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-p)(2)] (X = OMe (1), Me (2), H (3), F (4), Cl(5); 16-TMC = 1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane) are prepared by the reaction of [Ru-III(16-TMC)Cl-2]Cl with the corresponding alkyne and NaOMe in the presence of zinc amalgam. Low n(C=C) stretching frequencies are observed for 1-5 and are attributed to the sigma-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 Cr C bond distances (mean 2.076 and 1.194 Angstrom, respectively). The cyclic voltammograms of 1-5 show quasi-reversible Ru-III/II and Ru-IV/III oxidation 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 lambda(max) at 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 p(pi)(ArC=C) --> d(pi*)(Ru-III) charge transfer is proposed for this transition.