Tuning of electronic properties via labile N→B-coordination in conjugated organoboranes

Article abstract of DOI:10.1039/c6tc05418h

In this report we demonstrate that labile intramolecular N→B-Lewis pair formation can serve to tailor the properties of π-conjugated electronic materials. A series of boranes has been prepared by copper-catalyzed 1,3-dipolar azide-alkyne ‘click’-cycloaddi

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PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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ARTICLE
Tuning of electronic properties via labile N→B-coordination in
conjugated organoboranes
esiReceived 00th January 20xx,
Accepted 00th January 20xx
[
a]
[b]
[a]
Sandra Schraff, Yu Sun, Frank Pammer*
DOI: 10.1039/x0xx00000x
www.rsc.org/
In this report we demonstrate that labile intramolecular N→B-Lewis pair formation can serve to tailor the properties of π-
conjugated electronic materials. A series of boranes has been prepared by copper-catalyzed 1,3-dipolar azide-alkyne
‘
2
click’-cycloaddition with a bifunctional borane (B -H), which allowed to introduce electronically varied aryl-substituents on
the triazole ring (p-MeOPh, p-MePh, p-F
weight of M = 7.2 kDa (PDI = 2.1, DP
3
CPh), and to also prepare a polymer (PTrz) with a number-average molecular
n
n
≈ 8.2). The optical and electrochemical properties of the borans have been
11
1
investigated in detail, and complemented by DFT calculations and anion binding studies. B NMR and dynamic H NMR
show that the N→B-coordination persists in solution but is comparatively weak, and the boranes undergo rapid thermal
exchange between closed, N→B-coordinated and open, non-coordinated conformers. UV-vis-absorption spectra exhibit
only a minor impact of the derivatization, while fluorescence measurements show a direct correlation between the
-1
peripheral substituents and observed Stokes shifts (3200 to 10000 cm ), fluorescence lifetimes (τ = 23–11 ns) and
quantum yields (φ = 17.3–6.4%). Titration with cyanide shows that the strength of the N→B-bond can be incrementally
tuned by variation of the peripheral substituents, to allow for direct binding of cyanide to boron.
formation also increases the electron affinity of conjugated π-
systems. Intramolecular N→B-ladder formation yields more
well-defined and chemically robust compounds, and
Introduction
Interest into conjugated electronic materials containing tri- additionally, depending on the molecular architecture,
and tetracoordinate boron centers has steadily grown over the enforces planarization of the conjugated π-system. This can
1
past decade. Organoboranes have been tailored for a range of facilitate more effective π-stacking in the solid state, which is
2
potential application including use as chemical sensors and as considered beneficial for charge transport in organic
3
,4
14
active components in organic light emitting diodes and semiconductors.
organic photovoltaic cells.
5
,6
Exampes for labile
N
B-ladders
The effect of introduction of boron on electronic and optical
properties crucially depends on the chemical environment at
the boron center. Introduction of tricoordinate boron, either
H
N
N
O
O
N
SiMe
C6F5
3
N
B(C
N
6
F
5
)
2
B
B
B
Mes
1
,7
as pendant groups or embedded into larger conjugated
Fe
Fe
6 5
C F
Mes
H
1
systems leads to an increased electron affinity, due to
,8
z
contributions of the empty p -orbital to the LUMO. A related
A
B
C
D
Erker, 2011
Tucker/Shipman/Walsh Jäkle, 2014
2013
Wang, 2014
approach that has gained increasing traction in recent years is
the isosteric substitution of C=C-double bonds by covalent B-N-
1
,3,9,10
Mes Mes
B
moieties,
that equates isosteric substitution of C-C-single bonds.
The electron withdrawing effect of the coordinative N→B-
and of coordinative N→B-Lewis pair formation,
This work
1
1,12
OMe
Me
CF
3
N
N
R
N
R' =
N
1a
N
N
R
1
3
interaction in both inter- and intramolecular Lewis pair
2 n
-((CH )12-) -
B
Mes Mes
Chart 1. Examples for labile N→B-ladders and boryl-substituted triazole-containing
model systems as key building unit.
Intramolecular N→B-Lewis pairs featuring
a variety of
conjugated N-heterocycles in the backbone have been
1
5,16,17,22,23
18,19,20
thiazoles,
reported, including pyridines,
iso-
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2
1
16f,17,22
25
imidazole, azobenzene, benzo[d]imidazole,
16f
23
quinolines,
quinolines,
pyrimidines,
pyrazine,
(pK
a
40
= 2.2) or the ligands employed by Jäkle (
C
a
, pK (pyridine) =
2
4
26,27,28
40
benzo- 3.4 ) and Wang (D, pK (1H-benzo[d]imidazole) = 5.7 ). In the
a
2
9
27
thiadiazole, and 4H-1,2,4-triazole.
In the above examples energetically favorable five-
systems at hand, the N→B-coordination is therefore likely
or weakened compared to the examples shown in Chart 1.
1
6-28,23
1
5,29
six-membered
rings are formed, and the N→B-coordinated
conformation is considered to be highly stabilized. Rare
examples of labile intramolecular N,B-Lewis pairs have been
reported, however, wherein dynamic dissociation of the N→B-
bond is observed: Erker and coworkers showed that strongly
Results and Discussion
Chemical Synthesis
4
1
We chose the ditopic borane
2
B -H (Scheme 1a) as substrate
Lewis acidic B(C
6 5 2
F ) -groups can form labile intramolecular
for functionalization via CuAAC, as it would enable us to
generate more extended conjugated systems through
simultaneous construction of two N→B-moieties and also
N→B adducts with trialkylamines, even under formation of
3
0a
30
strained four-membered N→B-rings (
facile exchange between an N→B-coordinated closed and non-
coordinated open conformation, still functions as
A
,
Chart 1). Due to
allows the preparation of polymers.
according to a modified procedure, starting from 1,4-dibromo-
,5-diiodo benzene. In a first step, two ethynyl substituents
2
B -H was prepared
A
a
frustrated Lewis pair catalyst in dihydrogen activation. Labile
coordination in more favorable five-membered rings is
observed when less Lewis acidic boronic acid esters are
2
were introduced by Sonogashira coupling with trimethylsilyl-
acetylene, followed by twofold lithiation and addition of
3
1
32
28
introduced (B), and in sterically congested systems (C , D ).
Examples of labile intramolecular P→B-coordination have also
dimesitylboron fluoride (Mes
borane -TMS Subsequent deprotection to
achieved 86% yield with tetrabutylammonium fluoride (TBAF)
2
BF) to give the TMS-protected
3
3,34,35,36
B
2
.
B
2
-H was
been reported.
In our group we have an ongoing
interest in the study of intramolecular Lewis pairs, particularly
with respect to the effect of N→B-coordination on the
4
2
adsorbed on silica gel,
which worked better than
4
commercially available TBAF-solution (68% ).
1
1
5
electronic properties of conjugated π-systems.
In the
3
A series of triazole-containing model systems (Trz-CF , Trz-
previous studies we also found N→B-coordination to be
energetically favored such a degree, that the observable
electronic properties are governed by the N→B-ladderized
species.
Me, and Trz-OMe, Scheme 1b) and a polymer (PTrz) were then
prepared. In the synthesis of the model boranes, prolonged
heating of
2
B -H in the presence of copper(I) iodide, diisopro-
4
3
pylethylamine and excess amounts of aryl-azides were
required to effect complete conversion of the substrate. The
sluggish reaction may be owed to the steric hindrance
imposed by the dimesitylboryl-groups. Still, electronically
diverse azides are compatible with this preparative approach,
since p-anisyl- and p-trifluoromethylphenyl-azides worked
equally well. The boranes proved air-stable as solids but
exhibited sensitivity towards moisture in solution.
In this work, we demonstrate that labile N→B-coordination
can serve to tailor electrochemical and optical properties, as
well as anion binding behavior of π-conjugated organo-
boranes.
Structural considerations
The N-heterocyclic component of N→B-ladder-boranes
1
13
reported herein was generated via copper-catalyzed azide-
The borane-models were fully characterized by H, C, and
B NMR and high resolution mass spectrometry (HR MS, see
3
7
alkyne-cycloaddition (CuAAC). CuAAC of terminal alkynes
1
1
exclusively furnishes 1,4-disubstituted 1H-1,2,3-triazoles, and
Electronic Supporting Information, ESI, for additional
comments). Single crystals of Trz-Me suitable for analysis via X-
ray diffraction were also obtained in the form of colorless
prisms by slow evaporation of a solution of the borane in
dichloromethane/n-Hexane. Crystallization of Trz-OMe and
thereby generates
a suitable regiostructure for N→B-
coordination starting from ortho-boryl-ethynylarenes. The
reaction is also highly tolerant towards functional-groups, and
7
3
generally provides high yields under mild conditions, and
allows the preparation of macromolecular material via click-
Trz-CF
3
inadvertently furnished microcrystalline needles,
3
8
type polymerization.
Furthermore, chemically and
9
electronically divers organic azides can be readily prepared,
unsuitable for X-ray diffraction. A previously unreported
3
crystal structure of the precursor
and has been included in the ESI.
B
2
-H could also be attained
-H and 1,12-
which allows to tailor peripheral substituents on the triazole
ring with preparative ease. However, the 1,4-disubstitution on
the 1H-1,2,3-triazoles depicted in Chart 1 does not provide
The polymer PTrz was prepared by heating
B
2
diazidodecane in a toluene/THF-solvent mixture with copper(I)
iodide in combination with pentamethyldiethylenetriamine
‘
proper’ conjugation between the periphery of the molecule
and the central phenylene core, as the conjugation pathway
the triazole-ring invariably leads across two consecutive single
bonds at N1. Compared to fully conjugated systems, changes
to peripheral substituents are therefore expected to have
more indirect influence on the overall properties of a given
(
PMDETA) as catalyst - a catalyst system known to perform
3
7a
well in click-type polymer syntheses (Scheme 1b).
The
reaction was complete after heating overnight and gave PTrz
as a colorless solid with a number-average molecule weight of
n
M = 7.2 kDa (PDI = 2.1), as determined via gel-permeation
a a
lead structure. Furthermore, 1H-1,2,3-triazoles (pK = 1.2, pK
chromatography in chloroform relative to polystyrene
standards. Due to the comparatively high mass of the repeat
of the corresponding acid) are significantly less basic than
most common N-heteroarenes including 4H-1,2,4-triazoles
unit (≈875 g/mol), this corresponds to
a degree of
2
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ARTICLE
a)
polymerization (DP
n
) of 8.2. The polymer could be purified by
Br
Me Si-CCH,
Br
3
(
Ph
3
P)
2
PdCl
2
,
1) 2.1 Eq. n-BuLi
precipitation into hexanes, and dissolved readily in polar and
aromatic organic solvents such as dichloromethane, THF and
CuI, DIPA
THF, 0 °C - r.t.
2) 2.1 Eq. Mes
2
BF
I
I
Me
3
Si
SiMe
3
THF, -78 °C - r.t.
1
toluene, and could be unambiguously identified by H and
13
Br
Br
1
C
, 58%
1
1
NMR. B NMR showed a broadened signal at 3.1 ppm
attributable to tetracoordinate N→B-species. Analysis of PTrz
via thermal gravimetry (TGA) showed rapid degradation to set
in above 275 °C. The polymer loses ca. 45% of its mass
between 275 and 380 °C, followed by a second degradation
event at 445 °C, to leave 24% of residual mass. (see Figure S43
in the ESI).
BMes
2
BMes
2
TBAF SiO
2
Me Si
3
SiMe3
H
H
THF, r.t.
Mes
2
B
2
Mes B
2
B -TMS, 56%
B
2
-H, 86%
R
Analysis of the polymer by high-resolution MALDI-FT-MS
showed series of oligomeric species up to m/z = 5.25 kDa,
while species up to m/z = 7.2 kDa could be observed by low
resolution MALDI-TOF-MS. The high-resolution isotope
patterns of the two main series correspond to molecular ions
of 1:1 azide/alkyne addition products up to n = 6 after loss of
Mes
B
b)
2
N
3
R
N
N
N
N
N
cat. CuI, EtNiPr
2
,
N
THF, 70 °C,
-120 h
B
9
Mes
2
R
Trz-Me
Trz-OMe : R = -OMe, 68%
Trz-CF
3
: R = -CH , 29%
2
B -H
+
+
3
: R = -CF
3
,
81%
one methyl or mesityl-group (m/z = [M -CH ] or [M -Mes] ;
n
3
n
+
see Figure S42 in the ESI). The molecular ions ([M ] ) were also
Mes
B
2
n
N
3
N
3
N
N
observed, but at much lower intensity. Ionization of the
polymer therefore proceeds analogous to ionization of the
molecular systems for which similar fragmentation patterns
were observed (see Figures S33, S36, and S39 in the ESI).
1
2
N
(CH2)12
N
cat. CuI, PMDETA,
Toluene/THF, 15:1,
100 °C, 16 h
N
N
B
n
Mes
2
PTrz, 61%
n
= 7.2 kDa, PDI = 2.1, DPn = 8.2
M
2
Scheme 1. a) Synthesis of borane B -H and b) preparation of N→B-ladder boranes.
PMDETA = N,N,N’,N’,N’’-Pentamethyldiethylenetriamine
Structural investigations
The boranes under investigation herein can in principle adopt
twofold N→B-coordinated ‘closed’ conformations, as well as
1
1
‘half-open’, and non-coordinated ‘open’ ones (Scheme 2).
B
NMR spectra of all four boranes, recorded at 340 K, showed
signals at 3-4 ppm in the typical range for tetracoordinate
2
boron, while resonances attributable to tricoordinate species
could not be identified. These results signify that (half-) closed
conformers are predominant in solution.
This assumption is corroborated by the crystal structure of
Trz-Me. The borane crystallized in the triclinic space group P-
1
(2) with two crystallographically independent molecules in
the unit cell, that both adopt centro-symmetrical, twofold
N→B-coordinated geometries. The N→B-bond lengths of
1
.655(1) and 1.664(1) Å are in good agreement with the
analogous structure simulated by DFT calculations (Trz-Me’
N→B = 1.659 Å, vide infra), but are only somewhat longer than
the corresponding bonds in other BMes -containing ladder
:
2
8
2
boranes such as
D (1.658(3), 1.644(3) Å ) or other phenyl-
1
6b,d
pyridine based systems (1.638(4)-1.667(4) Å)
). The bond
lengths therefore do not give a direct indication of the
strength of the coordinative interaction. Additionally,
preferential N→B-coordination in the solid state may just be
attributable to packing effects, as the DFT calculations also
Figure 1. a) Side view, and b) top-down view of the solid state structure of Trz-Me.
Ellipsoids shown at 55% probability level. Hydrogen atoms, one equivalent of
dichloromethane, and a second, crystallographically independent molecule have been
omitted for clarity. Characteristic bond length [Å] and angles [°]: Structure 1: B-N3 indicate that the energetic differences between open and
Mes
Mes
1
.664(1), C1-B 1.645(1), B-C
1.637(1), B-C
1.636(1), C2-C4 1.457(1), N1-C6 closed conformers are relatively small, and N→B-conformers
Mes Mes
1
1
C
9
.440(1), N3-C4-C2-C1 6.1(1), N2-N1-C6-C7 40.9(1), C1-B-N3 93.4(1),
C
-B-C
may therefore be thermally populated. Furthermore,
tetracoordination and higher temperatures generally lead to
Mes
16.4(1). Structure 2: B-N3 1.655(1), B-C1 1.642(1), C2-C4 1.459(1), B-C 1.636(1), B-
Mes
1.635(1), N1-C6 1.442(1), N3-C4-C2-C1 -6.5(1), N2-N1-CR1-C7 -30.9(1), N3-B-C1
1
1
Mes
Mes
sharper
B
NMR signals due to slowed quadrupolar
3.5(1), C -B-C 115.4(1).
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4
4
relaxation. For these latter reasons, we were not able to low temperature (220 K → 208 K), in step with the coalescence
directly observe N→B-conformers at low temperature, while of the mesityl signal, and concurrently shifts e.g. in Trz-OMe
tricoordinate species might be present, but may not be from 8.68 ppm (340 K) to 9.14 ppm (208 K). Moreover, the
observable at all due to their generally more broadened resonance of the phenylene-protons remains almost
1
resonances. Furthermore, dynamic H NMR experiments show unaffected by changes in temperature. The shift of the triazole
that all three model systems indeed undergo rapid exchange signal is therefore likely attributable to rotation of the triazol
even well below ambient temperatures (Figure 3, see also ring itself, rather than to rotation of the mesitylgroups, which
Figures S22-S24 in the ESI): Rotation of the mesityl-groups is would be expected to affect both signals.
hindered. Consequently, the signal of the aromatic mesityl-
3
protons in Trz-Me, Trz-OMe and Trz-CF appears sharp only at
a)
Mes2
B
BMes2
BMes2
higher temperatures (340 K), but broadened under ambient
conditions (300 K). Should the boranes settle into a rigid,
closed conformation, the four aryl-protons of each BMes2-
group become magnetically inequivalent. This is indeed
observed at lower temperatures, where the aryl-CH signal
splits into two and four individual peaks below 238 K and
R
R
R
N
N
N
N
N
N
R
N
N
N
N
N
N
N
N
N
N
N
N
R
R
B
B
Mes
Mes2B
Mes
2
2
closed
half-open
open
b)
Mes2
BMes2
B
BMes2
R
R
R
N
N
N
N
N
R
N
N
N
N
4
5
N
N
N
2
20 K, respectively. From the signal separation at the
N
N
N
N
N
N
R
R
‡
B
B
Mes2
coalescence temperature activation barriers (ΔG TC) for the
rotation of the mesityl-groups of ca. 46 ±0.6 kJ/mol were
derived. (Table 1, see also section 2.3 of the ESI). More
comprehensive line-shape analyses of variable temperature
NMR spectra in the range of 208 to 253 K consistently
confirmed this estimate (Table 1): Through simulation of the
NMR spectra the exchange rates (k) at the respective
Mes
Mes2B
2
closed -2
half-open-2
open-2
c)
Mes2
B
Mes2
B
Mes2
B
N
Me
N
N
N
N
N
N
N
N
Me
Me
TSc
TSo
TS
Scheme 2. Conformers of Trz-ladders. a/b) Neutral boranes and dianions. R = Me, p-
temperatures were derived, which allowed to determine the MeOPh, p-MePh, and p-CF
3
Ph. c) Truncated systems for transitional state optimization.
‡
‡
activation enthalpy (ΔH ) and entropy (ΔS ) from the
corresponding Eyring plots (ln (k/T) vs. 1/T) (Figures S19-S21 in
the ESI).
These above values are comparable with the rotational
barriers in dimesitylborinic acid derivatives (Mes B-OR, R =
2
‡
‡
46
Me: ΔG = 48.5 ±1 kJ/mol, R = Ph: ΔG = 45.5 ±1 kJ/mol ), but
substantially lower than the barrier for the same process in
D
ΔG ≈ 70 kJ/mol , Chart 1). Activation barriers for N→B
‡
28
(
‡
A (ΔG = 58.2 ±1.7 kJ/mol ) and structural
‡ 31
B (ΔG = 43.9 kJ/mol ) have been found to be
30
dissociation in
fluctuation in
somewhat higher or comparable to those of the triazole-
systems reported herein.
1
Table 1. Activation parameters determined by variable temperature H NMR. Data
8
recorded in THF-d .
a
‡
b
‡c,d
‡d
‡d
T
C
ΔG TC
ΔG
[kJ/mol]
Trz-OMe 238(±3) 45.9(±0.6) 47.1(±2.3) +31.7(±1.6)
ΔH
ΔS
[
K]
[kJ/mol]
[kJ/mol]
[J/mol]
-65.0(±6.9)
-77.1(±5.9)
-81.0(±4.8)
Trz-Me
Trz-CF
238(±3) 46.2(±0.6) 46.7(±2.0) +28.6(±1.3)
238(±3) 46.1(±0.7) 46.6(±2.2) +27.6(±1.1)
3
a
b
c
d
Coalescence temperature at 238 K based on Δv. at 238 K. derived from Eyring
plots, see ESI.
The above results do not directly confirm that the boranes
fluctuate between open and closed conformations. However,
due to lower steric congestion around tricoordinate boron
centers, the exchange is likely to occur via N→B-dissociation,
2
.
8
as proposed for
D
The lability of the N→B-bond is further
corroborated by the strong temperature dependence of the
1
1
Figure 3. Examples for variable temperature H NMR sprectra of Trz-OMe. Recorded at
triazole-H resonance in H NMR experiments (Figure 3), which 500 MHz in THF-d8.
broadens upon cooling (340 K → 220 K) and sharpens again at
4
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Computational study
see Table S2 in the ESI). A quantification of this stabilizing
effect would require a more detailed analysis. However, this
phenomenon is related to the concept of ‘steric attraction’
that has been shown to distort the coordination geometry
To elucidate which conformers are energetically favored, and
to quantify the effect of varying the peripheral substituents,
quantum chemical calculations have been performed on
around metal centers through dispersion interactions of large
0
ligands, and can be in the order of 8-13 kJ/mol for Pd (PR
model systems (Trz-OMe’
triazole-capped borane (PTrz’) representing the repeat unit of
PTrz. For each borane structures of closed half-open and open
3
, Trz-Me’, Trz-CF ’) and a 1-methyl-
3
)
2
4
7
complexes.
,
Regardless of the predominant conformations of the neutral
boranes, the picture changes when dianionic systems are
conformers have been optimized at the B3LYP-D2/TZVP-level
in the gas phase, along with the corresponding singlet dianions
-
2
-
closed ’, ‘half-open ’ and ‘open ’ (Scheme 2a/b, Table 2).
2
-2
-2
compared. For PTrz’ , the half-open and open conformations
are highly favored over the closed form by -53
and -143 kJ/mol, respectively. The situation is only somewhat
different for the dianions of the aryl-triazoles. While half-open
conformations are unfavorable in all cases, the open
‘
For neutral systems, electronic transitions have also been
simulated by time dependent DFT (td-dft) at the same level of
theory.
-
2
-2
conformation is preferred by Trz-OMe’ and TrzMe’ , but
-
Table 2. Relative calculated Gibb’s energies of borane conformers in kJ/mol.
2
Optimization at B3LYP-D2/TZVP in the gas phase.
remains unfavorable for the Trz-CF ’ .
3
Charge
closed
half-open
open
Trz-OMe‘
Trz-Me‘
0
0
0
0
:=0
:=0
:=0
:=0
-4
-2
-3
≈0
-15
-2
Trz-CF
3
‘
-18
≈0
PTrz‘
Trz-OMe‘
Trz-Me‘
-2
-2
:=0
:=0
+20
+18
-20
-23
Trz-CF
3
‘
-2
-2
:=0
:=0
+35
-53
+14
PTrz‘
-143
The individual conformers were found to be surprisingly close
in energy. For Trz-Me’ and PTrz’ all three conformations are
effectively thermo-neutral (ΔG ≤ 5 kJ/mol), while for Trz-OMe’
o
c
‡
Figure 4. Molecular structures of conformers TS and TS , and transitional state TS .
and Trz-CF
0 kJ/mol more stable than either closed or half-open
conformations. Still, these energetic differences lie within the
3
’
open-conformations were actually about
Optimized at with B3LYP-D2/def2-TZVP, (pcm: THF).
2
What renders the above discussion so vital for the results
range of thermal vibrations, and conformations should either outlined herein, is the fact that conformational changes have a
be populated (Trz-Me’
,
PTrz’) or thermally accessible (Trz-CF
3
’,
similar impact on the electronic properties of a given system as
changing the peripheral substituent itself: A comparison of the
Trz-OMe’) under ambient conditions.
In order to evaluate whether rapid interconversion between Kohn-Sham frontier orbitals of Trz-CF3’ and Trz-OMe’ reveals
the conformers is possible, a simplified system (Scheme 2c, the differences in the orbital distribution between the two
Figure 4) was also studied. For this smaller molecule the closed derivatives to be much less pronounced than between the
c
o
(
TS ) and open
(
TS ) geometries were again of comparable individual conformers (Figure 5, see Figures S45 - S48 in the
‡
energy. A transitional state (TS ) was located that reassembles ESI). The HOMOs of the closed conformers consist of
the open geometry and is between 26 and 31 kJ/mol higher in contributions from the central diborylphenylene-unit and the
energy than either conformer. This result corroborates the electron-rich mesityl-groups, while the LUMOs are delocalized
findings from the dynamic NMR study of rapid interconversion throughout the phenyltriazole moieties. The electronic
in solution at ambient temperatures. The depiction of TS
o
configuration is almost reversed for the open conformers.
(Figure 4, bottom right) also gives an indication why (half)- Here, the HOMOs also extend onto the triazole rings, and the
open structures are where found so close in energy to the LUMOs are delocalized in the central phenylene ring and
closed conformers: Optimizations of non-N→B-coordinated dominated by contributions from the empty p -orbitals on the
z
4
1
moieties inadvertently converged on structures wherein steric boron centers, analogous to the precursor B2-H
strain is reduced through π-stacking of triazole- and mesityl-
Moreover, while the energy levels of the frontier orbitals
.
rings. The respective ring planes stand at angles between 11° show the expected stabilization when moving from electron
o
and 17° (15.4° for TS ), while the C4 carbon atom in the rich Trz-OMe’ to more electron deficient Trz-CF3’ (e.g. closed-
triazole ring and the Mesityl-ipso-carbon next to boron are Trz-OMe’ vs. closed-Trz-CF3’: ΔEHOMO(OMe→CF ) = -0.27 eV,
3
o
,
brought into close proximity (3.111 to 2.996 Å, 3.007 Å for TS
ΔELUMO(OMe→CF ) = -0.76 eV), this change is comparable to
3
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Journal Name
energetic differences between the conformers of an individual
borane Trz-CF open closed +0.57 eV,
open closed) = -0.18 eV) (see Table S3 in the ESI). The
(
3
’
:
ΔEHOMO
(
→
)
=
ΔELUMO
(
→
electronic properties of a given borane should therefore be
defined, both by the properties of the predominant conformer
in solution, and its response to external stimuli. This hypo-
thesis is supported by a series of experiments outlined in the
following.
Electrochemical analysis
In order to corroborate the hypotheses derived from DFT
calculations, electrochemical reduction potentials of the
boranes Trz-Me, Trz-CF3, Trz-OMe, of the polymer PTrz, and of
4
1
the trimethylsilyl-protected precursor B2-TMS were deter-
mined via cyclic voltammetry (CV, Figure 6a) and square-wave
voltammetry (SWV, Figure 6b) in THF.
The two tricoordinate boron centers in B2-TMS are very
effectively conjugated across the p-phenylene ring. Accor-
dingly, separate one-electron reductions to the radical anion
and dianion can be observed. The first, reversible reduction
+
showed a peak current centered on -2.00 V (SWV, vs. Fc/Fc ).
Reduction to the dianion proved irreversible in our hands, with
a peak current at -2.59 V. The redox potentials and the large
redox-splitting of ΔE = 0.59 V are in good agreement with the
data for the parent compound 1,4-(Mes B) -benzene (E = -
2
2
0/-1
48,49
4
+
1
.86 V, E-1/-2 = -2.55 V vs. Fc/Fc in DMF, 0.1 M NBu ClO
4
)
and the closely related 1,3-diborylphenylene derivative (E
+ 50
2.02 V, E-1/-2 = -2.64 V vs. Fc/Fc in THF, NBu PF ).
=
/-1
0
Figure 5. Kohn plots of open and closed conformers of Trz-OMe and Trz-CF
3
. Plots
-
4 6
generated using GaussView 5.0.8, isovalue 0.03. Hydrogen atoms have been omitted
The reduction potentials of the triazolyl-functionalized for clarity.
boranes appear cathodically shifted. For Trz-CF3 two individual
+
one-electron reductions at -2.29 and -2.37 V vs. Fc/Fc could
be resolved by square wave voltammetry. The other boranes
exhibit only a single reduction each, at -2.59 (Trz-Me) and -
2.61 V (Trz-OMe), at potentials that are effectively equal to the
second reduction of B2-TMS. Reduction of PTrz in THF solution
could be observed at -2.67 V (SVW, see Figure S2 in the ESI for
CV), while films of PTrz, measured against acetonitrile solution,
showed a sharp, fully reversible reduction wave at -2.53 V (see
Figure S3 in the ESI).
The important aspect of these results is the contrast
between the large redox-splitting in
Trz-CF , and the absence thereof in Trz-Me
Redox-splitting gets consecutively smaller with expansion of
B
2
-TMS, the smaller one in
3
,
Trz-OMe and PTrz
.
4
8,51,52,53
the π-system.
that in Trz-CF
more extended π-system than in
with findings from the DFT-study that showed closed
to be the most stable geometry. In contrast, for the dianions of
In the present case, this would indicate
the negative charge is delocalized throughout a
-TMS. This is in agreement
3
B
2
-
2
-
Trz-CF
3
-2
Trz-Me
, Trz-OMe and PTrz the open conformation is
predicted to be the most stable. The observation that these
boranes only take up electrons after the second reduction
2
potential of the precursor B -TMS is reached, corroborates the
impression that rather than stepwise reduction, two-electron
reductions occur with concurrent quantitative decoordination
2 3
Figure 6. a) Cyclic voltammograms of B -TMS, Trz-Me, Trz-CF , Trz-OMe, and b) square
-
and formation of the more stable open conformers.
2
wave voltammograms of Trz-Me, Trz-CF3, Trz-OMe, PTrz, and B2-TMS. * internal
+
standard Fc/Fc .
6
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ARTICLE
Optical properties and anion binding
UV-vis spectroscopy
trigger emission prior to complete relaxation of the molecules
resulting in the observed blue-shift.
Further analysis of the triazolyl boranes by UV-vis absorption
and fluorescence spectroscopy revealed very similar
absorption spectra (Figure 7a). All compounds show strong
absorption bands (λmax) around 290 nm, accompanied by
distinct shoulders toward longer wavelength. The shape of this
shoulder indicates that it consists of a superimposition of
multiple absorptions, possibly originating from different
conformers. Deconvolution of the spectra via Gaussian fit
allowed us to derive the longest wavelength absorption bands
(
λ
2
), that are centered on 335 nm for PTrz
OMe, and on 341 nm for Trz-CF , (see Figures S5-S10 in the
ESI), with corresponding onsets at 351 (PTrz), 358 nm (Trz-
Me Trz-OMe) and 367 nm (Trz-CF ), respectively.
, Trz-Me, and Trz-
3
/
3
In contrast to the minor differences in the optical absorption,
the fluorescence spectra of the triazolyl boranes vary
drastically (Figure 7b). PTrz shows a comparatively sharp
emission maximum at 374 nm, that corresponds to a Stokes
-1
shift of 3225 cm . The model boranes show very broad
emission bands with vibronic structuring and shortest
wavelength maxima at 412 nm (Trz-OMe), 474 nm (Trz-Me),
3
and 512 nm (Trz-CF ), that correspond to large Stokes shifts,
-1
ranging from 5500 to 10000 cm . Trz-OMe and Trz-Me
showed the highest fluorescence quantum yields (φ) of 17.3%
and 13.7%, and long fluorescence lifetimes of τ = 20 and
3
τ = 23 ns, while Trz-CF was less emissive (φ = 6.4%) with a
Figure 7. a) UV-vis absorption spectra and b) normalized fluorescence spectra of Trz-
lifetime of τ = 11 ns (see also Figure S11 in the ESI). The
similarity of the absorption and the disparity of the emission
spectra may be explained by different relaxation pathways
Me, Trz-CF
3 2 2
, Trz-OMe, B -TMS, and PTrz in THF. Excitation wavelengths: B -TMS:
3
35 nm, Trz-R: 330 nm.
before emission. The lower quantum yield and shorter To identify the origin of the individual absorption bands, TD-
fluorescence lifetime of Trz-CF
imply that this borane more DFT calculations have been performed on the individual
3
readily undergoes non-radiative relaxation and is therefore conformers (see Figure S44 and Table S4 in the ESI). According
likely less rigid than Trz-OMe and Trz-Me. One explanation to the calculations, the lowest energy transitions of open
may be photodissociation of N→B-bonds and subsequent conformers (excited state 1, ES1), are near exclusively HOMO-
conformational changes. An analogous intermolecular photo- to-LUMO in character, but are also only weakly populated with
dissociation has been described for pyridine-adducts of oscillator strengths (f) below of 0.05 (ES1 (open): e.g. Trz-CF ’:
3
8
d
boranes with moderate Lewis-acidity, and would agree with E = 3.15 eV / 394 nm, f = 0.024, Trz-OMe’: E = 3.23 eV / 384
the successively increasing Stokes shift when moving from Trz- nm, f = 0.033). Conversely, all open conformers show strongly
OMe to Trz-CF
The dynamic NMR study and the DFT calculations indicated excited state (ES3 (open): Trz-CF
3
.
populated transitions of comparable energy into the third
: E = 3.37 eV / 368 nm f =
3
’
that all thermally accessible conformers may be present in 0.133, Trz-Me’: E = 3.25 eV / 381 nm, f = 0.157, Trz-OMe’: E =
solution, and interconvert with a rate constant of ca. 30 Hz, 3.49 eV / 355 nm, f = 0.175, PTrz’: E = 3.50 eV / 354 nm, f =
even at 208 K. Consequently, absorption and emission likely 0.171, f = oscillator strength, see Table S4 in the ESI). These
represent superimposed spectra both different conformers. In transitions are largely HOMO-2-to-LUMO in character and
order to further elucidate this conjecture, UV-vis absorption represent charge transfers from the mesityl-groups to the
and fluorescence spectra of Trz-OMe and Trz-CF
recorded at temperatures between -20°C and 70°C in THF (see
3
have also diboryl-phenylene-moiety.
Transition into the first excited state of closed conformers is
Figures S12 and S13 in the ESI). These experiments showed also mainly HOMO-to-LUMO in character and represents a
only minor changes to the absorption spectra, while fluoresce- charge redistribution from the central core to the aryl-triazol-
ence spectra of both compounds exhibited blue-shifts of ca. moieties (Figure 5). Excitation into ES1 is generally lower in
2
0 nm upon heating. These results indicate that the relative energy than for the corresponding open conformers (ES1
ratios of the individual conformers do not change within the
closed): e.g. Trz-CF : E = 2.59 eV / 479 nm, f = 0.018, Trz-
(
3
’
observed temperature range, and hence the absorption OMe’: E = 3.04eV / 408 nm, f = 0.022). However, the
remains largely unaffected. Higher temperatures appear to transitions are only weakly populated, likely due to the limited
spatial overlap of the involved orbitals that hinders electronic
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coupling. The calculations predict no other strong absorptions
for closed conformers, with oscillator strengths among no
greater than 0.062 (ES9 of closed-Trz-OMe’: E = 3.40 eV /
365 nm, f = 0.062) for all four boranes.
All conformers also show multiple, complex absorptions that
largely involve orbitals ranging from HOMO-7 to LUMO+2 and
can mostly be considered as charge transfers from mesityl-
groups either to the aryl-triazolyl-moieties (closed) or to a
z
boron-p -dominated LUMO or LUMO+1 (open). These latter
transitions presumably compose the strong absorption band
2
below 325 nm. The origin of the λ -absorption band is less
clear. The strong absorption predicted for the open
conformers and the good agreement with the experimental
data, indicate that absorption from these species may
2
contribute to the λ -band. However, since all conformers are
presumably present in solution even at low temperature, we Scheme 3. Anion binding in N→B-boranes. Ar = p-MeOPh, p-MePh, p-F3CPh.
could not further corroborate this assignment experimentally.
Trz-OMe and Trz-Me showed only a limited response to the
-
addition of anions: Addition of a large excess of CN to Trz-
Anion binding studies
2
OMe resulted in partial suppression of the λ -band in the
More direct evidence for the variable strength of the N→B-
bonds was derived from anion binding experiments: Strong
absorption spectrum, while a weak shoulder band with an
absorption maximum at 370 nm emerged (Figure 8a,c). Only a
,54 marginal weakening of the fluorescence emission intensity was
observed (Figure 8e). Trz-Me showed a similar suppression of
nucleophiles such cyanide and fluoride coordinate even to
2
sterically
congested
Lewis-acidic
boron
centers.
Accordingly, addition of tetrabutylammonium cyanide (TBACN)
1
1
2
the λ -band and a 50% reduction of fluorescence intensity in
to Trz-CF
3
yields a sharpened signal at -13.6 ppm in the
B
-
the presence of a much smaller excess of CN (Figure 8b,d,f).
While these changes should be attributable to coordination of
NMR spectrum, that signifies coordination of cyanide to boron
5
Scheme 3, Figure S15 in the ESI). Notably, B NMR of Trz-
5
11
(
-
CN to boron, it must be pointed out that we were unable to
OMe and Trz-Me showed no response to addition of excess
cyanide. To further quantify this observation, all three boranes
were titrated with tetrabutylammonium cyanide (TBACN) and
the progress of the titrations was monitored by absorption and
1
observe this interaction directly by B NMR. In contrast, Trz-
1
-
3
CF , for which direct binding of CN to boron was observed,
-
exhibits a much stronger response to CN -addition (Figure 9).
-
Addition of up to ca. two equivalents of CN also causes
5
6
fluorescence spectroscopy (Figures 8 and 9).
2
suppression of the fluorescence emission and of the λ -
absorption band. Further addition of nucleophile then leads to
formation of a new emission band at 435 nm that is distinctly
blue-shifted compared to the emission of the neat borane Trz-
3 3
CF (512 nm). Cyanide binding constants for Trz-CF were
estimated as lgβ11 = 4±1 and lgβ12 = 8±1 for coordination of
-
one and two CN anions, respectively. The significant errors are
owed to the complex equilibria, and unknown optical
properties of the mono-adduct. Still, the anion affinity is
Table 3. Physicochemical data for ladder boranes and substrates.
Stokes
[
2
a]
[g]
CV[d]
1/2
SWV
[f]
swv[c]
λ
max
λ
λ
Onset
ε
λ
em
Ф
E
E
p
LUMO
[
b]
shift
-
1
-1
[
nm]
[nm]
/
[eV]
[nm]
[L∙mol cm ]
[nm]
/
[eV]
[%]
[V]
[V]
[eV]
-
1
[
cm ]
B
2
-TMS
276
265
346
342
335
3.58
3.62
3.70
393
390
358
32 660
n.d.
490
490
2.53
2.53
8430
8020
5580
n.d.
n.d.
17.3
-2.03
n.d.
-2.00, -2.59
n.d.
-3.10
n.d.
B
2
-H
4
4
12,
57
3.01,
2.71
Trz-OMe
Trz-Me
2
89
54 200
-2.61
-2.59
-2.51
2
2
2
86
90
76
335
341
334
3.70
3.64
3.71
358
367
352
43 600
20 500
n.d.
474
512
374
2.62
2.42
3.32
8760
9790
3225
13.7
6.4
-2.59
-2.39
-2.56
-2.29, -2.37
-2.66
-2.54
-2.73
-2.44
Trz-CF
PTrz
3
e
n.d.
-2.67 (-2.53)
SWV
[
a] Determined via Gaussian fit of the absorption spectra. [b] Derived from λmax and the shortest wavelength emission maximum. [c] Based on the SWV peak potential (E
of the first reduction and the HOMO-level of ferrocene (-5.1 eV, see ref. 58). [d] Half-potentials determined via cyclic voltammetry in THF-solution with 0.1 M [NnBu ][PF
scan-speed 100 mV/s. [e] Recorded as dip-coated film on a platinum electrode vs. 0.1 M [NnBu ][PF ] in NCMe-solution. [f] Peak potentials determined via square-wave
p
)
4
6
],
4
6
voltammetry in THF-solution with 0.1 M [NnBu
4 6
][PF ], scan-speed 100 mV/s. [g] Determined at λmax.
8
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Page 9 of 12
Journal Name
orders of magnitude lower than those of other functionalized
Jou Pr nl e aa ls oe fd Mo an to et r ai ad lj su s Ct hm ea mr g ii sn ts ry C
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DOI: 10.1039/C6TC05418H
ARTICLE
Conclusions
5
7
anion sensors (e.g. lgβ11 > 8, and lgβ12 = 16.6 for a twofold
We have demonstrated that CuAAC readily allows ethynyl-
56 groups in triarylboranes to be converted into 1,2,3-triazolyl-
BMes
2
-functionalized quaterthiophene). Further experiments
-
towards F .
indicated a similar binding behavior of Trz-CF
-
3
-
-
-
-
-
-
, SCN , OCN , OAc , Cl ,
substituents with electronically diverse substituents and a
suitable regiostructure for intramolecular N→B-coordination.
Binding of other anions (N
-
Br ) could not be observed.
3
, NO
2
, NO
3
1
1
B NMR and crystal data showed N→B-coordinated species to
be present both in solution and in the solid state, while
1
dynamic H NMR and DFT calculations indicate that dynamic
N→B-bond dissociated occurs even at low temperature. The
electrochemical and fluorescence properties, as well as anion-
binding studies show a direct correlation of the respective
physical property with the peripheral substituent, while
derivatization hardly affects the optical absorption. We
attribute these differences to the ability of the boranes to
dynamically revert, from closed, N→B-coordinated confor-
mations to open
, non-coordinated structures, as clearly
evidence by binding of cyanide to boron in Trz-CF
3
.
This work proves that labile N→B-coordination of 1H-1,2,3-
triazoles can be exploited to tailor the electronic properties of
organoboranes. Furthermore, since substituents on said 1H-
1,2,3-triazoles can readily be varied via CuAAC, this allows to
incrementally change the accessibility and Lewis acidity of a
given boron center.
Acknowledgements
This work was supported by the German Chemical Industry
Fund (FCI) (Liebig scholarship for F.P.) and the German Science
Foundation (DFG). We thank Prof. Martin Korth (Ulm
Figure 8. Emission and absorption spectra corresponding to the titration of Trz-OMe University) for discussions, Dr. Karlheinz Röcker (Ulm
-
5
(
a,c,e), and Trz-Me (b,d,f), with tetrabutylammonium cyanide. c(Trz-OMe) = 1.2*10 M, University) for support with fluorescence lifetime measure-
-
3
-
5
-
3
c(TBACN)= 4.8*10 M; c(Trz-Me) = 2.1*10 M c(TBACN)= 8.2*10 M. c) and d) show
ments, and Prof. Markus Enders (Univ. Heidelberg) for support
11
with B NMR and helpful discussions.
enlarged sections of a) and b), respectively.
Notes and references
(
1) (a) Y. Ren and F. Jäkle, Dalton Trans., 2016, 45, 13996-
1
3
4007; (b) F. Jäkle, Top. Organomet. Chem., 2015, 49, 297-
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in Comprehensive Inorganic Chemistry, Elsevier Ltd., 2013;
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(
Coord. Chem. Rev., 2006, 250, 1107-1121; (f) F. Jäkle in
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(
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-
5
-3
3 3
CF with tetrabutylammonium cyanide, c(Trz-CF ) = 4.0*10 M, c(TBACN)= 1.7·10 M.
d) Visible fluorescence response to TBACN addition. b) shows an enlarged section of a)
and additional titration points.
(
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Journal Name
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