Amidate Iridium(III) Bis(2-pyridyl)phenyl Complexes: Application

*To whom correspondence should be addressed. E-mail: [email protected] (Y.D.); [email protected] (M.Z.). Cite this:Organometallics 30, 1, 77-83...
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Organometallics 2011, 30, 77–83 DOI: 10.1021/om100714g

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Amidate Iridium(III) Bis(2-pyridyl)phenyl Complexes: Application Examples of Amidate Ancillary Ligands in Iridium(III)-Cyclometalated Complexes Wei Yang, Hao Fu, Qijun Song, Min Zhang,* and Yuqiang Ding* School of Chemical and Material Engineering, Jiangnan University, 1800 Lihu Road, Wuxi, Jiangsu Province 214122, People’s Republic of China Received July 21, 2010

The syntheses, structures, and photophysical properties of a series of iridium(III) bis(2-pyridyl)phenyl complexes, (ppy)2Ir(LX) (LX = amide ligands), are described. The complex (ppy)2Ir(Nphenylmethacrylamide) (5), bearing an acrylamidate ligand, has been further employed for polymerization to generate the metal-containing homopolymer poly((ppy)2Ir(N-phenylmethacrylamide)) (6). In addition, the complexes (ppy)2Ir(acetylaniline) (2) and (ppy)2Ir(N-phenylbenzamide) (3) have been structurally characterized by X-ray crystallography. It was found that the amidate ligand binds to the Ir center in an imine/alkoxide mode via a four-membered, nearly planar ring (N-Ir-O-C); the limited delocalization is supported by the lengthened C-O bond compared to the C-N bond length in the amidate backbone and no strong absorption around 1650 cm-1 in the infrared spectra. The photophysical and electrochemical properties of these complexes were then studied. The maximum emission wavelengths ranging from 510 to 548 nm as well as the ΔEp values varying from 2.55 to 2.77 V can be attributed to the different effects of ancillary amide ligands tolerating different electronic characters. The HOMO and LUMO energy levels and compositions of the iridium complexes were investigated by DFT calculations. The amidate ancillary ligands affect the absorption and emission energies of these complexes by tuning the HOMO energies.

Introduction Iridium(III)-cyclometalated complexes have attracted considerable attention due to their unique photophysical properties and applications in organic light-emitting diodes (OLEDs).1 These complexes can capture both singlet and triplet excitons and greatly enhance the internal phosphorescence quantum efficiency of OLEDs toward 100%.2 It was *To whom correspondence should be addressed. E-mail: yding@ jiangnan.edu.cn (Y.D.); [email protected] (M.Z.). (1) (a) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F. J. Am. Chem. Soc. 2001, 123, 4304–4312. (b) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125, 7377–7387. (c) Polson, M.; Fracasso, S.; Bertolasi, V.; Ravaglia, M.; Scandola, F. Inorg. Chem. 2004, 43, 1950–1956. (d) Muegge, B. D.; Richter, M. M. Anal. Chem. 2004, 76, 73–77. (e) You, Y.; Park, S. Y. J. Am. Chem. Soc. 2005, 127, 12438– 12439. (f) Hwang, F. M.; Chen, H. Y.; Chen, P. S.; Liu, C. S.; Chi, Y.; Shu, C. F. Inorg. Chem. 2005, 44, 1344–1353. (g) Thomas, K. R. J.; Velusamy, M.; Lin, J. T. Inorg. Chem. 2005, 44, 5677–5685. (h) Yang, C. H.; Cheng, Y. M.; Chi, Y. Angew. Chem., Int. Ed. 2007, 46, 2418–2421. (2) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys. 2001, 90, 5048–5051. (3) (a) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong, R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Inorg. Chem. 2001, 40, 1704–1711. (b) Ostrowski, J. C.; Robinson, M. R.; Heeger, A. J.; Bazan, G. C. Chem. Commun. 2002, 7, 784–785. (c) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971–12979. (d) Coppo, P.; Plummer, E. A.; Cola, L. D. Chem. Commun. 2004, 15, 1774–1775. (e) Li, C.; Su, Y.; Tao, Y.; Chou, P.; Chien, C.; Cheng, C.; Liu, R. Adv. Funct. Mater. 2005, 15, 387–395. r 2010 American Chemical Society

previously demonstrated that the cyclometalating ligands (e.g., 2-phenylpyridine, 2-phenylquinoline) govern the emission color of the resulting complexes in comparison to the ancillary ligands (e.g., acetylacetonate, picolinate).1a,3 However, this viewpoint has changed, since the emission colors of Ir(dfppy)2(LX) (LX = ancillary ligand, dfppy = 2-(4,6difluorophenyl)pyridyl) complexes can be tuned from blue to red by modulating the ancillary ligand.4 Therefore, the design and synthesis of both novel cyclometalating ligands and novel ancillary ligands provide potential strategies for phosphorescence color tuning. However, the development of C∧Ns (C∧N = cyclometalating ligand) has been hampered, since the synthesis of dichloride-bridged (C∧N)2IrCl2Ir(C∧N)2 is always difficult to achieve for steric and electronic reasons.4 Therefore, tuning the emission color by the ancillary ligand structure in the heteroleptic iridium complex of the already known cyclometalating ligand (C∧N) is considered a much easier, efficient, and competitive approach. Over the past few years, a number of contributions in the literature have described a bidentate, anionic ancillary ligand (LX), such as acetylacetonate (acac), functionalized β-diketonate, 2-picolinic acid, N-alkylsalicylimine,3a,5 pyrazolates, analogous triazolates,6a-d and amidinate.6e,f However, the modification of these ancillary ligands always requires complicated chemical reactions, and most of them are coordinated to the Ir atom via a five- or six-membered ring. Therefore, it is of great (4) You, Y.; Park, S. Y. J. Am. Chem. Soc. 2005, 125, 12438–12439. Published on Web 12/14/2010

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significance and interest to explore a new type of ancillary ligand, which could not only be readily synthesized and modified but also be coordinated to the Ir atom via a scarcely cyclic delocalized model. In the past several years, the subject of amide donors serving as ligands in coordination chemistry has attracted much attention for several reasons. (1) Amides can be easily synthesized in high yields from readily available starting materials. (2) Substituents on amide-based ligands can be varied to regulate both electronic and steric features of the resulting transition-metal complexes. (3) Binding modes to the metal center are controllable via the mediation of N-donor substituents or carbonyl-donor substituents on the amidate ligands.7 To date, a number of organometallic amidate complexes have been synthesized and structurally reported.7,8 Especially in recent years, complexes with Ti, Zr, Al, etc. have been used by Schafer and other researchers to investigate a wide range of applications, including reaction intermediates,8g catalysis for the hydroamination of alkynes8d,h,m and alkenes8j,l and for the transamidation of unactivated secondary carboxamides,8k and precursors for the polymerization of polar monomers,8c to name but few. To the best of our knowledge, there have been few reports, however, on the (5) (a) Yang, C.; Li, S.; Chi, Y.; Cheng, Y.; Yeh, Y.; Chou, P.; Lee, G.; Wang, C.; Shu, C. Inorg. Chem. 2005, 44, 7770–7780. (b) Kwon, T.; Cho, H. S.; Kim, M. K.; Kim, J.; Kim, J.; Lee, K. H.; Park, S. J.; Shin, I.; Kim, H.; Shin, D. M.; Chung, Y. K.; Hong, J. Chem. Commun. 2007, 3276–3278. (c) Xu, M.; Zhou, R.; Wang, G.; Xiao, Q.; Dua, W.; Che, G. Inorg. Chim. Acta 2008, 361, 2407–2412. (d) Lee, H. S.; Ahn, S. Y.; Huh, H. S.; Ha, Y. J. Organomet. Chem. 2009, 694, 3325–3330. (e) Ahn, S. Y.; Ha, Y. Mol. Cryst. Liq. Cryst. 2009, 504, 59–66. (6) (a) Song, Y.; Yeh, S.; Chen, C.; Chi, Y.; Liu, C.; Yu, J.; Hu, Y.; Chou, P.; Peng, S.; Lee, G. Adv. Funct. Mater. 2004, 14, 1221–1226. (b) Hwang, F.; Chen, H.; Chen, P.; Liu, C.; Chi, Y.; Shu, C.; Wu, F.; Chou, P.; Peng, S.; Lee, G. Inorg. Chem. 2005, 44, 1344–1353. (c) Zhou, G.; Ho, C.; Wong, W.; Wang, Q.; Ma, D.; Wang, L.; Lin, Z.; Marder, T. B.; Beeby, A. Adv. Funct. Mater. 2008, 18, 499–511. (d) Zhou, G.; Wang, Q.; Ho, C.; Wong, W.; Ma, D.; Wang, L.; Lin, Z. Chem. Asian J. 2008, 3, 1830–1841. (e) Peng, T.; Bi, H.; Liu, Y.; Fan, Y.; Gao, H.; Wang, Y.; Hou, Z. J. Mater. Chem. 2009, 19, 8072–8074. (f) Liu, Y.; Ye, K.; Fan, Y.; Song, W.; Wang, Y.; Hou, Z. Chem. Commun. 2009, 3699–3701. (7) (a) Huang, Y.; Huang, B.; Ko, B.; Lin, C. Dalton Trans. 2001, 1359–1365. (b) Lee, A. V.; Schafer, L. L. Eur. J. Inorg. Chem. 2007, 16, 2243–2255. (8) For examples, see: (a) Edema, J. J. H.; Meetsma, A.; Bolhuis, F.; Gambarotta, S. Inorg. Chem. 1991, 30, 2056–2061. (b) Cooper, A. C.; Huffman, J. C.; Caulton, K. G. Inorg. Chim. Acta 1998, 270, 261–272. (c) Baugh, L. S.; Sissano, J. A. J. Polym. Sci., Part A: Polym. Chem. 2002, 40, 1633–1651. (d) Li, C. Y.; Thomson, R. K.; Gillon, B.; Patrick, B. O.; Schafer, L. L. Chem. Commun. 2003, 2462–2463. (e) Zhang, Z.; Schafer, L. L. Org. Lett. 2003, 5, 4733–4736. (f) Ruck, R. T.; Bergman, R. G. Angew. Chem., Int. Ed. 2004, 43, 5375–5377. (g) Zhang, J.; Gunnoe, T. B.; Petersen, J. L. Inorg. Chem. 2005, 44, 2895–2907. (h) Thomson, R. K.; Zahariev, F. E.; Zhang, Z.; Patrick, B. O.; Wang, Y. A.; Schafer, L. L. Inorg. Chem. 2005, 44, 8680–8689. (i) Wang, H.; Chan, H.-S.; Okuda, J.; Xie, Z. Organometallics 2005, 24, 3118–3124. (j) Ayinla, R. O.; Schafer, L. L. Inorg. Chim. Acta 2006, 359, 3097–3102. (k) Hoerter, J. M.; Otte, K. M.; Gellman, S. H.; Stahl, S. S. J. Am. Chem. Soc. 2006, 128, 5177–5183. (l) Thomson, R. K.; Bexrud, J. A.; Schafer, L. L. Organometallics 2006, 25, 4069–4071. (m) Zhang, Z.; Leitch, D. C.; Lu, M.; Patrick, B. O.; Schafer, L. L. Chem. Eur. J. 2007, 13, 2012–2022. (n) Leitch, D. C.; Beard, J. D.; Thomson, R. K.; Wright, V. A.; Patrick, B. O.; Schafer, L. L. Eur. J. Inorg. Chem. 2009, 18, 2691–2701. (o) Jones, M. B.; Newell, B. S.; Hoffert, W. A.; Hardcastle, K. I.; Shores, M. P.; MacBeth, C. E. Dalton Trans. 2010, 39, 401–410. (9) (a) Monticelli, O.; Oliva, D.; Russo, S.; Clausnitzer, C.; P€ otschke, P.; Voit, B. Macromol. Mater. Eng. 2003, 288, 318–325. (b) Hakme, C.; Stevenson, I.; Fulchiron, R.; Seytre, G.; Clement, F.; Odoni, L.; Rochat, S.; Varlet, J. J. Appl. Polym. Sci. 2005, 97, 1522–1537. (c) Mehdipour-Ataei, S.; Sarrafi, Y.; Hatami, M.; Akbarian-Feizi, L. Eur. Polym. J. 2005, 41, 491–499. (d) Finocchiaro, P.; Consiglio, G. A.; Imbrogiano, A.; Failla, S.; Samperi, F.; Sebastiano, B.; Concetto, P.; Giuseppina, S. Eur. Polym. J. 2008, 44, 2639–2651. (e) Scholl, M.; Kadlecova, Z.; Klok, H. Prog. Polym. Sci. 2009, 34, 24–61. (f) García, J. M.; García, F. C.; Serna, F.; Pe~na, J. L. Prog. Polym. Sci. 2010, 35, 623–686.

Yang et al. Scheme 1. Synthesis of (ppy)2Ir(amidate) Complexesa

a Legend: 2, R1 = CH3, R2 = Ph; 3, R1 = Ph, R2 = Ph; 4, R1 = Ph, R2 = naphthyl; 5, R1 = isopropenyl, R2 = Ph.

use of amides to influence the photophysical properties of metal complexes. It is noteworthy that amides exhibit excellent performance in many fields: polyamide (nylon), especially aromatic polyamide, has outstanding stability (high melting point and decomposition temperature)9 and arylamines are used as superlative organic electron transport materials.10 Therefore, it is highly desirable to synthesize a class of thermally stable four-membered-ring iridium(III) complexes with excellent optical performance by employing N-aryl-substituted amides as ligands. In this paper, we report the first synthesis of amidate iridium(III) bis(2-pyridyl)phenyl complexes.Crystal structure analysis, a density functional theory (DFT) study, and measurement of electrochemical properties as well as photophysical properties for these complexes have been carried out. The ultimate goal is to investigate the influence of chelating amidate ligands on the photophysical properties of these cyclometalated complexes.

Results and Discussion Synthesis of (ppy)2Ir(amidate) Complexes. Scheme 1 illustrates the chemical structure and synthetic procedure of heteroleptic iridium (ppy)2Ir(amidate) complexes. The chloridebridged dimer (ppy)2Ir(μ-Cl)2Ir(ppy)2 (1) was synthesized according to the procedure described in the literature.11 Treatment of 1 with 2.5 equiv of amide derivatives in dichloromethane for 24 h in the presence of sodium methanolate at ambient temperature afforded (ppy)2Ir(amidate) complexes 2-5 in quantitative yields. Volatilization of the organic solvent CH2Cl2/hexane resulted in (ppy)2Ir(amidate) complexes as orange to yellow crystals of 2-5. An example of homopolymerization of complex 5 is shown in Scheme 2. The reaction was initiated by azobis(isobutyronitrile) (AIBN), and the mixture was stirred in THF at 70 °C for 4 h. The polymeric product 6 was obtained by repetitive precipitation with methanol. Crystal Structures of 2 and 3. The molecular structure of 2, determined by X-ray diffraction, is shown in Figure 1. It reveals that the Ir atom is approximately octahedrally coordinated to N-acetylaniline and two ppy ligands. The coordinating configuration of the “(ppy)2Ir” fragment in complex 2 is retained as that observed in the chloro-bridged dimer (ppy)2Ir(μ-Cl)2Ir(ppy)2.11b The formation of N(3)∧O(1) coordinated to Ir(1) is a nearly planar configuration (0.0193°); however, the benzene ring substituent on N(3) (10) (a) Liu, Y.; Liu, M. S.; Li, X.; Jen, A. K. Chem. Mater. 1998, 10, 3301–3304. (b) Wu, J.; Baumgarten, M.; Debije, M. G.; Warman, J. M.; Muellen, K. Angew. Chem., Int. Ed. 2004, 43, 5331–5335. (c) Ju, J. U.; Jung, S. O.; Zhao, Q. H.; Kim, Y. H.; Je, J. T.; Kwon, S. K. Bull. Korean Chem. Soc. 2008, 29, 335–338. (d) Kondakova, D. Y. J. Appl. Phys. 2008, 104, 084520–084529. (11) (a) Sprouse, S.; King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc. 1984, 106, 6647–6653. (b) Garces, F. O.; King, K. A.; Watts, R. J. Inorg. Chem. 1988, 27, 3464–3471.

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Figure 1. ORTEP diagram of 2 with thermal ellipsoids shown at the 30% probability level. The hydrogen atoms have been omitted for clarity. Selected bond distances (A˚) and angles (deg): Ir(1);N(1) = 1.999(1), Ir(1);N(2) = 2.016(1), Ir(1);N(3) = 2.146(1), Ir(1);C(11)=1.988(1), Ir(1);C(22)=1.940(1), Ir(1); O(1)=2.266(7), C(24)-O(1)=1.322(2), N(3)-C(24)=1.273(2); C(22)-Ir(1)-N(2)=79.0(5), N(3)-Ir(1)-O(1)=59.8(4), C(11)Ir(1)-N(1) = 80.5(5). Scheme 2. Synthesis of the Homopolymer poly[(ppy)2Ir(N-phenyl methacrylamide)] (6)

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Figure 2. ORTEP diagram of 3 with thermal ellipsoids shown at the 30% probability level. The hydrogen atoms have been omitted for clarity. Selected bond distances (A˚) and angles (deg): Ir(1);N(1) = 2.027(4), Ir(1);N(2) = 2.008(5), Ir(1);N(3) = 2.155(4), Ir(1);C(11) = 1.977(5), Ir(1);C(22) = 2.026(4), Ir(1);O(1) = 2.272(3), C(23)-O(1) = 1.261(6), N(3)-C(23) = 1.313(6); C(22)-Ir(1)-N(2) = 80.6(2), N(3)-Ir(1)-O(1) = 58.5(5), C(11)-Ir(1)-N(1) = 88.0(2).

Table 1. Effect of the Concentration of Reactants on the Molecular Weight entry

concn of AIBN (mol/L)a

concn of complex 5 (mol/L)b

103M (g/mol)

1 2 3 4 5

0.0396 0.0396 0.0396 0.0183 0.0609

0.038 0.075 0.100 0.038 0.038

6.07 9.32 15.40 8.90 5.78

a Concentration of AIBN; T = 343 K. b Concention of complex 5 [(ppy)2Ir(N-phenylmethacrylamide)]; T = 343K.

deviates from the plane of the four-membered ring at an angle of 31.3°. The bond length of Ir(1)-N(3) (2.146(1) A˚) is longer than those of Ir(1)-N(1) (1.999(1) A˚) and Ir(1)-N(2) (2.016(1) A˚), but all of these Ir-N bond lengths fall into the range of reported data of Ir-N (1.982-2.189 A˚).1a,12,13 The bond length of Ir(1)-O(1) (2.266(7) A˚) is much greater than the mean value of 2.088 A˚ reported in the Cambridge Crystallographic Database.14a In addition, the N(3)-Ir(1)-O(1) angle (59.8(4)°) is found to be wider than the reported value.8b The molecular structures of complexes 2 and 3 are approximately similar. The bond lengths and angles of crystalline 3 differ slightly from those of crystalline 2 (Figures 1 and 2). The four-membered ring N(3)-Ir(1)-O(1)-C(23) is nearly planar (0.0291°), and the benzene groups on N(3) and C(23) (12) Zhang, Y.; Baer, C. D.; Camaion, N. C.; O’Brien, P.; Sweigart, D. A. Inorg. Chem. 1991, 30, 1685–1687. (13) Neve, F.; Crispini, A.; Eur., J. Inorg. Chem. 2000, 1039–1043. (14) (a) Allen, F. H.; Davies, J. E.; Galloy, J. J.; Johnson, O.; Kennard, O.; Macrae, C. F.; Mitchell, E. M.; Mitchell, G. F.; Smith, J. M.; Watson, D. G. J. Chem. Inf. Comput. Sci. 1991, 31, 187–204. (b) Brown, C. J. Acta Crystallogr. 1966, 21, 442–455. (c) Hariharan, M.; Srinivasan, R. Acta. Crystallogr., Sect. C 1990, 46, 1056–1058.

Figure 3. UV-vis absorption and normalized PL emission spectra of complexes 2-6. The PL emission spectra are normalized to the maximum peak of complex 6 (λ ∼517 nm). Measurements for all the complexes have been carried out with CH2Cl2 solutions having a concentration of 10-5 mol/L.

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Table 2. Photophysical Properties of Iridium Complexesa abs wavelength λ (nm) soln luminescence λ (nm) ΦPL

2

3

4

5

6

251, 297, 346, 442 510 0.060

250, 298, 370, 430 542 0.089

248, 261, 291, 370, 418 548 0.095

262, 298, 341, 404, 463 545 0.155

262, 297, 341, 402, 464 517 0.289

a Absorption and emission spectra were recorded in spectroscopic grade dichloromethane at 298 K. Quantum yields of emission were measured in degassed dichloromethane solutions, using Alq3 in DMF (ΦPL = 0.116) as a reference.

Figure 4. Cyclic voltammograms of iridium complexes 2-5 (10-4 mol/L) with 0.1 mol/L of TBAPF6 at a scan rate of 100 mV/s. The CV scans of all iridium complexes are single scans. All potentials were referenced to SCE.

deviate from the plane at angles of 44.9 and 45.0°, respectively. In comparison to free amides (N-acetylaniline and N-phenylbenzamide),14b,c the coordinating process in complexes 2 and 3 significantly increases the bond length of C-O while decreasing the C-N bond length in the amide simultaneously, which is the result of resonance of the electrons in the chelating amidate ligand to Ir to form a four-membered ring. The partial double bond feature increases the bond length of the carbonyl group and decreases the length of the C-N bond, which is consistent with the results of the infrared (IR) spectra, which lack a strong absorption around 1650 cm-1. The number-average molecular weight (M) of the homopolymer 6 was measured by gel permeation chromatography (GPC), and the results are shown in Table 1. It is found that the concentrations of complex 5 and initiator (AIBN) have a significant influence on the M value; a higher concentration

Table 3. Electrochemical Data for Iridium Complexesa Ep (V) complex

oxidn

redn

ΔEp (V)

2 3 4 5

1.52 1.47 1.46 1.47

-1.03 -1.12 -1.16 -1.30

2.55 2.59 2.62 2.77

a All electrochemical data were determined at room temperature in CH3CN solution containing 0.1 mol/L TBAPF6.

of starting material 5 along with a relatively lower concentration of AIBN favors the polymerization (Table 1, entries 1-5). Photophysical Properties. The photophysical characterization of (ppy)2Ir(amidate) complexes 2-6 was studied as shown in Figure 3. All of these complexes give similar

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Table 4. HOMOs and LUMOs of the Iridium Complexesa

a

All the isovalues for the MO plots are 0.02.

absorption and emission spectra. The maximum absorption wavelengths of the complexes are observed at 251, 255, 250, 247, 262, and 262 nm, respectively (Table 2); intense bands from 250 to 330 nm in the ultraviolet part of the absorption spectra can be assigned to the spin-allowed ligand-centered π-π* transitions of the C ∧N ligands. The weak bands observed between 330 and 430 nm can be attributed to the 1 MLCT spin-allowed transitions. In addition, much weaker bands are also observed beyond 400 nm due to the contribution of 3MLCT transitions. All of the (ppy)2Ir(amidate) complexes 2-6 reported here give phosphorescence quantum yields (ΦPL) between 0.060 and 0.289 (Table 2). Complex 6 shows the highest ΦPL value, which is lower than that of (ppy)2Ir(acac).3a The wavelengths of solution photoluminescence (PL) of complexes 2-6 are observed around 520 nm, which fall into the green light region. These wavelengths of solution PL are very similar to those for (ppy)2Ir(acac).3a It is found that when unsaturated C-C substituents on the nitrogen atom or carbonyl group of the amidate ligand are increased, the maximum emission wavelengths of these complexes undergo a red shift (Table 2). In comparison to complex 5, the emission spectrum of polymeric complex 6 exhibits an obvious hypsochromic shift and a significant increase in the absorption intensity. The blue shift in the emission spectra can be attributed to the results of substituent modification: the vinyl group was transformed into an alkyl group on the amidate ligand after polymerization occurred. Furthermore, the polymerization affords the higher molecular weight complex 6, which contains more fluorophores at the same concentration and could result in stronger emission intensity.

Electrochemical Properties. Cyclic voltammetry (CV) was employed to study the electrochemical properties of these iridium complexes (Figure 4); the I-V response can provide information on the kinetics of the electron-transfer reaction and thermodynamics of the electrode-electrolyte interface. The cyclic voltammograms of the (ppy)2Ir(amidate) complexes are shown in Figure 4, at a Pt-disk working electrode in CH3CN solution (10-4 mol/L) with 0.1 mol/L tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. The relevant oxidation and reduction potentials are given in Table 3. The oxidation process in the CV of 2-5 in CH3CN is assigned to the Ir(III)/Ir(IV) couple. The first oxidation peaks of Ep for complexes 2-5 are observed at 1.52, 1.47, 1.46, and 1.47 V, respectively (Table 3). These close oxidation potentials obtained under identical conditions indicate that an increase in the number of conjugated double bonds does not affect the oxidative process of the iridium complexes very much. The first reduction peaks of these complexes appear at Ep -1.03, -1.12, -1.16, and -1.30 V, respectively. The reductive process is caused by the reduction of the amidate ancillary ligands, and the reduction potentials have no relation to the length of the conjugated system; the different Ep values are assigned to the different electrochemical environments of the amides. DFT Calculations. DFT calculations for complexes 2-5 were further carried out to ascertain the influence of the amide ligands on the photophysical properties. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels and compositions of the four iridium complexes were analyzed, and the results

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Table 5. Frontier Orbital Compositions (%) of Four Iridium Complexes from DFT MOs

2

3

4

5

HOMO

40.62 (ppy) 50.35 (Ir) 9.03 (acetylaniline) 92.21 (ppy) 5.14 (Ir) 2.65 (acetylaniline)

37.60 (ppy) 48.22 (Ir) 14.18 (N-phenylbenzamide) 90.63 (ppy) 4.86 (Ir) 4.51 (N-phenylbenzamide)

36.14 (ppy) 45.91 (Ir) 17.95 (N-naphthylbenzamide) 89.52 (ppy) 4.85 (Ir) 5.63 (N-naphthylbenzamide)

38.24 (ppy) 48.21 (Ir) 13.55 (N-phenylmethacrylamide) 91.05 (ppy) 4.99 (Ir) 3.96 (N-phenylmethacrylamide)

LUMO

ar given in Tables 4 and 5. The HOMO energy levels vary from -4.89 to -4.82 eV, while the LUMO energy levels are very close, with energies ranging from -1.40 to -1.44 eV. The LUMO distributions of 2-5 primarily reside on cyclometalated ligands (ppy) in comparison to the metal center and amidate ancillary ligands, which is similar to the case for the reported iridium analogues.15 The electronic populations confined to the metallic d orbital and ppy in the HOMOs of 2-5 are about 48% and 38%, repectively. However, the contribution of the amidate ligand is early 17.95% (Table 5), quite different from that of the complex of (ppy)2Ir(acac), in which the contribution of the ancillary ligand (acac) is almost negligible.15 These results can be ascribed to the far better partial p-bonding ability to the metal center of the amide ligands in comparison to that of the acac ligand. On the basis of these results obtained by DFT, the amidate ancillary ligands could also influence the energy of the HOMO in (C∧N)2Ir(LX) complexes, which is quite consistent with reported results.16

Conclusions In summary, we have successfully prepared and characterized a series of iridium complexes containing amidate ancillary ligands. X-ray crystallographic studies have been carried out for complexes 2 and 3. The high concentration of complex 5 and relatively low concentration of initiator favor polymerization and yield a higher molecular weight product. The electrochemical properties are influenced by the substituents on the amidate ancillary ligands. The emission spectra of complexes 2-6 are different, while their maximum absorption wavelengths are similar. The HOMO and LUMO energy levels and compositions of complexes 2-5 were investigated by DFT calculations. The results indicate the energy of the HOMOs of (C∧N)2Ir(LX) complexes could also be significantly influenced by amidate ancillary ligands. Thus, a color-tuning strategy can be achieved by mediating the substituents on the nitrogen atom or carbonyl group of the amidate ligands.

Experimental Section Materials. IrCl3 3 3H2O, acetylaniline, and other chemicals were obtained from commercial sources and used without further purification. All solvents were dried by common methods and freshly distilled prior to use. Amide derivatives (N-phenylbenzamide,17 N-naphthylbenzamide,18 N-phenylmethacrylamide19) were prepared according to the reported literature procedures. (15) Hay, P. J. J. Phys. Chem. A 2002, 106, 1634–1641. (16) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho, N. N.; Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 1713–1727. (17) Vogel, A. I. Elementary Practical Organic Chemistry; Longmans, Green: London, New York, Toronto, 1959; p 243. (18) DeRuiter, J.; Swearingen, B. E.; Wandrekar, V.; Mayfield, C. A. J. Med. Chem. 1989, 32, 1033–1038. (19) Wang, W.; Zhu, T.; Chen, G.; Li, C.; Di, H.; Ren, C.; Ding, Y. Synth. Met. 2008, 158, 1022–1027.

The chloro-bridged dimer (ppy)2Ir(μ-Cl)2Ir(ppy)2 was synthesized via the described procedure.11 General Experiments. All of the synthetic procedures involving Ir(III) species and sodium methanolate were carried out under a dry argon or nitrogen atmosphere by using a standard Schlenk tube; the main concern is the oxidative stability of intermediate complexes and moisture-sensitive sodium methanolate used in the reactions. Synthesis of (ppy)2Ir(amidate) Complexes 2-5. Complexes 2-5 were prepared via identical procedures. Herein, a typical procedure for the synthesis of (ppy)2Ir(acetylaniline) (2) is described as follows. [(ppy)2IrCl]2 (0.107 g, 0.10 mmol) and acetylaniline (0.037 g, 0.25 mmol) were dissolved in dichloromethane (10 mL); sodium methanolate (0.054 g, 1.0 mmol) was then added to the mixture to neutralize the hydrochloric acid generated by the reaction. The resulting mixture was stirred at room temperature for 24 h and was then taken to dryness in vacuo, and the residue was washed with water and diethyl ether successively to afford the yellow crude product, which was further recrystallized in CH2Cl2/ petroleum ether, and the desired yellow crystalline product 2 was obtained in 62% yield (0.078 g). IR (KBr): 3053 m, 2961 w, 2926 w, 1635 w, 1604 s, 1582 s, 1544 m, 1519 s, 1453 s, 1439 s, 1411 s, 1305 m, 1264 m, 1226 m, 1159 m, 1059 m, 1030 m, 1007 m, 964 m, 756 s, 732 m, 697 m, 670 m. 1H NMR (in CDCl3): δ 2.17 (s, 3H), 6.17 (d, J = 8 Hz, 2H), 6.52 (d, J = 11.2 Hz, 2H), 6.60 (t, J = 7.6 Hz, 1H), 6.69 (t, J=7.4 Hz, 1H), 6.77 (t, J=7.6 Hz, 1H), 6.84 (dd, J=7.6, 7.2 Hz, 2H), 7.02 (t, J=7.6 Hz, 2H), 7.17 (t, J = 6.4 Hz, 2H), 7.52 (dd, J = 7.6,7.2 Hz, 2H), 7.80 (t, J = 7.4 Hz, 2H), 7.92 (d, J=7.6 Hz, 2H), 8.80 (d, J=7.2 Hz, 1H), 8.89 (d, J=7.2 Hz, 1H). Anal. Found: C, 56.71; H, 3.86; N, 6.58. Calcd: C, 56.77; H, 3.81; N, 6.62. (ppy)2Ir(N-phenylbenzamide) (3). Orange-yellow crystal, yield 0.098 g (70%). IR (KBr): 3057 m, 1653 w, 1605 s, 1581 s, 1560 w, 1519 s, 1477 s, 1438 m, 1417 s, 1316 m, 1267 m, 1226 m, 1160 m, 1125 m, 1060 m, 1030 m, 932 m, 793 m, 755 m, 734 m, 695 m, 630 w, 612 w. 1H NMR (in CDCl3): δ 6.21 (d, J=7.6 Hz, 2H), 6.44 (d, J=7.6 Hz, 2H), 6.65 (t, J=7.6 Hz, 1H), 6.71 (t, J=7.2 Hz, 2H), 6.79 (t, J=10 Hz, 2H), 6.88 (m, 2H), 7.14 (t, J=7.6 Hz, 2H), 7.22 (t, J = 7.2 Hz, 1H), 7.34 (d, J = 8.0 Hz, 1H), 7.47 (d, J=7.6 Hz, 2H), 7.51 (dd, J=7.4, 7.2 Hz, 2H), 7.76 (dd, J = 7.6, 7.2 Hz, 2H), 7.82 (m, 3H), 8.91 (d, J = 7.2 Hz, 1H), 8.96 (d, J=7.6 Hz, 1H). Anal. Found: C, 60.21; H, 3.88; N, 6.09. Calcd: C, 60.33; H, 3.76; N, 6.03. (ppy)2Ir(N-naphthylbenzamide) (4). Orange-yellow crystal, yield 0.107 g (72%). IR (KBr): 3055 m, 1644 w, 1606 s, 1582 s, 1524 s, 1478 s, 1437 m, 1417 s, 1301 m, 1269 m, 1228 m, 1160 m, 1061 m, 1030 m, 916 m, 791 m, 755 m, 733 m, 695 m, 630 w, 612 w. 1 H NMR (in CDCl3): δ 6.04 (d, J = 8 Hz, 1H), 6.21 (d, J = 3.6 Hz, 1H), 6.37 (t, J = 7.6 Hz, 1H), 6.59 (d, J = 7.6 Hz, 1H), 6.64 (m, J = 7.2 Hz, 2H), 6.82 (t, J = 7.6 Hz, 2H), 7.05 (m, 3H), 7.12 (t, J = 7.6 Hz, 1H), 7.22 (t, J = 7.4 Hz, 2H), 7.31 (d, J = 7.2 Hz, 1H), 7.37 (d, J = 7.6 Hz, 1H), 7.45 (d, J = 7.6 Hz, 1H), 7.57 (m, 3H), 7.74 (m, 3H), 7.92 (t, J = 7.6 Hz, 3H), 9.06 (d, J = 7.6 Hz, 1H), 9.12 (d, J = 7.2 Hz, 1H). Anal. Found: C, 62.59; H, 3.91; N, 5.59. Calcd: C, 62.72; H, 3.78; N, 5.63. (ppy)2Ir(N-phenylmethacrylamide) 5. Orange crystal, yield 0.093 g (62%). IR (KBr): 3057 m, 1604 s, 1581 s, 1534 s, 1475 s, 1438 m, 1420 s, 1267 m, 1225 m, 1160 m, 1060 m, 1029 m, 929 m, 791 m, 755 m, 734 m, 694 m, 629 w. 1H NMR (in CDCl3):

Article δ 2.09 (s, 3H), 5.29 (s, 1H), 5.83 (s, 1H), 5.90 (d, J = 8 Hz, 2H), 6.52 (m, 2H), 6.72 (m, 4H), 7.02 (m, 2H), 7.45 (d, J = 7.6 Hz, 2H), 7.50-7.70 (m, 3H), 7.78 (d, J = 8 Hz, 2H), 7.85 (d, J = 7.2 Hz, 2H), 8.88 (d, J = 7.6 Hz, 2H). Anal. Found: C, 58.08; N, 6.36; H, 3.84. Calcd: C, 58.16; N, 6.46; H, 3.97. Synthesis of Polymer 6. AIBN (0.03 g, 0.2 mmol) and 5 (0.10 g, 0.15 mmol) were dissolved in THF (4 mL) in a round-bottom flask; the resulting mixture was stirred at 70 °C for 48 h. After the mixture was cooled to room temperature, methanol (100 mL) was added to precipitate the polymeric product. The crude product was obtained after filtration. Then it was further dissolved in THF (4 mL); the subsequent addition of methanol (100 mL) gave a deep yellow precipitate, which was then filtered and dried under reduced pressure to afforded the desired product 6 (yield 56%). The M value of the homopolymer 6 was measured by GPC. Theoretical Calculations. Calculations on the electronic ground state of the complexes were performed using B3LYP density functional theory.20 A 6-31G basis set was implemented for the first- and second-row elements H, C, N, and O, whereas the LANL2DZ basis set was employed for Ir. Ground-state B3LYP calculations were carried out using the Gaussian 03 suite of programs.21 (20) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A. J.; 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.; Pople, J. A. Gaussian 03, revision E.01; Gaussian, Inc., Wallingford, CT, 2004.

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Characterization Methods. IR spectra were recorded on a FTLA2000 spectrometer by dispersing samples in potassium bromide. NMR spectra were collected on a Bruker ACF-400 spectrometer with deuterated chloroform as solvent and tetramethylsilane as internal standard. Absorption spectra were measured using a UV/vis spectrophotometer (Model TU-1901). PL spectra were obtained using an RF-5301PC spectrofluorimeter (Shimadzu, Japan) connected to a photomultiplier tube with a xenon lamp as the excitation source. The PL quantum yields were measured in degassed dichloromethane solutions, using tris(8-hydroxyquinoline)aluminum (Alq3) in DMF (ΦPL = 0.116)22 as a reference. The electrochemical properties of the Ir(III) complexes were examined by CV and measured by using a standard one-compartment, three-electrode electrochemical cell in an IM6e electrochemical worksation (ZAHNER elektrik, Germany). A Pt-disk electrode was used as the working electrode, a Pt wire as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. Molecular weights of polymers were measured by a GPC (Waters 1515) using N,N-dimethylformamide as a solvent and calibrated against polystyrene standards. X-ray data for compounds 2 and 3 were collected using a Bruker SMART APEX II CCD area detector using monochromated Mo KR radiation (λ = 0.710 73 A˚). Crystal data of 2 and 3 were measured at 293(2) K.

Acknowledgment. This work was supported by the National Natural Science Foundation of China (Nos. 20571033 and 20971058) and by the Program for New Century Excellent Talents in University (NCET-06-0483). Supporting Information Available: CIF files giving crystallographic data for 2 and 3. This material is available free of charge via the Internet at http://pubs.acs.org. (22) Lytle, F. E.; Story, D. R.; Juricich, M. E. Spectrochim. Acta, Part A 1973, 29, 1357–1369.