Enlarging the π system of phosphorescent (C^C*) cyclometalated platinum(II) NHC complexes

Article abstract of DOI:10.1021/ic500971z

Cyclometalated (C^C*) platinum(II) N-heterocyclic carbene (NHC) complexes are emerging as a new class of phosphorescent emitters for the application in organic light-emitting devices (OLEDs). We present the synthesis of six new complexes of this class to

Full text of DOI:10.1021/ic500971z

Article
pubs.acs.org/IC
Enlarging the π System of Phosphorescent (C^C*) Cyclometalated
Platinum(II) NHC Complexes
Alexander Tronnier,† Alexander Pothig,†,§ Stefan Metz,‡ Gerhard Wagenblast,‡ Ingo Munster,‡
̈
̈
and Thomas Strassner*,†
†Physikalische Organische Chemie, Technische Universitat Dresden, 01069 Dresden, Germany
̈
‡BASF SE, 67056 Ludwigshafen, Germany
S
* Supporting Information
ABSTRACT: Cyclometalated (C^C*) platinum(II) N-hetero-
cyclic carbene (NHC) complexes are emerging as a new class of
phosphorescent emitters for the application in organic light-
emitting devices (OLEDs). We present the synthesis of six new
complexes of this class to investigate the influence of extended π
systems. Therefore, six different NHC ligands with a varying
number of additional phenyl substituents were used in
combination with the monoanionic acetylacetonate (acac)
ligand to obtain complexes of the general formula [(NHC)Pt-
(II)(acac)]. The complexes were fully characterized by standard techniques and advanced spectroscopic methods (195Pt NMR).
For all complexes the solid-state structure determination revealed a square-planar coordination of the platinum atom. Absorption
and emission spectra were measured in thin amorphous poly(methyl methacrylate) films at room temperature. Four compounds
emit in the blue-green region of the visible spectrum with quantum yields of up to 81%.
modifications of the phenylpyridine ligands on the photo-
physical properties have been carried out. By extending the π
system in the ligand backbone, energy levels can be tuned,21,22
and the emission wavelength can be red-shifted as far as the
near-infrared region of the spectrum. While the incorporation
of aryl systems with low rotational barriers (“soft” dihedrals)
bound to the central metal containing fragment often leads to
lower quantum yields or ligand-to-ligand charge transfer
(LLCT) emission, more rigid systems have shown superior
photophysical properties.23
Complexes of type B (Scheme 1) are a new class of
phosphorescent emitters with the carbene carbon atom
replacing the nitrogen atom of the C^N cyclometalated
complexes (A, Scheme 1).24 It has been shown that by
incorporation of a heterocyclic π system (C, Scheme 1) the
quantum yields of these complexes increased up to 90%.25,26
Therefore, we designed a series of complexes bearing ligands
with π systems of different sizes to investigate the influence of
those systems and kept acetylacetonate as the auxiliary ligand
because of its large highest occupied molecular orbital−lowest
unoccupied molecular orbital (HOMO−LUMO) gap to
facilitate a N-heterocyclic carbene (NHC)-centered, metal-
perturbed intraligand charge transfer (ILCT) or rather metal-
to-ligand charge transfer (MLCT) emission. The results are
compared to the unsubstituted complex of this class of emitters
(1, Scheme 1 type B with R = H, R′ = Me).25
INTRODUCTION
■
The development of new functional materials has contributed
much to the growing demand for energy conservation in
everyday life over the past decade.1 One of the key areas is the
modernization of display technologies and domestic lighting,2,3
where organometallic chemistry plays an important role. Third-
row transition metal complexes have been identified as
potential phosphorescent emitters for organic light emitting
diodes (OLEDs) with excellent photophysical properties based
on the spin−orbit coupling (SOC) induced by the transition-
metal atoms,4−7 which leads to higher quantum yields.
Especially iridium8,9 and platinum10−15 complexes have
attracted much attention in recent years because of their
extraordinary photophysical properties.16−18 It was found that
bi- or tridentate monoanionic ligands, for example, those based
on the 2-phenylpyridine fragment, could be cyclometalated to
the central metal atom forming (C^N) cyclometalated
complexes, often with β-diketonate auxiliary ligands (A, Scheme
1).12,19,20 In the past years extensive studies of the effect of
Scheme 1. General Structure of Cyclometalated Pt(II)
Acetylacetonate Complexes
Received: April 25, 2014
Published: May 27, 2014
© 2014 American Chemical Society
6346
dx.doi.org/10.1021/ic500971z | Inorg. Chem. 2014, 53, 6346−6356

Inorganic Chemistry
Article
naphthyl substituent in 5 is introduced via a palladium(0)
catalyzed coupling of bromonaphthalene and the commercially
available N-methyl-1,2-phenylenediamine, followed again by
ring closure with triethyl orthoformate and ammonium
tetrafluoroborate.
RESULTS AND DISCUSSION
■
Design, Synthesis, and Characterization. Ligands with
large π systems are known from a number of patents to be of
interest for phosphorescent iridium(III) emitters.27 All NHC
precursors of this work were designed with specific extensions
of the π system in mind regarding the number and position of
phenyl rings. To rule out steric or electronic effects from
functional groups, only unsubstituted phenyl rings were chosen.
Ligand precursor 2 has an additional phenyl ring at the N2
position compared to the 3-methyl-1-phenyl-imidazolium salt
that was used for the preparation of the reference complex
[(MeImiPh)Pt(acac)] 1 (acac = acetylacetonate).25 This
phenyl group with a low rotational barrier was incorporated
in 6, 7, and 8 as well, while these three ligands feature
additional extensions of the π system in the heterocyclic
backbone. The synthesis of all four compounds starts from a
commercially available diamine or α-diketone, which already
contains the ligand backbone and thus part of the π extension.
Both N-substituents in the heterocyclic ring are readily
introduced in one reaction step either by condensation with
aniline or palladium(0) coupling with an aryl halide (Scheme
2). Both routes would allow for further studies by variation of
For the synthesis of complexes 9−14 we followed a
previously developed route25 (Scheme 3): reaction of the
Scheme 3. Synthesis of Complexes 9−14
imidazolium salt with silver(I) oxide to form the silver(I) NHC
complex, transmetalation with dichloro(1,5-cyclooctadiene)-
platinum(II) (Pt(COD)Cl2) without further purification, and
treatment with excess acetylacetone and potassium tert-
butanolate (KOtBu) after solvent exchange. The crude product
was then washed with water and purified by flash chromatog-
raphy with methylene chloride on silica gel. The yield of
complex 9 turned out to be remarkably high (72%) for such a
tedious one-pot reaction. Since the reaction worked quite well
with this platinum(II) precursor we used the same route to
synthesize the other complexes (Scheme 4). Unfortunately the
Scheme 2. Synthesis of Ligand Precursors 2−8
Scheme 4. Synthesized Complexes 9−14
yields dropped significantly for the complexes with larger π
systems, and while they were still good for 10 (52%) and 12
(53%) and acceptable for 11 (19%) and 14 (11%), complex 13
could only be obtained in traces using dichloro(1,5-cyclo-
octadiene)-platinum(II). Therefore, we decided to use a
different platinum(II) precursor, dichlorobis(dimethyl sulfide)-
platinum(II), to obtain 13 in better yields of 18%.
All complexes (Scheme 4) were characterized by standard
techniques. The formation of the cyclometalated complexes
was verified by the disappearance of the imidazolium protons
and the formation of a specific coupling pattern for the
cyclometalated rings in the 1H NMR (see Supporting
Information, Figures S1−S6). The 195Pt NMR spectra for all
complexes show chemical shifts from −3335 ppm to −3402
ppm and are characteristic for complexes of this new class of
emitters. To assign the different NMR signals a detailed two-
dimensional (2D) NMR (COSY, HSQC, HMBC, ROESY)
investigation was conducted for complex 11 (see Supporting
the starting aniline or aryl halide source. The ring closure was
accomplished using different reagents. For the synthesis of 2
the intermediate was heated together with paraformaldehyde
and HCl, while in the case of 8 the chloromethyl methyl ether
was used. Reaction with triethyl orthoformate and ammonium
tetrafluoroborate led to compounds 6 and 7.
For the synthesis of 4 and 5 we used a different approach.
The synthesis of 4 involves an Ullmann-type copper(I)
catalyzed coupling of benzimidazole and iodobenzene followed
by methylation with iodomethane at the N2 position. The
6347
dx.doi.org/10.1021/ic500971z | Inorg. Chem. 2014, 53, 6346−6356

Inorganic Chemistry
Article
Figure 1. ORTEP29,30 representation of 9−11 in the solid state. Thermal ellipsoids are drawn at the 50% probability level.
Figure 2. ORTEP29,30 representation of 12−14 in the solid state. Thermal ellipsoids are drawn at the 50% probability level.
Table 1. Selected Bond Lengths [Å], Angles, and Dihedral Angles [deg] of Complexes 9−14
bonds [Å], angles [deg]
9
10
11
12
13
14
Pt(1)−C(1)
Pt(1)−C(3)
Pt(1)−O(1)
Pt(1)−O(2)
O(1)−Pt(1)−O(2)
C(1)−Pt(1)−C(3)
C(2)−N(1)−C(1)−Pt(1)
N(1)−C(1)−Pt(1)−O(1)
1.951(5)
1.985(5)
2.084(4)
2.032(3)
89.81(4)
80.1(2)
1.927(10)
2.003(10)
2.089(7)
2.060(7)
89.1(3)
1.927(4)
1.973(4)
2.094(3)
2.047(3)
89.42(12)
80.59(17)
0.6(4)
1.925(3)
1.906(8)
1.967(9)
2.072(6)
2.034(5)
90.6(2)
1.955(4)
1.981(2)
1.975(4)
2.086(3)
2.042(3)
89.38(12)
80.40(17)
3.1(5)
2.082(2)
2.0376(19)
89.71(8)
78.7(4)
79.87(10)
−4.0(3)
−179.30(18)
80.3(3)
−0.5(5)
−174.3(3)
−0.3(11)
−174.1(7)
−4.5(9)
−170.6(6)
177.9(3)
175.2(3)
Information, Figures S7−S10). This compound was chosen to
confirm the configuration of the cyclometalated ring because of
its good solubility in deuterated dimethyl sulfoxide (DMSO-
d6). We prefer to measure in DMSO-d6 over CDCl3 as we
observed partial decomposition of our complexes in CDCl3,
similar to a recently discussed case in the literature.28 The
chemical shifts for the carbene carbon atoms, and thus the
deshielding, depend strongly on the size of the NHC π system.
The signals are observed at 150 ppm for 9, 161 ppm for 10, 160
ppm for 11, 167 ppm for 13, and 155 ppm for 14 (no signal
detected for 12).
Solid-State Structure Determination. Single crystals
suitable for X-ray solid-state determination were obtained for
all complexes by slow evaporation of saturated methylene
chloride or methylene chloride/acetonitrile solutions. All
compounds are 16-valence-electron (16-VE) complexes and
exhibit a square-planar geometry around the tetracoordinated
platinum(II) center (Figures 1 and 2). For all complexes the
phenyl substituents at the N2 position with free dihedral angles
are twisted out of the plane of the central metallacycle (C1−
N1−C2−C3−Pt1) by 49.0−71.2 degrees. The shortest
platinum-to-ligand contact is the Pt(II) carbene carbon bond
(Pt−C1, 1.906−1.955 Å), which is slightly shorter than the Pt−
C3 bond to the cyclometalated aryl ring (1.967−2.003 Å). The
two Pt−O bonds are just over 2 Å, with the bond opposite of
the platinum carbene bond being always a little shorter (Pt1−
O1 2.072−2.094 Å, Pt1−O2 2.032−2.060 Å). The differences
in the bond lengths around the central metal atom mainly arise
from the σ-donating cyclometalated ring and the π-acceptor
qualities of the NHC (see Table 1).
Depending on the number of phenyl rings attached to the
NHC and especially at the N2 position the complexes exhibit a
different packing in the crystal (see Table 2 and Supporting
Information, Figures S11−S16). For 9 each complex has two
neighboring molecules, one where the acac moieties lie directly
above each other (O···H contact of 2.59 Å) with a separation of
3.25 Å and a Pt···Pt distance of 5.79 Å and another molecule,
which is slightly shifted, rotated about 180° and separated by
3.80 Å, while the corresponding Pt···Pt distance is only 4.74 Å.
The complexes are arranged in two stacks, which themselves
form a zigzag pattern throughout the crystal along the c axis and
meet at an angle of 55° (Supporting Information, Figure S11).
6348
dx.doi.org/10.1021/ic500971z | Inorg. Chem. 2014, 53, 6346−6356

Inorganic Chemistry
Article
Table 2. Crystal Data and Crystallographic Details for Complexes 9−14
complex
formula
9
10
11
12
13
14
C20H18N2O2Pt
513.46
C19H18N2O2Pt
501.44
C23H20N2O2Pt
551.50
C24H20N2O2Pt
563.50
C28H22N2O2Pt
613.57
C30H22N2O2Pt
637.59
fw [g/mol]
T [K]
198(2)
0.710 73
monoclinic
C2/c
198(2)
198(2)
198(2)
198(2)
0.710 73
monoclinic
P21/c
198(2)
wavelength [Å]
crystal system
space group
a [Å]
0.710 73
triclinic
0.710 73
triclinic
0.710 73
monoclinic
P21/c
0.710 73
triclinic
P1
̅
P1
̅
P1
̅
21.566(4)
7.8870(16)
20.3000(4)
90
7.6770(15)
10.391(2)
11.254(2)
73.86(3)
78.58(3)
72.26(3)
815.0(3)
2
7.7870(7)
11.3990(5)
12.1500(9)
62.580(4)
84.406(9)
78.739(6)
938.85(12)
2
10.8150(7)
15.8080(13)
11.7930(14)
90
11.274(2)
13.035(3)
18.632(3)
90
7.5370(6)
9.7140(11)
15.372(4)
93.589(13)
91.396(11)
98.144(7)
1111.3(3)
2
b [Å]
c [Å]
α [deg]
β [deg]
96.59(3)
90
99.603(7)
90
126.24(3)
90
γ [deg]
U [Å3]
3430.0(12)
8
1987.9(3)
4
2208.3(7)
4
Z
Dcalc [Mg/m3]
1.989
2.043
1.951
1.883
1.845
1.905
μ (Mo Kα)
8.197
8.622
7.494
7.081
6.383
6.346
[mm−1
]
crystal size [mm3]
0.50 × 0.39 × 0.22
1968
0.34 × 0.27 × 0.25
480
0.53 × 0.27 × 0.17
532
0.26 × 0.35 × 0.53
1088
0.58 × 0.41 × 0.37
1192
0.57 × 0.35 × 0.18
620
F(000)
refl. collected
independent refl.
GOF on F2
39 889
13 929
19 731
45 152
46 959
27 548
3511 [Rint = 0.042]
1.251
2978 [Rint = 0.118]
1.094
3825 [Rint = 0.033]
1.300
4059 [Rint = 0.027]
1.210
4448 [Rint = 0.079]
1.199
4574 [Rint = 0.047]
1.166
R1 [I > 2σ(I)]
wR2
0.0208
0.0532
0.0164
0.0142
0.0326
0.0249
0.0598
0.1209
0.0569
0.0339
0.0954
0.0552
data/restr/param
3511/0/228
2978/0/220
3825/0/256
4059/0/264
4448/0/300
4574/0/318
The structure of complex 10 is relatively planar, as the methyl
group at the N2 atom allows for an ordered structure that
forms only one stack along the a axis, wherein the complexes
form pairs (Supporting Information, Figure S12). These are
separated by 3.38 Å with a Pt···Pt distance of 3.30 Å, which is
slightly below the sum of the van der Waals radii, indicating a
weak metal−metal interaction. The distance to the next
platinum atom in the stack is 4.94 Å. Within the pairs, the
NHC moiety of one complex is oriented toward the acac ligand
of the second molecule. The solid-state structure of 11 also
forms a layered packing of two complexes, each in a distance of
just 3.37 Å with a Pt···Pt contact of 5.45 Å (Supporting
Information, Figure S13). They interact with the surrounding
molecules via their respective acac auxiliary ligands, which are
aligned on top of each other with an O···H contact of 2.67 Å.
Complexes 12 and 13 crystallize in a similar way as 9, with
two stacks again forming a zigzag pattern and a superstructure
of the two complexes oriented toward each other with a long
Pt···Pt distance of 6.20/5.04 Å (plane-to-plane 3.42/3.74 Å,
Supporting Information, Figures S14 and S15). Compound 14
has a similar molecular structure as 12 and 13 but forms layers
along the c axis (Supporting Information, Figure S16). Each
molecule has two different neighboring complexes in the next
layer. To one of them the respective NHC moieties are aligned
above each other, while for the other complex the acac ligands
interact (O···H contact 2.68 Å).
at room temperature (rt). To investigate intermolecular
interactions arising from the different π systems during the
photoluminescence process, efforts were made to create pure
amorphous emitter films (100%) from solution. Unfortunately
early crystallization was observed for 13 and 14, therefore no
emission data could be obtained. Absorption spectra of the
amorphous (9−12) and crystallized (13, 14) pure films show a
different number of peaks between 220 and 355 nm
(Supporting Information, Figure S17). The intense high-energy
transitions (<250 nm) might be ascribed to spin-allowed NHC-
centered 1π−π* transitions followed by mixed MLCT
transitions. While complexes 9−13 show similar spectra, the
spectrum of complex 14 is quite different since it does not
display a defined fine structure and shows only two bands (236
and 328 nm).
Furthermore, the absorption spectra were also measured in
thin poly(methyl methacrylate) (PMMA) films doped with 2
wt % emitter (Figure 3). The individual spectra are all very
similar compared to the measurements in pure films.
Because of the premature crystallization of 13 and 14
emission spectra for pure films could only be measured of 9−
12 (Supporting Information, Figure S18). While 9, 10, and 12
each show a single broad emission band in the green region,
indicating excimer formation or even direct Pt−Pt interaction,
the spectrum of 11 is typical for an ILCT emission showing
vibronic coupling. To investigate these effects in more detail
concentration-dependent measurements in PMMA films were
done for complex 10 (see Supporting Information, Figure S19).
For the different concentrations a change in the shape of the
curves was observed. The higher concentrations show a more
prominent emission toward 550 nm, similar to the emission in
the 100% film. The quantum yields obtained were remarkably
high for the pure emitter films: 3% (9), 14.2% (10, τ0 = 2.9 μs),
In summary the more planar complexes 10 and 11 with only
methyl groups at the N2 position are more densely packed and
show the shortest Pt···Pt contacts. The bond lengths and angles
around the central platinum atom found for these new
complexes are in good agreement with the few previously
published structures of this type.15,25,26,31−35
Photophysical Studies. The ultraviolet−visible (UV−vis)
absorption and emission spectra of all complexes were studied
6349
dx.doi.org/10.1021/ic500971z | Inorg. Chem. 2014, 53, 6346−6356

Inorganic Chemistry
Article
With the addition of another phenyl ring to the NHC the
emission becomes even more red-shifted for complex 13 (CIE
0.323; 0.628) with emissions at 515, 554, and 600 nm. The
color changes with increasing size of the π system to higher
wavelengths can be explained by the resulting stabilization of
the LUMO and thus a smaller energy gap between the singlet
ground state and the first excited emissive state. As can be seen
in the emission profiles, all complexes show vibronic coupling,
which is in good agreement with an NHC ligand-centered
ILCT process. The distance between the individual peaks is in
the range of 1200 to 1430 cm−1 and corresponds to NHC-
centered vibrations as determined by density functional theory
(DFT) calculations.
In Table 3 the photophysical results for the measurements in
PMMA are given. It is interesting to note that the highest
quantum yield was found for complex 13 with the largest π
system accessible for measurement. Even the incorporation of
only one phenyl ring at the N2 position of the NHC increases
the quantum yield from 7%25 for 1 to nearly 17% for 9. As
expected, the addition of a phenyl ring in the NHC backbone
instead of the N2 position has a greater effect due to a higher
contribution of π electrons to the frontier molecular orbitals
(FMOs) increasing the quantum yield to 40% (10). Further
extension can again increase the quantum efficiency but has a
strong impact on the emission color. Complexes 11 and 13
(with the larger π system) were found to have significantly
longer decay lifetimes (11, 183 μs; 13, 405 μs). The values for
9, 10, and 12 are similar to those measured before for this class
of emitters.25
Figure 3. Absorption spectra of complexes 9−14 (2 wt % in PMMA at
rt).
21.7% (11, τ0 = 66.4 μs), and 31% (12, τ0 = 9.2 μs) (τ0 = decay
lifetime).
The emission spectra of complexes 9−13 in 2 wt % PMMA
films (Figure 4) exhibit at least three distinct emission maxima.
Electrochemistry. For all complexes cyclic voltammetry
and differential pulse voltammetry experiments were conducted
in anhydrous dimethylformamide (DMF) under inert con-
ditions with 0.1 M n-Bu4NPF6 (TBAPF6) as supporting
electrolyte to improve the conductivity. All presented potentials
derived from differential pulse voltammetry are relative to the
ferrocene/ferrocenium (Fc/Fc+) redox couple used as an
internal reference. Ionization potentials (IP) and electron
affinities (EA) were calculated according to a relationship
developed by Forrest and co-workers.36,37 Complex 9, with
only one additional phenyl ring attached at the N2 position in
comparison to a reference complex 1 (Scheme 1, R = H, R′ =
Me),25 was found to have the most cathodic reduction potential
and thus the least electron affinity (EA = 1.31 eV,
quasireversible), while complexes 10−12, with a π extension
in the imidazole backbone, showed EAs of 1.44 eV (10,
reversible), 1.47 eV (11, irreversible), and 1.45 eV (12,
irreversible). Since the reduction is considered to involve the
LUMO, which is according to DFT calculations mainly
localized on the NHC ligand, the lower EA values of 10−12
can be attributed to the expanded π system of the cyclo-
metalated NHC ligands, which can more easily accommodate
Figure 4. Emission spectra of complexes 9−13 (2 wt % in PMMA at
rt).
The quantum yield of complex 14 was below the detection
limit of the quantum yield measurement system (ca. 1−2%).
́
For the complexes 9 (Commission Internationale de l’Eclairage
(CIE) 0.155; 0.110) and 10 (0.197; 0.238) with the smallest π
extension, the first emission peak lies in the deep blue region,
with the emission maximum at around 450 nm and additional
shoulders reaching into the green region of the spectrum. With
even larger π systems the emission is subsequently red-shifted
with the exception of 12, which remains a true blue emitter
(CIE 0.158; 0.147). The emission maximum for complex 11
(CIE 0.235; 0.458) with two additional phenyl rings compared
to the reference complex 1 (CIE 0.190; 0.190) is considerably
red-shifted to 476 nm, with weaker peaks at 508 and 550 nm.
Table 3. Photoluminescence Data (2 wt % in PMMA, rt) of the Cyclometalated Complexes 9−13
a
b
c
d
e
f
g
λexc [nm]
CIE x;y
λem [nm]
ϕ
τo [μs]
kr [103 s−1
]
knr [103 s−1
]
9
355
330
355
340
370
0.155;0.110
0.197;0.238
0.235;0.458
0.158;0.147
0.323;0.628
446
454
476
457
515
0.17
0.40
0.58
0.41
0.81
18.3
8.0
54.8
124.4
5.5
275.2
189.0
3.9
10
11
12
13
183.0
9.2
108.4
2.5
154.7
0.6
404.6
a
b
c
d
e
Excitation wavelength. CIE coordinates at rt. Maximum emission wavelength. Quantum yield at λexc, N2 atmosphere. Decay lifetimes (excited
f
g
by laser pulses (355 nm, 1 ns)) given as τo = τv/ϕ. kr = ϕ/τv. knr = (1 − ϕ)/τv.
6350
dx.doi.org/10.1021/ic500971z | Inorg. Chem. 2014, 53, 6346−6356

Inorganic Chemistry
Article
the negative charge. In this regard, the highest EAs were found
for the complexes with the greatest π extension, namely, 13
(1.76 eV, reversible) and 14 (2.46 eV, first cycle reversible;
second cycle irreversible). These observations are in line with
related Pt(II) systems that bear C^N cyclometalated
ligands.12,21,38 All compounds undergo an irreversible oxidation
with ionization potentials of 5.59 (9), 5.68 (10), 5.41 (11), 5.72
(12), 5.64 (13), and 5.40 eV (14). DFT calculations show a
participation of platinum d orbitals in the HOMO, which is
otherwise delocalized over the NHC ligand. Thus, a metal
oxidation (Pt(II) → Pt(III)) is possible, and the irreversibility
of the oxidation could be explained by nucleophilic attack of
solvents at the square-planar Pt(III) center, which usually
defines the redox properties of Pt(II).12 Although the
irreversibility of the electrochemical processes might be a
problem when using the data to estimate the HOMO−LUMO
separation, a decreasing gap is found with an expansion of the π
system at the NHC ligand as is consistent with the
expectations. The cyclic voltammograms are given in the
Supporting Information, Figures S20−S25.
Computational Studies. Geometry optimizations for the
singlet and triplet states were performed using DFT methods
(B3LYP/6-31G(d)). To gain further insight into the photo-
physical properties of the complexes, frontier molecular orbitals
(FMOs) (Supporting Information, Figure S26) and spin
densities (Supporting Information, Figure S27) were computed
for the optimized structures (based on Kohn−Sham orbitals) to
better understand the photophysical process and derive the
participation of the ligands and the platinum atom in the
emission process. The HOMO for each complex is mainly
composed of the cyclometalated aryl group and platinum d
orbitals, with some differing contributions from the imidazole
fragment and the acac ligand. When comparing the HOMOs of
the complexes it becomes obvious that the phenyl substituents
at the N2 atom seem to be taking no part. This is quite feasible
since the HOMO is of π orbital character and the phenyl
substituents are twisted out of the complex plane. We found
that with an increasing size of the π system the contribution
from the acac ligand to the FMOs becomes less notable.
With the changes in the ligand backbone (mainly an
extension of the π system) the calculated HOMO−LUMO
gap decreases significantly. The calculated S0−S1 gap follows
the order 9 (4.67 eV) > 10 (4.27 eV) > 12 (4.23 eV) > 11
(4.05 eV) > 13 (3.72 eV) and 14 (3.03 eV). This is in excellent
agreement with the experimentally observed emission maxima,
which are 446 nm (9), 454 nm (10), 457 nm (12), 476 nm
(11), and 515 nm (13). The energy gaps between the S0 and T1
states (both at their optimized geometries) were found to be
2.49 eV (9), 2.80 eV (10), 2.54 eV (11), 2.78 eV (12), 2.02 eV
(13), and 1.61 eV (14). With the exception of the value for
complex 9, these follow the same order as above.
influence of a phenyl ring at the N2 atom or at the backbone
of the ligand we found that the former is less pronounced. The
rigidity of the system plays an important role for the observed
quantum yields, as geometry changes between the singlet and
triplet state seem to reduce it (see Photophysical Studies).23
CONCLUSION
■
We present the synthesis and characterization of six new C^C*
cyclometalated platinum(II) NHC complexes and their ligands.
The solid-state structure determination revealed a square-planar
coordination of the central platinum atom for all complexes.
Five of the complexes were found to be phosphorescent
emitters with an emission in the blue to green region of the
visible spectrum, characteristic band structures, and good to
very good quantum yields of up to 81%. The number of phenyl
groups attached to the NHC ligand and their position exhibit a
significant effect on the photophysical properties. Quantum-
chemical DFT calculations are in good agreement with the
experimentally observed properties. Our results demonstrate
that changing the size of the π system and the right choice of
aryl substituents allows the tuning of the photophysical
properties of this class of novel C^C* cyclometalated
platinum(II) emitters.
EXPERIMENTAL SECTION
■
General Comments. Solvents of 99.5% purity were used
throughout this study. 1,4-Dioxane and DMF were dried using
standard techniques and stored under an argon atmosphere over
molecular sieves (4 Å). Dichloro(1,5-cyclooctadiene)platinum(II)39
was prepared following a modified literature procedure.25 Dichlorobis-
(dimethyl sulfide)platinum(II) was prepared according to a literature
procedure.40 Potassium tetrachloroplatinate(II) was obtained from
Pressure Chemicals Co. All other chemicals were obtained from
common suppliers and used without further purification. H, 13C, and
1
195Pt NMR spectra were recorded on a Bruker NMR spectrometer
1
(300 MHz, 500 MHz, 600 MHz). H and 13C NMR spectra were
referenced internally using the resonances of the solvent (1H: 7.26,
1
13C: 77.0 for CDCl3; H: 2.50, 13C: 39.43 for DMSO-d6). 19F NMR
spectra were referenced externally against trifluoromethylbenzene
(F3C−C6H5). 195Pt NMR spectra were referenced externally using
potassium tetrachloroplatinate(II) in D2O (−1617.2 (PtCl42−),
−2654.1 (PtCl2)). Shifts are given in ppm, and coupling constants J
are given in Hz. Elemental analyses were performed by the
microanalytical laboratory of our institute on a Hekatech elemental
analyzer. Melting points have been determined using a Wagner and
Munz Poly Therm A system and are not corrected.
Photoluminescence. The photoluminescence of the complexes
was measured in thin PMMA films doped with 2 wt % emitter and in
100% emitter films. The 2 wt % films were prepared by doctor blading
a solution of emitter (2 mg/mL) in a 10 wt % PMMA solution in
dichloromethane on a substrate with a 60 μm doctor blade. The film
was dried, and the emission was measured under nitrogen. The
excitation was carried out at a wavelength of 330−370 nm (Xe lamp
with monochromator), and the emission was detected with a
calibrated quantum-yield detections system (Hamamatsu, model
C9920−02). The phosphorescence decay was measured by excitation
with pulses of a third harmonic generation Nd:YAG laser (355 nm, 1
ns) and time-resolved photon counting in the multichannel scaling
(MCS) technique.
The DFT-optimized geometries are also in good agreement
with the data from the solid-state structure determination (see
Supporting Information, Tables S1−S6). Upon excitation to
the triplet state most complexes retain their geometry with the
exception of 9 and 12, which bend significantly out of planarity
(see Supporting Information, Figure S28).
The quite different photophysical properties of these six
complexes depend on the number of phenyl groups and the
size of the π system. With an increasing number of phenyl rings
the quantum yield generally increases, and the emission
maximum is red-shifted in agreement with a smaller
HOMO−LUMO gap of the complexes. Comparing the
Electrochemistry. Cyclic voltammetry and differential pulsed
voltammetry were performed using a computer-controlled Metrohm
Autolab PGSTAT12 potentiostat in a three-electrode single-compart-
ment cell with a platinum working electrode, a platinum wire counter
electrode, and a Ag/AgCl reference electrode. Anhydrous DMF
(distilled over P2O5) was used as the solvent under an inert
atmosphere, and 0.1 M TBAPF6 was used as the supporting
electrolyte. The measurements were conducted with 1 mM solutions
6351
dx.doi.org/10.1021/ic500971z | Inorg. Chem. 2014, 53, 6346−6356

Inorganic Chemistry
Article
of the corresponding complex at rt with a scan rate of 0.1 V/s. All
potentials were internally referenced to the ferrocene/ferrocenium
(Fc/Fc+) couple. The IP (HOMO) and EA (LUMO) levels were
calculated using the following equations: (1) IP [eV] = 1.4*(1.OxEonset
− Fc/Fc+Eonset) + 4.636 and (2) EA [eV] = 1.19*(1.RedE1/2 − Fc/
Fc+E1/2) + 4.78.37
reflux for 6 h, cooled to rt, and the yellow precipitate was filtered off,
washed with petroleum ether, triethyl orthoformate, and 3 mL of
dichloromethane (DCM). The crude product was further purified by
dissolving the dried solid in DCM, filtration, and removal of all volatile
compounds. Yield: 2.55 g, 59%. 1H NMR (DMSO-d6, 500.13 MHz): δ
10.23 (s, 1H, NCHN), 8.45 (d, J = 2.1 Hz, 1H, CHarom), 8.33 (d, J =
8.7 Hz, 1H, CHarom), 8.19 (d, J = 8.3 Hz, 1H, CHarom), 8.16 (m, 2H,
CHarom), 7.96 (d, J = 8.3 Hz, 1H, CHarom), 7.91 (dd, J = 2.1 Hz, J = 8.3
Hz, 1H, CHarom), 7.82 (t, J = 7.7 Hz, 1H, CHarom), 7.76 (d, J = 7.5 Hz,
1H, CHarom), 7.74 (m, 2H, CHarom), 4.22 (s, 3H, CH3) ppm. 13C NMR
(DMSO-d6, 125.75 MHz): δ 143.3 (NCHN), 133.0 (Ci), 132.7 (Ci),
131.9 (Ci), 130.9 (Ci), 130.6 (Ci), 130.5 (CHarom), 128.3 (CHarom),
128.0 (CHarom), 127.9 (CHarom), 127.8 (CHarom), 127.4 (CHarom),
126.9 (CHarom), 124.0 (CHarom), 122.2 (CHarom), 113.9 (CHarom),
113.5 (CHarom), 33.4 (CH3) ppm. 19F NMR (DMSO-d6, 282.4 MHz):
δ −148.89 ppm. mp 160−161 °C. Anal. Calcd for C18H15BF4N2
(346.1 g mol−1): C, 62.46; H, 4.37; N, 8.09. Found: C, 62.25; H, 4.14;
N, 8.05%.
1,3-Diphenyl-1H-benzo[d]imidazolium tetrafluoroborate (6). To
a mixture of 1,2-phenylenediamine (97.30 g, 882.0 mmol) and distilled
bromobenzene (283.10 g, 1785.0 mmol) in degassed toluene,
Pd2(dba)3 (8.00 g, 8.7 mmol), xantphos (15.30 g, 25.6 mmol),
distilled water (23.2 mL, 1287.0 mmol), and sodium tert-butanolate
(170.00 g, 1764.0 mmol) were added, and the resulting mixture was
heated to reflux for 5 h. After it was cooled to rt, the precipitate was
filtered off and washed with toluene. Flash chromatography with
toluene gave the corresponding intermediate. Part of the intermediate
(140.0 g, 537.8 mmol) was suspended in 1.62 L of triethyl
orthoformate under inert conditions before addition of ammonium
tetrafluoroborate (58.10 g, 537.8 mmol). The mixture was heated to
reflux for 18 h and then cooled to rt, and the precipitate was filtered off
and washed with petroleum ether and triethyl orthoformate. The crude
product was further purified by dissolving the dried solid in DCM,
filtration, and removal of all volatile compounds. Yield: 174.88 g, 91%.
1H NMR (DMSO-d6, 300.13 MHz): δ 10.57 (s, 1H, NCHN), 7.95 (m,
Synthesis. 1,3-Diphenyl-1H-imidazolium tetrafluoroborate (2).
The ligand precursor 2 was synthesized by modifying a known
literature procedure.41 Aniline (4.66 g, 50 mmol) was dissolved in 80
mL of methanol, and 40 wt % aqueous glyoxal solution (2.82 mL, 25
mmol) was added before the mixture was stirred for 72 h at rt. The
mixture was evaporated to dryness, and the residue was dissolved in 70
mL of ethyl acetate. Paraformaldehyde (0.94 g, 31.3 mmol) was added
before 4 M hydrochloric acid in 1,4-dioxane (9.4 mL, 37.5 mmol) was
slowly added. After it was stirred for 3 h, the reaction was quenched by
addition of a saturated sodium bicarbonate solution. The aqueous
phase was extracted twice with 100 mL of diethyl ether before 50 wt %
fluoroboric acid (5.5 mL) was slowly added to the aqueous phase. The
precipitated solid was collected, washed with water, and dried in vacuo.
1
Yield: 0.77 g, 10%. H NMR (DMSO-d6, 300.13 MHz): δ 10.34 (s,
1H, NCHN), 8.58 (d, J = 1.4 Hz, 2H, NCH), 7.92 (d, J = 7.6 Hz, 4H,
CCHarom), 7.72 (t, J = 7.6 Hz, 4H, CHarom), 7.64 (t, J = 7.6 Hz, 2H,
CHarom) ppm. 13C NMR (DMSO-d6, 75.475 MHz): δ 134.6 (Ci),
134.5 (NCHN), 130.1 (CHarom), 130.0 (CHarom), 122.0 (CHarom),
121.9 (CHarom) ppm. 19F NMR (DMSO-d6, 282.4 MHz): δ −148.9
ppm. Melting poing (mp) 132−134 °C. Anal. Calcd for C15H13BF4N2
(308.1 g mol−1): C, 58.48; H, 4.25; N, 9.09. Found: C, 58.23; H, 4.01;
N, 8.92%.
1-Phenyl-1H-benzo[d]imidazole (3). A literature procedure42 was
modified in a way that benzimidazole (1.77 g, 15.0 mmol),
iodobenzene (2.04 g, 10.0 mmol), potassium hydroxide (1.12 g, 20.0
mmol), and copper(I) oxide (0.22 g, 1.5 mmol) were suspended in 25
mL of dry DMSO and stirred under argon for 24 h at 120 °C. After it
was cooled, the reaction mixture was poured into 30 mL of water and
extracted with 3 × 50 mL of ethyl acetate. The combined organic
layers were washed with 50 mL of brine and dried over MgSO4. The
crude product was purified by flash chromatography using silica gel
with ethyl acetate as eluent. Yield: 1.31 g, 67%. Analytical data are in
agreement with the literature. 1H NMR (CDCl3, 300.13 MHz): δ 8.14
(s, 1H, NCHN), 7.89 (m, 1H, CHarom), 7.52 (m, 6H, CHarom), 7.35
(m, 2H, CHarom) ppm.
6H, CHarom), 7.79 (m, 8H, CHarom) ppm. 13C NMR (DMSO-d6,
75.475 MHz): δ 142.7 (NCHN), 132.9 (Ci), 131.1 (Ci), 130.6
(CHarom), 130.4 (CHarom), 127.7 (CHarom), 125.3 (CHarom), 113.7
(CHarom) ppm. 19F NMR (DMSO-d6, 282.4 MHz): δ −148.92 ppm.
1,3-Diphenyl-1H-naphtho[2,3-d]imidazolium tetrafluoroborate
(7). In an argon-flushed Schlenk tube, 2,3-diaminonaphthalene (26.5
g, 167.5 mmol) and distilled bromobenzene (57.85 g, 368.5 mmol)
were dissolved in 500 mL of dry toluene. Pd2(dba)3 (1.53 g, 1.7
mmol), xantphos (2.91 g, 5.0 mmol), and sodium tert-butanolate
(32.20 g, 335.0 mmol) were added to the solution. The mixture was
then heated to reflux for 12 h. After cooling to rt, water was added to
the reaction mixture, which was extracted, and the combined organic
phases were washed with water, dried over sodium sulfate, filtered, and
evaporated to dryness. Flash chromatography with DCM gave the
corresponding intermediate. The intermediate (43.30 g, 139.5 mmol)
was suspended in triethyl orthoformate (290.00 g, 1950.0 mmol)
under inert conditions before addition of ammonium tetrafluoroborate
(14.60 g, 24.7 mmol). The mixture was heated to reflux for 8 h and
then cooled to rt, and the precipitate was filtered off and washed with
petroleum ether and triethyl orthoformate. The crude product was
further purified by dissolving the dried solid in DCM, filtration, and
removal of all volatile compounds. Yield: 42.75 g, 75%. 1H NMR
(DMSO-d6, 500.13 MHz): δ 10.75 (s, 1H, NCHN), 8.61 (s, 2H,
CHarom), 8.30 (dd, J = 3.3 Hz, J = 6.4 Hz, 2H, CHarom), 8.05 (d, J = 7.8
Hz, 4H, CHarom), 7.87 (t, J = 7.8 Hz, 4H, CHarom), 7.80 (t, J = 7.6 Hz,
3-Methyl-1-phenyl-1H-benzo[d]imidazolium iodide (4). A known
literature procedure43 was modified in a way that compound 3 (388
mg, 2.0 mmol) and iodomethane (426 mg, 3.0 mmol) were dissolved
in 3 mL of tetrahydrofuran (THF) in a pressure tube and heated to
100 °C. The reaction was stirred for 24 h, and the resulting solid was
filtered off, washed with THF and diethyl ether, and dried in vacuo.
Yield: 616 mg, 92%. Analytical data are in agreement with the
literature. 1H NMR (DMSO-d6, 300.13 MHz): δ 10.12 (s, 1H,
NCHN), 8.16 (d, J = 8.0 Hz, 1H, CHarom), 7.77 (m, 8H, CHarom), 4.17
(s, 3H, CH3) ppm. 13C NMR (DMSO-d6, 75.475 MHz): δ 143.0
(NCHN), 133.1 (Ci), 131.8 (Ci), 130.8 (Ci), 130.3 (CHarom), 127.3
(CHarom), 126.8 (CHarom), 125.0 (CHarom), 113.8 (CHarom), 113.2
(CHarom), 33.4 (CH3) ppm.
3-Methyl-1-(naphthalene-2-yl)-1H-benzo[d]imidazolium tetra-
fluoroborate (5). In an argon-flushed Schlenk tube 2-bromonaph-
thalene (42.80 g, 206.7 mmol) and N-methyl-1,2-phenylenediamine
(25.00 g, 204.6 mmol) were dissolved in 500 mL of dry toluene.
Tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3, 1.88 g, 2.1
mmol), 9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene (xantphos,
3.56 g, 6.1 mmol), and sodium tert-butanolate (27.70 g, 285.4 mmol)
were added, and the resulting mixture was heated to reflux for 15 h.
After it was cooled to rt, water was added to the reaction mixture,
which was extracted, and the combined organic phases were washed
with water, dried over sodium sulfate, filtered, and evaporated to
dryness. Flash chromatography with cyclohexane, toluene, and ethyl
acetate gave the corresponding intermediate. Part of the intermediate
(3.10 g, 12.5 mmol) was suspended in triethyl orthoformate (26.00 g,
174.8 mmol) under inert conditions before addition of ammonium
tetrafluoroborate (1.30 g, 12.5 mmol). The mixture was heated to
2H, CHarom), 7.69 (dd, J = 3.3 Hz, J = 6.4 Hz, 2H, CHarom) ppm. 13
C
NMR (DMSO-d6, 125.75 MHz): δ 146.5 (NCHN), 133.1 (Ci), 131.4
(Ci), 130.6 (CHarom), 130.4 (CHarom), 130.2 (Ci), 128.4 (CHarom),
126.9 (CHarom), 125.4 (CHarom), 111.2 (CHarom) ppm. 19F NMR
(DMSO-d6, 282.4 MHz): δ −148.92 ppm. mp 256−258 °C. Anal.
Calcd for C23H17BF4N2 (408.2 g mol−1): C, 67.67; H, 4.20; N, 6.86.
Found: C, 67.33; H, 4.16; N, 6.78%.
7,9-Diphenyl-7H-acenaphtho[1,2-d]imidazolium chloride - BIAN
(8). The ligand precursor 8 was prepared according to a literature
procedure44 in two steps. In a 10 mL glacial acetic acid solution
6352
dx.doi.org/10.1021/ic500971z | Inorg. Chem. 2014, 53, 6346−6356

Inorganic Chemistry
Article
acenaphthenequinone (0.91 g, 5.0 mmol), aniline (1.16 g, 12.5 mmol,
1.14 mL), and zinc chloride (1.36 g, 10.0 mmol) were heated to reflux
and stirred for 1 h. After it was cooled to rt, the reaction mixture was
filtered and washed with acetic acid and diethyl ether. The dried solid
was suspended in 12 mL of water and heated to reflux with 12 g of
potassium carbonate for 2 h under intense stirring. The red solid was
collected by filtration, washed with water, and dried in vacuo. Then
0.33 g (1.0 mmol) of the obtained solid was heated to 100 °C in
chloromethyl methyl ether (2.01 g, 25.0 mmol, 1.90 mL) in a pressure
tube for 16 h. After it was cooled to rt the product was precipitated
with diethyl ether, filtered, washed with diethyl ether, and dried in
vacuo. Yield: 0.37 g, 96%. Analytical data are in agreement with the
literature. 1H NMR (DMSO-d6, 300.13 MHz): δ 10.21 (s, 1H,
NCHN), 8.23 (d, J = 8.1 Hz, 2H, CHarom), 8.13 (d, J = 7.5 Hz, 4H,
CHarom), 7.92−7.76 (m, 10H, CHarom) ppm.
(SP-4-3)-[3-Methyl-1-(naphthalene-3-yl)-1H-benzo[d]imidazol-2-
ylidene-κC2,κC2′](2,4-pentanedionato-κO2,κO4)platinum(II) (11). A
dried and argon-flushed Schlenk tube was charged with compound 5
(277 mg, 0.8 mmol) and silver(I) oxide (94 mg, 0.4 mmol). After
addition of 20 mL of dry 1,4-dioxane the reaction mixture was stirred
under argon in the dark at rt for 21 h. Pt(COD)Cl2 (299 mg, 0.8
mmol) and 10 mL of 2-butanone were added, and the mixture was
heated to 115 °C and stirred for another 21 h. Afterward, all volatiles
were removed under reduced pressure, and KOtBu (366 mg, 3.2
mmol), acetylacetone (0.33 mL, 3.2 mmol), and 20 mL of dry DMF
were added under argon. After this was stirred for 20 h at rt and for 6 h
at 100 °C, all volatiles were again removed under reduced pressure,
leaving the crude product, which was washed with water and purified
by flash chromatography with methylene chloride (silica gel KG60).
After drying in vacuo a yellow-green solid was obtained. Yield: 85 mg,
1
19%. H NMR (DMSO-d6, 600.16 MHz): δ 8.52 (d, J = 8.1 Hz, 1H,
(SP-4-3)-[1,3-Diphenyl-1H-imidazol-2-ylidene-κC2,κC2′](2,4-pen-
tanedionato-κO2,κO4) platinum(II) (9). A dried and argon-flushed
Schlenk tube was charged with compound 2 (246 mg, 0.8 mmol) and
silver(I) oxide (94 mg, 0.4 mmol). After addition of 20 mL of dry 1,4-
dioxane the reaction mixture was stirred under argon in the dark at rt
for 21 h. Pt(COD)Cl2 (299 mg, 0.8 mmol) and 10 mL of 2-butanone
were added, and the mixture was heated to 115 °C and stirred for
another 21 h. Afterward, all volatiles were removed under reduced
pressure, and KOtBu (366 mg, 3.2 mmol), acetylacetone (0.33 mL, 3.2
mmol), and 20 mL of dry DMF were added under argon. After this
was stirred for 20 h at rt and 6 h at 100 °C, all volatiles were again
removed under reduced pressure, leaving the crude product, which was
washed with water and purified by flash chromatography with
methylene chloride (silica gel KG60). After drying in vacuo a pale
CHarom), 8.22 (s, 1H, CHarom), 8.06 (s, 1H, CHarom), 7.98 (d, J = 7.6
Hz, 1H, CHarom), 7.76 (m, 2H, CHarom), 7.54 (dt, J1 = 1.1 Hz, J2 = 8.0
Hz, 1H, CHarom), 7.50 (t, J = 7.3 Hz, 1H, CHarom), 7.38 (m, 2H,
CHarom), 5.67 (s, 1H, CH), 4.29 (s, 3H, NCH3), 2.12 (s, 3H, CH3),
2.02 (s, 3H, CH3) ppm. 13C NMR (DMSO-d6, 150.91 MHz): δ 185.3
(CO), 184.9 (CO), 159.6 (NCN), 146.4 (CPt), 135.4 (Ci), 131.6 (Ci),
130.3 (Ci), 130.0 (Ci), 128.6 (CHarom), 127.7 (CHarom), 126.3
(CHarom), 124.8 (CHarom), 124.6 (CHarom), 124.4 (Ci), 124.2 (CHarom),
123.6 (CHarom), 111.6 (CHarom), 111.5 (CHarom), 108.1 (CHarom),
101.9 (CH), 31.2 (NCH3), 27.7 (CH3), 27.6 (CH3) ppm. 195Pt NMR
(DMSO-d6, 128.56 MHz): δ −3335 ppm. mp 245−246 °C. Anal.
Calcd for C23H20N2O2Pt (551.5 g mol−1): C, 50.09; H, 3.66; N, 5.08.
Found: C, 50.33; H, 3.72; N, 5.09%.
(SP-4-3)-[1,3-Diphenyl-1H-benzo[d]imidazol-2-ylidene-κC2,κC2′]-
(2,4-pentanedionato-κO2,κO4)platinum(II) (12). A dried and argon-
flushed Schlenk tube was charged with compound 6 (287 mg, 0.8
mmol) and silver(I) oxide (94 mg, 0.4 mmol). After addition of 20 mL
of dry 1,4-dioxane the reaction mixture was stirred under argon in the
dark at rt for 21 h. Pt(COD)Cl2 (299 mg, 0.8 mmol) and 10 mL of 2-
butanone were added, and the mixture was heated to 115 °C and
stirred for another 21 h. Afterward, all volatiles were removed under
reduced pressure, and KOtBu (366 mg, 3.2 mmol), acetylacetone (0.33
mL, 3.2 mmol), and 20 mL of dry DMF were added under argon.
After this was stirred for 20 h at rt and for 6 h at 100 °C, all volatiles
were again removed under reduced pressure, leaving the crude
product, which was washed with water and purified by flash
chromatography with methylene chloride (silica gel KG60). After
drying in vacuo a light yellow solid was obtained. Yield: 241 mg, 53%.
1H NMR (CDCl3, 300.13 MHz): δ 8.04 (d, J = 8.2 Hz, 1H, CHarom),
7.92 (dd, pseudo-triplet, J1 = 1.4 Hz, J2 = 7.4 Hz, JH,Pt = 27.0 Hz, 1H,
PtCCHarom), 7.57 (m, 6H, CHarom), 7.41 (t, J = 7.8 Hz, 1H, CHarom),
7.27 (t, J = 7.8 Hz, 1H, CHarom), 7.19 (dt, J1 = 1.4 Hz, J2 = 7.6 Hz, 1H,
CHarom), 7.12 (d, J = 7.4 Hz, 1H, CHarom), 7.09 (dt, J1 = 1.4 Hz, J2 =
7.6 Hz, 1H, CHarom), 5.31 (s, 1H, CH), 2.01 (s, 3H, CH3), 1.31 (s, 3H,
CH3) ppm. 13C NMR (CDCl3, 75.475 MHz): δ 185.4 (CO), 184.1
(CO), 148.0 (CPt), 136.5 (Ci), 136.2 (Ci), 131.5 (CHarom), 131.0 (Ci),
129.2 (CHarom), 129.1 (CHarom), 128.4 (CHarom), 126.2 (Ci), 124.5
(CHarom), 123.8 (CHarom), 123.7 (CHarom), 123.1 (CHarom), 111.8
(CHarom), 111.5 (CHarom), 110.9 (CHarom), 101.7 (CH), 27.6 (CH3),
27.1 (CH3) ppm. 195Pt NMR (CDCl3, 64.52 MHz): δ −3379 ppm. mp
231−233 °C. Anal. Calcd for C24H20N2O2Pt (563.5 g mol−1): C,
51.15; H, 3.58; N, 4.97. Found: C, 51.38; H, 3.63; N, 4.87%.
(SP-4-3)-[1,3-Diphenyl-1H-naphtho[2,3-d]imidazol-2-ylidene-
κC2,κC2′](2,4-pentanedionato-κO2,κO4)platinum(II) (13). A dried
and argon-flushed Schlenk tube was charged with 1,3-diphenyl-1H-
naphtho[2,3-d]imidazolium tetrafluoroborate 7 (327 mg, 0.8 mmol)
and silver(I) oxide (94 mg, 0.4 mmol). After addition of 20 mL of dry
1,4-dioxane the reaction mixture was stirred under argon in the dark at
rt for 21 h. Dichlorobis(dimethyl sulfide)platinum(II) (312 mg, 0.8
mmol) and 10 mL of 2-butanone were added, and the mixture was
heated to 115 °C and stirred for another 21 h. Afterward, all volatiles
were removed under reduced pressure, and KOtBu (366 mg, 3.2
mmol), acetylacetone (0.33 mL, 3.2 mmol), and 20 mL of dry DMF
were added under argon. After this was stirred for 20 h at rt and for 6 h
1
yellow solid was obtained. Yield: 296 mg, 72%. H NMR (CDCl3,
300.13 MHz): δ 7.81 (dd, pseudo-triplet, J1 = 1.9 Hz, J2 = 7.0 Hz, JH,Pt
= 26.4 Hz, 1H, PtCCHarom), 7.59 (m, 2H, CHarom), 7.45 (m, 3H,
CHarom), 7.38 (d, J = 2.1 Hz, 1H, NCCHarom), 7.03 (m, 4H, CHarom),
5.3 (s, 1H, CH), 1.98 (s, 3H, CH3), 1.36 (s, 3H, CH3) ppm. 13C NMR
(CDCl3, 75.475 MHz): δ 184.9 (CO), 184.0 (CO), 150.4 (NCN),
146.6 (CPt), 138.4 (Ci), 131.7 (CHarom), 128.7 (CHarom), 128.5
(CHarom), 126.8 (CHarom), 125.5 (Ci), 124.4 (CHarom), 123.4 (CHarom),
121.3 (CHarom), 114.5 (CHarom), 110.2 (CHarom), 101.6 (CH), 27.6
(CH3), 27.1 (CH3) ppm. 195Pt NMR (CDCl3, 64.52 MHz): δ −3402
ppm. mp 197−199 °C. Anal. Calcd for C20H18N2O2Pt (513.5 g
mol−1): C, 46.78; H, 3.53; N, 5.46. Found: C, 46.91; H, 3.31; N,
5.36%.
(SP-4-3)-[3-Methyl-1-phenyl-1H-benzo[d]imidazol-2-ylidene-
κC2,κC2′](2,4-pentanedionato-κO2,κO4)platinum(II) (10). A dried
and argon-flushed Schlenk tube was charged with compound 4 (269
mg, 0.8 mmol) and silver(I) oxide (94 mg, 0.4 mmol). After addition
of 20 mL of dry 1,4-dioxane the reaction mixture was stirred under
argon in the dark at rt for 21 h. Pt(COD)Cl2 (299 mg, 0.8 mmol) and
10 mL of 2-butanone were added, and the mixture was heated to 115
°C and stirred for another 21 h. Afterward, all volatiles were removed
under reduced pressure, and KOtBu (366 mg, 3.2 mmol),
acetylacetone (0.33 mL, 3.2 mmol), and 20 mL of dry DMF were
added under argon. After this was stirred for 20 h at rt and for 6 h at
100 °C, all volatiles were again removed under reduced pressure,
leaving the crude product, which was washed with water and purified
by flash chromatography with methylene chloride (silica gel KG60).
After drying in vacuo a yellow solid was obtained. Yield: 209 mg, 52%.
1H NMR (CDCl3, 300.13 MHz): δ 7.89 (m, 2H, PtCHarom, CHarom),
7.47 (d, J = 7.8 Hz, 1H, CHarom), 7.35 (m, 3H, CHarom), 7.13 (dt, J1 =
1.3 Hz, J2 = 7.6 Hz, 1H, CHarom), 7.03 (dt, J1 = 1.0 Hz, J2 = 7.4 Hz, 1H,
CHarom), 5.52 (s, 1H, CH), 4.26 (s, 3H, NCH3), 2.08 (s, 3H, CH3),
2.01 (s, 3H, CH3) ppm. 13C NMR (CDCl3, 75.475 MHz): δ 185.2
(CO), 160.7 (NCN), 148.1 (CPt), 135.5 (Ci), 131.3 (Ci), 131.2
(CHarom), 125.3 (Ci), 124.1 (CHarom), 123.7 (CHarom), 123.4 (CHarom),
122.7 (CHarom), 111.3 (CHarom), 110.8 (CHarom), 110.4 (CHarom),
102.0 (CH), 31.2 (NCH3), 28.0 (CH3), 27.9 (CH3) ppm. 195Pt NMR
(CDCl3, 64.52 MHz): δ −3395 ppm. mp 177−179 °C. Anal. Calcd for
C19H18N2O2Pt (501.4 g mol−1): C, 45.51; H, 3.62; N, 5.59. Found: C,
45.48; H, 3.74; N, 5.62%.
6353
dx.doi.org/10.1021/ic500971z | Inorg. Chem. 2014, 53, 6346−6356

Inorganic Chemistry
Article
at 100 °C, all volatiles were again removed under reduced pressure,
leaving the crude product, which was washed with water and purified
by flash chromatography with methylene chloride (silica gel KG60).
After drying in vacuo a yellow solid was obtained. Yield: 89 mg, 18%.
1H NMR (CDCl3, 300.13 MHz): δ 8.41 (s, 1H, CHarom), 8.06 (d, J =
PLATON,52 and the SHELXL-97 package.47,53 For the visualization,
Mercury54 and ORTEP-III29,30 were used, and for the preparation of
the Supporting Information, ENCIFER55 and the WinGX56 suite were
used.
Computational Details. All calculations were performed with the
Gaussian09 package.57 The density functional hybrid model
B3LYP58−62 was used together with the 6-31G(d)63−68 basis set. No
symmetry or internal coordinate constraints were applied during
optimizations. All reported intermediates were verified as true minima
by the absence of negative eigenvalues in the vibrational frequency
analysis. Harmonic force constants were calculated for all geometries
to verify them as ground states. In all cases platinum was described
using a decontracted Hay−Wadt (n + 1) effective core potential and
basis set.69−71 HOMO and LUMO FMOs were computed on the
singlet ground-state structure, while the spin densities were calculated
based on the ground-state structure of the optimized triplet state.
Approximate free energies were obtained through thermochemical
analysis, using the thermal correction to Gibbs free energy as reported
by Gaussian09. This takes into account zero-point effects, thermal
enthalpy corrections, and entropy. All energies reported in this paper,
unless otherwise noted, are free energies at standard conditions (T =
298 K, p = 1 atm), using unscaled frequencies. For visualization,
GaussView72 and CYLview73 were used.
8.0 Hz, 1H, CHarom), 7.96 (dd, J1 = 1.0 Hz, J2 = 7.4 Hz, 1H, CHarom),
7.82 (d, J = 8.4 Hz, 1H, CHarom), 7.75 (d, J = 7.8 Hz, 1H, CHarom),
7.67 (m, 2H, CHarom), 7.62 (m, 3H, CHarom), 7.50 (m, 3H, CHarom),
7.26 (t, 1H, CHarom), 7.12 (t, J = 7.3 Hz, 1H, CHarom), 5.33 (s, 1H,
CH), 2.02 (s, 3H, CH3), 1.33 (s, 3H, CH3) ppm. 13C NMR (CDCl3,
75.475 MHz): δ 185.5 (CO), 184.2 (CO), 166.8 (NCN), 147.9 (CPt),
136.7 (Ci), 136.4 (Ci), 131.4 (CHarom), 131.0 (Ci), 130.5 (Ci), 129.7
(Ci), 129.3 (CHarom), 129.2 (CHarom), 128.6 (CHarom), 128.1 (CHarom),
127.9 (CHarom), 126.0 (Ci), 125.8 (CHarom), 125.3 (CHarom), 123.9
(CHarom), 123.7 (CHarom), 111.8 (CHarom), 108.0 (CHarom), 107.3
(CHarom), 101.7 (CH), 27.5 (CH3), 27.1 (CH3) ppm. 195Pt NMR
(CDCl3, 64.52 MHz): δ −3336 ppm. mp 285−289 °C. Anal. Calcd for
C28H22N2O2Pt (613.6 g mol−1): C, 54.81; H, 3.61; N, 4.57. Found: C,
54.70; H, 3.56; N, 4.62%.
(SP-4-3)-[7,9-Diphenyl-7H-acenaphtho[1,2-d]imidazol-2-ylidene-
κC8,κC2′](2,4-pentanedionato-κO2,κO4)platinum(II) (14). A dried
and argon-flushed Schlenk tube was charged with compound 8 (305
mg, 0.8 mmol) and silver(I) oxide (94 mg, 0.4 mmol). After addition
of 20 mL of dry 1,4-dioxane the reaction mixture was stirred under
argon in the dark at rt for 21 h. Pt(COD)Cl2 (299 mg, 0.8 mmol) and
10 mL of 2-butanone were added, and the mixture was heated to 115
°C and stirred for another 21 h. Afterward, all volatiles were removed
under reduced pressure, and KOtBu (366 mg, 3.2 mmol),
acetylacetone (0.33 mL, 3.2 mmol), and 20 mL of dry DMF were
added under argon. After this was stirred for 20 h at rt and for 6 h at
100 °C, all volatiles were again removed under reduced pressure,
leaving the crude product, which was washed with water and purified
by flash chromatography with methylene chloride (silica gel KG60).
After drying in vacuo a red crystalline solid was obtained. Yield: 55 mg,
ASSOCIATED CONTENT
■
S
* Supporting Information
The crystallographic data for compounds 9−14 in CIF format,
packing diagrams for compounds 9−14, 2D NMR spectra,
cyclic and differential pulse voltammograms, FMOs and spin
densities as well as Cartesian coordinates for the optimized
structures. This material is available free of charge via the
Internet at http://pubs.acs.org
1
11%. H NMR (CDCl3, 300.13 MHz): δ 8.19 (d, J = 7.2 Hz, 1H,
AUTHOR INFORMATION
CHarom), 7.92 (dd, J1 = 1.4 Hz, J2 = 7.5 Hz, 1H, CHarom), 7.81 (m, 4H,
CHarom), 7.67 (m, 2H, CHarom), 7.59 (m, 3H, CHarom), 7.40 (dd, J1 =
7.0 Hz, J2 = 8.2 Hz, 1H, CHarom), 7.22 (dd, J1 = 1.4 Hz, J2 = 7.5 Hz,
1H, CHarom), 7.14 (d, J = 6.7 Hz, 1H, CHarom), 7.09 (dd, J1 = 1.4 Hz, J2
= 7.5 Hz, 1H, CHarom), 5.31 (s, 1H, CH), 2.00 (s, 3H, CH3), 1.34 (s,
3H, CH3) ppm. 13C NMR (CDCl3, 150.91 MHz): δ 185.0 (CO),
184.0 (CO), 155.3 (NCN), 146.9 (CPt), 137.4 (Ci), 131.8 (CHarom),
130.0 (Ci), 129.6 (Ci), 129.4 (CHarom), 129.1 (CHarom), 128.2
(CHarom), 127.7 (CHarom), 127.6 (CHarom), 127.4 (CHarom), 127.3
(CHarom), 126.0 (Ci), 125.8 (Ci), 125.7 (Ci), 124.3 (CHarom), 123.7
(CHarom), 123.6 (CHarom), 120.5 (CHarom), 111.8 (CHarom), 101.7
(CH), 27.6 (CH3), 27.1 (CH3) ppm. 195Pt NMR (CDCl3, 64.52
MHz): δ −3376 ppm. mp 312−315 °C. Anal. Calcd for C30H22N2O2Pt
(637.6 g mol−1): C, 56.51; H, 3.48; N, 4.39. Found: C, 56.12; H, 3.36;
N, 4.39%.
■
Corresponding Author
*E-mail: thomas.strassner@chemie.tu-dresden.de. Fax: 49 351
46339679. Phone: 49 351 46338571.
Present Address
§(A.P.) Anorganische Chemie, TU Munchen, Lichtenberg-
̈
strasse 4, 85747 Garching, Germany.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are very grateful to I. Stengel (BASF) for the measurement
of the cyclic and differential pulse voltammograms, and to Dr.
Fuchs (BASF) and Dr. Lennartz (BASF) for helpful
discussions. We also thank the ZIH for computation time on
their high-performance computing facility and the BMBF
(FKZ: 13N10477) for funding.
X-ray Crystallography. Preliminary examination and data
collection were carried out on an area detecting system (Kappa-
CCD; Nonius, FR590) using graphite monochromated Mo Kα
radiation (λ = 0.710 73 Å) with an Oxford Cryosystems cooling
system at the window of a sealed fine-focus X-ray tube. The reflections
were integrated. Raw data were corrected for Lorentz, polarization,
decay, and absorption effects. The absorption correction was applied
using SADABS.45 After merging, the independent reflections were
used for all calculations. The structure was solved by a combination of
direct methods46 and difference Fourier syntheses.47 All non-hydrogen
atom positions were refined with anisotropic displacement parameters.
Hydrogen atoms were placed in ideal positions using the SHELXL
riding model. Full-matrix least-squares refinements were carried out by
REFERENCES
(1) Crabtree, R. H. Organometallics 2011, 30, 17−19.
(2) Forrest, S. R. Nature 2004, 428, 911−918.
■
(3) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.;
Luessem, B.; Leo, K. Nature 2009, 459, 234−238.
(4) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151−154.
(5) Kwong, R. C.; Lamansky, S.; Thompson, M. E. Adv. Mater. 2000,
12, 1134−1138.
(6) Highly Efficient OLEDs with Phosphorescent Materials; Yersin, H.,
Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany,
2008; pp 438.
2
minimizing Σw(Fo − Fc2)2 with the SHELXL-97 weighting scheme
and stopped at shift/err < 0.001. Details of the structure
determinations are given in the Supporting Information. Neutral-
atom scattering factors for all atoms and anomalous dispersion
corrections for the non-hydrogen atoms were taken from International
Tables for Crystallography.48 All calculations were performed with the
programs COLLECT,49 DIRAX,50 EVALCCD,51 SIR92,46 SADABS,45
(7) Sasabe, H.; Kido, J. Chem. Mater. 2011, 23, 621−630.
6354
dx.doi.org/10.1021/ic500971z | Inorg. Chem. 2014, 53, 6346−6356

Inorganic Chemistry
Article
(8) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee,
H.-E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J.
Am. Chem. Soc. 2001, 123, 4304−4312.
(9) Tsuboyama, A.; Okada, S.; Ueno, K. In Highly Efficient Red-
Phosphorescent Iridium Complexes; Wiley-VCH: Weinheim, Germany,
2008; pp 163−183.
(10) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.;
Lipa, D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447−
457.
(11) Chan, S.-C.; Chan, M. C. W.; Wang, Y.; Che, C.-M.; Cheung,
K.-K.; Zhu, N. Chem.Eur. J. 2001, 7, 4180−4190.
(12) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.;
Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055−3066.
(13) Pomestchenko, I. E.; Luman, C. R.; Hissler, M.; Ziessel, R.;
Castellano, F. N. Inorg. Chem. 2003, 42, 1394−1396.
(14) Kalinowski, J.; Fattori, V.; Cocchi, M.; Williams, J. A. G. Coord.
Chem. Rev. 2011, 255, 2401−2425.
(36) D’Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.;
Polikarpov, E.; Thompson, M. E. Org. Electron. 2005, 6, 11−20.
(37) Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E.
Org. Electron. 2009, 10, 515−520.
(38) Ghedini, M.; Pugliese, T.; La Deda, M.; Godbert, N.; Aiello, I.;
Amati, M.; Belviso, S.; Lelj, F.; Accorsi, G.; Barigelletti, F. Dalton
Trans. 2008, 4303−4318.
(39) Drew, D.; Doyle, J. R. Inorg. Synth. 1990, 28, 346−349.
(40) Otto, S.; Roodt, A. J. Organomet. Chem. 2006, 691, 4626−4632.
(41) Clavier, H.; Correa, A.; Cavallo, L.; Escudero-Adan, E. C.;
Benet-Buchholz, J.; Slawin, A. M. Z.; Nolan, S. P. Eur. J. Inorg. Chem.
2009, 1767−1773.
(42) Huang, Y.-Z.; Miao, H.; Zhang, Q.-H.; Chen, C.; Xu, J. Catal.
Lett. 2008, 122, 344−348.
(43) Tennyson, A. G.; Rosen, E. L.; Collins, M. S.; Lynch, V. M.;
Bielawski, C. W. Inorg. Chem. 2009, 48, 6924−6933.
(44) Vasudevan, K. V.; Butorac, R. R.; Abernethy, C. D.; Cowley, A.
H. Dalton Trans. 2010, 39, 7401−7408.
(15) Hudson, Z. M.; Sun, C.; Helander, M. G.; Chang, Y.-L.; Lu, Z.-
H.; Wang, S. J. Am. Chem. Soc. 2012, 134, 13930−13933.
(16) Chou, P.-T.; Chi, Y. Chem.Eur. J. 2007, 13, 380−395.
(17) Murphy, L.; Williams, J. A. G. Top. Organomet. Chem. 2010, 28,
75−111.
(45) Sheldrick, G. M. SADABS, Version 2.10; University of
Goettingen: Goettingen, Germany, 2002.
(46) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27,
435.
(18) Rausch, A. F.; Homeier, H. H. H.; Yersin, H. Top. Organomet.
Chem. 2010, 29, 193−235.
(47) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr.
(19) D’Andrade, B. W.; Brooks, J.; Adamovich, V.; Thompson, M. E.;
Forrest, S. R. Adv. Mater. 2002, 14, 1032−1036.
2008, A64, 112−122.
(48) International Tables for Crystallography, Vol. C; Mathematical,
Physical and Chemical Tables; Wilson, A. J. C., Ed.; Kluwer (Springer-
Verlag): New York, 1992; pp 900.
(20) Wong, W.-Y.; He, Z.; So, S.-K.; Tong, K.-L.; Lin, Z.
Organometallics 2005, 24, 4079−4082.
(21) 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, 12403−12415.
(49) Hooft, R. W. W. Data Collection Software for Nonius-Kappa
CCD; Nonius B.V.: Delft, The Netherlands, 1999.
(50) Duisenberg, A. J. M. J. Appl. Crystallogr. 1992, 25, 92−96.
(51) Duisenberg, A. J. M.; Kroon-Batenburg, L. M. J.; Schreurs, A. M.
M. J. Appl. Crystallogr. 2003, 36, 220−229.
(22) Xu, X.; Zhao, Y.; Dang, J.; Yang, X.; Zhou, G.; Ma, D.; Wang, L.;
Wong, W.-Y.; Wu, Z.; Zhao, X. Eur. J. Inorg. Chem. 2012, 2012, 2278−
2288.
(52) Spek, A. L. Acta Crystallogr. 2009, D65, 148−155.
(53) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
Structures; University of Goettingen: Goettingen, Germany, 1997.
(54) Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.; Edgington, P. R.;
McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; van, d. S. J.;
Wood, P. A. J. Appl. Crystallogr. 2008, 41, 466−470.
(23) Rausch, A. F.; Murphy, L.; Williams, J. A. G.; Yersin, H. Inorg.
Chem. 2012, 51, 312−319.
(24) Egen, M.; Kahle, K.; Bold, M.; Gessner, T.; Lennartz, C.; Nord,
S.; Schmidt, H.-W.; Thelakkat, M.; Baete, M.; Neuber, C.; Kowalsky,
W.; Schildknecht, C.; Johannes, H.-H. Preparation of Transition Metal
N-heterocyclic Carbene Complexes for Use in Organic Light-Emitting
Diodes (OLEDs). WO2006056418A2, 2006.
(55) Allen, F. H.; Johnson, O.; Shields, G. P.; Smith, B. R.; Towler,
M. J. Appl. Crystallogr. 2004, 37, 335−338.
(56) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837−838.
(57) 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, Jr., J. A.;
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.;
(25) Unger, Y.; Meyer, D.; Molt, O.; Schildknecht, C.; Muenster, I.;
Wagenblast, G.; Strassner, T. Angew. Chem., Int. Ed. 2010, 49, 10214−
10216.
(26) Tronnier, A.; Risler, A.; Langer, N.; Wagenblast, G.; Muenster,
I.; Strassner, T. Organometallics 2012, 31, 7447−7452.
(27) Thompson, M. E.; Tamayo, A.; Djurovich, P.; Sajoto, T.;
Forrest, S. R.; Mackenzie, P. B.; Walters, R.; Brooks, J.; Li, X.-C.;
Alleyne, B.; Tsai, J.-Y.; Lin, C.; Ma, B.; Barone, M. S.; Kwong, R.
Organic Light-Emitting Devices Employing Organometallic Com-
pounds, Especially Luminescent Compounds with Carbene Ligands,
and Selected Compounds. WO2005113704A2, 2005.
(28) Baranoff, E.; Curchod, B. F. E.; Frey, J.; Scopelliti, R.; Kessler,
F.; Tavernelli, I.; Rothlisberger, U.; Gratzel, M.; Nazeeruddin, M. K.
Inorg. Chem. 2012, 51, 215−224.
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
(29) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
(30) Burnett, M. N.; Johnson, C. K. ORTEP-III; Oak Ridge National
Laboratory: Oak Ridge, TN, 1996.
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision B.01; Gaussian, Inc.: Wallingford, CT, 2009.
(58) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200−
1211.
(59) Becke, A. D. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−
3100.
(31) Tenne, M.; Unger, Y.; Strassner, T. Acta Crystallogr., Sect. C:
Cryst. Struct. Commun. 2012, 68, m203−m205.
(32) Tronnier, A.; Nischan, N.; Strassner, T. J. Organomet. Chem.
2013, 730, 37−43.
(33) Tronnier, A.; Nischan, N.; Metz, S.; Wagenblast, G.; Muenster,
I.; Strassner, T. Eur. J. Inorg. Chem. 2014, 2014, 256−264.
(34) Tronnier, A.; Metz, S.; Wagenblast, G.; Muenster, I.; Strassner,
T. Dalton Trans. 2014, 43, 3297−3305.
(60) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter
Mater. Phys. 1988, 37, 785−789.
(61) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett.
1989, 157, 200−206.
(35) Tronnier, A.; Poethig, A.; Herdtweck, E.; Strassner, T.
Organometallics 2014, 33, 898−908.
(62) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J.
Phys. Chem. 1994, 98, 11623−11627.
6355
dx.doi.org/10.1021/ic500971z | Inorg. Chem. 2014, 53, 6346−6356

Inorganic Chemistry
Article
(63) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
724−728.
(64) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257−2261.
(65) Hariharan, P. C.; Pople, J. A. Chem. Phys. Lett. 1972, 16, 217−
219.
(66) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213−
222.
(67) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209−214.
(68) Rassolov, V. A.; Pople, J. A.; Ratner, M. A.; Windus, T. L. J.
Chem. Phys. 1998, 109, 1223−1229.
(69) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270−283.
(70) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−310.
(71) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284−298.
(72) Dennington, R. I.; Keith, T.; Millam, J. M.; Eppinnett, W.;
Hovell, W. L.; Gilliland, R. GaussView, 3.09; Gaussian Inc.:
Wallingford, CT, 2003.
́
(73) Legault, C. Y. CYLview, 1.0b; Universite de Sherbrooke:
Sherbrooke, Canada, 2009.
6356
dx.doi.org/10.1021/ic500971z | Inorg. Chem. 2014, 53, 6346−6356