MLCT and LMCT transitions in acetylide complexes. structural, spectroscopic, and redox properties of ruthenium(II) and -(III) Bis(σ-arylacetylide) complexes supported by a tetradentate macrocyclic tertiary amine ligand

Ruthenium(II) complexes trans-[Ru(16-TMC)(C≡CC6H4X-p2] (X = OMe (1), Me (2), H (3), F (4), Cl (5); 16-TMC = 1,5,9,13-tetramethyl-l,5,9,13-tetraazacyclohexadecane) are prepared by the reaction of [RuIII(16-TMC)Cl2]Cl with the corresponding alkyne and NaOMe in the presence of zinc amalgam. Low v(C≡C) stretching frequencies are observed for 1-5 and are attributed to the σ-donating nature of 16-TMC. The molecular structures of 1, 3, and 5 have been determined by X-ray crystal analyses, which reveal virtually identical Ru-C and C≡ C bond distances (mean 2.076 and 1.194 Å, respectively). The cyclic voltammograms of 1-5 show quasi-reversible RuIII/IIand RuIV/IIIoxidation couples. Oxidative cleavage of the acetylide ligand in 3 by dioxygen affords [Ru( 16-TMC)(C≡CPh)(CO)]+(6). Ruthenium(III) derivatives trans-[Ru(16-TMC)(C≡CC6H4X-p)2]+are generated in situ by electrochemical oxidation in dichloromethane or by chemical oxidation of 1-5 with Ce(IV). Their UV-visible absorption spectra show a vibronically structured absorption band with λmaxat 716-768 nm. The vibrational progressions, which range from 1730 to 1830 cm-1, imply that the electronic transition involves distortion of the acetylide ligand in the excited state. An assignment of pπ(ArC≡C) →dπ*(RuIII) charge transfer is proposed for this transition.