New insights into catalytic hydrogenation by phosphido-substituted triruthenium clusters: confirmation of intact cluster catalysis by parahydrogen NMR

The phosphido-substituted triruthenium cluster Ru-3(CO)(9)(mu-H)(mu-PPh2) is shown to react with H-2 to form the trihydride cluster Ru-3(CO)(9)(H)(mu-H)(2)(mu-PPh2), which undergoes a number of re-arrangement reactions on heating to yield other phosphido-substituted triruthenium clusters. In the presence of alkyne substrates, heating the system leads to catalytic hydrogenation via CO loss and the formation of a Ru-3(eta(2)-PhC=CHPh)(CO)(8)(mu-H)(PHPh2) resting state, in a reaction affected by the polarity of the solvent. No mononuclear fragments are observed in the catalytic transformation, confirming directly that the phosphido ligand is able to exert a stabilising influence on the cluster core

High resolution imaging of plant tissues10

Connelly, A., Lohman, J. A. B., Loughman, B. C., Quiquampoix, H. and Ratcliffe, R. G. 1987. High resolution imaging of plant tissues by NMR.-J. exp. Bot. 38: 1713-1723.NMR images of living plant tissues were recorded at a 1H frequency of 200 MHz using a high resolution imaging technique that gave an in-plane pixel resolution of 50 μm × 50 μm or better. Images with interpretable contrast were obtained from germinating seeds, the roots of seedlings and the stems of young plants. The expected structural features of these tissues were readily observed including, in Mn2+ loaded tissue, the xylem vessels of maize root sections. Preliminary experiments on H2O-D2O exchange in maize roots, on the uptake of Mn2+ by maize roots and on the germination of seeds in situ demonstrate that the non-invasive method of NMR mini-imaging has the potential to complement existing techniques for physiological investigations in plant tissues. © 1987 Oxford University Press

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 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