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And Bis (Imidazoline Selone) Ligands for Visible-Light-Induced Oxidative Article pubs.acs.org/Organometallics Iridium Complexes Containing Bis(imidazoline thione) and Bis(imidazoline selone) Ligands for Visible-Light-Induced Oxidative Coupling of Benzylamines to Imines † ‡ † † ‡ ‡ Jaewon Jin, Hee-Won Shin, Joon Hyun Park, Ji Hoon Park, Eunchul Kim, Tae Kyu Ahn,*, † † ‡ Do Hyun Ryu,*, and Seung Uk Son*, , † ‡ Department of Chemistry and Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea *S Supporting Information ABSTRACT: Novel iridium(III) complexes containing bis- (N-heterocyclic carbene), bis(imidazoline thione) L2, and bis(imidazoline selone) L3 were prepared. The iridium complexes bearing L2 and L3 showed the significant absorption of visible light with maximum intensity at ∼460 nm. Bis(2-(2′-benzothienyl)pyridinato)iridium(III) complexes (Ir-6) with L3 showed excellent ability as a photosensitizer of visible light. Under blue LED irradiation with maximum emission at 460 nm, 0.25 mol % Ir-6 showed 94% conversion of benzylamine for 5 h at room temperature. Through mechanistic studies, it was suggested that the photoinduced oxidative coupling of benzylamine by Ir-6 follows a singlet oxygen pathway. The excellent performance of Ir-6 originated from the efficient visible light absorption at 460 nm and the enhanced triplet state due to the heavy-atom effect of L3. This work shows that bis(imidazoline thione) and bis(imidazoline selone) can be efficient ligands for tuning the optical properties of iridium(III) complexes. ■ INTRODUCTION in photoinduced oxidative coupling of benzylamines to imines Recently, chemical reactions triggered by sustainable energy in the presence of oxygen. sources such as solar energy and the relevant photoactive systems have attracted the attention of scientists.1 Over the past ■ RESULTS AND DISCUSSION decade, iridium(III) complexes bearing the 2-phenylpyridinato Scheme 1 shows the synthetic methods for new iridium 2 (ppy) ligand have shown interesting optical properties. complexes. First, the bis(ppy)iridium chloride dimer (ppy = 2- Numerous iridium(III) complexes with various geometric and phenylpyridinato) was prepared using the method described in electronic ligands have been prepared.3 In addition, two or the literature.10 Bis(N-heterocyclic carbene)-2AgBr (L1- more kinds of ligands have been introduced to iridium 2AgBr) was prepared by the reaction of the corresponding 4 − 11 complexes. The enhanced spin orbit coupling due to the imidazolium salt with silver oxide (Ag2O). The reaction of ff heavy-atom e ect of iridium has been shown to result in the [(ppy)2IrCl]2 with L1-2AgBr in 2-ethoxyethanol resulted in the ffi + 11,12 e cient mixing of singlet and triplet states and to promote formation of the complex [(ppy)2L1Ir] (Ir-1). Bis- intersystem crossing.2,3,5 The major studies of these iridium (imidazoline thione) L2 and bis(imidazoline selone) L3 were complexes have focused on the phosphorescent properties for prepared by the reaction of the corresponding imidazolium salt organic light emitting diode (OLED) devices.6 with elementary sulfur and selenium.13a Imidazoline thiones Due to their unique optical properties, iridium complexes and selones have been used as versatile ligands toward various 12 have recently been applied as photocatalysts and photo- transition metals. The reaction of [(ppy)2IrCl]2 with L2 and 1,7 + + initiators. For example, iridium complexes were applied as a L3 resulted in the formation of [(ppy)2L2Ir] or [(ppy)2L3Ir] 1 8 1 photosensitizer to generate a singlet oxygen ( O2), which can complexes (Ir-2, Ir-3). As observed in the literature, H NMR be used for various purposes, including photodynamic cancer peaks from protons in benzyl carbon and imidazoline rings of therapy (PDT).9 However, one of the drawbacks of iridium L2 and L3 shifted (0.3−0.5 ppm) downfield through 13 complexes is their low absorption of visible light. The shift of coordination. Moreover, X-ray photoelectron spectroscopy absorption bands to a visible light range can be induced (XPS) revealed that the S 2p and Se 3d orbital peaks of L2 and − through the screening of ligands. L3, respectively, shifted to higher energies by 1.6 2.0 eV in In this work, we report the preparation of iridium complexes containing bis(N-heterocyclic carbene), bis(imidazoline thi- Received: May 18, 2013 one), and bis(imidazoline selone) ligands and their properties Published: June 28, 2013 © 2013 American Chemical Society 3954 dx.doi.org/10.1021/om4004412 | Organometallics 2013, 32, 3954−3959 Organometallics Article Scheme 1. Synthesis of Iridium(III) Complexes Containing Bis(N-heterocyclic carbene), Bis(imidazoline thione), and Bis(imidazoline selone) Ligands Figure 1. UV/vis absorption (solid line) and normalized emission × −5 − (dotted line) spectra of 1 10 M Ir-1 Ir-6 in CH2Cl2. iridium complexes. (Figure S1, Supporting Information) Three − Table 1. Optical and Electrochemical Properties of further iridium complexes (Ir-4 Ir-6) were prepared using the Iridium(III) Complexes Ir-1−Ir-6 in CH Cl 2-(2′-benzothienyl)pyridinato ligand (btpy)14 instead of ppy. 2 2 Chart 1 summarizes the iridium complexes prepared in this oxidn λ ε λ Δ ° a b c d abs, (nm, em G es potential HOMO Ees work. compd M−1cm−1) (nm) (eV) (V) (eV) (eV) a Ir-1 380, 5900 473 2.68 1.16 −5.60 −2.92 Chart 1. Iridium(III) Complexes Used in This Study Ir-2 431, 4600 486 2.60 1.05 −5.49 −2.89 Ir-3 428, 2600 499 2.54 1.01 −5.45 −2.91 Ir-4 419, 9000 577 2.17 1.02 −5.46 −3.29 Ir-5 464, 9900 604 2.09 0.96 −5.40 −3.31 Ir-6 461, 11000 603 2.09 0.91 −5.35 −3.26 aΔ ° G es (the free energy of excited states above the ground states) values were calculated by the method in ref 19. bPotential (vs Ag/Ag+) of the first oxidative wave. cHOMO energy calculated by the potential of the oxidative wave.19 dExcited state energy calculated by HOMO + Δ ° G es. a − The counteranion, PF6 , is omitted. The analogue of Ir-1 having fluorine groups in the ppy ligand has been reported to show poor absorption in a range of visible light.11 Similarly, Ir-1 has shown very poor absorption of visible light (Figure 1 and Table 1) The introduction of L2 and L3, instead of L1 resulted in a significant red shift of the absorption − Figure 2. Cyclic voltammograms of Ir-1 Ir-6 in CH2Cl2. band to ∼430 nm. However, the absorption intensity and shift degree of Ir-2 and Ir-3 were not sufficient for the efficient absorption of visible light. In comparison to Ir-1, Ir-4 showed a 1), while those of Ir-2 and Ir-3 shifted to lower values: +1.05 red-shifted absorption at 419 nm. Moreover, Ir-5 and Ir-6 and +1.01 V, respectively. A similar trend was observed in the showed significant absorption (ε = 9900−11000 M−1 cm−1)at series Ir-4−Ir-6. These observations imply that L3 and L2 ∼460 nm and the a emission at ∼600 nm with a 140 nm Stokes provided a more electron rich effect, in comparison with L1. shift (Figure 1 and Table 1). Considering the significant absorption of visible light by Ir- Figure 2 shows the electrochemical properties of Ir-1−Ir-6 in 1−Ir-6, we investigated the photosensitizing performance of methylene chloride. The reduction peaks were overlapped with iridium complexes under visible light irradiation. It has been the reduction range of CH2Cl2, implying that the iridium reported that the photoinduced oxidative coupling of benzyl- complexes are relatively electron rich systems. The oxidation amines results in the formation of imines.15 Although a visible- potential of Ir-1 was observed at +1.16 V (vs Ag/Ag+) (Table light-induced system has been reported, the reaction temper- 3955 dx.doi.org/10.1021/om4004412 | Organometallics 2013, 32, 3954−3959 Organometallics Article ature was relatively high, at ∼80 °C.16 The system working at room temperature was relatively less explored.17 Table 2 summarizes the performance of Ir-1−Ir-6 in the visible-light- induced oxidative coupling of benzylamine to imine. Table 2. Visible-Light-Induced Oxidative Coupling of a Amines in the Presence of Iridium Complexes Ir1−Ir-6 Figure 3. Two main mechanisms for the photoinduced oxidative coupling of benzylamine to imine. b c singlet oxygen, which induces the oxidative coupling of entry cat./amt (mol %) amine yield (%) benzylamine to imine. Both mechanisms work depending on 1 Ir-1/0.1 benzylamine 20 the optical and electrochemical properties of the photo- 2 Ir-2/0.1 benzylamine 24 sensitizers. 3 Ir-3/0.1 benzylamine 36 In the photoredox pathway, the energy levels of the reactants 4 Ir-4/0.1 benzylamine 36 and photosensitizer should match well. In this work, the excited 5 Ir-5/0.1 benzylamine 45 state energy level of the photosensitizer should be located at an 6 Ir-6/0.1 benzylamine 70 energy level higher than the LUMO of oxygen. The HOMO 7 Ir-6/0.25 benzylamine 94 (90) energy level of benzylamine should be located at an energy level d 8 no cat. benzylamine NR higher than that of the photosensitizer or the oxidized e 9 Ir-6/0.25 benzylamine NR photosensitizer. On the basis of the oxidation potentials and 10 Ir-6/0.25 4-methoxybenzylamine 99 (92) emission spectra of iridium complexes, their excited state 11 Ir-6/0.25 4-methylbenzylamine 95 (92) energy levels were calculated in the range of −2.9 to −3.3 12 Ir-6/0.25 4-chlorobenzylamine 89 (83) eV,19,20 which matches well with the LUMO level (∼−3.6 eV) 13 Ir-6/0.25 4-fluorobenzylamine 76 (71) of oxygen.17,21 In comparison, the HOMO energy values of 14 Ir-6/0.25 1-phenylethylamine 61 (58) iridium complexes are higher than that (∼−6.5 eV) of 15 Ir-6/0.25 (2-thienyl)methylamine 60 (56) benzylamine.17,21 Thus, it can be reasoned that the reduction a Reaction conditions: 1 mmol of amine, 1 atm of O2, blue LED of oxidized iridium complexes by benzylamine is not favorable.
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