Fluorescent benzene-centered mono-, bis- and tris-triazapentadiene-boron complexes

Article abstract of DOI:10.1039/c5dt00908a

A series of novel benzene centered mono-, bis- and tris-1,3,5-triazapentadiene ligands 6a-e was synthesized and investigated with respect to their reactivity towards triphenylborane. The resulting blue-fluorescent boron complexes 14a-e with a six-membered

Full text of DOI:10.1039/c5dt00908a

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Fluorescent benzene-centered mono-, bis- and
tris-triazapentadiene–boron complexes†‡
Cite this: DOI: 10.1039/c5dt00908a
Christoph Glotzbach,a Nadine Gödeke,a Roland Fröhlich,a
Constantin-Gabriel Daniliuc,a Shohei Saito,b Shigehiro Yamaguchib and
Ernst-Ulrich Würthwein*a
A series of novel benzene centered mono-, bis- and tris-1,3,5-triazapentadiene ligands 6a–e was syn-
thesized and investigated with respect to their reactivity towards triphenylborane. The resulting blue-flu-
orescent boron complexes 14a–e with a six-membered ring chelate structure show excellent thermal
and chemical stability. All title compounds were completely characterized including X-ray diffraction
studies for 14a–c and 14e. Whereas the absorption spectra of all three classes of compounds are similar,
the fluorescence spectra show distinct differences. Thus, the emission spectra of 14a,b show Stokes shifts
of 4100–6700 cm−1 with low quantum yields both in solution and in the solid state. However, the more
bulky compounds 14c–e show markedly larger molar extinction coefficients and smaller bathochromic
shifts compared to 14a,b. For all compounds, we observe significantly more intense red-shifted fluor-
escence in the solid state compared to that in dichloromethane solutions. For the interpretation of the
absorption properties TD-DFT studies were performed based on DFT geometry optimizations.
Received 5th March 2015,
Accepted 14th April 2015
DOI: 10.1039/c5dt00908a
www.rsc.org/dalton
Introduction
Although 1,3,5-triazapentadienes are known since 1907,1–4
relatively little is known about their coordination chemistry
compared to their carbon analogues β-diimine- and the well-
known β-diketone ligands (Scheme 1).
1,3,5-Triazapentadienes are mostly used as ligands for tran-
sition metal ions,5,6 whereas coordination to main group
elements, such as Al,7 Ga,8 Tl9 and boron, is currently underre-
presented in the literature. Especially boron seems of interest
to us, since boron–nitrogen compounds are known to exhibit
intense fluorescence properties10,11 and are also used in
modern applications like OLEDs.12 In 1962 the first neutral
1 : 1 triazapentadiene–boron complex was synthesized by Milks
et al.13 and, a decade later, Mikhailov et al. started to inten-
sively study the synthesis of similar complexes.14 Since then,
only a small number of research groups including ours15–18
have studied this class of neutral triazapentadiene–boron com-
plexes. Species with such compositions and structural pro-
Scheme 1 (a) Backbone of
carbon analogues. (b) Extension of the monotriazapentadiene (I) to the
benzene-centered bis- (II) and tris-triazapentadiene (III).
a 1,3,5-triazapentadiene ligand and its
perties have not yet received intense attention, with some
aza-boron-dipyridomethenes and aza-boron-diquinomethene
systems showing the closest similarity to our compounds.19 In
most of the studies, the photophysical properties are either
not mentioned or are only of minor interest. This background
encouraged us to look into this matter more closely.
In this article we describe (a) the synthesis and full struc-
tural characterization of a series of novel benzene-centered
mono-, bis- and tris-1,3,5-triazapentadiene ligands and their
di-phenylboron complexes featuring an increasing number of
possible electronic and steric interactions, (b) their photo-
physical properties in solution and in the crystalline state as a
function of the size of the complexes, which are (c) supported
aOrganisch-Chemisches Institut, Westfälische Wilhelms-Universität, Corrensstr. 40,
48149 Münster, Germany. E-mail: wurthwe@uni-muenster.de
bInstitute of Transformative Bio-molecules, and Department of Chemistry, Graduate
School of Science, Nagoya University, Furo, Chikusa, Nagoya 464-8602, Japan
†Dedicated to the memory of Prof. Dr Paul von Ragué Schleyer.
‡Electronic supplementary information (ESI) available: Spectroscopic, crystallo-
graphic and quantum chemical details. CCDC 1011607–1011610. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5dt00908a
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by extensive quantum chemical calculations for a better under-
standing of their electronic structures in the ground state and
in the excited state. We were especially interested to know
whether the accumulation of up to three boron complexes at
the periphery of a benzene moiety exhibits intense electronic
interactions of these photochemically active subgroups.
Results and discussion
Synthesis
Mono-triazapentadiene ligands 6a,b: the synthesis of the
hitherto unknown secondary triazapentadiene ligand 6a,
which bears two hydrogen atoms at the terminal nitrogen
atom, was achieved by following a procedure developed by
Heße et al.,4,5 whereas the novel tertiary triazapentadiene
ligand 6b (with one hydrogen atom at the terminal nitrogen
atom) was synthesized by adapting a procedure of Häger et al.
(Scheme 2).16
Bis- and tris-triazapentadiene ligands 6c–e: the procedure
used for 6b is also the basis for the preparation of the bis-tria-
zapentadienes 6c–d20 and the novel three-armed tris-
triazapentadiene ligand 6e (Scheme 3).
Because of the incomplete reaction and its distinct basic
nature the free ligand 6e is quite difficult to isolate. Therefore
we decided to use the crude product 6e for further reactions
since we know from experience that the triazapentadiene
boron compounds are readily purified by either recrystalliza-
tion or via column chromatography on alumina oxide.15,21
Boron complexes 14a–e: the novel boron complexes derived
from these ligands were synthesized by heating a solution of
the respective triazapentadiene ligands 6a–e with 1 to 3 equi-
valents of triphenylborane in toluene under reflux for 5 hours
(Scheme 4).
Scheme 3 Synthesis of (a) bis- (6c, d) and (b) tris-triazapentadiene
ligands 6e.
After column chromatography on neutral alumina oxide,
boron complexes 14a–e featuring up to seven, thirteen and
nineteen cyclic units in close chemical neighbourhood were
obtained as yellow solids in yields from 47% to 77%. Recrystal-
lization afforded crystalline solids, which have melting points
in the range of 125–205 °C and show excellent thermal and
chemical stability.
tronic effects on the photophysical properties of the triazapenta-
diene boron complexes were the next aims of this study.
X-Ray structures
Single crystal X-ray diffraction analyses were carried out for
14a–c and 14e (Fig. 1; for 14a, see Fig. S4, ESI‡). For all three
types of compounds substantial deviations from planarity are
observed for the heterocyclic units due to the steric properties
of the aryl substituents at the B, N, and C-positions and due to
the sp3-hybridized boron atom (the dihedral angle C–N–B–N
amounts to 43–51°). Furthermore the peripheral aryl substitu-
ents are strongly tilted with respect to the heterocycle (34–54°
for the C-bound aryl groups, 44–64° for the N-bound aryl
groups). Individual differences of the tilt angles may be due to
the nature of the respective substitution patterns as well as
due to crystal packing effects in the solid state. In general, our
new boron compounds 14a–c and 14e may be described more
or less as propeller shaped molecules, which was postulated to
be a prerequisite to display the AIE effect in the solid state (see
below).22
With these boron compounds in hand, a closer look into
the structural properties and the influence of steric and elec-
Scheme 2 Synthetic pathways for secondary (6a) and tertiary (6b)
mono-triazapentadiene ligands.
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diene boron unit, we attempted to decrease the conformation-
al freedom of rotations of the ancillary substituents, thus
making the possibility for radiative decay from the excited
state dominant, which would result in an enhanced emission
compared to the mono triazapentadiene boron compounds
14a,b. These additional triazapentadiene boron units of com-
pounds 14c–e led in solution to relatively small bathochromic
shifts but a strong increase of the molar extinction coefficients
ε of the absorption (14c,d: λabs = 365 nm, ε365 nm = 19 000 M−1
cm−1 L; 14e: λabs = 363 nm, ε363
= 26 700 M−1 cm−1 L;
nm
Table 1), which goes in line with the higher number of
chromophores per volume in the bis- and tris-triazapentadiene
boron complexes compared to the mono-compounds, resulting
in more electronic transitions in the UV/vis-spectra. Similarly,
the fluorescence maxima (14c: λem,max = 542 nm; 14d: λem,max
=
539 nm; 14e: λem,max = 517 nm) are red-shifted compared to
the mono triazapentadiene boron complexes 14a,b (Fig. 3,
Table 1). The Stokes shifts of these three compounds 14c–e are
significantly larger compared to those of 14a,b. However, there
is no enhancement of the absolute quantum yields, which are
still below one percent (ΦF < 0.01). Thus, neither the differing
substituent effects (14b, 14d) nor the enhanced steric bulk
(14e) result in a strong increase of the fluorescence of the com-
plexes 14 in solution.
In contrast to the fluorescence of the solutions, the fluo-
rescence of the crystalline materials 14a–e is clearly visible to
the naked eye upon irradiating the sample with a handheld UV-
lamp as shown exemplarily in Fig. 4 for complexes 14a,c and e.
The solid state emissions of the mono triazapentadiene
Scheme 4 Synthesis of the triazapentadiene–boron complexes 14a–e.
Photophysical properties
Dichloromethane solutions of the boron complexes 14a,b boron complexes 14a and 14b are detected at λem = 600 nm
absorb light between λabs = 250–450 nm with 14a having its and λem = 590 nm, respectively. The bis-triazapentadiene
maximum at λabs,max = 263 nm and 14b at λabs,max = 307 nm. In boron complexes 14c,d and the tris-triazapentadiene boron
both cases the longest absorption maximum occurs at λabs
353 nm in the form of a broad shoulder with molar extinction state at the same wavelength (λem = 560 nm). Thus, all the
coefficients of ε353 nm = 1000 M−1 cm−1 L for 14a and ε353 nm
emissions of the crystalline compounds are significantly red-
=
compound 14e show their emission maximum in the solid
=
1300 M−1 cm−1 L for 14b. They also exhibit blue emission in shifted. While, as stated above, the quantum yields for 14a–e
the range of λem = 413–487 nm (Fig. 2, Table 1). In each case in solution are below one percent, the absolute quantum
three bands are observed for 14a at λem = 413 nm, 453 nm yields in the crystalline state are at ΦF = 0.02 for 14a, ΦF = 0.03
(fluorescence maximum) and 479 nm (very broad), for 14b at for 14b and 14c, ΦF = 0.04 for 14d and ΦF = 0.02 for 14e
λem = 414 nm (fluorescence maximum), 446 nm and 487 nm (Table 1).
(very broad). This leads to Stokes shifts of Δν = 4100 cm−1
This interesting behaviour of compounds 14a–e indicates
(14a) and Δν = 4200 cm−1 (14b). The absolute quantum different emission properties in solution compared to those in
yields of the emissions are below one percent (ΦF < 0.01). the solid state. The formation of excimers and the aggregation
This is consistent with the observation that the emission of induced emission effect (AIE), as first described in 2001 by the
14a and 14b cannot be detected by the naked eye as shown in research group of B. Z. Tang,22 are possible explanations for
Fig. 2.
these phenomena, which have to be studied in more detail
Interestingly, introducing an electron-donating group such before a conclusive interpretation can be made. Certainly,
as the p-methoxyphenyl group at the triazapentadiene unit steric bulk and therefore hindered rotation of the aryl substitu-
(14b) has only a minor influence on the photophysical pro- ents are possible reasons which have to be considered.23
perties. As seen from the UV-spectra, the exchange of a single
To demonstrate the different emission properties in solu-
substituent at one of the many aryl groups has only a very little tion and in the solid state by an additional qualitative experi-
effect on the spectroscopic properties. The background for ment, compound 14a was dissolved in acetonitrile and water
choosing different substitution patterns is mainly to demon- was added successively (Fig. 5).24 As stated above, the aceto-
strate the synthetic versatility of our experimental method.
nitrile solution of 14a is not emitting light visible to the naked
By increasing the steric bulk of this class of compounds by eye upon UV irradiation, but after addition of water 14a
introducing a second (14c,d) and a third (14e) triazapenta- precipitates due to its poor solubility in water and starts to
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Fig. 1 ORTEP plots (thermal ellipsoids with 50% probability) of the molecular structures of 14b, c and e. For detailed structural data for compounds
14a–c, and 14e see the ESI.‡
emit yellow light. Worth noting is the considerable red-shift in zations of 14a–c and 14e at the B3LYP25/def2-TZVP level using
the photoluminescence spectra of 14a from λem = 453 nm for a the GAUSSIAN 09 package of programs26 for the gas phase
solution in pure acetonitrile to λem = 550 nm for a suspension without consideration of solvent effects were performed using
in a 1 : 1 acetonitrile/water mixture, which is possibly due to the X-ray data as starting geometries. Subsequently, to study
aggregation resulting in the formation of excimers. One the light absorption of these molecules, excited singlet states
should keep in mind that these spectra are to be treated quali- and the respective transitions were evaluated using TD-DFT
tatively, since scatter light or other detection errors caused by calculations considering each of 20 states (Gaussian 09: td =
particles are likely to occur.
(root = 1, nstates = 20); see the ESI‡).27 The calculated and
experimental UV/vis spectra including the oscillator strengths
of the excited singlet states and the relevant Kohn–Sham mole-
cular orbitals of 14a, 14c and 14e are shown in Fig. 6–9.
Additional data are given in the ESI.‡
Quantum chemical-DFT calculations
To investigate the electronic structures of the mono-, bis- and
tris-triazapentadiene boron compounds, DFT geometry optimi-
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Fig. 2 (a) Photograph of compound 14a in solution and (b) the normalized absorption and emission spectra of 14a in CH2Cl2. (c) Normalized
absorption and emission spectra of 14b in CH2Cl2 and (d) the photograph of compound 14b in solution. Irradiation was performed with a UV lamp
with a wavelength of 366 nm.
Table 1 Photophysical data of the boron compounds 14a–e
εmaxb/M−1 cm−1
L
λemc/nm
Δνd/cm−1
ΦF
e
Compound
λabsa/nm
14a
14b
14c
14d
14e
353
353
365
365
363
1000
1300
19 000
19 000
26 700
453/600
414/590
542/560
539/560
517/560
4100
4200
6100
6100
6700
<0.01/0.02
<0.01/0.03
<0.01/0.03
<0.01/0.04
<0.01/0.02
a Longest wavelength absorption maxima recorded in dichloromethane using concentrations of about 10−5 M. b Molar extinction coefficient at
the longest-wavelength absorption maximum. c Fluorescence maxima. Wavelengths are given for solution/solid. d Stokes shift in cm−1 as
calculated from the longest wavelength absorption λabs and the shortest wavelength emission λem data (compare text and Fig. 2 and 3). e Absolute
fluorescence quantum yield determined by a calibrated integration sphere system. Quantum yields are given for the solution/solid state.
Fig. 3 (a) Absorption spectra of 14c, d and (b) their normalized emission spectra. (c) Absorption spectrum of tris-triazapentadiene boron complex
14e and (d) its normalized emission spectrum (10−5 M solutions in CH2Cl2).
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Fig. 4 Photographs of crystalline 14a, 14c and 14e under (a) daylight and (b) in a dark room under UV irradiation. Irradiation was performed with a
UV lamp with a wavelength of 366 nm. (c) Normalized emission spectra of 14a, 14c, and 14e in the solid state.
Fig. 5 Images of an acetonitrile/water (1 : 0–1 : 1) solution of 14a during UV irradiation (left). Normalized photoluminescence spectrum of 14a in a
1 : 0 (CH3CN/H2O) solution and a 1 : 1 (CH3CN/H2O) suspension (right). Irradiation was performed with a UV lamp with a wavelength of 366 nm.
For the mono-1,3,5-triazapentadiene–boron complex 14a, the terminal substituents and the electron pairs at the nitro-
the calculated UV/vis absorption spectrum matches quite well gen atoms of the triazapentadiene backbone (see the ESI‡)
with the experimental one (Fig. 6). The absorption band at the into the LUMO which is concentrated at the triazapentadiene
longer wavelength, λabs,calc = 343 nm (λabs,exp = 353 nm), is boron chelate.
described by the transitions involving S1, S2, S3 and S4 from
the occupied molecular orbitals located mostly at the aryl sub- experimentally observed absorption band at λabs,exp = 359 nm
stituents of the boron atom (HOMO−1, HOMO−2, HOMO−3) is reproduced by the calculations with a value of λabs,calc
For the bis-1,3,5-triazapentadiene–boron complex 14c the
=
and at the nitrogen lone pairs as well as at the substituents of 365 nm (Fig. 7). We assign it mainly to π–π* transitions invol-
the triazapentadiene backbone (HOMO, HOMO−4) to the ving the states S1 (HOMO−1 → LUMO, HOMO → LUMO,
LUMO. Thus, the photochemistry of this system might be HOMO → LUMO+1), S2 (HOMO → LUMO; HOMO−1 →
traced back to intramolecular charge transfer processes (ICT). LUMO) and S5 (HOMO−3 → LUMO; (HOMO−4 → LUMO) (see
The most intense absorption band at λabs,calc = 273 nm (λabs,exp the ESI‡).
= 263 nm) is predominantly described by the π–π* transitions
The absorption spectrum of the tris-1,3,5-triazapentadiene–
involving the states S7, S9 and S13, which can exclusively be boron complex 14e (Fig. 8) is qualitatively also similar to the
traced back to excitations from occupied molecular orbitals at spectra of 14a and 14c (Fig. 7). The calculated and the experi-
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Fig. 6 The experimental (black, y1-axis) and calculated (blue) UV/VIS spectra of 14a with the associated oscillator strength (y2-axis) (left). Tran-
sitions involving the excited states S1, S2, S3 and S4 and the relevant Kohn–Sham orbitals (right). For more detailed information see the ESI.‡
Fig. 7 The experimental (black, y1-axis) and calculated (blue) UV/VIS spectra of 14c with the associated oscillator strength (y2-axis) (upper left).
Kohn–Sham molecular orbitals of 14c. The degree of degeneracy of the energy levels is clearly visible within the HOMO/HOMO−1, HOMO−2/
HOMO−3 and HOMO−4/HOMO−5 pairs. For more detailed information, see the ESI.‡
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pied orbitals at the peripheral aryl substituents of the triaza-
pentadiene boron units (e.g. HOMO−3, HOMO−4, HOMO−5)
and the LUMO, LUMO+1 and LUMO+2 located at the three
triazapentadiene–boron cores of the molecule (Fig. 9) (see the
ESI‡ for additional Kohn–Sham molecular orbitals and calcu-
lation details). Not unexpectedly we noticed that the calculated
gas phase absorption features of 14c and 14e do not match
well with the experimental results at short wavelength when
compared to 14a and 14b. Certainly, steric bulk and conse-
quently more possible conformations and the lack of the
solvent in the calculations are reasons for this observation.
As the experimental and calculated data clearly indicate,
the mono-, the bis- and the tris-triazapentadiene boron com-
pounds behave unexpectedly similarly with respect to their
light absorption properties. This is indicated by the spectra
and the orbital pictures and also by the similar HOMO/LUMO
gaps which vary only in the range of ΔEHOMO–LUMO = 3.75–3.93
eV. Thus, due to the heavy steric hindrance, all three systems
exhibit only limited delocalization of electrons throughout the
molecule. In contrast, as shown above, the fluorescence pro-
perties (emission maxima and Stokes shifts) of these three
types of boron compounds differ significantly, especially in
the solid state. This indicates the importance of steric pro-
perties caused by intra- and intermolecular interactions.
Fig. 8 The experimental UV/VIS spectrum (black, y1-axis) and the cal-
culated excitation wavelengths of states S1–S20 for 14e with the associ-
ated oscillator strength (y2-axis).
mental spectra show their longest wavelength absorption at
λabs,exp = 363 nm and λabs,calc = 345 nm, which is represented
by calculated transitions involving the states S1 (λabs,calc
=
378 nm), S2 (λabs,calc = 362 nm), S3 (λabs,calc = 361 nm) and S5
(λabs,calc = 348 nm) involving usual π–π* excitations from the
three distinctly different orbitals HOMO, HOMO−1 and
HOMO−2 located at the individual triazapentadiene subunits
into their LUMO counterparts. The latter are characterized by
participation of one (LUMO), two (LUMO+1, LUMO+3,
LUMO+4) and three (LUMO+2) triazapentadiene boron units,
Experimental section
General procedures
which are electronically separated from each other. The Compounds 1–3, 5 and 7 are commercially available. The bis-
absorption band at lower wavelength at λabs,exp = 298 nm is triazapentadiene ligands 6c and 6d including their precursors
only partially represented by the accumulation of transitions (11c–13c, 11d–13d) were synthesized following a literature pro-
involving the states S8–S20 (λabs,calc = 335 nm) concerning occu- cedure.20 Melting points are uncorrected. Nuclear magnetic
Fig. 9 Kohn–Sham molecular orbital pictures of compound 14e.
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resonance spectroscopy (1H, 13C, 11B, 19F) was performed sulfate. Removing the solvent in vacuo gave compound 6a as a
on superconductive spectrometers: 300 MHz (1H: 300.1 MHz, yellow solid in 30% (3.38 g) yield: mp 118 °C. 1H NMR
13C: 75.5 MHz, 19F: 282.4 MHz, 11B: 96.3 MHz), 400 MHz (300 MHz; CDCl3): δ 7.93 (d, 2H, ArH), 7.67 (m, 2H, ArH), 7.43
(1H: 400.1 MHz, 13C: 100.6 MHz), 500 MHz (1H: 499.9 MHz, (m, 3H, ArH), 7.23 (d, 2H, ArH), 6.98 (m, 4H ArH), 4.83 (br, 2H,
13C: 125.7 MHz, 19F: 470.3 MHz, 11B: 160.4 MHz) and NH2), 2.40 (s, 3H, –CH3), 2.27 (s, 3H, –CH3) ppm. 13C NMR
600 MHz (1H: 599.8 MHz, 13C: 150.8 MHz, 19F: 564.4 MHz, 11B: (75 MHz; CDCl3): δ 161.0, 152.6 (CvN), 147.7, 141.1, 134.4
192.4 MHz). 1H NMR and 13C NMR: chemical shifts δ are given (Cq), 132.5, 131.1, 129.6, 129.2, 128.8, 128.6, 128.1, 126.9,
relative to TMS and referenced to the solvent signal. 19F NMR: 121.4 (CAr), 21.3, 20.7 (CH3) ppm. Anal. Calcd for C22H21N3:
chemical shifts δ are given relative to CFCl3 (external refer- C, 80.71; H, 6.46; N, 12.83. Found: C, 80.68; H, 6.40; N, 12.88.
ence), 11B NMR: chemical shifts δ are given relative to BF3·OEt2
Synthesis of ligand 6b. A solution of 2,4,6-trimethylaniline
(external reference). All signals in the 1H NMR and 13C spectra (1.1 mL, 7.8 mmol, 1.0 equiv.) in dry tetrahydrofurane (15 mL)
were assigned on the basis of relative intensities, coupling con- was cooled down to −78 °C and an equimolar amount of
stants, and GCOSY, GHSQC, GHMBC, 1D-COSY and 1D-NOE n-butyl lithium was added dropwise. After the reaction mixture
experiments. Mass spectra were recorded on a micrOTOF was stirred for 10 min, imidate 9 (3.1 g, 7.8 mmol, 1.0 equiv.)
using electron spray ionization. UV/VIS absorption spectra was slowly added and stirred overnight at room temperature.
were recorded with a resolution of 0.2 nm to 0.5 nm. If not The organic layer was washed with saturated sodium chloride
stated otherwise, dilute solutions with concentrations about solution (3 × 20 mL) before being dried with magnesium
10−5 M in a 1 cm square quartz cell in degassed dichloro- sulfate. The solvent was removed in vacuo and the crude
methane were used. Fluorescence lifetimes were measured product was recrystallized from a pentane/tetrahydrofurane to
with a Picosecond Fluorescence Measurement System equipped obtain 6b as yellow crystals in 25% (900 mg) yield: mp 176 °C.
with a picosecond light pulser (excitation wavelength 377 nm 1H NMR (400 MHz; C6D6): δ 7.52–7.42 (m, 2H, ArH), 7.05–7.02
with a repetition rate of 10 Hz). Absolute fluorescence (m, 4H, ArH), 6.92–6.80 (m, 4H, ArH), 6.74–6.66 (m, 4H, ArH),
quantum yields were determined with a calibrated integrating 6.15 (br, 1H, NH), 3.82 (s, 3H, –OCH3), 2.32 (s, 3H, –CH3), 2.30
sphere system. X-Ray diffraction (for more information please (s, 3H, –CH3), 2.29 (s, 3H, –CH3), 2.21 (s, 6H, Mes–CH3, ortho
)
see the ESI‡): Data sets were collected with a Nonius Kap- ppm. 13C NMR (100 MHz; C6D6): δ 161.4, 159.3 (CvN), 144.2,
paCCD diffractometer or a Rigaku CCD diffractometer with a 142.1, 140.5, 136.8 (Cq), 136.6 (CAr), 135.3, 134.7, 134.5,
rotating anode generator. The programs used: data collection, 133.0 (Cq), 132.9, 131.0, 130.1, 129.4, 129.3, 129.1, 128.9,
COLLECT (R. W. W. Hooft, Bruker AXS, 2008, Delft, The Neth- 127.3, 126.9, 113.6 (CAr), 55.5 (–OCH3), 21.6, 21.1, 21.0, 20.8
erlands) and CrystalClear (Rigaku Corp., 2000); data reduction (CH3) ppm. HRMS (ESI): calcd for C32H34N3O ([M + H]+)
Denzo-SMN (Z. Otwinowski, W. Minor, Methods Enzymol., 476.2696, observed 476.2695.
1997, 276, 307–326); absorption correction, Denzo (Z. Otwinowski,
Synthesis of ligand 6e. A solution of p-toluidine (2.73 g,
D. Borek, W. Majewski, W. Minor, Acta Crystallogr., Sect. A: 25.5 mmol, 7.5 equiv.) in dry tetrahydrofurane was cooled
Fundam. Crystallogr., 2003, 59, 228–234); structure solution down to −78 °C and n-butyl lithium (15.93 mL, 1.6 M,
SHELXS-97 (G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. 25.5 mmol, 7.5 equiv.) was added dropwise. After stirring the
Crystallogr., 1990, 46, 467–473); structure refinement solution for 30 min, a solution of compound 13e (3.10 g,
SHELXL-97 (G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. 3.4 mmol, 1.0 equiv.) in dry tetrahydrofurane was added
Crystallogr., 2008, 64, 112–122) and graphics, XP (Bruker slowly. After removal of the acetone/CO2-bath, the reaction
AXS, 2000). R-values are given for the observed reflections, mixture was stirred overnight and quenched with water after-
and wR2 values are given for all reflections. Supplementary wards. The crude product was filtered through deactivated
crystallographic data for 14a, 14b, 14c and 14e CCDC silica (80% cyclohexane, 20% ethyl acetate, Rf = 0.19) and
1011607–1011610 are given in the ESI.‡
directly used for further reactions after confirmation of the
mass via HRMS. HRMS (ESI): calcd for C75H70N9 ([M + H]+)
1096.5749, observed 1096.5735.
Synthesis of the starting materials and the mono- and
tris-triazapentadiene ligands
Synthesis of N-imidoylimidate 9. Ethyl 4-methoxybenz-
Synthesis of ligand 6a. A solution of imidoyl chloride 428 imidate hydrochloride29 (5.6 g, 30.0 mmol, 1.0 equiv.) was sus-
(8.44 g, 34.6 mmol, 1.0 equiv.) in diethyl ether (40 mL) was pended in dry dichloromethane (20 mL) and triethylamine
added dropwise to a suspension of benzamidine (4.16 g, (2.8 mL, 20.0 mmol, 2.0 equiv.) was added dropwise. After stir-
34.6 mmol, 1.0 equiv.) in diethyl ether (20 mL). The reaction ring for 1 h the reaction vessel was cooled down to 0 °C and
mixture was stirred for 2 h at room temperature and for 1 h imidoyl chloride 428 (7.3 g, 30.0 mmol, 1.0 equiv.) was added
under refluxing conditions. The resulting precipitate was fil- slowly. After the reaction mixture was stirred for 20 h, the pre-
tered off and sonicated in hot water to remove benzamidine cipitate was filtered off and washed several times with warm
hydrochloride. The residual triazapentadiene hydrochloride diethyl ether. The filtrate was evaporated and the crude
was separated and converted into 6a by adding a sodium product was purified by column chromatography on silica
hydroxide solution (2 N, 50 mL). The solution was extracted yielding 9 as yellow oil which was slowly crystallizing, giving 9
1
with dichloromethane several times until no solid remains. in 56% yield as a yellow solid: mp 85 °C. H NMR (300 MHz;
The combined organic layers were dried with magnesium CDCl3): δ 7.96–7.87 (m, 2H, ArH), 7.40–7.32 (m, 2H, ArH),
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7.23–7.16 (m, 2H, ArH), 6.96–6.87 (m, 2H, ArH), 6.75–6.67 Hz, 9H, –O–CH2CH3) ppm. 13C NMR (100 MHz; CD2Cl2):
(m, 2H, ArH), 6.62–6.54 (m, 2H, ArH), 4.26 (q, JHH = 7.0 Hz, 2H, δ 158.1, 157.5 (CvN), 147.6, 142.2, 137.4, 132.7 (Cq), 129.4,
H3CCH2O–), 3.74 (s, 3H, –OCH3), 2.38 (s, 3H, –CH3), 2.24 129.3, 128.3, 122.1 (CAr), 63.7 (–O–CH2CH3), 21.7, 21.1 (–CH3),
(s, 3H, –CH3), 1.31 (t, JHH = 7.1 Hz, 3H, H3CCH2O–) ppm. 14.6 (–O–CH2CH3) ppm. HRMS (ESI): calcd for C60H61N6O3
13C NMR (75 MHz; CDCl3):
δ
161.7 (–O–CvN), 158.5 ([M + H]+) 913.4800, observed 913.4806.
(vN–CvN), 156.6 (Cq–OCH3), 147.5, 140.7, 133.7, 131.8,
130.0 (Cq), 129.2, 128.9, 128.1, 124.0, 121.7, 113.7 (CAr),
63.0 (H3CCH2O–), 55.4 (–OCH3), 21.6, 21.1 (–CH3), 14.5
(H3CCH2O–) ppm. HRMS (ESI): calcd for C24H27N2O2
([M + H]+) 387.2067, observed 387.2067.
Synthesis of boron compounds 14a–14e based on mono-,
bis- and tris-triazapentadiene ligands 6
Synthesis of boron complex 14a. A Schlenk flask connected
to a reflux condenser under an argon atmosphere was charged
Synthesis of amide 11e. This compound was prepared by with triphenylborane (307 mg, 1.5 mmol, 1.0 equiv.) and tri-
modification of a literature procedure.30,31 A suspension of azapentadiene 6a (488 mg, 1.5 mmol, 1.0 equiv.). After adding
p-toluidine (16.1 g, 150.0 mmol, 5.0 equiv.) in dry toluene dry toluene (20 mL) to the solids the solution was refluxed for
(200 mL) and triethylamine (12.6 mL, 90.0 mmol, 3.0 equiv.) 3 h. After cooling down the reaction mixture the solvent was
was cooled to 0 °C. Then a solution of trimesoyl chloride removed in vacuo and the crude product was subjected to
(8.0 g, 30.0 mmol, 1.0 equiv.) in toluene (100 mL) was added column chromatography on neutral alumina with 99% cyclo-
dropwise over a period of 1 h. The solution was then stirred hexane and 1% ethyl acetate (Rf = 0.29). Title compound 14a
vigorously for 12 h before the solvent was evaporated in vacuo. was obtained as a yellow crystalline solid in 45% (322 mg)
The residue was dissolved in dichloromethane and washed yield: mp 205 °C. 1H NMR (300 MHz; C6D6): δ 7.92–7.85 (m,
with water (3 × 100 mL). Finally the organic layer was dried 4H, ArH), 7.74–7.65 (m, 4H, ArH), 7.42–7.33 (m, 4H, ArH),
over MgSO4 and the solvent was removed to give 11e as a color- 7.29–7.20 (m, 2H, ArH), 7.12–7.02 (m, br, 2H, NH, ArHH),
less solid in 95% (13.5 g) yield: mp 299 °C (297–298 °C31). 7.01–6.93 (m, 2H, ArH), 6.89–6.83 (m, 2H, ArH), 6.81–6.72
1H NMR (300 MHz; C2D6SO): δ 10.53 (s, 3H, NH), 8.69 (s, 3H, (m, 2H, ArH), 6.48–6.42 (m, 2H, ArH), 1.85 (s, 3H, –CH3), 1.70
ArH), 7.72 (d, JHH = 8.4 Hz, 6H, ArH), 7.20 (d, JHH = 8.3 Hz, 6H, (s, 3H, –CH3) ppm. 13C NMR (75 MHz; C6D6): δ 168.4, 163.3
ArH), 2.30 (s, 9H, –CH3) ppm. 13C NMR (75 MHz; C2D6SO): (CvN), 143.1, 140.3, 135.9, 135.7 (Cq), 134.9 (CAr), 134.6 (Cq),
δ 164.4 (CvO), 136.5, 135.6, 133.0 (Cq), 129.7, 129.2, 120.4 133.1, 132.3, 131.3, 129.2, 129.1, 129.0, 127.0 (CAr), 21.5, 21.0
(CAr), 20.6 (–CH3) ppm. HRMS (ESI): calcd for C30H27N3O3Na (–CH3) ppm. 11B NMR (96 MHz; C6D6): δ 0.4 (s, Bquart, ν1/2
([M + Na]+) 500.1945, observed 500.1945. 910 Hz) ppm HRMS (ESI): calcd for C34H31N3B ([M + H]+)
∼
Synthesis of imidoyl chloride 12e. Amide 11e (15.4 g, 492.2611, observed 492.2603. UV/VIS (dichloromethane): λabs
32.3 mmol, 1.0 equiv.) was added in several portions to thionyl (shape, ˜ν, ε) = 353 nm (sh, 28 329 cm−1, 1000 L mol−1 cm−1),
chloride (70.1 mL, 967.5 mmol, 30.0 equiv.) while stirring. The 263 nm (p, 38 023 cm−1, 4400 L mol−1 cm−1). Fluorescence
brown solution was then stirred for 3 h under refluxing con- (dichloromethane): λem (shape, ˜ν) = 479 nm (p, 20 876 cm−1),
ditions and the resulting SO2 and HCl were removed under 453 nm (p, 22 075 cm−1), 412 nm (p, 24 272 cm−1). Absolute
reduced pressure. The yellowish solid was recrystallized from a quantum yield (dichloromethane): ΦF = <1%.
tertiary butyl methyl ether/dichloromethane 1 : 1 mixture yield-
Synthesis of boron complex 14b. A Schlenk flask connected
ing 12e as a yellow crystalline compound in 71% (12.2 g) yield: to a reflux condenser under an argon atmosphere was charged
mp 133 °C. 1H NMR (300 MHz; CD2Cl2): δ 9.08 (s, 3H, ArH), with triphenylborane (207 mg, 0.9 mmol, 1.0 equiv.) and tri-
7.36–7.13 (m, 6H, ArH), 7.12–6.93 (m, 6H, ArH), 2.40 (s, 9H, azapentadiene 6b (404 mg, 0.9 mmol, 1.0 equiv.). After adding
–CH3) ppm. 13C NMR (75 MHz; CD2Cl2): δ 145.0 (–(Cl–)CvN), dry toluene (20 mL) to the solids the solution was refluxed for
141.2, 137.2, 136.2 (Cq), 133.3, 130.1, 121.1 (CAr), 21.3 (–CH3) 3 h. After cooling down the reaction mixture the solvent was
ppm.
removed in vacuo and the crude product was subjected to
Synthesis of N-imidoylimidate 13e. Ethyl 4-methyl- column chromatography on neutral alumina with 95% cyclo-
benzimidate hydrochloride 832 (9.1 g, 45.6 mmol, 3.0 equiv.) hexane and 5% ethyl acetate (Rf = 0.22). Title compound 14b
was suspended in dry dichloromethane (20 mL) and triethyl- was obtained as a yellow crystalline solid in 47% (280 mg)
amine (12.7 mL, 91.2 mmol, 6.0 equiv.) was added dropwise. yield: mp 125 °C. 1H NMR (300 MHz; C6D6): δ 8.26–8.18
At −15 °C, imidoyl chloride 12e (8.1 g, 15.2 mmol, 1.0 equiv.) (m, 2H, ArH), 7.91–7.83 (m, 4H, ArH), 7.76–7.68 (m, 2H, ArH),
was slowly added to the solution. After the reaction mixture 7.23–7.17 (m, 5H, ArH), 7.15–7.04 (m, 3H, ArH), 6.73–6.67
was stirred for 20 h, the precipitate was filtered off and washed (m, 2H, ArH), 6.64–6.53 (m, 4H, ArH), 6.35 (s, 2H, ArH), 3.06
several times with warm diethyl ether. The filtrate was evapor- (s, 3H, –OCH3), 2.33 (s, 6H, (–CH3)2), 1.81 (s, 6H, (–CH3)2), 1.80
ated and the crude product was purified by column chromato- (s, 3H, –CH3) ppm. 13C NMR (75 MHz; C6D6): δ 168.5, 167.8
graphy on deactivated silica with 90% cyclohexane and 10% (CvN), 162.7, 143.4, 142.8, 140.7 (Cq), 136.4 (CAr), 136.2, 136.0,
ethyl acetate (Rf = 0.20) yielding 13e as a yellow oil in 22% 135.6, 135.3 (Cq), 133.5, 131.8, 130.1, 129.5, 129.1, 129.2,
1
(3.1 g) yield: H NMR (400 MHz; CD2Cl2): δ 8.64 (s, 3H, ArH), 127.0, 126.6, 114.0 (CAr), 55.0 (–OCH3), 21.5, 21.4, 21.1, 21.0
7.22–7.16 (m, 6H, ArH), 7.02–6.94 (m, 12H, ArH), 6.68–6.61 (–CH3) ppm. 11B NMR (96 MHz; C6D6): δ 2.9 (s, Bquart, ν1/2
∼
(m, 6H, ArH), 4.32 (dq, JHH = 11.8, 7.1 Hz, 6H, –O–CH2CH3), 960 Hz) ppm. HRMS (ESI): calcd for C44H43N3OB ([M + H]+)
2.29 (s, 9H, –CH3), 2.27 (s, 9H, –CH3), 1.36 (dt, JHH = 8.8, 7.1 640.3501, observed 640.3507. UV/VIS (dichloromethane): λabs
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(shape, ˜ν, ε) = 353 nm (sh, 28 329 cm−1, 1300 L mol−1 cm−1), (B–CAr), 133.6, 132.0 (CAr,centralBenzene), 131.7 (Cq–Cl),
307 nm (p, 35 573 cm−1, 1800 L mol−1 cm−1). Fluorescence 130.9, 130.7, 130.3, 129.0, 128.7, 128.5, 128.2 (CAr), 127.5
(dichloromethane): λem (shape, ˜ν) = 494 nm (sh, 20 243 cm−1), (CAr,centralBenzene), 127.4, 126.6 (B–CAr), 21.2 (–CH3) ppm.
447 nm (p, 22 371 cm−1), 414 nm (p, 24 155 cm−1). Absolute 11B NMR (192 MHz, CD2Cl2): δ 2.6 (s, Bquart, ν1/2 ∼ 600 Hz)
quantum yield (dichloromethane): ΦF
=
<1%. Absolute ppm. HRMS (ESI): calcd for C72H57B2Cl2N6 ([M
+
H]+)
quantum yield (crystalline): ΦF = 3%.
1097.4224, observed 1097.4219. UV/VIS (dichloromethane):
Synthesis of boron complex 14c. A flask under an inert λabs (shape, ˜ν, ε) = 365 nm (p, 27 397 cm−1, 19 000 L mol−1
atmosphere was charged with triphenylborane (412 mg, cm−1), 297 nm (sh, 33 670 cm−1, 22 800 L mol−1 cm−1).
1.70 mmol, 2.0 equiv.) and bis-triazapentadiene 6c20 (619 mg, Fluorescence (dichloromethane): λem (shape, ˜ν) = 538 nm
0.85 mmol, 1.0 equiv.). After adding dry toluene, the reaction (p, 18 587 cm−1), 469 nm (p, 21 322 cm−1). Absolute quantum
mixture was heated under refluxing conditions for 4 h. After yield (dichloromethane): ΦF = <1%. Absolute quantum yield
that time, the solvent was removed under reduced pressure (crystalline): ΦF = 4%.
and the crude product was subjected to column chromato-
Synthesis of boron complex 14e. A flask under an inert
graphy on neutral alumina with 90% cyclohexane and 10% atmosphere was charged with triphenylborane (460 mg,
ethyl acetate (Rf = 0.44) to furnish the bis-triazapentadiene 1.86 mmol, 3.0 equiv.) and tris-triazapentadiene 6e (690 mg,
boron compound 14c in 54% (485 mg) yield: mp 187 °C. 0.63 mmol, 1.0 equiv.). After adding dry toluene, the reaction
1H NMR (600 MHz, CD2Cl2): δ 8.00 (t, JHH = 1.9 Hz, 1H, mixture was heated under refluxing conditions for 4 h. After
–NvCq–Cq–CH–Cq), 7.63–7.59 (m, 4H, ArH), 7.55–7.50 (m, 8H, that time, the solvent was removed under reduced pressure
B–ArH), 7.41 (dd, JHH = 7.8 Hz, JHH = 1.9 Hz, 2H, –NvCq–Cq– and the crude product was subjected to column chromato-
CH–CH–CH–Cq), 7.35–7.31 (m, 2H, ArH), 7.29–7.23 (m, 4H, graphy on neutral alumina with 90% cyclohexane and 10%
ArH), 7.20–7.10 (m, 12H, B–ArH), 6.99 (t, JHH = 7.8 Hz, 1H, ethyl acetate (Rf = 0.26) to furnish the tris-triazapentadiene
–NvCq–Cq–CH–CH–CH–Cq), 6.69 (ddt, JHH = 8.9, 7.4, 0.8 Hz, boron compound 14e as a yellow solid in 77% (768 mg)
1
8H, ArH), 6.65–6.60 (m, 4H, ArH), 6.51–6.46 (m, 4H, ArH), 2.13 yield: mp 191 °C. H NMR (600 MHz, CD2Cl2): δ 7.44 (s, 3H,
(s, 6H, –CH3), 2.11 (s, 6H, –CH3) ppm. 13C NMR (150 MHz, ArHcentralBenzene), 7.43–7.41 (m, 12H, B–ArH), 7.41–7.39 (m, 6H,
CD2Cl2): δ 166.9, 165.9 (CvN), 147.9 (B–Cq), 142.1, 142.0, ArH), 7.13–7.09 (m, 18H, B–ArH), 7.09–7.05 (m, 6H, ArH),
137.5, 136.9 (Cq), 135.9, 135.8 (Cq–CH3), 135.3 (B–CAr), 133.4, 6.67–6.60 (m, 12H, ArH), 6.56–6.49 (m, 6H, ArH), 6.05 (d, JHH
=
131.7 (CAr,centralBenzene), 130.7, 130.1, 128.8, 128.7, 128.5, 128.4, 8.0 Hz, 6H, ArH), 2.23 (s, 9H, –CH3), 2.11 (s, 9H, –CH3), 2.08
128.1 (CAr), 127.2 (CAr,centralBenzene), 127.0, 126.1 (B–CAr), 21.0, (s, 9H, –CH3) ppm. 13C NMR (150 MHz, CD2Cl2): δ 166.9,
20.9 (–CH3) ppm. 11B NMR (192 MHz, CD2Cl2): δ 2.4 (s, Bquart
,
165.2 (CvN), 147.7 (B–Cq), 142.4, 141.4, 140.8 (Cq), 136.1
ν1/2 ∼ 740 Hz) ppm. HRMS (ESI): calcd for C74H63B2N6 (Cq,centralBenzene), 135.9, 135.7 (Cq–CH3), 135.4 (B–CAr), 134.5
([M + H]+) 1057.5316, observed 1057.5307. UV/VIS (dichloro- (Cq–CH3), 133.3 (CAr,centralBenzene), 130.8, 129.2, 128.7, 128.4,
methane): λabs (shape, ˜ν, ε) = 365 nm (p, 27 397 cm−1, 19 000 L 128.2 (CAr), 127.0, 126.1 (B–CAr), 21.6, 21.1, 20.9 (–CH3) ppm.
mol−1 cm−1), 297 nm (sh, 33 670 cm−1, 20 700 L mol−1 cm−1). 11B NMR (192 MHz, CD2Cl2): δ −2.8 (s br, Bquart, ν1/2
∼
Fluorescence (dichloromethane): λem (shape, ˜ν) = 538 nm 3000 Hz) ppm. HRMS (ESI): calcd for C111H97B3N9 ([M + H]+)
(p, 18 587 cm−1), 469 nm (p, 21 322 cm−1). Absolute quantum 1588.8151, observed 1588.8167. UV/VIS (dichloromethane):
yield (dichloromethane): ΦF = <1%. Absolute quantum yield λabs (shape, ˜ν, ε) = 363 nm (p, 27 548 cm−1, 26 700 L mol−1
(crystalline): ΦF = 3%.
cm−1), 298 nm (p, 33 557 cm−1, 33 400 L mol−1 cm−1).
Synthesis of boron complex 14d. A flask under an inert Fluorescence (dichloromethane): λem (shape, ˜ν) = 517 nm
atmosphere was charged with triphenylborane (430 mg, (p, 19 342 cm−1), 479 nm (sh, 20 877 cm−1). Absolute quantum
1.8 mmol, 2.0 equiv.) and bis-triazapentadiene 6d20 (685 mg, yield (dichloromethane): ΦF = <1%. Absolute quantum yield
0.9 mmol, 1.0 equiv.). After adding dry toluene, the reaction (crystalline): ΦF = 2%.
mixture was heated under refluxing conditions for 4 h. After
that time, the solvent was removed under reduced pressure
and the crude product was subjected to column chromato-
graphy on neutral alumina with 90% cyclohexane and 10%
Conclusions
ethyl acetate (Rf = 0.35) to furnish the bis-triazapentadiene In summary, we have described the modular synthesis and full
boron compound 14d as a yellow solid in 49% (479 mg) yield: structural characterization including X-ray diffraction analysis
mp 120 °C. 1H NMR (600 MHz, CD2Cl2): δ 7.92 (td, JHH
=
of two novel mono-1,3,5-triazapentadiene–boron complexes
0.5 Hz, 1H, –NvCq–Cq–CH–Cq), 7.60–7.55 (m, 4H, ArH), 14a,b, two hitherto unknown bis-1,3,5-triazapentadiene–boron
7.52–7.47 (m, 8H, B–ArH), 7.39–7.33 (m, 4H, ArH), 7.31–7.24 complexes 14c,d and the first tris-triazapentadiene boron
(m, 4H, ArH), 7.19–7.07 (m, 12H, B–ArH), 6.98 (td, JHH = 7.7 complex 14e. In order to evaluate their photophysical pro-
5
Hz, JHH = 0.5 Hz, 1H, –NvCq–Cq–CH–CH–CH–Cq), 6.84–6.78 perties all compounds were investigated by absorption and
(m, 4H, ArH), 6.70–6.65 (m, 4H ArH), 6.65–6.61 (m, 4H, ArH), emission spectroscopy in solution as well as in the solid state.
6.47–6.41 (m, 4H, ArH), 2.11 (s, 6H, –CH3) ppm. The experimentally observed relatively similar absorption pro-
13C NMR (150 MHz, CD2Cl2): δ 167.3, 166.4 (CvN), 147.7 perties in solution of 14a,c,e were interpreted on the basis of
(B–Cq), 143.7, 141.9, 137.2, 137.0 (Cq), 136.4 (Cq–CH3), 135.4 quantum chemical TD-DFT calculations as a result of only
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limited electron delocalization involving the central benzene
ring and the neighboring triazapentadiene boron systems due
to the specific steric properties. The inspection of the relevant
Kohn–Sham molecular orbitals indicates that the photochem-
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transfer processes (ICT). In contrast, the emission properties of
the three classes of compounds differ significantly with respect
to emission maxima and Stokes shifts. Quite interestingly,
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Acknowledgements
Financial support from the Deutsche Forschungsgemeinschaft 14 (a) V. A. Dorokhov, B. M. Zolotarev, O. S. Chizhov and
(IRTG 1143 Münster-Nagoya and SFB 858) is gratefully
acknowledged. This work was partly supported by JSPS
Core-to-Core Program, A. Advanced Research Networks (S.Y.).
B. M. Mikhailov, Izv. Akad. Nauk. SSSR, Ser. Khim., 1977, 7,
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