Microwave-Accelerated C,N-Cyclometalation as a Route to Chloro-Bridged Iridium(III) Binuclear Precursors of Phosphorescent Materials: Optimization, Synthesis, and Studies of the Iridium(III) Dimer Behavior in Coordinating Solvents

Article abstract of DOI:10.1021/acs.inorgchem.0c01071

We present the results of our research on the use of microwaves as an unconventional heat source for the acceleration of iridium(III) chloro-bridged dimer preparation. The results enabled us to revise and improve known guidelines for the very quick and highly efficient synthesis of iridium(III) dimeric complexes in a very simple isolation manner. According to the developed methodology, the already known dimers containing ligands based on the 2-phenylpyridinato motif, as well as new ones stabilized with functionalized benzo[h]quinolinato and 2-phenoxypyridinato-based ligands, were efficiently synthesized. The scope of the incorporated ligands included compounds equipped with electron-donating (-Me,-OMe,-OPh,-NMe2), electron-withdrawing (-F,-Br,-CF3,-C6F5), and hole-transporting (-NPh2,-C6H4NPh2) groups. The obtained complexes were characterized by NMR, X-ray diffraction, and electrospray ionization mass spectrometry methods, and their behavior was examined in the presence of coordinating solvents such as dimethyl sulfoxide and acetonitrile. Investigation of the interactions between the above-mentioned solvents and dimers enabled us to confirm the ability of the former to cleave μ-chloride bridges, which enriches the knowledge in the field of organometallic chemistry. This knowledge can be particularly useful for the scientists working in the field of iridium-based materials, helping to avoid misinterpretation of the spectroscopic data.

Full text of DOI:10.1021/acs.inorgchem.0c01071

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Article
Microwave-Accelerated C,N-Cyclometalation as a Route to Chloro-
Bridged Iridium(III) Binuclear Precursors of Phosphorescent
Materials: Optimization, Synthesis, and Studies of the Iridium(III)
Dimer Behavior in Coordinating Solvents
Bartosz Orwat,* Myong Joon Oh, Maciej Zaranek, Maciej Kubicki, Rafał Januszewski,
and Ireneusz Kownacki*
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ABSTRACT: We present the results of our research on the use of
microwaves as an unconventional heat source for the acceleration of
iridium(III) chloro-bridged dimer preparation. The results enabled us
to revise and improve known guidelines for the very quick and highly
efficient synthesis of iridium(III) dimeric complexes in a very simple
isolation manner. According to the developed methodology, the
already known dimers containing ligands based on the 2-phenyl-
pyridinato motif, as well as new ones stabilized with functionalized
benzo[h]quinolinato and 2-phenoxypyridinato-based ligands, were
efficiently synthesized. The scope of the incorporated ligands included
compounds equipped with electron-donating (−Me, −OMe, −OPh,
−NMe2), electron-withdrawing (−F, −Br, −CF3, −C6F5), and hole-
transporting (−NPh2, −C6H4NPh2) groups. The obtained complexes were characterized by NMR, X-ray diffraction, and
electrospray ionization mass spectrometry methods, and their behavior was examined in the presence of coordinating solvents such
as dimethyl sulfoxide and acetonitrile. Investigation of the interactions between the above-mentioned solvents and dimers enabled us
to confirm the ability of the former to cleave μ-chloride bridges, which enriches the knowledge in the field of organometallic
chemistry. This knowledge can be particularly useful for the scientists working in the field of iridium-based materials, helping to
avoid misinterpretation of the spectroscopic data.
catalyst, which allowed the fabrication of red/green/blue
(RGB) pixel arrays and a multicolor OLED prototype made of
copolymer brushes, covalently bonded to the indium−tin oxide
(ITO) layer by irradiation of the monomer solution. The next
example related to this field is a new class of luminescent
mechanochromic iridium(III) complexes, also prepared from
chloro-bridged iridium(III) dimers as starting materials.8
Interestingly, the observed red shift of the final iridium
coordination compound phosphorescence, being the result of
conformational change caused by mechanical stimulation, may
be utilized in mechanical and chemical vapor sensors. Apart
from the above-mentioned lighting and sensing applications,
iridium compounds have been studied as biologically active
species. In this context, cationic iridium(III) complexes have
INTRODUCTION
■
Neutral or cationic C,N-cyclometalated iridium(III) complexes
have been studied mainly in the context of their use as
biosensors, photocatalysts, and phosphorescent emitters for
organic light-emitting diodes (OLEDs).1−3 Such compounds
might be synthesized in different ways, but the most popular
way proceeds via chloro-bridged binuclear iridium(III)
intermediates. Unfortunately, these intermediates have not
received much attention in the literature because they are
rarely target molecules for materials chemistry, despite their
involvement in the preparation of a vast number of
iridium(III) compounds. There are many papers in which
the use of iridium(III) dimers as intermediates for highly
advanced functional materials of desirable properties has been
reported,4−6 but here we will briefly discuss only the most
recent and relevant ones. For instance, various chloro-bridged
iridium(III) dimers have recently been applied in the synthesis
of methacrylate-functionalized triscyclometalated iridium(III)
complexes, to be used in light-mediated atom-transfer-radical
polymerization.7 The iridium(III) complexes played the role of
both a monomer and a polymer growth mediator/photo-
Received: April 11, 2020
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been examined as biosensors for DNA and RNA detection, as
well as conductors for the electrochemical sensing of interferon
gamma.9−11 Another noteworthy example is the decoration of
polyhedral oligomeric silsesquioxane with eight iridium(III)
cyclometalated moieties in the corner positions.12 Such
organic−inorganic hybrids have been found to be relatively
noncytotoxic and to effectively stain the cytoplasm of human
cervix epithelioid carcinoma cells, enabling its bioimaging
because of the strong photoluminescent properties. As in the
previously mentioned examples, synthesis of these interesting
compounds also proceeded via chloro-bridged iridium(III)
dimers. Finally, there are only a few reports illustrating the use
of dimers as such.13,14 One of them concerns a series of chloro-
bridged iridium(III) dimers based on the formyl-substituted 2-
phenylpyridinate ligands that have been studied as OLED
emitters. It was surprisingly found that, as the content of
formyl groups increased, the emission became brighter. This
observation has opened up new possibilities for the develop-
ment of iridium-based phosphorescent dopants because the
dimers have been considered to be poorly emissive
compounds.15,16
Considering the importance of chloro-bridged iridium(III)
dimers for the chemistry of iridium-based phosphorescent
materials, there are not many papers mainly devoted to their
preparation and properties. A significant development in the
field began in 1974, when Nonoyama reported the synthesis
and characterization of [Ir(μ-Cl)(bzq)2]2.17 Since then, many
reports on the synthesis of C,N-cyclometalated iridium(III)
complexes have been published, but as was already mentioned,
they very often have not been directly concerned with the
intermediate dimers. In some cases, the yields of these
precursors were not precisely given or they were not even
characterized but used as they were obtained, with the latter
being common practice in the research of cyclometalated
iridium compounds.16,18−21 The lack of the above-mentioned
experimental information makes evaluation of the synthesis
efficiency difficult, especially when the purity of the product
remains unknown. In addition, in most of the reported
procedures, the synthesis requires prolonged heating (up to 48
h) using conventional heat sources and relatively large
amounts of solvent, and sometimes even flash chromatography
purification of the product should be made.22−29 Even then,
the products were not always obtained with satisfactory yields.
An improvement in the latter aspect is especially in demand
because of the high price of iridium, which makes its
compounds very costly. The disadvantage of the other
methodologies is the need of using an excess of the ligand
precursors, which in the case of their sophisticated structures
might make them comparatively expensive to the starting
iridium material.30,31 However, there are a few examples of the
microwave irradiation protocols, whose application allowed
one to shorten the reaction times, raise the reaction
temperatures, increase the product yields, and improve the
overall reaction economy.32−35 Unfortunately, the reaction
conditions described in these reports are rather mutually
inconsistent, and there is still a lack of comprehensive data on
the microwave effect on the efficiency of reactions, leading to
the chloro-bridged iridium(III) dimers. On the basis of our
experience in the acceleration of various chemical trans-
formations by the means of microwave radiation,36,37 we
decided to examine the effect of such an unconventional heat
source on the formation of C,N-cyclometalated iridium(III)
dimers. We are convinced that the knowledge gained on this
subject will be extremely useful for those researchers who want
to improve the efficiency of their syntheses and to obtain both
known and novel dimers with high yields, as shown here.
Recently, we have published results on efficient stoichiometric
and catalytic methods for the synthesis of various 5-substituted
benzo[h]quinolines via C(sp2)−Br bond activation.38 The
ability of unmodified benzo[h]quinoline (bzqH) to cyclo-
metalate iridium(III) has already been confirmed per
analogiam to 2-phenylpyridine (ppyH).16,34 However, in
contrast to countless complexes based on the ppy motif, only
several examples of iridium(III) complexes with bzq have been
synthesized and characterized as potential electroluminescent
phosphors. In our recently published papers on the synthesis
and photophysical and emission properties of ionic and neutral
iridium complexes with a nonfunctionalized bzq ligand, we
have shown that this type of iridium coordination system
shows interesting electroluminescent properties, which can be
potentially useful in OLED technology.37,39 Therefore, it
would be beneficial for the development of an organometallic
phosphor field to extend the scope of complexes equipped with
new C,N-cyclometalating ligands, in particular taking into
account that our potential ligands include electron-donating
and -withdrawing substituents and groups known for their p-
type semiconductivity. In addition, our theoretical studies have
shown that some of these ligands might be useful in the
preparation of emitters for OLEDs.40 Therefore, we decided to
implement 5-functionalized benzo[h]quinolines in our re-
search and examine their reactivity in the iridium cyclo-
metalation process. Another innovative part of our work was
the obtainment of new dimers containing 6-membered
metallacyclic rings based on a 2-phenoxypyridinato motif, a
system in which the conjugation of two aromatic rings is
prevented. After the successful synthesis of all planned
complexes, they were subjected to spectroscopic character-
ization. We encountered some inconsistences in the inter-
pretation of analytical data reported in the literature, which
prompted us to get more deeply into the structural details of
compounds synthesized using X-ray diffraction (XRD), NMR,
and electrospray ionization mass spectrometry (ESI-MS)
techniques. As a result, we explained the behavior of the
iridium(III) dimers in specific solvents, which, if not known,
may lead to drawing the wrong conclusions in the related
research. All of the phenomena clarified here constitute an
important contribution to the chemistry of organometallic
iridium compounds.
RESULTS AND DISCUSSION
■
Optimization of the C,N-Cyclometalation Conditions.
As already mentioned above, the lack of a comprehensive
comparison of the methods allowing the synthesis of chloro-
bridged iridium(III) dimers using conventional and unconven-
tional heat sources prompted us to investigate this issue and to
find the optimal conditions for the efficient preparation of such
materials. Taking into account that the vast majority of the
methods for obtaining dimeric iridium(III) complexes are
based on the reactions of iridium(III) chloride hydrate with
the appropriate ligand precursors in an alcohol/water environ-
ment,16−35 this system was chosen as a model synthetic
protocol for our study. Of the many known examples of such
dimers, [Ir(μ-Cl)(ppy)2]2 was selected as the target molecule
because of its wide applicability and the availability of the
required ligand precursor.3 In summary, a composition
consisting of iridium(III) chloride hydrate, ppyH, and a 2-
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methoxyethanol/water (4:1) mixture was chosen as a model
reagent system (Scheme 1). We began testing the model
reaction by comparing the efficiency of the target dimer
formation depending on the mode of thermal treatment of the
reaction mixture. Therefore, the reactions were carried out
using an oil bath and a microwave reactor. On the basis of our
earlier work concerning the synthesis of phosphorescent
iridium(III) dopants and the results of the initial tests not
described here, the 10 min reaction time was chosen as
sufficient to illustrate the effect of the temperature and the
influence of microwave radiation on the yield of the product
formed.37 The data illustrating the observed trends are
presented in Chart 1. It has been clearly demonstrated that
the type of heat source has a significant impact on the reaction
outcome. In each reaction conducted at selected temperatures,
the yield of the product was higher in the case of the
microwave-assisted process. The reason might be that both the
iridium precursor and the reaction medium are highly polar,
making them susceptible to excitation by microwave
absorption. As one can see, the yield was strongly related to
the temperature of the reaction. In general, the yield increased
with increasing reaction temperature. A significant improve-
ment in the reaction yield was observed when the temperature
was elevated up to 150 °C, and when the process was carried
out at 170 °C, the yield improvement was much less
spectacular. The trend observed is consistent with the
literature data stating that 30 min of microwave heating at
190 °C led to an almost quantitative yield of the dimer.35
However, such harsh conditions can only be applied for
thermally and chemically stable ligands because not many
moieties and functional groups are able to survive intact in an
acidic alcohol/aqueous environment at a temperature close to
200 °C. Indeed, the authors of the cited paper admitted that
one of the quinoline-based ligands underwent degradation
under the reaction conditions, which prevented formation of
the corresponding dimer. Similarly, during some of our initial
tests on the formation of iridium(III) dimers stabilized with 5-
functionalized benzo[h]quinolines at 170 °C, the course of
Chart 1. Comparison of the Model Product Yields Obtained
in an Oil Bath and a Microwave Reactor
or a Lewis acid in the reaction system is essential for selectivity
of the process because it shifts the equilibrium toward dimer
formation.41,42 Considering the above, conducting the process
in a sealed microwave-pressurized vial is advantageous because
of trapping of the evolving HCl inside. However, HCl
consumption may occur under these conditions not only
because of the presence of ligand precursors of basic nature but
also as a result of the thermal reaction of HCl with alcohol,
leading to the corresponding alkyl halide and water.43 Such a
process should be facilitated along with elevation of the
reaction temperature. An additional advantage of the proposed
microwave-assisted protocol is the very simple product
isolation, which only requires filtration and rinsing. Heating
of the reaction mixtures at too high temperatures, resulting in
the formation of undesired byproducts as a consequence of
further cyclometalation or thermal degradation, implies the
need for their time-consuming purification, thus cutting out
the benefits of a quick and simple microwave-accelerated
process. Taking into account all of the aforementioned risks
associated with the change in the reaction temperature from
150 to 170 °C, paralleled with a relatively small increase in the
yield of the model complex, we decided that running the
process at 150 °C is the best compromise between the product
yield and simplicity of its isolation.
Our attention was drawn by the fact that in all reported
protocols on the preparation of chloro-bridged iridium(III)
dimers, the presence of water in the reaction system was
required. Interestingly, it has been shown that, even the other
solvents, such as acetonitrile (ACN), tetrahydrofuran, and
ethylene glycol, were scoped as the reaction medium in the
dimer formation with a significant admixture of water.35 The
large number of reports describing the synthesis of iridium(III)
dimers in the presence of water and the scarcity of protocols
carried out under anhydrous conditions strongly suggest the
importance of the role of water in the formation of such
complexes. It seems that the presence of water must be even
more important for microwave-assisted reactions because this
type of radiation is strongly absorbed by polar molecules and
stimulates them to collide with each other and with other
molecules in the reaction system. Therefore, we decided to
carry out the tests under the previously optimized conditions
(microwave reactor, T = 150 °C, t = 10 min, and Pmax = 200
W) with different alcohol/water ratios to evaluate how the
water content affects the yield of the product. The results are
presented in Chart 2.
1
undesirable side reactions was also noted, as evidenced by H
NMR analyses. For consideration of this issue, it is important
to remember that hydrogen chloride (HCl) is evolved as a
result of cyclometalation. At first sight, one could think of
introducing a HCl scavenger into the reaction system in order
to inhibit undesirable processes associated with the presence of
a strong acid and to shift the equilibrium toward the desired
dimer. On the other hand, it should be remembered that the
consumption of HCl would also promote further cyclo-
metalation, leading to the subsequent formation of triscyclo-
metalated iridium(III) species. Therefore, the presence of HCl
Scheme 1. Model Reaction of [Ir(μ-Cl)(ppy)2]2 Synthesis
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25% water content, showing that its further addition has no
beneficial effect. What is more, too high amount of water might
raise issues related to the solubility of the ligand, depending on
its lipophilic character, which might hamper dimer formation.
Another noteworthy observation was the effect on the
crystallinity of the product. When no or a low amount of
water was present in the reaction mixture, the isolated product
was in the form of an amorphous powder. When the reaction
was performed in the medium richer in water, the product
started to appear as a shiny, crystalline solid. This phenomenon
simplified isolation of the product because filtration was much
quicker. Considering the above-mentioned aspects, the
reaction at 150 °C for 10 min in a 1:3 water/2-methoxyethanol
mixture was confirmed to be the optimal conditions for the
microwave-accelerated preparation of dimers. We do not
recommend increasing the reagents/solvent ratio significantly
above the reported one because it resulted in the coformation
of insoluble byproducts in our tests.
Chart 2. Illustration of the Effect of the Water Content in
the Reaction Mixture on the Yield of [Ir(μ-Cl)(ppy)2]2
Synthesis of the Iridium(III) Dimers. The successful
optimization of the cyclometalation process encouraged us to
apply the developed methodology in the synthesis of the
already-known as well as novel chloro-bridged iridium(III)
dimers. The scope of the tested ligand precursors consisted of
various heterocyclic compounds, including 2-(2,4-
difluorophenyl)pyridine (2,4-diFppyH), the most intensively
studied ligand in the field of iridium-based blue emitters.
Fortunately, the F atoms remained intact under the reaction
conditions, and the corresponding dimer [Ir(μ-Cl)(2,4-
diFppy)2]2 was obtained with a very high yield of 94%. Of
particular interest was bzqH, a ppyH analogue, which seemed
to be a potentially attractive C,N-donating ligand for the
iridium dopant system because of its geometric similarity, more
rigid structure, and extended electron conjugation system.
Indeed, iridium(III) complexes bearing nonfunctionalized bzq
As one can see, the water content in the reaction medium
has a significant effect on the yield of the product. The
observed trend is clear: the higher the water content, the
higher the yield. The effect of the medium composition was
also observed by visual evaluation of the postreaction mixture.
When no water was added, distinct dark-green particles of
unreacted iridium(III) chloride were observed along with the
lemon-yellow product. Upon the addition of more water, the
color of the obtained solid became more yellow-orange and the
dark-green grains were no longer observed, suggesting higher
consumption of the starting iridium(III) chloride. We suppose
that the progress of the reaction might be controlled by the
solubility of iridium(III) salt in the solvent mixture. Because it
is an ionic material, the addition of a strongly polar solvent
(water) should increase its solubility and hence the availability
for cyclometalation. However, the yield reached a plateau at
Scheme 2. Scope and Yields of the Synthesized ppy- and bzq-Based Chloro-Bridged Iridium(III) Dimers
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Scheme 3. Alternative Synthetic Pathway for [Ir(μ-Cl)(5-Me2Nbzq)2]2
Scheme 4. Formation of Chloro-Bridged Iridium(III) Dimers from 2-Phenoxypyridine Derivatives
in the coordination sphere have shown interesting electro-
luminescent properties, which made them potentially useful in
OLED technology, as evidenced by us and other
groups.34,37,39,44 The preparation of [Ir(μ-Cl)(bzq)2]2 accord-
ing to the methodology developed here proved to be very
efficient; therefore, we focused on the synthesis of new
iridium(III) dimers using bzqH derivatives previously synthe-
sized and characterized in our group.38 As a result, we obtained
a series of new bzq-based chloro-bridged iridium(III) dimers
equipped with various functional groups (Scheme 2). The
scope of the substituents included electron-donating groups
(−OMe and −OPh), electron-withdrawing groups (−Br and
−C6F5), and p-type semiconducting moieties such as −NPh2
and −C6H4NPh2. The reactions proceeded very selectively,
with no C−Br and C−F bond activation or undesired
cyclometalation processes in the case of 5-Ph2NbzqH. This
is the first report on the synthesis of cyclometalated
iridium(III) complexes stabilized with functionalized bzq
ligands, which might be of great interest considering the
applicability and huge popularity of similar iridium(III)
dopants with modified ppy ligands. Because the highest
occupied molecular orbital (HOMO) of the cyclometalated
complex is most often localized on the C,N-cyclometalating
ligand, chemical functionalization of the latter is expected to
modify the electronic properties of the complex or improve
energy transfer due to the antenna effect. In addition, [Ir(μ-
Cl)(5-Brbzq)2]2 constitutes a convenient starting material for
further structure modification in the manner of C−Br bond
activation. All of the known and new dimers were obtained in
high to very high yields, which is advantageous because iridium
compounds are expensive and every percentage of yield is of
great importance.
In the testing of various 5-substituted benzo[h]quinolines
toward cyclometalation, we encountered a problem with the
reactivity of 5-Me2NbzqH, in whose case the expected product
was obtained with unsatisfactory yield (about 50%) and it was
not pure enough. Microwave irradiation was not the cause
because the outcome of the reaction carried out in an oil bath
was the same. It became evident that the issue was related to
the presence of the ligand’s functional group, which was
definitely characterized by the most basic properties among the
tested ones. Considering the chemical nature of the −NMe2
moiety, we suppose that it might be involved in HCl capture,
causing both further cyclometalation and transition of the
ligand into the salt form. Therefore, the obvious solution was
to eliminate the HCl source from the reagent system,
iridium(III) chloride in this particular case. Fortunately,
Baranoff et al. have reported a procedure for the obtainment
of cyclometalated dimers involving the reaction of [Ir(μ-
Cl)(COD)]2 (COD = 1,5-cyclooctadiene) with a stoichio-
metric amount of the C,N-donating ligand precursor. As a
result, COD ligands are eliminated from the metal
coordination sphere without the pH change effect.45 It is
noteworthy that it is a redox process in which iridium(I) in the
starting material is oxidized to iridium(III) and the released
COD acts as a scavenger of the H evolved in the C,N-
cyclometalation process. Implementation of this protocol
finally enabled the successful synthesis of [Ir(μ-Cl)(5-
Me2Nbzq)2]2 (Scheme 3). In this way, we expanded the
group of new dimers bearing the bzq ligand equipped with
electron-donating substituents (−OMe, −OPh, and −NMe2).
All of the examined ligands, based on the ppy and bzq cores,
had one feature in common; namely, they formed 5-membered
cyclometalated rings in the dimers’ structures. An interesting
report describing studies of the photophysical properties of
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Figure 1. Anisotropic-ellipsoid representations of the dimeric complexes [Ir(μ-Cl)(bzq)2]2 (left) and [Ir(μ-Cl)(5-(Ph2N)bzq)2]2 (right). Ellipsoids
are drawn at the 30% (left) and 20% (right) probability levels.
Figure 2. Anisotropic-ellipsoid representations of the monomeric complexes [Ir(ppy)2Cl(ACN)] (left) and [Ir(ppy)2Cl(DMSO)] (right).
Ellipsoids are drawn at the 50% probability level.
iridium(III) compounds containing 6-membered metallacyclic
systems has recently been published.46 In this paper, 2-
benzylpyridine was used as the C,N-cyclometalating ligand
precursor. The unique optoelectronic properties of the
complexes stabilized with this type of ligand result from the
lack of conjugation between two C- and N-donating aromatic
systems, caused by the presence of a methylene spacer. These
types of C,N-donating ligands are very promising for the
development of new iridium-based dopants because their
structure allows independent modification of their individual
HOMO and lowest unoccupied molecular orbital (LUMO)
energy levels by attaching substituents with different electronic
properties to the aromatic rings. The introduction of moieties
preventing conjugation between the ligand’s aromatic moieties
might be helpful in more precise and efficient tuning of the
emission of the complexes toward blue light.47−49 This
prompted us to synthesize analogous ligand precursors,
equipped with an electron-donating substituent on the
pyridine moiety or an electron-withdrawing substituent on
the phenyl ring, and to examine whether this class of
compounds can be used in the microwave-assisted synthesis
of chloro-bridged iridium(III) dimers. In order to check it, we
synthesized two O-bridged potential ligands, namely, 4-methyl-
2 - p h e n o x y p y r i d i n e ( P h O P y M e H ) a n d 2 - [ 4 -
(trifluoromethyl)]phenoxypyridine (CF3PhOPyH),50,51 and
examined them in the respective iridium(III) dimer formation
F
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1
Figure 3. Fragment of the H NMR spectrum of [Ir(μ-Cl)(ppy)2]2 in DMSO-d6.
(Scheme 4). Fortunately, the cyclometalation reaction
proceeded well under even milder conditions, enabling the
successful obtainment of the target iridium(III) dimers.
inner-coordination-sphere Ir atoms. It should be mentioned
that swapping of the C/N atom assignments always gave worse
refinement results.
In summary, it has been demonstrated that the previously
optimized reaction conditions allowed the quick and efficient
preparation of not only the already-known iridium(III) dimers
[Ir(μ-Cl)(ppy)2]2, [Ir(μ-Cl)(2,4-diFppy)2]2, and [Ir(μ-Cl)-
(bzq)2]2 but also a series of new dimers stabilized with 5-
substituted benzo[h]quinolinato ligands. Additionally, we
obtained new chloro-bridged iridium(III) dimers with the
use of ligands based on the 2-phenoxypyridine motif. The
functional groups of all of the new compounds make them
attractive precursors for application in the field of OLEDs and
bioimaging. All of the mentioned dimers were obtained in very
short reaction times (10 min) and high to very high yields,
which is advantageous for their further applications. After
successful preparation of the target compounds, we subjected
them to thorough characterization, which is discussed below.
XRD Crystallography. We managed to grow single crystals
of a few compounds suitable for XRD structural analysis, which
confirmed the proposed structures of the chloro-bridged
dimers (Figure 1). Moreover, the crystal structures of the
monomeric complexes derived from [Ir(μ-Cl)(ppy)2]2 by
cleavage with ACN or dimethyl sulfoxide (DMSO) molecules
are presented in Figure 2. XRD-suitable crystals of the dimeric
complexes were obtained by the slow evaporation of
dichloromethane ([Ir(μ-Cl)(bzq)2]2) and dichloromethane/
2-methoxyethanol/water ([Ir(μ-Cl)(5-Ph2Nbzq)2]2) solutions.
The crystals of the cleaved complexes were obtained by the
slow cooling of a hot saturated solution of [Ir(μ-Cl)(ppy)2]2 in
ACN ([Ir(ppy)2Cl(ACN)]) and diffusion of a saturated [Ir(μ-
Cl)(ppy)2]2 solution in DMSO into toluene ([Ir(ppy)2Cl-
(DMSO)]). Crystals of the monomeric forms were obtained
for a specific purpose, which is discussed in detail in the next
section. Crystallographic details of all of the resolved structures
are provided in Table S1. A very similar structure with an
identical formula, [Ir(ppy)2Cl(ACN)], was previously pub-
lished by Blasberg et al.52 Even though both dimeric complexes
are nonsymmetric, their structures are close to Ci symmetry,
with almost planar central Ir2Cl2 rings. All IrIII ions are six-
coordinated, in quite a regular octahedral fashion (cf. Table
S2a). In the Cambridge Structural Database (CSD),53 there
are almost 300 fragments with similar composition (μ-chloro-
bridged dimers with aromatic ligands, containing both Ir−N
and Ir−C bonds), and in almost all (with five exceptions), the
arrangement of N atoms around the Ir center is trans.54 Taking
this into account, together with the Ir−N and Ir−C bond
lengths and C−C−C and C−N−C angles, we assigned the
In the monomeric complexes [Ir(ppy)2Cl(ACN)] and
[Ir(ppy)2Cl(DMSO)], Ir atoms also have the regular
octahedral environment, with N atoms from two ppy ligands
in trans positions (Table S2b). The fifth and sixth coordination
positions are occupied by the solvent molecules and Cl anion.
It might be noted that even though for all metals DMSO is
coordinated with only a slight preference for a S atom over an
O atom (1865 vs 1135 hits in CSD), for iridium complexes,
the S atom is almost exclusively used as the coordination
center (65 vs 2 in CSD). ppy ligands are slightly twisted along
the central C−C bonds (cf. Table S2b). Crystal architectures
in all of the structures are determined by close-packing
requirements and by weak but numerous C−H···π and π···π
interactions. Many voids filled with diffused electron density
were found in the structure of [Ir(μ-Cl)(5-(Ph2N)bzq)2]2,
probably originating from the totally disordered solvent
molecules because of the complicated shape of the dimer
molecule. Similarly, in the crystal structures of monomeric
complexes, the ordered ACN molecule in [Ir(ppy)2Cl(ACN)]
or highly disordered water molecule in [Ir(ppy)2Cl(DMSO)]
filling the voids between the complex molecules were found.
Spectroscopic Analysis and Cleavage of [Ir(μ-Cl)-
(C^N)2]2. After the successful optimization and synthesis of
chloro-bridged iridium(III) dimers, we focused on their
spectroscopic characteristics. From the very beginning, our
attention was drawn by the appearance of a doubled set of
1
resonances originating from cyclometalated ligands in the H
NMR spectra of all of the obtained compounds dissolved in
DMSO-d6 (Figures 3 and S1−S10). The recorded spectro-
scopic images for these types of binuclear compounds seemed
unusual at first glance because such compounds bearing N-
donating atoms in the trans position, C-donating atoms in the
cis position, and two bridging Cl atoms in the cis position
should be characterized by chemically equivalent C,N-
cyclometalating ligands. In fact, for many reported dimers,
only one set of signals originating from the cyclometalating
ligands was observed in the 1H NMR spectra.26,55,56 However,
the mentioned spectra were recorded in nonpolar solvents
such as CD2Cl2 or CDCl3, which prompted us to conclude that
the differences in the spectroscopic images might be caused by
the polar and coordinative nature of DMSO. In our search for
the answer, we referred to the literature, where we found
misleading data. In 2011, Li et al. reported the synthesis of
[Ir(ppy)2(DMSO)2]PF6, for which the given 1H NMR
spectroscopic data set accurately reflected those obtained by
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Figure 4. 1H NMR titration of 3 mg of [Ir(μ-Cl)(ppy)2]2 dissolved in 0.7 mL of CD2Cl2 (top). Each subsequent spectrum was measured after the
addition of about 2 equiv of DMSO, followed by 5 min of shaking at room temperature.
us for [Ir(μ-Cl)(ppy)2]2 in DMSO-d6.57 The authors claimed
that a DMSO molecule was a sufficiently strong nucleophile to
push out the chloride anion from the internal to the external
coordination sphere, forming a [Ir(ppy)2(DMSO)2]+ cation.
However, in our opinion, such a cation with the retained
configuration of ppy ligands from the parent dimer should not
mentioned split of signals.24 In the face of the encountered
inconsistency, we decided to study in detail the behavior of
chloro-bridged iridium(III) dimers to explain what lies behind
the split of the NMR signals. We chose [Ir(μ-Cl)(ppy)2]2 as a
model compound and decided to examine its interaction with
DMSO as well as ACN. Unlike DMSO, the latter solvent is
more convenient for ESI, which enabled us to monitor its
interaction with the model dimer also by MS.
1
demonstrate the chemical inequivalence of ligands in the H
NMR spectrum. On the other hand, Soman et al. reported 1
year later exactly the same 1H NMR data set for [Ir(μ-
Cl)(ppy)2]2 in DMSO-d6 but without the explanation of the
Our study started from analysis of the literature on chloro-
bridged rhodium(III) and iridium(III) dimers, where we found
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some hints indicating that these compounds might undergo
solvation in coordinating solvents [ACN, N,N-dimethylforma-
mide (DMF), DMSO, and water], to create a monomeric
derivatives of the general formula [Ir(C^N ligand)2Cl-
(solvent)].15,23 However, it was not fully confirmed at the
time of these studies because of re-formation of the original
chloro-bridged dimers during the attempts of isolation.58
Considering the results described in the cited papers, it should
be assumed that conversion of the dimer to the solvated
monomer is an equilibrium reaction that shifts toward the
starting material when the coordinating solvent concentration
is lowered, e.g., during isolation. Therefore, it is obvious that
isolation of the solvated iridium species should be performed
in the presence of a large excess of the coordinating solvent at
every step of purification. Considering this condition,
crystallization seemed to be the best choice, although it has
been suggested that it is unlikely to do that.58 Therefore, the
crystal structures of mononuclear C,N-cyclometalated iridium-
(III) complexes, obtained as a result of chloro-bridged dimer
cleavage by a coordinating solvent, are really scarce.59 Either
way, we also successfully managed to obtain XRD-suitable
crystals of [Ir(ppy)2Cl(ACN)] and [Ir(ppy)2Cl(DMSO)].
Both ACN and DMSO cleaved the chloro-bridged iridium(III)
dimer and coordinated to the central atom with the formation
of Ir−N and Ir−S bonds. The measured Ir−N bond length was
2.108 Å, while the average Ir−S bond length was 2.346 Å. The
solved crystal structures clearly confirmed that the formed
species do not have any symmetry elements; thus, their
cyclometalated ppy ligands are inequivalent. We suppose that
this is the origin of the doubled set of signals from
cyclometalated ligands in the NMR spectra measured in
DMSO-d6 but that it is not the only one. The inequivalence of
the ppy ligands is enhanced by the difference in the trans
effects of the chloride ligand and coordinated solvent, which
asymmetrically affects C-donating atoms in the positions trans
to them. It is known that nitriles and sulfoxides are the ligands
characterized by a stronger trans effect than halides.60,61 Thus,
the coordinated solvent should make the opposing Ir−C bonds
weaker than the Ir−C bond in the trans position to the
chloride ligand. In the case of [Ir(ppy)2Cl(DMSO)], it is true,
and the Ir−C bond that is opposite to the Ir−S bond is longer
by 0.021 Å, on average, than the Ir−C bond that is opposite to
the Ir−Cl bond. On the contrary, in [Ir(ppy)2Cl(ACN)] both
Ir−C bond lengths are exactly the same (2.001 Å). This
indicates a stronger coordinative nature of DMSO to ACN, a
very helpful observation for the explanation of their behavior
during NMR analysis, described in the next paragraph. At this
point, we can assume that the high-field-shifted set of
resonances corresponding to one of the two ppy ligands
originates from the ligand in the trans position to the
coordinated DMSO molecule. It is clear that the differences
in the trans effect are very subtle because the Ir atoms are
chelated by ppy ligands, forming 5-membered cyclometalated
rings, and, therefore, the distance between elements of the ring
cannot be so easily varied. With the aid of XRD analysis, we
were able to identify the chemical process occurring during the
chloro-bridged iridium(III) dimer dissolution in coordinating
solvents, but there was still the question of linking the outcome
with the results of NMR spectroscopy.
performed in nondeuterated solvents, we expected to observe
what happens both with the signals originating from the ppy
ligands and with those corresponding to the coordinated
solvent molecules. To our surprise, we noticed a significant
difference between the NMR spectra of [Ir(ppy)2Cl(ACN)]
and [Ir(ppy)2Cl(DMSO)] in CD2Cl2. In the case of the
former, we observed only one set of signals originating from
ppy and a singlet in the high-field region (Figure S17). The
chemical shift of the singlet corresponded to a free ACN
molecule.62 Moreover, integration showed that the ppy/ACN
ratio was 1:1, the same as that in the crystal phase, because
there was an extra ACN molecule per [Ir(ppy)2Cl(ACN)]
molecule in the unit cell. Even the addition of a few extra
microliters of ACN to the NMR tube did not result in the
appearance of signals corresponding to [Ir(ppy)2Cl(ACN)].
1
Then we decided to measure the H NMR spectrum of [Ir(μ-
Cl)(ppy)2]2 in ACN-d3 because the crystals of [Ir(ppy)2Cl-
(ACN)] were obtained after heating of the dimer in pure
ACN. However, the spectrum was not interpretable because of
the low solubility of the analyte and did not show clear
evidence for dimer cleavage (Figure S18). Surprisingly, the
product of the cleavage could be crystallized, which proves that
it is much more stable in the solid phase than in solution. In
this way, we proved that the ability of ACN to cleave [Ir(μ-
Cl)(ppy)2]2 is very weak in solution, while the equilibrium
between the dimeric and monomeric forms is evidently
established, and the stability of [Ir(ppy)2Cl(ACN)] is
enhanced in the crystalline phase. In contrast, many more
signals were observed in the 1H NMR spectrum of
[Ir(ppy)2Cl(DMSO)] crystals dissolved in CD2Cl2 (Figure
S19). Even more than two sets of resonances originating from
ppy were observed along with some minor resonance lines.
This indicated that DMSO is a stronger ligand than ACN,
resulting in observation of both the chloro-bridged dimer and
its cleaved form. The presence of the minor signals could be
attributed to the coformed impurities because the crystal-
lization process took about 2 weeks on the bench.63 Therefore,
we decided to perform an NMR titration of [Ir(μ-Cl)(ppy)2]2
in CD2Cl2 using DMSO as a titrant to monitor the cleavage
process in a more transparent way. Fragments of the spectra
recorded at the starting point and after the addition of each
DMSO portion are presented in Figure 4. At the beginning,
only one set of signals originating from the coordinated ppy
was observed, which proved that all of the ligands in the
chloro-bridged dimer are chemically equivalent. After the
addition of 2 equiv of DMSO per dimer molecule (Ir/DMSO
ratio of 1:1), new signals appeared and the intensity of the
starting material’s signals decreased. All of the resonances were
earlier observed in the NMR spectrum of [Ir(ppy)2Cl-
(DMSO)] obtained from the crystallization process, which
proved that an equilibrium between the same two compounds
takes place during the crystallization and titration (Figure
S19). The integration of well-separated signals from the low-
field region of the spectra allowed one to get rough
information about the equilibrium between the cleaved and
noncleaved forms. After the addition of a stoichiometric
amount of DMSO, the molar ratio of [Ir(μ-Cl)(ppy)2]2/
[Ir(ppy)2Cl(DMSO)] reached 1:3.25. Considering the low
concentration of the reactants and the facts that DMSO is a
monodentate ligand and that the equilibrium was reached after
an arbitrarily assumed 5 min of shaking, we came to the
conclusion that the formation of a mononuclear complex is
much preferable in this system. What is important is that this
1
Following the above conclusions, our next step was H
NMR analysis of the crystals to check whether the obtained
spectra resemble those obtained after dissolution of iridium-
(III) dimers in DMSO-d6. Because the crystallizations were
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reaction occurs very smoothly and quickly even in a low
concentration of reagents; therefore, there is no need to use
such harsh conditions as refluxing in neat DMSO to obtain the
desired mononuclear complexes.59 Further the addition of
DMSO equivalents resulted in a shift of the equilibrium toward
[Ir(ppy)2Cl(DMSO)], and total consumption of the starting
material was observed after the addition of 10 equiv of DMSO.
The equilibrium might be monitored also by observation of the
Dimer cleavage might be observed during ESI-MS measure-
ment as well. This is a so-called “soft ionization” technique,
which means that a fragmentation is not preferred. Addition-
ally, in this type of analysis, the solution-phase composition is
retained in the gas phase. Both features of this technique are
very helpful in achieving our goal because they should permit
observation of molecular ions of the cleaved form of the
studied complexes. In this technique, it is necessary to use
polar solvents because the ionization process requires
formation of the charged analyte’s solution droplets. However,
not all polar solvents can be used because volatility is another
desired feature. High-boiling solvents such as DMF and
DMSO are not compatible with the ESI technique because
they contaminate the source and decrease the detector’s
sensitivity. Because of limitation of the use of high-boiling
solvents (such as DMSO) in the ESI-MS technique, measure-
ments were made using ACN, which also proved its ability to
cleave the chloro-bridged iridium(III) dimers. In line with our
expectations, we observed signals corresponding to both the
chloro-bridged dimer and its cleaved form, while the mass
peaks corresponding to the cleaved form were dominant in the
spectra. The ionization mechanism in positive mode involved
abstraction of a chloride anion, not a typical addition of
hydrogen, sodium, or potassium cations. During analysis of our
compounds, we noticed an important relationship that might
have a crucial impact on the interpretation of the results
obtained. The appearance of the recorded spectra was strongly
affected by the potential applied to the orifice plate that was
implemented in the driving software as a declustering potential
option (DP, also called cone potential). This parameter has a
significant impact on the additional acceleration of the analyte
ions in the medium-pressure region of the source, which causes
collision-induced dissociation with the background gas.66 As a
result, the MS spectra obtained may not fully reflect the actual
molecular structure of the examined substances. For example,
adjustment of DP allowed one to control the clusterization/
declusterization of polypeptides and the dissociation of ligands
from the complex molecules, as in the case of
[H3Ru4(CO)12]−.67,68 At the default value of DP, the m/z
values corresponding to [Ir(C^N)2(ACN)]+ and [Ir(C^N)2]+
in various ratios were observed. This confirmed that the
cleaved forms were present in the solution because the cations
produced by abstraction of the chloride anion and a weakly
bonded ACN molecule were observed. With increasing DP,
the abundance of [Ir(C^N)2(ACN)]+ decreased and almost
only [Ir(C^N)2]+ could be observed. Interestingly, with
decreasing DP, the abundance of the ions corresponding to
the above-mentioned cations decreased, and the m/z value
corresponding to [Ir(C^N)2(ACN)2]+ became dominant.
When we analyzed the same sample using a mass spectrometer
of a different manufacturer without the possibility of adjusting
the DP value, the signals corresponding to [Ir(C^N)2]+ were
dominant. This shows that the MS spectra obtained by the
ESI-MS technique might be misinterpreted depending on the
parameters of the spectrometer used (Figure 5). For example,
it is possible to assume that, in the coordinating solvent, the
chloro-bridged dimers undergo double coordination of the
solvent molecule together with a shift of the chloride anion to
an outer coordination sphere, by the observation of [Ir-
(C^N)2(solvent)2]+ as a dominant ion in the spectra. This
process might be even more flawed by the involvement of
much stronger ligands. Therefore, our recommendation is to
1
high-field region in the H NMR spectra, where the signals of
the cleaving ligand are present. The addition of the first
portion of the titrant to the [Ir(μ-Cl)(ppy)2]2 solution resulted
in the appearance of a singlet at 2.55 ppm, corresponding to
the presence of free DMSO, accompanied by two additional
singlets, the first upfield-shifted and second downfield-shifted
(Figure S20). The chemical shifts of these resonances in
relation to free DMSO were ca. 0.5 ppm, and the number of
protons determined on the basis of their integration was the
same and exactly corresponded to the number of protons
assigned to resonance lines derived from ppy ligands in
[Ir(ppy)2Cl(DMSO)]. The break of the chemical equivalence
of both protons in the ppy ligands and methyl groups of the
coordinated DMSO caused by the lack of symmetry is the
evidence of [Ir(ppy)2Cl(DMSO)] formation. Of course, the
coordinated DMSO-d6 molecules could not be observed in the
1H NMR spectra measured in DMSO-d6, but duplication of
the resonances assigned to ppy ligands confirms the formation
of [Ir(ppy)2Cl(DMSO-d6)] in such an environment. The same
behavior was observed for all of the complexes reported herein.
The described phenomena might provide an explanation of
why, in the given set of spectroscopic data of the substance
supposed to be [Ir(ppy)2(DMSO)2]PF6, the resonances
originating from coordinated DMSO molecules were not
observed.57 It also confirms that the best way to pull out
chloride from the inner coordination sphere of iridium(III) is
the use of silver(I) salts,64,65 and the employment of sodium
salts might be insufficient. In this way, we confirmed with no
doubt that iridium(III) dimers undergo equilibrium cleavage in
the presence of coordinating solvents (Scheme 5). Moreover,
we have shown that the coordinative nature of DMSO is much
stronger than that of ACN, in spite of the difference in the
steric hindrance of these solvent molecules. The dimers based
on 2-phenoxypyridine also underwent cleavage in the same
conditions, but the process was hampered by the low solubility
of the species in DMSO, especially in the case of [Ir(μ-
Cl)(F3CPhOPy)2]2. Heating of the sample in DMSO-d6 led to
formation of the cleaved form of the dimer, as confirmed by 1H
NMR analysis, but the desired signals were accompanied by
some others originating from coformed species. We suppose
that DMSO molecules can substitute the coordinated pyridyls,
leading to the formation of a complex mixture of species.
Therefore, NMR analysis of [Ir(μ-Cl)(F3CPhOPy)2]2 in
CDCl3 instead of DMSO-d6 was made.
Scheme 5. Cleavage of [Ir(μ-Cl)(C∧N)2]2 with a
Coordinating Solvent
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Figure 5. Simplified scheme of the electrospray ion source and relationship between the potential applied to the orifice during the measurement
and the observable ions in positive mode.
combine different types of analytical methods in the studies of
such processes to avoid misinterpretation of the results.
determined. Finally, ESI-HRMS characterization of the chloro-
bridged iridium(III) dimers in ACN not only confirmed
formation of the target molecules but also showed how DP
adjustment of the spectrometer used varies the spectroscopic
image of the analyzed complexes. Our work clearly
demonstrated that, depending on the applied potential, a
chloride-abstracted cation derived from [Ir(C^N)2Cl(ACN)]
containing zero-, one-, or two-coordinated solvent molecules
becomes dominant in the spectrum. We believe that all of the
phenomena explained in this work strongly enrich the
knowledge in the field of iridium chemistry, will help in the
reconsideration of the results previously reported in the
literature, and support interpretation of the experimental data
in the upcoming works.
CONCLUSIONS
■
In summary, in this work we proved the positive effect of
microwave irradiation on the formation of chloro-bridged
iridium(III) dimers and the importance of the presence of
water in this process. A series of known and novel dimers were
obtained in the optimized conditions, and the products were
characterized by NMR spectroscopy and MS, which provided
strong experimental evidence of the dimers’ cleavage with the
coordinating solvents. In the first stage of our work, we
compared the efficiency of model dimer formation in a
microwave reactor and an oil bath, which showed the beneficial
effect of the unconventional source of energy at all of the
tested temperatures. Moreover, the great impact of the
presence of water in the reaction medium was confirmed
and assumed to improve both the iridium(III) chloride
solubility and absorption of stimulating radiation. All of the
tests allowed the establishment of optimal conditions for the
obtainment of high-purity dimers with minimization of the
occurrence of side reactions, which enabled a very simple
isolation process, just filtration and rinsing. According to the
developed protocol, a series of known and novel iridium(III)
dimers were synthesized very quickly and in high to very high
yields. The scope of the tested ligands included compounds
based on 2-phenylpiridinato, benzo[h]quinolinato, and 2-
phenoxypiridinato cores, equipped with electron-donating
(−Me, −OMe, and −OPh), electron-withdrawing (−F, −Br,
−CF3, and −C6F5), and p-type semiconducting (−NPh2 and
−C6H4NPh2) groups. In the case of the ligand bearing a
−NMe2 substituent, an alternative synthetic pathway, avoiding
HCl involvement, was proposed. All of the new iridium(III)
dimers constitute attractive starting materials for the
preparation of new OLED emitters or active species for
bioimaging. The obtained dimers were characterized by means
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01071.
Procedures for the synthesis of ligands, precursors, and
complexes, NMR and ESI-HRMS spectra of the new
compounds, and X-ray analysis detailed data (PDF)
Accession Codes
CCDC 1895621,1895622, 1897304, and 1897305 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
Bartosz Orwat − Faculty of Chemistry and Center for Advanced
1
of H NMR spectroscopy and ESI-MS in DMSO and ACN,
́
Technology, Adam Mickiewicz University in Poznan, 61-614
which revealed the occurrence of a chemical reaction between
these solvents and the binuclear iridium complexes. A detailed
investigation resulted in a multithreaded explanation of the
process, identified as cleavage of the μ-chloride bridges in
which DMSO coordination appeared to be much more
favorable than ACN coordination. The cleavage was clearly
evidenced in solution during NMR titration as well as in the
solid phase using XRD structure determination of the model
dimer’s cleaved forms. Moreover, the crystal structures of the
two dimers stabilized with bzq and 5-(Ph2N)bzq ligands were
́
Poznan, Poland; Email: b.orwat@amu.edu.pl
Ireneusz Kownacki − Faculty of Chemistry and Center for
Advanced Technology, Adam Mickiewicz University in Poznan,
61-614 Poznan, Poland; orcid.org/0000-0002-5445-760X;
́
́
Email: irekk@o365.amu.edu.pl
Authors
Myong Joon Oh − Faculty of Chemistry and Center for
Advanced Technology, Adam Mickiewicz University in Poznan,
́
́
61-614 Poznan, Poland
K
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conductor for the electrochemical sensing of IFN-γ. Sci. Rep. 2017, 7,
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Advanced Technology, Adam Mickiewicz University in Poznan,
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Octopuses with a Polyhedral Oligomeric Silsesquioxane (POSS) Core
and Iridium(III) Polypyridine Arms: Synthesis, Aggregation Induced
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(13) Liao, G.; Peng, X.; Li, T.; Ye, Z.; Xiang, X.; Fu, C. The
Discovery of an Iridium(III) Dimer Complex as a Potent Antibacterial
Agent against Non-Replicating Mycobacterium smegmatis. Polymers
2018, 10, 297.
́
61-614 Poznan, Poland; orcid.org/0000-0003-0593-5545
Maciej Kubicki − Faculty of Chemistry, Adam Mickiewicz
́
́
University in Poznan, 61-614 Poznan, Poland; orcid.org/
0000-0001-7202-9169
Rafał Januszewski − Faculty of Chemistry and Center for
Advanced Technology, Adam Mickiewicz University in Poznan,
́
́
61-614 Poznan, Poland; orcid.org/0000-0002-5562-6647
Complete contact information is available at:
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https://pubs.acs.org/10.1021/acs.inorgchem.0c01071
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge financial support from the National
Science Centre, Poland (Grants 2013/11/B/ST5/01334 and
2018/29/N/ST5/02926).
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