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.