Cationic Boron Formazanate Dyes**

Article abstract of DOI:10.1002/anie.202015036

Incorporation of cationic boron atoms into molecular frameworks is an established strategy for creating chemical species with unusual bonding and reactivity but is rarely thought of as a way of enhancing molecular optoelectronic properties. Using boron formazanate dyes as examples, we demonstrate that the wavelengths, intensities, and type of the first electronic transitions in BN heterocycles can be modulated by varying the charge, coordination number, and supporting ligands at the cationic boron atom. UV-vis absorption spectroscopy measurements and density-functional (DFT) calculations show that these modulations are caused by changes in the geometry and extent of π-conjugation of the boron formazanate ring. These findings suggest a new strategy for designing optoelectronic materials based on π-conjugated heterocycles containing boron and other main-group elements.

Full text of DOI:10.1002/anie.202015036

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Accepted Article
Title: Cationic Boron Formazanate Dyes
Authors: Ryan R Maar, Benjamin D. Katzman, Paul D Boyle, Viktor N
Staroverov, and Joe B. Gilroy
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10.1002/anie.202015036
Angewandte Chemie International Edition
COMMUNICATION
Cationic Boron Formazanate Dyes
Ryan R. Maar†, Benjamin D. Katzman†, Paul D. Boyle, Viktor N. Staroverov, and Joe B. Gilroy*[a]
[a]
Dr. R. R. Maar, B. D. Katzman, Dr. P. D. Boyle, Prof. Dr. V. N. Staroverov, and Prof. Dr. J. B. Gilroy
Department of Chemistry and The Centre for Advanced Materials and Biomaterials Research (CAMBR)
The University of Western Ontario
1151 Richmond Street North, London, Ontario N6A 5B7 (Canada)
E-mail: joe.gilroy@uwo.ca
†
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Incorporation of cationic boron atoms into molecular
frameworks is an established strategy for creating chemical species
with unusual bonding and reactivity but is rarely thought of as a way
of enhancing molecular optoelectronic properties. Using boron
formazanate dyes as examples, we demonstrate that the
wavelengths, intensities, and type of the first electronic transitions in
BN heterocycles can be modulated by varying the magnitude of
charge, coordination number, and supporting ligands at the cationic
boron atom. UV-vis absorption spectroscopy measurements and
density-functional calculations show that these modulations are
caused by changes in the geometry and extent of -conjugation of
the boron formazanate ring. These findings suggest a new strategy
for designing optoelectronic materials based on π-conjugated
heterocycles containing boron and other main-group elements.
The ability of main-group elements to form stable
compounds with unusual structure, bonding, and reactivity has
powered a resurgence of synthetic main-group chemistry[1] and
challenged the supremacy of transition metals in catalysis,[2]
bond-[3] and small-molecule activation techniques,[4] and the
development of functional materials.[5] In the field of functional
materials in particular, incorporation of main-group elements into
π-conjugated frameworks is becoming a powerful strategy for
the development of new optoelectronic materials.[6]
Here, we demonstrate that the variation of cationic charge
and coordination number of boron atoms is a very effective
approach for tuning the optoelectronic properties of boron
formazanate complexes such as 7−14. This demonstration is the
first of its kind for BN heterocycles and can be extended to other
main-group elements and similar ligand families (e.g., dipyrrin,
aza-dipyrrin, and -diketiminates).
Owing to its electron-deficient nature, boron is often
combined with organic fragments to modulate the energies of
the frontier orbitals. The resulting compounds have been
suggested as promising candidates for use in organic
electronics.[7] To modulate the optoelectronic properties of these
molecules, suitable methods to enhance Lewis acidity at the
boron centre are required. The traditional approaches rely on
installing anti-aromatic scaffolds around boron atoms[8],[9] or
using electron-withdrawing substituents.[10] Another strategy
consists in varying the charge and coordination number of boron
atoms[11] and produces two-,[12],[13] three-,[14],[15] and four-
coordinate[16],[17] cations and dications such as 1+−62+.[11a, 18] The
method of charge variation is familiar within the catalysis
arena[19] and fundamental research on the structure and bonding
of cationic boron compounds[20] but remains relatively
unexplored in the field of optoelectronic materials.
Boron formazanate dyes 7‒10+, 12+, and 132+ were
synthesized according to Scheme 1 and characterized using
multinuclear NMR, FT-IR, and UV-Vis absorption spectroscopies,
as well as high-resolution mass spectrometry (Figures S1‒S24).
BPhF formazanate 7 was prepared by heating 1,5-(p-tolyl)-3-
phenylformazan at 60 °C with excess N(iPr)2Et and a mixture of
TMSCl and KBPhF3 in CH2Cl2/CH3CN for 36 h. The crude
product was purified via column chromatography to produce
complex 7 in 75% yield. This transformation was accompanied
by a colour change from red to purple, a loss of the NH
resonance in the 1H NMR spectrum of 1,5-(p-tolyl)-3-
phenylformazan ( = 15.51), and the appearance of broad
singlets at 2.9 ppm in the 11B{1H} NMR spectrum and
‒164.6 ppm in the 19F NMR spectrum of 7. Halide exchange
between complex 7 and BCl3 afforded BPhCl formazanate 8 as
a dark-purple solid in quantitative yield. This was confirmed by
the presence of a singlet at 2.4 ppm in the 11B{1H} NMR
spectrum and absence of signals in the 19F NMR spectrum for
complex 8. BPhCl formazanate 8 was converted to the three-
One extremely useful platform for developing the chemistry
of main-group elements is furnished by formazanate
ligands.[21],[22] Boron difluoride formazanate dyes display easily
tunable optoelectronic properties through substituent variation[23]
and find numerous applications as cell-imaging agents,[24]
25]
electrochemiluminescent emitters,[23e,
and precursors to a
wide variety of BN heterocycles with unprecedented structure
and functionality.[26]
1
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Scheme 1. Synthesis of boron formazanates 7‒14. DMAP
=
4-dimethylaminopyridine, OTf
=
trifluoromethanesulfonate, dppmo
=
bis(diphenylphosphino)methane dioxide.
coordinate (borenium) cation 9+ by treatment with an equimolar
amount of AlCl3 in CH2Cl2. Complex 9+ was isolated as an air-
and moisture-sensitive dark-purple solid in 98% yield. The
presence of a three-coordinate boron centre was confirmed by a
broad signal centred at 35.5 ppm in the 11B{1H} NMR spectrum
and a sharp singlet at 103.6 ppm in the 27Al{1H} NMR spectrum
indicative of an [AlCl4]‒ counterion. BPhCl formazanate 8 was
also used to synthesize the four-coordinate (boronium) cation
10+ via treatment with AgOTf (OTf = trifluoromethanesulfonate)
followed by the addition of one equivalent of 4-
dimethylaminopyridine (DMAP). The resulting dark-red solid was
isolated in 92% yield and displayed a broad singlet in the 11B{1H}
NMR centred at 1.9 ppm and a sharp singlet in the 19F NMR
spectrum at ‒77.5 ppm. Attempts to prepare 10+ by the addition
of DMAP to 9+ resulted in the regeneration of 8 and DMAP•AlCl3.
in π-electron delocalization via resonance. Further analysis
revealed that the sp3-hybridized boron atoms in complexes 8,
12+, and 132+ are displaced from the plane defined by the four
BCl2 formazanate 11[26c] served as a precursor to complexes
12+, 132+, and 14, the last of which has been reported
previously.[26c] Complex 11 was converted to the four-coordinate
boronium
cation
12+
by
treatment
with
lithium
dibenzoylmethanate in CH2Cl2 and isolated as a red solid in 94%
yield after recrystallization. Treatment of BCl2 formazanate 11
with two equivalents of AgOTf, followed by the addition of
bis(diphenylphosphino)methane dioxide (dppmo), afforded a
phosphine oxide-stabilized boron dication 132+ as an orange
powder in 88% yield. The 11B{1H} NMR spectra for 12+ and 132+
were centred at 1.9 and 0.0 ppm, respectively. The latter also
displayed sharp singlets in the 19F and 31P{1H} NMR spectra
(19F: ‒78.2 ppm; 31P{1H}: 54.9 ppm).
Single crystals of complexes 8, 9+, 12+, and 132+ were
analyzed by X-ray diffraction (Figure 1 and Tables S1 and S2).
The C-N and N-N bond lengths in complexes 8, 9+, 12+, and 132+
range from 1.334(7) to 1.356(7) Å and 1.307(2) to 1.325(6) Å,
respectively. These values fall in between the standard lengths
of the respective single and double bonds, suggesting that the π
electrons of the formazanate backbone are delocalized.[27]
Borenium cation 9+ in particular features a markedly shorter
average B-N bond length of 1.451(4) Å compared to complexes
8, 12+, and 132+ [1.545(5) Å]. This length reduction suggests a
bond order greater than 1 and can be linked to the fact that 9+
possesses an sp2-hybrized boron atom capable of participating
Figure 1. Experimental solid-state structures of four crystallized boron
formazanate complexes. Anisotropic displacement ellipsoids are shown at the
50% probability level. For clarity, hydrogen atoms are omitted and phenyl
groups are shown as wireframes.
2
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10.1002/anie.202015036
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COMMUNICATION
nitrogen atoms of the formazanate backbones (N1, N2, N3, N4)
by a minimum of 0.380(3) Å, whereas the boron atom in
complex 9+ lies within 0.041(4) Å of that plane. The planes
defined by the N-aryl substituents and the four nitrogen atoms of
the formazanate ligand are offset by at least 48.59(7) in 8, 9+,
12+, and 132+. Thus, the boron formazanate (CN4B) ring is
almost planar in 9+ and has a boat-like conformation in the other
three complexes (Figure 1).
these shifts indicate that the coordination number of the cationic
boron centres can exert a strong influence on the position of the
lowest-energy absorption maximum. Complexes 14, 12+, and
132+ feature λmax values of 549 nm (ε = 18000 M‒1 cm‒1), 540 nm
(ε = 5900 M‒1 cm‒1), and 505 nm (ε = 9400 M‒1 cm‒1),
respectively. This suggests that the lowest excitation energies of
boron formazanate complexes with a fixed coordination number
increase with increasing positive charge at boron.
UV-vis absorption spectroscopy data for 8‒10+ (in toluene)
and 12+‒14 (in CH2Cl2) are presented in Figure 2a and Table 1.
The choice of solvent was limited by the stability and solubility of
each compound. Complexes 8, 9+, and 10+ exhibit broad
absorption bands with molar extinction coefficients (ε) ranging
from 6300 to 12100 M‒1 cm‒1. Complex 8, which features a four-
coordinate neutral boron atom, has a maximum absorption
wavelength (λmax) of 521 nm (ε = 12100 M‒1 cm‒1), consistent
To gain a better understanding of these observations, we
investigated complexes 8−10+ and 12+−14 using approximate
density-functional theory (DFT). The computational methodology
is documented in the Supporting Information; the results are
summarized in Table 1 and Figure 2b. The calculations show
that, in all cases, the dominant orbital pair associated with the
lowest-energy electronic absorption band involves the highest
occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO). Moreover, the the frontier orbital
energies decrease with increasing cationic charge within each
series 8−10+ and 12+−14. All of the lowest-energy excitations in
8−10+ and 12+−14 are of π→π* type (Figure 2b). Among the six
complexes 8−10+ and 12+–14, only one (9+) has a flat six-
membered CN4B ring, while in the other five the same ring is in a
boat-like conformation. The difference is clearly seen both in the
experimental (Figure 1) and calculated geometries of these
complexes.
with other neutral four-coordinate boron adducts of
23b]
formazanates.[23a,
The low-energy absorption band of 10+
(λmax = 510 nm, ε = 8300 M‒1 cm‒1), a molecule with a four-
coordinate cationic boron atom, is moderately blue-shifted and
has a lower intensity relative to the neutral BPhCl adduct 8. By
contrast, the low-energy absorption band of three-coordinate
boron cation 9+ is strongly red-shifted (λmax of 597 nm, ε = 6300
M‒1 cm‒1) relative to complexes 8 and 10+. The magnitudes of
Figure 2. (a) UV-vis absorption spectra of 10‒6 M dry degassed solutions of complexes 8‒10+ and 12+−14. (b) Frontier molecular orbitals and their energies
(in eV) for solvated complexes 8‒10+ and 12+‒14 calculated using the PBE1PBE/DGDZVP2 SCRF=CPCM method.
3
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10.1002/anie.202015036
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COMMUNICATION
Table 1. Experimental and simulated spectroscopic properties of complexes 8‒10+ and 12+‒14 in solution. The theoretical values were calculated by time-
dependent DFT using the PBE1PBE/DGDZVP2 SCRF=CPCM method.
Experiment
ε (M‒1 cm‒1
Theory
Intensity
Complex
Supporting ligands at boron
Solvent
λmax (nm)
521
)
λmax (nm)
507
Transition
8
Ph, Cl
toluene
toluene
toluene
CH2Cl2
CH2Cl2
CH2Cl2
12100
6300
8300
5900
9400
18000
0.417
0.259
0.401
0.134
0.308
0.500
HOMO→LUMO
HOMO→LUMO
HOMO→LUMO
HOMO→LUMO
HOMO→LUMO
HOMO→LUMO
9+
Ph
597
637
10+
12+
132+
14
Ph, DMAP
dibenzoylmethanate
dppmo
510
502
540
569
505
514
(OMe)2
549
546
The four-coordinate complexes 8 and 10+ have nearly identical
geometries of the CN4B ring, almost superimposable HOMOs
and LUMOs (Figure 2b), and approximately equal max values.
The modest 11 nm blue shift exhibited by 10+ relative to 8 (in
toluene) may be attributed to the positive charge of the boron
atom. The three-coordinate complex 9+ has the same +1 charge
at boron as 10+ but a much greater λmax value (a red shift by 76
nm) and a lower intensity of the first transition. The most
plausible reason for this dramatic red shift is the flat geometry of
the CN4B ring in 9+, which enables the HOMO to extend into the
phenyl substituent at boron and reduces the HOMO–LUMO gap.
The three four-coordinate complexes 12+–14 have similar
nonplanar CN4B rings and look-alike HOMOs. As a result, the
maximum absorption wavelengths in this set vary over a
narrower range (44 nm in CH2Cl2) and are strictly correlated with
the charges at boron: λmax(14) > λmax(12+) > λmax(132+).
Acknowledgements
This work was supported by the Natural Sciences and
Engineering Research Council (NSERC) of Canada (R.R.M.:
CGS-D Scholarship; B.D.K.: CGS-M Scholarship; V.N.S.: DG,
RGPIN-2020-06420; J.B.G.: DG, RGPIN-2018-04240), the
Ontario Ministry of Research and Innovation (J.B.G.: ERA, ER-
14-10-147), and the Canadian Foundation for Innovation (J.B.G.:
JELF, 33977).
Keywords: Borenium, boronium, formazanate ligands, BN
heterocycles, optoelectronic materials.
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10.1002/anie.202015036
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Optoelectronic properties of boron formazanate dyes are usually tuned by varying the substituents of the formazan moiety. We show
that the wavelength and type of the lowest-energy electronic excitation of these compounds can also be modulated through variation
of the magnitude of charge, coordination number, and supporting ligand at cationic boron.
Institute and/or researcher Twitter usernames: @westernuchem, @GilroyGroup
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