“Hetero-Multifunctionalization” of Gallium Corroles: Facile Synthesis, Phosphorescence, Redox Tuning, and Photooxidative Catalytic Improvement

Article abstract of DOI:10.1002/cplu.201900667

Bromination of tris- and tetrakis-trifluoromethylated gallium corroles (3CF₃?Ga and 4CF₃?Ga) afforded tetrabrominated species 3CF₃?4Br?Ga and 4CF₃?4Br?Ga (yields: 20 % and 25 %) characterized by NMR, UV-vis spectroscopy, and mass spectrometry. Red-shifted absorption and emission bands were found; 3CF₃?4Br?Ga and 4CF₃?4Br?Ga displayed 5–12 nm shifts in their Soret bands and 8–17 nm shifts for their Q bands, compared to the respective nonbrominated species (3CF₃?Ga and 4CF₃?Ga). The respective Φ_F values were found to be 0.013 and 0.016; phosphorescence (lifetime=0.23 μs) was observed for 3CF₃?4Br?Ga (anaerobic, RT). The effect of tetrabromination on redox potentials (0.89 and 0.99 V) gave a 85 mV shift per Br atom in the reduction potential. 4CF₃?4Br?Ga allows for efficient catalytic photooxidative Br^? to Br₂ conversion compared to the β-octa-Br system (Br₈?Ga) structurally characterized here. This “hetero-multifunctionalization” approach, that is, is substitution with different sets of β-substituents, can help optimize porphyrinoid properties.

Full text of DOI:10.1002/cplu.201900667

Accepted Article
Title: “Hetero multifunctionalization” in Gallium Corroles: Facile
Synthesis, Phosphorescence, Redox Tuning, and Photooxidative
Catalytic Improvement
Authors: Xuan Zhan, Yael Zini, Natalia Fridman, Qiu-Cheng Chen,
David G. Churchill, and Zeev Gross
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“
Hetero multifunctionalization” in Gallium Corroles: Facile
Synthesis, Phosphorescence, Redox Tuning, and Photooxidative
Catalytic Improvement
Xuan Zhan , Yael Zini , Natalia Fridman , Qiu–Cheng Chen , David G. Churchill[a],[b],[c],[d]*, Zeev
[
a]
[a]
[a]
[a]
[
a],
Gross *
Abstract: Bromination of tris– and tetrakis–trifluoromethylated
When investigating the properties of porphyrinoids, it has been
found that corrole synthesis, for example, has been slow
regarding rational substituent placement/incorporation because
as stepwise formal substitution is imparted, the breakdown in
molecular symmetry affords eight different β positions. The
systems can also be difficult to access or predict if substitution
proceeds by a radical mechanism. In comparing the effects
imposed by different substituents, there is often a tendency to
compare between separate corrole systems that have different
central atoms but that bear the same β substituents, or a system
possessing the same central metal, and with a different number
of the same β–substituent or those with different attachments of
the same substituent. Therefore, while these analogues, or
constitutional isomers, help us establish important groundwork
principles, it is also important to seek more complexity regarding
β-position substitution. A particular challenge would be to have
different non–H substituents present around the macrocyclic ring.
Herein, we describe the results of studies aimed at generating
closely related compounds containing different non-H sets of
substituents at the β positions. Therefore, they can be both
exhaustively compared with each other, and also compared back
to the reference molecules that contain only one type of
gallium corroles (3CF
species 3CF –4Br–Ga and 4CF
characterized by NMR, UV–vis spectroscopy, and MS. Red shifted
abs and emission bands were found; 3CF –4Br–Ga and 4CF –4Br–
Ga displayed 5–12 nm shifts in their Soret bands and 8–17 nm shifts
3
–Ga and 4CF
3
–Ga) afforded tetra–brominated
3
3
–4Br–Ga (Yields: 20% and 25%)
3
3
for their Q bands, compared to the respective non–Br species (3CF
Ga and 4CF –Ga). Respective Φ
.016; phosphorescence (lifetime = 0.23 µs) was observed for 3CF
3
–
3
F
values were found to be 0.013 and
0
3
–
4Br–Ga (anaerobic, RT). The tetra–Br effect on redox potentials (0.89
and 0.99 V), gave a 85 mV per Br shift in the reduction potential.
-
4CF
3
–4Br–Ga allows for efficient catalytic photooxidative Br to Br
2
8
conversion compared to the β–octa-Br system (Br –Ga) structurally
characterized here. “Hetero multifunctionalization” is substitution with
different sets of β–substituents to help optimize porphyrinoid
properties.
Introduction
Robust research on corroles, well-known contracted and aromatic
tetrapyrrole–based macrocycles, continues to be produced and is
considered in many technological applications. Facile and
efficient synthetic methods to help obtain these systems cleanly
and in good yields have been discovered, developed and
optimized starting from about 1999.[1] In recent years, researchers
have aimed at new ways of tuning chemical and photophysical
properties to help achieve desired molecular properties for
particular applications. Such accomplishments are on the verge
of strongly impacting energy–related fields such as photo–
sensitizers, dye–sensitized solar cells, photocatalysis in driving
photo–synthesis of small organic molecules and medicinal
applications like photo–sensitizers in photodynamic therapy
substituent.
The differences in spectroscopic signals and
electrochemical properties, for example, will help gauge what is
possible when the system becomes saturated with different sets
of non-H substituents at the β positions.
The structure–function relationship in corrole complexes has
been increasingly studied in recent years; one important
approach for corrole functionalization is the different ways
substitution of β–protons of corroles have been made possible.
[3]
By replacing β–protons with certain groups such as halogens
[4]
and trifluoromethyl groups , these substituent effects are at the
“
core” of physical organic investigations and interrogations of
porphyrinoids. Therefore, while the bromine group is often
considered to be a precursor to easily allow further substitution
through, e.g., cross coupling, there is a need to synthetically
access systems that are especially closely related but that have
differences through the presence of two different sets of β-
substituents.
Consequently, our work herein relates to a concept we will term
as porphyrinoid “hetero multi–functionalization” (substitution with
different functional groups involving multiple groups of each type).
The incorporation of these sets of substituents may be made
possible through treating a precursor involving one set with a
second substituent that will be able to occupy remaining C–H
positions through facile processes which (i) do not synthetically
disrupt the first substituent set, and (ii) also which, when
compared to H- substitution, help macrocycles/porphyrinoids
access important physical or chemical properties. In this study,
(
PDT) due to ROS (reactive oxygen species) generation under
irradiation with specific frequencies of incident light. [2]
[
a]
X. Zhan, Y. Zini, Dr. N. Fridman, Q-C. Chen, Prof. Dr. D. G.
Churchill, Prof. Dr. Z. Gross
Schulich Faculty of Chemistry
Technion-Israel Institute of Technology
Haifa 32000 (Israel)
E-mail: chr10zg@technion.ac.il
[
[
b]
c]
Prof. Dr. D. G. Churchill
Department of Chemistry
Korea Advanced Institute of Science and Technology (KAIST)
Daejeon (Republic of Korea)
E-mail: david.churchill.korea@gmail.com
Prof. Dr. D. G. Churchill
Center for Catalytic Hydrocarbon Functionalizations
Institute for Basic Science (IBS)
Daejeon (Republic of Korea)
[
d]
Prof. Dr. D. G. Churchill
KAIST Institute for Health Science and Technology (KIHST), 291
Daehak–ro, Yuseong–gu, Daejeon 34141, Republic of Korea
E-mail: david.churchill.korea@gmail.com
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to lay a foundation for “hetero multi–functionalization,” we used
gallium corroles. Therefore, the combination of the two sets (two
groups, Scheme 1) (or more than two sets) will further help shape
the evaluation of novel and more sophisticated porphyrinoid
designs in the future.
experiences a respective downfield shift (from 9.43 to 9.74 ppm)
when going formally from species 3CF –Ga to 3CF –4Br–Ga; a
3 3
small downfield shift was also observed in the signals appearing
in the 19F–NMR spectrum once bromination was achieved. This is
exemplified by the fluorine chemical shift in the –CF
region: the three –CF peaks moved from –48.9 ppm, –50.5 ppm
and –53.3 ppm; to –48.3 ppm, –50.1 ppm and –53.0 ppm. The
3
signal
3
1
9
downfield shifts observed in the
F NMR spectrum are
Results and Discussion
rationalized by the electron withdrawing effect imparted by
multibromination. The C2v molecular symmetry was found to
In order to prepare gallium corroles that contain two sets of
substituents, in which both sets bear multiple substituents, we
considered bromination of a well defined multi–substituted
trifluoromethylated corrole system. In this study, we initiated our
work from the β-unsubstituted tpfc gallium framework and then
remain after bromination transformed 4CF
One effective piece of evidence is to track the collection of the
three –C
group para–F resonances present in the 19F–NMR
3 3
–Ga to 4CF –4Br–Ga.
6 5
F
spectrum; when inspecting for these three nuclei, the three para–
F resonance signals are grouped cleanly in a pattern with an
integration ratio of 1:2 (Figure S2). The HR–MS spectra also
substituted the –CF
3
groups onto β positions and isolated these
species.4c Then, we were able to impart bromination using Br
2
supports our expected structures for compounds 3CF
3
–4Br–Ga
without interfering with the already-attached -CF groups.
Specifically, we advanced our study with the gallium complexes
of tris–/tetra–trifluoromethylated tpfc (5,10,15–
tris(pentafluorophenyl) corrole) gallium corrole complexes: 3CF
Ga and 4CF –Ga (Scheme 1). Treating these corroles with
3
and 4CF –4Br–Ga.
3
3
–
3
excess bromine in methanol at room temperature allowed for the
production of species in which the β C–H rich region (at the 7,8,12,
and 13 β positions) was exhaustively converted to C-Br groups;
this resulted in the formation of our target compounds bearing
proximal tetra-bromine substitution, namely gallium (III) 5,10,15–
tris(pentafluorophenyl)–2,3,17–tris–trifluoromethyl–7,8,12,13–
tetrabromocorrolate (3CF
tris(pentafluorophenyl)–2,3,17,18–tetra–trifluoromethyl–
,8,12,13–tetrabromocorrolate (4CF –4Br–Ga). The reason we
3
–4Br–Ga) and gallium (III) 5,10,15–
7
3
Scheme 1. Synthesis of tetra–brominated tris–/tetra–CF
3
–substituted species:
choose bromine rather than other bromination reagents such as
NBS (N-bromosuccinimide) for this reaction is that relative higher
selectivity could be obtained for the β-bromination of corrole when
3
CF –4Br–Ga and 4CF –4Br–Ga.
3
3
[
5]
Br
conducted via silica gel column chromatography using a 5:1
volume ratio) mixture of n-hexane:ethyl acetate; several drops of
2
was used based on previous reports. Purification was
(
pyridine were also added to the reaction mixture. The first eluents
for both reactions were confirmed by multinuclear NMR
spectroscopic and mass spectrometric characterization
techniques. The respective isolated yields were found to be 20 %
and 25 %. Bromination at the available 18 position in 3CF
3
–4Br–
Ga to give 3CF –5Br–Ga was not observed, as predicted due to
3
steric and electronic considerations. The tetra-brominated
species were the only dominant products after silica gel
chromatography purification; no tribrominated corroles were
detected.
It was important to interrogate the signals in the H and 19F NMR
spectra to confirm both the molecular symmetry and level of
substitution/saturation of the products (Figure 1). The support for
the occupation of the 7,8,12,13 positions was made possible by
the observation of the diminution/absence of the signals assigned
to the β–pyrrole protons resulting from addition of bromine atoms
1
Figure 1. Comparisons of the pertinent regime in the H (400 MHz) and 19F (377
MHz) NMR spectra (CDCl ) of 3CF -Ga, 3CF -4Br-Ga, 4CF -Ga and 4CF -4Br-
Ga, with a focus on the β–pyrrole CH’s and the CF groups.
1
3
3
3
3
3
3
Another important characterization method for corrole complexes
is UV–vis spectroscopy. As shown in Figure 2, a typical Soret
band is around 430 nm and several Q bands were observed; all
data are provided in Table 1. Our previous work [ showed that
β-trifluoromethylation induced red shifts in terms of absorption
and emission spectra. Compared to the non-substituted complex
(
via electrophilic substitution). Namely, only one signal was
exhibited for 3CF –4Br–Ga, whereas no β–pyrrole CH group
signal appeared in the spectrum as acquired for compound 4CF
Br–Ga. In Figure 1, these spectra are overlayed to allow for a
convenient and clear comparision; the chemical shift of C18–H
3
4c]
3
–
4
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was recorded in toluene at room temperature and under aerobic conditions; the
excitation wavelength was 420 nm. (c) The emission spectra comparison of
compound 3CF –4Br–Ga under aerobic and N atmosphere in toluene was
3 2
(
4
tpfc)Ga, the tris/tetra-CF
3
3
substituted complex 3CF -Ga and
CF -Ga exhibited small red shifts of 3 and 6 nm in the Soret
3
band and more evident red shifts of 18 and 30 nm in the Q bands.
Red shifts of 20 and 32 nm were also observed for complexes
acquired at room temperature. (d) A time–resolved phosphorescence decay
2
curve with χ –fitting was recorded under N
2
atmosphere in toluene; the
excitation wavelength was 643 nm and the emission wavelength was 900 nm
when operating the decay assay.
3
3 3
CF -Ga and 4CF -Ga, relative to (tpfc)Ga, in terms of the
emission peaks (Table 1). Further bromination produced more
red shifts in terms of UV and emission spectra; the different tetra–
brominated gallium corrole species 3CF
Br–Ga display a small 5–12 nm shift in the Soret band and larger
shifts of 8–17 nm for the Q bands in these species, compared to
the respective non–brominated species 3CF –Ga and 4CF –Ga.
The fluorescent properties of the two new brominated compounds
were also probed; a miniscule amount of fluorescence was
observed as expected, for these two heavily brominated gallium
3 3
–4Br–Ga and 4CF –
4
Table 1. Comparison of photophysical properties of all
investigated gallium corrole complexes (recorded at room
temperature in toluene).
3
3
b
τ(ns)c
2.61
2.49
Complex
Soret band
Q–band
λ
em a(nm)
Φ
F
(
nm)
(nm)
(ε,×10 )
corroles (Figure 2b); the fluorescence quantum yields (Φ
F
)
4
4
(ε, ×10 )
decreased from 0.57 and 0.55 for the non–brominated species
3
CF
3
3
–Ga and 4CF
3 3
–Ga to 0.013 and 0.016 for 3CF –4Br–Ga and
tpfc-Ga
402,
424 (28.30)
571,
597
602,
657
0.28
0.57
4CF
–4Br–Ga, respectively. Tetra–bromination also induced a
(2.40)
red shift of 20–35 nm for the emission peak (λem,max) when going
from the 3/4 CF –Ga to the 3/4 CF –4Br–Ga species (Table 1).
Shortened fluorescence lifetimes were observed (Table 1)
compared to the non–brominated species. One important and
interesting finding was the discovery and quantification of room
temperature phosphorescence (λmax = 900 nm) (Figure 2c ) for
3
3
3
Ga
CF
3
–
–
405 (2.96),
427 (7.16)
590
(1.48),
615
623, 677
643, 698
634, 689
(2.02)
3CF
3
413 (1.40),
437 (5.96)
598
(0.69),
626
0.013
0.55
0.24
4.54
3
2
compound 3CF –4Br–Ga, under N . The phosphorescence
4Br–Ga
lifetime was determined to be 0.23 µs (Figure 2d); corrole
complexes which were reported to emit room temperature
phosphorescence in anaerobic toluene above 800 nm were
(0.88)
4CF
3
–
416 (4.67),
430 (6.36)
574
(1.56),
596
[4a],[6]
confined to several cases;
for example, the iridium
Ga
coordinated species (tpfc)Ir with two pyridine as the axial ligands
exhibited room temperature phosphorescence with a wavelength
at around 800 nm; the corresponding lifetime was 4.91 µs under
argon atmosphere. Another tetra-β-trifluoromethylated gold (III)
tpfc complex was reported to emit room temperature
phosphorescence at a wavelength of 835 nm. This finding
therefore enlightened us about how bromination in these systems
imparts the heavy atom effects to help establish room
temperature phosphorescence.
(2.03),
627
(3.39)
4CF
3
–
421 (4.42),
442 (6.10)
586
(1.36),
669
0.016
0.26
4Br–Ga
6
09
1.69),
44
2.42)
(
6
(
[
a] λem represents the wavelength of the emission peak (λex = 420 nm). [b]
Calculated fluorescence quantum yields in aerated toluene with TPP
tetraphenylporphyrin) as standard (Φ 0.11). [c] Experimental
(
a
F
=
fluorescence lifetimes (prompt) measured in aerated toluene (λex = 420
nm).
The observation of how bromination changes the photophysical
properties encouraged us to seek quantitative data as acquired
from electrochemistry. We found that certain substituents may
exert a profound influence on the basic electronic redox potentials
3
such as –CF substitution in gallium corroles that was recently
found to be dramatic; compared to non-substituted species
(
4
tpfc)Ga, the tris- and tetra-CF
3
substituted species 3CF
3
-Ga and
st
CF -Ga displayed positive shifts of 400 and 440 mV of the 1
3
st
oxidation potential, respectively (the 1 oxidation potential of 0.37
V for (tpfc)Ga was recorded).[4c] It is of great interest to probe the
effect from further bromination on redox properties; Herein,
additional CV (cyclic voltammetry) experiments (Figure 3, Table
Figure 2. (a) The UV–vis spectra of all investigated complexes at
concentration of 1.4 × 10 mol L . (b) The fluorescence emission spectra of
studied compounds under the same absorbance (0.05 OD) at 420 nm. All data
a
–5
-1
2) were conducted to help elucidate the electronic effects of tetra–
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bromination as well as assessing HOMO–LUMO energy levels.
Large positive redox shifts were evident, observed after tetra–
bromination was achieved. Specifically, whereas the 1 oxidation
Studies of porphyrinoid systems have recently entailed (i) fuels
research related to renewable energy sources, as well as (ii)
molecular catalysis/photocatalysis; these various applications
have been increasingly “hot” research topics in recent years with
regard to corrole systems in particular. Besides catalytic water
splitting reactions,[9] the aerobic oxidation of bromide–to–bromine
st
potentials appeared at 0.77 and 0.83 V for non–brominated
species 3CF
significantly to 0.89 and 0.99 V for the tetra–brominated species
CF –4Br–Ga and 4CF –4Br–Ga, affording respective positive
3 3
–Ga and 4CF –Ga, these features formally moved
3
3
3
2 2
was also considered important in the design of H /Br
fuel cells.[10]
shifts that break down to 30 and 40 mV per Br. These values are
less than those previously reported: a positive oxidation shift of
Compared to the previous reported fully β–brominated gallium
tpfc corrole (8Br-Ga) which exhibited excellent photocatalytic
performance in bromide to bromine conversion,[11] the complex
1 mV from the effect of bromination.[7] Surprisingly, more
5
significant positive shifts were seen, compared to complexes
CF –Ga and 4CF –Ga and 3CF –4Br–Ga and 4CF –4Br–Ga,
3
4CF –4Br–Ga, owing to a more positive oxidation potential
3
3
3
3
3
seemed to be more efficient in driving photooxidation reactions.
While the 8Br-Ga compound has been reported before,[11] its X-
ray study is now included for the first time having been
successfully obtained through this study (Figure 4). In the
structure, the tetra N-pyrrolyl pocket containing the central metal
reveals [Ga-N] bond lengths ranging from 1.920 to 1.956 Å; these
are considered normal Ga-N bond-length values in comparison to
previous reports.[4c],[12] One pyridine was observed as the axial
ligand coordinating to the Ga with Ga-N bond length of 2.033 Å,
close to that observed in other gallum corrole complexes reported
before.[4c],[12] Unlike trifluoromethylation which most likely induces
evident planar distortion as reported before by us,[4] the
macrocyclic distortion based on measurements of dihedral angles
suggested no evident planar distortion exists upon octa-
bromination of the macrocycle. The hetero multifunctionalized
which produced reduction shifts of 340 mV and 290 mV, once
tetra–bromination was achieved. The much larger effect on
reduction rather than oxidation (Table 2), a previously noted
phenomenon,[ reduces the HOMO–LUMO energy gap from 2.03
8]
V in 3CF -Ga to 1.77 V in 4CF –4Br–Ga.
3
3
3CF -Ga
3
1
0
5
0
5
3CF3-4Br-Ga
4CF -Ga
3
4CF -4Br-Ga
3
3
3
corrole with bromines and substituted -CF groups (4CF -4Br-Ga)
may exhibit a relatively more distorted planarity compared to that
found for the purely brominated species (8Br-Ga). Comparative
photo–catalytic experiments using these two fully β–substituted
-
0.0
0.5
1.0
Potential / V
3
complexes 8Br-Ga and 4CF –4Br–Ga were performed. Detailed
experimental procedures are described; additional data is
provided in the Supporting Information. Two types of photo–
bromination reactions were attempted. The experimental
approaches are as follows: (1) irradiation (LED, λmax = 450 nm,
Figure 3. CV traces of 3CF
Glassy carbon working electrode, and an Ag/AgNO
CH
3
-Ga, 4CF
3
–Ga, 3CF
3
–4Br–Ga, 4CF
ref. electrode. Solvent:
3
CN with TBAP. Scan rate of 100 mV s were applied.
3
–4Br–Ga.
3
–1
–2
power = 150 mW·cm ) a Pyrex vial of 5 mL–acetonitrile reaction
mixture charged with 0.10 M 47 % HBr (58 µL) and 25 µM catalyst;
Table 2. Halfwave potentials for the first redox features of studied corroles.
Glassy carbon working electrode, and an Ag/AgNO ref. electrode were
used. Solvent: CH CN with TBAP. Scan rate of 100 mV s was applied.
(
2) the same as above, but with 0.10 M phenol (49 mg) presented
3
–1
in the reaction mixture. The simplified reaction schemes for these
procedures are shown in Figure 5. The formed bromine in assay
3
1
was detected and quantified by UV–vis methods monitoring the
Complex
1st oxidation
potential / V
1st reduction
potential / V
(HOMO-
LUMO) / eV
–
(λmax = 269 nm, ε = 55000 L mol–1 cm , in
–1
absorbance of Br
acetonitrile). [4c] The photocatalytic results revealed compound
3
4
CF
3
–4Br–Ga possessed better catalytic efficiency (TOF = 980
–
1
–1
3
CF
3
-Ga
0.77
0.89
-1.26
-0.92
2.03
1.81
h ) than complex 8Br-Ga (816 h ). The photo–bromination of
phenol which produced ortho– and para– substituted phenol with
a ratio of ca. 1:1, i.e., assay #2 was further checked to compare
the photo–catalytic performance of these two gallium corrole
complexes. Since the scope of this photo-catalytic reaction was
limited in that it used only two catalysts, the reaction mechanism
for giving a ratio of 1:1 is at present unclear which demands more
research is required to make an understanding of the mechanistic
3CF
3
-4Br-
Ga
4CF
3
-Ga
0.83
0.99
-1.07
-0.78
1.90
1.77
4CF
3
-4Br-
process clear in these present cases.. Similarly, species 4CF
3
–
Ga
4Br–Ga possessed better catalytic efficiency with TOF
determined at 384 h– compared to that of 296 h for catalyst 8Br-
1
–1
Ga. The improved performance for species 4CF
one important factor, that this complex owns a larger 1 oxidation
3
–4Br–Ga lies in
st
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1;
inset: absorbance value at a wavelength of 450 nm. (right) CV traces of 8Br–
3
potential value: 0.99 V (4CF –4Br–Ga) > 0.75 V (8Br-Ga); this
3 3
Ga and 4CF –4Br–Ga. Glassy carbon working electrode, and an Ag/AgNO
reference electrode were used. Solvent: CH CN with TBAP. A scan rate of 100
3
mV s was applied.
suggests that the photo–catalyst with a more positive redox
potential tended to have a better photo–oxidation efficiency.
Besides, the absorbed optical density of these gallium complexes
at the wavelength (450 nm) of emitting LED lamp were also
–1
3
measured: 0.31 (4CF –4Br–Ga) > 0.29 (8Br-Ga); this small
Conclusions
improvement in terms of the absorbance may ultimately benefit
our photo-catalytic reactions. In other words, the designed
multifunctionalized photo–catalyst is better matched with the
wavelength of the emitting light source and is expected to perform
better. Plausible reaction mechanisms were suggested in our
previous studies and are only briefly recapped here: [11] the
excited state of the photo–sensitizer formed after irradiation can
donate an electron to oxygen to allow for the formation of
In summary, bromination is a simple and conveniently applied
substitution method; the degree to which bromine groups in a
given macrocyclic type helps impart phosphorescence to create
novel emitting materials, from non-phosphorescent starting
materials is an important issue for current and future research.
Our results, involved, two novel mixed poly-brominated gallium
3 3
corrole complexes 3CF –4Br–Ga and 4CF –4Br–Ga with two
superoxide (O
under acidic conditions. The formed H
bromine; then, Br undergoes reaction with phenol to produce
2
–) which then generates hydrogen peroxide (H
may oxidize bromide to
2 2
O )
sets of functional groups at the β-positions prepared via a facile
method; newly synthesized complexes were carefully
characterized by NMR spectroscopy, mass spectroscopic and
UV–vis techniques. Further bromination results in red shifts in
both the UV–vis absorption and emission spectra, compared to
2
O
2
2
mono–brominated phenols by way of electrophilic substitution.
Formation of disubstituted products is not promoted considering
the relative concentrations of HBr and phenol.
their
temperature phosphorescence was observed under the N
atmosphere for one brominated species 3CF –4Br–Ga in which
non–brominated
counterparts.
Interesting
room–
2
3
the phosphorescence (λmax, em = 900 nm) lifetime was measured
to be 0.23 µs. The redox properties were probed with cyclic
voltammetry and revealed that further bromination produces
positively shifted redox potentials, especially on the reduction side.
There was, at most, an 85 mV·Br atom–1 positive shift (reduction)
observed for these systems. Positive redox shifts for the
polybromination was observed resulting in the enhanced
3
performance of species 4CF –4Br–Ga in terms of photocatalytic
efficiency when compared to e.g. the previously reported
octabrominated species 8Br-Ga in photocatalytic of bromide–to–
bromine conversion. Our study offers well–defined examples and
a new mindset on the extent to which β substitution can be varied
and structure–function relationships can be revealed. We are
aiming at improved photo–catalytic efficiency and stability within
biomedical uses among other applications by introducing different
functional groups at the β-positions within the confines of
accessible synthetic routes, low cost systems, and alternatives to
precious metal containing systems.
Figure 4. (left) Top view of the X–ray structure of 8Br–Ga(pyridine) and (right)
determined dihedral angles and Ga–N bond lengths based on the X-ray
structural information.
Experimental Section
Instrumental operations and further experimental information was
described in the Supporting Information.
3 3
The corrole complexes 3CF -Ga and 4CF -Ga were synthesized based on
previously reported literature and used as starting materials for further
mixed substituted compounds here (below). [4c]
3
Synthesis of complex 3CF -4Br-Ga. A flask charged with a pyridine
solution (15 mL) of 3CF -Ga (20 mg) and excess bromine (1 mL) was
3
stirred at room temperature for 2 hours. Then, the dark brown reaction
suspension mixture was taken up into dichloromethane (25 mL) and
washed with distilled water three times. Then, after the organic phase was
filtered and evaporated, the dry reaction mixture was then separated and
Figure 5. (a) Reaction schemes described in this investigation; molecular
structures of catalysts and photo–catalytic performance in TOF (turnover
–1
frequency, h ). (b) (left) The comparison of the UV–vis spectra for the two
–5
-
gallium complexes in acetonitrile at the same concentration of 1.4 × 10 mol·L
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10.1002/cplu.201900667
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purified by silica gel chromatography using n-hexane: ethyl acetate (5: 1
by volume) and a few drops of pyridine as an eluent. The first eluent was
darkly colored and clearly isolated after workup as a green solid; it was
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D. Bläser, R. Boese, I. Goldberg, Org. Lett. 1999, 1, 599-602; c) Z. Gross,
N. Galili, Angew. Chem. Int. Ed. 1999, 38, 2366-2369; d) R. Paolesse, S.
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found to be our target product (isolated yield: 20 %). 3CF
NMR (400 MHz, CDCl ): δ = 9.74 (s, 1H), 7.45 (s, 2H, pyridine-H), 6.64 (m,
H, pyridine-H), 6.23 (m, 4H, pyridine-H). 19F-NMR (377 MHz, CDCl
): δ =
48.28 (m, 3F), -50.09 (m, 3F), -52.97 (m, 3F), -137.53 (m, 6F, ortho-F), -
50.86 (t, J = 22.22 Hz, 1F, para-F), -151.00 (t, J = 23.14 Hz, 1F, para-F),
151.57 (t, J = 25.25 Hz, 1F, para-F), -163.63 (m, 4F, meta-F), -163.89 (m,
3
-4Br-Ga: 1H-
3
4
3
-
1
-
2
F, meta-F). MS+ (APCI, positive mode) for C40HBr
381.5769 (calculated), 1381.5181 (observed). UV−vis (toluene) λmax (ε)
4
F
24GaN
4
: m/z =
[2]
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1
nm (× 10 cm M−1)]: 413 (1.40), 437 (5.96), 598 (0.69), 626 (0.88).
4
−1
[
9481-9490; c) R. D. Teo, J. Y. Hwang, J. Termini, Z. Gross, H. B. Gray,
3
Synthesis of complex 4CF -4Br-Ga. A flask charged with a pyridine
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solution (15 mL) of 4CF -Ga (20 mg) and excess bromine (1 mL) was
3
stirred at room temperature for 2 hours. Then, the dark brown reaction
suspension mixture was taken up into dichloromethane (25 mL) and
washed with distilled water three times. Then, after the organic phase was
filtered and evaporated, the dry reaction mixture was then separated and
purified by silica gel chromatography using n-hexane: ethyl acetate (5:1 by
volume) and a few drops of pyridine as an eluent. The first eluent was
darkly colored and clearly isolated after workup as a green solid; it was
18942.
[
[
3]
4]
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620; b) A, Mahammed, M. Botoshansky, Z Gross, Dalton Trans. 2012,
41, 10938-10940; c) A. Schechter, M. Stanevsky, A. Mahammed, Z.
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-4Br-Ga: 19F-
): δ = -48.93 (s, 6F), -49.50 (s, 6F), -136.72 – -
found to be our target product (isolated yield: 25 %). 4CF
NMR (377 MHz, CDCl
37.01 (m, 6F, ortho-F), -151.02 (t, J = 21.69 Hz, 2F, para-F), -151.46 (t,
3
133, 12899-12901.
3
a) K. Sudhakar, A. Mizrahi, M. Kosa, N. Fridman, B. Tumanskii, M.
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L. J. W. Shimon, I. Saltsman, A. Mahammed, M. Kosa, M. Baik, D. G.
Churchill, Z. Gross, Inorg. Chem. 2019, 58, 6184−6198; c) X. Zhan, P.
Yadav, Y. Diskin-Posner, N. Fridman, M. Sundararajan, Z. Ullah, Q.-C.
Chen, L. J. W. Shimon, A. Mahammed, D. G. Churchill, M.-H. Baik, Z.
Gross, Dalton Trans. 2019, 48, 12279–12286.
1
J = 21.73 Hz, 1F, para-F), -162.73 (m, 2F, meta-F), -163.26 (m, 4F, meta-
-
F). MS (APCI, positive mode) for C41Br
4 27
F
N
4
Ga: m/z = 1450.5721
calculated), 1450.5669 (observed). UV−vis (toluene) λmax (ε) [nm (× 104
cm M−1)]: 421 (4.42), 442 (6.10), 586 (1.36), 609 (1.69), 644 (2.42).
(
−
1
Crystal Data for 8Br-Ga: moiety formula: C42 , 0.5(C6),
5
H Br
8
F15GaN
5
C H
2
3
N, C2.50,0.5(C3), sum formula: C51 Br F15GaN , M = 1698.55;
H
8
8 6
[
5]
6]
a) R-B. Du, C. Liu, D-M. Shen, Q. Y. Chen. Synlett. 2009, 16, 2701–2705;
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Chem. Soc. 2010, 132, 9230-9231.
monoclinic, space group C2/c (15), a = 39.074(6) Å, b = 12.1109(19) Å, c
3
= 2.127 g cm-3, µ (mm-
=
)
27.670(4) Å, V = 10610.1 Å , Z = 8, T = 100 K, D
x
1
= 6.641, Data completeness = 0.746, θ(max) = 25.104, R (reflections) =
[
0
.0701 (4302), wR (reflections) = 0.1780 (9221), S = 0.908, Npar = 670,
2
CCDC code: CCDC1957515. CCDC1957515 contains the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre.
[
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Acknowledgements
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Z. G. acknowledges the support of this research by a grant
from the Israel Science Foundation. D. G. C. acknowledges
Z. G., the Schulich Faculty of Chemistry, Technion-Israel
Institute of Technology, and support from KAIST for facilitating his
sabbatical year.
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Keywords: β-substitution• corroles • oxidation •
phosphorescence • photocatalysis
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Entry for the Table of Contents (Please choose one layout)
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[
a]
[a]
A synthetic and electrochemical study
of a new conceptual class of corroles
enables assessment of how β–
Xuan Zhan , Yael Zini , Natalia
[
a]
[a]
Fridman , Qiu–Cheng Chen , David G.
Churchill[a],[b],[c],[d]*, Zeev Gross[a],
*
substitution better enables facile light–
assisted C–H activation processes
such as the mono-bromination of
phenol. The corroles feature 3 and 4
Page No. – Page No.
“
Hetero multifunctionalization” in
gallium corroles: facile synthesis,
phosphorescence, redox tuning, and
photooxidative catalytic improvement
3
CF groups and 4 Br groups. “Hetero
multifunctionalization” signifies β–
substitution to help optimize
porphyrinoid (photophysical)
properties.
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