AROMATIC HYDROGEN ISOTOPE-EXCHANGE REACTIONS CATALYZED BY IRIDIUM COMPLEXES IN AQUEOUS-SOLUTION

Sodium hexachloroiridate(III) and sodium hexachloroiridate(IV) have be en 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 degrees C. Under these conditions the rate of incorporation of deuteri um into benzene was significant (typically 15% D in 6 h): and reductio n to iridium metal was minimized. The active catalytic species was ide ntified as a solvated iridium(III) species, which is also postulated t o be the active catalyst in solutions containing hexachloroiridate(IV) . The kinetics of exchange in benzene catalysed by sodium hexachloroir idate(III) were elucidated, and found to be more complex than for the corresponding sodium tetrachloroplatinate(II) catalysed exchange, in t hat a two-term rate dependence was found for catalyst concentration an d 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. Isot opic labelling in all compounds was confined to the aromatic ring, and most substituted benzenes exhibited deactivation of the ortho positio ns, indicating that a dissociative pi-complex exchange mechanism was o perating. This was confirmed by exchange into naphthalene, where it wa s found that labelling was predominantly in the beta position. Facile exchange into nitrobenzene provided good evidence of homogeneous catal ysis, and not catalysis by precipitated metal.