Synthesis and photophysical properties of ruthenium(II) isocyanide complexes containing 8-quinolinolate ligands

A series of ruthenium(II) bis(8-quinolinolato)complexes bearing isocyanide ligands (RNC) have been synthesized by thereaction 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 forcomplexes 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]. Thestructures of 2a, 3a, and 4b have been determined by X-ray crystallography.These complexes exhibit an intense absorption band in the UV region (λ_max = 320-390 nm) with molar extinction coefficients(ε) on the order of 10⁴ dm³ mol⁻¹ cm⁻¹ and a moderately intense absorption with e on theorder 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 quinolinolateligands, 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, la-3a in dichloromethane solution exhibit orange-red luminescence(645-680 nm). In 77 K EtOH/MeOH glass, complexes 1-4 and 6 give intensestructured emission spectra (593-638 nm). The cyclic voltammograms (CV) of 1a-4a generally exhibit an irreversible or quasi-reversible Ru^III/II couple atthe potential range of 0.02-0.38 V vs Fc⁺/Fc, except in the case of la, where areversible Ru^III/II and a quasi-reversible Ru^IV/III (0.68 V vs Fc⁺/Fc) coupleare observed. The potential for the Ru^III/II couple increases with theπ-accepting ability of the isocyanide ligands. In the CV of 1b-4b, a reversibleRu^III/II (-0.16 to 0.065 V) and quasi-reversible Ru^IV/III (0.70-0.85 V) coupleare observed. Copyright © 2009 American Chemical Society.