Redox behaviour of pyrazolyl-substituted 1,4-dihydroxyarenes: Formation of the corresponding semiquinones, quinhydrones and quinones

Article abstract of DOI:10.1515/znb-2006-0304

Pyrazolyl-substituted 1,4-dihydroxybenzene and 1,4-dihydroxynaphthene derivatives have been synthesized by reaction of 1,4-benzoquinone and 1,4-naphthoquinone, respectively, with pyrazole. Cyclovoltammetric measurements have shown that 1,4-benzoquinone po

Full text of DOI:10.1515/znb-2006-0304

Redox Behaviour of Pyrazolyl-Substituted 1,4-Dihydroxyarenes:
Formation of the Corresponding Semiquinones, Quinhydrones
and Quinones
a
a
d
a
d
b
c
Hans-Wolfram Lerner , G u¨ nter Margraf , Tonia Kretz , Olav Schiemann , Jan W. Bats ,
c
a
Gerd D u¨ rner , Fabrizia Fabrizi de Biani , Piero Zanello , Michael Bolte , and
Matthias Wagnera
a
Institut f u¨ r Anorganische Chemie, Johann Wolfgang Goethe-Universit a¨ t Frankfurt am Main,
Marie-Curie-Straße 11, D-60439 Frankfurt am Main, Germany
Institut f u¨ r Physikalische Chemie, Johann Wolfgang Goethe-Universit a¨ t Frankfurt am Main,
b
Marie-Curie-Straße 11, D-60439 Frankfurt am Main, Germany
Institut f u¨ r Organische Chemie, Johann Wolfgang Goethe-Universit a¨ t Frankfurt am Main,
c
Marie-Curie-Straße 11, D-60439 Frankfurt am Main, Germany
Dipartimento di Chimica dell’Universita, Via Aldo Moro, I-53100 Siena, Italy
d
Reprint requests to Dr. Hans-Wolfram Lerner. E-mail: lerner@chemie.uni-frankfurt.de
Z. Naturforsch. 61b, 252 – 264 (2006); received November 25, 2005
Pyrazolyl-substituted 1,4-dihydroxybenzene and 1,4-dihydroxynaphthene derivatives have been
synthesized by reaction of 1,4-benzoquinone and 1,4-naphthoquinone, respectively, with pyrazole.
Cyclovoltammetric measurements have shown that 1,4-benzoquinone possesses the potential to ox-
idize 2-(pyrazol-1-yl)- and 2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene. The 2,5-bis(pyrazol-1-yl)-
1
,4-dihydroxybenzene reacts with air to give quantitatively black insoluble 2,5-bis(pyrazol-1-yl)-1,4-
quinhydrone. Black crystals of 2,5-bis(pyrazol-1-yl)-1,4-quinhydrone suitable for X-ray diffraction
were grown from methanol at ambient temperature (monoclinic C2/c). The poor yields of pyrazolyl-
substituted 1,4-dihydroxybenzene and 1,4-dihydroxynaphthene derivatives can be explained by the
formation of insoluble black quinhydrons in the reaction of benzoquinone and naphthoquinone with
pyrazole. The dianions of 2-(pyrazol-1-yl)- and 2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene react
with oxygen to give the corresponding semiquinone anions. 2,5-Bis(pyrazol-1-yl)-1,4-benzoquinone
shows two reversible one-electron reduction processes in cyclovoltammetric measurements, whereas
pyrazolyl-substituted 1,4-dihdroxybenzene and -naphthene derivatives undergo irreversibile electron-
transfer processes.
Key words: Quinhydrone, Semiquinone, Hydroquinone, Redoxactive Ligands, Crystal Structure
Introduction
Redox-active ligands can be used to influence the
electrochemical reactivity of transition metals since
their redox activity is expanded upon complexation.
The resulting complexes can undergo multi-electron
transfer reactions which are the sum of the oxidation
state changes of the metal center and the ligand [1, 2].
Due to their electrochemical reversibility, quinone
derivatives are candidates for redox-active ligands.
Scheme 1. Oxidation of hydroquinone.
Owing to their rigid structure and completely conju-
Oxidation of hydroquinone and reduction of benzo- gated π-system, hydroquinone ligands can be expected
quinone derivatives are known to play an important to contribute efficiently to the spin-spin couplings
role in biological redox processes [3]. The oxidation of between paramagnetic metal ions. In addition, the
2
−
the dianion of hydroquinone 1 to benzoquinone 1ox two diamagnetic compounds hydroquinone and benzo-
occurs in two one-electron steps via the semiquinone quinone as well as the paramagnetic semiquinone an-
−
radical anion 1sq (Scheme 1). However, this process ion are able to interact with the orbitals of transition
depends strongly on the pH value.
metals to different extents.
0
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H.-W. Lerner et al. · Redox Behaviour of Pyrazolyl-Substituted 1,4-Dihydroxyarenes
253
Fig. 1. Dinuclear complex A and coordi-
nation polymer B.
Scheme 2. Formation of compounds 2, 3, and 4. i = iii: 1,4-addition of pyrazole; ii = oxidation with 1,4-benzoquinone,
elimination of hydroquinone.
By adding additional coordination sites, para- azole were first described by Gauß [15]. As reported
quinones can be derivatized to generate bidentate lig- previously, pyrazole adds to 1,4-benzoquinone to give
ands. Promising members of this family of com- a mixture of mono-pyrazolyl adduct 2 and bis-pyrazol-
pounds are the derivatives of (pyrazol-1-yl)-1,4- di- yl adducts 3 and 4 (Scheme 2). Ballesteros and cowork-
hydroxybenzene[4 – 7]. Therefore, we investigated the ers found that the relative amount of products thereby
coordination behaviour of hydroquinone derivatives depends strongly on the reaction time [16].
II
with chelating pyrazolyl anchor groups towards Cu
ions. We have established 2,5-bis(pyrazol-1-yl)-1,4-
dihydroxybenzene as a redox-active bridging unit in
In contrast to the results of earlier studies, investiga-
tions of our group have shown that a reaction between
benzoquinone and hydroquinone 3 takes place to give
quinone 3ox and a black insoluble material. The addi-
tion of benzoquinone to pyrazole also produces a black
precipitate. However, the solution of this reaction con-
tains the hydroquinones 1, 2, 3, and 4. These hydro-
quinones have been identified by analytical HPLC. The
stoichiometry of the reactants benzoquinone and pyra-
zole does not play an important role in product ratio as
the reaction time does. However, under all the applied
reaction conditions the yields of the hydroquinones 2,
II
dinuclear and polynuclear Cu complexes A and B
(Fig. 1) [8 – 13].
However, when 2-(pyrazol-1-yl)-1,4-dihydroxy-
naphthalene is reacted with CuCl2 and lithium
bis(trimethylsilyl)amide, no formation of Cu com-
plexes is observed. Instead, a redox reaction takes
place in which [Cu (NH )Cl] and 2-(pyrazol-1-yl)-
,4-naphtoquinone are formed [14]. It may thus
be concluded that 2-(pyrazol-1-yl)-1,4-dihydroxy-
naphthalene not only acts as a proton source in the
protolysis of lithium bis(trimethylsilyl)amide, but is
II
I
3
1
3
and 4 were quite low, which can be explained with
the formation of the insoluble black quinhydrone 3qh.
The X-ray powder diffraction studies of this insoluble
material have shown exclusively the pattern of 3qh.
It is interesting to note that if this reaction is carried
out under inert gas, the sum of the yields of pyrazolyl-
substituted hydroquinones is higher than the yields if
benzoquinone reacts with pyrazole in air. We were in-
terested in the preparation of hydroquinone 3 in large
quantities. We therefore have optimized the reaction
conditions to produce 3 in higher yield. The best re-
sult was obtained when the reaction was carried out
II
also involved in the reduction of the Cu centers. The
purpose of the following paper is to investigate the
electrochemical behaviour and the solid-state struc-
tures of several (pyrazol-1-yl)-1,4-dihydroxybenzene
and (pyrazol-1-yl)-1,4-dihydroxynaphthalene deriva-
tives. Finally we report on the crystal structure and the
properties of 2,5-(bispyrazol-1-yl)-1,4-quinhydrone.
Results and Discussion
Syntheses
The reactions of 1,4-benzoquinone and 1,4- in an inert gas atmosphere (e.g. nitrogen) with a ra-
naphthoquinone with derivatives of pyrazole and tri- tio of benzoquinone to pyrazole of 1 : 1 in hot dioxane
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H.-W. Lerner et al. · Redox Behaviour of Pyrazolyl-Substituted 1,4-Dihydroxyarenes
Scheme 3. Synthesis of the semiquinone
−
−
radical anions 2sq and 3sq .
Scheme 4. Synthesis of the quinhydrone 2qh and 3qh.
by oxidation of 3 with 1,4-benzoquinone and of 5
or 7 with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ). 6ox has been prepared from 2,3-dichloro-1,4-
naphthoquinone and pyrazole according to a literature
procedure [15].
The hydroquinones 2, 3, 4, and 5 can be deproto-
nated twice with 2 equivalents of a strong base such
as KOtBu. The resulting dianions show a pronounced
sensitivity towards oxidation. The EPR spectra of iso-
Fig. 2. The hydroquinone derivatives 5 and 7.
2
−
2−
and 3 in the
propanol solutions of dianionic 2
presence of traces of oxygen show signals which can
−
−
be assigned to the semiquinone anions 2sq and 3sq
Scheme 3).
Generally, semiquinone radical anions are only sta-
(
ble in solutions with a high pH value. In less al-
kaline solutions, a disproportionation reaction takes
place from semiquinone to hydroquinone and quinone,
as shown in Scheme 4. Equimolar mixtures of hy-
droquinone and benzoquinone are known to form
intensively colored quinhydrones. These complexes
show a charge transfer between the electron-rich hy-
droquinone and the electron-attracting benzoquinone
Fig. 3. The quinone derivatives 3ox, 5ox, 6ox and 7ox.
(Scheme 4).
(
yield 30%). The 1,4-hydroxybenzene derivatives 2, 3,
Upon standing in methanol under atmospheric con-
ditions, the hydroquinone 3 is oxidized quantita-
tively to 2,5-bis(pyrazol-1-yl)-1,4-quinhydrone 3qh.
and 4 have been separated by preparative HPLC and
by crystallisation.
The 1,4-naphtohydroquinone 5 and the 1,4- Black crystals of the quinhydrone 3qh suitable for
hydroquinone7 have been obtained following a similar X-ray diffraction were obtained from methanol at
synthesis protocol as described above for the prepara- ambient temperature. It is interesting to note that
tion of 2, 3, and 4. The syntheses of the quinone deriv- the quinhydrone 3qh is nearly insoluble in organic
atives 3ox, 5ox, and 7ox were conveniently achieved solvents.
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H.-W. Lerner et al. · Redox Behaviour of Pyrazolyl-Substituted 1,4-Dihydroxyarenes
255
Scheme 5. Reduction of the quinone 3ox.
single two-electron oxidation at +1.25 V vs. S.C.E. in
CH Cl .
2
2
In contrast to the hydroquinone derivatives 2, 3,
and 5, the quinone 3ox shows two reversible one-
electron reduction processes. The reduction of 3ox can
be explained with the formation of the semiquinone an-
Fig. 4. Observed EPR spectra of the semiquinone an-
ions 3sq (left) and 2sq (right).
−
−
−
−
EPR spectra of the semiquinone anions 2sq and 3sq
−
2−
ion 3sq and the hydroquinone dianion 3 as shown
in Scheme 5.
Solid-state EPR spectroscopy has revealed the
diamagnetic nature of 2,5-bis(pyrazol-1-yl)-1,4-quin-
hydrone 3qh. The nature of this quinhydrone can also
be observed in its IR spectrum, which is a superposi-
tion of the individual IR spectra of 3 and 3ox. Sim-
ilar IR patterns were observed for the parent com-
Structures
The molecular structures of the compounds 3qh, 4,
5, 5ox, 6ox, and 7ox are shown in Figs 5 – 16. Selected
bond lengths and angles are listed in the corresponding
figure captions, details of the crystal structure analyses
are summarized in Table 3.
−
pound 1qh [17]. In the EPR spectrum of 2sq a four-
−
line signal is observed, whereas 3sq shows a three-
line signal (Fig. 4).
The crystal structure determinations of 2, 3, 3ox,
The multiplet splitting of the signals of the semi-
and 4 · (H O) have already been reported [4, 18, 19].
2
−
−
quinone anions 2sq and 3sq is caused by the cou-
We obtained single crystals suitable for X-ray diffrac-
tion of 2 from chloroform and carried out a redetermi-
nation of the structure at low temperature [20].
pling of an unpaired electron to protons attached to the
−
−
central benzene ring {2sq : 3 H; (3sq ): 2 H}. Cou-
plings with pyrazolyl H substituents are not resolved
for either semiquinone.
The quinhydrone 3qh crystallizes in the monoclinic
space group C2/c with a crystallographically imposed
Electrochemistry
The electrochemical behaviour of 2, 3, 3ox,
and 5 has been investigated by cyclovoltammetry in
dichloromethane solution. The cyclovoltammetric pro-
files exhibited by 2, 3, and 5 relate to oxidation
processes; as expected, all of them possess features of
chemical irreversibility on the cyclovoltammetric time
scale (Scheme 4). The processes for 3 and 5 are all mo-
noelectronic, whereas 2 shows a two-electron process.
The unsubstituted parent hydroquinone undergoes a
Fig. 5. Thermal ellipsoid plot of quinhydrone 3qh showing
the atom numbering scheme. The displacement ellipsoids are
˚
drawn at the 50% probability level. Selected bond lengths [A]
Table 1. Formal electrode potentials (in V, vs SCE) for the
oxidation processes of 2, 3, and 5 and for the reduction
processes of 3ox in dichloromethane solutions.
◦
and torsion angles [ ]: C(1)–C(2) 1.390(6), C(2)–C(3)
1
.351(6), C(4)–C(5) 1.381(5), C(4)–C(6) 1.446(5),
C(5)–C(6)#1 1.440(5), C(6)–O(1) 1.309(5), C(1)–N(2)
1.374(5), C(3)–N(1) 1.365(4), C(4)–N(1) 1.422(4),
Oxidation
Reduction
2
3
3
5
+0.94*, +0.94*
+0.90*, +1.10*
N(1)–N(2) 1.378(5), C(6)–C(4)–N(1)–N(2) −18.1(6).
Hydrogen bonds [ A˚ ]: O(1)–H(10) 0.84, H(10)···N(2)
◦
ox
−0.29, −0.87
1.95, O(1)···N(2) 2.704(4), O(1)–H(10)···N(2) 149.2 .
+0.75*, +0.95*
Symmetry transformations used to generate equivalent
*
Irreversible process.
atoms: #1 −x+1, −y+1, −z+1.
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H.-W. Lerner et al. · Redox Behaviour of Pyrazolyl-Substituted 1,4-Dihydroxyarenes
˚
Table 2. Selected bond lengths in [A] in the solid state
of 3 [4], 3qh and 3ox [18].
3
3qh
3ox
C(4)–C(5)
C(5)–C(6)
C(4)–C(6)
C(6)–O(1)
N(1)–C(4)
C(1)–N(2)
C(3)–N(1)
C(1)–C(2)
C(2)–C(3)
1.383
1.383
1.396
1.361
1.425
1.332
1.364
1.382
1.361
1.381
1.440
1.446
1.309
1.422
1.365
1.365
1.390
1.351
1.335
1.467
1.508
1.221
1.409
1.324
1.362
1.399
1.364
Fig. 7. Packing of the molecules of 3qh. Projection along the
b-axis.
ent. The molecular planes in 3qh are shifted against
each other in a stair-like manner. Remarkably, the IR
spectrum of 3qh as well as of 1qh features the charac-
teristic vibrations of the hydroquinones 3 and 1 and the
quinones 3ox and 1ox, but the crystal structures of 3qh
and 1qh show only one type of molecule. The distance
˚
in 3qh between the molecular planes is about 3.2 A and
thus in a range typical for charge transfer complexes
between two molecules.
The molecular structure of hydroquinone 4 (mon-
Fig. 6. Packing of the molecules of 3qh in the unit cell.
oclinic space group P2 /c is shown in Fig. 8.
1
inversion center at the midpoint of the benzene ring Due to steric repulsion, the two pyrazole rings are
(
Figs 5, 6, and 7). The C–C bond lengths of the central twisted off the hydroquinone plane with torsion
◦
C -ring and the C-N and C–O distances at this ring angles C(4)–C(3)–N(31)–N(32) of −28.81(16)
6
◦
are in between the values found in the structures of 3 and C(1)–C(2)–N(21)–N(22) of 113.47(13) . As
and 3ox (Table 2).
shown in Fig. 9, the hydrogen bond network in the
The pyrazolyl rings in 3qh are slightly twisted off solid-state structure of 4 is quite different from that
the hydroquinone plane with a torsion angle N(2)– in 4 · (H2O). One intramolecular hydrogen bond per
◦
N(1)–C(4)–C(6) of −18.1(6) . In the crystals of the molecule is established in 4 between O(4) and N(32)
˚
˚
quinhydrone 1qh, the hydrogen bond formation leads [O(4)–H(4) = 0.92(2) A, H(4)···N(32) = 1.79(2) A;
◦
to an infinite molecular chain along the molecular O(4)–H(4)···N(32)
=
148(2) ]. Furthermore,
axis [21]. In the crystals of 3qh, by contrast, the an intermolecular hydrogen bond occurs be-
˚
molecules form columns in which the benzoquinone tween O(1) and N(22)#1 [O(1)–H(1) = 0.88(2) A,
˚
rings are stacked with hydrogen bonds between the H(1)···N(22)#1 = 1.86(2) A; O(1)–H(4)···N(22)#1 =
◦
oxygen atoms of the two neighboring benzoquinone 157.8(19) ].
units (Fig. 6). In the solid-state structures of both quin-
2-(Pyrazol-1-yl)-1,4-dihydroxynaphthalene
(5)
hydrone 1qh [21] and 3qh intermolecular hydrogen crystallizes with two independent molecules in the
˚
bonds are established [3qh: O(1)–H(10) = 0.84 A, asymmetric unit in the triclinic space group P1
˚
◦
H(10)···N(2) = 1.95 A; O(1)–H(10)···N(2) = 149.2 ], (Fig. 10 represents one of two molecules of 5;
but the connectivity into infinite arrays is quite differ- in the caption bond lengths and angles of both
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H.-W. Lerner et al. · Redox Behaviour of Pyrazolyl-Substituted 1,4-Dihydroxyarenes
257
Fig. 8. Thermal ellipsoid plot of 2,3-bis(pyrazol-1-
yl)-1,4-dihydroxybenzene 4. The displacement ellip-
Fig. 9. Packing of the 2,3-bis(pyrazol-1-yl)-1,4-dihydroxy-
benzene molecules 4.
soids are drawn at the 50% probability level. Selected
˚
◦
bond lengths [A] and torsion angles [ ]: C(1)–C(2) morph of 5ox. The structure of orthorhombic poly-
.3980(17), C(2)–C(3) 1.4022(18), C(3)–C(4) 1.4053(18),
C(4)–C(5) 1.3953(18), C(5)–C(6) 1.386(2), C(1)–O(1)
.3547(16), C(4)–O(4) 1.3691(16), C(2)–N(21) 1.4231(16),
N(21)–N(22) 1.3616(14), N(21)–C(25) 1.3562(17),
1
morph of 5ox features two independent molecules
(
Fig. 12), which possess angles between the pyra-
1
◦
zole plane and the naphthoquinone plane of 18.8
◦
N(22)–C(23) 1.3323(17), C(23)–C(24) 1.4018(18), and 20.6 , respectively. In the quinone fragment of
C(24)–C(25)
1.3711(19),
C(3)–N(31)
1.4275(15), the orthorhombic polymorph of 5ox the C(4)–C(5)
N(31)–N(32) 1.3709(15), N(31)–C(35) 1.3607(16),
˚
and C(17)–C(18) bonds are about 0.04 A shorter
N(32)–C(33)
C(34)–C(35)
1.3340(18),
1.3745(18),
C(33)–C(34)
C(1)–C(2)–N(21)–N(22)
1.399(2),
than the C(2)–C3 and C(15)–C(16) bonds. This
may result from resonance in the segments O(2)–
1
13.47(13), C(4)–C(3)–N(31)–N(32) −28.81(16). Hy-
˚
drogen bonds [A]: O(1)–H(1) 0.88(2), H(1)···N(22)#1 C(5)–C(4)–C(3)–N(1) and O(4)–C(18)–C(17)–C(16)–
1
1
.86(2), O(1)···N(22)#1 2.6976(14), O(1)–H(1)···N(22)#1
N(3). Each molecule shows two intramolecular con-
tacts which approach the van der Waals contact dis-
◦
57.8(19) ; O(4)–H(4) 0.92(2), H(4)···N(32) 1.79(2),
◦
O(4)···N(32) 2.6142(15), O(4)–H(4)···N(32) 148(2) .
Symmetry transformations used to generate equivalent
atoms: #1 x, −y+3/2, z+1/2.
˚
˚
tances [H(4)···N(2): 2.43 A, H(13)···O(1): 2.40 A,
˚
˚
H(17)···N(4): 2.44 A, H(26)···O(3): 2.36 A]. As
shown in Fig. 13, stacks of molecules are found in the
molecules are listed). Intramolecular hydrogen crystallographic b-direction. Molecules within each
bonds are established between O(2) and N(12) stack are connected by a number of π ···π-contacts.
˚
˚
[
O(2)–H(2) = 0.84 A, H(2)···N(12) = 1.81 A; The packing of the molecules also shows three inter-
◦
O(2)–H(2)···N(12) = 146.6 ]. Due to this hydrogen molecular C–H···O interactions with H···O distances
bond, the pyrazole and naphthoquinone planes in 5 between 2.39 and 2.47 A˚ and C–H···O angles be-
◦
are nearly coplanar with a torsion angle C(2)–C(1)– tween 144 and 147 .
◦
N(11)–N(12) of −4.5(9) . In addition, intermolecular
2,3-Bis(pyrazol-1-yl)-1,4-naphthoquinone
(6ox)
hydrogen bonds are found between O(9) and O(2A)#1 crystallizes in the orthorhombic space group Pccn
˚
˚
[
O(9)–H(9) = 0.84 A, H(9)···O(2A)#1 = 2.05 A; (Fig. 14). Due to steric repulsion, the two pyra-
◦
O(9)–H(9)···O(2A)#1 = 168.1 ].
zole rings are twisted off the naphthoquinone
2
-(Pyrazol-1-yl)-1,4-naphtoquinone (5ox) was ob- plane with torsion angles C(1)–C(2)–N(21)–N(22)
◦
tained in two different polymorphs, one in the mon- of −128.75(10) and C(4)–C(3)–N(31)–N(32) of
◦
oclinic space group C2/c, Fig. 11, the other one in −141.32(10) .
the orthorhombic space group Pna2 , Fig. 12. The
The molecular structure of 7ox is shown in Fig. 15.
1
angle between the pyrazole plane and the naphtho- The compound crystallizes in the monoclinic space
◦
quinone plane is 20.7 in the monoclinic poly- group C2/c. The two Br-substituted pyrazole rings are
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H.-W. Lerner et al. · Redox Behaviour of Pyrazolyl-Substituted 1,4-Dihydroxyarenes
Fig. 11. Thermal ellipsoid plot of 2-(pyrazol-1-yl)-
1
,4-naphthoquinone 5ox (monoclinic polymorph). The
Fig. 10. Thermal ellipsoid plot of one from two inde-
pendent molecules in the asymmetric unit of 2-(pyrazol-
displacement ellipsoids are drawn at the 50% probability
level. Selected bond lengths [A] and torsion angles [ ]:
C(1)–C(2) 1.511(10), C(1)–C(10) 1.358(10), C(2)–C(3)
˚
◦
1
-yl)-1,4-dihydroxynaphthalene 5. The displacement
ellipsoids are drawn at the 50% probability level. Se-
1
.509(10), C(3)–C(4) 1.407(10), C(3)–C(8) 1.420(9),
C(4)–C(5) 1.414(10), C(5)–C(6) 1.402(10), C(6)–C(7)
.405(10), C(7)–C(8) 1.418(10), C(8)–C(9) 1.510(10),
C(9)–C(10) 1.490(10), C(1)–N(11) 1.447(9), C(2)–O(2)
.235(9), C(9)–O(9) 1.252(8), N(11)–N(12) 1.386(8),
N(11)–C(15) 1.384(9), N(12)–C(13) 1.364(9), C(13)–C(14)
˚
◦
lected bond lengths [A] and torsion angles [ ]: C(1)–C(2)
.383(9), C(1)–C(10) 1.398(9), C(2)–C(3) 1.418(8),
C(3)–C(4) 1.420(9), C(3)–C(8) 1.417(9), C(4)–C(5)
.357(9), C(5)–C(6) 1.438(9), C(6)–C(7) 1.352(9),
C(7)–C(8) 1.431(8), C(8)–C(9) 1.448(9), C(9)–C(10)
.365(9), C(1)–N(11) 1.441(8), C(2)–O(2) 1.368(7),
C(9)–O(9) 1.377(7), N(11)–N(12) 1.347(8), N(11)–C(15)
.360(8), N(12)–C(13) 1.307(9), C(13)–C(14) 1.394(10),
1
1
1
1
1
1
–
.439(10), C(14)–C(15) 1.365(11), C(2)–C(1)–N(11)–N(12)
159.2(6).
1
C(14)–C(15) 1.390(9), C(2)–C(1)–N(11)–N(12) −4.5(9).
˚
pyrazolyl nitrogen atoms. As shown in Fig. 16 the
Hydrogen bonds [A]: O(2)–H(2) 0.84, H(2)···N(12)
◦
1
.81, O(2)···N(12) 2.553(7), O(2)–H(2)···N(12) 146.6 ; quinone 7ox forms a layer structure in the solid state. It
O(9)–H(9) 0.84, H(9)···O(2A)#1 2.05, O(9)···O(2A)#1 is interesting to note that the molecular structure of 7ox
◦
2
.876(7), O(9)–H(9)···O(2A)#1 168.1 . Selected bond
is quite different from that of 3ox. In the quinone
derivative 3ox [18] the pyrazolyl rings are turned al-
˚
◦
lengths [A] and torsion angles [ ] of the second mole-
cule: C(1A)–C(2A) 1.391(9), C(1A)–C(10A) 1.393(9),
◦
most 180 about the axis.
C(1A)–N(11A)
C(3A)–C(4A)
C(4A)–C(5A)
C(6A)–C(7A)
C(8A)–C(9A)
C(1A)–N(11A)
1.436(8),
1.430(9),
1.362(10),
1.347(10),
1.453(9),
1.436(8),
C(2A)–C(3A)
C(3A)–C(8A)
C(5A)–C(6A)
C(7A)–C(8A)
C(9A)–C(10A)
C(2A)–O(2A)
1.453(9),
1.396(10),
1.392(10),
1.431(10),
1.376(9),
1.339(8),
1.342(8),
Conclusion
In summary, cyclovoltammetric measurements have
shown that 1,4-benzoquinone possesses a higher
C(9A)–O(9A) 1.376(8), N(11A)–N(12A)
N(11A)–C(15A) 1.365(8), N(12A)–C(13A) 1.338(9), oxidation potential than the pyrazolyl-substituted
C(13A)–C(14A) 1.373(10), C(14A)–C(15A) 1.372(10),
quinones 2ox, 3ox, and 5ox and is therefore able to
oxidize the pyrazolyl-substituted hydroquinones 2, 3,
and 5. Moreover, the hydroquinone 3 reacts with air
C(2A)–C(1A)–N(11A)–N(12A)
−1.4(8).
Hydrogen
˚
bonds [A]: O(2A)–H(2A) 0.84, H(2A)···N(12A) 1.83,
◦
O(2A)···N(12A) 2.568(7), O(2A)–H(2A)···N(12A) 145.0 ;
O(9A)–H(9A) 0.84, H(9A)···O(2)#2 1.96, O(9A)···O(2)#2 to give the black insoluble quinhydrone 3qh quanti-
◦
2
.798(7), O(9A)–H(9A)···O(2)#2 171.9 . Symmetry trans- tatively. In the syntheses of the hydroquinone deriva-
formations used to generate equivalent atoms: #2 x + 1,
−
tives 2, 3, 4, and 5 the corresponding quinhydrones are
generated as side products. The formation of these in-
soluble black quinhydrone derivatives in the reaction
of 1,4-benzoquinone or 1,4-naphtoquinone with pyra-
zole can explain the poor yields of 2, 3, 4, and 5. The
syntheses of the quinone derivatives 3ox, 5ox, and 7ox
were conveniently achieved by oxidation of 3 with
y+1, −z+2.
almost in a common plane as depicted in Fig. 16.
A structural motif similar to that of the quinone 7ox
has also been found for the hydroquinone deriv-
ative 3 [4, 19]. However, the hydroquinone 3 dis-
plays intramolecular hydrogen bonds between the
OH groups of the hydroquinone fragments and the
1,4-benzoquinone and of 5 or 7 with 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone.
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H.-W. Lerner et al. · Redox Behaviour of Pyrazolyl-Substituted 1,4-Dihydroxyarenes
259
Fig. 12. Thermal ellipsoid plot of 2-(pyrazol-1-yl)-1,4-
naphthoquinone 5ox (orthorhombic polymorph). The
displacement ellipsoids are drawn at the 50% probability
˚
◦
level. Selected bond lengths [A] and torsion angles [ ]:
C(1)–C(2) 1.490(3), C(1)–C(6) 1.399(3), C(1)–C(10)
1
.388(3), C(2)–C(3) 1.492(3), C(3)–C(4) 1.345(3),
C(4)–C(5) 1.454(3), C(5)–C(6) 1.483(3), C(6)–C(7)
.391(3), C(7)–C(8) 1.383(3), C(8)–C(9) 1.387(3),
C(9)–C(10) 1.389(3), C(3)–N(1) 1.408(3), C(2)–O(1)
.229(3), C(5)–O(2) 1.227(2), N(1)–N(2) 1.372(2),
N(1)–C(13) 1.374(3), N(2)–C(11) 1.321(3), C(12)–C(11)
.406(3), C(12)–C(13) 1.361(3), C(14)–C(15) 1.493(3),
C(14)–C(19) 1.388(3), C(14)–C(23) 1.397(3), C(14)–C(15)
.493(3), C(15)–C(16) 1.505(3), C(16)–C(17) 1.339(3),
C(17)–C(18) 1.464(3), C(18)–C(19) 1.484(3), C(19)–C(20)
.398(3), C(20)–C(21) 1.384(3), C(21)–C(22) 1.385(3),
C(22)–C(23) 1.384(3), C(16)–N(3) 1.410(3), C(15)–O(3)
1
Fig. 13. Packing of the 5ox molecules (orthorhombic poly-
morph) in the unit cell.
1
1
Method C: Reaction of one equivalent of 1,4-benzo-
quinone and one equivalent of pyrazole in a nitrogen at-
mosphere.
General protocol: 1,4-Benzoquinone and pyrazole were
heated in dioxane under reflux for 1 h. The hot reaction mix-
1
1
1
.216(3), C(18)–O(4) 1.234(3), N(3)–N(4) 1.374(2), tures were filtered and the filtrates were taken to dryness in
N(3)–C(26) 1.367(3), N(4)–C(24) 1.325(3), C(24)–C(25) vacuo.
1
1
.398(3), C(25)–C(26) 1.361(3), N(2)–N(1)–C(3)–C(2)
61.28(18), N(4)–N(3)–C(16)–C(15) 162.02(19).
The relative amounts of 1 – 4 in these reaction mix-
tures were determined by analytical HPLC (Merck C ;
1
8
Merck Hitachi L4000A UV detector, λ = 254 nm; flow
−1
Experimental Section
rate: 0.8 ml min ) with gradient elution (0.1 M trifluo-
roacetic acid / methanol). 2 mg of each of the dried filtrates
were redissolved in 2 ml 0.1 M trifluoroacetic acid / methanol
and used for analysis.
Reagents and solvents were obtained from Aldrich Chem-
icals. NMR spectra were run at ambient temperature. NMR:
Bruker AMX 250, Bruker DPX 250, Bruker AMX 400 spec-
trometers. Abbreviations: s = singlet; d = doublet; tr = triplet;
Preparation of 2, 3, and 4: Pyrazole (12.94 g, 190 mmol)
and 1,4-benzoquinone (20.54 g, 190 mmol) were heated in
dioxane for one hour in an atmosphere of nitrogen. The hot
solution was filtered, the filtrate was taken to dryness in
vacuo. The residue was dissolved in a minimum amount of
1
vtr = virtual triplet; nr = multiplet expected in the H NMR
spectrum, but not resolved; pz = pyrazolyl; hqui = hydro-
quinone. Infrared spectra were taken of solid samples in KBr
on a Nicolet Magna IR 550 spectrometer. ESI-MS: Fisons
CHCl . Addition of ethanol led to precipitation of 3. The
3
(now Micromass) VG Platform II. Elemental analyses (car-
mother liquor was taken to dryness in vacuo and 2 and 4 were
separated with preparative HPLC. Recrystallisation (CHCl3)
of the crude material afforded compound 3 (30%) as color-
less needles.
ried out at the Institut f u¨ r Organische Chemie, Universit a¨ t
Frankfurt): Foss-Heraeus CHN-O-Rapid.
Addition of pyrazole to benzoquinone
For quantitative separations, the dried filtrate was
redissolved and separated by HPLC (Nucleoprep,
The reaction protocol was performed under different con-
ditions:
6
50 mm × 50 mm, 20 µm, Macherey-Nagel, Ger-
many; Merck Hitachi L4000A UV detector, λ = 254 nm;
SepTech Refractive Index Monitor) with isocratic elution.
A typical separation was performed with a flow rate
Method A: Reaction of one equivalent of 1,4-benzo-
quinone and two equivalents of pyrazole in air.
−1
Method B: Reaction of one equivalent of 1,4-benzo- of 0.1 l min , using a three-solvent system (hexane/
quinone and one equivalent of pyrazole in air. ethylacetate/dichloromethane 1 : 1 : 2).
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H.-W. Lerner et al. · Redox Behaviour of Pyrazolyl-Substituted 1,4-Dihydroxyarenes
Fig. 14. Thermal ellipsoid plot of 2-(Pyrazol-1-yl)-1,4-
naphthoquinone 6ox. The displacement ellipsoids are drawn
˚
at the 50% probability level. Selected bond lengths [A]
◦
and torsion angles [ ]: C(1)–C(2) 1.4941(15), C(1)–C(10)
1
.4838(15), C(2)–C(3) 1.3568(15), C(3)–C(4) 1.5062(15),
C(4)–C(5) 1.4851(15), C(5)–C(6) 1.3963(15), C(5)–C(10)
.3971(16), C(6)–C(7) 1.3876(17), C(7)–C(8) 1.3891(19),
C(8)–C(9) 1.3896(17), C(9)–C(10) 1.3951(16), C(1)–O(1)
Fig. 16. Packing of the 7ox molecules in the unit cell.
2-(Pyrazol-1-yl)-1,4-dihydroxybenzene (2): 1H NMR
1
1
.2208(14), C(4)–O(4) 1.2180(14), C(2)–N(21) 1.4105(14), (250 MHz, CDCl ): δ = 4.68 (s, 1 H), 6.48 (dd, J = 2.25 Hz,
3
N(21)–N(22) 1.3685(13), N(21)–C(25) 1.3610(14), 1 H), 6.64 (dd, J = 8.75 Hz, 1 H), 6.90 (d, J = 2.5 Hz, 1 H),
N(22)–C(23) 1.3238(15), C(23)–C(24) 1.4057(19),
C(24)–C(25) 1.3627(17), C(3)–N(31) 1.4005(14),
N(31)–N(32) 1.3756(13), N(31)–C(35) 1.3735(15),
N(32)–C(33) 1.3208(16),
C(34)–C(35) 1.3579(18),
6
.95 (d, J = 3.75 Hz, 1 H), 7.71 (d, J = 1.25 Hz, 1 H), 7.92
(
d, J = 2.0 Hz, 1 H), 10.86 (s, 1 H). HPLC: τ = 14.57 min.
+
MS (ESI): m/z = 176 [M ]. C H N O (176.17): calcd.
9
8 2 2
C(33)–C(34) 1.4069(19),
C(1)–C(2)–N(21)–N(22)
128.75(10); C(4)–C(3)–N(31)–N(32) −141.32(10).
C 61.36, H 4.58, N 15.90; found C 61.61, H 4.48, N 15.62.
−
2,5-Bis(pyrazol-1-yl)-1,4-dihydroxybenzene (3): 1H
NMR (250 MHz, CDCl ): δ = 6.50 (dd, J = 2.0 Hz,
3
2
H), 7.12 (s, 2 H), 7.72 (d, J = 1.5 Hz, 2 H), 7.95 (d,
J = 2.25 Hz, 2 H), 11.13 (s, 2 H). IR (KBr): υmax = 1542,
−
1
1
538, 1535 cm . HPLC: τ = 10.86 min. MS (ESI):
+
m/z = 242 [M ]. C H N O (242.24): calcd. C 59.50,
1
2
10
4
2
H 4.16, N 23.13; found C 59.74, H 4.13, N 22.96.
,3-Bis(pyrazol-1-yl)-1,4-dihydroxybenzene (4): 1H
NMR (250 MHz, CDCl ): δ = 6.32 (dd, 2 H, J = 2.25 Hz),
2
3
6
.82 (d, 2 H, J = 2.5 Hz), 7.02 (s, 2 H), 7.79 (d, 2 H,
Fig. 15. Thermal ellipsoid plot of 7ox showing the
atomic numbering scheme. The displacement ellipsoids
are drawn at the 50% probability level. Selected bond
J = 2.0 Hz), 8.31 (s, 2 H). HPLC: τ = 17.77 min. MS (ESI):
m/z = 242 [M ]. C H N O (242.24): calcd. C 59.50,
H 4.16, N 23.13; found C 59.37, H 4.06, N 22.79.
+
12 10
4
2
˚
◦
lengths [A] and angles [ ]: Br(1)-C(14) 1.893(8), O(1)-C(1)
.233(11), C(1)-C(3)#1 1.482(14), C(1)-C(2) 1.489(12),
C(2)-C(3) 1.343(11), C(3)-N(11) 1.431(11), C(3)-C(1)#1
.482(14), N(11)-C(15) 1.374(12), N(11)-N(12) 1.378(10),
N(12)-C(13) 1.338(13), C(13)-C(14) 1.427(15), C(14)-C(15)
1
Oxidation of 3
1
Preparation of 3ox: A solution of 1,4-benzoquinone
1
1
1
1
1
1
1
1
1
.360(14), O(1)-C(1)-C(3)#1 123.1(8), O(1)-C(1)-C(2)
19.0(9), C(3)#1-C(1)-C(2) 118.0(7), C(3)-C(2)-C(1)
21.4(9), C(2)-C(3)-N(11) 120.0(9), C(2)-C(3)-C(1)#1
20.5(8), N(11)-C(3)-C(1)#1 119.4(7), C(15)-N(11)-N(12)
(0.110 g, 1 mmol) in 30 ml of dioxane was added drop-
wise to a stirred solution of 2,5-bis(pyrazol-1-yl)-1,4-di-
hydroxybenzene (0.242 g, 1 mmol) in 30 ml dioxane at
11.6(7), C(15)-N(11)-C(3) 129.6(8), N(12)-N(11)-C(3) r. t. After the mixture had been stirred for one hour, the or-
18.8(7), C(13)-N(12)-N(11) 106.0(8), N(12)-C(13)-C(14) ange mother liquor was filtered off the precipitate, taken
08.8(10), C(15)-C(14)-C(13) 107.8(8), C(15)-C(14)-Br(1) to dryness in vacuo, redissolved in a minimum amount of
26.7(8), C(13)-C(14)-Br(1) 125.4(8), C(14)-C(15)-N(11)
05.7(9).
dichloromethane, filtered, and taken to dryness again. Re-
crystallisation (dichloromethane) of this material afforded
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261
σ
α
µ
α
β
γ
θ
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H.-W. Lerner et al. · Redox Behaviour of Pyrazolyl-Substituted 1,4-Dihydroxyarenes
Table 4. Relative amounts of 1 – 4 in reaction mixtures A, B
and C.
Preparation of 7: 1,4-Benzoquinone (8.76 g, 81.04 mmol)
and 4-bromopyrazole (11.99 g, 81.58 mmol) were suspended
in 200 ml of carefully degassed ethanol. The mixture was
heated to reflux for 18 h, whereupon a brown solution
formed. When the solution was cooled to r. t., a brown precip-
itate formed. The solid material was filtered off and recrys-
tallized from hot ethanol. Yield: 2.27 g (5.67 mmol, 14%).
1
2
3
4
Retention time [min]
2.73 13.04 11.03 18.0
Concentration in sample reaction A [mmol/l] 1.78 1.27 0.76 0.59
Concentration in sample reaction B [mmol/l] 1.47 0.53 0.45 0.7
Concentration in sample reaction C [mmol/l] 1.47 1.25 1.08 0.61
1
6
H NMR (250 MHz, d -DMSO): δ = 10.33 (s, 2 H), 8.62
compound 3ox (0.170 g, 0.71 mmol, 71%) as orange blocks.
1
3
(
s, 2 H), 7.96 (s, 2 H), 7.52 (s, 2 H). C NMR (62.9 MHz,
1
H NMR (250 MHz, CDCl ): δ = 6.51 (dd, J = 1.5 Hz, 2 H),
3
6
d -DMSO): δ = 141.0, 140.1, 131.4, 125.8, 111.4, 93.3. MS
7
2
.38 (s, 2 H), 7.78 (d, J = 1.5 Hz, 2 H), 8.63 (dd, J = 2.5 Hz,
+
(
ESI): m/z = 400 [M] . C H Br N O (400.05): calcd.
−1
12
8
2
4 2
H). IR (KBr): υmax = 1663, 1661, 1659, 1603, 1589 cm .
C 36.03, H 2.02, N 14.01; found C 35.98, H 2.04, N 13.78.
+
MS (ESI): m/z = 240 [M ]. C12H N O (240.22): calcd.
8
4 2
C 60.00, H 3.36, N 23.32; found C 59.76, H 3.24, N 22.92.
Syntheses of the quinones 5ox, 6ox, and 7ox
Preparation of 3qh: A solution of 3 (2.66 g, 11 mmol) in
1
00 ml of ethanol was stirred for two days in air. Black 3qh
Preparation of 5ox: 5 (1.06 g, 4.71 mmol) and 1,4-benzo-
quinone (0.51 g, 4.71 mmol) were dissolved in 100 ml
of ethanol and stirred at r. t. for five hours under an at-
mosphere of nitrogen. After cooling the reaction mixture
was precipitated. After filtering the residue was washed
with 20 ml of dichloromethane. X-ray powder diffraction
of this insoluble material shows exclusively the pattern
of 3qh (2.27 g, 9.42 mmol, 86%). IR (KBr): υmax = 1654,
◦
to 0 C, 2-(pyrazol-1-yl)-1,4-dihydroxynaphthalene precipi-
−
600, 1591, 1536, 1524 cm . C12H N O (241.23): calcd.
9 4 2
1
1
tated. Recrystallisation (n-propanol) of this material afforded
compound 5ox (0.39 g, 1.74 mmol, 37.0%) as yellow nee-
C 59.75, H 3.76, N 23.23; found C 59.84, H 3.86, N 22.92.
Upon storing 3 for two weeks in air, black single crys-
tals of 3qh suitable for X-ray diffraction were obtained from
a 0.1 M solution of 3 in methanol.
+
+
1
dles. (Found: 5ox 225, requires: % 5ox 224). H NMR
(
1
8
250 MHz, CDCl ): δ = 6.17 (dd, J = 2.0 Hz, 1 H), 7.24 (s,
3
H), 7.40 (m, 2 H), 7.42 (d, J = 2.75 Hz, 1 H), 7.80 (m, 2 H),
+
.34 (d, J = 2.75 Hz, 1 H). MS (ESI): m/z = 225 [M+H] .
−
Syntheses of salts with the semiquinone radical anions 2sq
and 3sq
C H N O (224.21): calcd. C 69.64, H 3.60, N 12.49;
13 8 2 2
−
found C 69.73, H 3.49, N 12.25.
−
The orthorhombic polymorph was obtained by recrystalli-
sation from dichloromethane.
Formation of 2sq : 2 (0.002 g, 0.01 mmol) and potassium
tert-butoxide (0.011 g, 0.10 mmol) were dissolved in 1 ml
of isopropanol in an atmosphere of argon. The pale yellow
solution formed was transferred via syringe into a Schlenk
Preparation of 6ox: Pyrazole (23.49 g, 345 mmol)
and 2,3-dichloro-1,4-naphthoquinone (13.05 g, 57.5 mmol)
in 200 ml of ethanol were heated under reflux for 24 h.
◦
EPR tube and cooled to −78 C.
◦
−
After cooling the reaction mixture to 0 C, 2,3-bis(pyrazol-
Formation of 3sq : 3 (0.003 mg, 0.01 mmol) and potas-
1
-yl)-1,4-naphthoquinone precipitated. Recrystallisation of
this material from ethanol afforded compound 6ox (5.12 g,
.2 mmol, 17.70 mmol, 30.8%) as thin orange needles.
sium tert-butoxide (0.011 g, 0.10 mmol) were dissolved
in 1 ml isopropanol in an atmosphere of argon. The pale
yellow solution formed was transferred via syringe into a
Schlenk EPR tube and cooled to −78 C.
6
1
◦
H NMR (250 MHz, CDCl3): δ = 6.43 (dd, J = 1.8 Hz,
J = 2.5 Hz, 2 H), 7.54 (dd, J = 1.8 Hz, 2 H), 7.80 (dd, J =
0
2
.5 Hz, J = 2.5 Hz, 2 H), 7.85 (dd, J = 3.5 Hz, J = 5.0 Hz,
Syntheses of the hydroquinones 5 and 7
H), 8.23 (dd, J = 3.5 Hz, J = 5.0 Hz, 2 H). MS (ESI):
+
m/z = 290 [M] . C H N O (290.28): calcd. C 66.20,
H 3.47, N 19.30; found C 66.07, H 3.35, N 19.04.
Preparation of 5: Pyrazole (1.23 g, 18 mmol) and 1,4-
naphthoquinone (2.85 g, 18 mmol) in 300 ml of ethanol
were heated under reflux for 16 h. The hot solution was fil-
16 10
4
2
Preparation of 7ox: A solution of DDQ (0.23 g, 1 mmol)
tered. Addition of water to the filtrate resulted in the pre- in 20 ml of dioxane was added dropwise to a stirred so-
cipitation of 2-(pyrazol-1-yl)-1,4-dihydroxynaphthalene. Re- lution of 7 (0.41 g, 1 mmol) in 200 ml of dioxane at r. t.
crystallisation (dichloromethane) of this material afforded After the mixture had been stirred for one hour, the or-
compound 5 (1.40 g, 6.2 mmol, 34.4%) as white needles. ange mother liquor was filtered off the white precipitate. The
1
H NMR (250 MHz, CDCl ): δ = 6.51 (dd, J = 2.25 Hz, mother liquor was taken to dryness in vacuo, redissolved in
3
1
1
H), 6.94 (s, 1 H), 7.54 (m, 2 H), 7.76 (d, J = 2.25 Hz, a minimum amount of dichloromethane, filtered and taken to
H), 7.95 (d, J = 2.5 Hz, 1 H), 8.20 (m, 2 H). MS (ESI): dryness again. Recrystallisation (CH Cl ) of this material af-
2
2
+
m/z = 226 [M] . C13H N O (226.23): calcd. C 69.02, forded compound 7ox (0.18 mg, 0.45 mmol, 45%) as orange
10
2 2
H 4.46, N 12.38; found C 68.85, H 4.32, N 12.12.
crystals.
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263
1
6
H NMR (250 MHz, d -DMSO): δ = 8.51 (s, 2 H), MULABS [23] option [2, 3qh, 4, 5, 5ox(monoclinic),
+
7
.86 (s, 2 H), 7.40 (s, 2 H). MS (ESI): m/z = 398 [M] . 6ox, 7ox] in PLATON [24]. The structures were solved by
C H Br N O (398.01): calcd. C 36.21, H 1.52, N 14.08; direct methods using the program SHELXS [25] and refined
1
2
6
2 4 2
2
found C 36.23, H 1.66, N 14.08.
against F with full-matrix least-squares techniques using
the program SHELXL-97 [26]. All non-hydrogen atoms
were refined with anisotropic displacement parameters. Hy-
drogen atoms were located by difference Fourier synthesis
Electrochemistry
Anhydrous 99.9%, HPLC grade dichloromethane for and refined using a riding model. Crystallographic data
electrochemistry was purchased from Aldrich Chemicals. (excluding structure factors) for the structures reported in
The supporting electrolyte used was electrochemical grade this paper have been deposited with the Cambridge Crys-
NBu PF obtained from Fluka. Cyclovoltammetry was per- tallographic Data Centre as supplementary publication nos.
4
6
formed in a three-electrode cell with a platinum working CCDC 202035 [2], CCDC 202039 [3qh], CCDC 202037 [4],
electrode surrounded by a platinum spiral counterelectrode CCDC 202038 [5], CCDC 202040 [5ox(monoclinic)],
and the aqueous saturated calomel reference electrode (SCE) CCDC 202034 [5ox(orthorhombic)], CCDC 202036 [6ox]
mounted with a Luggin capillary. Either a BAS 100A or a and CCDC 281847 [7ox]. Copies of the data can be obtained
BAS 100W electrochemical analyzer was used as a polariz- free of charge on application to CCDC, 12 Union Road,
ing unit.
Cambridge CB2 1EZ (Telefax: +1223/336 033; E-mail:
deposit@ccdc.cam.ac.uk).
X-ray crystallography
Acknowledgements
Data collection: STOE IPDS II two-circle diffractometer
M.W. is grateful to the “Deutsche Forschungsgemein-
[
2, 3qh, 4, 5, 5ox(monoclinic), 6ox, 7ox], Siemens-SMART- schaft” (DFG) for financial support. G.M. wishes to thank
CCD three-circle diffractometer [5ox(orthorhombic)], the “Fonds der Chemischen Industrie” (FCI) and the
˚
graphite monochromated Mo-Kα radiation (λ = 0.71703 A), “Bundesministerium f u¨ r Bildung und Forschung” (BMBF)
T = 173(2) K. Empirical absorption corrections were per- for a Ph. D. grant. P.Z. gratefully acknowledges the financial
formed using SADABS [22] [5ox(orthorhombic)] or the support of the University of Siena.
[
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[
10] A. V. Prokofiev, E. Dahlmann, F. Ritter, W. Assmus,
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2
(CCDC 202035):
˚
G. Margraf, M. Wagner, Cryst. Res. Technol. 39, 1014
C H N O , T = 173(2) K, Mo-Kα , λ = 0.71073 A,
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no. of reflections 27809, no. of independent reflec-
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2
64
H.-W. Lerner et al. · Redox Behaviour of Pyrazolyl-Substituted 1,4-Dihydroxyarenes
tions 1086. R1 = 0.0391, wR2 = 0.0848[I > 2σ(I)]. [23] R. H. Blessing, Acta Crystallogr. Sect. A 51, 33 (1995).
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(1990).
1
1 1
˚
˚
a = 4.6642(5) A, b = 11.0730(10) A, c =
˚
˚ 3
1
5.3260(10) A, V = 791.54(12) A ; see also
ref. [4].
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[
[
21] T. Sakurai, Acta Crystallogr. B24, 403 (1968).
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Germany (2000).
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