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‘<sub>3</sub>)<sub>2</sub>Ru(CO)Cl{Ru} moiety, the coordination number at the metal (L = 4-ethylisonicotinate or a vacant coordination site) and the substituent R (R = <i><sup>n</sup></i><sup></sup>butyl, phenyl, 1-pyrenyl) have been varied. Structures of the enynyl complex Ru(CO)Cl(PPh<sub>3</sub>)<sub>2</sub>(η<sup>1</sup>:η<sup>2</sup>-<i><sup>n</sup></i><sup></sup>BuHCCHC⋮C<i><sup>n</sup></i><sup></sup>Bu), which results from the coupling of the hexenyl ligand of complex <b>1a</b> with another molecule of 1-hexyne, of the hexenyl complexes (<i><sup>n</sup></i><sup></sup>BuCHCH)Ru(CO)Cl(P<i><sup>i</sup></i><sup></sup>Pr<sub>3</sub>)<sub>2</sub> (<b>1c</b>) and (<i><sup>n</sup></i><sup></sup>BuCHCH)Ru(CO)Cl(PPh<sub>3</sub>)<sub>2</sub>(NC<sub>5</sub>H<sub>4</sub>COOEt-4) (<b>1b</b>), and of the pyrenyl complexes (1-Pyr-CHCH)Ru(CO)Cl(P<i><sup>i</sup></i><sup></sup>Pr<sub>3</sub>)<sub>2</sub> (<b>3c</b>) and (1-Pyr-CHCH)Ru(CO)Cl(PPh<sub>3</sub>)<sub>3</sub> (<b>3a</b>-<b>P</b>) 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 π-conjugation 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 π-system. ESR spectra of the electrogenerated radical cations display negligible <i>g</i>-value anisotropies and small deviations of the average <i>g</i>-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 <i><sup>n</sup></i><sup></sup>butyl to 1-pyrenyl.