Donor-substituted octacyano[4]dendralenes: Investigation of π-electron delocalization in their radical ions

Article abstract of DOI:10.1021/jo301194y

Symmetrically and unsymmetrically electron-donor-substituted octacyano[4]dendralenes were synthesized and their opto-electronic properties investigated by UV/vis spectroscopy, electrochemical measurements (cyclic voltammetry (CV) and rotating disk voltammetry (RDV)), and electron paramagnetic resonance (EPR) spectroscopy. These nonplanar push-pull chromophores are potent electron acceptors, featuring potentials for first reversible electron uptake around at -0.1 V (vs Fc⁺/Fc, in CH₂Cl₂ + 0.1 M n-Bu₄NPF₆) and, in one case, a remarkably small HOMO-LUMO gap (ΔE = 0.68 V). EPR measurements gave well-resolved spectra after one-electron reduction of the octacyano[4]dendralenes, whereas the one-electron oxidized species could not be detected in all cases. Investigations of the radical anions of related donor-substituted 1,1,4,4-tetracyanobuta-1,3-diene derivatives revealed electron localization at one 1,1-dicyanovinyl (DCV) moiety, in contrast to predictions by density functional theory (DFT) calculations. The particular factors leading to the charge distribution in the electron-accepting domains of the tetracyano and octacyano chromophores are discussed.

Full text of DOI:10.1021/jo301194y

Article
pubs.acs.org/joc
Donor-Substituted Octacyano[4]dendralenes: Investigation of
π‑Electron Delocalization in Their Radical Ions
Benjamin Breiten,† Markus Jordan,† Daisuke Taura,† Michal Zalibera,‡ Markus Griesser,‡
Daria Confortin,‡ Corinne Boudon,§ Jean-Paul Gisselbrecht,§ W. Bernd Schweizer,† Georg Gescheidt,*,‡
and Franco̧
is Diederich*,†
†Laboratory of Organic Chemistry, ETH Zurich, Honggerberg, HCI, CH-8093 Zurich, Switzerland
̈
‡Institute of Physical and Theoretical Chemistry, Graz University of Technology, Stremayrgasse 9, A-8010 Graz, Austria
§Laboratoire d’Electrochimie et de Chimie Physique du Corps Solide, UMR 7177 CNRS, Universite
́
de Strasbourg, 4 rue Blaise
Pascal, F-67081 Strasbourg Cedex, France
S
* Supporting Information
ABSTRACT: Symmetrically and unsymmetrically electron-
donor-substituted octacyano[4]dendralenes were synthesized
and their opto-electronic properties investigated by UV/vis
spectroscopy, electrochemical measurements (cyclic voltam-
metry (CV) and rotating disk voltammetry (RDV)), and
electron paramagnetic resonance (EPR) spectroscopy. These
nonplanar push−pull chromophores are potent electron
acceptors, featuring potentials for first reversible electron
uptake around at −0.1 V (vs Fc+/Fc, in CH2Cl2 + 0.1 M n-
Bu4NPF6) and, in one case, a remarkably small HOMO−
LUMO gap (ΔE = 0.68 V). EPR measurements gave well-
resolved spectra after one-electron reduction of the
octacyano[4]dendralenes, whereas the one-electron oxidized
species could not be detected in all cases. Investigations of the radical anions of related donor-substituted 1,1,4,4-tetracyanobuta-
1,3-diene derivatives revealed electron localization at one 1,1-dicyanovinyl (DCV) moiety, in contrast to predictions by density
functional theory (DFT) calculations. The particular factors leading to the charge distribution in the electron-accepting domains
of the tetracyano and octacyano chromophores are discussed.
phores.2e,f,10 Recently, we reported the first double addition of
1. INTRODUCTION
TCNE to the two adjacent CC triple bonds in symmetric, 1,4-
π-Conjugated donor−acceptor (D−A) chromophores1 have
dianilino-substituted buta-1,3-diynes under formation of donor-
recently attracted renewed interest in view of potential
substituted octacyano[4]dendralenes.11 These novel chromo-
applications in the fabrication of opto-electronic materials.2
phores, despite their pronounced nonplanarity, feature strong
The energy and intensity of their characteristic intramolecular
intramolecular CT interactions as well as a high propensity for
charge-transfer (CT) transitions as well as their third-order
reversible electron uptake. Initial electron paramagnetic
optical nonlinearities depend both on the strength of the
resonance (EPR) measurements provided evidence for the
electron donor and acceptor moieties and the nature of the
formation of charge-separated paramagnetic species pointing to
delocalized organic radical anions in which the spin population
connecting π-conjugated spacer.3 Numerous potent organic
electron acceptors are cyano-rich derivatives, taking advantage
mainly resides at cyano group-containing moieties.
of the strong electron-accepting power of the cyano group, as
Here, we report a comprehensive study of the opto-
compared to its low molecular weight.4 Tetracyanoethene
electronic properties of symmetrically and unsymmetrically
donor-substituted octacyano[4]dendralenes 1−3 and compare
(TCNE)5 and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ),6
together with their derivatives and analogues,7 are possibly the
these properties to those of related 1,1,4,4-tetracyanobuta-1,3-
most prominent and most widely used representatives.
dienes (TCBDs) 4 and 5. The report has a special focus on the
TCNE readily reacts with organo-donor-activated CC triple
radical ions of these systems that are investigated in a combined
bonds to provide, via a formal [2 + 2] cycloaddition followed
experimental (EPR) and computational approach.
by retro-electrocyclization, donor-substituted 1,1,4,4-tetracya-
nobuta-1,3-dienes.8 The scope of this versatile transformation is
Special Issue: Howard Zimmerman Memorial Issue
very broad.9 The nature of both the donor, which activates the
alkyne, and the electron-deficient olefin can be greatly varied,
yielding entire new families of nonplanar push−pull chromo-
Received: June 12, 2012
© XXXX American Chemical Society
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Scheme 1. Syntheses of Octacyano[4]dendralenes 1−3
Figure 1. ORTEP plots of the solid-state molecular structures of 1 (left) and 3 (right). T = 100 K. Thermal ellipsoids are drawn at the 50%
probability level; arbitrary numbering; hydrogen atoms are omitted for clarity. Selected dihedral angles in 1 [°]: C3−C4−C5−C6 56.2(5), C21−
C5−C6−C16 −91.6(5), C2−C3−C4−C26 88.3(6), C26−C4−C5−C21 60.4(6). Selected dihedral angles in 2 [°]: C7−C8−C9−C10 50.1(7),
C43−C7−C8−C48 68.5(9), C48−C8−C9−C53 56.0(9), C53−C9−C10−C58 66.2(8).
Scheme 1). For comparison, in the physical studies, we also
prepared by a one-pot protocol the octacyano[4]dendralenes
211 and 3, starting from buta-1,3-diynes 6 and 7, respectively.
The known TCBDs 8 and 9 served as additional control
compounds.8b
For octacyano[4]dendralenes 1 and 3, NMR spectra could
not be recorded due to the presence of paramagnetic species in
the sample, as confirmed by electron paramagnetic resonance
(EPR) spectroscopy. Therefore, we turned to X-ray crystallog-
raphy in order to establish their structures in an unambiguous
way.
2.2. X-ray Structures of Octacyano[4]dendralenes 1
and 3. Crystals suitable for X-ray diffraction were obtained by
layering solutions of 1 or 3 in CH2Cl2 with n-hexane and
subsequent slow evaporation of the solvents. Dendralene 1
crystallizes in the monoclinic space group P21/c. The molecular
structure (Figure 1 left) shows a syn-conformation of the donor
2. RESULTS AND DISCUSSION
2.1. Synthesis. Recently, we found that the regioselectivity
of the TCNE addition to a buta-1,3-diyne end-capped with a
ferrocenyl (Fc) and a N,N-dimethylanilino (DMA) donor can
be switched upon changing the pH.12 At neutral pH, DMA is
+
the better activator (Hammett constants σp : Fc, − 1.00; NMe2,
−1.70),13 and cycloaddition/retro-electrocyclization (CA/RE)
occurs at the adjacent triple bond, yielding TCBD 4 (98%
yield). Upon protonation of the anilino group, activation by Fc
dominates and leads, after neutralization, to the formation of
the regioisomeric TCBD 5 (83%). Interestingly, monoadduct 4
did not undergo a second CA/RE reaction to the desired bis-
adduct 1, even with an excess of TCNE at 80 °C and at
prolonged reaction time. On the other hand, the stronger
electron donor DMA in monoadduct 5 enabled the second
cycloaddition, and the unsymmetrically donor-substituted
octacyano[4]dendralene 1 was obtained in good yield (78%;
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Figure 2. UV/vis absorption spectra of 1−3 in CH2Cl2 at 298 K (c ∼ 10−5 M).
substituents in the solid state, with a torsion angle of the
dendralene backbone (C3−C4−C5−C6) of 56.2(5)°. A similar
syn-conformation had previously been observed for the solid-
state structure of octacyano[4]dendralene 2.11 In this geometry,
all eight cyano groups converge into one hemisphere, whereas
the Fc and the DMA donor moieties point into the other. The
angles between the donor planes (phenyl ring of DMA and
substituted Cp ring of Fc) and their adjacent dicyanovinyl
(DCV) acceptor groups are almost matching (23.1° and 23.2°)
and close to planarity, allowing efficient intramolecular CT
interactions. However, two of the three dihedral angles between
the four DCV substituents on the dendralene backbone are
close to perpendicularity (between 86.0(5)° and 95.0(5)°; see
Figure 1 left), leading to particular electronic effects (see
below). This sterical arrangement induces a certain helicity of
the dendralene backbone as well as a favorable dipolar
interaction between the nitrile group C42N43 of the DCV
next to the Fc substituent and the DCV moiety C6
C16(CN)2 adjacent to the anilino donor (d(N43···C6) =
3.42 Å). This interaction might actually be a reason for the syn-
alignment of the donor substituents.
adjacent DCV groups (30.3° and 22.3°) are, however, similar to
the corresponding angles in 1.
2.3. Electronic Absorption Spectra. The UV/vis spectra
of chromophores 1−3 display two bathochromically shifted
intramolecular CT bands with absorptions reaching into the
near-infrared (NIR) region (Figure 2). Support for the CT
character of these bands was obtained by protonation with
CF3COOH, which led to their disappearance for compound 2
and 3, while subsequent reneutralization restituted the
absorption bands, as previously described.14 Protonation of a
solution of 1 with CF3COOH shows only a reduction of the
CT band, while subsequent reneutralization restituted the
absorption bands. A complete disappearance of the CT band
could not be observed due to the presence of the ferrocene
donor, which is not protonated. The intense CT band at higher
energy (λmax around 500 nm) involves the transition from the
donors to the adjacent, nearly coplanar and hence strongly π-
coupled DCV acceptor.15 The low-intensity, low-energy CT
transition at around 700 nm involves weakly π-coupled donor
and acceptor moieties and most probably results from CT from
an anilino to the cross-conjugated TCBD moiety at the center
of the molecules.
2.4. Electrochemistry. The electron-transfer properties of
1, 2,11 and 3 were investigated by cyclic (CV) and rotating disk
voltammetry (RDV), and the corresponding potentials are
summarized in Table 1. For some derivatives, reproducible data
could only be observed on freshly polished working electrodes
due to electrode inhibition during redox processes.
The anilino donor in octacyano[4]dendralene 1 is
responsible for the one-electron irreversible oxidation at
+1.07 V, compatible with related potentials of DMA oxidation
in TCBD derivatives.8b However, the oxidation potential is 320
mV anodically shifted compared to that of the anilino moiety in
TCBD 4.12 The influence of the octacyano[4]dendralene
acceptor on the oxidation potential of the Fc moiety is less
pronounced, with a reversible one-electron oxidation at +0.57 V
(5: +0.49 V).12 The voltammogram of [4]dendralene 3 can be
interpreted in terms of an irreversible two-electron oxidation
connected with the precipitation of the oxidized species at the
electrode surface. Indeed, in the reverse scan, a reduction peak
Compound 3 crystallizes in the triclinic space group P1 with
̅
two CH2Cl2 molecules in the asymmetric unit. The solid-state
geometry of 3 (Figure 1 right), with bulky terminal triarylamine
donor moieties, differs substantially from the one of compound
1. Although the torsion angle (C7−C8−C9−C10) of the
central dendralene fragment of 3 is quite similar in 3 (50.1(7)°)
and in 1 (56.2(5)°), the molecule adopts an extended anti-
conformation, probably because of steric repulsion between the
two triarylamino groups in a syn-arrangement. The bulkiness of
these groups forces a denser arrangement of the DCV moieties
on the [4]dendralene scaffold of 3, with substantially smaller
dihedral angles between 50.1(7)° and 68.1(9)° as compared to
1. Furthermore, the angles between the aniline donors and the
second neighboring DCV moiety in 3 (C4−C7−C8−C48
−116.2(7)°; C53−C9−C10−C11 −115.7(6)°) are significantly
more obtuse than the corresponding angles in 1 (C31−C3−
C4−C26 −95.0(5)°; C21−C5−C6−C7 90.8(6)°). The angles
between the planes of the anilino moieties and their directly
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Table 1. Cyclic Voltammetry (CV; Scan Rate v = 0.1 V s−1)
ing oxidations present also nonreversible features. In the latter
case, the first oxidation proceeds at the ferrocene moiety (1, 4,
5) in a reversible fashion. When only the anilino substituent is
present (2, 3), the redox potentials are in line with amine
oxidation. For the triphenylamino derivative 3, oxidation is
irreversible, presumably owing to dimerization, also in line with
the observed precipitations (see above). Only in the case of 2,
reversible cyclovoltammetric curves were recorded. For EPR
investigations, radical ions can be formed either by chemical
reactions or by electrolysis.16 Both approaches were followed to
generate radical cations of 1−5; the compounds were dissolved
in CH2Cl2 and chemically oxidized with either phenyliodine-
(III) bistrifluoroacetate (PIFA) or NOSbF6; moreover, anodic
oxidation was performed on a Pt electrode. Although the strict
high-vacuum conditions utilized for the production of the
oxidized species often allow even the detection of paramagnetic
species connected with irreversible cyclovoltammograms, none
of the investigated molecules gave rise to clearly distinguishable
EPR spectra after oxidation. In the case of 1, 4, and 5, an
Fe(III) species is expected. It is well established that oxidized
ferrocenes give rise to EPR spectra only in the solid state at
temperatures close to 4 K with characteristic signals pointing to
the axial symmetry of the paramagnetic center (g∥ = 4.3−2.6
and g⊥ = 1.9−1.2, depending on the substitution and character
of the solid matrix).17 Therefore, it is not unexpected that we
were unable to observe the corresponding signals neither in
fluid nor in frozen (77 K) solution. Nevertheless, this underpins
that the ferrocene unit is the electroactive part of 1, 4, and 5 in
the oxidative regime. In the case of 3, we were not able to
detect an EPR spectrum upon chemical or electrochemical
oxidation mirroring the irreversible oxidation wave in the
cyclovoltammogram. Unexpectedly, also oxidation of 2 did not
lead to a discernible EPR spectrum.
and Rotating Disk Voltammetry (RDV); Solvent CH2Cl2
a
(+0.1 M n-Bu4NPF6)
CV
RDV
c
d
f
ΔEp
Ep
slope
b
e
compd
E° (V)
(mV)
(V)
E1/2 (V)
(mV)
1
+1.07
g
+0.57 (1e−)
−0.11 (1e−)
−0.63 (1e−)
−1.60 (1e−)
−1.78 (1e−)
+0.99 (2e−)
−0.08 (1e−)
−0.58 (0.5e−)
−0.66 (0.5e−)
−1.56 (1e−)
−1.71 (1e−)
80
80
100
70
60
70
70
60
60
70
70
+0.56 (1e−)
−0.12 (1e−)
−0.64 (1e−)
−1.59 (1e−)
−1.76 (1e−)
+0.98 (2e−)
−0.09 (1e−)
−0.62 (1e−)
−1.70 (2e−)
60
60
90
60
110
60
h
2
75
85
140
3
+0.93
+0.89 (2e−)
−0.08 (1e−)
−0.60 (1e−)
−1.50 (1e−)
−1.71 (1e−)
60
60
70
70
70
−0.08 (1e−)
−0.57 (1e−)
−1.47 (1e−)
−1.68 (1e−)
75
90
80
80
a
All potentials are given versus the Fc+/Fc couple used as the internal
b
standard. Eo = (Epc + Epa)/2, where Epc and Epa correspond to the
c
cathodic and anodic peak potentials, respectively. ΔEp = Epa − Epc.
Ep = irreversible peak potential. E1/2 = half-wave potential.
Logarithmic analysis of the wave obtained by plotting E versus
log[I/(Ilim − I)]. Very small amplitude signal due to electrode
inhibition. Taken from ref 11.
d
e
f
g
h
whose shape is in agreement with redissolution is observed at
+0.54 V. On the other hand, the four DCV moieties of 1 and 3
gave rise to four reversible one-electron reduction steps. The
first two reductions are strongly facilitated compared with
TCBD derivatives (−0.11 and −0.63 V for 1 and −0.08 and
−0.57 V for 3), revealing the lowest electrochemical HOMO−
LUMO gap (ΔE = 0.68 V for 1) ever observed for this type of
D−A chromophores.2e,f Therefore, an efficient CT can be
considered, leading to a stabilized charge-separated state.
2.5. Radical Ions of CT Chromophores 1−5. The
electrochemical measurements on 1−5 indicated that one-
electron reduction is reversible with the tetra- and octacyano
moieties being the electroactive parts, whereas the correspond-
In the case of one-electron reduction reactions, performed
chemically (Na metal in THF, cathodic reduction, see the
Experimental Section) well-resolved EPR spectra could be
recorded for 1−5. For TCBD derivatives 4 and 5, EPR spectra
with essentially matching patterns were obtained (Figure 3).
The dominating EPR spectral patterns consist of five lines
caused by the interaction of two virtually identical 14N nuclei
interacting with the unpaired electron (14N isotropic hyperfine
coupling constants, hfcs, of 0.134 and 0.151 mT for 4•− and
0.154 and 0.134 mT for 5•−; each for 1 N). Further splittings
are caused by 14N hfcs of 0.040 (1 N)/0.033 mT (1 N) and
Figure 3. Experimental EPR spectra and their simulations obtained after reduction with a Na mirror in THF: (a) 4•−, g = 2.0035; (b) 5•−, g =
2.0038. Virtually identical EPR spectra were obtained upon controlled potential reduction of 4 and 5 in CH3CN solutions (+ 0.1 M n-Bu4NPF6)
with polarization potentials set 100 mV more negative than the E1/2 of the first reduction step.
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Figure 4. Geometry of the 1,1,4,4-tetracyanobuta-1,3-diene fragment: (a) parent 4 according to X-ray structure analysis. Calculated geometry
(B3LYP/6-31G(d)) of (b) parent 4 and (c) of 4•− (bond lengths are given in pm).
Figure 5. Experimental EPR spectra together with their simulations attributed to (a) 1•−, g = 2.0029, 14N hfc = 0.116 mT (4 N); (b) 2•− g = 2.0030,
14N hfc = 0.118 mT (4 N); (c) 3•−, g = 2.0027, 14N hfc = 0.118 mT (4 N).
0.033 mT (2 N) for 4•− and 5•−, respectively. These data
clearly reveal that the spin and the charge are not evenly
distributed between the two DCV moieties. This experimental
finding is rather unexpected when compared with reference
acceptors 8 and 9 resembling 4 and 5 but lacking the ferrocene
substituents. In 8•− and 9•−, the DCV moieties are identical in
terms of electron distribution, mirrored by the 14N hfcs of 0.109
(8•−) and 0.117 (9•−) mT, each attributed to four virtually
equivalent nitrogen nuclei.8c Indeed, taking the X-ray crystal
structure12 of 4 as the starting point for geometry optimization
of the radical anion (B3LYP/6-31G(d)), the initial dihedral
angle C1−C2−C3−C4, representing the twist around the
central buta-1,3-diene bond, decreases from an s-cis-type
arrangement (99.60°) to s-trans (153.17°, Figure 4).
The theoretically determined hfcs connected with this
optimized geometry show almost identical values for the four
cyano nitrogen atoms. This is in clear contrast with the
experimental values, which show that the two DCV moieties
are distinctly inequivalent. To investigate the quality of the
theoretical prediction, two routes were pursued:
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Figure 6. (a) Sketch of the spin distribution in 2•− (as representative example for 1•−−3•−) and (b) the corresponding singly occupied orbital
including the calculated and experimental hfcs (the calculated data are averaged from the four almost equivalent values, cf. the Supporting
Information).
(i) We calculated (DFT) the geometry of 4 and compared it
with the experimental X-ray data (see the Supporting
Information). Remarkably, the two geometries show
significant deviations: The calculation leads to an s-trans
conformation, whereas the X-ray structure is rather close
to an s-cis arrangement of the butadiene (Figure 4). The
calculated hyperfine data for 4•− employing the X-ray
structure indicate substantial delocalization of the spin
3. CONCLUSIONS
The first example of an unsymmetrically donor-substituted
octacyano[4]dendralene (1) is reported. With respect to the
electronic properties, doubling of the 1,1,4,4-tetracyanobuta-
1,3-diene chromophore on going from 4, 5, and related
TCBDs8 to octacyano[4]dendralenes 1−3 leads to electron
acceptors with substantially enhanced electron-accepting
power, mirrored by the markedly shifted reduction potentials
toward less negative values in 1−3. Whereas tetracyano
derivatives 4 and 5 are reduced at −0.81 and −0.95 V vs
Fc+/Fc, respectively,12 octacyano[4]dendralenes 1−3 take up
the additional electron already at values of −0.1 0.02 V vs
Fc+/Fc. This is quite a remarkable shift, which should also be
analyzed in view of 8 and 9 (Scheme 1),8b closely related to 4
and 5 but carrying exclusively N,N-dimethylaniline substituents
as electron-donating groups. The corresponding reduction
potentials are −0.69 and −0.89 V vs Fc+/Fc in line with the
additional anilino substituent in 9 diminishing the electron-
accepting power.8b In view of the notorious contamination by
radical anion impurities as a result of their exceptional electron-
accepting power, the octacyano[4]dendralenes could not be
characterized by high-resolution NMR spectroscopy, but rather
X-ray crystal structures were obtained to unambiguously
support the structures of derivatives 1−3.
1
into the aniline fragment causing well distinguishable H
hfcs, which, however, do not correspond to the
experimental data.
(ii) We aligned the molecular arrangement of the relaxed
DFT structure to that of the parent X-ray structure and
calculated the hfcs in a single-point procedure. In this
case, the theoretical data match rather well with the
experimental counterparts.
Apparently, in the case of the strongly polarized push−pull
molecules, calculations experience several superimposed
complications. It has been established that in some cases
DFT calculations using hybrid functionals (ideal for the
calculation of EPR data18) overestimate delocalization.19
Moreover, owing to the diminished delocalization, solvent
and ion-pairing effects play a decisive role for the radical anions,
thus leading to confined electron delocalization. This subtle
interplay of effects will be addressed in a separate publication.
However, our investigation clearly shows that rather
remarkably, and in contrast to 9•−, the charge is localized at
the DCV moiety adjacent to the acetylenic triple bond in the
radical anions of 4 and 5 (see also the Conclusions).
In the case of the [4]dendralenes 1−3, one-electron
reduction leads to very well resolved EPR spectra. Based on
the identical electron-accepting chromophore in the center of
the molecules, the splitting patterns in the spectra are basically
matching (Figure 5).
They are dominated by nine equidistant lines spaced by
0.116 (1•−) or 0.118 (2•−, 3•−) mT and matching g-factors, in
line with a structure containing four (virtually) equivalent
nitrogen atoms. Here, the spin distribution and the hyperfine
data are very well predicted by the theoretical calculations. The
spin and the charge are distributed in the central
tetracyanobutadiene moiety of the radical anions 1•−−3•−
(Figure 6).
Comparing 4 and 9 renders unexpected aspects: The E1/2(red)
values of these two molecules are rather similar; nevertheless,
the topologies of these molecules in the solid state are clearly
different. Ferrocene derivative 4 (the same holds for its analog
5) displays an s-cis-type arrangement of the two DCV
moieties.12 In contrast, 9 is an (somehow distorted) s-trans
8c
isomer. The EPR spectra attributed to 9•− and 4•−(5•−)
reveal rather divergent spin distributions. The EPR pattern very
clearly shows the presence of four virtually equivalent nitrogen
atoms in the case of 9•−8c whereas the spin is confined to only
two N atoms (one DCV unit) in the case of 4•−. These finding
points to two prominent conclusions: (i) The s-cis orientation
established for parent 4 and 5 by X-ray structure analysis is
retained in the corresponding radical anions (and incorrectly
predicted by calculations, see above), and (ii) the shifts of
reduction potentials observed in the series of the TCBD
derivatives are based on electronic and topological factors. This
subtle interplay of effects could only be established by the
combination of electrochemical measurements and EPR
spectroscopy. However, in the octacyano[4]dendralenes, such
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that could be unambiguously assigned to the corresponding radical
cation.
effects are essentially ruled out by the fact that the central
tetracyanobuta-1,3-diene core dominates the first reduction,
whereas the “outer” DCV moieties serve as interfaces. These
effects will be further elaborated using optical spectroscopy.
Electrochemical Reduction/Oxidation. The radical anions of
the investigated compound were also prepared by the controlled
potential electrolysis of approximately 1 mM CH3CN sample
solutions, containing 0.1 M n-Bu4NClO4 as supporting electrolyte.
The solutions were purged by Ar for 15 min and transferred to the
three electrode EPR electrochemical flat cell (Pt mesh working
electrode, Pt wire the auxiliary electrode, and Ag wire the
pseudoreference electrode). Electrolysis was performed in situ in the
cavity of the EPR spectrometer using the Uniscan PG580 potentiostat
to set the electrode potential about 100 mV more negative/positive
than the E1/2 of the redox process under study. Also in this case, the
anodic oxidation failed to produce EPR spectra that could be assigned
to the corresponding radical cations.
4. EXPERIMENTAL SECTION
Compounds 2,11 4,12 5,12 7,20 8,8b and 98b were synthesized according
to literature procedures.
X-ray Analysis of Compounds 1 and 3. The X-ray intensity data
were measured with Cu Kα radiation (λ = 1.54178 Å) and a mirror
optics monochromator for 1 and with Mo Kα radiation (λ = 0.71073
Å) and a graphite monochromator for 3. Cell dimensions were
obtained based upon the refinement of the XYZ-centroids of
reflections above 20 σ(I). The structure was solved by direct methods
and refined using OLEX2 and SHELXS.21 All non-hydrogen atoms in
Quantum Chemical Calculations. Calculations were carried out
with the Gaussian 03 package.23 Geometry optimizations (vibrational
frequency of stationary points was checked) and single-point
determinations of the Fermi contacts (hfcs), were conducted at the
B3LYP24 level of theory with the basis set 6-31G(d).25
1 were refined anisotropically by full matrix least-squares using
2
experimental weights w = 1/[s2(Fo)2 + (0.1000 P)2 ], where P = (Fo
+
2 Fc2)/3. For compound 3, the low number of observed reflexions did
not allow to refine all non-H atoms anisotropically. H-atom positions
were calculated and included in the structure factor calculation.
Suitable crystals were obtained by layering of a solution of 1 in
CH2Cl2 with n-hexane and subsequent slow evaporation of the
solvents. C34H19FeN9, Mr = 609.43 crystal dimensions 0.01 × 0.04 ×
0.08 mm monoclinic, space group P21/c, Z = 4, a = 7.4036(5) Å, b =
40.272(3) Å, c = 9.8627(6) Å, β = 104.307(5)°, V = 2849.5(3) Å3, D =
1.421 g cm−3at 100(2) K. Numbers of measured and unique
reflections were 12734 and 4125, respectively (Rint = 0.146). Final
R(F) = 0.0603, wR(F2) = 0.1540 for 399 parameters and 2705
reflections with I > 2σ(I) and 2.19 <θ < 60.23° (corresponding R
values based on all 4125 reflections are 0.1535 and 0.1831,
respectively). CCDC deposition no. 883490.
[1,1,6,6-Tetracyano-3,4-bis(dicyanomethylene)-5-[4-
(dimethylamino)phenyl]-1,5-hexadien-2-yl]ferrocene (1). A
solution of 5 (129 mg, 0.27 mmol) and TCNE (69 mg, 0.54 mmol)
in 1,1,2,2-tetrachloroethane (25 mL) was heated at 80 °C for 6 d
under inert atmosphere. The solvent was evaporated and the residue
subjected to FC (SiO2; CH2Cl2; Rf = 0.29) to afford 1 (128 mg, 78%)
as a dark brown solid: mp >300 °C; IR (ATR) ν = 3126 (w), 2916
(w), 2848 (w), 2216 (m), 1605 (s), 1573 (w), 1517 (s), 1490 (s),
1467 (s), 1434 (s), 1412 (m), 1390 (s), 1336 (s), 1297 (s), 1245 (w),
1211 (s), 1176 (s), 1144 (m), 1110 (m), 1055 (m), 996 (w), 941 (m),
876 (w), 837 (m), 816 (s), 738 (m), 692 (m), 671 (m), 656 (m), 636
(s), 611 cm−1 (m); UV/vis (CH2Cl2) λmax (ε) = 355 (10 500), 496
(21 600), 668 nm (3 300 M−1 cm−1); HR-MALDI-MS (3-HPA) m/z
Suitable crystals of 3 were obtained by layering of a solution of 3 in
CH2Cl2 with n-hexane and subsequent slow evaporation of the
solvents. C52H28N10·2CH2Cl2, Mr = 962.70; crystal dimensions 0.02 ×
−
calcd for C34H19FeN9 609.119, found 609.117 (100, [M]−). X-ray:
see Figure 1. Because of consistent contamination with radical anion,
also in the absence of reducing agent, resolved NMR spectra could not
be obtained.
0.2 × 0.3 mm triclinic, space group P1, Z = 2, a = 8.480(3) Å, b =
̅
15.471(4) Å, c = 18.112(5) Å, α = 91.016°, β = 94.203(8)°, γ =
93.318(8)°, V = 2365.2(12) Å3 D = 1.352 g cm−3at 100(2) K. Crystal
structure contains disordered CH2Cl2 molecules. Numbers of
measured and unique reflections were 18025 and 8382, respectively
(Rint = 0.0882). Final R(F) = 0.0956, wR(F2) = 0.2149 for 625
parameters and 3665 reflections with I > 2σ(I) and 1.13 <θ < 25.77°
(corresponding R values based on all 8382 reflections are 0.2141 and
0.2696, respectively). CCDC deposition no. 883491.
3,4-Bis(dicyanomethylidene)-2,5-bis[4-(diphenylamino)
phenyl]hexa-1,5-diene-1,1,6,6-tetracarbonitrile (3). A solution
of 7 (51 mg, 0.095 mmol) and TCNE (98 mg, 0.77 mmol) in 1,1,2,2-
tetrachloroethane (5.0 mL) was heated at 95 °C for 5 d under inert
atmosphere. The solvent was evaporated and the residue subjected to
FC (SiO2; CH2Cl2) to afford 3 (12 mg, 18%) as a black solid: mp
>300 °C; IR (ATR) ν = 3062 (w), 2617 (w), 2220 (m), 1607 (s),
1583 (w), 1522 (s), 1480 (s), 1441 (s), 1323 (s), 1287 (s), 1179 (s),
1074 (m), 1001 (w), 900 (m), 824 (w), 754 (m), 719 (m), 693 (m),
634 (m), 618 cm−1 (m); λmax (ε) = 278 (25 600), 474 (25 800), 521
nm (26 300 M−1 cm−1); HR-MALDI-MS (3-HPA) m/z calcd for
C52H28N10− 792.2504, found 792.2516 (100, [M]−). X-ray: see Figure
1. Because of consistent contamination with radical anion, also in the
absence of reducing agent, resolved NMR spectra could not be
obtained.
EPR Measurements. EPR spectra were recorded at ambient
temperature. Spectra simulations were performed by means of
WinSim, a public domain program.22
Chemical Reductions. Reductions were performed in dry THF or
CH2Cl2 with the use of Na metal. THF was heated to reflux over a
Na/K alloy and stored over a Na/K alloy under high vacuum. Its deep
blue color in combination with benzophenone was used as an indicator
for rigorously water-free conditions. CH2Cl2 was dried by heating to
reflux over molecular sieves and stored under high vacuum (min. 10−5
mbar). Samples were prepared in a special three-compartment EPR
sample tube connected to the vacuum line. A Na metal mirror was
sublimated to the wall of the tube, and about 0.4 mL of THF or
CH2Cl2 were freshly condensed to dissolve the investigated
compound. The sample was successively degassed by three freeze−
pump−thaw cycles and sealed under high vacuum. Reductions were
performed by contact of the THF or CH2Cl2 solution of the parent
molecule with the Na metal mirror in the evacuated sample tube. The
sample tubes were either stored at 190 K (2-propanol/dry ice bath) or
immediately transferred into the microwave cavity of the EPR
spectrometer.
ASSOCIATED CONTENT
■
S
* Supporting Information
Calculated geometries and hfcs of 1•− to 4•−. X-ray
crystallography details and CIF files. This material is available
free of charge via the Internet at http://pubs.acs.org/
AUTHOR INFORMATION
■
Corresponding Author
Chemical Oxidation. The dry degassed CH2Cl2 sample solutions
were prepared by the procedure described above. Oxidation was
performed by the mixing of the parent compound solution with
selected oxidant (phenyliodine(III) bistrifluoroacetate (PIFA) or
NOSbF6) in the evacuated sealed EPR tube. Unfortunately, none of
the samples prepared in this way was able to provide an EPR signal
*E-mail: diederich@org.chem.ethz.ch, g.gescheidt-demner@
tugraz.at.
Notes
The authors declare no competing financial interest.
In Memoriam Howard E. Zimmerman
G
dx.doi.org/10.1021/jo301194y | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
(12) Jordan, M.; Kivala, M.; Boudon, C.; Gisselbrecht, J.-P.;
Schweizer, W. B.; Seiler, P.; Diederich, F. Chem. Asian J. 2011, 6,
396−401.
ACKNOWLEDGMENTS
■
This work was supported by the ERC Advanced Grant No.
246637 (OPTELOMAC) and by the FWF (Austria, project no.
P20019-N17). B.B. is grateful for a Kekule fellowship from the
́
Fonds der Chemischen Industrie (VCI). D.T. was the recipient
of a JSPS postdoctoral fellowship.
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F.; Schweizer, W. B.; Bernet, B.; Diederich, F. Chem. Commun. 2011,
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