A new bicyclic oxazaborines with a bridged nitrogen atom, their thermic rearrangement and fluorescence properties

Article abstract of DOI:10.1016/j.jorganchem.2011.11.004

Cyclic β-enaminones bearing secondary amino group react with 4-substituted benzenediazonium tetraphenylborates in dichloromethane to form substituted bicyclic [1,3,2λ⁴]oxazaborines The oxazaborines rearrange, on heating to 200 °C in the absence of solvent or in DMF or DMSO, to isomeric 2H-[1,2,4,3λ⁴]triazaborines. Previously prepared [1,3,2λ⁴]oxazaborines derived from acyclic β-enaminones bearing secondary amino group either did not undergo the rearrangement or with a lot of difficulties and with negligible yield. The fluorescence behaviour of the prepared triazaborines was observed. These compounds fluoresce in 2-methyltetrahydrofurane and in solid state under low temperatures.

Full text of DOI:10.1016/j.jorganchem.2011.11.004

Journal of Organometallic Chemistry 699 (2012) 75e81
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
A new bicyclic oxazaborines with a bridged nitrogen atom, their thermic
rearrangement and fluorescence properties
a
a,
b
a
a
ꢀ
*
ꢀ
ꢁ
ꢀ
Frantisek Josefík , Markéta Svobodová , Valerio Bertolasi , Petr Simunek , Vladimír Machácek ,
a
c
ꢀ
ꢀ
Numan Almonasy , Eva Cernosková
a Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic
b Università di Ferrara, Dipartimento di Chimica and Centro di Strutturistica Diffrattometrica, Via L. Borsari 46, 44100 Ferrara, Italy
c Joint Laboratory of Solid State Chemistry of IMC ASCR, v.v.i. and University of Pardubice, Faculty of Chemical Technology, University of Pardubice, Studentská 84,
532 10 Pardubice, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Cyclic
raphenylborates in dichloromethane to form substituted bicyclic [1,3,2
rearrange, on heating to 200 ꢀC in the absence of solvent or in DMF or DMSO, to isomeric 2H-[1,2,4,3l4
triazaborines. Previously prepared [1,3,2 -enaminones bearing
4]oxazaborines derived from acyclic
secondary amino group either did not undergo the rearrangement or with a lot of difficulties and with
negligible yield. The fluorescence behaviour of the prepared triazaborines was observed. These
compounds fluoresce in 2-methyltetrahydrofurane and in solid state under low temperatures.
Ó 2011 Elsevier B.V. All rights reserved.
b
-enaminones bearing secondary amino group react with 4-substituted benzenediazonium tet-
l
4]oxazaborines The oxazaborines
Received 12 July 2011
Received in revised form
2 November 2011
]
l
b
Accepted 3 November 2011
Keywords:
b
-Enaminones
Diazonium tetraphenylborates
[1,3,2
4]Oxazaborines
2H-[1,2,4,3
4]triazaborines
l
l
Fluorescence
1. Introduction
position, and diazonium tetraphenylborates in dichloromethane.
Depending on the structure of the starting “polarised ethylene” this
During the last several years, unsaturated heterocyclic
compounds containing a boron atom have invited interest either
from the aspect of basic research or possible applications [1,2]. An
introduction of the boron atom into an organic molecule substan-
tially changes its electronic characteristics, which opens the way to
the construction of optoelectronic materials for molecular elec-
tronics based on organic molecules. These materials could be used
for organic light-emitting diodes (OLEDs). Complexes of diphe-
nylboron compounds also belong among these compounds from
the perspective of this aspect. The preparation of these compounds
results from a reaction of N,O-chelating ligands with triphe-
nylborane [3e6] or with diphenylborinic acid [7], N,N-chelating
ligands with triphenylborane [8e13] or N,C-chelating ligands with
diphenylboron chloride [14].
reaction gave various products. b-Enaminones formed oxazabor-
ines I (R1 ¼ Me, Ph),
b-enaminoamides a mixture of oxazaborines I
(R1 ¼ NHPh), diazaborinones II, or triazaborines III (R1 ¼ CONHPh),
respectively, and
b
-enaminonitriles gave triazaborines III (R1 ¼ CN).
If, in the case of the b-enaminonitriles [17], water was present in
the reaction mixture, oxazaborine I (R1 ¼ NH2) was also isolated
(Scheme 1).
The aim of this work is the preparation of bicyclic compounds
containing the B(C6H5)2 group derived from cyclic enaminones 4
(Scheme 2) and investigation of their fluorescence properties.
2. Materials and methods
2.1. NMR spectroscopy
In our previous papers we described the synthesis and proper-
ties of heterocyclic compounds containing the BPh2 group [15,16].
NMR spectra were measured at 295 K using a Bruker AVANCE
500 spectrometer equipped with a 5 mm broadband probe with
a gradient of magnetic field in the direction of z axis operating at
the frequencies 500.13 MHz (1H), 125.77 MHz (13C), 50.69 MHz
The reactions start from simple
b-enaminones [15], b-enaminoa-
mides [16] or -enaminonitriles [17], unsubstituted in the
b
a-
(
15N) and a Bruker AVANCE III 400 operating at 400.13 MHz (1H),
* Corresponding author. Tel.: þ420 466 037 039; fax: þ420 466 037 068.
E-mail address: marketa.svobodova@upce.cz (M. Svobodová).
100.62 MHz (13C), 128.38 MHz (11B) and 40.55 MHz (15N). The 1H
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.11.004

76
F. Josefík et al. / Journal of Organometallic Chemistry 699 (2012) 75e81
diffractometer with graphite monochromated Mo Ka radiation and
Ph Ph
B
Ph Ph
B
R3
R2
Ph
R3
R2
corrected for Lorentz and polarization effects. The data sets were
integrated with the Denzo-SMN package [18] and corrected for
Lorentz, polarization and absorption effects (SORTAV [19]). The
structures were solved by direct methods (SIR97 [20]) and refined
using full-matrix least-squares. In all compounds all non-hydrogen
atoms were refined anisotropically and hydrogens in part iso-
tropically and in part included on calculated positions, riding on
their carrier atoms.
All the calculations were performed using SHELXL-97 [21] and
PARST [22] implemented in WinGX [23] system of programs. The
crystal data and refinement parameters are summarized in
Tables S1 and S2. Selected structural parameters are given in
Tables S3 and S4 (see Supplementary material).
O
N
N
N
R1
O
H
N
N
N
N
Ar
Ar
R1 = Me, Ph, NHPh, NH2
R2 = Me, Ph, CH2Ph
R2 = Me, Ph
R3 = H, Me
R3 = H, Me, 2,4-(OMe)2C6H3
II
I
Ph Ph
B
Complete crystallographic data for the 16 structures in this
paper have been deposited at the Cambridge Crystallographic Data
Centre.
Ar
R3
N
N
N
R2
R1
2.3. Spectroscopic studies
R1 = COMe, CONHPh, CN
R2 = Me, Ph
The used solvents, acetonitrile (MeCN), dibutyl ether (DBE), and
2-methyltetrahydrofuran (2-MeTHF), were of spectroscopic grade
(SigmaeAldrich). The absorption spectra were measured on Perkin
Elmer Lambda 35 UV/VIS Spectrophotometer. The fluorescence
spectra were measured using Hitachi Perkin Elmer LS 55 Spectro-
fluorimeter. The fluorescence spectra of the compounds were
recorded by excitation at the wavelengths of the longest absorption
maximum. Quantum yields of the fluorescence in the solid state
were not determined.
R3 = H, Me
III
Scheme 1. Diphenylboron heterocycles prepared from
mides and -enaminonitriles.
b-enaminones, b-enaminoa-
b
NMR spectra were calibrated in CDCl3 on internal standard (hex-
amethyldisiloxane ( 0.05) or tetramethylsilane ( 0.00)) and the
13C NMR spectra were calibrated on the central signal of the solvent
multiplet (
76.9). The 13C NMR spectra were measured in a stan-
d
d
d
2.4. Differential thermal analysis
dard way and by means of APT pulse sequence (spectral width
26.455 kHz, acquisition time 1.238 s, zero filling to 64 K and line
broadening 1 Hz prior Fourier transformation). The 15N NMR
spectra were calibrated on external neat 15N nitromethane placed
The thermal properties were investigated by a differential
thermal analysis (DSC) using calorimeter DSC 12E (Mettler Toledo).
The temperature and enthalpy calibration was made with the help
of pure In. Measurements were carried out in nitrogen atmosphere.
Stability of nitrogen flow was controlled by mass flow controller
FMA 5400/5500 (Omega). Empty aluminium pan was used as
a standard. Sample (approx. 3 mg) in aluminium sample pan was
studied in the range 30e220 ꢀC. Different heating/cooling rates (1,
5, 15, 20 ꢀC/min) were used.
in a coaxial capillary (d 0.0). The d (
15N) values were measured with
the help of techniques with inversion detection (gradient selected
2D 1He15N HMBC) processed in the magnitude mode. The gradient
ratio was 70:30:50.1. Experiments were performed with the NH
one-bond coupling 90 Hz, and NH long-range coupling 5 Hz,
2 K ꢁ 160 zero filled to 2 K ꢁ 1 K, sinebell squared in both dimen-
sions. The 11B NMR spectra were calibrated on external B(OCH3)3
placed in a coaxial capillary (d 18.1). In order to suppress the signals
2.5. Materials
of 11B nuclei from a NMR tube glass, the measurements were
carried out in Teflon sample tube liners (Aldrich) inserted into
5 mm tubes whose bottom part of about 25 mm length was cut off.
Lactim-methylethers were synthesized according to the litera-
ture procedures [24] from lactams and dimethyl sulphate. Diazo-
nium tetraphenylborates were prepared according to the literature
[15]. In the case of 4-bromobenzenediazonium tetraphenylborate,
the diazonium salt was washed only with ethanol and then it was
dried under vacuum for 0.5 h at room temperature. CAUTION:
diazonium tetraphenylborates in crystalline state can undergo
explosive decomposition even at a very slight mechanical stimu-
lation!!! Anhydrous dichloromethane was purchased from Fluka or
distilled over P2O5 before used.
2.2. X-ray measurements
The crystal data for compounds 5aec, 5eeh, 5j and 6aeg, 6i
were collected at room temperature using a Nonius Kappa CCD
2.6. Synthesis
2.6.1. Synthesis of 2- and 3-substituted diketones 3a, cef and 4b
The title compounds were synthesized according to the litera-
ture [25]. A mixture of diketone 1a or 1b (234 mmol), corre-
sponding lactim-methylethers (258 mmol) and a catalytic amount
of nickel(II) acetylacetonate (0.59 g, 2.3 mmol) was heated at
100 ꢀC for 24 h. Then the reaction mixture was cooled. The solid
products were filtered and crystallized. (In the case of 3e and 4b,
water (170 mL) was added and the reaction mixture was extracted
Scheme 2. The reaction route from diketones to enaminones.

F. Josefík et al. / Journal of Organometallic Chemistry 699 (2012) 75e81
77
with dichloromethane (3 ꢁ 90 mL). The combined organic layers
were dried over anhydrous sodium sulphate and concentrated
under reduced pressure to afford the crude product. The crude
product was purified chromatographically on silica gel.) The
compounds prepared according to the procedure are summarised
in Supplementary material.
monitored by TLC. The solvent was evaporated in vacuo and the
crude reaction mixture was chromatographed on silica gel with
dichloromethane as the mobile phase to give compounds 6ael.
The compounds prepared according to the above-mentioned
procedures are summarized in Supplementary material.
3. Results and discussion
2.6.2. Synthesis of enaminones 4a, cef
The title compounds were synthesized according to the litera-
ture [26]. Sodium (7.38 g, 0.315 mol) was dissolved in methanol
(168 mL). Diketone 3 (0.105 mol) was added and the reaction
mixture was refluxed. After 1 h, the solvent was distilled off under
vacuum and the crude product was purified by column chroma-
tography on silica gel and crystallization or vacuum distillation. The
compounds prepared according to the procedure are summarised
in Supplementary material.
The starting
synthesis, starting from corresponding
methylethers (Scheme 2). -Diketones 1a,b react with the lactim-
b
-enaminones were prepared by two-step
b-diketones 1 and lactim-
b
ethers in the presence of a catalytic amount of Ni(acac)2 at 100 ꢀC
to form 2-, or 3-substituted diketones 3, respectively [25]. The
diketones undergo deacetylation by sodium methanolate in
methanol (Ref. [26], for analogic reaction with ethanolate) to form
enaminones 4. In the case of the diketone 1a and 6-methoxy-
2,3,4,5-tetrahydropyridine, no product 3 was isolated but the
deacetylated enaminone 4 directly.
2.6.3. General procedure for preparation of oxazaborines 5ael
To a cold (5 ꢀC) solution of
b-enaminones 4aef (5 mmol) in dry
The prepared enaminones 4aef were subjected to the
reaction with substituted benzenediazonium tetraphenylborates
(Scheme 3). The reaction proceeded in dry dichloromethane for
approximately 1 day at room temperature or in dichloromethane/
toluene for 1 h at room temperature and 2 h at reflux. Solvent was
then evaporated in vacuo and the residue was subjected to column
chromatography on silica gel. Twelve new condensed oxazaborines
5aef with bridged nitrogen atom in yields of 28e80% were ob-
tained (Table 1). A suggestion of the mechanism of the oxazaborine
formation is depicted in the literature [15].
dichloromethane (15 mL) and toluene (25 mL, dried over sodium)
freshly prepared corresponding benzenediazonium tetraphe-
nylborate (5 mmol) was added. The reaction mixture was stirred
24 h at room temperature or 1 h at room temperature (after 1 h
no diazonium salt was present in the reaction mixture, detection
was performed by means of 4,5-dihydroxynaphthalene-2,7-
disulfonate) and then 2 h at reflux. The reaction mixture was
cooled to room temperature and solvents were evaporated in vacuo.
The crude residue was chromatographed on silica gel with
dichloromethane as the mobile phase to give compounds 5ael. The
compounds prepared according to the procedure are summarised
in Supplementary material.
As in the case of formerly studied oxazaborines I (R1 ¼ Me, Ph),
we have also studied the behaviour of compound 5 upon heating to
higher temperatures. Oxazaborines I underwent, at temperatures
about 200 ꢀC,
a thermal rearrangement to triazaborines III
(R1 ¼ COMe). The oxazaborines I with primary amino groups rear-
ranged relatively easily, whereas oxazaborines I with secondary
amino groups either did not undergo the rearrangement, or did so
in negligible yields only. The newly prepared derivatives 5 were
subjected to both melting at 200 ꢀC and heating in DMF or DMSO
under reflux. The products isolated were triazaborines 6 (Scheme 4,
Table 2). The triazaborines 6a and 6e were formed (according to
TLC) under solvent-free heating in very low, non-isolable yields
only. Upon extending the time of heating, the starting unreacted
oxazaborines 5a,b were still detected in the melt, and the amount
of unidentified decomposition products increased. In some cases,
the primary products of azo coupling to enaminones 7 (Scheme 5)
were identified in the reaction mixture together with the starting
components and the products of the rearrangement. The primary
2.6.4. General procedure for the rearrangement of oxazaborines
5ael
Method A. Oxazaborines 5a,b,e,f,h,i,l were heated at 200 ꢀC for
appropriate time. The reaction was monitored by TLC. The crude
reaction mixture was chromatographed on silica gel with
dichloromethane as the mobile phase to give compounds
6a,b,e,f,h,i,l.
Method B. Oxazaborines 5 (2 mmol) were heated at reflux in
DMF (10 mL) for appropriate time (see Table 2). The reaction was
monitored by TLC. The solvent was evaporated in vacuo and the
crude reaction mixture was chromatographed on silica gel with
dichloromethane as the mobile phase to give compounds 6ael.
Method C. Oxazaborines 5 (2 mmol) were heated at reflux in
DMSO (10 mL) for appropriate time (see Table 2). The reaction was
Ph Ph
B
N2
X
BPh4
H
O
N
O
1N
+
n = 1–3
n = 1–3
i) CH2Cl2, r.t., 24 h or
ii) CH2Cl2/Toluene, reflux
R
R
2N
3N
5a: R = Me, X = Me, n = 1
5b: R = Me, X = Me, n = 2
5c: R = Me, X = OMe, n = 2
5d: R = Me, X = Br, n = 2
5e: R = Me, X = Me, n = 3
5f: R = Me, X = OMe, n = 3
5g: R = Me, X = Br, n = 3
5h:R = Ph, X = Me, n = 1
5i: R = Ph, X = Me, n = 2
5j: R = Ph, X = OMe, n = 2
5k R = Ph, X = Br, n = 2
5l: R = Ph, X = Me, n = 3
4a: R = Me, n = 1
4b:R = Me, n = 2
4c: R = Me, n = 3
4d:R = Ph, n = 1
4e: R = Ph, n = 2
4f: R = Ph, n = 3
X
Scheme 3. The synthesis of oxazaborines.

78
F. Josefík et al. / Journal of Organometallic Chemistry 699 (2012) 75e81
Table 1
Table 2
The yields of oxazaborines 5ael.
The yields of triazaborines 6ael.
Oxazaborine
Room temperaturea
Reflux in CH2Cl2/tolueneb
Triazaborine Melting without solvent Heating in DMF
Heating in DMSO
Yieldc [%]
Yieldc [%]
Time [h]
Yielda [%]
Time [h] Yielda [%] Time [h] Yielda [%]
5a
5b
5c
5d
5e
5f
5g
5h
5i
62
80
48
48
77
35
52
48
44
62
44
52
52
6a
6b
6c
6d
6e
6f
6g
6h
6i
1
1
e
e
1
0
20
e
24
5
15
14
9
20
14
24
14
11
14
19
30
34
28
43
18
6
20
56
67
27
76
10
3.5
2
2
1.5
1
1.5
0.5
2.5
1.5
2
23
35
38
45
11
11
0
40
40
37
45
15
49
d
d
e
61
0
6
d
5
e
d
e
37
3
1
e
e
1.5
16
73
e
60
d
5j
5k
5l
6j
6k
6l
d
e
1
1.5
28
48
a
The reaction proceeded for 24 h.
a
After column chromatography and crystallization.
b
The reaction proceeded for 1 h at laboratory temperature and then for 2 h at
reflux.
c
After column chromatography and crystallization.
Not performed.
d
eOeB(Ph)2e splits and a bond of the same kind pNeBe(Ph)2e
between the nitrogen of the azo group and boron, is formed. There
are not relevant data in the literature capable of explaining the
direction and irreversibility of the rearrangement 5 / 6. We tried
to find out the heat of the rearrangement 5 / 6 by means of DSC. In
the case of 5i, where a significant amount of 6i is formed under
solvent-free heating, the endothermic peak of the melting overlaps
with the peak of the rearrangement, hence it is not possible to
evaluate the heat of the reaction. In the case of compound 5b, with
a lower melting point, only a small amount of the triazaborine 6b is
formed under solvent-free rearrangement; the major reaction is
the decomposition of the oxazaborine, so that, even in this case, it is
not possible to evaluate the heat of the reaction. The corresponding
curves of the dependencies of the heat flow on temperature are
shown in Fig. S1.
products were identified by means of TLC and 1H NMR. An
enhancement of the yields of triazaborines 6a and 6e was achieved
after performing the rearrangement in the solution. Higher yields
of 6b, 6f and 6h were obtained as well. On the other hand, solvent-
free melting was better in the case of oxazaborine 5i, and especially
5l. In the case of oxazaborine 5b the attempt to give rise to the
rearrangement by means of microwave irradiation was also made,
however no significant improvement of the yield of 6b, compared
to the conventional heating methods, was achieved. The course of
the rearrangement was followed by TLC on Silufol (dichloro-
methane as the mobile phase).
The rearrangement of oxazaborines 5 to triazaborines 6 is not
reversible. Upon prolongation of the reaction time it is not possible
to achieve the complete rearrangement to compound 6, because
the oxazaborines, at the relatively high temperatures of the rear-
rangement, decompose and decomposition products are formed,
besides the triazaborines. On heating the pure triazaborines 6a and
6d to 200 ꢀC for 1 h under inert conditions, not even traces of the
corresponding oxazaborine were detected (by means of TLC) and
no decomposition of the triazaborines took place; hence the tri-
azaborines are stable at this temperature.
All the products prepared were characterized by means of
multinuclear magnetic resonance and in many cases also by means
of X-ray diffraction (see Supplementary material). The 11B chemical
shifts are of some importance for identification of the compounds
studied: in the case of oxazaborines 5 the values of
positive, and for the compounds with a seven-membered anne-
d (
11B) are
lated ring (5e, 5f, 5g and 5l) these values are higher than 4 ppm. The
triazaborines 6 have values of
but, similar to the case of the oxazaborines, those with annelated
d (
11B) lower than the oxazaborines 5,
The rearrangement of the oxazaborines 5 to the triazaborines 6
proceeds probably via an intramolecular variant of the ANRORC
mechanism. The dative covalent bond between oxygen and boron
seven-membered rings have rather higher values of d (
11B)
compared to the ones with smaller annelated rings. The 15N
chemical shift values of nitrogens N2 and N3 (Table 3) explicitly
Ph Ph
i) 200 °C, solvent free or
B
n = 1–3
ii) DMF, reflux or
O
O
N
iii) DMSO, reflux
n = 1–3
R
R
1N
B
Ph
Ph
6a: R = Me, X = Me, n = 1
2N
N
3N
X
N
6b:R = Me, X = Me, n = 2
6c: R = Me, X = OMe, n = 2
6d:R = Me, X = Br, n = 2
6e: R = Me, X = Me, n = 3
6f: R = Me, X = OMe, n = 3
6g: R = Me, X = Br, n = 3
6h:R = Ph, X = Me, n = 1
6i: R = Ph, X = Me, n = 2
6j: R = Ph, X = OMe, n = 2
6k: R = Ph, X = Br, n = 2
6l: R = Ph, X = Me, n = 3
5a–l
X
Scheme 4. The rearrangement of oxazaborines to triazaborines.

F. Josefík et al. / Journal of Organometallic Chemistry 699 (2012) 75e81
79
Table 4
The values of chemical shifts 15N and 11B for compounds 6ael (for the notation of
nitrogens see Scheme 4).
Triazaborine
d [ppm]
15N1
15N2
15N3
11B
0.18
ꢂ0.16
ꢂ0.24
ꢂ0.24
0.53
0.58
0.56
0.18
ꢂ0.11
ꢂ0.18
ꢂ0.05
0.67
6a
6b
6c
6d
6e
6f
ꢂ178.8
23.3
ꢂ157.1
a
a
a
ꢂ190.1
22.2
19.3
24.8
22.8
19.8
26.0
ꢂ163.0
ꢂ169.4
ꢂ159.3
ꢂ159.1
ꢂ165.6
ꢂ155.0
ꢂ184.6
b
ꢂ179.3
b
6g
6h
6i
Scheme 5. The structure of the primary products of azo coupling.
ꢂ177.2
ꢂ186.9
23.0
ꢂ162.6
a
a
a
6j
b
evidence the presence of an azo group and not a tautomeric
6k
6l
ꢂ183.1
ꢂ177.6
ꢂ168.6
ꢂ159.8
15N) from ꢂ175 to ꢂ195
23.5
hydrazo group. Quaternary nitrogen has
d
(
a
in both the oxazaborines and triazaborines (Table 4).
Not measured.
b
Not detected.
ORTEP [27] views for compounds 5a, 5c, 5eeh, 5j and 6beg, 6i
and crystal data for all compounds are shown in Supplementary
material (Figs. S2eS15, Tables S1eS4). In structure 5b (Fig. 1) the
atom C6 was found disordered and refined over two positions with
occupation factors of 0.62 and 0.38, respectively. In structure 5c
(Fig. S3) the phenyldiazenyl group was found disordered, with two
opposite conformations, and refined with occupation factors of
0.55 and 0.45, respectively. In both compounds 5e and 5f (Figs. S4
and S5) the atom C6 was found disordered and refined over two
positions with occupation factors of 0.69/0.31 and 0.62/0.38,
respectively. In structure 6a (Fig. 2) the C4 atom was found disor-
dered and refined over two positions with occupation factors of
0.63 and 0.37, respectively. In structure 6i (Fig. S15) the bonded
atoms C4eC5 were found disordered and refined over two posi-
tions with occupation factors of 0.60 and 0.40, respectively.
All the boron heterocycles assume similar mixed boat-envelope
conformation (B/E) as evidenced by the puckering parameters [28]
in Tables S3 and S4. In compounds 5 the phenyldiazenyl moieties
display different conformations with respect to the C2eN2 single
bond, due to different steric demands of the substituents at the
boron heterocycle.
show somewhat smaller shifts (w10 nm) than compounds 6iel
(w15 nm).
All the chromophores studied did not exhibit any fluorescence
in solutions at room temperature, but, except for compounds 6k
and 6l, they fluoresce in 2-methyltetrahydrofuran in frozen state at
77 K (Fig. 4) and by recording the fluorescence from the powder
solid phase of the compounds (Fig. 5). In comparison with the
frozen state in the 2-methyltetrahydrofuran, the fluorescence
maxima of all the chromophores in solid state are bathochromically
shifted.
We found out that the triazaborines prepared, in contrast to the
oxazaborines, fluoresce, which substantiates the reason for the
synthesis of these compounds.
The absorption spectra of 6aed and 6iel were measured in
acetonitrile (MeCN), dibutyl ether (DBE) and 2-methyltetrahydro-
furan (2-MeTHF) (Fig. 3 and Table 5). Dependent on the solvent
polarity and the character of the substituent, the absorption
maxima are between 410 and 440 nm. The data in Table 5 indicate
that an enhancement of the solvent polarity causes a significant
hypsochromic shift of the absorption maxima. Compounds 6aed
Table 3
The values of chemical shifts 15N and 11B for compounds 5ael (for the notation of
nitrogens see Scheme 3).
Oxazaborine
d [ppm]
15N1
15N2
15N3
11B
5a
5b
5c
5d
5e
5f
5g
5h
5i
ꢂ187.3
ꢂ195.4
ꢂ195.2
ꢂ193.4
ꢂ185.9
ꢂ185.7
ꢂ184.6
ꢂ182.2
ꢂ187.3
ꢂ188.7
ꢂ187.5
ꢂ174.4
101.4
76.5
85.6
83.6
78.8
86.5
84.9
78.3
83.3
88.7
89.7
79.3
89.3
3.45
3.15
3.07
3.27
4.08
4.06
4.05
3.67
3.49
3.52
3.63
4.32
a
a
a
a
a
a
106.1
108.0
a
5j
5k
5l
a
106.2
Fig. 1. ORTEP view of compound 5b displaying the thermal ellipsoids at 30% proba-
a
Not detected.
bility. Both the disordered C6 and C60 atoms are shown.

80
F. Josefík et al. / Journal of Organometallic Chemistry 699 (2012) 75e81
Fig. 4. Excitation (Ex) and fluorescence (F) spectra of triazaborines 6aed and 6iel
measured in 2-MeTHF at 77 K.
Fig. 2. ORTEP view of compound 6a displaying the thermal ellipsoids at 30% proba-
bility. Both the disordered C4 and C40 atoms are shown.
Fig. 5. Relative fluorescence intensity of triazaborines 6aed and 6iel in solid state.
4. Conclusions
A series of twelve bicyclic oxazaborines with bridged quaternary
nitrogen atoms has been prepared by means of a reaction of
substituted benzenediazonium tetraphenylborates with b-enami-
Fig. 3. Absorption spectra of triazaborines 6aed and 6iel in MeCN.
nones derived from cyclic secondary amines (pyrrolidine, piperi-
dine, perhydroazepine). The reaction proceeds slowly in dry
dichloromethane at room temperature and faster at the boiling
point of the solvent mixture (dichloromethane/toluene). Upon
heating to 200 ꢀC under solvent-free conditions or in boiling DMF
or DMSO, the oxazaborines rearrange to isomeric triazaborines
through the BeO bond splitting and forming a new BeN bond with
the nitrogen of the azo group. Complete rearrangement cannot be
achieved due to the decomposition of the oxazaborines to
unidentified products upon lengthening of the reaction time. The
rearrangement of the oxazaborines to triazaborines was proved to
be irreversible. The structure of the prepared oxazaborines and
triazaborines was studied by means of 1H, 13C, 11B and 15N NMR
and X-ray. Heterocyclic compounds containing the BPh2 moiety,
easily obtainable by the method described here, possess in their
skeleton functional groups convenient for subsequent chemical
Table 5
The absorption (l
A) maxima (nm) of prepared compounds in DBE, MeCN and in 2-
MeTHF at room temperature (RT) and at 77 K (LT) and the fluorescence ( )
lF
maxima (nm) in 2-MeTHF at 77 K (LT) and in solid state.
Trizaborine
DBE
MeCN
2-MeTHF
Solid state
lA
lA
lA
lF
lF
RT
LT
SS
6a
6b
6c
6d
6i
6j
6k
6l
429
419
426
421
427
434
428
440
420
410
417
410
413
418
413
424
427
418
425
418
424
431
425
436
526
511
531
505
519
530
e
544
549
553
541
565
588
543
592
e

F. Josefík et al. / Journal of Organometallic Chemistry 699 (2012) 75e81
81
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preparation of other boron-containing heterocycles. Most of the
compounds prepared exhibit fluorescence properties in powder
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Acknowledgments
The work was financially supported by Ministry of Education,
Youth and Sports of the Czech Republic (project MSM 002162 7501).
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[15] M. Pesková, P. Simunek, V. Bertolasi, V. Machácek, A. Lycka, Organometallics
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graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
cccdc.cam.ac.uk/data-request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2011.11.004.
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