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.