Oxidation products of doubly trimethylene-bridged tetrabenzyl p-phenylenediamine paracyclophane

Article abstract of DOI:10.1021/jo401921f

We report synthesis and investigation of doubly trimethylene-bridged tetrabenzyl-p-phenylenediamine 1(Bz) in its singly and doubly charged redox states. The singly oxidized monoradical cation, which is a mixed-valence (MV) system with directly interacting charge-bearing units, shows broad and solvent-sensitive intervalence bands consistent with class II compounds according to the Robin-Day classification. The doubly oxidized diradical dication of 1(Bz) exists in the spin-paired singlet state with thermally accessible triplet state. It has similar conformations as the other dimeric p-phenylenediamines, such as derivatives 1(Me) and 1(Et), in both the solid-state and solution phases. The successful isolation of the single-crystalline 1(Bz)²⁺ diradical dications with two different in nature counteranions, relatively highly coordinating SbF₆^- and weakly coordinating carborane [undecamethylcarborane HCB₁₁Me₁₁^- (CB^-)], reveals the distinct effect of the nature of counterions on the structural features of diradical dication. Cyclic voltammetry measurements of 1(Bz) in dichloromethane reveal separation of the first and second oxidation potential by 0.12 V (2.8 kcal/mol), indicating relatively stable mixed-valence state in the dichloromethane, whereas in the acetonitrile both the first and second oxidation potentials overlap into one unresolved redox peak with minimal separation.

Full text of DOI:10.1021/jo401921f

Article
pubs.acs.org/joc
Oxidation Products of Doubly Trimethylene-Bridged Tetrabenzyl
p‑Phenylenediamine Paracyclophane
Almaz S. Jalilov,*,† Lu Han, Stephen F. Nelsen, and Ilia A. Guzei
Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin 53711-1396, United States
S
* Supporting Information
ABSTRACT: We report synthesis and investigation of doubly
trimethylene-bridged tetrabenzyl-p-phenylenediamine 1(Bz)
in its singly and doubly charged redox states. The singly
oxidized monoradical cation, which is a mixed-valence (MV)
system with directly interacting charge-bearing units, shows
broad and solvent-sensitive intervalence bands consistent with
class II compounds according to the Robin−Day classification.
The doubly oxidized diradical dication of 1(Bz) exists in the
spin-paired singlet state with thermally accessible triplet state.
It has similar conformations as the other dimeric p-phenylenediamines, such as derivatives 1(Me) and 1(Et), in both the solid-
state and solution phases. The successful isolation of the single-crystalline 1(Bz)2+ diradical dications with two different in nature
−
−
counteranions, relatively highly coordinating SbF6 and weakly coordinating carborane [undecamethylcarborane HCB11Me11
(CB−)], reveals the distinct effect of the nature of counterions on the structural features of diradical dication. Cyclic voltammetry
measurements of 1(Bz) in dichloromethane reveal separation of the first and second oxidation potential by 0.12 V (2.8 kcal/
mol), indicating relatively stable mixed-valence state in the dichloromethane, whereas in the acetonitrile both the first and second
oxidation potentials overlap into one unresolved redox peak with minimal separation.
from magnetic susceptibility measurements on crystals of less
methylated compounds, whereas N,N,N′,N′-tetramethyl-p-
phenylenediamine+ (TMPD+) neither dimerized nor spin-
paired because of steric interactions between its methyl groups.
It was found about 30 years later that π-dimers of TMPD+ can
be observed at much lower temperatures than that used by
Michaelis10 and also that the spins do pair in a low-temperature
1. INTRODUCTION
The evolution of concept of organic radical ions has a long-
standing history, which goes back at least 150 years.1 Since
then, radical ions have been of interest as important
intermediates of chemical reactions2 and crucial components
for the development of new magnetic3 and conducting organic
and polymeric materials.4 Despite considerable interest related
to the open-shell organic systems (radicals), the main focus of
most of the research efforts remained from the applied
perspective, and details of the basic properties of the open-
shell systems are still relatively scarce. One of the most well-
known properties of organic radicals is their spontaneous
dimerization to form σ-bonds (two-center, two-electron
bonds).5 However, many persistent radicals in which unpaired
spin is delocalized over several atoms have a tendency to
dimerize (π-dimerization) or to associate to form π-stacks in
condensed phases.6 The distances between the π-stacks in the
solid-state structures are usually longer than the conventional
covalent bonds and shorter than the sum of the van der Waals
radii.7 The main driving force for π-dimerization is known to
arise from the overlap of singly occupied molecular orbitals
(SOMO).8 Common features of the dimers include reversibility
of π-dimerization and lower bond strength than σ-dimerization.
There is a large amount of literature on π-stacking of radical
ions, extending all the way back to the 1943 hypothesis of
Michaelis9 that p-phenylenediamine radical cations (PD+
derivatives) with fewer methyl groups “polymerized” at low
temperature, deduced from the observed color changes. In
addition, spin pairing was shown to occur in the solid state
modification of TMPD+ClO4 crystals.11 Later, Kosower et
−
al.12 further investigated this phenomenon and discovered π-
dimerization of viologens by studying the methyl-substituted
radical cation MV+ (Chart 1).
Chart 1
Several authors have reported spontanous π-dimerization of
another series of the persistent radical ions almost at the same
time: π-dimerization and the question of bonding interactions
between TMPD radical cations by Kimura et al.13 and
Nakayama and Suzuki,14 2,3-dichloro-5,6-dicyano-p-benzoqui-
none (DDQ) anion radical by Yamagishi,15 and tetracyanoqui-
Received: August 29, 2013
Published: October 17, 2013
© 2013 American Chemical Society
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nodimethane (TCNQ) anion radicals by Boyd and Phillips.16
On the basis of X-ray structures of the stretched cyclobutane-
like dimers of tetracyanoethylene (TCNE) radical anion, Miller
and co-workers17−19 introduced the term exceptionally long
four-center carbon−carbon bonding to describe them. There-
after, π-dimers involving larger π systems have often been
described as having multicenter long bonds. Theoretical
discussions of the nature of their bonding interactions have
been done by Head-Gordon and Jung,20 Mota et al.,21 and Tian
and Kertesz.22 Grampp et al.23 in 2002 and Kochi and co-
workers24 in 2003 reported experimental details of the
energetic parameters of the π-dimerizations of the best-
known radical ions as well as neutral radicals. They observed
that the enthalpy changes (ΔHD) for π-dimerization (eq 1) are
mainly in the range −6 to −9 kcal/mol and the entropy
changes (ΔSD) are in the range −30 to −40 eu. Relatively small
binding energies (ΔHD) and rather large negative entropy
changes for π-dimers make these processes entropy-dominated:
temperature NMR studies showed that all three conformations
are present in detectable amounts near −60 °C (see Figure
2).27,29 Unexpectedly, the methyl- and ethyl-substituted
Figure 2. Relative distribution of three conformations of 1(Me)2+ at
−60 °C.27
compounds showed noticeably different relative populations
of these conformations, with more uns than anti for 1(Me)2+
(syn:anti:uns ratio of 54:17:29) but less uns than anti for
1(Et)2+ (syn:anti:uns ratio of 68:20:12).27 On the other hand,
22+ was present exclusively in its uns conformation.29 In this
work, we report the effect of the benzylic substitution 1(Bz) on
the structural and electronic states of similar class of
paracyclophanes in both the singly and doubly oxidized states.
KD
R + R ⇌ [R, R]
(1)
In order to overcome an entropic factor, one can imagine
that covalent linkage between the two interacting radicals or
radical ions might give a better view of the energetic and
binding effects in the π-dimers. Hunig and co-workers25 carried
̈
out early attempts to covalently link pairs of methylviologen
derivatives (Figure 1).
2. RESULTS AND DISCUSSION
2.1. Synthesis of Paracyclophane 1(Bz). Paracyclophane
1(Bz) was synthesized according to the procedure shown in
Figure 3 from the precursor paracyclophane 1(H), the
Figure 1. Doubly bridged viologen diradical dications studied by
Hunig and co-workers.25
̈
Recently we reported several works on the oxidation of
“dimeric” three-carbon-bridged PD-containing [5,5]-
paracyclophanes (see Chart 2).26 Singly oxidized monoradical
Figure 3. Synthetic route for preparation of 1(Bz).
synthesis of which was previously described.26 1(Bz) was easily
obtained as a yellow crystalline product by benzylation of 1(H)
with excess benzyl bromide under basic conditions in 60% yield
(see Figure 3). Oxidation of apparent paracyclophanes with 1
equiv of oxidant results in the blue, relatively stable
monoradical cation. By double oxidation with 2 equiv of
oxidant, maroon diradical dications were obtained (see the
Experimental Section for details).
Chart 2
2.2. Electrochemical Measurements. Cyclic voltammo-
grams (CVs) for compound 1(Bz) in two different solvents
(dichloromethane, panel A, and acetonitrile, panel B) recorded
at room temperature are shown in Figure 4, and the lowest
oxidation potentials are summarized in Table 1. Similarly to the
previously published PD-containing paracyclophanes [1(R),
where R = Me, Et, or iPr], two regions of the oxidation
potentials could be observed (Figure 4). The first region is
below 0.5 V and corresponds to E1/2(0/+1) and E1/2(+1/+2).
The second region is above 0.5 V and corresponds to E1/2(+2/
+3) and E1/2(+3/+4). The unique feature of 1(Bz) is that there
is a separation of the first and second oxidation peaks (ΔE°) in
dichloromethane by 0.12 V, which corresponds to 2.8 kcal/mol.
Due to the poor solubility of neutral 1(Bz) in acetonitrile, CV
cations of the paracyclophanes 1(R) (where R = Me, Et, or iPr)
and 2 are mixed-valence systems with directly interacting
charge bearing units.27−29 According to the Robin−Day
classification, these types of mixed-valence systems could be
in either charge- and spin-localized (class II) or delocalized
(class III) states.30
Additionally, the doubly oxidized diradical dicationic para-
cyclophanes, which exist almost exclusively in the π-dimeric
spin-paired singlet states, have three isolable conformations:
syn, anti, and an unsymmetrical conformation (abbreviated as
uns) having one PD+ ring syn and the other anti. Low-
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Figure 4. Cyclic voltammetry of (A) 1(Bz) in 0.1 M TBAP/CH2Cl2 solution and (B) 1(Bz)+[SbF6]− in 0.1 M TBAP/CH3CN solution, with Pt
working electrode and scan rate 100 mV·s−1.
Kcomp = [1(Bz)• −]2 /[1(Bz)2+][1(Bz)°]
Table 1. Cyclic Voltammetric Data for 1(Bz) in CH2Cl2 and
CH3CN
= exp[ΔE°(F/RT)]
(2)
compd and
solvent
Robin−
a
a
b
E1/2(0/+1)
E1/2(+1/+2)
ΔE°
Kcomp Day class
The magnitude of Kcomp is related to electronic couplings
between the charge-carrying units of the organic mixed-valence
compounds.31 Generally, the compounds having values of Kcomp
varying from several orders of magnitudes (>105), considered
as strongly coupled class III systems according to the Robin−
Day classifications of mixed valency.32 The comproportionation
equilibrium constants for 1(Bz) are Kcomp = 157 and ∼3 in
dichloromethane and acetonitrile, respectively, suggesting a
weak solvent-dependent electronic coupling between the two
charge-bearing PD units to form the class II mixed-valence state
in acetonitrile and stronger electronic coupling with possible
formation of the class III (or borderline between class II and III
states, that is, class II/III)33 mixed-valence state in dichloro-
methane (schematically shown in Figure 5). Using Mulliken−
Hush analysis for the estimation of electronic interactions,
Rosokha and Kochi34 showed that free TMPD radical cations
forms a class II self-exchange mixed-valence state with its
neutral counterpart. Solvent-sensitive electronic couplings for
various dinitroaromatic radical anions are also reported in the
literature.35 Previously we have observed both localized (class
II) and delocalized (class III) mixed-valence systems of various
alkyl-substituted and bridge-modified paracyclophane radical
cations.26−29
1(Bz) in
−0.01 (0.12)
0.12 (0.12)
0.13
157
II/III
CH2Cl2
c
1(Bz) in
CH3CN
−0.09 (0.09)
−0.09 (0.09)
∼0.00
∼3
II
a
Potentials are given in volts vs Ag/Ag+ in the presence of supporting
electrolyte 0.1 M TBAP (scan rate = 100 mV/s). The number in
b
c
parentheses is Eox − Ered. ΔE° = E°1 − E°2. Number taken according
to reported simulation data for 1(R).26
data of the more soluble 1(Bz)+[SbF6]− salt were recorded.
The first two oxidation peaks are overlapped into one
unresolved oxidation peak (Figure 4B) and could not be
distinguished, indicating that in acetonitrile the second electron
is only slightly harder to remove than the first. Therefore, the
relatively high degree of disproportionation for the monoradical
cation of 1(Bz) to the neutral and the diradical dication of
1(Bz) could take place in acetonitrile, in comparison to
dichloromethane. The second region of oxidation peaks, where
the third and fourth oxidation waves can be observed, are
irreversible in both solvents and less informative due to
absorption of the less soluble polycations in organic solvents;
see Figure 4.
The comproportionation equilibrium constants, Kcomp, for
1(Bz) in two different solvents at 298 K were determined by eq
2, where ΔE° is the difference in oxidation potentials for the
first and second oxidation processes.
Furthermore, according to Evans and co-workers,36 the
reason for the reduced values of ΔE°, known as potential
compression, is either through a significant geometric change of
Figure 5. Schematic representation of two different mixed-valence states in dimeric TMPD radical cations.
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redox species upon the first and second oxidation processes or
through ion pairing between oxidized radical cations and anions
of the supporting electrolytes. For 1(Bz) with the geometrically
flexible trimethylene bridge, we have shown that there is a
minor geometric change between the singly and doubly
oxidized species, 1(R)+ and 1(R)2+, respectively.29 Therefore,
we assume that specific ion-pairing interactions, particularly
radical cation−solvent interactions (vide infra) could be the
major contribution to the potential compression of 1(Bz),
resulting in the localized nature of mixed-valence species in the
more polar acetonitrile.
corresponds to the electronic transition from the highest
doubly occupied molecular orbital to the singly occupied
molecular orbital (HOMO to SOMO) within the singly
oxidized PD unit. The second strongest energy transition in
the near-IR region is relatively weak and structureless. These
are typical mixed-valence transition bands corresponding to the
single electron transfer between oxidized and neutral PD
units.26,29 For 1(Bz)+, the maxima and shape of the mixed-
valence band are strongly dependent on the nature of the
solvent environment, as shown in Figure 6.
As suggested in the previous section, we assign mixed-valence
bands in dichloromethane as borderline class II/III (delocal-
ized) and in acetonitrile as class II (localized) charge-transfer
bands.
2.3.2. Diradical Dication of 1(Bz). Maroon diradical
dication 1(Bz)2+ was formed by oxidizing 1(Bz) with 2 equiv
of oxidant in organic media. Vis/NIR spectrum of 1(Bz)2+ in
acetonitrile shows two distinct absorbances at 13600 cm−1 and
at 19100 cm−1.
2.3. Oxidation Products of 1(Bz). 2.3.1. Radical Cation
of 1(Bz). Oxidation of 1(Bz) in three different solvents with 1
equiv of the oxidant NO+SbF6− results in the optical spectra of
oxidation products of 1(Bz) shown in Figure 6. As for the
As shown in Figure 7, the optical spectrum of 1(Bz)2+ in
acetonitrile clearly resembles that of 1(Et)2+ with a slight red
shift in both of the absorbance maxima. 1(Me)2+ has a
considerably higher percentage of uns conformations, which
absorb at a lower energy than the syn and anti conformations,27
than 1(Et)2+. Consistent with this expectation, the optical
spectrum of 1(Me)2+ shows a clear bulge (around 12 500
cm−1) on the low-energy side of the first absorption band (see
Figure 7), which we attributed to the contribution of its uns
conformation to the spectrum. The lack of this bulge for
1(Bz)2+ suggests that 1(Bz)2+ also has a much smaller
percentage of uns conformations than 1(Me)2+.
Figure 6. Visible−near-IR spectra of 1(Bz)+[SbF6]− in various
solvents. We note the broadening of the spectrum in acetonitrile
and in acetone due to overlapping with the optical spectra of doubly
oxidized product.
According to the results for 1(Me)2+ and 1(Et)2+,27 we
assign two of the lowest electronic absorptions to the bonding
and antibonding combination of PD+ singly occupied molecular
orbitals (SOMOs), that is, HOMO to LOMO (lowest
unoccupied molecular orbital) and HOMO − 1 to LUMO
transitions of the formed dimeric (PD+)2 unit, respectively (as
schematically shown in Figures 8 and 9).
methyl and ethyl derivatives 1(Me) and 1(Et), respectively, the
monoradical cation of 1(Bz) has two lowest-energy absorption
bands in the visible and the near-IR region. Particularly, the
monocationic and dicationic oxidation states of 1(Bz) closely
resemble the optical spectra of 1(Et)+ and 1(Et)2+, respectively
(see Figures 6 and 7).26,27 The strongest electronic absorption
of 1(Bz)+ at 15 700 cm−1 with resolved vibrational structures
1
Variable-temperature H NMR spectra of 1(Bz)2+·2[SbF6]−
in CD3CN/CD3OD (2:1 mixture by volume) reveals broad
peaks for the protons of PD+ units at room temperature, which
sharpen up considerably upon lowering the temperature to −50
°C (see Figure 10). As we have shown previously, these types
of diradical dications have a singlet ground state with thermally
accessible triplet states.26 Unlike H NMR spectra of 1(Me)2+
1
and 1(Et)2+, the spectrum of 1(Bz)2+ was uninformative due to
poorer solubility at low temperatures and broader peaks as
shown in Figure 10. However, we could still clearly observe
aromatic protons around 6 ppm, which correspond to the syn
and anti conformations of the diradical dications.
It might be conceivable that relative distributions of the
conformational isomers (syn, anti, and uns) differ in solution
and solid phases. In order to get further insight into the nature
of conformational distribution in the solid state, we recorded
diffusion reflectance spectra of the solid diradical dication salt
1(Bz)2+·2[SbF6]−. Figure 11 shows comparison of the
spectrum in acetonitrile (at room temperature) with that of
the solid salt 1(Bz)2+·2[SbF6]−, which shows similar band
shapes in both media. It is clear from the lowest energy
absorption bands of the solid-state spectrum that, as in the
solution, it mainly consists of syn and anti conformations in
solid phase.
Figure 7. Comparison of room-temperature optical spectra of
1(Me)2+, 1(Et)2+, and 1(Bz)2+. Absorptions are scaled arbitrarily.
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Figure 8. Self-dimerization of TMPD radical cations to form diamagnetic diradical dication.
Figure 11. Visible−near-IR spectrum of 1(Bz)2+·2[SbF6]− in
acetonitrile (red) and reflectance spectra of solid 1(Bz)2+·2[SbF6]−
(blue). The reflectance spectrum of solid 1(Bz)2+·2[SbF6]− has two
strong lowest-energy absorptions at 13 600 and 19 100 cm−1, which is
similar to the optical spectrum in solution.
Figure 9. Simplified molecular orbital (MO) energy diagram of PD
dimer dication.
X-ray Crystallographic Data of Diradical Dications. Slow
diffusion of hexane into a maroon solution of 1(Bz)2+·2[SbF6]−
in acetone at −30 °C gives black single crystals suitable for X-
ray crystallography. The unit cell contains one dication, two
anions, and four molecules of solvent acetone, to form the
general structure of crystal packing as [(C46H48N42+)-
anti conformation, structurally similar to the methyl and ethyl
derivatives. The distance between the two nitrogen atoms in
contact with adjacent PD units (N−N as shown in Figure 12) is
2.9480(13) Å, shorter than the sum of the out-of plane van der
Waals radii of the two nitrogen atoms (3.10 Å). Additionally,
each π-stacked dimeric unit is stabilized by two counterions
−
(SbF6 )2(C3H6O)4]n. The π-dimeric diradical dication resides
−
on a crystallographic inversion center, thus the asymmetric unit
(symmetry-independent part of the lattice) contains half of the
dication, one anion, and two molecules of solvent acetone, as
shown in Figure 12. The π-dimeric diradical dication is in the
(SbF6 ) via the H-bond C15−H···F3, which has a distance of
2.579 Å, and also by two close solvent molecules through H-
bond C10−H···O2, which has a distance of 2.220 Å (Figure
12).
1
Figure 10. Variable-temperature H NMR spectrum of 1(Bz)2+·2[SbF6]− in CD3CN/CD3OD (2:1 mixture by volume). A strong temperature-
dependent singlet peak corresponds to the hydroxyl group of the solvent methanol.
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dichloromethane molecules is disordered over two positions
with a 75.5(6)% major component and was refined with
constraints and restraints. Both of the two independent
diradical dications 1(Bz)2+·2[CB]− are in their anti con-
formations. The presence of the two independent diradical
dications that differ in the conformational features within the
same unit cell gives us an opportunity to discuss the effect of
counterion on the weak π−π dimeric interactions of two
independent diradical dications within the same single crystal.
The most significant features of the structural details of the two
independent diradical dications of 1(Bz)2+·2[CB]− and
1(Bz)2+·2[SbF6]− are summarized in Table 2. For clarity we
will abbreviate the two independent isomeric structures of
1(Bz)2+2[CB]− within the same unit cell as 1(Bz)2+·2[CB]−
(A) and 1(Bz)2+·2[CB]− (B). As listed in Table 2, 1(Bz)2+·
2[CB]− (B) has fairly similar structural details to 1(Bz)2+·
2[SbF6]−, but 1(Bz)2+·2[CB]− (A) has longer π−π distances
between two PD+ units.
To compare structural details of the two independent
diradical dications, we identified the shortest intermolecular
distances between the diradical dications and the counter-
anions. By doing so, the four shortest H-bonding contacts C−
H···CH3 between methyl groups of the two counteranions
(CB−) and C−H hydrogens of the π−π dimeric PD+ radical
cations were identified as shown in Figure 13. For 1(Bz)2+·
2[CB]− (A) with two symmetrical N1−N2 distances of 2.982
Å, the two shortest H-bonding contacts C−H···CH3 are 2.844
and 3.050 Å. Similarly, the numbers for 1(Bz)2+·2[CB]− (B)
are d(N3−N4) = 2.950 Å and d(C−H···CH3) = 2.947 and
3.083 Å. Here one can observe that the shorter π−π
interactions between PD+ units of the diradical dication
1(Bz)2+·2[CB]− (B) have longer intermolecular counter-
anion−radical cation interactions (C−H···CH3). The opposite
was observed for longer π−π interactions in the diradical
dication 1(Bz)2+·2[CB]− (A), with shorter counteranion−
radical cation interactions.
Figure 12. X-ray structure of 1(Bz)2+·2[SbF6]−·4Me2C=O. Hydrogen
atoms are omitted for clarity except for hydrogen atoms of the PD
unit. Only the symmetry-independent atoms are labeled. Molecular
diagrams are drawn with 50% probability ellipsoids.
Upon slow diffusion of hexane at −30 °C to the blue
dichloromethane solution of monoradical cationonic salt of
1(Bz) and weakly coordinated counterion undecamethylcar-
borane HCB11Me11 (CB−), diradical dication can be isolated
−
as a product of the disproportionation reaction (eq 3).
Undecamethylcarborane (CB−), known as a weakly coordinat-
ing and inert anion, is used preferably for isolation of reactive
cations and strong oxidants.37
2 1(Bz)• +[CB]− → 1(Bz)2+·2[CB]− + 1(Bz)°
(3)
The asymmetric unit contains two halves of two symmetry-
independent dications [1(Bz)2+], two full anions (CB−), and
four full dichloromethane molecules. The two conformational
isomers of dications reside on crystallographic inversion
centers. Therefore the ratio between species is 2 dications:4
anions:8 dichloromethane solvent molecules to form a general
structure of [(C46H48N42+)2(CB−)4(CH2Cl2)8]n. One of the
3. CONCLUSIONS
Changing the R group from methyl and ethyl to benzyl in the
“dimeric” three-carbon-bridged PD+-containing [5,5]-
paracyclophanes significantly increases the separation of the
first and second oxidation peaks in dichloromethane. The singly
Table 2. Comparison of 1(Bz) Dication X-ray Structures
a
a
1(Bz)2+·2[SbF6−] ·4Me2C=O (N26)
1(Bz)2+·2[CB−]·4CH2Cl2 (A) (N27)
1(Bz)2+·2[CB−]·4CH2Cl2 (B) (N27)
NCCC conformations
PD conformations
dication symmetry
d(N−N) (Å)
gg, gg
gg, gg
gg, gg
anti, anti
Ci
anti, anti
Ci
anti, anti
Ci
2.948
2.982
2.950
b
d(Cq−Cq) (Å)
3.053
3.099
3.050
c
d(CH−CH) (Å)
3.106
3.161
3.111
c
d(CH−CH) (Å)
3.160
3.234
3.173
d[N−CH2(br)] (Å)
d(N−R) (Å)
d(CH2−R′) (Å)
1.469, 1.476
1.468, 1.481
1.516, 1.523
2.988
1.464, 1.468
1.468, 1.474
1.509, 1.520
3.098
1.463, 1.477
1.471, 1.473
1.509, 1.516
3.020
d(mean C6 planes) (Å)
d
PD ring displacement (Å)
0.93
0.65
0.76
NCCC twists (deg)
NCCC twists (deg)
−6.62, 65.69
66.62, −5.69
−66.74, 65.80
66.74, −65.80
−66.70, 65.74
66.70, −65.74
a
b
c
Unit cell contains two independent dications. Distance between pairs of quaternary ring carbons. Distance between the closest ring CH carbons.
Measured from the perpendicular to the average C6 plane.
d
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Figure 13. Fragment of unit cell of 1(Bz)2+·2[CB]−·4CH2Cl2 (A) and 1(Bz)2+·2[CB]−·4CH2Cl2 (B). Hydrogen atoms are omitted for clarity except
for hydrogen atoms of the PD unit. Molecular diagrams are drawn with 50% probability ellipsoids.
oxidized monoradical cation 1(Bz)+ reveals broad and solvent-
chromatography to give 3.4 g of yellow-white crystalline product
1
(60% yield): UV 263 nm in CH2Cl2; mp 190−194 °C; H NMR
(CDCl3, 300.137 MHz) δ, ppm 7.28 (m, 4H), 7.23 (s, 4H), 6.60 (s, 8
H), 4.25 (s, 8H), 3.16 (t, 8H), 1.61 (m, 4H); 13C NMR (CDCl3,
300.137 MHz): δ, ppm 141.5, 140.1, 128.6, 127.9, 127.0, 118.36, 58.3,
49.1, 21.6; HR-MS TOF MS ES+ calcd [M + H]+ = 656.384
monoisotopic, measd [M + H]+ = 656.3877 (<1 ppm).
sensitive intervalence band indicative of weak electron-localized
class II mixed-valence system. Furthermore, the doubly charged
diradical dication 1(Bz)2+ exhibit similar conformational
features as ethyl derivative 1(Et)2+ in both the solid state and
solution phases. The diradical dication 1(Bz)2+ was isolated as
single-crystalline ion-paired salts with two different counterions,
4.2. Oxidation Reactions. Radical cation and diradical dication
salts 1(R)+·[X]− and 1(R)2+·2[X]−, were prepared by oxidation of the
corresponding neutral compounds with stoichiometric (1:1 or 1:2)
amounts of corresponding silver or nitrosonium salts (Ag+CB− or
−
SbF6 and a less coordinating carborane anion, undecame-
−
thylcarborane HCB11Me11 (CB−). Luckily, the carborane
anion-containing salt 1(Bz)2+·2[CB]− crystallized in two
conformationally isomeric structures within the same unit
cell. These conformationally isomeric structures, 1(Bz)2+·
2[CB]− (A) and 1(Bz)2+·2[CB]− (B), differ in both intra-
molecular π−π distances (PD+−PD+ separations) and inter-
molecular diradical dication 1(Bz)2+−counteranion 2CB−
distances.
−
NO+SbF6 , respectively) in dichloromethane, with a small amount of
acetonitrile added for preparation of the dication in order to increase
the solubility of the diradical dication salt. The resulting dark blue
solution of radical cation or maroon solution of diradical dication salts
was precipitated by use of an excess amount of hexanes. Silver
undecamethylcarborane [Ag+CB−] was prepared by an analogous
method to the literature procedure from Cs+ [HCB11Me11]− and
Together with previous studies,26−29,38 this work shows that
molecular interactions of the organic radical ions are clearly
sensitive to the solvent and counterion environment, which is
also shown by theoretical studies on the spontaneous
dimerization of TMPD radical cations.39 We expect that deeper
understanding from the molecular prospective of the
intermolecular binding nature of the open-shell organic radicals
can provide the basis for future discoveries of novel organic
materials.
AgNO3.40 For the oxidation with NO+SbF6 , it is important to bubble
−
the resulting reaction mixture with inert gas for several minutes in
order to remove the product NO gas, which forms adducts with radical
cations. Single crystals of the dication diradical salt 1(Bz)2+·[SbF6]−
were prepared by dissolving the diradical dication salt in acetone, and
the resulting clear maroon solution was overlaid with a small amount
of the mixture of acetone and diethyl ether, which was again overlaid
with diethyl ether. Analogously, single-crystalline 1(Bz)2+·2[CB]− was
prepared by dissolving the monoradical cation salt in dichloromethane
and the resulting clear solutions were overlaid with a small amount of
the mixture of dichloromethane and hexanes, which was again overlaid
with hexane. The solvent mixtures were kept in a low-temperature
refrigerator at −80 °C for several days. Radical ions crystallize with
solvent molecules that easily evaporate, making them difficult to
handle and to weigh. Therefore, characterization of radical ions is
usually limited to X-ray structural analysis and UV−vis−NIR and
NMR spectroscopic analysis (for singlet dications).
4. EXPERIMENTAL SECTION
4.1. N,N′,N″,N‴ Tetrabenzyl-1,5,12,16-tetraaza[5,5]-
paracyclophane, 1(Bz). 1,5,12,16-Tetraaza[5,5]paracyclophane
1(H)26 (2.53 g, 8.5 mmol) was immersed in anhydrous tetrahydrofur-
an (THF, 40 mL). Under nitrogen, 1.68 g (70 mmol) of NaH was
added to the solvent. The mixture was heated under reflux for 1 h.
After that, 4.45 g (25.5 mmol) of benzyl bromide was added to the
mixture. The mixture was stirred for 1 h and evaporation was
conducted to get rid of extra benzyl bromide and THF. Methanol (300
mL) was added dropwise to the product that remained. The resultant
solution was filtered, and the residue was washed with hexane and left
to dry in air. The product was further separated by column
4.3. Spectral Measurements. Optical spectra were acquired on a
UV−vis spectrometer (200−1100 nm) and UV−vis−NIR spectropho-
tometer (200−3000 nm), by use of a capped quartz cuvette with side
arms. Low-temperature measurements were performed by cooling to
the corresponding temperature of solution in Dewar under inert
atmosphere and rapid scanning of the cold sample under survey scan
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The Journal of Organic Chemistry
Article
control mode (baseline correction was measured similarly with pure
solvent). Variable-temperature 1H NMR data for 1(Bz)2+·2[SbF6]−
were recorded in the solvent system of 2:1 mixture by volume of
CD3OD/CD3CN. At −50 °C, 1H NMR aromatic H shifts (δ) uns 7.25
(m), 6.20 (s), 5.97 (s), 5.76 (s), 5.18 (s), 4.66 (s), 3.8 (m), and 3.05
(s). None of the conformations present were clearly observed, due to
low solubility of the diradical dication at lower temperatures and to
broad signals. So we cannot estimate the amounts of syn, anti, and uns
conformations that might be present.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Additional text, 11 figures, and 12 tables showing comparison
of optical spectra of 1(Et)+ and 1(Bz)+ in CH2Cl2; UV−vis and
1H and 13C NMR spectral data on 1(Bz); X-ray structure
reports on crystal structures of 1(Bz)2+·2[SbF6]− and 1(Bz)2+·
2[CB]− (PDF); CIF files. This material is available free of
charge via the Internet at http://pubs.acs.org.
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Chem., Int. Ed. 2001, 40, 2540−2545.
AUTHOR INFORMATION
■
(18) Del Sesto, R. E.; Miller, J. S.; Lafuente, P.; Nuvoa, J. J. Chem.
Corresponding Author
*E-mail almaz.jalilov@gmail.com
Eur. J. 2002, 8, 4894−4908.
(19) Miller, J. S.; Nuvoa, J. J. Acc. Chem. Res. 2007, 40, 189−196.
(20) Head-Gordon, M.; Jung, Y. Phys. Chem. Chem. Phys. 2004, 6,
2008−2011.
Present Address
†(A.S.J.) Department of Chemistry, Northwestern University,
2145 Sheridan Rd., Evanston, IL 60208-3113.
(21) Mota, F.; Miller, J. S.; Novoa, J. J. J. Am. Chem. Soc. 2009, 131,
7699−7709.
Notes
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10649.
The authors declare no competing financial interest.
(23) Grampp, G.; Landgraf, S.; Rasmussen, K.; Strauss, S. Spectrochim
Acta A 2002, 58, 1219−1226.
ACKNOWLEDGMENTS
■
(24) Lu, J.-M.; Rosokha, S. V.; Kochi, J. K. J. Am. Chem. Soc. 2003,
125, 12161−12171.
We thank the National Science Foundation for support of this
work under GM 0647719. We also thank Professor J. Michl for
providing Cs+CB− salt, Professor R. Rathore for assistance with
reflectance spectral measurements, and Professor J. F. Berry for
electrochemical instrumentation.
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