Experimental Observation of Thermally Excited Triplet States of Heavier Group 15 Element Centered Diradical Dianions

Article abstract of DOI:10.1002/chem.201706060

One-electron reductions of Mes*As=Fl* (1; Fl=2,7-di-tert-butylfluorenylidene, Mes=2,4,6-tBu₃C₆H₂) and diarsaalkenes [1,2-b]-IF(=AsMes*)₂ (2; IF=indenofluorene) with potassium led to the isolation of the arsenic-

Full text of DOI:10.1002/chem.201706060

A Journal of
Accepted Article
Title: Experimental Observation of Thermally Excited Triplet States of
Heavier Group 15 Element-Centered Diradical Dianions
Authors: Yong Fang, Li Zhang, Cheng Cheng, Yue Zhao, Manabu Abe,
Gengwen Tan, and Xinping Wang
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Experimental Observation of Thermally Excited Triplet States of
Heavier Group 15 Element-Centered Diradical Dianions
Yong Fang,[a]† Li Zhang,[a]† Cheng Cheng,[a] Yue Zhao,[a] Manabu Abe,*[b] Gengwen Tan*[a] and Xinping
Wang*[a]
Abstract: One-electron reductions of Mes*As=Fl* (1; Fl* = 2,7-di-
tert-butylfluorenylidene, Mes* = 2,4,6-tBu3C6H2) and diarsaalkenes
[1,2-b]-IF(=AsMes*)2 (2; IF = indenofluorene) with potassium led to
the isolation of the arsenic-centered radical anion salts 1K and 2K,
respectively. The diradical dianion salts 2K2 and 3K2 were afforded
by the reduction of 2 and 2,8-tBu2-[2,1-b]-IF(=AsMes*)2 (3) with an
excess amount of KC8. The radicals have been investigated by
single-crystal X-ray crystallography, EPR and UV-Vis absorption
spectroscopy, along with theoretical calculations. The calculations
revealed that 2K2 and 3K2 feature open-shell singlet ground states
ligand to successfully stabilize the neutral arsenic radical B.[2k] In
comparison to the limited number of arsenic-centered
monoradicals, its diradicals are even scarcer. There have been
only two stable examples of arsenic-based diradicals (C[3j] and
D[5]) reported by Schulz and coworkers. It is noteworthy that C
and D only feature some diradical characters, and their triplet
states can not be thermally accessible at experimental
conditions. In addition, they can further undergo one-electron
oxidation to the corresponding radical cations E and F.[3i] Until
now, there have been no examples of stable arsenic-based
with singlet-triplet energy gaps of 2.1 and 1.0 kcal mol-1, respectively. radical anions and diradical dianions, although diarsene[6] and
They are readily thermally excited to triplet states as demonstrated
by EPR spectroscopy. The obtained diradicals represent the first
examples of heavier Group 15 element-centered diradicals with
experimentally observable triplet states.
phosphaarsene[7] radical anions have been investigated by
electron paramagnetic resonance (EPR) spectroscopy.
Main-group element-based radicals have attracted tremendous
attention in contemporary organometallic chemistry because of
their importance in biological chemistry, theoretical chemistry
and materials science.[1] Due to the limited number of
energetically accessible valence orbitals in main-group elements,
their radicals are highly reactive and unstable under ambient
conditions. Despite the challenges, a variety of stable main-
group element-centered radicals have been successfully
isolated with the aid of sterically demanding or -conjugated
ligands.1 Group 15 element-centered radicals have been widely
investigated, and phosphorus-centered radicals have been
studied the most intensively. Several monoradicals[2],[3] and
diradicals[3d, 3j, 3l] of the phosphorus element have been isolated
and fully characterized. However, these diradicals feature large
singlet-triplet energy gaps, and their triplet states could not be
thermally populated under experimental conditions.
Scheme 1. Isolable arsenic-centered radicals and diradicals
Arsaalkenes, compounds bearing the As=C double bond, are
expected to be readily reduced to the radical anions owing to the
lower-lying *(As=C) orbital when comparing to that of C=C
double bond in olefins. This is attributed to smaller overlap of 4p
orbitals of the As atom with 2p orbitals of the C atom.[8]
Considering this property of arsaalkenes and the scarcity of
arsenic-based radicals, we were interested in synthesizing
stable arsenic-based radical anions and diradical dianions
through the reduction of arsaalkenes. Herein, we report the one-
electron reduction of the arsaalkene Mes*As=Fl* (1; Fl* = 2,7-di-
In contrast, there have been a limited number of isolable
arsenic-centered radicals (Scheme 1). In 2013, Robinson and
•+
coworkers reported the first stable As2 radical cation A
stabilized by two N-heterocyclic carbene (NHC) ligands.[4] In
2014, Grützmacher and coworkers utilized the phosphinidene
[a]
Y. Fang, Dr. Li Zhang, C. Cheng, Dr. Y. Zhao, Dr. G. Tan and 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
tert-butylfluorenylidene,
Mes*
=
2,4,6-tBu3C6H2),
and
diarsaalkene [1,2-b]-IF(=AsMes*)2 (2; IF = indenofluorene),
affording the radical anions 1K and 2K, respectively. The
diarsaalkenes 2 and 2,8-tBu2-[2,1-b]-IF(=AsMes*)2 (3) could also
undergo two-electron reductions to yield the diradical dianions
2K2 and 3K2, respectively. The solid-state and electronic
structures of the obtained radicals were investigated by single
crystal X-ray diffraction analysis, EPR and UV-Vis absorption
E-mail: gengwentan@nju.edu.cn; xpwang@nju.edu.cn
Prof. Dr. M. Abe
[b]
Department of Chemistry, Graduate School of Science, Hiroshima
University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Japan
E-mail: mabe@hiroshima-u.ac.jp
†
These authors contribute equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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spectroscopy, in conjunction with density functional theory (DFT)
calculations.
bonds (1.855(3) Å) to the indenofluorene moiety are longer than
that in the neutral compound 1 (1.794(11) Å),[9] but shorter than
that in the one-electron reduction product 1K (1.891(3) Å),
indicating that the single electron is equally delocalized over the
two As=C double bond moieties.
The doubly reduced species 2K2 and 3K2 crystallize in the
triclinic space group P-1 and orthorhombic space group Pna21,
respectively (Figure 1). The two K+ cations in 2K2 coordinate to
18-c-6 and THF molecules, whereas they are trapped by 222-
cryptand in 2K2. The dianion of 2K2 has a symmetric center, and
the two As atoms have identical coordination environment, while
the two As atoms of 3K2 have slightly different distances to the
Arsaalkene
1 was synthesized according to published
procedures.[9] Diarsaalkenes 2 and 3 were obtained through the
reaction of the in situ prepared Mes*AsCl2 with the dilithium salt
of the corresponding indenofluorenes, following by treatment
with an excess amount of 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) (see supporting information (SI) for details). The redox
properties of 1-3 were evaluated by electrochemical studies
(Figures S1–3 in the SI). The cyclic voltammetry reveals that 1
has one reversible reduction peak at –1.78 V, while 2 and 3
feature two reversible reduction peaks (–1.43 and –1.14 V for 2;
–1.98 and –1.42 V for 3), versus the Ag/AgNO3 reference
electrode. The electrochemical results suggest that compound 1
can undergo one-electron reduction, while 2 and 3 can be one-
electron and two-electron reduced.
connected
C atoms (1.896(5) and 1.879(5) Å) of the
indenofluorene fragment. The distances of the As atoms to C
atoms of the indenofluorene moieties in 2K2 (1.888(5) Å) and
3K2 (av. 1.888(5) Å) are comparable with that in 1K (1.891(3) Å),
consistent with the result that both *(As=C) orbitals are
occupied by one single electron.
a)
b)
Scheme 2. Stable arsenic-centered radical anion and diradical dianion salts.
Accordingly, the reduction of 1 with a stoichiometric amount of
elemental potassium afforded the radical anion 1K as deep-blue
crystals in 45% yield (Scheme 2). The one-electron reductions of
2 and 3 were carried out with one molar equiv. of elemental
potassium as the reductant. However, only the reduction of 2 led
to the isolation of deep-brown crystals of 2K in moderate yield.
We were not able to isolate the crystals of one-electron
reduction product of 3. The two-electron reductions of 2 and 3
with an excess amount of KC8 (three molar equiv.) resulted in
the formation of the diradical dianions 2K2 and 3K2 as deep-
green crystals in 53% and 36% yields, respectively. All the
reduced species of 1-3 are highly moisture- and oxygen-
sensitive, but are stable under an inert atmosphere at ambient
temperature for days.
Crystals of 1K and 2K were harvested from the concentrated
THF solution at –30 oC (Figures S4 and S5).[10] The solvated K+
cation interacts with the fulvene moiety in a 5 mode with a
distance of 2.826 Å to the center of the fulvene plane. The As–C
bond (1.891(3) Å) to the fulvene group is largely elongated in
comparison to that in 1 (1.794(11) Å),[9] in line with one-electron
addition to the * orbital of the As=C double bond. In contrast to
1K, the K+ cation of 2K is coordinated by 18-c-6 and two THF
molecules, and there is no direct cation-anion interaction in the
structure. The anion moiety is centrosymmetric. The As–C
Figure 1. Ellipsoid drawings of the dianion moiety of 2K2 (a) and 3K2 (b) at
30% probability. Hydrogen atoms and THF solvent molecules are omitted for
clarity.
The molecular structures of all the reduced species could be
well reproduced by DFT calculations at the (U)WB97XD/6-31G(d)
level of theory. The singly occupied molecular orbitals (SOMOs)
of the radical anions 1K and 2K are mainly comprised of As=C
antibonding interactions (Figures S6 and S7). The Mulliken spin
density distribution reveal that the unpaired electron mainly
resides at the As center (0.62) with small contribution from the C
atoms of the fluorene moiety in 1K (Figure S8). Most of the spin
density in 2K is located at two As centers, and there is some
contribution from the central phenyl group and the C atoms of
the indenofluorene moieties (Figure S9).
In order to determine the ground state of 2K2 and 3K2, we
carried out DFT calculations on their closed-shell singlet (CS),
open-shell singlet (OS) and triplet (T) states, and the obtained
stationary points were characterized by frequency calculations.
For 2K2, the OS state[11] has the lowest energy, and the singlet-
triplet gap is 2.1 kcal mol–1,[12] which is substantially smaller than
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its phosphorus analogue (6.7 kcal mol–1).[3l] This suggests that
2K2 has a larger diradical character and its triplet state might be
relatively easy to be thermally achieved. The Mulliken spin
density distribution reveals that most of the spin density resides
at the As centers, meanwhile there is a small amount of density
located at the five-membered rings and the central benzene
moiety (Figure 2a).
than those in A (24 G)[4] and B (21 G)[2k], further supporting that
1K is an arsenic-centered radical. The EPR spectrum of 2K in
THF solution at room temperature shows relatively weak signals
owing to the coupling with two As nuclei (a(57As) = 29.1 G)
(Figure S11). The much smaller coupling constant in comparison
to that in 1K can be interpreted by equal electron delocalization
over two As centers. The EPR spectrum of the frozen solution at
88 K exhibits high-resolution anisotropic hyper-couplings (Figure
S11), and the g factors and hfc tensors were determined as
follows: gx = 2.0404, gy = 2.0284, gz = 2.0032, axx = ayy = 26 G,
azz = 124 G, indicating the spin density mainly resides at the two
As centers, consistent with the calculated result.
a)
a)
b)
b)
*
*
Figure 2. The Mulliken spin density distribution of 2K2 (a) and 3K2 (b) at the
open-shell singlet state.
Since the two As=C spin carriers reside at the meta positions
of the central benzene group, similar to the electronic structures
of meta-quinodimethanes, we expected that 3K2 might exhibit a
triplet ground state.[13] However, the calculations reveal that the
diradical dianion 3K2 also features an open-shell singlet ground
state with a singlet-triplet gap of 1.0 kcal mol–1,12 which is much
smaller than that of 2K2. The spin density mainly resides at the
two As atoms, and there is a negligible amount of spin at the
central benzene ring (Figure 2b), in strikingly contrast with that
of 2K2.
Figure 3. Experimental (black) and simulated (red) EPR spectra of 2K2 (a) and
3K2 (b) obtained in the solid state at room temperature. The peaks labeled
with stars are most probably attributed to the monoradical impurity. The insets
show the half-field signal.
The different electronic structures between 2K2 and 3K2 are
also reflected by the nucleus-independent chemical shift (NICS)
values (Table S1).[14] The NICS(0) value for the central benzene
ring increases from 2, 2K to 2K2, suggesting the aromaticity
decreases during the reduction processes. This is attributed to
the electron delocalization over the central benzene ring leading
to an increasing degree of the quinoidal structure upon reduction.
In contrast, the aromaticity of the central bezene ring in 3K2
strengthens in comparison to that in 3, but the NICS(o) value for
the flanking benzene rings increases substantially from 3 to 3K2
due to the presence of some spin density at these two benzene
moieties in the doubly reduced species. In addition, the NICS(0)
values of the five-membered rings decrease during reductions,
particularly for 2 and its reduced species, they change from
antiaromaticity in 2 (3.61) to aromaticity in the doubly reduced
form (–1.04).
The EPR spectra of 2K2 and 3K2 were measured in the solid
state at room temperature (Figure 3). Strikingly, for both
compounds, the half-field signal was observed, supporting its
spin-triplet state at the measurement conditions. Therefore,
compounds 2K2 and 3K2 represent the first stable heavier Group
15 element-based diradicals featuring observable thermally
excited triplet states. The broad peaks centered at g = 2.0054 for
3K2 were simulated, and D = 700 G was obtained from the
simulation. The average spin–spin distance was estimated to be
3.4 Å from the D value, which is much shorter than the distance
between two As centers (5.8 Å) in the crystal structure,
indicating the spin delocalization over the indenofluorene moiety.
The EPR spectrum of 2K2 could not be simulated most probably
due to overlapping with signals from monoradical impurity.[15]
The UV-Vis absorption spectra of all the reduced species
were measured in THF solutions under a nitrogen atmosphere
(Figure 4). For the radical anion 1K, one long wavelength
absorption was observed at 624 nm, which is assigned to
HOMO()-LUMO() electron transition according to time-
The EPR spectrum of 1K in THF solution at room temperature
reveals a quartet signal (g = 2.018) with isotropic hyperfine
coupling constant (hfc) of a(57As) = 41.8 G owing to the coupling
to one 57As (I = 3/2) nucleus (Figure S10), which is much larger
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(Figure S13). In terms of both 2K2 and 3K2, they originate from
HOMO()-LUMO() and HOMO()-LUMO() electron transitions,
respectively (Figures S14 and S15). The broad peak at 870 nm
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anion impurity, which is confirmed by TD-DFT calculations.
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Figure 4. The UV-Vis absorption spectra of 1K, 2K, 2K2 and 3K2 obtained in
THF solutions under nitrogen atmosphere.
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In summary, by utilizing the fluorene and indenofluorene
groups as the structural scaffolds, we successfully isolated the
arsenic-centered radical anions and diradical dianions. The high
stability of these radicals is most probably attributed to partial
electron delocalization over the fluorene and indenofluroene
moieties. Our work provides the first examples of stable arsenic-
centered radical anions synthesized through reduction of the
As=C double bonds of arsaalkenes. Strikingly, 2K2 and 3K2 are
the first heavier Group 15 element-based diradicals featuring
thermally accessible triplet states because of the small singlet-
triplet energy gaps. The synthesis of polyradicals based on the
R′E=CR2 (E = Group 15 elements) building block is currently
undergoing in our laboratory.
Acknowledgements
We thank the National Key R&D Program of China (Grant
2016YFA0300404, X.W.) and the National Natural Science
Foundation of China (Grants 21525102, 21690062, X.W. and
21601082, G.T.) for financial support. We are grateful to the
High Performance Computing Center of Nanjing University for
doing the numerical calculations in this paper on its IBM Blade
cluster system.
[4]
Keywords: Radical • Diradical • Group 15 element • Arsenic •
Triplet state
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10.1002/chem.201706060
Chemistry - A European Journal
COMMUNICATION
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COMMUNICATION
Stable Radical Anions: The stable
arsenic-centered radical anion and
diradical dianion salts have been
successfully synthesized through
reductions of the arsaalkene and
diarsaalkene precursors. The obtained
diradicals represent the first examples
of heavier Group 15 element-centered
diradicals featuring experimentally
observable triplet states.
Y. Fang, L. Zhang, C. Cheng, Y. Zhao,
M. Abe,* G. Tan,* X. Wang*
Page No. – Page No.
Experimental
Observation
of
Thermally Excited Triplet States of
Heavier Group 15 Element-Centered
Diradical Dianions
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