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)]PF6, [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 endo-methylene 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-dihydroanthracene)(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.