Design and Synthesis of Tris-Heteroleptic Cyclometalated Iridium(III

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Design and Synthesis of Tris-Heteroleptic Cyclometalated Iridium(III) Complexes Consisting of Three Different Nonsymmetric Ligands Based on Ligand-Selective Electrophilic Reactions via Interligand HOMO Hopping Phenomena Yosuke Hisamatsu,† Sarvendra Kumar,† and Shin Aoki*,†,‡,§ †

Faculty of Pharmaceutical Sciences, ‡Division of Medical Science-Engineering Cooperation, Research Institute for Science and Technology, and §Imaging Frontier Center, Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan S Supporting Information *

ABSTRACT: In this article we report on the successful synthesis and isolation of cyclometalated Ir complexes having three different nonsymmetric ligands based on ligand-selective electrophilic reactions via interligand HOMO (highest occupied molecular orbital) hopping phenomena. It was hypothesized that the electrophilic substitution reactions of bis-heteroleptic Ir complexes having 8-benzenesulfonamidoquinoline as an ancillary ligand, 5a and 7, would proceed at the 5 position of the quinoline ring of these Ir complexes to afford 18 and 19, because their HOMOs are localized on the quinoline rings, as predicted by density functional theory (DFT) calculations. In these products, the HOMO is transferred to one of two ppy ligands, in which the phenyl group is trans to the Ir−N (1 position of quinoline) bond, and hence, the iodination or formylation of 18 and 19 occurs at the 5′ position of the ppy ligand to provide 20a, 23, and 24. Furthermore, we carried out the functionalization of 20a using cross-coupling reactions to obtain tris-heteroleptic Ir complexes containing three different ligands in good yields with negligible diastereomer formation. Photochemical properties, especially dual emission, and response to pH change, of new dualemissive tris-heteroleptic cyclometalated Ir complexes, 21−24, are also reported.

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INTRODUCTION Cyclometalated iridium(III) (Ir(III)) complexes have received considerable attention due to their unique photochemical, electrochemical, and structural properties.1 These attractive properties have prompted us to investigate the use of the cyclometalated Ir complexes not only for organic light-emitting diodes (OLEDs) as phosphorescent emitters2 but also for broad applications including photoenergy conversion,3 nonlinear optics,4 photocatalyst,5 photosensitizers,6 metallodrugs,7 and bioimaging probes.8 Numerous Ir complexes suitable for the above purposes have been developed over the past decade, and almost all of the reported compounds are classified into IrL3, IrL2L′, and IrL2A (L and L′ are different cyclometalating ligands and A is an ancillary ligand).1−8 Tris-heteroleptic cyclometalated Ir complexes (IrLL′A or IrLL′L″) (L, L′, and L″ denote different cyclometalating © XXXX American Chemical Society

ligands) composed of an Ir atom and three different ligands represent a new class of highly functionalized Ir complexes, which may open new avenues for the fine tuning of their photochemical and electrochemical properties and fill a gap between a series of homoleptic and bis-heteroleptic Ir complexes. However, the synthesis of this class of Ir complexes suffers from a lack of easy and efficient synthetic methods. Examples of tris-heteroleptic Ir complexes remain rare and are limited to IrLL′A-type compounds, in which A is a symmetric ancillary ligand such as acetylacetonate (acac), bipyridine, and 1,10-phenanthroline ligands.9 The synthesis, isolation, and characterization of tris-heteroleptic cyclometalated Ir complexes bearing three different nonsymmetric ligands have scarcely been Received: October 17, 2016

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DOI: 10.1021/acs.inorgchem.6b02519 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

ligands.14 The major reaction product is the heteroleptic μcomplex 9, which is reacted with symmetric ancillary ligands (ligand C) to afford the tris-heteroleptic Ir complex 10. Although this reaction would be a useful method for preparing a variety of tris-heteroleptic Ir complexes having symmetric ancillary ligands, one of its problems is that the diastereomers are produced when nonsymmetric ligands are used as third ligands. Method 2 in Chart 3 is the synthesis of tris-heteroleptic Ir complexes 11 (IrLL′L″) using ligand-selective substitution of tris-cyclometalated Ir complexes.14 Using this method, we succeeded in the synthesis and separation of the trisheteroleptic Ir complex 17 containing three different cyclometalating ligands from the tris-cyclometalated Ir complex 16 (mer-Ir(tpy)2(F2ppy)) (Chart 4). In this work, we used method 3 in Chart 3, which involves the selective substitution of bis-heteroleptic Ir complex 12 (containing two cyclometalating ligands A and one ligand D as an ancillary ligand) by electrophilic reactions in the synthesis of tris-heteroleptic cyclometalated Ir complexes containing three different nonsymmetric ligands 13 (IrAA′D) or 15 (IrAA′D′) with negligible formation of their diastereomers. Although both ligand A and ligand D could react with electrophilic reagents, the reactivity of ligand D is higher than that of ligand A to afford 14, not 13. Electrophilic substitution reactions then proceed at one of two ligands A of 14 to give 15. The reactivity and selectivity of bis-heteroleptic Ir complexes 5a {Ir(ppy)2(8BSQ−)} and 7 {Ir(mppy)2(8BSQ−)} in electrophilic substitution reactions and those of the substituted products (18 and 19) were predicted based on their HOMO, as determined by density functional theory (DFT) calculations (Chart 5). First, the most reactive site of 5a was predicted by the HOMO of 5a located on the quinoline ring of its 8BSQ unit (indicated by a filled arrow). Indeed, the bromination of 5a and 7 occurs at the predicted sites to provide 18 and 19 (corresponding to 14 in Chart 3). Subsequently, the HOMO of 18 transferred to its ppy unit (indicated by an open arrow), whose phenyl group is trans to the Ir−N (1 position of quinoline) bond, where the selective iodination of 18 was expected to occur to provide 20a (corresponding to 15 in Chart 3). The different reactivity of the two ppy ligands of 18 and 19 allowed us to prepare trisheteroleptic Ir complexes having nonsymmetric ancillary ligands (as a racemic mixture of Δ and Λ isomers) in good yields. The transfer of HOMOs between ligands in the abovementioned Ir complexes 5a, 18, and 20a (or 7, 19, and 24) is referred to as “interligand HOMO hopping” in this article. We demonstrate that two successive cross-coupling reactions of 20a provide 21 and 22. Furthermore, the Vilsmeier−Haack reaction of bromo-substituted Ir complexes 18 and 19 gives 23 and 24 having a formyl group. The photochemical properties of new dual-emissive tris-heteroleptic Ir complexes 21−24 and pH responsive emission behavior of 22 are also described.

reported in the literature, although the synthesis of such Ir complexes has been attempted by some research groups and the isolated product is a mixture of some diastereomers.10 Meanwhile, we reported on regioselective substitution reactions of fac-tris-cyclometalated Ir complexes such as Ir(ppy)3 1 (ppy, 2-phenylpyridine), Ir(tpy)3 2 (tpy, 2-(4′tolyl)pyridine), and Ir(mppy) 3 3 (mppy, 2-(4′methoxyphenyl)pyridine) at the 5′ position (the para position with respect to the C−Ir bond) on 2-phenylpyridine-type ligands and their subsequent conversions to 4 with a variety of functional groups (Chart 1).11 Chart 1

Furthermore, dual-emissive bis-heteroleptic cyclometalated Ir complexes (e.g., 5−7) containing 8-methylsulfonamidoquinoline or 8-benzenesulfonamidoquinoline (8BSQ) ligands as an ancillary ligand were previously reported by our group (Chart 2).12,13 These Ir complexes exhibit two different emission peaks Chart 2

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RESULTS AND DISCUSSION Stepwise Electrophilic Reactions of Bis-Heteroleptic Ir Complex 5a via Interligand HOMO Hopping Phenomena. During the DFT calculation of bis-heteroleptic Ir complex 5a,12 we noticed that its HOMO is localized on the Ir atom and the quinoline ligand but not on the ppy ligands (Chart 5), prompting us to hypothesize that electrophilic reactions such as bromination would likely occur at the 5 position of its quinoline ring to give 18. Interestingly, the DFT calculation of 18 implied that its HOMO is partly transferred to a phenyl ring on one of

(dual emission) at ca. 475−500 (blue to green-color emission) and ca. 600 nm (red-color emission), respectively. For example, Ir complex 6 {Ir(F2ppy)2(8BSQ−)} having two 2-(4′,6′difluorophenyl)pyridine (F2ppy) ligands and a 8BSQ ligand exhibit a whitish-colored emission at room temperature.12 Our current research project includes the development of new three synthetic methods for preparing tris-heteroleptic Ir complexes, as shown in Chart 3. Method 1 in Chart 3 involves the Zn halide (e.g., ZnBr2)-promoted degradation of triscyclometalated Ir complex 8 containing two A and one B B


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Inorganic Chemistry Chart 3

complex 18 in 98% yield, the structure of which was confirmed by X-ray crystal structure analysis, as shown in Figure 1a (ORTEP drawing15) and Table 1. Subsequently, when 18 was treated with N-iodosuccinimide (NIS) (ca. 5.7 equiv), 20 was produced in 91% yield and its 1H NMR spectrum showed a single set of proton signals (Figure 2), indicating that only one diastereomer was produced. The X-ray crystal structure of 20 (as a racemic mixture of Δ and Λ isomers) revealed that this product is 20a, in which an iodine atom was selectively introduced at the 5′ position of the phenyl group of a ppy ligand that is trans to the Ir−N(quinoline) bond (Figure 1b and Chart 6), not 20b (representative crystallographic data of 18 and 20a are summarized in Table S1 in Supporting Information). These results serve to verify the reactivity and selectivity of 5a and 18, as predicted in Chart 5. It would be expected that the bromo- and iodo-substituted tris-heteroleptic Ir complex 20a would be a useful intermediate for introducing a variety of functional groups. We previously reported that fac-tris-cyclometalated Ir complexes that contain electron-withdrawing groups such as sulfonyl (4a) and formyl groups at the 5′ position of 3 exhibit a blue-colored emission.11b In addition, Ir complexes that contain three pyridyl groups such as 4b in Chart 1 was reported as a pH sensor.11d Therefore, sulfonyl group was introduced into 20a using a palladium-catalyzed cross-coupling reaction to give 21 (Ir(ppy)(Tsppy)(Br8BSQ−), Ts, tosyl) in 54% yield16 and a 4pyridyl group was then introduced to produce 22 (Ir(ppy)-

Chart 4

the two ppy ligands, which are located in a trans configuration relative to Ir−N (1 position of quinoline) (indicated by a bold arrow in 18 in Chart 5) rather than the other phenyl ring that is located in a cis position (indicated by a dashed arrow in Chart 5). Therefore, it would be expected that electrophilic substitutions of the ppy ligand of 18 would proceed at the 5′ position, as predicted above, to afford a tris-heteroleptic cyclometalated Ir complex such as 20a rather than the other possible product, 20b. Furthermore, the HOMO of 20a with an iodine atom at the 5′ position of ppy ligand returned to the quinoline ring. This concept was confirmed by the ligand-selective electrophilic reactions of 5a and the successive conversion to the trisheteroleptic cyclometalated Ir complexes 20a, 21, and 22 consisting of three different nonsymmetric ligands (Chart 6). The reaction of 5a with N-bromosuccinimide (NBS) (1.0 equiv) at room temperature gave the monobrominated Ir C


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Chart 6

(Tsppy)(Py8BSQ−), Py, 4-pyridyl) by the Suzuki−Miyaura cross-coupling reaction (Chart 6). Moreover, the formylation of the mono-bromo-substituted Ir complexes 18 and 19 (prepared from 7) was carried out with POCl3 and DMF (Vilsmeier−Haack reaction)11a,b to obtain 23 (Ir(ppy)(CHOppy)(Br8BSQ−) and 24 (Ir(mppy)(CHOppy)(Br8BSQ−) having a formyl group as an electron-withdrawing group in 23% and 98% yields, respectively (Chart 7). Photophysical Properties of Tris-Heteroleptic Ir Complexes. The UV−vis absorption spectra and emission spectra of 18 and 21−24 (10 μM) were measured in DMSO at 298 K (Figures 3 and 4), and their photochemical properties are summarized in Table 2. As shown in Figure 3, the UV−vis absorption spectra of these complexes show a weak absorption in the region of ca. 350−500 nm, which is assigned to spinallowed and spin-forbidden metal-to-ligand charge transfer (MLCT) transitions and spin-forbidden π−π* transitions. Emission spectra of Ir complexes 21−24 (10 μM) were measured in degassed DMSO at 298 K (excitation at 366 nm). These Ir complexes exhibit a dual emission at ca. 500 (a HE (high energy) emission band) and ca. 620 nm (a LE (lowenergy) emission band) like 5a, 6, and 7.12 The emission wavelength, luminescent quantum yields (Φ), and luminescent lifetime (τ) data are summarized in Table 2. Emission of 20a is very weak, possibly due to the introduction of iodine.11a,b As

shown in Figure 4, the emission maxima of HE bands of trisheteroleptic Ir complexes 21, 22, 23, and 24 are 487, 494, 488, and 480 nm, respectively, which are observed at wavelengths between those of two bis-heteroleptic Ir complexes 5a that contains two ppy ligands (∼496 nm) and 6 that contains two F2ppy ligands (∼474 nm). On the other hand, 21, 23, and 24 with a bromo group on the quinoline ligands induced a red shift (ca. 20 nm) in the emission maxima of their LE bands in comparison with those of 5a and 7 (Figure 4 and Table 2). The Φ values of 21, 22, 23, and 24 were determined to be 0.52%, D


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Figure 1. ORTEP drawings of single-crystal structures of (a) 18 and (b) 20a with 50% probability ellipsoids.

Table 1. Selected Bond Lengths (Angstroms) of 18 and 20a Ir−Npyridine Ir−C Ir−Nquinoline Ir−Nsulfonamide

18

20a

2.03, 2.04 2.00, 2.01 2.16 2.19

2.05, 2.05 2.00, 2.00 2.13 2.19

1.2%, 0.43%, and 0.38%, respectively. The emission decay curves for dual-emissive Ir complexes 18 and 21−24 provide a better fit for a biexponential decay equation using a 475 nm long wave pass filter (Figure S1 in Supporting Information). Shorter (ca. 1−2 μs) and longer (ca. 6−12 μs) luminescent lifetimes are characterized as being an emission for a HE emission band mainly derived from ppy cyclometalating ligands, and the LE emission band is mainly from 8BSQ.12 The longer luminescent lifetimes of the LE emissions of 18 and 21−24 were confirmed by using a 590 nm long wave pass filter. These emission properties correspond to our previously

Figure 3. UV−vis spectra of Ir complexes in DMSO at 298 K: (a) Compound 5a (plain dashed curve), 18 (plain curve), 21 (bold curve), and 22 (bold dashed curve); (b) Compound 23 (bold curve) and 24 (plain curve). [Ir complex] = 10 μM.

Figure 2. 1H NMR spectrum (aromatic region) of 20a in CDCl3. E


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Inorganic Chemistry

The 1931 Commission Internationale de L′Éclairage (x, y) coordinates were calculated from emission spectra of Ir complexes 5a, 6, 7, 18, and 21−24.18,19 As shown in Figure 5, the xy color coordinates indicate greenish-white-colored

Figure 5. Chromaticity diagram showing the xy color coordinates of 5a, 6, 7, 18, and 21−24 (CIE 1931). xy color coordinates of these complexes were calculated from their emission spectra in degassed DMSO.

emission of 21 and 23 and a near to white-colored emission for 24, whose x and y values are 0.27 and 0.40, respectively (CIE of pure white light x = 0.33 and y = 0.33). The tris-heteroleptic Ir complexes 21, 23, and 24 fill a gap in the emission color of bisheteroleptic Ir complexes 5a, 6, and 7. DFT Calculation Study for Emission Spectra of 23. Time-dependent (TD)-DFT (triplet states) calculations were performed using the Gaussian 09 software program20 (PBE1PBE, the LanL2DZ basis set for Ir and Br atoms, and the 6-31G basis set for H, C, S, O, N atoms) on the basis of the optimized ground state geometry.13e,f,21 Because the properties of tris-heteroleptic Ir complexes are similar, the TD-DFT calculation for 23 is shown as a representative example (Table 3 and Figure 6). TD-DFT calculations suggest that the lowest energy triplet excited state T1 of 23 (2.01 eV, 616 nm) was mainly composed of 3MLquinCT + 3LquinC + 3LCHOppyLquinCT + 3 NsulfonamideLquinCT (Nsulfonamide, a lone pair of nitrogen of sulfonamide) (Table 3). Furthermore, the second lowest energy state T2 of 23 (2.70 eV, 459 nm) was mainly composed of 3MLCHOppyCT + 3MLppyCT + 3LCHOppyC + 3LquinLCHOppyCT + 3LquinLppyCT. The calculation results suggest that T1 and T2

Figure 4. Normalized emission spectra of Ir complexes in degassed DMSO at 298 K (excitation at 366 nm): (a) 5a (plain dashed curve), 18 (bold dashed curve), 21 (bold curve), and 22 (plain curve); (b) 6 (plain curve), 7 (dashed curve), 23 (bold curve), and 24 (bold dashed curve). [Ir complex] = 10 μM.

reported dual-emissive Ir complexes 5a, 6, and 7.12 In addition, the emission spectra of 21, 23, and 24 were measured in frozen DMSO at 77 K (Figure S2 in Supporting Information), showing dual emission with a hypsochromic shift at 77 K, as summarized in Table S2 in Supporting Information.

Table 2. Luminescent Properties of Ir Complexes 5a, 6, 7, 18, and 21−24 (excitation at 366 nm) in Degassed DMSO at 298 K compound 5a: Ir(ppy)2(8BSQ−) 6: Ir(F2ppy)2(8BSQ−) 7: Ir(mppy)2(8BSQ−) 18: Ir(ppy)2(Br8BSQ−) 21: Ir(ppy)(Tsppy)(Br8BSQ−) 22: Ir(ppy)(Tsppy)(Py8BSQ−) 23: Ir(ppy)(CHOppy)(Br8BSQ−) 24: Ir(mppy)(CHOmppy)(Br8BSQ−)

λmax (emission, nm) 496, 474, 487, 498, 487, 494, 488, 480,

617 493, 512, 641 512, 511, 514, 505,

613 617 637 630 635 638

Φ (%)a 3.5 13 7.0 0.30 0.52 1.2 0.43 0.38

τ (μs)b 1.6, 1.4, 1.3, 1.2, 1.8, 1.7, 1.5, 2.0,

12c 9.3c 8.2c 6.1d 7.3d 12d 6.2d 7.0d

τ (μs)e 9.0f 11f 6.8f 5.4g 5.7g 8.2g 5.0g 7.0g

a [Ru(bpy)3]Cl2 (Φ = 2.8% in an aerated aqueous solution) was used as a reference (ref 17). bEmission decay curves provide a better fit for biexponential decay equation. cA 435 nm long wave pass filter was used. dA 475 nm long wave pass filter was used. eEmission decay curves provide a better fit for the single-exponential decay equation. fA 550 nm long wave pass filter was used. gA 590 nm long wave pass filter was used.

F


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Inorganic Chemistry

Table 3. Triplet Transition States and Character of the Transitions of Tris-Heteroleptic Ir Complexes 23 Were Calculated by TD-DFT with the PBE1PBE Functional (LANL2DZ/6-31G) on the Basis of the Optimized Ground State Geometry comp 23

energy/eV (nm)

configurationsa

T1

2.01 (616)

T2

2.70 (459)

HOMO → LUMO (60%) HOMO−1 → LUMO (29%) HOMO → LUMO+1 (20%) HOMO → LUMO+2 (21%)

state

main transition characterb 3

3

MLquinCT + LquinC + 3LCHOppyLquinCT + 3NsulfonamideLquinCT

3

MLCHOppyCT + 3MLppyCT + 3LquinLCHOppyCT + 3LquinLppyCT

a

Major contributions are defined as >10% contribution to the transition. bThe metal-to-ligand (M to L) charge transfer, ligand to ligand (L to L) charge transfer, intraligand charge transfer, and ligand centered are represented by MLxCT, LxLyCT, NsulfonamideLxCT, and LCx, respectively. Nsulfonamide depicts a lone pair of nitrogen of sulfonamide, quin is a quinoline ring of 8BSQ.

Figure 6. Selected molecular orbitals of 23 obtained from DFT calculation using the PBE1PBE functional with LANL2DZ and 6-31G basis sets.

the dual emission, a HE emission band (ca. 470−570 nm) from the ppy ligands and a LE emission band (ca. 580−700 nm) from btp ligand in the absence of Cu2+. Interestingly, the formation of the complex 26 due to complexation of the DPA unit with Cu2+ quenches the emission derived from the btp part, allowing ratiometric measurement (IHEB/ILEB) of Cu2+ (Chart S1 in Supporting Information). The similar behavior was described as for protonation of the DPA part of 25.13d Moreover, Williams and co-workers reported the emission quenching behavior of bis-terpyridyl Ir complex 27 bearing a pyridyl group, whose emission intensity was suppressed by protonation (Chart S2 in Supporting Information).24 These phenomena can be attributed to nonradiative deactivation processes, due to protonation of the pyridine ring. This is similar to that described for the meso-pyridyl BODIPY 28 (Chart S2 in Supporting Information), emission of which is quenched by the protonation of pyridine ring due to the contribution of a nonradiative charge transfer from the BODIPY part to the pyridinium cation, since the pyridinium cation is more easily reduced than the original pyridine unit.25 As described above, the protonated pyridine ring on the 5 position of the 8BSQ unit of H·22 contributes to the quenching of the HE emission band rather than the LE emission (Figure 7a and 7b). To study these phenomena, the excited triplet

states are assigned to the LE emission band and HE emission band, respectively, and that all three ligands on 23 contribute to its emission. pH-Dependent Emission Behavior of 22. The pHdependent emission spectra of 22, which contains a pyridine ring on the 8BSQ portion, in degassed DMSO/100 mM buffer (pH 4−9) (1/6) at 298 K are shown in Figure 7a (its UV−vis spectra are shown in Figure S3 in Supporting Information). The emission intensity of both HE and LE bands of 22 was decreased at pH < 7, without a significant change in the emission wavelength, possibly due to the protonation of the pyridine ring at the 5 position of the 8BSQ unit.22 The pH-dependent plot in emission intensity at both 483 and 637 nm at each pH solution suggests that the pKa value of H·22 is between 6 and 7 (Figure 7b), which is almost identical to that of 4b.11d,23 Figure 7c shows that the xy color coordinate of 22 moves from pH 4 to pH 9 and exhibits whitish emission at pH 4 (x and y values are 0.27 and 0.30, respectively) and blueish white at pH 6−9. Lippard and co-workers previously reported on a Cu2+ probe 25 based on Ir complex, which is composed of two ppy ligands and one btp (2-(2′-benzo[b]thienyl)pyridine) ligand containing a di(2-picolyl)amine (DPA) unit (Chart S1 in the Supporting Information).13d It was reported that 25 exhibits G


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Inorganic Chemistry

HOMO−3 → LUMO+1. On the basis of the experiment results, the T1 and T2 transition states of acid-free 22 could be assigned to the LE emission band and HE emission band, respectively (Figure 7a and Table 4). On the other hand, the molecular orbitals of H·22 suggest the T2 state (2.01 eV, 617 nm) is composed of HOMO → LUMO and HOMO−1 → LUMO.26 In addition, the HE emission band of H·22 could be assigned for the T3 state (2.30 eV, 538 nm) (HOMO−1 → LUMO+1 and HOMO → LUMO+1) and T4 state (2.58 eV, 480 nm) (HOMO−3 → LUMO and HOMO−2 → LUMO +1), respectively (Figure 8b and Table 4). As shown in Figure 8b, HOMO−1, LUMO, and LUMO+1 contain the pyridinium portion of H·22. A possible explanation for emission quenching by protonation is that nonradiative deactivation processes are facilitated by 3LLCT transitions between the ppy or Tsppy ligand and the protonated pyridine ring on 8BSQ, as is the case with 2724 and 28.25

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CONCLUSIONS We report herein on a new synthetic approach for preparing tris-heteroleptic Ir complexes that contain three nonsymmetric ligands via the use of ligand-selective electrophilic reactions of 5a via an interligand HOMO hopping phenomena as an application of the electrophilic substitution reactions of cyclometalated Ir complexes previously reported by us.11 Our findings indicate that the reactivity and selectivity of cyclometalated Ir complexes for electrophilic reagents is predicted and controlled by the localization of their HOMO, as calculated by DFT. First, bromination occurs at the 5 position of the quinoline ring of 5a to provide 18. Selective iodination of 18 then occurs at the 5′ position of the ppy ligand, whose phenyl group is trans to the Ir−N(quinoline) bond. Interestingly, the obtained bromo- and iodo-substituted tris-heteroleptic Ir complex 20a consisted of eight different atoms containing Ir, H, C, N, O, S, Br, and I atoms, indicating the diversity of this methodology. The functionalization of 20a using crosscoupling reactions was also demonstrated. The formylation of mono-bromo-substituted Ir complexes 18 and 19 gives 23 and 24 containing a formyl group as the electron-withdrawing group. These complexes exhibit a dual emission at 480−515 nm (HE emission) mainly from the ppy ligands and 630−640 nm (LE emission) mainly from Br8BSQ. A tris-heteroleptic Ir complex 22 containing a pyridyl ring on the 8BSQ unit was also synthesized. It was found that protonation of the pyridine ring induces the decrease in emission intensity of the HE band rather than the LE band. To the best of our knowledge, this represents the first example of the isolation and characterization of tris-heteroleptic Ir complexes that contain three different nonsymmetric ligands (as a racemic mixture of Δ and Λ isomers). The electrophilic reactions by tuning of the HOMO localization represent a powerful method for preparing a variety of tris-heteroleptic Ir complexes with negligible formation of their stereoisomers. We believe that the ligand-selective electrophilic reactions of the cyclometalated Ir complexes can be applied to the design and synthesis of a variety of cyclometalated Ir complexes and to the development of new class of highly functionalized trisheteroleptic Ir complexes suitable for a wide variety of applications.

Figure 7. pH-Dependent emission spectra of 22 (10 μM) in degassed DMSO/100 mM buffer (from pH 4 to 9) (1/6) at 298 K. (b) pHDependent change in emission intensity of 22 (10 μM) at 483 (closed squares) and 637 nm (closed circles). (c) Chromaticity diagram showing the xy color coordinates of 22 in (CIE 1931). Excitation at 366 nm; au is in arbitrary units.

states of the acid-free and protonated forms of 22 were calculated by TD-DFT calculation on the basis of the optimized ground state geometry. The lowest energy triplet excited state T1 of acid-free 22 (1.87 eV, 664 nm) suggests contribution of HOMO → LUMO (Figure 8a and Table 4). The second lowest energy triplet excited state T2 of acid-free 22 (2.62 eV, 473 nm) was mainly composed of HOMO → LUMO+1 and

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EXPERIMENTAL PROCEDURES

General Information. All reagents and solvents were of the highest commercial quality and used without further purification,

H


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Inorganic Chemistry

Figure 8. Selected molecular orbitals of (a) acid-free 22 and (b) H·22 obtained from DFT calculation using the PBE1PBE functional with LANL2DZ and 6-31G basis sets. unless otherwise noted. Anhydrous N,N-dimethylformamide (DMF) and CH2Cl2 were distilled from calcium hydride. All reactions were

carried out under an atmosphere of argon. DMSO (spectrophotometric grade, WAKO Pure Chemical Industries Ltd.) was used for the I


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Inorganic Chemistry

1H), 8.39 (dd, J = 1.5, 8.4 Hz, 1H), 8.16 (d, J = 8.7 Hz, 1H), 7.80− 7.77 (m, 2H), 7.72−7.66 (m, 3H), 7.63−7.55 (m, 2H), 7.44−7.41 (m, 1H), 7.26−7.14 (m, 3H), 7.09−7.02 (m, 3H), 6.98−6.90 (m, 3H), 6.84−6.72 (m, 4H), 6.36−6.33 (m, 1H), 6.13 (d, J = 8.1 Hz, 1H) ppm. ESI-MS (m/z) calcd for C37H26N4O2S79Br191Ir [M]+: 860.0560. Found: 860.0556. Ir Complex 19. Compound 19 was prepared as a yellow solid (12 mg, 55%) from 712 (20 mg, 24 μmol), NBS (4.2 mg, 24 μmol), and distilled CH2Cl2 (7.0 mL) using a procedure similar to that used for 18. Mp 211−213 °C. IR (ATR): ν = 3064, 2934, 2833, 1584, 1549, 1459, 1427, 1304, 1278, 1212, 1142, 1036, 859, 772, 607, 577 cm−1. 1 H NMR (300 MHz, CDCl3/Si(CH3)4): δ = 9.06 (br d, J = 6.3 Hz, 1H), 8.38 (dd, J = 1.2, 8.6 Hz, 1H), 8.07 (d, J = 8.7 Hz, 1H), 7.83 (dd, J = 1.2, 5.1 Hz, 1H), 7.68 (d, J = 8.7 Hz, 2H), 7.63−7.58 (m, 2H), 7.55−7.49 (m, 2H), 7.41 (d, J = 8.4 Hz, 1H), 7.26−7.23 (m, 1H), 7.18−7.13 (m, 3H), 7.01−6.91 (m, 4H), 6.65 (br t, J = 6.6 Hz, 1H), 6.52 (dd, J = 2.4, 8.7 Hz, 1H), 6.44 (dd, J = 2.4, 8.4 Hz, 1H), 5.81 (d, J = 2.7 Hz, 1H), 5.70 (d, J = 2.7 Hz, 1H), 3.58 (s, 3H), 3.55 (s, 3H) ppm. ESI-MS (m/z) calcd for C39H30N4O4S79Br191Ir [M]+: 920.0772. Found: 920.0772. Ir Complex 20a. N-Iodosuccinimide (0.192 g, 0.854 mmol) was added to a solution of 18 (0.130 g, 0.151 mmol) in CH2Cl2 (25 mL) in the dark. After stirring at room temperature for 24 h, the reaction mixture was concentrated under reduced pressure, and the resulting residue was purified by silica gel column chromatography (CHCl3) and recrystallized from hexanes/CH2Cl2 to afford 20a as a yellow solid (0.136 g, 91%). Mp > 300 °C. IR (ATR): ν = 3068, 1608, 1558, 1472, 1296, 1270, 1135, 1085, 859, 846, 785, 741, 688, 570, 553, 535 cm−1. 1 H NMR (300 MHz, CDCl3/Si(CH3)4): δ = 9.18 (d, J = 5.7 Hz, 1H), 8.41 (dd, J = 1.5, 8.6 Hz, 1H), 8.20 (d, J = 9.0 Hz, 1H), 7.81−7.64 (m, 5H), 7.61−7.56 (m, 3H), 7.24−7.19 (m, 3H), 7.08−6.91 (m, 7H), 6.82−6.77 (m, 2H), 6.12 (d, J = 8.1 Hz, 1H), 6.06 (d, J = 8.4 Hz, 1H) ppm. ESI-MS (m/z) calcd for C37H25N4O2S79BrI191Ir [M]+: 985.9527. Found: 985.9543. Ir Complex 21. To a mixture of Pd2(dba)3 (10 mg, 11 μmol) and Xantphos (13 mg, 22 μmol) in anhydrous toluene (1 mL), 20a (30 mg, 30 μmol), sodium p-toluenesulfinate (24 mg, 0.14 mmol), nBu4NCl (20 mg, 72 μmol), and Cs2CO3 (15 mg, 46 μmol) were added. The reaction mixture was heated at 80 °C for 21 h and then concentrated under reduced pressure. CHCl3 (100 mL) was added to the residue, and the resulting solution was then washed with H2O (50 mL). The organic layer was dried over Na2SO4, filtered, and concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography (hexanes/CHCl3 = 1/3, 1/5 to CHCl3) and recrystallized from hexanes/CH2Cl2 to afford 21 as a yellow solid (17 mg, 54%). Mp 284−286 °C. IR (ATR): ν = 3016, 2925, 1606, 1576, 1561, 1496, 1477, 1302, 1101, 858, 756, 715, 665, 588, 534 cm−1. 1H NMR (300 MHz, CDCl3/Si(CH3)4): δ = 9.24 (d, J = 6.3 Hz, 1H), 8.44 (dd, J = 1.5, 8.4 Hz, 1H), 8.43 (d, J = 9.0 Hz, 1H), 7.83−7.71 (m, 8H), 7.61−7.56 (m, 3H), 7.26−7.20 (m, 3H), 7.11− 7.09 (m, 3H), 6.96 (t, J = 8.1 Hz, 1H), 6.85−6.73 (m, 4H), 6.65 (t, J = 7.8 Hz, 2H), 6.52 (d, J = 8.4 Hz, 1H), 5.92 (dd, J = 0.8, 7.7 Hz, 1H), 2.36 (s, 3H) ppm. ESI-MS (m/z) calcd for C44H32N4O4S279Br191Ir [M]+: 1014.0649. Found: 1014.0648. Ir Complex 22. To a solution of 21 (7.0 mg, 6.9 μmol) in distilled THF (1 mL), 4-pyridineboronic acid pinacol ester (17 mg, 84 μmol), Pd(PPh3)4 (9.6 mg, 8.4 μmol), and degassed 2 M K2CO3 (0.5 mL) were added. The reaction mixture was heated at reflux for 45 h and then concentrated under reduced pressure. AcOEt (20 mL) was added to the residue and washed with H2O (20 mL) and brine (10 mL). The organic layer was dried over Na2SO4, filtered, and concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography (hexanes/AcOEt = 1/2 to AcOEt) and recrystallized from hexanes/CH2Cl2 to afford 22 as a yellow solid (5.7 mg, 81%). Mp 256−258 °C. IR (ATR): ν = 3042, 1599, 1575, 1560, 1513, 1478, 1459, 1295, 1140, 713, 665, 583 cm−1. 1H NMR (300 MHz, CDCl3/Si(CH3)4): δ = 9.27 (d, J = 5.4 Hz, 1H), 8.71 (br d, J = 4.5 Hz, 2H), 8.58 (d, J = 8.4 Hz, 1H), 8.23 (d, J = 8.7 Hz, 1H), 7.84− 7.81 (m, 4H), 7.76−7.68 (m, 2H), 7.63−7.51 (m, 4H), 7.39−7.37 (d, J = 5.7 Hz, 2H), 7.26−7.21 (m, 4H), 7.18−7.08 (m, 3H), 6.98−6.79 (m,

Table 4. Representative Triplet Transition States and the Character of the Transitions of Acid-Free 22 and H·22 Calculated by TD-DFT with PBE1PBE Functional (LANL2DZ/6-31G) on the Basis of Their Optimized Ground State Geometries state

energy/eV (nm)

configurationsa

22

T1 T2

1.87 (664) 2.62 (473)

H·22

T1

1.28 (965)

T2

2.01 (616)

T3

2.30 (538)

T4

2.58 (480)

HOMO → LUMO (92%) HOMO → LUMO+1 (63%) HOMO−3 → LUMO+1 (12%) HOMO−1 → LUMO (64%) HOMO → LUMO (33%) HOMO → LUMO (63%) HOMO−1 → LUMO (32%) HOMO−1 → LUMO+1 (59%) HOMO → LUMO+1 (29%) HOMO−3 → LUMO (61%) HOMO−2 → LUMO+1 (15%)

comp

a Major contributions are defined as >10% contribution to the transition.

measurement of photophysical data. IrCl3·3H2O and Pd(PPh3)4 were purchased from Kanto Chemical Co. N-Bromosuccinimide (NBS) and 4-pyridineboronic acid pinacol ester were purchased from WAKO Pure Chemical Industries, Ltd. N-Iodosuccinimide (NIS), sodium ptoluenesulfinate, 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos), and tetrabutylammonium chloride (n-Bu4NCl) were purchased from Tokyo Chemical Industry Co., Ltd. Tris(dibenzylideneacetone)dipalladium (Pd2(dba)3) was purchased from Sigma-Aldrich. Phosphoryl chloride (POCl3) was purchased from Nacalai Tesque. All aqueous solutions were prepared using deionized, distilled water. UV−vis spectra were recorded on a JASCO V-550 and V630bio spectrophotometers at 25 °C. Emission spectra were recorded on JASCO FP-6200 and FP-6500 spectrofluorometers. IR spectra were recorded on a PerkinElmer FT-IR spectrophotometer (Spectrum100) at room temperature. Melting points were measured using a Yanaco MP-J3Micro Melting Point apparatus and are uncorrected. 1H NMR spectra (300 MHz) were recorded on a JEOL Always 300 spectrometer. Tetramethylsilane (Si(CH3)4) was used as an internal reference for 1H NMR measurements in CDCl3. Mass spectral measurements were performed on a Varian 910-MS. The mass of some tris-cyclometalated Ir complexes are observed as [M]+ (rather than [M + H]+) in ESI mode (Varian 910-MS).11 Elemental analyses were performed on a PerkinElmer CHN 2400 series II CHNS/O analyzer. Elemental analyses of 18, 20a, 21, 23, and 24 were not carried out because the content of Ir and halogen atoms was >25%. Thin-layer chromatographies (TLC) and silica gel column chromatographies were performed using Merck Art. 5554 (silica gel) TLC plate and Fuji Silysia Chemical FL-100D, respectively. For measurement of luminescence spectra in aqueous solutions at given pH’s, buffer solutions (CHES, pH 9.0; EPPS, pH 8.0; HEPES, pH 7.0; MES, pH 6.0, 5.0, and 4.0) were used. The Good’s buffer reagents were obtained from commercial sources: MES (2-morpholinoethanesulfonic acid, pK a = 4.8), HEPES (2-[4-(2-hydroxyethyl)-1piperazinyl]ethanesulfonic acid, pKa = 7.5) (Dojindo), EPPS (3-[4(2-hydroxy-ethyl)-1-piperazinyl]propanesulfonic acid, pKa = 8.0) (MP Biomedicals, LLC.), and CHES (2-(cyclohexylamino)ethanesulfonic acid, pKa = 9.5) (Dojindo). Negligible hazard exists in all synthetic procedures. Ir Complex 18.27 N-Bromosuccinimide (45.4 mg, 0.255 mmol) was added to a solution of 5a12 (200 mg, 0.255 mmol) in distilled CH2Cl2 (1 mL) in the dark. After stirring at room temperature for 0.5 h, the reaction mixture was concentrated under reduced pressure, and the resulting residue was purified by silica gel column chromatography (CHCl3) and recrystallized from hexanes/CH2Cl2 to afford 18 as a yellow solid (215 mg, 98%). Mp > 300 °C. IR (ATR): ν = 3042, 1608, 1583, 1562, 1277, 1304, 1141, 1098, 1085, 866, 753, 738, 606, 552 cm−1. 1H NMR (300 MHz, CDCl3/Si(CH3)4): δ = 9.18 (d, J = 5.4 Hz, J


Article

Inorganic Chemistry

Measurements of UV−vis Absorption and Luminescence Spectra. UV−vis spectra were recorded on JASCO V-550 and V630bio UV−vis spectrophotometers and emission spectra recorded on a JASCO FP-6200 and FP-6500 spectrofluorometers, respectively. All samples for spectroscopic measurements were carefully purified by recrystallization twice from hexanes/CH2Cl2 prior to use in order to minimize contamination, and the same spectra were observed after recrystallization. Sample solutions in quartz cuvettes equipped with Teflon septum screw caps were degassed by bubbling Ar through the solution for 10 min prior to making the luminescence measurements. The phosphorescence quantum yields (Φ) were determined using [Ru(bpy)3]Cl2 (bpy 2,2-bipyridine) (Φ = 2.8% in an aerated aqueous solution).13f,17 Equation 1 was used to calculate the emission quantum yields, in which Φs and Φr denote the quantum yields of the sample and the reference compound. The ηs and ηr are the refractive indexes of the solvents used for the measurements of the sample and the reference (η 1.477 for DMSO and 1.333 for H2O). As and Ar are the absorbance of the sample and the reference, and Is and Ir stand for the integrated areas under the emission spectra of the sample and reference, respectively (all of the Ir compounds were excited at 366 nm for the luminescence measurements in this study).

4H), 6.76 (t, J = 7.4 Hz, 1H), 6.68 (t, J = 7.8 Hz, 2H), 6.54 (d, J = 8.4 Hz, 1H), 5.95 (d, J = 7.2 Hz, 1H), 2.36 (s, 3H) ppm. ESI-MS (m/z) calcd for C49H37N5O4S2191Ir [M + H]+: 1014.1887. Found: 1014.1892. Anal. Calcd C49H36IrN4O4S2·6CH2Cl2: C, 43.33; H, 3.17; N, 4.59. Found: C, 43.11, H, 2.93; N, 4.62. Ir Complex 23. Phosphoryl chloride (POCl3) (17 mg, 0.11 mmol) was added to distilled DMF (0.5 mL), and the resulting mixture was stirred at room temperature for 1 h. To the solution compound 18 (5.0 mg, 5.8 μmol) was added and heated at 80 °C for 15 h. The deepred-colored reaction mixture was cooled to 0 °C, and 1 M NaOH (1.0 mL) was then added. After stirring for 15 min at 0 °C, the reaction mixture was stirred at room temperature for an additional 1 h. The resulting yellow precipitate was dissolved in CHCl3 (20 mL). The obtained organic layer was separated and washed with H2O (20 mL) and then dried over Na2SO4, filtered, and concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography (CHCl3) and recrystallized from hexanes/CH2Cl2 to afford 23 as a yellow solid (1.2 mg, 23%). Mp 223−225 °C. IR (ATR): ν = 3054, 2927, 1677, 1583, 1279, 1302, 1141, 754, 606, 577 = cm−1. 1 H NMR (300 MHz, CDCl3/Si(CH3)4): δ = 9.81 (s, 1H), 9.19 (br d, J = 7.8 Hz, 1H), 8.43 (dd, J = 1.5, 7.2 Hz, 1H), 8.10 (d, J = 8.7 Hz, 1H), 7.96 (d, J = 1.5 Hz, 1H), 7.88−7.77 (m, 3H), 7.73−7.68 (m, 3H), 7.61−7.58 (m, 2H), 7.30−7.18 (m, 3H), 7.12−7.04 (m, 3H), 7.00− 6.85 (m, 4H), 6.81 (br t, J = 7.2 Hz, 1H), 6.58 (d, J = 7.8 Hz, 1H), 6.11 (br d, J = 7.5 Hz, 1H) ppm. ESI-MS (m/z) calcd for C38H27N4O3S79Br191Ir [M + H]+: 889.0588. Found: 889.0595. Ir Complex 24. Compound 24 was prepared as a yellow solid (5.0 mg, 98%) from 19 (5.0 mg, 5.4 μmol), POCl3 (20 mg, 0.13 mmol), and distilled DMF (1.0 mL) using a procedure similar to that used for 23. Mp 230−232 °C. IR (ATR): ν = 3065, 2930, 2834, 1583, 1357, 1425, 1141, 1035, 776, 605, 577, 535 cm−1. 1H NMR (300 MHz, CDCl3/Si(CH3)4) δ = 10.25 (s, 1H), 9.20 (d, J = 5.7 Hz, 1H), 8.43 (dd, J = 1.5, 8.4 Hz, 1H), 8.12 (d, J = 8.7 Hz, 1H), 7.93 (s, 1H), 7.82 (dd, J = 1.5, 4.8 Hz, 1H), 7.73−7.53 (m, 6H), 7.29−7.24 (m, 1H), 7.20−7.16 (m, 2H), 7.10−6.92 (m, 5H), 6.74 (t, J = 5.7 Hz, 1H), 6.56 (dd, J = 2.4, 8.4 Hz, 1H), 5.94 (s, 1H), 5.62 (d, J = 2.4 Hz, 1H), 3.57 (s, 3H), 3.52 (s, 3H) ppm. ESI-MS (m/z) calcd for C40H30N4O5S79Br191Ir [M]+: 948.0721. Found: 948.0730. X-ray Data Collection and Refinement. Single-crystal X-ray studies were performed on a Bruker APEX II CCD diffractometer equipped with a Bruker Instruments low-temperature attachment. Data were collected at 100 K using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The frames were indexed, integrated, and scaled using the SMART and SAINT software packages.28 An empirical absorption correction was applied to the collected reflections with SADABS29 using XPREP.30 All of the structures were solved by the direct method using the program SHELXS-97 and refined on F2 by the full-matrix least-squares technique using the SHELXL-97 program package.31 All non-hydrogen atoms in the structure were refined anisotropically. Crystal Data for 18. A yellow crystal of 18 was obtained by slow diffusion of hexane into its solution in CHCl3. C37H26BrIrN4O2S· CHCl3·0.5Hexane, Mr = 1025.24, monoclinic, P21/c, a = 9.8804(8) Å, b = 23.5726(19) Å, c = 17.0400(14) Å, α = 90°, β = 95.524(2)°, γ = 90°, V = 3950.3(6) Å3, Z = 4, ρcalcd = 1.724 g·cm−3, R = 0.0306 (for 5879 reflection with I > 2σ(I)), Rw = 0.0832 (for 7177 reflections), GOF = 1.075. CCDC1502791 contains the supplementary crystallographic data for the paper. Crystal Data for 20a. A yellow crystal of 20a was obtained by the slow diffusion of MeOH into its solution in CH 2 Cl 2 . C 37 H 25 BrIIrN 4 O 2 S, M r = 988.68, monoclinic, P2 1 /n, a = 10.9830(18) Å, b = 9.7438(17) Å, c = 30.330(5) Å, α = 90°, β = 99.433(3)°, γ = 90°, V = 3201.9(9) Å3, Z = 4, ρcalcd = 2.051 g·cm−3, R = 0.0323 (for 5319 reflections with I > 2σ(I)), Rw = 0.0811 (for 5900 reflections), GOF = 1.081. CCDC1502793 contains the supplementary crystallographic data for the paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/ cif.

Φs = Φr (ηs 2A r Is)/(ηr 2A sIr)

(1)

The luminescence lifetimes of sample solutions of Ir complexes (5.0 μM) in degassed DMSO at 298 K were measured on a Laser Flash Photolysis System (TSP1000-M-PL, Unisoku, Osaka, Japan) instrument by using THG (355 nm) of a Nd:YAG laser, Minilite I (Continuum, CA, USA) as the excitation source and long wave pass filters (475 or 590 nm). Luminescence decays contain both HE and LE emission bands using a 475 nm long wave pass filter. Luminescence decays of the LE emission band were observed using a 590 nm long wave pass filter. The signals were monitored with an R2949 photomultiplier. Data were analyzed using single or biexponential decay equations. Sample solutions in quartz cuvettes equipped with Teflon septum screw caps were degassed by bubbling Ar through the solution for 20 min prior to measuring the lifetime. Theoretical Calculations. Density functional theory (DFT) calculations were also carried out using the Gaussian 09 program20 (PBE1PBE functional, the LanL2DZ basis set for Ir, Br, and I atoms and the 6-31G basis set for H, C, F, S, O, N atoms).21 TD-DFT calculations were carried out based on optimized ground state geometries using the same functional and basis set.13e,f The molecular orbitals were visualized using Jmol software (an open-source Java viewer for chemical structures in 3D, http://www.jmol.org/).

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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.inorgchem.6b02519. Representative crystallographic parameters of 18 and 20a, luminescence decay curves of 18 and 21−24, emission spectra of 21, 23, and 24 at 77 K, UV−vis and emission spectra of 22 in aqueous solutions at different pH, structures and emission spectra of 25−28, and 1H NMR spectra of 18, 19, 20a, and 21−24 (PDF) (CIF) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail, [email protected]. ORCID

Shin Aoki: 0000-0002-4287-6487 Notes

The authors declare no competing financial interest. K


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ACKNOWLEDGMENTS This work was supported by grants-in-aid from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (Nos. 26860016 and 16K18851 for Y.H. and Nos. 24659055 and 24640156 for S.A.) and “Academic Frontiers” project for private universities: matching fund study from MEXT, and TUS (Tokyo University of Science) fund for strategic research areas. We thank Prof. Dr. Reiko Kuroda and Dr. Kyouhei Sato (Research Institute for Science and Technology, Tokyo University of Science) for providing CIE calculation software. We wish to acknowledge Mr. Taiki Itoh (Faculty of Pharmaceutical Sciences, Tokyo University of Science) and Ms. Tomoko Mastsuo (Research Institute for Science and Technology, Tokyo University of Science) for the X-ray single-crystal structure analysis. We appreciate the aid of Mrs. Fukiko Hasegawa (Faculty of Pharmaceutical Sciences, Tokyo University of Science) for measurement of mass spectra.

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REFERENCES

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(26) The emission peak suggesting the lowest energy triplet state T1 of H·22 (1.28 eV, 965 nm) was undetectable under our experimental conditions. (27) 1H NMR spectra of 18, 19, 20a, 21, 22, 23, and 24 are shown in Figures S5−S11 in Supporting Information. (28) Smart & SAINT Software Reference Manuals, Version 6.45; Bruker Analytical X-ray Systems, Inc.: Madison, WI, 2003. (29) Sheldrick, G. M. SADABS, Software for Empirical Absorption Correction, Version 2.05; University of Gö ttingen: Gö ttingen, Germany, 2002. (30) XPREP, 5.1 ed.; Siemens Industrial Automation Inc.: Madison, WI, 1995. (31) Sheldrick, G. M. SHELXL97, Program for Crystal Structure Refinement; University of Göttingen: Göttingen, Germany, 2008.

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Design and Synthesis of Tris-Heteroleptic Cyclometalated Iridium(III

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