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)₂Ru(CO)Cl{Ru} moiety, the coordination number at the metal (L = 4-ethylisonicotinate or a vacant coordination site) and the substituent R (R = ⁿbutyl, phenyl, 1-pyrenyl) have been varied. Structures of the enynyl complex Ru(CO)Cl(PPh₃)₂(η¹:η²-ⁿBuHCCHC⋮CⁿBu), which results from the coupling of the hexenyl ligand of complex 1a with another molecule of 1-hexyne, of the hexenyl complexes (ⁿBuCHCH)Ru(CO)Cl(PⁱPr₃)₂ (1c) and (ⁿBuCHCH)Ru(CO)Cl(PPh₃)₂(NC₅H₄COOEt-4) (1b), and of the pyrenyl complexes (1-Pyr-CHCH)Ru(CO)Cl(PⁱPr₃)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 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 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 ⁿbutyl to 1-pyrenyl.