CCCâPincerâNHC Osmium Complexes: New ... - ACS Publications

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CCC−Pincer−NHC Osmium Complexes: New Types of Blue-Green Emissive Neutral Compounds for Organic Light-Emitting Devices (OLEDs) Roberto G. Alabau,† Beatriz Eguillor,† Jim Esler,‡ Miguel A. Esteruelas,*,† Montserrat Oliván,† Enrique Oñate,† Jui-Yi Tsai,‡ and Chuanjun Xia*,‡ †

Departamento de Quı ́mica Inorgánica, Instituto de Sı ́ntesis Quı ́mica y Catálisis Homogénea (ISQCH), Centro de Innovación en Quı ́mica Avanzada (ORFEO-CINQA), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain ‡ Universal Display Corporation, 375 Phillips Boulevard, Ewing, New Jersey 08618, United States S Supporting Information *

ABSTRACT: Novel homoleptic and heteroleptic NHC carbene containing bis(tridentate) osmium(II) complexes have been designed, synthesized, and characterized, and their photophysical properties have been studied. The complex OsH6(PiPr3)2 (1) reacts with the tetrafluoroborate salts of 1,3bis(3-methylbenzimidazolium-1-yl)benzene and 1,3-bis(3methylimidazolium-1-yl)benzene, in dimethylformamide, under reflux to afford the hydride−carbonyl derivatives [OsH(C NHCCarylC benzimidazolium )(CO)(PiPr3 )2]BF 4 (2) and [OsH(CNHCCarylCimidazolium)(CO)(PiPr3)2]BF4 (3), as a result of the direct metalation of one of the NHC units of the salts, the activation of the C−H bond at the 6-position of the bridged aryl group, and metal carbonylation by solvent decarbonylation. In contrast to the BF4 salts, under the same conditions, the iodide salts of 1,3-bis(3-methylbenzimidazolium-1-yl)benzene, 1,3bis(3-methylimidazolium-1-yl)benzene, and 1,3-bis(3-methylbenzimidazolium-1-yl)-5-trifluoromethylbenzene undergo direct metalation of both NHC units and C−H bond activation of the bridged aryl group at the 2-position to give the respective osmium(IV) dihydrides [OsH2(CNHCCarylCNHC)(PiPr3)2]I (4a−6a), which by deprotonation with KtBuO yield the osmium(II) monohydrides OsH(CNHCCarylCNHC)(PiPr3)2 (7−9). The reactions of 7 with 1,3-bis(3-methylbenzimidazolium-1-yl)benzene tetrafluoroborate and of 9 with 1,3-bis(3-methylbenzimidazolium-1-yl)-5-trifluoromethylbenzene tetrafluoroborate lead to the homoleptic derivatives Os(CNHCCarylCNHC)2 (10, 11), whereas the reactions of 9 with the tetrafluoroborate salts of 1,3-bis(3methylbenzimidazolium-1-yl)benzene and 1,3-bis(3-methylimidazolium-1-yl)benzene generate heteroleptic Os(CNHCCarylCNHC)(CNHCCaryl′CNHC) (12) and Os(CNHCCaryl′CNHC)(CNHC′CarylCNHC′) (13). Treatment of 7 with 3,5-bis(3-methylbenzimidazolium-1-yl)-2,6-dimethylpyridine tetrafluoroborate affords the salt [Os(CNHCCarylCNHC)(CNHCCaryl′CNHC)]BF4 (14), with Caryl′ being a pyridinium group. Its deprotonation generates the neutral heteroleptic derivative Os(CNHCCarylCNHC)(CNHCCaryl′CNHC) (15). Complexes 10−13 and 15 are emissive in the blue-green spectral region with high quantum yields in the solid state, which reach 0.62 for 11. OLEDs using this compound as an emitting material show blue emission (CIE coordinates: (0.14, 0.26)). The brightness of the device reaches 10000 cd/m2 at 9.5 V. The maximum external quantum efficiency (EQE) was 19.2% at 1000 cd/ m2.

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states.5 Although a diverse array of pincer ligands bearing the NHC motif have been employed, those based on an NHC− aryl−NHC framework are attracting special interest,6 in particular ligands where the central aryl group is directly connected to the imidazole- or benzimidazole-based NHC fragments without any linker. They generate a flat backbone resulting from two rigid five-membered metalla rings. Platinum-group metals occupy a prominent place in the chemistry of these ligands. The Hollis group has prepared Ti-,7 Zr-,8 and Hf(CCC)9 complexes containing 1,3-bis(3-butylimi-

INTRODUCTION The broad applications of N-heterocyclic carbenes (NHCs) as organocatalysts1 and excellent ligands in coordination chemistry and catalysis2 have turned them into relevant tools in modern chemistry. However, they are more than that; promising results during the past few years underline a great potential of NHC complexes in various fields of material science,3 in particular for the stabilization of blue emitters that are especially sought to make organic light-emitting diodes.4 An important class of N-heterocyclic carbene ligands is that introducing a donor atom between two carbene edges. This type of ligand provides pincer coordination, developing abilities to stabilize less common coordination polyhedra and oxidation © 2014 American Chemical Society

Received: September 2, 2014 Published: September 17, 2014 5582

dx.doi.org/10.1021/om500905t | Organometallics 2014, 33, 5582−5596

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have several potential advantages over iridium- and platinumcontaining complexes. For example, strong back-bonding from osmium to the ligand can promote MLCT transition, usually resulting in high photoluminescence quantum yields and a short excited state lifetime. A short excited-state lifetime is normally preferred since it can reduce excited state degradation and triplet−triplet annihilation. Shallow HOMO energy levels make them very good hole traps in devices, making them ideal centers for direct charge trapping and recombination. Most osmium(II) complexes reported in the literature emit green to red in the visible spectrum.29 Emissive salts are candidates for light-emitting electrochemical cells, whereas neutral compounds offer opportunities for the fabrication of OLEDs by the vapor deposition method, which is currently the dominant commercial manufacturing process. In the search for blue-emitting neutral osmium complexes with a flat backbone that provide narrow emission line widths and short excited state lifetimes, which can result in better color purity and longer device lifetime, we have investigated the reactivity of the d2 hexahydride complex OsH6(PiPr3)2 toward CbenzimidazoliumCarylCbenzimidazolium and CimidazoliumCarylCimidazolium salts. In this paper, we show a general entry to neutral Os(CCC) pincer derivatives, including Os(IV) and Os(II) hydride species and homoleptic and heteroleptic bis(tridentate) Os(II) complexes, the photoluminescent properties of the latter, and the electroluminescence properties of an OLED device of one of them. To the best of our knowledge, this is the first report on neutral blue osmium complexes using a bis(tridentate) ligand structure.

dazol-1-yl-2-idene)-2-phenylene which, in addition to catalyzing the hydroamination−cyclization of unactivated alkene amines, undergo transmetalation to Rh and Ir to afford [M(μI)(CCC)]2.10 These dimers also act as efficient catalyst precursors for the hydroamination−cyclization of alkene amines and the hydrosilylation of alkynes (Rh).10a Transmetalation to the platinum sources PtX(η4-COD) (COD = 1,5cyclooctadiene; X = Cl, Br) yields the corresponding PtX(CCC) derivatives, which are blue emitting in methanol solution with photoluminescence quantum yields of about 0.014.11 The Chianese group has reported five- and sixcoordinate Ir(III) hydrido chloride complexes from 1,3phenylene-bridged bis(benzimidazolium) chlorides, which are active catalyst precursors for arene C−H borylation, alkane dehydrogenation, and olefin isomerization.12 The Braunstein group has studied the reactivity of CimidazoliumCarylCimidazolium salts toward Ir(I) complexes. Depending on the experimental conditions, mononuclear Ir(I), dinuclear Ir(I) and Ir(III), and pincer Ir(III) derivatives were obtained from the known precursor [Ir(μ-Cl)(η4-COD)]2.13 The pincer Ir(III) complexes include [Ir(CCC)2]X (CCC = 4,6-dimethyl-1,3-bis(3butylimidazol-1-yl-2-idene)-2-phenylene; X = I, PF6), which are blue emitting in acetonitrile solution with photoluminescence quantum yields of about 0.4,14 and six-coordinate hydrido iodide compounds related to those of the Chianese group. The consequence of the increased steric hindrance of N substituents on the catalytic activity in the transfer dehydrogenation of cyclooctane has been also evaluated for the hydride iodide derivatives.15 The Zhong group has described Ru(II) salts containing a CimidazolylideneCarylCimidazolylidene ligand and a neutral tridentate group, which exhibit low RuII/RuIII redox potentials,16 whereas the Hwang group has observed that related carbonyl species are efficient catalyst precursors for the hydrogen transfer from 2-propanol to ketones.17 The osmium pincer complexes are very scarce in comparison with the plethora of derivatives of this type reported with the rest of the platinum-group metals. The chemistry of the osmium pincer has been mainly focused on a few PCP,18 PNP,19 and POP20 compounds along with some NNN,21 CNN,22 CNO,23 and CNC24 derivatives. The Wong group has also reported Os(II) salts, [Os(CCC)(CO)(NN)]X, containing NHC-based pincer ligands such as 1,3-bis(3-methylimidazol-1-yl-2-idene)-2-phenylene and 1,3-bis(3-methylbenzimidazol-1-yl-2-idene)-2-phenylene and aromatic diamines, which are emissive in the red spectral region with quantum yields between 10−4 and 10−2.25 OLED technology has been considered as the next generation for display and lighting applications. OLEDs using phosphorescent emitters, i.e. PHOLEDs, have attracted tremendous attention due to their superior properties in comparison to OLEDs using fluorescent emitters.26 An internal quantum efficiency of 100% has been realized for red, green, and blue PHOLEDs.27 Red and green PHOLEDs have met the commercial requirements for display and lighting applications. However, blue PHOLEDs remain a challenge. Heavy-metal ions containing cyclometalated groups have been investigated as phosphorescent emitters, such as iridium(III), platinum(II), and osmium(II) complexes.28 Out of the three, osmium(II) is the least studied metal partially due to the difficulty of synthesis and ligand design. Furthermore, it is harder to realize blue emission from an osmium(II) complex than from both iridium and platinum due to its shallow HOMO energy levels. However, if blue can be realized, osmium(II) complexes do

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RESULTS AND DISCUSSION Osmium(II) Hydrido Carbonyl Complexes: Reactions with BF4− Salts. The cleanest synthetic strategy for the preparation of transition-metal NHC complexes is undoubtedly direct metalation.30 The method requires the presence at the starting complex of some strong Brønsted base, which affords labile ligands as a result of the deprotonation of the imidazolium salts. Neutral transition-metal polyhydride complexes are basic enough to produce the deprotonation of the imidazolium salts. According to this, complex OsH6(PiPr3)2 (1) reacts with N-alkyl- and N-arylimidazolium salts to give osmium polyhydride derivatives containing a monodentate or chelate NHC ligand with normal or abnormal coordination depending upon the bulkiness of the N substituents.31 In addition, this hexahydride has proven to activate σ bonds of a wide range of organic molecules,32 in particular when the substrate contains a heteroatom binding site.33 In the light of these precedents, we decided to start the work exploring the reactivity of 1 toward the tetrafluoroborate salts of 1,3-bis(3methylbenzimidazolium-1-yl)benzene and 1,3-bis(3-methylimidazolium-1-yl)benzene. Treatment of dimethylformamide solutions of 1 with 1.0 equiv of these salts, under reflux, for 40 min leads to the Os(II) hydride carbonyl complexes 2 and 3 (Scheme 1), as a result of a one-pot synthesis of three different reactions, including the direct metalation of one of the NHC units of the salt, the activation of the C−H bond at the 6position of the bridged aryl group, and metal carbonylation by solvent decarbonylation.34 Complexes 2 and 3 were isolated as white solids with moderate yields of 40% and 25%, respectively. Complex 2 has been characterized by X-ray diffraction analysis. The structure (Figure 1) confirms the success of the one-pot process. The coordination geometry around the osmium atom can be rationalized as derived from a distorted 5583


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The H···F interactions break apart in dichloromethane solution at room temperature. Accordingly, the 1H NMR spectra of 2 and 3 show the N2CH resonances at 9.40 and 9.12 ppm, respectively, as singlets instead of the expected triplets resulting from H−F spin coupling. The hydride ligand displays, at −8.30 ppm for 2 and −8.81 ppm for 3, a triplet with a H−P coupling constant of about 26 Hz. In the 13C{1H} NMR spectra, the metalated carbene resonances appear at 193.7 (2) and 187.4 ppm (3), whereas the metalated aryl signals are observed at 163.6 (2) and 162.1 ppm (3). As expected for the free rotation of the benzimidazolium or imidazolium moieties around the aryl−N bond, the 31P{1H} NMR spectra show singlets at about 23 ppm. Carbonyl bands at 1862 (2) and 1874 cm−1 (3) in the IR spectra are also a characteristic feature of these compounds. Os(IV) Dihydride and Os(II) Monohydride Complexes: Influence of the Anion of the Salts. The C−H bond activation of the central aryl group at the 6-position, with the second benzimidazolium of imidazolium moiety disposed in a para position, prevents the metalation of the latter. However, activation at the 2-position should facilitate the addition of the second NHC unit. Indeed, in the reactions of both 131a and IrH5(PPh3)238 with imidazolium salts, the nature of the anion markedly influences the stoichiometry of the resulting complexes, as well as the normal versus abnormal coordination mode of the NHC ligands. For the salts here used, the anion appears to determine the broken C−H bond position of the central aryl group, and therefore, the chelate or pincer nature of the resulting ligand. Thus, in contrast to the case for the BF4− counterparts the treatment of 1 with 1.0 equiv of the iodide salts of 1,3-bis(3-methylbenzimidazolium-1-yl)benzene, 1,3bis(3-methylimidazolium-1-yl)benzene, and 1,3-bis(methylbenzimidazolium-1-yl)-5-trifluoromethylbenzene in dimethylformamide, under reflux, for 20 min leads to the iodide salts of the Os(IV) dihydride pincer complexes 4a−6a (Scheme 2), which were isolated as white solids in about 40% yields, as a consequence of the C−H bond activation at the 2-position and the metalation of both NHC units. Iodide salts 4a−6a are fairly acidic, and therefore they can be easily deprotonated. Thus, the addition of 1.3 equiv of potassium tert-butoxide to tetrahydrofuran solutions of the dihydrides, at room temperature, affords the corresponding osmium(II) monohydride derivatives 7−9, which were isolated as yellow solids in about 90% yield. The hydride abstraction is reversible; the addition of 1.0 equiv of HBF4·OEt2 to diethyl ether solutions of 7−9, at room temperature, produces the quantitative precipitation of the corresponding tetrafluoroborate salts 4b−6b, which cannot be obtained by direct reaction between 1 and the tetrafluoroborate benzimidazolium or imidazolium salts. The formation of the dihydrides was confirmed by means of the X-ray structure of 4b. Figure 2 shows a view of the cation of the salt. As expected for the pincer coordination, the Os(CCC) skeleton is T-shaped with the osmium atom situated in the common vertex and CNHC−Os−CNHC and CNHC−Os−Caryl angles of 148.43(12)° (C(1)−Os−C(15)), 73.96(12)° (C(1)−Os−C(10)) and 74.48(12)° (C(15)−Os−C(10)). Thus, the coordination geometry around the metal center can be rationalized as a distorted pentagonal bipyramid with axial phosphines (P(1)−Os−P(2) = 163.75(3)°) and the hydride ligands lying in the equatorial plane along with the pincer. The Os−CNHC bond lengths of 2.087(3) Å (Os−C(1)) and 2.080(3) Å (Os−C(15)), as well as the Os−Caryl distance of

Scheme 1

Figure 1. ORTEP diagram of complex 2 (50% probability ellipsoids). Hydrogen atoms (except the hydride and H(15)) are omitted for clarity. Selected bond lengths (Å) and angles (deg): Os−C(1) = 2.141(5), Os−C(7) = 2.075(5), H(15)···F(1A) 2.60(6), H(15)··· F(4A) 2.59(6); P(1)−Os−P(2) = 161.57(5), C(1)−Os−C(7) = 76.4(2), C(1)−Os−C(41) = 177.8(3), C(7)−Os−H(01) = 162(3), C(7)−Os−C(41) = 101.4(3).

octahedron, with the phosphine ligands occupying trans positions (P(1)−Os−P(2) = 161.57(5)°). The perpendicular plane is formed by the aryl C(1) and carbene C(7) atoms of the generated chelate ligand, which acts with a bite angle of 76.4(2)°, the hydride trans to the carbene unit (C(7)−Os− H(01) = 162(3)°), and the carbonyl ligand disposed trans to the aryl group (C(1)−Os−C(41) = 177.8(3)°). The Os−C(1) bond length of 2.141(5) Å compares well with the Os−aryl distances found in other five-membered osmacycles resulting from ortho-metalation reactions,35 whereas the Os−C(7) bond length of 2.075(5) Å agrees well with those reported for Os NHC compounds with normal coordination of the NHC unit.31,36 In addition, we should mention the separations between the hydrogen atom H(15) of the benzimidazolium moiety and the fluorine atoms F(1A) and F(4A) of the tetrafluoroborate anion, 2.60(6) and 2.59(6) Å, which are slightly shorter than the sum of the van der Waals radii of the hydrogen and fluorine (rvdW(H) = 1.20, rvdW(F) = 1.47 Å),37 suggesting ion pairing. 5584


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Scheme 2

the metalated aryl carbon atom, which is observed between 170 and 160 ppm. The 31P{1H} NMR spectra contain a singlet at about 4 ppm, which is split into a triplet under off-resonance conditions. The existence of the Os(II) monohydride complexes was also confirmed by X-ray diffraction analysis, in this case through the structure of 7. Figure 3 gives a view of the molecule. The coordination around the osmium atom can be described as a distorted octahedron with trans phosphines (P(1)−Os−P(2) = 152.84(5)°). The perpendicular plane is formed by the pincer, which acts with CNHC−Os−CNHC and CNHC−Os−Caryl angles of 151.0(2)° (C(1)−Os−C(15)), 75.5(2)° (C(1)−Os−C(9)), and 75.5(2)° (C(15)−Os−C(9)), and the hydride. The Os− Figure 2. ORTEP diagram of the cation of complex 4b (50% probability ellipsoids). Hydrogen atoms (except the hydrides) are omitted for clarity. Selected bond lengths (Å) and angles (deg): Os− C(1) = 2.087(3), Os−C(10) = 2.079(3); Os−C(15) = 2.080(3); P(1)−Os−P(2) = 163.75(3), C(1)−Os−C(15) = 148.43(12), C(1)− Os−C(10) = 73.96(12), C(10)−Os−C(15) = 74.48(12).

2.079(3) Å (Os−C(10)), compare well with those of 2. The 1 H, 13C{1H}, and 31P{1H} NMR spectra of the salts 4a−6a and 4b−6b, in acetonitrile-d3, at room temperature are consistent with the high symmetry of this structure. The equivalent hydride ligands display a triplet between −4.5 and −7.0 ppm, with H−P coupling constants of about 14 Hz, in the 1H NMR spectra. In agreement with the dihydride nature of these compounds, the high-field resonance of 5b displays a 400 MHz T1(min) value of 115 ms at 213 K, which is consistent with the separation of 1.49 Å between the hydride ligands of 4b. In the 13 C{1H} NMR spectra, the most noticeable resonances are that due to the equivalent metalated NHC carbon atoms, which appear between 190 and 175 ppm, and that corresponding to

Figure 3. ORTEP diagram of complex 7 (50% probability ellipsoids). Hydrogen atoms (except the hydride) are omitted for clarity. Selected bond lengths (Å) and angles (deg): Os−C(1) = 2.053(6), Os−C(9) = 2.033(6), Os−C(15) = 2.039(6); P(1)−Os−P(2) = 152.84(5), C(1)− Os−C(15) = 151.0(2), C(1)−Os−C(9) = 75.5(2), C(9)−Os−C(15) = 75.5(2). 5585


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CNHC bond lengths of 2.053(6) Å (Os−C(1)) and 2.039(6) Å (Os−C(15)), as well as the Os−Caryl distance of 2.033(6) Å (Os−C(9)), agree well with those of 2 and 4b. The 1H, 13 C{1H}, and 31P{1H} NMR spectra of 7−9, in benzene-d6, at room temperature are consistent with the structure shown in Figure 3. In agreement with the presence of the hydride ligand in the compounds, the 1H NMR spectra contain a high-field resonance between −9 and −10 ppm, which appears as a triplet with a H−P coupling constant of about 34 Hz. In the 13C{1H} NMR spectra the resonance due to the equivalent metalated NHC carbon atoms is observed between 198 and 184 ppm, whereas that corresponding to the metalated aryl carbon atom appears at about 173 ppm. The 31P{1H} NMR spectra contain a singlet between 22 and 18 ppm, which is split into a doublet under off-resonance conditions. Homoleptic and Heteroleptic Os(II) Bis(tridentate) Complexes. Homoleptic osmium(II) derivatives containing two CCC pincer ligands can be prepared starting from the corresponding Os(II) CCC monohydride compounds (eq 1).

Figure 4. ORTEP diagram of complex 10 (50% probability ellipsoids). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Os(1)−C(1) = 2.042(8), 2.051(8), Os(1)−C(10) = 2.048(7), 2.057(7), Os(1)−C(15) = 2.026(8), 2.037(8), Os(1)− C(23) = 2.049(7), 2.037(7), Os(1)−C(32) = 2.045(7), 2.049(8), Os(1)−C(37) = 2.043(7), 2.032(8); C(1)−Os(1)−C(10) = 74.6(3), 74.5(3), C(10)−Os(1)−C(15) = 75.0(3), 75.5(3), C(23)−Os(1)− C(32) = 75.2(3), 75.5(3), C(32)−Os(1)−C(37) = 74.8(3), 75.1(3), C(1)−Os(1)−C(15) = 149.6(3), 149.9(3), C(23)−Os(1)−C(37) = 150.0(3), 150.6(3), C(10)−Os(1)−C(32) = 177.8(3), 178.6(3).

The Os(II) CCC monohydride complexes 7−9 are also useful starting materials to prepare osmium(II) derivatives containing two different CCC pincer ligands (Scheme 3). Thus, the treatment of dimethylformamide solutions of 9 with 1.0 equiv of the tetrafluoroborate salts of 1,3-bis(3-methylbenzimidazolium-1-yl)benzene and 1,3-bis(3-methylimidazolium-1yl)benzene, in the presence of 15 equiv of triethylamine, under reflux leads to the heteroleptic Os(II) bis(tridentate) derivatives 12 and 13 of the types Os(CNHCCarylCNHC)(CNHCCaryl′CNHC) and Os (CNHCCaryl′CNHC)(CNHC′CarylCNHC′), respectively, which can be also obtained by reaction of the respective monohydrides 7 and 8 with 1,3-bis(3-methylbenzimidazolium-1-yl)-5-trifluoromethylbenzene tetrafluoroborate, under the same conditions. Complexes 12 and 13 were isolated as pure yellow solids in 53% and 16% yields, respectively. The existence of this novel type of compound was confirmed by X-ray diffraction analysis through the structure of 12. Figure 5 gives a view of the molecule. The atom distribution around the metal center is as in 10, with the bridged 5trifluoromethylbenzene in the position of one of the phenyl groups of the homoleptic compound. The replacement does not generate significant changes in the structural parameters. Thus, the Os−CNHC (2.043(5)-2.046(5) Å) and Os−Caryl (2.037(5) and 2.061(5) Å) bond lengths as well as the CNHC−Os−CNHC (150.40(19) and 150.09(19)°) and Caryl− Os−Caryl (178.34(19)°) angles lie in the range of those of 10. As expected, the metalated NHC (193−179 ppm) and aryl (180−170 ppm) resonances in the 13C{1H} NMR spectra of 12 and 13, in benzene-d6, are also consistent with those of 10 and 11. This methodology using Os(II) CCC monohydride complexes as starting materials allows us to prepare even heteroleptic Os(CNHCCarylCNHC)(CNHCCaryl′CNHC) derivatives containing a bridging heterocycle (Scheme 4). Treatment of dimethylformamide solutions of 7 with 1.0 equiv of 3,5-bis(3-

Treatment of dimethylformamide solutions of 7 with 1.0 equiv of 1,3-bis(3-methylbenzimidazolium-1-yl)benzene tetrafluoroborate, under reflux, for 2 h leads to a mixture of the neutral homoleptic Os(CNHCCarylCNHC)2 complex 10 and the salt 4b. The addition of triethylamine to the reaction mixture increases the amount of 10 while it decreases that of 4b. Thus, in the presence of 15 equiv of the amine, complex 10 becomes the main metal species in the mixture. Under the same conditions, the reaction of 9 with 1,3-bis(3-methylbenzimidazolium-1-yl)5-trifluoromethylbenzene tetrafluoroborate yields 11. These compounds were isolated as pure yellow solids in 60% and 42% yields, respectively. Complex 10 has been characterized by X-ray diffraction analysis. The structure has two chemically equivalent but crystallographically independent molecules in the asymmetric unit. Figure 4 shows a drawing of one of them. The geometry around the osmium atom can be rationalized as a distorted octahedron where the pincer ligands occupy mer positions with CNHC−Os−CNHC angles of 149.6(3) and 149.9(3)° (C(1)− Os−C(15)) and 150.0(3) and 150.6(3)° (C(23)−Os−C(37)) and the metalated aryl groups are disposed trans (C(10)−Os− C(32)) = 177.8(3) and 178.6(3)°). The Os−CNHC (2.026(8)− 2.051(8) Å) and Os−Caryl (2.045(7)−2.057(7) Å) bond lengths compare well with those of 2, 4b, and 7. In agreement with the high symmetry of this type of compound, the 13C{1H} NMR spectra of 10 and 11 in benzene-d6 show a resonance for the four equivalent metalated NHC carbon atoms at about 192 ppm and a signal for the two equivalent metalated aryl carbon atoms at 171.1 ppm for 10 and 178.0 ppm for 11. 5586


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Scheme 3

group in the complex is strongly supported by the 1H NMR spectrum, in dichloromethane-d2, which contains an NH resonance at 10.60 ppm. In agreement with 12 and 13, the 13 C{1H} NMR spectrum shows two resonances for the metalated carbon atoms of the carbene moieties at 193.0 and 191.7 ppm and two resonances for the metalated carbon atoms of the bridging groups at 192.7 and 167.5 ppm. As expected, the pyridinium group is easily deprotonated. Thus, the addition of 1.3 equiv of potassium tert-butoxide to a tetrahydrofuran solution of 14 produces the abstraction of the NH proton to afford the neutral derivative 15, which was isolated as a yellow solid in 91% yield. In the 13C{1H} NMR spectrum, in benzened6, the metalated NHC resonances are observed at 193.7 and 192.3 ppm, whereas the signals due to the metalated carbon atoms of the bridging groups appear at 186.9 and 171.0 ppm. Photophysical Properties of the Bis(tridentate)−Os(II) Complexes. The absorption spectra of toluene solutions of the homoleptic complexes 10 and 11 and the heteroleptic derivatives 12, 13, and 15, at 298 K, show a similar profile (Figure 6). They exhibit two main bands in the visible region: an intense absorption between 341 and 363 nm and a less intense band at lower energies (396−405 nm). To gain deeper insight into the electronic properties of the transitions involved in the absorption process, the homoleptic complex 10 and the heteroleptic derivative 12 were studied by means of timedependent density functional theory (TD-DFT) under vacuum. The M06 functional developed by Truhlar and Zhao was used in these calculations.39 A very good agreement between the vertical excitation energies and the wavelengths of the absorption maxima in the experimental spectra was found (Table 1), which allows the accurate assignment of the

Figure 5. ORTEP diagram of complex 12 (50% probability ellipsioids). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Os−C(1) = 2.046(5), Os−C(16) = 2.046(5), Os−C(24) = 2.043(5), Os−C(38) = 2.043(5), Os−C(10) = 2.037(5), Os−C(33) = 2.061(5); C(1)−Os−C(10) = 75.35(19), C(10)−Os−C(16) = 75.05(19), C(24)−Os−C(33) = 74.92(19), C(33)−Os−C(38) = 75.20(19), C(1)−Os−C(16) = 150.40(19), C(24)−Os−C(38) = 150.09(19), C(10)−Os−C(33) = 178.34(19). 150.40(19), C(24)−Os−C(38) = 150.09(19), C(10)−Os−C(33) = 178.34(19).

methylbenzimidazolium-1-yl)-2,6-dimethylpyridine tetrafluoroborate, in the presence of 15 equiv of triethylamine, under reflux leads to 14 containing two different CCC pincer ligands, one of them with a bridging pyridinium. This salt was isolated as an orange solid in 30% yield. The presence of the pyridinium Scheme 4

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Figure 6. UV/vis spectra of complexes 10−13 and 15. All measurements were carried out in dilute solutions in toluene (5.5 × 10−5 M, except for 10 (2.4 × 10−4 M)) at 298 K.

Table 1. UV/Vis Experimental Dataa for Complexes 10−13 and 15 and Corresponding Computed TD-DFT Gas-Phase Vertical Excitation Energies for Derivatives 10 and 12 complex

obsd λ (nm) (ε (104 M−1 cm−1))

10

363 (1.91), 405 (0.87)

11 12

341 (5.11), 396 (1.93) 341 (4.04), 403 (1.51)

13

332 (2.44), 356 (2.81), 398 (1.09) 333 (2.87), 351 (3.13) 397 (1.20)

15

Figure 7. Computed HOMO-1, HOMO, and LUMO+2 of complexes 10 (a) and 12 (b) (isosurface value of 0.025 au).

calcd λ (nm)b 330 (0.3446), 336 (0.3953), 383 (0.0652), 386 (0.0624)

complex 12, its emission energy is between those of the homoleptic counterparts 10 and 11. Thus, it displays a band at 490 nm, both in the solid state and in toluene at room temperature. In toluene at 77 K, the emission is shifted toward the blue (466 nm). Complex 13, containing an imidazolylidene pincer ligand, contains an intense emission in the yellow region at about 580 nm, in the solid state. In agreement with the trend observed for these compounds, the emission is blue-shifted in toluene (489 nm, at room temperature and 469 nm, at 77 K). Complex 15, containing a pincer ligand with a heterocyclic bridge, is hardly emissive (Φ < 0.01) and shows the same behavior as 13. An emission wavelength can be estimated through the difference in energies between the optimized triplet excited state and the singlet state with the same geometry as the optimized triplet excited state.41 Keeping this in mind, we have computed the emission wavelengths in the gas phase for complexes 10 and 12. The obtained values, 531 nm for 10 and 523 nm for 12, are in good agreement with the experimental values. Thus, the S0−T1 transition can be attributed to metalto-ligand charge transfer with a remarkable π−π* HOMO to LUMO. Electroluminescence Properties of an OLED Device Based on 11. In comparison to the Ir and Pt analogues, there are relatively few reports on cyclometalated osmium complexes with pincer type carbene ligands.25 In fact, neutral bis-pincer osmium complexes with blue emission are unprecedented in the literature. Compound 11 showed light blue emission with a reasonably high photoluminescence quantum yield. Thus, it was chosen as an emitter for device fabrication. The device has the following structure: ITO/HAT-CN (10 nm)/TAPC (30 nm)/10% 11: mCP (30 nm)/TmPyPB (40 nm)/LiF/Al. The structure of the material is illustrated in Figure 9. The device was fabricated by high-vacuum ( 2σ(I)) and wR2 = 0.0652, 11200/2/597 data/restraints/parameters, GOF = 1.029, largest peak and hole 1.678 (close to osmium atom) and −1.196 e/Å3. Crystal data for 7: C40H60N4OsP2, mol wt 849.06, yellow, irregular block (0.10 × 0.08 × 0.06), monoclinic, space group P21/n, a = 15.091(5) Å, b = 16.749(5) Å, c = 15.335(5) Å, β: 92.462(4)°, V = 3873(2) Å3, Z = 4, Z′ = 1, Dcalcd = 1.456 g cm−3, F(000) = 1736, T = 100(2) K, μ = 3.408 mm−1. 30168 measured reflections (2θ: 3−58°, ω scans 0.3°), 9126 unique reflections (Rint = 0.072), minimum/ maximum transmission factors 0.682/0.842, final agreement factors R1 = 0.0498 (6555 observed reflections, I > 2σ(I)) and wR2 = 0.0936, 9126/0/441 data/restraints/parameters, GOF = 1.037, largest peak and hole 2.622 (close to osmium atom) and −2.190 e/Å3. Crystal data for 10: C44H34N8Os·0.75C5H12, mol wt 919.10, yellow, irregular block (0.10 × 0.05 × 0.03), monoclinic, space group P21/n, a = 21.403(2) Å, b = 17.8070(19) Å, c = 22.402(2) Å, β = 107.190(2)°, V = 8156.5(15) Å3, Z = 8, Z′ = 2, Dcalcd = 1.497 g cm−3, F(000) = 3692, T = 100(2) K, μ = 3.171 mm−1, 60342 measured reflections (2θ = 3−58°, ω scans 0.3°), 15144 unique reflections (Rint = 0.0962), minimum/maximum transmission factors 0.673/0.842, final agreement factors R1 = 0.0551 (10144 observed reflections, I > 2σ(I)) and wR2 = 0.1180, 15144/14/1016 data/restraints/parameters, GOF = 0.987, largest peak and hole 1.735 (close to osmium atom) and −2.133 e/Å3. Crystal data for 12: C45H33F3N8Os·CH3CN·2C7H8, mol wt 1158.32, colorless, irregular block (0.17 × 0.11 × 0.07), triclinic, space group P1̅, a = 14.5098(7) Å, b = 14.5147(7) Å, c = 14.6079(7) Å, α = 67.9870(10)°, β = 63.5090(10)°, γ = 69.7170(10)°, V = 2492.3(2) Å3, Z = 2, Z′ = 1, Dcalcd = 1.543 g cm−3, F(000) = 1168, T = 100(2) K, μ = 2.621 mm−1. 27927 measured reflections (2θ = 3−50°, ω scans 0.3°), 11593 unique reflections (Rint = 0.0362), minimum/ maximum transmission factors 0.756/0.862, final agreement factors R1 = 0.0428 (10083 observed reflections, I > 2σ(I)) and wR2 = 0.1060, 11593/9/586 data/restraints/parameters, GOF = 1.079, largest peak and hole 1.795 (close to osmium atom) and −1.077 e/Å3. Computational Details. The theoretical calculations were carried out on the model complexes by optimizing the structures at the M06DFT levels with the Gaussian 09 program.49 The basis sets used were LANL2DZ basis and pseudopotentials for Os and 6-31G(d,p) for the rest of the atoms. All minima were verified to have no negative frequencies. All calculations were carried in vacuo. For both 10 and 12 we performed TD-DFT calculations at the same level of theory, calculating the lowest 75 singlet−singlet and 8 singlet−triplet excitations at the ground state S0 and the lowest 8 singlet−singlet and 8 singlet−triplet excitations at the lowest excited triplet T1 optimized geometries. It has to be noted that the singlet−triplet excitations are set to 0 due to the neglect of spin−orbit coupling in the TD-DFT calculations as implemented in G09. Photophysical Studies. All manipulations of the organometallic compounds were carried out in the strict absence of oxygen and water. UV−vis spectra were recorded on an Evolution 600 spectrophotometer. Steady-state photoluminescence spectra were recorded with a Jobin-Yvon Horiba Fluorolog FL-3-11 spectrofluorometer. An IBH 5000F coaxial nanosecond flash lamp was used to measure the lifetimes.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for M.A.E.: [email protected]. *E-mail for C.X.: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Spanish MINECO (Projects CTQ2011-23459 and Consolider Ingenio 2010 (CSD200700006)), the DGA (E35), the European Social Fund (FSE), and Universal Display Corporation is acknowledged. We thank the Centro de Supercomputación de Galicia (CESGA) and the Instituto de Biocomputación y Fı ́sica de Sistemas Complejos (BIFI) for the generous allocation of computational resources.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF files giving normalized excitation and emission spectra for complexes 10−13 and 15, full ref 49, computational details, and positional and displacement parameters, crystallographic data, and bond lengths and angles of compounds 2, 4b, 7, 10, and 12. This material is available free of charge via the Internet at http://pubs.acs.org. 5594


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Organometallics

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