Blue-Green Luminescent Rhenium(I) Tricarbonyl Complexes with

May 9, 2012 - Tatsuhiko Mukuta , Peter V. Simpson , Jamila G. Vaughan , Brian W. Skelton ... Sara Muzzioli , David H. Brown , Stefano Stagni , and Mas...
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Blue-Green Luminescent Rhenium(I) Tricarbonyl Complexes with Pyridine-Functionalized N-Heterocyclic Carbene Ligands Xiao-Wei Li,† Hong-Yan Li,† Gao-Feng Wang,† Fei Chen,† Yi-Zhi Li,† Xue-Tai Chen,*,† You-Xuan Zheng,*,† and Zi-Ling Xue‡ †

Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People’s Republic of China ‡ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *

ABSTRACT: Five rhenium(I) tricarbonyl chloride complexes with pyridinefunctionalized N-heterocyclic carbenes, Re(CO)3(L)Cl (L = 3-methyl-1-(2pyridyl)imidazol-2-ylidene (1), 3-methyl-1-(2-picolyl)imidazol-2-ylidene (2), 3methyl-1-(2-pyridyl)benzimidazolin-2-ylidene (3), 3-methyl-1-(2-picoyl)benzimidazolin-2-ylidene (4), 1-methyl-4-(2-pyridyl)-1,2,4-triazoline-5-ylidene (5)), have been synthesized by silver carbene transmetalation and characterized by elemental analysis, 1H NMR, 13C NMR, and IR spectra. The molecular structures of 3−5 have been determined by single-crystal X-ray diffraction. The electrochemical and photophysical properties of complexes 1, 3, and 5 have been studied. In both degassed CH2Cl2 solution and the solid state at room temperature or 77 K, the emission wavelengths (465−511 nm) of complexes 1, 3, and 5 lie in the blue-green region, which are rare in contrast to the reported rhenium(I) tricarbonyl complexes. The photoluminescence lifetime decays of Re(I) complexes 1, 3, and 5 were measured, and the quantum yields were calculated by using the standard sample ([Ru(bpy)3]2+(Cl−)2 in degassed acetonitrile solution; Φstd = 0.094).

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INTRODUCTION In the past decade organic light-emitting diodes (OLEDs) based on transition-metal complexes1 have received considerable attention, owing to their high emission quantum efficiencies and rich excited-state behavior. Among these metallic complexes, rhenium complexes2 often exhibit extraordinary luminescent properties and play an important role in the photophysics and photochemistry of transition-metal complexes. In particular, the rhenium(I) tricarbonyl complexes of the general formula fac-Re(CO)3(L)X, where L is a bidentate diimine ligand and X is a halogen, have been intensively studied and applied in various areas such as electroluminescent materials in OLED devices,3 solar energy conversion,4 and photocatalytic CO2 reduction5 ever since the pioneering works of Wrighton and Morse in 1974.6 The luminescence properties of the reported rhenium tricarbonyl complexes Re(CO)3(L)X were directly connected to the nature of ligand L (L = 2,2′bipyridine,2f 2-(1-ethylbenzimidazol-2-yl)pyridine,7 pyridyltriazine,8 triazolopyridine,8 2,2′-dipyridylamine,9 bipyridine derivatives,10 etc.). Tuning of luminescence properties and emission colors of these complexes can be achieved by modification of the ligand structure. N-heterocyclic carbenes (NHCs) as useful ancillary ligands have become the focus of interest in organometallic and coordination chemistry.11 Their strong σ-donor abilities result in the formation of stable M−C bonds and have been mostly applied to homogeneous catalysts.12 In comparison, their © 2012 American Chemical Society

photoluminescence properties have rarely been investigated. Only a few metal NHC complexes based on, e.g., Pt,13 Ir,14 Re,15 and Ru16 have been explored and considered to have unique luminescence behaviors. For example, Unger et al.13a prepared platinum(II) tetracarbene complexes with a bis(triazolin-5-ylidene) ligand and successfully achieved a shift of emission wavelength from the ultra-blue/near-UV to the deep blue region. Thompson and co-workers14a have developed iridium(III) NHC complexes with high luminescent quantum yields and long lifetimes, which display luminescence in the blue region. Che et al.15 obtained a series of luminescent rhenium(I) carbonyl N-heterocyclic carbene complexes containing aromatic diimine ligands, which are emissive at room temperature with a maximum emission wavelength in the range 550−620 nm. Most rhenium(I) tricarbonyl complexes show a maximum emission wavelength in the range of 520−640 nm. In order to tune the emission color of rhenium(I) complexes, we have investigated their rhenium(I) tricarbonyl complexes employing an NHC as the ancillary ligand. Here we report the synthesis and structural characterization of several new rhenium(I) tricarbonyl complexes with pyridine-functionalized NHC ligands as well as their interesting luminescent properties. Received: July 17, 2011 Published: May 9, 2012 3829

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes to give rhenium(I) tricarbonyl chloride complexes 1−5 are shown in Scheme 1. 1−5 were prepared by reacting silver carbene

presented in Table S1 (see the Supporting Information). The molecular structures are shown in Figure 1, and selected bond distances and angles are given in Table 1. Table 1. Selected Bond Lengths (Å) and Angles (deg) for 3− 5

Scheme 1. Synthetic Routes to Complexes 1−5

3 Re1−C1 Re1−C2 Re1−C3 Re1−C4 Re1−N3 Re1−Cl1 C3−Re1−C1 C3−Re1−C2 C1−Re1−C2 C3−Re1−C4 C1−Re1−C4 C2−Re1−C4 C3−Re1−N3 C1−Re1−N3 C2−Re1−N3 C4−Re1−N3 C3−Re1−Cl1 C1−Re1−Cl1 C2−Re1−Cl1 C4−Re1−Cl1 N3−Re1−Cl1

complexes via the reaction of the carbene precursors L1−L5 and Ag2O with the metal precursor Re(CO)5Cl in CH2Cl2. The crude products were purified by column chromatography on silica gel, yielding pure complexes 1−5. 1 H NMR spectra of 1−5 show the absence of signals in the range 10−12 ppm assignable to the imidazolium C2−H, benzimidazolium C2−H, and triazolium C5−H proton signals of L1−L5 (Figure 1S; see the Supporting Information), indicating the coordination of the carbene carbon to the Re atom.17 The 1H NMR spectra of 2 and 4 display two doublet signals with an AM pattern: 2 at 5.75, 5.19 ppm and 4 at 6.28, 5.35 ppm, suggesting that the two hydrogen atoms of the CH2 linker are diastereotopic. The signals of carbene carbon atoms appear at 181−192 ppm in 13C NMR spectra. The IR spectra of these complexes show three intense absorption bands in the range 1860−2020 cm−1, which are attributed to the stretching vibrations of three carbonyl groups. Molecular Structures of Complexes 3−5. Single crystals suitable for X-ray crystallography studies of compounds 3−5 were obtained by slow evaporation of dichloromethane solutions of 3−5 in the air. The crystallographic data are

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Bond Lengths 1.925(8) 1.907(8) 1.949(8) 1.954(8) 1.954(10) 1.883(8) 2.127(7) 2.148(8) 2.194(6) 2.191(6) 2.489(2) 2.5156(19) Bond Angles 91.3(4) 88.0(3) 90.2(3) 89.7(3) 87.8(4) 91.0(3) 93.8(3) 92.2(3) 99.9(3) 96.1(3) 171.2(3) 172.7(3) 92.7(3) 95.6(3) 173.1(3) 175.7(3) 97.8(3) 91.5(3) 74.2(3) 81.3(3) 177.3(3) 178.6(2) 91.4(3) 91.0(2) 90.0(2) 89.5(2) 85.7(2) 88.8(2) 84.63(19) 85.50(17)

5 1.967(10) 1.905(8) 1.893(9) 2.139(8) 2.246(7) 2.482(3) 86.1(4) 87.9(4) 88.8(4) 93.7(3) 172.0(4) 99.2(4) 93.6(3) 97.9(4) 173.2(3) 74.1(3) 177.2(3) 93.9(3) 94.8(3) 86.0(2) 83.7(2)

For complexes 3−5, the rhenium atoms are in a sixcoordinate environment with the nitrogen atoms from the pyridine ring and the carbene carbon, together with three CO ligands and one chlorine atom. Two CO ligands are trans to the pyridine nitrogen atom and carbene carbon atom, respectively. In complexes 3 and 4, one CH2Cl2 molecule is cocrystallized. The Re−Ccarbene distances are 2.127(7), 2.148(8), and 2.139(8) Å in complexes 3−5, respectively, which are very close to those of the reported rhenium NHC complexes.18 The Re−Cl bond lengths in 3−5 are 2.489(2), 2.5156(19), and 2.482(3) Å, respectively, which fall in the region of known rhenium(I) carbonyl chloride complexes.2e,19 The Re−CO and Re−Npyridine bond distances are in the ranges 1.883(8)−1.967(10) and 2.191(6)−2.246(7) Å, respectively, which are in close agreement with those in analogous rhenium(I) complexes.2f The benzimidazole and triazole rings are approximately coplanar with the pyridine ring in 3 and 5, while benzimidazole and pyridine ring form a distorted dihedral angle (54.85°) in 4.

Figure 1. Molecular structures of complexes 3 (a), 4 (b), and 5 (c) with ellipsoids at the 30% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. 3830

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−4.74 − Eonset(Red) (Eonset(Red) = −0.99 V for 3 and −0.98 V for 5). These calculations give HOMO and LUMO energy levels of −5.97, −5.88, and −6.04 eV and −3.76, −3.75, and −3.76 eV for 1, 3, and 5, respectively. Photophysical Properties. The emissions of complexes 2 and 4 are too weak to be detected in both the solid state and solution at room temperature, while complexes 1, 3, and 5 show very interesting photoluminscent properties. This difference could be explained from the view of molecular structures. The imidazole, benzimidazole, and triazole rings are approximately coplanar with the pyridine ring in complexes 1, 3, and 5, leading to their rigidity. However, there is a methylene bridge between imidazole or benzimidazole and pyridine rings in 2 and 4, leading to a more flexible molecular structure with weaker emission. Ward et al.9 reported that the coordination ring of the ligand with Re(I) also can affect the luminescence. In complexes 2 and 4, coordination of the ligand at the Re(I) center takes place with formation of a six-membered chelate ring, as opposed to a five-membered ring for complexes 1, 3, and 5. This is likely to result in lower ligand field strength in the present cases. As a consequence, lower-lying, thermally accessible d−d MC levels could provide an efficient nonradiative path for disposal of the excitation energy in the Re(I) complexes reported here. This behavior is well-known for derivatives of [Ru(bipy)3]2+, in which the ligand field strength around the metal is reduced by steric distortions22 and has recently been demonstrated for a range of Re(I) tricarbonyl diimine complexes,23 although it should be noted that a range of other nonradiative decay pathways are in principle available.23,24 The absorption spectra of ligands L1, L3, and L5, together with the corresponding rhenium complexes 1, 3, and 5 in CH2Cl2 solution are given in Figure 3S (see Supporting Information). Figure 3 shows the excitation and emission spectra of complexes 1, 3, and 5 in degassed CH2Cl2 solution together with emission spectra of 1, 3, and 5 in the solid state at room temperature. The emission spectra of 1, 3, and 5 in CH2Cl2 at 77 K are shown in Figure 4S (Supporting Information). The photophysical data of Re(I) complexes 1, 3, and 5 are given in Table 2. The absorption spectra of complexes 1, 3, and 5 in CH2Cl2 exhibit two strong absorption peaks at approximately 220−285 nm that can be ascribed to intraligand π−π* transitions by comparison with the absorption spectra of free ligands (Figure 3S, Supporting Information). The low-energy broad bands at 355, 359, and 354 nm for complexes 1, 3, and 5, respectively,

The Ccarbene−Re−N bond angles in the five-membered chelating ring of 3 (74.2(3)°) and 5 (74.1(3)°) are smaller than those in the six-membered rings of 4 (81.3(3)°), which is possibly due to the presence of a methylene bridge between pyridine and the benzimidazole ring in 4. The CCO−Re−CCO bond angles in complexes 3−5 are in the range of 86.1(4)− 91.3(4)°, which are close to 90°. Electrochemistry. Cyclic voltammograms of Re complexes 1, 3, and 5 in Figure 2 show irreversible metal-centered

Figure 2. Cyclic voltammograms for compounds 1, 3, and 5 (5 × 10−4 M) in CH2Cl2 solutions of (Bu4N)PF6 (0.1 M) at a sweep rate of 0.1 V/s.

oxidation and ligand-based reduction in CH2Cl2 solution, which are consistent with the redox behavior of Pybm-based Re(I) diimine complexes reported in the literature.7 Complex 1 shows an irreversible anodic wave at E1/2 = +1.30 V with an onset oxidation potential of +1.23 V vs SCE and an irreversible cathodic wave at E1/2 = −1.07 V with an onset reduction potential of −0.98 V vs SCE. The anodic wave is associated with a Re(I)-based oxidation process (ReI/ReII), and the cathodic wave is associated with a ligand-based reduction process ([ReICl(CO)3(L)]/[ReICl(CO)3(L•)]−).20 Complexes 3 and 5 show similar redox behaviors. The energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are calculated from the onset oxidation (Eonset(Ox)) and reduction (Eonset(Red)) potentials with the formulas EHOMO = −4.74 − Eonset(Ox) (−4.74 V for SCE with respect to the zero vacuum level21 and Eonset(Ox) = +1.14 V for 3 and +1.30 V for 5) and ELUMO =

Figure 3. (a) Excitation and emission spectra of rhenium(I) complexes 1, 3, and 5 in degassed CH2Cl2 solution (1 × 10−3 M) under ambient conditions. (b) Emission spectra of rhenium(I) complexes 1, 3, and 5 in the solid state at room temperature. 3831

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Table 2. Photophysical Data of Re(I) Complexes 1, 3, and 5 at Room Temperature medium

abs λ/nm (ε/103 dm3 M−1 cm−1)

CH2Cl2 solid CH2Cl2 (77 K)

227 (55), 277 (16), 355 (6.7)

CH2Cl2 solid CH2Cl2 (77 K)

228 (61), 282 (25), 359 (8.9)

CH2Cl2 solid CH2Cl2 (77 K)

229 (45), 267 (17), 354 (6.4)

excitation λa/nm

emission λb/nm

Compound 1 312, 400 369 394, 420 Compound 3 330, 413 374 421 Compound 5 307, 401 369 372, 400

Φc (degassed)

504 465 470

0.028

502 480 472

0.034

511 465 476

0.030

τ

3.12, 12.76 ns (χ2 = 1.088) 3.26 μs (χ2 = 0.978)

32.75, 144.11 ns (χ2 = 1.056) 3.81 μs (χ2 = 0.991)

17.81, 57.04 ns (χ2 = 1.143) 4.76 μs (χ2 = 0.987)

Excitation wavelength. bEmission wavelength. cQuantum yields were calculated using [Ru(bpy)3]2+(Cl−)2 in degassed acetonitrile solution (Φstd = 0.094) as standard. a

order 5 > 1 ≅ 3. This trend could be due to the different electronic donor capacities of the N-heterocyclic carbenes. It is well-known that N-heterocyclic carbene is a strong electron donor, which could heighten the energy of the π* orbital, leading to the bathochromic shift of the emission peak. Because the electronic donor capacity of triazole-based carbene is weaker than those of imidazole- and benzimidazole-based carbenes, a longer emission wavelength was observed for 5. The luminescence quantum yields for complexes 1, 3, and 5 in deaerated dichloromethane were also measured. The quantum yield of complex 3 (Φ = 0.034) in deaerated CH2Cl2 solution is higher than those of 1 (Φ = 0.028) and 5 (Φ = 0.030). The imidazole, benzimidazole, and triazole rings are approximately coplanar with the pyridine ring in complexes 1, 3, and 5, which results in the rigidity of these complexes. However, complex 3 should be more rigid than 1 and 5 due to the presence of a benzimidazole ring, which results in the higher quantum yield of 3. For complexes 1, 3, and 5, the excited-state lifetimes have also been measured in the solid state at room temperature and in CH2Cl2 at 77 K. The decay lifetimes of these complexes are relatively short on the nanosecond time scale at room temperature, also in agreement with the reported data for Re(I) carbene complexes.15,27 As indicated in Figure 5S (Supporting Information), the lifetime curves of 1, 3, and 5 are composed of biexponential decays with lifetimes of 3.12 (45.79%) and 12.76 ns (54.21%) for 1, 32.75 (43.43%) and 144.11 ns (56.57%) for 3, and 17.81 (63.83%) and 57.04 ns (35.17%) for 5, respectively, and the equation is Fit = A + B1e(−t/τ1) + B2e(−t/τ2). The long-lived component is assigned to the emission from the MLCT state, and the shorter-lived component is ascribed the π−π* state or is due to the effect of a quencher such as O2 because the samples were measured in a normal atmosphere. Generally speaking, if there is a potential surface crossing from the higher π−π* state of the pyridinefunctionalized N-heterocyclic carbene to the lower MLCT state, one can expect a decay lifetime from the π−π* state shorter than that for the lower MLCT state on the basis of the energy gap law.26,28 Considering that the lifetimes in deaerated CH2Cl2 glass at the low temperature of 77 K are on the microsecond time scale (3−5 μs, Table 2 and Figure 5S (Supporting Information)) composed of single-exponential decay, the shorter lifetime measured at room temperature should be due to the effect of the O2 quencher.

should be assigned to a metal-to-ligand charge-transfer transition (MLCT) (dπ(Re)-π*(py-NHC)). It is obvious that all MLCT absorption bands of complexes 1, 3, and 5 are similar to those of the analogous Re(I) carbonyl complexes and displayed a small blue shift compared to those of the reported complexes, such as [Re(CO)3(bipy)Cl]2f (λmax 371 nm), [Re(CO)3(PNB)Cl]25 (PNB = 2-(2-pyridyl)-3(naphthylmethyl)benzimidazole, λmax 388 nm) and [Re(CO)3(PFB)Cl]25 (PFB = 2-(2-pyridyl)-3((pentafluorobenzyl)methyl)benzimidazole, λmax 394 nm). Figure 3a shows the excitation and emission spectra of complexes 1, 3, and 5 in deaerated CH2Cl2 solution. The MLCT band in the absorption spectra has resolved into two bands in the scanning excitation spectra: namely, the 1MLCT and 3MLCT states at 312 and 400 (1), 330 and 413 nm (3), and 307 and 401 nm (5), respectively. Upon excitation at either the π−π* absorption band or the MLCT absorption peaks, complexes 1, 3, and 5 in deaerated CH2Cl2 solution exhibit the same MLCT emission in the range of 502−511 nm, except for the concurrent change in emission intensities. This observation indicates that the potential surface crossing from the higher state π−π* to the lower MLCT state is very efficient, and the major contribution of the observed emission is from the MLCT state.26 At 77 K, complexes 1, 3, and 5 in deaerated CH2Cl2 glass show broad emission peaks at 470, 472, and 476 nm (Figure 4S, Supporting Information), respectively. As shown in Figure 3b, broad emission bands peaking at 465, 480, and 465 nm for complexes 1, 3, and 5 in the solid state were respectively obtained, which are very close to those at low temperature. These emissions are believed to originate from the MLCT. The different emission peaks observed in the solid state and deaerated CH2Cl2 solution suggest the rigidochromic effect, which was first proposed by Wrighton and co-workers.6 The emission maxima of complexes 1, 3, and 5 show a blue shift in contrast with the related Re(I) complexes with the emission bands in the range 520−640 nm, e.g. Re(CO)3ClL (L = 2-(1ethylbenzimidazol-2-yl)pyridine, 606 nm),7 [ReCl(CO)3(bpy)] (633 nm),19 [ReCl(CO)3(Bn-pyta)] (Bn-pyta = 1-benzyl-4-(2pyridyl)-1,2,3-triazole, 538 nm),19 owing to the relatively low conjugation of the NHC ligand with respect to previously used diimine systems, which is in agreement with their absorption spectra discussed above and work on similar complexes.27 This is particularly true if one considers the emission wavelength range for the neutral complexes bearing anionic ancillary ligands. In solution, the emission wavelengths increase in the 3832

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Hz, 4,5-imidazol-2-ylidene H), 8.32 (t, 1H, 3JH−H = 8.5 Hz, 5-H of Py), 8.27 (d, 1H, 3JH−H = 8.5 Hz, 3-H of Py), 7.68 (d, 1H, 3JH−H = 2 Hz, 4,5-imidazol-2-ylidene H), 7.51 (t, 1H, 3JH−H = 7 Hz, 4-H of Py), 3.93 (s, 3H, CH3). 13C NMR (DMSO-d6, 125 MHz, ppm): δ 199.35 (CO), 198.58 (CO), 191.28 (CO), 189.52 (Re-C), 153.65, 153.17, 142.76, 125.48, 124.42, 117.83, 113.26, 38.81 (CH3). Anal. Calcd for C12H9N3O3ClRe (464.88): C, 31.00; H, 1.95; N, 9.04. Found: C, 30.88; H, 1.88; N, 8.91. IR (KBr, cm−1): νCO 2019, 1920, 1860. Re(CO)3(L2)Cl (2). Elution with CH2Cl2/acetone (40/1) gave 2 as a white solid. Yield: 0.196 g (41%). 1H NMR (DMSO-d6, 500 MHz, ppm): δ 9.03 (d, 1H, 3JH−H = 5.5 Hz, 6-H of Py), 8.11 (t, 1H, 3JH−H = 8 Hz, 5-H of Py), 7.78 (d, 1H, 3JH−H = 8 Hz, 3-H of Py), 7.59 (t, 1H, 3 JH−H = 6 Hz, 4-H of Py), 7.54 (s, 1H, 4,5-imidazol-2-ylidene H), 7.42 (s, 1H, 4,5-imidazol-2-ylidene H), 5.75 (d, 1H, 2JH−H = 15 Hz, CHH linker), 5.19 (d, 1H, 2JH−H = 15.5 Hz, CHH linker), 3.92 (s, 3H, CH3). 13 C NMR (DMSO-d6, 125 MHz, ppm): δ 198.38 (CO), 197.05 (CO), 192.15 (CO), 181.36 (Re-C), 158.18, 155.74, 140.71, 125.95, 125.86, 123.29, 121.82, 55.75, 37.84 (CH3). Anal. Calcd for C13H11N3O3ClRe (478.90): C, 32.60; H, 2.32; N, 8.77. Found: C, 32.32; H, 2.16; N, 8.70. IR (KBr, cm−1): νCO 2010, 1898, 1864. Re(CO)3(L3)Cl (3). Elution with CH2Cl2 gave 3 as a yellow-green solid. Yield: 0.118 g (23%). 1H NMR (DMSO-d6, 500 MHz, ppm): δ 8.98 (d, 1H, 3JH−H = 5.5 Hz, 6-H of Py), 8.65 (d, 1H, 3JH−H = 8.5 Hz, Ar-H), 8.49 (d, 1H, 3JH−H = 7.5 Hz, Ar-H), 8.37 (t, 1H, 3JH−H = 8 Hz, 5-H of Py), 7.95 (d, 1H, 3JH−H = 8.5 Hz, 3-H of Py), 7.66−7.60 (m, 2H, Ar-H), 7.57 (t, 1H, 3JH−H = 6 Hz, 4-H of Py), 4.21 (s, 3H, CH3). 13 C NMR (DMSO-d6, 125 MHz, ppm): δ 201.94 (CO), 199.25 (CO), 198.34 (CO), 189.21 (Re-C), 154.38, 153.69, 143.07, 135.79, 130.79, 125.92, 125.89, 124.17, 114.43, 113.56, 113.20, 36.41 (CH3). Anal. Calcd for C16H11N3O3ClRe (514.94): C, 37.32; H, 2.15; N, 8.16. Found: C, 37.20; H, 2.05; N, 8.10. IR (KBr, cm−1): νCO 2016, 1924, 1878. Re(CO)3(L4)Cl (4). Elution with CH2Cl2 gave 4 as a white solid. Yield: 0.19 g (36%). 1H NMR (DMSO-d6, 500 MHz, ppm): δ 9.07 (d, 1H, 3JH−H = 5.5 Hz, 6-H of Py), 8.15 (t, 1H, 3JH−H = 7.5 Hz, 5-H of Py), 8.07 (m, 2H, Ar-H), 7.75 (d, 1H, 3JH−H = 8 Hz, 3-H of Py), 7.61 (t, 1H, 3JH−H = 6 Hz, 4-H of Py), 7.49−7.42 (m, 2H, Ar-H), 6.28 (d, 1H, 2JH−H = 16.5 Hz, CHH linker), 5.35 (d, 1H, 2JH−H = 15.5 Hz, CHH linker), 4.18 (s, 3H, CH3). 13C NMR (DMSO-d6, 125 MHz, ppm): δ 197.95 (CO), 196.81 (CO), 192.45 (CO), 191.68 (Re-C), 158.26, 155.59, 140.89, 134.73, 133.32, 126.15, 125.97, 124.34, 124.18, 111.91, 111.36, 52.14, 34.88 (CH3). Anal. Calcd for C17H13N3O3ClRe (528.96): C, 38.60; H, 2.48; N, 7.94. Found: C, 38.42; H, 2.39; N, 7.88. IR (KBr, cm−1): νCO 2019, 1925, 1887. Re(CO)3(L5)Cl (5). Elution with CH2Cl2/acetone (20/1) gave 5 as a yellow solid. Yield: 0.148 g (32%). 1H NMR (DMSO-d6, 500 MHz, ppm): δ 9.86 (s, 1H, NCHN), 8.90 (d, 1H, 3JH−H = 5 Hz, 6-H of Py), 8.42−8.36 (m, 2H, 3,5-H of Py), 7.60 (t, 1H, 3J = 6 Hz, 4-H of Py), 4.13 (s, 3H, CH3). 13C NMR (DMSO-d6, 125 MHz, ppm): δ 198.56 (CO), 198.09 (CO), 189.53 (CO), 189.00 (Re−C), 153.86, 150.55, 142.94, 140.62, 125.64, 114.19, 39.68 (CH3). Anal. Calcd for C11H8N4O3ClRe (465.87): C, 28.36; H, 1.73; N, 12.03. Found: C, 28.26; H, 1.68; N, 11.99. IR (KBr, cm−1): νCO 2020, 1935, 1879. X-ray Crystallography. A suitable single crystal of 1, 3, or 5 was mounted on the top of a glass fiber. Diffraction data were collected on a Bruker SMART CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.710 73 Å) at 298 K. Absorption corrections were applied using the SADABS program.36 The structures were solved by direct methods and refined by full-matrix least-squares refinement based on F2 using the SHELXL program.37 The hydrogen atoms were positioned in idealized positions and refined in the ridingmodel approximation. Other non-hydrogen atoms were refined with anisotropic thermal parameters.

CONCLUSION In this work, we have prepared and characterized Re(I) tricarbonyl complexes with pyridine-functionalized N-heterocyclic carbene ligands. Complexes 1, 3, and 5 show interesting photoluminescence properties, with the maximum emission wavelength (465−511 nm) lying in the blue-green region and moderate quantum yields of Φ = 0.028, 0.034, and 0.030, respectively, suggesting that these complexes are promising blue-green emitters and may be used in displays. Efforts are being made to use the rhenium complexes 1, 3, and 5 as electroluminescent materials in OLEDs.

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EXPERIMENTAL SECTION

General Procedures. All manipulations were performed under dry nitrogen using standard Schlenk glassware. The solvents CH2Cl2 were dried over CaH2 and distilled under nitrogen. The carbene precursors L1,29 L2,30 L3,31 and L432 were prepared according to the procedures previously reported. Other chemicals were available from commercial sources and used as received without further purification. NMR data were measured on a Bruker AM-500 spectrometer with TMS as internal standard (DMSO-d6). Elemental analyses (C, H, and N) were carried out on a Perkin-Elmer 240C analytic instrument. IR spectra were performed on a Nicolet NEXUS870 FT-IR spectrometer with KBr pellets in the range of 400−4000 cm−1. The solutions of these complexes and standard sample in CH2Cl2 and CH3CN were degassed with three freeze−pump−thaw cycles. UV−vis absorption and emission spectra were recorded on a UV-3100 spectrophotometer and Hitachi F-4600 luminescence spectrophotometer, respectively. The emission lifetimes were measured with an Edinburgh Instruments FLS920P fluorescence spectrometer. The quantum yield was calculated by comparison of the integrated intensity of the standard sample [Ru(bpy)3]2+(Cl−)2 in degassed acetonitrile solution33 (Φstd = 0.094) and the unknown sample in degassed CH2Cl2 solution according to the equation Φunk = Φstd(Iunk/Istd)(Astd/ Aunk)(ηunk/ηstd),34 where the subscripts unk and std refer to the unknown sample and standard sample, respectively, Φ is the luminescence quantum yield, I is the integrated intensity, A is the absorbance at the excitation wavelength, and η is the refractive index of the solvent. Cyclic voltammetric measurements were conducted on a Model CHI 660 D voltammetric analyzer with glassy carbon as the working electrode, a polished platinum wire as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and 0.1 M (Bu4N)PF6 as the supporting electrolyte at a scan rate of 0.1 V/s. Synthesis of 1-Methyl-4-(2-pyridyl)-1,2,4-triazolium Iodide (L5). A mixture of 4-(2-pyridyl)-1,2,4-triazole35 (1.00 g, 6.85 mmol) and iodomethane (1.94 g, 13.69 mmol) in acetone (50 mL) was stirred at 60 °C for 12 h. The reaction mixture was filtered, and the solid was washed three times with diethyl ether and dried under reduced pressure to afford L5 as a white powder. Yield: 1.68 g (85%). 1 H NMR (500 MHz, DMSO-d6, ppm): δ 10.96 (s, 1H, CH3NCHN), 9.99 (s, 1H, NCHN), 8.70 (d, 1H, 3JH−H = 4.5 Hz, 6-H of Py), 8.28 (t, 1H, 3JH−H = 6.5 Hz, 5-H of Py), 8.06 (d, 1H, 3J = 8 Hz, 3-H of Py), 7.73−7.70 (m, 1H, 4-H of Py), 4.18 (s, 3H, CH3). 13C NMR (DMSOd6, 125 MHz, ppm): δ 144.81 (CH3NCHN), 139.99 (NCHN), 136.80, 136.54, 136.14, 121.29, 110.45 (Py C), 34.51 (CH3). Anal. Calcd for C8H9N4I (288.09): C, 33.32; H, 3.12; N, 19.44. Found: C, 33.25; H, 3.03; N, 19.36. Synthesis of Rhenium(I) Complexes Re(CO)3(L)Cl (1−5). Solutions of L1−L5 (1.0 mmol) and Ag2O (0.232 g, 1.0 mmol) in 40 mL of CH2Cl2 were allowed to react with exclusion of light for 24 h at room temperature. Then Re(CO)5Cl (0.361 g, 1.0 mmol) was added. The mixture was stirred again for 36 h at 40 °C and filtered after cooling. The solvent was removed under vacuum; the crude product was purified by column chromatography on silica gel using dichloromethane and acetone as eluent. Re(CO)3(L1)Cl (1). Elution with CH2Cl2/acetone (30/1) gave 1 as a yellow solid. Yield: 0.153 g (33%). 1H NMR (DMSO-d6, 500 MHz, ppm): δ 8.84 (d, 1H, JH−H = 5.5 Hz, 6-H of Py), 8.44 (d, 1H, 3JH−H = 2

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving 1H NMR spectra of complexes 1−5 and ligand L5 in DMSO-d6, emission spectra of ligands L1, L3, and L5 in degassed CH2Cl2 solution, the absorption spectra of ligands L1, 3833

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Organometallics

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L3, and L5 and complexes 1, 3, and 5 in CH2Cl2 solution, the emission spectra of 1, 3, and 5 in CH2Cl2 at 77 K, photoluminescence lifetime decay curves of 1, 3, and 5 in the solid state at room temperature and in CH2Cl2 at 77 K, detailed X-ray structure determination procedures, a summary of crystallographic data (Table S1), and CIF files giving crystallographic data for 3−5. This material is available free of charge via the Internet at http://pubs.acs.org. Crystal data are also available from the Cambridge database (CCDC 828916−828918).

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 25 83596775. Fax: +86 25 83314502. E-mail: [email protected] (X.-T.C.); [email protected] (Y.-X.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the Natural Science Grant of China (Nos. 21071078 and 21021062 to X.-T.C., No. 20971067 to Y.-X.Z.) and the U.S. National Science Foundation (CHE-1012173 to Z.-L.X.).

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