One-electron oxidation-induced dimerising C-C coupling of a 2,5-diamino-1,4-benzoquinonediimine: A chemical and electrochemical investigation

Article abstract of DOI:10.1039/b408762c

The one-electron oxidation of N,N′,N″,N?-tetraneopentyl- 2,5-diamino-1,4-benzoquinonediimine 3 has been carried out using Ag(I) as an oxidising agent and led to the formation of the postulated nitrogen-based radical cation B. This unstable species can evolve by following two competing pathways, either a hydrogen atom abstraction from a THF solvent molecule, which leads to the N,N′,N″,N?-tetraneopentyl-2,5-diamino-1,4- benzoquinonemonoiminemonoiminium triflate 5, or a dimerisation of two carbon-based radical cations C, which affords the new dimer 4. An X-ray diffraction study of the latter established the presence of a centre of symmetry in the middle of the newly formed C_sp3-C_sp3 bond. Breaking of this bond upon reduction regenerates 3 under mild conditions. This type of chemically induced formation/breaking of a C-C single bond appears to be unprecedented in quinonoid chemistry. Electrochemical studies performed in THF confirm the proposed mechanism for the one-electron oxidation process.

Full text of DOI:10.1039/b408762c

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P A P E R
One-electron oxidation-induced dimerising C–C coupling
of a 2,5-diamino-1,4-benzoquinonediimine: a chemical and
electrochemical investigationw
a
a
b
a
Jean-philippe Taquet, Olivier Siri, Jean-Paul Collin, Abdellatif Messaoudi and
a
Pierre Braunstein*
a
Laboratoire de Chimie de Coordination (UMR 7513 CNRS), Universite´ Louis Pasteur,
4
, rue Blaise Pascal, 67070 Strasbourg cedex, France. E-mail: braunst@chimie.u-strasbg.fr
Laboratoire de Chimie Organo-Mine´rale (UMR 7513 CNRS), Universite´ Louis Pasteur,
, rue Blaise Pascal, 67070 Strasbourg cedex, France
b
4
Received (in Montpellier, France) 9th June 2004, Accepted 10th September 2004
First published as an Advance Article on the web 9th December 2004
0
00
000
The one-electron oxidation of N,N ,N ,N -tetraneopentyl-2,5-diamino-1,4-benzoquinonediimine 3 has been
carried out using Ag(I) as an oxidising agent and led to the formation of the postulated nitrogen-based
radical cation B. This unstable species can evolve by following two competing pathways, either a hydrogen
0
00
000
atom abstraction from a THF solvent molecule, which leads to the N,N ,N ,N -tetraneopentyl-2,5-diamino-
,4-benzoquinonemonoiminemonoiminium triflate 5, or a dimerisation of two carbon-based radical cations
C, which affords the new dimer 4. An X-ray diffraction study of the latter established the presence of a
centre of symmetry in the middle of the newly formed Csp bond. Breaking of this bond upon reduction
1
3
–Csp
3
regenerates 3 under mild conditions. This type of chemically induced formation/breaking of a C–C single
bond appears to be unprecedented in quinonoid chemistry. Electrochemical studies performed in THF
confirm the proposed mechanism for the one-electron oxidation process.
sent a large family of electroactive molecules with a well-
documented behaviour toward reduction, which leads to the
formation of radical anions semiquinones and/or aromatic
Introduction
The transfer of one electron from a strong electron donor to a
strong electron acceptor molecule has emerged over the last 15
years as a valuable concept for accomplishing novel reactions in
14
hydroquinones. We recently observed an unprecedented
reaction in quinonoid chemistry whereby reaction of a 6p +
1
2
organic and transition metal chemistry. Among them, oxida-
tive dimerisation of radical cations is generating increasing
interest, and this process plays a key role in the industrial
6p zwitterion with TCNE or TCNQ resulted in the regioselec-
tive, formal insertion of the olefin into a C–H bond of the
15
quinone ring. In contrast, very few examples of oxidation
reactions of quinonoid compounds have been reported in the
literature, such as the direct oxidative dimerisation resulting in
3
electropolymerisation of pyrroles and thiophenes. The oxida-
tive dimerisation induced by metal ions such as Ag(I), Tl(III) or
Sc(III) is also an efficient method to couple aromatic compounds
16–21
symmetrical bisquinones linked by Csp
our interest in the synthesis, electronic structure and coordina-
2
atoms.
As part of
4
such as benzenoid derivatives and porphyrins,
,5
6–12
allowing
the preparation of larger nanoscale arrays. A number of organic
one-electron oxidising agents, including triarylaminium salts
and tetracyanoolefinic compounds such as tetracyanoethylene
22–24
tion properties of new multifunctional quinonoid ligands,
we decided to explore the behaviour of the ‘‘potentially anti-
aromatic’’, acidichromic N,N ,N ,N -tetraneopentyl-2,5-dia-
in oxidation chemistry.
0
00 000
23a,b
(
TCNE) and 7,7,8,8-tetracyanodiquinodimethane (TCNQ), are
mino-1,4-benzoquinonediimine 3
also used efficiently in radical organic synthesis. In particular,
Clyburne and coworkers have recently coupled N-heterocyclic
carbenes 1 under mild conditions by tetracyanoethylene-in-
duced one-electron oxidation, which afforded the new air-
This molecule has been shown to contain the first example of
two separated, conjugated, and localized 6p-electron systems
that can be tuned by reversible protonation to become delo-
23b
calised. The corresponding N-benzyl derivative was found to
+
1
3
sensitive 1,2-dicationic dimers of type 2 [eqn. (1)]:
be a sensitive H chemosensor with large Stokes shifts (4120
nm) by comparison with conventional fluorophores.
2
3c
ð1Þ
The importance of quinonoid molecules in many areas of
1
chemistry and biology has been well-recognised. They repre-
4
We describe in this work the chemical and electrochemical
one-electron oxidation of 3 and the first regioselective oxidative
dimerisation reaction without dehydrogenation of the coupling
w Electronic supplementary information (ESI) available: computa-
tional details and cif file for compound 4 ꢀ 2H
2
O. See http://
www.rsc.org/suppdata/nj/b4/b408762c/
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
1
88
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 8 8 – 1 9 2

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site in quinonoid chemistry. The chemical reversal of this
reaction will also be discussed.
Results and discussion
Treatment of the tetraneopentyl derivative 3 in THF with 0.8
equiv. of AgOTf for 1 week in the absence of light resulted in a
colour change from yellow to red and precipitation of the pale-
1
yellow compound 4 in 34% yield (Scheme 1). Its H NMR
spectrum revealed the presence of an olefinic proton (d = 6.70)
3
and of a tertiary methine sp proton (d = 4.80). Furthermore,
the presence of two different signals for the neopentyl groups
clearly indicated a lower molecular symmetry of the dication in
4
the N–H protons, which is consistent with the presence of
compared to 3. We observed only one signal at d = 8.35 for
2
5
13
CbNH groups. The C spectrum (DEPT 135 experiment)
confirmed the presence of a sp carbon bearing only one proton
3
Fig. 1 Structure motif of the dicationic part of 4 in 4 ꢀ 2H
2
O (methyls
(
d
C–H = 40.88). Formation of a dicationic dimer was sup-
ported by electrospray mass spectrometry on the solid dis-
solved in CH CN, which showed in the positive ion mode
peaks at 416.3999 and 831.7947 amu corresponding to [4]
of t-Bu groups, CH protons and triflate counterions have been
2
omitted for clarity). Thermal ellipsoids enclose 50% of the electron
density.
3
2
+
+
and [4 ꢁ H] , respectively. Evaporation of the red THF fil-
trate, followed by the addition of hexane to remove soluble 3 in
excess, led in addition to a red precipitate. Comparison of its
between the two cycles. Examination of the bond distances
within the N(1)–C(3)–C(4)–C(5)–N(2) moiety shows an equal-
isation of the C–C and C–N bond distances, which is consistent
with the delocalisation of the 6p-electron system and the planar
1
13
23a,b
H and C NMR spectra with that of analogues
clearly
showed that 5 had formed (Scheme 1). The identity of this
compound as the monoprotonated form of 3 was then con-
firmed by the preparation of an authentic sample of 5 via direct
protonation of 3 with triflic acid.
2
3b
geometry of this moiety.
distances within the N(3)–C(6)–C(1)–C(2)–N(4) moiety reveal
In contrast, the C–C and C–N
two consecutive C–C single bonds and two CQN double
bonds, consistent with the presence of the sp carbon atom
3
Colourless crystals of compound 4 ꢀ 2H
2
O were obtained by
C(1). The C(2)–C(3) and C(5)–C(6) distances of 1.504(4) and
˚
slow evaporation of an acetone–chlorobenzene solution and
subjected to analysis by X-ray diffraction. However, owing to
the insufficient quality of the crystals, and despite numerous
attempts and changes in the crystallisation solvents, only the
structure motif could be determined, which, nevertheless,
clearly established a dicationic dimeric structure resulting from
1
.503(4) A, respectively, correspond to single bonds and ac-
count for the lack of conjugation between the two C
moieties.
We observed that AgBF was similarly effective for this
4
coupling reaction but with a significant reduction of the
reaction time (3 days instead of 1 week), and that the reaction
6
ring
C–C coupling of two C ring moieties without dehydrogena-
6
tion of the coupling site (Fig. 1). Selected bond lengths and
bond angles are reported in Table 1. There is a centre of
0
symmetry in the middle of the newly formed C(1)–C(1 ) bond
˚
whose length of 1.576(6) A corresponds to a single bond
Scheme 1 The proposed mechanism for the formation of 4 and 5
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 8 8 – 1 9 2
189

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˚
Table 1 Selected interatomic distances (A) and bond angles (1) in 4 ꢀ
2
2H O
0
0
0
C(1)–C(1 )
C(1)–C(2)
C(1)–C(6)
C(2)–C(3)
C(5)–C(6)
C(4)–C(5)
C(2)–N(4)
C(6)–N(3)
C(3)–N(1)
C(5)–N(2)
1.576(6)
1.518(4)
1.516(4)
1.504(4)
1.503(4)
1.392(4)
1.265(4)
1.263(4)
1.315(4)
1.324(4)
C(1 )–C(1)–C(2)
C(1 )–C(1)–C(6)
109.9(3)
110.0(3)
107.9(2)
115.2(3)
120.2(3)
125.7(3)
129.0(3)
C(2)–C(1)–C(6)
C(1)–C(2)–C(3)
C(2)–C(3)–C(4)
N(1)–C(3)–C(4)
N(4)–C(2)–C(1)
with either AgOTf or AgBF4 was further accelerated by
the addition of 1 equiv. of iodine (reaction in 1 day). Note
that direct treatment of 3 with only I resulted in no reaction
2
after 1 week.
By analogy with related studies,
(
1
9,26,27
a possible mechanism
Scheme 1) for this reaction could involve first the one-electron
oxidation of an amino nitrogen atom of 3, leading to the nitro-
gen-centred radical cation A. Proton migration to the more
2
basic proximal sp nitrogen atom would result in the protona-
Fig. 2 Representation of the frontier orbital SOMO for the radical
d
+
.
tion of the other vinamidine moiety and formation of a
cyanine-type structure stabilised by intramolecular electronic
delocalisation between the two nitrogen atoms (intermediate
cation [3]
from the solvent leads to 5. Nevertheless, the yield of the C–C
dimer 4 is comparable to those observed in the oxidative
2
3b
B). This postulated radical cation B could evolve by follow-
ing two competing pathways. The first one would involve
hydrogen atom abstraction from the solvent THF, a well-
1
1,12,33
coupling reactions of Zn(II) porphyrins.
The proposed mechanism for the formation of 4 and 5
suggests that electrochemical oxidation could also lead to the
formation of the dicationic dimer present in 4, as also observed
2
8–30
known reaction in radical chemistry,
benzoquinonemonoiminemonoiminium triflate 5 (the corre-
which leads to the
2
3b
11
sponding chloride has been previously characterised).
The
for the formation of meso,meso-coupled porphyrin arrays.
Thus, exploration of the electrochemical oxidation of 3 was
performed using cyclic voltammetry and controlled potential
second possibility consists in the delocalisation of the radical
onto the central electrophilic carbon atom (intermediate C),
which restores two CQN double bonds and maintains delo-
calisation of the positive charge between the NHR groups.
4 6
electrolysis in anhydrous THF containing N(n-Bu) PF as
supporting electrolyte, with a platinum net as working elec-
trode. The voltammogram of 3 (Fig. 3) shows an irreversible
oxidation wave at 1.08 V vs. SCE, resulting from a one-electron
oxidation process, and consequently leading to the formation
of a radical cation. The irreversibility of the electrochemical
process is consistent with an irreversible formation of the
radical cation in B by rapid proton migration in the radical
Dimerisation of two such radical cations [radical radical cation
(
1
RRC) mechanism], would then lead to the triflate salt of a
dicationic C–C dimer, 4. Interestingly, related s-dimeric struc-
tures have been isolated as intermediates in the oxidative
4
dimerisation of aromatic compounds (aminobenzenes).
Similar pathways involving hydrogen atom abstraction or
radical cations dimerisation have also been discussed for the
reactions of N-heterocyclic carbenes 1, but in this case only
hydrogen atom abstraction or only dimerisation of two radical
cations occurred, selectively depending on the one-electron
ꢁ
1
cation precursor A. Increasing the scan rate to 8 V s did not
allow to reach reversibility. A controlled potential electrolysis
during 2 h in anhydrous THF was realised at 1.3 V vs. SCE in
order to see whether the electrochemical oxidation leads to the
same ratio of products 4 and 5 as the chemical oxidation.
Unfortunately, no precipitation of 4 was observed and the
colour of the solution, which turned from yellow before
electrolysis to red, obviously indicated the formation of 5 by
hydrogen atom abstraction from THF, consistent with the
formation at the electrode of only a small amount of radical
cation, which disfavours its dimerisation in solution.
1
3
oxidant used.
We were unable to spectroscopically confirm the formation
of a radical cation [3]d (i.e., in A, B or C) so we turned to
computational studies in order to obtain some information on
its structure. Qualitative computations were performed on
+
d
+
31
[
CACAO program (computer aided composition of atomic
3]
¨
using the extended Huckel theory (EHT) with the
3
2
orbitals). Details on the calculated radical cation obtained
from 3 are given in the ESI and the most interesting result is the
high spin density concentrated on the central olefinic carbon
atoms. This is also consistent with the calculated net charge on
these carbon atoms, which increases from ꢁ0.296 e in the
Deprotonation of 4 was also attempted, in particular in
order to prepare a new neutral bis(aminotriimine) ligand of
interest in coordination chemistry. Thus, 4 was reacted in
CH Cl with NEt and after 5 min, the reaction mixture was
2
2
3
quenched with water, the organic phase was separated and the
solvent evaporated to dryness. The solid residue was taken up
in hexane and the yellow filtrate was slowly evaporated,
leading to the formation of pure yellow prisms that could be
d
+
neutral molecule 3 to ꢁ0.049 e in the radical cation [3] . The
shape of the singly occupied molecular orbital (SOMO) (Fig. 2)
clearly suggests that dimerisation of the radical cation will
occur through C–C bond formation.
1
separated and analysed by H NMR spectroscopy. Unexpect-
edly, the benzoquinonediimine 3 was recovered in 50% yield.
Formation of a white, still unidentified, compound in irrepro-
The yields obtained for 4 and 5, slightly in favour of 5, could
be explained by the relatively slow kinetics of the dimerisation
reaction. Indeed, the rate-determining step of the overall
reaction is presumably the formation of the radical cation
from 3 and, under our reaction conditions, at any given time
only a small amount of radical cations is generated, which
limits the dimerisation process. Thus, alternative reaction
pathways become possible and abstraction of a hydrogen atom
1
13
ducible yields was sometimes observed (its H and C NMR
data suggest a hydrolysis product of 4). The clean chemical
reduction of the dimer 4 was achieved by redox titration with 2
3
4
equiv. of benzophenoneketyl radical (ꢁ1.88 V vs. SCE),
previously generated by reaction of excess sodium with benzo-
phenone in THF, and 3 was obtained in quantitative yield.
1
90
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 8 8 – 1 9 2

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ꢁ1
4 6
Fig. 3 Cyclic voltammogram of 3 in anhydrous THF [0.1 M N(n-Bu) PF ] at a scan rate of 100 mV s .
This reaction, which consumes two electrons, clearly shows
that the formation of the C–C s-bond that led to 4 can be
reversed upon reduction. Cyclic voltammetry of 4 in anhydrous
THF or CH Cl showed only an irreversible reduction wave at
performed by the ‘‘Service de Microanalyse’’, Universite Louis
´
0
00
000
Pasteur (Strasbourg, France). The N,N ,N ,N -tetraneopenty-
lamino-1,4-benzoquinonediimine 3 was prepared according to
2
3a,b
the literature.
2
2
ꢁ
0.72 V vs. SCE. Controlled potential electrolysis for 0.5 h at
1.35 V vs. SCE of 4 in anhydrous THF or CH Cl allowed
ꢁ
2
2
Syntheses of 4 and 5
the observation of an oxidation wave of the product corre-
sponding to that of 3. Although new in quinonoid chemistry,
this type of chemically induced formation/breaking of a C–C
single bond is not unprecedented and Berke and coworkers
have recently observed this process in dimeric manganese
A mixture of 3 (1.00 g, 2.40 mmol) and AgOTf (0.49 g, 1.92
mmol) in dry THF (150 ml) was stirred at room temperature, in
the absence of light, for 1 week under nitrogen. Formation of a
silver mirror and a precipitate was observed. After filtration,
the solid was extracted with CH Cl and flash evaporation of
3
5
carbyne complexes. There is also an interesting analogy
between the behaviour of 4 in basic medium and the slow
2
2
the solvent and drying under vacuum afforded 4 as a cream
powder. The red filtrate of the reaction mixture was taken to
dryness. Addition of hexane removed excess 3 and afforded 5
decomposition in CHCl of s-dimeric 5,15-dioxoporphodi-
3
methenes, obtained by oxidation of Zn(II) octaethylporphyrin
with Tl(III), into the monomeric dioxoporphodimethene and an
as a red powder.
4 (0.37 g, 34% yield): H NMR (500.13 MHz, CDCl ) d: 1.00
3
3
1
unknown compound.
3
(
s, 36 H, CH
3.64 (m, 8 H, CH
H, NbCbC–H), 8.35 (br s, 4 H, N–H); C{ H} NMR
3
), 1.03 (s, 36 H, CH
3 2
), 3.30 (s, 8 H, CH –N), 3.58–
Conclusion
2
–NH), 4.80 (s, 2 H, NQC–C–H), 6.70 (s, 2
1
3
1
I
0
00
000
The Ag promoted oxidative coupling reaction of N,N ,N ,N -
tetraneopentyl-2,5-diamino-1,4-benzoquinonediimine 3, which
is itself the two-electron oxidation product of the aromatic
compound 6, resulted in the formation of a radical cation,
which evolves either by hydrogen atom abstraction from the
solvent or by a dimerisation reaction, the latter being the first
non-dehydrogenative and reversible C–C coupling in quino-
noid chemistry.
(
125.77 MHz, CDCl
3 3 3
) d: 27.48 (CMe ), 27.69 (CMe ), 32.60
(
6
CMe ), 33.01 (CMe ), 40.88 (NQC–C–H), 54.82 (CH –NH),
3
3
2
2
3.76 (CH –N), 93.08 (NbCbC–H), 151.85 (CbN), 156.25
(
CQN); HRMS (ESI) calcd. for C H N : m/z 416.3879
5
2
96
8
2+ +
[M]
,
found: 416.3999; 831.7679 [M ꢁ H] , found:
+
8
31.7947; 981.7278 [M + OTf] , found: 981.7595; anal. calcd
: C, 57.32; H, 8.55; N, 9.90; found: C,
7.00; H, 8.47; N, 10.02; UV-vis (CH Cl ) lmax: 368(br) nm
96 6 8 6 2
for C54H F N O S
5
2
2
(
log e = 4.64).
1
5
s, 18 H, CH
(0.48 g, 44% yield): H NMR (300.13 MHz, CDCl ) d: 1.03
3
(
(
(
3
), 1.04 (s, 18 H, CH
3 2
), 3.13 (s, 4 H, CH –N), 3.21
3
d, J
= 6.06 Hz, 4 H, CH –NH), 5.46 (s, 2 H, HC ), 7.69
2
sp2
HH
1
br t, 2 H, NH), 8.29 (br s, 1 H, NH); C{ H} NMR (75.5
3
1
MHz, CDCl ) d: 27.70 (CH ), 27.90 (CH ), 32.32 (CMe ),
3
3
3
3
3
3.89 (CMe
3
), 55.50 (CH
2
N), 58.59 (CH
2
N), 87.24 (HCsp ),
2
1
sistent with a rapid equilibrium between two tautomers, gen-
erating an average structure of higher symmetry in solution.
50.74 (Csp N), 152.47 (Csp N). The NMR data of 5 are con-
2
2
The extension of this reaction to other benzoquinonedi-
imines is in progress and should provide an easy entry into a
new class of aminoimine type ligands. Further studies on the
deprotonation of the C–C coupling product 4 are also in
progress in order to fully understand the mechanism of the
C–C bond breaking induced by deprotonation, as well as
studies of the influence of the nature of the one-electron
oxidising agent used on the reactivity of 3.
2
3b
Reaction of 5 with the benzophenoneketyl radical
A blue solution of benzophenoneketyl radical in 5 ml THF,
previously generated by reaction of benzophenone (0.07 mg,
0.4 mmol) with excess solid Na in 5 ml of dry THF, was added
dropwise to a suspension of 4 (0.23 g, 0.20 mmol) in dry THF
(150 ml) at room temperature. The yellow solution was taken
to dryness and 3 was recovered quantitatively as a yellow
Experimental
General details
1
powder. It was characterised by comparison of its H NMR
2
data with those of an authentic sample.
3b
All solvents were dried and distilled using common techniques
unless otherwise stated. All manipulations were performed
using standard Schlenk techniques under dry nitrogen atmo-
1
13
Electrochemical measurements
sphere. H NMR (300.13 or 500.13 MHz) and C NMR (75.5
or 125.77 MHz) spectra were recorded on a Bruker AC-300 or
AMX-500 instrument. ESI mass spectra were recorded on an
Autospec HF mass spectrometer. Elemental analyses were
Electrochemical experiments were performed with a three-
electrodes system consisting of a platinum working electrode,
a platinum wire counter electrode, and a standard reference
N e w J . C h e m . , 2 0 0 5 , 2 9 , 1 8 8 – 1 9 2
191

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Table 2 Crystallographic data for compound 4 ꢀ 2H
2
O
5
6
7
A. McKillop, A. G. Turrell, D. W. Young and E. C. Taylor, J.
Am. Chem. Soc., 1980, 102, 6504.
X. Peng, Y. Nakamura, N. Aratani, D. Kim and A. Osuka,
Tetrahedron Lett., 2004, 45, 4981.
(a) A. Nakano, N. Aratani, H. Furuta and A. Osuka, Chem.
Commun., 2001, 1920; (b) A. Tsuda, Y. Nakamura and A. Osuka,
Chem. Commun., 2003, 1096.
N. Aratani, A. Osuka, Y. H. Kim, D. H. Jeong and D. Kim,
Angew. Chem., Int. Ed., 2000, 39, 1458.
T. Ogawa, Y. Nishimoto, N. Yoshida, N. Ono and A. Osuka,
Angew. Chem., Int. Ed., 1999, 38, 176.
N. Yoshida, H. Shimidzu and A. Osuka, Chem. Lett., 1998, 55.
T. Ogawa, Y. Nishimoto, N. Yoshida, N. Ono and A. Osuka,
Chem. Commun., 1997, 337.
Formula
C
54
H
96
N
8
F
O
6 6
S
2
2
ꢀ 2H O
ꢁ1
Formula wt/g mol
Crystal system
Space group
1167.54
Monoclinic
P2 /n
1
˚
a/A
12.3030(3)
17.0200(4)
16.8770(6)
˚
b/A
˚
c/A
8
9
b/1
U/A
96.6510(11)
3510.21(17)
2
3
˚
1
1
0
1
Z
ꢁ
3
r
calc/g cm
1.105
0.141
ꢁ1
m(MoKa)/mm
F(000)
T/K
1
2
H. Shimidzu and A. Osuka, Angew. Chem., Int. Ed., 1997, 36,
135.
13 T. Ramnial, I. McKenzie, B. Gorodetsky, E. M. W. Tsang and J.
1260
173
Total reflect.
Unique reflect.
12183
12182
0.1451
A. C. Clyburne, Chem. Commun., 2004, 1054.
¨
S. Pataı and Z. Rappoport, The Chemistry of the Quinonoid
Compounds, Wiley, Chichester, 1988.
J.-p. Taquet, Q.-Z. Yang, O. Siri and P. Braunstein, unpublished
results.
14
15
16
R
int
saturated calomel electrode (SCE), versus which all potentials
are reported. All measurements were carried out under Ar, in
degassed THF (previously distilled from Na/benzophenone
L. S. Liebeskind and S. W. Riesinger, Tetrahedron Lett., 1991,
32, 5681.
17 J. H. P. Schutte, J. A. K. du Plessis, G. Lachmann, A. M. Viljoen
and H. D. Nelson, J. Chem. Soc., Chem. Commun., 1989, 1290.
P. Joseph-Nathan, M. E. Garibay and R. L. Santillan, J. Org.
Chem., 1987, 52, 759.
under N
0
2
) or CH Cl
6
PF solutions as the supporting electrolyte. An
2
2 2 2
(distilled from CaH under N ), using
1
1
8
9
.1 M N(n-Bu)
4
EG&G Princeton Applied Research Model 273A potentiostat
connected to a computer (Programme Research Electrochem-
istry Software) and a Brucker EI 30M potentiostat connected
to a printing table were used for the cyclic voltammetry
measurements.
A. R. Forrester, A. S. Ingram, I. L. John and R. H. Thomson, J.
Chem. Soc., Perkin Trans. 1, 1975, 12, 1115.
20 K. Chandrasenan and R. H. Thomson, Tetrahedron, 1971, 27,
2529.
J. F. Corbett, J. Chem. Soc. (C), 1970, 1912.
(a) P. Braunstein and O. Siri, Chem. Commun., 2002, 208; (b) P.
´
Braunstein, O. Siri, J.-p. Taquet, M.-M. Rohmer, M. Benard and
2
1
2
2
X-Ray data
R. Welter, J. Am. Chem. Soc., 2003, 125, 12246; (c) J.-p. Taquet,
O. Siri, P. Braunstein and R. Welter, Inorg. Chem., 2004, 43,
Diffraction data were collected on a Kappa CCD diffract-
ometer using graphite-monochromated MoKa radiation (l =
6
944–6953.
23 (a) O. Siri and P. Braunstein, Chem. Commun., 2000, 2223; (b) O.
Siri, P. Braunstein, M.-M. Rohmer, M. Benard and R. Welter, J.
˚
.71073 A). The relevant data are summarised in Table 2. Data
´
0
were collected using phi scans and the structures were solved by
Am. Chem. Soc., 2003, 125, 13793; (c) M. Elhabiri, O. Siri, A.
Sornosa-Tent, A.-M. Albrecht-Gary and P. Braunstein, Chem.
Eur. J., 2004, 10, 134; (d) P. Braunstein, A. Demessence, O. Siri
and J.-p. Taquet, C. R. Chim., 2004, 7, 909–913.
3
6,37
direct methods using the SHELX 97 software; the refine-
2
ment was by full-matrix least squares on F . No absorption
correction was used. All non-hydrogen atoms were refined
anisotropically with H atoms introduced as fixed contributors
˚
2
4
P. Braunstein, O. Siri, J.-p. Taquet and Q.-Z. Yang, Chem. Eur.
J., 2004, 10, 3817.
(
d
C–H = 0.95 A, U11 = 0.04). The insufficient quality of the
25 C. Rabiller, G. Ricolleau, M. L. Martin and G. J. Martin, New J.
Chem., 1980, 4, 35.
2
crystals precluded a completely satisfactory refinement of the
structure, which was therefore not deposited at the CCDC.
6
S. P. Rath, R. Koerner, M. M. Olmstead and A. L. Balch, J. Am.
Chem. Soc., 2003, 125, 11798.
2
7
A. L. Balch, B. C. Noll, S. M. Reid and E. P. Zovinka, J. Am.
Chem. Soc., 1993, 115, 2531.
Acknowledgements
28 P. Renaud, F. Beaufils, L. Feray and K. Schenk, Angew. Chem.,
Int. Ed., 2003, 42, 4230.
This work was supported by the Centre National de la
2
9
D. Crich, X. Huang and M. Newcomb, Org. Lett., 1999, 1
225.
Recherche Scientifique and the Ministe
tionale, de l’Enseignement Superieur et de la Recherche (Ph.D.
grant for J.-p. T.). We are also grateful to Prof. R. Welter
ULP Strasbourg) for the crystal structure analysis, to Dr. G.
Royal (Universite Joseph Fourier Grenoble) for preliminary
electrochemical studies on 3 and to Dr. M. Benard (ULP
`
re de l’Education Na-
´
30 S. Nakahama, S. Hino and N. Yamazaki, Polymer, 1971, 2
6.
(a) R. Hoffmann and W. N. Lipscomb, J. Chem. Phys., 1962, 36,
5
3
1
(
2
1
179; (b) R. Hoffmann and W. N. Lipscomb, J. Chem. Phys.,
962, 37, 2872; (c) R. Hoffmann, J. Chem. Phys., 1963, 39, 1397.
´
´
The parameters used and the Mulliken orbital populations
obtained for the metal atoms are available in the Electronic
supplementary information (ESI, Tables S1 and S2).
C. Mealli and D. M. Proserpio, J. Chem. Educ., 1990, 67, 399.
J. H. Fuhrhop, E. Baumgartner and H. Bauer, J. Am. Chem.
Soc., 1981, 103, 5854.
Strasbourg) for discussions on the theoretical analysis.
3
3
2
3
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