PHENOL CYCLOHEXADIENONE EQUILIBRIUM FOR ETA-2-COORDINATED ARENES

The effect of eta-2 coordination on an arene is explored in the context of the phenol-ketodiene equilibrium. Whereas for the free ligand this equilibrium heavily favors the phenol tautomer, we find that for the complexes [Os(NH3)5(2,3-eta-2-arene)]2+ (arene = phenol; 4-, 5-, 6-methylphenol; 4,5-dimethylphenol) the corresponding equilibrium constants approach unity (20-degrees-C). Starting with the phenolic isomer in methanol, conversion to 2,4-cyclohexen-1-one is kinetically favored over the formation of the 2,5-analogue, although the latter is the thermodynamically favored product. All tautomerization processes are stereocontrolled, with protonation and deprotonation occurring selectively at the ring face opposite the metal. Electrochemical studies indicate that the phenol-2,5-dienone equilibrium is not profoundly influenced by complexation of osmium(III). In addition to the mononuclear species, where L = phenol, two binuclear complexes are characterized of the form [{Os(NH3)5}2(mu-eta-2-eta-2-L)]4+ where L = 2,4- or 2,5-cyclohexadien-1-one.