Structure and Dynamics of Half-Sandwich Ruthenium(IV) Alkynyl Hydrido Complexes^†

The coordinatively unsaturated complex [Cp*Ru(PMeiPr2)2][BAr‘4] (1; BAr‘4 = B{3,5-C6H3(CF3)2}4) reacts with 1-alkynes in diethyl ether at 0 °C, furnishing the RuIV alkynyl hydrido derivatives [Cp*RuH(C⋮CR)(PMeiPr2)2][BAr‘4]. In an analogous fashion, the reaction of 1 with 1-alkyn-3-ols in diethyl ether at 0 °C leads to the 3-hydroxyalkynyl hydrido complexes [Cp*RuH(C⋮CC(OH)RR‘)(PMeiPr2)2][BAr‘4]. The complexes [Cp*RuH(C⋮CCOOMe)(PMeiPr2)2][BAr‘4] and [Cp*RuH(C⋮CC(OH)Ph2)(PMeiPr2)2][BAr‘4]·Et2O have been structurally characterized, and their crystal structures show a transoid disposition of the hydride and alkynyl ligands. These compounds are stereochemically nonrigid and undergo a rapid equilibrium between possible stereoisomers in solution, which has been studied by variable-temperature NMR spectroscopy and computer simulation. The dynamic NMR study for these processes indicates energy barriers of ca. 11 kcal mol-1, and moreover the differences in energy for the possible stereoisomers are very small (ca. 1 kcal mol-1). The alkynyl hydrido complexes rearrange to their more stable vinylidene or hydroxyvinylidene tautomers. Spontaneous dehydration of the latter leads to either vinylvinylidene or allenylidene complexes.