Structure and electronics in dimeric boron π expanded azine and salphen complexes

Article abstract of DOI:10.1039/c6pp00479b

Although boron-based fluorophores incorporating nitrogenous chelating ligands have received much attention, there has been little work on examples of boron-salphen and azine derivatives. In this report, we present several π expanded boron salphen type complexes, incorporating both bis(2-hydroxynaphthaldehyde)azine as well as ortho, meta and para variants of the analogous 2-hydroxynaphthaldehyde salphen compounds. For the azine, we observed only the formation of a single BF₂ adduct, while for the naphth-phen compounds dimeric BF₂ binding was observed. All new compounds were fully characterized via X-ray diffraction, and both DFT and TDDFT studies were carried out to probe the electronic structures of these fluorophores.

Full text of DOI:10.1039/c6pp00479b

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Structure and electronics in dimeric boron π
expanded azine and salphen complexes†
Cite this: DOI: 10.1039/c6pp00479b
Laura A. Crandall, Mahesh B. Dawadi, Tailon Burrell, Adwoa Odoom and
Christopher J. Ziegler*
Although boron-based fluorophores incorporating nitrogenous chelating ligands have received much
attention, there has been little work on examples of boron–salphen and azine derivatives. In this report,
we present several π expanded boron salphen type complexes, incorporating both bis(2-hydroxy-
naphthaldehyde)azine as well as ortho, meta and para variants of the analogous 2-hydroxynaphthalde-
Received 30th December 2016,
Accepted 9th February 2017
hyde salphen compounds. For the azine, we observed only the formation of a single BF adduct, while for
2
2
the naphth-phen compounds dimeric BF binding was observed. All new compounds were fully charac-
DOI: 10.1039/c6pp00479b
terized via X-ray diffraction, and both DFT and TDDFT studies were carried out to probe the electronic
structures of these fluorophores.
rsc.li/pps
We have been investigating new ligand systems to afford
dimeric BF2 fluorophores, and have turned our attention to
Introduction
Recent work in chromophore synthesis has focused on the the well-known salen class of chelates. Diphenol salen ((N,N′-
synthesis of organic fluorophores with large absorption cross bis(salicylaldehydo)ethylenediamine)) and its copper complex
2
6
sections and high quantum yields of emission for use in bio- were first synthesized in 1889 by Combes. Since the first syn-
1
–8
logical imaging, sensors, and organic electronics.
One of thesis of this class of ligands, the development of salen metal
the most studied classes of fluorophores for these applications derivatives has focused on their utility as ligands in chiral cata-
is the BODIPY (boron-dipyrromethene) compound due to its lysts for enantioselective syntheses. Although there are a large
2
7–31
ease of synthesis and modification, strong absorbance, and number of reports on metal based salen compounds,
9
–13
high quantum yield of emission.
More recently, dimeric there has been much less work on the boron adducts of these
3
2–34
BF chromophore compounds have been synthesized as new chelates.
There has been some work on the complexation
with salen type ligands, where the bridging unit is
dine and nindigo BF adducts, as well as our recently reported either a hydrazine or a phenylene diamine. The earliest report
2
fluorophores, including bis-BODIPY complexes, 1,8-naphthyri- of BF
2
2
6
–9,14–23
BOPHY fluorophore.
to have bathochromically shifted absorbance and emission and co-workers, who presented a mono-BF
profiles in comparison to the unsubstituted BODIPY zine–salen ligand system (Fig. 1, compound 1).
The bis-BODIPY complexes tend on a BF salen compound was presented in 1973 by Umland
2
2
adduct of a hydra-
3
5
More
9
,14–17
fluorophores.
The bis-BF 1,8-naphthyridine and BOPHY
2
fluorophores exhibit strong absorbance bands at around
4
0
30 nm and large quantum yields of emission (ϕ = 0.97 and
2
3,24
.92 respectively).
Like bis-BF2 1,8-naphthyridine and
2
nindigo has a strong red shifted absor-
BOPHY, the bis-BF
bance at around 740 nm. Although it is fluorescent, the bis-
2
5
BF nindigo adduct is less stable in solution and decomposes
2
2
into the mono-BF adduct over a period of a few days.
Department of Chemistry, University of Akron, Akron, OH, 44325-3601, USA.
E-mail: ziegler@uakron.edu
†
Electronic supplementary information (ESI) available: Spectroscopic, and
crystallographic data for compounds. CCDC 1523987–1523991 for 1–5. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c6pp00479b
2
Fig. 1 Azine, π expanded azine and π expanded salphen BF complexes.
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recently, dimeric phenylene bridged salphen complexes have Synthesis of salen compounds
3
6–38
been synthesized and investigated.
In this report, we inves-
complexes that
The ligand precursors to the compounds presented in this
tigate the synthesis of π expanded salphen–BF
2
report were synthesized according to previously established
incorporate 2-hydroxynaphthaldimines (Fig. 1) including
hydrazine and phenylene diamine analogues with ortho, meta
and para substitution patterns. We were also able to structu-
39,40
literature procedures.
A typical procedure is as follows: two
equivalents of salicylaldehyde or 2-hydroxynaphthaldehyde
and one equivalent of hydrazine or phenylene diamine were
dissolved in a minimum of ethanol with a few drops of acetic
acid. After stirring at room temperature for 30 min, the precipi-
tate was filtered and washed with cold methanol, and then
dried overnight.
2
rally characterize Umland’s mono-BF adduct, compound 1.
The incorporation of this group expands the π system by a
phenyl group on each end of the chromophore, which extends
their absorption/emission characteristics to the red end of the
spectrum. We have X-ray structural data on all adducts pre-
sented in this report, and have fully characterized their pro- Synthesis of 1–5
perties. In addition, we carried out DFT/TDDFT calculations
on these compounds to elucidate their electronic structures.
Compounds 1–5 were synthesized by reacting one equivalent
of the salen ligand precursor with a 10 mole excess of DIPEA
and a 15 mole excess of BF ·OEt in a minimum of dichloro-
3
2
methane at room temperature for 24 h. The reactions can be
seen in Scheme 1. Compounds 1–3 and 5 precipitated from
the reaction mixtures as yellow orange solids. The solids were
collected by vacuum filtration and washed with dichloro-
methane. After drying, the solids were suspended in water and
stirred for 30 min to remove any excess DIPEA, filtered, and
dried overnight. Compound 4 did not form a precipitate from
the reaction solution; the reaction mixture was washed with
Experimental
General information
All reagents and starting materials were purchased from
commercial vendors and used without further purification.
Column chromatography was performed on silica gel (Dynamic
Adsorbents, Inc., 63–200 μm). Deuterated solvents were purchased
from Cambridge Isotope Laboratories and used as received.
NMR spectra were recorded on 300 or 500 MHz spectro-
meters, chemical shifts were assigned in ppm relative to
4
water and the organic layer was dried over MgSO and the
volume reduced. The reaction mixture was then purified by
column chromatography on silica gel with dichloromethane as
1
13
residual solvent resonances ( H, C NMR spectra). High
resolution mass spectrometry experiments were performed on
a Bruker MicroTOF-III instrument. X-ray intensity data were
measured on a Bruker CCD-based diffractometer with dual
Cu/Mo ImuS microfocus optics (Cu Kα radiation, λ = 1.54178 Å,
Mo Kα radiation, λ = 0.71073 Å). Crystals were mounted on a
cryoloop using Paratone oil and placed under a steam of nitro- 19
gen at 100 K (Oxford Cryosystems). The data were corrected for
absorption with the SADABS program. The structures were
refined using the Bruker SHELXTL Software Package (Version
the eluting solvent.
1
1: Yield: 0.077 g (33.6%), H NMR (300 MHz; CDCl ) δ ppm:
3
10.55 (s, 1H), 9.21 (s, 1H), 8.64 (s, 1H), 7.70 (t, J = 7.3, 2H), 7.52
13
(
q, J = 7.9, 3H), 7.17–6.99 (m, 4H). C NMR (300 MHz;
d -CDCl ) δ ppm: 164.71, 161.05, 139.49, 134.97, 134.27,
6
3
133.45, 132.55, 132.06, 120.97, 120.31, 119.73, 117.53, 117.18.
11
3
F NMR (300 MHz; CDCl ) δ ppm: 131.44 (br. s). B NMR
4
(
M
500 MHz; DMSO) δ ppm: 7.27 (s). λ
358 nm, ε 2.1 × 10
max
−1
−1
2 2 2
cm . HRMS (ESI): m/z calc. for C14H12BF N O 289.0960,
+
found 289.1018 [M + H] .
: Yield: 0.056 g, 28%, H NMR (300 MHz; d -DMSO)
6
.1), and were solved using direct methods until the final an-
1
2
6
2
isotropic full-matrix, least squares refinement of F converged.
UV-visible spectra were recorded on a Hitachi UV-Vis
spectrophotometer (U-3010). Fluorescence excitation and emis-
sion data in solution were recorded on a Horiba Jobin-Yvon
FluoroMax-4 fluorescence spectrophotometer using quinine
δ ppm: 11.56 (s, 1H), 10.09 (s, 1H), 9.85 (s, 1H), 8.97 (d, J =
8.79 Hz, 1H), 8.60 (d, J = 8.49 Hz, 1H), 8.34 (d, J = 9.09 Hz, 1H),
8.12 (d, J = 9.09 Hz, 1H), 8.02 (d, J = 9.57 Hz, 1H), 7.95 (d, J =
7.89 Hz, 1H), 7.76–7.65 (m, 2H) 7.56–7.45 (m, 2H), 7.35–7.30
sulphate in 0.1 M H
held constant at 1 nm. The quantum yields in solution were
calculated using the following equation: Φ = Φst ([Grad]
)/(ηst ); ηst = 1.33; Φst = 0.52 and Grad is the gradi-
2 4
SO as a standard. All slit widths were
x
x
/
2
2
[Grad]st)(η
x
ent from the plot of integrated fluorescence intensity vs.
absorbance.
For the calculations, the starting geometries of compounds
1
–5 were taken from X-ray structures. All compounds were opti-
mized using the TPSSh hybrid (10% Hartree–Fock exchange)
exchange correlation functional with the 6-311G(d) basis set.
Solvent effects were modelled using the PCM approach.
Acetonitrile was used as a solvent in all of the single point
PCM-DFT and PCM-TDDFT calculations. All computations
4
1
were performed using the Gaussian 09 software package.
Scheme 1
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1
3
(
m, 2H). C NMR (300 MHz; d
6
-DMSO) δ ppm: 161.13, 160.47, comprised of a hydrazine-Schiff base nitrogen atom and a
1
1
1
60.01, 156.78, 140.14, 136.17, 134.69, 132.23, 131.69, 131.28, deprotonated phenolic oxygen atom. Only one equivalent of
29.21, 129.12, 128.04, 127.82, 124.99, 124.05, 121.71, 121.32, boron is coordinated by these two ligand systems; the second
18.84, 118.42, 108.41, 107.07. F NMR (300 MHz; d -DMSO) potentially chelating segment remains unreacted even in the
6
1
9
1
1
δ ppm: 131.86 (br. s). B NMR (500 MHz; d -DMSO) δ ppm: presence of an excess of BF3.
7
ε
6
−1
3
−1
.21 (s) λmax 340 nm, ε = 15 × 10
M
cm
λ
max 432 nm,
In compound 1, the B–O and B–N bonds measure
3
−1
−1
=
38
×
10
M
cm . HRMS (ESI): m/z calc. for 1.446(3) Å and 1.620(3) Å respectively. In compound 2, which
+
C H BF N O Na 411.1092, found 411.1129 [M + Na] .
was elucidated at lower resolution than 1, the B–O bond
2
2
15
2 2 2
1
3
: Yield: 0.043 g (64%), H NMR (300 MHz; CDCl
3
) δ ppm: measures 1.473(12) Å and the B–N bond measures 1.589(12) Å.
9
7
.29(br. s, 2 H), 8.09 (d, J = 8.5 Hz, 2H), 7.96 (d, J = 9.1, 2H), In both compounds, notably the B–O bonds are measurably
.79–7.76 (m, 2H), 7.70 (d, J = 7.9, 2H), 7.63–7.57 (m, 4H), 7.43 shorter than the B–N bonds. The B–F bonds in both com-
1
3
(t, J = 8.2, 2 H), 7.07 (d, J = 9.4, 2H). C NMR (500 MHz; pounds range between ∼1.36 and ∼1.38 Å. Both compounds
CDCl
3
) δ ppm: 163.87, 142.00, 131.66, 130.03, 129.67, 129.26, are planar; there is no observed rotation along the N–N bond
1
9
1
28.17, 127.92, 125.31, 120.00, 119.89. F NMR (300 MHz; of the azine or the C–C bond pendant to the Schiff base. This
1
1
CDCl
3
) δ ppm: 132.20 (br. s). B NMR (500 MHz; CDCl
3
)
,
is clearly due to the presence of an intramolecular hydrogen
bond between the OH and the Schiff base of the unligated
−
1
−1
δ ppm: 7.27 (s). λmax 344 nm, ε = 26 × 103 M
λmax 387 nm, ε = 25 × 10 M cm . HRMS (ESI): m/z calc. for portions of the compounds.
cm
3
−1
−1
+
C
28
H
18
B
2
F
4
N
2
O
2
Na 535.1368, found 535.1382 [M + Na] .
The ortho, meta, and para naphthylphen ligands can also
1
4: Yield: 0.134 g, (84%), H NMR (300 MHz; d
6
-DMSO) be readily generated from the corresponding diamines and
δ ppm: 9.85 (br. s, 2H), 8.61 (d, J = 8.49 Hz, 2H), 8.40 (d, J = 2-hydroxynaphthaldehyde using acetic acid as the catalyst
9
7
.06, 2H), 8.21 (s, 1H), 8.03 (d, J = 7.92, 2H), 7.89 (d, J = (Scheme 1). The BF
2
chemistry of the ortho and para salphen
.35 Hz, 2H), 7.81–7.69 (m, 3H), 7.57 (t, J = 7.62, 2H), 7.37 (d, ligand systems has been explored by Jiang, although the struc-
1
3
37
J = 9.09, 2H). C NMR, (300 MHz; d -DMSO) δ ppm: 162.06, tures of these compounds were not elucidated. Lu and co-
6
1
1
2
43.09, 141.53, 131.60, 130.16, 129.41, 127.76, 125.24, 124.17, workers also presented the synthesis of bis-BF adducts of
21.60, 119.68. F NMR (300 MHz; d -DMSO) δ ppm: 132.47 t-butyl modified forms of the meta and the para salphen. We
6
1
9
36
1
1
(br. s). B NMR (500 MHz; d -DMSO) δ ppm: 4.43 (br. s). were able to synthesize the bis-BF modified π expanded ortho,
6
2
3
−1
−1
3
λ
max 342 nm, ε = 20 × 10 M cm , λmax 400 nm, ε = 29 × 10
meta, and para 2-hydroxynaphthaldimine salphen analogues
−
1
−1
M
5
cm . HRMS (ESI): m/z calc. for C28
H
18
B
2
F
4
N
2
O
2
Na using excess of BF
characterized each resultant compound (3–5) by X-ray
-DMSO) methods, which can also be seen in Fig. 2. As expected, each
via a N,O
H), 8.03 (d, J = 7.32 Hz, 2H), 7.94 (s, 4H), 7.75 (t, J = 7.1 Hz, chelate; as in compounds 1 and 2, the B–O bonds are much
3
and DIPA (Scheme 1). We structurally
+
35.1368, found 535.1382 [M + Na] .
1
5
: Yield: 0.106 g (76%), H NMR (300 MHz; d
6
δ ppm: 9.80 (s, 2H), 8.62 (d, J = 8.2 Hz, 2H), 8.39 (d, J = 9.1 Hz, ligand coordinates with the two equivalents of BF
2
2
2
1
3
H), 7.58 (t, J = 7.3 Hz, 2H), 7.36 (d, J = 9.1 Hz, 2H). C NMR shorter (∼1.45 Å) than the B–N bonds (∼1.58–1.59 Å).
-DMSO) 161.85, 161.13, 142.28, 141.38, 131.64, Similarly, the B–F bonds measure ∼1.38 Å.
29.40, 27.78, 125.23, 125.03, 121.63, 119.68, 108.97. F NMR
(300 MHz; d
6
1
9
1
Unlike the mono-BF compounds 1 and 2, the bis adducts
2
1
1
6
(300 MHz; d -DMSO) 132.51 (br. s). B NMR (500 MHz; are not planar. Without the hydrogen bond to enforce planar-
3
−1
−1
d
6
-DMSO) 5.39 (br. s). λmax 343 nm, ε = 20 × 10 M cm
ity, these compounds can rotate along the C–C bonds pendant
3
−1
−1
λmax = 411 nm, ε = 35 × 10 M cm . HRMS (ESI): m/z calc. to the phenyl ring. In 4 and 5, the meta and para compounds
for C28
+
H
18
B
2
F
4
N
2
O
2
Na 535.1368, found 535.1382 [M + Na] .
respectively, the naphthyl groups deviate ∼14.4 and ∼30.2°
respectively from the planes of the phenyl rings. We surmise
that these deviations observed in the solid state result from
packing forces and that these rings can rotate freely in solu-
tion. In compound 3, however, the lack of planarity can be
Results and discussion
The first work on dimeric BF compounds began more than ascribed to the steric bulk of the ortho naphthyl units. The
2
3
5
four decades ago with the work of Umland and co-workers.
naphthyl rings are at angles ranging from ∼63.1° to ∼75.6° to
In a 1973 report, they presented the first salphen BF com- the central phenyl for the two equivalents in the solid state,
2
pound; a 1 : 1 complex (compound 1) was reported by these and the phenyl rings are oriented in opposite directions.
authors using benzene as the reaction solvent. More recently, a These compounds can be considered chiral, with both enan-
bis-BF
2
adduct of a t-butyl modified version of compound 1 tiomers present in the solid state. This type of chirality can be
3
6
was presented by Lu and co-workers. We were able to syn- considered as a variant of strain induced chirality, similar to
3
9,40
thesize the π expanded analogue of 1, compound 2, using that seen in polyarenes or the BINAP family of ligands.
dichloromethane as the solvent with DIEPA as the base. In We probed the absorption and emission spectra of com-
addition, we were able to reproduce the synthesis of com- pounds 1–5, which are shown in Fig. 3. Table 1 lists the photo-
pound 1 and elucidate its structure, as shown in Fig. 2. chemical parameters of these compounds. All of them exhibit
Both compounds exhibit nearly identical geometrical para- two absorption peaks; one at higher energy below 350 nm, and
meters. The BF2 units are coordinated by bidentate chelates a second peak at a lower energy depending on the structure of
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Fig. 2 The structures of 1–5 with 35% thermal ellipsoids. Hydrogen atoms on carbon atom positions have been omitted for clarity.
Fig. 4 DFT predicted (TPSSh functional/6-311G(d) basis set) frontier
orbital energy levels for 1–5.
shift to lower energies. With regard to emission, compounds 1
and 2, with heteroatoms unoccupied by BF coordination,
2
exhibit the lowest quantum yields of emission.
In order to gain greater insight into the electronic struc-
Fig. 3 The normalized absorption (solid line) and fluorescence emission
dotted line) spectra of 1 and 2 (top) and 3–5 (bottom) in acetonitrile.
Compounds were excited at the lowest energy maxima for emission spectra.
tures of these π expanded BF compounds, we carried out DFT
2
(
and TDDFT calculations to determine their orbital compo-
sitions and energies as well as to predict their spectra. Fig. 4
shows the relative energy levels for the compounds investi-
Table 1 Selected absorption and emission parameters for compounds 1–5 gated in this report. For compounds 1 and 2, the hydrazine
bridged compounds, the HOMO and LUMO orbitals are rela-
λ
max
ε
λ
ex
λ
em
Q.Y. Stokes
tively isolated. The HOMO–LUMO gap shrinks from ∼2.60 to
2.53 eV from 1 to 2, in agreement with the absorption trends.
Compound (nm) (M cm−1
−
1
)
(nm) (nm) (%)
shift (nm)
∼
4
1
2
3
4
5
358
432
338
400
411
2.1 × 10
3.8 × 10
2.5 × 10
2.9 × 10
3.5 × 10
358
368
338
400
411
570
574
452
462
499
<1
1
6
212
206
114
62
For 3–5, the HOMO and HOMO−1 as well as the LUMO
and LUMO+1 are closely spaced; in 3 and 4 they are nearly
degenerate. The HOMO–LUMO gaps decrease as one proceeds
from 3–5 (∼3.06, ∼3.10 and ∼2.78 eV), which also correlates to
the sequential bathochromic shifts in these compounds.
The structures of the frontier orbitals of 1 and 2 are shown
4
4
4
4
6
26
88
the compound. The azine bridged compound 2 shows the in Fig. 5. Both the HOMO and LUMO are π bond in character,
most red shifted absorbance, followed by the para (5), meta (4), and extend across the azine link due to the planar nature of
and ortho (3) phenylene bridged compounds. The extinction the structures of 1 and 2. This in part explains the clear red-
coefficients for all compounds rise as the absorption maxima shift in absorption of compound 2 relative to 3–5. Fig. 5
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ment for all of the compounds. In the hydrazine bridged com-
pounds, the bands arise from what can be characterized
as π to π* transitions. In compound 1, this is a HOMO−2 to
LUMO transition, while in 2 it is a HOMO to LUMO transition.
The expansion of the π system in 2 destabilizes the ground
state π orbital as well as stabilizes the LUMO, resulting in a
very significant red shift in the primary absorption band. In
compounds 3–5, the primary absorption bands can also be
described as π to π* transitions, with the electron moving from
an orbital with more naphthylaldimine character to an orbital
with a higher percentage of phenylene bridge composition.
However, due to the lack of planarity in 3–5, we do not observe
the similar degree of orbital stabilization/destabilization as
seen in 2.
Conclusion
In conclusion, we have prepared a series of π-expanded azine
and salphen BF fluorophores. The hydrazine bridged system 2
2
incorporates only one equivalent of boron, similar to that seen
for the previously synthesized compound 1. The phenylene-
bridged compounds all incorporate two equivalents of BF2.
The quantum yields of emission range from very low (<1%) to
Fig. 5 DFT predicted frontier orbital structures for 1–5.
displays the HOMO−1, HOMO, LUMO and LUMO+1 for 3–5. ∼25% for the para substituted compound 5. Our future work
In compounds 3 and 4, all four of the orbitals are primarily on these compounds will investigate their time resolved photo-
naphthylaldimine based, with some bridging phenylene physical behaviour.
character in all four orbitals. In contrast, the para substituted
phenylene compound 5 has a significant phenylene character
in the HOMO and LUMO, and much less in the LUMO+1 and
HOMO−1. In general, the degree of phenylene participation in
the frontier orbitals along with the ability to form a planar The authors would like to acknowledge the University of Akron
Acknowledgements
configuration both clearly correlate with the increase in red for support of this research.
shift of absorption and emission for 3–5. We carried out
TDDFT investigations into 1–5, and the experimental versus
predicted spectra for these compounds can be seen in Fig. 6.
Overall, we observe good agreement between theory and experi-
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