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⋮CC 6 H 4 X- 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 ν(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 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⋮CC 6 H 4 X- 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 λ 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 π (ArC⋮C) → d π * (Ru III ) charge transfer is proposed for this transition.