Synthesis and Photophysical Properties of Ruthenium (II) Isocyanide

Sep 16, 2009 - A series of ruthenium(II) bis(8-quinolinolato) complexes bearing .... Chi-Chiu Ko , Jacky Wai-Kit Siu , Apple Wai-Yi Cheung , and Shek-...
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Organometallics 2009, 28, 5709–5714 DOI: 10.1021/om900253h

5709

Synthesis and Photophysical Properties of Ruthenium(II) Isocyanide Complexes Containing 8-Quinolinolate Ligands Chi-Fai Leung,† Siu-Mui Ng,† Jing Xiang,† Wai-Yeung Wong,‡ Michael Hon-Wah Lam,† Chi-Chiu Ko,*,† and Tai-Chu Lau*,† †

Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong, People’s Republic of China, and ‡Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon, Hong Kong, People’s Republic of China Received April 3, 2009

A series of ruthenium(II) bis(8-quinolinolato) complexes bearing isocyanide ligands (RNC) have been synthesized by the reaction of [RuQ3] (Q=8-quinolinolate) with RNC in the presence of Zn/Hg. These complexes have the general formula [RuQ2(RNC)2] (1, R = tert-butyl; 2, R = 4-MeOPh; 3, R=4-ClPh; 4, R=2,4,6-Br3Ph). Both the yellow cis,cis,trans (a) and orange-red trans,trans,trans (b) isomers have been isolated for complexes 1-4. trans,trans,trans-[Ru(Tol-Q)2(tBuNC)2] (6, HTol-Q= 8-hydroxyl-5-tolylquinoline) has also been prepared from [Ru(PPh3)2Cl2]. The structures of 2a, 3a, and 4b have been determined by X-ray crystallography. These complexes exhibit an intense absorption band in the UV region (λmax = 320-390 nm) with molar extinction coefficients (ε) on the order of 104 dm3 mol-1 cm-1 and a moderately intense absorption with ε on the order of 103 dm3 mol-1 cm-1 at 400-492 nm. The intense absorption at 320-390 nm is assigned to the ligand-centered πfπ* transitions of the quinolinolate ligands, probably mixed with the πfπ* transitions of the isocyanide ligands. The lower energy absorptions at 400-492 nm are assigned to Ru(dπ)fπ*(Q) metal-to-ligand charge transfer (MLCT) transitions. Upon excitation at λ > 350 nm, 1a-3a in dichloromethane solution exhibit orange-red luminescence (645-680 nm). In 77 K EtOH/MeOH glass, complexes 1-4 and 6 give intense structured emission spectra (593-638 nm). The cyclic voltammograms (CV) of 1a-4a generally exhibit an irreversible or quasi-reversible RuIII/II couple at the potential range of 0.02-0.38 V vs Fcþ/Fc, except in the case of 1a, where a reversible RuIII/II and a quasi-reversible RuIV/III (0.68 V vs Fcþ/Fc) couple are observed. The potential for the RuIII/II couple increases with the π-accepting ability of the isocyanide ligands. In the CV of 1b-4b, a reversible RuIII/II (-0.16 to 0.065 V) and quasi-reversible RuIV/III (0.70-0.85 V) couple are observed. Introduction Luminescent transition metal complexes have attracted much attention in recent years because of their potential applications in optical sensing and sensitization, electroluminescent (EL) devices such as organic light-emitting diode (OLED) and light-emitting electrochemical cells (LEC), and photovoltaics.1,2 Since the first study of tris(8-quinolinolato)aluminum (AlQ3)-based multilayer EL devices,3 considerable attention has been focused on the study of various metal 8-quinolinolates and their derivatives.4-9 We have recently developed the synthesis of a series

of bis(8-quinolinolato)ruthenium(II) complexes of general formula [RuQ2L2] (L2 = (DMSO)2, COD, (4-Mepy)2, etc.)14 We report here the synthesis and photophysical properties of a series of bis(8-quinolinolato)ruthenium(II) complexes bearing isocyanide ligands, with the general formula [RuQ2(RNC)2]. Polypyridyl ruthenium(II) and osmium(II) bearing isocyanide or carbonyl ligands are known to possess interesting emissive properties.10-13

Experimental Section Materials and Physical Measurements. 8-Hydroxyl-5-tolylquinoline (HTol-Q), [RuQ3], and [Ru(Tol-Q)2(PPh3)2] were prepared according to literature methods.8,14,15 The 4-chloro- and

*Corresponding author. E-mail [email protected]. (1) Baranoff, E.; Collin, J. P.; Flamigni, L.; Sauvage, J. P. Chem. Soc. Rev. 2004, 33, 147. (2) Chou, P. T.; Chi, Y. Eur. J. Inorg. Chem. 2006, 3319. (3) Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (4) Chen, C. H.; Shi, J. Coord. Chem. Rev. 1998, 171, 161. (5) Wang, Y.; Zhang, W.; Li, Y.; Ye, L.; Yang, G. Chem. Mater. 1999, 11, 530. (6) Wang, L.; Jiang, X.; Zhang, Z.; Xu, S. Displays 2000, 47. (7) Khreis, O. M.; Gillin, W. P.; Sommeton, M.; Curry, R. J. Org. Electron. 2001, 2, 45. (8) Montes, V. A.; Pohl, R.; Shinar, J.; Anzenbacher, P., Jr. Chem.; Eur. J. 2006, 12, 4523. (9) Curry, R. J.; Gillin, W. P. Appl. Phys. Lett. 1999, 75, 1380.

(10) Chi, Y.; Chou, P. T. Chem. Soc. Rev. 2007, 36, 1421. (11) Li, E. Y.; Cheng, Y. M.; Hsu, C. C.; Chou, P. T.; Lee, G. H. Inorg. Chem. 2006, 45, 8041. (12) Villegeas, J. M.; Stoyanov, S. R.; Huang, W.; Lockyear, L. L.; Reibenspies, J. H; Rilema, D. P. Inorg. Chem. 2004, 43, 6383. (13) Stoyanov, S. R.; Villegeas, J. M.; Rilema, D. P. Inorg. Chem. Commun. 2004, 7, 838. (14) Leung, C. F.; Wong, C. Y.; Ko, C. C.; Yuen, M. C.; Wong, W. T.; Wong, W. Y.; Lau, T. C. Inorg. Chim. Acta 2009, 362, 1149. (15) Menon, M.; Pramanik, A.; Bag, N.; Chakravorty, A. J. Chem. Soc., Dalton Trans. 1995, 1417.

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2,4,6-tribromophenyl isocyanide ligands were synthesized by reported procedures.16 tert-Butyl isocyanide, 4-methoxyphenyl isocyanide, and 8-hydroxyquinoline were purchased from Aldrich. Other chemicals were of reagent grade and used without further purification. IR spectra were obtained from KBr discs by using a Bomen MB-120 FTIR spectrophotometer. UV/visible spectra were recorded on a Perkin-Elmer Lamda 19 spectrophotometer. Steady-state emission and excitation spectra were recorded on a Horiba Jobin Yvon Florolog-3-TCSPC spectrophotometer. Measurements of the EtOH/MeOH (4:1, v/v) glass samples at 77 K were carried out with samples contained in a quartz tube inside a quartz-walled Dewar flask. 1H NMR spectra were recorded on a Varian (300 MHz) FT-NMR spectrometer. The chemical shifts (δ ppm) were reported with reference to tetramethylsilane (TMS). Elemental analyses were done on an Elementar Vario EL III analyzer. Cyclic voltammograms were recorded on a PAR model 273 potentiostat, using ferrocene (Fc) as internal reference. A glassy carbon disk working electrode and a Ag/AgNO3 reference electrode were used. The supporting electrolyte was 0.1 M [nBu4N]PF6 in CH3CN. [nBu4N]PF6 (Aldrich) was recrystallized three times from ethanol and dried in vacuo at 120 °C for 24 h before use. Acetonitrile was first refluxed over calcium hydride and then distilled under argon. Synthesis of Complexes. cis,cis,trans- (1a) and trans,trans, trans-[RuQ2(tBuNC)2] (1b). A mixture of [RuQ3] (120 mg, 0.225 mmol), tert-butyl isocyanide (47 mg, 0.56 mmol), and a few pieces of Zn/Hg in 25 mL of ethanol was refluxed under argon for 18 h. After cooling to room temperature, the orange-red solution was filtered and the volatiles were then removed by rotary evaporation. The residue was dissolved in dichloromethane and loaded onto a silica gel column. A yellow band eluted with dichloromethane was collected and dried under reduced pressure to give a yellow crystalline solid (1a). The red band eluted with dichloromethane/acetone (10:1) was also collected and dried under reduced pressure to give a red crystalline solid (1b). 1a was recrystallized from dichloromethane/n-hexane. Yield: 43 mg (34%). Anal. Calcd for C28H30N4O2Ru: C, 60.53; H, 5.41; N, 10.09. Found: C, 60.31; H, 5.33; N, 10.16. UV/vis (CH2Cl2) λmax, nm (ε, M-1 cm-1): 345 (9280), 452 (6330). IR (KBr, cm-1): 2050 (νNC), 2123 (νNC), 2979 (νCH, tBuNC). 1H NMR (300 MHz, CDCl3): δ 1.32 (s, 18H, tBuNC), 6.80 (d, 3JHH = 8.10 Hz, 2H, Q), 7.08 (m, 4H, Q), 7.32 (t, 3JHH = 8.10, 2H, Q), 7.93 (dd, 3 JHH =8.40, 4JHH =1.20, 2H, Q), 8.33 (dd, 3JHH =4.65, 4JHH = 1.20, 2H, Q). 1b was recrystallized from dichloromethane/n-hexane. Yield: 50 mg (40%). Anal. Calcd for C28H30N4O2Ru: C, 60.53; H, 5.41; N, 10.09. Found: C, 59.97; H, 5.39; N, 10.04. UV/vis (CH2Cl2) λmax, nm (ε, M-1 cm-1): 377 (11 300), 396 (11 500), 489 (5840). IR (KBr, cm-1): 2098 (νNC), 2978 (νCH, tBuNC). 1H NMR (300 MHz, CDCl3): δ 1.02 (s, 18H, tBuNC), 6.76 (d, 3JHH =7.50, 2H, Q), 6.92 (d, 3JHH = 7.50, 2H, Q), 7.28 (m, 4H, Q), 7.96 (dd, 3 JHH =8.25, 4JHH =1.35, 2H, Q), 9.08 (dd, 3JHH =5.10, 4JHH = 1.20, 2H, Q) cis,cis,trans- (2a) and trans,trans,trans-[RuQ2(4-MeOPhNC)2] (2b). The complexes were synthesized by a similar procedure to that for 1a and 1b using 4-methoxylphenyl isocyanide (75 mg, 0.56 mmol). The yellow band eluted from a silica gel column with dichloromethane/acetone (10:1) was collected and dried in vacuo to give a yellow crystalline solid (2a). A red-orange band eluted with acetone was also collected and dried to give a reddish-orange crystalline solid (2b). 2a was recrystallized from dichloromethane/n-hexane. Yield: 54 mg (37%). Anal. Calcd for C34H26N4O4Ru: C, 62.28; H, 3.97; N, 8.55. Found: C, 62.09, H, 4.13, N, 8.36. UV/vis (CH2Cl2) λmax, nm (ε, M-1 cm-1): 322 (27 300), 444 (8030). IR (KBr, (16) Ugi, I.; Fetzer, U.; Eholzer, U.; Knupfer, H.; Offerman, K. Angew. Chem. 1965, 77 (11), 492.

Leung et al. cm-1): 2074 (νNC), 2127 (νNC). 1H NMR (300 MHz, CDCl3): δ 3.80 (s, 6H, MeO), 6.76 (m, 4H, Q þ 4-MeOPhNC), 6.87 (d, 3JHH = 7.20, 2H, Q), 7.16 (m, 8H, Q þ 4-MeOPhNC), 7.38 (t, 2H, 3JHH =8.10, Q), 8.0 (dd, 3JHH =8.53, 4JHH =1.35, 2H, Q), 8.44 (dd, 3JHH = 4.80, 4JHH = 1.50, 2H, Q). 2b was recrystallized from dichloromethane/n-hexane. Yield: 62 mg (42%). Anal. Calcd for C34H26N4O4Ru: C, 62.28; H, 3.97; N, 8.55. Found: C, 62.41, H, 4.06; N, 8.48. UV/vis (CH2Cl2) λmax, nm (ε, M-1 cm-1): 331 (25 100), 389 (18 200), 478 (5840). IR (KBr, cm-1): 2088 (νNC). 1H NMR (300 MHz, CDCl3): δ 3.72 (s, 6H, MeO), 6.62 (d, 4H, Q þ 4-MeOPhNC), 6.86 (m, 6H, Q þ 4-MeOPhNC), 7.03 (dd, 3JHH = 7.80, 4JHH = 1.20, 2H, Q), 7.34 (m, 4H, Q), 8.07 (dd, 3JHH =8.40, 4JHH =1.50, 2H, Q), 9.17 (dd, 3JHH = 4.80, 4JHH = 1.50, 2H, Q). cis,cis,trans- (3a) and trans,trans,trans-[RuQ2(4-ClPhNC)2] (3b). The complexes were synthesized by a similar procedure to that for 1a and 1b using 4-chlorophenyl isocyanide (77 mg, 0.56 mmol). The yellow band eluted from a silica gel column with dichloromethane/acetone (10:1) was collected and dried in vacuo to give a yellow crystalline solid (3a). A red-orange band eluted with acetone was also collected and dried to give a reddish-orange crystalline solid (3b). 3a was recrystallized from dichloromethane/n-hexane. Yield: 49 mg (32%). Anal. Calcd for C32Cl2H20N4Ru: C, 64.75, H, 3.40, N, 9.44. Found: C, 64.81, H, 3.21, N, 9.37. UV/vis (CH2Cl2) λmax, nm (ε, M-1 cm-1): 323 (23 700), 440 (8410). IR (KBr, cm-1): 2052 (νNC), 2116 (νNC). 1H NMR (300 MHz, CDCl3): δ 6.91 (d, 3JHH = 6.90, 2H, Q), 7.16 (m, 8H, Q þ 4-ClPhCN), 7.25 (m, 6H, Q þ 4-ClPhNC), 7.40 (t, 3JHH = 7.80, 2H, Q), 8.03 (dd, 3JHH = 8.40, 4JHH = 1.5), 8.43 (dd, 3JHH = 4.95, 4 JHH = 1.35, 2H, Q). 3b was recrystallized from dichloromethane/n-hexane. Yield: 57 mg (38%). Anal. Calcd for C32Cl2H20N4Ru: C, 64.75, H, 3.40, N, 9.44. Found: C, 64.82, H, 3.54, N, 9.37. UV/vis (CH3Cl) λmax, nm (ε, M-1 cm-1): 348 (25 100), 465 (6250). IR (KBr, cm-1): 2077(νNC). 1H NMR (300 MHz, CDCl3): 6.86 (m, 6H, Q þ 4-ClPhNC), 7.04 (d, 3JHH = 8.10, 2H, Q), 7.12 (m, 4H, 4-ClPhNC), 7.36 (m, 4H, Q), 8.10 (dd, 3JHH =8.40, 4JHH =1.50, 2H, Q), 9.14 (dd, 3JHH = 4.80, 4JHH = 1.50, 2H, Q). cis,cis,trans- (4a) and trans,trans,trans-[RuQ2(2,4,6-Br3PhNC)2] (4b). The complexes were synthesized by a similar procedure to that for 1a and 1b using 2,4,6-tribromophenyl isocyanide (191 mg, 0.56 mmol). The reaction mixture was filtered to give a red solid and a yellow filtrate. The solvent was removed and the residue was dissolved in dichloromethane and purified by silica gel column chromatography using dichloromethane/acetone (10:1) as eluent to give 4a as a yellow crystalline solid. The red solid was purified by alumina column chromatography using acetone as eluent to give an orange-red crystalline solid (4b). 4a was recrystallized from dichloromethane/n-hexane. Yield: 63 mg (26%). Anal. Calcd for Br6C32H16N4O2Ru: C, 35.95, H, 1.51, N, 5.24. Found: C, 35.81, H, 1.67, N, 5.43. UV/vis (CH3Cl) λmax, nm (ε, M-1 cm-1): 327 (24 200), 391 (18 600). IR (KBr, cm-1): 1997 (νNC), 2105 (νNC). 1H NMR (300 MHz, CDCl3): δ 6.93 (dd, 3JHH =7.80, 4JHH =1.20, 2H, Q), 7.21 (m, 4H, Q), 7.42 (t, 3JHH = 8.10, 2H, Q), 7.57 (s, 2H, 2,4,6-Br3PhNC), 8.06 (dd, 3 JHH =8.55, 4JHH =1.35, 2H, Q), 8.55 (dd, 3JHH =4.65, 4JHH = 1.35, 2H, Q). 4b was also recrystallized from dichloromethane/n-hexane. Yield: 76 mg (32%). Anal. Calcd for Br6C32H16N4O2Ru: C, 35.95, H, 1.51, N, 5.24. Found: C, 35.79, H, 1.64, N, 5.15. UV/ vis (CH3Cl) λmax, nm (ε, M-1 cm-1): 377 (23 100), 472 (14 200). IR (KBr, cm-1): 2040 (νNC). 1H NMR (300 MHz, CDCl3): δ 6.85 (dd, 3JHH = 7.80, 4JHH = 0.90, 2H, Q), 7.05 (dd, 3JHH = 8.25, 4JHH = 1.05, 2H, Q), 7.35 (m, 4H, Q), 7.49 (s, 2H, 2,4,6Br3PhNC), 8.09 (dd, 3JHH =8.70, 4JHH =1.50, 2H, Q), 9.12 (dd, 3 JHH = 4.80, 4JHH = 1.20, 2H, Q). trans,trans,trans-[Ru(PPh3)2(Tol-Q)2] (5). [Ru(PPh3)2Cl2] (120 mg, 0.17 mmol) was suspended in 25 mL of ethanol, and 8-hydroxyl-5-tolylquinoline (81 mg, 0.35 mmol) was added. The

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Organometallics, Vol. 28, No. 19, 2009

Table 2. Selected Bond Lengths (A˚) and Bond Angles (deg) for 2a, 3a, and 4b

Table 1. Crystal and Structural Determination Data for the Complexes 2a, 3a, and 4b

formula fw crystal system space group a/A˚ b/A˚ c/A˚ R/deg β/deg γ/deg volume/A˚3 Z Dcalc/g cm-3 μ(Mo KR)/mm-1 F(000) temperature/K λ/A˚ θmin, θmax/deg total, unique data R1(obsd/all) wR2(obsd/all) goodness-of-fit on F2

2a 3 H2O

3a 3 CH2Cl2

4b

C34H28N4O5Ru 673.689 monoclinic P 1 21/n 1 (no. 14) 9.8044(4) 14.0508(6) 21.6146(9) 90.00 92.02(0) 90.00 2975.77(20) 4 1.504 1.43 1639 293(2) 0.71073 3.12, 25.00 4982, 3623 0.0588/0.0868 0.1275/0.1350 1.113

C33H22Cl2N4O2Ru 607.632 triclinic P1 (no. 2)

Br6C32H16N4O2Ru 1069.02 monoclinic I 1 2/a 1 (no. 15)

8.3101(4) 13.0622(7) 15.2027(5) 78.156(4) 83.45(0) 28.703(4) 1579.11(13) 2 1.576 3.885 1162 293(2) 0.71073 3.22, 25.00 5251, 4357 0.0566/0.0681 0.1795/0.1795 1.047

23.0841(10) 6.9214(3) 23.5613(12) 90.00 112.32(1) 90.00 3482.52(30) 4 2.039 3.00 506 293(2) 1.54178 6.82, 54.97 2054, 1834 0.0631/0.0678 0.1690/0.1740 1.054

mixture was refluxed under argon for 3 h. The golden yellow crystalline solid formed was filtered, washed with ethanol, and then air-dried. Yield: 103 mg (55%). Anal. Calcd for C68H54N2O2P2: C, 74.64, H, 4.97, N, 2.56. Found: C, 74.45, H, 4.81, N, 2.68. trans,trans,trans-[Ru(Tol-Q)2(tBuNC)2] (6). A mixture of 5 (80 mg, 0.091 mmol), tert-butyl isocyanide (30 mg, 0.37 mmol), and a few pieces of Zn/Hg was refluxed under argon for 18 h to give an orange-red solution. The mixture was filtered and the solvent was removed by rotary evaporation. The orange-red residue was purified by silica gel column chromatography using ethyl acetate as eluent. The red-orange band was collected and dried in vacuo to give an orange-red crystalline solid, which was recrystallized from dichloromethane/n-hexane (6). Yield: 42 mg (48%). Anal. Calcd for C42H42N4O2Ru: C, 68.55, H, 5.75, N, 7.61. Found: C, 68.69, H, 5.85, N, 7.64. UV/vis (CH3Cl) λmax, nm (ε, M-1 cm-1): 309 (19 100), 420 (11 500), 492 (6860). IR (KBr, cm-1): 2098 (νNC). 1H NMR (300 MHz, CDCl3): δ 2.42 (s, 6H, Tol-Q), 2.59 (s, 18H, tBuNC), 6.83 (d, 2H, 3JHH = 8.10, Tol-Q), 7.17 (d, 2H, 3JHH = 6.30, Tol-Q), 7.27 (m, 8H, Tol-Q), 7.36 (t, 2H, 3JHH = 7.80, Tol-Q), 8.23 (d, 2H, 3JHH = 9.00, Tol-Q), 9.05 (dd, 2H, 3JHH = 5.10, 4JHH = 1.50, Tol-Q). X-ray Structure Determination. Crystals suitable for X-ray diffraction analysis were obtained for compounds 2a 3 H2O, 3a 3 CH2Cl2, and 4b. Crystal data and experimental details are given in Table 1. X-ray diffraction data were collected at 293 K on an Oxford CCD diffractometer using graphite-monochromated Mo KR radiation (λ = 0.71073 A˚) for 2a 3 H2O and 3a 3 CH2Cl2 and Cu KR (λ = 1.54178 A˚) for 4b in the ω-scan mode. Selected bond lengths and bond angles are given in Table 2. Absorption corrections were done by the multiscan method. The structures were resolved by the heavy-atom Patterson method or direct methods and refined by full-matrix least-squares using SHELX-97 and expanded using Fourier techniques.17 All non-hydrogen atoms were refined anisotropically. H atoms were generated by the program SHELXL-97. The positions of H atoms were calculated on the basis of riding mode with thermal parameters equal to 1.2 times that of the associated C atoms, and participated in the calculation of final R-indices. (17) Sheldrick, G. M. SHELX-97; University of G€ottingen: G€ottingen, Germany, 1998.

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2a

3a

4b

Ru(1)-O(1) Ru(1)-O(4) Ru(1)-N(1) Ru(1)-N(2) Ru(1)-C(8)

2.059(4) 2.074(4) 2.133(5) 2.105(5) 1.893(6)

Ru(1)-C(27) C(27)-N(4) C(8)-N(3) O(1)-O(5)

1.944(7) 1.192(8) 1.153(8) 2.866(8)

Ru(1)-C(27)-N(4) Ru(1)-C(8)-N(3) Ru(1)-O(1) Ru(1)-O(2) Ru(1)-N(1) Ru(1)-N(4)

173.77(54) 174.31(52) 2.087(5) 2.073(5) 2.111(6) 2.110(6)

N(1)-Ru(1)-O(1) N(2)-Ru(1)-O(2) Ru(1)-C(25) Ru(1)-C(26) C(25)-N(2) C(26)-N(3)

82.58(17) 80.82(17) 1.899(7) 1.915(7) 1.171(9) 1.164(8)

Ru(1)-C(25)-N(2) Ru(1)-C(26)-N(3)

174.86(56) 176.77(52)

N(1)-Ru(1)-O(1) N(4)-Ru(1)-O(2)

79.55(16) 79.81(17)

Ru(1)-O(1) Ru(1)-N(2)

2.098(7) 2.066(9)

Ru(1)-C(1) C(1)-N(1)

1.975(10) 1.157(12)

Ru(1)-C(1)-N(1)

178.11(83)

N(2)-Ru(1)-O(1)

80.1(3)

Results and Discussions Syntheses and Characterizations. The reaction of [RuQ3] with RNC in refluxing ethanol in the presence of Zn/Hg under argon produces [RuQ2(RNC)2] as a mixture of cis, cis,trans- (1a-4a) and trans,trans,trans-isomers (1b-4b) (Scheme 1), which can be readily separated by column chromatography. Similar methods have been used to synthesize bis(8-quinolinolato) and bis(acetylacetonato) ruthenium complexes containing various π-acid ligands such as diene, dmso, and pyridine.14,18 Attempts to synthesize the corresponding [Ru(Tol-Q)2(RNC)2] via [Ru(Tol-Q)3] according to Scheme 1 failed. Instead, the trans,trans,trans-isomer of the complex was synthesized from trans,trans,trans-[Ru(Tol-Q)2(PPh3)2] (5), which was prepared by the reaction of RuCl2(PPh4)3 with HTol-Q in ethanol.15 The reaction of 5 with an excess of tert-butyl isocyanide (tBuNC) gives the corresponding trans,trans,trans-[Ru(Tol-Q)2(tBuNC)2] (6) (Scheme 2). The IR spectra of the cis,cis,trans-isomers 1a-4a show two strong νNC in the range 2000-2150 cm-1, indicative of a mutually cis disposition of the isocyanide ligands.12,19-21 The trans,trans,trans-isomers 1b-4b and 6 exhibit only one single νNC at ∼2040-2098 cm-1 in the same region.22 The slightly lower νNC stretching frequencies observed in the complexes compared to that of the free ligands can be attributed to a π back-bonding interaction between ruthenium(II) and isocyanide. The stretching frequencies follow the order 4a (1997, 2105 cm-1) < 3a (2052, 2116 cm-1) < 2a (2074, 2127 cm-1) and 4b (2040 cm-1) 4-MeO-C6H4NC. The 1H NMR spectra of 1-4 and 6 reveal a 1:1 ratio of the 8-quinolinolate and the isocyanide ligands, consistent with the proposed formula [RuIIQ2(RNC)2]. (18) Bennett, M. A.; Heath, G. A.; Hockless, D. C. R.; Kovacik, I.; Willis, A. C. J. Am. Chem. Soc. 1998, 120, 932. (19) Cadierno, V.; Crochet, P.; Dı´ ez, J.; Garcı´ a-Garrido, S. E.; Gimeno, J. Organometallics 2004, 23, 4836. (20) Kawano, H.; Nishimura, Y.; Onishi, M. Dalton 2003, 1808. (21) Ruiz, J.; Riera, V.; Vivanco, M. J. Chem. Soc., Dalton Trans. 1995, 1069. (22) Linder, E.; Gespr€ags, M.; Gierling, K.; Fawzi, R.; Steimann, M. Inorg. Chem. 1995, 34, 6106.

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Leung et al.

Scheme 1. Ligand Substitution Reaction of [RuQ3] with Isocyanide

Scheme 2. Synthetic Routes for Compounds 5 and 6

X-ray Crystal Structures. Single crystals of 2a and 3a were obtained by slow diffusion of n-hexane into a dichloromethane solution of the complexes at 4 °C. Single crystals of 4b were obtained by slow evaporation of a chloroform/ ethanol solution of the complex. The crystal data and experimental details for these complexes are summarized in Table 1. Structures of cis,cis,trans-RuQ2(4-MeOPhNC)2 (2a) and cis,cis,trans-RuQ2(4-ClPhNC)2 (3a). The cis,cis,trans-configuration of 2a and 3a and the trans,trans,trans-configuration of 4b are revealed by X-ray crystallography. Perspective views of these complexes are shown in Figures 1-3. Selected bond distances and angles are listed in Table 2. Complexes 2a and 3a have a distorted octahedral geometry with the two 8-quinolinolate ligands in a cis,cis,trans-configuration. The two isocyanide ligands are cis to each other and trans to the N atoms of the quinolinolate ligands. Complex 4b adopts an octahedral geometry with an all-trans-configuration, consistent with a single νNC in IR spectrum. The Ru-C (isocyanide) and CtN (isocyanide) bond lengths in these complexes are in the range 1.893-1.973 and 1.153-1.192 A˚, respectively. The Ru-C-N(isocyanide) units are almost linear (173.77178.11°). These bonding parameters are similar to those reported for other ruthenium(II) isocyanide complexes.12,19-22 The bite angles of the quinolinolato ligands in these complexes are about 80°, and the Ru-N(Q) and Ru-O(Q) bond distances are in the range 2.059-2.087 and 2.105-2.133 A˚, respectively, which are typical of ruthenium(II) 8-quinolinolato complexes.14 Photophysical Properties of the [RuQ2(RNC)2]. Both cis, cis,trans- and trans,trans,trans-isomers exhibit an intense absorption band in the UV region (λmax = 320-390 nm) with molar extinction coefficients (ε) on the order of 104 dm3 mol-1 cm-1 and a moderately intense absorption with ε on the order of 103 dm3 mol-1 cm-1 at 400-492 nm (Figure 4). The intense absorption at 320-390 nm is assigned to the ligand-centered πfπ* transitions of the quinolinolate ligands, probably mixed with the πfπ* transitions of the isocyanide ligands, as much more intense absorptions are observed for complexes 2-4 and 6 with phenyl isocyanide ligands than for 1 with

Figure 1. ORTEP drawing of 2a 3 H2O (thermal ellipsoids are drawn at the 50% probability; solvent of crystallization (water) and hydrogen atoms are omitted for clarity).

tert-butyl isocyanide. The lower energy absorptions at 400492 nm are assigned to Ru(dπ)fπ*(Q) metal-to-ligand charge transfer (MLCT) transitions, probably mixed with the intraligand charge transfer (ILCT) transitions of the quinolinolate ligands, as the ILCT transitions of quinolinolate ligands of related complexes have also been reported to occur in this region.23 These low-energy absorption bands for the cis,cis,trans-isomers (1a-4a) are significantly blue-shifted compared to the corresponding trans,trans,trans-isomers (1b-4b) (see Experimental Section). This is attributed to (23) (a) Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F. Inorg. Chem. 1986, 34, 3858. (b) Donges, D.; Nagle, J. K.; Yersin, H. Inorg. Chem. 1997, 36, 3040. (c) Kunkely, H.; Vogler, A. Inorg. Chem. Commun. 1998, 1, 398. (d) Yersin, H.; Donges, D.; Nagle, J. K.; Sitters, R; Glasbeek, M. Inorg. Chem. 2000, 39, 770. (e) Czerwieniec, R.; Kapturkiewicz, A.; AnulewiczOstrowska, R.; Nowacki, J. J. Chem. Soc., Dalton Trans. 2001, 2756. (f) Cheng, Y.-M.; Yeh, Y.-S.; Ho, M.-L.; Chou, P.-T.; Chen, P.-S.; Chi, Y. Inorg. Chem. 2005, 44, 4594.

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Table 3. UV/Vis and Emission Data for Compounds 1-4 and 6 Absorption λabs / nm (ε / dm3 mol-1 cm-1) 1a 1b 2a 2b 3a 3b 4a 4b 6

345(9280), 452(6330) 377(11 300), 396(11 600) 489(5840) 322(27 300), 444(8030) 331(25 100), 389(18 200), 478(5840) 323(23 600), 440(8910) 348(25 100), 465(6250) 327(24 200), 391(18 600) 377(23 100), 472(14 200) 309(19 100), 420(11 500), 492(6860)

Emission λem / nm glass 77 K

Emission λem / nm CH2Cl2 298 K

614 625 599 610 595 606 586 600 638

665 655 645

Figure 2. ORTEP drawing of 3a (thermal ellipsoids are drawn at the 50% probability; solvent of crystallization (CH2Cl2) and hydrogen atoms are omitted for clarity).

Figure 5. Emission spectra of 1a, 2a, and 3a in dichloromethane solution at 298 K.

Figure 3. ORTEP drawing of 4b (thermal ellipsoids are drawn at the 50% probability; hydrogen atoms are omitted for clarity).

Figure 4. UV/vis spectra of 1a, 2a, and 3a in dichloromethane.

stronger π-back-bonding between Ru(II) and RNC in the cis, cis,trans-isomers (as reflected in the shorter Ru-C(isocyanide) bond distances), which would result in lower lying dπ(Ru)

orbitals and hence in a higher energy MLCT transition. In the trans,trans,trans-isomers the two RNC ligands would compete for the same dπ(Ru) orbitals for π-back-bonding. The energy of this band also increases with the π-accepting ability of the isocyanide ligand: 2,4,6-Br3-C6H2NC (4a: 391 nm; 4b: 472 nm)>4-Cl-C6H4NC (3a: 440 nm; 3b: 465 nm) >4-MeOC6H4NC (2a: 444 nm; 2b: 478 nm) > tBuNC (1a: 452 nm; 1b: 489 nm) (Table 3). This trend is in agreement with the MLCT [dπ(Ru)fπ*(Q)] assignment since the better π-accepting Br3C6H2NC would better stabilize the dπ(Ru) orbital, thus giving rise to a higher energy MLCT [dπ(Ru)fπ*(Q)] transition. The increased stabilization of the dπ(Ru) orbital with increasing π-accepting ability of the isocyanide ligand is also reflected in the RuIII/II redox potential (vide infra). Similarly, the red-shift in the lower energy absorption for 6 compared to that of 4b can be explained by a better π-conjugation in the tolyl quinolinolate ligand, which renders the π*(Q) orbital lower lying in energy and hence lower in MLCT [dπ(Ru)fπ*(Q)] energy. Upon excitation at λ>350 nm, 1a-3a in dichloromethane solution exhibit orange-red luminescence (645-680 nm) (Figure 5). In 77 K EtOH/MeOH glass, complexes 1-4 and 6 produce intense structured emission spectra (593638 nm) (Figure 6). In contrast to the ligand-centered (LC) emissions of other 8-quinolinolato complexes, which occur in a similar region and are insensitive to the nature of the ancillary ligands or the metal center,23 these emissions show a similar energy dependence on the isomeric forms, π-accepting ability of the isocyanide, and conjugation in the quinolinolate ligand to the MLCT [dπ(Ru)fπ*(Q)] absorption. This energy dependence strongly suggests the involvement of the MLCT state. Therefore, with reference to previous spectroscopic studies on related 8-quinolinolato complexes,23 they are also tentatively assigned to be derived from the MLCT [dπ(Ru)fπ*(Q)] excited state with ligand-centered character.

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Figure 6. Emission spectra of 1a, 1b, 2a, 3a, and 4a in ethanol/ methanol (4:1) glass at 77 K. Table 4. Electrochemical Data and Experimental Details for Compounds 1-4 and 6

1a 1b 2a 2b 3a 3b 4a 4b

RuIII/IIE1/2/ V (ΔE/mV)a

RuIV/IIIEpa/Va

0.02 (62) -0.16 (60) 0.17b 0.01 (63) 0.24b 0.03 (64) 0.38b 0.065 (75)

0.68 0.72

Figure 7. Cyclic voltammogram of 1a and 1b in 0.1 M nBu4NPF6 in acetonitrile. Scan rate is 50 mV s-1.

0.71 0.70 0.85

Versus Fcþ/Fc. b Quasi-reversible or irreversible couple and only Epa reported. All potentials were obtained in 0.1 M nBu4NPF6 in acetonitrile, except 4a, for which 0.1 M nBu4NPF6 in CH2Cl2 is used. a

Electrochemical Properties of RuQ2(RNC)2. The electrochemical data for compounds 1-4 are presented in Table 4. The cyclic voltammograms (CV) of the cis,cis,trans-isomers (1a-4a) generally exhibit an irreversible or quasi-reversible RuIII/II couple at the potential range 0.02-0.38 V vs Fcþ/Fc, except in the case of 1a, where a reversible RuIII/II and a quasi-reversible RuIV/III (0.68 V vs Fcþ/Fc) couple are observed. The potential for the RuIII/II couple increases with the π-accepting ability of the isocyanide ligands, 2,4,6-Br3-C6H2NC (4a: 0.38 V) > 4-Cl-C6H4NC (3a: 0.24 V) > 4-MeOC6H4NC (2a: 0.17 V) > tBuNC (1a: 0.02 V), in accordance with stabilization of Ru(II) through π-back-bonding with isocyanide. In the CV of the trans,trans,trans-isomers 1b-4b, a reversible RuIII/II (-0.16 to 0.065 V) and quasi-reversible RuIV/III (0.70-0.85 V) couple are observed. The potential for the RuIII/II couple in 1b-4b also increases with the π-accepting ability of the isocyanide ligand, and as expected from the weaker π-back-bonding between Ru(II) and isocyanide in these complexes, the potentials are lower than that of the corresponding cis,cis,trans-isomers 1a-4a. The potential of the RuIV/III couple is relatively insensitive to the nature of the isocyanide ligand, except in 4b, where the

Figure 8. Cyclic voltammogram of 2a and 2b in 0.1 M nBu4NPF6 in acetonitrile Scan rate is 50 mV s-1.

presence of three bromo substituents in the isocyanide ligand should make it much less electron-donating, and hence the potential of the RuVI/III couple is increased. In conclusion, we report here general synthetic routes for the preparation of a series of neutral ruthenium complexes of general formula [RuQ2(RNC)2]. These complexes are air stable and soluble in a variety of organic solvents, and they possess interesting photophysical properties. They are therefore potentially useful as materials for electroluminescent devices.

Acknowledgment. The work described in this paper was supported by the Research Grants Council of Hong Kong (CityU 2/06C) and the City University of Hong Kong (7002095). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.