Activation of H2 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)(PR3) [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)(PPh3) [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)(PPh3) and (PPh3)2H2Rh(µ-I)(µ-Br)Rh(CO)(PPh3) 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)(PPh3) 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)(PPh3) 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)(PMe3) 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)(PMe3) 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.