Structure and dynamics of half-sandwich ruthenium(IV) alkynyl hydrido complexes

The coordinatively unsaturated complex [Cp*Ru((PMePr2)-Pr-i)(2)][BAr'(4)] (1; BAr'(4) = B{3,5-C6H3(CF3)(2)}(4)) reacts with 1-alkynes in diethyl ether at 0 degreesC, furnishing the Ru-IV alkynyl hydrido derivatives [Cp*RuH(Cequivalent toCR)((PMePr2)-Pr-i)(2)] [BAr'(4)]. In an analogous fashion, the reaction of I with 1-alkyn-3-ols in diethyl ether at 0 degreesC leads to the 3-hydroxyalkynyl hydrido complexes [Cp*RuH(Cequivalent toCC(OH)RR')((PMePr2)-Pr-i)(2)][BAr'(4)]. The complexes [Cp*RuH(Cequivalent toCCOOMe)((PMePr2)-Pr-i)(2)][BAr'(4)] and [Cp*RuH(Cequivalent toCC(OH)Ph-2)((PMePr2)-Pr-i)(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.