Diastereoselective Synthesis and Photophysical Properties of Bis

Aug 24, 2017 - Chiral luminescent molecules have attracted great attention for their wide potential application in 3D optical displays,(1) encrypted t...
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Diastereoselective Synthesis and Photophysical Properties of BisCyclometalated Ir(III) Stereoisomers with Dual Stereocenters Li-Ping Li, Su-Yang Yao, Yan-Ling Ou, Lian-Qiang Wei, and Bao-Hui Ye* MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, People’s Republic of China S Supporting Information *

ABSTRACT: Diastereoselective synthesis of bis-cyclometalated Ir(III) stereoisomers Δ/Λ-[Ir(C∧N)2(D-pro)] and Δ/Λ-[Ir(C∧N)2(L-pro)] (where C∧N is 2-phenylpyridine (Hppy), (4,6difluorophenyl)pyridine (Hdfppy), and 2-phenylquinoline (Hpq) and pro is proline) with dual stereogenic centers at the metal and auxiliary ligand has been developed. The diastereomers Λ-L and ΛD, and Δ-L and Δ-D exhibit distinguishable photophysical properties in both solution and the solid state. The thermodynamically stable diastereomers Λ-[Ir(ppy)2(L-pro)], Λ-[Ir(dfppy)2(Lpro)], and Λ-[Ir(pq)2(L-pro)] emit a green emission at 524 nm (Φ = 3.5% and τ = 35 ns), a blue-green emission at 480 nm (Φ = 4.5% and τ = 59 ns), and a red emission at 588 nm (Φ = 6.5% and τ = 200 ns) in DCM solution, respectively, which are blue-shifted accompanied by a large quantum yield and long lifetime relative to the corresponding unstable diastereomers Δ-[Ir(ppy)2(L-pro)] at 537 nm (Φ = 2.3% and τ = 29 ns), Δ-[Ir(dfppy)2(L-pro)] at 489 nm (Φ = 2.8% and τ = 43 ns), and Δ-[Ir(pq)2(L-pro)] at 591 nm (Φ = 5.4% and τ = 192 ns). Similar cases were also observed in crystals, but the signals were significantly red-shifted with respect to those in solution. Single-crystal structural analyses show that the Δ-L and Λ-D diastereomers exhibit larger interligand repulsion and loose molecular packing with respect to the Δ-D and Λ-L diastereomers, resulting in energy and photophysical property differences. In addition, the isomers with Δ and Λ configurations at the metal center exhibit positive and negative circularly polarized luminescence (CPL) signals, respectively, indicating that the effects of the chiral carbon atoms in pro ligands on CPL signals are negligible.

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INTRODUCTION Chiral luminescent molecules have attracted great attention for their wide potential application in 3D optical displays,1 encrypted transmission and data storage,2 photoelectric devices,3 sensors,4 and biological probes.5 Ir(III) complexes with cyclometalated ligands, such as 2-phenylpyridine (Hppy) and its derivatives, have played an important role in organic light-emitting diodes (OLEDs) and bioimaging, due to their large Stokes shift, high thermal stability, good quantum yield, and tunable emissive properties through modification of the cyclometalated or ancillary ligands.6−8 In most cases, Ir(III) complexes used in OLED fabrication, such as fac-Ir(ppy)3, Ir(ppy)2(acac), and Ir(dfppy)2(picolinate) (FIrpic; Hdfppy is 4,6-difluorophenyl)pyridine), have a six-coordinate octahedral structure in racemic mixtures. However, these Ir(III) complexes exhibit intrinsic metal-centered chirality with Λ and Δ configurations. Although the racemic Ir(III) complexes emit light without net polarization, the enantioenriched complexes emit circularly polarized luminescence (CPL) with a polarization bias.9 Moreover, the molecular orientation and solidstate morphology exert a significant effect on the luminescent properties.10 As has been previously observed (Wallach’s rule), the enantiopure crystals often do not pack as efficiently as the © XXXX American Chemical Society

racemic mixture, leading to distinct photophysical properties even though they have the same chemical composition.11 The emission of Λ- and Δ-[Ru(phen)3](PF6)2 (phen is 1,10phenanthroline) is red-shifted about 10 nm relative to that of the racemic form with a significant difference in the quenching behavior toward an oxygen molecule.11b Interestingly, a drastic change in quantum yields between the homochiral aggregates of ΛΛ-[Ag{Ir(ppy-CF3)2}2(D-pen)(D-Hpen)] (Φem = 0.12) and ΔΔ-[Ag{Ir(ppy-CF3)2}2(D-pen)(D-Hpen)] (Φem = 0.034, ppy-CF3 is 2-[3,5-bis(trifluoromethyl)phenyl]pyridine and DH2pen is D-penicillamine) has been observed in methanol solution at room temperature.12a However, distinguishable photophysical properties between Ir(III) diastereomers have been scarcely investigated.12b Although various strategies have been developed to prepare enantiomeric Ir(III) complexes,13−15 efficient and facile methods to access the optically pure stereoisomers are still highly desired: in particular, for the thermodynamically unstable diastereomer.16 It is worth noting that the presence of dual stereogenic centers (chiral at metal center and chiral Received: June 1, 2017

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DOI: 10.1021/acs.organomet.7b00406 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

CPL signals were presented in ΔI, and no corrections were applied to the CPL spectra. The excitation wavelength was 360 nm. ΔI = IL − IR, where IL and IR indicate the output signals of left and right circularly polarized light. The magnitude of circular polarization in the excited state was defined as gPL = 2(IL − IR)/(IL + IR). Experimentally, the value of ΔI/I is calculated as ΔI/I = [(ellipticity)/(32980/ln 10)]/ (unpolarized PL intensity). General Procedures for Diastereoselective Synthesis of Λ-L and Δ-D Ir(III) Diastereomers. An appropriate amount of the precursor rac-[(Ir(C∧N)2Cl)2], L-Pro or D-Pro (11.5 mg, 0.1 mmol), and NaOMe (8.6 mg, 0.16 mmol) were added to MeOH (15 mL), respectively. The solution was stirred under a N2 atmosphere at 65 °C for 24 h. The solvent was removed, and then the crude product was purified by silica gel column chromatography, with a DCM/MeOH mixture (100/1 to 100/5 v/v) as eluent to get the target products Λ-L and Δ-D. For Λ-[Ir(ppy)2(L-pro)] (Λ-L-1) and Δ-[Ir(ppy)2(D-pro)] (Δ-D-1), rac-[(Ir(ppy)2Cl)2] was in 0.13 mmol. Yield: 55 mg, 87%. Anal. Calcd for C27H24IrN3O2: C, 52.75; H, 3.94; H, 6.85. Found: C, 52.53; H, 3.62; H, 6.54. 1H NMR (400 MHz, DMSO-d6): δ 9.03 (d, J = 5.5 Hz, 1H), 8.66 (d, J = 4.7 Hz, 1H), 8.16 (t, 2H), 7.95 (t, 2H), 7.71 (dd, J = 10.1, 8.2 Hz, 2H), 7.47 (t, 1H), 7.41 (t, 1H), 6.76 (t, 2H), 6.63 (t, 1H), 6.58 (t, 1H), 6.30 (d, J = 7.1 Hz, 1H), 5.88 (d, J = 7.4 Hz, 2H), 3.79 (dd, J = 14.5, 8.6 Hz, 1H), 2.24−2.10 (m, 1H), 2.05−1.90 (m, 1H), 1.78 (m, 1H), 1.61−1.48 (m, 1H), 1.45−1.28 (m, 2H). 13C NMR (101 MHz, DMSO-d6): δ 182.89, 169.06, 168.18, 152.81, 150.41, 148.18, 148.06,145.10, 144.62, 138.39, 138.18, 133.00, 132.26, 129.28, 129.08, 124.84, 124.50, 123.23, 123.04, 120.96, 120.38, 119.64, 119.54, 61.79, 48.18, 30.94, 26.36. CD for Λ-L-1 (Δε, M−1 cm−1, DCM): 264 (46), 284 (−13), 318 (+25), 382 (+17), 418 (+6), 500 nm (−8). CD for Δ-D-1 (Δε, M−1 cm−1, DCM): 264 (−38), 287 (+9), 317 (−21), 381 (−15), 418 (−5), 499 nm (+7). Single crystals were obtained by the evaporation of a DCM solution of the complex. For Λ-[Ir(dfppy)2(L-pro)] (Λ-L-2) and Δ-[Ir(dfppy)2(D-pro)] (ΔD-2), rac-[(Ir(dfppy)2Cl)2] was in 0.2 mmol. Yield: 51.4 mg, 75%. Anal. Calcd for C27H20F4IrN3O2: C, 47.23; H, 2.94; H, 6.12. Found: C, 47.20; H, 2.86; H, 6.32. 1H NMR (400 MHz, DMSO-d6): δ 9.08 (d, J = 6.0 Hz, 1H), 8.71 (d, J = 5.6 Hz, 1H), 8.28 (d, J = 7.2 Hz, 2H), 8.11 (t, 2H), 7.59 (m, 2H), 6.72 (t, 2H), 5.79 (d, J = 8.9 Hz, 1H), 5.31 (d, J = 11.2 Hz, 1H), 3.91−3.76 (m, 1H), 2.00 (d, J = 7.6 Hz, 2H), 1.78 (d, J = 6.6 Hz, 2H), 1.47 (dd, J = 23.1, 15.3 Hz, 3H). 13C NMR (101 MHz, DMSO-d6): δ 183.35, 171.30, 170.33, 154.53, 151.86, 149.98, 149.29, 146.88, 146.11, 139.96, 139.43, 134.02, 133.11, 130.64, 129.31, 125.87, 125.68, 124.55, 124.33, 122.24, 122.16, 121.12, 120.76, 63.49, 51.95, 32.65, 26.83. CD for Λ-L-2 (Δε, M−1 cm−1, DCM): 275 (−19), 305 (+41), 332 (+18), 363 (+30), 382 (+8), 396 (+9), 460 nm (−12). CD for Δ-D-2 (Δε, M−1 cm−1, DCM): 275 (+18), 305 (−39), 332 (−17), 363 (−28), 382 (−8), 396 (−9), 460 nm (+12). Light yellow single crystals were obtained by the evaporation of a DCM solution of the complex. For Λ-[Ir(pq)2(L-pro)] (Λ-L-3) and Δ-[Ir(pq)2(D-pro)] (Δ-D-3), rac-[(Ir(pq)2Cl)2] was in 0.1 mmol, and the products were further washed with 3 mL of EtOH. Yield: 54.4 mg, 76%. Anal. Calcd for C35H28IrN3O2: C, 58.81; H, 3.95; H, 5.88. Found: C, 58.65; H, 3.70; H, 5.61. 1H NMR (400 MHz, CDCl3): δ 8.74 (d, J = 8.8 Hz, 1H), 8.25 (d, J = 8.7 Hz, 1H), 8.18 (d, J = 8.7 Hz, 1H), 8.11 (d, J = 8.7 Hz, 1H), 8.04 (t, 2H), 7.89 (d, J = 7.6 Hz, 2H), 7.81 (d, J = 7.8 Hz, 1H), 7.76 (d, J = 7.9 Hz, 1H), 7.54 (m, 4H), 7.04 (dd, J = 18.5, 7.7 Hz, 2H), 6.91 (t, 1H), 6.77 (t, 1H), 6.57 (t, 1H), 6.20 (d, J = 7.6 Hz, 1H), 3.13 (dd, J = 15.3, 7.2 Hz, 1H), 2.35−2.24 (m, 1H), 2.04−1.95 (m, 2H), 1.75− 1.67 (m, 2H), 1.50−1.39 (m, 2H). 13C NMR (75 MHz, CDCl3): δ 180.94, 180.78, 171.80, 170.21, 151.02, 149.78, 149.17, 147.96, 145.84, 138.26, 137.89, 137.00, 134.41, 131.61, 130.66, 129.75, 129.26, 128.85, 127.47, 127.37, 127.07, 126.44, 126.17, 125.71, 125.45, 122.60, 121.55, 120.49, 117.55, 117.49, 115.6, 61.64, 48.34, 28.13, 25.32. CD for Λ-L-3 (Δε, M−1 cm−1, DCM): 260 (+45), 306 (+72), 358 (+67), 472 nm (−3). CD for Δ-D-3 (Δε, M−1 cm−1, DCM): 260 (−40), 306 (−67), 357 (−60), 472 nm (+7). General Procedures for Synthesis of Λ-D and Δ-L Ir(III) Diastereomers. The chiral precursors Λ-[Ir(dfppy)2(MeCN)2]-

ligand) in the same structure affords enantiomers and diastereomers, which provides the tremendous advantage of obtaining various isomers of one molecule featuring different properties. Unfortunately, there is no robust route to synthesize the stereoisomers with dual stereogenic centers selectively;9c,d,17 the resolution of stereoisomers with dual stereocenters is hard work and chiral HPLC techniques usually need to be used. In this context, we report on the synthesis of enantiomerically and diastereomerically pure Δ and Λ biscyclometalated Ir(III) isomers (Δ-D, Δ-L, Λ-D, and Λ-L; Scheme 1) by a chiral auxiliary ligand strategy with Scheme 1. Chemical Structures of Ir(III) Complex Isomers

commercially available optically pure proline (pro). In these processes, we accidentally found that the diastereomers exhibit luminescence changes distinguishable to the naked eye. The photophysical properties of these Ir(III) isomers show that the diastereomers have great differences in emission wavenumbers and quantum yields both in solution and as crystals. The absolute configurations of the diastereomers Δ-[Ir(ppy)2(Lpro)]·CHCl3 and Δ-[Ir(ppy)2(D-pro)]·CH2Cl2, and Λ-[Ir(dfppy)2(L-pro)]·CH2Cl2 and Λ-[Ir(dfppy)2(D-pro)]·CH2Cl2 have been determined by X-ray single-crystal diffraction measurements to figure out the difference in luminescent properties between these isomers.

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EXPERIMENTAL SECTION

Materials and Methods. The precursors rac-[(Ir(ppy)2Cl)2], rac[(Ir(dfppy)2Cl)2], rac-[(Ir(pq)2Cl)2], Δ-[Ir(ppy)2(MeCN)2](PF6), and Λ-[Ir(ppy)2(MeCN)2](PF6) (Hpq is 2-phenylquinoline) were prepared according to the literature.15e,18,19 Elemental analyses were carried out with an Elementar Vario EL analyzer. ESI-MS was obtained on a Thermo LCQ DECA XP mass spectrometer. 1H NMR spectra were recorded with a Varian Mercury-Plus 300 or a Bruker AV 400 spectrometer by using the solvent as an internal standard. Electronic absorption spectra were obtained on a PERSEE TU-1901 UV−vis spectrophotometer. Electrochemical measurements were carried out on a CHI-730C electrochemistry system in CH3CN. A three-electrode assembly comprising a glassy-carbon working electrode, a Pt auxiliary electrode, and a Pt-wire reference electrode were used. All experiments were carried out at room temperature using 0.1 mol/L n-Bu4NPF6 as the supporting electrolyte. The reference electrode was assumed to be stable and was referenced externally to ferrocene (0.39 V). CD spectra were measured on a JASCO J-810 CD spectropolarimeter. Emission spectra and lifetimes were determined on an FLS920 fluorescence lifetime and steady-state spectrometer at room temperature. Quantum yields of air-equilibrated DCM solution were determined on a FLSP920 fluorescence spectrophotometer with Ru(bpy)3Cl2 in airequilibrated water solution (Φ = 0.028) as a standard.20 The emission quantum yields of the crystals were measured in an integration sphere associated with Hamamatsu C9920-03G absolute PL quantum yield measurement system. The CPL spectra were measured on a JASCO CPL-300 spectrophotometer with “Standard” sensitivity at 50 nm min−1 scan speed with 0.1 nm resolution and response time of 8 s. The B

DOI: 10.1021/acs.organomet.7b00406 Organometallics XXXX, XXX, XXX−XXX

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Organometallics (PF6), Δ-[Ir(dfppy)2(MeCN)2](PF6), Λ-[Ir(pq)2(MeCN)2](PF6), and Δ-[Ir(pq)2(MeCN)2](PF6) were synthesized according to our previous method.15e For Λ-[Ir(dfppy)2(MeCN)2](PF6) and Δ-[Ir(dfppy)2(MeCN)2](PF6): yield, 33.6 mg, 84%. Anal. Calcd for C26H18F10IrN4P: C, 39.05; H, 2.27; H, 7.01. Found: C, 39.32; H, 2.32; H, 6.95. ESI-MS: m/z 655.11 [M − PF6]+. 1H NMR (400 MHz, CD3CN): δ 9.08 (d, J = 5.6 Hz, 2H), 8.35 (d, J = 8.4 Hz, 2H), 8.11 (t, 2H), 7.51 (t, 2H), 6.56 (d, J = 11.2 Hz, 2H), 5.53 (d, J = 8.8 Hz, 2H), 2.14 (s, 6H). 13C NMR (75 MHz, CD3CN): δ 165.13, 164.19, 161.67, 159.57, 152.28, 148.32, 140.86, 124.91, 124.57, 124.31, 114.50, 114.30, 99.71. CD for Λ-[Ir(dfppy)2(MeCN)2](PF6) (Δε, M−1 cm−1, DCM): 313 (−55), 343 (+32), 375 (+8), 426 nm (+10). CD for Δ[Ir(dfppy)2(MeCN)2](PF6) (Δε, M−1 cm−1, DCM): 313 (+46), 340 (−40), 381 (−9), 426 nm (−9). For Λ-[Ir(pq)2(MeCN)2](PF6) and Δ-[Ir(pq)2(MeCN)2](PF6): yield, 36.8 mg, 89%. Anal. Calcd for C34H26F6IrN4P: C, 49.33; H, 3.17; H, 6.77. Found: C, 49.15; H, 3.12; H, 6.63. ESI-MS: m/z 683.18 [M − PF6]+. 1H NMR (400 MHz, CD3CN): δ 8.81 (d, J = 8.9 Hz, 2H), 8.56 (d, J = 8.7 Hz, 2H), 8.25 (d, J = 8.8 Hz, 2H), 8.10 (d, J = 8.0 Hz, 2H), 7.89 (t, 2H), 7.84 (d, J = 7.8 Hz, 2H), 7.75 (t, 2H), 6.97 (t, 2H), 6.68 (t, 2H), 6.11 (d, J = 7.7 Hz, 2H), 1.96 (s, 6H). 13C NMR (101 MHz, CD3CN): δ 171.52, 149.29, 148.01, 145.74, 141.95, 134.05, 133.13, 131.46, 130.55, 129.69, 128.50, 127.76, 124.28, 119.01, 118.66, 108.70, 107.39. CD for Λ-[Ir(pq)2(MeCN)2](PF6) (Δε, M−1 cm−1, DCM): 310 (−90), 357 (+44), 472 nm (+11). CD for Δ-[Ir(pq)2(MeCN)2](PF6) (Δε, M−1 cm−1, DCM): 310 (−82), 357 (−44), 472 nm (−14). The chiral precursor Λ-[Ir(C∧N)2(MeCN)2](PF6) or Δ-[Ir(C∧N)2(MeCN)2](PF6) (0.1 mmol), D-Pro or L-Pro (17 mg, 0.15 mmol), and NaOMe (8.6 mg, 0.24 mmol) were added to MeOH (15 mL), respectively. The solution was stirred under a N2 atmosphere at 50 °C for an appropriate time (monitored by TLC). The solvent was removed, and the resulting material was dissolved in 30 mL of DCM and then washed with H2O (3 × 10 mL) to remove the unreacted pro. The combined organic extract was dried over Na2SO4, filtered, and concentrated under high vacuum to give the product. For Λ-[Ir(ppy)2(D-pro)] (Λ-D-1) and Δ-[Ir(ppy)2(L-pro)] (Δ-L-1), the reaction time was 8 h. Yield: 53.4 mg, 87%. Anal. Calcd for C27H24IrN3O2: C, 52.75; H, 3.94; H, 6.85. Found: C, 52.53; H, 3.62; H, 6.59. 1H NMR (400 MHz, DMSO-d6): δ 8.69 (d, J = 5.6 Hz, 1H), 8.57 (d, J = 5.3 Hz, 1H), 8.16 (t, 2H), 7.99 (t, 2H), 7.69 (t, 2H), 7.48 (t, 2H), 6.76 (t, 2H), 6.62 (t, 1H), 6.60 (t, 1H), 6.24 (d, J = 7.4 Hz, 1H), 5.86 (d, J = 7.8 Hz, 1H), 3.63 (d, J = 9.6 Hz, 1H), 2.32 (s, 1H), 2.09 (d, J = 10.2 Hz, 1H), 1.94−1.86 (m, 3H), 1.23 (m, 2H). 13C NMR (101 MHz, DMSO-d6): δ 184.23, 171.25, 170.33, 153.44, 151.86, 149.98, 149.29, 146.58, 146.41, 139.96, 139.70, 134.02, 133.39, 130.64, 130.31, 125.87, 125.68, 124.55, 124.33, 122.24, 122.16, 121.12, 120.76, 64.19, 51.95, 32.53, 27.93. CD for Λ-D-1 (Δε, M−1 cm−1, DCM): 258 (+38), 275 (−11), 287 (−10), 315 (+12), 361 (+13), 401 (+5), 457 nm (−5). CD for Δ-L-1 (Δε, M−1 cm−1, DCM): 258 (−40), 275 (+12), 287 (+11), 316 (−14), 361 (−12), 401 (−6), 457 nm (+6). Deep yellow single crystals were obtained by the evaporation of a CHCl3 solution of the complex. For Λ-[Ir(dfppy)2(D-pro)] (Λ-D-2) and Δ-[Ir(dfppy)2(L-pro)] (ΔL-2), the reaction time was 10 h. Yield: 61.3 mg, 89%. Anal. Calcd for C27H20F4IrN3O2: C, 47.23; H, 2.94; H, 6.12. Found: C, 47.15; H, 2.75; H, 6.02. 1H NMR (400 MHz, DMSO-d6): δ 8.71 (d, J = 5.2 Hz, 1H), 8.62 (d, J = 5.4 Hz, 1H), 8.26 (d, J = 7.0 Hz, 2H), 8.12 (s, 1H), 7.59 (d, J = 5.9 Hz, 2H), 6.68 (t, 2H), 5.72 (d, J = 6.7 Hz, 1H), 5.48 (d, J = 7.9 Hz, 1H), 5.27 (d, J = 9.1 Hz, 1H), 3.65 (dd, J = 17.7, 9.1 Hz, 1H), 2.05−1.86 (m, 4H), 1.69 (d, J = 9.3 Hz, 1H), 1.45 (m, 2H). 13C NMR (101 MHz, DMSO-d6): δ 182.88, 165.08, 161.98, 156.71, 152.65, 150.95, 149.01, 139.90, 139.68, 128.99, 128.64, 123.90, 123.68, 123.38, 123.19, 122.92, 122.73, 114.57, 114.44, 113.62, 113.46, 97.47, 97.19, 62.68, 50.56, 31.06, 26.70. CD for Λ-D-2 (Δε, M−1 cm−1, DCM): 306 (+40), 341 (+21), 361 (+26), 396 (+9), 460 nm (−12). CD for Δ-L-2 (Δε, M−1 cm−1, DCM): 305 (−53), 341 (−30), 363 (−40), 396 (−13), 460 nm (+18). Yellow single crystals were obtained by the diffusion of Et2O into a DCM solution of the complex. For Λ-[Ir(pq)2(D-pro)] (Λ-D-3) and Δ-[Ir(pq)2(L-pro)] (Δ-L-3), the reaction time was 24 h. Yield: 64.3 mg, 90%. Anal. Calcd for

C35H28IrN3O2: C, 58.81; H, 3.95; H, 5.88. Found: C, 58.71; H, 3.76; H, 5.72. 1H NMR (400 MHz, CDCl3): δ 8.53 (d, J = 8.4 Hz, 1H), 8.24 (dd, J = 15.2, 8.8 Hz, 2H), 8.11−8.05 (m, 2H), 7.97−7.92 (m, 1H), 7.89−7.81 (m, 2H), 7.79 (d, J = 7.9 Hz, 1H), 7.76−7.71 (m, 1H), 7.54 (dd, J = 19.4, 6.3 Hz, 4H), 7.01 (t, 1H), 6.93 (t, 2H), 6.73 (t, 1H), 6.58 (dd, J = 14.8, 7.5 Hz, 1H), 6.29 (d, J = 7.7 Hz, 1H), 3.45 (dd, J = 14.9, 10.3 Hz, 1H), 2.79 (d, J = 7.2 Hz, 1H), 2.56−2.46 (m, 2H), 1.90 (m, 2H), 1.32 (d, J = 8.1 Hz, 2H). 13C NMR (101 MHz, CDCl3): δ 174.32, 171.95, 169.21, 164.97, 163.45, 145.49, 142.63, 142.32, 139.86, 138.88, 131.68, 131.56, 130.03, 129.13, 128.40, 127.85, 125.27, 123.76, 123.22, 122.50, 122.27, 121.16, 120.04, 119.68, 118.97, 116.45, 116.07, 115.04, 114.07, 110.77, 109.02, 62.82, 37.02, 23.09, 20.24. CD for Λ-D3 (Δε, M−1 cm−1, DCM): 257 (−5), 273 (−18), 312 (+89), 354 (+44), 464 nm (−15). CD for Δ-L-3 (Δε, M−1 cm−1, DCM): 257 (+6), 273 (+18), 312 (−82), 357 (−40), 469 nm (+10). Single-Crystal X-ray Crystallography. The diffraction intensities for Ir(III) complex diastereomers were collected on an Oxford Gemini S Ultra CCD Area detector diffractometer with graphite-monochromated Cu Kα radiation (λ = 1.54178 Å). All of the data were corrected for absorption effects using the multiscan technique. The structures were solved by direct methods21 and refined by iterative cycles of leastsquares refinement on F2 followed by difference Fourier synthesis.22 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were included in the final structure factor calculation at idealized positions and were allowed to ride on the neighboring atoms. The crystal data and the details of data collection and refinement for the complexes are summarized in Table S1 in the Supporting Information.

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RESULTS AND DISCUSSION Diastereoselective Synthesis and Characterization of Ir(III) Stereoisomers. Inspired by Lusby’s and Meggers’ works,15 using chiral amino acids as a chiral auxiliary ligand to prepare enantiopure Ir(III) complexes, we also observed the reaction of [(Ir(ppy)2Cl)2] with L-pro and found that the reaction was diastereoselective and base-dependent.15e However, the synthetic yields of the previous reports were low ( 1 > 3, which is in line with the trend of increasing π-donating ability of the cyclometalated ligand. The oxidation potentials of the diastereomers Λ-L-2 and Δ-L-2, Λ-L-3 and Δ-L-3 are identical. Significantly, different oxidation potentials are observed for the diastereomers Λ-L-1 and Λ-L-1 (ΔE1/2 = 30 mV). Different steric hindrance in the diastereomers might be attributed to the observed differences in oxidation potentials. Similar cases for dinuclear Ir(III) diastereomers have also been reported.16b,27 X-ray Single-Crystal Structures. In order to gain a better understanding of structural distinctions among the diastereomers, which may lead to differences in photophysical properties, single-crystal structures of Δ-L-1 and Δ-D-1, and Λ-D-2 and Λ-L-2 were determined (see Table S1 in the Supporting Information). The single crystals of the stereoisomers crystallized in chiral space groups, and the absolute configurations at the metal centers and pro ligands have single chirality, as reflected by Flack parameters near zero. The structures of Ir(III) centers show a slightly distorted octahedral geometry with coordination of two carbon atoms in cis positions and two nitrogen atoms in trans positions from two cyclometalated ligands as well as an oxygen atom and a nitrogen atom from the pro ligand, as shown in Figure 3 and Figure S14 in the Supporting Information. The best equatorial plane is defined by two carbon atoms and nitrogen and oxygen atoms from pro; the Ir(III) ion slightly deviates from this plane. The Ir−N and Ir−C distances of the C∧N ligands are around 2.0 Å, which are consistent with reports for cyclometalated Ir(III) complexes.15,23,28 The five-membered chelate ring defined by the pro ligand and the Ir(III) ion is not planar with an envelope form, due to the carbons deviating from the plane. It is worth mentioning that the tetrahydropyrrole rings in the stable diastereomers Δ-D-1 and Λ-L-2 are located below the coordinated plane, while those in the unstable diastereomers ΔL-1 and Λ-D-2 sit above the coordinated plane. The puckering degree can be reflected by the torsion angle defined by Ir1− N3−C24−C25 (145.5° for Δ-D-1, 123.6° for Δ-L-1, 144.9° for Λ-L-2, and 123.9° for Λ-D-2), indicating that the tetrahydropyrrole rings in the stable diastereomers are greater apart D

DOI: 10.1021/acs.organomet.7b00406 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

the former are slightly longer than those of 9.928 and 10.01 Å for the latter. Such an arrangement leads to the hydrogenbonded chains intercalating each other into a 2D layer on the ab plane via π···π and C−H···π interactions in Δ-D-1 and Λ-L-2 diastereomers. This would decrease the amount of void space (1.9 to 0% and 3.0 to 1.2%) in Δ-D-1 and Λ-L-2 diastereomers. We rationalize that such different supramolecular organizations would reflect the photophysical properties of the diastereoisomers in their crystalline state. Photophysical Properties in Solution. Absorption and emission data of the complexes in DCM solution and in the solid state were collected and are summarized in Table 1. Absorption spectra of the complexes are shown in Figure 4 and

Figure 3. Crystal structures of Δ-D-1 (left) and Δ-L-1 (right) (solvent molecules omitted for clarity). Selected bond lengths (Å) and angles (deg): Δ-D-1, Ir1−C7 = 1.985(17), Ir1−C18 = 2.038(15), Ir1−N1 = 2.044(7), Ir1−N2 = 2.065(7), Ir1−O1 = 2.154(11), Ir1−N3 = 2.227(13), N1−Ir1−N2 = 172.8(3), N3−Ir1−O1 = 78.0(3); Δ-L-1, Ir1−C11 = 1.984 (19), Ir1−C22 = 2.008(12), Ir1−N1 = 1.999(14), Ir1−N2= 2.050(15), Ir1−N3 = 2.174(13), Ir1−O3 = 2.170(12), N1− Ir1−N2 = 172.9(6), O3−Ir1−N3 = 78.6(5).

from the Ir(III) centers with respect to the corresponding unstable diastereomer. It is also found that the distances between C1−H and C25−H (H···H 2.078 and 2.122 Å) in the unstable diastereomers are shorter than those of C22−H and N3−H (H···H 2.205 and 2.219 Å) in the stable diastereomers, which may lead to a large interligand repulsion and thermodynamic instability for the former diastereomer. We rationalize that the intramolecular steric interference between the ligands indicate the unequal formation of the diastereoisomers and result in energy and photophysical property differences. Examination of the molecular packing of the complexes shows a helical chain running along the b axis linking via the N···O hydrogen bond between the two neighboring molecules (see Figures S15−S18 in the Supporting Information). However, the supramolecular organizations between the diastereomers are different. The Ir···Ir distances between the neighboring molecules along the hydrogen-bonding helical chain are 8.179 and 8.226 Å in Δ-D-1 and Λ-L-2 diastereomers, which are longer than those of 7.102 and 6.882 Å in Δ-L-1 and Λ-D-2 diastereomers. The pitches of 10.431 and 10.219 Å for

Figure 4. Normalized absorption and emission spectra of Λ-L-1, Δ-L-1, and rac-[Ir(ppy)2(L-pro)] in air-equilibrated DCM at 298 K.

Figures S19 and S20 in the Supporting Information. It should be pointed out that the absorption spectra of the stereoisomers and the racemate are identical and are similar to those of the typical mononuclear Ir(III) complexes reported.29 The spectra show an intense band in the range of 250−280 nm depending on the cyclometalated ligand, which can be attributed to the spin-allowed π−π* transitions (LC) related to the cyclometalated ligand. The weaker absorption bands extended to the visible region could be attributed to the result of metal to ligand

Table 1. Photophysical Data for Ir(III) Complexes in the Solid State and in DCM Solution at Room Temperature emission crystal compound Λ-L-1 Δ-L-1 Δ-D-1 Λ-D-1 Λ-L-2 Δ-L-2 Δ-D-2 Λ-D-2 Λ-L-3 Δ-L-3 Δ-D-3 Λ-D-3

absorption λabsa (nm) 263, 266, 263, 266, 258, 259, 258, 258, 278, 278, 278, 278,

396, 405, 396, 405, 345, 345, 345, 345, 346, 347, 346, 347,

456 456 456 456 386, 386, 387, 387, 417, 418, 417, 418,

427 428 427 427 466 466 466 466

solution (DCM)

powder λemb (nm)

λemb (nm)

τd (ns)

ΦPLe (%)

kr/knrf (106 s−1)

λemb (nm)

τ (ns)

ΦPLe (%)

kr/knrf (106 s−1)

542 557 543 558 503, 538c 508 503, 538c 508 628 644 629 644

542 568 543 568 503, 537c 516,563c 503, 538c 516, 563c 635 657 635 657

30 25 31 25 57 35 57 35 57 52 56 52

4.1 2.1 4.1 2.1 3.5 2.0 3.5 2.0 1.8 1.3 1.8 1.3

1.37/32.0 0.84/39.2 1.32/30.9 0.84/39.2 0.61/16.9 0.57/27.9 0.61/16.9 0.57/27.9 0.32/17.2 0.25/19.0 0.32/17.5 0.25/19.0

524 537 523 537 480,503c 489 480,503c 489 588 591 588 591

35 29 35 29 59 43 59 43 200 192 200 191

3.5 2.3 3.5 2.3 4.5 2.8 4.5 2.8 6.5 5.4 6.5 5.4

1.00/27.6 0.79/33.7 1.00/27.6 0.79/33.7 0.76/16.2 0.65/22.6 0.76/16.2 0.65/22.6 0.33/4.7 0.28/5.0 0.33/4.7 0.28/5.0

a Measured with a concentration of 1 × 10−5 M. bλex 360 nm. cShoulder of the emission band. dτ = ∑τnAn2/∑τnAn, obtained by tail fitting (λex 360 nm). eThe quantum yields were measured using an integration sphere in the solid state and in air-equilibrated DCM relative to Ru(bpy)3Cl2 in airequilibrated water solution (Φ = 0.028). fThe radiative kr and nonradiative knr values in crystals and DCM solution were calculated according to the equations kr = Φ/τ and knr = (1 − Φ)/τ, from the quantum yields Φ and lifetime values τ.

E

DOI: 10.1021/acs.organomet.7b00406 Organometallics XXXX, XXX, XXX−XXX

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Organometallics (3MLCT) transitions.30 Moreover, the moderately intense band around 345 nm in pq complexes is likely to be a combination of spin-allowed metal to ligand charge transfer (1MLCT).31 The emission profiles of the complexes in DCM solution exhibit a vibronic resolution which is typical for Ir(III)-ppy complexes30 and can be assigned to a mixed 3MLCT/LC state. The photophysical behaviors of the enantiomers are identical in solution under our experimental conditions. However, the emission properties of the diastereomers are distinguishable, unlike the previous observations for Ir(III) stereoisomers,9,17 as shown in Figure 4 and Figure S15 in the Supporting Information. Λ-L-1 shows a brilliant green emission, with an emission maximum center at 524 nm, a luminescence quantum yield of 3.5%, and an emission lifetime of 35 ns (see Table 1). However, the emission maximum of Δ-L-1 is red-shifted to 537 nm, with a relatively small quantum yield of 2.3% and a short lifetime of about 29 ns. These results also encouraged us to compare the emission behavior of the racemate, and we found that the emission of rac-[Ir(ppy)2(L-pro)] is at 532 nm, with a quantum yield of 2.5% and a lifetime of about 35 ns. Moreover, the profiles and the maxima positions of the emission spectra for Λ-L-1 are identical in DCM solution at different concentrations (see Figure S16 in the Supporting Information), indicating that there is no significant aggregation-induced emission (AIE) effect on the emission properties under our experimental conditions. Analogously, Λ-L-2 exhibits a blue-green emission at 480 nm with a quantum yield of 4.5% and an emission lifetime of about 59 ns (see Table 1 and Figure S17 in the Supporting Information), and the emission maximum of Δ-L-2 is slightly red shifted to 489 nm with a relatively small quantum yield of 2.8% and a short lifetime of about 43 ns under the experimental conditions. Λ-L-3 and Δ-L-3 display red emissions at 588 and 591 nm, respectively, but a quantum yield of 6.5% and a lifetime of about 200 ns for Λ-L-3 are larger than those (Φ = 5.4% and τ = 192 ns) of Δ-L-3. Moreover, the radiative and nonradiative rate constants (kr and knr, respectively), which are simply evaluated as kr = Φ/τ and knr = (1 − Φ)/τ, can be calculated on the basis of the quantum yield and lifetime. It is found that Λ-D and Δ-L diastereomers have a significantly larger nonradiative rate constant in comparison to those of Λ-L and Δ-D diastereomers in DCM solution (see Table 1), suggesting that the former might induce a larger excited-state relaxation and result in an effective pathway for nonradiative decay. These differences may be attributed to a larger interligand repulsion in Δ-L and Λ-D diastereomers with respect to Δ-D and Λ-L diastereomers, resulting in energetic and photophysical properties. Photophysical Properties in Crystals. Emission properties of the Ir(III) stereoisomers have also been observed in crystals, as shown in Figure 5 and Table 1. The photophysical properties of the enantiomers are identical; however, those of the diastereomers are significantly distinguishable due to the different molecular arrangements in crystals. Λ-L-1 shows a broad green emission upon excitation at 360 nm, with a maximum emission wavelength at 542 nm, a luminescence quantum yield of 4.1%, and an emission lifetime of about 30 ns. The emission spectrum of Δ-L-1 is red-shifted to 568 nm with a yellow-green emission, a small quantum yield of 2.1%, and a short lifetime of about 25 ns. We also tried to get the crystals of the racemate for comparison; however, the diastereomers are self-resolved during the crystallization from solution.32 Similar emission behaviors are also observed for Δ-/Λ-L-2 and Δ-/Λ-L-

Figure 5. Normalized emission spectra of Λ/Δ-L-1 (a), Λ/Δ-L-2 (b) and Λ/Δ-L-3 (c) in the solid state under λex 360 nm excitation at 298 K.

3 complexes; the emission wavelengths of Λ-L-2 and Λ-L-3 are blue-shifted about 13 nm (λem 503 nm) and 22 nm (λem 657 nm) with respect to those of Δ-L-2 (λem 516 nm) and Δ-L-3 (λem 635 nm), respectively. The ΦPL and emission lifetime for Λ-L-2 (3.5% and 57 ns) and Λ-L-3 (1.8% and 57 ns) are considerably higher than those of the corresponding Δ-L-2 (ΦPL= 2.0% and τ = 35 ns) and Δ-L-3 (ΦPL= 1.3% and τ = 52 ns), respectively. As shown in Table 1, the emission maxima of Δ-L-1, Λ-L-1, Δ-L-2, Λ-L-2, Δ-L-3, and Λ-L-3 in crystals are 31, 18, 27, 23, 66, and 47 nm red-shifted relative to those in solution, respectively; this is consistent with the previous report for [Ir(ppy)2(pam)]+ (pam is 2-picolylamine).11c Furthermore, the luminescence quantum yield and emission lifetime for Δ-/Λ-L-3 in crystals are drastically reduced with respect to those in solution, and the nonradiative rate constants knr are significantly increased, indicating that luminescence decay turns out to be considerably fast and the presence of additional nonradiactive decay channels may be caused by aggregation in crystals. F

DOI: 10.1021/acs.organomet.7b00406 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Luminescence Mechanochromic Properties. Luminescent properties of mechanochromic compounds are strongly correlated with the molecular packing in the solid state. The distinct organization between the diastereomers may provide an opportunity to observe mechanochromism in stereoisomers, which has never been observed for Ir(III) diastereomers so far, although racemic and homochiral compounds with distinct molecular packings and different photophysical properties have been observed.11 Interestingly, Λ-L-1 and Δ-L-1 crystals exhibit visible mechanical stimuli-responsive color and emission changes visible to the naked eye, as shown in Figure 6. Upon

Figure 7. CPL spectra of the enantiomers (left) Δ-D-1 and Λ-L-1 and (right) and Δ-L-1 and Λ-D-1 in air-equilibrated DCM (1 × 10−6 M) at room temperature.

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CONCLUSIONS A diastereoselective synthesis approach for the four stereoisomers of bis-cyclometalated Ir(III) complexes with dual stereogenic centers at the metal center and auxiliary ligand has been developed in good yield and high purity. The diastereomers Λ-L and Λ-D, and Δ-D and Δ-L exhibit distinguishable photophysical properties. The stable diastereomers Λ-L and Δ-D show a blue-shifted emission with a large quantum yield and a long lifetime with respect to the corresponding unstable diastereomers Λ-D and Δ-L in both solution and the solid state due to the differences in steric repulsion between the ligands and molecular packing arrangement in crystals. The isomers with Δ and Λ configurations at the metal centers exhibit positive and negative CPL signals, respectively, and the effects of the chiral carbon atoms in pro ligands on CPL signal are negligible.

Figure 6. Photographs of Λ-L-1 and Δ-L-1 in crystalline and after grinding under (a) ambient and (b) 365 nm light.

mechanical grinding of the yellow crystals of Λ-L-1 in an agate mortar, the color changes from yellow to yellowish, while the yellow color in Δ-L-1 still remains. The color changes for Δ/ΛL-2 (yellow) and Δ/Λ-L-3 (red) upon grinding are indistinct. Moreover, the Λ-L diastereomer exhibits a brighter emission in comparison to the corresponding Δ-L diastereomer under UV lamp irradiation at 365 nm. The luminescence color is also modified after grinding. Λ-L-1 shows a featureless green emission profile with an emission maximum at 542 nm in both the crystalline and powder state (see Figure 5a). However, Δ-L-1 exhibits a blue shift from 568 to 557 nm accompanied by yellow-green to green emission after grinding. The emission profile also becomes narrow in the grinding state. Similar cases are also observed for Λ-/Δ-L-2 and Λ-/Δ-L-3 diastereomers. These may be attributed to the loose packing arrangement for the Δ-L diastereomer in crystals that is broken down after grinding, leading to a blue shift in the emission maximum center. Circularly Polarized Luminescence in Solution. To gain further insight into the possible effect of the stereogenic carbon atom on the optical properties, CPL properties of stereoisomers Λ-/Δ-L-1 and Λ-/Δ-D-1 were observed in DCM solution at room temperature. As shown in Figure 7, each pair of enantiomers has mirror-image profiles with a broad band in the range from 450 to 700 nm peaking around 540 nm. It is worth mentioning that the isomers with Δ configuration at the Ir(III) center show positive CPL signals, while those with Λ configuration at metal center exhibit negative CPL signals. No significant distinctions were observed between the diastereomers such as Δ-D-1 and Δ-L-1, and Λ-D-1 and Λ-L-1, suggesting that the CPL properties are mainly determined by the configuration at metal center and the effects of the chiral carbon atoms in pro ligands are negligible. The calculated dissymmetry factor gLum is on the order of 10−3 (see Figure S19 in the Supporting Information), which is similar to those reported for Ir(III) complexes.9

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00406. CD, 1H NMR, CV, and structures for the compounds (PDF) Accession Codes

CCDC 1544347−1544350 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, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for B.-H.Y.:[email protected]. ORCID

Bao-Hui Ye: 0000-0003-0990-6661 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSF of China (21272284 and 21571195). G

DOI: 10.1021/acs.organomet.7b00406 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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DOI: 10.1021/acs.organomet.7b00406 Organometallics XXXX, XXX, XXX−XXX

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Organometallics (32) Urban, R.; Krämer, R.; Mihan, S.; Polborn, K.; Wagner, B.; Beck, W. J. Organomet. Chem. 1996, 517, 191−200.

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DOI: 10.1021/acs.organomet.7b00406 Organometallics XXXX, XXX, XXX−XXX