Exchange couplings and hydrogen dynamics in ruthenium trihydride adducts with coinage-metal cations

Abstract

The reactions of AgBF ₄ and [Au(tht) ₂ ]PF ₆ with [Ru(η-C ₅ Me ₅ )H ₃ {P(C ₆ H ₁₁ ) ₃ }] 1 (tht = tetrahydrothiophene) led to the adducts [{Ru(η-C ₅ Me ₅ )H ₃ [P(C ₆ H ₁₁ ) ₃ ]} ₂ M]X (M = Ag, X = BF ₄ 3; M = Au, X = PF ₆ 4) similar to the previously reported [{Ru(η-C ₅ Me ₅ )H ₃ [P(C ₆ H ₁₁ ) ₃ ]} ₂ Cu]PF ₆ 2. Variable-temperature ¹ H NMR experiments were performed on 2–4 dissolved in organic liquids down to 130 K which complement previous experiments on 1. Like 1 and 2, 3 and 4 display at low temperatures exchange couplings between proton pairs in the trihydride sites which increase with temperature and which depend on the molecular structure. At higher temperatures a classical exchange between the hydride protons occurred the rate constants for which determined by lineshape analysis. As the coinage metal lowers the symmetry of the trihydride spin systems from the AB ₂ type in 1 to the ABC type in 2–4, the ¹ H NMR spectra also give information about the rate constants of an intramolecular metal transfer leading to an effective AB ₂ symmetry of the trihydride spins. The results indicate (i) the absence of kinetic hydrogen/deuterium isotope effects on the classical hydrogen-exchange processes within the margin of error in the temperature interval covered, (ii) only small effects of the presence of Lewis-acidic cation on the classical exchange dynamics, but (iii) important effects of these cations on the quantum-mechanical exchange couplings. In particular, these couplings increase with increasing electronegativity of the coinage metal by favouring dihydrogen configurations.