Palladium-Catalyzed C?H Bond Arylation of Cyclometalated Difluorinated 2-Arylisoquinolinyl Iridium(III) Complexes

Article abstract of DOI:10.1002/chem.202102006

The utility of C?H bond functionalization of metalated ligands for the elaboration of aryl-functionalized difluorinated-1-arylisoquinolinyl Ir(III) complexes has been explored. Bis[(3,5-difluorophenyl)isoquinolinyl](2,2,6,6-tetramethyl-3,5-heptanedionato)

Full text of DOI:10.1002/chem.202102006

Communication
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Chemistry—A European Journal
Palladium-Catalyzed CÀ H Bond Arylation of Cyclometalated
Difluorinated 2-Arylisoquinolinyl Iridium(III) Complexes
Marie Peng,[a] Jinqiang Lin,[b] Wei Lu,*[b] Thierry Roisnel,[a] Véronique Guerchais,*[a]
Henri Doucet,*[a] and Jean-François Soulé*[a]
chemical modifications of 2-phenylpyridine (ppy) core by add-
Abstract: The utility of CÀ H bond functionalization of
ing functional groups have been used as a tool to control the
metalated ligands for the elaboration of aryl-functionalized
luminescence properties.[1,5] Moreover, the introduction of
difluorinated-1-arylisoquinolinyl Ir(III) complexes has been
reactive functional groups on the cyclometalated ligands paves
explored.
Bis[(3,5-difluorophenyl)isoquinolinyl](2,2,6,6-
the way for bioimaging applications by tuning their photo-
physical properties and/or hooking up interactions with bio-
logical substrates.[6] The most widely used strategy for the
preparation of bis-cyclometalated C^N-based Ir(III) complexes
[Ir(C^N)2(L^X)] (L^X is a monoanionic ligand), involves a multi-
step synthetic procedure, starting by the synthesis of the
organic pro-ligands followed by their complexation using
IrCl3.nH2O as precursor (Figure 1, Pathway A). Although this
conventional synthetic method provides the expected Ir(III)
complexes in satisfactory reaction yields, the complexation
steps often require drastic conditions, which are sometimes not
compatible with some functionalities borne by the pro-ligands.
An alternative approach to functionalize such species is to
perform the CÀ C coupling directly on the complexes, using the
so-called “catalysis-on-the-complex” (Figure 1, Pathway B). Be-
sides expending the panel of functional groups that can be
incorporated on the C^N-ligands, this post-functionalization
strategy accelerates the synthesis of a library of novel metal
complexes: only three manipulations are required to prepare
three complexes, where formerly six steps were necessary (Figure 1
pathway A vs. B).
Following the seminal work of Williams on Suzuki cross-
coupling reaction of bromo-functionalized (bis-terpyridyl)
iridium(III) complexes (Figure 2A),[7] post modification Suzuki
reactions have become a straightforward approach to tune the
photophysical properties of the Ir(III) complexes.[8] A few
examples of Suzuki cross-coupling reactions onto the ancillary
ligand were also reported.[9]
Although Pd-catalyzed CÀ H bond arylation has emerged as
a suitable alternative to Suzuki cross-couplings for the synthesis
of poly(hetero)aryls –since no pre-functionalization is
required-[10] their applications for catalytic CÀ H functionalization
of cyclometalated ligands, has been barely explored. So far,
only one procedure of “catalysis-on-the-complex” for CÀ H bond
functionalization has been reported by some of us for the direct
arylation of thienyl rings of tris-homoleptic fac-Ir(C^N-thpy)3
[thpy=2,2’-thienylpyridine], and bis-cyclometalated [Ir(C^N-
thpy)2(acac)] (acac=acetylacetonate) (Figure 2B).[11] The aryla-
tion took place regiospecifically at the C5À H bond position of
the metalated thienyl rings, inducing a red-shifted of their
emission wavelengths. In contrast, CÀ H bond arylation of
metalated C-aryl ring in 2-arylpyridine-based Ir(III) complexes is
not reported yet. This is probably due to the lower reactivity of
tetramethyl-3,5-heptanedionato) iridium(III) undergoes Pd-
catalyzed CÀ H bond arylation with aryl bromides. The
reaction regioselectively occurred at the CÀ H bond flanked
by the two fluorine atoms of the difluoroaryl unit, and on
both cyclometalated ligands. This post-functionalization
gives a straightforward access to modified complexes in
only one manipulation and allows to introduce thermally
sensitive functional groups, such as trifluoromethyl, nitrile,
benzoyl, or ester. The X-ray crystallography, photophysical,
and electrochemical properties of the diarylated complexes
were investigated. Whatever the nature of the incorporated
substituted aryl groups is, all obtained complexes emit red
phosphorescence (622–632 nm) with similar lifetimes (1.9–
2 μs).
Since two decades, cyclometalated iridium(III) complexes have
become one of the most studied class of luminescent metal
complexes[1] for their potential applications ranging from
photocatalysts[2] to luminescent chemosensors[3] as well as from
new electroluminescent display materials to devices.[4] Much
research aimed at developing novel Ir(III) phosphorescent
complexes with excellent emission properties, in particular pure
color and high luminescence quantum efficiency. These photo-
physical assets of Ir(III) complexes relies on a strategic molecular
design and consequently on the convenient access to newly
functionalized C^N-ligands, a crucial and determining step in
the way to luminescent complexes. A plethora of examples of
[a] M. Peng, Dr. T. Roisnel, Dr. V. Guerchais, Dr. H. Doucet, Dr. J.-F. Soulé
Univ Rennes, CNRS UMR6226
3500 Rennes (France)
E-mail: veronique.guerchais@univ-rennes1.fr
henri.doucet@univ-rennes1.fr
jean-francois.soule@univ-rennes1.fr
[b] J. Lin, Prof. W. Lu
Department of Chemistry
Southern University of Science and Technology
Shenzhen, Guangdong (P. R. China)
E-mail: luw@sustech.edu.cn
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202102006
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Figure 1. Different approaches to prepare functionalized Ir(III) complexes.
questions remain pending about this strategy of catalysis-on-the
complex: i) is the regioselectivity still driven by the F atoms or
CÀ Ir bond will affect it? ii) will Ir(III) complexes be stable enough
under the required catalytic reaction conditions i.e. temper-
ature, presence of Pd catalyst, base, and additives?
Catalysis on the complex: We began our study by
preparing the Ir(III) complexes 3 and 4 via reported procedures
(Scheme 1).[14] Firstly, the pro-ligands 1-(2,4-difluorophenyl)
isoquinoline (1) and 1-(3,5-difluorophenyl)isoquinoline (2) were
obtained in 84% and 50% yields, via Suzuki coupling from the
appropriate difluorinated aryl boronic acids and 2-chloroisoqui-
noline using 2 mol% PdCl(C3H5)(dppb) [dppb=1,4-bis
(diphenylphosphino)butane)] as catalyst in the presence of
K2CO3 as the base in toluene-MeOH-water as a solvent mixture.
Secondly, the reaction of 1 and 2 with IrCl3·nH2O afforded the
corresponding cyclometalated μ-chloro-bridged dimers, which
then reacted with 2,2,6,6-tetramethylheptane-3,5-dione in the
presence of a base (K2CO3) to produce complexes 3[14] and 4 in
75% and 40% yield, respectively.
Having the neutral complex 3 in hands, we explored the
most suitable conditions for Pd-catalyzed CÀ H bond arylations
of the metalated difluorinated phenyl ring, namely (i) Pd-based
precursors (ii) additives (ligands or bases) and (iii) solvents,
using 4-bromobenzotrifluoride as the aryl source for this late-
stage coupling (Table 1). The optimized conditions, namely, 4
equivalents of 4-bromobenzotrifluoride, 5 mol% PdCl2(CH3CN)2
associated with dppb as phosphine ligand in the presence of 4
equivalents of KOPiv (KOPiv=potassium pivalate) as the base in
Figure 2. Pd-catalyzed arylation of cyclometalated Ir(III) complexes.
°
DMA (DMA=N,N-dimethylacetamide) at 125 C for 16 h, af-
forded the diarylated complex 5a in 89% isolated yield (Table 1,
entry 1). Noteworthy, a trace amount of the monoarylated
complex 5b was also detected (<5%) by NMR spectroscopy of
the crude mixture. A significant decrease in yield was observed
using the stable, well-defined PdCl(C3H5)(dppb) catalyst or
replacing PdCl2(CH3CN)2 by Pd(OAc)2 (Table 1, entries 2 and 3).
A diphosphine ligand with a smaller bite angles, dppe (dppe=
1,2-bis(diphenylphosphino)ethane), or mono-phosphine P(2-
furyl)3, gave lower yields in 5a with the additional presence of
the monoarylated complex 5b in 10–15% yield (Table 1,
entries 4 and 5). When a lower amount of aryl bromide was
employed, the yield of 5a dropped without affording 5b in a
yield higher than 15% -even using only 1 equivalent of 4-
aryl CÀ H bond compared to heteroaryl CÀ H bond in Pd
catalysis.[12] We have previously succeeded to perform Pd-
catalyzed direct arylation of the polyfluorinated aryl unit of 2-
(polyfluorophenyl)pyridine derivatives to prepare bis-cyclome-
talated luminescent Ir(III) complexes.[13] We showed that the
incorporation of an aryl group between the two fluorine atom
induces the enhancement the luminescence quantum yields of
the resulting cationic Ir(III) species to near-unity. However, this
synthetic approach required several manipulations to access
the targeted Ir-complexes. To speed up their preparation, we
decided to assess the reactivity of cyclometalated 2-arylpyridine
in Pd-catalyzed CÀ H bond arylations (Figure 2C). Several
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other solvents than DMA such as DMF, NMP (DMF=N,N-
dimethylformamide, NMP=N-methylpyrrolidone), or toluene
gave significantly lower yields in 5a (Table 1, entry 9–11). We
also tried our optimized conditions for the CÀ H bond arylation
of the pro-ligand 1 (Table 1, entries 12 and 13). We observed
the same regioselectivity (the arylation took place at the CÀ H
bond flanked by the two fluorine atoms while the orthoÀ CÀ H
bond to the isoquinoline remains untouched). However, a
longer reaction time (72 h) was required to obtain 5c in 66%
yield. We also performed a competitive reaction using 1
equivalent of 4-bromobenzotrifluoride, 1 equivalent of iridium
complex 3 and 2 equivalents of proligand 1 (Table 1, entry 14).
Surprisingly, the reaction was not complete and a mixture of
the diarylated complex 5a (12%) and the arylated proligand 5c
(31%) was obtained. This result suggests that the cyclometalla-
tion did not enhance the reactivity of the CÀ H bond but
prevented the coordination of the nitrogen atom of the
isoquinoline unit to Pd, which seems to deactivate the catalyst.
Kinetics studies have been performed on the reaction
between complex 3 and 4-bromobenzotrifluoride (4 equiva-
lents) in the presence of 5 mol% PdCl2(CH3CN)2 associated with
Scheme 1. Preparation of bis[2-(3,5-difluorophenyl)isoquinolinyl)](2,2,6,6-
tetramethyl-3,5-heptanedionato)-Ir(III) (3) and bis[2-(4,6-difluorophenyl)
isoquinolinyl](2,2,6,6-tetramethyl-3,5-heptanedionato)-Ir(III) (4).
Table 1. Control reactions of optimized conditions.
°
dppb using 4 equivalents of KOPiv in DMA at 125 C (Figure 3).
As expected, the reaction starts by the formation of the
monoarylated complex 5b, which is then quickly converted
into the diarylated complex 5a. The kinetic formation of 5b
shows a steady-state profile with a yield not exceeding 28%.
This profile suggests that the second arylation proceed faster
than the first one which might give a clue on the difficulty to
obtain the mono-arylated complex 5b in good yield.
Using these optimized reaction conditions, the direct
arylation of complex 3 with 4-bromobenzonitrile, 4-bromoben-
zophenone, and ethyl 4-bromobenzoate was investigated in
Entry
Deviation from standard conditions
Yield in 5a [%]
1
2
3
4
5
6
7
8
–
89
54
62
PdCl(C3H5)(dppb) as catalyst
Pd(OAc)2/dppb as catalyst
dppe as ligand
10 mol% tri(2-furyl)phosphine as ligand
3 equiv. of ArBr
1 equiv. of ArBr
KOAc as base
DMF as solvent
NMP as solvent
xylene as solvent
–
28[a]
30[a]
39[a]
24[a]
54[a]
19[a]
28[a]
7[a]
9
10
11
12[b]
13[b]
14[d]
19[c]
72 h
–
66%[c]
12% (31%)[c]
[a] 5b was also observed (<10–15% yield). [b] reaction performed with 1
(2 equiv.) instead of 3. [c] yield in 5c. [d] reaction performed with 3
(1 equiv.), 1 (2 equiv.), 4-bromobenzotrifluoride (1 equiv.).
bromobenzotrifluoride, due to the formation of Ullmann-type
aryl bromide homocoupling as the main side-product (Table 1,
entries 6 and 7).[15] Our studies clearly show that KOPiv as base
outperformed KOAc (Table 1, entry 8). Reactions carried out in
Figure 3. Kinetic profile of Pd-catalyzed CÀ H bond arylation of complex (3)
with 4-bromobenzotrifluoride. 19F NMR yield using fluorobenzene as internal
standard.
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order to verify the applicability of the above method to the
access to systems with sensitive functional groups (Scheme 2).
In all cases, the diarylation occurred, affording the bis-arylated
iridium complexes 6–8 in 88%, 55%, and 44% yield, respec-
tively.
7 and 8, respectively. Notably, the distance between two
iridium centers for adjacent complexes in the crystal packing
varies slightly from 9.726 Å to 9.587 Å for the parent complexes
3 and 4. A significant increase of Ir…Ir distance is observed of
10.088, 13.557 and 13.237 Å for the arylated complexes 5a, 7,
and 8, respectively. Notably, 7 exhibits the longer intermetallic
distance of 13.557 Å showing the dramatic influence of the
steric hindrance of the added p-benzophenone substituents.
Electrochemical studies: Depicted in Figures S1 are cyclic
voltammograms of complexes 3–8 recorded in deoxygenated
dichloromethane with 0.1 M nBu4NPF6 as supporting electro-
lytes and with redox potentials referenced against the ferro-
cene/ferrocenium (Fc0/+) oxidation couple. Complexes 3, 4, 5a,
6, and 8 show reversible oxidation waves at 0.60, 0.49, 0.63,
0.65, and 0.60 V vs. Fc0/+, respectively, while complex 7 with a
benzoyl group, exhibits a quasi-reversible oxidation wave at
0.61 V vs. Fc0/+. These oxidation waves are negatively shifted
from the Ir(III)/(IV) redox couple that is usually at around 1.0 V
X-ray crystal structures: Single crystals of complexes 3, 4,
5a, 7, and 8 suitable for X-ray analysis were grown by slow
diffusion of n-pentane into a CH2Cl2 solution of the respective
complexes. The X-ray crystal structures of the neutral complexes
4, 5a, 7, and 8 are depicted in Figure 4. Selected bond distances
and bond angles, as well as selected bite angles, are tabulated
in Table S1. All neutral complexes exhibited distorted octahe-
dral geometries, with the two C^N ligands adopting a C,C-cis,
and N,N-trans configuration similar to that of the archetype
complexes.[16] The IrÀ C and IrÀ N bond distances in all crystal
structures are in the same range. The introduced aryl pendant
group is not coplanar with the C^N ligand. The torsion angles
between the plane of the incorporated aryl group and that of
the cyclometalated phenyl ring of the two C^N ligands are
52.10(5)/43.00(6), 41.6(11)/48.1(10), and 43.4(13)/43.4(13) for 5a,
vs. Fc0/+ [13b]
and hence could be tentatively assigned to be the
,
oxidation of the difluorinated 2-arylisoquinoline ligand. The
oxidation couple of 3 (0.60 V) is positively shifted from that of 4
(0.49 V), which is in line with our previous report on the
phenylpyridyl Ir(III) analogues[13b] and signifies the importance
of the positions of the fluorine atoms, i.e. their electronic effects
on the cyclometalated ligands for Ir(III) complexes. Evidently,
the substituents conferred by the Pd(II)-catalyzed CÀ H bond
arylation of the cyclometalated difluorinated 2-arylisoquinoline
ligands only slightly shift the oxidation couples of 5a and 6–8
for 0–0.05 V from the unsubstituted complex 3. Among the
complexes under study,
4 and 7 show identifiable but
irreversible reduction wave at around 2.0 V vs. Fc0/+, and hence
the HOMO-LUMO gaps could be estimated to be around 2.6 eV,
which is slightly smaller than that (2.7–2.8 eV) for the phenyl-
pyridyl Ir(III) analogues.[13b]
Scheme 2. Post-functionalization of cyclometalated Ir(III) complex 3 via Pd-
catalyzed CÀ H bond arylation.
Electronic absorption and emission properties: The photo-
physical parameters of complexes 3–8 are listed in Table 2, and
the UV-vis absorption and emission spectra recorded in
deoxygenated CH2Cl2 solutions are depicted in Figure 5. The
salient features of the electronic absorption spectrum of these
complexes are comparable to those recorded for the typical
cyclometalated Ir(III) complexes.[1,2] The intense absorption
bands in the 230–350 nm range could be attributed to the π-π*
intraligand (IL) transitions of the difluorinated 2-arylisoquinoline
ligands. The relatively weaker absorption bands in 400–500 nm
Figure 4. .Solid-state structures of complexes 3, 4, 5a, 7, and 8. hydrogen
atoms are omitted for clarity. Thermal ellipsoids are at the 50% probability
level.
Figure 5. (a) Absorption and (b) normalized emission spectra of complexes
3–8 in deoxygenated CH2Cl2 (λex =365 nm, concentration ~1×10À 5 M) at
298 K.
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Table 2. Photophysical data of complexes 3–8.
Complex
Absorption[a]
Emission
Medium
λmax/nm (ɛ/104 MÀ 1 cmÀ 1
)
T [K]
λmax [nm]
τem [μs]
Φem[d] [%]
[b]
3
236 (6.06), 287 (3.84), 341 (1.86), 455 (0.66)
CH2Cl2
298
77
298
77
298
77
298
77
622
583, 625
616
591, 622
632
596, 634
627
586, 633
622
590, 636
627
2.0
8.4
5.0
6.0
1.9
5.6
2.0
5.5
1.9
4.4
2.0
6.8
52
MeTHF[c]
[b]
4
228 (5.31), 257 (3.89), 279 (sh, 3.29), 341 (1.86), 387 (0.84), 471 (0.34)
236 (6.07), 283 (4.48), 387 (sh, 0.83), 456 (0.43)
233 (8.29), 287 (5.67), 333 (3.94), 449 (0.59)
CH2Cl2
51
33
39
34
41
MeTHF[c]
[b]
5a
6
CH2Cl2
MeTHF[c]
[b]
CH2Cl2
MeTHF[c]
[b]
7
239 (6.37), 281 (5.06), 342 (2.98), 444 (0.45)
CH2Cl2
298
77
298
MeTHF[c]
[b]
8
233 (6.61), 287(4.64), 322 (3.31), 453 (0.51)
CH2Cl2
MeTHF[c]
77
586, 633
[a] UVÀ vis spectra were recorded in CH2Cl2 at a concentration of 10À 5 M at 298 K. [b] PL spectra, lifetimes, and quantum yields were recorded in deaerated
CH2Cl2 solutions at a concentration of 10À 5 M at 298 K. [c] PL spectra and lifetimes were recorded in 2-methyltetrahydrofuran at a concentration of 10À 5 M at
77 K. [d] Absolute quantum yields measured with an integrating sphere.
range tailing beyond 550 nm could be assigned to be metal-to-
ligand charge transfer (1/3MLCT) and ligand-to-ligand charge
transfer (1/3LLCT) transitions. Notably, the lowest-energy tran-
sition of complex 4 at 471 nm is red-shifted from that of
complex 3 at 455 nm, which is in line with the electrochemical
study mentioned above. Again, introduction of substituents by
the Pd(II)-catalyzed CÀ H bond arylation of the CÀ N-ligands
impart minor effects on the low-energy transitions of the series
complexes, except increased molar extinction coefficients in the
UV region. The HOMO-LUMO gaps estimated from the
electronic absorption spectra are around 2.6 eV (475 nm), which
is coincident with the energy gap estimated from the electro-
chemical measurements.
Complexes 3–8 in deoxygenated CH2Cl2 solutions emit
intense orange-yellow light, upon photo-excited at 350 nm.
Broad emission spectra (Figure 5b) were recorded for com-
plexes 3, 5a, and 6–8 with emission maxima, quantum yields,
and lifetimes of 622–627 nm, 33–52%, and 1.9–2.0 μs, respec-
tively, which are slightly red-shifted, but comparable with the
parameters recorded with the mono-fluorinated parent com-
plex [IrC^N-pic-F)2(acac)] (emission maximum at 600 nm in
CH2Cl2 with quantum yield and lifetime of 33% and 1.2 μs,
respectively).[17] As previously reported, the incorporation of aryl
groups has no significant impact on the emission wavelength.[13]
Complex 4 with F-substituents at 3,5-positions shows emission
In summary, Pd-catalyzed CÀ H bond arylation was success-
fully applied to the cyclometalated 2-(3,5-difluorophenyl)
isoquinoline of neutral heteroleptic Ir(III) complexes giving rise
to aryl-substituted complexes in good to excellent yields. The
arylation regioselectively occurred at the CÀ H bond of the 3,5-
difluorophenyl ring flanked by two fluorine atoms with a good
functional group tolerance (i.e, trifluoromethyl, cyano, ketone,
or ester). The arylation of the cyclometalated ligand seems to
proceed faster than on the proligand due to the absence of
interactions between the nitrogen atom of the isoquinoline unit
and Pd complex. The introduced pendent aryl groups have a
negligible impact on the emission properties, all post-function-
alized complexes remain good red emitters. In addition, the
incorporation of these aryl unit increases the intermetallic Ir···Ir
distance (up to 3.8 Å) between two adjacent complexes in the
crystal packing, an important parameter to reduce intermolecu-
lar quenching in optoelectronic devices. This catalysis-on-the-
complex strategy based on aryl CÀ H bond arylation simplifies
and opens new chemical space to design cyclometalated Ir(III)
complexes featuring target functions that are essential for their
applications in material science and biology especially for the
chemical modulation of biological properties of metal com-
plexes.
maximum at 616 nm but with a vibronically structured emission Acknowledgements
profile. The excitation spectrum (Figure S2) monitored at the
emission maximum for each of the complexes are identical to
their respective absorption spectrum, indicating that the
emissions essentially came from the cyclometalated Ir(III)
complexes. Interestingly, all these complexes under study show
We thank the CNRS, University of Rennes 1 and French Scientific
Ministry of Higher Education for providing financial support.
similar structured emission spectra (Figure S3) in the 77 K glassy Conflict of Interest
2-methyltetrahydrofuran solutions with peak maximum around
590/630 nm and lifetimes in the 4.4–8.4 μs, implying that the
emission could be the same parentage most likely due to the
difluorinated 2-arylisoquinoline ligands with little contribution
from the metal and the aryl-substituents on the difluorinated
phenyl-group.
The authors declare no conflict of interest.
Keywords: catalysis · post-synthetic modifications · iridium ·
luminescence · red emitters
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Manuscript received: June 7, 2021
Accepted manuscript online: July 8, 2021
Version of record online: ■■■, ■■■■
Chem. Eur. J. 2021, 27, 1–7
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COMMUNICATION
M. Peng, J. Lin, Prof. W. Lu*, Dr. T.
Roisnel, Dr. V. Guerchais*, Dr. H.
Doucet*, Dr. J.-F. Soulé*
1 – 7
Palladium-Catalyzed CÀ H Bond
Arylation of Cyclometalated Difluori-
nated 2-Arylisoquinolinyl Iridium(III)
Complexes
Catalysis-on-the-complex was
affected by CÀ Ir bond, as the reaction
occurred at the CÀ H bond flanked by
the two-fluorine atoms. All post-func-
tionalized Ir(III) exhibit red phosphor-
escence with moderate to good effi-
ciency.
developed to incorporate diversely
substituted aryl group on the ligands
of neutral cyclometalated Ir(III)
complexes. The regioselectivity of Pd-
catalyzed CÀ H bond arylation is not