Phosphorescent Cyclometalated Platinum(II) Imidazolinylidene Complexes

Article abstract of DOI:10.1002/ejic.202001077

We present the synthesis and characterization of six novel bidentate (Formula presented.) cyclometalated platinum(ii) complexes derived from saturated N-heterocyclic carbene precursors, namely 1-aryl-3-methyl-1H-4,5-dihydroimidazolium salts. The title compounds were then synthesized by a multi-step reaction, which includes an in situ generation of the silver carbene complex, followed by transmetalation to platinum and subsequent introduction of the β-diketonate ligand. Structural characterization by NMR experiments and solid-state structures prove the cyclometalation and the saturated backbone of the NHC motif. Photophysical and electrochemical properties of the platinum(ii) complexes were examined and studied in detail by DFT calculations. The title compounds are strongly emissive at room temperature in the sky-blue region of the visible spectrum and show quantum yields of up to 71 % in a PMMA matrix.

Full text of DOI:10.1002/ejic.202001077

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doi.org/10.1002/ejic.202001077
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Phosphorescent Cyclometalated Platinum(II)
Imidazolinylidene Complexes
Sergej Stipurin[a] and Thomas Strassner*[a]
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Dedicated to Prof. Dr. Siegfried Hünig on the occassion of his 100. birthday.
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We present the synthesis and characterization of six novel
ization by NMR experiments and solid-state structures prove
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bidentate C C cyclometalated platinum(ii) complexes derived
the cyclometalation and the saturated backbone of the NHC
motif. Photophysical and electrochemical properties of the
platinum(ii) complexes were examined and studied in detail by
DFT calculations. The title compounds are strongly emissive at
room temperature in the sky-blue region of the visible
spectrum and show quantum yields of up to 71% in a PMMA
matrix.
from saturated N-heterocyclic carbene precursors, namely 1-
aryl-3-methyl-1H-4,5-dihydroimidazolium salts. The title com-
pounds were then synthesized by a multi-step reaction, which
includes an in situ generation of the silver carbene complex,
followed by transmetalation to platinum and subsequent
introduction of the β-diketonate ligand. Structural character-
Introduction
Due to the continuously growing world population the global
energy demand increases constantly. The illumination of indoor
and outdoor spaces requires approximately 20% of the overall
energy consumption. This high demand paved the way for new,
more efficient light sources compared to the traditional light
bulb.[1] A promising technology are organic light emitting
diodes (OLEDs) using phosphorescent transition-metal com-
plexes, especially iridium(III) and platinum(II) compounds, which
are known for their outstanding photophysical properties.[2]
Because of the strong spin orbit coupling of the metal center,
these complexes can in principle reach 100% luminescence
efficiency.[2l,3] It has also been shown that strong donor ligands
and rigid coordination environments, e.g. cyclometalated
ligands like 2-phenylpyridine ðC^NÞ, suppress non-radiative
decay processes and improve the efficiency of these com-
pounds (Figure 1).[2j,3b,4]
Figure 1. Selected cyclometalated Pt(II) complexes.[2j,9o,11]
the more complex synthesis the cyclometalated carbene ligands
Since the first reports of Wanzlick[5] and Öfele[6] on N-
heterocyclic carbenes (NHCs) and the discovery of the first
isolated one by Arduengo,[7] NHCs attracted a large interest in
the field of synthesis of metal complexes and their applications
and today are a very common ligand class.[8] We and others
were able to introduce N-heterocyclic carbenes in emissive
metal complexes, resulting in highly efficient emitters.[9] Despite
(C C ) were developed as an advancement of the well-known
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derivatives of the 2-phenylpyridine because of their stronger
donor capability through the carbene carbon atom. It was
successfully applied next to platinum also with other transition
^
metals.[10] By variation and substitution of the C C ligand the
*
emission properties of phosphorescent complexes can be
customized. In combination with mono anionic bidentate
auxiliary ligands, this results in stable, soluble complexes which
can be processed for use in OLEDs.
[a] S. Stipurin, Prof. Dr. T. Strassner
NHCs with a saturated backbone, especially imidazolinyli-
denes, are also a long and very well-known class of carbenes.[12]
They are used in a wide range of organometallic compounds
and applications, especially in catalysis.[13] Perhaps the most
famous representatives are the ruthenium catalysts of Grubbs
and Hoveyda.[14] In comparison, the number of platinum
complexes is surprisingly low.[15] Also imidazolinylidene com-
plexes carrying a N-aryl substituent with an unsubstituted
ortho-position[16] and cyclometalated complexes are rare.[17] In
fact, to our knowledge only one publication describes this type
Physikalische Organische Chemie, Technische Universität Dresden
01069 Dresden, Germany
E-mail: thomas.strassner@chemie.tu-dresden.de
www.tu-dresden.de/mn/chemie/oc/oc3
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202001077
© 2021 The Authors. European Journal of Inorganic Chemistry published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution Non-Commercial NoDerivs License, which
permits use and distribution in any medium, provided the original work is
properly cited, the use is non-commercial and no modifications or adap-
tations are made.
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doi.org/10.1002/ejic.202001077
of cyclometalated NHC platinum complexes.[11b] Hiraki and
coworkers synthesized a complex (Figure 1), which is very
similar to our previous work based on imidazolylidenes,[18] but
they did not investigate its photophysical properties. Some-
aryl-1H-4,5-dihydroimidazoles 4a–4c. After a quick aqueous
workup and without further purification, they were immediately
converted to the stable 1-aryl-3-methyl-1H-4,5-dihydroimidazo-
lium salts 5a–5c via quaternization with an excess amount of
iodomethane. The crude products were triturated with diethyl
ether to yield the analytically pure ligand precursors.
The platinum(II)complexes 6aa–6cb were synthesized fol-
lowing a multi-step procedure developed in our laboratories.[11a]
The 4,5-dihydroimidazolium salts 5a–5c were reacted with
silver(I)oxide to give the respective silver(I)carbene complexes.
After the addition of Pt(COD)Cl2 the NHC ligand is trans-
metalated to the platinum and subsequently cyclometalated at
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times imidazolinylidenes are attributed
a stronger donor
capability compared to imidazole based carbenes.[15e,19] This
should help to further destabilize and therefore reduce the
accessibility of metal centered electronic states, which contrib-
ute to radiationless relaxation processes in a phosphorescent
complex in the triplet state.[3b] Shifting these levels to higher
energies should lead to a more efficient emitter with higher
quantum yields.
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°
Therefore, we present the synthesis of six novel C C
115 C. In the last step a β-diketone together with a base is
cyclometalated platinum(ii) complexes based on 1-aryl-3-meth-
yl-imidazolinylidene ligands. Following our studies on imidazo-
lylidene systems[9d,11a] we show and compare the effect of the
saturated backbone of the NHC on the photophysical properties
and structures of the platinum complexes.
added. After in situ deprotonation the respective β-diketonate is
transferred to the platinum center. Besides acetylacetonate
(acac), its mesityl substituted derivative (mesacac) was used in
this study, which already showed in previous reports a
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significant impact on the photophysical properties of C C
platinum complexes.[9d,21] The complexes 6aa–6cb were isolated
by flash column chromatography.
Results and Discussion
Synthesis
Structural characterization and thermal stability
The synthesis of the ligand precursors starts from aniline and its
derivatives 1a–1c according to a previously described proce-
dure (Scheme 1).[20] Following this procedure N-arylethylenedi-
amines 2a–2c were formed through a nucleophilic substitution
with 2-bromoethylamine hydrobromide. The crude products
were deprotonated in an alkaline medium, extracted and
purified by column chromatography. Subsequently the dia-
mines 2a–2c were formylated with p-nitrophenyl formate to
form 3a–3c and purified by column chromatography. The ring
closure was performed in an acidic medium to obtain the N-
All compounds were characterized by standard NMR techni-
ques, mass spectrometry and elemental analysis. All platinum
complexes were additionally investigated by 195Pt-NMR (see
Supporting Information). For all compounds, characteristic
signals both in the 1H and the 13C experiments are found for the
methylene groups. The protons generate multiplets in the area
of 2.5–4.5 ppm and the carbon atoms are easily identified by
negative signals in the area of 35–55 ppm in the DEPT 135
experiments. In the proton spectra of ligand precursors 5a–5c
a single-proton singlet appears at approximately 9.3 ppm for
the NCHN group where the carbene will be formed. Compared
to their corresponding imidazolium salts, the signals are shifted
upfield by about 0.5 ppm. The 13C signals for the attached
carbon atoms are located in the area of 155 ppm and are thus
shifted downfield by about 18 ppm.[22] The disappearance of
1
the NCHN signal in the H spectra of the platinum complexes
6aa–6cb indicates deprotonation of the imidazoline motif and
the formation of a carbene. The observed signals in the 195Pt
spectra in the area from À 3303 to À 3381 ppm and the
1
characteristic HÀ 195Pt hetero coupling in the proton spectra for
each compound confirm the formation of the desired com-
plexes and the cyclometalation into the adjacent phenyl ring. In
the acetylacetonate complexes 6aa, 6ba and 6ca the 13C
carbene resonances could be unequivocally assigned by 2D
NMR to be at approximately 173 ppm, which is a significantly
higher value compared to the reported corresponding
imidazole based complexes.[9d,11a] This shift and the ones of the
precursors 5a–5c are an indication of the stronger donor
capability of imidazolinylidenes.[19g] Also a dependence of R1 on
the chemical shift is observed. While the substitution of a
proton with a methyl group shifts the resonance of the carbene
carbon atom from 173.7 ppm for 6aa to 173.0 ppm for 6ca, the
chlorine atom moves it more upfield to 172.9 ppm for
Scheme 1. Synthesis of the ligand precursors and the Pt(II) complexes.
°
(i) BrCH2CH2NH2 · HBr, toluene, reflux. (ii) p-NO2C6H4OCHO, THF, 0 C. (iii)
°
PPSE, CH2Cl2, reflux. (iv) MeI, CH2Cl2, reflux. (v) Ag2O, 45 C. (vi) Pt(COD)Cl2, rt
°
°
to 115 C. (vii) β-diketone, KOtBu, rt to 100 C.
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compound 6ba. This trend is even clearer for the signals in the
the unit cell containing four molecules each. The crystal
structures are similar and show comparable values for all
characteristic bond lengths and angles (Figure 2 and Figure 3).
The values for PtÀ C and PtÀ O bonds are within the
expected range and comparable to structurally related motifs
featuring imidazole based carbenes.[9d,11a,18] Only the C(3)À C(4)
bonds are with 1.533(3) Å (for 6ba) and 1.536(3) Å (for 6ca)
significantly longer and display the single bond character. In
both complexes the central platinum atom forms a slightly
distorted square-planar coordination geometry. Exemplary, in
the chlorine substituted compound 6ba the acetylacetonate
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195Pt spectra (À 3381.4 ppm to À 3374.1 ppm to À 3348.6 ppm).
The analogous aryl substituted complexes 6ab, 6bb and 6cb
show the same characteristics.
The proposed molecular structure was additionally con-
firmed by solid-state structure determinations. Single crystals of
6ba, 6ca and 6cb (Tables S1–S3) were grown by slow diffusion
of n-pentane into a concentrated solution of each compound in
dichloromethane. At first the structures of the unsubstituted
acetylacetonate compounds are discussed. Both platinum(II)
complexes crystallize in the monoclinic space group P21/n with
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°
allows an angle of 90.14(6) allows for O(1)À Pt(1)À O(2), the five-
°
membered metallacycle forms an angle of 81.06(8) for C(2)À Pt-
t(1)À C(6). Noticeably, the imidazolinylidene motif forms an
almost planar cycle despite the sp3 hybridized carbon atoms of
the backbone which is demonstrated by the dihedral angle
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N(1)À C(4)À C(3)À N(2) of 3.9(2) . Both ligands remain almost
perfectly in the coordination plane with dihedral angles O-
°
(1)À Pt(1)À C(2)À N(1) and C(2)À N(2)À C(5)À C(6) under 4 . In the
methyl substituted complex 6ca the angles are found to be
comparable, but the corresponding dihedral angles are by
approximately one degree smaller. In the solid state the
molecules form face to face pairs, which align offset to each
other (see Supporting Information, Figure S1 and Figure S2).
The molecular pairs suggest no direct PtÀ Pt interaction. The
shortest intermolecular distance is found to be 6.8107(4) Å for
6ba respectively 6.8271(4) Å for 6ca, which is above their van
der Waals radii. In contrast the mesityl substituted platinum
complex 6cb crystallizes in the triclinic space group P-1. The
unit cell contains four molecules of the presented compound,
where two molecules form the asymmetric unit. It was also
found that solvent molecules co-crystallized in a disordered
way, which could not be properly described, so they were
omitted through the SQUEEZE function of PLATON.[24]
Figure 2. Mercury[23] illustration of solid-state structure of compound 6ba.
Thermal ellipsoids at 50% probability. Selected bond lengths [Å] and angles
°
[ ] for 6ba: Pt(1)À C(2) 1.9561(19), Pt(1)À C(6) 1.9851(19), Pt(1)À O(1)
2.0807(14), Pt(1)À O(2) 2.0486(13), C(3)À C(4) 1.533(3); C(2)À Pt(1)À C(6) 81.06(8),
O(1)À Pt(1)À O(2) 90.14(6); O(1)À Pt(1)À C(2)À N(1) 2.9(2), C(2)À N(2)À C(5)À C(6)
1.3(2), N(1)À C(4)À C(3)À N(2) 3.9(2).
The bond lengths from the metal center to the surrounding
atoms and the one in the backbone of the NHC are very similar
for the mesacac complex 6cb compared to the previously
discussed bonds of 6ba and 6ca (Figure 4). The bond angles
°
C(2)À Pt(1)À C(6) and O(1)À Pt(1)À O(2) are with 80.70(8) respec-
°
tively 88.91(5) slightly reduced. Remarkably are the changes of
the dihedral angles, especially O(1)À Pt(1)À C(2)À N(1) with
°
°
À 7.0(2) and N(1)À C(4)À C(3)À N(2) with À 8.1(2) . The introduc-
tion of the bulky groups results in a more distorted geometry
where the auxiliary ligand bends slightly out of the coordina-
tion plane. In the solid state the molecules form stacks, in which
the complexes are twisted so that the C C ligands partially lay
over each other and the shortest PtÀ Pt distance is found to be
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6.4293(5) Å (Figure S3).
Since the novel platinum complexes are potential com-
pounds for application in OLEDs and therefore vapor deposition
techniques, the thermal stability of all compounds 6aa–6cb
was investigated by thermogravimetric analyses (TGA) (Figur-
es S4–S9). Almost all complexes appear highly stable up to a
minimum of 280 C, only compound 6cb already shows a mass
loss of 5% at 204 C. This point of decomposition for
compounds with an unsubstituted C C ligand 6aa (284 C),
6ab (285 C) and the methyl derivative 6ca (288 C) was
Figure 3. Mercury[23] illustration of solid-state structure of compound 6ca.
Thermal ellipsoids at 50% probability. Selected bond lengths [Å] and angles
°
°
[ ] for 6ca: Pt(1)À C(2) 1.956(2), Pt(1)À C(6) 1.989(2), Pt(1)À O(1) 2.0902(15),
°
Pt(1)À O(2) 2.0500(14), C(3)À C(4) 1.536(3); C(2)À Pt(1)À C(6) 80.99(8), O(1)À Pt-
(1)À O(2) 89.74(6); O(1)À Pt(1)À C(2)À N(1) 1.1(2), C(2)À N(2)À C(5)À C(6) 0.0(3),
N(1)À C(4)À C(3)À N(2) 2.9(2).
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°
°
°
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Figure 5. Quantitative UV/Vis absorption spectra for all platinum complexes
in dichloromethane solutions (c=5×10À 5 molLÀ 1) at room temperature.
Figure 4. Mercury[23] illustration of solid-state structure of compound 6cb.
Thermal ellipsoids at 50% probability. Second molecule of the asymmetric
°
unit omitted for clarity. Selected bond lengths [Å] and angles [ ] for 6cb:
Pt(1)À C(2) 1.9556(19), Pt(1)À C(6) 1.9891(18), Pt(1)À O(1) 2.0908(13), Pt(1)À O(2)
2.0512(13), C(3)À C(4) 1.529(3); C(2)À Pt(1)À C(6) 80.70(8), O(1)À Pt(1)À O(2)
88.91(5); O(1)À Pt(1)À C(2)À N(1) À 7.0(2), C(2)À N(2)À C(5)À C(6) À 2.4(2), N-
(1)À C(4)À C(3)À N(2) À 8.1(2).
appears, which is very close together for all complexes
(280 nm–285 nm). For the unsubstituted platinum complex 6aa
it is found at 282 nm (280 nm for 6ab), for the chlorine
substituted 6ba at 285 nm (284 for 6bb) and for the methyl
substituted compound 6ca it is in between the two values at
284 nm (283 nm for 6cb). Two more broad bands appear
around 260 nm and 247 nm. Interestingly, while for the
acetylacetonate complexes 6aa and 6ba the higher energy one
is stronger, for the methyl substituted complex 6ca the band
with the lower energy is the dominant one, which is also the
trend for the aryl substituted complexes.
For the photoluminescence measurements poly(methyl
methacrylate) (PMMA) films of all title compounds with an
emitter load of 2 wt% were prepared. This polymer matrix
should best resemble the amorphous environment within a
PhOLED.[3b] All novel platinum complexes show photolumines-
cence in the sky-blue area with moderate to good photo-
luminescence quantum yields (PLQY) in the range from 24 to
71%. The phosphorescence decay time τ0 was measured to be
between 5 and 25 μs (Table 1).
observed in a narrow temperature range. For the chlorine
°
°
substituted complexes 6ba (302 C) and 6bb (304 C) the 5%
mass loss was moved to slightly higher temperatures.
Photophysical characterization
Absorption and emission spectra at room temperature were
recorded to investigate the photophysical properties of the
novel platinum(II) complexes. Additionally, the quantum yield,
colour coordinates and luminescence lifetimes were measured
of all six compounds.
The absorption measurements were performed in dichloro-
methane solutions with an analyte concentration of 5×
10À 5 molLÀ 1 (Figure 5). All title compounds show similar spectra
with several broad absorption bands in the region of 225–
450 nm, with the aryl substituted acetylacetonate complexes
6ab, 6bb and 6cb absorbing stronger. The absorptions start
near the UV area with a flat increase and two weaker transitions
before at approximately 280 nm the first significant band
The obtained emission spectra are given in Figure 6. The
complexes show emission maxima in the region from 470 to
490 nm. The wavelength depends slightly on the substitution in
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the C C ligand. For the acetylacetonate complexes 6aa, 6ba
Table 1. Photoluminescence data of all title compounds. PL in a PMMA matrix (2 wt% emitter load, λexc =305 nm).
[b]
[d]
[f]
Φ[a]
[%]
λem
[nm]
CIE[c]
x; y
τ0
kr[e]
knr
[μs]
[103 ·sÀ 1
]
[103 ·sÀ 1
]
6aa
6ab
6ba
6bb
6ca
6cb
29
71
38
70
24
42
470
486
473
476
481
490
0.168; 0.221
0.219; 0.376
0.172; 0.249
0.189; 0.306
0.180; 0.284
0.231; 0.398
23
5
25
7
23
8
44.0
195.0
40.9
136.7
43.0
107.8
79.7
66.7
58.6
136.1
177.0
128.2
[a] PLQY at room temperature and λexc =305 nm. [b] Emission wavelength with maximum intensity at room temperature. [c] CIE colour coordinates. [d] τ0
given as τ0 =τexp/Φ. [e] kr =Φ/τexp. [f] knr =(1-Φ)/τexp
.
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triple-zeta 6-311G* basis set and dispersion correction (D3BJ).
1
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3
Compared to the solid-state structure the combination PBE0/6-
311G* gave the most reliable results for bond lengths and
angles.
4
Time-dependent DFT calculations of the ground state
structure of the platinum complex 6ba in dichloromethane as a
solvent were used to simulate the respective absorption
spectrum and to assign the experimentally observed bands to
orbital transitions. The calculated and experimental spectra are
compared in Figure 7. While the broad low energy band at
approximately 310 nm and the intense bands in the area of
250–260 nm are described well through the calculated excited
states 2, 8 and 11, the significant band at approximately
280 nm is clearly underestimated by the state 3.
To visualize these processes, the respective natural tran-
sition orbitals (NTOs) were calculated (Figure 8). The low energy
excitation process described by the second state occurs from
orbitals which are spread mainly across the cyclometalated
ligand and the platinum center to orbitals almost exclusively on
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Figure 6. Emission spectra for title compounds in a PMMA matrix (2 wt%
emitter load, λexc =305 nm) at room temperature.
the acetylacetonate fragment. Consequently, this state can be
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defined as a LLCT (C C to acac) transition mixed with a MLCT
and 6ca a bathochromic shift is observed from the unsubsti-
tuted compound (470 nm) to chlorine (473 nm) to methyl
substitution (481 nm). For all three compounds a moderate
quantum yield was measured. Compared to analogous
imidazole based platinum complexes the emission maximum is
shifted to lower energies by about 30 nm, but in the case of no
substitution on the phenyl moiety the PLQY is raised to
approximately four times the value.[11a] The phosphorescence
decay time remains at the same level.
Interestingly, the “substitution shift” of the emission wave-
length is different for the mesacac complexes 6ab, 6bb and
6cb, here the chlorine derivative shows the highest energy
emission. The mesityl groups result in a bathochromic shift of
up to 16 nm and a significant increase of the quantum yield to
up to 71%. Also, the decay times are reduced to 5–8 μs. This
effect of the auxiliary ligand is also known from analogous
imidazolylidene complexes, but much stronger.[9d] While the
acetylacetonate compounds still show slightly structured
emission bands, the introduction of the mesacac leads to a
change (in the case of 6bb) or complete disappearance of the
vibrational structures, which is a hint for an increased metal-to-
ligand charge-transfer (MLCT) process during the emission,
which was also observed for the analogous imidazolylidene
compounds.[9d]
transition. The third state is dominated by a transition from the
metal center to the orbitals across both ligands (MLCT). For the
high energy transitions visualized by state 8 and 11 on the
other hand both the hole and the particle orbitals are
distributed over the whole molecule.
For all platinum(II) complexes the ground and excited state
structures were optimized. The results are given exemplarily for
the benchmark system 6ba in Figure 9 (see Figures S10–S14 for
all other complexes). The ground state structure with a singlet
multiplicity (Figure 9, left) closely resembles the molecular
geometry obtained by X-ray diffraction. For the optimized
excited triplet state two different structures on this level of
theory were found (Figure 9, right). Both show significant
distortions during the transition to the excited state. The
auxiliary ligand is bent either up or down out of the
coordination plane of the platinum center, with the second
structure slightly thermodynamically favored by 0.03 kcal/mol.
DFT Calculations
To better understand the photophysical properties and pro-
cesses in the presented novel platinum(II) complexes, density
functional theory calculations using the Gaussian16 program
package were employed. In all calculations, platinum was
described by a LANL2DZ ECP and basis set. For all applications,
a benchmark process was carried out based on the crystal
structure of 6ba. The hybrid functionals B3LYP and PBE0 and
the local GGA functional PBE were tested together with the
Figure 7. Comparison of experimental (green) and calculated (grey, 0.333 eV
half-width) absorbance spectrum. Oscillator strengths given as obtained
from calculation.
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Figure 10. Calculated frontier molecular orbitals for the ground state (left
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and center, isoval=0.03) and the spin density for the thermodynamically
favoured excited state (right, density=0.002) for 6ba.
auxiliary ligand on the emissive process. Almost no density is
located on the NHC ligand, so this state can be described as
MLCT/LCacac (ligand centered) state. Comparable results are also
obtained for the complexes 6aa and 6bb. This emissive state
was also observed in analogous imidazolylidene based
complexes.[9d]
Interestingly, for the other three investigated complexes in
this study emissive states with linear geometries were achieved
and the calculated spin densities suggest MLCT/LCMLCT=LCC^
C
*
emissive states as exemplary shown in Figure 11 for compound
6cb. Recent theoretical investigations of the triplet hypersur-
^
*
Figure 8. NTO analysis (isoval=0.03) of compound 6ba for the transitions to
excited states highlighted in Figure 7.
face of phosphorescent C C platinum complexes showed that
the choice of the functional in DFT simulations influences the
obtained calculated triplet geometries drastically and the
assignment of the emissive state should be done with care.[26c,27]
Regardless, both linear and bent triplet geometries in this study
lead to triplet emitting compounds 6ab and 6bb with the
same level of efficiency of up to 71%.
Figure 9. Optimized geometries for compound 6ba in the ground state (left)
and in the excited triplet states (right). Side views (bottom) relative to the
square-planar coordination of the platinum atom.
According to Kasha’s rule this should be the emissive state.[25]
This kind of behavior was already observed earlier for similar
compounds.[18,21a,26]
To generate a better insight into the emissive process, the
frontier molecular orbitals of the ground state and the spin
density of the emissive state were calculated (Figure 10,
Figures S15–S19 for all other complexes). While the highest
^
*
occupied molecular orbital is dominantly spread over the C C
ligand and the platinum center, the lowest unoccupied orbital
shifts almost completely to the acetylacetonate ligand. Some
electron density remains on the NHC motif and the metal atom.
The spin density also demonstrates the crucial influence of the
Figure 11. Emissive state for compound 6cb. Optimized geometry in the
excited triplet state (top left) and side view relative to the square-planar
coordination of the platinum atom (bottom left). Spin density for the excited
state (right, density=0.002).
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Electrochemistry
The energies of HOMOs and LUMOs of the complexes were
1
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3
4
5
6
7
8
9
calculated from the observed redox potentials using a relation-
ship published by Forrest and co-workers.[28] For the acetylacet-
onate complexes this results in HOMO energies between
À 5.04 eV and À 4.93 eV and a LUMO energy of À 1.36 eV. The
values for the aryl substituted β-diketonate complexes are
shifted again, here to more stabilized levels. For the HOMOs
energies of À 5.38 eV (6ab), À 5.43 eV (6bb) and À 5.23 eV (6cb)
are calculated. The LUMOs were found to have values of
À 1.75 eV, À 1.80 eV and À 1.74 eV, respectively, leading to
HOMO-LUMO gaps of 3.49–3.68 eV.
Additionally, the electrochemical behavior of all complexes
6aa–6cb was investigated using cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) (Figure 12, for detailed CV
and DPV spectra see Figures S20–S28). All presented potentials
are derived from the DPV experiments and refer to the first
observable redox event (Table 2). While the oxidation events for
all novel compounds were irreversible, the first reduction steps
appeared to be (quasi-)reversible. The oxidative potentials Eox of
the acac complexes 6aa (0.24 V), 6ba (0.31 V) and 6ca (0.24 V)
are apparently slightly affected by the substituent of the
cyclometalated motif, which is in agreement with the DFT
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calculations as the HOMOs are dominantly spread across the Conclusions
^
*
C C ligand. The reductive potentials Ered for these three
^
*
complexes on the other hand are all found at same value of
approximately À 2.88 V matching the high contribution of the
acac ligand to the LUMOs. The introduction of the mesacac
ligand leads to a shift of both the oxidative and the reductive
potentials to more positive voltage values (0.45 V–0.59 V for Eox,
À 2.56 V–À 2.51 V for Ered), but with the retention of the ligand
influence on the orbitals. Similar results were also found for
Phosphorescent C C
cyclometalated complexes with
imidazole-based ligands and their photophysical properties
have been studied for years. The emissive behavior of their
saturated analogues, on the other hand, is less known, only
being mentioned in patents.[17i,j]
In this manuscript we report the synthesis of novel
platinum(II) complexes based on bidentate C C cyclometalated
^
*
^
[9d–g,i,j]
*
structurally related C C platinum complexes.
NHC ligands with a saturated backbone and present a system-
atic study of their thermal stability, photophysical properties
and electrochemical behavior. Preparation of the carbene
ligand precursors was achieved starting from commercially
available aniline derivatives, allowing for different substitutions
in the phenyl motif. The proposed structures were confirmed by
NMR experiments as well as solid-state structures. Both
methods prove the single bond character of the backbone. TGA
experiments revealed a high stability of the complexes over a
broad temperature range. The photophysical properties of the
presented complexes were investigated in detail by UV/Vis
spectroscopy and photoluminescence measurements. The plat-
inum complexes showed strong emissions in the sky-blue
region of the visible spectrum, with photoluminescence
quantum yields ranging in the scope of 24–71% and phosphor-
escence decay time from 5 to 25 μs. The photoluminescence
measurements were combined with DFT calculations to assign
atomic electron transitions and to visualize the properties in the
ground and excited state of the complexes. While the HOMO of
the ground state is spread dominantly across the cyclometalat-
ing motif of the N-heterocyclic ligand and the platinum center,
the excited triplet state is strongly influenced by its proposed
geometry. For half of the platinum compounds a bend MLCT/
Figure 12. Cyclic voltammograms of all complexes 6aa–6cb. Measured in
DMF and internally referenced against Fc/Fc+.
Table 2. Redox potentials and calculated energies for HOMO, LUMO and band gap of all title compounds.
[a]
[b]
[c]
[d]
[e]
Eox
[V]
EHOMO
[eV]
Ered
[V]
ELUMO
[eV]
Eg
[eV]
6aa
6ab
6ba
6bb
6ca
6cb
0.24
0.56
0.31
0.59
0.24
0.45
À 4.93
À 5.38
À 5.04
À 5.43
À 4.94
À 5.23
À 2.88
À 2.54
À 2.88
À 2.51
À 2.87
À 2.56
À 1.36
À 1.75
À 1.36
À 1.80
À 1.36
À 1.74
3.58
3.63
3.68
3.63
3.57
3.49
[a] Derived from DPV data, referenced internally vs. Fc/Fc+. [b] EHOMO [eV]=À 1.4*EoxÀ 4.6.[28a] [c] Derived from DPV data, referenced internally vs. Fc/Fc+. [d]
ELUMO [eV]=À 1.19*EredÀ 4.78.[28b] [e] Eg = jEHOMOÀ ELUMO j.
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negative eigenvalues. UV/Vis spectra and electronic transitions
LCacac emissive state is indicated, whereas the linear emissive
1
2
3
4
5
were calculated using TD-DFT methods (singlet, nstates=50, CPCM,
solvent=dichloromethane) as implemented in the Gaussian16
package. Calculated geometries were visualized with GaussView.[39]
The coordinates of the optimized structures are given in the
Supporting Information.
states can be described as MLCT=LCC^ . The contributions of
C
*
the ligands to the frontier molecular orbitals were additionally
confirmed by voltammetry experiments.
6
7
8
9
Electrochemistry: Cyclic voltammetry and differential pulse voltam-
metry experiments were carried out employing a Biologic SP-150
potentiostat with a glassy carbon working electrode, a platinum
wire counter electrode and an Ag/Ag+ pseudo reference electrode.
All complexes were measured as 0.5 mM solutions in degassed,
anhydrous DMF along with 0.1 M supporting electrolyte (N(n-
Bu)4ClO4) at room temperature. The CV measurements were
conducted with a sweep rate of 100 mV/s while the DP voltammo-
grams were recorded with a scan rate of 50 mV/s. All measurements
were internally referenced against ferrocene/ferrocenium (Fc/Fc+).
Experimental Section
General considerations: Compounds 3a–3c, 4a–4c and the
platinum complexes were synthesized in flame dried Schlenk tubes
under argon atmosphere, all other compounds were synthesized in
dried standard laboratory glassware under air. Solvents of at least
99.0% purity were used in all reactions in this study. Dimethyl-
formamide (DMF) was dried using standard techniques and stored
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over
molecular
sieve
and
(3 Å).
bis-1,3-(2,4,6-
Dichloro(cycloocta-1,5-diene)platinum(II)[29]
trimethyl-phenyl)propan-1,3-dione[30] were prepared according to
modified known literature procedures. The solution of trimethylsilyl
Synthesis and Characterization: A detailed description of the
synthetic procedures is given below exemplarily for each class of
compounds. The syntheses and analytical data of the analogously
prepared compounds are given in the Supporting Information.
polyphosphate (PPSE) was prepared according to
a known
literature procedure.[31] All other compounds were obtained from
1
common suppliers and were used without further purification. H-,
13C- and 195Pt-NMR spectra were acquired on Bruker NMR
Avance300, Bruker DRX 500 and Bruker Avance600 NMR spectrom-
Compound 2a: In a 250 mL round-bottom flask, a mixture of
3.568 g 2-bromoethylamine hydrobromide (98%, 17.1 mmol) and
4.777 g aniline (51.2 mmol, 3 equiv) in 64 mL toluene is refluxed for
4 h. After cooling to room temperature, the mixture is treated with
80 mL of 20% aq. NaOH until the precipitate dissolves and the
aqueous phase is extracted with dichloromethane. The combined
organic phases are washed with distilled water and dried over
magnesium sulfate. The solvent is removed in vacuo and the
product is isolated by column chromatography (dichloromethane-
1
eters. H and 13C spectra were referenced internally (1H: 7.26 ppm,
13C: 77.16 ppm for CDCl3; 1H: 2.50 ppm, 13C: 39.43 ppm for DMSO-d6;
1H: 5.32 ppm, 13C: 54.00 ppm for CD2Cl2). 195Pt spectra were
referenced externally by using potassium tetrachloroplatinate(II) in
D2O (PtCl42À : À 1617.2 ppm). Chemical shifts are given in ppm,
coupling constants J in Hz. The spectra are given in the Supporting
Information (Figures S29–S88). EI mass spectra were recorded by
GC-MS coupling on an Agilent Technologies 6890N GC system
equipped with a 5973 N mass-selective detector (70 eV). ESI mass
spectra were recorded on a Bruker Esquire LC mass spectrometer
with an ion-trap detector. Positive and negative ions were detected.
Elemental analyses were performed by the microanalytical labora-
tory of our institute on a Hekatech EA 3000 Euro Vector elemental
analyzer. Melting points were determined by using a Wagner and
1
methanol, 7:3 to 1:9) as a highly viscous liquid (1.623 g, 70%). H-
NMR (300 MHz, CDCl3): δ (ppm)=7.23–7.13 (m, 2H, CHarom), 6.71 (t,
3J=7.3 Hz, 1H, CHarom), 6.65 (d, 3J=7.7 Hz, 2H, CHarom), 4.00 (br s, 1H,
NH), 3.19 (t, 3J=5.8 Hz, 2H, NCH2CH2N), 2.96 (t, 3J=5.8 Hz, 2H,
NCH2CH2N), 1.32 (br s, 2H, NH2). 13C-NMR (75 MHz, CDCl3): δ (ppm)=
148.5 (Ci), 129.4 (CHarom), 117.6 (CHarom), 113.1 (CHarom), 46.7
(NCH2CH2N), 41.3 (NCH2CH2N). MS (EI, 70 eV) m/z (%): 136 (17) [M]+,
106 (100) [MÀ CH2NH2]+. Due to partial decomposition during work-
up no satisfying elemental analysis was received.
Munz PolyTherm
A system and are given uncorrected. The
thermogravimetric analyses were conducted with a Netzsch TG209
F1 Libra using Al2O3 crucibles under argon atmosphere (sample
chamber evacuated and refilled twice prior to analysis, 100 ml/min)
Compound 3a: 1.623 g of 2a (11.9 mmol) are dissolved in 18 mL
°
dry THF and cooled down to 0 C. Under stirring 2.033 g 4-
°
and a heat rate of 5 K/min (25–500 C).
nitrophenyl formate (98%, 11.9 mmol, 1 equiv) is added in small
portions to the solution in a time period of 15 min. After 2 h of
stirring at the same temperature the solvent is removed in vacuo,
the residue is dissolved in ethyl acetate and washed with an
aqueous saturated NaHCO3 solution. The aqueous phase is
extracted with ethyl acetate once and the combined organic phases
are dried over magnesium sulfate. The solvent is removed in vacuo
and the crude product is purified by column chromatography with
ethyl acetate as eluent affording the product as a high viscous
Photoluminescence and absorption measurements: The 2 wt%
emitter films were prepared by doctor blading a solution of an
emitter in a 10 wt% PMMA solution in dichloromethane on a quartz
substrate with a 60 mm doctor blade. After drying of the film, the
emission was measured under nitrogen atmosphere. Excitation was
conducted in a wavelength range of 280–400 nm (Xe lamp with a
monochromator), and the emission was detected with a calibrated
quantum yield detection system (Hamamatsu, model C9920-02).
The phosphorescence decay was measured with an Edinburgh
Instruments mini-t by excitation with pulses of an EPLED (360 nm,
20 kHz) and time-resolved photon counting (TCSPC). Absorption
spectra were measured on a PerkinElmer Lambda 365 UV/Vis
spectrometer in dichloromethane solutions with an analyte concen-
1
liquid (1.487 g, 76%). H-NMR (300 MHz, CDCl3): δ (ppm)=8.21 (s,
1H, OCH), 7.23–7.12 (m, 2H, CHarom), 6.79–6.68 (m, 1H, CHarom), 6.67–
6.58 (m, 2H, CHarom), 5.97 (br s, 1H, NH), 3.60–3.50 (m, 2H,
NCH2CH2N), 3.36–3.28 (m, 2H, NCH2CH2N). 13C-NMR (75 MHz, CDCl3):
δ (ppm)=161.9 (OCH), 147.9 (Ci), 129.5 (CHarom), 118.1 (CHarom), 113.0
(CHarom), 43.9 (NCH2CH2N), 37.9 (NCH2CH2N). MS (EI, 70 eV) m/z (%):
164 (12) [M]+, 106 (100) [MÀ CH2NHCHO]+. Anal. Calcd for C9H12N2O:
C 65.83, H 7.37, N 17.06, found: C 65.77, H 7.52, N 17.12.
tration of 5×10À 5 molLÀ 1
.
Computational details: The Gaussian16[32] package was used to
perform all quantum chemical calculations employing the hybrid
functionals B3LYP[33] and PBE0[34] and the local GGA functional
PBE.[35] The 6-311G*[36] basis set were used. Platinum was described
by the LANL2DZ ECP and basis set.[37] Dispersion forces were
simulated by using the D3 dispersion correction with Becke-
Johnson damping (D3BJ).[38] All given structures were verified as
true minima by vibrational frequency analysis and the absence of
Compound 4a: 1.409 g of 3a (8.6 mmol) are dissolved in 30 mL of a
dichloromethane solution of PPSE and refluxed for 2 h. After
cooling to room temperature, the mixture is extracted with distilled
water and the combined aqueous phases were made alkaline with
Na2CO3 to pH 9–10. Afterwards the aqueous phase is extracted with
dichloromethane and the combined organic phases are washed
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doi.org/10.1002/ejic.202001077
with distilled water once. After drying over magnesium sulfate, the
solvent is removed in vacuo to afford the crude product, which is
immediately converted in the next reaction.
Conflict of Interest
1
2
3
4
5
The authors declare no conflict of interest.
Compound 5a: Crude 4a from the previous reaction and 4.100 g
iodomethane (99%, 28.6 mmol, 3.3 equiv) are dissolved in 29 mL
dichloromethane and refluxed for 16 h. The solvent is removed in
vacuo and the residue is washed with small portions of THF and
Keywords: Imidazolinylidene ligands
Phosphorescence · Platinum · Structure elucidation
· Carbene ligands ·
6
diethyl ether. After drying in vacuo, the product is afforded as a
7
8
9
1
°
light-coloured solid (1.046 g, 42% for two steps). M. p. 105 C. H-
NMR (300 MHz, DMSO-d6): δ (ppm)=9.34 (s, 1H, NCHN), 7.54–7.45
3
3
[1] F. G. Montoya, A. Peña-García, A. Juaidi, F. Manzano-Agugliaro, Energy
Build. 2017, 140, 50–60.
(m, 2H, CHarom), 7.37 (d, J=7.8 Hz, 2H, CHarom), 7.29 (t, J=7.3 Hz,
1H, CHarom), 4.45–4.27 (m, 2H, NCH2CH2N), 4.16–4.01 (m, 2H,
NCH2CH2N), 3.28 (s, 3H, NCH3). 13C-NMR (75 MHz, CDCl3): δ (ppm)=
155.1 (NCHN), 136.5 (Ci), 129.8 (CHarom), 125.9 (CHarom), 117.2 (CHarom),
50.4 (NCH2CH2N), 48.0 (NCH2CH2N), 35.1 (CH3). MS (ESI) m/z: 161
[MÀ IÀ ]+. Anal. Calcd for C10H13IN2: C 41.69, H 4.55, N 9.72, found: C
41.53, H 4.54, N 9.87.
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[2] a) M. Thompson, MRS Bull. 2007, 32, 694–701; b) K. Li, G. S. Ming Tong,
Q. Wan, G. Cheng, W. Y. Tong, W. H. Ang, W. L. Kwong, C. M. Che, Chem.
Sci. 2016, 7, 1653–1673; c) J. Song, H. Lee, E. G. Jeong, K. C. Choi, S. Yoo,
Adv. Mater. 2020, 1907539, 1–17; d) J. H. Lee, C. H. Chen, P. H. Lee, H. Y.
Lin, M. K. Leung, T. L. Chiu, C. F. Lin, J. Mater. Chem. C 2019, 7, 5874–
5888; e) H. T. Mao, G. F. Li, G. G. Shan, X. L. Wang, Z. M. Su, Coord. Chem.
Rev. 2020, 413, 213283; f) M. Sarma, K. T. Wong, Chem. Rec. 2019, 19,
1667–1692; g) H. Xu, R. Chen, Q. Sun, W. Lai, Q. Su, W. Huang, X. Liu,
Chem. Soc. Rev. 2014, 43, 3259–3302; h) T. Sajoto, P. I. Djurovich, A.
Tamayo, M. Yousufuddin, R. Bau, M. E. Thompson, R. J. Holmes, S. R.
Forrest, Inorg. Chem. 2005, 44, 7992–8003; i) V. Adamovich, J. Brooks, A.
Tamayo, A. M. Alexander, P. I. Djurovich, B. W. D’Andrade, C. Adachi,
S. R. Forrest, M. E. Thompson, New J. Chem. 2002, 26, 1171–1178; j) J.
Brooks, Y. Babayan, S. Lamansky, P. I. Djurovich, I. Tsyba, R. Bau, M. E.
Thompson, Inorg. Chem. 2002, 41, 3055–3066; k) J. Li, P. I. Djurovich,
B. D. Alleyne, M. Yousufuddin, N. N. Ho, J. C. Thomas, J. C. Peters, R. Bau,
M. E. Thompson, Inorg. Chem. 2005, 44, 1713–1727; l) H. Yersin, W. J.
Finkenzeller, Highly Effic. OLEDs with Phosphorescent Mater. (Ed.: H.
Yersin), Wiley-VCH, Weinheim 2008, pp. 1–89; m) C. Cebrián, M. Mauro,
Beilstein J. Org. Chem. 2018, 14, 1459–1481; n) Z. Chen, C. L. Ho, L.
Wang, W. Y. Wong, Adv. Mater. 2020, 32, 1–45.
Compound 6aa: 0.231 g 5a (0.8 mmol) and 0.093 g silver(I) oxide
(0.8 mmol, 0.5 equiv) are dissolved in 20 mL dry DMF and heated to
°
45 C for 4 h under exclusion of light. After adding 0.299 g Pt(COD)
Cl2 (0.8 mmol, 1 equiv), the mixture is stirred at room temperature
°
for 24 h hours and subsequently heated to 115 C for another
21 hours. The solution is cooled to room temperature, 0.160 g
acetylacetone (1.6 mmol, 2 equiv) and 0.180 g potassium tert-
butoxide (1.6 mmol, 2 equiv) are added and the mixture is stirred at
°
room temperature for 24 hours and afterwards at 100 C for 6 h. All
volatiles are removed in vacuo, dichloromethane is added to the
residue and the mixture is treated in an ultrasonic bath. The slurry
is filtered over a pad of celite, the solvent is removed in vacuo and
the crude product is purified by column chromatography with
dichloromethane as eluent. After removal of all volatiles under
reduced pressure, the analytically pure product is obtained after
[3] a) H. Yersin, Top. Curr. Chem. 2004, 241, 1–26; b) H. Yersin, A. F. Rausch,
R. Czerwieniec, T. Hofbeck, T. Fischer, Coord. Chem. Rev. 2011, 255,
2622–2652.
[4] Y. Chi, P.-T. Chou, Chem. Soc. Rev. 2010, 39, 638–655.
[5] H.-W. Wanzlick, H.-J. Schönherr, Angew. Chem. Int. Ed. Engl. 1968, 7,
washing the solid with small portions of n-pentane and diethyl
1
°
ether and drying in vacuo (0.127 g, 35%). Dec. pt.: 295 C. H-NMR
141–142.
(300 MHz, CDCl3): δ (ppm)=7.80–7.54 (m, 1H, PtCCHarom), 6.97 (td,
3J=7.5 Hz, 4J=1.2 Hz, 1H, CHarom), 6.83 (td, 3J=7.5 Hz, 4J=1.2 Hz,
[6] K. Öfele, J. Organomet. Chem. 1968, 12, P42–P43.
[7] A. J. Arduengo, R. L. Harlow, M. Kline, J. Am. Chem. Soc. 1991, 113, 361–
363.
3
4
1H, CHarom), 6.36 (dd, J=7.5 Hz, J=1.1 Hz, 1H, CHarom), 5.48 (s, 1H,
CHacac), 3.89–3.79 (m, 2H, NCH2CH2N), 3.79–3.68 (m, 2H, NCH2CH2N),
3.64 (s, 3H, NCH3), 2.05 (s, 3H, OCCH3), 1.96 (s, 3H, OCCH3). 13C-NMR
(151 MHz, CDCl3): δ (ppm)=185.4 (CO), 185.2 (CO), 173.7 (NCN),
149.8 (Ci), 130.5 (PtCCHarom), 124.0 (CHarom), 121.5 (PtCCHarom), 120.4
(CHarom), 107.2 (CHarom), 102.0 (CHacac), 53.3 (NCH2CH2N), 43.8
(NCH2CH2N), 34.2 (CH3, NHC), 28.1 (CH3, acac), 28.0 (CH3, acac). 195Pt-NMR
(64.52 MHz, CDCl3): δ (ppm)=À 3381.4 (s). MS (ESI) m/z: 807 [2 M-
acac]+. Anal. Calcd for C15H18N2O2Pt: C 39.74, H 4.00, N 6.18, found:
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Deposition Numbers 2024972 (for 6ca), 2024973 (for 6cb), and
2024974 (for 6ba) contain the supplementary crystallographic data
for this paper. These data are provided free of charge by the joint
Cambridge Crystallographic Data Centre and Fachinformationszen-
trum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/
structures.
Acknowledgements
We are very grateful to the ZIH Dresden for allocating computa-
tional time on their high performance computing system. We also
thank Dr. Ivana Císařová (Charles University Prague) for her help
with the solid state structure refinement of 6cb. Open access
funding enabled and organized by Projekt DEAL.
Eur. J. Inorg. Chem. 2021, 804–813
www.eurjic.org
812
© 2021 The Authors. European Journal of Inorganic Chemistry published
by Wiley-VCH GmbH

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Manuscript received: November 26, 2020
Revised manuscript received: December 20, 2020
Eur. J. Inorg. Chem. 2021, 804–813
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by Wiley-VCH GmbH