A heteroleptic cyclometalated iridium(iii) fluorophenylpyridine complex from partial defluorohydrogenation reaction: Synthesis, photophysical properties and mechanistic insights

Article abstract of DOI:10.1039/c2dt32800c

In this paper, a C-F bond activation reaction of a chloro-bridged iridium(iii) dimer (dfppy)₂Ir(μ-Cl)₂Ir(dfppy) ₂ (1) (dfppy denotes 2-(4,6-difluorophenyl)pyridyl) in the presence of sodium methoxide has been reported, lea

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A heteroleptic cyclometalated iridium(III)
fluorophenylpyridine complex from partial
defluorohydrogenation reaction: synthesis,
photophysical properties and mechanistic insights†
Cite this: Dalton Trans., 2013, 42, 4539
Liang Li, Feng Wu, Songlin Zhang,* Dawei Wang, Yuqiang Ding* and
Zhenzhong Zhu
In this paper, a C–F bond activation reaction of a chloro-bridged iridium(III) dimer (dfppy)2Ir(μ-Cl)2Ir-
(dfppy)2 (1) (dfppy denotes 2-(4,6-difluorophenyl)pyridyl) in the presence of sodium methoxide has
been reported, leading to the formation of a heteroleptic cyclometalated iridium(III) fluorophenylpyridine
complex 2. HPLC-mass analysis confirmed the release of formaldehyde in the product mixture. When
sodium benzyloxide was used as the base, complex 2 was also generated with the release of a benz-
aldehyde derivative. Complex 2 has been fully characterized by 1H-NMR, 19F-NMR and X-ray crystallo-
graphic methods, confirming the partial loss of one of the fluorine atoms on one of the cyclometalated
phenylpyridyl ligands. Photophysical studies of complex 2 show that it has a similar absorption spectrum
to that of Ir(III)(dfppy)3. However, the emission spectrum shows a red shift maximum emission band at
478 nm due to the loss of a single fluorine atom, highlighting the critical effect of fluorine on the photo-
luminescence of these Ir(III) complexes. Finally, intensive mechanistic studies including HPLC-mass analysis,
1H-NMR, and 19F-NMR studies demonstrate that the formation of complex 2 should involve a critical
β-hydride elimination of Ir(III)-alkoxide intermediate and the participation of Ir-hydride and/or Ir-fluoride
intermediates.
Received 28th September 2012,
Accepted 13th December 2012
DOI: 10.1039/c2dt32800c
www.rsc.org/dalton
introduction of fluorine atoms on the ppy backbone leads to
significant blue shift of the emission wavelengths compared to
Introduction
Organofluorine chemistry is an active field of research due to the Ir(ppy)3 complex.3
the interesting properties that fluorine furnishes to the carbon
However, owing to the inertness of C–F bonds towards
skeleton.1 As a consequence, these organofluorine compounds chemical reactions, activation of C–F bonds is challenging and
show a broad range of applications in pharmaceutical and has been the subject of many investigations7 including the
material science.2 For instance, 2-(2,4-difluorophenyl)pyridine transformation from C–F to C–H, C–C and other bonds. Tran-
(dfppy) has been widely used as a ligand in the synthesis of sition metal complexes have been found to possess a unique
cyclometalated iridium(III) complexes, which have been inten- ability to react with C–F bonds and a few examples, including
sively studied as emitting materials for organic light-emitting both stoichiometric and catalytic activation of aromatic C–F
devices (OLEDs).3–5 Ha et al.6 reported that Ir(ppy)2(dfppy) and bonds, are reported to occur with palladium,7e nickel,8
Ir(ppy)(dfppy)2 (ppy denotes 2-phenylpyridyl) exhibit emission rhodium,9 and iridium.10 For instance, Braun et al. reported
bands at 502 and 495 nm, respectively, and Thompson3 con- the selective C–F bond activation in the presence of C–Cl
firmed that Ir(dfppy)3 gave a maximum emission at 468 nm bonds by the treatment of 5-chloro-2,4,6-trifluoropyrimidine
together with a luminescent quantum efficiency of 0.43. The with [Ni(COD)2]/PCy3. The C–F activation product trans-[NiF(4-
C4N2ClF2)(PCy3)2] was obtained which can further be proto-
nated to obtain the C–H coupling product by hydrochloric
acid.8a The same group also reported that RhH(PEt3)3 or RhH-
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education,
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122,
(PEt3)4 could react with pentafluoropyridine to afford the C–F
activation intermediate Rh(4-C5NF4)(PEt3)3, which underwent
Jiangsu Province, China. E-mail: slzhang@jiangnan.edu.cn, yding@jiangnan.edu.cn;
Fax: +86-510-85917763; Tel: +86-510-85917763
further reaction with CO and CH3I to form the corresponding
†Electronic supplementary information (ESI) available: Details of preparation,
C–F activation/C–C coupling products.9 Milstein et al. reported
spectroscopic and photophysical data. CCDC 903296. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c2dt32800c
an interesting iridium(I)-mediated C–F activation reaction
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Scheme 1 The preparation of partially defluorinated complex 2.
involving the simultaneous cleavage of the P–C bond of the
phosphine ligand and P–F bond formation by reacting the
methyl iridium(I) complex Ir(CH3)(PEt3)3 with perfluorinated
benzene, which produced
a pentafluorophenyl iridium(I)
complex (C6F5)Ir(PEt3)2(PEt2F).10
Herein, we report an interesting and unexpected example of
iridium(III) complex-promoted C–F bond activation coupled
with C–H bond formation. Reaction of μ-chloro-bridged dimer
(dfppy)2Ir(μ-Cl)2Ir(dfppy)2 (1) with NaOCH3 produced an
unprecedented partially defluorinated product (dfppy)2Ir(fppy)
(2) [fppy denotes 2-(4-fluorophenyl)pyridyl)] (Scheme 1). Syn-
thesis, characterization and photophysical properties for
complex 2 are presented. The marked effect of the fluorine
atom on the ppy backbone on the photophysical properties of
the resulting iridium(III) complex has been recognized.
Additionally, a possible mechanism for the formation of
complex 2 is also suggested based on evidence derived from
HPLC-mass, 1H-NMR and 19F-NMR studies.
Fig. 1 ORTEP diagram of complex 2 with thermal ellipsoids shown at the 50%
probability level. Hydrogen atoms have been omitted for clarity. Selected bond
lengths (Å) and bond angles (°): Ir(1)–C(11) 2.007(12), Ir(1)–N(1) 2.032(10),
Ir(1)–C(22) 2.098(10), Ir(1)–N(2) 2.098(10), N(1)–C(1) 1.329(16), N(1)–C(5)
1.377(15), N(2)–C(16) 1.371(18), N(2)–C(12) 1.381(15), C(11)–Ir(1)–N(1) 80.4(5),
N(2)–Ir(1)–C(22) 79.0(6), F(1)–C(7)–C(8) 118.5(15), F(1)–C(7)–C(6) 118.7(17),
F(2)–C(9)–C(8) 119.2(15), F(2)–C(9)–C(10) 117.0(19), C(14)–C(15)–F(3) 125.5(15),
C(16)–C(15)–F(3) 115.2(16).
Finally, to unambiguously elucidate the structure, single
crystal X-ray diffraction analysis was done for complex 2, which
presented the most convincing and intuitional evidence, as
shown by Fig. 1. Complex 2 adopts an octahedral geometry
with the two dfppy ligands locating in a cis-C,C′ and trans-N,N′
manner. The other two coordination positions are occupied by
one partially defluorinated ligand 2-(4-fluorophenyl)pyridyl
ligand, forming a five-membered metallocycle (for more infor-
mation, see ESI† and the deposited crystal structure CCDC
903296). These intensive characterization studies confirm the
structure of complex 2, especially the partial loss of fluorine
atoms on the dfppy ligand.
Results and discussion
Synthesis and characterization of complex 2
Into a Schlenk tube containing 2-ethoxyethanol as solvent,
dichloro-bridged dimer 1 [(dfppy)2IrCl]2, generated from the
reaction of dfppy ligand with IrCl3·3H2O according to a
general procedure (see ESI†), and sodium methoxide were
added. The mixture was stirred at reflux for 48 h and gave a
yellow suspension. A purified yellow crystal of complex 2 was
Photophysical properties
obtained in 46.3% yield after work-up, thin layer chromato- UV-vis absorption and normalized photoluminescent emission
graphy (TLC), and recrystallization using n-hexane/CH2Cl2 spectra of complex 2 were determined at a concentration of
mixed solvent.
10−5 mol L−1 in CH2Cl2 solution at 298 K. The absorption spec-
1H-NMR signals (Fig. S1 in ESI†) provide us with further trum of complex 2 (Fig. 2) shows two intense absorption bands
structural information of 2: peaks with chemical shifts ranging appearing in the ultraviolet range of the spectrum at 239 and
from 5.74 to 6.60 and 7.50 belong to H atoms of the benzene 264 nm, respectively, which are assigned to the π → π* tran-
ring, and peaks with chemical shifts of 6.74, 6.88 and 7.56 to sitions for the complex. Additional very weak absorption
8.15 belong to H atoms of the pyridine ring. The integral bands at 357, 386 and 459 nm are assigned to the metal-to-
values of these peaks are 7 and 12, respectively, and the total ligand charge transfer transitions.3 Generally, this absorption
value is 19 which is consistent with the structure of complex 2. spectrum is consistent with relevant absorption bands for
Furthermore, the signals of 19F-NMR (Fig. S2 in ESI†) indi- related Ir(III) ppy or dfppy complexes.
cate that chemical shifts at −109.56 and −110.35 are attributed
However, the photoluminescent emission spectrum for
to the two F atoms of one 2-(2,4-difluorophenyl)pyridyl ligand. complex 2 (Fig. 3) shows a significant maximum emission at
Those of −109.70 and −111.58 are attributed to the two 478 nm with an excitation wavelength of 264 nm. Compared to
F atoms of the other 2-(2,4-difluorophenyl)pyridyl ligand. The iridium(III) complexes Ir(ppy)3, Ir(ppy)2(dfppy), Ir(ppy)(dfppy)2
single peak at −112.31 is attributed to the 4-F atom of the 2-(4- and Ir(dfppy)3 which show maximum emission wavelengths at
fluorophenyl)pyridyl ligand.
510,3 502,6 4956 and 468 nm3, respectively, complex 2 displays
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Scheme 2 The preparation of complex
2
with the release of m-nitro
benzaldehyde.
the product mixture was treated with Sawicki’s reagent
(3-methyl-2-benzothiazolinone hydrazone hydrochloride mono-
hydrate). Subsequent HPLC-mass analysis of the resulting solu-
tion shows that apparent ion peaks around 192 (Fig. S12 in
ESI†) are present which are assigned to formaldehyde-2-
(3-methyl-2-benzothiazolylidene)hydrazone, the condensation
product of formaldehyde and Sawicki’s reagent (Fig. S12 and
S13†). Moreover, under identical conditions, when sodium
methoxide was replaced by sodium m-nitrobenzyloxide
(Scheme 2), the expected m-nitrobenzaldehyde was observed as
Fig. 2 UV-vis absorption spectrum of complex
10−5 mol L−1 in CH2Cl2 solution at 298 K.
2 at a concentration of
a
1H-NMR signal at 9.895 ppm (Fig. S5 in ESI†). From these
results, it is proposed that sodium alkoxide or benzyloxide
reacted with the dimer to give the iridium alkoxide or benzyl-
oxide complex initially, and produced the aldehyde through a
subsequent β-H elimination process. Relevant studies invol-
ving this β-hydride elimination of Ir-alkoxide have already
been reported previously by Bergman et al., and Milstein
et al.13,14
Fig. 3 Normalized photoluminescent emission spectrum of complex 2 at a
concentration of 10−5 mol L−1 in CH2Cl2 solution at 298 K.
Further mechanistic evidence from spectroscopic studies is
1
presented below. Firstly, the H-NMR spectrum for one of the
bands of TLC analysis of the product mixture shows typical
signals at −15.517(20) and −15.871 ppm (Fig. S6 in ESI†),
which are assigned to the chemical shifts of the hydride
moiety of Ir–H bonds. Similar values were previously reported
for relevant Ir(III) 2-phenylpyridyl hydride complexes by the
Chong, Sung and Young group.15 Secondly, for another band
of TLC analysis, the 19F-NMR spectrum (Fig. S7 in ESI†) shows
characteristic signals of the fluorine atom in the Ir–F bond at
−61.982 and −61.986 ppm, respectively, as well as the conven-
tional C–F bonds on the aromatic rings. Finally, HPLC-mass
analysis of the product mixture (Fig. S8, S9, S10 and S11 in the
ESI†) shows four different ion peaks at 192, 174, 573 and 555,
which are assigned to 2-(2,4-difluorophenyl)pyridine, 2-(4-
difluorophenyl)pyridine, [(dfppy)2Ir]+ and [(dfppy)Ir(fppy)]+,
respectively. These studies indicate that iridium hydride and
iridium fluoride intermediates should be involved during the
formation of complex 2.
Based on these studies, a probable mechanistic proposal
for the formation of complex 2 is presented in Fig. 4. The
chloride-bridged dimer 1 reacts with sodium methoxide to
form Ir-alkoxide intermediate 3,11,16 which undergoes
β-hydride elimination to form Ir hydride intermediate 4 with
the release of formaldehyde. Intermediate 4 could undergo
reductive elimination to afford intermediate 5 and compound
an intermediate emission wavelength between that of Ir(ppy)-
(dfppy)2 and Ir(dfppy)3. In addition, a quantum yield of
0.14 has been determined for complex 2 at room temperature
in degassed dichloromethane solutions using tris(8-hydroxy-
quinoline) aluminum (Alq3) in DMF (ΦPL = 0.116) as a refer-
ence, which is indicative of a less efficient quantum effiecency
compared to a value of 0.43 for Ir(dfppy)3 reported by Thomp-
son et al. These results highlight the critical electronic effects
of the fluorine atoms of the phenylpyridine ligand on the
photoluminescence of these Ir(III) complexes3,11,12 and are
intrinsically consistent with the accepted consensus that the
introduction of fluorine atoms generally stabilizes the HOMO
orbital more than the LUMO ortbital of the iridium complexes,
leading to an increased HOMO/LUMO gap and shorter emis-
sion wavelengths.11c,e The lifetime of 9.28 μs for the excited
triplet emitting state for complex 2 has also been determined
using an excitation wavelength of 264 nm, which is much
longer than that for the related Ir(dfppy)3 complex (1.6 μs).
This possibly explains the smaller value of the quantum yield
for complex 2 due to a stronger triplet–triplet quenching
effect.
Mechanistic studies for the formation of complex 2
To provide some insights into the formation of complex 2, 6 (compound 6 has been detected by our HPLC-MS ion peaks
at around 192, vide supra). Similar reactivity has also been
determine the release of formaldehyde during this process, reported before.17 Oxidative addition of intermediate 5 with
some preliminary mechanistic studies were performed. To
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were measured using a UV-Vis spectrophotometer (Model
TU-1901). Photoluminescence spectra were obtained using an
RF-5301PC fluorescence spectrophotometer (Shimadzu, Japan)
connected to a photomultiplier tube with a xenon lamp as the
excitation source. The quantum efficiency measurement was
carried out at room temperature in degassed dichloromethane
solutions, using tris(8-hydroxyquinoline) aluminum (Alq3) in
DMF (ΦPL = 0.116) as a reference. Phosphorescence lifetime
measurement was performed on an Edinburgh instrument
LFS920 spectrometer equipped with a microsecond xenon
flash lamp as the excitation source in degassed dichloro-
methane solutions at room temperature. The data were ana-
lyzed by iterative convolution of the luminescence decay
profile with the instrument response function using the soft-
ware package provided by Edinburgh Instruments. NMR
spectra were collected on a Bruker AVANCE III 400 spectro-
meter with deuterated dichloromethane and deuterated
chloroform as solvents and tetramethylsilane as the internal
standard. X-Ray data for complex 2 were collected using a HER-
CURY-AFC8 CCD area detector using monochromated Mo-Kα
radiation (λ = 0.71073 Å). The crystal 2 was measured at 223(2)
K. Elemental analyses (C, H, N) were carried out on a Vario EL
III elemental analyzer (Elementar, Germany). Mass spectra
were recorded on a Waters MALDI SYNAPT QT mass spectro-
meter. HPLC analysis was performed on a high pressure
liquid chromatograph (Waters 515) using water and aceto-
nitrile as the mobile phase.
Fig. 4 The proposed mechanism for the formation of complex 2.
the C–F bond of compound 6 would generate complex 7. Trans-
metalation of complex 7 with complex 4 would deliver directly the
target complex 2, with the concurrent formation of complex 8.18
Conclusions
We report herein the synthesis and characterization of a
heteroleptic Ir(III) fluorophenylpyridyl complex 2. The results
demonstrate the involvement of a partial hydrodefluorination
of the cyclometalated fluorophenylpyridine ligand. Studies on
photophysical properties for complex 2 show that it has a
similar MLCT excitation process to analogous Ir(III)(dfppy)3 or
Ir(III)(ppy)3 complexes, but exhibits a notable blue shift relative
to Ir(III)(ppy)3 and red shift relative to Ir(III)(dfppy)3. In
addition, a mechanistic proposal has also been presented for
the formation of complex 2 based on a series of mechanistic
studies characterizing the relevant intermediates.
Reaction of (dfppy)2Ir(μ-Cl)2Ir(dfppy)2 with sodium methoxide
Two different bases were used to react with 1 and generated
the same product (dfppy)2Ir(fppy) (2) as shown in Schemes 1
and 2. Thus, herein, a typical procedure for the production of
2 is described as follows: [(dfppy)2IrCl]2 (0.183 g, 0.15 mmol)
and NaOCH3 (0.083 g, 1.54 mmol) were added into a 100 mL
three-necked flask. After being vacuumed by a vacuum pump
and refilled with dry nitrogen three times, ethoxy-ethanol
(10 mL) was then added to the mixture. The resulting mixture
was stirred at 373.15 K (100 °C) for 48 h, then dried in vacuo
and the residue was extracted with water and dichloro-
methane. The organic layer was separated by TLC to afford 2
as a yellow crude powder, which was further recrystallized in
hexane/dichloromethane and a desired yellow cuboid crystal
product 2 was obtained. IR (KBr): 3087 m, 1604 1479 s,
1430 m, 1405 s, 1295 1249 m, 848 831 m, 756 m. 1H-NMR
(400 MHz, CD2Cl2, 25 °C): δ 5.74 (dd, J = 2.4, 2.4 Hz, 1H), 5.94
(dd, J = 2.4, 2.4 Hz, 1H), 6.38 (m, 1H), 6.41 (m, 1H), 6.60 (td,
J = 8.8, 2.7 Hz, 2H), 6.74 (m, 2H), 6.88 (m, 1H), 7.50 (m, 1H),
7.56 (d, J = 0.9 Hz, 2H), 7.62 (d, J = 1.6 Hz, 1H), 7.70 (d, J =
3.4 Hz, 1H), 7.77 (d, J = 5.6 Hz, 1H), 7.82 (d, J = 8.2 Hz, 1H),
7.97 (d, J = 5.9 Hz, 1H), 8.15 (m, 2H). 19F-NMR (377 MHz,
CD2Cl2, 25 °C): δ −109.56 (d, J = 9.6 Hz), −109.70 (d, J =
10.0 Hz), −110.35 (d, J = 9.6 Hz), −111.58 (d, J = 9.9 Hz),
Experimental
General considerations
IrCl3·3H2O, m-nitrobenzaldehyde, sodium methoxide, sodium
borohydride and other chemicals were obtained from commer-
cial sources and used without further purification. All solvents
were dried and purified by known procedures and freshly dis-
tilled under nitrogen from appropriate drying agents prior to
use. All manipulations and reactions involving air and/or
moisture-sensitive organometallic compounds were performed
under an atmosphere of dry nitrogen using standard Schlenk
techniques.
Characterization methods
IR spectra were recorded on a FTLA2000 spectrometer by dis- −112.31 (s). Anal found C 53.13 H 2.59 N 5.55 Cald C 53.22
persing samples in potassium bromide. Absorption spectra H 2.57 N 5.64. MS (m/z): 746 [M + H+]+.
4542 | Dalton Trans., 2013, 42, 4539–4543
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Acknowledgements
We thank the National Natural Science Foundation of China
(NSFC Nos. 21202062 and 20971058), the Natural Science
Foundation of Jiangsu Province of China (BK2012108), the
General Financial Grant from the China Postdoctoral Science
Foundation (2012M520993) and the Fundamental Research
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