Rate Studies on the Release of Terminal Alkynes from Indenylruthenium(II) Vinylidene Complexes. Implications on the Mechanism of eta-1-Vinylidene into eta-2-Alkyne Isomerization

The indenyl-ruthenium(II) vinylidene complexes [Ru{=C=C(H)R}(?5-C9H7)(PPh3)2][PF6] (R = Ph (1), 4-MeO-C6H4 (2), 4-Me-C6H4 (3), 4-Ph-C6H4 (4), 4-F-C6H4 (5), 4-Cl-C6H4 (6), 4-I-C6H4 (7), 4-MeCO-C6H4 (8), 4-O2N-C6H4 (9), nBu (10), (eta-5-C5H4)Fe(eta-5-C5H5) (11, Fc) have been synthesized from [RuCl(C9H7)(PPh3)2] and RCCH, in a methanolic solution of NaPF6 at rt. The complexes, when dissolved in acetonitrile-d3, release the vinylidene ligand upon thermal activation in the form of the corresponding terminal alkyne, with formation of the solvato complex [Ru(NCCD3)(eta-5-C9H7)(PPh3)2][PF6] (1a). The reactions, followed by 31P{1H} NMR spectroscopy, exhibit first-order behaviour in the vinylidene substrates, and the activation parameters deltaH = 24 ± 1 kcal mol-1 and deltaS = -3 ± 2 cal mol-1 K-1 for complex 1 (36 - 54 °C). The Hammett plot derived from the aryl substituted complexes yields the reaction parameter rho = - 1.5, indicating a rate enhancement effect by electron donor substituents on the beta-carbon atom. Formation of the vinylidene complex 1 from PhCC(13)H as well as release of the alkyne proceed by exclusive 1,2-shift of the hydrogen atom, as determined by the reactions of [RuCl(eta-5-C9H7)(PPh3)2] with the 13C labelled alkyne and of the C(alfa)-enriched vinylidene complex [Ru{=13C=C(H)Ph}(eta-5-C9H7)(PPh3)2][PF6]. The vinylidene moiety undergoes rapid H/D exchange with D2O/H2O at rt, while the presence of a nitrogen base transforms complex 4 into the corresponding neutral acetylide derivative. The reactions of complexes 4, 10 and 11, performed in solvent mixtures CD3CN/H(D)2O, exhibit primary kinetic isotopic effects in the range kH/kD = 1.17 - 1.88, in agreement with an intramolecular 1,2-hydrogen shift mechanism characterized by a non-linear C-H(D)-C structure of the rate determining transition state.