Theoretical Mechanistic Study of Rhodium(I) Phosphine-Catalyzed H/D Exchange Processes in Aqueous Solutions

The mechanism of hydrogen isotope exchange between a catalytically active rhodium dihydride ([RhH 2 Cl(PR 3 ) 3 ]) and acidic water has been investigated by means of density functional calculations carried out for model complexes interacting with protonated water clusters. The protonation of hydride ligands by hydrated H 3 O + is shown to occur via dihydrogen-bonded adducts, which can easily transform to cationic hydrido-dihydrogen species ([RhH(H 2 )Cl(PR 3 ) 3 ] + ). The structure and possible internal rearrangements of this intermediate are characterized, and a low barrier (∼3 kcal/mol) for η 2 -H 2 rotation is found. The energy gap for the protonation/deprotonation process is estimated to be in the same range. The structures and the relative stabilities of the involved species suggest that the leaving proton in the deprotonation step may either shift back to the original water molecule or alternatively transfer to another H 2 O.