Luminescent Iridium(III) Pyridinium-Derived N-Heterocyclic Carbene Complexes as Versatile Photoredox Catalysts

Article abstract of DOI:10.1021/acs.inorgchem.7b00955

The development of novel luminescent iridium(III) complexes with highly tunable emission energy and versatile applications is of particular importance. In this Communication, a series of luminescent iridium(III) complexes supported by chromophoric pyridinium-derived N-heterocyclic carbene (NHC) ligands that display tunable emission from 516 to 682 nm were prepared. These complexes can be used as photocatalysts in photooxidation and photoreduction reactions and could have potential applications in pH sensing.

Full text of DOI:10.1021/acs.inorgchem.7b00955

Communication
pubs.acs.org/IC
Luminescent Iridium(III) Pyridinium-Derived N‑Heterocyclic Carbene
Complexes as Versatile Photoredox Catalysts
,†
†
‡
†
‡
Tsz Lung Lam,* Jing Lai, Rajasekar Reddy Annapureddy, Minying Xue, Chen Yang,
†
†
,§
Yunzhi Guan, Pingjian Zhou, and Sharon Lai-Fung Chan*
†Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, P. R.
China
‡
State Key Laboratory of Synthetic Chemistry, Institute of Molecular Functional Materials and Department of Chemistry, The
University of Hong Kong (HKU), Pokfulam Road, Hong Kong, P. R. China
*
S Supporting Information
were synthesized in 19−47% yield by refluxing a mixture of
Ir(C^N) (μ-Cl)] and L1−L8 in ethylene glycol for 12 h under
ABSTRACT: The development of novel luminescent
iridium(III) complexes with highly tunable emission
energy and versatile applications is of particular
importance. In this Communication, a series of lumines-
cent iridium(III) complexes supported by chromophoric
pyridinium-derived N-heterocyclic carbene (NHC) ligands
that display tunable emission from 516 to 682 nm were
prepared. These complexes can be used as photocatalysts
in photooxidation and photoreduction reactions and could
have potential applications in pH sensing.
[
2
2
argon followed by anion metathesis and chromatographic
13
workup (Scheme 1). A downfield C NMR signal in the range
Scheme 1. Synthesis and Chemical Structures of 1−10
yridinium-derived N-heterocyclic carbene (NHC) ligands
P
(pyridylidene), N-(2-pyridyl)-4-R-pyridine-2-ylidene (4-R-
iso-BIPY), first introduced by Bercaw and co-workers as a class of
1
robust ligands for Shilov Chemistry, are deemed to have the
of 174.3−188.1 ppm, characteristic of a metalated NHC carbenic
combined features of robustness of the metal−NHC bond and
rich photophysics of diimine ligands arising from the low-energy
π* orbitals. Nonetheless, the coordination chemistry of 4-R-iso-
3g,9,11,12
C resonance, was observed for 1−10.
Diffraction-quality
crystals were obtained for 1, 2, 5, and 10 (Figure S1). A facial
arrangement for pyridinic/pyrazolic N (i.e., C trans to N) is
1
,2
BIPY is largely unexplored. While there are a number of
reports on the analogous metal “bipyridinium-like” NHC
complexes, most of them focused on structural or catalytic
observed in all of these complexes. The Ir−C
distances
carbene
range from 1.962(3) to 1.976(6) Å (Table S1), which are
comparable to literature values with carbene trans to the pyridine
3
studies, except the recent work on iridium(III) N-methylbipyr-
3f,12b
ring.
Interestingly, analysis of 2, 5, and 10 suggests extensive
3f,g
idinium complexes by Coe and co-workers. In this regard, we
delocalization of π electrons from the −NR substituents into the
2
are attracted to using the 4-R-iso-BIPY ligands to design new
pyridylidene ring (Figure 1), which is evidenced from the
III
luminescent cationic Ir -NHC complexes because the former
could be easily prepared and structurally modified, leading to
easy access to diverse NHC ligands having tunable low-energy π*
orbitals. More importantly, cationic cyclometalated iridium
complexes, particularly those with diimine ligands, are
documented to have vast applications in electroluminescent
+
Figure 1. Resonance structures of [Ir(C^N) (4-R-iso-BIPY)] with and
4
5
6
2
devices, photoinduced hydrogen production, chemosensors,
without the −NR substituent.
6
b,7
2
bioimaging,
and photoredox catalysts for organic trans-
8
formations. The applied studies of their NHC counterpart,
2
9
−11
following: (i) sp hybridization of N atoms in the NMe and
however, have received less attention.
Herein we describe
2
NC H O groups (∠RNR = 115.2−119.0°); (ii) torsional angle
novel luminescent cationic cyclometalated iridium(III) com-
plexes supported by the 4-R-iso-BIPY ligands and their
applications as strong photoreductants and photooxidants in
photochemical reactions and for pH sensing.
4
8
between −NR and the pyridylidene ring close to 0° (1.6−7.9°);
2
1
(
iii) variable-temperature H NMR measurements of 5,
indicating a restricted rotation of the C−NMe bond with
ΔG = 14.9 kcal/mol (Figure S2); (iv) C NMR data of Ir−
2
⧧
13
The 4-R-iso-BIPY ligands (L1−L8) were prepared by a one-
pot reaction in fair-to-excellent yields (37−98%; Scheme S1).
Complexes [Ir(C^N) (4-R-iso-BIPY)]PF [1−10; C^N = 2-
rot
Received: April 20, 2017
Published: August 28, 2017
2
6
phenylpyridine anion (ppy) or 1-phenylpyrazole anion (ppz)]
©
2017 American Chemical Society
10835
DOI: 10.1021/acs.inorgchem.7b00955
Inorg. Chem. 2017, 56, 10835−10839

Inorganic Chemistry
Communication
Ccarbene of 2, 5, and 10 being less deshielded than that of 1 (188.1
ppm for 1 vs 174.3−178.8 ppm for 2, 5, and 10).
to 3.0 μs and quantum yields of up to 0.81 (Table 1). Alcoholic
glass (77 K) and solid (298 and 77 K) emission spectra were also
measured (Table S3 and Figures S7−S9). Complexes 1−8
display markedly blue-shifted emission at 77 K glass and, in
particular, a vibronic spectral feature was observed for 2−8 with
spacing of ca.1200−1500 cm− (Figures 2 and S7). A more
detailed photophysical study on 5, as a representative for 2−8,
was performed to elucidate these spectral changes. The 298 K
fluid emission of 5 is slightly sensitive to the polarity of the
The photophysical data of 1−10 are listed in Tables 1 and S3.
Figure 2 depicts UV−vis absorption spectra of 1 and 5 in
1
Table 1. Photophysical Data of 1−10
a
Em λmax/nm
Abs λmax/nm (ε × 10 / M cm−1
a
3
−1
b
)
(τ/μs; ϕ )
1
2
276 (33.5), 374 (9.0), 403 (sh) (6.0), 494 (0.8)
596 (0.1; 0.06)
solvents (Δλ < 7 nm; Figure S10), yet the large radiative rate
max
269 (36.5), 288 (33.4), 332 (33.2), 342 (35.6), 377 546 (0.9; 0.33)
16.8), 460 (2.1)
5
−1
(
constant, on the order of 10 s , is in favor of a predominate
MLCT excited state. Besides, considerable MLCT character
3
13
3
3
4
5
6
7
8
266 (42.0), 294 (40.4), 310 (37.9), 322 (sh) (32.9), 537 (1.0; 0.37)
67 (14.3), 459 (1.6)
3
is presumably preserved for 77 K glass emission, despite the
distinct vibronic emission profile, because the emission liftime (τ
276 (36.6), 286 (34.9), 333 (33.0), 343 (37.0), 376 533 (1.6; 0.53)
19.6), 459 (2.2)
(
=
5.0 μs) and quantum yield (ϕ = 0.94) are comparable to those
261 (37.9), 279 (36.0), 321 (33.6), 331 (37.2), 362 522 (2.1; 0.78)
18.1), 446 (2.4)
(
in fluid at 298 K. Variable-temperature emission measurements
on 5 in an alcoholic mixture revealed a significant spectral change
at ca. −130 °C, the temparature at which a liquid-to-glass
transition of the alcoholic mixture takes places (Figure S11). This
finding indicates that the change in the emission from 298 to 77
K is a rigidochromic effect in which the rigid medium at 77 K
hinders the solvent molecules from stabilizing the charge-transfer
excited state and results in a hypochromic shift in the emission. A
similar luminescence rigidochromism was reported for other
274 (38.0), 289 (36.2), 331(34.1), 342 (39.0), 372
20.1), 452 (2.5)
516 (2.2; 0.81)
(
257 (56.5), 277 (51.3), 311 (39.4), 337 (39.2), 494 682 (1.8; 0.19)
22.4)
(
257 (46.5), 278 (43.4), 382 (14.0), 411 (16.4), 474 607 (3.0; 0.04)
42.2), 520 (sh) (12.6)
(
9
1
315 (10.0), 385 (5.2), 485 (1.0)
586 (0.2; 0.02)
516 (2.4; 0.35)
0
320 (33.4), 330 (35.6), 359 (13.6), 430 (1.9)
a
−5
Measured in a degassed CH Cl solution at 2 × 10 M for 1−6 and
2
2
4
b,14
−6
b
8
−10 and 5 × 10 M for 7 at 298 K. Phosphorescence quantum
cationic iridium complexes.
The transient absorption
yields were measured using an integrating sphere.
difference spectrum of 5 is different from that of fac-Ir(ppy)3,
suggesting that the excited state of 5 is not associated with ppy
(
Figure S13). Furthermore, complexes [Ir(ppz) (4-R-iso-
2
t
BIPY)]PF (R = Bu for 9 and NMe for 10) emit at 586 and
6
2
5
2
16 nm (Table 1) respectively, which is equalvent to 286.3 and
22.8 cm− energy reduction with respect to the ppy analogues 1
1
and 5 (Figure S12). The marginal energy difference in emission
upon switching the cyclometalated ligand from ppy to ppz
indicates the emissive excited state of [Ir(C^N) (4-R-iso-
2
3
BIPY)]PF complexes in this work to be MLCT associated
6
4a
with pyridylidene rather than with cyclometalated ligands. This
assignment is in line with spin-density calculations of the T state
1
Figure 2. Left: UV−vis absorption spectra of 1 (black line) and 5 (red
line) in CH Cl at 298 K. Right: Emission spectra of 1 (black line) and 5
of 1a (model compound of 1) and 5 in which the spin density is
mainly contributed from the iridium (0.65 for 1a and 0.65 for 5)
and pyridylidene (1 for 1a and 1.04 for 5) with a minor
contribution from ppy (2ppy; 0.35 for 1a and 0.31 for 5; Figure
S14).
2
2
(
red line) in degassed CH Cl at 298 K (solid line) and in alcoholic glass
2 2
[
1:4 (v/v) methanol/ethanol] at 77 K (dashed line).
The cyclic voltammogram of 1−10 features an irreversible/
CH Cl . The strong absorption bands for 1 and 5 at λ < 300 nm
2
2
3
−1
−1
1
quasi-reversible oxidation peak (except 8, with an additional
[
ε = (33.5−37.9) × 10 M cm ] are assigned as ππ*
+/0
oxidation peak) with E from +0.51 to +0.97 V (vs Fc ) and an
pa
transitions of ppy and 4-R-iso-BIPY (Figure S3). The additional
intense absorption band at λ = 331 nm (ε = 37.2 × 10 M
cm ) for 5 is assigned as the ππ* transition of L5. The
moderately intense bands at 330−430 nm for 1 are assigned as an
admixture of singlet metal-to-ligand charge-transfer ( MLCT)
3
−1
irreversible reduction peak with E from −2.16 to −1.56 V (vs
pc
max
+/0
−1
1
Fc ) (Table S6 and Figure S15). The E values are +0.97 and
pa
+
0.82 V, while the E values are −1.75 and −2.13 V for 1 and 5,
pc
1
respectively. It is evident that the amino substituent on
pyridinium-derived NHC destabilizes both highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) but to a larger extent in LUMO. As inferred
from DFT calculations, where HOMO and LUMO are mainly
1
and ligand-to-ligand charge-transfer ( LLCT) transitions from
dπ(Ir) → π*(pyridylidene) and π(ppy)→ π*(pyridylidene),
1
whereas that at 347−413 nm for 5 is assigned as MLCT
transitions from both dπ(Ir) → π*(pyridylidene) and dπ(Ir) →
π*(ppy) according to time-dependent density functional theory
III
localized on the Ir ion and pyridylidene, respectively (Figure
III
IV
(
TDDFT) calculations (Figure S5 and Tables S4 and S5). The
S14), the oxidation is assigned to Ir to Ir with some
3
−1
−1
weak absorptions beyond 450 nm (ε < 0.8 × 10 M cm ) for 1
contribution from ppy, while the reduction occurs at the
3
−1
−1
IV/III
and beyond 420 nm (ε < 2.3 × 10 M cm ) for 5 are assigned
pyridylidene. The excited-state redox potentials E(Ir
*) and
1
3
III /II
as a combination of MLCT and MLCT transitions from dπ(Ir)
E(Ir * ) are estimated based on the E0−0 and electrochemical
data (Table S6). When the R group of the pyridylidene ligand
was changed, powerful one-electron photoreductant 5 with
and photooxidant 1 with
E(Ir * ) = +0.71 V vs Fc , which is more oxidizing than
→
π*(pyridylidene).
Complexes 1−8 display broad featureless emission spectra in
deaerated CH Cl at room temperature (Figures 2 and S6). The
IV/III
+/0
E(Ir
*) = −1.87 V vs Fc
2
2
III /II
+/0
emission maxima range from 516 to 682 nm with lifetimes of up
1
0836
DOI: 10.1021/acs.inorgchem.7b00955
Inorg. Chem. 2017, 56, 10835−10839

Inorganic Chemistry
Communication
2
+
II /I
[
Ru(bpm) ] [bpm = 2,2′-bipyrimidine; E(Ru * ) = +0.61 V vs
tosylic acid of increasing concentration (0−120 μM), it was
observed that the low-energy absorption band collapsed
gradually, while the emission intensity was attenuated by ca.
3
+
/0
15
Fc ], could be obtained.
Visible-light-driven thiol−ene reaction was chosen to
16
demonstrate the oxidizing nature of the excited state of 1. In
the presence of 0.3 mol % 1 and 0.5 equiv of p-toluidine as the
radical mediator, the reaction of benzylmercaptan with cyclo-
hexene under blue LED irradition for 1 h affords the
corresponding thiol−ene coupling product in 88% yield (entry
5
0% (Figure 4).
1
, Tables 2 and S7). The reaction is also compatible with
Table 2. Visible-Light-Catalyzed Radical Thiol−Ene
a
Additions Using 1 as a Photocatalyst
Figure 4. Left: UV−vis spectral change of 8 (20 μM) in a 0.05 M
n
[
Bu N]PF −MeCN solution containing various [TsOH] (0−120
4
6
μM). Inset: Plot of the absorbance at 455 and 300 nm against [TsOH].
Right: Emission spectra of 8 (20 μM) in a 0.05 M [ Bu N]PF −MeCN
solution in the presence of 0 and 120 μM TsOH.
n
4
6
In summary, a series of luminescent iridium complexes
supported by pyridinium-derived NHC ligands were synthe-
sized. Their electronic and photophysical properties can be
modified via a simple modification of pyridinium-derived NHC
ligands. These iridium complexes are versatile photocatalysts and
can be used in both photooxidation and -reduction reactions. We
also demonstrated that this class of complexes could have
potential application in pH sensing.
a
b
Refer to the Supporting Information for details. 0.1 mol %
c
2+
d
photocatalyst. [Ru(bpz)3] as the photocatalyst. [Ir-(dF-CF -
ppy) (dtbpy)] as the photocatalyst. [Ir(ppy) (dtbpy)] as the
photocatalyst.
3
+
e
+
2
2
internal/terminal alkene possessing other functionalities as well
as phenylacetylene that furnishes the respective thiol−ene
products in 54−81% yields (entries 2−6, Tables 2 and S7).
Indeed, these results were generally better than those obtained
from [Ru(bpz) ] [E(Ru * ) = +1.07 V vs Fc
6
ppy) (dtbpy)] and [Ir(ppy) (dtbpy)] [dF-CF -ppy = 2-(2,4-
difluorophenyl)-5-(trifluoromethyl)pyridine anion and dtbpy =
4
respectively, vs Fc
nature of the excited state (vide supra) and the one-electron-
reduced form [E(Ir ) = −2.13 V vs Fc ] empowers 5 to
mediate a visible-light-driven CO reduction. Under blue LED
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00955.
■
2+
II /I
+/0 15b
;
entries 1−
*
S
3
, Table 2] and comparable to those of [Ir-(dF-CF -
3
+
+
2
2
3
III /II
,4′-di-tert-butyl-2,2′-bipyridine; E(Ir * ) = +0.83 and +0.28 V,
Experimental procedures, computational details, Tables
S1−S11, Figures S1−S20, Scheme S1, X-ray crystal
structures of 1, 2, 5, and 10 (PDF)
+
/0 15b
;
entry 1, Table 2]. The highly reducing
III/II
+/0
Accession Codes
2
CCDC 1546477−1546479 and 1546489 contain the supple-
mentary 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.
irradiation, the reduction of CO using a 5 + [Co(TPA)Cl]Cl
protocol achieves TON(CO) up to 1900 in 75 h (Scheme 2).
2
Scheme 2. Visible-Light-Driven CO Reduction to CO Using a
2
5
(0.4 mM) + [Co(TPA)Cl]Cl (5 μM) Protocol
AUTHOR INFORMATION
Corresponding Author
E-mail: joeltl@hku.hk.
■
*
This protocol offers the following merits over the reported [fac-
ORCID
17
Ir(ppy) + [Co(TPA)Cl]Cl] protocol: (i) higher TON(CO)
3
Tsz Lung Lam: 0000-0002-6610-9338
Rajasekar Reddy Annapureddy: 0000-0001-5855-4688
Sharon Lai-Fung Chan: 0000-0003-0274-7258
(
i.e., 1900 vs 1500); (ii) higher efficiency upon solar-to-fuel
conversion (i.e., CO + H = 86 vs 33 μmol); (iii) a high CO
production rate is maintained even after 10 h of irradiation
2
(Figure S16).
Notes
Unlike 2−6, the amino substituent of 7 and 8 is intervened by
The authors declare no competing financial interest.
§
Deceased July 16, 2017.
an aryl group and not directly attached to the pyridylidene ring.
Such structural feature reduces the degree of delocalization of π
electron of the amino substituent toward the pyridylidene ring,
which, in turn, enhances the availability of π electron against the
local environment. Thus, it is envisioned that 7 and 8 could
potentially be applied for pH sensing. As a proof-of-concept,
when an acetonitrile solution of 8 (20 μM) was treated with
ACKNOWLEDGMENTS
■
This work is supported by The Hong Kong Polytechnic
University, the Hong Kong Research Grants Council (UGC;
PolyU 253038/15P; AoE/P-03/08) and RGC’s support for
German Academic Exchange Service.
1
0837
DOI: 10.1021/acs.inorgchem.7b00955
Inorg. Chem. 2017, 56, 10835−10839

Inorganic Chemistry
Communication
Zysman-Colman, E. Lessons learned in tuning the optoelectronic
properties of phosphorescent iridium(III) complexes. Chem. Commun.
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