Activation of H-2 by halogenocarbonylbis(phosphine)rhodium(I) complexes. The use of parahydrogen induced polarisation to detect species present at low concentration

Complexes of the form RhX(CO)(PR3)(2) [X=Cl, Br or I; R=Me or Ph] reacted w ith H-2 to form a series of binuclear complexes of the type (PR3)(2)H2Rh(mu -X)(2)Rh(CO)(PR3) [X=Cl, Br or I, R=Ph; X=I, R=Me] and (PMe3)(2)(X)HRh(mu-H )(mu- X)Rh(CO)(PMe3) [X=Cl, Br or I] according to parahydrogen sensitised H -1, C-13, P-31 and Rh-103 NMR spectroscopy. Analogous complexes containing mixed halide bridges (PPh3)(2)H2Rh(mu-X)(mu-Y)Rh(CO)(PPh3) [X, Y = Cl, Br o r I; X not equal Y] are detected when RhX(CO)(PPh3)(2) and RhY(CO)(PPh3)(2) are warmed together with p-H-2. In these reactions only one isomer of the products (PPh3)(2)H2Rh(mu-I)(mu-Cl)Rh(CO)(PPh3) and (PPh3)(2)H2Rh(mu-I)(mu- Br)Rh(CO)(PPh3) is formed in which the mu-iodide is trans to the CO ligand of the rhodium(I) centre. When (PPh3)(2)H2Rh(mu-Cl)(mu-Br)Rh(CO)(PPh3) is p roduced in the same way two isomers are observed. The mechanism of the hydr ogen addition reaction is complex and involves initial formation of RhH2X(C O)(PR3)(2) [R=Ph or Me], followed by CO loss to yield RhH2X(PR3)(2). This i ntermediate is then attacked by the halide of a precursor complex to form a binuclear species which yields the final product after PR3 loss. The (PPh3 )(2)H2Rh(mu-X)(2)Rh(CO)(PPh3) systems are shown to undergo hydride self exc hange by exchange spectroscopy with rates of 13.7 s(-1) for the (mu-Cl)(2) complex and 2.5 s(-1) for the (mu-I)(2) complex at 313 K. Activation parame ters indicate that ordering dominates up to the rate determining step; for the (mu-Cl)(2) system Delta H double dagger =52 +/- 9 kJ mol(-1) and Delta S double dagger -61 +/- 27 J K-1 mol(-1). This process most likely proceeds via halide bridge opening at the rhodium(III) centre, rotation of the rhod ium(III) fragment around the remaining halide bond and bridge re-establishm ent. If the triphenylphosphine ligands are replaced by trimethylphosphine d istinctly different reactivity is observed. When RhX(CO)(PMe3)(2) [X=Cl or Br] is warmed with p-H-2 the complex (PMe3)(2)(X)HRh(mu-H)(mu-X)Rh(CO)(PMe3 ) [X=Cl or Br] is detected which contains a bridging hydride trans to the r hodium(I) PMe3 ligand. However, when X=I, the situation is far more complex , with (PMe3)(2)H2Rh(mu-I)(2)Rh(CO)(PMe3) observed preferentially at low te mperatures and (PMe3)(2)(I)HRh(mu-H)(mu-I)Rh(CO)(PMe3) at higher temperatur es. Additional binuclear products corresponding to a second isomer of (PMe3 )(2)(I)HRh(mu-H)(mu-I)Rh(CO)(PMe3), in which the bridging hydride is trans to the rhodium(I) CO ligand, and (PMe3)(2)HRh(mu-H)(mu-I)(2)Rh(CO)(PMe3) ar e also observed in this reaction. The relative stabilities of related syste ms containing the phosphine PH3 have been calculated using approximate dens ity functional theory. In each case, the (mu-X)(2) complex is found to be t he most stable, followed by the (mu-H)(mu-X) species with hydride trans to PH3.