Synthesis of bis(semiquinone)s and their electrochemical and electron paramagnetic resonance spectral characterization

Article abstract of DOI:10.1021/jo981597s

The syntheses of three new bis(semiquinone)s (2(··2-), 3(··2-), and 5(··2-)) linked through carbon-carbon double bonds in a geminal fashion (2(··2-) and 3(··2-)), and through an sp³ carbon (5(··2-)), are presented, as well as the results of variable-temperature EPR (VT-EPR) spectroscopy on these biradicals and two previously reported bis(semiquinone)s, 1(··2-) and 4(··2-). We suggest that the potential difference in redox couples associated with a biradical is useful for qualitatively assessing changes in the exchange parameter within an isostructural series. The zero-field-splitting parameters for 1(··2-) 5(··2-) are consistent with their electronic structures: biradicals 1(··2-)-3(··2-) which have conjugating groups attached to the semiquinone rings have D-values less than 5(··2-), a bis(semiquinone) that lacks such a conjugating group. Also, the D-value of 3(··2-) is significantly less than those of 1(··2-) and 2(··2-), in agreement with larger interelectron separation in 3(··2-), a biradical with quinone-methide π-system delocalization. Changes in counterion Lewis acidity are not manifested in the D-values of the biradical dianion 4(··2-). The EPR spectrum of biradical 5(··2-) is consistent with the existence of at least two rotamers having different zero-field-splitting parameters. Biradicals 1(··2-)-4(··2-) gave linear Curie plots, consistent with J > 0 (ferromagnetic coupling) or J = 0. The temperature-dependent intensity of EPR signals of 5(··2-) are characteristic of antiferromagnetic coupling. Best fit results give J = -114 ± 6 cal/mol for the |D/hc| = 0.01309 cm^-1 rotamer of 5(··2-), and J = -76 ± 3 cal/mol for the |D/hc| = 0.01026 cm^-1 rotamer of 5(··2-).

Full text of DOI:10.1021/jo981597s

9462
J . Org. Chem. 1998, 63, 9462-9469
Syn th esis of Bis(sem iqu in on e)s a n d Th eir Electr och em ica l a n d
Electr on P a r a m a gn etic Reson a n ce Sp ectr a l Ch a r a cter iza tion †
David A. Shultz,* Andrew K. Boal, and Gary T. Farmer
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204
Received August 5, 1998
The syntheses of three new bis(semiquinone)s (2••2-, 3••2-, and 5••2-) linked through carbon-carbon
double bonds in a geminal fashion (2••2- and 3••2-), and through an sp3 carbon (5••2-), are presented,
as well as the results of variable-temperature EPR (VT-EPR) spectroscopy on these biradicals and
two previously reported bis(semiquinone)s, 1••2- and 4••2-. We suggest that the potential difference
in redox couples associated with a biradical is useful for qualitatively assessing changes in the
exchange parameter within an isostructural series. The zero-field-splitting parameters for 1••2-
-
5••2- are consistent with their electronic structures: biradicals 1••2--3••2- which have conjugating
groups attached to the semiquinone rings have D-values less than 5••2-, a bis(semiquinone) that
lacks such a conjugating group. Also, the D-value of 3••2- is significantly less than those of 1••2-
and 2••2-, in agreement with larger interelectron separation in 3••2-, a biradical with quinone-methide
π-system delocalization. Changes in counterion Lewis acidity are not manifested in the D-values
of the biradical dianion 4••2-. The EPR spectrum of biradical 5••2- is consistent with the existence
of at least two rotamers having different zero-field-splitting parameters. Biradicals 1••2--4••2- gave
linear Curie plots, consistent with J > 0 (ferromagnetic coupling) or J ) 0. The temperature-
dependent intensity of EPR signals of 5••2- are characteristic of antiferromagnetic coupling. Best
fit results give J ) -114 ( 6 cal/mol for the |D/hc| ) 0.01309 cm-1 rotamer of 5••2-, and J ) -76
( 3 cal/mol for the |D/hc| ) 0.01026 cm-1 rotamer of 5••2-
.
The preparation of paramagnetic ligands for the con-
struction of molecule-based magnets is an active area of
research.1 Several groups have synthesized novel nitrox-
ide-type bi- and triradicals,2 and we recently reported the
synthesis and characterization of a mixed nitroxide-
semiquinone biradical,3 as well as the preparation of
m-phenylene-linked and trimethylenemethane (TMM)-
type bis(semiquinone)s.4,5 The EPR spectroscopic features
of the two bis(semiquinone)s were consistent with the
proposed electronic structures. Herein, we report the
synthesis of three new bis(semiquinone)s (2••2-, 3••2-, and
Biradicals 1••2--4••2- were synthesized to test the
viability of the TMM- and m-phenylene-type strategies
for preparing high-spin bis(semiquinone)s. TMM-type bis-
(semiquinone)s 1••2- and 2••2- have adamantane and
norbornane ring systems that “cap” the exocyclic CdC
and provide steric protection of any spin density delo-
calized into the exocyclic CdC. Ultimately, we will use
ligands 1••2- and 2••2- to extend our study of the effects
of bond torsions on exchange coupling in TMM-type
biradicals.11,12 Bis(semiquinone) 3••2- is an analogue of
Yang’s biradical,13-16 and the semiquinone spins should
delocalize into the quinone-methide group.
5
••2-) linked through carbon-carbon double bonds in a
geminal fashion (2••2- and 3••2-) and through an sp3
carbon (5••2-), as well as the results of variable-temper-
ature EPR (VT-EPR) spectroscopy of all five compounds
(Chart 1).
The design criteria for high-spin ligands is identical
to those for design of organic biradicals, a topic that has
been discussed in some detail.6 Generally, the common
approach is to attach “spin-containing groups” to a linker
fragment known to couple unpaired electrons in a high-
spin or ferromagnetic fashion.7 Two common, successful
linkers are those in nondisjoint8 biradicals trimethylene-
methane (TMM) and m-xylylene, known triplet ground-
state molecules.9,10
* To whom correspondence should be addressed. Tel: (919) 515-6972.
email: shultz@chemdept.chem.ncsu.edu. Fax: (919) 515-8920. Web:
http://www2.ncsu.edu/ncsu/chemistry/das.html.
† Taken from the M.S. Thesis of A.K.B.
Finally, biradical 5••2- was examined to determine if
the CdC groups of 1••2--3••2- played a part in exchange
coupling beyond holding the semiquinone groups close
together in space.
(1) For general texts on molecular magnetism, see the following: (a)
Gatteschi, D. Molecular Magnetic Materials; Kluwer Academic Pub-
lishers: Amsterdam, 1991. (b) Kahn, O. Molecular Magnetism; VCH:
New York, 1993. (c) Molecular Magnetism: From Molecular Assemblies
to the Devices; Coronado, E., Delhaes, P., Gatteschi, D., Miller, J . S.,
Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1996;
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and Applications; M. M. Turnbull, T. Sugimoto, L. K. Thompson, Eds.;
ACS Symposium Series 644; American Chemical Society: Washington,
DC, 1996.
Syn th esis
Bis(orthoquinone)s 1 and 4 were prepared as described
previously.4,5 Compounds 2, 3, and 5 were prepared as
10.1021/jo981597s CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/24/1998

Synthesis and Characterization of Bis(semiquinone)s
J . Org. Chem., Vol. 63, No. 25, 1998 9463
Ch a r t 1
shown below. Methyl norbornane-7-carboxylate was pre-
pared using a modified procedure of Kwart:17 metalation
of 7-bromonorbornane with lithium 4,4′-di-tert-butylbi-
phenylide (LiDBB),18 followed by reaction of the alkyl-
lithium with CO2, and Fischer esterification gave ester
7 in excellent yield. Ester 7 was reacted with 2 equiv of
3-tert-butyl-5-lithiocatechol dimethyl ether,5 followed by
p-toluenesulfonic acid-promoted dehydration, and dem-
ethylation to give bis(catechol), 2-H4. Oxidation of 2-H4
with Fe´tizon’s reagent19 gave bis(quinone) 2.
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1339.

9464 J . Org. Chem., Vol. 63, No. 25, 1998
Shultz et al.
Three methods were used to prepare bis(semiquino-
ne)s. The first method is that reported by Mu¨ller:22
reduction of quinone to catecholate, followed by compro-
portionation with an equimolar portion of quinone, eq 1.
This method worked for bis(quinone)s 4 and 5, suggesting
CdC reduction or related reactivity in 1-3. Alternatively,
anthrasemiquinone (AQ-•) was used as a one-electron
reducing agent,23 eqs 2 and 3. Anthrasemiquinone reduc-
tion was the only method used that gave reproducible,
positive results for 3. Reductions of the bis(quinone)s that
gave alkali metal salts were carried out in the presence
of excess pentamethyldiethyltriamine (PMDTA), which
we found to effectively inhibit formation of semiquinone
aggregates.24 Finally, controlled-potential-coulometry
(CPC) was used for 1 and 2 because neither Na-metal
reduction nor reduction with AQ-• reproducibly gave
biradicals. CPC was carried out on THF solutions of bis-
(quinone)s using tetra-n-butylammonium hexafluoro-
phosphate as the supporting electrolyte, eq 4. Neither
EPR spectra nor the results of variable-temperature
studies were a function of the method (or the semiquinone
counterions) used for production of the biradicals.25
Bis(quinone) 3 was prepared in a similar fashion, by
reaction of 3-tert-butyl-5-lithiocatechol bis(methoxymeth-
yl) ether5 with ester 9 followed by removal of the TMS
group, elimination, deprotection, and oxidation of bis-
(catechol) 3-H4. Ester 9 was prepared by carboxylation
of the lithium reagent formed from 8,20 followed by
esterification and reprotection of the phenol group.
Condensation of 3-tert-butyl-5-lithiocatechol dimethyl
ether with ester 1021 provided a carbinol that underwent
electrophilic aromatic substitution upon exposure to
polyphosphoric acid to form, after demethylation, fluo-
rene derivative 5-H4. Oxidation provided bis(quinone) 5
in good yield.
Electr och em istr y
Figure 1 shows the cyclic voltammograms of 1-5 as
solutions in THF, and pertinent data are collected in
Table 1. All bis(quinone)s examined showed separate
waves for the first and second redox couples which
correspond to bis(quinone) to (quinone-semiquinone) and
(quinone-semiquinone) to bis(semiquinone) couples, re-
spectively. Bis(quinone)s 1, 2, and 5 exhibited nearly
identical CV waveforms with redox couples at ca. -0.73
and ca. -0.94 V vs Ag/AgNO3, and splittings of the redox
(20) Harrer, W.; Kurreck, H.; Reusch, J .; Gierke, W. Tetrahedron
1975, 31, 625.
(21) Davies, D. I.; Hey, D. H.; Summers, B. J . Chem. Soc. C 1970,
2653.
(22) Mu¨ller, E.; Gu¨nter, F.; Scheffler, K.; Ziemek, P.; Rieker, A.
Chem. Ber. 1965, 98, 134.
(23) (a) Chen, K.; Hirota, N. J . Am. Chem. Soc. 1972, 94, 5550. (b)
Takeshita, T.; Hirota, N. J . Am. Chem. Soc. 1971, 93, 6421.
(24) Shultz, D. A.; Boal, A. K.; Campbell, N. P. Inorg. Chem. 1998,
37, 1540.
(25) Reduction of 4 by eqs 1, 3, or 4 gave the same results.

Synthesis and Characterization of Bis(semiquinone)s
J . Org. Chem., Vol. 63, No. 25, 1998 9465
nondisjoint, biradicals, like 1••2--3••2- and perhaps 5••2- 26
.
A relationship between ∆E1/2 and J is reasonable since
charge and spin are delocalized over the same atoms, i.e.,
electrostatic repulsion between electrons in the SOMOss
a major contributor to ∆E1/2sshould scale with the
exchange energy for the same pair of electrons. The
ferromagnetic component of J is the exchange integral
(ka,b) whose magnitude is directly proportional to coex-
tensivity (the sharing of common atoms) of biradical
SOMOs,27 and ka,b should therefore be proportional to the
electron-electron repulsion responsible for ∆E1/2. Finally,
localizing spin in a region of space where overlap can
occur with the other radical moiety (as in 1••2--3••2- and
5
••2-) creating weak bonding interactions should increase
the antiferromagnetic component of J .27 Thus, an ob-
served increase in electron-electron repulsion in a series
of isostructural dianion biradicals should correlate with
J in the series. Based on values of ∆E1/2, |J |(3••2-) >
|J |(2••2-) ≈ |J |(1••2-). Since ∆E1/2 for 5 is nearly identical
to ∆E1/2 for both 1 and 2, and 5 lacks the π-topology
required for ferromagnetic coupling, J is expected to be
more negative (i.e., more antiferromagnetic) for 5••2- than
F igu r e 1. Cyclic voltammograms for bis(quinone)s 1-5 as 1
mM solutions in THF. Supporting electrolyte is tetra-n-
butylammonium hexafluorophosphate (100 mM). Scan rate )
200 mV/s.
for 2••2- or 1••2-
.
EP R Sp ectr oscop y
Ta ble 1. Cyclic Volta m m etr ic Da ta for Bis(qu in on e)s
Figure 2 shows the EPR spectra of 1••2--5••2- recorded
at 77 K in THF. All spectra are consistent with randomly
oriented triplet species28 along with doublet monoradical
impurities. The zero-field-splitting parameters for the
biradicals, obtained by simulation,29 are given in Table
2. The spectral shapes of 1••2--4••2- are invariant with
temperature changes, suggesting the lack of rotomers.11
1-5a
d
bis(quinone)
E1/2(1)b
E1/2(2)c
∆E1/2
1
2
3
4
5
-0.73
-0.72
-0.52
-0.67
-0.73
-0.94
-0.94
-0.77
-0.79
-0.93
-0.21
-0.22
-0.25
-0.12
-0.20
The zero-field splitting parameters for 1••2--5••2- are
consistent with their electronic structures: biradicals
1••2--3••2- which have conjugating groups attached to the
semiquinone rings have D-values less than either rota-
mer of 5••2-. Also, the D-value of 3••2- is significantly less
than those of 1••2- and 2••2-, in agreement with larger
interelectron separation in 3••2-, a biradical with quinone-
methide π-system delocalization.
Rotello’s group has shown differential response of spin
and charge density to hydrogen bonding interactions,30
so it is reasonable to expect a change in zero-field
splitting parameterssparticularly |D/hc|swhen counter-
ions of differing Lewis acidity are associated with the
semiquinone groups. However, the zero-field splitting
parameters of Na24••2-(PMDTA)n, Table 2, are nearly
identical to those of (n-Bu4N)24••,2-4 indicating that any
changes in counterion Lewis acidity are not well-
manifested in the D-value.31 It is interesting to note that
the D-value for 3••2- is nearly 2.7-times greater than that
of Yang’s biradical (ca. 0.0028 cm-1).32 The greater
a
0.5 mM bis(quinone) in THF, 100 mM n-Bu4NPF6 electrolyte;
b
potentials in volts vs Ag/AgNO3; scan rate ) 200 mV/s. Redox
potential for bis(quinone) T (quinone-semiquinone) couple; E1/2
) (Ep,c - Ep,a)/2. c Redox potential for (quinone-semiquinone) T
bis(semiquinone) couple; E1/2 ) (Ep,c - Ep,a)/2. ∆E1/2 ) E1/2(2) -
1/2(1).
d
E
couples of ca. -0.21 V. Any effect of the exocyclic double
bonds of 1 and 2 are not manifested in the CVs since the
redox potentials are nearly identical to those of 5sa
molecule that has the quinone groups attached geminal
on an sp3 carbon atom and delocalization beyond the
semiquinone rings is impossible.
Bis(quinone) 3, like 1 and 2, has two quinone groups
attached geminal on a CdC, but the double bond of 3 is
part of a quinone-methide π-system. This larger π-system
permits greater delocalization of charge and spin after
reduction than possible in reduced forms of 1 and 2.
Increased delocalization is clearly apparent in the more
positive E1/2(1) and E1/2(2) values for 3 compared to 1, 2,
or 5. It is important to note that ∆E1/2 for 3 is slightly
larger than in 1, 2, and 5. We interpret the larger ∆E1/2
as additional evidence of greater delocalization in 3 than
in 1 and 2. Greater delocalization of the semiquinone
charge should result in greater Coulombic repulsion upon
addition of the second electron to form the bis(semi-
quinone), hence the larger ∆E1/2. Bis(quinone) 4 has
quinone rings positioned about twice as far apart as in
1-3 and 5. The greater quinone-ring spatial separation,
in combination with the six-atom m-phenylene coupler,
has an overall effect of decreasing ∆E1/2 by nearly 2-fold.
It would be useful if ∆E1/2 were an electrochemical
“handle” on exchange coupling (J ) for isostructural,
(26) For an interesting and relevant discussion of the differences
in ∆E1/2 for disjoint and coextensive biradical dications, see: Bushby,
R. J .; McGill, D. R.; Ng, K. M.; Taylor, N. J . Chem. Soc., Perkin Trans.
2 1997, 1405.
(27) Kollmar, C.; Kahn, O. Acc. Chem. Res. 1993, 26, 259.
(28) Wasserman, E.; Snyder, L. C.; Yager, W. A. J . Chem. Phys.
1964, 41, 1763.
(29) Powder EPR spectra were simulated using: WINEPR SimFo-
nia, Shareware Version 1.25, Bruker Analytische Messtechnik GmbH,
Copyright ©1994-1996.
(30) Niemz, A.; Rotello, V. M. J . Am. Chem. Soc. 1997, 119, 6833.
(31) Li, Na, K, Rb, and Cs salts of 4••2- all have nearly identical
D-values; see Supporting Information for EPR spectra.

9466 J . Org. Chem., Vol. 63, No. 25, 1998
Shultz et al.
Ta ble 2. Zer o-F ield -Sp littin g P a r a m eter s a n d Resu lts of
VT-EP R Stu d ies for Bir a d ica ls 1••2--5••2- a
|D/hc|/10-4 |E/hc|/10-4
biradical
cm-1
cm-1
Curie plotb
1••2- c
2••2- c
3••2- e
4••2- d
5••2- c
94.7
91.0
86.0
45.3
130.9
102.6
6.30
5.75
6.70
0
-
-
linear
linear
linear
linear
curved, J ) -114 cal/mol
curved, J ) -76 cal/mol
a
Zero-field-splitting parameters determined by simulation.28
b
Curved Curie plots fit with eq 5; see text. c Biradical prepared
by CPC; n-Bu4N+ counterions. Biradical prepared by sodium
metal reduction in the presence of excess PMDTA. e Biradical
prepared by sodium anthrasemiquinone reduction in the presence
of excess PMDTA.
d
F igu r e 2. X-band EPR spectra of biradicals 1••2--5••2- as
frozen solutions in THF at 77 K. Dashed lines are simulated
spectra. Signals at g ) 2 are due to monoradicals. Insets: ∆ms
) 2 transitions. Counterions for 3••2- and 4••2-: Na+; counte-
rions for 1••2-, 2••2-, and 5••2-: n-Bu4N+. D-values for rotamers
of 5••2- were estimated as [(Z2(a) - Z1(a)]/2 and [(Z2(b) - Z1-
(b)]/2; see text.
F igu r e 3. X-band EPR spectra of biradical 5••2- as a frozen
solution in THF at 8, 15, 25, 50, 100, and 150 K. Signals at g
) 2 are due to monoradicals.
D-value of 3••2- is most likely due to its lower symmetry
which results in less delocalization of spin-density into
the quinone-methide π-system than might be expected.
If this supposition is correct, the C3-symmetric biradical
113-••33 should have a D-value smaller than that of 3••2-
and nearly equal to that of Yang’s biradical.
are 0.0088 cm-1, 0.0078 cm-1, 0.0075 cm-1, respectively,
in reasonable agreement with the two observed D-values.
As shown in Figure 3, the EPR spectrum of biradical
5••2- is consistent with the existence of at least two
rotamers having different zero-field splitting parameters.
We propose structures 5a ••2--5c••2- shown below as
possible rotomers. Calculated D-values34 for 5a ••2--5c••2-
Curie plots for the doubly integrated ∆ms ) 2 signals
of 1••2--4••2- and doubly integrated ∆ms ) 1 signals of
5••2- are shown in Figures 4 and 5, respectively, and are
summarized in Table 2. Biradicals 1••2--4••2- gave linear
(32) Mukai, K.; Ishizu, K.; Nakahara, M.; Deguchi, Y. Bull. Chem.
Soc., J pn. 1980, 53, 3363.
(33) We recently prepared a trinuclear metal complex with biradical
113-•• that exhibits D ≈ 0.005 cm-1, less than the D-value for 33-•• in
accord with our predictions; Shultz, D. A.; Lee, H.; Bodnar, S. H.,
manuscript in preparation.
(34) Sandberg, K. A.; Shultz, D. A. J . Phys. Org. Chem. 1998, in
press.

Synthesis and Characterization of Bis(semiquinone)s
J . Org. Chem., Vol. 63, No. 25, 1998 9467
F igu r e 5. Curie plots for biradical 5••2-. The data points were
collected between 8 and 150 K and represent normalized signal
intensities obtained by double integration of the ∆ms ) 1
signals. The data points correspond to the signals marked
Z1(a) and Z1(b) in Figure 2.
F igu r e 4. Curie plots for biradicals 1••2--4••2-. The data
points were collected between 8 and 150 K and represent
normalized signal intensities obtained by double integration
of the ∆ms ) 2 signals.
responses, consistent with J > 0 (ferromagnetic coupling)
or J ) 0.35 The intensity of ∆ms ) 1 signals of 5••2-
decreased with decreasing temperature consistent with
antiferromagnetic coupling. The data were fit to eq 5:35,36
Fitting the temperature-dependent intensities to a
Bleaney-Bowers-type equation gave J ) -76 cal/mol and
-114 cal/mol. Thus, the exocyclic CdC TMM-type cou-
plers in 1••2--3••2- are acting as spin couplers despite
expected large torsion angles.
-2J
RT
3 exp
(
)
C
T
IEPR
)
(5)
Exp er im en ta l Section
-2J
RT
[
]
1 + 3 exp
(
)
Gen er a l. Unless noted otherwise, reactions were carried
out in oven-dried glassware under an argon atmosphere. THF
was distilled under nitrogen from sodium benzophenone ketyl,
toluene was distilled from sodium metal under nitrogen, and
methylene chloride and acetonitrile were distilled from CaH2
under nitrogen. Benzene, methanol, and p-toluenesulfonic acid
hydrate were used as received from Fisher Scientific. tert-
Butyllithium (1.5 M in pentane) was used as received from
Acros Chemical. Other chemicals were purchased from Aldrich
Chemical Co. Electrochemical experiments and X-Band EPR
spectroscopy were performed as described previously.3 NMR
spectra were recorded at 300 MHz for 1H NMR and 75 MHz
for 13C NMR. Elemental analysis was performed by Atlantic
Microlab, Inc, Norcross, GA. Mass Spectra were obtained at
the NC State University Mass Spectrometry Facility.
where C is a constant and J is the exchange parameter.
Best fit results give J ) -114 ( 6 cal/mol for the |D/hc|
) 0.01309 cm-1 rotamer, and J ) -76 ( 3 cal/mol for
the |D/hc| ) 0.01026 cm-1 rotamer. Both J -values are
reasonably close to that of the related fluorenyl-bis-
(phenoxy) biradical, 12•• (J ) -65 cal/mol).37,38
Meth yl Nor bor n an e-7-car boxylate.17 In a 250 mL Schlenk
flask, 7-bromonorborane (3.7 g, 21.3 mmol) was dissolved in
10 mL of THF and cooled to -78 °C. To this was added a dark
green solution of LiDBB, formed by stirring DBB (12.5 g, 47
mmol) over excess lithium metal in 50 mL of THF for 3 h.18
During the addition of the LiDBB solution, the reaction
solution was first colorless and then turned red and finally
dark green. The resulting solution was stirred for 15 min at
-78 °C, followed by cannulating over a large excess of freshly
crushed CO2. The resulting light yellow solution was allowed
to warm to room temperature, and the THF was removed. Et2O
was added, and the organic layer extracted twice with 1 M
aqueous NaOH. The combined aqueous extracts were acidified
by the addition of concentrated HCl, followed by extraction in
to Et2O. The resulting organic fractions were combined,
washed once with H2O and once with saturated aqueous NaCl,
and finally dried over Na2SO4. The crude acid was then
esterified by refluxing overnight in acetone with K2CO3 (6 g,
42.7 mmol) and CH3I (9.1 g, 64 mmol, 4 mL). Acetone was then
removed, the mixture was dissolved in Et2O and H2O, and the
resulting organic layer was washed once with 1 M aqueous
NaOH, once with water, and finally with saturated aqueous
NaCl, followed by drying over Na2SO4. Solvent removal and
Kugelrohr distillation (100 °C, 200 mTorr) yielded the ester
as a sweet-smelling oil (2.9 g, 88% yield).
As expected, based on ∆E1/2 values and the lack of the
requisite π-topology for ferromagnetic coupling, J for 5••2-
is more negative than J -values for 1••2--3••2-. Thus, ∆E1/2
for biradical redox couples can be useful for qualitatively
assessing J within a series of isostructural compounds.
Con clu sion s
A series of bis(semiquinone)s have been prepared. The
difference between the first and second redox potentials
has been shown to be useful for qualitatively assessing
exchange coupling within a series of isostructural com-
pounds. The biradicals have zero-field-splitting param-
eters in accord with their structures, and the results of
VT-EPR studies indicate that coupling in 1••2--4••2- is
consistent with the π-topologies of their coupler groups.
Fluorenyl-bis(semiquinone), 5••2-, exists as at least two
rotamers in frozen THF. Intensities of signals from two
rotamers were recorded as a function of temperature.
(35) The Chemistry of Quinonoid Compounds, Vol. II; Berson, J . A.,
Ed.; J ohn Wiley & Sons: New York, 1988; pp 473-489.
(36) Bleaney, B.; Bowers, K. D. Proc. R. Soc. London 1952, A214,
451.
(37) Chandross, E. A.; Kreilick, R. J . Am. Chem. Soc. 1964, 86, 1263.
(38) Kopf, P. W.; Kreilick, R. W. J . Am. Chem. Soc. 1969, 91, 6569.
Bis(5-ter t-bu tyl-3,4-d im eth oxyp h en yl)n or bor n yl Ca r -
bin ol. In a 50 mL Schlenk flask, 4-bromo-6-tert-butylcatechol
dimethyl ether (820 mg, 2.99 mmol) was dissolved in 10 mL
of THF and cooled to -78 °C. tert-Butyllithium (3.9 mL of a
1.5 M solution, 5.9 mmol) was added dropwise, and the light

9468 J . Org. Chem., Vol. 63, No. 25, 1998
Shultz et al.
yellow, turbid solution was stirred for 1 h at -78 °C. A solution
of methyl norbornane-7-carboxylate (210 mg, 1.5 mmol) in 5
mL of THF was then added, and the reaction was stirred
overnight to give a faint yellow solution. The reaction was
quenched with saturated aqueous NH4Cl, transferred to a
separatory funnel, and washed once with saturated aqueous
NaCl. The organic portion was separated and dried over
Na2SO4, and the solvent was removed to yield a thick yellow
oil which was purified by radial chromatography (SiO2, 5%
Et2O:petroleum ether) to give a thick, colorless oil (610 mg,
dissolved in 200 mL each of methanol and Et2O with a catalytic
amount of concentrated HCl (3 drops). This mixture was
refluxed overnight under argon and cooled, and the methanol
was removed at the rotovap. The crude product, a thick yellow
oil, was dissolved in Et2O, transferred to a separatory funnel,
and washed once with saturated aqueous NaCl. The organic
portion was separated, dried over Na2SO4, and the solvent
removed in vacuo. This gave a yellow solid which was purified
by column chromatography (SiO2, 10% Et2O:petroleum ether)
1
to give a white solid (1.38 g, 75% yield). H NMR (CDCl3, 300
1
90% yield). H NMR (C6D6, 300 MHz) δ (ppm): 7.40 (s, 2H),
MHz) δ (ppm): 7.90 (s, 2H), 5.67 (s, 1H), 3.88 (s, 3H), 1.46 (s,
18H). 13C NMR (CDCl3, 75 MHz) δ (ppm): 167.6, 158.1, 135.8,
127.0, 121.2, 51.7, 34.3, 30.2. IR (film from CH2Cl2) ν (cm-1):
3575, 1697. Anal. Calcd for C16H24O3: C, 72.69; H, 9.15.
Found: C, 72.60; H, 9.20.
7.11 (s, 2H), 3.78 (s, 6H), 3.32 (s, 6H), 2.63 (s, 1H), 2.17-2.21
(bm, 4H), 1.78 (s, 1H), 1.64 (d, J ) 6.8 Hz, 2H), 1.51 (s, 18H),
1.23 (d, J ) 7.2 Hz, 2H), 1.14 (d, J ) 7.0 Hz, 2H). 13C NMR
(C6D6, 75 MHz) δ(ppm): 153.5, 147.9, 143.4, 142.1, 117.2,
110.2, 79.7, 60.1, 59.5, 55.5, 38.9, 35.5, 31.7, 31.0, 30.5. IR (film
from CH2Cl2) ν (cm-1): 3617, 3520. Anal. Calcd for C32H46O5:
C, 75.26; H, 9.08. Found: C, 75.34; H, 9.06.
Meth yl 3,5-Di-ter t-bu tyl-4-(tr im eth ylsilyloxy)ben zoa te,
9. A 100 mL Schlenk flask containing distilled diisopropyl-
amine (0.44 mL, 3.10 mmol) and 10 mL of THF was cooled to
-78 °C, and n-BuLi (1.0 mL of a 2.5M solution, 2.54 mmol)
was added. This solution was stirred for 10 min, followed by
the addition of the methyl ester from the previous reaction
(740 mg, 2.8 mmol) as a solution in 15 mL of THF, followed
by stirring while warming to room temperature. Trimethylsilyl
chloride (1.42 mL, 11.2 mmol) was added and the mixture
refluxed overnight. The reaction mixture was cooled and
quenched with 1 mL of H2O, transferred to a separatory
funnel, and sequentially washed once with H2O and saturated
aqueous NaCl, followed with drying over Na2SO4. The organic
layer was separated, and solvent removal gave the crude
product which was purified by column chromatography (Et3N-
deactivated SiO2, 10% Et2O:petroleum ether) to give a white
solid (0.800 g, 85% yield). 1H NMR (CDCl3, 300 MHz) δ
(ppm): 7.96 (s, 2H), 3.88 (s, 3H), 1.42 (s, 18H), 0.42 (9H). 13C
NMR (CDCl3, 75 MHz) δ (ppm): 167.6, 157.7, 141.1, 127.7,
122.1, 51.8, 35.3, 31.3, 3.9. IR (film from CH2Cl2) ν (cm-1):
1714. Anal. Calcd for C19H32O3Si: C, 67.81; H, 9.58. Found:
C, 67.92; H, 9.53.
Bis(5-ter t-bu t yl-3,4-d im et h oxyp h en yl)m et h ylen en or -
bor n a n e. The alcohol from the previous step (500 mg, 0.98
mmol) and p-toluenesulfonic acid hydrate (35 mg, 0.2 mmol)
were added to a 200 mL round-bottom flask with 100 mL of
benzene. The flask was connected to a Dean-Stark trap and
reflux condenser and refluxed with azeotropic removal of water
for 3 h. The reaction was cooled, transferred to a separatory
funnel, and washed once with saturated aqueous NaCl. The
organic portion was collected and dried over Na2SO4, and the
resulting solid was purified by radial chromatography (SiO2,
10% Et2O:petroleum ether) to give a white solid (460 mg, 95%
1
yield). H NMR (CDCl3, 300 MHz) δ (ppm): 6.75 (d, J ) 2.0
Hz, 2H), 6.54 (d, J ) 2.0 Hz, 2H), 3.88 (s, 6H), 3.75 (s, 6H),
2.74 (bm, 2H), 1.77 (d, J ) 6.6 Hz, 4H), 1.44(d, J ) 6.9 Hz,
4H), 1.36 (s, 18H). 13C NMR (CDCl3, 75 MHz) δ(ppm): 152.4,
149.1, 147.0, 141.8, 137.4, 127.5, 120.9, 112.3, 60.4, 55.7, 38.4,
35.1, 30.7, 29.1. Anal. Calcd for C32H44O4: C, 78.00; H, 9.00.
Found: C, 77.87; H, 9.07.
Bis(5-ter t-bu tyl-3,4-ben zoqu in on yl)m eth ylen en or bor -
n a n e, 2. The alkene from the previous reaction (50 mg, 0.1
mmol) was dissolved in 5 mL of CH2Cl2 in a 50 mL round-
bottom flask. The solution was cooled to -78 °C, BBr3 (380
mg, 1.52 mmol, 0.15 mL) was added, and the solution im-
mediately darkened to a deep purple color. The reaction was
then stirred for 12 h, and the bath temperature was kept below
-40 °C (allowing the reaction to warm to room-temperature
resulted in ca. 15% of the desired product and a difficult to
separate reaction mixture). The reaction mixture was poured
onto ice, transferred to a separatory funnel, washed once with
saturated aqueous NaCl (at which point the color dissipated),
dried over Na2SO4, and evaporated to dryness. The resulting
crude catechol, 2-H4, was dissolved in 10 mL of THF and
stirred with Fe´tizon’s reagent (300 mg, ca. 1.1 mmol Ag)
overnight yielding a dark red-green solution which was filtered
and evaporated. The dark red-green solid was purified by
column chromatography (SiO2, methylene chloride) to give 2
as a green solid (35 mg, 81% yield). 1H NMR (CDCl3, 300 MHz)
δ (ppm): 6.63 (d, J ) 2.2 Hz, 2H), 6.21 (d, J ) 2.0 Hz, 2H),
2.83 (s, 2H), 1.81 (d, J ) 7.7 Hz, 4H), 1.62 (d, J ) 7.0 Hz, 4H),
1.27 (s, 18H). 13C NMR (CDCl3, 75 MHz) δ (ppm): 180.1, 179.3,
164.1, 151.2, 150.4, 135.5, 127.6, 123.3, 39.8, 35.7, 29.2, 28.2.
IR (film from THF) ν (cm-1): 1685, 1657, 1615, 1560. Anal.
Calcd for C28H32O4: C, 77.75; H, 7.46. Found: C, 77.63; H,
7.51.
Bis(5′-ter t-bu tyl-3′,4′-bis(m eth oxym eth yloxy)p h en yl)-
3,5-d i-ter t-bu tyl-4-oxop h en ylen em eth a n e. In a 100 mL
Schlenk flask, 4-bromo-6-tert-butylcatechol dimethoxymethyl
ether (1.43 g, 3.75 mmol) was dissolved in 10 mL of THF and
cooled to -78 °C. tert-Butyllithium (5.0 mL of a 1.5 M solution,
7.50 mmol) was added dropwise, and the colorless, turbid
solution was stirred for 1 h at -78 °C. The TMS-protected
phenol from the previous reaction (600 mg, 1.79 mmol) was
then added as a solution in 10 mL of THF, and the reaction
allowed to warm to room temperature and stir overnight. The
next day, 5 mL of a deoxygenated 1 M aqueous NaOH solution
was added, and the reaction mixture was allowed to stir
another 48 h. The color slowly darkened from light yellow to
a deep orange-red of the quinone methide. The reaction was
transferred to a separatory funnel and washed once with satu-
rated aqueous NH4Cl and once with saturated aqueous NaCl,
followed by drying over Na2SO4 and solvent removal. This gave
an orange oil which was purified by radial chromatography
(SiO2, gradient elution; 100% petroleum ether f 20% Et2O:
petroleum ether) to give an orange solid (1.08 g, 84% yield).
1H NMR (CDCl3, 300 MHz) δ (ppm): 7.20 (s, 2H), 6.89 (s, 4H),
5.31 (s, 4H), 5.11 (s, 4H), 3.67 (s, 6H), 3.46 (s, 6H), 1.40 (s,
18H), 1.27 (s, 18H). 13C NMR (CDCl3, 75 MHz) δ (ppm): 186.2,
156.9, 149.4, 147.5, 146.8, 142.5, 135.3, 132.7, 129.2, 126.1,
118.8, 99.3, 95.4, 57.7, 56.5, 35.3, 30.6, 39.7. Anal. Calcd for
Meth yl 3,5-Di-ter t-bu tyl-4-h yd r oxyben zoa te. In a 100
mL Schlenk flask, 4-bromo-2,6-di-tert-butylphenol trimethyl-
silyl ether, 8 (2.5 g, 6.99 mmol), was dissolved in 25 mL of
THF and cooled to -78 °C. tert-Butyllithium (9.4 mL of a 1.5
M solution, 14 mmol) was added dropwise, and the resulting
light yellow solution was stirred for 1 h at -78 °C. This
solution was cannulated onto a large excess of dry ice in a 500
mL two-neck round-bottom flask, and the reaction was allowed
to be warmed to room temperature. The reaction was acidified
with 10 mL of 1 M HCl, transferred to a separatory funnel,
and washed once with saturated aqueous NaCl. The organic
portion was separated, dried over Na2SO4, and the solvent was
evaporated. This yielded the crude acid, which was then
C
43H62O9: C, 71.44; H, 8.65. Found: C, 71.17; H, 8.69.
Bis(5′-ter t-bu tyl-3′,4′-ben zoqu in on yl)-3,5-d i-ter t-bu tyl-
4-oxop h en ylen em eth a n e, 3. The quinone-methide from
above (75 mg, 0.1 mmol) was dissolved in 15 mL of methanol,
and concentrated hydrochloric acid (∼1 drop) was added. The
mixture was allowed to reflux overnight under argon, yielding
a dark red solution. Methanol was removed at the rotovap,
and the crude product was dissolved in Et2O, transferred to a
separatory funnel, washed once with saturated aqueous NaCl,
followed by drying over Na2SO4 and solvent removal, to yield
crude catechol, 3-H4. Oxidation was performed by dissolving
the catechol in 10 mL of THF and stirring with Fe´tizon’s
reagent (400 mg, ca. 1.4 mmol Ag) overnight. The dark red

Synthesis and Characterization of Bis(semiquinone)s
J . Org. Chem., Vol. 63, No. 25, 1998 9469
solution was filtered and the THF removed. The red-orange
solid was purified by column chromatography (SiO2, CH2Cl2)
to give the product, 3, as a red, glassy solid (47 mg, 87% yield).
1H NMR (CDCl3, 300 MHz) δ (ppm): 7.11 (s, 2H), 6.73 (d, J )
2.2 Hz, 2H), 6.37 (d, J ) 2.2 Hz, 2H), 1.29 (s, 36H). 13C NMR
(CDCl3, 75 MHz). δ (ppm): 186.3, 179.5, 178.9, 152.1, 151.4,
149.4, 144.1, 135.8, 135.2, 130.8, 128.6, 36.23, 36.17, 29.75,
29.45. IR (film from THF) ν (cm-1): 1661, 1614, 1565.
in 10 mL of CH2Cl2 in a 50 mL round-bottom flask and cooled
to -78 °C. Boron tribromide (1.4 g, 0.52 mL, 5.5 mmol) was
added in one portion, and the cooling bath was removed. The
reaction was stirred overnight and quenched by pouring onto
ice. The resulting solution was transferred to a separatory
funnel, washed once with saturated aqueous NaCl, separated,
and dried over Na2SO4. The resulting catechol, 5-H4, and
Fe´tizon’s reagent (1.2 mg, ca. 3 mmol Ag) were combined in a
50 mL round-bottom flask with 10 mL of THF, stirred
overnight, and filtered to yield a dark red-brown solution.
Solvent removal yielded a brown solid which was purified by
column chromatography (SiO2, CH2Cl2) to give 5 as an olive-
green solid (130 mg, 79% yield). 1H NMR (d6-acetone, 300
MHz) δ (ppm): 8.10 (d, J ) 7.5 Hz, 2H), 7.76 (d, J ) 7.5 Hz,
2H), 7.64 (dt, J ) 7.5, 2.1 Hz, 2H), 7.53 (dt, J ) 7.6, 1.8 Hz,
2H), 6.84 (d, J ) 2.2 Hz, 2H), 6.22 (d, J ) 2.3 Hz, 2H), 1.17 (s,
18H). 13C NMR (CDCl3, 75 MHz) δ(ppm): 180.3, 179.0, 154.2,
151.7, 142.8, 141.4, 133.6, 130.2, 128.9, 125.7, 125.2, 121.5,
65.3, 35.7, 29.2. IR (film from CH2Cl2) ν (cm-1): 1664, 1626.
HRMS (FAB) Calcd for C33H34O4 (M+ + 4H): 494.2457.
Found: 494.2448.
P r ep a r a tion of th e Bis(sem iqu in on e)s. Bis(semiquinone)
4••2- was prepared as described previously24 by first stirring 4
over a freshly prepared sodium mirror in THF/PMDTA until
the solution was colorless. This was then filtered into a THF/
PMDTA solution containing an equimolar amount of the 4,
resulting in an immediate color change to dark green. Bis-
(semiquinone) 3••2- was prepared by reduction with freshly
prepared sodium anthrasemiquinone, prepared according to
the literature procedure.23 The reduction was accomplished by
addition of 2 equiv of Na+AQ-• to THF/PMDTA to a THF
solution of 3, giving a dark purple solution. Both of these
operations were carried out in the glovebox.
HRMS (FAB) Calcd for
Found: 547.3399.
C
35H47O5(M+ + 5H): 547.3423.
2′-Bip h en ylyl-b is(5′′-ter t-b u t yl-3′′,4′′-d im et h oxyp h en -
yl)ca r bin ol. In a 50 mL Schlenk flask, 4-bromo-6-tert-butyl-
catechol dimethyl ether (1.22 g, 4.46 mmol) was dissolved in
15 mL of THF and cooled to -78 °C. tert-Butyllithium (5.8
mL of a 1.5 M solution in pentane, 8.8 mmol) was added
dropwise, and the colorless suspension was stirred for 1 h at
-78 °C. Methyl 2-phenylbenzoate, 10 (430 mg, 2.03 mmol),
was added as a solution in 5 mL of THF, and the reaction
mixture was warmed and stirred overnight at room temper-
ature. The resulting light yellow turbid solution was quenched
with 1 mL of saturated aqueous NH4Cl and transferred to a
separatory funnel, followed by washing once with saturated
aqueous NaCl. The organic layer was separated, dried over
Na2SO4, and evaportated to dryness. The resulting yellow oil
was purified by radial chromatography (SiO2, 10% Et2O:
petroleum ether) to give the product as a white solid (1.06 g,
1
93% yield). H NMR (d6-acetone, 300 MHz) δ (ppm): 7.31 (td,
J ) 7.4, 1.3 Hz, 1H), 7.24 (td, J ) 7.5, 1.4 Hz, 1H), 7.00-7.13-
(m, 5H), 6.84 (m, 4H), 6.71 (d, J ) 2.0 Hz, 2H), 3.83 (s, 6H),
3.66 (s, 6H), 1.25 (s, 18H). 13C NMR (d6-acetone, 75 MHz) δ
(ppm): 153.4, 148.0, 146.9, 143.8, 143.3, 142.7, 141.6, 133.2,
130.6, 130.1, 127.7, 127.0, 119.8, 111.8, 83.4, 60.5, 56.0, 35.7,
31.0. IR (film from CH2Cl2) ν (cm-1): 3555. Anal. Calcd for
C
37H44O5: C, 78.13; H, 7.79. Found: C, 78.20; H, 7.86.
For CPC reductions, THF solutions of ca. 1-10 mM in bis-
(orthoquinone) and 100 mM in TBAH were reduced at -1.2 V
vs Ag/AgNO3. The resulting solutions, generally dark green
in color, were transferred via syringe to a Schlenk flask (oven-
dried and Ar-purged), subjected to three freeze-pump-thaw
cycles, and transferred to the glovebox for EPR sample
preparation.
9,9-Bis(5′-ter t-bu tyl-3′,4′-d im eth oxyp h en yl)flu or en e. To
the alcohol from the previous step (300 mg, 0.53 mmol) in a
50 mL beaker was added approximately 10 g of polyphosphoric
acid, giving a thick purple gel. The mixture was heated with
stirring to about 90 °C on a hot plate. After 5 min, a white
precipitate was observed, and heating was maintained another
15 min during which time the color changed from purple to
brown with extensive precipatation. The suspension was cooled
and quenched by the addition of 100 mL of ice-water. The
brown suspension was transferred to a separatory funnel and
extracted twice with Et2O to give a yellow solution which was
washed once with saturated aqueous NaCl, separated, dried
over Na2SO4, and evaporated to dryness. The resulting yellow
solid was purified by radial chromatography (SiO2, 5% Et2O:
petroleum ether) to give the product as a white solid (250 mg,
Ack n ow led gm en t. We thank the National Science
Foundation (CHE-9634878) for support of this work.
D.A.S. also thanks the Camille and Henry Dreyfus
Foundation for a Camille Dreyfus Teacher-Scholar
Award. Partial funding for the Mass Spectrometry
Laboratory for Biotechnology Facility was obtained from
the North Carolina Biotechnology Center and the
National Science Foundation (CHE-9111391). We thank
1
86% yield). H NMR (d6-acetone, 300 MHz) δ (ppm): 7.89 (d,
Dr. Kay Sandberg for calculating D-values for 5a ••2-
5c••2-
-
J ) 7.4 Hz, 2H), 7.49 (d, J ) 7.5 Hz, 2H), 7.39 (t, J ) 7.2 Hz,
2H), 7.32 (t, J ) 7.2 Hz, 2H), 6.86 (d, J ) 2 Hz, 2H), 6.61 (d,
J ) 2.1 Hz, 2H), 3.78 (s, 6H), 3.59 (s, 6H), 1.24 (s 18H). 13C
NMR (CDCl3, 75 MHz) δ (ppm): 152.5, 151.9, 147.1, 142.2,
140.1, 127.5, 127.4, 126.2, 120.1, 119.7, 110.8, 65.6, 60.3, 55.7,
35.2, 30.7. Anal. Calcd for C37H42O4: C, 80.69; H, 7.68.
Found: C, 80.58; H, 7.65.
.
Su p p or tin g In for m a tion Ava ila ble: Spectral data (25
pages). This material is contained in libraries on microfiche,
immediately follows this article in the microfilm version of the
journal, and can be ordered from the ACS; see any current
masthead page for ordering information.
9,9-Bis(5′-ter t-bu tyl-3′,4′-ben zoqu in on yl)flu or en e, 5. The
cyclized product from above (200 mg, 0.36 mmol) was dissolved
J O981597S