Palladium-catalysed direct arylation of a tris-cyclometallated Ir(iii) complex bearing 2,2′-thienylpyridine ligands: A powerful tool for the tuning of luminescence properties

Article abstract of DOI:10.1039/c2cc16327f

Palladium-catalysed direct 5-arylation of metallated thiophenes of fac-Ir(NC^3′-thpy)₃ with aryl bromides via C-H bond functionalisation allows the synthesis of a variety of new Ir complexes in only one step (thpyH = 2,2′-thienylpyrid

Full text of DOI:10.1039/c2cc16327f

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COMMUNICATION
Palladium-catalysed direct arylation of a tris-cyclometallated Ir(III)
complex bearing 2,20-thienylpyridine ligands: a powerful tool for the
tuning of luminescence propertiesw
Kassem Beydoun,a Moussa Zaarour,a J. A. Gareth Williams,b Henri Doucet*a and
a
´
Veronique Guerchais*
Received 11th October 2011, Accepted 21st November 2011
DOI: 10.1039/c2cc16327f
Palladium-catalysed direct 5-arylation of metallated thiophenes
of fac-Ir(N^C30-thpy)3 with aryl bromides via C–H bond function-
alisation allows the synthesis of a variety of new Ir complexes in
only one step (thpyH = 2,20-thienylpyridine). The method offers
simple modification of the nature of the ligand and hence of the
photophysical properties of such complexes.
Scheme 1 Arylation of thiophenes by C–H bond activation.
access to the tris-cyclometalated complexes usually requires
drastic reaction conditions (B200 1C), which are incompatible
with ligands bearing thermally unstable functional groups.2
Therefore, to overcome these limitations, a straightforward
and selective method for the in situ functionalisation of iridium
complexes is highly desirable.
Palladium-catalysed C–H bond activation/functionalisation is a
powerful tool to access fine chemicals. However, it has apparently
never been applied to metallated ligands as a strategy to access
new luminescent materials. Charge-neutral, tris-cyclometallated
iridium(III) complexes of the form Ir(N^C)3 bearing 2-(hetero)-
arylpyridines, including 2,20-thienyl-pyridine (thpy), exhibit long
lifetimes and high emission quantum yields,1–3 making them
efficient phosphorescent components for OLEDs and other
applications (Fig. 1).4
One possible strategy would be the so-called ‘‘direct arylation’’,
involving regioselective C–H bond functionalisation of
thiophenes.8,9 In 1990, Ohta et al. reported the arylation of
thiophenes or furans with aryl halides, via a direct C–H bond
activation, in moderate to good yields using 5 mol% Pd(PPh3)4
as the catalyst (Scheme 1).10
Normally, the tuning of the absorption and emission properties
of such complexes requires the preparation of a variety of sub-
stituted ligands before complexation, and no straightforward
general route is available. Such ligands can be synthesized using
Stille,5 Negishi,6 or Suzuki7 cross-coupling reactions as the key
step before coordination to iridium(III). However, subsequent
Since these results, the palladium-catalysed direct arylation
of heteroaryl derivatives with aryl halides or triflates has
proved to be a powerful method for the synthesis of arylated
heterocycles.11 This reaction appears to be more attractive than
palladium catalysed Suzuki, Stille or Negishi cross-couplings,12
as no prior preparation of an organometallic derivative and its
transmetallation product using B(OR)3, XSnR3, or ZnX2 is
required. In this context, Ir(N^C30-thpy)3, 1,3 should represent
an attractive building block for accessing—in only one step—a
wide variety of new complexes which might display useful
luminescent properties (Scheme 2).
We report here that this approach offers straightforward
access to luminescent tris-cyclometallated iridium complexes
bearing 5-aryl-2-pyridylthiophene ligands. A substantial modu-
lation of the excited-state properties of the family of new
complexes 2c–9c has been achieved through the introduction
of the aryl groups.
Fig. 1 Generic structure of an Ir(N^C)3 complex incorporating 2,20-
thienylpyridine-based ligands.
a
´
Institut Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite
First, we attempted to promote the coupling of complex 1
with 4-bromobenzonitrile in the presence of Pd(OAc)2 as the
catalyst and KOAc as the base (Scheme 2).
´
´
de Rennes 1 ‘‘Organometalliques, Materiaux et Catalyse’’,
35042 Rennes Cedex, France. E-mail: henri.doucet@univ-rennes1.fr,
veronique.guerchais@univ-rennes1.fr; Tel: +33 (0)2 23 23 63 84
b Department of Chemistry, Durham University, South Road, Durham,
UK DH1 3LE
We selected a phosphine-free catalyst for such reactions in
order to avoid a possible decomposition of the iridium com-
plexes by ligand exchange with phosphines. We observed that
such phosphine-free reaction conditions, previously operative
w Electronic supplementary information (ESI) available: Procedures;
NMR data and additional absorption/emission data. See DOI:
10.1039/c2cc16327f
c
1260 Chem. Commun., 2012, 48, 1260–1262
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and 63% yield, respectively (Table 1, entries 2 and 3). On the
other hand, complex 3c from 4-bromonitrobenzene could not
be easily isolated in pure form owing to its very low solubility.
Only complex 3b could be purified by column chromatography.
To optimise the yield of this diarylated complex, the reaction
was stopped after 24 h instead of 48 h, giving 3b in 50%
yield (Table 1, entry 1). With 1- or 2-bromonaphthalene as the
coupling partners, the desired tris-arylated complexes 6c and 7c
were obtained, albeit in relatively low yields (Table 1, entries 4
and 5). The procedure thus seems to be more efficient when
electron-deficient aryl bromides are employed. This might be
the result of easier oxidative addition of such aryl bromides
to palladium. Finally, we examined the reaction with the two
heteroaryl bromides, 3-bromopyridine and 5-bromopyrimidine.
In both cases, the desired tris-heteroarylated complexes 8c and
9c were obtained in relatively good yields of 52% and 56%
respectively (Table 1, entries 6 and 7).
The photophysical properties of selected complexes 2a–c,
4c, 5c, and 8c were investigated by means of absorption and
emission spectroscopy in CH2Cl2 solution (Table 2). The
absorption spectra of the four new homoleptic complexes 2c,
4c, 5c and 8c are shown in Fig. 2, together with that of 1 for
comparison. It can be seen that the aryl-functionalised complexes
have similar absorption profiles to one another, but they differ
from 1 in that they display stronger, well defined bands in the
long-wavelength portion of the spectrum, one at around 420 nm
and a second at around 480 nm. The extinction coefficients, e, of
these bands increase with increasing electron-withdrawing ability of
the aryl para-substituent (viz. p-C6H4–CF3 o p-C6H4–CN). More-
over, by comparison of the trio of complexes 2a–c, the increase in
extinction coefficients of the new bands is seen to correlate with the
number of aryl groups introduced (overlaid absorption spectra for
these three complexes—with one, two and three cyanophenyls—
are shown in Fig. S1, ESIw). Notably, complex 2a shows two bands
in the 300–350 nm region, of which the higher-energy is evidently
associated with the non-functionalised thpy ligands and the lower
with the aryl-appended ligand.w
Scheme 2 Arylation of the thiophene unit of fac-Ir(thpy)3 by C–H
bond activation, leading to hetero- and homoleptic complexes.
on various thiophene derivatives,11g can be directly applied to
the metallated (double l) thiophene of fac-Ir(thpy)3 1. The
reaction of 4-bromobenzonitrile with 1 afforded the expected
tris-heteroarylated product 2c. Interestingly, both the reaction
time and ratio of reactants were found to have a dramatic
influence on the product distribution. The highest selectivity
and yield in favour of the formation of 2c were observed at
150 1C after 48 h. When we employed shorter reaction times
(3 or 8 h) and a lower reaction temperature (130 1C), complexes
2a and 2b could be obtained.
Then, the scope of this reaction was examined using a variety of
other aryl bromides and heteroaryl bromides (Table 1). Electron-
deficient para-substituted aryl bromides, 4-(trifluoromethyl)-
bromobenzene and 3,5-bis(trifluoromethyl)bromobenzene, gave
the desired 5-arylated thiophene complexes 4c and 5c in 48%
All of the complexes are luminescent in solution at room
temperature (Fig. 3). Compared to the starting species 1, the
complexes show a strong bathochromic shift of the well-resolved
0–0 band, up to 125 nm for 2c. The quantum yields are sub-
stantially lower than for 1 and the lifetimes somewhat shorter.
Increased non-radiative decay would be anticipated for the lower-
energy excited states, but an estimate of the radiative (kr) and non-
radiative (Sknr) decay constants from the quantum yield and
lifetime data suggests that not only is there an increase in Sknr, but
also a decrease in kr upon arylation (Table S1, ESIw). This effect
can be attributed to the more extended conjugated system in the
new complexes, such that the influence of the metal in promoting
intersystem crossing through the spin–orbit coupling effect is
reduced.13 A predominantly ligand-centred 3p–p* assignment
seems appropriate, supported by the lack of significant influence
of temperature on the emission maxima; spectra at 77 K are
shown in Fig. S2 (ESIw). The heteroleptic complexes 2a and 2b
show identical spectra to 2c, indicating that the lowest-energy state
(i.e. the emissive state) is associated with the functionalised ligand,
consistent with the difference in emission energy between 2c and 1.
In summary, we have demonstrated that 2,20-thienylpyridine
ligands coordinated to iridium(III) can be directly arylated with a
Table 1 Direct arylation of 1 with aryl bromides
Entry Aryl bromide
Product Yield (%)
1
2
3
4
5
6
7
4-Bromonitrobenzene
4-(Trifluoromethyl)bromobenzene
3,5-Bis(trifluoromethyl)bromobenzene 5c
2-Bromonaphthalene
1-Bromonaphthalene
3-Bromopyridine
3b
4c
50a
48
63b
20
6c
7c
8c
9c
26
52b
56
5-Bromopyrimidine
Conditions: catalyst: Pd(OAc)2 (0.05 mmol), aryl bromide (6 mmol),
1 (1 mmol), KOAc (6 mmol), DMAc, 130 1C, 48 h, under argon,
a
b
isolated yields. 24 h. 150 1C.
c
This journal is The Royal Society of Chemistry 2012
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Table 2 Photophysical data of selected Ir complexes in CH2Cl2 at 298 K
Complex
Absorption lmax/nm (e/Mꢀ1 cmꢀ1
)
Emission lmax/nm
t/ms
Flum ꢁ 102
40
0.84
0.81
1.1
2.0
1.7
1.9
1
300 (35 500), 343 (15 900), 398 sh (11 900)
303 (35 600), 346 (40 500), 416 (19 500), 485 sh (5290)
344 (57 400), 425 (27 000), 484 sh (9150)
340 (75 700), 426 (36 800), 486 sh (13 200)
322 (69 900), 411 (30 500), 476 sh (11 700)
322 (53 400), 411 (23 300), 477 sh (8700)
326 (42 900), 408 (19 100), 472 sh (8020)
548, 592, 643 sh
677, 742, 812 sh
676, 740, 813 sh
673, 738
652, 712, 795 sh
654, 714, 798 sh
645, 703, 784 sh
7.1
1.1
1.3
1.5
2.4
2.1
2.8
2a
2b
2c
4c
5c
8c
Notes and references
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Fig. 3 Emission spectra of complexes 1, 2c, 4c, 5c and 8c in CH2Cl2
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variety of aryl bromides using a Pd(OAc)2 catalyst. The thiophene
ring was regioselectively arylated at the C5 position. This method
provides facile access to arylated Ir complexes under mild reaction
conditions. Moreover, it should be noted that a range of func-
tionalities such as nitrile, nitro or trifluoromethyl on the aryl
bromide are tolerated. Such functional group tolerance allows the
easy modification of the electronic structures and, as a conse-
quence, the photophysical properties of the molecules. We anti-
cipate that these Pd-catalysed syntheses will provide a powerful
new tool for the preparation of novel light-emitting materials.
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K. B. is grateful to the CNRS and ‘‘Region Bretagne’’ for a
´
PhD grant. This work was supported by LEA CNRS MMC
Rennes-Durham and ANR COMET.
c
1262 Chem. Commun., 2012, 48, 1260–1262
This journal is The Royal Society of Chemistry 2012