Orthogonal Oriented Bisanthrancene-Bridged Bis(Triarylamine) Diradical Dications: Isolation, Characterizations and Crystal Structures

Article abstract of DOI:10.1002/asia.201801816

Two bis(triarylamine) diradical dication salts 1²⁺2[Al(OR_F)₄]^? and 2²⁺2[Al(OR_F)₄]^? bridged with an orthogonal oriented bisanthrancene motif were synthesized. Their solid state and electronic structures were investigated by various experimental approaches in conjunction with theoretical calculations. Superconducting quantum interference device measurements and density functional theory calculations reveal that both of 1²⁺ and 2²⁺ feature open-shell singlet ground states with rather small singlet-triplet energy gaps; thus, they can be regarded as nearly pure diradicals (biradicals). Their diradical characters are enhanced by the orthogonal configuration of the bisanthrancene moiety which demonstrates the importance of the geometry of bridging units in turning the electronic structures of bis(triarylamine) dications.

Full text of DOI:10.1002/asia.201801816

CHEMISTRY
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Accepted Article
Title: Orthogonal Oriented Bisanthrancene-Bridged Bis(Triarylamine)
Diradical Dications: Isolation, Characterizations and Crystal
Structures
Authors: Lei Wang, Li Zhang, Yong Fang, Yue Zhao, Gengwen Tan,
and Xinping Wang
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10.1002/asia.201801816
Chemistry - An Asian Journal
Artical
Orthogonal Oriented Bisanthrancene-Bridged Bis(Triarylamine)
Diradical Dications: Isolation, Characterizations and Crystal
Structures
Lei Wang,[a] Li Zhang,[a] Yong Fang,[a] Yue Zhao,[a] Gengwen Tan*[a] and Xinping Wang*[a]
Abstract:
Two
bis(triarylamine)
diradical
dication
salts
hydrocarbons bearing an orthogonal arranged bisanthrancene
spacer (Scheme 1).
Neutral precursors 1 and 2 were synthesized by palladium-
catalyzed Buchwald–Hartwig cross-coupling amination of 10,10'-
dibromo-9,9'-bianthracene[6] with the corresponding diarylamines
(see supporting information (SI) for details). From reactions of 1
12+2[Al(ORF)4]‒ and 22+2[Al(ORF)4]‒ bridged with an orthogonal
oriented bisanthrancene motif were synthesized. Their solid state
and electronic structures were investigated by various experimental
approaches in conjunction with theoretical calculations. SQUID
measurements and DFT calculations reveal that both of 12+ and 22+
feature open-shell singlet ground states with rather small singlet-
triplet energy gaps, thus they can be regarded as nearly pure
diradicals (biradicals). Their diradical characters are enhanced by
the orthogonal configuration of the bisanthrancene moiety which
demonstrates the importance of the geometry of bridging units in
turning the electronic structures of bis(triarylamine) dications.
and 2 with two molar equivalents of Ag[Al(ORF)4] (ORF
=
OC(CF3)3)[7] in CH2Cl2 at room temperature, the corresponding
dication salts 12+•2[Al(ORF)4]‒ and 22+•2[Al(ORF)4]‒ were
obtained in high yields as crystalline solids. Strikingly, the
dication salts of 12+ and 22+ are thermally stable under an inert
atmosphere.
Diradicals, species possess two unpaired electrons, have been
intensively studied not only attributing to their roles as important
reactive intermediates, but also playing fundamental significance
in nonlinear optics, molecular electronics and organic
spintronics.[1] In this context, Thiele’s and Chichibabin’s
hydrocarbons have been the subjects of interest for several
decades.[1] Chichibabin’s hydrocarbons exhibit characteristic
resonance structures between an open-shell diradical and a
closed-shell quinonoid form (Scheme 1).[2] Due to the high
reactivity, they readily reacts with oxygen, and also tend to
dimerize and/or polymerize.[1] Replacing the spin-bearing carbon
centers in Chichibabin’s hydrocarbons with nitrogen atoms
followed by two-electron oxidation will lead to stable π-
conjugated dicationic nitrogen analogues. [3]
In terms of improving the diradical character of the
bis(triarylamine) dications, different approaches have been
employed, including modifying the substituents of the peripheral
aryl groups and altering the lengths of bridging π-conjunction
units.[1k] Another practical strategy to access molecules with
pronounced diradical character is tethering geometrical
orthogonalization of singly occupied molecular orbitals.[4] One of
the facile methods to achieve this is utilizing perpendicular
linking units.[5] For instance, Tanaka et al. obtained a triplet
ground state bis(triarylamine) dication by applying an orthogonal
oriented spiro linker.[5b] Herein, we present the isolation and
characterization of two nitrogen analogues of Chichibabin’s
Scheme 1. Chichibabin’s hydrocarbon and bisanthrancene-bridged
bis(triarylamine) dications.
Crystals suitable for X-ray crystallographic studies were
obtained by cooling the concentrated CH2Cl2 solutions of salts
12+•2[Al(ORF)4]‒
and
22+•2[Al(ORF)4]‒
at
–30
oC.[8]
12+•2[Al(ORF)4]‒ crystallizes in the triclinic space group P−1,
while 22+•2[Al(ORF)4]‒ crystallizes in the monoclinic space group
P21/c. The stereoviews of dications 12+ and 22+ are illustrated in
Figure 1. Their important structure parameters are listed in Table
1. For both 12+ and 22+, the nitrogen atoms feature a perfect
trigonal planar geometry, and the angles between the NC3
planes and anthracene moieties are 69.4° and 79.6° for 12+ and
76.9° and 67.8° for 22+. The average N−Ci bond lengths to the
anthracene units are longer than those to the peripheral aryl
groups. Compared to the precursor 1, the C−N bonds between
the nitrogen atoms and the peripheral aryl rings of 12+ and 22+
are remarkably shortened, while the lengths of the C−N bonds
between the nitrogen atoms and the bisanthracene moieties are
almost unchanged. (Table S2 in SI) The lengths of the C–C
bonds between the two anthracene moieties are 1.502 Å and
1.497(3) Å for 12+ and 22+, respectively, which are typical for
[a]
Dr. L. Wang, Dr. L. Zhang, Y. Fang, Dr. Y. Zhao, Dr. G. Tan, Prof.
Dr. X. Wang
State Key Laboratory of Coordination Chemistry, School of
Chemistry and Chemical Engineering, Collaborative Innovation
Center of Advanced Microstructures, Nanjing University, Nanjing
210023, China
E-mail: gengwentan@nju.edu.cn; xpwang@nju.edu.cn
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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Table 1. Selected experimental and calculated bond lengths (Å), relative energy (kcal mol-1), and diradical character (y) for 12+ and 22+
.
[a]
ΔEX–OS
avg. N–Ar
1.400
1.447
1.406
1.406
1.400
1.456
1.410
1.410
avg. N–Ci
1.444
1.338
1.439
1.439
1.441
1.330
1.430
1.430
Cp1–Cp2
1.502
1.371
1.504
1.504
1.497
1.370
1.504
1.504
avg. Ci–Co and Cm–Cp
avg. Co–Cm
1.437
BLA[b]
0.030
0.058
0.031
0.031
0.032
0.060
0.026
0.027
y[d]
12+
X-ray
CS[c]
OS[d]
T[d]
1.407
1.481
1.415
1.415
1.406
1.482
1.419
1.418
18.19
0
1.423
1.446
0.998
0.024
1.446
22+
X-ray
CS[c]
OS[d]
T[d]
1.438
13.04
0
1.422
1.445
0.985
0.038
1.445
[a] Energy relative to the OS state. X=CS (closed-shell singlet), OS (open-shell singlet), or T (triplet). [b] BLA is defined as the difference of the average length
of Co‒Cm and Ci–Co, Cm–Cp. [c] Calculated at the B3LYP/6-31G(d) level. [d] Calculated at the UB3LYP/6-31G(d) level.
C−C single bonds, suggesting little quinonoid structural
contribution to the molecular structures. The geometry
configuration of the two anthracene rings is nearly perpendicular
(87.0° for 12+ and 88.2° for 22+), leading to a large dihedral angle
between the two NC3 planes. Reduced bond-length alteration
(BLA) has been considered as a practical parameter to estimate
the degree of diradical character in hydrocarbon diradicals.[2b, 9]
BLA is defined as the difference of the average length of Co‒Cm
vs those of Ci–Co and Cm–Cp in this work (Table 1). The BLAs
(0.030 for 12+ and 0.032 for 22+) are smaller than the previous
The diradical characters of 12+ and 22+ were further
investigated by variable temperature EPR spectroscopy and
SQUID measurements. At 298 K, the EPR spectra of the solid
samples of 12+ and 22+ show g factors of 2.0023 and 2.0042,
respectively, which are close to the value for free electron
(Figure 2). Unfortunately, the spectra cannot be simulated,
which is probably due to poor resolutions. Strikingly, 12+ and 22+
are EPR active even at 90 K and the forbidden resonances Δms
= ±2 were observed (Figure 2), indicating fairly good diradical
characters for 12+ and 22+.
3a, b]
reported diphenyl bridged bis(triarylamine) dications,[9b,
showing that the orthogonal configuration of the bisanthracene
unit significantly improve the diradical characters of the
bis(triarylamine) dications.
a)
a)
b)
b)
Figure 1. Thermal ellipsoid (50%) drawings of a) 12+ and b) 22+. Hydrogen
atoms were omitted for clarity.
Figure 2. Powder EPR spectra of (a) 12+ and (b) 22+ at 90 K.
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SQUID measurements were conducted at 2‒300 K on the
powder samples of 12+ and 22+. The χMT versus T plots show an
abrupt susceptibility increasing in the range of 2 K to 50 K and
reach a close value of 0.75 cm3 K mol-1 above 50 K(Figure 3).
The plots can be well fitted by the Bleaney−Bowers equation[10]
with Hamiltonian H = ‒ 2J S1 S2 (S1 = S2 = 1/2), and the singlet-
triplet energy gap was estimated to be 2J = −5.5 and −4.3 cm-1
for 12+ and 22+, respectively. The results show that 12+ and 22+
possess open-shell singlet ground states with rather small
singlet-triplet energy gaps, and thus can be regarded as having
almost degenerate singlet and triplet states.
UV-Vis spectra of 12+ and 22+ measured in CH2Cl2 solutions
at room temperature show the maximum absorptions at 1213
and 1406 nm, respectively (Figure 4b). Time-dependent-DFT
calculations on the open-shell singlet geometry of 12+ and 22+ at
the UB3LYP/6-31G(d) level indicate that the maximum
absorptions are due to HOMO−1 (α) → LUMO (α) and HOMO−1
(β) → LUMO (β) transitions (See Figure S1 in SI).
Figure 4. a) Spin densities of 12+-OS (top) and 22+-OS (bottom), calculated at
the UB3LYP/6-31G(d) level. b) UV-Vis spectra of 12+ and 22+ measured in
CH2Cl2 solutions (1 × 10-4 M) at room temperature.
In summary we have stabilized and isolated two examples
of bisantrance bridged bis(triarylamine) diradicals dications 12+
and 22+ featuring open-shell singlet ground states and the triplet
states are thermally accessible due to rather small singlet-triplet
energy gaps. The small singlet-triplet energy gaps are likely
attributed to the orthogonal configuration of the bianthracene
units. Our work demonstrates the important role of geometric
configuration of the spacer in tuning the electronic structures of
bis(triarylamine) dications. Exploration of the application of these
systems and other orthogonal configured diradicals is under way
in our laboratory.
Figure 3. χMT versus T curves for the powder of 12+ and 22+ in SQUID
measurements, and the fitting plots obtained with the Bleaney–Bowers
equation.
In order to further understand their electronic structures, we
carried out calculations for 12+ and 22+.[11] Full geometry
optimizations for closed-shell (CS), open-shell singlet (OS) and
triplet (T) states were performed at the (U)B3LYP/6–31G(d) level
and the obtained stationary points were characterized by
frequency calculations. The broken-symmetry approach was
applied for open-shell singlet calculations. The double-saddle-
like quinoidal structure of the bisanthracene moieties was
applied to the closed-shell initial geometries.[12] Relative
energies, bond lengths, and their alternations (BLAs) as well as
diradicals character values y are listed in Table 1. The bond
lengths and BLAs of optimized OS and T geometries are closer
to the values of crystal structures than those of CS states. The
energy gaps ΔET-OS = 8.4 cm-1 for 12+ and ΔET-OS = 13.4 cm-1 for
22+ are obtained after the approximate spin-projection (AP)
correction,[13] which support the open-shell singlet ground state
and are consistent with the SQUID measurements. The
occupation number of the lowest unoccupied natural orbital
(LUNO) of the OS state, i.e. the diradical character y, represents
the “degree” of the diradical character (y = 0 for closed-shell and
y = 1 for pure diradical).[9b] The diradical character values y of
12+-OS and 22+-OS are very closed to 1, indicating a significant
diradical character of 12+ and 22+. In addition, the spin density is
mainly delocalized over the nitrogen atoms and peripheral aryl
rings (Figure 4a).
Acknowledgements
We thank the National Key R&D Program of China (Grants
2016YFA0300404 and 2018YFA0306004, X.W.), the National
Natural Science Foundation of China (Grants 21525102 and
21690062, X.W., 21601082, G.T.), the Fundamental Research
Funds for the Central Universities (Grants 14380134 and
020514380152, G. T.) and China Postdoctoral Science
Foundation (Grant 2018M632280, L. Z.) for financial support.
The calculations were performed at the High-Performance
Computing Center of Nanjing University.
Keywords: Diradical • Dication • Orthogonal Configuration •
SQUID • DFT Calculations
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X-ray data for 1, 12+ and 22+ are listed in Table S1 in SI. CCDC
1898509 (1), 1884017 (12+·2[Al(ORF)4]-·3(CH2Cl2)), and 1884018
(22+·2[Al(ORF)4]-·(CH2Cl2)) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge
Crystallographic
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
With
the
orthogonal
oriented
L. Wang, L. Zhang, Y. Fang, Y. Zhao, G.
Tan,* X. Wang*
bisanthrancene as bridges, we have
synthesized and characterized two
stable bis(triarylamine) diradical cation
salts, which feature rather small
singlet-triplet energy gaps. This work
demonstrates the important role of
geometric configuration of the spacer
in tuning the electronic structures of
diradicals.
Page No. – Page No.
Orthogonal Oriented Bisanthrancene-
Bridged Bis(Triarylamine) Diradical
Dications: Isolation,
Characterizations and Crystal
Structures
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