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

The (tricyclic arene)ruthenium(0) complex [Ru(eta(4)-1,5-COD)(eta(6)-phenanthrene)] (4) (COD cyclooetadiene) is prepared by reduction of [Ru(acac)(2)(eta(4)-1,5-COD)] (3) (acac = acetylacetonato) with sodium phenanthrene in 15% yield. Similar treatment of 3 with sodium anthracene gives a mixture of [Ru(eta(4)-1,5-COD)(eta(6)-anthracene)] (6) and [Ru(eta(4)-1,5-COD)(eta(6)-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(eta(4) 1,5-COD)(eta(6)-phenanthrene)]PF6, [5]PF6, in 53% yield, whereas similar protonation of 7 gives an equilibrium mixture of a hydrido diene complex [RuH(eta(4)-1,5-COD)(eta(6)-9,10-dihydroanthracene)]PF6, [8]PF6, and an agostic cyclooctenyl complex [Ru(eta(1),eta(3)-C8H13)(eta(6)-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-d(6), have been derived from variable-temperature NMR experiments; Delta(H)degrees = -12 +/- 1 kJ mol(-1), DeltaGdegrees = 1 +/- 2 kJ mol(-1), and DeltaSdegrees = -44 +/- 4 J K-1 mol(-1). The large negative entropy is consistent with the agostic formulation. As expected, the eta(3)-cyclooctenyl complex can be trapped as [Ru(1-3-eta(3)-C8H13)(eta(6)-9,10-dihydroanthracene)(CO)JPF(6), [10]PF6, in 80% yield on exposure of the equilibrium mixture of [S]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.