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Reversible binding of dihydrogen to dinuclear ruthenium complexes containing chelating diphosphines Chau, Daniel Elliot Kwok-Yue

Abstract

The preparation of dinuclear mixed-valence ruthenium complexes of general formula Ru₂Cl₅(P-P)₂, previously reported for a range of chelating ditertiary phosphines including chiral systems, has been extended to include compounds of the diphosphine 1,4-bis(dicyclohexylphosphino)butane (P-P = DCYPB). The complexes are generally prepared by the reaction of RuCl₃(PR₃)₂(DMA)‧DMA, R = phenyl or p-tolyl, DMA = N,N-dimethylacetamide, with one equilvalent of the appropriate diphosphine. The H₂-reduction of Ru₂Cl₅(P-P)₂ complexes in DMA, or in toluene in the presence of an added base, generally gives the dinuclear Ru(II,II) complexes [RuCl(P-P)(μ-Cl)]₂, but the DCYPB species was not formed; the procedures with DCYPB (including studies in CH₂Cl₂) led to poorly characterised complexes which may be polynuclear polyhydrides and appear to contain an η²-H₂ ligand. ¹H and ³¹P{¹H}NMR spectroscopy was used extensively to study the systems. The previously reported reaction of the dinuclear ruthenium(II) complex, [RuCl(DPPB)(μ-Cl)]₂ (DPPB = 1,4-bis(diphenylphosphino)butane), with 1 atm H₂ at ambient temperature to give the molecular hydrogen complex (η²-H₂)Ru(DPPB)(μ-Cl)₃RuCl(DPPB), has been extend to the 1,3-bis(diphenylphosphino)propane (DPPP) system; a corresponding η²-H₂ species with the DCYPB ligand has been detected following reaction of Ru₂Cl₅(DCYPB)₂ with H₂. The η²-H₂ ligands in the DPPP and DCYPB systems have internuclear distances of 0.86 Å as estimated by ¹H NMR variable temperature spin-lattice relaxation data (T₁(min) = 12 ms at 300 MHz). With the DPPP system, a ¹JHD of 29.4 Hz was found for the η²-HD isotopomer, and the η²-H₂ ligand is replaceable by N₂. The reversible equilibrium conversion under H₂ of [RuCl(DPPB)₂(μ-Cl)]₂, 9, to (η²-H₂)Ru(DPPB)(μ-Cl)₃RuCl(DPPB), 9a , in CH₂Cl₂ has been studied at 0-25°C by UV-vis and ³¹P{¹H}NMR spectroscopy, and by stopped-flow kinetics; the equilibrium constant and the corresponding thermodynamic parameters, and the forward and reverse rate constants have been determined. The equilibrium is close to being thermally neutral (ΔH° ~ zero), while a ΔS° value of ~60 J/mol K indicates the involvement of solvent molecules. A rough estimate of -60 kJ/mol is made for ΔH° of the process RuII + H₂ → RuII(η²-H₂). Preliminary studies were made on the reaction of [RuCl(DPPB)₂(μ-Cl)]₂, 9, with O₂, which may give a peroxide, and the subsequent reaction of this product with H₂; the latter reaction results in the formation of water and regeneration of a mixture of 9 and 9a.

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