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Article
Mechanical Bond-Protected Air-Stable Radicals
Junling Sun, Zhichang Liu, Wei-Guang Liu, Yilei Wu, Yuping Wang, Jonathan C. Barnes,
Keith R. Hermann, William A. Goddard, Michael R. Wasielewski, and J. Fraser Stoddart
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06857 • Publication Date (Web): 14 Aug 2017
Downloaded from http://pubs.acs.org on August 15, 2017
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Mechanical Bond-Protected Air-Stable Radicals
Junling Sun,
#,1
Zhichang Liu,*
,#,1
Wei-Guang Liu,
3
Yilei Wu,
1
Yuping Wang,
1
Jonathan C. Barnes,
1,4
Keith R. Hermann,
1
William A. Goddard III,
3
Michael R. Wasielewski,
1,2
and J. Fraser Stoddart
1,
*
1
Department of Chemistry and
2
Argonne-Northwestern Solar Energy Research (ANSER) Center,
Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA
3
Materials and Process Simulation Center, California Institute of Technology, Pasadena,
CA 91125, USA
4
Department of Chemistry, Washington University, One Brookings Drive, St. Louis, MO 91125, USA
*E-mail: zhichang-liu@northwestern.edu, stoddart@northwestern.edu
MAIN TEXT
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ABSTRACT: Radical templation centered around a heterotrisradical tricationic inclusion
complex DB
•+
⊂DAPQT
2(•+)
, assembled from an equimolar mixture of a disubstituted 4,4'-
bipyridinium radical cation (DB
•+
) and an asymmetric cyclophane bisradical dication
(DAPQT
2(•+)
), affords a symmetric [2]catenane (SC•7PF
6
) and an asymmetric [2]catenane
(AC•7PF
6
) on reaction of the 1:1 complex with diazapyrene and bipyridine, respectively. Both
these highly charged [2]catenanes have been isolated as air-stable mono-radicals and
characterized by EPR spectroscopy. X-Ray crystallography reveals that the unpaired electrons
are delocalized in each case across two inner 4,4'-bipyridinium (BIPY
2+
) units forming a mixed-
valence (BIPY
2
)
•3+
state inside both [2]catenanes, an observation which is in good agreement
with spin-density calculations using density functional theory. Electrochemical studies indicate
that by replacing the BIPY
2+
units in homo[2]catenane HC
•7+
—composed of two mechanically
interlocked cyclobis(paraquat-p-phenylene) rings—with 0, 1, and 2 more highly conjugated
diazapyrenium dication (DAP
2+
) units, respectively, a consecutive series of 5, 6, and 7 redox
states can be accessed in the resulting SC•7PF
6
(0, 4+, 6+, 7+, and 8+), HC•7PF
6
(0, 2+, 4+, 6+,
7+, and 8+), and AC•7PF
6
(0, 1+, 2+, 4+, 6+, 7+, and 8+), respectively. These unique
[2]catenanes present a promising prototype for the fabrication of high-density data memories.
■ INTRODUCTION
Ever since the landmark discovery of the triphenylmethyl radical
1
by Moses Gomberg,
research on stable organic radicals
2
has attracted attention, not only on account of their exotic
electronic properties, but also because of their potential applications as spin-labels
3
and in
organic lithium batteries
4
as well as in conductive and magnetic materials.
5
To date, however,
most organic radicals experience a fleeting existence and readily undergo dimerization and/or
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3
oxidation. The synthesis and isolation of persistent radicals in crystalline forms remains a
challenge. In addition, molecular systems with adjustable number of accessible redox states are
quite difficult to achieve.
4,4'-Bipyridinium radical cations (BIPY
•+
) tend to form
6
(BIPY
•+
)
2
dimers in a ‘face-to-face’
manner in the solid state as a result of favorable radical-pairing interactions. Conversely, in a
dilute solution, (BIPY
•+
)
2
dimers are prone
7
to dissociate because of their low association
constants. Recently, however, we found
8
that a cyclobis(paraquat-p-phenylene) bisradical
dication (CBPQT
2(•+)
) and a BIPY
•+
radical cation are capable of assembling to afford a stable
trisradical tricationic inclusion complex BIPY
•+
⊂CBPQT
2(•+)
in MeCN, assisted by radical-
pairing interactions. This 1:1 inclusion complex has been investigated extensively and employed
in recognition motifs, either to template
9
the formation of, otherwise difficult to synthesize
highly energetic mechanically interlocked molecules (MIMs), or to enhance
10
the switching
performance of bistable MIMs. In particular, we have been able to synthesize
9
(Figure 1) an air-
and water-stable paramagnetic homo[2]catenane HC
•7+
from the trisradical complex
DB
•+
⊂CBPQT
2(•+)
. Up to six redox states of this [2]catenane can be accessed electrochemically.
Herein we demonstrate a molecular system—namely, a novel class of octacationic
[2]catenanes—which exhibits adjustable multiple accessible redox states. We report the radical
template-directed syntheses of two analogues (Figure 1) of HC
•7+
—namely, the asymmetric
[2]catenane AC
•7+
and the symmetric [2]catenane SC
•7+
—by incorporating simultaneously both
the less conjugated BIPY
2+
and the more highly conjugated 2,7-diazapyrenium (DAP
2+
) units
into the [2]catenane structures in order to modulate the number of the accessible redox states of
the resulting molecules. We show that these [2]catenanes, which exist as persistent air-stable
radicals, can exist in a consecutive series of 5 (SC
•7+
: 0, 4+, 6+, 7+, and 8+), 6 (HC
•7+
: 0, 2+, 4+,
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4
6+, 7+, and 8+), and 7 (AC
•7+
: 0, 1+, 2+, 4+, 6+, 7+, and 8+) redox states. We have characterized
these mixed-valence and other redox states by (i) electron paramagnetic resonance (EPR) and
UV-Vis-NIR spectroscopies, (ii) high-resolution mass spectrometry (HR-MS), (iii) single crystal
X-ray diffraction (XRD) analysis, and (iv) electrochemical means as well as (v) density
functional theory (DFT) calculations.
■ RESULTS AND DISCUSSION
Since the N−N distances in both DAP
2+
and BIPY
2+
dications are
11
essentially identical, and
DAP
2+
dications are known to undergo
12
the one-electron reduction to form the corresponding
DAP
•+
radical cations, DAP
2+
dications were selected as substitutes for the BIPY
2+
units in
HC
•7+
. The redox properties of the N,N'-dimethyl-2,7-diazapyrenium dication (MDAP
2+
) were
explored by variable scan-rate cyclic voltammetry (CV) and compared with those of 1,1'-
dimethyl-4,4'-bipyridinium (MV
2+
). CV of MDAP
2+
at 10 mV s
–1
reveals (Figure 2a, black
trace) that the reduction of MDAP
2+
to MDAP
•+
occurs at a potential of –450 mV, which is
similar to that of –480 mV for MV
2+
. As the scan rate is increased, the reduction wave remains
the same, but the intensity of the original oxidation wave at –370 mV gradually decreases, and
meanwhile a new oxidation wave appears at +40 mV. Eventually, at the scan rate of 1 V s
–1
(Figure 2a, purple trace), two oxidation waves reach the same intensity. These observations
indicate
13
that, in solution, the MDAP
•+
radical cations exist primarily as cationic radical dimers
(MDAP
•+
)
2
, wherein oxidation leads firstly to the formation of a single unpaired spin mixed-
valence dimer (MDAP
2
)
•3+
before it completely dissociates into two MDAP
2+
dications. When
the scan rate is slower than the time scale of the dissociation of (MDAP
2
)
•3+
, oxidation of
(MDAP
•+
)
2
is observed to occur as a single oxidation wave. Once the scan rate becomes faster
than the dissociation rate, however, two separate oxidation waves corresponding to
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