CˆC*-cyclometalated platinum(II) complexes with trifluoromethyl-acetylacetonate ligands – Synthesis and electronic effects

Journal of Organometallic Chemistry 730 (2013) 37e43

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C C*-cyclometalated platinum(II) complexes with trifluoromethyl-acetylacetonate ligands e Synthesis and electronic effects Alexander Tronnier, Nicole Nischan, Thomas Strassner* Physikalische Organische Chemie, Technische Universität Dresden, Bergstrasse 66, 01062 Dresden, Germany

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 June 2012 Received in revised form 21 July 2012 Accepted 26 July 2012

A series of C C*-cyclometalated platinum(II) trifluoromethyl-acetylacetonate complexes could be synthesized via a new synthetic route. They have been characterized by standard methods (NMR, EA) as well as three solid-state structures. Quantum chemical calculations on the HOMO and LUMO levels show significant differences to the previously reported acetylacetonate derivatives. The influence of the CF3groups on the photophysical properties is studied in detail. Ó 2012 Elsevier B.V. All rights reserved.

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Functional molecular materials based on platinum(II) complexes for the application as phosphorescent emitters in organic light emitting diodes (OLEDs) have found increasing interest during the last decade [1e9]. The ligands have been designed in a way that efficient phosphorescence could be observed for square-planar platinum(II) complexes [10e13]. Frequently those ligands contain aromatic N-donor or cyclometalated ligands [12,14]. Various heterocycles with aryl rings in a position which allows to cyclometalate them, e.g. 2-phenylpyridyl, 2-(20 -thienyl)pyridyl or 2phenylbenzothiazole have been used. These bidentate ligands generally interact with the metal via the nitrogen atom and the cyclometalated carbon atom (Scheme 1, NPy C*) [12,14]. Often these complexes contain additional bidentate monoanionic ligands based on the b-diketonate motif [12,13,15]. Changes to the b-diketonate backbone or its substituents have shown a significant effect on the emission properties and were used to tune the emission wavelengths [16]. For iridium(III) complexes it was already known [15,17e19] that the 2-pyridine motif can be replaced by a N-heterocyclic carbene ligand, leading to the C C* ligand shown above (Scheme 1), but for the square-planar platinum(II) complexes only one example was known. We could recently demonstrate that also platinum(II) complexes with

donating carbene carbon atoms are accessible from aryl substituted imidazolium compounds [11], although the synthesis has its difficulties which has been independently confirmed by another group [20]. Compounds with blue phosphorescent emission at room temperature are of general interest and we were interested how an electronic variation at the acetylacetonate ligand system would influence the emission properties [21]. There are examples in the literature where substitution of CH3 by CF3 leads to a completely different electronic structure [22e25]. To investigate whether this is only true for NPy C complexes or also in the case of the new C C* ligands, we decided to synthesize the analogous complexes with CF3-groups as described in Scheme 2. ˇ

1. Introduction

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Keywords: Platinum(II)eNHC complexes Cyclometalation Electronic effects DFT calculations

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* Corresponding author. E-mail address: [email protected] (T. Strassner). 0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2012.07.046

2. Experimental 2.1. General comments Solvents of at least 99.5% purity were used throughout this study. Dichloro(1,5-cyclooctadiene)platinum(II) [26] and the imidazolium salts 1aed [11] were prepared according to literature procedures. All other chemicals were obtained from common suppliers and used without further purification. 1,4-Dioxane and DMF were dried by standard procedures prior to use. 1H, 13C and 19F NMR spectra were recorded with a Bruker AC 300 and Bruker DRX500 P spectrometer. The 1H and 13C spectra were referenced internally to the resonances of the solvent (CDCl3), 19F spectra were referenced externally against trifluoromethylbenzene (F3CeC6H5).

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A. Tronnier et al. / Journal of Organometallic Chemistry 730 (2013) 37e43

(CArH); 120.6 (Ci); 114.5 (CArH); 110.4 (CArH); 93.5 ((CO)CH(CO)); 35.0 (NCH3) ppm. 19F NMR (300.13 MHz, CDCl3): d 75.17; 75.44 ppm. Decomposition >210  C. Anal. Calc. for C15H10N2O2F6Pt: C, 32.21; H, 1.80; N 5.01. Found: C, 32.17; H, 1.80; N, 4.75%. 0

Scheme 1. Cyclometalated ligands based on 2-phenylpyridine (NPy^C*) and 3phenylimidazolyl (C^C*).

Shifts d are given in ppm; coupling constants J in Hz. Elemental analyses were performed by the microanalytical laboratory of our institute using a Hekatech EA-3000 EuroVektor Elemental Analyzer. Melting and decomposition points were determined with a Wagner & Munz PolyTherm A melting point apparatus and are uncorrected.

0

2.2. General procedure 0.8 mmol of the imidazolium salt and 0.4 mmol silver(I)oxide (0.093 g) are suspended in 20 mL dioxane and stirred for 16 h at room temperature under an argon atmosphere. After addition of 10 mL butanone and 0.8 mmol dichloro(1,5-cyclooctadiene)platinum(II) (0.299 g) the mixture is heated to reflux for 16 h. The solvent is removed under reduced pressure and the residue is dissolved in 20 mL DMF. 3.2 mmol 1,1,1,5,5,5-hexafluoroace tylacetone (0.666 g; 0.45 mL) and 3.2 mmol potassium-tert-butanolate (0.359 g) are added. The reaction mixture is stirred for another 16 h at RT and then 6 h at 100  C. After removal of the solvent, the resulting residue was washed with water, followed by column chromatography with the eluent dichloromethane (silica gel KG60).

0

2.3.1. [3-Methyl-1-phenylimidazol-2-ylidene-C2,C2 ]platinum(II) bistrifluoromethylacetylacetonate (2a) The reaction of 0.229 g (0.8 mmol) 3-methyl-1phenylimidazolium iodide 1a follows the general procedure. Volatiles were removed in vacuo to yield an off-white solid (0.121 g, 27.0%). 1 H NMR (300.13 MHz, CDCl3): d 7.47 (d, 3JHH ¼ 7.3 Hz, 1H, CArH); 7.18 (d, 3JHH ¼ 2.1 Hz, 1H, CArH); 6.86e6.99 (m, 3H, CArH); 6.76 (d, 3 JHH ¼ 2.1 Hz, 1H, CArH); 6.17 (s, 1H, (CO)CH(CO)); 3.84 (s, 3H, NCH3) ppm. 13C NMR (125.75 MHz, CDCl3): d 171.3 (CO); 171.2 (CO); 146.4 (Ci); 145.2 (Ci); 131.8 (CArH); 124.8 (CArH); 124.4 (CArH); 121.2

N

I

2.3.3. [3-Methyl-1-(4-methylphenyl)imidazole-2-ylidene-C2,C2 ] platinum(II)bistrifluoromethyl-acetylacetonate (2c) The reaction of 0.240 g (0.8 mmol) 3-methyl-1-(4methylphenyl)imidazolium iodide 1c follows the general procedure described above. Volatiles were removed in vacuo to yield an off-white solid (0.103 g, 22.5%). 1 H NMR (300.13 MHz, CDCl3): d 7.25 (s, 1H, CArH); 7.14 (d, 3 JHH ¼ 2.1 Hz, 1H, CArH); 6.73e6.78 (m, 3H, CArH); 6.16 (s, 1H, (CO) CH(CO)); 3.84 (s, 3H, NCH3); 2.28 (s, 3H, CArCH3) ppm. 13C NMR (125.75 MHz, CDCl3): d 171.5 (CO); 144.7 (Ci); 144.1 (Ci); 134.1 (Ci); 132.5 (CArH); 125.1 (CArH); 121.1 (CArH); 120.2 (Ci); 114.4 (CArH); 110.1 (CArH); 93.5 ((CO)CH(CO)); 35.0 (NCH3); 21.4 (CCH3) ppm. 19F NMR (300.13 MHz, CDCl3): d 75.14; 75.45 ppm. Decomposition >210  C. Anal. Calc. for C16H12N2O2F6Pt: C, 33.52; H, 2.11; N 4.89. Found: C, 33.84; H, 2.15; N, 4.65%. 0

2.3. Synthesis of the metal complexes

N

2.3.2. [1-(4-Bromophenyl)-3-methylimidazole-2-ylidene-C2,C2 ] platinum(II)bistrifluoromethyl-acetylacetonate (2b) The reaction of 0.292 g (0.8 mmol) 1-(4-bromophenyl)-3methylimidazolium iodide 1b follows the general procedure described above. Volatiles were removed in vacuo to yield an offwhite solid (0.114 g, 22.2%). 1 H NMR (300.13 MHz, CDCl3): d 7.42 (s, 1H, CArH); 7.08 (d, 3 JHH ¼ 8.1 Hz, 1H, CArH); 7.03 (s, 1H, CArH); 6.74 (s, 1H, CArH); 6.66 (d, 3 JHH ¼ 8.1 Hz, 1H, CArH); 6.15 (s, 1H, (CO)CH(CO)); 3.81 (s, 3H, NCH3) ppm. 13C NMR (125.75 MHz, CDCl3): d 171.9 (CO); 171.8 (CO); 145.3 (Ci); 144.4 (Ci); 133.8 (CArH); 127.1 (CArH); 123.8 (Ci); 121.4 (CArH); 117.3 (Ci); 114.5 (CArH); 111.6 (CArH); 93.6 ((CO)CH(CO)); 35.0 (NCH3) ppm. 19F NMR (300.13 MHz, CDCl3): d 75.07; 75.28 ppm. M.p. 208e220  C. Anal. Calc. for C15H9N2O2F6BrPt: C, 28.23; H, 1.42; N 4.39. Found: C, 28.43; H, 1.35; N, 4.19%.

1. Ag2 O, Dioxane, RT 2. Pt(COD)Cl2 , Butanone, reflux 3. DMF, KOt Bu, RT - 100 °C

2.3.4. [1-(4-Methoxyphenyl)-3-methylimidazole-2-ylidene-C2,C2 ] platinum(II)bistrifluoromethylacetyl-acetonate (2d) The reaction of 0.253 g (0.8 mmol) 1-(4-methoxyphenyl)-3methylimidazolium iodide 1d follows the general procedure described above. Volatiles were removed in vacuo to yield a red solid (0.128 g, 27.2%). 1 H NMR (300.13 MHz, CDCl3): d 7.08 (d, 3JHH ¼ 2.1 Hz, 1H, CArH); 7.02 (d, 3JHH ¼ 2.7 Hz, 1H, CArH); 6.77 (d, 3JHH ¼ 8.5 Hz, 1H, CArH); 6.71 (d, 3JHH ¼ 2.1 Hz, 1H, CArH); 6.51 (d, 3JHH ¼ 8.5 Hz, 1H, CArH); 6.15 (s, 1H, (CO)CH(CO)); 3.81 (s, 3H, OCH3); 3.76 (s, 3H, NCH3) ppm. 13 C NMR (125.75 MHz, CDCl3): d 155.9 (Ci); 143.6 (Ci); 140.1 (Ci); 122.2 (Ci); 121.0 (CArH); 116.9 (CArH); 114.3 (CArH); 110.9 (CArH); 109.5 (CArH); 93.6 (CArH); 55.3 (OCH3); 35.0 (NCH3) ppm. 19F NMR

N N

CF3 O Pt O

O R

1a-d

F3 C

CF 3

O CF3

R

Scheme 2. Synthesis of the C^C*-cyclometalated complexes.

2a-d

2a: R=H 2b: R=Br 2c: R=CH 3 2d: R=OCH 3

A. Tronnier et al. / Journal of Organometallic Chemistry 730 (2013) 37e43

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(300.13 MHz, CDCl3): d 75.87; 76.06 ppm. M.p. 173  C. Anal. Calc. for C16H12N2O3F6Pt: C, 32.61; H, 2.05; N 4.75. Found: C, 32.21; H, 1.83; N, 4.61%. 2.4. Structure determination of compounds 2a, 2c and 2d Preliminary examination and data collection were carried out on an area detecting system (Kappa-CCD; Nonius, FR590) using graphite monochromated Mo Ka radiation (l ¼ 0.71073 Å) with an Oxford Cryosystems cooling system at the window of a sealed finefocus 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 [27]. After merging the independent reflections were used for all calculations. The structure was solved by a combination of direct methods [28,29] and difference Fourier syntheses [30]. All non-hydrogen atom positions were refined with anisotropic displacement parameters. Hydrogen atoms were placed in ideal positions using SHELXL riding model. Full-matrix least-squares refinements were P carried out by minimizing wðFo2  Fc2 Þ2 with the SHELXL-97 weighting scheme and stopped at shift/err <0.001. Details of the structure determinations are given in the Supplementary material. Neutral-atom scattering factors for all atoms and anomalous dispersion corrections for the non-hydrogen atoms were taken from International Tables for Crystallography [31]. All calculations were performed with the programs COLLECT [32], DIRAX [33], EVALCCD [34], SIR92 [28], SIR97 [29], SIR2004 [35], SADABS [27], PLATON [36,37] and the SHELXL-97 package [30,38]. For the visualization Mercury [39] and ORTEP-III [40] and for the preparation of the supporting material ENCIFER [41] and the WinGX suite [42] were used (Table 1). 2.5. Computational details All calculations were performed with Gaussian03 [43]. The density functional hybrid model B3LYP [44e48] was used together with the 6-31G(d) [49e54] basis set. No symmetry or internal

Fig. 1. ORTEP plot of complex 2a in the solid state. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å), angles and dihedral angles ( ): Pt(1)e C(1) 1.935(7); Pt(1)eC(5) 1.994(6); Pt(1)eO(1) 2.113(4); Pt(1)eO(2) 2.069(5); O(1)e Pt(1)eO(2) 89.50(17); C(1)ePt(1)eC(5) 80.3(3); C(4)eN(1)eC(1)ePt(1) 2.0(7); N(1)e C(1)ePt(1)eO(1) 173.9(5).

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 in order to verify them as ground states. In all cases platinum was described using a decontracted HayeWadt (n þ 1) ECP and basis set [55].

Table 1 Crystallographic details of complexes 2a, 2c and 2d. Complex

2a

2c

2d

Empirical formula fw T [K] Wavelength [Å] Crystal system Space group a [Å] b [Å] c [Å] a [ ] b [ ] g [ ] U [Å3] Z Dcalc [Mg/m3] m(MoKa) [mm1] Crystal size [mm3]

C15H10F6N2O2Pt 559.34 198(2) 0.71073 Monoclinic P 21/c 9.4040(14) 7.8410(8) 21.6220(12) 90 103.835(9) 90 1548.1(3) 4 2.400 9.145 0.64  0.39  0.12 1048 33,605 3176 [R(int) ¼ 0.0611] 1.175 0.0258 0.0631 3176/0/236

C16H12F6N2O2Pt 573.37 198(2) 0.71073 Monoclinic C 2/c 12.186(2) 20.680(4) 13.682(3) 90 90.69(3) 90 3447.7(12) 8 2.209 8.215 0.70  0.24  0.12 2160 37,943 3525 [R(int) ¼ 0.0683] 1.185 0.0294 0.0804 3525/0/246

C16H12F6N2O3Pt 589.37 198(2) 0.71073 Triclinic P 1 7.7570(8) 10.373(3) 11.3420(17) 96.71(2) 108.494(19) 96.280(19) 849.1(3) 2 2.305 8.347 0.76  0.37  0.16 556 13,477 2942 [R(int) ¼ 0.0594] 1.219 0.0241 0.0679 2942/0/255

F(000) Reflections collected Independent refl. Goof on F2 R1 [I > 2s(I)] wR2 Data/restr/param

Fig. 2. ORTEP plot of complex 2c in the solid state. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å), angles and dihedral angles ( ): Pt(1)e C(1) 1.938(6); Pt(1)eC(5) 1.980(6); Pt(1)eO(1) 2.103(4); Pt(1)eO(2) 2.074(4); O(1)e Pt(1)eO(2) 88.88(17); C(1)ePt(1)eC(5) 80.0(2); C(4)eN(1)eC(1)ePt(1) 1.6(7); N(1)e C(1)ePt(1)eO(1) 179.3(4).

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3. Results and discussion 3.1. Synthesis of the complexes The square-planar platinum(II) complexes 2aed could be synthesized via a recently discovered new route [11]. These compounds are some of the first examples where the ligand is a C C* cyclometalated carbene at platinum. Starting from different aryl-substituted imidazolium salts 1aed we could deprotonate the C2 carbon atom of the imidazolium core by Ag2O presumably forming a silver(I) carbene complex in situ which then was transmetalated to Pt(COD)Cl2 in butanone/dioxane. Addition of a base (potassium-tert-butanolate) in DMF together with a specific temperature program leads to the cyclometalation at the C2 position of the phenyl ring. We then added 1,1,1,5,5,5hexafluoroacetylacetone which occupies the other two positions at the platinum(II) center leading to neutral complexes. Contrary to a recent report in the literature we do not observe other intermediates [20]. The formation of 2aed could be followed by NMR spectroscopy, where the 1H NMR signal of the imidazolium salt (NeCH]N) disappears and the characteristic 13C NMR peak for the carbene carbon atom appears. To unequivocally determine the structures we set out to get single crystals suitable for solid-state structure determination and succeeded in the cases of 2a (Fig. 1), 2c (Fig. 2) and 2d (Fig. 3) shown below. Details of the single crystal structure determination are given below. ˇ

Fig. 3. ORTEP plot of complex 2d in the solid state. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å), angles and dihedral angles ( ): Pt(1)e C(1) 1.936(6); Pt(1)eC(5) 1.981(5); Pt(1)eO(1) 2.103(4); Pt(1)eO(2) 2.075(4); O(1)e Pt(1)eO(2) 89.38(15); C(1)ePt(1)eC(5) 80.6(2); C(4)eN(1)eC(1)ePt(1) 3.3(7); N(1)e C(1)ePt(1)eO(1) 172.7(5).

Fig. 4. a) Stacking arrangement of 2a along the b axis in the crystal. The shortest PtePt distance is 3.343 Å, and the PtePtePt angle is 146.6 . b) Zigzag conformation in the crystal.

Fig. 5. a) Stacking arrangement of 2c in the solid state with two complexes forming dimer-like structures. The shortest PtePt distance is 3.225 Å and the PtePtePt angle is 139.7. b) Stack conformation along the c axis.

A. Tronnier et al. / Journal of Organometallic Chemistry 730 (2013) 37e43

1,2

1

Absorption

2a 2b

0,8

2c 0,6

2d

0,4

0,2

0 200

250

300

350

400

450

λ / nm Fig. 6. Absorption spectra of the complexes 2aed (2wt% in PMMA, rt).

3.2. Solid-state structures Red colored single crystals suitable for solid-state determination could be obtained from saturated methylene chloride/ acetonitrile solutions. The structures of the platinum(II) complexes show a square-planar coordination of the platinum atom. The distance from the platinum atom to each of its four surrounding neighbor atoms is about 2 Å with slightly longer Pte O distances. The platinum carbene bond is the shortest contact which is in good agreement with previously reported structures of this general type [11,56]. Although the molecular structure of the three complexes is similar, they crystallize in different crystal systems (monoclinic, triclinic) and space groups (P 21/c, C 2/c, P 1) despite the same crystallization procedure at ambient conditions was used. For 2a the complexes pack in infinite stacks along the b axis of the crystal. In each stack two complexes are arranged in a parallel fashion with a distance of 3.30 Å and the NCH3 groups facing in opposite directions. The acetylacetonate ligand of one molecule and the NHC ligand of the next one lie in close enough proximity to allow for pep interactions, while the PtePt distance (3.342 Å) is below the sum of the van der Waals radii (3.5 Å respectively), an indication for a metalemetal interaction. The individual pairs of structures are oriented in a skewed fashion, which leads to a Pte

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PtePt angle of 146.6 (Fig. 4a) and a distance to the platinum atom of the next pair of 4.830 Å. The planes of two neighboring stacks meet at an angle of 58.6 forming a zigzag pattern throughout the crystal (Fig. 4b). The shortest PtePt distance was found for 2c (3.225 Å) with the metal atoms lying perpendicularly above each other (Fig. 5a). In this structure the CF3-acetylacetonate ligands lie on top of each other while the phenyleNHC ligands are flipped over (Fig. 5a), leading to an interaction of the imidazole rings with the aromatic system of their counterpart. Only the sterically demanding CF3-groups cause some twisting along the PtePt-axis (Fig. 5b). The complexes are arranged as pairs in parallel stacks along the c axis of the crystal with a PtePt distance of 4.726 Å between the pairs and a PtePtePt angle of 139.7. The solid-state structure of 2d is very similar to 2a, the only difference between them is the orientation of the stacks. 2d stacks along the a axis of the crystal and the planes of the complexes between different stacks are oriented in a parallel fashion not forming a zigzag conformation. The shortest PtePt contact in this structure is 3.269 Å and the PtePtePt angle is found to be 146.6 . 3.3. Photophysical data Although the absorption spectra are similar compared to the corresponding acetylacetonate complexes [11] we were surprised to find that the emission is very low at room temperature. The quantum yield for all complexes at room temperature was below the detection limit of the quantum yield measurement system (ca. 2e5%). The absorption maximum for all samples is the PMMA matrix at 220 nm followed by weaker bands at around 250 nm (pe p*) and 310 nm (MLCT) (Fig. 6). Especially for 2d a stronger absorption at 250 nm was observed. In general the bands are not as distinct as the ones observed before [11]. 3.4. DFT calculations The calculated geometries on the chosen level of theory (B3LYP/ 6-31G(d)) are in very good agreement with the bond distances, angles and dihedral angles found in the solid-state structures. On the basis of the optimized singlet ground states we computed the KohneSham frontier molecular orbitals (FMOs) to get a better understanding of the surprising photophysical behavior (Fig. 7).

Fig. 7. Frontier molecular orbitals of the complexes 2aed (top: LUMO, bottom: HOMO) given at an isovalue of 0.02 computed on the singlet ground state.

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A. Tronnier et al. / Journal of Organometallic Chemistry 730 (2013) 37e43

Fig. 8. LUMO of 2d (left) and the acac congener (right).

4. Conclusion ˇ

The shown FMOs suggest an inter-ligand charge transfer (ILCT) during the emission process with a significant metal contribution to both states (HOMO, LUMO). Since the phosphorescence occurs in general from the lowest lying triplet state an increased contribution of metal centered d-orbitals to the LUMO is counterproductive because of the population of slightly higher lying vacant platinum d-orbitals. Population of these states usually leads to lower quantum yields due to radiationless relaxation processes [57]. However we do not propose a nonradiative de-excitation process solely from metal orbital participation in the LUMO but believe that the spatial HOMOeLUMO separation, which is more distinct in these molecules compared to the acac complexes, (and especially the fluorine substitution of the acetylacetonate) is responsible for the low quantum efficiency. This is in line with a recent report by Abdou and coworkers, which studied the effect of fluorine substitution in luminescent gold(I) complexes [58]. They demonstrated a significant correlation between the photophysical properties and the number of fluorine atoms in the ligands. No luminescence was observed for the ligands with the maximum number of fluorine atoms. Since the CF3-acac ligand carries six fluorine atoms and changed the ordering of the unoccupied FMOs, an increased influence seems most likely. Fig. 8 shows the LUMO of 2d and the corresponding acetylacetonate complex. It is obvious that for 2d the LUMO is completely localized on the CF3eacac ligand, while the corresponding acac complex shows some part of the LUMO on the NHC ligand. Additionally the d-orbitals of the metal cannot interact with the orbitals on the NHC ligand. The fluorine atoms in 2d also lead to a larger number of knots, only one fluorine of each CF3-group in the complex plane can make a contribution to the p* orbitals. In our case, the CF3eacac ligands unfortunately shut down the emission, which was unexpected as not in every case those ligands lead to a decrease in quantum efficiency. E.g. quite recently Yun reported cyclometalated iridium complexes with a CF3eacac ligand [59], which showed good quantum yields in solution at room temperature.

Four new C C*-cyclometalated complexes with different substituents in the 4-position of the aromatic ring have been synthesized and characterized. They demonstrate that the recently published route to carbene-donor substituted cyclometalated platinum(II) complexes allows the use of different counteranions. The photophysical properties of the complexes are drastically changed by the CF3-groups on the acetylacetonate ligand. Using DFT calculations the structures and molecular orbitals have been calculated. From the difference of the HOMO/LUMO levels it can be explained why the emission is weaker than in the case of the corresponding acac complexes. Acknowledgments We thank the center of high-performance computing (ZIH) of the TU Dresden for providing computing time on their computer systems. We cordially thank Dr. Wagenblast (BASF) for the photophysical measurements. Appendix A. Supplementary material CCDC 892378 (2a), 892379 (2c) and 892380 (2d) contain the supplementary crystallographic data, which can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (þ44) 1223-336-033 or e-mail: [email protected]. Details of the quantum-chemical calculations are available from the author. References [1] L. Murphy, J.A.G. Williams, Top. Organomet. Chem. 28 (2010) 75. [2] A.F. Rausch, H.H.H. Homeier, H. Yersin, Top. Organomet. Chem. 29 (2010) 193. [3] G. Zhou, W.-Y. Wong, X. Yang, Chem. Asian J. 6 (2011) 1706.

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