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Luminescent Rhenium(I) Pyridyldiaminocarbene Complexes: Photophysics, Anion-Binding, and CO2‑Capturing Properties Chi-On Ng, Shun-Cheung Cheng, Wing-Kin Chu, Kin-Man Tang, Shek-Man Yiu, and Chi-Chiu Ko* Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China S Supporting Information *
ABSTRACT: A series of luminescent isocyanorhenium(I) complexes with chelating acyclic diaminocarbene ligands (N^C) has been synthesized and characterized. Two of these carbene complexes have also been structurally characterized by X-ray crystallography. These complexes show blue-to-red phosphorescence, with the emission maxima not only considerably varied with a change in the number of ancillary isocyanide ligands but also extremely sensitive to the electronic and steric nature of the substituents on the acyclic diaminocarbene ligand. A detailed study with the support of density functional theory calculations revealed that the lowestenergy absorption and phosphorescence of these complexes in a degassed CH2Cl2 solution are derived from the predominantly metal-to-ligand charge-transfer [dπ(Re) → π*(N^C)] excited state. The unprecedented anion-binding and CO2-capturing properties of the acyclic diaminocarbene have also been described.
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INTRODUCTION The unique and diverse electronic properties of N-heterocyclic carbene (NHC) ligands and the important catalytic applications of their transition-metal complexes have led to an extensive development and investigation of this family of complexes over the past 2 decades.1 On the basis of their unique electronic properties, NHC ligands have recently been used for the design of luminescent transition metal−NHC complexes2 for different applications including emissive materials for efficient organic light-emitting devices.3 We have also investigated the phosphorescent properties of a series of rhenium(I) diimine complexes with NHC ligands, which are converted from the highly luminescent isocyanorhenium(I) diimine complexes4 by reactions with suitable nucleophiles.5 While the NHC ligands in most of the reported luminescent transition metal−NHC complexes2,5 are spectator ligands, which can only alter or modify the emission properties through perturbation of the metal-centered orbitals by changing the metal−ligand interactions. Designs of carbene-containing luminescent transitionmetal complexes with an emissive excited state directly involving the orbitals of the carbene ligands are scarce.6 On the other hand, studies of luminescent carbene-containing transition-metal complexes have mainly been focused on those with cyclic carbene ligands, whereas corresponding studies on acyclic carbene complexes have been much less investigated. It is believed that the open acyclic carbene structure would only not lead to a much stronger sensitivity to the steric effects of the substituents due to the flexibility of the bond angle at the carbene carbon atom; however, it would also be significantly affected by changes of the microenvironment.1j These effects would render the properties of the acyclic carbene complexes © XXXX American Chemical Society
highly sensitive to changes of the microenvironment. In this study, we reported a series of luminescent isocyanorhenium(I) complexes with various bidentate acyclic diaminocarbene ligands (N^C). The photophysical, electrochemical, anionbinding, and carbon dioxide (CO2)-capturing properties of these complexes have also been described.
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EXPERIMENTAL SECTION
Materials and Reagents. [Re(CO)5Br] was obtained from Strem Chemicals, Inc. Dimethyl sulfate, silver(I) triflate, and thallium(I) triflate were obtained from Aldrich Chemical Company. Triethylamine, phosphorus(V) oxychloride (POCl3), and methanoic acid were purchased from Uni-Chem Reagent Chemical Company. 2-Chloroaniline, 2-aminopyridine, 2-aminothiazole, and 2-aminobenzothiazole were obtained from Acros Chemical Company. 4-Chlorophenyl isocyanide (4-ClC6H4NC), 2,6-dimethylphenyl isocyanide [2,6(CH 3 ) 2 C 6 H 3 NC], 2,6-diisopropylphenyl isocyanide {2,6[(CH3)2CH]2C6H3NC}, and 4-methylphenyl isocyanide [4-(CH3)CH6H4NC] were prepared based on literature procedures.7 fac[Re(CO)3(CNR)2Br], fac-[Re(CO)3(CNC6H4Cl-4)3](CF3SO3), and [Re(CNR)5I] were prepared by modified synthetic procedures reported for related rhenium complexes.4c Physical Measurements and Instrumentation. Photosubstitution reactions were performed in the previously described photoreactor.4b A Pen-Ray lamp (11SC-1; λ = 254 nm) was used as the excitation light source for the photoreaction. 1H and 13C NMR spectra were recorded on a Bruker AV600 NMR spectrometer with chemical shifts (δ, ppm) calibrated using tetramethylsilane (Me4Si). Fourier transform infrared (FTIR) spectra were recorded on a PerkinElmer Spectrum 100 FTIR spectrophotometer. All electrospray ionization Received: April 23, 2016
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DOI: 10.1021/acs.inorgchem.6b01017 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
metathesis reaction with ammonium hexafluorophosphate in methanol. Subsequent recrystallization from acetone−ether gave a paleyellow crystalline solid. Yield: 57.3 mg, 73.1 μmol, 45.7%. 1H NMR (600 MHz, CDCl3): δ 10.52 (1H, s, NH), 9.75 (1H, s, NH), 8.52 (1H, d, J = 5.3 Hz, 6-pyridyl H), 7.93 (1H, td, J = 7.2 and 1.3 Hz, 4-pyridyl H), 7.45 (5H, m, phenyl H and 3-pyridyl H), 7.35 (2H, d, J = 8.7 Hz, phenyl H), 7.30 (2H, d, J = 8.5 Hz, phenyl H), 7.11 (1H, t, J = 6.4 Hz, 5-pyridyl H). 13C NMR (151 MHz, CDCl3): δ 212.7, 192.1, 188.0, 187.0, 158.5, 151.5, 141.5, 138. 8, 137.1, 135.4, 130.2, 130.1, 128.5, 128.2, 120.6, 113.1, 107.6, 106.4. Positive-ion ESI-MS: m/z 639.5 ([M − PF6]+). IR (KBr disk; cm−1): 2168 ν(CN), 2033, 1967, 1940 ν(CO), 847 ν(P−F). Anal. Calcd for C22H14Cl2N4O3RePF6 (784.45): C, 33.68; H, 1.80; N, 7.14. Found: C, 33.87; H, 1.99; N, 7.42. fac-{Re(CO)3(CNC6H4CH3-4)(N^C2)}(PF6) (2). The complex was synthesized following the same procedure as that described for 1, except fac-[Re(CO)3(CNC6H4(CH3)-4)2(Br)] (93.5 mg, 160 μmol) was used instead of [Re(CO)3(CNC6H4Cl-4)2(Br)]. Yield: 60.0 mg, 76.4 μmol, 47.8%. 1H NMR (600 MHz, CDCl3): δ 10.25 (1H, s, NH), 9.53 (1H, s, NH), 8.52 (1H, d, J = 5.6 Hz, 6-pyridyl H), 7.81 (1H, td, J = 7.3 and 1.5 Hz, 4-pyridyl H), 7.44 (1H, d, J = 8.4 Hz, 3-pyridyl H), 7.24 (8H, m, phenyl H), 7.10 (1H, t, J = 6.1 Hz, 5-pyridyl H), 2.40 (3H, s, CH3), 2.40 (3H, s, CH3). 13C NMR (151 MHz, CDCl3): δ 212.6, 192.6, 187.9, 187.5, 158.4, 151.6, 141.6, 141.0, 139.5, 137.8, 130.4, 130.4, 126.9, 126.6, 120.5, 112.8, 107.6, 106.4, 21.4, 21.1. Positive-ion ESI-MS: m/z 599.6 ([M − PF6]+). IR (KBr disk; cm−1): 2168 ν(CN), 2028, 1958, 1937 ν(CO), 836 ν(P−F). Anal. Calcd for C24H20N4O3RePF6·0.5(CH3)2CO (772.65): C, 39.64; H, 3.00; N, 7.25. Found: C, 39.23; H, 3.25; N, 7.67. fac-{Re(CO)3(CNC6H4Cl-4)(N^C3)}(PF6) (3). The complex was synthesized following the same procedure as that described for 1, except 2-amino-1,3-thiazole (16.0 mg, 160 μmol) was used instead of 2-aminopyridine. Yield: 81.2 mg, 103 μmol, 64.7%. 1H NMR (600 MHz, CDCl3): δ 7.81 (2H, d, J = 8.0 Hz, phenyl H), 7.55 (1H, s, NH), 7.47 (1H, s, NH), 7.36 (2H, d, J = 8.0 Hz, phenyl H), 7.32 (2H, d, J = 8.1 Hz, phenyl H), 7.29 (1H, d, J = 3.5 Hz, 5-thiazolyl H), 7.19 (2H, d, J = 8.1 Hz, phenyl H), 6.75 (1H, d, J = 3.5 Hz, thiazolyl H). 13C NMR (151 MHz, CDCl3): δ 214.7, 194.8, 193.4, 189.4, 189.3, 161.2, 150.7, 138.9, 135.8, 130.0, 129.9, 128.8, 128.2, 127.8, 122.7, 111.5. Positiveion ESI-MS: m/z 645.5 ([M − PF6]+). IR (KBr disk, cm−1): 2154 ν(CN), 2012, 1945, 1905 ν(CO), 847 ν(P−F). Anal. Calcd for C20H12Cl2N4O3SRePF6 (790.48): C, 30.39; H, 1.53; N, 7.09. Found: C, 30.51; H, 1.63; N, 6.89. fac-{Re(CO)3(CNC6H4Cl-4)(N^C4)}(PF6) (4). The complex was synthesized following the same procedure as that described for 1, except 2-amino-1,3-benzothiazole (24.0 mg, 160 μmol) was used instead of 2-aminopyridine. Yield: 83.6 mg, 107 μmol, 66.6%. 1H NMR (600 MHz, CDCl3): δ 7.86 (3H, m, two phenyl Hs and NH), 7.70 (1H, d, J = 8.1 Hz, 7-benzothiazolyl H), 7.67 (1H, d, J = 7.8 Hz, 4-benzothiazolyl H), 7.47 (1H, t, J = 8.1 Hz, 6-benzothiazolyl H), 7.36 (3H, m, two phenyl Hs and NH), 7.32 (2H, d, J = 8.8 Hz, phenyl H), 7.27 (1H, t, J = 7.8 Hz, 5-benzothiazolyl H), 7.14 (2H, d, J = 8.8 Hz, phenyl H). 13C NMR (151 MHz, CDCl3): δ 217.4, 194.4, 193.6, 188.8, 188.1, 149.6, 138.7, 135.9, 131.7, 130.0, 129.9, 129.5, 128.9, 127.8, 126.8, 125.1, 123.2, 123.2, 122.2, 119.7. Positive-ion ESI-MS: m/z 598.6 ([M − PF6]+). IR (KBr disk, cm−1): 2157 ν(CN), 2015, 1948, 1915 ν( CO), 852 ν(P−F). Anal. Calcd for C24H14Cl2N4O3SRePF6 (840.54): C, 34.29; H, 1.68; N, 6.67. Found: C, 34.48; H, 1.91; N, 6.43. cis,cis-{Re(CO)2(CNC6H4Cl-4)2(N^C1)}(PF6) (5). A 50 mL THF solution of fac-[Re(CO)3(CNC6H4Cl-4)3](CF3SO3) (150 mg, 180 μmol) and 2-aminopyridine (22.1 mg, 54.1 μmol) was heated to 80 °C. It was irradiated with a Pen-Ray mercury lamp (11SC-1, 254 nm) in a water-cooled quartz jacket for 3 h.4b,c,17 After the solvent was evaporated by rotary evaporation, it was purified by column chromatography (silica gel, 9:1 dichloromethane−ethyl acetate). The PF6− salt was obtained by the metathesis reaction with ammonium hexafluorophosphate in methanol. Subsequent recrystallization from acetone−ether gave a pale-yellow crystalline solid. Yield: 42.4 mg, 54.1 μmol, 30.0%. 1H NMR (600 MHz, CDCl3): δ 10.32 (1H, s, NH), 9.51
mass spectrometry (ESI-MS) spectra were obtained by using a single quadrupole mass spectrometer (PE-SCIEX API 150 EX). The CHN analyses of all newly synthesized compounds were done on an Elementar vario MICRO cube elemental analyzer. The UV−vis absorption spectra were obtained by using a Hewlett-Packard 8452A diode-array spectrophotometer. Steady-state emission and excitation spectra were measured on a SPEX FluoroLog 3-TCSPC spectrofluorometer. The solutions for emission measurements were degassed with four successive freeze−pump−thaw cycles. Emission studies of the samples in a 77 K EtOH−MeOH (4:1, v/v) glassy medium were performed with diluted sample solutions in a quartz tube immersed in liquid nitrogen in a quartz optical dewar. The emission lifetimes were recorded on a Edinburgh Instrument Laser Flash Photolysis LP920 spectrometer with 355 nm output (third harmonic, ∼8 ns) of a Spectra-Physics Quanta-Ray Q-switched LAB-150 pulsed Nd:YAG laser as the excitation source. The emission lifetimes (τ) were determined by single-exponential fitting of the emission intensity decay traces according to the equation I(t) = I0 exp(−t/τ), where I(t) and I0 refer to the emission intensity at times t and 0, respectively. Luminescence quantum yields were measured by the optically dilute method8 using [Ru(bpy)3]Cl2 in aqueous solution (ϕem = 0.049 with 436 nm excitation) or quinine sulfate in aqueous sulfuric acid (0.5 M) as the standard solution. Cyclic voltammetry measurements were performed in 0.1 M nBu4NPF6−acetonitrile solutions at room temperature and recorded on a CHI 620 electrochemical analyzer (CH Instruments, Inc.), using ferrocene (FeCp2) as the internal reference. A glassy-carbon working electrode (3 mm diameter, CH Instruments, Inc.), a platinum wire counter electrode, and a Ag/ AgNO3 (10 mM in acetonitrile) reference electrode (CH Instruments, Inc.) were used. All solutions for electrochemical measurements were degassed by argon. The crystal structures were determined on an Oxford Diffraction Gemini S Ultra single-crystal X-ray diffractometer with graphitemonochromatized Cu Kα (λ = 1.54184 Å) or Mo Kα (λ = 0.7107 Å) radiation. Direct methods in the SHELXL-97 program10 on a PC were used to solve the structures. Using direct methods, Re and many nonH atoms were located. Other non-H atoms were found and located after successful refinement by full-matrix least squares with the SHELXL-97 program10 on a PC. In the final stage of least-squares refinement, all non-H atoms were anisotropically refined. The positions of the H atoms were calculated based on the riding mode with thermal parameters equal to 1.2 times that of the associated C atoms and participated in the calculation of the final R indices. Computational Details. The ground-state (S0) structures of selected complexes (1, 5, and 6) were optimized using density functional theory (DFT) with the m06 global hybrid meta-GGA functional.11 The Los Alamos National Laboratory (LANL2DZ) effective core potential basis sets were used for the Re atom,12 and the 6-311+G(d,p) basis sets were used for nonmetal atoms.13 Vibrational analyses were performed to ensure that all optimized structures are situated at a minimum, and population analyses were done to find the fractional contributions of different atoms to the molecular orbitals. On the basis of the optimized structures of the S0 state, timedependent DFT (TD-DFT)14 calculations were done to calculate the vertical excitation transition energies for the predicted absorption spectra. The solvent effect of dichloromethane (CH2Cl2) was taken into account by the polarized continuum model with integral equation formalism.15 All calculations were done with the Gaussian 09 package of programs.16 Synthesis. All manipulations of the reactions were carried out under an anhydrous argon atmosphere using standard Schlenk techniques. fac-{Re(CO)3(CNC6H4Cl-4)(N^C1)}(PF6) (1). A solution of fac[Re(CO)3(CNC6H4Cl-4)2(Br)] (100 mg, 160 μmol) and AgOTf (45.2 mg, 176 μmol) in tetrahydrofuran (THF; 30 mL) was refluxed for 18 h. After cooling to room temperature, the solution was filtered and 2-aminopyridine (15.1 mg, 160 μmol) was added. The mixture was then refluxed for an additional 18 h. After removal of the solvent, it was further purified by column chromatography (silica gel, 95:5 dichloromethane−ethyl acetate). The PF6− salt was obtained by a B
DOI: 10.1021/acs.inorgchem.6b01017 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Anal. Calcd for C38H24Cl5N7SRePF6·0.5(CH3)2CO (1148.18): C, 41.32; H, 2.37; N, 8.54. Found: C, 41.31; H, 2.55; N, 8.72. {Re(CNC6H4Cl-4)4(N^C4)}(PF6) (9). The complex was synthesized following the same procedure as that described for 6, except 2-amino1,3-benzothiazole (18.0 mg, 120 μmol) was used instead of 2aminopyridine. Yield: 58.2 mg, 49.8 μmol, 49.7%. 1H NMR (600 MHz, CDCl3): δ 12.52 (1H, s, NH), 10.86 (1H, s, NH), 8.18 (1H, d, J = 8.2 Hz, 7-benzothiazolyl H), 7.82 (1H, d, J = 8.1 Hz, 4benzothiazolyl H), 7.48 (1H, t, J = 8.1 Hz, 5-benzothiazolyl H), 7.46 (2H, d, J = 8.6 Hz, phenyl H), 7.42 (3H, m, two phenyl H and 6benzothiazolyl H), 7.35 (4H, d, J = 8.7 Hz, phenyl H), 7.29 (2H, d, J = 8.6 Hz, phenyl H), 7.20 (2H, d, J = 8.7 Hz, phenyl H), 7.18 (2H, d, J = 8.6 Hz, phenyl H), 7.14 (4H, d, J = 8.7 Hz, phenyl H), 6.66 (2H, d, J = 8.6 Hz, phenyl H). 13C NMR (151 MHz, CDCl3): δ 219.8, 175.2, 167.1, 157.7, 149.7, 138.8, 134.3, 133.7, 133.2, 131.3, 130.8, 130.7, 130.0, 129.9, 129.7, 129.6, 129.3, 129.2, 128.8, 128.0, 127.8, 127.7, 127.6, 127.4, 127.2, 126.8, 126.7, 125.9, 125.0, 122.7, 120.4. Positiveion ESI-MS: m/z 1024.3 ([M − PF6]+). IR (KBr disk, cm−1): 2131, 2052, 2005, 1945 ν(CN), 836 ν(P−F). Anal. Calcd for C42H26Cl5N7SRePF6 (1169.20): C, 43.14; H, 2.24; N, 8.39. Found: C, 43.41; H, 2.52; N, 8.27. {Re(CNC6H4(CH3)-4)5(pyNH2)}(PF6) (10). The complex was synthesized following the same procedure as that described for 6, except [Re(CNC6H4(CH3)-4)5(I)] (90.0 mg, 100 μmol) was used instead of [Re(CNC6H4Cl-4)5(I)]. Yield: 69.0 mg, 68.3 μmol, 68.1%. 1H NMR (600 MHz, CDCl3): δ 8.05 (1H, d, J = 4.4 Hz, 6-pyridyl H), 7.42 (1H, t, J = 7.7 Hz, 4-pyridyl H), 7.32 (8H, d, J = 8.3 Hz, phenyl H), 7.30 (2H, d, J = 8.3 Hz, phenyl H), 7.25 (8H, d, J = 8.3 Hz, phenyl H), 7.21 (2H, d, J = 8.3 Hz, phenyl H), 6.63 (1H, t, J = 5.7 Hz, 5-pyridyl H), 6.55 (1H, d, J = 8.3 Hz, 3-pyridyl H), 4.54 (2H, s, NH2), 2.40 (12H, s, CH3), 2.38 (3H, s, CH3). 13C NMR (151 MHz, CDCl3): δ 158.5, 153.8, 147.9, 139.3, 138.9, 137.7, 130.2, 130.1, 129.9, 126.2, 125.4, 114.4, 113.8, 113.1, 112.6, 108.8, 107.8, 21.3, 21.3. Positive-ion ESIMS: m/z 866.3 ([M − PF6]+). IR (KBr disk, cm−1): 2184, 2079, 2036 ν(CN), 832 ν(P−F). Anal. Calcd for C45H41N7RePF6 (1011.03): C, 53.46; H, 4.09; N, 9.70. Found: C, 53.54; H, 4.31; N, 9.52. {Re(CNC6H3(CH3)2-2,6)5(pyNH2)}(PF6) (11). The complex was synthesized following the same procedure as that described for 6, except [Re(CNC6H3(CH3)2-2,6)5(I)] (97.0 mg, 100 μmol) was used instead of [Re(CNC6H4Cl-4)5(I)]. Yield: 46.4 mg, 59.2 μmol, 59.1%. 1 H NMR (600 MHz, CDCl3): δ 8.74 (1H, d, J = 6.1 Hz, 6-pyridyl H), 7.50 (1H, dd, J = 8.6 and 6.1 Hz, 4-pyridyl H), 7.16 (4H, t, J = 7.5 Hz, phenyl H), 7.12 (8H, d, J = 7.5 Hz, phenyl H), 7.07 (3H, m, 2 phenyl H and 3-pyridyl H), 7.04 (1H, t, J = 8.6 Hz, phenyl H), 6.44 (1H, dd, J = 8.6, 6.1 Hz, 5-pyridyl H), 6.12 (2H, s, NH2), 2.42 (24H, s, CH3), 2.41 (6H, s, CH3). 13C NMR (151 MHz, CDCl3): δ 161.7, 161.2, 158.7, 154.4, 138.5, 134.8, 133.7, 130.6, 128.1, 127.8, 126.1, 113.4, 112.2, 19.0, 18.8. Positive-ion ESI-MS: m/z 866.3 ([M − PF6]+). IR (KBr disk, cm−1): 2153, 2050, 1997 ν(CN), 842 ν(P−F). Anal. Calcd for C50H51N7RePF6 (1081.16): C, 55.55; H, 4.75; N, 9.07. Found: C, 55.53; H, 4.61; N, 9.26. {Re(CNC6H3(CH(CH3)2)2-2,6)5(pyNH2)}(PF6) (12). The complex was synthesized following the same procedure as that described for 6, except [Re(CNC6H3(CH(CH3)2)2-2,6)5(I)] (125 mg, 100 μmol) was used instead of [Re(CNC6H4Cl-4)5(I)]. Yield: 41.2 mg, 52.3 μmol, 52.4%. 1H NMR (600 MHz, CDCl3): δ 8.81 (1H, d, J = 6.1 Hz, 6pyridyl H), 7.54 (1H, t, J = 7.7 Hz, 4-pyridyl H), 7.31 (4H, t, J = 7.8 Hz, phenyl H), 7.18 (10H, m, 9 phenyl H and 3-pyridyl H), 7.12 (2H, d, J = 7.7 Hz, phenyl H), 6.41 (1H, t, J = 6.0 Hz, 5-pyridyl H), 6.15 (2H, s, NH2), 3.33 (10H, m, CH(CH3)2), 1.13 (48H, d, J = 6.9 Hz, CH3), 1.06 (12H, d, J = 6.8 Hz, CH3). 13C NMR (151 MHz, CDCl3): δ 161.4, 158.8, 158.2, 154.4, 144.9, 143.6, 138.5, 128.8, 128.5, 127.9, 127.2, 126.8, 125.7, 123.5, 123.1, 30.1, 29.9, 22.6, 22.5. Positive-ion ESI-MS: m/z 866.3 ([M − PF6]+). IR (KBr disk, cm−1): 2157, 2050, 1983 ν(CN), 842 ν(P−F). Anal. Calcd for C70H91N7RePF6 (1361.69): C, 61.74; H, 6.74; N, 7.20. Found: C, 61.82; H, 6.70; N, 7.29.
(1H, s, NH), 8.58 (1H, d, J = 5.0 Hz, 6-pyridyl H), 7.87 (1H, td, J = 7.0 and 1.5 Hz, 4-pyridyl H), 7.52 (1H, d, J = 8.8 Hz, 3-pyridyl H), 7.41 (6H, m, phenyl H), 7.30 (2H, m, phenyl H), 7.26 (4H, m, phenyl H), 7.05 (1H, td, J = 5.9 and 0.9 Hz, 5-pyridyl H). 13C NMR (151 MHz, CDCl3): δ 215.7, 195.4, 191.8, 159.2, 159.0, 152.1, 151.2, 140.4, 135.5, 130.0, 129.8, 129.6, 129.4, 129.1, 128.2, 127.7, 126.4, 125.9, 121.2, 120.3, 119.5, 112.7. Positive-ion ESI-MS: m/z 749.2 ([M − PF6]+). IR (KBr disk, cm−1): 2152 ν(CN), 2092, 1972, 1914 ν(C O), 847 ν(P−F). Anal. Calcd for C28H18 Cl3N5O2 RePF6·0.5[(CH3CH2)2O] (931.07): C, 38.70; H, 2.49; N, 7.52. Found: C, 38.95; H, 2.83; N, 7.91. {Re(CNC 6 H 4 Cl-4) 4 (N^C 1 )}(PF 6 ) (6). To a solution of [Re(CNC6H4Cl-4)5(I)] (100 mg, 100 μmol) in THF (50 mL) was added TlOTf (39.0 mg, 110 μmol). The resulting solution was refluxed for 14 h. After filtration, 2-aminopyridine (11.3 mg, 120 μmol) was added to the filtrate. Thereafter, it was refluxed for 14 h. After removal of the solvent, it was purified by column chromatography (silica gel, 19:1 dichloromethane−ethyl acetate). The PF6− salt was obtained by the metathesis reaction with ammonium hexafluorophosphate in methanol. Subsequent recrystallization from acetone−n-pentane gave a greenish-yellow crystalline solid. Yield: 51.4 mg, 45.2 μmol, 45.1%. 1H NMR (600 MHz, CDCl3): δ 10.12 (1H, s, NH), 9.28 (1H, s, NH), 8.73 (1H, d, J = 5.4 Hz, 6pyridyl H), 7.78 (1H, td, J = 7.1 and 1.3 Hz, 4-pyridyl H), 7.39 (9H, m, phenyl H and 3-pyridyl H), 7.28 (2H, d, J = 8.6 Hz, phenyl H), 7.18 (8H, m, phenyl H), 7.01 (1H, t, J = 6.5 Hz, 5-pyridyl H), 6.66 (2H, d, J = 8.6 Hz, phenyl H). 13C NMR (151 MHz, CDCl3): δ 218.1, 175.3, 168.5, 158.3, 150.9, 139.3, 139.1, 134.2, 133.4, 133.1, 131.1, 130.8, 129.9, 129.9, 129.3, 129.2, 128.1, 127.5, 126.8, 125.8, 119.2, 111.7. Positive-ion ESI-MS: m/z 968.5 ([M − PF6]+). IR (KBr disk, cm−1): 2133, 2052, 2005, 1977 ν(CN), 847 ν(P−F). Anal. Calcd for C40H26Cl5N7RePF6 (1113.12): C, 43.16; H, 2.35; N, 8.81. Found: C, 43.42; H, 2.58; N, 8.88. {Re(CNC6H4Cl-4)4(N^C1 Me2)}(PF6) (7). To the triflate salt of 6 (100 mg, 89.8 μmol) in acetone (50 mL) was added potassium carbonate (49.6 mg, 359 μmol) and dimethyl sulfate (17.0 μL, 180 μmol). The resulting mixture was stirred at room temperature for 20 h. After extraction of deionized water and chloroform, the organic layer was dried with MgSO4. After removal of the solvent, it was purified by column chromatography (silica gel, 19:1 dichloromethane−ethyl acetate). The PF6− salt was obtained by the metathesis reaction with ammonium hexafluorophosphate in methanol. Subsequent recrystallization from dichloromethane−ether gave a reddish-orange crystalline solid. Yield: 89.3 mg, 78.4 μmol, 87.3%. 1H NMR (600 MHz, CDCl3): δ 8.89 (1H, d, J = 5.3 Hz, 6-pyridyl H), 7.92 (1H, t, J = 7.6 Hz, 4pyridyl H), 7.44 (4H, m, phenyl H), 7.35 (8H, m, phenyl H), 7.27 (2H, d, J = 8.2 Hz, phenyl H), 7.23 (2H, d, J = 8.5 Hz, phenyl H), 7.20 (3H, m, two phenyl H and 3-pyridyl H), 7.16 (1H, t, J = 6.4 Hz, 5pyridyl H), 7.02 (2H, d, J = 8.5 Hz, phenyl H), 4.06 (3H, s, CH3), 3.21 (3H, s, CH3). 13C NMR (151 MHz, CDCl3): δ 234.7, 178.7, 166.8, 161.2, 157.3, 151.6, 146.6, 139.8, 134.8, 134.3, 133.4, 132.8, 131.7, 131.1, 130.3, 130.0, 129.9, 129.6, 128.7, 128.0, 127.6, 127.0, 126.8, 126.7, 126.0, 120.4, 112.7, 50.8, 40.5. Positive-ion ESI-MS: m/z 996.5 ([M − PF6]+). IR (KBr disk, cm−1): 2130, 2052, 2002, 1966 ν(CN), 845 ν(P−F). Anal. Calcd for C42H30Cl5N7RePF6·(CH3CH2)2O (1178.23): C, 44.85; H, 2.99; N, 8.32. Found: C, 45.07; H, 3.21; N, 8.17. {Re(CNC6H4Cl-4)4(N^C3)}(PF6) (8). The complex was synthesized following the same procedure as that described for 6, except 2-amino1,3-thiazole (12.0 mg, 120 μmol) was used instead of 2-aminopyridine. Yield: 57.8 mg, 51.7 μmol, 51.6%. 1H NMR (600 MHz, CDCl3): δ 10.80 (1H, s, NH), 9.63 (1H, s, NH), 7.52 (1H, d, J = 3.7 Hz, 5thiazolyl H), 7.39 (8H, m, phenyl H), 7.25 (2H, d, J = 8.6 Hz, phenyl H), 7.20 (6H, m, phenyl H), 7.16 (2H, d, J = 8.6 Hz, phenyl H), 7.07 (1H, d, J = 3.7 Hz, 4-thiazolyl H), 6.63 (2H, d, J = 8.6 Hz, phenyl H). 13 C NMR (151 MHz, CDCl3): δ 218.6, 178.0, 168.2, 167.5, 158.8, 139.6, 139.4, 134.1, 133.1, 133.0, 133.0, 131.7, 130.5, 129.9, 129.9, 129.7, 129.6, 129.4, 129.3, 129.0, 127.9, 127.8, 127.6, 127.4, 127.2, 126.7, 125.7, 113.5. Positive-ion ESI-MS: m/z 974.3 ([M − PF6]+). IR (KBr disk, cm−1): 2134, 2054, 2005, 1960 ν(CN), 848 ν(P−F). C
DOI: 10.1021/acs.inorgchem.6b01017 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic complex precursors [Re(CNR)5(I)], fac-[Re(CO)3(CNR)2(Br)], and fac-[Re(CO)3(CNR)3](CF3SO4) were synthesized according to our previously reported procedures.4 By coordination of amino-substituted N-donor ligands, such as 2-aminopyridine, 2aminothiazole, and 2-aminobenzothiazole, to these synthetic precursor complexes, a series of luminescent isocyanorhenium(I) complexes with N,C-bidentate diaminocarbene ligands (N^C) was obtained. With the halorhenium(I) complex precursors [Re(CNR)5(I)] and fac-[Re(CO)3(CNR)2(Br)], coordination of 2-aminopyridine, 2-aminothiazole, and 2aminobenzothiazole was achieved by halide abstraction with silver triflate or thallium triflate, followed by the thermal ligand substitution reactions (Scheme 1). For [Re(CO)3(CNR)3](CF3SO4), coordination was performed by the photoligand substitution reaction (Scheme 1). Upon coordination of these ligands, the N,C-bidentate diaminocarbene ligands were formed by the nucleophilic attack on the isocyanide carbon with the amino substituent of the coordinated N-donor ligands.5a,18 However, the corresponding diaminocarbene ligands cannot be formed in the pentaisocyanorhenium(I) complexes with the electron-rich isocyanide ligands, such as p-tolyl isocyanide and 2,6-dimethylphenyl isocyanide, as illustrated by the formation of complexes 10−12. The N−H protons on the diaminocarbene ligand can be readily methylated by dimethyl sulfate in the presence of suspended potassium carbonate, as exemplified in the preparation of 7. The carbene ligands in complexes 1−9 are characterized by the very downfield 13C carbene signals with chemical shifts in the typical range of 212.6−234.7 ppm.5,18,19 In addition, the carbonyl and isocyanide ligands in these complexes are characterized by the CO and CN stretches in the ranges of 1905−2033 and 1945−2171 cm−1, respectively. For the tricarbonylisocyanorhenium carbene complexes (1−4), they show only one isocyano CN stretch and three carbonyl (C O) stretches in their IR spectra, which are consistent with the number of active IR stretches for tricarbonylisocyanorhenium(I) complexes arranged in a facial configuration.20 In the case of dicarbonyldiisocyano complex 5, it is characterized by two C N stretches and two CO stretches, in agreement with the cis,cis conformation.4b For tetraisocyano complexes (6−9), they are characterized by four IR-active CN stretches.4a,b In the case of pentaisocyanorhenium(I) complexes 10−12, they show three CN stretches. The identities as well as geometrical arrangements of the ligands in these complexes were unambiguously confirmed by the X-ray crystal structures of 6, 8, 11, and 12 (see below). It is interesting to note that the amine functional group on the pyridine ligand in some of the pentaisocyanorhenium(I) complexes ([Re(CNR)5(NH2py)]+ [R = C6H4(CH3)-4 (10), C6H3(CH3)2-2,6 (11), and C6H3(CH(CH3)2)2-2,6 (12)] do not react with the corresponding isocyanides. These pentaisocyanorhenium(I) complexes are characterized by the four symmetrical phenyl isocyanide 1H NMR signals and the absence of downfield carbene 13C signal in their 13C NMR spectra. This is in accordance with the reactivity study of the coordinated isocyanides, which show strong dependence on the electron richness of both the metal center and isocyanide ligand as well as the nucleophilicity of the nucleophiles.5,6,21 Although the stability of isocyanide ligands in 11 and 12 may be attributed to the steric effect of the methyl or isopropyl groups
Scheme 1. Synthetic Routes to Complexes 1−12
at the ortho positions, this explanation is not supported by the successful formation of the diaminocarbene ligand when we incorporated 2-aminopyridine into the complex of [Re(CO)3(CNC6H3(CH3)2-2,6)2(Br)]. As revealed by Hahn and Tamm in the study of metal complexes with the benzoxazol-2ylidene ligand,22 the reactivity of the coordinated isocyanide ligands toward nucleophilic attack can be predicted from the stretching frequency of the CN stretch because it reflected the electron richness of the isocyanide carbon. A close scrutiny of the CN stretches of the precursor complexes found that the stretching frequency of the highest-energy CN stretch is in the order of [Re(CO) 3 (CNR) 3 ](CF 3 SO 3 ) > [Re(CO)3(CNR)2(Br)] > [Re(CNR)5I], which is in line with D
DOI: 10.1021/acs.inorgchem.6b01017 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry the π-accepting ability of the other ancillary ligands. For [Re(CNR)5I], the CN stretching frequencies decreased upon the introduction of electron-donating alkyl substituents. As with the reported result,22 no nucleophilic attack was found in the rhenium(I) isocyano complexes with CN stretching frequencies less than ca. 2160 cm−1. X-ray Crystal Structure. Single crystals of 6, 8, 11, and 12 with quality suitable for X-ray crystal structure determination were obtained by the slow diffusion of pentane or diethyl ether into the concentrated dichloromethane or acetone solutions of the complexes. The perspective drawings of the complex cations of 6, 8, 11, and 12 are depicted in Figure 1. The crystal data and experimental details are given in Table S1. Selected bonding parameters of these complexes are summarized in Table 1. In the structures of 6 and 8, the ReI metal center adopted a distorted octahedral geometry with bite angles of 75.5° and 75.6° subtended by the bidentate pyridyl- and thiazolyldiaminocarbene ligands, which are similar to those observed in complexes with bipyridine23 and pyridylcarbene ligands.6b−i In the absence of the bidentate ligand, the ReI metal centers in the structures of 11 and 12 are almost octahedral, with L−Re−L′ bond angles close to 90° or 180°. Concerning the bonding parameters in these structures, the Re−C(carbene), Re− C(isocyanide), and CN(isocyanide) bond distances are in the ranges of 2.13−2.15, 1.95−2.04, and 1.14−1.18 Å, respectively, which are in the typical ranges reported for other rhenium(I) isocyanide4,24 and carbene complexes.5,19 In these structures, the isocyanide ligands are slightly bent, with the bond angles of CN−C in the range of 157.7−175.7°. The deviation from linearity can be explained by the π-backbonding interaction between the ReI metal center and isocyanide ligands. Because of the less rigid structure of the acyclic diaminocarbene ligands, the bond angles of N− C(carbene)−N of 6 (112.1°) and 8 (113.1°) are larger than those reported for NHC ligands (101−109°).1c,5,6,19,21,25 Electronic Absorption and Emission Properties. All of the complexes (1−12) dissolve in dichloromethane to give solutions with colors ranging from colorless to pale yellow to orange. The UV−vis absorption data of these solutions are collected in Table 2. These complexes show intense ligandcentered (LC) ππ* transitions of the substituted phenyl isocyanide and bidentate carbene ligands (N^C) with a molar absorptivity on the order of 104 dm3 mol−1 cm−1 in the highenergy UV region (λ < 330 nm). With reference to a previous spectroscopic study of the isocyanorhenium(I) diimine complexes4 and the computational study (see below), the lower-energy absorption shoulders are assigned to the admixture of metal-to-ligand charge-transfer (MLCT) transitions of dπ(Re) → π*(N^C) and dπ(Re) → π*(CNR). These assignments are consistent with a computational study on the selected complexes (1, 5, and 6), which shows that their highest occupied molecular orbitals (HOMOs) are mainly of dπ(Re) orbital character mixed with some π*(CNR) component, with the close-lying lowest unoccupied molecular orbital (LUMO) and LUMO+1 being π*(N^C) and π*(CNR), respectively (Table S2). It is likely that the lowest-energy absorption of these complexes is due to MLCT transitions of dπ(Re)→π*(N^C). This assignment is supported by the fact that the lowest-energy absorption shoulders of 10−12, corresponding to the MLCT dπ(Re) → π*(CNR) transitions, are considerably blue-shifted compared to the lowest-energy absorption of 6 (Figure 2). The MLCT transitions are red-
Figure 1. Perspective drawings of the complex cations of (a) 6, (b) 8, (c) 11, and (d) 12 with atomic numbering. H atoms have been omitted for clarity. Thermal ellipsoids are shown at the 30% probability level.
shifted when the ancillary carbonyl ligands are replaced by the isocyanide ligands (absorption energies in the order of 1 > 5 > 6; 3 > 8; 4 > 9). This is due to the weaker π-accepting ability of the isocyanide ligands than the carbonyl ligand. A similar trend was also observed in the isocyanorhenium(I) diimine complexes.4 E
DOI: 10.1021/acs.inorgchem.6b01017 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Table 1. Selected Bond Distances (Å) and Angles (deg) for 6, 8, 11, and 12 with Estimated Standard Deviations in Parentheses 6 Re(1)−C(1) Re(1)−C(2) Re(1)−C(3) Re(1)−C(4)
2.002(4) 2.009(4) 1.954(4) 2.032(4)
Re(1)−C(10) Re(1)−N(5) C(10)−N(6)
2.126(3) 2.186(3) 1.362(5)
C(10)−N(7) C(9)−N(6) C(11)−N(7)
1.333(5) 1.372(5) 1.430(5)
C(10)−Re(1)−N(5) C(1)−N(1)−C(17)
75.29(12) 163.8(5)
174.1(9) 165.4(5)
C(4)−N(1)−C(35) N(6)−C(10)−N(7)
173.4(5) 112.1(3)
Re(1)−C(1) Re(1)−C(2) Re(1)−C(3) Re(1)−C(4)
2.026(10) 2.019(9) 1.963(8) 2.016(10)
C(2)−N(2)−C(23) C(3)−N(1)−C(29) 8 Re(1)−C(8) Re(1)−N(5) C(8)−N(6)
2.147(9) 2.160(6) 1.382(10)
C(8)−N(7) C(7)−N(6) C(9)−N(7)
1.298(11) 1.359(10) 1.434(9)
C(8)−Re(1)−N(5) C(1)−N(1)−C(15)
75.6(3) 174.2(10)
157.7(8) 170.6(11)
C(4)−N(1)−C(33) N(6)−C(8)−N(7)
175.7(13) 113.1(7)
Re(1)−C(1) Re(1)−C(2)
2.033(8) 2.012(8)
C(2)−N(2)−C(21) C(3)−N(1)−C(27) 11 Re(1)−C(3) Re(1)−C(4)
1.999(8) 2.014(8)
Re(1)−C(5) Re(1)−N(6)
2.009(7) 2.242(6)
C(1)−N(1)−C(11) C(2)−N(2)−C(19)
170.1(8) 177.0(3)
177.4(3) 175.8(3)
C(5)−N(5)−C(43)
170.1(8)
Re(1)−C(1) Re(1)−C(2)
2.026(2) 2.040(2)
C(3)−N(3)−C(27) C(4)−N(4)−C(35) 12 Re(1)−C(3) Re(1)−C(4)
2.032(2) 2.036(2)
Re(1)−C(5) Re(1)−N(6)
1.952(2) 2.244(2)
C(1)−N(1)−C(11) C(2)−N(2)−C(23)
175.9(2) 175.6(2)
C(3)−N(3)−C(35) C(4)−N(4)−C(47)
170.9(2) 165.4(2)
C(5)−N(5)−C(59)
171.6(2)
introduction of two methyl groups into the N atoms of the diaminocarbene ligand. This is not caused by the electronreleasing effect of the methyl substituents but their steric repulsion, leading to a noncoplanar twisted diaminocarbene structure. This decreases delocalization of the lone-pair electrons from the N atom to the carbene and thus renders a lower-lying empty pπ(Ccarbene) orbital. The considerably more stabilized LUMO of 7 is further supported by the much less negative ligand-based reduction potential for 7 than 6 (see below). A similar effect on the empty pπ(Ccarbene) orbital was reported in the cyclic diaminocarbene ligand, with one of the N atoms twisted by a cyclic structure to restrict the π delocalization.28 In a 77 K EtOH−MeOH (4:1, v/v) glassy medium, complexes 5−9 show significantly blue-shifted 3MLCT emissions (Figure S1) relative to their emissions in the solution state at room temperature. This is attributed to the rigidochromism, which is typically observed for MLCT phosphorescence.29 For complex 5, the significantly longer emission lifetime is suggestive of the mixing of 3LC phosphorescence, while for complexes 1−4, their emissions in a 77 K glassy medium are only slightly blue-shifted but with much longer lifetimes compare to those in the solution state at room temperature. These characteristics are indicative of the predominant 3LC excited-state origin. Electrochemistry. The electrochemical properties of 1−9 in acetonitrile (0.1 M nBu4NPF6) were investigated by cyclic voltammetry. The electrochemical data are tabulated in Table 3. Figure 4 depicts the representative cyclic voltammograms. In the oxidative scan, 1−4 exhibit one irreversible oxidation wave with Epa + 1.64 to +1.69 V vs SCE, 5 shows a quasi-reversible oxidation couple with E1/2 at +1.32 V vs SCE, and 6−9 show
Upon excitation into the lowest-energy absorption band with λ > 350 nm, the carbene complexes 1−9 in dichloromethane solutions exhibit photoluminescence, with emission colors ranging from blue to red covering most of the visible region (Figure 3). With reference to the spectroscopic studies on the related rhenium(I) complexes20,26 and the observation of the emission energy trend in line with the lowest-energy absorption, the emissions of 5−9 are tentatively assigned to predominant 3MLCT [dπ(Re) → π*(N^C)] excited-state origins. For complexes 1−4, their emissions are ascribed to a mixed 3 MLCT/ 3 LC excited-state origin. As with the isocyanorhenium(I) diimine systems,4,5a the emission becomes red-shifted [emission energy: 1 (467 nm) > 5 (529 nm) > 6 (547 nm); 3 (499 nm) > 8 (590 nm); 4 (489 nm) > 9 (570 nm)] when the carbonyl ligands are replaced by isocyanide ligands. This is because the dπ(Re) orbital become less stabilized, as reflected by the cathodic shift of the metalcentered oxidation (see below) when carbonyl ligands are replaced by weaker π-accepting isocyanide ligands. It is interesting to note that, in contrast to diimine-based MLCT phosphorescent emitters,4d,27 which show a blue-shifted emission when the π conjugation of the diimine ligand is decreased, a lower-energy emission was observed for complexes with a less π-conjugated bidentate carbene ligand [emission energy: 1 (467 nm) > 4 (489 nm) > 3 (499 nm); 9 (570 nm) > 8 (590 nm)]. This trend can be rationalized from the heavy contribution of the empty pπ(Ccarbene) component, which become lower-lying in the presence of a less π-conjugated Nheterocycle because of the interaction with a lower-lying lonepair electrons, in the LUMO [π*(N^C)]. When the emissions of 6 (547 nm) and 7 (672 nm) are compared, a more pronounced red shift of the emission is noted upon the F
DOI: 10.1021/acs.inorgchem.6b01017 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 2. Photophysical Data of 1−12 complex
medium
1
CH2Cl2 CH3CN glassd CH2Cl2 CH3CN glassd CH2Cl2 CH3CN glassd CH2Cl2 CH3CN glassd CH2Cl2 CH3CN glassd CH2Cl2 CH3CN glassd CH2Cl2 CH3CN glassd CH2Cl2 CH3CN glassd CH2Cl2 CH3CN glassd CH2Cl2 CH2Cl2 CH2Cl2
2
3
4
5
6
7
8
9
10 11 12
emissiona λem/nm (τ0/μs) 467 459 445 448 441 434 499 494 498 489 487 478 529 529 470 547 556 488 672 670 539 590 593 489 570 608 504 f f f
(0.037) (0.091) (40.0) (0.132) (0.262) (12.0) (5.01) (3.20) (149) (0.72) (0.30) (86.4) (0.282) ( 1 (Figure S11 and Table S2), which is consistent with the trend of the oxidation potentials of these complexes (Table 3). For the calculated LUMOs, they are mainly π*(N^C) orbitals, but they are energetically very close to the LUMO+1 (
