Aromatic Hydrogen Isotope Exchange Reactions Catalyzed by Iridium Complexes in Aqueous Solution

Sodium hexachloroiridate (III) and sodium hexachloroiridate (IV) have been used as homogeneous catalysts for hydrogen isotope exchange between benzenoid compounds and water. The ideal solvent consisted of 50 mole % acetic acid/water, and the optimum temperature was found to be 160°C. Under these conditions the rate of incorporation of deuterium into benzene was significant (typically 15% D in 6 h), and reduction to iridium metal was minimized. The active catalytic species was identified as a solvated iridium(III) species, which is also postulated to be the active catalyst in solutions containing hexachloroiridate (IV). The kinetics of exchange in benzene catalysed by sodium hexachloroiridate (III) were elucidated, and found to be more complex than for the corresponding sodium tetrachloroplatinate (II) catalysed exchange, in that a two-term rate dependence was found for catalyst concentration and the reaction was inversely dependent on hydrogen ion concentration. The reaction was found to be independent of chloride ion concentration, this confirming that the active catalyst is a solvated species. Isotopic labelling in all compounds was confined to the aromatic ring, and most substituted benzenes exhibited deactivation of the ortho positions, indicating that a dissociative π-complex exchange mechanism was operating. This was confirmed by exchange into naphthalene, where it was found that labelling was predominantly in the β position. Facile exchange into nitrobenzene provided good evidence of homogeneous catalysis, and not catalysis by precipitated metal.