Bis(boryl anion)-Substituted Pyrenes: Syntheses, Characterizations, and Crystal Structures

Article abstract of DOI:10.1021/acs.organomet.7b00368

The two new diboranes 1 and 2 connected by a pyrene moiety at the 1,6- and 1,3-positions, respectively, were synthesized, and their two-electron-reduction reactions were investigated. The doubly reduced species 1^??2- is silent in electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopic measurements, suggesting a quasi-quinoidal structure with a diradical character of 1^??2-, which has a singlet-triplet gap of 6.6 kcal mol^-1 as determined by theoretical calculations. In contrast, the reduction product 2^??2- is EPR active and theoretical calculations indicate that 2^??2- has an open-shell singlet ground state with a singlet-triplet energy gap of 4.9 kcal mol^-1.

Full text of DOI:10.1021/acs.organomet.7b00368

Communication
pubs.acs.org/Organometallics
Bis(boryl anion)-Substituted Pyrenes: Syntheses, Characterizations,
and Crystal Structures
Ningning Yuan, Wenqing Wang, Yong Fang, Jiacheng Zuo, Yue Zhao, Gengwen Tan,*
and Xinping Wang*
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center
of Advanced Microstructures, Nanjing University, Nanjing 210023, People’s Republic of China
S
* Supporting Information
ABSTRACT: The two new diboranes 1 and 2 connected by a pyrene
moiety at the 1,6- and 1,3-positions, respectively, were synthesized,
and their two-electron-reduction reactions were investigated. The
doubly reduced species 1••2− is silent in electron paramagnetic
resonance (EPR) and nuclear magnetic resonance (NMR) spectro-
scopic measurements, suggesting a quasi-quinoidal structure with a
diradical character of 1••2−, which has a singlet−triplet gap of 6.6 kcal
mol−1 as determined by theoretical calculations. In contrast, the
reduction product 2••2− is EPR active and theoretical calculations
indicate that 2••2− has an open-shell singlet ground state with a
singlet−triplet energy gap of 4.9 kcal mol−1.
Scheme 1. Structurally Characterized Diboryl Dianions
he synthesis of stable main-group-element-based radicals
T
represents one of the most active research fields in
contemporary organometallic chemistry.1 Diradicals, molecules
with two unpaired electrons, have been attracting a great deal of
attention in the last decades owing to their importance in
understanding the nature of the chemical bonds and their
interesting physical properties (optical, electronic, magnetic,
etc.).2 They are envisioned to have promising applications as
functional materials in electronic devices, quantum information
processing systems, and organic spintronics.2 Three-coordinate
boranes have proven to be elegant spin carriers, and they have
been frequently utilized to construct diradicals owing to their
intrinsic electron deficiency.3 For instance, Kaim and co-
workers demonstrated that 1,4-phenylene- and 4,4′-dipheny-
lene-linked diboranes could undergo one-electron reduction,
affording their radical anions as detected in situ by EPR
spectroscopy.3a Rajca and co-workers reported a persistent
bis(boryl anion) diradical in solution featuring a triplet ground
state, but its solid state structure was not determined.4 Since
then, with the help of sterically encumbered and π-conjugated
ligands, stable boron-based monoradicals, stabilized by
delocalization of the unpaired electron on one or more boron
atom,s have been isolated,5 and diboryl dianions A−F have
been isolated and fully characterized (Scheme 1).
reminiscent of the scenario observed in bis(triarylamine)-based
diradical dications.9
Pyrene-based materials have widely applied as organic
electronics, and a variety of pyrene derivatives have been
synthesized up to now.10 The substitution of pyrene at different
positions of pyrene rings renders its derivatives significantly
different properties. Tethering two Mes2B units to pyrene
moiety at 2,7-positions followed by two-electron reduction
afforded the dianion A, which has some biradicaloid character
with a singlet−triplet energy gap of 13 kcal mol−1 as
determined by computational calculations.5l In order to
investigate how the Mes2B functional groups at the different
positions of pyrene can influence the electronic structures of
The dianions A5l and B6 feature an open-shell singlet ground
state as indicated by theoretical calculations, while D−F5m,7
have closed-shell structures. In addition, we recently reported
the first example of stable bis(boryl anion) diradical C having a
triplet ground state with a small singlet−triplet energy gap.8
These results suggest that the bridging unit can tune the
property of the obtained diradical dianion species of diborane,
Special Issue: Tailoring the Optoelectronic Properties of Organo-
metallic Compounds with Main Group Elements
Received: May 12, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00368
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Organometallics
Communication
the corresponding doubly reduced species, we synthesized the
precursors 1 and 2 with two Mes2B moieties at the 1,6- and 1,3-
positions, respectively (Scheme 2). Herein, we report two-
Scheme 2. Pyrene-Bridged Diboranes 1 and 2 and Their
Doubly Reduced Species
electron reduction of 1 and 2 affording the isolable diradical
dianions 1••2− and 2••2−, respectively, with significantly smaller
singlet−triplet gaps in comparison to that of A.5l
Figure 1. Top view of the molecular structure of the diradical dianion
1••2− (a) and its side view (b). All hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): B1−C1 1.500(4),
B1−C9 1.608(5), B1−C18 1.619(4), C1−C2 1.440(4), C1−C7A
1.474(4), C2−C3 1.358(4), C3−C4 1.440(4), C4−C5 1.392(4), C5−
C6 1.392(4), C6−C7 1.399(4), C7−C8 1.437(4), C4−C8A 1.435(4),
C8−C8A 1.435(5); C1−B1−C9 122.7(3), C1−B1−C18 120.1(3),
C9−B1−C18 116.9(3).
The neutral compounds 1 and 2 were synthesized by the
reactions of Mes2BF with 1/2 molar equiv of the corresponding
dilithium salts of pyrene, which were generated in situ through
transmetalation of the dibromopyrene precursors with 2 molar
equiv of nBuLi in THF (see the Supporting Information for
details). Compounds 1 and 2 were isolated as yellow solids, and
they were characterized by NMR spectroscopy and elemental
analysis. The cyclic voltammetry investigation of 1 and 2
revealed two nearly reversible reduction peaks at −2.24, −1.95
V and at −2.41, −1.89 V, versus the Ag/Ag+ reference electrode
(Figures S1 and S2 in the Supporting Information),
respectively, suggesting that their doubly reduced species
might be stable enough for isolation.
Encouraged by the electrochemical reduction results, we
further carried out two-electron chemical reduction of 1 and 2
in an attempt to isolate their doubly reduced species and study
their structural and electronic properties. The reduction of 1
and 2 with a slight excess amount of KC8 in the presence of 18-
crown-6 in THF at room temperature afforded the doubly
+
reduced dianion salts [(18-c-6)K(THF)2]2 ·12−•• and [(18-c-
+
6)K(THF)2]2 ·22−•• as blue crystals in 19.8% and 20.1% yields,
respectively. The molecular structures and electronic structures
of 12−•• and 2••2− were further studied by X-ray crystallog-
raphy, EPR spectroscopy, and UV−vis absorption spectrosco-
py, in conjunction with DFT calculations.
The dianion salt [(18-c-6)K(THF)2]2 ·1••2− crystallizes in
+
Figure 2. Top view of the molecular structure of the diradical dianion
2••2− (a) and its side view (b). All hydrogen atoms are omitted, and
two Mes moieties are drawn in a wireframe mode for the sake of
clarity. Selected bond lengths (Å) and angles (deg): B1−C8 1.504(7),
B1−C22 1.620(7), B1−C31 1.612(7), B2−C10 1.504(6), B2−C49
1.602(7), B2−C40 1.609(7), C7−C8 1.471(6), C8−C9 1.416(6),
C9−C10 1.391(6), C10−C11 1.480(6), C7−C16 1.429(7), C11−C16
1.432(6); C8−B1−C22 119.9(4), C8−B1−C31 122.4(4), C22−B1−
C31 117.2(4), C10−B2−C49 125.1(4), C10−B2−C40 116.3(4),
C49−B2−C40 118.2(4).
the triclinic space group P1. The structure of 1••2− is shown in
̅
Figure 1, it possesses a crystallographic inversion center in the
+
crystal. The dianion of [(18-c-6)K(THF)2]2 ·2••2− is shown in
Figure 2, there are two essentially identical molecules in the
same asymmetric unit. In contrast to the perfectly planar
geometry of the central moiety in A, the two boron atoms in
12−•• and 2••2− are not in the same plane with the central
pyrene moiety, and they reside at two different sides of the
pyrene plane (Figures 1b and 2b). The boron centers have an
almost planar configuration with the sum of angles around
boron atoms being 359.7° in 12−•• and 359.5 and 359.6° in
2••2−. The dihedral angles between the BC3 and pyrene planes
are 27.83° in 12−•• and 33.74 and 30.23° in 2••2−. The B−
C(pyrene) bonds (1.500(4) Å in 12−•• and 1.504(7) and
1.504(6) Å in 2••2−) are slightly shorter than those in A
(1.510(3) Å)5l but much longer than those in D (1.486(7) and
1.483(6) Å).5m In 12−••, the C−C bonds d and g−k in the
central six-membered ring have the property of aromatic C−C
bonds, whereas the bond b is more typical of a CC double
bond (1.360(4) Å). Therefore, 12−•• is best described as a
naphthalene substituted by two boron analogues of butadiene
at the 1,8- and 4,6-positions as shown in Scheme 2, which is
strikingly different from the structure of A.5l The C−C bonds
of the pyrene group in 22−•• feature an aromatic C−C bond
property, indicating limited electron delocalization over the
pyrene moiety.
B
DOI: 10.1021/acs.organomet.7b00368
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Communication
In order to elucidate the electronic structures of 1••2− and
2••2−, we further carried out EPR measurements and theoretical
calculations. The microcrystalline sample of 12−•• is EPR silent
at both room temperature and low temperatures, suggesting a
diamagnetic nature or relatively large singlet−triplet gap.
Comparison of the calculated energies at the upbe1pbe/6-
31G* level indicates that the open-shell singlet (OS) state has
the lowest energy with a OS−triplet gap of 6.6 kcal mol−1 after
an approximate spin projection (AP) method treatment.11
Moreover, the THF solution of 1••2− is also NMR silent at
room temperature and at 213 K except for the observation of
the proton signals of 18-c-6.12 Therefore, 12−•• is best described
as a dianion with some diradical character, similar to that of A.5l
In contrast, the crystalline sample of 2••2− is EPR active and
a quite broad resonance signal was observed along with a weak
Δms = 2 (half-field) transition at 90 K (Figure 3a), suggesting
wavelength absorptions (589, 659, and 787 nm) for 12−•• and
two absorptions (569 and 1066 nm) for 2••2− present in the
corresponding spectra. The absorptions at 787 and 1066 nm for
1••2− and 2••2−, respectively, attributed to the SOMO−LUMO
transition according to time-dependent DFT calculations
(Figures S4−S7 in the Supporting Information).
In summary, we have demonstrated that, by changing the
substitution positions of the diboryl groups from the 1,6- to the
1,3-positions of pyrene moiety, the electronic structures of the
obtained dianion 1••2− and 2••2− are strikingly different. 1••2−
features a singlet ground state and can be treated as a
naphthalene moiety substituted by two boron analogues of
butadiene at the 1,8- and 4,6-positions. The dianion 2••2− is
EPR active, and it has an open-shell singlet ground state as
determined by theoretical calculations and variable-temperature
EPR spectroscopy. Our work demonstrates that the singlet−
triplet gap of bis(boryl anion) diradicals can be tuned facilely by
changing the substitution positions at the bridging unit.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00368.
Experimental details, crystallographic data, cyclic voltam-
mograms of 1 and 2, and DFT calculation details (PDF)
Cartesian coordinates for the calculated structures (XYZ)
+
Figure 3. (a) EPR spectrum of [(18-c-6)K(THF)2]2 ·2••2− at 90 K in
Accession Codes
CCDC 1548469−1548470 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
the solid state. The inset shows the half-field signal. (b) Mulliken spin
density distribution of 2••2− in an open-shell singlet state.
13
a spin-triplet state of 2••2−
.
Variable-temperature EPR
measurements reveal a good linear correlation of ln(AT)
versus 1/T, where A is the absorption area of the half-field
signal, yielding a singlet−triplet gap of 4.8 kcal mol−1 (Figure
S3 in the Supporting Information).14 DFT calculations reveal
that the open-shell singlet is the ground state, and the singlet−
triplet gap is 4.9 kcal mol−1 after applying an AP method
treatment,11 in accordance with the experimental result.
Moreover, the Mulliken spin density distribution indicates
that the spin density is mainly localized on two boron centers
with a small amount on the pyrene group (Figure 3b).
AUTHOR INFORMATION
Corresponding Authors
*E-mail for G.T.: gengwentan@126.com.
*E-mail for X.W.: xpwang@nju.edu.cn.
■
ORCID
Gengwen Tan: 0000-0002-6972-2197
Xinping Wang: 0000-0002-1555-890X
Notes
The UV−vis absorption spectra of 1, 2, 1••2−, and 2••2− were
obtained in THF under a nitrogen atmosphere (Figure 4). In
comparison to their neutral precursors, there are three long-
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Grant 21525102, X.W.; Grant 21601082, G.T.), the Major
State Basic Research Development Program (Grant
2016YFA0300404, X.W.), and the Natural Science Foundation
of Jiangsu Province (Grant BK20140014, X.W.) 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.
G.T. acknowledges the National Postdoc Foundation for a
fellowship. N.Y. thanks the Promotion B project of Nanjing
University for a scholarship.
+
Figure 4. UV−vis spectra of 1, 2, [(18-c-6)K(THF)2]2 ·1••2−, and
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DOI: 10.1021/acs.organomet.7b00368
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