Thiophene-benzoquinones: Synthesis, crystal structures and preliminary coordination chemistry of derived anilate ligands

Article abstract of DOI:10.1039/c4ob01582g

2,5-Bis(thiophene) and 2,5-bis(ethylenedioxy-thiophene) (EDOT) derivatives of 3,6-diethoxy-1,4-benzoquinone (para isomers) were prepared by Stille coupling between the 2,5-dibromo-3,6-diethoxy-1,4-benzoquinone precursors and (n-Bu)₃Sn-R (R = 2-thiophenyl or 3,4-ethylenedioxy-2-thiophenyl) reagents. In a parallel series of experiments 2,6-bis(thiophene) and 2,6-EDOT-3,5-diethoxy-1,4-benzoquinone (meta isomers) were synthesized by the same strategy. The four compounds were structurally characterized. The thiophene derivatives show essentially planar conformation thanks to the conjugation and establishment of S?O 1,5-nonbonded interactions, while in the EDOT derivatives the thiophene moieties are twisted with respect to the benzoquinone ring because of the steric hindrance. TD-DFT calculations were performed on both para and meta thiophene isomers in order to explain the differences observed in the UV-Vis absorption spectra. The 2,5-derivatives are valuable precursors for thiophene containing anilate (An) ligands, as the first examples of electron rich substituent based anilates. The Cu(II) complex [Cu(Th₂An)(tbbpy)]·2H₂O (Th₂An = thiophene-anilate; tbbpy = 4,4′-bis(tert-butyl)-2,2′-bipyridine) was isolated and structurally characterized. The metal center lies within a square planar coordination geometry, while the ligands engage in a set of intermolecular contacts.

Full text of DOI:10.1039/c4ob01582g

Organic &
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PAPER
Thiophene-benzoquinones: synthesis, crystal
structures and preliminary coordination chemistry
of derived anilate ligands†
Cite this: Org. Biomol. Chem., 2014,
2, 8752
1
a,b
b
b
a
Matteo Atzori, Flavia Pop, Thomas Cauchy, Maria Laura Mercuri and
b
Narcis Avarvari*
2
,5-Bis(thiophene) and 2,5-bis(ethylenedioxy-thiophene) (EDOT) derivatives of 3,6-diethoxy-1,4-benzo-
quinone (para isomers) were prepared by Stille coupling between the 2,5-dibromo-3,6-diethoxy-1,4-
benzoquinone precursors and (n-Bu) Sn-R (R = 2-thiophenyl or 3,4-ethylenedioxy-2-thiophenyl)
3
reagents. In a parallel series of experiments 2,6-bis(thiophene) and 2,6-EDOT-3,5-diethoxy-1,4-benzo-
quinone (meta isomers) were synthesized by the same strategy. The four compounds were structurally
characterized. The thiophene derivatives show essentially planar conformation thanks to the conjugation
and establishment of S⋯O 1,5-nonbonded interactions, while in the EDOT derivatives the thiophene moi-
eties are twisted with respect to the benzoquinone ring because of the steric hindrance. TD-DFT calcu-
lations were performed on both para and meta thiophene isomers in order to explain the differences
observed in the UV-Vis absorption spectra. The 2,5-derivatives are valuable precursors for thiophene con-
taining anilate (An) ligands, as the first examples of electron rich substituent based anilates. The Cu(II)
Received 25th July 2014,
Accepted 5th September 2014
2 2 2
complex [Cu(Th An)(tbbpy)]·2H O (Th An = thiophene-anilate; tbbpy = 4,4’-bis(tert-butyl)-2,2’-bipyri-
DOI: 10.1039/c4ob01582g
dine) was isolated and structurally characterized. The metal center lies within a square planar coordination
geometry, while the ligands engage in a set of intermolecular contacts.
www.rsc.org/obc
(
M = Fe, Cr; X = Cl, Br, I) containing 3,6-dihalo-anilate ligands,
Introduction
in which the crucial role of the substituents has been evi-
Derivatives of the 2,5-dioxybenzoquinone framework contain- denced in the establishment of intermolecular halogen
ing various substituents at the positions 3 and 6 constitute a bonding interactions which influenced the magnetic pro-
7
well-known motif present in many natural products showing perties. These mononuclear complexes are valuable building
1
important biological properties such as anticoagulant, anti- blocks for the preparation, by reaction with a second metal
2
3
4
diabetic, antioxidative, anticancer activities, etc. Structural ion, of bi-dimensional layered molecular magnets formulated
II III
+
+
modifications of the natural products afforded related com- as A[Mn Cr (X An) ]·G (A = [(H O)(phenazine) ] or NBu ;
pounds with relevance in medicinal chemistry. On the other X = Cl, Br, I, H; G = CH COCH or H O), with ordering temp-
hand, the 2,5-dihydroxybenzoquinones may act in their depro- eratures depending on the substituents. More recently, we
tonated dianionic form as valuable ditopic chelating ligands have combined the paramagnetic tris(chloranilato)ferrate(III)
towards transition metals to provide the extensive family of ([Fe(Cl An) ] ) anion with the bis(ethylenedithio)-tetrathia-
2
3
3
3
4
5
3
3
2
8
3
−
2
3
6
anilate (An = anilate) based complexes. In this respect we have fulvalene (BEDT-TTF) donor in a series of three semiconducting
recently described a complete series of mononuclear paramag- radical cation salts [BEDT-TTF] [Fe(Cl An) ]·nCH Cl ·mH O (x =
x
2
3
2
2
2
III
3−
netic tris(haloanilato)metallate(III) complexes, [M (X An) ]
3, 5 or 6) obtained by electrocrystallization, showing different
donor : anion ratios. These haloanilate ligands X
coordination chemistry, together with their cyano ((CN) An
or nitro ((NO ) An ) analogues, contain only electron with-
2 2
2
3
9
2−
2
An used in
2
−
)
2
2
−
a
Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Cagliari,
drawing substituents. We have decided to introduce, at the 3,6
positions of the 2,5-dioxybenzoquinone ring, electron rich
substituents, which might provide additional properties to
the precursors and to the corresponding metal complexes. In
particular we were interested in thiophene and ethylenedioxy-
thiophene (EDOT) motifs due to their planar conjugated struc-
S.S. 554 – Bivio per Sestu, I09042 Monserrato (Cagliari), Italy
b
Laboratoire MOLTECH-Anjou UMR 6200, UFR Sciences, CNRS, Université d’Angers,
Bât. K, 2 Bd. Lavoisier, 49045 Angers, France. E-mail: narcis.avarvari@univ-angers.fr
†
Electronic supplementary information (ESI) available: Additional figures,
tables and crystallographic data as mentioned in the text. CCDC
015746–1015751. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4ob01582g
1
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ture, prone to promote intramolecular charge transfer (CT) that similar results were reported for the synthesis of 2,2,6,6-
towards the electron poor benzoquinone acceptor, and to tetramethyl-1-piperidinyloxy (TEMPO) bis-substituted dibromo-
1
1
provide an access to electroactive metal complexes. We benzoquinone para and meta derivatives.
describe herein the synthesis, solid state and spectroscopic The final assignment as para (2a) and meta (2b) isomers
characterization of 3,6-bis(thiophene) or EDOT derivatives of was provided by single crystal X-ray analysis. Suitable crystals
,5-diethoxybenzoquinone, as well as those of their 3,5-substi- of 2a were obtained by slow evaporation of a solution of 2a in
tuted-2,6-diethoxybenzoquinone isomers. Theoretical calcu- CH Cl . An ORTEP drawing and a summary of crystallographic
2
2
2
lations at the DFT level explain the differences observed in the data for 2a are reported in the ESI (Fig. S1 and Table S1†).
absorption properties. Moreover, the synthesis of 3,6-bis(thio- Although initially our main objective was to access electron
phene) and EDOT anilate ligands is described together with rich substituted para-anilates as precursors for anilate ligands,
preliminary coordination chemistry studies.
we decided to investigate the substitution with both para and
meta isomers in order to estimate the influence of the substitu-
ent position on the charge transfer properties. Accordingly, the
bis-Stille coupling reactions with 2a,b and the tin based pre-
cursors of thiophene and EDOT provided the bis functiona-
lized diethoxy-benzoquinones 3 and 4 in good yields as orange
Results and discussion
Syntheses and crystal structures description
The 1,4-benzoquinone derivatives 3a,b and 4a,b were syn- (3a), red (4a) or deep purple (3b and 4b) crystalline solids.
thesized by palladium catalyzed Stille type coupling reaction Stille type functionalization of the substrate 2a has been pre-
between (n-Bu)
enedioxy-2-thiophenyl) and either 2,5-dibromo-3,6-diethoxy- belonging to the family of fungal natural products asterri-
1
3
Sn-R (3a,b, R = 2-thiophenyl; 4a,b, R = 3,4-ethyl- viously described for the preparation of bis(indole) derivatives
1
2
,4-benzoquinone 2a or 2,6-dibromo-3,5-diethoxy-1,4-benzo- quinones, while Suzuki–Miyaura coupling has been reported
quinone 2b, respectively (Scheme 1).
for the mono or bis-functionalization of the dimethoxy
The dibromo derivatives 2a,b used as starting materials for analogue of 2a with phenyl-boronic acids.
1
3
the coupling reactions were synthesized according to the syn-
thetic procedure described by Wallenfels and Draber, orig- by slow evaporation of CH
Single crystals for compounds 3a,b and 4a,b were obtained
2
Cl or CH Cl –MeOH solutions.
2
2
2
1
0
inally reported for the preparation of 2a as the only isomer.
Compound 3a crystallizes in the triclinic system, space group
In our case, by treating 2,3,5,6-tetrabromo-1,4-benzoquinone 1 P1
ˉ, with one half independent molecule in the unit cell. The
(
bromanil) with KF in EtOH (Scheme 1), we observed the for- thiophene units are coplanar with the benzoquinone ring
mation of two isomers, the 2,5-dibromo-3,6-diethoxy-1,4- (Fig. 1), very likely as a result of the conjugation between the
benzoquinone 2a and the 2,6-dibromo-3,5-diethoxy-1,4-benzo- aromatic thiophene substituents and the benzoquinone
quinone 2b, in a ca. 50 : 50 ratio (89% as the total yield), in moiety, but also thanks to the establishment of intramolecular
1
0
14
contrast to what was previously reported. Although com- 1,5-nonbonded S⋯O interactions. Indeed, a very short S⋯O
pounds 2a and 2b have very close retention factors, they can distance of 2.66 Å is observed. The preference for the
be separated by column chromatography with an appropriate S⋯OvCquinone rather than S⋯O–Calkoxy interactions is in
1
4c
elution system (see the Experimental section). It is noteworthy agreement with previous theoretical calculations.
Scheme 1 Synthesis of thiophenyl (3a,b) and 3,4-ethylenedioxythiophenyl (4a,b) derivatives of 1,4-benzoquinone.
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Fig. 2 Two crystallographically independent molecules in the structure
of 3b; the disorder on the orientation of the thiophene substituent in
molecule A is shown.
The crystal packing of 4a is dominated by the parallel head-
to-tail arrangement of EDOT substituents along the c-axis
(
Fig. 3) that, in turn, do not allow for any type of short contact
between the benzoquinone moieties, as found in 3a.
Compound 4b crystallizes in the monoclinic P2 /c space
1
group with one independent molecule in the unit cell. Once
again, compound 4b does not show planar conformation, as
Fig. 1 (a) View of the crystal packing of 3a in the bc plane; (b) stacking
of 3a along the a-axis.
Compound 3b crystallizes in the monoclinic space group
P2 /a with two independent molecules in the unit cell. Both
1
independent molecules of the meta-substituted isomer do not
show fully planar structures, as found in the para isomer 3a,
but they are slightly twisted, with dihedral angles between the
plane formed by each thiophene and the benzoquinone ring
of ca. 173° and 165°, and ca. 173° and 161° for the A and B
molecules, respectively. Moreover, one of the thiophenes in
molecule A is disordered with two different orientations pre-
senting 0.64 and 0.36 site occupancy factors (Fig. 2). Now,
both types of possible intermolecular S⋯O nonbonded inter-
actions occur in the structure, with the S⋯OvC distances,
amounting at 2.63 Å in A and 2.62 Å in B, being shorter than
the S⋯O–C contacts of 2.71 Å in A and 2.76 Å in B.
Compound 4a crystallizes in the monoclinic C2/c space
group with one half of an independent molecule in the unit
cell. The EDOT moieties do not lie in the plane of the benzo-
quinone ring, as found for the analogous thiophene derivative
3
a, but show symmetry related dihedral angles of ca. 130°.
This non-planar conformation can be due either to the steric
hindrance between the 3,4-ethylenedioxy group and the
oxygen atoms of the benzoquinone moiety (Fig. S2†) or to
the stacking interactions between the EDOT substituents in
the molecular packing of 4a (vide infra).
Fig. 3 Portion of the crystal packing of 4a (a) in the ac plane and (b) in
the ab plane; the stacking of the EDOT substituents along the c axis is
highlighted.
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while 3b is not centrosymmetric. Both compounds show a
band centered at ca. λ = 390 nm with very similar extinction
3
−1
−1
coefficients (5905 and 6036 dm mol cm for 3a and 3b,
respectively) and one absorption band in the UV region with
comparable extinction coefficients and wavelengths: λ =
3
−1
−1
3
1
02 nm (ε = 18 300 dm mol cm ) and λ = 278 nm (ε =
3
−1
−1
6 680 dm mol cm ) for 3a and 3b, respectively, which can
be related to a π–π* transition involving the benzoquinone
ring.
DFT and TD-DFT calculations were performed in order to
rationalize the electronic spectra of compounds 3a and 3b and
to understand the difference in the charge transfer properties.
The equilibrium ground state geometries were optimized for
both isomers. There is no significant change with respect to
Fig. 4 Molecular structure of 4b.
the EDOT substituents form dihedral angles of ca. 131° and the starting geometries taken from the X-ray data. The isomer
29° with respect to the plane of the benzoquinone ring 3a converged to a C point group symmetry structure. The fron-
Fig. 4).
tier molecular orbital plots show clearly that the HOMO, of
This is very likely due to the steric hindrance (Fig. S3†).
type, is delocalized over the thiophene–(CvC)quinone
1
i
(
π
However, unlike the structure of 3b, now both thiophene units fragments, with some contribution from the oxygen atoms of
are oriented towards the CvO group. Some short intermole- the ethoxy groups, while the LUMO, also of π symmetry, is
cular contacts between ethylenedioxy and benzoquinone largely based on the electron poor CvO groups (Fig. 6, see
fragments can be observed in the packing (Fig. S3†).
also Table S3†), both of the orbitals being symmetric (g) with
respect to the inversion center. In the meta isomer 3b, which
does not present any symmetry element since the thiophene
rings point in opposite directions, the HOMO and LUMO are
delocalized over the same fragments as in 3a. It is then clear
that independent of any symmetry considerations, charge
transfer from the thiophene units to the central benzoquinone
ring might occur as HOMO→LUMO excitations.
UV-Vis spectroscopy and theoretical calculations
An interesting feature of these compounds is the possibility to
show charge transfer (CT) from the thiophene or EDOT units
to the benzoquinone ring. Interestingly, a solution of 3a in
acetonitrile has yellow colour, while that of 3b is violet.
Indeed, the UV-Vis spectra show the presence of a tail at
ca. λmax = 525 nm for 3a, with a very low extinction coefficient
Then, TD-DFT calculations, which are very well adapted for
15
3
−1
−1
such organic systems, were performed to rationalize the UV-
ε = 407 dm mol cm , and an absorption band centered at
max = 536 nm for 3b, with a much higher value of the extinc-
1
6
visible absorption spectra. The hybrid functional PBE0 with
a large basis set was used (see ESI† for details). Such a
methodology reproduces almost exactly all local π→π* tran-
sitions whereas a relatively large error (0.2 to 1 eV) has been
λ
3
−1
−1
tion coefficient (ε = 1896 dm mol cm ) (Fig. 5). These intri-
guing spectral features have an explanation related to
symmetry selection rules for the electronic transitions
1
7
observed for charge transfer and Rydberg type transitions.
(
vide infra), since the molecule 3a has an inversion center
Fig. 6 Frontier orbitals for 3a (HOMO top left and LUMO bottom left)
and 3b (HOMO top right and LUMO bottom right) with an isovalue
chosen to represent 35% of the function.
Fig. 5 UV-Vis spectra (200–800 nm) for 3a (black line) and 3b (red line)
in CH CN solution; the inset shows the charge transfer band region.
3
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Fig. 7 On the left side, experimental (solid lines) and simulated (dashed lines) absorption spectra of isomers 3a (in black) and 3b (in red). On the
right side, electronic transitions calculated for the isomers 3a (in black, up) and 3b (in red, bottom).
Therefore the diagnostic tool proposed by Peach and Tozer, improved using calculations in solutions (Fig. S4†), is
who have shown that, for a hybrid functional, if the spatial observed. Hence, the color is not accurately reproduced by this
1
5
overlap (noted Λ) between the ground state and the excited method. However, the difference between the absorption pro-
state wavefunction is below 0.3, the calculated excitation perties of both isomers is clearly in agreement with the calcu-
energy is likely to be inaccurate, has been used for every tran- lations. Indeed, the S
sition in order to evidence any possible issue.
0
to S
symmetry in the case of the para substituted isomer 3a,
An excellent agreement has been found between the experi- whereas higher energy excitations are related. The resemblance
mental and calculated transitions in the high energy region between both S to S excitations can be better seen with the
Fig. 7). Tables 1 and 2 give details, energies and spatial density difference plots. The thiophene moieties lose density
overlap of the calculated excitations for both isomers 3a and whereas the π* of the carbonyl groups is populated (Table 3
1
excitation is forbidden by
1
8
0
1
(
3
b.
Nevertheless, for the first excitation of 3b, calculated at
52 nm and observed at 536 nm, corresponding to a charge and 4b show similar features to the thiophene analogues.
and S4†).
UV-Vis spectra of the corresponding EDOT derivatives 4a
6
transfer, a difference in energy of 0.4 eV, which is not However, solutions of 4b in acetonitrile are deep red, while
those of 3b are violet, very likely because of the less efficient
conjugation in the former than in the latter, and hence the
Table 1 TD-DFT calculated energies, spatial overlap and assignment of
the most pertinent low-lying singlet to singlet electronic excitations of
weaker charge transfer. Compound 4a shows a weak tail-like
3
−1
−1
absorption at around λ = 540 nm (ε = 720 dm mol cm ),
which corresponds to the charge transfer from EDOT to the
benzoquinone ring, whereas the meta isomer 4b shows the
charge transfer band as a shoulder at around λ = 520 nm (ε =
3
a
λ
Wavenumber
Energy
(eV)
Oscillator
strength
−
1
Excitation
(nm)
(cm
)
Λ
3
−1
−1
1
812 dm mol cm ), hence blue-shifted with respect to 3b,
S
S
S
S
0
→S
0
→S
0
→S
0
→S
1
2
8
9
676
463
312
284
14 800
21 587
32 067
35 244
1.83
2.68
3.98
4.37
0.000
0.122
0.612
0.308
0.58
0.65 in agreement with the observed difference in color between
0.77
0.68
the two compounds. As for 3b this transition should be
allowed since there is no inversion center in the molecule.
Both 4a and 4b show two absorption bands centered at λmax
=
3
−1
−1
4
18 nm (ε = 4005 dm mol cm ) and λmax = 275 nm (ε =
Table 2 TD-DFT calculated energies, spatial overlap and assignment of
3
−1
−1
−1
the most pertinent low-lying singlet to singlet electronic excitations of 28 300 dm mol cm ) (for 4a) and λmax = 414 nm (ε = 3121
3
−1
3
−1
3
b
dm mol cm ) and λmax = 267 nm (ε = 26 595 dm mol
cm ) (for 4b), with similar λmax and extinction coefficients
−
1
λ
Wavenumber
Energy
(eV)
Oscillator
strength
−
1
(Fig. 8).
Excitation
(nm)
(cm
)
Λ
S
S
S
S
S
S
S
S
0
0
0
0
0
0
0
0
→S
→S
→S
→S
→S
→S10
→S11
→S12
1
2
4
8
9
652
440
429
293
288
274
263
261
15 333
22 739
23 297
34 094
34 673
36 514
37 995
38 366
1.90
2.82
2.89
4.23
4.30
4.53
4.71
4.76
0.066
0.050
0.050
0.088
0.161
0.410
0.122
0.071
0.62 Electrochemical studies
0.39
0.40
Compounds 3a,b and 4a,b were studied by cyclic voltammetry
0.58 in order to gain insight into their electrochemical properties.
0.40
0.62
0.33
Cyclic voltammograms of 3a and 3b show for both compounds
one irreversible oxidation peak at ca. 1.46 V and one reversible
0.60 reduction peak at ca. −0.50 V (Table S5†). The oxidation peak
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Table 3 One-electron density difference between the ES and GS. The isocontour value is set to represent 75% of the electron. The solid turquoise
white grid) zones indicate an increase (decrease) of density upon electronic transition
(
Excitation (Osc. str.)
3a
Excitation (Osc. str.)
3b
S
0
→S
1
(0.000)
0
S →S
0
S →S
0
S →S
1
2
4
(0.066)
(0.050)+
(0.050)
S
0
→S
2
(0.122)
ca. −0.55 V associated with the reduction of the benzoquinone
ring are present for both isomers (Table S5†). In addition, the
EDOT derivatives 4a and 4b, when compared to 3a and 3b,
undergo electropolymerization generating a polymeric redox
system characterized by a reversible oxidation peak at ca.
0.80 V, for which the intensity increases upon repetitive
cycling (Fig. 9).
Preliminary coordination chemistry studies
Hydrolysis under basic conditions of compounds 3a and 4a
followed by acidification with a concentrated acid affords the
expected 2,5-di(thiophen-2-yl)-3,6-dihydroxy-1,4-benzoquinone
5
1
4
a and 2,5-di(3,4-ethylenedioxythiophen-2-yl)-3,6-dihydroxy-
,4-benzoquinone 6a products, whereas compounds 3b and
b, under the same conditions, give rise to the mono-depro-
Fig. 8 UV-Vis spectra (200–800 nm) for 4a (black line) and 4b (red
line) in CH CN solution; inset shows the charge transfer band region.
3
may be associated with the oxidation of the thiophene moiety;
its position at lower values with respect to the oxidation poten-
tial of the unsubstituted thiophene (∼2.0–2.1 V vs. SCE) is in
1
9
the normal range for substituted thiophenes. The reduction
peak may be, instead, associated with a reversible reduction
process involving the benzoquinone moiety (Fig. S5†).
Cyclic voltammograms for 4a and 4b show similar features
to those for their thiophene analogues. An irreversible oxi-
dation potential centered at ca. 1.34 V associated with the oxi-
dation of the EDOT units, thus occurring at a slightly more
cathodic potential than the one for the unsubstituted EDOT
Fig. 9 Cyclic voltammogram (−0.40–1.40 V) for 4a (CH
3
CN, 0.1 mol
−3
−1
dm
[(n-Bu) N]PF , 100 mV s ). The inset shows the reversible
4 6
2
0
(
∼1.5 V vs. SCE), and a reversible reduction peak centered at reduction peak in detail.
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Scheme 2 Synthesis of compounds 5a,b and 6a,b.
tected compounds 2,6-di(thiophen-2-yl)-3-hydroxy-5-ethoxy- probably because of the limited conjugation possibilities of
,4-benzoquinone 5b and 2,6-di(3,4-ethylenedioxythiophen-2- the monoanions, in contrast to the dianions generated from
1
yl)-3-hydroxy-5-ethoxy-1,4-benzoquinone 6b (Scheme 2).
the para isomer (Scheme 3).
These results can be explained on the basis of the stability
The deprotonated compounds 5a and 6a can thus behave
of the corresponding anions. For the para-substituted deriva- as suitable ligands towards transition metal ions, in analogy to
tives 5a and 6a the deprotection of the ethoxy groups takes what is described in the literature for the electron withdrawing
place easily on both positions 3 and 6 because of the stability based analogues. It should be pointed out, however, that 5a
of the generated dianionic structure (Scheme 3). Instead, for and 6a are the first anilate ligands containing electron rich
the meta-substituted derivatives 5b and 6b only one of the two substituents. To prove their coordinating abilities, compound
ethoxy groups is removed, generating the monoanions. The 5a in its deprotonated dianionic form, hereafter thiophene-
2
−
deprotection of the second ethoxy group does not take place anilate (Th
2
An ), has been reacted with the Cu(II) complex
2
1
[
Cu(tbbpy)Cl ] (tbbpy = 4,4′-bis(tert-butyl)-2,2′-bipyridine),
2
Scheme 3 Resonance structures for the dianionic forms of 5a and 6a
above) and the monoanionic forms of 5b and 6b (below); note that a
(
and c are the prevailing forms of 5a and 6a and a is the prevailing form
of 5b and 6b.
Fig. 10 ORTEP drawing of 7 (thermal ellipsoids at the 30% probability
level).
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affording the heteroleptic square-planar complex [Cu(Th
2
An)- one presents the thiophene units ca. 180° rotated, with sulfur
(
tbbpy)]·2H O (7) (Fig. 10).
atoms interacting with the coordinating oxygen atoms through
2
Single crystal X-ray analysis shows that 7 crystallizes in the 1,5-nonbonded S⋯O interactions (2.74 and 2.77 Å).
monoclinic space group P2 /n with one independent molecule The metal centre exhibits a square-planar coordination geo-
1
of the complex and two crystallization water molecules in metry with the thiophene-anilate and tbbpy ligands co-
general positions in the asymmetric unit. The complex shows ordinated to the Cu(II) ion in the 1,2-bidentate coordination
an overall planar structure with thiophene substituents that are mode, with the O and N atoms respectively. The metal–oxygen
practically coplanar with the benzoquinone ring (dihedral and metal–nitrogen bond distances are 1.911(3)–1.914(3) and
angles of ca. 170°), with short 1,5-nonbonded S⋯O interactions 1.954(4)–1.961(4) respectively (Table 4), in the normal range
(
2.76–2.77 Å) between the sulfur atoms of the thiophenes and for heteroleptic Cu(II) complexes containing chloranilate and
2
2
the peripheral oxygen atoms of the ligand (Fig. 11).
2,2′-bipyridine ligands. The C–O bond distances are influ-
The thiophene units were found disordered in two different enced by the coordination to the metal center. The oxygen
orientations presenting 0.57 and 0.43 site occupancy atoms coordinated to the metal have C–O distances 0.06 Å
factors. One of the two orientations is the one described longer on average than those of the peripheral oxygen atoms
above (Fig. 11) where the sulfur atoms of the thiophenes interact which show a major double bond character (Table 4),
with the peripheral oxygen atoms of the ligand, while the other suggesting that the thiophene-anilate ligand is likely present
in the well-known o-quinone-like resonance structure (b and d
6
a
forms in Scheme 3).
The crystal packing of 7 is dominated by alternated head-to-
tail stacking interactions between the molecules of the
complex that run parallel to the a crystallographic axis
(
Fig. 12), with C⋯S and some C⋯C distances lower than the
sum of the van der Walls radii (S1⋯C10 3.47 Å, S2⋯C2 3.49 Å,
C1⋯C22 3.33 Å, C1⋯C21 3.39 Å).
Interestingly, the crystallization water molecules present in
the structure connect pairs of metal complexes of adjacent par-
allel columns via hydrogen bonding interactions with the per-
Fig. 11 Molecular structure of 7 with 1,5-nonbonded S⋯O interactions ipheral oxygen atoms of the thiophene-anilate ligands
highlighted.
(Fig. S6†).
Table 4 Selected bond distances (Å) for 7
Conclusions
Bonds
7
An elegant synthesis of thiophene and EDOT 2,5-disubstituted
anilate derivatives by Stille coupling is described, together
1.954(4) with the preparation, in parallel, of the corresponding 2,6-di-
substituted derivatives. The four compounds were character-
ized by single crystal X-ray diffraction, which emphasized the
1.236(5) planar conformation of the thiophene derivatives with the
Cu(1)–O(1)
Cu(1)–O(2)
Cu(1)–N(1)
Cu(1)–N(2)
C(19)–O(1)
C(20)–O(2)
C(22)–O(3)
C(23)–O(4)
1.911(3)
1.914(3)
1.961(4)
1.285(5)
1.302(5)
1.238(5)
establishment of S⋯O 1,5-nonbonded interactions. Very inter-
Fig. 12 (a) View of the crystal packing of 7 in the ac plane; (b) detailed view of the alternated head-to-tail disposition along the a crystallographic
axis.
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estingly, theoretical calculations at the TD DFT level show that
C
10
H
10Br
2
O
4
: 354.0; found: 353.7; elemental analysis calcd (%)
in the 2,5-bis(thiophene) derivatives (para isomers) the intra- for C H Br O : C 33.93, H 2.85; found: C 34.02, H 2.77. 2b:
1
0
10
2 4
1
3
molecular charge transfer from the thiophene units to the
benzoquinone ring is symmetry forbidden, while it is allowed
H NMR (300 MHz, CDCl
J 6.9); C NMR (125 MHz, CDCl ): δ = 173.8, 155.5, 118.8,
3
): δ = 4.46 (q, 4H, J 6.9), 1.43 (t, 6H,
3
13
3
in the 2,6-derivatives (meta isomers), in agreement with the 70.8, 15.9; MS (MALDI-TOF): m/z calcd for C H Br O : 354.0;
1
0
10
2 4
2 4
different colours of the respective solutions. The 2,5-derivatives found: 353.7; elemental analysis calcd (%) for C10H10Br O :
represent valuable precursors for the preparation of thiophene C 33.93, H 2.85; found: C 33.92, H 2.79.
or EDOT-anilate ligands by successive deprotection of the ethoxy
groups and deprotonation. These are the first representatives of (50 mg, 0.14 mmol), 2-(tributylstannyl)thiophene (135 mg,
anilate ligands containing electron rich, electroactive, substitu- 0.36 mmol), and [Pd(PPh ] as the catalyst (63 mg, 15%
2,5-Di(thiophen-2-yl)-3,6-diethoxy-1,4-benzoquinone 3a. 2a
3 4
)
ents. Their binding ability was proved in a preliminary coordi- mmol) were mixed in dry toluene (50 mL) and heated at reflux
nation chemistry experiment by the preparation and structural for 12 h under an argon atmosphere. The mixture was filtered
characterization of a square planar Cu(II) complex containing through Celite and silica gel, washed with toluene and then
one thiophene-anilate ligand and one 4,4′-bis(tert-butyl)-2,2′- CH Cl . The combined organic phases were evaporated and
2
2
bipyridine ligand. The strategy we describe herein opens the way purified by column chromatography on silica gel (pentane–
towards the preparation of a large variety of d- and f-metal com- CH Cl , 1 : 1) to give 3a as yellow crystals (yield 85%). Single
plexes based on these new electroactive anilate ligands. As per- crystals suitable for X-ray analysis were obtained by slow evap-
2
2
1
spective, the introduction of other functional substituents oration from a solution of 3a in CH
2
Cl
2
. H NMR (300 MHz,
3
4
through the same synthetic strategy is also envisaged.
CDCl ): δ = 8.10 (dd, 2H, J 3.9, J 1.2), 7.53 (dd, 2H), 7.15 (dd,
H, J 5.4, J 3.9), 4.47 (q, 4H, J 7.2), 1.50 (t, 6H, J 7.2);
3
3
4
3
3
2
1
3
C NMR (125 MHz, CDCl
3
): δ = 182.5, 151.8, 131.9, 130.6,
1
29.7, 126.8, 121.9, 70.3, 16.0; MS (MALDI-TOF): m/z calcd for
Experimental
General
C H O S : 360.5; found: 360.8; elemental analysis calcd (%)
for C18H O S : C 59.98, H 4.47, S 17.79; found: C 58.74,
16 4 2
1
8
16 4 2
NMR spectra were recorded on a Bruker Avance DRX 300 H 4.37, S 17.18.
spectrometer (300 MHz for 1H, 75 MHz for 13C). Chemical 2,6-Di(thiophen-2-yl)-3,5-diethoxy-1,4-benzoquinone 3b. 3b
shifts are expressed in parts per million (ppm, δ), downfield was obtained as violet shiny crystals according to the pro-
from TMS as an external reference. The following abbrevi- cedure described for 3a using 2b (50 mg, 0.14 mmol), 2-(tributyl-
ations are used: singlet (s), doublet (d), triplet (t), quartet (q), stannyl)thiophene (135 mg, 0.36 mmol), and [Pd(PPh ) ] as
3
4
doublet of doublets (dd). MS (MALDI-TOF) spectra were the catalyst (63 mg, 15% mmol) (yield 81%). Single crystals
recorded on a Bruker Biflex-IIITM equipped with a 337 nm N suitable for X-ray analysis were obtained by slow evaporation
laser. Elemental analyses were performed on a Flash 2000 from a solution of 3b in CH Cl . H NMR (300 MHz, CDCl ):
2
1
2
2
3
3
4
3
4
Fisher Scientific Thermo Electron Analyzer. Palladium cata- δ = 8.05 (dd, 2H, J 3.9, J 1.2), 7.56 (dd, 2H, J 5.1, J 1.2), 7.15
lyzed reactions were carried out under an argon atmosphere in (dd, 2H, J 5.1, J 3.9), 4.42 (q, 4H, J 7.2), 1.48 (t, 6H, J 7.2);
toluene (HPLC grade). Bromanil (1) was synthesized according
3
4
3
3
1
3
C NMR (125 MHz, CDCl ): δ = 182.5, 151.8, 131.9, 130.6,
3
2
3
to the literature. 2-(Tributylstannyl)thiophene was purchased 129.7, 126.8, 121.9, 70.3, 16.0; MS (MALDI-TOF): m/z calcd for
from Sigma-Aldrich and 2-(tributylstannyl)-3,4-ethylenedioxy- : 360.5; found: 360.8; elemental analysis calcd (%)
for C H O S : C 59.98, H 4.47, S 17.79; found: C 59.41,
18 16 4 2
C H O S
2
4
thiophene was synthesized according to the literature.
Cu(tbbpy)Cl ] (tbbpy = 4,4′-bis(tert-butyl)-2,2′-bipyridine) was H 4.41, S 17.77.
synthesized according to the literature method.
,5-Dibromo-3,6-diethoxy-1,4-benzoquinone 2a and 2,6- quinone 4a. 2a (100 mg, 0.28 mmol), 2-(tributylstannyl)-3,4-
dibromo-3,5-diethoxy-1,4-benzoquinone 2b. 1 (12 g, ethylenedioxythiophene (151 mg, 0.36 mmol), and [Pd(PPh
8.4 mmol) was partially dissolved in anhydrous ethanol as the catalyst (63 mg, 7.5% mmol) were mixed in dry toluene
800 mL) and KF (16 g, 275.4 mmol) was added under stirring (40 mL) and heated at reflux for 24 h under an argon atmos-
at room temperature. After 1.5 h the white solid was filtered phere. Then, 2-(tributylstannyl)-3,4-ethylenedioxythiophene
off and the solution was evaporated. The orange solid was (101 mg, 0.36 mmol) and fresh [Pd(PPh ] (63 mg, 7.5%
1
8
16 4 2
[
2
2
1
2,5-Di(3,4-ethylenedioxythiophen-2-yl)-3,6-diethoxy-1,4-benzo-
2
3 4
) ]
2
(
3 4
)
redissolved in ca. 400 mL of CCl and heated until reflux. The mmol) in dry toluene (10 mL) were added to the reaction
4
black impurities were filtered off and the orange solution evap- mixture and heated at reflux for another 16 h. The mixture was
orated to give a mixture of 2a and 2b in a ca. 50 : 50 ratio (9 g, filtered through Celite and silica gel, washed with toluene and
8
9% yield). 2a and 2b can be separated by column chromato- then CH Cl . The combined organic phases were evaporated
2 2
graphy on silica gel (pentane–CH
2
Cl
2
, 2 : 1, R
f
2a = 0.53, R
f
2b and purified by column chromatography on silica gel (CH
2 2
Cl ,
=
0.48; then CH
2
Cl
2
). Single crystals suitable for X-ray analysis then CH
2
2
Cl –EtOAc, 99%/1%) to give 4a as yellow-orange crys-
were obtained by slow evaporation from a solution of 2a in tals (yield 90%). Single crystals suitable for X-ray analysis were
1
3
CH
1
1
2
Cl
2
. 2a: H NMR (300 MHz, CDCl
3
): δ = 4.57 (q, 4H, J 6.9), obtained by slow evaporation from a solution of 8 in a CH
2
Cl
2
–
3
13
1
.43 (t, 6H, J 6.9); C NMR (125 MHz, CDCl
56.3, 116.0, 71.4, 16.0; MS (MALDI-TOF): m/z calcd for 4.31 (q, 4H, J 7.0), 4.22 (m, 8H), 1.33 (t, 6H, J 7.0); C NMR
3
): δ = 174.9, MeOH mixture. H NMR (300 MHz, CDCl
3
): δ = 6.54 (s, 2H),
3
3
13
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1
(
125 MHz, CDCl
3
): δ = 181.4, 154.8, 141.0, 140.7, 119.1, 105.3, several washing with fresh water (yield 83%). H NMR
1
02.5, 69.7, 64.7, 64.4, 15.8; MS (MALDI-TOF): m/z calcd for (300 MHz, CD COCD ): δ = 6.61 (s, 2H), 4.22 (m, 8H); MS
3 3
C
22
H
20
O
H
8
S
2
: 476.5; found: 477.2; elemental analysis calcd (%) (MALDI-TOF): m/z calcd for C18
: C 55.45, H 4.23, S 13.46; found: C 55.05, H elemental analysis calcd (%) for C14
15.25; found: C 50.91, H 2.86, S 14.68.
2,6-Di(3,4-ethylenedioxythiophen-2-yl)-3-hydroxy-5-ethoxy-
H
12
O
8
S
2
: 420.0; found: 419.5;
for C22
.13, S 13.23.
,6-Di(3,4-ethylenedioxythiophen-2-yl)-3,5-diethoxy-1,4-benzo-
20
O
8
S
2
H O S : C 51.42, H 2.88, S
8 4 2
4
2
quinone 4b. 4b was obtained as violet-red crystals according to 1,4-benzoquinone 6b. 6b was obtained as a dark violet solid
the procedure described above for 4a using 2b (100 mg, according to the procedure described for 6a using 4b (50 mg,
1
0
.28 mmol), 2-(tributylstannyl)-3,4-ethylenedioxythiophene 0.10 mmol) and NaOH (8.5 mg, 0.20 mmol) (yield 81%). H
(
1
302 mg, 0.72 mmol), and [Pd(PPh
5% mmol) (yield 87%). Single crystals suitable for X-ray ana- (q, 2H, J 7.2), 4.24 (m, 4H), 4.22 (m, 4H), 1.30 (t, 3H, J 7.2);
3
)
4
] as the catalyst (121 mg, NMR (300 MHz, CD
3
COCD
3
): δ = 6.71 (s, 1H), 6.63 (s, 1H), 4.28
3
3
1
3
lysis were obtained by slow evaporation from a solution of 4b
in a CH
6
7
1
3 3
C NMR (125 MHz, CH COCH ): δ = 185.3, 180.6, 154.3,
1
2
Cl
2
–MeOH mixture. H NMR (300 MHz, CDCl
3
): δ = 153.1, 143.5, 143.1, 143.0, 142.5, 133.7, 133.6, 130.3, 124.3,
3
3
.57 (s, 2H), 4.21 (q, 4H, J 7.0), 4.20 (m, 8H), 1.32 (t, 6H, J 105.0, 103.7, 70.6, 66.6, 66.5, 66.2, 66.1, 17.0; MS (MALDI-
13
.0); C NMR (125 MHz, CDCl
3
): δ = 179.1, 153.5, 141.1, 140.9, TOF): m/z calcd for C20
H
16
O
8
S
2
: 448.4; found: 449.2; elemental
20.7, 105.8, 103.5, 69.2, 64.7, 64.4, 15.7; MS (MALDI-TOF): analysis calcd (%) for C20
H O S : C 53.56, H 3.60, S 14.30;
16 8 2
m/z calcd for C H O S : 476.5; found: 477.2; elemental analy- found: C 52.93, H 3.52, S 13.96.
2
2
20 8 2
sis calcd (%) for C22
C 55.45, H 4.14, S 13.15.
,5-Di(thiophen-2-yl)-3,6-dihydroxy-1,4-benzoquinone 5a. 3a (60 mL) solution of 3a (100 mg, 0.28 mmol). The mixture was
50 mg, 0.14 mmol) was dissolved in CH CN (30 mL). NaOH stirred and heated at reflux temperature for 1.5 h. [Cu(tbbpy)-
11 mg, 0.28 mmol) dissolved in H O (10 mL) was added to the Cl ] (113 mg, 0.28 mmol) was dissolved in H O (15 mL) and
H
20
O
8
S
2
: C 55.45, H 4.23, S 13.46; found:
[Cu(Th
2
An)(tbbpy)]·2H
2
O 7. NaOH (22 mg, 0.56 mmol) was
dissolved in H
2
O (15 mL) and added drop-wise to a CH
3
CN
2
(
(
3
2
2
2
CH CN solution and the resulting mixture was stirred and added drop-wise to the blue solution containing the dianionic
3
heated at reflux temperature for 1.5 h. The violet solution was form of 5a. 7 precipitates immediately as a green solid. The
filtered and acidified using concentrated HCl. 5a precipitates mixture was filtered and the precipitate was washed several
by concentration of the CH CN–H O solution as a brown- times with CH CN, and then with Et O (yield 87%). Single
3
2
3
2
yellow solid together with some colourless impurities. The crystals suitable for X-ray analysis were obtained by slow
product can be purified by dissolution in a mixture of MeOH– diffusion of MeOH in a solution of 7 in DMF. ESI-MS, m/z
+
CH Cl (80 : 20) and filtration of the insoluble product (yield found (calcd) = 634.27 (634.10) [[Cu(Th An)(tbbpy)]-H] ; UV-Vis
2
2
1
2
−1
3
3
−1
9
3 3 3
0%). H NMR (300 MHz, CD COCD ): δ = 8.10 (dd, 2H, J 3.6, (CHCl solution, λmax/nm (ε/dm mol cm )): 305 (25 920),
J 0.9), 7.62 (dd, 2H, J 5.2, J 0.9), 7.18 (dd, 2H, J 5.2, J 3.6); 333 (18 720), 347 (16 174), 394 (6034), 634 (1634). Elemental
MS (MALDI-TOF): m/z calcd for C H O S : 304.3; found: analysis calcd (%) for C H CuN O S : C 57.34, H 5.11, N
4
3
4
3
4
1
4
8
4
2
32 34
2 6 2
3
2
04.4; elemental analysis calcd (%) for C14
H
8
O
4
S
2
: C 55.25, H 4.18; found: C 57.16, H 4.60, N 4.15.
.65, S 21.07; found: C 54.81, H 2.51, S 21.06.
,6-Di(thiophen-2-yl)-3-hydroxy-5-ethoxy-1,4-benzoquinone
X-Ray structure determinations
2
5
b. 5b was obtained as a brown solid according to the pro- X-ray diffraction measurements were performed on a Nonius
cedure described for 5a using 3b (50 mg, 0.14 mmol) instead Kappa CCD diffractometer, using graphite-monochromated
1
of 3a, and NaOH (11 mg, 0.28 mmol) (yield 87%). H NMR MoK radiation (λ = 0.71073 Å). The structures were solved by
α
3
4
25
2
(
(
3 3
300 MHz, CH COCH ): δ = 8.12 (dd, 1H, J 3.9, J 0.9), 8.08 direct methods (SHELXS-97 or SIR2004 ) and refined on F
dd, 1H, J 3.9, J 1.2), 7.76 (dd, 1H, J 5.1, J 1.2), 7.66 (dd, 1H, with full-matrix least squares (SHELXL-97), using the Wingx
3
4
3
4
26
3
4
3
4
3
27
J 5.1 J, 1.2), 7.20 (dd, 1H, J 5.4, J 4.2), 7.17 (dd, 1H, J 5.4, software package. The non-H atoms were refined with aniso-
4
3
3
13
J 4.2), 4.50 (q, 2H, J 7.2), 1.45 (t, 3H, J 7.2); C NMR tropic displacement parameters. A summary of the crystallo-
125 MHz, CD COCD ): δ = 188.2, 180.7, 151.4, 151.0, 134.8, graphic data and the structure refinement for 3a,b, 4a,b and 7
33.6, 133.2, 133.1, 132.8, 131.4, 128.6, 128.2, 126.1, 114.7, is reported in Table 5. CCDC 1015746 (2a), CCDC 1015747
1.5, 17.1; MS (MALDI-TOF): m/z calcd for C16 : 332.4; (3a), CCDC 1015748 (3b), CCDC 1015749 (4a), CCDC 1015750
(
3
3
1
7
12 4 2
H O S
found: 332.8; elemental analysis calcd (%) for C16
7.81, H 3.64, S 19.29; found: C 57.67, H 3.45, S 19.52.
,5-Di(3,4-ethylenedioxythiophen-2-yl)-3,6-dihydroxy-1,4-
benzoquinone 6a. 4a (50 mg, 0.10 mmol) was dissolved in
H
12
O
4
S
2
: C (4b), CCDC 1015751 (7), contain supplementary crystallo-
5
graphic data for this paper.
2
UV-Visible studies
CH CN (30 mL). NaOH (8.5 mg, 0.20 mmol) dissolved in H O UV-Vis absorption spectra (800–200 nm) were recorded in
3
2
(10 mL) was added to the CH
3
3
CN solution and the resulting CH CN solution on a Varian Cary 5 spectrophotometer.
mixture was stirred and heated at reflux temperature for 24 h.
The violet-reddish solution was filtered and acidified using
Computational details
concentrated HCl. 6a precipitates by concentration of the All calculations have been performed with the Gaussian09
2
8
CH
3 2
CN–H O solution as a yellow-brown solid together with program. Starting from the X-ray data and without forcing
some colourless impurities. The product can be purified by any symmetry, the gas phase ground state geometries have
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Paper
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a
Table 5 Summary of X-ray crystallographic data of 3a, 3b, 4a and 4b
3a
3b
4a
4b
7
Empirical formula
Formula weight
Crystal size (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
18
H
16
O
4
S
2
C
36
H
32
O
8
S
4
C
22
H
20
O
8
S
2
C
22
H
20
O
8
S
2
C
32
H
32CuN
2 6 2
O S
360.43
0.40 × 0.10 × 0.05
Triclinic
Pˉ1
4.891(1)
9.361(2)
9.693(4)
104.639(2)
101.792(2)
100.598(2)
407.06(2)
1
720.86
0.50 × 0.20 × 0.20
Monoclinic
476.50
476.50
0.26 × 0.24 × 0.04
Monoclinic
668.26
0.40 × 0.05 × 0.05
Monoclinic
0.60 × 0.20 × 0.20
Monoclinic
C2/c
20.441(2)
6.614(1)
17.168(1)
90
111.534(6)
90
2158.9(4)
4
P2
1
/a
P2
1
/c
1
P2 /n
7.6923(7)
46.482(6)
9.938(2)
90
112.199(11)
90
9.782(1)
14.556(1)
15.493(1)
90
98.14(11)
90
6.878(2)
17.926(8)
24.286(8)
90.00
92.51(2)
90
γ (°)
V (Å )
3
3290.1(8)
4
2183.9(3)
4
2991.5(5)
4
Z
T (K)
ρ (calc.) (Mg m
μ (mm
293(2)
1.470
0.347
293(2)
1.455
0.343
293(2)
1.468
0.282
293(2)
1.449
0.291
150(2)
1.484
0.918
−3
)
−
1
)
θ range (°)
GooF
4.38 to 26.49
1.022
2.21 to 25.00
1.040
3.36 to 30.50
1.051
2.99 to 27.50
1.038
2.42 to 24.20
1.042
R
wR
1
0.0588
0.1092
0.0589
0.1564
0.0494
0.1259
0.0686
0.1765
0.0599
0.1169
2
a
2
o
2 2
2 2 1/2
2
2
o
2
2
o
2
R
1
o
= ∑||F | − |F
c o
||/∑|F |, wR
2
= [∑[w(F
− F
c
) ]/∑[w(F
o
) ]] , w = 1/[σ (F
) + (aP) + bP], where P = [max(F
,0) + 2F
c
]/3.
been optimized by a density functional theory method with CRP-17453 Project “Nano Materiali Multifunzionali per Appli-
1
6
the hybrid PBE0 functional (with 25% of exact exchange) and cazioni nell’Elettronica Molecolare”, Fondazione Banco di Sar-
the augmented and polarized Pople type basis set 6-311++G- degna and INSTM. I. Freuze and C. Mézière are thanked for
(3df,2pd). The gas phase excited state energies were deter- the MS analyses.
mined at the same level by a linear response time-dependent
DFT method. To assess the error committed by ignoring the
solvent, the meta isomer ground state geometry and excited
state energies have also been computed with a polarizable con-
References
2
9
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Acknowledgements
This work was supported in France by the CNRS and the
University of Angers. The work in Italy was supported by
Regione Autonoma della Sardegna, L.R. 7-8-2007, Bando 2009,
8762 | Org. Biomol. Chem., 2014, 12, 8752–8763
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