Planar, Stair-Stepped, and Twisted: Modulating Structure and Photophysics in Pyrene- and Benzene-Fused N-Heterocyclic Boranes

Article abstract of DOI:10.1002/chem.202002118

Because of their rigidity, polycyclic aromatic hydrocarbons (PAHs) have become a significant building block in molecular materials chemistry. Fusion or doping of boron into PAHs is known to improve the optoelectronic properties by reducing the LUMO energy

Full text of DOI:10.1002/chem.202002118

Accepted Article
Accepted Article
Title: Planar, Stair-stepped, and Twisted: Modulating Structure and
Photophysics in Pyrene- and Benzene-Fused N-Heterocyclic
Boranes
Authors: Kelsie Krantz, Sarah Weisflog, Nathan Frey, Wenlong Yang,
Diane Dickie, Charles Ediwn Webster, and Robert J Gilliard
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To be cited as: Chem. Eur. J. 10.1002/chem.202002118
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Planar, Stair-stepped, and Twisted: Modulating Structure and
Photophysics in Pyrene- and Benzene-Fused N-Heterocyclic Boranes
Kelsie E. Krantz,[a] Sarah L. Weisflog,[a] Nathan C. Frey,[b] Wenlong Yang,[a] Diane A. Dickie,[a]
Charles Edwin Webster*,[b] and Robert J. Gilliard, Jr.*,[a]
[a] Kelsie E. Krantz, Sarah L. Weisflog, Dr. Wenlong Yang, Dr. Diane A. Dickie, Prof. Dr. Robert J.
Gilliard, Jr.
Department of Chemistry, University of Virginia
409 McCormick Rd./ PO Box 400319, Charlottesville, VA 22904 (USA)
E-mail: rjg8s@virginia.edu
[b] Nathan C. Frey, Prof. Dr. Charles Edwin Webster
Department of Chemistry, Mississippi State University
Box 9573, Mississippi State, MS 39762 (USA)
E-mail: ewebster@chemistry.msstate.edu
ABSTRACT: Because of their rigidity, polycyclic aromatic hydrocarbons (PAHs) have become a
significant building block in molecular materials chemistry. Fusion or doping of boron into PAHs
is known to improve the optoelectronic properties by reducing the LUMO energy level. Herein, we
report a comprehensive study on the syntheses, structures, and photophysical properties of a
new class of fused N-heterocyclic boranes (NHBs), pyrene- and benzene-linked in a "Janus-type"
fashion (2-4, 6-9, and 11). Remarkably, these examples of fused NHBs display fluorescent
properties, and collectively their emission spans the visible spectrum. The pyrene-fused NHBs all
display blue fluorescence, as the excitations are dominated by the pyrene core. In notable
contrast, the emission properties of the benzene-fused analogues are highly tunable and is
dependent on the electronics of the NHB fragments (i.e., the functional group directly bound to
the boron atoms). Pyrene-fused 2-4 and 11 represent the only molecules in which the K-region of
pyrene is functionalized with NHB units, and while they exhibit distorted (twisted or stair-stepped)
pyrene cores, benzene-fused 6-9 are planar. The electronic structure and optical properties of
these materials were probed by computational studies, including an evaluation of aromaticity,
electronic transitions, and molecular orbitals.
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INTRODUCTION
The dramatic increase in the synthesis of chemical compounds that contain N-heterocyclic
borane (NHB) (C2N2B-R, also called diazaborole) moieties may be traced to the seminal discovery
of boryllithium.[1] Since then, anionic NHBs[2] (i.e., boryl ligands) have been widely used as ligands
in main-group[3] and transition metal chemistry,[4] particularly in the synthesis of compounds
featuring new heteronuclear bonds. Additionally, diazaboroles and their polymeric analogues[5]
have been synthesized for their photoluminescent properties potentially useful for optoelectronic
devices.[6] Fusing multiple NHB units together in a "Janus-type" fashion, and thus adding an
element of bifunctionality, has received significantly less attention. These types of compounds are
extremely rare, mostly due to synthetic difficulty and thermodynamic instability.[7] Cowley and
coworkers isolated a cationic pyracene-fused NHB that displayed multielectron redox behavior
due to the ligand’s ability to readily undergo reduction (Figure 1a).[8] More recently, Kinjo et al.
reported boron-based tetraaminoethylene radical cations derived from the boryl-linked
tetraazaolefin (Figure 1b).[9] Last year, Weber et. al isolated and structurally characterized a
methylated benzene-fused bis(diazaborole).[10] Notably, this was the first example of a benzene-
fused bis(diazaborole) that was characterized structurally by X-ray diffraction. However,
photophysical data was unable to be obtained due to rapid decomposition of UV-vis samples.
Other examples of fused-NHBs with benzene[11] and quinone[12] cores have been incorporated
into field effect transistors, but their structural properties were not studied. Despite these
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advances, a comprehensive study of both the structural and photophysical properties of fused-
NHBs linked by polycyclic aromatic hydrocarbons (PAHs) is hitherto unknown.
Figure 1. a) Pyracene-fused N-heterocyclic borane with redox properties; b) boryl-linked tetraazaolefin that
oxidizes to the radical cation; c) methylated benzene-fused bis(diazaborole) d) This work: neutral pyrene-
and benzene-fused N-heterocyclic boranes featuring structural distortions and luminescent properties.
Owing to boron’s empty pz-orbital, the incorporation of boron into aromatic systems can result
in π-conjugation, important for inducing electronic properties useful in optoelectronics,[13] sensor
technology,[14] and fluorescent dyes.[15] Pyrene, a blue-chromophore, has served as a ubiquitous
building block in electronic devices.[16] In recent years, boron-doped pyrene-based molecules
have become popular with main-group chemists, with the goal of understanding structure-property
relationships in new functional materials.[17] For example, tethering boron to pyrene is known to
reduce the LUMO energy and increase the acceptor character by lowering the energy needed for
absorption and emission.[17a, 18] However, studies of compounds containing boron bound to pyrene
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have been limited due to challenging synthetic routes associated with modifying pyrene.[19]
Indeed, functionalization at the K-region of pyrene often requires multi-step syntheses with low
overall yields, and electrophilic substitution at the 1-, 3-, 6-, and 8-positions is preferred (i.e, where
the HOMO lies).[16, 20] Recently, we have been exploring doping PAHs with boron,[21] and attaching
main-group element fragments to the K-region of pyrene (Figure S1). This lead to the first pyrene-
fused N-heterocyclic germylenes (NHGe), which display extremely twisted molecular
structures.[22]
Herein we report the syntheses, molecular structures, photophysical properties, and
extensive computational studies of pyrene- and benzene-fused NHBs (Figure 1d). Remarkably,
the pyrene derivatives 2-4 and 11 are the first examples of compounds in which an NHB moiety
is fused to the K-region of pyrene, and they display twisted or stair-stepped structures. In contrast,
the benzene-fused compounds 6-9 are planar and possess tunable optical properties based on
the electronics of the NHB moieties. It is noteworthy that these are the first fused-NHBs that exhibit
fluorescent properties spanning the full visible spectrum.
RESULTS AND DISCUSSION
We began our studies by synthesizing a tetramine with sterically demanding N-(2,6-
diisopropylphenyl) groups. However, due to low yields in the multi-step synthetic route we
switched to the less bulky N-(tert-butyl) tetramine (1), prepared using a Buchwald-Hartwig
amination (Scheme 1).[23] Dehydrogenation of 1 and BH3•THF at 110 °C gave compound 2 as a
pale-yellow solid in 77% yield. In the 1H NMR spectrum, a downfield shift in the resonance (1.26
to 1.66 ppm) assigned to the N-(tert-butyl) protons represent the decrease in electron density at
the nitrogen atoms, where the lone pairs fill the p-orbital of boron. A resonance was not observed
in the 11B{1H} NMR spectrum, which was attributed to quadrupolar line broadening.
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Scheme 1. Synthesis of pyrene-fused N-heterocyclic boranes.
Yellow plate-shaped air- and moisture-sensitive single crystals of 2 suitable for X-ray
diffraction studies were obtained from a concentrated toluene solution at -37 °C. An overall twisted
structure of 2 was observed, comparable to our pyrene-fused NHGe,[22] but with smaller "twist
angles"[24] [C2–C2–C2ʹ–C1ʹ (40°) and N2–N1–N2ʹ–N1ʹ (52°)] (Figure 2a). However, the twist
angles of 2 are still substantial compared to known pyrene-fused molecules which span two
benzene rings (Figure 2b).[25]
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Figure 2. Molecular structure of 2: a) front view, b) side view. Thermal ellipsoids shown at 50% probability
and H atoms (except B-H) were omitted for clarity. Selected bond lengths (Å) and angles (°): B1–H1:
1.15(3); B1–N1: 1.421(7); B1–N2: 1.433(7); C2–C1–C2ʹ–C1ʹ: 40; N2–N1–N2ʹ–N1ʹ: 52.
Compound 3 was isolated by in-situ lithiation of 1 and subsequent salt elimination to give a
green solid in 32% yield (Scheme 1). A downfield shift in resonance from 1.26 to 1.79 ppm for the
1
N-(tert-butyl) protons was observed in the H NMR spectrum. In the 11B{1H} NMR spectrum, a
broad singlet at 25.7 ppm was attributed to the two chemically equivalent boron atoms. In the 19F
NMR spectrum, two broad resonances separated by 0.06 ppm (-125.64 and -125.70 ppm) were
observed in approximately 20:80 ratio. These peaks were assigned to both 10B (I = 3) and 11B (I
= 3/2) coupling to 19F. However, due to rapid quadrupolar relaxation the coupling constants could
not be determined.
Figure 3. Molecular structure of 3: a) front view, b) side view. Thermal ellipsoids shown at 50% probability
and H atoms were omitted for clarity. Selected bond lengths (Å) and angles (°): B1–F1: 1.335(4); B1–N1:
1.419(5); B1–N2: 1.423(5); C1–C2–C1ʹ–C2ʹ: 0; N1-N2-N1ʹ-N2ʹ: 0, C3–C2–C1–C7: 20.
Pale green plate-shaped air- and moisture-sensitive single crystals of 3 suitable for X-ray
studies were obtained from a concentrated toluene/hexane (1:1) mixture at -37 °C (Figure 3a).
Unlike the twisted structure of 2, 0° “twist angles” are observed, and the middle four rings of 3 are
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distorted out of plane in a stair-step fashion with a 20° C3–C2–C1–C7 torsion angle (Figure 3b).
The 6-membered rings of pyrene not interacting with nitrogen atoms remain coplanar. It is
noteworthy that 3 is the first structurally characterized NHB bearing a terminal boron monofluoride
unit.
Lithiation and subsequent salt elimination of 1 with PhBCl2 afforded a light orange solid of 4
in 47% yield (Scheme 1). The 1H NMR spectrum showed a downfield shift in resonance from 1.26
to 1.61 ppm which integrated to 36, and was attributed to the N-(tert-butyl) protons. In the 11B{1H}
NMR, a broad singlet at 33.1 ppm was assigned to the two chemically equivalent boron atoms.
Yellow rod-shaped air- and moisture-sensitive single-crystals of 4 suitable for X-ray diffraction
studies were obtained from a concentrated hexanes solution at -37 °C (Figure 4a). Compound 4
displayed a distorted pyrene core similar to 3 (Figure 4b), but with a larger torsion angle [28° for
C3–C2–C1–C7 plane]. Additionally, the boron-bound phenyl rings are twisted away from the 5-
membered rings, yielding a 60° C22–C21–B1–N2 torsion angle, while the geometry around the
boron atoms remains trigonal planar.
Figure 4. Molecular structure of 4: a) front view, b) side view. Thermal ellipsoids shown at 50% probability.
H atoms and co-crystallized solvent were omitted for clarity. Selected bond lengths (Å) and angles (°): B1–
C21: 1.573(4); B1–N1: 1.453(4); B1–N2: 1.448(5); C1–C2–C1ʹ–C2ʹ: 0; N1–N2–N1ʹ–N2ʹ: 0, C3–C2–C1–C7:
28; C22–C21–B1–N2: 60.
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We were then interested in obtaining fused boranes in which the entire molecule is planar, to
study how the linker and topography of the structure affects the photophysical properties. Due to
its tunable pH-dependent colorimetric properties, we targeted fused systems based on an N-alkyl
substituted benzene tetramine.[26] Using the tetramine (5)[27] and similar reaction conditions used
for making compounds 2-4, the benzene-fused compounds 6-8, (Scheme 2) were synthesized in
1
62, 48, and 50% yields respectively. In the H NMR spectra, a downfield shift from 1.29 to 1.65
(for 6), 1.43 (for 7), and 1.54 ppm (for 8) for the N-tert-butyl protons were consistent with the
11
formation new products. In the B{1H} NMR spectra, chemical shifts of 5.4 (for 7), 28.4 (for 8),
30.0 ppm (for 9) were attributed to the chemically equivalent boron environments. Similar to 2, no
19
11B{1H} NMR chemical shift was observed for 6 due to quadrupolar line broadening. In the F
NMR spectrum of 7, a standard 1:1:1:1 quartet resonance was observed at -151.8 ppm which
was attributed to the presence of two BF2 units (1JBF = 56.4 Hz). Compound 7 forms via loss of
LiH after the first deprotonation due to the resonance structure that can be obtained when the
nitrogen-based electrons are delocalized onto the benzene ring of 5. The loss of LiH prohibits all
four deprotonation events occurring, and accordingly, only two equivalents of LiF are eliminated.
Though the specific reasons for the mechanistic differences between 3 and 7 are unclear, this
underscores the ability of the two linking π-systems to facilitate the formation of diverse structural
arrangements.
Scheme 2. Synthesis of benzene-fused N-heterocyclic boranes.
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Air- and moisture-sensitive colorless rod-shaped crystals of 6 suitable for X-ray studies were
obtained from a concentrated solution in toluene at 110 °C and subsequent cooling to room
temperature (Figure 5a). Unlike 2, compound 6 displayed a completely planar structure (Figure
5b). The B1–H1 [1.140(17) Å] bond lengths were comparable to those in 2 [1.15(3) Å].
Figure 5. Molecular structure of 6: a) front view, b) view showing planarity of benzene. Thermal ellipsoids
shown at 50% probability and H atoms except for boron bound H were omitted for clarity. Selected bond
lengths (Å): B1–H1: 1.140(17); B1–N1: 1.426(2); B1–N2: 1.426(2); N1–C2: 1.4166(18); C1–C2: 1.395(2);
C1–C3ʹ: 1.398(2).
Pink cube-shaped crystals of 7 suitable for X-ray studies were obtained from a
concentrated THF solution at room temperature (Figure 6a). Similar to 6, compound 7 displayed
a planar structure (Figure 6b). The system, which indeed contains two BF2 units, can be best
described as a zwitterionic molecule with two positive charges delocalized over the benzene core
and a negative charge localized on each boron atom. This interpretation is further supported by
the bond lengths of C1–C2 [1.3968(12) Å] and C1–C3ʹ [1.5062(12) Å] which suggest that the π-
electrons are delocalized across the N1–C1–C2–C3–N2 atoms and not within the central 6-
membered ring. This description is also consistent with reported bis(boron difluoride) complex
[(BF2)2(μ-4-EtAp)] (Ap = azophenine),[28] platinum N’,N’,N’,N”-diboronazophenine complexes,[29]
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and the [(ttmgb)(BF2)2](BF4)2 (ttmgb = 1,2,3,4-tetrakis(tetramethylguanidinyl)benzene) salt.[30]
Additionally, the B–F bond lengths [1.3906(12) and 1.3853(12) Å] are slightly longer than the B1–
F1 bond [1.335(4) Å] in the pyrene complex 3.
Figure 6. Molecular structure of 7: a) front view, b) view showing planarity of benzene. Thermal ellipsoids
shown at 50% probability and H atoms were omitted for clarity. Selected bond lengths (Å): B1–F1:
1.3906(12); B1–F2: 1.3853(12); B1–N1: 1.5690(13), B1–N2: 1.5678(13), C1–N1: 1.3266(11); C1–C2:
1.3968(12); C1–C3ʹ: 1.5062(12).
Colorless plate-shaped single crystals of 8 suitable for X-ray studies were obtained from a
concentrated toluene solution at -37 °C (Figure 7a). The molecular structure showed a planar
benzene-fused NHB with orthogonal phenyl groups bound to boron (Figure 7b). In contrast to 4,
the phenyl groups in compound 8 are perpendicular to the boron plane, with a C13–C12–B1–N1
torsion angle of 93°.
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Figure 7. Molecular structure of 8: a) front view, b) view showing planarity of benzene. Thermal ellipsoids
shown at 50% probability and H atoms were omitted for clarity. Selected bond lengths (Å) and angles (°):
B1–N1: 1.4436(19), B1–N2: 1.440(2), B1–C12: 1.609(6); C13–C12–B1–N1: 93.
Probing the acidity of compound 6 was then of interest, and we explored this by the addition
of a strong base (Scheme 3). Two equivalents of n-butyllithium were added to a mixture of 6 in
THF. The absence of a resonance in the 7Li NMR spectrum supported the elimination of LiH rather
than the formation of a tetracoordinate Li borate salt. Further analysis of the reaction revealed
that the B–H substituent on 6 is more hydridic than acidic, and thus compound 9 was isolated as
a white crystalline solid (50% isolated yield). A broad resonance at 30.0 ppm was observed in the
11B{1H} NMR spectrum which was very similar to the shift of 8 (28.4 ppm).
Plate-shaped colorless single crystals of 9 suitable for X-ray analysis were grown from a
concentrated toluene solution at -37 °C (Figure 8a). Compound 9 exhibited a planar structure with
the n-butyl groups oriented trans to each other (Figure 8b). The B1–C12 bond length [1.589(8) Å]
was comparable to the B1–C12 bond [1.609(6) Å] of 8.
Figure 8. Molecular structure of 9: a) front view, b) view showing planarity of benzene. Thermal ellipsoids
shown at 50% probability and H atoms were omitted for clarity. Selected bond lengths (Å): B1–N1: 1.439(7),
B1–N2: 1.460(7), B1–C12: 1.589(8).
To gain insight into the photophysical properties of 2-4 and 6-9, UV-vis and fluorescence
studies were performed. Absorption maxima of 2-4 were exhibited at 395, 377, and 405 nm,
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respectively, assignable to π-π* transitions (Figure 10a). Emission maxima for the pyrene-fused
NHBs (2-4) were shown at 442, 456, and 457 nm, respectively (Figure 10b), which were all
hypsochromically shifted with respect to the tetramine starting material (1). Due to the high
contribution of the pyrene core in these excitations, they all emit blue fluorescence under UV-light.
The benzene-fused molecules (6-9) have absorption maxima at 326, 384, 387, and 320 nm,
respectively, which were all assignable to π-π* transitions (Figure 10c). Emission maxima for the
benzene-fused NHBs (6-9) were shown at 346, 415, 489, and 350 nm, respectively, and were
attributed to π*-π transitions (Figure 10d). Compounds 7 and 8 were bathochromically shifted with
respect to the tetramine starting material (5) while compounds 6 and 9 were hypsochromically
shifted. In addition to the experimental studies, the electronic structure and transitions of the
fused-NHBs were investigated by DFT calculations (B3LYP/BS1 level of theory), which supported
the aforementioned assignments. (see SI for details including natural transition orbitals). In the
benzene-fused NHBs, the color of emission varies significantly based on the substituent bound
to boron. This major difference is due to the NHB rings contributing more hole to particle (6, 88%-
30%, see Figure S78), while the NHB rings in the pyrene-fused compounds contribute much less
(2, 49%-24%). Notably, 6-9 all emit in the NIR region which is promising for optoelectronic
materials, specifically fluorescent probes.[31]
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Figure 10. a) UV-vis absorption of 1-4; b) emission of 1-4; c) UV-vis absorption of 5-9; d) emission of 6-9.
All samples were dissolved in toluene and ran at room temperature. Compounds 1-4 were excited at 380
nm and 5-9 were excited at 310 nm.
We then desired to explore the photophysical properties of a planar pyrene-fused NHB to
test if the fluorescence could be tuned in a manner similar to the benzene-fused NHBs. Therefore,
we replaced the N-(tert-butyl) substituents with less bulky N-methyl groups to limit steric
interactions with the pyrene-C-H. Compound 10 was isolated in 45% yield via Buchwald-Hartwig
amination (see SI for experimental details). Lithiation of 10 and successive salt elimination with
PhBCl2 gave 11 as a yellow solid in 66% yield (Scheme 3).
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Scheme 3. Synthesis of pyrene-fused N-heterocyclic borane (N-methyl).
Scheme 3. Synthesis of pyrene-fused N-heterocyclic borane (N-methyl).
Yellow rod-shaped single crystals of 11 were obtained from a concentrated toluene/THF
(10:1) mixture at room temperature (Figure 9a). As expected, the overall structure revealed a
more planar pyrene core, albeit not completely flat (Figure 9b). Examination of the space-filling
model (Figure S29) clearly shows that the methyl protons still interact with the hydrogen atoms
on pyrene to yield a slight distortion from planarity. The C3-C2-C1-C7 torsion angle in 11 (11°) is
significantly smaller than that in 4 (28°). The decrease in steric bulk at nitrogen also impacts the
rotation of the B-Ph group. Consequently, compound 11 displayed the smallest torsion angle for
the C20-C15-B1-N2 plane (49°) compared to the same angle in 4 (60°) and benzene-fused 8
(93°).
Figure 9. Molecular structure of 11: a) front view, b) side view. Thermal ellipsoids shown at 50% probability
and H atoms were omitted for clarity. Selected bond lengths (Å) and angles (°): B1–C15: 1.5669(19); B1–
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N1: 1.4291(18); B1–N2: 1.4272(18); C1–C2–C1ʹ–C2ʹ: 0; N1–N2–N1’–N2’: 0, C3–C2–C1–C7: 11; C22–
C21–B1–N2: 49.
The absorption (366 nm) and emission maxima (487 nm) of 10 were comparable to that of 1
(λabs = 369 nm, λem = 493 nm)[22] (Figure 11). Conversely, a significant difference in the appearance
of the absorption and emission features of 11 (λabs = 392 nm, λem = 419 nm) was observed in
contrast to 4. Additionally, the emission maximum was substantially blue-shifted from 4. Since
the blue emission color of 11 is not drastically different from 2-4, it supports the use of pyrene-
based molecules as blue emitters in organic light emitting diodes. However, this also highlights
the benefit of the benzene-fused materials (i.e., in terms of tunable optical properties by eye).
Figure 11. a) UV-vis absorption of 10 and 11; b) emission of 10 and 11. Samples were dissolved in toluene
and run at room temperature. Compound 10 was excited at 365 nm and 11 was excited at 380 nm.
In the pyrene-fused molecules, the HOMO and LUMO are all delocalized throughout the
pyrene core with minimal electron density at boron (Figure 12). However, benzene-fused 6, 8,
and 9 all have HOMOs that are localized on boron and the benzene core (Figure 13). Nodal
planes pass directly through the middle of the compounds, parallel to the boron atoms. In contrast,
the HOMO and LUMO of compound 7 are delocalized on the carbon and nitrogen backbone with
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no localization on boron, which is consistent with its zwitterionic structure. Unlike 6 and 9, the
LUMO of compound 8 is localized on the outer phenyl rings rather than the boron atoms and
benzene core. These results validate the boron-dependent electronics and the substantial
difference in emission colors observed under UV light. In the pyrene-fused NHBs (2-4, and 11)
the frontier orbitals are only pyrene-based whereas the HOMO/LUMOs of the benzene-fused
NHBs (6-9) include the 5-membered boron heterocyclic rings. Notably, the LUMO of compound
8 is the only example where the frontier orbitals are extended to the boron-bound substituent
outside of the ring (phenyl groups). Additionally, the fused NHBs possess large HOMO-LUMO
gaps that cover a broad range of energies (2.88-4.84 eV) that could be promising for materials
applications that require wide band gaps.[32]
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Figure 12. Frontier orbitals of pyrene-fused NHBs 2-4 and 11.
.
Figure 13. Frontier orbitals of benzene-fused NHBs 6-9.
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To further investigate the electronic contributions to aromaticity in the fused NHBs and how
this was affected by structural distortions, Nucleus Independent Chemical Shifts (NICS(0)) were
computed for each ring (Table 1). For the pyrene-fused NHBs, the NICS(0) values for B/B' and
C/C' rings in the pyrene core are consistent across all compounds. As expected, the C/C' rings
are the most aromatic in the pyrene core, and maintain their status as π-electron sextets as
supported by Clar’s theory.[33] However, the NICS(0) values for ring A/A', the 5-membered boron
containing rings, change dramatically throughout all molecules. These changes can be attributed
to the differences in the electronic and geometric structures influenced by nitrogen-alkyl group,
boron-functional group, and the fused aromatic core. An overall trend of more negative values for
the A/A' rings in the benzene-fused NHBs is observed in comparison to the pyrene-fused
analogues (with the exception of 7). This can be attributed to the electronic contribution and
delocalization of the HOMO on the NHB in 6, 8, and 9 unlike 2-4, and 11. Notably, the NICS(0)
values for ring A/A' in 7 are significantly higher than those found in the other compounds due to
the borate drastically altering the electronics and removing all aromatic character from the center
6-membered ring (E). Consequently, the NICS(0) value for ring E in 7 is positive, a major contrast
from any other molecule in this study. Compounds 6, 8, and 9 all retain aromaticity in ring E as
justified by the negative values.
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Table 1. NICS(0) values ( B3LYP/BS2 level of theory) of
compounds 2-11.
Compound
A/A'
B/B'
C/C'
D/D'
E
–
2
3
-6.39
-5.80
-4.84
-7.98
-1.70
-7.56
-7.87
–
-4.95 -10.47
-5.29 -10.78
-4.44 -10.56
–
–
–
4
-7.09
–
–
6
–
–
–
–
–
–
–
–
-11.77
4.08
7
–
8
-7.33 -11.44
9
–
–
-11.51
10
11
-5.07 -11.65
-4.44 -10.56
–
–
-4.84
-7.09
Conclusion
In conclusion, we have isolated, fully characterized, and investigated the photophysical
properties of the first pyrene-fused NHBs, which display unique twisted or stair-stepped
structures. We have also compared them to structurally related benzene-fused materials, which
are all planar. The tunable optical properties of the benzene-fused NHBs make them promising
candidates for functional materials due to their emission in the near-infrared region. The structural
properties of the fused-NHBs confirm that the aromatic core (pyrene vs. benzene) plays a
significant role in the overall electronic structure of the molecules. We believe that these new
synthetic routes to molecules that would otherwise be challenging to access, as well as the
tunability of this system, will spark new research efforts regarding the use of these compounds
as main-group functional materials. Moreover, similar strategies may be successful for unraveling
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10.1002/chem.202002118
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the chemistry of distorted aromatic systems that incorporate other p-block elements and these
efforts are currently underway in our laboratory.
EXPERIMENTAL SECTION
General Information. All manipulations involving air- and moisture-sensitive compounds were
carried out under inert atmosphere of purified argon using an MBRAUN LABmaster glovebox
equipped with a -37 °C freezer, or standard Schlenk line techniques. All solvents were purified by
distillation over sodium and benzophenone. Deuterated benzene was purchased from Cambridge
Isotope Laboratories, purified by distillation over sodium, and degassed via several freeze-pump-
thaw cycles. Glassware was oven-dried at 190 °C overnight. The NMR spectra were recorded at
room temperature on a Bruker Advance 600 MHz spectrometer. Proton and carbon chemical
shifts are reported in ppm and are referenced using the residual proton and carbon signals of the
13
deuterated solvent (1H; C6D6, δ 7.16; C; δ 128.1 ppm). Boron and fluorine chemical shifts are
reported in ppm and referenced using external standards (11B; BF3•Et2O, δ 0.0; 19F; C6F6, δ -164.9
ppm). The UV-visible and fluorescence spectra were recorded on a Cary 60 UV-vis
Spectrophotometer and a Cary Eclipse Fluorescence Spectrophotometer. Sample solutions were
prepared in toluene in 1 cm square air-free quartz cuvettes. Due to rapid decomposition at the
low concentrations needed for obtaining accurate quantum yield data, we were unable to report
these values for the fused-NHBs (this was also observed in Weber’s compounds).[34] Elemental
analyses were performed on a Perkin Elmer 2400 Series II analyzer. Similarly to other reported
fused-NHBs,[34] we were unable to obtain C,H,N-values for 3, 4, 8, and 11 due to air- and moisture-
sensitivity. All chemicals were purchased from commercial sources and used directly unless
noted otherwise. The 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (IPr•HCl)[35], 4,5,9,10-
tetrabromo-2,7-ditert-butylpyrene[36], and compound 1[22-23] were prepared according to the
literature procedures.
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Compound 2. To a solution of compound 1 (0.300 g, 0.501 mmol) in toluene (15 mL), BH3•THF
(1 M in THF, 1.75 mL, 1.75 mmol) was added under argon. The yellow slurry was heated to 110
°C for 2 days until a yellow solution was observed. The reaction was cooled to room temperature
and the pale-yellow solid was collected via filtration and washed with hexanes (5 mL) and the
remaining solid was dried under reduced pressure to leave compound 4 (0.239 mg, 77% yield).
Compound 4 can also be isolated from reaction of 2 with 2 eq. of BH3•THF in toluene. 1H NMR
(600 MHz, C6D6, 298 K) δ = 8.83 (s, 4H, CHpyr), 1.88 (s, 36H, N(C(CH3)3)), 1.66 ppm (s, 18H,
13
C(CH3)3). Unable to observe the BH due to the quadrupolar effect. C{1H} NMR (150.90 MHz,
C6D6, 298 K) δ = 132.3, 129.3, 125.7, 125.2, 117.5, 54.5, 34.8, 32.6, 30.2 ppm. 11B NMR (C6D6,
192.55 MHz): No shift was observed. Anal. Calcd for C40H60B2N4: C, 77.67; H, 9.78; N, 9.06%.
Found: C, 77.18; H, 9.75; N, 9.02%.
Compound 3. To a -78 °C solution of compound 1 (0.200 mg, 0.334 mmol) in Et2O (20 mL), n-
BuLi (2.5 M in hexanes, 0.56 mL, 1.40 mmol) was added slowly. The orange solution was slowly
warmed to room temperature and stirred for 20 hrs. The reaction mixture was cooled to -78 °C,
and BF3• Et2O (0.08 mL, 0.668 mmol) was added. The reaction was slowly warmed to room
temperature and the light green solution was stirred at room temperature for 22 hrs. The solvent
was removed under reduced pressure, and the product was extracted from the green residue with
toluene (30 mL). The mixture was filtered over celite to remove LiF salt. The solvent was removed
from the filtrate to leave a crude green solid. The solid was washed with hexane (3 x 5 mL) to
leave compound 3 as a light green solid (0.0698 g, 32% yield). Single crystals were obtained via
1:1 mixture of toluene/hexane at -37 °C. 1H NMR (600 MHz, C6D6, 298 K) δ = 8.57 (s, 4H, CHpyr),
1.79 (s, 36H, N(C(CH3)3)), 1.60 ppm (s, 18H, C(CH3)3). 13C{1H} NMR (150.90 MHz, C6D6, 298 K)
δ = 144.0, 131.0, 125.5, 117.2, 116.8, 55.1, 36.1, 33.2, 32.4 ppm. 11B NMR (C6D6, 192.55 MHz):
δ 25.7 ppm. 19F NMR (C6D6, 564.69 MHz): δ -125.7 ppm (d, J = 33.9 Hz, 1F).
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Compound 4. To a -78 °C solution of compound 1 (500 mg, 0.835 mmol) in Et2O (20 mL), n-BuLi
(2.5 M in hexanes, 1.40 mL, 3.51 mmol) was added slowly. The orange solution was slowly
warmed to room temperature and stirred for 17 hrs. The solvent was removed under reduced
pressure and the orange residue was dissolved in toluene (50 mL), and PhBCl2 (0.218 mL, 1.67
mmol) was added slowly. The reaction was stirred at room temperature for 21 hrs. The red mixture
was filtered over celite to remove LiCl salt, and the solvent was removed from the filtrate to leave
a crude red solid. The solid was washed with hexane (3 x 5 mL) to leave compound 4 as a pale
orange solid (303.8 mg, 47% yield). 1H NMR (600 MHz, C6D6, 298 K) δ = 8.65 (s, 4H, CHpyr), 7.96
(d, 4H, o-PhH), 7.37 (t, 4H, m-PhH), 7.29 (t, 2H, p-PhH), 1.64 (s, 18H, C(CH3)3), 1.61 ppm (s,
36H, N(C(CH3)3)). 13C{1H} NMR (150.90 MHz, C6D6, 298 K) δ = 144.1, 134.6, 134.3, 126.4, 116.7,
116.0, 56.4, 36.2, 35.2, 32.4 ppm. 11B NMR (C6D6, 192.55 MHz): δ 33.1 ppm.
Compound 5. Synthesis of compound 5 was modified from the reported literature.[27] In a Schlenk
tube 1,3-bis(2,6-diiso-propylphenyl)imidazolium chloride (IPr•HCl, 0.189 g, 0.444 mmol),
[Pd(OAc)2] (0.0513 g, 0.229 mmol), and NaOtBu (0.0646 g, 0.673 mmol) were stirred in toluene
(30 mL) at room temperature until a clear orange solution was observed. 1,2,4,5-
tetrabromobenzene (5.00 g, 12.7 mmol) and NaOtBu (5.98 g, 62.2 mmol) were then added to the
orange solution. Under positive argon pressure, tert-butylamine (5.61 mL, 53.3 mmol) was added,
and the resulting orange mixture was heated to 110 °C for 15 hrs. The mixture was cooled to
room temperature and filtered over celite to remove NaBr salt. The solvent was removed from the
filtrate to leave a brown solid. The crude solid was washed with hexane (3 x 20 mL) to leave
compound 5 as a tan solid (3.93 g, 85% yield). 1H NMR (600 MHz, C6D6, 298 K) δ = 6.68 (s, 2H,
CHbenz), 3.61 (s, 4H, NH), 1.29 ppm (s, 36H, N(C(CH3)3).
Compound 6. To a solution of compound 5 (1.00 g, 2.76 mmol) in toluene (100 mL), BH3•THF (1
M in THF, 5.79 mL, 5.79 mmol) was added dropwise. The resulting yellow mixture was heated to
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10.1002/chem.202002118
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110 °C for 2 days. The mixture was then cooled to room temperature and the product was
collected via filtration. The solid was washed with hexane (3 x 5 mL) to leave compound 5 as a
white crystalline solid (0.650 g, 62% yield). Single crystals suitable for X-ray studies were grown
from a refluxing toluene solution and slow cooling to room temperature. 1H NMR (600 MHz, C6D6,
298 K) δ = 7.66 (s, 2H, CHbenz), 1.65 ppm (s, 36H, N(C(CH3)3). Unable to observe the BH due to
13
quadrupolar effect. C{1H} NMR (150.90 MHz, C6D6, 298 K) δ = 130.7, 100.6, 67.8, 52.4, 30.9
ppm. 11B NMR (C6D6, 192.55 MHz): No shift was observed. Anal. Calcd for C22H40B2N4: C, 69.14;
H, 10.55; N, 14.66%. Found: C, 68.64; H, 10.62; N, 14.54%.
Compound 7. To a -78 °C solution of compound 5 (0.200 g, 0.552 mmol) in THF (40 mL) n-BuLi
(2.5 M in hexanes, 0.927 mL, 2.32 mmol) was added slowly. The red solution was slowly warmed
to room temperature and stirred for 20 hrs. The solution was cooled to -78 °C, and BF3• Et2O
(0.136 mL, 1.10 mmol) was added. The dark purple solution was stirred at room temperature for
24 hrs. The solvent was removed under reduced pressure, and the product was extracted from
the purple solid residue with toluene (40 mL). The mixture was filtered over celite to remove LiF
salts. The solvent was removed from the filtrate to leave a crude purple solid. The solid was
recrystallized from a concentrated THF solution at room temperature to leave compound 7 as a
1
purple crystalline solid (120 mg, 48% yield). H NMR (600 MHz, C6D6, 298 K) δ = 5.43 (s, 2H,
CHbenz), 1.43 ppm (s, 36H, N(C(CH3)3). 13C{1H} NMR (150.90 MHz, C6D6, 298 K) δ = 155.9, 128.9,
55.2, 29.0, 28.3 ppm. 11B NMR (C6D6, 192.55 MHz): δ 5.4 ppm. 19F NMR (C6D6, 564.69 MHz): δ
-151.8 ppm (q, J = 56.4 Hz, 2F). Anal. Calcd for C22H38B2F4N4: C, 57.92; H, 8.40; N, 12.28%.
Found: C, 58.03; H, 8.46; N, 11.84%.
Compound 8. To a -78 °C solution of compound 5 (0.500 g, 1.38 mmol) in Et2O (40 mL), n-BuLi
(1.6 M in hexanes, 3.62 mL, 5.79 mmol) was added slowly. The red solution was slowly warmed
to room temperature and stirred for 20 hrs. The solvent was removed under reduced pressure
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10.1002/chem.202002118
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and the red solid residue was dissolved in toluene (40 mL), and PhBCl2 (0.365 mL, 2.76 mmol)
was then added. The reaction was stirred at room temperature for 24 hrs. The reaction mixture
was filtered over celite to remove LiCl salt. The solvent was removed from the filtrate to leave a
crude pink solid. The solid was washed with hexane (3 x 5 mL) to leave compound 8 as a light
pink solid (0.373 g, 51% yield). Single crystals suitable for X-ray studies were obtained from a
concentrated toluene solution at -37 °C. 1H NMR (600 MHz, C6D6, 298 K) δ = 7.81 (s, 2H, CHbenz),
7.56 (d, 4H, o-PhH), 7.20 (m, 6H, m-PhH/p-PhH overlapped), 1.54 ppm (s, 36H, N(C(CH3)3)).
13C{1H} NMR (150.90 MHz, C6D6, 298 K) δ = 133.9, 130.1, 127.3, 127.0, 102.8, 54.5, 32.4 ppm.
11B NMR (C6D6, 192.55 MHz): δ No shift was observed, hidden behind probe.
Compound 9. To a -78 °C mixture of compound 6 (0.200 g, 0.5233 mmol) in THF (20 mL), n-
BuLi (1.6 M in hexanes, 0.719 mL, 1.15 mmol) was added slowly. The resulting green mixture
was warmed to room temperature and stirred for 4 hrs. The solvent was removed under reduced
pressure to leave a green residue. The product was extracted with toluene (5 mL) and
recrystallized at -37 °C to leave compound 9 as a white crystalline solid (0.130 g, 50% yield). 1H
NMR (600 MHz, C6D6, 298 K) δ = 7.64 (s, 2H, CHbenz), 1.77 (s, 36H, N(C(CH3)3), 1.73 (t, 4H,
CH2CH2CH2CH3), 1.68 (m, 4H, CH2CH2CH2CH3), 1.50 (m, 4H, CH2CH2CH2CH3), 1.02 ppm (t, 6H,
13
CH2CH2CH2CH3). C{1H} NMR (150.90 MHz, C6D6, 298 K) δ = 129.9, 103.0, 54.4, 32.1, 31.8,
26.6, 14.3 ppm. 11B NMR (C6D6, 192.55 MHz): 30.0 ppm. Anal. Calcd for C30H56B2N4: C, 72.88;
H, 11.42; N, 11.33%. Found: C, 72.41; H, 11.42; N, 11.16%.
Compound 10. In a Schlenk tube 1,3-bis(2,6-diiso-propylphenyl)imidazolium chloride (IPr•HCl,
0.0861 g, 0.202 mmol), [Pd(OAc)2] (0.0227 g, 0.101 mmol), and NaOtBu (0.0292 g, 0.304 mmol)
were stirred in toluene (40 mL) at room temperature until a clear orange solution was observed.
4,5,9,10-tetrabromo-2,7-ditert-butylpyrene (0.914 g, 1.45 mmol) and NaOtBu (0.667 g, 6.94
mmol) were then added to the orange solution. Under positive argon pressure, methylamine (2 M
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10.1002/chem.202002118
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in THF, 3.47 mL, 6.94 mmol) was added, and the resulting tan mixture was heated to 110 °C for
18 hrs. The mixture was cooled to room temperature and filtered over celite to remove NaBr salt.
The solvent was removed from the filtrate to leave a brown solid. The crude solid was washed
with hexane (3 x 5 mL) to leave compound 10 as an orange solid (0.281 g, 45% yield). 1H NMR
(600 MHz, C6D6, 298 K) δ = 8.43 (s, 4H, CHpyr), 4.02 (q, 4H, NH), 2.28 (d, 12H, NCH3), 1.60 ppm
(s, 18H, C(CH3)3). 13C{1H} NMR (150.90 MHz, C6D6, 298 K) δ = 147.8, 137.0, 129.0, 120.6, 116.5,
37.4, 35.6, 32.3 ppm.
Compound 11. To a -78 °C solution of compound 10 (200 mg, 0.464 mmol) in Et2O (40 mL), n-
BuLi (1.6 M in hexanes, 1.22 mL, 1.95 mmol) was added slowly. The dark red solution was slowly
warmed to room temperature and stirred for 20 hrs. The solvent was removed under reduced
pressure and the red residue was dissolved in toluene (40 mL), and PhBCl2 (0.121 mL, 0.929
mmol) was added slowly. The reaction was stirred at room temperature for 27 hrs. The dark yellow
mixture was filtered over celite to remove LiCl salt, and the solvent was removed from the filtrate.
The crude yellow-green solid was washed with hexanes (3 x 5 mL) to leave compound 11 as a
yellow solid (184 mg, 66% yield). Single crystals suitable for X-ray studies were obtained from a
concentrated toluene/THF mixture (10:1) at room temperature. 1H NMR (600 MHz, C6D6, 298 K)
δ = 9.05 (s, 4H, CHpyr), 7.73 (d, 4H, o-PhH), 7.46 (t, 4H, m-PhH), 7.38 (t, 2H, p-PhH), 3.94 (s,
13
12H, NCH3), 1.63 ppm (s, 18H, C(CH3)3). C{1H} NMR (150.90 MHz, C6D6, 298 K) δ = 146.1,
136.0, 135.1, 129.7, 128.8, 128.4, 126.1, 117.8, 113.4, 35.8, 35.7, 32.2 ppm. 11B NMR (C6D6,
192.55 MHz): δ 29.8 ppm.
ACKNOWLEDGEMENTS
We are grateful to the University of Virginia and the National Science Foundation (OIA-1539035
and CHE-1800201) for support of this work. Computations were performed at the Mississippi
State University High Performance Computing Collaboratory (HPC2) and the Mississippi Center
for Supercomputing Research.
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