Rate studies on the release of terminal alkynes from indenylruthenium(II) vinylidene complexes.: Implications on the mechanism of η1-vinylidene into η2-alkyne isomerization

The indenylruthenium(II) vinylidene complexes [Ru { =C=C(H)R} (eta(5)-C9H7)(PPh3)(2)][PF6] [R = Ph (1), 4-MeOC6H4 (2), 4-MeC6H4 (3), 4-PhC6H4 (4), 4-FC6H4 (5), 4-ClC6H4 (6), 4-IC6H4 (7), 4-MeCOC6H4 (8), 4-O2NC6H4 (9), Bu-n (10), (eta(5)-C5H4)Fe(eta(5)-C5H5) (11, Fc)] have been synthesized from [RuCl(eta(5)- C9H7)(PPh3)(2)] and RC CH, in a methanolic solution of NaPF6 at room temperature. The complexes, when dissolved in acetonitrile-d(3), release the vinylidene ligand upon thermal activation in the form of the corresponding terminal alkyne, with formation of the solvato complex [Ru(N CCD3)(eta(5)- C9H7)(PPh3)(2)][PF6] (1a). The reactions, followed by P-31{H-1} NMR spectroscopy, exhibit first-order behavior in the vinylidene substrates, and the activation parameters Delta H-double dagger = 24 +/- 1kcal mol(-1) and Delta S-double dagger = -3 +/- 2 cal mol(-1) K-1 for complex 1 (36-54 degrees C). The Hammett plot derived from the aryl-substituted complexes yields the reaction parameter rho = -1.5, indicating a rate enhancement effect by electrondonor substituents on the beta-C atom. Formation of the vinylidene complex 1 from PhC (CH)-C-13 as well as release of the alkyne proceeds by an exclusive 1,2 shift of the H atom, as determined by the reactions of [RuCl(eta(5)-C9H7)(PPh3)(2)] with the C-13-labeled alkyne and of the C-alpha-enriched vinylidene complex [Ru { = C-13=C(H)Ph}(eta(5)-C9H7)(PPh3)(2)] [PF6]. The vinylidene moiety undergoes rapid H/D exchange with D2O/H2O at room temperature, 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)(2)O, exhibit primary kinetic isotopic effects in the range k(H)/k(D) = 1.17-1.88, in agreement with an intramolecular 1,2 hydrogen shift mechanism characterized by a nonlinear C-H(D)-C structure of the rate-determining transition state.