| Summary: | Mixed-valence complexes of ruthenium of general formula Ru₂CL₅(P-P)₂, P-P = chiraphos, norphos, dppp, dppb or diop, have been prepared by reaction of RuCL₃P₂(DMA).DMA, P = PPh₃ or P(p-tolyl)₃,with one equivalent of the appropriate ditertiary phosphine. The solid state structure of the chiraphos derivative is shown to be a symmetrical triply chloro-bridged dimer by X-ray analysis. The extent of electron delocalisation in these mixed-valence complexes has been studied by means of their intervalence transfer bands in the near-infrared region. A low energy absorption is assigned to the dinuclear complex? however, additional absorptions at higher energy indicate more than one species is present in solution.
Investigations of the reaction of Ru₂CL₅(chiraphos)₂ with H₂ in DMA show this to be an autocatalytic reaction, but attempts to elucidate mechanistic details have been unsuccessful. The product generated in situ by reaction with H₂ is the ionic species, [Ru₂CL₅(chiraphos)₂]⁻DMAH⁺. The neutral complexes [RuCL₂(P-P)]₂, P-P = chiraphos, dppp, dppb, or diop, are isolated via such an ionic complex, or alternatively by reaction in toluene in the presence of added base. N.m.r. studies show the neutral complexes to have structure I, but the complexes readily adopt a triply chloro-bridged structure (II) in the presence of coordinating solvents (S). [Formula Omitted] The complexes [RuCL₂(P-P)]₂ P-P = chiraphos or diop, catalyse the asymmetric hydrogenation of prochiral alkenes. The nature of the phosphine and substrate are found to be significant in terms of the hydrogenation rate, % e.e., and product configuration. Hydrogenation of (Z)-ɑ-acetamidocinnamic acid with 97% e.e. has been achieved using the chiraphos derivative. An unusual increase in % e.e. of the hydrogenated substrate with decreasing temperature is observed.
The reaction of Ru₂CL₄(dppb)₂(acetone).acetone, 1̲, with H₂ has been investigated. In DMA, in the presence of added base, a hydrido-complex is generated, but has not been isolated pure. The reaction of 1̲ with H₂ in CH₂/CL₂/CH₃OH produces the dinuclear complex, Ru₂H₂(CO)CL₂(dppb)₂ (III). This product is also [Formula Omitted] formed in the absence of H₂, and is thought to arise via base promoted decarbonylation of methanol.
Addition of norbornadiene (nbd) to 1̲ produces RuCL₂(nbd) (dppb),
which has been characterised by ¹H⁻ and ³¹P-n.m.r., and elemental and X-ray analysis. The reaction of this complex with H₂ is complicated by initial dissociation of the norbornadiene ligand. However, one of the minor products is thought to be RuHCL(nbd)(dppb), a stable hydrido-alkene complex, even in the presence of H₂. Cationic complexes of ruthenium(ll) have been prepared from 1̲ and Ru₂CL₅(dppb)₂ by halide abstraction using AgPF₆. These are the dinuclear complexes [Ru₂CL₃(dppb)₂(S)n]⁺PF₆⁻ , n = 2, S = acetonitrile and n = L, S = acetone; and the mononuclear complexes [RuCL(dppb)(CH₃CN₃) ]+PF₆⁻ and [RuCL(dppb) (ƞ⁶-toluene) ]+PF₆⁻. The reaction of [RuCL(dppb)(CH₃CN)₃]+PF₆⁻ with H₂, results in reduction of the acetonitrile ligands to triethylamine. The catalytic properties of this complex have also been investigated for the hydrogenation of the nitrile and imine substrates. === Science, Faculty of === Chemistry, Department of === Graduate
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