Synthesis of and stereospecific hydride migration in cationic (tricyclic arene)(cyclooctadiene)ruthenium(II) complexes

The (tricyclic arene)ruthenium(0) complex [Ru(η⁴-1,5-COD)(η⁶-phenanthrene)] (4) (COD = cyclooctadiene) is prepared by reduction of [Ru(acac)₂(η⁴-1,5-COD)] (3) (acac = acetylacetonato) with sodium phenanthrene in 15% yield. Similar treatment of 3 with sodium anthracene gives a mixture of [Ru(η⁴-1,5-COD)(η⁶-anthracene)] (6) and [Ru(η⁴-1,5-COD)(η⁶-9,10-dihydroanthracene)] (7) in 3:1 molar ratio, from which only 7 can be isolated in a pure state. Protonation of 4 by HPF₆ yields the cationic hydridoruthenium(II) complex [RuH(η⁴-1,5-COD)(η⁶-phenanthrene)]PF₆, [5]PF₆, in 53% yield, whereas similar protonation of 7 gives an equilibrium mixture of a hydrido diene complex [RuH(η⁴-1,5-COD)(η⁶-9,10-dihydroanthracene)]PF ₆, [8]PF₆, and an agostic cyclooctenyl complex [Ru(η¹,η³-C₈H₁₃)(η⁶ -9,10-dihydroanthracene)]PF₆, [9]PF₆, in 4:1 molar ratio at 295 K in CD₂Cl₂, in which the endomethylene protons of the COD ligand and the agostic hydride in [9]PF₆ exchange rapidly on the NMR time scale, even at 193 K. Thermodynamic parameters for the equilibrium between [8]PF₆ and [9]PF₆ in acetone-d₆ have been derived from variable-temperature NMR experiments; ΔH° = -12 ± 1 kJ mol^-1, ΔG° = 1 ± 2 kJ mol^-1, and ΔS° = -44 ± 4 J K^-1 mol^-1. The large negative entropy is consistent with the agostic formulation. As expected, the η³-cyclooctenyl complex can be trapped as [Ru(1-3-η³-C₈H₁₃)(η⁶-9,10-di hydroanthracene)-(CO)]PF₆, [10]PF₆, in 80% yield on exposure of the equilibrium mixture of [8]PF₆ and [9]PF₆ to an atmosphere of CO. Addition of D₂O to the equilibrium mixture leads to selective facile deuteration of the hydride and endo-methylene protons of the 1,5-COD and cyclooctenyl ligands. Such endo-selective H/D exchange reaction is also observed for [5]PF₆.