Electronically Divergent Triscyclometalated Iridium(III) 2-(1-naphthyl)pyridine Complexes and Their Application in Three-Component Methoxytrifluoromethylation of Styrene

Article abstract of DOI:10.1055/s-0037-1611716

A systematic study of the photophysical and electrochemical properties of triscyclometalated homoleptic iridium(III) complexes based on 2-(1-naphthyl)pyridine (npy) ligands is presented. A systematic investigation of ligand substitution patterns showed an influence on the lifetime of the excited state, with slight changes in the absorption and emission spectral features. Specifically, the emission lifetime of a complex of an npy ligand substituted with a strongly electron-withdrawing trifluoromethyl group was longer than that of the corresponding complex with the electronically nonperturbed ligand (3.7 μs versus 1.5 μ s). Electronically complementary ligands and complexes with orthogonal configurations showed slightly shorter excited state lifetimes compared with unsubstituted npy (1.4-3.0 μs). All complexes displayed reversible or quasireversible redox-couple processes, with the complex of the trifluoromethylated ligand showing the highest ground-state oxidation potential E _1/2^ox [Ir(III)/Ir(IV) = 0.95 V vs. SCE in CH ₂ Cl ₂ ]. This study showed that these complexes can be used as efficient photoredox catalysts, as demonstrated by their application in a regioselective methoxytrifluoromethylation in which the npy complexes showed equal or better performance compared with the archetypical photo redox catalyst tris[2-phenylpyridinato]iridium(III).

Full text of DOI:10.1055/s-0037-1611716

SYNLETT
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Georg Thieme Verlag Stuttgart · New York
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Synlett
R. E. N. Njogu et al.
Letter
Electronically Divergent Triscyclometalated Iridium(III)
-(1-naphthyl)pyridine Complexes and Their Application in
Three-Component Methoxytrifluoromethylation of Styrene
2
Rachael E.N. Njogu ◊a,d
R
C1 R = H, R1 = H
Peter Fodran◊b
Yuxi Tianc,1
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0
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0-0
0
0
3-
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3
4
8-8
3
3
1
1
C2 R = OCH3, R = H
R1
C3 R = CF , R1 = H
3
1
N
Ir
C4 R = CH3, R = H
C5 R = CH3, R = CH3
C6 R = H, R = CH3
Lydia W. Njengad
David K. Kariukid
Amir O. Yusufd
Ivan Scheblykinc
Ola F. Wendt*a
1
R1
R
1
N
N
R
Synthesis
Photophysical properties
Electrochemistry
Photoredox catalysis
Carl-Johan Wallentin*b
R1
0
0
0
0-0
0
0
3-
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3-9
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a
Centre for Analysis and Synthesis, Department of Chemistry, P.O. Box 124, SE-22100
Lund University, Sweden
ola.wendt@chem.lu.se
b
Department of Chemistry and Molecular Biology, P.O. Box 462, SE-40530 University of
Gothenburg, Sweden
carl.wallentin@chem.gu.se
c
Chemical Physics, Department of Chemistry, Lund University, P.O Box 124, SE-22100, Sweden
Department of Chemistry, University of Nairobi, P.O Box 30197, KE-00100 Nairobi, Kenya
These authors contributed equally.
d
◊
Published as part of the Special Section 10th EuCheMS Organic Division Young Investigator Workshop
Received: 09.11.2018
Accepted after revision: 08.01.2019
Published online: 08.02.2019
DOI: 10.1055/s-0037-1611716; Art ID: st-2018-v0731-l
Abstract A systematic study of the photophysical and electrochemical
properties of triscyclometalated homoleptic iridium(III) complexes
based on 2-(1-naphthyl)pyridine (npy) ligands is presented. A systemat-
ic investigation of ligand substitution patterns showed an influence on
the lifetime of the excited state, with slight changes in the absorption
and emission spectral features. Specifically, the emission lifetime of a
complex of an npy ligand substituted with a strongly electron-with-
drawing trifluoromethyl group was longer than that of the correspond-
ing complex with the electronically nonperturbed ligand (3.7 μs versus
1.5 μs). Electronically complementary ligands and complexes with
orthogonal configurations showed slightly shorter excited state life-
times compared with unsubstituted npy (1.4–3.0 μs). All complexes dis-
played reversible or quasireversible redox-couple processes, with the
complex of the trifluoromethylated ligand showing the highest ground-
ox
state oxidation potential E1/2 [Ir(III)/Ir(IV) = 0.95 V vs. SCE in CH Cl ].
2
2
Carl-Johan Wallentin initiated his academic career 2004 as a PhD stu-
dent at Lund University (Sweden) under the directions of Prof. Kenneth
Wärnmark. The main efforts during his time as a PhD student focused
on various aspects of supramolecular chemistry, with particular empha-
sis on hydrogen bonding and its utilization in the construction of
discrete molecular aggregates. In 2011 he joined the research group of
Prof. Corey Stephenson, at that time at Boston University (USA), to
explore the upcoming field of photoredox catalysis. Thereafter, he
joined the Department of Chemistry and Molecular Biology at
Gothenburg University to establish his own group active within the
fields of method development and sustainable chemistry. Most recently
his efforts have been focusing on the engagement of carboxylic acids in
alkene difunctionalization, initiated by visible-light mediated genera-
tion of transient acyl radical intermediates.
This study showed that these complexes can be used as efficient pho-
toredox catalysts, as demonstrated by their application in a regioselec-
tive methoxytrifluoromethylation in which the npy complexes showed
equal or better performance compared with the archetypical photo-
redox catalyst tris[2-phenylpyridinato]iridium(III).
Key words photoredox catalysis, iridium catalysis, oxytrifluorometh-
ylation, styrene, naphthylpyridines, ligands
In recent years, the luminescent properties of triscyclo-
metalated Ir(III) complexes have been utilized in the devel-
2
,3
opment of light-emitting diodes,
dye-sensitized solar
4
5–7
cells, biological labels, and therapeutics , as well as in
sensor technology and the emerging field of photoredox
8,9
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R. E. N. Njogu et al.
Letter
catalysis.2,10–13 The ground- and excited-state physicochem-
ical properties (i.e. redox-, absorption- and emission char-
R
C1 R = H, R1 = H
1
C2 R = OCH3, R = H
R1
1
acteristics) of these complexes can be fine-tuned through
C3 R = CF3, R = H
1
careful ligand design.14–16 Modifications of the ligands can
N
Ir
C4 R = CH3, R = H
1
C5 R = CH3, R = CH3
R1
R
affect the energy of the low-lying emissive triplet excited
C6 R = H, R1 = CH
3
3
3
17
states such as the MLCT and π–π (3LC) states by influ-
encing the HOMO–LUMO energy gap. The effects of such
modifications are usually difficult to predict, but electron-
withdrawing groups have been reported to decrease the en-
ergy of the HOMO by increasing π-backdonation.18 Con-
versely, electron-donating groups have the opposite effect.
Inductive and mesomeric effects can also give rise to differ-
ent behaviors and, thus, fluoro and trifluoromethyl substit-
uents have been reported to influence the HOMO–LUMO
N
N
R
R1
Figure 1 Chemical structures of the Ir(III) complexes C1–6
ing trifluoromethyl group (C3), together with the applica-
tion of these complexes in the methoxytrifluoromethyla-
tion of styrene.
9
energy gaps in different ways. Ir(III) metal centers have a
strong spin–orbit coupling, which results in a rapid inter-
system crossing, producing a long-lived lower-energy
The ligands for complexes C1–6 were synthesized by
Suzuki–Miyaura cross-coupling of the appropriate 2-bromo-
pyridine and a small excess (1.1 equiv) of the corresponding
commercially available naphthylboronic acid.23 The ligands
were isolated in excellent yields (>90%). The desired homo-
leptic triscyclometalated complexes C1–6 were then
synthesized in one step by a slightly modified version of the
reported procedure,9 which provided the complexes in
moderate yields (39–50%).24 In general, complexes C1–6 are
air-stable up to 300 ℃ and show good solubility in both
polar and nonpolar solvents. The complexes were charac-
17
triplet state that shows enhanced redox abilities com-
pared with the ground state. Notably, Ir(III) complexes have
increased ligand-field-stabilization energies compared with
their Ru(II) counterparts, providing them with broader and
15
more-precise redox-tuning opportunities.
Lately, there has been a growing demand for Ir(III) com-
plex sensitizers that can efficiently utilize solar or artificial
low-energy light sources. Intensive efforts have been devot-
ed to identifying and developing complexes that have
strong absorption bands in the visible region of the electro-
19
1
13
magnetic spectrum. To this end, homoleptic tris-C,N iridi-
um complexes have been widely investigated, and these
efforts have primarily focused on the prototypical 2-phenyl-
terized by means of H and C NMR spectroscopy, HRMS,
elemental analysis and, in the case of complex C1, x-ray dif-
fraction (Figure 2).25 The H NMR spectra of C1-6 exhibited
well-resolved peaks consistent with the facial configura-
tion. The x-ray analysis of complex C1 confirmed its struc-
tural assignment, including the facial configuration. Thus,
1
16
pyridine (ppy) ligand. Although more-conjugated 2-(1-
naphthyl)pyridine (npy) ligands show promising proper-
20
ties, they remain underexplored.
An especially prominent utilization of visible light is in
photoredox catalysis, an emerging tool for organic transfor-
complex C1 is C -symmetric with a pseudooctahedral
geometry.
3
21,22
mations.
Typically, these transformations are triggered
by excitation of a photoredox-active catalyst that subse-
quently mediates single-electron-transfer (SET)-based pro-
cesses. Photoredox catalysts have various inherent limita-
tions stemming from their short lifetimes and the restricted
redox windows defined by their corresponding excited
states. Consequently, to realize the potential of photoredox
catalysis fully, new catalysts addressing these shortcomings
need to be developed.
Here, we report the synthesis and the complete charac-
terization of a new family of triscyclometalated Ir(III) com-
plexes, Ir(npy) , in which 2(1-naphthyl)pyridine ligand is
3
substituted by electron-donating methyl or methoxy
groups (Figure 1; C2, C4–6) or with an electron-withdraw-
Figure 2 Molecular structure of complex C1 at the 50 % probability
level.25 Hydrogen atoms are omitted for clarity. Selected bond lengths
(Å) and angles: Ir–N1 = 2.088(12), Ir–N2 = 2.135(11), Ir–N3 = 2.128(11),
Ir–C7 = 2.035(12), Ir–C22 = 2.002(17), Ir–C37 = 2.038(13),
N1–Ir–N2 = 96.7(5)°, N1–Ir–N3 = 95.8(5)°, N2–Ir–N3 = 99.0(4)°.
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R. E. N. Njogu et al.
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The UV–vis absorption [Figure 3(A)] and photolumines-
cence (PL) spectra [Figure 3(B)] for complexes C1–6 were
measured as solutions in CH Cl at room temperature (Table
The complexes exhibited strong orange-red photolumi-
nescence (588–610 nm) upon excitation at 400 nm [Figure
3(B)]. The broad and structureless features of the emission
bands indicate MLCT transitions. The complexes were red-
shifted in comparison with the corresponding free ligands.
Complexes C2, C3, C5, and C6 were red-shifted, and com-
plex C4 was blue-shifted compared with the parent com-
plex C1. The bathochromic shifts of the complexes relative
to C1 were 19 nm for C2, 8 nm for C3, 8 nm for C5 and 7 nm
for C6. The hypsochromic shift of C4 relatively to the parent
C1 was 3 nm. All complexes were substantially red-shifted
2
2
1). All the complexes exhibited similar spectral features;
this is fully consistent with the structural similarities of the
ligands. All complexes exhibited intense UV absorption
bands below 350 nm that were assigned to the spin-
1
allowed ligand-centered π–π* transition. This agrees with
the absorption features observed for the free ligands, which
did not show any distinctive absorptions at longer wave-
lengths. Absorption bands observed in the 350–400 nm
range were ascribed to a typical spin-allowed metal-to-
compared with Ir(ppy) , as expected for a more extended π-
3
1
15
ligand charge-transfer transition ( MLCT), in which an elec-
system, and the substituent effects in C1–6 are much
tron is promoted from a metal d orbital to the vacant π*
orbital on one of the ligands. The absorption observed
above 400 nm with minimal molar extinction coefficients
smaller and are difficult to rationalize.
The lifetimes of the luminescent triplet states of the
complexes in degassed solutions of CH Cl , prepared by
2
2
–1
–1
(
10–100 M ·cm ) could be attributed to both spin–orbit-
multiple freeze–pump–thaw cycles, are summarized in
Table 1. The substituents influenced the excited state life-
times despite the small changes in the absorption and
emission spectra. Specifically, the emission lifetime of C3,
substituted with a strongly electron-withdrawing CF3
group was the longest (τ = 3.7 μs), followed by the electron-
rich complex C1 (τ = 3.0 μs), whereas the remaining com-
plexes substituted with methyl groups had slightly shorter
lifetimes (τ = 1.4–2.6 μs). Typically, the electron-donating or
-withdrawing properties of the substituent affects the C,N-
ligand by either decreasing or increasing the electron densi-
3
3
26
coupling-enhanced π–π* and MLCT transitions. Complex
C3 exhibits a red shift in its absorption band despite the ex-
pected stabilization of the HOMO. This is opposite to previ-
27
ous observations. The MLCT transitions can be viewed as
photoinduced charge separations in which the metal center
is oxidized and the ligand is reduced. Typically, these excit-
ed species show enhanced redox potentials compared with
their ground states, a property that can be utilized in pho-
toredox-catalyzed transformations.26,28
29
ty in the ring system where the substituent is attached.
The long lifetime of C3 is in agreement with a HOMO stabi-
lization, but it must be noted that this is not in accord with
the observed absorption and emission spectra of C3; fur-
thermore, C2 which contains an electron-donating group,
also has a relatively long lifetime.30 In the context of pho-
toredox catalysis, the excited state of the complex should
persist for long enough to permit the reaction to take place.
Essentially, a longer lifetime increases the probability that
the catalyst will productively interact with substrates while
in its excited state.
Table 1 Photophysical and Electrochemical Data for Complexes C1–6
Complex
Emission
Electrochemistry (V)
31
λ
max
τ (μs)
E
1/2
E
III*/IV
485nm
Ir(ppy) a
519
591
610
603
588
603
602
1.5
1.5
3.0
3.7
2.6
1.4
2.6
0.77
0.73
0.66
0.95
0.64
0.62
0.64
–1.73
–1.36
–1.37
–1.11
–1.47
–1.44
–1.42
3
C1
C2
C3
C4
C5
C6
a
Approximated excited-state redox potential.32
Figure 3 (A) UV–VIS Absorption and (B) photoluminescence spectra of
C1–6 in CH Cl at room temperature
2
2
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R. E. N. Njogu et al.
Letter
Principally, in photoredox catalysis, the ground- and the
excited-state redox potentials of a photocatalyst are used in
philicity, binding selectivity, and affinities of biologically
active compounds.3
4–36
Thus, the development of mild and
22,28
SET-mediated transformations.
Consequently, the elec-
practical procedures for trifluoromethylation has become a
vital research endeavor. Currently, trifluoromethyl group
can be introduced by using either nucleophilic or electro-
philic reagents. A prominent example belonging to the lat-
ter group is Umemoto’s reagent, 5-(trifluoromethyl)diben-
zo[b,d]thiophenium tetrafluoroborate.
trochemical properties of the iridium complexes were
probed by cyclic voltammetry to establish the ground-state
redox potentials in degassed solutions of 0.1 M tetra-
butylammonium hexafluorophosphate (TBAH) as the
supporting electrolyte in CH Cl at a scan rate of 0.1 V/s.
2
2
All complexes exhibited reversible or, possibly, quasirevers-
ible oxidation waves [Figure 4(A)] during anodic scanning,
δEp = 100–140 mV, which can be attributed to the metal-
Yasu et al.34 reported that Umemoto’s reagent can be
used for dual functionalization of styrenes. In their report,
the authors suggested that the reaction proceeds through
18
centered Ir(III)/Ir(IV) redox couple (Table 1). Reversibility
of redox processes for complexes used as a photocatalyst is
an exergonic single-electron reduction of Umemoto’s
red
reagent (E1
= –0.37 V versus SCE in CH Cl ) by photo-
/2
2 2
28
III
ox
vital for reaction efficiency. The observed oxidation
potentials followed the expected trend that the electron-
deficient complex C3 exhibited the highest ground state
excited Ir (ppy) {E
[*Ir(III)/Ir(IV)] = –1.73 V versus SCE
3
1/2
in MeCN} to afford an electrophilic trifluoromethyl radical
that adds to styrene. The resulting radical adduct reduces
ox
IV
E1/2 [Ir(III)/Ir(IV) = 0.95 V versus SCE in CH Cl ], whereas
the oxidatively quenched Ir (ppy) , which turns over the
2
2
3
lower values of E1/2 were observed for the neutral and elec-
tron-rich complexes (0.62–0.73 V).
photoredox cycle and yields a benzylic cation that, in turn,
is trapped by nucleophilic MeOH. Given that our complexes
C1–6 have relatively low oxidation potentials (Table 1), we
reasoned that alkoxytrifluoromethylation of styrenes, as re-
ported by Yasu et al.34 would be an ideal reaction to test
their photoredox catalysis properties.
In their excited state, these complexes can act as energy
donors, electron acceptors, or electron donors. For any giv-
en reaction, the process that dominates is usually deter-
mined by both thermodynamic and kinetic factors associat-
ed with the reaction. Consequently, reductive initiation of
photoredox processes is typically more efficiently mediated
by catalysts that are strong reductants in their excited
states. It is not possible to determine these experimentally,
but they can be approximated by using the energy associat-
An initial attempt to reproduce the reported procedure
with Ir(ppy)3 as catalyst gave the product in 87% yield,
which is slightly lower than the reported yield of 99% (Table
2, entries 1 and 2).37 Such discrepancies can be expected, as
the reactions typically depend on the efficiency of irradia-
33
38
ed with their emission (Table 1). The excited-state reduc-
tion potentials for complexes C1–6 were all estimated to fall
in the range –1.11 to –1.47 V, which is distinctly higher
tion and, therefore, the reaction setup. When we used C1–
6 in our experimental setup (see Supplementary Informa-
tion) under the usual reaction conditions we obtained the
corresponding products in yields ranging from 56 to 100%.
Complexes C1, C2, and C3 gave yields toward the higher end
of this range (97–100%), and are therefore more efficient
*
than the value for Ir(ppy)3 {E red[*Ir(III)/Ir(IV)] = –1.73 V}.
This difference can be explained by the comparably higher-
energy emission of Ir(ppy) (519 nm versus 588–610 nm).
3
than is Ir(ppy) in this specific transformation. Complex C4
3
provided the product in the same yield as Ir(ppy) (87%).
3
Complexes C5 and C6 gave the product in lower yields of 79
and 56%, respectively. Note that the electronically neutral
C1, the most-electron-deficient complex C2, and the most-
electron-rich complex C3 all provided the product in simi-
lar yields. Common to these complexes are the relatively
high reduction potentials associated with the catalysts in
the turnover event of the catalytic cycle [Ir(IV)/Ir(III) =
0.62–0.95 V]. This indicates that the efficiency of the reac-
tion is coupled to the efficiency of oxidation of the key ben-
zyl radical intermediate. However, the other complexes C4–
6
are all potent reductants in their excited states, and they
have both reasonable reduction potential for the
a
Ir(IV)/Ir(III) couple (0.62–0.64 V) and more than adequate
lifetimes of their excited states. Taken together, these charac-
teristics do not differ substantially from those of the better-
performing catalysts C1, C2, and C3, as is especially evident
in the comparison with C2. It is therefore more likely that
catalyst deactivation provides a better explanation of the
slightly inferior performance of C4, C5, and C6. Deactivation
Figure 4 Cyclic voltammograms of complexes C1–6
Organofluorine compounds play an integral role in the
fields of materials science, pharmaceuticals, and agrochem-
icals. Notably, the trifluoromethyl moiety alters the lipo-
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R. E. N. Njogu et al.
Letter
of the catalyst is often observed in photoredox catalysis,
and it stems from the reactions between intermediate high-
energy radicals and the ligands of the catalysts.39 The low-
er-performing complexes C4, C5, and C6 in this work all
contain methyl substituents. Hydrogen-atom abstraction
from these substituents must be regarded as a very likely
side reaction that could initiate deactivation of the catalyst.
No byproduct associated with degradation of the catalysts
was observed, but this does not rule out radical-based
degenerative pathways for these catalysts. As such, in out-
methylation, complexes C1, C2, and C3 provide an addition
to the current range of photoredox catalysts, in terms of an
accessible redox window in the catalytic cycle.
In conclusion, six 2-(1-naphthyl)pyridine ligands and
their corresponding Ir(III) complexes C1–6 were synthe-
sized, and characterized by means of crystallographic, spec-
troscopic and electrochemical studies. The systematic mod-
ification of the ligands by the introduction of electron-
withdrawing or electron-donating groups with varying de-
grees of substitution and strength on the phenyl or pyridyl
ring was shown to permit tuning of the photophysics and
electrochemistry of the resulting complexes. The complexes
were found to be strongly luminescent (588–610 nm) and
performing Ir(ppy) as a catalyst in the methoxytrifluoro-
3
Table 2 Comparison of Synthesized Complexes C1–6 with Ir(ppy)3
CF3
BF4
S
OMe
(1.1 equiv)
CF3
catalyst (0.5 mol %)
CH2Cl2/MeOH (9/1)
4
h, RT,
blue LED light
CF3
S
S
+
CF3
IrIII
IrIV
*
CF3
IrIII
MeOH
OMe
CF3
CF3
–
H+
Entry
Catalyst
Conversion of styrenea,b (%)
Yielda,b (%)
122
2
Ir(ppy)
Ir(ppy)
C1
100
87
99
87
3
3
3
98
97
4
C2
100
100
87
100
100
87
5
C3
6
C4
7
C5
79
79
8
C6
65
56
a
Average of two experiments.
Determined by H NMR with an internal standard.
b
1
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R. E. N. Njogu et al.
Letter
to exhibit long triplet lifetimes of 1386–3692 ns. In addi-
tion, the complexes exhibited reversible or quasireversible
oxidation waves with E1/2 ranging from 0.62 to 0.95 V. High-
ly efficient oxytrifluoromethylation of styrene by using C1–
(14) Bossi, A.; Rausch, A. F.; Leitl, M. J.; Czerwieniec, R.; Whited, M.
T.; Djurovich, P. I.; Yersin, H.; Thompson, M. E. Inorg. Chem.
2013, 52, 12403.
(
(
15) Lowry, M. S.; Bernhard, S. Chem. Eur. J. 2006, 12, 7970.
16) Denisov, S. A.; Cudré, Y.; Verwilst, P.; Jonusauskas, G.; Marín-
Suárez, M.; Fernández-Sánchez, J. F.; Baranoff, E.; McClenaghan,
N. D. Inorg. Chem. 2014, 53, 2677.
6
as photoredox catalysts was achieved with up to quantita-
tive yields. Under the reaction conditions used, complexes
C1–4 showed a better or similar performance compared
with the archetypical photoredox catalyst Ir(ppy)3.
(17) Bae, H. J.; Kim, H.; Lee, K. M.; Kim, T.; Eo, M.; Lee, Y. S.; Do, Y.;
Lee, M. H. Dalton Trans. 2013, 42, 8549.
(18) Constable, E. C.; Ertl, C. D.; Housecroft, C. E.; Zampese, J. A.
Dalton Trans. 2014, 43, 5343.
Funding Information
(19) Takizawa, S.-y.; Shimada, K.; Sato, Y.; Murata, S. Inorg. Chem.
014, 53, 2983.
(20) Kondrashov, M.; Raman, S.; Wendt, O. F. Chem. Commun. 2015,
2
We would like to acknowledge funding from the International Science
Program (KEN 01), the Swedish Research Council (2014-04046, 621-
51, 911.
2
4
014-3935, 2017-00748), and the Carl Tryggers Foundation (CTS 16:
90) for a fellowship for P.F.()
(21) Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. J. Org. Chem. 2016,
81, 6898.
(
(
22) Koike, T.; Akita, M. Inorg. Chem. Front. 2014, 1, 562.
23) 2-(1-Naphthyl)pyridine (L1); Typical Procedure
Acknowledgment
A Straus flask equipped with a stirrer bar was charged with 2-
bromopyridine (1.0 equiv.), 1-naphthylboronic acid (1.1 equiv),
Pd(PPh3)4 (4 mol%), 2 M aq Na CO (2.5 mL/mmol), toluene
We thank Dr. Patrik Jarvoll (Karolinska Institute) for assistance with
NMR spectroscopy, Professor K. Börjesson and J. Mony (University of
Gothenburg) for their assistance with UV–vis and photoluminescence
measurements, and Professor E. Ahlberg (University of Gothenburg)
for fruitful discussions of electrochemical data.
2
3
(3.8 mL/mmol), and ethanol (1.3 mL/mmol). The mixture was
degassed by three freeze–pump–thaw cycles and then refluxed
under inert conditions overnight. The mixture was cooled to r.t.
and extracted with EtOAc (3 × 20 mL). The organic phases were
combined, dried (MgSO ), and concentrated. The pure ligand
4
was obtained by flash column chromatography [silica gel,
EtOAc–hexane (1:4)].
Supporting Information
Compound was isolated as viscous liquid that solidified upon
Supporting information for this article is available online at
standing. Yield: 1.6 g (99%).
https://doi.org/10.1055/s-0037-1611716.
S
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p
p
orti
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o
n
S
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p
orit
n
g Inform ati
o
n
1
H NMR (400 MHz, CDCl ): δ = 8.81 (ddd, J = 4.9, 1.9, 1.0 Hz, 1
3
H), 8.11 (dd, J = 8.5, 1.3 Hz, 1 H), 7.93 (dt, J = 7.2, 1.3 Hz, 2 H),
7
.82 (td, J = 7.7, 1.8 Hz, 1 H), 7.63–7.54 (m, 3 H), 7.54–7.46 (m, 2
References and Notes
13
H), 7.33 (ddd, J = 7.6, 4.9, 1.2 Hz, 1 H). C NMR (101 MHz,
CDCl ): δ = 159.27, 149.56, 138.51, 136.44, 133.96, 131.18,
(1) Current address: Key Laboratory of Mesoscopic Chemistry of
3
MOE, School of Chemistry and Chemical Engineering and
Jiangsu Key Laboratory of Vehicle Emissions Control, Nanjing
University, Nanjing, P. R. of China.
128.93, 128.39, 127.51, 126.52, 125.90, 125.62, 125.33, 125.09,
+
122.06. HRMS (ESI+): m/z: [M + H] calcd for C15H12N: 206.0970;
found: 206.095.
(
(
(
2) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151.
3) Jung, S.; Kang, Y.; Kim, H.-S.; Kim, Y.-H.; Lee, C.-L.; Kim, J.-J.; Lee,
S.-K.; Kwon, S.-K. Eur. J. Inorg. Chem. 2004, 3415.
(24) Iridium(III) Complexes C1–6; General Procedure
A Straus flask was charged with the ligand L1–6 (12.0 equiv),
Na CO (6.0 equiv), IrCl (1.0 equiv), and deionized H O (60 mL).
2
3
3
2
The mixture was degassed by three freeze–pump–thaw cycles,
and the sealed Straus flask was heated at 205 °C for 48 h. The
4) Ning, Z.; Zhang, Q.; Wu, W.; Tian, H. J. Organomet. Chem. 2009,
694, 2705.
mixture was then cooled and extracted with CH
2 2
Cl (3 × 20 mL).
(
(
5) Lo, K. K.-W.; Zhang, K. Y. RSC Adv. 2012, 2, 12069.
6) Zhao, Q.; Yu, M.; Shi, L.; Liu, S.; Li, C.; Shi, M.; Zhou, Z.; Huang,
C.; Li, F. Organometallics 2010, 29, 1085.
The extracts were dried (MgSO ), filtered through a pad of
Celite, and concentrated. The product was isolated by flash
chromatography [silica gel, EtOAc–hexane (1:3) (for ligand),
4
(
(
(
7) Yu, M.; Zhao, Q.; Shi, L.; Li, F.; Zhou, Z.; Yang, H.; Yi, T.; Huang, C.
then CH Cl (for complexes)].
2 2
Chem. Commun. 2008, 2115.
(25) CCDC 1875304 contains the supplementary crystallographic
data for compound C1. The data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/getstructures.
(26) Jiang, X.; Peng, J.; Wang, J.; Guo, X.; Zhao, D.; Ma, Y. ACS Appl.
Mater. Interfaces 2016, 8, 3591.
(27) Coppo, P.; Plummer, E. A.; De Cola, L. Chem. Commun. 2004,
1774.
(28) Arias-Rotondo, D. M.; McCusker, J. K. Chem. Soc. Rev. 2016, 45,
5803.
8) You, Y.; Han, Y.; Lee, Y.-M.; Park, S. Y.; Nam, W.; Lippard, S. J. J. Am.
Chem. Soc. 2011, 133, 11488.
9) Singh, A.; Teegardin, K.; Kelly, M.; Prasad, K. S.; Krishnan, S.;
Weaver, J. D. J. Organomet. Chem. 2015, 776, 51.
(
10) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti, F.
In Photochemistry and Photophysics of Coordination Compounds
II; Balzani, V.; Campagna, S., Ed.; Springer: Berlin, 2007, 143.
11) Narayanam, J. M. R.; Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40,
(
102.
(
(
12) Lang, X.; Chen, X.; Zhao, J. Chem. Soc. Rev. 2014, 43, 473.
13) Telitel, S.; Dumur, F.; Telitel, S.; Soppera, O.; Lepeltier, M.;
Guillaneuf, Y.; Poly, J.; Morlet-Savary, F.; Fioux, P.; Fouassier,
J.-P.; Gigmes, D.; Lalevée, J. Polym. Chem. 2015, 6, 613.
(29) Millett, A. J.; Habtemariam, A.; Romero-Canelón, I.; Clarkson, G.
J.; Sadler, P. J. Organometallics 2015, 34, 2683.
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G

G
Synlett
R. E. N. Njogu et al.
Letter
(30) Okada, S.; Okinaka, K.; Iwawaki, H.; Furugori, M.; Hashimoto,
basic Al O before addition. The microwave vial was sealed and
2
3
M.; Mukaide, T.; Kamatani, J.; Igawa, S.; Tsuboyama, A.;
Takiguchi, T.; Ueno, K. Dalton Trans. 2005, 1583.
carefully degassed in by three vacuum–nitrogen cycles. A
mixture of MeOH (0.2 mL) and CH Cl (1.8 mL), previously
2
2
(
(
(
31) Hofbeck, T.; Yersin, H. Inorg. Chem. 2010, 49, 9290.
32) Tucker, J. W.; Stephenson, C. R. J. J. Org. Chem. 2012, 77, 1617.
33) Juris, A.; Balzani, V.; Belser, P.; von Zelewsky, A. Helv. Chim. Acta
degassed by three freeze–pump–thaw cycles, was added. The
resulting mixture was irradiated with a blue LED light strip (7
W) at a distance of 2 cm with stirring for 4 h. The vial was then
opened, and the internal standard trimethoxybenzene was
added. The reaction mixture with the internal standard was
briefly stirred and then transferred to an NMR tube equipped
1
981, 64, 2175.
34) Yasu, Y.; Koike, T.; Akita, M. Angew. Chem. Int. Ed. 2012, 51,
567.
35) Kou, J.; Lu, C.; Wang, J.; Chen, Y.; Xu, Z.; Varma, R. S. Chem. Rev.
017, 117, 1445.
36) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Chem. Rev. 2013,
13, 5322.
(
(
(
(
9
with a sealed D O capillary, which was used for locking and
2
2
shimming of the spectrometer. The yield was determined by
using 128 scans with a relaxation time of 60 s. The reported
yield is based on the integration of the most upfield signals (δ =
2.27 and 2.43 ppm) of the product. The experiments were per-
formed twice, and the conversions and yields are reported as
average values.
1
37) Oxytrifluoromethylation of Styrene; General Procedure
The oxytrifluoromethylation was performed according to the
reported procedure; see ref. 33. An oven-dried microwave vial
was cooled to r.t. under high vacuum then charged with 5-(tri-
fluoromethyl)dibenzo[b,d]thiophenium tetrafluoroborate (42.0
mg, 1.1 equiv), the appropriate catalyst (0.5 mol%), and styrene
(38) Griffin, J. D.; Zeller, M. A.; Nicewicz, D. A. J. Am. Chem. Soc. 2015,
137, 11340.
(39) Devery, J. J. III.; Douglas, J. J.; Nguyen, J. D.; Cole, K. P.; Flowers, R.
A. II.; Stephenson, C. R. J. Chem. Sci. 2015, 6, 537.
(10.4 mg, 0.1 mmol), which had been filtered through a pad of
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G