Ruthenium complexes with vinyl, styryl, and vinylpyrenyl ligands:: A case of non-innocence in organometallic chemistry

We herein,describe a systematic account of mononuclear ruthenium vinyl complexes L-{Ru}-CH CH-R where the phosphine ligands at the (PR'(3))(2)Ru(CO)Cl={Ru} moiety, the coordination number at the metal (L = 4-ethylisonicotinate or a vacant coordination site) and the substituent R (R = (n)butyl, phenyl, 1-pyrenyl) have been varied. Structures of the enynyl complex Ru(CO)Cl(PPh3)(2)(eta(1):eta 2-nBuHC = CHC equivalent to(CBu)-Bu-n), which results from the coupling of the hexenyl ligand of complex la with another molecule of 1-hexyne, of the hexenyl complexes ((BuCH)-Bu-n = CH)Ru(CO)Cl((PPr3)-Pr-i)(2) (1c) and ((BuCH)-Bu-n = CH)Ru(CO)Cl(PPh3)(2)(NC5H4COOEt-4) (1b), and of the pyrenyl complexes (1-Pyr-CH=CH)Ru(CO)Cl((PPr3)-Pr-i)(2) (3c) and (1-Pyr-CH=CH)Ru(CO)Cl(PPh3)(3) (3a-P) have been established by X-ray crystallography. All vinyl complexes undergo a one-electron oxidation at fairly low potentials and a second oxidation at more positive potentials. Anodic half-wave or peak potentials show a progressive shift to lower values as pi-coniugation within the vinyl ligand increases. Carbonyl band shifts of the metal-bonded CO ligand upon monooxidation are significantly smaller than is expected of a metal-centered oxidation process and are further diminished as the vinyl CH = CH entity is incorporated into a more extended pi-system. ESR spectra of the electrogenerated radical cations display negligible g-value anisotropies and small deviations of the average g-value from that of the free electron. The vinyl ligands thus strongly contribute to or even dominate the anodic oxidation processes. This renders them a class of truly non-innocent ligands in organometallic ruthenium chemistry. Experimental findings are fully supported by quantum chemical calculations: The contribution of the vinyl ligand to the HOMO increases from 46% (Ru-vinyl delocalized) to 84% (vinyl dominated) as R changes from (n)butyl to 1-pyrenyl.