Abstract
Abstract: The effect of 1
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-i;2-arene)]2+
(arene= phenol; 4-, 5-, 6-methylphenol; 4,5-dimethylphenol) the corresponding equilibrium constants approach unity (20 °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 ster-
eocontrolled, 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(m-j)2:»;2-L)]4+ where L= 2,4- or 2,5-cyclohexadien-1 -one.
