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Cite This: Inorg. Chem. 2018, 57, 6853−6864

Metalated Ir(III) Complexes Based on the Luminescent Diimine Ligands: Synthesis and Photophysical Study Julia R. Shakirova,† Olesya A. Tomashenko,† Ekaterina E. Galenko,† Alexander F. Khlebnikov,† Pipsa Hirva,‡ Galina L. Starova,† Shih-Hao Su,§ Pi-Tai Chou,*,§ and Sergey P. Tunik*,† †

St. Petersburg State University, Institute of Chemistry, 7/9 Universitetskaya emb., 199034 St. Petersburg, Russia Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland § Department of Chemistry, National Taiwan University, Taipei, 10617 Taiwan, R.O.C. Downloaded via UNIV OF SUSSEX on July 17, 2018 at 08:43:56 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: A series of novel diimine (N∧N) ligands containing developed aromatic [2,1-a]pyrrolo[3,2-c]isoquinoline system have been prepared and used in the synthesis of Ir(III) luminescent complexes. In organic solvents, the ligands display fluorescence which depends strongly on the nature of solvents to give moderate to strong orange emission in aprotic solvents and shows a considerable blue shift and substantial increase in emission intensity in methanol. Insertion of electron-withdrawing and -donating substituents into peripheral phenyl fragment has nearly no effect onto emission parameters. The ligands were successfully used to prepare the metalated [Ir(N∧C)2(N∧N)]+ complexes (where N∧C = phenylpyridine (N∧C-1), p-tolylpyridine (N∧C-2), 2(benzo[b]thiophen-2-yl)pyridine (N∧C-3), 2-benzo[b]thiophen-3-yl)pyridine (N∧C-4), and methyl 2-phenylquinoline-4carboxylate (N∧C-5)) using standard synthetic procedures. The complexes obtained display moderate to strong phosphorescence in organic solvents; the emission characteristics is determined by the nature of emissive triplet state, which varies substantially with the variations in the structure and donor properties of the C- and N-coordinating functions in metalating ligands. TD-DFT calculations show that for complexes 1, 2, and 4 the emission originates from the mixed 3MLCT/3LLCT excited states with the major contribution from the aromatic moiety of the diimine ligand, whereas in 3 the emissive triplet manifold is mainly located at the N∧C ligand to give structured emission band typical for the ligand centered (LC) excited state. In the case of 5, the phosphorescence may be also assigned to the mixed 3MLCT/3LLCT excited state; however, the major contribution is attributed to the aromatic moiety of the metalating N∧C ligand.

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INTRODUCTION ∧

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donor or acceptor substituents into the ligand periphery. Changing electron-donicity of the ligand coordination functions affects energy of metal centered orbitals that in turn may result in switching between different emissive states which display different photophysical characteristics such as emission energy, radiative rate constants, and observed lifetime and hence quantum yield. In our recent publication26 we present a new type of fused heterocyclic aromatics based on {pyrido[2,1-a]pyrrolo[3,2c]isoquinoline} core, which are strongly luminescent and show emission response onto the presence of protic solvents and organic acids related to protonation of the nitrogen atom of the pyrrole ring. The nucleophilic properties of this nitrogen center prompted us to explore its ability to take part in coordination bonding, in particular with Au(I) ions,27 to give a series of linear L−Au−(alkynyl) complexes, which unexpect-

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Iridium complexes of the [Ir(N C)2(N N)] type are wellknown luminescent compounds,1−5 which found numerous applications as emitting components of OLED devices,4,6−12 sensors,13−18 probes in bioimaging,19−24 and nonlinear optical materials.25 These compounds display extremely versatile photophysical behavior determined by delicate balance between energy of frontier orbitals taking part in photoexcitation processes and their relative contribution into emissive excited states. The major triplet excited states responsible for phosphorescence in these Ir(III) complexes are (i) metalperturbed intraligand (3IL) transition (which may have alternative names LC (ligand centered) or ILCT (intraligand charge transfer) if it is a multidentate ligand), (ii) metal to ligands charge transfer (3MLCT), and (iii) interligand charge transfer (3LLCT). The ligand-centered photoactive states are commonly located at aromatic systems that make possible finetuning of emission properties through variations in aromatic core composition and size, as well as by insertion of either © 2018 American Chemical Society

Received: February 14, 2018 Published: May 29, 2018 6853

DOI: 10.1021/acs.inorgchem.8b00390 Inorg. Chem. 2018, 57, 6853−6864

Article

Inorganic Chemistry edly display independent ligand fluorescence without any impact of the coordinating heavy atom center. Evidently, coordination properties of the aromatic system can be strengthened by insertion of the second coordination function into position suitable for chelate cycle formation, considerably expanding the area of the ligand application. Accordingly, a novel core structure containing {pyrido[2,1-a]pyrrolo[3,2c]isoquinoline} fragment and adjacent pyridyl substituent was synthesized and applied as a bidentate N∧N ligand in preparation of luminescent metalated Ir(III) complexes, which have been fully characterized, including detailed photophysical study and DFT calculations.

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column chromatography on silica gel (DCM/MeOH from 1:0 to 0:1) to give the product as dark orange solid. Mp 250−251 °C; yield 143 mg, 67%. 1H NMR (CDCl3, 400 MHz): δ 7.06 (dd, J = 7.0, 5.2 Hz, 1H), 7.37−7.42 (m, 1H), 7.58−7.66 (m, 4H), 7.73 (d, J = 8.1 Hz, 2H), 7.87−7.98 (m, 3H), 8.42 (d, J = 4.1 Hz, 1H), 8.58 (d, J = 8.6 Hz, 1H), 8.93 (d, J = 6.9 Hz, 1H), 8.98 (d, J = 8.9 Hz, 1H), 9.42 (d, J = 8.2 Hz, 1H). 13C NMR ((CD3)2SO, 100 MHz): δ 109.9 (C), 119.2 (C), 120.6 (CH), 121.2 (CH), 121.7 (CH), 122.5 (CH), 123.7 (CH), 123.9 (C), 124.7 (q, J = 4.0 Hz, CH), 124.7 (q, J = 271.6 Hz, C), 124.8 (CH), 125.4 (CH), 127.1 (q, J = 32.2 Hz, C), 130.0 (C), 130.7 (CH), 131.2 (CH), 131.9 (CH), 132.4 (CH), 134.2 (C), 135.6 (CH), 136.7 (C), 142.6 (C), 146.8 (C), 148.0 (CH), 157.0 (C). HRMS (ESI) m/z: 440.1369 calcd for C27H17F3N3+ [M + H]+, found 440.1355. 3-(4-Methoxyphenyl)-2-(pyridine-2-yl)pyrido[2,1-a]pyrrolo[3,2-c]isoquinoline (N∧N-C). To a stirred refluxing solution of 1-(2-(2bromophenyl)-4-(4-methoxyphenyl)-5-(pyridin-2-yl)-1H-pyrrol-3-yl)pyridine-1-ium bromide (285 mg, 0.711 mmol) in acetonitrile (45 mL) under argon atmosphere was added a solution of AIBN (352 mg, 1.14 mmol, 3 equiv) and TTMSS (532 mg, 1.14 mmol, 3 equiv) in acetonitrile (21 mL) mixture through syringe pump for 24 h under reflux. After the reaction was completed, the solvent was evaporated under reduced pressure. The residue was dissolved in 1 M HCl and washed with benzene and ether. The solution was basified by 10% aqueous NaOH; the product was extracted with DCM and dried over K2CO3. The solvent was evaporated, the residue was purified by column chromatography on silica gel (DCM/MeOH from 1:0 to 0:1) and then on aluminum oxide (DCM/MeOH from 1:0 to 20:1) to obtain product as dark red solid. Mp 213−215 °C (DCM/MeOH); yield 176 mg, 59%. 1H NMR (CDCl3, 400 MHz): δ 3.91 (s, 3H), 7.00−7.02 (m, 3H), 7.23−7.27 (m, 1H), 7.34−7.39 (m, 3H), 7.40− 7.45 (m, 1H), 7.48−7.52 (m, 1H), 7.70−7.74 (m, 1H), 7.79−7.83 (m, 1H), 8.44−8.46 (m, 1H), 8.63−8.46 (m, 1H), 8.80−8.83 (m, 1H), 9.07−9.08 (m, 1H), 9.15−9.17 (m, 1H). 13C NMR (CDCl3, 100 MHz): δ 55.3 (CH3), 111.3 (C), 114.5 (CH), 119.2 (C), 120.5 (CH), 120.8 (CH), 122.9 (CH), 123.4 (CH), 123.5 (CH), 124.0 (CH), 124.2 (C), 125.2 (CH), 128.9 (C), 129.77 (C), 129.81 (CH), 130.8 (CH), 131.8 (CH), 132.8 (CH), 133.8 (C), 135.3 (CH), 136.9 (C), 147.1 (C), 149.6 (CH), 155.3 (C), 158.9 (C). HRMS (ESI) m/z: 402.1601 calcd for C27H20N3O+ [M + H]+, found 402.1597. Synthesis of Iridium Complexes. Bis(μ-chlorido) bridged dimeric precursors {(N∧C)2IrCl}2, where N∧C = phenylpyridine (N∧C-1), p-tolylpyridine (N∧C-2), 2-(benzo[b]thiophen-2-yl)pyridine (N∧C-3), 2-benzo[b]thiophen-3-yl)pyridine (N∧C-4), and methyl 2phenylquinoline-4-carboxylate (N∧C-5), were synthesized according to published procedures.31−33 General Procedure for the Synthesis of Cationic Complexes [(N∧C)2Ir(N∧N)]PF6. The heteroleptic biscyclometalated Ir(III) complexes with diimine ligands were synthesized according to a slightly modified published procedure.34 A solution of N∧N ligand (0.2 mmol) in DCM (10 mL) was added to the corresponding bis(μ-chlorido) bridged dimeric precursor (0.1 mmol) suspended in DCM/MeOH 1:1 mixture (20 mL). The reaction mixture was refluxed for 3 h. The resulting clear orange to deep red solutions were cooled down to room temperature, followed by the addition of excess of KPF6, and the mixture was stirred for additional 30 min. The reaction mixture was evaporated to dryness, dissolved in DCM, filtered, and evaporated once more. The crude products obtained were recrystallized to give the target compound. [(N∧C-1)2Ir(N∧N-A)]PF6 (1A). Needle orange crystals were obtained after recrystallization by slow gas phase diffusion of n-pentane into the concentrated acetone solution at room temperature (84%). 1 H NMR ((CD3)2CO, 400 MHz): δ 9.30 (d, J = 8.6 Hz, 1H), 9.15 (d, J = 6.9 Hz, 1H), 8.79 (d, J = 8.6 Hz, 1H), 8.55 (dd, J = 8.3, 0.6 Hz, 1H), 8.35−8.20 (m, 3H), 8.04 (d, J = 8.1 Hz, 1H), 8.01−7.93 (m, 2H), 7.89 (d, J = 7.7 Hz, 1H), 7.87−7.68 (m, 9H), 7.65 (ddd, J = 8.4, 1.6 Hz, 1H), 7.52 (ddd, J = 8.4, 7.1, 1.1 Hz, 1H), 7.22 (ddd, J = 7.3, 5.9, 1.3 Hz, 1H), 7.19−7.10 (m, 3H), 7.07−6.98 (m, 2H), 6.94−6.83 (m, 3H), 6.51−6.40 (m, 1H), 6.24 (dd, J = 7.5, 0.5 Hz, 1H). HRESI+MS: exp 872.2365 [M + ]; calcd 872.2365. Anal. Calcd for

EXPERIMENTAL SECTION

General Experimental Details. 1H, 1H−1H COSY (400 MHz), and 13C (100 MHz) NMR spectra were recorded on a Bruker 400 MHz Avance; chemical shift values were referenced to the solvent residual signals: 1H: CDCl3 (7.26 ppm), (CD3)2CO (2.05 ppm), and CD3OD (δ = 3.31 ppm); 13C: CDCl3 (77.00 ppm) and (CD3)2SO (39.51 ppm). Mass spectra were recorded on Bruker maXis HRMSESI-QTOF in the ESI+ mode. Melting points were determined on a melting point apparatus (Stuart SMP 30). Thin-layer chromatography (TLC) was conducted using aluminum sheets with 0.2 mm silica (fluorescent indicator, Macherey-Nagel). Microanalyses of organic ligands were carried out by using a Euro EA3028-HT instrument at the Research Park of St. Petersburg State University, Centre for Chemical Analysis. Microanalyses of Ir(III) complexes were carried out at the analytical laboratory of the University of Eastern Finland. Synthesis of the Diimine Ligands. The corresponding azirine precursors were synthesized according to the literature procedure (Scheme S1 and section S1 in Supporting Information).28−30 The synthetic routes for other precursors are shown in Schemes S2 and S3, and detailed synthetic procedures are described in Supporting Information sections S2 and S3. NMR spectra of all precursors are shown in Supporting Information section S4. 3-Phenyl-2-(pyridin-2-yl)pyrido[2,1-a]pyrrolo[3,2-c]isoquinoline (N∧N-A). A suspension of 1-benzyl-3-phenyl-2-(pyridin-2-yl)-1Hpyrido[2,1-a]pyrrolo[3,2-c]isoquinolin-4-ium bromide (146 mg, 0.269 mmol) and AlCl3 (359 mg, 2.69 mmol, 10 equiv) in benzene (20 mL) was refluxed for 7 h. The reaction mixture was then evaporated to dryness under reduced pressure. The solid residue was dissolved in the amount of aqueous 3% (wt) HCl sufficient for complete dissolution. This acidic solution was washed with diethyl ether (3 × 100 mL). The solution was then basified with aqueous 10% (wt) KOH and extracted with DCM (4 × 100 mL). The combined DCM phases were dried over K2CO3, filtered, and evaporated under reduced pressure to obtain the product as dark red solid. Mp 105−110 °C; yield 100 mg, 99%. 1H NMR (CDCl3, 400 MHz): δ 7.01 (ddd, J = 7.3, 4.8, 1.2 Hz, 1H), 7.21−7.26 (m, 1H), 7.30−7.34 (m, 1H), 7.38− 7.52 (m, 6H), 7.53−7.58 (m, 1H), 7.68−7.73 (m, 1H), 7.83−7.89 (m, 1H), 8.51 (d, J = 8.6 Hz, 1H), 8.63 (ddd, J = 4.8, 1.7, 0.9 Hz, 1H), 8.84 (d, J = 8.7 Hz, 1H), 9.05 (d, J = 6.9 Hz, 1H), 9.19 (d, J = 7.6 Hz, 1H). 13 C NMR (CDCl3, 100 MHz): δ 111.6 (C), 118.9 (C), 120.1 (CH), 120.5 (CH), 122.5 (CH), 123.2 (CH), 123.6 (CH), 123.8 (CH), 124.4 (C), 124.8 (CH), 127.0 (CH), 128.7 (CH), 128.9 (CH), 130.7 (CH), 130.7 (C), 131.5 (CH), 131.9 (CH), 135.1 (CH), 135.5 (C), 136.4 (C), 137.7 (C), 148.8 (C), 149.5 (CH), 156.3 (C). HRMS (ESI) m/z: 372.1495 calcd for C26H18N3+ [M + H]+, found 372.1493. 2-(Pyridin-2-yl)-3-(4-(trifluoromethyl)phenyl)pyrido[2,1-a]pyrrolo[3,2-c]isoquinoline (N ∧ N-B). A solution of azobis(isobutyronitrile) (AIBN) (241 mg, 1.47 mmol, 3 equiv) and tris(trimethylsilyl)silane (TTMSS) (366 mg, 1.47 mmol, 3 equiv) in acetonitrile (15 mL) was added through syringe pump during 12 h to a stirred refluxing acetonitrile (40 mL) solution of 1-(2-(2-bromophenyl)-5-(pyridin-2-yl)-4-(4-(trifluoromethyl)phenyl)-1H-pyrrol-3-yl)pyridin-1-ium bromide (294 mg, 0.489 mmol) under nitrogen atmosphere. The solvent was evaporated from the final reaction mixture under reduced pressure and the residue was purified by 6854

DOI: 10.1021/acs.inorgchem.8b00390 Inorg. Chem. 2018, 57, 6853−6864

Article

Inorganic Chemistry

(ddd, J = 5.6, 1.4, 0.7 Hz, 1H), 7.83 (dd, J = 8.3, 0.7 Hz, 1H), 7.80− 7.72 (m, 2H), 7.68−7.54 (m, 4H), 7.54−7.46 (m, 3H), 7.47−7.40 (m, 2H), 7.31−7.24 (m, 2H), 7.25−7.15 (m, 2H), 7.05 (dd, J = 7.9, 0.8 Hz, 1H), 6.93 (ddd, J = 8.2, 7.2, 0.8 Hz, 1H), 6.90−6.72 (m, 4H), 6.69 (ddd, J = 8.2, 0.9, 0.8 Hz, 1H), 6.42 (dd, J = 7.5, 0.7 Hz, 1H), 5.77 (d, J = 7.6 Hz, 1H), 4.24 (s, 3H), 3.74 (s, 3H). HRESI-MS: exp 1088.2760 [M+]; calcd 1088.2788. Anal. Calcd for C60H41F6IrN5O4P: C, 58.44; H, 3.35; N, 5.68; experimental: C, 58.26; H, 3.52; N, 5.73. [(N∧C-5)2Ir(N∧N-B)]PF6 (5B). Deep red crystals were obtained by slow evaporation of CH2Cl2/n-heptane solution at room temperature (79%). 1 H NMR (CD3OD, 400 MHz) δ 10.15 (d, J = 9.0 Hz, 1H), 9.30 (d, J = 8.7 Hz, 1H), 9.02 (s, 1H), 8.77 (d, J = 8.5 Hz, 1H), 8.71 (dd, J = 8.5, 1.1 Hz, 1H), 8.47 (d, J = 6.6 Hz, 1H), 8.39 (dd, J = 8.5, 0.8 Hz, 1H), 8.36 (d, J = 7.3 Hz, 1H), 8.27 (ddd, J = 8.6, 7.3, 1.0 Hz,1H), 8.03 (d, J = 5.6 Hz, 1H), 7.91 (dd, J = 8.1, 1.4 Hz, 1H), 7.81 (dd, J = 8.3, 0.6 Hz, 1H), 7.75−7.69 (m, 3H), 7.66 (ddd, J = 8.6 Hz, 6.6, 1.1 Hz, 1H), 7.60−7.42 (m, 4H), 7.32 (s, 1H), 7.28−7.15 (m, 3H), 7.13 (ddd, J = 7.5, 5.6, 1.2 Hz, 1H), 6.97 (d, J = 7.8 Hz, 1H), 6.92 (ddd, J = 8.3, 7.0, 0.6 Hz, 1H), 6.86 (ddd, J = 7.5, 7.6, 1.2 Hz, 1H), 6.83−6.74 (m, 2H), 6.68 (d, J = 7.5 Hz, 1H), 6.62 (d, J = 8.2 Hz, 1H), 6.33 (dd, J = 7.6, 0.5 Hz, 1H), 5.87 (d, J = 8.1 Hz, 1H), 4.24 (s, 3H), 3.74 (s, 3H). HRESI-MS: exp 1156.2626 [M+]; calcd 1156.2662. Anal. Calcd for C61H40F9IrN5O4P: C, 56.31; H, 3.10; N, 5.38; experimental: C, 56.48; H, 2.95; N, 5.46. XRD Experimental. Single crystals of 1A, 2A, 4B, and 5A were grown by the methods described above. A suitable crystal of C48H33N5IrPF6 (1A) was selected and studied on an Xcalibur Eos diffractometer. The crystal was kept at 100(2) K during data collection. A suitable crystal of C52.31H39.31Cl6.91F6IrN5P (2A) was selected and studied on an Xcalibur Eos diffractometer. The crystal was kept at 100(2) K during data collection. A suitable crystal of C109H66N10P2S4Ir2Cl6F18 (4B) was selected and studied on a SuperNova, Dual, Cu at zero, Atlas diffractometer. The crystal was kept at 100(2) K during data collection. A suitable crystal of C61H43Cl2F6IrN5O4P (5A) was selected and studied on an Xcalibur Eos diffractometer. The crystal was kept at 200(2) K during data collection. Using Olex2,35 the structures were solved with the Superflip36−38 structure solution program using charge flipping and refined with the ShelXL39 refinement package using least squares minimization. The unit cell of 2A contains disordered molecules of solvent (CHCl3) that have been treated as a diffuse contribution to the overall scattering without specific atom positions by SQUEEZE/ PLATON. Supporting crystallographic data can be obtained free of charge from The Cambridge Crystallographic Data Centre: 1A, CCDC 1559633; 2A, CCDC 1559545;4B, CCDC 1553868; 5A, CCDC 1555790. Photophysical Experiments. All photophysical measurements in solution were carried out in freshly distilled dichloroethane (DCE) and methanol. UV/vis spectra were recorded using a Shimadzu UV1800 spectrophotometer. The emission and excitation spectra in solution were measured with a Fluorolog-3 (JY Horiba Inc.) spectrofluorimeter. The absolute emission quantum yield in solution was determined by a comparative method using LED 365 nm pumping and coumarin 102 in ethanol (Φr = 0.764)40 as the reference with the refraction coefficients of dichloroethane, methanol, and ethanol equal to 1.42, 1.33, and 1.36, respectively. To calculate the quantum yield, we used the following equation:

C48H33F6IrN5P: C, 56.69; H, 3.27; N, 6.89; experimental: C, 56.63; H, 3.57; N, 6.41. [(N∧C-2)2Ir(∧N-A)]PF6 (2A). Plate like orange crystals was obtained after recrystallization by slow evaporation of CHCl3/n-heptane solution at room temperature (79%). 1 H NMR ((CD3)2CO, 400 MHz): δ 9.29 (d, J = 8.6 Hz, 1H), 9.14 (d, J = 6.7 Hz, 1H), 8.79 (d, J = 8.4 Hz, 1H), 8.59 (dd, J = 8.3, 1.3 Hz, 1H), 8.36−8.24 (m, 2H), 8.20 (d, J = 8.1 Hz, 1H), 7.99−7.88 (m, 3H), 7.83 (ddd, J = 5.6, 1.5, 0.8 Hz, 1H), 7.81−7.67 (m, 8H), 7.64 (ddd, J = 8.4, 7.6, 1.6 Hz, 1H), 7.55 (d, J = 7.9 Hz, 1H), 7.53 (ddd, J = 8.4, 7.0, 1.3 Hz, 1H), 7.21−7.07 (m, 4H), 7.03 (d, J = 8.3 Hz, 1H), 6.85 (dd, J = 8.0, 1.1 Hz, 1H), 6.73 (dd, J = 7.9, 1.1 Hz, 1H), 6.31 (s, 1H), 6.08 (s, 1H), 2.80 (s, 3H, −CH3), 2.13 (s, 3H, −CH3). HRESIMS: exp 900.2677 [M+]; calcd 900.2678. [(N∧C-3)2Ir(∧N-A)]PF6 (3A). Needle orange-red crystals were obtained by layering heptane over a concentrated acetone solution (80%). 1 H NMR ((CD3)2CO, 400 MHz): δ 9.25 (d, J = 8.5 Hz, 1H), 9.11 (d, J = 6.7 Hz, 1H), 8.98 (d, J = 5.8 Hz, 1H), 8.64−8.76 (m, 2H), 8.28 (ddd, J = 8.6, 7.3, 1.2 Hz, 1H), 8.12 (ddd, J = 7.6, 7.4, 1.4 Hz, 1H), 8.05 (d, J = 7.6 Hz, 1H), 7.94 (d, J = 5.5 Hz, 1H), 7.86−7.64 (m, 11H), 7.46 (d, J = 7.7 Hz, 1H), 7.35 (ddd, J = 8.4, 7.1, 1.3 Hz, 1H), 7.28 (ddd, J = 8.0, 7.2, 1.1 Hz 1H), 7.20 (ddd, J = 7.4, 5.8, 1.4 Hz, 1H), 7.23−7.14 (m, 3H), 7.14−7.09 (m, 1H), 7.07 (d, J = 8.3 Hz, 1H), 6.85−6.78 (m, 1H), 6.59 (d, J = 8.0 Hz, 1H), 6.14−6.02 (m, 2H). HRESI-MS: exp 984.1808 [M+]; calcd 984.1807. Anal. Calcd for C52H33F6IrN5PS2: C, 55.31; H, 2.95; N, 6.20; S, 5.68; experimental: C, 55.18; H, 3.13; N, 6.19; S, 5.73. [(N∧C-4)2Ir(∧N-A)]PF6 (4A). Needle orange crystals were obtained by slow gas phase diffusion of diethyl ester into the concentrated acetone solution at room temperature (72%). 1 H NMR ((CD3)2CO, 400 MHz): δ 9.29 (d, J = 8.7 Hz, 1H), 9.20 (d, J = 6.6 Hz, 1H), 8.75 (d, J = 8.5 Hz, 1H), 8.46−8.25 (m, 5H), 8.22 (d, J = 8.1 Hz, 1H), 8.09 (dd, J = 5.8, 0.7 Hz, 1H), 8.07−7.98 (m, 3H), 7.93 (dd, J = 5.8, 0.8 Hz, 1H), 7.86−7.70 (m, 8H), 7.66 (d, J = 7.8 Hz, 1H), 7.43−7.31 (m, 3H), 7.28−7.11 (m, 4H), 7.11−7.00 (m, 2H), 6.83 (ddd, J = 8.2, 7.1, 0.8 Hz, 1H). HRESI-MS: exp 984.1798 [M+]; calcd 984,1807. Anal. Calcd for C52H33F6IrN5PS2: C, 55.31; H, 2.95; N, 6.20; S, 5.68; experimental: C, 55.23; H, 3.16; N, 6.17; S, 5.78. [(N∧C-4)2Ir(N∧N-B)]PF6 (4B). Needle orange crystals were obtained by layering heptane over a concentrated dichloromethane solution (76%). 1 H NMR ((CD3)2CO, 400 MHz): δ 9.33 (d, J = 9.3 Hz, 1H), 9.12 (d, J = 6.6 Hz, 1H), 8.78 (d, J = 8.5 Hz, 1H), 8.48−8.33 (m, 4H), 8.30 (d, J = 8.1 Hz, 1H), 8.22 (d, J = 8.2 Hz, 1H), 8.16−7.99 (m J = 18.4, 15.0, 7.9 Hz, 8H), 7.97 (dd, J = 5.8, 0.9 Hz, 1H), 7.83 (ddd, J = 7.1, 6.7, 1.3 Hz, 1H), 7.80−7.70 (m, 2H), 7.67 (d, J = 7.7 Hz, 1H), 7.45− 7.32 (m, 3H), 7.28 (ddd, J = 7.5, 5.6, 1.4 Hz, 1H), 7.24−7.11 (m, 3H), 7.11−7.01 (m, 2H), 6.87 (ddd, J = 8.5, 7.1, 0.9 Hz, 1H). HRESI-MS: exp 1052.1638 [M + ]; calcd 1052.1680. Anal. Calcd for C53H32F9IrN5PS2: C, 53.17; H, 2.69; N, 5.85; S, 5.36; experimental: C, 52.81; H, 2.75; N, 5.90; S, 5.43. [(N∧C-4)2Ir(N∧N-C)]PF6 (4C). Needle orange crystals were obtained by layering heptane over a concentrated dichloromethane solution (74%). 1 H NMR ((CD3)2CO, 400 MHz): δ 9.34 (d, J = 6.8 Hz, 1H), 9.29 (d, J = 8.7 Hz, 1H), 8.74 (d, J = 8.5 Hz, 1H), 8.41 (d, J = 8.2 Hz, 1H), 8.39−8.26 (m, 4H), 8.22 (d, J = 8.1 Hz, 1H), 8.08 (d, J = 5.2 Hz, 1H), 8.06−7.98 (m, 3H), 7.90 (dd, J = 5.8, 0.8 Hz, 1H), 7.83 (ddd, J = 7.1, 6.8, 1.3 Hz, 1H), 7.76 (ddd, J = 8.2, 7.4, 1.5 Hz, 1H), 7.73 (d, J = 8.3 Hz, 1H), 7.70−7.58 (m, 3H), 7.44−7.29 (m, 5H), 7.24 (ddd, J = 7.4, 5.7, 1.3 Hz, 1H), 7.22−7.10 (m, 4H), 7.06 (ddd, J = 7.3, 6.0, 1.2 Hz, 1H), 6.82 (ddd, J = 8.2, 7.2, 0.9 Hz, 1H), 4.01 (s, 3H). HRESI-MS: exp 1014.1946 [M+]; calcd 1014.1912. [(N∧C-5)2Ir(∧N-A)]PF6 (5A). Deep red crystals were obtained by slow evaporation of CH2Cl2/n-heptane solution at room temperature (83%). 1 H NMR ((CD3)2CO, 400 MHz): δ 10.23 (d, J = 8.9 Hz, 1H), 9.40 (d, J = 8.8 Hz, 1H), 9.10 (s, 1H), 8.84 (d, J = 8.4 Hz, 1H), 8.74 (d, J = 6.7 Hz, 1H), 8.71 (dd, J = 8.5, 1.2 Hz, 1H), 8.47−8.33 (m, 3H), 8.09

Φs = Φr

ηs2A r Is ηr2A sIr

where Φs is the quantum yield of the sample, Φr is the quantum yield of the reference, η is the refractive index of the solvent, As and Ar are the absorbance of the sample and the reference at the wavelength of excitation, respectively, and Is and Ir are the integrated areas of emission bands. Fluorescence lifetimes were determined by the TCSPC (time-correlated single photon counting) method with a Fluorolog-3 (JY Horiba Inc.). A digital oscilloscope Tektronix TDS3014B (Tektronix, bandwidth 100 MHz), monochromator 6855

DOI: 10.1021/acs.inorgchem.8b00390 Inorg. Chem. 2018, 57, 6853−6864

Article

Inorganic Chemistry Scheme 1. Synthesis of Diimine Ligandsa

(i) Et3N, DCM, r.t., 2 days; (ii) KOH(Aq.), r.t., 12 h; (iii) K2CO3, BnBr, CH3CN, r.t., 2 h; (iv) K2CO3, LiCl, TBAB, Pd(OAc)2, DMF, 60 °C, 30 h; (v) AlCl3, benzene, reflux, 7 h; (vi) AIBN, TTMSS, CH3CN, reflux 24 h. a

Scheme 2. Synthesis of Iridium Complexes

■

MUM (LOMO, interval of wavelengths 10 nm), and photomultiplier tube Hamamatsu were used for the phosphorescence lifetime measurements. The lifetime data fit has been done using the JobinYvon software package and the Origin 8.1 program. Computational Details. All models were fully optimized with the Gaussian 09 program package41 at the DFT level of theory. The hybrid density functional PBE042 was utilized together with the basis set consisting of the def2-TZVPPD43 effective core potential basis set with triple-ζ-valence basis set with two sets of polarization and diffuse basis functions for Ir atoms, and the standard all-electron basis set 6-31G(d) for all other atoms. Absorption characteristics were investigated via simulating the singlet−singlet excitations with the TD-DFT method using the same density functional than with the optimizations. Solvent effects of dichloromethane were included into the simulations with the CPCM reaction field model. Emission properties were studied by optimizing the corresponding models in the triplet state and studying the changes in the appearance of the frontier molecular orbitals. Emission wavelengths were calculated by the total energy difference of the molecules in T1 and S0 electronic states in solution, which gave a reasonable estimation of the emission energetics.

RESULTS AND DISCUSSION

Synthesis. A new series of diimine (N∧N) chelating ligands based on {pyrido[2,1-a]pyrrolo[3,2-c]isoquinoline} heterocyclic system were prepared to use in the synthesis of iridium(III) bis-cyclometalated complexes [Ir(N∧C)2(N∧N)]+. Unsubstituted 3-phenyl-2-(pyridin-2-yl)pyrido[2,1-a]pyrrolo[3,2-c]isoquinoline (N∧N-A) was synthesized from 1-(2-(2-bromophenyl)-4-phenyl-5-(pyridin-2-yl)-1H-pyrrol-3-yl)pyridin-1ium bromide through four-step procedure including as a key stage Pd(II)-catalyzed direct intramolecular arylation 26 (Scheme 1, left). Trifluoromethyl-substituted (N∧N-B) and methoxy-substituted (N∧N-C) ligands were prepared by a modified procedure, which gave pyrido[2,1-a]pyrrolo[3,2c]isoquinolines directly from 1-(2-(2-bromophenyl)-4-phenyl5-(pyridin-2-yl)-1H-pyrrol-3-yl)pyridin-1-ium bromides under free radical conditions employing AIBN/TTMSS system44 (Scheme 1 right). The ligands and all the intermediate compounds were fully characterized using mass-spectrometry and 1H and 13C NMR spectroscopy (see pages S10−S37 in the Supporting 6856

DOI: 10.1021/acs.inorgchem.8b00390 Inorg. Chem. 2018, 57, 6853−6864

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Inorganic Chemistry

Figure 1. Perspective drawings of 1A, 2A, 4B, and 5A cations showing thermal ellipsoids at the 40% probability level with the atomic coloring list and schematic numbering of atoms coordinated to iridium(III) center. Hydrogen atoms are omitted for clarity.

Information). The data obtained are completely compatible with the structural patterns shown in Scheme 1. Interaction of these ligands with biscyclometalated bis(μchlorido) bridged iridium dimers in DCM/MeOH mixture (reflux for several hours) followed by the metathesis of chloride anion to PF6 led to formation of heteroleptic cationic iridium complexes [(N∧C)2Ir(N∧N)]PF6 in high yields (Scheme 2). X-ray Crystallographic Study. The solid-state structures of 1A, 2A, 4B, and 5A were determined by single-crystal XRD studies. Crystallographic data are summarized in Table S1; molecular structures of the central cationic fragments with the atom numbering scheme are shown in Figure 1. Structural parameters are given in Tables S2−S15; and selected bond lengths and angles are given in Table 1. Similar to related bis(cyclometalated) iridium(III) polypyridine complexes,23,45−48 the iridium(III) center of 1A, 2A, and 4B adopted a distorted octahedral geometry with the trans- angles at the metal center ranging from 170.11 to 176.69°. The Ir−C bonds are in cis-orientation and the bond lengths fall in the interval (1.969−2.029 Å) typical for this type of complexes. The Ir−N bonds for N∧N ligands are in trans-position relative to the carbon atoms of metalating ligands, which give slightly larger (2.124−2.200 Å) Ir−N bond distances compared to those to the nitrogen atoms of the N∧C ligands (2.034−2.057 Å) due to the well-known trans-effect of strong donating C− coordination center.23,45−48 In agreement with literature data15−19 the bite angles of the N∧C ligands (78.04−80.74°) exceed those of the N∧N ligand (75.81−76.33°). Complex 5A demonstrates significant deformation of octahedral environment at iridium center. This is in contrast to the other compounds of this series that is obviously due to intramolecular π−π interaction between aromatic systems of N∧N and N∧C ligands (the interplane distances are less than 3.3 Å), which also led to considerable reduction of the N2−Ir−

Table 1. Selected Bond Lengths for 1A, 2A, 4B, and 5A 1A Ir−N1 Ir−N2 Ir−C1 Ir−C2 Ir−N3 Ir−N4 C1−Ir−N4 C2−Ir−N3 N1−Ir−N2 N1−Ir−N3 N1−Ir−N4 N2−Ir−N3 N2−Ir−N4 N1−Ir−C1 N2−Ir−C1 N1−Ir−C2 N2−Ir−C2 N3−Ir−N4 C1−Ir−C2 C1−Ir−N3 C2−Ir−N4

2.034 2.046 2.014 2.029 2.155 2.200 170.11 176.24 176.69 85.43 96.34 97.87 84.71 80.74 98.75 96.66 80.03 75.81 82.79 94.50 106.99

2A

4B

Bond Length (Å) 2.034 2.057; 2.059 2.055; 2.002 1.969; 2.026 2.007; 2.140 2.130; 2.198 2.178; Angles (deg) 170.57 169.73; 174.12 173.63; 175.18 172.63; 90.67 90.83; 96.42 97.73; 94.14 95.13; 84.97 88.13; 80.59 79.14; 98.75 95.94; 94.94 95.44; 80.24 78.63; 75.99 76.23; 84.21 84.74; 95.03 95.06; 105.02 104.30;

2.054 2.056 1.970 1.990 2.124 2.177 170.73 172.23 172.53 91.13 96.03 94.83 90.13 79.04 95.64 95.94 78.04 76.33 83.14 94.63 106.44

5A 2.098 2.078 1.993 2.015 2.150 2.207 171.18 174.83 173.91 87.16 106.11 98.43 77.88 80.07 96.54 95.25 79.39 74.89 85.45 99.49 100.03

N4 (77.88°) and increase in the N1−Ir−N4 (106.11°) angles, shown in Figure 2. Other short intramolecular contacts in the molecule of 5A are also found: ortho-H of phenyl substituent in the N∧N ligand is additionally linked by C−H-π interaction (2.8 Å) with the quinoline fragment of N∧C ligand; α-H of quinoline fragment in the second N∧C ligand demonstrates the C−H···N hydrogen bonding (2.1 Å) with the pyrrole nitrogen of the N∧N ligand (Figure 2, left). 6857

DOI: 10.1021/acs.inorgchem.8b00390 Inorg. Chem. 2018, 57, 6853−6864

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Inorganic Chemistry

Figure 2. Two projections of the solid state structure of 5A showing short intramolecular contacts (left) and spatially close protons (right).

Figure 3. Overlay of the 1H−1H COSY (gray color)/NOESY(+) (blue and orange color) spectra of 5A in aromatic area with signal assignment; acetone-d6, 298 K. The NOESY cross-peaks, which are not related to adjacent aromatic protons, are marked with dashed-line circles.

Relative intensity and multiplicity of the signals fit well the molecular structures found in the solid state that is indicative of retention of these structural patterns in solution. The solid state structure of 5A features specific intramolecular interactions (see above) that prompted us to check their preservation in fluid media using full assignment of the ligand protons with the 1H−1H COSY spectrum and additional information revealed from the 1H−1H NOESY NMR spectrum (Figure 3).

Spectroscopic Properties. All complexes obtained were carefully characterized using appropriate spectroscopic techniques. The HR-ESI+ mass spectra demonstrate the dominant signals of singly charged cations with m/z values and isotopic patterns matching well the calculated ones for the corresponding molecular ions [(N∧C)2Ir(NN)]+ (Figures S1−S8). The 1H NMR spectra of all the complexes display three sets of signals corresponding to the N∧N-ligand protons and to the protons of inequivalent N∧C ligands (Figures S9−S16). 6858

DOI: 10.1021/acs.inorgchem.8b00390 Inorg. Chem. 2018, 57, 6853−6864

Article

Inorganic Chemistry Table 2. Photophysical Properties of Diimine Ligands in Solutiona ligand

solventb

N∧N-A

MeOH DCE MeOH DCE MeOH DCE

N∧N-B N∧N-C a

absorbance, λmax (nm) (ε × 10−4, M−1 cm−1) 303 290 301 292 303 289

(46.3), 326sh(19.0), 420(18.09) (52.5), 331 (43.9), 407 (15.7), 500 (14.8) (45.1), 325sh (16.6), 416 (17.6) (41.8), 329 (38.9), 404 (12.5), 499 (13.5) (44.5), 327sh (19.7), 422 (17.2) (45.92), 333 (40.78), 410 (14.23), 504 (13.33)

excitation λmax (nm) 303, 332, 303, 330, 302, 335,

325sh, 422 405, 502 327sh, 414 405, 498 328sh, 424 412, 506

emission λmax (nm)

QY

τ (ns)

513 630 508 626 537 656

0.95 0.02 0.84 0.08 0.06