Synthesis, NMR, X-ray and UV/Vis characterization and preliminary luminescence study of some boron β-iminoenolates having 6-aminocoumarin moiety

Article abstract of DOI:10.1016/j.jorganchem.2015.11.016

Five coumarin-based polarized ethylenes (enaminoketones, enaminoester and enaminoamide) were prepared using the condensation of the parent β-dicarbonyl compounds with 6-aminocoumarin. Reaction of the polarized ethylenes with an appropriate source of trivalent boron gave corresponding boron β-iminoenolates having either BF₂ or BPh₂ fragment. The prepared iminoenolates were characterized by means of multinuclear magnetic resonance in solution, single-crystal X-ray diffraction and UV/Vis spectroscopy. A preliminary luminescence study of the iminoenolates as well as their parent enamines was done. The compounds exhibited fluorescence in a solid state as well as in a frozen 2-methyltetrahydrofuran at 77 K. Exploratory tests showed promising AIE/AIEE properties of the tested compounds. On the other hand, no fluorescence in a solution state (with one exception) was observed.

Full text of DOI:10.1016/j.jorganchem.2015.11.016

Accepted Manuscript
Synthesis, NMR, X-ray and UV/Vis characterization and preliminary luminescence
study of some boron β-iminoenolates having 6-aminocoumarin moiety
Hana Doušová, Petr Šimůnek, Numan Almonasy, Zdeňka Růžičková
PII:
S0022-328X(15)30211-4
10.1016/j.jorganchem.2015.11.016
JOM 19303
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 15 May 2015
Revised Date: 2 November 2015
Accepted Date: 21 November 2015
Please cite this article as: H. Doušová, P. Šimůnek, N. Almonasy, Z. Růžičková, Synthesis, NMR, X-
ray and UV/Vis characterization and preliminary luminescence study of some boron β-iminoenolates
having 6-aminocoumarin moiety, Journal of Organometallic Chemistry (2015), doi: 10.1016/
j.jorganchem.2015.11.016.
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ACCEPTED MANUSCRIPT
Boron β-iminoenolates having coumarin-6-yl moiety on the nitrogen atom and BF2 or BPh2 fragment
fluoresce both in solid state and in frozen 2-methyltetrahydrofuran at 77 K. No fluorescence was
observed in solution. The fluorescence starts to appear in aggregation state after admixing their THF
solutions with water.

ACCEPTED MANUSCRIPT
Synthesis, NMR, X-ray and UV/Vis characterization and
preliminary luminescence study of some boron β-iminoenolates
having 6-aminocoumarin moiety
Hana Doušováa , Petr Šimůneka, , Numan Almonasya and Zdeňka Růžičkováb
a University of Pardubice, Faculty of Chemical Technology, Institute of Organic Chemistry and
Technology, Studentská 573, CZ 532 10, Pardubice, Czech Republic
bUniversity of Pardubice, Faculty of Chemical Technology, Department of General and Inorganic
Chemistry, Studentská 573, CZ 532 10, Pardubice, Czech Republic
Abstract: Six coumarin-based polarized ethylenes (enaminoketones, enaminoester and
enaminoamide) were prepared using the condensation of the parent β-dicarbonyl compounds with 6-
aminocoumarin. Reaction of the polarized ethylenes with an appropriate source of trivalent boron gave
corresponding boron β-iminoenolates having either BF2 or BPh2 fragment. The prepared iminoenolates
were characterized by means of multinuclear magnetic resonance in solution, single-crystal X-ray
diffraction and UV/Vis spectroscopy. A preliminary luminescence study of the iminoenolates as well
as their parent enamines was done. The compounds exhibited fluorescence in a solid state as well as in
a frozen 2-methyltetrahydrofuran at 77 K. Exploratory tests showed promising AIE/AIEE properties
of the tested compounds. On the other hand, no fluorescence in a solution state (with one exception)
was observed.
Keywords: Enaminone, boron, fluorescence, iminoenolate, NMR, X-ray.
1. Introduction
Luminescent organic molecules have received a great attention due to their applicability as materials
for organic light-emitting diodes (OLEDs) [1, 2], organic field-effect transistors (OFETs) [3],
fluorescent probes [4, 5] and many others. During the years a number of noteworthy luminophores
Corresponding author. Tel.: +420 466 037 039; fax: +420 466 037 068; e-mail: petr.simunek@upce.cz
1

have been introduced: e.g. coumarin and its derivatives (for recent review see e.g. ref. [6]),
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polyaromatics [7], porphyrins [8–11], xanthenes [12], boranils [13,14] etc. One way how to affect the
luminescent properties of an organic molecule can be an incorporation of a BR2 fragment into a
suitable bidentate ligand [1, 15]. Recently some interesting fluorescent boron difluoride complexes of
coumarins have been synthetized and characterized [16, 17]. BODIPY are another example of well-
known BF2-containing molecules having excellent fluorescence properties [4, 18–26]. Despite of their
advantages, BODIPY also suffer from some drawbacks which limit them in use, especially weak
luminescence in a solid state [15]. Searching for novel luminophores is then an important goal for
chemists. Difluoroboron complexes of β-enaminones I (Fig. 1) which belong among the family of β-
iminoenolate boron complexes [27] have recently attracted attention as a promising class of
fluorophores and number of them have been synthetized and characterized [28–38]. Xia et al. [32]
described excellent solution-state fluorescence for some heterocyclic β-iminoenolates. Shankarling et
al. [38] described synthesis, spectral and electrochemical characterization of some boron difluoride
complexes of benzoindoline-based β-enaminones. These compounds exhibit fluorescence both in a
solution and in a solid state, which is not too usual. Generally compounds I possess a weak
fluorescence in solution, but strong in solid state [29, 33] which is typical for substances having AIE
or AIEE (aggregation-induced emission/aggregation-induced emission enhancement) properties [33].
Substances with these properties can have a range of potential applications e.g. as OLED, sensors or
agents for photodynamic therapy [39]. To the best of our knowledge, the fluorescence properties of
diarylboron chelates of β-enaminones I are less examined, in comparison with their difluoro
analogues. Balaban et al. [40] described synthesis and spectral characterization of some dialkyl and
diphenyl boron chelates of enaminoketones. Ramos-Ortíz [41] prepared and characterized some
enaminone-based boronates with NLO properties. Catechol-based enaminone complexes of boron
have been prepared and characterized by Atwood et al. [42] Kunick et al. [31] described synthesis and
structure of some boron complexes of 4-anilinomethylene-1-benzazepine-2,5-diones. During the past
several years we have published synthesis, NMR and X-ray characterization, basic fluorescence and
DSC study and some interesting chemical behaviour for number of 2,2-diphenyl-1,3,2-oxazaborines
derived from enaminones and related compounds [28, 43–45].
Inspired by the above-mentioned, we decided to combine the enaminone-based oxazaborines (both
BF2 and BPh2) with another important fluorophore: coumarin. The resulting compounds could be
potential fluorophores having AIE or AIEE properties. In the present work we describe the synthesis,
NMR and X-ray characterization and preliminary fluorescence study of some novel coumarin-based
oxazaborines with general formula II (Fig. 1).
2

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Fig. 1. General formula of boron iminoenolates (left) and subjects studied here (right).
2. Materials and methods
2.1. General
NMR Spectra were measured using NMR spectrometers Bruker AVANCE III operating at 400.13
MHz (1H), 376.50 MHz (19F), 127.38 MHz (11B) and 100.12 MHz (13C) and Bruker Ascend™
operating at 500.13 MHz (1H), 470.66 MHz (19F), 160.48 (11B) and 125.12 MHz (13C). Proton spectra
in CDCl3 were calibrated on an internal TMS (δ = 0.00 ppm) and in DMSO-d6 on the middle signal of
the solvent multiplet (δ = 2.50 ppm). Carbon spectra were measured with broadband proton
decoupling in an ordinary way or using APT pulse sequence. Calibration of the carbon spectra was
done on the middle of the solvent multiplet (δ = 77.23 ppm in CDCl3 and 39.51 in DMSO-d6).
Fluorine-19 NMR spectra were measured without proton decoupling using α,α,α-trifluorotoluene as
the secondary external standard (δ = -63.9 ppm against CFCl3 as the primary standard) [46]. Boron-11
NMR were measured in 5 mm quartz NMR tubes (Norell) using B(OMe)3 as an external standard (δ =
18.1 ppm) [47]. All the pulse sequences were taken from the Bruker pulse sequence library. Elemental
analyses were performed on a Flash EA 2000 CHNS automatic analyser (Thermo Fisher Scientific).
HRMS were measured on a MALDI LTQ Orbitrap XL (Thermo Fisher Scientific) with 2,5-
dihydroxybenzoic acid (DHB) as the matrix. Melting points were measured on a Kofler hot-stage
microscope Boetius PHMK 80/2644 and were not corrected.
The absorption spectra were measured on a UV/Vis Perkin-Elmer Lambda 35 spectrophotometer at
room temperature. The emission spectra were measured on a Perkin-Elmer LS55 Spectrofluorimeter
equipped with a commercial low temperature accessory and a special commercial cuvette. The
fluorescence spectra in solid phase were recorded from the surface of the pressed powder in the special
cuvette. The spectra were corrected for the characteristics of the emission monochromator and for the
photomultiplier response and by excitation at the wavelengths of the absorption maxima.
The X-ray data for colorless crystals of 6b and light yellow crystals of 7a were obtained at 150 K
using an Oxford Cryostream low-temperature device on a Nonius Kappa CCD diffractometer with Mo
Kα radiation (λ = 0.71073 Å), a graphite monochromator, and the φ and χ scan mode. Data reductions
were performed with DENZO-SMN [48]. The absorption was corrected by integration methods [49].
Structures were solved by direct methods (Sir92) [50] and refined by full matrix least-square based on
3

F2 (SHELXL97) [51]. Hydrogen atoms were mostly localized on a Fourier difference electron density
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map, however, to ensure uniformity of treatment of crystal, all hydrogen atoms were recalculated into
idealized positions (riding model) and assigned temperature factors Hiso(H) = 1.2 Ueq (pivot atom) or
of 1.5Ueq (methyl). Hydrogen atoms in methyl moieties and in aromatic rings were placed with C–H
distances of 0.96 and 0.93 Å resp.
Rint = ∑ Fo - Fo,mean /∑Fo , GOF = [∑(w(Fo - Fc2)2)/(Ndiffrs - Nparams)] for all data, R(F) = ∑ Fo
-
2
2
2
2
½
2
2 2
½
Fc /∑ Fo for observed data, wR(F2) = [∑(w(Fo - Fc2)2)/(∑w(Fo ) )] for all data.
2.2. Material
All the solvents and reagents were used commercial without further treatments. The procedures for
synthesis of 6-nitrocoumarin (2) and 6-aminocoumarin (3) and their characterization data are in
Supporting Information. β-Dicarbonyl compounds 4 were used commercial without further treatments.
Diphenylborinic acid was prepared according to the literature procedure (see Supporting Information).
2.2.1. General procedure for the synthesis of 5a–c
The mixture of 6-aminocoumarin (3) (0.6 g, 3.7 mmol), β-diketone 4 (3.7 mmol) and p-
toluenesulfonic acid (6 mg, 0.03 mmol) in toluene (20 mL) was refluxed for 10–48 h. Water formed
during the reaction was removed in the form of azeotrope until only clear toluene distilled. The
volatile components were then evaporated under reduced pressure. Products 5a–c (white or yellow
solids) were purified either by recrystallization or firstly isolated by column chromatography and then
recrystallized. The following compounds were prepared in this manner:
6-{[(2Z)-4-Oxopent-2-en-2-yl]amino}-2H-chromen-2-one (5a). From pentane-1,3-dione (4a). Reflux
for 10 h; recrystallization from methanol. Yield: 0.36 g (40 %) of pale yellow solid. M.p.: 132–133 °C.
1H NMR (CDCl3/400 MHz): δH 2.01 (3H, s), 2.12 (3H, s), 5.25 (1H, s), 6.47 (1H, d, J = 9.5 Hz), 7.25–
7.33 (3H, m), 7.72 (1H, d, J = 9.5 Hz), 12.49 (1H, br s) ppm; 13C NMR (CDCl3/101 MHz): δC 19.8,
29.3, 98.3, 117.5, 117.6, 119.2, 123.3, 128.6, 135.3, 142.9, 151.6, 159.8, 160.4, 196.8. Anal. Calc. for
C19H15NO3: C, 69.12; H, 5.39; N, 5.76. Found: C, 68.91; H, 5.34; N, 5.66.
6-{[(2Z)-4-Oxo-4-phenylbut-2-en-2-yl]amino}-2H-chromen-2-one (5b). From 1-phenylbutane-1,3-
dione (4b). Reflux for 20 h; can be recrystallized from ethanol. Yield: 5.23 g (75 %) of light yellow
solid. M.p.: 157–158.5 °C. 1H NMR (CDCl3/400 MHz): δH 2.15 (3H, s), 5.95 (1H, s), 6.48 (1H, d,
J = 9.6 Hz), 7.30 (1H, m), 7.34–7.36 (2H, m), 7.43–7.51 (3H, m), 7.68 (1H, d, J = 9.6 Hz), 7.91–7.93
(2H, m), 13.13 (1H, br s) ppm; 13C NMR (CDCl3 /100 MHz,): δC 20.6, 95.0, 117.9, 118.0, 119.5,
123.6, 127.3, 128.6, 128.8, 131.4, 135.4, 139.8, 142.9, 152.0, 160.5, 161.9, 189.4 ppm. Anal. Calc. for
C19H15NO3: C, 74.74; H, 4.95; N, 4.59. Found: C, 74.56; H, 4.97; N, 4.80.
4

6-{[(3Z)-5-Oxohept-3-en-3-yl]amino}-2H-chromen-2-one (5c). From heptane-3,5-dione (4c). Reflux
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for 48 h; flash chromatography (silica gel, CHCl3/EtOAc, 3:2); recrystallization from methanol.
Yield: 0.88 g (52 %) of light yellow solid. M.p.: 73.5–74.5 °C. 1H NMR (CDCl3/400 MHz): δH 1.08
(3H, t, J = 7.5 Hz), 1.15 (3H, t, J = 7.5 Hz), 2.32 (2H, q, J = 7.5 Hz), 2.41 (2H, q, J = 7.5 Hz), 5.28
(1H, s), 6.47 (1H, d, J = 9.5 Hz), 7.28 – 7.34 (3H, m), 7.68 (1H, d, J = 9.5 Hz), 12.50 (1H, br s) ppm;
13C NMR (CDCl3/100 MHz,): δC 9.9, 12.5, 25.2, 35.5, 95.2, 117.6, 117.8, 119.3, 123.9, 129.2, 135.4,
143.0, 151.8, 160.5, 165.5, 201.0 ppm. Anal. Calc. for C16H17NO3: C, 70.83; H, 6.32; N, 5.16. Found:
C, 70.61; H, 6.11; N, 5.09.
2.2.2. Synthesis of enaminoamide 5d and enaminoester 5e
(Method A) A 50 mL flask was charged with 6-aminocoumarin (3) (1.1 g, 6.83 mmol), ethyl
acetoacetate (4d) (10.5 g, 80.7 mmol) and catalytic amount of acetic acid (0.03 mL). The reaction
mixture was heated gently in order to dissolve 6-aminocoumarin (3). The clear solution was then
stirred for 3.5 days at room temperature. Precipitated product 5e was filtered off and washed with
ethanol.
(Method B) A 250 mL flask fitted with a condenser was charged with 6-aminocoumarin (3) (3.68 g,
0.02 mol), ethyl acetoacetate (4d) (3 g, 0.02 mol), acetic acid (0.08 mL) and toluene (150 mL). The
reaction mixture was heated to boiling for 14 h. The water formed during the reaction was distilled off
as water-toluene azeotrope. After the reaction was finished, enaminoamide 5d precipitated upon
cooling and was removed by suction. Enaminoester 5e was isolated from the filtrate by evaporation
and recrystallization of the residue from ethanol.
(2Z)-N-(2-Oxo-2H-chromen-6-yl)-3-[(2-oxo-2H-chromen-6-yl)amino]but-2-enamide (5d). (Method
B) The precipitate was washed with DCM and hot ethanol. Yield: 2.78 g (36 %) of yellow-brown
solid; m.p.: 238–239 °C. 1H NMR (DMSO-d6/400 MHz): δH 2.07 (3H, s), 4.90 (1H, s), 6.46 (1H, d,
J = 9.5 Hz), 6.53 (1H, d, J = 9.5 Hz), 7.34 (1H, d, J = 9.0 Hz), 7.36–7.38 (1H, m), 7.41–7.44 (1H, m),
7.55 (1H, d, J = 2.8 Hz), 7.64 (1H, dd, J = 9.0 Hz, 2.5 Hz), 8.02–8.06 (2H, m), 8.10 (1H, d,
J = 2.5 Hz), 9.79 (1H, br s), 11.23 (1H, br s) ppm; 13C NMR (DMSO-d6/100 MHz,): δC 20.1, 90.1,
116.4, 116.5, 116.8, 117.0, 117.05, 118.7, 119.2, 121.8, 123.0, 127.2, 135.9, 136.4, 143.9, 144.5,
148.7, 150.1, 155.8, 159.9, 160.1, 168.3 ppm. Anal. Calc. for C22H16N2O5: C, 68.04; H, 4.15; N, 7.21.
Found: C, 67.92; H, 4.23; N, 7.38.
Ethyl (2Z)-3-[(2-oxo-2H-chromen-6-yl)amino]but-2-enoate (5e). Pale yellow solid. (Method A)
Yield: 0.72 g (39 %); m.p.: 117–118 °C. (Method B) Yield: 0.86 g (16 %); mp 112.5–113.5 °C.
Spectral data from both the methods were consistent. 1H NMR (CDCl3/400 MHz): δH 1.30 (3H, t,
J = 7.0 Hz), 1.99 (3H, s), 4.16 (2H, q, J = 7.0 Hz), 4.75 (1H, s), 6.46 (1H, d, J = 9.5 Hz), 7.20 (1H, d,
J = 2.3 Hz), 7.25 – 7.32 (2H, m), 7.66 (1H, d, J = 9.5 Hz), 10.40 (1H, br s) ppm; 13C NMR
(CDCl3/100 MHz,): δC 14.7, 20.4, 59.2, 87.3, 117.7, 117.8, 119.4, 123.2, 128.7, 136.1, 143.0, 151.5,
5

158.5, 160.7, 170.6 ppm. Anal. Calc. for C H NO : C, 65.92; H, 5.53; N, 5.13. Found: C, 65.68; H,
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15 15
4
5.46; N, 4.91.
2.2.3. General procedure for the synthesis of 2,2-difluoro-1,3,2λ4-oxazaborines 6
The procedure was adopted from ref. [52] The enaminone 5 (1.5 mmol) was dissolved or suspended in
dry DCM (10 mL) and triethylamine (3 mL, 3 mmol) was added afterwards. The reaction mixture was
stirred at room temperature for 20 min. Consequently, BF3 · Et2O (0.56 mL, 4.5 mmol) was added, the
mixture was refluxed for 3 h and then stirred at laboratory temperature for another 12–96 h. The crude
product was obtained by evaporation of the solvent or by filtration of the precipitate. The following
compounds were prepared using this procedure:
2,2-Difluoro-4,6-dimethyl-3-(2-oxo-2H-chromen-6-yl)-1,3,2λ4-oxazaborine (6a). Prepared from 5a,
reaction time 16 h, recrystallization from ethanol. Yield: 0.28 g (65 %) of yellow solid, m.p.: 217–219
°C. 1H NMR (CDCl3/400 MHz): δH 1.99 (3H, s), 2.23 (3H, s), 5.60 (1H, s), 6.50 (1H, d, J = 9.5 Hz),
7.34–7.37 (1H, m), 7.40–7.42 (2H, m), 7.70 (1H, d, J = 9.5 Hz) ppm; 13C NMR (CDCl3/100 MHz,): δC
21.6, 23.3, 99.2, 118.0, 118.2, 119.6, 125.5, 130.0, 136.0, 142.9, 153.6, 160.3, 172.0, 178.5 ppm.
HRMS (MALDI, m/z) Calcd. for C14H13BF2NO3 [M+H]+ 292.09511; Found 292.09531; Calcd. for
C14H12BFNO3 [M–F]+ 272.08888; Found 272.08907; Calcd. for C14H12BF2NNaO3 [M+Na]+
314.07705; Found 314.07735; Calcd. for C14H12BF2KNO3 [M+K]+ 330.05099; Found 330.05134.
2,2-Difluoro-4-methyl-6-phenyl-3-(2-oxo-2H-chromen-6-yl)-1,3,2λ4-oxazaborine (6b). Prepared from
5b, reaction time 24 h, purification by extraction with boiling ethanol. Yield: 1.61 g (83 %) of light
yellow solid, m.p.: 214–216.5 °C. 1H NMR (CDCl3/400 MHz): δH 2.13 (3H, s), 6.27 (1H, s), 6.50 (1H,
d, J = 9.5 Hz), 7.41–7.42 (2H, m), 7.47–7.51 (3H, m), 7.55–7.60 (1H, m), 7.72 (1H, d, J = 9.8 Hz),
7.99–8.01 (2H, m) ppm; 13C NMR (CDCl3/100 MHz,): δC 22.2, 95.9, 118.0, 118.2, 119.6, 125.5,
127.8, 129.0, 129.9, 133.1, 136.2, 142.9, 153.6, 160.3, 172.0, 172.3 ppm. Anal. Calc. for
C19H14BF2NO3: C, 64.62; H, 4.00; N, 3.97 %. Found: C, 64.35; H, 3.94; N, 3.85.
Crystallographic data for 6b: C19H14BF2NO3, M = 353.12, triclinic, P -1, a = 6.6760(2), b = 6.9680(3),
c = 17.9561(8) Å, α = 88.422(3), β = 79.445(3), γ = 76.136(3) °, Z = 2, V = 797.12(6) Å3, Dc = 1.471
g.cm-3, µ = 0.114 mm-1, Tmin/Tmax = 0.980/0.991; -7 ≤ h ≤ 8, -9 ≤ k ≤ 8, -23 ≤ l ≤ 22; 10245 reflections
measured (θmax = 27.30 °), 3515 independent (Rint = 0.0415), 2685 with I > 2σ(I), 235 parameters, S =
1.112, R1(obs. data) = 0.0498, wR2(all data) = 0.1040; max., min. residual electron density = 0.311, -
0.276 eǺ-3.
4,6-Diethyl-2,2-difluoro-3-(2-oxo-2H-chromen-6-yl)-1,3,2λ4-oxazaborine (6c). Prepared from 5c,
reaction time 12 h, purification by flash chromatography (silica gel, DCM) and subsequent
recrystallization from ethanol. Yield: 0.51 g (85 %) of white solid, m.p.: 133–135 °C. 1H NMR
(CDCl3/400 MHz): δH 1.12 (3H, t, J = 7.6 Hz), 1.26 (3H, t, J = 7.6 Hz), 2.24 (2H, q, J = 7.6 Hz), 2.49
6

(2H, q, J = 7.6 Hz), 5.62 (1H, s), 6.49 (1H, d, J = 9.6 Hz), 7.34–7.42 (3H, m), 7.70 (1H, d, J = 9.6 Hz)
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ppm. 13C NMR (CDCl3/100 MHz,): δC 10.6, 12.1, 27.3, 30.1, 95.5, 118.0, 118.1, 119.5, 125.7, 130.2,
135.7, 142.9, 153.6, 160.3, 176.8, 183.1 ppm. HRMS (MALDI, m/z) Calcd. for C16H17BF2NO3
[M+H]+ 320.12641; Found 320.12669; Calcd. for C16H16BFNO3 [M–F]+ 300.12018; Found
300.12040; Calcd. for C16H16BF2NNaO3 [M+Na]+ 342.10835; Found 342.10865; Calcd. for
C16H16BF2KNO3 [M+K]+ 358.08229; Found 358.08261.
2,2-Difluoro-4-methyl-3-(2-oxo-2H-chromen-6-yl)-6-[(2-oxo-2H-chromen-6-yl)amino]-1,3,2 λ4-
oxazaborine (6d). Prepared from 5d, reaction time 96 h, precipitated product washed with ether. Yield:
0.95 g (85 %) of yellow solid, m.p.: 253.5–255 °C. 1H NMR (DMSO-d6/400 MHz): δH 1.89 (3H, s),
5.19 (1H, br s), 6.52–6.55 (2H, m), 7.40–7.44 (2H, m), 7.48 (1H, d, J = 9.0 Hz), 7.58–7.62 (2H, m),
7.68 (1H, d, J = 2.5 Hz), 8.08–8.12 (2H, m), 10.78 (1H, br s) ppm; 13C NMR (DMSO-d6/125 MHz,):
δC 21.1, 82.2 (br), 116.6, 116.8, 117.0, 117.4, 119.0, 119.2, 120.2 (br), 125.4 (br), 126.8, 131.2, 133.0,
136.8, 143.9, 144.0, 150.6 (br), 152.2, 159.9, 163.8 (br), 167.3 (br) ppm. HRMS (MALDI, m/z) Calcd.
for C22H15BFN2O5 [M–F]+ 417.10526; Found 417.10564.
2.2.4. General procedure for the synthesis of 2,2-diphenyl-1,3,2λ4-oxazaborines 7
Method A: A flask fitted with a calcium chloride drying tube was charged with enaminone 5a–c
(2 mmol) in anhydrous DCM (20 mL). Afterwards triphenylborane (0.61 g, 2.5 mmol) was added
gradually under stream of argon. The reaction mixture was stirred for 2–7 days at laboratory
temperature, then the solvent was removed in vacuo. The crude product was isolated by column
chromatography of the residue followed by further purification.
Method B: the procedure is the same as in the Method A, but diphenylborinic acid (0.91 g, 5 mmol)
was added instead of triphenylborane.
The following compounds were prepared using the above-mentioned methodologies:
4,6-Dimethyl-3-(2-oxo-2H-chromen-6-yl)-2,2-diphenyl-1,3,2 λ4-oxazaborine (7a). Prepared from 5a
using Method A, reaction time 7 days; column chromatography (silica gel, DCM); product
precipitated upon extracting the residue from the chromatography with boiling ethanol. Yield: 0.58 g
(69 %) of white solid, m.p.: 163.5–164.5 °C. 1H NMR (CDCl3/400 MHz): δH 1.98 (3H, s), 2.09 (3H,
s), 5.45 (1H, s), 6.34 (1H, d, J = 9.5 Hz), 6.85 (1H, d, J = 2.3 Hz), 6.93 (1H, dd, J = 8.8 Hz, 2.5 Hz),
7.05 (1H, d, J = 8.5 Hz), 7.15 (6H, br s), 7.25–7.31 (4H, m), 7.40 (1H, d, J = 9.5 Hz) ppm; 13C NMR
(CDCl3/100 MHz,): δC 22.3, 23.7, 100.3, 117.2, 117.4, 118.7, 125.9, 126.5, 127.0, 130.5, 133.6, 139.4,
142.9, 152.4, 160.5, 169.4, 179.7 ppm. Anal. Calc. for C26H22BNO3: C, 76.68; H, 5.44; N, 3.44.
Found: C, 76.52; H, 5.48; N, 3.72.
Crystallographic data for 7a: C26H22BNO3, M = 407.26, monoclinic, P 21/c, a = 12.1011(9), b =
11.6340(7), c = 15.8901(8) Å, β = 108.900(5) °, Z = 4, V = 2116.5(2) Å3, Dc = 1.278 g.cm-3, µ = 0.082
7

mm-1, Tmin/Tmax = 0.981/0.989; -15 ≤ h ≤ 15, -13 ≤ k ≤ 15, -19 ≤ l ≤ 20; 23028 reflections measured
ACCEPTED MANUSCRIPT
(θmax = 27.50 °), 4805 independent (Rint = 0.0553), 3315 with I > 2σ(I), 280 parameters, S = 1.129,
R1(obs. data) = 0.0620, wR2(all data) = 0.1237; max., min. residual electron density = 0.332, -0.345
eǺ-3.
4-Methyl-3-(2-oxo-2H-chromen-6-yl)-2,2,6-triphenyl-1,3,2λ4-oxazaborine (7b). Prepared from 5b
using Method B, reaction time 5 days; column chromatography (silica gel, DCM); DCM was added to
the residue from the chromatography and the solution was extracted with saturated aqueous solution of
NaHCO3. Organic layer was dried over Na2SO4 and evaporated; the product precipitated from the
residue upon extraction with boiling cyclohexane. Yield: 1.64 g (70 %) of yellow solid, m.p.: 118–119
°C. 1H NMR (CDCl3/400 MHz): δH 2.13 (3H, s), 6.16 (1H, s), 6.35 (1H, d, J = 9.6 Hz), 6.93 (1H, d, J
= 2.5 Hz), 7.01 (1H, dd, J = 8.8 Hz, 2.5 Hz), 7.08 (1H, d, J = 8.8 Hz), 7.13–7.17 (6H, m), 7.34–7.44
(7H, m), 7.47–7.51 (1H, m), 7.92–7.94 (2H, m) ppm; 13C NMR (CDCl3/100 MHz,): δC 27.1, 97.3,
117.3, 117.5, 118.7, 125.8, 126.5, 127.0, 127.8, 128.8, 130.5, 132.3, 133.7, 134.3, 139.6, 142.9, 152.4,
160.5, 170.0, 172.9 ppm. Anal. Calc. for C31H24BNO3: C, 79.33; H, 5.15; N, 2.98. Found: C, 79.61; H,
5.42; N, 2.72.
4,6-Diethyl-3-(2-oxo-2H-chromen-6-yl)-2,2-diphenyl-1,3,2λ4-oxazaborine (7c). Prepared from 5c
using Method A, reaction time 6 days; column chromatography (silica gel, DCM); product
precipitated after standing overnight with cyclohexane; recrystallization from methanol. Yield: 0.45 g
(56 %) of white solid, m.p.: 134–134.5 °C. Method B gave 14% yield after 2 days using column
chromatography (silica gel, DCM) and recrystallization from methanol. 1H NMR (CDCl3/400 MHz):
δH 1.11 (3H, t, J = 7.5 Hz), 1.16 (3H, t, J = 7.5 Hz), 2.26 (2H, q, J = 7.6 Hz), 2.34 (2H, q, J = 7.5 Hz),
5.48 (1H, s), 6.34 (1H, d, J = 9.6 Hz), 6.86 (1H, d, J = 2.4 Hz), 6.93 (1H, dd, J = 8.8 Hz, 2.4 Hz), 7.03
(1H, d, J = 8.8 Hz), 7.11–7.18 (6H, m), 7.24–7.31 (4H, m), 7.41 (1H, d, J = 9.6 Hz) ppm; 13C NMR
(CDCl3/100 MHz,): δC 10.7, 12.5, 28.0, 30.4, 96.7, 117.1, 117.4, 118.6, 126.0, 126.4, 127.0, 130.7,
133.6, 139.1, 142.9, 152.3, 160.5, 174.2, 184.1 ppm. Anal. Calc. for C28H26BNO3: C, 77.25; H, 6.02;
N, 3.22. Found: C, 76.97; H, 5.98; N, 3.10.
3. Results and discussion
3.1.Synthesis
Starting β-enaminones 5 were prepared employing the condensation of 6-aminocoumarin 3 (prepared
from coumarin 1 via nitration and subsequent reduction of the formed 6-nitrocoumarin 2) with
appropriate β-dicarbonyl compounds 4a–d according to the Scheme 1. Enaminoketones 5a–c were
prepared using the standard methodology under acid catalysis with azeotropic removal of the reaction
water. The regioselective reaction in the case of benzoylacetone (synthesis of 5b) can be ascribed by
the well-known difference in the reactivity of acyl and benzoyl groups. Ethyl acetoacetate (4d) can
react either on its ester or ketone carbonyl group to provide in principle three products: enaminoester,
oxoamide and enaminoamide. Upon changing the reaction conditions we were able to affect the
8

composition of the reaction mixture. When performing the reaction in large excess of 4d, only the
ACCEPTED MANUSCRIPT
enaminoester 5e was isolated (Method A). On the other hand, using the similar conditions as in the
cases of the preparation of 5a–c (Method B), the enaminoamide 5d was the major product (Scheme 1).
Scheme 1. The synthesis of the starting enaminones.
The classic synthesis of boron β-iminoenolates is the reaction of β-enaminones with the appropriate
compounds of trivalent boron. 2,2-Difluoro-1,3,2-oxazaborines 6a–d were prepared by the reaction of
enaminones 5a–c and enaminoamide 5d with boron trifluoride ethyl etherate in refluxing
dichloromethane under basic conditions (Scheme 2). Similarly, their 2,2-diphenyl analogues 7a–c were
obtained upon treatment enaminones 5a–c with triphenylborane or triphenylborinic acid at laboratory
temperature (Scheme 2). Only a mixture of products was obtained in the reaction of enaminoester 5e
with both BF3 Et2O and Ph2BOH or Ph3B.
9

Scheme 2. The synthesis of target 1,3,2-oxazaborines 6 and 7.
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3.2.The absorption spectra
The absorption spectra of all the studied compounds in 1,4-dioxane (DO) are composed of one broad
band with a maximum in the region 308–380 nm and either of another one with a maximum (5d and
6b, d) or of a shoulder (5, 6a, c) in the region 250–290 nm. The spectra of the compounds 7 possess
another broad band at 271 nm. From the chemical structure of the studied compounds and their
absorption spectra, it is evident that alkyl size (R1, R2) has practically no (compounds 6a, c and 7a, c)
or only a small effect (compounds 5a, c) on the position of the absorption maximum in dioxane. The
replacement of the alkyl R1 by the phenyl group (compounds 5b, 6b, 7b) causes a considerable
bathochromic shift (Fig. 2, Fig. 3 left, Table 1). The transition from compounds 5 to 6 results in a slight
hypsochromic shift (1–12 nm). The substitution of the fluorine atoms by phenyl groups (6→7) results
in a bathochromic shift of 31–36 nm (Fig. 2, right and Fig. 3, left). Compound 5e exhibits a
bathochromic shift of 14 nm compared to 5a and a hypsochromic shift of 33 nm in comparison with 5b
(Table 1); a quite analogous relation exists between 6a,b and 6d (Fig. 2 right). The compounds 5d and
6d may consist from two, more or less conjugated, aminocoumarin chromophores ( Fig. 3 right).
Fig. 2. The absorption spectra of 5a–c,e (left) and 6a–d (right) in 1,4-dioxane.
10

Fig. 3. The absorption spectra of 7a–c (left) and 5d and 6d in 1,4-dioxane
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Table 1
Optical properties of the prepared compounds
Comp.
λA(max) [nm]
λF(max)/λPh(max)
λF(powder)
[nm]
(77 K) [nm]
DO
ε [103 mol−1
dm3 cm−1]
2-MTHF
2-MTHF
5a
5b
5c
5d
5e
6a
6b
6c
6d
7a
7b
7c
308
25.24
30.87
308
355
319
317
299
307
343
307
334
338
377
338
433/462,500,537 446
355
429
420,463,480
317
21.47
435/489,514
455/535
441
435
517
-
322
69.18
298
38.53
307
22.87
426/462,497
450*
399
402
343
22.86
308
26.74
428/463,498,542 427
331
33.66
421/479,511
457
481*
334/272
377/272
339/277
11.17/17.74
15.34/33.24
10.87/14.89
553
502
527*
484*
450*
* denotes a very weak emission
3.3.The luminescence spectra
With the exception of compound 5d (λF = 470 nm in dioxane), the studied compounds do not
fluoresce in a solution at room temperature. However, in 2-methyltetrahydrofuran (2-MTHF)
at 77 K all the investigated compounds exhibit more or less strong fluorescence. Furthermore,
a dual luminescence for some compounds of the series 5 and 6 was detected. The used
spectrophotometer LS55 makes possible to separate a luminescence with a lifetime shorter
than 10-6 s (i.e. fluorescence) from a luminescence with a lifetime longer than 10-3 s (i.e.
phosphorescence). The broadband with the maximum in the region at 450 nm recorded in the
fluorescence mode corresponds to the fluorescence from the first singlet ππ* state (for
example Fig. 4, left and Fig. 5, left). By the recording in the phosphorescence mode, a
bathochromaticlly shifted luminescence exhibiting a vibronic structure was detected (Fig. 4
left and Fig. 5 right). The phosphorescence spectra of some compounds 5 and 6, excepting 6d,
show very similar phosphorescence spectra (Fig. 5, right). On the contrary the structure of 6d
consists of two coumarin units and, probably, therefore, its phosphorescence spectrum is
bathochromically shifted. Very similar absorption and fluorescence band shapes, maxima and
11

ACCEPTED MANUSCRIPT
character of phosphorescence spectra of compounds 5 and 6 give evidence that chromophoric
system is formed by coumarin moiety.
The compounds of type 7 show only a broad fluorescence band in 2-MTHF at 77 K (Fig. 5,
right left). The longest absorption band in this series in DO at room temperature, in
comparison with the compounds 5 and 6, is bathochromically shifted by about 30–36 nm. This
could indicate a reordering of the singlet ππ* and triplet nπ* states; it means that for the series
of the compounds 7, the triplet nπ* has a higher excitation energy than the singlet ππ* and
does not apply in the deactivation cascade; compared to compounds 6, less probable
intersystem crossing for compounds 7 caused by absence of fluorine atoms may be considered,
too. All the studied compounds in a powder-state show more or less intense broadband
luminescence (Figs. 6, 7) with a maximum in the region 400–480 nm (5, 6) and at 553 and 527
nm (7a, b). Significantly, the compounds 5a, 6a, and 7a exhibit medium to strong fluorescence
emission, while the compound 6d exhibit the lowest fluorescence intensity of this series
(Figs.6, 7). The compounds 5c,d, and 6b,c show a relatively high fluorescence, however the
compounds 5b, 6d, 7b show very weak fluorescence (Fig. 6, 7). The compound 5e does not
exhibit any fluorescence in a solid state.
Fig. 4. The fluorescence and phosphorescence spectra of 5a (left) and 6d (right) in 2-MTHF at 77 K
12

ACCEPTED MANUSCRIPT
Fig. 5: The fluorescence spectra of 7a-c (left) and phosphorescence spectra of 5a and 6a, c,
d
(right) in 2-MTHF at 77 K
Fig. 6: The fluorescence spectra of 5a–d (left) and 6a–d (right) in solid state
Fig. 7: The fluorescence spectra of 7a-c in solid state
13

ACCEPTED MANUSCRIPT
Fig. 8. The fluorescence spectra of 6b (left) and 7b (right) in a solid (SS) and in an aggregated state
(THF/water solution)
The finding that some boron iminoenolates have appeared to be promising substrates for receiving
AIE/AIEE (aggregation-induced emission/aggregation-induced emission enhancement) properties [36]
encouraged us to perform an introductory study for selected representatives 6b and 7b. These
compounds do not exhibit fluorescence in THF, however, after the admixing of their THF solutions
with water (fw 90 %), a white turbidity and simultaneously a fluorescence start to appear (Figs. 8, 9).
Fig. 9. 10-4
solutions of compounds 6b (left) and 7b (right) without (top) and under UV (λ = 360 nm)
M
light (bottom): (a – THF, b – 50 % (v/v) water/THF, c – 99 % (v/v) water/THF).
The fluorescence intensity becomes stronger when used THF/water mixture with fw of 99 %. For
compound 7b, no significant changes were observed in the shape and position of spectra in the
aggregated state when compared with the solid state spectrum, but a bathochromic shift of the
fluorescence maxima, primarily in fw of 99 % was detected for compound 6b.
14

These changes in fluorescence properties in dependence on water content are documented by the
ACCEPTED MANUSCRIPT
photographs (Fig. 9). All the above-mentioned results indicate that compounds 6b and 7b are
AIE/AIEE active molecules.
3.4.NMR Study
It should be noted that difluoroboron iminoenolates are studied by means of 19F NMR relatively often.
The interpretations of these data are, however, sometimes questionable. A signal consisting of four
1:1:1:1 lines is sometimes interpreted as two 1:1 doublets meaning two non-equivalent fluorines split
by boron. Similarly, the presence of two 1:1 lines is occasionally explained as one 1:1 doublet meaning
two equivalent fluorines split by boron. Due to the spin number of boron-11 (I = 3/2), one boron atom
splits each neighbouring fluorine into four 1:1:1:1 lines [via 1J(11B–19F)] although distortion of the
signals due to the relaxation phenomena or absence of the splitting due to a broadening of the signals is
possible. In the case of the non-equivalence of the fluorines, an additional splitting into doublets can be
observed (due to the geminal 19F–19F coupling). Thus the above-mentioned presence of four 1:1:1:1
lines could be most commonly interpreted as two equivalent fluorines split by boron-11 coupling
(assuming that all the line distances are equal). Two 1:1 lines can be understood as two non-equivalent
fluorines where splitting is not observed owing to the broadening of the signals (due to either dynamics
of a molecule or the quadrupolar effect of boron or some other reasons). The whole situation can be
more complicated if an observable isotopic shift is present and the splitting of the fluorines by boron-10
(I = 3) into seven lines appears in the spectrum. Basic facts about boron NMR were published e.g.
several decades ago in ref. [54] Representative spectra for both the asymmetric and symmetric case are
shown in ref. [29]
Fluorine-19 NMR spectra of compounds 6a,b in CDCl3 consist of two broad signals of identical
intensity revealing the presence of two non-equivalent fluorine atoms (see Supporting Info, Figs. S35,
37). That indicates the lack of symmetry caused by a non-planarity of the molecules. In principle
(according to the above-mentioned paragraph) the fluorine signals should be split into doublets via
geminal coupling with neighbouring fluorine. Each line of the doublet should be split into another
1:1:1:1 multiplet via one-bond coupling with 11B (under the assumption of neglecting the 10B–19F
coupling). The absence of the splitting can be attributed to a molecular dynamics causing considerable
broadening of the signals. This assumption was supported by the fact that in other solvents (acetone,
acetonitrile) only one broad signal appeared. The dynamics of 6b was studied by means of 19F VT-
NMR study in CD3CN (Figs. 10, S46). At the laboratory temperature (293 K) the spectrum consists of
a broad singlet. Upon gradual cooling-down the sample, the appearance of the spectrum changes. At
283 K it reaches the coalescence point below which the fluorine atoms are further non-equivalent. At
263 K the spectrum can be described as a broad AB system where only geminal coupling between
neighbouring fluorines is observable. Upon cooling-down the sample to 238 K, each fluorine signal is
split into eight lines. This can be interpreted by the manifestation of both 19F–19F (with 2J(19F,19F)
15

about 95 Hz) and 19F–11B coupling (with 1J(19F,11B) about 14 Hz). The shoulders at the left edge of the
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signals are probably due to the 10B isotopomer. On the other hand, heating the sample results in the
sharpening the signal which is at 333 K sufficient for 19F–11B coupling to be observed (splitting into
approximately 1:1:1:1 multiplet with 1J(19F,11B) about 14 Hz). For the analysis of the splittings see
Figures 11 and S47. At the temperature interval 238–333 K the molecule of 6b thus changes from
asymmetrical case to symmetrical (Figure 10). Nice examples of both asymmetric and symmetric
boron iminoenolates described Gardinier et al. [29] Here the dynamic behaviour was not observed,
each molecule represented either the symmetric or the asymmetric case.
Fig. 10. 19F VT-NMR study of the compound 6b in acetonitrile-d3.
16

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Fig. 11. Left part: analysis of 19F NMR spectrum of 6b in CD3CN at various temperatures. Right part:
11B NMR spectrum of 6b in CD3CN at various temperatures.
Unlike 6a,b the spectrum of 6c in CDCl3 (Fig. S39) consists of two broad doublets with J ca 95 Hz.
This can be accounted for the geminal coupling between two non-equivalent fluorines. Due to the
broadening of the signals the considerably smaller boron-fluorine coupling is not observable under the
conditions of the measurement.
The values of the fluorine chemical shifts (Table 2) lie in the range common for these compounds [29,
39, 55–57] (approx. -130 – -140 ppm). Slightly lower values (about -122 – -124 ppm) described Xia et
al. [32] for difluoroboron chelates of some heterocyclic enaminoketones. No significant isotopic shifts
due to 10B were observed.
Boron signal in 11B NMR spectra of compounds 6a–c in CDCl3 is split into triplet with 1J(11B,19F)
about 14 Hz. The appearance of the signal is unaffected by temperature. The equivalence of both the
boron-fluorine coupling constants is probably due to only a slight difference between chemical
environments of both the fluorines which is reflected also by a small difference in their chemical shifts
(∆δ ~ 1 ppm at 238 K in CD3CN). This is also supported by the fact that 1J(11B,19F) for 6a–c are
comparable with those described in [29] for symmetric cases (about 15 Hz). Compared to B(OMe)3
the signals of compounds 6 are upfielded which is typical for four-coordinate boron compounds [54,
17

58]. Values of the chemical shifts range around zero, similarly to the values described for other
ACCEPTED MANUSCRIPT
difluoroboron iminoenolates [13, 29, 33, 56] and trifluoroborane adducts with four-coordinated
nitrogen [59].
Table 2
11B and 19F NMR parameters of compounds 6 and 7
11B
19F
6a
6b
6c
6d
7a
7b
7c
0.36, t, 1J(11B, 19F) = 15.2 Hz
0.68, t, 1J(11B, 19F) = 14.6 Hz
0.42, t, 1J(11B, 19F) = 15.3 Hz
0.49, br s
-134.0, -135.6 (2 × br s)
-134.5, -136.3 (2 × br s)
-134.0 (d, 95.8 Hz), -135.8 (d, 93.3 Hz)
-136.0, br s
5.29, br s
5.53, br s
4.89, br s
Measured in CDCl3 with exception of 6d (in DMSO-d6).
Compound 6d is insoluble in CDCl3. Both its 19F NMR and 11B NMR spectra in DMSO-d6 at
laboratory temperature consist of one broad singlet (see Figs. S41,42).
In general, 11B NMR signals of BPh2 fragment are shifted more downfield from those of BF2 fragment
[54]. Values in the range δ = 1.4–5.8 ppm can be extracted for similar O–B–N heterocycles from the
literature [13, 28, 43–45, 56]. Boron-11 NMR spectra of compounds 7 consist of a broad signal with
chemical shifts at the downfield edge of the range (δ = 4.9–5.5 ppm). This can be caused e.g. by rather
greater steric compulsion around the boron atom (two phenyl groups and coumarinyl fragment)
resulting in a decreased π-bonding to the adjacent phenyl rings, and therefore lowered shielding of the
central boron atom. It is supported by the fact that for similar O–B–N heterocycles, a change of the
substituent on the nitrogen atom from hydrogen to methyl led to a significant increasing in 11B shift (δ
from 1.8–2.5 to 3.7–4.4 ppm) [43, 44]. For 11B and 19F NMR data of compounds 6 and 7 see Table 2.
A comparison of 13C NMR spectra of the parent compounds 5 with the corresponding 6 and 7 reveals
a significant upfield shift for C3 carbon atom (numbering according to the ORTEPs) in the boron
heterocycles (∆δ 15.2–18.3). This suggests that more contributing resonance form of the oxazaborines
is the enol-imino than keto-enamino (Fig. 12, left). In the cases of compounds 6a,b and 7a the signals
of carbon C2 (and in some instances also C13 and/or C14) are split (see Figure S34 in the Supporting
Info). A possible explanation lies in a long-range coupling of the carbons with boron-11. To the best
of our knowledge, such observations are relative rare (due to the quadrupole character of boron and
resulting broadening the lines), for other examples see refs. [60, 61] The atypical shape of the
multiplets could be explained according to ref. [60] On the other hand, the splitting of the ipso carbons
(C15, C21) [which should be split into 1:1:1:1 quartet via 1J(11B–13C)] has never been observed.
Generally, the signals of the carbons were very broad and missed from the spectra (only in the case of
18

7b a very broad signal with δ ~ 147 ppm was found after applying the exponential window function
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with large line-broadening factor).
R
O
R
N
R
O
R
N
B
R4
R3
B
R4
R3
R1
R1
Fig. 12. Possible resonance forms of oxazaborines.
In the proton spectra of 6 and 7, the most striking change is the significant downfield shift of the
olefinic proton H2 in comparison with compounds 5. Compounds 6 possess ∆δ = 0.32–0.35 ppm, the
shift for compounds 7 is lower (∆δ = 0.2 ppm). This behaviour is typical for this kind of compounds
(see e.g. refs. [32, 55] It is probably related to the vicinity of the positively charged nitrogen.
3.5.Crystallography
Structures of 6b (Fig. 13) and 7a (Fig. 14) are composed of the difluoro or diphenylboron fragment and
anisobidentately bound ketiminate ligand. A boat-like conformation of the central ligand core is found
for 7a with the deviation of 0.179(2)Å for C2 and 0.621(2)Å for B1 atoms from the plane defined by
the N1, C1, C3 and O1 atoms. On the other hand, only the deviation of the boron atom of 0.307(2)Å
from the plane defined by all five core atoms of the ligand forming thus an envelope conformation is
found for 6b. The reason is a lower steric hindrance or electron density on the boron atom in 6b.
Another difference between both the compounds, caused probably by a deformation of the central
iminoenolate skeleton, is a tilting of the coumarin ring in 7a producing a lack of C(=O)O stacking. On
the other hand, this stacking can be found in 6b.
Fig. 13. The molecular structure (ORTEP 50% probability level) of 6b. Selected interatomic distances
[Å] and angles [°]: O1–B1 1.466(2), B1–N1 1.559(3), N1–C1 1.323(2), C1–C2 1.413(3), C2–C3
1.358(3), O1–C3 1.325(2), N1–C4 1.449(2), O2–C7 1.377(2), O2–C8 1.377(2), O3–C8 1.207(2); F1–
B1–F2 109.89(16), C3–O1–B1 122.48(15), O1–B1–N1 109.50(15), C1–N1–B1 121.26(15), C7–O2–
C8 121.94(15), O2–C8–O3 116.40(19).
19

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Fig. 14. The molecular structure (ORTEP 50% probability level) of 7a. Selected interatomic distances
[Å] and angles [°]: O1–B1 1.519(3), B1–N1 1.594(3), N1–C1 1.325(3), C1–C2 1.415(3), C2–C3
1.359(4), O1–C3 1.313(3), N1–C4 1.438(3), O2–C7 1.387(3), O2–C8 1.382(3), O3–C8 1.208(3); C15–
B1–C21 113.72(18), C3–O1–B1 116.38(17), O1–B1–N1 104.14(16), C1–N1–B1 117.05(18), C7–O2–
C8 121.6(2), O2–C8–O3 116.5(3).
Upon comparison of the relevant bond lengths of compounds 6b and 7a with those obtained for keto-
enamine arrangement [62] and with standard values [63] following facts can be retrieved:
A considerable shortening the C2–C3 and C1–N1 bonds as well as lengthening the C1–C2 and C3–O1
bonds.
The C2–C3 and C3–O1 bond lengths are close to those reported for C=C–O of enols (1.362 Å and
1.333 Å resp.).
The O–B bond length is significantly shorter than the N–B one.
With a respect to the above-mentioned as well as other angles and distances characteristic for boron
iminoenolate complexes, it can be stated that the π-electron density/a multiple bond character is
localized mainly between C1–N1 and C2–C3 atoms of the ligand respectively which means greater
contribution of enol-imino form against keto-enamino. Both the structures are similar to already
reported structures of boron diphenyl [43] or difluoro [29, 32, 34, 55, 64–66] iminoenolate complexes.
The conclusions are thus analogous to those made on the basis of 1H and 13C NMR in solution (vide
supra).
20

4. Conclusions
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The aim of the work was to combine two known luminophores: coumarin and boron iminoenolate and
investigate whether the combination would led to fluorescent molecules. Seven novel boron β-
iminoenolates, substituted with fluoride or phenyl group on the boron atom and coumarin-6-yl
fragment on the nitrogen, were prepared upon reacting the corresponding enamines with compounds
of trivalent boron (boron trifluoride diethyl etherate, triphenyl borane or diphenylborinic acid). The
compounds prepared were characterized by means of 11B, 19F, 13C and 1H NMR spectroscopy in
solution, single-crystal X-ray diffraction and UV/Vis spectroscopy. 19F VT-NMR study of the selected
substrate in the range 238–333 K revealed chemical exchange between the fluorines with the
coalescence point about 283 K. The preliminary fluorescence study revealed that all the studied
compounds (both the starting β-enaminones and the corresponding boron iminoenolates) show more or
less intense broad band luminescence in powder state with a maximum depending on the substitution
on the boron atom. With the exception of compound 5d, the studied compounds do not fluoresce in
solution at room temperature. On the other hand, all the investigated compounds exhibit more or less
strong fluorescence in a frozen 2-methyltetrahydrofuran at 77 K. Tests performed for compounds 6b
and 7b also revealed that these compounds are AIE/AIEE active. From the above mentioned it follows
that coumarin-6-yl-containing boron iminoenolates proved to be interesting luminophores. For
understanding the relations structure – photophysical characteristics – UV/Vis spectra of the studied
compounds, a detailed investigation of the luminescence characteristics including quantum yields and
a quantum chemical study of the characteristics of the excited states is now in progress in our
laboratory.
Acknowledgement
H.D. and P.Š. would like to thank to Faculty of Chemical Technology, University of Pardubice for
institutional support.
Appendix A. Supplementary material
CCDC 1050226 and 1050227 contain the supplementary crystallographic data for 6b and 7a,
respectively. Copies of this information may be obtained free of charge from the Cambridge
Crystallographic Data Centre via The Director, CCDC, 12 Union Road, Cambridge CB2 1EY, UK
(fax: +44-1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www: http:// www.ccdc.cam.ac.uk).
Additional experimental procedures as well as copies of NMR spectra are presented in the Supporting
Information file.
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