Cyclometalated Iridium(III) complexes of azadipyrromethene chromophores

Article abstract of DOI:10.1021/om4007032

Azadipyrromethenes are bidentate ligands that absorb in the red-orange region of the spectrum, with applications as tags, light harvesters, and sensitizers. Reported here are boron-transmetalation reactions that bind azadipyrromethenes to cyclometalated iridium(III). The new species absorb both near 590 nm and into the near-ultraviolet. Two examples have been crystallographically characterized. Both intra- and intermolecular aromatic stacking interactions are found. Cyclic voltammetry experiments show that azadipyrromethene complexes of iridium(III) undergo reduction and oxidation at potentials that depend little on the cyclometalating ligands on iridium. Density functional theory calculations indicate a LUMO that is azadipyrromethene- centered, whereas the HOMO may localize on either the Ir(C ^aN)₂⁺ fragment or the azadipyrromethene.

Full text of DOI:10.1021/om4007032

Article
pubs.acs.org/Organometallics
Cyclometalated Iridium(III) Complexes of Azadipyrromethene
Chromophores
Nihal Deligonul,† Amberle R. Browne,† James A. Golen,‡ Arnold L. Rheingold,‡ and Thomas G. Gray*,†
†Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States
‡Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093,
United States
S
* Supporting Information
ABSTRACT: Azadipyrromethenes are bidentate ligands that absorb in the red-orange region of the
spectrum, with applications as tags, light harvesters, and sensitizers. Reported here are boron-
transmetalation reactions that bind azadipyrromethenes to cyclometalated iridium(III). The new species
absorb both near 590 nm and into the near-ultraviolet. Two examples have been crystallographically
characterized. Both intra- and intermolecular aromatic stacking interactions are found. Cyclic
voltammetry experiments show that azadipyrromethene complexes of iridium(III) undergo reduction
and oxidation at potentials that depend little on the cyclometalating ligands on iridium. Density
functional theory calculations indicate a LUMO that is azadipyrromethene-centered, whereas the
+
HOMO may localize on either the Ir(C∧N)2 fragment or the azadipyrromethene.
In recent years, ring-fused chromophores analogous to
azadipyrromethenes have appeared.20,21 These dyes have longer
conjugated pathways and red-shifted optical spectra.
INTRODUCTION
■
Boron azadipyrromethenes (Figure 1) are blue pigments that
have been known since at least the Second World War.1−3 Only
The development of metallaazadipyrromethenes has lagged
behind that of boron chelates. Studies of copper, silver, and
gold(I) azadipyrromethenes appeared from this laboratory in
2007 and after.22−25 Bis(azadipyrromethene) complexes of
zinc(II) and mercury(II) are heavily chromophoric.26 Reports
followed of homoleptic bis(azadipyrromethenes) of M(II) (M
= Co, Ni, Cu) that are structurally and optically similar to the
zinc analogues.27−29 Boron azadipyrromethenes with distal 2-
pyridyl rings are reported as optical sensors of Hg2+, but the
discrete complex is uncharacterized.30 Complexes of heavier,
non-d10 metals are uncommon. Gray and collaborators31 have
reported fac-[Re(CO)3]+ complexes that are electroactive;
reductions are centered on the azadipyrromethene. Oppor-
tunities in metallaazadipyrromethene chemistry remain numer-
ous.
Many cyclometalated iridium(III) complexes phosphoresce
in colors across the spectrum.32−35 Emission at room
temperature typically persists for a few microseconds. Such
lifetimes are ideal for organic light-emitting diodes. Emission
originates from singlet and triplet excitons, but excited states
decay quickly enough to evade saturation. Thompson and co-
workers36 reported seven bis(cyclometalated) iridium(III)
dipyrromethene complexes that are redox active, with oxidation
and reduction centered mainly on the dipyrrin ligand. These
complexes are also luminescent and were applied in the
building of organic light-emitting diodes (OLEDs). Cyclo-
metalated Ir(III) complexes, both cations and anions, are the
luminactive sites in light-emitting electrochemical cells, metal
Figure 1. A boron azadipyrromethene chromophore.
in the past decade have they been appreciated for their excited-
state properties. O’Shea and collaborators4−7 have reported
+
reliable syntheses of azadipyrromethenes and their BF2
chelates. Absorption and luminescence spectra were disclosed
for boron complexes substituted with bromine or methoxy on
either phenyl set. An intense absorption band near 610 nm
responds to remote substituents; shorter-wavelength features
are less sensitive. Boron azadipyrromethenes fluoresce8 with
luminescence quantum yields commonly exceeding 20% if
heavy-atom substituents are absent.5,9 Zhao and Carriera10 have
produced chromophores rigidified by ring fusion along the
azadipyrromethene perimeter. One such species is a fluo-
rescence probe for cysteine.11 The low-energy absorption red-
shifts, but the fluorescence quantum yield is not enhanced
relative to that of an unconstrained analogue. Boron
azadipyrromethenes carrying trialkylammonium substituents
stain gram-positive bacteria and are photoinducible antimicro-
bial agents.12 Work on modified azadipyrromethenes has
illustrated pH and toxin sensing.13−15
Azadipyrromethenes have been embedded as repeat units in
polymers.16−18 Their electrochemical19 and light-harvesting
abilities have been surveyed for possible solar energy recovery.
Received: July 18, 2013
Published: January 16, 2014
© 2014 American Chemical Society
637
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Article
recorded with a Varian AS-400 spectrometer operating at 399.7 MHz.
Chemical shifts (δ) were determined in parts per million (ppm)
relative to trimethylsilane and referenced to the solvent residual peaks.
19F NMR spectra were measured using trichlorofluoromethane as an
internal standard in CDCl3. Microanalyses (C, H, and N) were
performed by Midwest Microlabs, LLC. Mass spectrometry was
performed at the University of Cincinnati Mass Spectrometry facility.
Ultraviolet−visible spectra were collected on a Cary 500 spectropho-
tometer in HPLC grade or better solvents.
Syntheses. [Ir(bt)2(H2O)2]OTf. Silver trifluoromethanesulfonate (57
mg, 0.22 mmol) was added to a solution of [Ir(bt)2(μ-Cl)]2 (80 mg,
0.10 mmol) dimer in 10 mL of a 1/1 (v/v) mixture of ethanol and
deionized water. The reaction mixture was then refluxed for 24 h at
100 °C. AgCl was removed by filtration, and the remaining solvent was
removed by rotary evaporation. The residue was redissolved in
dichloromethane and filtered through Celite. Dichloromethane was
removed by rotary evaporation, and the resulting residue was washed
with pentane. The isolated orange solids were dried under vacuum.
Yield: 93 mg (72%). 1H NMR (400 MHz, CDCl3): δ 8.68 (br s, 2H),
7.58 (d, 2H, J = 5.6 Hz), 7.64−7.52 (m, 6H), 6.87 (br s, 2H), 6.67 (br
s, 2H), 6.13 (br s, 2H). Anal. Calcd for C27H20F3IrN2O5S3·CH2Cl2: C,
40.59; H, 2.65; N, 3.51. Found: C, 40.46; H, 2.69; N, 3.14.
[Ir(ppy)2(LaBr2)] (1). To a solution of BF2·LaBr2 (67 mg, 0.10
mmol) in THF (9 mL) was added a solution of potassium hydroxide
(22 mg, 0.40 mmol) in deionized water (1 mL) under argon. The
mixture was stirred for 30 min, and then cis-[Ir(ppy)2(H2O)2]OTf (69
mg, 0.10 mmol) in THF (2 mL) was added to the reaction mixture.
After the mixture was stirred for 24 h at room temperature, the solvent
was evaporated under vacuum, and the resulting violet residue was
washed with 1 mL of water. The crude product was purified by column
chromatography (diethyl ether/hexanes, 1/9, v/v) to afford 1. Vapor
diffusion of pentane into a concentrated solution of 1 in THF gave
analytically pure 1 as a violet solid after filtration and vacuum drying.
Yield: 71 mg (64%). 1H NMR (400 MHz, CDCl3): δ 8.68 (d, 2H, J =
5.6 Hz), 7.59−7.52 (m, 10H), 7.31 (d, 4H, J = 7.2 Hz), 6.99−6.91 (m,
4H), 6.70 (t, 5H, J = 7.5 Hz), 6.40 (t, 5H, J = 7.4 Hz), 6.08 (t, 4H, J =
7.4 Hz), 5.36 (t, 2H, J = 7.6 Hz). HRMS (ESI): 1104.10159 (m/z
calcd for [M + H]+ 1104.10161). Anal. Calcd for C54H36Br2IrN5: C,
58.59; H, 3.28; N, 6.33. Found: C, 58.70; H, 3.42; N, 6.15.
ion sensors, and biological tags. Iridium(III) cyclometalates
have been evaluated as (pre)catalysts of photodriven water
oxidation.37,38 Common to most iridium(III) cyclometalates
are optically allowed absorption bands at or below 550 nm.
Long-wavelength photoaction is troublesome with cyclo-
metalated iridium(III). An intriguing prospect is joining the
visible excitability of azadipyrromethenes with the triplet-state
photoproperties of iridium(III). The synthesis of iridium(III)
cyclometalates is muddled by substitutional inertness and an
attendant need for harsh reaction conditions. Further, the
proximal arms might clash with the cyclometalating ligands on
iridium. Recent work from this laboratory has demonstrated
transmetalation from boron as an efficient means of installing
ligands on gold(I)39−53 and iridium(III).54 Bis(aquo)iridium-
(III) complexes react in the presence of base with boron
azadipyrromethenes. Chelation to iridium results. Structural
characterization of two complexes illustrates how the two
proximal phenyl moieties coexist with C∧N ligands in a
mononuclear complex. The azadipyrromethene dominates the
complexes’ optical properties, and these are rationalized with
density functional theory calculations.
EXPERIMENTAL SECTION
■
Materials. Solvents were purchased from Fisher Scientific and were
used as received. IrCl3·H2O and silver(I) triflate (AgOTf) were
purchased from Strem Chemicals. The ligands 2-phenylpyridine (ppy),
p-tolylpyridine (tpy), and 7,8-benzoquinoline (bzq) were purchased
from Acros; 2,4-difluorophenylpyridine (F2ppy), 2-phenylbenzothia-
zole (bt), 2,5-diphenyloxazole (dpo), and 2-phenylquinoline (pq)
were purchased from Sigma-Aldrich and used without further
purification (Chart 1). The azadipyrromethene La55 and BF2-chelated
Chart 1. Cyclometalating Ligands and the
Azadipyrromethene LaBr2
[Ir(tpy)2(LaBr2)] (2). To a solution of BF2·LaBr2 (74 mg, 0.11 mmol)
in THF (9 mL) was added a solution of potassium hydroxide (25 mg,
0.44 mmol) in deionized water (1 mL) under argon. The mixture was
stirred for 30 min, and then cis-[Ir(tpy)2(H2O)2]OTf (79 mg, 0.11
mmol) in THF (2 mL) was added to the reaction mixture. After the
mixture was stirred for 24 h at room temperature, the solvent was
evaporated under vacuum, and the resulting violet residue was washed
with 1 mL of water. The crude product was purified by column
chromatography (diethyl ether/hexanes, 1/4, v/v) to afford 2 as a
violet solid. Vapor diffusion of pentane into a concentrated solution of
2 in THF gave analytically pure product upon filtration. Yield: 72 mg
(57%). 1H NMR (400 MHz, CDCl3): δ 8.63 (d, 2H, J = 5.7 Hz), 7.67
(t, 2H, J = 6.1 Hz), 7.65 (d, 4H, J = 7.8 Hz), 7.58 (d, 2H, J = 8.2 Hz),
7.24 (s, 4H), 7.07 (t, 4H, J = 6.1 Hz), 6.95 (t, 2H, J = 7.5 Hz), 6.86 (d,
2H, J = 7.8 Hz), 6.72 (t, 4H, J = 7.9 Hz), 6.43 (br s, 4H), 6.23 (d, 2H,
J = 9.2 Hz), 5.30 (s, 2H), 2.17 (s, 6H). Anal. Calcd for C56H40Br2IrN5·
C5H12: C, 60.69; H, 4.34; N, 5.80. Found: C, 60.61; H, 4.30; N, 5.50.
[Ir(bzq)2(LaBr2)] (3). To a solution of BF2·LaBr2 (104 mg, 0.155
mmol) in THF (13.5 mL) was added a solution of potassium
hydroxide (34 mg, 0.62 mmol) in deionized water (1.5 mL) under
argon. The mixture was stirred for 30 min, and cis-[Ir(bzq)2(H2O)2]-
OTf (114 mg, 0.155 mmol) in THF (2 mL) was added to the reaction
mixture. After the mixture was stirred for 24 h at room temperature,
the solvent was evaporated under vacuum, and the resulting violet
residue was washed with 1 mL of water. The crude product was
purified by column chromatography (diethyl ether/hexanes, 2/3, v/v)
to afford 3 as a violet solid. Vapor diffusion of pentane into a
concentrated solution of 3 in chloroform gave analytically pure
azadipyrromethene (BF2·LaBr2) were synthesized following the
procedure previously reported by O’Shea and co-workers.5 The
brominated adduct (LaBr2) was prepared with N-bromosuccinimide.24
Cyclometalated Ir(III) chloro-bridged dimers, [Ir(C∧N)2(μ-Cl)]2,
were synthesized according to standard literature procedures.56−58
The bis(aquo) complexes [L2Ir(H2O)2]OTf were prepared according
to the method reported in the literature.58,59 1H NMR spectra were
1
product upon filtration and vacuum drying. Yield: 85 mg (48%). H
NMR (400 MHz, CDCl3): δ 9.01 (d, 2H, J = 5.3 Hz), 8.21 (d, 2H, J =
7.8 Hz), 8.04 (d, 2H, J = 7.0 Hz), 7.81 (d, 2H, J = 7.4 Hz), 7.65 (d,
638
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3H, J = 7.2 Hz), 7.50−7.51 (m, 5H), 7.47 (s, 3H), 7.40−7.36 (m, 4H),
7.29 (s, 2H), 6.80 (d, 2H, J = 7.7 Hz), 6.63 (t, 2H, J = 7.3 Hz), 6.39
(br s, 3H), 6.24 (t, 2H, J = 7.6 Hz), 5.09 (d, 2H, J = 7.4 Hz). Anal.
Calcd for C58H36Br2IrN5·H2O: C, 59.39; H, 3.27; N, 5.97. Found: C,
59.59; H, 3.06; N, 5.82.
violet solid. Analytically pure compound was isolated by vapor
diffusion of pentane into a concentrated solution of 7 in chloroform,
1
followed by filtration and vacuum drying. Yield: 80 mg (46%). H
NMR (400 MHz, CDCl3): δ 7.78 (d, 4H, J = 9.1 Hz), 7.67 (d, 4H, J =
8.1 Hz), 7.56−7.51 (m, 6H), 7.44 (t, 3H J = 7.5 Hz), 7.30 (t, 4H, J =
7.4 Hz), 7.07 (t, 2H, J = 8.1 Hz), 6.98 (d, 3H, J = 6.8 Hz), 6.89 (t, 4H,
J = 7.5 Hz), 6.62 (d, 4H, J = 6.9 Hz), 6.50 (t, 2H, J = 8.1 Hz), 6.27 (t,
2H, J = 7.7 Hz), 5.64 (d, 2H). Anal. Calcd for C62H40Br2IrN5O2·
0.5H2O: C, 59.57; H, 3.47; N, 5.60. Found: C, 59.56; H, 3.64; N, 5.44.
Electrochemistry. Tetrabutylammonium hexafluorophosphate
(Fluka) was recrystallized from ethyl acetate and ether and was
dried thoroughly under vacuum and stored in a nitrogen drybox.
Ferrocene (Aldrich) was purified via sublimation and stored under
nitrogen. Cyclic voltammetry experiments were performed in a
nitrogen-filled Vacuum Atmospheres drybox outfitted with a CH
Instrument Workstation at room temperature. A glassy-carbon
working electrode was polished with 0.05 μm alumina and was
cleaned and dried before use. A silver wire served as a quasi-reference
electrode, and a platinum wire was the counter electrode. Scans were
performed at a scan rate of 100 mV s−1.
Crystallography. Crystals of [Ir(ppy)2(LaBr2)] and [Ir-
(bt)2(LaBr2)] were affixed to nylon filament loops with Paratone-N
oil and cooled to 100 K. Data were collected using Bruker
diffractometers: for [Ir(ppy)2(LaBr2)], an APEXII (CCD) D8
platform; for [Ir(bt)2(LaBr2)], a Photon (CMOS) system. Structure
solution was achieved by direct methods, which revealed all non-
hydrogen atoms. Hydrogen atoms were treated as idealized
contributions. All aspects of data processing used standard routines
available in the Bruker and SHELXL libraries (Bruker AXS, Madison,
WI). Non-hydrogen atoms were refined anisotropically during the last
few refinements.
Calculations. Spin-restricted density functional theory computa-
tions were carried out with Gaussian09 rev. A.02.60 Geometries were
fully optimized, and converged structures were confirmed to be
potential energy minima by harmonic frequency calculations.
Calculations used the exchange and correlation functionals of Perdew,
Burke, and Ernzerhof61 and the TZVP basis set of Godbelt, Andzelm,
and co-workers for nonmetals.62 For iridium, the Stuttgart−Dresden
effective core potential and basis set were used;63 scalar relativistic
effects are included implicitly. Dielectric continuum solvation in
chloroform was included with the integral equation formalism of the
polarizable continuum model.64−67 Population analyses68 proceeded
with the AOMix software of Gorelsky.69,70
[Ir(F2ppy)2(LaBr2)] (4). To a solution of BF2·LaBr2 (60 mg, 0.09
mmol) in THF (9 mL) was added a solution of potassium hydroxide
(20 mg, 0.36 mmol) in deionized water (1 mL) under argon. The
mixture was stirred for 30 min, and cis-[Ir(F2ppy)2(H2O)2]OTf (68
mg, 0.09 mmol) in THF (2 mL) was added to the solution. After the
mixture was stirred for 24 h at room temperature, the solvent was
evaporated under vacuum, and the resulting violet residue was washed
with 1 mL of water. The crude product was purified by column
chromatography (diethyl ether/hexanes, 3/17, v/v) to afford 4 as a
violet solid. Analytically pure product was isolated by vapor diffusion
of pentane into a concentrated solution of 4 in chloroform, followed
1
by filtration and vacuum drying. Yield: 73 mg (69%). H NMR (400
MHz, CDCl3): δ 8.66 (d, 2H, J = 5.7 Hz), 8.01 (d, 3H, J = 8.9 Hz),
7.81 (d, 3H, J = 7.4 Hz), 7.58 (d, 4H, J = 7.1 Hz), 7.18−7.06 (m, 7H),
6.83 (t, 5H, J = 7.6 Hz), 5.95 (t, 4H, J = 9.6 Hz), 4.83 (dd, 4H, J = 11.1
Hz, J = 6.7 Hz). 19F NMR (376.1 MHz, CDCl3): δ −109.56 (q, 2F, J =
9.5 Hz), −111.92 (t, 2F, J = 12.5 Hz). Anal. Calcd for
C54H32Br2F4IrN5: C, 55.02; H, 2.74; N, 5.94. Found: C, 55.36; H,
2.91; N, 5.67.
[Ir(bt)2(LaBr2)] (5). To a solution of BF2·LaBr2 (51 mg, 0.08 mmol)
in THF (9 mL) was added a solution of potassium hydroxide (18 mg,
0.32 mmol) in deionized water (1 mL) under argon. The mixture was
stirred for 30 min, and then cis-[Ir(bt)2(H2O)2]OTf (64 mg, 0.08
mmol) in THF (2 mL) was added. After the mixture was stirred for 24
h at room temperature, the solvent was evaporated under vacuum, and
the resulting violet residue was washed with 1 mL of water. The crude
product was purified by column chromatography (diethyl ether/
hexanes, 2/3, v/v) to afford 5 as a violet solid. Vapor diffusion of
pentane into a concentrated solution of 5 in THF gave an analytically
pure violet solid upon filtration and vacuum drying. Yield: 54 mg
(56%). 1H NMR (400 MHz, CDCl3): δ 7.91 (d, 2H, J = 9.1 Hz), 7.50
(d, 2H, J = 8.1 Hz), 7.48−7.40 (m, 4H), 7.29 (d, 2H, J = 7.6 Hz), 7.19
(t, 6H, J = 7.4 Hz), 7.12 (d, 2H, J = 7.6 Hz), 6.88 (t, 2H, J = 6.2 Hz),
6.59 (t, 4H, J = 7.6 Hz), 6.47 (t, 4H, J = 7.3 Hz), 6.36 (d, 4H, J = 7.8
Hz), 6.09 (t, 2H, J = 6.0 Hz), 5.73 (d, 2H, J = 7.6 Hz). Anal. Calcd for
C58H36Br2IrN5S2: C, 57.10; H, 3.06; N, 5.74. Found: C, 57.34; H, 2.90;
N, 5.55.
[Ir(pq)2(LaBr2)] (6). To a solution of BF2·LaBr2 (67 mg, 0.10 mmol)
in THF (9 mL) was added a solution of potassium hydroxide (22 mg,
0.4 mmol) in deionized water (1 mL) under argon. The mixture was
stirred for 30 min, and cis-[Ir(pq)2(H2O)2]OTf (78.6 mg, 0.10 mmol)
in THF (2 mL) was added. After the mixture was stirred for 24 h at
room temperature, the solvent was evaporated under vacuum. The
resulting violet residue was washed with 1 mL of water. The crude
product was purified by column chromatography (diethyl ether/
hexanes, 1/1, v/v) to afford 6 as a violet solid. Vapor diffusion of
pentane into a concentrated solution of 6 in chloroform gave
analytically pure product upon filtration. Yield: 81 mg (67%). 1H
NMR (400 MHz, CDCl3): δ 8.41 (s, 2H), 8.32 (d, 2H, J = 9.6 Hz),
8.08 (d, 2H, J = 10.1 Hz), 7.87 (d, 2H, J = 7.9 Hz), 7.86 (d, 2H, J = 5.6
Hz), 7.52 (t, 2H, J = 6.3 Hz), 7.46 (t, 2H, J = 7.8 Hz), 7.20−7.14 (m,
10H), 6.79 (t, 2H, J = 6.2 Hz), 6.53 (t, 2H, J = 7.7 Hz), 6.48 (t, 2H, J =
6.9 Hz), 6.19 (d, 3H, J = 7.5 Hz), 6.12 (t, 3H, J = 7.4 Hz), 5.87 (d, 2H,
J = 7.3 Hz), 5.59 (d, 2H, J = 5.8 Hz). Anal. Calcd for C62H40Br2IrN5·
H2O·CHCl3: C, 56.24; H, 3.30; N, 5.21. Found: C, 56.47; H, 3.43; N,
5.03.
[Ir(dpo)2(LaBr2)] (7). To a solution of BF2·LaBr2 (95 mg, 0.14
mmol) in THF (9 mL) was added a solution of potassium hydroxide
(32 mg, 0.56 mmol) in deionized water (1 mL) under argon. The
mixture was stirred for 30 min, and then cis-[Ir(dpo)2(H2O)2]OTf
(116 mg, 0.14 mmol) in THF (2 mL) was added to the solution. After
the mixture was stirred for 24 h at room temperature, the solvent was
evaporated under vacuum, and the resulting violet residue was washed
with 1 mL of water. The crude product was purified by column
chromatography (diethyl ether/hexanes, 3/4, v/v) to afford 7 as a
RESULTS AND DISCUSSION
■
Syntheses. The aquo complex cis-[Ir(ppy)2(H2O)2]+ (ppy
= 2-phenylpyridyl), reported by Bernhard and co-workers, is
one member of a family of iridium(III) synthons.59 We have
demonstrated that reactions of (aquo)iridium(III) complexes
with boronic acids and esters yield chelated complexes in boron
to iridium transmetalations.54 The reaction is base-promoted,
with KOH and K3PO4 being effective. Addition of cis-
[Ir(ppy)2(H2O)2](OTf) to a solution of BF2·LaBr2 and KOH
in 10/1 (v/v) THF/H2O proceeded with a color change from
green to violet (Scheme 1). Silica gel column chromatography,
followed by vapor diffusion crystallization, yielded analytically
pure products. Isolated yields range from 46 to 69%. The
brominated azadipyrromethene was chosen for ease of
purification of complexes from residual free ligand. Complexes
of LaBr2 were separated from the free ligand by silica gel
column chromatography, whereas complexes of unbrominated
La required, in our hands, preparative thin-layer chromatog-
raphy followed by vapor diffusion crystallization (Supporting
Information). Also, the flanking bromo groups provided ready
handles for cross-coupling or other functionalization reactions.
Structures. Two representative compounds were charac-
terized by X-ray diffraction crystallography.71 Pentane vapor
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Scheme 1
diffusion into THF solution yielded violet blades of [Ir-
(ppy)2(LaBr2)] and [Ir(bt)2(LaBr2)]. The complex [Ir-
(ppy)2(LaBr2)] crystallizes in the centrosymmetric space
Figure 3. Crystal structure of [Ir(bt)2(LaBr2)] (50% probability
ellipsoids). H atoms and solvent of crystallization are omitted for
clarity. A partial atom labeling scheme is shown; carbon atoms are
unlabeled. Selected interatomic distances (Å): Ir1−C13, 2.014(4);
Ir1−C26, 2.024(4); Ir1−N1, 2.027(3); Ir1−N2, 2.074(3); Ir1−N5,
2.159(3); Ir1−N3, 2.197(3). Selected angles (deg): N5−Ir1−N3,
85.12(11); C13−Ir1−N1, 79.74(14); C26−Ir1−N2, 79.52(13); C31−
N4−C30, 128.1(3).
group P1. Crystals of [Ir(bt) (L Br )] are monoclinic, P2 /c,
̅
2
a
2
1
with two of each enantiomer per unit cell. A thermal ellipsoid
plot of [Ir(ppy)2(LaBr2)] appears as Figure 2 and that of
[Ir(bt)2(LaBr2)], it is 128.1(3)°. For comparison, the backbone
angle in BF2·LaBr2 is 119.5(2)° 5 and that in LaIrI(COD)
(COD = (1Z,5Z)-cycloocta-1,5-diene) is 125.3(3)°.72 The
greater meso-nitrogen distention in the iridium(III) complex
suggests steric hindrance between the azadipyrromethene
phenyls and the cyclometalating ligands. The azadipyrrome-
thene is more strained than when it is complexed to the larger
iridium(I).
Intramolecular π interactions are prominent in both
structures. The proximal arms of LaBr2 stack with the phenyl
rings of both 2-phenylpyridyl ligands in the structure of
[Ir(ppy)2(LaBr2)]. Centroid to centroid distances are 3.501 and
3.607 Å (Figure S2, Supporting Information). No stacking
interactions occur between neighbors within the unit cell. In
the structure of [Ir(bt)2(LaBr2)], stacking is both intra- and
intermolecular. The proximal phenyl groups stack with the
benzenoid rings of bt. Centroid to centroid distances within the
same complex are 3.536 and 3.599 Å (Figure S3, Supporting
Information). Intermolecular π stacking occurs between
proximal phenyls, with a centroid to centroid separation of
3.874 Å. Within one unit cell, a four-tiered π stack forms, as
shown in Figure 4. The structures prove that tetraarylazadi-
Figure 2. Crystal structure of [Ir(ppy)2(LaBr2)] (50% probability
ellipsoids). Hydrogen atoms are omitted for clarity. Carbon atoms are
unlabeled. Selected interatomic distances (Å): Ir1−C43, 2.017(9);
Ir1−C54, 2.024(9); Ir1−N5, 2.029(9); Ir1−N4, 2.040(8); Ir1−N1,
2.187(8); Ir1−N3, 2.183(7). Selected angles (deg): N1−Ir1−N3,
85.1(3); C43−Ir1−N4, 81.0(4); C54−Ir1−N5, 81.1(4); C4−N2−C5,
127.9(8).
[Ir(bt)2(LaBr2)] as Figure 3. Both enantiomers of each complex
occur in the unit cell, as does a THF of crystallization in the cell
of [Ir(ppy)2(LaBr2)]. Nitrogen atoms in the C∧N ligands are
trans to each other; azadipyrromethene nitrogens are trans to
carbon. Bonds between iridium and the azadipyrromethene
nitrogens are longer than those to carbon or nitrogen of the
cyclometalating ligands. Geometric parameters of the C∧N
ligands are unexceptional. The meso nitrogens of the
azadipyrromethene core dilate from an idealized sp2 geometry.
For [Ir(ppy)2(LaBr2)], the angle C−Nmeso−C is 127.9(8)°; for
Figure 4. Intermolecular π-stacking interactions in [Ir(bt)2(LaBr2)].
Distances between benzene ring centroids (yellow spheres) are given
in Å.
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pyrromethenes accommodate six-coordination if planar, chelat-
ing ligands are opposite each other. This favorable outcome is
not intuitive; the implication is that azadipyrromethene
complex synthesis is not obstructed by the jutting phenyl arms.
Optical Spectra. Electronic absorption spectra all show an
azadipyrromethene π−π* transition near 590 nm (Figure 5).
The boron complex shows two reversible reductions at E1/2 =
−0.7 and −1.5 V. Regardless of the C∧N ligands, iridium
complexes show one reversible and one irreversible reduction,
each at potentials more negative than the respective first or
second reductions of BF2·LaBr2. The second reduction is
irreversible for Ir complexes and varies with the cyclometalating
ligand. Iridium complexes undergo reversible or quasi-reversible
oxidations near +0.5 V that depend weakly on the cyclo-
metalating ligands. These have no clear counterpart in the
voltammograms of BF2·LaBr2, except for an irreversible event
near +0.6 V. The complexes ([Ir(tpy)2(LaBr2)], [Ir-
(pq)2(LaBr2)]) show irreversible oxidations at potentials
between +0.5 and +1.0 V. Combined, these measurements
indicate that the azadipyrromethene is the site of one-electron
reduction of the neutral complexes.
The two crystallographically characterized compounds were
chosen for density functional theory (DFT) computations;
both are representative. Figure 7 depicts a partial energy-level
diagram of [Ir(ppy)2(LaBr2)], with orbital plots inset at the
right. The highest occupied Kohn−Sham orbital (HOMO) has
40% Ir and 48% ppy character; percentages are of electron
density according to Mulliken.68 The HOMO-1 lies just 0.02
eV below the HOMO and is derived (97%) from LaBr2. The
lowest unoccupied Kohn−Sham orbital (LUMO) is also LaBr2-
centered (98%). The calculated energy gap between the
LUMO and the LUMO+1 is 40% that of the HOMO−
LUMO gap. Given the new compounds’ reducibility, the
calculated properties of the LUMO are unsurprising. Energy
levels and frontier orbital compositions of [Ir(bt)2(LaBr2)] are
similar to those of the ppy analogue, except that an
azadipyrromethene π orbital is the HOMO with a nearby
[Ir(bt)2]2+-centered HOMO-1. An energy-level diagram for this
compound appears as Figure S4 (Supporting Information).
Time-dependent DFT calculations were performed on
[Ir(ppy)2(LaBr2)] and [Ir(bt)2(LaBr2)], both in their opti-
mized, ground-state (singlet) geometries. Thus, excited singlets
or triplets are Franck−Condon states. For [Ir(ppy)2(LaBr2)],
the first singlet excited state results from a single-particle
LUMO ← HOMO-1 transition; both orbitals center on the
azadipyrromethene. For [Ir(bt)2(LaBr2)], the first excited
singlet derives from a combination of LUMO ← HOMO and
LUMO ← HOMO-1 transitions that mix through configuration
interaction. A state of mixed intraligand and metal−ligand to
ligand charge transfer is suggested. Triplet states and higher
singlet states show extensive configuration interaction. That is,
these states are built up of linear combinations of transitions
between orbitals.
Figure 5. Absorption spectra of iridium(III) complexes in chloroform
solution.
This feature is the optical hallmark of azadipyrromethenes; it
appears in the spectra of metal and boron com-
plexes.4−14,23−27,31 Overlapping charge-transfer bands of the
cyclometalated core appear at shorter wavelengths and extend
into the ultraviolet. No luminescence is observed at room
temperature upon excitation at 590 nm. Excitation at 260 nm
gives weak luminescence; emission spectra appear as given in
Figure S1 (Supporting Information). Attempted measurements
of luminescence lifetimes gave evidence of decomposition. The
azadipyrromethene ligand quenches the emission native to
cyclometalated iridium(III).
The new complexes were characterized by cyclic voltamme-
try. Voltammograms collected in dichloromethane appear as
Figure 6, along with that of the boron chelate BF2·LaBr2.
Potentials are referenced to the ferrocene/ferrocenium couple.
CONCLUSIONS
■
Cyclometalated iridium(III) complexes have been prepared
having a chromophoric azadipyrromethene ligand. The
common four-aryl geometry of azadipyrromethenes is
preserved, yet the ligand comports with six-coordinate
iridium(III). The syntheses disclosed here show that base-
assisted transmetalation from boron extends to complexes of
N∧N chelating ligands. We had earlier demonstrated its use in
preparing C∧N chelates.54 Visible spectra are dominated by
azadipyrromethene absorption bands. The new complexes are
electroactive, with reversible reductions and oxidations near
−1.3 and +0.5 V, respectively. Other redox events are
irreversible and depend on the cyclometalating ligands. DFT
calculations return a LUMO that is azadipyrromethene-
centered and is isolated in energy. An azadipyrromethene π
Figure 6. Cyclic voltammograms of iridium(III) azadipyrromethene
complexes in dichloromethane. Voltammograms were recorded at 100
mV s−1 and are referenced to internal Fc/Fc+.
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Figure 7. Partial Kohn−Sham orbital energy diagram of [Ir(ppy)2(LaBr2)] with continuum (IEFPCM) chloroform solvation. Frontier orbital images
appear at the right (contour level 0.03 au).
(2) Rogers, M. A. T. Nature 1943, 151, 504.
orbital is the HOMO or very near it. The azadipyrromethene
π−π* transition that colors BF2 complexes also governs the
optical properties of Ir(III) complexes.
+
(3) Knott, E. B. J. Chem. Soc. 1947, 1196−1201.
(4) Killoran, J.; Allen, L.; Gallagher, J. F.; Gallagher, W. M.; O’Shea,
D. F. Chem. Commun. 2002, 1862−1863.
(5) Gorman, A.; Killoran, J.; O’Shea, C.; Kenna, T.; Gallagher, W. M.;
O’Shea, D. F. J. Am. Chem. Soc. 2004, 126, 10619−10631.
(6) Hall, M. J.; McDonnell, S. O.; Killoran, J.; O’Shea, D. F. J. Org.
Chem. 2005, 70, 5571−5578.
(7) Grossi, M.; Palma, A.; McDonnell, S. O.; Hall, M. J.; Rai, D. K.;
Muldoon, J.; O’Shea, D. F. J. Org. Chem. 2012, 77, 9304−9312.
(8) Batat, P.; Cantuel, M.; Jonusauskas, G.; Scarpantonio, L.; Palma,
A.; O’Shea, D. F.; McClenaghan, N. D. J. Phys. Chem. A 2011, 115,
14034−14039.
(9) Loudet, A.; Burgess, K. Chem. Rev. 2007, 107, 4891−4932.
(10) Zhao, W.; Carreira, E. M. Angew. Chem., Int. Ed. 2005, 44,
1677−1679; Chem. Eur. J. 2006, 12, 7254−7263.
(11) Jiang, X.-D.; Zhang, J.; Shao, X.; Zhao, W. Org. Biomol. Chem.
2012, 10, 1966−1968.
Like the unsubstituted dipyrromethene complexes of
Thompson and co-workers,36 Ir(III) azadipyrromethenes
exhibit oxidative and reductive electrochemistry. Their
signature absorption features, a structureless absorption band
near 590 nm, is red-shifted from a similar band in the
dipyrromethene complexes, which appears near 480 nm. Unlike
dipyrromethene complexes, the new metallaazadipyrrome-
thenes are not luminescent in the visible region in room-
temperature solutions.
This work demonstrates rational syntheses of red-absorbing
iridium complexes. The electrooptical properties of azadipyrro-
methene ligands may carry over to other metal complexes and
to materials that assimilate them. Experiments that test these
conjectures are ongoing.
(12) Frimannsson, D. O.; Grossi, M.; Murtagh, J.; Paradisi, F.;
O’Shea, D. F. J. Med. Chem. 2010, 53, 7337−7343.
(13) McDonnell, S. O.; O’Shea, D. F. Org. Lett. 2006, 8, 3493−3496.
(14) Killoran, J.; O’Shea, D. F. Chem. Commun. 2006, 1503−1505.
(15) Gawley, R. E.; Mao, H.; Haque, M. M.; Thorne, J. B.; Pharr, J. S.
J. Org. Chem. 2007, 72, 2187−2191.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthesis and characterization of [Ir(typ)2(La)] (8), X-ray data
in CIF format, tables of crystallographic data, photophysical
properties, and optimized Cartesian coordinates. This material
is available free of charge via the Internet at http://pubs.acs.org.
(16) Yoshii, R.; Nagai, A.; Chujo, Y. J. Polym. Sci., Part A: Polym.
Chem. 2010, 48, 5348−5356.
(17) Gao, L.; Senevirathna, W.; Sauve,
5357.
́
G. Org. Lett. 2011, 13, 5354−
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
(18) Gao, L.; Tang, S.; Zhu, L.; Sauve,
́
G. Macromolecules 2012, 45,
7404−7412.
(19) Nepomnyashchii, A. B.; Broring, M.; Ahrens, J.; Bard, A. J. J. Am.
̈
Chem. Soc. 2011, 133, 8633−8645.
ACKNOWLEDGMENTS
(20) Gresser, R.; Hummert, M.; Hartmann, H.; Leo, K.; Riede, M.
Chem. Eur. J. 2011, 17, 2939−2947.
■
This work was supported by the U.S. Department of Energy,
Office of Basic Energy Sciences, Division of Materials Science
and Engineering, under Award DE-FG02-13ER46977 to
T.G.G. We thank the Republic of Turkey for a fellowship to
N.D. and Professor J. D. Protasiewicz, Case Western Reserve,
for access to instrumentation.
(21) Liu, H.; Mack, J.; Guo, Q.; Lu, H.; Kobayashi, N.; Shen, Z.
Chem. Commun. 2011, 47, 12092−12094.
(22) Teets, T. S.; Partyka, D. V.; Esswein, A. J.; Updegraff, J. B., III;
Zeller, M.; Hunter, A. D.; Gray, T. G. Inorg. Chem. 2007, 46, 6218−
6220.
(23) Teets, T. S.; Updegraff, J. B., III; Esswein, A. J.; Gray, T. G.
Inorg. Chem. 2009, 48, 8134−8144.
(24) Gao, L.; Deligonul, N.; Gray, T. G. Inorg. Chem. 2012, 51,
7682−7688.
REFERENCES
(1) Rogers, M. A. T. J. Chem. Soc. 1943, 590−596.
■
642
dx.doi.org/10.1021/om4007032 | Organometallics 2014, 33, 637−643

Organometallics
Article
(25) For a related manganese dipyrromethene complex, see:
Rausaria, S.; Kamadulski, A.; Rath, N. P.; Bryant, L.; Chen, Z.;
Salvemini, D.; Neumann, W. L. J. Am. Chem. Soc. 2011, 133, 4200−
4203.
(26) Teets, T. S.; Partyka, D. V.; Updegraff, J. B., III; Gray, T. G.
(56) Baranoff, E.; Curchod, B. F. E.; Frey, J.; Scopelliti, R.; Kessler,
Inorg. Chem. 2008, 47, 2338−2346.
F.; Tavernelli, I.; Rothlisberger, U.; Gratzel, M.; Nazeeruddin, M. K.
̈
(27) Palma, A.; Gallagher, J. F.; Muller-Bunz, H.; Wolowska, J.;
̈
Inorg. Chem. 2012, 51, 215−224.
McInnes, E. J. L.; O’Shea, D. F. Dalton Trans. 2009, 273−279.
(28) Bessette, A.; Ferreira, J. G.; Giguere, M.; Belanger, F.; Desilets,
̀ ́ ́
(57) Shin, I.-S.; Yoon, S.; Il Kim, J.; Lee, J.-K.; Kim, T. H.; Kim, H.
Electrochim. Acta 2011, 56, 6219−6223.
D.; Hanan, G. S. Inorg. Chem. 2012, 51, 12132−12141.
(29) Gresser, R.; Hoyer, A.; Hummert, M.; Hartmann, H.; Leo, K.;
Riede, M. Dalton Trans. 2011, 40, 3476−3483.
(30) Coskun, A.; Yilmaz, M. D.; Akkaya, E. U. Org. Lett. 2007, 9,
(58) Junge, H.; Marquet, N.; Kammer, A.; Denurra, S.; Bauer, M.;
Wohlrab, S.; Gartner, F.; Pohl, M.-M.; Spannenberg, A.; Gladiali, S.;
̈
Beller, M. Chem. Eur. J. 2012, 18, 12749−12758.
(59) McDaniel, N. D.; Coughlin, F. J.; Tinker, L. L.; Bernhard, S. J.
Am. Chem. Soc. 2008, 130, 210−217.
607−609.
(60) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
(31) Partyka, D. V.; Deligonul, N.; Washington, M. P.; Gray, T. G.
Organometallics 2009, 28, 5837−5840.
(32) Ulbricht, C.; Beyer, B.; Friebe, C.; Winter, A.; Schubert, U. S.
Adv. Mater. 2009, 21, 4418−4441.
(33) Baranoff, E.; Yum, J.-H.; Graetzel, M.; Nazeeruddin, M. K. J.
Organomet. Chem. 2009, 694, 2661−2670.
(34) Djurovich, P. I.; Thompson, M. E. In Highly Efficient OLEDs
with Phosphorescent Materials; Yersin, H., Ed.; Wiley-VCH: Weinheim,
Germany, 2008; pp 131−161.
(35) You, Y.; Nam, W. Chem. Soc. Rev. 2012, 41, 7061−7084.
(36) Hanson, K.; Tamayo, A.; Diev, V. V.; Whited, M. T.; Djurovich,
P. I.; Thompson, M. E. Inorg. Chem. 2010, 49, 6077−6084.
(37) Hull, J. F.; Balcells, D.; Blakemore, J. D.; Incarvito, C. D.;
Eisenstein, O.; Brudvig, G. W.; Crabtree, R. H. J. Am. Chem. Soc. 2009,
131, 8730−8731.
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision A.02; Gaussian, Inc., Wallingford, CT, 2009.
(61) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77,
3865−3868.
(38) Lalrempuia, R.; McDaniel, N. D.; Muller-Bunz, H.; Bernhard, S.;
̈
Albrecht, M. Angew. Chem., Int. Ed. 2010, 49, 9765−9768; Angew.
(62) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Can. J.
Chem. 1992, 70, 560−571.
Chem. 2010, 122, 9959−9962.
(39) Partyka, D. V.; Zeller, M.; Hunter, A. G.; Gray, T. G. Angew.
Chem., Int. Ed. 2006, 45, 8188−8191; Angew. Chem. 2006, 118, 8368−
8371.
(63) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987,
86, 866−872.
(64) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117−
(40) Partyka, D. V.; Esswein, A. J.; Zeller, M.; Hunter, A. D.; Gray, T.
129.
G. Organometallics 2007, 26, 3279−3282.
(65) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105,
2999−3093.
(41) Gray, T. G. Comments Inorg. Chem. 2007, 28, 181−212.
(42) Partyka, D. V.; Updegraff, J. B., III; Zeller, M.; Hunter, A. D.;
(66) Cances, E.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107,
̀
3032−3041.
Gray, T. G. Organometallics 2009, 28, 1666−1674.
(43) Gao, L.; Peay, M. A.; Partyka, D. V.; Updegraff, J. B., III; Teets,
T. S.; Esswein, A. J.; Zeller, M.; Hunter, A. D.; Gray, T. G.
Organometallics 2009, 28, 5669−5681.
́
(44) Vogt, R. A.; Peay, M. A.; Gray, T. G.; Crespo-Hernandez, C. E.
J. Phys. Chem. Lett. 2010, 1, 1205−1211.
(45) Peay, M. A.; Heckler, J. E.; Deligonul, N.; Gray, T. G.
Organometallics 2011, 30, 5071−5074.
(46) Partyka, D. V. Chem. Rev. 2011, 111, 1529−1595.
(47) Craig, S.; Gao, L.; Lee, I.; Gray, T.; Berdis, A. J. J. Med. Chem.
2012, 55, 2437−2451.
(48) Partyka, D. V.; Teets, T. S.; Zeller, M.; Updegraff, J. B., III;
̀
(67) Mennucci, B.; Cances, E.; Tomasi, J. J. Phys. Chem. B 1997, 101,
10506−10517.
(68) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833−1840.
(69) Gorelsky, S. I. AOMix: Program for Molecular Orbital Analysis,
version 6.82b; University of Ottawa, 2013; http://www.sg-chem.net.
Accessed December 16, 2013.
(70) Gorelsky, S. I.; Lever, A. B. P. J. Organomet. Chem. 2001, 635,
187−196.
(71) CCDC 937479 and 937480. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
(72) Deligonul, N.; Gray, T. G. Inorg. Chem. 2013, 52, 13048−13057.
Hunter, A. D.; Gray, T. G. Chem. Eur. J. 2012, 18, 2100−2112.
(49) Gao, L.; Niedzwiecki, D. S.; Deligonul, N.; Zeller, M.; Hunter,
A. D.; Gray, T. G. Chem. Eur. J. 2012, 18, 6316−6327.
(50) Heckler, J. E.; Zeller, M.; Hunter, A. D.; Gray, T. G. Angew.
Chem., Int. Ed. 2012, 51, 5924−5928.
(51) Partyka, D. V.; Zeller, M.; Hunter, A. D.; Gray, T. G. Inorg.
Chem. 2012, 51, 8394−8401.
́
(52) Vogt, R. A.; Gray, T. G.; Crespo-Hernandez, C. E. J. Am. Chem.
Soc. 2012, 134, 14808−14817.
(53) Lenker, H. K.; Gray, T. G.; Stockland, R. A., Jr. Dalton Trans.
2012, 41, 13274−13276.
(54) Maity, A.; Anderson, B. L.; Deligonul, N.; Gray, T. G. Chem. Sci.
2013, 4, 1175−1181.
(55) The ligand La is
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