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Synthesis and Photophysical Properties of Ruthenium(II) Isocyanide Complexes Containing 8-Quinolinolate Ligands

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posted on 12.10.2009, 00:00 by Chi-Fai Leung, Siu-Mui Ng, Jing Xiang, Wai-Yeung Wong, Michael Hon-Wah Lam, Chi-Chiu Ko, Tai-Chu Lau
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 14. 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 π→π* transitions of the quinolinolate ligands, probably mixed with the π→π* transitions of the isocyanide ligands. The lower energy absorptions at 400−492 nm are assigned to Ru(dπ)→π*(Q) metal-to-ligand charge transfer (MLCT) transitions. Upon excitation at λ > 350 nm, 1a3a in dichloromethane solution exhibit orange-red luminescence (645−680 nm). In 77 K EtOH/MeOH glass, complexes 14 and 6 give intense structured emission spectra (593−638 nm). The cyclic voltammograms (CV) of 1a4a generally exhibit an irreversible or quasi-reversible RuIII/II couple at the potential range of 0.02−0.38 V vs Fc+/Fc, except in the case of 1a, where a reversible RuIII/II and a quasi-reversible RuIV/III (0.68 V vs Fc+/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 1b4b, a reversible RuIII/II (−0.16 to 0.065 V) and quasi-reversible RuIV/III (0.70−0.85 V) couple are observed.