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

Abstract

Complexes of the form RhX(CO)(PR3)2 [X = Cl, Br or I; R = Me or Ph] reacted with H2 to form a series of binuclear complexes of the type (PR3)2H2Rh(μ-X)2Rh(CO)(PR 3) [X = Cl, Br or I, R = Ph; X = I, R = Me] and (PMe3)2(X)HRh(μ-H)(μ-X)Rh(CO)(PMe3) [X = Cl, Br or I] according to parahydrogen sensitised 1H, 13C, 31P and 103Rh NMR spectroscopy. Analogous complexes containing mixed halide bridges (PPh3)2H2Rh(μ-X)(μ-Y)Rh(CO)(PPh 3) [X, Y = Cl, Br or I; X ≠ Y] are detected when RhX(CO)(PPh3)2 and RhY(CO)(PPh3)2 are warmed together with p-H2. In these reactions only one isomer of the products (PPh3)2H2Rh(μ-I)(μ-Cl)Rh(CO)(PPh 3) and (PPh3)2H2Rh(μ-I)(μ-Br)Rh(CO)(PPh 3) is formed in which the μ-iodide is trans to the CO ligand of the rhodium(I) centre. When (PPh3)2H2Rh(μ-Cl)(μ-Br)Rh(CO)(PPh 3) is produced in the same way two isomers are observed. The mechanism of the hydrogen addition reaction is complex and involves initial formation of RhH2X(CO)(PR3)2 [R = Ph or Me], followed by CO loss to yield RhH2X(PR3)2. This intermediate 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)2H2Rh(μ-X)2Rh(CO)(PPh 3) systems are shown to undergo hydride self exchange by exchange spectroscopy with rates of 13.7 s-1 for the (μ-Cl)2 complex and 2.5 s-1 for the (μ-I)2 complex at 313 K. Activation parameters indicate that ordering dominates up to the rate determining step; for the (μ-Cl)2 system ΔH‡ = 52 ± 9 kJ mol-1 and ΔS‡ = -61 ± 27 J K-1 mol-1. This process most likely proceeds via halide bridge opening at the rhodium(III) centre, rotation of the rhodium(III) fragment around the remaining halide bond and bridge re-establishment. If the triphenylphosphine ligands are replaced by trimethylphosphine distinctly different reactivity is observed. When RhX(CO)(PMe3)2 [X = Cl or Br] is warmed with p-H2 the complex (PMe3)2(X)HRh(μ-H)(μ-X)Rh(CO)(PMe3) [X = Cl or Br] is detected which contains a bridging hydride trans to the rhodium(I) PMe3 ligand. However, when X = I, the situation is far more complex, with (PMe3)2H2Rh(μ-I)2Rh(CO)(PMe 3) observed preferentially at low temperatures and (PMe3)2(I)HRh(μ-H)(μ-I)Rh(CO)(PMe3) at higher temperatures. Additional binuclear products corresponding to a second isomer of (PMe3)2(I)HRh(μ-H)(μ-I)Rh(CO)(PMe3), in which the bridging hydride is trans to the rhodium(I) CO ligand, and (PMe3)2HRh(μ-H)(μ-I)2Rh(CO)(PMe 3) are also observed in this reaction. The relative stabilities of related systems containing the phosphine PH3 have been calculated using approximate density functional theory. In each case, the (μ-X)2 complex is found to be the most stable, followed by the (μ-H)(μ-X) species with hydride trans to PH3. © The Royal Society of Chemistry 1999