents
were purchased from Aldrich and were used as received.
Melting points were recorded on a capillary apparatus and are
uncorrected.
eV, k ) 12200, p ) 7.6.
consistent with the cyclic voltammetry peak splitting.
Thus, 7 has a peak splitting of about one-half of that of
1.
Inspection of the half-field region of the spectrum of 7
reveals signals consistent with transitions between the
ms ) -1 and ms ) +1 states of the triplet. The temper-
ature dependence of this signal allows for the singlet-
triplet energy separation J to be determined. The tem-
ø
perature dependence of øNT (where ) I/I300, where I is
N
the integrated signal intensity and I300 is the integrated
signal intensity at 300 K), was determined from the EPR
signal intensity for 7 (Figure 10). Fitting these data to
eq 110
Syn th esis of 1,3-Bis(1,5-d im eth yl-6-oxo-3-ver d a zyl)-
ben zen e (4). To a solution of 0.42 g (3.1 mmol) of isophthal-
dehyde in 5 mL of 100% ethanol, at 80 °C, was added 0.96 g
(8.1 mmol) of 2,4-dimethylcarbonohydrazide. After heating for
5 min, a white precipitate had formed. The mixture was stored
at 10 °C for 4 h and the white solid collected by filtration (63%).
ø
BT)
NT ) P + ke(-∆E/k
(1)
gives a singlet-triplet energy gap of 560 cm-1 (0.069 eV).
This value is lower than that found for 1, where the
activation energy has been determined to be 887 cm-1
(0.11 eV).10
1
Mp: 181 °C (dec). H NMR (DMSO-d6): δ 7.72 (1 H, s), 7.48-
7.36 (3 H, m), 5.68 (4 H, d), 4.90 (2 H, t), 2.94 (12 H, s). 13C
NMR (DMSO-d6): 154.5, 136.8, 128.1, 126.5, 126.1, 68.6, and
37.6 ppm. HRMS (CI): (m/z) C14H20N8O2 (M + H)+ calcd
335.1944, found 335.1938.
Con clu sion s
Into a small vial, 102.5 mg (0.306 mmol) of 1,3-bis(1,5-
dimethylhexahydro-6-oxo-1,2,4,5-tetrazin-3-yl)benzene and 732.6
mg (3.27 mmol) of lead oxide in 10 mL of dry THF were stirred
for 24 h under inert conditions. The lead oxide was removed
by filtering an EPR sample through a 0.2 µm Teflon syringe
filter, yielding a red solution. HRMS (CI): (m/z) C14H16N8O2
(M + H)+ calcd 329.147, found 329.147.
Syn th esis of 1,4-Bis(1,5-d im eth yl-6-oxo-3-ver d a zyl)-
ben zen e (5). In a round-bottom flask was placed 100 mg (0.31
mmol) of 1,4-bis(1,5-dimethylhexahydro-6-oxo-1,2,4,5-tetrazin-
3-yl)benzene29 in 6 mL of water. Na2CO3 (3 mL, 2 N) was added
together with 600 mg (1.8 mmol) of K3Fe(CN)6 in 5 mL of
water. The solution was stirred for 4 h and was then extracted
with CH2Cl2. The solvent was removed under vacuum. The
product (50 mg, 0.15 mmol, 48%) was collected as a red solid.
Mp: 250 °C (dec). UV-vis (CH3CN): λmax nm (log ꢀ): 415
(3.17), 277 (4.48), 200 (4.28). HRMS (CI): (m/z) C14H16N8O2
(M + H)+ calcd 329.3531, found 329.1487. LRMS (CI+): 329
(49%).
Electronic coupling between the radical centers in
substituted verdazyl biradicals is affected by substitution
at the C3 position. Aromatic spacers at C3 in 4-6 decrease
the magnitude of coupling between the two unpaired
electrons, compared to 1, as shown by cyclic voltammetry
and EPR spectral measurements. This effect is illustrated
in the observed electrochemistry, where two one-electron
oxidation waves (to produce a dication) are observed for
1, whereas a single two-electron oxidation wave is evident
in 5 and 6. Significant red shifts in the absorption
spectra, however, indicate structural perturbation by
these binding groups, although spectral broadening
makes unambiguous assignment of the causes for these
differences difficult.
Biradicals 4-6 all gave linear Curie plots with D
values ranging from 0.0043 to 0.0063 cm-1. A linear Curie
plot confirms that each member of this family exists as
a ground-state triplet or as a species with a degenerate
triplet/singlet energy level.21
Replacement of oxygen at C6 in 1 with sulfur in 7 does
affect the electronic structure of the biradical. The
absorption spectrum of 7 is red-shifted by about 50 nm
from that of 1. Electrochemical measurements indicate
two one-electron oxidation waves in 7, but scan reversal
reveals only one return wave, suggesting an unstable
Syn th esis of 2,5-Bis(1,5 d im eth yl-6-oxo-3-ver d a zyl)-
th iop h en e (6). To a solution of 1.1 g (8.9 mmol) of 2,4-
dimethylcarbonohydrazide in 10 mL of ethanol was added 0.50
g (3.6 mmol) of thiophenedicarboxaldehyde. The solution was
(35) Raphaelian, L.; Hooks, H.; Ottmann, G. Angew. Chem., Int. Ed.
Engl. 1967, 6, 363.
(36) Knorr, A. Funktionalisierte Fluoreszenzfarbstoffe auf Anthracen-
und Pyrenbasis; Knorr, A., Ed.; University of Regensburg: Regensburg,
1995.

9392 J . Org. Chem., Vol. 64, No. 26, 1999
Fico et al.
heated to reflux for 45 min and then cooled to room temper-
ature. A solid formed upon cooling which was separated by
filtration, yielding 0.84 g (69%) as a white powder. Mp: 242-3
The solution was cooled to room temperature and the product
collected by filtration, yielding 0.51 g (1.7 mmol, 34%) of
product as a brownish yellow powder. Mp: 271 °C. H NMR
1
1
°C. H NMR (DMSO-d6): δ 6.96 (s, 2 H), 5.83 (d, 4 H, J ) 5.7
(DMSO-d6): δ 5.60 (d, 4 H, J ) 6.1 Hz), 3.82 (t, 2 H, J ) 6.6
Hz), 3.31 (s, 12 H). 13C NMR (DMSO-d6): 173.7, 66.0, 43.7 ppm.
HRMS (CI): (m/z) C8H18N8S2 (M + H)+ calcd 291.43, found
291.12. LRMS (CI+): 291 (89%).
A vacuum-sealed solution of 0.024 g (0.087 mmol) of
2,2′,3,3′,4,4′,5,5′-octahydro-1,1′,5,5′-tetramethyl-3,3′-bis-1,2,4,5-
tetrazin-6,6′-thione and 0.072 g (0.25 mmol) of tetraphenyl-
hydrazine in 2 mL of degassed CHCl3 was heated in an oil
bath at 330 K for 48 h, turning first green and then dark
purple. UV-vis (CH3CN) λmax (log ꢀ): 284 (3.56).
Hz), 5.07 (t, 2 H, J ) 5.4 Hz), 2.92 (s, 12 H). 13C NMR
(DMSO-d6): 154.0, 140.9, 125.5, 65.5, and 37.9 ppm. UV-vis
(DMSO): λmax nm (log ꢀ): 318 (3.05). HRMS (CI): (m/z)
C
12H20N8O2S (M + H)+ calcd 341.42, found 341.151.
To a solution of 100 mg (0.30 mmol) of 2,5-bis(1,5-dimeth-
ylhexahydro-6-oxo-1,2,4,5-tetrazin-3-yl)thiophene in 7 mL of
water was added 600 mg of K3Fe(CN)6 (1.8 mmol) in 6 mL of
water and 3 mL of 2N Na2CO3. The solution was stirred for
approximately 6 h, and the resulting brown solution was
extracted with CHCl3. The solvent was removed under vacuum
and the resulting yellow-brown solid collected by filtration (48
mg, 0.14 mmol). Mp: 245 °C (dec). UV-vis (CH3CN): λmax nm
(log ꢀ): 357 (4.70, sh), 343 (4.78, sh), 317 (4.92), 300 (4.98, sh),
194 (5.02). HRMS (CI): (m/z) C12H14N8O2S (M + H)+ calcd
335.3773, found 335.103. LRMS (CI+): 335 (100%).
Syn th esis of 1,1′,5,5′-Tetr a m eth yl-6,6′-d ith io-3,3′-biver -
d a zyl (7). A solution of 0.67 g (5.0 mmol) of 2,4-dimethylthio-
hydrazide was combined with 0.60 g (2.3 mmol) of glyoxal
sodium bisulfate addition compound in 15 mL of distilled
water, and the resulting solution was heated to reflux for 2 h.
Ack n ow led gm en t. This work was supported by the
National Science Foundation and the Robert A. Welch
Foundation. We are grateful to Dr. David Brook for
helpful suggestions regarding the synthesis and spectral
interpretations included here. We would like to thank
Dr. David Shultz for his useful discussions and the use
of his equipment.
J O990962S