Transition metal catalyzed D2/H2O exchange: Distinguishing between the single and double exchange pathways

Isotopic exchange between D2 (g) and H2O (l) is catalyzed by the water soluble complexes Rh(TPPTS)3Cl and Rh(TPPMS)3Cl (where TPPTS = tris(3-sulfonatophenyl)phosphine, trisodium salt and TPPMS = 3-sulfonatophenyldiphenylphosphine, sodium salt). This isotope exchange has been monitored by gas chromatographic analysis of the gaseous headspace over the aqueous catalyst solution. Upon binding to the catalyst, the hydrogen isotopologues can either exchange one or both atoms with solvent. Kinetic schemes have been developed for both situations. Analysis of exchange data demonstrates that for both catalysts at pH ≥ 8 the D2 molecule only undergoes one exchange event per visit to the active site. At lower pH values this changes. Rh(TPPMS)3Cl shows evidence of double exchange at pH 7 and below. Rh(TPPTS)3Cl shows evidence of double exchange at pH 6 and below. Rate constants for both single and double exchange can be determined and have been analyzed in the pH range from 3 to 12. Mechanistic implications of the rate constants are discussed.

Rhodium complexes of water soluble phosphines catalyze H/D exchange between D2 and H2O. This reaction has been monitored and the data have been fit to a model where either one or both atoms of D2 can exchange during one visit to the active site. The ratio of this single to double exchange varies with pH and mechanistic implications of this finding are discussed. Figure optionsDownload as PowerPoint slide