Iridium(III) Complexes Bearing Tridentate Chromophoric Chelate

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Iridium(III) Complexes Bearing Tridentate Chromophoric Chelate: Phosphorescence Fine-Tuned by Phosphine and Hydride Ancillary Jia-Ling Liao,† Palanisamy Rajakannu,† Shih-Hung Liu,‡ Gene-Hsiang Lee,‡ Pi-Tai Chou,*,‡ Alex K.-Y. Jen,*,§ and Yun Chi*,†,§ †

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Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan ‡ Department of Chemistry and Instrumentational Center, National Taiwan University, Taipei 10617, Taiwan § Department of Materials Science and Engineering, Department of Chemistry, City University of Hong Kong, Hong Kong, SAR S Supporting Information *

ABSTRACT: Functional 2-pyrazolyl-6-phenylpyridine chelatesnamely, (pzpyphBu)H2 and (pzpyphCF3)H2 and phosphinesare successfully employed in the preparation of emissive Ir(III) metal complexes, for which the reaction with phosphine such as PPh3, PPh2Me, and PPh2(CH2Ph) afford corresponding Ir(III) complexes [Ir(pzpyphBu)(PPh3)2H] (1a), [Ir(pzpyphCF3)(PPh2R)2H] (2a−2c), R = Ph, Me, CH2Ph, which also show an equatorial coordinated hydride. In contrast, treatment with 1,2-bis(diphenylphosphino)benzene (dppb) and 1,2-bis(diphenylphosphino)ethane (dppe) yields the isomeric products [Ir(pzpyphBu)(dppb)H] (3a) and [Ir(pzpyphBu)(dppe)H] (3b), for which the distinctive, axial hydride undergoes rapid chlorination, forming chlorinated complexes [Ir(pzpyphBu)(dppb)Cl] (4a) and [Ir(pzpyphBu)(dppe)Cl] (4b), respectively. On the other hand, upon extensive heating of 2c, one of its coordinated PPh2(CH2Ph) exhibits benzyl cyclometalation and hydride elimination to afford [Ir(pzpyphCF3)(PPh2R)(PPh2R′)] (5c and 6c) R = CH2Ph and R′ = CH2(oC6H4) as the kinetic and thermodynamic products, respectively. Their structural, photophysical, and electrochemical properties are examined and further affirmed by the computational approaches.

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INTRODUCTION Tridentate chelates have recently emerged as versatile scaffolds in the assembly of functional transition-metal complexes.1−4 They are expected to react with various metal reagents to form stronger multidentate metal−ligand bonding. In contrast, lower denticity analogues provide monodentate or bidentate coordination mode, giving the higher lability and reduced thermodynamic stability. Hence, proper employment of tridentate chelate has led to the rapid advancement of research and applications such as homogeneous catalysis,5−7 organic light-emitting diodes (OLED),8−10 biological sensing and imaging,11,12 spin-crossover switches,13,14 and dye-sensitized solar cells (DSSCs).15 Furthermore, the third-row, late transition-metal ions, cf. Os(II), Ir(III), and Pt(II) metal ions, would afford complexes exhibiting fast intersystem crossing and giving ∼100% internal electroluminescence by harvesting both singlet and triplet excitons.16−20 Therefore, the corresponding metal complexes bearing tridentate chelates are expected to exhibit an enhanced emission efficiency and chemical stability, both of which are important to the phosphor designs demanded by commercial OLEDs. In addition, in the pursuit of efficient phosphors, another requirement is to attain the optimal structural assembly, in which the associated chromophoric and ancillary chelates should be capable of exerting great ligand field strength to © XXXX American Chemical Society

destabilize the metal-centered dd excited state that guides the radiationless deactivation processes, because of its repulsive potential energy surface, and to balance the positive charge of the central metal cation yielding the charge-neutral emitters.21 Accordingly, suitable chromophoric chelate consists of a family of dianionic tridentate entities involving 2-phenyl-6-(pyrazol-3yl) pyridine (e.g., L(1) of Scheme 1).9 Functionalization of this chelate with substituents of distinctive π-conjugation and electronic effect is expected to impose notable variation in photophysical characteristics (such as peak wavelength and emission lifetime) of the resultant phosphors. This class of dianionic chelates is distinctive from tridentate chelates such as 1,3-bis(benzimidazol-2-yl)benzene, bis(imidazol-2-ylidene) benzene, and tridentate polypyridyl chelate, employed by the research groups of Haga,22 De Cola,23 and Bernhard,24 from which higher emission efficiency was also recorded for the isolated Ir(III) complexes. Moreover, the monoanionic ancillary chelate can be the bidentate or tridentate heteroaromatic cyclometalate with either terminal pyridyl-based donor(s) (i.e., chelates L(2) and L(3))25 or imidazolylidene appendage(s) (i.e., chelates L(4), L(5), and L(6)).26−28 Since the C-donor of imidazolylidene and analogues29 is known to exert Received: April 4, 2018

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DOI: 10.1021/acs.inorgchem.8b00905 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Schematic Drawings of Chelates Suitable for Making Ir(III)-Based Phosphors

Scheme 2. Reactions Initiated from the Treatment of IrCl3·3H2O with Tridentate Chelate L(1) and Phosphine; General Protocol: (a) Addition of Phosphine, (b) Cyclometalation, (c) Isomerization, (d) Addition of Diphosphine, and (e) Chlorination

relatively stronger metal−ligand bonding (or ligand field strength) than that of the N-based donor such as pyridine, the imidazolylidene-coordinated Ir(III) emitters would exhibit better emission efficiency than those possessing only the Ndonor. This strategy is especially crucial to the blue emitters with higher emission energy.30−32 One recent report is the syntheses of bis-tridentate Ir(III) phosphors bearing both 2phenyl-6-(pyrazol-3-yl) pyridine class of chelate L(1) and diimidazolylidene pincer of ancillary L(6), and fabrication of the true-blue phosphorescent OLEDs using the same.33 Subsequent quantum chemical calculation unveils that the resulting bis-tridentate architecture is vital to reduce the ligand dissociation, which is essential for achieving the long-lived operational stability of OLEDs.34 However, there are almost no reports on the emissive Ir(III) complexes bearing both the class of tridentate chromophore such as L(1) and associated ancillaries deriving from simple phosphine or functional analogues (i.e., P+P, P^P, and P^C chelates), which are also strong field ligands and equally capable of suppressing the quenching of the dd excited states induced by thermal population. They have given successful preparation of highly efficient true-blue emitters.35 In fact, relevant emitters containing phosphine ancillaries have been widely employed in the assembly of Ir(III) emitters with

bidentate chromophoric chelates,36−41 as well as Pt(II) emitters with a single tridentate chromophore.42−47 Therefore, there is great interest in the exploration of their influence on the emerging Ir(III) phosphors bearing tridentate chromophoric chelates. Herein, we report the preparation, characterization, and photophysical properties of a new series of Ir(III) complexes, for which the tridentate chelates are functional chelate L(1) with substituents R = But and CF3, while phosphine ancillaries are noncyclometalating triphenylphosphine (PPh3), methyldiphenylphosphine (PPh2Me), 1,2-bis(diphenylphosphino)benzene (dppb), 1,2-bis(diphenylphosphino)ethane (dppe), and cyclometalating benzyldiphenylphosphine (PPh2(CH2Ph)). Particularly, all reactions with monodentate phosphines gave Ir(III) metal hydride complexes, e.g., route (a) in Scheme 2, which are an intermediate en route to emissive Ir(III) complexes bearing cyclometalating phosphine PPh2(CH2Ph), i.e., through routes (b) and (c). In sharp contrast, the reactions with diphosphine, such as dppb and dppe, only afford the axial-substituted hydride complexes, which are less stable and undergo chlorination to yield poorly emissive Ir(III) metal complexes via an alternative pathway from route (d) to route (e). Chemistry and photophysical B


Article

Inorganic Chemistry

949.3, [M+]. Anal. Calcd for C42H34F6IrN3P2: C, 53.16; H, 3.61; N, 4.43. Found: C, 52.98; H, 3.90; N, 4.59. Selected Crystal Data of 2b. C42H34F6IrN3P2; M = 948.86; monoclinic; space group = P21/n; a = 13.0516(4) Å, b = 18.3913(7) Å, c = 16.1776(5) Å; β = 103.4541(10)°; V = 3776.6(2) Å3; Z = 4; ρCalcd = 1.669 Mg m−3; F(000) = 1872; crystal size = 0.150 mm × 0.105 mm × 0.057 mm; λ(Mo Kα) = 0.71073 Å; T = 150(2) K; μ = 3.685 mm−1; 31 349 reflections collected, 8648 independent reflections (Rint = 0.0289), goodness of fit (GOF) = 1.137, final R1[I > 2σ(I)] = 0.0309 and wR2(all data) = 0.0660. Synthesis of 2c. Compound 2c was obtained by treatment of IrCl3· 3H2O (100 mg, 0.28 mmol) with (pzpyphCF3)H2 (100 mg, 0.28 mmol), PPh2(CH2Ph) (160 mg, 0.57 mmol) and NaOAc (233 mg, 2.8 mmol) in refluxing DGME for 6 h. Yield: 59% (184 mg, 0.16 mmol). Spectroscopic Data of 2c. 1H NMR (CDCl3, 500 MHz): δ 7.48 (s, 1H), 7.18 (t, J = 7.0 Hz, 2H), 7.14−7.10 (m, 4H), 7.08 (d, J = 8.0 Hz, 1H), 7.05−6.99 (m, 8H), 6.95−6.91 (m, 6H), 6.86 (d, J = 8.0 Hz, 1H), 6.83 (t, J = 7.5 Hz, 4H), 6.72 (s, 1H), 6.65 (d, J = 7.5 Hz, 4H), 6.59 (d, J = 7.5 Hz, 1H), 6.55−6.52 (m, 4H), 3.53 (dt, J = 14.0, 4.0 Hz, 2H), 3.18 (dt, J = 14.0, 4.0 Hz, 2H), − 13.74 (t, JPH = 17.0 Hz, 1H); 19F−{1H} NMR (CDCl3, 470 MHz): δ −60.16 (s, 3F), −62.86 (s, 3F); 31P−{1H} NMR (CDCl3, 202 MHz): δ 11.42 (s, 2P); MS (FD): m/z 1101.3, [M+]; Anal. Calcd for C54H42F6IrN3P2: C, 58.90; H, 3.84; N, 3.82. Found: C, 59.11; H, 4.02; N, 3.66. Selected Crystal Data of 2c. C54H42F6IrN3P2; M = 1101.04; triclinic; space group = P1̅; a = 12.3736(4) Å, b = 12.9173(4) Å, c = 15.9767(6) Å; α = 80.2808(11)°; β = 74.3945(10)°; γ = 68.2270(9)°; V = 2277.21(13) Å3; Z = 2; ρCalcd = 1.606 Mg m−3; F(000) = 1096; crystal size = 0.154 mm × 0.144 mm × 0.034 mm; λ(Mo Kα) = 0.71073 Å; T = 150(2) K; μ = 3.068 mm−1; 20877 reflections collected, 10 459 independent reflections (Rint = 0.0185), GOF = 1.068, final R1[I > 2σ(I)] = 0.0180 and wR2(all data) = 0.0420. Synthesis of 3a and 4a. Compound 3a was obtained by treatment of IrCl3·3H2O (70 mg, 0.43 mmol) with (pzpyphBu)H2 (147 mg, 0.43 mmol), dppb (190 mg, 0.43 mmol) and NaOAc (349 mg, 4.26 mmol) in refluxing DGME over a period of 1 h. Yield: 23% (96 mg, 0.10 mmol). Furthermore, CCl4 (0.1 mL, excess) was added to the solution of 3a (30 mg, 0.031 mmol) in CH2Cl2 (5 mL). The mixture was refluxed for 1 h, giving the chlorinated product 4a in 73% yield (23 mg, 0.023 mmol). Spectroscopic Data of 3a. 1H NMR (CD2Cl2, 500 MHz): δ 8.20− 8.17 (m, 2H), 8.12 (t, J = 7.5 Hz, 1H), 7.64 (t, J = 7.5 Hz, 1H), 7.56− 7.40 (m, 7H), 7.28−7.24 (m, 3H), 7.15−6.01 (m, 7H), 6.93 (d, J = 8.0 Hz, 1H), 6.88 (t, J = 8.0 Hz, 2H), 6.74−6.70 (m, 4H), 6.40 (s, 1H), 6.11−6.07 (m, 2H), 0.74 (s, 9H), − 8.65 (dd, JPH = 148.0, 15.0 Hz, 1H, Ir−H). 19F NMR (CD2Cl2, 470 MHz): δ −60.74 (s, 3F). 31 P−{H} NMR (CD2Cl2, 202 MHz): δ 38.81 (s, 1P), 17.69 (s, 1P). MS (FD): m/z 983.2, [M+]. Anal. Calcd for C49H41F3IrN3P2: C, 59.87; H, 4.20; N, 4.27. Found: C, 59.62; H, 4.52; N, 4.50. Selected Crystal Data of 3a. C49.50H42ClF3IrN3P2; M = 1025.45; triclinic; space group = P1̅; a = 14.9543(5) Å, b = 15.5734(5) Å, c = 20.7011(7) Å; α = 86.6142(11)°; β = 84.9730(12)°; γ = 67.1710(10)°; V = 4424.7(3) Å3; Z = 4; ρCalcd = 1.539 Mg m−3; F(000) = 2044; crystal size = 0.265 mm × 0.156 mm × 0.104 mm; λ(Mo Kα) = 0.71073 Å; T = 200(2) K; μ = 3.201 mm−1; 38 805 reflections collected, 20 293 independent reflections (Rint = 0.0258), GOF = 1.052, final R1[I > 2σ(I)] = 0.0289 and wR2(all data) = 0.0657. Spectroscopic Data of 4a. 1H NMR (CD2Cl2, 500 MHz): δ 8.10 (dd, J = 11.0, 8.0 Hz, 2H), 7.99 (t, J = 7.5 Hz, 1H), 7.63−7.56 (m, 2H), 7.52−7.48 (m, 4H), 7.41 (td, J = 8.0, 2.0 Hz, 2H), 7.36 (td, J = 8.0, 2.0 Hz, 2H), 7.30 (d, J = 7.5 Hz, 1H), 7.24 (dd, J = 12.0, 8.0 Hz, 2H), 7.16−7.10 (m, 3H), 7.02 (dd, J = 12.0, 8.0 Hz, 2H), 6.95 (d, J = 8.0 Hz, 1H), 6.84 (td, J = 7.5, 2.0 Hz, 2H), 6.80−6.75 (m, 3H), 6.67 (s, 1H), 6.60 (s, 1H), 6.29 (dd, J = 11.0, 8.0 Hz, 2H), 0.79 (s, 9H). 19 F NMR (CD2Cl2, 470 MHz): δ −60.80 (s, 3F). 31P−{H} NMR (CD2Cl2, 202 MHz): δ 25.83 (d, JPP = 6.4 Hz, 1P), 17.72 (d, JPP = 6.4 Hz, 1P). MS (FD): m/z 1017.3, [M + ]. Anal. Calcd for

properties of the isolated products are presented and discussed in the following sections.

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EXPERIMENTAL SECTION

General Information and Materials. Mass spectra were obtained on a JEOL AccuTOF GCX instrument operating in field desorption (FD) mode. 1H, 19F, and 31P NMR spectra were obtained using the Bruker Avance 500 NMR instruments. Elemental analyses were performed using the Elementar Vario EL III CHN-O rapid elementary analyzer. The 2-pyrazolyl-6-phenylpyridine chelates L(1), e.g., (pzpyphBu)H2, and (pzpyphCF3)H2, were prepared according to literature procedure.27 All reactions were conducted under a N2 atmosphere. Photophysical Measurement. Steady-state absorption and emission spectra were recorded by a Hitachi Model U-3900 spectrophotometer and an Edinburgh Model FLS920 fluorometer, respectively. To determine the photoluminescence quantum yield in solution, the samples were degassed by three freeze−pump−thaw cycles. Coumarin 530 in ethanol, with a quantum yield (Φ) of 0.58, was served as the standard for measuring the quantum yield in solution. The Φ value of the studied complexes in PMMA thin film was measured by an integrated sphere. Lifetime studies were performed by an Edinburgh FL 900 photon-counting system with EPL-375 diode laser as the excitation source. Cyclic Voltammetry. All electrochemical potentials were measured in a 0.1 M TBAPF6/CH3CN solution for oxidation and reduction reaction, and reported in volts using FcH/FcH+ as the reference; ΔEp is defined as Epa (anodic peak potential) − Epc (cathodic peak potential) and these data are quoted in units of mV. Glassy carbon electrodes were selected for the cathode and anode (i.e., oxidation and reduction processes). Synthesis of 1a. A mixture of IrCl3·3H2O (100 mg, 0.28 mmol), (pzpyphBu)H2 (98 mg, 0.28 mmol), PPh3 (150 mg, 0.57 mmol) and NaOAc (233 mg, 2.8 mmol) in 10 mL of DGME was heated under reflux for 5 h. After removal of solvent in vacuo, the residue was purified by silica gel column chromatography, using a mixture of ethyl acetate and hexane (1:6) as the eluent to afford a light green-emitting solid of 1a. Yield: 47% (142 mg, 0.13 mmol). Spectroscopic Data of 1a. 1H NMR (CD2Cl2, 500 MHz): δ 7.29− 7.26 (m, 12H), 7.14 (t, J = 7.0 Hz, 6H), 7.10−7.6 (m, 13H), 6.98 (s, 1H), 6.81 (d, J = 8.0 Hz, 1H), 6.76 (d, J = 8.0 Hz, 1H), 6.68 (d, J = 8.0 Hz, 1H), 6.60 (d, J = 8.0 Hz, 1H), 6.44 (s, 1H), 0.84 (s, 9H), −13.47 (t, JPH = 16.5 Hz, 1H, Ir−H). 19F NMR (CD2Cl2, 470 MHz): δ −60.38 (s, 3F). 31P−{H} NMR (CD2Cl2, 202 MHz): δ 13.18 (s, 2P). MS (FD): m/z 1061.3, [M+]. Anal. Calcd for C55H47F3IrN3P2: C, 62.25; H, 4.46; N, 3.96. Found: C, 61.85; H, 4.72; N, 4.05. Synthesis of 2a. Compound 2a was obtained by treatment of IrCl3· 3H2O (100 mg, 0.28 mmol) with (pzpyphCF3)H2 (92 mg, 0.28 mmol), PPh3 (150 mg, 0.57 mmol) and NaOAc (233 mg, 2.8 mmol) in refluxing DGME for 4 h. Yield: 39.1% (119 mg, 0.11 mmol). Spectroscopic Data of 2a. 1H NMR (CD2Cl2, 500 MHz): δ 7.28− 7.24 (m, 12H), 7.22−7.18 (m, 8H), 7.11 (t, J = 7.6 Hz, 12H), 7.05 (d, J = 8.0 Hz, 1H), 6.86−6.82 (m, 2H), 6.78 (d, J = 8.0 Hz, 1H), 6.38 (s 1H), −13.35 (t, JPH = 16.5 Hz, 1H, Ir−H). 19F NMR (CD2Cl2, 470 MHz): δ −60.64 (s, 3F), −63.23 (s, 3F). 31P−{H} NMR (CD2Cl2, 202 MHz): δ 11.30 (s, 2P). MS (FD): m/z 1073.2, [M+]. Anal. Calcd for C52H38F6IrN3P2: C, 58.21; H, 3.57; N, 3.92. Found: C, 58.26; H, 3.87; N, 3.95. Synthesis of 2b. Compound 2b was obtained by treatment of IrCl3·3H2O (100 mg, 0.28 mmol) with (pzpyphCF3)H2 (92 mg, 0.28 mmol), PPh2Me (0.2 mL, excess) and NaOAc (233 mg, 2.8 mmol) in DGME at 150 °C for 3 h. Yield: 75% (201 mg, 0.21 mmol). Spectroscopic Data of 2b. 1H NMR (CD2Cl2, 500 MHz): δ 7.52 (s, 1H), 7.25 (t, J = 7.9 Hz, 1H), 7.23−7.09 (m, 13H), 7.05−7.01 (m, 5H), 6.95 (dd, J = 7.9, 2.8 Hz, 2H), 6.93−6.89 (m, 4H), 6.63 (s, 1H), 1.54 (t, JPH = 3.6 Hz, 6H), −13.96 (t, JPH = 18.3 Hz, 1H, Ir−H). 19F NMR (CD2Cl2, 470 MHz): δ −60.43 (s, 3F), −63.02 (s, 3F). 31P− {H} NMR (CD2Cl2, 202 MHz): δ −6.77 (s, 2P). MS (FD): m/z C


Article

Inorganic Chemistry

λ(Mo Kα) = 0.71073 Å; T = 150(2) K; μ = 3.012 mm−1; 21 992 reflections collected, 11 034 independent reflections (Rint = 0.0171), GOF = 1.081, final R1[I > 2σ(I)] = 0.0214 and wR2(all data) = 0.0442. Spectroscopic Data of 6c. 1H NMR (d6-acetone, 500 MHz): δ 8.32 (d, J = 7.0 Hz, 1H), 7.52 (t, J = 8.0 Hz, 1H), 7.49 (s, 1H), 7.37 (d, J = 8.0 Hz, 1H), 7.33−7.30 (m, 2H), 7.22−7.11 (m, 6H), 7.03− 6.92 (m, 9H), 6.90−6.86 (m, 6H), 6.80 (td, J = 7.5, 2.5 Hz, 2H), 6.73 (s, 1H), 6.58 (d, J = 8.0 Hz, 2H), 6.49−6.44 (m, 4H), 4.19−4.14 (m, 1H), 3.90−3.84 (m, 1H), 3.68 (dd, J = 15.5, 7.5 Hz, 1H), 3.42 (dd, J = 15.5, 7.5 Hz, 1H); 19F NMR (d6-acetone, 470 MHz): δ −60.38 (s, 3F), −62.97 (s, 3F); 31P−{1H} NMR (d6-acetone, 202 MHz): 15.14 (d, J = 368.7 Hz, 1P), − 2.57 (d, J = 368.7 Hz, 1P); MS (FD): m/z 1099.2, [M+]; Anal. Calcd for C54H40F6IrN3P2: C, 59.01; H, 3.67; N, 3.82. Found: C, 49.89; H, 3.38; N, 3.46. Selected Crystal Data of 6c. C54H40F6IrN3P2; M = 1099.03; triclinic; space group = P1̅; a = 10.2669(3) Å, b = 11.1809(4) Å, c = 21.0187(8) Å; α = 92.6471(11)°; β = 95.3421(10)°; γ = 106.6811(11)°; V = 2294.49(14) Å3; Z = 2; ρCalcd = 1.591 Mg m−3; F(000) = 1092; crystal size = 0.141 mm × 0.125 mm × 0.085 mm; λ(Mo Kα) = 0.71073 Å; T = 150(2) K; μ = 3.045 mm−1; 18 102 reflections collected, 10 555 independent reflections (Rint = 0.0235), GOF = 1.089, final R1[I > 2σ(I)] = 0.0265 and wR2(all data) = 0.0603. Raman Spectral Measurement. Confocal Raman spectra of each complex were obtained with a Thermo Nicolet Almega XR dispersive Raman spectrometer (λex = 780 nm) and an Olympus Model BX51 microscope. The measurements were performed with an exposure time of 2 s, and all spectra were accumulated over an average of 30 scans. Computational Method. Calculations were performed with the Gaussian 09 program package.48 The geometry optimization of ground states of the 10 studied Ir(III) complexes are simulated with density functional theory (DFT) at the B3LYP/LANL2DZ (Ir) and B3LYP/6-31g(d,p) (H, C, N, F, Cl) levels using CH2Cl2 as the solvent. The optimized structures of studied Ir(III) metal complexes are then used to calculate the five lowest singlet (S0 → S5) and triplet optical electronic transitions (S0 → T5) using the time-dependent density functional theory (TD-DFT) method. The solvent effect is based on the polarizable continuum model (PCM), which is implemented in the Gaussian 09 program. For both singlet and triplet optical transitions, Mulliken population analysis (MPA) is applied to obtain the electron density distribution of each atom in specific molecular orbital of these Ir(III) metal complexes, as well as to calculate the metal-to-ligand charge transfer (MLCT) in each assignment during the singlet and triplet optical transitions. X-ray Structural Determination. Single-crystal X-ray diffraction data were recorded on a Bruker D8 VENTURE diffractometer equipped with Oxford Cryostream 800+ controller. The data collection was executed using the SMART program. Cell refinement and data reduction were made with the SAINT program. The structure was determined using the SHELXTL/PC program and refined using full-matrix least-squares. All non-hydrogen atoms were refined anisotropically, whereas all hydrogen atoms of hydrocarbyl fragments were placed at the calculated positions with fixed positional parameters, and hydride was independently located on the electron density map and included in the final stage of refinements.

C49H40ClF3IrN3P2: C, 57.84; H, 3.96; N, 4.13. Found: C, 57.53; H, 3.71; N, 3.97. Synthesis of 3b and 4b. Compound 3b was obtained by treatment of IrCl3·3H2O (120 mg, 0.34 mmol) with (pzpyphBu)H2 (118 mg, 0.34 mmol), dppe (136 mg, 0.34 mmol), and NaOAc (280 mg, 2.27 mmol) in refluxing DGME for 3 h. Yield: 19% (60 mg, 0.06 mmol). Furthermore, CCl4 (0.1 mL) was added to the solution of 3b (20 mg, 0.021 mmol) in CH2Cl2 (5 mL). The mixture was refluxed for 1 h, giving the chlorinated product 4b in 70% yield (15 mg, 0.015 mmol). Spectroscopic data of 3b. 1H NMR (CD2Cl2, 500 MHz): δ 8.03− 8.00 (m, 2H), 7.77−7.74 (m, 2H), 7.64 (t, J = 8.0 Hz, 1H), 7.44− 7.27 (m, 9H), 7.13 (t, J = 7.0 Hz, 1H), 7.07 (t, J = 7.0 Hz, 1H), 6.92 (t, J = 7.0 Hz, 2H), 6.86 (t, J = 7.0 Hz, 2H), 6.77 (d, J = 8.0 Hz, 1H), 6.62 (s, 1H), 6.58 (t, J = 8.5 Hz, 2H), 6.48 (t, J = 8.5 Hz, 2H), 6.34 (s, 1H), 3.24−3.16 (m, 1H), 2.86−2.72 (m, 2H), 2.57−2.49 (m, 1H), 0.76 (s, 9H), −9.67 (dd, JPH = 149.0, 15.5 Hz, 1H, Ir−H). 19F NMR (CD2Cl2, 470 MHz): δ −60.68 (s, 3F). 31P−{H} NMR (CD2Cl2, 202 MHz): δ 36.25 (s, 1P), 12.14 (s, 1P). MS (FD): m/z 935.3, [M+]. Anal. Calcd for C45H41F3IrN3P2: C, 57.81; H, 4.42; N, 4.49. Found: C, 57.63; H, 4.62; N, 4.62. Spectroscopic Data of 4b. 1H NMR (CD2Cl2, 500 MHz): δ 8.34− 8.30 (m, 2H), 8.12−8.08 (m, 2H), 7.73 (t, J = 8.0 Hz, 1H), 7.45− 7.35 (m, 7H), 7.32 (d, J = 8.0 Hz, 1H), 7.23 (d, J = 7.5 Hz, 1H), 7.18 (t, J = 7.5 Hz, 1H), 7.15 (t, J = 7.5 Hz, 1H), 6.96−6.87 (m, 5H), 6.74 (dd, J = 10.5, 8.0 Hz, 2H), 6.56 (dd, J = 10.5, 8.0 Hz, 2H), 6.49 (s, 2H), 3.12−2.71 (m, 4H). 19F NMR (CD2Cl2, 470 MHz): δ −60.85 (s, 3F). 31P−{H} NMR (CD2Cl2, 202 MHz): δ 10.01 (d, JPP = 6.1 Hz, 1P), 8.87 (d, JPP = 6.1 Hz, 1P). MS (FD): m/z 969.3, [M+]. Anal. Calcd for C45H40ClF3IrN3P2: C, 55.75; H, 4.16; N, 4.33. Found: C, 55.51; H, 3.99; N, 4.03. Conversion of 2c to 5c and 6c. A solution of 2c (150 mg, 0.14 mmol) and NaOAc (56 mg, 0.68 mmol) in decalin (10 mL) was heated at 180 °C for 48 h. The solvent was evaporated and the residue was purified by silica gel column chromatography, using a 1:8 mixture of ethyl acetate and hexane as the eluent, giving 5c (8%, 12 mg, 0.01 mmol) and 6c (30%, 45 mg, 0.04 mmol). Alternative Synthesis of 2c, 5c, and 6c. A mixture of [Ir(COD)(μ-Cl)]]2 (88 mg, 0.13 mmol), (pzpyphCF3)H2 (94 mg, 0.26 mmol), PPh2(CH2Ph) (145 mg, 0.52 mmol), and NaOAc (215 mg, 2.6 mmol) was heated in refluxing decalin (10 mL) for 12 h. After the removal of solvent in vacuo, the residue was purified by silica gel column chromatography using ethyl acetate and hexane (1:7) as eluent to afford three Ir(III) complexes, i.e., 2c (40%, 115 mg, 0.10 mmol), 5c (10%, 29 mg, 0.03 mmol), and 6c (10%, 30 mg, 0.03 mmol), respectively. Conversion of 5c to 6c. A solution of 5c (50 mg, 0.05 mmol) in decalin (10 mL) was heated at 180 °C for 36 h. The solvent was evaporated and the residue was purified by silica gel column chromatography using a 1:3 mixture of CH2Cl2 and hexane as the eluent to afford 6c (10%, 5 mg, 0.005 mmol), together with the recovered 5c (16%, 8 mg, 0.007 mmol). Spectroscopic Data of 5c. 1H NMR (d6-acetone, 500 MHz): δ 8.15 (d, J = 8.5 Hz, 1H), 8.13 (d, J = 7.5 Hz, 1H), 7.84 (t, J = 8.0 Hz, 1H), 7.80−7.77 (m, 2H), 7.63 (d, J = 7.5 Hz, 1H), 7.51 (t, J = 7.5 Hz, 1H), 7.48 (t, J = 7.0 Hz, 1H), 7.35−7.27 (m, 6H), 7.22 (t, J = 7.5 Hz, 2H), 7.18 (d, J = 8.0 Hz, 1H), 7.01 (s, 1H), 6.98 (d, J = 7.5 Hz, 1H), 6.93−6.86 (m, 6H), 6.75 (t, J = 8.0 Hz, 2H), 6.63 (t, J = 7.5 Hz, 1H), 6.48 (t, J = 8.5 Hz, 2H), 6.41−6.38 (m, 3H), 6.20 (d, J = 7.5 Hz, 2H), 5.96 (dd, J = 7.5, 4.5 Hz, 1H), 4.79 (dd, J = 15.0, 10.5 Hz, 1H), 3.99 (dd, J = 15.0, 13.0 Hz, 1H), 3.03 (dd, J = 15.5, 5.5 Hz, 1H), 2.82 (dd, J = 15.5, 5.5 Hz, 1H); 19F NMR (d6-acetone, 470 MHz): δ −60.69 (s, 3F), −62.69 (s, 3F); 31P−{1H} NMR (d6-acetone, 202 MHz): 8.47 (d, JPP = 13.1 Hz, 1P), −8.14 (d, JPP = 13.1 Hz, 1P); MS (FD): m/z 1099.2, [M+]; Anal. Calcd for C54H40F6IrN3P2: C, 59.01; H, 3.67; N, 3.82. Found: C, 49.89; H, 3.38; N, 3.46. Selected Crystal Data of 5c. C55H42Cl2F6IrN3P2; M = 1183.95; triclinic; space group = P1̅; a = 12.2720(6) Å, b = 12.4860(6) Å, c = 16.0893(8) Å; α = 82.6715(16)°; β = 85.3043(16)°; γ = 81.0960(15)°; V = 2410.9(2) Å3; Z = 2; ρCalcd = 1.631 Mg m−3; F(000) = 1176 crystal size = 0.198 mm × 0.183 mm × 0.138 mm;

■

RESULTS AND DISCUSSION Syntheses and Spectroscopic Characterization. As shown in Scheme 3, two tridentate chelates L(1) possessing distinctive tert-butyl and CF3 substituents at the para-position of phenyl fragment, i.e., (pzpyphBu)H2 and (pzpyphCF3)H2, are employed for this investigation, for which the introduced alkyl substituents are capable of fine-tuning the photophysical properties (vide infra). These chelates are synthesized using Suzuki coupling of 2-acetyl-6-bromopyridine with the aryl boronic acid, followed by Claisen condensation with ethyl D


Article

Inorganic Chemistry

NMR data, only one 31P NMR signal in the region between δ 13.18 and −6.77 is observed for Ir(III) complexes 1a and 2a− 2c. Hence, the phosphines and hydride occupy the axial and equatorial sites, relative to the tridentate chelate in all derivatives of 1a and 2a−2c. On the one hand, the chelating diphosphine of Ir(III) complexes 3a and 3b demands a ciscoordination arrangement, because of the inherent constraint. Hence, both complexes 3a and 3b exhibit two 31P NMR signals at δ 38.81 and 17.69 and at δ 36.25 and 12.14, as a result of two distinctive phosphine sites. The position of hydride also imposes a great difference in their chemical stabilities. This is revealed by the relative inertness of all Ir(III) complexes 1a and 2a−2c in solution upon exposure to air and moisture. In sharp contrast, the hydride complexes 3a and 3b undergo rapid chlorination of hydride in chlorinated solvents such as CCl4, CHCl3, and even CH2Cl2 to afford the corresponding chloro-substituted Ir(III) metal complexes 4a and 4b (cf. Scheme 5). This reaction is

Scheme 3. Structural Drawings of Chelates (pzpyphBu)H2 and (pzpyphCF3)H2

trifluoroacetate, and cyclization with hydrazine, giving pyrazole.27 With these chelates in hand, we then attempted the syntheses of iridium-hydride complexes [Ir(pzpyphBu)(PPh3)2H] (1a), [Ir(pzpyphCF3)(PPh2R)2H] (2a−2c), R = Ph, Me, CH2Ph, [Ir(pzpyphBu)(dppb)H] (3a), and [Ir(pzpyphBu)(dppe)H] (3b), by treatment of these tridentate chelates (pzpyphBu)H2 and (pzpyphCF3)H2 with IrCl3·3H2O in the presence of phosphine donor, such as PPh3, PPh2Me, PPh2(CH2Ph), dppb, and dppe. These reactions were typically conducted in refluxing diethylene glycol monomethyl ether (DGME) and in the presence of sodium acetate. Apparently, DGME served as the source of hydride for all Ir(III) complexes,49,50 while sodium acetate promoted cyclometalation via a concerted metalation and deprotonation reaction sequence.51 After that, the products were purified by routine silica gel column chromatography, followed by recrystallization. The structural drawings of Ir(III) complexes 1 − 3 are depicted in Scheme 4. Their identities were first confirmed by

Scheme 5. Structural Drawings of the Chloro-Substituted Ir(III) Complexes 4a and 4b

Scheme 4. Structural Drawings of Ir(III) Metal Complexes 1−3

akin to that reported for several metal hydride complexes conducted in chlorinated solvents.52,53 Moreover, among all reaction media tested, CCl4, which possesses the weakest C− Cl bonds among chlorinated solvents, displayed the highest reactivity. Their key spectroscopic features are the disappearance of hydride in their 1H NMR spectra, the retention of all other 1H NMR signals derived from both tridentate chelate L(1) and diphosphine and, ultimately, the correct molecular ion mass (M+), according to the field desorption (FD) mass analyses. Remarkably, a different reactivity pattern was noted for complex 2c, bearing two benzyl-substituted PPh2(CH2Ph) ligands than those with the noncyclometalating PPh3 and PPh2Me. It underwent concomitant C−H activation and hydride elimination to afford two structurally similar Ir(III) complexes [Ir(pzpyphCF3)(PPh2R)(PPh2R′)] R = CH2Ph and R′ = CH2(o-C6H4) (5c and 6c) in refluxing decalin over an extended period of 48 h (see Scheme 6). The formation of similar five-membered phosphine-containing metallacycle has been also documented in the literature.54,55 In all these

mass spectrometry, showing two monodentate phosphines or a single diphosphine chelate. Next, the presence of hydride is revealed by the 1H NMR signal in the high field region δ −13.35 to −13.96 for Ir(III) complexes 1a and 2a−2c, and at δ −8.65 to −9.67 for 3a and 3b, respectively. The hydride signal of 1a and 2a−2c splits into a triplet, with the JHP coupling constant being ∼17 Hz, which indicated the possession of two symmetrical arranged phosphines. The hydride of 3a and 3b appears as a doublet of doublet, for which the large and small JHP coupling constants of 148 and 15 Hz, suggest the coexistence of trans-P−Ir−H and cis-P−Ir−H coordination arrangements. In good consistency with the 1H

Scheme 6. Structural Drawings of Ir(III) Complexes 5c and 6c

E


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Inorganic Chemistry precedents, the addition of sodium acetate is essential for the cyclometalation reaction,56 except that this reaction is not stereospecific and the benzyl entity of 5c and 6c is located trans- or cis- to the second PPh2(CH2Ph) ligand, respectively. Preparation of 5c and 6c was also achieved by heating [Ir(COD)(μ-Cl)]]2 with (pzpyphCF3)H2, PPh2(CH2Ph), and sodium acetate in decalin for 12 h, albeit in low yields. It seems that switching the solvent from protic DGME to aprotic decalin not only suppressed the isolation of metal hydride 2c, but also induced a faster conversion to complexes 5c and 6c. Such media effect has been documented for several related reactions conducted in distinctive solvents.57 Finally, heating 5c under similar condition has induced a slow conversion to 6c at 180 °C over 36 h, but not for the reversed isomerization from 6c to 5c. The greater thermodynamic stability of 6c can be explained by either increased steric encumbrance between the equatorial PPh2 fragment and the phenyl group of the tridentate chelate in 5c, or reduced trans-influence of the transaligned C(bz)−Ir−N(py) linkage in 6c than that of the transC(bz)−Ir−P array of 5c, as both benzyl and phosphine groups are relatively strong field ligands, and should avoid to reside at the mutual trans-disposition for better stability. Importantly, one PPh2(CH2Ph) ligand of Ir(III) complex 2c retained its monodentate coordination in giving the mixture of 5c and 6c. This reactivity pattern is different from the similar reaction of iridium metal reagent with both tridentate chelate L(1) and bidentate carbene L(4), for which the pyrazolate entity of L(1) has turned to a bridging mode and led to formation of diiridium complex.58 Apparently, the stronger Ir−P bonding has prevented phosphine dissociation. Otherwise, it would yield a hypothetical, penta-coordinated intermediate, followed by dimerization. Structural Determination. Single-crystal X-ray diffraction (XRD) analyses were conducted on Ir(III) complexes 2b, 2c, 3a, 5c, and 6c for the unambiguous identification of their molecular structures. As indicated in Figures 1 and 2, complexes 2b and 2c are isostructural, in which the hydride is located on an equatorial plan defined by the tridentate chelate L(1), and dual phosphine ligands are located at the positions above and below the equatorial plane. The N(py)−Ir distance (2.056(3)−2.0662(16) Å) is slightly shorter than the N(pz)−Ir distance (2.089(3)−2.123(2) Å) due to the internal

Figure 2. Structural drawing of 2c with thermal ellipsoids shown at the 30% probability level. Selected bond distances: Ir−N(1) = 2.0662(16) Å, Ir−N(2) = 2.1234(16) Å, Ir−C(1) = 2.0492(19) Å, Ir−P(1) = 2.3181(5) Å, Ir−P(2) = 2.3088(5) Å, and Ir−H = 1.47(2) Å. Selected angles: C(1)−Ir−N(2) = 156.45(7)°, N(1)−Ir−H = 176.9(9)°, and P(1)−Ir−P(2) = 171.734(17)°.

constraint imposed on the L(1) chelate, while the P−Ir distances (2.3074 (10) - 2.3181(5) Å) are slightly unequal which can be attributed to the crystal packing effect. Overall, the bonding mode of the tridentate chelate L(1) is similar to that observed in the relevant Ir(III) complexes.59 A view of the molecular structure of 3a is depicted in Figure 3, showing a similar arrangement of L(1), but with hydride

Figure 3. Structural drawing of 3a with thermal ellipsoids shown at the 30% probability level. Selected bond distances: Ir−N(1) = 2.041(3) Å, Ir−N(2) = 2.125(3) Å, Ir−C(1) = 2.055(3) Å, Ir−P(1) = 2.2369(8) Å, Ir−P(2) = 2.3425(8) Å, and Ir−H = 1.58(3) Å. Selected bond angles: C(1)−Ir−N(2) = 156.64(12)°, P(2)−Ir−H = 170.7(12)°, N(1)−Ir−P(1) = 174.35(8)°, and P(1)−Ir−P(2) = 85.04(3)°.

located at the axial position. As a consequence, the P(2) atom showed an elongated Ir−P distance of 2.3425(8) Å, influenced by the trans-hydride, vs the shorter Ir−P(1) distance of 2.2369(8) Å. Finally, the structural diagram of isomeric Ir(III) complexes 5c and 6c are illustrated in Figures 4 and 5, respectively. Again, all metric parameters associated with the L(1) chelate are similar, and the most notable difference is the lengthening of the Ir−P(2) distance to 2.4156(6) Å, which is located opposite to the benzyl fragment of 5c, while all other Ir−P distances within both 5c and 6c span a slightly shorter

Figure 1. Structural drawing of 2b with thermal ellipsoids shown at the 30% probability level. Selected bond distances: Ir−N(1) = 2.056(3) Å, Ir−N(2) = 2.089(3) Å, Ir−C(1) = 2.055(4) Å, Ir−P(1) = 2.3074(10) Å, Ir−P(2) = 2.3156(10) Å, and Ir−H = 1.51(5) Å. Selected bond angles: C(1)−Ir−N(2) = 158.23(16)°, P(1)−Ir−P(2) = 173.01(3)°, and N(1)−Ir−H = 177.7(19)°. F


Article

Inorganic Chemistry

Figure 6. Ultraviolet−visible light (UV-vis) absorption and emission spectra of Ir(III) complexes 1a, 2a, 3a, and 4a recorded in CH2Cl2 solution (1 × 10−5 M) at room temperature (RT). Figure 4. Structural drawing of 5c with thermal ellipsoids shown at the 30% probability level. Selected bond distances: Ir−N(1) = 2.0405(18) Å, Ir−N(2) = 2.1249(18) Å, Ir−C(1) = 2.063(2) Å, Ir− P(1) = 2.2833(6) Å, Ir−P(2) = 2.4156(6) Å, and Ir−C(23) = 2.102(2) Å. Selected bond angles: C(1)−Ir−N(2) = 156.81(8)°, P(2)−Ir−C(23) = 176.09(6)°, N(1)−Ir−P(1) = 169.01(5)°, and P(1)−Ir−C(23) = 80.11(6)°.

peak onset extended beyond 420 nm is assigned to the spinforbidden 3π−π* plus MLCT transition processes, because of the lowered absorptivity. The UV-vis absorption spectra of Ir(III) complexes 5c and 6c are depicted in Figure 7, together with the spectrum of 2c as a reference. As can be seen, their similar spectral pattern indicates the dominance of tridentate chelate (i.e., pzpyphCF3) in determining the absorption spectra of these Ir(III) complexes; all possess two PPh2(CH2Ph) ligands, but with different coordination mode and orientation. Figure 6 also depicts the respective emission spectra of 1a− 4a recorded in degassed CH2Cl2, for which the emission profiles are very close to each other, as well as to those obtained from the spin-casted poly methyl methacrylate (PMMA) thin film (i.e., Figures S1− S3 in the Supporting Information). The observed vibronic fine structures confirm the dominant π−π* character at the lowest energy excited states of these Ir(III) complexes.60 Moreover, the Ir(III) metal complexes 1a and 2a possess dual PPh3 ligands and exhibit the first peak max. at 476 and 489 nm, respectively. This suggests that the CF3 substituent on the phenyl appendage of tridentate chelate L(1) of 2a induces a slightly smaller ligand-centered π−π* energy gap than that of the 4-tert-butyl-substituted phenyl appendage on L(1) chelate of 1a. Furthermore, the Ir(III) complexes 3a and 4a (also 3b and 4b) exhibit identical E00 at ∼480 nm, indicating that their emission energy is principally determined by this tridentate chelate L(1) and that the hydride-to-chloride substitution has negligible influence on the emission energy gap. On the other hand, these Ir(III) complexes show a profound variation in other properties such as emission quantum yield (Φ) and lifetime (τobs). First, Φ of 1a (50%) in CH2Cl2 solution is lower than that of 2a (Φ = 70%), which could be attributed to a larger quenching efficiency for the tert-butyl substituent, because of its greater rotational and vibrational freedoms than that of the CF3 substituent. Upon switching the hydride from an equatorial site to an axial site (i.e., 1a vs 3a), the results show a significantly reduction in Φ to 8% for 3a. This could be due to either the weakened Ir−H bonding interaction (vide infra) or the reduced steric shielding of hydride of 3a than that of 1a; the latter is well-shielded by two bulky PPh3 ligands residing above and below. The emission Φ value became even smaller upon conversion of 3a to the chloro complex 4a (0.3%), which can be understood by the inferior ligand field strength, because of the better π-donating property of the chloro substituent.

Figure 5. Structural drawing of 6c with thermal ellipsoids shown at the 30% probability level. Selected bond distances: Ir−N(1) = 2.055(2) Å, Ir−N(2) = 2.135(2) Å, Ir−C(1) = 2.055(3) Å, Ir−P(1) = 2.2877(8) Å, Ir−P(2) = 2.3573(7) Å, and Ir−C(17) = 2.087(3) Å. Selected bond angles: C(1)−Ir−N(2) = 157.03(10)°, N(1)−Ir− C(17) = 173.62(10)°, P(1)−Ir−P(2) = 169.05(3)°, and P(1)−Ir− C(17) = 79.29(8)°.

range of 2.2833(6)−2.3573(7) Å. The results exemplify the observed inferior stability of 5c than that of 6c discussed earlier. Photophysical Properties. Ultraviolet−visible light (UVvis) spectra of the representative Ir(III) complexes 1a, 2a, 3a, and 4a recorded in CH2Cl2 solution are depicted in Figure 6, and the numerical data are summarized in Table 1. All of these spectra exhibit intense absorptions below 350 nm with a molar extinction coefficient (ε) of >6 × 103 M−1 cm−1, which are ascribed to the π−π* transition over the L(1) chelate. The longer wavelength absorption in the region 360−420 nm with a lower extinction coefficient is attributed to the metal-toligand charge transfer (MLCT) transitions, while absorption G


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Inorganic Chemistry Table 1. Photophysical and Electrochemical Properties for the Studied Ir(III) Metal Complexes abs λmaxa (nm) [ε (× 103 M−1 cm−1)] 1a 2a 2b 2c 3a 3b 4a 4b 5c 6c

278 [35.8], 342 [7.9], 394 [2.7] 273 [40.1], 339 [6.3], 406 [1.2] 266 [35.3], 342 [6.3], 406 [1.4] 269 [45.1], 341 [8.0] 269 [25.3], 326 [12.2], 398 [2.9] 268 [31.1], 325 [11.0], 401 [2.6] 277 [29.7], 326 [11.2], 405 [2.5] 276 [26.3], 298 [12.2], 404 [2.3] 270 [25.8], 338 [7.6], 420 [1.1] 270 [40.8], 380 [2.6], 418 [0.9]

em λmaxb (nm)

Φb (%)

τobsb (μs)

kr (× 105 s−1)

knr (× 105 s−1)

Eoxpac (V)

Erepcc (V)

476, 512, 548 (475, 512, 545) 489, 524, 559 (486, 522, 558) 489, 526, 558 (487, 524, 560) 487, 524, 561 (484, 521,558) 479, 514, 547 (477, 512, 549) 480, 515, 550 (478, 513, 546) 478, 513, 545 (478, 514, 543) 480, 515 (477, 512, 549)

50 (48)

20.1 (25.4)

0.25

0.25

0.64

−2.73

70 (45)

21.5 (14.6)

0.32

0.14

0.74

−2.52

72 (47)

20.0 (13.6)

0.36

0.14

0.63

−2.50

61 (54)

16.5 (14.2)

0.37

0.24

0.73

−2.53

8.1 (13)

3.0 (14.2)

0.27

3.07

0.10

3.0 (11)

0.6 (10.5)

0.50

16.2

0.22

0.30 (55)

0.14 (26.8)

0.21

71.2

0.16 (36)

0.02 (18.8)

0.80

499

14 (20)

4.1 (11.2)

0.34

2.10

1.02 (0.99 [59])d 0.95 (0.91 [69])d 0.89

−2.70 (−2.66 [77])d −2.69 (−2.65 [72])d −2.13

38 (25)

5.3 (9.1)

0.72

1.17

0.72

495, 533, 578 (495, 530, 570) 495, 530, 567 (492, 527, 568)

−2.06 −2.32 −2.40

a Absorption spectra were measured in CH2Cl2 with a concentration of (1 × 10−5 M. bPhotoluminescent (PL) data were measured in degassed CH2Cl2, while data in parentheses were obtained from spin-casted PMMA thin film (5 wt %). cEpa and Epc are the anodic and cathodic peak potentials referenced to the FcH+/FcH couple. dData in parentheses indicated E1/2 and ΔEp, as defined by E1/2 = [(Epa + Epc)/2] in V and ΔEp = | Epa − Epc| (in mV).

the Supporting Information. All emission showed notable vibronic fine structure with similar peak positions, which is a clear indication of the π−π* dominated transition character at the excited state, and are mainly controlled by the tridentate class of chelate L(1). In addition, the cyclometalating counterparts 5c and 6c show a much faster nonradiative decay rate constant and become less efficient, which are reasonably attributed to the relatively weaker ligand field strength of the coordinated benzyl group than that exerted by the hydride in 2c. Electrochemical Properties. The electrochemical behavior of these Ir(III) complexes was studied using cyclic voltammetry in Table 1 and Figure S5 in the Supporting Information. As shown in Table 1, complexes 1a and 2a show ox an irreversible oxidation peak potential (Epa ) and an re irreversible reduction peak potential (Epc) at 0.64 and 0.74 V, and −2.73 V and −2.52 V, respectively. The slightly redshifted emission of 2a can be explained by the greater stabilization of the lowest unoccupied molecular orbital (LUMO) than that of the highest occupied molecular orbital (HOMO).27 For the series of Ir(III) complexes 2a, 2b, and 2c, there is almost no change for Erepc, bceause all of them possess a similar identical LUMO located on the L(1) chelate, while the larger variation of Eoxpa is caused by the distinctive phosphine donor. For the complexes 1a and 3a, there exists a much larger re change in Eox pa (0.52 V) than Epc (0.04 V). This observation is re verified by a similar change of Eox pa (0.42 V) and Epc (0.03 V) between 1a and 3b, and highlights the greater sensitivity of Eox pa to the coordination mode of phosphines, altering the electron density at the Ir(III) metal center. After conversion of 3a and 3b in forming 4a and 4b, both the oxidation and reduction potentials are shifted to more-positive (less-negative) regions, which are in agreement with the existence of more-stabilized HOMO and LUMO, which is a result of the poor electron donating property of chloride, relative to that of the hydride ligand. Finally, among the triads, 2c, 5c, and 6c all possess two PPh2(CH2Ph) fragments, so that the conversion from 2c to 5c

Figure 7. UV-vis absorption and emission spectra of Ir(III) complexes 2c, 5c, and 6c recorded in CH2Cl2 solution (1 × 10−5 M) at RT.

As depicted in Table 1, the associated radiative and nonradiative rate constant was calculated using the following equations: kr =

Φ τobs

and

k nr =

(1 − Φ) τobs

and the data showed that all complexes exhibit small change of radiative rate constants (kr), i.e., (2.1−7.2) × 104 s−1. In contrast, the nonradiative rate constants (knr) varied significantly from 1.4 × 104 s−1 for both 2a and 2b to 7.1 × 106 s−1 and 5.0 × 107 s−1 for 4a and 4b, respectively. The results unambiguously conclude that their emission efficiency is principally determined by the nonradiative decay process, for which the ligand field strength and/or the energy of upper lying metal centered dd excited state seem to be the major determining factors. Figure 7 also depicts the emission spectra of Ir(III) complexes 2c, 5c, and 6c in deaerated CH2Cl2 at RT, while that recorded in PMMA thin film are present in Figure S4 in H


Article

Inorganic Chemistry and 6c seems to induce less variation in Eox pa from 0.73 V to 0.89 and 0.72 V, but a very systematic change in Erepc from −2.53 V to −2.32 and −2.40 V. The results reveal a greater stabilization of LUMO induced by the cyclometalation and concomitant transformation from the hydride to the benzyl group. Computational Study. Time-dependent density functional theory (TD-DFT) study was conducted for gaining insight into the photophysical properties. The calculated energy in wavelengths and assignments of each transition of all studied Ir(III) complexes are listed in Table 2 and Tables Table 2. Calculated Wavelengths, Transition Probabilities, and Main Charge Characters of the Lowest Optical Transitions S1 and T1 for the Ir(III) Complexes 1a, 2a, 3a, and 4a in CH2Cl2

1a

2a

3a

4a

state

λ (nm)

T1

445.8

0

S1

370.4

0.045

T1

453.8

0

S1

377.7

0.0391

T1

450.9

0

S1

388

0.0371

T1

452.7

0

S1

397.6

0.0085

f

main assignments

metal-to-ligand charge transfer, MLCT (%)

HOMO → LUMO (44%) HOMO → LUMO (91%)

14.36


17.20


16.41


3.24

18.85

22.09

21.68

1.62

S1− S10 in the Supporting Information). Figure 8 and Figures S6−S15 in the Supporting Information depict the frontier orbitals involved in the lower-lying transitions. Particularly, the calculated emission energy of the S0 → S1 transition for 1a (370.4 nm), 2a (377.7 nm), 3a (388.0 nm), and 4a (397.6 nm) is close to the observed onsets of the experimental absorption peaks depicted in Figure 6. In addition, the calculated S0 → T1 transition energies, in terms of wavelength for 1a (445.8 nm), 2a (453.8 nm), 3a (450.9 nm), and 4a (452.7 nm) are also in good agreement with the trend of the onset of their phosphorescence spectra in Figure 6. Therefore, the current TD-DFT simulation works well in predicting the lowest Franck−Condon transition for both absorption and emission, based on the optimized ground state (S0) structure for the studied Ir(III) complexes that are composed of the L(1) class of chelate, and phosphine, hydride, and chloride ligands. The assignments for the lower-lying singlet S0 → S1 and triplet S0 → T1 optical transitions are derived mainly from the HOMO → LUMO process. For all titled Ir(III) complexes, the electron density distributions of HOMO are mainly localized at the central Ir atom (∼15% to 26%) and L(1) fragment, while LUMO are distributed at the L(1) fragment and very few in the central Ir(III) atom (2%−8%). Therefore, the S0 → S1 and S0 → T1 optical transitions are mainly assigned to the metal-toligand charge transfer (MLCT) mixed with the intraligand charge transfer (ILCT) transitions. Note that, for complexes 5c and 6c, because of the cyclometalation, the ancillary benzene

Figure 8. Frontier molecular orbitals HOMO and LUMO for the Ir(III) complexes 1a, 2a, 3a, and 4a in CH2Cl2. “Ir:” and “Cl:” denote the relative electron density distribution at the Ir and Cl atoms, respectively.

moiety has a trace contribution to HOMO (see Figures S14 and S15 in the Supporting Information). Nevertheless, because the transitions involve negligible ancillary ligands, these Ir(III) complexes should possess similar transition moments and emission gap, which are experimentally evidenced by the closer emission radiative lifetimes and peak wavelengths for all titled Ir(III) complexes. Despite similar transition properties, as mentioned early, their drastically different emission efficiencies are mainly controlled by the ancillaries (vide supra). Raman Spectroscopy. Last but not least, because of the weak scattering factors for hydrogen versus nearby Ir(III) atom, the hydride position could not be exactly located either by difference Fourier or by mean of least-squares refining in Xray crystallography. The typical IR ν(Ir−H) stretching band occurs in 2000−2200 cm−1.61−63 For a complementary study, alternatively, we used the Raman spectroscopy instead. In our previous study, the Ir−H bond stretching Raman frequencies are ∼2110−2200 cm−1.57 To provide further support of the trend of Ir−H bond strengths, both Raman experiments and calculations of corresponding vibrational frequencies of 1a, 2a, 2b, 2c, 3a, and 3b were performed with results shown in Figure I


Article

Inorganic Chemistry

and their location are pivotal to the photoluminescence. First, the emission energy is mainly determined by the tridentate chelate L(1), for which the CF3 (versus tert-butyl) substituent on phenyl fragment causes a slight red-shifting of photoluminescence, which is due to its greater capability in stabilizing the LUMOs relative to the HOMOs. Second, their emission efficiencies are mainly controlled by the ancillaries, among which the equatorial hydride of 1a and 2a−2c imposes a greater ligand field strength than that of the axial hydride of 3a and 3b. Hence, the latter exhibits inferior emission efficiency and underwent facile chlorination of hydride in giving the poorly emissive 4a and 4b. Lastly, heating of 2c, which bears two PPh2(CH2Ph) ligands, induces formation of both 5c and 6c, via C−H activation and H2 elimination in the presence of sodium acetate. Their higher emission efficiency implicates the possible application as phosphorescent dopant in the fabrication of OLEDs.

9, as well as Figures S16 and S17 in the Supporting Information. The experimental Ir−H bond stretching

■

Figure 9. Raman spectra of Ir(III) complexes in solid in the region of 1800−2200 cm−1. The asterisk symbol (*) symbolizes the corresponding peak position. −1

−1

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00905.

−1

frequencies are 2085 cm (1a), 2115 cm (2a), 2098 cm (2b), 2086 cm−1 (2c), 2059 cm−1 (3a), and 1996 cm−1 (3b), respectively, while the calculated Ir−H bond stretching frequencies of are 2135 cm−1 (1a), 2140 cm−1 (2a), 2093 cm−1 (2b), 2132 cm−1 (2c), 2075 cm−1 (3a), and 2064 cm−1 (3b), respectively (see Figures S16 and S17). Qualitatively speaking, the experimental data are in good accordance with the calculated Ir−H stretching frequencies. Because the structures of (2a, 2b versus 2c) and (3a versus 3b) are in the same scaffold of ancillary ligands (Scheme 4), it may be meaningful to discuss the trend of Raman frequencies by two separated classes. Upon geometry optimization, the Ir−H bond distance is calculated to be 1.624 Å for 2a, 1.629 Å for 2b, 1.626 Å for 2c, 1.621 Å for 3a, and 1.623 Å for 3b, which is consistent with the calculated ν(Ir−H) Raman frequencies, being on the order of 3a > 3b and 2a > 2c > 2b. For rationalization, dppb in 3a is a stronger electron withdrawing group than that of dppe in 3b. Also, both phenyl substituents in 2a and benzyl group in 2c have stronger electron-withdrawing ability than methyl group in 2b. As a result, the Ir−H bonding strength and hence Raman Ir−H stretching frequency are expected to be in the order of 3a > 3b and 2a > 2c > 2b. Experimentally, the observed Raman Ir−H stretching intensity trend is 3a > 3b and 2a > 2b > 2c. The latter of 2b > 2c seems to be opposite to the theoretical prediction. This controversy may result from the vibrational coupling and interference of combinational bands. Since current computational methodology cannot assess the vibrational coupling terms, this issue is pending resolution. Unfortunately, all these hydride complexes failed to undergo H/D exchange with D2O or CD3OD, even in the presence of added CF3CO2D or NaOD. Thus, confirmation of these Ir−H peaks by classical isotope labeling experiment was not conducted further.

UV/vis absorption and PL spectra in CH2Cl2 solution and in spin-casted PMMA thin film, results concerning the TD-DFT calculation and the Ir−H stretching frequencies, and 1H NMR spectra of all synthesized Ir(III) metal complexes (PDF) Accession Codes

CCDC 1827256−1827260 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.C.). *E-mail: [email protected] (P.-T.C.). *E-mail: [email protected] (A.K.-Y.J.). ORCID

Pi-Tai Chou: 0000-0002-8925-7747 Alex K.-Y. Jen: 0000-0002-9219-7749 Yun Chi: 0000-0002-8441-3974 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the funding from Ministry of Science and Technology of Taiwan, Featured Areas Research Program within the framework of the Higher Education Sprout Project administrated by Ministry of Education (MOE) of Taiwan, and City University of Hong Kong, Hong Kong SAR. We were grateful to the National Center for the HighPerformance Computing (NCHC) of Taiwan for the valuable computer time and facilities.

■

CONCLUSION In summary, syntheses and characterization of iridium(III) complexes (1−6) bearing a single tridentate chromophoric chelate L(1) and a range of distinctive phosphine and other ancillary, and the transformations from Ir(III) metal complexes 1−3 to 4−6 are reported. Our results indicate that all chelates J


Article

Inorganic Chemistry

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(21) Henwood, A. F.; Zysman-Colman, E. Lessons Learned in Tuning the Optoelectronic Properties of Phosphorescent Iridium(III) Complexes. Chem. Commun. 2017, 53, 807−826. (22) Yutaka, T.; Obara, S.; Ogawa, S.; Nozaki, K.; Ikeda, N.; Ohno, T.; Ishii, Y.; Sakai, K.; Haga, M. Syntheses and Properties of Emissive Iridium(III) Complexes with Tridentate Benzimidazole Derivatives. Inorg. Chem. 2005, 44, 4737−4746. (23) Darmawan, N.; Yang, C.-H.; Mauro, M.; Raynal, M.; Heun, S.; Pan, J.; Buchholz, H.; Braunstein, P.; De Cola, L. Efficient near-UV Emitters Based on Cationic Bis-Pincer Iridium(III) Carbene Complexes. Inorg. Chem. 2013, 52, 10756−10765. (24) Chirdon, D. N.; Transue, W. J.; Kagalwala, H. N.; Kaur, A.; Maurer, A. B.; Pintauer, T.; Bernhard, S. [Ir(N̂ N̂ N)(C∧N)L]+: A New Family of Luminophores Combining Tunability and Enhanced Photostability. Inorg. Chem. 2014, 53, 1487−1499. (25) Yang, L.; Okuda, F.; Kobayashi, K.; Nozaki, K.; Tanabe, Y.; Ishii, Y.; Haga, M. Syntheses and Phosphorescent Properties of Blue Emissive Iridium Complexes with Tridentate Pyrazolyl Ligands. Inorg. Chem. 2008, 47, 7154−7165. (26) Chung, L.-H.; Lo, H.-S.; Ng, S.-W.; Ma, D.-L.; Leung, C.-H.; Wong, C.-Y. Luminescent Iridium(III) Complexes Supported by NHeterocyclic Carbene-Based Ĉ Ĉ C-Pincer Ligands and Aromatic Diimines. Sci. Rep. 2015, 5, 15394. (27) Lin, J.; Chau, N.-Y.; Liao, J.-L.; Wong, W.-Y.; Lu, C.-Y.; Sie, Z.T.; Chang, C.-H.; Fox, M. A.; Low, P. J.; Lee, G.-H.; Chi, Y. BisTridentate Iridium(III) Phosphors Bearing Functional 2-Phenyl-6(imidazol-2-ylidene)pyridine and 2-(Pyrazol-3-yl)-6-phenylpyridine Chelates for Efficient OLEDs. Organometallics 2016, 35, 1813−1824. (28) Kuei, C.-Y.; Tsai, W.-L.; Tong, B.; Jiao, M.; Lee, W.-K.; Chi, Y.; Wu, C.-C.; Liu, S.-H.; Lee, G.-H.; Chou, P.-T. Bis-Tridentate Ir(III) Complexes with Nearly Unitary RGB Phosphorescence and Organic Light-Emitting Diodes with External Quantum Efficiency Exceeding 31%. Adv. Mater. 2016, 28, 2795−2800. (29) Strassner, T. Phosphorescent Platinum(II) Complexes with Ĉ C* Cyclometalated NHC Ligands. Acc. Chem. Res. 2016, 49, 2680− 2689. (30) Sajoto, T.; Djurovich, P. I.; Tamayo, A.; Yousufuddin, M.; Bau, R.; Thompson, M. E.; Holmes, R. J.; Forrest, S. R. Blue and near-UV Phosphorescence from Iridium Complexes with Cyclometalated Pyrazolyl or N-Heterocyclic Carbene Ligands. Inorg. Chem. 2005, 44, 7992−8003. (31) Chang, C.-F.; Cheng, Y.-M.; Chi, Y.; Chiu, Y.-C.; Lin, C.-C.; Lee, G.-H.; Chou, P.-T.; Chen, C.-C.; Chang, C.-H.; Wu, C.-C. Highly Efficient Blue-Emitting Ir(III) Carbene Complexes and Phosphorescent OLEDs. Angew. Chem., Int. Ed. 2008, 47, 4542−4545. (32) Elie, M.; Renaud, J. L.; Gaillard, S. N-Heterocyclic Carbene Transition Metal Complexes in Light Emitting Devices. Polyhedron 2018, 140, 158−168. (33) Kuo, H.-H.; Chen, Y.-T.; Devereux, L. R.; Wu, C.-C.; Fox, M. A.; Kuei, C.-Y.; Chi, Y.; Lee, G.-H. Bis-Tridentate Ir(III) Metal Phosphors for Efficient Deep-Blue Organic Light-Emitting Diodes. Adv. Mater. 2017, 29, 1702464. (34) Jacquemin, D.; Escudero, D. The Short Device Lifetimes of Blue PhOLEDs: Insights into the Photostability of Blue Ir(III) Complexes. Chem. Sci. 2017, 8, 7844−7850. (35) Chiu, Y.-C.; Hung, J.-Y.; Chi, Y.; Chen, C.-C.; Chang, C.-H.; Wu, C.-C.; Cheng, Y.-M.; Yu, Y.-C.; Lee, G.-H.; Chou, P.-T. En Route to High External Quantum Efficiency (∼12%), Organic True-BlueLight-Emitting Diodes Employing Novel Design of Iridium(III) Phosphors. Adv. Mater. 2009, 21, 2221−2225. (36) Wang, J.; Bai, F.-Q.; Xia, B.-H.; Zhang, H.-X. Efficient BlueEmitting Ir(III) Complexes with Phosphine Carbanion-Based Ancillary Ligand: A DFT Study. J. Phys. Chem. A 2011, 115, 11689−11695. (37) Park, S. W.; Kim, Y. S. Theoretical Study of New Blue Iridium Complexes Comprising a Bipyridine Derivative and Various Ancillary Ligands. J. Nanosci. Nanotechnol. 2014, 14, 5313−5316. (38) Alam, P.; Das, P.; Climent, C.; Karanam, M.; Casanova, D.; Choudhury, A. R.; Alemany, P.; Jana, N. R.; Laskar, I. R. Facile

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Iridium(III) Complexes Bearing Tridentate Chromophoric Chelate

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