A Phosphorescent C∧C* Cyclometalated Platinum(II

Article pubs.acs.org/Organometallics

A Phosphorescent C∧C* Cyclometalated Platinum(II) Dibenzothiophene NHC Complex Alexander Tronnier,† Anita Risler,‡ Nicolle Langer,‡ Gerhard Wagenblast,‡ Ingo Münster,‡ and Thomas Strassner*,† †

Physikalische Organische Chemie, Technische Universität Dresden, 01069 Dresden, Germany BASF SE, 67056 Ludwigshafen, Germany

‡

S Supporting Information *

ABSTRACT: Cyclometalated platinum(II) complexes with β-diketone ligands have recently attracted increasing interest as phosphorescent emitters for organic light-emitting devices (OLEDs). We present the synthesis of the new cyclometalated Pt(II) N-heterocyclic carbene complex [(NHC)PtII(acac)] (acac = acetylacetonate, NHC = 1-(dibenzo[b,d]thiophen-4-yl)-3-methyl-1H-imidazoyl). The complex was fully characterized by advanced spectroscopic methods (2D NMR, Pt NMR), elemental analysis, and solid-state structure determination, which revealed a square-planar coordination of the platinum atom. The photoluminescence properties were investigated in amorphous poly(methyl methacrylate) (PMMA) thin films. Quantum chemical density functional theory (DFT) computations on the singlet and triplet ground states have shown admixing of the Pt 5d orbitals with the frontier molecular orbitals (FMOs), explaining the good quantum yield.

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quantum yields and have been reported also in recent publications25 on this topic. So far, complexes of type C, bearing this motif in combination with imidazolium-based Nheterocyclic carbenes (NHCs), have only been mentioned in patents,26,27 while C∧C* cyclometalated platinum(II) βdiketone complexes with a dibenzo[b,d]thiophene group are unknown. We herein present the synthesis and photophysical characterization of the first complex of this type and its properties, as well as its solid-state structure.

cological and economical problems concerning growing worldwide energy demands and limited natural resources have led to increasing efforts in many scientific fields to tackle the challenge. Chemistry and especially organometallic chemistry1 has contributed much to the cause and is one of the key disciplines in the modernization of display technologies and domestic lighting to improve energy conservation. Transition-metal complexes play an important role as phosphorescent emitters in organic light-emitting devices (OLEDs)2 due to their higher spin−orbit coupling (SOC) and therefore increased quantum yields.3,4 Compounds containing iridium5,6 and platinum7−11 are of particular interest and have attracted much attention in recent years12−14 because of their extraordinary photophysical properties. Complexes with 2-phenylpyridine are a prominent example for C∧N cyclometalated complexes, often also together with βdiketonate auxiliary ligands (A, Scheme 1).8,15,16 The incorporation of heteroatoms such as boron17−19 and sulfur20−22 into the cyclometalated ligand or even the auxiliary ligand23,24 has allowed for further improvement and fine tuning of the photophysical properties. Complexes bearing the benzo[b]thiophen-2-yl (type B) moiety have shown good

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RESULTS AND DISCUSSION Synthesis and Characterization. The 1-(dibenzo[b,d]thiophen-4-yl)-3-methyl-1H-imidazolium iodide ligand precursor 1 was prepared in a way similar to that for the previously reported dibenzofuran congener (Scheme 2).28 Dibenzothiophene was lithiated with n-butyllithium (n-BuLi) at low temperature under an inert atmosphere. After metal− halide exchange with 1,2-dibromoethane the crude product was purified by recrystallization from an n-hexane/methylene chloride solution. 4-Bromobenzothiophene was coupled with imidazole in N-methyl-2-pyrrolidone (NMP) at 150 °C using copper(I) iodide as catalyst and potassium carbonate as base. Upon completion the reaction mixture was quenched with water and extracted with methyl tert-butyl ether (MTBE). The organic phases were combined, dried over sodium sulfate, and concentrated. After flash chromatography with methylene chloride/methanol the imidazole was obtained in 52% yield. In the final step 1-(dibenzo[b,d]thiophen-4-yl)-1H-imidazole

Scheme 1. General Structure of Cyclometalated Pt(II) Acetylacetonate Complexes

Received: August 2, 2012 Published: October 31, 2012 © 2012 American Chemical Society

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Scheme 2. Synthesis of 1

congener (155 ppm),28 the 13C carbene signal was observed at 150.4 ppm. Solid-State Structure Determination. Colorless single crystals of 1 could be grown from a concentrated methylene chloride solution at ambient temperature. We were able to verify the three-dimensional structure of the salt by a solid-state structure determination (see the Supporting Information, Figure S1). Additionally, we could also obtain yellow single crystals of the metal complex suitable for solid-state structure determination by slow evaporation of a saturated methylene chloride solution of 2 under ambient conditions. The structure shows a square-planar coordination of the central platinum atom as well as a planar conformation of the whole complex (Figure 1). The distance from the platinum atom to its four surrounding neighbor atoms is about 2 Å, although the Pt−O distances are slightly longer (2.091 and 2.056 Å) than the Pt−C bonds (1.954 and 1.975 Å). The distance between the metal and the cyclometalated carbon atom C5 is slightly longer than the carbene carbon−platinum bond. Very small dihedral angles including the central five-membered ring with the platinum(II) center and the thiophene ring are in good agreement with a planar structure (C1−N1−C4−C5 = −0.2°, S1−C14−C9−C8 = −0.4°, S1−C15−C4−N1 = −1.0°). While the C1−Pt1−C5 angle (80°) deviates significantly from the perfect 90° angle, the O1−Pt1−O2 angle of the auxiliary acetylacetonate ligand (90°) is ideal for the square-planar coordination geometry. As expected, the S−C bonds (1.749, 1.752 Å) are slightly longer compared to the O−C bonds (1.389, 1.402 Å) of the corresponding dibenzofuran complex. All data are generally in very good agreement with other reported structures of the (C∧C*)PtII(acac) type so far.28,29

was treated with methyl iodide in THF at elevated temperature to give 1 (98%, overall yield 36%). The synthesis of complex 2 could be accomplished in three steps: reaction of the imidazolium salt with silver(I) oxide, transmetalation with dichloro(1,5-cyclooctadiene)platinum(II) without further purification, and treatment with excess acetylacetone and potassium tert-butanolate after solvent exchange. The crude product was purified by flash chromatography in methylene chloride, giving complex 2 in 30% yield (Scheme 3). Scheme 3. Synthesis of 2

The complex was fully characterized by two-dimensional 1H and 13C NMR (COSY, HSQC, HMBC, NOESY) and 195Pt NMR spectroscopy to unambiguously assign all signals as well as elemental analysis (see the Supporting Information and Experimental Section). The formation of a cyclometalated species was proven by the disappearance of the C2-imidazolium proton (10.44 ppm) and a change in the shape of the signal for the hydrogen atom (7.71 ppm) adjacent to the cyclometalation site, which became a doublet. A characteristic long-range 1 H−195Pt coupling resulting in the formation of a pseudotriplet (7.97 ppm, 21.0 Hz) was observed (see the Supporting Information). In comparison to the dibenzo[b,d]furane

Figure 1. (a) ORTEP plot of 2 in the solid state. Thermal ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å), bond angles (deg), and dihedral angles (deg): Pt(1)−C(1), 1.954(4); Pt(1)−C(5), 1.975(4); Pt(1)−O(1), 2.091(3); Pt(1)−O(2), 2.056(3); O(1)−Pt(1)− O(2), 89.60(13); C(1)−Pt(1)−C(5), 80.47(18); C(4)−N(1)−C(1)−Pt(1), 1.3(5); N(1)−C(1)−Pt(1)−O(1), 176.3(3). (b) Orientation of 2 in the unit cell. Hydrogen atoms are omitted for clarity. Symmetry codes for Pt: (i) 1 − x, 1/2 − y, 3/2 + z; (ii) 1 − x, 1 − y, 1 − z; (iii) x, 1/2 − y, 1/2 + z. 7448

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implies that only a minor reorganization takes place between the ground state and excited state. Figure 3 shows the frontier molecular orbitals (FMOs) computed on the singlet ground state. Since the main emission

Figure 1b shows the three-dimensional orientation in the crystal. The molecules pack in infinite stacks with an intermolecular distance of 3.4 Å (complex plane) and a Pt− Pt distance of 3.413 Å. Each complex is rotated by 180° in the plane compared to its nearest neighbor. Thus, the acetylacetonate ligand of one complex lies directly above the NHC ligand of its neighbor, allowing for π−π interactions, while the Pt−Pt distance is below the sum of the van der Waals radii (3.5 Å, respectively), an indication of an interaction between the two metal atoms. The planes of two neighboring stacks meet at an angle of 71.5°, forming a zigzag pattern throughout the crystal. Photophysical Studies. To investigate intermolecular interactions during the photoluminescence process, efforts were made to create pure emitter films but were not successful due to early crystallization. Therefore, the absorption spectrum of this crystallized film shows no discernible fine structure. Furthermore, the absorption and emission spectra were measured in a thin PMMA film doped with 2 wt % emitter (Figure 2). The absorption spectrum shows a major absorption

Figure 2. Absorption (left) and emission (right) spectra of 2 (2 wt % in PMMA at room temperature).

peak of the amorphous PMMA matrix around 220 nm. Additional peaks are found at lower energies (260, 315, and 343 nm), which can be ascribed to mixed metal-to-ligand charge transfer (MLCT) and ligand (NHC) centered π−π* transitions.3 The emission with a quantum yield of 63% occurs in the blue-green region of the visible spectrum, where three different maxima at 474, 507, and 537 nm can be distinguished. The lifetime of the excited state (31.4 μs) is increased in comparison to the dibenzofuran version (23.0 μs) and is in the range known for the few complexes of this type presented so far.28 Computational Studies. Geometry optimizations for the singlet and triplet ground states were performed using DFT methods. To gain further insight into the photophysical properties of the complex, frontier molecular orbitals (FMOs) were computed on the optimized complex geometries (based on Kohn−Sham orbitals). A recently developed method to reliably predict the emission wavelength of transition-metal complexes (see Supporting Information) allowed us to predict the wavelength maximum to be 469 nm, which is in good agreement with the experimental maximum at 474 nm. The computationally optimized ground-state geometry for 2 is also in very good agreement with the experimental results from the solid-state structure characterization (Supporting Information, Table S3). There is no major difference between the optimized singlet and triplet ground-state geometries, despite some small changes in bond lengths around the central metal atom, which

Figure 3. Computed FMOs of the singlet ground state (B3LYP/631G(d)).

process is believed to be a LUMO−HOMO transition, FMOs are often used to describe the nature of the process (MLCT, ILCT, etc.). The calculations suggest a metal-perturbed intraligand transition on the NHC ligand, which is in good agreement with the observed high quantum yield and characteristic vibronic band structure of the emission spectrum. This is underlined by the marginal contribution from metalcentered orbitals to the HOMO (14.5%) and the LUMO (11.4%) according to Mulliken population analysis.

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CONCLUSION We present the synthesis and characterization of the first C∧C* cyclometalated platinum(II) NHC complex with a dibenzothiophene ligand. A solid-state structure determination revealed a square-planar coordination of the central platinum atom and the planarity of the whole complex. The complex shows a strong emission in the blue-green region of the visible spectrum, with a maximum at 474 nm and a characteristic band structure. The photophysical data obtained for this complex confirm our earlier findings for the dibenzofuran congener and allow for the assumption that the emission is mainly based on the NHC ligand with a significant metal contribution (MLCT/LLCT). We propose that the high 7449

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quantum yield is induced by the extended heterocyclic π system.

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with 1-(dibenzo[b,d]thiophen-4-yl)-3-methyl-1H-imidazolium iodide (1; 0.314 g, 0.8 mmol) and silver(I) oxide (0.095 g, 0.4 mmol). After addition of 20 mL of dry 1,4-dioxane the reaction mixture was stirred under argon in the dark at room temperature for 21 h. Dichloro(1,5cyclooctadiene)platinum(II) (0.299 g, 0.8 mmol) and 10 mL of 2butanone 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 potassium tert-butanolate (0.366 g, 3.2 mmol), acetylacetone (3.2 mmol), and 20 mL of dry DMF were added under argon. After the mixture was stirred for 20 h at room temperature 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). Drying under vacuum yielded a light yellow solid (0.135 g, 30%). 1H NMR (CDCl3, 600.16 MHz): δ 8.14 (d, J = 7.6 Hz, 1H, CHarom), 7.97 (d, pseudo-t, J = 7.9 Hz, JH,Pt = 21.0 Hz, 1H, PtCCHarom), 7.83 (d, J = 7.9 Hz, 1H, CHarom), 7.82 (d, J = 7.6 Hz, 1H, CHarom), 7.76 (d, J = 2.1 Hz, 1H, NCH), 7.43 (m, 2H, CHarom), 6.93 (d, J = 2.1 Hz, 1H, NCH), 5.54 (s, 1H, OCCH), 4.14 (s, 3H, NCH3), 2.11 (s, 3H, CH3), 2.0 (s, 3H, CH3) ppm. 13C NMR (CDCl3, 75.475 MHz): δ 185.1 (CO), 184.9 (CO), 150.4 (NCN), 141.0 (Ci), 137.3 (Ci), 136.0 (Ci), 134.3 (Ci), 128.2 (CHarom), 126.1 (CHarom), 125.4 (Ci), 124.7 (CHarom), 122.4 (CHarom), 121.8 (Ci), 121.2 (CHarom), 120.7 (CHarom), 116.9 (CHarom), 116.8 (CHarom), 102.1 (CH), 34.9 (NCH3), 27.9 (CH3), 27.8 (CH3) ppm. 195Pt NMR (CDCl3, 129 MHz): δ −3426.2 ppm. Mp: 297−299 °C. Anal. Calcd for C21H18N2O2PtS (557.53 g mol−1): C, 45.24; H, 3.25; N, 5.02; S, 5.75. Found: C, 45.38; H, 3.22; N, 4.97; S, 5.39. X-ray Crystallography. Preliminary examination and data collection were carried out on an area detecting system (KappaCCD; 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.30 After merging, the independent reflections were used for all calculations. The structure was solved by a combination of direct methods31,32 and difference Fourier syntheses.33 All nonhydrogen 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 minimizing ∑w(Fo2 − 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 ref 34. All calculations were performed with the programs COLLECT,35 DIRAX,36 EVALCCD,37 SIR92,31 SIR97,32 SADABS,30 PLATON,38 and the SHELXL-97 package.33,39 For visualization Mercury40 and ORTEP-III41,42 and for the preparation of supporting material ENCIFER43 and the WinGX44 suite were used. Computational Details. All calculations were performed with the Gaussian03 package.45 The density functional hybrid model B3LYP 46−50 and the gradient-corrected density functional BP8647,51,52 were used together with the 6-31G(d)53−58 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 in order to verify them as ground states. In all cases platinum was described using a decontracted Hay-Wadt(n+1) ECP and basis set.59−61 Approximate free energies were obtained through thermochemical analysis, using the thermal correction to Gibbs free energy as reported by Gaussian03. 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 GaussView62 was used.

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) was prepared according to a literature procedure. 28 Potassium tetrachloroplatinate(II) was obtained from Pressure Chemicals Co. All other chemicals were obtained from common suppliers and used without further purification. 1H, 13C, and 195Pt NMR spectra were recorded on a NMR spectrometer. 1H and 13C spectra were referenced internally using the resonances of the solvent (for CDCl3: 1H, 7.26 ppm; 13C, 77.0 ppm). 195Pt spectra were referenced using potassium tetrachloroplatinate(II) in D2O (−1617.2 ppm (PtCl42−), −2654.1 ppm (PtCl2)). Shifts are given in ppm and coupling constants J in Hz. Elemental analyses were performed by the microanalytical laboratory of our institute. Melting points have been determined and are not corrected. The photoluminescence of the complexes was measured in thin PMMA films doped with 2 wt % emitter or in 100% emitter films. The 2 wt % films were prepared by doctor-blading a solution of emitter (2 mg/L) 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 355−370 nm (Xe lamp with monochromator), and emission was detected with a calibrated CCD spectrometer. The phosphorescence decay was measured by excitation with pulses of a THG-NdYAG laser (355 nm, 1 ns) and time-resolved photon counting in the multichannel scaling (MCS) technique. 1-(Dibenzo[b,d]thiophen-4-yl)-3-methyl-1H-imidazolium Iodide (1). Commercially available dibenzothiophene (51.44 g, 0.28 mol) was dissolved in 460 mL of dry THF and slowly lithiated with 1.6 M n-BuLi in hexane (177 mL, 0.28 mol) at −40 °C under an argon atmosphere. The solution was warmed to 0 °C and was stirred for 6 h. Afterward the reaction mixture was cooled to −78 °C and a solution of 1,2-dibromoethane (103.23 g, 0.55 mol) in 40 mL of dry THF was added dropwise. The mixture was then stirred at room temperature overnight. After concentration under reduced pressure the crude brominated product was dissolved in methylene chloride and washed with water several times. The solution was concentrated again, and the resulting solid was recrystallized from an n-hexane/methylene chloride solution to give the brominated dibenzothiophene (71%). The 4bromobenzthiophene (51.04 g, 0.19 mol) was coupled with imidazole (26.42 g, 0.39 mol) for 18 h in 300 mL of dry NMP at 150 °C using copper(I) iodide (3.89 g, 0.01 mol) as catalyst and potassium carbonate (35.00 g, 0.07 mol) as base. Upon completion the reaction mixture was quenched with 600 mL of water. A 1500 mL portion of MTBE was added, and the resulting solid was filtered and washed with MTBE. The two phases were separated, and the aqueous phase was extracted three times with 200 mL of MTBE. The organic phases were combined, dried over sodium sulfate, and concentrated. After flash chromatography with methylene chloride/methanol on silica gel the dibenzothiophenylimidazole was obtained in 52% yield. In the final step 1-(dibenzo[b,d]thiophen-4-yl)-1H-imidazole (25.38 g, 0.10 mol) was stirred with methyl iodide (32 mL, 0.51 mol) in 250 mL of THF at 70 °C for 2 h to give 1 (98%, overall yield 36%). 1H NMR (CDCl3, 300.13 MHz): δ 10.44 (s, 1H, NCHN), 8.33 (d, J = 7.9 Hz, 1H, CHarom), 8.23 (m, 1H, CHarom), 8.15 (d, J = 7.9 Hz, 1H, CHarom), 7.88 (m, 1H, CHarom), 7.77 (s, 1H, NCH), 7.71 (t, J = 7.9 Hz, 1H, CHarom), 7.58 (m, 3H, CHarom), 4.38 (s, 3H, NCH3) ppm. 13C NMR (CDCl3, 75.475 MHz): δ 138.6 (Ci), 138.1 (Ci), 137.6 (NCHN), 134.9 (Ci), 133.3 (Ci), 129.1 (Ci), 128.4 (CHarom), 126.6 (CHarom), 125.7 (CHarom), 124.1 (CHarom), 123.9 (CHarom), 123.8 (CHarom), 123.0 (CHarom), 122.5 (CHarom), 121.3 (CHarom), 38.0 (CH3) ppm. Mp: 242−243 °C. Anal. Calcd for C16H13IN2S (392.26 g mol−1): C, 48.99; H, 3.34; N, 7.14; S, 8.17. Found: C, 49.24; H, 3.34; N, 7.20; S, 8.03. (SP-4-3)-[1-(Dibenzo[b,d]thiophen-4-yl)-3-methyl-1H-imidazol-2-ylidene-κC2,κC3′](2,4-pentanedionato-κO2,κO4)platinum(II) (2). A dried and argon-flushed Schlenk tube was charged 7450

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(24) Aoki, R.; Kobayashi, A.; Chang, H.-C.; Kato, M. Bull. Chem. Soc. Jpn. 2011, 84, 218. (25) Hudson, Z. M.; Blight, B. A.; Wang, S. Org. Lett. 2012, 14, 1700. (26) 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. WO2006056418A2, 2006. (27) Langer, N.; Kahle, K.; Lennartz, C.; Molt, O.; Fuchs, E.; Rudolph, J.; Schildknecht, C.; Watanabe, S.; Wagenblast, G. WO2009003898A1, 2009. (28) Unger, Y.; Meyer, D.; Molt, O.; Schildknecht, C.; Muenster, I.; Wagenblast, G.; Strassner, T. Angew. Chem., Int. Ed. 2010, 49, 10214. (29) Tenne, M.; Unger, Y.; Strassner, T. Acta Crystallogr., Sect. C 2012, 68, m203. (30) Sheldrick, G. M. SADABS, Version 2.10; University of Göttingen, Göttingen, Germany, 2002. (31) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (32) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115. (33) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112. (34) Wilson, A. J. C., Ed. International Tables for Crystallography; Kluwer: Dordrecht, The Netherlands, 1992; Vol. C (Mathematical, Physical and Chemical Tables). (35) Hooft, R. W. W. Data Collection Software for Nonius-Kappa CCD; Nonius BV., Delft, The Netherlands, 1999. (36) Duisenberg, A. J. M. J. Appl. Crystallogr. 1992, 25, 92. (37) Duisenberg, A. J. M.; Kroon-Batenburg, L. M. J.; Schreurs, A. M. M. J. Appl. Crystallogr. 2003, 36, 220. (38) Spek, A. L. Acta Crystallogr. 2009, D65, 148. (39) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Structures; University of Göttingen, Göttingen, Germany, 1997. (40) Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.; Edgington, P. R.; McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; van de Streek, J.; Wood, P. A. J. Appl. Crystallogr. 2008, 41, 466. (41) Burnett, M. N.; Johnson, C. K. ORTEP-III; Oak Ridge National Laboratory, Oak Ridge, TN, 1996. (42) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565. (43) Allen, F. H.; Johnson, O.; Shields, G. P.; Smith, B. R.; Towler, M. J. Appl. Crystallogr. 2004, 37, 335. (44) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837. (45) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; 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.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A. Gaussian 03, Revision D.01; Wallingford, CT, 2004. (46) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (47) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (48) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (49) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200. (50) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (51) Perdew, J. P. Phys. Rev. B 1986, 34, 7406.

ASSOCIATED CONTENT

S Supporting Information *

CIF files, text, tables, and figures giving experimental details, crystallographic data, detailed spectra, and Cartesian coordinates for the optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: 49 351 46339679. Tel: 49 351 46338571. E-mail: thomas. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Fuchs (BASF), Dr. Metz (BASF), and Dr. Lennartz (BASF) for helpful discussions. We also thank the ZIH for computation time at their high-performance computing facility.

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