Reactivity of Indenyl−Ruthenium(II) Vinylidene Complexes:  Selective Synthesis of Alkenyl−Phosphonio Derivatives via Nucleophilic Addition of Triphenylphosphine on Their η²-Alkyne Tautomers. Theoretical Study of the η¹-Vinylidene−η²-Alkyne Tautomerization

The activation of terminal alkynes with the halide derivatives [RuX(η 5 -1,2,3-R 3 C 9 H 4 )(CO)(PR 3 )] (R = Me, X = Br, PR 3 = PPh 3 ( 1 ), P i Pr 3 ( 3 ); R = H, X = I, PR 3 = P i Pr 3 ( 2 )) and AgBF 4 affords, in dichloromethane at room temperature, equilibrium mixtures containing the corresponding η 1 -vinylidene and η 2 -alkyne tautomers [Ru{=C C(H)R‘}(η 5 -1,2,3-R 3 C 9 H 4 )(CO)(PR 3 )][BF 4 ] ( 4a − c ) and [Ru(η 2 -HC⋮CR‘)(η 5 -1,2,3-R 3 C 9 H 4 )(CO)(PR 3 )][BF 4 ] ( 5a − c ), respectively. The reaction of 1 with AgBF 4 and phenylacetylene has been studied by variable-temperature 31 P{ 1 H} and 1 H NMR spectroscopy: at low temperature (−60 °C) the vinylidene complex [Ru{ C C(H)Ph}(η 5 -1,2,3-Me 3 C 9 H 4 )(CO)(PPh 3 )][BF 4 ] ( 4a ) is initially observed which upon warming (14 °C) forms an equilibrium with the π-alkyne derivative [Ru(η 2 -HC⋮CPh)(η 5 -1,2,3-Me 3 C 9 H 4 )(CO)(PPh 3 )][BF 4 ] ( 5a ). Treatment of this mixture with KO t Bu, in dichloromethane at room temperature selectively yields the neutral σ-alkynyl derivative [Ru(C⋮CPh)(η 5 -1,2,3-Me 3 C 9 H 4 )(CO)(PPh 3 )] ( 6 ) by displacement of the aforementioned equilibrium via deprotonation of the acidic vinylidene proton in 4a, while the addition of PPh 3 to this mixture stereoselectively affords the cationic alkenyl−phosphonio complex ( E )-[Ru{C(H) C(PPh 3 )Ph}(η 5 -1,2,3-Me 3 C 9 H 4 )(CO)(PPh 3 )][BF 4 ] [( E )- 7 ] via the nucleophilic attack of PPh 3 on the coordinated π-alkyne in 5a . In a similar fashion, compounds ( E )-[Ru{C(H) C(PPh 3 )R‘}(η 5 -1,2,3-R 3 C 9 H 4 )(CO)(PR 3 )][BF 4 ] (R = Me, PR 3 = PPh 3, R‘ = 1-cyclooctenyl [( E )- 13 ]; R = H, PR 3 = P i Pr 3, R‘ = Ph [( E )- 8 ], 1-cyclooctenyl [( E )- 14 ] can be selectively obtained by addition of PPh 3 to the corresponding reaction mixture. The monosubstituted alkenyl−vinylidene complexes [Ru{ C C(H)CH CRR‘}(η 5 -C 9 H 7 )(PPh 3 ) 2 ][BF 4 ] (R = R‘ = Ph ( 11a ); R = H, R‘ = (η 5 -C 5 H 4 )Fe(η 5 -C 5 H 5 ) [( E )- 11b ], 4-OMe-C 6 H 4 [( Z )- 11c ]) also react with triphenylphosphine, but in refluxing methanol, to afford the alkenyl−phosphonio derivatives ( EE )-[Ru{C(H) C(PPh 3 )CH CRR‘}(η 5 -C 9 H 7 )(PPh 3 ) 2 ][BF 4 ] ( 12a − c ) stereoselectively. The process also proceeds via an initial η 1 -vinylidene−η 2 -alkyne tautomerization followed by the nucleophilic attack of PPh 3 on the coordinated π-alkyne. The crystal structures of ( E )-[Ru{C(H) C(PPh 3 )Ph}(η 5 -1,2,3-Me 3 C 9 H 4 )(CO)(PPh 3 )][BF 4 ] (( E )- 7 ) and ( EE )-[Ru{C(H) C(PPh 3 )CH CH(η 5 -C 5 H 4 )Fe(η 5 -C 5 H 5 )}(η 5 -C 9 H 7 )(PPh 3 ) 2 ][BF 4 ] (( EE )- 12b ) have been determined by X-ray diffraction methods. Ab initio molecular orbital calculations on the η 1 -vinylidene to η 2 -alkyne tautomerization on the models [Ru(η 5 -C 9 H 7 )(PH 3 )L(C 2 H 2 )] + (L = CO, PH 3 ) are also reported.