Activation of Hydrogen on Iridium(I) Centers in Water: Solvent Effects, pH Effects, and Iridium−Hydride Exchange with D2O

The water-soluble iridium complexes trans -Ir(CO)L 2 X (X = Cl, OH; L = PPh 2 ( m -C 6 H 4 SO 3 K) (TPPMS), P( m -C 6 H 4 SO 3 Na) 3 (TPPTS)) have been used to extend the range of solvent effect for H 2 activation at an iridium(I) center and to examine the effect of pH changes on the rate of H 2 addition to an organometallic center. Hydrogen adds to the square-planar iridium(I) complexes in H 2 O and DMSO to give products analogous to those for trans -Ir(CO)(Cl)(PPh 3 ) 2 in organic solvents. The kinetic deuterium isotope effect, k H / k D = 1.1 (L = TPPMS) and 1.2 (L = TPPTS), for addition of H 2 to trans -Ir(CO)(Cl)L 2 in water is the same as that observed for trans -Ir(CO)(Cl)(PPh 3 ) 2 in toluene and indicates a common mechanism. More polar solvents accelerate the rate of H 2 addition to trans -Ir(CO)(Cl)L 2, indicating polarity in the transition state. Increasing the pH causes a decrease in the rate of H 2 addition for trans -Ir(CO)(Cl)(TPPMS) 2 and trans -Ir(CO)(Cl)(TPPTS) 2 in water. This pH effect, coupled with a shift of ν CO to higher frequency in water, is ascribed to protonation/hydrogen bonding to the iridium center. The hydride ligands undergo H/D exchange with D 2 O in a process that is quite dependent on the trans ligand.