Synthesis, Structure, and Luminescence Properties of Boron Complex with 4-Bromo-2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenoxide Ligand

Article abstract of DOI:10.1134/S1070363219110148

A boron complex with 4-bromo-2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenoxide ligand has been synthesized, and its photoluminescence properties in solution and in the solid phase have been examined. The structure of the complex has been determined by X-ray analysis. Non-classical intermolecular interaction (halogen bond) between the bromine and fluorine atoms has been detected in the crystal structure of the complex.

Full text of DOI:10.1134/S1070363219110148

ISSN 1070-3632, Russian Journal of General Chemistry, 2019, Vol. 89, No. 11, pp. 2246–2250. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Zhurnal Obshchei Khimii, 2019, Vol. 89, No. 11, pp. 1732–1737.
Synthesis, Structure, and Luminescence Properties
of Boron Complex with 4-Bromo-
2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenoxide Ligand
A. V. Rozhkova,* and A. S. Novikova
a St. Petersburg State University, St. Petersburg, 199034 Russia
*e-mail: iomcrozhkov@gmail.com
Received May 13, 2019; revised May 13, 2019; accepted May 16, 2019
Abstract—A boron complex with 4-bromo-2-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenoxide ligand has
been synthesized, and its photoluminescence properties in solution and in the solid phase have been examined.
The structure of the complex has been determined by X-ray analysis. Non-classical intermolecular interaction
(halogen bond) between the bromine and fluorine atoms has been detected in the crystal structure of the complex.
Keywords: boron complex, photoluminescence, halogen bond
DOI: 10.1134/S1070363219110148
Boron complexes with N,N-, N,O-, and O,O-chelating
ligands exhibit efficient photo- and electroluminescence
[1–3]. The emission range of organoboron complexes
covers the entire visible region, as well as the near-IR
region [4]. Unlike the most efficient modern emitters
based on platinum [5] and iridium compounds [6], boron
complexes are advantageous due to their much lower
cost, less laborious synthetic procedures, and the lack of
toxicity. Therefore, studies in the field of synthesis and
luminescence properties of boron compounds are continu-
ously developing. The color and efficiency of emission
of organoboron luminophores are determined mainly
by ligand environment of the boron atom. In addition,
luminescence properties of solid complexes are largely
influenced by intermolecular interactions in the crystal
structure. It was shown previously that boron complexes
with ligands containing an imidazolylphenoxide fragment
exhibit strong photo- and electroluminescence [7, 8].
hydroxide to obtain the corresponding potassium salt
which reacted with boron trifluoride–diethyl ether com-
plex at room temperature to afford complex 2 (Scheme 1).
The product was isolated in a good yield (72%) as a
yellow–green finely crystalline solid which is stable in
air and soluble in DMSO, DMF, and acetonitrile. By
recrystallization of 2 from acetonitrile we succeeded in
obtaining its single crystal suitable for X-ray analysis.
Complex 2 crystallizes as 1 : 1 solvate with aceto-
nitrile, where the components are linked to each other
through N1S···H2 hydrogen bond (Fig. 1). The boron coor-
dination sphere is a tetrahedron with the bond angles at the
boron atom ranging from 108.263(3)° to 109.2936(16)°.
The B1–N3 distance [1.55555(5) Å] corresponds to the
sum of the covalent radii of boron and nitrogen (1.55 Å
[10]); the B1–O1 bond [1.45025(5) Å] is slightly shorter
than the sum of the covalent radii of B and O (1.50 Å
[10]); and the B1–F1 [1.40407(3) Å] and B1–F2 bond
lengths [1.39656(4) Å] approach the sum of the covalent
radii of boron and fluorine (1.41 Å [10]). The dihedral
angle between the phenoxide and imidazole ring planes is
8.0°. Molecules 2 in crystal are packed in stacks through
π–π interactions between the imidazophenanthroline frag-
ments with a distance of 3.452 Å between the centroids
(Fig. 1). In addition, weak non-covalent intermolecular
interactions F2···Br1 like type I halogen bond [11] were
detected (Fig. 2). The F2···Br1 distance [2.9665(15) Å] is
Herein, we report the synthesis, structure, and pho-
toluminescence properties of a new boron complex with
a 4-bromo-2-(1H-imidazo[4,5-f][1,10]phenanthrolin-
2-yl)phenoxide ligand. The ligand, 4-bromo-2-(1H-
imidazo[4,5-f][1,10]phenanthrolin-2-yl)phenol (1) was
synthesized according to the known procedure [9] by
condensation of 1,10-phenanthroline-5,6-dione with
5-bromosalicylaldehyde in acetic acid in the presence of
ammonium acetate. Phenol 1 was treated with potassium
2246

SYNTHESIS, STRUCTURE, AND LUMINESCENCE PROPERTIES
2247
Scheme 1.
O
Br
H
N
N
O
O
OH
N
N
NH4OAc
N
+
AcOH, 120qC
N
Br
HO
1
Br
Br
H
H
N
N
N
N
N
(1) KOH, DMSO
(2) BF3·Et2O,
N
N
F2B
N
dimethoxyethane
HO
O
2
shorter than the sum of van der Waals radii of fluorine and
bromine (3.32 Å [12]); the C16Br1F2 angle is 156.54(10)°.
The existence of a bond critical point (3, –1) for the
corresponding atoms was confirmed by DFT quantum
chemical calculations and topological analysis of electron
density distribution in the framework of Bader’s quantum
theory of atoms in molecule (QTAIM). The results are
summarized in the table.
Laplacian, and near-zero positive total energy density at
the BCP, which are typical of non-covalent interactions.
The energy of the Br···F contact was estimated at 2.2 to
3.6 kcal/mol using different known correlations (see the
table). It should be noted that the calculations with both
functionals used (M06 and ωB97XD) predicted similar
electron densities at the BCPand Br···F interaction ener-
gies. The ratio –G(r)/V(r) = 1.43 >> 1 at the BCPindicates
negligible contribution of the covalent component to the
Br···F interaction [16].
In fact, QTAIM analysis revealed a bond critical
point (3, –1) for the intermolecular Br···F contact (type
I halogen–halogen bond). This interaction is character-
ized by a low electron density, positive electron density
The electronic absorption spectrum of complex 2
in acetonitrile contained bands assignable to π–π* and
Fig. 1. A fragment of crystal packing of complex 2; non-
hydrogen atoms are shown as thermal vibration ellipsoids
with a probability of 30%.
Fig. 2. Halogen bond C16–Br1···F2 in the crystal structure
of complex 2.
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2248
ROZHKOV, NOVIKOV
Calculated electron densities [ρ(r)], electron density Laplacians [2ρ(r)], total energy densities (Hb), potential energy densities
[V(r)], and kinetic energy Lagrangians [G(r)] at the bond critical point (3, –1) corresponding to Br···F type I halogen–halogen
intermolecular contact in the crystal structure of complex 2 and energies of the Br···F interaction
Eint, kcal/mol
ρ(r), 2ρ(r),
a.u. a.u.
Hb,
a.u.
V(r),
a.u.
G(r),
a.u.
Method/basis set
a
b
c
d
M06/DZP-DKH
0.010 0.048 0.002 –0.007 0.010
0.010 0.048 0.002 –0.007 0.010
2.2
2.2
2.7
2.7
2.5
2.5
3.6
3.6
ωB97XD/DZP-DKH
Eint = –V(r)/2 [13].
a
b
c
d
Eint = 0.429G(r) [14].
Eint = 0.58[–V(r)] [15]; this correlation was proposed especially for non-covalent interactions involving bromine atoms.
Eint = 0.57G(r) [15]; this correlation was proposed especially for non-covalent interactions involving bromine atoms.
n–π* transitions in the aromatic system of the ligand
(Fig. 3). The photoluminescence spectrum of 2 dis-
played two strong peaks with their maxima at λ 407 and
424 nm which can be assigned to transitions in an isolated
molecule of the complex, as well as a broad low-intense
band at λmax 570 nm which may arise from emission of
excimers. The solid state photoluminescence spectrum
of 2 is shifted red relative to the spectrum in acetonitrile
solution and is represented by a broad structureless band
with λmax 525 nm. The red shift is related primarily to
strong intermolecular interactions in the solid state (π–π
stacking and halogen bonding).
analysis of electron density distribution. The complex
shows photoluminescence in solution and in the solid
phase.
EXPERIMENTAL
The study was performed using the facilities of the
Magnetic Resonance Research Center, ChemicalAnalysis
and Materials Research Center, and Center for X-Ray
Diffraction Studies at the St. Petersburg State University.
4-Bromo-2-(1H-imidazo[4,5-f][1,10]phenanthrolin-
2-yl)phenol (1) was synthesized as described in [9].
Boron trifluoride–diethyl ether complex (Aldrich) was
used without further purification.
Thus, we have synthesized a new boron complex
containing a 4-bromo-2-(1H-imidazo[4,5-f][1,10]-
phenanthrolin-2-yl)phenoxide ligand. X-Ray analysis of
the new complex has revealed π–π stacking interactions
and Br···F halogen bonding in the crystal structure. The
formation of Br···F halogen bond has been confirmed
by quantum chemical calculations and Bader’s QTAIM
The 1H, 13C,11B, and 19F NMR spectra were recorded
in DMSO-d6 at room temperature on a Bruker ACF-400
spectrometer at 400, 125, 160.43, and 470.50 MHz, re-
spectively, using the residual proton and carbon signals
of the solvent as reference for 1H and 13C; the 11B and 19F
chemical shifts were measured relative to BF3·Et2O and
CFCl3, respectively, as external standards. Signals were
assigned with the use of two-dimensional GE-COSYand
GE-HSQC techniques. The IR spectrum was recorded
in KBr on a Shimadzu IR Prestige-21 spectrometer. The
mass spectrum was obtained on a Bruker microTOF
instrument (electrospray ionization from solution in
methanol; the most abundant isotope peak is given). The
electronic absorption spectra were recorded on a Shi-
madzu UV-1800 spectrophotometer. The luminescence
spectra were measured on a Horiba/JobinYvon Fluorolog
3 spectrofluorometer; λexcit = 360 nm.
Asingle crystal of 2 was obtained by slow evaporation
of a solution of 2 in acetonitrile at room temperature. The
X-ray diffraction data were obtained at 100 K on anAgile-
nt Technologies Supernova diffractometer equipped with
an Atlas CCD detector (CuKα radiation, λ = 1.54184 Å).
Ȝ, nm
Fig. 3. (1) Electronic absorption spectrum of complex 2 in
acetonitrile and its emission spectra in (2) acetonitrile and (3)
solid phase at room temperature; λexcit 360 nm.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 11 2019

SYNTHESIS, STRUCTURE, AND LUMINESCENCE PROPERTIES
2249
The structure was solved by the direct method and was
FUNDING
refined using SHELXL package [17] implemented in
OLEX2 software [18]. A correction for absorption was
applied using CrysAlisPro [19]. The positions of hydro-
gen atoms were calculated by SHELX algorithms. The
complete set of X-ray diffraction data for compound 2
was deposited to the Cambridge Crystallographic Data
Centre (CCDC entry no. 1914231).
This study was performed under financial support by the
Russian Science Foundation (project no. 17-73-10 078).
CONFLICT OF INTEREST
No conflict of interest was declared by the authors.
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